**1. Introduction**

There are a vast number of applications for biosensors ranging from medical monitoring and control, to release of drugs [1], and biosecurity [2]. The goblal market for biosensors in 2012 is estimated to reach 8.5 billion USD and projected to reach 16.8 billion by 2018 [3]. Po‐ rous silicon (p-Si) offers several advantages for its use as a biosensor such as a large specific surface area (of the order of 500 m 2 cm−3) [4], visible luminescence at room temperature [5] and biocompatibility [6]. The p-Si was accidentally discovered when, in 1956 at the U.S. Bell Laboratories, Arthur Uhlir Jr. and Ingeborg Uhlir observed a red-green film formed on the wafer surface while trying a new technique for polishing silicon (Si) crystalline wafers. At the time however, it was not considered an interesting material. But when Leigh Canham in 1990 [5] discovered its visible luminescence properties, researchers started studying its non‐ linear optical, electric and mechanical properties. These academic and technological efforts have permitted the fabrication of uniform porous layers with diameters as small as one nanometer, permitting an enormous inner surface density, which is useful for biosensing ap‐ plications. Several techniques exist to form this structure from a pure silicon crystalline wa‐ fer. The most popular is the electrochemical etching of crystalline silicon wafers (c-Si) [5]. Anodization begins when a constant current is applied between the c-Si wafer and the elec‐ trolyte by means of an electronic circuit controlling the anodization process [6].

Generally, p-Si is fabricated as shown in figure 1. We have a c-Si wafer (single crystalline) with the top face in contact with a hydrofluoric acid solution (HF) and where an immersed plati‐ num electrode is placed at certain distance and parallel to the wafer. In the bottom face of the wafer we find a flat metallic electrode that is in close electric contact. Between the two electro‐ des there is a controlled voltage supply with its negative pole connected to the platinum immersed electrode. A current is established from the anodic electrode (back of the wafer) and

© 2013 de la Mora et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 de la Mora et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the catodic electrode (platinum immersed). Modulating four variables: the intensity and interval of application of this current, the HF solution concentration, and the concentration and type of dopant previously applied to the c-Si wafer (type-n, type-p, or highly doped: type-p+ and type n+ ) it is then possible to control the porous size and p-Si layer geometrical parameters, as well as the number of layers. Dopant refers to a different element atom that replaces a percent‐ age of the Si atom inside the wafer and that is crystallographically compatible with it, but that presents an electron in excess (type n) or an electron lack (type p). This introduces a number of properties that modify the material behavior when an electric field is applied, mainly the resistivity, that will influence the etching process performance.

The electric current oxidizes the surface silicon atoms permitting a fluoride ion (formed in the HF solution because of the electrical current) attack on them generating the pores. By us‐ ing this electrochemical methodology it is also possible to create multilayer structures by al‐ ternating different current densities. For instance, if we start making the first layer with a current density J1 then the final porosity (and the refractive index) is going to be approxi‐ mately determined by this current density. The electrochemical reaction time determines the thickness. By switching the current density to a different value J2, the reaction mainly con‐ tinues at the crystalline silicon interface, leaving an almost intact first layer. Then the second layer will have a different refractive index and thickness (if we readjust the reaction time).

**Figure 1.** Experimental setup for porous silicon fabrication.

The figure 2 shows two structures that we fabricated from electrochemical etching of porous silicon. A luminescent monolayer made from p-type silicon wafers with a resistivity of 1-2 Ohm/cm (Fig. 2 a), and a multilayer prepared from p-type silicon wafers with a resistivity of 0.001-0.005 Ohm/cm (Fig. 2 b). Notice that by changing the dopant concentration, which is relate to the electrical resistivity used, the characterisitics of the p-Si structures differ. High resistivity crystalline silicon wafers give us higher porosities and small nanowires related to a given luminescent behavior. In turn, low resistivity allows us to achieve multilayers structures.

the catodic electrode (platinum immersed). Modulating four variables: the intensity and interval of application of this current, the HF solution concentration, and the concentration and type of dopant previously applied to the c-Si wafer (type-n, type-p, or highly doped: type-p+

