TSV by 355 UV laser for 4G component packaging with micro-electroforming
Introduction
Electronic devices tend to be smaller, thinner and more efficient. One such development was high density interconnection with micro holes to connect multilayer structures [1]. Through silicon via (TSV) was one of an effective technology to establish interconnections, such as three-dimensional integrated circuits (3D IC) [2]. In TSV process, via interconnection was formed by drilling, sidewall insulation, seed layer deposition, metal filling and etching [3]. 3D IC packaging has become as the best solution to solve the interconnect delay problem. The wafer stacking technology with vertical interconnection was used for large capacity memories or 3D processors. For high frequency application, the ground inductance and cross-talk suppression were the keys to the gain of radio-frequency and microwave circuits [4].
In the resent years, conventional drilling results in excessive tool wear and tool breakage. Ultraviolet (UV) laser was used for flexible printed circuit board (FPCB) drilling of 75–105 μm diameter via-holes. Laser drilling of microvias in epoxy-glass PCB was investigated by Kestenbaum et al. [5]. Laser drilling of very fine electronic via holes in common circuit board materials was studied by Kenney and Daily [6]. In 1996, Yilbas [7] considered three materials, such as stainless steel, nickel and titanium. The experimental study yielded tables of significance of each factor on the aspects that determined the quality of the holes. Takahash et al. [8] used laser to process the complementary metal–oxide–semiconductor (CMOS) chip. In 2011, Mezzapesa et al. [9] presented laser interferometry to measure the drilling depth. As a non-contact process, laser dicing could precisely cut the profile in clean edges without thermal and mechanical stresses. Lasers were used for industrial purpose including 10.6 μm CO2 laser, 1.06 μm YAG laser, and high intensity lasers such as excimer laser [10], [11], [12]. In long-wavelength lasers, the heat affected zone (HAZ) was generated by heat accumulation, resulting in poor dicing quality. However, due to the short pulse duration, smaller spot sizes and high peak power that tripling YAG with a shorter wavelength with 355-nm laser, it can remove material via vaporization, and result in a very small HAZ [13], [14], [15]. Photon energy of UV 355 laser is about 10–12.4 eV, whereas the silicon bond energy is about 1.791 eV. It means that the laser beam is capable of breaking the silicon bond directly without thermal effect. The wavelength of a traditional Nd:YAG laser is 1064 nm. When its frequency is modulated to increase, the wavelength can be modulated to shorten. The tripling Nd:YAG in frequency shortens its wavelength to 355 nm. Technically, laser photon is able to break a metallic bond energy at 7.7 eV, and a covalent bond at 6.36 eV. Through shortening the wavelength, the photon energy of UV 355 laser is capable of removing the material easily without thermal effect [16].
Although copper micro-electroforming was a well-known process for decades, completely void-free filling in deep and narrow holes was still a challenge. Because of interaction between electrolyte and via surface, distribution of current density varied a lot, and led to non-uniform deposition along the through-via surface [17], [18], [19]. In 1997, Romankiw [20] pointed out that some of the conditions in the micro-electroforming played an important factor, such as photoresist, components of the plating solution, and the plating seed layer. But the main factor affecting the thickness uniformity was current density. Maner et al. [21] reported that too high current density caused the coating defects due to the internal stress. Yeh [22] proposed a press-assisted micro-electroforming process to reduce the hydrogen generation, improved the via hole defects, and increased the deposition rate. In 2014, Chuang [23] presented supercritical electroplating technique to fill copper into through-wafer holes for fabrication of 3D integrated circuits and discussed the resistance relationship between pressure and current density. However, there were few academic researches on micro-electroforming combined with UV laser processing.
In this study, a 355-nm UV laser combined with micro-electroforming was applied to 4G high frequency component packaging. The nanosecond pulses laser was applied to drill TSV to form the via on the silicon wafers. Then, micro-Ni micro-electroforming was used to form the conductive channel with high aspect ratio. Optical microscope (OM) and scanning electron microscope (SEM) were applied to observe the surface morphology and cross section. Energy dispersive spectrometer (EDS) was employed to analyze the material elements. Then the conductive resistance was measured by a probe station and a LCR meter to analyze the optimal process parameters to package this chip. Finally, a network analyzer was used to measure the S parameters and observe the variations of resonance frequency in samples.
Section snippets
UV laser drilling process
A laser system includes converging lens, optical devices, and resonator. These components were used to focus the laser beam and melting the sample surface. The laser system in this study was shown in Fig. 1-(a) and the laser processing process was shown in Fig. 1-(b). The UV laser can effectively drill –through 550-μm thickness silicon chips. TSV of high aspect ratio on the 4G solidly mounted resonators (SMR) chip without HAZ effect was obtained. UV laser ray exhibits high directivity, narrow
UV laser drilling results
The laser power was related to the laser frequency and the power percentage. The results of laser drilling were shown in Fig. 6. The power percentage was changed and compared with different frequency. When the power percentage was high, the taper was small and good for micro-electroforming process. Generally, the taper of laser drilling was about 0–5°. The frequency and power percentage were considered to measure the effects on taper. The taper at 90% power percentage was better than 80% and
Conclusions
A 355-UV laser was used to drill about 100–150 μm micro-hole with the thickness of 550 μm on silicon substrate. Appropriate parameters for better drilling results of the laser were power percentage 90% and frequency at 30 kHz. The TSV taper was about 1.63°, and there were less HAZ effect around the TSV. Micro-electroforming was used to fill up the conductive via to make connections between chips. Different current densities were used to compare the filling results. The TSV resistance of 0.5 ASD
Acknowledgement
The authors gratefully acknowledge the sponsorship from Ministry of Science and Technology of Taiwan, ROC, under the project No. 103-2221-E-110-076-MY2.
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