Focused Ion Beam Exposure Research Articles

The role of chalcogen vacancies for atomic defect emission in MoS2

For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS2. Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10 nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale.

Investigation of local ion-stimulated carbon deposition to create vacuum field emission diodes

This paper presents the results of experimental studies of the influence of the technological parameters of a focused ion beam (FIB) on the process of local ion-stimulated deposition of carbon and tungsten when creating elements of vacuum nanoelectronics. The dependences are obtained illustrating the influence of the time of the FIB exposure at a point on the geometric parameters of the structures. Experimental samples of vacuum field-emission diodes based on semiconductor-metal-dielectric structures were fabricated by ion-stimulated carbon deposition. A technological process for creating field-emission diodes has been developed. The prospects of applying the FIB method for creating structures of vacuum field emission nanoelectronics are demonstrated.

Ion beam profiling from the interaction with a freestanding 2D layer.

Recent years have seen a great potential of the focused ion beam (FIB) technology for the nanometer-scale patterning of a freestanding two-dimensional (2D) layer. Experimentally determined sputtering yields of the perforation process can be quantitatively explained using the binary collision theory. The main peculiarity of the interaction between the ion beams and the suspended 2D material lies in the absence of collision cascades, featured by no interaction volume. Thus, the patterning resolution is directly set by the beam diameters. Here, we demonstrate pattern resolution beyond the beam size and precise profiling of the focused ion beams. We find out that FIB exposure time of individual pixels can influence the resultant pore diameter. In return, the pore dimension as a function of the exposure dose brings out the ion beam profiles. Using this method of determining an ion-beam point spread function, we verify a Gaussian profile of focused gallium ion beams. Graphene sputtering yield is extracted from the normalization of the measured Gaussian profiles, given a total beam current. Interestingly, profiling of unbeknown helium ion beams in this way results in asymmetry of the profile. Even triangular beam shapes are observed at certain helium FIB conditions, possibly attributable to the trimer nature of the beam source. Our method of profiling ion beams with 2D-layer perforation provides more information on ion beam profiles than the conventional sharp-edge scan method does.

Silicon dioxide mask by plasma enhanced atomic layer deposition in focused ion beam lithography

In this work, focused ion beam (FIB) lithography was developed for plasma enhanced atomic layer deposited (PEALD) silicon dioxide SiO2 hard mask. The PEALD process greatly decreases the deposition temperature of the SiO2 hard mask. FIB Ga+ ion implantation on the deposited SiO2 layer increases the wet etch resistivity of the irradiated region. A programmed exposure in FIB followed by development in a wet etchant enables the precisely defined nanoscale patterning. The combination of FIB exposure parameters and the development time provides greater freedom for optimization. The developed process provides high pattern dimension accuracy over the tested range of 90–210 nm. Utilizing the SiO2 mask developed in this work, silicon nanopillars with 40 nm diameter were successfully fabricated with cryogenic deep reactive ion etching and the aspect ratio reached 16:1. The fabricated mask is suitable for sub-100 nm high aspect ratio silicon structure fabrication.

Evaluating focused ion beam induced damage in soft materials

Focused ion beam (FIB) microscopy uses Ga+ ions to remove material from a sample for a variety of imaging and preparation techniques. While considerable work has examined the effects of FIB exposure on a number of materials, optimized FIB conditions for use with softer polymeric materials are yet to be determined. In this report we use phase contrast AFM to measure local changes in the elastic modulus of polycarbonate surfaces parallel to a sectioning FIB at varying beam energies. We show that polycarbonate surfaces exposed to lower FIB energies appear stiffer than the bulk material whereas surfaces exposed to the higher beam energies of up to 25keV are more representative of the bulk material. Energy dispersive spectroscopy (EDS) indicates that the polymer surfaces become stiffer because of Ga+ implantation from the FIB. Our experimental observations are supported by computer simulations showing an increase in the residual Ga+ concentration near-surface at lower FIB energies. A high energy FIB is therefore shown to be less invasive, producing a surface more representative of the bulk material, than using low energy FIB when sectioning polymers.

