ion-beam etching for the precise manufacture of optical coatings

Ion-beam etching for the precise manufacture ofoptical coatings

Daniel Poitras, J. A. Dobrowolski, Tom Cassidy, and Simona Moisa

We propose using ion-beam etching as an additional tool for the accurate control of the thickness of thinfilms during the manufacture of sensitive optical multilayer coatings. We use a dual ion-beam sput-tering system in the deposition and etch modes. In the deposition mode both the assist and sputteringion beams are used to produce dense films at deposition rates in the range of 0.1–0.3 nm�s. In the etchmode, only the assist ion beam is used to remove material at a rate of less than 0.1 nm�s. A very highprecision in the layer thicknesses can be obtained by alternating between deposition and etch modes.We observed that etching did not significantly affect the surface quality and the uniformity of thecoatings. We introduced etching into our current manufacturing process and demonstrated its potentialfor the fabrication of several optical multilayer systems with performances that are very sensitive to thethickness of their layers. © 2003 Optical Society of America

OCIS codes: 310.1620, 310.1860.

1. Introduction

Often considered to be a well-established technology,the field of optical coatings has in fact in the past10–15 years seen many significant developments.New applications in areas such as telecommunica-tions and compression of pulses have necessitatedthe development and implementation of enhancedmanufacturing methodologies. Not only have thedeposition processes become more stable, but manu-facturing techniques based on automation, subdivi-sion of layers, and real-time evaluation andreoptimization1 have also greatly increased the accu-racy of the process.2 The solutions submitted for theBow Lake manufacturing problem presented at the2001 OSA Optical Interference Coatings TopicalMeeting in Banff give a good idea of the current stateof the art of optical coatings manufacturing.3 Inthat exercise, the participants were asked to designand manufacture a multilayer coating with values fortransmittance T and reflectance R for s-polarizedlight incident at an angle of 7° that were as close aspossible to the specified target values. This prob-

The authors are with the Institute for Microstructural Sciences,National Research Council of Canada, 1200 Montreal Road, Ot-tawa, Ontario K1A 0R6, Canada. D. Poitras’s e-mail address [email protected].

Received 4 February 2003; revised manuscript received 27 Feb-ruary 2003.

0003-6935�03�194037-08$15.00�0© 2003 Optical Society of America

lem, which appeared to be easy at first, turned out tobe difficult because of the extreme sensitivity of thevarious solutions to errors in the thickness of somelayers. Figure 1�a� shows one of the designs submit-ted for the Bow Lake manufacturing problem. Theperformance of this coating would be excellent if allthe layers could be deposited accurately �Fig. 1�b��.However, even random thickness errors of less than�0.3 nm introduced in every layer of the system dur-ing manufacture significantly degrade the coating’sperformance. This effect was simulated numeri-cally by performing two series of 50 calculations inwhich random errors of less than �0.3 and �0.8 nmwere introduced into the individual layers. On thebasis of these data, two sets of upper and lower limitsfor the transmission and reflection were drawn �Fig.1�c��, within which all the experimentally measuredcurves produced by such manufacturing processesshould lie. In Fig. 1�d� the calculated performance ofthe solution is compared with the performance of themanufactured sample as measured by Maria Nadaland Michael Jacobson.3 Although the measuredperformance of the multilayer is good, it still differsfrom the calculated performance at many wave-lengths. This means that our current methodologyfor manufacturing optical multilayer coatings stillhas some shortcomings. Consider the following twopoints:

• Dividing the various layers into sublayers toimprove the thickness accuracy reduces but does not

1 July 2003 � Vol. 42, No. 19 � APPLIED OPTICS 4037

eliminate errors caused by overshooting the layerthickness.

• Assuming that the performance of the startingdesign was at a local minimum of the merit function,subsequent reoptimization of the remaining layerscan, at best, only reduce the performance degradationresulting from the small but finite thickness errorsmade during the deposition of the layers.

We propose that ion-beam etching be used to re-move material when in situ measurements show thatthe thickness of a deposited layer exceeds the desiredvalue. When etching is combined with real-time re-optimization, the final layer system will deviate less

from the optimum solution, and thus the resultingperformance should be better. We believe that thisis the first time in the published literature on man-ufacturing optical multilayer coatings that etching isused in this way. However, ion-beam etching hasbeen used in the field of optical coatings for charac-terization,4,5 pretreatment of substrates and fabrica-tion of micro-optical devices and gratings. What wepropose is the systematic use of ion-beam etching forthe more accurate control of the layers’ thicknessesduring the manufacturing process. We first showthat ion-beam etching does not significantly affect thesurface quality and the uniformity of the coatings.Then we use etching in the manufacture of threedifferent thickness-sensitive coatings.

