AU3111801A - Sol-gel process for producing synthetic silica glass - Google Patents

Description

WO 01/53225 PCT/US01/02325 SOL-GEL PROCESS FOR PRODUCING SYNTHETIC SILICA GLASS BACKGROUND OF THE INVENTION This invention relates generally to sol-gel processes for making 5 articles of silica glass and, more particularly, to sol-gel processes that utilize both an alkoxide precursor and a suspension of colloidal particles. Sol-gel processes are useful for making articles of synthetic silica glass having shapes close to that desired for the final product. This reduces costs associated with machining and polishing the articles. Typically, sol-gel processes 10 involve steps of (1) synthesizing a liquid silica solution, or sol, typically by hydrolyzing an alkoxide precursor or stabilizing a colloidal silica solution, (2) condensing the sol to yield a wet gel, (3) aging the wet gel to strengthen it, (4) drying the wet gel to produce a dry gel, and (5) sintering the dry gel to yield a dense glass article. 15 The gels yielded by this sol gel route typically are quite porous, with large pore (void) volumes and relatively low volume loading of silica. Because the porous gels are later sintered into dense glass articles, the large void volumes bring about a large amount of shrinkage during the sintering step and thus yield silica articles of reduced size. It therefore follows that reducing the void volume will 20 increase the amount of sintered silica glass that can be obtained from a single processing cycle and thus reduce manufacturing costs. Increasing the loading of silica in the sol is one way to achieve this. Three distinct processes have been used in the past to produce silica sols. One such process uses pure alkoxide precursors, but it typically yields a silica -1- WO 01/53225 PCT/USO1/02325 loading of only about 17 to 20%. A second such process uses colloidal silica in lieu of pure alkoxides. This latter process yields higher silica loading, but it requires stabilizing agents to be incorporated into the glass matrix. This can add to the cost of the process and can degrade the quality of the resultant glass. 5 A third process used in the past to produce silica sols uses a combination of an alkoxide and colloidal silica. Specifically, a hybrid colloidal silica/alkoxide sol is produced by adding fumed silica powder to an already hydrolyzed alkoxide of silica. The hydrolyzed alkoxide is produced initially by mixing together water and silica alkoxides, e.g., tetra-ethoxysilane (TEOS), which 10 are initially immiscible but which dissolve into each other when an alcohol reaction product is generated in sufficient quantity. Thus, hydrolysis is initiated and carried out by adding acidic water to an alkoxide and stirring the resulting mixture. After hydrolysis has been completed, the fumed silica powder, e.g., Aerosil OX-50, is added and the solution is violently stirred. 15 In this third process, the addition of a large amount of fumed silica powder to the hydrolyzed alkoxide solution typically yields a non-homogeneous sol incorporating large particles of aggregated silica powder and TEOS. These particles can weaken the resulting gel, which can lead to cracking during the drying stage. This problem can be overcome by adding the fumed silica powder only with 20 continuous ultra-sonication and by centrifuging the solution to remove aggregated particles, but this can add to the cost of the process. One significant application of such sol-gel processes is in making substrates for photoblanks, which are commonly used in the semiconductor industry. Such photoblanks typically include substrates formed of high-quality 25 synthetic silica coated with chromium and photoresist. The substrates typically have been made using a traditional soot deposition and densification process, in -2- WO 01/53225 PCT/USO1/02325 which a large cylindrical boule of very high quality silica is first grown and then core drilled to form a smaller square shape, most commonly 15 cm x 15 cm in size. Individual blanks then are sliced from the square core and polished. Sol-gel processes also have been proposed for making such 5 photoblank substrates. However, according to U.S. Patent No. 5,236,483 issued to Miyashita et al., substrates made using a sol-gel process exhibit inadequate optical transmission at 200 nm when OX-50 silica powder is used in the precursor sol. Optical transmission at this wavelength is a common measure of suitability of synthetic glass for use as photoblank substrates. Nevertheless, there is a strong 10 desire to use OX-50 powder in the sol, because of its relatively low cost. Synthetic glass suitable for use as photoblank substrates also must avoid warpage during the final sintering step. This sintering step typically causes shrinkage on of the order of 50%, on a linear scale, and warpage can occur when objects of high aspect ratios, i.e., length:thickness, undergo such large shrinkages. 