Aluminum nitride sintered bodies and their production method

Abstract: Disclosed are aluminum nitride sintered bodies having blackish hues, i.e. low lightness, useful in semiconductor production. Such an aluminum nitride sintered body is characterized by having a g-value of an unpaired electron in a spectrum of an electron spin resonance being not less than 2.0010. Alternatively the aluminum nitride sintered body is characterized by its ESR spectrum or its laser Raman spectrum. One method of producing the aluminum nitride sintered body is by sintering a raw material composed of powdery aluminum nitride having a carbon content of 500 - 5000 ppm at a temperature of not less than 1730°C to not more than 1920°C under a pressure of not less than 80 kg/cm 2 .

Description: Background of the Invention (1) Field of the Invention The present invention relates to aluminum nitride bodies and an apparatus for producing seminconductors using such aluminum nitride bodies as substrates. The invention also relates to aluminum nitride sintered bodies and its production process thereof as well as to an apparatus for producing seminconductors using such aluminum nitride sintered bodies bodies as substrates. (2) Related Art Statement In seminconductor-producing apparatuses such as etching apparatuses or chemical gas phase growing apparatuses, so-called stainless heaters or indirect-heating type heaters have been generally used. However, if such heat sources are employed, particles may be produced by the action of a halogen-based corrosive gas and a heat efficiency is poor. In order to solve those problems, NGK Insulators disclosed a ceramic heater in which a wire made of a high melting point metal is buried inside a dense ceramic substrate (JP-A 3-261131). This wire is spirally coiled inside the discoidal substrate, and terminals are connected to the opposite ends of the wire. It has been discovered that such a ceramic heater has excellent characteristics suitable particularly for use in the production of semiconductors. It is considered that nitride-based ceramics such as silicon nitride, aluminum nitride, and sialon are preferred as ceramics constituting substrates for the ceramic heaters. Further, a susceptor is sometimes placed on the ceramic heater, and a semiconductor wafer placed on the susceptor is heated. NGK Insulators, Ltd. disclosed that aluminum nitride is preferred as substrates for such ceramic heaters and susceptors (JP-A 5-101871). Particularly in the semiconductor-producing apparatuses, halogen-based corrosive gases such as ClF 3 gas are often used as an etching gas or a cleaning gas. The reason why aluminum nitride is preferred is based on confirmation that aluminum nitride has extremely high corrosion resistance against such halogen-based corrosive gases. A aluminum nitride sintered body itself generally exhibits white or whitish gray. However, it is desired that the substrate to be used as the above-mentioned heater or susceptor is black, because the black substrate has a greater radiation heat capacity and a better heating characteristic as compared with the white ones. Further, if the white or gray substrate is used in the above heater or the susceptor, uneven color is unfavorably likely to appear at the surface thereof. Therefore, some improvement has been demanded. Further, customers prefer blackish brown, or blackish gray substrates and the like having a high black degree and a low lightness than the whitish or gray substrates. In order to make an aluminum nitride sintered body black, it is known that an appropriate elementary metal or a metal compound (a blacking agent) is added into a powdery raw material, and a black aluminum nitride sintered body is produced by firing the resulting mixture (JP-B 5-64697). Such an additive, tungsten, titanium oxide, nickel, vanadium, etc. are known. EP-A-0743290 (only citable under EPC Article 54(3)) relates to aluminium nitride sintered bodies and production methods for such bodies. EP-A-0237072 and EP-A-0321975 relate to aluminium nitride sintered bodies containing relatively significant levels of metals other than aluminium. Disclosure of the Invention However, if an elementary metal or a metallic compound is incorporated into an aluminum nitride sintered body as a blacking agent, the content of metallic impurities in the aluminum nitride sintered body naturally becomes larger due to the incorporation of the additive. Particularly, if a Ia group element, a I element and/or a transition metal element exists in the aluminum nitride sintered body in the semiconductor-producing process, such element(s) adversely