aspect ratio dependent twisting and mask effects during plasma etching of sio 2 in fluorocarbon gas...

Aspect Ratio Dependent Twisting and Mask Effects During Plasma Etching of SiO2 in

Fluorocarbon Gas Mixture*

Mingmei Wang1 and Mark J. Kushner2

1Iowa State University, Ames, IA 50011 USA

[email protected]

2University of Michigan, Ann Arbor, MI 48109 [email protected]

http://uigelz.eecs.umich.edu

55th AVS, October 2008, Boston, MA

*Work supported by the SRC, Micron Inc. and Tokyo Electron Ltd.

AGENDA

Issues in high aspect ratio contact (HARC) etching.

Approaches and Methodologies

Electric field buildup due to charge deposition.

Feature twisting; trench to trench variation when etching at critical dimension (CD).

High energy electron (HEE) effects on feature twisting in SiO2 etching over Si.

Varied mesh resolution due to computing limitation.

Photo resist sputtering and redeposition.

Twisting and bowing during etch in features patterned with photo resist (PR) and hard mask (HM).

Concluding Remarks

MINGMEI_AVS08_AGENDA

University of MichiganInstitute for Plasma Science

and Engineering

CHALLENGES IN HARC ETCHING

Etched features for advanced micro-electronic devices have aspect ratios (AR) approaching 100.

Twisting, bowing and consequences of mask erosion challenge maintaining CD.

In this poster, results from a computational investigation of these processes are presented.

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Ref: ULVAC Technologies

Ref: Oxford Instruments

Mask Erosion

Twisting

Ref: JJAP, 46, p7873 (2007)

Bowing


and Engineering

HYBRID PLASMA EQUIPMENT MODEL (HPEM)

Electromagnetics Module: Antenna generated electric and magnetic fields.

Electron Energy Transport Module: Beam and bulk generated sources and transport coefficients.

Fluid Kinetics Module: Electron and Heavy Particle Transport.

Plasma Chemistry Monte Carlo Module:

Ion, Higher Energy Electron (HEE) and Neutral Energy and Angular Distributions.

Fluxes for feature profile model.

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and Engineering

MONTE CARLO FEATURE PROFILE MODEL

Monte Carlo techniques address plasma surface interactions and evolution of surface profiles.

Electric potential is solved using Successive Over Relaxation (SOR) method.

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Ions, HEE, radicals and neutrals

Mask

SiO2

Polymer

Si

-6 1510

Charged particles

++

+

+

+

+

-

+

+

+

--

-

-

--

--

+


and Engineering

SURFACE REACTION MECHANISM

Etching of SiO2 is dominantly through a formation of a fluorocarbon complex.

SiO2(s) + CxFy+(g) SiO2*(s) + CxFy

#(g)

SiO2*(s) + CxFy(g) SiO2CxFy(s)

SiO2CxFy (s) + CxFy+(g) SiFy(g) + CO2 (g) + CxFy

#(g)

Further deposition by CxFy(g) produces thicker polymer layers.

Sputtering of photo resist and redeposition.

PR(s) + CxFy+(g) PR(g) + CxFy

#(g)

PR(g) + SiO2CxFy(s) SiO2CxFy(s) + PR(s)

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and Engineering

FLUOROCARBON ETCHING OF SIO2

Plasma tends to be edge peaked due to electric field enhancement.

Plasma densities in excess of 1011 cm-3.

Ar/C4F8/O2 = 80/15/5, 300 sccm, 40 mTorr, RF 1 kW at 10 MHz, DC 200 W/-250 V.

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DC augmented single frequency capacitively coupled plasma (CCP) reactor.

DC: Top electrode RF: Substrate


and Engineering

10 MHz LOWER, DC UPPER: PLASMA POTENTIAL

LF electrode passes rf current. DC electrode passes combination of rf and dc current with small modulation of sheath potential.

Ar, 40 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V DC: 200 W, -470 V

ANIMATION SLIDE-GIFMINGMEI_AVS08_06


and Engineering

HIGH ENERGY ELECTRON (HEE) FLUXES

HEE fluxes increase with increasing RF bias power due to increase in plasma density.

40 mTorr, RF 10 MHz, DC 200 W/-250 V, Ar/C4F8/O2 = 80/15/5, 300 sccm

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HEE flux increases with increasing DC voltage.

HEE is naturally generated by RF oscillation (when VDC=0 V).

40 mTorr, RF 4 kW/1.5 kV at 10 MHz, Ar/C4F8/O2 = 80/15/5, 300 sccm


and Engineering

ION ENERGY ANGULAR DISTRIBUTIONS (IEADs)

IEADs for sum of all ions.

