Ion Beam Mixing
Encyclopedia
Ion Beam Mixing is 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.
effects; that is, impinging ions have high kinetic energies
that are transferred to target atoms on collision. Ion energies can be seen on the order of 1 keV
to 200 keV. When accelerated, such ion energies are sufficiently high to break intra-
and especially inter-molecular
bonds, and initiate relocations within an atomic lattice
.
The sequence of collisions is known as a collision cascade
.
During this ballistic process, energies of impinging ions displace atoms and electrons of the target material several lattice sites away, resulting in relocations there and interface mixing at the boundary layer. (Note that energies must be sufficiently high in order for the lattice rearrangements to be permanent rather than manifesting as mere vibrational responses to the impinging radiation, i.e. the kinetic energies must be above the threshold displacement energy
of the material.) If energies are kept sufficiently high in these nuclear collisions, then, compared to traditional high-dose implantation processes, ballistic ion implantation produces higher intrafilm alloy concentrations at lower doses of irradiation compared to conventional implantation processes.
At temperatures below 100 °C for most implanted materials, ion beam mixing is essentially temperature independent, but, as temperature increases beyond that point, mixing rises exponentially with temperature. This temperature dependence is a manifestation of incident ion beams effectively imparting the target species dependent activation energy to the barrier layer.
Ballistic ion beam mixing can be classified into two basic sub types, recoil mixing and cascade mixing, which happen simultaneously as a result of ion bombardment. In recoil mixing, atoms are relocated by single collision
events. Recoil mixing is predominately seen at large angles as a result of soft collisions, with the number of atoms undergoing recoil implantation varying linearly with ion dose. Recoil implantation, however, is not the dominant process in ion beam mixing. Most relocated atoms are part of a collision cascade
in which recoiled atoms initiate a series of lower energy lattice displacements, which is referred to as cascade mixing. Ion beam mixing can be further enhanced by heat spike effects
Ion mixing (IM) is essentially similar in result to interdiffusion, and hence most models of ion mixing involve an effective diffusion coefficient that is used to characterize thickness of the reacted layer as a function of ion beam implantation over a period of time.
The diffusion model, though, doesn’t take into account the miscibility of substrate and layer, so for immiscible or low miscibility systems it will overestimate the degree of mixing, while for highly miscible systems, the model will underestimate the degree of mixing. Thermodynamic effects are also not considered in this basic interdiffusion equation, but can be modeled by equations that consider the enthalpies
of mixing and the molar fractions
of the target species and one can thereby develop a thermodynamic effective diffusion coefficient reflecting temperature effects (which become pronounced at high temperatures).
concentrations using lower amounts of irradiation, and better control of band gap
variation and diffusion between layers. IM is also less cost-prohibitive than other modes of film preparation on substrates, such as chemical vapor deposition
(CVD) and molecular beam epitaxy
(MBE).
Disadvantages include the inability to completely direct and control lattice displacements initiated in the process, which can result in an undesirable degree of disorder in ion mixed samples, rendering them unsuitable for applications in which precise lattice orderings are paramount. Ion beams can not be perfectly directed nor the collision cascade controlled once IM effects propagate, which can result in leaking, electron diffraction
, radiation enhanced diffusion (RED), chemical migration and mismatch. Additionally, all ion mixed samples must be annealed.
Particle radiation
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....
in order to promote mixing
Mixture
In chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...
at the interface, and generally serves as a means of preparing electrical junctions, especially between non-equilibrium or metastable alloys and intermetallic
Intermetallics
Intermetallics or intermetallic compounds is a term that is used in a number of different ways. Most commonly it refers to solid-state phases involving metals. There is a "research definition" adhered to generally in scientific publications, and a wider "common use" term...
compounds. 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 another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
equipment can be used to achieve ion beam mixing.
Mechanism
The unique effects that stem from ion beam mixing are primarily a result of ballisticBallistics
Ballistics is the science of mechanics that deals with the flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.A ballistic body is a body which is...
effects; that is, impinging ions have high kinetic energies
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
that are transferred to target atoms on collision. Ion energies can be seen on the order of 1 keV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
to 200 keV. When accelerated, such ion energies are sufficiently high to break intra-
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
and especially inter-molecular
Intermolecular force
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together...
bonds, and initiate relocations within an atomic lattice
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
.
