COMPUTATIONAL INVESTIGATION OF DUAL-FREQUENCY POWER TRANSFER IN CAPACITIVELY COUPLED PLASMAS* Yiting...

COMPUTATIONAL INVESTIGATION OF DUAL-FREQUENCY POWER TRANSFER IN

CAPACITIVELY COUPLED PLASMAS*

Yiting Zhang and Mark J. KushnerDepartment of Electrical and Computer Engineering,

University of Michigan, Ann Arbor, 48109([email protected], [email protected])

Sang Ki Nam and Saravanapriyan Sriranman Lam Research Corp., Fremont, CA 94538

([email protected], [email protected])

June 17, 2013

* Work supported by Semiconductor Research Cooperation, National Science Foundation and the DOE Office of Fusion Energy Sciences.

AGENDA

· Dual frequency Capacitively Coupled Plasma (CCPs) and Ion Energy Angular Distributions (IEADs)

· Description of the model· IEADs and plasma properties for dual-frequency Ar

plasma· Voltage control vs. power control· Ratio of dual frequencies· Phase shift

· Concluding remarks

ICOPS_2013

University of MichiganInstitute for Plasma Science & Engr.

DUAL FREQUENCY CCP SOURCES· Dual frequency capacitively coupled discharges (CCPs) are

widely used for etching and deposition in the microelectronics industry.

· High frequencies produce higher electron densities at moderate sheath voltage and higher ion fluxes with moderate ion energies.

· A. Perret, Appl. Phys.Lett 86 (2005)University of Michigan

Institute for Plasma Science & Engr.

· LAMRC 2300 Flex dielectric etch tool

· Low frequencies contribute to the quasi-independent control of the ion flux and energy.

· Coupling between the dual frequencies may interfere with independent control of plasma density, ion energy and produce non-uniformities.

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ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD)

· Control of the ion energy and angular distribution (IEAD) incident onto the substrate is necessary for improving plasma processes.

· A narrow angle, vertically oriented IEAD is necessary for anisotropic processing.

· Edge effects which perturb the sheath often produce slanted IEADs.

● S.-B. Wang and A.E. Wendt, JAP● B. Jacobs, PhD Dissertation


· Ion velocity trajectories measured by LIF (Jacobs et al.)

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CONTROL OF IEADs

· As the size of transistors shrink, the critical dimension requirements of semiconductor fabrication become more stringent, and therefore more precise control of IEADs becomes important.


· SEM of a high aspect ratio profile

· This computational investigation addresses plasma dynamics and control of IEADs onto wafers in dual frequency CCPs.

· Controlling the voltage, power and phase difference between dual-frequencies to customize IEADs will be discussed.

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HYBRID PLASMA EQUIPMENT MODEL (HPEM)

Monte Carlo Simulation f(ε) or Electron Energy Equation

· 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’s

equations. · Plasma Chemistry Monte Carlo Module (PCMCM):

· IEADs in bulk, pre-sheath, sheath, and wafers.· Recorded phase, submesh resolution.

EETM

Continuity, Momentum, Energy, Poisson equation

FKM

Monte Carlo Module

PCMCMSe(r)

N(r)Es(r)


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REACTOR GEOMETRY

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· Capacitively coupled plasma with multi-frequency rf biases on bottom electrode.

· 2D, cylindrically symmetric. · Ar plasma: Ar, Ar(1s2,3,4,5),

Ar(4p), Ar+, e

· Base case conditions:· Ar, 30 mTorr, 1000 sccm· Voltage Control (VC):

2 MHz, 300 V; 60 MHz, 300 V· Power Control (PC): 2 MHz, 300 W; 60 MHz, 300 W

(Note: Y:X = 2:1)

· Real geometry aspect ratio

ELECTRON DENSITY, TEMPERATURE


· VC: 300 V, 2 MHz; 300 V, 60 MHz · PC: 300 W, 2 MHz; 300 W, 60 MHz

● Ar, 30 mTorr, 1000 sccm

· HF mainly contributes to ionization, and thus the current generated by HF is larger than the LF current.

