High purity synthetic silica and items such as semiconductor jigs manufactured therefrom

Abstract

Hollow ingots of transparent synthetic vitreous silica glass of external diameter greater than 400 mm and internal diameter greater than 300 mm are disclosed. The ingots are substantially free from bubbles or inclusions greater than 100 m in diameter, have no more than 100 ppB of any individual metallic impurity, and have chlorine concentration less than 5 ppM. Also disclosed are methods for producing such ingots, in which a porous soot body of density greater than 0.4 g/cm.sup.3 is deposited on an oxidation resistant mandrel. The soot body is dehydrated on a mandrel comprising graphite, carbon fiber reinforced carbon, silicon carbide, silicon impregnated silicon carbide, silicon carbide-coated graphite or vitreous silica, either under vacuum or in the presence of a reducing gas, and then sintered to transparent pore-free glass under vacuum or in an atmosphere of helium.

Claims

1. A process for the manufacture of an annular semiconductor jig of transparent synthetic vitreous silica suitable for use in a plasma etching environment, said jig having an external diameter greater than 400 mm and an internal diameter greater than 300 mm, wherein said jig is substantially free from bubbles or inclusions greater than 100 m in diameter; has no more than 100 ppB of any individual metallic impurity; and has chlorine concentration less than 380 ppB, the process comprising the steps of: depositing a porous soot body of density greater than 0.4 g/cm.sup.3 on an oxidation resistant mandrel of external diameter greater than 300 mm, said silica soot being generated during said depositing step by combustion of vapor of at least one of octamethylcyclotetrasiloxane, hexamethyldisiloxane, or decamethylcyclopentasiloxane; dehydrating said soot body on a mandrel comprising a material selected from graphite, carbon fibre reinforced carbon, silicon carbide, silicon impregnated silicon carbide, silicon carbide-coated graphite and vitreous silica, under vacuum; sintering the dehydrated soot body to transparent pore-free glass under vacuum to produce a hollow ingot; and slicing the hollow ingot to produce the annular jig, wherein, the hollow ingot is manufactured as a result of said depositing, dehydrating and sintering steps to a near-net-shape relative to a final shape of the annular jig without a secondary reflow or resizing process step, wherein the transparent vitreous silica of the annular jig has an annealing point (viscosity 10.sup.13 Poise) greater than 1,200 C.; and wherein the annular transparent synthetic vitreous silica jig is not doped with nitrogen and is not doped with fluorine.

2. A process for the manufacture of an annular transparent synthetic vitreous silica jig of external diameter greater than 400 mm and internal diameter greater than 300 mm and having a chlorine concentration of less than 5 ppM, the process comprising the steps of: feeding a chlorine-free silica precursor to the flame of at least one synthesis burner; depositing a porous soot body of density greater than 0.4 g/cm.sup.3 on an oxidation resistant mandrel of diameter at least 300 mm; dehydrating said soot body on a mandrel comprising a material selected from graphite, carbon fibre reinforced carbon, silicon carbide, silicon impregnated silicon carbide, silicon carbide-coated graphite and vitreous silica, under vacuum; sintering the dehydrated soot body to transparent pore-free glass under vacuum to produce a hollow ingot; and slicing the hollow ingot to produce the annular transparent synthetic vitreous silica jig, wherein, the hollow ingot is manufactured to a near-net-shape relative to a final shape of the annular transparent synthetic vitreous silica jig as a result of said depositing, dehydrating and sintering steps and without a secondary reflow or resizing process step, wherein the transparent vitreous silica of the annular jig has an annealing point (viscosity 10.sup.13 Poise) greater than 1,200 C.; and wherein the annular transparent synthetic vitreous silica jig is not doped with nitrogen and is not doped with fluorine.

3. A process according to claim 2, wherein the oxidation resistant mandrel and the mandrel used during the dehydration step are separate, the process further comprising the steps, subsequent to deposition and prior to dehydration, of removing said oxidation resistant mandrel and replacing the same with said mandrel upon which the dehydration and sintering steps takes place.

