Electron Beam Physical Vapor Deposition
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Electron-beam physical vapor deposition, or EBPVD, is a form of
physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...
in which a target
anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...
is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the target to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber (within line of sight) with a thin layer of the anode material.


Introduction

Thin-film deposition A thin film is a layer of materials ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...
is a process applied in the
semiconductor industry The semiconductor industry is the aggregate of companies engaged in the design and fabrication of semiconductors and semiconductor devices, such as transistors and integrated circuits. Its roots can be traced to the invention of the transistor ...
to grow electronic materials, in the
aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial, and military applications. Aerospace engineering consists of aeronautics and astron ...
industry to form thermal and chemical barrier coatings to protect surfaces against corrosive environments, in optics to impart the desired reflective and transmissive properties to a substrate and elsewhere in industry to modify surfaces to have a variety of desired properties. The deposition process can be broadly classified into
physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...
(PVD) and
chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (electro ...
(CVD). In CVD, the film growth takes place at high temperatures, leading to the formation of corrosive gaseous products, and it may leave impurities in the film. The PVD process can be carried out at lower deposition temperatures and without corrosive products, but deposition rates are typically lower. Electron-beam physical vapor deposition, however, yields a high deposition rate from 0.1 to 100 μm/ min at relatively low substrate temperatures, with very high material utilization efficiency. The schematic of an EBPVD system is shown in Fig 1.


Thin-film deposition process

In an EBPVD system, the deposition chamber must be evacuated to a
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
of at least 7.5
Torr The torr (symbol: Torr) is a Pressure#Units, unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (unit), atmosphere (101325 Pa). Thus one torr is exactly (≈ ). Historically, one torr was intended to be ...
(10−2 Pa) to allow passage of electrons from the
electron gun file:Egun.jpg, Electron gun from a cathode-ray tube file:Vidicon Electron Gun.jpg, The electron gun from an RCA Vidicon video camera tube An electron gun (also called electron emitter) is an electrical component in some vacuum tubes that produc ...
to the evaporation material, which can be in the form of an
ingot An ingot is a piece of relatively pure material, usually metal, that is Casting, cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedu ...
or rod. Alternatively, some modern EBPVD systems utilize an arc-suppression system and can be operated at vacuum levels as low as 5.0
Torr The torr (symbol: Torr) is a Pressure#Units, unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (unit), atmosphere (101325 Pa). Thus one torr is exactly (≈ ). Historically, one torr was intended to be ...
, for situations such as parallel use with magnetron sputtering. Multiple types of evaporation materials and electron guns can be used simultaneously in a single EBPVD system, each having a power from tens to hundreds of kilowatts. Electron beams can be generated by
thermionic emission Thermionic emission is the liberation of charged particles from a hot electrode whose thermal energy gives some particles enough kinetic energy to escape the material's surface. The particles, sometimes called ''thermions'' in early literature, a ...
,
field electron emission Field electron emission, also known as field-induced electron emission, field emission (FE) and electron field emission, is the emission of electrons from a material placed in an electrostatic field. The most common context is field emission from ...
or the anodic arc method. The generated electron beam is accelerated to a high kinetic energy and directed towards the evaporation material. Upon striking the evaporation material, the electrons will lose their energy very rapidly. The kinetic energy of the electrons is converted into other forms of energy through interactions with the evaporation material. The thermal energy that is produced heats up the evaporation material causing it to melt or sublimate. Once temperature and vacuum level are sufficiently high, vapor will result from the melt or solid. The resulting vapor can then be used to coat surfaces. Accelerating voltages can be between 3 and 40 kV. When the accelerating voltage is 20–25 kV and the beam current is a few
amperes The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
, 85% of the electron's kinetic energy can be converted into thermal energy. Some of the incident electron energy is lost through the production of X-rays and secondary electron emission. There are three main EBPVD configurations, electromagnetic alignment, electromagnetic focusing and the pendant drop configuration. Electromagnetic alignment and electromagnetic focusing use evaporation material that is in the form of an ingot, while the pendant drop configuration uses a rod. Ingots are enclosed in a
copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
crucible or hearth, while a rod will be mounted at one end in a socket. Both the crucible and socket must be cooled. This is typically done by
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
circulation. In the case of ingots, molten liquid can form on its surface, which can be kept constant by vertical displacement of the ingot. The evaporation rate may be on the order of 10−2 g/(cm2·s).


Material evaporation methods

Refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...
carbides like titanium
carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...
and borides like titanium boride and zirconium boride can evaporate without undergoing decomposition in the vapor phase. These compounds are deposited by direct evaporation. In this process these compounds, compacted in the form of an ingot, are evaporated in vacuum by the focused high-energy electron beam, and the vapors are directly condensed over the substrate. Certain refractory oxides and carbides undergo fragmentation during their evaporation by the
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
beam, resulting in a stoichiometry that is different from the initial material. For example, alumina, when evaporated by electron beam, dissociates into aluminum, AlO3 and Al2O. Some refractory carbides like
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A wide bandgap semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder a ...
and
tungsten carbide Tungsten carbide (chemical formula: ) is a carbide containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through sintering for use in in ...
decompose upon heating, and the dissociated elements have different volatilities. These compounds can be deposited on the substrate either by reactive evaporation or by co-evaporation. In the reactive evaporation process, the
metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
is evaporated from the ingot by the electron beam. The vapors are carried by the reactive gas, which is oxygen in case of metal oxides or
acetylene Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...
in case of metal carbides. When the thermodynamic conditions are met, the vapors react with the gas in the vicinity of the substrate to form films. Metal carbide films can also be deposited by co-
evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...
. In this process, two ingots are used, one for metal and the other for
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
. Each ingot is heated with a different beam energy so that their evaporation rate can be controlled. As the vapors arrive at the surface, they chemically combine under proper thermodynamic conditions to form a metal carbide film.


