Proton Beam Micromachining Research Articles

Formula omitted] treatment and vacuum effects in proton beam micromachining of PADC

Polyallyl diglycol carbonate (PADC) has been shown to be a suitable material as a thick resist for proton beam writing [Rajta, I., Baradács, E., Bettiol, A.A., et al., 2005. Optimisation of particle fluence in micromachining of CR-39. Nucl. Instrum. Methods B 231, 384.]. This material is commonly used as an etched track type particle detector. In most cases it is used to detect alpha particles in normal air conditions. However, to use this material as proton or alpha micromachinable resist, one needs to irradiate the material in vacuum. In this work, we investigated the effects of vacuum on the micromachinable properties of PADC. Our investigations proved that there were no drawbacks of the vacuum storage of the samples; so we concluded that PADC is a suitable material as a PBM resist in this respect, too. Another part of the current work concentrated on the effect of post-irradiation CO 2 treatment of the samples. Such a treatment increased the radiation sensitivity of PADC, i.e. reduced the required irradiation fluence. We have found that approximately 60% of fluence that of not-treated samples was sufficient to develop fully the radiation damaged structures.

Concept for processing of silicon check valves by proton beam micromachining

In this work, we present alternative methods for the formation of a simple a check valve for microfluidic applications. In two different approaches we exploited the characteristic features of the proton beam micromachining (PBM) technique and the selective formation and dissolution of porous Si around the implantation damaged areas. In the first case, we implanted 10 μm thick cantilever-type membrane of the valve normally to the crystal surface and at 30–60° to the sidewalls of the flow channel, which were also implanted at the same irradiation. During the porous Si formation we developed the sample 6–8 μm deeper than the implanting ion range damaged the crystal. Due to the isotropic nature of the porous Si etching, the thick sidewall blocks are still connected to the crystal while the thin membranes detached from the bottom, and they are only connected to one of the sidewalls. The other construction utilized the goniometer facility mounted on the microbeam chamber. We implanted the samples at 43° tilt, and developed the samples not as deep as the ion range. This way both the sidewalls and the membranes are attached to the bottom of the sample.

Proton beam micromachining on strippable aqueous base developable negative resist

Nowadays a significant amount of research effort is devoted to the development of technologies for the fabrication of microcomponents and microsystems worldwide. In certain applications of micromachining high aspect ratio (HAR) structures are required. However, the resist materials used in HAR technologies are usually not compatible with the IC fabrication, either because they cannot be stripped away or because they are developed in organic solvents. In the present work the application of a novel chemically amplified resist for proton beam micromachining is presented. The resist based on epoxy and polyhydroxystyrene polymers is developed in the IC standard aqueous developers. The exposed areas can be stripped away using conventional organic stripping solutions. In order to test the exposure dose sensitivity and the lateral resolution, various test structures were irradiated. Using this formulation 5–8μm wide lines with aspect ratio 4–6 were resolved.

Geometric control of fibroblast growth on proton beam-micromachined scaffolds.

Circular three-dimensional (3D) micropatterns with grooves and ridges of various sizes on the circumference of the structure were micromachined in polymethylmethacrylate, using proton beam micromachining. Fibroblasts were seeded in the center smooth nonpatterned surface of the circle. The circumference grooves could retard the outward spreading of cells after they became confluent in the central smooth surface. The fibroblasts eventually migrated across the grooves and ridges several days later. Wider and deeper grooves were more effective in retarding fibroblast spreading. Our results indicate that groove structures in cellular dimensions can effectively retard fibroblasts invasion. Proton beam micromachining, which has the unique advantage of being the only technique capable of manufacturing direct-write precise 3D microstructure at cellular dimensions, has great potential in generating 3D microscaffolds for studying cell behavior in a 3D microenvironment, which is important for tissue engineering.

