TRANSDUCERS ’01 EUROSENSORS XV The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 – 14, 2001 Formation of ultra-shallow p + /n junctions using BF 2 implantation for the fabrication of improved piezoresistive cantilevers E. Cocheteau, C. Bergaud , B. Belier*, L. Bary, and R. Plana LAAS/CNRS, 7 avenue du Colonel Roche, 31077 Toulouse Cedex 4, France bergaud@laas.fr * Institut d'Electronique Fondamentale, CNRS UMR 8622, Bat 220, Université de Paris-Sud, 91405 Orsay Cedex, France SUMMARY BF 2 implantation has been performed to obtain ultra-shallow p+/n junctions (<180 nm) for the fabrication of improved piezoresistive cantilevers. Due to the heavier mass of boron fluorine compared to boron, the junction depth can be significantly reduced and therefore sensitivity can likewise be increased (almost 80% of the theoretical maximum). Moreover, the presence of fluorine tends to reduce the well-known transient enhanced diffusion of boron thereby decreasing the junction motion upon annealing. Nonetheless, fluorine seems to impact the 1/f noise characteristics. Optimization of the thermal annealing could greatly improve these properties by recovering the silicon crystal. Keywords: BF 2 implantation, ultra-shallow junctions, piezoresistive cantilevers. INTRODUCTION Integrated piezoresistive cantilevers for atomic force microscopy have first been developed by Tortonese et al [1]. Their main advantage lies in that no optical detection is needed. This increases the compactness of the device with no need for precise alignment of the laser beam onto the cantilever. Moreover, arrays of piezoresistive cantilevers can be used in parallel and, therefore, higher throughputs can be achieved for atomic data storage devices [2], nanolithography techniques [3] and biosensors [4]. Recently, the development of nanoscale technologies has resulted in a new trend that supports a reduction in size since, for a given spring constant, smaller cantilevers have reduced mass leading in turn to higher resonance frequency and bandwidth measurements [5]. The difficulty in maintaining sensitivity when reducing the cantilever size is that the piezoresistive doped region should be made as shallow as possible. However, this may be cumbersome when the cantilever thickness reduces to less than one micrometer especially when forming ultra-shallow p + /n junctions. In this paper, ultra-shallow p + /n junctions (< 180 nm) have been obtained by implanting BF 2 instead of B. Due to its heavier mass, this molecule allows both the boron channeling effect and the corresponding energy of the implanted boron to be reduced (a 15-keV BF 2 implantation corresponds to a 3-keV B implantation since the BF 2 molecule is fifth times heavier than the boron ion). This considerably decreases junction depths compared to direct boron implantation. CANTILEVER FABRICATION Piezoresistive cantilevers are batch fabricated using standard micromachining techniques. The fabrication process used is similar to the one proposed by Tortonese et al [1] except that BF 2 is preferred to boron for implantation purposes and that a rapid thermal annealing (RTA) is performed instead of a conventional one. The starting material is a bonded SOI (silicon-on- insulator) wafer. The top layer is n-type, 1-5 Ω.cm, 15 μm-thick with a 1-μm-thick intermediate oxide layer. Thermal growth of a 400-nm-thick SiO 2 layer is performed and used as a masking layer for forming the silicon tips following the fabrication process proposed by Ravi et al. [6]. The next step consists in implanting the BF 2 with an energy of 15 keV and a dose of 5x10 14 cm -2 through a 5-nm-thick silicon dioxide layer. Electrical activation was achieved using Rapid Thermal Annealing (RTA at 950 °C for 15s) followed by a conventional annealing at 850 °C for 20 min. These conditions lead to a doping concentration of 10 19 cm -3 (extracted from the resistance measurement R=26.2 kΩ and knowing the junction depth x j =0.18