US20040154641A1

US20040154641A1 - Substrate processing apparatus and method

Info

Publication number: US20040154641A1
Application number: US10/150,748
Authority: US
Inventors: Garry Montierth
Original assignee: PCT Systems Inc
Current assignee: PCT Systems Inc
Priority date: 2002-05-17
Filing date: 2002-05-17
Publication date: 2004-08-12

Abstract

A method and apparatus for removing materials from a substrate contained in a closed, pressurized processing vessel. An exemplary method includes the steps of inserting a substrate to be processed into the processing vessel, closing the processing vessel gas-tight, and then pressurizing the processing vessel. A pressurized processing solution is introduced to the processing vessel while the processing vessel is maintained under pressure. The substrate is exposed to the processing solution, so that a processing step ensues and the desired material is removed from the substrate by reaction with the pressurized processing fluid. The processing is performed at greater than atmospheric pressure to maintain a high concentration of active components of the processing solution by increasing the solubility of active solution components, and to inhibit evaporation of volatile components of the processing solution.

Description

BACKGROUND OF THE INVENTION

The present invention relates to substrate processing. More specifically, the present invention relates to cleaning, etching, and stripping methods and apparatuses employed in the fabrication of semiconductor integrated circuits and devices.

Conventional integrated circuits are fabricated on semiconductor substrates. The fabrication process comprises a variety of steps, including the formation of photoresist films, which are used in a photolithography process to define parts of the integrated circuit. Following the photolithography process, a photoresist film must be removed (or “stripped”) before further processing can be performed. Various methods have been used to strip photoresist films from a substrate.

In one conventional method, a solution of sulfuric acid (H ₂SO₄) and hydrogen peroxide (H₂O₂) is applied to the photoresist film at a temperature of 120-140° C. The resulting Caro's acid (H₂SO₅) solution is an effective oxidizing solution, and because the photoresist film is an organic material, the Caro's solution operates to oxidize and remove the photoresist film. A drawback of this method is that H₂SO₄and H₂O₂are chemicals that are potentially harmful to individuals and the environment, especially at elevated temperatures. Since the conventional process vessels are generally open to the atmosphere, they vent fumes to the process tool air handling system, requiring large volumes of contaminated air to be collected and neutralized, and requiring safe disposal of the resulting salts. Further, it is becoming increasingly expensive to dispose of spent chemistries in an environmentally acceptable manner. An additional drawback of this conventional approach is that the removal rate is slow. A slow removal rate is undesirable, especially in a fabrication setting where removal rate has a direct and negative impact on throughput.

An alternative conventional approach that avoids the above chemical hazards and other problems associated with the above described stripping method is presented in U.S. Pat. No. 5,464,480. This patent describes use of ozonated water (i.e. ozone absorbed in deionized water) to remove organic materials, such as photoresist films, from a substrate. The concentration of ozone in the solution is increased by maintaining the solution below ambient temperature, e.g., within the range of 1° C. and 15° C. While this approach avoids problems associated with the previously described method, one of its primary drawbacks is that the removal rate is also low. The slow removal rate is attributable to the limited increase in solubility of ozone in water that is realized by maintaining the oxidizing solution below ambient temperature, and the reduction in reaction rate, which results from processing at sub-ambient temperatures.

A second type of wet processing step employed during the fabrication of semiconductor devices is the removal of inorganic material from a substrate. In one conventional etching process, silicon oxide is removed selective to an underlying material such as silicon through exposure to an aqueous hydrofluoric acid (HF) solution. The HF etches the oxide layer much more rapidly than other materials such as the underlying silicon. During this oxide removal process the concentration of HF can change, especially at elevated temperatures, altering the removal rate and selectivity exhibited of this wet processing step. One problem associated with conventional wet HF processing is nonuniformity of etching. For example, bubbles can become affixed to the surface of the substrate, blocking contact between the aqueous HF and the substrate in localized areas. A second problem sometimes associated with conventional HF wet processing is excessively high oxide removal rates, resulting in uneven etching and surface roughness.

A third type of wet processing frequently employed in the fabrication process is cleaning, and surface modification of wafers, wherein residual organic materials, particles and metals are removed from a substrate surface. One conventional cleaning sequence is sometimes referred to as the RCA washing method. This multi-step wet processing employs a series of five complementary chemical baths to remove the residual organic materials, particles and metals. In a first step, the substrate is subjected to a heated aqueous bath of H ₂SO₄and H₂O₂to remove residual organic materials, for example developed photoresist material remaining on a substrate surface. In a second step, the substrate is subjected to a dilute aqueous HF bath at room temperature to remove the oxide layer and impurities contained therein. In a third step, the substrate is subjected to a heated aqueous bath of ammonium hydroxide (NH₄OH) and H₂O₂, to remove particles and other contaminants. In a fourth step, the substrate is subjected to a heated aqueous bath of hydrochloric acid (HCl) and H₂O₂, to remove metals. Finally, in the fifth step, the substrate is again subjected to a bath of dilute hydrofluoric acid (HF) to remove the oxide layer formed by oxidation in the prior step, freeing metallic contaminants embedded in the oxide layer and permitting their removal, and rendering the surface of the wafer hydrophobic.

As true with the H ₂SO₄/H₂O₂organic stripping method described above, one drawback of the RCA washing method is that the chemicals used, i.e. H₂SO₄, NH₄OH, H₂O₂, HCl and HF, are potentially harmful to individuals and the environment, and are expensive to collect, neutralize, and dispose of safely. Additionally, because many of these chemical solutions are volatile or contain a dissolved volatile gas component in the aqueous phase and not contained in a closed vessel during processing, a change in concentration in the processing solution may occur over the course of processing, potentially altering the performance and outcome of the wet processing.

In “Ecofriendly Ozone-Based Wet Processes for Electron Device Fabrication”, Mitsubishi Electric ADVANCE, Kanegae proposes using ozonated water, and ozonated water containing small amounts of HF, as a substitute for the series of chemicals used in the conventional RCA washing method described above. This method is beneficial in that it reduces the number of potentially hazardous chemicals which must be employed during processing and then disposed of. However, this approach is undesirable in that its processing rates are no faster than those associated with the conventional sequences described above.

In another type of wet processing, surfaces of substrates being prepared as panels for liquid crystal displays (LCD's) are stripped using a mixture of organic solvents such as dimethyl sulfoxide (DMSO) and monomethanol amine (MEA) at elevated temperatures. This application for wet processing raises many of the same issues described above. Specifically, fumes and spent liquid chemistries generated during processing must be collected, treated, and disposed of at substantial expense. Moreover, over time the volatile components of the processing chemistry may be lost, undesirably altering the composition of the wet processing chemistry and the uniformity, repeatability and effectiveness of its performance.

Based on the above, there is a need in the art for improved methods and apparatuses for performing wet chemical cleaning, stripping, and etching steps during the fabrication of semiconductor devices.

SUMMARY OF THE INVENTION

The present invention is directed at methods and apparatus for removing materials from a substrate during, for example, the process of fabricating integrated circuits on a semiconductor substrate.

An embodiment of a method of removing a material from a surface of a substrate in accordance with the present invention comprises inserting the substrate including the material into an open processing vessel, closing the processing vessel gas-tight, and pressurizing the processing vessel to greater than atmospheric pressure. A pressurized processing solution is introduced into the pressurized processing vessel so that a surface of the substrate is exposed to the pressurized processing solution, and the substrate is processed to remove the material from the surface of the substrate by allowing the pressurized processing fluid to react with the material. The processing solution is maintained at higher than atmospheric pressure during introduction into the processing vessel and during at least part of the processing of the substrate.

An embodiment of a method of drying a substrate comprises positioning a substrate within a gas-tight processing vessel, and pressurizing the processing vessel to greater than atmospheric pressure. A pressurized rinsing liquid is introduced into the processing vessel, with at leas one of a component of the rinsing liquid or a pressurizing gas comprising a surface-tension lowering component concentrated at a surface of the rinsing solution. The substrate is submerged within the rinsing liquid; and the processed substrate is moved relative to the rinsing liquid such that a surface tension gradient is created between the rinsing liquid at a meniscus attached to the substrate surface and the remaining bulk portion of the rinsing liquid, the surface tension gradient drawing liquid from the substrate surface into the bulk rinsing liquid.

An embodiment of an apparatus for removing material from the surface of a substrate in accordance with the present invention comprises a processing vessel configured to receive and contain a substrate in a gas-tight sealed environment; and a source of a pressurized processing solution in fluid communication with the processing vessel through an inlet valve; A drain valve enables fluid communication of the processing vessel with a drain. A pressurized holding vessel is configured to maintain a pressurized processing solution having a concentration of a component greater than available in the solution at atmospheric pressure. A control valve is coupled between the processing vessel and the holding vessel for controlling flow of the processing solution from the holding vessel and to the processing vessel.

The identity of the material to be removed from the substrate will vary according to the particular application. In one application, the material to be removed may comprise an organic material such as a photoresist, and the processing solution may comprise ozone absorbed at high concentration in an acidic solution. The ozone concentration absorbed into the solution is greater than could occur at atmospheric pressure and at the same temperature because of the increased solubility of the gas in a pressurized solution, and because of inhibition of outgassing of ozone from the closed system. The higher concentration of ozone in the processing solution, at a given temperature, allows for faster processing rates than are achievable using conventional methods.

