J.F.L. Goosen

Comparison of techniques for measuring both compressive and tensile stress in thin films

Abstract New stress-measurement devices for measuring both tensile and compressive strain in single structures have been realized. The investigation is concentrated on the development of two techniques: (i) buckling and (ii) rotation. The buckling technique is based on the buckling of a beam when exceeding a critical strain level. Therefore, an array with different beam lengths is required. The rotation technique, on the other hand, converts the extension or contraction of the material into a rotation, which can be easily measured. These structures have been modelled, simulated, and tested experimentally, using thin polysilicon films. Both techniques have been shown to be promising methods for simple and accurate on-chip thin-film strain measurements.

Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability

This letter presents the application of electrostatic pull-in instability to study the size-dependent effective Young’s Modulus ? ( ~170–70?GPa) of [110] silicon nanocantilevers (thickness ~1019–40?nm). The presented approach shows substantial advantages over the previous methods used for characterization of nanoelectromechanical systems behaviors. The ? is retrieved from the pull-in voltage of the structure via the electromechanical coupled equation, with a typical error of ? 12%, much less than previous work in the field. Measurement results show a strong size-dependence of ?. The approach is simple and reproducible for various dimensions and can be extended to the characterization of nanobeams and nanowires.

The development of a low-stress polysilicon process compatible with standard device processing

When surface micromachined devices are combined with on-chip circuitry, any high-temperature processing must be avoided to minimize the effect on active device characteristics. High-temperature stress annealing cannot be applied to these structures. This work studies the effects of deposition parameters and subsequent processing on the mechanical properties of the polysilicon film in the development of a low-strain polysilicon process, without resorting to high-temperature annealing. The films are deposited as a semi-amorphous film and then annealed, in situ at 600/spl deg/C for 1 h, to ensure the desired mechanical characteristics for both doped and undoped samples. This low temperature anneal changes the strain levels in undoped films from -250 to +1100 /spl mu//spl epsi/. The best results have been obtained for an 850/spl deg/C anneal for 30 min which is used to activate the dopant (both phosphorus and boron). No further stress annealing was used, and 850/spl deg/C does not present problems in terms of thermal budget for the electrical devices. It is shown that these mechanical characteristics are achieved by forming the grain boundaries during subsequent low temperature annealing, and not during deposition. TEM (transmission electron microscopy) studies have been used to investigate the link between the structure and mechanical strain. This has shown that it is the formation of the grain boundary rather than the grain size which has a significant effect on strain levels, contrary to reports in the literature. Using the above-mentioned deposition process, a series of experiments have been performed to establish the flexibility in subsequent processing available to the designer. Therefore, by careful consideration of the processing, a low-temperature polysilicon process, which can be used to fabricate thin micromachined structures, has been developed.

Effects of size and defects on the elasticity of silicon nanocantilevers

The size-dependent elastic behavior of silicon nanocantilevers and nanowires, specifically the effective Youngs modulus, has been determined by experimental measurements and theoretical investigations. The size dependence becomes more significant as the devices scale down from micro- to nano-dimensions, which has mainly been attributed to surface effects. However, discrepancies between experimental measurements and computational investigations show that there could be other influences besides surface effects. In this paper, we try to determine to what extent the surface effects, such as surface stress, surface elasticity, surface contamination and native oxide layers, influence the effective Youngs modulus of silicon nanocantilevers. For this purpose, silicon cantilevers were fabricated in the top device layer of silicon on insulator (SOI) wafers, which were thinned down to 14 nm. The effective Youngs modulus was extracted with the electrostatic pull-in instability method, recently developed by the authors (H Sadeghian et al 2009 Appl. Phys. Lett. 94 221903). In this work, the drop in the effective Youngs modulus was measured to be significant at around 150 nm thick cantilevers. The comparison between theoretical models and experimental measurements demonstrates that, although the surface effects influence the effective Youngs modulus of silicon to some extent, they alone are insufficient to explain why the effective Youngs modulus decreases prematurely. It was observed that the fabrication-induced defects abruptly increased when the device layer was thinned to below 100 nm. These defects became visible as pinholes during HF-etching. It is speculated that they could be the origin of the reduced effective Youngs modulus experimentally observed in ultra-thin silicon cantilevers.

Pressure, flow and oxygen saturation sensors on one chip for use in catheters

This paper presents a combination sensor chip, which is to be fitted to a catheter for use in intervention therapy. The chip contains an absolute pressure sensor, a thermal flow sensor and a colour sensor to determine respectively: blood pressure, blood flow velocity and oxygen saturation in one location at the same time. The sensors where fabricated using epi-micromachining. Change in absorption at wavelengths 660 nm and 800 nm due to the oxygenation of the blood are measured using two stacked photodiodes that serve as a colour sensor.

