20 Optics & Photonics News ■ September2001

ptics has a long history, butthe wi de s pre ad use of t h i n -film coa ti n gs in optics is rel a-tively new. Optical coa ti n gs

a re now ad ays used wh en ever the opti c a lproperties of a specular su rf ace are to bea l tered . Th ey consist of one or more thinl ayers of m a terial that opera te by interfer-en ce co u p l ed with the natu ral opti c a lproperties of the materi a l s . Th ey ra n gef rom simple anti ref l ecti on coa ti n gs topulse length correctors in fem to s econ dl a s ers . Th ey may consist of a nything froma single layer to two hu n d red layers . This isa ra t h er abbrevi a ted account of the historyof the tech n o l ogy of m e a su rem ent in thin-film opti c s .

It is conven i ent to think of the devel op-m ent of s c i en ce or tech n o l ogy as a series ofadva n ces by particular indivi du a l s , on el e ading to another and forming a thre ad ofprogre s s . In re a l i ty the situ a ti on is mu chm ore com p l ex . In d ivi duals are mu ch lessi m portant than the ex i s ting state of t h esu bj ect . Over and over again we see dis-coveries and inven ti ons ign ored , evenri d i c u l ed , to be red i s covered later, of ten si-

mu l t a n eo u s ly and indepen den t ly by work-ers in different com mu n i ti e s . We of tens peak of devel opm ents as “before thei rti m e” and this is re a lly a good de s c ri pti on .Adva n ces tend to occur wh en they aren eeded , and if one indivi dual does notm a ke them , a n o t h er wi ll . S c i en ce andtech n o l ogy are re a lly qu i te unlike a singl et h re ad . Th ey are mu ch more like a net-work with indivi duals at the nodes andcon n ecti ons to other nodes and a gre a tdeal of redu n d a n c y. It is vi rtu a lly impo s s i-ble to de s c ri be the devel opm ents in det a i lin a linear account like this paper. Th i s ,t h erefore , is a qu i te sel ective and su bj ectiveaccount because it cannot be anything el s e .

Me a su rem ent tech n i ques form a ra t h ers pecial class in scien ce and tech n o l ogy.Th ey are ra rely an end in them s elve s , butt h ey en a ble other adva n ces to be made ,a n d , in fact , adva n ces depend on them .Th eir devel opm ent is of ten a re s ponse totech n o l ogical requ i rem en t s . Thus it is al-most impo s s i ble to sep a ra te the history ofthin-film measu rem ent from the historyof thin-film tech n o l ogy. The measu re-m ents became important as they were

n eeded for the furt h er devel opm ent of t h esu bj ect . The early story consists mu chm ore of i m provem ents in the unders t a n d-ing of thin films and their properti e s . Th em e a su rem ents that were made were of tenf a i rly cru de . As thin films became impor-tant in tech n o l ogy the need for mea-su rem ents of gre a ter acc u racy became a pp a ren t .

No one knows wh en interest in thinfilms bega n . The earliest thin film tech n o l-ogy prob a bly invo lved be a ten prec i o u sm etals and merc u ry amalga m s . Merc u rywas known as early as 1600 BC, and po s s i-bly even earl i er, and gold was known froma round 6000 BC. Be a ten gold was prob a-bly the first thin film material and nodo u bt the cra f t s m en invo lved knew alla bo ut the thickness of the film and how itrel a ted to the qu a n ti ty of gold requ i red fora given are a . No do u bt also the co l ors ofthin films, prob a bly of oil on water, wereob s erved .

Du ring the 16t h cen tu ry, an initi a lly se-c ret process was devi s ed in Ven i ce to makem i rrors . The coa ting was, a pp a ren t ly, a na m a l gam of ti n . This was perhaps the ear-

with what was known and could be meas-u red ra t h er than to introdu ce hypo t h e s e sthat were not veri f i a ble by ex i s ting ex peri-m ental re su l t s . He had a great abi l i ty to place measu rem ents in qu a n ti t a tivef ra m eworks and to draw gen eral con clu-s i ons from them . In Opti ck s he tri ed , n o ten ti rely su cce s s f u lly, to avoid the introdu c-ti on of s pec u l a tive con clu s i ons and to con-s tru ct a qu a n ti t a tive con text for his ob s er-va ti on s . It was clear that the co l ors in thinfilms had som ething to do with thei rt h i ck n e s s . To stu dy this furt h er requ i red aw ay of m e a su ring the thicknesses of t h ethin films and here Newton displayed hisgreat ex peri m ental abi l i ty. Ra t h er thanm e a su re an ex i s ting film of u n k n ownt h i ckness he found a way of c re a ting filmsof t h i cknesses that were known to an in-c red i ble degree of acc u rac y. A large plano-con c ave lens of acc u ra tely known rad ius ofc u rva tu re was placed on a flat glass su r-f ace . G eom etry then perm i t ted the thick-ness of the re su l ting film of air to be calcu-l a ted . This en a bl ed Newton to rel a te theex act thickness of a film of air to its co l orin wh i te ligh t . He ad ded fluids bet ween theglass su rf aces and so was able to va ry theden s i ty of his films. His measu rem en t swere so acc u ra te that Th omas Yo u n g, on ehu n d red ye a rs later, was able to use themto calculate the wavel engths corre s pon-ding to the co l ors of vi s i ble ligh t .

