Ceramic heater to heat wafers

Abstract: A ceramic heater (1) includes a ceramic substrate (2) having a heating surface (2a) for supporting and heating an object to be processed, a heating element (6) provided in the substrate (2), and embossed portions (4) on the heating surface (2a) which are to contacted with the objects. The number of the embossed portions (4) per unit area on the heat spot (3) is less than that on the rest of the heating surface (2a). When the average temperature of the heating surface (2a) is raised to reach a target temperature, the heat spot (3) having relatively high surface temperatures exist on the heating surface (2a).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic heater. 2. Description of the Related Art The semiconductor-producing apparatus is equipped with a ceramic heater to heat wafers, which are substrates, for manufacturing semiconductor films from raw material gases such as silane gas by thermal CVD or the like. A configuration so-called a dual zone heater is known as a ceramic heater. For the dual zone heater, an inner resistance-heating element and an outer resistance-heating element each comprising a high-melting point metal such as molybdenum are buried in a ceramic substrate. Separate current-introducing terminals are connected to the respective resistance-heating elements, and the inner resistance-heating element and the outer heating element are independently controlled by applying a given voltage to each resistance-heating element. For example, for an application to heating semiconductor wafers, controlling the temperature throughout the heating surface uniformly is necessary, and satisfying strict specifications such as temperature differences being within ±5°C throughout the heating surface under a use condition is demanded. However, a so-called heat spot may practically occur on the heating surface. Occurrence of the heat spot tends to significantly increase when a target temperature of the heating surface is not less than 500°C, and further not less than 600°C. The occurrence of the heat spot may cause a variation in thickness of the semiconductor film at the heat spot. The inventors further studied and found that when the semiconductor wafer is provided on the ceramic heater and a semiconductor film is formed thereon, an identical pattern of the semiconductor film, for example, as a spiral resistance-heating wire buried in the substrate of the ceramic heater may appear on the semiconductor wafer. Such a pattern shows that a pattern of the heat spot is occurred on the heating surface corresponding to the pattern of the resistance-heating element by an effect of the resistance-heating element buried in the substrate. EP-A-0929205 describes a ceramic heater, having an aluminium nitride substrate having a planar surface for support of the object being heated. Inside the substrate is a resistance heating element in a coil-shaped form. EP-A-0993024 describes an electrostatic chuck for holding wafers undergoing treatment. The chuck has a substrate and inner and outer electrodes concentrically buried in the substrate. On the support surface of the substrate there are embossed portions. The density of the embossed portions on the substrate surface is larger at the zone at which the inner electrode is located than at the zone at which the outer electrode is located. The object is to achieve a uniform attracting force. SUMMARY OF THE INVENTION It is an object of the present invention to provide a ceramic heater comprising a ceramic substrate having a heating surface for supporting and heating an object to be processed, and a heating element provided in the substrate, while preventing or reducing the adverse effect on the processed objects caused by a heat spot or heat spots on the heating surface. The present invention relates to a ceramic heater, as set out in claim 1. The form of placing of the heating element in the substrate is not restricted. Preferably, the heating element is buried in the substrate, but it can be provided on the surface of the substrate opposed to the heating surface. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference is made to the accompanying drawings, wherein: FIG. 1(a) shows an example of a pattern of a heat spot on a ceramic heater; FIG. 1(b) is a plane view showing a form of a heating surface 2a of a ceramic heater 1A; FIG. 2 is a plane view of a ceramic heater 1B; FIG. 3 is a longitudinal sectional view of a ceramic heater 1D; FIG. 4 is a plane view of a heating surface of the ceramic heater 1D showing a pattern of a heating element 6A with dotted lines; FIG. 5 is a plane view of the heating surface of the ceramic heater 1D showing a pattern of a heating element 6B with dotted lines; FIG. 6 is a longitudinal sectional view of a ceramic heater 1E; DETAILED DESCRIPTION OF THE INVENTION The present inventors conceived to arrange embossed portions on a heating surface to place a heat spot in a void space between the embossed portions while no embossed portion is provided on the heat spot, thereby preventing the heat spot from contacting with an object to be processed and acquiring air gaps between the heating surface and the objects. As a result, pressures in the gaps are equal to an interior pressure of a chamber. The pressure is usually low so that heat conduction from the heat spot to the objects is inhibited and temperature distributions of the objects become uniform. Thus, an adverse effect of the heat spot on the objects can be prevented. FIG. 1(a) is a plane view showing a position of the heat spot 3 on the ceramic