USE OF TEMPORALLY COINCIDENT ULTRAVIOLET-LASER PULSES OF 2 DIFFERENT WAVELENGTHS TO ABLATE ORGANIC POLYMERS

The etching of polymethyl methacrylate (PMMA), by pulsed ultraviolet laser radiation [20-ns full width at half maximum (FWHM), 193 nm] can be increased more than threefold by overlapping both spatially and temporally laser pulses of 308 nm (20 ns FWHM) along with the 193 nm pulses. At a constant fluence of 193 nm, increasing the fluence of the 308 nm pulses progressively increases the etch depth/pair of pulses even though the fluence of the 308 nm radiation is kept below the value at which it would etch the polymer by itself at 1 Hz repetition rate. When a time delay of +300 to -400 ns is introduced between the two pulses, the 193 nm pulse being set as time zero, the enhancement due to pairing the pulses is found to be a maximum at nearly total overlap. A similar enhancement in the etching of PMMA by pulsed 248-nm laser radiation can be achieved by mixing 308 nm pulses with the 248 nm, and in the etching of polyethylene terephthalate, by mixing 351-nm laser pulses with 308-nm laser pulses. Chemical analysis of the solid polymeric product from the laser ablation of PMMA by 248 and 308 nm alone, and by a mixture of 248 and 308 nm laser radiation shows that the mixed laser radiation etches in a way that is chemically similar to pulsed 248-nm laser light and the effect can probably be attributed to an enhancement of the photoablation process.