Newton recogn i zed the repeti ti on ofco l ors in gradu a lly increasing order as thet h i ckness of the film incre a s ed and formu-l a ted rules for iden ti f ying the order fromthe qu a l i ty of the co l or and hen ce thet h i ckness of the film inducing the co l or.This is the first det a i l ed account of a meas-u rem ent tech n i que for thin film thick n e s s .In deed it is the foreru n n er of the tech-n i ques used most of ten tod ay. The pri m a ryd i f feren ce is that we make ra t h er more pre-cise spectral measu rem ents of the qu a l i tyof the light ref l ected or tra n s m i t ted by thefilm because we have the tech n o l ogy to dos o. Even then we scarcely improve on New-ton’s acc u rac y. However, in spite of this ad-va n ce , m ore than two hu n d red ye a rs wereto pass before the tech n i que was to becom eo t h er than a curi o s i ty.

Not mu ch more progress seems to havebeen made in the measu rem ent of t h i nfilms in the 18t h cen tu ry. In deed the devel-opm ents in optics seem ed slow. This is of-ten attri buted to an inhibi ting ef fect ofNewton’s great work and a re s i s t a n ce byhis ad h erents to any mod i f i c a ti on of h i si de a s , but it is mu ch more likely that tech-

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September2001 ■ Optics & Photonics News 21

liest com m ercial app l i c a ti on of thin filmsand the mirrors were an immed i a tely su c-cessful produ ct . What tech n i ques wereu s ed ,i fa ny, to measu re thin film thick n e s sa re not known , but one can imagine that,wi t h o ut some con trol over thick n e s s , t h ecosts of produ cti on could not be stabl e .For several cen tu ries this was the majora pp l i c a ti on of thin films.

Re ad i ly ava i l a ble accounts of thin films tudies date from the 17t h cen tu ry. At thattime there was an increasing interest intech n o l ogy, trade and similar practi c a lm a t ters . Rel i gi on was loo s ening its inhibi t-ing grip on scien ce . The cl i m a te was be-coming more favora ble to scien tific dis-covery in a way that it had not been before .Zach a rias Ja n s en produ ced the first mod-ern micro s copes at the end of the 16t h cen-tu ry in Ho ll a n d . Th en in 1665, RobertHoo ke (1635-1703) publ i s h ed his boo kMi cro gra p h i a1 that reco u n ted his workwith the com pound micro s cope and wass om ething of a best sell er. Sa mu el Pepyswas evi den t ly fascinated by it. Hoo ke hadwi de scien tific intere s t s . In the opti c a lch a pter of Mi cro gra p h i a, he reported , forthe first ti m e , the ph en om ena inspired byhis use of the micro s cope . Pa rti c u l a rly rel-evant are his ob s erva ti ons of i n terferen cein thin films. These inclu ded floa ting oi lf i l m s , s oap bu bbles and thin plates ofm i c a . He discovered that there was a rel a-ti onship bet ween the co l or and the thick-ness of the film. Wh en the thick n e s sch a n ged so did the co l or. He was con-vi n ced that the co l or had som ething to dowith the beams of l i ght ref l ected from thef ront and rear su rf aces of the film. He too kthe vi ew that light was some kind of w avem o ti on .

Hoo ke was overs h adowed by the sligh t-ly yo u n ger Sir Is a ac Newton (1642-1727).Hoo ke had cri ti c i zed , s om ewhat unju s t ly,Newton’s first paper before the Royal Soc i-ety, wh i ch was on the su bj ect of the disper-s i on of l i ght into its co l ors by a pri s m .2

This led to an en m i ty bet ween the twom en that pers i s ted until Hoo ke’s de a t h . Si rE d mund Wh i t t a ker su ggests this en m i ty asa po s s i ble re a s on for the del ay in the pub-l i c a ti on of Newton’s Opti ck s3 u n til the ye a ra f ter Hoo ke’s de a t h . ( From yo ut h , Hoo keh ad su f fered from a deform a ti on of t h es p i n e , wh i ch he attri buted to long hoursbending over a lathe, and pain from thiscon d i ti on may have ex acerb a ted his qu i cktem per. Newton also had a rep ut a ti on ofbeing slow to for give . )

Newton’s scien tific style was to work

Sir Isaac Newton . (From the MIT Burndy Library,courtesy of AIP Emilio Segrè Visual Archives.)

Augustin Jean Fresnel. (From the Smithsonian In -stitution,courtesy of AIP Emilio Segrè Visual Archives.)

Thomas Young. (Courtesy of AIP Emilio Segrè VisualArchives.)

n o l ogy simply did not demand anyt h i n grad i c a lly new. Th ere was con s i dera bl eprogress in technical opti c s . For ex a m p l e ,ach rom a tic do u bl ets were inven ted mak-ing en ormous improvem ents to vi sual in-s tru m en t s . Va rious ideas of a wave theoryof l i ght were adva n ced but the wave theorywas not nece s s a ry for the current tech n o l-ogy and so it was largely ign ored by them a i n s tre a m . Co l or in thin films was al-w ays fascinating but it was not, at thats t a ge , of a ny practical use.

