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Ablation assistor enables low-fluence photoablation of carbon nanotubes

Use of a photoresist material and a low-fluence excimer laser can pattern carbon nanotubes cleanly and inexpensively.
2 February 2009, SPIE Newsroom. DOI: 10.1117/2.1200901.1436

Carbon nanotubes have been considered as a possible component in various microelectronic applications. Unfortunately, existing patterning methods limit progress toward this goal. Some researchers have patterned carbon nanotubes but either their methods were not compatible with the complementary metal-oxide semiconductor (CMOS) processes used for fabricating microelectronic components, or they could not precisely control the pattern geometry.1–3 Lu et al. used an oxygen plasma but this approach requires a vacuum etcher system, which limits the applicability of this process.1

Many scientists have studied excimer-laser photoablation—which uses UV light to etch surfaces—as a simplified method for patterning devices. With sufficiently high incident-laser fluence, photodissociation can etch material away. However, because of the chemical and physical stability of carbon nanotubes photoablation cannot be used for patterning, not even with high fluence (590mJ/cm2 with 500 pulses).4 Even if the technique were effective, this level is too high for many substances. For instance, silicon nitride—a commonly used hard material—may be damaged at a fluence of ~400mJ/cm2.

To get around these problems we used a photoablation-assistor material in the patterning process to ensure low-fluence exposure (see Figure  1). We coated carbon nanotubes, prepared on a silicon substrate, with a conventional photoresist, S1818 by MicroChem. Because the photoresist is in liquid phase, it fills the space under and above the nanotubes after the coating process. When both are exposed to low-fluence excimer-laser illumination, the physical force of the dissociated photoresist removes the nanotubes. Figure 1(d) shows the patterning after photoablation. Upon removal of the photoresist using acetone solution, we were able to produce clean patterning (see Figure 2).

Figure 1. Concept of photoresist-assisted excimer-laser photoablation of carbon nanotubes. (a) Carbon nanotubes are first prepared on a silicon substrate and then (b) coated with a photoresist. (c) After low-fluence laser illumination (d) the patterned nanotubes remain.

Figure 2. Scanning-electron micrographs of carbon nanotubes on silicon substrate show clean, sharp patterning.

This method can also pattern materials that are porous enough to incorporate the ablation assistor. Thus, it can be used in the fabrication of nanowire or chemical devices. To be viable, however, the assistor material must be easily photoablated. The threshold fluence of the assistor material should be low enough to prevent the substrate or underlying structures from being damaged. It should also be easy to remove cleanly after patterning, so that only the patterned material remains. Various photoresists could work. Many can be photoablated in low-fluence conditions, around 100mJ/cm2. Most importantly, the photoresist is a common chemical used in the high-volume production of microelectronic devices.

We have developed a novel patterning method that can be used for carbon nanotubes, nanowires, or porous materials. This process is CMOS-compatible and can therefore be applied in conventional fabrication facilities. Its low fluence ensures that there is no damage to the underlying structures or substrates. Because there is no development or etching step Required during patterning, the new method is economical. Finally, the pattern quality is clean and sharp. We are currently investigating the fabrication of carbon-nanotube devices using this process while also exploring other applications.

Kanti Jain, Junghun Chae 
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign (UIUC)
Urbana, IL

Kanti Jain is a professor of electrical and computer engineering. He received a bachelor's degree from the Indian Institute of Technology, Kharagpur, in 1969, and MS and PhD degrees from UIUC in 1970 and 1975, respectively. He was a postdoctoral fellow at the Massachusetts Institute of Technology between 1975 and 1977. From 1979 to 1988 he worked for IBM, where he invented and developed excimer-laser lithography, earning two Outstanding Innovation Awards. From 1989 to 1991 he was director of technology in advanced microelectronic-packaging systems for Raychem. He is also the founder of Anvik Corporation and has been its chief executive officer since 1992. Jain is a fellow of the Institute of Electrical and Electronics Engineers, the Optical Society of America, and SPIE, and a former member of the board of directors and the executive committee of SPIE. He is the author of the book ‘Excimer Laser Lithography,’ has published 62 papers, and holds 68 patents.

Junghun Chae is a postdoctoral research associate. He received his BS degree from the Korea Advanced Institute of Science and Technology (KAIST) in 1995 and his PhD in materials science and engineering from KAIST in 2002. He was a senior research engineer at LG.Philips LCD from 2002 to 2006, developing amorphous-silicon transistors, low-temperature transistors, and polysilicon transistors for thin-film transistor LCDs.