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 the ions differ in composition from 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 (10s of MeV) can cause nuclear transmutation.
Ion implantation equipment typically consists of an ion source, where ions of the desired element are produced, an accelerator, where the ions are electrostatically accelerated to a high energy, and a target chamber, where the ions impinge on a target, which is the material to be implanted. Thus ion implantation is a special case of particle radiation. Each ion is typically a single atom or molecule, and thus the actual amount of material implanted in the target is the integral over time of the ion current. This amount is called the dose. The currents supplied by implants are typically small (micro-amperes), and thus the dose which can be implanted in a reasonable amount of time is small. Therefore, ion implantation finds application in cases where the amount of chemical change required is small.
Typical ion energies are in the range of 10 to 500 keV (1,600 to 80,000 aJ). Energies in the range 1 to 10 keV (160 to 1,600 aJ) can be used, but result in a penetration of only a few nanometers or less. Energies lower than this result in very little damage to the target, and fall under the designation ion beam deposition. Higher energies can also be used: accelerators capable of 5 MeV (800,000 aJ) are common. However, there is often great structural damage to the target, and because the depth distribution is broad (Bragg peak), the net composition change at any point in the target will be small.
The energy of the ions, as well as the ion species and the composition of the target determine the depth of penetration of the ions in the solid: A monoenergetic ion beam will generally have a broad depth distribution. The average penetration depth is called the range of the ions. Under typical circumstances ion ranges will be between 10 nanometers and 1 micrometer. Thus, ion implantation is especially useful in cases where the chemical or structural change is desired to be near the surface of the target. Ions gradually lose their energy as they travel through the solid, both from occasional collisions with target atoms (which cause abrupt energy transfers) and from a mild drag from overlap of electron orbitals, which is a continuous process. The loss of ion energy in the target is called stopping and can be simulated with the binary collision approximation method.
Accelerator systems for ion implantation are generally classified into medium current (ion beam currents between 10 μA and ~2 mA), high current (ion beam currents up to ~30 mA), high energy (ion energies above 200 keV and up to 10 MeV), and very high dose (efficient implant of dose greater than 1016 ions/cm2).[ citation needed ]
All varieties of ion implantation beamline designs contain certain general groups of functional components (see image). The first major segment of an ion beamline includes a device known as an ion source to generate the ion species. The source is closely coupled to biased electrodes for extraction of the ions into the beamline and most often to some means of selecting a particular ion species for transport into the main accelerator section. The "mass" selection is often accompanied by passage of the extracted ion beam through a magnetic field region with an exit path restricted by blocking apertures, or "slits", that allow only ions with a specific value of the product of mass and velocity/charge to continue down the beamline. If the target surface is larger than the ion beam diameter and a uniform distribution of implanted dose is desired over the target surface, then some combination of beam scanning and wafer motion is used. Finally, the implanted surface is coupled with some method for collecting the accumulated charge of the implanted ions so that the delivered dose can be measured in a continuous fashion and the implant process stopped at the desired dose level.
Semiconductor doping with boron, phosphorus, or arsenic is a common application of ion implantation. When implanted in a semiconductor, each dopant atom can create a charge carrier in the semiconductor after annealing. A hole can be created for a p-type dopant, and an electron for an n-type dopant. This modifies the conductivity of the semiconductor in its vicinity. The technique is used, for example, for adjusting the threshold of a MOSFET.
Ion implantation was developed as a method of producing the p-n junction of photovoltaic devices in the late 1970s and early 1980s,along with the use of pulsed-electron beam for rapid annealing, although it has not to date been used for commercial production.
One prominent method for preparing silicon on insulator (SOI) substrates from conventional silicon substrates is the SIMOX (separation by implantation of oxygen) process, wherein a buried high dose oxygen implant is converted to silicon oxide by a high temperature annealing process.
Mesotaxy is the term for the growth of a crystallographically matching phase underneath the surface of the host crystal (compare to epitaxy, which is the growth of the matching phase on the surface of a substrate). In this process, ions are implanted at a high enough energy and dose into a material to create a layer of a second phase, and the temperature is controlled so that the crystal structure of the target is not destroyed. The crystal orientation of the layer can be engineered to match that of the target, even though the exact crystal structure and lattice constant may be very different. For example, after the implantation of nickel ions into a silicon wafer, a layer of nickel silicide can be grown in which the crystal orientation of the silicide matches that of the silicon.
Nitrogen or other ions can be implanted into a tool steel target (drill bits, for example). The structural change caused by the implantation produces a surface compression in the steel, which prevents crack propagation and thus makes the material more resistant to fracture. The chemical change can also make the tool more resistant to corrosion.
