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Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by radio frequency (RF) alternating current (AC) frequency or direct current (DC) discharge between two electrodes, the space between which is filled with the reacting gases.
A plasma is any gas in which a significant percentage of the atoms or molecules are ionized. Fractional ionization in plasmas used for deposition and related materials processing varies from about 10−4 in typical capacitive discharges to as high as 5–10% in high-density inductive plasmas. Processing plasmas are typically operated at pressures of a few millitorrs to a few torr, although arc discharges and inductive plasmas can be ignited at atmospheric pressure. Plasmas with low fractional ionization are of great interest for materials processing because electrons are so light, compared to atoms and molecules, that energy exchange between the electrons and neutral gas is very inefficient. Therefore, the electrons can be maintained at very high equivalent temperatures – tens of thousands of kelvins, equivalent to several electronvolts average energy—while the neutral atoms remain at the ambient temperature. These energetic electrons can induce many processes that would otherwise be very improbable at low temperatures, such as dissociation of precursor molecules and the creation of large quantities of free radicals.
The second benefit of deposition within a discharge arises from the fact that electrons are more mobile than ions. As a consequence, the plasma is normally more positive than any object it is in contact with, as otherwise, a large flux of electrons would flow from the plasma to the object. The difference in voltage between the plasma and the objects in its contacts normally occurs across a thin sheath region. Ionized atoms or molecules that diffuse to the edge of the sheath region feel an electrostatic force and are accelerated towards the neighboring surface. Thus, all surfaces exposed to the plasma receive energetic ion bombardment. The potential across the sheath surrounding an electrically isolated object (the floating potential) is typically only 10–20 V, but much higher sheath potentials are achievable by adjustments in reactor geometry and configuration. Thus, films can be exposed to energetic ion bombardment during deposition. This bombardment can lead to increases in the density of the film, and help remove contaminants, improving the film's electrical and mechanical properties. When a high-density plasma is used, the ion density can be high enough that significant sputtering of the deposited film occurs; this sputtering can be employed to help planarize the film and fill trenches or holes.
A simple DC discharge can be readily created at a few torr between two conductive electrodes, and may be suitable for deposition of conductive materials. However, insulating films will quickly extinguish this discharge as they are deposited. It is more common to excite a capacitive discharge by applying an AC or RF signal between an electrode and the conductive walls of a reactor chamber, or between two cylindrical conductive electrodes facing one another. The latter configuration is known as a parallel plate reactor. Frequencies of a few tens of Hz to a few thousand Hz will produce time-varying plasmas that are repeatedly initiated and extinguished; frequencies of tens of kilohertz to tens of megahertz result in reasonably time-independent discharges.
Excitation frequencies in the low-frequency (LF) range, usually around 100 kHz, require several hundred volts to sustain the discharge. These large voltages lead to high-energy ion bombardment of surfaces. High-frequency plasmas are often excited at the standard 13.56 MHz frequency widely available for industrial use; at high frequencies, the displacement current from sheath movement and scattering from the sheath assist in ionization, and thus lower voltages are sufficient to achieve higher plasma densities. Thus one can adjust the chemistry and ion bombardment in the deposition by changing the frequency of excitation, or by using a mixture of low- and high-frequency signals in a dual-frequency reactor. Excitation power of tens to hundreds of watts is typical for an electrode with a diameter of 200 to 300 mm.
Capacitive plasmas are usually very lightly ionized, resulting in limited dissociation of precursors and low deposition rates. Much denser plasmas can be created using inductive discharges, in which an inductive coil excited with a high-frequency signal induces an electric field within the discharge, accelerating electrons in the plasma itself rather than just at the sheath edge. Electron cyclotron resonance reactors and helicon wave antennas have also been used to create high-density discharges. Excitation powers of 10 kW or more are often used in modern reactors.
High density plasmas can also be generated by a DC discharge in an electron-rich environment, obtained by thermionic emission from heated filaments. The voltages required by the arc discharge are of the order of a few tens of volts, resulting in low energy ions. The high density, low energy plasma is exploited for the epitaxial deposition at high rates in low-energy plasma-enhanced chemical vapor deposition reactors.