Th en at the beginning of the 19t h cen-tu ry Th omas Young (1773-1829) ad-va n ced the theory of the interferen ce ofl i ght as a con s equ en ce of l i ght as a wave . Inhis Ba kerian lectu re to the Royal Soc i ety in1801 he stated : “Wh en two undu l a ti on sf rom different ori gins coi n c i de ei t h er per-fect ly or very nearly their joint ef fect is acom bi n a ti on of the moti ons bel on ging toe ach .” This idea immed i a tely ex p l a i n ed allthe ef fects ob s erved by Newton and oth-ers ,i n cluding the ch a n ge of co l or with in-c i den ce that had been “ very em b a rra s s i n gon every other su ppo s i ti on .”

Young should have been hailed as ah ero. In s te ad , in the Ed i n bu rgh Revi ew h ewas attacked by an anonymous wri ter,n ow iden ti f i ed as Hen ry Bro u gh a m , l a terLord Bro u gh a m , a law yer and su b s equ en t-ly Lord Ch a n cell or. The age seems to havebeen pattern ed by great qu a rrels wi t hbe a uti f u lly sculptu red insu l ting ph ra s e s ,using eru d i te ex pre s s i on s , hu rl ed by thep a rticipants at each other in publ i c . Th i swas a kind of du elling with words in ana ge wh en du elling with lethal we a pon swas not unknown . Young had , u n fortu-n a tely, c ri ti c i zed in print a mathem a ti c a l

work of Bro u gh a m’s , who was now takinghis reven ge .“ Feeble lu c u bra ti on s” and “a smu ch use as the Indian theory of the el e-phant or of the camel ” were some of t h em i l der cri ti c i s m s . All en tertaining stu f f , butpoor Young had , as a re su l t , great difficultyin get ting any recogn i ti on of his theory.

What re a lly saved Yo u n g’s theory fromobl ivi on was the almost simu l t a n eo u s ,a n dcert a i n ly indepen den t , work of Au g u s ti nJean Fre s n el (1788-1827). Fre s n el had de-vel oped his own theory of the prop a ga ti onof l i ght wh i ch com bi n ed the idea of w ave sand interferen ce along the same lines asYo u n g. Fre s n el had a su pporter in Do-m i n i que Fra n ç ois Jean Ara go (1786-1853)wh o, a l t h o u gh disappoi n ted that Yo u n gevi den t ly had pri ori ty, j oi n ed forces wi t hhim to prom o te the theory. The great su c-cess in pred i cting the bri ght spot in thed i f f racti on pattern of an op a que obj ectwas a convincing dem on s tra ti on of t h epower of the new theory. Th eir initial su p-po s i ti on that light was a lon gi tudinal vi-bra ti on could not explain po l a ri z a ti ona n d ,a l t h o u gh it was difficult to con ceive al a teral vi bra ti on in a three - d i m en s i on a lm ed iu m , n evert h eless the early re sults onpo l a ri z a ti on , n o t a bly by Éti enne Lo u i sMa lus (1775-1812), m ade it clear that thevi bra ti on had to be tra n s vers e . The theoryt h en perm i t ted the devel opm ent by Fre s-n el of the well - k n own ex pre s s i ons for re-f l ecti on at a su rf ace , the coef f i c i ents thatwere to bear his name. By the end of t h ef i rst qu a rter of the 19t h cen tu ry the tra n s-verse wave theory of the prop a ga ti on ofl i ght was firm ly establ i s h ed . Even tu a llyJames Cl erk Ma x well (1831-1879) showed4

that light waves were el ectrom a gn etic and

the fundamental theory that su pport sthin-film optics was then com p l ete .

Fre s n el tu rn ed his atten ti on to interfer-en ce ef fects in thin unsu pported films. Th es tory is told by Kn i t t l .5 Fre s n el unders toodvery well the maximum interferen ce ef fectof a qu a rterw ave layer but was puzzled bythe en ormous differen ce bet ween his cal-c u l a ted re sults for s-po l a ri z a ti on atobl i que inciden ce and measu red re su l t swh en the unsu pported film was on eh a l f w ave in thick n e s s . He could see no wayof obtaining the measu red re sult of zeroref l ect a n ce . It was Si m é on Denis Poi s s on(1781-1840) who poi n ted out to Fre s n elthat it was nece s s a ry to inclu de a com p l etes eries of beams ref l ected back and fort hbet ween the two su rf ace s . Fre s n el had beeni n cluding on ly two be a m s . Poi s s on alsowon dered wh et h er there could be a con d i-ti on for a qu a rterw ave layer to give zero re-f l ect a n ce . He found that this was po s s i bl eon ly in an asym m etric arra n gem ent andderived the well - k n own con d i ti on for a qu a rterw ave anti ref l ecti on coa ti n g,nf = √(n0nsu b) .

