Full length articleHigh-efficiency microdrilling of glass by parallel transient and selective laser processing with spatial light modulator
Introduction
For the manufacture of interposers in semiconductor packages and biochips, a highly efficient processing technology is required to fabricate microholes with high aspect ratios in glass [1], [2], [3]. For example, interposers require multiple through holes with diameters of 10–100 µm and depths of 30–1000 µm to be machined with precise position control [4], [5], [6]. Among the various available processing methods for glass [7], [8], [9], [10], laser processing, which involves only a few steps and is a noncontact method [4], [6], [11], [12], [13], is effective in processing microstructures with high aspect ratios. In particular, ultrashort pulse laser (USPL) processing [13], [14], [15], [16] has attracted considerable attention because it facilitates highly precise processing of material. However, the drilling of deep holes requires the irradiation of several pulses per point [17], [18], and a higher USPL repetition rate (>200 kHz) results in unstable processing [17] whereas the processing efficiency is highly dependent on the repetition rate; therefore, methods to improve the processing efficiency are required.
The parallel laser writing technique, which utilizes a diffractive optical element (DOE) or spatial light modulator (SLM), improves the processing efficiency [19], [20], [21], [22], [23], [24]. Although this method does not reduce the processing time of a single point, it reduces the effective processing time per point through simultaneous processing. Another strategy for improving the processing efficiency is to reduce the processing time of a single hole. Table 1 presents current research on high-speed drilling of a single hole. Transient and selective laser (TSL) processing [25] is a novel high-speed drilling method using two kinds of lasers (Fig. 1(a)). In this method, a filament in which electrons are excited is formed by irradiating a single USPL pulse. Further, this region selectively absorbs a continuous wave (CW) laser with a wavelength that generally penetrates glass, evaporating the material thermally. Finally, the TSL processing creates a hole at high speed, whereas maintaining the microstructure. In TSL processing, only a single USPL pulse is used to process a single hole while USPL processing requires several pulses to drill a deep hole (Fig. 1(b) and 1(c)), enabling more efficient drilling than USPL alone, particularly when the repetition rate is not very high. However, when processing numerous holes, the machining efficiency is limited by the USPL repetition rate or TSL processing time. For example, when the processing time is 40 µs, TSL processing has the potential to drill a hole at a rate of approximately 25,000 holes/s, and the maximum machining efficiency is limited by the USPL repetition rate when it is smaller than 25 kHz. Therefore, to further improve the machining efficiency, the effective processing time for TSL processing must be reduced.
In this study, we realize parallel TSL processing to further improve the machining efficiency. As TSL processing uses a USPL and CW laser of different wavelengths, it is necessary to appropriately split and focus these lasers in parallel TSL processing. In this study, we investigate the generation of holograms for parallel TSL processing and consider the relative positions of the beams suitable for parallel TSL processing in order to realize two-point parallel TSL processing (Fig. 1(d)).
Section snippets
Parallel TSL processing setup
The optical system used for parallel TSL processing is depicted in Fig. 2(a). A CW fiber laser (redPOWER® QUBE, SPI Laser) with a wavelength of 1070 nm and the maximum power of 500 W is employed as the CW laser in this system. Although the laser is randomly polarized, the SLM we use is designed to modulate only one polarization. Therefore, the laser is split into s-polarized and p-polarized waves by a polarizing beam splitter (PBS in Fig. 2(a)), and only the s-polarized waves are transmitted
Pulse-energy dependence
Fig. 5 presents the high-speed observation results of parallel TSL processing at different pulse energies. Immediately after USPL irradiation, laser absorption commences at 0 µs within the glass. With the decrease in the USPL pulse energy, absorption is limited to the vicinity of the USPL focal plane (Fig. 5(c) and 5(d)), which corresponds to the electronic excitation region formed by the USPL. The initial absorption region is longer when the pulse energy is higher (Fig. 5(a) and 5(b)),
Conclusion
Parallel TSL processing using an SLM successfully produced two 10-µm diameter and 100-µm depth holes simultaneously at a processing time of 50 µs. To stabilize the process, the sample surface needs to be arranged according to the filament. The hole shape can be controlled by the processing time, and the hole depth can be controlled by the light intensity if the average light intensity of the CW laser in the holes is greater than 8.4 MW/cm2. Using a high-power CW laser, the simultaneous drilling
Funding
Japan Society for the Promotion of Science KAKENHI [JP20J11607].
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: This study was conducted as part of the Social Cooperation Programs of the University of Tokyo “Creation of Hightech Glass,” financially supported by AGC Inc.
Acknowledgement
The authors would like to acknowledge the support from the Japan Society for the Promotion of Science and Leadership Development Program for Ph.D. of the University of Tokyo. This study was conducted as part of the Social Cooperation Programs of the University of Tokyo “Creation of Hightech Glass,” financially supported by AGC Inc.
We would like to thank Dr. Keiichi Nakagawa for providing the high-speed camera. We greatly appreciate the guidance of Prof. Hayasaki and Dr. Hasegawa for using the
Data statement
Data that support the findings of this study are available in the article.
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