PHY644
Basic Semiconductor Fabrication Lab.
Silicon (Si) wafer:
pre-doped with n-type dopant
1|P a g e
A. Wet Cleaning
Objective:
Cleaning the surface of Si wafer from native oxide and other contaminants such as organic and inorganic
particles and ions.
Equipment and Chemical:
Buffered oxide etchant (BOE)
De-ionized (DI) water
Dryer – spin dryer or blower (air or inert gas)
Method:
1. Immerse wafer in BOE for 60 seconds.
2. Rinse under flowing DI water.
3. Observe hydrophobicity.
4. Repeat (1-3) if wafer is hydrophilic.
5. Dry immediately.
6. Visual inspection – bare eyes and microscope – check for colour and particles
7. Thickness measurement – 300-400 Å
8. Keep in dry cabinet.
2|P a g e
B. Dry Oxidation
Objective:
Grow thin oxide layer on Si wafer
Equipment and Chemical:
Cleaned Si wafer
Oxidation furnace
Oxygen gas
Nitrogen gas
Method:
1. Place wafer in furnace.
2. Turn on furnace switches.
3. Open nitrogen gas valves -11 slm.
4. Set 200oC at temperature master control.
5. Increase temperature gradually until 1000oC.
Note: Increment of 200oC each time and wait 10 minutes after temperature has reach each set point.
6. When 1000oC is reached, wait 10 minutes for the system to reach equilibrium.
7. Close nitrogen gas valves.
8. Open oxygen gas valves– 12 slm.
9. Wait 60 minutes.
10. Close oxygen gas valves.
11. Open nitrogen gas valves – 11 slm.
12. Set 27oC at temperature master control.
13. Wait 5 minutes.
14. Close nitrogen gas valves.
15. Wait until temperature reach 27oC.
16. Turn off furnace switches.
17. Take out wafer.
18. Measure thickness – 800-1000 Å.
19. Calculate oxide thickness.
20. Calculate oxide growth rate from dry oxidation process.
21. Determine uniformity and non-uniformity.
3|P a g e
C. Wet Oxidation
Objective:
Grow thick oxide layer on Si wafer
Equipment and Chemical:
Cleaned Si wafer
Oxidation furnace with water bubbler
Oxygen gas
Nitrogen gas
Note:
Preparation prior to previous oxide growth from dry oxidation process.
Apply the method in A (Wet cleaning).
Method:
1. Place wafer in furnace.
2. Turn on furnace switches.
3. Open nitrogen gas valves -11 slm.
4. Set 200oC at temperature master control.
5. Increase temperature gradually until 600oC.
Note: Increment of 200oC each time and wait 10 minutes after temperature has reach each set point.
6. When 600oC is reached, turn on the water bubbler heater.
7. Set 800oC at temperature master control. Follow Note in (5.)
8. When 1000oC is reached, wait 10 minutes for the system to reach equilibrium.
9. Close nitrogen gas valves.
10. Assure that water has boiled and water vapor is produced.
11. Open oxygen gas valves – 12 slm.
12. Open water bubbler valves.
Note: Close the valve that let oxygen gas flow directly into the furnace. Let oxygen gas flow through
the water bubbler.
13. Wait 60 minutes.
14. Turn off water bubbler switches.
15. Close oxygen gas valves.
16. Open nitrogen gas valves – 11 slm.
17. Set 27oC at temperature master control.
18. Wait 5 minutes.
19. Close nitrogen gas valves.
20. Wait until temperature reach 27oC.
21. Turn off furnace switches.
22. Take out Si wafer.
23. Measure thickness – 4000-5000 Å.
24. Calculate oxide thickness.
25. Calculate oxide growth rate from wet oxidation process.
26. Determine uniformity and non-uniformity.
4|P a g e
D. Formation of windows (oxide wet etching)
Objective:
Form windows for dopant diffusion
Equipment and Chemical:
Si wafer coated with thick oxide layer
Spin coater
Hot plate
UV exposure
Positive Photoresist (PR)
Developer solution
Buffered oxide etchant (BOE)
De-ionized (DI) water
Acetone
Dryer – spin dryer/ air blower/ inert gas blower
Method:
