On the origin and magnitude of surface stresses due to metal nanofilms.

James Bowen,David Cheneler

doi:10.1039/c5nr08789a

Abstract

Metallisation is a vital process for micro- and nanofabrication, allowing the controlled preparation of material surfaces with thin films of a variety of metals. The films are often subjected to further processing, including etching, patterning, chemical modification, and additional lamination. The extensive applications of metallised substrates include chemical sensors and nanoelectronics. Here, we report an experimental study of the metallisation of silicon cantilevers with nano-films of chromium and titanium. Analysis of the stress distribution throughout the cantilever showed that metallisation causes a constant stress along the length of the beam, which can be calculated from interferometric quantification of the beam curvature. The structure of the metal/silicon interface was imaged using electron microscopy in an attempt to ascertain the physical origin of the stress. A theoretical model is constructed for the stressed beam system, and it is shown that there is no single parameter that can describe the change in stress. The resultant structure after deposition varies significantly for each metal, which gives rise to a variety of stress directions and magnitudes.

Highlights

Many silicon based dynamic micro electromechanical systems (MEMS), for instance resonant pressure sensors, require metal deposition in order to be integrated with their control circuitry
Cantilevers with a range of lengths have been coated with chromium and titanium with different thicknesses have been analysed using transmission electron microscope (TEM), energydispersive X-ray spectroscopy (EDX) and energy loss spectroscopy (EELS) in order to ascertain the physical origin of this stress
It was shown that there is no single parameter that can describe the change in stress as a function of the metal or film thickness

Summary

Introduction

Many silicon based dynamic micro electromechanical systems (MEMS), for instance resonant pressure sensors, require metal deposition in order to be integrated with their control circuitry. These devices frequently comprise of compliant components such as beams or diaphragms. Deposition of metal on these structures affects their mechanical behaviour in a manner that is difficult to predict due to the presence of stresses induced in the system during deposition. Such stresses arise even when the conditions are controlled as to prevent thermal effects. Stoney reported the first consideration of stress in deposited films[5]

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