Silicon oxide dissolution

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. 

Presence of HF results in the dissolution of silicon oxide and activates the surface. Fluoride species such as HF and F also react directly with the bare silicon surface. 

When the surface is completely covered by an oxide film, dissolution becomes independent of the geometric factors such as surface curvature and orientation, which are responsible for the formation and directional growth of pores. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus have intrinsically identical structure in all orientations. Also, on the oxide covered surface the rate determining step is no longer electrochemical but the chemical dissolution of the oxide.1... 

Vapor phase dissolution (VPD) is commonly used for surface and contamination analysis of semiconductor wafers [374-379]. HF vapor is used to remove a silicon oxide or native silicon layer. A drop of hydrofluoric acid or deionized water (with a volume of 50 to 200 jxL) is placed on the surface and rolled around the surface to dissolve the metals. The small drop is then analyzed by ICP-MS by using either a direct injection nebulizer, a micronebulizer, or ETV. The ability of ICP-MS to measure several elements rapidly in a small volume of solution is essential. 

As for the case of pore formation, although kinetics of silicon dissolution in the electropolishing regime have been studied by a number of groups [149-152], the detailed reaction mechanism is not well understood. The properties of electrochemical-ly formed silicon oxides have been reviewed recently [34, 153] and will not be discussed here. 

Chemical etching is a process for removal of silicon dioxide films through dissolution in solutions. Dissolution of silicon oxides, in the context of this book, is related to the anodic behavior of silicon electrodes. However, the dissolution of anodic oxides is not well studied. In contrast, there is a wealth of information on the dissolution of other types of oxides. Much of this information must also be applicable, at least the qualitative and mechanistic nature, to that of anodic oxides. Also, because oxides of different types are commonly used in device fabrication, compiling the etch rate data of these oxides and those of silicon (presented in Chapter 7) in the same volume would be useful in practice. Additionally, because silica-water interaction, which has been extensively investigated in the geological field, is fundamental to the etching of silicon oxides, some of the results from the investigations on the dissolution of rocks and sands are also included. 

Dissolution of silicon oxides is geographically important since silica is one of the most abundant minerals in the Earth s crust. At least nine different Si02 phases are found in natural and engineered earth systems in the form of quartz, cristobalite,... 

FIGURE 4.1. Range of dissolution rates of silicon oxides in different systems. 

The etch rate of anodic oxide can be determined by methods similar to those for thermal oxide or deposited oxides. It may also be estimated from the anodic current of the oxidized electrode. The anodic i-V curve of a silicon electrode typically shows a passivation-like peak above which the dissolution occurs through a two-step process the formation of oxide film followed by the chemical dissolution of the oxide. The steady-state anodic current measured at an anodic potential above the peak potential indicates the dissolution rate of the anodic oxide. Thus, the passivation current, /p, listed in Table 5.5 can be used for estimation of the etch rate of the oxide film formed at the anodic potentials. (A current density of 1 mA/cm corresponds to a silicon etch rate of 3.1A/S or to a silicon oxide etch rate of about 7 A/s.) For example, in 1% HF solution, ip is 5mA/cm, and thus the etch rale of the oxide film formed at a potential anodic of the first current peak is about 35 A/s. hi 2M KOH solution at room temperature, ip == 0.002mA/cm equivalent to an etch rate of about 0.014 A/s. These numbers appear to be in general agreement with the data in Table 4.1. 

FIGURE 4.27. Dissolution rate of the anodix oxide formed at 7V in 0.1 M [F] at pH 4.5 as a function of distance from the silicon/oxide interface. Data from Ref. ... 

In HF-Based Solutions. The overall reaction involving the dissolution of silicon oxide in HF solution can be expressed as... 

A New Model. The results of the studies on anodic oxide films (see section 5.9 and chapter 3 on passive film and anodic oxides) show that anodic oxide properties (oxidation state, degree of hydration, 0/Si ratio, degree of crystallinity, electronic and ionic conductivities, and etch rate) are a function of the formation field (the applied potential). Also, they vary from the surface to the oxide/silicon interface, which means that they change with time as the layer of oxide near the oxide/silicon interface moves to the surface during the formation and dissolution process. The oxide near the silicon/oxide interface is more disordered in composition and structure than that in the bulk of the oxide film. Also, the degree of disorder depends on the formation field which is a function of thickness and potential. The range of disorder in the oxide stmcture is thus responsible for the variation in the etch rate of the oxide formed at different times during a period of the oscillation. The etch rate of silicon oxides is very sensitive to the stmcture and composition (see Chapter 4). 

Reaction paths (1) and (11) in Fig. 5.70 account for the anodic reactions onp-Si and illuminated n-Si in HF solutions at high light intensities. Path (1) is involved in the exponential region at an anodic potential much lower than Vp responsible for direct dissolution of silicon and dissolution valence of 2, while path (11) is involved at a potential above Vp responsible for the indirect dissolution of silicon through formation and dissolution of oxide and for the dissolution valence of 4. At a potential that is lower... 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

In a slightly different angle, Kendall reasoned that beeause the 111 surfaee is oxidized thermally more rapidly than other low-index surfaees, the silicon surface ean be covered with a silicon oxide (or a hydrated silieon oxide) during etehing in aqueous solution, mueh faster than other planes. The formation of the oxide film passivates the (111) plane and blocks the dissolution reaetions. This model implies that the etch rate of a (111) surface should be similar to that of silieon oxide in KOH solu-... 

The etch rate of the (111) surface, although much smaller than those of the (100) and (110) planes, shows definite values, in the range of 2-10 A/s in KOH solutions. It is still much larger than the dissolution current density on a passivated surface in KOH (a dissolution rate of 2-10 A/s is equivalent to a current density of several milliamperes per square centimeter). In alkaline solutions, the dissolution rate of silicon oxide is less than 0.01 A/s (see Chapter 4), which is several orders of magnitude smaller than the etch rates of a (111) surface. Thus, it is unlikely that the silicon surface of any orientation is covered by Si02 during etching. 

FIGURE 9.2. Conceptual illustration on the role of silicon oxide in different electrochemical phenomena of silicon in terms of surface coverage and rates of formation and dissolution. 

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