Post on 27-Dec-2015
Plato ProbeDeposition 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 tungstenDC Voltages applied – DC currents recorded
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)
A
V
Plasma
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
• CCP
• Argon / 2Pa
• Ne= 1.2x1015 m-3,
• Te= 2.3eV,
• As layer capacitance decreases, RF Voltage drop increases
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No Layer Solid Line 200kHz; Dashed Line
400kHz
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3 Micron Layer Solid Line 200kHz; Dashed
Line 400kHz
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10 Micron Layer Solid Line 200kHz; Dashed
Line 400kHz
Layer Capacitance
Plato Probe Overview
Blocking capacitor
SheathImpedance
V probe
I probe
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