(a) Thirty observations on the oxide thickness of individual silicon wafers are shown in table 6E.23. Use these data to set up a control chart on oxide thickness and a moving range chart. Does the process exhibit statistical control? Does oxide thickness follow a normal distribution?

(b) Following the establishment of the control charts in part (a), 10 new wafers were observed. The oxide thickness measurements are as follows:
(c) Suppose the assignable cause responsible for the out-of-control signal in part (b) is discovered and removed from the process. Twenty additional wafers are subsequently sampled. Plot the oxide thickness against the part (a) control limits. What conclusions can you draw? The new data are shown in Table 6E.25.

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TABLE 6E.23 Data. for Exercise 6.65 Oxide Oxide Wafer Thickness Wafer Thickness 45.4 18.6 49.5 44.0 50.9 6 55.2 45.5 52.8 45.3 46.3 53.9 12 49.8 46.9 14 49.8 45.1 16 17 58.4 51.0 18 2 47.1 45.7 60.6 51.0 53.0 56.0 47.2 48.0 55.9 50.0 47.9 53.4 19 20 21 23 24. 25 26 27 28 29 30 10 13 TABLE 6E.25 Additional Data for Exercise 6.65 art (c) Oxide Oxide Wafer Thiekness Wafer Thickness 43.4 2 46.7 44.8 50.0 2 46.7 2 61.2 46.9 4 51.3 4 44.9 5 49.2 5 4.2 53.3 7 48.4 17 .1 474 51.3 45.6 20 45 13 46.5 8 50. 53.7 18 19 10