US20070246064A1

US20070246064A1 - Method of treating a substrate

Info

Publication number: US20070246064A1
Application number: US11/766,762
Authority: US
Inventors: David Jackson
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-05-03
Filing date: 2007-06-21
Publication date: 2007-10-25
Also published as: US20040045578A1

Abstract

A method of cleaning a substrate within a controlled environment includes placing the substrate into a high pressure vessel. The high pressure vessel is then supplied with a dense fluid under pressure. The dense fluid is contacted with the substrate for a selected period of time to at least partially remove a contaminant contained on the substrate. After the selected period of time, the vessel is depressurized to at least partially convert the dense fluid into a vapor. The vapor is then subjected to an energy field to form a plasma within the vessel which is used to treat the substrate for a second selected period of time. The thus cleaned substrate is then removed from the vessel.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional application of U.S. patent application Ser. No. 10/428,793 entitled METHOD AND APPARATUS FOR SELECTIVE TREATMENT OF A PRECISION SUBSTRATE SURFACE, filed on 2 May 2003, which claimed the benefit of similarly named U.S. Provisional Application No. 60/377,197 filed on 3 May 2002, each of which are hereby incorporated herein by reference.

BACKGROUND OF INVENTION

As technology has advanced, the system performance requirements and complexity of manufactured and assembled precision instruments and devices have increased, while the size of individual components and assemblies have decreased. This continuing miniaturization process has magnified the susceptibility of precision substrates and surfaces to contamination. Specific effects of contamination depend on the type of substrate, materials used, and system in which the device is used.
Thus, an important element of any precision manufacturing or assembly process is the removal of contaminants (oils, particulates, moisture, etc.) from the surfaces of precision substrates. Precision substrate surfaces include those in the manufacturing and assembly of semiconductors, fiber optic, optoelectronic, medical, and sensor devices, fabrics, textiles, and instruments, among many other contaminant-yield sensitive devices. The contamination of these surfaces usually results from external sources such as process equipment, personnel, process reaction by-products, chemical impurities and assembly residues.
For example, contaminants in contact with a precision substrate surface may inhibit the movement of a microelectromechanical system (MEMS) component such as a gear or moveable mirror, interfere with the transmission of light, prevent uniform electrodeposition of a metal, prevent wetting of a bonding agent, decrease adhesion strength between bonding interfaces, or produce shorts in microscopic electronic interconnects.
More than 80% of the yield loss of volume-manufactured integrated circuits is attributable to particle contamination. As device geometries continue to shrink and wafer size increases, particulate matter and residues will have an ever-increasing impact on device yields. Current cleaning technologies become less effective with the growing demand for removing sub-micron (<1 micrometer) contamination.
Selection of an appropriate combination of cleaning techniques for precision substrate surface cleaning must include consideration of the type of soil to be removed, substrate composition and properties, and the desired level of cleanliness. Precision substrate surface cleaning involves a wide range of substrate materials of composition, including metals, fibers, colorants, pigments, polymers, plastics, epoxies, and sealants, and usually have stringent cleanliness requirements such low particle counts, no surface residues, wetability, and surface brightness. Furthermore, a wide range of contaminations exist, including particle contamination, chemical contamination, biological contamination, ionic contamination, molecular contamination and outgassing or offgassing contamination. In addition, the precision substrate surfaces can exhibit a variety of surface geometries (e.g., tubing, insulated wires, small orifices, surface topography) that can make efficient cleaning and drying very difficult to achieve. To. date, no known universal and effective dry cleaning alternative method for the variety of precision substrate surface preparations exists. To properly develop such a universal alternative cleaning methodology, each alternative must be evaluated based on all of the above cleaning factors as well as cleanliness requirements, material compatibility, and cost effectiveness specific to the desired surface cleaning application.
For example, aqueous cleaning may be effectively used to clean simple geometric surfaces, but the cleaning liquid may become entrapped in crevices and hidden cavities of more complex components, thus potentially affecting the operation of the precision device if not thoroughly removed in subsequent drying steps. Furthermore, aqueous cleaning techniques are not compatible with new substrate materials and shrinking geometric features, which leads to effects such as corrosion or oxidation or may cause substrate damage during drying operations due to capillary force pressure within microvias and cavities.

It important to discuss the specific types of contaminations and precision substrate surfaces encountered and cleaning energies required to perform the myriad number of precision cleaning operations, and addressed by the present invention. The following is a generalized categorization of common surface contaminations and substrates encountered in precision cleaning applications. Surface contaminations may be classified and typified into four major groups according to Table 1.

TABLE 1


Contamination Classification Scheme

	Class A - Thick Films (viscous liquids and solids
	having micrometer level thickness)
	Type A1 - Cross linked and bonded films
	(i.e., optical coatings, polymers, plasma etch
	residues)
	Type A2 - Viscous organic films (i.e.,
	hydrocarbons, stains, blood)
	Class B - Thin Films (viscous liquids having
	monolayer and nanometer level thickness)
	Type B1 - Organic films (i.e., resist residue,
	finger oils, trace hydrocarbons, biological,
	haze)
	Type B2 - Inorganic films (i.e., water,
	minerals, ionics, oxides)
	Class C - Particulates (inorganic and organic solids
	and semi-solids)
	Type C1 - Macroscopic (i.e., particle sizes >10
	micron)
	Type C2 - Microscopic (i.e., particles sizes
	between 0.5 and 10 microns)
	Type C3 - Nanoscopic (i.e., particle sizes <0.5
	microns)
	Class D - Outgassing Compounds (condensed
	vapors)
	Type D1 - Organic outgassing compounds
	(i.e., organic gases and vapors)
	Type D2 - Inorganic outgassing compounds
	(i.e., inorganic gases and water vapor)

Precision Substrate Surfaces may be classified and typified according to Table 2.

TABLE 2


Substrate Surface Classification Scheme

	Class A - Anisotropic Surface
	Type A1 - Planer (i.e., diced or whole disk drive,
	semiconductor, optical and MEMS wafers)
	Type A2 - 3-Dimensional (i.e., optical benches,
	CMOS image sensor, implantable device)
	Class B - Isotropic Surface
	Type B1- Planer (i.e., photodiode, LCD, lead
	frame, optical lenses, polyimide film, IC test
	socket pad)
	Type B2 - 3-Dimensional (i.e., optical fiber,
	hollow tube, CMOS image sensor)

Cleaning is defined as the removal of unwanted substances (Table 1) from a substrate surface or subsurface (Table 2). The process of removing unwanted substances involves breaking bonds, chemical and physical, using a combination of mechanical, physical, and chemical energy. Furthermore, cleaning performance is measured in terms of a combination of the aforementioned cleaning energies, level of contamination, and cleaning time required to meet a certain cleanliness level. The mechanics of cleaning are complex due to the many variables as discussed above, thus an innumerable variety of conventional wet and dry combinational cleaning methodologies exist.

Furthermore, cleaning energies may be classified and typified according to Table 3.

TABLE 3


Cleaning Energies

	Class A - Mechanical Energy
	Type A1 - Shear stress
	Type A2 - Acoustic
	Class B - Physical Energy
	Type B1 - Heat
	Type B2 - Surface tension
	Type B3 - Viscosity
	Class C - Chemical Energy
	Type C1 - Solubilization
	Type C2 - Oxidation

