10 Diffusion 10

10 Diffusion 10 - Dopants are introduced into the silicon...

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Unformatted text preview: Dopants are introduced into the silicon during the growth of the ingot (lecture 5) during processing using surface masks to define lateral position (this lecture & lecture 13) Methods of doping include codeposition of dopant with layer (lecture 12) diffusion from an intermediate surface source (this one) Diffusion This lecture covers how dopants move in silicon Dopant control Log (dopant density) P B Log (dopant density) P B Substrate doping level Entire process may have >20 high temperature steps. Dopant profile shifts with each. No matter how the dopant is introduced, control of profile lies with subsequent thermal processing ! before after Junction Depth vs. Technology Node How can we attain such shallow junctions?- Furnace diffusion?- Ion implantation?- Rapid thermal annealing? 10- 20 15- 30 20- 40 26- 52 30- 60 36- 72 50- 100 Minimum junction depth (nm) 50 70 100 130 150 180 250 Linewidth (nm) (necessary for control of effective channel length) Diffusion mechanisms Needs vacanci es in lattice Interstitial Substitutional e.g. S, Fe, Cu, Au, O e.g. B, P, As, Sb, Al Why are some elements interstitial diffusers and some substitutional? Impurities can move through the lattice interstitially (between the atoms) or substitutionally (replacing substrate atoms) Where do vacancies come from? Vacancy Surface Schottky Defect Frenkel Defect V Si Si Si + V I Si Si Si + # of vacancies is given by Boltzmann statistics: k T E D e fe c t e- # at room T? # at diffusion T? Try these calculations! = atomic density Which takes most energy to form? For Si, atomic density = 5 x 10 22 cm-3 1- D diffusion model Equal probability of jumping L or R t 25 100 + 25 150 + 100 150 + 100 100 + 25 25 50 150 + 50 150 + 150 150 + 50 50 100 200 + 100 200 + 100 100 200 200 + 200 200 400 400 800 x x t=0 t=2 t=n Jump rate (substitutional) kT E kT E Defect Bond...
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This note was uploaded on 05/11/2010 for the course EEE EEE-530 taught by Professor Kozicki during the Spring '10 term at ASU.

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10 Diffusion 10 - Dopants are introduced into the silicon...

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