WO2001094076A1

WO2001094076A1 - Chemical-hydrodynamic etch planarization

Info

Publication number: WO2001094076A1
Application number: PCT/US2001/018218
Authority: WO
Inventors: Joseph Levert; Daniel Towery; Fan Zhang
Original assignee: Honeywell International Inc.
Priority date: 2000-06-08
Filing date: 2001-06-05
Publication date: 2001-12-13
Also published as: AU2001275265A1

Abstract

The present invention relates to the planarization of surfaces, typically surfaces of integrated circuit wafers during fabrication, by non-contact means. The surface to be planarized ('wafer') is mounted in a suitable chuck or holder and caused to undergo relative motion in close proximity with, but not touching, an opposing, proximate surface. The introduction of etchant chemicals into the gap between the two surfaces results in removal of materials from the surface of the wafer. Several configurations of wafer and proximate surface are presened as well as etchant chemicals. Advantages of the present invention include reducing shearing forces on the wafer that tend to delaminate multilayer wafers. Also, etchants lacking abrasive particles tend to be easier to reclaim and recycle, (or easier to dispose of in an environmentally satisfactory manner), resulting in reduced costs for etchant chemicals.

Description

CHEMICAL-HYDRODYNAMIC ETCH PLANARIZATION

BACKGROUND OF THE INVENTION

Technical Field The present invention relates to chemical etch planarization of surfaces and to chemical compositions and apparatus especially suited thereto. More particularly, the present invention relates to chemical etch planarization of surfaces achieved by the relative motion of surfaces in close proximity, generating thereby hydrodynamic effects in the gap between the proximate surfaces useful for controlling the planarization process and without the need for abrasive components of the etching slurry and without the need for mechanical contact of the etched surface with a polishing pad.

Description of Related Art

Modern designs for integrated circuits ("ICs") typically consist of multiple layers of material into which patterns are etched. Commonly, the layers consist of conducting, insulating and semiconductor material etched by means of photolithography (given by way of illustration, not intending to exclude thereby other arrangements of material or other means of patterning or etching). The near-universal trend in the manufacture of integrated circuits is to increase the density of components fabricated onto a given area of wafer, to increase the performance and reliability of the ICs, and to manufacture the ICs at lower cost with less waste and fewer defective products generated by the manufacturing process. These.goals lead to more stringent geometric and dimensional requirements in the manufacturing process. In particular, etching precise patterns into a layer is facilitated by the layer having a surface as nearly planar as feasible at the start of the etching process. For the common case of patterning by means of photolithography, a planar surface permits more precise location and dimensioning for focusing the incident radiation onto the surface to be etched than would be possible with a surface having deviations from planarity. Similar conclusions apply for electron beam or other means of etching. That is, deviations from planarity of the surface to be etched reduce the ability of the surface to support precisely positioned and precisely dimensioned patterns, hi the following description of the present invention we focus on the typical etching, planarization and patterning processes as practiced in the manufacture of ICs. However, this is by way of illustration and not limitation, as those skilled in the art will appreciate that the techniques of the present invention for producing planar surfaces will have applicability in increasing the precision of etching by means other than photolithography. In addition, the present invention is not limited to the field of IC manufacture and will find applicability in other areas of technology requiring planar surfaces.

A common method for the planarization (that is, made flat and smooth) of integrated circuit surfaces during the manufacturing process is chemical mechanical polishing ("CMP") depicted schematically in Figure 1. CMP typically involves a wafer to be planarized, 1, being pressed firmly against a polishing pad, 2 by means of force, 6, directed substantially perpendicular to the surface of the wafer to be planarized. Typically, the wafer, 1, will be caused to rotate as depicted by 3 in Figure 1, while the polishing pad will itself rotate (4 in Figure 1). Figure 1 depicts the polishing pad and wafer rotating in the same direction (for example, clockwise when viewed from above as in Figure 1). However, this is merely for purposes of illustration and counter-rotation of wafer and polishing pad is also practiced in one embodiment of conventional CMP. In addition to the rotation of the wafer depicted by 3 in Figure 1, the wafer, 1, maybe caused to oscillate in the plane of the surface being polished, substantially perpendicular to the plane of the applied force, 6 (This oscillatory motion is not depicted in Figure 1).

Typically, wafer, 1, is held firmly by a retaining ring fixed to a rotating wafer carrier, commonly gimbaled. The CMP process typically uses an abrasive slurry, 5, continuously introduced (dripped) onto the polishing pad, 2, throughout the planarization process. The abrasive slurry, 5, may also contain chemicals capable of reacting with the material to be removed from the surface of wafer, 1, the reaction products leaving the wafer's surface and typically residing on and in the polishing pad. Thus, conventional CMP typically employs both mechanical abrasion and chemical reactions to remove material from the surface of wafer 1 to achieve a planar surface.

The polishing pad, 3, is typically made of polyurethane or fibers impregnated with polyurethane, although other materials may also be used. The polishing pad is typically attached to a rigid, temperature controlled platen and rotated as depicted schematically in Figure 1.

Several significant drawbacks occur in the practice of conventional CMP, many of which relate to the use of polishing pad, 2. The polishing pad accumulates abrasive slurry and excess chemical reactive materials as well as material removed from the wafer both by abrasion and chemical reaction. Thus, the polishing pad requires an additional process commonly referred to as "pad conditioning" typically performed concurrently with the planarization of the wafer depicted in Figure 1. Various conditioning implements maybe brought to bear on the surface of the polishing pad at the same time as CMP is being performed, the conditioning implements being in contact with a portion of the polishing pad removed from the location of the wafer. (Pad Conditioning implements are not depicted in Figure 1). Pad conditioning (or co-processing) relates to the process of removing contaminants from the polishing pad to avoid degradation in performance from one wafer to the next or, in some cases, during the processing of a single wafer. Without pad conditioning, the removal rate, uniformity and planarity of the wafer material is frequently unstable from wafer to wafer making it impossible to use conventional CMP in practical IC production processes.

