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 silicon surface-micromachining, for example PolyMUMPs?, it is common to include a metal layer in addition to the doped polysilicon layers. However, the residual stresses of any metal layers do not necessarily match the residual stresses within the polysilicon layers, leading to non-uniform residual stresses within structures. This is most visible in cantilevers, which exhibit significant curvature. This can be either beneficial or detrimental, depending on the application. In either case, it is important to be able to predict the radius of curvature for the cantilever, as well as the final height of the tip. While the importance of this phenomenon has been recognized for some time, the equations available in the literature only reflect specific cases. For example, they consider layers of equal thickness, or non-uniform stresses with a uniform material. Unfortunately, this does not cover all cases, including the particularly important case of PolyMUMPs?. In this paper, we develop, from minimum energy considerations, a completely general equation for predicting both (1) the uniform strain of a cantilever and (2) the radius of curvature of a cantilever. These equations apply to an arbitrary number of layers, each with arbitrary material parameters and residual stress profiles. These results are checked against finite element simulations. The resulting model are also checked against POLY2-METAL cantilevers fabricated using PolyMUMPs?.

Curl and Effective Height of Micromachined Bi-Layer Cantilevers Under Differing Residual Stresses

Polymethyl methacrylate (PMMA) is a transparent thermoplastic with important applications as a positive resist for various radiation sources. For photo-patterning, PMMA is used with wavelengths shorter than 250 nm, as that is the commonly accepted upper limit of effectiveness. However, we have shown patterning of non-amplified PMMA films at 254 nm. Data for the etch depth as a function of dose (0 to 10 hours), developer temperature (20deg C to 30deg C), and etch time was collected. Developer speeds of up several microns a minute are possible, and the selectivity of exposed over unexposed PMMA can reach nearly 650. This demonstrates the feasibility of PMMA exposure using deep UV at 254 nm.

Exposure and Development of Poly (Methyl Methacrylate) Using 254nm Light Source and IPA/Water

When structures fabricated using surface-micromachining contain non-uniform stresses, those structures will deflect vertically during the release step. In this paper, we show how those deflections can be harnessed, and optionally amplified, to assemble out-of-plane components. This method of assembly introduces no new materials or steps into the fabrication process. Structures can be raised to 90° with respect to the substrate, or, with the addition of mechanical interlocks, to any arbitrary angle. In addition to reporting on the assembly of several surface-micromachined optical device using this method, this paper shows a simple free-space optical system. The optical system operates in the plane of the substrate surface, but light is coupled into the system perpendicularly to the substrate surface.

Non-uniform residual stress assembly for free-space optical systems

Polydimethylglutarimide PMGI-based resists are finding increasing
use in microelectromechanical systems MEMS as both sacrificial
and structural materials. PMGI-based resists are commercially
available and were originally designed for use in bilayer lift-off applications.
Literature on deep-UV exposure and development of PMGI films is
limited to films less than 2.5 m in thickness, and use only tetramethylammonium
hydroxide TMAH-based developers. We investigate the
exposure and development of PMGI films greater than 6 m in thickness
using the two main classes of developer for PMGI, TMAH, and tetraethylammonium
hydroxide TEAH-based developers. At these thicknesses,
a nonuniform dose through the film due to the optical absorption of the
PMGI leads to large gradients in the dissolution properties. We report
etch rates as a function of surface dose and development time. Additionally
a model is developed to provide a basic predictor of development
depth and other important data for fabrication process planning and
development.

R.W. Johnstone

Papers

Curl and Effective Height of Micromachined Bi-Layer Cantilevers Under Differing Residual Stresses

Exposure and Development of Poly (Methyl Methacrylate) Using 254nm Light Source and IPA/Water

Non-uniform residual stress assembly for free-space optical systems

Exposure and development of thick polydimethylglutarimide films for MEMS applications using 254-nm irradiation

Exposure of Thick Polymethylglutarimide Films for Structural MEMS