The con s tru cti on of ach rom a tic len s e srequ i red acc u ra te determ i n a ti on of ref rac-tive indices and to this end Jo s eph vonFra u n h ofer (1787-1826) dispers ed su n-l i ght using a pri s m . His ex peri m ent wassimilar to one carri ed out by Newtonm ore than a hu n d red ye a rs earl i er, butFra u n h ofer used a slit inste ad of a circ u l a rhole as en tra n ce apertu re , and so fo u n dthe dark lines that bear his name. Wo ll a s-ton had , in fact , ob s erved a few of the linese a rl i er, but Fra u n h ofer cataloged severa lhu n d red and named the principal on e s .L a ter, Fra u n h ofer con s tru cted the first dif-f racti on gra ting and made measu rem en t sof the wavel ength of l i gh t . In 1817 he de-vel oped an ac i d - b a s ed accel era ted agi n gtest for optical glass so that he could iden-tify com po s i ti ons with en h a n ced envi ron-m ental re s i s t a n ce . In the co u rse of t h i swork he ob s erved that the ref l ect a n ce ofs ome glasses was redu ced and correct ly at-tri buted this to the form a ti on of a thinfilm of redu ced index on the su rf ace .6 Th ef i rst anti ref l ecti on coa ti n gs were born . Th et h eory of the anti ref l ecti on coa ti n g, t h a n k sto Poi s s on and Fre s n el , was also com p l ete .However, practical app l i c a ti on of this dis-covery would be a long time in com i n g.

The 19t h cen tu ry was marked by gre a tprogress in optics and in all bra n ches ofs c i en ce . A major practical adva n ce fromthe thin film point of vi ew was the devel-opm ent of ch emical silveri n g. Credit is

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22 Optics & Photonics News ■ September2001

Figure 1.The transmittance and reflectance of an unsupported film of gold 90 nm thick.The transmit-tance peaks at 500 nm and so the color of the light transmitted will be predominantly green.The reflect-ed light is yellow (or minus-blue).

given to Ju s tus von Liebig (1803-1873) butit was thanks to the pion eering work ofJean Bern a rd Léon Foucault (1819-1866),who app a ren t ly devel oped the process in-depen den t ly, that it was incre a s i n gly ap-p l i ed to the con s tru cti on of ref l ecting tel e-s copes du ring the second half of the 19t h

cen tu ry. Gradu a lly the spec u lum met a lpri m a ries gave way to silvered gl a s s , a n dthe machines nece s s a ry to con s t a n t ly re-work the large specula disappe a red fromthe ob s erva tori e s .

Also abo ut the middle of the 19t h cen-tu ry, s p ut teri n g, a process for the depo s i-ti on of thin films, h ad been discovered7 bySir Wi lliam Robert Grove (1811-1896),who was ex peri m en ting with ga s eous dis-ch a r ge s . Grove began his career in law,m oved to phys i c s , retu rn ed to law, a n den ded up a high co u rt ju s ti ce . A few ye a rsa f ter Grove’s discovery Mi ch ael Fa rad ay(1791-1867) reported on met a llic depo s i t sthat had been produ ced by an el ectri c a ld i s ch a r ge .8 The paper does not de s c ri bethe process in detail but it appe a rs to be atype of ex p l oding wi re , po s s i bly inclu d i n gs ome el em ent of a rc eva pora ti on . Al-t h o u gh there were some ex peri m ents laterin the cen tu ry by others , little seems toh ave been made of t h em . Nevert h el e s sthese processes were the foreru n n ers to theen ormous growth of thin film tech n o l ogyin the 20t h cen tu ry.

An intere s ting con tri buti on to thinfilms comes from Mi ch ael Fa rad ay in hisBa kerian lectu re of 1 8 5 7 .“ Be a ten gold leafis known in films esti m a ted at the1 / 2 8 2 , 0 0 0t h of an inch in thick n e s s ; t h eya re tra n s lu cen t , tra n s m i t ting green ligh t ,ref l ecting yell ow, and absorbing a por-ti on .” The thick n e s s , 90 nm in tod ay ’su n i t s , was known acc u ra tely from themass per unit are a . One can spec u l a te thatperhaps this same ex peri m ent may havebeen carri ed out some thousands of ye a rse a rl i er. F i g u re 1 shows the calculatedtra n s m i t t a n ce and ref l ect a n ce of a 90 nmt h i ck gold film; it transmits in the greenand ref l ects yell ow and there is cert a i n ly ana b s orbed porti on .

In 1891 the first ed i ti on of Dennis Tay-l or ’s famous book On the Ad j u s tm ent andTe s ting of Tel e scopic Obje ctive s was pub-l i s h ed .9 “As rega rds the tarnish wh i ch weh ave above allu ded to as being noti ce a bl eu pon the flint lens of an ord i n a ry obj ec-tive after a few ye a rs of u s e , we are verygl ad to be able to re a s su re the own er ofsu ch a flint that this film of t a rn i s h , gen er-a lly loo ked upon with su s p i c i on , is re a lly a

very good fri end to the ob s erver, i n a s mu chas it increases the tra n s p a rency of his ob-j ective .” Tayl or devel oped a met h od of a r-ti f i c i a lly producing the tarnish by ch em i-cal etch i n g.1 0 At last Fra u n h ofer ’s discov-ery was being app l i ed , but it is almost cer-tain that Tayl or knew nothing of i t .

