The focused ion beam (FIB) is a powerful tool for fabrication, modification, and characterization of materials down to the nanoscale. Starting with the gallium FIB, which was originally intended for photomask repair in the semiconductor industry, there are now many different types of FIB that are commercially available. These instruments use a range of ion species and are applied broadly in materials science, physics, chemistry, biology, medicine, and even archaeology. The goal of this roadmap is to provide an overview of FIB instrumentation, theory, techniques, and applications. By viewing FIB developments through the lens of various research communities, we aim to identify future pathways for ion source and instrumentation development, as well as emerging applications and opportunities for improved understanding of the complex interplay of ion–solid interactions. We intend to provide a guide for all scientists in the field that identifies common research interest and will support future fruitful interactions connecting tool development, experiment, and theory. While a comprehensive overview of the field is sought, it is not possible to cover all research related to FIB technologies in detail. We give examples of specific projects within the broader context, referencing original works and previous review articles throughout.

AES

Auger electron spectroscopy

AFM

Atomic force microscopy

API

Application programming interface

APT

Atom probe tomography

ASIS

Atomic-size ion source

BCA

Binary collision approximation

BI

Backscattered ion

CAD

Computer-aided design

CMNT

Colloidal micro Newton thrusters

CMOS

Complementary metal–oxide–semiconductor

CNT

Carbon nanotube

COMB

Charge optimized many body

DAC

Digital-to-analog converter

DC

Direct current

DFT

Density functional theory

EAM

Embedded atom method

EBIT

Electron beam ion trap

EBL

Electron beam lithography

EBSD

Electron backscatter diffraction

ECR

Electron cyclotron resonance

EDS

Energy-dispersive X-ray spectroscopy

ESI

Electrospray ionization source

EELS

Electron energy loss spectroscopy

ESEM

Environmental scanning electron microscopy

ETD

Everhardt–Thornley detector

EUV

Extreme ultraviolet

ESA

Excited surface atom

FEB

Focused electron beam

FEBID

Focused electron beam induced deposition

FEBIE

Focused electron beam induced etching

FEEP

Field emission electric propulsion

FIB

Focused ion beam

FIBID

Focused ion beam induced deposition

FIBIE

Focused ion beam induced etching

FIM

Field ion microscopy

FinFET

Fin field-effect transistor

FMR

Ferromagnetic resonance

FOV

Field of view

GAP

Gaussian approximation potential

GUI

Graphical user interface

FWHM

Full width at half maximum

GFIS

Gas field-ionization source

GIS

Gas injection system

hBN

Hexagonal boron nitride

HIBL

Helium ion beam lithography

HIM

Helium ion microscope

HRTEM

High resolution transmission electron microscopy

HSQ

Hydrogen silsesquioxane

IBIC

Ion beam induced charge

IC

Integrated circuit

ICD

Image charge detector

ICP

Inductively coupled plasma

IIAES

Ion induced Auger electron spectroscopy

IL

Ionoluminescence

ILIS

Ionic liquid ion source

kMC

Kinetic Monte Carlo

LE-FIB

Low energy focused ion beam

LJ

Lennard-Jones-type

LMAIS

Liquid metal alloy ion source

LMIS

Liquid metal ion source

LoTIS

Low temperature ion source

MC

Monte Carlo

MCP

Micro channel plate

MD

Molecular dynamics

MFM

Magnetic force microscopy

ML

Machine learning

MS

Molecular statics

MEMS

Micro-electro-mechanical systems

MOTIS

Magneto-optical trap ion source

MRAM

Magnetic random access memory

NAIS

Nano-aperture ion source

NEMS

Nano-electro-mechanical systems

NIL

Nanoimprint lithography

NSOM

Near-field optical microscopy

NV

Nitrogen vacancy

PEMFC

Proton exchange membrane fuel cell

PFIB

Plasma focused ion beam

PI

Primary ion

PIXE

Particle induced X-ray emission

PMMA

Poly(methyl methacrylate)

