Metal/dielectric transmission interference filters with low reflectance 2 Experimental results

Metal@dielectric transmission interference filterswith low reflectance. 2. Experimental results

Brian T. Sullivan and Kari L. Byrt

5684 APPLIED OPTICS @ V

The successful fabrication of metal@dielectric multilayer filters requires not only accurate control of theindividual layer thicknesses, but also a good knowledge of the optical constants of the materials used inthe filters. In the case of metal films, it is also essential to know whether any transition layers areformed at the interfaces and, if so, how their thicknesses and optical constants depend on the depositionconditions. An automatic, real-time process control, magnetron sputtering deposition system wasmodified to permit the manufacture of metal@dielectric filters using optical monitoring techniques. Toillustrate the performance of this system, two bandpass filters, a short-wavelength pass filter, and aneutral density filter were produced, all having a low reflectance for light incident on one side. Themetal layers used in these filters consisted of either Ni orAg. TheAg films could be protected from the O2

plasma using thin Ni or Si films. Good agreement was obtained between the calculated and measuredspectral transmittance and reflectance curves.

1. Introduction

For a given class of applications, metal films areindispensable in optical filters. However, there are anumber of concerns associated with the design anddeposition of metal@dielectric filters. First, for thedesign of these filters, it is important that the opticalconstants of the metal films be well known.1–3 Thisis especially true for those materials whose opticalconstants depend on the thickness of the layer.Second, it is important that the metals chosen bestable in their intended ambient environment. Thiscan be achieved by protecting the metal@dielectricstack by cementing the multilayer coatings or byusing robust thin films to protect the metals. How-ever, the use of protective layers complicates thedeposition of metal@dielectric filters and so a materialrequiring these layers should be used only if no othermaterial is suitable. Finally, to fabricate themetal@dielectric filters successfully, one has to be ableto deposit accurately the desired thickness of themetal layer and to take into account any interfacelayers that may form between the metal layer and theadjacent dielectric layers.

The authors are with the Institute for Microstructural Sciences,National Research Council of Canada, Montreal Road, Ottawa,Ontario K1A0R6, Canada.Received 29 November 1994; revised manuscript received 8

February 1995.0003-6935@95@255684-11$06.00@0.

r 1995 Optical Society of America.

ol. 34, No. 25 @ 1 September 1995

Transmission interference filters with low reflec-tance are an important class of filters based onmetal@dielectric coatings. The application and de-sign of these filters, along with a review of previousliterature dealing with their fabrication, has beenpresented in a companion paper.4 In this paper wedescribe the fabrication of metal@dielectric filtersusing reactive sputter deposition and optical monitor-ing techniques for controlling the layer thicknesses.Several low-reflectance filters described in the compan-ion paper have been fabricated to illustrate the suc-cess of this technique.In Section 2 the sputter deposition process used to

fabricate these filters is described. In Section 3 weinvestigate two metals, Ni and Ag, to determine theirsuitability for use in sputter-depositedmetal@dielectricfilters. In addition the optical constants of opaquelayers of sputtered Ni and Ag were determined usingellipsometry, and these results were checked using insitu transmittance versus layer thickness measure-ments. In Section 4 we discuss the optical monitor-ing of the metal@dielectric filters, with emphasis onthe determination of any interfacial layers that maybe formed. In Section 5 experimental results for fourdifferent filters using Ni and Ag are presented. Thisis followed in Section 6 by a discussion on the stabilityof these filters. Some conclusions are presented inSection 7.

2. Experimental

In order to obtain stable filters, it is important toselect a deposition process for the desired materials

that results in dense or bulklike coatings. One suchprocess that has been used successfully at the Na-tional Research Council of Canada 1NRCC2 is magne-tron sputtering. A previously described automatedsputter deposition system, with real-time processcontrol, was used for the deposition of themetal@dielectric filters.5 Special features of this depo-sition system include a wideband optical monitor foraccurate in situ transmittance measurements and theability to mount three different sputtering targets inthe chamber at any one time. In addition, a sub-strate holder that can carry as many as six 1-in.-12.54-cm2-diameter substrates was constructed and in-stalled for this investigation. The substrates on thisholder can be rotated under computer control so thatsix different filters can be automatically deposited inone deposition run without having to break thevacuum. These substrates can also be used as opti-cal monitor chips for the manufacture of difficultfilters in order to improve the sensitivity of thedeposition control.Briefly, the automated real-time process control

works as follows.6,7 For a given layer, a thin-filmdesign program,8 TFDesign, transfers the desiredmaterial, thickness, and process parameters to acomputer that controls the multilayer deposition.The dielectric or metal layer is then deposited afterwhich the transmittance of the multilayer coating ismeasured from 380 to 860 nm. The layer thicknessjust deposited is then determined from this measure-ment. If, within a specified tolerance, the thicknessdeposited is less than that desired, the deposition ofthe same material is continued. If the thicknessdeposited is greater than that desired, the thick-nesses of the remaining layers to be deposited can bereoptimized in an attempt to recover the originaldesired filter spectral performance. The remainderof the multilayer system is deposited in a similarmanner. As this process has been fully automated,the deposition of even quite complex multilayer coat-ings can take place overnight with no operator inattendance.The reproducibility of this deposition process con-

