Research On MEMS Reliability Issues

There is growing interest in characterization of devices in micro scale,especially for microsystem.This leads to the concern in the long-term reliability of MEMS under diverse stringent conditions.The goal of this work is to study the main reliability issues of polycrystalline silicon(Poly-Si) and poly-SiC based MEMS.Specific physical-failure models and appropriate design solutions for their long-term reliability are investigated as following:1.In our study,Polycrystalline 3C-SiC films,are grown by low-pressure chemical vapor deposition(LPCVD) at 800℃using single precursor methylsilane on 100mm Si(100) wafers.Residual stress,strain and strain gradient are characterized as functions of deposition pressure,temperature and dichlorosilane as an additional silicon source.The analysis suggests that the change in stress values is due to a combination of effects,in particular thermal mismatch,grain size effect and chemical composition.In addition,n-type doped poly-SiC films is released by adding ammonia into the precursor.Various materials properties such resistivity,residual stress,strain,strain gradient as well as crystalinity and surface morphology are investigated for n-type doped poly-SiC films.The results are then discussed based on the site competition between nitrogen and carbon in order to explain the resistivity variation with NH₃ and DCS fraction;average bond length changes with Si/C ratio and nitrogen incorporation are responsible for stress variation in the films;and the enhancement of crystallinity may be due to reduction in C-C bond density in the grain boundaries upon N incorporation.The films with optimized electrical and mechanical properties are suitable for MEMS applications.2.We investigated poly-Si and poly-SiC electrodes corrosion behavior separately.For poly-Si electrodes,we present a detailed study of the effect of geometry and surface termination on the morphological and chemical evolution of polycrystalline silicon electrodes at high relative humidity(RH) and applied bias.The corrosion test structures consist of two poly-silicon electrodes isolated from the substrate by a silicon nitride layer.Anodic oxidation is observed on electrodes with native oxide layers.Damage to the cathode is observed within 20 hours under 89%RH when the anode to cathode area ratio is higher than unity.The unusual phenomenon is attributed to the surface electrolyte-driven electrochemical reactions in the system.Understanding of the corrosion mechanism has enabled the selection of surface passivation pretreatments to reduce the observed corrosion.In particular,the effects of hydrogen-terminating and alkyltrichlorosilane-based self-assembled monolayer coatings have been investigated.It is found that although both approaches help reduce the rate of corrosion at both electrodes,SAMs have a longer lasting impact.This is attributed to the greater stability of SAMs and their ability to form on silicon nitride isolation layers.For poly-SiC electrodes,we provide the first report of corrosion occurring at the anode of polycrystalline 3C-silicon carbide MEMS electrodes under high relative humidity and applied voltage.This unusual phenomenon is determined to be electrochemical in nature.Electrode pair and Canary wire corrosion behavior are investigated to yield a detailed evaluation of the stability and oxidation rate during corrosion.The effects of film stress on anodic oxidation are discussed, and suggestions to prevent damage due to corrosion are presented.3.We use a low thermal budget and simple way to treat SiC film surface to enhance its realiability.Low energy Ar+ ion bombardment on poly-SiC films is shown to tailor both the film stress and corrosion resistance in high humidity environment.The film average stress decreases as ion energy and bombarded time increase,and large positive strain gradient is reduced,while film thickness and electrical properties are affected minimally.The film stress and corrosion behavior affected by Ar⁺ ion bombardment are explained by ion peening, thermal sparks models and surface energy,surface stress effects.4.The time-dependent assessment of two contacting polycrystalline silicon surfaces is realized using a microinstrument that allows for in situ surface analysis. The evolution in contact resistance,morphology and chemistry is probed as a function of contact cycle.Initially,the contact resistance is found to decrease and then increase with impact cycle.Upon prolonged cycling,the fracture of Si grains is observed which grow to form a wear crater.The electrical,morphological and chemical analyses suggest that the wear of rough polysilicon surfaces due to impact proceeds through three distinct phases,namely plastic deformation of asperities, adhesive wear,and grain fracture.In this article,several materials and devices issues that can arise during the processing,the operation and the reliability of MEMS were discussed.The work could promote the commercialization of some poly-Si and poly-SiC base MEMS.