Plasma Chemistry Study in an
Inductively Coupled Dielectric Etcher
Chunshi Cui, John Trow, Ken Collins,
Betty Tang, Luke Zhang, Steve Shannon, and
Yan Ye
Applied Materials, Inc.
October 26, 2000
Dielectric Etch Organization
10/28/2008
Content
„
„
Introduction
Experimental Set-up
– Inductively-coupled dielectric etcher
– Plasma diagnostics
– Langmuir probe measurements
– Ion Mass Spectrometry (IMS)
– Optical Emission Spectrometry (OES)
„
Effects of Different Parameters
–
–
–
–
–
–
„
Source Power
Pressure
Surface temperatures
Carrier Gas flow and type
Location, Wafer type
Plasma sources
Summary
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Introduction (1)
„
An independent plasma source in conjunction with
capacitively coupled rf bias has allowed
– Independent control of density and ion energy
– In-situ dry clean of process chamber
– These have been the major focus until recently for plasma
source development
„
Control of chemical species continues to be a
major challenge
– This is the key to high selectivity of dielectrics to mask and
underlayer with high etch rate and wide process window
– This requires control of both gas phase and surface reactions
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Introduction (2)
„
A selective dielectric etch involves careful balance of polymer
deposition, chemical etching and physical sputtering.
„
The etch relies on proper combination of fluorocarbon radicals and
ions (CxFy).
„
Progress has been made in characterization of CxFy species using
LIF, laser absorption and etc.
„
Etch rate and selectivity need to be correlated to the fluxes of these
radicals and ions to the wafer.
„
This study is a survey of CxFy ions by Hiden IMS and CxFy
radicals by OES in an ICP dielectric etcher in conjunction with
plasma measurements by Langmuir probe.
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Dissociation Paths in C4F8 Plasmas
H. Kazum et al., Plasma Sources Sci. Technol. 5 (1996), 200-209.
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
CxFy Reaction Paths on Si Surface
Si Surface Reaction
C2 + SiF + other by-products
Si + CxFy
Adsorption of
Reactants
CxFy
Desorption of
Reactants
CxFy
Desorption of
By-Product
C2 + SiF
• Reaction Path:
1. Adsorption of reactants
2. Desorption or “boiling off” of reactants
3. Surface reaction to form C2, SiF and other by products
4. Desorption of by-products
• Reaction Rate is determined by Rate-Limiting (slowest Step).
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Inductively Coupled Dielectric Etcher
Plasma Uniformity Control
with Dual-Antenna Source
Temperature Control
of all “Plasma” Surfaces
Si Roof acting as RF
Window & Electrode
Movable
Ion Mass Spec.
or
Langmuir probe
Plasma Region
S/N
N/S
SiC Collar
Temp Controlled
Si Hot Ring
S/N
N/S
OES Window
Gas
Feed
DC
Electrostatic Ceramic Chuck
C. Cui
Dielectric Etch Organization
Bias
Hot Si Ring
GEC 2000 Presentation
Plasma Measurements
„
Diagnostics
Langmuir probe :
Te and ne
Hiden Ion Mass Spec. (IMS):
Relative spectra of ion species
Optical Emission Spec. (OES): Relative spectra of excited neutrals
„
Experimental Parameters
Total source power
Pressure
C4F8 flow
Carrier gas
Si roof temperature
„
=
=
=
=
=
0 - 1500W
7 - 80mT
15 - 30 sccm
He, Ar, Xe
100 - 200C
Baseline conditions are shown by the blue numbers above.
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Effect of Source Power: OES
At low source power (<1000 W at 30mT), the spectra are dominated
by Ar emission lines with very low C2 or SiF. This is an characteristics
of E-mode plasma (capacitive coupling from the bias power).
Change pf E- to H-mode (ICP) happens between 1000 to 1500W.
The threshold power for mode change is strongly dependent on
pressure, gas chemistry and chamber geometry.
„
„
„
2000
7000
Intensity (arb.)
C2
6000
RIE
500Wout
1000Wout
1500Wout
5000
Ar lines
4000
C2
1000
RIE
500Wout
1000Wout
1500Wout
C3
3000
C2
2000
SiF
F
1000
0
200
300
C. Cui
Dielectric Etch Organization
400
500
Wavelength (nm)
600
0
600
700
800
Wavelength (nm)
GEC 2000 Presentation
Effect of Pressure: Langmuir Probe
„
„
Plasma density (Ne) decreases with increasing pressure.
Electron temperature (Te) decreases with increasing pressure.
Te vs. Pressure
15C4F8/100Ar/1400Wout
Electron Density vs. Pressure
15C4F8/100Ar/1400Wout
2.E+11
Ne (cm-3)
4
Electron Density (cm-3)
ElectElectron Temperature
(eV)
5
Te (eV)
3
2
1
0
2.E+11
1.E+11
5.E+10
0.E+00
0
10
20
30
40
Pressure (mT)
50
60
0
10
20
30
40
50
60
Pressure (mT)
(Data were collected using Smart Probe by Scientific Systems.)