trol is demonstrated in Fig. 11a2, where the transmit-tance of an 18-layer coating, consisting of alternatingNb2O5 and SiO2 layers, is shown for three separatedeposition runs. The accuracy of the process controlis demonstrated in Fig. 11b2, where, for one of thesemultilayer coatings, themeasured, desired, and deter-mined transmittances are all shown to be in goodagreement. Here, the determined transmittancecurve is based on the theoretically determined depos-ited layer thicknesses. For good layer thicknesscontrol, the determined and measured transmit-tances should always be in good agreement after eachlayer is deposited. A substantial difference betweenthese curves indicates that the determined layerthicknesses or the optical constants of the materialsare incorrect.The metals were deposited at an argon partial

pressure of 3 mTorr while the oxide films werereactively deposited at the same pressure but with an

Ar@O2 flow ratio of 1:1. The substrate-to-target dis-tance was ,10 cm. The deposition rates for themetal and oxide materials were ,0.2 and ,0.1 nm@s,respectively. For this investigation, four metal tar-gets were used: Nb, Ni, Ag, and Si, of which anythree could be mounted in the chamber at any onetime.The transmittance and reflectance measurements

were performed after deposition either on a Perkin-Elmer 330 or a Lambda@19 spectrophotometer. Thereflectance measurements were made with respect toone of two reference surfaces: a fused-quartz samplewith a roughened back surface or an aluminummirror. These relative reflectance measurementswere then scaled by the actual reflectance of thereference sample to give absolute reflectance valueswith an uncertainty of 61%.

3. Metal Layer Deposition

Two metals of particular interest for metal@dielectricfilters are Ni and Ag.4 In the visible spectral region,Ni has a fairly neutral transmittance whereas Ag hasthe highest known metallic reflectance. To deter-mine the optical constants of these metals, opaquelayers were deposited on glass slides and were thenmeasured on a J. A. Woollam spectroscopic ellipsom-eter at four different angles over the visible spectralregion. The optical constants of the opaque metalwere obtained by assuming a substrate@ambientmodel when analyzing the ellipsometric data. In

Fig. 1. 1a2 Reproducibility and 1b2 accuracy of the automateddeposition control system for an 18-layer all-dielectric multilayercoating. In this and in similiar figures, the term desired refers tothe theoretical curve calculated based on the desired layer thick-nesses, the term measured refers to the experimentally measuredcurve, and the term determined refers to the theoretical curvecalculated based on the layer thicknesses determined during thedeposition run.

1 September 1995 @ Vol. 34, No. 25 @ APPLIED OPTICS 5685

Fig. 21a2, the measured in situ transmittances ofsputteredNi layers are shown for thicknesses rangingfrom 2 to 20 nm, in increments of ,2 nm. Alsoshown 1dotted curves2 are the theoretical transmit-tance fits based on the optical constants of the opaqueNi layer. The determined thicknesses are consistentwith those expected on the basis of the deposition rateand times for the different Ni layers. However, thefits for thicknesses below 10 nm are not too good,indicating that either the optical constants of Ni varywith thickness or that the substrate@ambient modelused is not appropriate. The measured and theoreti-cal curves for the sputtered Ag layers are shown inFig. 21b2 for thicknesses ranging from 10 to 70 nm, inincrements of,10 nm. For wavelengths higher than500 nm, the agreement between the two sets of curvesis good indicating that forAg films thicker than 10 nmthe optical constants do not vary significantly withthickness.To obtain stable dielectric films, energetic deposi-

tion processes such as reactive sputtering, reactiveion plating, or ion-assisted evaporation are oftenused. However, the presence of atomic oxygen, gen-erated from a plasma source, for example, can signifi-cantly affect a previously deposited metal layer byforming a metal–oxide interface layer. This is illus-trated in Figs. 31a2 and 31b2 for Ni andAg, respectively,where oxygen is added to the chamber in preparationfor the reactive sputter deposition of a dielectric layer.In Fig. 31a2 the in situ transmittance of a thin Ni

layer is shown in curves 112 to 142 for elapsed O2

Fig. 2. Theoretical fits to in situ transmittance measurements ofequally spaced thicknesses of 1a2 nickel and 1b2 silver thin films.The thicknesses vary 1a2 from 2 to 20 nm for the Ni films and 1b2from 10 to 60 nm for the Ag films. The theoretical fits were basedon the optical constants of thesemetals determined from ellipsomet-ric measurements of an opaque layer.