4. A process according to claim 2, wherein said oxidation resistant mandrel and said mandrel used during the dehydration step are the same, and wherein said oxidation resistant mandrel is not removed after soot deposition, but is retained and used to support the soot body during the subsequent dehydration and sintering processes.

5. A process according to claim 2, further comprising the step of annealing said hollow ingot to achieve a fictive temperature less than 1,100 C.

6. A process according to claim 2, further comprising the step of machining said hollow ingot into the annular jig, with minimal wastage of synthetic vitreous silica material.

7. A process according to claim 2, wherein the diameter of the mandrel used for sintering and the duration of the deposition process are both selected such that the hollow ingot product after sintering has an external diameter greater than 400 mm and internal diameter greater than 300 mm, and a ratio of external to internal diameter of not greater than 1.33.

8. A process according to claim 2, wherein said chlorine-free silica precursor material is delivered to said synthesis flame in the form of vapor.

9. A process according to claim 8, wherein said chlorine-free silica precursor material comprises at least one siloxane.

10. A process according to claim 9, wherein said at least one siloxane includes octamethylcyclotetrasiloxane, hexamethyldisiloxane, or decamethylcyclopentasiloxane.

11. A process according to claim 8, wherein said chlorine-free silica precursor material comprises at least one cyclic polymethylsiloxane.

12. A process according to claim 2, wherein the chlorine concentration of the annular transparent synthetic vitreous silica jig is less than 380 ppB.

13. A process according to claim 2, wherein the annular transparent synthetic vitreous silica jig is not doped with aluminum.

14. A process for the manufacture of an annular transparent synthetic vitreous silica jig of external diameter greater than 400 mm and internal diameter greater than 300 mm and having a chlorine concentration of less than 5 ppM, the process consisting of the steps of: feeding a chlorine-free silica precursor to the flame of at least one synthesis burner; depositing a porous soot body of density greater than 0.4 g/cm.sup.3 on an oxidation resistant mandrel of diameter at least 300 mm; dehydrating said soot body on a mandrel comprising a material selected from graphite, carbon fibre reinforced carbon, silicon carbide, silicon impregnated silicon carbide, silicon carbide-coated graphite and vitreous silica, under vacuum; sintering the dehydrated soot body to transparent pore-free glass under vacuum to produce a hollow ingot; and slicing the hollow ingot to produce the annular transparent synthetic vitreous silica jig, wherein, the hollow ingot is manufactured to a near-net-shape relative to a final shape of the annular transparent synthetic vitreous silica jig as a result of said depositing, dehydrating and sintering steps and without a secondary reflow or resizing process step, wherein the transparent vitreous silica of the annular jig has an annealing point (viscosity 10.sup.13 Poise) greater than 1,200 C., wherein the annular transparent synthetic vitreous silica iig is not doped with nitrogen and is substantially free of fluorine.

15. A process according to claim 14, wherein said chlorine-free silica precursor comprises at least one siloxane.

Description

(1) The invention is hereinafter described in more detail by way of example only, with reference to the accompanying figures, in which:

(2) FIG. 1 is a schematic diagram of a deposition facility suitable for use in a method according to the invention; and

(3) FIG. 2 is a plot of annealing point (viscosity 10.sup.13 Poise) versus the hydroxyl (OH) concentration for a series of chlorine-free and chlorine-containing synthetic vitreous silica samples.

EXAMPLE

(4) An example of the new process will now be described, as it may be applied to an ingot of dimensions suitable for manufacture of a standard blank for the manufacture of a semiconductor jig, i.e. a hollow ingot of outside diameter 420 mm, and inside diameter 353 mm.

