Plasma-enhanced chemical vapor deposition

  Plasma Enhanced Chemical Vapor Deposition (PECVD) is a process mainly to deposit thin films from a gas state (vapor) to a solid state on some substrate. There are some chemical reactions involved in the process which occur after creation of a plasma of the reacting gases. The plasma is generally created by RF (AC) frequency or DC discharge between two electrodes where in-between place is filled with the reacting gases.

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Discharges for processing

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 milliTorr 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 K, equivalent to several eV 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.

A 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 voltage between the plasma and the objects it contacts is normally dropped 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 a 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 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 so that significant sputtering of the deposited film occurs; this sputtering can be employed to help planarize the film and fill trenches or holes.

Reactor types

A simple direct-current (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 alternating-current (AC) or radio-frequency (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 10's 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 a dual frequency reactor. Excitation power of tens to hundreds of watts is typical for a 200-mm to 300-mm-diameter electrode.

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.

Film examples

Plasma deposition is often used in semiconductor manufacturing to deposit films onto wafers containing metal layers or other temperature-sensitive structures. Silicon dioxide can be deposited from dichlorosilane or silane and oxygen, typically at pressures from a few hundred 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); this hydrogen has an important influence on UV absorption, stability and mechanical stress, and electrical conductivity.

Oxide can also be deposited from tetraethylorthosilicate (TEOS) 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. 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 oxide film with good conformality over complex surfaces, the latter resulting from intense ion bombardment and consequent sputtering of the deposited from vertical onto horizontal surfaces.

References

• Smith, Donald (1995). Thin-Film Deposition: Principles and Practice. MacGraw-Hill. 
• Lieberman and Lichtenberg (1994). Principles of Plasma Discharges and Materials Processing. Wiley. 
• Dobkin and Zuraw (2003). Principles of Chemical Vapor Deposition. Kluwer. 

Links

• Plasma Enhanced Chemical Vapor Deposition, PECVD