M. Parameswaran

A planar self-sacrificial multilayer SU-8-based MEMS process utilizing a UV-blocking layer for the creation of freely moving parts

This work presents the first-reported freely moving parts, including out-of-plane hinges and rotating parts, fabricated in SU-8 that require no post-development bonding step. The technology used is a PolyMUMPs®-like SU-8 process developed by the authors, called the planar self-sacrificial multilayer SU-8 (PSALMS) process. The PSALMS process allows the independent patterning of planar SU-8 layers, so that the SU-8 acts as both the structural and sacrificial material. Each layer is a bilayer consisting of a standard SU-8 layer below a layer of SU-8 modified to heavily absorb UV by the addition of SC1827 resist. The use of this bilayer structure creates a greatly increased processing window, in which an exposure can expose the uppermost bilayer without affecting underlying bilayers. The ability to independently pattern layers allows for the creation of overhanging structures and freely moving parts. So far the PSALMS process has incorporated four structural layers, which allows for the creation of structures such as gears and out-of-plane hinges. A description of the underlying fabrication principle and processing details is presented in this paper. Also presented are gears and hinges that have been fabricated and display proper functionality.

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.

Control of the out-of-plane curvature in SU-8 compliant microstructures by exposure dose and baking times

The effect of processing conditions on the curvature of SU-8 cantilevers up to 6400 µm long is determined by experimental methods. Our observations suggest that a zero curvature condition can be achieved by controlling the exposure dose and post-exposure baking time for any given SU-8 thickness. The curvature of SU-8 cantilevers processed with different exposure doses and post-exposure baking times has been measured and reveals that the gradient of crosslinking density throughout the SU-8 film is the cause of out-of-plane stress. A general model for the SU-8 cantilever curvature is developed which explains the behavior of SU-8 structures that have been processed with different conditions and can be used to predict the general effect on the SU-8 cantilever curvature for different SU-8 thicknesses and process parameters.

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.

Microwave-induced, thermally assisted solvent bonding for low-cost PMMA microfluidic devices

We present a low-cost bonding method for polymethylmethacrylate (PMMA) microfluidics that combines elements of solvent bonding, thermal bonding and microwave bonding. Rather than using specialized equipment, we take household equipment and combine it to produce an effective bonding method that borrows from food packaging technologies for selective heating in a microwave. A poor solvent for PMMA is applied between two halves of a microfluidic system and clamped together using miniature binder clips. Excess solvent from the channels is then drawn out via capillary action and avoids channel clogging during the bonding process. Placing the whole apparatus in a commercial microwave will heat up the thin metal clips and cause the solvent to dissolve and bond the PMMA interface. The whole bonding process takes only a few minutes, and results in high bond strengths.

A hybrid sequential deposition fabrication technique for micro fuel cells

Micro fuel cell systems have elicited significant interest due to their promise for instantly rechargeable, longer duration and portable power. Most micro fuel cell systems are either built as miniaturized plate-and-frame or silicon-based microelectromechanical systems (MEMS). Plate-and-frame systems are difficult to fabricate smaller than 20 cm3. Existing micro fuel cell designs cannot meet the cost, scale and power requirements of some portable power markets. Traditional MEMS scaling advantages do not apply to fuel cells because the minimum area for the fuel cell is fixed by the catalyst area required for a given power output, and minimum volume set by mass transport limitations. We have developed a new hybrid technique that borrows from both micro and macro machining techniques to create fuel cells in the 1–20 cm3 range, suitable for cell phones, PDAs and smaller devices.

An introduction to surface-micromachining

An Introduction to Surface-Micromachining provides for the first time a unified view of surface-micromachining. Building up from the basic building block of microfabrication techniques, to the general surface-micromachining design, it will finish with the theory and design of concrete components. An Introduction to Surface-Micromachining connects the manufacturing process, microscale phenomena, and design data to physical form and function. This book will be of interest to mechanical engineers looking to scale down into micromachining and microelectronics designers looked to move horizontally to micromachining.

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.