Post on 16-Dec-2015
ION ENERGY DISTRIBUTIONS IN INDUCTIVELY COUPLED PLASMAS HAVING
A BIASED BOUNDARY ELECTRODE*
Michael D. Logue and Mark J. KushnerDept. of Electrical Engineering and Computer Science
University of Michigan, Ann Arbor, MI 48109, USA mdlogue@umich.edu, mjkush@umich.edu
Hyungjoo Shin, Weiye Zhu, Lin Xu, Vincent M. Donnelly, and Demetre J. Economou
University of Houston, Department of Chemical & Biomolecular Engineering, Houston, TX 77204
November 2011
GECNov2011
* Work supported by SRC and US Dept. of Energy Office of Fusion Energy Sciences
AGENDA
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
Control Of Ion Distribution Functions
Description of the Model and Geometry
Ion Energy Distributions (IEDs) and Plasma Parameters for Pulsed ICP
Pulsed ICP With Constant DC Boundary Electrode (BE) Bias
Pulsed ICP With Pulsed DC Boundary Electrode (BE) Bias
Concluding Remarks
CONTROL OF ION DISTRIBUTION FUNCTIONS
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
In plasma materials processing there is a need to control the ion energy distributions (IEDs) to surfaces with increasing precision.
A recent development controlling of IEDs in inductively coupled plasmas (ICPs) is use of a boundary electrode.
A dc or pulsed dc bias to the boundary electrode shifts the quasi-dc plasma potential and in turn the peak in the IED incident onto grounded surfaces.
Using results from a computational investigation, the effect of pressure and BE bias (both dc and pulsed dc) on plasma parameters and IEDs will be discussed.
Example: Simulated IEDs and etch profiles for Ar/Cl2=80/20, 10 mTorr, 300 W peak ICP power, 100 W peak bias power, and 5 kHz pulse frequency, Duty Cycle=50%
Agarwal, J. Vac. Sci. Technol. A, Vol 29 (2011)
DESCRIPTION OF HPEM
Modular simulator that combines fluid and kinetic approaches.
Resolves cycle-dependent phenomena while using time-slicing techniques to advance to the steady state.
GECNov2011
University of MichiganInstitute for Plasma Science & Engr.
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GECNov2011
Azimuthal antenna currents – retain only E, Brz
Plasma currents Collisional ion currents Kinetically derived non-local electron currents capture
nonlocal effects.
University of MichiganInstitute for Plasma Science & Engr.
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Electron Energy Distributions – Electron Monte Carlo Simulation
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GECNov2011
Cycle dependent electrostatic fields
Phase dependent electromagnetic fields
Electron-electron collisions using particle-mesh algorithm
Phase resolved electron currents computed for wave equation solution.
Captures long-mean-free path and anomalous behavior.
University of MichiganInstitute for Plasma Science & Engr.
HPEM-EQUATIONS SOLVED - ,, rf
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GECNov2011
University of MichiganInstitute for Plasma Science & Engr.
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GECNov2011
University of MichiganInstitute for Plasma Science & Engr.
PLASMA CHEMISTRY MONTE CARLO MODULE
Pseudo-Particles are launched at random times during the rf cycle based on ion source functions vs position.
Trajectories are integrated based on stored electric fields interpolated as a function of time during pulse period.
Null collision techniques are used to account for elastic and inelastic collisions.
Since transit time of ions may exceed pulse period, the stored electric fields during pulse are treated as being periodic.
For species K
For (r,z) mesh cell I,J
Get Source Function forSpecies KDetermine # of particlesTo launch
Launch particle at randomtime during rf cycle
Follow particles until a surfaceIs hit. Include acceleration fromFields and particle collisions
Collect statistics on particlesThat hit specified surface
For particle 1:Total Particles
Go to top of particle loop
Go to top of mesh cell loop
Go to top of species loop
BOUNDARY ELECTRODE ICP (BE-ICP): EXPERIMENT
BE-ICP is a cylindrical plasma driven by a solenoidal coil.
The biased electrode is at the top boundary consisting of annular rings.
Ion energies measured by retarding field energy analyzer (RFEA).
Electron, ion densities, temperatures: Langmuir probe
Argon, 10s mTorr, 40 sccm
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
PULSED BE-ICP
In the BE-ICP system both the ICP power and the DC bias on the boundary electrode can be independently pulsed.
This can allow for greater tunability of the plasma parameters as well as the IED
Investigate effects of using pulsed ICP power with and without pulsed dc bias on the boundary electrode
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
Time = 1/
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Boundary ElectrodeDC Bias(t)
ΔtBias
BOUNDARY ELECTRODEICP (BE-ICP): MODEL
Model representation of ICP system.
RFEA is modeled as a flat, grounded metal. Gridded structure is not be resolved at this scale.
Cylindrically symmetric model resolves pump as annular port.
Integration in time to a pulse-periodic steady state.
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
Te, Φ, ne DURING PULSE, ΔtBias = 42-60μs
Electron density peaks to 1012 cm-3. Plasma potential controls the IED.
University of MichiganInstitute for Plasma Science & Engr.
Animation Slide
Te Plasma Potential ne
GECNov2011
Te, PLASMA POTENTIAL DURING PULSE PERIOD
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
Experiment
Te and both have “overshoot” at beginning of pulse period to re-establish plasma.
Modulation of affords opportunity to shape IED.
Applying pulsed BE bias will further modulate , and so IED.
Duty cycle = 20%, PRF = 10 kHz, P(pulse average) = 120 W.
Model
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
IEDs – PULSED ICP WITH DC BOUNDARY
Experiment
A
B
IEDs consist of 2 peaks: High energy peak corresponding to large during plasma on period.
Low energy peak due to smaller (and falling) during plasma off period.
Vast majority of ionization occurs during plasma on part of cycle (20 s)
Ions “launched” during plasma on period require tens of s to reach substrate.
High energy peak is broadened as ions sample dynamics of during transport to substrate.
• Model
A
B
Time
Po
wer
(t)
Boundary ElectrodeDC Bias (t)
A
B
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
IEDs – PULSED ICP WITH PULSED DC BOUNDARY
Experiment
Energy of peak is independent of pressure as is determined by BE voltage.
Magnitude of peak decreases at high pressure due to collisionality, and populates intermediate energies.
Lower energy peak decreases as pressure increases due to increased collision frequency
Model predicts larger low energy component (very sensitive to acceptance angle of RFEA vs energy.)
• Model
A
A
B
B
B
Time
Po
wer
(t)
Boundary ElectrodeDC Bias (t)
A
B
ΔtBias
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
IEDs – PULSED ICP, PULSED DC BOUNDARY IN EARLY
AFTERGLOW
Experiment
The positions of the peaks in the IEADs are determined by the plasma potential during the plasma on period, and the voltage of the BE.
The magnitude of the peak is determined by the length of time the BE is on.
Slight peak around 1.5 V in model due to non-zero minimum plasma potential.
A
B
Time
Po
wer
(t)
Boundary ElectrodeDC Bias (t)
ΔtBias
• Model
B
B
B
A
A
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
IEDs – PULSED ICP WITH PULSED DC BOUNDARY IN
LATE AFTERGLOW
Experiment
By moving pulse of BE to the late afterglow, the low and high energy peaks are more distinctively separate.
The magnitude of the high energy peak is controlled by the length of the BE pulse.
A
B
Time
Po
wer
(t)
Boundary ElectrodeDC Bias (t)
• Model
ΔtBias
A
A
B
B
B
CONCLUDING REMARKS
University of MichiganInstitute for Plasma Science & Engr.GECNov2011
The use of a dc biased boundary electrode in ICP allows for control of the IED due to the shifting of the plasma potential by the dc bias.
Positive biases result in the increase of the IED peak energy by nearly the applied bias while negative biases resulted in a small, capped decrease in the IED peak energy.
Pulsing the dc bias on the boundary electrode in the ICP afterglow creates a narrower peak in the IED centered at approximately the applied bias potential. The height of the peak is determined by the length of the BE pulse.
This allows tuning of the energy and height of the peak energy of the IED.