Monitoring of periodic multilayers by the level method

Advanced algorithms for monochromatic monitoring of complex optical coatings

The paper proposes to use combinations of two algorithms for the on-line prediction of the termination times of layer depositions in the case of complex optical coatings with large numbers of layers with significantly different layer thicknesses. Notch filters with very thin and thick coating layers are considered as examples of coatings where such combinations of algorithms can be useful. Computational manufacturing experiments are used to estimate expected levels of thickness errors and the strength of the error self-compensation effect for various notch filter designs. It is shown that the estimates obtained can be used to select the optimal combination of the considered algorithms.

Pre-production estimates for the choice of strategy and algorithm for monochromatic monitoring of the production of optical coatings

The paper proposes an approach for the pre-production selection of the most promising combination of a monochromatic monitoring strategy and a monitoring algorithm used for predicting termination of layer deposition. The approach is based on the previously introduced estimate of the strength of the expected error self-compensation effect and the new estimate, to the best of our knowledge, of the strength of the cumulative effect of thickness error growth presented in this paper. The application of the proposed approach is demonstrated using computational manufacturing experiments with various types of optical coating designs.

Production of ultra-steep dichroic filters with broad band optical monitoring

The feasibility of using direct broad band optical monitoring control in the fabrication of the ultra-steep dichroic filters based on resonant structures is investigated. Using computational manufacturing and deposition experiments, the role of the errors self-compensation effect is clarified by comparing the results of direct broad band optical monitoring and time monitoring. The errors correlation strength of ultra-steep dichroic filter is analyzed and it shows that the correlation calculated by the current model is not strong. The relationship between errors correlation and errors self-compensation effect for the ultra-steep dichroic filter is discussed.

Noble Infrared Optical Thickness Monitoring System Based on the Algorithm of Phase-Locked Output Current–Reflectivity Coefficient

With the rapid development of modern science and technology, the application of infrared (IR) optical thin films is increasingly widespread, including in aerospace, autonomous driving, military development, and the fight against Coronavirus Disease 2019 (COVID-19), in which thin film devices play an important role. Similarly, with the improvement of user requirements, the precision requirements for the preparation of optical films are also developing and improving. In this paper, the IR optical thickness monitoring system is studied with the noble algorithm introducing the phase-locked amplifier current and single film reflectivity coefficient. An optical control system is developed to monitor the IR thin film accurately, which aids the IR narrow-band filter film research.

Advantages and challenges of optical coating production with indirect monochromatic monitoring

In this paper, we present our recent studies on raising the quality of optical coating production with an indirect monochromatic monitoring system. Preproduction error analysis and computational manufacturing are used to estimate potential advantages of application of indirect optical monitoring. It is then demonstrated that a key issue for realization of this advantage is accurate specification of tooling factors for layer thicknesses on test glasses. The tooling factors are precalibrated using single layer depositions and then are corrected using results of reverse engineering for the first production run. It is found that a gradual variation of tooling factors of low index layers is the main error factor in the first deposition run. Finally, we redeposit our coating with a modified monitoring strategy, taking into account this factor. The new experimental results show excellent correspondence with the theoretical spectral performance.

Automated construction of monochromatic monitoring strategies

We focus our efforts on development of an advanced monochromatic monitoring strategy to assist the optical coating engineer in finding a single wavelength or a sequence of monitoring wavelengths that meet simultaneously several practical demands, namely, specified input and output swing values, specified amplitude of a monitoring signal variation, and the distance between trigger point and the last signal extremum. Additionally, the most important demand is that the number of different monitoring wavelengths must be as small as possible. Manual construction of such a monitoring strategy is almost impossible because of a large number of conditions to be satisfied. We propose an algorithm that automatically generates a monitoring spreadsheet so that all demands can be satisfied as closely as possible. We consider six typical design problems and obtain a series of solutions for each of them. Then, we provide computational simulations of deposition processes assuming that they are controlled by monochromatic monitoring with the monitoring strategy generated by our algorithm, and we demonstrate how an optical coating engineer can select design solutions that exhibit the highest production yields.

An optical monitoring method for depositing dielectric layers of arbitrary thickness using reciprocal of transmittance

An approach extracting information of both optical monitoring signal and phase thickness of deposited layer on a trace diagram is proposed. Realtime fitting and calculation are performed to get both practical thickness and refractive index of deposited layer with the assist of quartz crystal monitoring for keeping steady rate of deposition. Monitoring error of thickness using this approach is analyzed. It was used to obtain the refractive indices and thickness of Ge layer and SiO layer in in situ measurement mode, and the results were compared with those of ex-situ spectral measurement using infrared spectrometer. The effectiveness of the proposed monitoring method was verified by fabricating narrow bandpass filter consisting of quarter-wave and non-quarter-wave layers.

