טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentOphir Gal
SubjectSurface Analysis of Contact Interface in RF MEMS Switches
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Eugen Rabkin
Dr. Lior Kogut
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

The technological development of high frequency communications and radars has led to the need for miniature radio-frequency (RF) switches with the ability to combine high-powered transmissions with a low power requirement.

The structure of a switch working electrostatically is based on a bar, or cantilever, suspended above the surface, which is capable of moving downwards and creating an electrical circuit, and thereby closing the switch.

The aims of this research were twofold: First, to uncover the relationship between the theoretical contact model of a typical switch and the metallurgical properties of the surface; and second, to study the micro-structure and composition of the contact area, correlating it with the electrical performance of the switch.

In this study the switches were characterized from three aspects: 1) electrical characteristics (I-V curve measurements), 2) surface characterization by a high resolution scanning electron microscope (HRSEM) aided by the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and atomic force microscope (AFM), and  3) micro-structure characterization.

Characterization of the switches using TOF-SIMS revealed the presence of aluminum traces in the contact area. The amount of aluminum decreased with increasing the distance from the contact spot. We put forward the hypothesis relating the source of the aluminum to the alumina substrate.

Characterization of the contact area with a HRSEM showed that there is a change in the micro-structure after activating the switches - beginning with the grain growth and through the presence of elevated hillocks at the grain boundaries.

We believe that Joule heating in the contact area causes release of the aluminum from within the alumina. In order to strengthen this hypothesis, thermal treatment of pristine switches at two different temperatures was performed.

Comparison of the four aforementioned characteristics (TOF-SIMS, HRSEM, EDX, AFM) led to a conclusion that the current flowing at the time the switch is active causes heating of the contact area and diffusion of the aluminum from the substrate to the surface. Also, we hypothesized that the area in the vicinity of the grain boundaries was harder than the grain interior because of aluminum-induced solute hardening.