Ronald B. Zmood

Prototype feedback-controlled bidirectional actuation system for MEMS applications

We have successfully developed a one degree-of-freedom microsuspension system, with active position control, as a paradigm of a micromagnetic bearing. This system integrates an electromagnetic actuator, a position sensor, and a feedback control system that provides active position control. This paper discusses the design and fabrication details of the microelectromechanical system (MEMS) components: the beam mass structure integrated with a drive coil and metallized targets, spacer plate, and sensor coils. It also discusses their integration with millimagnets and electronics. Noncontact magnetic bearings based on this principle have the potential of overcoming the tribo-physical issues associated with active MEMS devices.

Selective electroless copper plating on silicon seeded by copper ion implantation

Abstract We report on the successful use of copper (self) ion implantation into silicon to seed the electroless plating of copper on silicon (100) surfaces. Copper ions were implanted into silicon to doses of 5 × 1014−6.4 × 1016 ions cm-2 using a metal vapour vacuum arc ion implanter at extraction voltages of 10 kV and 20 kV. A copper film was then deposited onto implanted silicon using a commercial electroless plating solution. The ion energy was kept low enough to facilitate a low critical ‘seed’ threshold dose which was measured to be 2 × 1015 Cu ions cm-2. Test patterns were made using polyimide to study the adaptability of this technqiue to forming thick structures. Plated films were studied with Rutherford backscattering spectrometry, scanning electron microscopy (SEM), profilometry, energy-dispersive X-rays and Auger electron spectroscopy. The adhesion of films was estimated by a ‘Scotch tape test’. The adhesion was found to improve with increasing dose. However, high internal stress. Detailed examinations of the top and bottom of the film establish that delamination takes place at the amorphous-crystalline interface of the implanted silicon. SEM results show that the films grow first as isolated islands which become larger and eventually coalesce into a continuous film as the platingtime is increase.

Design and fabrication of a micro magnetic bearing

A micro magnetic bearing actuator has been designed using electromagnets. In this design, the rotor position is actively controlled in the radial directions and passively supported in the axial direction. A micro position sensor, along with a proportional plus derivative (PD) control system, constitutes the feedback network, which ensures that the rotor is actively suspended in the radial directions. The circular stator has four control coils which are sandwiched between two stator end plates. The diameter of the stator and rotor are 2.1 and 2.6 mm respectively, while the thickness is fixed at 250 µm. The air gap between the stator and rotor has been fixed at 10 µm. The stator and rotor plates were fabricated using Permalloy electroplating, while the control coils of the stator were hand-wound using conventional coil winding techniques. The bearing components were assembled using normal micro assembly techniques. The details of the fabrication and assembly techniques employed for the micro bearings are presented, along with the test results.

Wide-band low-input-impedance trans-impedance amplifiers for instrumentation applications

In this paper we investigate trans-impedance amplifiers which can be used to minimize the influence of stray capacitances on the operation of signal conditioners used for instrumentation applications such as inductive microsensors for micromachines. The effect of stray capacitances can be minimized when using this type of amplifier by ensuring that its input impedance is small. A technique for realizing very low input impedances over a wide bandwidth for these amplifiers is studied both analytically and experimentally for transistors having a wide range of gain - bandwidth products . In addition an improved circuit configuration which further reduces the amplifiers input impedance at high frequencies is investigated by computer simulation.

Smart magnetic structures for MEMS

Abstract By exploiting the special properties of magnetic materials, magnetic microelectromechanical system (MEMS) technology offers many challenging opportunities for useful device development in the future. This article discusses some of the magnetic materials used in MEMS devices and methods of fabricating them. Some key design issues are addressed, and applications of these technologies to electromagnetic devices developed at RMIT and to thermally controlled magnetic devices are examined.

A simplified approach for accurate estimation of transverse magnetic forces in micro-actuators having inclined pole faces

This paper proposes a fringing range method for accurately evaluating transverse magnetic forces in micro-actuators having inclined pole faces. In this method, the fringing flux in an air gap is assumed to be uniformally distributed in a range that is represented by a fringing coefficient, which is then adjusted systematically to ensure the magnetic forces are accurately determined under defined conditions of actuator physical symmetry. Although this approach can be generalized to other cases, this paper focuses on its application to the case of semi-infinite inclined pole faces. It is shown that the fringing range method can be used with good accuracy if the pole thickness is much larger than the air-gap length. This method was applied in designing a micro magnetic bearing actuator whose diameter is 2.6 mm.

Versatile microsensor for micromachine applications

Most conventional eddy current sensors are used for position measurements where the target approaches the sensor coil along its axial direction. In this paper we investigate a versatile eddy current micro sensor that can not only be used for measuring displacements parallel to the axis of the sensor coil but also for transverse displacement measurement in micro magnetic bearings for micro machines. In the transverse mode, the sensor coil impedance variation has been typically found to be less than when used in the axial mode. A number of approaches and techniques have been investigated and will be discussed to overcome the problem of low sensitivity. As a result a versatile eddy current sensor has been developed which is suitable for transverse position measurement in micro magnetic bearings. Experimental results shown that micro sensor sensitivities up to 20mV/micrometers can be achieved when the target moves between two sensor coils. The performance of the micro sensor suggests that it will have considerable scope for application in the micro machine are.

Modeling and control of a micro magnetic bearing

A prototype micro magnetic bearing actuator having an outside diameter of 2.6 mm has been fabricated at Royal Melbourne Institute of Technology and reported previously with an emphasis on fabrication issues. The discussion of this paper will focus on the design and control of the micro magnetic bearing with a particular emphasis on control system design and analysis. To this end, a simplified dynamic model for the micro actuator has been developed and used in determining controller parameters, leading to a successfully suspended non-rotating rotor in the micro magnetic bearing system.

Magnetic MEMS used in smart structures which exploit magnetic materials properties

Magnetic MEMS technology, by exploiting the special properties of magnetic materials, offers many challenging possibilities for useful device development in the future. In this paper we explore some of the magnetic materials used in MEMS devices, and methods of fabricating them. Some of the key design issues are briefly addressed and applications of this technology to electromagnetic devices developed at RMIT and to thermally controlled magnetic devices, which are of increasing interest, are examined.

Micro displacement sensing system and its application to micro magnetic bearings

In this paper we present a novel displacement sensing system where the sensing direction is in the plane of the planar sensor coils. Particular emphasis in this work is given to the design and micro fabrication of the sensor coils. It has been found that the position of the sensor coils is extremely important as the location of the sensor coils relative to the target significantly affects the sensitivity of the resultant sensing system. Extensive experiments have been carried out and show that best sensitivity is achieved when the sensor coil is located so that the overlap area between the rotor and the sensor coil turns changes most rapidly with the rotor displacement. Following the preliminary analysis and experiments, new optimized sensor coils have been designed and micro fabricated using UV lithography and electroplating, as detailed in the paper. Performance testing of the resultant sensing system has been carried out and is reported in the paper.