Investigation Of As Se Chalcogenide Glass In Precision .

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Investigation of As40Se60 chalcogenide glass in precision glassmolding for high-volume thermal imaging lensesJeremy Huddleston, Jacklyn Novak, William V. Moreshead, Alan Symmons, Edward FooteLightPath Technologies, Inc., 2603 Challenger Tech Ct, Ste 100, Orlando, FL, USA 32826ABSTRACTThe growing demand for thermal imaging sensors and cameras has focused attention on the need for larger volumesof lower cost optics in this infrared region. A major component of the cost of thermal imaging lenses is thegermanium content. As40Se60 was developed as a moldable, germanium-free chalcogenide glass that can serve as alow cost alternative to germanium and other infrared materials. This material also has promising characteristics forimproved optical performance, especially with regard to reduced thermal sensitivity. As40Se60 has found acceptanceas a material to be diamond turned or polished, but it is only now emerging as a legitimate candidate for precisionglass molding. This paper will review chalcogenide molding and characterize As40Se60 for widespread use in highvolume thermal imaging optics. The relative advantages and disadvantages of As40Se60 as compared to otherchalcogenide glasses will also be discussed.Keywords: Precision glass molding, chalcogenide, thermal imaging, infrared materials, LWIR lenses1. INTRODUCTIONOver the past decade, the prices for thermal imaging sensors have dramatically decreased through uncooledmicrobolometer technology. The resulting cost savings has significantly increased the demand for thermal imagersand expanded their applications in the commercial market. This in turn has increased the demand for longwaveinfrared (LWIR) lenses, and driven the search for high volume, low cost methods of manufacturing LWIR lenses.The technological roadmap for thermal imaging systems is following the same path that visible imaging systemshave followed in the recent past; from large SLR type cameras to small handheld cameras and finally to cell phonecamera systems. The enabling optical technology for the visible region was injection molding of plastic lenses.Although there are many plastic materials to choose from for the visible spectrum, these polymers absorb longwaveinfrared light, and are therefore inadequate for thermal applications operating in the LWIR (8-12µm) band.Although crystalline materials such as Ge, ZnS, and ZnSe, transmit well in the LWIR band, they are not moldableand therefore they are not well-suited to high-volume, low-cost production. In recent years, chalcolgenide materialshave grown in popularity for LWIR lens applications. The moldability of chalcogenide glass uniquely qualifies itfor the high-volume demand of commercial longwave IR applications.Although precision glass molding (PGM) of chalcogenides has already started the trend towards low cost optics inthe longwave infrared, the most commonly used chalcogenides in PGM to date have been compositions containinggermanium, such as Ge28Sb12Se60 (Vitron IG5) and Ge22As20Se58 (Umicore Gasir 1). The push for lower costmaterials and improved optical performance has caused heightened interest in the germanium-free compositionAs40Se60 (Vitron IG6). However, this material’s availability, manufacturability, and safety must be assessed, all ofwhich could limit its upside potential. This paper will discuss the relative advantages and disadvantages of As40Se60as compared to the germanium-containing Ge28Sb12Se60.Infrared Technology and Applications XLI, edited by Bjørn F. Andresen,Gabor F. Fulop, Charles M. Hanson, Paul R. Norton, Proc. of SPIE Vol. 9451,94511O · 2015 SPIE · CCC code: 0277-786X/15/ 18 · doi: 10.1117/12.2177026Proc. of SPIE Vol. 9451 94511O-1

