Self-aligned channel stop for trench-isolated island

DETAILED DESCRIPTION In accordance with the present invention, the need to provide a separation region between the trench and a device region within the island, which results in an unwanted increase in occupation area, is obviated by channel stop formation technique through which the stop is self-aligned with the trench sidewall, so that there is no alignment tolerance between the stop and the trench wall.
Moreover, the width of the stop is controlled by lateral diffusion, which can be smaller than the width of a line defined by a mask, since that width is the minimum mask width plus twice the lateral diffusion of the layer defined by the mask.
In accordance with the present invention, an initial masking layer, e.
g.
oxide, through which the trench pattern is to be formed in the semiconductor island layer, is used as a doping mask for introducing a channel stop dopant into a surface portion of the semiconductor layer where the trench is to be formed.
The lateral diffusion of the dopant beneath the oxide and adjacent to the trench aperture defines the eventual size of the channel stop, such that its lateral dimension is less than its depth, since an inherent characteristic of such lateral diffusion is that the dimension of the lateral diffusion beneath the mask is less than the diffusion depth into the semiconductor layer exposed by the mask aperture.
The semiconductor layer is then anisotropically etched to form a trench to a prescribed depth, usually intersecting the underlying semiconductor substrate.
Because the etch goes through only a portion of the channel stop diffusion, leaving that portion which has laterally diffused beneath the oxide mask, the channel stop is self-aligned with the sidewall of the trench.
The trench may be then oxidized and filled with polysilicon material to complete the trench isolation process.