The science of thin film is one of the oldest arts and its concurrently one of the newest sciences. If we consider the art of gold beating, thin film dates back to the metal ages. The Egyptians were the early practitioners of gold beating and gilding. They used gold's great malleability to hammer it to extremely thin leaf while its magnificence and resistance to chemical attack have made it a smart choice for durable ornamentation and protection purposes. Now-a-days, gold leafs can be machine beaten in the range of 0.05-0.1 Âµm by skilled craftsman and makes it almost invisible sideways. Therefore it is quite understandable why gold beaters were called upon to provide the first metal sample to be observed under TEM. Although not as old as gold beating, thin film technology such as mercury and fire gilding were related to gold beating. All these techniques were used for experimentation and process development purpose in the prehistoric era. Practitioner at that day were concerned with the price and purity of gold, surface preparation, uniform application of film, good adhesion to the substrate, reaction between the film and substrate, appearance, color, resistance to wear and safety. The modern thin film technology that we have today addresses the same generic issues but with more insight due the advances in micro and nano level technologies.
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Thin films are thin material layers that vary from fraction of a nanometer to a few micrometers in thickness. Thin film consists of a layer of material having two dimensions much larger than the other one. The thickness of thin film is a lot less than its lateral dimension as shown in figure 1. The thickness of the film is a lot less than the thickness of the substrate on which it is deposited as shown in figure 2.
Figure 1: Thin film thickness
Figure 2: Thin film and substrate thickness
The first evaporated thin films were obtained by Faraday in 1857 . After that evaporated thin films were confined to academic research. During this time thin films were developed for various applications and film properties. In recent years thin films were being used in industries for various purposes. Examples of thin film applications are as follows :
Microelectronic integrated circuits
Magnetic information storage systems
Wear resistant coatings
Corrosion resistant coatings
The reasons for popularity of thin film in the above mentioned applications are low cost, scale dependent physical properties (optical filters) and small devices (I.C.'s magnetic storage). The mechanical properties of thin film are different from those of the bulk material because of the nanostructure of the thin film and its adhesion to a substrate. Thin films possess good thermal stability and reasonably high hardness but they are very fragile. For thin films to be used in mechanical devices the following properties play an important role:
Usually thin films possess very high yield strength allowing them to handle high residual stresses. The residual stress becomes relieved during processing or operation by means of plastic deformation, thin film fracture or interfacial delamination. The mechanical properties of thin film can be obtained by means of tensile test of freestanding films, microbeam cantilever deflection technique and nanoindentation. By means of nanoindentation the elastic modulus, hardness, interfacial adhesion and film fracture toughness can be measured. In nanoindentation a sharp diamond indenter is pushed into the specimen, both force and indentation depths are measured. Failure of the thin film can occur due to inhomogeneous plastic flow. Thin film debonding is usually due to tensile stresses, low interfacial strength as shown in figure 3(a). Thin film buckling is due to compression stresses and interfacial strength also plays an important role as shown in figure 3(b). The microstructure of the interface between coating and substrate determine the mechanical properties of such a system. Composition, phases present, porosity, grain size, grain shape, defect types and density all play important role in determining the mechanical properties. The time independent properties are: elasticity, plasticity and fracture. The dependent properties are: viscoelasticity, creep/viscoplasticity and fatigue strength.
Figure 3: Failure of thin film due to stresses (a) Failure due to tensile stress (b) Failure due to compressive stress
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The tensile properties of thin film can be evaluated using micro-tensile apparatus, bulge techniques, centrifugal methods, X-ray and electron diffraction techniques. Hence the properties of thin films are different than its bulk materials, mechanical properties of thin films can be obtained from micro-tensile or indentation tests and thin films having thickness less than 200nm need a modeller in order to obtain its mechanical properties. The thickness of thin film is comparable to its microstructural dimension, tf (thickness of thin film) â‰ˆ d = grain size, tf â‰ˆ dislocation spacing = 1/âˆšÏ.
Although this article is about the mechanical properties of thin films, we will also discuss briefly on processing techniques, characterization and emerging applications of thin films in the subsequent sections.
2.0 PROCESSING TECHNIQUES OF THIN FILMS
Essentially all thin film deposition, processing and characterization techniques require a vacuum or some type of reduced pressure environment. The process of applying thin film on a substrate or a previously deposited layer is called "Thin film deposition". The term thin is sort of a relative term, but in most deposition techniques the layer thickness is controlled within a few tens of nanometers. The deposition techniques of thin films fall into two broad categories: physical or chemical. The deposition techniques will be discussed briefly in this section.