5686 APPLIED OPTICS @ Vol. 34, No. 25 @ 1 September 1995

plasma exposure times of 0, 5, 10, and 15 min,respectively. After the initial 5-min exposure to theO2 plasma, there is an ,2% increase in the transmit-tance across the visible region. Longer exposure tothe O2 plasma did not result in any further changes inthe transmittance. The initial increase, and thesubsequent stabilization, in the transmittance of theNi layer on exposure to the O2 plasma is consistentwith the formation of a thin, stable, nickel oxide 1NiO2layer.For the optical monitoring of the deposition of

metal@dielectric filters, it is important to know theoptical constants and thickness of this NiO interfacelayer. A semiopaque NiO layer was deposited byreactively sputtering theNi target in anAr@O2 plasma.Ellipsometric measurements of this semiopaque layerwere then used to obtain the optical constants of thereactively sputtered NiO. For comparison, anothersemiopaque NiO layer of the same thickness wasobtained by sequentially depositing and then oxidiz-ing a number of ,1-nm-thick Ni layers. The mea-sured in situ transmittances of these two semiopaqueNiO layers were in good agreement, indicating thatthe stoichiometry of the reactively sputtered NiO isclose to that of the NiO layer formed on exposure of aNi layer to an O2 plasma. It should be noted thatthese NiO films have high absorption in the visibleregion.Theoretical fits to the measured data in Fig. 31a2

were made to determine the NiO interface layer

Fig. 3. 1a2 The in situ transmittance of a Ni thin film is shown incurves 112 to 142 for elapsed O2 plasma exposure times of 0, 5, 10, and15 min, respectively. Two theoretical fits for the as-deposited Nifilm are based on 1i2 an 8.5-nmNi film only and 1ii2 a 7.8-nmNi and a4.1-nmNiO. The theoretical fit for the O2 plasma-exposed Ni filmgave thicknesses of 6.6 nm for the Ni and 6.0 nm for the NiOlayers. 1b2 The in situ transmittance of an ,25-nm-thick Ag thinfilm is shown in curves 112 to 152 for elapsed O2 plasma exposuretimes of 0, 5, 10, 15, and 20 min, respectively.

thickness. The fit to the measured transmittance ofthe as-deposited Ni film 1before exposure to the O2plasma2 was poor when the opaque Ni optical con-stants were used 3curve 1i24. However, the theoreticalfit to the O2 plasma-exposed Ni film using a two-layermodel of Ni and NiOwas quite good 3curve 1iii24. Eventhough there was no physical basis for it, the sametwo-layer model of Ni and NiO was used for theas-deposited Ni film, and this provided a good fit3curve 1ii24. When the theoretical results of the as-deposited and plasma-exposed models are compared,there appears to be a decrease of 1.1 nm in the Nilayer and an increase of 1.9 nm in the NiO layer afterexposure of the as-deposited Ni layer to the O2plasma. As a check, one can theoretically estimatethe ratio of the oxide layer thickness, toxide, to theoriginal metal layer thickness, tmetal, by the relation9

toxidetmetal

5Moxide

roxide

rmetal

Mmetal

, 112

where Moxide and Mmetal are the atomic or molecularweights of the oxide and metal materials, respec-tively, and roxide and rmetal are the densities of the oxideand metal materials, respectively. This relation as-sumes that the oxide layer expands only in thedirection perpendicular to the surface. For Ni,MNi 5 58.69 amu and rNi 5 8.90 g@cm3 whereas forNiO,MNiO 5 74.69 amu and rNiO 5 6.67 g@cm3. FromEq. 112 this then gives tNiO@tNi 5 1.70. A metal layerof thickness tNi 5 1.1 nm after complete oxidizationshould then result in an oxide layer of thicknesstNiO 5 1.87 nm. This is in good agreement with thethickness estimate that was obtained by fitting thetransmittance curves.In Fig. 31b2, the in situ transmittance of an ,25-nm-

thick Ag layer is shown in curves 112 to 152 for elapsedO2 plasma exposure times of 0, 5, 10, 15, and 20 min,respectively. It is clear that even after 20 min thetransmittance is still changing, indicating that oxy-gen is still diffusing through the Ag layer. Hence,unlike Ni, no stable oxide layer is formed on thesurface of aAg layer in an O2 plasma.Since Ag is the metal of choice for a number of

metal@dielectric filters, it would be useful to find aprotective layer that could be deposited over a Aglayer before O2 is added to the plasma. The protec-tive barrier that was first tried for Ag was Ni, basedon its stable oxide layer. Figure 41a2 shows thetransmittance of 1i2 an as-deposited Ag layer, 1ii2 a Aglayer overcoated with an ,2-nm-thick Ni layer, and1iii2 the Ni overcoated Ag layer after 20-min exposureto an O2 plasma. The transmittance of the protectedAg layer after exposure to the O2 plasma is close tothat of the as-deposited Ag layer, indicating that Ni isable to protect the Ag layer from the reactive atomicoxygen present in the plasma. A number of experi-ments established that a minimumNi layer thicknessof ,1.5 nm was required to protect the Ag layer fully.This minimum layer thickness is consistent with theestimated thickness decrease of 1.1 nm in a Ni layeron exposure to an O2 plasma.

