I am following this question.

maytham

Irradiation by UV or laser at suitable doses can cross-link the Polystyrene. The material you are etching is now cross-linked PS with different bonds which may or may not be more resistant to the plasma depending on the plasma chemistry used.

For instance, UV exposure are sometimes used to cross-link PS and enhance its etch resistance in oxygen-based capacitively-coupled plasmas. For references, you could read the work of Guarini or Chevalier on PS-b-PMMA.

Have a nice day.

Thanks Philippe Bezard for the answer. Yes we have done the research, the cross linking are much more obvious and created graphitic materials. We also come across the term zwitter characteristic. Nevertheless, why the evolution from polymer chains, later graphite, graphene (only short lifted) and later Diamond like carbon? Why the sequences? or why benzene ring is the one prone to irradiation and later reconstruct?

My best guess would be the heating due to the laser beam. sp2-hybridation is very likely to happen on an amorphous carbon network in the 500 - 700K range and sp2 is much more resilient to dry-etching than sp3. See for instance:

A. Horn, H atom impact induced chemical erosion reaction at C : H film surfaces, Chemical Physics Letters 231 (1994) 193-198

or W. Jacob, Surface reactions during growth and erosion of hydrocarbon films, Thin Solid Films 326 (1998) 1–42

But once again, that is just my best guess, I am no chemistry guy :)

Thanks Philippe:

It is good enough, as it extended the diacussion, maybe later we will get somebody that able to give good direction like you did.