Stress Test And Analysis Of Sensitive Structures Based On Piezoresistive Effect

The Micro-electro-mechanical products can not be fabricated by using one-step molding technology and have characteristics of different materials, different manufacture process, different connecting technology, as well as components with small size, low stiffness and strength, and high assembly accuracy. Low stress and process temperatures are the major advantages of adhesive bonding, which made adhesive bonding favorable for the miniature parts assembly. At the same time, the thermal cure required for adhesive bonding of miniature parts assembly causes thermal mismatch that induces extra stresses and strains, which may adversely affect the performance and reliability of the advice. In order to minimize or eliminate the assembly induced stress and optimize the device fabrication process, it is critical to carry out the quantitative measurement of the induced stress and study the stress influenced factors.This paper focus on adhesive bonding induced stress of a miniature part with piezoresistors based on the piezoresistive effect. The micro-machining technology combined with wet etching and dry etching method was designed to verify the possibility of miniature parts integrated with diffused resistors. Firstly, the piezoresistive effect theory used for testing stress was presented and the whole preliminary design was defined, including the choice of piezoresistance coefficient, the arrangement sites and dimension of resistors, then the technological process. The adaptability and compatibility of process used in the design process must be considered. Finally, the fabrication of miniature parts with diffused resistors was achieved.Adhesive bonding assembly system composed of components with different materials, usually including miniature parts, adhesive and substrate. The coefficient of thermal expansion of these materials was often different with each other, which would induce stress in miniature parts in the high-temperature adhesive cured process which may affect the performance of miniature parts. This paper targeted on stress test after adhesive cured, then designed miniature part samples with diffused resistors as testing carriers to study the surface stress affected by different adhesive and thermal treatment systematically. Meanwhile, the in-situ stress difference after assembly based on the piezoresistive effect was also studied.Miniature parts had sensitive bars (sensitive structures) as the key sections. While in the process of adhesive bonding, the stress applied on the miniature parts could affect the output performance of the whole miniature advice. The thermal stress was related to the coefficient of thermal expansion and the Young modulus of materials. This paper studied the relationship between packaging induced thermal stress and packaging materials by finite element software, especially analyzed the stress distribution of miniature parts and the effect of coefficient of thermal expansion and Young modulus of adhesive on stress. Meanwhile, the stress distributions of miniature parts with adhesive on different positions and with different substrate materials were also analyzed. According to test and finite element analysis results, the factors that influence the stress of adhesive bonding were studied and corresponding optimized strategy was present.