Hello Xianling
You might want to try prism coupling technique to measure the refractive
index of film. The basic assumption of this method is you already know exact
thickness of film. Even thought, this technique is widely used for thin film
rather than thick film, I don't see any restriction in thickness range.

Good Luck

Sung

-----Original Message-----
From: X Chen [mailto:xianling@gpu.srv.ualberta.ca]
Sent: Friday, August 13, 2004 7:12 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] refractive index measurement of thick films

Hello MEMS talkers,

Does anyone know a way to measure refractive index of thick films? I used
spectrophotometer to measure the VIS spectral transmission of 50 micron
SU-8 on Borofloat wafer. However, the transmittance modulation is just too
weak for reliable index fitting. With a large film thickness, light
coherence is lost due to finite bandwidth and surface vaiation. Reducing
film thickness to a few microns using SU-8 thinner will solve this problem
but I doubt the index will be different. I need to have a precise
dispersion curve (index vs. wavelength) for optical devices. The index
mentioned in publised papers varies greatly and is available only for a
few individual wavelengths. This index variation could be caused by
different processing conditions. Therefore, the index should ideally be
measured for my own process. But I would appreciate if someone can give me
a dispersion curve for SU-8 in VIS-NIR range.

Best regards,

Xianling Chen