Siebe Bouwstra

Atomic force microscopy probe with piezoresistive read-out and a highly symmetrical Wheatstone bridge arrangement

Abstract We have developed a new generic platform for the fabrication of multipurpose microprobes with integrated piezoresistive read-out, built-in background filter and silicon tip. The probe fabrication is based on SOI wafers with buried boron etch-stop layers, which allow us to realize probes with fully encapsulated resistors and integrated silicon tips. The dimensions of the resistors are well defined and leak-current is eliminated. Probes with a force constant in the range of 0.8–4 N/m and with resonant frequencies in the range of 40–80 kHz have been fabricated. The probes typically display a deflection sensitivity of (ΔR/R)z−1=2.4×10−7 A−1, and a force sensitivity (ΔR/R)F−1=2.7×10−6 nN−1. The change in resistance of the piezoresistors is detected by a highly symmetrical on-chip Wheatstone bridge arrangement. The measured noise level in the Wheatstone bridge is in good agreement with the calculated noise limit and a minimum detectable cantilever deflection of 0.3 A has been predicted for a measurement bandwidth of 10 Hz. The symmetrical bridge configuration has been compared with a nonsymmetrical setup, and it is concluded that the symmetrical Wheatstone bridge significantly decreases nonlinearities in the output response. Finally, the probe has successfully been used for atomic force microscopy (AFM) imaging.

AFM probes with directly fabricated tips

Different fabrication technologies for atomic force microscopy (AFM) probes have been investigated, emphasizing the fabrication of versatile tip shapes, which can be integrated with micromachined cantilevers. By successive reactive ion etching processing steps, novel high-aspect-ratio tip shapes have been produced and integrated with AFM cantilevers. The performance of the fabricated tips has been investigated by scanning test structures in an atomic force microscope.

Electroplating and characterization of cobalt-nickel-iron and nickel-iron for magnetic microsystems applications

The magnetic properties of pulse reverse (PR) electroplated CoNiFe and DC electroplated NiFe are presented. CoNiFe is a very promising material for magnetic microsystems due to the possibility of achieving a high saturation flux density (Bs) and a low coercivity (Hc). A new bath formulation has been developed, which by means of PR electroplating makes it possible to deposit high Bs CoNiFe with a low residual stress level. The magnetic properties have been determined using a new simple measurement setup that allows for wafer level characterization. The results have been validated by comparison to measurements performed with a vibrating sample magnetometer (VSM).

Opto-mechanical accelerometer based on strain sensing by a Bragg grating in a planar waveguide

Abstract In this paper we present an opto-mechanical sensor based on a Bragg grating as the strain-sensing element. The motivation for choosing this alternative way of strain sensing is that the sensed information is directly encoded into a wavelength, which is an absolute parameter insensitive to typical intensity and phase noise. Recently, it has been shown that fibre-optic strain sensors based on this technique are capable of resolving dynamic strain down to 0.6 × 10 −9 ( Hz ) −1 2 . To demonstrate that this new detection principle can also be used for high-performance microsensors, we have chosen to fabricate a silicon opto-mechanical accelerometer based on strain sensing by a Bragg grating in a planar waveguide. The fabrication of the accelerometer includes KOH wet etching of (110) silicon, controlling stress in PECVD glass waveguides and direct UV writing of Bragg gratings.

Design and fabrication of compliant micromechanisms and structures with negative Poisson's ratio

This paper describes a new way to design and fabricate compliant micromechanisms and material structures with negative Poissons ratio (NPR). The design of compliant mechanisms and material structures is accomplished in an automated way using a numerical topology optimization method. The procedure allows the user to specify the elastic properties of materials or the mechanical or geometrical advantages of compliant mechanisms and returns the optimal structures. The topologies obtained by the numerical procedure require practically no interaction by the engineer before they can be transferred to the fabrication unit. Fabrication is carried out by patterning a sputtered silicon on a PECVD-glass with a laser micromachining set-up. Subsequently the structures are etched into the underlying PECVD-glass and the glass are underetched, all in one two-step RIE process. The components are tested using a probe placed on an xy-stage. This fast prototyping allows newly developed topologies to be fabricated and tested within the same day.

Fabrication and characterization of truly 3-D diffuser/nozzle microstructures in silicon

We present microfabrication and characterization of truly three-dimensional (3-D) diffuser/nozzle structures in silicon. Chemical vapor deposition (CVD), reactive ion etching (RIE), and laser-assisted etching are used to etch flow chambers and diffuser/nozzle elements. The flow behavior of the fabricated elements and the dependence of diffuser/nozzle efficiency on structure geometry has been investigated. The large freedom of 3-D micromachining combined with rapid prototyping allows one to characterize and optimize diffuser/nozzle structures.

Compliant electro-thermal microactuators

This paper describes design, microfabrication and characterisation of topology optimised compliant electro-thermal microactuators. The actuators are fabricated by a fast prototyping process using laser micromachining and electroplating. Actuators are characterised with respect to displacement, force and work, by use of image analysis. Four different actuators are presented. These actuators are capable of displacements of 30 /spl mu/m and forces of 15 mN. The most recent actuator designs function in reasonable accordance with design predictions.

Tantalum oxide thin films as protective coatings for sensors

Reactively sputtered tantalum oxide thin-films have been investigated as protective coating for aggressive media exposed sensors. Tantalum oxide is shown to be chemically very robust. The etch rate in aqueous potassium hydroxide with pH 11 at 140/spl deg/C is lower than 0.008 /spl Aring//h. Etching in liquids with pH values in the range from pH 2-11 have generally given etch rates below 0.04 /spl Aring//h. On the other hand patterning is possible in hydrofluoric acid. Further, the passivation behaviour of amorphous tantalum oxide and polycrystalline Ta/sub 2/O/sub 5/ is different in buffered hydrofluoric acid. By ex-situ annealing in O/sub 2/ the residual thin-film stress can be altered from compressive to tensile and annealing at 450/spl deg/C for 30 minutes gives a stress-free film. The step coverage of the sputter deposited amorphous tantalum oxide is reasonable, but metallisation lines are hard to cover. Sputtered tantalum oxide exhibits high dielectric strength and the pinhole density for 0.5 /spl mu/m thick films is below 3 cm/sup -2/.

Conformal coating by photoresist of sharp corners of anisotropically etched through-holes in silicon

A new photoresist treatment is presented yielding conformal coating of three-dimensional silicon structures, including the sharp corners of through-holes obtained by anisotropic etching in (100)-silicon. Resist reflow from these corners is avoided by replacing the conventional baking procedure with a vacuum treatment. The investigated photoresist is Shipleys Eagle 2100 ED, a negative-working electrodepositable photoresist. Additionally, the vacuum treatment allows the photoresist to be used for lift-off processes. Electrical frontside to backside interconnections suitable for solder bonding have been realized applying the latter technique.

Silicon microphones - a Danish perspective

Two application areas of microphones are discussed, those for precision measurement and those for hearing instruments. Silicon microphones are under investigation for both areas, and Danish industry plays a key role in both. The opportunities of silicon, as well as the challenges and expectations, are discussed. For precision measurement the challenge for silicon is large, while for hearing instruments silicon seems to be very promising.