Electron-beam physical vapor deposition, or EBPVD, is a form of physical vapor deposition in which a target

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...

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 is a process applied in the

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...

industry 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 ast ...

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 (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 (subst ...

(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 The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...

min Min or MIN may refer to: Places * Fujian, also called Mǐn, a province of China ** Min Kingdom (909–945), a state in Fujian * Min County, a county of Dingxi, Gansu province, China * Min River (Fujian) * Min River (Sichuan) * Mineola (Amtr ...

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 a ...

of at least 7.5

Torr The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one " millimeter of merc ...

(10⁻² Pa) to allow passage of electrons from the electron gun to the evaporation material, which can be in the form of an ingot or rod. Alternatively, some modern EBPVD systems utilize an arc-suppression system and can be operated at vacuum levels as low as 5.0

, 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,

field electron emission Field electron emission, also known as field emission (FE) and electron field emission, is emission of electrons induced by an electrostatic field. The most common context is field emission from a solid surface into a vacuum. However, field emissio ...

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 elect ...

, 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 with the symbol Cu (from la, cuprum) 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 pink ...

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 (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

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 material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...

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 t ...

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 ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

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 semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...

and

tungsten carbide Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, 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 ...

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 (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...

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 ( systematic name: ethyne) is the 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 unstable in its pure ...

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 surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...

. In this process, two ingots are used, one for metal and the other for

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

. 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.

The 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 electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

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–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 low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wid ...

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 types ...

and cleaning, sputtering the target and controlling the microstructure 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. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

into compressive stress. Ion bombardment also increases the density of the film, changes the grain size and modifies amorphous films to polycrystalline films. Low-energy ions are used for the surfaces of semiconductor films.

Advantages of EBPVD

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 and

tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates b ...

industries, and electronic and optical films for semiconductor industries and thin-film solar applications.

Disadvantages of EBPVD

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