Octiv RF Power monitoring technology “

Talk Outline Impedans VI technology Introduction to power monitoring Need for VI Probes Why IV sensors are needed Why IV sensors are not easy How the Octiv’s work Digital oscilloscope & Spectrum Analyser Data analysis Comparison of Ion flux: Langmuir Probe v’s Octiv VI probe Summary

Impedans Octiv VI Patented Technology Impedans has filed patents in a number of global regions for two novel technologies for VI probes – Octiv. The Patents include optical sensors being developed by Impedans - Moduli The first patent allows us to measure each waveform, isolate their harmonics and inter-modulation components and allows reconstruction of the waveforms (a spectrum analyser and oscilloscope hybrid). The second is very powerful in plasma analysis as we produce the real and imaginary I(V) (Current v Voltage ) characteristics rather than just the I(t) (Current v Time) or I(f), (Current v Frequency) characteristic.

Introduction to Power Monitoring Traditional RF power sensors are based on directional coupler technology and were developed in the 1940’s Couplers measure a forward wave and reflected wave in a transmission line Watts Forward = Vf 2 / Z o (scalar values only are required) Watts Reflected = Vr 2 / Z o Where Z o is the characteristic impedance of the transmission line Directional Coupler VfVf VrVr

Introduction to Power Monitoring This directional coupler technology has become standard in power monitoring due to its simplicity. It is still a common technology in plasma monitoring Draw backs It applies only to a transmission line of limited impedance range eg. 50Ω 2VWSR. In multi-frequency and non-linear loads we need to know magnitude and phase of V f, V r and Z o at all frequencies. No longer simple – most coupler system do not work in plasma applications Monitoring power is no longer enough we need to know what is happening at the wafer

Need for new technology Knowing the exact shape of the current and voltage waveforms at the surface of the wafer is a very powerful diagnostic. The simplest way to do this is to have a well characterised and calibrated VI probe mounted after the match unit. This is even more important in pulsed, multi-frequency and frequency tuned RF systems.

VI Probes To monitor the power and other plasma parameters in plasma applications and with multi-frequency applications we need to monitor V and I as complex parameters in the full frequency domain. We can determine line Impedance We can accurately measure the local waveform. We can transpose the waveform onto the wafer surface. Drawbacks – analysing the data becomes very complex

Why IV sensors are needed – technical trends in plasma Non-linear: The plasma produces harmonics Multi-frequency: Couplers often use filters to remove harmonics and the sensors cannot measure a wide frequency range. Multi-frequency Simultaneous: When two frequencies are present the non-linear plasma load produces inter-modulation between the two frequencies. These inter-modulation components add to the complexity of the RF measurement and waveform reconstruction. Frequency Agile: In some systems one or two of the frequencies are not fixed but can move in order to facilitate matching of the power to the load. Pulsed Power: In a growing number of applications one or more of the RF power supplies are pulsed. This introduces further issues in that line impedance, plasma parameters and power supply frequency can change dramatically in microsecond timescales

Why are IV sensors not easy Existing data capture technology comprises two separate approaches which we can call a) Oscilloscopes and b) Spectrum Analysers. (We exclude simple continuous data collection which would require gigabytes of data storage per second). a) Oscilloscopes repeatedly measure a waveform at a single frequency, - the trigger frequency. Data at frequencies not synchronised with the trigger are averaged out and lost. b) Spectrum analysers measure in the frequency domain. All the frequency data is recorded but the individual waveforms are lost.

Current data analysis A) Oscilloscope B) Spectrum Analyser V I V I

How does the Octiv Capture Multiple Waveforms Octiv uses a simple loop to pick up the current from the RF magnetic field It uses a capacitor to pickup voltage from the E Field. Pick-up imperfections are calibrated out. The current and voltage is digitized with 14 bit accuracy and fed to an FPGA where a one shot signal is collected in a few microseconds Analogue Front End Pick-ups Data Capture Digitising

Waveform Signal – Time domain Figure 1 Voltage Current The Octiv’s high speed FPGA collects a single shot of current and voltage waveform and performs a Fast Fourier Transform (FFT). Example below is data for a 400kHz and 13.56MHz signal

