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Handbook of Sputter Deposition Technology
Fundamentals and Applications for Functional Thin Films, Nanomaterials, and MEMS
By Kiyotaka Wasa, Isaku Kanno, Hidetoshi Kotera
ElsevierCopyright © 2012 Elsevier Inc.
All rights reserved.
Thin Films and Nanomaterials
Hideaki Adachi and Kiyotaka Wasa
1.1 Thin Films and Nanomaterials 4
1.2 Thin Film Devices and MEMS 10
1.2.1 Thin Film Devices 10
1.2.2 Thin Film MEMS 12
1.3 Thin Film Deposition 14
1.3.1 Physical Vapor Deposition, PVD 14
Vacuum Deposition 14
Pulsed Laser Deposition 16
Molecular Beam Epitaxy 17
Miscellaneous PVD Processes 22
1.3.2 Chemical Vapor Deposition, CVD 23
Thermally Activated CVD 23
Plasma-Assisted CVD 24
1.4 Characterization of Thin Films 25
1.5 Sputtering as a Nanomaterial Engineering 28
Thin films are fabricated by the deposition of material atoms on a substrate. A thin film is defined as a low-dimensional material created by condensing, one-by-one, atomic/molecular/ionic species of matter. The thickness is typically less than several micron-meters. The thin films are typically deposited by a thermal evaporation of source materials in vacuum. Figure 1.1 shows the typical evaporation process for the deposition of thin films.
Historically, thin films have been used for more than a half century in making electronic devices, optical coatings, instrument hard coatings, and decorative parts. In 1960s, thin film transistors (TFT) and/or thin film integrated circuits (ICs) were extensively studied. These trials were not used in practice due to the drift of the TFT. After the developments of Si-(MOS) (Metal Oxide Semiconductor) IC in 1970s, thin film materials were used only for passive devices. The real market of thin films could not be developed like Si-IC. A variety of new materials was developed in academic phase such as a diamond-like carbon (DLC), high-Tc superconductors. The thin film technology is a well-established material processing technology. However, the thin film technology is still being developed on a daily basis, since it is a key in the twenty-first century development of new functional materials such as nanometer materials and/or man-made superlattices.
Thin film materials and devices are also available for minimization of toxic materials, since the quantity used is limited only to a surface and/or thin film layer. Thin film processing also saves energy consumption in production and is considered to be an environment-benign material technology.
Thin film technology is both an old and a current key material technology. Thin film materials and deposition processes have been reviewed in several publications. Among the earlier publications, the Handbook of Thin Film Technology is still notable, even though 40 years have passed since the book was published and many new and exciting developments have occurred in the intervening years.
At the beginning of this chapter, the features of the thin films are reviewed in relation to their applications for thin film devices and/or thin film micro-electromechanical systems (MEMS). Secondly, fundamentals of thin film fabrication processes are explained for a better understanding of the sputtering deposition.
1.1 Thin Films and Nanomaterials
Thin films are grown by the deposition of material atoms on a substrate. Typical thin film growth process on a substrate by the deposition material atoms is shown in Fig. 1.2.
The thin film growth exhibits the following features:
1. The birth of thin films of all materials starts with a random nucleation process followed by nucleation and growth stages.
2. The nucleation and growth stages are dependent upon various deposition conditions, such as growth temperature, growth rate, and substrate surface chemistry.
3. The nucleation stage can be modified by external agencies, such as electron or ion bombardments.
4. Film microstructure, associated defect structure, and film stress depend on the deposition condition of the nuclear stage.
5. Crystal phase and crystal orientation of the thin films are governed by the deposition conditions.
The basic properties of thin films, such as film chemical composition, structural propperties, film thickness, are controlled by the deposition conditions. The thin films exhibit unique properties that cannot be observed in bulk materials:
1. Unique material properties resulting from the atomic growth process on the growing substrates.
2. Size effects including quantum size effects characterized by the thickness, crystal orientation, and multilayer aspects.
Bulk materials are usually sintered from powder of source materials. The particle size of the powder is of the order of 1 m in diameter. Thin films aaaare synthesized from ultrafine particles, i.e., atoms or a cluster of atoms. Ultrauniform compound materials are possibly synthesized from the atomic collisions between the adatoms on a substrate surface.
Another consequence of the thin film growth process is the phenomenon of solubility relaxation. The atomistic process of growth during codeposition allows doping and alloying of films. Since thin films are formed from individual atomic, molecular, or ionic species, which have no solubility restrictions in the vapor phase, the solubility conditions between different materials are considerably relaxed. This allows the preparation of multicomponent materials, such as alloys and compounds over an extended range of compositions as compared to the corresponding bulk materials. It is thus possible to have tailor-made materials with desired properties, which adds a new and exciting dimension to materials technology. An important example of this technology of tailor-made materials is the formation of hydrogenated amorphous Si films for use in solar cells. Hydrogenation has made it possible to vary the optical band gap of amorphous Si from 1 to about 2 eV and to decrease the density of dangling bond states in the band gap so that doping a (n and p) is made possible.
