The influence of the substrate thermal conductivity on scanning thermochemical lithography

We present a joint experimental and computational study of the role of the substrate thermal conductivity on scanning thermochemical lithography (SThL) of thin organic films. We aim this study at lithography of the luminescent conjugated polymer poly(p-phenylene vinylene) (PPV) from its soluble precursor poly(p-xylene tetrahydrothiophenium chloride) (PXT), but our results provide relevant insights into the SThL of thermosensitive polymers in general, and into a wide range of nanoscale thermal and thermochemical processes in thin films. As high thermal conductivity substrates we used gold films on silicon, and indium-tin oxide (ITO) films on glass, successfully patterning PPV on both substrates. We find that a higher probe temperature (>300 °C instead of ≈250 °C) is necessary for lithography of PXT films on ITO compared to those on fused silica (for the same scanning speed and comparable precursor thickness). Surprisingly, however, our experiments show that minimum feature sizes are nearly independent of the underlying substrate. While a lateral resolution (full width at half maximum, FWHM) of 37 nm was achieved previously on fused silica for a 40 nm thick PXT film, we obtain here a FWHM of 36 nm for a 35 nm thick PXT layer on ITO. We compare our experiments with finite element simulations and gain further insight into the possibilities of thermochemical lithography, the necessary minimum probe temperature and the highest attainable resolutions. The model shows that for high thermal conductivity substrates there should be a region of unconverted polymer near the polymer-substrate interface. Our experiments demonstrate that patterned features are able to adhere to the substrate despite this unconverted layer, thus allowing SThL to work on very high thermal conductivity substrates such as gold. Our model builds on this experimental finding and accounts for the experimental lack of dependence of lateral size with substrate conductivity, i.e. it predicts that the minimum feature size increases only slightly for increasing thermal conductivities of the substrates.