材料表征教学资料 sem.ppt

Dr. Di Wu, Nanjing University1 The Scanning Electron Microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM instead of a light microscope. Scanning Electron Microscope Dr. Di Wu, Nanjing University2 A Brief Introduction Electron microscopy takes advantage of the wave nature of rapidly moving electrons. While visible light has wavelengths from 4,000 to 7,000 Angstroms, electrons accelerated to 10,000 eV have a wavelength of 0.12 Angstroms. Optical microscopes have their resolution limited by the diffraction of light to about 1000 diameters magnification. Electron microscopes, so far, are limited to magnifications of around 1,000,000 diameters, primarily because of spherical and chromatic aberrations. Scanning electron microscope resolutions are currently limited to around 10-15 Angstroms, though, for a variety of reasons. Dr. Di Wu, Nanjing University3 A Brief Introduction The scanning electron microscope generates a beam of electrons in a vacuum. That beam is collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the sample by electromagnetic deflection coils. The primary imaging method is by collecting secondary electrons that are released by the sample. The secondary electrons are detected by a scintillation material that produces flashes of light from the electrons. The light flashes are then detected and amplified by a photomultiplier tube. By correlating the sample scan position with the resulting signal, an image can be formed that is strikingly similar to what would be seen through an optical microscope. The illumination and shadowing show a quite natural looking surface topography. Dr. Di Wu, Nanjing University4 A Brief Introduction There are other imaging modes available in the SEM. Specimen current imaging using the intensity of the electrical current induced in the specimen by the illuminating electron beam to produce an image. It can often be used to show subsurface defects. Backscatter imaging uses high energy electrons that emerge nearly 180 degrees from the illuminating beam direction. The backscatter electron yield is a function of the average atomic number of each point on the sample, and thus can give compositional information. Scanning electron microscopes are often coupled with x-ray analyzers. The energetic electron beam - sample interactions generate x-rays that are characteristic of the elements present in the sample. Many other imaging modes are available that provide specialized information. Dr. Di Wu, Nanjing University5 Advantages of SEM The SEM has a large depth of focus, which allows a large amount of the sample to be in focus at one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Preparation of the samples is relatively easy since most SEMs only require the sample to be conductive. The combination of higher magnification, larger depth of focus, greater resolution, and ease of sample observation makes the SEM one of the most heavily used instruments in research areas today. Dr. Di Wu, Nanjing University6 History of SEM The first, true scanning electron microscope (SEM) was developed and described in 1942 by Zworykin. The instrument described eventually consisted of an inverted column (electron gun at the bottom), three electrostatic lenses and electromagnetic scan coils placed between the second and third lenses. A photomultiplier tube detected the scintillations on a phosphor screen caused by the secondary electron emissions. At Cambridge University in 1948, C. W. Oatley began construction of an SEM based on Zworykins. Graduate student D. McMullan described this work in a doctoral dissertation where they had claimed a resolution of 500 Angstroms. Further work, reported by K. C. A. Smith, made large changes to the electron optics. The electrostatic lenses were replaced with electromagnetic coils, a double deflection scanning system was added as were stigmator coils. Dr. Di Wu, Nanjing University7 History of SEM The next major improvements were made to the signal collection process. The original phosphor screen-photomultiplier used by Zworykin was improved by the addition of a light pipe. The light pipe allowed for direct optical coupling between the scintillator and the photomultiplier tube, greatly improving efficiency. This was accomplished by T. E. Everhart and R. F. M. Thornley, and the detector arrangement carries their names as the Everhart-Thornley detector. All of these improvements were combined in one instrument. Once again at Cambridge University, R. F. W. Pease and W. C. Nixon created the SEM V, utilizing the inverted column, electromagnetic lenses, double deflection scan system, stigmation coils and the Everhart-Thornley detector. This instrument became the basis for the first commercial SEM, the Cambridge Scientific Instruments Mark I, first available in 1965. Dr. Di Wu, Nanjing University8 Structure of a SEM Dr. Di Wu, Nanjing University9 Structure of a SEM Dr. Di Wu, Nanjing University10 Optical Microscope -vs- TEM -vs- SEM Dr. Di Wu, Nanjing University11 The Electron Source 电子枪是扫描电镜用以探测样品的“光”源。