WO2009032459A1

WO2009032459A1 - Apparatus using electrosprayed fluids for cleaning surfaces with reduced residual contaminants, and method related thereto

Info

Publication number: WO2009032459A1
Application number: PCT/US2008/072038
Authority: WO
Inventors: John F. Mahoney; James K. Finster; Julius Perel
Original assignee: Ehd Technology Group, Inc.
Priority date: 2007-08-02
Filing date: 2008-08-01
Publication date: 2009-03-12
Also published as: TW200917348A

Abstract

The present invention is directed to a system and method for removing surface contaminants with electrosprayed beams of microdroplets, with reduced residual contaminants attributable to the microdroplets themselves. The system and method may be used for cleaning surfaces, or even texturizing, etching or coating the surfaces. In one embodiment, the system includes a source configured to generate a beam of clusters to said surface, said source having an opening, a feed system configured to feed a liquid to said opening, and a treatment system configured to remove impurities from the liquid. The source includes a device configured to generate an electric field to exert electrostatic forces higher than a surface tension of the liquid at the opening. A method of the invention includes providing a source configured to generate a beam of clusters from an opening, feeding a liquid to the opening and removing impurities from the liquid, wherein the source generates an electric field to exert, electrostatic forces higher than a surface tension of the liquid at the opening.

Description

APPARATUS USING ELECTROSPRAYED FLUIDS FOR CLEANING SURFACES WITH REDUCED RESIDUAL CONTAMINANTS, AND METHOD RELATED THERETO

FIELD OF INVENTION

[0001] The present invention relates in general to an apparatus and method for cleaning surfaces, and in particular to the removal of organic films, particulate matter and other contaminants from surface of semiconductor wafers, with reduced residual impurities and contaminants.

BACKGROUND OF INVENTION

[0002] As nanoscale geometries become the norm in semiconductor, MEMS, storage, advanced packaging, and other technical fields, new technologies are developed to address the removal of submicron contaminants from critical surfaces. Electrohydrodynamics (EHD) create controlled-velocity nanodroplets that exhibit properties well-suited for such cleaning challenges. Electrohydrodynamic atomization involves the use of a conducting fluid that is broken up and dispersed into a beam of charged nanodroplets. To initiate an EHD beam, an electrostatic stress is applied to a meniscus of the conducting fluid that exceeds the surface tension forces which hold the meniscus intact.

[0003] In a typical EHD system, a small fluid reservoir holds a conductive process chemistry fluid to be sprayed, and an electrical contact in the reservoir applies a potential to the fluid. A pneumatic controller applies a controlled pressure to the fluid in the reservoir, resulting in a flow of fluid from the reservoir, through a capillary tube, and into an electrostatic field at a spraying end of the capillary (or virtual nozzle). EHD nanodroplet generation is controlled electrically, through reservoir charging levels and electric field manipulation at the nozzle. Speed and size of the nanodroplets can be varied, resulting in a wide range of process settings to match the nanodroplets to the contaminants and substrate. Nanodroplets can be created for ideal coupling of momentum transfer to particles. EHD apparatus and method are described in U.S. Patent No. 6,033,484 entitled APPARATUS FOR CLEANING CONTAMINATED SURFACES USING ENERGETIC

CLUSTER BEAMS (Mahoney), the entire disclosure of which is hereby incorporated by reference, and U.S. Patent No. 5,796,111 entitled METHOD AND APPARATUS FOR CLEANING CONTAMINATED SURFACES USING ENERGETIC CLUSTER BEAMS (Mahoney), the entire content of which is hereby incorporated by reference. Examples of known electrosprayed liquids include water, alcohols (methanol, 2-propanol (IPA), ethanol), glycerol, hydroxylamine, n-methylpyrrolidone (NMP), and sulfuric acid.

[0004] While EHD sprayed liquid solutions are effective for cleaning surfaces and substrates, precursor solid or gaseous impurities which exist in solution prior to atomization at the nozzle can become entrapped within and distributed among the droplets after formation. Upon impacting a substrate or surface, droplet breakup by fragmentation and evaporation can release the entrained impurities leading to an undesirable source of contamination.

