1. Plato Probe “
Deposition Tolerant Langmuir Probe

2. Talk Outline Standard Langmuir probe Impedans diagnostic technique
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

3. Standard Langmuir Probe
Standard Langmuir probes biased at low frequency (DC)
Conductive probe tip – often tungsten
DC 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)
Plasma A V

4. 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
Layer deposited
Probe Tip

5. 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

6. 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

7. 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

8. RF biased techniques: Booth / Braithwaite

9. Plato Probe
The Impedans RF biased probe technique for for use in systems depositing insulating layers

10. 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

11. RF Current and Voltage waveforms

12. RF Current and Voltage waveforms

13. RF Current and Voltage waveforms

14. RF Current and Voltage waveforms
Black – Total current to the electrode
Blue – Conduction current

15. IV characteristic generation

16. 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

17. Plato Probe Overview CCP Argon / 2Pa Ne= 1.2x1015 m-3, Te= 2.3eV,
As layer capacitance decreases, RF Voltage drop increases

18. Plato Probe Overview Blocking capacitor I probe Layer Capacitance
V probe
Sheath
Impedance
V sheath

19. 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