1. Oxidation
Farshid Karbassian

2. Outline Oxide Layer Applications Types of Oxidation Modeling
Dry Oxidation
Wet Oxidation
Modeling
C-V Measurement

3. Oxide Layer Applications
Name of the Oxide
Thickness (Å)
Application
Time in Application
Native
15-20
Undesirable -
Screen
~200
Implantation
Mid 70s to present
Masking
~5000
Diffusion
1960s to mid 70s
Field & LOCOS
Isolation
1960s to 90s
Pad
1960s to present
Sacrificial
<1000
1970s to present
Gate
30-120
Barrier
STI
1980s to present
Nitride stress buffer
Defect removal
Gate dielectric

4. Oxide Applications: Native Oxide
Purpose: This oxide is a contaminant and generally undesirable. Sometimes used in memory storage or film passivation.
Comments: Growth of native oxide layer at room temperature takes 3-4 hours up to about 12 Å.
p+ Silicon substrate
Silicon dioxide (oxide)

5. Oxide Applications: Gate Oxide
Purpose: Serves as a dielectric between the gate and source-drain parts of MOS transistor.
Gate oxide
Transistor site
p+ Silicon substrate
Source
Drain
Gate
Comments: Common gate oxide film thickness range from about 30 Å to 50 Å. Dry oxidation is the preferred method.

6. Oxide Applications: Field Oxide
Purpose: Serves as an isolation barrier between individual transistors to isolate them from each other.
Field oxide
Transistor site
p+ Silicon substrate
Comments: Field oxide thickness ranges from 2,500 Å to 15,000 Å. Wet oxidation is the preferred method.

7. Oxide Applications: Barrier Oxide
Purpose: Protect active devices and silicon from follow-on processing.
Comments: Deposition to several hundred Angstroms thickness.
Barrier oxide
Diffused resistors
Metal
p+ Silicon substrate

8. Oxide Applications: Pad Oxide
Purpose: Provides stress reduction for Si3N4
Comments: Very thin layer of oxide is deposited.
Passivation Layer
ILD-4
ILD-5
M-3
M-4
Pad oxide
Bonding pad metal
Nitride

9. Oxide Applications: Implant Screen Oxide
Purpose: Sometimes referred to as “sacrificial oxide”, screen oxide, is used to reduce implant channeling and damage. Assists creation of shallow junctions.
Ion implantation
Screen oxide
High damage to upper Si surface + more channeling
Low damage to upper Si surface + less channeling
p+ Silicon substrate
Comments: Thermally grown

10. Oxide Applications: Insulating Layer between Metals
Purpose: Serves as protective layer between metal lines.
Comments: Deposition
Passivation layer
ILD-4
ILD-5
M-3
M-4
Interlayer oxide
Bonding pad metal

11. LOCOS Process Cross section of LOCOS field oxide
(Actual growth of oxide is omnidirectional)
1. Nitride deposition
Pad oxide
(initial oxide)
Silicon
2. Nitride mask & etch
Nitride
3. Local oxidation of silicon
SiO2 growth
4. Nitride strip
SiO2

12. Selective Oxidation and Bird’s Beak Effect
Silicon oxynitride
Nitride oxidation mask
Bird’s beak region
Selective oxidation
Pad oxide
Silicon substrate
Silicon dioxide

13. STI Isolation Cross section of shallow trench isolation (STI)
Silicon
Trench filled with deposited oxide
Sidewall liner
1. Nitride deposition
Pad oxide
(initial oxide)
2. Trench mask and etch
Nitride
4. Oxide planarization (CMP)
5. Nitride strip
Oxide
3. Sidewall oxidation and trench fill
Oxide over nitride

14. Pre-oxidation Cleaning
Crystallization of silicon dioxide is very undesirable, since it is not uniform and crystal boundaries provide easy paths for impurities and moisture.
Therefore, pre-oxidation wafer cleaning is performed to eliminate crystallization.

15. Pre-oxidation Cleaning
Pre-oxidation cleaning is performed to remove particles, organic and inorganic contaminants, native oxide and surface defects.

16. RCA Cleaning RCA Standard Cleaning I (SC-1)
NH4OH:H2O2:H2O 1:1:5 – 1:2:7 (70-80OC)
DI water
RCA Standard Cleaning II (SC-2)
HCl:H2O2:H2O 1:1:6 – 1:2:8 (70-80OC)
RCA: Radio Corporation of America

17. RCA Cleaning
When wafers are submerged in RCA I solution, particles and organic contaminants oxidize, and their byproducts are either gaseous (e.g. CO), or soluble in the solution (e.g. H2O).
In RCA II, H2O2 oxidizes the inorganic contaminants and HCl reacts with the oxides to form soluble chlorides, which allows desorption of contaminants from the wafer surface.

18. HF etching
Native oxide on Si is of poor quality and needs to be stripped, especially for the gate oxide which requires the highest quality.
This is performed either in HF:H2O solution or in HF vapor etcher.
After native oxide stripping, some F atoms bind with Si atoms and form Si-F bonds on the silicon surface.

19. Thermal Oxidation Process Flow Chart
Wet Clean
Chemicals
% solution
Temperature
Time
Oxidation Furnace
O2, H2 , N2 , Cl
Flow rate
Exhaust
Temperature profile
Inspection
Film thickness
Uniformity
Particles
Defects

20. Thermal Oxidation
Depending on the quality and thickness which is required for the oxide layer, wet or dry oxidation may be used.
Former is faster, but latter is cleaner and makes a better interface.

21. Dry Oxidation
In dry oxidation, pure oxygen gas (5s at least) is used. At high temp. O2 molecules diffuse across an existing oxide layer to reach the Si/SiO2
interface.

22. Dry Oxidation

23. Diffusion of Oxygen Through Oxide Layer
TEM image of Si/SiO2 Si
SiO2
O, O2
Oxide-silicon interface
Oxygen-oxide interface
Oxygen supplied to reaction surface

24. Horizontal Diffusion Furnace

25. Vertical Diffusion Furnace

26. Horizontal and Vertical Furnace

27. Vertical Furnace Process Tube
Heater 1
Heater 2
Heater 3
Thermocouple measurements
Temperature
controller
Profile TCs
Control TCs
Overtemperature TCs
System TC

28. Si/SiO2 Interface Silicon Dangling Bond Si Si-SiO2 Interface Oxygen
Interface State Charge (Positive)
SiO2

29. Consumption of Silicon during Oxidation
Before oxidation
After oxidation

30. Wet Oxidation
At high temp. H2O dissociates and form hydroxide, HO, which can diffuses in the SiO2 layer faster than O2 .
A wet oxidation system may have a boiler or a bubbler or maybe it is a pyrogenic steam system, which is more common.

31. Wet Oxidation System
Pyrogenic Steam System

32. Wet Oxidation System
Bubbler System

33. Dry Oxidation Vs. Wet Oxidation

34. Deal/Grove (Kinetic) Model
Assumptions:
Temperature: oC
Pressure: atm
SiO2 thickness: μm
x2 + A x = B(t + τ) ;
τ = time for initial oxide thickness d0
A = 2 D /κ
B = 2 D C0 / C1
x = [B t A2 + d02 + A d0]0.5 – A / 2

35. Oxide Measurement
Color chart

36. Oxide Measurement
C-V Measurement

37. Any questions?