Highly Reliable MEMS Temperature Sensors for 275 $^{ \circ}\hbox{C}$ Applications—Part 2: Creep and Cycling Performance

The second part of this paper presents an experimental reliability assessment of microelectromechanical-system bimorph cantilever beams for high-temperature sensing applications. The sensor, presented in Part I, is shown to operate up to 275 °C and to have a quality factor of over 5000 at room temperature, a response time of under 600 μ s, and an inherent capacitance uncertainty of under 0.13 fF. In this part, the reliability of the bimorph component of the sensor is tested at sustained high temperatures (up to 275 °C), as well as throughout thermal cycling. The cantilevers are initially thermally annealed to relax the film stresses. During the anneal of the device, there is a decrease in room-temperature tip displacement from 120 to 70 μm and a decrease in the high-temperature tip displacement from 40 to 10 μm. After the thermal anneal, the devices show less than 3 μm of further creep after three months (over 2000 h) at 200 °C and over 120 h at 275 °C. Over 20 samples have been successfully tested up to 100 million cycles without failure. Record results are demonstrated to over 1.2 billion thermal cycles from fully relaxed to fully deflected states. Consisting of over 1000 beam profile measurements and dozens of individual fabricated chips, this represents one of the largest reliability studies of bimorphs to date.