Fabrication of micro-optical components in polymer using proton beam micro-machining and modification

Proton beam micro-machining (PBM) is a direct write lithographic technique that utilizes a high energy (MeV) sub-micron focused proton beam to machine or modify a material, usually a polymer. The technique has been developed in recent years at the Research Centre for Nuclear Microscopy, National University of Singapore where structures with feature sizes of well below 1 μm have recently been demonstrated. The PBM technique has several desirable features that make it suitable for rapid prototyping of micro-optical components. Structures made using PBM have very smooth side walls, high aspect ratio, and a scale that can be easily matched to existing optical fiber technology (0.1–1000 μm). Furthermore, PBM can also be used to modify the optical properties of polymers, particularly if the end of range is used. In this paper we demonstrate the use of proton beam micro-machining and modification for manufacturing micro-optical components in positive and negative resist. The structures that are fabricated can be used for both rapid prototyping and for large scale replication.

Proton beam micromachining dose normalization for SU-8 using ionoluminescence detection

Proton beam micromachining (PBM) allows the production of small, intricate, high aspect ratio structures with smooth sidewalls by direct writing MeV protons beams in resist materials such as SU-8 and PMMA. The process depends on the correct incident dose of protons, and conventional normalizing methods using RBS have been utilized previously. However for accelerated (sensitive) resists such as SU-8, the yield of backscattered ions, particularly for small structures, is insufficient for accurate dose measurement. We have used the more prolific ion induced photon emission from SU-8 as a dose normalizing signal for PBM. The SU-8 emits radiation at a wavelength of 560 nm under proton irradiation, and the yield per incident proton depends on the thickness of the resist layer. The photon yield per incident proton has a maximum value at a dose of 25 nC mm −2. The photon normalization method has been used to good effect to micromachine a complex shape with resulting good edge definition.

Proton beam micromachining on PMMA, Foturan and CR-39 materials

In this paper we investigate further the potential of proton beam micromachining (PBM) on three different materials: the polymers PMMA and CR-39, and the photowritable glass Foturan. A focused beam of 2 MeV protons delivered by the nuclear microprobe of ATOMKI was used to pattern these materials. The parameters of PBM and the obtained structures are presented.

The National University of Singapore high energy ion nano-probe facility: Performance tests

The Research Centre for Nuclear Microscopy, National University of Singapore incorporates three state-of-the-art beam lines, connected to a high brightness High Voltage Engineering Europa 3.5 MV Singletron accelerator. One of these lines is a NEC (National Electrostatics Corporation, USA) ion channeling facility, utilising broad beam ion beam analysis techniques for advanced materials research. The other two lines are microbeam facilities; one is designed for nuclear microscopy of biomedical samples and advanced materials, where relatively high currents (>50 pA) are required, and the other for proton beam micromachining (PBM) and materials modification, where lower currents can be utilised. The resolution performances of the two microbeam lines have been measured, and the results are as follows: (1) The nuclear microscope line incorporates the Oxford Microbeams OM2000 endstation with the OM50 quadrupole lenses configured in the high excitation triplet mode. This line has achieved the world’s best performances for analytical applications of 290 × 400 nm for a 50 pA current of 2 MeV protons. (2) The PBM line, which is the first of its kind worldwide, utilizes the new generation of compact (OM52) quadrupole lenses (Oxford Microbeams Ltd.) also configured in a high excitation, triplet configuration. This facility, which has superior demagnification properties, has achieved the world’s best performances for low current applications. Spot sizes of 35 × 75 nm have been measured using direct scanning transmission ion microscopy for beam currents of 10,000 protons per second.

Proton Beam Nano-Machining: End Station Design and Testing

AbstractA new nuclear nanoprobe facility has been developed at the Centre for Ion Beam Applications (CIBA) in the Physics Department of the National University of Singapore. This facility is the first of its type dedicated to proton beam micromachining on a micron as well as a nano scale. The design and performance of the facility, which is optimized for 3D lithography with MeV protons, is discussed here. The system has been designed to be compatible with Si wafers up to 6”.The production of good quality high aspect ratio microstructures requires a lithographic technique capable of producing microstructures with smooth vertical sidewalls. In proton beam micromachining, a high energy (e.g. 2 MeV) proton beam is focused to a sub-100 nm spot size and scanned over a resist material (e.g. SU-8 and polymethylmethacrylate (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 nanometer sized polymer structures. 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. Since proton beam micromachining is a fast direct write lithographic technique it has high potential for the production of high-aspect-ratio nano-structures.

