coatings generally consist of thin films deposited on solid substrates from
a liquid solution. These thin films generally find applications in modifying the
reflectivity of the substrate' s surface, altering its rigidity, or
modifying its surface chemistry.
films are generally deposited by dip-coating, but may also be deposited
using spin coating or other thin film deposition techniques.
To achieve uniform defect-free deposition, the substrate must be free
of dust and other particles and it must be uniformly wetting to the sol-gel
solution used to deposit the film. The
most common application is to deposit the film on a substrate that is
completely wetting to the solution being applied.
coatings are frequently based on organo-silane precursor molecules,
containing a hydrolysable silicon termination and an organic segment. These molecules are generally designed to cross-link and polymerize
as the coating dries and cures. Generally,
their polymerization includes the possibility of their grafting via silanol
bonds to exposed sites on the glass surface. As such, it is interesting to clean a bare glass substrate to expose
its native silanol groups. Other
substrates may be used for sol-gel deposition. These may include polymer and ceramic substrates. Their cleaning will be briefly addressed at the end of the chapter.
The cleaning procedures will be presented in two separate categories,
the first dealing with particle removal, and the second with exposing the
active silanol sites at the glass surface. Subsequent
curing and drying of the coating, designed to give the coating its required
mechanical and optical properties, will not be discussed in this section.
II. Particle Removal
To achieve removal of
particles from the substrate, an easily implemented and
efficient process is the use of ultrasonic cavitation in
an aqueous surfactant solution. Despite its ubiquity
and common use, it is helpful to exercise care in applying
this technique. The basic approach is simple.
An aqueous surfactant solution, generally with
concentration in the range 1-5% is prepared. The
sample is placed in the surfactant solution and the latter
is immersed in an ultrasound bath for a period of time
ranging from one minute to fifteen minutes. This cleaning
is followed by rinsing and drying of the substrate.
ultrasonic baths operate at a frequency of 40 kHz.
This generates a standing wave pattern in the ultrasound bath,
cleaning the sample vigorously in certain spots and leaving it virtually
unaffected in other areas. This may be easily visualized by placing a sheet of aluminum
foil in the ultrasound bath. After
less than a minute's exposure, the sheet held up in front of a light source
will show small perforations at regularly spaced intervals, indicating both
the presence of a standing wave pattern and that the cleaning impacts
received by the sheet occur primarily at fixed locations. Some ultrasound baths have a 2 kHz sweep on the base 40 kHz frequency to mitigate this effect
However, this reduces the areas with no cleaning without eliminating
the non-uniform cleaning. Two
solutions may be envisaged. The
first is to move the sample in the ultrasound bath.
The second, more effective , is to generate white noise from
a multitude of frequencies injected into the ultrasound bath.
This effect is achieved in Crest ultrasonic systems, where a system
of two transducers is used to generate the ultrasonic agitation.
These transducers operate at a basic frequency and variations in
their phase difference serve to generate white noise, resulting in uniform
cleaning properties throughout the ultrasonic
Dr William R. Birch presently works for Corning in the
Surface Modification Group. His research focuses on the cleaning and coating of glass
surfaces, with a particular emphasis on Corning code 1737 glass,
generally used for flat panel displays. He obtained his Ph.D. in Physics from Carnegie Mellon University
in 1994, examining the molecular structure of surfactant monolayers
adsorbed on the native oxide layer on silicon wafers. Following a postdoctoral experience in surface optical tweezers
at the Institut Curie in Paris, he joined Corning' s Fontainebleau
Research Center in 1995. Industrial requirements for low cost products and a high degree
of reliability led to the development of the GAPS coated slides product,
part of the Corning Microarray Technology platform.
7 bis des Valvins
Corning Science Products
Advanced Display Products
second tip is that the cavitation size decreases with
increasing frequency. Thus, an ultrasonic bath
operating at 68 kHz will generate smaller impacts with a
higher density. This will contribute to less
damage of fragile structures and particles with smaller
sizes will be removed. This information
becomes important when the particle sizes to be removed
become of the order of one micron and if the sample to be
cleaned bears a fragile surface structure.
low frequencies, in the range 35 - 45 kHz, have been found to be appropriate
for cleaning a wide range of industrial components, where the material is
solid and rapid cleaning is desired.
Ingredient for good cleaning
efficient cleaning in an ultrasonic bath, three essential
ingredients must be controlled: the presence of a
surfactant solution at adequate concentration, the
operating temperature, and the degassing of the
solution. The first of these is required to
efficiently transmit the ultrasonic cavitation
energy. The presence of surfactant also
influences the cavitation threshold, which must be below
the minimum amount of energy available in the
bath. The temperature of the solution has a
strong influence on the cleaning efficiency.
Finally, degassing of the solution is critical to
achieving uniform cleaning. In fact, gas
bubbles will absorb cavitation energy, leading to poor
cleaning efficiency. This may be again
observed using a thin sheet of aluminum foil dipped into
the ultrasound bath. For a uniform "white
noise" ultrasonic bath, the foil is emerges uniformly
puckered. The presence of areas where the foil
has remained smooth indicates an incomplete degassing of
the solution. Degassing is achieved by leaving
the ultrasonic bath running for an extended period of
time. Last, but not least, ultrasonic
agitation is efficient for heating liquids and stirring
a more practical level, ultrasonic cleaning may be
practiced as follows. Deionized water is mixed
with a standard ultrasonic cleaning surfactant, such as
Chem-Crest 14, from Crest ultrasonics, in the ratio of 3%
surfactant by weight. This is poured into the
ultrasound bath. Ultrasonic agitation and
heating (if present) are started. The
temperature may be set to 45°. The solution
is then allowed to warm up and degas for thirty minutes to
a surfactant is chosen for cleaning the glass
substrate. This surfactant should be chosen
according to the substrate and the desired cleaning
effect. Alkaline surfactant solutions may
cause a slight etching of the glass surface.
