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MUF material contains a polymer matrix that absorbs ultrasound, and filler particles that scatter ultrasound. Typically a lower acoustic frequency having lower resolution must be used to image flip chips having MUF.

In a flip chip having molded underfill, the ultrasound must travel twice - as a downward- moving pulse, and later as an upward-moving echo - through the encapsulant material on top of the silicon [Figure 1]. (The encapsulant material in the attachment layer, changed by the capillary flow process, is less absorbing than this layer.)

Sonoscan’s laboratories have seen hundreds of molded underfill samples. Most can be imaged acoustically, generally with a lower-resolution transducer, but with good results. A few flip chips are encapsulated with an underfill material that is especially attenuating, but these can usually still be imaged with meaningful results, and have been very useful in giving clues about transducer design changes that can provide both good penetration and good resolution. A few mold compounds have proved to be so attenuating that meaningful details may not be observed.

Broadly speaking, however, considerable success has been achieved in finding methods to image molded underfill flip chips. In part, success has been a matter of designing a new transducer with the right parameters to image a given molded flip chip design. Sonoscan routinely designs and manufactures customized transducers to meet the specific requirements of customer parts of all kinds that need something other than a standard transducer. The company develops and produces all of its transducers above 50 MHz. Knowing how to turn out a transducer to meet given parameters has been useful in imaging molded underfill samples; in turn, these samples have provided new insights into transducer design.

Inspecting diverse 200mm and 300mm wafers

As of mid-2013, 300mm wafers are used in producing chips for Silicon on Insulator (SOI), Chip-on-Wafer, and Backside Illumination (BSI), the latter for camera applications. There are numerous applications for 200mm wafers. The most exciting may be MEMS applications, many of which use the 200mm diameter. The versatile nature of MEMS devices is evident in their recent use in medical sensing applications, including DNA sequencers.

As die sizes and feature sizes shrink, the critical dimensions of the structural defects such as cracks and bubbles also shrink. For 200mm and 300mm wafers today, acoustic micro imaging tools need two chief capabilities: high spatial resolution in the acoustic data collection process, and high

throughput rate to image large numbers of high- population wafers. (In roughly four years, when 450mm wafers come on line, these capabilities will be even more important.)

To meet this challenge, Sonoscan has developed a multi-diameter automated wafer inspection system having multiple ultrasonic transducers and multiple stages. The AW system, shown in Figure 2, handles both 200mm and 300mm wafers. A single machine can have stages for both 200mm and 300mm wafers, and can image two wafers simultaneously. The system has a library of recipes for the various wafer types it may be required to image. It can operate as a stand-alone unit, or can be controlled by the host computer through its SECS/GEM interface.

Two wafers are scanned simultaneously on the system’s two stages. A robotic arm unloads the wafers from one or more loadports containing FOUPs or other carriers. During the scanning process, the robotic arm performs other pre- and post-scan functions on individual wafers. These functions are designed to achieve maximum overall throughput.

The accelerometers, pressure sensors and other sensors made with MEMS technology typically have an internal cavity as well as a bondline around the cavity to ensure its hermeticity. The key interest is in the bondline, which may contain voids or channels that could leak and destroy the cavity’s hermeticity. Even though the bondline on some newer MEMS designs is as thin as 6 microns, discontinuities and interruptions in the bondline are still imaged.

Figure 3 is the acoustic image of a portion of a bonded MEMS wafer. The dark regions are the bondline surrounding and sealing the cavity.

Issue IV 2013 37

Figure 2. Sonoscan’s AW system can image 200mm and 300mm wafers and find defects down to 5 microns in size

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