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 THE 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 Hydrohobicity |
| ||Type D—Surface Friction |
| ||Type B—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/O3 and 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° F. [60° 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 CO2 spray 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 CO2 immersion 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 CO2 cleaning processes, CO2 snow and CO2 pellets, CO2 snow 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. CO2 snow 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 CO2 gas. Furthermore, CO2 snow 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.
CO2 snow 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 CO2 pellet 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/O3 cleaning produces surfaces that meet critical cleanliness requirements. The UV/O3 cleaning 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/O3 cleaning does not remove inorganic contaminants or particulates. Moreover, UV/O3 process 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/O3 cleaning 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/O3 treatment 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; and
5. Optical connector—selective removal of polishing residues and films from an end face in preparation for use; and
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.