Plasma-enhanced chemical vapor deposition (PECVD) is a

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 ...

process used to deposit thin films from a gas state (

vapor In physics, a vapor (American English) or vapour (Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R ...

) to a

solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...

state on a substrate.

Chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

s are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...

(RF)

alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...

(AC) frequency or

direct current Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...

(DC) discharge between two

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

s, the space between which is filled with the reacting gases.

Discharges for processes

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 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 ...

, 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 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 ...

of the deposited film occurs; this sputtering can be employed to help planarize the film and fill trenches or holes.

Reactor types

A simple DC discharge can be readily created at a few

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 The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). Definition One volt is defined as the electric potential between two point ...

, 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.

Origins

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. Several internal STL publications were followed in 1964 by French, British and US patent applications. An article was published in the August 1965 volume of Solid State Electronics. 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.

Film examples and applications

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 deposition

Silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

can be deposited using a combination of silicon precursor gasses like dichlorosilane or

silane Silane (Silicane) is an inorganic compound with chemical formula . It is a colorless, pyrophoric gas with a sharp, repulsive, pungent smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental ...

and oxygen precursors, such as

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and

nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...

, typically at pressures from a few millitorr to a few torr. Plasma-deposited

silicon nitride Silicon nitride is a chemical compound of the elements silicon and nitrogen. (''Trisilicon tetranitride'') is the most thermodynamically stable and commercially important of the silicon nitrides, and the term ″''Silicon nitride''″ commonly re ...

, formed from

and

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

, 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 Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

, which can be bonded to

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

(Si-H) or

(Si-NH); this hydrogen has an important influence on IR and UV absorption, stability, mechanical stress, and electrical conductivity. This is often used as a surface and bulk passivating layer for commercial multicrystalline silicon photovoltaic cells. 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 A silanol is a functional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in all alcohols. Silanols are often invoked as intermediates in organosilicon c ...

, and can be unstable in air. 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.

References