Pad conditioning is typically performed with a diamond-impregnated ring or disk tools pressed against the rotating polishing pad. This process removes from the polishing pad material removed from the wafer surface including CMP reaction products, abraded materials and unconsumed abrasive, reactive slurry, 5. Pad conditioning is thus necessary to prevent material build-up on the pad and the attendant degradation in performance. However, diamonds may occasionally fall from the pad conditioning disk onto polishing pad, 2, resulting in scratches on the surface of wafer, 1. Following a typical CMP, a cleaning operation is typically required of the polishing pad, 2, to remove as many contaminants resulting from the planarization process as possible. This post-CMP cleaning is typically performed by scrubbing with a mechanical brush with the application of specialized cleaning chemicals. Such post-CMP cleaning of the polishing pad increases the complexity of the overall CMP process requiring additional process tools, processing time and additional consumable items such as the cleaning chemicals. Thus, while CMP has generally been successful in planarizing surfaces, it is a costly and complicated process with numerous processing parameters that have been difficult to control precisely in typical manufacturing environments.

While CMP has been successfully employed in many planarization processes, it has several disadvantages which the present invention intends to reduce or eliminate. As noted above, CMP is a rather complex and costly multi-step process having many processing parameters that are generally difficult to control in a practical manufacturing environment. In addition, CMP is a mechanical process subjecting the wafer (typically a multi-layer IC) to shear stresses. Some of the IC layers may consist of films having low dielectric constant, that are often mechanically weak relative to conventional dielectrics, tending to delaminate under the shear stress of CMP. Application of shear stress is contraindicated for such layers and may result in damage. The downward force, 6, causing contact between the wafer, 1, and polishing pad, 2, typically will be sufficient to cause a small amount of deflection in the surface of polishing pad, 2. Polishing by means of a deflected pad will typically result in removal of material from the surface being polished partially from lower regions of the surface which are not readily accessible to contact by a flat polishing pad. Thus, polishing with a deflected pad will require a longer time and the removal of more material to achieve planarity than would use of a nondeflected, flat polishing pad.

Abrasive or other particles from slurry, 5, may contaminate the surface of wafer, 1 or result in scratches therein. Both are undesirable. The presence of solid material in slurry, 5, makes reclaiming or recycling the slurry impractical and complicates the processing of the waste from CMP planarization. The present invention intends to reduce or eliminate some or all of these disadvantages in conventional CMP planarization, resulting thereby in improved planarization.

One form of chemical etching involves the application of etching reagents to a spinning surface, spin etch planarization ("SEP") as described in a prior patent application commonly assigned herewith (09/356,487) and incorporated herein by reference. The rotational motion of the substrate undergoing etching results in centrifugal forces on the etching reagent and consequent etchant dispersal and flow over the surface. SEP thus results in non-contact chemical etching and planarization. SEP offers several potential advantages over conventional CMP. Among these are the possibility of reclaiming for reuse chemical reagents not consumed by SEP processing, thereby reducing waste of reagents and lowering processing costs. Contaminated, reacted or otherwise non-reusable reagents are typically liquids in SEP, lacking the significant amount of dissolved solids generally found in conventional CMP. Therefore such SEP by-products are generally more easily treated for recycling or environmentally satisfactory disposal. Further lowering the cost of SEP over conventional CMP is the relatively less complex machinery and associated equipment required by SEP.

The present invention relates to chemical-hydrodynamic (non-contact) planarization ("CHP") as a method for removing material and forming a highly planar surface. A polishing pad or other surface is brought into close proximity with the surface to be etched but not in contact therewith. We denoting this nearby (non-contacting) surface as the "proximate surface." Rotation or other relative motion of the etched surface and the proximate surface causes hydrodynamic effects in the gap between the proximate surfaces, leading to different and improved methods for controlling the planarization process. The present invention relates to configurations for CHP and to apparatus, systems and chemical compositions specially adapted thereto. It is envisioned that the present invention will be primarily applicable to non-abrasive planarization processes as a modification and improvement of spin-etch planarization. However, the present invention is not limited to purely chemical planarizations and the several embodiments of the present CHP invention can also be used in connection with abrasive-containing etchant solutions. Abrasive-containing CHP does not typically result in the advantages of a particle-free slurry as described above. However, in particular cases the hydrodynamic effects alone may be sufficiently advantageous as to compel the use of CHP along with abrasive slurries All-liquid chemical etching (or polishing) of copper is typified by the work of

Tytgat and Magnus (US Patent Nos. 4,981,553 and 5,098,517). In this work, a solution of chemicals capable of etching the substrate (typically copper) at the required rate and uniformity is described along with typical conditions of use. Typically, the surface to be etched is dipped, immersed or otherwise bathed in the etching solution for the appropriate amount of time.

Chemical etching has been used for planarization in the work of Cibulsky et. al. (US Patent No. 5,759,427) in combination with a mechanical head contacting and rubbing the surface of the substrate during processing. In this work, a chemical etching solution and rotating planarizing head are brought into contact with the surface to be planarized. Electropolishing is another means to planarize surfaces, particularly multilevel metallizations as typically found in ICs as in the work of Contolini and co-workers (5,096,550 and Solid State Technology. Vol. 40 (6), pp. 155-162 (1997)). This work relates to the anodic leveling of embedded copper lines making use of an electrolytic bath and a rotating, submerged anode. Electropolishing techniques are useful supplements to the CHP of the present invention as described in detail below.

