Sensors and Actuators A, 43 (1994) 339-345 Microlever with combined scanning force microscopy integrated sensor/actuator functions for 339 J. Brugger”, N. Blamf, Ph. Renaudb and N.F. de Rooija ‘Institute zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Microtechnology (ZMT), Universi8y of Neuch&el, Bmguet 2, Zoo0 Neuchdtei (Switzerland) “Swiss Center for Electronics and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Mic ro re c hno lo g y (CSEM ) Inc., M aladi& 71, 2ooO Neuchlftel (Switzerland) (Received August 13, 1993; accepted December 2. 1993) Abstract A novel silicon microfabricated sensor bead for the scanning force microscope (SFM) is presented. The force sensor consists of a cantilever and an adjacent counter-electrode forming the two plates of a capacitor. Force- induced cantilever deflections are monitored by capacitive detection. Typical lever dimensions of 800 PmX 40 km and a gap of 3 pm yield an active sensing capacitance C=O.l pF. The expected sensitivity in terms of vertical cantilever motion is AC/&=10 fF/m. In addition to the sensing capability, the microlever can also be z-actuated by applying controlled voltages. This allows both the tip-to-sample distance and the cantilever/system compliance to be adjusted. Expressions are derived for the amplitude of cantilever deflections under electrostatic actuation in the static and dynamic modes as pertinent to applications of SFTvlin the contact and non-contact modes. The microlever is fabricated using silicon bulk- and surface-micromachining techniques including fusion bonding and sacrificial layer etching. First measurements of the static and dynamic deflections of cantilevers are analysed and show promising results. The reported device basically represents a module of an SFM microsystem with integrated cantilever deflection sensor and adjustment capability. 1. Introduction In scanning force microscopy local surface forces acting between a sample and a sharp tip are probed via the deflection of a tiny cantilever beam [l]. The cantilever motions, usually in the nanometre and sub- nanometre range, are sensed with different measuring techniques including assembled optical, tunnelling or capacitive set-ups [2]. All these detection principles require external physical components to be aligned, somewhat complicating the system design. In consideration of an integrally designed scanning force microscopy (SFM) sensor head, a piezoresistive cantilever [3] and a piezoelectric lever [4] have been operated successfully. Furthermore, a bimorph structure (metal/piezoelectric zinc-oxide) providing integrated three-dimensional tip actuation has also been presented [5]. Such sensing and actuated cantilevers are of great interest for miniaturized scanning probe systems, not least for industrial applications such as IC surface profiling. In particular, they enable the scanning of the tip versus the fixed sample while having the sensor incorporated in the probe, thus substantially simplifying the design of a stand-alone SFM microsystem. The capacitive method for cantilever deflection mea- surements has proven to be an interesting alternative to other methods [6-g]. The fabrication and operation of such a miniaturized capacitive probe are inherently critical. Strict instrumental requirements have to be ensured in order to control and minimize stray ca- pacitance effects for a high-resolution system. Accurate machining and alignment of the capacitor plates and gap are crucial for successful operation. Silicon micromachining is a promising technique to batch-fabricate reproducibly such devices with high accuracy. In this paper we present a fully micromachined capacitive microlever for the SFh4. The device has the interesting asset of incorporating both a cantilever- deflection sensing and adjustment capability in a single probe. While profiling a sample surface in the quasistatic mode, minute cantilever deflections induce a change in the capacitance gap between two silicon beams, which can be measured via interface electronics. Al- ternatively, when vibrating and scanning the microlever in a non-contact dynamic mode, the frequency shift due to force gradients can be measured. Silicon is well suited for resonant methods due to its inherently high Q-factor [lo]. The electrostatic actuation capability by applying controlled voltages allows for an induced cantilever bending, thus permitting the tip-sample fine approach and eventually an autonomous feedback system. Fur- 0924-4247/94/$07.00 Q 1994Elsevier Science S.A. All rights reserved SSDI 0924-4247(93)00701-S