15 Photoblanks typically can have high aspect ratios of about 25. It should be appreciated from the foregoing description that there remains a need for a sol-gel process for producing a gel having a high loading of silica, so that larger volumes of silica glass can be produced, without the added expense and inconvenience of requiring a stabilizing agent and/or significant 20 additional processing equipment or steps. It should also be appreciated that there remains a need for a low-cost sol-gel process that can be used to produce photoblank-quality glass that is high in optical transmission and that avoids warpage. The present invention fulfills these needs. -3- WO 01/53225 PCT/USO1/02325 SUMMARY OF THE INVENTION The present invention resides in an improved sol-gel process for producing a synthetic silica glass article, in which a sol is formed having a high loading of silica without the need for a stabilizing agent and without the need for 5 significant additional processing equipment or steps such as continuous ultra sonication, centrifugation, or violent mixing of the sol. More particularly, the process includes forming a sol by introducing an aqueous colloidal suspension into an organic silicon alkoxide solution and then allowing the organic silicon alkoxide to hydrolyze into a sol containing fine aggregates of silica particles. The sol then 10 undergoes gelation, to form a wet gel, which is in turn dried and sintered to produce a dense glass article. By mixing the colloidal suspension with the alkoxide solution before hydrolysis has occurred, the suspended particles are broken down by chemical reaction. The agglomeration of the colloidal particles into particulates of excessive size, e.g., greater than about 10 microns, is avoided. 15 In a preferred application of the process, the aqueous colloidal suspension includes fumed silica powder, e.g., Aerosil OX-50 or Aerosil 200. When Aerosil OX-50 is used, its mole ratio to water preferably is in the range of about 1:4 to 1:8, and most preferably about 1:5. The sol has a silica loading of greater than about 20%, and preferably in the range of about 34 to 40%. The 20 organic silicon alkoxide solution can take the form of a tetra-alkoxy-silane, preferably tetraethoxysilane, tetramethoxysilane, tetrapropoxysilane, and mono and bi-substituents of such silanes. The mole ratio of the silicon alkoxide to water in the sol preferably is in the range of about 1:4 to 1:20, and most preferably about 1:6 to 1:10. 25 Alternatively, the aqueous colloidal suspension can incorporate titania, zirconia, erbia, alumina, or combinations thereof, or it can include colloidal -4- WO 01/53225 PCT/US01/02325 metal particles or colloidal particles of glass and/or metal having an outer coating of gold, silver, rhodia, platinum, or combinations thereof. When the process of the invention is used to make a silica photoblank substrate, the silica is initially purified before mixing with the organic silicon 5 alkoxide solution. This purification ensures that the silica photoblank substrate exhibits good optical transmission at a wavelength of 200 nm. Purification can be accomplished by heating the fumed silica powder in a chlorine-containing atmosphere and then rehydrating the chlorine-treated fumed silica powder. The chlorine-containing atmosphere can be selected from the group consisting of 10 chlorine gas (C1 2 ) and thionyl chloride (SOCl2). In another feature of the invention, useful when the process is used to make a silica photoblank substrate, the step of sintering includes placing a flat weight on the substrate, to prevent warping. The flat weight, which can be formed of a material that includes silicon carbide and/or boron nitride, preferably is 15 initially passivated in a chlorine-containing atmosphere, to minimize the transfer of impurities to the photoblank substrate. Other features and advantages of the present invention should become apparent from the following detailed description of the preferred processes, which discloses by way of example the principles of the invention. 