**Ultrafast Fabrication of Metal Nanostructures Using Pulsed Laser Melting**

Bo Cui

*Waterloo Institute for Nanotechnology (WIN) University of Waterloo Canada* 

### **1. Introduction**

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

Ultrafast Fabrication of Metal Nanostructures Using Pulsed Laser Melting 115

changing, the oscillating electrons re-radiate their kinetic energy, unless they undergo frequent collisions with the atoms – in this case energy is transmitted to the lattice (absorbed) and the

For absorbing media the refractive index is a complex number with the form n=n+ik. The

( 1) ( 1) *n k <sup>R</sup> n k* 

<sup>4</sup> <sup>1</sup> ( 1) *<sup>n</sup> <sup>R</sup>*

*I dz*

where I is the intensity of the incident light (also termed irradiance, in W/cm2). The inverse of is referred as the absorption length. For normal incidence the power density (in W/cm3)

Both n and k (thus ) are generally functions of wavelength and temperature. For a semiconductor such as silicon (see Fig. 2 (a)), the absorption coefficient increases with decreasing wavelength gradually near its indirect band-gap at 1.1 eV (1.13 µm), but increases rather abruptly near its direct band-gap at 3.4 eV (0.36 µm) because no phonons are involved for direct transition. At photon energies significantly exceeding Eg, the optical

Fig. 2 (b) depicts the light-coupling parameters for Al and Au. Metals, due to their large density of free electrons close to the Fermi level, have large absorption coefficients (corresponding to absorption lengths -1 around only 10 nm) over the whole spectrum. Their reflectivity is more difficult to determine than that of semiconductors. Generally, their reflectivity is high above a certain critical wavelength that is related to its electron plasma frequency p given by (4Ne2/me)N, where N is electron density and me is electron mass. Below the critical wavelength, which is in the UV or visible part of the spectrum, R decreases sharply. For Au and Cu, the plasma frequency is reduced (which causes their

At higher light intensities and/or higher temperatures, the optical properties can be modified by three mechanisms: lattice heating, free carrier generation (for semiconductors only) and excitation. At elevated temperatures, more phonons are present, leading to an increase of lattice-carrier collision. As a result, the reflectivity of most metals decreases with increasing temperature, and this effect makes metals susceptible to thermal runaway.

2 2

*n k*

 

(3)

(4)

(1)

(2)

external field is weakened. Re-radiation of the energy is the cause of reflection.

The emissivity, or the fraction of incident radiation absorbed, is then

reflectivity at normal incidence is (n=1 for air or vacuum)

2 2

2 2

1 4 *dI k*

( ) (1 ) *<sup>z</sup> z I Re*

response of semiconductors tends towards that of metals.

yellow color) by transition from d-band states.

**2.1.2 High light intensity and/or temperature regime** 

**2.1.1 Low light intensity and temperature regime** 

The absorption coefficient is defined as

deposited at depth z is

within ~100 ns. It can also anneal the patterned metallic nanostructures. More importantly, the current method is not limited to ductile metals, and hard metals like nickel can be structured readily.

Various applications have been developed, and here we will present laser assisted direct imprint, wafer planarization, via-hole filling, transfer printing, and nano-tip formation. Other important applications using pulsed laser nanofabrication, such as self-perfection by liquefaction (SPEL) that removes nanostructure fabrication defects (notably line edge roughness) (Chou 2008, Xia 2010), and nano-channel fabrication using sealing by lasermelting (Xia 2008), will not be presented.
