The MOS planox process

In MOS integrated circuits, active areas are distinguished from inactive areas by the difference in threshold voltage. This difference is usually achieved by using vastly different oxide thicknesses in the respective areas. A result of this practice is that: (a) photoengraving of thick oxide is necessary; (b) the surface of the wafer presents large steps in the oxide (1.5 µ typical). Where metallization runs across these steps weak spots can occur unless the shape of the step is carefully controlled or very thick metal used.