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Cleaning of Substrate Surfaces
60 CHAPTER 4 4. CLEANING OF SUBSTRATE SURFACES
Surfaces exposed to the atmosphere are generally contaminated. Any unwanted material and/or energy on a surface is regarded as a contaminant. Any manipulation at all contributes to the generation of contaminations. Surface contamination can be gaseous, liquid, or solid in its physical state and may be present as a film or in particulate form. Furthermore, it can be ionic or covalent and inorganic or organic in its chemical character. It can originate from a number of sources, and the first contamination is often a part of the process creating the surface itself. Sorption phenomena, chemical reactions, leaching and drying procedures, mechanical treatment, as well as diffusion and segregation processes can give rise to various compositions of surface contamination. Most scientific and technical investigations and applications, however, require clean surfaces. So, for example, before a surface can be coated with thin films, it must be clean; if it is not clean, then the film will not adhere well or may not adhere at all. In the case of coating clean often means absence of detrimental impurities. In optical coating applications, even very minute contaminations of special materials can interfere with the functioning of sensitive devices whereas other materials may have practically no influence. Therefore, the cleaning of surfaces is a very important but also a very difficult and delicate operation. The adhesive forces holding small particles onto surfaces may be quite strong in terms of bond strength per unit area. The mechanical energy available to break such bonds is relatively small in most cleaning processes. Consequently, in a given cleaning process, a strong effort is necessary to reduce the adhesive bonds between contaminant and surface as much as possible if the cleaning process is to be at all effective. The reduction of these adhesive bonds is affected by the choice of the cleaning fluid and the conditions under which it is used, particularly the temperature. Pure water, for instance, may be relatively ineffective in removing the contaminants from a given surface, whereas hot water with detergent is quite effective in the same process. There are a number of diverse cleaning techniques used in various scientific and industrial cleaning problems. Probably the universally used basic cleaning method is scrubbing or polishing a surface. This is used with various cleaning fluids, and for all degrees of cleaning abrasion, ranging from the lightest touch of a non-abrasive brush to rigorous brushing. The latter procedure is a relatively harsh treatment used only to remove gross contaminants from unfinished parts. It is wrong, however, to assume that all the contaminants can be removed in a single cleaning operation. In many cases, particulate material can tenaciously adhere to a surface, and surprising forces can be developed between a surface and a small particle. The cleaning tech-
61 niques will only be partially effective and, in general, will probably be less and less effective as particle size decreases. In the case of glass and crystal surfaces, the undesired materials on the surface may be mainly water-containing oxidic polishing residues, inorganic and organic dust particles, or films consisting of oil and grease or a combination of these.
4.1 CLEANINGPROCEDURES Cleaned surfaces can be classified into two categories: atomically clean surfaces and technologically clean surfaces. Surfaces of the first category are required for special scientific purposes and these can only be realized in ultra high vacuum. With the exception of these very sophisticated products, practical coating applications require only technologically clean or slightly better qualities of surface. In accordance with a principle described in ref. [1 ], the degree of surface cleanliness must meet the following two criteria: it must be good enough for subsequent processing and it must be sufficient to ensure the future reliability of the product for which that surface will be used. A further distinction must be made between cleaning methods that are applicable in the atmosphere and those that are applicable only in vacuum. In all cases where handling of the parts and the use of solvents is required, cleaning cannot be performed in a vacuum. If cleaning in vacuum, e.g. by heating operations and particle bombardment is used, then this is generally conducted inside the deposition system.
4.1.1 CLEANINGWITH SOLVENTS Cleaning with solvents is a widespread procedure that is always included whenever cleaning of glass surfaces is discussed. In this process, various cleaning fluids are used. A distinction must be made between demineralized water or aqueous systems such as water with detergents, diluted acids or bases and non-aqueous solvents such as alcohols, ketones, petroleum fractions and chlorinated or fluorinated hydrocarbons. Emulsions and solvent vapours are also used. The type of solvent used depends on the nature of the contaminants. There are many papers on the cleaning of glass by solvents; some examples are given in refs. [2-10]. However, one has to consider the far-reaching new regulations to preserve the stratospheric ozone layer, given in the Montreal Protocol, now nearly ten years old. These regulations prohibit the use of fluorinated and chlorinated hydrocarbons, FCHC, also as cleaning fluids. Exceptions can be made only when using completely closed and controlled cleaning systems. Information is available, for instance, in the various OzonAction newsletters [3]. Instructions how to replace FCHC in cleaning procedures can be found in the proceedings of cleaning seminars, e.g. [39], or obtained directly from the producers of cleaning machines.
62 4.1.1.1
RUBBINGAND IMMERSION CLEANING
Perhaps the simplest method of removing superficial dirt from glass is to rub the surface with cotton wool dipped in a mixture of precipitated chalk and alcohol or ammonia. There is evidence, however, that traces of chalk can be left behind on such surfaces so that after that treatment the parts must carefully be washed off in pure water or alcohol. This method is best suited as a precleaning operation, i.e. as the first step in a cleaning sequence. Rubbing a lens or a mirror substrate with a lens tissue saturated in solvent is almost a standard cleaning procedure. It takes advantage of solvent extraction and imparts a high liquid shearing force to attached particles when the fibres of the tissue pass over the surface. The resulting cleanliness is dependent on the presence of contaminants in the solvent and in the lens tissue. Recontamination is avoided by discarding each tissue after one pass over the surface. A very high level of surface cleanliness is achievable with this cleaning operation. A simple and often used cleaning technique is that of immersion or dip cleaning. The basic equipment employed in dip cleaning is easy to construct and inexpensive. An open tank of glass, plastic or stainless steel is filled with a cleaning fluid and the glass parts, which are clamped with a pair of tweezers or are inserted in a special holder, are then dipped in the fluid. Agitation may or may not be employed. The wet parts are taken out of the tank after a short time and may then be dried by rubbing with a towel of pure cotton free of washing-powder contaminants. The parts are subsequently inspected by dark-field illumination. If the degree of cleaning is not sufficient, the operation can be repeated by further dipping in the same fluid or other cleaning fluids. In addition to physical cleaning, chemical reactions can also be exploited for cleaning purposes. Various acids with strengths ranging from weak to strong, as well as mixtures such as chromic and sulphuric acid are used. All acids, with the exception of hydrofluoric acid, must be used hot, i.e. between 60 and 85~ to produce a clean glass surface. Silica is not readily dissolved by acids, apart from hydrofluoric acid, and the surface layer on aged glass is invariable finely divided silica. Higher temperatures may aid the dissolution of silica so that a new surface is created on the treated glass. Acid cleaning cannot be used for all types of glass. This is especially true for glasses having a high barium-or lead oxide content, such as some optical glasses. These are leached by even mild acids, producing a loose silica surface film. According to a report in [10], a cold diluted mixture of 5% HF, 33% HNO3, 2% Teepol | and 60% H20 should be an excellent universal fluid for cleaning glass and silica. Caustic solutions exhibit a detergency and the ability to remove oils and greases. The lipids and fatty materials are saponified by the bases to soaps. These watersoluble reaction products can readily be rinsed off the clean surface. It is generally desirable to limit the removal process to the contaminant layer, but a mild attack of the substrate material itself is often tolerable and ensures that the cleaning process is complete. Attention must be paid to unwanted, stronger etching and leaching effects. Such processes may destroy the surface quality and should therefore be
63 avoided. The chemical resistance of inorganic and organic glass can be found in the glass catalogues. Single and combined immersion cleaning processes are mainly used to clean small pieces.
