Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
1999-02-19
2001-04-03
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S553000, C338S223000
Reexamination Certificate
active
06211496
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention is related to a molybdenum disilicide (MoSi
2
) based ceramic composite heating element, more particularly to the heating element of a heat-treatment furnace, in which severe temperature-distribution is required, such as an oxidation, diffusion or low-pressure chemical-vapor-deposition (LP-CVD) furnace for producing semiconductors. In addition, the present invention is related to a production method of the molybdenum-disilicide based ceramic composite heating element.
2. Description of Related Art
Heretofore, an Fe—Cr—Al based, metallic heating element has been used in a heat-treatment furnace for producing semi-conductors, such as an oxidizing, diffusion or LP-CVD furnace. In a recent development of the rapid thermal processing, which pursues micro-devices and enhances cost competence in the production of semiconductor devices, the MoSi
2
heating element has been used. That is, since the heat resistance of the MoSi
2
heating element is superior to that of a metallic heating element, the MoSi2 heating element can be used at a surface power density as high as approximately ten times that of the metallic heating element. For example, the permissible level of the surface powder density of the metallic heating element at 1000° C. is around 2 W/cm
2
, while that of the MoSi
2
heating element at 1000° C. is 20 W/cm
2
. Furthermore, by means of using the molybdenum-disilicide heating element and omitting a liner tube, which is usually made of silicon carbide and installed inside the heating element and the heat capacity of the heat treatment furnace is dramatically decreased. Therefore, only an inner tube made of quartz is installed inside the heating element as a processing tube. This feature readily leads to increasing the temperature-elevation and lowering rates. Specifically, in the case of a diffusion furnace, one heat-treatment time can be shortened by 60% or more, i.e., the conventional 200 minutes or more can be shortened to 80 minutes or less.
When the MoSi
2
heating element is operated in a temperature of 1400° C. or more and under ambient atmosphere, since a dense protective film consisting of silica (SiO
2
) is formed on the surface of the heating element, the heating element can be stably operated. However, in a temperature range of 400 to 1200° C., where the oxidation, diffusion or LP-CVD furnace is operated, low-temperature oxidation, which is a characteristic of the MoSi
2
heating element, occurs and the polycrystalline material is pulverized, since the protective dense silica film is not formed. This is the so-called “pest” phenomenon. Under the present situation, the life of the MoSi
2
heating element is, therefore, unsatisfactory for the users.
The present applicant proposed in Japanese Unexamined Patent Publication (kokai) No. 8-143,365 published Jun. 4, 1996 to form on the surface of the heating element a silicon carbide (SiC) film by CVD. The effects attained by this film are, however, not yet wholly satisfactory.
SUMMARY OF INVENTION
It is, therefore, an object of the present invention to provide a molybdenum-disilicide based ceramic composite heating element, in which the low-temperature oxidation of MoSi
2
is prevented to satisfactorily prolong the life of the heating element used for the low-temperature heating, such as in a heat-treatment furnace for producing semiconductor.
It is another object of the present invention to provide a method for producing the molybdenum-disilicide composite-ceramic heating element.
Regarding the characteristic pest phenomenon of the MoSi
2
heating element, there is a report by Kurokawa in the 22nd Society for Corrosion Engineering (1995) pages 63-81, organize by Japan Corrosion Protection Society. According to this report, the “pest” phenomenon is considered to be caused by a volume expansion due to the formation of MoO
3
and high vapor pressure of MoO
3
at flaw of grain boundary in MoSi
2
sintered material. It seems, therefore, advisable to produce fully dense material free from flaws such as cracks and pores to prevent the “pest” phenomenon. It also seems to be important to form on the surface of the sintered material at a dense and stable oxidation protective coating of SiO
2
.
As the present inventors have reported on a prevention of “Low Temperature Oxidation by (Direct) Current Heating” in MoSi
2
based ceramic composite for infrared radiation source regarding the present invention in Journal of The Japan Institute of Metals, Vol. 61(3) pages 247-248, 1997, on the other hand, it is observed that the protective silica film is fractured by charging with electricity and the low-temperature oxidation thus goes on. Although the formation of protective silica film on the surface mentioned above must be an important method for preventing the low-temperature oxidation, it should in fact be called a life-prolonging method rather than a “pest” avoiding method.
The present inventors considered that the main reason for the pest phenomenon resides in the fact that, when the MoO
3
is formed in the grain boundaries of MoSi
2
, an intergranular fracture (a separation of grains) occurred. The present inventors then designed the following molybdenum-disilicide based ceramic composite. According to the conventional measure for preventing the pest phenomenon, a dense and stable protective SiO
2
film is formed on the surface of sintered material. According to the present invention, this measure is applied microscopically to the grain boundaries, so as to decrease the area of the grain boundaries, where the MoSi
2
grains are in contact with one another, to as low a level as possible. The resultant material is a composite of MoSi
2
and a silica-bearing oxide phase or a glass phase, which phase has a relatively low melting point as compared with the MoSi
2
Such composite has a micro-structure such that as much as possible of the silica-bearing oxide phase or the glass phase is existing at the grain boundaries of the MoSi
2
grains, thereby preventing the “pest” oxidation.
In accordance with the present invention, there is provided a based ceramic composite of MoSi
2
having a network structure and silica-bearing oxide phase or glass phase having a relatively low melting point as compared with the MoSi
2
and distributing in a net-like form along the boundaries of the MoSi
2
grains.
The molybdenum-disilicide based ceramic composite heating element having long life at low-temperature heating according to the present invention consists essentially of molybdenum disilicide grains having a network structure and a secondary phase consisting of at least one material selected from the group consisting of a silica-bearing oxide and a glass having a relatively low melting point as compared with said molybdenum disilicide, and further said secondary phase distributes in a net-like form along the boundaries of the said molybdenum disilicide grains.
The silica-bearing oxide phase or glass phase is preferably from 20 to 45% by volume.
According to an embodiment of the present invention, there is provided a heating element used in a heat-treatment furnace for producing semiconductors, in which the outer processing tube, i.e., the liner tube is omitted, and, further the heating element faces directly the quartz tube, in which semiconductor wafers are heat-treated. The impurities of the heating element according to this embodiment are limited to 0.05 mass % or less of Fe, 0.01 mass % or less of Cu, 0.05 mass % or less of Na and 0.05 mass % or less of K, and 0.16 mass % or less of the total of these impurities.
A method for producing the ceramic composite heating element according to the present invention by with a processing of sintering, bending and welding, is characterized in that: from 20 to 45% by volume of a clay mineral powder is added to the MoSi
2
powder, followed by blending with water; and, the blended mixture is shaped, dried and sintered in non-oxidizing atmosphere at a temperature of from 1250 to 1550° C., thereby forming the above mentioned net-like microstruc
Jiang Wan
Uchiyama Tetsuo
Dahbour Fadi H.
Kabushiki Kaisha Riken
Kubovcik & Kubovcik
Walberg Teresa
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