Orthogonal and fine lithographic structures attained from the next generation proton beam writing facility

A second generation proton beam writing (PBW) system has been built at the Centre for Ion Beam Applications at the National University of Singapore for fabrication of high aspect ratio 3D nano lithographic structures. System improvements and a few lithographic structures obtained with this facility are presented in this paper. Through accurate alignment of the magnetic quadrupole lenses and the electrostatic scanning system, orthogonal beam scanning has been achieved. The earlier constrain of limited beam scan area has been overcome by adopting a combination of beam and stage scanning as well as stitching. With these improvements smallest ever Ni structure of 65 nm in width has been fabricated using nickel electroplating on a proton beam written PMMA sample in the second generation PBW facility. Using this improved PBW facility, we have also demonstrated the fabrication of fine lithographic patterns with 19 nm line width and 60 nm spacing in 100 nm thick negative high resolution hydrogen silsesquioxane resist. Future possible system improvements leading to finer resolution will be discussed briefly.     

Replication of sub-100 nm structures using h- and s-PDMS composite stamps

Soft-UV-NIL as replication technique was used to replicate sub-100 nm structures. The aim of this work is the stamp production and the replication of structures with dimensions smaller than 100 nm in a simple manner. Composite stamps composed of two layers, a thin hard PDMS layer supported by a thick soft PDMS (s-PDMS) layer are compared to common s-PDMS stamps regarding the resolution by using a Siemens star (star burst pattern) as test structure. The master is fabricated by electron beam lithography in a 140 nm thick PMMA resist layer. The stamp is molded directly from the structured resist, without any additional anti sticking treatment. Therefore the resist thickness determines the aspect ratio, which is 1.5 at the resolution limit. The replication is done in a UV-curing cycloaliphatic epoxy material. The employed test structure provides good comparability, the resolution limit at a glance, and it integrates a smooth transition from micro- to nanostructures. Therefore it is a capable structure to characterize the UV-NIL.     

Prediction and optimization of electrospinning parameters for polymethyl methacrylate nanofiber fabrication using response surface methodology and artificial neural networks

Since the fiber diameter determines the mechanical, electrical, and optical properties of electrospun nanofiber mats, the effect of material and process parameters on electrospun polymethyl methacrylate (PMMA) fiber diameter were studied. Accordingly, the prediction and optimization of input factors were performed using the response surface methodology (RSM) with the design of experiments technique and artificial neural networks (ANNs). A central composite design of RSM was employed to develop a mathematical model as well as to define the optimum condition. A three-layered feed-forward ANN model was designed and used for the prediction of the response factor, namely the PMMA fiber diameter (in nm). The parameters studied were polymer concentration (13–28 wt%), feed rate (1–5 mL/h), and tip-to-collector distance (10–23 cm). From the analysis of variance, the most significant factor that caused a remarkable impact on the experimental design response was identified. The predicted responses using the RSM and ANNs were compared in figures and tables. In general, the ANNs outperformed the RSM in terms of accuracy and prediction of obtained results.     

Resist evaluation for proton beam writing,Ni mold fabrication and nano-replication

Proton beam writing (PBW) is a new direct-write technique which has shown great potential to fabricate structures down to 20 nm level in resist material. Protons can be accelerated up to a high energy (3.5 MeV) at Centre for Ion Beam Applications. Because the mass of a proton is much larger than the mass of an electron (m_p:m_e = 1,800:1), the energy of the secondary electrons is very small compared with secondary electrons generated by electron beam lithography. Therefore, a proton will travel along a straight path into resist and secondary electrons will only expose the resist within several nanometers around the path of the proton. PBW is capable of fabricating structures with very straight, vertical and smooth sidewalls without proximity effect. This is very important when combining PBW with Ni electroplating and nanoimprinting as well as injection molding. High quality Ni molds with smooth and vertical side walls are critical in nanoimprint lithography and injection molding. In our experiments, several new resists including AR-P 3250, a mixture of AR-P 3250 and AR 300-12, and ma-N 2401 are tested with PBW for the production of high aspect ratio Ni molds and thermoplastic replication with these molds. High aspect ratio structures (up to 7) are fabricated at a width of 500 nm in Ni molds. The structures are transferred to plastic via nanoimprinting and injection molding.     

