Experimental analysis of lithium niobate CMP for room temperature bonding

Abstract

Lithium niobate (LN, LiNbO3) is a type of artificial crystal exhibition piezoelectricity, pyroelectric and ferroelectricity, which has been widely used in electronic components. To use in electronic components, a LN wafer has to be bonded to a Si substrate. Bonding of Si and LN allows for ease of micromachining a high quality electrically-sensitive thin film, as well as creating mechanically-rigid diaphragms. For this reason, Si combined with LN is desirable for microelectromechanical system (MEMS) devices such as a pressure sensors, microfluidic devices and optical data storage systems. However, the large difference in thermal expansion coefficient between Si and LN causes serious thermal stresses during the thermal-pressure bonding process which has generally been used. Therefore room temperature bonding would be the best candidate for making a strong and stress-free interface between Si and LN. Room temperature bonding requires a lower surface roughness and lower defect contact on the LN wafer surface than thermal bonding does. A chemical mechanical polishing (CMP) process produces a thin LN wafer with a high quality surface suited for room temperature bonding and with a suitale thickness that affects the sensitivity of the devices. Here LN wafers were polished using a colloidal silica slurry, resulting in a high material removal rate (MRR) and a fine surface quality under a condition of low pH, high abrasive concentration and low flow rate. The polishing mechanism of LN was investigated by mechanical, chemical and thermal analysis.