Making WEDM Industry-ready for Slicing Semiconductor Ingots for Solar PV Applications
Makarand M Kane, Prof. S. V. Kulkarni, Prof. Himanshu Bahirat, Prof. Suhas S. Joshi
Semiconductor wafers are extensively used in the solar photovoltaic and microelectronics industries. Previous phase of research at IIT Bombay has proved that, Wire Electric Discharge Machining (WEDM) can reduce kerf (material) loss from 40% (in conventional wire-sawing technique) to 15%. WEDM is also called spark-erosion because the sparks act as the heat source for erosion. Being a non-contact process, WEDM can be used to slice hard semiconductors like Silicon Carbide (SiC) as well, which are difficult to cut with wire-sawing. Our efforts are aimed at making the WEDM industry-ready. In this direction, we have developed the following systems and solutions.
Design of Customizable Miniature WEDM
A completely customizable miniature WEDM (mini-WEDM) is developed whose details are given in Table 1. This is first of its kind “miniature or table-top” WEDM, which is smaller in size (by ≈ 60%) and lesser in cost (by ≈30%) as compared to commercial WEDMs. Fig. 1 shows a photograph of the mini-WEDM designed and built at IIT Bombay with an image of spark erosion of silicon. A novel "Pulsed Current Reference Scheme" is proposed for the pulse generator to improve the efficiency by 30%. Although the pulse generator handles small amount (≈ 100 W) of power, the efficiency improvement can save up to ₹ 900,000/- per month, considering production of thousands of wafers per day.
Table 1 Specification of the mini-WEDM
Size of the machine
1200 × 600 × 600 mm
Maximum travel of CNC
Minimum step of CNC
Pulse generator ratings
200 V, 20 A, 10 kHz
2. Electrical Supply Schemes for Multi-Wire EDM
Presently, slicing of semiconductor ingots is done with multi-wire saw which produces several thousand wafers per day. WEDM with single wire cannot match this throughput. However, multi-wire EDM (MWEDM) is not yet commercially available as there are several engineering challenges involved, one of which is as follows.
With conventional electrical supply schemes in MWEDM, very high tension (≈ 85-90% of the wires' failure strength) needs to be applied to the wires, which can cause wire-breakage and hinder the process. If this tension is not used, the wires would vibrate due to strong wire-wire electromagnetic forces, which would affect the surface quality of the cut wafers. Hence, there is a trade-off between the tension applied to the wires and the flatness of the cut wafers achievable with MWEDM. This is a critical engineering constraint to realize MWEDM. We have proposed two novel electrical supply schemes such that, the required tension can be reduced to 20% of that required in the case of conventional schemes.
The research work is accepted for publication in peer-reviewed journals: IEEE Transactions on Plasma Science, IET Power Electronics, and Procedia CIRP (Elsevier). Plus, two patents are also being filed.
Utility and Future Scope
Besides its use for cutting silicon ingots, the mini-WEDM can be utilized for manufacturing small parts for watches, medical equipment, molds for plastic components, etc. Especially, it is a useful product for small scale industries and small engineering colleges, who cannot afford WED machines in the market, which are large in size and costly. It is also beneficial for research institutes to explore WEDM for cutting novel materials.
The novel supply schemes for MWEDM can take the technology to the next level so that, solar and microelectronics industries can easily adopt it. Besides these, MWEDM is also useful for slicing metals for sputtering targets, cutting rare-earth magnetic materials, etc. Therefore, both technologies have applications beyond the semiconductor industry as well.
We are looking to transfer the technologies to enterprises so that, they are put to appropriate use.