Composite material, heat-absorbing component, and method for producing the composite material

Abstract

In a known composite material with a fused silica matrix there are regions of silicon-containing phase embedded. In order to provide a composite material which is suitable for producing components for use in high-temperature processes for heat treatment even when exacting requirements are imposed on impermeability to gas and on purity, it is proposed in accordance with the invention that the composite material be impervious to gas, have a closed porosity of less than 0.5% and a specific density of at least 2.19 g/cm.sup.3, and at a temperature of 1000 C. have a spectral emissivity of at least 0.7 for wavelengths between 2 and 8 m.

Claims

1. A composite material comprising: a matrix of fused silica in which regions of a phase containing silicon in elemental form have been embedded, wherein the composite material is impervious to gas, has a closed porosity of less than 0.5% and a specific density of at least 2.19 g/cm.sup.3, and, at a temperature of 1000 C., has a spectral emissivity of at least 0.7 for wavelengths between 2 and 8 m.

2. The composite material according to claim 1, wherein the matrix has pores therein with a maximum pore dimension of less than 10 m.

3. The composite material according to claim 1, wherein the phase containing silicon in elemental form is present in a weight fraction that is not more than 5%.

4. The composite material according to claim 1, wherein the matrix consists essentially of fused silica having a hydroxyl group content of not more than 30 ppm by weight.

5. The composite material according to claim 1, wherein the phase containing silicon in elemental form consists essentially of silicon having a metallic purity of at least 99.99% and wherein the matrix possesses a chemical purity of at least 99.99% SiO.sub.2 and a cristobalite content of not more than 1%.

6. The composite material according to claim 1, wherein the phase containing silicon in elemental form exhibits non-spherical morphology with maximum dimensions of on average less than 20 m.

7. A heat-absorbing component, comprising: at least one surface formed from a composite material comprising a matrix of fused silica in which regions of a phase containing silicon in elemental form have been embedded, said matrix being impervious to gas, having a closed porosity of less than 0.5% and a specific density of at least 2.19 g/cm.sup.3, and, at a temperature of 1000 C., having a spectral emissivity of at least 0.7 for wavelengths between 2 and 8 m.

8. The component according to claim 7, wherein the component is a reactor, fitting, or component configured to be used in an oxidizing or heat-treating operation, in epitaxy, or in chemical vapour deposition.

9. The component according to claim 7, wherein the component is a plate, ring, flange, dome, crucible, or solid or hollow cylinder.

10. A method for producing a composite material, said method comprising: forming a porous shaped body from a powder mixture comprising amorphous fused silica powder and a powder containing silicon in elemental form and/or from a mixed powder comprising amorphous fused silica, interspersed with a phase containing silicon in elemental form and, compacting the shaped body so as to produce the composite material, wherein the forming of the shaped body comprises a slipcasting process comprising producing a suspension that comprises the powder mixture and/or the mixed powder in a liquid, consolidating the suspension by removal of liquid so as to form a green body, and forming the shaped body from the green body by drying.

11. The method according to claim 10, wherein the suspension comprises a powder mixture of fused silica powder and powder containing silicon in elemental form, and wherein the powder containing silicon in elemental form consists essentially of silicon having a metallic purity of at least 99.99%, and has a particle size distribution with a D.sub.97 of between 1 to 20 m and a D.sub.10 of 2 m, and the powder containing silicon in elemental form constituting a volume fraction in the powder mixture of not more than 5%.

12. The method according to claim 11, wherein the powder containing silicon in elemental form is mixed into the liquid comprising the fused silica powder.

13. The method according to claim 10, wherein the fused silica powder comprises amorphous particles having particle sizes not more than 200 m, and wherein SiO.sub.2 particles having particle sizes between 1 m and 60 m make up a largest volume fraction of the fused silica powder.

14. The method according to claim 13, wherein the fused silica powder particles have a particle size distribution with a D.sub.50 of less than 50 m, and have an SiO.sub.2 content of at least 99.99% by weight.

15. The method according to claim 10, wherein the fused silica powder particles are produced by wet grinding of initial granular SiO.sub.2.

16. The method according to claim 10, wherein the shaped body is heated to a sintering temperature below the melting temperature of silicon.

17. The composite material according to claim 1, wherein the phase containing silicon in elemental form exhibits non-spherical morphology with maximum dimensions of on average between 3 and 20 m.

18. The method according to claim 13, wherein the amorphous particles have particle sizes not more than 100 m.

19. The method according to claim 13, wherein the fused silica powder particles have a particle size distribution which is characterized by a D.sub.50 of less than 40 m, and have an SiO.sub.2 content of at least 99.99% by weight.