Additional approaches to non-contact planarization include the work of Ahern (5,040,336), in which a non-contact polishing system using a rotating motion for polishing semiconductor substrates is described. Jet flow polishing techniques of Mil'shtein and Fournier (Materials Letters. Vol. 9 (4), pp. 133-135 (February 1997)) describe liquid etchant freely flowing past a wafer, lacking the gap-induced hydrodynamic effects of CHP. Float polishing of quartz is described in the work of Soares et. al. (Applied Optics, Vol. 33 (1), pp. 89-95 (January 1994)). The present invention uses hydrodynamic effects generated by the relative motion of surfaces in close proximity to effect and control planarization. The present invention employs in a preferred embodiment only chemical etchants in a liquid form contacting the surface undergoing planarization. However, the present invention is not inherently limited to abrasive-free etchants. The etching systems, procedures and conditions for use described in the present invention provide adequate control of the etching process to achieve adequate planarization in reasonable amounts of time while reducing or eliminating many of the drawbacks associated with conventional CMP as described above.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to the planarization of surfaces, typically surfaces of integrated circuit wafers during fabrication, by non-contact means. The surface to be planarized

("wafer") is mounted in a suitable chuck or holder and caused to undergo relative motion in close proximity with, but not touching, an opposing, proximate surface. The introduction of etchant chemicals into the gap between the two surfaces results in removal of materials from the surface of the wafer. The properties of the etchant, gap and relative motion of the surfaces are chosen such that diffusion of etchant to the wafer surface tends to control the removal of material from the wafer. Thus, protruding regions of the wafer are preferentially etched, leading to planarization. Several configurations of wafer and proximate surface are presented as well as etchant chemicals. The etchant may, but need not, be free of abrasive particles. Advantages of the present invention include reduced shearing forces on the wafer that tend to delaminate multilayer wafers. Also, etchants lacking abrasive particles tend to be easier to reclaim and recycle, (or easier to dispose of in an environmentally satisfactory manner), resulting in reduced costs for etchant chemicals. Other advantages of the present invention in comparison with conventional CMP or other techniques are described elsewhere herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures herein are schematic depictions, not drawn to scale.

Figure 1: Schematic depiction of conventional chemical-mechanical -planarization ("CMP").

Figure 2: Cross-sectional, magnified schematic depiction of liquid-solid interface, boundary layer, flow and diffusion of reagents during planarization.

Figure 3: Schematic cross-sectional side view of one embodiment of wafer, carrier, turntable and hydrodynamic boundary layer.

Figure 3 A: Schematic top view of relative rotation of wafer and proximate surface.

Figure 4: Schematic perspective view of an alternative embodiment of wafer and moving proximate surface.

Figure 5: Schematic side view of an alternative embodiment of wafer and moving proximate surface.

Figure 6: Schematic side view of an alternative embodiment of wafer and moving proximate surface.

Figure 7: Schematic depiction of electropolishing embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description and figures, similar reference numbers are used to identify similar elements. The chemical-hydrodynamic planarization ("CHP") of the present invention makes use of elementary chemical and physical processes related to those previously described in connection with SEP (09/356,487), with specific and improved embodiments making use of hydrodynamic phenomena deriving from substantially parallel moving surfaces in close proximity. Most embodiments of this invention have the wafer in close proximity to a single surface with relative motion between the wafer and the proximate surface. (With respect to the laboratory either or both the wafer the proximate surface(s) may be in motion so long as relative motion results that is suitable for effecting the hydrodynamic control of the present invention.)

One embodiment of the present invention (Figure 6) utilizes a plurality of moving surfaces in proximity with the wafer. In such a multi-surface embodiment, each of the proximate surfaces should preferably be substantially parallel to the wafer surface over at least a limited area.

The present invention in connection with CHP may make use of either abrasive slurries or all-liquid etchant (that is, etching reagents lacking abrasive particles). It is envisioned in the practice of the present invention that all-liquid etching will be the preferred mode of planarization. However, it is understood herein that the use of abrasive slurries in connection with the planarization systems of CHP is included within the scope of the present invention.

The basic non-contact planarization processes using an all-liquid etching solution (i.e. no abrasive particles to form a slurry) fall into several general classifications of chemical and physical mechanisms, used singly or in combination, as follows: a) Diffusion controlled reactions to etch preferentially protruding regions of the surface, thereby facilitating planarization. b) Balanced oxidation and reduction of oxide to facilitate uniform removal of material from successive surface layers. c) Self-galvanic microcouples on the surface being etched, facilitating uniform galvanic action on a very fine dimensional scale for uniform removal of material and avoidance of pitting. d) Selection of chemical etchants and additive chemicals to assist in achieving selective removal of multiple layers of different materials without losing planarization.

a) Diffusion Controlled Reactions Important control of the process of planarization is achieved by making use of diffusion-limited reactions, enhanced by the configurations of CHP described herein. That is, the physical contact of the reagent(s) used in the practice of the present invention is used to affect and control the chemical etch processes occurring on the surface to be planarized. A combination of reagents, diluents (inert solvents carrying the reactive etching species), temperatures and other conditions, are selected such that diffusion of the reagents through solution and to the reaction sites on the surface determine the rate of planarization. Thus, diffusion in the direction normal to the surface is an important reaction-controlling mechanism in this embodiment of the present invention.

It is generally understood in the flow of liquids over surfaces that a substantially stationary boundary layer occurs at the liquid-solid interface, and liquid flow increases in velocity (in a direction parallel to the surface) with increasing distance away from the surface until achieving the flow rate of the bulk liquid in the absence of a surface. This is generally true for smooth as well as rough surfaces as depicted schematically in Figure 2. The relatively higher regions of the surface to be etched tend to encounter more rapidly moving fluids. In the practice of the present invention, the moving fluid is the etching reagent(s). Thus, under diffusion-limited reaction conditions, the higher regions of the surface to be etched encounter more etching reagent(s). This etches the higher regions more rapidly than the recessed regions as the elevated regions encounter continuously replenished etchant while recessed regions, less accessible to etchant, react more slowly. Additionally, the reaction products of elevated region etching tend to diffuse downward onto the surface to be etched further slowing etching in such regions and leading to the desired planarization effects. Thus, if the most diffusion-inaccessible regions of the surface lack sufficient etchant for complete reaction, preferential etching of elevated surface regions results as only such regions encounter sufficient etchant. Specifically, 14 in Figure 2 depicts schematically a typical gradient of fluid velocities increasing with distance from the surface of the solid, 12. Thus reagent species, 13, more readily flow past the upward-projecting portions of the surface (schematically 16 in Figure 2) continually replenishing the fluid in contact with such elevations with etching reagent. Lower regions, 12 in Figure 2, do not typically contact faster flowing portions of the reagent stream, 14, as it moves across the surface, 12, of the wafer to be planarized. Under the reaction conditions of the present invention, reaction rates leading to planarization are typically diffusion-limited. Thus, the relatively higher fluid flow in the vicinity of region 16 in comparison with region 12 tends to more rapidly etch region 16, facilitating planarization. The near-stagnant fluid in the lower regions (adjacent the surface) requires that the active chemical etching species diffuse vertically downward for a substantial distance, which occurs rather slowly under typical reaction conditions obtaining during planarization. This slow diffusion process typically limits the total supply of etchant at the reaction sites thereby limiting the etch rate.

