The influence of the substrate thermal conductivity on scanning thermochemical lithography
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(a) Schematic representation of the experiment set-up: The hot Wollaston wire probe is scanning across a sample. (b) Chemical structure of PXT and PPV. (c) Cross-section through the layer and the coordinate system that is used later for the simulations. Point A marks r = z = 0 and point B is at the polymer-substrate interface. The PXT layer is 35 nm thick and the interlayer 140 nm thick. The point where the probe touches the air-polymer interface it at r 0 = 122.5 nm.
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Image (a) shows a tapping-mode AFM (TM-AFM) image of lines written at 350 °C and 10 μm/s demonstrating the ability of SThL to create straight and reproducible lines. The initial precursor film for all experiments on ITO was ≈35 nm thick. (b) Tapping mode AFM image of features patterned at 400 °C and 150 μm/s and vertical profile along the white line. The line profile is the result of the probe scanning twice across the surface (trace and retrace), leading to a line spacing of 320 nm. (c) TM-AFM image of features patterned at 380 °C and 20 μm/s. The resolution is 36 nm (FWHM), albeit the lines are no longer straight and well-connected to the substrate. (d) TM-AFM image of PPV lines written across a silicon oxide (SiO2)–gold interface with an initially 20 nm thick precursor layer. The evaporated gold layer is ≈200 nm thick. The lines were written at 400 °C at 20 μm/s across the interface and are only hardly visible in the image due to the small height of the features (≈15 nm) compared to the interface step. The interruption of the line near the SiO2 interface is an artifact that results from the large radius of curvature along the patterning direction, causing the tip to touch the step before the patterned line reaches it. A gradient image (i.e., an image of the height gradient at each point) of (d) for better visualization of the lines is shown in the supplementary information.
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Finite element modeling. (a) Simulated temporal evolution of the temperature at point B as defined in Fig. 1. (b) Simulated temperature distribution along the z-axis (r = 0) at steady state. (c) Conversion ratio along r = 0 belonging to the temperature distribution shown in (b). (d) Vertical distance dz of the conversion boundary from the substrate for different tip temperatures Ttip . The red line marks an estimate of the largest possible dz (dz max ) which still ensures that the structure will not be washed away during the rinsing step. (e) Plot of along the air-polymer interface. (f) Plot of dr = FWHM−2 r 0 for different Ttip . The red circles indicate the expected smallest dr which follow from dz = dz max = 11.7 nm for the different substrates. Note the overall increase of the minimum feature size as a function of k. (g) Surface plots of the temperature and conversion ratio in both spatial dimensions in the case of an ITO substrate and a 350 °C hot tip.
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