Andrew H. Cannon

Wafer bonding using microwave heating of parylene intermediate layers

This paper describes a novel wafer bonding technique using microwave heating of parylene intermediate layers. The bonding is achieved by parylene deposition and thermal lamination using microwave heating. Variable frequency microwave heating provides uniform, selective and rapid heating for parylene intermediate layers. The advantages of this bonding technique include short bonding time, low bonding temperature, relatively high bonding strength, less void generation and low thermal stress. In addition, the intermediate layer material, parylene, is chemically stable and biocompatible. This bonding technique can be used for structured wafers also because parylene provides a conformal coating. Therefore, this is a very attractive bonding tool for many MEMS devices. The bonding strength and uniformity were evaluated using diverse tools. Fracture mechanisms and the effects of bonding parameters and an adhesion promoter were also investigated. The bonding with a structured wafer was also successfully demonstrated.

Nanomaterial transfer using hot embossing for flexible electronic devices

We demonstrate hot embossing to pattern carbon nanotubes (CNTs) on flexible substrates. Patterns of CNTs grown on both microtextured and flat silicon templates were transferred into polymer substrates, with good replication of both the CNT patterns and surface relief features. The transferred CNTs formed a highly entangled network with electrical resistance of 1kΩ–9MΩ, depending on growth and embossing conditions. The electrical properties showed a strong sensitivity to both light and temperature. This dry transfer process shows promise for high throughput manufacturing of nanomaterial-based flexible electronic devices.

Casting metal microstructures from a flexible and reusable mold

This paper describes casting-based microfabrication of metal microstructures and nanostructures. The metal was cast into flexible silicone molds which were themselves cast from microfabricated silicon templates. Microcasting is demonstrated in two metal alloys of melting temperature 70 ◦ C or 138 ◦ C. Many structures were successfully cast into the metal with excellent replication fidelity, including ridges with periodicity 400 nm and holes or pillars with diameter in the range 10‐100 μm and aspect ratio up to 2:1. The flexibility of the silicone mold permits casting of curved surfaces, which we demonstrate by fabricating a cylindrical metal roller of diameter 8 mm covered with microstructures. The metal microstructures can be in turn used as a reusable molding tool. (Some figures in this article are in colour only in the electronic version)

Wafer bonding using microwave heating of parylene for MEMS packaging

This paper describes a novel wafer bonding technique using microwave beating of parylene intermediate layers. The bonding is achieved by parylene deposition and thermal lamination using microwave heating. Variable frequency microwave heating provides uniform, selective, and rapid heating for parylene intermediate layers. The advantages of this bonding technique include short bonding time, low bonding temperature, relatively high bonding strength, less void generation, and low thermal stress. In addition, the intermediate layer material, parylene, is chemically stable and biocompatible. This bonding technique can be used for structured wafers as well because parylene provides a conformal coating. Therefore this is a very attractive bonding tool for many MEMS devices. The bonding strength and uniformity were evaluated using diverse tools. Fracture mechanisms and the effects of bonding parameters and an adhesion promoter were investigated as well. The bonding with a structured wafer was also successfully demonstrated.

Molding ceramic microstructures on flat and curved surfaces with and without embedded carbon nanotubes

This paper explores micromolding fabrication of alumina ceramic microstructures on flat and curved surfaces, the transfer of carbon nanotube (CNT) micropatterns into the ceramic and oxidation inhibition of these CNTs through ceramic encapsulation. Microstructured master mold templates were fabricated from etched silicon, thermally embossed sacrificial polymer and flexible polydimethylsiloxane (PDMS). The polymer templates were themselves made from silicon masters. Thus, once the master is produced, no further access to a microfabrication facility is required. Using the flexible PDMS molds, ceramic structures with mm scale curvature having microstructures on either the inside or the outside of the curved macrostructure were fabricated. It was possible to embed CNTs into the ceramic microstructures. To do this, micropatterned CNTs on silicon were transferred to ceramic via vacuum molding. Multilayered micropatterned CNT–ceramic devices were fabricated, and CNT electrical traces were encapsulated with ceramic to inhibit oxidation. During oxidation trials, encapsulated CNT traces showed an increase in resistance that was 62% less than those that were not encapsulated. The processes described here could allow fabrication of inexpensive 3D ceramic microstructures suitable for high temperature and harsh chemical environments.

