1. DEVELOPMENT OF ION ENERGY ANGULAR DISTRIBUTION THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS*
Yiting Zhanga, Nathaniel Mooreb, Walter Gekelmanb
and Mark J. Kushnera
(a) Department of Electrical and Computer Engineering,
University of Michigan, Ann Arbor, 48109 ,
(b) Department of Physics, University of California,
Los Angeles, 90095
, )
September 2011
* Work supported by National Science Foundation and Semiconductor Research Corp.

2. University of Michigan Institute for Plasma Science & Engr.
AGENDA
Introduction to dual frequency capacitively coupled plasma (CCP) sources and Ion Energy Angular Distributions (IEAD)
Description of the model
Plasma properties for 2 MHz / 30 MHz
Ar Plasma properties
Ar/O2 Plasma Properties
Uniformity and Edge Effect
Concluding Remarks
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_01

3. DUAL FREQUENCY CCP SOURCES
Capacitively coupled discharges (CCPs) are widely used for etching and deposition of microelectronic industry.
High driving frequency achieve higher electron densities at moderate sheath voltage and higher ion fluxes with moderate ion energies.
A low frequency contributes the quasi-independent control of the ion flux and energy.
However, the non-uniformity problems arise with increases of the driving frequency.
 A. Perret, Appl. Phys.Lett 86 (2005)
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_02

4. ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD)
Control of the ion energy and angular distribution (IEAD) at the substrate provides the potential for improving plasma processes.
A narrow angular IEAD at the substrate with the majority ion flux perpendicular to the substrate is desired for anisotropic processing.
Edge effects produce slanted IEADs.
S.-B. Wang and A.E. Wendt,
J. Appl. Phys., Vol 88, No.2
B. Jacobs, PhD Dissertation
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_03

5. University of Michigan Institute for Plasma Science & Engr.
GOALS
Results from a computational investigation of ion transport through RF sheaths will be discussed.
Investigate the motion of ion species in the RF pre-sheath and sheath region of CCPs using sub-meshing technique to provide finer resolution at different phase of RF source.
Comparison to experimental results from laser induced fluorescence (LIF) measurements by Low Temperature Plasma Physics Laboratory at UCLA.
Assessment of O2 addition to Ar plasmas.
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_04

6. HYBRID PLASMA EQUIPMENT MODEL (HPEM)
EETM
FKM
PCMCM
Se(r)
Monte Carlo Simulation f(ε) or Electron Energy Equation
Continuity, Momentum, Energy, Poisson equation
Monte Carlo Module
N(r)
Es(r)
Electron Energy Transport Module　（EETM):
Electron Monte Carlo Simulation provides EEDs of bulk electrons.
Separate MCS used for secondary, sheath accelerated electrons.
Fluid Kinetics Module (FKM):
Heavy particle and electron continuity, momentum, energy and Poisson equations.
Plasma Chemistry Monte Carlo Module (PCMCM):
IEADs in bulk, pre-sheath, sheath, and wafers
Recorded phase, submesh resolution
M.Kushner, J. Phys.D: Appl. Phys. 42(2009)
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_05

7. University of Michigan Institute for Plasma Science & Engr.
REACTOR GEOMETRY
Inductively coupled with 2-freq CCP on substrate
2D, cylindrically symmetric.
Base conditions
ICP Power: 400kHz,300 Watt
High Freq RF: 10 MHz
300 Watt 300 Volt
Low Freq RF: 2MHz
100 Watt 150 Volt
Specify power, adjust voltage.
Main Species in Ar
Ar , Ar*, Ar+, e
Main Species in Ar/O2
O2 ,O2*, O2+, O, O*,O+, O-
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_06

8. University of Michigan Institute for Plasma Science & Engr.
PLASMA PROPERTIES
Majority of power deposition that produces ions comes from inductively coupled coils.
Ion acceleration is produced by capacitive coupling.
Plasma distribution determines local sheath thickness, potential and ion mixing ratio at wafer.
Te peaks near coil where E-field is largest.
Electro-static waves due to double layers.
Ion Density (cm-3)
Ar/O2 =0.8/0.2,
20mTorr, 300 SCCM
Freq=2 MHz, 300 Watt
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9. PULSED LASER-INDUCED FLUORESCENCE (LIF)
A non-invasive optical technique for measuring the ion velocity distribution function.
Ions moving along the direction of laser propagation will have the absorption wavelengths Doppler-shifted from λ0,
Ion velocity parallel to the laser obtained fromΔλ=λ0-λL=v//λ0/c
University of Michigan
Institute for Plasma Science & Engr.
B. Jacobs, PRL 105, (2010)
YZHANG_MIPSE2011_08

