1. Time-resolved diagnostics of pulsed plasmas etching processes
10/05/2000
Time-resolved diagnostics of pulsed plasmas etching processes
G. Cunge
Laboratoire des Technologies de la Microélectronique CNRS (France)

2. Plasma etching principles: example of Si etching in Cl2/O2:
10/05/2000 Si
Mask
SiOClx layer Cl
SiCl4
Cl2/O2 plasma
Cl2 + e- 
SiO2
The Si surface is etched by Cl atoms  formation of SiCl4 volatile etch products
(process strongly assisted by energetic ions bombardment)
Plasma etching can be Anisotropic (vertical direction only)
The ion bombardment is directional + Formation of passivation layers on the sidewalls
Plasma etching can be Selective (Si is etched but SiO2 is not)

3. Time resolved diagnostics in reactive plasmas: challenges
10/05/2000
The time-resolution needed depends strongly on the application:
1) Real time monitoring of reactive plasmas
100 ms time resolution is good enough to monitor the variation of radical’s densities in real time.
Challenge: highly sensitive diagnostic techniques are needed !
2) Measurement of radical’s sticking coefficient  on surfaces (pulsed plasmas)
We monitor the decay of radicals’ densities in the afterglow:
kloss=kwall+kgas
The radicals’ density must be measured with a time resolution < 100 µs
Challenge: fast acquisition system synchronized with plasma pulses

4.  Fast acquisition systems synchronized with plasma pulses
Time-resolved measurements in pulsed plasmas: basic principles and apparatus
Plasma diagnostics provide a signal (typically a current).
Goal: capture signal variations during one pulsing period (100 µs at 10 kHz)
Modulation
Plasma
Potentiel V p
RF power
Detector
+ amplifier
Trigger
IT(t)
Analog/Digital or
Photon counter
 Fast acquisition systems synchronized with plasma pulses
CW signal
(photodiode,
Langmuir probes….etc)
Pulses
(Mass spec,
PMT…etc)
A/D converter
(16 bit, 1 Ms/s)
Multichannel
analyzer

5. Experimental set-up
10/05/2000
10/05/2000
We are using an industrial ICP reactor from AMAT designed to etch 300 mm wafers but modified to host plasma and surface diagnostics
LED
for UVAS
(Cl2, SiClX)
Diode Laser
(T)
Mass Spectrometer (Cl, SiCl3)
Plasma reactor (ICP)
Reactor connected to XPS (by robotized vacuum transfer chamber)
 chemical composition of the wafer surface + reactor wall coatings
XPS 5

6. Detection of not optically accessible species by modulated beam threshold ionization Mass Spectrometry

7. Mass spectrometry: principle
Plasma
radicals
Energy analyzer
( eV)
Mass Filter
( amu)
Ionization chamber
(e eV)
Chaneltron
+ counting system

8. Mass spectrometry: principle
Plasma
radicals
Energy analyzer
( eV)
Mass Filter
( amu)
Ionization chamber
(e eV)
Issue: remove background
Chaneltron
+ counting system

9. The issues associated with MS: beam and background
to turbo pump
(80 l.s-1) e-
5-70 eV Ar
plasma
to energy filter,
mass filter and
ion counting Ar
Ar+
Ar atoms enter the MS head through an orifice and are ionized by an electron beam.
Hypothesis: the Ar+ signal intensity is proportional to the Ar density in the plasma
However, a large part of the signal originates from the residual pressure of Ar in the MS head  this background signal must be subtracted to capture the beam component
 modulated beam MS

10. Introduction: Modulated Beam MS
Why : to subtract the background signal MS
ionization chamber
plasma
to energy filter
to mass filter
to ion counting
to pump
to pump
H. Singh et.al., JVST A 17(5), 1999
+ Measurement of beam+background and background alternatively
Need of multiple pumping stages to reduce the background
Hypothesis : the beam does not influence the background

11. Modulated beam Mass Spectrometry
The tuning fork resonant (10Hz) chopper
+ differential pumping (Hiden) e-

12. Modulated beam Mass Spectrometry
The tuning fork resonant (10Hz) chopper
+ differential pumping e-

13. Modulated beam Mass Spectrometry
The tuning fork resonant (10Hz) chopper
+ differential pumping e-
Chopper closed: beam blocked, measure background

