Chemical Physics of Thin Film Deposition Processes for Micro- and Nano-Technologies

Metallic thin films are deposited with electroplating and electroless processes by electrolysis: metallic ions in an aqueous solution are reduced to metal atoms.

Techniques used to deposit layers of materials generally depend on the size of the chemical entity used as a starting building block or precursor. For example, smaller entities (i.e., organometallic molecules) are preferably used under their gas form; larger ones (particles) require using other approaches, including Langmuir-Blodgett technique, layer-by- layer assembly or electrodeposition. When preformed nanosized objects are used to fabricate multilayer heterostructures soft-solution processing methods are preferred (electrochemical means will not be considered in that chapter). The choice of the method is dictated by the stability of the particles toward compression, oxidation, solubilization, chemicals, etc. At the nanometer scale, electrostatic, van der Waals, hydrophobic/hydrophilic, charge-transfer, π-π interactions, metal ligand coordination and hydrogen bonding become the predominant forces promoting an adsorption. The term “self” in self-assembly accounts for the fact that the building block units carry physical and chemical characteristics determining the “gluing” when interacting with an appropriate surface, It necessarily implies that the object is free of moving in a fluid (liquid) in order to reach the surface onto which it adsorbs. Such an object will be called a colloid, organic (polymers, surfactants) or inorganic (semiconducting or metallic nanoparticles, exfoliated sheets of a layered compound), with a size varying from few nanometers to a micrometer in one dimension at least.

Liquid phase epitaxy (LPE) has been applied for growing mainly semiconductor materials [1, 2]. It may be required, when electrical and crystallographic quality of wafers which are cut from bulk crystals is not good enough for making active devices directly on wafers, to grow same materials as substrates but better quality on wafers (homoepitaxy). Epitaxial growth can be also applied for making stacked multi-layer structures of several layers for such as integrated circuits or solar cells. In order to combine more than two different materials heteroepitaxy can be performed. Much efforts are made recently, using LPE techniques, to grow crystalline semiconductor layers on cheap substrates, ceramics or glass, for aiming to fabricate solar cells with higher energy conversion efficiency.

The development of inorganic functional thin films is driven by applications in electronics, solar technology, optics and other high-tech fields. A wide range of film compositions can be manufactured by gas phase or liquid phase deposition methods. Due to the high apparative costs of gas phase methods, the use of the sol-gel processing offers advantages by sufficiently inexpensive film technologies. Additionally, purity and stability of the precursors, homogeneity of mixed precursors, comparably low processing temperatures to transfer gel films into pure inorganic films can be used to generate high-performance thin films.

The application of metal-organic chemistry has played a major role in the development of thin film deposition by Chemical Vapour Deposition (CVD) and Sol-Gel techniques [1‐2]. The success of chemical synthesis routes is largely attributed to the availability of molecular compounds that can be transformed via solution (Sol-Gel) [3‐6] or gas phase (CVD) [7,8] reactions into high-purity coatings of desired ceramics or composites. In contrast to the solid-state reactions, the reactions in vapor or liquid phase allow a controlled interaction of atoms or molecules to form uniform films or particles. Further, the flexibility to combine different ligand or metal combinations allows the precursor designing to meet the demands of the target material. Assembling all the phase-forming elements in a single molecular source augments the advantages of chemical processing and simultaneously reduces the process parameters. In addition, the molecule-to-material transformation requires much lower temperatures than those required for the conventional (mixing, grinding and calcining) methods [9]. The clear practical implications of nanostructured materials [10,11] with a precise control over composition, size, size distribution and morphology has led to an upsurge of research activity in the synthesis and chemical processing of molecular precursors [12‐17]. Among the various inorganic compounds—halides, nitrates, acetates, carboxylates, ß-diketonates, alkyls, alkoxides—used in the synthesis of metal oxides, metal alkoxides (M(0R)_n) are especially attractive as precursors [18‐24]. Some of their salient features include high purity, easy transformation into oxides with formation of volatile byproducts, ability to form homogeneous solution in different solvents and conditions and more importantly the facile formation of heterometal species useful for the synthesis of multicomponent materials [25]. The present article is intended to provide a brief account of the recent developments in the field of heterometal alkoxide chemistry and their applications in obtaining nanocrystalline thin films.

Chemical Vapour Deposition (CVD) processes constitute an important technology for manufacturing thin solid films. Applications include various films on wafers in the IC-industry, decorative coatings, anti-reflection and spectrally selective coatings on optical components, and anti-corrosion and anti-wear layers on mechanical tools. CVD is very versatile and offers good control of film structure and composition, excellent uniformity, and the capability of conformal deposition on highly irregularly shaped surfaces. CVD processes have been reviewed in e.g. refs. [l]-[6].

