Patterning with a focused ion beam (FIB) is an extremely versatile fabrication process that can be used to create microscale and nanoscale designs on the surface of practically any solid sample material. Based on the type of ion-sample interaction utilized, FIB-based manufacturing can be both subtractive and additive, even in the same processing step. Indeed, the capability of easily creating three-dimensional patterns and shaping objects by milling and deposition is probably the most recognized feature of ion beam lithography (IBL) and micromachining. However, there exist several other techniques, such as ion implantation- and ion damage-based patterning and surface functionalization types of processes that have emerged as valuable additions to the nanofabrication toolkit and that are less widely known. While fabrication throughput, in general, is arguably low due to the serial nature of the direct-writing process, speed is not necessarily a problem in these IBL applications that work with small ion doses. Here we provide a comprehensive review of ion beam lithography in general and a practical guide to the individual IBL techniques developed to date. Special attention is given to applications in nanofabrication.

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Direct-Write Ion Beam Lithography

Focused ion beam (FIB) milling is widely used in fields such as the semiconductor industry and materials science research. The direct writing and small feature size also make FIB milling attractive for rapid prototyping of novel photonic structures. In this manuscript, we describe in detail a FIB milling procedure which enables high-resolution fabrication of complex micro- and nanostructures with precise geometry control. Two different procedures (for 2D and 3D structures) are described and implemented on the tip of a glass optical fiber for fabricating diverse structures embedded on or below the tip surface. The procedures described here can be easily adjusted and implemented on any conductive or non-conductive substrate.

Focused ion beam milling for prototyping 2D and 3D photonic structures

A hologram is a recording of the interference between an unknown object wave and a coherent reference wave. Providing the object and reference waves are sufficiently separated in some region of space and the reference beam is known, a high-fidelity reconstruction of the object wave is possible. In traditional optical holography, high-quality reconstruction is achieved by careful reillumination of the holographic plate with the exact same reference wave that was used at the recording stage. To reconstruct high-quality digital holograms the exact parameters of the reference wave must be known mathematically. This paper discusses a technique that obtains the mathematical parameters that characterize a strongly divergent reference wave that originates from a fiber source in a new compact digital holographic camera. This is a lensless design that is similar in principle to a Fourier hologram, but because of the large numerical aperture, the usual paraxial approximations cannot be applied and the Fourier relationship is inexact. To characterize the reference wave, recordings of quasi-planar object waves are made at various angles of incidence using a Dammann grating. An optimization process is then used to find the reference wave that reconstructs a stigmatic image of the object wave regardless of the angle of incidence.

Characterization of the reference wave in a compact digital holographic camera.

Focused ion beam milling of optical fiber sensors is a very recent field that first started with the milling of apertures on fiber tips, but it is the assessment of the viability and potential of applying this technology to the creation of optical fiber sensors that is the main goal of this thesis. To achieve this, an initial round-up of instances where focused ion beam was applied to optical fibers, whether as a mere investigative tool or as the core technique of a fabrication procedure, was performed. This allowed a global view of what had been done before and what could still be done and how. In the recent past, focused ion beam has been successfully used to fabricate structures on optical fibers but the question if the technology had further potential or was simply a publication gimmick, remained. This work shows that focused ion beam has a promising future in creating optical fiber structures with a quality and resolution that cannot be replicated with other techniques. Detailed procedures for preparing optical fibers for focused ion beam milling are described throughout the thesis. Microwire fabrication and dynamic chemical etching of tapered fiber tips are just two of the techniques used to prepare intermediate fiber structures for efficient milling. Taking advantage of suspended microwires, focused ion beam milling was used to cleave said microwires and create a suspended microwire cantilever structure, on a fiber end, for vibration sensing. Such a cleave, due to the abnormal configuration of the microwire structure, is easily achieved with focused ion beam but would be a challenge using conventional techniques. Further, the milling of Fabry-Perot microcavities on tapered fiber tips came as a need from the field of optogenetics and neural signal recording. Microcavity probes capable of sensing temperature and refractive index simultaneously were milled and characterized. These probes are currently being tested for optical recording of neural activity in rat brains. Finally, a push towards even smaller details was achieved with the fabrication of fiber Bragg gratings on tapered fiber tips. Such geometric, corrugation-based gratings allow for high-temperature operation in extreme environments and their fabrication led to the limit of the focused ion beam system available in our lab.

