Field emission from lateral silicon micro tip for detection of micromechanical resonators


  • Contact Researcher: Benoit CHARLOT, Dr.
  • Hosted LIMMS Japanese Laboratory: Hiroshi TOSHIYOSHI Lab --- thematiques

Project Overview

  • Keywords

  • Context :
GHz range micromechanical resonators are usually of small dimensions (some microns) and the relative mechanical displacement is then very small (in the order of nano to picometer). Conventional RF-MEMS filters use capacitive coupling for excitation as well as detection.
  • Objectives :
However, capacitive signal current produced by the MEMS oscillators is very weak in spite of high frequency resonance because of a very small value of the capacitance variation and large parasitic capacitive couplings. Furthermore, direct capacitive coupling has disadvantages such as difficulty in impedance matching and very small fan-out. This is the reason why for the moment, despite their high measured Q factors, these devices haven’t succeed in replacing traditional LC oscillators. In order to obtain an electron flow in the vacuum with reasonable voltage we use the strong electric field near the apex of a microtip to extract the electrons from the silicon to vacuum. Microtips are made by micromaching in silicon. The process is a combination of Deep reactive ion etching and TMAH etching on a Silicon on Insulator wafer. The tips are created in the plane so that the electron flow occurs between two adjacent tips at a distance typically of 3 to 5μm. The curve shows an exponential relation between the current and the voltage difference in accordance with the Fowler Nordheim theory[3].
  • Methods :
The project targets two different configurations to use the field emission current as displacement sensor for the micro resonator.
A first solution is the diode configuration where the distance between two tips is modulated by the mechanical displacement of the mechanical resonator. The Fowler Nordheim current depends on the electric field between anode and cathode a thus by the distance when bias at constant voltage. For this configuration we have first implemented a square resonator to be actuated within a Lame mode [4]. In this vibration mode, mechanical losses in anchors are low because anchors are coincident with vibration nodes, which allow reaching very high quality factors.

A second possible solution, is to use a triode configuration. In this case, the two tips (cathode and anode) are fixed and a third mobile structure (the gate), attached to a resonator will modulate the electron flow by squeezing the electron channel. Two fixed electrodes, acting as an electrostatic lens will be used to focus the electron flow. This configuration is analog to a diaphragm inserted close to a lens (electrostatic lens in our case). Finite element simulations have been implemented in order to compute the electrons trajectory between the tips and to evaluate the influence of both the position of the electrostatic lens and the resonator. The continuation of this work will be done using dedicated software that deals with space charge effects, which is not the case here.
  • References :
  1. Yamashita, K.; Sun, W.; Kakushima, K.; Fujita, H.; Toshiyoshi, H.; “A lateral field-emission RF MEMS device”, Transducers ‘05, pp. 1096-99.
  2. Yamashita, K.; Sun, W.; Kakushima, K.; Fujita, H.; Toshiyoshi, H.; IVNC2005, pp. 29.
  3. Fowler, R.H.; Nordheim, L.W.; Proc. R. Soc. London. A119, 173 (1928)
  4. Bhave, S. A; Gao, D.; Maboudian, R.; Howe, R. T. ; Fully-Differential Poly-SiC Lamé-Mode Resonator And Checkerboard Filter ; MEMS 2005, pp. 223-226.