QMS

Quadrupole mass spectrometer

RBS

Rutherford backscattering spectrometry

SDD

Silicon drift detector

SE

Secondary electron

SEM

Scanning electron microscopy

SFIM

Scanning field ion microscope

SI

Secondary ion

SII

Single ion implantation

SIMS

Secondary ion mass spectrometry

SNMS

Secondary neutral mass spectrometry

SNR

Signal to noise ratio

SPE

Single photon emitter

SPM

Scanning probe microscopy

SSPD

Superconducting single-photon detector

STEM

Scanning transmission electron microscopy

STIM

Scanning transmission ion microscopy

SQUID

Superconducting quantum interference device

TDDFT

Time-dependent density functional theory

TEM

Transmission electron microscopy

TIC

Total ion counter

TOF

Time-of-flight

UHV

Ultra-high vacuum

YBCO

Yttrium barium copper oxide

YIG

Yttrium iron garnet

YVO

Yttrium orthovanadate

YSZ

Yttrium stabilized zirconia

ZBL

Ziegler–Biersack–Littmark

ZPL

Zero-phonon line

The technological origin of the focused ion beam (FIB) instruments we use today lies in outer space, or more precisely, in the application of ion beams for spacecraft propulsion. In space, thrust can only be generated by ejecting matter, the so-called reaction mass, which must be carried along with the spacecraft. In addition to chemical thrusters based on combustion, ion thrusters have emerged as an important tool for high-precision movement. The positively charged ions that are generated by field ionization or by electrospraying are accelerated by electric or magnetic fields and then neutralized before being ejected in the opposite direction to that of the intended motion. (Neutralization is important here because otherwise the spacecraft would accumulate negative charge and thus attract the ejected positive ions.) Different types of thrusters based on liquid metal ion sources are currently being tested in space for ultra-precise position control of satellites, e.g., for the LISA gravitational wave interferometer.1 One of these thruster technologies, the electric field emission propulsion system (part of the LISA Pathfinder mission2 and more recent CubeSat launches3) is in fact very similar to the heart of many of our ground-based FIB instruments.

Whereas ion thrusters enable the exertion of forces in the micronewton range for the navigation of space objects, our ground-based FIB instruments enable fabrication, modification, and characterization of micrometer- to nanometer-sized objects.

The leading example of FIB processing is still the site-selective preparation of samples for high-resolution imaging techniques, in particular, for transmission electron microscopy (TEM) and atom probe tomography (APT) and for 3D volume imaging using scanning electron microscopy (SEM). Several reviews have already been devoted to these important, well-established applications.4–6 

However, since focused ion beams can be used to modify any material down to the nanoscale in a variety of ways, from targeted doping to structural modification and geometric shaping, the FIB is a powerful tool in all areas from basic research to technology. Our focus is, therefore, on these novel and advanced applications of the FIB.

This document presents the state of the art of FIB research and development today and discusses future perspectives. It is organized as follows: Sec. II gives an overview of FIB instrumentation, starting with the generation and control of the focused ion beam, followed by detectors and other complementary tools and accessories. Section III summarizes the theoretical approaches that can be used to describe various aspects of ion–matter interactions, including the binary collision approximation (BCA), molecular dynamics (MD), kinetic Monte Carlo (kMC) techniques, density functional theory (DFT), and continuum modeling. The last of these enables modeling over the longest length- and timescales, including treatment of ion- and electron-induced surface chemistry. The wide range of applications of the FIB is discussed in Sec. IV, which is organized according to the various experimental techniques (subtractive processing, defect engineering, imaging and tomography, elemental analysis, gas-assisted processing, and several other emerging directions). For each application field, a selection of examples is discussed. As a service to the community, summary tables with references have been compiled in order to provide a more comprehensive (albeit non-exhaustive) literature review of each of these application fields. These tables can be used as a starting point to help the reader identify new FIB opportunities for their own research.

Central to this document is the roadmap in Sec. V, which highlights future perspectives for FIB research and development based on the state of the art in instrumentation, theory, and applications previously described. Here, key drivers for the FIB in different areas of science and technology have been identified and linked to FIB-specific challenges and the steps needed for future developments. We hope that this roadmap can serve as an incubator for future developments and will provide inspiration for scientific and technological breakthroughs, as well as serve as a unique resource for funding agencies and industry.

The creation of a finely focused beam of ions presents several engineering challenges. Moreover, the beam requirements can vary widely depending on the application, sometimes with conflicting specifications. For example, for many applications, a high-current beam is desired to enable efficient milling (i.e., material removal by sputtering). However, high-current sources tend to deliver ions with a large energy spread, resulting in strong chromatic aberrations. Because of this (and other reasons, such as spherical aberrations from the lenses, as discussed later), it is challenging to build a high-current high-resolution source. Similar conflicting scenarios present themselves in many other areas of focused ion beam (FIB) instrumentation.

