|A publication of the National Electronics Manufacturing Center of Excellence||
Materials used in high-tech electronics on Navy ManTech projects have been fully characterized to understand how electronic structure and properties are essential; the performance of these devices can be enhanced through various processing techniques. Crystalline structure, interfacial and surface roughness, layer thickness, crystalline defects, phase identification, elemental and impurity analysis are some of the important material properties to study. These material characteristics can be analyzed using techniques that can measure bulk properties or surface properties (Figure 3-1). Each of these techniques will be discussed in more detail in this article.
Both solar and fuel cells are constructed similarly using a heavily doped electron (n-type) semiconductor layer sandwiched with a heavily doped hole (p-type) semiconductor layer. Semiconductor materials used in these devices are grown as thin films with a preferred orientation for their atomic planes. X-ray diffraction (XRD) is a powerful technique that uses Bragg’s law of diffraction to determine crystalline structure as well as atomic plane spacing. Incident x-rays enter the material at a specific angle and collide with an atom. The elastically scattered x-rays exit the sample at the same angle, resulting in constructive and destructive interference that is characteristic of the material. By searching a database, the orientation of the crystal can be determined from the x-ray diffraction data. Orientation of the semiconductor layer grown on another semiconductor layer is strongly influenced by the atomic plane spacing mismatch. With x-ray diffraction, scientists can determine if the crystal structure will lead to different performance of these devices.
For layered structures, fitting the x-ray reflectivity (XRR) to the Fresnel equation can also provide information relating to the individual film thickness, density, and roughness at the interfaces. Below the critical angle total external reflection occurs while above the critical angle, reflections from the interfaces result in constructive and destructive interference. This information is important for determining and maintaining the quality of multilayered films. Also, film thickness, density, and roughness may play an important role in electronic transport across the film interface.
For exposed surfaces, atomic force microscopy (AFM) can be used to image an entire surface plane in all three dimensions. This technique uses a laser beam focused on a cantilevered, fine-tipped probe. By monitoring the reflected laser beam using a series of photodiodes, any deflection of the cantilever arm indicates a van der Waals interaction between the tip of the probe and the sample surface. As the probe approaches the sample surface from a distance, strong attractive van der Waals forces pull the probe closer to the sample. As the probe nears the sample surface, strong repulsive van der Waals forces push the probe away from the sample. These forces can be measured and correlated to height differences across the plane of the sample. Rastering the probe across the entire surface provides the surface roughness and an atomic level imaging of surface topology that would not be possible with x-ray reflectivity.
AFM has several modes of operation: contact, non-contact, and intermittent (tapping). In contact mode, the cantilever tip is touching the sample surface which results in strong, repulsive van der Waals forces between the cantilever tip and sample. As a result, the sample will be damaged as the cantilever tip is dragged across the surface. In non-contact mode, the cantilever tip is operated above the resonance frequency for amplitudes of a few nanometers. As a result, a strong, attractive van der Waals force is present with the cantilever tip less than 10 nanometers away from the surface. This can provide imaging without any damage to the sample. Intermittent (tapping) mode is a cross between contact and non-contact modes where the cantilever tip is gently tapping across the surface. This mode is performed near the resonance frequency of the cantilever arm. Both strong attractive and repulsive forces are experienced by the cantilever tip as it approaches the sample. This is a compromise between the two extremes which results in minimal damage to the sample.
Electron microscopy is a valuable technique with high resolution for fracture analysis, elemental analysis, and phase identification. The scanning electron microscope (SEM) has an electron gun that provides a focused, collimated beam on the sample. Secondary and back scattered electrons are ejected from the surface of the sample to a nearby detector providing surface imaging on a resolution of one to 10 nanometers (depending on the instrument). Samples must be conductive or coated with conductive material to avoid surface charging.
When high-tech devices are under repeated use, failures may occur which must be examined under high magnification. Brittle and ductile failures can be easily identified by observing surface failure regions. Cross-sectioning is often used to provide even greater information for devices under repeated use.