[0005] Moreover, certain combinations in electrode/electrolyte chemistry can also cause the formation of non-volatile molecular species that flow downstream to the nozzle to become entrained in droplets formed in the atomization region. [0006] Another source of contamination relates to process-induced contaminants generated by interaction of the droplets with "background" gases and substrate material. Where the substrate material is a silicon wafer, "defect" mapping equipment, such as a KLA-Tensor SP2 indicates that the EHD beam can create "light point defects" (LPD) attributable to the above sources of contamination. [0007] It is therefore desirable to provide an improved EHD sprayed solution apparatus and method for cleaning surfaces that minimize residual surface impurities including those that arise from precursor solids or gaseous impurities in the sprayed solution prior to atomization and those generated by interaction of beam droplets with background gases and substrate material.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to a system and method for removing surface contaminants with electrosprayed beams of microdroplets, with reduced residual contaminants attributable to the microdroplets themselves. The system and method may be used for cleaning surfaces, or even texturizing, etching or coating the surfaces. In one embodiment, the system includes a source configured to generate a beam of clusters to said surface, said source having an opening, a feed system configured to feed a liquid to said opening, and a treatment system configured to remove impurities from the liquid. The source includes a device configured to generate an electric field to exert electrostatic forces higher than a surface tension of the liquid at the opening.

[0009] In a more detailed embodiment of the invention, the treatment system includes a degasser to remove gases from the liquid, a filter to remove particulates from the liquid, and a pump to recirculate the liquid between the feed system and the treatment system. Moreover, the feed system includes a reservoir that is constructed of low particle shedding and chemically-resistant material, and the pump recirculates the liquid at a high volumetric flow rate that exceeds flow rate to the opening.

[0010] In another more detailed embodiment, the system includes an electrode that charges the liquid. In accordance with a feature of the present invention, the electrode may contain mono- atomic metallic elements, binary metallic alloys, tertiary metallic alloys, quaternary metallic alloys, and/or vitreous carbon, and selected chemistry between the electrode and the liquid can result in the formation of various species that can be effectively treated by the treatment system in reducing residual surface contaminants that are attributable to the electrosprayed liquid. [0011] A method of the invention includes providing a source configured to generate a beam of clusters from an opening, feeding a liquid to the opening and removing impurities from the liquid, wherein the source generates an electric field to exert, electrostatic forces higher than a surface tension of the liquid at the opening.

[0012] In a detailed embodiment of the invention, removing impurities from the liquid comprises degassing the liquid, filtering the liquid and recirculating the liquid through a degasser and a filter. Moreover, the invention includes heating components, including an enclosure in which the beam of clusters are generated, providing electrostatic plates inside near the beam to attract ionic species within the beam, and/or facilitating condensation near the beam of gaseous species within the beam.

[0013] In a more detailed embodiment of the invention, the enclosure can be heated to at least 150 degrees Centigrade for at least 30 minutes, and panels can be cooled cryogenically to facilitate condensation. Moreover, chemistry of the liquid and a charging electrode can be selected so that oxidation reactions at the electrode result in formation of gaseous products or insoluble layers that remain on the electrode rather than enter the solution. The polarity of the voltage applied to the electrode can also affect the species that are formed. [0014] These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. It is understood that selected structures and features have not been shown in certain drawings so as to provide better viewing of the remaining structures and features.

BRIEF DESCRIPTION OF THE DRAWINGS [0015] FIG. 1 illustrates electrochemical reactions between a solution and an electrode within a reservoir of an EHD system. [0016] FIG. 2 illustrates formation of a nanocluster from an EHD beam droplet. [0017] FIG. 3 is a diagram showing an embodiment of a system in accordance with the present invention. [0018] FIG. 3 a is a diagram showing an alternate embodiment of a system in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention recognizes that a source of contamination prior to atomization of an EHD sprayed solution involves droplets entrained with separated contaminant molecules that are introduced into the EHD solution through a variety of sources including contaminated starting solution, and particles shedding off containment members and conduits within an EHD apparatus. [0020] The present invention also recognizes that another source of contamination prior to atomization involves chemical interactions between the solution to be sprayed and the electrode that applies voltage and charges the solution. As illustrated in FIG. 1 , a solution 10 doped with a conductive additive AC is contained in an electrolytic cell or insulated reservoir 12. Adjoined paired molecules AC exist in the solution as non-disassociated electrolyte or solute molecules which form anion-cation pairs. Adjoined paired molecules AC disassociate into solution anions A and solution cations C, either of which depending on the particular chemistry of the additive AC and the polarity of an immersed charging electrode 14 can interact with the material M of the electrode in forming new molecules MA or MC.