14. Modulated beam Mass Spectrometry
The tuning fork resonant (10Hz) chopper
+ differential pumping e-
Chopper closed: beam blocked, measure background
Chopper open: measure beam+background

15. Modulated beam Mass Spectrometry
Example in O2 plasma (cw)
finish to close
finish to open
start to open
start to close
O beam
O backgrd
1 chopping period
The tuning fork resonant (10Hz) chopper
+ differential pumping e-
Chopper closed: beam blocked, measure background
Chopper open: measure beam+background

16. Modulated beam Mass Spectrometry
Example in O2 plasma (cw)
finish to close
finish to open
start to open
start to close
O beam
O backgrd
O2 beam
O2 backgrd
The tuning fork resonant (10Hz) chopper
+ differential pumping e-
The nature (reactive or stable species)
of the beam influences strongly the background level.

17. Beam influence on the background ? (2/2)
Background level vs. pressure
finish to close
start to close
O2 beam
O2 backgrd
The background is mostly due to the beam, so it is crucial to estimate the background as soon as the beam is shut down.
How can we measure those curves I(t) during the choping periode ?

18. Time resolved measurements with Hidden internal boxcar
The issue of ghost pulses
(improvement : the dead band time)
Boxcar
(Hiden Analytical) RF
power
Ions are detected by a channeltron  each ion is detected as a burst of e- current  the output signal is the number of burst averaged over a time window (dwell).
chopper driver
ghost pulse
matching box
channeltron MS

19. Time resolved measurements with Hidden internal boxcar
The issue of ghost pulses
(improvement : the dead band time)
Dead Band Time
Boxcar
(Hiden Analytical) RF
power
chopper driver
ghost pulse
matching box
channeltron MS
Ghost pulses are corrected with the introduction of a dead band time.

20. Time resolved measurements with Hidden internal boxcar
(Hiden Analytical) RF
power
chopper driver
matching box
channeltron MS
For time-resolved measurements, (periodic) the system use a boxcar, which integrate the signal over a time-window (gate width). This signal is accumulated over several periods to improve S/R ratio (trigger is needed). The gate is then shifted to sample the signal at a later time in the cycle.
time
trigger
Mesure a t
Mesure a t+T
Mesure a t+2T

21. Time resolved measurements with Hidden internal boxcar
(Hiden Analytical) RF
power
chopper driver
matching box
channeltron MS
For time-resolved measurements, (periodic) the system use a boxcar, which integrate the signal over a time-window (gate width). This signal is accumulated over several plasma pulses to improve S/R ratio (trigger is needed). The gate is then shifted to sample the signal at a later time in the cycle.
time
trigger
Mesure a t
Mesure a t+T
Mesure a t+2T
Issue: long measurement / huge loss of information !

22. pulse & delay generator
Improvement : replace boxcar by the pulse counter
time
trigger
Mesure a t
pulse & delay generator
Boxcar
(Hiden Analytical)
Pulse counter
Pulse counter
 signal/noise420 RF
power
chopper driver
matching box
Boxcar
 signal/noise20 MS
Thanks to the pulse counter, signal to noise ratio is strongly enhanced. Thus, fast synchronized acquisitions are achievable.

23. Modulated beam Mass Spectrometry
Example in O2 plasma (cw)
finish to close
finish to open
start to open
start to close
O beam
O backgrd
1 chopping period
The tuning fork resonant (10Hz) chopper
+ differential pumping e-
With the multichannel pulse counter, we capture the entire signal during each chopping period
 Excellent S/N ratio

24. Time-resolved measurements in pulsed plasma by MS
Plasma pulses must be synchronized with chopper oscillations
 plasma pulsing frequency > chopper frequency
chopper
plasma
30 Hz
10 Hz

25. Time-resolved measurements in pulsed plasma by MS
Plasma pulses must be synchronized with chopper oscillations
 plasma pulsing frequency > chopper frequency
chopper
plasma
30 Hz
10 Hz
The chopper signal at 10 Hz is used to trigger a burst generator that produces pulses at N x 10 Hz to pulse the plasma

