2. Measurement Process Characterization
2.6. Case studies
2.6.1. Gauge study of resistivity probes

2.6.1.2.

Analysis and interpretation

Graphs of probe effect on repeatability

A graphical analysis shows repeatability standard deviations plotted by wafer and probe. Probes are coded by numbers with probe #2362 coded as #5. The plots show that for both runs the precision of this probe is better than for the other probes.

Probe #2362, because of its superior precision, was chosen as the tool for measuring all 100 ohm.cm resistivity wafers at NIST. Therefore, the remainder of the analysis focuses on this probe.

Plot of repeatability standard deviations for probe #2362 from the nested design over days, wafers, runs

The precision of probe #2362 is first checked for consistency by plotting the repeatability standard deviations over days, wafers and runs. Days are coded by letter. The plots verify that, for both runs, probe repeatability is not dependent on wafers or days although the standard deviations on days D, E, and F of run 2 are larger in some instances than for the other days. This is not surprising because repeated probing on the wafer surfaces can cause slight degradation. Then the repeatability standard deviations are pooled over:

• K = 6 days for K(J - 1) = 30 degrees of freedom
• L = 2 runs for LK(J - 1) = 60 degrees of freedom
• Q = 5 wafers for QLK(J - 1) = 300 degrees of freedom

The results of pooling are shown below. Intermediate steps are not shown, but the section on repeatability standard deviations shows an example of pooling over wafers.

Pooled level-1 standard deviations (ohm.cm) Probe Run 1 DF Run 2 DF Pooled DF 2362. 0.0658 150 0.0758 150 0.0710 300

Graphs of reproducibility and stability for probe #2362

Averages of the 6 center measurements on each wafer are plotted on a single graph for each wafer. The points (connected by lines) on the left side of each graph are averages at the wafer center plotted over 5 days; the points on the right are the same measurements repeated after one month as a check on the stability of the measurement process. The plots show day-to-day variability as well as slight variability from run-to-run. Earlier work discounts long-term drift in the gauge as the cause of these changes. A reasonable conclusion is that day-to-day and run-to-run variations come from random fluctuations in the measurement process.

Level-2 (reproducibility) standard deviations computed from day averages and pooled over wafers and runs

Level-2 standard deviations (with K - 1 = 5 degrees of freedom each) are computed from the daily averages that are recorded in the database. Then the level-2 standard deviations are pooled over: L = 2 runs for L(K - 1) = 10 degrees of freedom Q = 5 wafers for QL(K - 1) = 50 degrees of freedom as shown in the table below. The table shows that the level-2 standard deviations are consistent over wafers and runs.

Level-2 standard deviations (ohm.cm) for 5 wafers Run 1 Run 2 Wafer Probe Average Stddev DF Average Stddev DF 138. 2362. 95.0928 0.0359 5 95.1243 0.0453 5 139. 2362. 99.3060 0.0472 5 99.3098 0.0215 5 140. 2362. 96.0357 0.0273 5 96.0765 0.0276 5 141. 2362. 101.0602 0.0232 5 101.0790 0.0537 5 142. 2362. 94.2148 0.0274 5 94.2438 0.0370 5 2362. Pooled 0.0333 25 0.0388 25 (over 2 runs) 0.0362 50

Level-3 (stability) standard deviations computed from run averages and pooled over wafers

Level-3 standard deviations are computed from the averages of the two runs. Then the level-3 standard deviations are pooled over the five wafers to obtain a standard deviation with 5 degrees of freedom as shown in the table below.

Level-3 standard deviations (ohm.cm) for 5 wafers Run 1 Run 2 Wafer Probe Average Average Diff Stddev DF 138. 2362. 95.0928 95.1243 -0.0315 0.0223 1 139. 2362. 99.3060 99.3098 -0.0038 0.0027 1 140. 2362. 96.0357 96.0765 -0.0408 0.0289 1 141. 2362. 101.0602 101.0790 -0.0188 0.0133 1 142. 2362. 94.2148 94.2438 -0.0290 0.0205 1 2362. Pooled 0.0197 5

Graphs of probe biases

A graphical analysis shows the relative biases among the 5 probes. For each wafer, differences from the wafer average by probe are plotted versus wafer number. The graphs verify that probe #2362 (coded as 5) is biased low relative to the other probes. The bias shows up more strongly after the probes have been in use (run 2).

Formulas for computation of biases for probe #2362

Biases by probe are shown in the following table. The reader can download the following data as a text file. Differences from the mean for each wafer Wafer Probe Run 1 Run 2 138. 1. 0.0248 -0.0119 138. 281. 0.0108 0.0323 138. 283. 0.0193 -0.0258 138. 2062. -0.0175 0.0561 138. 2362. -0.0372 -0.0507 139. 1. -0.0036 -0.0007 139. 281. 0.0394 0.0050 139. 283. 0.0057 0.0239 139. 2062. -0.0323 0.0373 139. 2362. -0.0094 -0.0657 140. 1. 0.0400 0.0109 140. 281. 0.0187 0.0106 140. 283. -0.0201 0.0003 140. 2062. -0.0126 0.0182 140. 2362. -0.0261 -0.0398 141. 1. 0.0394 0.0324 141. 281. -0.0107 -0.0037 141. 283. 0.0246 -0.0191 141. 2062. -0.0280 0.0436 141. 2362. -0.0252 -0.0534 142. 1. 0.0062 0.0093 142. 281. 0.0376 0.0174 142. 283. -0.0044 0.0192 142. 2062. -0.0011 0.0008 142. 2362. -0.0383 -0.0469

How to deal with bias due to the probe

Probe #2362 was chosen for the certification process because of its superior precision, but its bias relative to the other probes creates a problem. There are two possibilities for handling this problem: Correct all measurements made with probe #2362 to the average of the probes. Include the standard deviation for the difference among probes in the uncertainty budget. The better choice is (1) if we can assume that the probes in the study represent a random sample of probes of this type. This is particularly true when the unit (resistivity) is defined by a test method.