Ian G. Foulds

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.

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.

Design and characterization of ultra-stretchable monolithic silicon fabric

Stretchable electronic systems can play instrumental role for reconfigurable macro-electronics such as distributed sensor networks for wearable and bio-integrated electronics. Typically, polymer composite based materials and its deterministic design as interconnects are used to achieve such systems. Nonetheless, non-polymeric inorganic silicon is the predominant material for 90% of electronics. Therefore, we report the design and fabrication of an all silicon based network of hexagonal islands connected through spiral springs to form an ultra-stretchable arrangement for complete compliance to highly asymmetric shapes. Several design parameters are considered and their validation is carried out through finite element analysis. The fabrication process is based on conventional microfabrication techniques and the measured stretchability is more than 1000% for single spirals and area expansions as high as 30 folds in arrays. The reported method can provide ultra-stretchable and adaptable electronic systems for distributed network of high-performance macro-electronics especially useful for wearable electronics and bio-integrated devices.

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.

New Movable Plate for Efficient Millimeter Wave Vertical on-Chip Antenna

A new movable plate concept is presented in this paper to realize mm-wave vertical on-chip antennas through MEMS based post-processing steps in a CMOS compatible process. By virtue of its vertical position, the antenna is isolated from the lossy Si substrate and hence performs with a better efficiency as compared to the horizontal position. In addition, the movable plate concept enables polarization diversity by providing both horizontal and vertical polarizations on the same chip. Through a first iteration fractal bowtie antenna design, dual band (60 and 77 GHz) operation is demonstrated in both horizontal and vertical positions without any change in dimensions or use of switches for two different mediums (Si and air). To support the movable plate concept, the transmission line and antenna are designed on a flexible polyamide, where the former has been optimized to operate in the bent position. The design is highly suitable for compact, low cost and efficient SoC solutions.

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.

A versatile multi-user polyimide surface micromachinning process for MEMS applications

This paper reports a versatile multi-user micro-fabrication process for MEMS devices, the “Polyimide MEMS Multi-User Process” (PiMMPs). The reported process uses polyimide as the structural material and three separate metallization layers that can be interconnected depending on the desired application. This process enables for the first time the development of out-of-plane compliant mechanisms that can be designed using six different physical principles for actuation and sensing on a wafer from a single fabrication run. These principles are electrostatic motion, thermal bimorph actuation, capacitive sensing, magnetic sensing, thermocouple-based sensing and radio frequency transmission and reception.

Three-Axis Magnetic Field Induction Sensor Realized on Buckled Cantilever Plate

This work presents the fabrication and characterization of a three-axis induction sensor consisting of one planar microcoil, fixed on the substrate, and two microcoils fabricated on Bbuckled cantilever plates (BCP) oriented perpendicularly to the substrate and each other. The BCP allows an out-of-plane translation while preserving a direct connection to the substrate, which aids the routing of electrical lines. The fabricated sensor is integrated on a single substrate, allowing interaction and integration with other systems. The devices are fabricated using a MEMS polymer fabrication process. Different microcoil configurations are realized with 17-30 turns, 5 μm track width, and 15-20 μm track pitch. The sensor showed up to 6.8 nT/√Hz resolution to magnetic fields within a frequency range of 40 Hz to 1 MHz. The BCP concept provides a strikingly simple method to fabricate a three-axis field sensor that can readily be integrated with electronic circuits, and the sensors performance can easily be adjusted within a wide range by changing the dimensions of the coils.

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.