Technical Issues - Layer Error Analyzer

Let's assume that you have developed reliable inverse-synthesis techniques which let you correct optical coating designs. If you can show that there are systematic thickness-dependent variations, you should be able to modify new designs and thereby achieve higher coating yields.

To address these issues, DESIGN 2.60 adds the Layer Error Analyzer (below). It's accessed via Design.. Error Analyzer. Like the FSPlot Module, you can return to the DESIGN Main Menu by pressing Esc or just clicking on the main screen. (If you forget and close the window, grid values are retained.) The Layer Analyzer requires that all layers are in physical thickness and that the design contains no repeated groups. The menu option is not available as follows: Interactor, Stack Mode, plotting vs. thickness, ellipsometry.

Click + Reference Design to clear the grid and add the theoretical coating design and + Best Fit Design to insert the reverse-engineered (inverse-synthesis) design. Then select the material you are interested in and click either Layer Thickness (monitor corrections depending on thickness of each layer) or Total Thickness (tooling factor corrections depending on total thickness of each layer). You can choose to plot % Deviation (centered about 0.0) or Correction Factor (centered about 1.0). As shown above there is a poor correlation with Total Thickness, but as can shown below there is an excellent correlation with individual Layer Thickness.

Switch to Correction Factor mode (Calculate Menu) and then click Quadratic Fit to determine a functional relationship. This can be seen below.

The remaining task is to apply the correction factors. WE DON'T REALLY WANT TO MODIFY THE DESIGN! What we want to modify are the thickness settings transferred to the coating chamber. Such modifications can be implemented in FilmStar BASIC in DESIGN (ThickList.bas, Maxtek DCM-220.bas), CRYSTAL, or MONITOR. What is needed in all cases is a Correction Factor function. Click Create BASIC Code in the Calculate Menu in order to generate the following:

Disclaimer...No claim is made that these are the only possible systematic errors. We look forward to extending and modifying the Layer Error Analyzer according to users' experiences.

Interestingly a Corrected Design gives a spectrum which is essentially a mirror image of the design with errors. This is most clearly illustrated by the flat laser output coupler.