Sensors on Silicon Allow On-Wafer Process Control

By accessing the very technology it helped create, semiconductor manufacturers are combining sensors, power sources and communications technology to improve process control during lithography, etching, RTP and other processes. Researchers from the University of California at Berkeley, including Mason Freed (now with OnWafer Technologies in Pleasant Hills, Calif.), Michiel Kruger, Costas Spanos and Kameshwar Poolla, revealed several proof-of-concept wafer sensor designs that measure film thickness and temperature in real time. They reported their findings in the August 2001 issue of the IEEE Transactions on Semiconductor Manufacturing. In-chamber wafer sensors allow faster uniformity characterization during equipment design, reduced use of dummy wafers for process calibration and faster detection of equipment problems.

The Berkeley researchers developed six sensor wafers, three that optimized the sensor and three wireless sensors that optimized the power, communications and isolation technologies. The first sensor was a polysilicon film-thickness sensor capable of reporting film thickness with an accuracy of <50 Å and repeatability of <15 Å. It was designed to function inside a polysilicon plasma etch chamber and on DUV lithography tracks. Using a van der Pauw probe fabricated on the gate poly layer of a 13-mask CMOS process, a wet SiO₂ etch selectively etched back the dielectric to the poly sensor. Testing using a very fast etchant (XeF₂) led to a second sensor design that added a guard-ring around the wafer edge. Extra poly reduced the loading effect and etch rate. The new design required only two masking layers for a two-week design turnaround vs. 12 months previously. Since poly resistivity is a function of small temperature changes, the final design used a buried van der Pauw structure next to each film thickness sensor, allowing compensation for resistivity variations due to temperature change.

The next studies led to a temperature sensor capable of reporting temperature at up to four locations within 1°C at up to 120°C. However, the initial design, which used a thermistor to measure temperature, an alkaline watch-cell battery power source and optically based communication, delivered an accuracy of only 5°C. The sensor used a resistance-to-frequency conversion circuit and a timer chip to generate a pulse train input to a visible LED. The wafer-mounted thermistor's resistance determined the frequency of the pulse train. External flash frequency determined the wafer temperature.

To correct for visible light interference, the second iteration used modulated infrared communications. A microprocessor-based scheme with integrated 8-bit A/D converter and standard IrDA communications protocol allowed data transfer at 9600 baud to an external detector. The chips, battery and communication were connected to the wafer using silver paint, for compatibility with the on-chip aluminum interconnects. An external amplifier matched the impedance of the thermistor and A/D converter input. This second design allowed more accurate temperature reading (within 3°C), but the desire to eliminate the external amplifier and expand the usable temperature range led the researchers to develop a surface-mounted sensor module whose voltage output is proportional to temperature. An aluminum/nickel layer and solder provided electrical connections. The presence of batteries limited the temperature ranges
to 120°C. An IR-transparent epoxy allowed sensor operation in a plasma etcher.