Nanolithography

In the fabrication of transistors, organic light emitting diodes (OLEDs), and energy storage devices, there has been a high demand for the thin film layers to be patterned, etched and coated. Lithography combines those processes and can create millions of devices in batch. During the past decades many nanolithography techniques have been developed to write micro- and nanostructures on a wide variety of materials.

Photolithography is the process of transferring geometric shapes on a mask to the surface of a silicon wafer. The steps involved in the photolithographic process are wafer cleaning; barrier layer formation; photoresist application; soft baking; mask alignment; exposure and developing; and hard-baking. First a photoresist layer is spin coated on silicon wafer, and mask aligner uses optical radiation to image the mask on the silicon wafer coated with photoresist layers. After wet chemical development, on negative photoresists the exposed area is polymerized and rendered insoluble to developer solution, while on positive photoresists the exposure decomposes a development inhibitor and the developer only dissolve the exposed area. Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light from excimer lasers with wavelengths of 248 and 193 nm, which allow minimum feature sizes down to 50 nm.

Because of its optical diffraction limit, conventional optical or ultraviolet photolithography is anticipated to reach its resolution limit at a wavelength of 157 nm, beyond which significant issues arise in terms of availability of light sources, masks, and the need for new photoresist materials. The use different forms of radiation, including extreme UV, x-ray, electron beams, and ion beams, to offer higher resolution, are growing in importance. Electron beam lithography (EBL) is a mask-less techniques that allow us to create patterns at the nanoscale. The EBL working principle is relatively simple and very similar to photolithography: A focused beam of electron is scanned across a substrate covered by an electron-sensitive material (resist) that changes its solubility properties according to the energy deposited by the electron beam. Areas exposed, or not exposed according to the tone of the resist, are removed by developing. A typical EBL process includes: CAD design, conversion and proximity correction, sample preparation, machine calibration, exposure and developing.

A typical EBL tool consisting of a chamber, an electron gun, a column containing all the electron optics needed to focus, scan, and turn on or turn off the electron beam. Reprinted from Ref 1*.

Photolithography has been well developed since its invention in 1959, but its resolution is subject to limitations set by the optical diffraction according to the Rayleigh equation. Additionally, it can’t be easily adopted for non-planar surfaces and it has little variation in the photoresists selections. A number of non-photolithographic techniques have been demonstrated for fabricating high quality micro/nanostructures. Soft lithography generates micropatterns of self-assembled monolayers (SAMs) by contact printing, or forms microstructures in materials by embossing (imprinting) and replica molding. For example, replica molding duplicates the information represented in a master by casting the liquid prepolymer of an elastomer, poly(dimethylsiloxane) (PDMS) as a widely used example, against the master that has a patterned relief structure in its surface. The features replicated can be smaller than 100 nm.

Scanning probe lithography (SPL) techniques utilize tools, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to visualize underlying surfaces with the highest spatial resolution. Taking advantage of the sharpness of the tips, and the localized strong tip-surface interactions, SPL has also been used to manipulate atoms on metal surfaces and to fabricate nanopatterns on metal ad semiconductor surfaces.

Conventional lithographic methods become increasingly inefficient and more expensive as the size scale of device gets ever smaller, especially at a minimum feature size of less than 45 nm. Block copolymer lithography serves as a good alternative by connecting two incompatible polymers at their ends that leads to a fascinating class of self-assembling materials. Nanoporous materials can be generated by selective removal of one component from a self-assembled block copolymer. These materials exhibit the nanopore size and pore topology and can be used as nanolithographic masks, separation membranes and nanomaterial templates. In linear AB diblock copolymers the following four equilibrium morphologies have been identified: lamellar, hexagonally packed cylindrical, bicontinuous gyroid, and BBC spherical. The principal spacings in these ordered structures are between 5 and 50 nm.

References:
1. Altissimo, Matteo. "E-beam lithography for micro-/nanofabrication." Biomicrofluidics 2010.
2. Xia, Younan. "Soft lithography." Angewandte Chemie International Edition 1998.
3. Hillmyer, Marc A. "Nanoporous materials from block copolymer precursors." Block copolymers II. Springer, Berlin, Heidelberg, 2005.

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