Study On Stress Gradient Measurement Method Of SU - 8 Film

SU-8negative photoresist has been widely used in MEMS. Because of its unique optical properties, mechanical properties and chemical properties, and because the photolithography technology of SU-8is cheaper than LIGA (lithographie, galvanoformung and abformung) technology, nowadays SU-8negative photoresist has been widely used in MEMS (Micro-Electro-Mechanic system) sensors and actuators. SU-8films’mechanical properties are very important if excellent MEMS devices are wanted, especially when SU-8film is used as structure layer in MEMS devices. Similar to usual materials used in MEMS devices such as polysilicon, mechanical properties of SU-8include Young’s modulus, stress, stress gradient, Poisson’s ratio, etc.This paper presents a table tennis racket-shaped structure to measure SU-8film’s stress gradient. A large number of simulating results from CoventorWare show that the error of stress gradient is less than1%if the maximum deflection of circular film is less than1/4. of the circular film’s thickness. The validity of the theory has been verified by CoventorWare software.A SU-8MEMS structure fabrication process based on sacrificial layer technology is also presented by this paper. No crack has been seen in the test structure, and also the test structure doesn’t adhere to the substrate. Stress gradient of SU-8film can be calculated after measuring the radius of curvature of the test structure by use of non-contact microscopic interferometry. The noncontact measuring method will not affect the test structure and it suits both the electric and insulating material measurement and with a good repetition.In this paper, we fabricated SU-8films of a table tennis racket-shaped structure and cantilever beam respectively, than measuring and calculating their stress gradient to have a composition. Experiments indicated that the presented measuring method is simple to operate, and has the advantages of high accuracy and good repeatability.This paper also measured the residual stress of SU-8film by micro-rotating-structures based on sacrificial layer technology. Measure the amount of deflection of the released SU-8test structure’s pointer by a microscope photograph of the deformation, then calculate the stress of SU-8film.