The present invention relates to a method and apparatus for measuring temperature. The invention is particularly useful for measuring the temperature of a silicon or other semi-conductor materials during the processing thereof, and is therefore described below with respect to this application, although it will be appreciated that the invention could advantageously be used in other applications as well.
Rapid Temperature Processing (RTP) of silicon or other semi-conductor materials requires accurate measurement and control of temperature. In a typical RTP cycle, heating of the wafer to 1,200.degree. C. takes place in seconds, making accurate control of the wafer temperature very crucial. The needed repeatability of temperature control from wafer to wafer is of the order of 1.degree. C. Present temperature measurement techniques use a pyrometric based technology which has many drawbacks, including the following:
1. The roughness of the wafer surface affects its emissivity; therefore a remote pyrometric measurement depends on the accuracy with which this emissivity is known. Cumbersome calibrations with embedded thermocouples in test wafers are needed to know the wafer emissivity with sufficient accuracy.
2. Deposited, uncontrolled layers of oxides, nitrides, polycrystalline, silicon, etc. affect the emissivity of the wafer surface. Because of interference effects which depend strongly on the thickness of these layers, again cumbersome thermocouple calibrations are needed for each single wafer or for a wafer in a batch, to ensure temperature measurement accuracy.
A so-called "Ripple Technique" has been recently described, Schietinger et al., Mat.Res.Soc. Symp.Proc., Spring 1991, which takes advantage, (while using the pyrometric method), of the AC ripple of heating lamps as the signature of the reflected component of the radiation from the wafer surface to monitor the wafer emissivity. The disadvantage of this method is that its accuracy is a function of temperature: it is more accurate for high temperatures than for low temperatures.