Thermal Oxidation

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Thermal Oxidation of Silicon WafersMay 9, 2012Ozan Sarıkaya, Naman Singh Negi, Öncü Güneş Atar


Outlook

• Introduction• Oxidation• Deal Grove Model• Inspection• Contamination• Impurity Redistribution• Anisotropy Effect• Conclusion


Place of Oxidation in Process Scheme

*Mechanical and structural properties of RF magnetron sputter-deposited silicon

carbide films for MEMS applications, Atul Vir Singh


Why do we oxide Si wafers?

Silicon dioxide used as:• Capacitor dielectric • Isolation material

• Masking material during - diffusion - etching processes

• Cleaning method to reclaim perfect silicon surface

Part of finished device

Used intermittently


Properties of

Thermal is amorphous.

• Excellent Electrical Insulator Resistivity > 1E20 ohm-cm Energy Gap

~ 9 eV

• High Breakdown Electric Field > 10MV/cm

• Stable and Reproducible Si/ interface

http://inst.eecs.berkeley.edu/~ee143/fa08/lectures/Section%204%20-%20Thermal%20Oxidation.pdf


Wet vs. Dry Oxidation

Wet Oxidation

• 1000 - 1200 °C• Preferred for thicker oxides• Oxide layer grows faster• More dangling bonds(dirtyinterface, causes leakages ofcurrent)• Yields lower density

oxide(lower dielectric strength)

Dry Oxidation

• 800 - 1000 °C• Used for thinner oxides• Oxide layer grows slower,

hence impractical for thicker growth

• Less dangling bonds


Deal Grove Model

(for large value of oxide thickness ‘x’)

(for small values of oxide thickness ‘x’)

Gas Silicon Oxide

Silicon

SiO2/Si Interface

B= Parabolic Rate ConstantB/A= Linear Rate Constant


Factors Affecting the Oxidation Rate

• Temperature

• Wet and dry conditions

• Pressure

• Crystal orientation

• Dopants

• Addition of chlorine


Inspection Techniques

After the oxidation process the wafer is inspected in the following three steps:

1. Surface inspection: scanning of the surface using UV light

2. Oxide thickness: • SEM• Interference• Ellipsometers• Color Comparison• Fringe counting

3. Oxide cleanliness: Capacitive-voltage techniques to detect the total number of ionic contaminates present in the oxide


Color Chart to Determine Oxide Thickness

Color Silicon Dioxide Thickness(nm)

Color Silicon Dioxide Thickness(nm)

Silver < 270 Yellow < 2000Brown < 530 Orange-red < 2400Yellow-brown < 730 Red < 2500Red < 970 Dark red < 2800Deep blue < 1000 Blue < 3100Blue <1200 Blue-green < 3300Pale blue < 1300 Light green < 3700Very pale blue < 1500 Orange-yellow < 4000Silver < 1600 Red < 4400Light yellow <1700

http://matec.org/ps/library3/secure/modules/033/LA4/M033LA4.html




Contamination•Critical for oxide quality

•Mobile ion contamination

•Especially crucial for MOS structures

•Sodium ion in gate oxides

•Solution: Add chlorine to oxidizing gas

•In the form of HCl, Cl2 or C2HCl3 (TCE), C2H3Cl3 (TCA)

•Problem – TCA is poisonous at high temperature, TCE is

carcinogenic

•Immobilization of sodium ions

•By-product: increased rate constants


Impurity Redistribution

• Also known as – Segregation

• Dopant atoms diffuse to different locations during

oxidation

• Boron, Gallium deplete from the surface

• Phosphorus, Arsenic and Antimony pile up at the surface

• Governed by segregation (m) and diffusion

coefficients

• For m>1, oxide rejects impurity, accumulation at

interface

• Other metallic impurities also experience segregation

• Al, Ca: segregated into oxide (quality problems)

• Ni, Cu: diffuse into bulk (defects, lifetime issues)


Crystal Orientation / Anisotropy

• Number of bonds at the surface depends on crystal orientation• Influences growth rate and quality

[Franssila, 2004]

http://www.inems.com/mems_course_area


Conclusions

• (Usually) Stands in between cleaning and etching in process sequence

• Non-uniform oxide thickness• Presence of contaminants• Redistribution of dopants

• Imperfections during oxidation influence the quality of further steps.

Determine the process quality


Thank You!

Any questions?


References

Campbell S., The Science and Engineering of Microfabrication, 2001Franssila S., Introduction to Microfabrication, 2004Senturia S., Microsystem Design, 2002Jaeger R., Introduction to Microelectronic Fabrication, 2001Pierret R., Field Effect Devices, 1990

Thermal Oxidation

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