# Improving the Lot Fabrication Stability and Performance of Silica Optical Films during PECVD
## 3 Results and Discussion  
### RI of the GeO\({}_{2}\)-SiO\({}_{2}\) Optical Film  
Figure 1 shows the RI maps and histograms for the 345 wafers with their specification limits and targets for RI-B, RI-C, RI-T, RI-R and RI-L. According to the requirements for silica optical waveguides, the lowest specification limit for the RI was set at 1.4620, the highest specification limit was set to 1.4634 and the RI target was set to 1.4627. The data presented in Figure 1 suggests the wafers were very close to having a normal distribution. The RI for RI-B, RI-R and RI-L were closest to the RI target. The RI for RI-C was lower than the RI target and the RI for RI-T was higher than the RI target. Cp is a measure of the potential capability of the process. Cp = (USL \(-\) LSL)/(6 \(\times\) StDev), where USL is the upper specification limit, LSL is the lower specification limit and StDev is the standard deviation. According to statistical process control (SPC) theory, the benchmark can be divided into several grades. Each grade represents the precision or potential capability of the process. In many industries, the Cp is divided into 4 grades: Cp \(\geq\) 1.33 means that the process is stable and the deviation can be reduced; 1.00 \(\leq\) Cp \(<\) 1.33 means that there is a risk of defective products in the process, so the specific reasons need to be checked and adjusted in time; 0.83 \(\leq\) Cp \(<\) 1.00 means that the process is unstable and that the specific reasons need to be found and adjusted in time; Cp \(<\) 0.83 means that the process is unsatisfactory and that it will need to analyzed and corrected [16]. RI-C, RI-T, RI-R and RI-L all returned values of more than 1.33, but the Cp index for RI-B was 1.32, suggesting the quality of the fabrication was a mildly variable, though the process was still under control. The Cp can be compared to the Cp. This is another measure of the potential capability of the process, however it is based on the minimum CPU and CPL. CPU is a potential capability measure based on the upper specification limit. CPL is a potential capability measure based on the lower specification limit. The Cpk benchmark has the same status as the Cp [16] and they are approximately equal, with the fabrication process being centered between the specification limits. Figure 1: _Cont._  
Figure 2 shows the RI between/within capability sixpack report for the 23 lots, and Figure 3 shows the RI between/within capability sixpack report for the 345 wafers. For the data in Figures 2 and 3, \(p\) (the probability value test) < 0.05. This means that the RI of the 23 lots (or 345 wafers) did not have a normal distribution for some reason (possibly temperature, relative humidity, material variation, process variation or machine aging). Looking at the data in Figures 2 and 3, the RI for the first six lots is very close to the RI target of 1.4627. It was the subsequent lots that began to deviate, with a maximum RI of 1.4632. The Cp/Cpk of the 23 lots and 345 wafers, however, exceeded 1.33, which indicates a capable process overall. Figure 1: Refractive index (RI) maps and histograms for the 345 wafers with their specification limits and target for RI-B, RI-C, RI-T, RI-R and RI-L. LSL = 1.4620, USL = 1.4634 and the target = 1.4627. Figure 3: RI between/within capability sixpack report results for the 345 wafers. Figure 2: RI between/within capability sixpack report results for the 23 lots. In this case, there was only a one-to-one mapping between the PLC process capability and the RI in the short term, with it becoming unstable in the longer term. In Figure 3, the tests for specific causes were set to \"six points in a row, all increasing or all decreasing\". When the number of wafers exceeded 120 [120 pcs/15 (pcs/lot) = 8 days], the RI value was higher than the UCL (upper control limit). In other words, after eight days, the Cp of the RI was below 1.33 and the UCL of the RI exceeded 1.4632. At this point, the SiH\({}_{4}\) flow needed to be adjusted. The main reason for this may have been aging of the PECVD machine.