(5) A suitable deposition facility is shown schematically in FIG. 1. This shows a horizontal deposition lathe, which is used to support a cylindrical substrate (1) made of an oxidation-resistant refractory material, such as alumina, silicon carbide etc., of diameter 350 mm. The lathe is provided with a linear array of 15 coaxial burners (2) made from quartz glass, each separated from its neighbours by a distance of 100 mm. These burners are fed with OMCTS vapour, in nitrogen as carrier gas, surrounded by a flow of hydrogen, which is again surrounded by a flow of oxygen. The flames of these burners are directed at the rotating substrate, and are caused to oscillate in an axial direction with amplitude of 200 mm. Alternatively, it is possible to arrange that the rotating substrate is oscillated in a similar manner. As the soot body accumulates, end-burners (3), also fed with hydrogen and oxygen, are directed at the tapered ends of the soot body, causing densification of the soot in the end regions, and minimising the risk of crack propagation from the ends of the body. Also, as the soot body grows in size, the distance between the burner array and soot body (4) is maintained at a constant value, in the region of 150 mm, either by raising the mandrel, or by lowering the burner assembly.

(6) Analysis of the soot from a similar experiment has shown that the deposition conditions chosen yield an average density of 0.6 g/cm.sup.3, so deposition of silica soot is maintained for a period of 21 hours, during which the soot is deposited to a diameter of 566 mm.

(7) The achievement of near net shape being an object of the process, the duration of the deposition process is adjusted to ensure deposition of the appropriate quantity of silica soot. This is aided by the provision of on-line weight measurement using load cells (5), so that as well as monitoring the diameter of the body using video camera (6), or a suitable laser gauge, it is possible to monitor the weight, and thus also the density of the deposited silica soot.

(8) On completion of the deposition process, the soot body is allowed to cool, and the mandrel is removed and replaced with a high purity graphite mandrel (<10 ppM ash), of diameter 347 mm, chosen to yield the required internal diameter of hollow ingot after sintering. The assembly is mounted in a vacuum furnace, resistively heated with graphite heating elements. The furnace is evacuated to a pressure of less than 0.5 torr (67 Pa), and back-filled with nitrogen. The pressure is again reduced to less than 0.5 torr (67 Pa) and the temperature is raised to 1,100 C., and dehydration of the soot body is commenced. After 6 hours at 1,100 C., the temperature is raised to 1,200 C., and held for a further period of 12 hours. The temperature is then raised progressively to 1,500 C. to effect sintering to a pore-free glass, and then the furnace is allowed to cool.

(9) Under the conditions used, the axial shrinkage of the soot body on sintering is approximately 10%, and the final external and internal diameters of the glass cylinder so generated are approximately 425 mm and 348 mm respectively, permitting machining of the required blank (420353 mm), with only small loss of material. By refining the operating parameters, it is anticipated that an even better match of dimensions can be achieved.

(10) It is thus clear that by appropriate choice of the diameter of the mandrel used for sintering and of the duration of the deposition process it is possible to ensure that the hollow ingot product after sintering will have appropriate inside and outside diameters such as will yield the desired product dimensions with minimal machining losses, and a hollow ingot product of near net shape is thus achieved via the combination of processes described.

(11) It appears beneficial in terms of increased viscosity of the glass, and etch-resistance, to achieve a low fictive temperature, and this may be attained by controlled slow cooling over a temperature range 1,200 C. down to 950 C. This may be effected in the above vacuum furnace (with corresponding loss of vacuum sintering capacity), or alternatively in a separate annealing oven. The ultimate fictive temperature achieved is dependent on cooling rate and duration of the annealing operation. The fictive temperature of a sample of the glass may be measured via the intensity of laser Raman scattering at a wavelength of 606 cm.sup.1 as described by C. Pfleiderer, et al. (J. Non-Cryst. Solids, 159 (1993), 145-153). It has proved beneficial to achieve a fictive temperature of less than 1,100 C., preferably less than 1,075 C., but for this high viscosity glass this requires an annealing cycle lasting several days.

(12) After annealing, the graphite mandrel is removed, and samples are taken for chemical analysis, OH and fictive temperature measurement. Typically analysis of potential contaminant metals reveals none at a concentration greater than 10 ppB (limit of detection), and as expected the material is substantially chlorine-free. Analysis of chlorine by nuclear activation has indicated Cl<380 ppB (limit of detection). Likewise, the glass is expected to be substantially free from fluorine.

