WO2002014579A1

WO2002014579A1 - Method for depositing a glass layer on a substrate

Info

Publication number: WO2002014579A1
Application number: PCT/US2001/018490
Authority: WO
Inventors: Heather D. Boek; Pushkar Tandon
Original assignee: Corning Incorporated
Priority date: 2000-08-10
Filing date: 2001-06-06
Publication date: 2002-02-21
Also published as: AU2001268246A1

Abstract

Flames hydrolysis deposition processes for depositing a presintered layer of glass on a substrate (56) may involve utilizing a deposition rate that is lower than conventional deposition rates, a traverse speed of the substrate (56) with respect to the flame that is lower than conventional traverse speeds, a ratio of traverse distance to substrate dimension that is less than conventional ratios of traverse distances to substrate dimension, or a combination of these techniques. The processes of the invention are useful for producing thin layers of glass for planar optical devices having improved optical characteristics, as compared with conventional techniques involving deposition of a porous soot layer which is subsequently sintered.

Description

METHOD FOR DEPOSITING A GLASS LAYER ON A SUBSTRATE

U.S. Patent application number 09/461,082, filed on December 14, 1999 is hereby incorporated by reference herein as if set forth in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to a method for depositing a thin layer of glass on a substrate.

2. Technical Background

Planar optical devices used in optical communications are generally comprised of a plurality of thin glass layers deposited on a planar substrate. The substrate is typically a substantially flat amorphous or crystalline dielectric material such as a glass or silicon wafer. The planar optical devices include a waveguide pattern generally defined by a patterned core material disposed on an undercladding layer, and an overciadding layer disposed over the patterned core material and portions of the undercladding on which the patterned core material is not located. The patterned core material is surrounded by the undercladding and overciadding, and has an index of refraction that is slightly higher than the indices of refraction of the undercladding and overciadding in order to achieve efficient propagation of light along the waveguide. In most cases, the thickness of the core material in a planar optical device is about the same order of magnitude as the wavelength of the light propagated along the waveguide, with a typical thickness for the core material being on the order of about 5 or 6 micrometers. The thickness of the overciadding layer and undercladding layer are not as critical, and are typically thicker overciadding layer and undercladding layer are not as critical, and are typically thicker than the core. The core layer may be deposited directly on a substrate, such as a silica wafer, with the substrate serving as the undercladding if it has appropriate optical characteristics. Alternatively, a separate undercladding layer may be formed on the substrate, such as in the case of a silicon wafer.

The glass layers of a planar optical device may be prepared using a suitable deposition process such as physical vapor deposition (PND) processes (e.g., sputtering or electron-beam epitaxy) or chemical vapor deposition (CVD) processes such as flame hydrolysis deposition (FHD), atmospheric pressure chemical vapor deposition (APCVD), low-pressure chemical vapor deposition (LPCVD), or plasma-enhanced chemical vapor deposition (PECND). After the core layer has been deposited on the substrate or undercladding layer, photolithographic techniques are generally employed to configure the core layer into a desired pattern. These techniques generally involve use of a photoresist layer that is deposited over the core layer and is selectively cured or decomposed by exposure to radiation through a mask to form a pattern of photoresist material that is insoluble to a photoresist solvent. The remaining soluble portions of the photoresist are removed with the solvent to expose areas of the core layer that are to be removed by an etchant such as hydrofluoric acid. After the core layer has been etched to produce a desired core pattern, the patterned photoresist is removed such as with a stripping solvent, and the overciadding is deposited over the patterned core material and exposed areas of the undercladding or substrate to complete the planar optical device. A flame hydrolysis deposition process commonly employed to deposit a core layer during the manufacture of planar optical device involves deposition of soot particles on a planar substrate to form a porous layer having a very large surface area. During the deposition processes, dopants such as POCl₃ , BC1₃ , GeCl₄ or organometallic precursors, are used to deposit along with silica to adjust the refractive index of the core layer to a value needed to effectively propagate light along the waveguides. In order to function as a waveguide core, the porous soot deposit must be sintered and densified to form a solid layer of glass. The sintering or densification step is conducted at a temperature that is sufficiently high to cause the dopants to volatilize at the surfaces of the deposit and cause the dopants in the deposit to diffuse or migrate toward the surfaces. As a result, dopant concentration gradients and compositional inhomogeneities are created in the deposit during sintering and densification. These compositional inhomogeneities cause the densified glass layer to have a nonuniform refractive index and a nonuniform expansion coefficient that causes surface roughness. The surface roughness and nonuniform refractive index result in inferior optical properties. If the layers could be deposited presintered during lay-down, the problems could be minimized or avoided.