The properties of thin films are governed by the deposition method. The thermal evaporation is a well-known process. The deposition process using the irradiation of energetic species is known as sputtering. Bunsen and Grove first observed sputtering phenomena in a gas discharge tube over 150 years ago. The cathode electrode was disintegrated by the discharge. Since that time, the basic level of understanding of the sputtering process has been fairly well developed. It was known that the disintegration of the cathode materials was caused by irradiation of the cathode surface by highly energetic ions. The removed particles, called sputtered species, were composed of highly energetic atoms. Their energy ranges were 1–10 eV, which was higher than those of the other deposition processes such as the thermal evaporation in vacuum. The energetic sputtered species lower the synthesis temperature. Typical example of lowering the growth temperature is diamond growth at room temperature. The known bulk diamonds are synthesized at high pressure (~50,000 psi) and high temperature (2000°C). The deposition of energetic carbon ions (~10–100 eV) enables the growth of cubic diamond crystallites at room temperature as shown in Fig. 1.3. It is possible to synthesize a hexagonal diamond at room temperature. Natural diamonds are cubic phase which is stable on the earth. Hexagonal diamonds cannot not be grown under thermodynamic equilibrium conditions; rather they are grown under nonthermal equilibrium conditions. The growth of hexagonal diamonds suggests that the thin film process provides exotic materials of nonthermal equilibrium phase.
Similar to the growth of hexagonal diamonds, varieties of exotic materials are synthesized based on the thin film processes such as superconducting cuprate of layered oxide perovskite compounds with high-transition temperature, Tc spin-dependent tunneling magnetoresistance (TMR) effect, stressed perovskite ferroelectric thin films with high-Curie temperature, thickness and/or crystalline size effects on dielectric constants of perovskite ferroelectrics, layered ferroelectric perovskite thin films with a giant permittivity, and/or with pseudopyroelectric effects. The stress affects the superconducting critical temperature for both metal superconducting thin films and the high-Tc cuprate thin films. Thin films further exhibit variety of interesting performance including intrinsic Josephson junctions in the high-Tc curates and giant magnetoresistance (GMR) effects in multilayer.
The thin film process is also available for the fabrication of the nanometer materials. Nanomaterials are defined as follows: materials or components thereof in alloys, compounds, or composites having one or more dimensions of nanometer size (1 nm = 10-9 m = 10 A). The nanomaterials are classified into three types:
1. Zero-dimensional nanomaterials have all three dimensions of nanometer size (e.g., quantum dots).
2. One-dimensional nanomaterials have two dimensions of nanometer size (e.g., quantum wires).
3. Two-dimensional nanomaterials have one dimension of nanometer size (e.g., thin films, superlattices).
The phenomenological dimensionality of nanomaterials depends on the size relative to physical parameters such as quantum confinement regime (≤100 atoms), mean free path of conduction electron (<10 nm), mean free path of hot electron (≤1 nm), Bohr excitation diameter (Si = 8.5 nm, CdS = 6 nm, GaAs = 196 nm), de Broglie wavelength (<1 nm). The three types of nanomaterials have been successfully synthesized by the thin film deposition processes such as codeposition, layerby-layer deposition in an atomic scale, and nanolithography. The typical structure of the nanometer superlattices produced by the thin film process are shown in Fig. 1.4.
The current progress in thin film research is much indebted to the atomic observation technology including the scanning tunneling microscope (STM) developed by Binnig and Rohrer.
Table 1.1 summarizes the interesting phenomena expected in thin film materials and devices.
1.2 Thin Film Devices and MEMS
1.2.1 Thin Film Devices
Since the latter part of the 1950s, thin films have been extensively studied in relation to their applications for making electronic devices. In the early 1960s, Weimer proposed TFT composed of CdS semiconducting films. He succeeded in making a 256-stage TFT decoder, driven by two 16-stage shift resistors, for television scanning, and associated photoconductors, capacitors, and resistors. Although these thin film devices were considered as the best development of both the science and technology of thin films for an integrated microelectronic circuit, the poor stability observed in TFTs was an impediment to practical use. The bulk silicon carbide (SiC) MOS devices were successfully developed at the end of 1960s. Thus, in the 1960s, thin film devices for practical use were limited to passive devices such as thin film resistors and capacitors. In the 1970s, several novel thin film devices were proposed, including thin film surface acoustic wave (SAW) devices, and integrated thin film bulk acoustic wave (BAW) devices, and thin film integrated optics.
Excerpted from Handbook of Sputter Deposition Technology by Kiyotaka Wasa, Isaku Kanno, Hidetoshi Kotera. Copyright © 2012 Elsevier Inc.. Excerpted by permission of Elsevier.
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