Focused ion beam technique for micro/nanoscale fabrication: Progress over the last decade

Focused ion beam machining and deposition for nanofabrication

Superconducting quantum interference devices (SQUIDs), which are the most ultrasensitive magnetic sensors, are generally made by microfabrication of superconducting thin films. The bicrystal junction, which is often used in high-temperature superconducting (HTS) Josephson junctions (JJs), has problems expensive bicrystal substrates, and the fact that JJs can only be arranged in a fixed straight line [1-2]. Because the HTS thin film is a type-II superconductor, magnetic fluxes penetrates the HTS thin film and moves for the Lorentz force, which increases the flux noise at low frequencies.In this study, we investigated a method to fabricate low-noise SQUID magnetic sensors by Ga focused ion beam (FIB) irradiation of high-temperature superconducting thin films. When HTS thin film is irradiated with FIB, point defects are introduced into the HTS crystal structure, which leads to changing into normal conduction. The dependence of the critical current change ratio IC/IC0 on the dose was shown in Fig.1. The dependence of the normal resistance RN are also depicted. The thickness of the YBa2Cu3O7-δ (YBCO) film was 50 nm and it was protected by a thin Au layer. IC/IC0 decreased with irradiation fluence and changed into normal conduction at 2.0×1017 ions/cm2; after this point RN increased steeply.HTS nanobridge JJ, were fabricated by FIB irradiation. Figure 2 shows the V-I characteristics of a nanobridge with an HTS film thickness of 50 nm and a bridge width of 120 nm. The critical current was reduced to 264.5 μA and shows a clear Shapiro-steps under microwave irradiation with frequency of 10 GHz. As a method to reduce the magnetic flux noise, we also investigated the introducing of normal conducting pinning sites (anti-dots) into HTS thin films by FIB irradiation. We have confirmed that anti-dots with diameters of several hundred nm can be formed by point irradiation of HTS thin films with FIB. In the future, we will confirm SQUID operation at a nanobridge JJ with an YBCO film thickness of 50 nm and investigate the improvement of SQUID noise characteristics by introducing anti-dots.  Fig. 1. Dependence of IC/IC0 and RN on FIB irradiation fluence.  Fig. 2. V-I characteristics of the HTS nanobridge with a width of 120 nm.

ABSTRACTWe studied the effects of phosphorus (P) on Ni nanocrystalline morphology formed by focused ion beam (FIB) irradiation for Ni-P amorphous alloy thin films. The P content in the amorphous alloy was varied from 8 to 12 wt.%. The nanocrystals induced by the FIB irradiation for Ni-11.8, 8.9, 7.9 wt.% amorphous alloy had an f.c.c. structure and showed unique crystallographic orientation relationships to the geometry of the focused ion beam, that is, {111}f.c.c. parallel to the irradiated plane and <110>f.c.c. parallel to the projected ion beam direction, respectively. The Ni nanocrystals precipitated like aggregates with decreasing of the P content. These results represent that the P content does not affect crystallographic orientation relationships, while influences the precipitation distribution of Ni nanocrystals generated by the FIB irradiation.

Selective area epitaxy of GaAs using very low energy Ga+ focused ion beam deposition combined with molecular beam epitaxial growth

Bulk single crystals exhibit the intrinsic properties of a given compound, but studying their anisotropic magnetotransport properties is challenging. Focused Ion Beam (FIB) milling at room temperature has been previously used to guide the electrical current path along a defined crystal direction or to extract microcrystals where the electrical current flows along a known direction. However, some materials, such as bismuth, melt under FIB irradiation. Bismuth, known for its unique properties, including very large magnetoresistance and a highly anisotropic Fermi surface, reacts to room‐temperature Ga + FIB irradiation, forming droplets on its surface. Therefore, a novel microfabrication approach based on cryogenic FIB milling is developed here. By using a Peltier stage or an integrated cryogenic module, surface melting is mitigated below −30 °C. Microscale slabs are extracted, either parallel or perpendicular to the single crystal surface, then shaped and electrically contacted in a chip for magnetotransport characterization. The large magnetoresistance observed along with Shubnikov–de Haas oscillations with single periodicity when current is applied perpendicular to the c axis, highlights the success of the approach. These results enable FIB microfabrication of single crystals that are sensitive to FIB irradiation and investigation of the anisotropic magnetotransport properties in microcrystals.

