Impact of design factors and environment on the ESD sensitivity of MEMS micromirrors

Abstract

In this paper, electrostatic discharge (ESD) tests on electrostatically actuated torsional micromirrors are performed in various pressure conditions and the ESD failure levels are reported for the first time in literature. The influence of device design factors of the micromirror, such as the torsional hinge width and the actuation bottom electrode area, on the ESD sensitivity is investigated. HBM ESD stress tests are performed in standard atmospheric pressure and in vacuum. These design parameters and the ambient pressure conditions are shown to affect the ESD failure levels of the MEMS micromirrors. Two distinct failure signatures are observed for micromirrors, viz., pull-in and sticking to the bottom electrode or getting destroyed and broken under the impact of the ESD stress.

Introduction

Microelectromechanical systems (MEMS) are increasingly entering the market as an enabling technology for more and more interesting applications in diverse fields such as next-generation communication, multimedia, consumer electronics, automotive and aviation, space and defence, healthcare and cognitive systems. In this context it is important that sufficient research work is carried out to study and characterize the reliability of MEMS components before they are integrated into commercial products [1]. Electrostatic discharge (ESD) as a cause of failure in MEMS has received increasing interest in recent times. ESD failure levels have been measured and reported in various MEMS devices [2], [3], [4]. Since MEMS are designed to operate in various environments, it is very useful to study the ESD failures in MEMS under those conditions. In the present paper, for the first time in literature, the ESD failure levels of MEMS micromirror devices have been measured in different ambient pressure conditions. This paper also investigates the effect of certain design parameters on the ESD sensitivity of MEMS micromirror devices. Based on previous results regarding the ESD failure levels in electrostatically actuated torsional micromirrors [4], new micromirror test structures with improved designs were processed and have been tested for ESD robustness. Human Body Model (HBM) ESD tests performed on these devices indicate an influence of parameters like the mechanical stiffness, the actuation electrode area and the ambient pressure conditions on the ESD failure levels. The results of these tests will be discussed in the following sections.

The MEMS devices used for the ESD tests together with all their design variations are described in the next section. The concept of the HBM ESD testing technique and the test conditions used for the MEMS are described in Section 3. Following that, the results of the ESD tests are described in Section 4. Section 5 presents some analysis of the results and inferences. This is then followed by the main conclusions of this work.