ION IMPLANTATION

                 ION  implantation

Ion implantation is a materials engineering process by which ions of
a material are accelerated in an electrical field and impacted into a
solid. This process is used to change the physical, chemical, or
electrical properties of the solid. Ion implantation is used in
semiconductor device fabrication and in metal finishing, as well as
various applications in materials science research. The ions alter the
elemental composition of the target (if the ions differ in composition
from the target), stopping in the target and staying there. They also
cause many chemical and physical changes in the target by transferring
their energy and momentum to the electrons and atomic nuclei of the
target material. This causes a structural change, in that the crystal
structure of the target can be damaged or even destroyed by the
energetic collision cascades. Because the ions have masses comparable
to those of the target atoms, they knock the target atoms out of place
more than electron beams do. If the ion energy is sufficiently high
(usually tens of MeV) to overcome the coulomb barrier, there can even
be a small amount of nuclear transmutation.



Contents
1 General principle
2 Application in semiconductor device fabrication
2.1 Doping
2.2 Silicon on insulator
2.3 Mesotaxy
3 Application in metal finishing
3.1 Tool steel toughening
3.2 Surface finishing
4 Other applications
4.1 Ion beam mixing
5 Problems with ion implantation
5.1 Crystallographic damage
5.2 Damage recovery
5.3 Amorphization
5.4 Sputtering
5.5 Ion channelling
6 Hazardous materials
6.1 High voltage safety
7 See also
8 References
9 External links

General principle

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 implanters are typically small
(microamperes), 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).
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


Doping

•The introduction of dopants in a semiconductor is the most common application of ion implantation. • Dopant
ions such as boron, phosphorus or arsenic are generally created from a gas source, so that the purity of the source
can be very high. •These gases tend to be very hazardous. 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 ptype
dopant, and an
electron for an ntype
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 pn
junction of photovoltaic devices in the late
1970s and early 1980s,[2] along with the use of pulsedelectron
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
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