afford serious problem upon the semiconductor wafer or the semiconductor-producing apparatus, even if the content thereof is very small (For example, this may cause a defect in the semiconductor). Therefore, it has been demanded to decrease the "lightness" of the aluminum nitride sintered body without incorporating the above blacking agent therein. "Lightness" is explained below. It is an object of the present invention to decrease the "lightness" of an aluminum nitride sintered body and to approach its color to black without incorporating a metal or a metallic compound or a heavy metal or a heavy metal compound, for example, a sintering aid or a blacking agent thereinto. Further, it is another object of the present invention to provide a semiconductor-producing apparatus having a radiation efficiency and a high commercial value by using a substrate with such an increased degree of black. The present invention provides an aluminium nitride sintered body according to claim 1. Preferably a semiconductor-producing apparatus has the aluminum nitride sintered body as a substrate. The present invention also provides a process for producing an aluminium nitride sintered body, according to claim 6. Brief Description of the Drawings Fig. 1 is a spectrum chart of a X-ray diffraction analysis of an aluminum nitride sintered body of one embodiment of the present invention; Fig. 2 is a spectrum chart of a X-ray diffraction analysis of an aluminum nitride sintered body as a comparative example; Fig. 3 is an electron microphotograph showing a ceramic tissue of an aluminum nitride sintered body according to another embodiment of the present invention; Fig. 4 is an electron microphotograph showing a ceramic tissue near a grain boundary of a grain of an AlN crystalline phase of the aluminum nitride sintered body according to another embodiment of the present invention; Fig. 5 is an electron microphotograph showing a ceramic tissue near a grain boundary of a grain of an AlN crystalline phase of the aluminum nitride sintered body as the comparative example; Fig. 6 is an electron microphotograph showing a ceramic tissue in the state a grain of a (AlN) x (AL 2 OC) 1-x phase is produced in a matrix composed of particles of an AlN crystalline phase; and Fig. 7 is an electron microphotograph showing the ceramic tissue of Fig. 6 in which the matrix and the grain of the (AlN) x (AL 2 OC) 1-x phase are enlarged. Best mode for carrying out the invention According to the present invention, the "lightness" means the following. The color of the surface of an object is expressed by three attributes in perception of a color, i.e., the hue, the lightness, and the chroma. Among them, the lightness is a scale for indicating an attribute in visual sight so as to judge whether a reflecting percentage at the surface of the object is large or small. The ways of indicating the scales of these three attributes are specified in JIS Z 8721. The lightness V is determined based on the achromatic color a standard that the lightness of ideal black is 0, and that of ideal white is 10. A color area between the ideal black and the ideal white is divided into 10 zones indicated by symbols of N 0 to N 10 which vary stepwise by an equal degree of the lightness in color perception. The lightness of an actual aluminum nitride sintered body is determined by comparing the color of the surface of the sintered body with color chips corresponding to N 0 to N 10, respectively. At that time, the lightness is principally determined up to the first decimal place, and O or 5 is taken as a figure at the first decimal place. The relative density of the aluminum nitride sintered body is a value defined by a equation of (relative density = bulky density/theoretical density), and the unit of the relative density is %. As a semiconductor-producing apparatus using the aluminum nitride sintered body according to the present invention as a substrate, a ceramic heater in which a resistive heater is buried in a substrate made of the aluminum nitride sintered body, a ceramic electrostatic chuck in which an electrostatically chucking electrode is buried in a substrate, a heater with an electrostatic chuck in which a resistive heater and an electrostatically chucking electrode are buried in a substrate, and an active type apparatus, for example, a high frequency wave generating electrode apparatus in which a plasma-generating electrode is buried in a substrate may be recited. Further, various semiconductor-producing apparatuses such as a susceptor for placing a semiconductor wafer thereon, a dummy wafer, a shadow