2. Experimental Methodology

A dual ion-beam sputtering system �Spector, Veeco-IonTech, Fort Collins, Colorado� was used for boththe deposition and the etching of the samples �Fig. 2�.In this system, a 16-cm-diameter argon-ion beam isincident onto the target, and it sputters material to-ward a moving substrate. At the same time, a 12-cm-diameter assist ion beam consisting of a mixtureof argon and oxygen ions bombards the substrate todensify the forming films. One can easily imple-ment an etch mode by turning off the sputtering ionbeam and by removing material from the substratewith the assist ion beam. The materials that wereused in this study were silicon dioxide �SiO2�, tanta-lum pentoxide �Ta2O5�, Inconel �a Ni–Cr–Fe alloy�,and amorphous silicon �a-Si�. The 2� � 4� B270glass substrates were mounted along the radius of a12� diameter circular holder �1 in. � 2.54 cm�. Toenhance the uniformity of the coatings, masks of dif-ferent profiles for each of the different deposition ma-terials were placed between the sputtered beam andthe substrate holder, which was rotating at 600 rpm.The thicknesses of the layers were estimated from

Fig. 1. Typical solution to the Bow Lake problem. �a� Refractive-index profile. �b� Target curves and calculated performance rep-resenting the silhouette of the Rocky Mountains �T target� andtheir reflection in Bow Lake �R target� near Banff. For clarity,only 1�5 of the target points are shown in the graph. �c� Expecteddegradation in the performance when random errors of less than�0.8 nm �black area� or less than �0.3 nm �gray area� are intro-duced in the thickness of the layers. �d� Measured performance ofthe corresponding manufactured solution submitted for the BowLake problem. �Measurements were done at the National Insti-tute of Standards and Technology �Gaithersburg, Maryland� andat Optical Data Associates �Tucson, Arizona�.�

Fig. 2. Schematic representation of the dual ion-beam sputteringdeposition process.

4038 APPLIED OPTICS � Vol. 42, No. 19 � 1 July 2003

direct transmittance measurements of the coatedsample.2 Before deposition, the chamber and sam-ple were heated at 200 °C for 2 h, and the chamberwas pumped down to a base pressure of 7 � 108

Torr. To prevent any oxygen depletion at the sur-face of the growing oxide films, we used a mixture ofargon and oxygen in the assist ion source during thedeposition and etching of oxide materials.

A variable-angle spectrometric ellipsometer�VASE, J. A. Woollam Co., Lincoln, Nebraska� and aspectrophotometer �Lambda-19, Perkin-Elmer� wereused to determine the optical constants of the filmsand to evaluate the performance of the coatings.The low reflectance measurements of the black-layercoatings presented in Subsection 3.D.1 were per-formed relative to a 1% reference transmission. The

Fig. 3. Rate of ion-beam etching and deposition of SiO2 and Ta2O5 films.

Fig. 4. Surface roughness �rms� values estimated by an AFM for three different ion-beam-etched Ta2O5 films and for an uncoated quartzsurface.


uniformity of the coatings was evaluated on a setupthat consisted of a broadband optical-fiber lightsource �BBS 1550 A-TS, AFC Technologies Inc.� andan optical spectrum analyzer �HP70951B, HewlettPackard�. An atomic force microscope �AFM� �Nano-scope III, Digital Instruments� was used for theroughness measurements.

3. Results

A. Deposition and Etch Rates

Figure 3 shows thickness versus time data for thedeposition and etching of small amounts of SiO2 andTa2O5. We see that the etch rate is kept low toimprove control of the thickness of the material re-moved. The fact that nonzero thickness intersectswere observed with zero-second deposition indicatesthat additional adjustments need to be introduced toallow for substrate shutter opening and closing times,as well as other software and hardware delays. Forthese first results, we did not attempt to fully opti-mize these parameters in order to further reduce theetch rate.

B. Surface Roughness

In a second series of experiments, we deposited andetched single layers of SiO2 and Ta2O5 on polishedquartz. We then used an AFM to observe a non-etched surface as well as coated surfaces from which5, 10, and 30 nm of material were removed by etch-ing. In Figs. 4�a�–4�d� the measurements of surfaceroughness are given for a polished quartz substrate,for single layers of Ta2O5 from which 5 and 30 nmwere removed by etching, and for a Fabry–Perot filterwith a 30-nm-etched cavity layer. These resultssuggest that the ion-beam sputtering process reducesthe surface roughness of polished quartz and thatetching does not significantly affect the surface qual-ity of the films.

