1. N MOS Fabrication Process
Sun Tiefeng: 2008054238
Bhuvan Adhikari 2009002702

2. INTRODUCTION
 Photolithography
 Etching
 NMOS Fabrication process
• Gate
• Source and Drain
• Vias
• Metal Contacts

3. Part 1
What is photolithography
What is etching
Part 2
How photolithography and etching are
used when we make MOSFET

4. What is photo-lithography
Photolithography
a process used in micro-fabrication to
pattern parts of a thin film or the bulk of
a substrate.
It uses light to transfer a geometric
pattern from a photo-mask to a light-
sensitive chemical "photo-resist", or
simply "resist," on the substrate.

5. TYPES OF RESIST
• Positive resists
the resist is exposed with UV
light wherever the underlying
material is to be removed.
•Negative resists :
the negative resist remains on
the surface wherever it is
exposed

6. What is etching
To produce circuit features, the resist
patterns must be transferred into the
underlying layers comprising the device.
The pattern transfer is accomplish by an
etching process that selectively removes
unmasked portions of a layer

7. The two types of etching processes
Wet etch processes
can lead to undercutting
result in an isotropic etch profile
the vertical etch rate is equal to
the horizontal etch rate

8. The two types of etching processes
Dry etching
One of the most widely used
Use plasma generated free
radicals
Remove material only from the
area dictated by photo-resist Provide the ability to
control sidewall anisotropic
pattern etching

9. N-MOS Fabrication Process
Si-substrate
Fig. (1) Pure Si single crystal
-----------------------
-------------------------
--------------------------
--------------------------
Fig. (2) P-type impurity is lightly
doped

10. N-MOS Fabrication Process
Thick SiO2
(1 µm)
-----------------------
-------------------------
--------------------------
--------------------------
Fig. (3) SiO2 Deposited over si surface
Photoresist
Thick SiO2
(1 µm)
-----------------------
-------------------------
--------------------------
--------------------------
Fig. (4) Photoresist is deposited
over SiO2 layer

11. N-MOS Fabrication Process
UV Light
Mask-1
Photoresist
Thick SiO2
(1 µm)
-------------------------------
----------------------------------
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
----------------------------------
Mask-1 is used to expose the SiO2
where S, D and G is to be formed.
Fig. (5) Photoresist layer is
exposed to UV Light through a
mask

12. N-MOS Fabrication Process
Polymerised
Photoresist
-------------------------------
----------------------------------
Thick SiO2
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - (1 µm)
----------------------------------
Fig. (6) Developer removes unpolymerised photoresist. It
will cause no effect on Si surface

13. N-MOS Fabrication Process
Thick SiO2
(1 µm)
-------------------------------
----------------------------------
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
----------------------------------
Fig. (7) Etching [HF acid is used] will remove SiO2 layer
which is in direct contact with etching solution

14. N-MOS Fabrication Process
Thick SiO2
(1 µm)
-------------------------------
----------------------------------
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
----------------------------------
Fig. (7) unpolymerised photoresist is also etched away
[using H2SO4]

15. N-MOS Fabrication Process
Thin SiO2
(0.1 µm)
Thick SiO2
(1 µm)
-------------------------------
----------------------------------
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
----------------------------------
Fig. (8) A thin layer of SiO2 grown over the entire chip surface

16. N-MOS Fabrication Process
Polysilicon layer
(1 – 2 µm)
-------------------------------
---------------------------------- Thin SiO2
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Thick SiO2 (0.1 µm)
(1 µm)
----------------------------------
Fig. (9) A thin layer of polysilicon is grown over the entire chip
surface to form GATE

17. N-MOS Fabrication Process
Photoresist
Polysilicon
layer
------------------------------- Thin SiO2
---------------------------------- (0.1 µm)
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Thick SiO2
---------------------------------- (1 µm)
Fig. (10) A layer of photoresist is grown over polysilicon layer

18. N-MOS Fabrication Process
UV Light
Mask-2
-------------------------------
----------------------------------
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
----------------------------------
Mask-2 is used to deposit
Polysilicon to form gate.
Fig. (11) Photoresist is exposed to UV Light

19. N-MOS Fabrication Process
Polysilicon
Thin SiO2
(0.1 µm)
Thick SiO2
(1 µm)
-------------------------------
----------------------------------
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
----------------------------------
Fig. (12) Etching will remove that portion of Thin SiO2 which is
not exposed to UV light

20. N-MOS Fabrication Process
Polysilicon used as GATE
(1 – 2 µm)
Thin SiO2
(0.1 µm)
Thick SiO2
(1 µm)
-------------------------------
----------------------------------
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
----------------------------------
Fig. (13) Polymerised photoresist is also stripped away

21. N-MOS Fabrication Process
GATE
SOURCE DRAIN
Thin SiO2
(0.1 µm)
Thick SiO2
(1 µm)
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- --
----------------------------------
Fig. (14) n+ Doping to form SOURCE and DRAIN

22. N-MOS Fabrication Process
Step - Metallization
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Fig. (15) A thick layer of SiO2 (1 µm) is again grown.

23. N-MOS Fabrication Process
Step - Metallization
UV Light
Mask-3
Mask-3 is used to make contact cuts for S, D and G.
Photoresist
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Fig. (16) Photoresist is grown over thick SiO 2. Selected areas of the poly GATE and SOURCE and
DRAIN are exposed where contact cuts are to be made

24. N-MOS Fabrication Process
Step - Metallization
Mask-3
Photoresist
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Fig. (17) The region of photoresist which is not exposed by UV light will become soft. This
unpolymerised photoresist and SiO2 below it are etched away.

25. N-MOS Fabrication Process
Step - Metallization
Mask-3
Photoresist
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Fig. (18) The contact cuts are formed for S, D and G (hardened photoresist is stripped away).

26. N-MOS Fabrication Process
Step - Metallization
Metal (1µm)
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Fig. (19) Metal (aluminium) is deposited over the surface of whole chip (1 µm thickness).

27. N-MOS Fabrication Process
Step - Metallization
Photoresist
Metal (1µm)
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Fig. (20) Photoresist is deposited over the metal.

28. N-MOS Fabrication Process
Step - Metallization
UV Light
Mask-4
Photoresist
Metal (1µm)
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Mask-4 is used to deposit metal in contact cuts of S, D and G.
Fig. (21) UV Light is passed through Mask-4 (with a aim of removing all metal other than metal in
contact-cuts).

29. N-MOS Fabrication Process
Step - Metallization
Mask-4
Photoresist
Metal (1µm)
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- -- Thick SiO2 Thick SiO2
(1 µm) (1 µm)
----------------------------------
Fig. (22) Photoresist and metal which is not exposed to UV light are etched away.

30. N-MOS Fabrication Process
Step - Metallization
SOURCE DRAIN
GATE
- - - - - - - - - - - - -- ---- - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - - - - - - -- - - - - -- - - - - - - - - - - -
-- - - - - - - - - - - - - n+ - - - - - -n+ - - - - - - - - - -
-- --
----------------------------------
Fig. (23) Final n-MOS Transistor

31. THANK YOU