An energy dispersive spectroscopy (EDS) detector can be attached as an accessory to the SEM. This detector can be used for elemental analysis of the sample. Phase identification can be made based on microstructure features and quantification of the elements present. Using a backscattered electron (BSE) detector, differences in atomic number (Z) can be identified by contrast in imaging. Atoms with larger cross sectional areas (high Z) have a greater tendency for elastic scattering to produce backscattered electrons. On the other hand, atoms with smaller cross sectional areas (low Z) have a lower tendency for elastic scattering to produce backscattered electrons. Therefore, the intensity of backscattered electrons will appear brighter for high Z atoms and darker for low Z atoms. During heating, materials in these devices will undergo various phase transformations. Using a BSE detector, one can determine phase transformation changes under different processing conditions.
Another electron microscope with even greater capabilities is the transmission electron microscope (TEM). Sample preparation is a time-consuming process which requires thinning the sample to hundreds of nanometers thick. The transmission electron microscope also has a collimated electron beam, but it passes through the sample providing a density based image below. Higher voltages are needed for TEM (than SEM) to enable the electrons to penetrate the sample and provide a higher resolution (0.2 nm to 0.5 nm). At this resolution, crystallographic defects can be observed such as dislocations (missing rows of atoms in a plane), stacking faults (a layered interruption in regularly ordered stack of atoms, i.e., ABCABABC), and twinning (boundary plane where two crystals of the same kind meet). Also, transmitted electron diffraction patterns can be obtained to understand the bulk crystallographic structure of the sample. Instead of being limited to an EDS detector, the TEM can utilize an electron energy loss spectroscopy (EELS) detector which can identify the element and the different oxidation states. An EELS detector can more easily detect lighter elements while EDS can be used to detect heavier elements.
Impurity levels are often important for layered structures. Secondary ion mass spectroscopy (SIMS) is an essential tool for examining trace elements in semiconductor thin films. The basic principle of this technique is to ion beam sputter the sample surface to examine the secondary ions. SIMS can operate using two modes: static and dynamic. In static mode, the sputtering rate is done slowly to examine an atomic monolayer on the surface. This mode allows for surface analysis with minimal sample damage. In dynamic mode, the sputtering rate is much faster allowing depth profiling and bulk analysis causing surface damage to the sample. As the primary ion source is sputtered onto the sample surface, some of the primary ions will ionize on the surface to produce secondary ions. The secondary ionization efficiency depends on the surface atoms. For electropositive surface atoms, an oxygen primary ion source will yield high secondary ionization efficiency. For electronegative surface atoms, a cesium primary ion source will yield high secondary ionization efficiency. Generally speaking, electropositive and electronegative atoms are on the left side and the right side of the periodic table, respectively. Therefore, it is important to use the appropriate primary ion source when analyzing the sample. The detection limit for trace element analysis is between 1012 to 1016 atoms per cubic centimeter.
In summary, there are several techniques available to study the effects of processing on material properties in high efficiency electronics. However, all of these tools have their pros and cons which can determine which one will be more appropriate for the application. First, sample preparation must be considered prior to analysis. Some techniques require the sample to be sputter coated or sectioned. This will result in irreversible damage to the sample, but will yield information which may otherwise be unknown. Next, there are several different modes to operate each instrument which may or may not be applicable to the analysis. Knowledge of the sample is paramount in selecting the proper mode for each technique. Last, consistent analysis between samples is crucial to provide reliable data. Changing the mode, voltage setting, scanning area, or magnification between samples will make it difficult accurately compare all the data. It is recommended to determine the most suitable settings for analyzing the sample and keep them consistent. If a change is needed due to changes in sample composition, then it should be noted and documented for future reference.
The EMPF utilizes the latest equipment and techniques to study material properties such as those discussed here. For more information, please contact the EMPF at 610.362.1320, via email at firstname.lastname@example.org or visit the website at www.empf.org.
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