A com p l etely uncon n ected series of de-vel opm en t s1 1 in the 19t h cen tu ry re su l tedin the Lippman emu l s i on that was used inco l or ph o togra phy and was essen ti a lly am et a l - d i el ectric interferen ce stru ctu rewith a half-wave repeat spac i n g.

Thu s , by the end of the 19t h cen tu ry,a p a rt from the Lippmann emu l s i on ,t h i n -film optics con s i s ted in the main of s i lver-ing of ref l ectors and tarnishing of obj ec-tive s . In both these app l i c a ti ons it is un-l i kely that film thickness was an issue northat any acc u ra te measu rem ents were car-ri ed out on the films. Process con d i ti on swere likely rel a ted direct ly to sati s f actoryperform a n ce .

However, the gro u n dwork for su b s e-qu ent measu rem ents of thin films was be-ing laid. In 1862, Armand Hi ppo lyte Lo u i sF i zeau (1819-1896) de s c ri bed an interfer-om eter that was to become of i m port a n cein the measu rem ent of su rf aces and, bym e a su ring step hei gh t s , the thicknesses off i l m s . F i zeau fri n ge s , of ten call ed “f ri n ge sof constant thick n e s s ,” a re produ ced be-t ween two su rf aces that are close toget h erbut define a wed ged film in bet ween .Fri n ges run ac ross the wed ge forming con-tour lines of the thickness of the interven-ing film. If one of the su rf aces is distorted

or has a film step, the irreg u l a ri ty reve a l si t s el f in the distorted shape of the fri n ge sand can be measu red . If a broad mon o-ch rom a tic source is used , F i zeau fri n ge s ,provi ded they are of l ow order, can bevi ewed by the eye by focusing on the su r-f ace under measu rem ent (Fig. 2 ) . F i ze a ualso poi n ted out the import a n ce of s m a llangular size and narrow spectral width ofs o u rce for the produ cti on of h i gh - orderf ri n ge s , c a rri ed out the first ex peri m en t srel a ting fri n ge vi s i bi l i ty and spectral wi d t hof s o u rce , and inspired Édo u a rd Steph a n(1837-1923) to con s tru ct the first stell a ri n terferom eter at l'Ob s erva toi re de Ma r-s ei ll e . It was also at Ma rs ei lle that Ch a rl e sFa bry (1867-1945) and Jean Ba pti s te Gas-p a rd Gu s t av Al f red Pérot (1863-1925)produ ced their famous mu l ti p l e - beam in-terferom eter.1 2 The Fa bry - P é rot étalon isthe basic stru ctu re used in thin-film nar-row-band filters .

In 1887, Ot to Hei n ri ch Wi en er (1862-1 9 2 7 ) , prob a bly best known for his workon standing optical wave s , devi s ed a newsu rf ace interferom eter by making thep l a tes in Fize a u’s arra n gem ent para ll el andd i s persing the light ref l ected from a nar-row strip ac ross them by imaging the stri pon the en tra n ce slit of a spectrogra ph ( F i g. 3 ) . To improve the en er gy gra s p, t h el i ght source should be con cen tra ted on thea rea of the sample under measu rem en tand the path differen ce bet ween su rf aceand referen ce should therefore be kepts m a ll to avoid the ef fect of the finite aper-tu re on high - order fri n ge s . Usu a lly the two

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September2001 ■ Optics & Photonics News 23

Figure 2. Schematic of arrangement for viewing low-order Fizeau fringes (left) and alternative arrange-ment for high-order fringes (right).

c a l c u l a ted , in 1891, t h ech a n ge in po l a ri z a ti onof an incident be a mi n du ced by obl i que re-f l ecti on at a su rf acecoa ted with a thin film

and so devi s ed the measu rem ent tech-n i que now known as ell i p s om etry.1 3 Th eadva n t a ge of this tech n i que is that ab-s o lute measu rem ents of ref l ect a n ce areavoi ded (although the redu cti on of the re-sults is invo lved , tod ay it is inva ri a bly per-form ed by com p uter ) .

It now starts to become difficult to keeptrack of devel opm en t s . The net work ofd i s coveries and adva n ce s ,e ach com bi n i n gwith others to prom o te sti ll more ad-va n ce s , becomes impo s s i bly com p l ex . Al-most all the nece s s a ry fo u n d a ti on for them odern measu rem ent of thin films was inp l ace . But the tech n o l ogy of thin films wass ti ll in its infancy and the demand form e a su rem ents was largely missing.

