Infrared Physics & Technology 52 (2009) 109–112

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Infrared Physics & Technology

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Broad band antireflection coating on zinc sulphide simultaneously effectivein SWIR, MWIR and LWIR regions

A. Ghosh *, A.S. UpadhyayaInstrument Research and Development Establishment, Dehra Dun 248 008, India

a r t i c l e i n f o

Article history:Received 2 June 2008Available online 25 March 2009

Keywords:Antireflection coatingGradient index conceptE-beam evaporationCoating materials: Thorium fluoride andzinc sulphideEquivalent layer system

1350-4495/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.infrared.2009.03.002

* Corresponding author. Tel.: +91 135 2787167; faxE-mail address: amitav59@hotmail.com (A. Ghosh

a b s t r a c t

In recent years multi-spectral imagery is steadily growing popularity. Multi-channel imaging whichincludes short-wave infrared (SWIR), mid-wave infrared (MWIR) and long-wave infrared (LWIR) systemsare useful for threat detection, tracking, thermal signature detection and terrain analysis. In this paper, abroad band antireflection coating on ZnS substrate, simultaneously effective in SWIR, MWIR and LWIR isreported. The coating design approach was evolved using gradient index concept, where refractive indexvaries gradually from incident media to the ZnS (n = 2.2) substrate. The gradient index profile depicted by4th degree polynomial n(t) = �0.45t4 + 1.9t3 � 2.7t2 + 1.9t + 1,where n(t) is the refractive index at the dis-tance t from ambient, and t is the thickness in micron. The profile is best approximated by eight discretestep index layers, whose first layer is thorium fluoride (n = 1.42; lowest index stable material available).Other seven layers are replaced by two equivalent layer system of real materials thorium fluoride andzinc sulphide. Final 15 layers design is deposited by e-beam evaporation. The maximum layer thicknesswas restricted around 0.7 lm to overcome the stress problem in the film. This 15 layers coating hasshown average transmission 95% in 0.9–10.5 lm spectral band having peak 99% at 9 lm.

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1. Introduction

Multi-spectral imagery system provides information which isnot available by only exploiting a particular band of the electro-magnetic spectrum. Regions of the spectrum are selected by sensorbands to optimize collection for certain categories of informationmost evident in short-wave infrared (SWIR), mid-wave infrared(MWIR) and long-wave infrared (LWIR) regions. Simultaneous dig-ital outputs can be obtained from above mentioned three bands. Byusing fusion of the images, obtained in these three bands, break-through can be achieved for threat detection, tracking, thermal sig-nature detection and terrain analysis. A single optical system canbe designed and developed for these three bands, having commonoptical components. Zinc sulphide’s transmission spectrum from0.4 lm to 11 lm is free from major absorption, with the materialeasily available in large quantity and high purity to suit for mostelectro optical applications. Investigation on general optical prop-erties of zinc sulphide has been extensively carried out over theyears by different authors [1,2]. Very good surface hardness,robustness and ease of fabrication makes zinc sulphide a strongcandidate for optical elements in SWIR (0.9–2.5 lm), MWIR(3–5 lm) and LWIR (7.5–10.5 lm) bands.

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: +91 135 2787128.).

Refractive index of zinc sulphide is 2.2, so it transmits only 73%incident radiation in uncoated condition. So antireflection coatingis a critical requirement for zinc sulphide substrate, to make it use-ful for any optical system. The present paper describes design anddevelopment of antireflection coating on zinc sulphide substrates,which is effective in our region of interest.

The conventional design approach of antireflection coating bydestructive interference method is effective only for limited wave-length region. However, a broad band antireflection coating can bedeveloped by multilayer stack but they give only partial success tothe above limitation. Moreover, quarter–quarter coating will alsolead to stress development in the film due to large film thicknessof infrared coating. So to overcome these two problems a noncon-ventional method must be followed for development of antireflec-tion coating. A more versatile approach would be to avoid sharprefractive index variation between air and substrate interface. Var-ious inhomogeneous profiles have been studied already for perfectantireflection coating [3,4]. To generate the inhomogeneous profilea film is deposited on the substrate, whose refractive index variescontinuously from substrate to air medium and this is called thegradient index approach [5,6]. The gradient index profile passeslight of any wavelength without reflection and produce wide bandantireflection coating [7]. Coevaporating two materials may generatethe gradient index profile with independently controlled rates ofevaporation or by mechanical modulation of deposition using shut-ters with continuously adjustable aperture above the evaporation

Fig. 2. Approximated step index variation profile.