A potential approach to extend the current areal density of magnetic recording beyond 300 Gb/in/sup 2/ is physical patterning of the media. Among all the techniques that have been employed to-pattern the continuous media into nanometer scale discrete tracks and bits, the focused ion beam (FIB) nanofabrication is one of the most straightforward techniques that does not require the use of various types of masks and molds. There has been much interest in nano-patterning of hard and soft magnetic materials and perpendicular recording media using the FIB, but very few of them has been focused on the milling processes. In this work, the FIB patterning of commercial post-sputtering longitudinal media is demonstrated. Special efforts have been put on optimizing the milling process through controlling the magnification, milling depth, and the ion beam scan mode. It was found that the parallel milling mode was effective in reducing the re-deposition at small depth and thus led to smaller nano-structures. The FIB processed nanostructures have been characterized using AFM and MFM. Patterns with square islands of 50 nm and spacing of 10 nm have been obtained. It is interesting to note that, in both serial and parallel milling modes at large depth, the re-deposition has accumulated at the junctions of the original dots and formed new dots. As dots formed by re-deposition have already lost their magnetic properties, they should be suppressed as much as possible. In the contrast to this, the re-deposition was almost eliminated in the parallel-milling mode at small depth. It can be seen clearly from the MFM observation that the in-plane magnetization has been pertained for the patterned structures at dimensions as small as 50 nm and single domain structures were observed. The structures obtained in this work can be used to study magnetic recording at contact mode and at very high recording density.

The combination of focused ion beam (FIB) micro-processing and the lift-out technique using the micro pick-up system was applied to the preparation of TEM specimens of irradiated SiC/SiC composites. The deformation caused by microstructural evolution was observed in the pyrolitic carbon interphase, and several helium bubbles and cavities were detected in the CVI matrix.

Micro-patterning of chemical functionality of anthracene-bis-resorcinol film using focused ion beam

A concept of all-UHV (ultra-high vacuum) resistless process for III-V compound semiconductors is described. This new technology is based on focused ion beam (FIB) doping and molecular beam epitaxy (MBE) in a clean-UHV environment. The submicron-diameter focused ion beams (FIBs) of multiple ion species, which are emitted from a field emission liquid metal ion source (LMIS) are powerful tools for maskless etching, doping, and deposition. Recent developments in FIB processes for III-V compound semiconductors are described, including multicomponent liquid metal ion sources, a focused ion beam implanter, and the physical characteristics of implanted GaAs. Second, preliminary tests of a new compound crystal growth system consisting of MBE layer growth linked with the maskless FIB doping are described. This new technique has been actually used to fabricate planar structures suitable for electrical and optical device integration. Third, innovative UHV processes using electron and photon beams are describ...

Finely focused ion beam technology in III-V compound semiconductors

Focused Ion Beam or FIB systems have been used in integrated circuit production for some time. The ability to combine rapid, precision focused ion beam sputtering or gas-assisted ion etching with focused ion beam deposition allows for rapid-prototyping of circuit modifications and failure analysis of defects even if they are buried deep within the chip's architecture. Inevitably, creative TEM researchers reasoned that a FIB could be used to produce site specific parallel-sided, electron transparent regions, thus bringing about the rather unique situation wherein the specimen preparation device often was worth as much as the TEM itself. More recently, FIB manufacturers have concentrated on improving the resolution and imaging characteristics of these instruments, resulting in a more general-purpose characterization tool. The Micrion 2500 FIB system used in this study is capable of 4 nm imaging resolution using either secondary electron or secondary ions, both generated by a 50 kV liquid metal gallium ion source.

Silver nanostructures are directly grown on the apex of commercially available silicon AFM probes using our area-selective electroless deposition: the apex of a silicon AFM probe is irradiated using a focused ion beam (FIB), and then the FIB-irradiated AFM probe is exposed to a pure AgNO3 aqueous solution. With this method, a silver nanostructure selectively grows on the tip apex where the native oxide layer has been removed in response to FIB irradiation. Silver ions are reduced by the electrons flowing from the silicon probes into the solution through the FIB-irradiated area owing to the difference in Fermi energy between silicon and the solution. The morphology of the growing silver depends on the concentration of both AgNO3 and the electrons. The growth of a gold nanoflower is also demonstrated on the apex of a silicon AFM probe.

First-order gain-coupled distributed feedback lasers have been fabricated in the (Ga,In)As/(Al,Ga)As material system by maskless patterning with focused ion beam implantation. The laser devices are operating at 77 K and room temperature at emission wavelengths between 1.0 and 0.7 μm. The structures obtained are thermally stable up to annealing temperatures of 800 °C. A full in situ processing of gain-coupled distributed feedback laser structures was for the first time successfully demonstrated. This process includes molecular beam epitaxy, grating patterning by focused ion beam, and epitaxial overgrowth. All processed lasers show single mode emission at all operating conditions as expected for gain-coupled lasers.

Sculpting on polymers using focused ion beam

Localized Ga implantation by means of a focused ion beam and subsequent deep reactive ion etching is used to fabricate microstructures or nanostructures in Si. It is found that the area irradiated with the focused ion beam acts as an etch stop in reactive ion etching. The etch rate ratio between the unimplanted and Ga-implanted area increases to 2.56 with an increasing Ga ion dose up to 1.59 × 1016 ions cm−2. However, the etch rate ratio decreases with further increase in ion dose and the surface of the etched Si microstructures is concave due to the effect of ion sputtering during focused ion irradiation. This technique is promising for the fabrication of 3D nanostructures in Si.