An innovative NEMS pressure sensor approach based on heterostructure nanowire X. Xu * , B. Bercu, F. Lime, L. Montès IMEP-LAHC, Grenoble Institute of Technology, Minatec, 3 Parvis Louis Néel, 38016 Grenoble, France article info Article history: Received 23 March 2009 Received in revised form 22 May 2009 Accepted 16 July 2009 Available online 22 July 2009 Keywords: Mechanical stress/strain Nanowire Tunnel junction Finite element method Pressure sensor Ultra-thin membrane NEMS abstract Mechanical stress is increasingly applied in microelectronics. For instance, strained silicon technology is widely used to improve carrier mobility and then driver current for advanced MOS transistors. For micro- electromechanical systems, piezoresistive effects are universally employed in pressure sensors. In this paper, we present an original method for studying mechanical stress impact on the property of nano- devices placed on ultra-thin membranes, which has several advantages compare with the conventional four-point-bending method. Using this architecture, we have studied an innovative Nano electro- mechanical system (NEMS) pressure sensor to investigate its property in static and dynamic modes respectively. We have determined the optimal orientation and position of a nanowire on the membrane. We simulated the electrical transport behavior in the hetero-junction nanostructure by interrupting the nanowire with a dielectric adopting tunnel junction approach. We show that a large improvement in pressure measurement sensitivity can be obtained relying on the direct tunneling current. We also inves- tigate the mechanical stress impact on the potential barrier height that leads to the variation of the tun- nel current and dynamic multi-bends of this nanostructure in its dynamic deformation modes. Finally, our work helps to understand the electrical and mechanical properties of the nanostructure under the influence of large mechanical stress and to design innovative NEMS pressure sensors. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Mechanical stress influence on the device electrical property is the popular issue in recent years. For instance, the carrier mobility and therefore drive current are significantly improved by the strained silicon technology for advanced MOS transistors. On the other hand, in the domain of MEMS, piezoresistive effects are used to design a pressure sensor. To investigate the interaction between the mechanical stress and the electrical properties of microelectronic devices and nano- structures, the choice of the mechanical stress generation method is paramount. Compare with the conventional four-point-bending method [1] and the AFM tip method to apply mechanical stress on nanostructure [2], our ultra-thin membrane technique shows several advantages. These include not only the ability to induce heavier mechanical stresses, but also a unique way to obtain dy- namic deformations with frequencies up to several MHz. In this paper, we present an innovative NEMS pressure sensor based on (i) nanowire (ii) tunnel junction and (iii) heterostructure on a membrane. The finite element method (FEM) simulations show a large variation of tunnel current with applied pressure in static mode. We investigate the electrical behavior with the tunnel junction thickness variation in the nanowire and the large mechan- ical stress impact. In the dynamic mode, the nanowire can be mul- ti-bended under the different frequency excitations. Finally, we conclude by demonstrating the large improvement in measurement sensitivity when we use the hetero-junction nanostructure. 2. Ultra-thin membrane technique Ultra-thin membranes are fabricated by deep reactive ion etch- ing (DRIE) on silicon on insulator (SOI) wafer for a highly accurate membrane area and thickness [3]. The membrane area can be var- ied from square micrometers up to square millimeters and its thickness is of the order of a few hundred nanometers up to a few micrometers. In this study, the area of the membrane is 150 lm  150 lm. This membrane is made of three layers: one sil- icon layer of 300 nm sandwiched between two silicon dioxide lay- ers of 400 nm to reduce the residual stress. We have studied two main kinds of membrane actuation: static and dynamic modes. Using a vacuum chamber placed under the membrane, a static deformation is induced on the membrane cor- responding to the differential pressure on the upper and lower sur- faces (up to 3 atm). As far as dynamic actuation is concerned, piezoelectric device can be used to be the oscillation source. By placing a membrane cover on this piezoelectric device which is dri- ven by an AC voltage, in this way, the membrane can vibrate. The 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.07.018 * Corresponding author. E-mail addresses: xux@minatec.inpg.fr (X. Xu), bercu@minatec.inpg.fr (B. Bercu), montes@minatec.inpg.fr (L. Montès). Microelectronic Engineering 87 (2010) 406–411 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee