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Plato Probe
“
Deposition Tolerant
Langmuir Probe
Talk Outline
Standard Langmuir probe
•
•
•
•
Issues in deposition systems
RF biased probe techniques
Sobolewski Method
Booth / Braithwaite Probe
Impedans diagnostic technique
• Calibration
• RF current and voltage waveforms
• IV characteristic generation
The Plato probe overview
Summary
Standard Langmuir Probe
Standard Langmuir probes biased at low
frequency (DC)
Conductive probe tip – often tungsten
DC Voltages applied – DC currents recorded
Plasma
A
Current drawn from the plasma by probe –
returned to the plasma through conductive
chamber wall
Probe Voltage biased from negative potential up
to 5 or 10V past the plasma potential
Tip is positively biased to clean (energetic
electron bombardment heats ‘white’ hot)
V
Langmuir Probes: Issues in deposition systems
• Probe tip becomes coated during plasma process
• Insulating coatings prevent DC current flow – probe fails
• Standard cleaning method does not work
• Even if probe is clean, insulated reactor walls problematic
• Conductive layer increase tip area – analysis complicated
• Langmuir probe immune to coatings highly desirable
Ceramic
Probe Tip
Layer deposited
RF biased techniques: Sobolewski Method
• ICP sustains plasma, RF
bias controls ion energy
• RF bias is capacitively
coupled through blocking
capacitor
• Capacitive probe measures
Voltage (with dc
component)
• Inductive pickup measures
Current (no DC component)
Note: Blocking capacitor
behaves in a similar way
to insulating layer
RF biased techniques: Sobolewski Method
• Current through RF biased sheath
• At turning points of RF waveform
dV/dt=0
• At most negative point Ie=0 also
• RF bias is capacitively coupled (no
net current)
• Constant ion current (DC current)
recovered
RF biased techniques: Booth / Braithwaite
• RF biased planar probe with guard-ring
• Planar sheath ensured
• Pulse modulated RF bias
charges/discharges external
capacitance
• By monitoring voltage and current the
probe IV characteristic is determined
RF biased techniques: Booth / Braithwaite
Plato Probe
The Impedans RF biased probe technique for for use
in systems depositing insulating layers
Calibration
• RF biased electrode in ICP plasma
• Measured IV waveforms using
oscilloscope
• Stray impedance calibrated using 2 port
network theory
• Short and open circuit terminations of the
electrode surface
• Frequency domain calibration as stray
impedance is a function of frequency
• Langmuir probe used to validate results
RF Current and Voltage waveforms
RF Current and Voltage waveforms
RF Current and Voltage waveforms
RF Current and Voltage waveforms
• Black – Total current to the electrode
• Blue – Conduction current
IV characteristic generation
Plato Probe Overview
• Capacitively coupled rf bias applied to tip
• 200 micron recess around the tip
circumference
• Probe front end fully replaceable when
coated layer exceeds 50 micron
• 2 frequencies used to check /
compensate for deposited layer
Plato Probe Overview
3 Micron Layer
No Layer
Solid Line 200kHz; Dashed Line 400kHz
Solid Line 200kHz; Dashed Line 400kHz
0.00016
0.00016
0.00014
0.00014
0.00012
0.00012
0.0001
0.0001
0.00008
0.00008
0.00006
0.00004
0.00006
0.00002
0.00004
-40
0.00002
-30
-20
-30
-20
-10
0
-0.00002
10
-0.00004
0
-40
0
-10
-0.00002 0
10
10 Micron Layer
-0.00004
Solid Line 200kHz; Dashed Line 400kHz
0.00016
0.00014
0.00012
•
CCP
•
Argon / 2Pa
0.00008
•
Ne= 1.2x1015 m-3,
0.00006
•
Te= 2.3eV,
•
As layer capacitance decreases,
RF Voltage drop increases
0.0001
0.00004
0.00002
-40
-30
-20
0
-10
0
-0.00002
-0.00004
10
20
Plato Probe Overview
Blocking capacitor
I probe
Layer
Capacitance
V probe
Sheath
Impedance
V sheath
Summary
RF Biased probes can be used to
measure plasma parameters in
deposition environments
Technique developed by Impedans
allows much of IV characteristic to
be recovered
Plato probe allows operation even
with layer thicknesses of up to 50
microns present
2 frequencies used to check /
compensate for deposited layers