Harry J. Whitlow

Detection efficiency of time-of-flight energy elastic recoil detection analysis systems

Abstract The detection efficiency of recoils with masses ranging from H up to Nb at energies from 0.05 to 1 MeV per nucleon has been investigated for Time-of-Flight Energy Elastic Recoil Detection (ToF-E ERD) systems. It is observed that the detection efficiency for the ToF-E detector telescope depends on the stopping power in the carbon foils, which in turn relies upon the recoil mass and energy. Furthermore, the limits of this behaviour depend on the setting of the discriminator thresholds. The detection efficiency of a time detector could be fitted to a universal curve that can be described by a simple empirical formula as a function of recoil electronic stopping power in the carbon foil. This formula can be used to predict the detection efficiency by recoil energy for N, O and other elements, for which it may not be easy to prepare suitable reference samples containing only that element.

Angular distribution of particles sputtered from Cu, Pt and Ge targets by keV Ar+ ion bombardment☆

Abstract Measurements of angular distributions of material sputtered from polycrystalline copper and platinum and amorphous germanium targets by irradiation with argon ions are reported. The beam energy was varied from 1.25 keV to 320 keV (for Cu and Pt upwards from 10 keV only). All targets yielded an angular distribution more outward peaked than the cosine predicted by collision-cascade theory. The germanium result were well-fitted by cos n distributions with n varying from 1.3 at the lowest to 1.6 at higher energies. The copper and platinum targets yielded distributions of the same general shape superimposed with distinct shoulders indicating preferential ejection along nonrandomly distributed close packed directions typical for a polycrystalline textured target. The measured distributions for germanium are in excellent agreement with the results from recent computer simulations of sputtering.

Multivariate analysis method for energy calibration and improved mass assignment in recoil spectrometry

Abstract Heavy ion recoil spectrometry is rapidly becoming a well established analysis method, but the associated data analysis processing is still not well developed. The pronounced nonlinear response of silicon detectors for heavy ions leads to serious limitation and complication in mass gating, which is the principal factor in obtaining energy spectra with minimal cross talk between elements. To overcome the above limitation, a simple empirical formula with an associated multiple regression method is proposed for the absolute energy calibration of the time of flight-energy dispersive detector telescope used in recoil spectrometry. A radical improvement in mass assignment was realized, which allows a more accurate and improved depth profiling with the important feature of making the data processing much easier.

Lithography of high spatial density biosensor structures with sub-100 nm spacing by MeV proton beam writing with minimal proximity effect

Metal electrode structures for biosensors with a high spatial density and similar to85 nm gaps have been produced using focused megaelectronvolt (MeV) proton beam writing of poly-(methyl methacrylate) positive resist combined with metal lift-off. The minimal proximity exposure and straight proton trajectories in (similar to100 nm) resist layers for focused MeV proton beam writing are strongly indicative that ultimate electrode gap widths approaching a few nanometres are achievable.

Mass resolution of recoil fragment detector telescopes for 0.05–0.5 A MeV heavy recoiling fragments

Abstract The factors governing the mass resolution for 0.05–0.5 A MeV recoil nuclei have been investigated for detector telescopes in which carbon-foil time zero detectors and ion-implanted silicon detectors are used to determine the time of flight and energy respectively. Experimentally determined second moments of the mass distribution have been compared with theoretical estimates based on literature data. The experimental mass resolution is in reasonably good absolute agreement with theoretical estimates. For low energy ( A MeV) particles the mass resolution is dominated by the contribution from the silicon detector and thus largely independent of timed flight length. In fact for detection of very low energy (0.1 A MeV) recoil nuclei timed flight lengths of less than 0.22 m are sufficient.

Linear Energy Transfer of Heavy Ions in Silicon

Researchers performing radiation testing on electronic components often rely on semi-empirical prediction codes for determining the linear energy transfer (LET) (or electronic stopping force) of ions, without paying much attention to their reliability. However, it is seen that estimations calculated with different codes can have over 10% discrepancies, especially in the case of heavy ions with higher LET (e.g., xenon). As a consequence of the modern component fabrication techniques this has become an important issue when studying the radiation durability of electronics. In order to clarify this inconsistency, LET measurements for 131Xe and 82Kr in silicon have been undertaken and obtained results are presented, discussed and compared with earlier predicted data.

