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Looking acoustically into MEMS devices

Posted: 09 Jun 2010 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS devices? MEMS acoustic imaging? gap type defects?

Non-destructive internal inspection of MEMS bonded wafer pairs via acoustic micro imaging is useful in finding, characterizing and eliminating anomalies and defects. During product development, acoustic inspection is helpful is modifying processes to avoid defects. During production, acoustic inspection spots rejects and identifies process drift. Inspection is typically performed before the dicing of the MEMS wafer pair.

The ultrasonic transducer that scans the wafer pair pulses UHF ultrasound into the top surface and receives the return echoes. Pulse-echo occurs thousands of times per second as the transducer moves across the surface. Each scanned x-y coordinate yields one pixel in the acoustic image which, in the high resolution typically used for MEMS wafers, consists of millions of pixels.

In many of the electronics items (such as plastic IC packages) that are imaged acoustically, there are multiple depths of interest, and the return echo signals used to make the acoustic image may be restricted to (gated on) a specific depth such as the die face or the lead frame depths. MEMS wafer pairs may have one or several depths of interest at material interfaces. Gating on a specific interface may be needed, but refocusing of the ultrasound is not necessary. The user of the acoustic micro imaging system is usually interested in the seal around each MEMS device, although he may also image the MEMS cavity itself to observe, for the example, the etching underneath the beam in an accelerometer. To see the extent of the beam freed by etching, the wafer pair would be imaged from the back side.

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