X-ray beams

Grangeat P. Description of a 3-D reconstruction algorithm for diverging X-ray beam., Radiol. Soc. North. America Conf Proc., Nov.1985. 

The volume of defects is calculated using intensity evaluation. Considering the polychromatic radiation of microfocus X-ray tubes the X-ray beam is represented by an energy dependent intensity distribution Io(E). The intensity Ip behind a sample of thickness s is given by integrating the absorption law over all energies ... 

The X-ray beam often leaves the tube envelope through a special beam window. This window also must be joined with the vacuum envelope. 

The laminography method was developed initially for medical applications as a non-computer layer-by-layer visualization of the human body [1,2]. In this case an inclined initial X-ray beam projects an image of a specific layer of the object to the detector surface with defocusing of the other layers during a synchronous rotation of the object and the detector (Fig. 1). 

As object we used a polished diamond oriented with its flat table vertically. Figure 4 shows a radiograph taken with the table plane parallel to the X-ray beam. The magnification is about 50 times. The inset shows the contrast along an horizontal trace, which is also indicated. 

The system PLC handles the sequential control of the system, while the actual manipulation of the fish block in the X-ray beam is done by joystick from the operators control station. 

The fish block will be moved through the X-ray beam and the resulting image is studied on the high resolution monitor. The operator has the ability to judge a block as acceptable, rejectable or downgrade able via push-button. 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monocluomatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also infonnation conceming the chemical enviromnent of the atoms in the near-surface region. Thus, it is probably the most popular and usefiil surface analysis teclmique. There are a number of fonus of photoelectron spectroscopy in conuuon use. 

One of the more recent advances in XPS is the development of photoelectron microscopy [ ]. By either focusing the incident x-ray beam, or by using electrostatic lenses to image a small spot on the sample, spatially-resolved XPS has become feasible. The limits to the spatial resolution are currently of the order of 1 pm, but are expected to improve. This teclmique has many teclmological applications. For example, the chemical makeup of micromechanical and microelectronic devices can be monitored on the scale of the device dimensions. 

Schdmaiic illustration of an X-ray scattering experiment. The X-ray beam travels a different distance when ered by an electron at the origin compared to an electron situated at r... 

The physical techniques used in IC analysis all employ some type of primary analytical beam to irradiate a substrate and interact with the substrate s physical or chemical properties, producing a secondary effect that is measured and interpreted. The three most commonly used analytical beams are electron, ion, and photon x-ray beams. Each combination of primary irradiation and secondary effect defines a specific analytical technique. The IC substrate properties that are most frequendy analyzed include size, elemental and compositional identification, topology, morphology, lateral and depth resolution of surface features or implantation profiles, and film thickness and conformance. A summary of commonly used analytical techniques for VLSI technology can be found in Table 3. 

X-Ray Methods. In x-ray fluorescence the sample containing mercury is exposed to a high iatensity x-ray beam which causes the mercury and other elements ia the sample to emit characteristic x-rays. The iatensity of the emitted beam is directly proportional to the elemental concentration ia the sample (22). Mercury content below 1 ppm can be detected by this method. X-ray diffraction analysis is ordinarily used for the quaUtative but not the quantitative determination of mercury. 

Soft x-rays with wavelengths of 1—10 nm ate used for scanning x-ray microscopy. A zone plate is used to focus the x-ray beam to a diameter of a few tens of nanometers. This parameter fixes and limits the resolution. Holographic x-ray microscopy also utilizes soft x-rays with photoresist as detector. With a strong source of x-rays, eg, synchrotron, resolution is in the 5—20-nm range. Shadow projection x-ray microscopy is a commercially estabflshed method. The sample, a thin film or thin section, is placed very close to a point source of x-rays. The "shadow" is projected onto a detector, usually photographic film. The spot size is usually about 1 ]lni in diameter, hence the resolution cannot be better than that. 

Step 1. A single crystal with the largest dimension equal to about 0.01 mm to 0.3 mm is mounted on a glass fiber which in turn is mounted on a copper pin. The copper pin is placed on a goniometer head which in turn is placed on the goniometer (Fig. 13). The crystal is positioned manually in the center of the goniometer. In this position, the crystal is always in the center of the monochromated incident x-ray beam (whose diameter is about 1.0 mm). 

Step 2. The computer opens a shutter, bathing the crystal in a monochromatic x-ray beam. The computer rotates the crystal for about one minute and the rotation diffraction image is stored on the detector and then read into the computer memory. When the operator examines the image and is confident that the sample is indeed a single crystal, the experiment can proceed. 

Microdiffraction. By concentrating the incident x-ray beam on a small portion of a sample it is possible to get a complete diffraction pattern of very small regions of a sample. Of course, the intensity from such small regions is weak and an area detector that can coUect a large portion of the diffraction pattern at one time makes this appHcation practical. A typical region size is about 50 p.m in diameter. 

Radiographic tests are made on pipeline welds, pressure vessels, nuclear fuel rods, and other critical materials and components that may contain three-dimensional voids, inclusions, gaps or cracks that are aligned so that the critical areas are parallel to the x-ray beam. Since penetrating radiation tests depend upon the absorption properties of materials on x-ray photons, the tests can reveal changes in thickness and density and the presence of inclusions in the material. 

X-rays that are reflected from adjacent planes travel different distances (see Figure 18.6c), and Bragg showed that diffraction only occurs when the difference in distance is equal to the wavelength of the x-ray beam. This distance is dependent on the reflection angle, which is equal to the angle between the primary beam and the planes (see Figure 18.6c). 

In X-Ray Fluorescence (XRF), an X-ray beam is used to irradiate a specimen, and the emitted fluorescent X rays are analyzed with a crystal spectrometer and scintillation or proportional counter. The fluorescent radiation normally is diffracted by a crystal at different angles to separate the X-ray wavelengths and therefore to identify the elements concentrations are determined from the peak intensities. For thin films XRF intensity-composition-thickness equations derived from first principles are used for the precision determination of composition and thickness. This can be done also for each individual layer of multiple-layer films. 

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