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Pb(Zr,Ti)O₃ (PZT) is a piezoelectric perovskite oxide that is a solid solution of PbZrO₃ and PbTiO₃. The Zr/Ti ratio at the B-site determines the crystal structure and material properties. A large piezoelectric property emerges around Zr:Ti = 52:48, known as the morphotropic phase boundary (MPB), which is an almost vertical line in the temperature versus composition phase diagram, where a transition occurs between the tetragonal and rhombohedral ferroelectric phases. The piezoelectric property of PZT can further be tuned by impurity doping to make it “hard” or “soft,” which denotes the feasibility of the dipole movement. Because PZT is a ferroelectric material with switchable polarization by an external electric field, a poling process allows the use of polycrystalline PZT materials with high piezoelectric properties. PZT has a piezoelectric constant nearly an order of magnitude higher than that of AlN [3]. Because of the high piezoelectric properties of PZT, an exemption has been applied to PZT from the RoHS Restriction of Hazardous Substances since 2003.

AlN is a III-V compound with a wultzite structure which shows two polarities since the basal plane is determined between alternating layers of Al- and N-. Thus AlN thin film in polycrystalline form may have no piezoelectricity when two polarities are equally distributed [4, 5]. AlN is known for its piezoelectric tolerance to high temperature (> 1000 °C), and can be applied in harsh environments that require robust thermal stability [6]. AlN has a relatively low piezoelectric constant (e_31,f) compared with PZT, which leads to a lower output pressure of ultrasound from AlN-based pMUT. However, because AlN has a lower dielectric constant ε₃₃ than PZT, the sensitivity of AlN-based pMUT is high [3]. As has been reported recently, doping can significantly modify the properties of AlN. Wang et al [7]. reported an Sc: AlN-based pMUT that showed a 20 % increase in the electromechanical coupling coefficient (k_t²) and a triple increase in the output pressure with respect to the value of an AlN-based pMUT with the same geometry and fill factor under the same measurement conditions. Moreover, Fichtner et al. reported that AlN also exhibits ferroelectric behavior upon Sc doping [8].

Fig. 2. 
Schematics of PZT [14], AlN [15] crystal and PVDF [16] structures

Polyvinylidene fluoride (PVDF), a semi-crystalline polymer polymerized with H₂C=CF₂ monomers, is well known for its higher piezoelectric coefficient than that of other piezoelectric polymers. The piezoelectric performance is enhanced by defects generated during the connection between two CF₂ groups or CH₂ groups. Defects induce an increase in polarization, which leads to an improvement in the piezoelectric response. The β-phase has the best piezoelectric performance of the five known phases of PVDF (α, β, γ, δ, and ε) because all dipole moments are aligned. Studies have been conducted where further control of the crystal orientation of PVDF was achieved by adding trifluoroethylene (TrFE) [9]. Qualcomm fabricated fingerprint sensors that successfully conducted under-display ultrasonic detection using PVDF-TrFE [10-12]. Even though the material shows a relatively poorer piezoelectric response than PZT, it has a dielectric constant similar to that of AlN [13]. Thus, with the help of a charge amplifier, the PVDF-based pMUT could be implemented in commercial devices [10-12].

To fabricate a pMUT, piezoelectric membranes must have sufficient empty space for vibration. Unlike cMUT, empty spaces in pMUT do not have to be between the device layers. Thus, after fabricating piezoelectric films through methods like sputtering [7, 18, 19], sol-gel [20] and spin coating [21], void fabrication can be performed at the final stage using a deep reactive ion etching (DRIE) method, as shown in Fig. 3. The piezoelectric thin film with top and bottom electrodes must be integrated onto a passive layer, mostly Si, to convert the lateral motion (31 mode) of the piezoelectric layer into vertical bending motion. Because the thickness of the Si passive layer must be precisely controlled, silicon on insulator (SOI) wafers are usually used, where the buried oxide layer works as an etch-stop layer during DRIE of the backside handling Si.

Fig. 3. 
Typical MEMS fabrication flow of pMUT using DRIE for ultrasound cells. Reproduced with permission from Ref. [22].

