In a message dated 7/1/99 2:29:01 AM Pacific Daylight Time,
[log in to unmask] writes:

<<     I have been asked by my company to investigate a method of
      monitoring the level of bicarbonate in (sodium carbonate) developer
      solutions. I have a couple of questions I hope someone can answer:

      * How is the bicarbonate formed in the first place and what variables
      affect its' concentration?
      * What effects does this bicarbonate have on developmeny (if any).
      * Is there any way of subsequently removing the bicarbonate?
      * Finally, is anyone already measuring bicarbonate level and what
      system works best for you?

      Thanks for your time.

      David Albin
  >>

First must thank John Nelson for his confidence, this is my area of
expertise, if it exists, so we dive in.

Photoresist is acidic, or is a "Lewis Acid", which means that although it may
not donate acidity, it does the inverse, and absorbs base, or alkalinity, or
in other words, Sodium Carbonate, and turns it into Sodium Bicarbonate.

(To make sure we are all on the same page, Sodium Bicarbonate (hereinafter
called Bicarb, to save typing) is half neutralized Sodium Carbonate)

The reaction, if we assume Resist has the formula HR (acids donate Hydrogen,
so HR is resist in the acidic form), and this format-less media can handle
it, is:

  Na2CO3 +   HR   --> NaHCO3  +  NaR
Carbonate   Resist        Bicarb    Dissolved Resist

Titration is a way to determine Bicarb content, but pH is a DIRECT readout of
the ratio of Carbonate to Bicarb, so why bother with the titration.

The titration to determine Bicarb, is actually interfered with by the
presence of dissolved resist, so, in many ways pH is a more accurate way to
determine bicarb content...

At the end of the day, one has to ask, why do they care what the bicarb
content is?
Frankly, you concern yourself with the makeup of the carbonate solution
initially, and watch the pH of the developer sump, and go to sleep, actual
bicarb content is of concern only to accountants and graduate students, not
to production people.

You ESPECIALLY do not want to consider removing bicarb from solution (to do
so, you simply add caustic soda, which converts bicarb back to
carbonate--don't do this) because to do so allows the resist (or in your case
soldermask?) to build in concentration, and this, in my experience, IS A BIG
PROBLEM.  If my memory is functioning this AM, the rule of thumb is each
ml-square foot of resist should consume 1 US gallon of 1% Sodium Carbonate
solution.

Turns out, we have found, that simply high levels of Sodium, from any source,
and to a lesser extent, Potassium, cause resist (and presumeably soldermask)
insolubility, and thus allowing high levels of dissolved developer can
seriously "foul up" developing quality.

Me thinks that there are production/quality issues that someone thinks is due
to bicarb present.  If you are using a properly made up developer solution,
and controlling the sump by pH, bicarb is not going to be a problem.

Why not tell us what the problem that someone thinks is being caused by the
bicarb, and tell us about your process (developer make up, batch/feed &
Bleed, control methods) and we may be able to cut to the chase and give real
help, instead of simply ways to collect more data.

(Does this message fall into the category of more than you EVER wanted to
know about developing?)

Rudy Sedlak
RD Chemical Company (Incidently, David, I suspect we share a common agent in
California...Advanced Microsystems)