Waferbond technologies and quality assessment

Several wafer bond technologies like direct and anodic bonding without intermediate layer or thermo-compression wafer bonding with intermediate layers like low melting frit glasses, eutectic materials or polymers were developed during the last years and are nowadays extensively used in industrial applications. The paper will describe some important aspects of new Ti-Si and Laser assisted transmission Si to Si bonding techniques and their application for the fabrication of an ultrasonic transducer array. Main aspects for the reliability of wafer bonded devices are the yield after bonding, the bond strength and the hermeticity as well as the suitability for the packaging processes (e.g. moulding) and the influence of electrical and mechanical properties of the bonded elements by the bond process. The paper will focus on methods to characterize the bond strength and the hermeticity of bonded compounds. The main method to evaluate the bond strength is the micro chevron test. This test requires special designed test samples and is destructive, but also standard chips with bonded caps can be used for strength measurements. For anodic bonding between transparent glass and silicon a non destructive bond strength test structure can be used. This non-destructive test structure based on structure fabricated on one surface of the bond partners and has dimensions of several 10 to 100 μm. After the anodic bonding the length of the not bonded gap around this structure can be used to calculate the bond strength. The hermeticity can be tested by helium leakage test or by using resonant structure inside of the cavities which change the frequency depending on the pressure inside of the cavity.