In this work, physical and optical properties of ZnS films grown at different evaporation conditions have been
studied. ZnS 3000 nm thick films have been deposited on Ge substrates at 200°C, 120°C and without substrate heating.
In addition, evaporation rates of 4, 2 and 1 nm/s have been considered. The structural and morphological properties of
the films have been analysed by XRD and AFM, respectively and the refractive index in the 2.4-11.5 microns range has
been determined from transmittance spectra through reverse synthesis. From this analysis, the most suitable evaporation
conditions for ZnS thin films deposition have been defined in terms of film properties and intended applications on
thermal IR multilayer coatings.
Afterwards, adhesion properties of ZnS films deposited under the optimised conditions have been analysed. ZnS
films deposited at 120°C and 4 nm/s peeled off when subjected to MIL-F-48616 standard surface durability testing. The
use of a MgO bonding layer to enhance the ZnS film adherence to the substrate has been proposed and its effect on the
ZnS film properties has been studied. Finally, the mechanical stability of the ZnS coating under MIL-F-48616 standard
testing has been confirmed for films grown onto MgO coated substrates.
High performance narrow bandpass filters for the mid-infrared range of the spectrum have been designed, prepared and characterized. The filters must assure an optimal rejection with a mean transmission lower than 0.02%T and a maximum transmission lower than 0.1%T out of the pass band from 1 to 6 microns. The pass band must be centered in 4.52±0.04 microns, have a full width at half maximum of 0.09±0.01 microns and a maximum transmittance higher than 75% when measured at normal incidence. A 75-layer theoretical design meeting all the requirements is obtained by computer optimization using experimental values for the optical constants of the layers; SiO and Ge are the low and high index materials respectively. A tolerance analysis of the design indicates that the rejection band performance is very stable, but the pass band is very sensitive to thickness deviations in a group of seven layers, defined as the critical part of the coating. The filters are prepared by evaporation in high vacuum; the deposition rate and final thickness of each layer are controlled by a quartz-crystal monitor. Filters with the required performance are obtained applying a method based on the previous deposition and optical characterisation of the critical part of the coating. Finally, the parameters of the transmittance measurements have been optimized to assure the necessary accuracy of the spectra, including the blocking of the pass band to reliably define the rejection parameters.
A technology to process uncooled polycrystalline PbSe IR detectors on interference filters has been developed. Thus, the lead salt natural spectral response can be modified as required. PbSe is deposited, processed and sensitized, following a unique method, on an interference filter made up of a sapphire or silicon substrate and a Ge/SiO multilayer structure. Unlike standard polycrystalline PbSe processing methods, we deposit PbSe by sublimation in vacuum. As-deposited, PbSe is not sensitive to infrared light. In order to turn it photosensitive it is necessary to expose the films to specific thermal treatments. We have developed a very efficient sensitization process during which substrates are submitted to temperatures as high as 450 ºC. In this work we demonstrate that we are able to process a PbSe detector directly on top of an interference filter. Also, we present preliminary results regarding the compatibility of our technology with standard photolithography and dry etch techniques. Results obtained pave the way for the development of uncooled multicolor medium-wave infrared detectors.