The proper selection and application of these cleaning energies (Table 3) is critical to efficiently remove of the many varieties of contaminants encountered in precision cleaning applications, reducing cleaning time, meeting cleanliness requirements, and selectively treating without damage to a precision substrate surface.
As can be seen from Tables 1, 2 and 3, removal of the various types of contaminants from precision substrate surfaces necessitates the use of several cleaning energies. For example, particles (i.e., Table 1, Type C1, C2, and C3 contaminants) contained on a wafer surface (i.e., Table 2, Type A1 substrate surface) require shearing action (i.e., Table 3, Type A1 cleaning energy) for complete removal. However, plasma reacted resist residues (i.e., Table 1, Type B1 contaminants) on that same wafer would require additional cleaning energy in the form of oxidative cleaning species (i.e., Table 3, Type C2 cleaning energy) to achieve both a particle and residue free substrate surface. Also, other cleaning energies (i.e., solubility, thermal, and shear) may be required to rinse and dry the cleaned substrate using, for example, a conventional wet and dry cleaning method.
In conventional processes, several wet and dry cleaning techniques are employed to achieve the desired quality and performance of the cleaning process. This is so because each cleaning technique delivers a certain and usually fixed performance profile—that is a type of cleaning energy and effectiveness for a certain type and level of contamination and for a certain type of substrate surface. Also because wet and dry methods are different chemically and physically, special rinsing and drying techniques must be included in the methodology.
For example, a common technique used to achieve various levels of cleanliness, be it organic, inorganic and particulate cleanliness, involves the combinational use of various wet and dry cleaning and drying technologies. Examples of combinational cleaning and drying processes include organic solvent cleaning (i.e., Table 3, Type A1 and C1 cleaning energies) followed by nitrogen drying (i.e., Table 3, Type A1 and B1 cleaning energies); oxidative hydroxylamine cleaning (i.e., Table 3, Type C2 cleaning energy) followed by deionized water flushing (i.e., Table 3, Type A1 and C1 cleaning energies) and alcohol drying (i.e., Table 3, Type C1 and B3 cleaning energies); and plasma cleaning (i.e., Table 3, Type C2 cleaning energy) followed by ozonated water residue removal and alcohol drying (i.e., Table 3, Type A2, B3, and C2 cleaning energies).
As can be seen, conventional cleaning and drying methods can be fairly extensive and most often involve combinations of wet and dry chemistries, techniques and equipment. However, conventional cleaning methods are becoming increasingly problematic for precision substrate surfaces as device geometries shrink and new manufacturing materials are used. For example, issues such as microscopic and contaminant-related defects caused by stiction and capillary collapse are becoming more prevalent in optical and IC wafer fabrication. Furthermore, highly energetic cleaning processes such as vacuum plasmas may damage substrates and especially microscopic features present on a surface while removing unwanted surface contaminations. Still moreover, to achieve nano-scale levels of cleanliness without damage to the substrate surface and to modify said substrate surface to prepare for following operations, a many iterations of wet and dry processing is required, each process enabling the next produce a biocompatible surface, produce a hydrophobic surface, or to create a barrier film. Surface modifications can only be properly performed on a precision substrate once its surface is free of hydrocarbons, particles and other contaminating residues. As such, there is a present need for a dry combinational cleaning and surface treatment method that can produce a clean surface first and then modify said cleaned surface to produce additional beneficial surface properties as identified in Table 4.
To address this need, the present inventor has developed a completely dry substrate surface cleaning and modification method using a unique combination of state-of-the-art dry cleaning and surface modification technologies. Candidate technologies were identified, studied, and evaluated to determine the performance characteristics and limitations for each.
As a result of this work, it has been discovered that using various dry cleaning and surface modification techniques in certain combinations, called instant surface treatment methods herein, allows for complete treatment of a substrate surface without resorting to conventional wet cleaning and drying methods described above. The present method provides the entire range of cleaning energies (i.e., Table 3) required for the various contaminations (i.e., Table 1) and substrate surfaces (i.e., Table 2) encountered in most precision substrate surface cleaning. Moreover, a cleaning technique was chosen (i.e. low pressure plasma) which serves as a follow-on surface modification technique (i.e., Table 4), thus increasing the utility of the present invention. The present method is highly selective and an instant method may be constructed so as not to damage delicate features found on precision substrate surfaces. In many applications, only the affected substrate surface may be treated which minimizes re-contamination and materials compatibility problems using the techniques described herein. Furthermore, the present invention can treat nearly all types of contamination typically found on various precision substrate surfaces and can produce a physicochemically modified surface which is necessary for subsequent manufacturing operations such bonding, plating, coating, assembly, or for direct use.

SUMMARY OF INVENTION

The present invention employs various combinations of solid cryogenic carbon dioxide spray cleaning, liquid and supercritical carbon dioxide immersion cleaning, atmospheric plasma, and ultraviolet/ozone cleaning. Furthermore, the present invention teaches the use of a new dry cleaning technique called electrohydrodynamic (EHD) cleaning for the removal of nanoscopic surface contaminations. Although very attractive, these individual techniques have discrete characteristics, that is application and performance limitations, which prevent them individually from properly treating a substrate surface and may even damage a surface. For example, physical damage to a substrate surface may be caused by a particular technique due to excessive cleaning energies required (i.e., high spray pressure, high plasma energy level, presence of oxidizing chemistries) and excessive treatment periods required to achieve a certain surface cleanliness level. The individual technologies employed in combination in the present process. Moreover, many times the native substrate surface must be chemically activated to insure good adhesion or wetting for follow-on manufacturing processes. Finally, water reduction and pollution prevention are a major concern for the precision device manufacturing industries.
Most conventional wet and dry combinations create significant waste by-products, pose worker exposure dangers, and consume tremendous amounts of water resources. Much interest exists to develop alternative precision substrate cleaning and drying methods to replace hazardous chemicals such as organic solvents, acids and hydrogen peroxide. Technological advances in this area such as Microelectromechanical Systems (MEMS) and shrinking line widths and deep trenches with high aspect ratios require advanced cleaning and drying technologies. Industry utilizes or has proposed various techniques to remove plasma reacted or patterned organic photoresists (i.e., Table 1, Type A1 contamination) and particles (i.e., Table 1, Type C2 and C3 contamination), rinse and dry a semiconductor wafer. An example of a typical conventional and combinational cleaning technique for semiconductor substrates follows.
With respect to cleaning wafers to remove an organic photoresist contamination, commercial cleaning systems have been developed which employ ozone and water to replace dangerous or ecologically-unsafe chemical processes such as sulfuric acid-hydrogen peroxide mixtures, toxic organic solvents, and amine-based cleaning agents. One such system, called the SMS DI03 photoresist strip process (Legacy Systems Inc., Fremont, Calif.), uses an ozone generator and diffuser located in a tank of chilled (5 C) deionized water which is circulated into a tank containing the wafers. Ozone is a powerful oxidizer that is used to mineralize organic contamination. following ozone treatment, water rinsing and drying are performed. However, complete drying of precision substrates following cleaning by wet methods is limited due to hydration of small capillaries, vias and interstices that may be present. Moreover, a lack of substrate surface selectivity can be limiting in many applications, because the entire precision device is subjected to the combinational cleaning method that complicates cleaning, drying and compatibility issues. Drying methods typically employ an alcohol rinse to overcome some of these issues. For example, techniques include the use of an isopropyl alcohol (IPA) vapor dryer, full displacement IPA dryer, and others. These IPA-type dryers often rely upon a large quantity of a solvent such as isopropyl alcohol and other volatile organic liquids to facilitate drying of the semiconductor wafer. An example of such a technique is described in U.S. Pat. No. 4,911,761, and its related applications, in the name of McConnell et al. and assigned to CFM Technologies, Inc. McConnell et al. Generally describes the use of a superheated or saturated drying vapor as a drying fluid. This superheated or saturated drying vapor often requires the use of large quantities of a hot volatile organic material. The superheated or saturated drying vapor forms a thick organic vapor layer overlying the rinse water to displace (e.g., plug flow) such rinse water with the drying vapor. The thick vapor layer forms an azeotropic mixture with water, which will condense on, wafer surfaces, and will then evaporate to dry the wafer. A limitation with this type of drying technique is its use of the large solvent quantity, which is hot, highly flammable, and extremely hazardous to health and the environment. Another limitation with such a drying technique is its cost, which is often quite expensive. In fact, this dryer needs a vaporizer and condenser to handle the large quantities of hot volatile organic material.
As line size becomes smaller and the complexity of precision manufactured devices increases, it is clearly desirable to have an all dry cleaning and surface treatment technique, including both method and apparatus, that selectively removes unwanted organic films ad particles, prevents additional particles, and does not introduce compatibility problems for the manufactured device. The complete selective cleaning technique may also include a step of drying the precision substrate, without other adverse results. A further desirable characteristic includes reducing or possibly eliminating re-contamination of precision surfaces during cleaning and handling. The aforementioned conventional technique fails to provide such desired features, thereby reducing the yield of good precision devices.