Nickel and copper electroplating of proton beam micromachined SU-8 resist

Proton beam micromachining (PBM) has been shown to be a powerful technique to produce three-dimensional (3D) high-aspect-ratio microstructures (Watt et al., 2000). Potential commercial applications of PBM, which is a fast direct write technique, will become feasible if the fabrication of metallic molds or stamps is realised. Metallic components can be produced by electroplating a master from a microstructure produced in resist. The production of high-aspect-ratio metallic stamps and molds requires a lithographic technique capable of producing smooth and near 90° sidewalls and a one to one conversion of a resist structure to a metallic microstructure. PBM is the only technique capable of producing high-aspect-ratio microstructures with sub-micron details via a direct write process. In PBM, SU-8 (Lorenz et al., 1997) resist structures are produced by exposing the SU-8 resist with a focused MeV proton beam followed by chemical development and a subsequent electroplating step using Ni or Cu. The data presented shows that PBM can successfully produce high-aspect-ratio, sub-micron sized smooth metallic structures with near 90° sidewall profiles.

Proton beam micromachined resolution standards for nuclear microprobes

The quest for smaller spot sizes has long been the goal of many nuclear microprobe groups worldwide, and consequently there is a need for good quality resolution standards. Such standards have to be consistent with the accurate measurement of state-of-the-art nuclear microbeam spot sizes, i.e. 400 nm for high current applications such as Rutherford backscattering spectrometry and proton-induced X-ray emission, and 100 nm for low current applications such as scanning transmission ion microscopy or ion beam-induced charge. The criteria for constructing a good quality nuclear microprobe resolution standard is therefore demanding: the standard has to be three dimensional with a smooth surface, have an edge definition better than the state-of-the-art beam spot resolutions, and exhibit vertical side walls. Proton beam micromachining (PBM) is a new technique of high potential for the manufacture of precise 3D microstructures. Recent developments have shown that metallic microstructures (nickel and copper) can be formed from these microshapes. Prototype nickel PBM resolution standards have been manufactured at the Research Centre for Nuclear Microscopy, NUS and these new standards are far superior to the 2000 mesh gold grids currently in use by many groups in terms of surface smoothness, vertical walls and edge definition. Results of beam resolution tests using the new PBM standards with the OM2000 microprobe end station/HVEE Singletron system have yielded spot sizes of 290 nm×450 nm for a 50 pA beam of 2 MeV protons.

Proton beam micromachining: electron emission from SU-8 resist during ion beam irradiation

Proton beam micromachining (PBM) is a direct write lithographic technique that uses a focused beam of MeV protons to pattern a resist material. The most common resist material used in the PBM process is SU-8 which is usually spin coated onto various substrates. The method used to ensure that the correct dose is delivered to the sample during irradiation is Rutherford backscattering spectrometry (RBS). There are however limitations to using the RBS signal for normalizing the dose in highly sensitive resist materials such as SU-8. The limited number of backscatter events means that normalizing the dose for every pixel is not possible. The secondary electron yield for SU-8 is at least an order of magnitude higher than that for backscattered ions. With an appropriate detector these signals can be essentially used for ion detection and thus used to accurately monitor ion dose. In this paper we investigate the secondary electron yield from SU-8 polymer resist layers of varying thickness on silicon. It is shown that the signals produced during MeV ion irradiation can be directly related to the ion dose and used for dose normalization during PBM.

