1. Mason Freed, Costas Spanos, Kameshwar Poolla
Spatially Resolved Heat Flux Sensor on a Silicon Wafer for Plasma Etch Processes
SFR Workshop
May 24, 2001
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.
5/24/2001

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
Plasma
Chemical heating
Ion flux heating
Chuck
cooling
Wafer
Chuck
5/24/2001

3. Sensor Geometry: Modified Gardon Gauge
Membrane
Top View T
 sensitivity depends on diameter squared D
Incident heat flux
Antenna
Base
Membrane T
Heat flow within thin dielectric membrane
Membrane thickness=w
Membrane
Side View
Heat flow within membrane
Incident heat flux (q ) T
Heat sink b
5/24/2001

4. Discrimination of Ionic / Chemical Heating
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: 
 antenna structure keeps this added material from adversely affecting the sensitivity
Router,exposed
Router,covered
Rinner,covered2
Vchemical
Vionflux + – + –
Rinner,exposed
Rinner,covered
Router,covered2
5/24/2001

5. First Process – High Built-In Stress Gradient
Warped Antenna
Etch
Holes
Membrane
5/24/2001

6. First Process – Printability Problems
Open Circuit
“Gunk”
Polysilicon
central
resistor
Metal
Wires
5/24/2001

7. Second Process
Moved to polysilicon antenna structure, from previous aluminum/oxide/photoresist stack
Enlarged wires to allow better printability
Switched to wet Aluminum etchant, to avoid aluminum/polysilicon selectivity problem with Cl-based plasma etch
Modified layout to place etch holes closer together, and more evenly spaced
Modified layout to allow better matching between the inner and outer temperature-sense resistors
5/24/2001

8. Problems Solved
5/24/2001

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.
5/24/2001