Deposited films are patterned according to array structures such as 1D, 2D row-column, and 2D accommodated with application-specific integrated circuits (ASIC). The 1D array structure cannot operate the imaging process in a direction parallel to the array because membrane cells in the same array are controlled simultaneously through a single top electrode pad. Thus, the 1D array requires the mechanical movement of the device along the direction of the arrays during scanning. 2-D arrays not only reduce the extra time required for moving the transducer but also allow more options for scanning with beam steering and focusing on 2-dimensional control. However, 2-D arrays pose problems in complex imaging, scanning, and system organization caused by numerous interconnections between each cell and electrode. Row-column addressing can solve this problem by reducing the interconnections without decreasing the number of cell elements [23]. For example, for N×N arrays, the number of wires can be lowered to 2N. To achieve ultrasound imaging with better control of each cell element, ASIC can be organized to have pMUT and complementary metal-oxide semiconductor (CMOS) integration by having a one-on-one connection between each cell and the CMOS transistor, as shown in Fig. 4(c). Because CMOS is vulnerable to high temperatures of approximately 525 °C [24], CMOS and piezoelectric diaphram MEMS are fabricated separately and bonded to form interconnections through flip chip bonding, which provides a large number of shorter electric contacts compared with wire bonding. In flip chip bonding, thermocompression is used depending on the eutectic temperature of the bonding material, and the thermosonic method is used when the system requires a much lower temperature and pressure [25].

Fig. 4. 
pMUT arrays (a) 1D (Reproduced with permission from Ref. [18]), (b) 2D row-column [20], (c) 2D on Asic (Reproduced with permission from Ref. [26].

For fingerprint sensors, the standard resolution is 500 DPI, the same as that required by FBI-compliant systems [27]. To enhance the lateral resolution of the fingerprint sensor, the cell size and pitch length should be diminished. This interferes with the acoustic output and fill factor for the CMOS-combined pMUT because the area for interconnection remains the same whereas the active area decreases. Jiang et al [28]. reported an increase in the fill factor and output pressure of an ultrasonic fingerprint sensor using a rectangular cell design. Figs. 5(c) and (d) show the output pressure of the rectangular and circular pMUT at each resonance frequency for a hydrophone distance of 220 μm and 400 μm. The measured acoustic surface pressures for each pMUT are 70.8 and 41.9 kPa, which are caused by the three times higher fill factor.

Tanaka. et al [29]. adopted a hexagonal array to upgrade the imaging system and simplify the imaging operation. The hexagonal arrangement allows the densification of operating elements with a relatively smaller pitch and larger cell size, which contributes to the enhancement of the imaging resolution and sound pressure. Moreover, only two types of distance information are required to perform beamforming, whereas for the orthogonal arrangement, this number is three. A time delay should exist for each input signal of the orthogonal array, which is difficult to achieve for such an arrangement because the elements have a common electrode in either the row or column direction.

Fig. 5. 
(a) Rectangular and (b) circular cell design and their output pressure measurements, (c) and (d), respectively. (a-d) Ref. [28]. Structural comparison of (e) hexagonal and (f) orthogonal cell arrangement. Reproduced with permission from Ref. [29].

Tang et al [26]. reported an ultrasonic fingerprint sensor on a chip containing pMUT combined with CMOS ASIC through eutectic bonding. This AlN pMUT provided a 591 × 438 DPI resolution, comprising 110 × 56 rectangular cells with a pitch of 43 μm. A polydimethylsiloxane (PDMS) coupling layer was placed on an ultrasonic sensor for acoustic impedance matching. The surface and inner tissue of the fingerprint, known as the epidermis and dermis, respectively, were scanned to provide images, as shown in Fig. 6(a). Ultrasonic waves exhibit a 100 % reflection caused by the air-filled valley part of the fingerprint, resulting in a dark pattern in the image. Ultrasound penetrates the ridge and is reflected near the dermis by an impedance mismatch. Analyzing the received echoes with a time delay separates the epidermal and dermal images in a single operation.

Fig. 6. 
(a) Dermal and (b) epidermal images acquired using pMUT. (b) High-resolution ultrasound sonograph of a human fingertip. (c) Anatomy cross-section of human fingertip. (a-c) Reproduced with permission from Ref. [26]. (d) PMDS fingerprint phantom and its image, acquired under (e) glass layer and (f) OLED display. (d-f) Reproduced with permission from Ref. [17].

To use fingerprints for biometric authentication in smartphones, Peng et al [17]. reported a piezoelectric polymer-based pMUT for under-display fingerprint sensing. 10 μm thick and 1 × 1 mm² P(VDF-TrFE) was set as a single element with E-solder as the backing layer and electrode. The PDMS fingerprint phantom was scanned by the mechanical movement of the transducer. Images with a resolution of 500 × 500 DPI resolution and 70.7 μm lateral resolution were acquired with a glass layer placed between the pMUT and phantom. The lateral resolution of the acquired image through the OLED touch screen was 68.9 μm, providing a similar quality of image regardless of whether the phantom was under the glass or display screen, as shown in Figs. 6(e) and 6(f). pMUT-based fingerprint-imaging technologies are listed in Table 2.