Ion beam

Last updated August 27, 2024

An ion beam is a beam of ions, a type of charged particle beam. Ion beams have many uses in electronics manufacturing (principally ion implantation) and other industries. There are many ion beam sources, some derived from the mercury vapor thrusters developed by NASA in the 1960s. The most widely used ion beams are of singly-charged ions.

Units

Ion current density is typically measured in mA/cm², and ion energy in electronvolts (eV). The use of eV is convenient for converting between voltage and energy, especially when dealing with singly charged ion beams.^[1]

Broad-beam ion sources

Most commercial applications use two popular types of ion source, gridded and gridless, which differ in current and power characteristics and the ability to control ion trajectories.^[1] In both cases electrons are needed to generate an ion beam. The most common types of electron emitter are hot filament and hollow cathode.

Gridded ion source

In a gridded ion source, DC or RF discharge are used to generate ions, which are then accelerated and decimated using grids and apertures. Here, the DC discharge current or the RF discharge power are used to control the beam current.

The ion current density $j$ that can be accelerated using a gridded ion source is limited by the space charge effect, which is described by Child's law: $j\approx {\frac {4\epsilon _{0}}{9}}{\sqrt {\frac {2e}{m}}}{\frac {(\Delta V)^{\frac {3}{2}}}{d^{2}}},$ where $\Delta V$ is the voltage between the grids, $d$ is the distance between the grids, and $m$ is the ion mass.

The grids are spaced as closely as possible to increase the current density, typically $d\sim 1\ \mathrm {mm}$ . The ions used have a significant impact on the maximum ion beam current, since $j\propto m^{-{1}/{2}}$ . All else being equal, the maximum ion beam current with krypton is only 69% of the maximum ion current of an argon beam; with xenon the ratio drops to 55%.^[1]

Gridless ion sources

In a gridless ion source, ions are generated by a flow of electrons, without grids. The most common gridless ion source is the end-Hall ion source, with which the discharge current and the gas flow are used to control the beam current.

Applications

Ion beam etching or sputtering

One type of ion beam source is the duoplasmatron. Ion beams can be used for sputtering or ion beam etching and for ion beam analysis.

Ion beam application, etching, or sputtering, is a technique conceptually similar to sandblasting, but using individual atoms in an ion beam to ablate a target. Reactive ion etching is an important extension that uses chemical reactivity to enhance the physical sputtering effect.

In a typical use in semiconductor manufacturing, a mask can selectively expose a layer of photoresist on a substrate made of a semiconductor material, such as a silicon dioxide or gallium arsenide wafer. The wafer is developed, and for a positive photoresist, the exposed portions are removed in a chemical process. The result is a pattern left on the surface areas of the wafer that had been masked from exposure. The wafer is then placed in a vacuum chamber, and exposed to the ion beam. The impact of the ions erodes the target, abrading away the areas not covered by the photoresist.

Focused ion beam (FIB) instruments have numerous applications for characterization of thin-film devices. Using a focused, high-brightness ion beam in a scanned raster pattern, material is removed (sputtered) in precise rectilinear patterns revealing a two-dimensional, or stratigraphic profile of a solid material. The most common application is to verify the integrity of the gate oxide layer in a CMOS transistor. A single excavation site exposes a cross section for analysis using a scanning electron microscope. Dual excavations on either side of a thin lamella bridge are utilized for preparing transmission electron microscope samples.^[2]

Another common use of FIB instruments is for design verification and/or failure analysis of semiconductor devices. Design verification combines selective material removal with gas-assisted material deposition of conductive, dielectric, or insulating materials. Engineering prototype devices may be modified using the ion beam in combination with gas-assisted material deposition in order to rewire an integrated circuit's conductive pathways. The techniques are effectively used to verify the correlation between the CAD design and the actual functional prototype circuit, thereby avoiding the creation of a new mask for the purpose of testing design changes.

Materials science use sputtering for extending surface analytical techniques such as secondary ion mass spectrometry or electron spectroscopy (XPS, AES) so that they can depth profile them.

Biology

In radiobiology a broad or focused ion beam is used to study mechanisms of inter- and intra- cellular communication, signal transduction and DNA damage and repair.

Medicine

Ion beams are also used in particle therapy, most often in the treatment of cancer.

Space applications

Ion beams produced by ion and plasma thrusters on board a spacecraft can be used to transmit a force to a nearby object (e.g. another spacecraft, an asteroid, etc.) that is irradiated by the beam. This innovative propulsion technique named Ion Beam Shepherd has been shown to be effective in the area of active space debris removal as well as asteroid deflection.

High-energy ion beams

High-energy ion beams produced by particle accelerators are used in atomic physics, nuclear physics and particle physics.

As weapon

Ion beams can theoretically be used to make a weapon, but this has not been demonstrated. Electron beam weapons were tested by the U.S. Navy in the early 20th century^{[ citation needed ]}, but the hose instability effect prevents them from being accurate at a distance of over approximately 30 inches.

Related Research Articles

Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fabrication and in metal finishing, as well as in materials science research. The ions can alter the elemental composition of the target if they stop and remain in the target. Ion implantation also causes chemical and physical changes when the ions impinge on the target at high energy. The crystal structure of the target can be damaged or even destroyed by the energetic collision cascades, and ions of sufficiently high energy can cause nuclear transmutation.