Working at Standard Telecommunication Laboratories (STL), Harlow, Essex, R C G Swann discovered that RF discharge promoted the deposition of silicon compounds onto the quartz glass vessel wall. [1] Several internal STL publications were followed in 1964 by French, [2] British [3] and US [4] patent applications. An article was published in the August 1965 volume of Solid State Electronics. [5]
Swann attending to his original prototype glow discharge equipment in the laboratory at STL Harlow, Essex in the 1960s. It represented a breakthrough in the deposition of thin films of amorphous silicon, silicon nitride, silicon dioxide at temperatures significantly lower than that deposited by pyrolytic chemistry.
Plasma deposition is often used in semiconductor manufacturing to deposit films conformally (covering sidewalls) and onto wafers containing metal layers or other temperature-sensitive structures. PECVD also yields some of the fastest deposition rates while maintaining film quality (such as roughness, defects/voids), as compared with sputter deposition and thermal/electron-beam evaporation, often at the expense of uniformity.
Silicon dioxide can be deposited using a combination of silicon precursor gasses like dichlorosilane or silane and oxygen precursors, such as oxygen and nitrous oxide, typically at pressures from a few millitorr to a few torr. Plasma-deposited silicon nitride, formed from silane and ammonia or nitrogen, is also widely used, although it is important to note that it is not possible to deposit a pure nitride in this fashion. Plasma nitrides always contain a large amount of hydrogen, which can be bonded to silicon (Si-H) or nitrogen (Si-NH); [6] this hydrogen has an important influence on IR and UV absorption, [7] stability, mechanical stress, and electrical conductivity. [8] This is often used as a surface and bulk passivating layer for commercial multicrystalline silicon photovoltaic cells. [9]
Silicon dioxide can also be deposited from a tetraethylorthosilicate (TEOS) silicon precursor in an oxygen or oxygen-argon plasma. These films can be contaminated with significant carbon and hydrogen as silanol, and can be unstable in air[ citation needed ]. Pressures of a few torr and small electrode spacings, and/or dual frequency deposition, are helpful to achieve high deposition rates with good film stability.
High-density plasma deposition of silicon dioxide from silane and oxygen/argon has been widely used to create a nearly hydrogen-free film with good conformality over complex surfaces, the latter resulting from intense ion bombardment and consequent sputtering of the deposited molecules from vertical onto horizontal surfaces[ citation needed ].
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 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 can be an unwelcome source of wear in precision components. However, the fact that it can be made to act on extremely fine layers of material is utilised in science and industry—there, it is used to perform precise etching, carry out analytical techniques, and deposit thin film layers in the manufacture of optical coatings, semiconductor devices and nanotechnology products. It is a physical vapor deposition technique.
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp, chemiluminescence. Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them.
Electrodeless plasma excitation methods include helicon plasma sources, inductively coupled plasmas, and surface-wave-sustained discharges.
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.
The Madison Symmetric Torus (MST) is a reversed field pinch (RFP) physics experiment with applications to both fusion energy research and astrophysical plasmas.
A capacitively coupled plasma (CCP) is one of the most common types of industrial plasma sources. It essentially consists of two metal electrodes separated by a small distance, placed in a reactor. The gas pressure in the reactor can be lower than atmosphere or it can be atmospheric.
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral. During the process, the plasma generates volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target.
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 polymers. PVD is characterized by a process in which the material transitions from a condensed phase to a vapor phase and then back to a thin film condensed phase. The most common PVD processes are sputtering and evaporation. PVD is used in the manufacturing of items which require thin films for optical, mechanical, electrical, acoustic or chemical functions. Examples include semiconductor devices such as thin-film solar cells, microelectromechanical devices such as thin film bulk acoustic resonator, aluminized PET film for food packaging and balloons, and titanium nitride coated cutting tools for metalworking. Besides PVD tools for fabrication, special smaller tools used mainly for scientific purposes have been developed.
