Ion beam mixing

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Ion beam mixing is the atomic intermixing and alloying that can occur at the interface separating two different materials during ion irradiation. [1] 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.

Contents

Mechanism

The unique effects that stem from ion beam mixing are primarily a result of ballistic 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.

Analysis

The degree of mixing of a film scales with the ion mass, with the intensity of any given incident ion beam, and with the duration of the impingement of the ion beam on a target. The amount of mixing is proportional to the square roots of time, mass and ion dose. [2]

At temperatures below 100 °C for most implanted materials, ion beam mixing is essentially independent of temperature 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. [3]

Ballistic ion beam mixing can be classified into two basic subtypes, recoil mixing and cascade mixing, which take place 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. [3] Ion beam mixing can be further enhanced by heat spike effects [4]

Ion mixing (IM) is essentially similar in result to interdiffusion, 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. [3]

The diffusion model does not 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).

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 solute concentrations using lower amounts of irradiation, and better control of band gap variation and diffusion between layers. [3] [5] The cost of IM is also less prohibitive than that of 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 cannot 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. [6] Additionally, all ion mixed samples must be annealed.

See also

Related Research Articles

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Neutron radiation Ionizing radiation that presents as free neutrons

Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new isotopes—which, in turn, may trigger further neutron radiation. Free neutrons are unstable, decaying into a proton, an electron, plus an electron antineutrino with a mean lifetime of 887 seconds.

Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to normal levels of background radiation. The term irradiation usually excludes the exposure to non-ionizing radiation, such as infrared, visible light, microwaves from cellular phones or electromagnetic waves emitted by radio and TV receivers and power supplies.

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.

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Stopping power (particle radiation)

In nuclear and materials physics, stopping power is 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.

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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.

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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.

The threshold displacement energy is the minimum kinetic energy that an atom in a solid needs to be permanently displaced from its lattice site to a defect position. It is also known as "displacement threshold energy" or just "displacement energy". In a crystal, a separate threshold displacement energy exists for each crystallographic direction. Then one should distinguish between the minimum and average over all lattice directions threshold displacement energies. In amorphous solids it may be possible to define an effective displacement energy to describe some other average quantity of interest. Threshold displacement energies in typical solids are of the order of 10 - 50 eV.

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.

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.

Radiation materials science describes the interaction of radiation with matter: a broad subject covering many forms of irradiation and of matter.

A Primary Knock-on Atom or PKA is an atom that is displaced from its lattice site by irradiation; it is, by definition, the first atom that an incident particle encounters in the target. After it is displaced from its initial lattice site, the PKA can induce the subsequent lattice site displacements of other atoms if it possesses sufficient energy, or come to rest in the lattice at an interstitial site if it does not.

Ion track

Ion tracks are damage-trails created by swift heavy ions penetrating through solids, which may be sufficiently-contiguous for chemical etching in a variety of crystalline, glassy, and/or polymeric solids. They are associated with cylindrical damage-regions several nanometers in diameter and can be studied by Rutherford backscattering spectrometry (RBS), transmission electron microscopy (TEM), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS) or gas permeation.

Neutron embrittlement, sometimes more broadly radiation embrittlement, is the embrittlement of various materials due to the action of neutrons. This is primarily seen in nuclear reactors, where the release of high-energy neutrons causes the long-term degradation of the reactor materials. The embrittlement is caused by the microscopic movement of atoms that are hit by the neutrons; this same action also gives rise to neutron-induced swelling causing materials to grow in size, and the Wigner effect causing energy buildup in certain materials that can lead to sudden releases of energy.

Igor Serafimovich Tashlykov

Igor Serafimovich Tashlykov was a USSR and Belarusian physicist, Doctor of Physical and Mathematical Sciences (1989), Professor (1992), member of the New York Academy of Sciences (1994), and Belarusian Physical Society (1995). Places of work, positions: Research Institute of Applied Physical Problems (APP) of the Belarusian State University, Belarusian State Technological University, Maxim Tank Belarusian State Pedagogical University (BSPU).

References

  1. ↑ Ion-solid interactions, Cambridge Solid-State Science series, ch11, p295
  2. ↑ B. M. Paine and R. S. Averback, Ion beam mixing: basic experiments, Nucl. Instrum. Methods Phys. Res. B 7/8, 666 (1985)
  3. 1 2 3 4 Nastasi, Michael (17–29 July 2004). "Ion Beam Mixing" (PDF). Radiation Effects in Solids. Erice, Sicily, Italy: Los Alamos National Laboratory Materials Science and Technology Division. Retrieved 2 May 2007.
  4. ↑ K. Nordlund and M. Ghaly and R. S. Averback (1998). "Mechanisms of ion beam mixing in metals and semiconductors". J. Appl. Phys. 83 (3): 1238–1246. Bibcode:1998JAP....83.1238N. doi:10.1063/1.366821.
  5. ↑ Abedrabbo, S.; Arafah, D.E.; Gokce, O.; Wielunski, L.S.; et al. (May 2006). "Ion Beam Mixing for Processing of Nanostructure Materials". Journal of Electronic Materials . 35 (5): 834–839. doi:10.1007/BF02692536. S2CID   98541376 . Retrieved 2 May 2007.
  6. ↑ Abedrabbo, Sufian; Arafah, D.E.; Salem, S. (May 2005). "Ion Beam Mixing of Silicon-Germanium Thin Films". Journal of Electronic Materials . 34 (5): 468–473. doi:10.1007/s11664-005-0053-1. S2CID   95064618 . Retrieved 2 May 2007.
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