DOPED FUSED SILICA COMPONENT FOR USE IN A PLASMA-ASSISTED MANUFACTURING PROCESS AND METHOD FOR PRODUCING THE COMPONENT

Abstract

Doped quartz glass components for use in a plasma-assisted manufacturing process contain at least one dopant which is capable of reacting with fluorine to form a fluoride compound, and the fluoride compound has a boiling point higher than that of SiF.sub.4. The doped quartz glass component has high dry-etch resistance and low particle formation, and has uniform etch removal when used in a plasma-assisted manufacturing process. The doped quartz glass has a microhomogeneity defined by (a) a surface roughness with an R.sub.a value of less than 20 nm after the surface has been subjected to a dry-etching procedure as specified in the description, or (b) a dopant distribution with a lateral concentration profile in which maxima of the dopant concentration are at an average distance apart of less than 30 μm.

Claims

1-12. (canceled)

13. A doped quartz glass component for use in a plasma-assisted semiconductor manufacturing process containing at least one dopant that is capable of reacting with fluorine to form a fluoride compound, wherein the fluoride compound has a boiling point higher than that of SiF.sub.4, and characterized in that the doped quartz glass has a microhomogeneity defined by (a) a surface roughness with an R.sub.a value of less than 20 nm after the surface has been subjected to a dry-etching procedure as specified in the description, or (b) a dopant distribution with a lateral concentration profile in which maxima of the dopant concentration are at an average distance apart of less than 30 μm.

14. The component according to claim 13, characterized in that the surface has an R.sub.a value of less than 15 nm or the maxima of the dopant concentration are at an average distance apart of less than 20 μm.

15. The component according to claim 13, characterized in that the dopant or dopants are present in a total dopant concentration ranging from 0.1 wt. % to 5 wt. %.

16. The component according to claim 13, characterized in that the dopant or dopants are present in a total dopant concentration ranging from 0.5 to 3 wt. %.

17. The component according to claim 13, characterized in that the doped quartz glass contains at least one dopant compound with a dopant selected from the group consisting of: Al, Sm, Eu, Yb, Pm, Pr, Nd, Ce, Tb, Gd, Ba, Mg, Y, Tm, Dy, Ho, Er, Cd, Co, Cr, Cs, Zr, In, Cu, Fe, Bi, Ga and Ti.

18. The component according to claim 14, characterized in that aluminium is the dopant and Al.sub.2O.sub.3 is the dopant compound, and in that the total dopant concentration is in the range of 0.5 to 3 wt. %.

19. The component according to claim 13, characterized in that the doped quartz glass is made from synthetically produced SiO.sub.2 raw materials.

20. A method of producing a doped quartz glass component according to claim 13, for use in a plasma-assisted manufacturing process, comprising the following method steps: (a) providing a slip containing SiO.sub.2 particles in an aqueous liquid, (b) providing a doping solution containing a solvent and at least one dopant in dissolved form, (c) bringing together doping solution and slip to form a dispersion, in which a solid containing the dopant is precipitated, (d) drying the dispersion to form granular particles containing SiO.sub.2 and the dopant, and (e) sintering or fusing the granular particles to form the doped quartz glass component, characterized in that the SiO.sub.2 particles in the slip are aggregates or agglomerates of SiO.sub.2 primary particles and have an average particle size of less than 30 μm.

21. The method according to claim 20, characterized in that for bringing together the doping solution and slip, the doping solution is atomised to form a spray mist and this is supplied to the dispersion.

22. The method according to claim 20, characterized in that when the doping solution and slip are brought together, the latter is kept in motion.

23. The method according to claim 20, characterized in that before the doping solution and slip are brought together, the latter is adjusted to a pH value greater than 12.

24. The method according to claim 20, characterized in that the SiO.sub.2 primary particles are produced pyrogenically and preferably have an average particle size of less than 100 nm.

25. The method according to claim 20, characterized in that the sintering of the granular particles takes place in a nitrogen-containing atmosphere by gas pressure sintering.

