1. EFFECT OF BIAS VOLTAGE WAVEFORMS ON ION ENERGY DISTRIBUTIONS AND FLUOROCARBON PLASMA ETCH SELECTIVITY* Ankur Agarwal a) and Mark J. Kushner b) a) Department of Chemical and Biomolecular Engineering Email: aagarwl3@uiuc.edu b) Department of Electrical and Computer Engineering Email: mjk@uiuc.edu University of Illinois Urbana, IL 61801, USA http://uigelz.ece.uiuc.edu 51 st AVS Symposium, November 2004 * Work supported by the NSF, SRC and VSEA

2. University of Illinois Optical and Discharge Physics AGENDA  Introduction  Bias Voltage Waveforms  Approach and Methodology  Ion Energy Distribution Functions  Fluorocarbon Etch Selectivity  Etching Recipes  Summary ANKUR_AVS04_Agenda

3. University of Illinois Optical and Discharge Physics HIGH ETCH SELECTIVITY  High etch selectivity is a necessary characteristic for semiconductor manufacturing.  Prevents erosion of photoresist and/or underlying films.  Permits over-etching to compensate for process nonuniformities. ANKUR_AVS_01  Low Etch Selectivity  Substrate damage  Improper etch stop layer  High Etch Selectivity  Little Substrate damage  Proper etch stop layer

4. University of Illinois Optical and Discharge Physics ETCH MECHANISM  CF x and C x F y form a polymeric passivation layer which regulates delivery of etch precursors and activation energy.  Chemisorption of CF x produces a complex at the oxide-polymer interface. ANKUR_AVS_02  Low energy ion activation of the complex produces polymer.  The polymer layer is sputtered by energetic ions  The complex formed at the oxide- polymer interface undergoes ion activated dissociation to form volatile etch products (SiF 3, CO 2 ).

5. University of Illinois Optical and Discharge Physics ACHIEVING HIGH SELECTIVITY ANKUR_AVS_03 Ref: S.-B. Wang and A.E. Wendt, J. Vac. Sci. Technol. A, 19, 2425 (2001)  High etch selectivity is achieved by controlling the ion energy distribution at the substrate.  Sinusoidal bias: Broad ion energy distribution does not discriminate thresholds (narrow process window).  Tailored bias: Produce a narrow ion energy distribution which discriminates between threshold energies (broad process window).  Ion activation scales inversely with polymer thickness, while polymer thickness scales inversely with bias. Sinusoidal Bias

6. University of Illinois Optical and Discharge Physics VALIDATION OF REACTION MECHANISM ANKUR_AVS_04  The reaction mechanism has been validated with experiments by Oehrlein et al using C 4 F 8, C 4 F 8 /Ar, C 4 F 8 /O 2. 1  Larger ionization rates result in larger ion fluxes in Ar/C 4 F 8 mixtures. This increases etch rates.  With high Ar, the polymer layers thins to submonolayers due to less deposition and more sputtering and so lowers etch rates. Ref: A. Sankaran and M.J. Kushner, J. Vac. Sci. Technol. A, 22, 1242 (2004) 1 Li et al, J. Vac. Sci. Technol. A, 20, 2052 (2002)

7. University of Illinois Optical and Discharge Physics CUSTOM BIAS VOLTAGE WAVEFORMS  Ion Energy Distribution (IED) traditionally controlled by varying the amplitude of a sinusoidal voltage waveform.  Resultant IED – broad; both high and low energy ions  Specially tailored non-sinusoidal bias voltage waveform  Narrow IED at the substrate  Peak of IED can be positioned to achieve desired selectivity ANKUR_AVS_05  Synthesized voltage Waveform:  Periodic  Short voltage spike  Ramp down Ref: S.-B. Wang and A.E. Wendt, J. Vac. Sci. Technol. A, 19, 2425 (2001)  The “10% Waveform

8. University of Illinois Optical and Discharge Physics INTEGRATED MODELING  HPEM (Hybrid Plasma Equipment Model) is the reactor scale model platform.  Low pressure (<10’s Torr)  2-d and 3-d versions  Address ICP, CCP, RIE  HPEM is linked to profile simulators – MCFPM (Monte Carlo Feature Profile Model) to predict the evolution of submicron features.  2-d and 3-d  Fluxes from HPEM ANKUR_AVS_06  An integrated reactor and feature scale modeling hierarchy was developed to model plasma processing systems.

9. University of Illinois Optical and Discharge Physics HYBRID PLASMA EQUIPMENT MODEL ANKUR_AVS_07  A modular simulator addressing low temperature, low pressure plasmas.  Electro-magnetic Module:  Electromagnetic Fields  Magneto-static Fields  Electron Energy Transport Module:  Electron Temperature  Electron Impact Sources  Transport Coefficients  Fluid Kinetics Module:  Densities  Momenta  Temperature of species  Electrostatic Potentials

10. University of Illinois Optical and Discharge Physics MONTE CARLO FEATURE PROFILE MODEL ANKUR_AVS_08  Monte Carlo based model to address plasma surface interactions and evolution of surface morphology and profiles.  Inputs:  Initial material mesh  Etch mechanisms (chemical rxn. format)  Energy and Angular dependence  Gas species flux distribution used to determine the launching and direction of incoming particles.  Flux distributions from equipment scale model (HPEM)

11. University of Illinois Optical and Discharge Physics DYNAMIC SIMULATION – REACTOR SCALE  Transformer-coupled plasma (TCP) reactor geometry  To accelerate ions to the wafer, a rf bias voltage is applied.  Base case conditions:  Ar/C 4 F 8 = 75/25, 100 sccm  15 mTorr, 500 W  200 V p-p, 5 MHz  “10%” Voltage Waveform ANKUR_AVS_09

