Wavelength dependence of the excimer laser etching characteristics of a polymeric resist

The direct etching by excimer laser light of thin films of a polymeric resist, PMTM, has been studied at wavelengths of 157, 193, 222, and 248 nm. The energy absorbed per unit ablated volume near the etching threshold fluence indicates that a wavelength-dependent reaction mechanism causes ablation of the film. When laser wavelengths shorter than 222 nm are used at fluences close to threshold no evidence of heating of the polymer is observed. At 222 nm we observe thermal effects which cause melting and flow of the polymer while at 248 nm trapped pockets of volatile gas, which can subsequently explode, appear to be formed below the (molten) surface.