For example, Optical 1789, from NGL Cleaning in
Switzerland, gives a solution of pH 9 at 3%
concentration. This solution was found to
lightly etch a Pyrex glass surface. To avoid
etching, an acidic surfactant may be used.
Some of these surfactants are highly efficient at
eliminating dust and organic surface residues.
Should the primary focus be on the removal of particles, a
charged polymer surfactant may be used. Such
an example is BYK 154, from BYK-Chemie, Wesel, Germany.
These charged polymer surfactants, designed for dispersing
pigments in paints, are considered to be highly efficient
at suspending particles in solution. The
chosen cleaning surfactant is diluted into pure water (see
below) at the manufacturer's recommended concentration
(typically 2-5%, by weight). This solution
chosen is then poured into a glass beaker or other rigid
container. Plastic containers should not be
used, as these will not transmit ultrasonic energy
effectively. Container wall materials should
be rigid, such as glass or metal. The
container filled with the cleaning surfactant solution is
then placed in the ultrasonic bath, the ultrasonic
agitation is activated, and the solution is allowed to
heat and degas for 30 minutes to one hour.
water in this context is defined as deionized water that
has received a subsequent organic removal, such as that
from an activated carbon cartridge. Water
purifying systems are available from Millipore, Barnstead,
or Elga, and the production of 18 megaohms x cm
resistivity water free of organics is a routine laboratory
One caveat to be noted is that high resistivity (18 megaohms x cm)
water is deionized (free of ions), but the organic
molecule contaminants have not necessarily been
removed. In fact, the organic contamination is
not measured by the resistivity of the water.
While ions are generally not considered strong
contaminants for glass surfaces, the organic molecules
contained in water may deposit on the glass substrate,
hence re-contaminating the surface in a random manner.
specification for pure water, particularly as relating to
the cleaning of glass surfaces, implies the absence of
organic molecules that may strongly adsorb to the glass
surface, rendering it non-uniform and
"contaminated." This implies that
the water should not come into contact with materials that
may release organic molecules. A simplistic
approach would then be to only used cleaned inorganic
containers (such as pyrolysed glass (see section on dry
cleaning below)), or carefully cleaned (possibly with
Hellmanex, as described in wet cleaning section below)
metal containers. This restriction may be
lifted for some plastic containers. Nalgene
plastic bottles may be used to store and dispense pure
water, provided they are "cured" with pure water
for several days before use. This curing
involves filling the bottles with pure water and replacing
this water twice per day for one week. The
reasoning behind this procedure is that the majority of
molecules that could leach out of the plastic bottles in a
short time are removed by the water curing.
The cured bottle is not expected to contaminate pure water
during short storage times. This has not been
confirmed by specific measurements. One final
caveat regarding pure water is to avoid storage or
stagnation. This is due to the water being a
suitable medium for the growth of organisms.
Essentially, the water will grow organisms in a similar
manner to a piece of bread being allowed to
mold. This is particularly critical for water
purifying systems, which should be kept running and not be
switched off for more than 24 or 48 hours. If
they are switched off, they may require complete cleaning
substrate to be cleaned is placed in the cleaning surfactant solution under
ultrasonic agitation for a period ranging from five to fifteen minutes.
A beaker of pure water is used to rinse the substrate.
This may be combined with ultrasonic agitation to accelerate the
process. A rinsing time of five
to ten minutes should be adequate. This
rinsing is repeated with a second beaker of pure water.
Finally, the sample is allowed to soak in a third beaker of pure
water for about 2 minutes. This
last step is designed to remove small molecules that may have leached into
the slightly porous glass surface. Following
rinsing, the sample must be dried. This
is best achieved by using heated circulated air that has been filtered to
remove dust. It is important
that the drying be achieved as rapidly as possible, since water has a
tendency to adsorb contaminants from ambient air and re-deposit them on the
substrate as it dries. In the
absence of heated circulated air, compressed nitrogen gas may be used to
blow-dry the surfaces. This
must be done with care to avoid leaving drying streaks on the surface.
blow-dry a surface, the procedure outlined below should be used.
This is important to avoid drying streaks and surface inhomogeneities
that may affect the sol-gel coating deposited on the surface.
slide is held, using blue polypropylene forceps from Nalgene (reference
6320-0010, Nalge Nunc International, Rochester, NY 14625), as shown in
figure 1, below:
1: Handling of microscope slide during drying with plastic forceps. Slide should be held vertically.
after removal from the final rinse, the slide is blow dried using clean
compressed air (for example, approximately 3 bar pressure, emitted through a
nozzle with diameter about 3 mm). The slide is held quasi-vertically and its
reverse face (if there is a preferred face) is blow-dried from the top down
using a jet of compressed air directed at an angle of about 45° from the
vertical. The reason for drying
the reverse face first is to maintain a water film on the front face,
avoiding water streaks. The
front face is dried later, after the possibility of the reverse face
generating water streaks has been minimized.
The drying of the reverse face is shown schematically in figure 2,
below. It is important that the
water film be chased down the front of the slide as it dries, leaving no
drops or streaks. The drying
should begin by chasing the water trapped between the plastic forceps and
2: Blow-drying of microscope slide with a flow of compressed nitrogen.