[0021] Where the new molecules are a volatile, gaseous species, the molecules rise harmlessly above the solution and can be pumped out. However, where the electrode/solution chemistry results in new molecules of a non-volatile species, they are transmitted by the solution to the nozzle providing a source of undesirable contaminants in the electrosprayed droplets. These non-volatile contaminants deposit on substrates that intercept the beam downstream of the atomization region. For example, where the electrode material M is gold, the conductive additive is hydrochloric acid (HCL), and the gold electrode is connected to a source of positive voltage, AuCl molecular species result from electrolytic reactions between the electrode and the conductive additive, where the coalescence thereof within the droplets yield AuCl nanoclusters that are liberated after evaporation of in-flight beam droplets.

[0022] Other electrode/electrolyte chemistry results in formation of nonvolatile, insoluble species that form insoluble layers that remain on the electrode rather than entering the solution. For example, if the electrode is silver and the conductive additive is HCl, resulting molecular species AgCl that forms insoluble layers that remain on the electrode.

[0023] The present invention further recognizes that another source of contamination occurs after atomization where background gas molecules in an EHD process chamber can combine with the liquid beam droplets to create acidic material leading to surface residue. [0024] Regardless of the source of contamination, contaminated EHD sprayed liquid (or solution, used interchangeably herein) can leave behind undesirable residual impurities on the very substrate the EHD sprayed liquid solution is intended to clean. As illustrated in FIG. 2, droplets 20 entrained with separated contaminant molecules 22 evaporates and shrinks in size, the molecules 22 coalesce into a non-volatile nanocluster 24. Upon further evaporation of the droplet, the nanocluster 24 is liberated impacting substrate 26 and remains thereon as surface residual contaminant 28.

[0025] In accordance with a feature of the present invention, an embodiment of a system 30 that generates a multiply charged electrosprayed beam consisting of droplets having nanometer and micrometer dimensions, with reduced surface residual contaminants is shown in FIG. 3. A charged droplet beam 32 is generated by delivering a conductive solution 34 from a sealed, pressurized reservoir 36 along a capillary tube 38 to its an end or tip 40 (hereafter referred to as nozzle) having a small bore, preferably with a diameter in the range between about 25-100 microns, preferably between about 25 to 50 microns. The tip 40, the capillary tube 38 and extractor electrodes 37 defining an electric field in an atomization region are collectively referred to an EHD droplet source 39. [0026] Typically, the solution 34 which may be an organic or inorganic solution mixture is made conductive by addition of chemicals that impart electrolytic activity, for example, acids or bases. The electrosprayed fluid can consist of a single component organic or inorganic liquid or a mixture of one or more chemically different components. Examples of electrosprayed liquids include but are not limited to: hydrogen peroxide, TMAH, nitric acid, phosphoric acid, hydrofluoric acid and ammonium hydroxide. Many of the aforementioned chemicals can be combined in many ways to prepare solution mixtures sufficient to generate stable EHD beams. In some applications, the conductivity of the liquid may be too low or too high to achieve the desired beam properties of particle size and velocity. In these cases, amounts of added acidic or basic chemical agents are increased or lowered to achieve the desired beam properties. Conductive additives can also include volatile salts (e.g. ammonium acetate).

[0027] The solution is charged by applying voltage to a conducting (metallic, carbon) wire or electrode 42 immersed in the solution 34. Applying pressure supplied by gas source 44, as regulated by pressure/flow controller 46, above the solution in the reservoir 36 causes the solution to flow through the capillary delivery tube 38. Upon arrival at the nozzle 40 housed within an EHD process chamber 50 defined by a vacuum enclosure 52, the solution mixture is subjected to a high electrical field defined by the extractor electrodes 37 at atomization region 54 which disperses the charged liquid creating a beam of electrosprayed micron and submicron-sized droplets 32. As discussed below, pre-spray management of solution electrolysis and post-spray management of the beam environment advantageously minimize if not avoid residue contaminant (defects) buildup on target substrate 56 during surface cleaning operations using apparatus 30.

