Khairudin Mohamed

A review of roll-to-roll nanoimprint lithography

Since its introduction in 1995, nanoimprint lithography has been demonstrated in many researches as a simple, low-cost, and high-throughput process for replicating micro- and nanoscale patterns. Due to its advantages, the nanoimprint lithography method has been rapidly developed over the years as a promising alternative to conventional nanolithography processes to fulfill the demands generated from the recent developments in the semiconductor and flexible electronics industries, which results in variations of the process. Roll-to-roll (R2R) nanoimprint lithography (NIL) is the most demanded technique due to its high-throughput fulfilling industrial-scale application. In the present work, a general literature review on the various types of nanoimprint lithography processes especially R2R NIL and the methods commonly adapted to fabricate imprint molds are presented to provide a clear view and understanding on the nanoimprint lithography technique as well as its recent developments.PACS81.16.Nd

Review on Thermionic Energy Converters

Thermionic energy converter (TEC) is a heat engine that generates electricity directly using heat as its source of energy and electron as its working fluid. Despite having a huge potential as an efficient direct energy conversion device, the progress in vacuum-based thermionic energy converter development has always been hindered by the space charge problem and the unavailability of materials with low work function. It is only recently that researchers have started to look back into this technology as recent advances in manufacturing technology techniques have made it possible to solve these problems, making TECs a viable option in replacing current energy production systems. The focus of this paper is to review the challenges of producing efficient and practical TECs, along with recent findings and developments in mitigating these challenges. Furthermore, this paper looked into potential applications of TECs, based on recent works and technologies, and found that, with certain improvements, it can be applied in many sectors.

Investigation of a nanofabrication process to achieve high aspect-ratio nanostructures on a quartz substrate

This work investigates the development of a nanofabrication process to achieve high aspect-ratio nanostructures on quartz substrates using electron beam lithography (EBL) patterning and fluorinated plasma etching processes. An imaging layer of a poly(methyl methacrylate) bi-layer resist was spun coated on quartz substrate and exposed by an e-beam with the designed patterns of sub-100 nm feature sizes using a Raith-150 EBL patterning tool. Additive pattern transfer was employed by depositing a 40 nm thick Nichrome layer on the resist pattern using a metal evaporator which was later lifted off by soaking in acetone. Nichrome was employed as an etch mask and an Oxford Plasmalab 80Plus reactive ion etcher was used for the etching process. The etching process was carried out in a gas mixture of CHF(3)/Ar with a flow rate ratio of 50/30 sccm, pressure of 20 mTorr, radiofrequency power of 200 W and at room temperature. These etching process parameters were found to achieve a 10 nm min(-1) etch rate and tall vertical side walls profile. An aspect-ratio of 10:1 was achieved on 60 nm feature size structures.

Three-Dimensional Patterning Using Ultraviolet Nanoimprint Lithography

Although an extensive number of publications have been reported on nanoimprint lithography (NIL) techniques, the ability of NIL for three-dimensional (3-D) patterning has not been fully addressed in terms of the mold fabrication and imprint processes. Developing technologies for patterning 3-D and multilevel features are important because they eliminate multiple steps and complex interlevel alignments in the fabrication process of nanoscale devices and structures. The semiconductor industries through the International Technology Roadmap for Semiconductor (ITRS) organization have identified NIL (ITRS 2003; ITRS 2008), especially ultraviolet curable nanoimprint lithography (UV-NIL) as a strong candidate for the next generation lithography (NGL) technology for nodes down to 5 nm. Three dimensional NIL (3D-NIL) will have a variety of practical applications including generating patterns for MEMS and NEMS devices, on-chip optics, antireflection structures and in biochip reactors. This chapter explores the 3-D patterning capability using an UV-NIL technique. The 3-D features and multilevel mold design, fabrication, and imprint processes have been studied and analysed and outcome will be presented and discussed. In the UV-NIL technique, a transparent mold with micro/nanostructure patterns on its surface is allowed to be printed and replicated on UV curable polymer without the need of high applied pressure or temperature. UV-NIL has the potential to fabricate micro/nanostructures with high resolution, high reproducibility, low cost, high throughput and in addition is capable of 3-D patterning. Its resolution is not limited by diffraction as in optical lithography or electron scattering as in electron beam lithography (EBL) and there is no expense that scales with reducing dimensions as in other nanolithography technologies. The UV-NIL technique is especially useful in avoiding processes that require high pressure and high temperature cycles. Mold pattern writing, pattern transfer and imprint are the three major areas that are described here. The master molds or dies are made using a high-resolution but low-throughput EBL technique. The pattern transfer process for making the mold is achieved using a reactive ion etching (RIE) technique and subsequent imprint lithography is employed for the replication of the micro/nanostructures onto the daughter molds.

