A Novel Sensor for Monitoring Plasma Reactor Chamber Walls: Simultaneous Monitoring of Ion Flux and Films Deposited on Chamber Walls:

1
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 Universit
y 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.
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.

3
Multiple Total Internal Reflection - Fourier
Transform Infrared (MTIR-FTIR) surface sensor

• 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
  crystals 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.

4
Experimental Setup - Lam TCP 9400
(13.56MHz)
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.

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.

7
Wall Cleaning/Conditioning Step Influences the
Ion Fluxes in the Subsequent Etching Steps
SF6 cleaning step
SF6O2 cleaning step

• Ion flux and its variation with time depends on
  the wall cleaning step
• If plasma reactor walls are cleaned/conditioned
  with SF6O2 Ion Flux remains steady for a
  longer time compared to conditioning with SF6
  only.

8
Relation Between the Ion Flux, Gas Phase
Composition and Wall Deposits
Ion Flux
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.

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.