Method for preparing titania-silica glass with uniform hydroxyl concentration

Abstract

A process for forming a titania-silica glass body, the process comprising exposing a titania-doped silica soot body to a first heat treatment by heating the body to a first temperature T1 of about 800°C to about 1100°C for a first duration t1 calculated using the following formula: Equation (I): c is the characteristic length of the body (cm), and α is the thermal diffusivity of the body (cm 2 / sec). The process further includes exposing the body to a second heat treatment by heating the body to a second temperature T2 of about 1050°C to about 1250°C, wherein after the second heat treatment, the peak-to-valley difference in hydroxyl concentration among multiple segments of the body is about 70 ppm or less. [Equation 1] TIFF2026502008000008.tif9114

Description

[Technical Field]

[0001] This application claims the benefit of priority to Dutch Patent Application No. 2034196, filed February 21, 2023, which claims the benefit of priority to U.S. Provisional Application No. 63 / 440,718, filed January 24, 2023, the contents of which are relied upon and incorporated herein by reference in their entirety.

[0002] The present disclosure is directed to silica-titania glasses having uniform hydroxyl concentrations. The glass articles may be suitable for use in extreme ultraviolet lithography applications. [Background technology]

[0003] Extreme ultraviolet (EUV) lithography uses optical elements to illuminate, project, and shrink pattern images to form integrated circuit patterns. The use of extreme ultraviolet radiation is beneficial in that smaller integrated circuit features can be achieved. Optical elements for EUV lithography are currently made from low thermal expansion glasses, such as silica-titania glasses. This glass is traditionally made by a flame hydrolysis process in which high-purity precursors are injected into a flame to form glass particles, which are then deposited on a glass body.

[0004] In EUV lithography systems, this glass is typically coated with a reflective surface to form a reflective mirror. Furthermore, the glass must be able to meet the strict thermal expansion requirements of the EUV lithography system. Specifically, the glass must be able to maintain its surface shape (known as "morphology") when exposed to temperature changes in the system. Temperature-stable glass is required to avoid any induced distortions in the wavefront characteristics of EUV projection optics. Summary of the Invention

[0005] Embodiments of the present disclosure include methods of manufacturing glass bodies that can advantageously maintain their morphology during operation of an EUV lithography system. Thus, according to embodiments of the present disclosure, the glass bodies reduce or prevent any wavefront characteristics of EUV projection optics.

[0006] According to a first embodiment, the present disclosure is directed to a process for forming a titania-silica glass body, the process comprising: providing a titania-doped silica soot body having a structure represented by the following formula:

number

[0007] According to a second embodiment, the present disclosure is directed to a process for forming a titania-silica glass body, the process comprising: providing a titania-doped silica soot body having a structure represented by the following formula:

number

[0008] While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter herein, it is believed that the description will be better understood from the following specification when taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a flowchart of a process for forming a glass body according to embodiments disclosed herein. [Figure 2A] FIG. 2 is a schematic diagram of a system for producing loose soot particles in the process of FIG. 1 according to embodiments disclosed herein. [Figure 2B] 2 shows a molded body obtained by the process of FIG. 1. [Figure 2C] 2 shows a molded body obtained by the process of FIG. 1. [Figure 3] 2 is a flowchart of the fixing process of the process of FIG. 1. [Figure 4A] 4 is a plot of temperature versus time for an exemplary bonding process according to the process of FIG. 3. [Figure 4B] 1 is a plot of temperature versus time for a traditional bonding process. [Figure 4C] 4C is a plot of hydroxyl concentration versus radial position for both the exemplary bonding process of FIG. 4A and the traditional bonding process of FIG. 4B. [Figure 4D] 4 is a plot of temperature versus time for another exemplary bonding process according to the process of FIG. 3. [Figure 5A] 1 illustrates an exemplary glass body according to embodiments disclosed herein. [Figure 5B] FIG. 5B shows a cross-sectional view of a sample of the glass body of FIG. 5A according to embodiments disclosed herein. [Figure 5C] FIG. 5B illustrates another cross-sectional view of the glass body sample of FIG. 5A having an outer peripheral edge according to embodiments disclosed herein. [Figure 6] 1 is a plot of hydroxyl concentration along a sample of a glass body according to embodiments disclosed herein. DETAILED DESCRIPTION OF THE INVENTION

[0010] As used herein, "ppm" means parts per million by weight.

[0011] As used herein, "atm" means atmosphere.

[0012] FIG. 1 depicts a process 100 for producing a titania (TiO) glass body suitable for use in EUV lithography applications. As discussed further below, the produced glass has a uniform hydroxyl (OH) concentration throughout the body. Because thermal expansion characteristics depend on hydroxyl concentration, uniform distribution of hydroxyls is necessary to achieve uniform thermal expansion throughout the glass body. Having uniform thermal expansion throughout the glass body prevents the glass body from expanding when exposed to different temperature environments, which is beneficial, for example, in lithography applications. Thus, the bodies disclosed herein are suitable for use, for example, in EUV lithography applications. The glass may be ultra-low expansion glass (ULE® glass) manufactured by Corning Incorporated.

[0013] EUV lithography technology relies on an optical projection system to expose a reflective photomask to EUV light, which then directs the light reflected from the photomask onto a thin photosensitive layer deposited on the surface of a semiconductor wafer. This technique is commonly used in the manufacturing process of semiconductor devices. EUV lithography systems operate at a wavelength of approximately 13.5 nm. This extremely short wavelength poses several challenges to the design of EUV systems. For example, the reflective coating on the mirror body in an EUV system cannot reflect all light at such low wavelengths. Approximately 30% of the light is absorbed by the reflective coating rather than reflected. The absorbed light generates unwanted heat in the mirror body, causing it to thermally expand or contract. Such changes in the mirror body can then deform the reflective coating on the mirror body, leading to distortion of the wavefront of the reflected light. The wavefront distortion can lead to reduced resolution of the EUV system and errors in the pattern formed on the photosensitive layer.

[0014] Therefore, mirror bodies must be able to maintain their shape and form even when exposed to the demanding thermal loads of EUV systems. Silica-titania glasses, such as ULE® glass, are currently the material of choice for mirror bodies in EUV systems.

[0015] It has also been recently shown that a higher level of uniformity in silica-titania glasses in EUV systems reduces any expansion or contraction of the glass. More specifically, with such a higher level of uniformity, the glass maintains its overall shape when exposed to temperature changes in EUV systems. Embodiments of the present disclosure are directed to producing glass bodies with such uniformity. Specifically, embodiments of the present disclosure are directed to producing glass bodies with uniform hydroxyl concentrations.

[0016] Referring to FIG. 1 , step 110 of process 100 involves generating soot particles. More specifically, step 110 involves forming the soot particles as loose soot particles and then collecting the loose soot particles. FIG. 2 depicts a schematic diagram of a system 200 for generating loose soot particles using a chemical vapor deposition process. As shown in FIG. 2 , system 200 includes a first reservoir 220 containing a silica precursor 224 and a second reservoir 230 containing a titania precursor 234. First reservoir 220 includes an inlet 222 at or near the base of the reservoir for introducing a carrier gas, such as nitrogen. The carrier gas forms a vapor stream with silica precursor 224. Similarly, second reservoir 230 includes an inlet 232 at or near the base of the reservoir for introducing a carrier gas, such as nitrogen. The carrier gas in the second reservoir 230 forms a vapor stream having a titania precursor 234 .

