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.

                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.

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.