Ultrafast Fabrication of Metal Nanostructures Using Pulsed Laser Melting

Abstract

Nanoimprint lithography (NIL) is a mechanical molding process. Two formats of NIL are commonly used: thermal NIL and UV-curing NIL. In thermal NIL, the resist is typically a thermoplastic or thermoset polymer that becomes soft at temperatures well above its glass transition temperature, thus it can be imprinted by a rigid mold. NIL has demonstrated low cost and high throughput patterning (Chou 1996, Schift 2008) with a high resolution of sub-5 nm (Austin 2004, Hua 2004). However, due to their very high melting temperature, direct patterning of metal or silicon using an NIL-like process is very challenging. Previously, metal structure fabrication by transfer-printing or imprinting without melting has been demonstrated. For instance, Kim et al used a hard silicon mold to press into a metal film on a substrate with a high pressure of 290 MPa that fractured the metal film at the mold pattern edge; and they then peeled off the metal and transfer-printed it to another substrate, achieving sub-micrometer resolution (Kim 2000). Yu et al fabricated Ag metal electrodes for organic light emitting devices by transfer-printing with a polydimethylsiloxane (PDMS) stamp, which peels off the portion of the metal film that is in contact with the protruded PDMS patterns (Yu 2007). However, it has a low resolution of 13 μm and a low yield that depends on the peel direction. Buzzi et al, Pang et al and Hirai et al applied ultra-high pressure of several hundred MPa to directly imprint a solid metal at room temperature (Buzzi 2008, Pang 1998, Hirai 2003). Chen et al and Chuang et al used a mold having a sharp geometry to deform or imprint (penetrate) metal thin films (<50 nm) deposited on a soft polymer bottom layer at pressure of 10-20 MPa and temperature slightly lower than the glass transition temperature of the bottom polymer (Chen 2006, Chuang 2008). The above methods suffer from poor patterning resolution and are limited to ductile metals because, during the patterning, the metals are in the hard solid phase. In this chapter, we present a method of direct patterning of metal or silicon nanostructures using a pulsed laser that can melt the metal or silicon. Like the aforementioned method, the most prominent feature of this technique is that it is a one-step patterning process – it replaces the steps of resist patterning in lithography, subsequent pattern transfer by etching or liftoff, and resist removal all by one single simple step. In addition, as molten metal or silicon has very low viscosity (on the same order as water), this technique can pattern them