[0017] The silica precursor 224 can include, for example, SiCl4 and / or octamethylcyclotetrasiloxane (OMCTS). The titania precursor 234 can include, for example, TiCl4, titanium isopropoxide (TPT), titanium tetraisopropoxide (TTIP), and / or tetraisopropyl titanate (TIPT).

[0018] To prevent saturation of the silica vapor and titania vapor streams, bypass flows of carrier gas are also introduced into system 200 at inlets 226 and 236. The silica vapor stream then passes through distribution system 242 and into manifold 248, and the titania vapor stream passes through distribution system 244 and into manifold 248.

[0019] The silica vapor stream and the titania vapor stream are then mixed in manifold 248 to form a two-stream mixture. As further shown in FIG. 2 , the two-stream mixture flows into furnace 250. More specifically, the two-stream mixture passes through fume line 252 and enters burner 254, which is mounted on the top of furnace 250. The two streams then meet with a fuel / oxygen mixture in burner 254 to combust and oxidize the mixture. The fuel may be natural gas. The oxidation and combustion of the mixture forms loose soot particles 260, which are cooled and directed into collection chamber 264. The soot particles 260 include silicon dioxide and titanium dioxide. More specifically, the silicon dioxide and titanium dioxide in the particles mix at the atomic level to form Si-O-Ti bonds.

[0020] In some embodiments, the soot particles 260 are directed upward through a tube 270 rather than downward into the collection chamber 264. The tube 270 may be a quartz tube that carries the soot particles 260 in the vapor stream to one or more filter bags 272. The soot particles 260 are removed from the vapor stream by the filter bags 272 and then deposited in one or more collection chambers 264'. For example, the soot particles 260 fall downward from the filter bags 272 and into the collection chamber 264'. To prevent excessive accumulation of soot particles 260 on the bag, N pulses may be periodically applied to the filter bags 272. In some embodiments, the collection chamber 264' is a stainless steel hopper. The soot particles 260 can then be further collected from the collection chamber 264' and deposited in a barrel, where the soot particles 260 can be stored until further use.

[0021] The soot particles 260 produced are spherical with a substantially uniform distribution of SiO2 and TiO2 within the particle. In addition to SiO2 and TiO2, the composition of the soot particles 260 may also include CO and CO2, which may be incorporated into the particle due to the fuel in the burner 254. The size of each soot particle 260 may vary depending on the conditions of the burner 254, but generally, the soot particles 260 have an average diameter of about 20 nm to about 500 nm, or 50 nm to about 400 nm, or 60 nm to about 300 nm, or 50 nm to about 100 nm.

[0022] The soot particles 260 may cool to about 200°C or less, or about 175°C or less, or about 150°C or less, or about 125°C or less, or about 100°C or less, or about 75°C or less, or about 50°C or less, or about 25°C or less, or about 20°C or less before reaching the collection chamber 264, 264'.

[0023] Referring again to FIG. 1, in step 120 of process 100, soot particles 60 are removed from collection chamber 264, 264′ and deposited in the mold to a concentration of approximately 0.50 g / cm 3 or more, or approximately 0.55 g / cm 3 or more, or approximately 0.60 g / cm 3 or more, or approximately 0.65 g / cm 3 or more, or approximately 0.70 g / cm 3 or more, or approximately 0.75 g / cm 3 or more, or approximately 0.80 g / cm 3 or more, or approximately 0.85 g / cm 3 Additionally or alternatively, the compact may have a density of about 1.20 g / cm or greater. 3 or less, or about 1.15 g / cm 3 or less, or about 1.10 g / cm 3 or less, or about 1.00 g / cm 3 or less, or about 0.95 g / cm 3 or less, or about 0.90 g / cm 3 or less, or about 0.85 g / cm 3 or less, or about 0.80 g / cm 3or less, or about 0.75 g / cm 3 or less, or about 0.70 g / cm 3 In an embodiment, the compact has a density of about 0.50 g / cm 3 ~Approx. 1.20g / cm 3 , or approximately 0.60 g / cm 3 ~Approx. 1.10g / cm 3 , or approximately 0.80 g / cm 3 ~Approx. 1.00g / cm 3 , or approximately 0.90 g / cm 3 ~Approx. 0.95g / cm 3 The compact has a density of about 5% or less, or about 4% or less, or about 3% or less, or about 2% or less, or about 1% or less, or about 0.75% or less, or about 0.50% or less, or about 0.25% or less, or about 0.20% or less, or about 0.15% or less, or about 0.10% or less, or about 0.05% or less, or about 0.02% or less, or about 0.01% or less, or about 0.00% from the average density across the body. While FIG. 2B shows an exemplary cylindrical compact and FIG. 2C shows an exemplary rectangular compact, the compact may include other shapes beyond those specifically depicted herein. As shown in FIGS. 2B and 2C, the compact has a length L and a height H. Note that the length L is also the diameter of the cylinder in FIG. 2B.

[0024] In embodiments, the body length L can be from about 0.02 m to about 1.30 m, or from 0.04 m to about 1.20 m, or from 0.06 m to about 1.00 m, or from 0.08 m to about 0.80 m, or from 0.10 m to about 0.60 m, or from 0.20 m to about 0.40 m. Furthermore, in some embodiments, the body height H is about 0.05 m to about 0.50 m, or about 0.06 m to about 0.40 m, or about 0.08 m to about 0.20 m, or about 0.10 m to about 0.20 m, or about 0.25 m to about 0.50 m, or about 0.25 m to about 0.40 m, or about 0.25 m to about 0.30 m, or about 0.20 m to about 0.50 m, or about 0.20 m to about 0.40 m, or about 0.20 m to about 0.30 m. However, it should be noted that the body length L and height H may vary and are not limited by the embodiments disclosed herein. It should also be noted that in some embodiments, the body length L is greater than the height H, while in other embodiments, the height H is greater than the length L.

[0025] Referring again to process 100, the compact is then bonded in step 130. After bonding, the body is melted and then annealed in step 140 to relieve any internal stresses within the body. The relieved internal stresses allow for better cutting and machining of the body, such as slicing the body into multiple slices. In some embodiments, the body is annealed for a duration of about 100 hours or more, or about 200 hours or more, or about 250 hours or more. The maximum annealing temperature can be about 750°C to about 1200°C, or about 800°C to about 1100°C, or about 900°C to about 1000°C. Once the annealing step is complete, the body is ready for slicing.

[0026] In a traditional bonding process (during step 130 of process 100), the compact is placed in a bonding furnace and heated. The traditional heating process involves heating the compact to a temperature of about 900°C to about 1100°C, and then slowly heating the body to a temperature of about 1100°C to about 1300°C at a heating rate of about 1°C / hr to about 36°C / hr. However, during these heating steps in such a traditional bonding process, the body may experience temperature differences between the inner and outer portions of the body. These temperature differences can cause hydroxyl concentration differences throughout the body, thus producing a body with a non-uniform hydroxyl concentration.

