Jon Orloff

Solid hydrogen at 342 GPa: no evidence for an alkali metal

Solid hydrogen, an electrical insulator, is predicted to become an alkali metal under extreme compression, although controversy surrounds the pressure required to achieve this. The electrical conductivity of hydrogen as a function of pressure and temperature is of both fundamental and practical interest—metallic hydrogen may be of relevance to planetary interiors, and has been suggested as a potential high-temperature superconductor. Calculations, suggest that depairing (destruction of the molecular bond) should occur around 340 GPa, accompanied by the formation of an alkali metal at this pressure, or at substantially higher pressures,. Here we report that solid hydrogen does not become an alkali metal at pressures of up to 342 ± 10 GPa, achieved using a diamond anvil cell. This pressure (which is almost comparable to that at the centre of the Earth) significantly exceeds those reached in earlier experiments—216 GPa (ref. 6) and 191 GPa (ref. 7)—at which hydrogen was found to be non-metallic. The failure of solid hydrogen to become an alkali metal at the extreme pressures reported here has implications for our current theoretical understanding of the solid-state phase.

Fundamental limits to imaging resolution for focused ion beams

This article investigates the limitations on the formation of focused ion beam images from secondary electrons. We use the notion of the information content of an image to account for the effects of resolution, contrast, and signal‐to‐noise ratio and show that there is a competition between the rate at which small features are sputtered away by the primary beam and the rate of collection of secondary electrons. We find that for small features, sputtering is the limit to imaging resolution, and that for extended small features (e.g., layered structures), rearrangement, redeposition, and differential sputtering rates may limit the resolution in some cases.

Focused ion beam sputter yield change as a function of scan speed

In many of the applications of focused ion beams, such as integrated circuit sectioning and TEM sample preparation, considerable volume of materials may need to be removed. Thus optimizing the sputter yield is important. For very rapid scan speeds at normal incidence, each pass of the beam removes a thickness of material which is much smaller than the beam diameter. In this case, this milling yield corresponds to the yield at normal incidence. However, if the scan speed is slowed down so that the thickness removed per pass is comparable to the beam diameter, then locally under the beam the ions are not normally incident even though the beam is normal to the surface. The milling yield of Si and SiO2, for example, increases by a factor of seven to eight in going from normal incidence at 0° to 75°–85°. Thus the material removal rate can be significantly increased by reducing the scan speed. We have measured the milling yield of Si and SiO2 as a function of scan speed in one axis by milling boxes, typically 7...

Study of precursor gases for focused ion beam insulator deposition

The electrical properties of insulators formed by focused ion beam induced deposition of various siloxane precursor gases have been compared. Leakage current and breakdown field have been measured by forming metal-insulator-metal structures. It was found that the focused ion beam induced deposition of metal on top of the insulator can substantially degrade the quality of the insulator. We found that the resistivity of the insulator material depends on the deposition yield (e.g., the amount of Ga implantation) as well as on the chemical nature of the precursor gas. From the precursor gases studied, the new compound pentamethylcyclopentasiloxane shows the best performance. Compared to the commercially used tetramethylcyclotetrasiloxane compound, an improvement in resistivity by two orders of magnitude (∼8×1011 versus ∼6×109 Ω cm) and a factor of about 1.5 in breakdown field (650 vs 440 V/μm) could be achieved.

Development of a high brightness gas field ion source

We have investigated the emission properties of gas field ionization emitters built up from electrochemically dc-etched W〈111〉 and W〈100〉 single crystal wires. A standard thermal field method was used to confine the ion emission to a single spot at the apex of the emitter. The angular current density obtained for these kinds of tips with a small end radius (100–200 nm) is in the range of 5 μA/sr. For H2 and Ne, maximum ion current was observed in the temperature range between 18 and 21 K. We have measured the ion current density for different gases (H2, He, and Ne) as a function of extraction voltage and gas pressure. The linear dependence of the angular current density on the gas pressure shows that current densities of 10 μA/sr and above can be expected with this type of emitter.

