Optics are used almost anywhere in the military. From vision systems and target designators used by troops on the ground, through guidance systems utilized in both manned and unmanned aircraft, to reconnaissance and surveillance packages carried by satellites in Earth orbit. These optical systems are usually in a hard and variable temperature and humidity and in contact with abrasive materials such as sand and salt. The use of coatings deposited by different methods of the deposition is a definitive requirement for all these systems. These coatings, in addition to physically protect sensitive components and sensors against stressful conditions, should be such as to allow components and sensors to provide the proper function for which they are designed. Regarding the military status, all these tasks occur when the inappropriate operation and damage of a piece can lead to military defeat. On the other hand, the cost to use these coatings should also be taken into account.
Optical thin film consists of one or multi layers of nanometer to a few micrometers of different materials. In order to achieve the proper function, the process of thin film deposition should be such that the sequence, uniformity, material thicknesses and indices of refraction of these layers are correctly and accurately implemented.
The processes commonly used in optical thin-film coatings include reactive magnetron sputtering, thermal evaporation, which are physical vapor deposition mechanisms (PVD) and low pressure chemical vapor deposition (LPCVD) processes. Each of the deposition methods has unique advantages, such that none of these methods are suitable for making all parts and sensors. In most military applications, the key parameters are those that should be considered for proper deposition. The first of these is coating hardness. Hard coatings resist damage due to repeated cleaning or abrasion from particulates like sand. Here, there is a progression from evaporation, which produces the least dense and softest films; through to sputtering and LPCVD which both produce highly densified, hard coatings.
The thin film created by the thermal evaporation method is less densified compared to the thin film created by the sputtering method. The higher density, which means lower porosity, causes less water molecules can penetrate to the thin film structure in humid environment. The absorption of moisture changes the refractive index of the thin film and shifts its performance diagram towards the longer wavelengths. This phenomenon is called “wet / dry shift”. This phenomenon is not a problem for high-bandwidth coatings such as Anti-reflection Coatings, but in the case of low-bandwidth coatings, such as mid-pass and notch-filter (which only reflect a specific wavelength in the laser beam and pass through the rest), it has significant effects. These are all coatings widely employed in military systems, including target designators, multispectral imaging sensors, and countermeasures. However, for the particular spectral range (mid-wave to long-wave IR) and materials involved in DSI military optics, wet/dry shift is not an issue in evaporative films, and all three technologies deliver acceptable performance for this metric.
Another important issue is the internal stresses of the deposited thin films. Multi layers coatings usually exhibit tension stresses to tensile stress. In general, higher the compressive stress leads to the more durable coating, unless there is so much tension that affects the adhesion to the substrate. In these situations there are several problems for thicker coatings. The thermal evaporation method is less energy process than the other two methods, and the thin films which are deposited by this method have the least stress.
Surface roughness and bulk scattering properties are other features that should be considered. The scattering results in light leakage and reduces optical efficiency, reducing image contrast and signal-to-noise ratio. In the case of surface roughness, the thin film deposited by the sputtering method has a greater surface roughness than the thin film deposited by the thermal evaporation method.
By increasing the layers in a multi-layer coating, the acceptable tolerance for the thickness and refractive index of the layers decreases. These all increases the cost of the deposition of the layers. Obviously, the purpose of determining a specific coating is to perform the duties and performance intended to cover it, but costs must also be taken into account.
The two key parameters in making narrowband filters that are often used in the military are the center wavelength (CWL) and full-width at half-max (FWHM) which should be considered in thin film deposition process. Another key parameter for the proper operation of the coatings in military applications is the out of band blocking (OBB).
For mid-infrared wavelength and long infrared wavelength filters used for military applications, the CWL and half-power point (HPP) tolerance is typically 0.5% wavelength, but to make more precise filters, with less tolerance, costs are rising. Similarly, slopes of 2% or greater are standard; smaller (steeper) values are more difficult. Consequently, if the distance between the wavelengths to be transmitted or reflected is high, there is no need for strict values that lead to higher costs, for the mentioned parameters.
Out of band blocking (OBB) is also costly. This may be specified to start at the first 1% or 0.1% point on either side of the bandpass, or sometimes at a given wavelength away from the half-power point, or even at a particular wavelength. It may be specified as a maximum value, average value, or total integrated power within the blocking range. In most military systems, what really need blocking is only certain key wavelengths or wavelength bands. Considering these, it simplifies the fabrication process and reduces costs.
Another feature that is discussed in the optical coatings is flatness. Virtually all optics vendors nominally specify component flatness before coating. Intrinsic mechanical stresses in optical coatings (which are usually denser and harder) can reduce the nominal flatness of a surface by several times. In addition, when it comes to flatness, there are other important parameters to consider. First, if the optical coating is to be used only in optical transmission, the flatness should not be expressed and instead of the corresponding characteristic is the amount of distortion of the transition wave front which is less sensitive to the mechanical stresses of the thin films deposition.
Flatness of the deposited thin film surface is related to the type of coating. This problem is seen more when two layers of different materials with different thicknesses deposited on both sides of an optical thin film which due to increased mechanical stresses, it reduces the surface flatness significantly.
Generally speaking, military optics has many common points with commercial optics, but in the same environmental and operational conditions, the requirements of military plans are more precise and to rigorously hit the target with those parameters is more critical. With the knowledge of what types of parameters are critical for various applications and which deposition method should be used to achieve the best performance, the cost of the thin film deposition can be reduced for the fabrication of the military systems.
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David Favot, Staff Engineer, Deposition Sciences, Inc., (Santa Rosa, CA)