The slide should be held vertically and the water chased down the
face of the slide. No streaks
should be generated.
water is chased to the bottom of the slide.
Any water near the edges is chased from the middle towards the edges.
It is important to avoid that the water thus accumulated on
the edges of the slide be chased back onto the front face of the slide, thus
forming streaks. To achieve
this, the jet of compressed air must be halted before it passes the edge of
the slide. The angle of the air
jet must then be tilted to become more parallel with the glass slide and its
position adjusted so that it blows past the edge of the slide.
This will blow the water droplets accumulated on the edges off the
slide. This is shown in figure
3, below for the lower edge of the slide.
3: Chasing water that has accumulated on the lower edge of a slide.
The stream should be directed past the edge of the slide, at an angle
of about 30 to 45° with the edge of the slide. The water should be blown
off the slide directly, without spreading across the slide.
similar concept applies for the vertical edges of the slide, where the
compressed air jet is held almost horizontally, and allowed to blow past the
edge of the slide. This is done
holding the jet at a similar angle with respect to the front surface of the
slide and position with respect to the edge of the slide.
The water drops being blown off the edge of the slide indicate
success of this blowing procedure. These
drops do not come back onto the front of the slide, nor do they have any
tendency to spread over the back of the slide.
They are drawn towards the edge of the slide and are blown off the
edge of the slide.
Once the reverse face of the slide has been dried of its water film
in this manner, the front face is dried in a similar manner.
Maintaining the compressed air blowing past the lower and side edges
of the slide while drying the front face will have contributed to minimizing
the residual water film on the front side.
Water streaks are to be avoided.
Care must be taken not to chase water drops on the front or the
reverse surface of the slide. If necessary, any water traces remaining on the edges or back
of the slide may then be dried off by touching a piece of absorbent paper.
Two parting comments. First of all, tap water, should be avoided.
It may form deposits in ultrasonic baths and on the substrates to be
cleaned. Secondly, the use of
industrial compressed air should be avoided, as it generally contains
significant quantities of oil from the compressor.
Spray cleaning provides an alternative process for removing
particulate contaminants from flat surfaces (Pul84).
This technique is reported to effectively remove particles with sizes
above 5 microns. Pressures of
350 kPa are reported in early work. More
recent work reports a high degree of cleaning efficiency using pressures of
6.9 MPa (Sto78), with 99.9 % particle removal above 5 microns in 5-10
seconds. This paper compares
this particle removal efficiency to ultrasonic cleaning, removing 20-60% of
particles this size in 2-10 minutes. Recent
work (Awa96) suggests that ultrasonic cleaning may remove one micron size
particles with 95 % efficiency and 0.5 microns with 84 % efficiency.
One may infer that spray cleaning is rapid and efficient for
particles greater than five microns, while ultrasonic cleaning may remove
particles sizes as low as micron and sub-micron.
The above procedures are designed to remove dust particles.
To avoid recontamination of the surfaces, they should be practiced in
a clean room environment or under a laminar flow hood.
Some of the particulate matter deposited on a glass surface may be in
the form of glass chips. A
fraction of these may have fused with the glass surface and may not be
removable. If this is the case,
the glass chip particles may be considered as forming an integral part of
Wettability of the Substrate
This section will deal with generating a substrate that is wettable
to the sol-gel solution to be deposited.
In the case of glass, the procedures outlined below also serve to
expose the native siloxane sites at the surface.
These sites are important for the anchoring of the sol-gel coating to
the substrate via siloxane bonds. These
bonds may be formed between the silanol groups in the hydrolyzed groups of
the sol-gel precursor molecules and the silica or alumina sited on the
The glass substrate may have a variety of differing compositions,
ranging from sodalime glass, to borosilicate glass, to aluminoborosilicate
glass, to pure silica.
glass, commonly known as window glass, is generally the most commonly used
substrate. Microscope slides
are commonly made from this glass, either using a float glass process or a
draw glass process. In the case
of a float glass process, the glass is cooled over a bath of molten tin,
enriching its "float" side with tin oxide.
Both the float and draw glass forming processes result in flat glass
sheets that are smooth on a molecular scale, requiring no further polishing.
Sodalime glass contains about 13% sodium oxide.
This component is highly soluble in water, reacting to form sodium
hydroxide. This reaction occurs
in ambient air. The humidity in
the air will generate a coating of sodium hydroxide, coating the surface of
the glass. This layer may
interfere with adhesion to the glass surface and it is best removed by
rinsing in water (Oma89). Another
effect of forming sodium hydroxide is its reaction with carbon dioxide in
air, leading to the formation of a whit sodium carbonate powder on the glass
surface, also referred to as "blooming."
Borosilicate glass, commonly known also as Pyrex, is used more
rarely, since its forming process does not lend itself to the cheap
production of large flat sheets of glass.
Aluminoborosilicate glass, such as Corning code 1737F glass, is a
glass with a high degree of durability.
The Corning product, manufactured with a "fusion-draw" process, provides
a substrate with elevated chemical durability, a high stability against
deformations up to 700° C, and excellent optical properties.
It is produced for flat screen applications, where a wide sheet of
smooth and flat glass with minimal thickness is required, in the range 0.9
to 1.1 millimeters.
silica substrates generally provide an expensive option.
Manufactured by such processes as flame hydrolysis of precursor
molecules, they are sintered to form a solid blow and subsequently polished
to form a smooth surface. The
removal of polishing residue from these surfaces presents a considerable
challenge, and may result in an increase of their surface roughness.