C. Coating Uniformity

During the deposition mode, profiled shadow maskswere introduced to intercept the beam of sputteredatoms in order to enhance the uniformity of the re-sulting films. During the etch mode, however, wechose not to introduce a mask in front of the etchingion beam in order to avoid any contamination of thefilms. The uniformity of the ion-beam etching thusrested entirely on the angular-intensity profile of theassist ion beam. For that reason, we expected theetching mode to change the uniformity of the opticalcoatings.

To quantify the effect of the ion-beam etching onthe uniformity of the coatings, we chose to etch thespacer layers of narrowband Fabry–Perot filters andto measure the peak position at various locations onthe filter. The peak position of this type of filterdepends mostly on the cavity thickness and is thussensitive to changes in uniformity. We then com-pared the uniformity of filters with nonetched cavi-ties with the uniformity of filters with cavities fromwhich 5 and 30 nm of material was removed by ion-

beam etching. Fabry–Perot filters with both SiO2and Ta2O5 spacers were deposited, but since Ta2O5spacers are more sensitive to metric thicknesschanges, only the results obtained with these filtersare shown in Fig. 5. Figure 5�a� shows the variationin peak position as a function of Y for X � 20 mm onthe substrate, and the three-dimensional diagrams inFigs. 5�b� and 5�c� show the difference between thetransmission peaks for the etched and nonetched fil-ters as a function of X and Y when 5 or 30 nm ofmaterial is removed by ion-beam etching. One cansee from this figure that a 5-nm etch barely affectedthe uniformity of the Fabry–Perot filter, while a30-nm etch had a more detrimental effect. Thechanges in uniformity due to a 5-nm and a 30-nm etchshown in Figs. 5�b� and 5�c� correspond to maximumsof 0.3% and 1.3% variations in thickness, respec-tively. These results confirmed that etching doesaffect the uniformity of the optical coatings, but they

Fig. 5. Effect of ion-beam etching on the uniformity of Fabry–Perot filters deposited on a 2� � 4� glass substrate, as measured bythe position of the transmission peak. �a� Peak position as afunction of the longitudinal position on the sample. �b�, �c� Vari-ation in the peak position at different locations on two Fabry–Perotfilters from which 5 and 30 nm of material was removed by ion-beam etching.


also suggest that the change in uniformity is smallwhen the total thickness removed by etching is small.

D. Examples

The following results were chosen to illustrate thepotential of the etch mode during the manufacture ofoptical multilayer coatings. The manufacturing ap-proach was almost identical to the one used previ-ously in our group2 �with subdivision of layers andreoptimization�, except that this time etching wasintroduced whenever overshooting occurred duringthe deposition of selected layers of the system.Three different examples were investigated: ablack-layer coating, a ten-layer normal-incidencebeam splitter, and a special filter with a target trans-mittance curve that corresponds to the silhouette of atemple in Kyoto, Japan.

1. Black-Layer CoatingThe first example is a five-layer black-layer coatingmade of Inconel and SiO2 deposited onto an opaquefilm of Inconel �Fig. 6�. The objective was to deposita coating on Inconel with a reflectance lower than

0.1% between 400 and 450 nm for unpolarized lightincident at 27°. Using our previous manufacturingmethodology, without etching, we could fabricate fil-ters with R lower than 0.5%, but could not reduce Rbelow 0.1%.6 The main problem arose from the coat-ing’s sensitivity to errors in the different layer thick-nesses, particularly for the metal layers, whichdictated the lower limit of R. It was necessary toprotect the Inconel layers with thin layers of a-Si toprevent their oxidation during the SiO2 deposition.Errors in the SiO2 layers resulted in the shifting ofthe spectrum along the wavelength axis. Monitor-ing the thickness of the last dielectric layers by trans-mission measurements was difficult because of thesignificant decrease in the signal after the metallayer depositions �see Fig. 6�b��. For a more detaileddescription of the monitoring process, the interestedreader is referred to the original publication.6 Fig-ure 6�c� shows the deterioration of the performance ofthe black-layer coating resulting from a �5-nm errorin the last layer’s thickness.

Fig. 6. Black-layer coating. �a� Refractive-index profile of thesystem. �b� Calculated transmittance of the monitoring glass af-ter deposition of the last layer without and with a �5-nm thicknesserror. �c� Calculated performance of the completed black-layercoating without and with a �5-nm error in the thickness of the lastSiO2 layer. �d� Measured performances of black-layer coatingsfabricated without and with the use of ion-beam etching.

Fig. 7. Normal-incidence beam splitter. �a� Refractive-indexprofile. �b� Calculated performance and effect of random thick-ness errors of less than �0.3 nm in all the layers. �c� Measuredtransmittance of three coatings produced without etching andwithout reoptimization, without etching but with reoptimization,and with etching but without reoptimization.