From the above, it is seen that a method and apparatus for cleaning and precision drying semiconductor integrated circuits that is dry, safe, easy, and reliable is desirable. There is a present need for a all dry and enabling combinational method which can produce any desired level of cleanliness down to the nanoscopic scale, and starting with various levels and types of contamination. Moreover, a robust and all dry cleaning method is desired to achieve the desired surface or substrate cleanliness and surface energy. Still moreover, a cleaning method is desired which optimizes the capabilities of each technique to achieve a stepwise reduction in contamination levels without causing damage to the precision surfaces. Finally, a non-toxic and environmentally friendly dry cleaning method is desired to eliminate pollution, reduce hazardous waste by-product generation, reduce water usage and eliminate worker exposure to toxic, corrosive, or carcinogenic cleaning chemicals.

TABLE 4


Surface Modification Types

	Type A - Adhesion Promotion
	Type B - Hydrophilic Properties
	Type C - Oleophobicity and
	Hydrophobicity
	Type D - Surface Friction
	Type E - Barrier Films
	Type F - Biocompatibility

Still moreover, the aforementioned conventional cleaning approaches do not have the capability of modifying or treating precision surface once cleaned. Again this is due to the inherent incompatibilities between the conventional surface cleaning and modification techniques. Referring to Table 4 above, surface modification schemes may include etching away a thin layer of native and clean surface to increase wetability or to promote adhesion strength during subsequent manufacturing operations such as adhesive bonding. Moreover, surface modification also may include depositing small amounts of organic or inorganic molecules onto a cleaned surface to decrease friction, invention are described in the following sections. This discussion includes an assessment of benefits and performance limitations associated with each dry cleaning technique.
The present invention is a combinational method which enables the removal of most levels and types of macroscopic, microscopic and nanoscopic contaminants, thick films, thin films, absorbed contaminants, interstitial residues and particles as described in Table 1 herein. The present method and exemplary treatment apparatuses taught herein have been developed as a result of the present inventors understanding and exploitation of the relationships between the various dry cleaning mechanisms and performance profiles for plasma, dense fluid, UV/O₃and EHD dry cleaning and surface preparation treatments. A brief discussion of each technique used in the present invention follows.
Vacuum and Atmospheric Plasma Cleaning and Modification Technique
Vacuum and atmospheric plasma cleaning uses an electrically charged gas containing ionized atoms, electrons, highly reactive free radicals, electrically neutral species, and ultraviolet radiation. Plasmas are produced in a multi-stage process by passing an electric current through the process gas. The resulting plasma is highly reactive with surface contaminants. Plasmas can be used in a wide range of temperature and pressure conditions; however, cold plasmas (those with temperatures less than 140.degree. F. [60.degree. C.]) are most often used for cleaning applications. Normal operating pressures for vacuum plasma cleaning processes range from 1 to 500 millitorr. Vacuum chamber plasmas may be used to treat large surfaces and entire substrates, whereas atmospheric and enhanced capillary discharge plasmas may be used selectively to treat only a portion of a precision substrate surface.
In general, for thick and thin film contaminations plasma cleaning can produce extremely clean surfaces in minutes. Since the cleaning medium is a gas, hidden areas of complex parts can be cleaned better (albeit, rather slowly) than line-of-sight processes, such as carbon dioxide spray cleaning. Most plasma gases are selective in their cleaning ability, removing either organic contaminants or inorganic contaminants, but not both. Therefore, gas selection and mixing is critical. For example oxygen mixtures may be used for hydrocarbon cleaning, hydrogen mixtures for oxide removal and fluorinated mixtures recalcitrant carbonaceous residues or highly cross-linked polymers. In addition, some types of energetic plasma, and especially vacuum plasmas, can cause erosion of critical dimensions on metal or epoxy surfaces if the conditions are not carefully controlled. Moreover, long treatment times are required for thick film contaminants and for complete removal of all carbonaceous residues.
Finally, following combinational surface cleaning techniques described herein, gaseous or vaporous admixtures may be injected into a dense fluid-plasma process chamber under low pressure and plasma energy conditions to produce a chemically modified clean surface. For example, this surface may be processed to have a thin fluorocarbon film, a Teflon coating, which provides a low friction abrasion barrier for the cleaned surface. Furthermore, the cleaned surface may be activated to prepare for cell growth or protein bonding (for example an implant surface) or for enzyme bonding (for example a platinum sensor surface). Polymeric precision devices such as medical instruments, optical elements, and other critical medical devices, once cleaned, can be treated to produce highly wettable, low permeable, clean, and sterile substrate surfaces using the present invention.
Exemplary plasma cleaning and modification systems suitable for use in the present invention are available from 4th State Inc., Belmont, Calif. (Vacuum Plasma) and SurFx Technologies, LLC, Los Angeles, Calif. (Atmospheric Plasma).
An aspect of the present invention is to exploit the various plasma techniques to first chemically alter a surface contaminant or weaken adhesion forces between organic thick film contaminant and substrate surface—thereby changing its state so that a combinational and secondary technique such as CO₂spray or liquid immersion may be used much more effectively and efficiently. Moreover, a low pressure plasma surface modification treatment may immediately follow the cleaning treatments to physicochemically modify a cleaned surface to produce a clean and modified surface which is wettable, bondable, biocompatible, or exhibits enhanced surface characteristics such as sterility, impermeability, low friction, or enhanced light reflectance. This would not be possible without first using the first combinational surface cleaning operation. Moreover, this aspect of the present invention may be combined into a single process tool in which a precision substrate surface is cleaned and modified in a single operation.
Dense Fluid Surface Cleaning Technique
Liquefied gases and supercritical fluid cleaning technologies (i.e., Dense Fluids) use the enhanced solvent characteristics of compounds that are heated and pressurized to near or above their unique critical points. Such fluids have the solvent power of a conventional liquid cleaning agents and the transport properties of a gas. Carbon dioxide is the most-frequently used dense fluid, due to the low temperature and pressure of its critical point. Dense phase carbon dioxide can effectively remove oils, lubricants, and other organic contaminants. A typical dense fluid cleaning process consists of three steps: gas pressurizing and heating, extraction of contaminant, and gas-contaminant separation. Once the extracting fluid has cooled and/or de-pressurized below its critical point, the solubility of the fluid decreases, the contaminants can be readily removed, and more than 90 percent of the gas can be recovered for reuse. Cleaning with dense fluids is well suited for many precision substrate surface cleaning applications because of its compatibility with a wide variety of materials and because no solvent residue remains on component surfaces after cleaning. However, dense fluids may not be compatible with some elastomers (such as Viton) and causes swelling and cracking in some polymer materials. Additional limitations of dense fluid cleaning include the difficulty in removing high molecular weight hydrocarbons or highly cross linked organic molecules, and the possibility of damaging delicate components as a result of the high system pressure.
An aspect of the present invention is to exploit the various plasma techniques to first chemically alter a surface contaminant or weaken adhesion forces between said contaminant and substrate surface—thereby changing its state so that a combinational and secondary technique such as liquid CO₂immersion may be used much more effectively and efficiently. Moreover, plasma treatment may immediately follow the secondary treatment to chemically modify a clean surface. This would not be possible without first using the combinational cleaning operation.
Alternatively, dense fluids may be expanded or condensed to form solid sprays that may be used as physical cleaning agents—cryogenic dense fluid sprays. Solid carbon dioxide (CO₂) and argon (Ar) ice spray cleaning processes may be used to remove organic contaminants and particulates by an impact/flushing method. Of the two distinct CO₂cleaning processes, CO₂snow and CO₂pellets, CO₂snow is more suited for typical precision particle and thin film residue cleaning applications and pellets are more suited for gross particle and thick film contaminant removal. CO₂snow is formed when liquid carbon dioxide is allowed to rapidly expand through a nozzle. This creates solid particles of CO₂(i.e., snow) entrained in a stream of pressurized CO₂gas. Furthermore, CO₂snow may be compressed into larger pellets and used as a more aggressive treatment media. Argon ice spray cleaning is similar to snow cleaning, with an argon ice particle spray formed through the combination of argon gas with liquid nitrogen.
The solid particles contact contaminant particles on the substrate and remove them through a cryo-kinetic and momentum transfer process as well as thin film solubilization. Following impact, the solid particles then transform into a gas (sublime) and thus do not add any volume to the waste stream.
CO₂snow and Ar spray cleaning are non-abrasive processes and are typically used as a final clean following other more aggressive pre-cleaning steps herein, for example a plasma or CO₂pellet spray pre-treatment technique. Snow has some ability to remove molecular films of organic contaminants but cannot remove heavy amounts of organic contaminants and cleaning is typically restricted to line-of-sight. Argon ice cleaning is even more restricted to fine particle contamination removal. Other potential limitations of dielectric solid spray cleaning include thermal shock concerns and condensation build-up, which can inhibit cleaning. These latter two drawbacks can be overcome by proper design of a cleaning method as addressed using the present invention, which limits exposure of a substrate to long treatment periods or to excessively high spray pressures.
In still another example of a dense fluid treatment step, dense fluid sprays comprising pressurized and superheated carbon dioxide and trace amounts of water vapor (steam), the subject of a PCT application by the present inventor, have been found by the present inventor to be very effective for removing tenacious contaminants such as waxy or grainy buffing and polishing compounds. This type of dry dense fluid steam spray is suitable as a gross pre-clean prior to snow spray cleaning operations above.
