Hi Andrea,

What thickness tolerance do you require?  How are you measuring the
thickness of your membranes?

The wafer itself is a critical factor in your process.  The TTV (total
thickness variation) of the wafer will manifest itself as thickness
variation in the membrane (i.e. if some areas of your wafer are 305um thick
and others are 300um, when you etch 286um into the silicon you will have
membranes which are 19um and 14um thick, respectively).  Using an etch stop
layer can mitigate the effects of TTV.

One way to improve etch rate uniformity might be to reduce your etch rate
(lower temperature), stir the etch bath, tightly control the temperature of
the etchant, or use a surfactant.  There is much literature on the use of
surfactants in improving etched surface quality.  I hope this helps.

Best regards,

--
Brian C. Stahl
Graduate Student Researcher
UCSB Materials Research Laboratory
brian.stahl@gmail.com / bstahl@mrl.ucsb.edu
Cell: (805) 748-5839
Office: MRL 3117A


On Sat, Oct 9, 2010 at 10:04 AM, Andrea Mazzolari wrote:

> Hi all,
>
> i need to realize silicon membranes having thickness 14um and lateral size
> 5x5 mm in 4'' silicon wafers, starting thickness is 300um.
> I can not use any stop layer or SOI wafers.
>
> I already realized some membranes using KOH etch, but thickness is not so
> uniform (thickness variation is about 4-5um).
> I tried to use magnetic agitation, but it did not helped much.
>
> Any suggestion on how to improve thickness uniformity ? I tried also
> ultrasonic agitation, but membranes went broken along the (100) planes.
>
> Best regards,
> Andrea