4.1.1.2 VAPOURDEGREASING Vapour degreasing is a process that is primarily useful for removing grease and oil films from surfaces. In glass cleaning, it is often used as the last step in a sequence of various cleaning operations. A vapour-degreasing apparatus consists essentially of an open tank with heating elements in the bottom and water-cooled condensing coils running around the top perimeter. The cleaning fluid may be isopropyl alcohol or one of the chlorinated and fluorinated hydrocarbons. The solvent is vaporized and forms a hot, high-density vapour, which remains in the equipment because the condenser coils prevent vapour loss. The precleaned cold glass pieces, in special holders, are immersed in the dense vapour for periods ranging from 15 sec to several minutes. Pure cleaning fluid vapour has a high solvency for fatty substances, and when it condenses on cold glass a solution is formed with the contaminant, which drips off and is replaced by more pure condensing solvent. The process runs until the glass is so hot that condensation ceases. The greater the thermal capacity of the glass, the longer the vapour will continue to condense, washing the immersed surface. A glass cleaned in this way shows static electrification. This charge must be eliminated by a treatment in ionized clean air to prevent the attraction of dust particles from the atmosphere which adhere very strongly because of the electric forces. Vapour degreasing is an excellent way to obtain highly clean surfaces. The efficiency of cleaning can be checked by determining the coefficient of friction, in addition to dark-field-inspection, contact-angle and thin-film-adhesion measurements methods. High values are typical for clean surfaces [7].
4.1.1.3 ULTRASONICCLEANING Ultra sonic cleaning provides a valuable method of removing stronger adherent contaminants. This comparatively recent process produces an intense physical cleaning action and is therefore a very effective technique for breaking loose contaminants that are strongly bonded to a surface. Inorganic acidic, basic and neutral cleaning fluids are used as well as organic liquids. The cleaning is performed in a stainless steel tank containing the cleaning fluid and equipped with transducers on the bottom or at the side walls. These transducers convert an oscillating electrical input into a vibratory mechanical output. Glass is chiefly cleaned at frequencies between 20 and 40 kHz. The action of these sound waves gives rise to cavitation at the glass surface/cleaning liquid interface. The instantaneous pressure generated by
64 small imploding bubbles may reach about 1000 atm. It is obvious, therefore, that cavitation is the prime mechanism of cleaning in such a system, although detergents are sometimes used to assist in emulsification or dispersion of released particles. In addition to other factors, an increase in power input will provide a higher cavitation density at the surface, which in turn increases the cleaning efficiency. It is also a very fast process: cleaning cycle times are between a few seconds and a few minutes. Ultrasonic cleaning is used to remove pitch and polishing-agent residues from optically worked glass. Since it is often also used in cleaning sequences to produce surfaces with very low residual contamination levels ready for thin-film deposition, cleaning facilities are often located in a clean room rather than in a manufacturing area.
4.1.1.4 SPRAY CLEANING The spray-cleaning process uses the shearing forces exerted by a moving fluid on small particles to break the adhesion forces holding the particles to the surface [11,12]. The particles will be suspended in the turbulent fluid and carried away from the surface. In general, the same types of liquids that are used for immersion cleaning can also be used for spray cleaning. The more viscous and dense the cleaning liquid, the more momentum will be imparted to the attached contamination particle, assuming a constant stream velocity of the liquid. Increasing the pressure and the corresponding liquid velocity results in an increase in cleaning efficiency. Pressures of about 350 kpa are used. With a narrow fan-spray nozzle, the nozzle-to-surface distance should not exceed one hundred nozzle diameters to obtain optimal results. High-pressure spraying of organic liquids causes problems with surface cooling followed by unwanted water-vapour condensation which can leave surface spots. This can be prevented either by using a surrounding nitrogen atmosphere or using a water spray, which shows no spotting, instead of an organic liquid. High-pressure liquid spraying is a very powerful and effective method to remove particles as small as 5 gm. In some cases also, high-pressure air or gas jets are very effective.
4.1.2 CLEANINGBY HEATING AND IRRADIATION Placing substrates in a vacuum causes evaporation of volatile impurities. The effectiveness of this process also depends on how long the substrates remain in the vacuum and on the temperature as well as on the type of contaminant and on the substrate material. Under high-vacuum conditions at ambient temperature, the influence of partial pressure on desorption is negligible. Therefore, desorption is produced here by heating. Heating the glass surfaces causes a more or less strong desorption of adsorbed water and various hydrocarbon molecules, depending on the temperature. The applied temperatures are in the range between 100 and 350~ and the required heating time is between 10 and about 60 minutes. Only in the case of
65 ultra-high-vacuum applications is it necessary to use heating temperatures higher than 450~ in order to obtain atomically clean surfaces. Cleaning by heating is particularly advantageous in all those cases where, because of desired special film properties, film deposition is performed at higher substrate temperatures. But, as a consequence of heating, polymerization of some hydrocarbons to larger aggregates and decomposition to carbonaceous residues may also occur. This sometimes makes such heat treatments problematical. However, treatment with high-temperature flames, for instance a hydrogen-air flame, shows good results, although the surface temperature in such a process reaches only about 100~ In a flame, various kinds of ions as well as impurities and molecules of high thermal energy [13,14] are present. It is assumed [7] that the cleaning action of a flame is similar to that of a glow discharge in which highly energetic, ionized particles strike the surface of the parts to be cleaned. According to this model, removal of material from a glass surface in a flame may occur because of the high-energy particles which impart their energy to the adsorbed contaminants. Particle bombardment and surface recombination of ions will liberate heat and may in this way also help to desorb contaminant molecules. A relatively new technique for cleaning surfaces is the use of ultraviolet radiation to decompose hydrocarbons. Exposure times of about 15 hours in air produced clean glass surfaces [ 15]. If properly precleaned surfaces are placed within a few millimetres of an ozone-producing uv source, clean surfaces are produced in even less than one minute [16]. This clearly demonstrates that the presence of ozone increases the cleaning speed. As for the cleaning mechanism, it is known that the contaminant molecules are excited and/or dissociated under the influence of uv. Furthermore, it is also known that the production and the presence of ozone produces highly reactive atomic oxygen. It is now assumed that the excited contaminant molecules and the free radicals, produced by the dissociation of the contaminant, react with atomic oxygen and form simpler and volatile molecules like H20, CO, and N2. An increase in temperature was found to increase the reaction rates [ 17].
