1. Device Fabrication Example
Group:- 2

2. pn junction Diode Fabrication
Start:-
The starting point is a flat, damage-free , single- crystal, Si wafer.
Common dopants are boron for P-type layers and phosphorus, antimony and Arsenic for N-type layers.
Assume the wafer is p-type, having been uniformly doped with boron during the formation of the crystal.

3. 1.Oxidation:-
The process of oxidation consists of growing a thin film of silicon dioxide on the surface of the silicon wafer.
It will serve as a diffusion barrier.
The oxide thickness must be comfortably greater than the projected masking thickness.

4. 2.Lithography#1:-
This process performed to open “diffusion” holes in the oxide that will eventually become the positions of the pn junction diodes.

5. 3.Phosphorus Diffusion
After a proper clean-up the wafer is nest inserted into a phosphorus pre-deposition Furnace.
The formation of n+ -p junctions is surface regions not protected by the oxide. (The + in n+ is indicate a very high doping.

6. 4.Metallization(sputter Al)
Evaporation of Al yields a thin metal film over the entire surface of the wafer due to connect the device to the ‘’outside world’’

7. 5.Lithography #2:-
It is performed to remove excess metal external to the area of the diffused junction.

8. To produce commercial diodes, a diamond-edged saw would be used to cut the wafer into pieces containing a single device.

11. n-well process Fabrication Steps
Typically use p-type substrate for
nMOS transistors
Requires n-well for body of pMOS
transistors

12. Oxidation
Blank wafer covered with a layer of SiO2 using oxidation

13. Photoresist Spin on photoresist
Photoresist is a light-sensitive organic polymer
Softens where exposed to light

14. Lithography
Expose photoresist through n-well mask

15. Etch
Etch the uncovered oxide using HF (Hydroflouric acid)

16. Strip Photoresist
Etch the remaining photoresist using a mixture of acids

17. n-well
n-well is formed using either diffusion or ion implantation

18. Strip Oxide
Strip off remaining oxide using HF. Subsequent steps use the same photolithography process

19. Polysilicon Deposit thin layer of oxide.
Chemical Vapor Deposition (CVD) of silicon layer
Forms many small crystals called polysilicon
Heavily doped to be good conductor

20. Polysilicon Patterning
Use same lithography process to pattern polysilicon

21. Self-Aligned Process
Cover with oxide to define n diffusion regions

22. N-diffusion
Pattern oxide using n+ active mask to define n diffusion regions

23. N-diffusion cont.
Diffusion or ion implantation used to create n diffusion regions

24. N-diffusion cont.
Strip off the oxide to complete patterning step

25. P-Diffusion
Similar set of steps form p+ diffusion regions for pMOS source and drain and substrate contact

26. Contacts Cover chip with thick field oxide
Etch oxide where contact cuts are needed

27. Metallization Sputter on aluminum over whole wafer
Remove excess metal leaving wires

28. Detailed Mask Views Six masks n-well Polysilicon n+ diffusion
p+ diffusion
Contact
Metal
Fabrication and Layout

30. 3D Structure

31. Thanks