OctivRF Power monitoring technology“

Talk Outline

• Impedans VI technology

• Introduction to power monitoring

• Need for VI ProbesWhy IV sensors are neededWhy IV sensors are not easy

• How the Octiv’s workDigital oscilloscope & Spectrum AnalyserData 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 / Zo (scalar values only are required)

• Watts Reflected = Vr 2 / Zo

• Where Zo is the characteristic impedance of the transmission lineDirectional

Coupler

Vf Vr

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 Vf , Vr and Zo 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

Captu

reD

igit

isin

g

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!

13.56

MHz

400 kHz

2 MHz intermodulat

ed

13.56

MHz

400 kHz

27 MHz

FFT

Next we use the ‘digital’ oscilloscope

• We next break up the frequency spectrum into user selected ranges FR1 = 350-450kHz, FR2 = 13.0-14.0MHz.

• We now search for the strongest signal in each range Fr1 and Fr2.

• All the data is now sent to two or more digital oscilloscopes one triggered at Fr1 and the second at Fr2 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 13.56 triggered Oscilloscope in Red, averaged over 100 data sets (about 1ms)

• The input data was normalised to

• V1-5= 1,0.3,0.2,0.1,0.05 and

• I1-5 = 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

F1

F2

Fn

I

F1

F2

Fn

AFE

ADC

Spectrum Analysis D

IGITAL SCOPES

Mechanical Outline 75mm2

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

Real Current to Electrode

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

-40 -30V -20V -10V 10V 20V 30V 40V

Current – Voltage Characteristic

Ion Flux = 10mA

Electrons

Comparison of ion flux measured with Langmuir Probe and Octiv

Real Current to ElectrodePlasma

13.56MHz

Octiv IV Probe

Langmuir Probe ALP

Match unit

Capacitively coupled plasma

Comparison of ion flux measured with Langmuir Probe and Octiv

0

0.6

1.2

1.8

2.4

3

3.6

4.2

4.8

5.4

6

0 20 40 60 80 100 120-0.4

0.6

1.6

2.6

3.6

4.6

5.6

6.6

7.6

J+ Alp

J+ Octiv

A/m2

A/m2

Power W

2 Pa in Argon

0

.6

1.2

1.8

2.4

3

3.6

6

0 20 40 60 80 100 120-0.4

0.6

1.6

2.6

3.6

4.6

5.6

6.6

7.6

4.2

4.8

5.4J+ Alp

J+ Octiv

A/m2

A/m2

11 Pa in Argon

Comparison of ion flux measured with Langmuir Probe and Octiv

Comparison of ion flux measured with Langmuir Probe and Octiv

5-100W; 1-11Pa Argon; 13.56MHz

J+Octiv = 1.28 x J+ALP

R2 = 0.943

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.6 1.2 1.8 2.4 3 3.6

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.