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PHOTON EFFECTS IN DAMAGE OF POROUS LOW- k SIOCH DURING PLASMA CLEANING *

PHOTON EFFECTS IN DAMAGE OF POROUS LOW- k SIOCH DURING PLASMA CLEANING * Juline Shoeb a) and Mark J. Kushner b) a) Department of Electrical and Computer Engineering Iowa State University, Ames, IA 50011 jshoeb@eecs.umich.edu b) Department of Electrical Engineering and Computer Science

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PHOTON EFFECTS IN DAMAGE OF POROUS LOW- k SIOCH DURING PLASMA CLEANING *

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  1. PHOTON EFFECTS IN DAMAGE OF POROUS LOW-k SIOCH DURING PLASMA CLEANING* Juline Shoeba) and Mark J. Kushnerb) a) Department of Electrical and Computer Engineering Iowa State University, Ames, IA 50011 jshoeb@eecs.umich.edu b) Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor, Ann Arbor, MI 48109 mjkush@umich.edu http://uigelz.eecs.umich.edu Nov. 2011 * Work supported by Semiconductor Research Corporation

  2. University of Michigan Institute for Plasma Science & Engr. AGENDA • Low-k Dielectrics • Modeling Platforms • Low-k Damage During Ar/O2 And He/H2 Plasma Clean • Damage Reduction Using He/H2 Plasmas • Photon and Interconnectivity Influence On Low-k Damage • H2O Uptake and Low-k Degradation AVS_02

  3. The capacitance of the insulator contributes to RC delays in interconnect wiring. • Low-k porous oxides, such as C doped SiO2 (CHn lining pores) reduce the RC delay. • Porosity  0.5, Interconnectivity  0.5. • Plasmas may remove hydrophobic -CH3 groups. Free radical sites adsorb H2O and increase k. • Desire to maintain low-k value by minimizing -CH3 damage. POROUS LOW-k DIELECTRICS AVS_03 Ref: http://www.betasights.net/wordpress/wp-ontent/uploads/2011/01/renesas_edram_mims.jpg

  4. University of Michigan Institute for Plasma Science & Engr. • Typical porous SiO2 has CH3 lining pores with Si-C bonding – referred to as SiOCH. • Ave pore radius: 0.8-1.1 nm • Porosity: up to 50% • Etching, damage, cleaning, sealing and H2O uptake of SiOCH is modeled as multistep process • Etch Ar/C4F8/O2 CCP • Damage/Clean Ar/O2 or He/H2 ICP • Low-k H2O Uptake • Sealing To Prevent H2O Uptake LOW-k PLASMA DAMAGE Mask Porous Low-k SiCOH Si AVS_04 Ref: http://www.betasights.net/wordpress/wp-ontent/uploads/2011/01/renesas_edram_mims.jpg

  5. O atoms can abstract H from –CH3 groups and remove -CH3: O+ Si-CH3 (s)  Si-CH2(s) + OH(g) O + Si-CH2(s) Si(s) + CH2O(s) O + CH2O(s)  CO(g)+ H2O(g). O atoms can cause Si-C bond scission and remove –CH3 groups: O+ Si-CH3 (s)  -CH3(s) + Si(s) + O(g) O+ -CH3 (s)  -CH2O(s) + H(g) O + CH2O(s)  CO(g)+ H2O(g).  H removes -CH3 as CH4(g) and abstracts H forming Si-CHx-1 groups: H+ Si-CH3 (s)  -Si(s) + CH4 (g) H + Si-CHx(s)  Si-CHx-1(s) + H2(g). University of Michigan Institute for Plasma Science & Engr. LOW-k DAMAGE: O2 AND H2 PLASMAS Ref: M.F.A.M. van Hest et al., Thin Solid Films 449 40 (2004) O. V. Braginsky et al.,Journal of Aplied Physics 108 073303 (2010) A. M. Urbanowicz et al., Journal of The Electrochemical Society, 157 5 H565-H573 (2010). AVS_05

  6. University of Michigan Institute for Plasma Science & Engr. PHOTON GENERATION AND DAMAGE: Ar/O2, He/H2 ICP • Photons penetrate into the porous SiCOH, are adsorbed by SiO2 andbreak Si-CH3 bonds producing adsorbed •CH3(ads) which enhances demethylation rate (-CH3 removal): hv + Si-CH3(s)  -Si•(s)+ •CH3(ads) hv + SiO2(s)  SiO2*(s). • Ar/O2 Plasmas: e + O O(1D), O(3s), O(5s), O(5p)+ e O(3s) O + hν(130 nm) O(3s) O(1D) + hν(164 nm) O(5p) O(5s)+ hν(777 nm) O(5s)O + hν(136 nm) • He/H2 Plasmas: e + He He*+ e e + He He**+ e e + He*He*+ e He** He + hν(~100 nm) Ref: J. Lee and D. B. Graves, J. Phys. D 43, 425201 (2010). AVS_06