Figure 6�d� depicts the best result of many differentdeposition attempts obtained without the use of etch-ing and the result obtained on the first attempt withion-beam etching. Although the measured perfor-mance is still worse than the calculated theoreticalcurve, it is below 0.1% over most of the spectral range,and this comfortably met our requirements. We be-lieve that the difference between the calculated andmeasured performance is mainly due to a differencebetween the experimentally obtained optical con-stants and those used in the calculations.

2. Ten-Layer Normal-Incidence Beam SplitterThe next example is a ten-layer normal-incidencebeam splitter filter made of Ta2O5 and SiO2 �Fig. 7�a��with a transmittance of 55% in the spectral range of450–650 nm �Fig. 7�b��. The coating was designedto be sensitive to changes in the thicknesses of thelayers. For example, we observed that a 0.5-nm er-ror in the last layer had a marked effect on the flat-ness of the transmittance curve. Figure 7�b� showsthe effect on the performance of the coating with�0.3-nm random errors in all the layers of the sys-tem.

Table 1 compares the nominal thicknesses of layersin the design with the estimated thicknesses duringthree different deposition runs. In the first two ex-periments, etching was not used, and the results cor-respond to runs without and with reoptimization.In the third deposition run, ion-beam etching withoutreoptimization of the remaining layers was used.Also, for each deposition run, the table shows thedifferences between the estimated and the nominalthickness of the layers. One can see that the thick-ness errors for most of the layers in the coatings arebelow 1 nm in the conventional deposition approach

without etching, except for two layers that have er-rors of 1.6 and 2.1 nm. The thickness errors aresimilar when reoptimization without etching is used,but the thickness values of the remaining layers arecorrected to reduce the effect of errors on the perfor-mance of the coating. When ion-beam etching isused to correct the thickness of every layer, the thick-ness errors are reduced to values below 0.1 nm �seeSection 4 for a more detailed discussion of the deter-mination of the thickness errors�. From the mea-sured performances of the filters produced in thethree deposition runs shown in Fig. 7�c�, we can con-clude the following:

• Although in the coating deposited without re-optimization and without etching only two layers hadthickness errors greater than 1 nm, the measuredperformance of the system was relatively poor.

• Real-time reoptimization without etching im-proves the performance of the coating significantly,but the ability to recover from mistakes depends onthe position within the system of the error-sensitivelayers.

• The ion-beam-etched coating has the best per-formance by far, which suggests that the thicknessestimates in Table 1 must be close to reality.

3. Kyoto FilterIn the previous two examples, ion-beam etching wasused on each layer of the system. This approachallows an extremely high manufacturing precision�etching and deposition modes can be repeated untilthe desired tolerance for the film thickness isreached�, but it can significantly affect the time re-quired for the deposition of the coatings �see Table 1�.For the third example, we chose a complex filter that

Table 1. Normal-Incidence Beam Splitter: Nominal and Measured Metric Thickness Valuesa

Layer

DesignNo Etching

No Reoptimization No Etching Reoptimization Etching

Material Nominal Measured Error Reoptimized Measured Error Measured Error

Substrate Glass — — — — — — — —1 Ta2O5 10.29 11.89 1.60 10.29 10.27 0.02 10.26 0.032 SiO2 39.15 39.33 0.17 39.13 40.45 1.32 39.11 0.043 Ta2O5 119.64 119.61 0.03 121.19 121.43 0.24 119.66 0.024 SiO2 307.89 308.68 0.79 310.02 310.07 0.05 307.96 0.075 Ta2O5 78.47 78.82 0.35 81.59 81.48 0.11 78.52 0.056 SiO2 125.67 125.67 0 125.60 126.83 1.23 125.70 0.037 Ta2O5 49.06 51.19 2.13 46.10 46.48 0.38 48.99 0.078 SiO2 80.24 80.26 0.02 88.41 88.25 0.16 80.19 0.069 Ta2O5 57.83 58.22 0.39 56.38 56.80 0.42 57.84 0.0110 SiO2 128.48 129.37 0.89 131.06 131.04 0.02 128.41 0.07Ambient Air — — — — — — — —

rms error — — — 0.94 — — 0.61 — 0.05Deposition time

�hh:mm:ss�— 02:28:26 02:32:44 03:36:32

MF 0.0484 1.105 0.343 0.184

aThe rms values of the thickness errors and the final merit function values are also shown. Also given are the estimated depositiontimes, which do not include the pumping time. �All thickness values are expressed in nanometers.�


is composed of 26 layers, and we decided to use ion-beam etching only on a few selected critical layers.The target transmittance curve for this filter repre-sents the silhouette of the Shogun Palace in Kyoto,Japan.7 The refractive-index profile, the targetcurve, and the calculated performance of the coatingare shown in Figs. 8�a�–8�c�. The last layer is alsoone of the most thickness-sensitive layers of this sys-

tem. That means that the performance of the mul-tilayer cannot be improved without ion-beam etchingif the nominal thickness of this layer is exceededduring the deposition.

