Nanocharacterization of the adhesion effect and bending stiffness in optical MEMS

Highlights

• The MEMS structures electroplated from gold are suspended at 3 μm above the silicon substrate. Different configurations of gold micromembranes are investigated in order to determine the proper geometry that is less sensitive to a thermal gradient and that provides small adhesion force between flexible part and substrate. The influence of temperature on stiffness was investigated by changing the temperature within the range of 20–100 °C. Investigations were performed in an environmental controlled chamber with an atomic force microscope XE 70.

• Different configurations of microbridges are investigated in order to determine the proper geometrical configuration that is less sensitive to a thermal gradient. A constant cross-section beam is considered as a reference structure and it is compared with the other tested samples fabricated in the same geometrical dimensions but with some additional rectangular holes performed on the flexible plate. The scope of the fabricated rectangular holes is to reduce the temperature influence on the behaviour of clamp-clamp beam for applications where a thermal gradient occurs.

• The investigated micromembranes were also numerically analyzed with the Finite Element method using ANSYS Workbench 13.0 software. The numerical values of stiffness and adhesion are compared with analytical and experimental results.

• The research results are useful for designers to predict the behaviour of material and structurefor optical or thermal applications in order to improve the MEMS reliability and lifetime.

Abstract

The scope of this paper is the reliability design and testing of flexible MEMS components as clamp-clamp beams for the out-of-plane displacement. The field of implementation of such structures is in optical relevant applications such as the optical microsensors or optical microswitches. Moreover these structures can be successfully implemented in RF switches or in the other MEMS applications. The research studies presented in this paper consider the analytical and numerical analysis follow by the experimental validation. The mechanical and tribological characteristics such as the sample static response under an applied force and the adhesion effect between the flexible structure and substrate are investigated. The samples under test are fabricated from a reflective material – gold. Experimental investigations are performed by atomic force microscopy in order to determine the response of the gold microbridges under an applied force. Moreover, to identify the proper geometry that is less sensitive to a thermal gradient, different geometrical configurations of microbridges are tested under different temperatures. An etalon structure is considered as a reference beam and it is compared with the other samples fabricated in the same geometrical dimensions but with some additional rectangular holes performed on the flexible plate. The scope of holes is to reduce the temperature influence on the mechanical behaviour of clamp-clamp beam from application where a thermal gradient occurs. During numerical analysis and experimental investigations, the temperature of samples is increased from 20 °C to 100 °C and the sample response is monitored. A comparison between numerical and experimental results is provided at the end of paper. The research results are useful for designers to predict the behaviour of material and structure from optical or thermal applications in order to improve the reliability and the MEMS lifetime.

Introduction

Nowadays, the current trend is to combine sensors, actuators and the electrical interface on the same chip in order to obtain a functional system usually called Microelectromechanical Systems – MEMS [1].

This is useful implemented in many day-to-day applications such as vibrational sensors, in-car insurance sensors, lab-on-chip from medical applications, seismic sensors and so on.

The goal of optical MEMS is to integrate into a single device the mechanical, electrical and optical properties. One of the MEMS applications in optical systems are communication networks, optical white light interferometry, displays of mobile phones, variable optical attenuators, optical spectrometers or bar code readers. The MEMS components from optical applications are designed to have high mobility and to be less sensitive to a thermal gradient. These functional characteristics depend on stiffness that is given by the geometrical configuration of structure and is influenced by the material behaviour.

The scope of research work presented in this paper is the reliability design and testing of flexible MEMS components as clamp-clamp beams for the out-of-plane displacement considering different thermal gradients applied on structures. The field of implementation of such structures is in the optical relevant applications such as the optical microsensors or optical microswitches where an additional prestress is provided by a thermal gradient.

Among the main causes of failure of these microstructures are excessive tension that occurs due to bending effect and stiction. Stiction is one of the most important and inevitable problems of microbridges failure under large deflections [2]. Stiction means that the two structures are bonded to each other, never apart in controlled or unintentionally into contact with each other [3]. This phenomenon of stiction is crucial in small sizes structure, because it is caused by interfacial forces relatively high compared to the restoring force. The control of these opposite forces is therefore crucial. Since the reduction of interfacial force is done by mainly chemical processing [4], the control of the contact area and surfaces conditions are important for stiction [5].

Although a lot of mechanical reliability assessments have been conducted, it is difficult to discuss general theory about the reliability of MEMS components, because all the assessments are made by various methods and conditions. Most often, the operating temperature of the MEMS components as microbridges, microcantilevers or micromembranes can be significantly higher than the ambient temperature. In these conditions the temperature introduces in microstructures a residual stress and additionally deforms the microcomponents with an influence on the mechanical response of the system. In other, to integrate the microbridges in a MEMS device with high reliability it is necessary to determine the real stiffness of the flexible structure. The microbridge stiffness is closely linked to the material properties and is influenced by geometric characteristics of the structure.

Specialized literature includes biaxial stress analysis on characterizing microbridges with rectangular and circular geometry and analysis of elastic modulus of homogeneous structure [2], [6], [7], [8]. The static investigations of the micromembranes with measurement of their load – deflection behaviour have been already reported [9], [10]. Finite element analysis (FEA) is a very useful tool to simulate microbridges deformation, to calculate the stiffness and to view the stress distribution in samples [11]. Currently, MEMS researchers do not have enough information to predict the output responses of flexible MEMS microstructures and intense efforts should focus in this respect [9], [12], [13].

In this work the analytical formula to compute the microbridges stiffness as a function of the geometrical parameters is provided in Section 2 of paper. The numerical analysis of microbridges stiffness was performed at different temperatures by the thermo-mechanical module of ANSYS Workbench 13 Software as it is presented in Section 3 of paper. The experimental investigations of microbridges deflections under a mechanical force performed by using an atomic force microscope (AFM) in order to measure the stiffness of samples are included in Section 4 of paper. This section also presents the pull-off force measurements between the flexible part of microbridges and substrate as well as the temperature influence on stiffness and adhesion. The results and discussions are presented in Section 5. The paper is ended with important conclusions as including in Section 6 of paper.

Scientific and experimental details for each evaluation were very different because the reasons for those evaluations are geared toward direct interest of each researcher such as: product development, materials science, etc. The research results obtained in this paper are useful for designers to predict the behaviour of material and structure from optical application and to increase the MEMS lifetime.