4.1.3 CLEANINGBY STRIPPING LACQUER COATINGS The use of strippable adhesive or lacquer coatings to remove dust particles from a surface is a very special and somewhat unconventional cleaning technique. It is preferably used to clean small pieces such as, for example, laser-mirror substrates. It can be concluded from published results [ 18] that even very small dust particles that have become embedded in the adhesive coating can be effectively removed from the surface. It was found that among various commercially available strip coatings, nitrocellulose in amyl acetate is best suited for stripping dust without leaving a residue. However, probably dependent on the type of coating used, sometimes small amounts of organic residue remain on the surface after stripping. If this happens, the stripping operation may be repeated or the surface may be cleaned with an organic solvent, possibly in a vapour degreaser.
66 The basic cleaning procedure is quite simple. The thick lacquer coating is applied to the precleaned surface with a brush or by dipping. The parts are then allowed to dry completely. In a subsequent operation, performed in a laminar flow box to prevent recontamination, the lacquer film is stripped off. Stripping is easier if a wire loop is embedded in the coating. Attempts to strip off the film in vacuum prior to thin-film deposition were only partly successful because of the difficulty in detecting surface residues inside the evacuated system.
4.1.4 CLEANINGIN AN ELECTRICAL DISCHARGE This type of cleaning is the one most widely used in practice. It is performed in the coating plant at reduced pressure immediately prior to film deposition. There are various experimental arrangements in use to sustain a glow discharge for cleaning [7, 19, 20]. Generally, the discharge burns between two, only negligibly sputtered, aluminium electrodes, which are positioned near the substrates. Oxygen and sometimes argon are normally used to form the necessary gas atmosphere. It seems, however, that mild cleaning is effective only when oxygen is present. Typical discharge voltages are in the range between 500 and 5000 Volts. The substrates are immersed into the plasma, without being a part of the glow-discharge circuit. Only precleaned substrates are treated. A glass surface immersed in the plasma of a glow discharge is bombarded by electrons, mainly positive ions and activated atoms and molecules. Therefore, the cleaning action of a glow discharge is very complex. The contributions of the individual phenomena, which have been quite well investigated, depend strongly on the various electrical and geometrical parameters and on the discharge conditions [21 ]. Numerous of processes are responsible for the beneficial action of glow-discharge treatment of substrates before film deposition, as now considered. Particle bombardment and surface recombination of ions with electrons transfer energy to the substrate and cause heating. It is possible to obtain temperatures up to 200~ Heat and the bombardment with electrons as well as with low-energy ions and neutral atoms favour the desorption of adsorbed water and some organic contaminants. The impact of activated oxygen leads to chemical reactions with organic contaminants, resulting in the formation of low-molecular-weight and therefore volatile compounds. Furthermore, the surface is modified chemically through the addition of oxygen and/or the sputtering of easily removable glass components such as alkali atoms. Physical modifications may occur by bombardment-induced surface disorder, prenucleation by sputtered and deposited foreign material [22] and deposited polymerized or even carbonized hydrocarbon from residual-gas components. The most important parameters in glow-discharge cleaning are the type of applied voltage (ac or dc), the value of discharge voltage, the current density, type of gas and gas pressure, duration of treatment, type of material to be cleaned, shape and arrangement of the electrodes and position of the parts to be cleaned. It is easy to see that it is impossible to establish universally approved data for an optimal cleaning
67 process for all types of deposition plants and substrate materials. In commercially available coating systems, there is often only a simple rod-shaped aluminium glow discharge electrode. In a dc-discharge, this electrode acts as the cathode. The walls of the plant represent the anode and are grounded. The insulating substrates are placed near the anode. In such an arrangement, the walls of the vacuum chamber receive the greatest amount of discharge current and only a smaller portion bombards the substrates. Gas desorption of the walls is thus improved, decreasing the time required for evacuation. Deposition of a minute quantity of material sputtered from the walls may contaminate the cleaned substrates. To obtain a controlled gas atmosphere, the plant is first pumped down to high vacuum and then filled up with oxygen to the desired discharge pressure. To prevent recontamination and to improve the cleaning efficiency, special arrangements have been developed and carefully tested. These are described mainly in refs. [19] and [21]. In the region of the positive column, there are equal numbers of lower energy ions and electrons with a Maxwellian velocity distribution. Immersion of insulating substrates in this zone of the glow discharge plasma enables a careful cleaning with low-energy electrons. However, because of the water vapour content of the residual gas, regeneration of adsorbed water layers can often not be avoided. Substrate cleaning in high vacuum by bombardment with ions generated in special ion guns is seldom used for industrial coating processes. Ion-beam technologies, however, offer interesting possibilities not only in cleaning but also in polishing and machining of optical surfaces, as well as for modifying the optical constants of subsurface layers [23, 24]. This may be interesting for the production of integrated optical devices [25, 26].
4.1.5 CLEANINGCYCLES Surface cleaning is performed by various methods, such as washing with solvents, heating, stripping and plasma treatment. Each has its range of applicability. Solvent cleaning has the greatest range of utility but is inadequate in many cases, particularly where the solvents themselves are contaminants. Heating is useful up to the temperature limits of the surfaces to be cleaned. Plasma treatment provides a cleaning method where contaminant bond strengths exceed the temperature limits of the system. The plasma energy can be much higher than that achieved thermally and still not damage surfaces because of the low thermal flux. No one method, however, has all the desired features of simplicity, low cost and effectiveness. To achieve optimum cleanliness of substrate surfaces, combinations of the various cleaning methods must be used. As an example, a part is frequently vapour degreased before spray cleaning. The vapour degreaser removes the oil film but is not effective on the particulate materials. The spray cleaner, however, is quite effective in removing these materials, but might not be if a residual oil film had been left on the surface. Only after removing this oil film, can the maximum effectiveness of the spray cleaning be realized. Cleaning fluids are frequently incompatible
68 with one another and it is necessary to completely remove one cleaning fluid from the surface before proceeding to another cleaning fluid. These few examples show the necessity of using sequences of cleaning operations. There is no universal approach to cleaning cycles. They are quite varied, and are specifically tailored to the particular requirements of the surfaces being cleaned and the contaminants that exist upon them. There are, however, some general guidelines to follow when establishing a cleaning sequence. Precleaning of glass parts usually starts with immersion cleaning in detergent solutions, assisted by rubbing, wiping or ultrasonic agitation, followed by rinsing in demineralized water and/or alcohol. It is important to get the parts dry without allowing solution sediment to remain on the surface because it is often hard to remove later [27]. In a cleaning operation, the sequence of cleaning liquids must be chemically compatible and mixable without precipitation at all stages. A change from acidic solutions to caustic solutions requires rinsing with plain water in between. The change from aqueous solutions to organic fluids always requires an intermediate treatment with a mixable co-solvent such as alcohol or special dewatering fluids. Corrosive chemical agents from the fabrication process as well as corrosive cleaning agents may remain on the surfaces for only a very short time. The last steps in a cleaning cycle must be performed extremely carefully. In a wet operation, the final rinsing fluid used must be as pure as possible and, generally, it should be as volatile as is practical. The choice of the best cleaning cycle often requires an empirical approach, e.g. [34 and 40]. Finally, it is important that cleaned surfaces are not left unprotected. Proper storage and handling before further treatment by film deposition are stringent requirements.
4.1.6 CLEANING OF ORGANIC GLASS Cleaning of organic glass and plastic materials requires special techniques and handling because of their low thermal and mechanical stability. Organic glass surfaces may be covered with low molecular weight fractions, surface oils, antistatic films, finger prints, etc.. Most of the contaminants can be removed by an aqueous detergent wash or by other solvent cleaning possibly associated with mild liquid etching [36]. However, care must be taken with cleaning fluids because they may be absorbed into the polymer structure causing it to swell and possibly to craze on drying. With organic polymers cleaning means that the surface must be modified in such a way that it does not represent an area of insufficient adhesion at the interface substrate surface / film formed after film deposition. Therefore frequently cleaning of plastics may mean simply the modification of the surface so that the contaminant initially present is afterwards no longer considered as a contaminant. The proper treatment in a glow discharge plasma is very effective for that purpose since, in addition to micro roughening, it also causes chemical activation and cross-linking
69 [37]. In particular cross-linking is advantageous because it increases the surface strength of the polymer and reduces the amount of undesirable low molecular weight components [38]. Proper cleaning fluids and short cleaning time as well as carefully established energy limits and proper doses in particle bombardment or in radiation treatment are important for optimum results.
4.2
METHODSFOR CONTROL OF SURFACE CLEANLINESS
The methods suitable for control of surface cleanliness are mainly those discussed in Section 3.1.2. Compositional changes on and near the surface of glass (i.e. to a depth of about 20/~) can be measured with Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), ion-scattering spectroscopy (ISS) or secondary-ion-mass spectrometry (SIMS). Coupling these methods with sputteretching, yields highly detailed compositional profiles of the intermediate glass surface in the thickness range from 20 to ~ 2000 A [35]. Measurement of the average composition through to the far surface, that is to about 10,000 A, is now routinely available with electron microprobe analysis (EMPA), energy-dispersive Xray analysis (EDX) in the scanning electron microscope (SEM), or with infraredreflection spectrometry (IRRS). The related merits of these glass characterization techniques have been already reviewed [28]. These highly specialized techniques are mainly used to check the efficiency of a newly established cleaning sequence and to troubleshoot when sudden problems arise in a cleaning cycle. In daily practice, however, the use of such excellent inspection methods is much too time-consuming and expensive. There are also many other cheaper cleanliness tests such as inspection of the breath figure, the atomizer test [29] and the water-break test [30]. All of these qualitative tests are based on the wettability, which is generally high on clean glass surfaces. Oil- and grease-contaminated, and therefore hydrophobic, glass shows a grey breath figure consisting of individual, relatively large water droplets with large contact angles to the surface. Clean glass shows a black breath figure and the water film consists of droplets with contact angles approaching zero. Exposure of a dry and clean glass to a water spray from an atomizer yields a formation of very fine water droplets, which in areas of surface contamination coalesce to larger aggregates. Similar behaviour is responsible for the break of a continuous water film in contaminated surface regions. The contact angle of water droplets and droplets of other materials on a surface can even be a quantitative measure of wettability [31 ]. The frictional resistance to solids rubbed on glass is a further sensitive measurement of the cleanliness of the surface. Clean glass has an abnormally high friction coefficient, which is near Its = 1 [7]. The presence of an adsorbed monomolecular layer of a fatty acid, such as stearic acid, however, is sufficient to have a marked lubricating effect [7]. The corresponding coefficient of friction is then only Its = 0.3.
70 To detect residual particles (organic and inorganic dust) on a glass surface, light scattering is a valuable tool. Inspection of a cleaned surface with a simple dark-field lamp or in a microscope with dark-field illumination is often used to assess the result of cleaning. The light-scattering effect can also be used for a quantitative measurement, as can be seen in ref. [81 ] of Chapter 3.
4.3 MAINTENANCE OF CLEAN SURFACES The stability and maintenance of a cleaned surface is often more critical than the final surface state which is achieved after the cleaning process. Storage in an ultraclean, controlled environment, a very expensive but most effective measure is usually seldom required. Instead of using a universal protection device, it is often easier and cheaper to identify the undesirable contaminants and to eliminate them from the storage environment. Such contaminants are usually the airborne ones, including various types of dust particles, atmospheric condensates of chemical vapours, and, last but not least, water vapour. The use of some preventive measures is, therefore, well worth considering. Contact with dust may be reduced drastically by storing the parts in a closed container or in a clean bench. Adsorbance of hydrocarbon vapours can he minimized by storing the substrates in freshly oxidized aluminium containers, which preferentially absorb the hydrocarbons. A disadvantage of this technique is the extra steps involved in periodically stripping and reoxidizing the metal surface of the container. The ultraviolet/ozone technique, mentioned above, may also be used to keep oxidic surfaces clean in an ambient environment. In general, however, cleaned surfaces should at least be stored in clean glass or plastic containers. Cleaning and maintenance of glass cleanliness also depends strongly on the microstructure of the surface. Contaminants from polishing and subsequent operations may be hidden in micropores and fine flaws. These may then, in the presence of adsorbed water vapour, attack the glass network. Water vapour favours corrosion of the surface and advanced corrosion requires repolishing of the glass surfaces. Such effects are strong with some special types of glass such as those with a high content of lanthanum oxide [32,33]. Even glasses with a very high chemical durability, such as soda lime borosilicate compositions, exhibit the problem of maintenance of a cleaned surface state to some degree. Often glass surfaces are sealed in a hermetic device. In some cases, these surfaces are contaminated by the sealing operation, which frequently involves heating of other non clean surfaces. Before sealing, all interior surfaces should be as clean as possible. Highly unstable, water-vapoursensitive surfaces are generally stored in vacuum desiccators. However, the best precaution for preventing recontamination, beside cleanliness of operator and environment, is to avoid long intervals between cleaning and film deposition. The cleaned parts should be mounted into the substrate holders immediately after cleaning and the whole device should be placed carefully and quickly in the coating plant.
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and H. Karow, Fabrication methods for precision optics, Wiley, NY, 1993 p. 560. H. Bach, Glastechn. Ber., 56 (1983) 1; 56 (1983) 29; 56(1983)55. J. Koutsky, see ref. [1 ], Vol. 1, p. 351. D.M. Mattox, Thin Solid Films, 53 (1978) 81. Mitsuharu Konuma, Chapter 8 in: Film Deposition by Plasma Techniques, Springer, Berlin, New York 1992: and K. Telgenbtischer, J. Leiber, Schltisselbericht des BMFT Forschungsvorhaben Plasmapolymerisation, F6rderkennzeichen 13 N 5616, 19. April 1993. Proc. Seminar: Reinigung vor der Beschichtung, OTTI-Technologie Kolleg. Regensburg (Germany) 1997. F. Scheerer, Handbticher der FCkW-freien Reinigung, Leica, Wetzlar, (Germany) 1995.
Surfaces exposed to the atmosphere are generally contaminated. Any unwanted material and/or energy on a surface is regarded as a contaminant. Any manipulation at all contributes to the generation of contaminations. Surface contamination can be gaseous, liquid, or solid in its physical state and may be present as a film or in particulate form. Furthermore, it can be ionic or covalent and inorganic or organic in its chemical character. It can originate from a number of sources, and the first contamination is often a part of the process creating the surface itself. Sorption phenomena, chemical reactions, leaching and drying procedures, mechanical treatment, as well as diffusion and segregation processes can give rise to various compositions of surface contamination. Most scientific and technical investigations and applications, however, require clean surfaces. So, for example, before a surface can be coated with thin films, it must be clean; if it is not clean, then the film will not adhere well or may not adhere at all. In the case of coating clean often means absence of detrimental impurities. In optical coating applications, even very minute contaminations of special materials can interfere with the functioning of sensitive devices whereas other materials may have practically no influence. Therefore, the cleaning of surfaces is a very important but also a very difficult and delicate operation. The adhesive forces holding small particles onto surfaces may be quite strong in terms of bond strength per unit area. The mechanical energy available to break such bonds is relatively small in most cleaning processes. Consequently, in a given cleaning process, a strong effort is necessary to reduce the adhesive bonds between contaminant and surface as much as possible if the cleaning process is to be at all effective. The reduction of these adhesive bonds is affected by the choice of the cleaning fluid and the conditions under which it is used, particularly the temperature. Pure water, for instance, may be relatively ineffective in removing the contaminants from a given surface, whereas hot water with detergent is quite effective in the same process. There are a number of diverse cleaning techniques used in various scientific and industrial cleaning problems. Probably the universally used basic cleaning method is scrubbing or polishing a surface. This is used with various cleaning fluids, and for all degrees of cleaning abrasion, ranging from the lightest touch of a non-abrasive brush to rigorous brushing. The latter procedure is a relatively harsh treatment used only to remove gross contaminants from unfinished parts. It is wrong, however, to assume that all the contaminants can be removed in a single cleaning operation. In many cases, particulate material can tenaciously adhere to a surface, and surprising forces can be developed between a surface and a small particle. The cleaning tech-
61 niques will only be partially effective and, in general, will probably be less and less effective as particle size decreases. In the case of glass and crystal surfaces, the undesired materials on the surface may be mainly water-containing oxidic polishing residues, inorganic and organic dust particles, or films consisting of oil and grease or a combination of these.
4.1 CLEANINGPROCEDURES Cleaned surfaces can be classified into two categories: atomically clean surfaces and technologically clean surfaces. Surfaces of the first category are required for special scientific purposes and these can only be realized in ultra high vacuum. With the exception of these very sophisticated products, practical coating applications require only technologically clean or slightly better qualities of surface. In accordance with a principle described in ref. [1 ], the degree of surface cleanliness must meet the following two criteria: it must be good enough for subsequent processing and it must be sufficient to ensure the future reliability of the product for which that surface will be used. A further distinction must be made between cleaning methods that are applicable in the atmosphere and those that are applicable only in vacuum. In all cases where handling of the parts and the use of solvents is required, cleaning cannot be performed in a vacuum. If cleaning in vacuum, e.g. by heating operations and particle bombardment is used, then this is generally conducted inside the deposition system.
4.1.1 CLEANINGWITH SOLVENTS Cleaning with solvents is a widespread procedure that is always included whenever cleaning of glass surfaces is discussed. In this process, various cleaning fluids are used. A distinction must be made between demineralized water or aqueous systems such as water with detergents, diluted acids or bases and non-aqueous solvents such as alcohols, ketones, petroleum fractions and chlorinated or fluorinated hydrocarbons. Emulsions and solvent vapours are also used. The type of solvent used depends on the nature of the contaminants. There are many papers on the cleaning of glass by solvents; some examples are given in refs. [2-10]. However, one has to consider the far-reaching new regulations to preserve the stratospheric ozone layer, given in the Montreal Protocol, now nearly ten years old. These regulations prohibit the use of fluorinated and chlorinated hydrocarbons, FCHC, also as cleaning fluids. Exceptions can be made only when using completely closed and controlled cleaning systems. Information is available, for instance, in the various OzonAction newsletters [3]. Instructions how to replace FCHC in cleaning procedures can be found in the proceedings of cleaning seminars, e.g. [39], or obtained directly from the producers of cleaning machines.
62 4.1.1.1
RUBBINGAND IMMERSION CLEANING
Perhaps the simplest method of removing superficial dirt from glass is to rub the surface with cotton wool dipped in a mixture of precipitated chalk and alcohol or ammonia. There is evidence, however, that traces of chalk can be left behind on such surfaces so that after that treatment the parts must carefully be washed off in pure water or alcohol. This method is best suited as a precleaning operation, i.e. as the first step in a cleaning sequence. Rubbing a lens or a mirror substrate with a lens tissue saturated in solvent is almost a standard cleaning procedure. It takes advantage of solvent extraction and imparts a high liquid shearing force to attached particles when the fibres of the tissue pass over the surface. The resulting cleanliness is dependent on the presence of contaminants in the solvent and in the lens tissue. Recontamination is avoided by discarding each tissue after one pass over the surface. A very high level of surface cleanliness is achievable with this cleaning operation. A simple and often used cleaning technique is that of immersion or dip cleaning. The basic equipment employed in dip cleaning is easy to construct and inexpensive. An open tank of glass, plastic or stainless steel is filled with a cleaning fluid and the glass parts, which are clamped with a pair of tweezers or are inserted in a special holder, are then dipped in the fluid. Agitation may or may not be employed. The wet parts are taken out of the tank after a short time and may then be dried by rubbing with a towel of pure cotton free of washing-powder contaminants. The parts are subsequently inspected by dark-field illumination. If the degree of cleaning is not sufficient, the operation can be repeated by further dipping in the same fluid or other cleaning fluids. In addition to physical cleaning, chemical reactions can also be exploited for cleaning purposes. Various acids with strengths ranging from weak to strong, as well as mixtures such as chromic and sulphuric acid are used. All acids, with the exception of hydrofluoric acid, must be used hot, i.e. between 60 and 85~ to produce a clean glass surface. Silica is not readily dissolved by acids, apart from hydrofluoric acid, and the surface layer on aged glass is invariable finely divided silica. Higher temperatures may aid the dissolution of silica so that a new surface is created on the treated glass. Acid cleaning cannot be used for all types of glass. This is especially true for glasses having a high barium-or lead oxide content, such as some optical glasses. These are leached by even mild acids, producing a loose silica surface film. According to a report in [10], a cold diluted mixture of 5% HF, 33% HNO3, 2% Teepol | and 60% H20 should be an excellent universal fluid for cleaning glass and silica. Caustic solutions exhibit a detergency and the ability to remove oils and greases. The lipids and fatty materials are saponified by the bases to soaps. These watersoluble reaction products can readily be rinsed off the clean surface. It is generally desirable to limit the removal process to the contaminant layer, but a mild attack of the substrate material itself is often tolerable and ensures that the cleaning process is complete. Attention must be paid to unwanted, stronger etching and leaching effects. Such processes may destroy the surface quality and should therefore be
63 avoided. The chemical resistance of inorganic and organic glass can be found in the glass catalogues. Single and combined immersion cleaning processes are mainly used to clean small pieces.