The second protective barrier that was tried for Agwas based on a thin Si layer since it is known to form astable oxide. Figure 41b2 shows the transmittance of1i2 an as-deposited Ag layer, 1ii2 a Ag layer overcoatedwith an ,2–3-nm-thick Si layer, and 1iii2 the Siovercoated Ag layer after 20-min exposure to an O2plasma. In this case, all three transmittance curvesare nearly superimposed on each other, indicatingthat the Si protective layer works well and does notsignificantly change the desired transmittance. Theactual thickness of the Si layer was not determinedaccurately. However, the typical oxide layer formedon Si has a thickness tSiO2 < 2–3 nm. Using Eq. 112withMSi 5 28.09 amu, rSi 5 2.33 g@cm3,MSiO2 5 60.08amu, and rSiO2 5 2.19 g@cm3 gives tSiO2@tSi 5 2.27.With this theoretical estimate, the minimum Si thick-ness required is of the order of 1–2 nm.No significant interface layer is observed in trans-

Fig. 4. Protected Ag layers. 1a2 and 1b2 Performance of Ag layersprotected with ,2-nm-thick Ni and ,3-nm-thick Si layers, respec-tively. The dashed, thick, and solid curves correspond to the insitu transmittances of the as-deposited layers, after the depositionof the protective layer and after a 20-min exposure to the O2

plasma, respectively. 1c2 Fabry–Perot filter of the Ag@SiO2@Ag@SiO2

type: theoretical transmittance 1solid curve2 and measured trans-mittances of filters with protective layers based on Ni and Si.


mission measurements when either the Ni or Agmetal films are deposited on top of a dielectric layer.Therefore, a protective layer is required only when adielectric layer is to be deposited on top of aAg layer.It is important that the protective layer, and the

surface oxide layer that forms on it after exposure toan O2 plasma, do not seriously interfere with theperformance of the metal@dielectric filter. That is,the protective layer should be as thin as possible andits oxide should be nonabsorbing. A sensitive filterto test this requirement is a Fabry–Perot structureconsisting of a SiO2 spacer layer sandwiched betweentwo Ag layers. The presence of any absorbing oxideinterface layers will greatly decrease the peak trans-mittance of this metal@dielectric Fabry–Perot filter.In Fig. 41c2, the theoretical transmittance of thisFabry–Perot filter is shown along with the measuredtransmittances of two experimental filters that useprotective layers of either Ni or Si after deposition ofthe Ag layers. It is quite evident from the peaktransmittances of the measured filters that the Si is abetter choice than Ni for use as a protective layer.This is not surprising since the O2 plasma-formedinterface layer of Si is SiO2, which is nonabsorbing,whereas the interface layer of Ni is NiO, which ishighly absorbing. In addition to Si, a number ofother materials might also be suitable as protectivelayers, provided that their oxides are transparent andthat they form a stable barrier against further oxida-tion.

4. Deposition Strategy

Several issues have to be considered when devising adeposition strategy for metal@dielectric filters: 112the optical constants of the metal layers may varywith thickness, 122 the formation of a surface oxideinterface layer on a material after exposure to an O2plasma, and 132 the formation of an interface layerbetween a metal layer and a previously depositeddielectric layer.Concerning the variation of optical constants of the

metal layers with thickness, it is clear from Fig. 21a2that the optical constants of a thick Ni layer do notadequately describe the measured transmittance ofthe thin Ni layers. Depending on the specificationsof the metal@dielectric filter, this may or may not be aproblem. In terms of optical monitoring of the metallayers, however, it is important to get a good theoreti-cal fit to the measured transmittance or else system-atic thickness errors may occur. In practice, sincethe deviation between the theoretical fit and themeasured transmittance averages out across the vis-ible spectrum 3see Fig. 21a24, it is possible to get goodthickness control when depositing the Ni metal layer.After this metal layer is partially oxidized and an-other transmittance measurement is made, it is thenpossible to improve the theoretical fit to the measureddata by adjusting the thicknesses of both the metaland interface layers. Although these layer thick-nesses may not be physically realistic, this approachdoes allow for a reasonably accurate optical monitor-ing of the remaining layers to be deposited.


The second issue related to the accurate depositionof metal@dielectric filters is whether the depositedmetal layers are significantly affected by anO2 plasma.As discussed in Section 3, some metals and semicon-ductors, such as Ni or Si, form a thin stable oxidelayer. Other materials, such as Ag, do not and as aresult the entire layer can be rapidly oxidized. Forthe latter case, the solution is to deposit, without anyO2 present, a thin protective layer 1of ,2–3-nm thick-ness2 of a material such as Ni or Si that will form astable protective barrier layer.Whether or not protective layers are used, it is