Description

EXEMPLARY EMBODIMENT

[0076] The invention will be explained in more detail below with the aid of an exemplary embodiment and a drawing. The individual figures show the following:

[0077] FIG. 1 an etch profile in a sample made of an Al.sub.2O.sub.3-doped quartz glass according to the invention after carrying out a standard dry-etching program in a plasma-etching reactor,

[0078] FIG. 2 an etch profile in a reference sample made of pure, undoped quartz glass (reference sample) after carrying out the standard dry-etching program,

[0079] FIG. 3 a graph showing the erosion rate as a function of the acceleration voltage of the plasma-etching reactor,

[0080] FIG. 4 a graph showing the erosion rate as a function of the Al.sub.2O.sub.3 concentration of the quartz glass,

[0081] FIG. 5 a graph showing the erosion rate as a function of the CF.sub.4 concentration in the etching gas of the plasma-etching reactor,

[0082] FIG. 6 a graph showing the relative erosion rate as a function of the internal pressure in the etching chamber of the plasma-etching reactor for various samples,

[0083] FIG. 7 a graph showing the chemical occupancy of the surface of etched samples as a function of the etching period,

[0084] FIG. 8 an etch profile in a comparative sample made of an Al.sub.2O.sub.3-doped quartz glass after carrying out the standard dry-etching program,

[0085] FIG. 9 a scanning electron microscope image of a sample surface made of pure, undoped quartz glass (reference sample) after carrying out the standard dry-etching program in the plasma-etching reactor,

[0086] FIG. 10 a scanning electron microscope image of a sample surface made of Al.sub.2O.sub.3-doped quartz glass according to the invention after carrying out the standard dry-etching program in the plasma-etching reactor,

[0087] FIG. 11 a scanning electron microscope image of the surface in a comparative sample made of an Al.sub.2O.sub.3-doped quartz glass after carrying out the standard dry-etching program,

[0088] FIG. 12 the surface of the comparative sample of FIG. 11 in higher magnification,

[0089] FIG. 13 an embodiment of a reactor for carrying out a plasma-assisted manufacturing process, and in particular for carrying out dry-etching procedures, in a schematic diagram,

[0090] FIG. 14 a sketch explaining the method of determining a lateral dopant concentration profile,

[0091] FIG. 15 an image produced by energy-dispersive X-ray spectroscopy (EDX) of the surface of a test sample made of a flame-fused and Al.sub.2O.sub.3-doped quartz glass after carrying out the standard dry-etching program in the plasma reactor, and

[0092] FIG. 16 a graph showing the lateral, two-dimensional relative concentration distribution of the elements, Si, Al, C, oxygen (O) and fluorine (F) within the test sample along the measuring line drawn in on FIG. 15.

Production of a Doped Quartz Glass Component

[0093] In a conventional soot deposition process, using octamethylcyclotetrasiloxane (OMCTS) as a starting substance, SiO.sub.2 primary particles with average particle sizes of less than 100 nm were synthesised, which agglomerated together in a reaction zone to form secondary particles in the form of more or less spherical aggregates or agglomerates. These secondary particles, which were made up of different numbers of primary particles and had an approximate average particle size (D.sub.50 value) of less than 10 μm, will also be referred to below as “SiO.sub.2 particles”. Table 1 gives typical properties of the SiO.sub.2 particles.

TABLE-US-00001 TABLE 1 Tapped density 0.03-0.05 m.sup.2/g Residual moisture 0.02-1.0% Primary particle size 94 nm D.sub.10 3.9 +/− 0.38 μm D.sub.50 9.4 +/− 0.67 μm D.sub.90 25.6 +/− 10.4 μm

[0094] A slip was prepared, composed of these discrete, synthetically produced SiO.sub.2 particles with an average particle size (D.sub.50 value) of around 10 μm in ultrapure water.

[0095] By adding a concentrated ammonia solution, the pH value was adjusted to 14. The alkaline suspension was homogenised and filtered.

[0096] In addition, an aqueous doping solution of AlCl.sub.3 in ultrapure water was produced, homogenised and likewise filtered.