Frequency domain Spectrum of Voltage FFT, showing fundamental frequencies at 400kHz (near zero) and 13.56MHz (near the centre). We also see harmonics, aliased harmonics and inter-modulation structure, with a little noise. This is a clean spectrum! MHz 400 kHz 2 MHz intermodulated MHz 400 kHz 27 MHz FFT

Next we use the ‘digital’ oscilloscope We next break up the frequency spectrum into user selected ranges FR1 = kHz, FR2 = MHz. We now search for the strongest signal in each range F r1 and F r2. All the data is now sent to two or more digital oscilloscopes one triggered at F r1 and the second at F r2 and more frequencies if needed. A second data set is collected and the process repeated Each oscilloscope gets all the raw data so no information is lost and all the instruments are located inside the FPGA chip – low cost.

In Oscilloscope mode These figures shows the Average magnitude (FFT) of the fundamental and first 4 harmonics of the voltage (top) and current (bottom) at 13.56MHz (Spectrum Analyser in Blue and triggered Oscilloscope in Red, averaged over 100 data sets (about 1ms) The input data was normalised to V 1-5 = 1,0.3,0.2,0.1,0.05 and I 1-5 = 1.0,0.3,0.2,0.1,0.05 It is seen that noise, inter-modulation and aliased signals cancel in oscilloscope mode i.e. we reject unwanted data. V I

Octiv data analysis (FPGA) V V F1F1 F2F2 FnFn I F1F1 F2F2 FnFn AFEAFE ADCADC Spectrum Analysis DIGITALSCOPESDIGITALSCOPES

Mechanical Outline 75mm 2 Octiv VI Probe with N-Type connectors All this power in a small footprint

Octiv Non-linear: Each waveform contains the fundamental amplitudes and phases of all signals required to reconstruct that waveform captured at 14bit accuracy. Multi-frequency: Each digital oscilloscope captures each individual waveform separately. The sensor is broadband. No need for external filters. Multi-frequency Simultaneous: Multiple oscilloscopes are pipelined simultaneously to capture each individual waveform separately. Frequency Agile: The spectrum analyser is run every few microseconds to establish where the signal is and this is used to trigger the oscilloscope. If the frequency changes the waveform is still synced. Pulsed Power: The oscilloscopes can also be triggered from an external trigger in pulsed mode to capture pulsed data at a one to two microsecond resolution.

Plasma Parameter measurement As well as being the most sophisticated way to capture multiple frequency waveforms as a function of time, the Octiv can also reconstruct the Current – Voltage characteristic, I(V).

Measuring Plasma Parameters with OCTIV VI Probe Plasma 13.56MHz OCTIV VI Probe Match unit ICP 10MHz

Measured Current-Voltage to electrode in plasma –Impedans method Patent pending

Algorithm to determine IV characteristic

Example of IV characteristic -20mA -10mA 10mA 20mA 30mA 40mA 50mA 60mA 70mA V-20V-10V10V20V30V40V Current – Voltage Characteristic Ion Flux = 10mA Electrons

Comparison of ion flux measured with Langmuir Probe and Octiv Plasma 13.56MHz Octiv IV Probe Langmuir Probe ALP Match unit Capacitively coupled plasma

Comparison of ion flux measured with Langmuir Probe and Octiv J+ Alp J+ Octiv A/m 2 Power W 2 Pa in Argon

J+ Alp J+ Octiv A/m 2 11 Pa in Argon Comparison of ion flux measured with Langmuir Probe and Octiv

5-100W; 1-11Pa Argon; 13.56MHz J +Octiv = 1.28 x J +ALP R 2 = J +Octiv Linear fit Ion Flux measured by ALP

Conclusions Octiv VI probe is the most advanced technique for measuring V(t) and I(t) waveforms in plasma systems Measures up to 32 frequencies of up to 5 independent waveforms Unique feature is pulsed operation Measures Ion Flux to the wafer Our roadmap aims to extend our measurement of key plasma parameters at the wafer to electron temperature, electron density. Example of a comparison of Ion Flux measured by Octiv and LP shows good correlation.