Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Plasmas in Deposition Processes 91
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CHAPTER 3
Surface Preparation for Film and Coating
Deposition Processes
3.1 Introduction 93
3.2 External Cleaning 95
3.2.1 Gross Cleaning 95
3.2.2 Specific Cleaning 98
3.2.3 Application of Fluids 107
3.2.4 Rinsing 112
3.2.5 Drying, Outgassing, and Outdiffusion 115
3.3 Evaluating and Monitoring of Cleaning 117
3.3.1 Cleaning Tests 118
3.3.2 Particle Detection on Smooth Surfaces 120
3.3.3 Particle Detection on Rough Surfaces 120
3.4 Recontamination in the Ambient Environment 121
3.4.1 Ambient Environment 121
3.4.2 Handling 122
3.4.3 Storage 123
3.5 In Situ Cleaning 124
3.5.1 Sputter Cleaning 125
3.5.2 Ion Scrubbing 126
3.5.3 Reactive Plasma Cleaning 126
3.6 Recontamination in the Deposition System 129
3.7 Some Surface Modification Processes 131
3.7.1 Ex Situ Surface Modification 131
3.7.2 In Situ Surface Modification 132
3.1 Introduction
Surface preparation [1] includes surface treatments (surface modification) to change the
properties of the surface in a desirable way [2] as well as cleaning, which is the reduction of
surface contamination to an acceptable level. One objective of any surface preparation
procedure is to produce as homogeneous a surface as possible. Reproducible surface
preparation, as well as the associated handling and storage techniques, are obtained by having
Copyright © 2010 Peter M. Martin. Published by Elsevier Inc.
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93
94 Chapter 3
and following appropriate written specifications and procedures. In order to have reproducible
surface preparation processes it is generally necessary to have a reproducible and
homogeneous starting surface.
Surface modification can include surface treatments such as roughening or smoothing the
surface, making a harder surface by plasma treatment, or by activating a polymer surface by
plasma treatment. Care must be taken to ensure that the surface preparation processes do not
change the surface in an undesirable or in an uncontrolled manner.
Cleaning is used so that desirable processing and film/coating properties can be obtained. As a
practical matter a ‘clean’ surface is one that contains no significant amounts of undesirable
material; thus what constitutes a clean surface depends on the requirements. The requirements
range from those concerned with monolayer coverage and atomically clean surfaces, to gross
cleaning such as used for fusion welding. General contamination such as a hydrocarbon
surface layer can cause a low nucleation density of the depositing film on a surface, cause poor
overall adhesion of a film to a surface, and prevent good electrical contact in the case of
deposited electrical contacts. Local contamination (film or particle) can result in locally poor
adhesion of a film to a surface giving pinholes in the film. Figure 3.1 shows how pinholes are
formed in deposited thin films by particulate contamination on the surface, inclusions in the
surface or by surface features. Cleaning should address local surface conditions such as
porosity, embedded particles, steps, roughness, etc., that affect film properties, produce
pinholes, and local loss of adhesion.
External cleaning takes place outside the deposition system as a separate process in a
controlled environment. It includes gross cleaning to remove large amounts of contaminants
often by removing some of the substrate surface, and specific cleaning, which is directed
toward removing a specific contaminant such as particulates, or hydrocarbons. A clean
processing environment, and proper handling and storage after the external cleaning are
important to minimize recontamination of the cleaned surface before it is placed in the
deposition chamber. For example, to control recontamination by particulates, a filtered air
cleanroom can be used. Problems with recontamination are lessened if the cleaning procedure
is integrated into the deposition line, the prepared surfaces are used immediately, and/or if
there is a final in situ cleaning process in the deposition system.
The objective of cleaning processes is to facilitate the fabrication of an acceptable product in
the most reproducible and economical way. Cleaning processes should be as simple and
effective as possible in order to meet the processing requirements. Elaborate cleaning processes
are often expensive and self-defeating. Often there is a tradeoff between the various stages of
the cleaning process, handling/storage, and previous as well as subsequent processing. This
can mean that simple changes in one stage make complex changes in another step unnecessary.
The processing sequence as a whole should be considered when developing a cleaning process.