Substrate

The substrate on which the film deposition takes place is ultrasonically cleaned and fastened to the substrate holder. The substrate holder is attached to the manipulator shaft. The manipulator shaft moves translationally to adjust the distance between the ingot source and the substrate. The shaft also rotates the substrate at a particular speed so that the film is uniformly deposited on the substrate. A negative bias DC
voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
of 200–400 V can be applied to the substrate. Often, focused high-energy electrons from one of the electron guns or infrared light from heater lamps is used to preheat the substrate. Heating of the substrate allows increased
adatom An adatom is an atom that lies on a crystal surface, and can be thought of as the opposite of a surface vacancy. This term is used in surface chemistry and epitaxy, when describing single atoms lying on surfaces and surface roughness. The word ...
–substrate and adatom–film diffusion by giving the adatoms sufficient energy to overcome kinetic barriers. If a rough film, such as metallic nanorods, is desired substrate cooling with water or
liquid nitrogen Liquid nitrogen (LN2) is nitrogen in a liquid state at cryogenics, low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, mobile liquid whose vis ...
may be employed to reduce diffusion lifetime, positively bolstering surface kinetic barriers. To further enhance film roughness, the substrate may be mounted at a steep angle with respect to the flux to achieve geometric shadowing, where incoming line of sight flux lands onto only higher parts of the developing film. This method is known as glancing-angle deposition (GLAD) or oblique-angle deposition (OAD).


Ion-beam-assisted deposition

EBPVD systems are equipped with ion sources. These ion sources are used for substrate
etching Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other type ...
and cleaning,
sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and c ...
the target and controlling the
microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymer ...
of the substrate. The ion beams bombard the surface and alter the microstructure of the film. When the deposition reaction takes place on the hot substrate surface, the films can develop an internal tensile stress due to the mismatch in the coefficient of thermal expansion between the substrate and the film. High-energy ions can be used to bombard these ceramic thermal barrier coatings and change the
tensile stress In continuum mechanics, stress is a physical quantity that describes forces present during deformation. For example, an object being pulled apart, such as a stretched elastic band, is subject to ''tensile'' stress and may undergo elongati ...
into
compressive stress Compressive stresses are generated in objects when they are subjected to forces that push inward, causing the material to shorten or compress. These stresses occur when an object is squeezed or pressed from opposite directions. In everyday life, ...
. Ion bombardment also increases the density of the film, changes the grain size and modifies amorphous films to
polycrystalline A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. S ...
films. Low-energy ions are used for the surfaces of semiconductor films.


Advantages

The deposition rate in this process can be as low as 1 nm per minute to as high as few micrometers per minute. The material utilization efficiency is high relative to other methods, and the process offers structural and morphological control of films. Due to the very high deposition rate, this process has potential industrial application for
wear Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in ...
-resistant and thermal barrier coatings in aerospace industries, hard coatings for
cutting Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force. Implements commonly used for wikt:cut, cutting are the knife and saw, or in medicine and science the sca ...
and
tool A tool is an Physical object, object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many Tool use by animals, animals use simple tools, only human bei ...
industries, and electronic and
optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
films for semiconductor industries and thin-film solar applications.


Disadvantages

EBPVD is a line-of-sight deposition process when performed at a low enough pressure (roughly <10−4 Torr ). The translational and rotational motion of the shaft helps for coating the outer surface of complex geometries, but this process cannot be used to coat the inner surface of complex geometries. Another potential problem is that filament degradation in the electron gun results in a non-uniform evaporation rate. However, when vapor deposition is performed at pressures of roughly 10−4 Torr (1.3 hPa) or higher, significant scattering of the vapor cloud takes place such that surfaces not in sight of the source can be coated. Strictly speaking, the slow transition from line-of-sight to scattered deposition is determined not only by pressure (or mean free path) but also by source-to-substrate distance. Certain materials are not well-suited to evaporation by EBPVD. The following reference materials suggest appropriate evaporation techniques for many materials:
Vacuum Engineering & Materials Co., Inc.

Kurt J. Lesker Company
Also see Oxford'
Evaporation Guide for the Elements


See also

* Electron-beam technology


References


See also

* D. Wolfe, Thesis (Ph.D), Thesis 2001dWolfe,DE, Synthesis and characterization of TiC, TiBCN,TiB2 /TiC and TiC/CrC multilayer coatings by reactive and ion beam assisted, electron beam-physical vapor deposition (EB-PVD) The Pennsylvania State University, 1996. * * {{cite journal , first=D. , last=Wolfe , author2=J. Singh , title=Surface and Coatings Technology , volume=124 , year=2000 , pages=142–153 Electron beam Thin film deposition