Since the late 1980s there have been spectacular developments in micromechanical or microelectro-mechanical (MEMS) systems which have enabled the exploration of transduction modes that involve mechanical energy and are based primarily on mechanical phenomena. As a result an innovative family of chemical and biological sensors has emerged. In this article, we discuss sensors with transducers in a form of cantilevers. While MEMS represents a diverse family of designs, devices with simple cantilever configurations are especially attractive as transducers for chemical and biological sensors. The review deals with four important aspects of cantilever transducers: (i) operation principles and models; (ii) microfabrication; (iii) figures of merit; and (iv) applications of cantilever sensors. We also provide a brief analysis of historical predecessors of the modern cantilever sensors.

Cantilever transducers as a platform for chemical and biological sensors

This manuscript describes the use of water-soluble polymers for use as sacrificial layers in surface micromachining. Water-soluble polymers have two attractive characteristics for this application: 1) They can be deposited conveniently by spin-coating, and the solvent removed at a low temperature (95-150 degrees C), and 2) the resulting layer can be dissolved in water; no corrosive reagents or organic solvents are required. This technique is therefore compatible with a number of fragile materials, such as organic polymers, metal oxides and metals-materials that might be damaged during typical surface micromachining processes. The carboxylic acid groups of one polymer-poly(acrylic acid) (PAA)-can be transformed by reversible ion-exchange from water-soluble (Na+ counterion) to water-insoluble (Ca2+ counterion) forms. The use of PAA and dextran polymers as sacrificial materials is a useful technique for the fabrication of microstructures: Examples include metallic structures formed by the electrodeposition of nickel, and freestanding, polymeric structures formed by photolithography.

Water‐Soluble Sacrificial Layers for Surface Micromachining

▪ Abstract Polysilicon surface micromachining is advancing significantly and many new applications are moving beyond the prototyping phase. Recent technical successes are leading to excitement concerning various uses of devices in optical, wireless, sensor, and many other areas. Incorporation of state-of-the-art integrated circuit (IC) fabrication methods, such as planarization by chemical mechanical polishing (CMP), has enabled extension to a five-level technology. This has opened significant design space, especially for microactuator applications. Recent advancement of in situ microdiagnostics for materials and surface properties has enhanced our understanding of device reliability and performance and will allow devices to operate near well-known materials limits. New IC-compatible materials will further enhance the capabilities of microsystems in terms of performance, reliability, and operation in harsh environments.

IC-Compatible Polysilicon Surface Micromachining

Core/shell nanoparticles were first reported in the early 1990s with a simple spherical core and shell structure, but the area is gradually diversifying in multiple directions such as different shapes, multishells, yolk/shell etc., because of the development of different new properties of the materials, which are useful for several advanced applications. Among different sub-areas of core/shell nanoparticles, yolk/shell nanoparticles (YS NPs) have drawn significant attention in recent years because of their unique properties such as low density, large surface area, ease of interior core functionalization, a good molecular loading capacity in the void space, tunable interstitial void space, and a hollow outer shell. The YS NPs have better properties over simple core/shell or hollow NPs in various fields including biomedical, catalysis, sensors, lithium batteries, adsorbents, DSSCs, microwave absorbers etc., mainly because of the presence of free void space, porous hollow shell, and free core surface. This review presents an extensive classification of YS NPs based on their structures and types of materials, along with synthesis strategies, properties, and applications with which one would be able to draw a complete picture of this area.

Yolk/shell nanoparticles: classifications, synthesis, properties, and applications

Microactuators and their technologies

Silicon dioxide sacrificial layer etching has become a major surface micromachining method to fabricate microsensors and microactuators often made of polycrystalline silicon. An overview of the materials available in integrated circuit manufacturing is given, and the etch mechanism and sacrificial layer etch kinetics are reviewed. Selectivity issues important for the proper choice of layers and etchants are addressed discussing the chemical attack of aluminum during long sacrificial layer etching, as an example. Various etchants known from other studies are compared: concentrated and dilute HF, buffered HF (BHF), nitric acid based etchants known as P-etch, R-etch, S-etch, as well as mixtures of HF and HCl, and vapor HF. `Pad-etch', an acetic acid/ammonium fluoride/ethyleneglycole solution is shown to have an enhanced selectivity against aluminum. Some device examples such as arrays of deflectable micromirrors demonstrate the versatile application of sacrificial oxide etching in surface micromachining.

https://iopscience.iop.org/article/10.1088/0960-1317/7/1/001/pdf

Silicon dioxide sacrificial layer etching in surface micromachining

Thermally actuated micromirrors fabricated using a standard CMOS process and one subsequent anisotropic silicon etch step are presented. The device consists of a mirror plate supported by bimorph cantilever beams. Even short beams show a large deflection effect. A theoretical analysis valid for n-morph bending beams has been derived and found to be consistent with experimental and computer-simulation results.

Thermally actuated CMOS micromirrors

A low-cost process approach for fabricating electrostatic aluminum actuators is presented. The second metallization step of a CMOS process is done in two successive passes. The layer thickness of each pass is chosen to the mechanically desired value to fonn the aluminum actuator. The total metal thickness after both passes corresponds to the standard thickness of metal 2. Thus, the second CMOS metallization can be used for circuitry as well as for mechanical devices. Using the first metallization as sacrificial layer, the actuator is released in a post-processing etch. As an example, deflectable micro mirrors are presented. Using coupled finite element methods, electric fringing fields and stress could be included in the theoretical analysis.

F.-P. Steiner

Papers

Silicon dioxide sacrificial layer etching in surface micromachining

Thermally actuated CMOS micromirrors

Double Pass Metallization For Cmos Aluminum Actuators

REVIEW ARTICLE: Silicon dioxide sacrificial layer etching in surface micromachining