Stephen F. Bart

Microfabricated electrohydrodynamic pumps

Abstract Pumping is often cited as a general application which motivates the development of microfabricated motors and other actuators. In that spirit, this paper studies microfabricated electrohydrodynamic fluid pumps. In electrohydrodynamic (EHD) pumping, fluid forces are generated by the interaction of electric fields and charges in the fluid. In contrast to forces generated by mechanical pumping using an impeller or bellows, EHD pumping requires no moving parts and consequently offers the possibility of simplified fabrication and high reliability. This paper discusses electrohydrodynamic pumping and issues concerning its use in micronsize scale systems. The fundamental operating principles of the EHD pump are outlined and examples of configurations which meet the requirement for inducing free electric charge are shown. The possible performance achievable in micron size-scale regimes is indicated. Issues concerning fluid conductivity, instability and surface tension are addressed. A microfabricated structure which demonstrates the EHD pumping of a highly insulating silicone oil is described. The structure consists of an array of 10 μm by 235 μm highly doped, LPCVD polycrystalline silicon electrodes patterned over silicon nitride. The electrode array is excited with a traveling wave of electric potential. Pumping results are qualitatively described. This paper describes a study of electrohydrodynamic pumping, including issues concerning its application to micron size-scale systems. The fundamental operating principles of EHD pumps are outlined and the possible performance achievable in micron size-scale regimes is indicated. Surface and bulk instabilities are addressed. Finally, the preliminary results of an EHD pumping experiment with a microfabricated structure are described. A practical requirement for EHD pumping is the induction of free electric charge in the volume of the fluid to be pumped or on its interface with another material. Charge accumulation on a material interface is readily achievable; however, if one material is a fixed rigid wall, such as the wall of a conduit, no pumping can take place. Consequently, the practical application of microfabricated EHD pumps may require the induction of free charge in the volume of the fluid, possibly by temperature-induced conductivity gradients. A further constraint on the usefulness of EHD pumping is its reduced effectiveness with conducting fluids. Hence its usefulness in many situations, including biological environments, may be limited. Another possible difficulty may be fluid instabilities; however, these instabilities may be useful for mixing and cooling purposes. Yet, it remains to be seen if they can compete with molecular diffusion in micron-size scale systems. Finally, it seems clear that surface tension will be a dominant force in virtually any micron-scale system with a liquid surface. In spite of these difficulties, electrohydrodynamic interactions may prove to be a reasonable way to achieve pumping without moving parts.

Design considerations for micromachined electric actuators

Abstract This paper provides a perspective on the design and fabrication of surface-micromachined actuators. An analysis of electromagnetic to mechanical energy conversion indicates that electric drive is preferable to magnetic drive for these microactuators. Planar rotary and linear microactuators can be fabricated by selectively etching multi-layer thin-film sandwiches, with the potential for gaps on the order 1 μm and lateral dimensions on the order of 300 μm. Prototype designs are presented for rotary variable-capacitance and induction micromotors. Rotor speeds of 2.4 × 10 5 rad s −1 (2.3 × 10 6 rpm) and accelerations of 2.9 × 10 9 rad s −2 appear feasible for both. In conclusion, some of the research problems in developing a microactuator technology are identified, including warpage of microstructural films due to residual stress, friction between micromachined surfaces and electric breakdown in small gaps.

A study of three microfabricated variable-capacitance motors

Abstract This paper discusses the design, microfabrication, operating principles and experimental testing of three types of rotary variable-capacitance micromotors. The advantages and disadvantages of these motors are discussed. The three motor types are top-drive, side-drive and harmonic side-drive. In this work, the micromotors are surface micromachined using heavily-phosphorus-doped polysilicon for the structural material, deposited oxide for the sacrificial layers and LPCVD nitride for electrical isolation. Frictional forces associated with electric pull-down forces on the rotor are dominant in the side-drive and harmonic side-drive motors fabricated and tested to date. Air drive and electric excitation have been used in studying these effects. Side-drive micromotors have been successfully operated by a three-phase electrical signal with the rotors air-levitated. With air levitation, successful operation is achieved at bipolar excitations greater than 80 V across 4 μm air-gap motors having eight rotor and twelve stator poles, with only half of the stator poles excited. Motor operation is sustained indefinitely.

Electric micromotor dynamics

The dynamometry technique uses a strobe flash which is triggered from a phase excitation signal after a known time delay. This acts essentially as a video shutter allowing the position of the rotor as a function of the time delay to be recorded and measured. A dynamic model is developed that includes an electrostatic drive term, a velocity-dependent viscous drag term, and a Coulomb friction term that is dependent on the square of the drive voltage and the sign of the velocity. From the position-versus-time data, coefficients for this model are estimated using nonlinear least square error estimation. It is shown that both viscous drag and Coulomb friction terms are required if the model is to closely fit all the experimental data. The motor dynamics are shown to have a weak, if any, dependence on the rotor-bushing apparent area of contact. >

Principles in design and microfabrication of variable‐capacitance side‐drive motors

