Porous silicon etched layer

L.M. Sorokin, N.S. Savkina, V.B. Shuman, A.A. Lebedev, G.N. Mosina, J. Hutchinson, Features of the structure of a porous silicon carbide layer obtained by electrochemical etching of a 6H-SiC substrate, Tech. Phys. Lett., 28, 935-938 (2002). 

M Navarro, JM Lopez-Villegas, J Samitier, JR Morante, J Bausells, A Merlos. Electrochemical etching of porous silicon sacrificial layers for micromachining applications. J Micromech Microeng 7 131-132, 1997. 

Electrochemically etched porous silicon is known to introduce lattice distortion and strain (Lei et al. 2004) as well as dangling bonds and as such was already demonstrated to be used as an effective gettering side (Poponiak 1975 Borisenko and Dorofeev 1983 Shieh and Evans 1993). Multilayers of porous silicon etched into low-cost and low-quality Si substrate were reported to be a good diffusion barrier for contaminants from the bulk (Bilyalov et al. 2001). Several examples of such porous srhcon multilayer structure, used not as a sacrificial layer but rather embedded in between low-quality, highly doped p+, electrically inactive substrates and higher-quality active Si layer grown epitaxially on top, are reported (Bilyalov et al. 2001 Kuzma-Filipek et al. 2009 Radhakrishnan et al. 2012). 

In Kuzma-Filipek et al. (2009), the authors show an example of effective gettering by means of electrochemically fabricated multilayer of porous silicon etched into UMG Si substrates, intrinsically contaminated with transition metals impurities. The test structures consisted of a 15-layer stack of a total thickness of 1,300 nm on top of which 300 nm epitaxial silicon was grown by CVD. The structure was exposed to high-temperature treatment and the metal trapping effect was monitored by... 

There are essentially three different ways how to prepare nanometer sized silicon particles. The porous silicon is, as already mentioned, prepared by anodic etching of silicon wafers in an HF/ethanol/water solution [6, 7]. The microporous silicon has typically a high porosity of 60-70 vol.%, and it consists of few nm thin wires which preserve the original orientation of the wafer. The thickness of the wires varies within the PS layer and the material is very brittle. Free standing PS films can be prepared by application of a high current density after the usual etching of the desired thickness of the PS. 

Historically, the first reports of porous silicon layers were by Uhlir [59] and Turner [60]. These authors reported on the electropolishing of silicon and noted that under certain conditions a porous layer was formed at the silicon surface. The first models for porous layer formation assumed that the layer was formed on the silicon substrate by a deposition process thought to involve the reduction of divalent silicon to amorphous Si via a disproportionation reaction in solution [61]. Subsequently, Theunissen [62] showed that the porous structure was the result of a selective etching process within the silicon, contradicting the silicon deposition model. 

Although the dissolution process results in the formation of a porous structure, electrode impedance measurements [72, 73] have shown that the etching process is not limited by mass transport, even for thick porous silicon layers [74]. Figure 12 shows a plot of potential as a function of time during pore formation in p-Si(lOO),... 

Based on these characteristics, porous silicon may be described as a random array of channel-like pores or etch tunnels growing in <100) directions. For the case of n-type silicon these channels are isolated from each other and, for etching in the dark, the pore spacing is approximately equal to the depletion layer width at a planar surface [83-86]. For the case of p-type silicon the channels are interconnected. The... 

Although most work related to pore formation in silicon has involved electrochemical etching of silicon in HF solutions, porous silicon layers have also been formed by chemical etching and by spark erosion in vacuum. 

The formation of porous layers in silicon by chemical etching in HF/HNO3 solutions was first reported at about the same time as electrochemical etching [101-104]. These so-called stain etch films are characterized by rough or porous surfaces, typically < 500 A in thickness. Recent work has shown that these stain etch films exhibit strong visible photoluminescence, similar to the emission observed from electrochemically etched porous silicon layers. 

Internal reflection infrared spectra measured in situ during etching of silicon in HF solutions exhibit characteristic Si - H modes, although the Si - H spectrum is broad because of interaction of the surface Si-H groups with the electrolyte. No electrochemical or chemical intermediate species have been detected [112]. Infrared spectra of porous silicon layers after drying reveal characteristic Si-H and Si-H2 peaks similar to the spectra obtained for hydrogen on Si(lOO) 2x1 surfaces [112]. 

S. Billat, M. Thonissen, R. Arens-Fischer, M. G. Berger, M. Kruger, and H. Luth, Influence of etch stops on the microstructure of porous silicon layers. Thin Solid Films 297, 22, 1997. 

D. Dimova-Malinovska, M. Sendova-Vassileva, N. Tzenov, and M. Kamenova, Preparation of thin porous silicon layers by stain etching, Thin Solid Films 297, 9, 1997. 

Electroless deposition of the catalytic Pt or Pt-Ru layer was proposed for the preparation of electrodes in microdirect methanol fuel cells.53 A porous silicone substrate is prepared by the anodic etching in HF-ethanol-water (1 1 1) solution. After the etching, at the surface of porous silicon substrate, a thin film of titanium is sputtered and then a film of Pt or Pt-Ru alloy with thickness of about 150-200 nm was electroless deposited. The electrodes prepared in this way helped in minimization of the fuel cell size and increased the reactive area of the catalyst over the silicon electrode surface. 

A variety of mammalian cells have been successfully cultured onto porous silicon surfaces. The first publications on this topic by Bayliss et al. demonstrated that attachment of Chinese hamster ovary (CHO) cells proceeded on porous silicon surfaces to a similar extent as on bulk silicon (Bayliss et al. 1997a, b). This was also confirmed with the neuronal cell line B50 (Bayliss et al. 2000). Cell viability in these studies was determined using two colorimetric assays, the MTT based on enzymatic reduction of a tetrazolium salt to a purple formazan and the neutral red uptake assay. B50 and CHO cells were cultured on bulk silicon, porous silicon, glass, and polycrystalline silicon. Both viability assays suggested that the neuronal cells showed preference for porous silicon above the other surfaces, while CHO cells showed the lowest viability on the porous silicon surface (Bayliss et al. 1999, 2000). The surfaces of the porous silicon used in these early studies were not modified post-etching, and it was not until a study utilized porous silicon surfaces with an oxide layer for cell culture that surface chemistry was found to play a crucial factor (Chin et al. 2001). Rat... 

Crystalline silicon is a strong but brittle material. The introduction of porosity often lowers hardness, stiffiiess, and fracture strength (see handbook chapter Mechanical Properties of Porous Silicon ), and if the stmcture becomes too weak, it cannot often survive common material processing techniques without alteration. Examples include air drying (see handbook chapter Drying Techniques Applied to Porous Silicon ), reduction of particle size via communition (see handbook chapter Milling of Porous Silicon Microparticles ), and oxidation of layers (see handbook chapter Oxidation of Mesoporous Silicon ). The properties of electrochemically etched layers can depend not only on etch parameters but how the material was dried. The properties of microparticles can be sensitive to how they were milled. 

Trucks G, Raghavachaii K, Higashi G et al (1990) Mechanism of HF etching of silicon surfaces - a theoretical understanding of hydrogen passivation. Phys Rev Lett 65 504-507 Unagami T (1980a) Formation mechanism of porous silicon layer by anodization in HF solution. J Electrochem Soc 127 476-483... 

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