We manufactured the Kyoto filter in Fig. 8�a� usingreoptimization without, as well as with, the use ofion-beam etching of selected layers. A preliminarygoal for the tolerance of the thickness values was setto be �1 nm for the first 21 layers and �0.1 nm for thelast five layers of the system. In addition, we etchedthose critical layers for which small deposition errorslead to significant changes in the merit function dur-ing reoptimization. The results are shown in Fig.8�d�. This use of ion-beam etching on the most crit-ical layers, along with the use of reoptimization, pre-vented the deterioration of the performance,particularly toward the end of the deposition process.In addition, the total deposition times �not includingan initial 3-h pump-down and bake-out time� were5:21:12 and 5:51:28 h for the coatings produced with-out and with etching, respectively.

4. Discussion and Conclusions

We have demonstrated that ion-beam etching is auseful tool for increasing precision in manufacturingoptical coatings and that it can be used without sig-nificantly affecting the surface quality and the thick-ness uniformity of the coatings. However, it wouldbe wrong to suggest that etching alone can solve allthe problems. For example, the utility of ion-beametching is limited during the deposition of insensitivelayers—layers whose optical construction parame-ters are difficult to determine accurately immediatelyafter their deposition. Ion-beam etching will notsolve all the problems related to very sensitivelayers—layers that are not particularly difficult tomeasure accurately but that require extreme accu-racy for the performance of the finished multilayer.It has been shown previously that this problem isbest handled by real-time reoptimization.8 Also,ion-beam etching will not solve problems related tosmall discrepancies in the theoretical and experimen-tal values of the optical constants of the materialsused in the multilayer. Throughout our experi-ments we have made the assumption that the opticalconstants of the materials are accurately known. Inthe future, this problem might be best handledthrough the simultaneous in situ ellipsometric deter-mination of the optical constants of the depositedlayers.9

Table 1 shows that for the process that used ion-beam etching the thickness errors were estimated tobe less than 0.1 nm for all the layers. These esti-mates, of course, are not exact, considering that theerrors in transmittance measurements are estimatedto be 0.1% by the National Institute for Standardsand Technology and the Institute for National Mea-surement Standards,3,10 and that some of the layersare insensitive. On many occasions during the man-ufacture of the coatings, we had to measure the spec-tral transmittance and determine the thickness ofthe last layer several times before choosing the most

Fig. 8. Filter with a transmittance that corresponds to the sil-houette of a temple in Kyoto. �a� Refractive-index profile of thefilter. �b� Target transmittance as a function of wavelength. �c�Calculated transmittance of the system shown in �a� above. �d�Measured transmittance of two filters fabricated without and withion-beam etching. The thickness of the remaining layers werereoptimized in both instances.


reliable thickness value, which yielded the lowestmerit function value during curve fitting. The thick-ness determination would certainly benefit frommore measurement data. For example, this could bedone by combining transmittance with reflectancemeasurements, by measuring transmittance at sev-eral angles of incidence, or by combining photometricspectra with in situ ellipsometric11 or phase measure-ments.12 In addition, much work remains to be doneon enhancing the insensitivity of thin-film solutionsto thickness errors and on the investigation of howreoptimization might affect the thickness sensitivityof coatings during their manufacture.

At no time during the experiments did we see aneed to introduce a correction for the variation in therefractive index at the surface of the ion-beam-etchedfilms. The refractive index at the surface is likely tochange, but the results obtained so far suggest thatfor the filters manufactured this variation is not sig-nificant. This is not surprising in view of the factthat the thickness of material removed is so smalland that we start with materials of very low porosity.However, in the future, we might have to considersmall accidental refractive-index inhomogeneitiesduring the design and reoptimization processes.

For coatings with sensitive layers, thickness uni-formity will have to be improved in both the deposi-tion and the etching modes. This will necessitatethe development of ion sources with better uniformityand more sensitive methods for the tracking of uni-formity changes.13

Preliminary results of this research were presentedat the Society of Vacuum Coaters’ Forty-Fifth AnnualTechnical Conference held in Lake Buena Vista, Flor-ida, 13–18 April 2002.14

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ion-beam etching for the precise manufacture of optical coatings

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