20. The method according to claim 11, wherein the D.sub.97 of the particle size distribution is greater than 3 m.

Description

WORKING EXAMPLE

[0074] The invention is elucidated in more detail below by means of working examples and a drawing. As single figure,

[0075] FIG. 1 shows a flow diagram to illustrate the production of one embodiment of the fused silica component of the invention for use in semiconductor fabrication, on the basis of one procedure according to the invention;

[0076] FIG. 2 shows a diagram with the normal emissivity as a function of the wavelength at different temperatures for a sample with 5% by weight Si phase;

[0077] FIG. 3 shows a diagram of the normal emissivity as a function of the wavelength at different temperatures for a sample with 2% by weight Si phase;

[0078] FIG. 4 shows a diagram of the normal emissivity as a function of the wavelength at different temperatures for a sample with 1% by weight Si phase, and

[0079] FIG. 5 shows a diagram of the normal emissivity as a function of the wavelength of two different samples from the prior art, at room temperature, for comparison.

[0080] The method of the invention is elucidated by way of example hereinafter, using the production of a heat insulation ring made of fused silica for an RTP reactor for the treatment of a wafer, with reference to FIG. 1.

[0081] Sample 1

[0082] For a batch of 10 kg of base slip 1 (SiO.sub.2/water slip), in a drum mill having a fused silica lining and a capacity of approximately 20 litres, 8.2 kg of amorphous granular fused silica 2, obtained by fusing natural raw silica material and having particle sizes in the range between 250 m and 650 m, are mixed with 1.8 kg of deionized water 3 having a conductivity of less than 3 S. The granular fused silica 2 has been purified beforehand in a hot chlorinating process; it is ensured that the cristobalite content is below 1% by weight.

[0083] This mixture is ground on a roller bed at 23 rpm, using fused silica grinding balls, for a period of 3 days until the base slip 1 is homogeneous and has a solids content of 78%. In the course of the grinding procedure, as a result of SiO.sub.2 passing into solution, there is a lowering in the pH to approximately 4.

[0084] The grinding balls are subsequently removed from the resultant base slip 1, and an admixture is made, in the form of silicon powder 4 having a metallic purity of 99.99%, in an amount such as to give a solids content of 83% by weight.

[0085] The silicon powder 4 consists of substantially non-spherical powder particles with a narrow particle size distribution, whose D.sub.97 is approximately 10 m and whose fine fraction, with particle sizes of less than 2 m, has been removed beforehand. The silicon powder 4 is dispersed uniformly in the base slip 1 by continuous mixing.

[0086] The slip filled with the silicon powder 4 is homogenized for a further 12 hours. The homogeneous slip 5 obtained in this way has a solids content of 83%. The weight fraction of the silicon powder as a proportion of the overall solids content is 5%, and the volume fraction, owing to the similar specific densities of SiO.sub.2 and Si, is likewise almost 5%more precisely, 4.88%. The SiO.sub.2 particles 2 in the fully homogenized slip 5 exhibit a particle size distribution characterized by a D.sub.50 of about 8 m and by a D.sub.90 of about 40 m.

[0087] The slip 5 is cast into a pressure casting mould in a commercial pressure casting machine and dewatered via a porous polymeric membrane, to form a porous green body 6. The green body 6 has the shape of a ring for an RTP reactor for the treatment of wafers.

[0088] For the purpose of removing bound water, the green body 6 is dried in a ventilated oven at about 90 C. for five days and, after cooling, the resulting porous blank 7 is worked mechanically almost to the final dimensions of the fused silica ring 8 to be produced.

[0089] For the sintering of the blank 7, it is heated in a sintering oven, under air, to a heating temperature of 1390 C. over the course of an hour, and is held at this temperature for 5 h. Cooling takes place with a cooling ramp of 1 C./min to an oven temperature of 1000 C., and thereafter without regulation, with the oven closed.

[0090] The resulting fused silica ring 8 is superficially abraded to give an average surface roughness Ra of approximately 1 m. It consists of a gas-impervious composite material having a density of 2.1958 g/cm.sup.3, in which non-spherical regions of semimetallic Si phase, separated from one another in a matrix of opaque fused silica, are distributed homogeneously, the size and morphology of these Si phase regions corresponding largely to those of the Si powder employed. The maximum dimensions are on average (median value) in the range from about 1 to 10 m. The composite material is stable in air to a temperature of up to about 1200 C.