In some applications, for example prosthetic devices such as artificial joints, it is desired to have surfaces very resistant to both chemical corrosion and wear due to friction. Ion implantation is used in such cases to engineer the surfaces of such devices for more reliable performance. As in the case of tool steels, the surface modification caused by ion implantation includes both a surface compression which prevents crack propagation and an alloying of the surface to make it more chemically resistant to corrosion.
Ion implantation can be used to achieve ion beam mixing, i.e. mixing up atoms of different elements at an interface. This may be useful for achieving graded interfaces or strengthening adhesion between layers of immiscible materials.
Ion implantation may be used to induce nano-dimensional particles in oxides such as sapphire and silica. The particles may be formed as a result of precipitation of the ion implanted species, they may be formed as a result of the production of an mixed oxide species that contains both the ion-implanted element and the oxide substrate, and they may be formed as a result of a reduction of the substrate, first reported by Hunt and Hampikian. nm on up to 20 nm and with compositions that can contain the implanted species, combinations of the implanted ion and substrate, or that are comprised solely from the cation associated with the substrate.Typical ion beam energies used to produce nanoparticles range from 50 to 150 keV, with ion fluences that range from 1016 to 1018 ions/cm2. The table below summarizes some of the work that has been done in this field for a sapphire substrate. A wide variety of nanoparticles can be formed, with size ranges from 1
Composite materials based on dielectrics such as sapphire that contain dispersed metal nanoparticles are promising materials for optoelectronics and nonlinear optics.
|Implanted Species||Substrate||Ion Beam Energy (keV)||Fluence (ions/cm2)||Post Implantation Heat Treatment||Result||Source|
|Produces Oxides that Contain the Implanted Ion||Co||Al2O3||65||5*1017||Annealing at 1400 °C||Forms Al2CoO4 spinel|
|Co||α-Al2O3||150||2*1017||Annealing at 1000 °C in oxidizing ambient||Forms Al2CoO4 spinel|
|Mg||Al2O3||150||5*1016||---||Forms MgAl2O4 platelets|
|Sn||α-Al2O3||60||1*1017||Annealing in O2 atmosphere at 1000 °C for 1 hr||30 nm SnO2 nanoparticles form|
|Zn||α-Al2O3||48||1*1017||Annealing in O2 atmosphere at 600 °C||ZnO nanoparticles form|
|Zr||Al2O3||65||5*1017||Annealing at 1400 °C||ZrO2 precipitates form|
|Produces Metallic Nanoparticles from Implanted Species||Ag||α-Al2O3||1500, 2000||2*1016, 8*1016||Annealing from 600 °C to 1100 °C in oxidizing, reducing, Ar or N2 atmospheres||Ag nanoparticles in Al2O3 matrix|
|Au||α-Al2O3||160||0.6*1017, 1*1016||1 hr at 800 °C in air||Au nanoparticles in Al2O3 matrix|
|Au||α-Al2O3||1500, 2000||2*1016, 8*1016||Annealing from 600 °C to 1100 °C in oxidizing, reducing, Ar or N2 atmospheres||Au nanoparticles in Al2O3 matrix|
|Co||α-Al2O3||150||<5*1016||Annealing at 1000 °C||Co nanoparticles in Al2O3 matrix|
|Co||α-Al2O3||150||2*1017||Annealing at 1000 °C in reducing ambient||Precipitation of metallic Co|
|Fe||α-Al2O3||160||1*1016 to 2*1017||Annealing for 1 hr from 700 °C to 1500 °C in reducing ambient||Fe nanocomposites|
|Ni||α-Al2O3||64||1*1017||---||1-5 nm Ni nanoparticles|
|Si||α-Al2O3||50||2*1016, 8*1016||Annealing at 500 °C or 1000 °C for 30 min||Si nanoparticles in Al2O3|
|Sn||α-Al2O3||60||1*1017||---||15 nm tetragonal Sn nanoparticles|
|Ti||α-Al2O3||100||<5*1016||Annealing at 1000 °C||Ti nanoparticles in Al2O3|
|Produces Metallic Nanoparticles from Substrate||Ca||Al2O3||150||5*1016||---||Al nanoparticles in amorphous matrix containing Al2O3 and CaO|
|Y||Al2O3||150||5*1016||---||10.7± 1.8 nm Al particles in amorphous matrix containing Al2O3 and Y2O3|
|Y||Al2O3||150||2.5*1016||---||9.0± 1.2 nm Al particles in amorphous matrix containing Al2O3 and Y2O3|
Each individual ion produces many point defects in the target crystal on impact such as vacancies and interstitials. Vacancies are crystal lattice points unoccupied by an atom: in this case the ion collides with a target atom, resulting in transfer of a significant amount of energy to the target atom such that it leaves its crystal site. This target atom then itself becomes a projectile in the solid, and can cause successive collision events. Interstitials result when such atoms (or the original ion itself) come to rest in the solid, but find no vacant space in the lattice to reside. These point defects can migrate and cluster with each other, resulting in dislocation loops and other defects.
