Arvind Sankaran 1 and Mark J. Kushner 2 1 Department of Chemical Engineering 2 Department of Electrical and Computer En

1. AVS 2002Nov 3 - Nov 8, 2002Denver, ColoradoINTEGRATED MODELING OF ETCHING, CLEANING AND BARRIER COATING PVD FOR POROUS AND CONVENTIONAL SIO2 IN FLUOROCARBON BASED CHEMISTRIES* Arvind Sankaran1 and Mark J. Kushner2 1Department of Chemical Engineering 2Department of Electrical and Computer Engineering University of Illinois, Urbana, IL 61801, USA email: asankara@uiuc.edu mjk@uiuc.edu http://uigelz.ece.uiuc.edu *Work supported by SRC, NSF and SEMATECH

2. AGENDA • Low dielectric constant materials • Surface reaction mechanism and validation • Fluorocarbon etching of SiO2/Si • Ar/O2 etching of organic polymer • High aspect ratio etching of porous and non porous SiO2 • Integrated Modeling: Ar/O2 strip of polymer and IMPVD • Concluding Remarks University of Illinois Optical and Discharge Physics AVS03_AS_02

3. LOW DIELECTRIC CONSTANT MATERIALS • The increase in the signal propagation times due to RC delay has brought the focus onto low dielectric constant (low-k) materials (inorganic and organic) • Inorganics such as porous silica (PS) are etched using fluorocarbon chemistries; organics are etched using oxygen chemistries. University of Illinois Optical and Discharge Physics AVS03_AS_03

4. GOAL FOR INTEGRATED MODELING • Plasma processing involves an integrated sequence of steps, each of which depends on the quality of the previous steps. University of Illinois Optical and Discharge Physics CFDRC_0503_05

5. SURFACE REACTION MECHANISM - ETCH • CFx and CxFy radicals are the precursors to the passivation layer which regulates delivery of precursors and activation energy. • Chemisorption of CFx produces a complex at the oxide-polymer interface. 2-step ion activated (through polymer layer) etching of the complex consumes the polymer. • Activation scales as  1/L and the L scales as  1/bias. • In Si etching, CFx is not consumed, resulting in thicker polymer layers. • Si reacts with F to release SiFx. University of Illinois Optical and Discharge Physics AVS03_AS_05

6. SURFACE REACTION MECHANISMS - STRIP • Ar/O2 is typically used for polymer stripping after fluorocarbon etching and resist removal. • Little polymer removal is observed in absence of ion bombardment suggesting ion activation. • For SiO2 etching in mixtures such C4F8/O2, the fluorocarbon polymer is treated as an organic. Resists are treated similarly. University of Illinois Optical and Discharge Physics AVS03_AS_06

7. MONTE CARLO FEATURE PROFILE MODEL (MCFPM) • The MCFPM predicts time and spatially dependent profiles using energy and angularly resolved neutral and ion fluxes obtained from equipment scale models. • Arbitrary chemical reaction mechanisms may be implemented, including thermal and ion assisted, sputtering, deposition and surface diffusion. • Energy and angular dependent processes are implemented using parametric forms. • Mesh centered identity of materials allows “burial”, overlayers and transmission of energy through materials. University of Illinois Optical and Discharge Physics INTELTALK_AS_17

8. MODELING OF POROUS SILICA • MCFPM may include “two phase” materials characterized by porosity and average pore radius. • Pores are incorporated at random locations with a Gaussian pore size distribution. Pores are placed until the desired porosity is achieved with/without interconnects. • Interconnected structures can be addressed. University of Illinois Optical and Discharge Physics AVS03_AS_07

9. TYPICAL PROCESS CONDITIONS • Process conditions • Power: 600 W • Pressure: 20 mTorr • rf self-bias: 0-150 V • C4F8 flow rate: 40 sccm • The fluxes and energy distributions are obtained using the HPEM. University of Illinois Optical and Discharge Physics AVS03_AS_08

10. BASE CASE ION AND NEUTRAL FLUXES • Self-bias = - 120 V. Decrease in neutral and ion fluxes along the radius have compensating effects. • Ions have a narrow energy and angular distribution, in contrast to neutrals. University of Illinois Optical and Discharge Physics AVS03_AS_09

11. VALIDATION OF REACTION MECHANISM: C4F8 • The mechanism was validated with experiments by Oehrlein et al using C4F8, C4F8/Ar and C4F8/O2.1 • Threshold for SiO2 etching was well captured at self-bias  -40 V. Polymer formation is dominant until the threshold bias • As polymer thins at higher biases, the etching proceeds. 1 Li et al, J. Vac. Sci. Technol. A 20, 2052, 2002. University of Illinois Optical and Discharge Physics AVS03_AS_10

