Polymeric Mechanical Sensor with Integrated Strain Gauge Readout Based on a Piezoresistive Carbon Black-Polymer Composite Material L. Gammelgaard, P. Rasmussen, M. Calleja, A. Boisen
[email protected]; phone +45 4525 5787; fax: +45 4588 7762 Department of Micro- and Nanotechnology, Technical University of Denmark, Bldg. 345e, DK-2800, Lyngby, Denmark
Summary: We present an SU-8 cantilever strain sensor with an integrated piezoresistor based on a conductive composite of SU-8 and carbon black particles. The polymer composite is structured by UV-photolithography and has been integrated into a polymeric micro cantilever. The polymer composite is piezoresistive with a gauge factor around 15 to 20. Keywords: cantilever, polymer composite, piezoresistive readout Cantilever-based sensors offer a platform for direct measurements with highly sensitive, label-free molecular recognition on small sample volumes. The cantilever sensor principle has a wide range of applications in real time local monitoring of chemical and biological interactions1-3. A reason for the growing interest in making cantilever sensor systems is the possibility of making such measurements on a portable device. The principle of cantilever sensors is to detect the bending of the cantilever that typically is induced by a change in surface stress at one side of the cantilever. This deflection can then be monitored by optical techniques. This optical readout method though very sensitive, is difficult to integrate into a micro-liquid handling system. To overcome this kind of problems we have previously developed cantilever-based sensors with piezoresistive readout4. Previously, these cantilever sensors have been made of silicon-based material with piezoresistors of poly-silicon. Recently, cantilevers based on the polymer SU-85 with piezoresistors of Au have been fabricated6-8. The Au piezoresistors have shown a modest gauge factor around 1-2 and in order to increase the sensitivity of the polymer cantilevers a new piezoresitive material is necessary. In this work, we have realized for the first time an SU-8 cantilever with integrated readout based on a piezoresistive SU-8/Carbon black composite material. This novel composite material can be processed by standard UV-lithography and is compatible with most cleanroom processes. The SU-8/Carbon composite has been produced by ultrasonic mixing and can be spun on wafers forming thin-film layers less than 2 µm thick. A cheap and fast process sequence for the polymer cantilevers have been developed.
An atomic force microscope (AFM) have been used to make surface measurements showing a clear connection between surface roughness and carbon loading. For carbon loading less than 20% (by weight) very smooth surfaces have been demonstrated. The deflection sensitivity of the polymeric cantilevers with the integrated piezoresistive readout have been measured to be 2.2-3.0⋅10-6 nm-1. The gauge factor of an SU-8/Carbon composite with 16.6% (by weight) carbon particles have been demonstrated to be between 15-20 thus making this material more strain sensitive than silicon due to the low Young’s modulus of SU-8. When integrated into a micro-fluidic system the sensor can be made very small and portable making it interesting for ‘point of care’ analysis. Moreover, this polymer composite might in the future open up for completely new application in micro technology and sensor systems.
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Fig. 1. Optical image of the micromachined cantilever with integrated piezoresistive readout and four gold electrodes. The cantilever is 200 µm long, 200 µm wide and approximately 7 µm thick. Fig. 3. Repeated endpoint deflection of the cantilever. The resistance is measured as a function of the cantilever deflection in steps of 5 µm. The sudden drop in resistance seen at the second peak is due to the needle being removed from the cantilever.
Figure 2 AFM image of an SU-8/Carbon composite surface with 16.6% (by weight) carbon particles. A peak to valley value of only 6 nm can be seen on the graph below.
Fig. 4. The relative change in resistance as a function of the deflection. The three lines represent the three steps in the deflection experiment; deflection, release, and second deflection