In another application for the present invention, material to be removed from the substrate may comprise an inorganic material, for example a metal or dielectric. In such an embodiment, the processing solution may comprise an acidic or basic solution subjected to greater than atmospheric pressure. The resulting concentration of acid or base in the pressurized solution may be maintained at high temperatures, thereby enhancing reaction and increasing removal rate. Due to the closed nature of the system employed for the wet processing in accordance with embodiments of the present invention, the increased removal rate occurs without corresponding loss of volatile solution components through evaporation or outgassing associated with conventional high temperature wet processing systems.

In still another potential application for the present invention, the surface of a substrate may be cleaned to remove trace amounts of combinations of different types of material, such as metal, organic, and inorganic contamination. In such a cleaning application, a multiple series of complementary wet processing chemistries may be applied to the wafer surface. Some or all of the steps may occur in the same processing vessel, and some or all of these steps may take place at elevated pressures or pressure profiles.

A further understanding of the nature and advantages of the inventions disclosed herein may be realized by reference to the remaining portions of the specification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic drawing of an embodiment of a processing apparatus in accordance with the present invention. [0019]
FIG. 2 is a simplified schematic drawing of an exemplary ozonating apparatus that can be used to generate a pressurized, high-concentration ozone solution and which may be used in the processing apparatus shown in FIG. 1., according to an embodiment of the present invention. [0020]
FIG. 3 is a simplified schematic drawing of an embodiment of an apparatus in accordance with the present invention which includes a holding vessel and a processing vessel (such as that shown in FIG. 1) which may be used to maintain the processing solution under pressure during removal from the processing vessel. [0021]
FIG. 4 is a simplified schematic diagram of an embodiment of an apparatus in accordance with the present invention showing the introduction of processing fluid into a processing vessel through spray nozzles. [0022]
FIG. 5 plots the concentration of dissolved ozone in an aqueous solution versus temperature, at three different pressure conditions. [0023]
FIG. 6 is a simplified flow diagram illustrating an embodiment of a processing method in accordance with the present invention which can be used to process substrates using the processing apparatus shown in FIG. 1. [0024]
FIG. 7 is a simplified flow diagram illustrating an embodiment of a processing method in accordance with the present invention which shows the “pressurized” ozone processing sequence. [0025]
FIG. 8 is a simplified flow diagram illustrating an embodiment of a multi-sequence substrate cleaning process. [0026]
FIG. 9 is a simplified flow diagram illustrating an alternative embodiment of a multi-sequence substrate cleaning process.[0027]

