One technique for manufacturing semiconductor devices involves depositing epitaxial layers on a silicon wafer. The use of epitaxial layers provides several advantages, including the ability to alter dopant levels and types. Additionally, one may use a buried layer, also called a diffused subcollector, that is precisely located beneath an active device formed by later fabrication steps. Such precise location requires registration between the buried layer and the active device and the present invention provides a method for registrating these later device fabrication steps with the buried layer.
FIG. 1 illustrates a conventional semiconductor structure 2 comprising a monocrystalline silicon wafer substrate 4 on which an oxide mask 6 has been deposited. Substrate 4 has a &lt;100&gt; crystallographic orientation. As shown in FIG. 1, gap 8 in oxide mask 6 exposes a portion 10 of surface 12 of substrate 4. A doped region 14 is created at gap 8 by conventional means, typically by diffusing or implanting dopant into the exposed, oxide-free surface 10. The dopant is preferably driven into substrate 4 at an elevated temperature, such as 900.degree. C-1200.degree. C.
FIGS. 2-4A illustrate further fabrication steps in accordance with the prior art. As shown in FIG. 2, a delineation oxide layer 16 is formed on top of doped region 14, delineation oxide layer 16 is typically about 1000 .ANG. to 4000 .ANG. A thick. FIG. 3 illustrates the results produced by an oxide stripping procedure in which both oxide mask 6 and delineation oxide layer 16 have been stripped away from substrate 4. As shown in FIG. 4, after the oxide stripping procedure, the periphery of doped region 14 is defined by oxidation delineation step 18. As is seen in FIG. 4, step 18 has a quarter-circle or fillet shape.
One problem particular to substrates having a &lt;100&gt; crystal orientation is that, as epitaxial layers are deposited, the distinctiveness of the delineation step softens or is obscured; this softening or obscuring of the delineation step is often referred to as pattern wash-out. A washed-out delineation step is illustrated in FIG. 4A. The problem caused by the indistinct nature of the washed-out delineation step is that accurate registration with periphery 22 of doped region 14 becomes difficult or impossible.