A more interesting source of silica surfaces is the native oxide
layer found on silicon wafers, commercially used in the semiconductor
industry. These offer the
advantages of a high quality smooth silica surface, coupled to the
inconvenience of poor mechanical strength of the silicon wafer substrate.
For glass surfaces, it is important to note that the cleaning
procedures described below are effective for removing residual amounts of
hydrocarbon contamination. The
procedures are designed to remove a thickness of organic contamination of
the order of one monolayer, with an equivalent thickness of the order of 1
nanometer or 1 gram per 1000 square meters of surface.
The presence of organic films on the substrate with thickness of the
order of 0.1 microns or greater will generate considerable cleaning
difficulties. As such, it is
necessary to remove all gross contamination by solvent or surfactant
solution cleaning before applying these procedures.
In particular, this includes the removal of plastic protective films
or paints. If a plastic film is
removed, residual glue left on the glass surface must also be removed
Other substrates to be used may include polymer surfaces.
The activation of their surface energy may be achieved using a
UV/ozone or a plasma process, as outlined below.
Early reports of glow discharge activation (Pul84) suggested a
mechanism involving the creating of new chemical functional groups at the
polymer surface, generated through ion and electron bombardment.
This is accompanied by a micro roughening of the surface, requiring
the use of carefully controlled energy limits and cleaning times to obtain
reproducible results. This
process is generates random surface chemical functions and is generally
influenced by the composition of the surrounding gaseous environment. It is highly effective for increasing the surface wettability
of the polymer substrate. Over
time, the surface of the polymer will "heal", reducing its
wettability, through a mechanism where high energy groups at the surface of
the polymer are replaced by low energy groups from the interior through a
re-organization of the structure.
increased wettability of the substrate will facilitate the deposition of a
sol-gel coating from a liquid phase. UV/ozone
or plasma treatment may render any polymer surface, including fluorocarbons
such as Teflon, highly wetting to polar aqueous and alcoholic solutions.
The binding of the sol-gel coating to the surface after
deposition and curing will need to be verified.
Liquid chemical treatments may also be used to increase the
wettability of the polymer surface. One
example is the application of sol-gel coatings onto polymer surfaces
following their exposure to caustic soda solution.
The alkaline solution hydrolyses ester groups at the polymer surface,
increasing the wettability and allowing binding of the sol-gel coating to
the polymer substrate.
The cleaning procedures outlined in this section will be divided into
three broad categories: acid solution cleaning, alkaline solution cleaning,
and dry cleaning. The first two
categories involve liquid cleaning, whereby particle removal may be achieved
at the same time as surface activation.
The third category will have little or no impact on particle removal,
but will generally provide a rapid and simple way of activating surfaces.
The alkaline cleaning generally involves a light etching of the glass
surface. The acid cleaning,
although it may leach components from the glass surface, generally does not
involve an etching of the glass.
The advantages and disadvantages of these cleaning procedures may be
summarized as follows. The
liquid cleaning procedures are simple to set up and rapid to use.
However, they generate considerable waste products and require either
the use of significant operator time, or the use of sophisticated machines.
As such, they are well suited to the cleaning of occasional samples
in research laboratories.
strong caveat for these techniques is the requirement of rinsing and drying
the samples. The latter step is
delicate and invariable compromises the obtaining of a uniform, clean and
dry surface on the substrate.
(Pul84), is not discussed. This
technique relies on the condensation of vapor from a freshly-distilled
solvent. It may remove dust and
organic contaminants. It will
not generally provide a glass surface that is uniformly wettable to water,
due to residual organic molecules that remain adsorbed to the glass surface.
The dry cleaning procedures require the investment in suitable
equipment. They have the
advantage of being able to clean large quantities of samples with little or
no waste products and little operator input.
They also have the distinct advantage of directly generating a clean,
uniformly wettable and dry surface on the substrate.
This is one of the most commonly used procedures for cleaning glass
and silica surfaces in small research laboratories.
It has the advantage of being easy to set up, requiring the purchase
of basic chemicals and the use of a fume hood.
However, it generates significant toxic waste products and is thus
rarely scaled up for industrial applications.
The basic concept is that of mixing a concentrated acid with a strong
The original formulation was the use of a saturated potassium
dichromate solution in concentrated sulfuric acid. The solution, known as chromerge, was prepared by completely
dissolving 20 grams of potassium dichromate (K2Cr2O7)
in 90 grams of water. To this
was slowly added 900 grams of concentrated sulfuric acid. The mixing reaction is highly exothermic, exposing the
operator to a high risk from spattering.
This solution could be re-used as long as it remained brown, rather
than green, indicating the oxidation state of the dichromate.
The sample was dipped in the chromerge for 20 minutes,
followed by 20 minutes in a 1:1 concentrated hydrochloric acid/water
solution to remove chromium ions from the surface.
A pure water rinse and the drying of the sample surface follow this.
The latter should be practiced as described in the particle removal
section above, taking care not to allow water streaks across the surface.
The use of chromerge has been all but completely banned in most
research and industrial establishments. This is because the chromium ion is highly toxic to the environment
and poses a severe waste disposal problem, even in small quantities. Further, the hexavalent chromium present in the above solutions is
considered by regulatory agencies to be a potent human carcinogen. The addition of chloride or halogens to chromerge solutions can
generate the highly toxic and volatile carcinogen, chromyl chloride. This mandates the use of chromerge under a chemical fume hood.