DETAILED DESCRIPTION OF THE INVENTION

A. Apparatus [0028]
FIG. 1 shows a [0029] processing apparatus 10, according to an embodiment of the present invention. Processing apparatus 10 comprises a pressurized processing vessel 100, within which a substrate 102 or plurality of substrates in a substrate boat 104 is/are processed. Substrate boat or holder 104 may include a mechanism for rotating the wafers to enhance the uniformity and effectiveness of wet processing which occurs.
In the particular embodiment shown in FIG. 1, [0030] substrates 102 are completely submerged in a processing solution 106, which as explained in more detail below, may comprise a high-concentration oxidant dissolved in an aqueous or acidic or basic solution, or organic solvent. The dissolved oxidant or acid or base is prevented from outgassing or evaporating from processing solution 106 by maintaining the processing solution under pressure both during and after introduction to processing vessel 100. As used in the instant patent application, the term “under pressure” means any pressure above atmospheric pressure. A drain valve 108 controls the amount of processing solution 106 in processing vessel 100 and gas phase valves 110 and 132 control the gas pressure in processing vessel 100 by regulating the flow of gas into vessel 100 from first and second gas sources 111 and 133 respectively. Gas sources 111 and 133 may contain an inert gas or a gas comprising one or more components sought to be incorporated into the processing solution, or a combination of inert and active gases.
Optionally, the wet processing apparatus may further comprise a circulation loop [0031] 112 having inlet 112 a, outlet 112 b, pump 114, filter 116, and other structures such as liquid flow meters. Pump 114 pumps processing solution 106 through circulation loop 112, so that processing solution 106 is subjected to dynamic flow during processing. The embodiment of the wet processing apparatus shown in FIG. 1 may further achieve a desired level of mixing and fluid movement within the chamber 100 by operation of magnetically-actuated stirrer structure 130, or alternatively a strirrer that is directly actuated by a rotating shaft or other structure.
While the embodiment shown in FIG. 1 includes a circulation loop, this is not required by the present invention. Examples of other vessel architectures useful for wet processing in accordance with the present invention include static tanks, overflow tanks including weir structures for receiving vessel overflow, and quick dump rinsing (QDR) tanks having moveable floors or doors enabling rapid loss of fluid out from the tank, with the fluid either recirculated or disposed of. [0032]
And while the embodiment of FIG. 1 shows the same vessel receiving both the inlet pressurizing gas and the pressurized processing solution, this is also not required by the present invention. Alternative embodiments in accordance with the present invention could employ a gas-tight housing of a first material encompassing a liquid processing vessel formed from a different material. An example of such an embodiment could be an apparatus wherein a quartz processing vessel is desired to contain the processing liquid due to its extremely low leachable contamination properties. However, quartz is a relatively brittle material and may not withstand high applied gas pressures. Accordingly, such a quartz processing vessel could be positioned within a gas-tight enclosure comprising stainless steel or another rigid material that is capable of containing the high applied pressure. [0033]
The apparatus shown in FIG. 1 also includes a [0034] vacuum pump 150 in communication with vessel 100 through valve 151. Vacuum pump 150 may allow evacuation of air from closed vessel 100 prior to introduction of a gas from gas sources 111 and/or 132 to pressurized vessel 100. Such evacuation can ensure a homogenous gas environment in the vessel prior to introduction of the pressurized processing solution, and may also allow transfer of processed substrates to and from vessel 100 under evacuated conditions utilizing a load lock or similar structure.
The apparatus of FIG. 1 further includes a [0035] temperature control structure 118 in thermal communication with the pressurized flow of processing solution 106 to regulate the temperature of the processing solution. By controlling the temperature of the pressurized processing solution, structure 118 can effect a variety of desirable processing results. For example, where temperature control structure 118 is configured to increase the temperature of the pressurized flow of processing solution, the rate of wet chemical reactions may be enhanced. A corresponding reduction in processing time increases process throughput and lowers operating costs, without the outgassing or evaporation and loss of volatile processing solution components associated with conventional high temperature processing. Elevated temperatures usefull for processing in accordance with embodiments of the present invention may range to about 200° C. or even higher depending upon the requirements of a particular application.
Conversely, [0036] temperature control structure 118 may be configured to reduce the temperature of the pressurized flow of processing solution. In such alternative embodiments, reduced temperature of the pressurized processing solution may suppress unwanted competing reactions or reaction mechanisms, while allowing desired reactions to occur at increased rates due to elevated concentration of components of the processing solution. For example, at elevated temperatures reactions leading to metal corrosion may be thermodynamically favored. Therefore, in order to avoid unwanted corrosion of metal during wet processing, it may prove advantageous to maintain the temperature of the pressurized processing solution at sub-ambient temperature, allowing the increased concentration of solution components to drive the desired wet processing. Reduced temperatures useful for processing in accordance with the present invention include any temperature above the freezing point of the processing solution or components thereof, which for water is 0° C., −95° C. for acetone, and may be even lower for other organic solvents.
In addition, the temperature of the processing solution need not be maintained constant during wet processing in accordance with embodiments of the present invention. A changing temperature profile may be employed to maximize effectiveness of the processing, depending upon the characteristics and requirements of a particular application. [0037]
In the specific embodiment of FIG. 1, the temperature control structure is positioned in circulation loop [0038] 112. However in alternative embodiments, for example an apparatus not featuring a circulation loop, the temperature control structure may be in direct thermal communication with processing vessel 100. Moreover, while FIG. 1 shows a single temperature control structure responsible for both heating and chilling the pressurized flow of processing solution (i.e. a Peltier heater/chiller), this is also not required by the present invention. In accordance with alternative embodiments, separate heating and chilling structures could be employed to control the temperature of the processing solution.
Still further additionally, the apparatus may include a [0039] megasonic unit 120 in acoustic communication with the processing vessel. Acoustic energy communicated from megasonic unit 120 to the contents of the vessel may aid in the transfer of processing solution components across the hydrodynamic boundary layer present on the surface of a submerged substrate, thereby enhancing reactivity and decreasing processing times. The use of megasonic cleaning in substrate processing is described in detail in U.S. Pat. No. 5,279,316, entitled “Multiprocessing sonic bath system for semiconductor wafers”, coassigned with the instant application and hereby incorporated by reference for all purposes. In one particular embodiment, application of a pressurized flow of processing solution into a closed processing vessel may result in sufficient pressurization of the vessel to effect the high concentrations and rapid removal rates of the present invention.
The apparatus of FIG. 1 further includes a gas waste/[0040] remediation unit 136 in communication with processing vessel 100 through gas valve 138. Waste/remediation unit 136 allows for the destruction of waste effluent from vessel 100 prior to its being exhausted into the environment.
Referring now to FIG. 2, there is shown an exemplary embodiment of an [0041] ozonating apparatus 20, which can be used to generate a pressurized, high-concentration ozone (O₃) solution according to an embodiment of the present invention. Oxygen (O₂) is introduced into ozone generator 200 via an inlet 202 from oxygen source 201. Ozone generator 200 receives the flow of oxygen and in turn produces a gas containing a substantial amount of ozone at outlet 204, which is coupled through a gas filter and gas phase ozone sensor to inlet 206 of venturi injector 208. The ozone-containing gas is introduced into venturi injector 208, which is disposed in a pressurized ozonating loop 210. Venturi injector 208 functions to raise the velocity and lower the pressure of the passing liquid solution. A quasi-vacuum is thus created in the liquid solution in the middle of venturi 208 and educes ozone-containing gas from ozone generator 200 into the liquid flowing through ozonating loop 210, thereby forming an ozonated solution. The ozonated solution exits outlet 212 of venturi injector 208 and is introduced into an inlet 214 of a surge vessel 216. Prior to introduction of the ozonated solution, surge vessel 216 is partially filled with deionized (DI) water. Whereas DI water would most commonly be employed as the liquid material, other solvents may also be used, depending on the specific application. For example, in alternative embodiments in accordance with the present invention nonaqueous solutions or acidic or basic aqueous solutions may be used. While the structure shown in FIG. 2 includes an ozonation loop, this is not required by the present invention. In one alternative embodiment, the ozone generating apparatus could be in fluid communication with a water stream that is flowed into the processing vessel and then drains out without being recirculated. In still another alternative embodiment, an ozone generating apparatus for use in the present invention could simply provide an outlet for ozone containing gas to the head space of a vessel containing processing solution.
Surge vessel [0042] 216 is maintained under pressure and operates to separate the ozonated solution from the gas phase above the surface of the liquid. Ozonating loop 210 is also maintained under pressure. The ozonated solution circulates around loop 210 by use of a pump 218, which receives the solution from an outlet 220 of surge vessel 216, via an optional filter 283. An optional temperature control structure 219 may also be placed in loop 210 to heat or cool the ozonated solution. The concentration of ozone dissolved in the ozonated solution increases as the solution is pumped around ozonating loop 210 and educed with more ozone. Specifically, as the ozonated solution circulates from surge vessel 216 through ozonating loop 210 and back to surge vessel 216, and as more and more ozone is educed into the solution by operation of venturi injector 208, a peak concentration of ozone is eventually absorbed in the solution. At a given temperature, this peak concentration is higher than can be achieved by a system operating at atmospheric pressure with the same ozone concentration in the inlet gas stream.
[0043] Ozonating loop 210 further comprises static mixing structure 280 downstream of injector 208. Static mixing structure 280 enhances interaction between the gas phase ozone and the liquid processing solution, resulting in additional quantities of ozone being introduced into solution. Ozonating loop 210 also includes a contactor structure 282 positioned downstream of the static mixer that enhances the residence time that the liquid processing solution is in contact with ozone gas, further enhancing dissolution of ozone in the liquid phase such that a condition of saturation of the solution with dissolved ozone is approached. While the embodiment shown in FIG. 2 includes both a static mixer and a contactor structure, neither is required by the present invention, and one or both could be omitted and the apparatus would remain within the scope of the present invention.
As discussed in detail below in connection with advantages offered by the present invention, in certain applications it may be disfavored for the pressurized processing solution to contain bubbles of gas phase material such as ozone. Accordingly, in the embodiment shown in FIG. 1 [0044] ozonating loop 210 further includes cyclonic degasser/separator 284 positioned downstream of contactor 282. Cyclonic degasser 284 serves to remove bubbles introduced into the liquid processing solution as a result of the injection, mixing, and contacting processes just performed. Such a degassing structure may not be required in alternative embodiments of the present invention where gases are dissolved in the pressurized processing solution without the formation of bubbles. For example, in certain embodiments in accordance with the present invention, ozone may be introduced into the processing solution across a membrane utilizing a membrane gasifier structure. Unlike the venturi injector structure shown in FIG. 1, in an alternative embodiment utilizing a membrane gasifier, bubbles of ozone would not be introduced into the solution to require subsequent separation/removal in bubble-sensitive applications.
FIG. 3 shows another embodiment of an apparatus in accordance with the present invention, which integrates the pressurized processing vessel of FIG. 1 with the ozonation apparatus of FIG. 2. At the conclusion of a wet processing stage, [0045] processing solution 106 may be transferred from processing vessel 100 to a holding vessel 300 held under sufficiently less pressure than processing vessel 100, allowing the transfer of processing fluid to be accomplished. In the embodiment shown in FIG. 3, the holding vessel may comprise the surge vessel of the ozonation apparatus shown in FIG. 2. Alternatively however, holding vessel 300 may comprise a separate and independent holding vessel maintained under sufficient pressure to ensure the high-concentration characteristic of the processing solution.
[0046] Valves 310, 311, and 110 control the relative pressure between processing vessel 100 and holding vessel 300 to effectuate the transfer to holding vessel 300 via valve 311. Although not shown in FIG. 3, one or more pumps may be employed to effect or assist in the transfer of fluids between the processing and holding vessels. For example, the pressure in the holding vessel could be maintained at either a higher or lower pressure than in the processing vessel.
In the embodiment shown in FIG. 3, the head space of holding [0047] vessel 300 is in gaseous communication with the head space of processing vessel 100 through valve network 340. In this embodiment, the processing vessel receives a flow of pressurized ozone-containing gas from the holding tank 300, such that during wet processing the gaseous environment remains enriched in ozone to facilitate dissolution of the gaseous ozone in the processing solution. Upon completion of processing, valve network 340 is shut off and processing solution 106 is transferred from processing vessel 100 to holding vessel 300. After processing solution 106 has been transferred to holding vessel 300, processing vessel may be vented and substrates 102 removed from processing vessel 100. In the manner described, the ozone and other components of the processing solution may be maintained in solution for subsequent use, and disposal or remediation of these solution components is not required. Alternatively, direct connection could be made between a second ozone generator and the processing vessel to supply a fresh ozone-containing gas stream.
While the embodiment shown in FIG. 3 includes a single holding vessel, an alternative embodiment in accordance with the present invention could utilize two or more holding vessels in selective fluid communication with the processing vessel. Such a configuration would be of particular value in performing multi-step wet processing in accordance with embodiments of the present invention, wherein a substrate is exposed to a sequence of complementary processing solutions. Examples of such multi-step wet processing are discussed below in connection with FIGS. 8 and 9. [0048]