For a deposit to be presintered, the sintering rate must be faster than the deposition rate or, in other words, the characteristic time of sintering must be less than the characteristic time of soot deposition. This has been achieved by increasing the flame temperature of the flame hydrolysis burner. Increased flame temperature can be achieved by increasing the flow of gases to the burner and changing the ratio of oxygen to methane flow such that this ratio is closer to the stoichiometric ratio. However, there is a limited process window where the flame is stable since high back-pressure and flashback are encountered close to the stoichiometric ratio. Another problem is that a given increase of the flame temperature at the burner does not provide a proportionate increase of the substrate temperature or a proportionate decrease of the characteristic time of sintering. Accordingly, raising the flame temperature at the burner is not an efficient way of producing presintered deposits during flame hydrolysis deposition, and accurate control of presintering and product consistency is difficult to achieve. These problems are especially prevalent at higher distances between the substrate and the burner, such as distances which are needed in many cases to incorporate sufficient amounts of certain types of dopants, such as GeO ₂ and P₂ O₅ .

Although increasing the rate of dopant introduction to the burner as flame hydrolysis deposition progresses to create a negative dopant concentration gradient from the exposed surface toward the substrate to compensate for volatilization of dopant during densification can provide some benefit, more uniform dopant concentrations and properties can be achieved if the deposited materials are presintered to eliminate or at least significantly reduce the need for post-deposition heat treatment to densify the deposited material. The presintered material does not require subsequent heat treatment or requires substantially less post-deposition heat treatment to densify the material. The presintered material has a significantly lower surface area and is substantially non- porous. As a result, substantially less volatilization will occur during any subsequent heat treatment if needed. As a result, a glass layer having more uniform dopant concentrations and properties (even at a microscopic level) can be achieved by presintering during flame hydrolysis deposition. Another advantage of presintering as compared with increasing dopant concentrations during deposition is that it is significantly easier to control the deposition process, since dopants are introduced to the burners at a constant rate. Presintering also provides advantages as compared with other processes which attempt to reduce volatilization and development of nonuniform dopant concentrations and nonuniform properties during post-deposition heat treatment to densify the deposited material, such as those processes in which volatilization during heat treatment is inhibited by exposure to an atmosphere containing vaporized dopant. In particular, a presintering process that eliminates or very significantly reduces the need for post-deposition heat treatment would be more desirable than a process requiring a post-deposition heat treatment step performed in a controlled atmosphere containing a vaporized dopant. Therefore, there exists a need for improved flame hydrolysis deposition processes in which presintering can be more easily and more precisely controlled to fabricate a thin glass layer of consistent quality with very uniform thickness and very uniform refractive index, which results in better device performance. SUMMARY OF THE INVENTION In accordance with an aspect of the invention, a process for depositing a layer of glass on a substrate includes arranging a flame hydrolysis burner near the substrate so that glass soot formed at the flame hydrolysis burner is directed toward its surface. Fuel, oxygen and reactive material is supplied to the flame hydrolysis burner, and is reacted in its flame to form a glass soot. The rate at which the reactive material is supplied to the flame hydrolysis burner and the time per pass of the substrate over the flame is selected so as to result in the soot being sintered substantially simultaneous with being deposited on the substrate. In accordance with another embodiment of the invention, the reactive material supply rate and the time per pass of the substrate over the flame satisfy the relationship: μά_p lσ < t_p ρ_p πd _p D² /24Ψ, where μ is the viscosity of the soot particles, σ is the surface energy of the soot particles, t is the time per pass of the substrate over the flame, W is the soot weight per pass, d is the soot particle diameter, p „ is the soot particle density, and D is the diameter of the substrate.

In accordance with another embodiment of the invention, a process for depositing a layer of glass on a substrate includes arranging a flame hydrolysis burner near the substrate so that glass soot formed at the flame hydrolysis burner is directed toward its surface, supplying fuel, oxygen and reactive material to the flame hydrolysis burner and reacting them in the flame of the burner to form a glass soot, and moving the surface of the substrate with respect to the burner so that the flame traverses the surface at a speed which results in a characteristic time of sintering of the soot which is less than or equal to the characteristic time of deposition of the soot. The invention provides improved processes for depositing glass layers on substrates, with improved uniformity of thickness and refractive index.

Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.

It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic illustration of equipment for the application of a thin glass layer onto a substrate using a flame hydrolysis deposition process in accordance with the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is directed to flame hydrolysis deposition processes for forming a thin glass layer having a very uniform thickness and a very uniform refractive index by adjusting process conditions so that the soot or very fine particles of glass produced at the burner flame form sintered agglomerates at the surface of a substrate, whereby the deposited material is a substantially solid mass of glass that cannot be further densified, or can only be slightly densified, by subsequent heat treatment. Sintering of the soot as it is deposited, or substantially simultaneous with its deposition on a substrate is is included within the meaning of "presintering" as used within this document. With a typical flame hydrolysis deposition process used for forming a thin glass layer, a porous layer of soot is deposited on a substrate. The substrate carrying the soot deposit is then placed in a furnace which, in one embodiment, may have an oxygen, helium, air or oxygen-helium atmosphere, and is heated at a temperature of about 1,400°C to sinter the soot. With the process of this invention, subsequent heat treatment to densify the deposited material may be eliminated. However, with the process of this invention subsequent heat treatment may be desirable in some cases. For example, in some cases the edges of a presintered deposit may require a subsequent densification treatment to achieve uniform thickness across the entire deposited layer. Unlike the reduction in thickness associated with densification of a soot deposit, however, the presintered deposits of the invention will exhibit a much smaller decrease in thickness when subjected to densification heat treatment conditions. Accordingly, the presintered deposits of this invention may be characterized in terms of their relative inability to be further densified upon heat treatment at conditions normally employed for densifying soot deposits.

Very thin glass layers having very uniform dopant concentrations and very uniform refractive indices may be produced by raising the temperature of the flame during flame hydrolysis deposition. While this process has been very successful, changes in flame conditions do not necessarily translate into proportionally increased temperatures of the material being deposited, especially at higher standoff distances between the substrate and the burner, such as those distances needed to incorporate sufficient amounts of certain types of dopants (e.g., GeO ₂ , P₂ O₅).

It has been determined that presintering can be achieved during flame hydrolysis deposition by adjusting the process so that the characteristic time of sintering is less than or equal to the characteristic time of deposition. The inventors have made a significant contribution to the art by developing a mathematical expression for the characteristic time of deposition. The characteristic time of sintering Ts may be expressed mathematically as

T_*- μά_p /σ , and the inventors have determined that the characteristic time of deposition To may be expressed mathematically as

where μ is the viscosity of the soot particles, σ is the surface energy of the soot particles, t_p is the time per pass of the substrate over the flame, W is the soot weight per pass, d ^ is the soot particle diameter, p _p is the soot particle density, and D is the diameter of the substrate, and K is a constant which depends on the geometry involved. In the case of a circular wafer and assuming spherical soot particles, for example, K will equal 24. The mathematical expressions for the characteristic time of sintering and the characteristic time of deposition provide both a qualitative and quantitative understanding of presintering during flame hydrolysis deposition. For example, the mathematical expressions provide an understanding of why presintering can be achieved by increasing the flame temperature during flame hydrolysis deposition. Assuming that all other parameters are substantially unchanged (e.g., time per pass, soot weight per pass, and weight per diameter), the characteristic time of deposition remains substantially unaffected by an increase in flame temperature. However, an increase in flame temperature will cause a decrease in the viscosity of the soot particles, and an increase in the surface energy of the soot particles, which results in a decrease in the characteristic time of sintering. If the increase in the flame temperature is sufficient, the characteristic time of sintering can be decreased so that it is less than or equal to the characteristic time of deposition, thereby causing presintering of the deposited material.

The mathematical expressions developed by the inventors suggest that presintered deposits can be obtained by reducing the deposition rate or by increasing the temperature of the material during deposition. These observations based on the development of the mathematical expressions for characteristic time of sintering and characteristic time of deposition provide the basis for the development of alternative techniques for achieving presintering during flame hydrolysis deposition. In accordance with an aspect of the invention, presintering during flame hydrolysis deposition is achieved by significantly reducing the rate at which reaction gases such as organometallics, SiCl₄ , TiCl₄ , etc. are introduced to the flame hydrolysis burner. This change does not significantly affect the characteristic time of sintering. However, by reducing the rate at which reactive gases are introduced to the flame hydrolysis burner, the soot weight per pass (W) is decreased, which in turn causes the characteristic time of deposition to increase. As is apparent from the mathematical expressions, the rate at which reactive materials are introduced to the burner can be decreased, whereby the characteristic time of deposition is increased so that the characteristic time of sintering is less than or equal to the characteristic time of deposition, thereby achieving presintering during flame hydrolysis deposition.

