FEATURE u Test & Measurement


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design time by as much as 70%. “The vision of CHIPS is an ecosystem of


discrete modular, reusable IP blocks, which can be assembled into a system using existing and emerging integration technologies,” writes Andreas Olofsson in program information posted on the DARPA website. Still, there are challenges to making the chiplet concept work, including how to verify and test the individual chiplets from a variety of third-party vendors. Integrating multiple chiplets into stacked, 3D packages also requires high-density interconnections, all of which are potential sources of failure.


In comparison to other 3D package types,


for example, stacked dies with through-silicon vias (TSV) require much smaller, finer-pitch solder bumps that create additional challenges in defect detection. Given the combined value of the chiplets, interposer and other components, a single defective chiplet or poor interconnection can render the entire 3D package non-functional. This is driving the requirement for 100% inspection during manufacturing, ideally with non-destructive testing methods.


NON-DESTRUCTIVE TESTING OF 3D PACKAGES The challenge today is to perform 100% inspection with relatively high throughput, to identify and remove 3D packages or components that do not meet quality requirements. Among the available non- destructive methods, scanning acoustic microscopy (SAM) is the most widely used technique for testing and failure analysis involving stacked dies or wafers. SAM uses 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 analysed


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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. 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 a defect, inhomogeneity or a boundary inside material, is partly scattered and will be detected. The transducer transforms the reflected sound pulses into electromagnetic pulses, which are displayed as pixels with defined gray values thereby creating an image. 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. Images from different depths can be combined into a single scan as well, called Tomographic Acoustic Micro Imaging (TAMI). For even higher throughputs, up to four transducers can simultaneously scan. Multiple transducers can be used on a single substrate and the images then stitched together, or multiple transducers can simultaneously scan multiple substrates. “Scanning Acoustic Microscopy provides non-destructive imaging of defects and delaminations in die and package materials,” says Lisa Logan, Applications Manager Scanning Acoustic Microscopes for PVA TePla Analytical Systems, a company that designs and manufactures advanced SAMs. “SAM is particularly useful for inspection of small, complex, three-dimensional devices,” adds Logan. “The equipment is highly sensitive to the presence of delaminations and air gaps


at sub-micron thicknesses.” The most common defects in 3D packaging


are delamination, cracks in substrate, die tilt, misalignment and void in micro-bumps, bump defects, solder bridging, popcorn cracks, voids in underfill and voids and delamination in TSVs. The resolution of microscopic image depends on the acoustic frequency, the material properties and aperture of the transducer. The frequency of the ultrasonic signals generated for 3D package inspection is typically from 15MHz to 300MHz. Transducers, the heart of all SAM systems,


play such a critical role that manufacturers like PVA TePla Analytical Systems design and manufacture the transducers used in the equipment in a proprietary thin-film technology process. The frequency of the ultrasonic signals can


even be increased into the GHz range, which makes it possible to detect defects even in the sub-micron-range. PVA TePla’s high-resolution, GHz frequency SAM tool, for example, successfully detects voids in TSVs of 5-micron diameter and 50-micron depth, immediately after plating. According to Logan, several leading suppliers


of programmable logic devices have already evaluated and purchased high-resolution SAM equipment for non-destructive analysis of next generation 3D products to scan for packaging anomalies.


“3D-chip manufacturers are trying to push the limits on what they can detect, in terms of defects,” says Logan. “So, today, the evaluation of scanning acoustic microscopy equipment often comes down to which equipment delivers the highest resolution at fastest throughput speeds for 100% inspection.”


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