important that the modification of a layer’s thicknessafter deposition and the formation of any interfacelayer be taken into account in the modeling of themultilayer. Otherwise, the automated real-time pro-cess control outlined above will not work properly.To accomplish this the thin-film program TFDesignwas modified to permit the operator to specify in amultilayer filter design any interface layers that maybe formed during deposition. For example, whereverthere is a Ni layer in a metal@dielectric filter, a NiOinterface layer with zero thickness should be insertedbetween this metal layer and the next dielectric layer.During the manufacture, the metal layer will bedeposited and its thickness determined. There arenow two possible options for handling the subsequentoxide surface layer that forms when O2 is introducedinto the chamber in preparation for the next dielectriclayer. The first option is for the program to skip thisinterface layer, deposit the next dielectric layer, andthen determine the thicknesses of the metal, inter-face, and dielectric layers simultaneously. The otheroption is to ‘‘deposit’’ the interface layer as a dielectriclayer. In this case, an O2 plasma is established inthe chamber for a specified period of time. Then,since zero thickness was initially specified for theinterface layer, no actual deposition will take place,but an optical monitor measurement will still bemade. The thicknesses of the metal and interfacelayers can be determined from this transmittancemeasurement. After this the next dielectric layercan be deposited in the usual manner. In order toensure an unambiguous determination of the metaland interface layer thicknesses, this latter option waschosen for all the filters discussed in this paper.The third issue concerns the formation of an inter-

face layer between a metal layer and a previouslydeposited dielectric layer. A number of experimentswere carried out in which Ni or Ag metal layers weredeposited onto one or more dielectric layers. Ini-tially, some of these experiments indicated that therewas a significant interface layer present between themetal and dielectric layers. For example, on onefilter, even though the transmittance was expected todecrease by ,20% across the entire visible spectrumafter the deposition of a Ni layer, it actually increasedseveral percent before it began to decrease. Althoughit was obvious that an interface layer was present, itwas not clear how it was being formed. It waseventually discovered that this ‘‘interface’’ layer arose

from a metal oxide being first deposited instead of thedesired metal. This problem occurred because themetal target surface became poisoned with an oxidelayer during the deposition of the dielectric layers andthe assigned presputtering time to clean the targetsurface was not adequate. This problem was subse-quently eliminated by increasing the presputteringtime and by monitoring the target bias voltage until itindicated that a clean surface was present prior to thedeposition of the metal layer. However, the firstthree filters presented in Section 5 were depositedbefore this problem was fully identified. Hence, forthe modeling of these filters, interface layers wereinserted before each metal layer. After the problemassociated with the poisoning of the metal targets hadbeen solved, further experiments indicated that nosignificant interface layers were present between themetal layer and previously deposited dielectric layers.It should be clear from the above discussions that

the design of a metal@dielectric filter, once depositionconsiderations are taken into account, is an iterativeprocedure. The first step is to design the coating,neglecting the formation of any interface layers.The next step is to understand how a given metallayer reacts to adjacent layers or to subsequentdeposition processes. This usually requires a num-ber of experiments, and, when interface layers areformed, these layers must be characterized in termsof their thicknesses and optical constants. The final

Table 1. Construction Parameters of the Double-CavityFabry-Perot Filter

Layer Type Material

MetricThickness 1nm2

Desired Determined

— Substrate Quartz 1 mm1 Nb2O5 53.7 53.12 SiO2 91.9 91.63 Nb2O5 46.0 45.84 SiO2 213.0 212.65 Nb2O5 50.2 50.16 SiO2 97.6 97.37 Nb2O5 60.6 60.58 SiO2 102.6 103.09 Nb2O5 59.3 59.110 SiO2 99.1 99.511 Nb2O5 59.9 59.912 SiO2 203.4 202.913 Nb2O5 42.1 42.214 SiO2 70.6 70.215 Nb2O5 16.3 16.816 Interface NiO — 10.417 Ni 7.3 7.218 Interface NiO — 2.019 Nb2O5 41.7 41.820 SiO2 112.3 115.421 Interface NiO — 7.722 Ni 7.6 7.423 Interface NiO — 2.624 Nb2O5 32.8 33.325 SiO2 47.3 49.2— Medium Air — —

step is to include these interface layers in the finaldesign and to optimize the thickness of other layers toachieve the desired optical performance. It mayturn out that several iterations are required in prac-tice in order to achieve metal@dielectric filters withthe desired specifications. The need for further itera-tions may arise, for example, from the inaccurateoptical constants used for the thin metal or metaloxide layers. In this case, one must empiricallydetermine the optimum metal thickness to deposit inorder to meet the desired optical performance.

5. Experimental Results

Four examples were chosen to demonstrate the accu-racy with which the complex metal@dielectric filterscan be deposited. The initial design of these trans-mission filters with low reflectance is discussed in thecompanion paper.4 As mentioned above, the designswere modified to include any necessary protectivelayers or any interface layers that may form duringdeposition. These modified designs were then re-fined to allow for the interface layers that wereexpected to form during the deposition. The refinedmodified designs for the four filters are listed inTables 1–4. These tables show the material anddesired thickness for each layer along with the theo-retically determined layer thicknesses of the depos-ited layer. Ni was used in the designs of the firstthree filters and so no protective layers were required.As mentioned above, there was a problem associatedwith the Ni-based filters, which resulted in a NiOlayer being deposited prior to the Ni layers. To takethis into account, these filters were modeled with aninterface layer inserted between previously depositeddielectric layers and the Ni layers. In addition, this

Fig. 5. Double-cavity Fabry–Perot filter 1see Table 12. Desired,measured, and determined 1a2 transmittance and 1b2 reflectancecurves.