Surface Preparation for Film and Coating Deposition Processes 95
Figure 3.1: Pinhole formation. [From Society of Vacuum Coaters Education Guides to Vacuum
Coating Processing (2009) – Atomistic Film Growth and Resulting Film Properties – Pinholes,
with permission.]
3.2 External Cleaning
3.2.1 Gross Cleaning
In general, gross cleaning takes some of the substrate surface with it and can change the surface
chemistry or morphology in an undesirable manner. After the gross contamination is removed,
it is important to control the type and amount of contaminants to which the surface is exposed.
3.2.1.1 Stripping
In some cases, such as removing deposited film material from fixtures, or reprocessing parts
with defective coatings, large amounts of undesirable material must be removed from a surface
and the process is called stripping. Stripping is usually done by mechanical abrasion or by
chemical etching.
96 Chapter 3
3.2.1.2 Abrasive Cleaning
The removal of gross contamination by abrasive cleaning includes the use of:
abrasive surfaces, e.g. sandpaper, emery paper, steel wool, Scotch-brite
TM
and
Soft-scour
TM
scouring pads
abrasive powders in a paste or fluid carrier, e.g. SiC, Al
2
O
3
, diamond, precipitated
calcium carbonate (CaCO
3
), CeO, and Snow-Floss
TM
(diatomaceous earth with the
calcium carbonate removed, leaving a friable silica network)
impacting particles entrained in a high-velocity gas or liquid stream: vapor honing,
glass bead blasting, liquid honing, grit blasting, sand blasting, etc.
abrasives combined with an etchant to provide chemical–mechanical abrasion and
polishing.
Abrasive particles can be used wet or dry. Commercially available abrasive particles include:
aluminum oxide
diamond
cerium oxide
glass beads
crushed glass
silicon carbide
fractured cast iron grit
sodium bicarbonate
silica sand.
Dry glass bead blasting is a commonly used cleaning technique but, as with other grit abrasive
techniques, can leave shards of glass embedded in soft surfaces. The amount of grit embedded
depends on how long the glass beads have been used, i.e. how much they have been fractured.
Water-soluble particles can be used for abrasive cleaning and allow easy removal of the
water-soluble embedded particles. For example, the Prophy-jet
TM
dental abrasive unit uses
5 m sodium bicarbonate (baking soda) particles entrained in a high-velocity water stream.
Grit blasting uses grit such as fractured cast iron, alumina, or silica, of varying sizes and
shapes accelerated in a gas stream to deform and gouge the surface. Particles can be entrained
in a high-velocity gas steam by using a siphon system or a pressure system such as used in
sand blasting equipment. In addition to removing gross contamination, grit blasting roughens
Surface Preparation for Film and Coating Deposition Processes 97
the surface. The Society of Automotive Engineers (SAE) has specifications on grit size and
type, e.g. SAE Specification J444 for cast iron grit in the range G10 (2820 m maximum size)
to G325 (120 m maximum size), which gives the percentage by weight allowed on standard
screens. Bombardment of a surface by grit is like shot peening and places the surface in
compressive stress which may produce unacceptable distortion of the surface. Blasting can
also be done with plastic media, which is less damaging than the harder grits; however, it can
leave organic contamination on the surface.
Wet blasting uses an air-blasted slurry of fine abrasives in chemically treated water. It is
sometimes called liquid honing since it is usually used when dimensional tolerances have to be
maintained. Abrasion cleaning can be very mild, such as the use of CeO polishing slurry in a
high-throughput wash system to preclean glass before the standard wash cycle [3].
3.2.1.3 Wet Chemical Etching
Chemical etching can be used to remove surface material along with the contaminants. This is
a very useful technique for getting the surface into a known condition. Pickling is a term used
to denote the removal of the large amounts of oxides (scale) that are formed on metals during
the metal fabrication process. The process generally involves an alkaline clean before acid
pickling in order to get uniform wetting and etching. For example, aluminum and aluminum
alloys can be pickled by immersion in various combinations of sulfuric, nitric, hydrofluoric,
and chromic acids; copper and copper alloys can be pickled in combinations of sulfuric and
oxidizing acids, and iron and steel are pickled in sulfuric or hydrochloric acid solutions.
Chemical etchants can be highly selective in their action. This can result in preferential etching
of grain boundaries and when etching a two-phase system. Mild pickling is called a bright dip.
