Low temperature bonding of interface acoustic waves resonators on silicon wafers

Abstract

Surface acoustic wave (SAW) devices are widely used for high frequency wireless telecommunications. The use of interface acoustic waves (IAW) (1), also termed boundary waves, is promising to push back the limitations set by the sensitivity of the surface to disturbances. The IAW principle consists in guiding elastic waves at the interface between two materials, one of which at least is piezoelectric to allow for excitation and detection. Interface waves vanish in the depth of the substrates from both sides of the interface with the acousto-electric energy confined at the interface. From this perspective, the realisation of high frequency passive devices based on the coupling of piezoelectric transducers with semiconductor components is attractive. We developed a process which enables the bonding of a 3 inches processed lithium niobate wafer with a silicon substrate. In order to prevent thermal strain due to thermal expansion coefficients mismatch in the stack, a low temperature wafer bonding (125°C) is performed by enhancing energy surfaces thanks to chemical surface activation. Aluminium interdigital transducers (IDT) are buried inside the lithium niobate wafer. The problems inherent to surface pollution are sought to be solved by the self-encapsulation of the component. IDT resonators with 10 µm, 20 µm and 30 µm pitch were manufactured. Back side collective wet etching of vias through the Si wafer in KOH gives access to the pad contacts for measurements. We use a finite element analysis/boundary integral method (FEA/BIM) model to discuss the response of devices in Y-cut LiNbO3 wafers bonded on (100) silicon.