Dear JOSH:
I have read with interest the responses to your question regarding the
use of sodium and/or potassium hydroxide in conjunction with sodium
and/or potassium carbonate as a means of developer replenishment. I
also reveiwed the related artical, "Fine Lines in High Yield (Part
XXVI): Replenishment Chemistry and Process Control for Aqueous
Development", on page 32 of the October issue of CircuiTree.
First, the use of free hydroxide (be it sodium or potassium hydroxide)
has been incorporated into commercially available liquid carbonate based
developers for at least 15 years. Often the presence of free hydroxide
in these liquid developers has been criticized, wrongly in most cases,
as "dulling" or attacking solder in LPI developing, contributing to
resist sidewall degradation, and/or being generally undesirable. I say
wrongly, and contrary to the CircuiTree artical, because free hydroxide
is not present is any significant concentration in a working developing
solution. When carbonate, either sodium or potassium, is mixed with
water, the equilibrium established is so far to the left, there is
essentially no free KOH nor bicarbonate present. (There are several
references support this statement. See "Quantitative Analytical
Chemistry", 3rd Edition, James S. Fritz, George H. Schenk, Jr., Allyn
and Bacon, Inc., Boston;, "...mixtures of hydroxide and bicarbonate
cannot exist in solution because they immediately react and form
carbonate." In addition, the dissaciation consant, Kb for CO3 is
1.8/10(4). If you are so inclined to calculate the concentration of
free hydroxide in the equilibrium of carbonate, you will note it is more
than three powers of ten lower in concentration than carbonate.) Hence
I believe it is misleading to state KOH and K2CO3 are two bases which
are consumed by the carboxylic acid groups on the photopolymer during
developing. It is more reasonable that as carbonate is "develops" the
photopolymer resulting in the conversion of carbonate to bicarbonate,
KOH in the solution reacts with the bicarbonate to form carbonate and
water. (Rudy Sedlak included the actual chemical reaction in his
response.)
So what is the significance here?! Does the presence of free hydroxide
improve the control, pH or otherwise, of the developer? NO!! But it
CAN reduce the consumption of carbonate by "regenerating" bicarbonate in
the system back to carbonate; and hence yield some cost advantages.
What is most important in controlling the developer is control of the pH
and TOTAL carbonate concentration. The pH of a devloper solution is
determined by the ratio of carbonate/bicarbonate. Control the pH and
you control the RATIO of carbonate/bicarbonate, BUT NOT THE TOTAL
CARBONATE. The only variables that govern pH control are the quality of
the pH meter/pH probe and rate of additions of new chemistry. The only
variable that governs total carbonate concentration is the accuracy of
the water/carbonate ratio added. The point that Rudy makes in your
second question is absolutely critical! If you added carbonate,
carbonate/hydroxide, or hydroxide alone to your system, the resist
loading would become unnacceptably high, and developing quality would
suffer.
If you have more questions regarding automated developing specific to
your line, feel free to contact me.
Regards,
Ted Stern
Circuit Research Corp.
612-479-6525
[log in to unmask] wrote:
>
> Recently, I heard of a dry film photoresist developer system that uses sodium
> hydroxide as a pH stabilizer. Apparently, the system doses the developer
> sump with carbonate based on a panel counter, and sodium hydroxide based on
> solution pH. The sodium hydroxide is used straight out of the drum, probably
> at around 40 % strength. I'm told this controls pH in a very tight range,
> and reduces consumption of carbonate.
>
> I have some questions regarding a system of this type.
>
> 1. What would the advantage be of using both chemicals? Cost savings don't
> seem to be too likely, since you are replacing one chemical with another.
>
> 2. What are the potential dangers of using sodium hydroxide in a developer?
> What effects could it have on photoresist?
>
> 3. Does the addition of sodium hydroxide cause bicarbonate to be converted
> to carbonate?
>
> 4. How many people are using this type of system in production?
>
> Thank you in advance for your assistance.
>
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