Using Laser Edge Isolation for Scribing During Crystalline Silicon Solar Cell Production

Lasers are used for improving cell performance or decreasing manufacturing costs in view of the constant growth in c-Si solar cell production. Laser processing has developed rapidly in the following areas:

• Laser-fired contacts (LFC)
• Laser-grooved buried contact (LGBC)
• Metal/emitter wrap-through (M/EWT).

Laser Edge Isolation Process

One of the most commonly used laser processes in c-Si solar cell production is laser edge isolation. The reason for the popularity of edge isolation is the ion doping/diffusion step of the c-Si cell production process, where a shallow (~µm’s) layer of the bulk p-type silicon is integrated with negatively doped ions. The doped area envelops the whole wafer and produces electrical shunting between front electrical contact and the back (figure 1) without the need for isolation scribe.

Figure 1. Without edge isolation groove, current shunt occurs between front and back contact through the ion diffusion layer.

Basically, laser edge isolation is performed by scribing a groove around a solar cell’s perimeter very close to the edge of the wafer. To achieve a good outcome, the groove depth has to extend for a certain distance beyond the ion diffusion layer. The general groove dimensions have a width and depth of 20to 40µm x 10 to 20µm, respectively.

Pulseo Lasers

It is recommended that for superior-quality edge isolation scribing, the newest Q-switched diode-pumped solid state (DPSS) laser technology, such as the Pulseo® 355-20 can be used.

The Pulseo laser is a very suitable tool with a <23ns short pulse width and a 355nm wavelength. It’s been proven that the Pulseo 355-20 integrated with high- speed scanning galvanometer technology is capable of edge isolation times in the 1–2 second range for 156mm wafer. According to the system optical design, isolation scribes are machined at speeds ranging between 500 and 1000mm/sec or higher (figure 2).

Figure 2. Depth profile and microscope photo of edge isolation groove machined with Pulseo 355-20 laser system.

In the case of laser edge isolation, the Pulseo UV laser is a better option as it provides excellent accuracy because of the short pulse period and the extremely shallow absorption depth in silicon of the 355nm wavelength. Sometimes, the 532nm wavelength laser is also recommended for similar cases. An environmentally-friendly option is the Pulseo 532-34, which is a powerful Q-switched laser tool. This tool can be easily integrated with a galvo scanner to achieve high- speed cornering with limited “burn-in”.

At120kHz pulse repetition frequency and more, a scribe floor can be easily machined even with high speeds and small focus spot sizes. The shorter pulse widths can be matched with low energy / high rep rate laser output so as to provide clean and efficient isolation scribes.

Conclusion

The Spectra-Physics group of Pulseo lasers matches the high power and high repetition rate Q-switched DPSS laser products (Table1). The Pulseo lasers possess high peak power, high-quality manufacturing, and short-pulse width, thereby making them perfect for challenging precision- dependent industrial applications.

Table 1. Pulseo Laser Specifications

This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.

For more information on this source, please visit Spectra-Physics.