C O N F I D E N T I A L
Etch Product Business Group
Effects of Dual Bias Frequency on SiO2 Contact Hole Etching in Very High Frequency Fluorocarbon Plasmas
Wonseok Lee, Shawming Ma, Jang Gyoo Yang, Daniel Hoffman and Yan Ye
Dielectric EtchApplied Materials
PEUG Meeting, Mar 10, 2005
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Outline
Requirement of High Aspect Ratio (HAR) contact etch– Polymer deposition and ion energy distribution
Dual bias effect on ion energy distribution– Bias frequency selection– Simulation of ion energy distribution
Process results of high frequency source/dual bias chamber– Profile control– Process window
Conclusion
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Why High Ion Energy Necessary for HARC?
Typical requirements for advanced HAR etching process- Highly selective process (to mask and sub-layer) : thick polymer generation- vertical profile is necessary : need easy penetration of reactive ion through polymer layer
: Polymer layer: Reactive layer
2000 AVS, ASET
Oxide etching surface
High selectivity Thick polymer deposition over the reactive layer Decreases net ionenergy penetrating through polymer layer low etch rate & tapered profileHigh aspect ratio Ion energy decreases at the bottom of hole due to ion scattering and shading effect from hole entrance low etch rate & tapered profileHigh incident Ei is necessary for high etch rate and vertical prHigh incident Ei is necessary for high etch rate and vertical profile in HARofile in HAR
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Vrf Behavior with VHF and 13.56MHz Bias
- Measuring tool : z-scan(z-Ware)- Wafer type : Si dummy wafer - Measuring plasma condition : HARC etching condition with PET process
- Measuring Vrf at 10sec processing time
Measuring plasma condition : HARC etching condition with PET process
Source power
0 1000 2000 3000 4000 50000
200
400
600
800
1000
1200
1400
1600
1800
1850Watts
Waf
er V
olta
ge (-
V)
Bias Power(W)
0Ws 100Ws 200Ws 300Ws 400Ws 500Ws 600Ws 700Ws 800Ws 900Ws 1000Ws 1500Ws 2000Ws
The efficiency of The efficiency of VVrfrf rise withrise withbias power starts to get gentlebias power starts to get gentle
over reasonable source and bias powersover reasonable source and bias powers
Low
High
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Candidates for Increasing Ion Energy
- Increase the ratio of Area_ground/Area_powered cathode: Asymmetric sheath-> area ratio improvements already obtained.
- Increase RF bias generator capacity and try higher bias power: Vrf can only increase 1.4X with 2 X power increaseSome customers restrict available maximum power.Match box and ESC durability at very high power
- Low frequency (2MHz): Limited plasma striking capability at low pressure
More limited RIE mode plasma at low pressure
- Dual frequency (13.56MHz/2MHz): Best candidate for HAR process regime
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Enabler DFB Chamber Overview
Very high frequency source (>100MHz) giving very low DC bias on the top electrode
Controllable plasma density and neutral dissociation for different applications
Two additional knobs –CSTU and NSTU to tune etch rate and CD uniformities
Very wide, easily tunable and usable operating windows
Ion Energy Distribution tunability by DFB
~
~
Polymer/plasma confinement
2.0 MHz RF Bias Power
Charged species tuning unit (CSTU)
Neutral species tuning unit (NSTU)
Wafer Temperature Control
>100 MHz VHF Source
~
13.56 MHz RF Bias Power
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Ion density/power input
100% 1.8 MHz
50% 1.8 MHz
100% 13.56 MHz
Ion Energy
Ion Energy Distribution
By Total Bias Power
By B
ias
Pow
er R
atio
13
25
60
>100
100
Plasma Technology for Critical EtchDual Bias Frequency (1.8MHz + 13.56MHz) for Ion Energy Distribution (IEDF) Tunability
Frequency (MHz)
Application of 2 & 13 MHz bias enables ion energy tuning and maintainsplasma density decoupling
The width of distribution is tunable by changing the bias power ratio of 2 frequenciesThe center of distribution is tunable by changing the total bias power
Dual bias frequency (2.0MHz + 13.56MHz) can provide the right ion energy distribution needed for future critical etch applications
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Tunable Ion Energy Distribution by Dual Frequency Bias (2.0/13.56MHz)
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200 400 600 800 1000 1200 1400
0.02
0.04
0.06
0.08
Energy (Volts)
Den
sity
(arb
uni
ts)
.
