Research Of The Effects Of Nanoscale Surface Roughness On The Electrical Performances Of RF MEMS Switches

Radio frequency(RF) micro electromechanical system(MEMS) switches have been used in remote communication,remote sensing monitoring,radar and other traditional area,and begin to be used in 5G technology,artificial intelligence,the Internet of Things and other emerging fields due to the advantages of small size,high yield,low insertion loss,high isolation and so on.However,the electrical performances of the RF MEMS switches are affected by the nanoscale surface roughness.Unfortunately,the systematic research of the influence factors of nanoscale surface roughness and its effects on the electrical performances of RF MEMS switches have not been presented so far.Therefore,this dissertation studied the effects of deposition parameters on the nanoscale surface roughness of the deposition thin film,and proposed a novel method to calculate the electrical performances of RF MEMS switches considering the nanoscale surface roughness.The main contents of this dissertation are as follows:1.The effects of deposition parameters on the nanoscale surface roughness of copper thin films were studied based on Newton Equations of Motion.The effects of deposited film thickness,deposition temperature,deposition rate,and thermal reflow temperature on the nanoscale surface roughness of the copper films were researched by molecular dynamic(MD) simulation and compared with experimental results.The optimal parameters were proposed to obtain both the best film quality and yield.The explicit relationships between the deposition parameters and nanoscale surface roughness of copper thin films were given.Compared with traditional experimental method,MD simulation used in this dissertation is low cost and time consuming,and it can explain some problems during the deposition process and provide guideline for the thin film deposition.2.Effects of nanoscale surface roughness on the transmission performances of coplanar wave guide(CPW) were analyzed based on Maxwell’s equations.The threshold values of CPW transmission line impedance,reflection and rough coefficient were defined.The electrical performance expressions of transmission line considered surface roughness were proposed.This dissertation made up for the deficiency of the model of traditional transmission line,and proposed a judgement for the consideration of surface roughness of RF transmission line.3.The expression to calculate the capacitance of RF MEMS switch parallel plate capacitors was proposed based on Current Continuity Theorem and Gauss Distribution.The RF MEMS switch was simplified as a parallel plate capacitor with a rough electrode,a dielectric layer and a smooth electrode.The capacitance of above mentioned parallel plate capacitors was derived based on the capacitance of an asperity and a flat electrode.The surface roughness model proposed in this paper is close to the experimental results compared with previous model,which decreases the error between the design and experimental results of capacitance and improves the design efficiency of RF MEMS switches.4.The complete expressions to calculate the S-parameters of RF MEMS switches considering nanoscale surface roughness were given on the basis of Maxwell’s equations.The S-parameters of RF MEMS switches considering nanoscale surface roughness were given based on the expression CPW transmission line and parallel plate capacitor.The accuracy of the proposed expressions of S-parameters were validated by finite element method(FEM) simulation and experimental results.The expressions proposed in this paper fill in the gap about the S-parameters of RF MEMS switches when nanoscale surface roughness is considered,which provides a guide line for the design and improves the design efficiency of RF MEMS switches.