电子 从阴极发射，通过一个电场加速，并被聚焦。 电子“光”源的形状、电子的束流和加速电压是扫 描电镜性能和分辨率的主要决定因数。 半导体、材料科学和生命科学研究要求扫描电镜 具有高空间分辨率、高速成像和可靠的操作。这 就要求电子枪具有如下特性： Dr. Di Wu, Nanjing University12 The Electron Source 小尺寸电子源 电子能量发散低 高亮度（亮度：单位立体角内的束流） 发射电流低噪音和高稳定性 操作简便 廉价 Dr. Di Wu, Nanjing University13 电子束从灯丝（filament）发射，灯丝可由各种 材料制成。最常见的灯丝是钨灯丝、LaB6灯丝和 场发射灯丝。 The Electron Source tungsten hairpin gun LaB6 gun field emission gun cold FE gun thermal (Schottky) FE gun thermionic gun Dr. Di Wu, Nanjing University14 Thermionic Gun 热发射电子枪便宜、耐用，并且不需要超高真空。以 前，热发射电子枪用于绝大多数电镜。 热发射电子枪中，电子从被加热的灯丝发射，并被加 速向阳极运动。发散的电子束从阳极孔中出射。 Dr. Di Wu, Nanjing University15 Thermionic Gun Thermionic emission is the escape of electrons from a heated surface. Electrons are effectively evaporated from the material. To escape from the metal, electrons must have a component of velocity at right angles to the surface and their corresponding kinetic energy must be at least equal to the work done in passing through the surface. This minimum energy is known as the work function. If the heated surface forms a cathode, then at a given temperature T (0K) the maximum current density emitted is given by the Richardson/Dushman equation. Dr. Di Wu, Nanjing University16 Richardson/Dushman equation: where A and B are constants. The most important parameter for thermionic emission is that the work function should be as low as possible to use a cathode at an acceptable temperature. Thermionic Gun Dr. Di Wu, Nanjing University17 Thermionic Gun In a diode structure, electrons leaving the cathode surface lower the electric field at the surface. A stable condition exists when the field is zero as any further reduction would repel electrons back to the cathode. This stable regime is known as space- charge-limited emission and is governed by the Child Langmuir equation. Dr. Di Wu, Nanjing University18 Thermionic Gun Child-Langmuir equation: where P, is a constant which is a function of the geometry of the system. However, if the voltage becomes sufficiently high, the Richardson limit for current is reached when the emission becomes temperature limited. Dr. Di Wu, Nanjing University19 Tungsten Hairpin Gun A bent tungsten wire filament, with a diameter of around 100 micrometers, is spot welded to metal posts. These posts are embedded in a ceramic holder and extend out the other side to provide electrical connections. Dr. Di Wu, Nanjing University20 Tungsten Hairpin Gun In operation, the filament will be heated by passing an electrical current through it. Optimum filament temperature for the thermionic emission of electrons is around 2700 degrees Kelvin. The anode, which is positive with respect to the filament, forms powerful attractive forces for electrons. This causes electrons to accelerate toward the anode. Dr. Di Wu, Nanjing University21 Tungsten Hairpin Gun The accelerating voltage, generally between -500 Volts and -50,000 Volts DC, is applied to the Wehnelt cylinder. The anode is connected to electrical ground. Resistive self-biasing is usually used where an adjustable bias resistance connects the filament to the accelerating voltage. The biasing brings the filament slightly more positive than the Wehnelt. Dr. Di Wu, Nanjing University22 Tungsten Hairpin Gun The left figure depicts the equipotential lines between the various parts of an electron gun. Electrons leaving the filament will be accelerated along the gradient towards the most positive area, the anode. This beam of electrons will be focused by the shape of the field gradient to a cross-over just before the anode, forming the first optical image of the source and ensuring that a larger percentage of the electrons will pass through the aperture of the anode. equipotential lines of a tungsten gun Dr. Di Wu, Nanjing University23 Tungsten Hairpin Gun Without