Overview

In Chapter 1 it was shown that the defect level, DL, after test was given by the theoretical relationship

Both the 'goodness' of the fabrication process, the yield, Y, and the effectiveness of the testing procedure, FC, are involved in this result.

Modelling of the number of good die on a wafer, which is principally dependent upon die size and process goodness and not upon circuit complexity, has been extensively studied, since with accurate modelling the yield, and hence the cost, of new circuits may be forecast. Also, once the detailed modelling parameters have been determined, the quality of the production lines can be maintained and possibly improved. However, the available modelling theory and available parameter values usually lag the latest production process, and as a result the yield of most production lines has historically tended to be higher than that predicted by modelling theory. (This is also true of reliability predictions for most products, where actual reliability, except for catastrophic occurrences, tends to be somewhat higher than predicted.)

Wafer yield Y, where Y has some value between 0 and 1, is normally considered to be a function of (i) the density of defects per unit area of the wafer, D ₀, (ii) the area of the die, A, and (iii) an empirical parameter, ?, which is specific to a particular fabrication process or model being used. Many equations have been proposed for Y, all of which have a statistical...