Fig. 6. Short-wavelength pass filter 1see Table 22. Desired,measured, and determined 1a2 transmittance and 1b2 reflectancecurves.

Table 2. Construction Parameters of the Short-Wavelength Pass Filter

Layer Type Material


Desired Determined

— Substrate Quartz 1 mm1 Nb2O5 91.0 91.02 SiO2 126.0 125.73 Nb2O5 79.1 79.14 SiO2 117.6 117.05 Nb2O5 77.3 77.36 SiO2 117.9 117.57 Nb2O5 73.4 73.48 SiO2 123.6 123.79 Nb2O5 68.7 68.610 SiO2 127.6 127.711 Nb2O5 67.8 67.712 SiO2 124.6 124.713 Nb2O5 72.0 71.914 SiO2 117.2 117.415 Nb2O5 76.7 76.616 SiO2 112.3 112.317 Nb2O5 78.6 78.518 SiO2 114.3 114.419 Nb2O5 78.0 78.020 SiO2 107.2 107.221 Nb2O5 3.8 0.022 Interface NiO — 10.423 Ni 6.9 6.824 Interface NiO — 5.225 Nb2O5 39.5 35.226 Interface NiO — 2.327 Ni 1.0 0.628 Interface NiO — 2.029 SiO2 81.2 80.3— Medium Air — —


interface layer also helped improve the theoreticaltransmittance fit since the optical constants used forthe thin Ni layers were not entirely accurate.In Figs. 5–8, the determined theoretical curve is

based on the determined thicknesses of the depositedlayers whereas the desired theoretical curve is basedon the desired layer thicknesses.

A. Fabry–Perot 1Double-Cavity2 Bandpass Filter

This filter was designed to have a 50% peak transmit-tance at a wavelength of 560 nm and a low reflectance

Table 3. Construction Parameters of the Neutral Density Filter

Layer Type Material


Desired Determined

— Substrate Quartz 1 mm1 Nb2O5 3.0 3.02 SiO2 136.3 136.63 Nb2O5 14.0 13.94 SiO2 69.0 67.85 Interface NiO — 6.76 Nickel 9.6 9.77 Interface NiO — 2.38 Nb2O5 31.0 27.09 Interface NiO — 3.210 Nickel 6.3 5.511 Interface NiO — 2.412 Nb2O5 24.6 24.313 Interface NiO — 2.714 Nickel 2.5 2.315 Interface NiO — 1.716 Nb2O5 19.0 22.417 SiO2 71.6 72.6— Medium Air — —

Fig. 7. Neutral density filter 1see Table 32. Desired, measured,and determined 1a2 transmittance and 1b2 reflectance curves.

for light incident from the air side. The modifieddesign of this filter is listed in Table 1. This filteressentially consists of a Nb2O5@SiO2 double-cavityfilter with a low-reflectance stack attached to it.The agreement between the desired and measured

transmittance and reflectance curves, shown in Figs.51a2 and 51b2, respectively, is good. However, itshould be pointed out that, although there is goodagreement between the measured and determinedtransmittance curves, the agreement between thecorresponding reflectance curves is not quite as good.

Table 4. Construction Parameters of the Single-CavityFabry-Perot Filter

Layer Type Material


Desired Determined

— Substrate Quartz 1 mm1 Ag 32.0 37.32 Protective Si 3.0 1.03 Interface SiO2 — 0.04 SiO2 122.6 119.75 Ag 41.3 28.96 Protective Si 3.0 0.07 Interface SiO2 — 0.08 SiO2 63.8 56.79 Ni 4.0 3.5— Medium Air — —

This is an indication that there is a problem accu-rately modeling the thin Ni and NiO layers, whichleads to the discrepancy between the determined andmeasured reflectance curves.The curves in Fig. 5 correspond to the third fabrica-

tion iteration for this filter. In successive attemptsat deposition, the thicknesses of the interface layersused in the design were adjusted to match the thick-nesses that were previously determined.

B. Short-Wavelength Pass Filter

The transmittance of this filter was designed to be40% from 400 to 600 nm and 0% from 600 to 800 nmwhile also maintaining a low reflectance for lightincident from the air side. The modified design ofthis filter is listed in Table 2. This filter was basedon a Nb2O5@SiO2 short-wavelength pass design with alow-reflectance stack attached to it.The agreement between the desired and measured

transmittance and reflectance curves, shown in Figs.61a2 and 61b2, respectively, is good. As in the firstexample, it is difficult to model thin Ni and NiO layerssince the determined andmeasured reflectance curvesare not in good agreement.

C. Neutral Density Filter

The design of this neutral density filter is listed inTable 3. This filter was designed to have a transmit-tance of 10% and an average reflectance of less than

Fig. 8. 1a2 and 1b2 Conventional single-cavity Fabry–Perot filter. Desired, measured, and determined transmittance and reflectancecurves. The construction parameters of this filter are the same as those listed in Table 4 except that the last Ni layer is omitted. 1c2 and1d2 Low-reflectance single-cavity Fabry–Perot filter 1see Table 42. Desired, measured, and determined transmittance and reflectancecurves.