[0091] In visual terms, the matrix is translucent to transparent. When viewed under a microscope, it exhibits no open pores, and at most closed pores with maximum dimensions of on average less than 10 m; the porosity as calculated on the basis of the density is 0.37%, assuming a theoretical matrix density of 2.2 g/cm.sup.3 and a theoretical Si phase density of 2.33 g/cm.sup.3.

[0092] The incorporated Si phase contributes to the opacity and also has consequences for the thermal properties of the composite material overall. This composite material exhibits high absorption of thermal radiation at high temperature. This is shown by the diagram of FIG. 2, with the spectral profile of the emissivity of this material.

[0093] The emissivity at room temperature is measured in a customary way, using an Ulbricht sphere. This allows measurement of the directional hemispherical spectral reflectance R.sub.dh and of the directional hemispherical spectral transmittance T.sub.dh, from which the normal spectral emissivity is calculated.

[0094] The measurement at elevated temperature in the wavelength range from 2 to 18 m takes place by means of an FTIR spectrometer (Bruker IFS 66v Fourier Transform Infra-red (FTIR)), to which a BBC sample chamber is coupled via an additional optical system, on the basis of the aforementioned BBC measurement principle. This sample chamber, in the half-spaces in front of and behind the sample mount, has temperature-conditionable black-body surrounds and a beam exit opening with detector. The sample is heated to a predetermined temperature in a separate oven, and for measurement is moved into the beam path of the sample chamber, with the black-body surrounds set to the predetermined temperature. The intensity captured by the detector is composed of an emission component, a reflection component and a transmission component that is, of intensity emitted by the sample itself, intensity impinging on the sample from the front half-space and reflected by said sample, and intensity which impinges on the sample from the rear half-space and is transmitted by said sample. Three measurements must be carried out in order to determine the individual parameters of emissivity, reflectance and transmittance.

[0095] The diagram of FIG. 2, and also the further diagrams of FIGS. 3 and 4, each show the profile of the normal emissivity e as a function of the measurement wavelength (in m and in logarithmic plotting), over the wavelength range from 2 to 18 m and for different temperatures of the sample body between room temperature and 1200 C.

[0096] FIG. 2 shows that the emissivity of sample 1 with 5% by weight Si phase in the wavelength range from 2 to about 4 m is heavily dependent on the temperature of the sample body. The greater the heating of the sample body, the higher the emission in this wavelength range, with no substantial difference being evident any longer between 1000 C. and 1200 C. In the case of the samples heated to 1000 C. and 1200 C., the normal emissivity in the whole of the wavelength range between 2 and 8 m is above 0.75; at the wavelength of 3 m it is 0.79.

[0097] In principle, the emissivity increases essentially with the wavelength, but exhibits a pronounced minimum at approximately for measurement radiation of around 9 m. The minimum can be attributed to reflection by the fused silica of the matrix.

[0098] The effect of the incorporation of Si phase, and the production technique via the slipcasting route, are apparent, in particular, from the following phenomena: [0099] As the temperature of the sample body goes up, there is an increase in the emission in the wavelength range between 2 and 5 m. At the maximum measurement temperature of 1200 C., the maximum emission in this wavelength range is achieved as well. At the sample body temperature of 1000 C., the emission in the entire wavelength range between 2 and 8 m is more than 70%. In this wavelength range, therefore, the material exhibits low reflection and low transmission. [0100] In the wavelength range around 2.72 m, which is characteristic for the absorption and emission of hydroxyl groups in fused silica, there is no noticeable effect. This is made clear in particular by comparison of the spectral emission profile of the measurement sample at room temperature (as shown for sample 1 in FIG. 2) with the profile of the spectral emission for the comparative sample in accordance with FIG. 5. The production of the comparative sample C1as elucidated in more detail later on belowdiffers primarily, apart from small differences in the sintering conditions (see Table 1), in that the base slip contains no added Si. The diagram of FIG. 5 shows that the material of the comparative sample, in the wavelength range between 0.25 m and about 3.5 m, has a pronounced emission band at 2.72 m, which is attributable to the hydroxyl group content of this material. This emission band is completely absent for the composite material of sample 1 (and also, moreover, in the case of samples 2 and 3, as shown by FIGS. 3 and 4). From the respective emission measurements at room temperature, taking account of error tolerances, the hydroxyl group content for samples 1 to 3, averaged over the sample thickness, is not more than 20 ppm by weight. The fact that a low hydroxyl group content is obtained in spite of non-reactive drying of the green body (without use of drying reagents, such as fluorine or chlorine), shows that the Si phase consumes hydroxyl groups during the sintering of the composite material to high density.

