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Figure 3. Acoustic image of a small area of a MEMS wafer. Blue arrows indicate separations of the bondline from the substrate; the red arrow points out thinning of the bondline

the systems sends back return echoes from material interfaces. The strongest echoes are returned by the interface between a solid (silicon) and the air in a gap (meaning a crack, non-bond, void, bubble, etc.), even when the gap is as thin as 200Å. In a single polished wafer without metallization, such defects and anomalies are typically the only material interfaces within the bulk of the silicon.

The blue arrows indicate areas where the seal is not bonded to one of its substrates - a condition that destroys the integrity of the cavity. The red arrows indicates an area where the bondline has been thinned laterally. There is no breach, but the bondline may be expected to be thinner and more vulnerable to stresses at this point.

Two new SEMI standards make it easier to evaluate the hermeticity of a MEMS bondline. SEMI MS8-0309 (“Guide to Evaluating Hermeticity of MEMS Packages”) provides guidelines for evaluating bondline integrity with acoustic micro imaging. SEMI MS10-0912 (“Test Method to Measure Fluid Permeation Through MEMS Packaging Materials”) describes how to measure the permeability of various bondline materials, and how to measure acoustically the thickness of a bondline to determine its long-term reliability.

Figure 4. Acoustic image of one portion of a direct bonded wafer pair shows size variation in the voids between the wafers

Single wafers that will be used in SOI, BSI and other applications are sometimes imaged before bonding in order to spot surface cracks and subsurface damage. The ultrasound pulsed by

The defects most frequently imaged in wafers bonded for SOI and BSI applications are bubble- like voids, (Figure 4) contaminants or particles between the two wafers. A particle causes local upward curvature of one wafer; both bubbles and particles can cause the silicon above the defect to collapse during wafer thinning.

The contact bonding of these wafer pairs can also be evaluated earlier, after bonding but before annealing. Because the AW system uses a non- immersion system to couple the transducer to the wafer, there is reduced danger of water ingression between the wafers; unannealed wafers can thus be imaged and, if the contact bonding is not acceptable, separated and reprocessed.

300mm wafers for 2.5 D devices (including chip-on-wafer assemblies with interposers) are widely imaged on the AW system, and present their own challenges. The typical chip on wafer arrangement consists of a flip chip connected by its solder bumps to an interposer, which is in turn is connected by larger solder ball to the substrate.

The two layers of underfill tend to attenuate ultrasound, meaning that a lower frequency transducer is in order. Lower frequency means lower resolution, but the deeper solder balls are larger than the solder bumps above, and respond well to a lower frequency.

When an AW system has finished scanning a wafer, the output takes two forms: the quantitative data enumerating the anomaly/defect locations, and an acoustic image of the whole wafer. The acoustic image may be referred to, but it can hardly be viewed in its entirety because it displays defects down to 5 microns in size in a wafer that may be 300,000 microns in diameter.

Engineers may look at a specific region of the image because (for example) there is a history of the tool touching this region and causing contamination. If the quantitative data for a wafer type has previously proven to be reliable, that data alone is generally used as the guide for removal of defect die.

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38 Issue IV 2013

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