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Sol-Gel

June 2000

COATINGS : AN INTRODUCTION TO THE CLEANING PROCEDURES


by William R. BIRCH



I. Introduction
 

Sol-gel 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. 
The 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. 

Sol-gel 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

II.1 Ultrasonic cleaning

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.

Standard 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 
bath.
 

MEET THE AUTHOR

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.

Contact address
Corning S.A.
7 bis des Valvins
77210 Avon
France
Email birchw@corning.com

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A 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.   

Generally, 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. 

II.1.1 Ingredient for good cleaning

For 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 liquids.

On 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 one hour.   

II.1.2 Surfactants choice

Next, 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.

II.1.3 Water purity

Pure 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 installation.   
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.

The 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 and disinfecting. 

 

II.1.4 Procedures

The 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. 
To 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.

The slide is held, using blue polypropylene forceps from Nalgene (reference 6320-0010, Nalge Nunc International, Rochester, NY 14625), as shown in figure 1, below:

 

 

 

 

 

 

 

 

 

 

 

Figure 1: Handling of microscope slide during drying with plastic forceps.  Slide should be held vertically.

Immediately 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 the slide.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 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.

The 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.

 
 

 

 

 

 

 

 

 

 

 

 

Figure 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.

A 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.

II.2 Spray Cleaning

                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 the substrate.

III. 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 substrate.

III.1 Glasses

The glass substrate may have a variety of differing compositions, ranging from sodalime glass, to borosilicate glass, to aluminoborosilicate glass, to pure silica.

Sodalime 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.   

Finally, 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

III.2 Polymers

                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.   
The 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.

IV. Cleaning procedures

                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.   
A 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.   
Vapor-degreasing (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. 

IV.1 Acid solution cleaning

                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 oxidizing agent.

                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 air.

                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 containers.

                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.

IV.2 Alkaline solution cleaning

                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. 

The 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 environments.   

IV.3 Dry cleaning

                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. 

The dry cleaning 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 laboratories.

                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.   
Some 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 surfaces. 

Other 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 fluorocarbon molecules.   

The 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.   

This section will briefly discuss the influence of surface cleaning on the glass surface and its application.
 

A glass surface is not inert. The 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 examined.   
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.   
All 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. 
The 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 wettability changes.

                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).

VI Conclusions

                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 contamination. 

 
REFERENCES
 

Awa96     Awad Sami, Ultrasonic Cavitations and Precision Cleaning, Precision Cleaning 1996: November; 12-17. x

Bir95       Birch 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. 

Bir94       Birch 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. 

Gro83      Grosskopf Klaus, Cleaning of sensitive glass surfaces, Proc. SPIE-Int. Soc. Opt. Eng. (PSISDG, 0277786X, Symp. Opt. Surf. Technol.) 1983; 381: 98-104.

Lel79       Lelah Michael D and Marmur A, Wettability of Soda-Lime Glass: The Effect of Cleaning Procedures, Ceramic Bulletin 1979;58: 1121-1124 

Oma89    O'Mara Richard, Contamination of Glass, A Label Adhesion Problem, ASTM Standardization News, 1989; November: 34-38 

Pul84       Pulker Hans, Chapter 4, "Cleaning of Substrate Surfaces," in Coatings on Glass, New York: Elsevier 1984; 52-63, and references therein. 

Sto78       Stowers Irving F, Advances in cleaning metal and glass surfaces to micron-level cleanliness, Journal of Vacuum Science Technology 1978;15:751-754. 

Vig85       Vig John R, UV/Ozone Cleaning of Surfaces. Journal of Vacuum Science Technology A 1985; 3: 1027-1035 

Vig87       Vig John R, "UV/Ozone Cleaning of Surfaces." In Treatise on Clean Surface Technology, K. L. Mittal, ed. New York: Plenum Press, 1987.

 

 

RELATED LINKS

 

Companies

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Chemicals & Hazards

 

Cleaning techniques

  • UV-Ozone 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 references.
    Copyright notice
    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.
  • Uvocs
    UV/Ozone cleaning
  • Advanced Plasma Surface Cleaning Techniques (Introduction)
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    Online paper by Mario A. Hozbor, Andreo C. Greene,1994 
  • Plasma cleaning: Suppliers list
    From Powesourcing.com
  • Laser cleaning suppliers list
    From Micromagazine.com Buyers Guide

 

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