J.L. Sanchez

Micromachining using deep ion beam lithography

Abstract In recent years the process combining deep X-ray lithography with electroforming and micromoulding (i.e. LIGA), has become an important technique for the production of high aspect-ratio microstructures for the fabrication of micro-electromechanical systems (MEMS). The aim of this paper is to investigate the potential of high energy ion microbeams for carrying out similar micromachining, and in particular for overcoming the geometrical restrictions which are inherent in deep x-ray lithography. Using a scanned 2.0 MeV proton beam of approximately 1 micron diameter, we produced latent microstructures in high molecular weight PMMA resist. These resist microstructures were subsequently developed using a multi-component developer which is highly specific in the removal of exposed resist, while leaving unexposed or marginally exposed material unaffected. A suitable range of exposures has been established, and factors affecting the geometrical fidelity of the produced microstructure have been investigated. The relative advantages and limitations of this technique vis a vis deep X-ray lithography are discussed.

Resist materials for proton micromachining

Abstract The production of high aspect ratio microstructures is a potential growth area. The combination of deep X-ray lithography with electroforming and micromolding (i.e. LIGA) is one of the main techniques used to produce 3D microstructures. The new technique of proton micromachining employs focused MeV protons in a direct write process which is complementary to LIGA, e.g. micromachining with 2 MeV protons results in microstructures with a height of 63 μm and lateral sub-micrometer resolution in PMMA resist. The aim of this paper is to investigate the capabilities of proton micromachining as a lithographic technique. This involves the study of different types of resists. The dose distribution of high molecular weight PMMA is compared with three other types of resist: First the positive photo resist AZ P4620 will be discussed and then PMGI SF 23, which can be used as a deep UV, e-beam or X-ray resist. Finally SU-8, a new deep UV negative type of chemically amplified resist will be discussed. All these polymers are applied using the spin coating technique at thicknesses of between 1 and 36 μm

Micromachining using focused high energy ion beams: Deep Ion Beam Lithography

Abstract The combination of deep X-ray lithography with electroforming and micromoulding (i.e., LIGA) has been shown to offer high potential for the production of high aspect-ratio microstructures. The LIGA technique, employing synchrotron light and a suitable X-ray mask, allows production of 3D microstructures in PMMA with aspect ratios around 100. Here we demonstrate that the novel technique of Deep Ion Beam Lithography (DIBL), a direct process utilizing a focused beam of MeV ions scanned in a predetermined pattern over a suitable resist material, can produce three dimensional microstructures with sub-micrometer feature sizes. Microstructures extending up to 100 μm from the substrate with aspect ratios approaching 100 can be produced. Multiple exposures at different ion energies allow production of multilayer structures in single resist layers of SU-8, a newly developed, chemically accelerated, negative tone, near UV, photoresist.

The use of proton microbeams for the production of microcomponents

Abstract The recently developed process of high-energy ion beam micromachining (proton micromachining) is discussed. Proton micromachining is a novel process for the production of high aspect-ratio 3D microstructures. The sub-micron lateral resolution and the well-defined range of an MeV proton microbeam are utilized to make lithographic structures in suitable polymers (e.g., SU-8, PMMA). Sub-micron structures with a depth of tens of microns and aspect-ratios approaching 100 have been achieved. The use of different energies for multiple exposures allows the production of intricate 3D multi-layer structures in a single polymer layer, and because no mask is needed the process offers a wide range of possible geometries for the production of non-prismatic or even rounded features. The throughput of the technique does not compare favourably with conventional (masked) processes for high volume batch production of microcomponents. On the other hand, significant applications of high-energy ion beam micromachining may be developed, e.g. for the rapid production of prototypes, the research into the characteristics of microstructures, and the manufacture of molds, stamps and X-ray masks.

A high resolution beam scanning system for deep ion beam lithography

Abstract The technique of Deep Ion Beam Lithography (DIBL) allows the production of high aspect-ratio microstructures in suitable polymer resists (e.g. PMMA) with complex three-dimensional geometries in a fast, direct write process. In conjunction with micromoulding and electroforming, the DIBL-technique may prove extremely useful for the production of microcomponents, micromachines and microelectromechanical systems (MEMS). The present scanning system (OM-DAQ) in use at the Singapore Nuclear Microscope facility is limited to a 256 × 256 pixels raster scan. This system, while adequate for analytical applications, has limitations when machining high resolution structures using DIBL. A new scanning system has been developed in order to overcome this limitation. The new scanning system is based on a DAC PC-card that allows flexible scanning with a resolution of up to 4096 × 4096 pixels. The new system allows the beam to be scanned in specific patterns, which are designed to achieve optimal resolutions. The use of the new scanning system provides a mechanism for translating high resolution digital images into high resolution three dimensional microstructures. Using this system we have produced submicron (300 nm) walls with an aspect ratio approaching 100, three dimensional complex microstructures with smooth walls and corners, and multiple microstructures exposed by repetitive scanning.

