Study On In-Situ Extracting Method For Mechanical Properties Of CMOS MEMS Multilayered Films

Micro-Electro-Mechanical Systems, or MEMS, is a state-of-the-art technology that developed from microelectronics and combines some mechanical, thermal, optical, electromagnetic, and fluidic elements with electronics. CMOS MEMS process is recently widely applied to MEMS and has fabricated a large variety of MEMS devices and systems. It is well known that MEMS devices are highly dependent on factors such as the Young’s modulus and residual stress of the thin films, etc, which determine both the structure and the functionality of MEMS devices. However, for MEMS materials in the form of thin films based on CMOS MEMS technology, even the same material in the same process will show different properties under different conditions, so the properties of materials should be accurately assessed. But the materials of thin films have relatively small dimensions that are difficult to handle experimentally using the classical macroscopic characterization techniques. Besides, structures are often composed of multilayered thin films in CMOS MEMS process. Various methods, which have been applied to study the Young’s modulus and residual stress for the single layer film, are not easily extended to multilayered films. In this sense, in order to monitor mechanical properties of MEMS thin films, to predict and optimize the performances of MEMS devices, and to assure reliability, consistency and stability of the process, it is very important to develop the process-compatible in-situ test structures and extracting methods for properties of materials for multilayered thin films. For in-situ test structures and extracting methods of the described kind, a catalog of requirements may contain the following criteria, among others:process for test structures and the monitored process should be compatible; test structures should be simple in geometry, minimal micromechanical structured, robust and repeatable; measurement should be simple and direct.In this paper, in-situ test structures for measuring Young’s moduli and residual stresses of each individual layer for multilayered materials are present, and corresponding extracting methods are proposed. The main work and innovations of this dissertation are described as following:First, dynamic models of micro beams of multilayered thin films with different widths are proposed on the basis of the Euler-Bernoulli Beam Theory. Models of resonant frequency are introduced not only for the doubly-clamped beams that are initially flat, but also for the ones that are buckled due to the residual stresses. Taking into consideration of the deflection of multilayered cantilevers with residual stresses, models of resonant frequency for multilayered cantilevers are also presented.Second, a novel extracting method for Young’s moduli and residual stresses of multilayered thin films by measuring resonant frequencies of doubly-clamped beams is developed. A new extracting method is also presented, which determines Young’s moduli and residual stresses of multilayered thin films by measuring resonant frequencies of cantilever beams and doubly-clamped beams. What’s more, a novel extracting method for Young’s moduli and residual stresses of multilayered thin films is also developed, by measuring resonant frequencies of cantilever beams and doubly-clamped beams, together with measuring deflections of cantilever beams. All the methods have a wide range of adaptability, for the reason that both the initially flat doubly-clamped beams and the buckled ones are applied in those methods.Third, regarding the models and methods above as the foundation, test structures are designed. As the frequencies and deflections of bilayer structures are measured, Young’s moduli and residual stresses for both polysilicon and gold layer are obtained. Young’s modules for polysilicon layer is 148.7±5.5GPa, while residual stress for polysilicon layer is-15.42±1.15MPa, and Young’s modules for gold layer is 78.2±5.5GPa, while residual stress for gold layer is 18.51±4.81MPa. Test structures are simple and small. It is easy to perform measurement and suitable for the in-situ monitor to MEMS process.All the test structures we put forward are simple and small in geometry. Fabrication of test structures is compatible with CMOS MEMS process. The measuring methods are simple, direct, reliable and repeatable. So it is capable to meet the requirements of in-situ measurements, and can be applied in MEMS process line to provide direct monitoring of the fabrication.