Acid cleaning of metals can have the detrimental effect of introducing hydrogen into the
surface and embrittling metals and ceramics [4]. If hydrogen embrittlement is a concern,
either do not use an acid or the etched part should be high-temperature vacuum fired after
etching.
Etching removes surface layers such as oxides, eliminates or blunts surface cracks in brittle
materials, and removes difficult-to-remove contaminates. Common etchants for glass include
sodium or ammonium bifluoride (100 g of ammonium bifluoride salt to 800 ml deionized
water), trisodium phosphate, which is a mild etchant, and hydrofluoric acid, which is a very
strong etchant. Hydrofluoric (HF) acid solution is a common etchant for silicon and can leave
a silicon surface either hydrogen-terminated or hydroxyl-terminated. The silicon surface is
hydrophobic if hydrogen-terminated and hydrophilic if hydroxyl-terminated.
When using etchants for cleaning, care must be taken to prevent selective removal of surface
constituents that are important to further processing. For example, etching glass-bonded
alumina ceramics in HF results in selective removal of the glass [Ca–Mg–Al–Si–O] phase
which can weaken the ceramic surface and result in poor adhesion. Etchants can change the
98 Chapter 3
surface chemistry. For example, acid etching a soda lime glass surface, which is normally
basic, leaches the sodium from the surface and makes the surface acidic, which changes its
wetting properties [5].
Sometimes chemical etching does not remove some constituents from a surface and leaves a
smut that must be removed by another etching step. For example, etching copper-containing
aluminum alloys with NaOH leaves a copper smut and/or a silicon smut on the surface. The
copper smut can be removed by an HNO
3
etch and a copper/silicon smut can be removed with
a HNO
3
/HF etch. In some cases an etchant can be devised that etches all the constituents
uniformly. For example, in etching Al:Cu:Si alloys a concentrated nitric acid (100 ml) plus
ammonium bifluoride (6.8 g) etch is used. The etching mechanism is solution of the copper,
oxidation of the aluminum and the silicon, then etching of the resulting oxides. The etchant
actually etches silicon more rapidly than the aluminum.
3.2.2 Specific Cleaning
3.2.2.1 Solvent Cleaning
Some contaminants can be removed from surfaces by solvents that dissolve (take into solution)
the contaminant. Polar solvents such as water and water–alcohol mixtures are used to dissolve
ionic materials that are polar contaminates. Non-polar solvents such as the chlorinated
hydrocarbon (chlorofluorocarbon (CFC)) solvents are used to remove non-polar contaminates
such as grease. Often there is a mixture of solvents used to dissolve both polar and non-polar
contaminates. Solvents can vary greatly as to their ability to dissolve contaminants and their
effectiveness needs to be determined by determining the solubility parameter for specific
contaminants. The solubility parameter is the maximum (saturation) amount of a specific
contaminant that can be dissolved in a specific amount of the solvent. Many non-chlorinated
hydrocarbon-based or petroleum-based materials are used as solvents.
Chlorinated hydrocarbon non-polar solvents such as trichloroethylene are often preferred to
hydrocarbon-based or petroleum-based solvents because of their lower flammability (i.e.
higher flashpoint as determined by ASTM D-1310-63). However, there is concern with the
toxicity, carcinogenic properties, and ozone-depleting action of some of the materials and they
should be used in closed recirculating systems.
Chlorinated solvents can react with water to form HCl, which can react with metals,
particularly aluminum, producing corrosion. For example, chlorinated solvents can react with
Al, Mg, Be, and Zn (white metals) to form inorganic salts. Often stabilizers are added to the
chlorinated solvents to reduce their tendency to react with water and form acids. If stabilizers
are not used the pH of the cleaner should be monitored to keep a pH in the 6–7 range [6].If
there is a possibility of solvent trapping due to incomplete rinsing, particularly in stressed
metal joints, chlorinated solvents should not be used since chloride residues enhance stress
Surface Preparation for Film and Coating Deposition Processes 99
Figure 3.2: Recycling with VOCs. [From SVC Education Guides to Vacuum Coating Processing
(2009) – Surface Preparation: Enclosed and Closed-Loop Cleaning Systems, with permission.]
corrosion. Cleaners containing chlorine-based oxidants can present the same stress corrosion
problem.