4.1.1.2 VAPOURDEGREASING Vapour degreasing is a process that is primarily useful for removing grease and oil films from surfaces. In glass cleaning, it is often used as the last step in a sequence of various cleaning operations. A vapour-degreasing apparatus consists essentially of an open tank with heating elements in the bottom and water-cooled condensing coils running around the top perimeter. The cleaning fluid may be isopropyl alcohol or one of the chlorinated and fluorinated hydrocarbons. The solvent is vaporized and forms a hot, high-density vapour, which remains in the equipment because the condenser coils prevent vapour loss. The precleaned cold glass pieces, in special holders, are immersed in the dense vapour for periods ranging from 15 sec to several minutes. Pure cleaning fluid vapour has a high solvency for fatty substances, and when it condenses on cold glass a solution is formed with the contaminant, which drips off and is replaced by more pure condensing solvent. The process runs until the glass is so hot that condensation ceases. The greater the thermal capacity of the glass, the longer the vapour will continue to condense, washing the immersed surface. A glass cleaned in this way shows static electrification. This charge must be eliminated by a treatment in ionized clean air to prevent the attraction of dust particles from the atmosphere which adhere very strongly because of the electric forces. Vapour degreasing is an excellent way to obtain highly clean surfaces. The efficiency of cleaning can be checked by determining the coefficient of friction, in addition to dark-field-inspection, contact-angle and thin-film-adhesion measurements methods. High values are typical for clean surfaces [7].
4.1.1.3 ULTRASONICCLEANING Ultra sonic cleaning provides a valuable method of removing stronger adherent contaminants. This comparatively recent process produces an intense physical cleaning action and is therefore a very effective technique for breaking loose contaminants that are strongly bonded to a surface. Inorganic acidic, basic and neutral cleaning fluids are used as well as organic liquids. The cleaning is performed in a stainless steel tank containing the cleaning fluid and equipped with transducers on the bottom or at the side walls. These transducers convert an oscillating electrical input into a vibratory mechanical output. Glass is chiefly cleaned at frequencies between 20 and 40 kHz. The action of these sound waves gives rise to cavitation at the glass surface/cleaning liquid interface. The instantaneous pressure generated by
64 small imploding bubbles may reach about 1000 atm. It is obvious, therefore, that cavitation is the prime mechanism of cleaning in such a system, although detergents are sometimes used to assist in emulsification or dispersion of released particles. In addition to other factors, an increase in power input will provide a higher cavitation density at the surface, which in turn increases the cleaning efficiency. It is also a very fast process: cleaning cycle times are between a few seconds and a few minutes. Ultrasonic cleaning is used to remove pitch and polishing-agent residues from optically worked glass. Since it is often also used in cleaning sequences to produce surfaces with very low residual contamination levels ready for thin-film deposition, cleaning facilities are often located in a clean room rather than in a manufacturing area.
4.1.1.4 SPRAY CLEANING The spray-cleaning process uses the shearing forces exerted by a moving fluid on small particles to break the adhesion forces holding the particles to the surface [11,12]. The particles will be suspended in the turbulent fluid and carried away from the surface. In general, the same types of liquids that are used for immersion cleaning can also be used for spray cleaning. The more viscous and dense the cleaning liquid, the more momentum will be imparted to the attached contamination particle, assuming a constant stream velocity of the liquid. Increasing the pressure and the corresponding liquid velocity results in an increase in cleaning efficiency. Pressures of about 350 kpa are used. With a narrow fan-spray nozzle, the nozzle-to-surface distance should not exceed one hundred nozzle diameters to obtain optimal results. High-pressure spraying of organic liquids causes problems with surface cooling followed by unwanted water-vapour condensation which can leave surface spots. This can be prevented either by using a surrounding nitrogen atmosphere or using a water spray, which shows no spotting, instead of an organic liquid. High-pressure liquid spraying is a very powerful and effective method to remove particles as small as 5 gm. In some cases also, high-pressure air or gas jets are very effective.
4.1.2 CLEANINGBY HEATING AND IRRADIATION Placing substrates in a vacuum causes evaporation of volatile impurities. The effectiveness of this process also depends on how long the substrates remain in the vacuum and on the temperature as well as on the type of contaminant and on the substrate material. Under high-vacuum conditions at ambient temperature, the influence of partial pressure on desorption is negligible. Therefore, desorption is produced here by heating. Heating the glass surfaces causes a more or less strong desorption of adsorbed water and various hydrocarbon molecules, depending on the temperature. The applied temperatures are in the range between 100 and 350~ and the required heating time is between 10 and about 60 minutes. Only in the case of
65 ultra-high-vacuum applications is it necessary to use heating temperatures higher than 450~ in order to obtain atomically clean surfaces. Cleaning by heating is particularly advantageous in all those cases where, because of desired special film properties, film deposition is performed at higher substrate temperatures. But, as a consequence of heating, polymerization of some hydrocarbons to larger aggregates and decomposition to carbonaceous residues may also occur. This sometimes makes such heat treatments problematical. However, treatment with high-temperature flames, for instance a hydrogen-air flame, shows good results, although the surface temperature in such a process reaches only about 100~ In a flame, various kinds of ions as well as impurities and molecules of high thermal energy [13,14] are present. It is assumed [7] that the cleaning action of a flame is similar to that of a glow discharge in which highly energetic, ionized particles strike the surface of the parts to be cleaned. According to this model, removal of material from a glass surface in a flame may occur because of the high-energy particles which impart their energy to the adsorbed contaminants. Particle bombardment and surface recombination of ions will liberate heat and may in this way also help to desorb contaminant molecules. A relatively new technique for cleaning surfaces is the use of ultraviolet radiation to decompose hydrocarbons. Exposure times of about 15 hours in air produced clean glass surfaces [ 15]. If properly precleaned surfaces are placed within a few millimetres of an ozone-producing uv source, clean surfaces are produced in even less than one minute [16]. This clearly demonstrates that the presence of ozone increases the cleaning speed. As for the cleaning mechanism, it is known that the contaminant molecules are excited and/or dissociated under the influence of uv. Furthermore, it is also known that the production and the presence of ozone produces highly reactive atomic oxygen. It is now assumed that the excited contaminant molecules and the free radicals, produced by the dissociation of the contaminant, react with atomic oxygen and form simpler and volatile molecules like H20, CO, and N2. An increase in temperature was found to increase the reaction rates [ 17].
