Dielectric thin-film-based multilayer coatings arewidely used in optics and electronics. Various coatingtechnologies, such as chemical vapor deposition(CVD), physical vapor deposition (PVD), ion beamsputtering, magnetron reactive sputtering, andplasma-ion-assisted deposition, are currently ap-plied [1–5]. Atomic layer deposition (ALD) is similarto CVD, however, with marking differences [6,7]. Thereactants are introduced into the reaction chambersuccessively, and the reaction is surface controlledwith the film growing with a (sub)monolayer thick-ness per cycle. Hence ALD has received increasinginterest as coating technology, because it offers tightcontrol of the film thickness [6,7]. Additionally, ALDof various dielectric material films is possible at low
substrate temperatures (below 200 °C), making itinteresting for coating polymers.
The design of optical coatings such as filters andantireflective coatings based on multilayer systemsrequires detailed knowledge of the optical propertiesof the materials. The dispersion curves and thegrowth rates of the dielectric films must be accu-rately determined. Unfortunately, several factorswill affect these properties. The deposition techniqueand conditions (e.g., temperature and pressure) un-iquely determine the characteristics of the films[4]. In situ monitoring of the film growth based oncomplex simulation algorithms are generally appliedto produce specialty optics [8–10]. Even under tightin situ control, it is difficult to technically meetstringent tolerances.
We present a thorough characterization of Al2O3and TiO2 thin films deposited by ALD using spectro-scopic ellipsometry, x-ray reflectometry, and trans-mission electron microscopy (TEM). The films were
grown on silicon wafer, BK7 glass, polycarbonate(PC), quartz, and poly(methyl methacrylate) sub-strates. The films were deposited in the 80–200 °Ctemperature range, and the changes in refractive in-dices and growth rates were determined. Multilayercoatings for narrow-bandpass filters (NBPFs) andantireflection coatings using ALD are demonstrated,whereby the target is achieved without in situmonitoring.
2. Experiment
A. Atomic Layer Deposition
Deposition of thin Al2O3 and TiO2 layers was carriedout in a commercial hot-wall flow-type ALD reactor(SUNALE R75, Picosun, Finland). Trimethylalumi-num [AlðCH3Þ3 (TMA)], titanium(IV) isopropoxide(Ti½OCHðCH3Þ2�4 [TiOP]) and H2O or 30%H2O2 wereused as aluminum, titanium, and oxygen reactantsources, respectively. The TiOP precursor was heatedto 45–60 °C and delivered through a booster system.The pulsing times were 0:1 s for TMA, 0:5 s for TiOP,and 2 s for H2O or H2O2 with N2 as carrier gas at aflow rate of 200 sccm (sccm denotes cubic centimetersper minute at standard temperature and pressure).The purging time after each pulse was set to 4 s.Purging was done with N2 gas with a flow rateof 200 sccm.
B. Ellipsometry
Spectroscopic ellipsometry measurements in the370–1000nm spectral range were done with a J. A.Woollam M2000 ellipsometer equipped with a rotat-ing analyzer and a compensator. The experimentaldata were analyzed with the WVase32 software pro-vided with the equipment. Transmittance spectrawere recorded with the Woollam M2000 equipmentand with a NanoCalc 2000 spectrometer. The spec-tral range of the spectrometer was 280–850nm;the UV VIS–NIR source and the transmitted lightwere coupled by optical fibers to the detector. Back-ground spectra were recorded prior and after thetransmittance measurements to assure no intensityvariation of the incident light.
C. Coating Design
Coating designs were calculated using commerciallyavailable software (IMD). An ideal model withoutsurface roughness was applied. The effect of thick-ness variation of individual layers on the optical per-formance of the NBPF has been evaluated.
D. Transmission Electron Microscopy
Microstructure analysis of the interface between thinfilms and substrates was performed by cross-sectional TEM. The samples for TEM were madeby standard methods. They were glued togetherface-to-face with epoxy resin then mechanically po-lished to a thickness of approximately 100 μm,dimpled from one side to get a thickness of approxi-mately 20 μm at the center, followed by ion millingusing a Gatan precision ion polishing system. TEM
investigations were carried out with a Jeol 1010equipment operating at 100keV.
3. Results and Discussion
A. Characterization of Al2O3 and TiO2 Films
Al2O3 and TiO2 thin films of 10 to 400nm thicknesswere deposited in the 80 to 250 °C temperaturerange. The film thicknesses determined through el-lipsometry were in excellent agreement with theTEM data. The growth rates of Al2O3 and TiO2 werein the range of 1–1.25 and 0:3–0:7Å0=cycle. Higherdeposition rates were obtained when H2O2 was usedas a precursor. The growth rates (g) did not dependsignificantly on the substrate material; however, thelinear fit (t ¼ yþN � g) of the film thickness (t) ver-sus number of cycles (N) showed a considerable var-iation (up to 5nm) for the y-intercept values betweenthe substrates, indicating that the substrate materi-al has a significant effect on the initial “seed” film.This effect is related to the incubation time of thecoating process and is also temperature dependent.Depending on the substrate material and surfaceroughness, the precursor has a higher affinity andmore precursor may adsorb on the surface to forma saturated monolayer. In subsequent cycles, thesubstrate influence on the adsorption is minimal.