[0027] An embodiment of the present disclosure replaces the heating step of a traditional bonding process with process 300 of Figure 3. As shown in Figure 3, according to an embodiment of the present disclosure, bonding of a compact (step 130 of process 100) involves exposing the compact to a first heat treatment step and a second heat treatment step with a ramp-up step between these steps. More specifically, step 310 of Figure 3 involves a first heat treatment step during which the compact is held at a first temperature T1 of about 800°C to about 1100°C, or about 825°C to about 1075°C, or about 850°C to about 1050°C, or about 875°C to about 1025°C, or about 900°C to about 1000°C, or about 925°C to about 975°C, or about 950°C to about 1000°C. The body can be maintained at a first temperature T1 such that the first temperature T1 is constant throughout the first heat treatment, with any temperature varying by no more than about 5°C, or no more than about 4°C, or no more than about 3°C, or no more than about 2°C, or no more than about 1°C, or no more than about 0.5°C from the average temperature during the first heat treatment. Due to such a constant temperature, the first heat treatment can also be referred to as an isothermal hold.

[0028] During the first heat treatment, the body is subjected to a thermal treatment of the following formula:

number

[0029] In some exemplary embodiments, the first duration t1 is from about 30 minutes to about 35 hours, or from about 1 hour to about 30 hours, or from about 1.5 hours to about 27 hours, or from about 2 hours to about 25 hours, or from about 4 hours to about 22 hours, or from about 6 hours to about 20 hours, or from about 8 hours to about 18 hours, or from about 10 hours to about 15 hours, or from about 12 hours to about 16 hours, or from about 30 minutes to about 10 hours, or from about 1 hour to about 8 hours, or from about 15 hours to about 30 hours. In some exemplary embodiments, these durations are for a body having a length L of 0.25 m and a height H of 0.25 m.

[0030] Heating of the body during the first heat treatment step can be in an inert environment in the presence of an inert gas. "Inert gas" means a gas that does not chemically react with the body. In some embodiments, the body can be exposed to a vapor doping process during the first heat treatment step, as discussed further below.

[0031] Without wishing to be bound by theory, it is believed that the first heat treatment (step 310) disclosed herein advantageously generates uniform pressure and temperature in the body by allowing gases to be purged and removed from the body, ultimately helping to achieve the uniform hydroxyl concentration disclosed herein. Additionally, the first heat treatment also sinters the glass body, albeit to a lesser extent. However, it should be noted that the majority of the glass sintering occurs during the second heat treatment step.

[0032] 3 , step 320 of process 300 includes a ramp-up step during which the temperature is increased from a first temperature T1 to a second temperature T2. That is, the ramp-up step transitions from a first thermal treatment step to a second thermal treatment step. The increase in temperature from T1 to T2 during the ramp-up step can be at a rate of about 20° C. / hour or greater, or about 25° C. / hour or greater, or about 30° C. / hour or greater, or about 35° C. / hour or greater, or about 40° C. / hour or greater, or about 45° C. / hour or greater, or about 50° C. / hour or greater, or about 55° C. / hour or greater, or about 60° C. / hour or greater, or about 65° C. / hour or greater, or about 70° C. / hour or greater, or about 75° C. / hour or greater, or about 80° C. / hour or greater. In some embodiments, the increase in temperature from T1 to T2 during the ramp-up step can be at a rate of about 20°C / hr to about 120°C / hr, or about 30°C / hr to about 100°C / hr, or about 40°C / hr to about 80°C / hr, or about 50°C / hr to about 100°C / hr, or about 50°C / hr to about 80°C / hr. The total duration of the ramp-up step can be about 0.5 hours to 18 hours, or about 1 hour to 16 hours, or about 1.5 hours to about 14 hours, or about 2 hours to about 12 hours, or about 2.5 hours to about 12 hours, or about 3 hours to about 10 hours, or about 4.5 hours to about 8 hours, or about 5 hours to about 6 hours.

[0033] Step 330 of process 300 includes a second heat treatment during which the body is held at a second temperature T2 of about 1050° C. to about 1250° C., or about 1100° C. to about 1200° C., or about 1125° C. to about 1200° C., or about 1150° C. to about 1200° C., or about 1100° C. to about 1150° C., or about 1125° C. to about 1175° C., or about 1125° C. to 1150° C. The body can be held at the second temperature T2 such that the second temperature T2 is constant throughout the entire second heat treatment, with no temperature varying by more than about 5° C., or more than about 4° C., or more than about 3° C., or more than about 2° C., or more than about 1° C., or more than about 0.5° C. from the average temperature during the second heat treatment. Due to such a constant temperature, the second heat treatment may also be referred to as an isothermal hold.

[0034] The body can be held at the second temperature T2 for a second duration t2 of about 6 hours or more, or about 10 hours or more, or about 1 day or more, or about 2 days or more, or about 3 days or more, or about 4 days or more, or about 5 days or more, or about 6 days or more, or about 7 days or more, or about 8 days or more, or about 9 days or more, or about 10 days or more. In some embodiments, the maximum duration can be about 20 days or less, or about 18 days or less, or about 16 days or less, or about 14 days or less, or about 12 days or less, or about 10 days or less, or about 8 days or less, or about 6 days or less, or about 4 days or less, or about 2 days or less, or about 1 day or less.

[0035] In an embodiment, the second duration t2 is greater than the characteristic sintering time, which is the time required to fully sinter the body during consolidation in step 130 of process 100. The characteristic sintering time is calculated using the following formula:

number

[0036] After the second heat treatment is complete, the body is a fully dense titania-doped silica optical glass element, substantially free of any inclusions or voids. Therefore, duration t2 must be long enough to completely cement the titania-doped silica soot compact into a fully dense titania-doped silica glass optical element.

[0037] Heating of the body during the second heat treatment step can be in an inert environment in the presence of an inert gas. In some embodiments, the body can be exposed to a vapor doping process during the second heat treatment step, as discussed further below.

[0038] Without wishing to be bound by theory, it is believed that the majority of the glass sintering to fully bond the body occurs during the second heat treatment (step 330) disclosed herein. Because the second heat treatment is performed at a constant temperature, the temperature of the body is uniform throughout the body during glass sintering. Thus, for example, the inner and outer portions of the body are at the same temperature during glass sintering. This advantageously results in the uniform hydroxyl concentration disclosed herein.

[0039] After the first heat treatment step is completed, the body has a uniform and consistent hydroxyl concentration throughout the body. However, temperature changes during the ramp-up step may cause the hydroxyl concentration to become non-uniform. This uniform and consistent hydroxyl concentration throughout the body returns after the second heat treatment step. However, it should be noted that without the initial uniformity from the first heat treatment step, the body would not have this uniform and consistent hydroxyl concentration after the second heat treatment step. In other words, including the first heat treatment helps achieve a uniform and consistent final hydroxyl concentration after the second heat treatment step.

