Nanoimprint lithography (NIL) is a nonconventional lithographic technique for high-throughput patterning of polymer nanostructures at great precision and at low costs. Unlike traditional lithographic approaches, which achieve pattern definition through the use of photons or electrons to modify the chemical and physical properties of the resist, NIL relies on direct mechanical deformation of the resist material and can therefore achieve resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in conventional techniques. This Review covers the basic principles of nanoimprinting, with an emphasis on the requirements on materials for the imprinting mold, surface properties, and resist materials for successful and reliable nanostructure replication.

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Nanoimprint Lithography: Methods and Material Requirements**

Nanoimprint is an emerging lithographic technology that promises high-throughput patterning of nanostructures. Based on the mechanical embossing principle, nanoimprint technique can achieve pattern resolutions beyond the limitations set by the light diffractions or beam scatterings in other conventional techniques. This article reviews the basic principles of nanoimprint technology and some of the recent progress in this field. It also explores a few alternative approaches that are related to nanoimprint as well as additive approaches for patterning polymer structures. Nanoimprint technology can not only create resist patterns as in lithography but can also imprint functional device structures in polymers. This property is exploited in several non-traditional microelectronic applications in the areas of photonics and biotechnology.

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/48920/d4_11_r01.pdf?sequence=2

Recent progress in nanoimprint technology and its applications

Nanofabrication by electron beam lithography and its applications

We investigate poly(methylmethacrylate) (PMMA) development processing with cold developers (4–10 °C) for its effect on resolution, resist residue, and pattern quality of sub-10 nm electron beam lithography (EBL). We find that low-temperature development results in higher EBL resolution and improved feature quality. PMMA trenches of 4–8 nm are obtained reproducibly at 30 kV using cold development. Fabrication of single-particle-width Au nanoparticle lines was performed by lift-off. We discuss key factors for formation of PMMA trenches at the sub-10 nm scale.

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Sub-10 nm electron beam lithography using cold development of poly(methylmethacrylate)

A new UV-curable liquid resist based on cationic polymerization of silicone epoxies has been developed for UV-assisted nanoimprint lithography. Uniform films with thicknesses ranging from below 50 nm to over 1 μm can be easily spin-coated using a suitable undercoating layer on a substrate. Patterns with feature sizes ranging from tens of micrometers to 20 nm are imprinted at room temperature with a pressure of less than 0.1 MPa.

Room-Temperature, Low-Pressure Nanoimprinting Based on Cationic Photopolymerization of Novel Epoxysilicone Monomers.

A new process has been developed to fabricate T-shaped gates and Γ-shaped gates for high performance metal–semiconductor field effect transistors and high electron mobility transistors using a bilayer of Shipley UVIII DUV resist and poly(methylmethacrylate). The two resists are separated by a 20–30 nm thick layer of aluminum and after patterning by electron beam lithography a two-stage development technique is used to remove the aluminum and to produce well-defined resist profiles. The process can be used to fabricate T-shaped and Γ-shaped gates with footwidth sizes as small as 50 nm and headwidth to footwidth ratios in excess of 40:1 for T gates and 35:1 for Γ gates. The ability to fabricate gates with these dimensions arises from the fact that the UVIII resist is considerably more sensitive to electron beam exposure than PMMA. Further benefits derived from using a UVIII: PMMA bilayer are better control of footwidth dimensions and shorter electron beam patterning times compared to bilayers of PMMA with copolymers of PMMA. This article describes process optimization and the relationship between feature size and exposure dose.

Electron beam lithography process for T- and Γ-shaped gate fabrication using chemically amplified DUV resists and PMMA

In this article, we report a procedure for the fabrication of ultrashort T gates using high resolution electron beam lithography and a PMMA/LOR/UVIII resist stack. The intermediate lift-off resist (LOR) layer improves the quality of gate lithography, and consequently, device yields. It is unaffected by wet chemical gate recessing procedures and we report the application of the procedure to the fabrication of pseudomorphic and metamorphic high electron mobility transistors (pHEMTs) with 50 nm T gates. Fabricated pHEMTs had a gm of 600 mS/mm and ft of 200 GHz. Metamorphic HEMTs had a gm of 1500 mS/mm and ft of 350 GHz. We believe these are the fastest transistors of their kind in the world.

Fabrication of ultrashort T gates using a PMMA/LOR/UVIII resist stack

Nanoimprint lithography is capable of patterning substrates with high definition patterns at relatively high patterning speeds. In this article we describe the fabrication of high resolution “T” gate resist profiles by imprint lithography. The fabrication of high resolution stamping tools and the imprinting process itself are critical to the success or failure of this technique and they are described in the article. Two different techniques were used to fabricate stamping tools. The first involved pattern definition by high resolution electron beam lithography followed by electroforming. The second involved pattern definition by electron beam lithography followed by a two stage silicon etching process. Imprinted T gate resist profiles with footwidths less than 100 nm in length were obtained on gallium arsenide substrates for the purpose of producing metallized gates for a self-aligned gate process.

Fabrication of T gate structures by nanoimprint lithography

T-gates are commonly used in high frequency low noise transistors on III–V materials since they provide a combination of short gate length and low gate resistance. Nanoimprint lithography can produce minimum pattern feature sizes equivalent to those attainable by high resolution electron beam lithography and it has potential advantages in terms of speed and cost. The imprint lithography step must be reliable and compatible with existing device process flows. In this article we describe a bilayer resist imprinting procedure for the fabrication of 120 nm T-gates for high electron mobility transistors. The results of transistor dc characterization are also presented and are similar to those obtained for transistors fabricated on the same material with gates realized by electron beam lithography.

Fabrication of high electron mobility transistors with T-gates by nanoimprint lithography

A novel process has been developed for the fabrication of sub-100 nm T-shaped gates for high performance MESFETs and HEMTs using a bilayer of Shipley UVIII DUV resist and polymethylmethacrylate (PMMA). The process is reliable and gives well defined metallised gates. Ratios of gate cross-section to gate length in excess of 20:1 have been achieved.

Y. Chen

Papers

Electron beam lithography process for T- and Γ-shaped gate fabrication using chemically amplified DUV resists and PMMA

Fabrication of ultrashort T gates using a PMMA/LOR/UVIII resist stack

Fabrication of T gate structures by nanoimprint lithography

Fabrication of high electron mobility transistors with T-gates by nanoimprint lithography

Fabrication of T-shaped gates using UVIII chemically amplified DUV resist and PMMA