1. 11/8/2000 1 Spatially Resolved Heat Flux Sensor Array on a Silicon Wafer for Plasma Etch Processes SFR Workshop November 8, 2000 Mason Freed, Costas Spanos, Kameshwar Poolla Berkeley, CA 2001 GOAL: Design, build, and test an array of heat flux sensors on a silicon wafer, with external electronics..

2. 11/8/2000 2 Motivation Plasma etch processes are highly sensitive to wafer temperature, in terms of etch rate, selectivity, and anisotropy Heat delivered to the wafer has two principle sources: ion flux bombardment, and exothermic chemical etch reactions It is very difficult to measure these two quantities, spatially resolved, without using wafer-mounted sensors

3. 11/8/2000 3 Methods for Constructing Heat Flux Sensors Simple, “layered” heat flux gauge: Problem: for semiconductor dimensions and materials,  T is very small: Dielectric, thermal conductivity  Temperature Sensors t Incident heat flux (q  )

4. 11/8/2000 4 Possible Solution: Thermopile Use series connection of many thermocouples to “amplify” temperature difference, giving a measurable output voltage. - from Holmberg, Diller 1995

5. 11/8/2000 5 Possible Solution: Thermopile Benefits –Sensitivity increases linearly with number of thermocouples –Can use 100s or 1000s of them  1000X amplification Problems –Sensor size is proportional to number of thermocouples –Typical thermocouple materials are not part of standard CMOS process  can’t easily combine with electronics –CMOS thermocouples fabricated from n-poly / p-poly are an order of magnitude less sensitive –Assumes no conduction along thermocouple leads – may not be a good assumption

6. 11/8/2000 6 Possible Solution: Gardon gauge “Rotate” the heat flow to travel laterally instead of vertically  increase the effective dielectric thickness  depends on diameter squared! TT Heat flow within thin dielectric membrane Membrane Top View Membrane Side View Heat flow within membrane Incident heat flux (q  ) TT Heat sink D w

7. 11/8/2000 7 Discrimination of Ion Flux / Etch Exothermicity Use two heat flux sensors, one with an exposed layer of etched material (“exposed” in diagram) and the other without this material (“covered”) Place sensors into Wheatstone bridge arrangement:   etched material must be low conductivity to avoid “shorting” the thermal path across the membrane V ionflux +– V chemical +– R outer,exposed R outer,covered R inner,exposed R inner,covered R inner,covered2 R outer,covered2

8. 11/8/2000 8 Proposed heat flux sensor geometry Add antenna to “funnel” heat through the center, maximizing the temperature difference  T Heat flow within membrane Incident heat flux (q  ) TT Heat sink  now, a factor 10X higher  now the conductivity of the top etched material doesn’t affect the operation of the sensor b

9. 11/8/2000 9 2002 and 2003 Goals Demonstrate heat flux sensor in plasma etch environment, with external electronics, by 9/30/2002. Design wireless heat flux sensor wafer and demonstrate it in plasma etch environment, by 9/30/2003.