Optical Monitors - Performance

Impact of Optical Monitoring on Product Performance & Manufacturing Yield

Target Film Stack Characteristics
The graphs below show the target transmission and optical density spectra of the high performance steep edge notch filter used in this example.

General design features include;

        Material: TiO2 & SiO2, ebeam deposition with IAD
        Film stack design
            Demanding 34 layer film stack
            with non-QW termination
        Passband monitoring

Coating Scheme Design: Optimisation for Optical Monitoring

The growth of a film stack requires an endpoint for each and every layer to be achieved. Depending upon the nature of the film stack, the effect of any ‘cut point’ errors can rapidly become compounded with each subsequent layer, and consequently, the target product can soon become unmanufacturable as the number of required layers increases. This is the main reason why optical monitoring is crucial to achieving high performance optical coatings.

In a coating system there are many possible ways to achieve a given film stack design including decisions over test glass changes, monitoring wavelengths, cut point algorithms, etc, etc. All of these factors have an impact upon the achievable accuracy of each layer and upon the ultimate performance of the entire film stack. For this reason, Intellevation has developed a unique simulation capability, FilmSimulator, integrated directly with its optical monitoring hardware.

FilmSimulator Screen

The film stack design is achieved using one of a number of commercially available film stack design packages. This is then imported into Intellevation’s FilmMaker package and processed through FilmSimulator during the optimisation process. Uniquely, FilmSimulator enables the coating engineer to optimise the coating scheme for their particular coating chamber, optical monitor and target product specification combination.

Cut-Point Accuracy

The following graphs show the cut-point accuracy achieved on a film by film basis during the film stack deposition. Using an optimised Quartz Crystal Monitoring scheme, cut-point accuracies of ± 1% can be achieved. This can readily be reduced to less than ± 0.25% using Optical Monitoring.

	Best cut point accuracy achieved with Quartz Crystal Monitoring
	Cut point accuracy achieved with Optical Monitoring

Impact on Product Performance and Manufacturing Yield

One of the most important features of many steep edge optical filters is the precise position of the band edge. In this case a band edge repeatability of ± 0.7nm is required.

The following spectra show the performance of film stacks produced over many growth runs using Quartz Crystal Monitoring compared with those produced using Intellevation’s IL551 Optical Monitor. The Quartz Crystal controlled growth runs show a spread in band edge positions of greater than 3nm resulting in a low process yield.

	Quartz Crystal Monitoring Band Edge Spread > 3 nm
	Optical Monitoring Band Edge Position ± 0.1 nm

However, use of the Optical Monitor enables the band edge position to be controlled reliably to around ± 0.1 nm resulting in a significant increase in process yield. This improvement comes about because of the IL551's ability to simultaneously combine non-qw cutting with optical autocompensation which progressively minimises cut point erors. These combined features result in a highly reproducible output filter characteristic and therefore a significant increase in process yield.

Conclusions

Band edge position repeatability achieved using IL551 Optical Monitor ± 0.1nm.
             16X improvement over Quartz Crystal Monitoring

Autocorrection achieved using Optical Monitor on NON-QW cut points!

Optical Monitor achieves true optical thickness even if refractive index variations occur during process. By comparison, the quartz crystal monitor is insensitive to these in-process variations and therefore is unable to make any correction.

Optical Monitor compensates for early / late cut on QW and non-QW processes.

Simulation of complete process using FilmSimulator and FilmCharacter.

The Optical Monitor benefits are applicable over a wide range of precision coatings, giving significant improvements in repeatability and process yield.

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