EE 245: Introduction to MEMS
Lecture 8: Surface Micromachining I
CTN 9/22/09
Copyright © 2009 Regents of the University of California
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
66
Diffusion Modeling
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
67
Diffusion Modeling (cont.)
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
68
Diffusion Modeling (Predeposition)
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
69
Diffusion Modeling (Limited Source)

This
** preview**
has intentionally

**sections.**

*blurred***to view the full version.**

*Sign up*
EE 245: Introduction to MEMS
Lecture 8: Surface Micromachining I
CTN 9/22/09
Copyright © 2009 Regents of the University of California
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
70
Diffusion Modeling (Limited Source)
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
71
Two-Step Diffusion
•
Two step diffusion procedure:
ª
Step 1
: predeposition (i.e., constant source diffusion)
ª
Step 2
: drive-in diffusion (i.e., limited source diffusion)
•
For processes where there is both a predeposition and a
drive-in diffusion, the final profile type (i.e.,
complementary error function or Gaussian) is determined by
which has the much greater Dt product:
(Dt)
predep
» (Dt)
drive-in
Ö
impurity profile is complementary
error function
(Dt)
drive-in
» (Dt)
predep
Ö
impurity profile is Gaussian (which
is usually the case)
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
72
Successive Diffusions
•
For actual processes, the junction/diffusion formation is only
one of many high temperature steps, each of which
contributes to the final junction profile
•
Typical overall process:
1. Selective doping
(
Implant
→
effective (Dt)
1
= (
Δ
R
p
)
2
/2
(Gaussian)
(
Drive-in/activation
→
D
2
t
2
2. Other high temperature steps
(
(eg., oxidation, reflow, deposition)
→
D
3
t
3
, D
4
t
4
, …
(
Each has their own Dt product
3. Then, to find the final profile, use
in the Gaussian distribution expression.
(
)
i
i
i
tot
t
D
Dt
∑
=
EE C245
: Introduction to MEMS Design
LecM 4
C. Nguyen
8/20/09
73
The Diffusion Coefficient
(as usual, an Arrhenius relationship)
⎟
⎠
⎞
⎜
⎝
⎛
−
=
kT
E
D
D
A
o
exp