4a_Advanced Aluminum Metallization_complete

4a_Advanced Aluminum Metallization_complete - Advanced...

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Department of Metallurgical and Materials Engineering Advanced Aluminum Metallization S. Gupta Associate Professor Dept. of Metallurgical and Materials Engineering The University of Alabama
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Department of Metallurgical and Materials Engineering Contents ± Introduction ± Silicon Diffusion and Junction Spiking ± Electromigration in Aluminum Thin Films ± Hillock Formation ± Step Coverage and Planarization
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Department of Metallurgical and Materials Engineering Introduction ± Advantages of aluminum for IC interconnects: ² Excellent adherence to silicon and SiO 2 ² Ease of forming Schottky and ohmic contacts to silicon ² Compatibility with CVD oxide or nitride depositions ² Excellent bondability ² Good thermal conductivity ² High electrical conductivity ² High optical reflectivity ² Can be used as a single metal system
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Department of Metallurgical and Materials Engineering Introduction (contd.) ± Aluminum metallization for VSLI must exhibit the following characteristics: ² Reliable contact over shallow sub-micron-depth junction ² Reliable contact in a very small contact area ² Reliable contact over high aspect-ratio features ² A smooth hillock-free coating ² Acceptable electromigration resistance of sub-micron metallization linewidths
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Department of Metallurgical and Materials Engineering Limitations of Aluminum Metallization ± Limitations of pure aluminum metallization arise from: ² Solubility of silicon in aluminum ² Rapid self-diffusion of aluminum at modest temperatures Expected for temperatures over half of melting point (in K) Melting point of pure Al = 660 o C = 933 K Half of melting point = 467 K Therefore, at typical device processing or device operating temperatures of 200 o C = 473 K, expect rapid self-diffusion. This leads to : ³ Silicon migration ³ Hillock growth ³ Electromigration ³ Grain growth and film topographical changes.
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Department of Metallurgical and Materials Engineering Al-Si Phase Diagram ± Solubility of silicon in Al indicated by retrograde silicon solubility curve A-B ± Si precipitation occurs at temperatures below solubility curve. Precipitates are saturated with Al, therefore p-type (Al is an acceptor). 0.25 400 0.5 450 0.8 500 1.59 577 Si (wt. %) Temperature ( o C)
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Department of Metallurgical and Materials Engineering Al-Si Phase Diagram
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Department of Metallurgical and Materials Engineering Silicon Diffusivities in Magnetron Sputtered Al Films 2.85 x 10 -8 2.30 x 10 -8 4.08 x 10 -8 560 1.60 x 10 -8 9.23 x 10 -9 1.89 x 10 -8 500 7.90 x 10 -9 4.12 x 10 -9 1.07 x 10 -8 450 2.67 x 10 -9 1.69 x 10 -9 4.93 x 10 -9 400 Si Diffusivity Al+ 0.014 at% Sn (cm 2 /sec) Si Diffusivity Al+ 0.013 at% In (cm 2 /sec) Si Diffusivity pure Al (cm 2 /sec) Temperature ( o C)
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Department of Metallurgical and Materials Engineering Junction Spiking ± Silicon dissolves into the aluminum metal ± Migrates along metallization line ± Aluminum replaces migrating silicon and spikes junction ± Driving force is solubility of silicon in aluminum ± Mean silicon migration distance where D = silicon diffusivity, and t = time.
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This note was uploaded on 12/30/2010 for the course MTE 550 taught by Professor S.gupta during the Spring '08 term at Alabama.

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4a_Advanced Aluminum Metallization_complete - Advanced...

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