Computational Modeling of Direct Print Microlithography
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ABSTRACT Using a combination of experiment and simulations, we have studied the equilibrium shapes of liquid microstructures on flat but chemically heterogeneous substrates. The surface patterns, which define regions of different surface energy, induce deformations of the liquid-solid contact line, which in turn can either promote or impede capillary break-up and bulge formation. We study numerically the influence of the adhesion energies on the hydrophilic and hydrophobic surface areas, the pattern geometry and the deposited fluid volume on the liquid surface profiles. INTRODUCTION In the last decade many efforts have been made both to reduce the minimum feature size of electronic devices and to increase the throughput and reduce fabrication costs. Several groups have explored printing techniques such as gravure offset printing, screen printing, inkjet-printing and micro-contact printing for lithography or direct deposition of semiconductor- or polymerbased thin film transistors and light emitting diodes [1-11]. We are investigating wet printing techniques for the transfer of liquid inks from a chemically patterned surface onto an unpatterned target substrate. There are five major technological challenges involved in this process: (a) the fabrication of the printing plates, (b) the deposition and distribution of the ink on these stamps, (c) the control of the behavior of the ink patterns between ink deposition and printing, (d) the printing and (e) the stability of the printed ink patterns. In this article we focus on aspects of the behavior of liquid microstructures on chemically heterogeneous surfaces and the transfer process during printing. PRINTING PLATE FABRICATION The selective distribution of the ink on the imaging areas of the printing plate is achieved with the aid of a hydrophobic self-assembled monolayer (SAM) [12], which repels the ink from the non-imaging areas. There are several approaches to stamp fabrication. For example one can homogeneously coat a substrate like a silicon wafer with an SAM of octadecyltrichlorosilane (OTS) and subsequently remove the monolayer from the imaging areas by exposure to deep-UV light (X,= 193 nm) through a chromium mask [13]. Alternatively, conventional photolithography can be used to pattern the SAM by first spin-coating photoresist on the SAM, exposing it to UV light through a chromium mask, developing the resist, removing the monolayer by oxygen plasma treatment and lastly removing the remaining photoresist. Since the mechanical resistance of SAMs to scratches is not very high and because scratches render the stamp surface hydrophilic in undesired locations, scratched stamps have to be reprocessed from the beginning or disposed. This problem can be partially overcome by depositing and patterning a 50 nm thick gold layer on the silicon surface by wet chemical etching and coating the gold with an SAM of hexadecanethiol (HDT). HDT forms a chemical bond with gold but does not adhere to the bare silicon regions. In case of surface scratches, which only damage the HDT la
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