* Work supported by Tokyo Electron Ltd. and Semiconductor Research Corp.

SURFACE MODIFICATION OF POLYMER PHOTORESISTS TO PROTECT PATTERN TRANSFER

IN FLUOROCARBON PLASMA ETCHING*

Mingmei Wanga) and Mark J. Kushnerb)

a)Iowa State University, Ames, IA 50011 USA

[email protected]

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

http://uigelz.eecs.umich.edu

62th GEC, October 2009, Saratoga Springs, NY

*Work supported by Tokyo Electron Ltd. and Semiconductor Research Corp.

MW_GEC2009

AGENDA

Consequences of ion induced cross-linking on etch rates and photoresist (PR) CD control.

Description of the model

Scaling of mixing and implantation with Ion Energy Distributions

Strategies to control PR erosion

VUV induced degradation and cross-linking of PR

Si extraction (SiFx) and deposition on PR and CFx polymer

Concluding Remarks

University of MichiganInstitute for Plasma Science & Engr.MW_GEC2009

Ions

PR

SiO2

University of MichiganInstitute for Plasma Science & Engr.

CxFy+

CF

Cx-1Fy-1

+

O+

O

+

Si+

O,F

Ar+

+

O,F

Bulk Plasma

Substrate (SiO2, Si or PR)

Polymer

O2+

F2+

Si Ar

Small ions accelerated by the sheath implant into the wafer surface forming weakly bonded or interstitially trapped species causing mixing and damage during plasma etching.

PR sputtering and ion-induced composition changes change PR facets which affect profile during high aspect ratio (HAR) etching.

Develop computational infrastructure to investigate implantation effects.

IMPLANTATION and MIXING DURING PLASMA ETCHING

MW_GEC2009


D. Humbird, D. B. Graves et. al., J. Vac. Sci. Technol. A, Vol. 25, 2007

MOLECULAR DYNAMICS SIMULATION on MIXING

Mixing of Si crystal due to Ar+ bombardment was investigated using MD simulation.

Scaling of amorphous layer thickness with ion energy showed a good correlation.

Amplification faces difficulties due to huge amount of calculations.

DESCRIPTION OF MODEL


Hybrid Plasma Equipment Model (HPEM)

Monte Carlo Feature Profile Model (MCFPM)

Plasma Chemistry Monte Carlo Model (PCMCM)

SourcesFieldsTransport coefficients

FluxesEnergy angular distributions

Sputtering Yields Range of Ions

Implantation / Mixing

SiSiO2

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Within one cell: out= in exp(-a/)

Where in = incident energy; out = left energy.

a = Actual length that the particle travels.

= Calculated stopping range f(in).

*n = mixing step; N = allowed maximum mixing step.

IMPLANTATION MODEL

*R = Random number

ImplantSurface reaction

Stopping range

= f(in)

a Implant

Move to next cell

/in R*

Exchange identity

Mixing

Start

Gas-solid surface

interaction

End

No

Yes

No

Yes

Yes

No

Mixing

in

a out

Implant

Ar+,F+,Si+

C+,O+

SiO2,Si or PR

Pushed out n<N*

MCFPM Mesh

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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.

Example reaction of surface dissociation.

M(s) + CxFy+(g) M(s) + Cx-1Fy-1(g) + C(g) + F(g)

Ions on PR sputter, produce cross-linking and redeposit PR.

PR(s) + Ar+(g) PR2(s) + Ar(g) + H(g) + O(g)

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

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

SURFACE REACTION MECHANISM


*PR2 = cross-linked PR

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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.

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

DC: Top electrode RF: Substrate



ION ENERGY ANGULAR DISTRIBUTIONS (IEADs)

Peak of ion energy ranges from 300 to 1200 eV for 1 – 4 kW bias power.

Angle distribution spreads from -10 to 10 degree .

Stopping range in surface materials ranges from 0 to 70 Å.

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IMPLANTING and MIXING DEPTH vs ENERGY

Only polymer deposition occurs at 1 eV.

Sputtering, implanting and deposition coexist at 10 eV.

Depth of implantation and mixing increases with increasing ion energy (100 eV~10 keV).

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Etching rate for SiO2 increases with increasing ion energy.

Balance between sputtering and cross-linking (more resistive to etching) on PR (PMMA) surface results in similar etching rate for all energies.

Surface roughness of SiO2 increases as etching proceeds due to micro-masking.

Etching selectivity (SiO2/PR): 100 eV = 6; 500 eV = 18; 1000 eV = 23.

ETCHING SELECTIVITY vs ENERGY

(a) (b) (c)

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PR EROSION vs ASPECT RATIO

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Cross-linking of PR due to ion bombardment protects PR.

Selectivity to SiO2 is 10.

As AR increases, PR is eroded slowly.

For AR>16, PR is depleted.

Other strategies are needed to better retain CD.