Microcantilever deflection compensation with focused ion beam exposure

We report a silicon microcantilever deflection compensation technique using a focused ion beam (FIB). We find that microcantilevers fabricated in the top silicon layer of a silicon-on-insulator wafer often have a residual deflection up out of the plane of the silicon, after release by hydrofluoric (HF) acid in which the underlying silicon dioxide is removed. We investigate compensating the out-of-plane deflection with a dose of Ga ions in the anchoring region of each microcantilever. Using this technique, deflection compensation is performed on an array of twenty-two 300 µm long and 0.65 µm thick microcantilevers. The average deflection is reduced from 919 nm ± 232 nm after HF release to –9 nm ± 34 nm after FIB compensation.

Ultralow Superharmonic Resonance for Functional Nanowires

Functional nanowires, made from materials such as zinc oxide, offer the promise of energy scavenging and precise sensing due to their vibrational properties, but their high intrinsic resonance frequencies (in the kilohertz to megahertz range) have limited the applications in nanotechnology. In this paper, we describe a method for introducing a new type of resonance at ultralow frequencies in ZnO nanowires. By using in situ ion implantation, nanodevice assembly, electronic signal generation, mechanical measurement, and electron beam characterization, we have achieved resonance at frequencies two orders of magnitude lower than the natural resonance frequency. Through both experimental investigation and theoretical simulation, we show that electric charge imbalance arising from focused ion beam exposure is responsible for the creation of this unprecedented superharmonic resonance behavior in ZnO nanowires.

Impact of fluence-rate related effects on the sputtering of silicon at elevated target temperatures

In this work we show how ion-beam-induced epitaxial recrystallization plays a role in focused ion-beam (FIB) sputtering of silicon at elevated temperatures. The sputtering process is the key to all high-precision machining of microstructures and nanostructures by FIBs. A fluence-rate effect observed for the sputtering of silicon at elevated temperatures arises from competition between stabilizing interactions between populations of defects produced by consecutive ion impingement (damage buildup) and dynamic self-annealing. By high-resolution transmission electron microscopy analysis we show that the damage, produced by exposure of silicon to a 50 kV focused gallium (Ga) ion beam at elevated target temperatures, departs quite substantially from the expected damage based on the distribution of energy within the substrate due to nuclear stopping. An amorphous layer observed at room temperature is completely absent at higher temperatures. In contrast to FIB exposure at room temperature the implanted layers contain only point defects complexes and dislocations, thus suggesting that defect annealing takes place but it is incomplete. Correlating FIB sputtering experiments and high-resolution transmission electron microscopy, we discuss the lower sputtering yield at elevated target temperatures as the result of a higher surface binding energy of crystalline Si in comparison to amorphous silicon.

Magnetic strip patterns induced by focused ion beam irradiation

Focused ion beam exposure was used to locally alter the magnetic properties of a continuous Co∕Pd multilayer film with perpendicular magnetic anisotropy. The saturation magnetization, coercivity, and magnetic anisotropy of the films can be tuned by Ga irradiation depending on exposure dose. As a result, a periodic strip pattern consisting of 80nm wide exposed strips which are magnetically soft, separated by 170nm wide magnetically hard, unexposed areas was created. Due to strong magnetostatic coupling between the strips, a number of magnetic domain configurations could be stabilized and these have been observed by magnetic force microscopy and magneto-optic Kerr effect measurements. The magnetic domain configurations and their reversal behavior were investigated by micromagnetic simulations as a function of exposure dose and strip period.

Synthesis of nanowires in room temperature ambient: A focused ion beam approach

We present a focused ion beam based technique for the synthesis of nanowires in room temperature ambient without using any additional material source. Based on the idea of a catalytic approach gallium antimonide and antimony nanowires were grown with diameters of about 20nm. We suppose that the intense focused ion beam exposure forms catalytic particles alloyed by the constituents of the growing nanowire in situ. In contrast to a broad class of techniques for nanowire growth neither heating of the sample nor any additional material source is required, thereby being compatible with on-chip microelectronics.

Fabrication of Magnetic Nanostructures for MRAM using Electron Beam and Focused Ion Beam Exposure of HSQ