.
.
.
.
.
.
.
.
.
.Freq
uenc
y Mix
(13MHz /
2MHz)
100% / 0%
80% / 20%
60% / 40%
40% / 60%
20% / 80%
0% / 100%
By properly mixing2MHz and 13MHz,the IEDF width canbe controlled whilemaintaining constant average energy
IEDF Overlay–13MHz / 2MHz Mixing FractionConstant Vpp Waveform Across Sheath
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Other Frequency Considerations
2 or 13MHz/VHF (and 2MHz/>100MHz)– Even though we top-launch, our >100MHz high voltage point is on
the wafer (by design, typically Vwafer/Vroof~3)– The >100MHz rf voltages on the wafer are very low, so the major
effect is source modifies 2 or 13MHz distribution by only relative density impact
2/27MHz or 2/60MHz– Complex: the 27MHz/60MHz contributes to both density creation and
to ion energy adjustment– This leads to less orthogonality.
2/13MHz bias with >100MHz source– 2/13 achieves the “dialable” energy distribution– >100MHz independently adjusts the density effect
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Window of Deep Etch Comparison
green is (n,Emin, Emax) space; red is achievable by Enabler
Single Frequency Bias withVHF source
Dual Frequency Bias withVHF source
10 mT 30 mT
vlow vhigh, n,( ) eminopt emaxopt, nopt,( ),vlow vhigh, n,( ) eminopt emaxopt, nopt,( ),vlow vhigh, n,( ) eminopt emaxopt, nopt,( ), vlow vhigh, n,( ) eminopt emaxopt, nopt,( ),
10 mT 30 mT
Large fractions of the(n, Emin, Emax)
volume are achieved
We can just achieve the lowerend of this space,
and only at lower pressures
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Blanket Thermal Oxide Etching with DFB
Baseline Process(single frequency bias)
Bias Ratio(13.56MHz/2MHz)
Power Ratio (2MHz/13.56MHz)
Cotour Map
Etch Rate(A/min) 3820 3729 3624 3530Uniformity(1sigma,% 1.5 1.52 2.3 1.59
Uniformity(M-m,%) 3.02 3.03 4.91 3.08Bias Power(W)
(13.56MHz-2MHz)Power Ratio
(2MHz/13.56MHz)
Contour Map
Etch Rate(A/min) 3452 3342 3239 3065Uniformity(1sigma,% 3.41 3.55 3.99 4.25
Uniformity(M-m,%) 6.66 6.24 7.1 7.65
13.56 only 7 3 1.7
0 0.14 0.33 0.6
1 0.6 0.33 0.14
1 1.67 3 7
-1 0 1 2 3 4 5 6 7 83000
3100
3200
3300
3400
3500
3600
3700
3800
3900
2
3
4
5
6
7
8
9
Etc
h R
ate
(A/m
in)
Ratio of [P2MHz/P13.56MHz]
Mm
Uni
form
ity(%
)
As 2MHz fraction increases, uniformity is getting worse buttends to be saturated at 7% Mm over 1:1 ratio and also
etch rate drops due to lower density.
