Silicon hole formation

Positive working silicon-containing resist with 0.8 jum resolution and O2 RIE selectivity greater than five was used for fine via hole formation in a polyimide film by O2 RIE process. 

K. Morisawa, M. Ishida, S. Yae, and Y. Nakato, Electrochemical metal deposition on atomically nearly-flat silicon surfaces accompanied by nano-hole formation, Electrochim. Acta 44, 3725, 1999. 

Stimulated desorption, whether photon, electron, or ion induced, is an inelastic sputtering process as it is the energy associated with the formation of a core hole that results in the emission of the element in question. Indeed, the formation of F " and Cl ions on electron irradiation of Aluminum and Silicon surfaces is accepted to arise through core hole formation followed by ejection through the Coulom-bic repulsion induced. In the case of ion-irradiated surfaces, it has been suggested that stimulated emission arises from Auger electrons formed in relatively distant neighbors (Williams 1981). 

Russell et al. first observed the formation of multilayered parallel orientations of lamellae in films of symmetric polystyrene-Woc -poly(methyl methacrylate) (PS-b-PMMA) on silicon, referred to as surface-induced ordeting. No preferential orientation was observed in toluene-cast films, but parallel orientation was observed after thermal annealing. This was accompanied by film thickness forming in steps, also known as island and hole formation or terracing. Parallel orientation from selective interactions with walls also occurs for films of cylinders.In all of these films, films were formed on surfaces with a preferential interaction with one of the blocks. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Palladium and platinum are also used as carrier lifetime controllers in Si. Pd creates an electron trap at Ec - 0.22 eV and a hole trap at Ev + 0.32 eV in Si (Chen and Milnes, 1980). Pt induces a single electron trap at Ec + 0.28 eV (Chen and Milnes, 1980). All of these levels are passivated by atomic hydrogen (Pearton and Haller, 1983) suggesting that hydrogen might be profitably used during silicide formation to passivate electrically active levels near the silicon-silicide interface. 

Many theories on the formation mechanisms of PS emerged since then. Beale et al.12 proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. Smith et al.13-15 described the morphology of PS based on the hypothesis that the rate of pore growth is limited by diffusion of holes to the growing pore tip. Unagami16 postulated that the formation of PS is promoted by the deposition of a passive silicic acid on the pore walls resulting in the preferential dissolution at the pore tips. Alternatively, Parkhutik et al.17 suggested that a passive film composed of silicon fluoride and silicon oxide is between PS and silicon substrate and that the formation of PS is similar to that of porous alumina. 

This is the regime of anodic current densities below JPS. A hole approaching the interface initiates the divalent electrochemical dissolution of a silicon surface atom at the emitter. The dissolution proceeds under formation of H2 and electron injection, as shown in Fig. 4.3. The formation of PS structures is confined to this region. 

Under anodic potentials in acidic electrolytes free of fluoride, silicon is passivated by formation of an anodic oxide under comsumption of four holes (h+), according to the reaction ... 

Divalent dissolution is initiated by a hole from the bulk approaching the silicon-electrolyte interface which allows for nucleophilic attack of the Si atom (step 1 in Fig. 4.3). This is the rate-limiting step of the reaction and thereby the origin of pore formation, as discussed in Chapter 6. The active species in the electrolyte is HF, its dimer (HF)2, or bifluoride (HF2), which dissociates into HF monomers and l ions near the surface [Okl]. The F ions in the solution seem to be inactive in the dissolution kinetics [Se2], Because holes are only available at a certain anodic bias, the Si dissolution rate becomes virtually zero at OCP and the surface remains Si-H covered in this case, which produces a hydrophobic silicon surface. 

At higher anodic potentials an anodic oxide is formed on silicon electrode surfaces. This leads to a tetravalent electrochemical dissolution scheme in HF and to passivation in alkaline electrolytes. The hydroxyl ion is assumed to be the active species in the oxidation reaction [Drl]. The applied potential enables OH to diffuse through the oxide film to the interface and to establish an Si-O-Si bridge under consumption of two holes, according to Fig. 4.4, steps 1 and 2. Details of anodic oxide formation processes are discussed in Chapter 5. This oxide film passivates the Si electrode in aqueous solutions that are free of HF. 

In some ways electropolishing and electrochemical pore formation can be understood as the two sides of the same coin. In the first case the rate-limiting species in the chemical reaction is HF, while in the second it is the supply of holes from the electrode. If we assume a rough silicon wafer surface and a reaction that is... 

That the assumption of such spatially separated sites is justified has been demonstrated by experiments using evaporated metal films, acting as catalytic sites [LilO]. In an electrolyte composed of aqueous HF, H202 and ethanol, stain film formation has been observed under and close to evaporated thin films of Au, Pt and Pd, while silicon samples free of metal films showed no PS formation. The metal is assumed to act as a cathodic site, where H202 is reduced to H20 under injection of two holes into the silicon VB. These holes are consumed by the for-... 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

A specific feature of macropore formation in n-type silicon is the possibility of controlling the pore tip current by illumination and not by applied bias. This adds another degree of freedom that is not available for mesopore or macropore formation on p-type substrates. The dark current density of moderately doped n-type Si electrodes anodized at low bias is negligible, as shown in Fig. 4.11, therefore all macropore structures discussed below are formed using illumination of the electrode to generate the flux of holes needed for the dissolution process. Illumination, however, is not the only possible source of holes for example, hole injection from a p-doped region is expected to produce similar results. 

An interesting question is whether such well-ordered pore arrays can also be produced in other semiconductors than Si by the same electrochemical etching process. Conversion of the macropore formation process active for n-type silicon electrodes on other semiconductors is unlikely, because their minority carrier diffusion length is usually not large enough to enable holes to diffuse from the illuminated backside to the front. The macropore formation process active in p-type silicon or the mesopore formation mechanisms, however, involve no minority carrier diffusion and it therefore seems likely that these mechanisms also apply to other semiconductor electrodes. 

---