By: Kun Li, William O’Neill, Jack Gabzdyl
Institute for Manufacturing, University of Cambridge
The micro-machining of silicon components for use in the MEMS fabrication and microelectronics industry is well-established, and the demand for semiconductors started to recover after the 2008 downturn. Laser micro-drilled interconnect via holes is an application that has been applied in high volume manufacturing since 1995. In 2006, the laser held 70% of the microvia market because of its broad processing capabilities with a wide range of materials. Meanwhile the photovoltaic (PV) industry has experienced enormous growth over last few years, and it forecasts to grow to $100 billion between 2008 and 2013 (Lux Research, NY). As the most important material, c-Silicon has 77% of market share of world production of solar cells NanoMarket web (2009). Additionally, many novel high efficiency solar cell concepts (Emitter Wrap Through & Metal Wrap Through) are only feasible with laser technology, since it satisfies the requirement of drilling a few thousand holes (50-100µm in diameter) per second in c-Si.
Most Si machining was conventionally done with a diode pump solid state (DPSS) UV laser because of better absorption of UV wavelength, but the DPSS laser is both complex and expensive. The development of a master oscillator power amplifier (MOPA) based high brightness Yb based fiber laser configuration has provided tunable pulse parameters (20-200ns pulse duration), peak powers approaching 14kW, and pulse repetition rates up to 500kHz. These high peak powers allow the 1065nm laser to drill the Si using a wavelength that is transmissive to Si by turning the material molten and increasing absorption. Fiber lasers have the additional benefits of cost reduction, flexibility and reliability.
With the pulse shaping capability of the 1um fibre laser, Si drilling of different depth and quality can be achieved with the same pulse energy. The fast rising, high peak, short pulses generated vaporisation dominated ablation, leaving a limited resolidified molten layer. While longer, lower pulses are more desirable for efficient drilling, which resulted melt expulsion dominated mechanism. Fig.1 and fig.2 are the examples of pulse shaping and its effects on Si drilling. The longer pulse duration enhances laser matter interaction time, which allows heat diffuse further into the material, hence ablating a deeper hole.