Design, fabrication, and test of an on-chip micro flow cytometer with integrated out-of-plane microlenses

A micro flow cytometer with an integrated three-dimensional hydro-focusing unit and out-of-plane microlenses was successfully fabricated and tested. The entire system was fabricated with SU-8 ultra-violet lithography process. In the hydro-focusing unit, sheath flows pass through a trapezoid-shaped chamber with three 30° slopes to focus the sample flow in both horizontal and vertical directions. As an essential component in the on-chip optical detection system, integrated out-of-plane microlens was embedded in the sidewall of the fluid outlet channel in the detection area. A pre-aligned optical fiber holder was fabricated on chip to fix the output optical fiber in a position aligned to the microlens. Optical simulation and analysis were also conducted using commercial software Zemax. Numerical simulation results confirmed that the use of microlens substantially improved the detection efficiency by focusing the fluorescent light from the sample cells into the output optical fiber. Preliminary cell counting experiment was performed using the fabricated micro flow cytometer system and the experimental results proved the feasibility of the integrated micro flow cytometer design.     

LIGA micro-openings for coherence characterization of X-rays

Infinitely thin opaque screens serving for diaphragms as defined in the visible-light optics are not feasible for operation with X-ray beams due to their high penetrability. Micro-openings of predicted sidewall shape in a gold layer up to 140 μm thick which would provide low transparency, are proposed. The proposed micro-openings were made using the LIGA technique and tested successfully at photon energies of up to 25 keV. The micro-openings can be used as targets for coherence X-ray pattern or, if long interference tails are avoided by means of the advanced sidewall shape, as X-ray beam collimators.     

Fabrication and analysis of the reflowed microlens arrays using JSR THB-130 N photoresist with different heat treatments

This paper reports that the fabrication of the reflowed microlens by the negative tone JSR THB-130 N photoresist can be treated with different thermal treatments using hotplate and oven. The different disk or thin cylinder arrays with diameters of 40–70 μm and thickness of about 7.4 μm were patterned using photolithography technology, and baked at 220°C by two kinds of thermal treatments using hotplate and oven to form reflowed microlens arrays. The spot size of the refractive microlens was then measured by optical microscopy and the total focal length of refractive microlens was simulated by curve fitting the lens profiles. The resolution of the microlens arrays approaches to 400 dpi as coated with Hexamethyldisilizane material. The smallest spot size of about 2.72 μm at the nominal 40 μm microlensis is obtained by the oven heat treatment, and the shortest total focal length of about 150 μm at the nominal 40 μm microlens is achieved by the hotplate heat treatment. The reduced spot size and total focal length of the microlens could improve the density and performance of optical devices and imaging systems.     

Fabrication of a high aspect ratio (HAR) micropillar filter for a magnetic bead-based immunoassay

A microfluidic chip has been realized for investigating immune cell (U937) activation with lipopolysaccharide (LPS) and subsequent pro-inflammatory cytokine (Interleukin-6, IL-6) detection (Ruffert et al. Proc. EMBL Conference Microfluidics 2012a, p 184; Proc. NanoBioTech Montreux, Poster Sessions B 2012b, pp 17–18). The microfluidic chip comprises two compartments: one compartment for the on-chip cell cultivation, differentiation, and stimulation, while the second one hosts superpara-magnetic beads (Ø 2.8 μm) conjugated to anti-IL-6 antibodies for capturing the LPS-induced IL-6. The two compartments are separated by a micropillar-based filter with a spacing of 2 μm. This filter allows the induced cytokines to infuse into the bead compartment (i.e. the magnetic immunoassay compartment), while preventing the magnetic beads and cells to cross over to the other compartment. To fulfill this requirement, a high aspect ratio pillar array was demonstrated as key element of this study and functionally characterized. The pore size of the filter is given by the lateral distance between the single pillars, which are fabricated by molding microfluidic structures in polydimethylsiloxane, using a master mold made of the expoy-based photoresist SU-8?. An aspect ratio of 5:1 could be achieved with SU-8? bars featuring the dimensions 10 µm × 2 μm (height × width).     