the Wehnelt and anode, electrons emitted from the filament would tend to stay in the area of the filament. This forms a space charge or a cloud of electrons whose mutual repulsion resists any further emission from the filament. The anode, being at ground potential, is more positive than the filament and attracts the electrons away from the filament - providing the primary acceleration for the electron beam. Dr. Di Wu, Nanjing University24 Tungsten Hairpin Gun By adding the grid, or Wehnelt, we have a way of controlling the space charge of the filament, shaping the beam and increasing the beam current. Adjusting the bias resistance, and thereby the voltage differential between the filament and the grid, allows the beam current to be adjusted from a small de-focussed beam current, through a focussed maximum current, to cut-off. Cut-off is that point at which the more strongly negative fields of the grid prevent any electrons from reaching the anode by reversing the gradient completely around the filament. Dr. Di Wu, Nanjing University25 LaB6 Gun LaB6 cathodes are becoming quite common. They use a single crystal LaB6 rod, of approximately 1 mm in diameter. This cathode can not be heated as directly as a tungsten filament, so a special mount or separate heater is used to provide the 1700 - 2100 degrees Kelvin required. The tip of the rod is polished to a point, then a small angled flat is usually polished at the point. The flat provides a defined area for emission. Without the flat, or if the cathode material evaporates past the flat, emission occurs from a broad undefined area around the point and resolution is decreased. Dr. Di Wu, Nanjing University26 LaB6 Gun A LaB6 electron gun uses the same basic cathode / grid / anode configuration used in a tungsten wire gun. Because of the high reactivity of the borides at the cathode temperatures used and their brittle nature, the mounting and heating of the cathode is crucial. Dr. Di Wu, Nanjing University27 LaB6 Gun Two large posts compressively hold pyrolytic graphite blocks which in turn hold a piece of LaB6 single crystal. The posts are made of a molybdenum- rhenium alloy which maintains a high modulus of elasticity at high temperatures. This means that the clamping force will not relax during operation. Dr. Di Wu, Nanjing University28 LaB6 Gun The graphite blocks both mechanically support and resistively heat the LaB6 crystal. Electron emission results from passing an electrical current through the posts and graphite blocks. Because of the inherent characteristics of pyrolytic graphite, very little heat is conducted to the supporting posts. Dr. Di Wu, Nanjing University29 LaB6 Gun The standard LaB6 single crystal has a 90 conical tip with a flat emitting surface of several microns in diameter at the apex. This tip shape provides the optimum performance and lifetime. Generally, a sharper cone angle and smaller flat provides higher brightness but shorter lifetimes. Dr. Di Wu, Nanjing University30 Field Emission Gun Electron paths from a field-emission source tip size: 100 1000 Dr. Di Wu, Nanjing University31 Field Emission Gun In the field emission gun, a very strong electric field (109 V/m) is used to extract electrons from a metal filament. Temperatures are lower than that needed for a thermionic gun. This gives a much higher source brightness than in thermionic guns, but requires a very good vacuum. Dr. Di Wu, Nanjing University32 Source Brightness In electron microscopy, the source brightness is defined as beam current density per unit solid angle. Brightness Jc is the current density (in A/cm2) and is the convergence angle (in degrees). The unit of brightness are thus A/cm2sr, where sr is a steradian. Dr. Di Wu, Nanjing University33 Field Emission Gun The application of a high voltage between a fine point cathode and a contract surface can, by a tunneling effect, give sufficient energy to an electron so that it escapes from the surface. This phenomena is known as high-field or Fowler/Nordheim emission. It should not be forgotten that the electric field around a point is greatly enhanced relative to the apparent average electric field between the electrodes. Dr. Di Wu, Nanjing University34 Field Emission Gun The current density (A/m2) emitted by such a point is given by Where E is the electric field at the emitter, the work function and k a constant approximately equal to 1. With fields of the