Simple optical characterisation for biomimetic micromachined silicon strain-sensing structure Dedy H.B. Wicaksono * a , Grégory Pandraud a , Patrick J. French a a Electronic Instrumentation Lab., Dept. of Microelectronics, Delft University of Technology, Mekelweg 4, 2628CD, Delft, The Netherlands; ABSTRACT This paper presents an on-going work to develop micromachined silicon-based strain sensor inspired from the campaniform sensillum of insects. We present simple optical setup for the characterisation of a membrane-in-recess structure as an early stage in mimicking the natural sensor. The microstructure is a 500 nm-thick SiO 2 /SiN circular membrane, burried 13 µm from the surface of a 3x3 mm, 525 µm thick Si-chip. The chip was attached to a 45x10x0.525 mm Si beam. The simple optical characterisation setup is based on imaging the reflected laser beam from the biomimetic structure. Since an optical cavity between the membrane and the Si beams beneath was formed, ideal flat-parallel Fabry- Perot interferometer equation was applied to interpret the results semi-quantitatively. We obtained 2-D interference fringe pattern having 3 orders of maxima from the middle to the edge of the circular apperture, as a result of an initial internal membrane stress. The pattern changed non-linearly as we applied flexural strain from behind the beam up to 50 µm, most probably caused by nonlinear deflection of the membrane (i.e. the membrane did not deflect similarly as the beam beneath it). This phenomena might explain one of the strain-amplifying properties of this biomimetic strain sensing microstructure. Keywords: Biomimetics, Strain Sensing, MEMS, Optical characterisation, Fabry-Perot interferometer 1. INTRODUCTION Nature has long been an inspiration for engineer. Many of today’s world engineering masterpieces and work resemble those similar structures or systems found in nature [cf. 1-3]. However, it was not until recently that the sensor society started to take inspirations from high-performance sensors found in nature (see [4] for a comprehensive overview of current emerging development of nature-inspired sensors). Campaniform sensillum is a kind of strain sensor found in insects, e.g. the Blowfly (Calliphora vicina). The campaniform sensillum is basically an opening in the cuticle (with a size of 5 to 10 µm in diameter for the circular shape one) covered by membrane layers (figure 1). The shape of the opening is generally ellipse and sometimes almost circular [5,6]. Deformation in the insect’s cuticular layer is sensed by the campaniform sensillum using mechanical coupling, transduction and encoding mechanism to transfer the environmental information to the insect’s nervous system. Previous work [7] showed that the mechanical coupling mechanism was resolved into discrete components: a cap surrounded by a collar, a joint membrane and an annulus-shaped socket septum with a spongy compliant zone (the spongy cuticle). The coupling mechanism is a mechanical linkage which transforms the stimulus into two deformations in different directions: monoaxial transverse compression of the dendritic tip of a sensory neuron cell, which acts as a transducer, and vertical displacement of the cap. The natural campaniform sensilla, regardless of the high Young modulus of the exocuticle layer of the insect (k ≈ 10 9 Nm -2 ), can still detect minute changes of strain. These sensors are as sensitive to displacement in that stiff structure as the receptors in the human ear are to sound [8]. This sensitivity is among others due to their unique membrane-in-recess microstructure. The membrane located inside a blind-hole amplifies the strain. For a rather detail discussion on the sensing mechanism, refer to [7], as well as to [9] for our previous report on the design and development strategy for a biomimetics MEMS-based strain sensor inspired from Campaniform sensillum. * d.h.b.wicaksono@ewi.tudelft.nl ; phone +31-(0)15 278 3342; fax +31-(0)15 278 5755 Third Intl. Conf. on Experimental Mechanics and Third Conf. of the Asian Committee on Experimental Mechanics, edited by Quan, Chau, Asundi, Wong, Lim, Proc. of SPIE Vol. 5852 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.621919 788 Downloaded from SPIE Digital Library on 21 May 2010 to 131.180.130.114. Terms of Use: http://spiedl.org/terms