Focused ion beam milling of optical fiber microstructures for sensing applications

Facile synthesis, structural, optical and optoelectrical characterizations of promising novel InSbS3 thin films for photovoltaic applications

Fiber Bragg gratings and long-period fiber gratings are widely used in optical communication systems and sensing applications. These structures are fabricated by UV-induced index changes along the core of a fiber, with fine control over the grating period and refractive index profile. Further flexibility and control over these structures can be achieved with focused ion beam (FIB) techniques, by fabricating fiber gratings through nano- and micro-machining. These FIB techniques enable the fabrication of gratings with larger index contrast and stronger interaction with environmental samples (ideal for chemical sensing applications). The sub-micron milling of the fibers allows modulating the refractive index with air voids, which can be filled with the sample fluid to be monitored. In this work we explore the design and fabrication of novel fiber grating configurations for sensing applications, inscribed on single-mode fibers using FIB. A single Fabry-Perot cavity is fabricated by milling a hole through the fiber (cladding and core), as well as a long-period grating based on a cascade of partially-etched-through cavities.

Fabrication of optical fiber gratings through focused ion beam techniques for sensing applications

In the initial step of the current work’s scenarios, the AlxSe70Te30-x (x = 0, 15, and 30 wt.%Al) chalcogenide glasses were synthesized. The alloy-derived thin films with a thickness (∼200 nm) were prepared. The optical spectra (transmittance T(λ) and reflectance R(λ)) were measured in the range of (300–2500 nm). Calculation of the optical parameters included determining the energy bandgap, Eg and tail energy, Ee. The results revealed a decrease in the bandgap energy of the direct transition and an increase in the tail energy with increasing Al-content. The X-ray dispersive analysis (EDX) detected the presence of the elemental components in the three studied systems. As well, by analyzing X-ray diffraction patterns (XRD) and s canning electron microscopy (SEM), the amorphous state of the films was confirmed. In addition, differential scanning calorimetry (DSC) at a heating rate of 5 K/min was used to determine the pre-crystallization and crystallization parameters. For the study of DC conductivity, the temperat ure dependence of sheet resistance was measured and its related parameters were extracted. While, the study of AC conductivity began by measuring the temperature dependence of capacitance and conductance at 5 kHz. Electrical conductivity has been studied in a temperature range of (300–500 K). It was found that there are two types of conduction pathways. The activation energies, Mott parameters, barrier potential energy, and density of localized states around the Fermi level were all evaluated using Mott's variable range hopping model as a benchmark. The dielectric constant, loss factor, complicated electrical modules, and impedance spectroscopy parameters were all shown to be substantially dependent on temperature and Al-content. 

Extended study into the prominence of aluminum content in controlling optical parameters, thermal properties, electrical conductivity and dielectric behavior of amorphous Al-Se-Te thin films for optoelectronic applications

Zinc-induced changes on structural pathways, optical parameters, optical constants extracted by Kramers-Kronig Formulas, Photoluminescence Spectra and photovoltaic characteristics of n-Cd50Zn S50/i-AgSe/p-Si solar cells

A low-cost Rayleigh backscattering sensing configuration is used to obtain high-resolution and real-time 2D maps of the directional surface strain on a fiberglass board. This approach has potential applications in 2D/3D structural health monitoring.

Ammar Alqahtani

Papers

Fabrication of optical fiber gratings through focused ion beam techniques for sensing applications

Extended study into the prominence of aluminum content in controlling optical parameters, thermal properties, electrical conductivity and dielectric behavior of amorphous Al-Se-Te thin films for optoelectronic applications

Zinc-induced changes on structural pathways, optical parameters, optical constants extracted by Kramers-Kronig Formulas, Photoluminescence Spectra and photovoltaic characteristics of n-Cd50Zn S50/i-AgSe/p-Si solar cells

2D directional surface strain mapping through distributed optical fiber sensors