Programmable proximity aperture lithography with MeV ion beams

A novel MeV ion beam programmable proximity aperture lithography system has been constructed at the Accelerator Laboratory of the University of Jyvaskyla, Finland. This facility can be used to fabricate three dimensional microstructures in thick (<100μm) polymer resist such as polymethylmethacrylate. In this method, MeV ion beams from the 1.7 MV pelletron and K130 cyclotron accelerators are collimated to a beam spot of rectangular shape. This shape is defined by a computer-controlled aperture made of a pair of L-shaped Ta blades which are in close proximity to the sample to minimize the penumbra broadening. Here the authors report on development of the system, the controlling software, the calibration procedures, investigations of multiple scattering effects, and present illustrative results using 3MeV He2+4 ion beams for lithography and 56MeV N3+14 ion beams for creating patterns of regions with ion tracks.

Thin detectors for the CHICSi /spl Delta/E-E telescope

A pilot series of 10 /spl mu/m to 15 /spl mu/m thin silicon detectors has been made for the /spl Delta/E-E telescopes in the CHICSi detector system. This system will operate at the CELSIUS heavy ion storage ring in Uppsala, Sweden. /spl Delta/E-E telescopes provide isotope identification and energy determination of fragments from nuclear collisions. The thin detectors are made as p-i-n diodes in thin etched membranes in 280 /spl mu/m thick silicon wafers. The membranes are made with anisotropic etching using 25 wt.% tetramethylammonium hydroxide (TMAH) solution. The etch speed of this solution is very uniform across a wafer. As a result detectors with uniform thickness can be produced. The etch depth varies with less than /spl plusmn/0.3 /spl mu/m over a wafer and the surface microroughness is in the range from 2 to 4 nm. Each detector has a 10.0 mm/spl times/10.0 mm active area on a 10.2 mm/spl times/10.2 mm membrane surrounded by a 1.1 mm wide supporting frame. The detectors have leakage currents in the active area of approximately 0.5 nA at 20 V. The breakdown voltage of the detectors is above 100 V. Evaluation experiments with telescopes consisting of a thin detector in combination with a thick detector have shown excellent isotope separation capabilities. Mass separation of /sup 6/Li and /sup 7/Li is clearly observable.

Measurements of the mean energy-loss of swift heavy ions in carbon with high precision

Abstract Z1 dependent effects in heavy ion stopping around stopping maximum under conditions of equilibrium charge have been measured with a time of flight-energy elastic recoil detection analysis (ToF-E ERDA) set-up. The ToF section was used to tag individual recoil atoms with their energy prior to entering the stopping foil and the exit energy was measured subsequently using a Si p–i–n charged particle detector. The mean energy-loss and average energy of recoils have been determined for many elements with Z1=3–26 in a continuous energy range from ∼0.2 to ∼0.9 MeV per nucleon and from ∼0.03 to 0.1 MeV per nucleon for 79Au. The stopping power and energy dependence were in good agreement with most literature data. However, considerable discrepancies in the energy dependence were observed with semi-empirical SRIM-2000 estimates around the stopping power maximum for some of the elements. This technique, which allows direct measurement of Z1 effects in stopping foils to be performed with high-precision, showed that the dependence of the experimentally determined stopping number (Lexp) with the stopping parameter ξ clustered around a trend line with Z1 dependent deviations that exceeded the estimated uncertainty. The deviations were more significant for large ξ and were 15% between 9Be and 7Li at ξ=10.

High‐resolution recoil spectrometry for separate characterization of Ga and As in AlxGa(1−x)As structures

Mass and energy‐dispersive recoil spectrometry, where the recoil energy is derived from the recoil time of flight, has been used to characterize the depth distribution of Al, Ga, and As in an AlxGa(1−x)As quantum‐well structure. Signals characterizing the Al, Ga, and As distribution with good separation between Ga and As (average crosstalk <2%) could be obtained from depths less than 560 nm from the surface. The depth resolution of the As signal at the surface was 16 nm FWHM, which is considerably better than achieved using a silicon particle detector (34 nm).