Chromerge cleaning, as described in the paragraph above, was found to
be adequate for generating a uniformly wettable surface on the native oxide
layer of a silicon crystal. This
cleaned substrate, while still wet, showed uniform deposition of a
charge-adsorbed monolayer of surfactant (Bir94). To obtain a uniform dry surface, this monolayer was burned
off with UV/ozone radiation. This
gave excellent results for the deposition of a monolayer of anionic
surfactant from a thinning film of surfactant solution (Bir95). Further, the re-hydration of this surfactant layer showed a
relatively uniform thickness, indicating uniform surface properties of the
hydrated substrate surface.
As a replacement to the highly successful but toxic chromerge
solution, Godax laboratories (Takoma Park, MD, USA) has developed a product
called Nochromix .
This is reportedly dispensed in a powder form, costing approximately
$2.50 per bag. It is added to
concentrated sulfuric acid. As the solution turns brown, the solution is spent and a new
package may be added to the sulfuric acid.
Due to the hygrosopic nature of sulfuric acid, it is important to
keep concentrated sulfuric acid solutions closed to air while not in use. This avoids their dilution with water following prolonged exposure to
Prior to the commercial existence of Nochromix , an efficient
alternative to chromerge was developed. This consisted of
mixing two parts concentrated sulfuric acid with one part
hydrogen peroxide solution, providing the strong oxidizing
agent. The mixing process is strongly exothermic,
resulting in an extremely hot final solution. Cleaning
with this mixture is recommended at 90° C for 20 minutes.
This is followed by a rinse in pure water and careful
drying as described above. This solution removes
hydrocarbons from the surface of the substrate with vigor.
Care should be taken when using the hot cleaning solution.
It should be used under a fume hood, together with a
protection against spills from the breaking of glass
Verification of a successful cleaning procedure following
acid cleaning is essential. The sample should be
completely wettable to water during the pure water rinse.
Wettability of a surface to water in this context implies
that a film of water on the surface does not dewet.
Dewetting is observed when dry patches are formed where
the liquid film has receded from an area of surface and
they are characterized by the presence of a liquid film
with thickness of order one millimeter adjacent to a
"dry" zone (by visual observation). This may be
further verified, if necessary, by halting the water flow
and holding the substrate vertically. The water film will
then thin. As it thins, light interference fringes should
be observed. If dry patches are formed before the
interference fringes are seen, the glass is not completely
wetting in these areas and the cleaning has failed. On
completion this test, the glass should immediately be
re-wet with water and dried carefully as described above.
Finally, these acid cleaning procedures are not effective
for the complete removal of fluorocarbon molecules, such
as perfluoro-silanes grafted to a glass surfaces. Further,
poly-dimethyl siloxane (PDMS, or silicone oil)
contaminants are not well removed using these acid
cleaning solutions. PDMS contamination is frequently found
in environments where vacuum pump oils or silicon rubber
seals are used.
The PDMS molecules adsorb strongly to glass surfaces and
are generally not removed by surfactant solution cleaning
or by acid cleaning.
Care should be taken that no PDMS or fluorocarbon
silane molecules come into contact with the acid cleaning
solutions, as they may contaminate the solutions to a
point where they will no longer render glass hydrophilic
and may increase its hydrophobicity.
These solutions, generally made from sodium hydroxide (or
potassium hydroxide) dissolved in water, are designed to
lightly etch the glass surface. Solutions above pH 9 begin
to etch glass surfaces. The manufacture of an aqueous NaOH
solution is a relatively simple procedure. Its etching of
the glass ensures the removal of surface residues,
resulting in a clean surface. Glass may cleaned by
exposing it to a saturated aqueous sodium hydroxide
solution at room temperature for fifteen minutes. Should
the glass be coated with a non-wetting layer, this
exposure may need to be longer for the solution to attack
the glass under the non-wetting layer. Alcoholic solutions
of NaOH or KOH may be used to improve the wetting
properties of the solution.
cleaning with alkaline solution should be followed by a
five minute dip in 1M hydrochloric acid to neutralize the
alkali attack on the glass surface. This should be
followed by a twenty minute dip in pure water. The
sample should then be checked for surface cleanliness by
wettability, as described in the above section, followed
by a careful drying in air, as described in the
particulate removal section.
An efficient alternative to concentrated sodium hydroxide solution is
the use of a surfactant known as Hellmanex.
Developed by Hellma (Müllheim, Germany), this surfactant was
designed to clean quartz spectroscopy cells, without altering their optical
properties and leaving no residue. Its
recommended use is a one hour dip at room temperature or a 30 minute dip at
30° C for a 2% solution by weight in water. In practice, ten to twenty minutes under ultrasonic agitation have
given satisfactory results. Hellmanex
surfactant yields an alkaline solution of small molecule surfactant. The alkaline solution contributes to a light etching of the glass
surface, while the surfactant ensures wetting of the substrate, even if
coated with hydrophobic contaminants. The
use a small molecule surfactant is believed to contribute to the absence of
any surface residue after rinsing. Following
cleaning, it may be useful to soak the substrate in pure water for twenty
minutes to leach out any small molecules absorbed into the glass surface. This is followed by a pure water rinse and a careful drying, as
described in the particle removal section, above. We have found this cleaning solution to exhibit excellent cleaning
properties, coupled with a simple application process. Another similar alkaline small surfactant molecule cleaning powder is
sold under the trade names of Alconox.
