Porous Silicon Integrated Photonic Devices for Biochemical Optical Sensing

Abstract

IntroductionIn the last few years, great efforts have been spent in the development of integrated microsystems, devices of few square centimeters in size including microsensors, microfluidic components, reaction chambers, detectors, and so on.More than a simple ensemble of devices, this is a new research field that combines the properties and characteristics of different materials to find innovative and affordable solutions in applications such as sensing, biotechnology, analytical chemistry.The device miniaturization not only means lower costs through mass production, but also improvement in terms of analysis time, simplicity of use and decrease in consumption of materials (reagents and analytes) (Chandrasekaran et al., 2007).The integrated devices are largely made of silicon but can also include a microfluidic systems; for this reason, their technology is based both on the techniques used in integrated circuit manufacturing and on "soft" fabrication methods (Xia & Whitesides, 1998).In this chapter, we describe the fabrication and the characterization of integrated photonic devices based on nanostructured silicon for biochemical optical sensing.The porous silicon (PSi) is fabricated by electrochemical etching of doped crystalline silicon in an aqueous solution of hydrofluoridric acid.It can be simply described as a network of air holes in a silicon matrix: its dielectric properties, and in particular the refractive index, depend on the content of void, which can be accurately controlled by tuning the process parameters, so that different structures (Fabry-Perot interferometer, Bragg mirror, optical microcavity, aperiodic multilayered sequences) showing good quality optical responses can be obtained.Like other porous materials, PSi is an ideal platform for biosensing due to its high specific surface area (~100 m 2 cm -3 ) which assures an efficient interaction with the species to detect.However, the integration of PSi sensing structures in a microsystem is not straightforward: its surface instability and the low compatibility with alkaline treatments, frequent in devices fabrication, are severe limitations in this application field.In this chapter, we analyze these technological limits and propose solutions that have led to the realization of innovative and high-performant integrated devices using porous silicon as functional platform in bio-analysis experiments. Properties of porous siliconPSi is a very versatile material due to its peculiar morphological, physical, and chemical properties: evidence of this is the huge number of papers about PSi features and devices www.intechopen.comCrystalline Silicon -Properties and Uses 276 based on this nanostructured material that appear in the literature every year.One reason for this clear success is the easy fabrication of sophisticated optical multilayers, such as one-dimensional photonic crystals, by a simple, but not trivial, computer-controlled electrochemical etching process. Porous silicon fabrication by electrochemical etchingPorous silicon was discovered in 1956 by the Uhlirs at Bell Labs, USA, during a study on the electropolishing of crystalline silicon in an HF-based solution.They observed the formation of a deposit "tentatively supposed to be a Si suboxide" (Uhlir, 1956).The scientific community was not much interested in porous silicon until to 1990 when Leigh Canham, working at the Defence Research Energy in England, demonstrated an efficient tunable room temperature light output from the material (Canham, 1990).In the years later, thousands of papers were published on porous silicon and its potential applications in microelectronics, optoelectronic devices, chemical and biological sensing.