ABSTRACTGiant magnetoresistive (GMR) materials-based magnetic random access memory (MRAM) has become attractive due to non-volatility, speed and density1. The vertical MRAM (VMRAM) design model shows good signal level and high speed and density potential. The memory cell for the VMRAM model is a ring shaped magnetic material multilayer, which ensures high repeatability and low switching energy. This GMR structure, however, is difficult to pattern as it contains materials such as Ni, Fe, Co, and Cu, which are difficult to dry etch because they lack volatile etch products. This work shows that it is possible to overcome the difficulties associated with etching GMR materials by using hydrogen silsesquioxane (HSQ) as an etch mask. We have used HSQ in a direct-write electron-beam lithography system with a dose of 600 μC/cm2, and in a Ga+ ion focused ion beam (FIB) system with a dose of 12 μC/cm2. Both are followed by development and an argon plasma etch at 10mTorr and 100W RIE power. The HSQ layer provides high resolution as well as good etching resistance. Electron-beam exposed HSQ shows a 1:1.5 selectivity over the GMR film stack and the FIB exposed HSQ showed an improved etch selectivity of 1:1. Ring shaped GMR structures with a 75/225 nm (ID/OD) have been fabricated, which corresponds to a memory density of 4Gb/in2.

Advanced nanoscale material processing with focused ion beams

We present an approach for the generation of metallic Ga dots and In nano-crystallites which, in contrast to conventional bottom-up or top-down processes, is based on a subtractive self-organization process relying on material decomposition induced by focused ion beam exposure. The diameters of the Ga dots range from 120to850nm with an aspect ratio of about 0.4 at a dot density of up to 7×107∕cm2. Two-dimensional ordered arrays of freestanding dots were fabricated by a site control technique relying on prepatterned holes and an irradiation mediated migration and agglomeration. By the analogous technique we generated indium nanocrystallites on the (100) InAs surface with sizes ranging from about 50nm to a few microns obviously due to a preferential loss of arsenic atoms during Ga+ focused ion beam irradiation. The influence of the ion dose, the beam energy, and the dose rate on the surface evolution was investigated by atomic force microscopy, scanning electron microscopy, Auger electron spectroscopy, and x-ray diffraction measurements.

A novel focused-ion-beam lithography process for sub-100 nanometer technology nodes

A focused-ion-beam (FIB) machine is a versatile tool used extensively in the integrated circuit (IC) industry for conducting failure analysis, prototype fabrication, and device repair. Lithography can also be performed using FIB for direct patterning of photoresists without using a mask. The FIB technique has several advantages over other maskless processes, such as direct-write e-beam lithography; ion-beam patterning of the photoresist offers higher resist sensitivity and lesser backscattering/proximity effects than the e-beam patterning. In this paper, we investigated a new FIB lithography process, called NERIME (negative resist image by dry etching). The NERIME process is a single-layer resist scheme, which implements a Ga + FIB exposure of DNQ/novolak-based resists, followed by a near-ultraviolet (NUV) flood exposure, silylation, and oxygen dry etching. The NERIME process could not only yield both positive and negative images, but also achieve a nanometer resolution down to 65 nm and a high aspect ratio for the processed patterns. The principle of the negative image formation was also investigated by TEM and found to be due to the creation of a thin gallium oxide mask layer during etching. The NERIME process could be utilized for specific CMOS processing steps for the next generation technology nodes, such as high-resolution lithography over topographical surfaces.

Size and site controlled Ga nanodots on GaAs seeded by focused ion beams

We present an approach for the generation of uniform metallic nanodots which, in contrast to conventional bottom up or top down processes, is based on a subtractive self-organization process relying on material decomposition induced by focused ion beam exposure. Nearly pure gallium dots were fabricated on GaAs (100) using a 50 keV Ga focused ion beam at normal incidence and a subsequent rapid thermal annealing. A dose of 2.5×1016 ions/cm2 can be considered as a threshold dose for the development of dots on GaAs (100). The diameters of these dots range from 120 to 850 nm with an aspect ratio of about 0.4 and a dot density of up to 7.23×107/cm2. Two-dimensional ordered arrays of embedded as well as freestanding Ga dots were fabricated by a site control technique relying on pre-patterned holes and an irradiation mediated migration and agglomeration.

Novel dry-developed focused ion beam lithography scheme for nanostructure applications

Focused ion beam (FIB) lithography has certain advantages over the rival direct-write electron beam lithography in terms of resist sensitivity, scattering and proximity effects. Combining the FIB lithography with both top surface imaging (TSI) and reactive ion etching (RIE) will further strengthen its advantages towards anisotropic processing of thicker resist layers in comparison to those used by the conventional lithography processes. Some of the inherent limitations to the FIB lithography, such as low-penetration depth and substrate damage, could also be eliminated. Our recently developed NERIME (negative resist image by dry etching) process combines these advantages due to the incorporation of focused Ga + ion beam (Ga + FIB) exposure, near UV exposure, silylation and dry development process steps. The work described in this paper follows our investigations into the NERIME process for nanostructure applications and also outlines a simplified (two-step) process, which incorporates Ga + FIB exposure and oxygen dry development. The two-step NERIME process is a negative working TSI system for DNQ/novolak based resists. In this paper, nanometer resist patterns as small as 30 nm and having a high aspect ratio of up to 15 were resolved using the NERIME and the simplified two-step NERIME processes.