Volatile organic compounds (VOCs) are those that have boiling points below 138
◦
C. The
discharge of VOCs into the environment is regulated by local, state, and federal laws. In order
to comply with these regulations it may be necessary to recycle the material by condensation
of the vapors or to thermally destroy the vapors by burning before they are released into the
atmosphere. Figure 3.2 is one such recycling system. Many clothes dry-cleaning
establishments use VOCs with intensive recycling.
Some solvents for removal of greases are: N-methyl-2-pyrrolidone-based solvents such as
GAF NMP
TM
and GAF M-PYROL
TM
(22–24), terpene-based solvents such as Bioact EC-7
TM
and Genesolv 2000
TM
, and n-propyl bromide (nPB), as well as the Fluoroinerts
TM
, which do
not contain chlorocarbon bonds. Reports indicate that the terpenes may be as effective as CFCs
in many instances, although they have a greater tendency to leave residues. Terpenes suffer
from the fact that they have low flash points (about 49
◦
C) and reduced lower explosive limits
(LELs).
Supercritical fluids (SCF) can be used as solvents. There is a good correlation between the
ability of a simple solvent to dissolve materials and its density. Solvents can be densified most
easily when they are in the supercritical state, i.e. no distinction between the liquid and vapor
state. Carbon dioxide (CO
2
) has been shown to have a Hildebrand solubility parameter that can
vary from 0 in the gas to 10 under high-pressure supercritical conditions. Values of 6–8 are
typical, which is about the same as hexane and carbon tetrachloride. Supercritical CO
2
fluid
(SCF-CO
2
– critical point 31
◦
C, 74 bar pressure) has the advantage that it is stable, has low
100 Chapter 3
toxicity and minimal cost, and is a solvent for many organic materials. Liquid CO
2
is used in
some clothes dry-cleaning systems.
3.2.2.2 Alkaline Cleaners
Alkaline cleaners (generally silicate and phosphate based) are saponifiers which convert
organic fats to water-soluble soaps. Alkaline cleaners have a pH of about 11 and are generally
used hot. After using alkaline cleaners the surface should be neutralized by an acid dip before
the water rinse since alkali salts adhere strongly to surfaces. Clean oxide surfaces strongly
adsorb hydrocarbons, and detergents or solvents normally do not completely remove the
hydrocarbons; alkaline or oxidative cleaners must be used to remove the remaining
hydrocarbons. For example, carbonized hydrocarbon contaminants on glass can be removed
by cleaning in a saturated water solution of KOH at 75
◦
C. Strong alkaline cleaners can etch
aluminum and oxide surfaces, particularly glasses, so solution strength (pH), temperature, and
exposure times should be carefully controlled.
Alkaline cleaners are used to clean the oil and oxide off steel drills. After rinsing in an acid
rinse they are rinsed in a final rinse containing a flash rust inhibitor to protect the surfaces from
oxidation as they dry.
3.2.2.3 Detergent (Soap) Cleaners
Detergent cleaning is a comparatively mild cleaning technique. In detergent cleaning, the
detergent surrounds particles, taking them into suspension without actually dissolving the
material. This action is assisted by wetting agents and surfactants that loosen the particles from
the surface. Liquid dishwasher soap is an excellent detergent for many applications such as
cleaning polymer surfaces. A major problem with soaps is that metal ions, such as the calcium
and magnesium, which are found in hard water, make the soaps insoluble thus leaving a
residue. Deionized (DI) water should always be used for residue-free detergent cleaning. Many
detergents contain phosphates that can be environmentally harmful and subject to pollution
regulations.
3.2.2.4 Solution Additives
When cleaning a surface with a fluid, the surface energies [7] of both the solid and the liquid,
as well as the interfacial energy between the two, are important in the wetting and spreading of
the fluid on the surface and the ability to displace particles and other contaminants from the
surface. Wetting agents reduce the surface energy of fluids. Table 3.1 shows the effect of some
additives on the surface tension of water.
Surfactants are the generic name for surface-active agents that reduce the interfacial energy of
materials in contact. Surfactants used with water have both hydrophobic (water-hating) and
hydrophilic (water-loving) groups. They dissolve in water by virtue of their hydrophilic groups
and lower the surface energy of water to about 30 mJ/m
2
. The surfactant collects at the