Hi Tao,

A period of 1.5um should be fairly easy to achieve with that laser, even
with the crossed exposure (are you after a 2-D array of holes or
pillars?).  Many things could cause your problem, but my best two guesses
are:
1)  Reduce your intensity by spreading out your beam.  If your features
are gone after 3s develop, you are way over-exposed for a positive resist
like 1805.  Since your time is already short, you have to reduce
intensity.
2)  There is probably non-uniformity in your optical field.  Issues like
spatial filtering, uniform power in both beams, minimizing stray laser
scatter (coherent), and having a minimum number of optical elements in the
beam path can help here.

Good Luck!!

---Chris Striemer       [email protected]


>Date: Thu, 27 May 2004 15:10:03 -0700
>From: tao liu 
>Reply-To: General MEMS discussion 
>To: [email protected]
>Subject: [mems-talk] About laser holography
>
>Hi,
>
>I use HeCd laser to do holography with two-beam interference.  The
>wavelength is 446nm.  My sample is PR1805/SiO2/GaAs.  The period is
>1.5um.  The sample is exposed twice to form a crossed grating.  After
>exposure and development, I can always see the non-uniformity on the
>sample surface: some area has good exposure, some area has
>under-exposure, and some area has over-exposure.  Because the time for
>a single exposure is only ~ 3s and the development is also only ~ 3s,
>I suspect that this is caused by the very short development time.  (I
>cannot use long development time for my present conditions.  Otherwise
>all the patterns will be lost.)  Can anybody give me some hints for
>that?  Thanks.
>
>Best regards,
>
>Tao