· With voltage control, larger HF power (2926 W total) generates higher ne. With power control, relatively low HF (51 V) due to higher efficiency of electron heating generates lower electron density.

· Uniformity increases with higher ionization.

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Ar+ IEAD FROM BULK TO SHEATH· VC :300 V, 2 MHz; 300 V, 60 MHz

· The lower ne with PC produces a thicker time averaged sheath thickness.

· Longer ion transit time in thick sheath makes the IED curve smoother and lower in energy.

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· IED on wafer

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· PC :300 W, 2 MHz, 300 W 60 MHz

DC bias -123 V

DC bias -256 V

MIN MAX

HIGH FREQUENCY VOLTAGE· The plasma impedance changes little, total power increases

with VHF2.

· With higher HF voltage, plasma density increases significantly.· Higher power correlates with better uniformity.


● Ar, 30 mTorr, 1000 sccmICOPS_2013 MIN MAX

· Changing HF/LF voltage ratio will strongly affect the IEADs. With higher HF, self generated DC bias becomes more negative, and the total IEAD shifts to higher level due to higher sheath potential during anodic LF cycle.

· Broadening in high energy peaks due more HF modulation. The energy width is almost independent of HF amplitude.


HIGH FREQUENCY VOLTAGE


DC bias- 256 V

DC bias- 355 V

DC bias- 459 V

· Increasing LF (300 V to 600 V) has little effect on ne and Te since electron heating scales with 2.

· Majority of additional power results in ion acceleration.


LOW FREQUENCY VOLTAGE


· Increasing LF voltage shapes IED (e.g., ΔE) with little change in plasma properties.

· During the cathodic LF cycle, increase in sheath potential accelerates ions to higher energy.

· During anodic LF cycle, sheath potential is dominated by HF which is unchanged – and so modulation of IED persists.


LOW FREQUENCY VOLTAGE


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- 256 V, DC - 307 V, DC - 371 V, DC

HIGH FREQ POWER

· With increase in HF power 300 W to 900 W, HF voltage changes by only 30 V.

· Amplitude is always small compared to LF, and so width of IEAD does not significantly change.

· Increase in ne with power reduces sheath width which then modulates IEAD at HF.



MIN MAX ICOPS_2013

LOW FREQ POWER

· The plasma density and uniformity change little with LF power.

· At low frequency, LF voltage increases nearly linear with power.

· The LF power is mainly deposited into sheath – the width of IED increases with LF power.



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IED DEPENDENCE ON ΔPHASE

● Ar, 30 mTorr, 1000 sccmICOPS_2013

· Energy of HF modulated peaks in IED depend on relative phase between LF and HF.

· Shift of energy of peaks depends on value of high frequency due in part to change in sheath thickness.

MIN MAX


· Phase difference between LF and HF modulates sheath potential and electron dynamics during rf period.

· The sheath thickness (scales with [e]-1/2 ) is larger in cathodic LF cycle, and so brings about less modulation in high energy peak of IED.

· By dynamically controlling phase difference, a smooth time averaged IED can be produce without HF modulation.

300V 2 MHz300V 60 MHz

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SHEATH vs ΔPHASE

CONCLUDING REMARKS


· For dual frequency CCPs sustained in Ar plasma,· With higher frequency , the current generated by HF is

greater than the LF current. · Increasing HF voltage will increase the plasma density

as well as shift the total IEADs to higher energies. · Increasing LF voltage will mainly deposit power within

sheath, and therefore extend the IEAD energy width with little change in composition of fluxes.

· Changing phase between HF and LF in high density, thin sheath plasma will modify time averaged IEADs significantly.

· With knowledge of the relationship between IEADs and settings of dual frequency rf biases, precise customization and control of IEADs can be achieved.

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COMPUTATIONAL INVESTIGATION OF DUAL-FREQUENCY POWER TRANSFER IN CAPACITIVELY COUPLED PLASMAS* Yiting...

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