20 BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows the optical transmission of a photoblank substrate made by the process defined in Example 8, below, as compared to the optical transmissions of two industry standard photoblank glasses, Dynasil 1100 and Suprasil 2. -5- WO 01/53225 PCT/USO1/02325 DETAILED DESCRIPTION OF THE PREFERRED PROCESSES The preferred process of the invention efficiently produces high quality silica glass articles of increased size, without the added expense and inconvenience of requiring special stabilizing agents and/or additional processing 5 equipment or steps in the sol synthesis stage. In the process, high loadings of colloidal particles, e.g., fumed silica powder, are added to an acidic water medium, to produce an aqueous slurry. This slurry then is added to a silicon alkoxide solution and the mixture is slowly stirred together, during which time the silicon alkoxide hydrolyzes and the slurry is broken down by chemical reaction. This 10 produces a hydrolyzed sol incorporating a suspension of very fine aggregates of colloidal particles. By mixing the colloidal particle slurry with the alkoxide before hydrolysis has occurred, agglomeration of the colloidal particles into particulates of excessive size is avoided without the need for special measures such as violent 15 shaking, continuous ultra-sonication, centrifugation, or adding of special stabilizing agents. The process of the invention can achieve substantially higher loadings by weight over what can be achieved using a standard pure alkoxide process. A sol that is homogeneous is considered necessary for the production of a gel having the desired strength and uniformity. Experimental data indicates 20 that the maximum particle size that can be tolerated in such a sol is about 10 microns. The process of the invention produces a sol having this characteristic, with reduced expense and process complexity. One factor that contributes to the creation of a sol having the desired high loading is the weight ratio of the colloidal particles to water. Generally, the 25 higher this ratio, the smaller the size of the colloidal aggregates in the final sol. -6- WO 01/53225 PCT/USO1/02325 This is considered counter-intuitive and contrary to previous teachings. In the past, higher fumed silica loading has been associated with massive precipitated particles, requiring centrifugation. In one particularly preferred application of the process of the 5 invention, the colloidal particles take the form of Aerosil OX-50, a common fumed silica powder. Using the process of the invention, sols having silica loadings in the range of 34 to 40% can be achieved. This is roughly double what can be achieved using a standard pure alkoxide process. In this preferred process, the mole ratio of the OX-50 powder to water in the aqueous colloidal suspension is preferably in the 10 range of about 1:4 to about 1:8, and most preferably about 1:5. The organic silicon alkoxide solution preferably comprises a tetra-alkoxy-silane such as tetraethoxysilane, tetramethoxysilane, tetrapropoxysilane, and mono- and bi substituents of such silanes. The mole ratio of the tetra-alkoxy-silane to water in the sol preferably is in the range of about 1:4 to about 1:20, and most preferably in 15 the range of about 1:6 to about 1:10. Alternatively, the aqueous colloidal suspension can incorporate other forms of fumed silica (e.g., Aerosil 200), titania, zirconia, erbia, alumina, or combinations thereof, or alternatively it can incorporate colloidal metal particles or colloidal particles of glass and/or metal having an outer coating of gold, silver, 20 rhodia, platinum, or combinations thereof The process of the invention can be better understood by reference to the following illustrative examples. -7- WO 01/53225 PCT/USO1/02325 Example 1 About 72 grams of Aerosil OX-50 silica powder was slowly added to about 108 grams of acidified de-ionized water at a pH of 2.0, to form a viscous paste. The paste's OX-50:H 2 0 mole ratio was about 1:5. The acid was selected 5 from hydrochloric acid (HCl), nitric acid (HNO 3 ), acetic acid (CH 3 COOH), sulfuric acid (H 2

