Ghazal,

In this type of system, the HeNe beam is normal to the sample and you
need to measure only the intensity of the reflected beam.  The beam is
usually directed through an optical port at the top of the chamber via
a fiber or mounted directly on the chamber.  A photodiode is generally
adequate for measurement.  The beam is reflected both from the top
surface of the film being etched and also (assuming the film is
transparent at 633nm) from the surface of the wafer below the film.
As the film is etched, you can observe the constructive and
destructive interference fringes between the two reflected beams (by
measuring intensity) as a function of time.  Presumably you know the
refractive index of the film being etched, and with this information,
you can calculate the the amount of the material being removed, if
desired.  This is not necessary, of course, as when the film is
completely etched, the beam intensity will be constant.  A HeNe laser
is used probably because it is inexpensive and has good beam
characteristics.

Brad Cantos
[email protected]
http://holage.com

LinkedIn: http://www.linkedin.com/in/bradcantos

On Mar 8, 2009, at 8:59 AM, Hakemi, Ghazal [SAS] wrote:

> Dear Brent,
>
> Thank you for your reply, I think Your method would suit my purpose.
> The only thing I don t understand is the setup: how do you manage to
> put laser and detector outside the chamber and get an image? Are
> these two components just kept outside the plasma range? Is that
> what you are trying to do? Why do you use HeNe laser? Is it because
> it is not disrupted by the frequency in the chamber?
> Also one more question:  what type of photodetector do ypu use? is
> it a CCD?
>
> Many thanks again.
>
> Regards.
>
> Ghazal