10. Ar+ IEAD FROM BULK TO SHEATH
IEAD changes significantly through sheath from bulk plasma.
In the bulk plasma and pre-sheath, the IEAD is essentially thermal and broad in angle.
In the sheath, ions are accelerated by the E-field in z direction and the angle narrows.
Ar, 20mTorr, 300 SCCM
HF=30 MHz 100Watt
LF=2 MHz 300Watt
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_09

11. University of Michigan Institute for Plasma Science & Engr.
IEAD NEAR EDGE OF WAFER
IEADs are separately collected over center, middle and edge regions.
Non-uniformity near the edge region - IEAD has broader angular distribution.
Maximum energy consistent regardless of radius.
Center
Middle
Edge
０.5 mm above wafer
Ar, 20mTorr, 300 SCCM
HF=30MHz 100Watt
LF=2 MHz 300Watt
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_10

12. PEAKS IN ION ENERGY DISTRIBUTION vs PHASE
IEAD properties differ during the RF period.
Argon ions are most energetic shortly after the maximum in accelerating field.
Experiments show similar trend.
B.Jacobs, W.Gekelman, PRL 105, (2010)
Ar/O2=0.8/0.2,
0.5 mTorr, 50 SCCM
LF600kHz, 425W
HF=2MHz, 1.5kW
Phase refers to HF
Ar, 20mTorr, 300 SCCM
HF=30MHz 100Watt
LF=2 MHz 300Watt
Phase refer LF
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_11

13. University of Michigan Institute for Plasma Science & Engr.
IEAD UNDER DIFFERENT RF PHASES
B. Jacobs, PhD Dissertation (2010)
IEADs far above wafer are independent of phase, and slowly drifting.
In the pre-sheath, small ion drifts cause the IEAD to slightly change vs phase.
Ar/O2=0.8/0.2,
0.5 mTorr, 50 SCCM
HF600kHz, 425W
LF=2 MHz, 1.5kW
Sheath ~3.6 mm
LIF measured 4.2 mm above wafer
Ar/O2 =0.8/0.2,
20mTorr, 300 SCCM
Freq=2 MHz
IEAD 4 mm above wafer
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_12

14. University of Michigan Institute for Plasma Science & Engr.
IEAD UNDER DIFFERENT RF PHASES
Due to periodic acceleration in sheath, development of IEAD depends on phase.
During low acceleration phases, IEAD drifts in sheath.
During high acceleration phase, IEAD narrows as perpendicular component of velocity distribution increases.
B. Jacobs, W. Gekelman, PRL 105, (2010)
Ar/O2=0.8/0.2,
0.5 mTorr, 50 SCCM
HF600kHz, 425W
LF=2 MHz, 1.5kW
Sheath ~3.6 mm
LIF measured 1 mm above wafer
Ar/O2 =0.8/0.2,
20mTorr, 300 SCCM
Freq=2 MHz
IEAD 0.5 mm above wafer
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_13

15. University of Michigan Institute for Plasma Science & Engr.
O2 ADDITION TO AR
With increasing O2, negative ion ( O2-, O-) formation increases the sheath potential for fixed power.
IEAD for Ar+ extends in energy and narrows in angle.
Ar+ IEAD on wafer
20 mTorr, 300 SCCM.
Freq=2 MHz, 300 W.
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_14

16. University of Michigan Institute for Plasma Science & Engr.
CONCLUDING REMARKS
In the pre-sheath, IEAD is thermal and broad in angle. When the ion flux is accelerated through the sheath, the distribution increases in energy and narrows in angle.
Edge Effect can be observed clearly by using the high resolution afforded by sub-meshing. Multiple peaks in IEADs come from IEADs alternately accelerated by rf field during the whole RF period.
Increasing O2 changes the sheath properties – a narrower IEAD achieved when percentage of O2 increase from 5% to 20%.
University of Michigan
Institute for Plasma Science & Engr.
YZHANG_MIPSE2011_15