MEMS high frequency resonator in 3D technology for Atomic

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					 MEMS high frequency resonator in 3D technology for Atomic Force sensor

    Benjamin Walter, Emmanuelle Algré, Marc Faucher, Bernard Legrand and Lionel Buchaillot.
   Institut d’Electronique, de Microélectronique et de Nanotechnologie - IEMN CNRS UMR 8520,
  ISEN Department , Cité Scientifique - Avenue Poincaré, BP 60069 - 59652 Villeneuve d'Ascq Cedex - FRANCE

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Topics: integration of technologies, etching

Abstract—In order to implement high
frequency Atomic Force Microscopy (AFM),
we purpose a MEMS design. Some specific 3D
technologies on SOI (Silicon On Insulator)
have been developed. We will focus on two
topics: self aligned sacrificial gap and in-plane
sharp tips using anisotropic wet and dry
etches of silicon.
                      I. INTRODUCTION
    We present an AFM probe device which is based on the
bulk mode of a silicon ring resonator. The actuation and the
detection of this resonator use an electrostatic transduction.
Thus, this transduction allows us to excite the elliptic mode
which deforms the silicon ring in an ellipse. Then the
intensity of the signal readout can be quantified by the
equivalent electrical output resistance, expressed by [1,2]:
                                    √ .
                                                           (1)     Figure 1 : Fabrication process leading to self standing AFM ring probes

where K and M are the effective stiffness and mass of the
resonator, d is the capacitive gap size, Q is the resonator’s
quality factor, Vp is the DC polarization voltage and L and
h are the electrodes’ length and height, respectively. From
this equation, it is evident that about a hundred nanometers
capacitive gaps are needed to reduce the equivalent output
resistance to reasonable values.
                     II. SACRIFICIAL GAP
    The fabrication process was run on SOI (Silicon On
Insulator) wafer with 5µm-silicon-layer on 2µm-oxide that
acts as a sacrificial layer. Then a 300 nm Low Temperature
oxide (LTO) layer was deposited in order to perform
electrical insulation of the resonator signal and excitation
coplanar access lines (Fig. 1a). The ring, its anchoring and
the lateral tips dimensions were defined by a Bosch-type
anisotropic plasma etching of the top silicon layer (Fig. 1b).
The next step consisted in defining a sacrificial oxide for the
electrostatic gap (Fig. 1c). Then a 6 µm Low pressure
chemically vapor deposited (LPCVD) polysilicon layer was
deposited and etched to form the electrodes, using the 300
nm LTO oxide as an etch stop barrier (Fig. 1e). Finally, a
backside Bosch process was performed in order to define an
AFM holder and resonator backside. The resonator was              Figure 2 : Scanning electron micrograph of a 110µm radius resonator-
released using wet etching of the 300 nm LTO and the              based probe with tip length of 60µm.
sacrificial lateral oxide gap.
    Once the release was done, each resonator could
individually be picked up from the wafer thanks to the
backside etching design.
                         III.     IN-PLANE TIPS
    In order to use this resonator like an AFM probe, we
have to fabricate sharp tips in the same plane than the
resonant mode. This conducts us to develop a batch process
of in-plane tips compatible with the entire process describe
below. The following process can easily take place at the
beginning of the previous process [3].
    The first step stay the same, the resonator pattern is
etched (Fig. 3a). But this time, the tip is not well defined at
the end of the ring, the silicon edge is pattern at an angle θ
with respect to the [110] direction of the silicon and extends
beyond the future location of the tip. After deposition of a
400-nm-thick layer of LPCVD low-temperature oxide (Fig.
3b), photoresist is patterned to have openings above the tip
region. Next, an anisotropic plasma CHF3/CF4 removes the
oxide from above the exposed regions while leaving an
oxide side wall, which runs around the perimeter of the           Figure 4 : Scanning electron micrograph of an in-plane tip
exposed silicon (Fig. 3d). The photoresist is removed, and
finally the wafer is etched in a 80°C 25%wt TetraMethyl
Ammonium Hydroxide (TMAH) enough time to make
appear the {111} silicon planes. By removing the remaining
oxide, the tip is formed at the end of the resonator (Fig. 3f).                           ACKNOWLEDGMENT

                                                                      The authors would like to acknowledge all the IEMN
                                                                  clean room staff for their constant technical support. The
                                                                  project is funded by ‘ANR PNANO’, project IMPROVE-


                                                                  [1]   S. Pourkamali, and F. Ayazi, “Soi-based hf and vhf single-crystal
                                                                        silicon resonators with sub-100 nanometer vertical capacitive gaps,”
                                                                        TRANSDUCERS ‘03, Boston,MA, pp. 837–840.
                                                                  [2]   Z Hao, S. Pourkamali, and F. Ayazi, “VHF single-crystal silicon
                                                                        elliptic bulk mode capacitive disk resonators – part I : design and
                                                                        modeling,” Journal of Microelectromechnical Systems, vol. 13 no.
                                                                        6, pp. 1043–1053, December 2004
                                                                  [3]   R.P Ried, H. J. Mamin, B. D. Terris, L-S. Fan, and D. Rugar, “6-
                                                                        MHz 2-N/m piezoresistive atomic-force-microscope cantilevers with
                                                                        INCISIVE tips,” Journal of Microelectromechnical Systems, vol. 6
                                                                        no. 4, pp. 1043–1053, December 1997

Figure 3 : Fabrication process leading to in-plane tips