1.1. Overrview of Precission Glass MooldingPrecision glassgmolding, PGM, is a mannufacturing proocess used to makemhigh quallity lenses and optical compoonents.The generaal nature of thhe process is thhe compressionn molding of glassgpreformss at high tempeerature under highlyhcontrolled conditions. A more detailedd overview of thet process cann be found in Schaub,et al.1 A brief summmary ofpfollowws. The PGM processpstarts withw the manuffacturing of toooling designedd specifically forf thethe PGM processproduct to be manufactuured. This tooliing typically consistscof a toop mold, a botttom mold andd ancillary toolling toform the outsideodiametter or other feeatures of the component. AdditionalAtoolling may be required to aliggn theindividual mold halves. TheT customizeed tooling is then inserted intto the glass moolding machinee. A glass prefform isthen inserted into the tooling stack. Thee top mold is thhen reintroduceed and the systtem is evacuateed. The toolingg stackand the glaass preform aree then heated att a controlled rate.rA schemaatic of the proceess is given in Figure 1.The final processing teemperature is dependent onn the individual glass type. The preformm is then put undercompressioon in order to beginbforming the glass. The amount of loadd applied to thee glass is contrrolled throughoout themolding cyycle; the load is removed onceothe cycle is completed. The tooling stack is then cooled, typicaally bypurging thee system with an inert gas. Inn order to cost effectively maanufacture the lenslthis coolinng cycle is optiimizedfor the fasttest possible cyycle time. Oncce the final prooduct is cool enougheto handdle, the componnent is removeed andthe processs is repeated.FFigure1 - Precission Glass Moldiing SequenceProc. of SPIE Vol. 9451 94511O-2

1.2. History of Chalcogenide GlassChalcogenide glasses are amorphous compounds based on the chalcogen elements: sulfur (S), selenium (Se), orTellurium (Te). One or more of the chalcogens is usually paired with at least germanium (Ge) or arsenic (As) forchemical stability. Other elements such as antimony (Sb) may be added to the composition to achieve desiredproperties. These glasses transmit primarily in the mid-wave infrared (MWIR) and longwave infrared (LWIR)wavebands, making them suitable for thermal imaging applications. As opposed to traditional crystalline lensmaterials for LWIR, such as Ge, ZnS, ZnSe, the moldability of chalcogenide glass uniquely qualifies it for the highvolume demand of commercial applications.1.2.1. Early Use for MWIR ApplicationsThe earliest published work on non-oxide glasses was a paper written by Carl Schulz-Sellack in 1870.2,3 His workshowed that As40S60 chalcogenide glass was transparent in the infrared region. There was very little further researchdone on the subject until 1950 when the same glass composition was investigated by Rudolf Frerichs and the resultswere published in a paper titled “New optical glasses transparent in infrared up to 12µm.”2,3 Frerichs’ paper renewedinterest in this novel material and other groups began to research the glass. In the 1950s, As40S60 began beingproduced on an industrial scale and products utilizing the material were manufactured. Servo Corporation not onlymanufactured the glass, but also was the first to introduce a product that was made with this glass. The product wasused for the detection of overheated wheel bearings on railroad cars.2 The main appeal of chalcogenide glass is itsinfrared transparency, which makes it ideal for thermal applications.1.2.2. Development for LWIR ApplicationsThe transmission window of sulfide glasses only extends to about 11µm, limiting it to MWIR applications. Othermaterials needed to be investigated in order to produce a glass with LWIR transparency. In his book, ChalcogenideGlasses for Infrared Optics, Hilton discusses in detail how a program at Texas Instruments (TI) was funded by theU.S. Air Force in 1966 to research infrared materials for optics. Previous research on chalcogenides had focused onelectronic rather than optical properties. Hilton states that the best composition, TI 1173 (Ge28Sb12Se60), wasselected from the germanium-antimony-selenium system. Later research programs also investigated glasses in thegermanium-arsenic-selenium system. TI produced small quantities of systems using its TI 1173 glass that were usedin Air Force and Navy aircraft.2 In 1977, Hilton left TI and soon founded a new company, Amorphous Materials,Inc. (AMI). Since then, they have developed and produced several “AMTIR” glasses of different compositions.Hilton notes that “during the period from 1950 to [2010], only three compositions have been produced in tonquantities: arsenic trisulfide, TI 1173 (Amtir 3), and TI 20 (Amtir 1).”21.3. Chalcogenide Composition LandscapeThe significance of this last point is that out of the many compositions that have been developed for researchpurposes, only a select few have met the practical requirements of optical performance and manufacturabilitymaking them suitable for production. To highlight this point, Table 1 below shows most of the commonchalcogenide compositions available from primary glass suppliers.Proc. of SPIE Vol. 9451 94511O-3

