FACTORS AFFECTING THE SEALING EFFICIENCY OF LOW-k DIELECTRIC SURFACE PORES USING SUCCESSIVE He AND Ar/NH 3 PLASMA TREATMENTS Juline Shoeb a) and Mark J. Kushner b) a) Department of Electrical and Computer Engineering Iowa State University, Ames, IA b) Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor, Ann Arbor, MI October 2009 * Work supported by Semiconductor Research Corporation JULINE_GEC09

 Low-k Dielectrics  Modeling of Porous Low-k Sealing  Sealing Mechanism  Surface Site Activation by He plasma pre-treatment  Sealing by Ar/NH 3 Treatment  Sealing Efficiency Dependence  Porosity and Interconnectivity  Treatment time and Pore Radius  Concluding Remarks AGENDA University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

POROUS LOW-k DIELECTRIC  Metal interconnect lines in ICs run through dielectric insulators.  The capacitance of the insulator contributes to RC delays.  Porous oxides, such as C doped SiO 2 (with CH n lining pores) have a lower dielectric constant which reduces the RC delay.  Porosity is  0.5. Inter- connected pores open to surface may degrade k- value by reactions with plasma species. Ref: University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

GOALS AND PREMISES OF SEALING MECHANISM  To prevent the degradation of low-k materials pores open to the surface should be sealed.  He plasma followed by NH 3 plasma treatment has been shown to seal pores in SiCOH.  He + and photons break Si-O bonds and remove H atom from CH n lining pores Ref: A. M. Urbanowicz, M. R. Baklanov, J. Heijlen, Y. Travaly, and A. Cockburn, Electrochem. Solid-State Lett. 10, G76 (2007). PlasmaTreatment Time (s) Function He200Surface Activation NH 3 30 ( Post-He) Sealing  Subsequent NH 3 exposure seals the pores by adsorption reactions forming C-N and Si-N bonds.  Reaction mechanisms and scaling laws for He-NH 3 plasma sealing of porous SiCOH have been developed based on results from a computational investigation.  Experimental results by others were used to build the mechanism. University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

MODELING : LOW-k PORE SEALING  Hybrid Plasma Equipment Model (HPEM)  Plasma Chemistry Monte Carlo Module (PCMCM)  Monte Carlo Feature Profile Model (MCFPM) Energy and angular distributions for ions and neutrals He PLASMA Porous Low-k Coils Wafer Substrate Metal Plasma Ar/NH 3 PLASMAS University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

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  ).  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.  Cells identities changed, removed, added for reactions, etching deposition. PCMCM Energy and angular distributions for ions and neutrals HPEM MCFPM Provides etch rate And predicts etch profile University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

 80 nm wide, 80 nm thick porous SiCOH.  Average pore radius: nm  Process integration steps: 1.Ar/C 4 F 8 /O 2 CCP: Etch 10:1 Trench 2.O 2 ICP: Remove CF x polymer and PR. 3.He ICP: Activate Sites 4.NH 3 ICP: Seal Pores INITIAL LOW-k PROFILE, PROCESS INTEGRATION University of Michigan Institute for Plasma Science & Engr. Hard Mask Porous Low-k SiCOH Si JULINE_GEC09

AR/O 2 PLASMAS: POLYMER REMOVAL AND CH 3 GROUP ETCHING  Polymer Sputtering Ar + (g) + P(s)  P * (s) + Ar(g) Ar + (g) + P * (s)  CF 2 (g) + Ar(g) O + (g) + P(s)  CF 2 (g) + O(g) O 2 + (g) + P(s)  CF 2 (g) + O 2 (g)  Polymer Etching O (g) + P(s)  P*(s) + O (g) O + (g) + P * (s)  COF(g) + O (g) O 2 + (g) + P * (s)  COF(g) + O2(g)  CH n Group Etching M + (g) + CH n (s)  CH n-1 (s) + H(g) + M(g) O(g) + CH n-1 (s)  CO(g) + H n-1 (g) O + (g) + CH n-1 (s)  CO(g) + H n-1 (g) O 2 + (g) + CH n-1 (s)  CO(g) + H n-1 O(g) University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

SURFACE ACTIVATION IN He PLASMA  He + and photons break Si-O bonds and remove H from CH 3 groups.  Bond Breaking He + (g) + SiO 2 (s)  SiO(s) + O(s) + He(g) He + (g) + SiO(s)  Si(s) + O(s) + He(g) He + (g) + P(s)  P * (s) + He(g) hν + SiO 2 (s)  SiO(s) + O(s) hν + SiO(s)  Si(s) + O(s)  Activation He + (g) + CH n (s)  CH n-1 (s) + H(g) + He(g) hν + CH n-1 (s)  CH n-2 (s) + H(g) He + (g) + CH n (s)  CH n-1 (s) + H(g) + He(g) hν + CH n-1 (s)  CH n-2 (s) + H(g)  Reactive sites assist sealing in the subsequent Ar/NH 3 treatment. University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

SEALING MECHANISM IN Ar/NH 3 PLASMA  N/NH x species are adsorbed by activated sites forming Si-N and C-N bonds to seal pores.  Further Bond Breaking M + (g) + SiO 2 (s)  SiO(s) + O(s) + M(g) M + (g) + SiO(s)  Si(s) + O(s) + M(g)  N/NH x Adsorption NH x (g) + SiO n (s)  SiO n NH x (s) NH x (g) + Si(s)  SiNH x (s) NH x (g) + CH n-1 (s)  CH n-1 NH x (s) NH x (g) + P * (s)  P(s) + NH x (s)  SiNH x -NH y /CNH x -NH y compounds seal the pores where end N are bonded to Si or C by Si-C/Si-N NH y (g) + SiNH x (s)  SiNH x -NH y (s) NH y (g) + CH n-1 NH x (s)  CH n-1 NH x -NH y (s) University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

