ECE6450L3-Diffusion Chap 3

ECE6450L3-Diffusion Chap 3 - ECE 6450 - Dr. Alan Doolittle...

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Unformatted text preview: ECE 6450 - Dr. Alan Doolittle Georgia Tech Lecture 3 Diffusion Reading: Chapter 3 ECE 6450 - Dr. Alan Doolittle Georgia Tech Impurity Diffusion: Pfann patented the idea of using diffusions in Si and Ge in 1952. Diffusions are most commonly used for: 1.) Bases, emitters, and resistors in bipolar technology 2.) Form source and drain regions and dope polysilicon gate/interconnect lines in MOS technology. When to use it and when not to use it: 1.) Use when damage from Ion Implantation leads to unacceptable decreases in Minority carrier lifetime, electrical junctions need to be very deep, or a cheap easy solution is needed 2.) Do not use it for ultra-shallow junctions, majority carrier devices (use ion implantation instead) or total impurity dose is critical (ex. MOSFET channel) Diffusions sources include: 1.) Chemical source in a vapor at high temperature. 2.) Doped oxide source (either deposited at high temperature or as a Spin on polymer). 3.) Diffusion/annealing from an Ion implanted source. i ECE 6450 - Dr. Alan Doolittle Georgia Tech Impurity Diffusion: Traditional Tube Furnace Baffles used to mix gases Wafers Held in a quartz boat Hot Furnace ECE 6450 - Dr. Alan Doolittle Georgia Tech Ficks first law states that impurities flow (with flux J) toward a decrease in concentration, (1) The diffusion coefficient, D, also called diffusivity, or diffusion constant, characterizes a particular impuritys resistance to flow when exposed to an impurity gradient. We do not measure impurity gradients or impurity fluxes. These quantities are difficult to obtain. Thus, using the law of conservation of matter, (2) This second law simply states that the total change in flux leaving a volume equals the time rate of change in the concentration in the volume. x t x C D J = ) , ( x J t t x C = ) , ( Impurity Diffusion: ECE 6450 - Dr. Alan Doolittle Georgia Tech Plugging (1) into (2), one can rewrite Ficks first law as Ficks second law, (3) In certain special cases, D is independent of x, (4) We will examine various solutions of this differential equation later. More generally in 3D: (5) Note that generally, D=f( T , x , and even C). = x t x C D x t t x C ) , ( ) , ( 2 2 ) , ( ) , ( x t x C D t t x C = ( ) C D t C = Impurity Diffusion: ECE 6450 - Dr. Alan Doolittle Georgia Tech What distinguishes one impurity from another is its diffusivity....
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This note was uploaded on 08/23/2011 for the course ECE 6450 taught by Professor Doolittle during the Fall '10 term at University of Florida.

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ECE6450L3-Diffusion Chap 3 - ECE 6450 - Dr. Alan Doolittle...

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