An Efficient Depth-tracking Method for Live Control of Laser Drilling Process with Optical Coherence Tomography

Laser drilling has been widely used for fabricating holes in the semiconductor industry due to high throughput and small heat-affected zone. However, it produces varying depths owing to uncertain external conditions and needs a live control at a few kHz rate to handle the fast material removal rate. Optical coherent tomography holds a capacity of in situ acquiring raw interferogram at a high rate (>80 kHz), but the depth extraction is slow (<0.5 kHz) due to the involved heavy Fast Fourier Transform (FFT). To address it, an efficient depth-tracking algorithm is proposed to save the FFT. It searches the depth in the raw interferogram locally with a known last depth given the two truths that only one depth exists and the adjacent depths would not change significantly. The proposed algorithm was proved to expedite the measuring rate to around 3 kHz with a sub-pixel tracking precision. To further secure the rate from the interrupt of operation system, the tracking process is parallelly implemented with a stabilized 50 kHz in a field programmable gate array. The tests were conducted on probe cards with depth variations introduced by offsetting laser focus. The proposed method exhibited the capacity of maintaining a uniform depth.