**4. Applications of pulsed laser assisted nanofabrication**

In this section we will present laser assisted direct imprint, wafer planarization, via-hole filling, transfer printing, and nano-tip formation.

### **4.1 Laser-assisted direct imprint (LADI)**

The most straightforward application of laser assisted nanofabrication is LADI (Cui 2010, Chou 2002). In the LADI process, a single laser pulse passed through a transparent quartz mold and melted a thin layer of the substrate material, or the film material if it was different from the substrate. The characteristic time constant for electron-phonon collision to reach thermal equilibrium between excited electron gas and the lattice in metals is on the order of picoseconds, which is negligible compared to the laser pulse duration. The molten layer was then embossed by the quartz mold. Once the liquid layer re-solidified, the mold was separated from the substrate. As can be seen, the most prominent feature of LADI is that it is a one-step patterning process – it replaces the steps of resist patterning in photolithography or nanoimprint lithography, subsequent pattern transfer by etching or liftoff, and resist removal all into one single step. Besides LADI of metal or silicon, one can also imprint a polymer resist that absorbs UV light thus gets heated by the laser pulse (Xia 2003, Xia 2010). Fig. 7 shows SEM images of 200 nm period Al, Au, Cu and Ni gratings with ~100 nm linewidth imprinted by LADI. Only one pulse was used to melt and imprint those metals at a fluence of 0.22, 0.53, 0.24 and 0.41 J/cm2 for Al, Au, Cu and Ni, respectively. We found that multiple pulses up to 50 pulses had insignificant effects to the imprint results. As shown in the figure, although 100 nm features in metal were imprinted, the resolution was not near

Ultrafast Fabrication of Metal Nanostructures Using Pulsed Laser Melting 125

fine features on the mold were faithfully duplicated into the silicon. Reflection measurement, as shown in Fig. 8c, indicates that the silicon remained in liquid state for

Fig. 8. LADI of single crystal silicon wafer. (a ) Quartz mold. The ~10 nm wide notches were caused by the reactive ion etching trenching effect during mold fabrication; (b) Imprinted pattern in silicon showing the notches were faithfully duplicated; (c) The reflectivity of a HeNe laser beam from the silicon surface versus the time, when the silicon surface is

c

irradiated by a single laser pulse with 1.6 J/cm2 fluence and 20 ns pulse duration. Molten Si,

High-performance large-scale integrated circuits (ICs) require several levels of interconnect. Planarization processes which smooth and flatten the surface of an IC at various stages of fabrication are essential for high-resolution photolithography. Besides being smooth and flat, good film step-coverage without internal voids is also crucial for interconnection. Present IC interconnects are fabricated by the damascene process that consists of four major steps: patterning trenches in low-k dielectric materials by photolithography, sputtering metal plating base, copper electrical plating to fill the trench, and chemical mechanical polishing that planarizes the surface. The step-coverage of electrodeposited metal film, though better than other PVD (physical vapor deposition,

about 220 ns, roughly 10 times the pulse duration.

becoming a metal, gives a higher reflectivity.

**4.2 Laser-assisted wafer planarization** 

the sub-10 nm level as had been achieved in LADI of crystalline silicon (see below). This is partially due to two factors: (1) silicon expands 8.6% when re-solidified, which would help to press the un-solidified silicon to fill the void near the sharp corners; whereas Al, Au, Cu and Ni shrink 11 - 12% during re-solidification, which draws the un-solidified metals back from the sharp corners and thus results in an unfilled corner and rounded profile; and (2) the surface tension of molten silicon (0.78 N/m at its melting point) is lower than molten metals, so it is easier to imprint. We believe that here the volume shrinkage plays a major role, due to which the trench cannot be fully filled no matter how high a pressure is applied to overcome the surface tension. Besides front-side irradiation (laser beam passes from the mold to the substrate), metal can also be patterned by laser irradiation from the substrate side if the metal is deposited on a transparent substrate with a film thickness low enough (e.g. <300 nm) to be melted entirely.

