Transcript Plasma Diagnostics

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Plasma Diagnostics
Monday Afternoon Tutorial for UC-DISCOVERY
Major Program Award on
Feature Level Compensation and Control
Eray S. Aydil
Chemical Engineering Department
University of California Santa Barbara
12/01/2003
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Central Problem in Plasma Etching
• To understand how externally controlled variables affect the process outcome
through the internal plasma parameters.
Externally controlled
variables
pressure,
gas flow rate
and composition,
rf power,
rf-bias power,
wafer temperature
Internal plasma
parameters
ion flux, J+
radical fluxes, Gi
ion energy, E
Figures of merit
(process outcome)
etch rate,
anisotropy
selectivity,
uniformity,
reproducibility
• Plasma diagnostics are experimental methods based on various electrical and
spectroscopic techniques that allow the measurement of internal plasma
parameters.
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Ion and etchant fluxes impinging on the wafer
surface determines the etch rates and profile
evolution in plasma etching processes.
+ +
ER = f (J+, E, Gi, T)
Example: SF6/O2 etching of Si
ER = f (J+, E, GF, GO, T)
Would like to measure or
estimate J+, E, GF, GO
Passivating oxide layer
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Langmuir Probes
I
I=Iesat
www.hidenanalytical.com
www.staldertechnologies.com
I=Iionsat
I ionsat  a(V p  V probe)
a
2
e
ni eA
2M i

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1
2
Vp
V
Vf
 eV p  V probe 
I e  I esat exp 

kT
e


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On Wafer Ion Flux Probe Measurements
Heavily Doped
Conducting Si wafer
Measurement Probe
R
V
Kapton
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rf filter
Bias Voltage
Si
 Probe mounted on 8” heavily doped
Si wafer.
 Probe biased at ~ -70 V with respect
to the Si wafer
 Ion current determined by measuring
the voltage drop across a known
resistance.
 Both reference and measurement
probe are isolated from ground using
a floating power supply.
 Plasma sees the same surfaces during
etching of a wafer.
 Probe and reference are etched but
measurement is not affected.
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Ion flux measurements in SF6/O2 plasmas
2
Ion Current Density (mA/cm )
4.0
3.5
 Measurements were done in plasmas
containing SF6, O2, HBr, Cl2, NF3
and probe worked well for extended
periods of time.
3.0
 I-V probe
 ion flux probe
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
100
Effect of TCP power and pressure
50% O2, 80 sccm total flow
Pressure (mTorr)
Ion current, mA/cm2
4
5 mTorr
15 mTorr
3
25 mTorr
2
1
0
0
200
400
600
800
TCP power, W
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Measuring radical concentrations in a plasma
Line of Sight
Appearance Ionization
Mass Spectrometry
Time,
Cost,
Footprint
Laser Induced
Fluorescence
UV Absorption
IR Absorption
Optical Emission
Spectroscopy with
Actinometry
Accuracy
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Optical Emission Spectroscopy
$ 25 K
Imaging
Spectrographs with
CCDs
$ 10 K
Monochromator &
PMT
Cost
Integrated
spectrographs and
data acquisition
$3K
$ 0.5 K
Photodiode and
narrow pass filter
10
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Resolution, nm
0.1
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Optical Emission Spectra
5
40 SF6 / 40 O2 / 5 Ar
4
Cl 2 /Ar pla sm a
3
Em ission Intensity
Optical emission (a.u.)
O
F
Ar
2
1
0
300
400
500
600
Wavelength (nm)
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700
800
Ar
75 0.4 nm
Cl
74 9.2 nm
Ar 75 1.5 nm
Ar pla sm a
7 35
7 40
7 45
7 50
7 55
7 60
W aveleng th (nm )
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Optical Emission

X  e  X  e
ke
X  X
*
u
kem
*
u
*

dn X *
 kex n X ne  kem n X *  0
dt
kex n X ne
nX * 
kem
I  S (  )kem n X *  S (  )kex n X ne
u

hn

e
ground state

2
kex 
me

 ex (  )  f (  )d
0
Emission intensity depends on nx, ne and Te
Emission intensity is not a measure of X concentration
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Optical Emission Actinometry
J. Coburn and M. Chen, J. Appl. Phys. 51, 3134(1980).
I X  S (  X )kex ,X nX ne
I Ar  S (  Ar )kex ,Ar nAr ne
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IX
n
 X
I Ar
n Ar
S (  X )kex ,X

S (  Ar )kex ,Ar
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Actinometry Requirements
 Excitation to the emitting states of X and actinometer
(e.g., Ar) must have similar magnitude cross sections and
thresholds.
 ex, X  ex, Ar then  is a weak f(Te).
 ex, X  ex, Ar then  is f(Te) which must be determined.
 Emitting state must only be populated by electron impact
excitation of the ground state.
Cl  e   Cl*  e 

