Trench filling device isolation

Optimum device isolation is not only restricted to the deep trench isolation between devices, but also calls for shallow trenches to allow isolation between the base and the subcollector reach-through diffusion. This trench reaches down to the subcollector only. A third trench shape option is that of wide, deep trenches. These would be formed outside of the immediate device areas and serve (after filling with SiO ) as the bed on which to locate the wiring lines which interconnect the different devices and circuits. Locating wires on such SiO2 areas reduces the wiring capacitances significantly. 

In the case of device applications, the filling material is some form of dielectric material, such as oxides, polymers, or polycrystalline silicon. (Metal filled trenches could serve as buried interconnecting device lines). As suggested previously, generally two types of trenches may have to be filled up - narrow ones for device isolation, and wider ones to benefit other circuit functions. The process requirements are somewhat different for each. However, independent of the trench width, the initial processing is the same. 

To achieve the lowest possible delay a bipolar switching transistor developed by IBM minimizes parasitic resistances and capacitances. It consists of self-aligned emitter and base contacts, a thin intrinsic base with an optimized collector doping profile, and deep-trench isolation (36). Devices must be isolated from each other to prevent unwanted interactions in integrated circuits. While p—n junctions can be used for isolation, IBM s approach etches deep trenches in the siUcon wafer which are filled with Si02 to provide electrical insulation. 

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