[0040] The first heat treatment step, the second heat treatment step, and / or the ramp-up step may be performed under vacuum pressure, where the body is not actively doped with hydroxyl. These embodiments may be referred to as "no OH doping" because the heat treatment step and / or the ramp-up step are performed in an atmosphere that is free (or essentially free) of water and steam. In contrast, in other embodiments, the first heat treatment step, the second heat treatment step, and / or the ramp-up step may be performed in the presence of steam, also referred to herein as steam doping. As discussed further below, the body is doped with hydroxyl during steam doping, resulting in the resulting glass body having a relatively high hydroxyl concentration. Conversely, a "no OH doping" process produces a glass body with a relatively low hydroxyl concentration.

[0041] In yet other embodiments, the shaped precursor may be actively dried during the first heat treatment step, the second heat treatment step, and / or the ramp-up step. During such active drying embodiments, the body is exposed to a desiccant while being heated. In embodiments, the desiccant may be a halide, such as chlorine and / or fluorine, or carbon monoxide. Actively drying the body during the heat treatment steps and / or the ramp-up step produces a glass body with a reduced hydroxyl concentration, even lower than that of the "no OH doping" process.

[0042] It is further noted that in the "no OH doping" and vapor doping processes, the precursor may not be exposed to a halide agent. Thus, in these embodiments, the final glass body may be halide-free (i.e., may contain less than 100 ppm halide) such that the glass body is halide-free.

[0043] As discussed above, the body may be exposed to a vapor doping process during the first heat treatment step, the second heat treatment step, and / or the ramp-up step, which allows the body to be bonded in the presence of vapor. This increases the hydroxyl concentration in the resulting glass body. The vapor doping process involves exposing the body to a vapor-containing atmosphere in a bonding furnace to load the body with hydroxyl groups. The vapor-containing atmosphere may contain only vapor or vapor in combination with an inert gas. The partial pressure of the vapor may be about 1.0 atm or less, or about 0.50 atm or less, or about 0.40 atm or less, or about 0.30 atm or less, or about 0.20 atm or less, or about 0.10 atm or less, or about 0.05 atm or less. In embodiments, the partial pressure of the vapor is greater than about 0.00 atm, or greater than or equal to about 0.01 atm, or greater than or equal to about 0.05 atm, or greater than or equal to about 0.08 atm, or greater than or equal to about 1.0 atm, or greater than about 0.00 atm to about 1.0 atm, or about 0.05 atm to about 1.0 atm, or about 0.25 atm to about 0.8 atm, or about 0.50 atm to about 0.75 atm, or greater than about 0.00 atm to about 0.25 atm, or greater than about 0.00 atm to about 0.20 atm. In embodiments where the body is exposed to a "no OH doping" process (as opposed to a vapor doping process), it is noted that the partial pressure of the vapor in the consolidation furnace is 0.0 atm (note that while 0.0 atm can only be achieved in theory, in practice, due to practical limitations, partial pressures of about 0.02 atm or less can be achieved for an "OH doping only" process). During the vapor doping process, the vapor partial pressure must be constant in the consolidation furnace, and any pressure difference must be no more than ±2% of the absolute pressure in the furnace. It is also noted that longer vapor doping exposure times result in higher hydroxyl concentrations in the body.

[0044] In some embodiments, the body is exposed to a steam-containing environment during the first heat treatment step, the ramp-up step, and the second heat treatment step. In other embodiments, the body is exposed to a steam-containing environment during the second heat treatment step (but not during the first heat treatment step and the ramp-up step). In still other embodiments, the body is exposed to a steam-containing environment during the first heat treatment step and the ramp-up step, and during the hold time t of the second heat treatment step. hold and then exposed to a steam-free environment for the remaining duration of the second heat treatment step (t rem ) is followed by exposure of the body to the vapor-laden environment for a retention time t hold is equal to or greater than the first duration t1 of the first heat treatment step. For example, the holding time t hold is about 30 minutes to about 35 hours, or about 1 hour to about 30 hours, or about 1.5 hours to about 27 hours, or about 2 hours to about 25 hours, or about 4 hours to about 22 hours, or about 6 hours to about 20 hours, or about 8 hours to about 18 hours, or about 10 hours to about 15 hours, or about 12 hours to about 16 hours, or about 30 minutes to about 10 hours, or about 1 hour to about 8 hours, or about 15 hours to about 30 hours. rem The steam duration is the total fixation time (second duration t2) minus t hold In some exemplary embodiments, t rem The vapor duration is about 25 hours to about 10 days, or about 30 hours to about 8 days, or about 32 hours to about 6 days, or about 35 hours to about 4 days, or about 30 hours to about 2 days.

[0045] The use of steam in the vapor doping process disclosed above offers many advantages, including a high hydroxyl concentration in the glass, which reduces viscosity, promotes a low fictive temperature, and avoids seed formation in the glass.

[0046] Additional embodiments of the above-disclosed vapor doping process are disclosed in US Pat. No. 9,580,350, which is incorporated by reference in its entirety.

[0047] In embodiments, the vapor doping process begins during the second heat treatment step, thus exposing the body to a vapor-containing atmosphere during the second heat treatment step. Additionally, in some embodiments, the vapor doping process begins during the second heat treatment step, but only after equilibrium is reached.

[0048] In some embodiments, the bonding process includes the vapor doping process disclosed above (during at least one of the first heat treatment step, the second heat treatment step, and the ramp-up step), and during the second heat treatment step, the body is held at a temperature T2 of about 1100° C. to about 1150° C., or about 1125° C. In other embodiments, the bonding process includes the "no OH doping" process disclosed above, and during the second heat treatment step, the body is held at a temperature T2 of about 1125° C. to about 1175° C., or about 1150° C.

[0049] Upon completion of the bonding process, which includes the steps of process 300, the compact is now converted into a bonded glass, which is then annealed as discussed above with reference to process 100 of FIG.

[0050] 4A shows a plot of temperature versus time for an exemplary bonding process including a first heat treatment step and a second heat treatment step according to embodiments disclosed herein. In this example, the first heat treatment step is at a temperature T1 of 1030° C., and the second heat treatment step is at a temperature T2 of 1125° C. Furthermore, in this illustrative example, the ramp-up step between the first and second heat treatment steps is at a heating rate of 50° C. / hour. It should also be noted that this exemplary bonding process includes the vapor doping process described above.

[0051] In comparison, Figure 4B shows a plot of temperature versus time for a traditional bonding process. As discussed above, this traditional process involves slowly heating the body at a heating rate of about 1°C / hr to about 36°C / hr. This traditional process does not include an isothermal hold or ramp-up step for both the first and second heat treatment steps.

[0052] FIG. 4C shows the modeled hydroxyl concentrations of glass samples produced using the exemplary bonding process of FIG. 4A compared to the traditional bonding process of FIG. 4B. The hydroxyl concentrations of the glass samples were measured over a 0.127 m × 0.127 m area of ​​each sample. As shown in FIG. 4C, the glass samples produced by the exemplary bonding process had a more uniform hydroxyl concentration along the measured radial length of the sample. In comparison, the glass samples produced using the traditional bonding process had a much lower hydroxyl concentration at the radially outer edge than at the center of the measured portion of the sample. The process for measuring hydroxyl concentration is discussed further below.