In the following, the various components of the FIB instrument are discussed. We start by describing the different ion sources that are used (Sec. II A). The source defines many properties of the final instrument, including the achievable spot size. Then we address beam transport via the ion optical column (Sec. II B), and subsequently detectors and analytics (Sec. II C); these elements, which are used to steer, shape, and detect the ions, must be designed in such a way as to ensure the best possible end performance of the particular source being used. In the sample chamber, there are several other components that can be incorporated, including specialized sample stages for in situ or in operando experiments, micro-/nanomanipulators, and gas injectors. These are discussed in Sec. II D. Next, we address considerations concerning experiments with radiaoactive samples (Sec. II E). Finally, we outline software needs and correlative approaches for beam control, automation, and multi-modal analysis (Sec. II F). An overview of these topics can also be found in the book chapter by Note.7 

In order to achieve high spatial resolution in a FIB instrument, an ion source with high brightness is required.8 Analogous to the definition in electron microscopy, the brightness B of an ion source is a measure of the compactness and directionality of the ion beam, according to
B = I A Ω ,
(1)
where the emission current I from a source area A is emitted into a solid angle Ω.9,10 In practice, this means that ions are suitable for microscopy and nanofabrication if they are emitted from a highly localized area into a well-defined direction. The reduced brightness is derived from the above definition by taking into account the acceleration voltages of the FIB instrument; representative values for the different ion sources are given in Table I. In general, a smaller (virtual) source size means a higher brightness and, consequently, a higher achievable lateral resolution. However, for the final resolution of the instrument, additional parameters such as the extractable ion current, energy spread, and of course the performance of the ion optics are also of importance.
TABLE I.

FIB sources compared according to key parameters. Only common ion species are listed. All published or commercially available ion species can be found in Fig. 1. Superscript q marks different charge states (from 1 to 3) obtained from LMIS/LMAIS; for plasma sources even higher charge states are obtained. Subscript n for LMIS/LMAIS indicates the possible emission of polyatomic clusters comprising up to ten ions (or even more). Parameters for ionic liquid ion source (ILIS) indicated by * are estimated values from Ref. 11. For better comparison, the lateral spatial resolutions have been converted to FWHM values assuming a Gaussian beam profile resulting in an error-function-like edge profile.

Source Common ion species Reduced brightness (A m−2 sr−1 V−1) Min. energy spread (eV) Max. current (nA) Lateral resolution FWHM (nm) Key applications
LMIS/LMAIS  (Li, Si, Ga, Ge, In, Sn, Sb, Au, Pb, Bi, …) 1. . n q +  1 × 10 6 (Ref. 12 2–40 (Refs. 12 and 13 100  2–2.8 (Refs. 14 and 15 Surface patterning and cross-sectioning, volume imaging, local doping and implantation, SIMS, mask edit, ion thruster 
ILIS  EMI+, BMI+, BF 4 , I, …, cluster ions  5 × 10 6* (Ref. 11 7–10 (Refs. 16 and 17 0.75* (Ref. 11 50*–30 000 (Ref. 11 Ion thruster, reactive ion etching, SIMS 
PFIB  (Xe, Ar, Kr, N, O …)q+  1 10 × 10 3 (Ref. 18 7–10 (Ref. 18 2500 (Ref. 19 20–100 (Refs. 18–20 High rate sputtering, volume imaging, implantation of gaseous elements 
GFIS  (He, Ne)+  1 4 × 10 9 (Refs. 21–23 0.25–1 (Refs. 21 and 24 0.15 (Ref. 23 0.5/4.0 (Refs. 25–27 High resolution imaging and nanostructuring, mask repair, implantation of gaseous elements 
LoTIS/MOTIS  (Li, Cs, Cr, Rb)+  1 240 × 10 5 (Ref. 28 0.2–2 (Refs. 29 and 30 25 (Ref. 31 4.9 (Ref. 28 SIMS, implantation 
NAIS  Hq+, Arq+  4 × 10 5 (Ref. 32 0.9–2.3 (Ref. 33 0.2–20 (Ref. 32 2000 (Ref. 32 SEM as an ion source, proton beam writing 
Source Common ion species Reduced brightness (A m−2 sr−1 V−1) Min. energy spread (eV) Max. current (nA) Lateral resolution FWHM (nm) Key applications
LMIS/LMAIS  (Li, Si, Ga, Ge, In, Sn, Sb, Au, Pb, Bi, …) 1. . n q +  1 × 10 6 (Ref. 12 2–40 (Refs. 12 and 13 100  2–2.8 (Refs. 14 and 15 Surface patterning and cross-sectioning, volume imaging, local doping and implantation, SIMS, mask edit, ion thruster 
ILIS  EMI+, BMI+, BF 4 , I, …, cluster ions  5 × 10 6* (Ref. 11 7–10 (Refs. 16 and 17 0.75* (Ref. 11 50*–30 000 (Ref. 11 Ion thruster, reactive ion etching, SIMS 
PFIB  (Xe, Ar, Kr, N, O …)q+  1 10 × 10 3 (Ref. 18 7–10 (Ref. 18 2500 (Ref. 19 20–100 (Refs. 18–20 High rate sputtering, volume imaging, implantation of gaseous elements 
GFIS  (He, Ne)+  1 4 × 10 9 (Refs. 21–23 0.25–1 (Refs. 21 and 24 0.15 (Ref. 23 0.5/4.0 (Refs. 25–27 High resolution imaging and nanostructuring, mask repair, implantation of gaseous elements 
LoTIS/MOTIS  (Li, Cs, Cr, Rb)+  1 240 × 10 5 (Ref. 28 0.2–2 (Refs. 29 and 30 25 (Ref. 31 4.9 (Ref. 28 SIMS, implantation 
NAIS  Hq+, Arq+  4 × 10 5 (Ref. 32 0.9–2.3 (Ref. 33 0.2–20 (Ref. 32 2000 (Ref. 32 SEM as an ion source, proton beam writing 

