Scanning electron microscopy
Scanning electron microscopy (SEM) is a technique where electrons from an electron gun are accelerated by a high voltage (5 – 50 kV) towards the sample surface where they cause emission of secondary electrons and scattering of electrons from the sample surface. These primary electrons can also be backscattered from the surface. The secondary electrons are collected by a detector and converted to an electrical signal that can be displayed on a monitor or be computerized. The electron beam is focused to a small spot and scanned over the sample so that an image of the surface geometry can be recorded. The resolution of a SEM reproduction is not only instrument dependent but also dependent on the sample material. The limit is set by how well the beam can be focused and by scattering processes in the sample surface. Typical values of this limit are 10 – 20 Å, implying that smaller features than that cannot be detected. The image contrast can arise from several different phenomena of which one is the topographical contrast that means that it is more probable to detect electrons scattered from sample surfaces near the detector than from more distant surfaces. This kind of contrast gives images that are easy to interpret. Sample preparation is uncomplicated, the samples should be clean and preferably electric conducting and nonmagnetic. Insulating surfaces cause problem with saturated charges. This requires either lower acceleration voltage for the beam (with consequently lower sensitivity) or an auxiliary coating of the surface by a thin conductive film, e.g. a gold film. For thin film analysis the SEM is a suitable tool for imaging morphology of film surfaces and microstructure of film cross sections.
Electron spectroscopy for chemical analysis
The electron spectroscopy for chemical analysis (ESCA), also known as X-ray photoelectron spectroscopy (XPS), is a surface sensitive tool for element analysis where the sample is irradiated by monochromatic X-ray photons. The photons cause emission of electrons with characteristic kinetic energy depending on different elements present in the sample surface. Detection of these energies can give a qualitative as well as a quantitative analysis of the elements in the sample. Also the chemical state of the sample atoms can be determined through the chemical shift, i.e. the change in binding energy of an electron when the atom is bound to another atom. With sputtering and analysis by turns depth profiles of the sample composition can be obtained.
X-ray diffraction (XRD) is a versatile material analysis technique for crystalline materials. Some examples of what XRD can be used for are: determination of lattice constants, identification of unknown substances, phase analysis and measurements of grain sizes and intrinsic stress.
The idea behind x-ray diffraction is that a crystal, with its regularly repeating structure, will diffract electromagnetic radiation with a wavelength of the same size as the crystal’s inter-atomic distance, just like an optical grating will diffract visible light. One way to understand the x-ray diffraction is to regard the atom planes in the crystal as a stack of semi-transparent mirrors. The diffraction can now be treated like reflections in the atom planes where every plane reflects a part of the radiation so that there will be several reflections. These reflections will interfere constructively when they are in phase, i.e. the difference in path length equals an integer multiple of a wavelength. This occurs only when the angle of incidence satisfies Bragg’s law: 2d sinθ =nλ , where d is the distance between two adjacent atom planes, θ is the Bragg angle, n is an integer, and λ is the x-ray wavelength. For all other angles there will be a destructive interference and no reflection.
As there are several sets of atom planes with different spacing in a crystalline substance there will be strong reflections in several directions for a polycrystalline sample. Every strong reflection has two properties, diffraction angle and intensity, and this data can be compared with databases and an unknown substance and its crystal structure can be determined.
In the case of very thin films the intensity of the reflections from the film can be so weak that they will drown in the background radiation, e.g. from the substrate. This problem can be avoided by using a grazing incidence method, GI-XRD, which means that the incidence x-rays have a very small angle with respect to the surface and this increases the intensity so that the technique becomes more surface sensitive.
By the use of a Goebel mirror parallel incidence x-rays are obtained that gives higher intensity and simplifies GI-XRD. For an ordinary XRD this also makes it possible to analyze non-flat samples.
The stylus profilometer
A stylus profilometer is used to measure the surface roughness and the film thickness. The principle is to move a stylus with a very small load over the surface and acoustically record the stylus vertical position as a function of the horizontal position.
To get a reliable measurement of the thickness of a thin film there must be a quite distinct step from the original substrate surface to the film. Otherwise the step cannot be distinguished from the surface roughness if this is too high. Such a step can be obtained by masking an area before the deposition or by etching afterwards. For a good profilometer a typical value of the vertical resolution is 5 Å, but this can limit the possibility to measure large vertical variations. Typical maximum measurable film thicknesses are about 15 μm for commercial profilometers.
Analysis of the mechanical properties of thin films
Among different mechanical properties of thin films, the hardness is one of the most important. The hardness is however not a property that can be unambiguously determined, especially not for thin films. First of all the hardness value is dependent on the measuring technique. All techniques include an indenter of a hard material (e.g. diamond) that is pressed into the tested material by a fixed load. The hardness value is then calculated from the load and area (real or projected) or the depth of the indent. The indenter can have different shapes (e.g. pyramids) and different load ranges that give results that not can be compared directly. For thin films the influence of the substrates complicates the hardness measurements and to decrease this influence, microhardness measurement techniques are employed, where very small loads (0.01 – 10 N) are used. Two of the most common microhardness techniques are micro Vickers and Knoop. Both techniques use pyramidal indenters but Knoop uses an elongated pyramid that gives shallower indentations than the Vickers indenter do. Despite the small loads the microhardness measurements give for thin films the hardness of the system film and substrate. To determine the hardness of the film only it can either be calculated from the hardness of the coated substrate and the hardness of the uncoated substrate using a model, e.g. the Jönsson-Hogmark model, or be obtained by a nanoindentation technique. In such techniques extremely small loads (a few mN) are used so that the sizes of the indentations become very small because of the big contribution of elastic deformation. By nanoindentation the indents can be in the order of 100 nm and this is small enough to avoid influence of the substrates for µm thick films, but the indentations can not be measured optically like in a conventional microhardness tester. In a nanoindenter the relation between the load and the displacement (depth) of the indenter is recorded continuously during the whole load and unload cycle. From these load/unload curves not only the film hardness can be obtained but also the elastic (or Young’s) modulus, i.e. the ability of a material to withstand elastic deformation. The values of these two properties can be calculated using a model by Oliver and Pharr.