1. Saurabh J. Ullal, Anna R. Godfrey, Eray S. Aydil
A Novel Sensor for Monitoring Plasma Reactor Chamber Walls: Simultaneous Monitoring of Ion Flux and Films Deposited on Chamber Walls:
SFR Workshop
November 8, 2000
Berkeley, CA
Saurabh J. Ullal, Anna R. Godfrey, Eray S. Aydil
Department of Chemical Engineering
University of California Santa Barbara
Santa Barbara, CA 93106
Erik A. Edelberg, Linda Braly, John Daugherty, Vahid Vahedi
Lam Research Corporation
Fremont, CA 94538
2001 GOAL: to build and demonstrate Langmuir probe based on-wafer ion flux probe array using external electronics 9/30/2001.
11/08/2000

2. Motivation and Introduction: The need for a “wall” sensor
Plasma reactor walls play a crucial role in determining the ion fluxes in a plasma etching reactor, especially at low pressures.
This well-known plasma-wall interaction can cause problems ranging from process drifts in semiconductor manufacturing to irreproducible data in fundamental studies employing plasma diagnostics.
No fundamental understanding of reactions occurring on the reactor walls; no clear metrics to quantify the condition of the reactor wall.
Terms such, as “wall conditioning” and “reactor seasoning” have become accepted language in plasma science and technology literature to describe the art of avoiding such effects.
We have developed a surface probe based on multiple total internal reflection Fourier transform infrared (MTIR-FTIR) spectroscopy that can be used as a diagnostic technique to monitor the films and adsorbates on the plasma reactor walls and applied it to silicon etching in Cl2/O2 plasmas.
We used this probe in conjunction with ion flux sensors to understand the effect of walls on the ion flux incident onto the wafer surface during plasma etching.
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3. Multiple Total Internal Reflection - Fourier Transform Infrared (MTIR-FTIR) surface sensor
From FTIR
Spectrometer
To HgCdTe
Detector
Chamber
Wall
Internal Reflection
Crystal
Plasma
IR radiation from a spectrometer is directed onto one of the beveled edges of an internal reflection crystal (IRC). The IR beam undergoes multiple total internal reflections from the crystal’s surface and emerges from the opposite beveled edge. After interacting with the IRC surface the IR radiation is collected and directed to a detector. In this way, IR spectra of films and species that are adsorbed on to the walls and the IRC are recorded.
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4. Experimental Setup - Lam TCP 9400®
(13.56MHz)
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5. On-Wafer Ion Flux Measurements in a Cl2 Discharge in Lam TCP 9400® 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.
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6. SixClyOz Film Deposits on the Reactor Wall During Etching of Si with Cl2/O2 Plasma
SiO2 film is deposited on reactor walls from the reaction of SiClx with O even in the absence of O2 in the feed gas: quartz window or walls can be the source of Si and O.
Sensor is sensitive to even a few Å of oxide on the walls.
Sufficiently long SF6/O2 plasma removes the oxide film from the walls.
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7. Wall Cleaning/Conditioning Step Influences the Ion Fluxes in the Subsequent Etching Steps
SF6 cleaning step
SF6+O2 cleaning step
Ion flux and its variation with time depends on the wall cleaning step
If plasma reactor walls are cleaned/conditioned with SF6+O2 : Ion Flux remains steady for a longer time compared to conditioning with SF6 only.
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8. Relation Between the Ion Flux, Gas Phase Composition and Wall Deposits
Cl & SiClx
SiO2 on the Walls
Ion Flux monitored using ion flux probe
SiClx and Cl concentrations monitored using optical emission
Wall deposition monitored using the MTIR-FTIR probe
Oxygen plasma oxidizes the surface of the wafer and probe
Cl2 plasma (no bias power) etches the oxide layer slowly compared to the Si.
Drift in Ion Flux is due to changing wall conditions and plasma composition.
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9. Summary
A novel diagnostic technique based on (MTIR) FTIR has been developed and used to monitor walls during plasma etching of Si with Cl2/O2 plasmas.
Chlorinated silicon oxide (SixClyOz) film is deposited on the reactor walls during Si etching with Cl2/O2 plasmas.
The chamber can be reset to a reproducible state by removing this film using a SF6 plasma.
Ion flux bombarding the wafer surface depends on the conditions of the reactor walls most likely due to the effect of walls on the neutral composition.
Ion flux bombarding the wafer surface depends on the concentration of SiClx reaction products in the reactor and is a function of time during etching.
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.
Simultaneously monitor ion fluxes and wall conditions as well as gas phase composition to understand the effects of walls and gas composition on ion fluxes by 9/30/2003.
11/08/2000