Polycristalline silicon nanowires based TFTs compatible with CMOS technology for chemical integrated sensors
Résumé
Thanks to their high surface to volume ratio, silicon nanowires (SiNWs) are good candidates as sensitive units for fabrication of high sensitive chemical sensors. SiNWs can be prepared by one of two approaches, “top-down“ and “bottom up”. In a bottom up strategy synthesis methods most developed are layer-by-layer self-assembly, Vapor Liquid Solid (VLS) and using matrix template. The main drawbacks of these synthesis methods for a 3D integration are the difficulty in control of size and positioning of the nanowires. In this case, nanowires need to be selectively collected and manipulated to be assembled in a planar layout. For the “top down” approach several advanced nanopatterning techniques were developed such as e-beam, atomic force microscopy (AFM), deep UV and nanoimprint lithographies, to obtain SiNWs. The main drawbacks of these advanced lithographic tools with nanometer size resolution rest on the high cost generated, and more generally the low throughput capability unsuitable with mass production. Because SiNWs synthesis can be compatible with the reproducible and reliable performances of the established silicon technology, SiNWs based sensor integration will allow a lower manufacturing cost, in addition to the advantageous electronic features of embedded detection and signal processing in silicon technology. The intrinsic reliability of the well-known semiconductor CMOS (Complementary Metal Oxide Semiconductor) process also guarantees reproducible and reliable performances.
In our study, polycrystalline silicon nanowires are synthesized following the top-down approach using a classical fabrication method commonly used in microelectronic industry: the sidewall spacer formation technique. The feasibility of these polycristalline SiNWs with a curvature radius as low as 50 nm was previously demonstrated (1). This method allows the fabrication of parallel SiNWs network controlled over a large range of doping levels. The resistivity dependence with the P- or N-type in-situ doping level is both related to the nanowires size dependent structural quality and doping specie (2). Feasibility of back gate or top gate N- or P-channel polycrystalline SiNWs TFTs (fig. 1, 2 and 3), and charged chemical species detection (fig. 4) using such TFTs were also demonstrated (3).
Our results show the great flexibility in design of planar SiNWs array by direct patterning technique using conventional lithographic tools. The polycristalline silicon NWs make them good candidates for the fabrication of electrically controlled thin film devices (resistors and transistors), in particularly for both chemical sensing and electronics applications. Results show the full compatibility of the nanospacer polycristalline SiNWs technology with the existing silicon CMOS technology, using nanowires as potential sensitive units for integrated gas sensors applications. Indeed, field effect behaviour observed in polycristalline SiNWs based transistors is promising to amplify chemical species detection and for electronics conditioning sensing signal using back gate and top gate configurations respectively. In addition, thanks to silicon surface functionalization possibilities, such results offer a great potential for further developments of integrated SiNWs based (bio)chemical sensors and their implementation in electronic systems.
1. F. Demami, L. Pichon, R. Rogel, A. C. Salaun, Mat. Sc. Eng. , 012014 (2009)
2. R. Rogel, E. Jacques, L. Pichon, A. C. Salaun, IEEE Trans. Elect. Dev., 61(2), 598-604 (2014)
3. A. C. Salaun, L. Pichon, G. Wenga, EUROSENSORS 2014 Procedia Engineering (2014). To be published.