 Cl2  e   Cl*  Cl  e 
 Cl m  e   Cl*  e 
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Etch Rate (mm/min)
Example: Use of OES and ion flux measurements in
SF6/O2 etching of Si
mm
-2 -1
-40 V
0.8
0.6
-20 V
0.4
0.2
0.0
1.5
6
5
1.0
4
16
1
1.0
2
3
0.5
2
1
0.0
0
20
40
60
0
80
Pressure (mTorr)
10 mT
25 mT
40 mT
75 mT
800W TCP/-20 V rf-bias/40 SF6/40 O2/150 sec
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•
Etch rate has a maximum at some
intermediate pressure (~25 mTorr).
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F Density (a.u.)
Ion Flux (10 cm s )
0
1.2
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Example: Absolute measurements of Cl and Cl2
concentrations in Cl2 plasma
Donnelly, J. Vac. Sci. Technol. A 14, 1076 (1996)
Malyshev, Donnelly, Kornblit, and Ciampa, J. Appl. Phys. 84, 137 (1998)
Ullal, Singh, Daugherty, Vahedi and Aydil J. Vac. Sci. Technol. A 20, 1195 (2002).


A number of emission lines for Cl2, Cl and Ar studied for
suitability (Donnelly, et al.)

305 nm Cl2 emission (Eth = 8.4 or 9.2 eV)

822 nm Cl emission (Eth = 10.5 eV)

750.4 nm Ar emission (Eth = 13.5 eV)
The Ar emitting state has unusually low  for excitation from
Arm but threshold does not match the Cl2 or Cl thresholds

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 must be corrected for Te dependence
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From emission intensities to absolute concentrations
In the limit power 0;
dissociation  0
nCl2  ng = Pg/kBTg



2
, Ar
Cl 2
I Ar

-3
g
n Ar 
power 0
Repeat zero-power extrapolation
at different pressures to
determine Cl2 (Te)
Determine Cl concentration by
mass balance or
Use nCl at single point to
determine Cl,Ar
nCl
α Cl (Te ) 
n Ar
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3.5
n C l fr om ac tino me tr y
3.0
2
n C l fr om ac tino me tr y
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 Cl

n

I
P = 10 mTorr, no wafer, Q = 100 sccm Cl2
Cl or Cl2 Concentration ( x10 #/cm )

 I Ar 


 I Cl  High Power
n C l fr om mass bala nce
2.5
2.0
1.5
1.0
0.5
0.0
0
200
600
400
T C P P o w er (W )
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Studying the Effect of Walls on the Cl2 Dissociation
Using OES and Actinometry
5 mTorr
10 mTorr
100
% of undissociated Cl2
% of undissociated Cl2
100
75
50
25
0
5 mTorr
10 mTorr
75
50
25
0
0
100
200
300
400
500
600
700
800
T CP Po wer(W )
Alumina reactor walls: high
Cl sticking probability g~1
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0
100
200
300
40 0
500
600
700
800
TC P P ow er (W )
SiO2 covered walls: low Cl
sticking probability g~0.03
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Mass Spectrometry
http://www.mcb.mcgill.ca/~hallett/GEP/PLecture1/MassSpe_files/image011.gif
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Line of Sight Threshold Ionization Mass Spectrometry
• Threshold ionization can be used for detecting ALL
radicals in a plasma
• Density of radicals is obtained at the substrate plane
Principle of TIMS
O + e  O+ + 2e
: 13.6 eV (E1)
O2 + e  O+ + O + 2e
: 19.0 eV (E2)
• Since E1 > E2, an electron energy scan can differentiate the
two products
• E2-E1 is typically equal to the bond energy of the bond that is
broken during dissociative ionization
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Dissociation on the ionizer filament also produces radicals which must be
distinguished from the radicals in the beam extracted from the plasma
Beam
Filament
(Thoria Coated Iridium)
Id
If
Ionization cage
(3-5 V DC bias)
eEmission
Control
Ie
1800 K
Ie
(thermal dissociation
into radical species)
A
To Bessel Box
Molecules are thermally dissociated on the filament and
ionized resulting in a spurious background signal.
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25-200 mTorr
Plasma
1 mm Sampling Orifice
Heated Electrode
4 mm
1st stage
183 mm
Radical/ion beam
~10-5 Torr
4 mm
Beam Chopper
Turbo Pump
200 l/s
2nd stage
~10-9
Torr
~10-7 Torr
to Turbo Pump
900 l/s
Hiden
QMS
3rd stage
to Turbo Pump
60 l/s
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c/s (a.u.)
Beam-to-Background Ratio
8x10
5
7x10
5
6x10
5
5x10
5
4x10
5
3x10
5
2x10
5
1x10
5
• Pure O2: Beam-toAr
O2
O2 beam
chopper closed
• For radicals, the
chopper open
O2 background
0
20
40
Time (s)
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background ratio
3.2 at 25 mTorr
and 2.0 at 200
mTorr.
80
beam-tobackground ratio
will depend on the
sticking probability
of the radical.
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O atom detection in O2 plasma
4
O + e  O+ + 2e
: 13.6 eV
O2 + e  O+ + O + 2e
: 19.0 eV
m/e = 16
Chopper open
Direct
ionization
c/s (a.u.)
10
Chopper closed
3
10
2
Direct and dissociative
ionization
10
1
10
12
14
16
18
20
22
Electron Energy (eV)
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26
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O in the beam = Signal w/chopper open – Signal w/chopper closed
2.0x10
c/s (a.u.)
1.5x10
1.0x10
5.0x10
4
O + e  O+ + 2e
: 13.6 eV
O2 + e  O+ + O + 2e
: 19.0 eV
d issociativ e and direct
ionizatio n
4
4
3
0.0
12
lin ear fit
(on ly direct ionization )
14
16
18
20
22
24
26
Electron Energy (eV)
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Quantifying the Mass Spectrometer signal
S    I e    nionizer
where,
S
: QMS signal in c/s