Finally, glass surface may be etched by exposure to a solution of
fluoride ions, such as that generated by an aqueous solution of hydrofluoric
acid. These solutions have been
included in this section, despite their being acidic, because they rely on
etching the glass surface to achieve cleaning. The fluoride ions rupture siloxane bonds.
These cleaning solutions are generally dilute hydrofluoric acid, of
the order of a few percent in HF concentration. The concentration and cleaning time should be adjusted to the glass
composition in order to achieve cleaning without excessive etching. The solutions are generally used at room temperature. More concentrated HF solutions are used to dissolve
macroscopic pieces of glass. These
concentrated HF solutions should not be used to clean glass surfaces, since
their rapid etching increases the surface roughness of the glass. Great care should be exercised when handling hydrofluoric acid, even
in dilute solutions. It is
extremely toxic and skin exposure (or inhalation) can be fatal. The acid penetrates the skin and migrates in towards the
bones. During its passage, it
neutralizes nerve endings, causing no pain or feeling. On reaching the bone, a decalcification takes place.
This may lead to a necrosis and a possible amputation.
Exposure of skin over an area approximately larger than the palm of a
hand may result in heart failure. Thus,
the use of hydrofluoric acid is not recommended outside of well-controlled
This category encompasses a several processes that clean a glass
surface without touching it or significantly altering its surface
composition. The machines used
and their safety protection require a significantly larger capital
investment than the above chemical cleaning procedures for cleaning small
numbers of glass samples. They
rely on high temperatures or the use of plasma to degrade and desorb
contaminant hydrocarbons from the glass surface.
procedures present the significant advantage of being non-contact cleaning
and resulting directly in a clean and dry substrate surface, ready to
receive a subsequent coating. They
generate few waste products and require minimal operator input, making them
highly attractive to the scaling up required by industrial processes. Their efficiency at cleaning different types of contaminants
from a variety of substrates makes them attractive for use in research
One of the most basic processes for cleaning glass surfaces is the
use of pyrolysis. This involves
heating the glass to a temperature above 300° C, generally 500 ° C for
over 30 minutes. A typical
cycle will consist of a rise in temperature from room temperature to 500° C
over four hours, followed by five hours at 500° C and a return to room
temperature over five hours. The
slow increases and decreases in temperature are designed to avoid glass
breakage from thermal shock. This
process generates few waste products. It
relies on the degradation and desorbtion of organic contaminants from the
surface of the glass. It has
been found to be effective in increasing the wettability of glass surfaces
contaminated with any organic pollution, including such molecules as PDMS or
fluorocarbons. This process may
generate soot residue and is not effective in removing this soot. When using this process, it is important that the residual
contamination on the glass surface be of the order of nanometer thickness or
less. This is in order to avoid
macroscopic residues on the glass surface, such as carbon or other inorganic
matter. This technique does not
remove dust particles from the glass surface. Care should be used when pyrolyzing sodalime glass, where the
presence of moisture may leach alkalis from the glass, generating a sodium
hydroxide residue on the glass surface. This is generally not a problem when using air with ambient humidity,
provided that air circulation around the samples is limited.
A more efficient process is the use of plasma to clean substrates. This process is suitable for industrialization and has been used in
some large industrial processes. The
cleaning may involve an oxygen plasma to directly oxidize hydrocarbons, or
an argon plasma to degrade them and desorb them. Such processes are suitable for cleaning glass surfaces and for
activating polymer surfaces. The
use of a plasma bears the distinct advantage of being able to penetrate
inside complex structures. Generating
plasma requires a specifically designed reactor. Generally, the specific gas to be used is injected to form an
atmosphere at reduced pressure. Electromagnetic
radiation (RF or microwave frequency) is then coupled into the enclosure,
generating the plasma.
plasma systems operate at atmospheric pressure, removing the need for a
vacuum-sealed enclosure. Plasma
cleaning has an interesting feature, whereby its, since its
"effective" temperature (its kbT equivalent thermal
energy) is of several thousand degrees. This is achieved by coupling energy into the gaseous phase without
strongly increasing the substrate temperature. As an example, this process may clean carbon residue from surfaces
without excessive heating.
The plasma cleaning techniques also find application in the
activation of other inorganic substrate surfaces and organic polymer
dry cleaning techniques
One reported technique for cleaning glass surfaces is the use of
laser cleaning (Wür86). This
relies on an infrared high energy beam to irradiate the surface and desorb
contaminants. While no
wettability data are given in the report, the technique is reported to
remove organic contaminants that have diffused into the glass surface, lying
at depths of order 10 angstroms.
Rendering glass surfaces hydrophilic and highly wettable may also be
achieved using ultraviolet (UV) radiation and ozone, as described by John R.
Vig (Vig85, Vig87). This
technique may be considered as an evolution of corona discharge cleaning,
whose use dates back to 1956 (Pul84).
UV/ozone cleaning is most easily implemented using a
low-pressure mercury lamp, emitting radiation at the wavelengths of 184.9
and 253.7 nm. The first of
these generates ozone from oxygen , while the second combines with ozone to
oxidize hydrocarbon contamination at the glass surface. Such a lamp may be obtained from BHK Inc. (Claremont, CA, USA).
It will need to be mounted in an enclosure allowing the sample to be
positioned 1-3 cm from the lamp and protecting the user from UV radiation. The production of ozone in large quantities requires that the lamp be
mounted in an area ventilated to a fume hood.
