Plato Probe “ Deposition Tolerant Langmuir Probe

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

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

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

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

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