Design and fabrication of a freeform microlens array for uniform beam shaping

Freeform optics has become a practical solution to solving number of problems in modern optical design. In this paper, we proposed a fabrication method using the combination of ultraprecision diamond machining and microinjection molding to achieve high volume and low cost freeform microlens manufacturing. The freeform microlens array discussed in this research is capable of redistributing a collimated light into a pre determined, in this case, a uniform pattern. The optical design, slow tool servo diamond machining, microinjection molding process and optical measurement were discussed. The simple optical design provided a platform for freeform microlens calculation. Slow tool servo diamond broaching was selected to fabricate the mold insert. After the mold insert was fabricated, microinjection molding machine was utilized to replicate the optical geometry into plastic substrates. The freeform microlens array that was fabricated in this research could achieve light re-distribution at the target with approximately 80% uniformity. The research conducted in this paper can be readily implemented in optical industry.     

Multi-scale,multi-depth lithography using optical fibers for microfluidic applications

This paper proposes and demonstrates a method for multi-scale, multi-depth three-dimensional (3D) lithography. In this method, 3D molds for replicating microchannels are fabricated by passing a non-focused laser beam through an optical fiber, whose tip is immersed in a droplet of photopolymer. Line width is adjustable from 1 to 980 µm using eight kinds of optical fibers with different core diameters. The height of line drawing can be controlled by adjusting the distance between the tip of the optical fiber and a substrate. The surface roughness (R_a, R_z) of a single line and plane was evaluated. The method was employed to fabricate a 3D mold of a microchannel containing tandem chambers, which was then successfully replicated in PDMS. Multi-scale, multi-depth 3D lithography can provide a simple, flexible tool for producing PDMS microfluidic devices.     

Simulation and fabrication of a branch-channel rhombic micromixer for low pressure drop and short mixing length

The planar micromixers merit easy integration with the microsystem but encounter the problems of high pressure drop and long mixing length for high mixing efficiency over 90 %. In this article, an advanced branched rhombic micromixer (BRM) have been designed and investigated by 3D numerical simulations and experiments. Polydimethylsiloxane (PDMS) molding process was used for chip fabrication of the experiment. The CFD-ACE⁺ software was applied for modeling simulations. Simulation results showed that this optimum geometry design of four-mixing-unit BRM with the branch position at 700 μm high and 100 μm wide can effectively improve the mixing efficiency over 95 % at Reynolds number (Re) 120 with a rather low pressure drop about 9,000 Pa and a short mixing length of 5.5 mm as well. The high mixing efficiency at low pressure drop is attributed to the greatly increased contact interface area and chaotic-convection vortices as well as the low flow resistance with branched channels. The mixing verification is performed by the flow visualization system via the popular rhodamine B dye (10 mM) added DI method for PDMS BRM chips.     

“LIGA2.X” process for mass production of single polymeric LIGA micro parts

The new modification of the LIGA process “LIGA2.X” is a promising process chain to replicate high accuracy single polymeric LIGA micro parts in large scale to feasible costs. The advantages of this new approach compared to the standard LIGA process are the reduction of fabrication costs, the elimination of any rework after replication, industry suitable mold technology for injection molding using a new developed demolding concept, optimized injection molding parameters for every single cavity, shot volumes of the parts below 0.5 mm³ and a new freedom in the arrangement of structure cavities in multicavity molds. Aspects covered in this paper are the introduction of the complete process chain of the new “LIGA2.X” process and the used mold concept for the micro-män 50 micro injection molding machine. Furthermore replicated single polymeric LIGA microparts fabricated using the new process chain are shown. To conclude, potential process improvements and future work will be outlined.     

