1. On-Wafer Ion Flux Sensors
SFR Workshop
November 8, 2000
Berkeley, CA
Tae Won Kim, Saurabh Ullal, Baosuo Zhou, Eray Aydil
Department of Chemical Engineering
University of California Santa Barbara
Santa Barbara, CA 93106
2001 GOAL: to build and demonstrate Langmuir probe based on-wafer ion flux probe array using external electronics 9/30/2001.
11/8/2000

2. Motivation and Goals
Variation of ion bombardment flux and its spatial distribution with plasma conditions is critical to plasma etching.
Ion flux uniformity at the wafer determines the uniformity of etching and etching profile evolution.
There have been almost no measurements of the ion flux or ion flux distribution across the wafer as a function of both r and q in realistic etching chemistry.
Design, build and demonstrate an on-wafer ion flux analyzer with external electronics capable of mapping J+ (r,q) on a wafer.
11/8/2000

3. On-Wafer Ion Flux Probe Array
10 probes on 3” wafer
Evaporated metal on PECVD SiO2 on Si wafer. Lines insulated by PECVD SiO2
External Electronics based on National Instruments SCXI platform
The array is scanned at a rate of 1000 Samples/sec (100 Samples/probe/sec)
Lab View Interface
11/8/2000

4. Ion Flux Uniformity As a Function of Power
Ion Flux as a function of r and q over the whole wafer is determined by extrapolating between the probes.
Ion flux uniformity was measured in an inductively coupled plasma reactor in Ar discharge to demonstrate the probe operation. Qar = 8 sccm, P = 50 mTorr, Probe bias = -70V.
11/8/2000

5. Observation of Plasma Instabilities and “Hot Spots” with the Ion Flux Array
At high pressures and high power a region of high ion flux (“hot spot”) develops at the edge of the wafer at seemingly random positions. Qar = 200 sccm, P = 50 mTorr, Probe bias = -70V.
In fact, this “hot spot” rotates and moves around the edge on a time scale that depends on the plasma conditions.
11/8/2000

6. Spatiotemporal Variation of the Hot Spot
Plasma Instability:
Spatiotemporal Variation of the Hot Spot
t = 0 s
t = 1.5 s
Ion flux at a particular location on the wafer surface fluctuates at constant plasma operating conditions (Ar 168 sccm, 500 mTorr, =200 W,)
t = 3.2 s
t = 4.5 s
11/8/2000

7. Single point Ion Flux Measurements in Cl2 plasma in a Lam TCP reactor
Goal: extend the measurements to a commercial reactor and realistic chemistry.
Challenging because of lack of ground reference and presence of rf bias.
Probe mounted on 8” Si wafer.
External electronics.
Ion current determined by measuring the voltage drop across a known resistance.
Both reference and measurement probe are isolated from ground (using a floating power supply.)
Measurement Probe -75V with respect to reference)
Heavily Doped
Si wafer (Reference)
11/8/2000

8. On-Wafer Ion Flux Measurements in a Cl2 Discharge in Lam TCP Reactor
Ion Flux in Cl2 plasma increases as a function of exposure time to Cl2 plasma until it finally saturates.
Changes in chamber wall conditions is likely to be responsible for the drift.
Exposure to SF6 plasma resets the chamber back to reproducible condition.
Ion flux behavior as a function of time is reproducible.
11/8/2000

9. Summary
Designed and build an on-wafer ion flux probe array with external electronics.
Demonstrated the use of the array for
mapping ion flux uniformity in an Ar plasma.
measuring spatiotemporal variation of the ion flux in presence of a plasma instability.
Completed preliminary experiments in a commercial reactor.
2002 and 2003 Goals
Build and demonstrate 8” on-wafer ion flux probe array in industrial plasma etcher with external electronics by 9/30/2002.
Integration of Si-based IC with sensor arrays. Characterize and test integrated MEMS ion sensor array. 9/30/2003.
11/8/2000