Ar/C4F8/O2 = 80/15/5, 300 sccm, 40 mTorr, 10 MHz, DC 200 W/-250 V, RF 4 kW.(AR = 7 12 16 22)

Animated Slide-GIF


STRATEGY to ELIMINATE PR EROSION

In DC-CCP, large fluxes of Si (in addition to VUV fluxes) may be incident on wafer and PR.

Deposition of Si and formation of Si-C layers may improve PR selectivity.

Si easily extracts one or two F from polymer CxFy to promote further polymer deposition.

MW_GEC2009

STRATEGY to ELIMINATE PR EROSION


Step 1: PR and CFx Activation

PR(s) + VUV PR*(s) + PR(g) CxFy(s) + Si(g) CxFy*(s) + SiFx(g)

Step 2: Deposition of Si, CFx, Passivation

PR*(s) + Si(g) PR(s) + Si(s) PR*(s) + CxFy(g) PR(s) + CxFy(s) Si(s) + CxFy(g) Si(s) + CxFy (s) Si(s) + F(g) SiFx(s)

Step 3: VUV Photoablation, activation

CxFy(s) + VUV CxFy*(s) + CxFy(g)

Step 4: Further Deposition

SiFx(s) + CxFy(g) SiFx(s) + CxFy(s) CxFy*(s) + CxFy(g) CxFy(s) + CxFy(s)

Polymer (CxFy) PR (PMMA) Si or SiO2

Bond C-F (methyl)

C-F(ethyl)

C-F(i-

propyl)C-C C-C

C-H(methyl

)

C-H(ethyl)

C-H(i-

propyl)C=O Si-Si Si-O

ΔH (eV/bond)

4.77 4.77 4.73 3.60 3.60 4.47 4.25 4.12 7.72 2.25 4.77

* Organic Chemistry, Michigan State University

VUV BOND BREAKING and PHOTOLYSIS

Average Bond Energy*

MW_GEC2009


VUV resonant radiation from Ar produces lines at ~105 nm (11.8 eV).

Photon energy able to break all “first bonds” in PMMA, polymer, Si, SiO2.

Isotropic VUV fluxes are onto and absorbed in top layers of features.

Interactions of VUV with PR are important in PR erosion and surface activation.


IEADs on TOP and BOTTOM ELECTRODES

(Top electrode) (Bottom electrode)

Ion energy increases with increasing DC power on top electrode.

On bottom electrode, ion energy is almost unchanged when varying DC power.

AR, HF 500 W at 60 MHz, LF 4 kW at 5 MHz, 40 mTorr, 300 sccm.

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0 500 1000 1500 200010

15

20

25

30

S

i Flu

x / T

ota

l Io

n F

lux

(%)

DC Power (W)

0 500 1000 1500 20000.0

0.2

0.4

Ph

oto

n F

lux

/ To

tal I

on

Flu

x (%

)

DC Power (W)


0 500 1000 1500 2000

5

10

15

20

Total Ion

Flu

x (x

1015

cm

-2s-1

)

DC Power (W)

Si

FLUXES at WAFER CENTER

At wafer center Si/Ion flux increases with DC power.

Photon/ion flux does not have clear correlation with DC power.

• AR, HF 500 W, LF 4 kW, 40 mTorr, 300 sccm.

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PROTECTING PR WITH VUV and Si FLUXES


Without Si and VUV exposure, PR is slowly etched (~8 nm/min).

Cross linking by VUV flux has a small effect.

Si flux ultimately increases polymer deposition and Si-C rich layer.

Combination of VUV and Si induces more polymer deposition.

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• Ar/C4F8/O2=80/15/5, HF 500 W, LF 4 kW, DC 2 kW, 40 mTorr, 300 sccm.


VUV FLUX vs PR ETCHING


Increasing VUV flux induces more cross-linking and activated surface sites.

Cross-linked PR is more resistive to etch.

With highly cross-linked PR at high VUV flux, polymer deposition dominates.

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Si FLUX vs PR ETCHING

Si deposition and its promotion of polymer deposition protects PR from sputtering and erosion.

Sensitivity of balance of PR etching and deposition with respect to Si flux is being investigated.


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CONCLUDING REMARKS

Implantation has been investigated as damage mechanism and hardening of PR through cross linking.

PR hardening scales similarly to sputtering – weak effect.

Mixing at interfaces increases with ion energy.

Consequences of Si fluxes sputtered from dc electrodes studied in concert with VUV fluxes.

High VUV fluxes (~1014 cm-2s-1) produce highly cross-linked PR surface.

Si fluxes produce Si-C hardened surface and promote CFx deposition.

Net effect is preservation of PR.


* Work supported by Tokyo Electron Ltd. and Semiconductor Research Corp.

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Transcript of * Work supported by Tokyo Electron Ltd. and Semiconductor Research Corp.