heater 1, and FIG 1(b) is a plane view of the ceramic heater 1A after fabricating the embossed portions as viewed from a side of a heating surface 2a. A substrate 2 of the ceramic heater 1A is, for example, a disk, and one of main faces of the substrate 2 constitute the heating surface 2a. The heat spot 3 occurs on the heating surface 2a at a target temperature. The target temperature herein means a maximum temperature to be reached during a treatment of the object. When a number of the embossed portions 4 are fabricated on the heating surface 2a according to the present invention, the embossed portions are arranged to place the heat spot 3 in the void spaces 5 and 5A between the embossed portions. In the present invention, the number of the embossed portions per unit area on the heat spot is set to be less than that on the rest of the heating surface. A ratio of (the number of the embossed portions per unit area on the heat spot)/(the number of the embossed portions per unit area on the rest of the heating surface) is preferably less than 1/3 to acquire the effect of the present invention. In a most preferred embodiment of the present invention, the embossed portions are arranged to place the heat spot in the void space between the embossed portions, while no embossed portion is provided on the heat spot. In a preferred embodiment of the present invention, the heating element is buried in a planar form in the substrate. When the heating element is projected on the heating surface in a direction perpendicular to the heating surface, the embossed portions are fabricated on void spaces of the heating element. FIG. 2 shows a heater representing this embodiment. In the ceramic heater 1B shown in FIG. 2, a pattern of the heat spot corresponding to a pattern of a heating element 6 buried in the substrate appears on the heating surface. Accordingly, the embossed portions 4 are arranged not to be above the heating element 6 on the heating surface 2a. That is, when the heating element 6 is projected on the heating surface in a direction perpendicular to the heating surface, no embossed portion exists on an area in which the heating element 6 exist, while the area in which the heating element 6 exists is located on the spaces 5B between the embossed portions. In the present invention, the number of the embossed portions per unit area above the heating element is set to be less than that on the rest of the heating surface. A ratio of (the number of the embossed portions per unit area above the heating element)/(the number of the embossed portions per unit area on the rest of the heating surface) is preferably less than 1/3 to acquire the effect of the present invention. In a most preferred embodiment of the present invention, the embossed portions are arranged to place the heating element in the void space between the embossed portions, while no embossed portion is provided above the heating element. In another preferred embodiment of the present invention, a plurality of heating elements are buried in the substrate. Each of the heating elements is buried along each of respectively corresponding planes. When the heating elements are projected on the heating surface in a direction perpendicular to the heating surface, they overlap each other to exist nearly in the same position. The ceramic heater 1D shown in FIG. 3 represents the above embodiment. A plurality of, for example, two lines of heating elements 6A and 6B are buried in the substrate 2. Each of the heating elements 6A and 6B is buried in the substrate to stretch along a plane A or B, respectively, which planes are parallel to each other. Each of the heating elements 6A and 6B is connected to an electric power source I 1 or 12 via a terminal 9 or 10, respectively. FIG. 4 is a plane view of the substrate 2 as viewed from the side of the heating surface 2a showing a two-dimensional pattern of the heating element 6A with a dotted line. The dotted line is a pattern of the heating element 6A when the heating element 6A is projected on the heating surface 2a in a direction C perpendicular to the heating surface 2a. The embossed portions 4 are not fabricated on the area in which the heating element 6A exists. The area in which the projected heating element 6A exists is located on the space 5B between the embossed portions 4. FIG. 5 is a plane view of the substrate 2 as viewed from the side of the heating surface 2a showing a two-dimensional pattern of the heating element 6B with a dotted line. The dotted line is a pattern of the heating element 6B when the heating element 6A is projected on the heating surface 2a in a direction C perpendicular to the heating surface 2a. The embossed portions 4 are not fabricated on the area in which the projected heating element 6B exists. The area in which the projected heating element 6B exists is located on the space 5B between the embossed portions 4. In this embodiment, when the heating elements 6A and 6B are projected on the heating surface 2a in a direction C perpendicular to the heating surface 2a, the projected heating elements 6A and 6B overlap each other to exist nearly in the same position. Moreover, according to the present invention, when each of the heating elements 6A and 6B is projected on the heating surface 2a, the embossed portions 4 do not exist on the area of the heating surface 2a in which the projected heating element 6A or 6B exists. That is, the area in which each of the projected heating elements 6A and 6B exists is placed on space 5B between the embossed portions. In another preferred embodiment