**6. Conclusion**

134 Recent Advances in Nanofabrication Techniques and Applications

viscosity of molten Au and Cu is around 4 cp (only 5 that of wafer (Yaws 1998)), leading to L1~ 5 m. To pattern isolated metal mesas, the liquid flows from four sides to fill a square-shaped hole in the mold, so the maximum patternable mesa size should be roughly doubled to ~10 m, which qualitatively agrees with the experiment (17 m). Moreover, it is interesting to see how far molten SiO2 can flow under similar conditions. For example, its viscosity at 2200 K is 8107 cp (a very tacky "liquid"), 7 orders higher than that of Au and Cu, leading to a calculated L1 of only ~1 nm if assuming a top 200 nm SiO2 layer is molten. This explains why the quartz mold can be used to pattern a metal at temperature slightly higher than its melting point, though the mold could not be used repeatedly for >>10 times.

Fig. 16. Dynamics of molten metal flowing into trenches during LADI. Assume that grating line-width and trench-width are both equal to L, and trench depth and molten layer

Next, to estimate the inertial force which decides how fast the steady flow develops, it is assumed that the liquid metal is inviscid (μ = 0) for simplicity. The applied pressure is to accelerate the flow along the x-direction with acceleration given by (note that the effective pressure is doubled for a grating with equal line and trench width, and z is the direction

*a*

On the other hand, the volume of the liquid changes at a rate

2 4 ( /2) *F P hz P*

(12)

*L dh <sup>Q</sup> dt* (13)

*L dt* (14)

(15)

*m hz L L*

The velocity is then u=at at time t, and the flow-rate Q=uh=ath (ignoring the z-dimension).

2

2 *P L dh th*

4

<sup>2</sup> 2 ~ ln 2 *P P <sup>L</sup>* 

thickness are both equal to h0.

perpendicular to the paper):

Therefore, solving the equation

gives (h=h0, h0/2 at t=0, )

In this chapter, five nanofabrication processes using pulsed excimer laser were described. These methods share the common advantage of being orders faster than most other fabrication techniques. They are also very suitable to patterning metals, which are more difficult to process than semiconductors by conventional lithographies and etching.

The first was laser-assisted direct imprinting, which produced 200 nm-period Cu, Ni and Al gratings over about 1 mm2 area within ~100 ns. LADI of W failed due to its high thermal stress. The second was wafer planarization using a flat and smooth "mold". With this technique, Cu surface was planarized by laser melting under pressure, which also squeezed the molten film to fill completely voids under the film. Cu conducting lines embedded in a dielectric matrix were created by an additional etching step. Third, a similar process could also fill 100 nm-wide and 500 nm-deep via-holes with Si and Cu. Fourth, sub-100 nm diameter Cr dots were melt-transferred to the receiving substrate using a laser-induced

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nanotransfer printing. Fifth, Cr tips with apex diameter as small as 10 nm and aspect ratio up to 10:1 were achieved by laser melting and boiling through the proposed mechanism of an electro-hydrodynamic instability. Such tips could be employed as field emitters for flat panel display, or as tips for scanning probe microscopy.

For laser-assisted direct imprint, planarization, and via-hole filling, the resolution is determined by the balance between the surface tension and the applied pressure. Whereas the maximum patternable feature size is limited by the liquid viscosity and friction at the interfaces. Strong adhesion between the mold and the molten material rules out clean separation thus sets another constraint on processable materials. To scale up the process to full wafer patterning, the challenge is the availability of pulsed laser with large enough beam size and the very high pressure normally needed.
