Having done SiN etching using this method for several years, I can advice
you that this method does in fact work very well with high selectivity to Si
and SiOx (>100:1).  Etch rate is approximately as published for LPCVD SiN
~45A/min.  For PECVD nitride it is faster, assume by a factor of 25-50%
depending on the quality of the PECVD nitride and growth rate and
temperature.  Need to have an effective tank for this though -- etching
vessel needs to have a closed lid, because the water concentration needs to
not change in order for the etch rate to remain stable.  Any further
questions, send me an email directly.

---------- Forwarded message ----------
> From: avinash balakrishnan 
> To: [email protected]
> Date: Thu, 24 Nov 2005 14:12:01 -0800 (PST)
> Subject: [mems-talk] Silicon Nitride Etching
> Hi all,
>
>    I am interested in chemical etching silicon nitride. I read somewhere
> in the forum that 85 % phosphoric acid at 160 C is helpful. Can any one tell
> me how long should i etch to get a decent microstructure. References or
> paper links can be very helpful.
>
> Avinash



Having done such a construction several years ago as an undergrad, you need
to remember that dopant concentrations are *very* important.  Low
concentrations of boron or phosphorus (near intrinisc region) have positive
TCR, but for negative TCR you need heavily doped (e.g. 10^20, to degenerate
levels) doping of boron, so you will need a thick implant mask (e.g. thick
resist or preferably a >1um thick SiO2 layer) for the rest of the wafer
before you send it to implant to prevent ions from penetrating the rest of
the wafer.  Additionally, you do not want the implant energy to be too large
because you want to confine the penetration of the ion to the top layer of
the polysilicon that you are using as the resistor, so you probably want to
use 30keV, maybe a bit higher, because you are doping to degeneracy, but do
*not* exceed 40keV or penetration will exceed 4kA p-Si and probably will get
into the underlying insulating layers and possibly the substrae itself.  You
want the highest level you can probably get from the vendor doing your
implant for Boron because you are going to material practical saturation
,and since boron is a interstital impurity (vs. phosphorus as
substitutional) you don't have to use as much energy to get it into the
lattice sites.  Don't forget that once you ge the wafers back from implant,
you need to do a faily long anneal to diffuse the boron adequately and
evenly through the poly-Si, I would suggest 3 hours at 1100C if your thermal
budget for the rest of the wafer can tolerate this, remember this is not so
much an activation anneal (e.g. one that RTP would be practical for) but
rather a diffusion one.


---------- Forwarded message ----------
> From: Tao 
> To: General MEMS discussion 
> Date: Fri, 25 Nov 2005 05:49:09 -0500
> Subject: [mems-talk] Related questions in Piezoresistive application
> Dear Memebers,
>    I am going to use piezorisistive effect. I will dope Boron in a
> n-type Si wafer to make four resistors and build a Wheatstone bridge.
> The question is that:
> 1) The Boron resistors will be made by ion implantation. In order to get
> good pn-junctions between resistor and substrate (n-type PHO wafer), how
> to choose the wafer with a proper dopants (PHO) concentration? At the
> same time, how to choose the ion implantation condition? (energy, source
> concentration (cm^-2), the final concentration in the resistors ( cm^-3))..
> 2) In order to insulate the Boron-resistors, someone applies an inverse
> voltage to the substrate to increase the barrier. Do you think it's
> necessary? In my view, the inverse voltage is not necessary. But I am
> not sure. I wonder under what condition, I don't need need to apply that
> voltage?
>
> Thanks in advance!
> Happy thanksgiving!
>
> HTao
>
>
>
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