SO

4 ), and combinations thereof. About 208 grams of tetra-ethoxysilane (TEOS) was added to this paste, to produce a two-phase mixture having a

TEOS:H

2 0 mole ratio of about 1:6. After about 90 minutes of slow mixing, the mixture became a white, single-phase solution, or sol. This sol was ultra-sonicated 10 for about 5 minutes and then centrifuged for about 30 minutes at 3000g. No settling was observed, and the sol easily flowed through 10-micron sieve filter paper. This shows that the process can be performed effectively without the need for such special measures such as violent shaking, continuous ultra-sonication, centrifugation, or adding of special stabilizing agents. 15 At this stage, the silica loading of the sol was about 34% silica by weight. The median particle size, as observed by diluting the sol in alcohol and measuring with a Horiba L-900 laser particle size analyzer, was 1.75 microns (see Table 1). Depending on the end application, the sol could be further concentrated, using an evaporating device, to achieve 50% by weight of silica (a volume 20 reduction of about 36%, carried out at 60* C and reduced pressure). This mixture did not settle when centrifuged at 3000g for 20 minutes. The mixture is flowable for casting and is free of any stabilizing agents. This sol can then be gelled, dried and sintered according to methods described in the prior art, to yield high quality synthetic silica glass. -8- WO 01/53225 PCT/USO1/02325 TABLE 1 Example Median particle size (microns) 1 1.75 2 6.5 5 3 25.5 4 35.0 5 250 to 1000 6 1.75 7 40 10 Example 2 About 120 grams of Aerosil OX-50 powder was slowly added to 180 grams of deionized water at a pH of 2.0, to form a viscous paste. The paste's Si:H 2 0 mole ratio was about 1:5. About 208 grams of TEOS was added to this paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:10. 15 This mixture was then slowly mixed together and, after about 90 minutes, a white, single-phase liquid was produced. This single-phase liquid was ultra-sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. No settling was observed, and the sol flowed smoothly through a filter paper of 10-micron mesh size. The median particle size, as observed by diluting the sieved sol in alcohol and 20 measuring with a Horiba L-900 laser particle size analyzer, was 6.5 microns (see Table 1). In this example, the silica loading of the mixture was about 35% silica by weight. -9- WO 01/53225 PCT/USO1/02325 Example 3 About 60 grams of Aerosil OX-50 powder was slowly added to about 180 grams of deionized water at a pH of 2.0, to form a viscous paste. The paste's Si:H 2 0 mole ratio was about 1:10. About 208 grams of TEOS was added to this 5 paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:10. This mixture was then slowly mixed together and, after about 90 minutes, a white, single-phase liquid was produced. This single-phase liquid was ultra-sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. Some settling was . observed, and the mixture failed to flow smoothly through a filter paper of 10 10 micron mesh size. This mesh filter screened out about 5 to 25% by mass of the silica. In this example, the silica loading of the mixture typically was in the range of about 18.5 to 22.0% silica by weight. The median particle size, as observed by diluting the sieved sol in alcohol and measuring with a Horiba L-900 laser particle size analyzer, was 25.5 15 microns (see Table 1). This is substantially larger than the desired median particle size of less than about 10 microns. This example reveals the detrimental effect of reducing the mole ratio of Aerosil OX-50 silica powder to water to a value less than about 1:8. Example 4 20 About 300 grams of Aerosil OX-50 powder was slowly added to about 450 grams of deionized water at a pH of 2.0, to form a viscous paste. The paste's Si:H 2 0 mole ratio was about 1:5. About 208 grams of TEOS was added to this paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:25. This mixture was then slowly mixed together and, after about 90 minutes, a 25 white, single-phase liquid was produced. This single-phase liquid was ultra -10- WO 01/53225 PCT/USO1/02325 sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. Some settling was observed, and the liquid failed to flow smoothly through a filter paper of 10 microns mesh size. The median particle size, as observed by diluting the sieved sol in 5 alcohol and measuring