 CCP for trench etch.  Ar/C 4 F 8 /O 2 = 80/15/5  40 mTorr, 300 sccm  10 MHz  5 kW  CF x polymers deposited on side-walls efficiently seal the open pores.  Depending on the application polymers:  Removed by O 2 plasmas (followed by other pore sealing process)  Maintained for pore sealing Ar/C 4 F 8 /O 2 CCP TRENCH ETCH Animation Slide-GIF Hard Mask Porous Low-k SiCOH Si University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

He PLASMA PRE-TREATMENT  Ion density: 3.8 x cm -3.  Porous low-k exposed for up to 300 s to the plasma.  250V substrate bias assisted ablating H and Si-O bond breaking.  Conditions: He, 10 mTorr, 300 W ICP, 250V Bias University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

PORE-SEALING BY SUCCESSIVE He AND NH 3 /Ar TREATMENT  Surface pore sites are activated by 200s He plasma treatment.  Successive 30 s NH 3 treatment seals the pores forming Si-N and Si-C bonds.  Initial Surface Pores University of Michigan Institute for Plasma Science & Engr. Animation Slide-GIF  He Plasma Site Activation  Ar/NH 3 Plasma Pore Sealing JULINE_GEC09

POLYMER REMOVAL: Ar/O 2 PLASMA TREATMENT  Oxygen Ion density: 3.8 x cm -3.  56 s exposure to plasma.  250V substrate bias assisted in removal of polymer deep in trench.  Conditions: Ar/O 2, 10 mTorr, 300 W ICP, 250V Bias University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

POLYMER REMOVAL FROM SIDEWALLS  Keeping the mask, the trench was exposed to Ar/O 2 plasmas for polymer removal.  Ion bombardment activates/removes the polymers. CH n groups are also etched during this process due to the presence of oxygen species.  Neutral oxygen and Ar + activate polymer while ions remove it by etching or sputtering reactions Ar + (g) + P(s)  P * (s) + Ar(g) Ar + (g) + P * (s)  CF 2 + Ar(g) O(g) + P(s)  P * (s) + O (g) O + (g) + P(s)  CF 2 + O (g) O + (g) + P * (s)  COF + O(g) University of Michigan Institute for Plasma Science & Engr. Animation Slide-GIF Hard Mask Porous Low-k SiCOH Si JULINE_GEC09

POLYMER REMOVAL & CH 3 DEPLETION  Polymer removal in Ar/O 2 plasmas may also remove CH 3 groups as O diffuses into the porous network.  Ar/O 2 plasma exposure sufficient to remove all polymer also results in CH 3 removal.  An optimum strategy may be to leave traces of polymer in recesses (that is, shorter Ar/O 2 plasma exposure) to prevent CH 3 removal. University of Michigan Institute for Plasma Science & Engr. Hard Mask Porous Low-k SiCOH Si JULINE_GEC09

SEALING: WITH POLYMER REMOVAL  Mask was removed to expose tops of the low-k material.  He plasma activated the tops and the polymer free side-walls of the trench.  Successive NH 3 plasma exposure seals the top and side-walls through C-N and Si-N bonding. University of Michigan Institute for Plasma Science & Engr. Si Animation Slide-GIF Activation Sealing Activation Sealing JULINE_GEC09

SEALING EFFICIENCY: PORE RADIUS  Sealing efficiency decreases with increasing pore size dropping below 75% > 1.0 nm.  Sidewall efficiency is less sensitive to pore radius due to some unremoved polymer.  C-N and Si-N are “first bonds.”  Sealing requires N-N bonding, which has limited extent.  Too large a gap prevents sealing. University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

SEALING: TREATMENT TIME DEPENDENCE  Without He plasma treatment, Ar/NH 3 plasmas seal only 40% of pores.  Sealing efficiency increases with He treatment time for 200s, then saturates.  200 s treatment breaks nearly all side-wall/surface Si-O bonds and activates most CH 3 groups.  Sealing efficiency of pores increases for 20s of Ar/NH 3 treatment, then saturates – all dangling bonds on the surface are passivated. University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

SEALING EFFICIENCY: ASPECT RATIO  Sealing efficiency on sidewalls decreases with increasing aspect ratio.  Ability for ions to penetrate into features to activate sites decreases with larger aspect ratio.  Sealing improves with longer treatment but some sites are shadowed and will not be sealed. University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09

CONCLUDING REMARKS  Integrated porous low-k material sealing was computationally investigated  Ar/C 4 F 8 /O 2 Etch  Ar/O 2 Clean  He Pretreatment  Ar/NH 3 sealing  Si-N and C-N bonds formed by adsorption on active sites followed by one N-N bond linking C or Si atoms from opposite pore walls.  Pore sealing efficiency is independent of porosity and interconnectivity, while dependent on both He and NH 3 plasma treatment time.  The sealing efficiency degrades when with pore radius > 1 nm and with increasing aspect ratio. University of Michigan Institute for Plasma Science & Engr. JULINE_GEC09