Surface silicon, fluoride electrolytes

A number of different fluoride salts have been used as electrolytes for electropolishing silicon. Not only must the fluoride salt be sufficiently water soluble but the anode reaction product, the corresponding fiuosilicate, must also be readily soluble in water. Potassium fluoride is highly water soluble but the fiuosilicate is not. At a critical anodic etch rate silicon will passivate in KF solutions due to the precipitation of SiFg on the surface. Ammonium fluoride and ammonium fiuosilicate are both sufficiently water soluble. The acids HF and SiFg are even more water soluble. 

Although surface reactions at the silicon/electrolyte interface have been studied for many years, the nature of the interactions between the silicon surface and fluoride containing electrolytes are only recently becoming understood. Recent characterizations using various spectroscopic and microscopic techniques have resulted in new insight into the properties of silicon surfaces on an atomistic scale. Significant issues remain to be resolved, however, such as the reaction mechanisms, the processes leading to pore formation, and the nature of surface oxides. 

A. Belaidi, M. Safi, F. Ozanam, J. N. Chazalviel, and O. Gorochov, Surface chemistry during porous-silicon formation in dilute fluoride electrolytes, J. Electrochem. Soc. 46, 2659, 1999. 

H. Gerischer and M. Lubke, The electrochemical behaviour of n-type silicon (lll)-surfaces in fluoride containing aqueous electrolytes, Ber. Bunsenges. Phys. Chem. 91, 394, 1987. 

The first models for the electrochemical dissolution process of silicon in HF assumed a fluoride-terminated silicon surface to be present in electrolytes containing HF [Ge6, Du3[. However, by IR spectroscopy it was found that virtually the whole surface is covered by hydride (Si-H) [Ni3[. No evidence of Si-F groups is found in IR spectra independent of HF concentration used [Ch9[. This is surprising insofar as the Si-F (6 eV) bond is much stronger than the Si-H (3.5 eV) bond, and so it cannot be assumed that Si-F is replaced by Si-H during the electrochemical dissolution. This led to the conclusion that if a silicon atom at the surface establishes a bond to a fluorine atom it is immediately removed from the surface. 

For anodic oxidation of a silicon electrode in fluoride-free electrolytes, the reaction of step 2 is only expected during the first seconds of anodization, until all Si-H surface groups are replaced by Si-OH. 

A bare surface of silicon can only exist in fluoride containing solutions. In reality, in these media, the electrode is considered to be passive due to the coverage by Si— terminal bonds. Nevertheless, the interface Si/HF electrolyte constitutes a basic example for the study of electrochemical processes at the Si electrode. In this system, the silicon must be considered both as a charge carrier reservoir in cathodic reactions, and as an electrochemical reactant under anodic polarization. Moreover, one must keep in mind that, according to the standard potential of the element, both anodic and cathodic charge transfers are involved simultaneously (corrosion process) in a wide range of potentials. 

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

The specific adsorption of an anion at the oxide/electrolyte surface, which changes the surface charge, may be viewed as a surface complexation reaction. Thus, fluoride ions that are adsorbed at the silicon oxide surface centers form Si-F complex ... 

Illumination with light having a wavelength larger than the band gap of silicon generates a photocurrent under an anodic potential on an n-Si electrode but has essm-tially no effect on p-Si, as would be expected from the basic theories of semiconductor electrochemistry. However, the photocurrent may not be sustained because of the formation of an oxide film, which passivates the silicon surface to various degrees depending on the electrolyte composition. In solutions without fluoride species, the photocurrent is only a transient phenomenon before the formation of the oxide film. In fluoride solutions, in which oxide film is dissolved, a sustained photocurrent can be obtained. 

---