Yipeng Lu

Ultrasonic fingerprint sensor using a piezoelectric micromachined ultrasonic transducer array integrated with complementary metal oxide semiconductor electronics

This paper presents an ultrasonic fingerprint sensor based on a 24 × 8 array of 22 MHz piezoelectric micromachined ultrasonic transducers (PMUTs) with 100 μm pitch, fully integrated with 180 nm complementary metal oxide semiconductor (CMOS) circuitry through eutectic wafer bonding. Each PMUT is directly bonded to a dedicated CMOS receive amplifier, minimizing electrical parasitics and eliminating the need for through-silicon vias. The array frequency response and vibration mode-shape were characterized using laser Doppler vibrometry and verified via finite element method simulation. The arrays acoustic output was measured using a hydrophone to be ∼14 kPa with a 28 V input, in reasonable agreement with predication from analytical calculation. Pulse-echo imaging of a 1D steel grating is demonstrated using electronic scanning of a 20 × 8 sub-array, resulting in 300 mV maximum received amplitude and 5:1 contrast ratio. Because the small size of this array limits the maximum image size, mechanical scanning was used to image a 2D polydimethylsiloxane fingerprint phantom (10 mm × 8 mm) at a 1.2 mm distance from the array.

Modeling, Fabrication, and Characterization of Piezoelectric Micromachined Ultrasonic Transducer Arrays Based on Cavity SOI Wafers

This paper presents high fill-factor piezoelectric micromachined ultrasonic transducer (PMUT) arrays fabricated via a novel process using cavity SOI wafers. The simple three-mask fabrication process enables smaller diameter PMUTs (25 μm) and finer pitch than previous processes requiring through-wafer etching. PMUTs were fabricated with diameters from 25 to 50 μm, resulting in center frequencies from 13 to 55 MHz in air. Two types of devices, having different piezoelectric layers, lead zirconium titanate (PZT), and aluminum nitride (AlN), were fabricated and characterized. Comparing 50-μm diameter devices, the PZT PMUTs show large dynamic displacement sensitivity of 316 nm/V at 11 MHz in air, which is ~20x higher than that of the AlN PMUTs. Electrical impedance measurements of the PZT PMUTs show high electromechanical coupling k2t = 12.5% and 50-Ω electrical impedance that is well-matched to typical interface circuits. Immersion tests were conducted on PZT PMUT arrays. The fluid-immersed acoustic pressure generated by an unfocused 9 × 9 array of 40-μm diameter, 10-MHz PZT PMUTs, measured with a needle hydrophone 1.2 mm away from the array, was 58 kPa with a 25 Vpp input. Beam forming based on electronic phase control produced a narrow, 150-μm diameter, focused beam over a depth of focus >0.2 mm and increased the pressure to 450 kPa with 18 Vpp input.

A High Fill-Factor Annular Array of High Frequency Piezoelectric Micromachined Ultrasonic Transducers

This paper presents a 1.2-mm diameter high fill-factor array of 1261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz in fluid for intravascular ultrasound imaging. At 1061 transducers/mm2, the PMUT array has a 10-20 times higher density than previous PMUT arrays realized to date. Aluminum nitride (AlN)-based PMUTs described in this paper are fabricated using a process compatible with the fabrication of inertial sensors, radio frequency (RF) resonators, and CMOS integrated circuits. The PMUTs are released using a front-side sacrificial etch through etching holes that are subsequently sealed by a thin layer of parylene. Finite element method and analytical results, including resonant frequency, pressure sensitivity, output acoustic pressure, and directivity are given to guide the PMUT design effectively, and are shown to match well with measurement results. Due to the PMUTs thin membrane (750-nm AlN/800-nm SiO2) and small diameter, a single 25-μm PMUT has approximately omnidirectional directivity and no near-field zone with irregular pressure pattern. PMUTs are characterized in both the frequency and time domains. Measurement results show a large displacement response of 2.5 nm/V at resonance and good frequency matching in air, high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz, when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused high-output pressure acoustic beam.

Pulse-echo ultrasonic fingerprint sensor on a chip

A fully-integrated ultrasonic fingerprint sensor based on pulse-echo imaging is presented. The device consists of a 24×8 Piezoelectric Micromachined Ultrasonic Transducer (PMUT) array bonded at the wafer level to custom readout electronics fabricated in a 180-nm CMOS process. The proposed top-driving bottom-sensing technique minimizes signal attenuation due to the large parasitics associated with high-voltage transistors. With 12V driving signal strength, the sensor takes 24μs to image a 2.3mm by 0.7mm section of a fingerprint.