Some Aspects on the Mechanical Analysis of Micro-Shutters

An array of individually addressable micro-shutters is being designed for spectroscopic applications. Details of the design are presented in a companion paper. The mechanical design of a single shutter element has been completed. This design consists of a shutter blade suspended on a torsion beam manufactured out of single crystal silicon membranes. During operation the shutter blade will be rotated by 90 degrees out of the array plane. Thus, the stability and durability of the beams are crucial for the reliability of the devices. Structures were fabricated using focused ion beam milling in a FEI 620 dual beam machine, and subsequent testing was completed using the same platform. This allowed for short turn around times. We performed torsion and bending experiments to determine key characteristics of the membrane material. Results of measurements on prototype shutters were compared with the predictions of the numerical models. The data from these focused studies were used in conjunction with experiments and numerical models of shutter prototypes to optimize the design. In this work, we present the results of the material studies, and assess the mechanical performance of the resulting design.

Focused chromium ion beam

With the goal of expanding the capabilities of focused ion beam microscopy and milling systems, the authors have demonstrated nanoscale focusing of chromium ions produced in a magneto-optical trap ion source. Neutral chromium atoms are captured into a magneto-optical trap and cooled to 100 μK with laser light at 425 nm. The atoms are subsequently photoionized and accelerated to energies between 0.5 and 3 keV. The accelerated ion beam is scanned with a dipolar deflector and focused onto a sample by an einzel lens. Secondary electron images are collected and analyzed, and from these, a beam diameter is inferred. The result is a focused probe with a 1 standard-deviation radius as small as 205±10 nm. While this probe size is in the useful range for nanoscale applications, it is almost three times larger than is predicted by ray-tracing simulations. Possible explanations for this discrepancy are discussed.

Analytical model of a gas phase field ionization source

High resolution focused ion beam technology is based on the use of field ionization sources. The most widely used source by far is the liquid metal ion source (LMIS) and most focused ion beam systems today employ it. The gas phase field ionization source (GFIS) is hardly employed today except in field ionization microscopy, but present day technology would allow its unique properties vis a vis the liquid metal ion source, including smaller virtual source size and energy spread, and the ability to produce ions from H, He, and heavier noble gases, to be exploited. If the GFIS is used as an ion source for a focusing column it would be useful to be able to calculate its emission properties (current versus voltage) as a function of emitter shape (a parameter not variable in a LMIS). Such a calculation had not been done precisely based on realistic emitter shapes. In this work, a theoretical description of the mechanism for ion production in a GFIS is presented. The comparison of the result with the experimenta...

A Study of Optical Properties of Gas Phase Field Ionization Sources

Publisher Summary The unique properties of gas field ionization source (GFIS)—small source size and the ability to produce light-weight noble gas ions—make it a valuable candidate for use in focused ion beam (FIB) systems for certain applications that supplement those of FIBs based on the liquid–metal ion source (LMIS). This chapter investigates the GFIS source optical properties based on the sphere‐on‐orthogonal cone (SOC) model that well represents thermally annealed field emitter geometry. The chapter presents two alternative spherical and chromatic aberration integrals. They reduce to the usual forms under special circumstances. In addition, the derived spherical aberration involves only up to the second‐order derivative of the potential in the integrand, which, thus, eases the numerical calculation of the potential distribution for general emitter topography. One important feature of field ionization is that the critical distance of ionization, though very small in magnitude, plays an essential role in determining the aberration coefficients so that the results of field electron emission cannot be simply borrowed in deriving source optical properties. Two methods to evaluate the GFIS virtual source size are reviewed in the chapter—the addition in quadrature and direct ray tracing.

Fracture tests of etched components using a focused Ion beam machine

Many optical MEMS device designs involve large arrays of thin (0.5 to 1 (mu) m) components subjected to high stresses due to cyclic loading. These devices are fabricated from a variety of materials, and the properties strongly depend on size and processing. Our objective is to develop standard and convenient test methods that can be used to measure the properties of large numbers of witness samples, for every device we build. In this work we explore a variety of fracture tests configurations for 0.5 (mu) m thick silicon nitride membranes machined using the Reactive Ion Etching (RIE) process. Testing was completed using an FEI 620 dual focused ion beam milling machine. Static loads were applied using a probe, and dynamic loads were applied through a piezo-electric stack mounted at the base of the probe. Results from the tests are presented and compared, and application for predicting fracture probability of large arrays of devices are considered.