[0097] The doping solution was supplied in the form of a spray mist to the slip, which was agitated by stirring. To produce the spray mist, the doping solution was atomised using a spray nozzle, the operating pressure being set at 2 bar and the flow rate at 0.8 I/h. The spray mist thus produced contained drops having an average diameter of between 10 μm and 40 μm. Owing to the high pH of the slip, an immediate precipitation of the dopant occurred in the form of Al(OH).sub.3. The solid particles adsorbed on the existing surfaces of the SiO.sub.2 particles and were thereby immobilised, such that a coagulation of the solid particles or a sedimentation was prevented. The slip to which the dopant had been added was then homogenised by stirring for a further 2 hours. With this procedure, it was ensured that an optimally homogeneously doped SiO.sub.2 slip was obtained.

[0098] The doped SiO.sub.2 slip was frozen and further processed by frost granulation to form a granular material. The granular material slurry obtained after thawing was washed multiple times with ultrapure water and the excess water was decanted off each time.

[0099] The granular material slurry that had been freed from ammonia and purified was then dried at a temperature of around 400° C. The dried granular material typically had grain sizes ranging from 300 μm to 600 μm. It was welded into a plastic mould and pressed isostatically at 400 bar to form a granular material blank.

[0100] The granular material blank was treated in a chlorine-containing atmosphere at approximately 900° C. for approximately 8 hours. As a result, impurities were removed from the blank and the hydroxyl group content was reduced to approximately 3 ppm by weight.

[0101] The purified granular material blank had a cylindrical shape with a diameter of 30 mm and a length of 100 mm. Its average density was approximately 45% of the density of the doped quartz glass. It was pre-sintered by heating to a temperature of 1550° C. in a vacuum furnace and then sintered by gas pressure sintering under argon to form a cylinder of Al.sub.2O.sub.3-doped, transparent quartz glass. The gas pressure sintering process was performed in a gas pressure sintering furnace with an evacuable sintering mould made of graphite. The interior of the sintering mould was of cylindrical configuration and was delimited by a base and a side wall having an annular cross-section.

[0102] In this way, glass samples with average Al.sub.2O.sub.3 concentrations of between 1 and 2.7 wt. % were prepared. For carrying out measurements, plates with a thickness of approximately 1 mm and lateral dimensions of between 13 mm×13 mm and 28 mm×28 mm were cut therefrom and polished.

Plasma-Etching Tests

[0103] Dry-etching tests were performed on samples of the Al.sub.2O.sub.3-doped quartz glass and a sample of commercially available quartz glass. For this purpose, a dry-etching reactor was employed, as explained above with the aid of FIG. 13.

[0104] The surface[s] of the samples to be measured were polished, such that they had an initial average roughness (R.sub.a value) of approximately 3 nm, and were partially masked with polyimide tape. The samples were then treated for a period of 0.5 hours to 3 hours together with a reference sample made of commercially available, non-doped quartz glass of high homogeneity (“Spectrosil 2000” from Heraeus Quarzglas GmbH & Co. KG) in order to measure the etch stage (also referred to below as the “erosion stage”) and the surface roughness.

Surface Profile and Erosion Rates

[0105] The relative erosion rate of the aluminium-doped samples compared with the reference sample varied as a function of the aluminium oxide concentration of the sample, the chamber pressure, the inductive power coupled to the plasma, and the bias voltage that was applied.

[0106] The graph of FIG. 2 shows the surface profile thus obtained for the reference sample of undoped quartz glass after carrying out the standard dry-etching procedure explained above over a total etching period of 1 h. The etch depth H (in nm) is plotted against the position coordinate P (in μm). On the right-hand side of the graph is the masked surface region of the sample; on the left-hand side is the etched and roughened region of the sample. This shows that a pronounced erosion stage has formed, with a height of approximately 1360 nm; the R.sub.a value of the eroded surface was approximately 15 nm.

[0107] As a comparison, FIG. 1 shows the profile curve of a sample doped with 2.7 wt. % Al.sub.2O.sub.3, which was treated together with the reference sample. This sample shows an erosion stage of approximately 560 nm and thus an erosion rate approximately 59% lower than that of the reference sample. The R.sub.a value of the eroded surface was approximately 10 nm, and therefore was even somewhat lower than for the reference sample.