with a Horiba L-900 laser particle size analyzer, was 35 microns (see Table 1). This is substantially larger than the desired median particle size of less than about 10 microns. This example reveals the detrimental effect of having a TEOS:water mole ratio of less than 1:20. Example 5 10 About 24 grams of Aerosil OX-50 powder was slowly added to about 36 grams of deionized water at a pH of 2.0, to form a viscous paste. The paste's Si:H 2 0 mole ratio was about 1:5. About 208 grams of TEOS was added to this paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:2. This mixture was then slowly mixed together, but a single-phase liquid never was 15 attained. Instead, the mixture was grainy and incorporated a collection of particles about 250 to 1000 microns in diameter (see Table 1). This mixture was centrifuged for 30 minutes at 3000g, and all of the particles appeared to settle out. The mixture failed to flow smoothly through a filter paper of 10-micron mesh size. This sol was unacceptable. This example reveals the detrimental 20 effect of having a TEOS:water mole ratio greater than 1:4. -11- WO 01/53225 PCT/USO1/02325 Example 6 About 72 grams of Aerosil OX-50 powder was slowly added to about 108 grams of deionized water at a pH of 2..0, to form a viscous paste. The paste's Si:H 2 0 mole ratio was about 1:5. About 208 grams of TEOS was added to this 5 paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:6. This mixture was then slowly mixed together and, after about 90 minutes, a white, single-phase liquid was produced. This single-phase liquid was ultra-sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. The liquid flowed smoothly through a filter paper of 10-micron mesh size. 10 At this stage of the process, the sol's silica loading was about 34% silica by weight. The mixture was further concentrated by evaporating alcohol under reduced pressure at about 600 C. About 116 grams of ethanol was evaporated to provide a final silica loading of 48.5% by mass. This resultant sol smoothly flowed through filter paper with a 10-micron mesh size. The median 15 particle size, as observed by diluting the sieved sol in alcohol and measuring with a Horiba L-900 laser particle size analyzer, was 1.75 microns (see Table 1). No settling was observed when this sol was centrifuged for 30 minutes at 3000g. As in all the other examples, no stabilizing agents were used. This example shows that higher silica loadings, up to about 50% by 20 mass, can be prepared by evaporation of the chemically mixed sol. -12- WO 01/53225 PCT/USO1/02325 Example 7 About 72 grams of Aerosil OX-50 powder was slowly added to about 108 grams of de-ionized water at a pH of 2.0, to form a viscous paste. The paste's Si:H 2 0 mole ratio was about 1:5. The paste was ultra-sonicated for about 5 minutes 5 and then diluted in alcohol and slowly stirred. The median particle size of this solution was then analyzed using the Horiba L900 particle size analyzer and observed to be 40 microns (see Table 1). In this example, the agglomerate size of the Aerosil OX-50 powder in an aqueous solution was significantly larger than the acceptable size of about 10 10 microns. Comparison with Examples 2, 3 and 6 indicates than the hydrolysis reaction of these previous Examples is effective in reducing the agglomerate size of the OX-50 particles to an acceptable level, i.e., less than 10 microns in size. Example 8 In this example, the process of the invention is used to make a 15 synthetic silica photoblank substrate. For this application, the silica powder must first be purified, to remove metal particle impurities that are considered to reduce the optical transmission of the glass at the short (200 nm) wavelengths of interest. This purification is accomplished by heating the powder to 1000 C for 5 hours in a synthetic silica furnace, after which the heated powder is exposed for 1 hour to a 20 gas mixture containing chlorine and nitrogen, or containing thionyl chloride (SOCl 2 ) and nitrogen. This chlorination step removes any trace metal impurities in the powder. The powder then is rehydrated by exposing it to nitrogen gas bubbled through deionized water. -13- WO 01/53225 PCT/USO1/02325 About 72 grams of the purified OX-50 silica powder is slowly added to 108 grams of deionized water at a pH of 2.0 (Si:H 2 0 mole ratio = 1:5), to form a viscous paste. To this paste is added 208 grams of tetra-ethoxysilane (TEOS)