Dense fluid cleaning systems, including gas, solid, liquid and supercritical fluid systems, suitable for use in the present invention are available from The Defiex Corporation, Valencia, Calif.
An aspect of the present invention is exploit the unique solvency and surface scouring characteristics provided by the aforementioned dense fluid immersion and spray cleaning techniques. However, to use this technique effectively, it has been found that the surface contaminant must be first treated to eliminate or reduce contaminant characteristics such as dryness, lack of solubility, organic cross linking, tackiness, and thickness.
The above dry cleaning techniques may be used in various combinations to chemically and physically treat a precision substrate surface to remove thick and thin film contaminants efficiently down to the 50 angstrom level and particle residues to the 0.2 micron level—considered microscopic level cleaning. However, to continue treating to below these levels, which is to the nanoscopic level, additional combinational methods must be employed. These are described in the following sections.
Electrohydrodynamic Surface Cleaning Technique
Electrohydrodynamic (EHD) cleaning is a vacuum cleaning technique that utilizes microscopic and energetic cluster beams to remove sub-micron residues adhering to a native substrate surface. Electrostatically charged micro droplets or clusters having a pre-determined chemistry and composition, velocity, energy and size are directed at a precision substrate surface under vacuum conditions. Micro clusters are extremely effective for removal of sub-micron level contaminations, without leaving a residue, but are highly directional and can be easily blocked by thick films or large particles (i.e., >5 microns) and complex topography present on a substrate surface. For example, microscopic mirrors on an optical wafer will occlude the micro cluster beam. An EHD cleaning system suitable for use in the present invention is available from Phrasor Scientific, Duarte Calif.
An aspect of the present invention is to exploit the combinations of plasma and dense fluid above to first chemically clean and modify a precision substrate surface in preparation for nano-scale cleaning treatments using EHD. The EHD technique is enabled by the combinational techniques that precede it.
Ultraviolet Light with Ozone Surface Cleaning Technique
The ultraviolet/ozone (UV/O₃) cleaning process involves the exposure of a contaminated precision surface to ultraviolet light in the presence of ozone. Cleaning occurs when contaminant molecules are excited or dissociated by the absorption of short-length UV light. At the same time, the ozone breaks down into atomic oxygen, which then reacts with the excited contaminant molecules and free radicals to form simpler, volatile molecules, such as carbon dioxide and water vapor.
UV/O₃cleaning produces surfaces that meet critical cleanliness requirements. The UV/O₃cleaning process has been used successfully to remove very thin organic films from a number of different surfaces in precision cleaning applications. This process is relatively inexpensive to set up and operate and, since it has no moving parts, is easy to maintain. However, UV/O₃cleaning does not remove inorganic contaminants or particulates. Moreover, UV/O₃process has a line-of-sight cleaning limitation and the possibility of staining, discoloration, or corrosion of surfaces that can result from improper wavelengths or exposure times. Therefore, this step is used as a polishing step following the above dry cleaning combinational techniques. UV/O₃cleaning systems suitable for use in the present invention are available from Jelight Company, Inc., Irvine, Calif.
An aspect of the present invention is to first use a plasma and dense fluid combinational cleaning technique to first chemically clean and modify a precision substrate surface. However, the aforementioned chemically and physically pretreated substrate surface still contains molecular levels of contaminants, both films and residues. For example, the clean and modified surface can then be exposed to a UV/O₃treatment for a few seconds, which rapidly produces a molecularly clean surface and enhances the outgassing of absorbed films and gases from a substrate surface. This would not be possible, without first using the first combinational cleaning operation described herein.
Thus, the alternative combination of dry surface cleaning and modification technologies discussed above and used in the present invention are very attractive but have limitations due to varying levels of cleaning performance, line-of-sight effectiveness, and potential damage (i.e., plasma etching) to substrate surfaces if contacted for an extended treatment period or if used at excessive energy levels. However, if used in certain combinations, an instant cleaning method may be established for removing a variety of contaminants from precision substrate surfaces based on the nature of and interaction between contamination and surfaces. The nature of the various contaminants and substrates; the contaminant-substrate and contaminant-contaminant adhesion forces present must be fully understood. Furthermore, the various interactions between the cleaning method and the substrate and substrate features present thereon must be understood. Once all of these interrelationships and discrete cleaning parameters are understood, an instant and enabling relationship may be established which selectively removes a contaminant or group of contaminants from a substrate, in pre-determined and discrete steps, without damaging the various substrate features which may be present, for example patterned resists, microvias, microstructures, and beneficial coatings. Moreover, the present invention uniquely and easily lends itself to being performed in a single process chamber or integration within a staged or in-line cluster tool. This is beneficial since it reduces re-contamination of precision substrate surfaces during handling.
An aspect of the present invention is to ascertain and apply an instant and enabling dry surface treatment combination which eliminates wet processing and rinsing and drying steps, increases cleaning and modification tool productivity, decreases equipment cost, and size, reduces pollution, and improves substrate cleanliness, quality and yield.
The present invention illustrates a method in which an instant enabling, dry, and selective cleaning combination is established using four dry techniques described above; plasma, dense fluids, electrohydrodynamic cleaning and UV/Ozone cleaning. Using two or more of these techniques sequentially or simultaneously as an instant combination, virtually any type of contamination may be effectively and selectively removed from a substrate without damaging the precision substrate surface.
The present invention may be used in the field of precision manufactured and assembled devices and precision test apparatus such as wafers, dies, CMOS image sensors, fiber optic connectors, optical fibers, optical benches, optics, IC test socket pads, flexible polyimide gold circuits, PCB rework, lead frame bond pads, and photodiodes, among many others. The invention is illustrated in various examples as follows.
1. Patterned MEMS die—selective removal of residues, particles and absorbed gases from complex topography;
2. Polyimide gold circuit film—selective removal of a protective organic coating from gold metallized layers in preparation for platinum electroplating;
3. Optical fiber—selective removal of acrylate polymer, fingerprints, and particles from stripped or spliced optical fiber surfaces and surface energy increase in preparation for bonding;
4. Optical filter—selective removal of a proprietary organic film and particles and surface modification in preparation for coating;
5. Optical connector—selective removal of polishing residues and films from an end face in preparation for use;
6. PCB rework—selective removal of out-of-spec electronic component, preparation of bond pad area, and bonding of new spec component; and
7. IC Test Socket Cleaning—selective surface re-conditioning of IC socket test pads.
However it will be recognized that the invention has a much wider range of applicability. Merely by way of example, the invention can also be applied to selective cleaning of disk drive read-write heads, diced wafers, image sensors, optical sensors, implantable medical devices, lead frames, LCDs, OLEDs, photodiodes, and many other precision devices and surfaces.
Moreover, medical substrates such as boroscopes, polyester grafts, polyurethane blood filters may be treated using the present invention to remove residues, particles, biological contaminant and may be treated using plasma techniques to improve wetability and biocompatibility. Still moreover, the plasma-dense fluid cleaning-modification combination may be used to clean and treat commercial textiles and fabrics to remove complex surface soils and to brighten fabric fiber surfaces, respectively.
The present invention provides a safe, robust, and selective method and apparatus to treat a precision substrate surface using an enabling combination of atmospheric plasma, dense fluids, UV/O₃, and electrohydrodynamic (EHD) cleaning techniques, which when used in various combinations described herein, an entire spectrum of surface residue and particle cleaning performance to the nanometer level, and is better understood by reference to the following figures and detailed discussion that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—Performance profiles for the exemplary dry cleaning techniques; Plasma, Dense Fluids, EHD, and UV/O₃.
FIG. 2—Exemplary combinational dry cleaning approach for multi-layered contamination on a substrate surface.
FIG. 3—Exemplary flow diagram showing the various instant dry cleaning methods possible using the present invention.
FIG. 4—Exemplary dry cleaning method options matrix that correlates the contaminants, substrate surface, and enabling dry cleaning techniques.
FIGS. 5 a and 5 b—Exemplary photomicrographs at 500.times. magnification showing before and after surface cleaning large and small particles using vacuum plasma and dense fluid spray cleaning treatments, respectively.
FIGS. 6 a and 6 b—Exemplary photomicrographs at 500.times. magnification showing before and after surface cleaning of large and small particles using an EHD spray.
FIGS. 7 a and 7 b—Exemplary photomicrographs at 2500.times. magnification showing before and after surface cleaning of large and small particles using an EHD spray.
FIGS. 8 a and 8 b—Exemplary photomicrographs at 2500.times. magnification showing before and after surface cleaning of small particles using an EHD spray.
FIG. 9—Exemplary polyimide-gold flexible circuit substrate selective cleaning application.
FIG. 10—Exemplary in-line surface treatment apparatus for the substrate of FIG. 9.
FIG. 11—Exemplary printed circuit board substrate selective cleaning application.
FIG. 12—Exemplary cluster cleaning and assembly apparatus for the substrate of FIG. 11.
FIG. 13—Exemplary IC socket test substrate selective surface cleaning application.
FIG. 14—Exemplary optical device selective surface cleaning application.
FIG. 15—Exemplary fiber optic connector selective surface cleaning application.
FIG. 16—Exemplary MEMS wafer selective surface cleaning application.
FIG. 17—Exemplary combinational cluster cleaning tool for performing sequential treatments of a precision substrate surface.