Table 1 - Landscape of CommerciallyCAvvailable Chalcoggenide Glasses. The Arsenic-freeeand Germaanium-free comppositions are higghlighted for theeir trade-offs.Note that each glass suppplier in Table 1 has its owwn brand namme associated withw each commposition, givinng thember of manuffacturable commpositions avaailable than acttually exist. A fewimpressionn that there are a larger numimportant points can be taken from thhis table. Firstt, all of the coompositions suuitable for the LWIR band containcost are merely slightsvariationns of the ratioss in the GeAsSSe family. Finally, only two of theselenium. Secondly, moS 60 and As40See60, stand out asa containing only one of thhe elements geermanium or arrsenic.compositioons, Ge28Sb12SeThis last pointphas signnificant cost immplications; bothbrelated too the rising coost of germaniium, as well as theintangible cost of impleementing safetty procedures for arsenic-coontaining mateerials, in orderr to maintain a safemanufacturing work enviironment. For these reasons, we have choseen to focus on comparing theese two chalcoggenidegee As40Se60.compositioons: the arseniic-free Ge28Sb121 Se60 and the germanium-fre1.4. Challcogenide Glasss for PGMSince 20055, LightPath haas conducted PGMPtrials witth several diffeerent materials, and the arsennic-free Ge28Sbb12Se60(IG5 / TI 1173 / AMTIIR 3) composiition was founnd to meet thee optical perfoormance requirrements for thhe vastmlity. LightPathh has branded this compositiion formajority of applications, while providiing favorable manufacturabilD2”, which cann also be founnd in the LighttPath catalog inn Zemax. Ann assortment off theseits moldedd lenses as “BDlenses can be seen in Figuure 2.Figure 2 - Molded chalcogenide glass lensesProc. of SPIE Vol. 9451 94511O-4

The advantages of chalcogenides over crystalline materials such as germanium have been well documented.However, the choice between the various chalcogenide compositions has been largely subjective, based on differentglass suppliers or molders marketing their particular product offerings. As discussed in the previous section, thearsenic-free Ge28Sb12Se60 is a good baseline material for benchmarking the germanium-free As40Se60 composition.As such, the advantages and disadvantages of As40Se60 glass relative to Ge28Sb12Se60 will be the focus of theremainder of this paper.2. GLASS CHARACTERIZATION AND QUALIFICATIONThe high volume production PGM process is critically dependent on a thorough understanding of important glasscharacteristics, and ensuring that these remain constant from batch to batch. Qualification of a new glass thereforeinvolves measuring these key characteristics on glass from several batches or boules. Detailed descriptions andexplanations of the techniques used for glass characterization can be found in a previous paper in reference 4, “AnInvestigation of Material Properties for a Selection of Chalcogenide Glasses for Precision Glass Molding.”.Although not all are discussed in detail in this paper, Table 2 shows a summary of the properties measured duringqualification of As40Se60 compared to Ge28Sb12Se60.Table 2 - Key Properties of Select Chalcogenides: Arsenic-free Ge28Sb12Se60 and Germanium-free As40Se60PropertyWavelength Range (µm)Index @ 10µmWavelength Dispersiondo /da. (8 -12µm)GezsSb12 e601 -16(typical absorption peak at 12.5pm)1 -182.60232.7777-3.67 x 10-3-2.77 x 10-3Thermal Constantdn/dT (x10-6/ C)-7nillOcTg ( C)285185CTE (x10-6/ C)14.520.9Density (g /cm3)4.684.63Hardness (Vickers)189142Proc. of SPIE Vol. 9451 94511O-5