[0028] In accordance with a feature of the present invention, the solution 34 contained in the reservoir 36 can be ultrapure with impurities at concentration levels of parts-per-billion or less. Suitable starting solutions, including chemical solutions from J.T. Baker sold under the trademark ULTREX, may be mixed with other solutions and/or electrolytes for use as the solution 34 in the reservoir 36.

[0029] Further in accordance with a feature of the present invention, the reservoir 36 is constructed of low particle shedding and chemically resistant materials, including PFA, TEFLON, PVDF, glass, and the like. Moreover, the apparatus 30 includes a treatment system 57 that in the embodiment of FIG. 3 includes a recirculating pump 58 that pumps, preferably continually, solution 34 through a degassing chamber or degasser 60 and an inline filtering unit 62 with a retention rating no greater than about 0.01 micron. The filtering unit 62 returns filtered and outgassed solution back to the reservoir 36. The degasser serves to eliminate dissolved gases from EHD process solutions that can act as precursors for droplet/particle nucleation. Constant recycling of solution at high volumetric flow rates at the point-of-usage, such as immediately prior to the inlet of the capillary tube 38 or the outlet of the capillary at the nozzle 40, should minimize traces of particles and dissolved gases immediately prior to the low volumetric flow usage by EHD spraying at the nozzle. Advantages are present when the recycling rate exceeds the usage rate or flow rate to the nozzle. For example, for a single nozzle embodiment, the recycling may be on the order of milliliter/min whereas the usage rate may be on the order of microliter/min. [0030] Nonexhaustive examples of gases that can be removed from the solution by the degasser

60 include COx, SOx, and POx. These gases can contribute to formation of carbolic, sulphuric and phosphoric acids, respectively, in the gas phase leading to acidic nanodroplets which tend to have lengthy evaporation times (e.g., see Y. Ye et al., "Condensation-Induced Particle Formation During Vacuum Pump Down," J. Electrochem. Soc. 140, 1463, (1993)). [0031] To reach the EHD droplet source39, the solution 34 exits the reservoir 36 through the capillary tube 38 and is delivered to the nozzle 40. Flow of the solution through the capillary tube is regulated by the flow controller 46, using gas, for example, dry, ultrapure nitrogen gas, from gas source 44. A filter 66 located between the gas source 44 and an input of the flow controller 46 removes particles from the gas, and a second filter 68 at the exit of the flow controller prevents particles that might originate in the flow controller or the gas source from entering the reservoir 36.

[0032] As also recognized by the present invention, various types of contaminant species, including non-volatile species, can be avoided with selected electrode/solution (inclusive of electrolyte) combinations and chemistry, as follows: [0033] a. Solution and electrode materials are chosen so that oxidation reactions at the electrode result in the formation of gaseous products which rise to the surface of the solution and are harmlessly pumped away by the treatment system 57. Examples of such solution and electrode materials are peroxides, including hydrogen peroxide, in combination with an acid, including nitric acid, sulfuric acid and acetic acid, capable of disassociating in a polar solvent, such as water or glycerol, where a positive polarity is applied to a gold electrode, hi one embodiment, for example, the acid is nitric acid.

[0034] b. Solution and electrode materials are chosen so that oxidation reactions at the electrode result in formation of insoluble layers that remain on the electrode rather than enter the solution. Any suitable acid may be used that is capable of disassociating in a polar solvent to form charged ions which are capable of forming a salt with the metal of the electrode that is insoluble in the polar solvent. The electrode is any metal that can form a salt with a disassociated acid ion that is insoluble in a polar solvent. For example, using a silver electrode in the presence of HCl results in formation of silver chloride (AgCl) at the electrode with positive polarity which is insoluble in water or isopropanol (IPA). [0035] The conducting electrode can be composed of different materials, including but not limited to:

[0036] a. a mono-atomic metallic element, e.g., gold, silver, tantalum, platinum; [0037] b. any metallic alloy, including but not limited to binary, tertiary and quaternary metallic alloys; and [0038] c. vitreous carbons.

