The Effect of Back-chamber Volume on the Surface micromachined Acoustic Sensor

We present a study of the back-chamber volume effects on sensitivity and frequency response of the surface micromachined MEMS acoustic sensor. A surface micromachined acoustic sensor, unlike conventional bulk micromachined sensor, has a limited volume of back-chamber formed under the surface of the substrate. An acoustic back-chambers are formed by isotropic etching of the silicon substrate under sensing structures. However, due to structural limitation and process, it is difficult to fabricate sufficient depth. Consequently, the volume of the back-chamber, which is extremely smaller compared to the bulk micromachined acoustic sensor, affects acoustic sensor performance. We fabricated three surface micromachined acoustic sensors which have different membrane diameters, and measured the membrane displacement characteristics while increasing the volume of the back-chamber by controlling the XeF2 etching cycle. As a result, the back-chamber volume of surface micromachined acoustic sensor should not be less than a certain size in order to obtain the desired performance and it depends on the structural parameters such as membrane size and sensing gap.