Monitoring error compensation in general optical coatings

Error compensation in optical monitoring was demonstrated many years ago for narrow bandpass and edge filters. It is shown here that the compensation effects can apply to a broader range of more general coatings. This is illustrated with a very broadband antireflection coating design of 20 layers and a ramp design of 13 layers. The choice of monitoring wavelength and monitoring strategy are important, and suggestions are made concerning those choices. The results are derived from and demonstrated by computer simulations of the monitoring processes. The importance of monitoring directly on only one part and with one wavelength throughout the process in order to obtain the benefits of error compensation is emphasized.

Dual wavelengths monitoring for optical coatings

A new monitoring method based on the use of dual wavelengths monitoring is proposed. Firstly, the sensitivity of each layer in an optical coating for the monitoring wavelength is calculated by admittance equations. Then two appropriate monitoring wavelengths are chosen to make sure that every layer has a sensitive terminal point. The thickness error of the layer can be compensated. For quarter-wave multilayer and nonquarter-wave multilayer optical coatings, the advantage of this new monitoring method has been demonstrated by both the theoretical analyses and experimental results.

Computational experiments on optical coating production using monochromatic monitoring strategy aimed at eliminating a cumulative effect of thickness errors

We present an algorithm for the automatic generation of a monitoring spreadsheet that enables the most effective application of the termination level correction algorithm proposed in our previous publication. On a whole the presented monochromatic monitoring strategy entirely eliminates a cumulative effect of thickness errors in optical coating production using direct optical monitoring. The effectiveness of the new monitoring strategy is demonstrated by computational manufacturing experiments in which such error factors as instability of deposition rates, errors in measured transmittance data, shutter delays, and variations of layer refractive indices from their theoretical values are simulated.

Elimination of cumulative effect of thickness errors in monochromatic monitoring of optical coating production: theory

We propose an algorithm for correcting deposition termination levels that allows elimination of the cumulative effect of errors in previously deposited layers. For the application of this algorithm at least one monitoring signal extremum should be registered during a layer deposition. We also derive a theoretical relation for the estimation of errors in layer refractive indices based on the results of on-line monitoring measurements. At least two monitoring signal extrema are required for its application.

Statistical approach to choosing a strategy of monochromatic monitoring of optical coating production

We presents what we believe to be a new approach to choosing a sequence of monitoring wavelengths for monochromatic monitoring of optical coating production. The new approach is based on a preproduction estimation of expected levels of errors in thickness of layers of a deposited coating. It is demonstrated that the proposed monitoring strategy reduces the effect of accumulation of thickness errors. An advantage of the new monitoring strategy becomes especially noticeable when the number of monitored layers is equal to several dozens.

Variation of band-edge position with errors in the monitoring of layer termination level for long- and short-wave pass filters

Optical monitoring of periodic thin-film stacks by the termination of each layer at the same constant photometric level has certain advantages. One of these principal advantages is the error compensation effect in the vicinity of the monitoring wavelength. In this study, we examine, by simulation, the effect of an error in the knowledge of the absolute value of the photometric termination level on the probable stability in the manufacture of the edge position of a blocked band. The results include equations that allow the determination of the appropriate values of parameters associated with the optimum termination levels to minimize the effects of such errors.

Reflectance diagram-aided technique for optical coating design and monitoring

An amplitude reflectance diagram is a useful tool for visualizing the relationships between the performance of an optical coating and the optical parameters. This paper gives a brief description of the construction and characterization of the reflectance diagrams for different indices. Then, applications of reflectance diagramaided techniques to the design of both phase dispersion-induced narrow reflection and transmission filters, as well as to the optical monitoring of the symmetric multilayers of types (A2BA) and (AB2CBA), are illustrated. Some experimental results of phase dispersion-induced narrowband filters are reported.

Optical thickness monitoring sensitivity improvement using graphical methods

Semidirect level monitoring has error compensation capabilities of potential benefit to optical coatings of certain types. A review of the principles of level monitoring using circle diagrams shows how to design a level monitoring scheme for various cases, including the use of precoated monitoring chips. The film thickness sensitivities for various optical monitoring strategies differ considerably. Optimum level trigger point monitoring offers improved sensitivity of change in reflectance vs change in optical thickness. This procedure is also expected to give small thickness errors when optically monitored. Optimum level trigger point monitoring, and its application, is fully described.