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A deep back chamber micro-electro-mechanical system (MEMS) acoustic sensor with two sacrificial layers based on surface micromachining on a GaAs substrate is presented. As it is designed to be implemented on only the front side of a substrate in order to satisfy the need for a simple monolithic integrated process, this sensor has bottom electrode anchors fabricated using the first sacrificial layer for the formation of a back chamber. Each bottom electrode anchor, with a rectangular area of 50 mum times 50 mum, enables the bottom electrode to act as a stiff back plate because it make the bottom electrode fix onto a substrate through the patterning of the first sacrificial layer. The circular-shape MEMS acoustic sensor proposed in this paper has a membrane diameter of 1.0 mm and a back chamber total depth of 11.5 mum. It shows a pull down voltage of 9.7 V and a zero-bias sensor capacitance (CS0) of 2.9 pF at 1 KHz. Additionally, the sensor has an open-circuit sensitivity of 0.08 mV/Pa at 1 KHz with a bias of 1.5 V.