ring, a tube for generating a high frequency plasma, a dome for generating a high frequency plasma, a high frequency wave-permeable window, an infrared radiation-permeable window, a lift pin for supporting a semiconductor wafer, and a shower plate may be recited. The thermal conductivity of the aluminum nitride sintered body is preferably not less than 90 W/m·K when in use for the substrate in the heating member such as the ceramic heater, the heater with the electrostatic chuck, or the semiconductor wafer-holding susceptor. During the course of investigating aluminum nitride sintered bodies, the present inventors have succeeded in producing a blackish gray to blackish brown aluminum nitride sintered body containing almost no metallic element such as a sintering aid or a blacking agent other than aluminum and having a extremely low lightness with a black color of not more than lightness N 4 specified in JIS Z 8721. Since this aluminum nitride sintered body exhibits the black having not more than the lightness N 4 specified in JIS Z 8721 as a preferred embodiment, the sintered body has a large radiation heat amount and an excellent heating characteristic. Therefore, such a sintered body is suitable as a substrate constituting a heating material for a ceramic heater, susceptor, etc. In addition, since the content of the metal element(s) other than aluminum can be made extremely small, it is not feared that a semiconductor wafer is contaminated with such a metallic element. In particular, it is not feared that the sintered body will afford an adverse effect upon the semiconductor wafer or an apparatus therefor. Furthermore, a variation in color at the surface of the aluminum nitride sintered body according to the present invention is not conspicuous, and appearance of the aluminum nitride sintered body is extremely excellent. In addition, since the sintered body has a high black degree, its commercial value is largely enhanced. More specifically explaining, the present inventors prepared a raw material composed of powdery aluminum nitride containing 500 ppm to 5000 ppm of carbon, molding it, and sintering the molding at not less than a temperature 1730°C under pressure of not less than 80 kg/cm 2 by the hot press. By so doing, the inventors succeeded in producing the above-mentioned blackish brown to blackish gray aluminum nitride substrate-having a small lightness. The raw material composed of the powdery aluminum nitride containing 500 ppm to 5000 ppm of carbon may be produced by any one of the following methods. (1) To powdery aluminum nitride is added a given amount of a carbon source to adjust the content of carbon in the powder to 500 ppm - 5000 ppm. (2) A raw material composed of the powdery aluminum nitride having the carbon content of 500 ppm to 5000 ppm is produced by mixing plural kinds of aluminum nitride powders having different carbon contents at a given ratio. In this case, three or more kinds of aluminum nitride powders may be mixed. However, as a preferred embodiment, a raw material composed of the powdery aluminum nitride having the carbon content of 500 ppm to 5000 ppm is produced by mixing a first aluminum nitride powder having a lower carbon content with a second aluminum nitride having a higher carbon content at a given ratio. In this way, the present inventors succeeded in stably producing the aluminum nitride sintered body having low lightness by sintering the aluminum nitride powder containing carbon at a given ratio in a given temperature range under high pressure. If the content of carbon is less than 500 ppm, the lightness of the sintered body increases, whereas if the content of carbon is more than 5000 ppm, the relative density of the aluminum nitride sintered body is decreased to less than 92 %, and its hue is gray. It was clarified that if the firing temperature is less than 1730°C, the densification of the sintered body is not sufficient so that the aluminum nitride sintered body may be white and its lightness may rise to not less than 7. On the other hand, if the firing temperature of the above powder is more than 1920°C, a polytype phase is produced so that the lightness of the aluminum nitride sintered body may rise. When the firing temperature was in a range of 1750°C to 1900°C, the lightness of the aluminum nitride sintered bodies were particularly decreased. If the firing pressure is less than 80 kg/cm 2 , it was clarified that an AlN-Al 2 CO crystalline phase is produced or a polytype phase is produced in addition to AlN-Al 2 CO crystalline phase so that the lightness of the aluminum nitride sintered body may