Molecular Materials

Low molecular weight organic resists initially were not suitable for e-beam lithography because thin films made of them on SiO2 were polycrystalline (Yoshiiwa 1996). Recently, organic materials have been described that form amorphous films and are sensitive to electrons. The third allotrope of carbon, C60, has been shown to be a negative tone e-beam resist, leading to 20 nm structures (Tada 1997). The draw back of this material was its low solubility and relatively poor sensitivity toward electrons compared to the polymeric materials. However, these limitations have been overcome to some extent by the chemical modification of C60 (Fig. 16.1(a)), which has improved the solubility, inhibited the tendency to form polycrystalline materials (Robinson 1998) and achieve sensitivities toward electrons which are almost comparable to PMMA.

Fig. 16.1. Chemical structures of (a) C60 derivatives (b) triphehylene derivatives (Ref. Robinson 1999,2000), (c) a 4,4',4''-tris(allylysuccinimido)-triphenylamine (from Ref. (Yoshiiwa 1996), and (d) a calixarene derivative (from Ref. Sailor 2004)

Fig. 16.1. Chemical structures of (a) C60 derivatives (b) triphehylene derivatives (Ref. Robinson 1999,2000), (c) a 4,4',4''-tris(allylysuccinimido)-triphenylamine (from Ref. (Yoshiiwa 1996), and (d) a calixarene derivative (from Ref. Sailor 2004)

Another approach to overcoming the polycrystallinity problem has been the use of liquid crystalline materials based on triphenylenes (Fig. 16.1(b)), which also have large extended ^-electron framework and high carbon content like the C60 derivatives, which are thought to be important in terms of the sensitivity and etch durability (Robinson 1999). These liquid crystalline materials were able to surpass the comparative resolution of PMMA, but the sensitivity was not comparable (Robinson 2000). Fig. 16.2 illustrates the e-beam patterned structures that can be created with these triphenylene derivatives.

30 kV X 150K Tilt Angle 40°

20C

) nm

(b)_

SMHB

M

20 nm

30 kV X 150K Tilt Angle 0°

100C

■ ■ ■ ■ nm

Fig. 16.2. Scanning electron micrographs of e-beam written structures to thin films of hexapentyloxytriphenylene on SiO2 surfaces after washing to leave the crosslinked triphenylene structures (adapted from Ref. Robinson 1999)

Fig. 16.2. Scanning electron micrographs of e-beam written structures to thin films of hexapentyloxytriphenylene on SiO2 surfaces after washing to leave the crosslinked triphenylene structures (adapted from Ref. Robinson 1999)

Other classes of molecular materials that have received interest as e-beam resist materials are calixarenes (Schock 1997), 1,3,5-tris[4-(4-toluenesulfonyloxy) phenyl)-benzene (Yoshiiwa 1996) and 4,4',4''-tris(allylysuccinimido)-triphenylamine (Yoshiiwa 1996) (Fig. 16.1(c)). The calixarenes appear to be making some running in terms of sensitivity and resolution of features especially when used as a chemically amplified resist (Sailor 2004) (Fig. 16.1(d)). The limitations of these molecular materials are that a thin film of several 10 to 100s of molecules thick is formed by drop casting or spin coating of a solution onto the SiO2 surface, followed by solvent evaporation. Thus, resolution of the structures formed is limited by the thickness of the film, as a result of electron scattering effects in the multilayer film (Rai-Choudhury 1997). The electron scattering results in the electron exposure to propagate radially away from the focus of the original e-beam incident source, and scales with the thickness of the resist film. In addition, a number of processing steps are required pre- and post- e-beam exposure (Chen 1993). The use of self-assembled monolayers, i.e. one molecule thick, as resists overcomes the back-scattered electron problem, as well as making processing potentially simpler.

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