Fig. 9. Aging of metal@dielectric interference filters. Transmittance and reflectance curves of 1a2 a double-cavity Fabry–Perot filter, 1b2 ashort-wavelength pass filter, 1c2 a neutral density filter, immediately after deposition and after 12 months.

1% across the visible spectrum. As can be seen inFig. 7, the actual measured transmittance was closeto 13% across the visible spectrum with an averagereflectance of less than 2%.Because of time constraints, this was the only

fabrication attempted for this filter. From Fig. 21a2, athickness error of ,2 nm can be estimated in one ofthe Ni layers to explain this difference of,3%. Fromthe fabrication results of the above two filters it isclear that probably only one more iteration would beneeded to meet the desired filter specifications.

D. Fabry–Perot 1Single-Cavity2 Bandpass Filter

The basic structure of the Fabry–Perot filter of thisexample is Ag@SiO2@Ag@SiO2@Ni 1see Table 42.This filter was designed to demonstrate that theaddition of an ,4-nm-thick Ni layer at the air inter-face can reduce the Commission Internationalede l’Eclairage luminous reflectance from 64% to 5%.Of course, this is at the expense of reducing the peak


transmittance from 51% to 27%. In this filter, thinSi layers were used to protect the Ag layers fromoxidizing. Note in Table 4 that the SiO2 interfacelayers, which form after the Si protective layers arepartially oxidized, are sometimes determined to havezero thickness. This is not realistic, indicating thatthe optical constants used for the thin Si layers maybe inaccurate. However, it is clear from Fig. 41c2that the oxidized Si protective layers should notchange the overall transmittance significantly and soerrors in the determination of the Si layer interfaceshould not affect the overall performance of thisfilter.The transmittance and reflectance of the high-

reflectance version of this filter 1with no final Nilayer2 are shown in Figs. 81a2 and 81b2, respectively,whereas the transmittance and reflectance of thelow-reflectance version 1with a final Ni layer2 areshown in Figs. 81c2 and 81d2, respectively. The mea-sured, desired, and determined transmittance and

reflectance curves are in reasonably good agreementwith each other, except for a shift in the peak wave-length position.This peak wavelength shift resulted from a lack of

sensitivity of the measured transmittance to thethickness of the SiO2 spacer layer. As the thicknessof a layer is first determined after depositing ,95% ofthe desired thickness, an overestimation in the layerthickness at this point could result in a thinner thandesired layer. This is what happened to the SiO2spacer layer in this filter, and, since it is not easy tocompensate for errors in this layer thickness, thisresulted in a shift of the peak transmittance towardlower wavelengths. There were also similar prob-lems with the determination of the second SiO2 layerthickness except now errors in this layer affect boththe overall final transmittance and reflectance. Amore accurate in situ thickness determination of thislayer was not possible until the final Ni layer wasdeposited. However, at this stage of the deposition,adjusting the Ni layer thickness can compensate onlythe final transmittance or reflectance, but not both.Hence, to deposit this filter with reasonable accu-

racy it was necessary to deposit the SiO2 layersstrictly based on timing. This meant depositing aSiO2 layer in one step, based on its deposition rate,and then performing a transmittance measurementto get an estimate of its final thicknesses. The layerthicknesses deposited this way are accurate to,65%,which is sufficient to obtain the agreement observedbetween the desired and measured curves in Fig. 8.Clearly, better thickness monitoring methods are

required for this filter. This could involve the deposi-tion of the SiO2 layers onto precoated monitor slidesto increase the sensitivity of the thickness determina-tion routine or to rely on quartz crystal monitoring.Alternatively, transmittance, reflectance, or ellipso-metric measurements at oblique angles would alsoassist in improving the thickness determination.

6. Stability

The transmittances and reflectances of the filterswith Ni layers were measured immediately afterdeposition and then 12months later. The results areshown in Figs. 91a2–91c2 corresponding to the filters1a2–1c2 described in Section 5. As can be seen, thechange over this time period is less than 1% and 2%for the transmittance and reflectance measurements,respectively. These changes are nearly within theuncertainties associated with the spectrophotometermeasurements. Hence, there is no significant changein the performance of these metal@dielectric filtersafter a year. Since the last filter 1d2, based on Aglayers with Si protection layers, was deposited onlyrecently, there is no similar aging data available.However, a single Ag layer protected with a thin Ni

layer, which was subsequently oxidized, was mea-sured as-deposited and 12 months later. The resultsare shown in Fig. 10, and it is evident from the ,7%increase in transmittance at the wavelength of 400nm that these layers have changed somewhat over

this time period. Similar data for single Ag layersprotected with thin oxidized Si layers are not yetavailable.

