In a conventional semiconductor device, dielectric isolation is performed by LOCOS isolation, trench isolation or the like. Recently, according to a combination of a deep trench provided by the trench method and a SOI structure, it is possible to make a semiconductor device with complete dielectric isolation.
The SOI-type semiconductor device is realized by forming a plurality of semiconductor monocrystalline regions of a silicon or the like on a insulating layer such that they are dielectrically isolated each other, and forming a transistor element thereon. Therefore, when a MOS integrated circuit is formed by employing the SOI structure, latch-up can be prevented since a well structure is not necessary.
On the other hand, since the SOI-type semiconductor device is made by forming elements on the insulating layer and isolating the elements by an insulator inserted therebetween, it is not affected by .alpha. rays which are radiated from a radioactive element such as uranium, thorium or the like that is in very small quantity included in a package or the like. Therefore the device can be radiation-proof. Because the SOI structure has some advantages as mentioned above, it has been employed in making a semiconductor device.
In the semiconductor integrated circuit device, the elements such as a transistor, a resistor or the like are not always uniformly disposed in the device, and it may include an element isolated from the other elements due to the design of a circuit or the patterning of a layout. When the silicon oxide film is polished, the resistance to the polishing in the isolated element is less than that in the other elements. Therefore, even if the polishing of the silicon oxide film is properly stopped, a part of the isolated element may be polished, thereby forming an uneven surface.