Self-assembly for three-dimensional integration of functional electrical components

Current microelectronics manufacturing and packaging rely on pick-and-place methods, which are serial manufacturing processes. Self-assembly (SA) is a parallel manufacturing process that can be three dimensional and as such could improve the manufacture of systems that require diverse integration of sensors, actuators, electronics and power sources. This paper describes SA of millimeter-scale parts in which functional electronic components and electrical interconnects were cast into 5 mm cubes of polymethylmethacrylate. Surface forces induced both gross and fine alignment of the cubes. The cubes were bonded using low-melting-temperature alloy, resulting in a self- assembled three-dimensional circuit. This technique could be expanded for assembly of systems having more than 104 components. The ultimate goal is to combine a large number of diverse active components to allow the manufacture of systems having dense integrated functionality.

Microstructured metal molds fabricated via investment casting

This paper describes an investment casting process to produce aluminum molds having integrated microstructures. Unlike conventional micromolding tools, the aluminum mold was large and had complex curved surfaces. The aluminum was cast from curved microstructured ceramic molds which were themselves cast from curved microstructured rubber. The aluminum microstructures had an aspect ratio of 1:1 and sizes ranging from 25 to 50 μm. Many structures were successfully cast into the aluminum with excellent replication fidelity, including circular, square and triangular holes. We demonstrate molding of large, curved surfaces having surface microstructures using the aluminum mold. (Some figures in this article are in colour only in the electronic version)

Hydrophobicity of curved microstructured surfaces

This paper presents measurements and models for how the macroscopic curvature of microstructured polymers affects hydrophobicity. Flexible polymer substrates were fabricated with arrays of regular microstructures. The interaction of liquid drops with these surfaces was analyzed for flat substrates and substrates flexed into either positive or negative cylindrical shapes. Liquid droplet static contact angle and dynamic slide angle were measured for a range of surfaces. An increase in substrate curvature corresponded with decreased slide angle for liquid droplets suspended on the surface asperities. This phenomenon is investigated in terms of solid–liquid contact line and the periodicity of surface microstructures. We present a model that can be used to understand the observed phenomena and to design microstructure geometries for hydrophobicity.

Flexible microdevices based on carbon nanotubes

This work reports the fabrication and testing of flexible carbon nanotube microdevices made using hot embossing material transfer. Both micro-plasma and photodetector devices were made using as-grown unpurified multi-wall carbon nanotubes printed on PMMA substrates. Optical detectors were fabricated by attaching metal wires and monitoring the resistance as a function of light exposure. The electrical resistance of the nanotubes showed a strong sensitivity to light exposure which was also enhanced by heating the devices. While such processes in MWCNTs are not fully understood, the addition of thermal energy is believed to generate additional free charge carriers in the nanotubes. The plasma-generating microdevices consisted of a thin layer of thermoplastic polymer having the CNT electrode on one side and a metal electrode on the reverse side. The devices were electrically tested under atmospheric conditions with 0.01–1 kV ac and at 2.5 kHz, with the plasma igniting near 0.7 kV. The fabrication of these flexible organic devices demonstrates the ability to pattern useful carbon nanotube microdevices in low-cost thermoplastic polymers.

Visualizing contact line phenomena on microstructured superhydrophobic surfaces

The authors introduce a direct method for determining droplet solid-liquid-vapor interfacial geometry on microstructured surfaces. A heated liquid metal droplet of size 18 μl is deposited onto a microstructured surface and then freezes, preserving the microstructure impressions onto the metal surface. Postsolidification microscopy can measure contact line geometry and identify wetting state. This approach can be used to visualize contact line on curved and opaque microstructured surfaces.