Several ingred i ents were nece s s a ry forthin film tech n o l ogy to grow. Th ere had tobe su i t a ble met h ods of depo s i ti on , but ,most of a ll , t h ere had to be a need . Th ebest depo s i ti on tech n i ques requ i red vac u-um and there were great stri des in vac u-um equ i pm ent in the 1930s. The ava i l a bi l-i ty of good vacuum equ i pm ent en co u r-a ged thin film re s e a rch . Alu m i n i z i n g, p i o-n eered by John Stron g, rep l aced silveri n gfor astron omical mirrors . An ti ref l ecti oncoa ti n gs became mu ch more attractive .1 4

A com m on tech n i que used for determ i n-ing film thickness at this time was thewei ght met h od . A given wei ght of m a teri-al was com p l etely eva pora ted from a ther-

su rf ace s , referen ce and te s t , a re silvered ,the referen ce parti a lly, so that the fri n ge sa re narrow. The re sult is a series of d a rklines running ac ross the spectru m , e achreprodu c i n g, in wavel en g t h , the shape ofthe profile of the test su rf ace along them e a su red stri p. The fri n ges in thisa rra n gem ent are usu a lly de s c ri bed as“f ri n ges of equal ch rom a tic order,” and theWi en er interferom eter was to become veryi m portant in the assessment of film thick-nesses by measu ring the hei ght of the stepat the ed ge of the film.

Paul Ka rl Lu dwig Dru de (1863-1906)s t a rted his career as a firm bel i ever in them echanical theory of l i ght prop a ga ti onand on ly gradu a lly shifted in favor of t h en ew el ectrom a gn etic theory. In deed , a tone point he showed that both the el ectro-m a gn etic and mechanical theories wereequ iva l ent in terms of the set of d i f feren-tial equ a ti ons governing the prop a ga ti on .It was simply a case of a different interpre-t a ti on of the para m eters . O n ce firm ly inthe el ectrom a gn etic camp, he derived rel a-ti onships bet ween the micro s tru ctu re ofthin films and their el ectrical and opti c a lproperti e s . In the co u rse of his studies he

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24 Optics & Photonics News ■ September2001

Table 1. Banning’s colors for monitoring high-in-dex layers16 compared with Newton’s soap bubblecolors.3

F i g u re 3. Rough layout of the modified interfe rometer suggested by W i e n e rproducing fringes of equal chromatic order.The dark fringes run across the spec-trum as shown, reproducing the profile of the test surface.

mal source . The film thickness was thenc a l c u l a ted from esti m a tes of the thick n e s sd i s tri buti on of the coa ti n g. Ma rtin Ha n sCh ri s tian Knu d s en (1871-1949), ra t h ere a rl i er,1 5 h ad dem on s tra ted the cosine lawfor eva pora ti on source s .

The actual tri gger for an en ormous ex-p a n s i on in thin film tech n o l ogy was Worl dWar II. Optical instru m ents of a ll kindswere requ i red in great qu a n ti ti e s . Th ei rperform a n ce , e s pec i a lly in poor ligh ti n gcon d i ti on s , could be gre a t ly improved byadding anti ref l ecti on coa ti n gs . All parti c i-pants in the war immed i a tely and vi rtu a llys i mu l t a n eo u s ly began to add anti ref l ecti oncoa ti n gs to bi n oc u l a rs , ra n gef i n ders ,bom b s i ghts and the like . All opera ted un-der con d i ti ons of great sec rec y. Du ring thewar little was publ i s h ed on thin film coa t-i n gs , but after the war en ded there was af l ood of p u bl i c a ti ons that has never abat-ed . The su bj ect had now become main-s tre a m .

It is likely that the great ex p a n s i on inoptics and espec i a lly thin film opti c swould have occ u rred wi t h o ut the war butit might have been a little slower. The ti m ewas ri ght and the su bj ect dem a n ded it.The com p l ex i ty of optical instru m en t a-ti on had re ach ed the stage at wh i ch coa t-i n gs were nece s s a ry. Alu m i n i zed mirrorsand anti ref l ected lenses were on ly the be-gi n n i n g. O f co u rse there were , and are sti ll ,con ti nuing military requ i rem en t s . Butt h ere appe a red a trem en dous demand fora ll kinds of f i l ters ,e s pec i a lly for the vi s i bl eand the infra red in a va ri ety of a pp l i c a-ti on s , a n a lysis in ch emical manu f actu re ,el i m i n a ti on of heat in film proj ectors , en-

Banning Newton Optical thicknessfor green light

Bluish white Black*: very black spots and within these other spots of an intense blackness

↓ Blue: Obscure and dark sky color

White White: perfect whiteness l/4,first-order maximum

↓Yellow Yellow:pretty good and lively

↓ Red:a brighter color inclining to orange...a tincture of scarlet next to violet

Magenta Violet: intense and deep with l/2,first-order minimumlittle or no redness in it

↓Blue Blue: bright sky color

↓Greenish white Green: little and dilute seeming 3l/4,second-order maximum

rather a greenish white than a green

↓Yellow Yellow: intense bright and copious yellow…

best of all the yellows

↓ Red: fair and lively scarlet…best of all the reds

Magenta Purple: purple much inclined to red l, second-order minimum

↓Blue Blue: excellent blue of a bright sky color

↓Green Green: copious and pure, deep and lively 5l/4,third-order maximum

* N ew t o n ’s films were unsupported while Banning’s were on a substrate so with ze ro film thickness Newton would see blackwhile Banning would see white.

h a n ced ref l ectors for optical instru m en t s ,a n ti ref l ecti on coa ti n gs for camera len s e s ,beam splitters for interferom eters , l a r gea pertu re po l a ri zers . The list went on andon . As tron om ers soon found the en or-mous adva n t a ges in con trast afforded byi n terferen ce filters wh en looking at faint,d i f f u s e , m on och rom a tic obj ects aga i n s tthe bri ght night sky. L a ter the laserbro u ght a com p l etely new set of requ i re-m ents for coa ti n gs . Co l or tel evi s i on re-qu i red dich roic beam splitters for cam-era s . Tod ay optical com mu n i c a ti on de-mands even more .