Still other embodiments in accordance with the present invention may utilize high flow rates to effect wet processing. In accordance with another alternative embodiment of the present invention, FIG. 4 shows a schematic illustration of an [0049] exemplary processing apparatus 40 comprising a pressurized processing vessel 400 within which a substrate 402 or a plurality of substrates 402 in substrate holder 404 is/are processed. Processing vessel 400 may be evacuated to reduce the pressure to sub-atmospheric, then re-pressurized with only an inert gas such as nitrogen, or may contain a reactive gas, such as ozone, or may contain a combination of gases such as CO₂and ozone. Vessel 400 may also contain a liquid processing solution, similar to that explained above, so that substrates 402 may be contacted with the liquid processing solution by total immersion, partial immersion, or wetting resulting from spraying the substrate surface with liquid droplets or mists. One or more nozzles 406 are disposed in processing vessel 400 and configured, so that they may direct (i.e. spray) a processing solution towards surfaces of substrates 402 that are to be processed. Nozzles 406 are coupled to a processing solution supply line 408. While the embodiment shown in FIG. 4 includes nozzles, alternative embodiments in accordance with the present invention may simply include orifices or holes through which processing solution is sprayed.
Processing [0050] solution supply line 408 enters/exits processing vessel 400, via a processing solution line inlet/outlet 410. A processing solution control valve 412 controls flow of the processing solution through supply line 408 and nozzles 406. A pressure control valve 414 is also coupled to processing vessel 400 to control the gas/liquid pressure in processing vessel 400. And, a drain valve 418 is coupled to the bottom of processing vessel 400 to control draining of liquids (e.g. processing solutions or solutions used to clean processing vessel 400) from processing vessel 400.
The apparatus shown in FIG. 4 may utilize high liquid flow rates to enhance the rate and/or effectiveness of processing that occurs on the substrates. For example, turbulence may be imparted to the fluid flowing through the supply line, such that liquid within the vessel exhibits a sufficiently turbulent flow to promote interaction between the substrate surface and the processing solution. Such turbulence may be desirable in stripping applications, amongst others. [0051]
In such high flow velocity embodiments, it may also be possible to utilize a drop in pressure between the sprayed ozonated liquid flow and the wafer surface to induce even more rapid and effective processing. For example, the ozone content of the gas produced by the ozone generator of FIG. 2 may be limited by physical constraints. However, the percentage of ozone in the gas or the ozone concentration that is ultimately outgassed from the pressurized processing solution when a pressure drop occurs may substantially exceed that contained in the originally-generated gas. For example, as a flow of ozonated processing solution is forced through a nozzle, just such a pressure drop occurs, resulting in the outgassing and formation of bubbles containing an enhanced concentration of ozone. Ozone outgassing from the solution in the manner indicated not only elevates the concentration of ozone in bubbles that can be delivered close to the substrate surface when the substrate is submerged, but can also increase the gas phase concentration in the processing vessel to beyond the concentration of ozone in the initially generated gas. This enriched ozone-gas phase proximate to the surface of the wafer provides yet another mechanism for bringing the ozone into contact with the wafer surface for stripping, etching, or cleaning purposes. [0052]
In still other alternative embodiments in accordance with the present invention, the wafers need not be immersed in processing solution. In one alternative approach, a substrate may be suspended over a processing solution, such that the vapor phase overlying the liquid interacts with and processes the substrate. In accordance with still another alternative approach, a processing solution may be sprayed in the form of liquid droplets onto the wafer surface. The nozzles could be configured to produce relatively large drops, such that momentum transferred to the surface of the wafer by impingement of the large drops further promotes interaction between material on the wafer surface and the processing solution, thereby enhancing wet processing. Alternatively, a pulsating shower spray could be employed for this purpose. Where the substrate comprises fragile structures that can be damaged by impact of the liquid, a fine mist yielding only low momentum transfer may be preferable. Where the substrate is not completely immersed in the processing fluid, the pressure drop caused by forcing fluid through the nozzles may lead to an enhanced ozone concentration in the gas phase in the processing vessel, causing more rapid substrate processing if the pressurized processing fluid is close enough to saturation prior to entering the nozzle. The specific needs of a particular application will determine this aspect of the present invention. [0053]
Embodiments in accordance with the present invention may further utilize rotation of the substrates during processing to enhance processing. Substrate rotation during processing may accomplish a number of useful functions. Rotation of the substrate may reduce the thickness of the hydrodynamic or acoustic boundary layer present on the surface of a submerged wafer, promoting mass transfer and other interaction between the processing fluid and components thereof, and the substrate. For processing of non-submerged substrates, rotation of the substrate may reduce the thickness and increase uniformity of the liquid layer present on the wafer surface, promoting mass transfer of a component from the gas phase to the wafer surface. Rotating the wafer also enables the uniform distribution of energetic processes such as spraying or the application of ultrasonic or megasonic energy over the entire surface of a substrate being processed either in a submerged or nonsubmerged state. [0054]
Still other alternative embodiments in accordance with the present invention could utilize a laminar or plug flow, rather than turbulent flow, of pressurized processing fluid within the processing vessel to achieve wet processing of submerged semiconductor wafers or substrates. Such embodiments would be particularly suited for cleaning substrates by uniformly carrying away small amounts of contamination with a minimum of mixing or possible reabsorption onto the substrate surface. Enhancement of wet processing in such laminar flow embodiments would be associated with elevated concentrations of processing solution components and elevated processing temperatures enabled by pressurization. [0055]
B. Methods [0056]
Henry's law teaches that the concentration of a component gas dissolved in a liquid phase increases as the concentration of that component increases in the gas phase above the liquid, and as temperature decreases. Therefore, as the concentration of ozone in the gas phase is increased, the ozone dissolved in the liquid phase increases at a given temperature and pressure. Reducing the temperature increases the dissolved ozone content in the liquid phase at a given pressure and gas phase concentration. This is shown in FIG. 5, which plots dissolved ozone concentration versus temperature of an aqueous solution at three different pressures: 0 psig, 14.7 psig, and 29.4 psig at a nearly constant gas phase ozone concentration. [0057]
Dalton's law teaches that the concentration of a gas dissolved in a liquid increases as the total gas pressure in the gas phase increases. Under ideal conditions, this increase is directly proportional. Thus, if saturation occurred at 100 ppm of ozone dissolved in DI water at 20° C. and atmospheric pressure (1 [0058] atm 0 psig) at a certain gas phase ozone concentration, approximately a two-fold increase in ozone concentration (i.e. 200 ppm) can be achieved in the liquid by raising the gas pressure to ˜14.7 psig, which is about one atmosphere of pressure greater than atmospheric pressure with the same ozone concentration in the gas phase. If the pressure is raised even further to ˜29.4 psig, the ozone concentration would increase to about 300 ppm and so on. These numbers are exemplary only and are not meant to indicate saturation concentrations precisely at any specific set of processing conditions, but based on behavior of an ideal gas and are therefore only approximations. Greater concentrations of ozone will arise as the pressure is increased even further. Eventually however, the one-to-one proportionality between pressure and concentration deviates from the ideal case. Therefore, to maximize dissolved ozone concentration in the liquid phase, system pressure and gas phase concentration are maximized. This characteristic is also reflected in FIG. 5, wherein the concentration of dissolved ozone increases with increased pressure, even though the concentration of ozone in the gas phase produced by the ozone generator remained substantially constant.
Embodiments of methods in accordance with the present invention take advantage of the relation between pressure, temperature, and solubility to enhance wet processing performance. By performing wet processing at elevated pressures, the solubility of active components in the processing solution is maintained, allowing for optimization of such wet processing parameters as removal rate and selectivity. [0059]
Specifically, the processing apparatuses shown and described above in conjunction with FIGS. [0060] 1-4 may be employed in a number of wet processing applications, including but not limited to stripping of organic materials such as developed photoresist films, and removal of residual organic or inorganic particles from substrates. In such an application, processing solution may comprise a high-concentration of oxidant (e.g. ozone) dissolved in deionized water, such as the ozonated solution produced by the apparatus of FIG. 2.
The processing apparatuses [0061] 10 of FIGS. 1-4 may also be used to etch inorganic materials such as silicon, silicon dioxide, or metals from substrates. In this type of application, the processing solution may comprise an acidic solution such as HF for an oxide etch or HCl for removing metals, or may comprise a basic solution such as NH₄OH for removing inorganic particles. Any of these solutions could also optionally contain a high concentration of an oxidant such as ozone. The possible candidates of processing solutions described above are only exemplary and therefore should not be construed as limiting. Other solutions may be used, depending on, for example, the material to be removed from the substrate. For example, the processing solution may comprise an undeveloped organic photoresist material, photoresist developer material, or another type of organic substance utilized in the fabrication of semiconductor devices.
In the above-described embodiments, the processing solution is maintained under pressure during introduction into processing vessel and at least during part of the time the substrates are being processed. However, the processing solution may also be maintained under pressure during preparation and prior to introduction into the processing chamber. Because the processing solution is not allowed to equilibrate to atmospheric pressure, high concentrations of components from the gas phase may be maintained in the oxidant solutions, and volatile components in non-oxidant solutions retained and prevented from loss through outgassing. Additionally, loss due to evaporation of other components is also prevented. [0062]
Referring now to FIG. 6, there is shown an [0063] exemplary method 60 of processing substrates using, for example, apparatus 30 in FIG. 3, according to an embodiment of the present invention. Initially, in step 600 substrates 102 are placed into processing vessel 100. Then, in step 602 vessel 100 is closed gas-tight. In optional step 603 (optional indicated by dashed box), the vessel 100 may be evacuated prior to the introduction of pressurized gas in the following step. In step 604, an inert gas such as nitrogen or a non-inert gas such as oxygen or ozone is introduced under pressure into the closed processing vessel. As a result of this inflow of gas into the closed vessel, vessel 100 becomes pressurized.
Pressures of up to about sixty atmospheres or even higher may be imposed. The upper pressure of 60 ATM is merely exemplary, and the present invention is not limited to operating within any particular pressure range. Factors to be considered in determining pressures for operation of the present invention include but are not limited to, solubility of active components in the particular processing solution, and the expected cost of manufacturing, operating, and maintaining the wet processing equipment. For the latter factor, application of relatively high pressures (above 10 ATM for example) may substantially increase the cost of the structures required to safely and effectively maintain the processing solution under pressure. [0064]
While FIG. 6 shows an embodiment wherein the pressurized processing solution is maintained under pressure during processing, it is not required that this elevated pressure remain constant. Depending upon the requirements of a particular application, a changing pressure profile may be applied during the course of a wet processing step. In certain embodiments, a changing pressure profile may optimize processing parameters based upon solubility characteristics of dissolved solution components. In other embodiments, a changing pressure profile may best allow for replenishment of solution components as they are consumed during processing. Similarly, temperature gradients may also be employed to enhance processing. [0065]
In step [0066] 606, after processing vessel 100 is pressurized processing solution 106 is introduced under pressure into vessel 100, for example from the holding tank/surge vessel shown in FIG. 3. While being maintained under pressure, substrates 102 are then processed in step 608 for a time necessary to remove the unwanted material from the substrates 102.
After [0067] substrates 102 have been processed, in step 610 processing solution 106 may be maintained under pressure and flowed to a pressurized holding tank pending removal of substrates 102 from vessel 100. This allows processing solution 106 to be reused to process a subsequent batch of substrates. Alternatively, processing solution 106 may be removed from the processing vessel via a drain valve.
In an optional following step [0068] 612, once the pressurized processing solution has been removed from the chamber, substrates 102 may be rinsed with DI water to remove any residual materials from the wafers and from the processing chamber. The rising step 612 may be optional where a dilute processing solution was used, or where only low-levels of contamination are expected.
Further optionally, in [0069] step 614 the processed wafers can be dried in accordance with embodiments of the present invention. For example, processed wafers have conventionally been dried by creating and exploiting a differential or surface tension gradient between the surface tension of fluid comprising the meniscus formed on the substrate surface versus the surface tension of the bulk fluid in the liquid phase. As the substrate is removed from the liquid into the gas phase, fluid clinging to the substrate surface in the region of the meniscus is pulled back into the bulk fluid. Generally this has been accomplished by introducing a solvent vapor into the gas phase above the liquid covering the substrates. As the solvent starts to go into solution of both the fluid of the liquid surface generally as well as the fluid of the meniscus, the localized concentration of that component increases and reduces the localized surface tension between that fluid and wafer at the liquid gas interface. Because the volume of the fluid of the meniscus is so small, a more rapid concentration increase occurs in the meniscus area than in the surface of the bulk fluid. The localized increased concentration causes a localized reduction in the surface tension between the effected fluid and the substrate surface. The wafer is either raised out of the bulk fluid, or the fluid drained from the substrate. Thus the differential in localized surface tension of the fluid leads to fluid being drawn from the meniscus area into the bulk fluid.