This paper presents a detailed discussion of the critical issues it the design and fabrication of polysilicon, rotary, variable‐capacitance, side‐drive, electric micromotors. Three different side‐drive motor architectures with stator pole number to rotor pole number ratios of 3:1; 3:2, and 2:1 are considered. For each architecture, output torque characteristics of typical microfabricated motors are simulated using two‐dimensional finite‐element solutions in the plane of the substrate. The 3:2 design is shown to provide superior torque coverage with higher minimum torque values as compared to the other two designs. An examination of the contribution of the axial fringing fields shows that, for typical micromotors, the rotor–stator capacitance is more directly a function of the rotor–stator thickness and not of the vertical rotor–stator pole‐face overlap. Furthermore, since the rotor–stator capacitance is not very sensitive to a vertical offset between the rotor and the stator, electric forces tending to vertically align the rotor to the stator are significantly smaller than would be predicted from a simple parallel‐plate capacitance calculation. A standard and a localized oxidation of silicon (LOCOS)‐based side‐drive micromotor fabrication process are described. The standard process is used as a case study to provide a detailed discussion of practical issues that need to be considered in the development of a polysilicon surface‐micromachined motor fabrication process. Specific motor design examples are described and a brief history of our experimental findings is presented. Typical 3:2 micromotors have been operated with bipolar excitations as low as 37 V across 1.5 μm gaps and at speeds as high as 15 000 rpm.

An analysis of electroquasistatic induction micromotors

Abstract This paper studies the steady-state operation of the electroquasistatic induction micromotor (IM). A rotary pancake IM compatible with surface micromachining serves as an example. A model is developed to predict the electric potential, field and free charge within the IM. The model also predicts the motive torque and transverse force of electric origin acting on its rotor. The torque is balanced against bushing friction and windage to determine rotor velocity. Here, the bushing friction is modeled as a function of the transverse force acting on the rotor. Finally, an equivalent circuit model is developed, which described important aspects of the electromechanical operation of the IM. The model is used to study IM performance and its dependence on IM dimensions and material properties. For example, IM performance is predicted to be a complex function of axial IM dimensions and a strong function of rotor conductivity. The study also reveals that IM performance can differ significally from that of the variable-capacitance micromotor (VCM). For example, the dependence of motive torque and transverse force on axial dimensions can be significanly different in some IM operating regimes, allowing the possibility of improved performance over the VCM. IM and VCM dependences on micromotor geometry, velocity and material properties can also be significanlty different. The excitation and control requirements reflect the difference between a synchronous (VCM) and an asynchronous (IM) motor, as well as the possibility of obtaining an axially stable rotor position for certain IM material parameters.

Operational characteristics of microfabricated electric motors

Abstract We report on the operational characteristics of LOCOS-based microfabricated radial-gap electric motors through lifetime tests, transient measurements, modeling, and parameter extraction. We have found that the reduction of static friction (stiction) in the bearings by the incorporation of a silicon nitride film permits these micromotors to spin in normal air ambients. Frictional drag from the bearing, which results from the electric-based side-pull of the rotor, is found to be the dominant rotor-retarding force and to lead to motor wearout after approximately 10 000 rotor revolutions. Furthermore, the frictional coefficient of the nitride-on-polysilicon micromotor bearing is determined to be 0.36 ± 0.04.

A LOCOS process for an electrostatic microfabricated motor

Abstract A novel process based on local oxidation of silicon (LOCOS) for fabricating planar electric micron-scale radial-gap motors is described. Issues relating to the design, fabrication and performance of these micromotors are discussed, with particular emphasis given to the LOCOS process. Functioning variable-capacitance motors with rotors of 50 μm radii are described in detail and qualitative results of their operation with air levitation are presented. Finally, the integration of electronics with the micromotors on the same substrate is discussed. We have discussed the main issues involved in designing and fabricating electric variable-capacitance motors. Radial-gap motors have been selected for controllable instability and ease of fabrication. Furthermore, by examining the fabrication requirements of high-precision thick structures, we conclude that planarization is required unless greatly increased process complexity can be tolerated. The LOCOS-based process has thus been developed because it is inherently planar, unlike standard surface micromachining processes. Micron-scale motors and other structures have been successfully fabricated using the new LOCOS-based process. These motors were spun electrostatically up to speeds over 10 00O rpm, without visible signs of wear, when levitated by an externally generated air bearing. We are investigating the use of reduced-friction coatings such as Si 3 N 4 [2] or Teflon [19] in the bearing structure to eliminate the need for external air levitation. The inherent planarization of the motor not only facilitates the fabrication but may also facilitate the development of certain applications that require some form of direct cover or package. Starting and load transients must be sensed and controlled for stable VC motor performance. Closed-loop control, however, is virtually impossible witho

Toward the design of successful electric micromotors

Various characteristics of electric micromotors are considered as well as the relations between those characteristics, in an attempt to organize the process by which successful micromotors can be designed. Special attention is paid to electromechanical characteristics. The resulting organization illuminates the interdisciplinary nature of the physics behind micromotor analysis and design and suggests specific research problems which should be pursued to support micromotor design, fabrication, and performance. A simple design example which considers the impact of friction torque on the top speed of both variable-capacitance and induction rotary micromotors is provided for illustration.<<ETX>>

Extraction of compact models for MEMS/MOEMS package-device codesign

MEMS package requirements are by their nature application specific. MEMS devices are often inherently sensitive to stress induced by their packages sand often need direct access to the environment. Therefore, understanding the influence of packaging on MEMS is critical to a successful coupled package-device co-design. Here, an automated package-device interaction simulator has been developed. The simulator uses Finite Element Method models for both the package and the device analysis and ties the result of the simulations together through parametric models. This so- called Compact Model Extraction is an efficient way to solve complex problems. Several examples illustrate the use of this technique.