Abstract

The lithographic patterning of conjugated polymer thin films is important for both device applications such as light emitting diodes and for microlithography. Previous patterning methods which include selective photoelectrochemical deposition and scanning electrochemical microscopy were limited by either the small size of the pattern or the need for photoconductive substrates. In this presentation, we demonstrate that conjugated polymers from the polythiophene series can be patterned directly (without resist) by using the 325nm ultraviolet (UV) radiation from a helium cadmium (He-Cd) laser. These results are an extension of the previous work by Abdou et al [1] and Hu et al [2] on laser irradiation effects on the polythiophenes. Four polymers: polythiophene (PTh), poly (3-methylthiophene) (P3MT), poly (3-butylthiophene) (P3BT) and poly (3-hexylthiophene) (P3HT) were prepared. Both PTh and P3MT were deposited electrochemicallv onto indium tin oxide (ITO) coated glass substrates. The substituted polythiophenes, P3BT and P3HT were synthesized chemically by Grignard coupling and oxidative polymerisation and were dissolved in chloroform to form dilute solutions. Thin films of P3BT and P3HT about 100nm thick were obtained by the dip coating technique. Lithographic patterning was performed using a custom-built UV laser writing system. The He-Cd laser has an output power of 15mW and the power intensity at focus is 286kWcm⁻². After line patterns were written, the samples were observed by optical microscopy and atomic force microscopy (AFM). It was found that at sufficiently high incident fluences, all four polymers can be ablated directly into grooves several microns wide and up to 500µm long. The groove widths generally changed slightly with fluence but decreased rapidly near the respective cutoff fluences where no grooves could be formed (Fig. 1). At fluences below about 10⁵ Wem⁻², cross-linking was observed in both P3BT and P3HT Films. This renders the irradiated areas insoluble in chloroform and after development in this solvent, line structures 1-6µm wide were found on the glass substrate. Some of these line structures had rippled sidewalls and were attributed to solvent infusion effects. The significance of the present results is that both positive and negative tone lithographic patterning have been demonstrated. The ablation of PTh and P3MT in particular overcomes the difficulty of patterning these insoluble and infusible polymers. The mechanisms of the observed patterning processes will be explained and the linewidth trends shown in Fig.1 and other experiments will be discussed. This direct-write process should have potential applications in polythiophene based devices and conductive resists.

© 1998 IEEE