Ozone in significant quantities is considered an environmental
pollutant and the use of a catalyst at the exit of the fume hood may be
recommended. Alternatively, a
complete UV/ozone cleaning equipment may be purchased from UVOCS (Montgomeryville,
PA, USA). The distance from the
sample to the lamp is critical, since wavelength used to oxidize
hydrocarbons is absorbed by the ozone. One to three centimeters are found to give good results.
A non-optimized cleaning system will result in exposure times of 15
to 30 minutes to obtain a wettable glass surface.
Optimizing the system may reduce this time to below two minutes. This process provides a high degree of versatility and ease of use.
It cleans substrates without contact in relatively short times
without strong heating. The
sample temperature generally does not rise above 60° C. The system may be used to activate the surface of polymer substrates.
Its effectiveness on fluorocarbons is lower than on hydrocarbon
surfaces, presumably due to its oxidation mechanism and the UV resistance of
UV/ozone cleaning will effectively remove organic contamination layers with
thickness on the order of one nanometer. For coatings with thickness of order one micron, only the surface of
the coating will likely be rendered hydrophilic. For this reason, it is important to pre-clean the substrate with
organic solvents or surfactant solutions to degrease and remove macroscopic
contaminants. This cleaning
process will not remove dust or other particles from the substrate surface.
V. Influence of Cleaning
The previous section described a variety of procedures for rendering
a glass substrate hydrophilic and exposing its silanol groups for reaction
with the molecules in the sol-gel coating. While all these procedures are designed to obtain a hydrophilic glass
surface, wettable to aqueous and alcoholic solutions, the cleaning processes
impact the glass surface, leaving different states for the glass surface. The glass surface may be altered in its chemical composition, as well
as its roughness. Exposure to
aqueous solutions may leach components from the glass surface, significantly
altering its surface chemical properties. Etching solutions may significantly increase the physical roughness
of glass surfaces, from a few angstroms rms for fire-polished glass, to nm
or even micron roughness.
section will briefly discuss the influence of surface cleaning on the glass
surface and its application.
A glass surface is not inert.
cleaning procedure used will generally alter the glass surface. A clear example of this is the influence of acid cleaning on sodalime
glass surfaces. The sodalime
glass contains 13 % sodium oxide. This
component, as well as the calcium oxide in the glass, are leached from the
surface region by exposure to the acid solution. An acid-cleaned sodalime glass surface is closer to a silica surface
than the as-formed glass surface. Conversely,
cleaning a sodalime glass surface under a UV/ozone lamp will have little or
no influence on the glass surface composition. If pyrolysis is used, where the sodalime glass is taken to 500° C,
just below its softening point of about 550° C, this may re-generate the
glass surface composition to a state close to that found on the as-formed
surface. Lelah and Marmur
(Lel79) describe the wettability of sodalime glass surfaces, measured by
observing the spreading rate of water drops, following a combination of
liquid cleaning and pyrolysis, covering several cleaning procedures.
Depending on the liquid cleaning procedure used, the wettability of
the sodalime glass is either determined primarily by the final pyrolysis
step, or is influenced by the prior liquid cleaning. Differences in the glass surface behavior following heat cleaning may
be associated with a leaching of soluble components, such as sodium oxide,
where the heat treatment is not sufficiently long or at sufficiently high
temperature to re-generate the original glass surface composition.
We have found that the contamination of glass surfaces following
cleaning may depend strongly on the procedure used to clean the glass.
For this experiment, three typical glass surface compositions were
used. Sodalime glass, in the
form of microscope slides from Erie Scientific; aluminoborosilicate glass,
in the form of Corning code 1737F glass; and silica surfaces, in the form of
the native oxide layer on silicon wafers, supplied by Unisil Corporation
(p-doped or n-doped four-inch wafers from standard production), were
The three glass
species were cleaned using chromerge and were then passivated in
hydrochloric acid, as described in the "Acid Cleaning" section
above. These samples were
placed in contaminated octane liquid and the contact angle of sessile water
drops measured as a function of immersion time. While the contact angle of each deposited water drop remained
constant, the water drops deposited at later times showed higher contact
angles. This increase was seen
over a period of five to seven minutes, after which the contact angle values
were fairly constant. Following
removal from octane and blow-drying, water sessile drop contact angles on
the substrates were of the order of 30-50°. This indicates a contamination of the substrates.
The three acid-cleaned substrates behaved in a similar manner. However, when the substrates were cleaned by pyrolysis, it was found
that the silica surface was contaminated more strongly (the water contact
angle under octane went up from about 20° to about 60°), while the
sodalime glass remained wettable to water, indicating no strong adsorption
of contamination from the octane. The
pyrolysed aluminoborosilicate glass showed a similar contamination behavior
to the acid-cleaned substrate.
of these surfaces were hydrophilic following acid or pyrolysis cleaning.
However, the difference in their susceptibility to contamination
indicates significant differences in their surface composition.
Lelah and Marmur (Lel79) report differences in the wettability of
cleaned glass, following exposure to ambient air, varying with the technique
used to clean the sodalime glass.
For coating applications, we have generally found a trend in quality,
with silicon wafers providing the best substrate, followed a close second by
Corning code 1737 glass, and finally sodalime glass. All three glass species provide high quality substrates for the
adhesion of silane or sol-gel (silanol-based) coatings. As an example, when used as a substrate for silane coatings, the
native oxide layer on silicon surfaces has consistently shown the highest
coating quality results.