DETAILED DESCRIPTION

FIG. 1 shows performance profiles for the exemplary dry cleaning techniques; Plasma, Dense Fluids, EHD, and UV/O₃used in the present invention. Referring to FIG. 1, performance profiles for plasma (2), dense fluid (4), EHD (6), and UV/O₃(8) dry cleaning and surface preparation treatments are represented a Gaussian distribution curves. The performance profiles represent generalized upper and lower limits of cleaning efficiency for a certain class of contaminants. A generalized boundary condition (10) exists which demarks the transition from macro and microscopic layers (12) to nanoscopic layers (14) of contamination, and to the rough and porous native substrate surface. Furthermore, the individual treatment groups bisected by the boundary condition (10) may change in sequence, or may be used selectively. For example, plasma (2) and dense fluids (4) are used in combination with the present invention to address macroscopic and microscopic contaminations, as well as surface modification treatments (Table 4). Following plasma and dense fluid surface pre-treatments, EHD (6) and UV/O₃(8) may be used selectively to address nanoscopic and molecular contamination concerns, respectively.
As shown in FIG. 1, plasma cleaning provides various Table 3, Class B and C cleaning energies and dense fluids provide various Table 3, Class A, B, and C cleaning energies. Thus these two combinational techniques can provide a range of cleaning energies suitable for most Table 1, Class A and B contaminations and can be used efficiently in various forms and combinations to remove nearly 100% of particles as small as 0.5 micrometer and residues down to the molecular level. At this point (10), nanoscopic contaminations in the form of nano-sized particles, molecular films, and outgassing compounds are exposed on a substrate surface. More efficient dry cleaning techniques must be employed for nanoscopic and molecular contaminants, but without a preceding surface treatment, these contaminants remain encapsulated in thicker contaminating films and larger particles. Light obscuration, particle hideout, capillary forces, Van der Waals force, and stiction shield or hold these low level contaminations to the surface and prevent various cleaning energies from effectively accessing and removing these nanoscopic contaminations. EHD cleaning provides Table 3, Class A1 cleaning energy required to remove very small particles and UV/O₃provides Table 3, Class C2 cleaning energy to remove absorbed gases and vapors from the native surface. As shown in FIG. 1, EHD and UV/O₃combinational cleaning provides a range of effective cleaning performance in a range from 0.5 micrometers down to 0.01 micrometers, including the removal of molecular contaminants.
FIG. 2 shows the exemplary combination of dry cleaning techniques, conditions of use, and sequencing to remove various layers of contamination. Referring to FIG. 2, a contaminated surface may be characterized as follows; thick polymeric films (16) can be on the order of several microns thick, followed by viscous oily films and large particles (18), followed by very small particles (20), and finally monolayer films and outgassing contaminants (22) on or within the pores of a rough native substrate surface (24). At the surface level, microscopic pores and capillaries (26) present on the substrate surface (24) entrap very small particles and absorbed molecules and vapors.
Thus as characterized in FIG. 2, it can be seen that each upper contaminant layer encapsulates the layer below it. Moreover, the physicochemistry of each layer may be different in terms of quantity, physicality, and bonding energies. The present invention utilizes the aforementioned dry cleaning techniques in various combinations and under limited contact periods in an enabling sequence to remove the various layers shown in FIG. 2 with increasing precision. For example, as shown in FIG. 2, atmospheric plasma (28) may be used very efficiently and selectively in a 2 minute exposure period to reduce a polymeric contaminant layer (16), which lowers bonding energy and increases surface area. Following this, a dense fluid spray (30) is used selectively to remove the reduced contaminants and particles freed by the plasma treatment (28). If desired and required, EHD (32) may be used to remove nanoscopic residues freed by the dense fluid treatment (30). Finally, as a polishing step, UV/O₃(34) may be used remove or flash off the remaining monolayers of absorbed gases and vapors.
Furthermore, following plasma and dense fluid combinational surface cleaning treatments described above, a native and clean surface is exposed. A second plasma treatment (36) may be again selectively applied, although using the same dense fluid-plasma cleaning treatment device, to the substrate surface to modify a portion of the substrate surface, for example the removal of oxides (i.e., Table 1, Type B2 contaminant) using a hydrogen plasma gas mixture, in preparation for bonding (i.e., Table 4, Type A modification). Moreover, as discussed herein, the additional of special admixture gases during a plasma treatment will impart beneficial surface properties to a properly cleaned and activated surface such as increased impermeability or permeability, low friction, biocompatibility, brightness or other physicochemical surface features. Thus the techniques are arranged and applied in a specific and enabling sequence to achieve a desired level of cleanliness and surface modification.

FIG. 3 is an exemplary flow diagram showing the various exemplary instant dry cleaning (Methods 1-7) and cleaning-modification (Method 8) methods possible using the present invention. Referring to FIG. 3, the four dry techniques employed produce several possible instant precision substrate surface cleaning and modification methods as shown in Table 5.

TABLE 5


Instant Surface Cleaning Methods

	Exemplary Instant Method 1:
	Step 1 - Plasma Cleaning (38)
	Step 2 - Dense Fluid Cleaning (40)
	Step 3 - EHD Cleaning (42)
	Step 4 - UV/O₃Cleaning (44)
	Exemplary Instant Method 2:
	Step 1 - Plasma Cleaning (38)
	Step 2 - Dense Fluid Cleaning (40)
	Step 3 - Plasma Surface Modification (38)
	Exemplary Instant Method 3:
	Step 1 - Plasma Cleaning (38)
	Step 2 - Dense Fluid Cleaning (40)
	Step 3 - UV/O₃Cleaning (44)
	Exemplary Instant Method 4:
	Step 1 - Plasma Cleaning (38)
	Step 2 - Dense Fluid Cleaning (40)
	Exemplary Instant Method 5:
	Step 1 - Dense Fluid Cleaning (40)
	Step 2 - Plasma Cleaning (38)
	Step 3 - Dense Fluid Cleaning (40)
	Exemplary Instant Method 6:
	Step 1 - Dense Fluid Cleaning (40)
	Step 2 - EHD Cleaning (42)
	Exemplary Instant Method 7:
	Step 1 - Dense Fluid Cleaning (40)
	Step 2 - UV/O₃Cleaning (44)
	Exemplary Instant Method 8:
	Step 1 - Plasma Cleaning (38)
	Step 2 - Dense Fluid Cleaning (40)
	Step 3 - Plasma Modification (38)

