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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Mesoporous silica low-k dielectric films: Deposition, characterisation and integration Chris Murray Chemnitz University of Technology, Center of Microtechnologies, Germany

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Contents Where is Chemnitz? Why low-k? Sol-gel route to mesoporous silica Characterisation techniques Integration Conclusions

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Wheres Chemnitz?

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Why low k? The RC delay problem C LL C V =C LG WS P T Upper Metal Layer Interconnect Lower Metal Layer R=2 L/PT C=2(C LL +C V ) = 2k 0 (2LT/P+LP/2T) RC=2 k 0 (4L 2 /P 2 +L 2 /T 2 ) M. Bohr, Tech. Digest IEEE Int. Electron. Devices Meeting 1995 p. 241

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Why Low k?

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Metals Exchanging Cu for Al reduces delay by about 35% although many problems exist for integration.

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Roadmap (1999 ITRS, International SEMATECH) Interconnect Technology Requirements - Near Term

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Dielectrics A range of dielectrics are becoming commercially available Morey & Asthana, Solid State Technology, Vol. 2, No. 6, June 1999 p.71,

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Introduction of pores to lower k By making SiO 2 porous, the effective k value is reduced

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Sol Gel Processing Sol Gel Science, Brinker & Scherrer, Academic Press, 1990, p. 1

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien General Sol-Gel Process Flow for Porous SiO 2

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Mesoporous xerogel low-k film production Precursor = TEOS, water, solvent, catalyst hydrolysis / condensation reactions Spin on Post-spin on gelation Drying / Annealing 450C Hydrophobisation with HMDS Pre-spin on gelation Precursor & catalyst mixing n-type 150 mm Si wafers Si-O-Si porous network forms after spin-on Replace hydrophylic surface -Si-OH groups with hydrophobic -Si-O-Si(CH 3 ) 3 5 process variations = range of dielectric constants

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Film properties Nanoglass is a trademark of Allied Signal / Honeywell

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Film Characterisation Atomic Force Microscopy TEM of Pt imprint of xerogel film very smooth film surface maximum height R max < 8 nm mean roughness R a < 1 nm

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien

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Ellipsometric Porosimetry (EP) n porosity AND, toluene absorption pore size and distribution, pore interconnectedness pumpPC P adsorptive laser sample detector OP=55% FP=54% CSA=260 m 2 cm -3

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Laser Induced Surface Acoustic Waves (LSAW) Laser Pulse SAW Piezoelectric pickup V film thickness, density, elastic modulus and Poissons ratio Velocity (m s -1 ) Frequency (MHz)

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Nanoindentation Can measure elastic modulus hardness Cant measure porosity !! Careful sample and substrate effects for films < 1m

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Porosity Results LSAW & RBS agree to within experimental error EP up to 20% lower than RBS How realistic is the reference density? * Film reference density used for LSAW and RBS porosity = 2.26 g cm -3 RBS measured thermally grown dense SiO 2 = 2.24 0.1 g cm -3

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Skeletal Density Combine LSAW or RBS mean film density with EP open porosity skeletal density skeleton = 1.4 - 1.7 g cm -3

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Surface Area & Pore size r (mean) increases and pore surface area decreases as porosity increases i.e. fewer, larger pores Elastic Modulus & Hardness Modulus and hardness correlate well with porosity and each other Overestimation of nanoindentation modulus by >20% due to substrate effects Refinements needed

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Patterning of xerogel thin films using PECVD SiO 2 cap layer Trench etch: ICP using CF 4 / CHF 3 trench depth: 500 nm cap: 50 nm PECVD SiO profile angles > 85 no undercut of cap layer bowing was observed at large open areas Si xerogel cap MOCVD TiN Trench after MOCVD TiN deposition Si Cu TiN SiO 2 Xerogel Si xerogel Etched 0.2 m trenches

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Outlook Porous silica materials posess lowest k values Many integration issues remain to be resolved Davies & Corbett Solid State Tech., Vol. 43, Issue 4, April, 2000

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien How do we define porosity? skeleton pore

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Can measure film thickness refractive index full and open porosity pore size distribution pore connectivity cumulative surface area Cant measure elastic modulus !! Careful full porosity calculated using reference refractive index value of dense SiO 2 toluene cannot penetrate pores of radius < 0.3 nm Ellipsometric Porosimetry Non-destructive Simple vacuum equipment Non-hazardous adsorptive

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Can measure average film density elastic modulus Cant measure Pore size distribution surface area !! Careful Need a film skeletal density value for porosity calculations e.g. dense SiO 2 Laser Induced Surface Acoustic Waves Non-destructive / non-invasive No patterned substrates or vacuum chamber required Only technique which provides density/porosity and mechanical characteristics

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Measurement Capabilities Proven Possible Proven for dense films

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Technische Universitt Chemnitz, Zentrum fr Mikrotechnologien Conclusions Porosity depends on choice of reference values used in calculations Cant use skeleton = SiO2 ? Can we still use n skeleton = n SiO2 EP shows pores are interconnected LSAW mean film density correlates with RBS LSAW or RBS mean film density combined with EP open porosity gives film skeletal density skeleton = 1.4 -1.7 g cm -3 Elastic modulus values low, hard to measure with accuracy Techniques should refine themselves as the sample database grows