[0053] 4D shows another plot of an exemplary bonding process including a first heat treatment step and a second heat treatment step according to embodiments disclosed herein. In this example, the first heat treatment step is at a temperature T1 of 1030° C., the second heat treatment step is at a temperature T2 of 1125° C., and the ramp-up step is at a heating rate of 50° C. / hour (similar to the embodiment of FIG. 4A). However, in this embodiment, the body is heated for a holding time t hold No exposure to steam during the retention time t hold After completion of the second heat treatment step, the body is then subjected to a second heat treatment step for a duration of t rem In the exemplary embodiment of FIG. hold is approximately 25 hours of duration, and t rem The duration is about 2.5 days.

[0054] For purposes of this disclosure, hydroxyl concentration was measured by segmenting the body into multiple segments and measuring the hydroxyl concentration of each segment, as discussed below with reference to Figures 5A-5C. Figure 5A shows body 10 produced by process 100 of Figure 1 using exemplary bonding process 300 of Figure 3. Body 10 is thus the resulting body after annealing step 140 of process 100. Body 10 may be an ingot or substrate to which one or more layers are applied during downstream processing.

[0055] As discussed above, body 10 is a titania-doped silica glass. The silica concentration in body 10 can be about 80% by weight or greater, or about 85% by weight or greater, or about 90% by weight or greater, or about 92% by weight or greater, or about 95% by weight or greater, or about 97% by weight or greater, or about 98% by weight or greater, or about 99% by weight or greater, or about 85% by weight to about 97% by weight, or about 90% by weight to about 95% by weight. The titania concentration in body 10 can be about 1.0% by weight to about 15.0% by weight, or about 6.0% by weight to about 12.0% by weight, or about 6.0% by weight to about 8.5% by weight, or about 6.5% by weight to about 8.0% by weight, or about 7.0% by weight to about 7.7% by weight, or about 6.5% by weight to about 7.8% by weight.

[0056] Body 10 has a length L', a width W', and a height H', as shown in Figure 5A. In some embodiments, length L' and width W are each greater than height H'. For example, length L' and width W' can each be about 500 mm or less, or about 450 mm or less, or about 400 mm or less, or about 350 mm or less, or about 300 mm or less, or about 250 mm or less, or about 200 mm or less, or about 150 mm or less, or about 100 mm or less, or about 75 mm or less, or about 50 mm or less, or about 25 mm or less, or about 20 mm or less, or about 15 mm or less. Additionally or alternatively, the length L' and width W' of the glass body 10 are each about 15 mm or more, or about 20 mm or more, or about 25 mm or more, or about 50 mm or more, or about 75 mm or more, or about 100 mm or more, or about 150 mm or more, or about 200 mm or more, or about 250 mm or more, or about 300 mm or more, or about 350 mm or more, or about 400 mm or more, or about 450 mm or more, or about 500 mm or more. In some embodiments, both the length L' and the width W' are about 150 mm, or about 152 mm, or about 179 mm. However, it is contemplated that in some embodiments, the length L' may differ from the width.

[0057] Furthermore, height H' can be less than each of length L' and width W'. In some embodiments, height H' is about 400 mm or less, or about 350 mm or less, or about 300 mm or less, or about 250 mm or less, or about 200 mm or less, or about 150 mm or less, or about 100 mm or less, or about 75 mm or less, or about 50 mm or less, or about 25 mm or less, or about 20 mm or less, or about 15 mm or less, or about 10 mm or less, or about 5 mm or less. Additionally or alternatively, height H' is about 5 mm or more, or about 10 mm or more, or about 15 mm or more, or about 20 mm or more, or about 25 mm or more, or about 50 mm or more, or about 75 mm or more, or about 100 mm or more, or about 150 mm or more, or about 200 mm or more, or about 250 mm or more, or about 300 mm or more, or about 350 mm or more, or about 400 mm or more. In some embodiments, height H' is about 63 mm, or about 150 mm, or about 152 mm.

[0058] As also discussed above, the body 10 can be sliced ​​into multiple samples. FIG. 5A shows the smallest dimension (i.e., characteristic length L) of the body 10. c 5A shows exemplary samples 15 of body 10 that form subportions of the body along the length H' of body 10. Each sample 15 may be considered a body, a substrate, or a wafer. In the embodiment of FIG. 5A, height H' is less than each of length L' and width W', such that height H' is the smallest dimension. Thus, height h' of sample 15 extends along height H' of body 10. In the embodiment of FIG. 5A, body 10 includes multiple samples along its height H'. However, in other embodiments, it is contemplated that one sample 15 extends along the entire height H' of body 10 (or along the entire smallest dimension of the body, if the smallest dimension is not height H'). In these embodiments, body 10 includes only one sample 15, such that one sample 15 forms the entire body 10.

[0059] While FIG. 1 depicts body 10 and sample 15 as square components with flat surfaces, embodiments contemplate that body 10 and / or sample 15 may include other shapes. For example, the outer shape of body 10 and / or sample 15 may be circular or oval, or asymmetric. Furthermore, body 10 and / or sample 15 may be curved to form a concave or convex structure. In one exemplary embodiment, body 10 is formed from a single sample 15 (such that the single sample 15 extends across the entire length, width, and height of body 10), and body 10 has a concave structure. Sample 15 may be a reticle, a photomask, a mirror, and / or a photomask holder.

[0060] Each sample 15 has a substantially uniform hydroxyl concentration and titania concentration across the length and width of the sample. To determine the uniformity of the sample throughout the body, each sample is divided into segments across the length and width of the sample. For example, FIG. 5B shows sample 15 divided into segments 20 across the cross-sectional length L' and width W' of sample 15. The concentrations of one or more constituents (e.g., hydroxyl, titania) can then be determined for each segment 20 to determine the uniformity of each of these constituents along the sample 15. For example, to determine the uniformity of the hydroxyl concentration across the cross-section of sample 15, the hydroxyl concentration can be measured for each segment 20. As discussed further below, the concentration of one or more constituents is determined throughout the entire thickness h' of each segment 20.

[0061] While FIG. 5B depicts segments 20 extending along the entire length L' and width W' of sample 15, it is contemplated that the portion of sample 15 including segments 20 may be less than the entire cross-sectional length L' and width W'. For example, as shown in FIG. 5C, sample 15 may include an outer peripheral edge 17 where no segments 20 are formed. Thus, outer peripheral edge 17 may be the gap between the end of segment 20 and the outer edge of sample 15. In embodiments, the outer peripheral edge may extend a length L''' of about 2 mm to about 20 mm, or about 4 mm to about 16 mm, or about 5 mm to about 16 mm, or about 8 mm to about 14 mm, or about 10 mm to about 12 mm. In some embodiments, length L''' is about 12.5 mm or about 12.7 mm.

