Specific Process Knowledge/Characterization/Measurement of film thickness and optical constants
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Measurement of Film Thickness and Optical Constants
Thickness measurement of thin films can be done by optical measurement methods. At DANCHIP we have two techniques for this: Ellipsometry (ellipsometer) and Reflectivity measurement (FilmTek+Optical Profiler). If the thin film is not transparent for light then you must find another way to measure the thickness. You can ex. etch a pattern down to the next layer or substrate and then measure the etch step by profilometry. See description further down.
Measurement if the optical constants of a thin film is measured together with the thickness of the film either by ellipsometry (using the ellipsometer),or by refraction (using the FilmTek).
Comparison of the different metodes
| FilmTek 4000 | Ellipsometer | Optical Profiler | Etch/stylus metod | |
|---|---|---|---|---|
| Generel description | Thin films up to 250 µm, Especially good for thick thin films and for wafer mapping | Good for very thin films down to a few Å | Thin films up to 20µm, especially good for measuring in a small point (down to 4µm) | Technique for thickness measurement for non-transparent films |
| Method | Reflection | Ellipsometry | Reflection | Etch combined with stylus profiling |
| Film thickness range | <250 µm (for silicon oxides > ~75nm, thinner layer can be measured using the UV light source) | 20 Å to ~2µm (for silicon oxide) | ~30nm-20µm (down to 10nm when using the base without the microscope) | ~100nm-several micrometers |
| Film thickness accuracy | Very dependent of how good the model fits (if the fit is good it could be within 1% for a single layer) | Very dependent of how good the model fits. | Very dependent of how good the model fits.(Not so good fitting posibility) | Depends on how good the end-stop works |
| Index range | not any limits | not any limits | not any limit | No index measurement |
| Index accuracy | not known | not known | not know | not relevant |
| Wavelength range | 400-1000 nm (with UV source down to 250nm) | 210-1690 nm | 350-950 nm | Not relevant |
| What kind of thin films can be measured | Any film that is transparent to the light in the given wavelength range
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Any film that is transparent to the light in the given wavelength range
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In principle any film that is transparent to the light in the given wavelength range. It is limited by the refractive index files in the software and only one model (Cauchy)
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Limited by the etching method and the stylus profling instrument. |
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Film thickness measurement of Polysilicon film (non-transparent >~4µm): Etch + profilometry method
Can be used to find the thickness for a polysilicon layer on top of a silicon oxide layer. This will however etch away part of the polysilicon layer so you need to use a test wafer.
Procedure:
- Etch the polySi away on ½ the wafer in one of the RIE2:
- Place the wafer on a 4" Al-carrier.
- Place ½ a silicon wafer on top.
- Put on the Al-carrier ring to make sure the ½ silicon wafer does not fall off.
- Transfer it to the RIE chamber.
- Select the a Si etch recipe (ex. OH_PolyA, make sure the etch time is long enough).
- Start the etch.
- Watch the end point.
- When the end point occures about the recipe.
- Take out the wafer.
- Measure the etch step in one of the profilers.