All-dielectric narrow-bandpass ~NB! filters that canbe used for a wavelength division multiplexer arebecoming the most popular key components usedin current optical-fiber communication systems.Compared with those of the other technologies, thedemonstrated advantages of the wavelength-division-multiplexer devices made by interference filters havetotal passivity, high thermal stability, and lowpolarization-dependent losses.1,2 These filters areonstructed on the basis of an all-dielectric Fabry–erot interferometer or etalon,3 which consists of two
dentical parallel reflectors separated by a spacerith an optical thickness of an integral number ofalf-waves. The reflector is a multilayer stack withhe alternating high- and low-refractive-index layers,
When this research was performed, R.-Y. Tsai [email protected]!, C. S. Chang, C. W. Chu, T. Chen, F. Dai, D. Lin, S. F. Yan,and A. Chang were with the Industrial Technology Research In-stitute, Opto-Electronic and Systems Laboratories, Building 51,195-8 Chung Hsing Road, Section 4, Chutung, Taiwan 310. R.-Y.Tsai is now with the Walsin THz Technology Corporation, 397,Hsin Shu Road, Hsin Chuang, Taipei Hsien, Taiwan.
Received 19 June 2000; revised manuscript received 2 January2001.
and the thickness of each layer is a quarter-wave.By controlling the order number of the spacer, theradiant reflectance of the reflectors, and the disper-sion of the phase shift upon reflection of the reflec-tors, one can easily set the peak wavelength andbandwidth of the NB filter discretionally.4 Mean-while, by means of controlling the number of cavity,the passband flatness and sharpness of the transmis-sion band can be determined.3
Ta2O5 and SiO2 films have been widely used inthin-film coatings as high- and low-refractive-indexlayers, respectively, because of their desirable opticalproperties and environmental stabilities.5 Ta2O5and SiO2 films are usually prepared with variouspreparation techniques such as electron-beam depo-sition,6 ion-assisted deposition,7,8 and sputtering.9–11
However, the characteristics of these films stronglydepend on fabrication methods and substrate tem-peratures. Films prepared by reactive direct-current ~dc! magnetron sputtering from tantalumand silicon targets under appropriate experimentalconditions exhibit decreased porosity, increased re-fractive index, smooth surfaces, and increased resis-tance to adverse environments.9–11 The depositionrate can be enhanced by introduction of the oxygendirectly to the substrate but not to the target wherethe reactant pressure is maintained at a low level toavoid the oxidation of the target surface and to in-
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crease the sputtering rate. For these reasons vari-ous attempts have been proposed to apply reactive dcmagnetron sputtering for the manufacturing of NBfilters. From these attempts, one successful methodis the ion-assisted dc magnetron sputtering tech-nique, which possesses the above-mentioned generaladvantages and has the potential for mass productionwith large area uniformity.12 For the ion-assisted dcmagnetron sputtering the oxygen gas is introducedand ionized through the ion gun and toward the sub-strate. Since the reactivity of the ionized oxygen ishigher than that of neutral oxygen, a stoichiometricoxide coating with perfect optical performance can beobtained under a lower oxygen-flow rate.
Our aim in this research is to study the tempera-ture stability of NB filters prepared by reactive ion-assisted bipolar dc magnetron sputtering. Two3-cavity NB filters centered at 1550 nm with high-index half-wave spacers were made, and 78–102 lay-ers of Ta2O5 and SiO2 were presented. Puretantalum and silicon targets were used for the depo-sition of Ta2O5 and SiO2 films, respectively. Toavoid the oxidation of the targets, pure argon was fedinto the target as the sputtering gas and pure oxygenwas introduced through the ion gun and toward theBK7 glass substrate. Characteristics of the NB fil-ter and its component Ta2O5 and SiO2 films wereevaluated by the optical performance, surface rough-ness, moisture resistance, and temperature stability.