2.1. Optical Performannce PropertiessSeveral of the key glass properties primmarily affect thhe optical perfoormance of thee final lens. Thhese parameterrs willbe discusseed in this sectioon, while the propertiespprimmarily affecting manufacturingg will be discussed in sectionn 2.2.2.1.1. TrransmissionFrom Tablle 2 we see thaat the wavelenggth ranges for bothbAs40Se60 overoGe28Sb12SeS 60 cover the fullf MWIR to LWIRLband, makking them suitaable for thermaal imaging appplications. Figgure 3 shows thet measured transmissionto bothofmaterials forf uncoated witnesswsampless. Comparing the uncoated transmissiontreemoves any diifferences due to ARcoatings, butb does include differencess in reflectancee from the difffering refractiive indices (Frresnel loss) off thesematerials, which is appparent in the SWIR and MWIRMregionss of the plot. The absorptiion band locaated atapproximaately 12.5 µm is due to oxide impurities in thet form of Gee-O bonds.5 Thhe size of this absorptionabannd mayvary betweeen different glassgsuppliers due to their processingptechniques, incluuding whether or not they haave anextra puriffication step in their manufaccturing processs. As40Se60 doees not contain germanium,gsoo no Ge-O bondds canbe formed and the absorpption at 12.5µmm is avoided.Figuure 3 - Transmiission of 5mm thicktwindows of Ge28Sb12Se606 and As40Se600fractive Index2.1.2. RefrThe refracctive index of As40Se60 is higher than thaat of Ge28Sb12Se60 and thereefore more opptical power maym beachieved ono a given lenss for the same sag and slope limitations. However,Hthis differencedis smmall compared to theindex diffeerences between other LWIIR materials, anda therefore does not consstitute a signifficant advantage forAs40Se60. It will be showwn in section 4 that approximmately the samme nominal perfformance can beb achieved foor bothlens materiials.Proc. of SPIE Vol. 9451 94511O-6

2.1.3. Wavvelength DispeersionFigure 4 shhows the overrlaid plots of reefractive indexx vs. wavelenggth for As40Se60o Ge28Sb12Se600. The6 compared towavelengthh dispersion, oro change in reefractive indexx with wavelenngth (dn/dλ) isi indicated byy the slopes off thesecurves. Inn the MWIR, thhe dispersion ofo these two maaterials is apprroximately equual, but the disppersion in the LWIRband is slightly lower foor As40Se60. Thhough small, thist difference represents an incremental addvantage in noominalperformancce for As40Se600. However, diffractivedelemments are oftenn added to challcogenide lensees to compensaate forwavelengthh distortion. In most cases, this obviates anya perceived advantageato lower material dispersion, whhich isverified in the design studdy of section 4.4Figure 4 - Wavelength dispersion of GeG 28Sb12Se60 (lleft axis) and AsA 40Se60 (right axis)2.1.4. Theermal SensitivittyThe thermal coefficient ofo refractive inndex, or dn/dTT, is the changee in refractive index over temmperature. Figgure 5shows the measured datta for the refrractive index vs.v temperaturre of As40Se60. The lower thermal consttant ofAs40Se60 (332ppm/ C) theeoretically enaables a higher intrinsic operaating temperatuure range relattive to Ge28Sb12Se60.The practiccal effect of thhis constant on MTF performance over tempperature has beeen shown6 (byy the present authorsain a previoous paper) to beb inadequatelyy described byy first-order opptical theory, anda lacks a connsistent performmancecriterion accross the indusstry, even for riigorous MTF analyses.aAsseessing this impaact will be the primary focus of thedesign studdy in section 4.FFigure5 - Reffractive index vs.v Temperaturre of Ge28Sb12SeS 60 (left axis) and As40Se60 (right axis)Proc. of SPIE Vol. 9451 94511O-7

