The fairly recent availability of commercial focused ion beam (FIB) microscopes has led to rapid development of their applications for materials science. FIB instruments have both imaging and micromachining capabilities at the nanometer–micrometer scale; thus, a broad range of fundamental studies and technological applications have been enhanced or made possible with FIB technology. This introductory article covers the basic FIB instrument and the fundamentals of ion–solid interactions that lead to the many unique FIB capabilities as well as some of the unwanted artifacts associated with FIB instruments. The four topical articles following this introduction give overviews of specific applications of the FIB in materials science, focusing on its particular strengths as a tool for characterization and transmission electron microscopy sample preparation, as well as its potential for ion beam fabrication and prototyping.

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Focused Ion Beam Microscopy and Micromachining

The application of focused ion beam (FIB) technology in microfabrication has become increasingly popular. Its use in microfabrication has advantages over contemporary photolithography or other micromachining technologies, such as small feature resolution, the ability to process without masks and being accommodating for a variety of materials and geometries. An overview of the recent development in FIB microfabrication technology is given. The emphasis will be on direct milling, or maskless techniques, and this can distinguish the FIB technology from the contemporary photolithography process and provide a vital alternative to it. After an introduction to the technology and its FIB principles, the recent developments in using milling techniques for making various high-quality devices and high-precision components at the micrometer scale are examined and discussed. Finally, conclusions are presented to summarize the reviewed work and to suggest the areas for improving the FIB milling technology and for future research.

https://iopscience.iop.org/article/10.1088/0960-1317/14/4/R01/pdf

Recent developments in micromilling using focused ion beam technology

An array of very uniform cylindrical nanopores with a pore diameter as small as 25 nm has been fabricated in an ultrathin micromachined silicon nitride membrane using focused ion beam (FIB) etching The pore size of this nanosieve membrane was further reduced to below 10 nm by coating it with another silicon nitride layer This nanosieve membrane possesses adequate mechanical strength up to several bars of transmembrane pressure, and it can withstand high temperatures up to 900 C In addition, it is inert to many aggressive chemicals such as hot concentrated potassium hydroxide (KOH), piranha (H2SO4 + H2O2), and nitric acid (HNO3)

Silicon nitride nanosieve membrane

The advent of high-harmonic generation in gases 30 years ago set the foundation for attosecond science and facilitated ultrafast spectroscopy in atoms, molecules, and solids. We explore high-harmonic generation in the solid state by means of nanostructured and ion-implanted semiconductors. We use wavelength-selective microscopic imaging to map enhanced harmonic emission and show that the generation medium and the driving field can be locally tailored in solids by modifying the chemical composition and morphology. This enables the control of high-harmonic technology within precisely engineered solid targets. We demonstrate customized high-harmonic wave fields with wavelengths down to 225 nanometers (ninth-harmonic order of 2-micrometer laser pulses) and present an integrated Fresnel zone plate target in silicon, which leads to diffraction-limited self-focusing of the generated harmonics down to 1-micrometer spot sizes.

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Tailored semiconductors for high-harmonic optoelectronics

An overview of the existing two-dimensional carrier profiling tools using scanning probe microscopy includes several scanning tunneling microscopy modes, scanning capacitance microscopy, Kelvin probe microscopy, scanning spreading resistance microscopy, and dopant selective etching. The techniques are discussed and compared in terms of the sensitivity or concentration range which can be covered, the quantification possibility, and the final resolution, which is influenced by the intrinsic imaging resolution as well as by the response of the investigated property to concentration gradients and the sampling volume. From this comparison it is clear that, at present, none of the techniques fulfills all the requirements formulated by the 1997 Semiconductor Industry Association roadmap for semiconductors [National Technology Roadmap for Semiconductors (Semiconductor Industry Association, San Jose, CA, 1997)]. Most methods are limited to pn-junction delineation or provide a semiquantitative image of the differen...

Status and review of two-dimensional carrier and dopant profiling using scanning probe microscopy

In this article, some limitations of the processing of structures with dimensions in the nanometer range by focused ion beams will be discussed. In order to enable exact depth control of nanometer structures, the effective sputter yield of silicon was determined as function of the ion dose. At ion doses below 1016 cm−2, the effective sputter yield is not constant and the volume of the area processed increases due to the implantation of ions. Material removal can be measured for doses above 2×1016 cm−2 and it reaches equilibrium for doses of about 3×1017 cm−2. This dose dependence of the effective sputter yield becomes especially effective in beam tail regions with low ion intensity. The shape of nanostructures is further determined by combining the beam shape and the angle dependence of the sputter yield which was experimentally determined. Using this approach with a Gaussian beam shape, a comparison of simulated and measured sidewall angles has shown good agreement for trench structures. Only sidewall regions close to the surface and to the bottom of deep structures show slight deviations. At the surface, non-Gaussian beam tails lead to unintentional sputtering at the corners of the processed area. At the bottom, forward scattered ions lead to higher sputter erosion.

