The present invention relates to a method of manufacturing a MOS device having a structure where an insulating film is deposited in a field region.
A semiconductor device such as a memory device using a MOSFET shown in FIG. 1 is known which has a coplanar (or LOCOS) structure to increase packing density and improve reliability. The semiconductor device comprises a p-type silicon substrate 1, a field oxide film 2 selectively formed in the field region, a gate oxide film 3 formed on an element formation region around the field oxide film 2, a gate electrode 4 formed on the gate oxide film 3, and an ion-doped layer 5 which functions as a channel stopper and which is formed in the silicon substrate 1 under the field oxide film 2. The coplanar structure has advantages in that indentation of the element surface is small since part of the field oxide film 2 is formed in the silicon substrate 1, and in that excellent step coverage is obtained in the aluminum wiring which connects semiconductor elements, thereby providing a highly reliable semiconductor LSI. However, lateral extrusions B of this structure which are called bird's beaks are formed in an oxide film, as shown in FIG. 1, adversely affecting microminiaturization of the element.
The narrower the effective channel width Weff of the MOSFET is, the greater the adverse effect. The actual thickness of the gate oxide film 3 becomes greater than a desired thickness thereof, thus increasing variation in the threshold voltage. In the worst case, the thickness of the gate oxide film becomes substantially the same as that of the field oxide film. As a result, the source-drain path is open. As shown in FIG. 2, the threshold voltage VT of the MOSFET is increased when the effective channel width Weff is decreased, thus resulting in the so-called narrow channel effect.
FIG. 3 is a schematic sectional view of a MOSFET having a BOX (Buried-Oxide Isolation) structure which solves the problem of the bird's beak encountered in the coplanar structure. The MOSFET comprises a silicon substrate 1, a silicon oxide film 6 deposited by low-temperature epitaxial growth in a groove with vertical walls, a gate oxide film 3 formed on the element formation region surrounded by the silicon oxide film 6, a gate electrode 4 formed on the gate oxide film 3, and an ion-doped layer 5 which functions as a channel stopper and which is formed in the silicon substrate beneath the silicon oxide film 6. Since the silicon oxide film 6 is formed in this MOSFET without involving high-temperature thermal oxidation, the bird's beaks are not formed. Therefore, the above structure has an advantage in that a MOSFET is formed which has a desired effective channel width. However, we found by experiments that when the gate electrode 4 extends onto the surface portion of the silicon oxide film 6, the threshold voltage VT of the MOSFET decreases with a decrease in the effective channel width Weff, as shown in FIG. 4.
Generally, the electrical characteristics of the MOSFET must not depend on the effective channel width.