An ultra thin package for an electric acoustic sensor chip of a micro electro mechanical system is provided. A substrate has a first substrate surface and a second substrate surface opposite to the first substrate surface. At least one conductor bump is formed on the second substrate surface. An electric acoustic sensor chip having a first chip surface and a second chip surface opposite to the first chip surface is provided. The first chip surface is electrically connected to the conductor bump. The conductor bump is positioned between the second substrate surface and the first chip surface to create a space. The conductor bump is used for transferring a signal from the sensor chip to the substrate. An acoustic opening passing through the substrate is formed.