In contrast to the near-stagnation of reagent in the regions near the surface, rapid flow of reagents just above the protruding regions continually replenishes the etchant species in these local regions. The relatively small vertical distance from the flowing etchant to the protruding portions of the surface allows a greater cumulative supply of etchant species to reach these regions by diffusion. Thus, greater reaction rates at protruding regions of surface are expected, resulting in greater etch rates for protruding regions than for lower surface regions. Surface planarization follows as the protruding regions are etched more rapidly. When operating under diffusion-limited conditions, the physical properties of the reagent solution affecting diffusion become important as well as the chemical properties. Thus additives controlling viscosity, surfactants, wetting agents, and other diffusion-altering additives, all have a role in affecting the diffusion properties of the reagent solution. Temperature also affects diffusion as well as affecting some chemical reactions and is, therefore, also a useful parameter to control in some embodiments of the present invention. Figure 3 shows in schematic cross section one embodiment of an apparatus for performing CHP. This apparatus makes use of planetary kinematics analogous to that employed in many mechanical polishing or conventional CMP applications. Wafer, 1, is held rigidly in a wafer chuck, 3, attached to carrier, 10 by means of rotary bearings, 7. A gimbaled joint, 8, connects carrier, 10, to loading arm, 9, through which vertical force , 6, is applied to maintain the desired spacing between wafer, 1, and turntable (pad), 4. The loading shaft, 9, is prevented from moving in any direction except vertical as depicted in Figure 3. The gimbaled joint, 8, permits the unit of wafer/chuck/carrier (1/3/10) to vary in inclination to maintain a substantially uniform etchant film thickness, 13, between wafer, 1, and turntable, 4. The inclination depicted in Figure 3 is exaggerated for purposes of clarity in depiction, not to indicate that inclinations as large as that depicted in Figure 3 actually occur in the practice of CHP. The depictions in the figures herein are schematic only and not to scale.

It is worthwhile to note that the gimbaled joint, 8, is not aligned with the wafer center. Control of this offset controls the angle of the wafer with respect to the turntable and thereby controls the thickness of the etchant film between the wafer and the turntable. The thickness of the etchant film controls the velocity gradient of the boundary layer which, in turn, controls the rate of diffusion. The rate of diffusion has a significant effect on the planarization ability. Figure 3 A depicts the wafer and turntable in the present invention to be rotating in the same direction (as also depicted in Figure 1). Conventional CMP may make use of both same-sense-rotation ("parallel" as depicted in Figures 1, 3 and 3 A) as well as opposite-sense- rotation ("anti-parallel" or "counter-rotating"). However, in the practice of the present invention it is preferred that parallel rotation be employed to maintain a nearly uniform velocity profile of etchant beneath the wafer over time. Parallel rotation tends to generate over time a uniform boundary film thickness of etchant against the wafer surface between the wafer and the turntable. This uniform boundary film, in the course of time, tends to generate uniform shearing forces at all locations over the wafer surface. Uniform shearing forces and boundary layer velocity gradients allow for more precise control of the etching process. Uniform shearing avoids the generation of abnormally large shearing forces at some locations on the wafer surface. Such abnormally large shearing forces may delaminate layers of the wafer at the location of such large forces, essentially destroying the utility of the entire wafer. Thus, the generation and maintenance of uniform shearing forces by CHP using apparatus of the type depicted in Figure 3 leads to an improved etching process with more uniformity and better process control.

Another embodiment of a CHP system according to the present invention is depicted in Figure 4. Figure 4 utilizes a continuous flexible belt, 11, in place of the turntable. The close proximity of belt, 11, to wafer, 1, results in the desired gap in which the hydrodynamic boundary layer of etchant may be formed. Moving belt, 11, in direction 17 while oscillating wafer, 1, in direction 3 exposes the entire surface of the belt approximately uniformly to the surface of the wafer. It is preferred in embodiments of the present invention that the wafer not reside predominantly adjacent to the same region of the turntable or belt. While there is an absence of direct, mechanical contact between belt and wafer, diffusion of reaction products away from the region in which they are generated is not expected to be rapid. Thus, a form of etchant "aging" in the region directly adjacent the wafer may occur as reaction (etching) products tend to collect in such regions. Therefore, preferable practice of the present invention makes us of as much of the turntable surface as is feasible. An additional embodiment of the present invention as related to a system for performing CHP is depicted in Figure 5. Turntable, 19, in this embodiment consists of a compliant disk or bladder using internal fluid (liquid or gas) to maintain uniform "contact" with wafer. "Contact" as noted in Figure 5 is intended to indicate an approximate uniform spacing between wafer, 1, and bladder, 19, short of direct mechanical contact. In addition, the wafer, 1, may be moved laterally (in the direction perpendicular to the plane of the page) to reduce etchant aging analogous to the motion depicted in Figure 4. In all cases, the uniform motion of the wafer with the turntable (proximate surface) generates the substantially uniform boundary layer required for effective CHP.

