Conventionally, heater units having a resistance heating element embedded in a metallic base have been widely used in susceptors and electrostatic chucks for semiconductor processing. The resistance heating element typically consists of a sheath heater including a sheath tube made of stainless steel. Typically, a stainless steel sheath heater is cast into an aluminum base, clamped between a pair of aluminum or stainless plates, or interposed between a pair of plates which are joined together by welding along the outer edge.
FIG. 5 shows a conventional cast heater unit 10 including a base 11 having a stainless steel sheath heater 12 cast therein. The stainless steel sheath heater 12 typically comprises a stainless steel sheath pipe, a resistance heater wire received in the sheath pipe, and an insulator which electrically insulates the resistance heater wire from the sheath pipe. The sheath heater 12 is bent into a desired shape, and is cast into the aluminum base 11. The aluminum base 11 is then machined and polished to a desired finish.
FIG. 6 shows a conventional clamped heater unit 13. In this case, the metallic base consists of a lower base 14 and an upper base 15, each made of an aluminum or stainless steel plate. A sheath heater 12 is received in a groove 14a formed in the lower base 14, and the upper base 15 is placed closely over the lower base 14. The upper and lower bases 14 and 15 are then firmly joined with each other by fastening them together with threaded bolts.
FIG. 7 shows a conventional welded heater unit 16. In this case also, the metallic base consists of a lower base 17 and an upper base 18, each made of an aluminum or stainless steel plate. Likewise, a sheath heater 12 is placed in a groove 17a formed in the lower base 17, and the upper base 18 is placed closely over the lower base 17. The upper and lower bases 17 and 18 are then firmly joined with each other by welding them together along their edge 19.
A cast heater unit demonstrates a favorable heat conductivity owing to the close contact between the metallic base and the heating element. However, the metallic base is typically made of aluminum or an aluminum alloy which has a relatively low melting point (typically about 540.degree. C.), and the cast heater unit using such a metallic base is therefore unsuitable for use in a high temperature environment exceeding 500.degree. C. in temperature. Also, because the casting temperature of aluminum is higher than 700.degree. C., the sheath pipe must be made of a high melting point material such as stainless steel and Inconel. Because the material of the sheath pipe differs in thermal expansion coefficient from the aluminum base, the resulting thermal stress may cause deformation to the heater element under a high temperature condition, and the repeated thermal stress may cause damage to the resistance heater wire and warping of the heater unit.
A clamped heater unit using an aluminum base can be used only in a same temperature range as a cast heater unit. But, because the sheath heater is retained by the base only physically, the repeated thermal stress is less likely to cause damage to the resistance heater wire. On the other hand, a certain gap is created in the groove, and this gap impairs heat transfer. In particular, the contact between the sheath heater and the base, and between the upper and lower bases is not very intimate, and this results in a poor thermal conductivity and localized temperature rises. Localized temperature rises are known to impair the durability of the resistance heater wire.
A welded heater unit is not much different from a clamped heater unit in these respects, and has the additional problem of distortion due to the heat at the time of welding.