Because ion implantation causes damage to the crystal structure of the target which is often unwanted, ion implantation processing is often followed by a thermal annealing. This can be referred to as damage recovery.
The amount of crystallographic damage can be enough to completely amorphize the surface of the target: i.e. it can become an amorphous solid (such a solid produced from a melt is called a glass). In some cases, complete amorphization of a target is preferable to a highly defective crystal: An amorphized film can be regrown at a lower temperature than required to anneal a highly damaged crystal. Amorphisation of the substrate can occur as a result of the beam damage. For example, yttrium ion implantation into sapphire at an ion beam energy of 150 keV to a fluence of 5*1016 Y+/cm2 produces an amorphous glassy layer approximately 110 nm in thickness, measured from the outer surface. [Hunt, 1999]
Some of the collision events result in atoms being ejected (sputtered) from the surface, and thus ion implantation will slowly etch away a surface. The effect is only appreciable for very large doses.
If there is a crystallographic structure to the target, and especially in semiconductor substrates where the crystal structure is more open, particular crystallographic directions offer much lower stopping than other directions. The result is that the range of an ion can be much longer if the ion travels exactly along a particular direction, for example the <110> direction in silicon and other diamond cubic materials.This effect is called ion channelling, and, like all the channelling effects, is highly nonlinear, with small variations from perfect orientation resulting in extreme differences in implantation depth. For this reason, most implantation is carried out a few degrees off-axis, where tiny alignment errors will have more predictable effects.
Ion channelling can be used directly in Rutherford backscattering and related techniques as an analytical method to determine the amount and depth profile of damage in crystalline thin film materials.
In fabricating wafers, toxic materials such as arsine and phosphine are often used in the ion implanter process. Other common carcinogenic, corrosive, flammable, or toxic elements include antimony, arsenic, phosphorus, and boron. Semiconductor fabrication facilities are highly automated, but residue of hazardous elements in machines can be encountered during servicing and in vacuum pump hardware.
High voltage power supplies used in ion accelerators necessary for ion implantation can pose a risk of electrical injury. In addition, high-energy atomic collisions can generate X-rays and, in some cases, other ionizing radiation and radionuclides. In addition to high voltage, particle accelerators such as radio frequency linear particle accelerators and laser wakefield plasma accelerators other hazards.
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 exploited 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.
Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with a well-defined orientation with respect to the crystalline substrate. The new layers formed are called the epitaxial film or epitaxial layer. The relative orientation of the epitaxial layer to the crystalline substrate is defined in terms of the orientation of the crystal lattice of each material. For epitaxial growth, the new layer will be crystalline and will all have a single orientation relative to the substrate; amorphous growth or multicrystalline growth with random crystal orientation does not meet this criterion.
A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.
Nanoparticle are particles between 1 and 100 nanometres (nm) in size with a surrounding interfacial layer. The interfacial layer is an integral part of nanoscale matter, fundamentally affecting all of its properties. The interfacial layer typically consists of ions, inorganic and organic molecules. Organic molecules coating inorganic nanoparticles are known as stabilizers, capping and surface ligands, or passivating agents. In nanotechnology, a particle is defined as a small object that behaves as a whole unit with respect to its transport and properties. Particles are further classified according to diameter.
Elastic Recoil Detection Analysis (ERDA), also referred to as forward recoil scattering, is an Ion Beam Analysis technique in materials science to obtain elemental concentration depth profiles in thin films. This technique is known by several different names. These names are listed below. In the technique of ERDA, an energetic ion beam is directed at a sample to be characterized and there is an elastic nuclear interaction between the ions of beam and the atoms of the target sample. Such interactions are commonly of Coulomb nature. Depending on the kinetics of the ions, cross section area, and the loss of energy of the ions in the matter, Elastic Recoil Detection Analysis helps determine the quantification of the elemental analysis. It also provides information about the depth profile of the sample.
