Illumination etching

FIGURE 3 Steady-state current-potential characteristics of an n-type GaN/Pt electrochemical cell under dark and illuminated etching conditions [26],... 

W. P. Gomes and H. H. Goossens analyze the kinetics of the electrochemical reactions used for etching gallium arsenide. The authors describe means by which information about the mechanisms may be obtained. Based on the understanding of the anodic dissolution of n-type and p-type GaAs, both in the dark and under illumination, etching mechanisms for different oxydants with and without current are derived. 

Under certain (limited) circumstances a mere physisorption of the functional polymers might work, in particular, if only transient surface coatings are employed. For example, the spin-coating of a clean Si-wafer by a photoresist is sufficient to prepare a photoreactive layer that can be used to generate a pattern by mask-illumination, etch this into the Si to laterally stmcture the substrate, and then dissolve the polymer away again. 

This class of CCD has a higher quantum yield than a front-illuminated similar CCD. The origin of this improvement is as follows. The silicon substrate for the chip is etched down, on the face opposite the electrodes, to reach a thin layer (—15 pm), and the light illuminates the array from the back, opposite from the usual way (Fig. 4). It was noted in Sec. 5 that electrode gates that are used for the readout process overlap the pixel surface. Since these electrodes have a given thickness and are laid on the front face, photoelectrons have to travel across a... 

There is also an etched layer of Si on the surface under illumination as illustrated in Figure 18. This etched layer is mainly due to photo-induced corrosion. As a result of the photo induced dissolution the top surface of PS layer recedes with time. The rate of dissolution depends on doping, F1F concentration, current density and illumination intensity. 

Figure 18. Three layers are resulted after formation of PS on illuminated n-Si sample an etch layer, a micro PS layer and a macro PS layer. The walls of the macro pores may also be fully or partially covered with micro PS.

Illumination generates holes within the material of PS and causes photo corrosion of PS that is much faster than that in the dark. Depending on illumination intensity and time, the pore walls in a PS can be thinned to various extents by the photo induced corrosion. This corrosion process is responsible for the etched crater between the initial surface and the surface of PS as illustrated in Figure 28. It is also responsible for the fractal structure of the micro PS formed under illumination. 

For doping-dependent anodic etch stops in HF, a general hierarchy of dissolution is observed [La5] illuminated n-doped and n+-doped areas are most easily dissolved, followed by p+-doped areas. Next likely to be dissolved are p-type areas. Moderately n-type doped areas kept in the dark are least likely to be etched. This hierarchy corresponds to the potential shift of the I-V curve in the regime of PS formation [Gal, Zh5]. 

But even in a homogeneously doped material an etch stop layer can be generated by an inhomogeneous charge carrier distribution. If a positive bias is applied to the metal electrode of an MOS structure, an inversion layer is formed in the p-type semiconductor. The inversion layer passivates in alkaline solutions if it is kept at the PP using a second bias [Sm5], as shown in Fig. 4.16b. This method is used to reduce the thickness variations of SOI wafers [Og2]. Illuminated regions... 

A prerequisite for all etch-stop techniques discussed so far is an electrical connection to an external power supply. However, if the potential required for passivation in alkaline solutions is below 1 V, it can be generated by an internal galvanic cell, for example by a gold-silicon element [As4, Xil]. An internal galvanic cell can also be realized by a p-n junction illuminated in the etchant, as discussed in the next section. Internal cells eliminate the need for external contacts and make this technique suitable for simple batch fabrication. 

An initially flat silicon electrode surface will develop a surface topography if the photocurrent varies locally. This variation can be caused by a lateral variation of the recombination rate or by a lateral variation of the illumination intensity. The photoelectrochemical etching of a silicon electrode is related to the etch-stop techniques discussed in Chapter 3. While different etching rates for different areas of the electrode may be obtained by electrical insulation or by a different doping density, the etch rate may also be altered by a difference in illumination intensity. Basically four photoelectrochemical etching modes are possible for homogeneously doped substrates ... 

The etch rate of an illuminated area on p-type Si is reduced under cathodic potential in alkaline solutions, while an area kept in the dark shows the OCP etch rate [Ve2]. 

An illuminated area on n-type Si is anodically passivated in alkaline solutions for potentials in excess of PP, whereas an area kept in the dark is not passivated and is therefore etched with the OCP etch rate. 

Due to the anodic shift of the OCP potential with illumination [Be9], a p-type Si electrode under anodic bias in HF is preferably etched in the dark areas. 

The experimentally observed parabolic increase in pore depth and linear decrease in concentration shown in Fig. 9.18 c indicate that Eq. (9.6) is valid [Le9]. The macropore growth rate decreases linearly with l according to Eq. (9.6). If, therefore, a constant pore diameter is desired for a macropore array, a decrease in etching current or illumination intensity, respectively, with time is required. 

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. 

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