Steve-

You will get many opinions about this subject since the decision of the
"best" etch is structure specific.

I have developed etch processes using all three of the etches that you
mentioned which are currently running
in our MEMS fab.  Here is my opinion.

EDP would be my choice for the P+ etch stop since the etch rate of the mask
layer in EDP is low and the
selectivity of the etch to the P+ etch stop is much higher in EDP.  Of
course,  EDP is a lower ph etch which means
that diffusion effects are present in deep cavities.   You must be careful
of overloading the solution with etch products
since silicates will form on the etch surface.  The diffusion effects,
which can result in areas
of an etched feature being 1-10 microns higher than other areas on the same
die,  are not harmful since they are removed
once the etch stop is reached.

KOH does not have a good selectivity to the P+ etch stop unless a low
concentration of KOH is used or something (alcohol)
is added to the KOH.  The advantages of using KOH over EDP such as better
111:100 selectivity and less diffusion effects are
compromised by adding alcohol or by lowering the concentration and the
selectivity is still not as good as EDP's.

TMAH is similar to EDP in the low etch rate of oxide used for the etch mask
but at 25% it does not suffer from as much diffusion
or loading problems.  Unfortunately,  the selectivity of the TMAH to P+ is
worse than EDP.

So,  if you do not have a safety concern with running EDP,  it would be my
choice.  If you are not in a mass production mode,
then the KOH alcohol option may be easier to handle since you can watch the
wafers more closely negating the selectivity
advantage of the EDP.




Steve Forgrieve  on 02/19/99 07:24:31 AM

Please respond to Steve Forgrieve ; Please
      respond to [email protected]


To:   [email protected]
cc:    (bcc: Dan W Chilcott/DELCO)

Subject:  Boron Etch Stop





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I am using a boron etch stop to build a hinge on silicon. What is the
best anisotropic etchant to use. I am considering KOH, TMAH, EDP, etc.

--
Steve Forgrieve
Department of Electrical Engineering
The University of Edinburgh
King's Buildings
Mayfield Rd
Edinburgh EH9 3JL
(Int +44) 131 650 5665 (enquiries)
email: [email protected]



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I am using a boron etch stop to build a hinge on silicon. What is the best
anisotropic etchant to use. I am considering KOH, TMAH, EDP, etc.
-- 
Steve Forgrieve
Department of Electrical Engineering
The University of Edinburgh
King's Buildings
Mayfield Rd
Edinburgh EH9 3JL
(Int +44) 131 650 5665 (enquiries)
email: [email protected]
 

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