Device Fabrication Example

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 nMOStransistors • 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 (Hydroflouricacid)

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. PolysiliconPatterning • 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