Fig. 3. Fifteen layer design of antireflection coating.

110 A. Ghosh, A.S. Upadhyaya / Infrared Physics & Technology 52 (2009) 109–112

sources [8]. But these two methods are very difficult and cum-bersome. Alternatively, the same refractive index profile may begenerated by depositing an equivalent system comprising a largenumber of thin layers of high and low refractive index materials[9–11]. Here equivalent system method is followed to develop anefficient, durable coating on zinc sulphide substrates.

2. Design approach

Graded transition layers are regarded as excellent antireflectiondevice for normal incidence. Some studies have been carried out tofind the inhomogeneous layer profile. Southwell suggested that aquantic profile act as efficient antireflection coating [10]. Anotherefficient profile consists of a half period of an exponential sine[12]. For this work various index profiles were studied. Startingfrom a linear equation, it was seen that a fourth degree polynomialsuits best for our application. By our experience and repeated trialsthe index profile achieved as n(t) = at4 + bt3ct2 + dt + e, wherea = �0.45, b = 1.9, c = �2.7, d = 1.9, e = 1, and t is the coating thick-ness in micron. The index variation is shown in Fig. 1. It is not veryeasy to control the deposition and achieve this index variationexperimentally, during deposition cycle. So this profile is bestapproximated by eight discrete step index layers, whose first layeris thorium fluoride (n = 1.42; lowest index stable material avail-able). More number of such layers will yield a closer approxima-tion to the gradient index profile, but would be comprised of alarge number of actual layers which would be cumbersome to de-posit and also not cost effective. Again it is observed that over alltransmission is also not improved much with more steps. Further,less than eight layers used give a poor approximation to the gradi-ent index (Fig. 2).

This eight steps gradient index profile is then optimized to obtainthe best yield in transmission in SWIR, MWIR and LWIR regions.Extensive studies have been carried out to find the most suitablecoating materials. It was found that thorium fluoride (n = 1.42)and ZnS (n = 2.2) is the most suitable combination for low and highindex material for our application. Both the materials have very lowabsorption, very good compatibility with each other that gives sta-ble high performance antireflection coating. In the above step gradi-ent index model, the first layer next to air is thorium fluoride, andeach of the other seven hypothetical layers is replaced by two layerof available coating material i.e. thorium fluoride and zinc sulphide,based on the two layers equivalent concept of Southwell [13]. Theoptical thickness (NT << k) of the layers are given by

TH ¼ TðN2 � N2LÞ=ðN

2H � N2

L Þ and TL ¼ T � TH

Fig. 1. Inhomogeneous index variation.

where N is the refractive index of thin hypothetical layer of thick-ness T, k the reference wavelength, TH and TL are the thicknessesof high and low index film of refractive index NH and NL. In the de-sign NH = 2.2 (ZnS) and NL = 1.42 (ThF4) have been chosen and final-ly a 15 layer design equivalent to eight layer stipulated index profileis achieved. Index profile is shown in Fig. 3. Theoretical transmis-sion curves of step index design and 15 layer design are superim-posed in Fig. 4.

Fig. 4. Theoretical transmission curve from both designs.

Table 1Sample test record for low pass filter. Substrate: ZnS; diameter: 25 mm; coating materials: ZnS/ThF4.