Versatile CMOS-MEMS integrated piezoelectric platform

We present the extension of the InvenSense fabrication platform to piezoelectric transduction. The newly proposed CMOS-MEMS Integrated Piezoelectric Platform inherits the wafer bonding advantages of its predecessor, leverages existing semiconductor infrastructure, and is applicable to a wide range of applications.

11.8 Integrated ultrasonic system for measuring body-fat composition

An accurate, low-power, and highly integrated solution for accurate assessment of body fat is presented that addresses a growing consumer interest in economical and easy-to-use solutions for monitoring personal health and fitness. Unlike the prevalent present solution that estimates body fat percentage from an impedance measurement integrated in a weight scale and gives only a global index with accuracy compromised by a host of factors including skin moisture and metabolic activity, the reported approach uses ultrasound for an accurate measure of the actual thickness of the fat and muscle layers [1].

Short-range and high-resolution ultrasound imaging using an 8 MHz Aluminum Nitride PMUT array

Ultrasound imaging uses costly bulk piezoelectric transducers and high voltage (200V+) electronics. Low-cost and low-voltage ultrasound transducers would enable many new applications in healthcare, biometrics, and personal health-monitoring. Here, we demonstrated short-range (~mm) and high-resolution (<;100 μm) imaging based on piezoelectric micromachined ultrasonic transducers (PMUTs) and a 1.8 V interface ASIC. The PMUTs use piezoelectric Aluminum Nitride (AlN), which has the advantages of low-temperature (<;400 °C) deposition and compatibility with CMOS fabrication but has a relatively low piezoelectric constant (e31=-0.5 C/m2), making detection of ultrasound signals from tiny (50 μm) PMUTs a challenging task. To solve this problem, we developed an ASIC with a low-noise analog front-end pre-amplifier that is impedance matched to the PMUT. Furthermore, a novel beam-forming and scanning method was demonstrated to achieve a sub-100μm focus size and 70 μm scanning step. Pressure map measurement from phased PMUT array and pulse echo imaging results were demonstrated using 1-D and 2-D phantoms.

High frequency piezoelectric micromachined ultrasonic transducer array for intravascular ultrasound imaging

This paper presents a 1.2 mm diameter high fill-factor array of 1,261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz for intravascular ultrasound (IVUS) imaging and other medical imaging applications. At 1061 transducers/mm2, the PMUT array has a 10-20× higher density than the best PMUT arrays realized to date. The PMUTs utilize a piezoelectric material, AlN, which is compatible with CMOS processes. Measurements show a large voltage response of 2.5 nm/V and good frequency matching in air, a high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused, high output pressure acoustic beam.

Pulse-Echo Ultrasound Imaging Using an AlN Piezoelectric Micromachined Ultrasonic Transducer Array With Transmit Beam-Forming

This work demonstrates short-range and high-resolution ultrasonic imaging using 8 MHz aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT) arrays, which are compatible with complementary metal-oxide semiconductor circuitry and wafer-level mass manufacture. Because AlN has a low dielectric constant, the PMUTs have low capacitance and a custom 1.8 V interface application-specified integrated circuit with on-chip charge-pump (1.8 to 32 V) is capable of providing sufficient output current to drive the PMUT array. Transmit beam-forming is used to produce a 90 μm focused acoustic beam-width. A pressure map measured with a needle hydrophone agrees with finite element method-simulations. Finally, 1-D and 2-D pulse-echo imaging was conducted using metal targets.

Monolithic 591×438 DPI ultrasonic fingerprint sensor

This paper presents a 591×438 DPI ultrasonic fingerprint sensor. The sensor is based on a piezoelectric micromachined ultrasonic transducer (PMUT) array that is bonded at wafer-level to complementary metal oxide semiconductor (CMOS) signal processing electronics to produce a pulse-echo ultrasonic imager on a chip. To meet the 500 DPI standard for consumer fingerprint sensors, the PMUT pitch was reduced by approximately a factor of two relative to an earlier design. We conducted a systematic design study of the individual PMUT and array to achieve this scaling while maintaining a high fill-factor. The resulting 110×56 PMUT array, composed of 30×43μm2 rectangular PMUTs achieved a 51.7% fill-factor, three times greater than that of the previous design. Together with the custom CMOS ASIC, the sensor achieves 2 μV/Pa sensitivity, 13 kPa pressure output, 75μm lateral resolution, and 150μm axial resolution in a 4.6 mm × 3.2 mm image.