ION BEAM MACHINING (IBM)
In IBM method, the metal is removed by bombarding the work with accelerated ions. These ions collide with the surface atoms of the workpiece and knock them out of the surface. Thus the metal is removal occurs an atomic scale and hence the process is also called: ion etching, ion milling or '0° polishing. Unlike other thermoelectric processes (EBM, LBM, PAM and EDM) no large scale heating of the workpiece surface takes place during the process.
The schematic arrangement of IBM is basically simple and is similar to EBM. At the top end of the vacuum chamber is the ion beam generating apparatus, which consists of an electron gun which discharges free electrons into the vacuum chamber filled with argon gas. The gas gets ionized by the electrons. A stream of ions is thus generated and is directed on to the target material, which is clamped to a table at the other end of the vacuum chamber. The work table can be oscillated and rotated. This makes it possible to subject different points on the work surface to the ion beam. It is not necessary to work in an absolute vacuum. About 10-4 torr is suitable.
Depending upon the type of power used, there are two systems of IBM. One system uses a d.c. source. It is simple and less expensive but can be used only for conductive materials. The more universal system uses radio frequency equipment. It is more expensive but can be used for any material.
The method is similar to chemical etching in the need for masking. However, no undercutting takes place in this method. Also, powerful etchants that can propagate along cracks and degrade the masking material, are not needed in this technique. Metal removal rates will depend upon the workpiece material (for example for aluminium it is twice that for tungsten). The MRR vary upto 2 to 10-4 mm/min. The accuracy of etching is very high. Tolerances in the range of ± 5 * 10'4 mm are possible.
Regarding applications of IBM, the method is mainly used in micromachining of electronic components, like computer memories, figuring optical surfaces and for the precision fabrication of fine wire dies in refractory materials. Typical materials which can be etched by IBM include glass, aluminium, quartz, crystals, silica, agates, porcelains, cermets, mixtures of quartz and asbestos, and numerous metals and oxides including rare earth metals. The method holds greatest promise in the field of electronics. It can be used to etch multilayered structures (for example to form a terminal) without the risk of incompatibility. This is an important advantage since multilayered I.C. circuits are becoming increasingly common. The method has proved extremely valuable in etching surfaces of specimens prior to studying their microstructure. The material microstructure is clearly revealed, since the method does not alter the work surface. Also, the etched surface is equally well suited to both optical and electron microscope. The method is also used for the deposition of thin film of material, particularly in electronic industries. Typically, the material to be deposited is made the anode in a low-
pressure argon or other rare gas atmosphere. Gaseous ions bombard the cathode, sputtering its material on the substrate. Deposition of thin coatings on such products like razor blades is also common.
The advantages of IBM are:
MRR are easily controlled, no residue, no undercutting as with CHM, no etchants are required, in figure etching resolution is limited only by the resolution of the mask and the method is almost universal.
The main drawbacks of the method are:
MRR are very slow, it is relatively expensive and thermal or radiation damage may occur in some materials.
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