At 01:52 PM 11/21/96 -0600, you wrote:

>Another problem is that we got the 30% concentration and 60 C
>temperature from another paper on KOH etching, but really don't know
>if those are ideal for our purposes.  I've spent the past few days
>combing a number of journals, hoping to find an article with a
>parametric survey for KOH with respect to the <111> surface quality,
>but haven't had any luck.
>
>Next question:  Is KOH really the etchant we want to use for this?
>Primarily we need very very clean <111> surfaces, and we need the
>spacing of the facets to be very very regular.  Our mask is good down
>to better than 0.01um (don't ask), so if we could hold that tolerance
>we'd be set.
>
>Any advice or references would be a great help.
>
>Thanks,
>
>Tom Benedict                          McDonald Observatory at Mount Locke
>tom@astro.as.utexas.edu               Department of Astronomy
>Tel: 512-471-3337                     University of Texas at Austin
>Fax: 512-471-6016                     Austin, Texas  78712
>


Dear Tom,

I'm working on micromachining of hybrid submounts for applications in the
field of planar integrated photonics. The idea is to use {111} surfaces as
optical mirrors. Depending on the wavelength, a RMS value of 15-40nm is
required in order to limit the scattering loss due to topographic roughness.

Most articles on this topic are studies about the {100} surface roughness,
not the roughness of {111} sidewalls. My experience is in agreement with
most of the observations decribed in the article of T.Kwa et al. in
J.Electrochem.Soc.:
* the sidewall roughness is especially dependant on the O2-content in the
silicon if high temperature processing is applied to the wafer (e.g.
deposition of a masking layer by LPCVD). Hence, a high quality PECVD masking
or low O2-content silicon is preferable.
* cleanliness is a very important issue throughout the whole process (e.g.
resistance decay of DI-water).
* isotropic etching just prior to the KOH-based process also decreases the
roughness of the {111} planes.

Kind regards,

Johan.
 _________________________________________

  Ir.Ing. Johan E. van der Linden

  University of Gent (Belgium)
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