Ammonia Sensors Based on Suspended Silicon Nanowires

International audienceResistors based on parallel suspended polycrystalline silicon nanowires are fabricated using the classical top down CMOS silicon technologies. Results show potential use of silicon nanowires for charged chemical species (ammonia) detection. Resistors are promising as low-cost manufacturing gas sensors, for very high sensitive ammonia detection over a large concentration range. Motivation and results 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" or "bottom up". In a bottom up strategy the individual base elements (atoms, molecules…) of the system are linked together to form larger subsystems, without using high cost lithographic tools. The main drawbacks of these synthesis methods 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. The "top down" approach starts from bulk materials and scales down the patterned areas. In this way, several advanced high cost lithographic tools with nanometer size resolution are needed, and the low throughput capability is unsuitable with mass production. In this work, 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 [1]. Assets of this technological process rest on low cost lithographic tools, classical silicon planar technology compatibility, and the possibility to get numerous parallel nanowires with precise location on the substrate. Because SiNWs act as sensitive units, specific designs are developed to allow large sensing areas. Two types of resistors (fig. 1 and 2) based on grounded or suspended SiNWs are studied as gas (ammonia) sensors. Resistors process fabrication is compatible with a mass production planar layout. Ammonia molecules adsorbed on the surface of undoped SiNWs act as electron donors (reducing agents) resulting to an increase of the current into the nanowires. The ammonia detection is checked by measuring the relative response of the sensors defined by: S g =(I-I g)/I where I and Ig are the current values for devices in vacuum and reactive ambience respectively. First measurements presented in fig.3 are carried out at room temperature, in a vacuum chamber, under controlled ammonia/nitrogen (NH3:N2) mixture varying from 2 ppm to 700 ppm. For each type of sensors, response to ammonia detection follows a linear behaviour (fig. 3). Results highlight that Sg is higher for suspended polycrystalline SiNWs based devices, related to a high surface to volume ratio of the sensitive units. In addition, relative sensitivity, Sg/[NH3], of such sensors is linearly dependent on the surface to volume ratio of the sensitive units (fig. 4).These results show potential use of these SiNWs based sensors for charged chemical species (ammonia) detection in a fully compatible silicon CMOS technology.The effects of the shape of sensitive silicon nano-objects (nanowires, nanoribbons…), silicon crystal quality, and the low detection limit of the sensors will also be discussedReferences[1] F. Demami, L. Pichon, R. Rogel, A. C. Salaun, Fabrication of polycrystalline silicon nanowires using conventional UV lithography, Materials Science and Engineering, 6 (2009) 012014