Nitride Overcoat

FIGURE 12.18 AFM scan (above) and cross section across the dashed line (below) of a transistor area on a wafer with oxide etch back after planarization and nitride strip. 

FIGURE 12.19 Schematic cross section of a wafer with continuous nitride overcoat. 

The nitride layer could alternatively be structured with additional lithography and etching steps in such a way as to leave the protective overcoat only in isolation areas (Fig. 12.20) [33,34]. In this case, planarity is further improved at the expense of the increased process complexity. Using a high-selectivity slurry in combination with a patterned nitride overcoat allows complete elimination of dishing in large isolation areas (Fig. 12.21). 

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Cutiongco, E. C., Li, D., and Chung, Y. W., "Tribological Behavior of Amorphous Carbon Nitride Overcoats for Magnetic Thin-film Rigid Disks, 7. Tribol., Vol. 118, 1996, pp. 543-548. 

FIGURE 12.21 Optical microscope picture of wafer surface with a patterned nitride overcoat after planarization and nitride strip. Notice the complete elimination of dishing (color uniformity in the green isolation area). 

Metal is then deposited into the opened vias (openings) in the oxide layer and over its surface. During the subsequent photolithography process, it is patterned to form the desired electrical interconnections. These two steps are repeated for each succeeding level to produce additional levels of interconnections. Finally, a protective overcoat of oxide/nitride is applied (passivation), and vias are opened so that the wires eonnectlng the IC chip to its carrier package can be bonded to output pads. 

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