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

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Outlook

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

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Place of Oxidation in Process Scheme

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

carbide films for MEMS applications, Atul Vir Singh

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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

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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

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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

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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

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Factors Affecting the Oxidation Rate

• Temperature

• Wet and dry conditions

• Pressure

• Crystal orientation

• Dopants

• Addition of chlorine

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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

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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

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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

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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)

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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

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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

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Thank You!

Any questions?

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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