Field evaporation and GCIB processing of electrodes ß Field evaporation theory ß Multiscale Molecular Dynamics of Cu-tip Field Evaporation ß Surface processing with GCIB ß Experimental studies of GCIB ß Future applications Z. Insepov1), D. Swanson2), A. Kirkpatrick2), A. Hassanein1) 1) ANL, 2) EPION Corp. Argonne National Laboratory A Laboratory Operated by The University of Chicago
Field Evaporation Theory In a sufficiently high positive electric field, surface atoms can be desorbed from the surface at low temperature in the form of multiply charged positive ions. This effect have been used in field ion microscopy (FIM) for a long time. F ev n 1 Ê 3.6n = ÁL + Â Ii - nf - nr0 Ë i r0 Copper 2 ˆ V A L, ev f, ev r 0, _ I 1, ev I 2, ev I 3, ev E aff, ev F 1+, GV/m 3.50 4.60 1.25 7.73 20.29 36.83 1.23 30
Multiscale MD Simulation Method E tip =E m sin(p/2 t/t m ) t m =200 ps, E m =1,10,100 GV/m f=1.25 GHz, T=300-1300K + = The MD region : & r i ( t) = - 1 m Thermal balance: r dt(, t) r = DT dt The eq.of motion : 2 r d ui ó ik = 2 dt dx u + dx ( r, t) ˆ Â j i U r The Finite - Difference Region (Mesh) : Ê ó ik = -áktd ik + Ke lld ik + 2mÁe Ë Ê d ik ˆ + xe lld ik + 2h Á & e ik - & e ll Ë 3 e ik k 1 Ê ui Á 2 Ë dxk k i ij ij c - thermal diffusivity, K - bulk modulus, x - bulk viscosity, a - therm.expans.coeff, m -shear modulus, h - shear viscosity. ik -e ll d 3 ik ˆ
MD of Field Evaporation @1GV/m Electric Field: 1GV/m 3% of the critical value for Cu 30 GV/m T=300K
MD of Field Evaporation @ 1GV/m Electric Field: f=1.25ghz 1GV/m 3% of the critical field for Cu 30GV/m T=1100K
MD of Field Evaporation: 10GV/m Electric Field: f=1.25ghz 10GV/m 30% of the critical field for Cu 30 GV/m T=650K
MD of Field Evaporation: 10GV/m & variable update for surface charges Electric Field: f=1.25ghz 10GV/m, 30% of critical field for Cu 30 GV/m T=800K Automated update for the Charged surface atoms
MD of Field Evaporation: 100 GV/m Electric Field: f=1.25ghz 100GV/m, 3 times of the critical field for Cu 30 GV/m; T=800K Automated update for the charged surface atoms Ecr for cooperative evaporation could be estimated from the movie and it is 36 GV/m
Surface processing with Gas Cluster Ion Beams (GCIB) Conventional ions deep penetration thermal spike along stopping path normal sputtering distribution >1000 Å Gas cluster ions clusters have high total energy but low energy per atom extremely shallow penetration into target surface energy deposition into localized region lateral sputtering distribution inherent smoothing action < 100 Å
Cluster impact produces extreme transient conditions Cluster ion impact Ar + Shock wave generation Lateral sputtering Crater formation Lateral sputtering and crater formation target target target On metal surfaces, craters were hemispherical. The Lateral sputtering effect (LSE) was predicted by Molecular Dynamics simulation in 1992 and then was confirmed experimentally. It occurs due to interaction of rarefaction nano-scale shock-waves with an open target surface. The LSE effect dramatically enhances the surface atomic diffusivity which eventually would lead to the surface smoothing.
Surface smoothing by GCIB--example SiC Pre 54 A RMS Post 2.7 A RMS Pre 57.3 A RMS Post 5.7 A RMS
Atomic scale surface smoothing by GCIB AFM image of Ta film surface After GCIB Ra = 4Å Initial Ra =12Å, (Masked during processing)
GCIB surface enhancement Initial Ra = 3.8 Å Glass After GCIB Ra = 2.5 Å
GCIB Smoothing of Cu Pre-GCIB: Z = 210.6A Ra = 13.9A Rms = 17.9A Post-GCIB: Z = 67.0A Ra = 5.5A Rms = 6.9A
GCIB Smoothing of PVD Cu 30 25 20 Ra (A) 15 10 5 0 0 5 10 15 20 GCIB Process Time (arb. units)
GCIB smoothing of Cu with N 2 Pre-GCIB: Z = 72A Ra = 6.5A Post-GCIB: Z = 36A Ra = 3.4A
GCIB cleaning of ECD CuOx 30 25 CuOx etching with N2 GCIB 1E15 / cm2 dose 20 native oxide thickness CuOx thickness (A) 15 10 5 0 0 5 10 15 20 25 30 35 GCIB Energy (kv)
What can GCIB do? Smoothing to atomic levels Surface densification Reactive chemical modification of surfaces Assisted deposition of thin films High rate etching and cleaning without causing roughening Precise uniform reproducible etching and thinning Spatially variable etching and thinning Ultra-shallow implantation in semiconductors and metals
New Challenge for future generation of RF accelerators A Field Evaporation model has been proposed and simulated by Molecular Dynamics method Our modeling has revealed a new cooperative evaporation effect of a tip at a high electric field To reduce the surface roughness, we have proposed a new surface smoothening process by GCIB that can reduce the surface roughness up to atomic scale We plan to dramatically increase surface smoothness, make surfaces more dense, and to overall increase the critical field for the vacuum breakdown