Wafer-level experimental study of residual stress in AlN-based bimorph piezoelectric micromachined ultrasonic transducer

Abstract

During the last decade, piezoelectric micromachined ultrasonic transducer (pMUT) have raised a great interest for various applications. In particular, AlN material has been used to build bimorph structures made of two AlN layers sandwiched between metallic electrodes. This kind of pMUT offer good performances for both sensing and actuating because of the multi-electrodes scheme of the design. Yet, this pMUT architecture is expected to be particularly sensitive to residual stress within the AlN layers. At the wafer level, stress dispersion is likely to cause important performance variations. In this paper, we characterize bimorph pMUT working in the 50-250 kHz range to investigate how much stress variations at the wafer level (8″) impact critical pMUT features such as resonant frequency, sensitivity and quality factor. Drive sensitivity up to 216 nm/V are obtained with a single electrode scheme. Two approaches are combined, respectively based on pMUT simulation and curvature measurement, to assess the contribution of AlN residual stress to the overall tension of the diaphragm, ranging from −300 to 900 N m −1 . The resulting dispersion of resonant frequency is higher than 100%. We highlight how to obtain satisfactory yield.

Curvature and Stress Effects on the Performance of Contour‐Mode Resonant ΔE Effect Magnetometers

“…For the MOKE‐imaged device, initial stress components were tuned until a good match was obtained with σ 0,11 = 45 MPa, σ 0,22 = 75 MPa, and σ 0,12 = 5 MPa, which is in the range of stress that may be expected for sputtered films. [ 36,38,50 ] The z ‐component of the displacement vector field, after letting the initial stress go to equilibrium, is shown in comparison with the measured profile in Figure a, and it can be seen that measurement and simulation match very well. The displacement is asymmetric relative to the xy ‐coordinate axes, which is caused by the nonzero in‐plane shear component σ 12 of the equilibrium stress.…”

“…Furthermore, it is unsurprising that residual stress may vary across a wafer in sputtered films, as evidenced by studies that have quantified stress variations in AlN across wafers. [ 36–38 ] We do not know the degree to which such wafer location‐based stress nonuniformity in the AlN film, compared to the subsequent chip‐by‐chip deposition of the magnetic multilayer, contributes to causing residual stress differences between the completed sensor devices. One way or another, the stress nonuniformities evidently led to significant variation in the performance of these magnetometers.…”

Neuromorphic object localization using resistive memories and ultrasonic transducers

“…pMUT sensors are arranged in a printed circuit board, separating the two receivers of about 10 cm, with the emitter between the receivers. In this work, each membrane is a suspended bimorph structure made of two 800 nm-thick piezoelectric AlN layers sandwiched between three 200 nm-thick Molybdenum (Mo) layers, covered by a 200 nm-thick top SiN passivation layer, as reported in 62 . Inner and outer electrodes are patterned on the bottom and the top Mo layers, while the middle Mo electrode is not patterned and used as the ground, resulting in a membrane with four electrodes pairs.…”