As will be explained in further detail, during flame hydrolysis deposition, a substrate mounted on a chuck is moved relative to the flame at a predetermined traverse speed along a predetermined path to achieve a uniform deposit thickness over the entire surface of the substrate. During deposition, the wafer passes over the flame of the flame hydrolysis burner a multitude of times. In accordance with another aspect of the invention, it has been determined that presintering during flame hydrolysis deposition can be achieved by decreasing the traverse speed of the chuck and substrate relative to the flame hydrolysis burner. By decreasing the traverse speed of the chuck, and the substrate carried by the chuck, with respect to the flame hydrolysis burner, the average temperature at the location of the substrate at which material is being deposited is higher than it would be at a higher traverse speed. As a result, the material being deposited is at a higher temperature during deposition, which causes the viscosity of the material being deposited to decrease and the surface energy of the material being deposited to increase, thereby decreasing the characteristic time of sintering. Therefore, by decreasing the traverse speed of the substrate relative to the flame hydrolysis burner, it is possible to decrease the characteristic time of sintering so that the characteristic time of sintering is less than or equal to the characteristic time of deposition, and presintering occurs during flame hydrolysis deposition.

During a typical flame hydrolysis deposition process for forming a glass layer on a substrate, the substrate, mounted on a chuck, makes several passes with respect to the flame of the burner. In general, for each pass the traverse distance of the substrate (e.g., a silicon wafer) with respect to the flame hydrolysis burner (typically stationary) is more than twice the diameter of the substrate. For example, for a circular silicon wafer having a diameter of 100 millimeters, a typical traverse distance is about 260 millimeters. At the end of each pass the flame is about 80 millimeters away from the first edge of the substrate. Before the next pass the position of the substrate is adjusted with respect to the flame in a direction perpendicular to the traverse direction of the substrate with respect to the flame as the flame passes across the surface of the substrate. After the adjustment, the substrate is moved toward the flame, passes over the flame and then past the flame so that a second edge of the substrate that is opposite of the front edge of the substrate is about 80 millimeters from the flame. In other words, the material from the flame hydrolysis burner is only being deposited on the wafer less than 40% of the time. The purpose of having a traverse distance that is about 260% of the diameter of the substrate surface is to ensure uniform deposition from edge to edge across the substrate surface. In accordance with another aspect of the invention, it has been determined that another technique which may be employed to achieve presintering during flame hydrolysis deposition is to reduce the traverse distance. By reducing the traverse distance, the flame from the flame hydrolysis burner is located in close proximity to the substrate for a longer period of time per pass. As a result, the average temperature of the substrate increases, which in turn causes the average temperature of the material being deposited to increase, whereby the viscosity of the material being deposited decreases and the surface energy of the material being deposited increases. As a result, the characteristic time of sintering is reduced. In some cases, it may be possible to achieve presintering during flame hydrolysis deposition by sufficiently reducing the traverse distance (i.e. , the distance that the substrate moves relative to the flame hydrolysis burner during each pass of the substrate with respect to the flame) so that the characteristic time of sintering is less than or equal to the characteristic time of deposition. However, more typically it will be desirable or necessary to reduce the traverse speed of the substrate relative to the flame hydrolysis burner while also reducing the traverse distance of the substrate relative to the flame hydrolysis burner during each pass.

An example of equipment which may be used in connection with the embodiments of the invention described above is shown in Fig. 1, which schematically illustrates a system 30 for deposition a core material on a wafer substrate. The process illustrated takes place under a suitable hood 32 with an exhaust outlet 34. A line or slot burner 36 receives a fuel such as a mixture of methane (CH<t) and an oxidant (O₂) in a gas conduit 38 with injection oxygen being supplied to burner 36 through conduit 40. A reactive material is injected into a conduit 42 from a reservoir 44 using a metered pump or other conventional equipment illustrated at 46. A carrier gas, such as nitrogen, is supplied to a heated vaporizing line 50 through conduit 48. The nitrogen carrier gas and vaporized reactive material is supplied to the burner conduit 38 through conduit 52 with the burner 36 forming a line of soot 54, which is applied to the exposed surface 56 of wafer substrate 60 held in a vacuum chuck 70. Chuck 70 has a hollow spindle 72 coupled to a lathe (not shown) for rotating the chuck in the direction indicated by arrow A in Fig. 1 during the deposition process. The chuck and burner are translated with respect to each other in x and y directions, as indicated by arrows B & C, typically by the movement of the burner over the surface of the chuck which rotates about spindle 72. A vacuum source (not shown) is coupled to spindle 70 for holding the wafer substrate 60 to chuck 70.