In the field of ion sources, the first breakthroughs came in the 1970s with the development of the liquid metal ion source (LMIS) and the gas field-ionization source (GFIS). While the latter evolved from field ion microscopy (FIM) dating back to the 1950s34 and was first employed for scanning transmission ion microscopy (STIM) of biological specimens,35,36 the LMIS was originally developed for space thruster applications.37–39 

The LMIS consists of a capillary or a sharp needle tip, wetted with a molten metal. Through a combination of surface tension and applied electric field, the so-called Taylor cone40 is formed, resulting in the extraction of ions from the source apex by field evaporation. The most common form of LMIS is based on gallium due to its low melting temperature and low vapor pressure. In fact, the Ga-LMIS is still the most used source for FIB instruments, not only because of its high brightness, but also because of its high source stability.

Directly related to the LMIS is the liquid metal alloy ion source (LMAIS), which through the use of a variety of alloys enables access to a much wider range of ion species,12,15,41–49 but with strongly varying source lifetimes from minutes to months. Based on the same source principle, the ionic liquid ion source (ILIS) is wetted with a compound that dissociates into molecular anions and cations such as the ionic liquids C6N2H11-BF4 (EMI-BF4), C6N2H11-GaCl4 (EMI-GaCl4), or C8N2H15-I (BMI-I).16,50–55 The ILIS is, thus, capable of producing beams of molecular ions, both positively and negatively charged, but has not yet been implemented commercially.

The GFIS operates at low temperature, producing ions from the field ionization of adsorbed gas atoms, and is characterized by the highest brightness and hence the highest deliverable spatial resolution of all sources developed to date.21,25,56 This is a consequence of the atomically sharp emitter, which consists of only three atoms at its tip, the so-called trimer, whereby each atom emits a beamlet and thus forms a virtual source with a size in the Angstrom range. The trimer must be formed by the user and has a typical lifetime between a few days and a few weeks. The source gases that are widely used for high-resolution imaging and high-resolution (metal-free) milling are He and Ne. Operation of the GFIS with H2,57 N2,58,59 Xe,60 and Kr61 has also been demonstrated. A closely related source technology is the atomic-size ion source (ASIS) source,62,63 which uses field emission of adatoms (Au and Ag have been demonstrated) deposited onto a tip made from refractory metal or another inert material with a high field ionization strength. High resolution is to be expected due to the single atomic ionization site employed in this technology. However, among other issues, it is the limited lifetime of the ASIS which has hindered its application in actual FIB instruments so far.

Another source option offering nonmetallic ions is the plasma ion source of the so-called plasma focused ion beam (PFIB) instrument. Plasma ion sources achieve high currents of up to 2 μA and have a long lifetime, but lower brightness than LMIS sources. The plasma can be generated by electron impacts, as in a duoplasmatron,18,64 by inductive coupling of alternating currents in a radio frequency antenna [an inductively coupled plasma (ICP)],65 or by microwaves in an electron cyclotron resonance (ECR) ion source.66 

In addition to the systems mentioned above, most of which are commercially available, there are other less common types of ion source. One example is the so-called cold atom ion source, of which there are two varieties: the magneto-optical trap ion source (MOTIS) and the low temperature ion source (LoTIS). These use magneto-optical trapping (in combination with laser-cooling in the case of the LoTIS) to generate a trapped cloud of atoms or an intense atomic beam with a (transverse) temperature in the microkelvin regime.31,67–72 The atoms in the trap or beam are then field- or photoionized to produce an isotopically pure, singly charged ion beam of high brightness and low energy spread.29,30,73–75 MOTIS