5.7 Tolerance and Matching of Electrical Parameters

A number of uncertainties during fabrication (e.g., as to the exact furnace temperature or the ion implantation dose) have as a result significant tolerances in the process parameters, for example, 5 percent for the oxide thickness or 10 percent in the body doping. These, in turn, imply corresponding tolerances in the electrical parameters characterizing the various devices. ^[*] For example, for a typical process, tolerances for the threshold voltage might be 0.1 V; for the parameter K ?, 10 percent; for the body effect coefficient ?, 10 percent; for capacitances per unit area, 5 percent; for resistances, 20 percent. ^{[ ]} Geometric layout dimensions will also exhibit tolerances; for example, channel lengths and widths may have tolerances of 0.2 ?m.

Despite the above facts, designers often use among other pieces of information sets of "typical" electrical parameters, like those in Examples 5.1 and 5.2. These do, indeed, provide some rough estimate of what can be expected, and in so doing they serve a useful purpose. At the same time, it should be realized that only if all fabrication parameters involved happen to be at their mean value can one expect to find devices characterized by typical electrical parameters; and this is extremely unlikely. Relying on precise electrical parameter values for proper integrated circuit operation is absurd. Thus, tolerances must be taken into account in professional design.

The problem with using independent tolerance values, such as those given above, is that...