The measu rem ent processes for thee a rly coa ti n gs were ru d i m en t a ry. Alu-minizing requ i red simply an op a que film.An ti ref l ecti on coa ti n gs were con tro ll ed byexamining their ref l ected co l or — prec i s elythe tech n i que for film thickness determ i-n a ti on devi s ed by Newton some 250 ye a rse a rl i er! This tech n i que was used even form ore com p l i c a ted mu l ti l ayer coa ti n gs .Ma ry Banning de s c ri bed the tech n i que inan important 1947 paper.1 6 Ta ble 1 showsBa n n i n g’s co l ors for zinc su l f i de films( h i gh index) com p a red with Newton’s co l-ors for soap bu bbl e s .3

Gre a ter acc u racy requ i red ph o toel ec-tric detectors and mon och rom a tic ligh t ,ech oing what Fizeau had said some 100ye a rs earl i er. Ha rry D. Po l s ter1 7 reported in1952 his use of a ph o toel ectric sys tem tom e a su re the va ri a ti on of ref l ect a n ce of am on i tor chip at one wavel ength du ri n gthe depo s i ti on of a filter. Four layers co u l dbe depo s i ted in sequ en ce on each fre s hsu rf ace . This tech n i que is essen ti a lly theone used tod ay for the produ cti on of awi de ra n ge of optical coa ti n gs .

In 1959, G ü n ter Sa u erbrey propo s edthe use of qu a rtz crystal oscill a tors tom on i tor film thickness depo s i ti on . Su chc rystals were used in large qu a n ti ties to as-su re the frequ ency of el ectronic rad i otra n s m i t ters by coupling from their natu-ral shear oscill a ti on into an el ectrical sig-nal by the piezo - el ectric ef fect . Th ey werecon t a i n ed in plug-in cans and in the earlyd ays workers found it conven i ent to sawopen the cans around the crystals and sim-p ly mount them in the vacuum sys tem sothat the crystal face was arra n ged tow a rd sthe source of va por. Th ey were inclu ded asthe tuning el em ent in an el ectronic oscill a-tor circ u i t . As the va por con den s ed on thec rystal the natu ral frequ ency va ri ed andthe ch a n ge in oscill a tor frequ ency was am e a su re of the mass of m a terial depo s i ted .It is intere s ting that for some ye a rs previ-

o u s ly the normal tech n i que for depo s i ti n gthe met a l , u su a lly go l d , el ectrodes on thef aces of the crystal was vacuum depo s i-ti on . The final tuning of the oscill a tor wasach i eved by adding ad d i ti onal film materi-al until the frequ ency was correct . Le s l i eHo lland de s c ri bes this process in some de-t a i l .1 8 It seems unlikely that it did not oc-cur to anyone that this frequ ency ad ju s t-m ent might be converted into a way ofm e a su ring the amount of depo s i ted mate-ri a l , but for some re a s on it was not su g-ge s ted , at least in pri n t . Po s s i bly the needwas not yet app a ren t . By the mid 1960scom m ercial microb a l a n ces inten deds pec i f i c a lly for the mon i toring of t h i n - f i l mdepo s i ti on were re ad i ly ava i l a ble and to-d ay the tech n i que is in wi de s pre ad use. If i tis not alw ays the pri m a ry thickness mon i-toring tech n i que it is inva ri a bly used forthe measu rem ent of depo s i ti on ra te .

Constant demands for improved per-form a n ce meant that ch a racteri z a ti on ofthin films after depo s i ti on became veryi m port a n t . The battery of tech n i ques thatwere devel oped in the second half of t h e1 9t h cen tu ry was at last dep l oyed . Newones were cre a ted . It is impo s s i ble in anaccount of this size to cover them . Ol iverHe avens in his book publ i s h ed in 19551 9

m en ti ons over 25 different tech n i ques form e a su ring the optical properties of t h i nsolid films. Tod ay the nu m ber is cert a i n lym ore than 100. Herm a n’s book on opti c a ld i a gn o s tic tech n i ques for thin film pro-ce s s i n g2 0 s tretches to 783 page s . Why one a rth do we have so many tech n i qu e s ?Th ere is no do u bt some redundancy but am a j or re a s on is that we do need them . Weh ave learn ed that the thin films we use inoptics can be far from the simple thin slabsof h om ogen eous bu l k - l i ke material thatform ed the ori ginal model . Films po s s e s scom p l i c a ted micro s tru ctu re s , ex h i bit va ri-a ti ons in stoi ch i om etry, a re inhom oge-n eous and anisotrop i c , i n teract stron glywith the envi ron m en t , and most of t h ei rp a ra m eters are different from those ofsimilar bulk materi a l s . Frequ en t ly there isno corre s ponding bulk ph a s e . One prop-erty affects another and they are ex trem elys en s i tive to depo s i ti on con d i ti on s . Toach i eve the con ti nual increases in per-form a n ce requ i red of optical coa ti n gs , a llthese factors and their interacti ons mu s tbe unders tood , con tro ll ed , and improved .We are learning all the ti m e . For ex a m p l e ,thanks to Ha ruo Ta k a h a s h i ’s 1995 con tri-buti on ,2 1 we now apprec i a te the interac-ti ons bet ween therm a l , m echanical and

