In this newsletter, we’re talking about how Multitrator can solve problems in the analysis of solutions used in the semi-conductor fabrication industry.

I learned a new word the other day. It’s “fab” as in “wafer fab”, the name they give to the fabrication facilities where semi-conductor wafers are made. I’ve seen pictures of these places - high tech clean room facilities, all the workers in space suits, and everything handled by robots. However, behind the scenes in the analytical lab., where the critical etching solutions are analysed, things may not always be so up to date. From some of the analytical methods I’ve seen, they’re reminiscent of the 1940’s or 1950’s: complex manual methods using visual endpoint indication, slow, and prone to operator error. Luckily, we can offer a modern Multitrator makeover.

In newsletter #8, we showed how Multitrator could analyse one of the complex acid mixtures used to selectively etch away unwanted areas of the wafers. This acid mixture is comprised of nitric, phosphoric and acetic acids. We proposed a two-step automated titration sequence, where the sulphuric content was analysed by a titration with barium chloride, followed by a titration with sodium hydroxide to give two endpoints. Endpoint results from both titrations are sent to a dedicated spreadsheet, which computes the amount of each of the acids present in the mixture. Recently, we developed a simpler method where we only require a single titration with sodium hydroxide to determine all three components.

Endpoint #1     Endpoint #2     Endpoint #3     

Phosphoric acid (pKa1=2.12)     Phosphoric acid (pKa2=7.21)     Phosphoric acid (pKa3=12.30)    

Nitric acid (dissociated)       Acetic acid (pKa=4.76)          

This latest method relies on applying a little know-how to get a stoichiometric third endpoint for phosphoric acid. If you’d like to know the exact details, send us an email at labrat@multitrator.com <mailto:labrat@multitrator.com>. This method can also be adapted to controlling the manufacture of sodium tripolyphosphate, as well as checking the quality of ammonium phosphate fertilizers.

We’ve just finished work on another etch mixture called “polyetch”. This is a diabolical mixture, real “China Syndrome” stuff, as it contains hydrofluoric acid as well as nitric and acetic acids. Our challenge was to devise a rational titration approach which could improve accuracy, precision and productivity. From an examination of the dissociation constants, it was anticipated that two endpoints would be obtained in an acid-base titration:

Endpoint #1     Endpoint #2     

Nitric acid (dissociated)       Hydrofluoric acid (pKa=3.17)    

        Acetic acid (pKa=4.76)  

However, when we came to titrate the mixture with aqueous sodium hydroxide, only one endpoint corresponding to the total acid content of the mixture resulted. Time to call in the heavy artillery. This time we dissolved the sample aliquot in isopropanol and titrated with cyclohexylamine in isopropanol. Success! Two endpoints with very good precision. But we still needed a way to determine the hydrofluoric acid independent of the acetic component.

The solution came in the form of a titration which can only be performed thermometrically. The fluoride is titrated with aluminium nitrate. The reaction is the exothermic precipitation of sodium di-potassium hexafluoride. The precision of the determination is excellent, and the titration is fast, taking only a minute or so. This is an ideal titration where the concentration of fluoride is higher than approximately 0.5%.

If the two titrations are performed sequentially, the results can be sent to the same line of the same spreadsheet, so that all three components and the total acidity of the mixture can be computed automatically. The two burettes can be set up side-by-side, and the same thermometric probe is used for both titrations.

Once again, the application notes for these two procedures are available by writing to us at labrat@multitrator.com <mailto:labrat@multitrator.com>.

If you have a problem in titrating complex mixtures, contact us. There’s a good chance we have a Multitrator thermometric solution for you!

All the best until next time,

Tom Smith

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