4.1.3 CLEANINGBY STRIPPING LACQUER COATINGS The use of strippable adhesive or lacquer coatings to remove dust particles from a surface is a very special and somewhat unconventional cleaning technique. It is preferably used to clean small pieces such as, for example, laser-mirror substrates. It can be concluded from published results [ 18] that even very small dust particles that have become embedded in the adhesive coating can be effectively removed from the surface. It was found that among various commercially available strip coatings, nitrocellulose in amyl acetate is best suited for stripping dust without leaving a residue. However, probably dependent on the type of coating used, sometimes small amounts of organic residue remain on the surface after stripping. If this happens, the stripping operation may be repeated or the surface may be cleaned with an organic solvent, possibly in a vapour degreaser.
66 The basic cleaning procedure is quite simple. The thick lacquer coating is applied to the precleaned surface with a brush or by dipping. The parts are then allowed to dry completely. In a subsequent operation, performed in a laminar flow box to prevent recontamination, the lacquer film is stripped off. Stripping is easier if a wire loop is embedded in the coating. Attempts to strip off the film in vacuum prior to thin-film deposition were only partly successful because of the difficulty in detecting surface residues inside the evacuated system.
4.1.4 CLEANINGIN AN ELECTRICAL DISCHARGE This type of cleaning is the one most widely used in practice. It is performed in the coating plant at reduced pressure immediately prior to film deposition. There are various experimental arrangements in use to sustain a glow discharge for cleaning [7, 19, 20]. Generally, the discharge burns between two, only negligibly sputtered, aluminium electrodes, which are positioned near the substrates. Oxygen and sometimes argon are normally used to form the necessary gas atmosphere. It seems, however, that mild cleaning is effective only when oxygen is present. Typical discharge voltages are in the range between 500 and 5000 Volts. The substrates are immersed into the plasma, without being a part of the glow-discharge circuit. Only precleaned substrates are treated. A glass surface immersed in the plasma of a glow discharge is bombarded by electrons, mainly positive ions and activated atoms and molecules. Therefore, the cleaning action of a glow discharge is very complex. The contributions of the individual phenomena, which have been quite well investigated, depend strongly on the various electrical and geometrical parameters and on the discharge conditions [21 ]. Numerous of processes are responsible for the beneficial action of glow-discharge treatment of substrates before film deposition, as now considered. Particle bombardment and surface recombination of ions with electrons transfer energy to the substrate and cause heating. It is possible to obtain temperatures up to 200~ Heat and the bombardment with electrons as well as with low-energy ions and neutral atoms favour the desorption of adsorbed water and some organic contaminants. The impact of activated oxygen leads to chemical reactions with organic contaminants, resulting in the formation of low-molecular-weight and therefore volatile compounds. Furthermore, the surface is modified chemically through the addition of oxygen and/or the sputtering of easily removable glass components such as alkali atoms. Physical modifications may occur by bombardment-induced surface disorder, prenucleation by sputtered and deposited foreign material [22] and deposited polymerized or even carbonized hydrocarbon from residual-gas components. The most important parameters in glow-discharge cleaning are the type of applied voltage (ac or dc), the value of discharge voltage, the current density, type of gas and gas pressure, duration of treatment, type of material to be cleaned, shape and arrangement of the electrodes and position of the parts to be cleaned. It is easy to see that it is impossible to establish universally approved data for an optimal cleaning
67 process for all types of deposition plants and substrate materials. In commercially available coating systems, there is often only a simple rod-shaped aluminium glow discharge electrode. In a dc-discharge, this electrode acts as the cathode. The walls of the plant represent the anode and are grounded. The insulating substrates are placed near the anode. In such an arrangement, the walls of the vacuum chamber receive the greatest amount of discharge current and only a smaller portion bombards the substrates. Gas desorption of the walls is thus improved, decreasing the time required for evacuation. Deposition of a minute quantity of material sputtered from the walls may contaminate the cleaned substrates. To obtain a controlled gas atmosphere, the plant is first pumped down to high vacuum and then filled up with oxygen to the desired discharge pressure. To prevent recontamination and to improve the cleaning efficiency, special arrangements have been developed and carefully tested. These are described mainly in refs. [19] and [21]. In the region of the positive column, there are equal numbers of lower energy ions and electrons with a Maxwellian velocity distribution. Immersion of insulating substrates in this zone of the glow discharge plasma enables a careful cleaning with low-energy electrons. However, because of the water vapour content of the residual gas, regeneration of adsorbed water layers can often not be avoided. Substrate cleaning in high vacuum by bombardment with ions generated in special ion guns is seldom used for industrial coating processes. Ion-beam technologies, however, offer interesting possibilities not only in cleaning but also in polishing and machining of optical surfaces, as well as for modifying the optical constants of subsurface layers [23, 24]. This may be interesting for the production of integrated optical devices [25, 26].
4.1.5 CLEANINGCYCLES Surface cleaning is performed by various methods, such as washing with solvents, heating, stripping and plasma treatment. Each has its range of applicability. Solvent cleaning has the greatest range of utility but is inadequate in many cases, particularly where the solvents themselves are contaminants. Heating is useful up to the temperature limits of the surfaces to be cleaned. Plasma treatment provides a cleaning method where contaminant bond strengths exceed the temperature limits of the system. The plasma energy can be much higher than that achieved thermally and still not damage surfaces because of the low thermal flux. No one method, however, has all the desired features of simplicity, low cost and effectiveness. To achieve optimum cleanliness of substrate surfaces, combinations of the various cleaning methods must be used. As an example, a part is frequently vapour degreased before spray cleaning. The vapour degreaser removes the oil film but is not effective on the particulate materials. The spray cleaner, however, is quite effective in removing these materials, but might not be if a residual oil film had been left on the surface. Only after removing this oil film, can the maximum effectiveness of the spray cleaning be realized. Cleaning fluids are frequently incompatible
68 with one another and it is necessary to completely remove one cleaning fluid from the surface before proceeding to another cleaning fluid. These few examples show the necessity of using sequences of cleaning operations. There is no universal approach to cleaning cycles. They are quite varied, and are specifically tailored to the particular requirements of the surfaces being cleaned and the contaminants that exist upon them. There are, however, some general guidelines to follow when establishing a cleaning sequence. Precleaning of glass parts usually starts with immersion cleaning in detergent solutions, assisted by rubbing, wiping or ultrasonic agitation, followed by rinsing in demineralized water and/or alcohol. It is important to get the parts dry without allowing solution sediment to remain on the surface because it is often hard to remove later [27]. In a cleaning operation, the sequence of cleaning liquids must be chemically compatible and mixable without precipitation at all stages. A change from acidic solutions to caustic solutions requires rinsing with plain water in between. The change from aqueous solutions to organic fluids always requires an intermediate treatment with a mixable co-solvent such as alcohol or special dewatering fluids. Corrosive chemical agents from the fabrication process as well as corrosive cleaning agents may remain on the surfaces for only a very short time. The last steps in a cleaning cycle must be performed extremely carefully. In a wet operation, the final rinsing fluid used must be as pure as possible and, generally, it should be as volatile as is practical. The choice of the best cleaning cycle often requires an empirical approach, e.g. [34 and 40]. Finally, it is important that cleaned surfaces are not left unprotected. Proper storage and handling before further treatment by film deposition are stringent requirements.