(TEOS:H

2 0 mole ratio = 1:6). The resulting two-phase mixture is slowly mixed 5 together and, after 90 minutes, becomes a white single-phase liquid. The single phase liquid is ultra-sonicated for 5 minutes and filtered through 10-micron mesh filter. After ultra-sonication, 12 ml of a base (ammonia-water, pH=10.0) is added. The sol then is cast into a square-shaped graphite mold having a size of about 27 cm x 27 cm. The graphite mold with the cast sol is introduced into an autoclave, 10 and the sol gels within about 2 hours after which the autoclave is sealed. The gel remains within the autoclave for 20 additional hours, to complete the gelation step. After the gelation step has been completed, the gel is topped off with a liquid composed of 88% ethanol and 12% deionized water and the topped-off gel then is aged. Specifically, the topping off liquid is pumped into the autoclave to fill 15 the autoclave chamber, and the chamber is then immediately drained, leaving the gel inside the mold topped off with the liquid. The temperature of the autoclave then is ramped up to 60*C, over a span of 6 hours, and held at this temperature for 42 hours. The topping-off liquid, including a portion that remains on the floor of the autoclave chamber following its drainage from the chamber, is in an amount 20 sufficient to maintain a saturation pressure of about 450 mm Hg at 60*C. This completes the in-situ aging step. After the aging step has been completed, the remaining topping liquid is drained and pure isopropanol is pumped into the autoclave at 450 mm Hg, while the temperature is maintained at 60*C. About 1.5 liters of isopropanol are added 25 for an autoclave volume of about 20 liters. The temperature of the autoclave then is ramped to 215*C, over a span of 20 hours, and in turn to 240'C, over a span of 22 hours. The autoclave's pressure is allowed to equilibrate (typically at about 42 -14- WO 01/53225 PCT/USO1/02325 atmospheres) at 240 C for 1 hour. The pressure inside the autoclave then is slowly released over a span of 5 hours, at a rate of about 100 mm Hg/min., while maintaining the temperature at 240*C. The autoclave then is cooled to room temperature, and the mold is removed. After drying is complete, a crack-free 5 monolithic aerogel is removed from the mold. The size of the gel after drying is about 25 mm x 25 mm x 2.5 mm. This monolithic gel then is sintered between two silicon carbide (SiC) plates according the sequence set forth in Table 2. Before being used in the sintering step, the two SiC plates are passivated by subjecting them to a chlorine 10 containing atmosphere at 900'C for 5 hours. Suitable passivating atmosphere include chlorine gas (C 2 ) or thionyl chloride (SOCl 2 ). This sintering step yields a flat glass substrate, of size 15 cm x 15 cm x 1.5 cm. TABLE 2 Step Temp. Rate Hold (OC) ( 0 C/hr) (hours) 15 1 20 500 0 2 800 50 5 3 950 50-100 1 4 1050 50 27-44 5 1350 25 6 20 6 1200 25 2 7 10 150 End The flat glass substrate then is subjected to a high-temperature treatment, to dissolve inclusions. As taught by U.S. Patent No. 5,236,483 to Miyashita, et al., this high-temperature treatment can be accomplished by heating -15- WO 01/53225 PCT/USO1/02325 the substrate to 1800'C and maintaining that temperature for 12 minutes. This provides an inclusion-free glass substrate having an optical transmission comparable to the best UV-grade synthetic silica commercially available, i.e., Suprasil*, available from Heraeus Quarzschmelze G.m.b.H. Corporation, and 5 Dynasil*, available from Dynasil Corporation of America. Figure 1 depicts the optical transmission of a glass substrate produced by the process of Example 8, as compared with the optical transmission of Suprasil and Dynasil substrates. Glass substrates made in accordance with the invention meet the Semiconductor Equipment Manufacturing Industry (SEMI) standards for 10 transmission in the UV region (-200 in). It should be appreciated from the foregoing description that the present invention provides an improved sol-gel process for producing a synthetic silica glass article, in which a sol is formed having a silica loading as high as 34 to 40%. This high loading is achieved by introducing an aqueous colloidal suspension 15 into a silicon alkoxide solution and the mixture is slowly stirred together, during which time the mixture hydrolyzes and the colloidal suspension is broken down by chemical reaction. This produces a hydrolyzed sol incorporating a suspension of very fine aggregates of colloidal particles, having particle sizes less than about 10 microns. The need for a stabilizing agent and/or for continuous ultra-sonicating, 20 centrifugation, or violently stirring of the sol is eliminated. Although the invention has been described in detail with reference only to the presently preferred process, those of ordinary skill in the art will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims. -16-

Claims (24)

1. A process for producing a synthetic silica glass article, comprising: combining an aqueous colloidal suspension with a silicon alkoxide solution to produce a mixture; 5 allowing the mixture to hydrolyze into a sol containing fine aggregates of colloidal particles and then to gel into a wet gel; drying the wet gel to produce a dry gel; and sintering the dry gel to produce a dense silica glass article.

2. A process as defined in claim 1, wherein the aqueous colloidal suspension includes fumed silica powder.

3. A process as defined in claim 2, wherein the fumed silica powder is Aerosil OX-50 powder having a mole ratio to water in the aqueous colloidal suspension in the range of about 1:4 to about 1:8.

4. A process as defined in claim 3, wherein the mole ratio of Aerosil OX-50 powder to water in the aqueous colloidal suspension is about 1:5.

