Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
2002-05-29
2004-11-30
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C324S557000, C174S258000, C174S262000, C174S264000
Reexamination Certificate
active
06825555
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hot plate which is used mainly in the semiconductor industry and has superior temperature-rising/dropping property.
BACKGROUND ART
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.
SUMMARY OF THE INVENTION
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 10
−7
Pa·m
3
/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 10
−7
Pa·m
3
/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 10
−7
Pa·m
3
/sec (He) or less by measurement with a helium leakage detector.
In the hot plate of the present invention, the leakage quantity thereof is 10
−7
Pa·m
3
/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/m·k 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 mm
2
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, “Semiconductor World 1992, November, p. 112 to 115”.
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 10
−7
Pa·m
3
/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/cm
2
).
The leakage quantity is preferably from 1×10
−8
to 1×10
−12
Pa·m
3
/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 10
−6
Pa·m
3
/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 10
−6
Pa·m
3
/sec (He) is generated. In addition, in JP Kokai Hei 10-279359, firing i
Hiramatsu Yasuji
Ito Yasutaka
Ibiden Co. Ltd.
Nelms David
Tran Mai-Huong
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