Ion implantation is a low-temperature process by which the components of one element are accelerated into the solid surface of the plate, thereby changing its physical, chemical or electrical properties. This method is used in the manufacture of semiconductor devices and in the decoration of metals, as well as in the study of materials science. Components can change the elemental composition of the plate if they stop and remain in it. Ion implantation also causes chemical and physical changes when atoms collide with a target at high energy. The crystal structure of the plate can be damaged or even destroyed by energy cascades of collisions, and particles of sufficiently high energy (10 MeV) can cause nuclear transmutation.
General principle of ion implantation
Equipment usually consists of a source where the atoms of the desired element are formed, an accelerator, where they electrostatically accelerate to high energy, and a target chamber, where they collide with the target, which is the material. Thus, this process is a special case of particle radiation. Each ion is usually an individual atom or molecule, and thus the actual amount of material implanted into the target is an integral over time of the ion current. This number is called the dose. The currents supplied by implants are usually small (microamps), and therefore the amount that can be implanted in a reasonable amount of time is small. Therefore, ion implantation is used in cases where the number of necessary chemical changes is small.
Typical ion energies range from 10 to 500 keV (from 1600 to 80,000 aJ). It is possible to use ion implantation at low energies in the range of 1 to 10 keV (160 to 1600 aJ), but the penetration is only a few nanometers or less. Power below this leads to very slight damage to the target and falls under the designation of ion beam deposition. And higher energies can also be used: accelerators capable of 5 MeV (800,000 aJ) are common. Nevertheless, great structural damage is often done to the target, and since the depth distribution is wide (Bragg peak), the net composition change at any point on the target will be small.
The ion energy, as well as various types of atoms and the composition of the target, determine the depth of penetration of particles into a solid. A monoenergetic ion beam usually has a wide depth distribution. The average penetration is called the range. Under typical conditions, it will be between 10 nanometers and 1 micrometer. Thus, low-energy ion implantation is particularly useful in cases where it is desired that a chemical or structural change be near the surface of the target. Particles gradually lose their energy when passing through a solid, both from random collisions with target atoms (which cause sharp energy transfers), and from light braking from overlapping electronic orbitals, which is a continuous process. The loss of ion energy in the target is called a stop and can be modeled using the method of ion implantation of the binary collision approximation.
Accelerator systems are usually divided into medium current, high, high energy and a very significant dose.
All varieties of ion implantation beam designs contain certain general groups of functional components. Let's look at some examples. The first physical and physico-chemical fundamentals of ion implantation include a device known as a source for generating particles. This device is closely connected with displaced electrodes for extracting atoms into the beam line and most often with some means of selecting specific types for transport to the main section of the accelerator. The selection of the "mass" is often accompanied by the passage of the extracted ion beam through the magnetic field with the exit path limited by blocking holes or "gaps" that allow only ions with a certain value of the product of mass and velocity. If the target surface is larger than the diameter of the ion beam and the implanted dose is preferably evenly distributed over it, then some combination of beam scanning and plate movement is used. Finally, the target is connected to some method of collecting the accumulated charge of the implanted ions, so that the delivered dose can be measured continuously and the process stops at the desired level.
Application in the manufacture of semiconductor devices
Doping with boron, phosphorus or arsenic is a common application of this process. During ion implantation of semiconductors, each dopant atom can create a charge carrier after annealing. You can build a hole for the p-type dopant and an n-type electron. This changes the conductivity of the semiconductor in its vicinity. The technique is used, for example, to adjust the MOSFET threshold.
Ion implantation was developed as a method for obtaining the pn junction of photovoltaic devices in the late 1970s and early 1980s along with the use of a pulsed electron beam for fast annealing, although until now it has not been used for commercial production.
Silicon on the insulator
One of the known methods for preparing this material on dielectric substrates (SOIs) from conventional silicon substrates is the SIMOX process (separation by oxygen implantation), in which high-dose fall-in air is converted to silicon oxide due to the high-temperature annealing process.
Mesotaxia
This is the term for the growth of a crystallographically coincident phase under the surface of the main crystal. In this process, ions are implanted with a sufficiently high energy and dose into the material to create a layer of the second phase, and the temperature is controlled so that the structure of the target is not destroyed. The crystalline orientation of the layer can be designed to fit the target, even if the exact lattice constant can vary greatly. For example, after implantation of nickel ions into a silicon wafer, a silicide layer can be grown in which the orientation of the crystals coincides with the values of silicon.
Metal finish
Nitrogen or other ions can be implanted into a target made of tool steel (for example, drills). The structural change provokes surface compression in the material, which prevents the propagation of cracks and, thus, makes it more resistant to fracture.
Surface finish
In some applications, for example for prostheses, such as artificial joints, it is desirable to have a target that is very resistant both to chemical corrosion and to wear due to friction. Ion implantation is used to construct the surfaces of such devices for more reliable operation. As in the case of tool steels, target modification caused by ion implantation includes both surface compression, which prevents crack propagation, and alloying to make it more chemically resistant to corrosion.
Other applications
Implantation can be used to achieve mixing of ion beams, that is, the blending of atoms of various elements at the interface. This may be useful to achieve graded surfaces or to enhance adhesion between layers of immiscible materials.
Nanoparticle formation
Ion implantation can be used to induce nanoscale materials in oxides such as sapphire and silicon dioxide. Atoms can be formed by precipitation or the formation of mixed substances that contain both an ion-implanted element and a substrate.
Typical ion beam energies used to produce nanoparticles range from 50 to 150 keV, and ion fluence ranges from 10-16 to 10-18 square meters. see Can be formed a wide variety of materials with sizes from 1 nm to 20 nm and with compositions that may contain implanted particles, combinations that consist solely of a cation bound to the substrate.
Substances based on dielectrics, such as sapphire, which contain dispersed metal ion implantation nanoparticles, are promising materials for optoelectronics and nonlinear optics.
Problems
Each individual ion produces many point defects in the target crystal upon impact or implantation. Jobs are lattice points not occupied by the atom: in this case, the ion collides with the target atom, which leads to the transfer of a significant amount of energy to it, so that it leaves its site. This target itself becomes a shell in a solid and can cause successive collisions. Internodes occur when such particles stop in a solid, but do not find free space in the lattice for living. During ion implantation, these point defects can migrate and cluster with each other, which leads to the formation of dislocation loops and other problems.
Amorphization
The amount of crystallographic damage may be sufficient for the complete transition of the target surface, that is, it should become an amorphous solid. In some cases, complete amorphization of the target is preferable than a crystal with a high degree of imperfection: such a film can re-grow at a lower temperature than is required for annealing a severely damaged crystal. Amorphization of the substrate can occur as a result of a change in the beam. For example, when yttrium ions are implanted in sapphire at a beam energy of 150 keV to a fluence of 5 * 10-16 Y + / sq. cm, a vitreous layer is formed with a thickness of approximately 110 nm, measured from the outer surface.
Spraying
Some of the collision events cause atoms to be ejected from the surface, and thus ion implantation will slowly etch the surface. The effect is noticeable only for very large doses.
Ion channel
If a crystallographic structure is applied to the target, especially in semiconductor substrates, where it is more open, then specific directions stop much less than others. The result is that the radius of action of the ion can be much larger if it moves exactly along a certain path, for example in silicon and other cubic diamond materials. This effect is called ion channeling, and, like all such effects, are highly nonlinear, with small deviations from the ideal orientation, leading to significant differences in the depth of implantation. For this reason, most are performed a few degrees off axis, where tiny alignment errors will have more predictable effects.