Preparation of thin lithium niobate layer on silicon wafer for wafer-level integration of acoustic devices and LSI

This paper describes UV adhesive bonding of lithium niobáte (LN) and silicon (Si), and the following polishing of LN, which are key technologies in wafer-level integration processes for LN acoustic devices on LSI. Five UV adhesive candidates were investigated in terms of bonding-induced stress and removability by O₂ plasma treatment. The latter is important because the UV polymer is used as a sacrificial layer in the above processes. Based on the results, we selected one usable UV adhesive, and obtained bending-free LN/Si hybrid substrates, overcoming a large difference in the coefficient of thermal expansion between LN (7.5 (c-axis) - 14.4 (a-axis) × 10^-6 /K) and Si (2.6 × 10^-6 /K). The LN substrate on the Si substrate was thinned and surface-polished by an experimentally obtained recipe. Finally, a mirror-finished LN layer with a thickness of ca. 10 μm was successfully obtained without noticeable cracks. It was confirmed that this thin LN layer survived in the fabrication process of surface acoustic wave (SAW) devices.