Essential design and fabrication considerations for the reliable performance of an electrothermal MEMS microgripper

Over the past years, microelectromechanical systems (MEMS) have become a vital component within a wide range of technologies, making the study of their performance and operational reliability a very critical aspect for their correct functionality. Microgrippers are one such type of MEMS devices with one of their key applications being the manipulation of biological tissues and cells. This paper presents a microgripper design based on the `hot and cold arm' electrothermal actuation mechanism that is suitable to study the deformability properties of human red blood cells under healthy and pathological conditions. The main scope of this paper is to highlight a number of failure mechanisms that are typical of surface micromachined MEMS microgrippers. These include out-of-plane buckling of the hot arm, stress concentrations, device stiction, and residual stresses. The studied polysilicon microgripper structures were designed and fabricated in line with the specifications of the commercial fabrication process PolyMUMPs (TM). The microgripper design was numerically modelled and electrothermomechanically studied using finite element analysis in CoventorWarer (R). Experimental testing on the fabricated structures was used to demonstrate reliable microgripper performance as well as instances of the considered failure mechanisms. Results for the reliable microgripper performance show that the microgripper arms deflected as expected when actuated, with the obtained simulation and experimental results in good agreement. The investigated failure mechanisms have led to the identification of essential design and fabrication considerations whose thorough investigation, with the aid of appropriate modelling approaches, is essential to define improvement solutions and best practices to mitigate the failure or malfunction of the microgripper during operation.