C. Cui
/ Alex Paterson
Dielectric Etch Organization
GEC 2000 Presentation
Effect of Pressure: Ion Mass Spec.
„
„
„
C+ and F+ decrease while CF+ increases with pressure.
CF2+ and CF3+ slightly increase but much less than CF+.
Higher pressure can reduce dissociation due to lower Ne and Te
(Langmuir probe data) but not effective enough in the highly
dissociation plasma.
C
F
CF
CF2
CF3
C2F4
CxFy Ion Count
CF2CF3C2F4
3%3% 0%
CF3C2F4
CF2 4% 1%
13%
1.E+07
1.E+06
1.E+05
5mT
1.E+04
0
C. Cui
20
40
Pressure (mT)
Dielectric Etch Organization
CF
82%
50mT
CF
94%
60
GEC 2000 Presentation
Effect of Radial Distance: Ion Mass Spec.
„
„
O+/O2+ ratio decreases with radial distance, possibly due to charge
exchange between O+ and O2 and wall recombination of O and O+.
O+/O2+ ratio decreases with pressure indicating reduced O2 dissociation.
O+ / O2+
2
O+/O2+(3.5cm from wall)
O+/O2+ (at wall)
1.5
1
0.5
0
0
C. Cui
Dielectric Etch Organization
10
20
30
Pressure (mT)
40
GEC 2000 Presentation
Effect of Si Surface Temperature: OES
Below 200C, C2*/Ar* increases with Si temperature meaning the
rate-limiting-step is desorption of polymer from Si surface. Above
200C, the trend reverses due to the limitation of CxFy absorption.
Si surface acts like a good polymer “sink” or “reflector” but not a
F scavenger.
„
„
9.00
8.00
5.00
C2/Ar*10
4.00
F/Ar*10
3.00
2.00
C. Cui
SiF/Ar*10
6.00
Intensity
Normalized
7.00
Si Roof
1.00 Si Ring@300oC
0.00
100
150
200
Dielectric Etch Organization
Si Ring
Si Roof@120oC
250
300
350
400
450
Silicon Temperature (C)
500
GEC 2000 Presentation
Effect of C4F8 Flow: Ion Mass Spec.
„
C4F8 flow of 15 and 30sccm results in similar ion mass spectra
meaning not an effective knob for ion species control in an
already highly dissociated plasma.
Baseline
15C4F8/100Ar/in
probe
15C4F8/100Ar
C4F8/Ar
CF2
12%
304F8/100Ar
C4F8/Xe
C2F4
CF3
5%1%
CF2
8%
CF
82%
C. Cui
Dielectric Etch Organization
Higher Flow
30C4F8/100Ar/in probe
C2F4
CF3
4%1%
CF
87%
GEC 2000 Presentation
Effect of Carrier Gas: He, Ar and Xe
„
The different ionization threshold and mass of He, Ar and Xe affect
fundamentally plasma generation and loss resulting in different plasma
density and electron energy distribution function (EEDF).
„
Langmuir probe measurement shows that the EEDF is non-Maxwellian in
C4F8/Xe mixture with depleting high energy tails. The average electron
energy is lowest in Xe and highest in He while the electron density has
opposite trend.
„
The low ionization energy threshold of Xe (~11eV) allows the high
ionization rates with less hot electrons and results in a high density but
low electron energy plasma.
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
EEDF in C4F8/Ar and C4F8/Xe Plasmas
C4F8/Xe
C4F8/Ar
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Effect of Carrier Gas:
Electron Density and Average Energy
C4F8 Mixed With Noble Gases
8
6
4
He
Ne
Ar
Xe
2
0
0
20
40
60
80
100
120
140
Mass of Noble Gas
ne (10^11cm3)
Av Elect Energy (eV)
Plasma Potential / 10 (Volts)
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Effect of Carrier Gas: CxFy Ion Mass Spec.
„
„
„
Percentage of F is 50 times less in C4F8/Xe than in C4F8/He.
Percentage of heavy CxFy ions is far more in Xe than in Ar & He.
These are due to low dissociation in C4F8/Xe which has the
lowest average electron energy.
Nomalized by Total Ion Count
1
C4F8/He
C4F8/Ar
0.1
C4F8/Xe
0.01
0.001
C
C. Cui
Dielectric Etch Organization
F
CF
CF2
CF3
C2F4
GEC 2000 Presentation
Effect of Carrier Gas: OES Normalized By F*
„
„
C2*/F* ratio is 8 times higher in C4F8/Xe than in C4F8/Ar.
This demonstrates, again, less dissociation in C4F8/Xe.