Micro-PIXE analysis of trace element concentrations of natural rubies from different locations in Myanmar

Abstract The trace element concentrations found in geological samples can shed light on the formation process. In the case of gemstones, which might be of artificial or natural origin, there is also considerable interest in the development of methods that provide identification of the origin of a sample. For rubies, trace element concentrations present in natural samples were shown previously to be significant indicators of the region of origin [S.M. Tang et al., Appl. Spectr. 42 (1988) 44, and 43 (1989) 219]. Here we report the results of micro-PIXE analyses of trace element (Ti, V, Cr, Fe, Cu and Ga) concentrations of a large set ( n = 130) of natural rough rubies from nine locations in Myanmar (Burma). The resulting concentrations are subjected to statistical analysis. Six of the nine groups form clusters when the data base is evaluated using tree clustering and principal component analysis.

VIRTUAL PIXE AND RBS LABORATORY

Abstract An interactive software package for a complete simulation of Particle Induced X-ray Emission (PIXE) and Backscattering Spectra (BS) is being developed. The user is in a position to define all experimental parameters such as incident ions (protons, deuterons or He ions), their energy, excitation and detection geometry, detector parameters and filters as well as sample composition and thicknesses of a number of layers. This is all done through an easy to operate interactive user interface. Simulated PIXE and RBS spectra are immediately displayed on the screen and can be saved either as bitmaps and/or files suitable for further processing. Each output comes with a complete set of experimental parameters, absolute and relative line intensities (including all major K and L lines), peak to background ratio and detection limits for all elements of interest. The program has also a number of utility routines to calculate various fundamental parameters such as photon cross sections, K and L shell ionization and production cross sections, energy loss, and detector efficiency. All these routines use the state-of-the art data base sources. The program operates on a personal computer under the MS Windows operating system. The simulation is fast and the program is easy to operate. The package will be useful in many ways. Firstly, it will be an excellent teaching tool for researchers/students without experience with PIXE/RBS. Secondly, it will be of immense help in planning and optimizing PIXE and/or RBS experiments. The user can ‘run’ a simulated experiment for any sample/experimental parameters and test various settings/scenarios to achieve optimal results without actually carrying out the experiment, thereby saving a lot of expensive machine time that would otherwise have been spent on trial and error experiments.

PARAMETRIZATION OF THE TOTAL PHOTON MASS ATTENUATION COEFFICIENTS FOR PHOTON ENERGIES BETWEEN 100 EV AND 1000 MEV

Abstract A convenient way of generating mass attenuation coefficients for data analysis in various fields of X-ray spectroscopy is by means of semi-empirical schemes. However, the validity of most of the existing schemes is limited to relatively narrow energy intervals (typically 1–50 keV) and their accuracy is usually poor especially in regions near absorption edges. A few years ago, we use a new semi-empirical scheme that enabled us to fit, for the first time, photon data for all elements in the very wide energy range, i.e. between 0.1 and 1000 keV. The scheme was found flexible enough to provide excellent fit to the most recent compiled photon cross section data in the entire energy range. It was based on fitting data to an exponential function with the addition of a Klein–Nishina term to account for dominant incoherent scattering at higher energies. In the present work, our former scheme is extended beyond the pair-production threshold (1.02 MeV) all the way to 1000 MeV. An interactive MS Windows based computer program for generating, printing and displaying mass attenuation coefficients based on the proposed scheme has been developed.

Fluence dependence of IBIC collection efficiency of CMOS transistors

Abstract IBIC (Ion Beam Induced Charge) imaging of deep structures of semiconductor devices with a focused MeV light ion beam has been shown to offer significant advantages over the established EBIC (Electron Beam Induced Current) technique, because the large range and the small lateral straggling of the ion beam allows direct imaging of buried device structures. The technique is limited by the accumulation of radiation damage that reduces the charge collection efficiency. We report on measurements of the beam fluence dependence on IBIC collection efficiency for proton and alpha particle beams at 2000 keV energy. A HCF4007 CMOS transistor array was used in these measurements. The influence of surface passivation layers on charge collection efficiency and its evolution with ion dose is discussed.

Nuclear microscopy study of fish scales

Abstract Fish scales are useful to ichthyologists for purposes of classification, identification, age determination and history study. So far, all studies of fish scales, for whatsoever purposes, have relied on the use of optical or electron microscopes to study their form and structure. Employing the NUS nuclear microscope, we measured the concentrations of Sr, Zn, Fe, Mn, Ni, Cu, Cr, As, Se, Br and Pb in the scales of 17 fish species. The objective was to test the hypotheses that the trace elements in scales could be used for species differentiation, and that the fish scale could potentially be used as a monitor for the environment. Our preliminary results gave positive indications.