U.S. patent application number 11/766762 was filed with the patent office on 2007-10-25 for method of treating a substrate.
Invention is credited to David P. Jackson.
Application Number | 20070246064 11/766762 |
Document ID | / |
Family ID | 31997151 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070246064 |
Kind Code |
A1 |
Jackson; David P. |
October 25, 2007 |
METHOD OF TREATING A SUBSTRATE
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.
Inventors: |
Jackson; David P.; (Saugus,
CA) |
Correspondence
Address: |
DUFAULT LAW FIRM, P.C.
920 LUMBER EXCHANGE BUILDING
TEN SOUTH FIFTH STREET
MINNEAPOLIS
MN
55402
US
|
Family ID: |
31997151 |
Appl. No.: |
11/766762 |
Filed: |
June 21, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10428793 |
May 2, 2003 |
|
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|
11766762 |
Jun 21, 2007 |
|
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60377197 |
May 3, 2002 |
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Current U.S.
Class: |
134/1.2 ;
257/E21.226; 257/E21.228 |
Current CPC
Class: |
H05K 3/26 20130101; H01L
21/02052 20130101; B08B 7/0035 20130101; B08B 7/0021 20130101; H01L
21/02046 20130101; C25D 5/34 20130101 |
Class at
Publication: |
134/001.2 |
International
Class: |
C25F 1/00 20060101
C25F001/00 |
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.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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-US-00001 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)
[0009] Precision Substrate Surfaces may be classified and typified
according to Table 2. TABLE-US-00002 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)
[0010] 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.
[0011] Furthermore, cleaning energies may be classified and
typified according to Table 3. TABLE-US-00003 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
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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).
[0016] 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.
[0017] 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.
[0018] 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
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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-US-00004 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
[0024] 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.
[0025] 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/O.sub.3 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
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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).
[0030] 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 CO.sub.2 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
[0031] 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.
[0032] 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 CO.sub.2 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.
[0033] 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
(CO.sub.2) 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 CO.sub.2 cleaning
processes, CO.sub.2 snow and CO.sub.2 pellets, CO.sub.2 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. CO.sub.2 snow is
formed when liquid carbon dioxide is allowed to rapidly expand
through a nozzle. This creates solid particles of CO.sub.2 (i.e.,
snow) entrained in a stream of pressurized CO.sub.2 gas.
Furthermore, CO.sub.2 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.
[0034] 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.
[0035] CO.sub.2 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
CO.sub.2 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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
[0040] 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.
[0041] 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
[0042] The ultraviolet/ozone (UV/O.sub.3) 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.
[0043] UV/O.sub.3 cleaning produces surfaces that meet critical
cleanliness requirements. The UV/O.sub.3 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/O.sub.3
cleaning does not remove inorganic contaminants or particulates.
Moreover, UV/O.sub.3 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/O.sub.3 cleaning systems suitable for use in the present
invention are available from Jelight Company, Inc., Irvine,
Calif.
[0044] 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/O.sub.3 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 1. Patterned MEMS die--selective removal of residues,
particles and absorbed gases from complex topography;
[0050] 2. Polyimide gold circuit film--selective removal of a
protective organic coating from gold metallized layers in
preparation for platinum electroplating;
[0051] 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;
[0052] 4. Optical filter--selective removal of a proprietary
organic film and particles and surface modification in preparation
for coating;
[0053] 5. Optical connector--selective removal of polishing
residues and films from an end face in preparation for use;
[0054] 6. PCB rework--selective removal of out-of-spec electronic
component, preparation of bond pad area, and bonding of new spec
component; and
[0055] 7. IC Test Socket Cleaning--selective surface
re-conditioning of IC socket test pads.
[0056] 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.
[0057] 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.
[0058] 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/O.sub.3, 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
[0059] FIG. 1--Performance profiles for the exemplary dry cleaning
techniques; Plasma, Dense Fluids, EHD, and UV/O.sub.3.
[0060] FIG. 2--Exemplary combinational dry cleaning approach for
multi-layered contamination on a substrate surface.
[0061] FIG. 3--Exemplary flow diagram showing the various instant
dry cleaning methods possible using the present invention.
[0062] FIG. 4--Exemplary dry cleaning method options matrix that
correlates the contaminants, substrate surface, and enabling dry
cleaning techniques.
[0063] FIGS. 5a and 5b--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.
[0064] FIGS. 6a and 6b--Exemplary photomicrographs at 500.times.
magnification showing before and after surface cleaning of large
and small particles using an EHD spray.
[0065] FIGS. 7a and 7b--Exemplary photomicrographs at 2500.times.
magnification showing before and after surface cleaning of large
and small particles using an EHD spray.
[0066] FIGS. 8a and 8b--Exemplary photomicrographs at 2500.times.
magnification showing before and after surface cleaning of small
particles using an EHD spray.
[0067] FIG. 9--Exemplary polyimide-gold flexible circuit substrate
selective cleaning application.
[0068] FIG. 10--Exemplary in-line surface treatment apparatus for
the substrate of FIG. 9.
[0069] FIG. 11--Exemplary printed circuit board substrate selective
cleaning application.
[0070] FIG. 12--Exemplary cluster cleaning and assembly apparatus
for the substrate of FIG. 11.
[0071] FIG. 13--Exemplary IC socket test substrate selective
surface cleaning application.
[0072] FIG. 14--Exemplary optical device selective surface cleaning
application.
[0073] FIG. 15--Exemplary fiber optic connector selective surface
cleaning application.
[0074] FIG. 16--Exemplary MEMS wafer selective surface cleaning
application.
