Robert W. Johnstone

Theoretical limits on the freestanding length of cantilevers produced by surface micromachining technology

To determine the maximum possible length of freestanding micromachined cantilevers, in this paper we provide a theoretical analysis of three important forces on cantilevers, namely acceleration, Casimir and Coulomb forces. The analysis provides theoretical limits to cantilever lengths separate from the well-known effects of surface adhesion and capillary collapse. This analysis offers an insight into the problem of in-use stiction in microstructures, which is a major source of functional failure in dynamic micromechanical systems. In this paper we conclude with a table that lists the maximum free standing length of microstructures that would offer reliable operation without stick–slip motion, excluding the possible effects of surface adhesion.

Deep-UV patterning of commercial grade PMMA for low-cost, large-scale microfluidics

Although PMMA can be exposed using a variety of exposure sources, deep-UV at 254 nm is of interest because it is relatively inexpensive. Additionally, deep-UV sources can be readily scaled to large area exposures. Moreover, this paper will show that depths of over 100µm can be created in commercial grade PMMA using an uncollimated source. These depths are sufficient for creating microfluidic channels. This paper will provide measurements of the dissolution depth of commercial grade PMMA as a function of the exposure dose and etch time, using an IPA:H2O developer. Additionally, experiments were run to characterize the dependence of the dissolution rate on temperature and agitation. The patterned substrates were thermally bonded to blank PMMA pieces to enclose the channels and ports were drilled into the reservoirs. The resulting fluidic systems were then tested for leakage. The work herein presents the patterning, development and system behaviour of a complete microfluidics system based on commercial grade PMMA.

Automated assembly of hingeless 90° out-of-plane microstructures

A novel design for hingeless out-of-plane microstructures is presented. These structures can be assembled to 90° by a single-point actuation, which can be provided by, for example, a microelectronics wirebonder or a microprober station. Both wirebonders and microprober stations are commonly available to microfabrication facilities, and therefore the assembly method described here introduces a practical and economical approach to the creation of out-of-plane structures. The microstructure designs can be used in many types of microfabrication processes, and in particular have been fabricated using both PolyMUMPs and an SU-8 technology developed at Simon Fraser University. In addition to the fabricated devices, we will present the results of finite element analysis (FEA). Also reported here are tests for positional repeatability and reliability.

Deep-UV exposure of poly(methyl methacrylate) at 254 nm using low-pressure mercury vapor lamps

Poly(methyl methacrylate) (PMMA) is a transparent thermoplastic with important applications as a positive resist for various radiation sources. When used as a photoresist, PMMA is typically used with wavelengths shorter than 240 nm, as that is the commonly accepted upper limit of effectiveness. However, the authors have shown patterning of nonamplified PMMA films at 254 nm, which is significant because 254 nm radiation can be produced using inexpensive low-pressure mercury vapor lamps. Data for the etch depth as a function of exposure dose (0–12 h), developer temperature (20–35 °C), and etch time were collected. Dissolution rates of up to many microns a minute are possible, and the dissolution rate ratio of exposed over unexposed PMMA can reach over 3000. This demonstrates the feasibility of PMMA exposure using deep-UV at 254 nm.

Polydimethylglutarimide (PMGI) as a sacrificial material for SU-8 surface-micromachining

SU-8 is finding increased use as a structural polymer MEMS material due to its biocompatibility, mechanical properties and low cost. The goal of this work is to expand the use of SU-8 through the creation of SU-8-based surface-micromachining processes that use polydimethylglutarimide (PMGI) as a sacrificial layer. PMGI is a deep-UV positive resist, used mainly for bilayer lift-off processes. PMGI is a good sacrificial layer candidate, as it is spinable at a wide variety of thicknesses, is photopatternable and has a glass transition temperature greater than the processing temperatures required for SU-8. PMGI is shown to be useful as a sacrificial layer for SU-8 surface micromachining processes with one freestanding layer with patterned metal, single-layer devices with more than one thickness, and two layer devices. Two classes of devices were fabricated with the developed processes. The first class of devices are compliant mechanisms, including bent-beam actuators, thermal isolation platforms and out-of-plane grippers. The second class of devices fabricated are freely moving devices such as hinged plates and gears, which require the use of true kinematic joints, such as scissor hinges, staple hinges and pin joints.

Buckled cantilevers for out-of-plane platforms

In this paper, we show how surface-micromachined buckled cantilevers can be used to construct out-of-plane structures. We include the relevant theory necessary to predict the height and angle of plates attached to buckled cantilevers, as well as the mechanical stresses involved in assembly. These platforms can be assembled to any angle between 0° and 90° with respect to the substrate by changing the attachment point and the amount of deflection. Example devices were fabricated using PolyMUMPs™ and assembled. Using these devices, the deflection of the buckled cantilevers was verified, as well as the placement for raised platforms.

Non-uniform residual stresses for parallel assembly of out-of-plane surface-micromachined structures

This note describes a method for the parallel self-assembly of out-of-plane surface-micromachined structures that uses non-uniform residual stresses, inherent in many surface-micromachining processes. The residual stresses are used to achieve a one-time only actuation capable of lifting and assembling raised structures. Theory is provided to calculate the deflection and stiffness of bi-layer cantilevers. Devices for amplifying the vertical deflection are demonstrated and used to assemble large arrays of devices.

Self-assembly of surface-micromachined structures using electrostatic attraction

The paper extends the work done using micro-fabricated hinges in surface micromachining to create fully 3D devices. These devices include free-space micro-optic systems and various sensors. While these applications are interesting, the assembly process is difficult. We present the basic theory and process necessary to perform the assembly using electrostatic interactions. The process is easy and reliable. We were able to lift early prototype mirrors with voltages as low as 35 volts.

Fuse-tethers in MEMS

During the fabrication of freestanding micromechanical structures, the structures must often be attached to the substrate to prevent movement, particularly during the release process. The attachments are then removed, freeing the structures from the substrate when they are to be used. Tethers are long thin beams that mechanically anchor freestanding structures to the substrate during fabrication, but are easily broken afterwards. This paper focused on fuse-tether designs and the associated technique used to break the tethers, Joule heating. The breaking characteristics of two fuse-tether designs were investigated using different current pulses. For each design, the current pulse that produced the most desirable electrical and mechanical break was chosen for reliability testing. The reliability tests resulted in a 100% success rate. However, molten silicon splattered undesirably in 20% of the cases. In addition to empirical testing, ANSYS® was used to simulate the Joule heating process. The ANSYS® model produced results that closely matched the break characteristics observed in the empirical tests. This research demonstrated that a fuse-tether can be severed reliably with the Joule heating technique, and the fuse-breaking characteristics can be predicted by modeling.

New design for surface micromachined bistable and multistable switches

Microsystems often require switches or mechanisms to provide two stable states. In answer to this need, we design a novel micromechanical bistable switch based on the locking mechanism commonly used in extension ladders. This switch was designed and fabricated through the multiuser MEMS processes (MUMPs). Actual performance of the switch was videotaped, analyzed, and compared with theory and simulation. This design was fully functional on the first design iteration, and can easily be extended to provide multiple stable states. We outline the design, simulation, and results from the testing of the microfabricated system.