The chuck diameter is from 7 to 12 inches, typically made of fused quartz, or a metal such as INCONEL^®. The flame temperature of burner 36 typically is greater than 1000°C with the wafer surface temperature ranging from 600° to 900°C for sintering the soot 54 and forming a doped glass layer onto the surface of the wafer. The deposition of from about 5 to 8 μ layer of doped glass forming the core of the planar lightwave circuit requires multiple passes of the burner and chuck depending upon the speed of the traverse of the burner with respect to the wafer substrate, the spacing, and the flame temperature. The process typically takes about two hours to complete. In a conventional flame hydrolysis deposition process used for depositing a thin glass layer, a typical flow rate for the reactive gases is about 0.06 ml/min. In the conventional process, the materials are deposited in the form of a porous layer of soot that requires subsequent heat treatment to densify the deposited material. It has been determined that a conventional process can be modified by reducing the rate at which the reactive materials are introduced to the burner, hydrolyzed and deposited on the substrate, so that the characteristic time of sintering is equal to or less than the characteristic time of soot deposition, whereby presintering is achieved, and the need for subsequent heat treatment for densification is eliminated or significantly reduced. It has been found that conventional processes can typically be modified to achieve presintering by reducing the rate at which reactive materials are introduced to the burner and deposited on the substrate to about 0.04 mL/min or less, with a preferred rate being from about 0.03 to about 0.035 mL/min.

With conventional flame hydrolysis processes used for producing a thin layer of glass on a substrate, the traverse speed is typically about 5 cm/sec. In such processes, the material is deposited in the form of fine soot particles which form a porous layer that must be subjected to a post-deposition heat treatment to densify the layer. It has been found that such processes can be modified to achieve presintering of the material being deposited so that subsequent heat treatment for densification is eliminated or substantially reduced, by reducing the traverse speed. More specifically, it has been found that presintering can be achieved by modifying such conventional processes by decreasing the traverse speed to about 2 cm/sec or less, and more preferably about 1 cm/sec or less. In most conventional processes for producing a thin glass film on a substrate using flame hydrolysis deposition, the traverse distance of the substrate with respect to the flame is about 260% of the dimension of the substrate in the direction that the substrate is being traversed. In these conventional processes, the material is deposited in the form of fine soot particles which form a porous layer that must be subjected to subsequent heat treatment to densify the deposited material. It has been found that such conventional processes can be modified to achieve presintering by decreasing the traverse distance so that the characteristic time of sintering is less than the characteristic time of deposition. More specifically, it has been determined that presintering may be achieved by reducing the traverse distance to less than 150% of the traverse dimension of the substrate, and more preferably less than about 130% of the traverse dimension of the substrate.

The above techniques may be employed singularly or in any combination to achieve presintering of the deposited material. For example, presintering may be achieved by concurrently reducing the rate at which reaction gases are introduced to the flame hydrolysis burner and reducing the traverse speed of the substitute with respect to the burner as described above, by concurrently reducing the rate at which reaction gases are introduced to the flame hydrolysis burner and reducing the traverse distance of the substrate with respect to the burner per pass as described above, by concurrently reducing the traverse speed and the traverse distance, or by concurrently reducing the rate at which reaction gases are introduced to the burner while reducing the traverse speed and traverse distance.

The presintered glass layers of this invention exhibit significantly more uniform thickness and uniform refraction index than glass layers prepared using a conventional process in which a porous soot deposit is subsequently sintered. Micro-scale roughness, was also much better for these cores than for cores manufactured by typical conventional commercial processes. This micro-scale roughness is closely related to chemical zonation in the core and the device performance. In the preconsolidated core, there is less interfacial area for germania to migrate or diffuse across, hence the glass has less chemical zonation. A comparison between sooty and presintered cores, including average and maximum roughness as measured by AFM for unetched cores, is shown in the following table:

It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.

