Raster scanning

Here is the raster scan of the s o we were discussing. Let me knowwhsl you think. 

In the spectrum acquisition mode the probe is either fixed in the spot mode or raster scanned over a small area at high magnification and a complete spectrum acquired. A typical spectrum is shown in Figure 2. 

The results shown in Figure 6 above are an example of this mode of analysis, but include additional information on the chemical states of the Si. The third most frequently used mode of analysis is the Auger mapping mode, in which an Auger peak of a particular element is monitored while the primary electron beam is raster scanned over an area. This mode determines the spatial distribution, across the surface, of the element of interest, rather than in depth, as depth profiling does. Of course, the second and third modes can be combined to produce a three-dimensional spatial distribution of the element. The fourth operational mode is just a subset of the third mode a line scan of the primary beam is done across a region of interest, instead of rastering over an area. 

The STM uses this eflFect to obtain a measurement of the surface by raster scanning over the sample in a manner similar to AFM while measuring the tunneling current. The probe tip is typically a few tenths of a nanometer from the sample. Individual atoms and atomic-scale surface structure can be measured in a field size that is usually less than 1 pm x 1 pm, but field sizes of 10 pm x 10 pm can also be imaged. STM can provide better resolution than AFM. Conductive samples are required, but insulators can be analyzed if coated with a conductive layer. No other sample preparation is required. 

Usually, in AFM the position of the tip is fixed and the sample is raster-scanned. After manual course approach with fine-thread screws, motion of the sample is performed with a piezo translator made of piezo ceramics like e. g. lead zirconate tita-nate (PZT), which can be either a piezo tripod or a single tube scanner. Single tube scanners are more difficult to calibrate, but they can be built more rigid and are thus less sensitive towards vibrational perturbations. 

Figure 40. Operating modes for electron beam systems left — raster scan coupled with continuous table motion right — vector scan, step and repeat.

Figure 41. Comparison of vector scan and raster scan writing schemes.

Several other successful machines have also been reported using raster scanning. The IBM EL-1 is a step-and-repeat type system that exposes a 5 mm x 5 mm field. These small fields are then stitched together to form the complete circuit. The EL-1 systems use a 2.5 /im x 2.5 fim square beam and are used to manufacture custom circuits. 

In the classical contact mode (Fig. 6a) AFM measures the hard-sphere repulsion forces between the tip and the sample. As a raster-scan drags the tip over the sample surface, the detector measures the vertical deflection of the cantilever, which indicates the local sample height. A feedback loop adjusts the position of the cantilever above the surface as it is scanned and monitors the changes in the surface height, generating a 3D image—a decisive advantage of AFM over TEM [3]. 

To avoid any flicker in the image, the electron beam is scanned across and down the screen, many times per second, following a predetermined set of parallel lines, the method being known as raster scanning. The phosphor dots are the picture elements or pixels and light up as the beam scans across each one. In colour televisions and monitors additive mixing of the three colours of red, green and blue produces the... 

D/A converters to generate raster. scan voltages. An important point to be noted is that D/A converters have finite step sizes. A typical D/A converter has a full range of 10 V, with 12-bit accuracy. Each step is 20 V/4096=4.88 mV. By using it directly to drive the x, y piezo with a typical piezo constant of 60 A/V, each step is about 0.3 A. If amplification is installed, the step size becomes larger. 

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