Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 1 6 TABLE 1 – MATERIALS AND DIMENSIONS FOR MODULES IN THE QUAD DIAPHRAGM PMUT CELL MODEL Stack from top Materials and modules Length × width Thickness Pt top-electrodes 10 µm × 10 µm 50 µm Thin-film PZT active layer 40 µm × 40 µm 0.5 µm Not seen Pt bottom-electrode 10 µm × 10 µm 100 µm Ti passive layer 10 µm × 10 µm 1 µm Vacuum cavity 10 µm × 10 µm 5 µm PI substrate 42 µm × 42 µm 8 µm Fig. 1 — The quad diaphragmPMUT cell in (a) planar viewwith key dimensions and materials, and (b) top and bottom angle views with thickness and cavity dimensions. (a) (b) TABLE 2 – MECHANICAL AND DAMPING PROPERTIES OF THE 40 MHz QUAD DIAPHRAGM PMUT MODEL Material Mechanical Damping (viscoelastic) Density Bulk velocity Shear velocity Bulk attenuation Shear attenuation Polyimide 1082 kg/m 3 3500 m/s 2000 m/s 9 dB/cm at 10 MHz 13 dB/cm at 10 MHz Titanium 4480 kg/m 3 6100 m/s 3100 m/s 0.3 dB/MHz/cm 1.2 dB/MHz/cm Platinum 21,400 kg/m 3 3260 m/s 1730 m/s 0.3 dB/MHz/cm 0.9 dB/MHz/cm U ltrasound iswidelyused for nonde- structive evaluation (NDE), struc- tural health monitoring (SHM), acoustic emission, sound navigation ranging, and in sensors for automobiles, medicine, and many other applications. Ultrasound fingerprint sensors [1,2] for user authentication and ultrasound buttons [3,4] are now replacing mechanical buttons. These next-generation, small-form-factor sensors, a few cm 2 or even smaller than 10 mm 2 , have been achieved through ad- vances in piezoelectric micromachined ultrasonic transducers (PMUTs) that can be positioned on either a flexible poly- mer or a silicon substrate to form an ar- ray [5-8] . In particular, flexible array sensors via the piezoelectric effect. The flexible PMUT model described in this article was designed to have a resonance fre- quency close to 40 MHz. It had a quad diaphragm structure—a single PMUT cell with four moving diaphragms. Each diaphragm had 10 × 10 µm planar di- mensions, and consisted of, from the top, concentric layers of 50 nm thick platinum (Pt) top electrode/1 µm thick PZT active layer/50 nm thick Pt bottom electrode/1 µm thick titanium (Ti) pas- sive layer/5 µm deep cavity surrounded by a polyimide (PI) substrate. Figure 1 shows schematics of the single PMUT cell model with dimensions, and Table 1 displays dimensions and materials for corresponding modules. FINITE ELEMENT ANALYSIS MODELING PZT 8 and materials with the prop- erties shown in Fig. 2 and Table 2 were used for this FEA study [12,13] . It should be noted that full characterization of the elastic and piezoelectric proper- ties of PZT thin films has not yet been achieved. Moreover, the properties of a PZT thin film may change during the complex microfabrication process. Consequently, the original properties can be applied to complex geometries for NDE and SHM, the human body [9] , and for medical monitoring and assistive devic- es [10,11] . Kim et al. [8] have recently shown how finite-element analysis (FEA) can be applied to develop an optimized, robust design of a 10 MHz, flexible, PMUT array sensor. This article presents an improved PMUTarray sensor design for ahigher res- onance frequency of 38 MHz. PMUT MODEL Piezoelectric micromachined ul- trasonic transducers (PMUTs) have a typical structure of, from the top, a top electrode, PbZr 0.52 Ti 0.48 O 3 (PZT) active layer, bottom electrode, elastic passive layer, a cavity wrapped by a substrate, and a substrate. The d31 coefficient of PZT leads to stretching and contraction in the radial and/or width directions, depending on its shape, when activat- ed by top and bottom electrodes. The PZT motions impose bending moments on the coupled elastic passive layer so that the diaphragm creates and propa- gates acoustic waves in the surrounding medium. When the returning acous- tic waves hit and bend the diaphragm, the PZT layer experiences mechanical stress and outputs an electric potential