[0075] FIG. 17--Exemplary combinational cluster cleaning tool for
performing sequential treatments of a precision substrate
surface.
DETAILED DESCRIPTION
[0076] FIG. 1 shows performance profiles for the exemplary dry
cleaning techniques; Plasma, Dense Fluids, EHD, and UV/O.sub.3 used
in the present invention. Referring to FIG. 1, performance profiles
for plasma (2), dense fluid (4), EHD (6), and UV/O.sub.3 (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/O.sub.3 (8) may be used selectively to address
nanoscopic and molecular contamination concerns, respectively.
[0077] 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/O.sub.3 provides Table 3, Class
C2 cleaning energy to remove absorbed gases and vapors from the
native surface. As shown in FIG. 1, EHD and UV/O.sub.3
combinational 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.
[0078] 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.
[0079] 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/O.sub.3 (34) may be used remove or
flash off the remaining monolayers of absorbed gases and
vapors.
[0080] 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.
[0081] 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-US-00005 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/O.sub.3
Cleaning (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/O.sub.3 Cleaning (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/O.sub.3 Cleaning (44) Exemplary Instant
Method 8: Step 1 - Plasma Cleaning (38) Step 2 - Dense Fluid
Cleaning (40) Step 3 - Plasma Modification (38)
[0082] 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.
[0083] 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.
[0084] 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).
[0085] 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.
[0086] FIG. 5a 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.
[0087] Referring to FIG. 5b, a dense fluid spray was used to clean
a plasma treated surface as in FIG. 5a. As can been seen in FIG.
5b, 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.
[0088] FIGS. 6a and 6b 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.
6a, 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. 6b,
following cleaning using EHD treatment it can be seen that all the
smaller particles visible in FIG. 6a 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. 6a and 6b, 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. 5a and 5b
enable a much more selective process such as EHD. Moreover, and not
shown here, a more selective technique such UV/O.sub.3 would 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. 7a and 7b.
[0089] FIGS. 7a and 7b give exemplary photomicrographs at
500.times. and 2500.times. magnification, respectively, showing the
EHD cleaned surface of FIG. 6b. As can be seen in FIG. 7a, 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. 7b. 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).
[0090] UV/O.sub.3 cleaning 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. 7b
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/O.sub.3 cleaning. This is illustrated in
FIGS. 8a and 8b below.
[0091] FIGS. 8a and 8b 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. 8a, 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. 8b, 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/O.sub.3 cleaning to remove
molecular and outgassing contaminants present on the EHD cleaned
surface because there are no shielding particles and residues
present.
[0092] 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.
[0093] 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.
[0094] 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
N.sub.2/O.sub.2 vacuum 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).
[0095] 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/O.sub.2 for 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.
[0096] 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.
[0097] 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.
[0098] 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.
[0099] 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.
[0100] 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.
[0101] 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.
[0102] 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).
[0103] 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.
[0104] 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.
[0105] 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.
[0106] 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.
[0107] 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/O.sub.3 treatments 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).
[0108] 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.
[0109] 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/O.sub.3 treatment 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.
[0110] 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
[0111] Remove Gross Polishing Agents and Water Residues:
[0112] 1. CO.sub.2 steam flush--200 C., 120 psi, 2-3 minutes
[0113] Remove Fine Particle and Thin Film Residues:
[0114] 2. CO.sub.2 snow spray--80 psi, 5-30 seconds
[0115] Degrade Surface Stains:
[0116] 3. Vacuum plasma treatment--100 mTorr, 200 watts, Ar/O.sub.2
atmosphere, 5 minutes
[0117] Remove Plasma Degraded Contaminant Residues:
[0118] 4. CO.sub.2 snow spray--80 psi, 5-30 seconds
Implantable Polyester Graft Substrate Cleaning, Biocompatibility,
and Sterilization Method
[0119] Remove Extractable Contaminants:
[0120] 1. Supercritical CO.sub.2 extraction--2500 psi/60 C, 60
minute extraction cycle
[0121] Degrade (Oxidize) Surface Residues and Biological
Activity:
[0122] 2. Vacuum plasma treatment to degrade monomers and reduce
surface particle adherence--500 mTorr, 200 watts, N.sub.2/O.sub.2
atmosphere, 5 minutes
[0123] Remove Plasma Degraded Contaminants:
[0124] 3. Liquid CO.sub.2 rinse to wash plasma treated particles,
1200 psi, 25 C., 20 minute cycle
[0125] Activate Surface to Improve Biocompatibility:
[0126] 4. Vacuum plasma treatment to degrade monomers and reduce
surface particle adherence--500 mTorr, 200 watts, Ar/O.sub.2
atmosphere, 5 minutes
Soiled Inspection Boroscope Cleaning and Sterilization Method
[0127] Remove Gross Biological Fluids and Residues:
[0128] 1. Water rinses and drain--30 C., 20 psi, 2-3 minutes
[0129] 2. CO.sub.2 steam flush--250 F., 120 psi, 2-3 minutes
[0130] Degrade Adhering Thick Films and Biological Soils:
[0131] 3. Vacuum plasma treatment--100 mTorr, 200 watts, Ar/O.sub.2
atmosphere, 5-60 minutes
[0132] Remove Plasma Degraded Residues and Large Particles:
[0133] 4. Liquid CO.sub.2 spray--900 psi, 25 C., 10 minutes
[0134] Remove Small Particles:
[0135] 5. CO.sub.2 snow spray--80 psi, 5-30 seconds
[0136] 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.
* * * * *