Laser polishing

Fig. 2 Surface morphology of a typical CVD diamond film (a) before and (b) after laser polishing.

During laser polishing thin layers are ablated to smooth a glass surface. Lasers are often used to polish fused silica glass parts [494] and to cut glasses [290]. 

The two ends of the laser diode in Figure 9.11 are polished to increase internal reflection. As a consequence of the cavity geometry the laser beam is, unlike that of most lasers, highly divergent. 

Early injection lasers were small rectangular parallelepipeds made by cutting a wafer of GaAs. Feedback was provided by mirrors polished on two edges or by cleaving. The wafer had ap—n junction incorporated into it and broad area or stripe contacts were provided. Laser stmctures have since evolved to satisfy a wide range of appHcation specific requirements. 

Semiconductor laser diodes are widely used in CD players, DVDs, printers, telecommunication or laser pointers. In the structure, they are similar to LEDs but they have a resonant cavity where laser amplification takes place. A Fabry-Perot cavity is established by polishing the end facets of the junction diode (so that they act as mirrors) and also by roughening the side edges to prevent leakage of light from the sides of the device. This structure is known as a homojunction laser and is a very basic one. Contemporary laser diodes are manufactured as double heterojunction structures. 

A focused laser beam of 0.2 joules per pulse is capable of releasing rare gases from well defined 10-100 pm size spots on a polished surface. As a result, it is possible to extend rare gas mass spectrometry to the region of less than 1 pg samples. The technique was first applied to the study of complex lunar samples by George Megrue [2]. 

Optical examination of etched polished surfaces or small particles can often identify compounds or different minerals hy shape, color, optical properties, and the response to various etching attempts. A semi-quantitative elemental analysis can he used for elements with atomic number greater than four by SEM equipped with X-ray fluorescence and various electron detectors. The electron probe microanalyzer and Auer microprobe also provide elemental analysis of small areas. The secondary ion mass spectroscope, laser microprobe mass analyzer, and Raman microprobe analyzer can identify elements, compounds, and molecules. Electron diffraction patterns can be obtained with the TEM to determine which crystalline compounds are present. Ferrography is used for the identification of wear particles in lubricating oils. 

Coppeta et al. [10] made slurry film measurements during using laser-induced fluorescence. By addition of a fluorescent dye to the polishing slurry film thickness was experimentally from the fluorescence intensity of the lubrication film as measured through a transparent substrate. Film thickness measurements were in good agreement with those of Levert et al. [7,8]. This technique can also be used to study slurry transport across the wafer surface, diameter variation in lubrication film thickness, and slurry mixing effects [11]. 

The necessary pump powers can be achieved either by other lasers (e.g. nitrogen lasers, solid-state lasers or even focussed He-Ne- or Ar+-gas lasers) or by flash-lamps. The simplest practical arrangement is a square spectrophotometer cell, polished on all sides, containing the dye solution which is pumped by a nitrogen laser whose beam is focussed into a line parallel to and directly behind one of the cell windows. Then the Fresnel reflection from the two adjacent windows gives enough feedback in most cases, so that no additional resonator mirrors are needed and the dye laser oscillation starts. 

Figure 10.13 Voltammetry of 1.0 mM dopamine and 1.0 mM ascorbic acid in 0.1 M H2S04. Dashed line is polished GC solid line is after three 25-MW/cm2 laser pulses in situ. [Adapted from Ref. 46.]...

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