[0101] Using the slipcasting process elucidated above, further composite materials were produced, with the composition and individual process parameters varied experimentally. Table 1 reports these parameters and the results measured on the samples.

[0102] FIG. 3 shows the spectral profile of the emissivity of sample 2 (with 2% by weight Si phase), and FIG. 4 shows the profile for sample 3 (with 1% by weight Si phase). The comparison of the respective maximum values of the emissivity in the wavelength range from 2 to 4 m shows the surprising outcome that the measurement sample with the smallest Si phase content (sample 3, with 1% by weight) shows the highest emissivity of all of the samples measured, with 0.85 at 3000 nm and 1000 C. This can be attributed to the fact that with a small Si phase content, lower-lying layers of material make a greater contribution to the emission than for a higher level. With an Si phase content of less than 1% by weight, therefore, even higher emissivities are likely in principle.

[0103] In the course of its intended use in an RTP reactor, the ring of composite material produced in this way surrounds a wafer that is to be treated. The internal diameter of this ring corresponds to the outer diameter of the wafer. The heating element of the RTP apparatus is generally configured as an array of IR emitters which are located in a plane above and/or below the combination of wafer and ring. The ring of composite material diminishes the effect of excessively rapid cooling at the wafer edge, and so contributes to uniform temperature distribution over the entire wafer surface.

[0104] In principle the composite material is predestined for applications where high heat absorption and low heat reflection, or a particularly homogeneous temperature distribution, are important factors. It may take a wide diversity of geometric forms, such as the form of a reactor, apparatus, carrier tray, bell, crucible or protective shield, or else the form of more simple components such as tubes, rods, plates, flanges, rings or blocks. Other examples include the following: [0105] The use as a heat store element for the thermal conditioning of semiconductor components or displays, including in particular in short-duration oxidizing and heat-treating processes. [0106] The use as a reactor or dome in the context of high-speed epitaxy, for both homo and hetero processes. [0107] The use as heat protection and cladding element especially with respect to IR radiation between 2 and 8 m at high temperatures. [0108] The use for artistic or design applications.

Comparative Example 1 (Sample C1)

[0109] The high density, low porosity and high emissivity of the composite material of the invention are attributable substantially to the nature, size and distribution of the Si phase inclusions.

[0110] This is shown by comparison with a commercial opaque fused silica ring without corresponding inclusions of Si phase. A material of this kind and its production are described in DE 44 40 104 A1. Apart from the use of Si-containing starting material to produce the base slip, and slight differences with regard to the sintering conditions, the production of this material corresponds to that of Example 1. It has a density of about 2.16 g/cm.sup.3 and a closed porosity of 2.5% and it acts primarily as a diffuse reflector, this being evident from the fact that it possesses a virtually constant direct spectral transmission of less than 10% in the wavelength range from 190 to 2650 nm with a path length of just 1 mm.

[0111] The diagram of FIG. 5 shows the spectral emissivity of this material (from two measurement runs) in the wavelength range between 0.25 m and 18 m at room temperature. In the visible wavelength range to approximately 2 m, the emissivity is less than 10%, a fact which can be attributed primarily to the high reflectivity of the opaque fused silica. At a wavelength of around 2.7 m, the absorption or emission band already mentioned earlier on above is apparent, this band being assignable to hydroxyl groups in the fused silica, amounting to around 300 ppm by weight in the case of this fused silica.

TABLE-US-00001 TABLE 1 Sample C1 1 2 3 4 5 Production process/ Slip/ Slip/ Slip/ Slip/ Slip/ Slip/ liquid H.sub.2O H.sub.2O H.sub.2O H.sub.2O H.sub.2O H.sub.2O Sintering 1435/3 1390/5 1390/5 1390/5 1350/24 1350/48 temperature/time ( C./h) Si phase content 0 5 2 1 2.5 2.5 (% by weight) Si phase particle 10 10 10 10 12 size: D97 Hydroxyl group 300 <20 <20 <20 <20 <20 content (ppm by weight) Density (g/cm{circumflex over ()}3) 2.16 2.1930 2.1945 2.1958 2.1966 2.1998 Porosity (%) 2.5 0.37 0.4 0.5 0.30 0.15 Emissivity @ 0.21 0.79 0.79 0.85 0.75 0.81 1000 C./3000 nm

[0112] The figures for the emissivity are based on the composite material as a whole. For samples 4 and 5, these values were not determined (n.d.). In column C1, the data for the above comparative example are reported.