How scanning acoustic microscopy (SAM) aids hybrid bonding test

Hybrid bonding—a significant advancement in chip packaging technology—is becoming vital in heterogeneous integration, which enables semiconductor companies to merge multiple chiplets with diverse functions, process nodes, and sizes into a unified package. It vertically links die-to-wafer or wafer-to-wafer via closely spaced copper pads, bonding the dielectric and metal bond pads simultaneously in a single bonding step.

However, the enhanced reliability and mechanical strength of its interconnects compared to traditional bump-based interconnections don’t come without challenges. For instance, to successfully transition to high-volume manufacturing with high yields, it requires advanced metrology tools that can quickly identify defects such as cracks and voids within the bonded layers.

PVA TePla OKOS, a Virginia-based manufacturer of industrial ultrasonic non-destructive (NDT) systems, claims to have a solution based on scanning acoustic microscopy (SAM). A non-invasive and non-destructive ultrasonic testing method, SAM is quickly becoming the preferred technique for testing and failure analysis involving stacked dies or wafers, according to Hari Polu, president of PVA TePla OKOS.

SAM utilizes ultrasound waves to non-destructively examine internal structures, interfaces, and surfaces of opaque substrates. The resulting acoustic signatures can be constructed into 3D images that are analyzed to detect and characterize device flaws such as cracks, delamination, inclusions, and voids in bonding interfaces. The images can also be used to evaluate soldering and other interface connections.

Figure 1 SAM is becoming a preferred technique for testing and failure analysis involving stacked dies or wafers. Source: PVA TePla OKOS

SAM—an industry standard for inspection of semiconductor components to identify defects such as voids, cracks, and delamination—has been adapted to facilitate 100% inspection of hybrid bonded packages, says Polu.

How it works

In hybrid bonding, various steps must be reliably performed to ensure quality. The process starts with manufacturing the wafers or dies in a semiconductor fab before the chips are bonded together. The next key steps include the preparation and creation of the pre-bonding layers, the bonding process itself, the post-bond anneal, and the associated inspection and metrology at each of the step.

However, in conventional SAM techniques, wafers are held horizontally in a chuck and processed in a water medium. That, in turn, could lead to water ingress, which could cause significant issues in the next step of assembly. On the other hand, by re-designing the chuck in a vertical orientation, engineers can use gravity to eliminate any concern over water ingress while also using other water management technologies.

Here, SAM directs focused sound from a transducer at a small point on a target object. The sound hitting the object is either scattered, absorbed, reflected, or transmitted. As a result, the presence of a boundary or object and its distance can be determined by detecting the direction of scattered pulses as well as the time of flight. Next, samples are scanned point by point and line by line to produce an image.

Figure 2 SAM stands ready to deliver 100% non-destructive inspection of vertically stacked and bonded die-to-wafer or wafer-to-wafer packages to help facilitate the adoption of hybrid bonding. Source: PVA TePla OKOS

It’s important to note that scanning modes range from single-layer views to tray scans and cross-sections and that multi-layer scans can include up to 50 independent layers. The process can extract depth-specific information and apply it to create 2D and 3D images. Then, the images are analyzed to detect and characterize flaws like cracks, delamination, and voids.

The AI boost

Polu is confident that advancements in artificial intelligence (AI)-based analysis of the data collected from SAM inspection of wafer-to-wafer hybrid bonding will further automate quality assurance and increase fab production. “Innovations in the design of wafer chucks, array transducers, and AI-based analysis of inspection data are converging to provide a more robust SAM solution for fabs involved in hybrid bonding,” he said.

So, when fabs take advantage of the higher level of failure detection and analysis, the production yield and overall reliability of high-performance chips improve significantly. “Every fab will eventually move toward this level of failure analysis because of the level of detection and precision required for hybrid bonding,” Polo concluded.

Especially when the stakes are higher than ever because one bad wafer, die, or interconnection could cause the entire package to be discarded down the line.

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