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EETE DECEMBER 2012

DESIGN & PRODUCTS MEMS TECHNOLOGIES an electrical stimulus is applied to one of the top electrodes to excite the piezoelectric layer Device performance where transverse piezoelectric coefficient e31 is in addition to overcoming the higher native utilized to generate a bulk acoustic wave in the frequency challenge for quartz crystals and entire device. the resonator then vibrates later- traditional MEMs we also studied known ally, and the mechanical motion is transduced quartz limitations such as activity dips, ag- through the piezoelectric layer to be sensed by ing, vibration sensitivity, etc. the other top electrode. the resonance frequen- Activity Dips: one of the known issues cy of the fundamental mode is determined in for crystal oscillators is activity dips that can Equation 1 by the device’s lateral length l, the cause intermittent failures. these failures effective elastic constant of the combined reso- Fig. 3: Three-dimensional model of affect both the frequency and the resistance nator body Eeff, and the effective mass density. (i.e., the Q) of crystal resonators. Activity pMEMS resonator with a microshell. dips are usually caused by interfering modes (e.g., by high overtone flexure modes) and are strongly influenced by the crystal’s drive level and load reactance. these activity dips are not present for MEMs oscillators as the bulk acoustic wave is dispersed in since the MEMs resonators are designed to different directions in the resonator body, the suppress undesired modes over tempera- energy in the desired length direction can be ture and process variations that can impair maximized by designing the resonator’s thick- crystal-based oscillators. ness and width to minimize loss from other Shock and Vibration Sensitivity: in directions. Figure 2 shows a typical 107 Mhz addition, as part of the improved reliability, Fig. 4: Singulated wafer level resonator with measured Q value of 7453 and il MEMs oscillators demonstrate enhanced of -12.4dB. packaged pMEMS resonators on a semiconductor-grade shock and vibration the pMEMs resonators are fabricated by grain of rice. resistance. standard quartz devices are a cMOs-compatible process with wafer level fragile since the crystal is housed within packaging (WLP). The fabrication process starts a metal or a ceramic package, allowing with an sOi wafer deposited with piezoelectric the crystal to be fractured by a shock and electrode materials. Once the electrodes of 50-100g. Manufacturers have to then are patterned, the resonator body is defined implement specific storage, packing and by patterning the piezoelectric layer and scs shipping protocols for crystal devices to stack. avoid careless handling. afterwards a cap wafer is bonded to the pMEMs oscillator resistance to shock device wafer, and a pad metal layer is depos- Fig. 5: Teardown of a pMEMS oscillator and vibrations were evaluated. the ited to form the hermetic seal of the wafer level in a plastic QFN package. devices were easily able to survive more packaged device. Figure 3 shows the three- than 1500g of shock and 20g of vibra- dimensional view of the 550×450×200μm3 resonator with WLP. tion test as shown in figure 6. A few diced WLP pMEMS resonators on a grain of rice – see the small size of pMEMs resonators results in better reli- figure 4 - put things in perspective. ability, i.e., less mass results in better vibration/shock sensitivity. these pMEMs resonators are wire bonded to an ic die to In addition to meeting military grade shock / vibration specifica- form an oscillator. Figure 5 shows the teardown of the pMEMs tions, pMEMs devices also remain functional after shock tests oscillator plastic package. of 70,000g. Frequency Stability: long-term frequency stability was also measured by subjecting these plastic QFN packaged pMEMS Fig. 2: The S21 plot of a typical pMEMS resonator. Fig. 6: pMEMS devices passed military vibration and shock testing. 36 Electronic Engineering Times Europe December 2012 www.electronics-eetimes.com


EETE DECEMBER 2012
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