26. Cl atoms detection in pulsed discharge
Plasma pulses must be synchronized with chopper oscillations
 plasma pulsing frequency > chopper frequency
chopper
plasma
The multichannel pulse counter capture the signal during the entire chopping period

27. Cl atoms detection in pulsed discharge
Plasma pulses must be synchronized with chopper oscillations
 plasma pulsing frequency > chopper frequency
chopper
plasma
The multichannel pulse counter capture the signal during the entire chopping period
Calibration on absolute density by comparison with Ar signal:

28. Summary What about the ions ?
By combining diagnostics, we can monitor the densities of almost all the radicals even in complex chemistries (here SiCl4/Cl2)
SiClX
ClX
What about the ions ?

29. Time resolved ion flux measurement in pulsed plasmas with a capacitively-coupled rf planar probe

30. Well known technique introduced by Braithwaite et al in 1996
Principle (in CW plasma)
Well known technique introduced by Braithwaite et al in 1996
Principle: fed a planar probe with by RF bursts through a blocking capacitor
Planar
Probe
(1 cm2)
Blocking
capacitor
Pulsed rf
Plasma
(CW)
-40 V
Vprobe
RF ON: capacitor charges (DC self bias about-40 V)
RF OFF: e- can’t reach the probe polarized at -40V
 Capacitor discharges by collecting ions Bohm flux
Measurement of the capacitor discharge’s current in OFF period  ion flux

31. Current (ion flux) measurement in CW plasmas
Potentiel V
Plasma
We use a direct current measurement system through a 1 k serial resistor
(Booth et al, Rev. Sci. Instrum. 71, 2722)
- RF ON: RF signal propagates through diodes
- RF OFF: capacitor discharge’s current flow through resistor and is measured by the A/D (triggered by probe pulses)
A/D
Trig
LP filter V I
Blocking capacitor
Ions only
Ions + e-
Pulsed RF generator

32. Current (ion flux) measurement in pulsed plasmas
Issue: several plasma pulses are needed to charge blocking capacitor to - 40 V
 Probe pulsing frequency < Plasma pulsing frequency (but synchronized)
A/D
Plasma
Potentiel V p LP
filter
Freq.
divider
Trig
 Use a frequency divider triggered by the plasma pulses to trigger probe pulses at f/10

33. Current (ion flux) measurement in pulsed plasmas
Issue: several plasma pulses are needed to charge blocking capacitor to - 40 V
 Probe pulsing frequency < Plasma pulsing frequency (but synchronized)
Probe: 200 Hz
Plasma: 2 kHz
voltage (V)
Probe RF
Time variations of ion flux during one plasma pulse
A/D
Plasma
Potentiel V p LP
filter
Freq.
divider
Trig

34. Typical pulsing frequency; 1-20 kHz
Experimental set-up
10/05/2000
10/05/2000
We are using an industrial ICP reactor from AMAT designed to etch 300 mm wafers but modified to host plasma and surface diagnostics
Mass Spectrometer (Cl, Br)
Pulsed Plasma reactor
(ICP): DPS
LED
for UVAS
(Cl2)
D2 lamp
For VUVAS
(HBr, Br2)
System equipped with pulsed RF generators / Operation at constant peak power
Typical pulsing frequency; 1-20 kHz 34

35. Results and discussion (1):
Impact of the pulsing parameters on the radicals densities

36. Cl and Cl2 kinetics in Cl2 plasmas pulsed at low frequency
15 Hz Cl2 discharge
Plasma ON: e- + Cl2  Cl + Cl + e-
Plasma OFF (low Te): Cl + wall  Cl2
RF ON Cl
Cl2
Wall

37. From low to high frequency pulsing
15 Hz Cl2 discharge
Important conclusions:
Reactive radicals (Cl) are :
- Produced only during ON period
- Lost continuously on the reactor walls
Timescale for significant radical density variations: several milliseconds
For pulsing frequencies > 1 kHz (T< 1 ms) the radical density is not anymore modulated during the pulse

