MTL Annual Report

Figure 1: A 3-axis force sensor with polysilicon strain gauges on the flexure beams.

Multi-axis force-sensing at the micro-scale is necessary for a wide range of applications in biology, materials science, and nanomanufacturing.  A three-degree-of-freedom force-sensor (Figure 1) was designed that is capable of accurately and precisely measuring the adhesion forces (nanoNewtons) between biologically active surfaces.  This force sensor is positioned and actuated using a Hexflex nanopositioner and Lorenz force actuators.  The fabricated device is shown in Figure 2.

In order to design high-accuracy, high-precision, multi-axis MEMS force sensors, a closed-form model was developed to optimize the strain-sensitivity of the MEMS force-sensor.  This model first sets constraints on the system due to package size, fabrication techniques, desired degrees of freedom, and force range.  The layout of the flexure system is optimized to meet the kinematic and manufacturing constraints of the MEMS force-sensor.  The geometry of the flexures is set to maximize the strain at the sensor locations.

Figure 2: Fabricated 3-axis force sensor.

This model was incorporated into a thermal/electric model to fully characterize all of the inputs to the system.  The resolution of the force-sensor is a function of the noise from the strain-sensors, the noise in the electronics, the thermomechanical noise, and the sensitivity of the strain-sensors to a force input.  Based on this model, the dominant noise sources are identified and the sensor system is optimized to reduce these noise sources.  The thermal/electric model is also used to determine the major factors limiting accuracy of the force-sensor.  In most cases, the drifts in both the electronics and sensors caused by fluctuations in room temperature were the major sources of accuracy errors.  Therefore, an environmental enclosure with closed-loop control over temperature was designed to reduce the thermal variation.  Overall, the final design of the force sensor is capable of producing sub-nanoNewton-resolution force measurements with nanoNewton-level accuracy.