Patrick J. French

Fabrication of a CMOS compatible pressure sensor for harsh environments

The fabrication and characteristics of CMOS compatible absolute pressure sensors for harsh environments are presented in this paper. The sensor which was fabricated using post-processing surface micromachining consists of 100 circular membranes with a total capacity of 14 pF. PECVD SiC was used due to its good mechanical properties, but since SiC has high resistivity, aluminium layers were used for electrodes. The stiction problems were avoided by using polyimide PI2610 as a sacrificial layer. The pressure sensors were fabricated and the change of capacitance over full pressure range, 5 bar, was 3.4 pF.

Shape based Monte Carlo code for light transport in complex heterogeneous tissues

A Monte Carlo code for the calculation of light transport in heterogeneous scattering media is presented together with its validation. Triangle meshes are used to define the interfaces between different materials, in contrast with techniques based on individual volume elements. This approach allows to address realistic problems in a flexible way. A hierarchical spatial organisation enables a fast photon-surface intersection test. The application of the new environment to evaluate the impact of the trabecular structure of bone on its optical properties is demonstrated. A model of the trabecular micro structure recovered from microCT data was used to compute light distribution within tissue. Time-resolved curves across a spherical bone volume were computed. The work presented enables simulation of radiative transport in complex reality-based models of tissue and serves as a powerful, generic tool to study the effect of heterogeneity in the field of biomedical optics.

Sacrificial oxide etching compatible with aluminum metallization

This paper reports on sacrificial oxide etching with very high selectivity to aluminum metallization using mixtures of 73% HF and IPA. Etch rate ratios up to 680 have been achieved even for (slow etching) thermal oxide. Thermal oxide etch rates up to 1.8 /spl mu/m/min. are reported. Thick polysilicon accelerometers with aluminum metallization and thermal sacrificial oxide have been made as well as full aluminum microstructures using plasma oxide as sacrificial layer.

Electrochemical etch stop engineering for bulk micromachining

Microelectromechanical systems (MEMS) may be fabricated with several micromachining techniques, the oldest and most widespread technique being that of bulk-micromachining. Although there are alternative etching techniques, wet anisotropic etching is still favoured by industry in many cases because of its simplicity and low costs. Electrochemically controlled etching allows for reproducible fabrication of structures with a uniform thickness. Recently, new etch stop techniques based on galvanic element formation in an alkaline etchant have been reported. Galvanic element formation has been observed earlier, though the concept never received much attention in the literature on bulk-micromachining. This paper discusses the theory of electrochemical etching and galvanic element formation in a wet etchant in general and also its applications to etch stop engineering for bulk-micromachining.

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.

A novel method for nanoprecision alignment in wafer bonding applications

Wafer bonding has been identified as a promising technique to enable fabrication of many advanced semiconductor devices such as three-dimensional integrated circuits (3D IC) and micro/nano systems. However, with the device dimensions already in the nanometre range, the lack of approaches to achieve high precision bonding alignment has restricted many applications. With this increasing demand for wafer bonding applications, a novel mechanical passive alignment technique is described in this work aiming at nanoprecision alignment based on kinematic and elastic averaging effects. A number of cantilever-supported pyramid and V-pit microstructures have been incorporated into the outer circumference area of the to-be-bonded Si chips, respectively. The engagement between the convex pyramids and concave V-pits and the compliance of the support cantilever flexures result in micromechanical passive alignment which is followed by direct bonding between the Si chips. The subsequent infrared (IR) and scanning electron microscopy (SEM) inspections repeatedly confirmed the achievement of alignment accuracy of better than 200 nm at the bonding interface with good bonding quality. The impact and potential applications of the developed alignment technique are also discussed.

Reflow of BPSG for sensor applications

Borophosphosilicate glass (BPSG) has been investigated for both the surface planarization of plasma-etched and refilled trenches before etchback and structural layer deposition to enable the fabrication of genuine surface-micromachined double clamped beams and the smoothing of bulk-micromachined surfaces up to mirror quality for optical applications. Sufficient reflow has been obtained without an excessive reduction of the etch rate in HF using low-pressure chemical vapour deposition (LPCVD) with 8 sccm BCl3 flow at 950 degrees C.

Miniature 10 kHz thermo-optic delay line in silicon

The scanning delay line is a key component of time-domain optical coherence tomography systems. It has evolved since its inception toward higher scan rates and simpler implementation. However, existing approaches still suffer from drawbacks in terms of size, cost, and complexity, and they are not suitable for implementation using integrated optics. In this Letter, we report a rapid scanning delay line based on the thermo-optic effect of silicon at λ = 1.3 μm manufactured around a generic planar lightwave circuit technology. The reported device attained line scan rates of 10 kHz and demonstrated a scan range of 0.95 mm without suffering any observable loss of resolution (15 µm FWHM) owing to depth-dependent chromatic dispersion.

Macroporous silicon formation for micromachining

This paper presents a new technique of micromachining using macro porous silicon. Macro porous silicon is made by electrochemical etching in hydrofluoric acid. The etch rate and the morphology of the etched surface as a function of etch parameters, (current density, applied voltage and HF concentration) are investigated. Optimization of these parameters makes it possible to fabricate a micromechanical structures such as 45 micrometer deep, 3 micrometer wide and 8 micrometer pitch trenches. Furthermore the diameter of the pore is easy to control by adjusting the current density. During the pore formation an increase in the current density leads to an increase in the pore diameter. This does not effect the diameter of existing pores. This connection of the pores under the structure can be achieved. In this way, various kinds of single crystal silicon micromachined structures can be fabricated.

Biomimetic strain-sensing microstructure for improved strain sensor: fabrication results and optical characterization

This paper describes the fabrication, and early optical characterization results of a new biomimetic strain-sensing microstructure. The microstructures were inspired from the campaniform sensillum, a highly sensitive strain sensor found in the exocuticle layer of insects. We investigate the natural strain-sensor characteristics by mimicking some of its simplest structural features. Blind-hole- and membrane-structural features were combined and fabricated as membrane-in-recess microstructure. To investigate the strain-sensing (or strain-amplifying) property of the microstructure, an optical characterization setup was devised based on the interference pattern formed by reflected laser beams from different surfaces of the microstructures. Preliminary qualitative analysis of the results obtained shows unsimilar intensity level changes as a function of spatial location on the membrane, thus indicated the biomimetic microstructures strain-amplifying property. This property could be utilized for future improvement of currently available planar-based conventional strain sensors.