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Practical Aspects of Infrared Technology

Our June issue outlined the vast field of infrared technology and the role that coating materials and processes play in enabling that technology. We reviewed the IR spectrum, applications by spectral region, user disciplines, and coating and substrate materials. Several messages were intended as take aways; one is that different portions of the IR spectrum, which stretches from the Near IR beginning at ~800 nm to the thermal IR out to 14 µm (LWIR), employ specific transparent coating materials. Special applications that require operation to wavelengths longer than 50 µm involve an even different set of materials for transparency. We distinguished between sensing reflected IR energy in the NIR and SWIR solar energy regions, from sensing thermal emission at longer wavelengths. As more sensitive cooled and non-cooled detectors and IR imaging materials are developed, more applications are being introduced and satisfied. Therefore, in addition to the previously dominant  military application market, IR imaging devices are being applied today to medical, scientific, and even automotive devices.

Earth’s atmosphere provides windows through which IR energy is transmitted. Water and CO2 absorption occurs between these NIR, MWIR, and LWIR bands.  Figure 2 shows the transmitted energy wavelength regions for the atmosphere. Clear and water bands are evident. IR instruments, depending on their functions, are designed to operate in the transmitted regions. Remote sensors for DoD, NOAA, and NASA missions operate in the “water free” bands. Medical systems are not restricted by atmospheric absorptions.

Infrared Applications Know No Bounds! Evidenced by this Hubble IR image of a nebula agitated by a young aggressive star. Photo Credit: NASA/ESA/Hubble

 

Figure 2 - Transmitted energy wavelength regions for the atmosphere.

The ever-expanding range of applications requires mature coating materials preparation and deposition technology. In this issue, we explore more detailed process parameters. The reader is referred to Tables 2 and 3 of the June 2012 CMN issue where IR coating materials properties were listed. Lenses and windows used for the short wave (SW) and long wave (LW) thermal IR range, including the span 3 to 14 µm, are silicon, germanium, zinc sulfide (ClearTran), ZnSe, CaF2 and more recently calcogenides. Additional materials to cover the SW region are included in Table 1 below.

Table 1. IR materials listed in decending index value. dn/dT indicates the stability of index with temperature. From (from “Selecting Infrared Optical Materials” by Jay Vizgaitis, University of Arizona, Optics 521, December 14, 2006.

Calcogenide IR Glasses
Calcogenides are alloys composed of non-oxide elements / compounds.  AMTIR and GASIR listed in Table 1 are types of calcogenide glass; calcogenide IR glasses are manufactured by several suppliers and given different names. Calcogenide glasses are based on elemental S, Se, or Te compositions, with Ge, As, or Sb added to modify optical, thermal, and mechanical properties. Compositions that transmit to visible wavelengths are S-based; IR transmitters are based on Se or Te alloys. Being O-free, constant transmittance is exhibited from at least 2 µm to 11 µm.  These materials are currently used to make optical components for the two atmosphere bands, 3-5 µm and 8-12 µm. As40Se40 transmits to 12+ µm (Schott IR6).

The conventional IR window materials listed above can be used in combination to produce achromatic optical lens systems. The introduction of calcogenide materials now permits lenses, domes or other complex 3-D shapes to be molded to final shape. This reduces manufacturing costs and waste because complex shapes can be preformed in a mold thereby apporaching near net figures. Some compositions can be drawn into fiber optics for the IR, an application used in medical laser surgery. Uncertain global availability of materials such as Ge, for example, is not a problem with calcogenides. Significant disadvantages accompany the application of calcogenide optics that are otherwise not problematic with the conventional substrates. Their low glass transition temperature limits the deposition parameters for optical coating. They are soft and subject to scratching during finishing and handling.  Also, adhesion of coatings requires careful design and processing to be successful. Table 2 lists the glass transition temperatures, thermal expansion coefficients, and refractive index at wavelength 10 µm for Schott calcogenide glasses.

Table 2.Some properties of Schott calcogenide IR glasses (Schott.com).

The high refractive indices of  calcogenide glasses require AR coatings to maximize transmission.  The usual layer components for AR coatings on calcogenides are chosen from ZnS, Ge, YF3, or another fluoride compound such asYbF3 or the mixture IRX.  ThF4 has been replaced with the non-radioactive aforementioned fluoride compounds.  IR coatings are deposited by thermal evaporation using resistance heating for ZnS and ZnSe and the fluorides. However,  low-power E-beam can also be used for the fluoride compounds.  If ZnS is used, the substrate temperature must be kept at 175° C to prevent the possibility of deforming the calcogenide substrate by approaching its Tg. But, you may also recall from previous CMN discussions, that when ZnS is deposited on substrates at temperatures >~175° C, the quantity of material deposited as a growing solid film layer is less than expected. This phenomenon is the result of two events that occur to create an error in deposited thickness, and in relating the thickness on the substrate to that on the crystal monitor. Most IR coating chambers use crystal thickness monitoring because of its simplicity to implement and its affordability over IR optical monitoring. The condensation rate evidenced as low “sticking coef” is reduced as substrate temperature is increased, and may decrease to as low as 50%. The second event that must occur is that surface energies must be favorable for nucleation on the crystal and substrate. This conditioning is encouraged by pre-coating the crystal (and perhaps the substrate) by a precursor layer of deposited ZnS. Armed with this information, the next step is to determine whether the first layer in the coating design should be a fluoride, sulfide, or Ge. The answer depends on the calcogenide composition, but a fluoride first layer is suggested.  Here IRX works well because it grows dense amorphous film layers.

The economical availability of uncooled microbolometer IR detectors and arrays has opened up the field of applications of IR optics and their requisite optical coatings. Lenses and windows made of IR materials that once were exclusive to military devices are being adapted to commercial applications. Microbolometer sensors for the thermal IR region are now used in applications including medical, scientific, fire fighting and even in automotive night time imagers. The coating industry has kept pace with the pressure to develop coating materials and processes for IR components. Improved coating materials such as YbF3 are under continual development and evaluation. High energy processes are being applied to fluoride-compound deposition. In the early stages of the introduction of ion-assisted deposition processes and sputtering techniques, great improvements in the mechanical and optical properties of oxide-compound deposition were realized.

Some of that technology is being applied to fluoride compound deposition. Recall that oxide compounds absorb at wavelengths larger than ~7 / 8 µm. The favorite low-index material, silicon dioxide, is not usable in the thermal IR.  Fluoride compounds provide the low-index component of coating designs. Typically, they are soft, underdense,  and have high tensile stress. IRX was designed to overcome these problems. Alternatively, the incorporation of high-energy processes, that include ion-assisted bombardment or plasma energizing of the growing film, produces more densely compacted layers with higher hardness and potentially lower stress, depending strongly on deposition conditions. Thus, it is now possible to include fluoride compounds in coatings that must survive harsh environmental impact such as rain and dust with minimal damage. The precaution is to lower the ion energy to prevent the loss of fluorine, and as a result, increase absorption. Generally, a slight composition change will be evidenced more at short wavelengths than in the IR.

Practitioners of IR coating technology are aware that it is learned through required study, observation and experience to perfect. The materials need to be processed differently than the oxide materials used in the SWIR and visible regions. The substrates are different, and in themselves require special finishing and handling considerations to prevent damage, to promote adhesion, and to achieve coatings durable to environmental stresses such as humidity, salt water, and erosion by impacting rain and sand. There is more on these topics addressed in previous CMN articles.