4.1.6 CLEANING OF ORGANIC GLASS Cleaning of organic glass and plastic materials requires special techniques and handling because of their low thermal and mechanical stability. Organic glass surfaces may be covered with low molecular weight fractions, surface oils, antistatic films, finger prints, etc.. Most of the contaminants can be removed by an aqueous detergent wash or by other solvent cleaning possibly associated with mild liquid etching [36]. However, care must be taken with cleaning fluids because they may be absorbed into the polymer structure causing it to swell and possibly to craze on drying. With organic polymers cleaning means that the surface must be modified in such a way that it does not represent an area of insufficient adhesion at the interface substrate surface / film formed after film deposition. Therefore frequently cleaning of plastics may mean simply the modification of the surface so that the contaminant initially present is afterwards no longer considered as a contaminant. The proper treatment in a glow discharge plasma is very effective for that purpose since, in addition to micro roughening, it also causes chemical activation and cross-linking
69 [37]. In particular cross-linking is advantageous because it increases the surface strength of the polymer and reduces the amount of undesirable low molecular weight components [38]. Proper cleaning fluids and short cleaning time as well as carefully established energy limits and proper doses in particle bombardment or in radiation treatment are important for optimum results.
4.2
METHODSFOR CONTROL OF SURFACE CLEANLINESS
The methods suitable for control of surface cleanliness are mainly those discussed in Section 3.1.2. Compositional changes on and near the surface of glass (i.e. to a depth of about 20/~) can be measured with Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), ion-scattering spectroscopy (ISS) or secondary-ion-mass spectrometry (SIMS). Coupling these methods with sputteretching, yields highly detailed compositional profiles of the intermediate glass surface in the thickness range from 20 to ~ 2000 A [35]. Measurement of the average composition through to the far surface, that is to about 10,000 A, is now routinely available with electron microprobe analysis (EMPA), energy-dispersive Xray analysis (EDX) in the scanning electron microscope (SEM), or with infraredreflection spectrometry (IRRS). The related merits of these glass characterization techniques have been already reviewed [28]. These highly specialized techniques are mainly used to check the efficiency of a newly established cleaning sequence and to troubleshoot when sudden problems arise in a cleaning cycle. In daily practice, however, the use of such excellent inspection methods is much too time-consuming and expensive. There are also many other cheaper cleanliness tests such as inspection of the breath figure, the atomizer test [29] and the water-break test [30]. All of these qualitative tests are based on the wettability, which is generally high on clean glass surfaces. Oil- and grease-contaminated, and therefore hydrophobic, glass shows a grey breath figure consisting of individual, relatively large water droplets with large contact angles to the surface. Clean glass shows a black breath figure and the water film consists of droplets with contact angles approaching zero. Exposure of a dry and clean glass to a water spray from an atomizer yields a formation of very fine water droplets, which in areas of surface contamination coalesce to larger aggregates. Similar behaviour is responsible for the break of a continuous water film in contaminated surface regions. The contact angle of water droplets and droplets of other materials on a surface can even be a quantitative measure of wettability [31 ]. The frictional resistance to solids rubbed on glass is a further sensitive measurement of the cleanliness of the surface. Clean glass has an abnormally high friction coefficient, which is near Its = 1 [7]. The presence of an adsorbed monomolecular layer of a fatty acid, such as stearic acid, however, is sufficient to have a marked lubricating effect [7]. The corresponding coefficient of friction is then only Its = 0.3.
70 To detect residual particles (organic and inorganic dust) on a glass surface, light scattering is a valuable tool. Inspection of a cleaned surface with a simple dark-field lamp or in a microscope with dark-field illumination is often used to assess the result of cleaning. The light-scattering effect can also be used for a quantitative measurement, as can be seen in ref. [81 ] of Chapter 3.
4.3 MAINTENANCE OF CLEAN SURFACES The stability and maintenance of a cleaned surface is often more critical than the final surface state which is achieved after the cleaning process. Storage in an ultraclean, controlled environment, a very expensive but most effective measure is usually seldom required. Instead of using a universal protection device, it is often easier and cheaper to identify the undesirable contaminants and to eliminate them from the storage environment. Such contaminants are usually the airborne ones, including various types of dust particles, atmospheric condensates of chemical vapours, and, last but not least, water vapour. The use of some preventive measures is, therefore, well worth considering. Contact with dust may be reduced drastically by storing the parts in a closed container or in a clean bench. Adsorbance of hydrocarbon vapours can he minimized by storing the substrates in freshly oxidized aluminium containers, which preferentially absorb the hydrocarbons. A disadvantage of this technique is the extra steps involved in periodically stripping and reoxidizing the metal surface of the container. The ultraviolet/ozone technique, mentioned above, may also be used to keep oxidic surfaces clean in an ambient environment. In general, however, cleaned surfaces should at least be stored in clean glass or plastic containers. Cleaning and maintenance of glass cleanliness also depends strongly on the microstructure of the surface. Contaminants from polishing and subsequent operations may be hidden in micropores and fine flaws. These may then, in the presence of adsorbed water vapour, attack the glass network. Water vapour favours corrosion of the surface and advanced corrosion requires repolishing of the glass surfaces. Such effects are strong with some special types of glass such as those with a high content of lanthanum oxide [32,33]. Even glasses with a very high chemical durability, such as soda lime borosilicate compositions, exhibit the problem of maintenance of a cleaned surface state to some degree. Often glass surfaces are sealed in a hermetic device. In some cases, these surfaces are contaminated by the sealing operation, which frequently involves heating of other non clean surfaces. Before sealing, all interior surfaces should be as clean as possible. Highly unstable, water-vapoursensitive surfaces are generally stored in vacuum desiccators. However, the best precaution for preventing recontamination, beside cleanliness of operator and environment, is to avoid long intervals between cleaning and film deposition. The cleaned parts should be mounted into the substrate holders immediately after cleaning and the whole device should be placed carefully and quickly in the coating plant.
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