Advanced semiconductor fabrication QA: a blue CHIPS investment

The CHIPS and Science Act of 2022, the largest publicly funded R&D programme in US history, is designed to return the country to domestic chipmaking dominance and combat supply chain issues that can affect a wide range of essential products such as computers, automobiles, smartphones, appliances, and defence systems.

The legislation appropriates $52 billion to support semiconductor research and manufacturing, with the majority slated to build, expand, and modernise domestic facilities and equipment for fabrication, assembly, testing, or R&D. In addition, the legislation earmarks approximately $200 billion more for investments in semiconductor and other scientific research, technology, education, training, and tax credits in a public-private partnership.

One key to the successful reshoring and ramp up of domestic semiconductor fabrication, however, will be sufficient inspection and quality assurance (QA) particularly of the most advanced chips. Due to the CHIPS and Science Act as well as global chip demand, there is an accelerating demand for non-destructive failure analysis and reliability testing methodologies that can detect minute semiconductor manufacturing defects such as voids, cracks, and the delamination of different layers.

This is spurring investment in Scanning Acoustic Microscopy (SAM), a non-invasive, non-destructive, ultrasonic testing method capable of highspeed 100% inspection. SAM testing is already the industry standard for 100% inspection of semiconductor components for identifying defects within microelectronic devices.

Recent advances in SAM also facilitate the detection of much smaller defects than previously possible, which will be crucial to ensure chip quality as new fabs come online and begin production.

“Advanced SAM systems make it possible to move to a higher level of failure analysis because of the level of detection and precision involved. In the past, detecting a 500-micron defect was the goal; now it is a 50-micron defect. With this type of testing, we can inspect materials and discover flaws that were previously undetected,” said Hari Polu, President of OKOS, a Virginia-based manufacturer of industrial SAM ultrasonic non-destructive testing systems. The company serves the semiconductor fab, electronics manufacturing, aerospace, metal/alloy/composite manufacturer, and end-user markets.

In fact, SAM seems to address at least one of the elements identified by the National Institute of Standards and Technology (NIST) as necessary for the US to lead the global semiconductor manufacturing industry once again.

NIST’s recent report titled strategic opportunities for US semiconductor

Manufacturing identifies seven Grand Challenges that need critical attention from a metrology perspective to achieve the vision of a US-led global semiconductor industry.

In addition, the report identifies 32 Path Forward Elements that describe potential strategies for addressing the challenges.

For one of the grand challenges, Advanced Metrology for Future Micro-electronics Manufacturing, SAM essentially answers one of the Path Forward Elements, namely “rapid, high-resolution, non-destructive techniques for characterising defects and impurities and correlating them with performance and reliability.”

Scanning acoustic microscopy

Scanning acoustic microscopy (SAM) functions by directing 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. By detecting the direction of scattered pulses as well as the ‘time of flight’, the presence of a boundary or object can be determined as well as its distance.

To produce an image, samples are scanned point by point and line by line. Scanning modes range from single layer views to tray scans and cross-sections. Multi-layer scans can include up to 50 independent layers. Depth-specific information can be extracted and applied to create two-and three-dimensional images that can be analysed to detect and characterise flaws such as cracks, inclusions, and voids.

Smaller manufacturers and independent testing labs are likely to have a tabletop SAM model that provides a scan envelope of over 300 mm with a maximum scan velocity of 500 mm/s and accuracy and repeatability of +/- 5.0 micron. Software allows using saved data to virtually re-scan, view, and analyse data for simultaneous real-time analysis or post collection review. Often, such tabletop units are used to analyse data for failure analysis, product inspection, quality control, R&D, process validation, as well as to determine product reliability, in process quality control, and vendor qualification.

As requirements rise to accommodate testing for higher levels of production, semiconductor fabs will often utilise a larger system with the capability of high-speed inspection. The challenge, however, is to perform this inspection at extremely high throughput with 100% inspection to identify and remove components that do not meet quality requirements. This necessitates more advanced equipment that can simultaneously inspect several layers, often on multiple channels, scanning multiple samples in handling trays in an automated fashion to accelerate the process.

According to Polu, SAM can also be custom designed to be fully integrated into high volume manufacturing systems. Advanced inspection technologies detect minute flaws in semiconductor wafers for 100% inspection of all materials. Semiconductor fabs now have access to 100% inspection of wafers, panels, and singulated components in trays.

Fortunately, recent advancements in SAM technology have significantly improved throughput speeds and defect detectability. When high throughput is required for 100% inspection, ultra-fast single or dual gantry scanning systems are utilised along with multi-head transducers can also be used to simultaneously scan for higher throughput.

As important as the physical and mechanical aspects of conducting a scan, the software is critical to improving the resolution and analysing the information to produce detailed scans.

Multi-axis scan options enable A, B, and C-scans, contour following, off-line analysis, and virtual rescanning of chips. This results in highly accurate internal and external inspection for defects and thickness measurement via the inspection software.

Various software modes can be simple and user friendly, advanced for detailed analysis, or automated for production scanning. An off-line analysis mode is also available for virtual scanning.

Polu estimates that OKOS’ software-driven model enables them to drive down the costs of SAM testing while delivering the same quality of inspection results. Consequently, this type of equipment is well within reach of even modest semiconductor testing labs.

“Every semiconductor fab and R&D lab will eventually move towards a higher level of failure analysis because of the stringent detection and precision requirements today,” says Polu. “The cost advantages and time savings of SAM equipment make this possible.”

With labs in Santa Clara, CA, and Manassas, VA, OKOS offers contract analysis and testing services that meet existing industrial and military standards. The service also provides customers with the ability to vet technology and feasibility prior to investing in equipment.

Today, for the semiconductor fabs ramping up production capacity for the CHIPS and Science Act and global demand, SAM offers the best value due to the exceptional level of failure analysis detail it provides, compared to conventional methods. As a result, advanced SAM systems are now considered essential tools in semiconductor fabs, R&D, and Quality Assurance labs.