|Nowadays it is not always clear and easy to make the decision how to deposit thin films, because there is such a competition among the alternative techniques, which have been developed to be more and more versatile.
Whether to evaporate, sputter, or chemically deposit a material, depends not only on the material properties and intended applications, but also on the process complexity and operation expenses of the deposition technique.
In some cases a hybrid process needs to be established in order to take advantage of the features of different techniques.
Thermal evaporation is probably the simplest physical-vapor deposition (PVD) process for producing thin films, in which source atoms or molecules (evaporant) receive thermal energy from the heating system to form the vapor phase and subsequently condense on a substrate. This process involves either vaporization when a solid first melts and then transforms into vapor or sublimation when solid-vapor transformation occurs directly. High deposition rates, high vacuum condition and general applicability to all classes of materials are the main reasons for the popularity of this technique.
There are generally two kinds of evaporation sources ― electrically heated and electron-beam heated. Electrically heated evaporation source relies on the Joule heating using resistance or induction heaters, which heat the whole evaporants to its evaporation temperature. The sources can have very different configurations such as wire sources, sheet sources, sublimation furnaces and crucible sources. A key issue with such evaporation sources is that they should not contaminate, react with or alloy with the evaporants, or release gases at the evaporation temperature.
In this respect as well as the energy input, electron-beam heating becomes certainly the preferred evaporation technique. In the e-beam evaporation electrons are thermionically emitted from heated filaments, accelerated by a negative potential on the cathode, and directed to the evaporant charge at ground potential due to the presence of a transverse magnetic field. The purity of the evaporant is assured because only a small amount of the charge melts or sublimes so that the effective crucible is the unmelted skull material surrounded.
Instead of thermal evaporation which is caused by absorption of thermal energy, atoms can also leave a solid source material by means of sputtering, i. e. surface bombardment with energetic particles. Similar to evaporation the emitted atoms in sputtering process travel through a reduced pressure ambient and deposit atomically on a substrate. The source material, also called target, serves as the cathode, to which a DC or RF power supply is connected whereas the substrate serves as the anode, which may be floating, grounded or biased.
After the vacuum chamber is filled with a working gas, typically argon, an electrical discharge (plasma) can be initiated by applying a voltage between the cathode and the anode. The positively ionized gas atoms in the plasma are accelerated towards the target due to the potential drop in the vicinity of the target surface and strike out atoms which pass through the plasma and condense on the substrate to form the desired thin films.
There are several variants of sputtering process, namely DC, RF, reactive and magnetron sputtering. These terms are however about different aspects and what in practice used are usually hybrids of them. There are a number of advantages of sputtering technique. Except for high rate and large area it enables also deposition of alloys and composites with components having very different vapor pressure. The films exhibit in general low surface roughness, high density, high lateral homogeneity and good adhesion to the substrate.
Furthermore, sputtering targets of almost all technical materials are nowadays commercially available, no matter metals, semiconductors or oxides, fluorides, borides and nitrides. These materials can normally be manufactured in a variety of shapes, for instance as rectangular and circular disks or as toroids. These properties make sputtering a very popular technique both for scientific research and industrial production. The coating service offered by ALINESON is mostly carried out by magnetron sputtering.
Pulsed laser deposition
Pulsed laser deposition (PLD) is another PVD process, which becomes more and more attractive nowadays for growing high-quality epitaxial thin films. Originally it was classified as an unconventional variant of evaporation process, since PLD involves also vaporization of materials except the “heating system” is a high-power laser source. Nowadays PLD is rather considered as an individual deposition technique due to its substantial difference in configuration and application compared to evaporation.
In PLD process materials are ablated from a solid target by high-power pulsed laser, usually with ultraviolet wavelength. The ablation process produces a transient, highly luminous plasma plume which contains neutrals, ions, electrons etc. The plasma plume expands away from the target surface and interacts with the chamber atmosphere until it reaches the substrate, where the films are deposited. Several advantages make PLD a favorable technique for growing dielectrics and superconductors. It has the capability of highly stoichiometric transfer of materials from target to substrate, which allows growth of complex multicomponent films with small pieces of bulk material. Moreover, the usage of external energy results in an extremely clean process with background gas being either inert or reactive. The conducting oxides as well as dielectric and ferroelectric thin films offered by ALINEASON are mainly realized by this method.
Metal-organic vapor-phase epitaxy
Beside the abovementioned PVD processes, chemical vapor deposition (CVD) is also a very widely used technique for thin film growth. Instead of material transfer from condensed-phase evaporant or target, CVD uses gaseous reactants (precursors) at moderate pressure for the thin film formation.
CVD is a complex process and involves generally several sequential steps. During the process the substrate is placed under constant gas flow of precursors. Chemical reactions in the gas phase produce new reactive species and by-products in the reaction zone. These initial reactants and their products are then transported to the substrate surface through chemical or physical adsorption. Heterogeneous reactions between these reactants are catalyzed by the surface and lead to nucleation and film growth. Volatile by-products of the surface reactions leave the surface by desorption and are transported away from the reaction zone.
Among a variety different CVD processes, metal-organic vapor-phase epitaxy (MOVPE), also called metal-organic chemical vapor deposition (MOCVD), becomes nowadays the dominant technique for making optoelectronic devices based on compound semiconductors. This technique is utilized by ALINEASON for offering high-quality epitaxial films of III-V semiconductors such GaAs and GaN.