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The following method may be used to accurately determine (within a couple of
wavelengths) the thickness of your Si wafers (~200 um, I would call them
wafers rather than membranes!).

Place the sample normal (perpendicular) to the source in an IR
spectrophotometer. With a small collimated spot size,(less than the sample
diameter) record the spectrum of the sample over the wavelength interval
equivalent to about 10 sample thicknesses. Record the spectrum in a region
of the optical spectrum where silicon does NOT show large variations in
absorption. This is necessary in order that the refractive index remains
relatively constant, and its value within the wavelength window in which you
are working, may be easily be determined from a reference book. Note the
positions (in wavength (microns) ) of the maxima AND minima from the
spectrum. Plot a graph of (1/wavelenth) in microns, versus the maxima/minima
order number m. In other words count the first local max on your spectrum as
number one, then the next local min as two, and the next local max as
three....etc..etc...

Plot the graph of 1/wavelength in microns on the Y axis, against the order
number m, on the X axis. This should yield a straight line graph which has a
slope = 4nd.  In this case d, is the thickness of your sample in microns,
and n is the refractive index. The refractive index may be looked up in a
reference book.

Again, it is most important that you record the spectrum over a relatively
flat region of the spectral absorption profile of the sample. This
information is easily and readily available in most books on the optical
properties of solids.

I would suggest that for the thicker samples (> 10um) you shoud consider
using a Tenco profilometer, or a Talysurf profilometer.

For the thinner samples (~ 1um) the preferred and most accurate means of
determining the thickness, is to use a prism coupled laser beam into the
sample, and determine the thickness of the sample using m-line spectroscopy.
This method can also be used to determine the thickness of other thin films
which are optically transparent at the guided wavelength. The thickness can
be determined to within the wavelength of the probe source, and is VERY
precise and repeatable. However, some skill is required in acheiving a good
coupling point for the probe beam, and the prism must have a refractive
index which is very well characterised at the probe wavelength, as well as
being significantly higher in magnitude than the sample under investigation.

If I can be of any further assistance to you please let me know. Good luck!


Karl H. Cazzini (Ph.D)
Conifer Group & Associates,
116 Cochituate Road,
Framingham MA 01701
USA



                                                        Karl.



-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Dr.
Roumiana Paneva
Sent: Tuesday, October 27, 1998 10:20 AM
To: [email protected]
Subject: Si membrane thickness determination


Dear colleagues,

I am looking for an equipment for precise nondestructive thickness
determination of Si membranes with thicknesses from 1um up to about 20 um.
The membrane size is larger than 200um. The equipment should allow
measurements on 6" (and 5") wafers, has a x-y-table for measurements over
the wafer and a spot size smaller than 200um. I would prefer to measure the
real thickness of the membrane and not to use a profilometer or change of
microscope's focus to calculate the membrane thickness from the wafer
thickness. Information about IR spectrometer with a spot size smaller than
200um or another type of measuring system is deeply appreciated.

        Dr. Roumiana Paneva

        X-FAB GmbH
        Haarbergstasse 61
        D-99097 Erfurt
        Deutschland
        Tel: (+49) 361 42 053 21
        FAX: (+49) 361 42 053 11




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