6.2Scanning Electron Microscope (SEM) A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with electrons in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition. The electron beam is generally scanned in a raster scan pattern, and the beam's position is combined with the detected signal to produce an image. Specimens can be observed in high vacuum, low vacuum and an environmental SEM specimens can be observed in wet condition. 2 The most common mode of detection is by secondary electrons emitted by atoms excited by the electron beam. The number of secondary electrons is a function of the angle between the surface and the beam. On a flat surface, the plume of secondary electrons is mostly contained by the sample, but on a tilted surface, the plume is partially exposed and more electrons are emitted. By scanning the sample and detecting the secondary electrons, an image displaying the tilt of the surface is created. 3 Principles & Capacities of SEM The types of signals produced by a SEM include: secondary electrons (SE), back-scattered electrons (BSE), characteristic X-rays, light (cathodoluminescence) (CL), specimen current and transmitted electrons.4 Secondary electrons (SE) Secondary electron detectors are standard equipment in all SEMs, but it is rare that a single machine would have detectors for all possible signals. The signals result from interactions of the electron beam with atoms at or near the surface of the sample. In the most common or standard detection mode, secondary electron imaging (SEI), the SEM can produce very high-resolution images of a sample surface, revealing details less than 1 nm in size. Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample.5 Back-scattered electrons (BSE) Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. BSE are often used in analytical SEM along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. BSE images can provide information about the distribution of different elements in the sample. 6 Characteristic X-rays Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher-energy electron to fill the shell and release energy. These characteristic X-rays are used to identify the composition and measure the abundance of elements in the sample. 7 Sample Preparation All samples must be in an appropriate size to fit in the specimen chamber and are generally mounted rigidly on a specimen holder called a specimen stub. For conventional imaging in the SEM, specimens must be electrically conductive, at least at the surface, and electrically grounded to prevent the accumulation of electrostatic charge at the surface. Metal objects require little special preparation for SEM except for cleaning and mounting on a specimen stub.8 Nonconductive specimens tend to charge when scanned by the electron beam, and especially in secondary electron imaging mode, this causes scanning faults and other image artifacts. They are therefore usually coated with an ultrathin coating of electrically conducting material, deposited on the sample either by low-vacuum sputter coating or by high-vacuum evaporation. Conductive materials in current use for specimen coating include gold, gold/palladium alloy, platinum, osmium, iridium, tungsten, chromium, and graphite.9 A spider coated in gold, having been prepared for viewing with an SEM. 10 Low-voltage micrograph (300 V) of distribution of adhesive droplets on Post-It note.No conductive coating was applied: such a coating would alter this fragile specimen. Schematic Diagram of an SEM 11 In a typical SEM, an electron beam is thermionically emitted from an electron gun fitted with a tungsten filament cathode. Normally used in thermionic electron guns Has the highest melting point and lowest vapour pressure of all metals. Can be heated for electron emission. Low cost. The electron beam, which typically has an energy ranging from 0.2 keV to 40 keV, is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within of the specimen (interaction volume). 12 SEM Images 13 SEM image of normal circulating human blood. This is an older and noisy micrograph of a common subject for SEM micrographs: red blood cells. SEM image of a photoresist layer used in semiconductor manufacturing taken on a field emission SEM. These SEMs are important in the semiconductor industry for their high-resolution capabilities. 6.3Infrared Spectroscopy Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer.14 6.3.1FTIR (Fourier Transform Infrared Spectroscopy) Fourier transform infrared spectroscopy (FTIR) is a technique which is used to obtain an infrared spectrum of absorption, emission, photoconductivity or Raman scattering of a solid, liquid or gas. An FTIR spectrometer simultaneously collects spectral data in a wide spectral range. The term Fourier transform infrared spectroscopy originates from the fact that a Fourier transform (a mathematical process) is required to convert the raw data into the actual spectrum. The Components of FTIR Spectrometers A common FTIR spectrometer consists of a source, interferometer, sample compartment, detector, amplifier, A/D convertor, and a computer. The source generates radiation which passes the sample through the interferometer and reaches the detector. Then the signal is amplified and converted to digital signal by the amplifier and analog-to-digital converter, respectively. Eventually, the signal is transferred to a computer in which Fourier transform is carried out. Figure below is a block diagram of an FTIR spectrometer. 16 Block diagram of an FTIR spectrometer The goal of any absorption spectroscopy is to measure how well a sample absorbs light at each wavelength. The most straightforward way to do this, is to shine a monochromatic light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength. FTIR technique shines a beam containing many frequencies of light at once, and measures how much of that beam is absorbed by the sample. The beam described above is generated by starting with a broadband light source one containing the full spectrum of wavelengths to be measured. 18 The light shines into a Michelson interferometera certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference. Different wavelengths are modulated at different rates, so that at each moment, the beam coming out of the interferometer has a different spectrum. 19 Schematic of the Michelson interferometer Computer processing is required to turn the raw data (light absorption for each mirror position) into the result of light absorption for each wavelength. The processing required a common algorithm called the Fourier transform. The raw data is sometimes called an "interferogram". 20 In FTIR Analysis, is a failure analysis technique that provides information about the chemical bonding or molecular structure of materials, whether organic or inorganic. It is used in failure analysis to identify unknown materials present in a specimen, and is usually conducted to complement Energy Dispersive X-Ray (EDX) analysis. 21 Unlike SEM inspection or EDX analysis, FTIR spectroscopy does not require a vacuum, since neither oxygen nor nitrogen absorb infrared rays. FTIR analysis can be applied to minute quantities of materials, whether solid, liquid , or gaseous. When the library of FTIR spectral patterns does not provide an acceptable match, individual peaks in the FTIR plot may be used to yield partial information about the specimen.22 A scan of an FTIR Spectrum Plot 23