Design and simulation of a high-performance Z-axis SOI-MEMS accelerometer

This paper presents a new micromachined z-axis accelerometer as well as a new method to sense the out-of-plane displacement capacitively via comb finger arrays. The new design built the z-accelerometer using eight folded beam suspension to minimize the off axis sensitivities in both the x- and y- directions. The proposed method implements the sensing electrode as a comb finger arrays surrounding the sensor. This method enables the realization of the sensor by bulk micromachining process, increases the sense capacitance and reduces the off-axis sensitivity. This process allows building the micromachined accelerometer with large inertial mass. This work introduces the design and simulation for this accelerometer. The introduced method results in a high sense capacitance as well as high sensitivity. The simulated sense capacitance is 19.6627 pF. The sensor sensitivity is 2.037 μm/g with a very small total noise equivalent acceleration of 3.096 μg/ $ \sqrt {Hz} $ .     

Parallelizing Complex Streaming Applications on Distributed Scratchpad Memory Multicore Architecture

Multicore processors can provide sufficient computing power and flexibility for complex streaming applications, such as high-definition video processing. For less hardware complexity and power consumption, the distributed scratchpad memory architecture is considered, instead of the cache memory architecture. However, the distributed design poses new challenges to programming. It is difficult to exploit all available capabilities and achieve maximal throughput, due to the combined complexity of inter-processor communication, synchronization, and workload balancing. In this study, we developed an efficient design flow for parallelizing multimedia applications on a distributed scratchpad memory multicore architecture. An application is first partitioned into streaming components and then mapped onto multicore processors. Various hardware-dependent factors and application-specific characteristics are involved in generating efficient task partitions and allocating resources appropriately. To test and verify the proposed design flow, three popular multimedia applications were implemented: a full-HD motion JPEG decoder, an object detector, and a full-HD H.264/AVC decoder. For demonstration purposes, SONY PlayStation \(^{\circledR }\) 3 was selected as the target platform. Simulation results show that, on PS3, the full-HD motion JPEG decoder with the proposed design flow can decode about 108.9 frames per second (fps) in the 1080p format. The object detection application can perform real-time object detection at 2.84 fps at \(1280 \times 960\) resolution, 11.75 fps at \(640 \times 480\) resolution, and 62.52 fps at \(320 \times 240\) resolution. The full-HD H.264/AVC decoder applications can achieve nearly 50 fps.     

Fabrication of micro pore optics with smooth sidewall using X-ray lithography

Micro pore optics (MPO) as an X-ray imaging system is perfectly suited for the applications in space telescopes due to its light-weight and high-resolution properties. We report the fabrication of MPO samples by LIGA process focusing on its sidewall surface used as mirrors for X-ray reflection. An intermediate mask is fabricated and used to obtain the working mask in order to avoid the UV exposure to a very thick photo resist layer. Around 400 μm-thick nickel MPO plate is obtained with the aspect ratio of the square pore and sidewall of 8 and 32, respectively. The root mean square roughness of the sidewall surface is below 10 nm in a 5 × 5 μm² region. Some striations are found on the sidewall surface originating from the jagged edge of the chromium coating on the UV mask.     

A direct and neural controller performance study with beam wandering mitigation control in free space optical link

The beam wander on the detector plane is one of the main causes for major power loss which severely degrades the performance of Free Space Optical (FSO) link. Designing a suitable controller to correct the beam motion at a faster rate to increase the beam stability becomes significant. This paper presents an investigation on the performance of two types of controller designed for aiming a laser beam to be at a particular point under dynamic disturbances. The first design is based on the Taguchi’s method (direct controller) while the second is the Artificial Neural Network (ANN) method (neural-controller). These controllers process the beam location information and generate the necessary outputs to mitigate the beam wandering. The pipelined-parallel architecture for both controllers are developed in a Field Programmable Gate Array (FPGA) and installed at the receiver station. Evidence of the suitability and the effectiveness of the proposed controller in terms of prediction exactness, prediction error, reduction of focal point wander, response to impulse and scintillation are provided through experimental results from the FSO link established for the horizontal range of 0.5 km at an altitude of 15.25 m.     

