Figure 1 shows a plot of the Reduced Si and Oxygen concentration of a Wafer of Silicon with a 1000 A overlayer of SiO2. Where the two lines cross is said to be the time which in this case, the SiO2 layer has been completely etched away. This time is about 420 seconds. 1000 A in 420 seconds corresponds to an etch rate of 143 A/sec. By calibarting the depth sacle to an etch rate of 143 A/sec. and analysing a second sample, in this case I just used the data from the calibratoin experiment, the correct depth of the SiO2 layer can be obtained form where the lines cross in the graph in Figure 2.
