3. Production Process Characterization
3.5. Case Studies
3.5.1. Furnace Case Study

3.5.1.3.

Identify Sources of Variation

The next part of the analysis is to break down the sources of variation.

Box Plot by Run

The following is a box plot of the thickness by run number.

Conclusions From Box Plot

We can make the following conclusions from this box plot.

1. There is significant run-to-run variation.

2. Although the means of the runs are different, there is no discernable trend due to run.

3. In addition to the run-to-run variation, there is significant within-run variation as well. This suggests that a box plot by furnace location may be useful as well.

Box Plot by Furnace Location

The following is a box plot of the thickness by furnace location.

Conclusions From Box Plot

We can make the following conclusions from this box plot.

1. There is considerable variation within a given furnace location.

2. The variation between furnace locations is small. That is, the locations and scales of each of the four furnace locations are fairly comparable (although furnace location 3 seems to have a few mild outliers).

Box Plot by Wafer

The following is a box plot of the thickness by wafer.

Conclusion From Box Plot

From this box plot, we conclude that wafer does not seem to be a significant factor.

Block Plot

In order to show the combined effects of run, furnace location, and wafer, we draw a block plot of the thickness. Note that for aesthetic reasons, we have used connecting lines rather than enclosing boxes.

Conclusions From Block Plot

We can draw the following conclusions from this block plot.

1. There is significant variation both between runs and between furnace locations. The between-run variation appears to be greater.

2. Run 3 seems to be an outlier.