Free Etch Process

Russell and Garnis found that an etching solution recommended for the precleaning of aluminum prior to resistance welding gave excellent results. This solution consisted of nitric acid and sodium sulfate (N—S). In later modifications, a P etch was developed containing sulfuric acid, sodium sulfate, nitric acid, and ferric sulfate. 

When aluminum was treated, the presence of nitric acid resulted in the production of oxides of nitrogen. These oxides are toxic and must be vented. In an effort to eliminate the necessity for venting the toxic etching fumes, a new etchant composition called P2 was developed, which does not give off any appreciable fumes and produces good bond strength 

Degreasing or solvent cleaning may be carried out prior to using the P2 etch, by using the procedure described in Section 6.2.1. The composition of P2 etching solution is given in Table 6.3. 

In 1996, Ciitchlow and Brewis conducted an extensive review of 41 mechanical, chemical, electrochemical, or other identified treatments specifically designed to modify the surface of aluminum to enhance bond durability. They listed a number of useful conclusions that indicate the direction for future research into these aluminum pretreatment techniques. 

These surface modifications methods may be combined with a range of chemical add-ons, such as primers, coupling agents, or hydration inhibitors, to stabilize the surface during storage or to further enhance bond durability. A number of useful analytical techniques were identified, ranging from wettability, optical inspection methods, to more complex techniques such as Auger electron spectroscopy and electron spectroscopy for chemical analysis. 

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Product desorption is a crucial step in the etch process. A free radical can react rapidly with a solid surface, but unless the product species has a reasonable vapor pressure so that desorption occurs, no etching takes place. For instance, when an aluminum surface is exposed to fluorine atoms, the atoms adsorb and react to form AIF3. However, the vapor pressure of AIF3 is 1 torr at 1240 C thus etching is precluded at ambient temperatures. 

In CF4 plasma etching at 0.5 to 1 Torr pressure, the active species are neutral free radicals like F atoms and CF3 (28). Novolak resins have fairly strong CF4 plasma etch resistance (29), which, because of widespread use of the dry etching process, is one of the required features for resins used as resist materials for IC manufacturing processes. 

Evidence from investigations outside the field of catalysis tends to support the more recent model. Specifically, a great deal of work outside of catalysis shows that free radicals are responsible for many etching processes. For example, several studies designed to demonstrate the existence of free radicals show that methylene radicals will cause volatilization of a number of metals. Also, modem research into the mechanism of etching in plasmas (dry etching for integrated... 

Experiments conducted by the same group for the thermal etching of silver in an oxygen atmosphere suggest that evaporation of metal plays little role in the thermal etching process. The thermodynamic model appears to best explain the observations. That is, identical faceted surfaces formed both in the case of suppressed and free evaporation. 

To explain the particles that formed in both the ethylene/oxygen and hydrogen/oxygen mixtures, it was postulated that they form in the gas phase and that the overall etching process takes place in three steps. First, free radicals are formed homogeneously in a boundary layer adjacent to the surface. Second, these radicals interact with metal atoms in the surface. This interaction results in the formation of volatile intermediates. Third, the metastable, volatile intermediates interact in the gas phase so that metal particles are formed and stable product molecules released. Individual metastable species presumably interact with each other and also with particles formed from multiple collisions. The larger particles interact with each other as well. 

The etching processes in all alkali hydroxides are similar to those in a KOH solution, namely, free H2O and OH are the active agents. It has been suggested that the difference among solutions such as NaOH, LiOH, NH4OH, RbOH and CsOH is mainly... 

Next, the sacrificial layer is patterned and holes are etched into the oxide using established lithography and etching processes. These holes will be filled and thus act as anchor points on the left end of the two cantilevers formed later (Fig. 5.3.1 e). In the next step, the functional polysilicon layer is deposited (Fig. 5.3.1b). The thickness of this layer determines the mechanical properties of the movable beam. The thicker it is, the stiffer the beam will be in the z axis, which is desirable for structures intended to move only in the xy direction. But its thickness is limited by the capabilities of the deposition process used. The functional layer is next patterned and etched (Fig. 5.3.1c). Depending on the thickness of the polysilicon layer, specific trench etch processes (as described later on) may be required, especially when this layer is rather thick. Finally, the sacrificial layer is removed (Fig. 5.3.1 d). This is typically done with wet or vapor phase etches to dissolve the silicon dioxide and leave parts of the functional structures free-standing and movable. When using wet etching, special care has to be taken to prevent Stic-... 

The reaction mechanism of the etch process is not fully clear. Several researchers proposed physical models for silicon anisotropic etching from the viewpoints of energy band gap [4, 5] and Gibbs free energy [6]. Over these models, one equation describing the simplified reaction mechanism is addressed as follow ... 

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