Bram Krijnen, Koen Swinkels, Dannis Brouwer, Leon Abelmann & Just Herder
A 3-DoF micro-electromechanical (MEMS) stage has been designed with an innovative integrated feedback system based on thermal sensors. The stage is integrated in the device layer of a silicon-oninsulator-wafer, which means that no assembly is required and the stage can be fabricated using only a single mask. The range of motion is over 160 µm in two directions and 325 mrad of rotation, which exceeds the range of motion of existing MEMS stages by far.
From the 1980s on there has been a strong increase in the number of applications that use actuation or sensing based on micro-electromechanical systems (MEMS). One of the first examples is an accelerometer integrated in IC technology. Many applications have been reported since, such as digital micromirror devices for projectors, pressure sensors, gyroscopes, and flow sensors. Due to their small size and low cost, MEMS are also becoming increasingly popular in consumer products, for example in the Nintendo Wii for motion sensing, in digital cameras for image stabilisation, and in smartphones for sports tracking and navigation. MEMS are all around us, nowadays.
MEMS applications do not only benefit from their small volume and low cost, they can also provide superior performance. By scaling down from the macro- to the microscale, the mass of structures (m ~ r 3) decreases more rapidly than their stiffness (k ~ r), which inherently means a higher eigenfrequency and a faster response time. This opens up a range of interesting applications for MEMSbased positioning stages. Here, the design, fabrication, and experimental evaluation of a miniature planar positioning stage with integrated feedback are presented. The complete system has a wafer surface area of only 6×6 mm2 and is able to position an end-effector with an in-plane range of motion of 160 μm in x- and y-direction and a rotation of 325 mrad. Stage motion with three DoFs (degrees of freedom) is realised by means of an eccentric connection of three singleDoF shuttles using leaf springs (Figure 2). The positions of the single-DoF shuttles determine the position and rotation of the 3-DoF stage. The single-DoF shuttles each consist of two electrostatic actuators, two flexure mechanisms and a position sensor for feedback. Actuation is provided by electrostatic comb-drives which are straight-guided by flexure mechanisms. Two flexures are used to prevent rotation of the shuttle. Since a comb-drive actuator can only apply an attractive force, two comb-drives are used per shuttle, to enable motion in opposite directions. The position of each shuttle is measured by a thermal displacement sensor.
The sensor consists of two heaters that are resistively heated. Heat is conducted through air towards the ‘cold’ shuttle. Therefore, the temperature of the heaters changes when the overlap changes and thus the stage position changes. This results in a measurable change in the electrical resistance of the heaters, due to the PTC (positive temperature coefficient) effect in silicon.