Polysilicon-Filled Trenches

With this optimization technique, topography is reduced after the planarization step. The STI trenches are filled with polysilicon instead of Si02 [39]. After trench, etching, the first patterned hard mask is stripped and a new continuous oxide/nitride layer is deposited for electrical isolation between the substrate and the trench polysilicon. After CMP, the dishing in isolation areas is counteracted by a high-temperature oxidation step, which oxidizes the upper part of the remaining polysilicon in the trenches. As the volume of the Si02 is 

FIGURE 12.25 REM image of a wafer after selective oxide deposition. 

FIGURE 12.26 Schematic cross sections showing the major steps in STI formation with polysilicon-filled trenches. 

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Most poly-Si CMP processes involve patterned oxide filled with polysilicon. The generation mechanism and physical appearance of the scratches for polysilicon CMP are the same as in the oxide CMP described earlier. Most Polysilicon CMP processes are rather selective toward silicon oxide. More specifically, the polysilicon removal rate is usually much higher than that for oxide. Such slurries usually contain low concentration of abrasive, which usually translates to low defect density. At low magnification, scratches on the poly-Si film after CMP usually look like a continuous line rather than chatter marks. At higher magnification, the scratch usually looks like irregular trench with multifaceted walls. 

Next, the sacrificial layer is patterned and holes are etched into the oxide using established lithography and etching processes. These holes will be filled and thus act as anchor points on the left end of the two cantilevers formed later (Fig. 5.3.1 e). In the next step, the functional polysilicon layer is deposited (Fig. 5.3.1b). The thickness of this layer determines the mechanical properties of the movable beam. The thicker it is, the stiffer the beam will be in the z axis, which is desirable for structures intended to move only in the xy direction. But its thickness is limited by the capabilities of the deposition process used. The functional layer is next patterned and etched (Fig. 5.3.1c). Depending on the thickness of the polysilicon layer, specific trench etch processes (as described later on) may be required, especially when this layer is rather thick. Finally, the sacrificial layer is removed (Fig. 5.3.1 d). This is typically done with wet or vapor phase etches to dissolve the silicon dioxide and leave parts of the functional structures free-standing and movable. When using wet etching, special care has to be taken to prevent Stic-... 

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