7. Conclusions

Metal@dielectric transmission interference filters withlow reflectance have been successfully fabricated byreactive sputter deposition using optical monitoring.Prior to fabrication, the metals used in these filtershad to be investigated to examine their stability whenexposed to an O2 plasma, which was necessary for thedeposition of dielectric layers. Ni and Si were ob-served to form stable oxides, however, Ag layersdeteriorated rapidly when exposed to the O2 plasma.Since no oxide interface layers were observed whenthe metal layers were deposited on top of dielectriclayers, it is likely that the interface formation isrelated to the presence of highly reactive atomicoxygen generated in the plasma. In the case of aAg-based filter, a thin layer of Si was required toprotect theAg layer during deposition.Even once the interface layer’s optical constants

and thickness were determined for a given metal orsemiconductor material, several iterations were re-quired to producemetal@dielectric filters thatmatchedthe desired filter specifications to within 1 or 2%.One reason for this was the inaccurate optical con-stants for some of the thin metal layers. Anotherproblem was the lack of sensitivity of the measuredtransmittance to some dielectric layer thicknesses.Both problems lead to a lack of agreement betweenthe theoretical transmittance based on the deter-mined thicknesses of the layers of the system and themeasured transmittance. This indicates that thereis a problem with the modeling of the depositionprocess. This has to be resolved if complex opticalmetal@dielectric filters are to be manufactured in asingle deposition run without any iterations.The in situ transmittance measurements provided

valuable insights into the different aspects of themetal@dielectric fabrication process. With this newunderstanding, it should be possible to produce thesefilters using other thickness control methods such asquartz crystal monitoring.

The results of this research were first presented atthe 1993 Annual Meeting of the Optical Society of

Fig. 10. Transmittance of a Ag layer protected with a NiO layerjust after deposition and after 12 months.


America.10–12 A brief summary of some of this studyhas also been presented at the International Sympo-sium on Optical Interference Coatings, Grenoble,6–10 June 1994.9The authors thank J. A. Dobrowolski and L. Li for

many useful discussions and helpful comments.

References and Notes1. V. A. Koss, A. Belkind, D. E. Aspnes, L. Nazar, J. A. Dobrowol-

ski, F. C. Ho, K. Memarzadeh, J. A. Woollam, F. Bernoux, J. P.Piel, and J.-L. Stehle, ‘‘Uncertainty of multiple determinationof optical constants and layer thicknesses of thin films. Caseof TiO2@Ag@TiO2 coating on glass,’’ in Optical Materials Tech-nology for Energy Efficiency and Solar Energy Conversion VII,C. Granqvist and C. M. Lampert, eds., Proc. Soc. Photo-Opt.Instrum. Eng. 1016, 199–206 119882.

2. K. Memarzadeh, J. A. Woollam, andA. Belkind, ‘‘Ellipsometricstudy of ZnO@Ag@ZnO optical coatings. Determination oflayer thicknesses and optical constants,’’ in Optical MaterialsTechnology for Energy, Efficiency and Solar Energy, C. M.Lampert, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 823, 54–61119872.

3. V. T. Bly and J. T. Cox, ‘‘Infrared absorber for ferroelectricdetectors,’’ Appl. Opt. 33, 26–30 119942.

4. J. A. Dobrowolski, L. Li, and R. Kemp, ‘‘Metal@dielectrictransmission interference filters with low reflectance. 1.Design,’’ Appl. Opt. 34, 5673–5683 119952.


5. J. A. Dobrowolski, J. R. Pekelsky, R. Pelletier, M. Ranger, B. T.Sullivan, andA. J. Waldorf, ‘‘A practical magnetron sputteringsystem for the deposition of optical multilayer coatings,’’ Appl.Opt. 31, 3784–3789 119922.

6. B. T. Sullivan and J.A. Dobrowolski, ‘‘Deposition error compen-sation for opticalmultilayer coatings. I. Theoretical descrip-tion,’’ Appl. Opt. 31, 3821–3835 119922.

7. B. T. Sullivan and J.A. Dobrowolski, ‘‘Deposition error compen-sation for optical multilayer coatings. II. Experimental re-sults—sputtering system,’’ Appl. Opt. 32, 2351–2360 119932.

8. TFDesign is a NRCC developed, noncommercial program.9. J. H. Underwood, E. M. Gullikson, and K. Nguyen, ‘‘Tarnish-

ing of Mo@Si multilayer x-ray mirrors,’’ Appl. Opt. 32, 6985–6990 119932.

10. B. T. Sullivan, ‘‘Automated process control of optical coatings,’’in Annual Meeting Technical Digest, Vol. 16 of OSA 1993Technical Digest Series 1Optical Society of America, Washing-ton, D.C., 19932, p. 139.

11. L. Li, J. A. Dobrowolski, and B. T. Sullivan, ‘‘Low reflectanceinterference coatings with arbitrary transmittance,’’ in An-nual Meeting Technical Digest, Vol. 16 of OSA 1993 TechnicalDigest Series 1Optical Society of America, Washington, D.C.,19932, p. 114.

12. J. A. Dobrowolski, ‘‘Black bandpass interference filters,’’ inAnnual Meeting Technical Digest, Vol. 16 of OSA 1993 Techni-cal Digest Series 1Optical Society of America, Washington,D.C., 19932, p. 115.

Metal/dielectric transmission interference filters with low reflectance 2 Experimental results

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