M.B.H. Breese

Proton beam writing

Proton beam (p-beam) writing is a new direct-writing process that uses a focused beam of MeV protons to pattern resist material at nanodimensions. The process, although similar in many ways to direct writing using electrons, nevertheless offers some interesting and unique advantages. Protons, being more massive, have deeper penetration in materials while maintaining a straight path, enabling p-beam writing to fabricate three-dimensional, high aspect ratio structures with vertical, smooth sidewalls and low line-edge roughness. Calculations have also indicated that p-beam writing exhibits minimal proximity effects, since the secondary electrons induced in proton/electron collisions have low energy. A further advantage stems from the ability of protons to displace atoms while traversing material, thereby increasing localized damage especially at the end of range. P-beam writing produces resistive patterns at depth in Si, allowing patterning of selective regions with different optical properties as well as the removal of undamaged regions via electrochemical etching.

ION BEAM LITHOGRAPHY AND NANOFABRICATION: A REVIEW

To overcome the diffraction constraints of traditional optical lithography, the next generation lithographies (NGLs) will utilize any one or more of EUV (extreme ultraviolet), X-ray, electron or ion beam technologies to produce sub-100 nm features. Perhaps the most under-developed and under-rated is the utilization of ions for lithographic purposes. All three ion beam techniques, FIB (Focused Ion Beam), Proton Beam Writing (p-beam writing) and Ion Projection Lithography (IPL) have now breached the technologically difficult 100 nm barrier, and are now capable of fabricating structures at the nanoscale. FIB, p-beam writing and IPL have the flexibility and potential to become leading contenders as NGLs. The three ion beam techniques have widely different attributes, and as such have their own strengths, niche areas and application areas. The physical principles underlying ion beam interactions with materials are described, together with a comparison with other lithographic techniques (electron beam writing a...

Three-dimensional silicon micromachining

A process for fabricating arbitrary-shaped, two- and three-dimensional silicon and porous silicon components has been developed, based on high-energy ion irradiation, such as 250 keV to 1 MeV protons and helium. Irradiation alters the hole current flow during subsequent electrochemical anodization, allowing the anodization rate to be slowed or stopped for low/high fluences. For moderate fluences the anodization rate is selectively stopped only at depths corresponding to the high defect density at the end of ion range, allowing true three-dimensional silicon machining. The use of this process in fields including optics, photonics, holography and nanoscale depth machining is reviewed.

Magnetism in MoS2 induced by proton irradiation

Molybdenum disulphide, a diamagnetic layered dichalcogenide solid, is found to show magnetic ordering at room temperature when exposed to a 2 MeV proton beam. The temperature dependence of magnetization displays ferrimagnetic behavior with a Curie temperature of 895 K. A disorder mode corresponding to a zone-edge phonon and a Mo valence higher than +4 has been detected in the irradiated samples using Raman and x-ray photoelectron spectroscopy, respectively. The possible origins of long-range magnetic ordering in irradiated MoS2 samples are discussed.

Focusing of MeV ion beams by means of tapered glass capillary optics

We present evidence of the focusing effects of fine glass capillary optics for MeV He ion beams. The glass capillary optics are formed by a puller as to have inlet diameters of about 1 mm and outlet diameters of submicrons. The total length of the optics is about 50 mm. Impingent MeV ions to such optics are reflected by the inner wall several times, in a very similar process to the so-called surface channeling. The majority of incident ions are lost by the dechanneling, or large-angle scattering process, however, a part of them, actually about 1% more or less, is emitted through the outlet without significant energy loss. Compared with the conventional micro-ion beam facilities, the present method is certainly simple and lowcost, thus providing an easy method of submicron Rutherford backscattering spectrometry or particle induced x-ray emission analyses. In addition, if the ion species are extended to heavier elements, the present method provides versatile maskless ion implantation techniques.

Three-dimensional microfabrication in bulk silicon using high-energy protons

We report an alternative technique which utilizes fast-proton irradiation prior to electrochemical etching for three-dimensional microfabrication in bulk p-type silicon. The proton-induced damage increases the resistivity of the irradiated regions and acts as an etch stop for porous silicon formation. A raised structure of the scanned area is left behind after removal of the unirradiated regions with potassium hydroxide. By exposing the silicon to different proton energies, the implanted depth and hence structure height can be precisely varied. We demonstrate the versatility of this three-dimensional patterning process to create multilevel free-standing bridges in bulk silicon, as well as submicron pillars and high aspect-ratio nanotips.

A theory of ion beam induced charge collection

The ion beam induced charge technique can image the depletion regions of microelectronic devices through their thick metallization and passivation layers, and buried dislocation networks in semiconductor material by measuring the number of charge carriers created by a focused MeV light ion beam scanning over the sample surface. In this paper it is shown how the charge pulse height can be calculated in terms of the ion type and energy, and the minority carrier diffusion length. The effect of surface layers, depletion layers, ion induced damage, and ion channeling on the measured charge pulse height are also considered. It is shown how this approach can be used to simulate the charge pulse height reduction due to ion induced damage.

Microcircuit imaging using an ion‐beam‐induced charge

Ionizing radiation such as photons, keV electrons, or MeV ions can generate electron‐hole pairs in semiconducting material. The high penetrating power of MeV light ions allows them to generate electron‐hole pairs from deep within intact microelectronic devices, so images can be formed of the device active areas with very little degradation of the spatial resolution of the focused MeV ion beam. Furthermore, the ion‐beam‐induced charge (IBIC) image contrast is not strongly affected by the energy loss through the overlying device layers. This article is the first to demonstrate the capability of a nuclear microprobe to generate IBIC images of the active regions of devices through the passivation and metallization layers. The effect of the carrier generation volume on IBIC resolution is assessed. The ability of IBIC to align the major crystal axes of semiconductor samples is shown, and the effect of ion‐induced damage on IBIC image contrast is considered.

Micron-scale analysis of SiC/SiCf composites using the new Lisbon nuclear microprobe

Abstract A new nuclear microprobe has now been commissioned at ITN, Lisbon. This paper first describes the layout and modes of operation of the new microprobe, which is located on a 3.1 MV single-ended Van de Graaff accelerator. Within two days of first commissioning the microprobe, a spatial resolution of ∼1.5 μm was achieved for backscattering analysis, and a resolution of ∼1.0 μm for transmission work. The steps taken to produce this resolution, and the remaining factors, which further limit it are discussed. The first results from this microprobe for the spatially resolved analysis of SiC/SiCf ceramic composites are presented here. These materials have applications in fusion technology and structural changes were investigated after exposure to lithium orthosilicate and lithium titanate breeder materials in fusion relevant conditions. Ti and Cr rich precipitates could be found in the samples that were exposed to lithium titanate.

XAFCA: a new XAFS beamline for catalysis research

A new X-ray absorption fine-structure (XAFS) spectroscopy beamline for fundamental and applied catalysis research, called XAFCA, has been built by the Institute of Chemical and Engineering Sciences, and the Singapore Synchrotron Light Source. XAFCA covers the photon energy range from 1.2 to 12.8 keV, making use of two sets of monochromator crystals, an Si (111) crystal for the range from 2.1 to 12.8 keV and a KTiOPO4 crystal [KTP (011)] for the range between 1.2 and 2.8 keV. Experiments can be carried out in the temperature range from 4.2 to 1000 K and pressures up to 30 bar for catalysis research. A safety system has been incorporated, allowing the use of flammable and toxic gases such as H2 and CO.