Force measurements on U-shaped electrothermal microactuators: applications to packaging M Boutchich*, T J Mamtora, G J McShane, I Haneef, D F Moore † , and J A Williams Department of Engineering, University of Cambridge, Cambridge, UK The manuscript was received on 15 March 2007 and was accepted after revision for publication on 10 July 2007. DOI: 10.1243/09544062JMES664 Abstract: The current paper critically reviews the prospects for the electrothermal actuation of elastic fixtures used as packaging elements for opto-electronic components. A convenient design methodology is presented together with a practical scheme for both prototyping out- of-plane bimorph actuators and measuring the vertical forces that they can deliver. A test bench has been assembled capable of measuring both the displacement and the restoring force delivered by such actuators which are patterned using laser micromachining of a bilayer consisting of 500 nm titanium tungsten (Ti-W) and 3 mm silicon nitride (SiN) thin films on a sili- con substrate. An analytical model is derived to predict the dependence of the restoring force on the input electrical power and topology of the actuator. Experimental results are presented for bilayer actuators made of Ti-W/SiN in which attainable forces are of the order of 25 mN for input powers of 70 mW. An approximate theoretical model correlates well on the measured results of restoring force for different actuator geometries and supply currents. A packaging prototype was successfully tested using 550 mm long U-shape actuators with a gap width of 200 mm. These were able to move macroscopic components with rotations of up to 38. Keywords: electro-thermal, actuator, elastic modulus, bilayer, restoring force, packaging 1 INTRODUCTION Thin film technology is used to fabricate microme- chanical beams used in a wide range of microelec- tromechanical systems (MEMS) devices and sensors including commercial accelerometers and scanning probe microscopes [1]. Typical film thick- nesses are in the range 50 nm to 2 mm and manufac- turing processes from complementary metal-oxide- semiconductor (CMOS) technology have been adapted, with slight modifications, for these MEMS applications. With the aim of reducing the assembly costs of microsystems, there are opportu- nities at the high end of this thickness range to use micromachined beams as clipping structures, i.e. as microclips, to hold optical and electrical com- ponents in position on micro-optical benches. For example, commercially available low-stress silicon nitride (SiN) [2] has been used to clip optical fibres in V-grooves etched in the surface of silicon wafers and so interface a single-mode fibre to a laser [3]. Silicon-rich SiN obtained by chemical vapour deposition (CVD) is a high strength material with an elastic modulus of the order of 200 GPa making strong fixtures a possibility from relatively thin films. In addition, such films are patternable by both conventional lithography and reactive ion etching. Some of the mechanics of these relatively simple structures are well understood but it would be highly desirable to be able to fabricate more complex clipping geometries combined with remo- tely controllable actuators which would allow fine adjustments to be made to the position of the clipped components without manual intervention. Furthermore, new CVD materials are becoming available – for example SiC and bi-layers of SiN and titanium tungsten (Ti-W) – with potentially superior mechanical properties. Rapid prototyping methods, such as laser machining of surface *Corresponding author: NXP Semiconductors, 75 Kapeldreef, Leuven 3000, Belgium. email: mohamed.boutchich@nxp.com † Deceased SPECIAL ISSUE PAPER 87 JMES664 # IMechE 2008 Proc. IMechE Vol. 222 Part C: J. Mechanical Engineering Science