Corning code 1737 glass comes a close second, and sodalime glass shows
slightly poorer results. The
coatings used for this evaluation were self-assembled monolayers, where
defects generated by substrate inhomogeneities rapidly lead to measurable
Mechanically polished glass surfaces present very different cleaning
issues from as-formed (or fire-polished) glass surfaces. The polishing process relies on using first coarse, then
progressively finer, abrasive powders to grind the surface to the desired
smoothness. Some of the powder
may remain embedded in the glass surface and latent scratches may appear as
the glass is cleaned and the powder removed.
Further, the polishing process may generate microscopic cracks in the
surface of the glass. These cracks may be amplified during the cleaning process.
Critical parameters for limiting the increase in roughness and
defects in the surface of polished glass include the hardness of the glass
and its chemical durability. The latter is primarily determined by the soluble components
in the glass composition. The
cleaning of such glass surfaces is described in (Gro83).
A large variety of cleaning procedures exit that render glass
wettable and free of particles (Lel79, Pul84).
Reviewing the literature, one rapidly learns that the cleaning
processes used vary with the subsequent application of the glass surface. In fact, some coating applications may tolerate a significant level
of organic contamination of the glass surface. In these circumstances, a less aggressive cleaning procedure may
enable the use of less toxic and less aggressive cleaning agents.
Finally a common point in all glass cleaning procedures is that the
surface should be coated or otherwise used as soon as possible after
cleaning (Pul84). Glass
surfaces, and more especially clean and wettable glass surfaces, have a high
surface energy. Thus, they have
a tendency to adsorb particulate and organic contamination from the ambient
environment. Storing the
surfaces in freshly oxidized aluminum containers may reduce the adsorption
of organic contaminant molecules from the ambient air (Pul84). Particulate
contamination will generally create unacceptable defects. Adsorbed organic contaminant molecules, generally less than a full
monolayer coverage, of order nm thickness, will generate a heterogeneous
wettability. This may lead to
non-uniform coatings, in particular if deposited from liquid media. We generally work with ambient contaminants.
The glass surfaces are never perfectly clean in ambient air. It is generally recommended that the glass surface be used within a
few minutes following its cleaning. Our
experiments generally work despite the presence of ambient contaminants on
the glass substrate. It is
desirable to minimize the risk of coating defects caused by random ambient
Awa96 Awad Sami, Ultrasonic Cavitations and Precision Cleaning, Precision Cleaning
1996: November; 12-17. x
WR, Garoff S, Knewtson M, Suter RM, Satija S, The Molecular and
Microscopic Structure of Precursing Thin Films of Anionic Surfactant on
the Silicon Oxide/Silicon Surface. Langmuir 1995; 11: 48.
WR, Garoff S, Knewtson M, Suter RM, Satija S, The Molecular Structure of
Autophobed Monolayers and Precursing Films of a Cationic Surfactant on the
Silicon Oxide/Silicon Surface, Colloids and Surfaces 1994;89: 145.
Klaus, Cleaning of sensitive glass surfaces, Proc. SPIE-Int. Soc. Opt. Eng.
(PSISDG, 0277786X, Symp. Opt. Surf. Technol.) 1983; 381: 98-104.
Lelah Michael D and Marmur A, Wettability of Soda-Lime Glass: The
Effect of Cleaning Procedures, Ceramic Bulletin 1979;58: 1121-1124
O'Mara Richard, Contamination of Glass, A Label Adhesion Problem,
ASTM Standardization News, 1989; November: 34-38
Pulker Hans, Chapter 4, "Cleaning of Substrate Surfaces,"
in Coatings on Glass, New York: Elsevier 1984; 52-63, and references
Stowers Irving F, Advances in cleaning metal and glass surfaces to
micron-level cleanliness, Journal of Vacuum Science Technology
Vig John R, UV/Ozone Cleaning of Surfaces. Journal of Vacuum Science
Technology A 1985; 3: 1027-1035
Vig John R, "UV/Ozone Cleaning of Surfaces." In Treatise on
Clean Surface Technology, K. L. Mittal, ed. New York: Plenum Press, 1987.
Ultrasonic Cleaning Equipment
This is an online directory of ultrasonic cleaning equipment
companies, vendors and manufacturers located in North
Complete line of ultrasonic cleaning equipment and
Filters and other engineering products and services for
cleaning fluids and materials.
water purification products. Replacement filters and
membranes for most laboratory water systems. Complete
laboratory water purification systems
The complete NALGENE Product Catalog online
Additives for Coatings and Paints
Chemicals & Hazards
cleaning (pdf file 3 Mo)
by John Vig 45 pages, 175 references. (pdf file 3Mo)
Chapter 6 of "Handbook of semiconductor wafer cleaning
technology. Science, Technology & Applications" Ed.
by W. Kern, Noyes Publications, 1993
The UV/Ozone cleaning procedure has been shown to be a
highly effective method of removing a variety of
contaminants from silicon and compound semiconductor wafers,
as well as from many other types of surface. John Vig
describes here the technique, the underlying mechanisms, the
variables involved, safety considerations and details for
construction of a UV/Ozone cleaning facility. 45 pages, 175
Although the book in which this chapter appears is
copyrighted, the author is an employee of the U.S.
Government and performed this work as part of his official
duties. The work is, therefore, not subject to copyright
protection by the publisher.
Plasma Surface Cleaning Techniques (Introduction)
From ONRL National Laboratory
for cleaning & etchning
Cleaning and Drying: Alternatives to Conventional Technologies
Online paper by Mario A. Hozbor, Andreo C. Greene,1994
cleaning: Suppliers list
cleaning suppliers list
From Micromagazine.com Buyers Guide