Solving problems in the analysis of solutions used in the semi-conductor fabrication industry, using modern titration technology instead of complex manual methods using visual endpoint indication
"Fab" as in "wafer fab" is the name given to the fabrication facilities where semi-conductor wafers are made. Pictures of these places show 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.
Some analytical methods are reminiscent of the 1940s or 1950s: complex manual methods using visual endpoint indication, slow, and prone to operator error.
Stirling says it can offer a modern Multitrator makeover.
It has developed a simple method which only requires a single titration with sodium hydroxide to determine all three components.
This latest method relies on applying a little know-how to get a stoichiometric third endpoint for phosphoric acid.
This method can also be adapted to controlling the manufacture of sodium tripolyphosphate, as well as checking the quality of ammonium phosphate fertilizers.
The company has 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. The 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. However, when it came to titrating 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 Stirling dissolved the sample aliquot in isopropanol and titrated with cyclohexylamine in isopropanol. Success: two endpoints with very good precision.
But it 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.
The application notes for these two procedures are available from Stirling (links above).
