1. Consequences of Implanting and Surface Mixing During Si and SiO 2 Plasma Etching* Mingmei Wang 1 and Mark J. Kushner 2 1 Iowa State University, Ames, IA 50011 USA mmwang@iastate.edu 2 University of Michigan, Ann Arbor, MI 48109 USA mjkush@umich.edu http://uigelz.eecs.umich.edu 36th ICOPS, June 2009, San Diego, CA *Work supported by Tokyo Electron Ltd. Micron Inc. and the SRC. MINGMEI_ICOPS09_01

2. AGENDA  Implanting, mixing and photoresist (PR) erosion  Processes  Molecular ion dissociation on surfaces  Small ion penetration and mixing  PR cross-linking  Description of Model  Scaling of Mixing and Implantation  Concluding Remarks MINGMEI_ICOPS09_02 University of Michigan Institute for Plasma Science & Engr.

3. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_03 CxFy+CxFy+ C F C x-1 F y-1 + O+O+ O + Si + O,F Ar + + O,F Bulk Plasma Substrate (SiO 2, Si or PR) Polymer O2+O2+ F2+F2+ Si Ar  Small ions accelerated by the sheath implant into the wafer surface forming weakly bonded or interstitially trapped species.  Implanting causes surface mixing which produces damage during plasma etching. C O F Si Ar Diamond structure of Si IMPLANTATION AND MIXING DURING PLASMA ETCHING

4.  Photoresist defines the features to be etched – usually a hydrocarbon polymer.  PR is sputtered by energetic ions. Frag- mented PR segments are more easily eroded.  Profile of high aspect ratio (HAR) features can be modified due to PR erosion. C H O PHOTORESIST (PR) EROSION University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_04 Ions PR SiO 2 Bulk Plasma PR

5.  Consequences of implantation and mixing are poorly characterized in modeling of plasma etching.  Usually only included in compute-intensive molecular dynamics simulations.  Incorporate implantation, mixing, and sputtering into Monte-Carlo Feature Profile Model coupled to equipment scale plasma models.  Investigate:  Scaling of implantation, mixing, and etching selectivity.  Degradation and cross-linking of PR surface due to energetic bombardment.  Goals  Characterize mixing damage during etching.  Develop strategies to preserve pattern transferring while minimizing damage. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_05 APPROACHES AND GOALS OF INVESTIGATION

6. DESCRIPTION OF MODEL University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_06 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 Sputtering Yields Range of Ions Implantation / Mixing Si SiO 2

7. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_07 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 Implant Surface reaction Stopping range = f(  in ) aa Implant Move to next cell  /  in  R* Exchange identity Mixing Start Gas-solid surface interaction End No Yes No Yes No j=1 2 3 4 5 i=12345 Mixing   in a  out Implant Ar +,F +,Si + C +,O + SiO 2,Si or PR Pushed out n<N*

8.  Etching of SiO 2 is dominantly through a formation of a fluorocarbon complex.  SiO 2 (s) + C x F y + (g)  SiO 2 *(s) + C x F y (g)  SiO 2 *(s) + C x F y (g)  SiO 2 C x F y (s)  SiO 2 C x F y (s) + C x F y + (g)  SiF y (g) + CO 2 (g) + C x F y (g)  Further deposition by C x F y (g) produces thicker polymer layers.  Example reaction of surface dissociation.  M(s) + C x F y + (g)  M(s) + C x-1 F y-1 (g) + C(g) + F(g)  Sputtering of PR and redeposition.  PR(s) + Ar + (g)  PR2(s) + Ar(g) + H(g) + O(g)  PR(s) + C x F y + (g)  PR(s) + C x F y (g)  PR(g) + SiO 2 C x F y (s)  SiO 2 C x F y (s) + PR(s) SURFACE REACTION MECHANISM University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_08 *PR2 = cross-linked PR

9. FLUOROCARBON ETCHING OF SiO 2  Plasma tends to be edge peaked due to electric field enhancement.  Plasma densities in excess of 10 11 cm -3.  Ar/C 4 F 8 /O 2 = 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 University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_09

10. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_10 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 ranges from 0 to 70 Å.

11. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_11 STOPPING RANGE IN PR, POLYMER, Si, AND SiO 2  Stopping range increases with increasing energy.  At specific energy, implanting depth: PR(PMMA*) > SiO 2 > Si > Polymer *PMMA = Polymethylmethacrylate  Data from SRIM (the Stopping and Range of Ion in Matter) Stopping range = f(  incident )

12. MINGMEI_ICOPS09_12 (a) (b) University of Michigan Institute for Plasma Science & Engr.  Etch rates, degree of mixing and depth of implantation increase with bias power. IMPLANTING AND MIXING DEPTH vs BIAS POWER  After 5 s  After same etch level

13. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_13 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).

14. 3 nm 30 nm University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_14 ETCHING SELECTIVITY  Ar/C 4 F 8 /O 2 = 80/15/5, 4 kW, 300sccm, 40mTorr, DC 200W/-250V.  Etch stop occurs at Si surface due to low reaction rate with CF x.  Etching selectivity for SiO 2 /PR(PMMA) is around 10.  The roughness of SiO 2 surface is due to non-uniform polymer deposition. Micro-masking Micro-trenching

15. Structure of PR(PMMA) University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_15A  PR molecule is degraded due to energetic ion sputtering.  Degraded PR segments are more easily eroded.  Newly formed dangling bonds are recombined (cross-linking), and cross-linked PR forms a “hard crust”.  Sputtering yield is calculated using SRIM. MECHANISM FOR DEGRADATION AND CROSS-LINKING OF PR CCH 2 CH 3 …CH 2 CCH 2 CH 3 CCH 2 … CH 3 C O OC O OC O O

16. CCH 2 CH 3 …CH 2 CCH 2 CH 3 CCH 2 … CH 3 C O OC O OC O O DEGRADATION OF PR (SPUTTERING) Broken Bond University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_15B

17. CCH 2 CH 3 …CH 2 CCH 2 CCH 2 … CH 3 C O OC O OC O O H University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_15C Broken Bond DEGRADATION OF PR (SPUTTERING)

18. CCH 2 CH 3 …CH 2 CCH 2 CH 3 CCH 2 … CH 3 C O CH 2 OC O OC O CH 3 O H H INTER-MOLECULE CROSS-LINKING University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_15D New Bond Broken Bond

19. CCH 2 CH 3 …CH 2 CCH 2 CCH 2 … CH 2 C O OC O OC O CH 3 O H H H H University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_15E New Bond Broken Bond INTER-MOLECULE CROSS-LINKING

20. CCH 2 CH 3 …CH 2 CCH 2 CCH 2 … CH 2 C O OC O CH 3 O C OCH 2 O H H H H University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_15F New Bond Broken Bond INTER-MOLECULE CROSS-LINKING

21. Molecule B Molecule A MINGMEI_ICOPS09_15G New Bond Broken Bond INTRA-MOLECULE CROSS-LINKING

22. Molecule B MINGMEI_ICOPS09_15H New Bond Broken Bond INTRA-MOLECULE CROSS-LINKING Molecule A

23. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_16  Etching rate for SiO 2 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 SiO 2 increases as etching proceeds due to micro- masking.  Etching selectivity (SiO 2 /PR): 100 eV, 6; 500 eV, 18; 1000 eV, 23. ETCHING SELECTIVITY vs ENERGY (a)(b)(c)

24. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_17 ETCHING SELECTIVITY vs RF BIAS POWER  At similar etching level of PR, aspect ratio (AR) of the trench increases with bias power.  Etching selectivity increases when PR is cross-linked.  Si is damaged during over- etch by implantation and mixing.  Ar/C 4 F 8 /O 2 = 80/15/5, 300 sccm, 40 mTorr, 10 MHz, DC 200 W/-250 V. 4 kW SiO 2 Si PR 1 kW4 kW No mixing, no sputtering Si AR = 9 14 13

25. University of Michigan Institute for Plasma Science & Engr. MINGMEI_ICOPS09_18 CONCLUDING REMARKS  Algorithms have been incorporated into the MCFPM to predict implanting and mixing.  Depth of implanting and mixing increases with increasing bias power and ion energy.  More damage is obtained at higher etching rate.  PR surface is cross-linked due to sputtering.  At higher bias power and ion energy, etching selectivity for SiO 2 /PR is better.  Strategies to be developed for high power processing:  Protect pattern transferring at higher etching rate.  Reduce damage during over-etch.