Scanning Acoustic Microscopy:
Taking it from the Lab to High Throughput Fab
The demand for equipment that can perform non-destructive imaging and materials analysis has increased significantly for silicone ingots, wafers, integrated circuits (ICs), MEMS and other electronic packages produced by the billions in automated production environments.
Today, however, the continuously evolving high volume production requirements are rapidly migrating beyond the products that were recently considered standardized in the industry. New device designs, packaging methods, shrinking dimensions, bonded wafer interfaces and demand for increase product yields are driving market demands for significantly improved capabilities and sophistication of production equipment. These challenges also include higher levels of automation integration for component handling, improved clean room performance requirements as well as scanning of ever smaller components and interface connections.
For these reasons Scanning Acoustic Microscopy (SAM) technology continues to advance and is now rapidly becoming the technique of choice.
SAM utilizes ultrasound waves to non-destructively examine internal structures, interfaces and surfaces of opaque substrates. The resulting acoustic signatures can be constructed into a 3 dimensional images which are analyzed to detect and characterize device flaws such as cracks, delamination, inclusions and voids in bonding interfaces, as well as to evaluate soldering and other interface connections.
“Using ultrasound provides a clear advantage in ensuring good adhesion and mechanical integrity of devices,” says Peter Hoffrogge, Product Manager of PVA TePla Analytical Systems GmbH, a company that designs and manufactures advanced Scanning Acoustic Microscopes.
Compared to alternative techniques like X-ray that are used to evaluate volumes and densities, ultrasound looks at interfaces, says Hoffrogge.
“In the example of a sinter connection on a power device, the gaps are only a few nanometers,” says Hoffrogge. “With X-ray you don’t get any contrast, so you can’t tell whether the die has adhesion through the interlay or not. With ultrasound, it is easy to see.”
The challenge today is to perform this inspection at extremely high throughput with 100% inspection to identify and remove components that do not meet quality requirements. Often, these defects can occur in different layers of the device.
This necessitates more advanced equipment that simultaneously inspect several layers, often on multiple channels scanning multiple samples in handling trays in automated fashion to accelerate the process.
However, as with other inspection systems, increasing throughput requirements traditionally has required sacrificing image resolution.
Fortunately, Hoffrogge says today’s advanced inspection equipment can overcome these limitations.
Much of today’s equipment is custom designed to be completely, integrated into other high volume manufacturing systems. This includes models specifically designed for inspection of items such as crystal ingots, wafers and electronics packages in a range of standard sizes.
For items with more unique product geometries or sizes, Hoffrogge says equipment can be semi-customized to meet the requirement of the application based on established, common components.
The unique characteristic of acoustic microscopy is its ability to image the interaction of acoustic waves with the elastic properties of a specimen. In this way the microscope is used to image the interior of an opaque material.
Scanning acoustic microscopy works 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 (scattered at 180°) or transmitted (scattered at 0°).
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.
Pre-Developed, Integrated Systems
Today, SAM equipment exist that have been pre-developed to handle standardized items such as bonded wafer inspection of MEMS, CMOS imaging sensors, etc. These equipment test for inclusions or delaminated areas in the bonding interfaces and other defects.
“Typically damage inspection is performed in late stage of production to make sure the device is error free, 100% flawless. It is typically done before dicing,” says Hoffrogge.
According to Hoffrogge, PVA TePla’s bonded wafer inspection tool, AUTO WAFER, is optimized for high throughput with 4 transducers and automated wafer handling.
Cassette loading systems facilitate quick loading (open load port, SMIF, FOUP or customized input and output cassettes), robots for handling 5-12 inch wafers, integrated scanners for wafer tracking, along with other features required of such systems.
For volume inspection of single crystal ingots (e.g. Si, Ge, GaAs) a multiple transducer scanning system (4 heads) is used to estimate the 3-dimensional location of defects inside the crystal volume.
In this way, the tool is capable of analyzing voids and inclusions, estimating their depth and size simultaneously. The tool can inspect 5-12 inch Si ingots up to 400 mm thickness and a weight of 75 kg. The defect resolution may reach down to 100 μm voids in Silicon.
Hoffrogge says there are a significant number of systems in production performing inline inspection of sensitive electronic devices transported in JEDEC-Trays.
Regardless of the type of component inspected, each system includes integrated data analysis and automation software, GEM/SECS interface for fab host communication and other key features.
For applications with unique product geometries and sizes, companies like PVA TePla can build on core platform components that utilize the latest technology.
When even higher throughput is required, up to 4 transducers can simultaneously scan for higher throughput. Multiple transducers can be used on a single substrate and the images then stitched together, or multiple transducers can simultaneously scan multiple substrates.
Throughput can also be increased by incorporating ultra-fast single or dual gantry scanning systems or 6-axis robots. Other possible add-ons include rotation axes (flipping), vacuum chucks and customized water tanks.
“We have a large portfolio of core components which have become standard for us that are used or modified to provide rapid solutions to our customers,” says Hoffrogge. “We basically have the complete value chain available in house, so we can speed the development process from the conception of a project through to a complete process qualification, providing maximum value to our customers.”
Even transducers, the heart of all SAM systems, are manufactured internally to meet specific scanning requirements. Transducers are adapted to each application or specific customer device and inspection requirement, ensuring the highest level of defect detection.
PVA TePla Analytical Systems, for example, designs and manufacture transducers in a very wide frequency range from 3 to 2000 MHz. The company performs this work in-house, utilizing proprietary thin film technology it has developed over many years.
“We are able to manufacturer transducers that are within the standard cost and lead times of the industry,” says Hoffrogge. “We have all the equipment for manufacturing and testing in house, so we don’t have to rely on third parties for items such as optical components where they often have very significant lead times.”