Dissolution anodic, silicon

The fundamental and applied electrochemistry of the silicon/electrolyte interface is presented in an authoritative review by Dr. Gregory Zhang, with emphasis in the preparation of porous silicon, a material of significant technological interest, via anodic dissolution of monocrystalline Si. The chapter shows eloquently how fundamental electrokinetic principles can be utilized to obtain the desired product morphology. 

The formation of etch pits and tunnels on n-Si during anodization in HF solutions was reported in the early 1970 s. It was found that the solid surface layer is the remaining substrate silicon left after anodic dissolution. The large current observed on n-Si at an anodic potential was postulated to be due to barrier breakdown.5,6 By early 80 s7"11 it was established that the brown films formed by anodization on silicon substrate of all types are a porous material with the same single crystalline structure as the substrate. 

The electrochemical reactions and processes involving the anodic dissolution of silicon in HF solutions have been extensively studied in the past. Table 5 provides a summary for the characteristics of the anodic processes that are relevant to the formation of PS (details are documented in Ref.1.)... 

The surface of silicon during anodic dissolution is dynamically terminated by hydrogen the dissolution of silicon atoms proceeds by first forming a Si-H bond. 

Formation of porous silicon is an anodic dissolution process, which consists of carrier transport in the semiconductor, electrochemical reactions at the interface, and mass transport of the reactants and reaction products in the electrolyte. There are a... 

Uhlir and Turner2 3 first reported in the late 1950 s that silicon surface can be covered with a brown film during anodization in HF solutions and suggested that the film was a subfluoride (SiFi)x grown on the surface during the anodic dissolution. Later, Memming and Schwandt proposed that the... 

In the positive branch of the i/V graph, anodic dissolution process will remove material from silicon crystals. The conditions for optimal etching of silicon have been extensively explored for micromachining or surface polishing in the fabrication of electronic devices. Most generally, the etch rate of silicon in HE solutions is isotropic among the various crystalKne orientations. The etch rate of silicon at room temperature at the open-circuit potential (OCP) is very low, on the order of 10 nm s , which is equivalent to 100 nA cm , in aqueous HE solutions. 

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

FIGURE 5.61. Mechanism of the anodic dissolution of silicon in low HF concentrations at higher anodic potentials (tetravalent dissolution). After Memming and Schwandt. " ... 

According to Seidel et al. the dissolution at OCP is an electrochemical process with concurrent anodic dissolution of sihcon and reduction of hydrogen ions. The oxidation of silicon gives out electrons which are consumed for the reduction of hydrogen. Both OH" and H2O are the active species in that OH" is involved in silicon dissolution and H2O in hydrogen evolution ... 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Electroless deposition of Au in KAu(CN)2 -I- HF can be controlled by both the kinetic process and the diffusion process. The deposition is a two-step process, with initial diffusion-limited deposition of the intermediate species, followed by surface-limited reduction of this species. For electroless deposition of Pt, it has been reported that the rate-determining step is the deposition on n-Si, whereas it is the dissolution of silicon on p-Si. Electroless copper deposition does not occur on Si02-covered silicon surface due to the lack of anodic dissolution of silicon In a non-HF solution, the deposition of copper on a bare silicon surface results in the formation of oxide aroimd the metal particles. In HF solutions, the deposition of copper proceeds very slowly in the dark on both p-Si and n-Si samples due to the lack of carriers. The... 

The anodic dissolution of silicon in these solutions occurs by the following reactions... 

A high etch rate is only obtained when both rates of oxidation and dissolution are high. The etch rate can be increased either by addition of effective oxidizing agents into the solution or by anodic polarization which increases the surface concentration of carriers. As shown by the i-V curve (Fig. 5.1) the anodic dissolution of silicon in HF solutions consists of three regions. At potentials below the current peak, nonuniform anodic... 

Porous silicon (PS) is a material that is formed by anodic dissolution of silicon in HF solutions. The formation of PS was first reported in the late 1950s in studies on electropolishing of silicon. Since then, particularly after 1990 when luminescence of PS was discovered, numerous investigations have been undertaken. These investigations have revealed that PS has extremely rich morphological features with properties that are very different from those of silicon and the formation process of PS is a very complex function of many factors such as HF concentration, type of silicon, current density, and illumination intensity. 

In this chapter, the conditions for the formation of PS, the relation between the formation conditions and PS morphology, and the mechanisms for the formation of PS and morphology are discussed. The various aspects of surface condition, nature of reactions, and reaction kinetics that are fundamentally involved in the anodic dissolution of silicon are discussed in Chapters 2-5. 

A quantitative description of the diverse morphological features of PS requires the integration of the aspects discussed above as well as the fundamental reaction processes involved in silicon/electrolyte interface structure, anodic dissolution, and anodic oxide formation and dissolution as detailed in Chapters 2-5. Any mathematical formulation for the mechanisms of PS formation without such a global integration would be limited in the scope of its validity and in the power to explain details. In addition, a globally and microscopically accurate model would also require the full characterization of all of the morphological features of PS in relation to all of the... 

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