Proton beam micromachining: a new tool for precision three-dimensional microstructures

Proton beam micromachining (PBM) is a novel technique for the production of high aspect-ratio three-dimensional (3D) microcomponents. PBM is a direct write process in which a focused beam of MeV protons is scanned in a pre-determined pattern over a suitable resist material (e.g. PMMA or SU-8) and the latent image formed is subsequently chemically developed. One strategy for full exploitation of the advantages offered by PBM is the conversion of the microstructures produced in the resist to metallic components by electrolytic plating. The metallic structures may then be used in a bulk production process, e.g. microstamping or micromolding. In this paper we describe the introduction of a beam blanking system to improve the quality of microstructures, and present data showing that electrolytic Ni plating of proton beam micromachined resist structures result in well defined and smooth metallic microstructures.

High precision 3D metallic microstructures produced using proton beam micromachining

A crucial step in the development of mechanically strong microstructures is the conversion of structures made from resist material of low hardness and strength, to harder and more durable metallic microstructures. The implementation of a post lithographic process step such as electroplating offers the possibility of producing metallic structures. In proton beam micromachining (PBM) a focused MeV beam is scanned in a predetermined pattern over a resist (e.g. PMMA or SU-8), which is subsequently chemically developed. The proton beam in resist follows an almost straight path, enabling the production of microstructures with well-defined rectangular side walls. If the resist layer is laid down with a thickness of typically 50% of the proton range on a conductive substrate, then the end of range straggling and resultant end of range beam broadening does not occur in the resist, but in the substrate. The conducting substrate acts as a seed layer for plating. In this current work, smooth well-defined metallic microstructures with a height of 10 μm are produced using electrolytic Ni plating. One spin-off application is that the plated Ni structures, which have excellent side wall definition, exhibit properties that are far superior to the current 2000 lines per inch gold grid resolution standard used by many nuclear microscopy groups worldwide.

A LabVIEW™-based scanning and control system for proton beam micromachining

LabVIEW™ is steadily gaining in popularity as the programming language of choice for scientific data acquisition and control. This is due to the vast array of measurement instruments and data acquisition cards supported by the LabVIEW™ environment, and the relative ease with which advanced software can be programmed. Furthermore, virtual instruments that are designed for a given system can be easily ported to other LabVIEW™ platforms and hardware. This paper describes the new LabVIEW™ based scanning and control system developed specifically for proton beam micromachining (PBM) applications. The new system is capable of scanning figures at 16-bit resolution with improved sub-microsecond scan rates. Support for electrostatic beam blanking and external dose normalization using a TTL signal have been implemented. The new software incorporates a semi-automated dose calibration system, and a number of novel dose normalization methods. Limitations of the current beam scanning hardware are discussed in light of new results obtained from micromachining experiments performed in SU-8 photoresist.

Sub 100 nm proton beam micromachining: theoretical calculations on resolution limits

Proton beam micromachining is a novel direct-write process for the production of three-dimensional (3D) microstructures. A focused beam of MeV protons is scanned in a pre-determined pattern over a suitable resist material (e.g. PMMA or SU-8) and the latent image formed is subsequently developed chemically. In this paper calculations on theoretical resolution limits of proton beam micromachined three-dimensional microstructures are presented. Neglecting the finite beam size, a Monte Carlo ion transport code was used in combination with a theoretical model describing the delta-ray (δ-ray) energy deposition to determine the lateral energy deposition distribution in PMMA resist material. The energy deposition distribution of ion induced secondary electrons (δ-rays) has been parameterized using analytical models. It is assumed that the attainable resolution is limited by a convolution of the spread of the ion beam and energy deposition of the δ-rays.

Focused high energy proton beam micromachining: A perspective view

Micromachining techniques utilising optical, UV and X-ray photons, as well as electrons, low energy heavy ions and high energy light ions (protons), are briefly reviewed. The advantages and disadvantages of each process are discussed. High energy ion beam micromachining (proton micromachining) is a new process which exhibits a unique feature; direct-write 3-dimensional micromachining at submicron resolutions. Although this technique may not compete with conventional mask processes for producing high volume batch production of microcomponents, high energy ion beam micromachining may have a significant role in rapid prototyping, research into the characteristics of microstructures, and the manufacture of molds, stamps and thick masks. Several examples of high energy proton micromachining are presented to illustrate the potential of the technique.