Yet another embodiment is depicted in Figure 6 in which compliant rollers, 20, provide the flexibility to generate the hydrodynamic boundary layer required between wafer and turntable. In this embodiment, there are a plurality of proximate surfaces, each maintaining a local region of substantially parallel boundary layer and gap with the wafer surface. This embodiment offers several more degrees of process control as each roller, 20, can be adjusted as to speed, material, temperature, etc. independent of the others.

b Balanced Oxidation and Reduction: Examples from the planarization of copper, tantalum, tantalum nitride (Cu/Ta/TaND.

As in SEP, effective planarization making use of CHP of the present invention involves a combination of chemical species and chemical reactions. One such reaction is the oxidation of the surface to form an oxide in combination with reaction with a co-reactant selected so as to reduce or otherwise remove the oxide thus formed.

Oxidation by a suitable oxidizing species uniformly oxidizes the copper surface thereby "passivating" the metal. The oxide or similar passivation film partially protects the underlying metal layer (typically copper in the present example) which thereby limits further oxidation of the metal. Accelerated local oxidation of the metal frequently results in pitting and/or loss of surface planarity.

As the passivation firm is formed, reaction with a co-reactant occurs. The co-reactant is chosen so as to remove the passivation film by reduction or some other chemical mechanism. The co-reaction to remove the passivation film needs merely to produce a reaction product that dissolves and is removed by the chemical solution in the vicinity of the surface. The newly exposed metal surface is again exposed to oxidation, formation of a passivation layer and removal by co-reactant. This cycle recurs many times during planarization and is helpful in maintaining planarity in the practice of the present invention.

c Self-Galvanic Microcouple

Microscopic differences in the surface structure and chemical environment of materials lead to different regions of the same surface having different electrochemical properties. Regions of pure metal, grain boundaries, other defects or dislocations are sufficient to provide regions having different electrochemical potentials. Thus, in'the presence of an electrolyte (i.e. the etchant) self-galvanic microcouples arise connecting such regions as anode and cathode, leading to electrochemical removal of material, typically by means of an oxidation/reduction reaction set. These non-uniform surface regions leading to self-galvanic couples occur substantially uniformly over the surface and have microscopic dimension (hence, "microcouples"). Thus, such galvanic couples lead to material removal on a very fine scale, avoiding removal of large amounts of material from any localized area. Large scale galvanic couples, when present, commonly result in deleterious pitting of the surface. Microcouples, on the other hand, can be employed in a process of electropolishing. Electropolishing has been employed for integrated circuit planarization as in the references cited above.

It is envisioned in the practice of the present invention that electropolishing may optionally be used in conjunction with CHP. That is, the CHP removal of metal films can be enhanced with electropolishing generated by an applied potential in combination with specific chemical etchants and electrolytes suitable for electropolishing the particular surface under consideration (for example, Cu Ta/TaN).

Electropolishing may be employed for virtually all metals and alloys and substances having suitable electrical conductivity. A typical electropolishing system is depicted in Figure 7. During electropolishing, the workpiece (wafer) to be electropolished, 22, is electrically connected to the anode (+) terminal of a dc power supply, 21, and the cathode (-) terminal connected to an inert electrode, 23. Alternatively, the power supply can be pulsed or reverse pulsed. Both workpiece and inter electrode are immersed into a bath of etchant, 24, typically an acidic solution of etchants such as phosphoric acid. Application of a current leads to preferential dissolution of points and projections on the workpiece, 22, leading to a smoothing (planarization) of the surface. The formation of a viscous boundary layer at the surface of the workpiece also contributes to the preferential dissolution of peaks and projections, enhancing planarization. Control of solution chemistry, temperature, current density and time leads to reasonably precise process control. By choosing appropriate solution electrolytes, higher removal rates for selected surface components (such as Ta or TaN) can be achieved. Electrochemical planarization can be used to enhance surface smoothness in conjunction with the CHP processes and equipment of the present invention.

d Additive Chemicals

In combination with any other embodiment of the present invention, additive chemicals may be introduced into the reagent mixture for the purpose of modifying (typically slowing) reaction rates and/or adjusting diffusion rates by adjusting solution viscosity. Other uses for chemical additives include enforcing a more uniform chemical reactivity over a wider surface area and assisting in allowing preferential removal of one type of metal in preference to another when processing bimetallic or multimetallic layers. Such chemical inhibitors are chosen to ensure that the material removal is done without loss of planarization.

The present invention is not limited to a single step. Multiple steps are included within the scope of the present invention. Examples include application of multiple chemical reactive solutions, possibly including an initial passivation step followed by application of a reagent mixture which equally passivates and dissolves the surface yielding thereby a controlled, smooth planar surface. This procedure could typically be followed by a final etching step to remove and passivate material, followed by a final rinse (typically with de- ionized water) for cleaning. Other embodiments of the present invention relate to the chemical etch planarization of surfaces in which more than one substance is exposed to the etching reagent(s). In this embodiment, the reagent mixture may contain surfactant chemicals that preferentially bind to one (or some) of the exposed substances or selectively alter the chemical properties of one (or some) of the surface constituent materials. Preferential etching follows, typically resulting in selective planarization of the surface in this embodiment of the present invention.

Practical industrial applications may require the reagent mixture to contain other additives to inhibit premature reaction, stabilize the mixture, increase shelf life of the reagent mixture, reduce volatility, inhibit toxicity, inhibit photodegredation, and the like. Such additives are known to those skilled in the art and are not otherwise specified in detail herein.

Another class of additives are those that affect the viscosity of the etchant with minimal effects on the etchant's chemical etching capability. These viscosity modifiers (such as glycols) affect the thickness and velocity distribution of the boundary layer. Modifying the boundary layer assists in modifying the diffusion-controlled reaction mechanism to achieve planarization of non-planar surfaces.

The present invention relates to planarization of surfaces, such as wafers for integrated circuits, in which mechanical abrasion of the surface with a polishing pad or abrasive particles is absent.