38. From low to high frequency pulsing
15 Hz Cl2 discharge
Important conclusions:
Reactive radicals (Cl) are :
- Produced only during ON period
- Lost continuously on the reactor walls
Timescale for significant radical density variations: several milliseconds
1 kHz Cl2 discharge
For pulsing frequencies > 1 kHz (T< 1 ms) the radical density is not anymore modulated during the pulse
RF OFF
 Only time-averaged values will be considered in the following

39. Impact of the pulsing frequency and duty cycle on the Cl density
Cl atoms density
(Cl2 plasma / 20 mTorr / 800 W)
The duty-cycle has a strong impact on the Cl atoms density
The pulsing frequency has no effect !
Pulsed plasmas with small duty cycle = reduced chemical reactivity
(fragmentation of feedstock gas by electron impact collisions is reduced)

40. When duty cycle increases  Longer ON period  higher Cl density
Impact of the pulsing frequency and duty cycle on the parent gas fragmentation
~ 0.1 eV
3 eV Te
RF ON
RF OFF
Radical loss
Radical production Cl e-
Cl2
Walls
Plasma
Potentiel V p
RF power
Cl atoms are produced only in the ON period but are lost continuously on the reactor walls
When duty cycle increases  Longer ON period  higher Cl density

41. When duty cycle increases  Longer ON period  higher Cl density
Impact of the pulsing frequency and duty cycle on the parent gas fragmentation
RF ON
RF OFF
RF power Te
Plasma
3 eV
Plasma
Potentiel V
~ 0.1 eV p Cl e-
Cl2
Radical production
Walls Cl
Cl2
Walls Cl
Cl2
Radical loss
Radical loss
Cl atoms are produced only in the ON period but are lost continuously on the reactor walls
When duty cycle increases  Longer ON period  higher Cl density
When frequency increases total ON time (averaged over many cycles) is unchanged  no effect on Cl density
The ratio of atomic to molecular densities (Cl/Cl2) can be controlled by the duty cycle

42. Results and discussion (2):
Impact of pulsing parameters on the ion flux

43. Impact of the duty cycle on the ion flux: (1) Electropositive plasma
He plasma 10 mTorr / 700 W / 1kHz
In the afterglow, ion flux decay is due to Te drop (UBohm ) and ambipolar losses (ni )
Rise and fall times of the ion flux  10 µs << pulsing period (1000 µs)
In ON period the ion flux  reaches steady state and is high even at 10%DC

44. Impact of the duty cycle on the ion flux: (1) Electronegative plasma
Cl2/SiCl4 plasma
10 mTorr / 700 W / 1kHz
Rise time and decay time of the ion flux are longer than in He
(attributed to negative ion formation in the afterglow)
 In ON period, the ion flux strongly depends on the duty cycle
and remains very small below 25 % DC
Important conclusion for the design of pulsed etching process

45. Results and discussion (3): Impact of pulsing parameters on the IEDF

46. Without bias power, the ions are accelerated towards the wafer by Vp.
Time-averaged IEDF in pulsed ICP plasmas (no bias)
IEDF are recorded at wafer surface by a SEMION multigrid analyzer from IMPEDANS (see D.Gahan’s presentation)
Ions from ON period
Ions from
OFF period
RF ON
RF OFF Te Vp
3 eV
15 V
Te ~ 0.2 eV
Vp ~ 1 V
Without bias power, the ions are accelerated towards the wafer by Vp.
In pulsed plasmas Vp is rapidly modulated between 15 V (ON period) and 1 V (OFF period)
 Bimodal IEDF
In a plasma pulsed with a small duty cycle the wafer is bombarded mostly by low energy ions  key point to reduce damages

47. IEDF in synchronous pulsed plasmas (with bias power)
Cl2 plasma 1000 W / 50 Wbias CW or pulsed at 1 kHz
Ions from ON period (ICP and bias)
Bi-modal IEDF (corresponding to ions from ON and OFF periods)
The ion energy in the ON period increases rapidly when the DC is decreased