100
150Xe
150Ar
10
1
Dielectric Etch Organization
O(777.54)
F(703.75)
H(656.29)
C2(563.55)
C2(516.52)
CO(313.44)
SiF(440.05)
C. Cui
Si(288.16)
0.1
GEC 2000 Presentation
Effect of Carrier Gas: Process
„
„
„
C4F8/Ar or C4F8/Xe process was applied to the pre-patterned via holes.
Sharp facet of via tops with C4F8/Xe implies that physical sputtering is
dominant over chemical etching of the holes.
Rounded facet with C4F8/Ar suggests that isotropic etching maybe by
excessive F atoms shown clearly by high C2*/F* ratio in OES spectra.
15C4F8/150Xe
C. Cui
Dielectric Etch Organization
15C4F8/150Ar
GEC 2000 Presentation
Summary
„
„
„
„
„
„
Chemical species were experimentally investigated in an C4F8based ICP as a function of many parameters.
Clear E- to H-mode transition from low to high ICP power was
observed accompanied by drastic change in chemical species
spectra. In typical ICP operation, CxFy molecules are highly
dissociated and the plasma is characterized by C2* rich OES
and CF+ dominated IMS spectra.
High pressure can slightly reduce dissociation but not effective
enough to change the nature of high dissociation.
Hot Si surface inside chamber plays more a role of polymer
“sink” or “reflector” but a less role of F atom scavenger.
The most effective knob among all is the type of carrier gas. Using
Xe as a carrier gas can dramatically reduce the dissociation of
fluorine-carbon molecules in an ICP oxide etcher.
Dissociation control using different plasma generation mechanisms
are currently in progress.
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Future Perspective
„
Gas phase and surface reaction mechanisms of CxFy plasmas
need to be thoroughly studied.
„
Semiconductor industry faces serious challenges due to feature
shrinking and low-k/Cu Dual-Damascene in interconnect circuitry.
„
This plasma community can really help the industry with new
innovations in terms of chemical species control
–
–
–
–
Different plasma generation mechanisms
Introduction of new chemistry
Introduction of new surfaces
Many more ……
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Ion Mass Spectra vs. Wafer Type
„
„
„
CF+ ions are the dominant reactive ion species.
Minor loading effect of CxFy+ observed from Si to patterned SiO2 wafer.
More release of by-product CO+ seen with patterned SiO2 wafer.
Normalized by Total Ion Count
0.2
C4F8/Ar Si Wafer
0.15
C4F8/Ar Patterned Oxide Wafer
0.1
0.05
0
C
F
CF
CF2
CF3
Reactive Ions
C. Cui
Dielectric Etch Organization
C2F4
CO
SiF
Byproduct ions
GEC 2000 Presentation
Challenges due to Device Evolution
„
Feature Shrink beyond 0.1μm
VLSI device features shrinks rapidly, leading to thinner mask
thickness higher aspect ratio (HAR) of dielectric patterns. This
requires highly selective etching to dielectric materials to mask.
„
Evolution of Materials:
SiO2 & Al
low-k & Cu
The VLSI industry has clearly taken the direction of Low-k & Cu Dual
Damascene (DD) for lower response (RC) time in interconnect
circuitry. This has introduced more challenges for dielectric etch due
to more complex structures and constantly evolving low-k materials.
C. Cui
Dielectric Etch Organization
GEC 2000 Presentation
Optical Emission in ICP and CCP
In ICP, CxFy plasmas are characterized by C2 rich OES which is a
symbol of high dissociation.
Typically, C2*/Ar* ratio is many times higher in ICP than CCP plasmas
C2*/Ar* ratio is used as a empirical method to guide process
development for selective etch.
„
„
„
OES from an ICP Chamber
OES Emission from an MERIE Chamber
40000
200000
Ar (750)
35000
160000
30000
C2 (516)
Intensity (arb.)
25000
20000
F (703)
15000
140000
120000
100000
80000
F (703)
60000
10000
C2 (516)
40000
5000
20000
0
C. Cui
Dielectric Etch Organization
Wavelength (nm)
862
821
780
739
698
616
574
533
492
450
408
366
325
283
240
852
811
770
730
689
648
607
567
526
485
444
403
363
322
281
240
0
657
Intensity (arb.)
Ar (750)
180000
Wavelength (nm)GEC 2000 Presentation
Probe Data Interpretation
„
The total probe current was used. No correction was made for ion
current.
„
The Electron Energy Distribution Function (EEDF) was calculated
from the Druyvestyn formula.
f 0( ε)
=
8 .m e
2
1
d
.
.
I
3 Area d V 2
qe
„
The electron density and the average electron energy were
calculated from integrating the EEDF.
„
The effective electron temperature for a non - Maxwellian EEDF is
T eff =
C. Cui
Dielectric Etch Organization
2.
3
E av
GEC 2000 Presentation