Graduate Student Presentations

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1 Graduate Student Presentations Dang, Huong Chip packaging March 27 Call, Nathan Thin film transistors/ liquid crystal displays April 4 Feldman, Ari Optical computing April 11 Guerassio, Ian Self-assembly alternative to lithography April 11 Leenheer, Andrew Next Generation Lithography April 18 Porpora, Daniel Carbon nanotube based microelectronics April 18 Rance, Will 3-d chip architectures April 25 Szymanski, Scott* Transition metal thin film interconnects April 25

2 Where were we? Si solar Cell Two Levels of Masks - photoresist, alignment Etch and oxidation to isolate thermal oxide, deposited oxide, wet etching, dry etching, isolation schemes Doping - diffusion/ion implantation Metallization - Materials deposition, PVD, CVD

3 Doping Reminder Goal of Doping: Substitution of atoms with excess or deficiency of valence electrons e.g. B or P substituting for Si Diffusion doping is typically done in two steps: (Almost all doping is now ion implantation) Predeposition - Fixed concentration at surface, desired dose diffuses in Drive in - Source at surface removed, additional diffusion to get desired distribution Spin on Anneal Strip Anneal

4 A few questions Phosphorus is 1) an n-type dopant 2) a p-type dopant Phosphorus diffuses 1) Interstitially 2) by exchanging places with Si atoms 3) on vacancy sites If you expose photoresist through a mask and the dark regions in the mask do not have photoresist on them after you develop, the photoresist was 1) positive 2) negative 3) neutral

5 Drive-in - estimating the profile Fick s law - You need the PDE, but you also need the boundary conditions! C(z,0) = 0, Z 0 dc(0,t)/dz = 0 C(,t) = 0 0 C( z, t) dz = Solution: C(z, t) = Q T 2 -z Q T 4Dt πdt = e constant We can model the drive in step from our homework, here after predep we had a sheet resistance of 12Ω/ and depth of 1.1µm. This gave a carrier concentration of 5x10 19 /cm 3 and a surface concentration of 5.5x10 15 /cm 2 Characteristic Length Scale - Diffusion Length

6 What about the diffusion Coefficient? Use first three terms in Fair s vacancy model. D = D o + n n i D + n n i 2 D 2 I told you to assume n~n i ~10 19 /cm 3 Is this reasonable? From Campbell table 3.2 (1100C=1373K) D o = 3.9cm 2 /s e -(3.66/k1373) = 1.43 x cm 2 /s D - = 4.4cm 2 /s e -(4.0/k1373) = 9.13 x cm 2 /s D 2- = 44cm 2 /s e -(4.37/k1373) = 4.00 x cm 2 /s D = 1.56 x cm 2 /s

7 Simulations Suprem-IV is a process simulation tool developed at Stanford University Phosphorous drive-in Log10(Phosphorus) Suprem calculation Gaussian Approximation Depth in microns

8 Suprem simulation of boron predep and drive-in Boron Diffusion Log10(Boron) Depth in microns Boron Predep 1100C 30 min. Boron drivein 1100C 30 min. Boron drivein 1100C 60 min. Boron drivein 1100C 60 min 200 angstrom oxide cap Boron predep in gas at 5 x /cm 3 concentration followed by drive-ins. Effect of oxide cap on profile near the surface Boron Diffusion Why 5x10 20 /cm 3? 1) Damage threshold 2) Solubility limit 3) B partial pressure 1) Dimensional argument Log10(Boron) Depth in microns

9 Solid Solubility, what is it? 1100C 5x10 20 /cm 3

10 Oxide is an effective anti-diffusion barrier for Si VLSI? 1) For boron but not for phosphorus 2) For phosphorus but not for boron 3) It works well for both 4) It depends

11 Final Topic on Diffusion: Oxide How fast do dopants diffuse through oxide? Diffusivity important, Solubility important Consider D o of Boron Si prefactor 0.37cm 2 /s Activation Energy 3.46eV SiO 2 prefactor cm 2 /s Activation Energy 3.53eV Now D o of Phosphorous Si prefactor 3.9 cm 2 /s Activation Energy 3.66eV SiO 2 prefactor 0.19 cm 2 /s Activation Energy 4.03eV Oxide is often used as a diffusion mask- how thick does it need to be? Oxide is used for isolation - does it isolate? What is the thermal load? Oxide is also a gate dielectric with heavily B doped polysilicon gates - diffusion through gate is an issue M Metal O S Oxide Silicon Doped polysilicon

12 Suprem-IV Wet Oxide then Diffusion Oxide antidiffusion barrier 20 Log10(Boron) min wet O2 at 1000C, 30 min boron predep at 1100C 30 minute boron predep at 1100C 0 Effect of oxide cap on profile near the surface Depth in microns Substrate is P doped at 1 x /cm 3, Wet oxide growth at atmospheric pressure for 60 minutes at 1000C, Boron predep from 30 minutes at 1100C in gas with a concentration of 5 x /cc.

13 Simulation of predep and drive-in to find junction depth 1000 C P predep in p-type wafer doped at 1x10 17 /cm C drive in. How long to get a 4.0µm deep junction? Junction Depth Log10(concentration) Depth (um) Predep (20 min., 1000C) Drivein (3 hr, 1100C, oxide cap) Drivein (6 hr, 1100C, oxide cap) Drivein (9 hr, 1100C, oxide cap) Drivin (12hr, 1100C, oxide cap) Boron (after all anneals) Log10(concentration) Junction Depth Depth (um)