12. VALIDATION: C4F8/Ar and C4F8/O2 • Larger ionization rates result in larger ion fluxes in Ar/C4F8 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. • O2 etches polymer and reduces its thickness. Etch rate has a maximum with O2, similar to Ar addition. University of Illinois Optical and Discharge Physics AVS03_AS_11

13. PROFILE COMPARISON: MERIE REACTOR V. Bakshi, Sematech • Process conditions • Power: 1500 W CCP • Pressure: 40 mTorr • Ar/O2/C4F8: 200/5/10 sccm University of Illinois Optical and Discharge Physics AVS03_AS_12

14. VALIDATION OF POROUS SiO2 ETCH MODEL • Two porous substrates • 2 nm pore radius, 30% porosity • 10 nm pore radius, 58% porosity • Process conditions • Power: 1400 W (13.56 MHz) • Pressure: 10 mTorr • rf self-bias: 0-150 V • 40 sccm CHF3 • Etch rates of P-SiO2 are higher than for NP-SiO2 due to lower mass densities of P-SiO2. Exp: Oehrlein et al, J. Vac. Sci.Technol. A 18, 2742 (2000) University of Illinois Optical and Discharge Physics AVS03_AS_13

15. WHAT CHANGES WITH POROUS SiO2? • The “opening” of pores during etching of P-SiO2 results in the filling of the voids with polymer, creating thicker layers. • Ions which would have otherwise hit at grazing or normal angle now intersect with more optimum angle. • An important parameter is L/a (polymer thickness / pore radius). • Adapted: Standaert, JVSTA 18, 2742 (2000) University of Illinois Optical and Discharge Physics INTELTALK_AS_30

16. EFFECT OF PORE RADIUS ON HAR TRENCHES 4 nm 10 nm 16 nm • With increase in pore radius, L/a decreases causing a decrease in etch rates. • Thicker polymer layers eventually lead to mass corrected etch rates falling below NP-SiO2. There is little variation in the taper. University of Illinois Optical and Discharge Physics AVS03_AS_15

17. HAR PROFILES: INTERCONNECTED PORES 100% 0% 60% Interconnectivity University of Illinois Optical and Discharge Physics INTELTALK_AS_40

18. EFFECT OF PORE RADIUS ON CLEANING • Larger pores are harder to clean due to the view angle of ion fluxes. • Unfavorable view angles lead to a smaller delivery of activation energy, hence lower activated polymer sites. 4 nm 16 nm • Ar/O2=99/1, 40 sccm, 600 W, 4 mTorr University of Illinois Optical and Discharge Physics ANIMATION SLIDE AVS03_AS_17

19. CLEANING INTERCONNECTED PORES • Cleaning is inefficient with interconnected pores. • Higher interconnectivity leads to larger shadowing of ions. 0% 60% 100% • Ar/O2=99/1, 40 sccm, 600 W, 4 mTorr Interconnectivity University of Illinois Optical and Discharge Physics ANIMATION SLIDE CHEME_AS_19

20. EFFECT OF ASPECT RATIO ON STRIPPING • Cleaning decreases with increasing aspect ratios. • Pores at the top of the trench are stripped better due to direct ions (view angle). • Pores near the bottom see ions reflected from the bottom of the trench and are cleaned better. 1 3 Aspect Ratio 5 • Ar/O2=99/1, 40 sccm, 600 W, 4 mTorr University of Illinois Optical and Discharge Physics ANIMATION SLIDE AVS03_AS_19

21. EFFECT OF PORE RADIUS ON Cu DEPOSITION • Larger pores require longer deposition times for conformal coverage. • This produces thicker bottom and open field films. • Voids are created or initiated by larger pores. NP 4 nm 10 nm 16 nm • Surrogate study for seed layer deposition and barrier coating. University of Illinois Optical and Discharge Physics AVS03_AS_20

22. EFFECT OF INTERCONNECTIVITY ON Cu IMPVD • Interconnected pores need to be sealed to avoid pin-hole formation. • Pore sealing by Cu IMPVD ineffective at larger interconnectivities. • Thicker layers to seal pores produces trench narrowing, which can lead to pinch off. 0% 30% 60% 100% Interconnectivity University of Illinois Optical and Discharge Physics AVS03_AS_21

23. CONCLUSIONS • Etching of PS obeys scaling laws as that of SS. Etch rate increases for smaller pores and slows for larger pores (at high porosities). • L/a determines etch rate variation of P-SiO2. Polymer filling increases the net thickness. • Stripping is inefficient for interconnected pore networks and for larger pores due to the unfavorable view angles for the ion fluxes. Low aspect ratio pores are better cleaned. • Cu IMPVD is non-conformal for closed pore networks with larger pores. Pin-hole formation and trench narrowing is seen for interconnected networks. University of Illinois Optical and Discharge Physics AVS03_AS_22