The Cu Ta/TaN slurries, or etchants, described herein are improved formulations of chemicals typically used for conventional CMP including abrasive slurries, metal etchants and cleaners, chemical polishes, brighteners, and pickling solutions, etc. Such chemical mixtures are typically comprised of one or more of the following constituents:

TABLE A

Mineral Acids Organic Acids Strong Bases Mineral Salts Organic Salts pH buffers Oxidizing Agents Corrosion Inhibitors Chelating Agents Surface Modifying Agents Liquid polymers Surfactants Solution Stabilizers Solvents (including water) fri general, CHP carried out according to the present invention using the etchants described herein requires a method of introducing the etchant onto the wafer surface and preferably a method for transporting the etchant across the wafer surface. The CHP methods of the present invention using etchants described herein may, but need not, include abrasives in the etchant mixture and avoid mechanical contact between the wafer surface and another surface or body, such as a polishing pad. Embodiments of the present invention functioning without contact between the wafer and (for example) a polishing pad are described herein and included within the scope of the present invention. Hydrodynamic effects occurring in the gap between the wafer and the proximate surface increase the flexible modes of controlling the etching process.

In the non-contact CHP processes described herein, the wafer does not actually contact an opposing surface such as a polishing pad. This is depicted schematically in Figure 3 in which wafer, 1, is held in a suitable mount or chuck, 3, in close proximity to a nearby surface, 2, in order to create a boundary region, 13. The embodiments of Figures 4, 5, and 6 omit depicting the boundary layer and gap between the wafer and proximate surface for purposes of graphical clarity. The existence of such a gap and boundary layer of etchant is presumed. The boundary region, 13, captures the etchant solution therein, maintaining the etchant in close proximity with the surface of wafer, 1. The thickness of boundary region, 18 , (that is the gap between opposing body, 2, and wafer, 1), will typically be in the range of 10- 100 μm. The creation of a boundary region has proven to be a useful means of controlling the motion of the etchant solution over the wafer surface in this embodiment of the present invention.

Several process parameters may be adjusted to give acceptable planarization by means of CHP including, variation of the distance of the opposing body from the wafer surface, the type and speed of relative motion of the wafer and the opposing body, the surface texture and material of the opposing body. The opposing body may be of any material that provides the desired rigidity, durability, shape and chemical properties. The surface of the opposing body may be processed or coated to provide the desired surface texture, consistent with the other processing parameters of CHP. The etchant solution may, but need not, contain abrasive particles. The motion of the boundary layer of etchant solution, 13, over the wafer, in proximity to the opposing surface, will provide a controllable degree of mechanical etching effected by the fluid-borne motion of abrasives over the wafer surface. Thus, mechanical abrasion will occur even in the instance that the abrasive particles are chemically inert with respect to the surface being etched (i.e. do not themselves interact chemically with the wafer surface). Alternatively, abrasive particles may be chemically active, reacting with the wafer surface undergoing etching to remove material by chemical modes as well as mechanical abrasion. The use of a combination of inert and reactive abrasive particles is also included within the scope of the present invention.

Heating the etchant solution is another parameter that may be adjusted as a means for controlling the removal and rate of removal of material from the surface of the wafer. Temperature control of the reaction may be achieved by introducing the etchant onto the wafer preheated to the desired temperature, by heating the opposing body, and or by maintaining the opposing body at a constant temperature. For a sufficiently narrow boundary region, controlling the temperature of the opposing body will substantially affect the temperature of the etchant solution in contact with the wafer. Thus, temperature adjustment and control of the etchant solution and/or temperature control of the opposing body are within the scope of the present invention. Improved planarization via CHP is achieved in some embodiments by means of etching solutions as described below. Abrasive particles are optionally included and may optionally react with the surface of the wafer being etched as well as provide mechanical removal of wafer material. Many suitable abrasive materials could be used, including conventional abrasives (SiO₂, Al₂O₃ and the like), and various nonconventional abrasives that are comprised of metals, solid elemental particles (for example carbon), polymer particles, oxides, carbides, fluorides, carbonates, borides, nitrides, or hydroxides of several metals, including, but not limited to, Al, Ag, Au, Ca, Ce, Cr, Cu, Fe, Gd, Ge, La, hi, Hf, Mn, Mg, Ni, Nd, Pb, Pt, P, Sb, Sc, Sn, Tb^ Ti, Ta, Th, Y, W, Zn, Zr, or mixtures thereof. These particles may be coated with a thin layer of another material, including but not limited to those described above. The potential advantages of the use of coated particles are expected to include decreasing cost by coating a less dense, inactive and inexpensive particle, such as SiO₂, with a chemically active, and often more dense and expensive active material such as CeO₂. The effective density of such particles will be less than solid particles comprising all chemically reactive material, and thus more stable in terms of particle settling according to Stokes Law which predicts a larger settling velocity for particles having a higher density.

Similarly for a given wt% of solids, slurries comprised of coated abrasive particles (typically less dense) will have a greater number of particles in a given volume of fluid and thereby present more effective abrasive surface area in contact with the wafer surface. It is envisioned in the practice of the present invention that many of the particle systems described herein will be produced by means of the "sol" method. This typically involves growing the particles to their final size in solution. By growing the particles entirely in solution and remaining in solution for use (that is, never dried) there is no sintering or "necking" of the particles that will result in large agglomerate, which may be damaging to the sensitive IC layers, or underlying structures. Having avoided agglomeration, these particles are introduced into solvent systems very readily and at lower cost than conventional abrasives that typically must undergo additional and expensive particle size reduction and powder dispersion processing. The practice of the present invention makes use of several particle size distributions. A bi-modal particle size distribution, or a multi-modal particle size distribution, or a broad Gaussian particle size distribution, may all be employed in the practice of the present invention with typical particle sizes in the range 4 nm to 5μm. It is envisioned in the practice of the present invention that particle sizes will not be a critical parameter, particularly in terms of increasing removal rates and reducing defects and scratches. We note elsewhere herein typical components of the etching reagents useful in the practice of the present invention. Practical industrial applications may also require the reagent mixture to contain other additives to inhibit premature reaction, stabilize the mixture, increase shelf life of the reagent mixture, reduce volatility, inhibit toxicity, inhibit photodegradation, and the like. Such additives are known to those skilled in the art and art not otherwise specified in detail herein.