2.1 Physical Deposition
2.2 Chemical Deposition
3.0 FILM FORMATION AND STRUCTURE OF THIN FILMS
4.0 CHARATERIZATION OF THIN FILMS
In the early days of its use, interest on thin film was centered on optical applications which demanded the measurement of optical properties and film thickness. With the rapid growth of thin film utilization in MEMs and NEMs, it is now required to fully understand the intrinsic characteristics of the thin film. The increasingly interdisciplinary nature of new applications necessitates the characterization and measurement of other properties. In many cases, existing techniques used for characterizing and testing bulk materials were borrowed and modified for thin films such as X-ray diffraction, electron spectroscopy, mass spectroscopy, microscopy, nuclear scattering, mechanical testing etc. Table 1 shows a partial list of techniques employed in characterization of thin films.
Table 1: Analytical techniques used in thin film technology
These analytical techniques have exceptional structural resolution and chemical analysis capabilities over small lateral and even depth dimension. Some of these techniques only sense and provide useful information about the first few layers of atoms while some other can dig deeper. A lot of these techniques are also non-destructive and leaves the thin film harmless. All these techniques mostly utilize an ion, electron or photon beam which interacts and excites the film in such a way that some combination of secondary beams are created that carries valuable chemical and morphological information. These advanced techniques allow us to characterize thin films with the same degree of ease and precision that is associated with bulk materials which is quite astonishing.
Some techniques associated with determination of film thickness, film structure and chemical composition will be briefly discussed in this section. Specific mechanical property evaluation will be discussed in the next section.
4.1 Film Thickness
Thin film properties and behavior is largely dependent on the thickness of the film. Advanced applications of thin films require maintenance of accurate and reproducible film thickness. Therefore, it is very important to accurately measure the thickness of film. Different type of films and wide range of applications gave rise to different techniques for thickness measurement. Only a few of them are capable of run-time monitoring of film thickness during the growth process. Thickness measurement techniques can be broadly classified as optical and mechanical technique. Table 2 shows a summary of techniques used in measurement of film thickness.
Table 2: Summary of some film thickness measurement techniques
Type of Technique
Multiple beam FET
A step and reflective coating required
Multiple beam FECO
A step, reflective coating and spectrometer required; accurate but time consuming
80 nm-10 Âµm
Non-destructive; For transparent films on reflective surface
40 nm-20 Âµm
Non-destructive; For transparent films
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For SiO2 on Si
1 nm-5 Âµm
For transparent films; Analysis is complicated
> 2 nm
Simple and rapid
Accuracy is dependent on knowledge of film density
A few %
Large film thickness causes non-linear behavior
4.2 Film Morphology and Structure
Understanding the morphology and structure of thin film is a very important issue. Film surface morphology, including grain size and shape, identification of constituents, presence of inclusion, voids, cracks, delamination etc. are of concern in thin film technology. Evaluating the cross sectional views of multilayered structures, film-substrate interaction also provide great insight into advanced thin film technologies. Diffraction pattern also provides important crystallographic information which is of great interest for thin film scientists. Some of the most common structural characterization methods that are used in thin film technology are:
Scanning electron microscopy (SEM)
Transmission electron microscopy (TEM)
Atomic force microscopy (AFM)
X-ray diffraction etc.
Figure 4 shows the basic working principle of these techniques.
Figure 4: Schematic of structural characterization techniques (AFM, XRD, TEM and SEM)
4.3 Film composition (Chemical Characterization)
Chemical characterization of thin films involves the evaluation of surface atoms, interior atoms and other compound as well as their spatial distributions in different directions. Some of the most popular and common chemical characterization methods are:
Auger electron spectroscopy (AES)
X-ray photoelectron spectroscopy (XPS)
Secondary ion mass spectroscopy (SIMS)
Energy dispersive X-ray (EDX)
Rutherford backscattering (RBS) etc.
AES, XPS and SIMS are surface analytical techniques as they can detect ions or electrons emitting form surface layers that are less than 1.5 nm deep and can detect almost all the elements of periodic table. EDX and RBS usually scan the whole thickness of film and a portion of the substrate also. Only XPS and AES (to a much lesser extent) are capable of readily providing information on the chemical bonding and valance states.
5.0 MECHANICAL PROPERTIES OF THIN FILMS