Diffusion etching process

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. 

Rate vs. temperature curves similar to that shown in Fig. 14 for the A surfaces have been obtained for the dissolution of germanium in HF + H2O2 + H2O etchants (4). These curves were attributed to two consecutive reactions taking place on the surface. Each reaction was assumed to take place on a distinct fraction of the surface. Such a mechanism is unlikely for an etching process. In addition, one of the activation energy values associated with two segments of the Ge rate curves is well below 10 kcal /mole, indicative of diffusion control, and the other is well above 10 kcal /mole, indicative of activation control. It appears thus more likely that the curve of Fig. 14 and those reported for Ge (4) represent a transition from diffusion to activation-controlled dissolution rather than two consecutive chemical reactions. 

Contrary to the observations reported by Bagley et al. (7) on the etching of an organosilsesquioxane in a barrel reactor, the PBTMSS etching process does not proceed beyond the surface layer, probably because much higher elasticity of the PBTMSS network causes collapse of the micropores through which the active species diffuse into the film. 

This table clearly shows that for all three crystal faces studied, etching at a diffusion-limited rate leads to a flat surface (Table 2, cases A, B, C, and G, H, I). This is explained by the fact that for a diffusion-limited process, the etching rate is the same over the entire surface and is hence unaffected by solid-state factors. 

Today, a typical process flow for advanced ICs consists of 300 to 500 steps, 30% of which are wafer cleaning steps." Many process steps during IC fabrication may introduce contamination, which must be cleaned before the next process step. For example, in processes such as steam oxidation, resist etching, and ion implantation, metallic contamination typically introduces a surface concentration of 10 to lO Vcm. The need for wafer cleaning can be separated into three areas (1) preparation of the wafer surfaces for oxidation, diffusion, deposition, and metallization (2) preparation for the application of photoresist and (3) removal of photoresist after the etching process." ... 

Locally confined etchant can also be used to preferentially etch small holes on silicon. In this technique an active etchant is generated through a reduction reaction at the tip of a fine electrode which is positioned near the silicon surface. In situations where diffusion-controlled process limits the in-depth etch rate, deep etching can obtained using a centrifugal force. ... 

The HF—HNO3 etching system was systematically investigated by Schwartz and Robins [97, 119]. They found that in the low HF concentration region, the etching process is a diffusion-controlled process etch rates of different silicon materials in... 

Briefly, during the etching process, when the thickness of the neutral p-layer becomes thinner than the diffusion length of the electrons, a npn bipolar transistor is built up right under the silicon/liquid interface, as demonstrated in [27]. In such situations a large current, injected by the silicon/liquid interface, can flow through the reverse-biased pn junction - hence the typical current peak exhibited in all the records of the etching process. 

Industrial applications of diffusion dialysis for acid recovery include recovery of acids from waste acid from surface treatment of steel and stainless steel, from alumilite treatment, from the etching process of aluminum and titanium, and in other hydrometallurgy industries. The recovery rate of acid can be controlled by changing the flow velocity of the waste solution and water in the dialyzer, in which solutions flow counter-current. For hydrochloric acid, as explained before,... 

Mechanical characteristics are an important consideration in the design of NbsSn conductors. The behavior during bending and under tensile stress is compared below for a tape clad on both sides with copper and a tape clad with stainless steel and copper. The NbsSn/Nb substrate ( lO mNb and 10 fim NbsSn) was prepared by liquid diffusion followed by an etching process to reduce ac losses [" ]. The copper and stainless steel cladding were bonded by soldering with an Ag-Sn eutectic. The thickness of the copper was 24 /xm for the... 

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