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

SURFACE MODIFICATION OF POLYMER PHOTORESISTS TO PROTECT PATTERN TRANSFER IN FLUOROCARBON PLASMA ETCHING * Mingmei Wang a) and Mark J. Kushner b) a) Iowa State University, Ames, IA 50011 USA mmwang@iastate.edu b) University of Michigan, Ann Arbor, MI 48109 USA mjkush@umich.edu

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

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  1. 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 mmwang@iastate.edu b)University of Michigan, Ann Arbor, MI 48109 USA mjkush@umich.edu http://uigelz.eecs.umich.edu 62th GEC, October 2009, Saratoga Springs, NY *Work supported by Tokyo Electron Ltd. and Semiconductor Research Corp. MW_GEC2009

  2. University of Michigan Institute for Plasma Science & Engr. 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 MW_GEC2009

  3. Ions Bulk Plasma + + + CxFy+ Ar+ PR Si+ O2+ O+ F2+ Cx-1Fy-1 Polymer O,F SiO2 Ar Si O,F F O C Substrate (SiO2, Si or PR) University of Michigan Institute for Plasma Science & Engr. IMPLANTATION and MIXING DURING PLASMA ETCHING • 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. MW_GEC2009

  4. University of Michigan Institute for Plasma Science & Engr. MOLECULAR DYNAMICS SIMULATION on MIXING D. Humbird, D. B. Graves et. al., J. Vac. Sci. Technol. A, Vol. 25, 2007 • 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. MW_GEC2009

  5. Hybrid Plasma Equipment Model (HPEM) Sources Fields Transport coefficients Si Plasma Chemistry Monte Carlo Model (PCMCM) SiO2 Fluxes Energy angular distributions Monte Carlo Feature Profile Model (MCFPM) University of Michigan Institute for Plasma Science & Engr. DESCRIPTION OF MODEL Sputtering Yields Range of Ions Implantation / Mixing MW_GEC2009

  6. End Start Gas-solid surface interaction Surface reaction No Yes Implant Stopping range  = f(in) No Implant Yes Mixing a Move to next cell No /in  R* Yes Exchange identity University of Michigan Institute for Plasma Science & Engr. IMPLANTATION MODEL Ar+,F+,Si+ C+,O+  Pushed out in n<N* out a Mixing Implant SiO2,Si or PR MCFPM Mesh 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. *R = Random number MW_GEC2009

  7. University of Michigan Institute for Plasma Science & Engr. 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. • 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) *PR2 = cross-linked PR MW_GEC2009

  8. FLUOROCARBON ETCHING of SiO2 University of Michigan Institute for Plasma Science & Engr. • DC augmented single frequency capacitively coupled plasma (CCP) reactor. • DC: Top electrode RF: Substrate • 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. MW_GEC2009

  9. University of Michigan Institute for Plasma Science & Engr. 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. MW_GEC2009

  10. University of Michigan Institute for Plasma Science & Engr. 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). MW_GEC2009

  11. (a) (b) (c) University of Michigan Institute for Plasma Science & Engr. ETCHING SELECTIVITY vs ENERGY • 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. MW_GEC2009

  12. University of Michigan Institute for Plasma Science & Engr. PR EROSION vs ASPECT RATIO • 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 MW_GEC2009

  13. University of Michigan Institute for Plasma Science & Engr. 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

  14. University of Michigan Institute for Plasma Science & Engr. 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) MW_GEC2009

  15. University of Michigan Institute for Plasma Science & Engr. VUV BOND BREAKING and PHOTOLYSIS Average Bond Energy* * Organic Chemistry, Michigan State University • 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. MW_GEC2009

  16. University of Michigan Institute for Plasma Science & Engr. 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. MW_GEC2009

  17. University of Michigan Institute for Plasma Science & Engr. 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. MW_GEC2009

  18. University of Michigan Institute for Plasma Science & Engr. 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. • Ar/C4F8/O2=80/15/5, HF 500 W, LF 4 kW, DC 2 kW, 40 mTorr, 300 sccm. MW_GEC2009

  19. University of Michigan Institute for Plasma Science & Engr. 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. • Ar/C4F8/O2=80/15/5, HF 500 W, LF 4 kW, DC 2 kW, 40 mTorr, 300 sccm. MW_GEC2009

  20. University of Michigan Institute for Plasma Science & Engr. 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. • Ar/C4F8/O2=80/15/5, HF 500 W, LF 4 kW, DC 2 kW, 40 mTorr, 300 sccm. MW_GEC2009

  21. University of Michigan Institute for Plasma Science & Engr. 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. MW_GEC2009

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