Tables 1-12 following are examples of reagent mixtures usefully employed in the practice of the present invention for planarizing copper surfaces or other surfaces as indicated on the Tables.

TABLE 1: AQUEOUS PEROXIDE - PHOSPHORIC ACID REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer Reducer Other Additives a) H₂O₂ H₃PO₄ HCl, aliphatic alcohols b) H₂O₂ H₃PO₄ HCl,

Agidol (butylated hydroxytoluene) c) H₂O₂ .H3PO4 HCl, Agidol-2,

d) H₂O₂ H₃PO₄ HCl, 2,6-di-tert-butyl-

4[(dimethylamino) methyljphenol

e) H₂O₂ H₃PO₄ [1] HCl; H₃PO₄, (HPO₄)^2", PO₄ ^3" f) H₂O₂ H₃PO₄ HCl, 2,6-di-tert- -4N,N-dimethyl aminomethylphenol g) H₂O₂ H₃PO₄ borax h) H₂O₂ H₃PO₄ various additives

TABLE 2: AQUEOUS PEROXIDE - SULFURIC ACID REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer Reducer Other Additives

a) H₂O₂ H₂SO₄ / H₃PO₄ Ethylene glycol,

ZnSO₄ b) H₂O₂ H₂SO₄ MeOH, Poly(oxy ethylene)lauryl ether,

Malic acid c) H₂O₂ H₂SO₄ HOOC(CX₂)„COOH with X=OH, amine, H n=l-4

d) H₂O₂ H₂SO₄ 3% tartaric acid

1% ethylene glycol e) H₂O₂ H₂SO₄ 1,2,4-triazole,

1,2,3-triazole, tetrazole, nonionic surfactant f) H₂O₂ H₂SO₄ C₂H₅OH, aliphatic alcohols, nonionic surfactant

g) H₂O₂ H₂SO₄ Triflouroethanol, Laprol 602^® surfactant, aliphatic alcohols h) H₂O₂ H₂SO₄ aliphatic alcohols i) H₂O₂ H₂SO₄ SiF₆, Organic salt surfactant j) H₂O₂ H₂SO₄ various additives TABLE 3: AQUEOUS PEROXIDE MINERAL ACID REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer " Reducer Other Additives

a) H₂O₂ HNO₃ alcohols,

HOOC(CX₂)„COOH X=OH, amines, H n=l-4

b) H₂O₂ HNO₃ various additives

TABLE 4: AQUEOUS NITRIC ACID REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer Reducer Other Additives

a)H₂O₂/HNO₃ H₃PO₄ MeOH b)H₂O₂/HNO₃ Triflouroethanol,

Laprol 602^®

Surfactant, aliphatic alcohols c)HNO₃ H₃PO₄ PVA d)HNO₃ H₂SO₄ diphenylsulfamic acid. aliphatic alcohols e)HNO₃ H₂SO₄ HCl f)HNO₃ H₂SO₄ various additives g)HNO₃ BTA (benzotriazole)

TABLE 5: AQUEOUS PEROXIDE ORGANIC ACID REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer Reducer Other Additives

a) H₂O₂ Oxalic acid Sodium oxalate, Benzotriazole, Sodium Lignosulfonate b) H₂O₂ other organic various additives acids

TABLE 6: AQUEOUS DILUTE MINERAL ACID REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Acid Other Additives

a) H₃PO₄ various additives

TABLE 7: AQUEOUS CONCENTRATED ACID REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer Acids Other Additives

a) H₃PO₄/Acetic/H₂SO₄ b) H₃PO4/Acetic/HNO₃ c) H₃PO₄/Acetic/HNO₃/ H₂SO₄

Glycol, Gelatine Carboxymethyl- cellulous, amines, surfactants, heavy metal salts including Cu and Ta. d) H₂O₂ H₃PO /Acetic/H₂SO Glycol, Gelatine Carboxymethyl- cellulous, amines, surfactants, heavy metal salts including Cu and Ta.

e) H₂O₂ H₃PO₄ H₂SO₄ 100ml propylene glycol, 100ml 2-ethyl- hexylamine, 25 ppm cr.

f) H₃PO₄/Acetic/HNO₃ nonionic surfactant

g) H₂O₂ H₃PO₄/Acetic/HNO₃/ H₂SO₄ various additives h) H₃PO₄/HNO₃ TABLE 8: AQUEOUS DILUTE ACID - METAL SALT REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer Acid Metal Salt Other Additives a) HCl CuCl b) HCl CuCl KC1 c) HCl FeCl various additives d) H₂O₂ H₂SO₄ CuCl n-propanol e) HCl CuCl various additives f) H₂O₂ H₂SO₄ CuCl various additives g) HCl FeCl₃ glycerol h) HNO₃ HCl FeCl₃ i) HCl FeCl₃

J) HCl ^' FeCl₃ various additives k) HCl FeCl₃ CuCl₂, SnCl₂

1) HCl FeCl₃ ethanol

TABLE 9: AQUEOUS OXIDIZER - SALT REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Oxidizer 2nd Oxidizer Base Salt Other Additives a) NaClO₃ NH₄F CuSO₄ Na EDTA salt of wetting agent b) FeNO₃ various additives c) (NH₄)₂S₂O₈ various additives d) CuNH₄Cl₃ NH₄OH various additives e) Na₂S₂O₃ K₂S₂O₅ various additives

TABLE 10: AQUEOUS BASE REAGENT SOLUTIONS FOR PLANARIZATION OF COPPER

Base Oxidizer Other Additives a) NH₄OH/KOH H₂O₂ various additives b) NH₄OH H₂O₂ various additives c) NH₄OH (NH₄)₂S₂O₈ various additives d) NH₄OH Cu(NO₃)₂

TABLE 11 : AQUEOUS ACID REAGENT SOLUTIONS FOR PLANARIZATION OF

TANTALUM AND COPPER

Oxidizer Acid Other Additives a) HNO₃ HF various additives b) H₂O₂ HF various additives c) HNO₃ HF lactic acid, various additives d) H₂O₂ HF H₂SO₄

TABLE 12: AQUEOUS BASE REAGENT SOLUTIONS FOR PLANARIZATION OF

TANTALUM AND COPPER

Base Acid Other Additives a) NaOH b) NaOH H₂O₂ c) KOH H₂O₂ d) NH₄OH H₂O₂ e) NH₄OH

Remove oxide film after planarization with rinse of dilute sulfuric acid or other solutions.

