K. Ansari

Fabrication of high aspect ratio 100nm metallic stamps for nanoimprint lithography using proton beam writing

We report a way of fabricating high-quality void-free high-aspect-ratio metallic stamps of 100nm width and 2μm depth, using the technique of proton beam writing coupled with electroplating using a nickel sulfamate solution. Proton beam writing is a one-step direct-write process with the ability to fabricate nanostructures with high-aspect-ratio vertical walls and smooth sides, and as such has ideal characteristics for three-dimensional (3D) stamp fabrication. Nanoindentation and atomic force microscopy measurements of the nickel surfaces of the fabricated stamp show a hardness and side-wall roughness of 5GPa and 7nm, respectively. The fabricated 100nm 3D stamps have been used to transfer test patterns into poly(methylmethacrylate) films, spin coated onto a silicon substrate. Proton beam writing coupled with electroplating offers a process of high potential for the fabrication of high quality metallic 3D nanostamps.

Proton beam writing: a progress review

A new direct write 3D nano lithographic technique has been developed at the Centre for Ion Beam Applications (CIBA) in the Physics Department of the National University of Singapore. This technique employs a focused MeV proton beam which is scanned in a predetermined pattern over a resist (e.g. PMMA or SU-8), which is subsequently chemically developed. The secondary electrons induced by the primary proton beam have low energy and therefore limited range, resulting in minimal proximity effects. Low proximity effects coupled with the straight trajectory and high penetration of the proton beam enables the production of 3D micro and nano structures with well-defined smooth side walls to be directly written into resist materials. In this review the current status of proton beam writing will be discussed; recent tests have shown this technique capable of writing high aspect ratio walls up to 160 and details down to 30 nm in width with sub-3 nm edge smoothness.

Stamps for nanoimprint lithography fabricated by proton beam writing and nickel electroplating

In the emerging fields of nanoscience and nanotechnology, the demands for low-cost and high-throughput nanolithographic techniques have increased. Nanoimprint lithography is considered as one of the candidates showing high potential for nanofabrication, and here we report a fabrication process that utilizes high-quality nickel stamps with micron features down to sub-100 nm, made using proton beam writing coupled with nickel sulfamate electroplating. The fabricated stamps have a high aspect ratio, with smooth and vertical sidewalls. Nanoindentation and atomic force microscopy (AFM) measurements of the features on the surface of the stamps indicate a hardness of 5 GPa and a sidewall roughness of 7 nm. The stamps have been used for nanoimprint lithography on polymethylmethacrylate (PMMA) substrates and the imprinted patterns show a high degree of reproducibility.

Fabrication of enclosed nanochannels in poly(methylmethacrylate) using proton beam writing and thermal bonding

We report a technique for fabricating enclosed nanochannels in poly(methylmethacrylate) (PMMA) using proton beam writing coupled with thermal bonding. Using proton beam writing, straight-walled high-aspect-ratio channels can be directly fabricated through a relatively thick PMMA resist layer spin coated on a Kapton film. By thermally bonding the fabricated structures onto bulk PMMA, peeling off the Kapton substrate, and bonding the remaining exposed side to PMMA, enclosed high-aspect-ratio nano/microchannels can be fabricated. Such enclosed channels can be incorporated into fluidic polymeric devices, and the process is compatible with the fabrication of multilevel three-dimensional fluidic chips with vertical interconnects.

Improvement in proton beam writing at the nano scale

Here we report on the progress of 3D nano machining using MeV protons. In proton beam (p-beam) writing a proton beam is typically focused down to a sub 100 nm spot size and scanned over a resist material (e.g. Su-8 or PMMA). Currently the scanning is performed using a magnetic scan coil which has an intrinsically long settling time. A new scanning system is introduced which employs electrostatic scanning and allows an increase in writing speed up to 2 orders of magnitude.

Porous-silicon-based Bragg reflectors and Fabry-Perot interference filters for photonic applications

Visible light emission from the porous silicon (PSi) formed by anodic etching of Si in HF solution has raised great interest in view of possible applications of Si based devices in optoelectronics. In particular, multilayers consisting of periodic repetition of two PSi layers whose refractive indices are different can be exploited to design interference filters for controlling the emission wavelength as well as for the spectral narrowing of the wide emission band of Psi. Fabry-Perot optical microcavities with an active layer of λ\2 or λ sandwiched between two Bragg reflectors, consisting of alternating layers of high and low refractive indices are fabricated on heavily doped p-type silicon. We have investigated the optical properties of these microstructures using reflectivity and photoluminescence measurements at various temperature.

Micromachining using a focused MeV proton beam for the production of high-precision 3D microstructures with vertical sidewalls of high orthogonality

The production of high aspect ratio microstructures requires a lithographic technique capable of producing microstructures with vertical sidewalls. There are few techniques (eg proton beam micromachining, LIGA and Stereolithoghaphy) capable of producing high aspect ratio microstructures at sub-micron dimensions. In Proton Beam Micromachining (PBM), a high energy (eg 2 MeV) proton beam is focused to a sub-micron spot size and scanned over a resist material (eg SU-8 and PMMA). When a proton beam interacts with matter it follows an almost straight path, the depth of which is dependent on the proton beam energy. These features enable the production of multilevel microstructures with vertical sidewalls of high orthogonality. Proton beam micromachining is a fast direct write lithographic technique; in a few seconds a complicated pattern in an area of 400 x 400 micrometers 2 can be exposed down to a depth of 150 micrometers . These features make proton beam micromachining a technique of high potential for the production of high-aspect-ratio-structures at a much lower total cost than the LIGA process, which requires a synchrotron radiation source and precision masks. Research is currently under way to improve the process that employs the SU-8 negative photo-resist as a mold to electroplate Ni. Experiments have shown that post-bake and curing steps are not required in this SU-8 process, reducing the effects of cracking and internal stress in the resist. Plated Ni structures can be easily produced which are high quality negative copies of the SU-8 produced microstructures.

Fabrication of silicon molds with multi-level, non-planar, micro- and nano-scale features

A method for single-step fabrication of arbitrary, complex, three-dimensional (3D) silicon structures from the nano- to millimeter-scale at multiple levels on non-planar, curved, or domed surfaces is reported. The fabrication is based on focused or masked ion beam irradiation of p-type silicon followed by electrochemical anodization. The process allows fabrication of a wide range of surface features at multiple heights and with arbitrary orientations by varying the irradiated feature width, ion type, energy fluence, and subsequent anodization conditions. The technology has achieved depth resolution of 10 nm as step heights and is capable of creating lateral features down to 7 nm at high aspect ratios of up to 40, with surface roughness down to 1 nm scaled up to full wafer areas. The single-step ability has seamlessly interfaced a network of complex, integrated micro- to nano-structures in 3D orientations with no alignment required. The final template has been converted to a master copy for nano-imprinting lithography of 3D fluidic structures and optical components.