order of 109 V/m, current densities can attain 1012 A/m2 but the actual current is quite small due to the small surface of the emitter. Dr. Di Wu, Nanjing University35 Field Emission Gun Field emission results from electrons tunneling past the work function of the metal tip helped by the high electrical field gradients. EF EV EF EV vacuum metalvacuum metal V Dr. Di Wu, Nanjing University36 Field Emission Gun While electrons are emitted from the surface, their apparent source is a single point beneath the surface. That is, because of the electrical fields present, electrons tend to be emitted tangential to the surface which, in a hemispherical tip, results in an apparent source at the focus of the hemisphere. This apparent source will actually not be a point because of thermally caused differences in the electrons momentum. Dr. Di Wu, Nanjing University37 Field Emission Gun It was first suggested in 1954 that a heated tungsten point, rather than a bent tungsten wire, might produce a smaller source size and higher brightness. This cathode actually incorporates both thermionic and field emissions and is referred to as a Schottky or T-F (thermal - field) cathode. Dr. Di Wu, Nanjing University38 Field Emission Gun The emission process itself depends on the work function of the metal, which can be affected by absorbed gases. This is the reason a very high vacuum is required. Refresh (flash heating with a large current) is required for a cold field emission gun when absorbed gases prevent further emission. Sustaining high electrical field gradients is also essential to emission, so a tip that is well worn might not emit electrons at all. Dr. Di Wu, Nanjing University39 Field Emission Gun As with the tungsten filament gun, the voltage difference or bias between the first anode and the accelerating voltage on the cathode determines the emission current. The second anode is at ground potential and the voltage difference from here to the cathode determines the acceleration given to the electrons. The shape of the anodes is carefully selected to minimize aberrations. Dr. Di Wu, Nanjing University40 SCHOTTKYCOLD FIELDLaB6TUNGSTEN Source Size (nm)153104104 Energy Spread (eV)0.3 - 1.00.2 - 0.31.01.0 Brightness (A/cm2SR)5 x 108109107106 Short-Term Beam Current Stability (%RMS) 1 year1 year 1000 hrs 100 hrs Comparison Among Electron Sources expensive cheap Dr. Di Wu, Nanjing University41 The Schottky emitter combines the high brightness and low energy spread of the cold field emitter with the high stability and low beam noise of thermal emitters. As its emitting area is approximately 100 times larger than that of the cold field emitter, it can deliver the much higher probe currents necessary for analytical applications. Furthermore, it achieves a similar low energy spread as the CFE source, but at an emission current level more than 50 times higher. Because of their many advantages, Schottky emitters are increasingly used for high precision imaging and measurement. Advantages of Schottky Emitters Dr. Di Wu, Nanjing University42 Electron-Specimen Interaction Dr. Di Wu, Nanjing University43 Electron-Specimen Interaction Dr. Di Wu, Nanjing University44 Scattering of Electrons Inelastic ScatteringInelastic Scattering During inelastic scattering, energy is transferred to the electrons surrounding the atoms and the kinetic energy of the energetic electron involved decreases. A single inelastic event can transfer a various amount of energy from the beam electron ranging from a fraction to many keVs. The main processes include phonon excitation, plasmon excitation, secondary electron excitation, continuum X-ray generation, and ionization of inner shells. In all processes of inelastic scattering, energy is lost, though different processes lose energy at varying rates. Dr. Di Wu, Nanjing University45 Scattering of Electrons Elastic ScatteringElastic Scattering As the name implies, elastic scattering results in little ( axis. Dr. Di Wu, Nanjing University231 EBSD Application Example I Small orientation maps were collected of individual large grains or clusters of large grains. It is clear that all the large grains are twinned. The twinning characteristics vary from grain to grain, but typically they have 2-fold or 3-fold structures. Dr. Di Wu, Nanjing University232 EBSD Application Example I The microstructure of the matrix was characterized using automated EBSD mapping. Th