The optical constants of the Al2O3 films have beenparameterized by the Cauchy function with an ex-tinction coefficient k ¼ 0. The Cauchy, Lorenz, andTauc–Lorenz formalisms have been applied to theanalysis of the TiO2 film thicknesses and optical con-stants. The Tauc–Lorenz formalism improved the el-lipsometric fit and decreased the mean square errorvalue. It yielded nonzero extinction coefficient below400nm, however, of the order of 10−3, showing mini-mal absorbance of the deposited films. This value isin accordance with previously reported data [11]showing applications for optical filters in the230–280nm spectral range. Further analysis of theALD coatings at shorter wavelengths is necessaryto prove that these are of comparable quality andmay also be used for such applications.
The experimental dispersion curves are depictedin Fig. 1. The Al2O3 refractive index has minimal de-pendence on the deposition temperature and showslow dispersion, with values of 1:65� 0:05 throughoutthe visible and NIR spectral range. In contrast, theTiO2 refractive index at 633nm increases from ∼2:1to ∼2:5 with increasing substrate temperature from80 to 200 °C. Similar behavior of the TiO2 films wasobserved with other deposition techniques [4] and re-ported also for ALD films [12,13]. The TEM micro-graph in Fig. 2 of a TiO2=Al2O3 bilayer depositedat 120 °C shows that both films are amorphous. Athigher temperature (>165 °C), the anatase phaseis formed, and one can observe a relative increaseof the refractive index (∼0:15) for the films depositedaround 150–200 °C compared with the 120 °C deposi-tion [12]. Nonetheless, the optical constants of the
films grown on Si wafer did not vary significantlyfrom the ones on BK7 glass (∼0:03).
B. Narrow Bandpass Filter
Using the refractive indices of alumina and titaniadetermined for the ALD process employing H2O2,we designed a dichroic filter with a bandpass trans-
mittance at 470nm and FWHM of 4nm. The filter ismade of 15 layers of the HðLHÞ3ð2LÞðHLÞ3H type,where H corresponds to high and L to low refractiveindex material. The corresponding target film thick-nesses are 50:1nm for H (TiO2) and 72:4nm for L(Al2O3). Theoretical modeling has shown that thebandpass wavelength can be shifted significantlythrough slight variation of the (2L) alumina filmthickness. Figure 3 illustrates the color-coded trans-mittance of filters as function of this (2L) layer thick-ness. The calculations show that a thicknessvariation of the spacer alumina layer from 120 to170nm shifts the bandpass wavelength from 445to 490nm, while the FWHM is <5nm.
Experimentally, we produced a set of five filterswith various (2L) layer thicknesses. The layer thick-ness offset amounts ∼6nm corresponding to 50 cy-cles of Al2O3 deposition (up to �12nm deviationfrom the target thickness). Samples were succes-sively removed from the reaction chamber and keptunder standard laboratory conditions until the lar-gest Al2O3 thickness was reached. Then the ALDprocess was concomitantly finished on all samples.Figure 4 depicts the transmittance spectra of the ob-tained dichroic filters. The wavelength position of thefilters shifts from 469 to 485nm with increasingthickness of the spacer (2L) layer as controlled byALD. The calculated transmittance of the band-passes reaches 92% with a sideband transmittanceof 1–3%. The experimental peak transmittancemeasured at ∼3nm spectral resolution is in the60–70% range.
The film thicknesses of the layers (15 variables)were determined through a fit to the transmittancespectra for each sample, whereas the optical con-stants were kept constant (see Table 1). The presenceof the interference peaks in the NBPF samples makeit possible to obtain an excellent fit to the experimen-tal transmittance data. The calculated film thick-nesses show some variation between samples foreach layer. This nonuniformity is low and could befurther reduced through optimization of the reactionchamber for optical coatings. Additionally, the bot-tom layers have a lower thickness than the corre-sponding upper ones, although the same numberof cycles was applied indicating the influence ofthe seed material on the growth rate and possiblyslight variation in the deposition conditions. Anotherexplanation is possibly the formation of refractiveindex gradients at increasing film thicknesses asreported previously for TiO2 [13].