[0062] The segments 20 may be adjacent segments spanning a particular length and width of the sample 15 (so that no gaps are formed between adjacent segments). As discussed above, this particular length and width (through which all segments 20 extend) may be equal to or less than the length L' and width W' of the sample 15. In embodiments, the segments 20 are adjacent segments spanning a length and width (through which all segments 20 extend) such that the length and width of the sample 15 are, respectively, about 25 mm or more, or about 30 mm or more, or about 40 mm or more, or about 50 mm or more, or about 60 mm or more, or about 75 mm or more, or about 100 mm or more, or about 125 mm or more, or about 150 mm or more, or about 175 mm or more, or about 180 mm or more, or about 190 mm or more, or about 200 mm or more, or about 250 mm or more.

[0063] If the sample 15 includes a flat surface, the segments 20 are formed along a flat plane, as shown in Figure 5B. However, if the sample 15 includes a concave or convex surface, the segments 20 are formed along a curved surface of the sample 15.

[0064] As shown in FIG. 5B, each segment 20 has a length L'' and a width W'', each of which is approximately 12.7 mm. However, it is contemplated that in other embodiments, the length L'' is not equal to the width W''. It should also be noted that in some embodiments, the length L'' and width W'' of the segment 20 may be equal to the length L''' of the peripheral edge 17.

[0065] The height of each segment 20 is the height h' of the sample 15, as discussed above. Thus, in an embodiment, the height h' is about 7.62 mm.

[0066] As discussed above, the concentration of one or more constituents may be determined within each segment 20. Thus, for example, the concentration of hydroxyl may be determined for each adjacent segment 20 within the sample 15. If each segment 20 has a length and width of 12.7 mm, the concentration of the constituent is determined at a frequency of 12.7 mm across the cross-section of the sample 15. For example, the concentration of hydroxyl is measured at a frequency of 12.7 mm across the cross-section of the sample 15.

[0067] The hydroxyl concentration of each segment 20 is measured using transmission Fourier transform infrared spectroscopy ("FTIR"). As used herein, "transmission" means that light is directed through the glass body to be measured to determine the hydroxyl concentration (as opposed to determining the hydroxyl concentration using light reflected from the glass body to be measured). Thus, "transmission" requires a non-scattering surface. When the sample 15 is loaded into the FTIR for measurement, beam alignment and background measurements may first be performed. The FTIR then measures the base absorption peak of the hydroxyl, thereby measuring the peak height relative to the background signal, which is the straight line between the points surrounding the absorption peak. The absorption peak height is then divided by the thickness h' of the sample 15 to determine the absorption coefficient β OH The hydroxyl concentration is then calculated using the following formula: C=β OH / ε×MW OH / D glass×106 Using the absorption coefficient β OH where C is the concentration of hydroxyl in ppm for a particular segment 20 and β OH is the absorption coefficient of the glass, and ε is 3670 cm -1 is the molar absorptivity of hydroxyl relative to the absorption peak at a wavenumber of MW OH is the molecular weight of hydroxyl (g / mol), and D glass is the density of hydroxyl (g / cm 3 ) The FTIR analysis disclosed above is further disclosed in KM Vis, et al., "Quantitative infrared spectroscopic measurement of hydroxyl concentration in silica glass," J. Non-Crystalline Solids, 203 (1996) 27-36, which is incorporated herein by reference. As discussed above, the hydroxyl concentration is measured for each segment 20 of the sample 15, and is measured through the entire thickness h' of each segment 20. The measurement of the hydroxyl concentration is then repeated across all segments 20 of the sample 15.

[0068] One or more segments 20 may have a different hydroxyl concentration than one or more other segments 20. However, in embodiments, segments 20 each have substantially the same hydroxyl concentration regardless of where the segment is located on substrate 10.

[0069] Additionally, the average hydroxyl concentration along the length L' and width L' of sample 15 can be determined by averaging together the hydroxyl concentrations of the individual segments 20. According to embodiments disclosed herein, the average hydroxyl concentration throughout sample 15 can be from about 0 ppm to about 2000 ppm, or from about 200 ppm to about 1900 ppm, or from about 300 ppm to about 1800 ppm, or from about 400 ppm to about 1700 ppm, or from about 500 ppm to about 1750 ppm, or from about 600 ppm to about 1600 ppm, or from about 700 ppm to about 1500 ppm, or from about 800 ppm to about 1400 ppm, or from about 900 ppm to about 1600 ppm. The range may be from about 00 ppm to about 1300 ppm, or from about 1000 ppm to about 1200 ppm, or from about 1000 ppm to about 1100 ppm, or from about 600 ppm to about 1500 ppm, or from about 600 ppm to about 1400 ppm, or from about 600 ppm to about 1300 ppm, or from about 700 ppm to about 1000 ppm, or from about 50 ppm to about 200 ppm, or from about 75 ppm to about 150 ppm, or from about 80 ppm to about 125 ppm. In some embodiments, the average hydroxyl concentration throughout Sample 15 is about 400 ppm or less, or about 350 ppm or less, or about 300 ppm or less, or about 250 ppm or less, or about 200 ppm or less, or about 150 ppm or less, or about 100 ppm or less, or about 90 ppm or less, or about 80 ppm or less, or about 75 ppm or less, or about 70 ppm or less, or about 60 ppm or less, or about 50 ppm or less.

[0070] In some particular embodiments, the maximum hydroxyl concentration between segments 20 can range from about 1000 ppm to about 1400 ppm, or from about 1000 ppm to about 1300 ppm, or from about 1000 ppm to about 1200 ppm, or from about 1000 ppm to about 1100 ppm, or from about 1050 ppm to about 1100 ppm, or from about 1060 ppm to about 1090 ppm. The minimum hydroxyl concentration between segments 20 may, in some particular embodiments, range from about 600 ppm to about 1300 ppm, or from about 800 ppm to about 1200 ppm, or from about 900 ppm to about 1100 ppm, or from about 1000 ppm to about 1100 ppm, or from about 1050 ppm to about 1100 ppm, or from about 1060 ppm to about 1080 ppm, or from about 100 ppm to about 500 ppm, or from about 200 ppm to about 400 ppm.

[0071] It should be noted that the hydroxyl concentration within segment 20 depends on whether the above-disclosed adhesion process includes a vapor doping process. As discussed above, such a vapor doping process produces a substrate with a higher hydroxyl concentration.

[0072] The difference between the highest and lowest average hydroxyl concentrations among different segments 20 is also determined. More specifically, the segment 20 with the highest hydroxyl concentration is compared to the segment 20 with the lowest hydroxyl concentration. The difference between the highest and lowest hydroxyl concentrations is then calculated. This difference between the highest and lowest concentrations in the sample 15 is referred to as the peak-valley (PV) difference in concentration. The smaller the PV difference, the more uniform the concentration is within a particular sample.

