Improved throughput of deep high-aspect-ratio trenches using split-beam laser ablation

A 20W 355nm DPSS Q-switched nanosecond pulse width laser, with external beam-splitting optics, was used to simultaneously ablate two 600μm deep, 140μm wide, 13.4mm long blind trenches in silicon using a five line wide cut strategy, achieving a 1.22x throughput improvement compared with a single-beam 20W laser configuration. Improved split-beam throughput was achieved because overhead time consisting of non-cut time during galvanometer retrace and turn-around movements and the time taken to ablate shoulder formations, were found to be approximately independent of laser power. With this split-beam approach, where two identical trenches are simultaneously cut, overhead time is split between the two trenches when cut time/die is calculated, halving the effective overhead time/die, and thereby improving throughput. Specific throughput improvement depends upon cut strategy, trench size, and insertion loss of the beam-splitting optics. Beam splitting optics consisted of a half-wave plate, Glan-laser polarizing prism, and mirror. Making use of the linear polarization characteristic of laser light, rotation of the half-wave plate was used to adjust the relative power in each beam, and thereby equalize the ablation rate of each beam. Adjustment of the mirror angle determined the separation between the two trenches.