12. University of Illinois Optical and Discharge Physics REACTANT FLUXES ANKUR_AVS_10  Polymer formation – Low energy process  Polymer sputtering and etch activation – High energy  15 mTorr, 500 W, 200 V p-p, Ar/C 4 F 8 = 75/25, 100 sccm  Dominant Ions: Ar +, CF 3 +, CF +  Dominant Neutrals: CF, C 2 F 3, F

13. University of Illinois Optical and Discharge Physics ION ENERGY DISTRIBUTION FUNCTIONS  Custom waveform produces constant sheath potential drop resulting in narrow IED.  Sheath transit time is short compared to pulse period  Energy depends on instantaneous potential drop.  As duration of positive portion of waveform IEDs broaden in energy.  15 mTorr, 500 W, 200 V p-p, Ar/C 4 F 8 = 75/25, 100 sccm ANKUR_AVS_11 V dc : 42 46 56 64 75 -73

14. University of Illinois Optical and Discharge Physics IEAD vs CUSTOM BIAS WAVEFORMS  As duration of positive portion of waveform is increased, IEDs broaden in energy.  Waveforms attain form as sinusoidal waveform  Increasing waveform beyond 50% narrows the IEDs again as dc characteristic is obtained.  15 mTorr, 500 W, 200 V p-p, 5 MHz, Ar/C 4 F 8 = 75/25, 100 sccm ANKUR_AVS_12 V dc : -73 -25 -21 -19 -12 13

15. University of Illinois Optical and Discharge Physics IEAD vs CUSTOM BIAS VOLTAGE  The peak energy of the IEAD is controlled by amplitude and frequency.  IED broadens at higher biases due to thickening of sheath and longer transit times.  IED still narrower compared to sinusoidal voltage waveform.  15 mTorr, 500 W, Ar/C 4 F 8 = 75/25, 100 sccm ANKUR_AVS_13

16. University of Illinois Optical and Discharge Physics ETCH PROFILES – CUSTOM VOLTAGE WAVEFORM ANKUR_AVS_14 5 %8 %10 %12 % ANIMATION NEXT SLIDE  X % indicates percent of cycle with positive voltage  Low X % have IEADs which produce etch stops.

17. University of Illinois Optical and Discharge Physics ETCH PROFILES – CUSTOM VOLTAGE WAVEFORM ANKUR_AVS_14 5 %8 %10 %12 % ANIMATION SLIDE MASK SiO 2 Si  X % indicates percent of cycle with positive voltage  Low X % have IEADs which produce etch stops.

18. University of Illinois Optical and Discharge Physics FLUOROCARBON PLASMA ETCH SELECTIVITY  Maximum Etch Rate for the 10 % waveform.  12 % waveform:  Broader IED  Lower Etch Rates  Lower Selectivity  In a regime where selectivity is higher, custom waveform enables higher etch rates  For same etch rates lower selectivity with sin waveform. ANKUR_AVS_15

19. University of Illinois Optical and Discharge Physics ETCH PROFILES – CUSTOM VOLTAGE PEAK-TO-PEAK ANKUR_AVS_16 400 V 500 V 1000 V 1500 V  XXX V indicates amplitude of bias  Increasing bias increases etch rate and reduces selectivity. ANIMATION NEXT SLIDE

20. University of Illinois Optical and Discharge Physics ETCH PROFILES – CUSTOM VOLTAGE PEAK-TO-PEAK ANKUR_AVS_16 ANIMATION SLIDE 400 V500 V1000 V1500 V  XXX V indicates amplitude of bias  Increasing bias increases etch rate and reduces selectivity. MASK SiO 2 Si

21. University of Illinois Optical and Discharge Physics FLUOROCARBON PLASMA ETCH SELECTIVITY  Increasing bias voltage increases etch rates.  Loss of selectivity with increasing bias voltages. ANKUR_AVS_17

22. University of Illinois Optical and Discharge Physics ETCHING RECIPES  Multi-component recipes:  Main-etch: Non selective; High bias  Over-etch: Selective; Low bias  Traditionally, gas mixture is changed to obtain a selective etch.  Controlling chemical component  Clearing of gases is determined by residence time  Finite selectivity  Custom tailored voltage waveform  Controlling physical component  Change amplitude – immediate control  “Infinite” selectivity ANKUR_AVS_18

23. University of Illinois Optical and Discharge Physics ETCHING PROFILES – RECIPE ANKUR_AVS_19 ANIMATION NEXT SLIDE 200 V (Slow, selective) 1500 V (Fast, non-selective) 1500/200 V (Fast, selective) 1500/1000/100/200 V (Fast, selective)

24. University of Illinois Optical and Discharge Physics ETCHING PROFILES – RECIPE ANKUR_AVS_19 ANIMATION SLIDE 200 V (Slow, selective) 1500 V (Fast, non-selective) 1500/200 V (Fast, selective) MASK SiO 2 Si 1500/1000/100/200 V (Fast, selective) 1847 s713 s1377 s1356 s

25. University of Illinois Optical and Discharge Physics SUMMARY  Higher etch selectivity was obtained by controlling ion energy distribution.  Flux, Energy and Angular distribution optimized to attain high etch selectivity  Special tailored voltage waveform was synthesized.  Short voltage spike followed by ramp down  Results in a narrow IED over wide range of voltages and frequency.  New etching recipe  Based only on bias voltage amplitude without changing gas chemistry.  Excellent control over selectivity demonstrated. ANKUR_AVS_20