Peak in ion energy increases with increasing rf bias power while IEAD narrows.

Higher energy ions increase maximum positive charging of feature.

40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 10 MHz, DC 200 W/-250 V.

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and Engineering

HEE energy increases with increasing rf bias power.

Narrower angular distribution (-2 ０ ~ 2 ０ ) than for ions.

Peak at maximum energy with long tails.

40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 10 MHz, DC 200 W/-250 V.

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HEE ENERGY ANGULAR DISTRIBUTIONS


and Engineering

HEE EFFECTS ON TWISTING: FINE MESH Atomic scale mesh size (~3 Å).

Ions hitting the surface deposit charge. Electrons may scatter. Statistical composition of fluxes into small features produces occasional twisting.

Twisting occurs randomly without considering HEE (3/20).

HEE neutralizes charge effectively deep into the trench. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1 kW at 10 MHz, DC 200 W.

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Without HEE With HEE

Different random seedsDifferent random seeds

Aspect Ratio = 1:25

Coarse mesh (~5 nm) with photo resist erosion on the top.

Bowing occurs at later stage of etching due to reflection from sloped profile of eroded PR.

HEE fluxes improve feature profiles.

Trench to trench differences due to small opening (75nm) to the plasma and statistican nature of fluxes.

40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5 kW at 10 MHz.

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Aspect Ratio = 1:20

HEE EFFECTS on TWISTING: COARSE MESH

Without HEE

With HEE


and Engineering

HEE ENERGY ANGULAR DISTRIBUTIONS

HEE energy increases with increasing DC voltage.

Narrower angular distribution is obtained at high voltage with longer tails.

At low energy region (<500 eV), low DC voltage causes broader angular distribution and lower particle density.

40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1.5 kV at 10 MHz.


and EngineeringMINGMEI_AVS08_12

TWISTING ELIMINATION: DC VOLTAGE

Two group of profiles are selected from 21 cases with different random seed number generators.

HEE neutralizes positive charge deep into the trench.

Higher HEE energy and flux produce better profiles and higher etch rates:

VDC=0 V, twisting probability=7/21.



40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1.5 kV at 10 MHz.

Aspect Ratio = 1:20MINGMEI_AVS08_13

Dif

fere

nt

ran

do

m

see

ds


and Engineering

PHOTO RESIST SPUTTERING and PROFILE BOWING

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Aspect Ratio = 1:30ANIMATION SLIDE-GIF


and Engineering

Time sequence of feature etching.

Photo resist is eroded during process broadening view-angle to plasma.

Bowing occurs at later stage of etching as view-angle and slope of PR increases.


PHOTO RESIST SPUTTERING and PROFILE BOWING

MINGMEI_AVS08_14

Aspect Ratio = 1:30 University of MichiganInstitute for Plasma Science

and Engineering

Time sequence of feature etching.

Photo resist is eroded during process broadening view-angle to plasma.

Bowing occurs at later stage of etching as view-angle and slope of PR increases.


MASK MATERIAL EFFECTS

(AR=30)

(AR=40)

(AR=30)

Hard mask is not etched or sputtered easily.

PR has an etching selectivity of ~10 over SiO2.

Bowing occurs at the middle height of trench with the hard mask.

Bowing occurs right under the PR layer.


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and Engineering

Ions & HEE

E-Field

BOWING MECHANISM

With hard mask, as etch depth increases, ions with a small incident angle hit the side wall.

Statistical deposition of charge produces deflection of narrow angle ions.

With photo resist etching, ions hitting PR surface reflect to the side wall of trench.


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PROPOSED METHODS OF BOWING ELIMINATION

Deposit a protective layer onto PR.

Sputtering protective layer away at later stage of etching.

Multiple layers of mask materials (upper PR, lower hard mask).

Increase HEE flux and energy to further neutralize positive charge on trench bottom and side walls.

Control ion energy as the etch proceeds to utilize selectivity difference between PR and SiO2 etching.

PR

HM

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Many methods have been proposed to address bowing.

CONCLUDING REMARKS

HEE effects on eliminating twisting in HARC etching have been computationally investigated in fluorocarbon plasmas.

Statistical nature of ion fluxes into small features produce lateral electric fields which deflect ions.

HEE neutralizes positive charge deep into the trench to eliminate ion trajectory change and accelerate etching.

Photo resist sputtering leads to bowing at top of feature profile.

Bowing occurs at middle of feature in HARC (AR~40) etching.

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aspect ratio dependent twisting and mask effects during plasma etching of sio 2 in fluorocarbon gas...

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