VLSI fabrication -Twin Tub Processes.ppt

twin tub

1. Twin Tub Processes
•Twin-tub CMOS technology provides the
basis for separate optimization of the p-
type and n-type transistors.
•One can optimize independently for
threshold voltage, body effect, and the
gain associated with n- and p-devices.

2. Contd…
A. Starting material: an n+ or p+ substrate with lightly
doped -> "epitaxial" or "epi" layer -> to protect "latch up"
B. Epitaxy"
a. Grow high-purity silicon layers of controlled
thickness
b. With accurately determined dopant concentrations
c. Electrical properties are determined by the dopant
and its concentration in Si
C.Process sequence
a. Tub formation
b. Thin-Oxide construction
c. Source & drain implantations
d. Contact cut definition e. Metallization

3. •The main advantage of this process is that
the threshold voltage, body effect
parameter and the transconductance can
be optimized separately. The starting
material for this process is p+ substrate
with epitaxially grown p-layer which is also
called as epilayer.
 

4. Twin Tub Process: N-well / P-well
First place wells to provide properly-doped
substrate for n-type, p-type transistors:

5. Twin Tub Process: Polysilicon
Pattern polysilicon before diffusion regions:

6. Twin Tub Process: N+/ P+ Diffusion
Add diffusions, performing self-masking:

7. Start adding metal layers:
Twin Tub Process: Contact / Via / Metal

9. Twin-well CMOS process cross section

10. Twin Tub CMOS Process Cross Section

11. ---Struggle gives Strength and Dignity....