Anodic Bonding of CMUTs with Inverted Voltage-Temperature Sequence for Enhanced Bond Integrity

Super resolution volumetric imaging requires a large aperture, high frequency and large bandwidth Row-Column Addressed (RCA) transducer. RCA Capacitive Micromachined Ultrasound Transducer (CMUT) arrays are a promising solution. To ensure pressure uniformity along the array, a large CMUT array requires a metal electrode, and therefore, anodic bonding is a promising technique for manufacturing such an array. However, previous research has demonstrated that vertical short circuits are induced by high bonding voltages, decreasing the array yield. Thus, lowering the bonding voltage prevents vertical short circuits and decreases the bond strength by reducing the magnitude of the total electrical charge extracted from the wafer stack during the anodic bonding process. This work suggests using a temperature ramp method for fabricating CMUT arrays where the conventional ramping sequence in anodic bonding is inverted. This change means that the voltage is applied before heating the wafer stack to improve the bond strength without increasing the number of vertical short circuits, allowing for the fabrication of high-frequency lambda/2 pitch CMUT arrays.