I. Pattern transfer (pn junction)
1. Place Si wafer on spin coater.
2. Spin at 1800 rpm, 30 seconds
3. Drop 3 ml of positive PR (while spinning).
4. Pre-exposure (soft) bake 110oC, 5 minutes
5. Cool down wafer.
6. UV exposure 90 seconds – mask is transparent at proposed windows.
7. Post-exposure (hard) bake 110oC, 10 minutes
8. Cool down wafer.
9. Immerse in developer solution – dissolve exposed PR.
10. Observe colour changes at proposed windows - red to blue.
11. Rinse under flowing DI water.
12. Dry immediately.
13. Measure thickness at windows – 4000 Å (thick oxide)
II. Oxide etching
14. Immerse in BOE – etching of oxide layer at windows. Note immerse time and calculate etching rate.
15. Observe colour changes at windows, blue to silver.
16. Rinse under flowing DI water – observe hydrophobicity.
17. Dry immediately.
18. Observe under microscope – observe window profile/structure
19. Measure thickness at windows – 300-400 Å (bare silicon)
20. Immerse wafer in acetone – dissolve/strip off PR
21. Rinse under flowing DI water.
22. Dry immediately.
5|P a g e
E. Doping
Objective:
Form pn junction at windows
Equipment and Chemical:
Si wafer with formation of windows
Spin coater
Hot plate
Dopant solution
Diffusion furnace
Nitrogen gas
Oxygen gas
Method:
I. Spin coating of dopant
1. Place Si wafer on spin coater.
2. Spin at 2500 rpm, 30 seconds
3. Drop 3 ml of positive dopant solution (while spinning).
4. Heat on hot plate (soft) bake 150oC, 5 minutes
5. Measure sheet resistance (Rs) at windows.
II. Dopant diffusion
6. Load dopant-coated Si wafer into diffusion furnace.
7. Turn on furnace switches.
8. Open nitrogen gas valves -11 slm.
9. Set 200oC at temperature master control.
10. Increase temperature gradually until 1000oC.
Note: Increment of 200oC each time and wait 10 minutes after temperature has reach each set point.
11. When 1000oC is reached, wait 10 minutes for the system to reach equilibrium.
12. Close nitrogen gas valves.
13. Open oxygen gas valves– 12 slm.
14. Wait 60 minutes.
15. Close oxygen gas valves.
16. Open nitrogen gas valves – 11 slm.
17. Set 27oC at temperature master control.
18. Wait 5 minutes.
19. Close nitrogen gas valves.
20. Wait until temperature reach 27oC.
21. Turn off furnace switches.
22. Take out Si wafer.
23. Measure sheet resistance (Rs) at windows.
6|P a g e
F. Metallization
Objective:
Form metal contact
Equipment and Chemical:
Doped Si wafer
Physical vapour deposition (PVD) chamber
Aluminium foil
Tungsten filament
Spin coater
Hot plate
UV exposure
Positive Photoresist (PR)
Developer solution
De-ionized (DI) water
Metal etchant
Acetone
Dryer – spin dryer/ air blower/ inert gas blower
Method:
I. Metal deposition
1. Wrap aluminium on tungsten filament.
2. Load doped wafer in PVD chamber.
3. Open rough valve to decrease chamber pressure to 1.0 x 10-3 torr.
4. Close rough valve.
5. Turn on high vacuum (turbo) pump to decrease further until chamber pressure reach 1.0 x 10-5 torr.
6. Turn on high power supply to heat filament.
7. Observe the evaporation process. Turn off high power to filament once finished.
8. Turn off the vacuum system.
9. Take out wafer.
10. Observe colour on wafer – metallic silver
II. Metal etching
(a) Pattern transfer (metal)
11. Place Si wafer on spin coater.
12. Spin at 1800 rpm, 30 seconds
13. Drop 3 ml of positive PR (while spinning).
14. Pre-exposure (soft) bake 100oC, 5 minutes
15. Cool down wafer.
16. UV exposure 90 seconds – mask is opaque at windows.
17. Post-exposure (hard) bake 120oC, 10 minutes
18. Cool down wafer.
19. Immerse in developer solution – dissolve exposed PR.
20. Observe colour changes at pattern background - red to metallic silver
21. Rinse under flowing DI water.
22. Dry immediately.
7|P a g e
23. Observe under microscope – observe window profile/structure
(b) Metal etching
24. Heat metal etchant to 100oC.
25. Immerse wafer in metal etchant – etching of metal layer at pattern background.
Note immerse time and calculate etching rate.
26. Observe colour changes at pattern background – metallic silver to blue
27. Rinse under flowing DI water.
28. Dry immediately.
29. Observe under microscope – observe window profile/structure
30. Immerse wafer in acetone – dissolve/strip off PR
31. Rinse under flowing DI water.
32. Dry immediately.
8|P a g e