3D Microfabrication Using Emulsion Mask Grayscale Photolithography Technique

Recently, the rapid development of technology such as biochips, microfluidic, micro-optical devices and micro-electromechanical-systems (MEMS) demands the capability to create complex design of three-dimensional (3D) microstructures. In order to create 3D microstructures, the traditional photolithography process often requires multiple photomasks to form 3D pattern from several stacked photoresist layers. This fabrication method is extremely time consuming, low throughput, costly and complicated to conduct for high volume manufacturing scale. On the other hand, next generation lithography such as electron beam lithography (EBL), focused ion beam lithography (FIB) and extreme ultraviolet lithography (EUV) are however too costly and the machines require expertise to setup. Therefore, the purpose of this study is to develop a simplified method in producing 3D microstructures using single grayscale emulsion mask technique. By using this grayscale fabrication method, microstructures of thickness as high as 500μm and as low as 20μm are obtained in a single photolithography exposure. Finally, the fabrication of 3D microfluidic channel has been demonstrated by using this grayscale photolithographic technique.

Synthesis and Gas Sensing Properties of SnO2 Nanostructures by Thermal Evaporation

Tin oxide nanostructures (NS) were grown on silicon substrates by thermal evaporation method with three different parameters. These parameters were temperatures (650 °C, 750 °C and 850 °C), nickel catalyst concentrations (0, 5 and 10 milimoles) and tin powder source to substrate distances (2 cm, 4 cm and 6 cm). The parameters were found to affect the size and morphology of the synthesized nanostructures. Formation of nanospheres (NSs), nanoneedles (NNs) and nanowires (NWs) of tin oxide were observed by Scanning Electron Microscope (SEM) at different synthesis conditions. Synthesis temperature was found to have most pronounced effect on the size and morphology of the nanostructures. Catalyst concentration has affected the porosity and growth of the nanostructures. The distance between source and substrate affected the nanostructures predominately on distribution and particle size. Energy dispersion X-ray (EDX) analysis confirms the presence of tin and oxygen in all nanostructures at all synthesis conditions. X-ray diffraction (XRD) proves the formation of tin oxide phase in all samples. Significant formation of tin oxide nanowires was observed at 850 °C. Gas sensing properties of SnO2 nanowires (NW) toward ethanol (C2H5OH) gas at 450°C with different volume concentration was measured. It was found SnO2 NW had good sensing properties for C2H5OH at 100 ppm compared to measurements made at 25-50 ppm.

Patterning of multi-leveled microstructures on flexible polymer substrate using roll-to-roll ultraviolet nanoimprint lithography

The recent developments of flexible electronics, biochips, optical devices and micro/nano-electro-mechanical-systems (MEMS/NEMS) have featured various complex three-dimensional or multileveled micro/ nano structures in its designs. However, fabricating these structures using the existing technologies such as photolithography and electron beam lithography (EBL) are time consuming and involved high process costs. Nevertheless, the production of these microstructures at high volume manufacturing scale has led to the demand for a simpler, low-coat and high-throughput technique for patterning process. In the present work, multi-level microstructures (3-levels) with minimum feature size of approximately 50 μm are continuously patterned onto flexible polymer substrate using in-house designed roll-to-roll ultraviolet nanoimprint lithography (R2R-UV-NIL) system. Using a commercially available 50μm-thick polyethylene terephthalate (PET) film as the flexible substrate and SU8-2002 photopolymer as the imprint resist, continuous patterning of the multi-level structures has been demonstrated at speed of 100 mm/min using R2R-UV-NIL imprinting tool. Ten imprints were produced consecutively, where the confocal laser scanning microscopy (CLSM) measurements of the imprints demonstrated the potential of the R2R-UV-NIL technique to replicate multi-level structures, albeit the pattern waviness or plane flatness issue due to the deformation of the soft PDMS mold. With further process optimization and usage of a harder mold material, the R2R-UV-NIL is a promising technique and tool for fabricating complex 3D and multi-level microstructures on flexible substrate for future applications.

