Influence of surface layer properties on the thermo-electro-mechanical characteristics of a MEMS/NEMS piezoresistive cantilever surface stress sensor

Abstract

In the past decade, micro/nano cantilever platforms have been actively explored for realizing variety of MEMS/NEMS sensor and actuator systems. In treatise, a majority of the cantilever platform based devices reported are non-monolithic structures with additional functional layers for improving their performance metrics. In this paper, for the first time, we elucidate the impact of an additional thin film Au layer which is typically used in the case of piezoresistive micro/nano cantilever surface stress sensors as a bio-interface for improving their chemical/biological selectivity and sensitivity. In this study, unlike the earlier reports in addition to the electro-mechanical response, we have also considered the phenomenon of thermal drift inaccuracy which has been seldom considered in the modeling and design stages of piezoresistive cantilever devices. The major component of inaccuracy i.e. multi-morph deflection induced due to the combined effect of multi-layered structure of a piezoresistive micro/nano cantilever sensor with a thin Au film and joule heating in the dc-biased piezoresistor is modeled and investigated. Here, the prime focuses are the following: (i) to investigate the effect of Au immobilization layer thickness and coverage on the thermo-electro-mechanical response and (ii) for a fixed sensor geometry, to identify an optimal Au immobilization layer thickness and coverage to nullify the multi-morph induced thermal drift. Premise has been investigated using a multi-physics modeling and simulation software in two phases. To validate the modeling approach, simulation results have been compared with experimental data reported in the literature. In phase-I, influence of the addition of Au immobilization layer on the sensor response is investigated. Results show that an addition of a 50 nm Au immobilization layer atop silicon monolithic structural layer with an integrated piezoresistor and a silicon dioxide isolation layer reduces resonant frequency by 15.23%, causes a change in multi-morph deflection by 2.83 times with a new behavior of reversal in cantilever bending direction and change in thermal sensitivity by 41.47%. In phase-II, impact of Au layer thickness and coverage on the sensor is analyzed. Results depict that apart from Au layer thickness, Au layer coverage area is also critical in governing the sensor response and by careful selection of the two parameters thermal drift component due to multi-morph deflection can be completely nullified.