Chemical beam epitaxy (CBE) forms an important class of deposition techniques for semiconductor layer systems, especially III-V semiconductor systems. This form of epitaxial growth is performed in an ultrahigh vacuum system. The reactants are in the form of molecular beams of reactive gases, typically as the hydride or a metalorganic. The term CBE is often used interchangeably with metal-organic molecular beam epitaxy (MOMBE). The nomenclature does differentiate between the two processes, however. When used in the strictest sense, CBE refers to the technique in which both components are obtained from gaseous sources, while MOMBE refers to the technique in which the group III component is obtained from a gaseous source and the group V component from a solid source.
Nanochemistry is the combination of chemistry and nanoscience. Nanochemistry is associated with synthesis of building blocks which are dependent on size, surface, shape and defect properties. Nanochemistry is being used in chemical, materials and physical, science as well as engineering, biological and medical applications. Nanochemistry and other nanoscience fields have the same core concepts but the usages of those concepts are different.
Gas Cluster Ion Beams (GCIB) is a technology for nano-scale modification of surfaces. It can smooth a wide variety of surface material types to within an angstrom of roughness without subsurface damage. It is also used to chemically alter surfaces through infusion or deposition.
Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by 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.
Ion beam mixing is the atomic intermixing and alloying that can occur at the interface separating two different materials during ion irradiation. It is applied as a process for adhering two multilayers, especially a substrate and deposited surface layer. The process involves bombarding layered samples with doses of ion radiation in order to promote mixing at the interface, and generally serves as a means of preparing electrical junctions, especially between non-equilibrium or metastable alloys and intermetallic compounds. Ion implantation equipment can be used to achieve ion beam mixing.
Static secondary-ion mass spectrometry, or static SIMS is a secondary ion mass spectrometry technique for chemical analysis including elemental composition and chemical structure of the uppermost atomic or molecular layer of a solid which may be a metal, semiconductor or plastic with insignificant disturbance to its composition and structure. It is one of the two principal modes of operation of SIMS, which is the mass spectrometry of ionized particles emitted by a solid surface upon bombardment by energetic primary particles.
Rutherford backscattering spectrometry (RBS) is an analytical technique used in materials science. Sometimes referred to as high-energy ion scattering (HEIS) spectrometry, RBS is used to determine the structure and composition of materials by measuring the backscattering of a beam of high energy ions impinging on a sample.
Melting-point depression is the phenomenon of reduction of the melting point of a material with reduction of its size. This phenomenon is very prominent in nanoscale materials, which melt at temperatures hundreds of degrees lower than bulk materials.
The vapor–liquid–solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. The growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid–solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.
A collision cascade is a set of nearby adjacent energetic collisions of atoms induced by an energetic particle in a solid or liquid.
Stopping and Range of Ions in Matter (SRIM) is a group of computer programs which calculate interaction of ions with matter; the core of SRIM is a program Transport of ions in matter (TRIM). SRIM is popular in the ion implantation research and technology community and also used widely in other branches of radiation material science.
Silver nanoparticles are nanoparticles of silver of between 1 nm and 100 nm in size. While frequently described as being 'silver' some are composed of a large percentage of silver oxide due to their large ratio of surface to bulk silver atoms. Numerous shapes of nanoparticles can be constructed depending on the application at hand. Commonly used silver nanoparticles are spherical, but diamond, octagonal, and thin sheets are also popular.
The binary collision approximation (BCA) signifies a method used in ion irradiation physics to enable efficient computer simulation of the penetration depth and defect production by energetic ions in solids. In the method, the ion is approximated to travel through a material by experiencing a sequence of independent binary collisions with sample atoms (nuclei). Between the collisions, the ion is assumed to travel in a straight path, experiencing electronic stopping power, but losing no energy in collisions with nuclei.
Radiation materials science describes the interaction of radiation with matter: a broad subject covering many forms of irradiation and of matter.
The MIAMI facility is a scientific laboratory located within the Ion Beam Centre at the University of Huddersfield. This facility is dedicated to the study of the interaction of ion beams with matter. The facilities combine ion accelerators in situ with Transmission Electron Microscopes (TEM): a technique that allows real-time monitoring of the effects of radiation damage on the microstructures of a wide variety of materials. Currently the laboratory operates two such systems MIAMI-1 and MIAMI-2 that are the only facilities of this type in the United Kingdom, with only a few other such systems in the world. The MIAMI facility is also part of the UKNIBC along with the Universities of Surrey and Manchester, which provides a single point of access to a wide range of accelerators and techniques.
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