The optical analysis of the NBPFs on a section ofthe samples coated on both sides of the substrate(without a mask) was carried out to assess the uni-formity of the coating on the front and back sides.The transmittance spectra (see Fig. 5 for selectedspectra of samples 1–3) show that a double-side coat-ing nearly perfectly suppresses the transmittancepeaks around 470nm. Additional layers may beadded to the coating sequence to enhance the trans-mittance above 600nm and produce edge filters.
Fig. 1. Dispersion curves of (a) Al2O3 and (b) TiO2 as functions ofsubstrate temperature determined by spectroscopic ellipsometry.The black curves correspond to the refractive indices measuredon the Si wafer, whereas the gray curves depict the correspondingdata on BK7 glass. The ALD deposition was done with H2O as pre-cursor for oxygen source (plain curves) or H2O2 (curves withsquares, shown only for 120 °C). Note the different scale of the re-fractive index.
Fig. 2. TEM micrograph of a TiO2=Al2O3 bilayer on the Si wafer.The native SiO2 layer of ∼1:5nm can be observed. The scale bar is200nm.
Coating processes applied to dichroic filters re-quire tight in situ control of the layer thicknessand continuous process adjustment. Although in thisALD process no in situ measurement was applied,the target bandpass filter could be achieved, andthe bandpass wavelength could be efficiently shiftedto obtain a variety of filters. The capability of ALD tosimultaneously coat both sides of the substrate withuniform films may offer additional advantages forproducing NBPFs.Recently, significant efforts were directed to devel-
op commercially available ALD tools adapted toin situ ellipsometry monitoring. Although ALDtools equipped with quartz microbalance have beenavailable, the optical (ellipsometric, transmittance/
reflectance) control of the film thicknesses will rein-force the application of ALD for optical coatings. Asignificant advantage of ALD coating technologyversus sputtering and ion-assisted deposition techni-ques is the capability of large batch reactor chambers(up to 1m length, ∼80 cm diameter are currentlyavailable). The uniformity of the ALD films is∼1–2%, even in such large coating chambers. None-theless, complex optical elements will require evenmore stringent control of the optical thicknesses toachieve the target function. Hence optical monitoringfor each or several optical elements must be probablyimplemented during the ALD process to indentifyturning points [10]. Once errors are identified, cor-rection can be carried out in a perfectly controlledmanner on an atomic scale by ALD. It may be alsopossible to develop ALD chambers with correctioncapability (i.e., process a fewmore cycles) for each op-tical element, such as by shielding the other optics.Alternatively, it may become attractive to combineALD tools with faster (CVD, sputtering, PVD) equip-ment and use ALD for the extremely thin layers andfor corrective purposes in the optical coating in-dustry.
C. Antireflection Coating
Broadband antireflective coatings were deposited onPC, BK7 glass, and quartz substrates. The antireflec-tive coatings are made of five alternating layers(LHLHL) where L corresponds to Al2O3 and H toTiO2 with layer thicknesses in the range of 14 to80nm. Selected transmittance spectra are depictedin Fig. 6. The samples coated on both sides showabove 98% transmittance. The antireflective coatingson the front and back sides improve the transmit-tance by 6–7% compared to uncoated substrates.The advantage of ALD over other coating technolo-
Fig. 3. (Color online) Calculated transmittance of the target NBPF as function of the (2L) layer thickness. The bandpass wavelengthshifts to higher wavelength with increasing thickness.
Fig. 4. Transmittance spectra of the NBPF optics of the five di-chroic filters. The spectra are offset by one for clarity. The dotedcurves correspond to the experimental data, and the plain curvescorrespond to the calculated spectrum. The bandpass wavelengthis shifted from 469 to 485nm.
gies is that the film deposition occurs on both sidessimultaneously, and therefore the coating time is re-duced. Single-side-coated samples were obtainedwith a protection mask. The transmittance of thesesamples is improved by 3–4% compared to the un-coated substrates.The transmittance of the uncoated substrates (PC
88%, BK7 92%, quartz 92% at 500nm) will affect thetransmittance of the coated samples. The transmit-tance at 500nm of the double-side-coated PC reaches∼94%while coatedBK7andquartz show∼96% trans-mittance at this wavelength. Figure 6 shows thatthe coating on the PC substrate produced an in-creased transmittance toward the UV VIS region
(400–550nm), while the deposition on the BK7 andquartz samples produced highest transmittance inthe VIS IR region, although the PC and BK7 sampleswere coated simultaneously, and the quartz samplesseveralmonths later. Asmentioned inSubsection3.A,the film thickness will also depend on the substratematerial. The coating parameters were optimizedfor the BK7 substrate because of considerable diffi-culty in determining the optical constants and growthrates on PC and quartz.
