Download Octiv Technical

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 / Zo (scalar values only are required)
• Watts Reflected = Vr 2 / Zo
• Where Zo is the characteristic impedance of the transmission line
Vf
Vr
Directional
Coupler
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
V
A) Oscilloscope
I
V
B) Spectrum Analyser
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
Digitising
voltage from the E Field.
• Pick-up imperfections are
calibrated out.
Data Capture
Analogue Front
End
Pick-ups
• 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
Waveform Signal – Time domain
• 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
Voltage
Current
Figure 1
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!
400
kHz
13.5
6
MHz
13.5
6
MHz
400
kHz
2 MHz
intermodulated
27
MHz
FFT
Next we use the ‘digital’ oscilloscope
• We next break up the frequency spectrum into user selected ranges FR1 = 350450kHz, 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)
V
• 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.
I
Octiv data analysis (FPGA)
Spectrum Analysis
F1
F2
V
A
F
E
A
D
C
Fn
F1
I
F2
Fn
V
D
I
G
I
T
A
L
S
C
O
P
E
S
Mechanical Outline 75mm2
All this power in a small footprint
Octiv VI Probe with N-Type connectors
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
Match unit
13.56MHz
RealICPCurrent to Electrode
Plasma
OCTIV VI
Probe
10MHz
Measured Current-Voltage to electrode in plasma
–Impedans method Patent pending
Algorithm to determine IV characteristic
Example of IV characteristic
Electrons
70mA
60mA
50mA
40mA
Current – Voltage
Characteristic
30mA
20mA
10mA
Ion Flux = 10mA
-40
-30V
-20V
-10V
-10mA
-20mA
10V
20V
30V
40V
Comparison of ion flux measured with
Langmuir Probe and Octiv
13.56MHz
Match unit
Octiv IV
Probe
Langmuir Probe
ALP
Real Current to Electrode
Plasma
Capacitively coupled plasma
Comparison of ion flux measured with
Langmuir Probe and Octiv
2 Pa in Argon
A/m2
A/m2
6
7.6
5.4
4.8
4.2
J+ Alp
6.6
J+ Octiv
5.6
4.6
3.6
3
3.6
2.4
2.6
1.8
1.6
1.2
0.6
0.6
0
0
20
40
60
Power W
80
100
-0.4
120
Comparison of ion flux measured with
Langmuir Probe and Octiv
A/m2
11 Pa in Argon
A/m2
6
7.6
5.4
4.8
4.2
J+ Alp
6.6
J+ Octiv
5.6
4.6
3.6
3
3.6
2.4
2.6
1.8
1.6
1.2
0.6
.6
0
0
20
40
60
80
100
-0.4
120
Comparison of ion flux measured with
Langmuir Probe and Octiv
5-100W; 1-11Pa Argon; 13.56MHz
5
4.5
J+Octiv = 1.28 x J+ALP
Linear fit
4
2
R = 0.943
J+Octiv
3.5
3
2.5
2
1.5
1
0.5
0
0
0.6
1.2
1.8
2.4
Ion Flux measured by ALP
3
3.6
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.