Efficient Depth Measurement for Live Control of Laser Drilling Process with Optical Coherence Tomography

• Published in: Photonics Vol. 11; no. 8; p. 743
• Main Authors: Zhao, Jinhan, Zhang, Chaoliang, Ding, Yaoyu, Bai, Libing, Cheng, Yuhua
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.08.2024
• Subjects: Acoustics; Algorithms; Closed loops; closed-loop control; Depth measurement; depth-tracking; Drilling; Fast Fourier transformations; Feedback control; Field programmable gate arrays; field-programmable gate array; Fourier transforms; Heat affected zone; hole depth; Laser arrays; Laser drilling; Lasers; Light; Machine learning; Manufacturing; Material removal rate (machining); Measurement techniques; Microelectromechanical systems; Optical Coherence Tomography; optical coherent tomography; Silicon nitride; Tomography; Tracking
• ISSN: 2304-6732; 2304-6732
• DOI: 10.3390/photonics11080743

Laser drilling is widely used for fabricating holes in the semiconductor industry due to high throughput and a small heat-affected zone. However, it produces varying depths owing to uncertain external conditions and requires live control at the rate of a few tens of kHz to handle the fast material removal rate. Optical coherent tomography is capable of in situ acquiring a raw interferogram at a high rate (>80 kHz), but the depth extraction is slow due to the involved heavy Fast Fourier Transform (FFT). To address this, 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 do not change significantly. The proposed algorithm was proven to expedite the measuring rate six times with sub-pixel tracking precision. To further secure the rate against the interrupting of the system, the tracking process is parallelly implemented in a field-programmable gate array. The closed-loop control tests were conducted on probe cards with depth variations introduced by offsetting laser focus. The proposed method maintained a uniform depth, with variations reduced by 80% compared to traditional methods.