  7. University of Michigan Institute for Plasma Science & Engr. LOW-k DEGRADATION: WATER VOLUME • Since H2O has a high k (~80), water adsorption can seriously degrade k of porous SiCOH. • Even a small percentage of H2O addition degrades the low-k. • Only 2.5% of water volume makes the k as high as solid SiO2 (~3.9). • Degradation of k and adsorbed water volume are related: Ref: T. Kikkawa, S. Kuroki, S. Sakamoto, K. Kohmura, H. Tanaka, and N. Hata, Journal of The Electrochemical Society, 152(7), G560-G566 (2005). AVS_07

  8. LOW-k DAMAGE BY H2O UPTAKE AND SEALING • O2 Plasma : • O2 plasmas remove CH3 groups and increases the k (water adsorption from humid air). • He Plasma Power • Increase in power of He plasma improves surface activation. • A better activated surface seals the pores better (blocks water uptake) during NH3 plasma treatment. Ref: K. Maex, M. Baklanov, D. Shamiryan, F. Iacopi, S. H. Brongersma, K. Maex, and Z. S. Ya novitskaya, J. Appl. Phys. 93, 8793 (2003). Ref: A. M. Urbanowicz, D. Shamiryan, A. Zaka, P. Verdonck, S. De Gendt and M. R. Baklanov, J. Electrochem. Soc. 157, H565 (2010). Iowa State University Optical and Discharge Physics AVS_08

  9. University of Michigan Institute for Plasma Science & Engr. SEALING MECHANISM IN Ar/NH3 PLASMA • N/NHx species are adsorbed by activated sites (generated by He treatment) forming Si-N and C-N bonds to seal pores. • Further Bond Breaking M+ + SiO2(s)  SiO(s) + O(s) + M M++ SiO(s)  Si(s) + O(s) + M • N/NHx Adsorption NHx + SiOn(s)  SiOnNHx(s) NHx + Si(s)  SiNHx(s) • NHx + CHn-1 (s)  CHn-1NHx(s) • SiNHx-NHy/CNHx-NHy compounds seal the pores where end N are bonded to Si or C by C-N/Si-N NHy + SiNHx(s)  SiNHx-NHy(s) NHy + CHn-1NHx(s)  CHn-1NHx-NHy(s) AVS_09

  10. University of Michigan Institute for Plasma Science & Engr. MODELING : PLASMA DAMAGE OF LOW-k He/H2 or Ar/O2 PLASMA DAMAGE HUMID AIR (H2O) Coils Energy and angular distributions for ions and neutrals Plasma Metal Porous Low-k Substrate Wafer • Plasma Chemistry Monte Carlo Module (PCMCM) • Hybrid Plasma Equipment Model (HPEM) • Monte Carlo Feature Profile Model (MCFPM) AVS_10

  11. University of Michigan Institute for Plasma Science & Engr. MONTE CARLO FEATURE PROFILE MODEL (MCFPM) • The MCFPM resolves the surface topology on a 2D Cartesian mesh to predict etch profiles. • Each cell in the mesh has a material identity. (Cells are 4 x 4 A ). • Gas phase species are represented by Monte Carlo pseuodoparticles. • Pseuodoparticles are launched towards the wafer with energies and angles sampled from the distributions obtained from the PCMCM. • Adsorption of photons and photon-surface interactions considered. • Cells identities changed, removed, added for reactions, etching, and deposition. HPEM PCMCM Energy and angular distributions for ions and neutrals MCFPM Provides etch rate And predicts etch profile AVS_11

  12. University of Michigan Institute for Plasma Science & Engr. • Ar/O2 Plasmas: • Major fluxes to the substrate (cm-2 s-1): O1.0 x 1018 O22.0 x 1018 O+2.0 x 1015 O2+4.0 x 1015 Ar+ 5.0 x 1014 • He/H2 Plasmas: • Major fluxes to the substrate (cm-2 s-1): H6.0 x 1017 H23.0 x 1018 H2(v=1)2.0 x 1016 H2(v=2)2.0 x 1016 H+2.0 x 1015 H2+8.0 x 1013 LOW-k DAMAGE :PLASMA REACTOR • H2/He Plasma • Conditions: H2/He = 25/75, Ar/O2 =5/95, 10 mTorr, 300 W ICP AVS_12

