P. Diomede, D. J. Economou and V. M. Donnelly Plasma Processing Laboratory, University of Houston

Transcription

1 P. Diomede, D. J. Economou and V. M. Donnelly Plasma Processing Laboratory, University of Houston Acknowledgements: DoE Plasma Science Center, NSF Presented at the 57 th AVS Conference, Albuquerque, NM

2 Introduction / Motivation Control of the energy of ions bombarding a substrate in contact with plasma is critical for plasma processing. The ionergy must be high enough to drive anisotropic etching, but not too high to induce substrate damage and/or loss of selectivity. As device dimensions continue to shrink, precise control of the ionergy distribution (IED) becomes increasingly important. 2

3 Goal and Approach Goal: Develop methodologies to achieve tailored ionergy distribution functions (IEDFs). Approach: Use combination of modeling/simulation and experiments. Modeling is semi-analytic. Simulation is PIC using XPDP1 (V. Vahedi, G. DiPeso, C. K. Birdsall, M. A. Lieberman, and T. D. Rognlien, Plasma Sources Sci. Technol., 2, 261 (1993); Verboncoeur, Alves, Vahedi, Birdsall, J. Comp. Phys., 104, 321 (1993)). 3

4 Electrode Immersed in a Plasma Plasma density and electron temperature are not affected by the applied potential Bulk Plasma (n 0, T e ) Sheath Electrode (Target) Blocking capacitor, C b Applied rf, V rf 4

5 Semi-analytic Model (1) Schematic of the sheath region Electrode Sheath Pre-sheath Plasma (bulk) x I d I e I i n 0, T e 1. Electrode immersed in semi-infinite plasma of givelectron (ion) density and electron temperature. 2. Electron, ion and displacement currents flow through the sheath. 3. Non-linear sheath capacitance C s is calculated from the electric field at the electrode, E. E Cs = ε 0 A V s V=V S V=V 1 V=0 2n kt e(v V ) V ε kt V 1 e s 1 s 1 2 E = [exp( ) + 2 ] / 0 e 1 A. Metze et al., J. Appl. Phys., 60, 3081 (1986). P. Miller and M. Riley, J. Appl. Phys., 82, 3689 (1997). T. Panagopoulos and D. Economou, JAP, 85, 3435 (1999). 5

6 Semi-analytic Model (2) Equivalent circuit model, A. Metze et al., J. Appl. Phys., 60, 3081 (1986). V rf C b I T V T C T V P Subscripts T and G refer to target and ground electrodes, respectively. d d C b (Vrf V T ) + C T (VP V T ) + IT = dt dt d d C T (VP V T ) + CG VP + IT + IG = 0 dt dt 0 I G C G dv dt d V = d ( V V τ T i p ) Ions respond to a damped potential V d Desired voltage V rf is applied through blocking capacitor, C b. Given n 0, T e, V rf and C b, calculate V d, V T and V p. 6

7 Semi-analytic Model (3) Having determined V d, find ionergy distribution P(E). P( E ) = 1 2π dv d d( ωt ) 1 E= ev d A. Metze et al., J. Appl. Phys., 60, 3081 (1986). E. Kawamura et al., Plasma Sources Science & Technology, 8, R45 (1999). 7

8 Tailored voltage waveforms: Spikes (1) Target voltage and Ar + IEDF, 500 khz F. L. Buzzi et al. PSST, 18 (2009) PIC simulation: = m -3, T e = 2eV 8

9 Tailored voltage waveforms: Spikes (2) Semi-analytical model: = m -3, T e = 2 ev, C B = 5 µf, A S /A T = 5 9

10 Tailored voltage waveforms: Spikes (3) PIC simulation: = m -3, T e = 2eV, f = 10MHz 10

11 Tailored voltage waveforms: Spikes (4) Semi-analytical model: = m -3, T e = 2 ev, C B = 5 µf, A S /A T =1 11

12 Tailored voltage waveforms: Staircase (1) Target voltage and Ar + IEDF, 500 khz F. L. Buzzi et al. PSST, 18 (2009) PIC simulation: = m -3, T e = 2eV 12

13 Tailored voltage waveforms: Staircase (2) Semi-analytical model: = m -3, T e = 2 ev, C B = 5 µf, A S /A T =5 13

14 Tailored voltage waveforms: Square Wave (1) Experiments (dashed line): H 3+ ions, 195 khz P.Kudlacek et al. JAP 106 (2009) PIC simulation: = m -3, T e = 0.15 ev 14

15 Tailored voltage waveforms: Square Wave (2) Semi-analytical model: = m -3, T e = 0.15 ev, C B = 5 µf, A S /A T =5 15

16 Tailored voltage waveforms: Square Wave (3) PIC simulation: = m -3, T e = 0.15 ev, f = MHz 16

17 Tailored voltage waveforms: Square Wave (4) Semi-analytical model: = m -3, T e = 0.15 ev, C B = 5 µf, A S /A T =1 17

18 Tailored voltage waveforms Square Wave with blocking capacitor (1) Experiments (dashed line): H 3+ ions, C B = 166 pf, 27.7 khz P.Kudlacek et al. JAP 106 (2009)

19 Tailored voltage waveforms: Square Wave with blocking capacitor (2) Semi-analytical model = m -3, T e = 0.15 ev, C B = 500 pf, A S /A T =1 19

20 Tailored voltage waveforms: Square Wave + slope with blocking capacitor (1) Experiments (solid line): H 3+ ions, C B = 166 pf, 33.3 khz P.Kudlacek et al. JAP 106 (2009)

21 Tailored voltage waveforms: Square Wave + slope with blocking capacitor (2) Semi-analytical model = m -3, T e = 0.15 ev, C B = 1.66 nf, A S /A T =1 21

22 Summary The energy distribution of ions bombarding the substrate can be tailored by applying voltage waveforms with special shapes (e.g., spikes, staircase, square wave). Semi-analytic model can rapidly identify voltage waveforms that result in tailored IEDFs. PIC simulation is useful for verifying and fine tuning such waveforms. 22