Fig. 7. SEM images of 200 nm-period metal gratings patterned by LADI. (A) Al, laser fluence 0.22 J/cm2; (B) Au, 0.53 J/cm2; (C) Cu, 0.24 J/cm2; and (D) Ni, 0.41 J/cm2. Scale bar is 1 m.

We have also attempted to imprint tungsten. We found that it could be melted readily by a single laser pulse, but it could not be patterned with well defined gratings (yet grating pattern was still noticeable) using a quartz mold. This is likely because: (1) tungsten's melting point is 1800 K higher than that of quartz, hence quartz will be melted (softened) during LADI of W; (2) molten tungsten's surface tension is relatively high, nearly twice that of Cu, thus more difficult to imprint; (3) solid tungsten has a high Young's modulus E, three times that of Cu, leading to >3 higher thermal stress (equal to E(L/L), with L/L being thermal expansion between room temperature and melting point). Because of the high thermal stress, W film was found to crack after the LADI process.

LADI of single crystal silicon wafer (not thin film) is shown in Fig. 8. Due to the thermal expansion upon re-solidification that helps fill the sharp corners of the mold pattern, 10 nm

the sub-10 nm level as had been achieved in LADI of crystalline silicon (see below). This is partially due to two factors: (1) silicon expands 8.6% when re-solidified, which would help to press the un-solidified silicon to fill the void near the sharp corners; whereas Al, Au, Cu and Ni shrink 11 - 12% during re-solidification, which draws the un-solidified metals back from the sharp corners and thus results in an unfilled corner and rounded profile; and (2) the surface tension of molten silicon (0.78 N/m at its melting point) is lower than molten metals, so it is easier to imprint. We believe that here the volume shrinkage plays a major role, due to which the trench cannot be fully filled no matter how high a pressure is applied to overcome the surface tension. Besides front-side irradiation (laser beam passes from the mold to the substrate), metal can also be patterned by laser irradiation from the substrate side if the metal is deposited on a transparent substrate with a film thickness low enough

Fig. 7. SEM images of 200 nm-period metal gratings patterned by LADI. (A) Al, laser fluence 0.22 J/cm2; (B) Au, 0.53 J/cm2; (C) Cu, 0.24 J/cm2; and (D) Ni, 0.41 J/cm2. Scale bar is 1 m. We have also attempted to imprint tungsten. We found that it could be melted readily by a single laser pulse, but it could not be patterned with well defined gratings (yet grating pattern was still noticeable) using a quartz mold. This is likely because: (1) tungsten's melting point is 1800 K higher than that of quartz, hence quartz will be melted (softened) during LADI of W; (2) molten tungsten's surface tension is relatively high, nearly twice that of Cu, thus more difficult to imprint; (3) solid tungsten has a high Young's modulus E, three times that of Cu, leading to >3 higher thermal stress (equal to E(L/L), with L/L being thermal expansion between room temperature and melting point). Because of the high

D

LADI of single crystal silicon wafer (not thin film) is shown in Fig. 8. Due to the thermal expansion upon re-solidification that helps fill the sharp corners of the mold pattern, 10 nm

thermal stress, W film was found to crack after the LADI process.

A B

(e.g. <300 nm) to be melted entirely.

C

fine features on the mold were faithfully duplicated into the silicon. Reflection measurement, as shown in Fig. 8c, indicates that the silicon remained in liquid state for about 220 ns, roughly 10 times the pulse duration.

Fig. 8. LADI of single crystal silicon wafer. (a ) Quartz mold. The ~10 nm wide notches were caused by the reactive ion etching trenching effect during mold fabrication; (b) Imprinted pattern in silicon showing the notches were faithfully duplicated; (c) The reflectivity of a HeNe laser beam from the silicon surface versus the time, when the silicon surface is irradiated by a single laser pulse with 1.6 J/cm2 fluence and 20 ns pulse duration. Molten Si, becoming a metal, gives a higher reflectivity.
