The present invention relates to a hot plate which is used mainly in the semiconductor industry and has superior temperature-rising/dropping property.
Hitherto, a heater, a wafer prober and the like wherein a base material made of metal such as stainless steel or aluminum alloy is used has been used in semiconductor producing/examining devices and so on, examples of which include an etching device and a chemical vapor deposition device.
However, such a heater made of metal has the following problems.
First, the thickness of the heater plate has to be as thick as about 15 mm since the heater is made of metal. Because in a thin metal plate a bend, a strain and so on are generated on the basis of thermal expansion resulting from heating so that a silicon wafer put on the metal plate is damaged or inclined. However, if the thickness of the heater plate is made thick, a problem that the heater becomes heavy and bulky arises.
The temperature of a face for heating an object to be heated such as a silicon wafer (referred to as a heating face hereinafter) is controlled by changing the voltage or current amount applied to resistance heating elements. However, since the metal plate is thick, the temperature of the heater plate does not follow the change in the voltage or current amount promptly. Thus, a problem that the temperature is not easily controlled is caused.
Thus, JP Kokai Hei 11-40330 suggests a ceramic substrate (hot plate) wherein a nitride ceramic or a carbide ceramic, which has a high thermal conductivity and a great strength, is used as a substrate and resistance heating elements formed by sintering metal particles are set up on the surface of a plate made of this ceramic.
The hot plate having such a structure is usually put into a supporting case. At the time of cooling after heating is performed, a coolant is caused to flow into the supporting case in order to make cooling rate high. In this way, the ceramic substrate is rapidly cooled.
Although the hot plates using these ceramics are superior to heaters-made of metals in temperature-rising/dropping property, performance required for temperature-dropping property at the time of cooling with a coolant and the like is not sufficiently satisfied.
The inventors made eager investigations in order to solve the above-mentioned problems. As a result, the inventors have found the following: the reason why the temperature-dropping property of such a hot plate is insufficient is that, because of insufficient sinterability, a cooling gas is released to the outside through the sintered body at the time of cooling so that cooling thermal efficiency deteriorates; and thus the above-mentioned problems can be solved by adjusting the degree of the sintering in such a manner that the sintered body has a leakage quantity of 10xe2x88x927 Paxc2x7m3/sec (He) or less by measurement with a helium leakage detector.
Specifically, the inventors have found that, by: firstly oxidizing the surface of raw material particles of a nitride ceramic and the like; adding an oxide thereto; and successively performing a sintering step under pressure and the like step, the leakage quantity thereof can be made as small as 10xe2x88x927 Paxc2x7m3/sec (He) or less by measurement with a helium leakage detector.
Furthermore, the inventors have also found that, in this case, the helium leakage quantity and breakdown voltage at the time of raising temperature have a correlation. Thus, the present invention has been made.
That is, the present invention is a hot plate comprising: a ceramic substrate; and a resistance heating element formed on the surface of the ceramic substrate or inside the ceramic substrate,
wherein the ceramic substrate has a leakage quantity of 10xe2x88x927 Paxc2x7m3/sec (He) or less by measurement with a helium leakage detector.
In the hot plate of the present invention, the leakage quantity thereof is 10xe2x88x927 Paxc2x7m3/sec (He) or less by measurement with a helium leakage detector. When the leakage quantity is in such a degree, the ceramic substrate mentioned above is sufficiently densely sintered. Thus, the ceramic substrate can give a thermal conductivity of 150 W/mxc2x7k or more. Therefore, the ceramic substrate mentioned above has excellent temperature-rising/dropping property. Additionally, at the time of cooling, a gas as a coolant does not permeate the ceramic substrate; therefore, the ceramic substrate has high cooling thermal efficiency and particularly has superior temperature-dropping property.
Since the above-mentioned ceramic substrate is superior in mechanical property, no warp is generated in the ceramic substrate and the ceramic substrate is also superior in breakdown voltage and Young""s modules at a high temperature.
In the case of measuring the above-mentioned leakage quantity, the same sample as the above-mentioned ceramic substrate is prepared to have a diameter of 30 mm, an area of 706.5 mm2 and a thickness of 1 mm, and set in a helium leakage detector. Thereafter, the leakage quantity of the above-mentioned ceramic substrate can be measured by measuring a flow amount of helium passing through the above-mentioned sample.
The helium leakage detector measures the partial pressure of helium at the time of leakage but does not measure the absolute value of the gas flow amount. The helium partial pressure values of the samples of which the leakage quantities are known are measured in advance, and unknown leakage quantity is calculated by simple proportional calculation on the basis of the helium partial pressure detected at the measurement. The detailed measurement principle of the helium leakage detector is described in a monthly journal, xe2x80x9cSemiconductor World 1992, November, p. 112 to 115xe2x80x9d.
That is, if the above-mentioned ceramic substrate is sufficiently densely sintered, the above-mentioned leakage quantity is a considerably small value. On the other hand, if the sinterability of the above-mentioned ceramic substrate is insufficient, the above-mentioned leakage quantity becomes a large value.