The fabrication of microelectrode array biochip on PET using plate-to-plate NIL

Polyethylene terephthalate (PET) films are useful for various applications due to the characterization of PET were flexibility, thermal resistance and chemical resistant. PET Films are widely used in both commercial and industrial today since they are highly demand in the market. In this study, microelectrode array biochip was fabricated on flexible PET film using (plate-to-plate) nanoimprint lithography (NIL). The PET films acted as substrate while PDMS as the mould. The Polydimethylsiloxane (PDMS) mould was attached on top of the PET film and additionally clamped between two pieces of glass. A mild force was applied to it for REM (replica moulding). A study was carried out upon the surface profiling of the PDMS mould and PET to monitor the possibility of deformation after applying force upon the samples. The experiment shall present the finding of required force to REM the microelectrode array biochip pattern on the PET film.Polyethylene terephthalate (PET) films are useful for various applications due to the characterization of PET were flexibility, thermal resistance and chemical resistant. PET Films are widely used in both commercial and industrial today since they are highly demand in the market. In this study, microelectrode array biochip was fabricated on flexible PET film using (plate-to-plate) nanoimprint lithography (NIL). The PET films acted as substrate while PDMS as the mould. The Polydimethylsiloxane (PDMS) mould was attached on top of the PET film and additionally clamped between two pieces of glass. A mild force was applied to it for REM (replica moulding). A study was carried out upon the surface profiling of the PDMS mould and PET to monitor the possibility of deformation after applying force upon the samples. The experiment shall present the finding of required force to REM the microelectrode array biochip pattern on the PET film.

Synthesis and fidelity study of ultraviolet-curable hydrogen silsesquioxane analogue as an elastomeric stamp

An analogue of hydrogen silsesquioxane (HSQ) with an epoxy-terminated inorganic polydimethylsiloxane spacer at tethered positions was prepared as a stamp for soft lithography. Nuclear magnetic resonance (NMR) spectroscopy and wide angle X-ray diffraction (WAXD) results proved the existence of a crystallite cage silsesquioxane structure embedded in an amorphous matrix. The fluid analogue was ultra-violet cured whose curing characteristic was closely related to catalyst concentration, post-bake time, and post-bake temperature. Spectroscopic analysis showed that the pattern was successfully replicated onto the HSQ analogue surface with high fidelity down to the sub-micron scale. The electron beam dosages affect the features of the replicated soft mould. The surface profile and roughness were almost identical between the hard and soft mould leading to a respectable surface roughness (Ra and Rq) of 3.0–4.5 nm and aspect ratio as high as 7.5. Synergistic effect of the rigid cage structure of silsesquioxane with a flexible organic spacer offers an efficient master mould filling.

Evaluation of satellite and meteorology data to estimate solar irradiance over five cities in Malaysia

In this paper, meteorology and satellite data are evaluated to estimate global and direct solar irradiance over Malaysia. The meteorology data consist of direct solar irradiance were for 2013 and the satellite data were global solar irradiance from year 2000 till 2004. The estimated global solar irradiance were then compared with observe satellite data and their performance were judge by using statistical indicators, including root mean squared error and mean percentage error. The 5 cities studied in Malaysia includes Kota Kinabalu (KK), Kuching (KC), Ipoh (IP), Alor Setar (AS) and Kuantan (KU). The yearly average of hourly solar irradiance for all 5 cities throughout a year lies between 193 W/m2 and 208 W/m2. The highest solar irradiance has been estimated at 216 W/m2 in the month of March due to the dry season, and the minimum solar irradiance is predicted in the month of December is 164 W/m2 that coincides with the rainy season in Malaysia. Results show that the proposed global and direct solar irradiance models present good regression coefficient (R2) value of 0.9661 and 0.9549 respectively. The average diffuse solar irradiance cast over these cities were 3.0% (correction factor) of global solar irradiance. Since Malaysian government are focusing on renewable and clean energy, these models and analyses will benefit the government and education department for further development in solar technologies.