In rapid thermal processing of semiconductor wafers a wafer is supported in a processing chamber by a plurality of pointed quartz pins, usually three. The low thermal conductivity of the quartz as well as the small contact area at the support point minimizes heat transfer from the wafer into the support rods thereby assuring uniform wafer temperature. The wafer is rapidly heated by thermal radiation from a single or an array of tungsten-halogen lamps which are arranged to provide uniform temperature throughout the wafer.
It is extremely important that the wafer temperature be continually monitored during wafer processing. It is also desirable to monitor the thickness of films which are deposited as the wafer surface during processing. An ideal temperature and thickness sensor has the following characteristics: fast response time to provide accurate and repeatable real time temperature independent of changing wafer conditions, must not disturb the wafer temperature distribution, be insensitive to wafer processing gases and pressures, and inexpensive.
Different temperature dependent physical phenomena have been used to measure the temperature of wafers during processing. One of the most popular temperature sensor, based on lattice vibrations, is the thermocouple. Although the thermocouple is fairly accurate, its must contact the wafer. There are several problems with the contacting technique. First, thermocouples are metallic and serves as heat sinks on the wafer producing temperature non-uniformities. The thermocouple material contacting the wafer contaminates the wafer. Finally, unless the thermocouple is welded or bonded onto the wafer it is difficult to ensure a reliable contact. Thus, thermocouples, although used extensively for calibration purposes are generally not used during actual processing. The most widely used temperature measurement technique in rapid thermal processing is pyrometry. Pyrometry, however, has the significant limitation that its measurements are strongly dependent on the emissivity of the wafer. The emissivity of the wafer is dependent on several factors including film thickness, surface roughness, surface material, and temperature and is difficult predict. Other method of temperature measurement based on emission (black body radiation) are multi-wavelength pyrometry, infrared spectral correlation, and ellipsometry. Another temperature dependent phenomena is thermal expansion. In one such temperature measurement technique, a grating is placed on the wafer and the moire pattern created by the interference of a laser light grating and an etched grating is studied to measure the local thermal expansion, and thus the temperature. Thermal expansion of the thickness of the wafer and the length of the wafer are also used to measure the temperature. The wide range of temperature measurement schemes reflects the fact that temperature monitoring and control is currently a very important issue in semiconductor processing.