: product of m/e-ratio dependent factors
Ie
: electron current of the ionizer

: cross-section of the ionization process
nionizer : number density of neutrals in the ionizer (nbeam+ nbackground)
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Calibration
•
•
CH4 (m/e =16) is used for calibration.
QMS signal for CH4 is measured for a known pressure of the gas in the
plasma chamber under plasma-off condition
nO 
•


 S OO

 CH4  CH4
S

 CH4  CH4
 


O  O
 



 n
 CH 4


CH4 calibration must be done right after the O concentration measurements to
avoid the effect of drifts in the SEM sensitivity
Singh, Coburn, and Graves, JVST A 17, 2447 (1999).
Singh, Coburn, and Graves, JVST A 18, 299 (2000).
Agarwal, Quax, van de Sanden, Maroudas and Aydil, JVST A 22, in press (2004).
Agarwal, Hoex, van de Sanden, Maroudas and Aydil, Appl. Phys. Lett, in press (2003).
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Absolute O atom concentrations in O2/Ar discharge
25 mTorr
50 mTorr
75 mTorr
100 mTorr
150 mTorr
200 mTorr
2.50
-3
nO (x10 ) (m )
2.00
19
1.50
1.00
0.50
0.00
0
10 20 30 40 50 60 70 80 90 100
Percent O2 in Feed Gas
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Example: N2* (metastable A3Su+ state) and N
concentrations in N2 plasma
N2* + e  N2+ + 2e
N2 + e  N2+ + 2e
counts/s (a.u.)
•Can N2* be detected
and absolute
concentrations of N and
N2* measured?
10
5
10
4
10
3
Chopper
Open
Chopper
Closed
2000
10
2
10
1
10
counts/s (a.u.)
•In plasma assisted MBE
of GaN, N2* may be
preferred over N as the
nitrogen precursor.
: 9.4 eV (??)
: 15.6 eV
1000
0
10
12
14
Beam-Plasma on
Beam-Plasma off
16
0
12
13
14
Electron Energy (eV)
18
20
Electron Energy (eV)
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Probable Franck-Condon Transitions
26
22
B 2Su+
10
5
20
18
A 2 u
20
15
10
0
0
X 2Sg
+
5
c
0
9.4 eV
E (eV)
16
20
15
14
12
a
15
10
8
10
5
1 +
2 X Sg
10
5
25
0
0
6
4
B g
Transition ‘a’: ~11 eV
Transition ‘b’: ~12 eV
Transition ‘c’: ~14 eV
3
20
15
A 3S +
u
20
b
15
10
5
0
0
0.4 0.8 1.2
2.0
r (Å)
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Eion = 14.53 eV
24
2.8
3.6
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0.20
1.2
0.18
1.0
0.16
0.8
0.14
0.6
-3
-3
1.4
19
0.22
*
1.6
0.12
0.4
0.10
0.2
0
50
100
150
Pressure (mTorr)
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200
N2 (10 m )
0.24
Atomic N (10 m )
1.8
19
Absolute N2* and N Concentrations
0.08
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Summary
 Simultaneous with the emergence of plasma processing as an enabling
technology, a variety of plasma diagnostic methods have been
developed over the last two decades to measure internal plasma
properties.
 Ion current probe
 OES and actinometry
 Line of sight threshold ionization mass spectrometry
 Ease of implementation range from methods that take ~days-week to
“Ph.D. lifetime.”
 To save time and money the first ask “What do we want to measure
and how accurately do we want to measure it?”
 Measurement of radical concentrations over the wafer and ion flux
impinging on its surface help in process development and improve
fundamental understanding of etching and deposition processes.
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Acknowledgements
Sumit Agarwal (now @ University of Massachusetts)
Jun Belen (UCSB)
Dr. Sergi Gomez (UCSB)
Bram Hoex (now @ Eindhoven Univ. of Technology)
Guido Quax (now @ Eindhoven Univ. of Technology)
Saurabh Ullal (now Lam Research Corporation)
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