A common window material for applications that require the simultaneous use of visible and infrared wavelengths is zinc sulfide, which offers a high transmittance between 400 nm and 12 μm. Depending on the manufacturing process, zinc sulfide can, however, exhibit large scattering losses (>10%) which degrade the imaging quality in particular in the visible spectral range. In this contribution, the different sources for light scattering such as volume imperfections, surface roughness, and subsurface damage are analyzed individually for hot isostatic pressed chemical vapor deposited zinc sulfide and correlated to the structural properties resulting from the fabrication process.
Modern optical systems are often required to function in severe environmental conditions for prolonged periods without suffering from performance degradation. Essential parts of such electo-optical systems are windows, domes and other optical elements. These elements are almost always coated with an efficient, multispectral and highly durable antireflection coating. The complexity further increases when these coatings are applied on curved surfaces, such as hemispherical domes that are usually used in order to allow a wider field of regard. The durability of the coated optical component is dependent upon many different parameters such as deposition method and process parameters and the adhesion between layers and substrate. However, one very important parameter, which can have a significant impact on the durability and optical performance, is the stress state of the applied anti-reflection layers. This subject is mostly left untreated mainly due to the difficulty of characterization and modeling techniques and lack of thin film mechanical constants which sometimes significantly differ from the bulk constants of the same material. The stress state of the optical part is mainly determined by the mechanical properties of the coating materials and substrate, geometrical shape of the part and the thickness of the layers. In this work, both analytical and experimental approaches were used for characterization of stress distribution in thin optical films. Various single and multiple thin films were deposited using the electron beam evaporation technique onto Si 6'' wafers. The film thickness was measured using an ellipsometer and the bi-axial module of the different films was studied by measuring the change in radius of curvature of the Si substrate and applying Stoney's equation. The results were compared to the analytical Timoshenko solution of stress state in a single and multiple film structure. Knowing the stress state of a multi-layer coating would allow the engineer to design a part with the desired stress state which alongside the optical design, is expected to bring about a significant improvement in the durability and optical performance.
The design of a modern optical system often raises new challenges for manufacturers of high end optical components. One such challenge, which has become more and more common, is the requirement for highly durable hemispherical domes to allow for wide field of view. There are many difficulties to overcome before the final product can be made. In this paper we present some of the major difficulties of developing such domes made from ZnS grown by chemical vapor deposition (CVD). First, the CVD process which introduces the challenge of removing the grown raw dome from the graphite mold without causing cracking and breakage is discussed. Then, the challenges introduced by the electron beam (EB)-gun evaporation method, most commonly used for evaporating the anti-reflection coating, are presented. Amongst these challenges, the mounting of the dome in-side the coating chamber, the coating uniformity over the dome's curvature and the coating's environmental durability are the most difficult problems to overcome. The paper presents how computerized modeling along with experimental procedures can be combined to minimize the difficulties in the production processes and improve the overall product quality and yield.
In this paper we report an on going research to correlate between optical coating survivability and military
(MIL) standards. For this purpose 8 different types of coatings were deposited on 1" substrates of sapphire, multi-spectral
ZnS (MS-ZnS), germanium, silicon and BK7. All coatings underwent MIL standard evaluation as defined by customer
specifications and have passed successfully. Two other sets were left to age for 12 months at two different locations, one
near central Tel-Aviv and one by the shoreline of the Mediterranean Sea. A third set was aged for 2000 hours at a special
environmental chamber simulating conditions of temperature, humidity and ultra-violet (UV) radiation simultaneously.
Measurements of optical transmission before and after aging from all 3 sets reveal, in some cases, major transmission
loss indicating severe coating damage. The different aging methods and their relation to the MIL standards are discussed
in detail. The most pronounced conclusion is that MIL standards alone are not sufficient for predicting the lifetime of an
external coated optical element and are only useful in certifying the coating process and comparison between coatings.
In the current complex battle field, military platforms are required to operate on land, at sea and in the air in all weather
conditions both day and night. In order to achieve such capabilities, advanced electro-optical systems are being
constantly developed and improved. These systems such as missile seeker heads, reconnaissance and target acquisition
pods and tracking, monitoring and alert systems have external optical components (window or dome) which must remain
operational even at extreme environmental conditions. Depending on the intended use of the system, there are a few
choices of window and dome materials. Amongst the more common materials one can point out sapphire, ZnS,
germanium and silicon. Other materials such as spinel, ALON and yittria may also be considered.
Most infrared materials have high indices of refraction and therefore they reflect a large part of radiation. To minimize
the reflection and increase the transmission, antireflection (AR) coatings are the most common choice. Since these
systems operate at different environments and weather conditions, the coatings must be made durable to withstand these
extreme conditions. In cases where the window or dome is made of relatively soft materials such as multispectral ZnS,
the coating may also serve as protection for the window or dome.
In this work, several antireflection coatings have been designed and manufactured for silicon and multispectral ZnS. The
coating materials were chosen to be either oxides or fluorides which are known to have high durability. Ellipsometry
measurements were used to characterize the optical constants of the thin films. The effects of the deposition conditions
on the optical constants of the deposited thin films and durability of the coatings will be discussed. The coatings were
tested according to MIL-STD-810E and were also subjected to rain erosion tests at the University of Dayton Research
Institute (UDRI) whirling arm apparatus in which one of the coatings showed no rain drop impact damage at all.
Multi-spectral (WC)-ZnS has high transparency in a wide spectral range, from 0.4μm to 12μm. However, the relatively low hardness of WC-ZnS components limits their use especially for air-borne military systems where conditions such as particle or rain erosion may occur. In this work, a protective coating for WC-ZnS components was developed. The protective coating reduces damage caused by rain drop impacts without affecting the transparency of the uncoated component. An anti-reflective (AR) coating was evaporated over the protective coating to improve the transmission of the component. The microstructure of the protective coating was found to be of columnar grains, tens of nanometers in diameter. Rain erosion tests of the coated WC-ZnS samples conducted at the University of Dayton Research Institute (UDRI) whirling arm apparatus showed that they fully comply with the commonly accepted OCLI specification #6040012 for rain erosion resistant coating for forward looking infra-red (FLIR) grade ZnS. The damage threshold velocity (DTV), measured at the Cambridge University Multiple Impact Jet Apparatus (MIJA), increased from 120m/sec for an AR coated WC-ZnS sample to 178m/sec for a WC-ZnS sample coated with a 17μm protective coating and an AR coating. In this paper we show that the improved rain erosion protection was achieved without causing any change in the transparency of a WC-ZnS substrate. We also thoroughly discuss how the protective coating thickness and WC-ZnS sample thickness affect the damage caused by rain erosion.