Measurement of Oxide Etch Rate of SOI Structure using near IR Microscopy

Abstract

The sacrificial oxide etching is one of the key steps in fabrication of micromechanical devices on bonded silicon on insulator (SOI) wafers. The determination of the etch rate of buried oxide layers in bonded SOI wafers from scanning electron microscopy (SEM) cross-sections is a rather tedious and time consuming task. However, the knowledge of the etch front propagation is essential for ensuring the success of etch release and thus the device operation. In this paper a fast and non-destructive method for measurement of sacrificial oxide layer etch rates is described. The pattern and time dependencies of SOI structure etching in concentrated hydrofluoric acid are studied by near infrared microscopy. The etch length of the SOI can be accurately measured with IR microscope. The measurement results were also verified by SEM cross-sections. The observed variation in etch rates can be explained by the SOI interface quality and not by any aspect ratio dependency caused by varying structure layer thickness. In the used etch times for etching of the thermal oxide the etch-rate seems to be limited by the bonded interface quality and not by transport limitations. The results of these experiments and some application examples are shown. Other possible applications, limitations and error sources of this method are discussed.