MEMS is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size, and MEMS devices generally range in size from 20 micrometres to a millimetre, although components arranged in arrays can be more than 1000 mm². They usually consist of a central unit that processes data and several components that interact with the surroundings.

Photolithography is a process used in the manufacturing of integrated circuits. It involves using light to transfer a pattern onto a substrate, typically a silicon wafer.

A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronics industry.

Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuits (ICs) such as computer processors, microcontrollers, and memory chips. It is a multiple-step photolithographic and physico-chemical process during which electronic circuits are gradually created on a wafer, typically made of pure single-crystal semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.

In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and can be an unwelcome source of wear in precision components. However, the fact that it can be made to act on extremely fine layers of material is utilised in science and industry—there, it is used to perform precise etching, carry out analytical techniques, and deposit thin film layers in the manufacture of optical coatings, semiconductor devices and nanotechnology products. It is a physical vapor deposition technique.

Time of flight (ToF) is the measurement of the time taken by an object, particle or wave to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a way to learn about the particle or medium's properties. The traveling object may be detected directly or indirectly. Time of flight technology has found valuable applications in the monitoring and characterization of material and biomaterials, hydrogels included.

Electron cyclotron resonance (ECR) is a phenomenon observed in plasma physics, condensed matter physics, and accelerator physics. It happens when the frequency of incident radiation coincides with the natural frequency of rotation of electrons in magnetic fields. A free electron in a static and uniform magnetic field will move in a circle due to the Lorentz force. The circular motion may be superimposed with a uniform axial motion, resulting in a helix, or with a uniform motion perpendicular to the field resulting in a cycloid. The angular frequency of this cyclotron motion for a given magnetic field strength B is given by

Dry etching refers to the removal of material, typically a masked pattern of semiconductor material, by exposing the material to a bombardment of ions that dislodge portions of the material from the exposed surface. A common type of dry etching is reactive-ion etching. Unlike with many of the wet chemical etchants used in wet etching, the dry etching process typically etches directionally or anisotropically.

The gridded ion thruster is a common design for ion thrusters, a highly efficient low-thrust spacecraft propulsion method running on electrical power by using high-voltage grid electrodes to accelerate ions with electrostatic forces.

Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures on various materials.

<span class="mw-page-title-main">Focused ion beam</span> Device

Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a scientific instrument that resembles a scanning electron microscope (SEM). However, while the SEM uses a focused beam of electrons to image the sample in the chamber, a FIB setup uses a focused beam of ions instead. FIB can also be incorporated in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams. FIB should not be confused with using a beam of focused ions for direct write lithography. These are generally quite different systems where the material is modified by other mechanisms.

Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral. During the process, the plasma generates volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target.

Neutral-beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. Neutral-beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices. To date, all NBI systems were based on positive precursor ion beams. In the 1990s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative-ion-based NBI systems at LHD (H⁰, 180 keV) and JT-60U (D⁰, 500 keV). The NBI designed for ITER is a substantial challenge (D⁰, 1 MeV, 40 A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), ion cyclotron resonance heating (ICRH), and lower hybrid resonance heating (LH).

In semiconductor electronics fabrication technology, a self-aligned gate is a transistor manufacturing approach whereby the gate electrode of a MOSFET is used as a mask for the doping of the source and drain regions. This technique ensures that the gate is naturally and precisely aligned to the edges of the source and drain.

Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module in fabrication, and every wafer undergoes many etching steps before it is complete.

<span class="mw-page-title-main">Sputter deposition</span> Method of thin film application

Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV. The sputtered ions can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber. Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film.

<span class="mw-page-title-main">Plasma (physics)</span> State of matter

Plasma is one of four fundamental states of matter characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly in stars, but also dominating the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.

Ion milling is a specialized physical etching technique that is a crucial step in the preparation of material analysis techniques. After a specimen goes through ion milling, the surface becomes much smoother and more defined, which allows scientists to study the material much easier. The ion mill generates high-energy particles to remove material off the surface of a specimen, similar to how sand and dust particles wear away at rocks in a canyon to create a smooth surface. Relative to other techniques, ion milling creates much less surface damage, which makes it perfect for surface-sensitive analytical techniques. This article discusses the principle, equipment, applications, and significance of ion milling.

Glossary of microelectronics manufacturing terms

References

1 2 3 Harold R. Kaufman and the staff of Kaufman & Robinson Inc. (2011). Applications of Broad-Beam Ion Sources: An Introduction (PDF). Fort Collins, Colorado: Kaufman & Robinson, Inc. ISBN 978-0-9852664-0-0.
↑ Giannuzzi, Lucille A., Stevie, Fred A. Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques, and Practice, Springer 2005 – 357 pages

External links

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[KRI-1] 1 2 3 Harold R. Kaufman and the staff of Kaufman & Robinson Inc. (2011). Applications of Broad-Beam Ion Sources: An Introduction (PDF). Fort Collins, Colorado: Kaufman & Robinson, Inc. ISBN 978-0-9852664-0-0.

[2] Giannuzzi, Lucille A., Stevie, Fred A. Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques, and Practice, Springer 2005 – 357 pages

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