In accordance with embodiments of the present invention, this drying process can take place at elevated pressures. Drying at elevated pressures in accordance with embodiments of the present invention can alter the surface tension parameters at the interface between the substrate and the processing solution. For example, elevated pressures can drive greater concentrations of the surface tension lowering component into the processing solution, enhancing the range of the gradient and/or the speed at which the gradient is formed. Alternatively, pressurized drying in accordance with embodiments of the present invention may allow varieties and combinations of surface tension lowering materials to be present in the processing fluid and in the surrounding gas phase at concentrations unavailable at atmospheric pressure. These surface tension lowering components may enable substrate drying to take place at speeds and efficiencies not possible at ambient pressure. Alternatively, a second solution with greatly reduced surface tension, such as an alcohol, could be put onto the surface of the rinsing fluid. The surface tension differential or gradient between the two fluids could be used for drying. [0070]
In a [0071] final step 616, the apparatus and substrates are readied for additional processing. Where the wet processing step just described is part of a series of complementary wet or dry processing steps, substrates 102 may remain in the vessel to await repressurization and renewed exposure to pressurized processing solutions, or to reactive pressurized gases in the case of an additional dry processing step. Alternatively, the substrates 102 may be transferred to another processing vessel for additional wet and/or dry processing.
Transfer of processed substrates to and from the processing vessel can be accomplished in a number of ways. In one exemplary method, [0072] substrates 102 are moved out of processing solution 106 into a load lock (not shown in the figures) coupled to vessel 100. Once sealed and pressure-separated from vessel 100, substrates 102 may then be removed from the load lock. Alternatively, after processing, the pressure could be increased with a second gas and the fluid drained to accomplish drying of the substrates. Further alternatively, after substrates have been processed, the pressure can be released and the substrates removed from the processing vessel at atmospheric pressure while the vessel continues to hold the processing solution. The substrates could then be moved to a separate drying structure to remove residual fluid if necessary.
In an exemplary application, processing apparatuses described so far may be used to strip an organic material (e.g. developed photoresist) from surfaces of substrates, for example utilizing a pressurized aqueous solution comprising ozone and an organic acid such as acetic acid, or simply ozone in a DI water solution. A [0073] method 70 used to perform this exemplary application is shown in FIG. 7.
In [0074] initial step 700 of method 70, substrates 102 are transferred into processing vessel 400 of FIG. 4. In step 701, vessel 400 is sealed gas tight. In optional step 702, the air is evacuated from the vessel. In optional step 703, an ozone rich gas may be introduced into the processing vessel, thereby raising the pressure to above atmospheric. This pressurized gas could come directly from an ozone generator, or could come from outgassing in another vessel. In step 704, a pressurized processing liquid may be introduced into vessel 400 such that a pressure drop occurs. For example, the processing fluid could be pumped at 100 psig and the process vessel could start out at 25 psig and reach 50 psig or even higher, resulting in ozone outgassing and enhanced ozone concentration in the vessel in the gas phase. Optionally, the resulting partially depleted liquid can be drained from the vessel or reclaimed for reuse, and fresh pressurized processing fluid with a high ozone concentration could continue to be introduced.
In step [0075] 705, the introduction of pressurized solution continues via spray nozzles that wet the wafer surfaces with a thin film of liquid. Optionally, prior to or during this processing step, the wafers could be rotated or spun with the wafers held in a horizontal or vertical orientation, with the result of thinning the solution layer that is formed on the substrate surface. In the processing step 705, the condition of elevated ozone gas concentration in the gas phase, together with a thin, highly ozonated liquid layer, may result in substantial enhancement of diffusion of ozone to the surface of the substrate, increasing the reaction rate. Processing step 705 of method 70 can take place at elevated temperatures to enhance reaction rates, at depressed temperatures to enhance dissolved concentrations of solution components, or at ambient temperature for ease of processing. A temperature profile changing with time could also be employed. Owing to the elevated pressure during processing step 705, higher concentrations of dissolved ozone may be present in the water layer, and more highly concentrated amounts of ozone may be present in the gas phase, enhancing wet processing performance as compared with conventional processing at atmospheric pressure. Gases other than ozone or mixtures of gases could alternatively be employed in this embodiment. For example, carbon dioxide or a mixture of carbon dioxide and ozone could be employed in accordance with embodiments of the present invention to form a pressurized processing solution
The processing flow shown and described in conjunction with FIG. 7 may conclude with additional solution transfer, wafer rinsing, wafer drying, and wafer transfer steps as have been previously described. [0076]
Another potential application for methods and apparatuses in accordance with the present invention is wafer cleaning. Such a cleaning process may comprise, for example, removing from the substrate surface organic materials, particles and metals residual from a prior etching or stripping step. A processing vessel, as is shown in the processing apparatuses of FIGS. [0077] 1-4 may be used in performing the cleaning method. In either case, the process would be similar to the respective methods described in FIGS. 6 and 7. Where processing apparatus 10 is used to perform a post-stripping cleaning process, FIG. 8 shows an overall view of several steps performed in such an exemplary multi-sequence cleaning process 80.
In a first series of [0078] steps 82, a first processing solution comprising a pressurized ozonated aqueous sulfuric acid H₂SO₄may be applied to remove residual organic contamination and some metal contaminants. In a second series of steps 84, a second processing solution comprising pressurized ozonated aqueous dilute hydrofluoric acid (HF) may be applied to remove oxide and metallic contaminants incorporated in the oxide. In a third series of steps 86, a third processing solution comprising pressurized aqueous ozonated ammonium hydroxide may be applied to remove inorganic particles. In a fourth series of steps 88, a fourth processing solution comprising pressurized aqueous ozonated hydrochloric acid (HCl) may be applied to remove additional metal contaminants. Finally, in a fifth series of steps 90, a fifth processing solution comprising pressurized aqueous ozonated dilute hydrofluoric acid (HF) may be applied for removing oxide formed in the fourth series of steps, as well as for removing additional metallic contaminants.
The process flow illustrated in FIG. 8 shows sequential exposure of substrates to different processing chemicals within the same vessel. In order to facilitate sequential wet processing utilizing more than one type of chemistry, substrates present and the processing vessel in which they are housed may be rinsed and/or dried between processing stages in order to remove residual contamination and prevent cross-contamination. [0079]
While the process flow illustrated in FIG. 8 shows sequential exposure of substrates to different processing chemicals within the same vessel, this is not required by the present invention. A method in accordance with embodiments of the present invention may employ one or more substrate transfer steps between different pressurized processing vessels that are devoted to application of a specific processing chemistry, thereby minimizing the risk of cross-contamination between different pressurized chemical solutions. In addition, while FIG. 8 shows a series of pressurized wet processing steps being performed, this is also not required, and some of the processing steps in a sequence could be performed under atmospheric pressure. [0080]
And while the process flow illustrated in FIG. 8 shows a multi-step process wherein substrates are exposed to successive wet processing chemistries, this is also not required by the present invention. One or more dry processing steps could be performed within the processing vessel in sequence with wet processing steps, and the method would remain within the scope of the present invention. For example, following a wet processing step a substrate could be exposed to a pressurized flow of HF gas in the absence of a liquid processing solution. At the completion of such a dry processing step, the substrate could be subjected to a second wet processing step under pressure. [0081]
Moreover, while the process flow illustrated in FIG. 8 illustrates application of five different processing chemicals in a particular sequence, the present invention is not limited to this number of processing chemicals, or to this particular sequence of steps. Depending upon the requirements of a particular application, alternative embodiments in accordance with the present invention may utilize all, some, or additional processing chemistries applied to the substrate in a different order. For example, inorganic acids which may be utilized as components of processing solutions in accordance with embodiments of the present invention include but are not limited to HF, HCl, H[0082] ₂SO₄, H₂CO₃, HNO₃, H₃PO₄, Aqua Regia, chromic and sulfuric acid mixtures, sulfuric and ammonium persulfate mixtures, and various combinations thereof. Examples of organic acids which may be utilized as components of processing solutions in accordance with embodiments of the present invention include but are not limited to acetic acid, formic acid, butyric acid, propionic acid, citric acid, and sulfonic acid. Examples of oxidants which may be utilized as components in processing solutions in accordance with embodiments of the present invention include but are not limited to oxygen, ozone, hydrogen peroxide, and other peroxides. Other possible oxidants include oxygen radicals and hydroxy radicals generated from ozone or oxygen gas through exposure to an electrical discharge or UV radiation. Examples of bases which may be utilized as components of processing solutions in accordance with embodiments of the present invention include but are not limited to NH₄OH, NaOH, and KOH.
FIG. 9 shows a simplified flow chart of an alternative embodiment of a [0083] multi-step cleaning method 900 in accordance with the present invention, which avoids the use and consequent need to dispose of many of the hazardous chemicals of FIG. 8. Specifically, this replacement process for the simplified RCA washing method is shown in FIG. 9 and utilizes pressurized ozonated water and pressurized ozonated water containing a small amount of hydrofluoric acid in place of the multiple cleaning chemistries just described. In first step 902 of such an alternative multi-step washing method, an ozonated water solution removes and oxidizes residual organics. In second step 904, an ozonated water solution containing a small amount of HF removes the oxide layer and contaminants incorporated therein. In a third step 906, the ozonated water may be activated with megasonic energy and is used to remove particles. In a fourth step 908 the ozonated water HF solution removes metals and the new oxide layer.
While the multi-sequence cleaning process described and illustrated in conjunction with FIGS. 8 and 9 utilizes basic, acidic and oxidizing components, processing methods and apparatuses in accordance with embodiments of the present invention are not limited to these types of processing solution components. In accordance with other embodiments of the present invention, pressure may be applied to maintain elevated concentrations of a reducing agent such as hydrogen gas or reactions within a processing solution to cause the desired surface changes. Such a reducing agent may serve to passivate or alter surface properties of a substrate, for example by minimizing the formation of an oxide layer, or replacing hydrophilic SiO bonds with hydrophobic SiH. [0084]
Another possible category of component for a processing solution in accordance with embodiments of the present invention is an additive which suppresses undesirable side reactions. Surfactants and wetting agents represent yet another class of components of a processing solution available for use in connection with embodiments of the present invention. Such wetting agents and surfactants serve to lower the surface tension of fluids at the interface with the substrate surface. The reduction in surface tension facilitates interaction between the processing solution and contamination present within the varied topography of the substrate. In addition to allowing wetting agents and surfactants to be maintained at a high concentration, the elevated pressures associated with the methods and apparatuses in accordance with the present invention may also directly facilitate the wetting process itself. For example, high concentrations of wetting agents or surfactants at atmospheric pressure may lead to excessive foaming, but the foaming may be minimized or reduced at the elevated pressures under which processing is conducted in accordance with embodiments of the present invention, resulting in more uniform and effective wet processing. [0085]
The reactivity of pressurized processing solutions in accordance with embodiments of the present invention, and hence the rate of removal of material, can be further increased utilizing techniques in addition to pressurization. For example, the addition of acoustic energy from a megasonic unit to a pressurized flow of processing solution may result in enhanced wet processing, prior to, during, or after the application of the processing solution under pressure. [0086]
Increasing the concentration of ozone dissolved in a DI water solution utilizing embodiments of the present invention can increase the rate of removal of organic photoresist material from the surface of a substrate. For example, with a silicon wafer coated with a conventional positive photoresist, an ozonated aqueous processing fluid containing 80 ppm ozone at 5° C. at atmospheric pressure in a relative quiesent tank removed photoreist at a rate in the range of 400-500 Å/min. By increasing the ozone concentration in the processing solution to 160 ppm at 5° C. by raising the pressure to just over 15 psig, the photoresist removal rate increased to between about 800-1000 Å/min. As discussed in detail elsewhere in this invention description, the reduction in temperature of a pressurized processing solution can also enhance processing by suppressing undesirable competing reactions and reaction mechanisms. [0087]
Maintaining a processing solution above ambient temperature can also enhance wet processing. For example, wet processing conditions of atmospheric pressure and 5° C. containing about 80 ppm of ozone dissolved in DI water in a relatively quiesent tank resulted in a photoresist removal rate of between about 400-500 Å/min. Raising the solution temperature to 20° C. at atmospheric pressure resulted in a photoresist removal rate of between about 700-800 Å/min. [0088]
In accordance with other alternative embodiments of the present invention, the reactivity of pressurized processing solutions, and hence material removal rates, can also be enhanced by increasing the flow rate of the processing solution. Moreover, the combined effect of temperature and flow rate may provide a cumulative benefit when wet processing is conducted at elevated pressures in accordance with embodiments of the present invention. For example, a flow of processing solution pressurized to 1.8 atm and maintained at 70° C. contained dissolved ozone of approximately 65 ppm. Application of this pressurized solution under the same conditions with a flow having a Reynolds number of about 200,000 or greater resulted in photoresist stripping at a rate of between about 2000-4000 Å/min, with even higher stripping rates possible under different conditions. For example, developed photoresist material that is implanted with ions may be removed at different rates than non-implanted resist material. Also, different resist materials may be removed at different rates [0089]
