Etching Mechanisms

FIGURE 7.15. The bias dependence of the etch rates for (a) p(lOO) and n(lOO), and (b) p(lll) and n(lll) low-doped Si. The data have an experimental spread of approximately 20%. The open-circuit (OCP) and passivation (PP) potentials are marked. Note that the OCP is the same for the n(lll) and p(lll) samples. After Glembocki et al. (Reproduced by permission of The Electrochemical Society, Inc.) 

With the identification of H2O and OH as the reactants and H2 and Si(0H)2(0 )2 as the reaction products, the overall reaction during etching in KOH solutions has been established  

The details of this reaction are discussed in more detail in Chapter 5. As far as the etching kinetics is concerned, according to Seidel et the dependence of etch rate on KOH concentration in the concentration range of 10 to 60% can be best fitted by 

Glembocki ef a/. proposed a model based on the activities of H2O and OH. According to them, water exists in two forms, hydrated and free, and it is the free water and free hydroxyl ions that are responsible for etching reactions. The corresponding rate equation is 

The difference between n-Si and p-Si at the cathodic potentials indicates the effect of carriers on the etching process. The hydrogen evolution may either obtain the electron directly from the dissolving surface silicon atom or from the semiconductor. For n-Si, cathodic bias provides a high concentration of surface electrons for the hydrogen reaction resulting in a decrease of the dissolution rate. However, for p-Si electrons are the minority carriers, which are not available at a cathodic bias, and the etch rate remains more or less constant at cathodic potentials. 

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Another view of the Si(lOO) etching mechanism has been proposed recently [28], Calculations have revealed that the most important step may actually be the escape of the bystander silicon atom, rather than SiBr2 desorption. In this way, the SiBr2 becomes trapped in a state that otherwise has a very short lifetime, pennitting many more desorption attempts. Prelimmary results suggest that indeed this vacancy-assisted desorption is the key step to etching Si(lOO) with Br2. 

Etch Mechanisms. Most wet etches for the compound semiconductors employ oxidation of the semiconductor followed by dissolution of the oxide. For this reason, many wet etches contain the oxidant hydrogen peroxide, although nitric acid can also be used. One advantage of wet etching over dry is the absence of subsurface damage that is common with dry etching. Metal contacts placed on wet-etched surfaces exhibit more ideal characteristics than dry-etched surfaces. 

Figure 1(a) shows the etch rates of niobium oxide pillar and Si film, and the etch selectivity of Si to niobium oxide as a function of CI2 concentration. The etch condition was fixed at coil rf power of 500 W, dc-bias to substrate to 300 V and gas pressure of 5 mTorr. As the CI2 concentration increased, the etch rate of niobium oxide pillar gradually decreased while Si etch rate increased. It indicates that the etch mechanism of niobium oxide in Cl2/Ar gas is mainly physical sputtering. As a result, the etch selectivity of Si film to niobium oxide monotonously increased. The effect of coil rf power on the etch rate and etch selectivity was examined as shown in Fig. 1(b). As the coil rf power increased, the etch rates of niobium oxide and Si increased but the etch rate of niobium oxide showed greater increase than that of Si. It is attributed to the increase of ion density with increasing coil rf power. Figure 1 (c)... 

A Dektak siuface profilometer was used to measure the etch rates. The profiles of the etched films were observed by field emission scanning electron microscopy (FESEM). In addition, x-ray photoelectron spectroscopy PCPS) was utilized to examine the existence of possible etch products or redeposited materials, and to elucidate the etch mechanism of Co2MnSi magnetic films in a CVOa/Ar plasma. 

Fig. 13.1. Electrochemical etching of tungsten tips, (a) A tungsten wire, typically 0.5 mm in diameter, is vertically inserted in a solution of IN NaOH. A counterelectrode, usually a piece of platinum or stainless steel, is kept at a negative potential relative to the tungsten wire, (b) A schematic illustration of the etching mechanism, showing the "flow" of the tungstate anion down the sides of the wire in solution. (Reproduced from Ibe et al., 1990, with permission.)...

A second observation bearing on the selective etching mechanism is shown in Fig. 3.4. In this figure the rate of accumulation of carbon caused by CFj bombardment is plotted as a function of ion dose for Si and oxidized Si surfaces. It can be... 

These results led the workers to suggest that catalysis actually leads to the removal of surface nickel atoms, primarily due to local heating which takes place at the reaction site. Furthermore, during the catalytic process, the nickel atom is temporarily part of a liquid- or gas-phase intermediate. Once the catalytic process is complete, the authors postulated that the free nickel atom readsorbed onto the bulk nickel, adsorbed onto the inert support, remained as nickel sol in the liquid, or continued to act as a catalyst. It was claimed that this model explained several observations, such as the differences between unsupported and supported nickel. The supported metal has a greater surface area upon which the metal can readsorb, so it tends to leave fewer atoms in the product liquid. The model also explains the observation that the reaction vessel became coated with a thin film of nickel after lengthy use. This postulated etching mechanism is similar to the recent model discussed above, whereby etching results from free-radical-surface interactions. 

Fig. 8.15. Model for etching mechanism of acidic and alkaline solutions on polar ZnO faces [109]. The dangling bonds at the surfaces exhibit partially positive (<5+ for Zn) or partially negative (

While the microscopic etch mechanism of ZnO single crystals in alkaline and acid solutions is well understood [109,117], a detailed understanding of the etching behavior of compact polycrystalline films is still not available. In the following we will discuss the relation between etching behavior of... 

We conclude that the microscopic etch mechanism must be the same for single crystals and sputter deposited, polycrystalline ZnO Al. For the latter, the tendency for crater formation is masked by inhomogeneous chemical or physical properties like porosity, composition or, in case of dynamic deposition, multilayered ZnO Al films. This multilayer structure results from the fact that structural properties of ZnO Al deposited by a sputter process varies depending on the position of the film relative to the race track of the sputter target [131,132]. This dependence is important for the etch rate of in-line sputter deposited films [133]. 

A technique that combines both physical and chemical etching mechanisms is chemically assisted ion beam etching (CAIBE). In CAIBE, an inert gas ion beam is directed at a sample which is situated in a... 

Etching of spheres and hemispherical pits to their limiting form is desirable in the determination of etching mechanisms. 

The model system comprising the reaction sequence from initial 0-atom attack to steady-state erosion of a hydrocarbon surface can serve as a benchmark for fundamental atom-surface interactions at hyperthermal collision energies and for etching mechanisms of materials. Within this model system, there is still much to learn. It is likely that when a hyperthermal oxygen atom strikes a saturated hydrocarbon surface, it will either abstract a hydrogen atom or it will scatter inelastically. The subsequent reaction sequence becomes murky. Very little is known about the mechanisms of oxidation, surface roughening, or material loss. In fact, even the sticking... 

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