A particular instant dry cleaning method is chosen for a particular surface cleaning application and is based on the type of substrate, contaminants, cleaning time, and desired level of cleanliness.
FIG. 4 is a dry cleaning method options matrix that correlates the contaminants, substrate surface, and enabling dry cleaning methods described herein and in Table 5. Moreover, FIG. 4 provides four exemplary precision cleaning applications and shows the optimal instant method from Table 5 for each application.
Referring to FIG. 4, the exemplary contaminants described in Table 1 herein form the column (46) and the exemplary substrate surfaces of Table 2 herein form the row (48) of a cleaning options matrix (50). The cleaning options matrix (50) comprises the various cleaning techniques suitable to address the specific contaminant (46) on a particular substrate surface (48). From this matrix, an instant surface cleaning and treatment method is created based on the real-world contaminant-substrate surface application. Referring to FIG. 4, surface treatment application examples (52) comprise the following; the topside of an IC wafer to remove RIE etch residues and particles in preparation of a resist coating (54), the interior of a CMOS image sensor to remove stains, solder flux, and particles in preparation for hermetic sealing (56), the gold bonding pads on an organic polyimide flexible circuit to remove resist and particles in preparation for platinum electroplating (58), and the exposed optical fiber to remove finger oils, particles and stripping residues in preparation for adhesive bonding (60).
As shown in FIG. 4, an instant method (62) as described in FIG. 3 above was used which encompassed the nature and level of contamination present and to meet the cleanliness objectives for each cleaning application. For the IC wafer application, exemplary instant method 1 described above in Table 5 and FIG. 3 (64) met the cleaning objective. For the CMOS image sensor application, exemplary instant method 3 described above in Table 5 and FIG. 3 (66) met the cleaning objective. For the polyimide-gold pad cleaning application, exemplary instant method 4 described above in Table 5 and FIG. 3 (68) met the cleaning objective. Finally, for the optical fiber cleaning application, exemplary instant method 5 described above in Table 5 and FIG. 3 (70) met the cleaning objective.
FIG. 5 a is an exemplary photomicrograph at 500.times. magnification showing a surface that has been treated with a vacuum plasma at 100 mTorr with a nitrogen-oxygen atmosphere. As can be seen in the figure, plasma treatment does not remove particles on a surface. Because plasma energy is predominantly oxidative, the process is rather slow for large and inorganic particle contaminations. Various sized particles can be seen in the figure including a large 10 micrometer sized particle (72), numerous 1 micron particles (74), and a 0.5 micron particle (76). An extended plasma treatment could eventually remove these particles from the surface through aggressive oxidative destruction, but an extended treatment is inefficient and, more importantly, would also attack and damage the native substrate surface and any delicate features such as micromotors and microoptics. Thus a short plasma treatment is beneficial if selectively used to remove thick and thin organic film contamination in preparation for a follow-on thin film and particle cleaning techniques.
Referring to FIG. 5 b, a dense fluid spray was used to clean a plasma treated surface as in FIG. 5 a. As can been seen in FIG. 5 b, a dense fluid spray treatment produces a particle clean surface (78) at 500.times. magnification. Also seen in the figure is the rough surface topography showing the presence of small pits (80), ridges (82), and valleys (84). Surface features such as these, and as depicted graphically in FIG. 2, (24) and (26), present a major challenge to conventional surface cleaning approaches, including dense fluid sprays. These surface features hide small particles and residues from the impacting cleaning media such as snow. A dense fluid spray is generally effective for removing small particles present in on the ridges (82) and valleys (84), however the pits (80) represent a significant challenge. Increasing the dense fluid spray duration may dislodge small particles and residues trapped within these surface depressions, however this would require an extended surface treatment or increased spray pressure and risk the possible freezing or encapsulating the small particles within the pit or possibly damaging delicate surface features with an increased duration or impact pressure. Thus, similar to vacuum plasma treatment, dense fluid sprays may be used selectively herein to remove surface thin films and particles in preparation for additional surface treatments, such as a follow-on plasma surface modification, or possibly a nanoscopic residue removal using more selective substrate treatment techniques.
FIGS. 6 a and 6 b are exemplary photomicrographs at 500.times. magnification showing before and after surface cleaning a particle debris field using an EHD spray, respectively. Referring to FIG. 6 a, a massive number of inorganic particles are present on the surface, including innumerable sub-micron particles still invisible at this magnification, hundreds of 0.5 to 5 micron particles (86), and up to a large 20 micron particle (88). Referring to FIG. 6 b, following cleaning using EHD treatment it can be seen that all the smaller particles visible in FIG. 6 a have been removed. However, many 1 micron particles (90) and all of the larger particles; 5 micron (92), 10 micron (94), and a large 20 micron particle (96) remain. Moreover, large particles (98) can be seen hiding within a large surface groove (100). As is clearly demonstrated by comparing FIGS. 6 a and 6 b, EHD treatment is ineffective for removing large particles from a substrate surface and very selective for removing small particle contaminations. Thus the exemplary plasma and dense fluid surface treatment techniques depicted in FIGS. 5 a and 5 b enable a much more selective process such as EHD. Moreover, and not shown here, a more selective technique such UV/O₃would be similarly enabled by the aforementioned surface treatment techniques because it suffers from same performance limitations described herein such as residue obscuration effects. This phenomenon is exemplified in the discussion that follows using FIGS. 7 a and 7 b.
FIGS. 7 a and 7 b give exemplary photomicrographs at 500.times. and 2500.times. magnification, respectively, showing the EHD cleaned surface of FIG. 6 b. As can be seen in FIG. 7 a, the surface debris field contains numerous and variously sized particles having diameters generally greater than 0.5 microns, and in particular, a very large 20 micron particle (102) in the center of the figure. A close-up of the area bounded by the rectangle (104) is shown in FIG. 7 b. Examination of this EHD cleaned surface at 2500.times. clearly shows the phenomenon of spray obscuration, which causes an impingement spray such as EHD micro cluster sprays to be effectively blocked by larger particles and residues present on a surface. The large particle (104) shields a swath of small particles (106) at its base, having diameters of 0.2 microns and smaller. However, sub-micron particles such as those (106) hidden at the base of the large particle (104) are not present in exposed regions of the surface (108).
UV/O₃cleaning is even more selective with respect to particle cleaning and line-of-sight cleaning limitations. Since only UV light oxidation cleaning mechanisms are involved, substrate surfaces present under a large particle such as shown in FIG. 7 b would not be exposed to UV light energy and thus not cleaned. Thus it is extremely beneficial to first clear the surface of thick films and large particles to enable selective surface cleaning process steps EHD and UV/O₃cleaning. This is illustrated in FIGS. 8 a and 8 b below.
FIGS. 8 a and 8 b are exemplary photomicrographs at 2500.times. magnification showing before and after surface cleaning of sub-micron particles using an EHD spray, respectively. As shown in FIG. 8 a, numerous sub-micron particles are present on an exposed and relatively planar surface (110) as well as several particles (112) contained within a small surface trench (114). The surface, once pre-cleaned using a plasma and dense fluid to remove thick film residues and large particles, is properly prepared for a much more selective treatment using EHD. As shown in FIG. 8 b, the surface (116) and trench (118) are free of sub-micron particles following EHD treatment. Moreover, this surface may be efficiently and effectively treated with UV/O₃cleaning to remove molecular and outgassing contaminants present on the EHD cleaned surface because there are no shielding particles and residues present.
Having thus described the particular cleaning performance limitations and enabling and overlapping benefits of using the plasma, dense fluids and EHD cleaning treatments herein, following is a discussion of exemplary precision substrate treatment applications and apparatuses using the present dry cleaning method.
FIG. 9 is a graphic representing a portion of a polyimide-gold flexible circuit substrate that has been separated from a roll of material containing hundreds of these substrates (a 3M product). The exemplary substrate is predominantly organic having a gold circuit trace (120) and gold bonding pad (122) encapsulated between two sheets of thin polyimide polymer (124). Moreover, the gold bonding pads (122) contain a thin film of organic resist (126) on their surfaces. The presence of this contaminant (126) prevents the deposition of platinum onto the gold pad (126) surface. Therefore it must be removed prior to electrodeposition. In the example illustrated here; a flexible polyimide film (124) with electrical circuit tracing (120) have gold bonding pads (126) is a Table 2, Type B1 substrate having a portion thereon containing a thin film of cured resist coating which is a Table 1, Type A1 contaminant. The precision substrate surface thus described comprises approximately 98% organic film (polyimide) and 2% inorganics (gold). Without surface treatment, the gold pads cannot be electroplated with platinum.
A conventional surface treatment procedure for cleaning this substrate involved cutting from the roll stock, discrete precision substrate portions. Each substrate portion is then immersed in a N₂/O₂vacuum plasma (200 watts/100 mTorr) for 5 minutes. It was found that following plasma treatment of the discrete portions, the bond pads were still heavily contaminated with organic plasma etch residues, a Table 1, Type B1 contaminant. As such various mineral acid wash and water rinse cycles were used to remove residual “plasma contamination”. It was determined that vacuum plasma treatment produces a Table 1, Type B1 contamination as a by-product, which is probably caused by treating predominantly organic substrates such as polyimide in a high energy environment such as vacuum plasma. A nitrogen gas spray was used to dry the plasma and acid treated surfaces. A final plasma treatment was then used to “polish” the treated surfaces. However, the multi-stepped conventional technique thus described still produces variable surface treatment quality due to plasma residue formation. This is manifested in the cleaned product as sporadic and porous electrodeposits of platinum on the gold pads (122).
Using the present invention, It was determined that if a first and single plasma treatment is immediately followed by a short and selective dense fluid spray (treating only the gold pad surfaces), the precision substrate surfaces could be plated with platinum, thereby eliminating the corrosive acid washes and polluting rinse steps, and repetitive and re-contaminating plasma treatment step. Furthermore, it was determined that an atmospheric plasma using He/O₂for 2 minutes, followed by a 2 second snow spray, could also be used to selectively treat the gold pads (122) for platinum electroplating. Thus, using atmospheric plasma allows for the treatment of an entire roll of the exemplary precision substrates without having to cut discrete substrates from a roll of material. This makes the new surface treatment process much more efficient than the old method.
Thus the present example is another illustration of the enabling combination of plasma and dense fluids. A short 1 to 5 minute selective exposure of a precision substrate surface to an atmospheric oxygen plasma, followed by a 1 to 5 second spray of snow particles produces a precision clean surface which can be reproducibly electroplated with platinum. The present method reduces time, minimizes process steps, eliminates pollution, and improves surface cleaning quality. Moreover, the new method enables improved automation and in-line surface inspection, which are not easily done using conventional wet and dry cleaning combinations discussed herein. For example, a plasma-dense fluid cleaned surface may be immediately examined following treatment using an in-line surface inspection technique such as optically stimulated electron emission (OSEE) as a quality control step. This is illustrated in the discussion that follows using FIG. 10.
