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 three techniques for this: Ellipsometry (ellipsometer), Reflectivity measurement (FilmTek+Optical Profiler) and prism coupling (Prism Coupler). 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), by refraction (using the FilmTek) or by prism coupling (using the prism coupler).
Comparison of the different metodes
| FilmTek 4000 | Ellipsometer | Prism couple | 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 | Best for thin films with refractive index <2.02 and not too thin | 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 | Prism Coupling | Reflection | Etch combined with stylus profiling |
| Film thickness range | <250 µm (for silicon oxides > ~75nm) | 20 Å to ~2µm (for silicon oxide) | ~1-15 µm (silicon oxide@633nm) | ~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. | ±(0.5%+50Å) | 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 | <2.02 | not any limit | No index measurement |
| Index accuracy | not known | not known | ±0.001 | not know | not relevant |
| Wavelength range | 400-1000 nm (with UV source down to 250nm) | 210-1690 nm | 633nm or 1550 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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Thin films with refractive index below 2.02 | 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.