| Description |
Semiconductors were first introduced in the 19th century and now form the base material for electronic components that range from children's toys to spacecraft. High purity germanium (HP Ge) is one of the most essential materials in the semiconductor industry due to its particular applications: optical devices in the IR range and detectors for gamma-ray spectroscopy, which have been employed at Homeland Security and border control to detect nuclear materials. In the machining process of HP Ge, an inner diameter blade (IDB) and multi-wire saw (MWS) have conventionally been used for slicing the HP Ge ingot into wafers. Some inevitable drawbacks of both processes include a relatively big kerf loss and microcracks in the subsurface of the wafers that occur due to mechanical stresses that result from abrasive machining. This research investigates wire electrical discharge machining (wire EDM), which has been considered as a promising technique because of its ability to minimize the kerf loss by using very small diameter wires (12.7-200 μm). Moreover, wire EDM as a noncontact machining technique can slice ingots without creating microcracks by carefully selecting appropriate levels for the discharge energy. Wire EDM requires sufficient electrical conductivity of the workpiece. However, the resistivity (32.8 Ωcm) of HP Ge is high for wire EDM. To temporarily enhance the conductivity of the specimens, metals were deposited on the HP Ge. Machining experiments were performed to determine the correlation between the slicing rate and locally enhanced conduction of HP Ge through various discharge energy. From the results, the obtained maximum slicing rate is 7.7 mm2/min with Al coating, which is improved as much as 24 times over uncoated HP Ge. Increasing the discharge energy increased not only slicing rate but also kerf loss. Additional slicing experiments at reverse polarity (positive wire and negative workpiece) were performed with P-type Ge (0.01 Ωcm), which proved that the polarity should be considered as the main parameter for machining semiconductor with EDM. This research also seeks to create a locally conductive layer on the intrinsic Si (3.2 x 105 Ωcm) with powder mixed in the dielectric fluid. Al metal is deposited on the specimens as a trigger to generate sparks because the intrinsic Si is non-machinable with EDM. Among powders, boron powder is selected due to its role as a typical dopant for semiconductors. In results, the obtained maximum slicing rate (0.009 mm2/min) is 4.5 times higher than non-powder EDM. Local and temporal enhancement of conduction improves machinability of the high resistance of semiconductors with wire EDM. It will likely play a great role in manufacturing microdevices for the semiconductor industry. |
