Reverse engineering with witness chips

OptiRE allows you to export and process multi-can measurements recorded in the course of the deposition process also in the case of indirect optical monitoring (monitoring with witness chips).

Distribution of layers on witness chips are specified in the monitoring spreadsheet (Results --> Monitor) in OptiLayer module (important!)

You can start the reverse engineering with witness chips by specifying the number of chips, materials and thicknesses of chips through Data--> Witness Chips...

Example. a 25-layer edge filter operating at the angle of incidence of 45 degrees and exhibiting high reflectance in the range 583-683 nm and high transmittance in the range 670-710 nm. Spectral reflectance and transmittance of the filter are shown in Fig. 2 and its refractive index profile is shown in Fig. 3.

The design comprises layers of high-index material Nb2O5 with n=2.2 and low-index material SiO2 with n=1.47. Materials abbreviations are H and L, respectively.

indirect optical monitoring

Fig. 1. Specification of the witness chips in OptiRE.

edge filter spectral properties

Fig. 2. Spectral reflectance and transmittance of the 25-layer edge filter.

dichroic filter thicknesses

Fig. 3. Refractive index profile (layer thicknesses) of the 25-layer edge filter.

In order to monitor the deposition of the edge filter, four witness chips were chosen. The distribution of design layers is shown in Fig. 4. Chips 1 and 3 contain H-layer only. The first layers on Chips 1 and 3 are H-layers and the next layers are L-layers. 

indirect monitoring

Fig. 4. Distribution of layers of the 25-layer edge filter on witness chips. Chips 1 and 3 accumulate H-layers only.

multi-scan bbm measurements on chips

Fig. 5. Loading measurement data corresponding to witness chips shown in Fig. 4.

You can load the measurement data on witness chips through mutli-scan measurement option  Data--> Muli-scan measurements

You can load the data one by one though Load button and arrows,  chip numbers are specified in the corresponding column.

In the Fitting window (View --> Data Fitting), you can observe fitting of measurement data on chips by model data:

monitoring with witness chips

Fig. 6. Fitting of multi-scan data by model data on witness chips. Final curves correspond to layer 11 (chip 1), layer 12 (chip 2), layer 24 (chip 3), and layer 25 (chip 4).

The multi-scan measurements data corresponding to the witness chips can be processed in the same way as conventional measurement data. 

characterization witness glasses

Fig. 7. Errors in layer thicknesses obtained by rando errors model.

Typically, it is necessary to compare the data corresponding to the model coating with the theoretical data (nominal spectral curves). This can be done through View--> RE Spectra.

optical coatings oblique incidence

Fig. 8. In the current example, it is important to compare model and theoretical transmittance at AOI=45°. The wavelength range can be different from the wavelength range where multi-scan (BBM) records was taken.

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