optical properties that toget h er determ i n ethe way our filter ch a racteri s tics ch a n gewith tem pera tu re . This has led direct ly tothe incred i bly small tem pera tu re coef f i-c i ents we now ach i eve in our dense wave-l ength divi s i on mu l ti p l exing be a m s p l i t-ters . Our processes tod ay may appear su-perf i c i a lly like those of 20 ye a rs ago but theproperties of the layers , p a rti c u l a rly interms of s t a bi l i ty and ulti m a te perform-a n ce ,a re va s t ly improved . The first essen-tial to understanding is measu rem ent andwi t h o ut this vast array of m e a su rem en ttech n i ques all the immense improvem en t sof the past dec ades would have been im-po s s i bl e .

References1 . R .H o o ke, M i c rog ra p h i a, or Some Phy s i o l ogical Descri p -

tions of Minute Bodies Made by Magnifying Glasses withO b s e rvations and Inquiries there u p o n. 1 6 6 5 ,L o n d o n : Jo.M a rtyn and Ja.A l l e s t ry.

2 . I .N ew t o n ,N ew theory of light and colours. P h i l o-sophical Transactions of the Royal Society, 8 0,3 0 7 5 -87 (1672).

3 . I .N ew t o n , O p t i ck s or A Treatise of the Reflections,R e -f ra c t i o n s, Inflections and Colours of Light. 1 7 0 4 ,L o n d o n :The Royal Society.

4 . J . C. M a x we l l , A Treatise on Electricity and Magnetism.1 8 7 3 :C l a rendon Pre s s .

5 . Z .K n i t t l ,F resnel historique et actuel.Optica A c t a ,2 5, 167-73 (1978).

6 . J .F r a u n h o fe r,Versuche über die Ursachen des A n-l a u fens und Mattwe rdens des Glases und die Mittel,denselben zuvo r z u ko m m e n , in Joseph von Fraun-h o fer's Gesammelte Schriften. 1 8 8 8 ,Verlag derKoniglich Bayerischen Akademie der W i s-s e n s c h a f t e n :M ü n c h e n .

7 . W. R . G rove, On the electro-chemical polarity of gas-e s . Philosophical Transactions of the Royal Society,B 1 4 2, 87-101 (1852).

8 . M .F a r a d ay, Deflagrations of gold (and other metals)- heat - pre s s u re, & c. Philosophical Transactions ofthe Royal Society, 1 4 7, 152-8 (1857).

9 . H . D.Tay l o r,On the Adjustment and Testing of Te l e s c o p i cO b j e c t i v e s. 1 8 9 1 :T Cooke & Sons.

1 0 . H . D.Tay l o r, L e n s e s .1 9 0 4 . United Kingdom.P a t e n t2 9 5 6 1 .

1 1 . R .G ü t h e r,The Berlin scientist and educator W i l h e l mZ e n ker (1829-1899) and the principle of color se-l e c t i o n .P roceedings of the Society of Photo-OpticalInstrumentation Engineers,3 7 3 8, 20-9 (1999).

1 2 . C. F a b ry,A . Pe ro t ,T h e o ry et applications d'une nou-velle méthode de spectroscopie interfére n t i e l l e.A n nChim Phy s ,P a r i s , 7th series,1 6, 115-44 (1899).

1 3 . P.K .L .D r u d e, Über die Reflexion und Bre c h u n gebener Lichtwellen beim Durchgang durch eine mitOberflächenschichten behaftete planparallele Platte.Annalen der Phy s i k ,L e i p z i g , 4 3, 126-57 (1891).

1 4 . J .S t ro n g , J .O p t .S o c.A m . , 2 6 73-4 (1936).1 5 . M .K nu d s e n ,Annalen der Phy s i k , 4th Series, 1 9 1 5 . 4 8,

1 1 1 3 - 2 1 .1 6 . M .B a n n i n g ,J .O p t .S o c.A m . , 3 7 , 792 (1947).1 7 . H . D. Po l s t e r, O p t .S o c.A m . , 4 2, 21-5 (1952).1 8 . L .H o l l a n d ,Vacuum Deposition of Thin Films. 1 9 5 6 ,L o n-

d o n :C h apman and Hall.1 9 . O. S .H e ave n s , Optical Pro p e rties of Thin Solid Films.

1 9 5 5 ,L o n d o n :B u t t e r wo rths Scientific Publications.20. I. P. Herman, Optical Diagnostics for Thin Film Process -

ing. 1996,San Dieg o, London:Academic Press.21. H.Takahashi, Appl.Opt., 34 (4),667-75 (1995).

Angus Macleod is with Thin Film Center Inc., Tucson,A r i zo n a . He can be reached at angus@thinfilmcenter.com.

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September2001 ■ Optics & Photonics News 25