Study On Thermally Induced Packaging Effect On Distributed RF MEMS Phase Shifters

Distributed RF MEMS phase shifters have a wide application prospect in phased-array radar and communication systems with their significant advantages such as wide-band, true-time, low insertion.In RF application area, performance stability and reliability are critical issues for MEMS devices. One of the major factors is the thermal stress/strain induced in packaging process. In a die-attaching process, a chip fabricated with MEMS structures is attached to a packaging substrate with certain adhesive material. So a packaged MEMS device generally consists of various materials with different coefficients of thermal expansion (CTE). The stress/stain induced by CTE mismatch during the packaging process will have great influences on the performances of the devices.Previous research works mainly focused on the packaging effect of conventional MEMS devices. However, RF MEMS devices may be even more sensitive to thermal packaging effect. The distributed packaging stress/strain may have more complex effect on the distributed structure of this kind of devices. And the microwave signal itself is distributed and highly dependent on the geometric parameters of the devices.In this paper, the influences of thermally induced packaging effect on a distributed RF MEMS phase shifter are studied from two aspects of theory and experiments. The main work and innovations of this dissertation are described as following:1. A method of dividing the device-package system of a distributed RF MEMS phase shifter into certain basic elements is presented. The device section includes a coplanar waveguide (CPW) transmission line and a number of MEMS switches which make up a distributed MEMS transmission line (DMTL). The package section is considered as a multi-layer structure composed of chip substrate, adhesive layer and package substrate which are all modeled as beam-like elements.2. The mechanical and electrical models for a MEMS switch, electro-magnetic models for a DMTL and thermal-mechanical coupling model for a beam-like element are established respectively. The overall models for the device-package system of a distributed RF MEMS phase shifter are established by using simplified analysis method, distributed nodal analysis method and lumped nodal equivalent circuit method respectively. Then the influences of the thermal packaging effects on the RF performances, especially phase shift characteristic, of a distributed RF MEMS phase shifter are analyzed. Finite element method (FEM) simulations validate the models and analysis methods.3. Using high resistance silicon (HRS) and gallium arsenide (GaAs) substrates respectively, a number of distributed RF MEMS phase shifters with different structures and different positions on the chip are designed and fabricated. The chips are packaged using a die-attaching process. Then the phase shift characteristics of the devices before and after packaging process are measured. All the results of modeling analysis, FEM simulation and measurement indicate that the thermally induced packaging effects have much greater impact on the phase shift characteristics of a distributed RF MEMS phase shifter. And the variations of the phase shift of the device before and after packaging process are intensively related to their locations on the chip, which corresponds to the distributed feature of the thermally induced packaging stress on the surface of the chip.4. Base on studies above, a novel method for measuring the thermal packaging stress by using a distributed RF MEMS phase shifter is preliminarily discussed. According to its dependency on the variation of phase shift of the distributed RF MEMS phase shifter before and after the packaging processing and the increment of actuation voltage of MEMS switch, the thermal packaging stress on the chip surface can be measured in situ and real time.The work of this dissertation has a good reference value for the design and practical applications of distributed RF MEMS phase shifters.