MEMS (microelectromechanical systems) are microelectronic devices with small movable structures used in everything from projectors to gas sensors and gyroscopes. Over the past few years these types of device have infiltrated all nature of electronic devices. Think about the simple act of the screen on your smart phone changing orientation when you move, this was accomplished with a MEMS sensor.
The nature of the internal structure, within the devices, require very sophisticated assembly technics for the device to operate properly. Many times these devices need to be packaged in a high vacuum environment and the activation of getter materials to maintain vacuum levels.