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	300-950 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 ex: Silicon Oxide Silicon nitride PolySilicon Photoresists SU8 Other polymers Very thin layers of metals (<20 nm) and many more	Any film that is transparent to the light in the given wavelength range ex: Silicon Oxide Silicon nitride Very thin layers of PolySilicon Photoresists SU8 Other polymers Very thin layers of metals (<20 nm) and many more	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) ex: Silicon Oxide Silicon nitride PolySilicon Photoresists Other polymers Very thin layers of metals (<20 nm) and many more	In principle any that can be etched and stoped without overetching in the under laying material
Substrate size	small samples (at least 3mmx3mm) 50 mm wafers 100 mm wafers 150 mm wafers (any sample size below 150mmx150mm larger samples might also be possible, please ask	small samples (at least 1cmx2cm) 50 mm wafers 100 mm wafers 150 mm wafers (any sample size below 150mmx150mm larger samples might also be possible, please ask	small samples (at least 1cmx1cm) 50 mm wafers 100 mm wafers 150 mm wafers (any sample size below 150mmx150mm)	small samples (at least 3mmx3mm) 50 mm wafers 100 mm wafers 150 mm wafers (any sample size below 150mmx150mm larger samples might also be possible, please ask	Limited by the etching method and the stylus profling instrument.
Allowed materials	All materials - some needs a carrier - see the manaul for instructions	All materials	Almost all materials - NO sticky materials	All materials	Any material that can/may be etched and allowed that chosen etch equipment/bath

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 RIE's:
  • 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.