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Uniformity Tuning by CSTU
Baseline ProcessDual Frequency Bias (75% 2 MHz) vs CSTU
Outer CSTU(A) 4.5 5 5.5 6
Cotour Map
Etch Rate(A/min) 3306 3245 3279 3336Uniformity(1sigma,% 4.26 3.21 3.7 3.81
Uniformity(M-m,%) 7.22 5.22 6.82 5.95
Inner CSTU(A) 4.5 5 5.5 6
Contour Map
Etch Rate(A/min) 2996 3028 3066 3109Uniformity(1sigma,% 10.7 9.72 8.75 8.27
Uniformity(M-m,%) 14.78 12.35 11.52 11.83
4.0 4.5 5.0 5.5 6.0 6.53000
3100
3200
3300
3400
3500
3600
3700
3800
3900
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Etc
h R
ate
(A/m
in)
Outer CSTU (A)
Mm
Uni
form
ity(%
)
4.0 4.5 5.0 5.5 6.0 6.52800
2900
3000
3100
3200
3300
3400
3500
6
8
10
12
14
16
Etc
h R
ate
(A/m
in)
Inner CSTU (A)
Mm
Uni
form
ity(%
)
Outer CSTU
Inner CSTU
Outer CSTU looks more effective totune the etch rate uniformity
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Polymer Deposition Shape With DFB
P13.56MHz/(P13.56MHz+ P2MHz)
Pre 100% 71% 19%
Internal Data
Higher ratio of [2MHz/13.56MHz]– Less sidewall polymer deposition, – Reduce necking and bowing. – Etch front improved with the mixed bias powers
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Pattern Wafer Etching with DFB0.18µm hole/248nm resist/TEOS
0 1 2 3 4 5 660
80
100
120
140
160
180
0.40
0.45
0.50
0.55
0.60
0.65
CD
(nm
)
Ratio of [2MHz/13.56MHz]
BT
Rat
io (%
)
Btm CD
Top CD
BT Ratio
Top/Btm CD & BT Ratiovs. DFB Ratio
0 1 2 3 4 5 63000
3500
4000
4500
5000
5500
6000
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Etc
h R
ate
(A/m
in)
Ratio of [2MHz/13.56MHz]
Sel
ectiv
ity to
PR
Etch rate & Selectivity to PRvs. DFB Ratio
As 2MHz fraction increase (up to 1:3)- Profile is getting more vertical(Btm CD/BT ratio increase)
- No significant effect on etch rate- Selectivity to resist increases
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Striation Comparison with DFB0.18µm hole/248nm resist/TEOS
P13.56MHz/(P13.56MHz+ P2MHz)
100% 78% 47% 26% 16%57% 37%
Data Shown with Customer Permission
More 2MHz Fraction
No impact on striation (resist integrity)
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0.15µm hole/248nm resist/TEOS
Pattern Wafer Etching with DFB
TEOS Etch Rate Facet Sel. PR Bottom CD
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
4550
4600
4650
4700
4750
4800
4850
4900
4950
Center Edge Avg
TEO
S E
tch R
ate
@0.1
5um
(A/m
in)
2.0/13.56MHz Bias Ratio(Ptotal
=3800W)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
5.5
6.0
6.5
7.0
7.5
8.0 Center Edge Avg
Face
t Sel
ectivi
ty t
o P
R @
0.1
5um
2.0/13.56MHz Bias Ratio(Ptotal
=3800W)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8767880828486889092949698
100102
Center Edge Avg
Bott
om
CD
@0.1
5um
(nm
)
2.0/13.56MHz Bias Ratio(Ptotal
=3800W)
Almost same trend as 0.18um etching characteristics
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Etch Stop Window and Etching Profilevs DFB Mix (x% 13 MHz power)
E
100% 73% 50%
50% increase
37%23%,
More power
incr
easi
ng
O2
Vdc
Wider etch stop window
Getting more vertical
Internal Data
P13.56MHz/(P13.56MHz+ P2MHz)
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0.15um 0.20um 0.25um 0.30um
13.56MHzonly
Mixed DFB50%
Hole size
PositiveRIE lag(DFB)
ReverseRIE lag(Single)
Etch stop
RIE Lag (ARDE) vs DFB Ratio
0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.321.501.551.601.651.701.751.801.851.901.952.002.052.10
Etch
ing
Dep
th (u
m)
Hole Size (um)
13.56 only bias13.56/2.0 mixed bias (50/50%)
Internal Data
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Optimized Process Comparison Between 13.56MHz Single and 13.56/2MHz Dual frequency Bias
13.56MHz only 13.56MHz/2.0MHz Mixed
Top CD 152nmBtm CD 93nmBow CD 172nm
Top CD 152nmBtm CD 112nmBow CD 165nm
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Conclusion
Dual bias 2/13.56 MHz frequency with VHF source is capable of modulating ion energy distribution while maintaining similar iondensity
Combination of VHF source(>100MHz) and 2/13.56 MHz dual frequency bias has wider process window – Larger bottom/top CD ratio – Wider etch stop window– Better control of microloading/RIE lag– Some selectivity improvement– No striation dependence on the bias power ratio