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. We have various techniques for this: Ellipsometry (with the Ellipsometer VASE) and reflectivity measurements (with the FilmTek or optical profilers) or X-ray diffractometer.

If the thin film is not transparent to light or there is insufficient reflectance at the interfaces in your sample, you must find another way to measure the thickness. You can for instance etch a pattern down to the next layer or substrate and then measure the etch step by profilometry. Or if you deposit a layer by PVD you may be able to physically mask part of your sample or an adjacent dummy sample to create a sharp step that can be measured by profilometry (etching will usually create a sharper step, but a physical mask can give you a quick-and-dirty estimate). Another option not described further on this page is to cleave the sample to create a chip whose cross-section can be imaged in an SEM .

Measurement if the optical constants of a thin film is measured together with the thickness of the film either by ellipsometry or by refraction (using the FilmTek). Some of these methods also allow you to indirectly determine, e.g., the surface roughness of the layers in your sample, or, by X-ray reflectivity, the density of the layers.

Comparison of the different methods

	FilmTek 4000	Ellipsometer	Optical Profiler	XRD SmartLab	Etch/mask + stylus method
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)	Good for very thin films down to a few Å	Technique for thickness measurement for non-transparent films
Method	Reflection	Ellipsometry	Reflection	X-ray reflection	Etch or physical mask 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 ~10 µm (for silicon oxide)	~30 nm-20 µm (down to 10nm when using the base without the microscope)	few nm to approx. 100 nm (somewhat thicker for low Z materials)	~100 nm to 1 mm
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 possibility)	Very good, depends on the model fit	Depends on end-stop accuracy (of the etch) or step sharpness (if a physical deposition mask is used)
Index range	not any limits	not any limits	not any limit	No index measurement	No index measurement
Index accuracy	not known	not known	not know	not relevant	not relevant
Wavelength range	400-1000 nm (with UV source down to 250nm)	210-1690 nm	350-950 nm	Cu K_α radiation (1.5406 Å)	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 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	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	Any film that is transparent to the X-rays including thin layers of metals directly on Si wafers	Any that can be etched and stopped without overetching in the underlying material or any material deposited with PVD and partly physically masked.
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 (smallest spotsize 125µm) 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 3mmx3mm) 50 mm wafers 100 mm wafers 150 mm wafers (any sample size below 150mmx150mm larger samples might also be possible, please ask	up to 150 mm wafers (less signal if the sample is smaller than ~1x1 cm)	Limited by the etching method and the stylus profiling instrument.
Allowed materials	All materials - some need a carrier - see the manual for instructions	All materials	All materials	All materials	Any material that can/may be etched and allowed that chosen etch equipment/bath or that can be deposited by PVD using a physical mask
Comments		If no model exists for your layer structure, developing a model may require some work. The instrument has many models available already.		Measurement and analysis quite time consuming. Can determine layer roughness and density including density gradients	Direct measurement.