[0039] As understood by one of ordinary skill in the art, selection of electrode, solution, and/or electrolyte can be made to avoid production of impurities through unwanted chemical reactions.

Moreover, by applying a negative voltage rather than a positive voltage to power the electrode, other combinations for generating gaseous or insoluble species are available. Furthermore, the foregoing discussion for reducing contaminants in the solution by controlling electrochemical activity using an immersed electrode generally applies equally well to conducting capillaries. In the embodiment of an EHD apparatus as illustrated in FIG.3a, a metallic capillary tube 38a obviates an immersed electrode.

[0040] In accordance with the present invention, the solution as it arrives at the nozzle 40 for electrospraying should meet the following criteria:

Component Concentration Specification

Sulfate no greater than about 20 ppb;

Phosphate no greater than about 10 ppb; and/or

Metals no greater than about 0.1 ppb.

[0041] Because background gas molecules in an EHD process chamber can combine with liquid beam droplets to create acidic material leading to surface residue, the apparatus and method of the present invention minimizes background gases prior to and following initiation of the EHD spray. As illustrated in FIG. 3, the EHD process chamber 50 that is defined by a vacuum enclosure 52 into which the solution from the reservoir is delivered via the capillary tube upon increased pressurization above the solution in the reservoir, has heating components for heating the enclosure structure and cooling panels inside the chamber to facilitate collection by condensation of unwanted gaseous species arising from the electrosprayed liquid. In the illustrated embodiment, heater straps 70 are positioned outside the enclosure to heat the enclosure structure. Moreover, i cryogenic cooled panels 72 are positioned inside the enclosure in a generally surrounding configuration around the atomization region 54 in close proximity to the electrosprayed beam 32. [0042] In accordance with the present invention, a method of treating an EHD process chamber to reduce substrate surface residual contaminants includes:

[0043] A. Heating the enclosure structure 52 of the EHD process chamber 50 to a temperature of approx. 150 degrees centigrade for a period greater than approx 30 minutes to outgas and purge internal surface walls and internal process components of the chamber 50. [0044] B. Applying voltage to the electrostatic plates 73 to enhance the electrostatic collection of ionic species arising from beam droplets

[0045] C. Providing cooled surfaces within the chamber 50 to further assist in trapping undesirable gaseous species arising from beam droplets by means of condensation. [0046] Under typical operating conditions, Acts A, B and C are initiated before initiation of electrospraying at the nozzle, although Acts B and C may be initiated after initiation of Act A 10 and/or after initiation of electrospraying at the nozzle. Moreover, Act B may be more effective than Acts C and A, and Act C may be more effective than Act A.

[0047] The preceding description has been presented with reference to presently preferred . _<. embodiments of the invention. Workers skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structure may be practiced without meaningfully departing from the principal, spirit and scope of this invention.

[0048] Accordingly, the foregoing description should not be read as pertaining only to the 20 precise structures described and illustrated in the accompanying drawings, but rather should be read consistent with and as support to the following claims which are to have their fullest and fair scope.

-I I-

25

Claims

WHAT IS CLAIMED IS:

1. A system to remove contaminants from a surface with reduced residual contaminants attributable to the system, the system comprising: a source configured to generate a beam of clusters to said surface, said source having an opening; a feed system configured to feed a liquid to said opening; a treatment system configured to remove impurities from the liquid; wherein the source includes a device configured to generate an electric field to exert, upon liquid fed to a vicinity of said opening, electrostatic forces higher than a surface tension of said liquid.

2. A system of claim 1, wherein the treatment system includes a degasser configured to remove gases from the liquid.

3. A system of claim 1, wherein the treatment system includes a filter configured to remove particulates from the liquid.

4. A system of claim 1, wherein the treatment system includes a pump configured to recirculate the liquid between the feed system and the treatment system.

5. A system of claim 1, wherein the feed system includes a reservoir.

6. A system of claim 5, wherein the reservoir is constructed of low particle shedding material.

7. A system of claim 5, wherein the reservoir is constructed of a chemically -resistant material.

8. A system of claim 4 wherein the pump is configured to recirculate the liquid at a high volumetric flow rate that exceeds flow rate to the opening.

