A Study On Extraction Method And In-situ Testing Structure Of Piezoelectric Coefficient In MEMS Piezoelectric Thin Film

With the development and improvement of the process of Micro Electro-Mechanical System?MEMS?,more and more micromechanical structures are designed and fabricated.Most of these moveable structures are very small in dimensions,thus thin-film structures are required and applied.However,differences of mechanical behavior are huge between thin-film structures and macroscopical ones.So classical mechanical parameters are no more valid.Hence,in-situ testing is crucial for controlling and supervising mechanical behavior of thin-film structures.With the MEMS devices becoming integration and microminiaturization,piezoelectric films are paid closed attention corresponding to the improved demands.Because design and working life of devices depend on piezoelectric property,it is important for MEMS technology to extract piezoelectric coefficients accurately.And it becomes a focus to study on transverse piezoelectric coefficient d₃₁.But the extracting models of piezoelectric coefficient is not completed.Therefore a demand for an accurate in-situ testing method of piezoelectric coefficient is urgent now.In-situ testing methods of the mechanical parameters of polysilicon thin-film materials and the characteristic parameters of piezoelectric film materials in the surface micromachining process were studied on in the thesis principally.The main testing structures were multi-layer cantilevers,which were simple and direct.In addition,the process required was compatible with the mature process and the testing operation was easy and repeatable.First extraction of Young's modulus of each layer was realized,then the transverse piezoelectric coefficient d₃₁ of piezoelectric thin film was extracted by bring Young's modulus of each layer into the solution procedure.The veracity of extraction of transverse piezoelectric coefficient d₃₁ can be improved due to the definition of Young's moduli in advance.The main content is:first,to extract Young's moduli in thin films,it was divided into two research directions for resonance modeling.One was the Euler–Bernoulli beam structure,the other was Timoshenko beam structure.Meanwhile,two situations,buckled and unbuckled multi-layer cantilevers in origin,were discussed respectively when the Euler–Bernoulli beam structure was modeling.Second,for the extraction of piezoelectric coefficient of piezoelectric thin films,there were two methods to be compared and verified with each other.One was energy method proposed by this thesis,the other was classical balance method.Third,finite element simulation was done in the thesis using the testing structures proposed and the errors were all less than 8%.It confirmed the reliability of the two models proposed.Meanwhile,layout design,implementations of tape-out and in-situ testing on mechanical parameters were also done in the thesis.The errors were all less than 9%,which proved the of the validity of the theoretical model proposed.