[0073] The PV difference in hydroxyl concentration of segment 20 in sample 15 can be about 70 ppm or less, or about 60 ppm or less, or about 55 ppm or less, or about 50 ppm or less, or about 45 ppm or less, or about 40 ppm or less, or about 35 ppm or less, or about 30 ppm or less, or about 25 ppm or less, or about 20 ppm or less, or about 15 ppm or less, or about 10 ppm or less, or about 5.0 ppm or less, or about 2.5 ppm or less, or about 1.0 ppm or less, or about 0.0 ppm. Additionally or alternatively, the PV difference in hydroxyl concentration of segments 20 can be about 0.0 ppm or more, or about 1.0 ppm or more, or about 2.5 ppm or more, or about 5.0 ppm or more, or about 10 ppm or more, or about 15 ppm or more, or about 20 ppm or more, or about 25 ppm or more, or about 30 ppm or more, or about 35 ppm or more, or about 40 ppm or more, or about 45 ppm or more, or about 50 ppm or more. In some embodiments, the PV difference in average hydroxyl concentration of segments 20 is within the range of about 0.0 ppm to about 60 ppm, or about 10 ppm to about 50 ppm, or about 15 ppm to about 45 ppm, or about 20 ppm to about 40 ppm, or about 10 ppm to about 30 ppm.

[0074] As discussed above, the PV difference in hydroxyl concentration between segments 20 is very low, thus providing a homogeneous and uniform glass body 10. Due to such low PV difference, the glass body 10 will maintain its shape within an EUV system. Note that embodiments of the present disclosure include the PV ranges disclosed above with various maximum and average hydroxyl concentrations. Thus, for example, the PV ranges disclosed above can be for embodiments with relatively high hydroxyl concentrations and for embodiments with relatively low hydroxyl concentrations.

[0075] The PV difference in titania concentration of segment 20 in sample 15 can be about 0.0100 wt% or less, or about 0.0090 wt% or less, or about 0.0080 wt% or less, or about 0.0070 wt% or less, or about 0.0060 wt% or less, or about 0.0050 wt% or less, or about 0.0040 wt% or less, or about 0.0035 wt% or less, or about 0.0030 wt% or less, or about 0.0025 wt% or less, or about 0.0020 wt% or less, or about 0.0015 wt% or less, or about 0.0010 wt% or less. In embodiments, the PV difference in titania concentration of segments 20 ranges from about 0.0010 wt % to about 0.0050 wt %, or from about 0.0015 wt % to about 0.0045 wt %, or from about 0.0020 wt % to about 0.0040 wt %, or from about 0.0025 wt % to about 0.0035 wt %, or from about 0.0030 wt % to about 0.0050 wt %, or from about 0.0010 wt % to about 0.0030 wt %, or from about 0.0010 wt % to about 0.0025 wt %, or from about 0.0010 wt % to about 0.0020 wt %.

[0076] The PV difference in titania concentration in the glass body 10 is very low, thus providing a homogeneous glass body 10 having not only a uniform hydroxyl concentration but also a uniform titania concentration.

[0077] The titania concentration of each segment 20 is calculated based on the measured refractive index of each segment 20. As is well known in the art, the titania concentration in a glass body correlates with the refractive index of the glass body. Therefore, for purposes of this disclosure, the refractive index is measured to determine the titania concentration of the glass bodies disclosed herein. More specifically, an optical interferometer operating at a wavelength of 633 nm is used to measure the refractive index. Specifically, the optical interferometer is a Zygo Verifire HD manufactured by Zygo Corporation, which has a pixel size resolution of 270 microns and operates at a wavelength of 633 nm. The optical interferometer is configured so that the pixels are square, measuring 270 microns by 270 microns, and each pixel extends through the entire thickness h' of the sample 15. The refractive index is measured at each pixel within the segment 20 through the entire thickness of the pixel. The refractive index measured for each pixel within the segment 20 is then averaged together to determine the average refractive index of each segment 20. The refractive index measurement is then repeated across all segments 20 of the sample 15.

[0078] Figure 6 shows an exemplary plot of the distribution of hydroxyl concentrations in four samples sliced ​​from the same body. As shown in Figure 6, each of the four samples has a uniform hydroxyl concentration along the length and width of the sample, with very little concentration difference. Furthermore, all four samples have a uniform hydroxyl concentration between different samples. Therefore, the entire body from which the samples were sliced ​​also has a uniform hydroxyl concentration.

[0079] Illustrative Examples In a first illustrative example, a glass was produced by the following consolidation process without OH doping: (i) heating the body to a temperature of 1030°C for 1440 minutes during a first heat treatment to achieve a consolidation furnace vapor partial pressure of 0 atm; (ii) increasing the temperature from 1030°C to 1150°C at a rate of 50°C during a ramp-up step to achieve a consolidation furnace vapor partial pressure of 0 atm; and (iii) heating the body to 1150°C for 1440 minutes during a second heat treatment to achieve a consolidation furnace vapor partial pressure of 0 atm. The resulting body (after annealing step 140 of process 100) was then segmented into multiple segments across a 127 mm x 127 mm cross-section of the body. Each segment had a length (L'') and width (W'') of 12.7 mm and a height (h') of 7.62 mm. The average hydroxyl of the segments was 135 ppm, and the maximum hydroxyl concentration of the segments was 155 ppm. Additionally, the PV hydroxyl concentration of the segments was 40 ppm.

[0080] In a second illustrative example, a glass was produced by the following consolidation process involving a vapor doping process: (i) heating the body to a temperature of 1030°C for 1440 minutes during a first heat treatment to achieve a consolidation furnace vapor partial pressure of 0.33 atm; (ii) increasing the temperature from 1030°C to 1125°C at a rate of 50°C during a ramp-up step to achieve a consolidation furnace vapor partial pressure of 0.33 atm; and (iii) heating the body to 1125°C for 1440 minutes during a second heat treatment to achieve a consolidation furnace vapor partial pressure of 0.33 atm. The resulting body (after annealing step 140 of process 100) was then segmented into multiple segments across the 127 mm x 127 mm cross-section of the body. Each segment had a length (L'') and width (W'') of 12.7 mm and a height (h') of 7.62 mm. The average hydroxyl of the segments was 800 ppm, the maximum hydroxyl concentration of the segments was 835 ppm, and the PV hydroxyl concentration of the segments was 60 ppm.

[0081] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments of the present disclosure without departing from the spirit and scope of the disclosure. Thus, the present disclosure is intended to cover such modifications and variations insofar as they come within the scope of the appended claims and their equivalents.

Claims

1. 1. A process for forming a titania-silica glass body, said process comprising: The titania-doped silica soot body may be prepared according to the following formula: [Equation 1] exposing the body to a first heat treatment by heating the body to a first temperature T1 of about 800°C to about 1100°C for a first duration t1 calculated using the formula: ｃ is the characteristic length of the body (cm), and α is the thermal diffusivity of the body (cm ２ / seconds), and exposing the body to a second heat treatment by heating the body to a second temperature T2 of about 1050°C to about 1250°C for a second duration t2; wherein after said second heat treatment, the peak-to-valley difference in hydroxyl concentration among a plurality of segments of said body is about 70 ppm or less, said hydroxyl concentration being measured using transmission Fourier transform infrared spectroscopy, said plurality of segments including all adjacent segments across the length and width of said body, said length being about 25 mm or more, and said width being about 25 mm or more.