increase. For a reason mentioned later, this pressure is preferably not less than 150 kg/cm 2 and more preferably not less than 200 kg/cm 2 . Judging from the ability of the actual production apparatus, this pressure is preferably not more than 0.5 ton/cm 2 . The aluminum nitride sintered body preferably exhibits a black color having a lightness N 3 specified in JIS Z 8721. Addition of a metal element other than aluminum to powdery aluminum nitride should be avoided, and such a metal element is not more than 100 ppm. The "metal element other than aluminum" means metal elements belonging to Groups Ia-VIIa, VIII, Ib, and IIb and part of Groups IIIb and IVb (Si, Ga, Ge, etc.) in the Periodic Table. Further, the present inventors confirmed that high purity aluminum nitride sintered bodies each having not more than a lightness N 4 can be produced even by the hot isostatic press under the same condition as given above so long as the above temperature and pressure requirements are met. The present inventors investigated the reason why the aluminum nitride sintered body produced as mentioned above had a high black degree and low lightness. As a result, the present inventors has discovered the following fact, and come to accomplished the present invention. More specifically, in the samples having the blackish brown or the blackish gray with not more than the lightness N 4, the main crystalline phase was AlN, and AlON was produced as an auxiliary crystalline phase, but no other crystalline phase could not be found. With respect to this, the X-ray diffraction analysis was effected. For example, as shown in Fig. 1, it was clarified that peaks of carbon clearly appeared. This shows that a carbon phase was produced in addition of the main AlN crystalline phase and the AlON crystalline phase. In this X-ray diffraction analysis chart, a peak corresponding to a c-axis face was not detected. This means that the number of laminated layers of a laminated structure made of carbon atoms is only a few, and thus the thickness of the carbon layer structure is extremely thin. It is considered that the carbon exists near the grain boundary of the AlN crystalline phase. When the above sample having the low lightness was heated, for example, at 1850°C in nitrogen atmosphere, as is seen from an X-ray diffraction chart shown in Fig. 2, the AlN crystalline phase remains, but none of the AlON phase and the carbon appear and were detected. This is considered in that oxygen in AlON phase and the carbon are solid solved into the AlN crystal grains. For example, the N-sites are substituted by oxygen atoms so that a color center may be formed to absorb a light in a short wavelength range. The microstructure of the above-mentioned aluminum nitride is shown in Fig. 3. As shown in Fig. 3, very small AlON crystals exist in the AlN crystal grain, almost no grain boundary is seen between the crystalline phases, and a grain boundary portion at which the crystals contact is dense and has no gap. Fig. 4 is an electron microphotograph of an aluminum nitride sintered body falling in the scope of the present invention in which a grain boundary portion of a crystal composed of AlN is enlarged. Fig. 5 is an electron microphotograph of a yellowish white sample in which a grain boundary portion of a AlN crystal is enlarged. No different phase is seen between the AlN crystals. As the process for the production of aluminum nitride powder, the reducing/nitriding process and a directly nitriding process are known. According to the present invention, the aluminum nitride sintered body having the low lightness can be produced by employing the powdery raw material prepared by either one of these processes. Chemical reactions used in these processes are given below. Reducing/nitriding process: Al 2 O 3 + 3C + N 2 → 2AlN + 3CO Direct nitriding process: i) Al(C 2 H 5 ) + NH 3 → AlN + 3C 2 H 6 (Gas phase process) ii) 2Al + N 2 → 2AlN When the aluminum nitride powder is to be sintered, carbon is added into the powder at a given ratio, and the mixture is heated and sintered under high pressure. At that time, Al 2 O 3 existing near the surface of the aluminum nitride powder is reduced with the carbon added, thereby producing AlN. It is considered that during when this reduction proceeds according to the below-mentioned formulae (1), (2) and (3), a band is formed on the surface of the ALN grain so that the band may continuously absorb a visible light over a wide range to lower the lightness. However, the carbon phase must remain near the grain boundary at the point of this time. It is considered that if the firing