Chemical Vapor Deposition (CVD) of oxides is a very large branch in the field of CVD processes and is always a large part in books about CVD [1].

Selective area deposition has received much attention in IC technology in the past forty years. Its advantage in IC technology is that one saves a mask and a full sequence of lithography, etching, resist removal and cleaning. In Selective Chemical Vapor Deposition (CVD) the selectivity is obtained by the different chemical behavior of reactants with different surfaces. The advantage of selective CVD is the self-alignment with respect to the previous pattern, which allows for tight design-rules in this phase of the IC production. Selective epitaxial Silicon deposition was investigated in the sixties of the last century. Later selective Tungsten, selective epitaxial SiGe, selective IH-V and II-VI compounds and recently selective deposition of Copper became intensively researched subjects. In these cases of selective deposition one etches a window in a dielectric to the substrate and then deposits the required layer. Due to nucleation matters it starts to grow immediately on the substrate material whereas the nucleation on the dielectric is retarded. However, in nature one never gets advantages for free. Selectivity loss, reaction with the substrate material, facetting, lateral overgrowth on the dielectric and pattern-density dependency are major problems.

These lectures cover an aspect of energy-assisted chemical vapour deposition (CVD) that involves the use of light, either in the UV or visible, to bring about some beneficial change in the deposition process. This can entail an enhancement in deposition rate or an improvement in film quality such as density, composition or reduced defect concentration. The range of materials covered in these lectures will include oxides, semiconductors and metals. The interaction of light with either the precursor vapour or the substrate can enable a stimulation of the precursor reaction by a number of different mechanisms that will be explored in these lectures. The practical realisation of photo-assisted CVD can be complex but improved light sources and reactor designs now offers a wider choice in achieving a practical system. The choice of light sources, precursors and reaction chambers will be covered in some depth.

In the early 1980s, a large number of scientific works have been dedicated to laser processing of materials, in particular in the field of microelectronic materials and devices for various applications such as wafer marking, substrate surface cleaning, doping and oxidation of silicon, etching and deposition of thin films, exposure or removal of photo-resists, and recrystallization of silicon on insulator substrates [1‐3]. The desirability of using a focused laser beam for maskless fabrication and alteration of integrated circuits has also been recognized [4].

Today’s ICs technology is based on MOSFET whose dimensions are shrunken as integration increases. It is wellknown that this elementary device in its present configuration (a channel and a single gate) could not be used any more for channel lengths lower than about 30 nm [1]. In fact, beyond this limit, two main problems will appear due to a small size effect. The first one is the dopant number (near unity) fluctuation in the channel from one device to another which will affect the device characteristics reliability. The second problem is the appearance of extra physical phenomenon such as ballistic transport or tunnel current flow through the oxide gate. Moreover, if the circuit integration increases but if the commutation still needs to exchange a great number of electrons (presently roughly 10⁴ electrons for a MOSFET), the energy dissipated in the interconnection layout will increase drastically. Consequently, in order to be able to keep on integration beyond this size limit, MOSFET configuration has to be modified [2] or new components, based on new physical phenomena and involving a lower number of electrons for switching must be designed to replace MOSFET in ICs. Single Electron Transistors (SET), whose principle is based on Coulomb blockade effect [3], are now considered as ideal devices to replace FET in memories.

Hyperthermal atoms are deposited upon a substrate in thin film deposition processes. Even when the atoms in the vapor phase are not intentionally accelerated to the substrate, the vapor phase atoms are attracted to the substrate surface with a potential of a few electron volts (eV) because of the interaction between the incoming atom and the substrate. Some deposition processes such as ion beam assisted deposition(IBAD), ion beam deposition (IBD), sputter deposition(S), and plasma enhanced chemical vapor deposition (PECVD)result in ions striking the substrate with energies from 10 to over 100 eV as shown in figure 1 [1].

Thin films deposition techniques are generally classified in two main groups: Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD). The last one encompasses sputtering and evaporation. They are applied, dependent on particular requirements of the production technology. Obviously, they have their specific advantages and simultaneously introduce given limitations. Below, selected PVD techniques will be classified and described in details. Special emphasis will be given to the group of sputtering techniques whereas high vacuum evaporation and its modifications go beyond the frame of this paper.

The kinetics of nitriding has been the focus of much research since the introduction of the process [1‐3]. Due to unusually advantageous combination of properties the nitriding of an austenitic AISI 304 stainless steel is widely studied. It is shown that ion nitriding provided by ion beam/plasma techniques at elevated temperature (∼400°C) is a potential candidate to overcome the problem of enhancing surface hardness and wear resistance of an austenitic stainless steel without decreasing their corrosion resistance. Gas and liquid nitriding need temperatures between 500 and 600°C, which are detrimental to the corrosion resistance due to structural transformation of alloys.