Characterization thin-film properties

The technological importance of thin films in snch areas as semicondnctor devices and sensors has led to a demand for mechanical property infonnation for these systems. Measuring the elastic modnlns for thin films is mnch harder than the corresponding measurement for bnlk samples, since the results obtained by traditional indentation methods are strongly perturbed by the properties of the substrate material. Additionally, the behaviour of the film under conditions of low load, which is necessary for the measnrement of thin-film properties, is strongly inflnenced by surface forces [75]. Since the force microscope is both sensitive to surface forces and has extremely high depth resolntion, it shows considerable promise as a teclnhqne for the mechanical characterization of thin films. 

Acoustic wave (AW) devices are ideally suited to thin film characterization due to their extreme sensitivity to thin film properties [10]. The sensitivity of AW devices to a variety of film properties (see Chapter 3), such as mass density, viscoelasticity and conductivity, makes them versatile characterization tools. The ability to rapidly monitor changes in device responses resulting from changes in thin film properties permits their use for monitoring dynamic processes such as film deposition, chemical modification (e.g., photo-polymerization, corrosion), and diffusion of species into and out of films. 

In this chapter, we explore the current and potential future applications of AW devices for materials characterization and process monitoring. Because of the limited mass of material that can be applied to the AW device surface, the majority of these applications deal with the chemical and physical characterization of thin-film properties. This thin film focus should not be thought of as a limitation of AW devices, but rather as a useful capability — the direct measurement of properties of materials in thin-film form. Since material properties can depend on the physical form (e.g., film, bulk) of the material (see Section 4.3.1.3), AW devices are uniquely suited to directly characterize thin-film materials. These considerations also indicate that even though it is possible to use AW thin-film data to predict bulk material properties, such extrapolations should be performed with care. 

In conclusion, the results presented in this chapter demonstrate the extreme versatility of AW devices for the characterization of materials. The inherent sensitivity of AW properties to the mechanical and electrical properties of thin films can be used to advantage to directly monitor a wide variety of film properties. Since the properties and behavior of thin-film materials can be very different from those of similar bulk materials, this ability to directly measure thin film properties can be a significant advantage in materials research and development. The ability to use thin films instead of bulk samples has the added advantage that the time required to perform an evaluation of dynamic processes such as diffusion and corrosion can be greatly decreased. The number of applications of AW devices to thin-film characterization continues to increase, and is limited only by the ingenuity of AW device researchers and developers. 

The details of thin-film formation by PVD or CVD on the atomic and molecular scale are unknown in most cases, but such knowledge would be very helpful to design new processes and to tailor film properties. Information is lacking due to the high reactivities, short lifetimes, and low concentrations of the relevant transient gas-phase species. Furthermore, many of the thin-film properties in statu nascendi are unknown due to the experimental difficulties of thin-film characterization during deposition, particularly with non-crystalline films and if established methods such as RHEED and FEED cannot be applied. Among the film properties, mechanical stress in thin films can lead to unwanted (uncontrolled) instabilities and peel-off phenomena. Therefore, in situ diagnostic methods have been developed to... 

Raman spectroscopy and Fourier Transform Raman spectroscopy (FTRS) [314,315] can detect vibrational motion in polymers but are less commonly employed in polymer blend characterization than FTIR nevertheless they offer utility in characterization of crystalline polymer morphology, conjugated polymers, thin film properties and surface modification as well as in... 

Physical Properties. Raman spectroscopy is an excellent tool for investigating stress and strain in many different materials (see Materlals reliability). Lattice strain distribution measurements in siUcon are a classic case. More recent examples of this include the characterization of thin films (56), and measurements of stress and relaxation in silicon—germanium layers (57). 

Films or membranes of silkworm silk have been produced by air-drying aqueous solutions prepared from the concentrated salts, followed by dialysis (11,28). The films, which are water soluble, generally contain silk in the silk I conformation with a significant content of random coil. Many different treatments have been used to modify these films to decrease their water solubiUty by converting silk I to silk II in a process found usehil for enzyme entrapment (28). Silk membranes have also been cast from fibroin solutions and characterized for permeation properties. Oxygen and water vapor transmission rates were dependent on the exposure conditions to methanol to faciUtate the conversion to silk II (29). Thin monolayer films have been formed from solubilized silkworm silk using Langmuir techniques to faciUtate stmctural characterization of the protein (30). ResolubiLized silkworm cocoon silk has been spun into fibers (31), as have recombinant silkworm silks (32). 

XRD is an excellenr, nondestructive method for identifying phases and characterizing the structural properties of thin films and multilayers. It is inexpensive and easy to implement. The future will see more use of GIXD and depth dependent measurements, since these provide important information and can be carried out on lab-based equipment (rather than requiring synchrotron radiation). Position sensitive detectors will continue to replace counters and photographic film. 

The 10 volumes in the Series on characterization of particular materials classes include volumes on silicon processir, metals and alloys, catalytic materials, integrated circuit packaging, etc. Characterization is approached from the materials user s point of view. Thus, in general, the format is based on properties, processing steps, materials classification, etc., rather than on a technique. The emphasis of all volumes is on surfaces, interfaces, and thin films, but the emphasis varies depending on the relative importance of these areas for the materials class concerned. Appendixes in each volume reproduce the relevant one-page summaries from the Encyclopedia and provide longer summaries for any techniques referred to that are not covered in the Encyclopedia. 

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

The area of colloids, surfactants, and fluid interfaces is large in scope. It encompasses all fluid-fluid and fluid-solid systems in which interfacial properties play a dominant role in determining the behavior of the overall system. Such systems are often characterized by large surface-to-volume ratios (e.g., thin films, sols, and foams) and by the formation of macroscopic assembhes of molecules (e.g., colloids, micelles, vesicles, and Langmuir-Blodgett films). The peculiar properties of the interfaces in such media give rise to these otherwise unlikely (and often inherently unstable) structures. 

A comprehensive work on the electrodeposition chemistry and characterization of anodically synthesized CdTe thin films has been presented by Ham et al. [98]. In this work, along with the electrolytic anodic synthesis of CdTe by using Cd anodes in alkaline solutions of sodium telluride, an electroless route of anodizing a Cd electrode held at open circuit in the same solution was also introduced. The anodic method was expected to produce CdTe with little contamination from Te on account of the thermodynamic properties of the system the open-circuit potential of Cd anodes in the Te electrolyte lies negative of the Te redox point, so... 

The work on carbon nitride solids is strongly related to research on diamondlike carbon (DLC) materials [5, 6]. DLC materials are thin film amorphous metastable carbon-based solids, pure or alloyed with hydrogen, which have properties similar to that of crystalline diamond (high hardness, low friction coefficient, high resistance to wear and chemical attack). This resemblance to diamond is due to the DLC structure, which is characterized by a high fraction of highly cross-linked sp -hybridized carbon atoms. To obtain this diamond-like structure... 

---