temperature exceeds and becomes higher than 1950°C, the production of the Al 2 O phase proceeds according to a formula: Al 2 O + C → Al 2 OC to decrease the carbon phase, and accordingly the relatively unstable band on the surface of the AlN grain formed according to the formulae (1), (2) and (3) decreases. Further, it is considered that if the holding time is too long, the carbon similarly decreases. (1) Al 2 O 3 + C → Al 2 O 2 + C0 (2) Al 2 O 3 + 2C → Al 2 O 2 + 2 CO (3) Al 2 O 3 + 3C → Al 2 O 2 + 3 CO The aluminum nitride sintered body according to the present invention ordinary possesses the main crystalline phase of AlN, an auxiliary crystalline phase of AlON and a carbon phase. It is preferable that the sintered body contains substantially no (AlN)x(Al 2 OC) 1-x phase, and has not more than the lightness N 4 specified in JIS Z 8721. That is, when powdery aluminum nitride is sintered under pressure of about 80 to 100 kg/cm 2 , a gray aluminum nitride sintered body having a lightness N 4 to N 5 is produced in some cases. The X-ray diffraction analysis result and other spectrum analysis results of the resulting crystalline phase revealed that the resulting matrix fundamentally includes the main crystalline phase of AlN, the auxiliary crystalline phase and the carbon phase. On the other hand, if a pressure of not less than 150 kg/cm 2 is employed, the lightness of the sintered body further decreases so that the lightness of not more than N 4 can be stably obtained. The microstructure of the fundamental matrix, etc. did not differ between the above two cases. However, as shown in electron microphotographs of Figs. 6 and 7, it was made clear that the (AlN) x (Al 2 OC) 1-x phase was slightly produced in the gray product. A very small gap was formed between the AlN crystalline phase and the (AlN) x (Al 2 OC) 1-x phase, so that light was scattered in this gap and the scattered light rose the lightness. Therefore, the lightness of the aluminum nitride sintered body can be further decreased to not more than N 4, and furthermore to not more than N 3.5 by preventing the production of the (AlN) x (Al 2 OC) 1-x phase. The following can be favorably used as a carbon source to be added to powdery aluminum nitride. (1) A resin containing carbon. For example, a flying powdery organic resin composed of such as a phenol resin. (2) Powdery carbon such as carbon black, graphite or the like (3) An intermediate product of aluminum nitride produced during a course of the reducing/nitriding process or the like and having a high concentration of carbon. The aluminum nitride powder and the carbon source can be mixed by dry type mixing such as a dry type bag, a ball mill or a vibration mill, or by a wet type mixing using an organic solvent. The aluminum nitride sintered body according to the present invention has a large heat radiation amount and excellent heating characteristic. Further, since the sintered body has almost no conspicuous variation in color at the surface and is blackish brown or blackish gray, its commercial value is high. Accordingly, the aluminum nitride sintered body can be used suitably particularly for various heating apparatuses. Furthermore, since the aluminum nitride sintered body according to the present invention does not use a sintering aid or a blacking agent as a feed source of a metal element besides aluminum and the content of the metal atoms beside aluminum is suppressed to not more than 100 ppm, it causes no contamination. Therefore, the sintered body is most suitable as a material for high purity processing. In particular, the sintered body will not afford any serious adverse effect upon the semiconductor wafer or its apparatus itself in the semiconductor-producing process. In the following, more specific experimental results will be explained. (Experiment A) Each of aluminum nitride sintered bodies in Test Runs A1 to A12 in Tables 5 and 6 was actually produced as follows. As a raw material of aluminum nitride, high purity powder produced by the reducing/nitriding process or the direct nitriding process was used. In each powder, the content of each of Si, Fe, Ca, Mg, K, Na, Cr, Mn, Ni, Cu, Zn, W, B and Y was not more than 100 ppm, and no other metal was detected besides those metals and aluminum. With respect to each Test Run, the content of carbon is shown in Tables 5 and 6. A preliminarily molded body having a discoidal shape was produced by uniaxially press molding each raw material powder. The molding was fired in a sealed state by the hot press. The firing temperature, the holding time period at this firing temperature and the