Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
1999-05-17
2002-09-10
Paik, Sang (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S543000, C219S548000, C252S506000
Reexamination Certificate
active
06448538
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electric heating element and, more particularly, to an electric heating element having a structure comprising a ceramic insulating substrate and an electrically heat-generating material film, said film being fused to the surface of said electric insulating ceramic substrate.
The present invention also relates to a structure of an electrostatic chuck and, more particularly, to a structure of an electrostatic chuck capable of quickly and precisely controlling the temperature of an electrically chucked material to be treated, such as a semiconductor substrate.
TECHNICAL BACKGROUND
In the field of electric heating elements, it is known that a planar heating element having less temperature variation can be obtained by forming a heater circuit on a ceramic plate having high thermal conductivity. Such a heater, referred to as a ceramic heater, is required to have the following characteristics.
(1) High adhesion strength between the circuit and the ceramic material
(2) Heater circuit material having high oxidation resistance and applicability at high temperatures
(3) High heat generation density of the heater, namely a high value of resistance of the heater circuit. Most importantly, possibility of production of large heaters with a low cost.
However, there are only the following two types available at present.
(1) A heater comprising a circuit made of an electric-heating metal and a previously sintered ceramic plate, said circuit being baked on said ceramic plate.
This type has such a structure that a circuit pattern is formed by sintering a paste made by mixing glass into a powder of a noble metal such as platinum, platinum alloy or silver. This type has the following drawbacks.
(1) This type is limited to the type wherein the circuit pattern is baked only on one side of the ceramic substrate (single-side baking). Because the surface with the circuit formed thereon is exposed, it is necessary to insulate this portion depending on the application.
(2) Adhesion strength of the electric-heating circuit is low and tends to peel off.
(3) Maximum operating temperature is limited to the melting point of glass used as the binder, with the operating temperatures 400 to 500 C. at most, and operation at a high temperatures above 1000 C. is prohibited.
(2) A heater made by integrally baking an electric-heating circuit at the same time when the ceramic substrate is sintered.
This type has a structure obtained by printing a circuit pattern of a powder paste of a metal having high melting point such as tungsten on a green sheet of a ceramic substrate, laminating another green sheet on the printed circuit and integrally sintering them under pressure. The resultant structure is a structure wherein an electric-heating circuit is incorporated between the ceramic plates (double-side baking).
Although this structure eliminates the drawback of the type (1) , namely exposure of the electric-heating circuit, there arise the following problems.
(1) Because the circuit must be covered by the ceramic, the circuit cannot be formed near the peripheral edge of the element, resulting in lower temperatures near the edges. Thus, it is difficult to achieve uniform temperature distribution.
(2) This type of thin planar shape is subject to a warp during sintering. Pressurized sintering is required to obtain a heater element without warp.
This method essentially involves the problem of deformation taking place during sintering of the ceramic material, and it is difficult to obtain a large-sized sintered article without deformation. A three-dimensional structure cannot be produced. This method requires it to use a die, leading to extremely high costs when producing articles in a small lot.
(3) Electric-heating metals are limited to high melting point metals such as tungsten and molybdenum, which do not melt at the sintering temperature of the ceramic. Tungsten and molybdenum have a drawback of tendency to oxidize, and the ceramic material that encloses the electric-heating circuit is required to be free of defects and completely air-tight. It is difficult to use in the air atmosphere at a high temperature over a long period of time. Tungsten and molybdenum have another problem that the electric resistance and heat generation density of these metals are low. The ceramic heater has such problems as described above.
Meanwhile, it is well known that suicides represented by molybdenum disilicide (MoSi
2
) have very high oxidation resistance and can be used in electrical heating operation at high temperatures in the air atmosphere.
Largest drawback of the silicide heat-generating material is that it is very brittle. Because of the brittleness, silicide is usually mixed with glass powder and the mixture is sintered to form a plate or rod having greater mechanical strength. However, use of glass as a binder gives rise to a problem with regard to the heat resistance. Also silicide itself has an intrinsic problem of softening at high temperatures, causing the heater element to deform and droop.
In the field of electrostatic chucks, on the other hand, plasma processing of semiconductors is required to be more minute and have higher accuracy as the scale of circuit integration increases.
In order to achieve extreme miniaturization and higher accuracy of plasma processing, the temperature of plasma processing is a very important factor. In the producing facilities in use at present, however, silicon wafers to be processed are only cooled to prevent overheat (etching process) and accordingly film forming process (CVD) is carried out at a lower temperature leaving the temperature rise during the process without intervention.
The present situation is as described above, which does not mean that the importance of temperature control is not recognized, but because there is no method available for controlling the temperature economically at a desired rate. Although precise temperature control is possible in a laboratory without economical considerations in terms of productivity, there is no method of quick and precise temperature control applicable to production lines, capable of quickly setting an optimum temperature for individual film material to be processed without decreasing the productivity.
Solving the problems described above requires a method of quickly regulating the temperature according to the speed of the production process. Namely, it is necessary to quickly and continuously regulating the temperature without decreasing the production speed.
Besides the plasma processing, there are such demands as quickly heating up to a predetermined temperature and quickly cooling down after heating, in order to increase the rate of operation of the facilities.
Such demands also call for quickly and continuously regulating the temperature.
In the case of a vacuum processing, on the other hand, moisture is adhered on the surface of the object to be treated. In order to quickly attain the desired vacuum degree, the object may be heated but there is no method of quickly heating only the object.
Under these circumstances, the present invention has been made for solving the above problems and an object is to provide an electric heating element having a novel structure which: 1) can be applied to either double-side baking type or single-side baking type of the electric-heating circuit by using a ceramic material which has previously been sintered as the substrate, 2) can solve the problem of deformation of the ceramic during sintering without requiring pressurization, 3) assures high adhesion strength between the circuit and the ceramic, 4) has excellent oxidation resistance and can be used in the air atmosphere at a high temperature, 5) allows it to produce large-sized articles or those having three-dimensional structures, and 6) has a high electrical resistance and a high wattage density.
The present invention also provides an electrostatic chuck having a novel structure capable of adsorbing and fixing semiconductor substrates and other objects to be treated, and quickly and precisely contr
Marshall Gerstein & Borun
Paik Sang
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