In-situ determination of the coefficient of thermal expansion of polysilicon thin films using micro-rotating structures

• In-situ test structure for determining the coefficient of thermal expansion (CTE)
• Analysis of the behavior of the test structure
• Uses of standard integrated circuit test equipment
• The CTE measured for the polysilicon thin film

A simple method for in situ determination of the coefficient of thermal expansion (CTE) of polysilicon thin films by using micro-rotating structures is presented. The structures are heated electrically and deflect due to the thermal expansion. An analytical expression is derived to relate the CTE of the polysilicon thin film with the lateral displacement, geometrical parameters and the average temperature of the test structure. The lateral displacement is designed to be a constant value (2 μm), while the average temperature of the test structure can be obtained from the measurement of resistance of the test structure. Instead of an optical or visual readout, electrical input and electrical readout are utilized. In the experiments, a current–voltage measurement system only is required and all measurements can be carried out in atmosphere. Finite element analysis and experimental results with surface micromachined polysilicon thin films are used to demonstrate the effectiveness of the proposed method. The average value of the obtained CTE is (2.76 ± 0.09) × 10− 6 K− 1 with temperature ranging from 450 K to 500 K.