pressure in the firing stage were varied as shown in Tables 5 and 6. The main crystalline phase and other crystalline phase of each aluminum nitride sintered body were measured by the X-ray diffraction analysis. Further, the relative density of the sintered body was calculated by bulk density/theoretical density, and the bulk density was measured by the Archimedean method. Since no sintering aid having a large density was not contained, the theoretical density of the sintered body is 3.26 g/cc. The content of carbon in the sintered body was measured by an elemental analysis. The hue of the sintered body was visually determined, and the lightness of the sintered body was measured according to the above-mentioned process. These results are also shown in Tables 5 and 6. In Test Run Al falling outside the scope of the present invention, the content of carbon was 150 ppm, the firing temperature 1800°C, and the pressure 200 kg/cm 2 . In the resulting sintered body, a crystalline phase other than the AlN crystalline phase was AlON alone, and no carbon phase was detected by the X-ray diffraction analysis. The hue of the sintered body was gray. In Test Run A2 falling inside the scope of the present invention, the content of carbon was 500 ppm, the firing temperature 1800°C, and the pressure 100 kg/cm 2 . In the resulting sintered body, the AlON crystalline phase and the carbon phase were detected. The hue of the sintered body was blackish gray, and the lightness was N 3.5. In Test Run A3 falling outside the scope of the present invention, the content of carbon was 750 ppm, the firing temperature 1700°C, and the pressure 200 kg/cm 2 . In the resulting sintered body, no carbon phase was detected. The hue of the sintered body was white. In Test Run A4 falling inside the scope of the present invention, the content of carbon was 750 ppm, the firing temperature 1750°C, and the pressure 150 kg/cm 2 . In the resulting sintered body, the AlON crystalline phase and the carbon phase were detected. The hue of the sintered body was blackish gray, and the lightness was N 4. In Test Run A5 falling inside the scope of the present invention, the content of carbon was 750 ppm, the firing temperature 1850°C, and the pressure 50 kg/cm 2 . In the resulting sintered body, the (AlN) x (Al 2 OC) 1-x phase was detected in addition to the AlON crystalline phase and the carbon phase. Accordingly, the hue of the sintered body was gray, but as mentioned above, it was confirmed that the black coloring of the matrix conspicuously proceeded. Excellent results were obtained with respect to Test Runs A6 and A7 falling inside the scope of the present invention. In Test Run A8 falling outside the scope of the present invention, the content of carbon was 750 ppm, the firing temperature 1950°C, and the pressure 150 kg/cm 2 . In the resulting sintered body, a polytype existed besides the AlN crystalline phase. The hue of the sintered body was milky white, and the lightness was N8. In Test Runs A9, A10 and A11 falling inside the scope of the present invention, excellent test results were obtained. In Test Run 12 falling in the scope of the present invention, the content of carbon was 10000 ppm, the firing temperature 1800°C, and the pressure 200 kg/cm 2 . In the resulting sintered body, the carbon phase was produced, and the black coloring of the matrix conspicuously proceeded. However, since the porosity increased, the lightness of the entire sintered body accordingly increased to N5. Among them, an X-ray diffraction chart of the sintered body of Test Run A6 is shown in Fig. 1. Respective peaks representing the AlN, AlNO and carbon phases are confirmed. Further, Fig. 3 is an electron microphotograph showing a ceramic tissue of a sintered body of Test Run A6. Fig. 4 shows the ceramic tissue near the grain boundary. With respect to the sintered bodies in Test Runs A2, A4, A7, A9, A10 and A11, similar X-ray diffraction charts and crystalline tissues were confirmed. Fig. 6 shows a ceramic tissue of the sintered body in Test Run A5, and Fig. 7 shows an enlarged view thereof. With respect to this tissue, the X-ray diffraction analysis result of the matrix portion and the analysis result of a visible light absorption spectrum were similar to those of Test Run A6. However, the (AlN) x (Al 2 OC) 1-x phase looking black exists in this matrix, so that a slight gap exists between this crystalline grain and the AlN crystal and the light is scattered in this gap to make the sintered body white. The matrix tissue of this sintered body is fundamentally the same as that of the aluminum nitride sintered body