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April 1997

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Subject:
From:
Ted Stern <[log in to unmask]>
Reply To:
Date:
Wed, 23 Apr 1997 10:01:10 -0700
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Dear Steve:

The only important difference I am aware of between sodium carbonate and
potassium carbonate is related to the concentration at which they are
employed.  DuPont literature recommends a 1.0% by weight sodium
carbonate solution for developing.  Note that the molecular weight of
sodium carbonate is 105.99 and the molecular weight of potassium
carbonate is 138.21.  The carbonate ion exhibits a molecular weight of
60.01 and therefore comprises a larger percentage of the total molecular
weight of the sodium salt versus the potassium salt; hence a 1% by
weight solution of sodium carbonate is STRONGER than a 1% by weight
solution of potassium carbonate.  For example:

SODIUM CARBONATE

	10 g/L  x  1 mol Na2CO3/105.99g  =  0.094 M CO3

POTASSIUM CARBONATE

	10 g/L  x  1 mol K2CO3/138.21g  =  0.072 M CO3

Note:  A 1% by weight solution is equivelent to 10 g/L.  Keep in mind
the g/L realtionship to % by weight is NOT LINEAR as the salt content
increases in concentration (ex:  40% by weight K2CO3 = @540 g/L)!!!

Keeping this in mind, it may be necessary to make corrections at make-up
to operate at the desired concentration.  It is also my understanding
that sodium (Na+) and potassium (K+) are "spectators" and do not
participate chemically in developing.

We have performed numerous SEM sidewall studies on virtually all types
of resists developed in both sodium and potassium carbonate.  It has
been our experience that the pH of the developing solution is the
critical variable (within concentration ranges of 8-12 g/L) regardless
of the salt employed.  Please feel free to call me at 612-479-6525 if
you have additional questions.

Ted Stern
Circuit Research Corp.

Steve wrote:
> 
> Besides solubility what are the differences between potassium and sodium
> carbonate developing solutions for aqueous dry films.  Does the potassium
> ion play a role  vs. Na?  Interested in finding out diiferences, if any?
> 
> Thanks

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