[0108] The graph of FIG. 3 shows an example of the dependence of the erosion rates on bias voltage for a sample of quartz glass doped with 1.5% Al.sub.2O.sub.3, compared with the reference sample. Here, the erosion rate v.sub.E (in μm/h) is plotted against the bias voltage By in (V). It was shown that, over the entire bias voltage range between 0 V and 300 V, the erosion rate of the aluminium-doped sample was significantly lower than that of the reference sample. At higher bias voltages, however, the relative difference between the erosion rates decreased. This is interpreted as follows.

[0109] During the plasma treatment, fluorine from the fluorocarbon plasma reacts with aluminium in the Al.sub.2O.sub.3-doped quartz glass, resulting in a surface layer on the glass which contains aluminium fluoride as well as silicon dioxide. In addition, the fluorine reacts with the silicon in the glass and forms silicon fluoride (SiF.sub.4). While SiF.sub.4 is gaseous at ambient temperature and therefore escapes from the surface immediately, AlF.sub.3 is solid and remains on the surface, thereby preventing further erosion and reducing the erosion rate. At higher bias voltages, the energy of the ions (principally argon ions) reaching the glass surface is higher and leads to increased sputtering of the surface, including the sputtering of the AlF.sub.3 formed by chemical reaction. Thus, at higher bias voltages the erosion rate of the aluminium-doped sample approaches that of pure quartz glass.

[0110] The graph in FIG. 4 shows the dependence of the erosion rate v.sub.E (in μm/h) on the initial Al.sub.2O.sub.3 concentration C.sub.Al (in wt. %). The erosion rates were determined for the reference sample (C.sub.Al=0) and for samples with weighed Al.sub.2O.sub.3 concentrations of 1, 1.5 and 2 wt. %. The following etch parameters were used: plasma gas composition: 90 vol. % argon and 10 vol. % CF.sub.4,

induction power: 600 W,
bias voltage (DC bias): 100 V,
chamber pressure: 2.8 Pa.

[0111] It is shown that the erosion rate decreases with the aluminium oxide concentration and, in the sample with the highest concentration (C.sub.Al=2 wt. %), it falls to approximately 40% based on the erosion rate of the reference sample.

[0112] The graph of FIG. 5 shows the dependence of the erosion rate v.sub.E (in μm/h) on the plasma gas composition, or more precisely on the proportion of CF.sub.4 in the plasma gas C.sub.CF4 (in vol. %; the rest is argon) and on the dopant concentration (for Al.sub.2O concentrations of 0; 1.0; 1.5; 2.0 and 2.5 wt. %) for the following etch parameters:

induction power: 600 W,
bias voltage: 100 V DC,
chamber pressure: 2.8 Pa.

[0113] The highest erosion rate is obtained for a composition of the plasma gas with approximately 10 vol. % CF.sub.4 and 90 vol. % argon. The relative erosion rates of the aluminium-doped quartz glasses compared with the reference sample were lowest for the highest CF.sub.4 content in the test, of 80 vol. %. This is in line with the theory that the erosion rate reduction is most significant when the plasma is rich in fluorine which is available for a chemical reaction with the aluminium in the glass, to form a masking of dense AlF.sub.3, and that the reduction in the erosion rate is less pronounced when the plasma is rich in argon, which increases the sputtering rate of the AlF.sub.3 on the sample surface.

[0114] FIG. 6 shows the dependence of the erosion rate v.sub.E (in μm/h) on the chamber pressure for quartz glass samples doped with 1.5 wt. % and with 2.5 wt. % Al.sub.2O.sub.3. In the graph, the development of the relative erosion rate (in μm/h—based on the erosion rate of the reference sample) is plotted against the bias voltage By (in V) for different chamber pressures (1 Pa and 6 Pa). At sufficiently low pressures and high bias voltages, no significant difference in the erosion rates is shown between the aluminium-doped glasses and the reference sample. At a low chamber pressure of 1 Pa, the sample with the weighed Al.sub.2O.sub.3 concentration of 1.5 wt. % shows a reduced erosion rate effect only at bias voltages of less than approximately 50 V. At a high chamber pressure of 6 Pa, however, both of the samples with the weighed Al.sub.2O.sub.3 concentrations of 1.5 wt. % and 2.5 wt. % show a lower relative erosion rate up to bias voltages of approximately 400 V. Thus, the effect of the doping on the etch rate depends on both the bias voltage and the chamber pressure. At lower chamber pressures, it is assumed that the flow of ions to the sample surface is higher, which would lead to a more intense sputtering of the aluminium fluoride masking. Thus, at lower chamber pressures the masking effect, which leads to a reduced erosion rate, would be less pronounced than at high chamber pressures.