The sequence of collisions is known as a collision cascade
Collision cascade
A collision cascade is a set of nearby adjacent energetic collisions of atoms induced by an energetic particle in a solid or liquid....
.
During this ballistic process, energies of impinging ions displace atoms and electrons of the target material several lattice sites away, resulting in relocations there and interface mixing at the boundary layer. (Note that energies must be sufficiently high in order for the lattice rearrangements to be permanent rather than manifesting as mere vibrational responses to the impinging radiation, i.e. the kinetic energies must be above the threshold displacement energy
Threshold displacement energy
The threshold displacement energy T_d is the minimum kinetic energythat an atom in a solid needs to be permanentlydisplaced from its lattice site to adefect position.It is also known as "displacement threshold energy" or just "displacement energy"....
of the material.) If energies are kept sufficiently high in these nuclear collisions, then, compared to traditional high-dose implantation processes, ballistic ion implantation produces higher intrafilm alloy concentrations at lower doses of irradiation compared to conventional implantation processes.
Analysis
There are some important relationships to note concerning ion beam mixing of films. First, the degree of mixing of a film scales with the ion mass, with the intensity of any given incident ion beam, and with the amount of time the ion beam is left to impinge on a target. The amount of mixing is proportional to the square roots of time, mass and ion dose.At temperatures below 100 °C for most implanted materials, ion beam mixing is essentially temperature independent, but, as temperature increases beyond that point, mixing rises exponentially with temperature. This temperature dependence is a manifestation of incident ion beams effectively imparting the target species dependent activation energy to the barrier layer.
Ballistic ion beam mixing can be classified into two basic sub types, recoil mixing and cascade mixing, which happen simultaneously as a result of ion bombardment. In recoil mixing, atoms are relocated by single collision
Elastic collision
An elastic collision is an encounter between two bodies in which the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter...
events. Recoil mixing is predominately seen at large angles as a result of soft collisions, with the number of atoms undergoing recoil implantation varying linearly with ion dose. Recoil implantation, however, is not the dominant process in ion beam mixing. Most relocated atoms are part of a collision cascade
Collision cascade
A collision cascade is a set of nearby adjacent energetic collisions of atoms induced by an energetic particle in a solid or liquid....
in which recoiled atoms initiate a series of lower energy lattice displacements, which is referred to as cascade mixing. Ion beam mixing can be further enhanced by heat spike effects
Ion mixing (IM) is essentially similar in result to interdiffusion, and hence most models of ion mixing involve an effective diffusion coefficient that is used to characterize thickness of the reacted layer as a function of ion beam implantation over a period of time.
The diffusion model, though, doesn’t take into account the miscibility of substrate and layer, so for immiscible or low miscibility systems it will overestimate the degree of mixing, while for highly miscible systems, the model will underestimate the degree of mixing. Thermodynamic effects are also not considered in this basic interdiffusion equation, but can be modeled by equations that consider the enthalpies
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
of mixing and the molar fractions
Molar mass distribution
In linear polymers the individual polymer chains rarely have exactly the same degree of polymerization and molar mass, and there is always a distribution around an average value. The molar mass distribution in a polymer describes the relationship between the number of moles of each polymer species...
of the target species and one can thereby develop a thermodynamic effective diffusion coefficient reflecting temperature effects (which become pronounced at high temperatures).
Advantages and disadvantages
Advantages of ion beam mixing as a means of synthesis over traditional modes of implantation include the process' ability to produce materials with high soluteSolution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
concentrations using lower amounts of irradiation, and better control of band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
variation and diffusion between layers. IM is also less cost-prohibitive than other modes of film preparation on substrates, such as chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
(CVD) and molecular beam epitaxy
Molecular beam epitaxy
Molecular beam epitaxy is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...
(MBE).
Disadvantages include the inability to completely direct and control lattice displacements initiated in the process, which can result in an undesirable degree of disorder in ion mixed samples, rendering them unsuitable for applications in which precise lattice orderings are paramount. Ion beams can not be perfectly directed nor the collision cascade controlled once IM effects propagate, which can result in leaking, electron diffraction
Electron diffraction
Electron diffraction refers to the wave nature of electrons. However, from a technical or practical point of view, it may be regarded as a technique used to study matter by firing electrons at a sample and observing the resulting interference pattern...
, radiation enhanced diffusion (RED), chemical migration and mismatch. Additionally, all ion mixed samples must be annealed.