H. Fouad

Benzophenone doped polydimethylsiloxane: self developable composite resist system for its use in a direct write laser lithography application

This paper reports a benzophenone doped polydimethylsiloxane (PDMS) composite resist system, for micro patterning using direct write laser lithography for its use in lab-on-chip (LOC) applications. A 248 nm excimer laser with a 20 ns pulse width is used for microfabrication of doped-PDMS. The effect of two major aspects viz. resist composition and laser processing parameters on the quality of fabricated microstructures is studied and optimized. The lithographic analysis reveals that the doped-PDMS shows self developable sensitivity at lower threshold fluence, 250 mJ cm−2. The optimized composition ratio 10: 1: 0.3 (wt%) of the PDMS monomer: curing agent: Benzophenone (P:C:B) is used for processing and analysis. Comprehensive analysis on the effect of laser ablation parameters (fluence, frequency and number of laser pulses) on etching performance (etch rate, geometry of micropattern and quality of surface) is presented. Increase in etch rate with fluence (250–650 mJ cm−2) is observed and considered to be in a working range. Simultaneously, increase in surface roughness as a function of fluence >650 mJ cm−2 is observed which can be associated with rapid rise in the photothermal decomposition of the composite resist. However pulse repetition rate (PRR) at 1, 5 and 10 Hz does not offer any significant effect on etch rate. The surface quality at a higher PRR is deprived due to redeposition of ablated material which concludes 1 Hz as a suitable working frequency. The deterioration of surface quality with increasing PRR is associated with the formation of a heat affected zone, due to cumulative heating, as the increase in temperature is 362 °C at 5 Hz and 624 °C at 10 Hz, above Ts ~ 1099 °C for 1 Hz. However, the number of pulses and etch rate are inversely related due to the plume effect. The overall study provides a guideline for precise control on fast prototyping direct write laser lithography processes used in LOC applications.