ALD allows the antireflective coating of complexand high aspect ratio substrates. Hence such antire-
Table 1. Individual Layer Thicknesses of the Narrow-Bandpass Filter Samples Calculated through a Fit of the Transmittance Spectra(see Fig. 4)a
aThe variable parameters total 15 layer thicknesses.
Fig. 5. Selected transmittance spectra of the NBPF optics [sam-ples 1 (thick curve), 2 (medium curve), and 3 (thin curve)] recordedfrom a portion of the substrate where the coating has been appliedon both sides. Such coatings would be an approach to produce edgefilters.
Fig. 6. Transmittance spectra of the antireflective coating sam-ples. The uncoated reference spectra (plain curve) are included forPC, BK7 glass (BK7), and quartz (Q). The single-side coating isdepicted by “s” and filled symbols, and the double-side coatingis depicted by “d” and open symbols. The spectra of the quartz sam-ples are offset by one for clarity.
flective coatings are promising for optics with curvedsurfaces, such as lenses andmicroarrays, without ad-ditional mechanical repositioning of the samples toachieve uniform coating. The excellent film thicknesscontrol of ALD makes it possible to deposit very thinlayers (<20nm) without in situ control. Since severalprecursors can be connected simultaneously to theALD equipment, there is no delay in depositingthe next layer of a different material. Through theALD control, there is no mixing of the layer materi-als, and the layer transition is sharp. Hence,although the grow rate in ALD is relatively lowcompared to other coating techniques, the totaldeposition time can be minimized.
4. Conclusions
ALD is a promising technique for producing high-quality optical coatings on various substrates. Anti-reflective coatings and NBPFs can be realized if adetailed knowledge of the optical properties andgrowth rates of the materials are available. The cap-ability of ALD to simultaneously coat both surfaces ofthe substrate may be used in the design of the opticalelements, providing considerable advantage overother coating technologies. Improvement of the opti-cal elements and design may be possible using lowerrefractive index materials. Characterization of opti-cal coating materials in the deep UV spectral rangeis in progress to broaden the application field ofALD.
Financial support of the Bundesministerium fürBildung und Forschung (BMBF) (project FKZ13N9711) is highly acknowledged. The authors arethankful to S. Hopfe for TEM sample preparation.
References
1. U. Schulz, U. B. Schallenberg, and N. Kaiser, “Antireflectioncoating for plastic optics,” Appl. Opt. 41, 3107–3110 (2002).
2. V. Janicki, D. Gäbler, S. Wilbrandt, R. Leitel, O. Stenzel, N.Kaiser, M. Lappschies, B. Görtz, D. Ristau, C. Rickers, andM. Vergöhl, “Deposition and spectral performance of an inho-mogeneous broadband wide-angular antireflective coating,”Appl. Opt. 45, 7851–7857 (2006).
3. M. Yang, A. Gatto, and N. Kaiser, “Design and deposition ofvacuum-ultraviolet narrow-bandpass filters for analyticalchemistry applications,” Appl. Opt. 45, 1359–1363 (2006).
4. F. Flory and L. Escouba, “Optical properties of nanostructuredthin films,” Prog. Quantum Electron. 28, 89–112 (2004).
5. H. Takashashi, “Temperature stability of thin-film narrow-bandpass filters produced by ion-assisted deposition,” Appl.Opt. 34, 667–675 (1995).
6. M. Knez, K. Nielsch, and L. Niinistö, “Synthesis and surfaceengineering of complex nanostructures by atomic layer depo-sition,” Adv. Mater. 19, 3425–3438 (2007).
7. D. Riihelä, M. Ritala, R. Matero, and M. Leskel, “Introducingatomic layer epitaxy for the deposition of optical thin films,”Thin Solid Films 289, 250–255 (1996).
8. R. R. Willey, “Simulation of errors in the monitoring of narrowbandpass filters,” Appl. Opt. 41, 3193–3195 (2002).
9. R. R. Willey, “Monitoring thin films of the fence post designand its advantages for narrow bandpass filters,” Appl. Opt.47, C147–C150 (2008).
10. R. R. Willey, Practical Design and Production of Optical ThinFilms (Marcel Dekker, 2002).
11. J. H. Correia, A. R. Emadi, and R. F. Wolffenbuttel, “UV band-pass optical filter for microspectrometers,” ECS Transactions4, 141–147 (2006).
12. J. Aarik, A. Aidla, A. A. Kiisler, T.Uustare, andW. Sammelselg,“Effect of crystal structure of TiO2 films grown by atomic layerdeposition,” Thin Solid Films 305, 270–273 (1997).
13. A. Kasikov, J. Aarik, H. Mändar, M. Moppel, M. Pärs, andU. Uustare, “Refractive index gradients in TiO2 thin filmsgrown by atomic layer deposition,” J. Phys. D 39, 54–60(2006).