Powerful polymeric thermal microactuator with embedded silicon microstructure

A powerful and effective design of a polymeric thermal microactuator is presented. The design has SU-8 epoxy layers filled and bonded in a meandering silicon (Si) microstructure. The silicon microstructure reinforces the SU-8 layers by lateral restraint. It also improves the transverse thermal expansion coefficient and heat transfer for the bonded SU-8 layers. A theoretical model shows that the proposed SU-8/Si composite can deliver an actuation stress of 1.30?MPa/K, which is approximately 2.7 times higher than the unconstrained SU-8 layer, while delivering an approximately equal thermal strain.

Design Overview of a Resonant Wing Actuation Mechanism for Application in Flapping Wing MAVs

This paper shows the design and analysis of the actuation mechanism for a four winged flapping wing MAV. The design is set up to exploit resonant properties, as exhibited by flying insects, to reduce the energy expenditure and to provide amplitude amplification. In order to achieve resonance a significantly flexible structure has to be incorporated into the design. The elastic structure used for the body of the MAV is a ring type structure. The ring is coupled to the wings by a compliant amplification mechanism which transforms and amplifies the ring deflection into the large wing root rotation. After initial sizing, the structures are analyzed by finite elements (eigenvalue and transient analysis). Based on the initial analysis, the structures are realized to be tested later. The wings are first analyzed independent of the structure in order to tune wing hinge stiffness to efficiently generate lift, exploiting passive wing pitching. The wings are tuned by using a quasi-steady aerodynamic model. The tuned wings are tested to judge if manufactured wings reflect the predicted performance. The ring-shaped thorax structure is combined with the wings to test resonant performance of the assembled structure. A test setup is built to quantify lift production. Lift is tested by suspending the prototype on a flexible beam and measuring changes in deflection when the model is actuated. Significant lift is produced using the current prototype. Kinematic patterns present during resonant actuation show correct timing of wing rotation.

Polymeric Thermal Microactuator With Embedded Silicon Skeleton: Part I—Design and Analysis

This paper presents the modeling of a new design of a polymeric thermal microactuator with an embedded meander-shaped silicon skeleton. The design has a skeleton embedded in a polymer block. The embedded skeleton improves heat transfer to the polymer and reinforces it. In addition, the skeleton laterally constrains the polymer to direct the volumetric thermal expansion of the polymer in the actuation direction. The complex geometry and multiple-material composition of the actuator make its modeling very involved. In this paper, the main focus is on the development of approximate electrothermal and thermoelastic models to capture the essence of the actuator behavior. The approximate models are validated with a fully coupled multiphysics finite element model and with experimental testing. The approximate models can be useful as an inexpensive tool for subsequent design optimization. Evaluation, using the analytical and numerical models, shows that the polymer actuator with the embedded skeleton outperforms its counterpart without a skeleton, which is in terms of heat transfer and, thus, response time, actuation stress, and planarity.

Multi-parameter sensor system with intravascular navigation for catheter/guide wire application

Interventional radiology is a medical speciality, which uses medical tools such as guide wires and catheters to diagnose and treat vascular diseases. To navigate these tools to the place of intervention, X-ray imaging is extensively used, creating an important health risk to the medical staff and to the patient. To reduce the radiation dose, an electromagnetic navigation system is currently being developed. Once the correct position has been attained with the guide wire, the catheter can be brought into place. In many cases, the intervention radiologist requires a number of measurements to assess the situation and the treatment required. To achieve this, a multi-sensor chip has been developed for blood flow, pressure and oxygen saturation level, with dimensions suitable for catheter applications. The localisation system and the measurement system will be presented in this paper.

Uncertainty-based Design Optimization of a Micro Piezoelectric Composite Energy Reclamation Device

In this paper uncertainty-based design optimization of a micro energy reclamation device is presented. The goal is to optimally design a Microelectromechanical Systems based device to extract maximum power from externally introduced vibrations. This microstructure consists of an array of piezoelectric composite cantilever beams connected to a free standing mass. Each cantilever beam undergoes deformation when subjected to external base vibrations. This deformation induces a mechanical strain in the beam resulting in the conversion to electric voltage due to the piezoelectric effect. In case of microstructures, uncertainties in geometry as well as material properties are large and therefore may have signicant effects on the mechanical behavior. In the present paper uncertainties in geometry and material properties are considered. A description of uncertainties via bounds on the uncertainty variables is adopted. Uncertainty-based design optimization is carried out using the anti-optimization technique.