of the present invention, a plurality of heating elements are buried in the substrate. Each of the heating elements is buried along each of respectively corresponding planes. When the heating elements are projected on the heating surface in a direction perpendicular to the heating surface, they exist in mutually different positions (positions without overlapping). In this case, the embossed portions are fabricated on the space of the heating surface between each of the heat spot since the heat spot appear on the heating surface along shapes corresponding to each of the buried heating elements. The ceramic heater 1E shown in FIG. 6 represents the above embodiment. A plurality of, for example, two lines of heating elements 6C and 6D are buried in the substrate 2. Each of the heating elements 6C and 6D is buried in the substrate to stretch along a plane A or B, respectively, which planes are parallel to each other. Each of the heating elements 6C and 6D is connected to an electric power source 11 or 12 via a terminal 9 or 10, respectively. In this embodiment, when the heating elements 6C and 6D are projected on the heating surface 2a in a direction C perpendicular to the heating surface 2a, the projected heating elements 6C and 6D exist in mutually different positions (positions without overlapping) on the heating surface 2a. When the heating elements 6C and 6D are projected on the heating surface 2a according to the present invention, the embossed portion do not exist on the area of the heating surface 2a in which the projected heating element 6C or 6D exists, and the area in which each of the projected heating elements 6C and 6D exists is located on the space 5D between the embossed portions. A height of each of the embossed portions 4 is preferably 5 - 500 µm from the standpoint of inhibiting an effect of the heat spot on the objects. In the present invention, although a diameter  of each of the embossed portions can be varied, the diameter  is preferably 0.2 - 10 mm from the standpoint of a uniform temperature distribution of the object. In addition, each of the embossed portions may be varied in a two-dimensional shape and a two-dimensional size. For example, a shape of a surface of the embossed portions which are to contact with the object may be polygonal such as triangle, tetragonal or hexagonal, ring or strip. Moreover, the number of the embossed portions is not particularly restricted. However, 0.0025-0.32 pieces/mm 2 is the particularly preferred number of the embossed portions per unit area from the standpoint of ensuring a uniform suction force against the object, especially against a substrate for manufacturing a semiconductor wafer or a semiconductor of a LCD panel, throughout the surface of the object. The dimensions of each of the embossed portions, such as the height, are measured with a dial gauge or a three-dimensional shape-measuring device. Generally, a material of the ceramic heater 2 is not restricted, an aluminum nitride base ceramic, a composite material containing aluminum nitride, an alumina base ceramic, a composite material containing alumina, and a composite ceramic of alumina and aluminum nitride are preferred from the standpoint of further reducing a generation of particle. A material of the embossed portions is not particularly restricted, but an aluminum nitride base ceramic, a composite material containing aluminum nitride, an alumina base ceramic, a composite material containing alumina, and a composite ceramic of alumina and aluminum nitride are preferred from the standpoint of further reducing a generation of particle. The embossed portions are fabricated with a blast process, a CVD process or the like. A material of the heating element is also not restricted. A conductive ceramic or a metal may be used, and preferably a high-melting point metal, and more preferably molybdenum, tungsten and an alloy thereof may be used. In the present invention, a hollow space can be provided between the heating surface and the heating element in the substrate of the ceramic heater and a plurality of heat conductors can be provided in the space to transverse the space. It had not been known that the heat conduction is facilitated by providing heat conductors at some areas in the air gap, as well as the heat conduction in the substrate is inhibited by forming the air gap in the substrate, thereby inhibiting the generation of the heat spot. In a case where the aforementioned space does not exist in the substrate and the heat spot having a relatively high surface temperature exist on the heating surface when the average temperature of the heating surface is raised to reach the target temperature, it is particularly advantageous to form the aforementioned space and heat conductors according to the present invention. In this case, the ceramic heater is designed so that the heat conductors are arranged in the space between the heat spot and are not located on the heat spot when the heat conductors and the heat spot are projected on the heating surface in a direction perpendicular to the heating surface. In a preferred embodiment of the present invention, the number of the heat conductors per unit area on the heat spot is less than that on the rest of the heating surface. A ratio of (the number of the heat conductors per unit area on the heat spot)/(the number of the heat conductors per unit area on the rest of the heating surface) is preferably less than 1/3 to acquire the effect of the present invention. More preferably, the heating elements are buried in planar forms in the substrate, and