of multi unit sites or contiguous units in a multi million gate layout design functional units for use in improving the fault coverage of the chip may be made from the backfill units without changing the size of the die the backfill units are free to be programmed at the metal level to be predetermined logical units after the integrated circuit is completely designed the backfill multi unit sites can be used to implement relatively minor changes in the netlist to improve the fault coverage as described in more detail below the changes in the netlist only require changes at the metal level which includes a relatively inexpensive metallization process the changes in the netlist may also be implemented in the original metallization process for no additional cost hence the need to provide an expensive new mask set for an integrated circuit revision process is avoided the netlist is typically edited manually with the backfill multi unit sites as needed to improve the fault coverage the multi unit sites locations and interconnections to nodes are selected with existing simulation tools with reference now to fig4 a method and tool for implementing fault coverage improvement for an integrated circuit such as an asic is described the method includes the use of a fault coverage.