2. 2. The process of claim 1, wherein the peak-to-valley difference in hydroxyl concentration is about 60 ppm or less.

3. 2. The process of claim 1, wherein the peak-to-valley difference in hydroxyl concentration is about 50 ppm or less.

4. The thermal diffusivity α is 0.003 cm ２ The process according to any one of claims 1 to 3, wherein the kinetic energy of the ion exchange reaction is 1 / sec.

5. The process of any one of claims 1 to 4, wherein after the second heat treatment step, the body has a silica concentration of about 80% by weight or greater.

6. The process of any one of claims 1 to 5, wherein after the second heat treatment, the body has a titania concentration of from about 6.0% to about 12.0% by weight.

7. 7. The process of any one of claims 1 to 6, further comprising increasing the temperature of the body from the first temperature T1 to the second temperature T2 at a rate of about 20°C / hour or greater.

8. 8. The process of claim 7, wherein the rate is about 30°C / hour or greater.

9. 9. The process of claim 8, wherein the rate is about 45°C / hour or greater.

10. 8. The process of claim 7, wherein the rate is from about 30°C / hr to about 100°C / hr.

11. The process of any one of claims 1 to 10, wherein the length is about 50 mm or more and the width is about 50 mm or more.

12. 12. The process of claim 11, wherein the length is about 100 mm or greater and the width is about 100 mm or greater.

13. 13. The process of any one of claims 1 to 12, further comprising exposing the body to an atmosphere having a partial pressure of about 0.50 atm or less during at least one of the first heat treatment and the second heat treatment.

14. 14. The process of claim 13, wherein the partial pressure is about 0.20 atm or less.

15. 15. The process of claim 14, wherein the partial pressure is less than about 0.02 atm.

16. 16. The process of claim 15, wherein the atmosphere is substantially free of water and steam.

17. 13. The process of any one of claims 1 to 12, further comprising exposing the body to an atmosphere containing steam and having a partial vapor pressure of from about 0.10 atm to about 1.00 atm during at least one of the first heat treatment and the second heat treatment.

18. 13. The process of any one of claims 1 to 12, further comprising exposing the body during the second heat treatment to an atmosphere containing steam and having a partial steam pressure of about 0.10 atm or less.

19. The body is subjected to a holding time t during the first heat treatment and the second heat treatment. ｈｏｌｄ and the body is not exposed to steam during the second heat treatment for a time t ｒｅｍ and the time t ｒｅｍ However, the holding time t ｈｏｌｄ the remainder of the second heat treatment after the holding time t ｈｏｌｄ is greater than or equal to the first duration t1 of the first heat treatment.

20. 20. The process of any one of claims 1 to 19, wherein the plurality of segments has an average hydroxyl concentration of about 1500 ppm or less.

21. 21. The process of claim 20, wherein the average hydroxyl concentration of the plurality of segments is less than or equal to about 400 ppm.

22. 22. The process of claim 21, wherein the average hydroxyl concentration of the plurality of segments is less than or equal to about 200 ppm.

23. The process of any one of claims 1 to 22, wherein the first temperature T1 is from about 850°C to about 1050°C.

24. The process of any one of claims 1 to 23, wherein the second temperature T2 is from about 1100°C to about 1200°C.

25. 25. The process of claim 24, wherein the second temperature T2 is from about 1125°C to about 1175°C.

26. 26. The process of any one of claims 1 to 25, wherein after the second heat treatment, the peak-to-valley difference in titania concentration among the plurality of segments of the body is about 0.0100 wt% or less.

27. The body is about 0.50 g / cm ３ The process of any one of claims 1 to 26, wherein the densities are equal to or greater than 10 ...

28. The density is about 0.65 g / cm ３ 28. The process of claim 27.

29. The density is about 0.50 g / cm ３ ~Approx. 1.20g / cm ３ 29. The process of claim 28, wherein

30. 28. The process of claim 27, wherein the body has a density variation of about 5% or less.

31. The process of any one of claims 1 to 30, further comprising forcing the titania-doped silica soot into a mold.

32. The process of any one of claims 1 to 31, wherein the glass body is a photomask.

33. 1. A process for forming a titania-silica glass body, said process comprising: The titania-doped silica soot body may be prepared according to the following formula: [Equation 2] exposing the body to a first heat treatment by heating the body to a first temperature T1 of about 800°C to about 1100°C for a first duration t1 calculated using the formula: ｃ is the characteristic length of the body (m), and α is the thermal diffusivity of the body (cm ２ / seconds), and exposing the body to a second heat treatment by heating the body to a second temperature T2 of about 1050°C to about 1250°C for a second duration t2; raising the temperature of the body from the first temperature T1 to the second temperature T2 at a rate of about 20°C / hour or greater.

34. 34. The process of claim 33, wherein the rate is about 30°C / hour or greater.

35. 35. The process of claim 34, wherein the rate is about 45°C / hour or greater.

36. 34. The process of claim 33, wherein the rate is from about 30°C / hr to about 100°C / hr.

37. The thermal diffusivity α is 0.003 cm ２ 37. The process of any one of claims 33 to 36, wherein the kinetic energy of the ion exchange reaction is 1 / second.

38. 38. The process of any one of claims 33 to 37, wherein the second temperature T2 is maintained at a constant temperature throughout the second heat treatment, the temperature fluctuating by no more than about 5°C.

39. 39. The process of any one of claims 33-38, further comprising exposing the body to an atmosphere having a partial pressure of about 0.50 atm or less during at least one of the first heat treatment and the second heat treatment.

40. 40. The process of claim 39, wherein the partial pressure is about 0.20 atm or less.

41. 41. The process of claim 40, wherein the partial pressure is less than about 0.02 atm.

42. 42. The process of claim 41, wherein the atmosphere is substantially free of water and steam.

43. 39. The process of any one of claims 33-38, further comprising exposing the body to an atmosphere containing steam and having a vapor partial pressure of from about 0.10 atm to about 1.00 atm during at least one of the first heat treatment and the second heat treatment.

44. 39. The process of any one of claims 33 to 38, further comprising exposing the body during the second heat treatment to an atmosphere containing steam and having a partial vapor pressure of about 0.10 atm or less.

45. The body is subjected to a holding time t during the first heat treatment and the second heat treatment. ｈｏｌｄ and the body is not exposed to steam during the second heat treatment for a time t ｒｅｍ and the time t ｒｅｍ However, the holding time t ｈｏｌｄ the remainder of the second heat treatment after the holding time t ｈｏｌｄ is greater than or equal to the first duration t1 of the first heat treatment.

46. The process of any one of claims 33 to 45, wherein the first temperature T1 is from about 850°C to about 1050°C.

47. The process of any one of claims 33 to 46, wherein the second temperature T2 is from about 1100°C to about 1200°C.

48. 47. The process of claim 46, wherein the second temperature T2 is from about 1125°C to about 1175°C.

49. The process of any one of claims 33 to 48, wherein the glass body is a photomask.

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