Limitations of focused ion beam nanomachining

Low energy focused ion beam (FIB) systems are used for the modification of integrated circuits, the preparation of TEM samples, the processing of structures in the sub -100 nm range, and for failure analysis. Focused gallium ion beams with diameters down to <10 nm (Full Width at Half Maximum) allow high resolution secondary electron surface inspection as well as local removal and deposition of material with high accuracy by physical sputtering or ion induced chemistry. Besides beam diameter and shape, gallium implantation and defect generation limit the application of FIB micro machining towards minimum resolution and sensitive analysis. Even when the desired structural dimensions are achieved, functionality of the sample may be hindered by implanted gallium and introduced defects. In this paper, vertical gallium distribution and lateral contamination for highly focused ion beam applications are investigated. SIMS analysis and Monte Carlo simulation are used for the determination for doses ranging from FIB inspection (1/spl middot/10/sup 14/ cm/sup -2/) to sputter removal at high doses (up to 1/spl middot/10/sup 18/ cm/sup -2/). The measurements revealed surface concentrations of 1/spl middot/10/sup 18/ cm/sup -3/ up to 6/spl middot/10/sup 21/ cm/sup -3/ and deep ranging tails. At doses exceeding 1.10/sup 17/ cm/sup -2/, the surface concentration reaches 6/spl middot/10/sup 21/ cm/sup -3/ and saturates, which is in agreement with simulations. Lateral contamination is determined by gallium background implantation due to inspection before processing and by non scanned neutrals. Defect generation was investigated by TEM. Even for typical FIB inspection at low doses (30 keV, 1/spl middot/10/sup 14/ cm/sup -2/) an amorphous layer is generated with a thickness of 50 nm.

Defects and gallium-contamination during focused ion beam micro machining

Focused ion beams (FIB) have drawn considerable interest as a tool for micromachining in the sub-micrometer regime with major applications in failure analysis and circuit repair. With shrinking dimensions, the demands on the precision of FIB-machining are increasing. In this paper, focused ion beam enhanced etching of silicon has been investigated using iodine as an etchant which leads to an increase in the material removal rate of silicon by a factor of up to 30 compared to sputter erosion. The influence of current density, dwell time, and loop time on the removal rate will be discussed and compared to model calculations. By secondary electron microscopy, the maximum slope of the generated structures has been determined. Finally, the application of this technique to the formation of thin lamellas for TEM inspection will be shown.

Local material removal by focused ion beam milling and etching

A chemical delineation technique suitable for two dimensional dopant profiling has been investigated and improved The formation of equi-concentration lines in p-doped areas during chemical etching due to drastic changes of the etching rate as function of dopant concentration has been studied Formation of p-n junction, degeneration of the semiconductor, and formation of recombination centers are discussed as possible cause of these changes In addition, a new etching procedure for delineation of equi-concentration lines in n-doped regions was successfully tested

Improved delineation technique for two dimensional dopant profiling

Scanning probe microscopy (SPM) enables high resolution imaging of sample surfaces. Probes with tip radii of a few nanometer facilitate the measurement of topography and surface roughness with sub nanometer vertical resolution. Lateral movement is precisely controlled and enables scanning with sub nanometer resolution and accurate positioning of the probe at a predefined position on a previously imaged surface. The integration of a controllable electron source into the tip of a silicon probe additionally allows the exposure of well defined areas with low energy electrons. Electron emission as well as electron dose can be controlled by variation of the gate voltage. Potential areas of application are lithography, electrical testing, and surface analysis by electron beam based spectroscopy, like inverse photoemission spectroscopy. In this work, field emitter structures were integrated into the tip of silicon probe sensors using focused ion and electron beam techniques. Material processing by focused particle beams has already been used for the repair of damaged or melted tips in field emitter arrays (FEA) and for the processing of emitter gate structures intended for use as electron sources in vacuum electronics. The focused ion beam was used for the processing of gate aperture by physical sputtering. The insulating layer was partially removed as damage and gallium contamination induced into substrate and insulator material during FIB processing deteriorate emission performance. Different methods, e.g. wet etching, in situ etching with gases, for removing residual insulator material and gallium contamination were investigated. Platinum emitters with diameters less than 100 nm were fabricated into the gate opening by electron beam induced deposition. Leakage current emerging after deposition of emitter structure was investigated and assigned to the deposition of precursor material at the side walls during deposition process. Different shapes of emitters have been fabricated to increase stability and improve electrical properties of the field emitters. Geometry of the silicon probes was adapted to the integration of an emitter gate structure with an diameter in the range of micrometers. Processing of the silicon probes was altered to replace the apex by plateaus with diameters between 1 /spl mu/m to 15 /spl mu/m. Figure 2 shows a flattened probe with a plateau diameter of approx. 3 /spl mu/m. The silicon probes were thermally oxidized with a thickness of 1 /spl mu/m and then deposited by platinum with a thickness of 0.3 /spl mu/m. Electrical contacts for gate and emitter voltage were processed by optical lithography. Measured electron emission currents for emitter gate structures processed by focused ion and electron beams were about 0.5 /spl mu/A/tip for a gate voltage of 70 V. The turn-on voltage was about 40 V.

http://ieeexplore.ieee.org/iel5/8668/27467/01222992.pdf

S. Petersen

Papers

Limitations of focused ion beam nanomachining

Defects and gallium-contamination during focused ion beam micro machining

Local material removal by focused ion beam milling and etching

Improved delineation technique for two dimensional dopant profiling

Integration of field emitters into scanning probe microscopy sensors using focused ion and electron beams