the heat conductors are formed in the space between the heating elements when the heat conductors and the heating elements are projected on the heating surface in a direction perpendicular to the heating surface. (Preferred Embodiment) (Experiment 1) Ceramic heaters schematically shown in FIGS. 3-5 were manufactured. Specifically, aluminum nitride powder obtained by a reduction nitriding process were used, an acrylic resin binder was added therein, and then the mixture was granulated by a spray dryer, thereby obtaining granulated grains. Three sheets of molded bodies were sequentially made by uniaxially press-molding the granulated grains, and then each of the three layers of molded bodies was stacked on top of another to make them a single piece. Coil-like resistance-heating elements 6A and 6B made of molybdenum were buried in the uniaxially press-molded body. The molded body was placed in a hot press mold, which was hermetically sealed. The molded body was heated at a heating rate of 300°C/hr. while the pressure was reduced in a temperature range from the room temperature to 1,000°C. After the temperature reached at 1,000°C, the pressure was simultaneously increased with increase in the temperature. The maximum temperature was 1,800°C, and the molded body was held and fired at the maximum temperature under 200kgf/cm 2 for 4 hours in nitrogen gas to obtain a sintered body. Then the sintered body was subjected to a machine work and a finish machining, thereby a heater was prepared. The diameter and the thickness of a substrate 2 were set to 240mm and 18mm, respectively. The clearance between the resistance-heating elements 6A and 6B was set to 5.5mm. The surface of the substrate 2 was sandblasted to fabricate the embossed portions 4. The height, the diameter and the density of the embossed portions 4 were set to 20 µm, 3 mm and 1.5 pieces/cm 2 , respectively. The average temperature of the heating surface of the heating apparatus was set to about 650°C by controlling power supply from each AC power source. The pressures in the chamber were set to high vacuum, 3 torr, 30 torr and 50 torr. Then, a silicon wafer was placed still on the heating surface, and silane gas was supplied to form a semiconductor film on the silicon wafer. The semiconductor film was made of, for example, TiN, SiO 2 , low-K material, Si 3 N 4 , Ta 2 O 3 or the like, and was used as a metal wiring film, insulating film, barrier film, electrode, capacitor, or the like. A surface state of the thus obtained silicon surface was observed. As a result, when the pressure in the chamber was high vacuum or 3 torr, a pattern of the semiconductor film corresponding to the heating elements was not observed at all. When the pressure was 30 torr or 50 torr, a pattern of the semiconductor film corresponding to the heating elements was observed in some degree. Thus, a further detailed investigation was conducted, and a thickness distribution was measured between the adjacent patterns to give a variation of not more than 1%. This level was not more than a half of the film thickness distribution over the entire wafer, so it is practically not a problem. As has been described above, according to the present invention, a ceramic heater comprising a ceramic substrate having a heating surface for supporting and heating an object to be processed, a heating element provided in the substrate, and embossed portions on the heating surface which are to contact with the objects is provided, thereby preventing an adverse effect on the objects caused by a heat spot on the heating surface. The present invention also consists in use of the ceramic heaters of the present invention in production of semiconductor devices, and in semiconductor device production apparatus including a ceramic heater in accordance with the invention.

Claims: A ceramic heater comprising a ceramic substrate (2) having a heating surface (2a) for supporting and heating an object to be processed, and a heating element (6) provided in the substrate, characterized by embossed portions (4) on the heating surface (2a) which are to contact with the object, wherein a heat spot (5A,5B) having a higher surface temperature than adjacent surface regions exists on the heating surface when the average temperature of the heating surface is raised to reach a target temperature and the number of the embossed portions per unit area on the heat spot is less than that on the rest of the heating surface. A ceramic heater as claimed in claim 1, wherein the heating element (6) is provided in a planar form in the substrate, and the pattern of the hot spot corresponds to the pattern of the heating element projected onto the heating surface in a direction perpendicular to the heating surface. A ceramic heater as claimed in claim 2, wherein a plurality of said heating elements (6A,6B) are buried in said substrate at respective planes, and the patterns of heating elements, projected on said heating surface in a direction perpendicular to said heating surface, overlap each other to exist nearly in the same positions. A ceramic heater as claimed in claim 2, wherein a plurality of said heating elements (6C,6D) are buried in said substrate at respective planes and when the patterns of heating elements, projected on said heating surface in a direction perpendicular to said heating surface, exist in mutually different areas.

Inventors: YAMAGUCHI SHINJI, YAMAGUCHI, SHINJI, Yamaguchi, Shinji, c/o NGK Insulators Ltd.
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 2001
Main classification: H01L21
IPC codes: H05B3, H01L21