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Removing field failures at wafer level (Part 1)

Posted: 29 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:wafer inspection system? acoustic data? MEMS?

Sonoscan's new high-throughput automated wafer inspection system provides device makers a way to find internal structural anomalies and thus have more control over the reliability of their products.

Specifically, the automated system inspects:
???Bonded wafers (SOI and MEMS)
???Temporarily bonded handle-and-device wafers
???Single wafers before bonding or other operations
???Wafer-level packaging (WPL) / chip-on-wafer

The system, called the AW300, nondestructively collects acoustic data from two wafers or two bonded wafer pairs simultaneously and produces a single high-resolution acoustic image of each wafer. On unpatterned wafers, the acoustic data and the image identify defective regions. On patterned wafers, overlaying a wafer map gives the data needed to identify defective devices for removal after the wafer has been diced and before any additional work is performed. The high degree of automation in the system minimises the need for human attention and maximises throughput.

This information not only cuts processing costs but also, in some procedures, avoids further losses. For example, if the roof of a void disintegrates during a wafer thinning operation, the particles can scratch subsequent wafers. In more extreme situations such a mishap can lead to the breakdown of the polishing instrument.

The analytical data provided also gives engineers insight into processing problems. For example, in inspecting production lots of MEMS wafer pairs that have previously been largely free of anomalies, voids may begin to appear. The acoustic data gives engineers an early warning of a process problem. The acoustic data may also point to the root cause of the process problem.

Figure 1: Sonoscan's AW300 system. Two loadports are at right, corresponding to the two transducers and stages inside the system.

The three basic wafer-related functions of the AW300 are handling, scanning, and reporting. As the name implies, the system (figure 1) was designed to handle 200mm and 300mm wafers. Its sister instrument, the AW200, was designed to 200mm wafers, as well as 150mm and 125mm wafers. Wafers smaller than 125mm are imaged on Sonoscan's laboratory C-SAM system, or its semi-automated P-300 system, both of which have universal wafer stages.

Across this range of wafer sizes are wafer assemblies of various constructions and functions. At the moment, the 300mm wafer pairs in production are predominantly SOI and chip-on-wafer (including BSI) wafer pairs. LED wafers are currently 100mm or 150mm in diameter, moving up from 50mm and perhaps eventually moving to 200mm. MEMS wafers generally run up to 200mm in diameter. Other changes in wafer diameters are likely to occur, perhaps most notably the expansion to 450mm. Within these and other bonded wafer applications there are specific structural anomalies that can lead to field failures. Specific AW series imaging protocols have been developed to find and report which die, 3D structures, or MEMS packages have such anomalies.

Figure 2: Layout of key features.

Technically, the AW300 is an advanced multi-scanner and multi-transducer system for high-throughput reflection-mode acoustic micro imaging. A top view of its layout is shown in figure 2. The robotic arms removes a wafer from one of the carriers, aligns the wafer, and places it on one of the two stages. With two stages and multiple transducers, both target wafers are scanned simultaneously. Ultrasound is pulsed into the wafer structure by the scanning transducer and the return echo signals are received by the transducer. The pulse-echo sequence occurs several thousand times a second as the transducer is scanning just above the surface of the single wafer or the top wafer of a bonded pair. The transducer is acoustically coupled to the wafer by Sonoscan's proprietary Waterfall Transducer, which maintains a constant column of water between the transducer and the surface of the wafer. It makes it unnecessary to plunge an entire wafer into a water bath. Placing bonded wafers into a water bath creates the risk of water ingressionwater traveling by capillary action between the two wafers. Water ingression can cause false bond interpretation as well as contamination and separation of temporary bonds.

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