Integrated Nanowire Logic Circuitry on Deformable Nonplanar Substrates

While a significant amount of recent research and development has focused on flexible electronics and improvements in thin film transistor performance on flexible substrates, less attention has been given to electronics application requiring deformable layouts or extreme, complex surface shapes and curvatures. As flexible electronics technology matures, a growing number of applications require stretchable or extreme nonplanar systems approaches. Examples of these applications include electronic eye cameras, hemispherical or curvilinear electronics, prosthetic or medical sensors, and stretchable LEDs, displays, photovoltaics, sensors, or logic devices utilizing thin silicon. In all of these applications it is critical to maintain the performance of the active electronic devices as the substrate is deformed due to changes in shape or external applied forces. A key issue related to this is the adjustment in resistive properties of nanomaterial-elastomer composites as the elastomeric substrates is stretched, thereby necessitating different approaches for the design and manufacture of deformable electronic systems.

Recently, Shin et al. reported on the integration of Tin Oxide (SnO₂) nanowires (NWs) as switching transistor logic elements for stretchable electronic systems. The use of nanowires for such electronic systems provides many potential benefits in comparison to other nanomaterials, including improved performance due to superior semiconductor properties, expanded integration approaches, and low cost processing compatible with low temperature flexible substrate materials. SnO₂ NWs offer specific advantages in terms of ease of synthesis, high quality semiconducting properties, and stability during exposure to various gas molecules. For this approach, SnO₂ NWs were first grown on silicon dioxide (SiO₂) surfaces by chemical vapor deposition employing the vapor-liquid-solid (VLS) mechanism using a 2nm gold film as the catalyst. The field effect transistor (FET) switching devices were fabricated by first spin-coating a SiO₂ passivated silicon wafer with 300nm of polyimide after which titanium-gold (TiAu) gate electrodes were patterned. The surface was then coated with a second polyimide layer after which the NWs were contact transferred to the polyimide surface using the sliding transfer technique. The NWs were all aligned with typical density on the order of 2-3 NWs/µm. The FET and inverter device fabrication was completed by patterning TiAu electrodes thereby defining the drain voltage, ground, and output voltage contacts, as well as a drain load resistor using the NWs. A plasma etch step was used to remove the polyimide below the NWs in the FET device, leaving them suspended. To characterize the performance of the NW FET circuits under various deformation conditions, a hemispherically molded PDMS film was fabricated, then stretched to form a planar, pre-stressed elastomeric substrate onto which the NW circuits were transferred.

Characterization of the SnO₂ NW FETs on the planar surface demonstrated excellent low-voltage switching characteristics and a nominal FET channel mobility of ~80 cm²/V-s. Because the regions where the FET devices were located were relatively large in comparison to the width of the metal interconnects between the devices, when the PDMS substrate was released and deformed to the original hemispherical shape, the majority of the stress was accommodated by the interconnects, which adopted arc-shaped geometries. This effect was modeled by the authors, which further confirmed that the electrodes remained within the elastic regime of mechanical behavior during deformation. Further characterization of the NW FET devices and interconnects under a range of deformations illustrated that while the shape and stress fields over the circuit elements and interconnects varied, the device performance was barely affected by the deformation conditions.

Thus, a facile approach to synthesize quality FET logic circuitry utilizing SnO₂ NWs has been demonstrated. The authors further demonstrated that the circuits can be integrated on deformable flexible substrates where the design of interconnects can accommodate the stress fields resulting from a range of deformation conditions. The scaling of such a flexible electronics system design has implications towards a range of emerging applications.

Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network

• Shin G, Bae MY, Lee HJ, Hong SK, Yoon CH, Zi G, Rogers JA, Ha JS. 2011. SnO₂ nanowire logic devices on deformable nonplanar substrates. ACS Nano. 5(12): 10009-10016. http://dx.doi.org/10.1021/nn203790a

Figure reprinted with permission from Shin G, Bae MY, Lee HJ, Hong SK, Yoon CH, Zi G, Rogers JA, Ha JS. 2011. SnO₂ nanowire logic devices on deformable nonplanar substrates. ACS Nano. 5(12): 10009-10016. http://dx.doi.org/10.1021/nn203790a. Copyright 2011 American Chemical Society.