9. A system of claim 1, wherein the system includes an electrode adapted to apply a charge to the liquid.

10. A system of claim 9, wherein the electrode extends into the liquid in the reservoir.

11. A system of claim 9, wherein the electrode is a capillary member through which the liquid is transported to the source.

12. A system of claim 9, wherein the electrode is constructed of at least one material selected from a group consisting of: mono-atomic metallic elements, binary metallic alloys, tertiary metallic alloys, quaternary metallic alloys, and vitreous carbon.

13. A system of claim 1, wherein the liquid transported by the feed system is characterized by at least one from a group consisting of: a sulfate concentration no greater than about 20 ppb; a phosphate concentration no greater than about 10 ppb; and metals concentration no greater than about 0.01 ppb.

14. A system of claim 1, wherein the liquid comprises an ultrapure liquid.

15. A method using electrosprayed liquid for removing contaminants from a substrate surface, the method comprising:

5 providing a source configured to generate a beam of clusters to said surface, said source having an opening; feeding a liquid to said opening; removing impurities from the liquid; and wherein the source also generates an electric field to exert, upon liquid fed to a vicinity of 10 said opening, electrostatic forces higher than a surface tension of said liquid.

16. A method of claim 15, wherein removing impurities from the liquid comprises degassing the liquid.

15 17. A method of claim 15, wherein removing impurities from the liquid comprises filtering the liquid.

18. A method of claim 15, wherein removing impurities from the liquid comprises recirculating the liquid through a degasser and a filter. 0

19. A method of claim 15, further comprising providing a vacuum chamber configured to enclose the beam and the electric field.

20. A method of claim 19, further comprising heating the vacuum chamber to at least 150 5 degrees Centigrade for at least 30 minutes.

21. A method of claim 19, further comprising applying voltage to electrostatic plates inside the chamber to attract ionic species within the beam.

22. A method of claim 19, further comprising at least one surface inside the chamber configured for condensation of gaseous species within the beam.

23. A method of claim 18, further comprising: (a) heating the vacuum chamber to at least 150 degrees Centigrade for at least 30 minutes prior to initiation of electrospraying;

(b) applying voltage to electrostatic plates inside the chamber to attract ionic species within the beam; and

(c) cooling a surface inside the chamber to facilitate condensation of gaseous species from the beam.

24. A method of claim 15, further comprising providing an electrode extending into the liquid.

25. A method of claim 24, further comprising selecting liquid and electrode chemistry from the group consisting of:

(a) Solution and electrode materials that react at the electrode to form gaseous products, and

(b) Solution and electrode materials that react at the electrode to form insoluble layers that remain on the electrode rather than enter the solution.

26. A method of claim 25, wherein the electrode in chemistry (a) comprises gold, and the solution in chemistry (a) comprises an acid capable of disassociating in a polar solvent.

27. A method of claim 26, wherein the solution further comprises a peroxide.

28. A method for claim 26, wherein the acid is selected from the group consisting of nitric acid, sulfuric acid and acetic acid

29. A method for claim 26, wherein a positive voltage is applied to the electrode.

30. A method of claim 25, wherein the acid in chemistry (b) includes an acid capable of disassociating in a polar solvent to form charged ions capable of forming a salt with a metal of the electrode, the salt being insoluble in the polar solvent.

31. A method of claim 25, wherein the electrode material in chemistry (b) includes a material that can form a salt with a disassociated acid ion, the salt being insoluble in a polar solvent.

32. A method of claim 31, wherein the electrode comprises silver and the acid is HCl

33. A method of claim 32, wherein a positive voltage is applied to the electrode.

34. A method of claim 15, further comprising providing an electrode to charge the liquid, wherein the electrode comprises at least one material selected from the group consisting of: mono- atomic metallic elements, metallic alloys, and vitreous carbons.

35. A method of claim 34, wherein the electrode comprises a metallic alloy selected from the group consisting of binary metallic alloys, tertiary metallic alloys, quaternary metallic alloys and combinations thereof.