48. IEDF in synchronous pulsed plasmas (with bias power)
Cl2 plasma 1000 W / 50 Wbias CW or pulsed at 1 kHz
Ions from ON period (ICP and bias)
At 20 % DC the ions have 5 times the energy of those in the CW plasma for the same bias power !
Bi-modal IEDF (corresponding to ions from ON and OFF periods)
The ion energy in the ON period increases rapidly when the DC is decreased

49. IEDF in synchronous pulsed plasmas (with bias power)
Key point:
Pbias (W)  Ii x Vbias
When Ii drops at constant power,
Vbias increases
 Ion energy increases

50. IEDF in synchronous pulsed plasmas (with bias power)
Key point:
Pbias (W)  Ii x Vbias
When Ii drops at constant power,
Vbias increases
 Ion energy increases
Ion flux decreases
Ion energy increases
In synch. pulsed plasma at low duty cycle the wafer is bombarded by very energetic ions

51. Summary and conclusions
RF power
Plasma
Impact of pulsing on plasma chemistry
Duty cyle
 Radical flux control
 New domains of plasma chemistry
Drawback: reduced polymerisation

52. Summary and conclusions
Plasma
RF power
RF power (ion acceleration)
Accessible cw
IEDF
Duty cyle
 Radical flux control
 New domains of plasma chemistry
CW Plasmas:
Restricted ion energy range

53. Summary and conclusions
RF power
Plasma
Accessible cw
IEDF
Duty cyle
 Radical flux control
 New domains of plasma chemistry
Pulsed plasma:  Access low energy
 reduced damages

54. Summary and conclusions
Plasma
RF power
RF power (ion acceleration)
IEDF CW
1 kHz 20% nm
Pulsed
Accessible cw
Duty cyle
 Radical flux control
 New domains of plasma chemistry
Synchronised pulsing:  Access high energy
 Improved profile control

55. Summary and conclusions
Plasma
RF power
RF power (ion acceleration)
IEDF CW
1 kHz 20% nm
Pulsed
Accessible cw
By pulsing the plasma, the range of operation of ICPs can be broaden from Downstream etching conditions to CCP etching conditions

57. Impact of the pulsing frequency and duty cycle on the Cl2 density
20 mTorr Cl2 at 800 W
The duty-cycle has a strong impact on the Cl2 density
The pulsing frequency has no effect ! CW
Cl2 residence time in the reactor (500 ms) >> pulsing period (< 1 ms)
Each Cl2 molecule experiences many plasma pulses
Cl2 dissociation fraction is driven by the total rf ON time during Cl2 lifetime
This total ON time is fixed by the duty cycle and not by the pulsing frequency

58. Pulsed ICP plasmas: Timescales
RF OFF
RF ON
Plasma
Potentiel V p
RF power
The timescales for significant variations of ion flux and neutral flux are different: ne
Ar plasma
pulsed 10 kHz
(Kushner et al)
SiCl4/Cl2 plasma
pulsed 100 Hz
Gives hope to control independently ion and neutral fluxes

59. In ON period the ion flux is  independent of the pulsing frequency
Impact of the duty cycle on the ion flux (1 kHz): (1) electropositive plasma
He plasma 10 mTorr / 700 W
Rise and fall times of the ion flux  10 µs << pulsing period (1000 µs)
In ON period the ion flux  reaches steady state and is high even at 10%DC
In ON period the ion flux is  independent of the pulsing frequency
Note: in afterglow, ion flux decay rate is due to ambipolar losses (ni ) and Te decay (UBohm )

60. pulse & delay generator
Time resolved measurements with Hidden internal boxcar
pulse & delay generator
Boxcar
(Hiden Analytical) RF
power
chopper driver
matching box
Ions are detected by a chaneltron  each ion is detected as a burst of e- current  the output signal is the number of burst averaged over a time window (dwell).
channeltron MS
For time-resolved measurements, (periodic) the system use a boxcar, which integrate the signal over a time-window (gate width). This signal is accumulated over several plasma pulses to improve S/R ratio (trigger is needed). The gate is then shifted to sample the signal at a later time in the cycle.
time
trigger
Mesure a t
Mesure a t+T
Mesure a t+2T