Silicon nanowires based sensors for NH3 detection

International audienceOwing to their physical and electrical properties, semiconducting nanowires are the subject of intense research activities. In particular, silicon nanowires (SiNWs) are full compatible with the high reliable well known CMOS (Complementary Metal Oxide Semiconductor) silicon technology. In addition, thanks to their high surface to volume ratio, SiNWs are good candidates 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. 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. The "top down" approach starts from bulk materials and scales down the patterned areas. In this way, several advanced lithographic tools with nanometer size resolution rest on the high cost generated, and more generally the low throughput capability is unsuitable with mass production. In this work, undoped silicon nanowires are fabricated following these tow approaches: i) Au-catalyst vapour liquid solid (VLS) SiNWs [1] (bottom-up), and ii) suspended sidewall spacer (SS) polycristalline SiNWs (top down) [2]. SiNWs are integrated into resistors used as gas (ammonia) sensors. Fabrication of two types of SiNWs based devices uses classical silicon microelectronic technologies [3]. Specific design is developed for VLS SiNWs based resistors to be compatible with a mass production planar layout. Fabrication of SS-polySiNWs based devices uses classical lithographic tools. 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. Then, ammonia detection is performed by measuring current into SiNWs based resistors. Measurements are carried out at room temperature, in a vacuum chamber, under controlled ammonia/nitrogen (NH3:N2) mixture varying from 2 ppm to 700 ppm. Results highlight that the relative response, Sg, defined as Sg=(I-Ig)/I where I and Ig are the current values for devices in vacuum and reactive ambient respectively, is higher for suspended polycristalline SiNWs based devices, probably related to a high structural defects density. For both sensors, response to ammonia detection follows a linear behaviour for ammonia concentration lower than 350ppm, and relative sensitivity, S=Sg/NH3, is 99.5%/ppm. These results show potential use of these nanowires for charged chemical species detection in a fully compatible silicon CMOS technology. [1] Ni L., Demami F., Rogel R., Salaun A.C., Pichon L., Materials Science and Engineering 6, 012013 (2009). [2] Demami F., Pichon L., Rogel R., Salaun A. C., Materials Science and Engineering 6, 012014 (2009). [3] Demami F., Ni L., Rogel R., Salaun A.C., Pichon L., Sensors and Actuators B: Chemical, 170, 159 (2012