C. Advantages [0090]
Embodiments of methods and apparatuses in accordance with the present invention may offer a number of advantages over conventional wet chemical processing techniques. One possible important advantage is that elevated pressures allow higher concentrations of active volatile components of the processing solution to be maintained in solution, where they are available to react with material on the wafer. For example, utilizing embodiments in accordance with the present invention, the concentration of a component of the processing solution may be within the range of about 0.1 and 5000 ppm in a temperature range of between about −60° C. and 200° C. The increased concentration of acidic, basic, oxidizing and other active solution components resulting from the elevated pressures in accordance with the present invention may enhance the extent and/or speed of reaction between the processing solution and material present on the wafer surface. [0091]
A second possible advantage conferred by embodiments of methods and apparatuses in accordance with the present invention is that the sealed environment allows processing to take place at elevated or depressed temperatures, thereby further enhancing the effectiveness and/or speed of processing. Specifically, certain components of the processing solution are volatile and may be lost by evaporation from an open processing vessel during wet processing at elevated temperatures or may outgas and be lost. For example, a processing solution utilized in accordance with embodiments of the present invention may include volatile organic components such as DMSO, MEA, n-methylpyrrolidone (NMP), catechol, and others. By performing wet processing in a sealed environment, these volatile components are retained in the liquid and are therefore available to interact with a substrate, even when the wet processing reaction is taking place at an elevated temperature. Examples of other volatile components of processing solutions that may be retained under pressure include but are not limited to water, ozone, carbon dioxide, HF, HCl, F[0092] ₂, Cl₂, NH₃, and other soluble and insoluble gases.
A third possible advantage of methods and apparatuses in accordance with embodiments of the present invention relate to the ability to create and utilize wet processing solutions comprising ratios of solution components that are unavailable at ambient pressure. For example, one embodiment of a pressurized processing solution in accordance with the present invention utilizes an aqueous mixture of ozone and HF. In this processing solution, the HF functions as an acid to dissolve exposed silicon oxide. Conversely, the ozone component functions as an oxidant to generate silicon oxide from exposed silicon. For conventional wet processing conducted at atmospheric pressure, it is difficult to maintain ozone in solution at sufficient concentrations to match the rate of removal of the oxide by HF. In accordance with embodiments of the present invention however, at elevated pressures it is possible to introduce sufficient quantities of ozone into solution such that the rate of oxide removal by the HF processing solution component approximately matches the rate of oxide formed by the ozone processing solution component. The ability to create wet processing solutions comprising heretofore inaccessible ratios of solution components adds flexibility and new capabilities for wet processing techniques. [0093]
A fourth possible advantage conferred by embodiments of methods and apparatuses in accordance with the present invention is the ability to control the level of bubble formation during processing. Bubbles of gas can form on the surface of a substrate as a result of the evolution of gas during a particular fabrication process step. For example, the presence of gas bubbles can be detrimental during performance of a variety of processing steps, including but not limited-to electroplating or electrochemical etching, stripping, and removal of silicon oxide with HF. [0094]
During wet processing, bubbles may inhibit the transfer of active components from either the liquid or gas phase (bubble) to the substrate. This inhibited transfer can lead to non-uniform processing. In such cases, bubble formation is undesirable and is preferably inhibited or suppressed. As a further illustration, in substrate cleaning applications the goal is for the processing solution to contact the entire surface area of the substrate, reacting with and removing the small amounts of contamination. However, bubbles can lodge within surface features such as high aspect ratio trenches, rendering these regions inaccessible to the processing solution. Moreover, impurities sought to be removed may accumulate at the liquid/gas interface of the bubble, rather than being carried away from the surface of the wafer by the processing solution, thereby inhibiting removal of the contaminants with the processing solution or increasing the tendency of the contaminants to reattach to the substrate surface. In still other applications, the presence of gas bubbles is undesirable owing to their compressibility and tendency to absorb and dissipate acoustic energy from megasonic units intended to enhance cleaning. [0095]
In other certain wet processing applications, bubble formation may be desirable. For example, in photoresist stripping applications, the formation, coalescence, collapse, and mere presence of bubbles can promote physical and chemical interaction between the processing fluid and the photoresist layer, resulting in enhanced removal of the photoresist. One possible mechanism for this activity is the creation of localized mixing or turbulence that can aid in the dislodging of films or particles on substrate surfaces, or aid in the minimization in chemical concentration profiles. From the perspective of reaction kinetics, finely divided bubbles dispersed in the liquid phase may contain an active gas phase component at a much higher concentration than that active component occurs in the liquid phase. By bringing high concentrations of small bubbles of the active component into close proximity to the substrate surface, the active gas component can more easily diffuse across the remaining relatively thin fluid layer adjacent to the substrate surface, allowing more active component to come into contact with the substrate surface and be available for reaction. [0096]
Advantageously, embodiments of methods and apparatuses for wet processing in accordance with the present invention can inhibit or promote the localized formation of bubbles in the processing solution. Specifically, the high pressures employed during processing in accordance with the present invention serve to effectively elevate the boiling point of the processing solution and components thereof, or that even when formed, bubbles tend to be smaller and shorter-lived, resulting in greater processing effectiveness. In cases where bubbles are being generated locally, increased pressure causes more of the liberated/formed gas to be dissolved or redissolved in the liquid, delaying the onset of stable bubble formation. Further, once the bubble is formed due to a localized pressure differential or aggressive agitation of the solution, the resulting bubble can be more rapidly reabsorbed when reexposed to the bulk fluid pressure. [0097]
When advantageous, bubbles can be formed from the saturated, or nearly saturated pressurized processing fluid by introducing localized pressure drops in the processing equipment. For example, this could allow the formation of gas bubbles containing higher concentrations of ozone than are present in the gas space above the fluid. As discussed above, the elevated concentration of active solution components in these gas bubbles can lead be to enhanced mass transfer and enhanced processing rates. [0098]
A fifth possible advantage conferred by embodiments of methods and apparatuses in accordance with the present invention is efficient consumption of resources and environmental compatibility. During conventional wet processing, ozone, NH[0099] ₃, HF, and other components dissolved in the processing solution outgas and must continuously be replenished in the solution, at substantial effort and cost in the form of precursor materials consumed. Additional expense is associated with treatment and remediation of the resulting effluent to allow its safe release into the environment.
By contrast, wet chemical processing of wafers in accordance with embodiments of the present invention is performed in a substantially closed system. The processing solution is maintained under pressure, and thus ozone and other potentially volatile solution components remain dissolved within the solution to react with the substrate rather than requiring constant replenishment and disposal. Thus in the apparatus shown in FIG. 3, maintenance of the processing solution under pressure substantially reduces the loss of volatile components through outgassing or evaporation, obviating the need to collect and remediate the lost materials. [0100]
Furthermore, while in certain embodiments the system pressure is reduced to atmospheric to facilitate removal of the substrate from the processing vessel, this is not required by the present invention. In alternative embodiments, upon cessation of processing the vessel could be evacuated and the processed wafers transferred to another evacuated chamber for continued wet or dry processing. In such embodiments, ozone and other volatile solution components can easily be kept segregated from the ambient air, allowing treatment at a point of maximum concentration. By contrast, atmospheric processing in an open vessel may require expensive and complicated equipment to capture volatile component fumes released during the processing sequence that become mixed with ambient air, increasing expenditures for capital equipment and elevating operating costs. [0101]
D. Additional Alternative Embodiments [0102]
While particular embodiments of the present invention have been shown and described above, one should understand that the present invention is not limited to these particular embodiments. Variations of these embodiments would remain within the scope of the inventive concept expressed herein. [0103]
For example, while embodiments in accordance with the present invention have been described so far in connection with pressurized wet processing for stripping, cleaning, and etching applications utilizing aqueous solutions, other liquids may be used. Liquids such as chlorinated, fluorinated, and other non-aqueous solvents, or combinations thereof, may also be used. Even a combination of immiscible liquids may be advantageously processed utilizing the proper combination of temperature, pressure, and mixing parameters to ensure interaction between the substrate surface and the liquids of the processing fluid. In one alternative embodiment in accordance with the present invention, the liquid may comprise a mixture of O[0104] ₃and CO₂in DI water. The carbonic acid resulting from dissolution of the CO₂lowers the pH, promoting solubility of ozone and allowing more ozone to be dissolved in solution, or stabilizing in solution ozone that has already been dissolved. CO₂dissolved in a pressurized processing solution in accordance with embodiments of the present invention can also enable wet processing utilizing supercritical or near-supercritical fluids or conditions. In such supercritical or near-supercritical applications, pressures of between about 50 and 100 ATM are commonly utilized.
And while embodiments of methods in accordance with the present invention have been described above in connection with pressurization of a closed processing vessel through the application of a pressurized flow of gas, the present invention is not limited to this particular step. In accordance with alternative embodiments of the present invention, the introduction of a pressurized processing solution into a closed processing vessel alone may be sufficient to induce pressurization of the vessel for pressurized solutions containing a significant degree of saturation of a dissolved gas component. The application of elevated temperatures during this inflow of pressurized liquid processing solution may further speed pressurization of the processing vessel. [0105]
In accordance with still other embodiments of the present invention, the processing solution exhibiting reduced or depleted components may be completely or partially drained from the processing vessel without the escape of enriched outgassed components. The depleted processing solution that is removed may be replaced with an inflow of component-rich pressurized processing solution. Utilization of a processing vessel that is gas-tight but not liquid tight may further enable this cycle of replacement of depleted material. [0106]
In accordance with still other alternative embodiments of the present invention, the air within a processing vessel could be evacuated and then replaced with one or more gases prior to introduction of the pressurized processing fluid. The replacement gases could be inert, or could contain active chemical species such as chlorine, fluorine, CO[0107] ₂, NH₃, or ozone that are intended to be adsorbed into the processing solution. It is also possible to introduce a series or a combination of different gases into the evacuated chamber to prepare for introduction of the processing solution under pressure. Alternatively, hydrocarbon gases could be introduced separately or in combination with other gases.
In accordance with still other alternative embodiments of the present invention, the processing solution may comprise an organic solvent. Examples of organic solvents which may be utilized in conjunction with the present invention include but are not limited to PR, PR developer, spin on glass, acetone, xylene, stoddard solvent, n-butylacetate, ethoxyethyl acetate, 2-methoxyethyl, NMP, TMAH, IPA, chlorinated solvents, fluorinated solvents, hydrocarbon solvents, sulfonic acid, and phenolic organic strippers. [0108]
The above discussion has focused upon utilization of a pressurized processing solution containing elevated concentrations of a dissolved gas component due to the application of high pressure. However, alternative embodiments in accordance with the present invention may also contain concentrations of undissolved gases that may interact with and process a substrate surface. Turbulence and other physical phenomena may promote both the introduction of the undissolved gases into the processing fluid, and interaction between the undissolved gases and the substrate surface. [0109]
While embodiments of the present invention described so far relate to wet chemical processing of substrates during the manufacture of semiconductor devices, for example substrates comprising silicon, SiGe, GaAs, Si, GaAs, GaInP, and GaN to name a few. However, the present invention is not limited to this particular application, and other materials may be subjected to processing under pressure. Examples of candidates for wet chemical processing utilizing the present invention include, but are not limited to, hard disks and hard disk substrates, optical devices such as mirrors, lenses, or waveguides, and substrates utilized in the fabrication of micro-electrical mechanical systems (MEMS), and liquid crystal display devices. [0110]
Furthermore, while embodiments in accordance with the present invention have been described above primarily in connection with wet processing applications wherein a component is introduced into a processing solution under pressure, the present invention does not require this. In accordance with alternative embodiments of the present invention, a wet processing solution may simply be maintained in the processing vessel at elevated pressures in accordance with the present invention, avoiding potential unwanted loss of the solution component through outgassing or evaporation. [0111]
Although the invention has been described in terms of preferred methods and structures, it will be understood to those skilled in the art that many modifications and alterations may be made to the disclosed embodiments without departing from the invention. Hence, these modifications and alterations are intended to be considered as within the spirit and scope of the invention as defined by the appended claims. [0112]

