The Further Development of Fringe Electric Field Theory for the Design of a Vertical Comb Drive to be used as a Force-Compensation Mechanism in an Interfacial Force Microscope

• Author / Creator

  Gallagher, Else E

• The overall objective of the research presented here is to begin the design of a comb drive that is to be used as a force-compensation mechanism in an interfacial force microscope. More specifically, the objective of this research is to choose the type of comb drive that has the most potential to further the measurement of interfacial forces, and fabricate test specimens of such comb drives that are then used in two studies that are intended to confirm, at least in part, that comb drives have potential to aid in the measurement of interfacial forces, and to further develop the electrostatic theory used to design comb drives in general. In the first study, the lower limit of the spring stiffness that comb drives can easily be fabricated with is explored, as the mechanical resistance they provide will affect the sensitivity of the force-compensation system. The objective of the second study is to examine how the fringe electric fields around the comb teeth (and thus more of the dimensions of the comb teeth) should be included in the calculation of the electrostatic force between the teeth so that the performance of the comb drives may be predicted more accurately. Comb drives are an attractive type of force-compensator because they can be made out of common materials and their electrodes can be automatically aligned with each other during their manufacture. This research focuses on comb drives that have springs that are designed to be compliant in the direction perpendicular to the substrate they are machined on (or vertically) rather than in the direction parallel to the substrate (or laterally), as vertically-oriented springs can be fabricated with a lower stiffness more easily. Vertically-offset comb teeth are designed to complement such springs so that electrostatic forces can be applied to the movable combs in both the upwards and downwards directions, and thus both attractive and repulsive interfacial forces on a probe attached to the movable combs can be compensated for. This research further focuses on vertical comb drives that have opposing comb teeth that are offset by a constant amount along their length, without a ground plane, so that a basic geometry can be considered for the modelling of the electric fields around them. A simple process that can produce comb drives capable of bi-directional vertical electrical actuation was chosen from the literature that created the vertical offset between its combs by etching down the tops of some of their teeth. It was learned that this could be done using only photoresist to mask the rest of the teeth, which had the advantage of protecting the sides of the teeth as well as their tops during the etch. The lower limit of the spring stiffness that the comb drives could be fabricated with was explored by fabricating several comb drives with different spring designs and applying loads to their movable combs through a series of weights and voltages, and measuring the resulting displacements of their springs. The comb drives with the lowest spring stiffnesses are promising candidates for aiding in measuring interfacial forces. Traditionally, an estimate of the net electrostatic force in comb drives that does not include the fringe electric fields around the tops and bottoms of their teeth has been used to choose the dimensions of the teeth. The electrostatic forces in several fabricated comb drives were thus measured – again by measuring the displacements of their movable combs as voltages were applied to them, after the stiffnesses of their corresponding springs had been determined. The results are compared to mathematical models of the fringe electric fields of increasing complexity to estimate the amount of complexity required for an accurate prediction of the electrostatic force. The measurement of the electrostatic forces in the fabricated comb drives also indicated that the current design will only generate about half of the electrostatic force for any given voltage than that predicted by a model that does not include the fringe fields around the comb teeth.

• Subjects / Keywords

  • Microfabrication
  • Fringe electric field
  • Vertical comb drive

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R30K26H34

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.