The field of cantilever-based sensing emerged in the mid-1990s and is today a well-known technology for label-free sensing which holds promise as a technique for cheap, portable, sensitive and highly parallel analysis systems. The research in sensor realization as well as sensor applications has increased significantly over the past 10 years. In this review we will present the basic modes of operation in cantilever-like micromechanical sensors and discuss optical and electrical means for signal transduction. The fundamental processes for realizing miniaturized cantilevers are described with focus on silicon- and polymer-based technologies. Examples of recent sensor applications are given covering such diverse fields as drug discovery, food diagnostics, material characterizations and explosives detection.

http://iopscience.iop.org/article/10.1088/0034-4885/74/3/036101/pdf

Cantilever-like micromechanical sensors

Lab-on-a-chip devices

In this Review, the interplay between materials and microfluidics is examined, with the discussion focused on how recent advances in materials fabrication have expanded the frontiers of microfluidic platforms and how the new microfluidic capabilities are, in turn, furthering materials design.

Interplay between materials and microfluidics

We report the successful fabrication and testing of 3D printed microfluidic devices with integrated membrane-based valves. Fabrication is performed with a low-cost commercially available stereolithographic 3D printer. Horizontal microfluidic channels with designed rectangular cross sectional dimensions as small as 350 μm wide and 250 μm tall are printed with 100% yield, as are cylindrical vertical microfluidic channels with 350 μm designed (210 μm actual) diameters. Based on our previous work [Rogers et al., Anal. Chem. 83, 6418 (2011)], we use a custom resin formulation tailored for low non-specific protein adsorption. Valves are fabricated with a membrane consisting of a single build layer. The fluid pressure required to open a closed valve is the same as the control pressure holding the valve closed. 3D printed valves are successfully demonstrated for up to 800 actuations.

/pdf/3d-printed-microfluidic-devices-with-integrated-valves-3uvp3g9m5s.pdf

3D printed microfluidic devices with integrated valves

Casimir effect on the critical pull-in gap and pull-in voltage of nanoelectromechanical switches is studied. An approximate analytical expression of the critical pull-in gap with Casimir force is presented by the perturbation theory. The corresponding pull-in parameters are computed numerically, from which one can notice the nonlinear effect of Casimir force on the pull-in parameters. The detachment length has been presented, which increases with increasing thickness of the beam.

Casimir effect on the pull-in parameters of nanometer switches. Microsys Technol

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.

/pdf/buckled-cantilevers-for-out-of-plane-platforms-whax4knrn2.pdf

Buckled cantilevers for out-of-plane platforms

Reliable microfabrication processes and materials compatible with complementary metal-oxide semiconductor (CMOS) technology are required by industry for the mass production of complex and highly miniaturized lab-on-a-chip systems. Photopolymers are commonly used in the semiconductor industry, and are suitable for the integration of multilayer structures onto CMOS substrates. This paper describes a novel photopolymer bonding process compatible with CMOS technology for the fabrication of three-dimensional monolithic microfluidic devices. The process consists of the formation of a conformal adsorbate film (CAF) approximately 15 nm thick on a patterned photopolymer layer (KMPR), thereby increasing the number of open polymer chains at the bonding interface and acting as an ultra-thin adhesive layer. This thin adhesive layer is made of the same photopolymer as the microfluidic structures, but has a substantially lower crosslinking density so it will be able to make better bonds during a thermocompressive bonding step. This CAF treatment substantially improves the bonding yield between two patterned and previously crosslinked photopolymer layers because both optimum structure strength (to resist deformation during bonding) and bonding strength from epoxy crosslinking can be achieved. We demonstrate high bonding yields of up to 99% of the useful area of the substrate after three successive bonding steps. With this technique, up to six layers have been bonded in a single device. Unlike previously reported methods the quality of bonding is mostly decoupled from soft-bake parameters and crosslinking level of the previously patterned layers. Three differentbonding processes were characterized to describe the bonding mechanism and the differences between the presented method and the partial-crosslinking bonding method. Capillary filling experiments were performed in microchannels of multilayer structures built with the CAF technique, without any observable leakage between layers.

/pdf/multilayer-bonding-using-a-conformal-adsorbate-film-caf-for-4th1pdjokr.pdf

Multilayer bonding using a conformal adsorbate film (CAF) for the fabrication of 3D monolithic microfluidic devices in photopolymer

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.

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

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.

Fuse-tethers in MEMS

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.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

R.W. Johnstone

Papers

Buckled cantilevers for out-of-plane platforms

Multilayer bonding using a conformal adsorbate film (CAF) for the fabrication of 3D monolithic microfluidic devices in photopolymer

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

Fuse-tethers in MEMS

Self-assembly of surface-micromachined structures using electrostatic attraction