1. Field of the Invention
The present invention relates generally to an apparatus for heat-treating various substrates such as a semiconductor substrate (hereinafter referred to as "wafer") using light-irradiation, and more specifically to, an apparatus for controlling a heat-treatment of a wafer in a closed loop by measuring the temperature of the wafer undergoing the heat-treatment in a light-irradiation heating furnace and a temperature measuring device used in such an apparatus.
2. Description of the Related Art
In a process of manufacturing a semiconductor wafer, various kinds of films and impurity regions should be formed in a wafer. In these steps, the wafer is heat-treated in a light-irradiation heating furnace. As integration of a semiconductor device increases and integration of circuitry fabricated on a single wafer intensifies, the manufacturing process of the wafer has become very much complicated. It is particularly important to control the temperature of the surface of a wafer undergoing a heat-treatment, in order to maintain product yield. The temperature of the surface of a wafer must therefore be measured accurately.
A conventional technique of measuring the temperature of a wafer surface is disclosed in Japanese Patent Laying-Open No. 63-148623. According to the technique, a thermocouple is used for measuring the temperature of a wafer surface. The thermocouple is directly contacted to the wafer surface, or fixed to the surface of the wafer by heat resisting adhesive.
According to this conventional approach, the metal wire of the thermocouple directly contacts the surface of the wafer to be measured. The direct contact allows the temperature of the wafer to be measured accurately.
On the other hand, according to this method, the wafer is contaminated by the metal of the thermocouple because its metal wire directly contacts the wafer. Further, metal components evaporated from these elements contaminate the wafer surface inside the light-irradiation heating furnace. As is well known, a thermocouple having a long and narrow line shaped external form can render its contact to the wafer unstable. In order to avoid such instability, the thermocouple should be fixed to the wafer surface by heat resisting adhesive, etc. Contamination of the wafer surface is therefore inevitable, and this temperature measuring technique is not applicable to wafers provided in fabrication of semiconductor devices.
Another conventional technique is disclosed, for example, in Japanese Utility Model laying-Open No. 60-179829. Referring to FIG. 1, an apparatus for heat-treating a wafer in accordance with this conventional technique includes a light-irradiation heating furnace 10 formed of quartz glass, a plurality of lamp heaters 20 provided above and below the light-irradiation heating furnace 10, a reflection plate 22 provided at the rear of the lamp heaters 20, a susceptor 36 for holding a wafer 30 in the light-irradiation heating furnace 10, and a furnace wall 52 for sealing one end of the light-irradiation heating furnace 10. Provided on the top of the light-irradiation heating furnace 10 are a guide tube 58 formed of quartz glass, a lens 60 positioned above the reflection plate 22, a light receiving sensor 62 provided at a position at which light is collected by the lens 60, a glass tube 64 having the light receiving sensor 62 on its axis line and disposed at a predetermined angle relative to the guide tube 58, a reference heat source 66 provided at one end opposing the light receiving sensor 62 and having its temperature kept constant, a rotating light shielding plate 68 for shielding alternately the axis line of the tube 58 and the axis line of the glass tube 64 in accordance with its rotation, a light shielding plate driver 72 for driving the rotating light shielding plate 68, and a measurement circuit 70 for finding the temperature of the wafers 30 from an output of the light receiving sensor 62.
Provided at the light-irradiation heating furnace 10 are a gas inlet 54 for introducing N.sub.2 gas, etc. into the light-irradiation heating furnace 10, and a gas outlet 56 through which the gas is exhausted. The susceptor 36 is carried by an arm 50, and the arm 50 extends externally from the light-irradiation heating furnace 10 through an opening provided at the furnace wall 52. The furnace wall 52 and the arm 50 move as one body, and, therefore, the wafer 30 is in/out from the light-irradiation heating furnace 10.
The conventional device shown in FIG. 1 operates as follows. Light emitted from the lamp heater 20 reaches the wafer 30 directly or reflected by the reflection plate 22, thereby heating the wafer 30. The wafer 30 generates radiation (infrared) light in accordance with its temperature. The radiation light is introduced into the guide tube 58, and collected by the lens 60 onto the light receiving surface of the light receiving sensor 62. Radiation light emitted from the reference heat source 66 is also collected onto the light receiving surface of the light receiving sensors 62 at that time. The rotating movement of the rotating light shielding plate 68 allows light from the wafer 30 and light from the reference heat source 66 to enter alternately into the light receiving sensor 62.
The light receiving sensor 62 alternately produces an output in accordance with the light emitted from the wafer 30, and an output in accordance with the light emitted from the reference heat source 66 and apply the same to the measuring circuit 70. The measuring circuit 70 samples an output of the light receiving sensor 62 at a timing in synchronization with the rotation of the rotating light shielding plate 68. The sampled values are separated into the values representing the temperature of the wafer 30 and the values representing the temperature of the reference heat source 66, and comparison is conducted between these values. Based on the result of the comparison, the relation between the temperature of the wafer 30 and the temperature of the reference heat source 66 is found, and the temperature of the wafer 30 can be kept constant by controlling the lamp heater 20 according to the obtained relation.
An apparatus as described above can measure the temperature of the wafer 30 on a non-contact basis. It is not necessary to make the thermocouple contact the surface of the wafer 30. Contamination of the surface of the wafer 30 by the metal wire of the thermocouple or adhesive can be avoided as a result. However, this method also bears problems yet to be solved as follows.
Wafers with different film structures or impurity concentrations are different in their emissivity. It is necessary to find the emissivity of each wafer before heat-treatment, in order to measure the temperature of the wafer accurately. This requires a number of operations which are cumbersome. The emissivity of the wafer 30 also changes with a change in air temperature. Therefore, the accurate temperature of the wafer can only be found within a considerably narrow range of temperatures.
Moreover, there exists another problem as follows. Some kind of change can takes place in the wafer when the heating is going on, for example, a change in composition according to, for example, the formation of an oxide film or a change in particle diameter due to the crystal state of polycrystalline silicon. Such a change can be responsible for a change in the emissivity of the wafer 30. It is therefore extremely difficult to always measure its accurate temperature when such a wafer undergoes heat-treatment.
A third conventional technique is disclosed in U.S. Pat. No. 4,924,073. U.S. Pat. No. 4,924,073, is incorporated herein by reference.
According to U.S. Pat. No. 4,924,073, controlling the temperature of a wafer when the wafer is heat-treated can be performed accurately both at the time of activation and resumption of the treatment after the treatment is once interrupted. The approach only requires a relatively simple apparatus structure.
The apparatus disclosed in U.S. Pat. No. 4,924,073 does not measure the temperature of the wafer when the wafer is actually undergoing the heat treatment. It is therefore impossible to find accurately by this method what temperature the wafer has when undergoing the treatment. Prior to a heat-treatment product wafer of a treatment using a dummy wafer must be conducted repeatedly. Different kinds of dummies should be prepared depending upon kinds of product wafers. Such preparation steps should be reduced.