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[SI-LIST] Re: Dielectric Constant - thin materials - DC to 10GHz

From: "George Tang" <gtang@xxxxxxxx>
To: <Farah.Haddadin@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
Date: Mon, 1 Dec 2003 11:12:43 -0800

Hi Farah,

Yes, you do need to have 2 fixtures, one for low frequency, and another for
high frequency.  For low frequency, you can measure capacitance of two
square parallel plates separated by the dielectric material under test.  Cut
the parallel plates along with the dielectric material into a 10cm by 10cm
square, or another dimension that results in a capacitance value which fall
within the range of your measuring equipment.  Use the following equation to
calculate the dielectric constant, Er:

C = (Er * Eo * plate-area) / dielectric thickness

Most capacitance meters measure capacitance at a few KHz, which is fairly
close to DC.

For GHz measurement, the best way is to measure the wave-length of a
standing wave.  Cut your parallel plate along with the dielectric material
into a 6 inch long by 100 mil wide strip, (assuming the dielectric thickness
is 1mm).  This will make the parallel plate characteristic impedance
approximately 50 ohms.  The exact characteristic impedance is not critical,
but it makes the measurement easier and more accurate if Zo is greater than
40 ohms.  At the center of the parallel plate (3 inch from each end), solder
two coaxial cables with the center wires connected to the top plate and the
shields connected to the bottom plate, while the 2 cables are orthogonal
from each other to minimize coupling.  Measure S21 on a network analyzer by
doing a sweep from 100 MHz to 3 GHz.  Capture the lowest frequency minimum
on the S21 curve.  That's the fundamental resonance (lambda/2) of the
standing wave.  Calculate the wave length of that frequency in air and in
the dielectric material.  Use the following equation to calculate Er:

lambda (in dielectric) = lambda (in free space) / sqrt(Er)

As you sweep to higher frequency range, you will see another S21 minimum at
approximately 3x the frequency.  Do the same calculation using 1.5 lambda
for your standing wave.  You can see the variation of Er as a function of
frequency.

Regards,

George

LSI Logic Corp.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Haddadin, Farah
Sent: Monday, December 01, 2003 8:51 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Dielectric Constant - thin materials - DC to
10GHz




Hello everyone,

I am looking for a fixture that is capable of measuring permittivity of
thin materials from DC - 10GHz.  There are two issues here:

1)  The wide band of frequency:  do I have to create two fixtures: one
for low frequency and one for high?
2)  The thickness of the material to be tested:  I was told that the
thickness of the material should roughly be: 20/sqrt(Er) [mm].  For
example, for a permittivity of 4, the thickness of the material under
test should be 10 mm.  This is a conservative thickness estimator, but
the thickness of the materials of my interest is  0.1mm - 1mm.

I would greatly appreciate it if someone can point me to an existing
fixture or methodology to test such thin materials for the frequency
range mentioned above.  Also, is there any reading material on the web
on how to create fixtures?

Thank you,
Farah



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[SI-LIST] Re: Dielectric Constant - thin materials - DC to 10GHz
From: Haddadin, Farah

[SI-LIST] Re: Dielectric Constant - thin materials - DC to 10GHz

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