Claims

What is claimed is:

1. A method of removing a material from a surface of a substrate, said method comprising the steps of:

inserting the substrate including the material into an open processing vessel;

closing the processing vessel gas-tight;

pressurizing the processing vessel to greater than atmospheric pressure;

introducing a pressurized processing solution into the pressurized processing vessel so that a surface of the substrate is exposed to the pressurized processing solution; and

processing the substrate to remove the material from the surface of the substrate by allowing the pressurized processing fluid to react with the material, wherein said processing solution is maintained at higher than atmospheric pressure during introduction into the processing vessel and during at least part of processing of the substrate.

2. The method of claim 1 wherein the material comprises an organic material.

3. The method of claim 2 wherein the organic material comprises a photoresist.

4. The method of claim 1 wherein the processing solution comprises an oxidant.

5. The method of claim 4 wherein the oxidant is selected from the group consisting of ozone, oxygen, hydrogen peroxide, and mixtures of ozone and hydrogen peroxide.

6. The method of claim 4 wherein the oxidant is at least one of an oxygen radical and a hydroxy radical generated from exposure to UV radiation or an electrical discharge.

7. The method of claim 4 wherein the oxidant comprises ozone absorbed in deionized water.

8. The method of claim 4 wherein the oxidant comprises ozone absorbed in a solution selected from the group consisting of an acidic solution, a basic solution, and an organic solvent.

