Jeff R. Mollinger

Magnetic-field measurements using an integrated resonant magnetic-field sensor

The present paper introduces a magnetic-field sensor based on a resonating single-crystal silicon structure. The excitation of the resonator is achieved by the Lorentz force generated by a sinusoidal current flowing through a rectangular coil deposited on the surface of the structure. The amplitude of the vibration, which is proportional to the magnetic field, is detected by sensing capacitors. Because of the high-quality factor of the resonator, a lower detection limit of 1 nT, or even smaller might be realised when the device is vacuum-packaged. This paper describes the working principle, the fabrication procedure as well as open- and closed-loop measurement results.

Quality factor of torsional resonators in the low-pressure region

Abstract This paper reports the calculation of the Q -factor of a torsional resonator in the low-pressure region if the vibration occurs in free space or close to another surface. The calculation is based on the model used by Christian but a modified Maxwell-Boltzmann distribution, the Maxwellian-Stream distribution, is applied to describe the velocity distribution of the gas particles colliding with the resonator. The result is an increased damping factor or lower quality, which matches much better with published measurements.

Through-wafer interconnect technology for silicon

This paper presents a novel method for creating through-wafer interconnects via an anisotropically etched groove in a (100)-silicon wafer. The idea is based on the realization of interconnection lines on the inclined sidewalls of the anisotropically etched grooves, which transfer the metalization to the back side of the wafer without open through-holes. The process itself is compatible with standard semiconductor technology and can be applied at the wafer level resulting in low packaging costs. All post interconnect processes are developed independently and can be added to any IC fabrication process. They are performed at the back side of the wafer at the packaging step, so during this processing the front side of the wafer can be protected from scratches and pollution. The key feature of the method presented is the coating of the anisotropically etched grooves with a polyimide and an electrodeposited photoresist. Further, methods to improve the photoresist uniformity over three-dimensional structures are discussed. Copper interconnects have been realized to show the feasibility of this through-wafer technique for front-to-back electrical interconnections. The thickness of the copper interconnects has been increased by copper electroplating to reduce their electrical resistance further and to increase their mechanical strength.

Low-cost plastic sensor packaging using the open-window package concept

Abstract This paper presents a low-cost transfer mould packaging concept for sensors, based on a direct-mould principle. The advantage of this new packaging concept, which we call the open-window package, is that up to the moulding step the whole assembly process is compatible with the standard lead-frame processing. The open-window package concept allows plastic sensor packaging with one or more environmental access paths to be created in a single moulding step. As an application of this new packaging concept, a plastic sensor package with a single access path has been developed for commercial production. The performances of this new low-cost package are satisfactory and lifetime performances show promising results. Furthermore, packaging of other types of sensors is in preparation.

Patterning of polyimide and metal in deep trenches

Abstract This paper presents a method to spin coat and pattern photo-sensitive polyimide in deep grooves and cavities. The polyimide can be used as dielectric layer to isolate metal interconnects from each other and from the substrate. The advantage of using a polyimide base layer for the metallization is that it smoothens the corners of the grooves and cavities to facilitate a good patterning of subsequent metal layers with standard resist process. Another advantage of polyimide over oxide or nitride for passivation is that relatively thick polyimide layers can be applied to minimize the capacitive coupling of the metallization to the substrate. The coating is performed in two steps. In the first step the wafer is spin coated in a conventional way in saturated solvent atmosphere and after that the wafer is rotated again, upside down to improve the uniformity of the layer. The preliminary coating with polyimide facilitates a uniform coating with standard photoresist for patterning metal layers. Additional improvement in the corner coverage is achieved by smoothening all obtuse corners by a short etch dip of the wafer in TMAH prior to polyimide coating.

Versatile tool for characterising long-term stability and reliability of micromechanical structures

Micromechanical devices have a wide range of applications in the near future. Therefore it is important to study their long-term behaviour under various conditions and environments. We introduce atomic force microscopy as a comprehensive tool in reliability study of MEMS devices. Mechanical properties of micromechanical structures can be characterised with high accuracy. Real-time monitoring of these properties during accelerated ageing tests gives information about the long-term stability of the structures. The AFM is a versatile instrument for predicting long-term operating of micromachined devices in a convenient way without building complicated experimental set-ups.

Reliability of silicon nitride as structural material in MEMS

Micromechanical devices have just entered out everyday life and have a wide range of applications in the near future. Therefore a strong need arises for elaborate study on reliability of micromechanical structures under various conditions and environments. Silicon nitride is one of the basic structural materials in MEMS devices due to its good mechanical properties. Extreme aspect ratio structures like long cantilevers or large membranes are often built of thin silicon nitride films. They serve mostly as mechanical supporting components forming multilayer structures. Precise characterization of the mechanical properties is required for proper design and reliable device operation. Atomic force microscope was utilized for measurement of Youngs modulus and spring constant of silicon nitride cantilever beams. Static and dynamic accelerated aging tests were conducted in order to predict the long-term behavior of the structures. Two reversed failure mechanisms were found during the aging tests, both substantially effecting the device operation.

Continuous Electrodeless Dielectrophoretic Separation in a Circular Channel

We present a novel continuous electrodeless separation structure based on dielectrophoresis (DEP). The non-uniform electric field is generated by applying voltage over a circular channel. Driven by the electro-osmotic flow, the particles with different dielectric properties move continuously to the different location across the channel as they flow due to the different DEP force, thus continuously separated into the different outlets. The finite element modelling and simulation results show it can separate particles of different dielectric properties in both spatial and time domain. Compared with the previously reported dieletrophoretic separation using electrode arrays [1-10], this structure is more easily fabricated, mechanically robust and chemically inert. And compared with the previously reported electrodeless dielectrophoretic separation methods [11-14], this structure achieves higher throughput and continuous separation.

Environment-induced failure modes of thin film resonators

Resonant mode micromechanical devices have great potentials due to their high sensitivity and easy signal processing. As they are also sensitive to environmental effects, vacuum packaging is often required, which largely increases the costs. The current study focuses on such environment induced reliability problems and degradation processes. Stiffening effect was observed on thin silicon nitride and silicon carbide cantilever beams in air. The resonance frequency gradually increases in time. When the cantilever is subjected to mechanical shock or large deflection, the resonance frequency suddenly drops, and then increases again. Air, increased humidity, argon rich and nitrogen rich atmosphere influence the stiffening and the shock response behavior. The effects are explained with the surface oxidation model. The oxide layer introduces stress in the structure increasing the overall stiffness, while mechanical shocks crack the layer. Silicon resonators gather airborne particles from the atmosphere due to electrostatic charging. The extra mass results in decrease of the resonant frequency. All these processes lead to unstable resonance frequency and thus to failure of the resonant mode device. Tests in inert environment suggest cheap atmospheric packaging solution to obtain reliable operation and yet good performance.

Time of flight technique used for measuring position and orientation of laparoscopic surgery tools

An ultrasound wireless positioning system is developed to localize the laparoscopic instruments inside the patients body. The cross correlation method applied for measuring time of flight of ultrasound bursts yields 1 mm resolution in determining the instrument position. The 90/spl deg/ or respectively 180/spl deg/ phase-shift is introduced after the first half of the ultrasound burst.