In the present invention, for example, by conducting the process of: oxidizing the surface of particles of a non-oxide ceramic such as a nitride ceramic at first; successively adding an oxide thereto; and carrying out sintering under pressure, a sintered body in which an oxide layer of the nitride ceramic and the like is integrated with the added oxide is formed. Such a sintered body has an extremely small leakage quantity of 10xe2x88x927 Paxc2x7m3/sec (He) or less in the measurement with the helium leakage detector.
In addition, when the formed body before the sintering is pressed as uniformly as possible by cool isostatic press (CIP), the sintering advances more uniformly and the sintering density is made higher. Thus, the leakage quantity becomes far smaller. The pressure upon the CIP is preferably from 50 to 500 MPa (0.5 to 5 t/cm2).
The leakage quantity is preferably from 1xc3x9710xe2x88x928 to 1xc3x9710xe2x88x9212 Paxc2x7m3/sec (He) by measurement with the helium leakage detector. This is because thermal conductivity at high temperature can be ensured and further cooling thermal efficiency becomes high at the time of cooling.
Incidentally, although aluminum nitride sintered bodies wherein a small amount of ALON crystal phase exists are disclosed in JP Kokai Hei 9-48668, JP Kokai Hei 9-48669, and JP Kokai Hei 10-72260 and so on, no metal oxide is added therein and they are manufactured by reductive nitrogenation method. Therefore, no oxygen exists on the surface and the sinterability is inferior. As shown in the comparative example, a relatively high leakage quantity of about 10xe2x88x926 Paxc2x7m3/sec (He) is generated. In JP Kokai Hei 7-153820, although yttria is added, the surface of aluminum nitride raw material powder is not fired in advance. Thus, as is also clear from the comparative example, the sinterability thereof is inferior, and a relatively large leakage quantity such as about 10xe2x88x926 Paxc2x7m3/sec (He) is generated. In addition, in JP Kokai Hei 10-279359, firing is performed under low temperature and atmospheric pressure, thus the leakage quantity becomes high as well. In JP Kokai Hei 10-158002 discloses an AlN substrate used for a substrate on which a semiconductor is mounted but not for a semiconductor producing/examining device, unlike the present invention. Furthermore, in JP Kokai Hei 10-167859, since the amount of yttria is as small as 0.2% by weight and sinterability is insufficient, the leakage quantity becomes high.
As described above, any conventional technique has not yet realized a semiconductor producing/examining device using a sintered body whose helium leakage quantity can be adjusted to 1xc3x9710xe2x88x927 Paxc2x7m3/sec (He) or less just like the one in the present invention.
Theses citations neither describe nor suggest any isotropic press such as cool isostatic press (CIP). Thus, the helium leakage quantity thereof has not yet been reduced.
The oxide to be added is preferably an oxide of an element which constitutes a nitride ceramic and the like. This is because the oxide is the same material as the surface oxide layer of the nitride ceramic and is very easily sintered together. In order to oxidize the surface of the nitride ceramic, it is desired to heat the nitride ceramic at 500 to 1000xc2x0 C. in oxygen or the air for 0.5 to 3 hours.
The average particle diameter of the nitride ceramic powder used in the sintering is preferably from about 0.1 to 5 xcexcm. This is because the powder is easily sintered.
Furthermore, it is desired that the ceramic powder contains 0.05 to 50 ppm (by weight) of Si and 0.05 to 80 ppm (by weight) of S. This is because these would cause the oxidized film on the surface of the nitride ceramic to be combined with the added oxide.
Other firing conditions will be described in detail in a later-described process for manufacturing a hot plate.
The ceramic substrate obtained by using the above-mentioned process to perform sintering desirably contains 0.05 to 10% by weight of oxygen. If the amount is less than 0.05% by weight, the sintering does not advance so that fracture is caused in the grain boundary and the thermal conductivity drops. On the other hand, if the oxygen amount is more than 10% by weight, the oxygen is unevenly precipitated in the grain boundary so that fracture is caused at the grain boundary. Thus, the thermal conductivity drops so that the temperature-rising/dropping property deteriorates.
In the present invention, it is desired that the ceramic substrate constituting the hot plate comprises a nitride ceramic containing oxygen and further the pore diameter of the maximum pore is 50 xcexcm or less. The porosity thereof is desirably 5% or less. It is also desirable that no pores are present in the ceramic substrate mentioned above or, if present, the pore diameter of the maximum pore is 50 xcexcm or less.
If no pores are present, breakdown voltage is especially high at high temperature. Conversely, if pores are present, fracture toughness becomes high. Thus, which is designed depends on required properties.
The reason why fracture toughness becomes high on the basis of the presence of pores is unclear, but it is presumed that the reason is based on stop of development of cracks by the pores.