  13. University of Michigan Institute for Plasma Science & Engr. PHOTON EFFECTS: O2 PLASMAS Without Photons • 130 nm photons in Ar/O2 plasmas deeply penetrate into the low-k (~100 nm), breaking Si-CH3 bonds to enhance removal of -CH3. Photon 1014 cm-2s-1 Photon 1015 cm-2s-1 Animation Slide AVS_13

  14. DAMAGE: Ar/O2 AND He/H2 (PHOTON FLUX) Photons form O2 plasmas penetrate ~100 nm but for He/H2 plasmas its ~20 nm . Overall O2 plasmas cause ~3 times more damage. University of Michigan Institute for Plasma Science & Engr. Ar/O2 Clean Model He/H2 Clean Experiment Ref: M. A. Worsley, S. F. Bent, S. M. Gates, N. C. M. Fuller, W. Volksen, M. Steen and T. Dalton, J. Vac. Sci. Technol. B 23, 395 (2005). Animation Slide AVS_14

  15. DAMAGE: Ar/O2 AND He/H2 (INTERCONNECTIVITY)  Ar/O2 Clean Interconnectivity 40% • A higher interconnectivity enables more damage. Model Interconnectivity 100% Experiment Animation Slide Ref: M. A. Worsley, S. F. Bent, S. M. Gates, N. C. M. Fuller, W. Volksen, M. Steen and T. Dalton, J. Vac. Sci. Technol. B 23, 395 (2005). AVS_15

  16. University of Michigan Institute for Plasma Science & Engr. LOW-k DAMAGE: DURING POLYMER CLEAN • A CFx polymer layer was deposited. • Polymer was then cleaned by Ar/O2 and He/H2 plasmas with a ~20s exposure. • During clean, some etching of -CH3 radicals occurred. • Photons produce Si-C scission and •CH3(ads) which is separated from Si. • •CH3(ads) can be etched by H2O present in humid air. CFx Depos-ition Ar/O2 Clean He/H2 Clean AVS_16

  17. Photons break Si-CH3 bonds during clean. •CH3(ads) is then etched by H2O in humid air: H2O+ •CH3(ads)CH4(g) + OH. Si reacts with and adsorbs H2O through H bonding H2O + -Si(s) -SiOH(s) + H H2O + SiOH(s)SiOH-H2O(s). Pore sealing by forming hydrophobic Si-NHy or CHx-NHy compounds can block such water uptake. University of Michigan Institute for Plasma Science & Engr. LOW-k INTERACTIONS: H2O UPTAKE AFTER CLEAN  He/H2 Plasma Clean  Humid Air Exposure After He/H2 Plasma Clean Ref: J. Proost, E. Kondoh, G. Vereecke, M. Heyns, and K. Maex, J. Vac. Sci. Technol. B 16, 2091(1998). Animation Slide AVS_17

  18. University of Michigan Institute for Plasma Science & Engr. LOW-kDEGRADATION: Si-OH AND SiOH-H2O • Total k degradation is distributed between chemisorbed H2O (SiOH) and hydrogen bonded H2O (SiOH-H2O). • -OH from Si-OH requires T > 400C to thermally remove while hydrogen bonded H2O can be removed at T ~200C. • k degrades more for Ar/O2 clean because more -CH3 groups are etched. AVS_18

  19. Pore sealing by successive He and Ar/NH3 plasmas produce a hydrophobic –NHx layer. H2O uptake is reduced, thereby limiting low-k degradation. University of Michigan Institute for Plasma Science & Engr. LOW-k INTEGRITY: NH3 PLASMA SEALING  NH3 Plasma Sealing  Post-Sealing Humid Air Exposure Animation Slide AVS_19

  20. University of Michigan Institute for Plasma Science & Engr. WATER VOLUME,DIELECTRIC CONSTANT • After critical amount of H2O adsorption (~10% volume), pore openings are blocked by Si-OH and H bonded H2O. • Water uptake following sealing Ar/NH3 plasma is nominal as hydrophobic –NHx layer prevents H2O molecules from entering the network. • Increase in water volume directly correlates to increase in dielectric constant. • Saturated k-value exceeds that of SiO2. AVS_20

  21. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS • Ar/O2 plasmas cause more damage to low-k SiOCH than He/H2 plasmas. • Photons can break Si-CH3 bonds and accelerate -CH3 removal process, more so in Ar/O2 plasmas than He/H2 plasmas • High interconnectivity enables more damage by providing pathways for radicals and enabling deeper penetration of photons. • -CH3 removal produces free radical sites that adsorb H2O and degrade the k value. • Sealing of pore openings using –NHx hydrophobic layers can be an effective way to maintain low-k integrity. • CO plasmas are recently used to enhance PR ash rate and also to minimize C depletion. AVS_21

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