A method for aliging a shallow trench isolation is provided. An aligning mark which is deeper than a prior technique is formed in a provided substrate. A trench is formed and an aligning trench is formed in the position over the aligning mark. A thick oxide layer is deposited on the semiconiductol substrate, in the trench and in the aligning trench. After a portion of the thick oxide layer removed, another portion of the thick oxide layer is removed by etching back. A gate oxide layer is formed on a substrate comprising the trench and the aligning trench. A polysilicon layer with the step-height profile in the position over the aligning mark is formed on the gate oxide layer.

In order to isolate a plurality of MOS and bipolar devices provided on the same chip, a plurality of first and second trenches are provided on a semiconductor substrate. Each of the first trenches is filled with silicon oxide containing no impurity and is used to isolate the MOS devices. On the other hand, the second trenches are formed within the first trenches. Each second trench is filled silicon oxide containing phosphorous and boron and is used to isolate the bipolar devices. The inner surface of each second trench is coated with a silicon nitride film for preventing boron (or phosphorous) from being diffused into the surrounding region.

A method, using multi-trench formation techniques, to define the respective depths of trenches having different widths. The method includes forming a buffer oxide layer and a polishing stop layer, in sequence, above a semiconductor substrate. Then, the buffer oxide layer, the polishing stop layer and the semiconductor substrate are defined to form at least one narrow trench. Thereafter, the buffer oxide layer, the polishing stop layer and the semiconductor substrate are again defined to form at least one wide trench whose depth is less than that of the narrow trench. Alternatively, the wide trench may be etch-defined first, followed by the narrow trench. However, in both cases the depth of the wide trench will be less than that of the narrow trench. Subsequently, an oxide layer is formed, which fills the narrow and wide trenches. Next, a portion of the oxide layer and a portion of the polishing stop layer are removed to form a planarized surface. Finally, the polishing stop layer and the buffer oxide layer are removed.

A laterally insulated buried zone of increased conductivity is fabricated in a semiconductor substrate. First, a reference layer is formed on a substrate with a buried zone of increased conductivity. Then the reference layer is patterned. A trench is produced in the substrate, and the insulation material used for filling the trench is applied to the structure thus produced. A planar surface is thereby formed in that the growth rate in the trench is faster than the growth rate on the reference layer adjacent the trench. Here, the reference layer is chosen such that the growth rate of the insulation material on the reference layer is at least a factor of two less than the growth rate of the insulation material on the surface of the trench which is to covered. This trench surface to be covered will usually be composed of substrate material. However, intermediate layers may also be provided.

A buried interconnect can be incorporated into the starting semiconductor on insulator wafer during the early stages of the circuit fabrication process flow for use with semiconductor devices. The buried interconnect provides an additional interconnect layer enabling an overall reduction in the silicon real estate occupied by interconnections. The buried interconnect has low resistance and can prevent the formation of unwanted PN junctions through the use of silicides. The buried interconnect and its fabrication method include an S0I wafer that has an oxidation layer formed on top of a semiconductor layer by oxidation, followed by an nitride layer formed on top of the oxide layer which then is selectively etched to form two trenches with regions of different depths. Some regions of the trenches are etched to remove all of the semiconductor layer in the trench to expose the buried oxide layer. In other regions, a thin layer of semiconductor is left at the bottom of the trenches. Next, all of the exposed surfaces of the trenches are oxidized and the oxide at the trench bottom is removed to expose the underlying semiconductor material. The underlying semiconductor material is then silicided to form a buried interconnect. The wafer, including the trenches, is subsequently covered with oxide and chemical-mechanical polishing is used to remove excess oxide outside the trenches.