[0115] To support the theory that the reduction in the erosion rate of the aluminium-doped samples is attributable to an AlF.sub.3 enrichment of the eroded surface, X-ray photoelectron spectroscopy measurements were performed on the eroded surfaces. A result of these measurements is shown by the graph of FIG. 7, from which the development of the relative molar concentrations C (mole %) of aluminium, fluorine and silicon can be seen for a test sample with a weighed Al.sub.2O.sub.3 content of 0.6 wt. % over the etching period t (in min). The measurements started only after a preliminary 10-minute sputtering of the surface to remove impurities (e.g. from carbon). The samples were treated with the plasma gas for 15 minutes, 30 minutes, 60 minutes and 120 minutes. A separate sample was produced for each measurement period. It was shown that the surface was enriched with aluminium and fluoride in the course of the plasma treatment and an approximately constant concentration was reached after approximately 30 minutes. The initial aluminium (oxide) concentration of approx. 1.6 mole % was increased to approximately 10 mole % by the plasma treatment, and the fluoride concentration rose to approximately 20 mole %. The Al:F ratio of 1:2 does not quite correspond to the molar ratio of 1:3 that would be expected of a pure AlF.sub.3 species, but shows that a chemical reaction took place between Al and F and an enrichment of these two species occurred on the surface. At the same time, the relative molar concentration of silicon was reduced, which can be explained by the enrichment with Al and F and by the chemical reaction of fluorine with silicon, resulting in volatile SiF.sub.4.

[0116] It has been shown that the quartz glass prepared by the method according to the invention has, after the plasma-etching treatment, a surface with a roughness that is significantly lower than the surface roughness of aluminium-doped samples produced according to the prior art. For example, a sample doped with approximately 0.9 wt. % Al.sub.2O.sub.3 was prepared by the method described in the above-mentioned US 2008/0066497 A1 (melting a powder mixture and depositing molten glass particles on a carrier by the Verneuil method). The treatment of this quartz glass with plasma conditions similar to those for the samples described with the aid of FIGS. 1 and 2 led to a significantly rougher surface, as shown by the erosion profile in FIG. 8 (etch depth H (in nm) and position coordinate P (in μm)). The unmasked part of the sample (left-hand side) shows valleys with a depth of more than 1000 nm and a surface roughness with an R.sub.a value of 160 nm. This is a measure of the marked change in the surface over the course of the lifetime of the component as a result of etch removal, which makes it difficult to take proper and reproducible account of the parameter of “surface roughness” with regard to particle generation in semiconductor fabrication.

[0117] In Table 2, the R.sub.a values of the etched surfaces are compiled for the reference quartz glass as described with the aid of FIG. 2, for the example according to the invention as described with the aid of FIG. 1, and for the comparative example as described with the aid of FIG. 8.

TABLE-US-00002 TABLE 2 Comparative Reference Example example (FIG. 2) (FIG. 1) (FIG. 8) Roughness depth R.sub.a (nm) 15 10 260 Relative change in 1 0.7 17 roughness depth R.sub.a based on reference R.sub.a

[0118] After the dry-etching treatment, the surface in the comparative example displays an average roughness depth R.sub.a which is higher by a factor of 17 compared with the surface of the undoped but highly homogeneous reference quartz glass. In comparison, after the dry-etching treatment, the surface of the doped quartz glass according to the invention displays an average roughness depth R.sub.a which is lower by a factor of 0.7 compared with the reference quartz glass.

