How to Compare Pre- and Post Process Measurements on Semiconductor Wafers ("Pre- and Post Maps")

Different sets of wafer data (maps) can be compares by the difference and ratio method. One map can be subtracted from another map by subtracting individual measurements point by point. The resulting map will visualize the difference between the two maps.

This is a simple method, however it often requires two maps, which are identical regarding the individual coordinates of sites. In WAFERMAP such differences can be calculated without the necessity of identical grids (only the wafer diameter must be the same). This is possible because different grids will be transformed to the grid with higher density of sites before comparison.

It is also possible to calculate the ratio between two files in the same way. Using the difference method both maps should have almost the same mean value. Therefore one map must be normalized to the mean value of the second map.

Example:

Titanium was sputtered with 600Å thickness on 200mm prime wafers. Then the wafer was annealed in a RTP chamber at approximately 700°C in pure nitrogen. The sheet resistance was mapped before („premap“) and after the RTP step („postmap“).

The ratio was calculated between both maps (in WAFERMAP select „Multi-File-Operations“→ „Compare“→ select the files in the 2 windows on the left → choose „X/Y“ in the center → click on „OK”).

The result shows a „signature“ of the RTP chamber.

Premap after sputtering

Mean: 14.81 Ohms/sq
Max: 15.42 Ohms/sq
Min: 14.35 Ohms/sq
Std.Dev: 2.32 %
Range: 1.07 Ohms/sq
Interval: 1.0 %

Postmap after RTP

Mean: 8.99 Ohms/sq
Max: 9.38 Ohms/sq
Min: 8.6 Ohms/sq
Std.Dev: 2.35 %
Range: 0.78 Ohms/sq
Interval: 1.0%

Ratio

Mean: 1.65
Max: 1.68
Min: 1.61
Std.Dev: 0.87 %
Range: 0.07
Interval: 0.5 %