Paper MN-MoM10
An Opto-Thermo-Mechanical MEMS Sensor for Direct Thermal Imaging

Monday, October 20, 2008, 11:20 am, Room 206

Infrared imaging plays a critical role in many applications ranging from night vision, environmental monitoring and astronomy. The paper describes a room-temperature compensated opto-thermo-mechanical un-cooled infrared imaging system with a direct color display. The sensor consists of an array of sensing elements suspended from the substrate using bimorph beam elements. Infrared radiation incident on a sensing element is absorbed and leads to a rise in temperature. The heat conducted from the sensing element to the bimorph beam elements leads to a deformation of the beam elements and results in a displacement of the sensing element in a direction perpendicular to the plane of the sensing element. Thus the lateral positions of the sensing elements is influenced by the infrared energy received by the sensing elements in addition to the room temperature. The lateral movement of sensing element is converted into a color image by interference of reflected light from the sensing element and another parallel element with an air gap suitable for creating a constructive interference in the visible spectral range. This second parallel element is also mounted using similar bimorph beam elements and is made of a material transparent to infrared radiation. Thus the transverse position of the transparent element does not depend on the incident infrared radiation but only the ambient temperature. This way it is possible to cancel the effect of room temperature on the interference pattern. The design considerations for the above device are described here. Various configurations of arranging the elements and the bimorph are discussed along with their relative merits. Simulations were conducted using Ansys© software. For an incident radiation of 100 W/m², the rise in temperature of 0.3mm x 0.3mm sensing element was about 5 degrees C. The sensitivity of the device was found to be of the order of 4 nm³/W. The time constant of the device was found to be about 2 seconds.