Interest in glass for Integrated Circuit (IC) packaging and interposer applications has accelerated in recent years, due to its favorable mechanical and electrical properties compared to current advanced materials. This report describes our recent results on Through Glass Via (TGV) drilling, utilizing a novel process employing a single laser source with an engineered pulse duration, pulse repetition frequency, and average power to rapidly form TGVs in 50 and 100 μm thick glass. The process forms TGVs with a ∼10 μm diameter with zero taper, smooth sidewalls and minimal splash; the dimensions of these TGVs meet the requirements for next-generation interposers to replace through silicon vias. Unlike Bessel beam-based processes, this process is compatible with high bandwidth beam steering technologies (galvanometers and Acousto-Optic Deflectors (AODs)), enabling an industrially viable throughput in high-density drill patterns of more than 10000 vias per second. The formation dynamics of the TGVs are elucidated using multiphase simulations and in-situ spectroscopic methods. Stress mitigation in the fully formed TGVs is explored through annealing studies; an alternate approach utilizes heating of the glass substrate during the laser processing to minimize stress formation during the drilling process. Both methods are shown to mitigate embedded stress and avoid cracking.
As communication networks move toward higher frequency bands, thin glass substrates are advancing toward industrial production in packaging and interconnect applications as high-frequency, low-loss materials. While current techniques for the formation of through glass vias (TGVs) allow for efficient drilling of <40 μm diameter holes, there are currently limited commercially viable options for smaller TGVs. Presented herein is a unique approach to forming high aspect ratio TGVs in 10-20 μs for 50 and 100 μm thick glass. This is accomplished by using a high-power quasi-continuous wave (QCW) laser with a simple Gaussian beam profile focusing scheme. Crucially, this approach is compatible with high bandwidth beam steering technologies, i.e., the combination of galvanometers and acousto-optic deflectors (AODs), allowing for simple scaling to industrially viable throughputs of tens of thousands of vias per second for high-density drill patterns. The TGVs have straight, smooth sidewalls, and high uniformity. Birefringence image microscopy is used to further assess the finer quality aspects of the TGVs formed; considerable residual stress embedded around the TGVs was found after laser drilling, which could cause cracking in subsequent process steps. It is demonstrated that the stress can be significantly reduced by either annealing the glass substrate after drilling or drilling at elevated temperatures to mitigate the embedded stress.
The recently reported copper ablation study using ultrafast IR lasers with unusually high burst repetition rates (∼ GHz) that claims “an order of magnitude” efficiency enhancement compared to non-burst processes due to “ablation cooling” warrants further investigation both experimentally and through modeling the process. We experimentally reproduce a subset of these results, compare it to the known best non-burst pulse results, and find that within our experimentally accessible parameter range, there is indeed an up to ∼ 3.5x benefit when punching (i.e. drilling holes) with 864 MHz pulse bursts. However, this efficiency increase does not translate from punching to milling (machining an area), which we find to be less than half as efficient as an optimized non-burst process, while also delivering worse process quality. We conclude that a hydrodynamic picture is needed to understand the discrepancy between punching and milling efficiency for a ∼ GHz burst process.
This paper presents a new CO2 laser technology for precision microfabrication applications. The laser produces short (microsecond) pulses at very high pulse repetition frequencies (PRFs). In contrast, most commercial CO2-laser micromachining applications employ one of two type of CO2 lasers: RF-excited with external pulse modulation, and TEA lasers. The laser technology presented here produces pulses sharing some of the characteristics of the TEA CO2 laser, but is capable of delivering them at much higher PRFs (20-100 kHz). Microfabrication applications to date are primarily microdrilling in common electronic circuit board and IC packaging materials, including unreinforced, glass-fiber reinforced, and particle-filled epoxies. These materials are processed using pulse energies lower than those generally used by conventional CO2 laser designs, and at speeds typically 1.5 to three times as fast as achieved by conventional CO2 laser drills.
Laser drilling has emerged in the last five years as the most widely accepted method of creating microvias in high-density electronic interconnect and chip packaging devices. Most commercially available laser drilling tools are currently based on one of two laser types: far-IR CO2 lasers and UV solid-state lasers at 355 nm. While CO2 lasers are recognized for their high average power and drilling throughput, UV lasers are known for high precision material removal and their ability to drill the smallest vias, with diameters down to about 25 –30 micron now achievable in production. This paper presents an overview of techniques for drilling microvias with the lasers.
Laser drilling has emerged in the last five years as the most widely accepted method of creating microvias in high- density electronic inter connect and chip packaging devices. Most commercially available laser drilling tools are currently based on one of two laser types: far-IR CO2 lasers and UV solid state lasers at 355 nm. While CO2 lasers are recognized for their high average power and drilling throughput, UV lasers are known for high precision material removal and their ability to drill the smallest vias, with diameters down to about 25-30 micrometers now achievable in production. This paper presents a historical overview of techniques for drilling microvias with UV solid state lasers. Blind and through via formation by percussion drilling, trepanning, spiralling, and image projection with a shaped beam are discussed. Advantages and range of applicability of each technique are summarized. Drivers of throughput scaling over the last five years are outlined and representative current-generation performance is presented.
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