according to the present invention, and the black coloring proceeded when considered relatively. However, the lightness of the sintered body was raised to N 5 due to the above scattered light. Next, the sintered body in Test Run A6 was experimentally thermally treated in nitrogen atmosphere. When this sintered body was thermally treated at 1850°C for 2 hours, only an outer peripheral portion of the sintered body was changed to a yellowish white color, but the hue and the lightness of a central portion did not change. The X-ray diffraction analysis of the yellowish white-colored portion revealed that its main crystalline phase was an AlN crystalline phase, and the peaks corresponding to the AlON phase and carbon, respectively disappeared and none of these peaks were detected. No change was observed with respect to the relative density and the lattice constant ratio. It is considered that near the surface of the sintered body in Test Run 6, oxygen in the nitrogen atmosphere, oxygen in the AlON phase and carbon were solid-solved in the AlN crystal grains. It is considered that such reactions more slowly proceed inside the sintered body. (Experiment B) Aluminum nitride sintered bodies in Test Runs B1 to B15 in Tables 7, 8 and 9 were actually produced in the same manner as in Test Runs A1-A12. As a raw material of aluminum nitride, high purity powders produced by the reducing/nitriding process or the direct nitriding process were used. In each powder, the content of each of Si, Fe, Ca, Mg, K, Na, Cr, Mn, Ni, Cu, Zn, W, B and Y was not more than 100 ppm, and no other metal was detected besides those metals and aluminum. In Test Run B1, powdery aluminum nitride (carbon content: 500 ppm) obtained by the reducing/nitriding process. In the other Test Runs. an additive having a larger content of carbon was added to one having a smaller content of carbon. As such an additive, a resin was used in some Test Runs, and aluminum nitride powder having a larger content of carbon was used in the other Test Runs. As the above resin, phenol resin powder was used, and its addition amount is shown. As the above alumina nitride powder, the one obtained by the reducing/nitriding process was used, and its content of carbon and an addition amount thereof are shown. The entire content of carbon (ppm) of each mixed raw material powder is shown. A preliminarily molded body having a discoidal shape was produced by uniaxially press molding each raw material powder. The molding was fired in a sealed state by the hot press. The firing temperature, the holding time period at this firing temperature and the pressure in the firing stage were varied as shown in Tables 7, 8 and 9. The main crystalline phase and other crystalline phase of each aluminum nitride sintered body in Test Runs were measured by the X-ray diffraction analysis. Further, the relative density, the hue and the lightness of the sintered body were measured in the same manners as in Test Runs A1-A12. Results are shown in Tables 7, 8 and 9. In Test Run B1 falling in the scope of the present invention, a blackish gray aluminum nitride sintered body was obtained. In each of Test Runs B2, B3, B4, B5 and B6 falling in the scope of the present invention, a blackish gray aluminum nitride sintered body was obtained. In Test Run B7 falling outside the scope of the present invention, since the firing temperature was as low as 1650°C, densification of a sintered body did not proceeded, and no carbon phase was produced, so that the hue of the sintered body was white. In Test Run B8 falling in the scope of the present invention, blackish gray sintered bodies were obtained. In Test Run B11 falling outside the scope of the present invention, since the firing temperature was too high, a polytype was except the AlN crystalline phase, and the hue of the sintered body was milky white. In Test Runs B1, B2 and B13 falling in the scope of the present invention, aluminum nitride sintered body having the blackish gray hue were obtained. In Test Run 15 falling in the scope of the present invention, the entire content of carbon in the entire aluminum nitride raw material was 7600 ppm, the AlON crystalline phase and the carbon phase were produced besides the AlN crystalline phase. However, since the sintering degree of the sintered body progressed, the relative density was only 88.0% and the lightness was N 5. (Wafer-heating test) A plate having a diameter of 210 mm and a thickness of 10 mm was prepared from the aluminum nitride sintered body of Sample A6 according to the present invention, and was placed in a vacuum chamber equipped with a heating