Ion beam lithographic fabrication of ordered VO2 nanoparticle arrays

AbstractLong-range ordered arrays of vanadium dioxide nanoparticles are fabricated by pulsed laser deposition in a patterned layer of poly(methyl methacrylate) resist. The two- dimensional arbitrary pattern is created by focused ion beam exposure of the resist, followed by pulsed laser deposition and thermal annealing. Interaction of light with the nanoparticles is controlled by their geometrical arrangement as well as by the difference in optical properties displayed between the metallic and semiconducting phases of VO2. Arrays like this open opportunities to study optical resonances and interactions for nanoparticles in close proximity, in the framework of the metal-semiconductor phase transition in VO2.

Recovery of shifted MOS parameters induced by focused ion beam exposure

The effects of focused ion beam (FIB) exposure on MOS transistors within a circuit were examined. It was found that FIB exposure does not cause parameter shifts as long as the gate is connected to the drain of other MOS transistors. However, the threshold voltage (V/sub t/) does shift during isolating the gate using a FIB. Further FIB exposure on MOS transistors with a floating gate is shown to cause larger shifts. Thermal annealing was studied to recover shifted V/sub t/. We demonstrated that a 400/spl deg/C-450/spl deg/C anneal could recover shifted V/sub t/ almost completely. Ninety percent recovery can be reached by annealing at 400/spl deg/C-450/spl deg/C for 1-2 hours, and V/sub t/ shifts can be reduced to about 10 mV.

Study of the chemical and morphological evolution of the GaAs surface after high fluence focused ion beam exposure

The impact of 50 keV focused Ga + ion bombardment on semi-insulating GaAs has been investigated at various ion fluences in the range of 6.25 × 10 10–1.25 × 10 17 ions/cm 2. As a result, the formation of spherical calotte shaped dots consisting of nearly pure gallium has been observed. The diameter of the calottes is in the range of 100 nm to more than 1 μm with an almost constant aspect ratio of about 0.4. The Ga dot formation promotes decomposition of the Ga-rich surface far below temperatures that have been reported for the thermal decomposition of undisturbed GaAs. The material decomposition occurs exclusively in the implanted regions, leading to micelle like structures on the irradiated surface. In this work, we present an experimental study of the morphological, chemical and electrical evolution of the GaAs surface under focused ion beam exposure by in situ FIB-secondary electron microscopy combined with atomic force microscopy, Auger electron spectroscopy, optical microscopy, and electrical measurements.

Current density profile extraction of focused ion beams based on atomic force microscopy contour profiling of nanodots

An approach to the profile extraction of nanoscale focused ion beams is presented. It is based on contour profiling of dots patterned at various ion doses by focused ion beam exposure. While the surface contour depends on the spatial variation of the beam-solid interaction, at no single ion dose the contour reflects the current density profile of the beam itself, due to effects such as target swelling, redeposition, and angle-dependent sputtering yield. Instead, we monitor the surface deviation relative to the unimplanted case as a function of dose for the radial positions of interest, and determine scaling factors for the dose such that the scaled curves coincide for all radial positions in the regions of small milling depths. We apply the method to beam shape determination of 10 and 50 keV focused ion beams using silicon and GaAs as targets and atomic force microscopy as a contour profiling technique. Despite the different irradiation response of silicon and GaAs, the beam profiles evaluated on these substrate materials agree excellently, which demonstrates that our approach is substrate independent. It allows profile extractions of nanoscale focused ion beams with high accuracy and high sensitivity even in the tails of the beams.

Highly Resolved Maskless Patterning on InP by Focused Ion Beam Enhanced Wet Chemical Etching

The fabrication of high resolution patterns on InP-substrates by focused ion beam implantation and selective wet chemical etching has been investigated. InP acts as positive resist for Ga+ focused ion beam exposure and development by HF. The values of contrast and sensitivity have been determined to 12 and 1.5×1013 cm-2, respectively. By this technique short period line gratings with a linewidth of 30 nm have been fabricated.