FEATURE NON CONTACT MEASUREMENT & INSPECTION
SCANNING ACOUSTIC MICROSCOPY T
PVA TePla Analytical Systems describe how scanning acoustic microscopy is rapidly becoming the technique of choice for examining the internal structures of products
he 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. However, the continuously evolving
high volume production requirements are rapidly migrating beyond the products that were recently considered standardised in the industry. New device designs, packaging methods, shrinking dimensions, bonded wafer interfaces and demand for increased 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 and 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 utilises ultrasound waves to non-destructively examine internal structures, interfaces and surfaces of opaque substrates. The resulting acoustic signatures can be constructed into three dimensional images which are analysed to detect and characterise 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, 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.
22 JUNE 2018 | INSTRUMENTATION
With ultrasound, it is easy to see.” The challenge today is to perform
this inspection at extremely high throughput with 100 per cent 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-customised to meet the requirement of the application based on established, common components.
ACOUSTIC SCANNING 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 focussed sound from a transducer at a small point on a target object.
“In the example of a sinter connection on a power device, the gaps are only a few nanometers. 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"
For items with more unique geometries or sizes, tools can be semi- customised to meet the requirement of the application based on established, common components
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 exists that has been pre-developed to handle standardised 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 per cent flawless. It is typically done before dicing,” says Hoffrogge. According to Hoffrogge, PVA TePla’s bonded wafer inspection tool, AUTO WAFER, is optimised for high throughput with four transducers and automated wafer handling. Cassette loading systems facilitate quick loading (open load port, SMIF,
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