Organic semiconductor strain sensors

Metallic foil or semiconductor piezoresistors are frequently used as strain sensors for shape, strain, or force monitoring applications. The sensors are typically connected in a Wheatstone bridge configuration and mounted on the surface or body to be tested. Recently strain sensors based on amorphous or microcrystalline silicon have been demonstrated on both rigid [1] and flexible [2] substrates. We report here the first strain sensors using an organic semiconductor as the active element. Thin film semiconductor strain gages can provide important advantages compared to metal foil or conventional semiconductor strain sensors. The large sheet resistance available for thin film semiconductors allows sensors with dramatically reduced size and power consumption compared to metallic foil sensors and low processing temperature allows fabrication of semiconductors sensors on a range of surfaces including flexible polymeric substrates. In addition, the availability of thin film transistors allows the fabrication of sensor arrays with integrated select and/or decode electronics. For applications where measuring the stain in a soft or flexible substrate is important, the large stiffness mismatch between an inorganic semiconductor sensor element and the substrate can be a problem. For example, the stiffness of a one micron thick silicon layer is similar to that of a fifty micron thick polyimide film. Figure 1 shows the results of a simulation of the stress distribution for a thin, high Young's modulus (200 GPa) material on a thicker, low Young's modulus (5 GPa) substrate. For this case the stress in the high modulus material is not representative of the stress present in the substrate. In addition, there is a stress concentration in the low modulus material at the edge of the high modulus structure (not easy to see in figure 1 due to the scaling) that may lead to non-reversible plastic substrate deformations that limit sensor application and repeatability. Figure 2 shows a simulation for a thin material on a substrate with matched Young's modulus (both 5 GPa). For this case the thin material stress is similar to that of the substrate and the stress concentration at the material edge is minimized. We have used a doped organic semiconductor as the active element for low Young's modulus strain sensors. The sensor cross-section is shown in figure 3. For these sensors 2 nm thick Ti and 20 nm thick Au were deposited on 50 micron thick polyimide substrates by thermal evaporation and patterned to form sensor electrodes and wiring. Next, a 50 nm thick pentacene layer was deposited, again by thermal evaporation. The pentacene layer was then doped p-type by exposure to a 1 % solution of ferric chloride in water. The doped pentacene film was then patterned using an aqueous polyvinyl alcohol photolithography step and oxygen reactive ion etching. The maximum process temperature used to fabricate the organic strain sensors is 110°C. Figure 4 shows completed 300 μm × 300 μm devices. To test the organic semiconductor strain sensors we supply power to the Wheatstone bridge and measure the differential bridge output as a function of strain. This sensor configuration shows maximum strain sensitivity for strain applied at 45° with respect to a line connecting the bridge power contacts and for simplicity we use cylinders of various diameter to provide the strain as a function of bending radius. Figure 5 shows the zero-strain offset bridge differential output voltage for bending diameters of 90, 76.6, 50.4, and 41.7 mm and (plotted as a function of strain). The strain sensitivity is about 75 mV%o. These results indicate that strain sensors with mechanical characteristics matched to low Young's modulus substrates and fabricated at low temperature are possible.