Handbook Of Thin Film Deposition Processes And Techniques Materials And Processing Technology-adds 1
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A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, integrated passive devices, LEDs, optical coatings (such as antireflective coatings), hard coatings on cutting tools, and for both energy generation (e.g. thin-film solar cells) and storage (thin-film batteries). It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.
In addition to their applied interest, thin films play an important role in the development and study of materials with new and unique properties. Examples include multiferroic materials, and superlattices that allow the study of quantum phenomena.
Nucleation is an important step in growth that helps determine the final structure of a thin film. Many growth methods rely on nucleation control such as atomic-layer epitaxy (atomic layer deposition). Nucleation can be modeled by characterizing surface process of adsorption, desorption, and surface diffusion.[1]
It is useful in the manufacture of optics (for reflective, anti-reflective coatings or self-cleaning glass, for instance), electronics (layers of insulators, semiconductors, and conductors form integrated circuits), packaging (i.e., aluminium-coated PET film), and in contemporary art (see the work of Larry Bell). Similar processes are sometimes used where thickness is not important: for instance, the purification of copper by electroplating, and the deposition of silicon and enriched uranium by a CVD-like process after gas-phase processing.
Here, a fluid precursor undergoes a chemical change at a solid surface, leaving a solid layer. An everyday example is the formation of soot on a cool object when it is placed inside a flame. Since the fluid surrounds the solid object, deposition happens on every surface, with little regard to direction; thin films from chemical deposition techniques tend to be conformal, rather than directional.
Chemical solution deposition (CSD) or chemical bath deposition (CBD) uses a liquid precursor, usually a solution of organometallic powders dissolved in an organic solvent. This is a relatively inexpensive, simple thin-film process that produces stoichiometrically accurate crystalline phases. This technique is also known as the sol-gel method because the 'sol' (or solution) gradually evolves towards the formation of a gel-like diphasic system.
Atomic layer deposition (ALD), and its sister technique molecular layer deposition (MLD), uses gaseous precursor to deposit conformal thin film's one layer at a time. The process is split up into two half reactions, run in sequence and repeated for each layer, in order to ensure total layer saturation before beginning the next layer. Therefore, one reactant is deposited first, and then the second reactant is deposited, during which a chemical reaction occurs on the substrate, forming the desired composition. As a result of the stepwise, the process is slower than CVD, however it can be run at low temperatures, unlike CVD.
Physical deposition uses mechanical, electromechanical or thermodynamic means to produce a thin film of solid. An everyday example is the formation of frost. Since most engineering materials are held together by relatively high energies, and chemical reactions are not used to store these energies, commercial physical deposition systems tend to require a low-pressure vapor environment to function properly; most can be classified as physical vapor deposition (PVD).
Electrohydrodynamic deposition (electrospray deposition) is a relatively new process of thin-film deposition. The liquid to be deposited, either in the form of nanoparticle solution or simply a solution, is fed to a small capillary nozzle (usually metallic) which is connected to a high voltage. The substrate on which the film has to be deposited is connected to ground. Through the influence of electric field, the liquid coming out of the nozzle takes a conical shape (Taylor cone) and at the apex of the cone a thin jet emanates which disintegrates into very fine and small positively charged droplets under the influence of Rayleigh charge limit. The droplets keep getting smaller and smaller and ultimately get deposited on the substrate as a uniform thin layer.
Stress and relaxation of stresses in films can influence the materials properties of the film, such as mass transport in microelectronics applications. Therefore precautions are taken to either mitigate or produce such stresses; for example a buffer layer may be deposited between the substrate and film.[18] Strain engineering is also used to produce various phase and domain structures in thin films such as in the domain structure of the ferroelectric Lead Zirconate Titanate (PZT).[20]
The usage of thin films for decorative coatings probably represents their oldest application. This encompasses ca. 100 nm thin gold leaves that were already used in ancient India more than 5000 years ago. It may also be understood as any form of painting, although this kind of work is generally considered as an arts craft rather than an engineering or scientific discipline. Today, thin-film materials of variable thickness and high refractive index like titanium dioxide are often applied for decorative coatings on glass for instance, causing a rainbow-color appearance like oil on water. In addition, intransparent gold-colored surfaces may either be prepared by sputtering of gold or titanium nitride.
Thin-film technologies are also being developed as a means of substantially reducing the cost of solar cells. The rationale for this is thin-film solar cells are cheaper to manufacture owing to their reduced material costs, energy costs, handling costs and capital costs. This is especially represented in the use of printed electronics (roll-to-roll) processes. Other thin-film technologies, that are still in an early stage of ongoing research or with limited commercial availability, are often classified as emerging or third generation photovoltaic cells and include, organic, dye-sensitized, and polymer solar cells, as well as quantum dot,[29] copper zinc tin sulfide, nanocrystal and perovskite solar cells.[30][31]
The use of spin coating in organic electronics and nanotechnology is widespread and has built upon many of the techniques used in other semiconductor industries. The relatively thin films and high uniformity required for effective device preparation, as well as the need for self-assembly and organisation to occur during the casting process, do however necessitate some differences in method.
For most standard spin coating techniques the objective is to keep the substrate spinning until the film is fully dry. As such this will mainly depend upon the boiling point and vapour pressure of the solvent that is used but also on the ambient conditions (temperature and humidity) that the spin coating is performed in. For most solvents (such as the list below) a spin coating duration of 30 seconds is usually more than adequate and is therefore recommended as a starting point for most processes.
Control of this process is critical. In practice, this means that when spin coating nanoparticles for OE/nano applications, the properties of the film don't just depend upon its physical characteristics (thickness, uniformity) but also very strongly upon the processing (drying time and conditions). By comparison, in many more general spin coating processes, such as applying a photoresist, the end result is broadly independent of the exact application route. For this reason, variations on the usual technique are often used in organic electronics and nanotech - especially in a research environment.
Many researchers will have faced issues coating films where the solute has poor or marginal solubility in the solvent used. This is especially common when using high molecular weight materials, or when trying to replace the solvent in an established system. Solutions with poor solubility can produce films with unwanted precipitation, especially as solvent evaporation drives the concentration above the solubility limit, leading to comets or particulates in the final film. Precipitation can be avoided by keeping solutions below the solubility limit of the material, but this may lead to deposited films being too thin.
For these types of materials, final film thickness will not always be proportional to the inverse square of the spin speed - so film thickness can be difficult to predict, and final films are not always level. For a more in-depth explanation of the reasons behind this, please refer to the spin coating film thickness equations. Due to their diverse range of behaviours, non-Newtonian solutions can present a significant challenge when it comes to the deposition of highly-uniform films.
Thin-film products typically incorporate very thin layers of photovoltaicly active material placed on a glass superstrate or a metal substrate using vacuum-deposition manufacturing techniques similar to those employed in the coating of architectural glass. Presently, commercial thin-film materials deliver about 4-5 watts per ft² of PV array area (under full sun). Thin-film technologies hold out the promise of lower costs due to much lower requirements for active materials and energy in their production when compared to thick-crystal products. 2b1af7f3a8