(1) Field of the Invention
The present invention relates to heaters for use in semiconductor-producing apparatuses, etc.
(2) Related Art Statement
The semiconductor-producing apparatus is provided with a ceramic heater for heating wafers as substrates in forming thin semiconductive films with a raw gas such as silane gas by hot CVD.
A heater structure so-called a two-zone heater is known as the ceramic heater. In the two-zone heater, an inner resistance heat generator and an outer resistance heat generator made of a high melting point metal such as molybdenum are buried in a ceramic substrate, separate current-introducing terminals are connected to the resistance heat generators, respectively, and given voltages are applied to them, respectively, to independently control the inner and outer resistance heat generators.
Further, in JP-A 5-326112, a resistance heat generator of a ceramic heater is constituted by plural circuit patterns made of a high melting point metal, and the circuit patterns are so arranged that they may supplement one another""s defect portions. More specifically, one of the plural circuit patterns is overlapped with the other at a folded portion, a returning portion or the like of the other.
Particularly, when the heater is to be used for heating semiconductor wafers, the temperature of a heating surface of the heater needs to be entirely uniformly controlled. It is required that the heater satisfy a severe requirement that the temperature is within variations of xc2x15xc2x0 C. over the whole heating surface under a use condition, for example. The ceramic heater of the two-zone controlling system can ordinarily satisfy such a requirement. However, as a result of making further investigations, the inventors found it difficult to maintain a given uniformly heating characteristic particularly as the temperature rises. For example, in even a ceramic heater which maintains uniformity in the temperature at a heating surface over a temperature range of room temperature to 500xc2x0 C., for example, variations in temperature at the heating surface sometimes rapidly become greater at its heating surface when the temperature is in a range of 600xc2x0 C. or higher.
The present inventors advanced investigations on the possible limit in controlling such uniform heating. For example, as shown in FIG. 6, a heater 3A is housed in a chamber 1. The heater 3A includes a planar substrate 5 and a layer of a heating element, 7, for heating the substrate 5. An object 2 to be heated is placed on a placing surface 5a of the substrate 5. The heating element layer 7 contacts a back surface 5b of the substrate 5. The heating element layer 7 is sandwiched between the substrate 5 and the insulating plate 6, and the substrate 5 and an insulating plate 6 are tightened with bolts 8A. A reference numeral 6a denotes a contact surface of the heating element layer. A cylindrical supporting member 10 is integrally joined to a rear surface 6b of the insulating plate 6. A base seat 11 is fixed to a lower end portion of the supporting member 10, and the base seat 11 is fixed to an outer wall (not shown) of the chamber. Reference numerals 4 and 9 denote a gate valve and an electrode cable, respectively.
However, since the diameter of the supporting member 10 is very large, although slightly smaller than that of the insulating plate 6, the area of the supporting member 10 is large. The dimensions of the outer configuration of the supporting member 10 is far larger than those of the entire configuration of the insulating plate 5, the heating element layer 7 and the substrate. As a result, the whole volume of the heater 3A becomes very large, so that much heat transmitted to the supporting member 10 from the substrate 5 constantly escapes. In this state, as the temperature of the substrate 5 rises, cold spots are likely to occur in the substrate 5 due to slight non-uniformity, etc. in producing the constituting members, thereby deteriorating uniformly heating ability. Further, since the gate valve 4 is arranged near the peripheral portion 5C of the substrate 5, heat escapes from near the gate valve 4 at the peripheral portion 5c of the substrate 5, resulting in a cold spot.
In order to solve the problems that the supporting member 10 has a large volume and that heat escapes from the supporting member 10 non-uniformly, the present inventors contrived and examined a heater 3B as schematically shown in FIG. 7. In the heater 3B, an additional insulating plate 14 is laminated on a side of a rear side 6b of an insulating plate 6, and is tightened with bolts 8B. A cylindrical supporting member 13 is provided under the insulating plate 14, and fixed to a base seat 11. The base seat 11 is attached to an outer wall (not shown) of a chamber. In this heater, since the insulating plate 6, a heating element layer 7 and a substrate 5 are attached to the seat of the chamber 11 with the small-sized supporting member 13, the volume of the entire heater 3B can be reduced. However, in this heater, heat escapes from the heating element layer 7 to the insulating plate 14 and the supporting member 13 as shown by an arrow A, a cold spot still occurs when the placing surface of the substrate becomes high temperatures.
It is an object of the present invention to provide a heater for heating on a substrate an object to be heated, having a structure which effectively prevents cold spots at a placing surface of the substrate and thereby enhances uniformity in temperature of the placing surface.
It is another object of the present invention to reduce the entire volume of the heater, while maintaining uniformity of the temperature of the placing surface of the substrate.
The heater according to the present invention includes a substrate having a placing surface for placing an object to be heated thereon, a layer of a main heating element for heating the substrate and the object, a layer of an auxiliary heating element, and an insulating body interposed between the main heating element layer and the auxiliary heating element layer. An area of the auxiliary heating element layer is smaller than that of the main heating element layer, and while the substrate and the object are heated with heat generated from the main heating element layer, heat escaping from the substrate is supplemented with heat generated from the auxiliary heating element layer.
These and other features of the invention will be appreciated upon reading the following description of the invention when taken in connection with the attached drawings, with the understanding that some modifications, variations and changes could be easily made by the skilled person in the art to which the invention pertains.