2.2. Mechhanical PropeertiesBeyond thhe optical charracteristics of As40Se60, otheer mechanical properties weere measured for validatingg glassquality andd compatibilityy with manufaccturing requiremments.2.2.1. CommpositionThe compoosition of As400Se60 was meassured using Prroton-Induced X-RayXEmissioon (PIXE) on fine-ground diiscs ofthe chalcoggenide glass, annd the results area shown in TableT3.Table 3 - Measured Coomposition of Ass40Se60 Glass2.2.2. Theermal PropertieesThe thermmal properties of a glass muust be well unnderstood for compatibilitycw the PGMwithM process. LighhtPaththoroughlyy investigates these propertties in order to choose thee best processsing temperatuures, materialss, andtechniquess. Dilatometry was used to measure the coefficient of thermal exppansion (CTE)), dilatometric glasstransition temperaturetTg(dil), and the softening point. TheT results aree shown in Figuure 6.gFigurre 6 - Dilatomeetry curve of AsA 40Se60 sampleeThe shape of the curve anda the values of CTE, Tg(dil)), and the softeening point aree all dependennt on the heatinng rateused in thee experiment, whichwwas 3 CC/min for this sample.sFrom thet curve, we findfthat the CTTE from 25-1000 C is20.9x10-6 / C,/Tg(dil) is 1775 C, and the softeningspoint is 210 C.Proc. of SPIE Vol. 9451 94511O-8

Referencinng Table 2, we see that the CTTE of As40Se600 is slightly higgher than Ge28SbS 12Se60, whichh has implicatioons onboth moldiing and the opptomechanical design of lenss assemblies. DueD to the elevvated temperattures of the moldingprocess, evven moderate differences inn CTE can havve significant effects on preess yields. Thhe geometries of thepreforms anda molds mustt all be analyzeed and balanceed for optimal formingfof the final lens shappe and proper releaserfrom the mold.mAlso, thee most commonnly used lens housinghmateriaal for LWIR appplications is aluminum,awhich hasa CTE of 23ppm/ C. Thhis means thatt As40Se60 is a closer match to Al than Gee28Sb12Se60 whhich can improve thethermal staability of the fuull lens assembbly.In the casee of dilatometrry, the Tg(dil) is the temperatuure where the CTECdeparts frrom the typicaal linear relatioonship.The Tg cann also be deterrmined using differentialdscaanning calorimetry (DSC) annd, in this case, it is determinned byfinding thee onset of a chhange in heat flow.fA DSC curvecfor As40SeS 60 is shown in Figure 7. TheT heating ratee usedwas 10 C/mmin in this expperiment.Figurre 7 - Determinnation of the Tg from the DSCC curve of an AsA 40Se60 samplleThe glass transition temmperature of As40Se60 is 100 C lower thann that of Ge28SbS 12Se60. This lower Tg can be anadvantage in the moldingg process, sincee lower moldinng temperaturees are required. Molds are exppected to last longer,and in genneral glass inteeractions with molds are exppected to be leess. However, this lower Tg could be a limmitingfactor for somesapplicatioons if high temmperature stabillity is an issue.2.2.3. DennsityDensity caan be importaant when the weight of ann optical asseembly is critiical, e.g. spacce or head-moountedapplicationns. As shown inn Table 2, the density of As40gnificantly diffferent from thaat of Ge28Sb12SeS 60. In4 Se60 is not sigthis case, thhe germanium-free glass doees not present ana advantage orr disadvantagee.Proc. of SPIE Vol. 9451 94511O-9

2.2.4. HardnessThe microhardness of the chalcogenide glasses was measured with a Vickers diamond indenter. While the valuesreported for both glasses in Table 2 are lower than traditional crystalline materials, hard carbon (diamond-like)coatings have been developed to improve the material’s durability when required for certain applications. As40Se60 issignificantly softer than Ge28Sb12Se60, while this does not have a significant impact on the molding performance, itdoes have ramifications a

1.4. Cha l Since 200 5 (IG5 / TI majority o its molded lenses can Table 1 - each glass sup that there ar points can be Secondly, m o ns, Ge28 Sb 12 S oint has sign cost of imple ring work env i ns: the arsen i co genide Gla s, LightPath h a 1173 / AMTI f applications, lenses as BD be seen in Fig u L andscape of C and Germ a plier in Tabl e a .

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