Denuded zone

The two types of hot eorrosion eause different types of attaek. High-temperature eorrosion features intergranular attaek, sulfide partieles and a denuded zone of base metal. Metal oxidation oeeurs when oxygen atoms eombine with metal atoms to form oxide seales. The higher the temperature, the more rapidly this proeess takes plaee, ereating the potential for failure of the eomponent if too mueh of the substrate material is eon-sumed in the formation of these oxides. 

Low-temperature eorrosion eharaeteristieally shows no denuded zone, no intergranular attaek, and a layered type of eorrosion seale. 

Intergranular corrosion is encountered in many metal systems, often associated with the presence of precipitates at grain boundaries. In the case of stainless steels, one widely accepted theory states that the precipitation of chromium carbides leads to a chromium-denuded zone which undergoes rapid corrosion. 

Irradiation for 60 min caused more damage. Partial detachment and desquamation of endotheUocytes from the basal membrane took place (Fig. 30.1c). Thrombocytes, fibrin threads and erythrocytes were found on the surfaces of denudated zones. 

Plate 1. Denuded zone around a small iron smelter without cleaning of stack gases as a result of high ambient concentrations of SOj. Heavy soil erosion after loss of protective vegetation could not be prevented by terraces. Typical for careless industry in the Fifties. Biersdorf, W. Germany, 1959 (Photograph by courtesy of H. van Haut). 

Finally, application of the thermochemical and kinetic databases to determine the denuded zone width of the intrinsic gettering annealing has been demonstrated. 

During a polymer flood, because of polymer adsorption, a polymer denuded zone forms at the front of polymer slug. If a surfactant slug is injected ahead of a polymer slug, however, adsorption sites are occupied by surfactant. In some cases, polymer loss is reduced to an insignificant level owing to the so-called competitive adsorption, discussed earlier. Thus, a polymer denuded zone may... 

The trend here is for simpler lifetime measurement methods and a better understanding of the mechanisms within the device or material, giving rise to the measured values. For example, for the denuded zone/precipitated interior intrinsic gettering mechanism, a high t and low is clearly an indicator of a successful process. 

All the considerations for influencing the precipitation behaviour so far aim at well defined denuded zones which ensure the trap density Ny in the device active areas to be extremely small and the diffusion length to be well controlled. It is possible to calculate the profile of interstitial oxygen (fig.14) near the surface of the wafers. In connection with material of suitable precipitation rates it is possible to tailor wafers very well to specific device needs. Moreover, it can be engineered in a way that the final device needs can be satisfied for different manufacturing processes /25/. Fig.15 gives an example, how the depth of the denuded zone can be varied due to different preanneals. 

Andrews 724/ and more recently Watanabe et al /26/ developed models for the denuded zone formation within the limitations of measurement tools for oxygen profiles. For MOS processing, which essentially has process temperatures around and below 1000°C, a three step process is required to set up a denuded zone. One for the out-diffusion of oxygen, one for nucleation of SiOx preci-... 

With controlled precipitation, intrinsic gettering takes place during processing. Because of the ability of metallic impurities to decorate other defects, specifically oxygen precipitates, these impurities are removed out of the denuded zone into the bulk of the wafer with beneficial effects for device performance. 

Oxygen precipitates are recombination centers for minority carriers. They then are limiting the diffusion length Lq. A silicon wafer with a denuded zone and a high concentration of precipitates is electrically an inhomogeneous system. 

Tayloring a denuded zone depth results in tayloring diffusion length and trap density Ny. This has influence on different aspects for device performance. 

As the denuded zone concept relies essentially on a two layer system other applications seem to be possible, which heavily depend on material with layered differences in lifetime (diffusion length). One of the areas is in the field of --particle sensitivity. 06 -particles emitted from the device package or from the environment create electron-hole pairs, which may lead to soft errors in memories. This can be suppressed if the carriers generated recombine at the oxygen precipitates rather than being able to diffuse to the memory cells. 

Figure 15. Denuded zone depths for different process conditions.

Figure 16. Effective diffusion length vs. depth of denuded zone. Note There is a geometrical limit of the maximum diffusion length.

Fig.20 gives a comparison between cross sectional measurements and SPV measurements, which are non-destructive. In essence, there exists correlation between the measurements of the depth of the denuded zone on the cleavage face and the diffusion length. SPV thus could be developed into a tool for non-destructively measuring DZ depths. 

Reflection electron microscopy (REM) is used to observe denuded zones created at the terrace edges. By analyzing the denuded zones as a funetion of temperature and time, the diffusivity may be calculated. This technique is limited to studying surfaees where islands may be easily distinguished, as on the Si(lOO) surface where the 2x1 and 1x2 reconstructions that alternate in successive layers are clearly visible [95Doil]. The resolution of the technique is on the order of 0.5 pm... 

Denuded Zone Depth and area of a silicon wafer that contains no oxygen precipitates or interstitial... 

A method was developed for determining the surface diffusion coefficient and activation energy of Ge adatoms on (001). That is, Ge self-assembled quantum dots which were grown on a relaxed SiGe buffer-layer nucleated preferentially over a network of buried 60° dislocations. The surface sites over the buried dislocations acted as sinks for Ge adatoms. The pre-exponential term in the diffusion constant could also be determined by using Pick s first law, and the observation that the total incident flux of Ge adatoms which impinged on the denuded zones equaled the average rate of volume increase of self-assembled quantum dots over dislocations. The diffusion of Ge adatoms on Si (001) could be described by ... 

Prior to the start of this erosion study, an optimum test time was determined that represented erosion wear conditions localized only within the carbide denuded region (upper carburized surface layer, exclusive of the carbide precipitated and substrate regions). In order to study the influence of carburization on erosion wear, it was important that the wear scar for this investigation was contained within the upper carburized surface layer (denuded zone). The optimum test time to achieve this was determined to be 8 min (valid for temperatures, 25-900°C). The erosion behavior test parameters for the pre-carburized coating and 25Cr-35Ni-Fe samples are shown in Table 25.3. 

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