Page 26 - PCMI Journal July 2018

David Allen | Emeritus Professor | Cranfield University | UK Photochemical Machining: Where has it come from, and where is it going?
• Will the etching process remain the same?
I believe that the spray etching process will remain fundamentally the same, but process control will need to improve as future tolerances are reduced. Recycling efficiency will need to improve progressively over the next decade to reduce costs of waste etchant disposal.
The extraction of dissolved metals such as nickel and chromium may even become economically feasible as waste disposal costs continue to rise. Increasing use of computer hardware and software will also be utilised to improve uniformity of etching.
I also think that the electrolytic etching process may be re-examined as a method for etching corrosion-resistant materials due to Health & Safety considerations in handling toxic etchants and increased environmental legislation pressures for environment-friendly disposal of waste etchants. However, in the future, better control of current density distribution will be required to obtain uniform etching across a sheet of metal in comparison to that obtained by Salfret in the 1950s. This can be made possible by sophisticated computer modelling of the current density distribution of each component design.
• Would it be worth the cost to tailor-make appropriate quality (fit-for-purpose) water and benefit accordingly?
Water quality is rarely constant as it varies from season to season and district to district as it is dependent on both local weather conditions and local geology influencing water hardness. Water is required as the main diluent for many chemical processing solutions including metal cleaners, photoresist developers and photoresist strippers. Process water is also required for post-clean rinsing, post-development rinsing and post-stripping rinsing. Therefore, the quality of the water used may be matched to its specific application dependent on its source. For instance, when cleaning metal [Bridges, 2008] recommended the following sequence:
1. alkaline chemical cleaner
2. vigorous tap water rinse
3. mild acid rinse
4. vigorous tap water rinse and
5. a final rinse in deionised (DI) water to prevent any contamination from the tap water
rinse (4) drying on the metal surface during the final drying stage.
Thus, DI or distilled water may be economically viable for some process stages even if it does cost more than tap water due to the savings made by reducing the number of defective parts produced.
On the other hand, for post-DFR development rinsing, it has been recommended that hard tap water is much preferred to soft tap water or DI water to achieve straighter resist sidewalls [Bridges, 2007].
Issue 131 July 2018 PCMI Journal 25