Passivation, sidewall

These processes are considerably more complex in actual CMOS fabrication. First, the lower layers of a CMOS stmcture typically have a twin-tub design which includes both PMOS and NMOS devices adjacent to each other (see Fig. 3b). After step 1, a mask is opened such that a wide area is implanted to form the -weU, followed by a similar procedure to create the -weU. Isolation between active areas is commonly provided by local oxidation of sihcon (LOCOS), which creates a thick field oxide. A narrow strip of lightly doped drain (LDD) is formed under the edges of the gate to prevent hot-carrier induced instabiUties. Passivation sidewalls are used as etch resists. A complete sequence of fabrication from wafer to packaged unit is shown in Figure 10. 

Nearly all pit models have been based on transport equations which strictly apply in solutions much more dilute than those usually found in pits, which exceed 1 M and often approach saturation in the metal chloride salt. The fundamental shortcoming of dilute solution transport theory is that it accounts only for interactions between ions and solvent molecules, and not between pairs of ions. Ion-ion interactions are manifested, for example, by deviations of the solution conductivity from values predicted by dilute solution theory, which become appreciable at concentrations as low as 0.01 M." This section will examine specific inaccuracies resulting from the dilute solution approximation, and point out cases where the use of concentrated solution transport models is tractable. Dilute and concentrated solution approaches will be compared in the context of a simple example of a one-dimensional pit with passive sidewalls. The metal and electrolyte solution were taken to be aluminum in 0.1 M NaCl. There are no cathodic reactions or homogeneous reactions in the pit, and the solution composition at the pit mouth is that of the bulk solution. This example was described in more detail in an earlier publication. This example is chosen because of its simplicity and since the behavior of the dilute solution model may be familiar to readers. 

Again, the emission pattern from a glass substrate is also shown for reference. Figure 17.18 shows the fluorescence output as a function of incidence angle for the passivated sample. At the surface-plasmon incidence angle, the total fluorescence enhancement compared to the reference is 12 (normalized to the 3.1% fill-fraction of the bottom surface of the nanoapertures), which is comparable to the enhancement obtained under full interior surface coverage (Figure 17.14). Therefore, the fluorescence enhancement (per unit area) is comparable for fluorophores on the bottom as for fluorophores on the sidewalls with backside detection, as before with individual apertures. 

Blauw, M.A. van der Drift, E. Marcos, G. Rhallabi, A. Modeling of fluorine-based high-density plasma etching of silicon trenches with oxygen sidewall passivation. J. Appl. Phys. 2003, 94, 6311-6318. 

The anisotropic nature of the etching by RIE and DRIE is a result of the directionality of the impinging ions. Sometimes it is useful to etch very deep channels or holes. Using conventional DRIE, some tapering of the sidewall profile is expected. However, an etching process has been developed where sequential etch/passivate steps are performed. This etching process, called the Bosch process, uses conventional DRIE methods as process gases for a short period of time, interrupts... 

A high-density decoupled plasma is most important to make this approach work, first to achieve sufficient sidewall passivation in a comparatively low-pressure regime, second to obtain a high fluorine concentration and thus high etch rates, and third to achieve a low plasma potential, close to ground potential. With a plasma potential close to ground, ion energy flow to the wafer surface is con-... 

A possible mechanism of formation of a sidewall passivation film in the case of silicon etching in a HCI/O2/BCI3 plasma is shown in Fig. 18 [77]. SiCf Hv-type byproducts are sputtered away by ion bombardment from the bottom of the trench. A portion of the sputtered flux strikes and sticks on the sidewalls on the trench. Oxygenation of the byproducts on the sidewalls results in a silicon dioxide type of film that resists etching. The sidewalls do not receive any appreciable ion bombardment and hence, depending on conditions, a rather thick inhibitor film may be formed. 

Fig. 18. Model of formation of sidewall passivation film during RIE of silicon in a HCI/O2/BCI3 plasma. Material sputtered away from the feature bottom (I) redeposits on the sidewall (2) where it is oxidized (3,4) to form a passivation film. After [77. ...

Complex boundary conditions especially when the surface is covered by thin films (sidewall passivation or damaged layers in plasma (Fig. 41), salt films in electrochemical engineering). 

Among the available anisotropic process schemes and recipes, the Bosch process which is named after the German company that developed and patented the technique is recognized as a standard process for ICP system. The Bosch approach is based on a variation of the Teflon-film sidewall passivation technique which avoids the recombination of active species in the gas phase. The deposition and etching steps are performed subsequently to control the sidewall profile of the 3D structures. In the deposition step, the precursor gas (CF4, C4Fg, CHF3, C2F6, or... 

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