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Electrical Discharge Machining Promises High-quality, Lower-cost Substrates

Wire electrical discharge machining is a versatile, economic approach to boule processing. Its small kerf width means that it wastes substantially less material than the incumbent technology – the wire saw – and no microcracks are produced during processing, explain Dinesh Rakwal and Eberhard Bamberg from the University of Utah.

Our community prides itself on its cutting-edge technologies. However, we are also dependent on a tool that has been refined and adapted over hundreds of years – the wire saw.

This piece of machinery is used by substrate manufacturers to cut thin slices of material from GaAs, InP and SiC boules, and offers a combination of simplicity, speed and effectiveness. However, sawing has some drawbacks: it wastes material and creates microcracks in these brittle semiconductors that can only be removed with additional processing.

Fortunately, these weaknesses can be addressed with wire electrical discharge machining (WEDM), a well established technique that has recently been applied to semiconductor machining through our efforts at the University of Utah.

This non-contact process begins by bringing a wire held at several hundred volts into close proximity with the boule. Arcing occurs when this gap falls to just a few microns. This transfers energy from the wire to a small region of the boule, which then undergoes local heating and evaporation. A small cut is created in the boule, which can be deepened by repeating the arcing process to eventually produce a thin, circular slice of material.

The distance between the wire and the boule must be controlled precisely. Short circuits can occur when the boule and the wire get too close together, but if they are too far apart the processing speed plummets. Fortunately, the gap can be monitored by the WEDM s operating voltage. A fall in voltage reveals that the wire is too near to the boule and the cutting speed must be reduced to increase the separation distance. Similarly, a rising voltage indicates that the wire is too far from the boule and a faster cutting speed is needed.

Dielectrics, such as deionized water or oil, are directed into the gap between the wire and the boule. This increases the strength of the electric field that is produced by the wire, leading to improved focusing of the electrical discharge. The liquid also helps to carry away the molten material that is generated during the cutting process.

Any substrate manufacturer considering a switch from wire saws to WEDM will want to process material quickly, without compromising quality. The maximum cutting speed depends on the material that is being processed and it is possible to speed up the procedure by increasing the discharge energy – which is governed by the voltage across the gap, the discharge current and the timing of the electrical pulses. However, microcracks start to appear when the energy is too high, because material is removed by thermal shocks rather than by melting. With germanium we have concluded that electrical discharge energies of less than 0.111 mJ are needed to prevent microcracks from forming.

The boule s electrical conductivity profoundly affects the WEDM processing speeds. High resistances reduce current flow between the wire and the boule, decrease the energy of the pulses that are used to melt the semiconductor and ultimately slow down the procedure. Processing material with a resistivity of 0.01 Ω cm, such as germanium doped to 1 × 1018 cm–3, is relatively easy. However, slow machining rates are inevitable at resistivities of 1 Ω cm. At 20 Ω cm, which is found in optical-grade undoped germanium that is used for infrared diffraction gratings, cutting speeds are incredibly slow, but we are aiming to address this with a high-voltage pulse generator.

Current flow is also limited by the contact resistance between the semiconductor and the metallic contacts that close the electrical loop. Adding metallic coatings on the semiconductor surface can cut contact resistance and lead to faster machining.

One of WEDM s strengths over wafer sawing is more-efficient material usage. This is seen in a reduction of the kerf – the width of material that is lost during a single cut. Wafer sawing produces 200–250 m kerfs, which are more than twice as wide as the narrowest kerfs produced with our WEDM processes for germanium. With WEDM, the kerf width depends on the diameter of the wire and the discharge energy. We have produced 138, 118 and 94 µm kerfs with 0.111 mJ pulses and 100, 75 and 50 µm diameter wires, respectively (figure 1). This kerf width equates to 24–32% material wastage for germanium substrate processing, compared with 40–45% for wire sawing. Thinner wafers will benefit even more, allowing WEDM to lead to greater cost savings.

WEDM has a downside – it coats substrates with a thin "recast" layer that contains material from the boule, wire and dielectric (figure 2). This layer needs to be removed before subsequent wafer processing, and can be etched away with acids. We have found that the recast layer on germanium is 7–8 µm thick and it can be etched away in 10 s with a mixture of 60% acetic, 25% nitric and 15% hydrofluoric acid.

Another application for WEDM is boule shaping. This includes machining of the substrate reference flat, which involves the removal of extraneous material as large pieces. The surplus material, which is estimated to be 13–15% of the boule, can be put into the melt after it is etched clean.

The standard boule-shaping technique – cylindrical grinding – is inferior to WEDM because it produces deep microcracks in the material. Grinding is also less efficient because it generates dust that is contaminated with abrasives from the grinding wheel and coolant. Recovering the precious semiconductor from this slurry is far from easy.

The versatility of WEDM enables it to be applied to post-growth processing, such as the dicing of individual devices from a wafer. Traditional machining methods struggle to process thick wafers and complex shapes, but WEDM can perform free-form dicing of any shape. Unlike laser machining, WEDM can handle a variety of wafer thicknesses without compromising the edge s shape or its quality.

Further reading
D Rakwal et al. 2008 Journal of Materials Processing Technology doi:10.1016/j.jmatprotec.2008.08.027.


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