Optical thickness monitoring sensitivity improvement using graphical methods

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.

Indirect broadband optical monitoring with multiple witness substrates

We present an indirect broadband optical monitoring approach based on using several witness substrates that are brought to a measurement position in a special sequence. Different witness substrates are used to monitor not groups of successive design layers but specially chosen design layers. An attractive feature of the presented monitoring approach is the ability to reliably control thin dielectric and metal layers. Considered examples demonstrate a good accuracy of the proposed approach.

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.

Synthesis and research of the optimum conditions for the optical monitoring of non-quarter-wave multilayers

Many optical filtering problems require the use of assemblies of layers with thicknesses that bear no obvious relationship to each other. Here we present the results obtained for a number of examples in which optical monitoring is performed with a change of control wavelengths for each layer of the stack. For this, it is necessary to determine for each layer the different wavelengths that provide an extremum of transmittance when the required thickness is achieved. We show that this leads, in some cases, to making the benefit of error compensation analogous to the well-known method used in the production of quarter-wave stacks. Because ion-assisted deposition and ion-plating techniques are suitable from the point of view of refractive-index reproducibility, optical monitoring can be used at a good level of performance. However, the production of high-quality optical thin films needs more than just the choice of a monitoring process. In particular, problems of uniformity are critical for high-performance coatings. Here we show how uniformity can be determined for each material involved.

Deposition error compensation for optical multilayer coatings. I. Theoretical description

The manufacture of complicated optical coatings consisting of many layers of different thicknesses can be a challenge, especially if the deposition technique does not produce dense layers. Deposition errors in a layer can affect not only the desired performance of a multilayer, but can also lead to a complete breakdown of the monitoring and control of subsequent layers. The best chance to achieve the desired optical performance of a multilayer involves deposition error compensation. In this process, the construction parameters of a completed layer are evaluated to determine if any deposition errors have occurred and then the remaining layers of the multilayer system are reoptimized to compensate for any errors made. This paper describes a versatile deposition error compensation program developed at the National Research Council of Canada for the simulation and real-time control of the manufacture of multilayers composed of dielectric or absorbing films. To model porous layers, an effective medium theory approach is used to relate the optical constants of the layer in vacuum and air to the microstructure of the layer. In the simulation mode, random errors are applied to the thickness and porosity of the layers and measurement errors are also included. The best monitoring strategy for the manufacture of a given multilayer is established on the basis of statistical information obtained from a number of these simulations. In this paper the results of calculations on the effectiveness of various monitoring strategies are presented for a sharp edge filter produced by three different physical vapor deposition methods. An extensive list of references to previous papers dealing with sources of errors during deposition is also provided.

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.