The electric current oxidizes the surface silicon atoms permitting a fluoride ion (formed in the HF solution because of the electrical current) attack on them generating the pores. By us‐ ing this electrochemical methodology it is also possible to create multilayer structures by al‐ ternating different current densities. For instance, if we start making the first layer with a current density J1 then the final porosity (and the refractive index) is going to be approxi‐ mately determined by this current density. The electrochemical reaction time determines the thickness. By switching the current density to a different value J2, the reaction mainly con‐ tinues at the crystalline silicon interface, leaving an almost intact first layer. Then the second layer will have a different refractive index and thickness (if we readjust the reaction time).

The figure 2 shows two structures that we fabricated from electrochemical etching of porous silicon. A luminescent monolayer made from p-type silicon wafers with a resistivity of 1-2 Ohm/cm (Fig. 2 a), and a multilayer prepared from p-type silicon wafers with a resistivity of 0.001-0.005 Ohm/cm (Fig. 2 b). Notice that by changing the dopant concentration, which is relate to the electrical resistivity used, the characterisitics of the p-Si structures differ. High

resistivity, that will influence the etching process performance.

**Figure 1.** Experimental setup for porous silicon fabrication.

) it is then possible to control the porous size and p-Si layer geometrical parameters, as well as the number of layers. Dopant refers to a different element atom that replaces a percent‐ age of the Si atom inside the wafer and that is crystallographically compatible with it, but that presents an electron in excess (type n) or an electron lack (type p). This introduces a number of properties that modify the material behavior when an electric field is applied, mainly the

type n+

142 State of the Art in Biosensors - General Aspects

and

After the electrochemical etching stage, the surface of p-Si is hydrogen-terminated; this per‐ mits to immobilize large amounts of biomolecules [7]. It is possible to control several param‐ eters of p-Si such as; pore size and consequently the refractive index, thickness, morphology, etc. by modifying the anodization conditions [6, 11]. Porosity can be measured by gravimet‐ rical means. That is, the original crystalline silicon wafer is weighed first, then p-Si is formed and the wafer is weighed again, finally the p-Si layer is removed by adding KOH (Potassi‐ um hydroxide) and the wafer is weighed once more. With these three measurements is pos‐ sible to determine the porosity. To measure the thickness, SEM (scanning electronic microscopy) techniques are normally used giving the best resolution and accuracy. Refrac‐ tive index is usually determined by optical interference methods, where the refractive index can be estimated by taking adjacent maxima or minima from interference fringes coming from the p-Si sample.

**Figure 2.** Crossectional SEM images of porous silicon nanostructures. A luminescent monolayer (a) and a multilayer (b). These strucutures were prepared at CIE-UNAM porous silicon laboratory.

There are other methods for obtaining p-Si such as the photoelectrochemical [5], the chemi‐ cal vapour etching [8], the metal-assisted etching [9], and the 'stain etching' procedure [10]. The last two techniques mentioned do not require an electrical bias. In the stain etching pro‐ cedure the power supply action is replaced by the chemical oxidant action of nitric acid. The reaction control is performed trough the addition of other additives. The results are less ho‐ mogeneous than for the first process described, but they still permit to have the material quality compatible with several applications. As an example in figure 3 we show SEM im‐ ages of a p-SI monolayer obtained by metal assisted etching of gold nanostructures and sub‐ sequent chemical attack of an HF/H202 electrolyte.

**Figure 3.** SEM images of gold nanostructures (a,b) used to fabricated a porous silicon monolayer. We show the sur‐ face (c) and the cross sectional (d) images. These structures were prepared at CIE-UNAM porous silicon laboratory.

The p-Si material can be prepared either in powder or wafer permitting to elaborate devices that can be dispersed in a given medium or reused [12]. Furthermore p-Si is a material that allows the fabrication of high quality photonic crystals [13] by applying the method descri‐ bed before to obtain multilayers structures. Such characteristics therefore allow several bio‐ sensing approaches usign this porous material [1].