9. The method of claim 4 wherein the oxidant is ozone at a concentration greater than about 0.1 ppm in a temperature range of between about −95° C. and 200° C.

10. The method of claim 4 wherein the oxidant is ozone at a concentration greater than about 0.1 and 5000 ppm in a temperature range of between about −95° C. and 200° C.

11. The method of claim 1 wherein the processing solution comprises an acid.

12. The method of claim 11 wherein the acid comprises an organic acid

13. The method of claim 12 wherein the organic acid is selected from the group consisting of acetic acid, formic acid, butyric acid, propionic acid, citric acid, oxalic acid, and sulfonic acid.

14. The method of claim 1 wherein the material comprises an inorganic material.

15. The method of claim 14 wherein the inorganic material is selected from the group consisting of silicon, polysilicon, silicon oxide, silicon nitride, TiN, GaAs, GaN, InP, GaInP, SiGe, Al, Cu, AlCuSi, W, silver, gold, molybdenum, titanium, titanium-tungsten, silicon carbide, alumina, and other metals.

16. The method of claim 1 wherein the pressurized processing solution comprises an acid selected from the group consisting of hydrofluoric acid, hydrochloric acid, sulfuric acid, Caro's acid, phosphoric acid, nitric acid, chromic acid, aqua regia, sulfuric/ammonium persulfate, buffered oxide etch, and carbonic acid.

17. The method of claim 16 wherein the pressurized processing solution further comprises ozone.

18. The method of claim 1 wherein the processing solution comprises a base.

19. The method of claim 18 wherein the base is selected from the group consisting of ammonium hydroxide, tetramethyl ammonium hydroxide, sodium hydroxide, potassium hydroxide, n-methylpyrrolidone, and monomethanol amine (MEA).

20. The method of claim 1 wherein the processing solution is pressurized to about 50 ATM or less

21. The method of claim 1 wherein the processing solution is pressurized to about 10 ATM or less.

22. The method of claim 1 wherein the processing solution is pressurized to between about 50 and 100 ATM.

23. The method of claim 1, further comprising a step of:

maintaining the processing solution above room temperature during the step of processing.

24. The method of claim 1, further comprising a step of:

maintaining the processing solution above a freezing temperature of the processing solution during the step of processing.

25. The method of claim 1, further comprising a step of:

after the step of processing, transferring the processing solution to a holding tank, while maintaining the processing solution under pressure.

26. The method of claim 1, further comprising:

removing the pressurized processing solution from the processing vessel after completion of processing;

introducing a second pressurized processing solution into the pressurized processing vessel to contact the substrate; and

processing the substrate to remove additional material from the surface of the substrate by allowing the second pressurized processing fluid to react with the additional material,

wherein said second processing solution is maintained at a pressure higher than atmospheric pressure prior to introduction into the processing vessel during introduction into the processing vessel and during at least part of processing of the substrate.

27. The method of claim 1, further comprising applying sonic energy to the pressurized processing vessel during the processing step.

28. The method of claim 1, further comprising:

evacuating the sealed processing vessel after the closing step; and

pressurizing the evacuated processing chamber by the introduction of a flow of gas prior to introduction of the pressurized processing solution.

29. The method of claim 28, wherein the evacuated processing chamber is pressurized by a flow of an inert gas.

30. The method of claim 28, wherein the evacuated processing chamber is pressurized by a flow of a gas that will react with the material.

31. The method of claim 1, further comprising:

pressurizing the processing solution to above atmospheric pressure prior to introduction into the processing vessel.

32. The method of claim 1, wherein the step of processing further comprises inducing turbulence in the pressurized flow of processing solution.

33. The method of claim 1, wherein the step of processing further comprises maintaining a laminar or a plug flow in the pressurized processing solution.

34. The method of claim 1, further comprising the step of rotating the substrate during the processing step to reduce a thickness of a liquid layer present over the substrate surface.

35. The method of claim 1, wherein a pressure of the processing vessel is changed during the processing step.

36. The method of claim 1, wherein the pressurized processing solution experiences a pressure drop upon introduction to the pressurized processing vessel, such that outgassing of a solution component occurs.

37. The method of claim 1, wherein the substrate is submerged in the processing solution.

38. The method of claim 1, wherein the processing solution is introduced into the processing vessel as a spray of droplets to wet a surface of the substrate.

39. The method of claim 1, wherein the surface of the substrate is suspended over the processing solution and thereby exposed to vapors from the processing solution.

40. The method of claim 1, wherein the substrate is rotated during at least part of the processing step.

41. The method of claim 1 wherein:

the step of closing the processing vessel comprises sealing a separate enclosure gas-tight around the processing vessel; and

the step of pressurizing the processing vessel comprises pressurizing the separate enclosure.

42. The method of claim 1 wherein the processing step comprises enhancing bubble formation in the processing solution.

43. The method of claim 1 wherein the processing step comprises suppressing bubble formation in the processing solution.

44. The method of claim 1 further comprising the step of drying the substrate after the processing step by generating a surface tension gradient between a meniscus of the processing solution on the substrate surface and a remaining bulk portion of the processing solution as the substrate is moved relative to the pressurized processing solution.

45. The method of claim 44 wherein the pressure induces a surface tension lowering component to be concentrated in the meniscus to generate the surface tension gradient.

46. The method of claim 1 wherein the processing solution further comprises a component selected from the group consisting of a reducing agent, a wetting agent, a surfactant, and a surface modifying reactant.

47. A method of drying a substrate comprising:

positioning a substrate within a gas-tight processing vessel;

pressurizing the processing vessel to greater than atmospheric pressure;

introducing a pressurized rinsing liquid into the processing vessel to submerge the substrate, the rinsing liquid comprising a surface-tension lowering component concentrated at a surface of the rinsing solution; and

moving the processed substrate relative to the rinsing liquid such that a surface tension gradient is created between a meniscus on the substrate surface and a remaining bulk portion of the rinsing liquid, the surface tension gradient drawing liquid from the substrate surface into the remaining bulk rinsing liquid.

48. The method of claim 47 wherein the surface tension lowering component is concentrated in the meniscus from a pressurized gas present in the gas tight processing vessel.

49. An apparatus for removing a material from a surface of a substrate, comprising:

a processing vessel configured to receive and contain a substrate in a gas-tight sealed environment;

a pressurized processing solution source in fluid communication with the processing vessel through an inlet valve, the pressurized processing solution source configured to containing a pressurized processing solution having a concentration of a component greater than available in the solution at atmospheric pressure; and

a drain valve enabling fluid communication of the processing vessel with a drain.

50. The apparatus of claim 49 further comprising:

a pressurized holding vessel configured to maintain a pressurized processing solution under pressure when not being used in the processing vessel; and

a control valve coupled between the processing vessel and the holding vessel for controlling flow of the processing solution from the holding vessel and to the processing vessel.

51. The apparatus of claim 49 further comprising a megasonic unit in acoustic communication with the processing vessel and configured to apply sonic energy to the processing vessel.

52. The apparatus of claim 49 further comprising a temperature control unit in thermal communication with the processing vessel and configured to apply or remove thermal energy from the processing vessel.

53. The apparatus of claim 49 further comprising a circulating loop including a pump configured to transfer the pressurized processing solution out of the processing vessel through a circulating outlet port and into the pressurized vessel through a circulating inlet port.

54. The apparatus of claim 49 further comprising a stirrer within the vessel to circulate fluid within the vessel.

55. The apparatus of claim 49 further comprising a vacuum pump in communication with the processing vessel through a vacuum valve.

56. The apparatus of claim 49 further comprising an ozonating apparatus in fluid communication with the pressurized processing vessel and configured to cause ozone to be dissolved in the pressurized processing solution.

57. The apparatus of claim 56 wherein the ozonating apparatus comprises:

an oxygen source;

an ozone generator in fluid communication with the oxygen source; and

an injector structure in fluid communication with the ozone generator, the injector structure configured to receive a flow of an ozone-containing gas from an outlet of the ozone generator and to cause ozone from the ozone-containing gas to be dissolved within a flow of the pressurized processing fluid.

58. The apparatus of claim 57 further comprising:

a surge vessel in selective fluid communication with the processing vessel through a surge valve; and

an ozonation loop in fluid communication with the surge vessel and with the injector structure.

59. The apparatus of claim 57 wherein the injector structure comprises a venturi injector.

60. The apparatus of claim 57 wherein the injector structure comprises a membrane gasifier.

61. The apparatus of claim 57 further comprising a static mixer to promote interaction between the ozone and the processing solution.

62. The apparatus of claim 57 further comprising a contact column to lengthen a contact time between the ozone and the processing solution.

63. The apparatus of claim 57 further comprising a degasser to remove bubbles from the processing solution.

64. The apparatus of claim 49 further comprising a nozzle configured to direct a pressurized flow of a processing solution into contact with a substrate present within the processing vessel.

65. The apparatus of claim 64 wherein the processing vessel is configured to submerge the substrate within the processing solution and the nozzle is immersed within the processing solution.

66. The apparatus of claim 64 wherein the nozzle is configured to spray droplets of processing solution onto the substrate.

67. The apparatus of claim 49 wherein the vessel is configured to support the substrate within a vapor phase produced from the processing liquid.