5. A process as defined in claim 2, wherein: the sol has a silica loading of greater than about 20%; and the gel is substantially free of agglomerated colloidal silica particles having a particle size greater than about 10 microns.

6. A process as defined in claim 5, wherein the sol has a silica loading in the range of about 34% to about 40%. -17- WO 01/53225 PCT/USO1/02325

7. A process as defined in claim 1, wherein: the sol is free of a stabilizing agent; and the process is free of steps of violently mixing the sol and/or continuously ultra-sonicating the sol.

8. A process as defined in claim 1, wherein the aqueous colloidal suspension comprises titania, zirconia, erbia, alumina, or combinations thereof.

9. A process as defined in claim 1, wherein the aqueous colloidal suspension comprises colloidal metal particles.

10. A process as defined in claim 1, wherein the aqueous colloidal suspension comprises colloidal particles of glass and/or metal having an outer coating of gold, silver, rhodia, platinum, or combinations thereof.

11. A process as defined in claim 1, wherein the organic silicon alkoxide solution comprises a tetra-alkoxy-silane.

12. A process as defined in claim 11, wherein the mole ratio of the silicon alkoxide to water in the sol is in the range of about 1:4 to about 1:20.

13. A process as defined in claim 11, wherein the mole ratio of the silicon alkoxide to water in the sol is in the range of about 1:6 to about 1:10.

14. A process as defined in claim 11, wherein the tetra-alkoxy silane comprises tetraethoxysilane, tetramethoxysilane, tetrapropoxysilane, and mono- and bi-substituents of such silanes. -18- WO 01/53225 PCT/USO1/02325

15. A process as defined in claim 1, wherein: the synthetic silica glass article is a photoblank substrate; and the process further comprises purifying a prescribed amount of fimed silica powder, and combining the purified fumed silica powder with de-ionized 5 water to produce the aqueous colloidal suspension.

16. A process as defined in claim 15, wherein purifying is effective to purify the fumed silica powder sufficiently to ensure that the photoblank substrate exhibits high optical transmission at a wavelength of 200 nm.

17. A process as defined in claim 16, wherein the purifying includes heating the fumed silica powder in a chlorine-containing atmosphere, and rehydrating the chlorine-treated fumed silica powder.

18. A process as defined in claim 17, wherein the chlorine containing atmosphere used in the step of heating includes a gas selected from the group consisting of chlorine gas (Cl 2 ) and thionyl chloride (SOC12).

19. A process as defined in claim 1, wherein: the synthetic silica glass article is a photoblank substrate; and sintering includes placing a flat weight on the substrate, to prevent warping.

20. A process as defined in claim 19, wherein the flat weight used in sintering is formed of a material that includes silicon carbide and/or boron nitride. -19- WO 01/53225 PCT/USO1/02325

21. A process as defined in claim 20, wherein the flat weight used in sintering is produced by passivating the weight in a chlorine-containing atmosphere, to prevent transferring impurities to the photoblank substrate.

22. A process for producing a synthetic silica glass article, comprising: forming a mixture of silicon alkoxide, colloidal silica, and water, wherein the mixture is substantially free of a stabilizing agent and has a silica 5 loading greater than about 30%, and wherein forming is performed without continuous ultra-sonicating or violently mixing the mixture; allowing the mixture to hydrolyze into a sol containing fine aggregates of silica particles and substantially free of agglomerated colloidal silica particles having a particle size greater than about 10 microns, and then allowing the 10 sol to gel into a wet gel; drying the wet gel to produce a dry gel; and sintering the dry gel to produce a dense silica glass article.

23. A process as defamed in claim 22, wherein: forming includes forming an aqueous colloidal suspension, forming an organic silicon alkoxide solution, and 5 mixing the aqueous colloidal suspension with the organic silicon alkoxide solution; the aqueous colloidal suspension includes Aerosil OX-50 fumed silica powder in a mole ratio to water in the range of about 1:4 to about 1:8; the organic silicon alkoxide solution comprises a tetra-alkoxy-silane 10 in a mole ratio to water in the range of about 1:4 to about 1:20; and the sol has a silica loading in the range of about 34% to about 40%. -20- WO 01/53225 PCT/USO1/02325

24. An article made according to the process of claim 1. -21-