[0119] A plurality of etching tests show that, regardless of the specific parameters of the dry-etching treatment, the ratio of the average roughness depths of doped quartz glass according to the invention and reference quartz glass is typically and preferably in the range of 0.5 and 3 and particularly preferably in the range of 0.7 to 2 for test samples treated at the same time.

[0120] FIG. 9 is an SEM image of the surface of the reference sample after a standard dry-etching procedure in a 10,000× magnification. The lateral distance between peaks and valleys of the roughness profile is approximately 1 μm.

[0121] FIG. 10 likewise shows a 10,000× magnification of the surface of a plasma-treated test sample made of quartz glass according to the invention, which is doped with 0.5 wt. % Al.sub.2O.sub.3. The lateral distance between peaks and valleys of the roughness profile is in the same range as for the reference sample.

[0122] The SEM image of FIG. 11 likewise shows a 10,000× magnification of the surface of a comparative sample made of quartz glass doped with 0.9 wt. % Al.sub.2O.sub.3 which has been plasma-treated with the aid of the standard dry-etching procedure and produced with the aid of the Verneuil method as in US 2008/0066497 A1. It can be seen that partial regions of the surface are similar to the surfaces shown in FIGS. 9 and 10 but that other partial regions have a different structure, which emphasises the greater inhomogeneity of a plasma-treated sample prepared in this way.

[0123] FIG. 12 is an image of the comparative sample at a lower magnification of approximately 610×, from which it can be seen that the lateral distances between valleys and peaks of the surface profile are approximately 200 μm on average.

[0124] In summary, these investigations show that a doped quartz glass produced by the production method described above using a doped slip made of pyrogenically produced SiO.sub.2 particles leads to at least a twofold reduction in the plasma erosion rate if the plasma conditions are such that the physical sputtering of the surface is minimised. In particular, the bias voltage on the sample should not be too high, and a chamber pressure greater than approximately 2 Pa has a favourable effect.

[0125] FIG. 15 shows measurement results for the chemical microhomogeneity of a flame-fused, aluminium-doped quartz glass after carrying out the standard dry-etching treatment in a plasma reactor. The flame fusion takes place by melting a powder mixture and depositing the molten glass particles on a carrier by the Verneuil method. The macroscopic measurement of the aluminium concentration in the test sample before the dry-etching treatment gave approximately 0.7 at. %.

[0126] After the dry-etching treatment, microscopic measurements of the chemical composition were performed by energy-dispersive X-ray spectroscopy (EDX). The dark regions of the image correspond to the topographic peaks determined profilometrically in roughness measurements, and the light regions to the topographic valleys. In the light region 51a, EDX analysis gives the following chemical composition (in at. %):

oxygen: 52.3%, silicon: 26.0%, carbon: 20.7%.

[0127] The light region 51b thus contains aluminium in a negligible quantity if at all. For the dark region 51b, the following chemical composition is obtained (in at. %):

oxygen: 46.5%, carbon: 26.9%, silicon: 20.0%, aluminium: 3.7%, fluorine: 2.8%.

[0128] The dark region 51a thus displays an aluminium enrichment caused by the plasma erosion process. A transition region between the light and dark regions (51a, 51b) having a longitudinal extension of approximately 30 μm is symbolised in FIG. 15 by an ellipsis with the reference sign 51c, and a measurement line for the line scan of FIG. 16 by the reference sign 51d.

[0129] In the line scan of FIG. 16, the pulse number “N” of the EDX elemental concentration measurements (in relative units) is plotted on the y axis against the position coordinate “x” along the measurement line 51d drawn in on FIG. 15. The concentration profiles for Si, Al, O, C, and F are labelled with the relevant chemical element symbols. A plurality of element symbols in brackets, such as (Al, C, F), (C, F) and (Al, O) show profile regions in which the profiles of the elements mentioned in each case overlap. It is apparent that, in the transition region 51c, the aluminium concentration increases significantly over a distance of approximately 30 μm. This test sample displays inadequate microhomogeneity accompanied by low dry-etch resistance. It is proof of the fact that, in the production of doped quartz glass for use in plasma-assisted manufacturing processes, the manufacturing method used plays a crucial part in adjusting the microhomogeneity.