Ion implantation

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An ion implantation system at LAAS technological facility in Toulouse, France. Ion implantation machine at LAAS 0521.jpg
An ion implantation system at LAAS technological facility in Toulouse, France.

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.

Materials science interdisciplinary field which deals with the discovery and design of new materials; primarily concerned with the physical and chemical properties of solids

The interdisciplinary field of materials science, also commonly termed materials science and engineering is the design and discovery of new materials, particularly solids. The intellectual origins of materials science stem from the Enlightenment, when researchers began to use analytical thinking from chemistry, physics, and engineering to understand ancient, phenomenological observations in metallurgy and mineralogy. Materials science still incorporates elements of physics, chemistry, and engineering. As such, the field was long considered by academic institutions as a sub-field of these related fields. Beginning in the 1940s, materials science began to be more widely recognized as a specific and distinct field of science and engineering, and major technical universities around the world created dedicated schools of the study, within either the Science or Engineering schools, hence the naming.

Crystal structure Ordered arrangement of atoms, ions, or molecules in a crystalline material

In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.

Collision cascade a set of nearby adjacent energetic (much higher than ordinary thermal energies) collisions of atoms induced by an energetic particle in a solid or liquid

A collision cascade is a set of nearby adjacent energetic collisions of atoms induced by an energetic particle in a solid or liquid.


General principle

Ion implantation setup with mass separator Ion implanter schematic.png
Ion implantation setup with mass separator

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.

Ion source Device that creates charged atoms and molecules (ions)

An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

Particle accelerator device to propel charged particles to high speeds

A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.

Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam.

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.

In physics, the electronvolt is a unit of energy equal to approximately 1.6×10−19 joules in SI units.

Ion beam deposition

Ion beam deposition (IBD) is a process of applying materials to a target through the application of an ion beam.

Bragg peak Path length of maximum energy loss of ionizing radiation

The Bragg peak is a pronounced peak on the Bragg curve which plots the energy loss of ionizing radiation during its travel through matter. For protons, α-rays, and other ion rays, the peak occurs immediately before the particles come to rest. This is called Bragg peak, after William Henry Bragg who discovered it in 1903.

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.

Stopping power (particle radiation) retarding force acting on charged particles, typically alpha and beta particles, due to interaction with matter, resulting in loss of particle energy

Stopping power in nuclear and materials physics is defined as the retarding force acting on charged particles, typically alpha and beta particles, due to interaction with matter, resulting in loss of particle energy. Its application is important in areas such as radiation protection, ion implantation and nuclear medicine.

Binary collision approximation

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.

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. [1]

Application in semiconductor device fabrication


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, [2] along with the use of pulsed-electron beam for rapid annealing, [3] although it has not to date been used for commercial production.

Silicon on insulator

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.

Application in metal finishing

Tool steel toughening

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.

Surface finishing

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.

Other applications

Ion beam mixing

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-induced nanoparticle formation

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. [4] [5] [6] 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. [7] [8] [9] [10] [11] [12] [13] [14] [15] 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 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.

Composite materials based on dielectrics such as sapphire that contain dispersed metal nanoparticles are promising materials for optoelectronics and nonlinear optics. [11]

Implanted SpeciesSubstrateIon Beam Energy (keV)Fluence (ions/cm2)Post Implantation Heat TreatmentResultSource
Produces Oxides that Contain the Implanted IonCoAl2O3655*1017Annealing at 1400 °CForms Al2CoO4 spinel [7]
Coα-Al2O31502*1017Annealing at 1000 °C in oxidizing ambientForms Al2CoO4 spinel [8]
MgAl2O31505*1016---Forms MgAl2O4 platelets [4]
Snα-Al2O3601*1017Annealing in O2 atmosphere at 1000 °C for 1 hr30 nm SnO2 nanoparticles form [15]
Znα-Al2O3481*1017Annealing in O2 atmosphere at 600 °CZnO nanoparticles form [9]
ZrAl2O3655*1017Annealing at 1400 °CZrO2 precipitates form [7]
Produces Metallic Nanoparticles from Implanted SpeciesAgα-Al2O31500, 20002*1016, 8*1016Annealing from 600 °C to 1100 °C in oxidizing, reducing, Ar or N2 atmospheresAg nanoparticles in Al2O3 matrix [10]
Auα-Al2O31600.6*1017, 1*10161 hr at 800 °C in airAu nanoparticles in Al2O3 matrix [11]
Auα-Al2O31500, 20002*1016, 8*1016Annealing from 600 °C to 1100 °C in oxidizing, reducing, Ar or N2 atmospheresAu nanoparticles in Al2O3 matrix [10]
Coα-Al2O3150<5*1016Annealing at 1000 °CCo nanoparticles in Al2O3 matrix [8]
Coα-Al2O31502*1017Annealing at 1000 °C in reducing ambientPrecipitation of metallic Co [8]
Feα-Al2O31601*1016 to 2*1017Annealing for 1 hr from 700 °C to 1500 °C in reducing ambientFe nanocomposites [12]
Niα-Al2O3641*1017---1-5 nm Ni nanoparticles [13]
Siα-Al2O3502*1016, 8*1016Annealing at 500 °C or 1000 °C for 30 minSi nanoparticles in Al2O3 [14]
Snα-Al2O3601*1017---15 nm tetragonal Sn nanoparticles [15]
Tiα-Al2O3100<5*1016Annealing at 1000 °CTi nanoparticles in Al2O3 [8]
Produces Metallic Nanoparticles from SubstrateCaAl2O31505*1016---Al nanoparticles in amorphous matrix containing Al2O3 and CaO [4]
YAl2O31505*1016---10.7± 1.8 nm Al particles in amorphous matrix containing Al2O3 and Y2O3 [4]
YAl2O31502.5*1016---9.0± 1.2 nm Al particles in amorphous matrix containing Al2O3 and Y2O3 [5]

Problems with ion implantation

Crystallographic damage

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.

Damage recovery

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.

Ion channelling

A diamond cubic crystal viewed from the <110> direction, showing hexagonal ion channels. Diamond structure.png
A diamond cubic crystal viewed from the <110> direction, showing hexagonal ion channels.

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. [16] 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.


Hazardous 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 voltages and particle accelerators

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.

See also

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Sputtering is a process whereby particles are ejected from a solid target material due to bombardment of the target by energetic particles, particularly gas ions in a laboratory. It only happens when the kinetic energy of the incoming particles is much higher than conventional thermal energies. This process can lead, during prolonged ion or plasma bombardment of a material, to significant erosion of materials, and can thus be harmful. On the other hand, it is commonly used for thin-film deposition, etching and analytical techniques. Sputtering is done either using DC voltage or using AC voltage. In DC sputtering, voltage is set from 3-5 kV and in RF sputtering, power supply is set at 14 MHz. Due to the application of an alternating current, the ions inside the plasma oscillate resulting in an increase in the levels of plasma.

Epitaxy crystal growth process

Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate.

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Nanoparticle particle with size less than 100 nm

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Sputter deposition physical vapor deposition (PVD) method of thin film deposition by sputtering

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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.

Vapor–liquid–solid method

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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.

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