mechanism having an infrared lamp unit. A silicon wafer having a diameter of 8 inches was placed on this plate, and thermocouples were fitted to the plate and the silicon wafer to simultaneously measure each of the temperatures thereof. As the infrared lamp unit, twenty of 500 W infrared lamps having an infrared peak around a wavelength of 1 µm and fitted to an reflecting plate made of aluminum were used, and these infrared lamps and the reflecting plate were arranged outside the vacuum chamber. The infrared beam radiated from each infrared lamp was, directly or indirectly after being reflected from the reflecting plate, led to the aluminum nitride sintered plate through a circular quartz window (diameter: 250 mm, thickness: 5 mm) provided at the vacuum chamber to heat the plate. In this heating apparatus, each infrared lamp was lit to elevate the temperature of the plate from room temperature to 700°C in one minute, and the plate was kept at 700°C for one hour. Thereafter, the infrared lamps were put out, and the plate was gradually cooled. Consequently, the consumption power of the infrared lamps was 8600 W at the maximum, and the temperature could be stably controlled. Further, measurement of the temperature of the silicon wafer revealed that when the temperature of the plate was kept at 700°C, the temperature of the wafer was 611°C. (Heating experiment in comparative example) Next, an aluminum nitride powder obtained by the reducing/nitriding process and containing 750 ppm of carbon was used, and a discoidal molding was produced by pressing this powder under pressure of 3 tons/cm 2 by the cold isostatic press. This molding was fired at 1900°C for 2 hours to produce a white aluminum nitride sintered body having a density of 99.4%. By using this sintered body, a silicon wafer-heating experiment was effected in the same manner as mentioned above. As a result, the consumption power was 10 kw at the maximum, and the time period required to realizing the temperature elevation was longer by around 2 minutes. Further, the infrared lamps were more likely to be wire-cut upon reception of a heat cycle of temperature rising-temperature descending between room temperature and 700°C. Furthermore, measurement of the temperature of the silicon wafer revealed that when the plate was held at 700°C, the temperature of the silicon wafer was 593°C. Thus, it was clarified that as compared with the invention example, the temperature of the silicon wafer was lower.

Claims: An aluminium nitride sintered body, having a main crystalline phase of AlN, a minor crystalline phase of AlON and a carbon phase, wherein the amount of carbon atoms is in the range 500 to 9800ppm and the content of each metal element other than Al is not more than 100ppm. An aluminium nitride sintered body according to claim 1, wherein AlN and AlON are the only crystalline phases. An aluminium nitride sintered body according to claim 1 or 2, which has lightness as defined in JIS Z 8721 of N4 or less. An aluminium nitride sintered body according to any one of claims 1 to 3, wherein the carbon atom content is in the range 500 to 5000ppm by weight. Apparatus for producing semiconductor devices having, as a substrate, an aluminium nitride sintered body according to any one of claims 1 to 4. A process for producing an aluminium nitride sintered body, comprising the steps of moulding a raw material composed of aluminium nitride powder having a carbon content of 500ppm to 10,000ppm, and sintering the moulded powder at a temperature of not less than 1730°C to not more than 1920°C under a pressure of not less than 8MPa (80 kg/cm 2 ), wherein the aluminium nitride powder has a content of not more than 100ppm of each metal element other than Al. A process according to claim 6, wherein said raw material composed of aluminium nitride powder is produced by adding carbon or a compound as a carbon source to aluminium nitride powder. A process according to claim 6, wherein said raw material composed of aluminium nitride powder is produced by a reducing/nitriding process. A process according to claim 6, wherein said raw material composed of aluminium nitride powder is produced by mixing at least first and second aluminium nitride powders having different contents of carbon. A process according to claim 9, wherein said second aluminium nitride powder is produced by a reducing/nitriding process.

Inventors: BESSHO YUKI, KOBAYASHI HIROMICHI, MORI YUJIMASA, BESSHO, YUKI, KOBAYASHI, HIROMICHI, MORI, YUJIMASA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1996
Main classification: C04B35
IPC codes: H01L23, C04B35