(13) The viscosity of samples 6043 mm in size may be measured by the bending beam method as described in ASTM C 598 93, and the Annealing Point (viscosity 10.sup.13 Poise) is typically found to be 1,200 C. The OH-content is determined by measuring the IR absorption according to the method of D. M. Dodd et al. (Optical determinations of OH in fused silica, J. Appl. Physics (1966), p. 3911), and is typically in the range 10-20 ppM. The fictive temperature can be reduced to 1,050-1,100 C. by using an appropriate annealing schedule.

(14) The viscosity of products made in this way is surprisingly high; however, on studying the viscosity of a range of other glasses, it was found to fit a trend. FIG. 2 shows a plot of a range of chlorine-free synthetic silica glasses, made from OMCTS (by direct deposition, and by soot and sinter methods) with controlled OH concentration. As may be seen, the Annealing Point (viscosity 10.sup.13 Poise) rises with diminishing OH concentration, and for the current sample was found to be 1,202 C.

(15) Also shown on this plot is the viscosity of a single specimen of synthetic vitreous silica sintered from a soot body which had been dehydrated in an atmosphere containing chlorine prior to sintering under vacuum, so that the OH content was 1 ppM and residual chlorine was approximately 2,000 ppM. This glass exhibited an annealing point of only 1,084 C., dramatically lower than the figures achieved in the present investigation.

(16) Intense examination of a polished annular section of the above ingot of thickness 100 mm typically reveals no bubbles or inclusions of size greater than the detection limit of the apparatus (10 m) in a volume of 100 cm.sup.3.

(17) This example demonstrates the principle of the new process, which provides a novel and economically viable route for the manufacture of blanks for making annular semiconductor jigs of extreme chemical purity, typically having all metallic impurities less than 100 ppB, substantially free from bubbles and inclusions greater than 100 m, and of high viscosity and excellent etch resistance.

(18) It is thought that the high viscosity of the glass made in the present work may arise in part from the fact that the soot body is dehydrated and subsequently sintered in intimate contact with a large diameter graphite mandrel, and within a vacuum furnace with graphite heating elements and carbon-based insulation materials. The heat treatment processes described above thus occur in a strongly reducing environment. It is furthermore to be expected that high viscosity glass with good plasma-etch resistance will also be achieved if, prior to sintering, the heat treatment includes heating in a reducing atmosphere, for example in the presence gases such as hydrogen, carbon monoxide, ammonia, a hydrocarbon gas or an appropriate organic or organosilicon vapour, such as the vapour of a siloxane or a silazane.

(19) It should be noted that with the current process higher levels of doping by nitrogen, by carbon, or by both can be achieved than with prior art processes because, following sintering, no further hot working of the material is necessary in the manufacture of the final product.

(20) The above example employed OMCTS as precursor introduced to the synthesis flame in the form of vapour. Alternatively, other siloxanes may be used alone or as mixtures of vapours. Again alternatively, it is possible to use other chlorine-free silicon-containing precursors, for example alkoxysilanes, either pure or as mixtures. In further embodiments, any of these precursors may be fed to the synthesis flame as a spray of atomised liquid droplets using burners adapted for atomisation via conventional methods, including gas atomisation, and ultrasonic atomisation, etc.

(21) In further variations of the above process, it is possible to manufacture a large diameter hollow body of synthetic vitreous silica of near net shape doped with one or more rare-earth metals, optionally in the presence of aluminium as a co-dopant. This may be achieved using a chlorine-free silica precursor of silica, preferably a siloxane compound fed as vapour to the flame, and by feeding the dopant metals likewise in the form of vapours of appropriate compounds. However, it may also be achieved by feeding the silica precursor, preferably a siloxane, in the form of a spray of atomised liquid, with the dopant metal or metals being fed as an aqueous solution of the appropriate salts in the form of an emulsion of microdroplets of the aqueous phase dispersed in the silica precursor, as envisaged in GB 1003468.4 and PCT/EP2011/052923. An alternative doping method involves immersion of the porous soot body in a solution of one or more salts of aluminium and/or one or more rare earth metals, before drying, calcining and sintering to pore-free glass.