In addition to the chemicals listed in Tables A and 1-12, various additives maybe included. Such additives may include (but are not limited to) the following:

TABLE B

1) Buffering Agents; including borax, zinc sulfate, copper carbonate.

2) Alcohol-Like Reagents; including low molecular weight alcohols, glycols, phenols, aliphatic alcohols and polyvinyl alcohols. 3) Surfactants; including anionic, cationic and nonionic surfactants.

4) Surface Modifying Agents; including substances that preferentially adhere to one surface material in preference to others and modify the chemical activity (increasing or decreasing) at such adsorption sites, thereby effecting the CHP processes.

5) Solution Stabilizers; including polyvinyl alcohols or other additives capable of suppressing or retarding the spontaneous decomposition of oxidizing reagents in the etchant solution, thereby promoting predictable rates of chemical reactions.

6) Wetting Agents; substances that modify the ability of other chemical reagents to adhere and react with the etched surface during CHP.

The etchant solutions described herein may be applied in a sequence of steps using different solution compositions and/or conditions for each step, or applied in a single CHP processing step. A sequence of steps may include, for example, an initial passivation step followed by treatment with a reagent composition that equally passivates and dissolves the surface resulting in a controlled, smooth, planar surface. A final etching step may then be applied followed by a final rinse with a cleaning solution, typically de-ionized water. The above steps are by way of example only and the processes of the present invention are not inherently limited to such steps.

It is envisioned that CHP performed by reagents lacking abrasive particles permits the reagents to be used once and disposed, or recycled and re-used. An important element in the controllability of the present CHP approach to planarization is that the etchant solutions have diffusion-limited reaction rates with the surface being planarized. A combination of diffusion-limited planarizing reactions and fine hydrodynamic control allow the process engineer to fine-tune the planarization by controlling the hydrodynamic boundary layer. The thickness of this hydrodynamic boundary layer as well as the shear rate typically controls the diffusion, hence controlling the reaction rate on the surface. Also, the elevated "high spots" on the surface being planarized project further into the shearing boundary layer hereby encountering more etchant reactants. Thus, the high spots are removed chemically at a more rapid rate than recessed, diffusion-inaccessible regions of the surface. Thus, a smooth and planar surface is the result. The hydrodynamic effects caused by the relative motion of the proximate surfaces permits a much greater degree of process control than that resulting from immersion in a bath. Also, the present CHP includes as process parameters the separation and relative motion of the proximate surface from the wafer. Thus, adjustment of boundary layer depth and relative motion permits process controls not possible with conventional CMP or SEP.

Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein.

Claims

CLAMS

We Claim:

1) A system for the planarization of surfaces comprising: a) a first surface; and, b) a second surface in close proximity to said first surface and approximately parallel therewith and having a gap between said first surface and said second surface, wherein said first surface and said second surface are mounted such as to undergo relative parallel motion; and, c) an etchant in said gap between said first surface and said second surface wherein said etchant preferentially removes material from raised portions of said first surface during relative motion of said first surface and said second surface, thereby planarizing said first surface.

2) A system as in claim 1 wherein said relative motion of said first surface and said second surface is rotation.

3) A system as in claim 2 wherein said relative motion further comprises linear translation of said first surface with respect to said second surface concurrent with said rotation.

4) A system as in claim 1 wherein said relative motion comprises rotation of said first surface in proximity to said second surface, wherein said second surface comprises a moving belt.

5) A system as in claim 4 wherein said relative motion further comprises linear translation of said first surface parallel to said second surface.

6) A system as in claim 1 wherein said relative motion comprises rotation of said first surface in proximity to said second surface, wherein said second surface comprises a rotating, compliant bladder. 7) A system as in claim 6 wherein said relative motion further comprises linear translation of said first surface relative to said second surface.

8) A system as in claim 1 wherein said relative motion comprises rotation of said first surface in proximity said second surface, wherein said second surface comprises a plurality of compliant rollers.

9) A system as in claim 1 wherein said gap is in the range of approximately 1.0 micrometers to approximately 100 micrometers.

10) A system as in claim 1 wherein said etchant comprises chemicals removing material from said first surface under diffusion-controlled reaction conditions.

11) A method of planarizing a surface comprising: a) causing a first surface and a second surface to undergo relative motion with a gap there between; and, b) introducing an etchant in said gap wherein said etchant preferentially removes material from raised portions of said first surface during said relative motion, thereby planarizing said first surface.

12) A method as in claim 11 wherein said etchant contains abrasive particles.

13) A method as in claim 11 further comprising electropolishing said first surface following said etching steps (a) and (b).

14) A method as in claim 11 further comprising controlling said removal of material by said etchant by controlling the speed of relative motion of said first surface and said second surface.

15) A method as in claim 11 further comprising controlling said removal of material by said etchant by controlling the viscosity of said etchant. 16) A method as in claim 11 further comprising controlling said removal of material by said etchant by controlling the geometry of said first surface and said second surface.

17) A method as in claim 11 further comprising controlling said removal of material by said etchant by controlling the temperature of said etchant.

18) A method as in claim 11 further comprising imposing a galvanic potential on said first surface.

19) A system as in claim 2 further comprising a gimbaled fixture firmly retaining said first surface wherein the gimbal of said fixture is offset from the centerline of said first surface.