FIG. 10 shows an exemplary in-line reel-to-reel surface treatment and inspection method and apparatus for the substrate discussed in FIG. 9. The exemplary polyimide-gold substrate (128) is supplied from 3M Company on a roll (130). This roll (130) contains hundreds of discrete precision substrates graphically depicted in FIG. 9. Construction of an in-line selective cleaning and inspection system using the present invention is described as follows. A machine is constructed using a reel-to-reel device comprising the roll of reeled source material (130) which is fed through a slotted mounting fixture (132) which presents a portion (e.g., exposed gold pads) of the precision substrate surface to a first treatment comprising an atmospheric plasma treatment device (134) which is directed (136) at said portion of said substrate surface, immediately followed by a second selective surface treatment comprising a dense fluid snow spray (138) which is directed (140) at the same substrate surfaces treated by the first treatment step. Optionally, the treated substrate surface may be inspected for residual organic resist residues using an optically stimulated electron emission analysis probe (142) which is directed (144) at the same substrate surface treated using the plasma and dense fluid sprays.
Again referring to FIG. 10, the rolled material (130) may be fed continuously or indexed (stop and go) as indicated by the arrow (146) to present the portion (128) of the roll material (130) requiring a surface treatment and optional inspection. The treated substrates (148) are rolled onto a take-up reel (150) and a clean interleaf barrier film (152) from a supply reel (154) may be rolled up with the treated substrates to protect treated surfaces (148) from being recontaminated. Also as shown in FIG. 10, an in-line nitrogen gas ionizer (156) may be used to deionize treated substrate surfaces to prevent electrostatic charge attraction of atmospheric contaminants during handling and storage.
In another example application shown in FIG. 11, an electronic printed circuit board substrate (158), containing many electronic components (160) requires selective substrate surface cleaning. In this application, a discrete electronic component, in this example a 0201 chip resistor (162), must be first removed, and the underlying surface must be cleaned and prepared for placement of a new component. The portion of the substrate surface to be cleaned and treated is represented by the circle (164) and discussed more fully below. The exemplary process is described as follows. The malfunctioning chip resistor (162) contains a silicone conformal coating (166), both of which are removed together using a thermal de-bonding technique (168), for example an infrared laser. Following thermal de-bond, the underlying substrate surface (170) is exposed and contains residual silicone conformal coating particles and soldering flux residues (172) on the surface (170) and on the tin bonding pads (174). A dense fluid spray (176) is used to remove residual silicone coating and flux residues from the immediate vicinity (178) and on the exposed surfaces of the tin bonding pads (174) to produce particle and residue free bonding pad surfaces (180). However, the cleaned bonding pad surfaces (180) still contain a thin film of oxide, which must be removed to provide proper wetting and good adhesion during the subsequent new component soldering operation. An atmospheric plasma surface treatment using an Argon-Hydrogen gas mixture (182) is employed to remove oxide contamination from, the cleaned bond pads (180) to produce a residue clean and oxide free bonding pad surface (184). Finally, the cleaned and treated pad surface (184) is ready for bonding the new electronic component. At this point, the surface may be optionally inspected using OSEE as above. A new electronic component (186) is thermally bonded (188) to the clean and treated bond pads (184) using an infrared laser and is coated with a small quantity of UV-curable silicone conformal coating (190), which may then be cured using a UV lamp.
Thus the present example illustrates a dry and very selective surface cleaning, treatment and inspection method which eliminates the need for treating the entire substrate (158) using conventional wet surface cleaning techniques such as aqueous immersion cleaning, acid oxide removal techniques, water rinsing, and hot air drying. Moreover, the present surface treatment method may be directly integrated into a soldering rework tool, producing a much more efficient production tool and process. This capability is illustrated in the following discussion using FIG. 12.
FIG. 12 is an exemplary cluster cleaning and assembly apparatus for performing the method described in FIG. 11. As shown in FIG. 12, a cluster cleaning and rework tool may comprise a hexagonal workstation (192), having at its center robotic transfer robotics (194). Surrounding the substrate transfer robotics (194), five selective treatment modules may be positioned as follows; a thermal de-bond module (196), a surface treatment module (198), a bonding module (200), a conformal coating module (202), and UV curing module (204). Integrated with the exemplary workstation (192) is incoming substrate conveyor (206) and an outgoing processed substrate conveyor (208). Having thus described the basic components for a cleaning and rework tool, following is a description of the operation of such a tool.
Referring to FIG. 12, an un-processed electronic board substrate (158) having a malfunctioning or out-of-spec electronic device (162) mounted thereon is conveyed (210) into the workstation (192) using an incoming conveyor (206). A substrate transfer robot (194) moves the electronic board substrate (158) through a series of rework and substrate treatment steps as described in FIG. 11. The electronic board (158) is moved counterclockwise through the workstation (192) as follows; to the thermal de-bond module (196), to the dense fluid-plasma surface treatment module (198), to the thermal bonding module (200), to the coating module (202), and to the UV curing module (204). Finally, the processed substrate (212) containing a new electronic device (186) and conformal coating (190) is conveyed (214) from the workstation (192) using an outgoing conveyor (208).
Having thus described in detail two exemplary precision substrate treatment applications and apparatuses using the present dry cleaning method, the following discussion provide additional and more general examples of precision substrate surface applications for the present invention using FIGS. 13, 14, 15 and 16.
FIG. 13 is an exemplary substrate comprising an IC socket test apparatus. As shown in FIG. 13, an IC socket test apparatus contains an array of gold plated test pads (216) that are affixed to a base (218), and to which is connected to a circuit test apparatus (not shown). A device such as a BGA chip containing a similar pattern and number of sockets (not shown) is contacted to the topside surface (220) of the IC test pads (216). Following this, the IC test socket apparatus tests the BGA for electrical or logic performance. This operation is performed thousands of times in production and requires periodic cleaning to remove oxides, particles and other contaminants that build-up over time. Cleaning of the contacts becomes necessary because contact resistance increases significantly as contaminating residues levels increase, producing false signals or misinterpretation of test results. As such, the present invention, and specifically the atmospheric plasma-dense fluid spray method described herein using a nitrogen-hydrogen plasma gas mixture followed by a dense fluid spray treatment will selectively clean and reconditioning of the gold contacting surfaces of an IC test socket substrate.
FIG. 14 is an exemplary selective substrate cleaning application comprising an optical bench. An optical bench is assembled using a housing (222), into which are assembled various optics, electronics and mounting fixtures. For example an optical cable (224) may be stripped to expose a bare optical fiber, which is bonded to a v-groove block assembly (226) and mated to a photodiode device (228). The exemplary optical bench thus described will convert light signals traveling down the optical fiber into electrical signals. The topside surface (230) of the v-groove block assembly (226) and exposed fiber (232) must be cleaned of stripping debris and other contaminations to provide for proper adhesive bonding of the fiber (232) with the photodiode assembly (228). The device as described is very difficult if not impossible to clean using conventional cleaning methods. The present invention, and specifically the atmospheric plasma-dense fluid spray method described herein using a nitrogen-oxygen mixture plasma gas mixture followed by a dense fluid spray treatment will selectively clean all surfaces of the exemplary optical bench, as well as prepare the interior of the assembly (222) for subsequent sealing.
FIG. 15 is an exemplary selective cleaning application comprising a fiber optic connector substrate. A fiber optical connector (234) contains a ferrule assembly (236), which comprises a ceramic body housing a cladded optical fiber. As shown in the figure, the front side of the ferrule (236) has critical surface features exposed including a ceramic face (238), optical fiber cladding face (240), and the optical fiber face (242). Following operations such as polishing, these faces become heavily contaminated with polishing particles and residues. The present method using a plasma and a dense fluid spray in combination may be used to selectively clean these critical surface features without scratching, etching or otherwise damaging the surfaces and optical transmission performance of this device.
FIG. 16 is an exemplary selective cleaning application comprising a MEMS wafer substrate. As shown in the figure, discrete dies (244) manufactured on the wafer substrate (246) using micromachining as well as more conventional IC circuit manufacturing techniques require frequent cleaning in between manufacturing steps. As shown in the topside figure (248), surface features such as trenches, vias, gears, and beams present significant surface cleaning challenges requiring a multiplicity of cleaning energies and techniques. The instant method comprising plasma, dense fluid, EHD, and UV/O₃treatments herein will provide the energies necessary to remove all varieties common patterning, micromachining, and plasma etching residues encountered in the MEMS manufacturing process without damaging delicate surface features as depicted in the figure (248).
Finally, having thus described more general examples of precision substrate surface applications for the present invention, the following discussion in relation to FIG. 17 describes a cluster cleaning tool for performing all possible instant dry cleaning methods described herein using the present invention.
FIG. 17 is a graphic showing an exemplary combinational cluster-cleaning tool for performing sequential dry surface treatments using the present invention. As shown in the figure, a cluster-cleaning tool may comprise a hexagonal workstation (250), having at its center robotic transfer robotics (252). Clustered about the substrate transfer robotics (252) are five selective treatment modules as follows; a vacuum or atmospheric plasma treatment module (254), a dense fluid solid, liquid or supercritical fluid treatment module (256), an EHD treatment module (258), a UV/O₃treatment module (260), and an inspection module (262), which can be a vision inspection system, OSEE system, and other possible surface inspection techniques. Integrated with the exemplary workstation (250) is an incoming substrate conveyor (264) and an outgoing processed substrate conveyor (266). The system thus described may be programmed to perform any of the instant dry substrate surface cleaning methods described herein.
Additional real-world examples of use and instant dry cleaning and modification methods developed using the present invention are as follows:
Lapped and Polished Sapphire Wafer Cleaning and Stain Removal Method
Remove Gross Polishing Agents and Water Residues:
1. CO₂steam flush—200 C., 120 psi, 2-3 minutes
Remove Fine Particle and Thin Film Residues:
2. CO₂snow spray—80 psi, 5-30 seconds
Degrade Surface Stains:
3. Vacuum plasma treatment—100 mTorr, 200 watts, Ar/O₂atmosphere, 5 minutes
Remove Plasma Degraded Contaminant Residues:
4. CO₂snow spray—80 psi, 5-30 seconds
Implantable Polyester Graft Substrate Cleaning, Biocompatibility, and Sterilization Method
Remove Extractable Contaminants:
1. Supercritical CO₂extraction—2500 psi/60 C, 60 minute extraction cycle
Degrade (Oxidize) Surface Residues and Biological Activity:
2. Vacuum plasma treatment to degrade monomers and reduce surface particle adherence—500 mTorr, 200 watts, N₂/O₂atmosphere, 5 minutes
Remove Plasma Degraded Contaminants:
3. Liquid CO₂rinse to wash plasma treated particles, 1200 psi, 25 C., 20 minute cycle
Activate Surface to Improve Biocompatibility:
4. Vacuum plasma treatment to degrade monomers and reduce surface particle adherence—500 mTorr, 200 watts, Ar/O₂atmosphere, 5 minutes
Soiled Inspection Boroscope Cleaning and Sterilization Method
Remove Gross Biological Fluids and Residues:
1. Water rinses and drain—30 C., 20 psi, 2-3 minutes
2. CO₂steam flush—250 F., 120 psi, 2-3 minutes
Degrade Adhering Thick Films and Biological Soils:
3. Vacuum plasma treatment—100 mTorr, 200 watts, Ar/O₂atmosphere, 5-60 minutes
Remove Plasma Degraded Residues and Large Particles:
4. Liquid CO₂spray—900 psi, 25 C., 10 minutes
Remove Small Particles:
5. CO₂snow spray—80 psi, 5-30 seconds
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

