For 25 years, thermite evaporation onto thin polymer webs such as polyester (PET) and polypropylene (PP) has produced a large number of barrier packaging films, decorative films, capacitor films and some window films. Wide web processing experience combined with deposition techniques such as electron beam evaporation, magnetron sputtering and plasma enhanced chemical vapor deposition has created a large number of new and exciting coating materials, including oxides and nitrides of most elements. More specifically, the combination of these coatings into composite coating stacks enables new products such as low emissivity, solar heat reflection, architectural glass films, electrochromic devices, and high-performance optical reflectors. With these technologies, unique coatings enable properties such as transparent electrodes, flexible glass barriers for moisture and gases, and amorphous soft magnetic materials for security devices.
To build functional components using vacuum coated mesh, the flexible substrates used for vacuum coating should follow strict quality standards . Table 32.1 surveys most substrates in use today. Many different properties are important for further functionality after coating, for example, mechanical tensile strength, Young's modulus, surface finish, optical clarity (e.g. transparency haze), and resistance to corrosion and UV radiation. Many special additional surface treatments also become indispensable in obtaining the necessary product properties. Table 32.2 lists most coating materials that have been used commercially to date. The recent technical availability of low-cost SiO2 A12O3 coatings has created very interesting building blocks for coating stacks. 2
Since the 1980s, tool coatings by physical vapor deposition (PVD) technology have been a field based on the work of early pioneers and an industry has developed around PVD tool coatings .
Primarily all PVD hard coating technologies are reactive processes where the metal species is vaporized and a gas is fed into the coating chamber which reacts with the metal species to form the desired compound. Furthermore, all successful hard coating processes today involve ion-assisted deposition of hard coatings. Ionizing radiation during deposition ensures a fully dense, well-adhered hard coating. Prior to deposition, all substrates were sputter etched to remove any oxide layer and heat was used to increase the substrate temperature to a range of 450 to 500°C when using high speed steel (HSS) tools.
There are four basic types of equipment used to deposit PVD tool coatings today, all of which fall under the broad category of ion plating. The four types differ in the evaporation or sputtering of the source material; how the plasma is generated; and the number and type of ions, electrons, and gas atoms that make up the plasma. The four PVD hard coating techniques are low voltage electron beam evaporation, cathodic arc deposition, triode high voltage electron beam evaporation,10 and balanced and unbalanced magnetron sputtering.
While these four PVD processes share many similarities, there are also many differences that can have an impact on the types of films that can be deposited in these systems. Each method has advantages and disadvantages, and no one method is suitable for all applications. The low-voltage electron beam evaporation process is very efficient at ionizing the evaporated reactant gas atoms and is estimated to have set the ionization level at 50%. Low-voltage electron beam deposition is a very consistent process that produces very hard and smooth TiN, which has become the industry standard.
Sputter deposition is one of the most complex methods and in many cases more expensive. However, sputter deposition allows better control over the composition of multi-element thin films and greater flexibility in the types of materials that may be deposited.
The main deposition variables that determine film growth kinetics, microstructural evolution, and therefore physical properties of films grown from the vapor phase by techniques such as thermal or electron beam evaporation, chemical vapor deposition, and sputter deposition are the chemical state of the precursors; incident Precursor fluxes, kinetic energies, and trajectories; film growth temperatures; and fluxes and chemical states, surface cleanliness, crystallinity, and orientation of incident contaminants and substrates. These represent properties within which the filmmaker can adjust the deposited material. It is important to know that in some cases the flux of contaminants competes with the flux of the thin film material for incorporation during the deposition process and it is highly dependent on base pressure, pumping speed and reactor vacuum of the design system (for example, whether substrate load locking is used to circumvent repeated air exposure), while substrate surface cleanliness also affects pre-deposition processing.
The kinetic energy of the incident flux during thermal evaporative growth is determined by the following equation. The temperature of the evaporation source is about 0.1 eV. However, in plasma or ion beam deposition techniques, the kinetic energy of the incident flux can increase to hundreds of eV. Low-energy (typically <100 eV) ion irradiation during vapor-phase thin film growth has been shown13,14 to be useful in changing the physical properties of the deposited layer in a controlled manner through trapping, preferential, sputtering, enhanced atomic diffusion, and dynamic collisional mixing. . Different methods of forming the gas phase are widely described. 15 While plasma and ion beam sputtering deposition techniques are emphasized, a brief overview of other vapor deposition techniques related to the topic of the review helps to establish a proper perspective on the formation of multicomponent thin films by plasma and ion beam sputtering deposition. The advantages and disadvantages of the techniques discussed in this review may vary depending on the specific material being incorporated into the thin film.
Advances in thin film research are a striking example of the interplay between fundamental research and practical applications. The need for improved methods and performance helps drive new discoveries that in turn open up even more opportunities for applications. Electronic devices, coatings, displays, sensors, optical devices and many other technologies depend on the deposition of thin films. Even though established methods exist to produce high-quality films, there is still interest in alternatives that may be cheaper, more reliable, or capable of producing films with novel or improved properties.