Run No 1 2 3 4 5 6Average transmission (%)0.9–10.5 lm 94.5 94 94 95 95 95.5

Durability and environmental testTest specificationAdhesion 1 pull by scotch tape OkAbrasion 50 rubs by cheese cloth OkHumidity 24 h; 95–100% (RH); 49 ± 1 �C Ok Ok OkTemperature cycle 2 h at 70 �C and �40 �C with 30 min change over Ok Ok

Fig. 5. (a) Transmission curve up 400 nm to 2 lm (Lambda 950). (b) Transmission curve above 2–12 lm (Spectrum GX)

A. Ghosh, A.S. Upadhyaya / Infrared Physics & Technology 52 (2009) 109–112 111

3. Experimental process

Before actual deposition of coating on the substrate, the de-signed coating deposition is checked for stress development. Thedesigned coating was deposited on a thin micro slide of thickness0.3 mm. The bending was not observed in the slide after the depo-sition; this confirms that the stress is not predominant enough tospoil the coating stability. The coating plant used for this coatingis Balzers BAK 600. Coating materials are supplied by Umicorematerials AG with purity 99.99%.

Water clear, polished zinc sulphide flat of diameter 25 mm andthickness 5 mm was chosen as the substrate. Substrate cleaningperforms a key role in coating stability. Substrate was cleaned inthree steps by ultrasonic cleaning followed by vapour degreasingfor half an hour. Initially the substrates were cleaned for 10 minin ultrasonic cleaner by soap solution, and then 10 min by deion-ized water and finally 20 min by isopropyl alcohol.

The ultimate vacuum of the chamber before coating was2 � 10�6 mbar. To obtain the uniform coating, the substratewas rotated with respect to the central point of coating cham-ber. Substrates were heated upto 145 ± 5 �C for 5 h inside thevacuum chamber. Deposition parameters like rate, layer se-quence and layer thickness were registered and controlled bya microprocessor interface with the coating unit. Materials weredegassed thoroughly before starting of deposition process. Boththe materials were evaporated by electron beam gun and tomonitor the thickness of each layer, quartz crystal monitorwas used. The rate of deposition is a critical parameter respon-sible for the stability of coatings. The rate of deposition wascarefully controlled, for thorium fluoride the rate was 1.5 nm/sand for zinc sulphide it was 1 nm/s with a tolerance of±0.1 nm/s.

4. Results and discussion

The transmission characteristic of coated substrate of zinc sul-phide was measured using double beam spectrophotometer(supplied by ‘Perkin–Elmer’ model No Lambda 950) and FourierTransform Infrared Spectrophotometer (supplied by Perkin–Elmermodel Spectrum GX). Spectrum GX FTIR spectrophotometer cannot measure transmission accurately below 1.3 lm wavelength.So the transmission below 2 lm was measured by double beamspectrophotometer and 2–12 lm by FTIR spectrophotometer. Thetransmission curves are recorded into two parts. This 15 layers coat-ing has shown average transmission 95% in 0.9–10.5 lm spectralband having peak 99% at 9 lm, which are shown in Fig. 5a and b. Agood agreement is found between theoretical and practical results,as shown in figure. The coating is found to be stable to environmen-tal test as per specification of MIL – F – 48616. Six different sampleswere coated in separate coating cycles and the durability and envi-ronmental stability tests were performed on all the samples. Resultof these tests is shown in Table 1. From these results it can be seenthat this coating is reproducible and also environmentally stable.

Acknowledgments

The authors are thankful to Mr. Ikbal Singh, Chief Designer(Optical Instrumentation), IRDE Dehra Dun and Mr. S.S. Sundaram,Director IRDE, Dehra Dun for their encouragement and valuablesuggestion in carrying out the above work.

References

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[3] J.A. Dobrowolski, D. Poitras, et al., Appl. Opt. 41 (2002) 3075.[4] D. Poitras, J.A. Dobrowolski, Appl. Opt. 43 (2004) 1286.[5] W.H. Southwell, SPIE 3133 (1997) 65.[6] J.R. Jacobsson, SPIE 2046 (1993) 2.[7] J.R. Jacobsson, Physics of Thin Film, vol. 8, Academic Press, Berlin, 1975. 51.[8] L. Nouvelot et al., SPIE 1782 (1992) 229.

[9] W. Ganning, SPIE 1019 (1988) 204.[10] W.H. Southwell, Opt. Lett. 8 (1983) 584.[11] R.R. Willey, SPIE 2046 (1993) 69.[12] B.G. Bovard, Appl. Opt. 32 (1993) 5427.[13] W.H. Southwell, Appl. Opt. 24 (1985) 457.