1. A method of treating a substrate within a single controlled environment comprising subjecting a surface of the substrate with atmospheric plasma to physicochemically modify a first contaminant, the atmospheric plasma formed in the absence of a thermal energy source, and then treating the surface with a dense fluid to remove the modified first contaminant from the substrate surface.

2. The method of claim 1 wherein the dense fluid includes a non-supercritical fluid.

3. The method of claim 2 wherein the non-supercritical fluid includes liquid carbon dioxide, solid carbon dioxide or solid argon.

4. The method of claim 1 and further comprising treating the surface of the substrate with a sub-atmospheric plasma after removing the modified first contaminant to prepare the surface for a subsequent application.

5. The method of claim 1 and further comprising treating the surface of the substrate with ultraviolet light in the presence of ozone to remove a second contaminant.

6. The method of claim 5 wherein the first contaminant includes inorganic matter and the second contaminant consists essentially of organic matter.

7. A method of treating a substrate comprising contacting a surface of the substrate with a non-supercritical dense fluid to remove a first contaminant and then contacting the surface with a plasma to remove a second contaminant or prepare the surface for a subsequent application.

8. The method of claim 7 wherein the plasma includes atmospheric plasma formed in the absence of a thermal source.

9. The method of claim 8 wherein the atmospheric plasma comprises ultraviolet light in the presence of ozone.

10. The method of claim 7 wherein the plasma includes sub-atmospheric plasma.

11. The method of claim 7 wherein the non-supercritical dense fluid includes liquid carbon dioxide, solid carbon dioxide or solid argon.

12. The method of claim 7 wherein the first contaminant includes inorganic matter and the second contaminant consists essentially of organic matter.

13. The method of claim 7 wherein the second contaminant consists essentially of organic matter.

14. A method of cleaning a substrate within a controlled environment comprising:

placing the substrate into a high pressure vessel;

supplying the vessel with a dense fluid under pressure;

contacting the dense fluid with the substrate for a selected period of time to at least partially remove a contaminant from the substrate;

depressurizing the vessel to at least partially convert the dense fluid into a vapor;

forming a plasma within the vessel by subjecting the vapor to an energy field;

treating the substrate with the plasma for a selected period of time; and

removing the thus cleaned substrate from the vessel.

15. The method of claim 14 wherein the dense fluid includes a non-supercritical dense fluid.

16. The method of claim 15 wherein the non-supercritical dense fluid includes liquid carbon dioxide.

17. The method of claim 14 wherein treating the substrate with the plasma neutralizes biological matter contained on the substrate.

18. The method of claim 14 wherein the plasma includes an atmospheric.

19. The method of claim 18 wherein the plasma is formed in the absence of a thermal energy source or heat source.

20. The method of claim 14 wherein the vapor is subjected to an electrical energy field to form the plasma.