Plasma activation is a method of surface modification employing plasma processing, which improves surface adhesion properties of many materials including metals, glass, ceramics, a broad range of polymers and textiles and even natural materials such as wood and seeds. Plasma functionalization also refers to the introduction of functional groups on the surface of exposed materials. It is widely used in industrial processes to prepare surfaces for bonding, gluing, coating and painting. Plasma processing achieves this effect through a combination of reduction of metal oxides, ultra-fine surface cleaning from organic contaminants, modification of the surface topography and deposition of functional chemical groups. Importantly, the plasma activation can be performed at atmospheric pressure using air or typical industrial gases including hydrogen, nitrogen and oxygen. Thus, the surface functionalization is achieved without expensive vacuum equipment or wet chemistry, which positively affects its costs, safety and environmental impact. Fast processing speeds further facilitate numerous industrial applications.
Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer.
High-power impulse magnetron sputtering is a method for physical vapor deposition of thin films which is based on magnetron sputter deposition. HIPIMS utilises extremely high power densities of the order of kW⋅cm−2 in short pulses (impulses) of tens of microseconds at low duty cycle of < 10%. Distinguishing features of HIPIMS are a high degree of ionisation of the sputtered metal and a high rate of molecular gas dissociation which result in high density of deposited films. The ionization and dissociation degree increase according to the peak cathode power. The limit is determined by the transition of the discharge from glow to arc phase. The peak power and the duty cycle are selected so as to maintain an average cathode power similar to conventional sputtering (1–10 W⋅cm−2).
Plasma-immersion ion implantation (PIII) or pulsed-plasma doping is a surface modification technique of extracting the accelerated ions from the plasma by applying a high voltage pulsed DC or pure DC power supply and targeting them into a suitable substrate or electrode with a semiconductor wafer placed over it, so as to implant it with suitable dopants. The electrode is a cathode for an electropositive plasma, while it is an anode for an electronegative plasma. Plasma can be generated in a suitably designed vacuum chamber with the help of various plasma sources such as electron cyclotron resonance plasma source which yields plasma with the highest ion density and lowest contamination level, helicon plasma source, capacitively coupled plasma source, inductively coupled plasma source, DC glow discharge and metal vapor arc. The vacuum chamber can be of two types - diode and triode type depending upon whether the power supply is applied to the substrate as in the former case or to the perforated grid as in the latter.
A microplasma is a plasma of small dimensions, ranging from tens to thousands of micrometers. Microplasmas can be generated at a variety of temperatures and pressures, existing as either thermal or non-thermal plasmas. Non-thermal microplasmas that can maintain their state at standard temperatures and pressures are readily available and accessible to scientists as they can be easily sustained and manipulated under standard conditions. Therefore, they can be employed for commercial, industrial, and medical applications, giving rise to the evolving field of microplasmas.
Plasma is one of four fundamental states of matter characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly in stars, but also dominating the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.
Plasma polymerization uses plasma sources to generate a gas discharge that provides energy to activate or fragment gaseous or liquid monomer, often containing a vinyl group, in order to initiate polymerization. Polymers formed from this technique are generally highly branched and highly cross-linked, and adhere to solid surfaces well. The biggest advantage to this process is that polymers can be directly attached to a desired surface while the chains are growing, which reduces steps necessary for other coating processes such as grafting. This is very useful for pinhole-free coatings of 100 picometers to 1-micrometer thickness with solvent insoluble polymers.
Plasma-activated bonding is a derivative, directed to lower processing temperatures for direct bonding with hydrophilic surfaces. The main requirements for lowering temperatures of direct bonding are the use of materials melting at low temperatures and with different coefficients of thermal expansion (CTE).
An excimer lamp is a source of ultraviolet light based on spontaneous emission of excimer (exciplex) molecules.
Low-energy plasma-enhanced chemical vapor deposition (LEPECVD) is a plasma-enhanced chemical vapor deposition technique used for the epitaxial deposition of thin semiconductor films. A remote low energy, high density DC argon plasma is employed to efficiently decompose the gas phase precursors while leaving the epitaxial layer undamaged, resulting in high quality epilayers and high deposition rates.