To connect to the transistor's terminals, aluminum is deposited on the surface and down into the via holes. Another lithographic masking step is performed to mask off the areas where aluminum should remain, then a metal etch is performed to remove the excess aluminum. This leaves the metallization contacting only those regions which are required by design.

Another oxide layer is deposited on top of the aluminum to electrically isolate it from subsequent steps. Note that each time a deposition or etch process step is performed, the surface contours get more developed and are often exaggerated.

These surface contours create obstacles or steps which make it difficult to lay down subsequent metal layers. They also make it difficult to spin on photoresist evenly and control the depth of focus for exposure. For these reasons, it is highly desirable to smooth the surface between process steps....

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