The reasons why the pore diameter of the maximum pore is desirably 50 xcexcm or less in the present invention are as follows: if the pore diameter is over 50 xcexcm, it is difficult to keep high breakdown voltage property at high temperature, particularly at 200xc2x0 C. or higher; and gas is easily released at the time of cooling, so that cooling thermal efficiency deteriorates.
The pore diameter of the maximum pore is desirably 10 xcexcm or less. This is because the amount of warp becomes small at 200xc2x0 C. or higher.
The porosity and the pore diameter of the maximum pore are adjusted by pressing time, pressure and temperature at the time of sintering, or additives such as SiC and BN. Since SiC or BN obstructs sintering, pores can be produced.
When the pore diameter of the maximum pore is measured, 5 samples are prepared. The surfaces thereof are ground into mirror planes. With an electron microscope, ten points on the surface are photographed with 2000-5000 magnifications. The maximum pore diameter is selected from the each photograph obtained by the photographing, and the average of the 50 shoots is defined as the pore diameter of the maximum pore.
The porosity is measured by Archimedes"" method. This is a method of crushing a sintered body to pieces, putting the pieces into an organic solvent or mercury to measure the volume thereof, then, obtaining the true specific gravity of the pieces from the weight and the volume thereof, and calculating the porosity from the true specific gravity and apparent specific gravity.
The diameter of the ceramic substrate constituting the hot plate of the present invention is desirably 200 mm or more. It is especially desirable that the diameter is 12 inches (300 mm) or more. This is because semiconductor wafers having such a diameter become main currents of the next-generation semiconductor wafers. This is also because the problem about the warp to be solved by the present invention is not easily caused in the ceramic substrate having a diameter of 200 mm or less.
The thickness of the above-mentioned ceramic substrate is desirably 50 mm or less, especially desirably 25 mm or less.
This is because: if the thickness of the ceramic substrate is over 25 mm, the thermal capacity of the ceramic substrate maybe too large; and particularly when a temperature-controlling means is set up to heat or cool the substrate, temperature following character may drop on the basis of the large thermal capacity.
This is also because the problem about the warp of the ceramic substrate is not easily caused in the ceramic substrate having a large thickness, such as a thickness over 25 mm.
The thickness of the ceramic substrate is optimally 5 mm or less. The thickness of the ceramic substrate is desirably 1 mm or more.
The ceramic substrate of the present invention is used at 150xc2x0 C. or higher, desirably 200xc2x0 C. or higher.
The material of the ceramic substrate constituting the hot plate of the present invention is not especially limited. However, nitride ceramics and carbide ceramics are preferred. Examples of the nitride ceramics mentioned above include metal nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, titanium nitride and the like.
Examples of the carbide ceramics mentioned above include such as silicon carbide, titanium carbide, tantalum carbide, tungsten carbide, zirconium carbide and the like.
As the above-mentioned ceramic material, an oxide ceramic may be used. Examples of the oxide ceramic mentioned above include metal oxide ceramics such as alumina, zirconia, cordierite, mullite and the like.
Aluminum nitride is particularly preferred among these nitride ceramics since its thermal conductivity is highest, that is, 180 W/mxc2x7K.
In the present invention, it is desired that the ceramic substrate contains an oxide.
As the oxide mentioned above, for example, an alkali metal oxide, an alkali earth metal oxide or a rare earth oxide can be used. Among these sintering aids, CaO, Y2O3, Na2O, Li2O and Rb2O are particularly preferred. Alumina or silica may be used. The content thereof is desirably from 0.5 to 20% by weight. If the content is less than 0.5% by weight, the leakage quantity may not be made to 10xe2x88x927 Pa m3/sec (He) or less.
The added oxide is optimally silica in the case of silicon nitride.
In the present invention, the ceramic substrate preferably contains 5 to 5000 ppm of carbon. The ceramic substrate can be blackened by incorporating carbon. This is because, when the substrate is used as a heater, radiant heat can be sufficiently used.
Carbon may be amorphous or crystalline. When amorphous carbon is used, a drop in the volume resistivity at high temperature can be prevented. When crystalline carbon is used, a drop in the thermal conductivity at high temperature can be prevented. Therefore, crystalline carbon and amorphous carbon may be used together dependently on use. The carbon content is preferably from 50 to 2000 ppm.
When carbon is incorporated into the ceramic substrate mentioned above, carbon is preferably incorporated in the manner that its brightness will be N4 or less as a value based on the rule of JIS Z 8721. The ceramic having such a brightness is superior in radiant heat capacity and shielding property.
The brightness N is defined as follows: the brightness of ideal black is made to 0; that of ideal white is made to 10; respective colors are divided into 10 parts in the manner that the brightness of the respective colors is recognized stepwise between the brightness of black and that of white at equal intensity intervals; and the resultant parts are indicated by symbols N0 to N10, respectively.
Actual brightness is measured by comparison with color chips corresponding to N0 to N10. One place of decimals in this case is made to 0 or 5.