Claims

The invention claimed is:

1. A process for depositing a layer of glass on a substrate, comprising: arranging a flame hydrolysis burner near a substrate so that glass soot formed at the flame hydrolysis burner is directed toward a surface of the substrate; supplying fuel, oxygen and reactive material to the flame hydrolysis burner; reacting the fuel, oxygen and reactive material in a flame of the flame hydrolysis burner to form a glass soot; and selecting a rate at which the reactive material is supplied to the flame hydrolysis burner and a time per pass of the substrate over the flame which results in the soot being sintered substantially simultaneous with being deposited on the substrate.

2. The process of claim 1, wherein the time per pass of the substrate over the flame is selected so as to result in a characteristic time of sintering which is less than or equal to the characteristic time of deposition.

3. The process of claim 2, where said surface of the substrate is moved with respect to the flame of the flame hydrolysis burner so that the flame traverses the surface of the substrate at a speed of about 2 cm/sec or less.

4. The process of claim 1, wherein said surface of the substrate is moved with respect to the flame of the flame hydrolysis burner so that the flame passes over the surface of the substrate and past an edge of the surface of the substrate a plurality of times, the substrate having a dimension in a direction in which the substrate is moved with respect to the flame, the distance that the substrate is moved with respect to the flame or each pass being about 150% or less of said dimension of the substrate.

5. The process of claim 2 wherein said surface of the substrate is moved with respect to the flame of the flame hydrolysis burner so that the flame passes over the surface of the substrate and past an edge of the surface of the substrate a plurality of times, the substrate having a dimension in a direction in which the substrate is moved with respect to the flame, the distance that the substrate is moved with respect to the flame for each pass being about 150% or less of said dimension of the substrate.

6. The process of claim 3, wherein the rate at which the reactive material is supplied to the flame hydrolysis burner is from about 0.03 mL/min to about 0.035 mL/min.

7. The process of claim 2, wherein the flame traverses the surface of the substrate at a speed of about 1 cm/sec or less.

8. The process of claim 3, wherein the distance that the substrate is moved with respect to the flame for each pass is about 150% or less of said dimension of the substrate.

9. The process of claim 4, wherein the distance that the substrate is moved with respect to the flame for each pass is about 130% or less of said dimension of the substrate.

10. The process of claim 8, wherein the rate at which the reactive material is supplied to the flame hydrolysis burner is from about 0.03 mL/min to about 0.035 mL/min.

11. The process of claim 1 wherein the reactive material supply rate and the time per pass of the substrate over the flame satisfy the relationship μd_p lσ < t_{p P p} τtd _p Ω² l2W, where μ is the viscosity of the soot particles, σ is the surface energy of the soot particles, t is the time per pass of the substrate over the flame, W is the soot weight per pass, d_p is the soot particle diameter, p _p is the soot particle density, and D is the diameter of the substrate.

12. A process for depositing a layer of glass on a substrate, comprising: arranging a flame hydrolysis burner near a substrate so that glass soot formed at the flame hydrolysis burner is directed toward a surface of the substrate; supplying fuel, oxygen and reactive material to the flame hydrolysis burner; reacting the fuel, oxygen and reactive material in a flame of the flame hydrolysis burner to form a glass soot; and moving said surface of the substrate with respect to the flame hydrolysis burner so that the flame traverses said surface of the substrate at a speed which results in a characteristic time of sintering of the soot which is less than or equal to the characteristic time of deposition of the soot.

13. The process of claim 12, wherein said surface of the substrate is moved with respect to the flame of the flame hydrolysis burner so that the flame passes over the surface of the substrate and past an edge of the surface of the substrate a plurality of times, the substrate having a dimension in a direction in which the substrate is moved with respect to the flame, the distance that the substrate is moved with respect to the flame for each pass being about 150% or less of said dimension of the substrate.

14. The process of claim 12, wherein the flame traverses the surface of the substrate at a speed of about 1 cm/sec or less.

15. The process of claim 13, wherein the flame traverses the surface of the substrate at a speed of about 1 cm/sec or less.

16. The process of claim 13, wherein the distance that the substrate is moved with respect to the flame for each pass is about 130% or less of said dimension of the substrate.

17. The process of claim 16, wherein the flame traverses the surface of the substrate at a speed of about 1 cm/sec or less.