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

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

IMPLANTATION and MIXING DURING PLASMA ETCHING Ions PR SiO2 CxFy+ C F Cx-1Fy-1 + O+ O Si+ O,F Ar+ 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. University of Michigan Institute for Plasma Science & Engr. MW_GEC2009

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

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

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

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 University of Michigan Institute for Plasma Science & Engr. MW_GEC2009

FLUOROCARBON ETCHING of SiO2 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. University of Michigan Institute for Plasma Science & Engr. MW_GEC2009

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

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

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

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) University of Michigan Institute for Plasma Science & Engr. Animated Slide-GIF MW_GEC2009

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

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

VUV BOND BREAKING and PHOTOLYSIS Average Bond Energy* Polymer (CxFy) PR (PMMA) Si or SiO2 Bond C-F (methyl) (ethyl) (i-propyl) C-C C-H C=O Si-Si Si-O ΔH (eV/bond) 4.77 4.73 3.60 4.47 4.25 4.12 7.72 2.25 * 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. University of Michigan Institute for Plasma Science & Engr. MW_GEC2009

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

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

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

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

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

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