Keywords

Introduction

The incredible explosion of solid-state technologies in the last 30 years has made the consumer electronics as we know it nowadays: sophisticated electronic equipment are largely available in entertainment, communications, and office productivity. The ability of material science in fabricating, and also mixing together, very different materials, both organic and inorganic, has launched what is expected to be the next technology revolution: the integrated microsystems. Micro (or even nano)-opto-electro-mechanical systems, M(N)OEMS, or micro total analysis systems, micro-TAS, are the acronyms that can be found so often in scientific production (Gad-el-Hak 2010). These devices will combine sensing, processing, actuation, and power management functions in order to achieve multispectral functionality, adaptability in response to a changing environment, and real-time data analysis. Beyond its peculiar and exciting electrooptical and chemical properties, porous silicon has shown, from the beginning of its discovery, very interesting features for integrated microsystems fabrication and applications. Both macroporous structures from n-type silicon and mesoporous layers created from heavily doped silicon are currently used in a wide set of devices, from chemical and biological analytical sensors to micro fuel cell, and many other microsystems can be found in the academic literature and industrial patents. In the following updated review, the range of micromachined devices realized via sacrificial porous silicon is illustrated; porous silicon-based optical or electrical transducers as sensing part of microsystems devoted to biological and chemical monitoring are then presented; finally a range of diverse functions demonstrated within devices is summarized.

Sacrificial Use of Porous Silicon in Microsystems

Macroporous silicon technology found its principal application in integrated microsystems as sacrificial layer: multilayered and suspended structures, such as bridges, membranes, and cantilevers, often require fabrication, almost always by isotropic etching and removing, by alkaline-based water solution, of a porous layer. The thickness of this layer can be up to 100 μm, or more, which is very much greater with respect to those obtained by thin film deposition techniques (<10 μm): in this sense, porous silicon passive layer is an exclusive technology. In Table 1 are reported some references of porous silicon sacrificial layers together with the functionalities of the resulting microsystems.

Table 1 Porous silicon as sacrificial layer
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Silicon on insulator technology and thermal insulation are other important fields where porous silicon morphology plays a key role, and thus, it is often used in complex microsystems: thermal properties of porous silicon layers can be strongly modulated by changing the porosity, i.e., the amount of air present in the silicon volume. On the other hand, pores can be completely filled by silicon dioxide, so that a nanocrystalline film can be transformed by thermal oxidation in an oxide layer preserving the desired geometry. Quite a few works on this subject can be found in the literature (Bomchil et al. 1988; Perichon et al. 2001; Friedberger et al. 2001; Lysenko et al. 2002; Mondal et al. 2009). Chapters in this handbook of relevance include “Oxidation of Mesoporous Silicon,” “RF Electrical Isolation with Porous Silicon,” and “Thermal Isolation with Porous Silicon.”

Porous Silicon-Based Sensing Microsystems

Mesoporous silicon is by far the most intriguing material for optical and electrical monitoring of chemical and biological molecular interactions. The integration of porous silicon transducers in microsystems is not trivial or straightforward: each step of the fabrication process (photolithography, etching, bonding/sealing, inlet/outlet) should be designed and realized, just preserving the physical and chemical characteristics of the sensing material. In case of porous silicon, pores accessibility, surface chemical features, and signal readout (both optical and electrical) should be maintained, if not optimized in microsystems fabrication. The utilization of biological bioprobes, which can recognize specific analytes in complex mixtures, thus enhancing the selectivity of the sensor systems, makes things even more difficult: biological molecules work in the so-called physiological conditions that could not match technological requirements. On the other hand, microsystems can really boost sensing features through electronics and microfluidic circuits. Onboard electronics allow signal amplification, restoration, storage, and transmission, which correspond to higher sensitivity, low drift, and possibility of actuation. Microfluidics ensure small volume consumption, safe operations, rapid analysis time, and protection of the transducer against the environment. For all these reasons, a great effort has been spent in porous silicon integration in microsystems for substance monitoring: Table 2 reports many successful applications of chemical and biological sensing that can be found in literature together with the method of transduction exploited in the device. Chapters in this handbook of particular relevance include “Porous Silicon Gas Sensing,” “Porous Silicon Immunoaffinity Microarrays,” and “Porous Silicon Optical Biosensors.”

Table 2 Porous silicon sensing microsystems
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Microdevices and Microsystems Incorporating Porous Silicon

Spongelike or coordinated ensembles of pores have been attractive for a lot of on-microsystems applications that span a wide range of interesting fields. Porous silicon membranes for hydrogen storage and release have been studied and characterized; mechanical properties of these membranes have been used in integrated microphones; microturbine and field emission devices are among the most advanced MEMS structures that can be obtained by porous silicon micromachining technique: from all those experiments, it results to the conclusion that researchers’ fantasy and imagination are the real limits for porous silicon application in microsystems. Very recently, a burst of porous silicon-based microsystems for biomedical applications appeared in specialized literature: from basic biologic studies, such as cell culturing, to biomedical devices, such as microneedles array or smart patch for drug delivery, have been presented. Table 3 shows the references on this heterogeneous subject. The chapters in this handbook on related topics include “Porous Silicon in Immunoisolation and Bio-filtration,” “Porous Silicon Photonic Crystals,” “Porous Silicon and Micro Fuel Cells,” and “Porous Silicon Based Mass Spectrometry.”

Table 3 Active functions of porous silicon in microdevices and microsystems
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