Large tuning ratio high aspect ratio variable capacitors using leveraged bending

Variable capacitors are a key component in Radio Frequency Micro Electro-Mechanical Systems (RF MEMS). They comprise fixed and flexible electrodes. Deformation, or actuation, of the flexible electrode changes the capacitance of the capacitor. This way, electrical properties of high frequency circuits can be modified. Traditionally, variable capacitors are based on a planar layout architecture, while a newer, vertical-wall, quasi three-dimensional approach theoretically enables increased device performance. Such devices depend on high aspect ratios, i.e. relatively high micro structures with very thin walls and gaps. A few vertical-wall variable capacitors made of nickel or gold have been fabricated to date, using deep X-ray lithography and subsequent electroplating (Achenbach et al. 2006; Klymyshyn et al. 2007, 2010) as the fabrication approach. They feature, amongst others, excellent quality factors of Q ≤ 95 at 5.6 GHz with 50 Ω reactance, but suffer from a very limited tuning range of the capacitance value (tuning ratio of, e.g., 1.38:1). The devices presented here exploit the same architecture and materials selection, resulting in similar, excellent Q-factors, but feature a different electrode layout approach, referred to as leveraged-bending. This layout is based on pulling a flexible electrode sideways, towards a fixed electrode, increasing the capacitance when actuating the variable capacitor. The leveraged bending approach theoretically enables infinitely high tuning ratios for components with perfect structure accuracy. To date, a significantly increased tuning ratio of 1.9:1 has been demonstrated. Limiting factors are an electrically non-ideal layout geometry chosen as a compromise to increase the fabrication yield, and structure deviations of ~1.6 μm from CAD layout to the electroplated component. Electrostatic actuation requires voltages between 0 and 72 V for capacitance values on the order of C = 0.3 pF at device dimensions of about 1.5 mm overall length, 5–10 μm gap and wall widths, and 100 μm metal height. Device performance measured with a vector network analyzer is in 97 % agreement with simulation results based on two-dimensional electrostatic-structural coupling (ANSYS Multiphysics) and three-dimensional electromagnetic field simulations (Ansoft HFSS). These simulations also indicate that an optimized gap geometry will allow to reduce the actuation voltage required by up to 40 %.     

Low cost method for hot embossing of microstructures on PMMA by SU-8 masters

Polymer based microfabrication technologies are used extremely in Bio-MEMS, especially in Microfluidic devices in recent years. In this paper, a novel method for fabrication of microstructures on a polymeric material using hot embossing lithography process is presented. The proposed method involves usage of low cost materials and procedure with respect to previous methods and can be processed in a short time. The master is made from SU-8 on an inexpensive glass substrate which is patterned by standard lithography. The embossing pressure can be increased in our master as the glass substrate used in this paper is more robust than Silicon. Master robustness and SU-8 to glass adhesion is optimized by some substrate pretreatments and SU-8 baking time and temperatures. Microchannels are replicated on a Polymethylmetacrylate (PMMA) stamp which is a plexiglass sheet with thickness of 1 mm. Significant embossing parameters including temperature, pressure and time are discussed and optimum values are determined. Microchannels are imprinted by depth of 50 μm and minimum width of 15 μm and aspect ratio more than 3. The microchannels are sealed by a PMMA cap using thermal annealing bonding.     

Fabrication of gold nanoparticle pattern using imprinted hydrogen silsesquioxane pattern for surface-enhanced Raman scattering

This is the first report of surface-enhanced Raman scattering (SERS) substrate fabrication using a combination of imprinted hydrogen silsesquioxane (HSQ: HSiO_3/2) patterns and self-assembly of gold nanoparticles (AuNPs). To assemble the AuNPs inside the imprinted HSQ pattern, it is important to understand the interactions between AuNPs and AuNPs, and those between AuNPs and HSQ. The authors investigated the effects HSQ surface charges on the self-assembly of AuNPs. It was found that the negatively charged AuNPs were successfully assembled according to the geometry of the negatively charged HSQ pattern. In addition, it was shown that the SERS substrate fabricated from an HSQ consisting of an inorganic polymer was suitable for organic chemical analysis, by comparing it with a substrate fabricated using an organic polymer.