Diffusion #2.ppt

Diffusion #2 ECE/ChE 4752: Microelectronics Processing Laboratory Outline Objectives Double Diffusions Concentration-Dependent Diffusion Diffusion in Silicon Lateral Diffusion Objectives Discuss the concept of double diffusions, an important part of how we fabricate our CMOS transistors in the lab. Introduce some “second-order” diffusion effects. Outline Objectives Double Diffusions Concentration-Dependent Diffusion Diffusion in Silicon Lateral Diffusion After p-well Diffusion After NMOS Source/Drain n+ Diffusion Notation: p-well Pre-dep Boron doping Pre-Dep: tpp @ Tpp = Dpp, Cspp tpp = p-well pre-dep time Tpp = p-well pre-dep temperature Dpp = p-well diffusion constant at pre-dep temperature Cspp = surface concentration for p-well pre-dep Notation: p-well Drive-in tpd @ Tpd = Dpd tpd = p-well drive-in time Tpd = p-well drive-in temperature Dpd = p-well diffusion constant at drive-in temperature Notation: n+ Source/Drain Pre-dep Phosphorus doping Pre-Dep: tnp @ Tnp = Dnp, Csnp; Dp1 tnp = n+ source/drain pre-dep time Tnp = n+ source/drain pre-dep temperature Dnp = n+ source/drain diffusion constant at pre-dep temperature Csnp = surface concentration for n+ source/drain pre-dep Dp1 = boron diffusion constant at source/drain pre-dep temperature Notation: n+ Source/Drain Drive-in tnd @ Tnd = Dnd; Dp2 tnd = n+ source/drain drive-in time Tnd = n+ source/drain drive-in temperature Dnd = n+ source/drain diffusion constant at drive-in temperature Dp2 = boron diffusion constant at source/drain drive-in temperature Profile: After p-well Diffusion Profile: After n+ Source/Drain Diffusion New Well Profile There is a pn-junction xj1 where ND(x) = NA(x) There is a new pn-junction xj2 where NA(x) = Csub (where xj2 xj0) Example Suppose we want to design a p-well CMOS diffusion process with a well depth of xj2 = 2.5 mm. Assume the n-type substrate doping is 1015 cm-3. Example (cont.) If we start with a boron pre-dep with a dose of 5 × 1013 cm-2, followed by a 1-hr drive-in a