2. Experiment
Three-cavity NB filters made with Ta2O5 and SiO2films as high- and low-refractive-index layers, respec-tively, were prepared by reactive ion-assisted bipolardc magnetron sputtering. The schematic view of thehomemade reactive ion-assisted bipolar dc magne-tron sputtering system is shown in Fig. 1. The coat-ing chamber consists of three 3-in. ~7.62 cm! AJAsputter guns and one Commonwealth Mark II EndHall ion source. The sputter guns were uniformly
Fig. 1. Schematic view of the homemade reactive ion-assistedbipolar dc magnetron sputtering system.
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distributed at the same radial distance from the cen-ter of the chamber of 14 cm. Pure tantalum andsilicon targets were used as starting materials for thedeposition of Ta2O5 and SiO2, respectively. An un-heated BK7 glass substrate with dimensions of 25mm 3 25 mm 3 1 mm was located at the upper centerof the chamber with a distance of 40 cm from eachsputter gun center. The experiment was performedat a constant power with an advanced energy dcpower supply AE 2500 connected with a Sparc LE.The duty cycle was 50 kHz. The ratio of positive andnegative cycle time was 4:1 and its amplitude ratiowas 6.6:1. The ion gun was operated at constantanode currents of 3.2 and 2.5 A for the deposition ofTa2O5 and SiO2, respectively. The chamber pres-sure was pumped down to 3 3 1026 Torr before dep-osition. Mass flow-controlled pure argon ~99.999%!was delivered to the sputter gun, ion gun, and hol-low cathode with flow rates of 60, 20, and 15 stan-dard cm3ymin ~sccm!, respectively. Pure oxygen~99.999%! with flow rates of 20 and 8 sccm wereintroduced through the ion gun and toward the sub-strate for the deposition of Ta2O5 and SiO2, respec-tively. The total working pressure was ;2 3 1023
Torr. The layer thickness of the filter was opticallydirect monitored with the turning value method.13
It was found that the theoretical accuracy of peakwavelength and the overall performance of completeNB filters were extremely good. The optical moni-toring system operated with the use of a fiber-opticsystem with collimating optics. A Santec ModelTSL-200 tunable laser with a wavelength accuracy of,0.01 nm was used as the light source, and aHamamatsu Model G5832-01 near-infrared photo-diode with a 3-mm-diameter opening was used as adetector. A sample was placed between fiber colli-mators to measure the normal-incidence transmis-sion spectra. A single-mode fiber ~9 mm! was used athe input end and a multimode fiber ~125 mm! at the
output end. The rotating speed of the sample was;900 runsymin.
Transmission spectra from 400 to 2000 nm of thesingle-layer films were measured with a HitachiModel U4001 spectrophotometer. The optical con-stant and thickness of each film were calculated withthe homogeneous envelope technique.14 Themoisture-resistance test applied to the films con-sisted of soaking the films in water at an ambienttemperature for 2 h and measuring the shift of thetransmission spectra before and after the test. Thesurface topographies of single-layer films and multi-layer filters were evaluated in air with a Nanoscope IIatomic force microscope from Digital Instruments.Transmission spectra of NB filters were obtainedwith a HP Model 83438A broadband erbium-amplified spontaneous emission source and a HPModel 71452B optical spectrum analyzer.Temperature-dependent wavelength shifts of NB fil-ters were measured in situ by the combination ofoptical spectrum analyzer and erbium-amplifiedspontaneous emission source, when samples were
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heated from room temperature to 100 °C within ahomemade vacuum chamber.
3. Results and Discussion
A. Single-Layer Films
Transmission spectra of single-layer Ta2O5 and SiO2films prepared by reactive ion-assisted bipolar dcmagnetron sputtering from tantalum and silicon tar-gets are shown in Figs. 2 and 3, respectively. Inputpowers of the sputter gun for Ta2O5 and SiO2 are 450and 300 W, respectively, and the deposition time is 30min. Oxygen gas was ionized by passage throughthe ion gun and directed toward the substrate. The
Fig. 3. Transmission spectra of SiO2 films prepared by reactiveon-assisted bipolar dc magnetron sputtering from the silicon tar-et with the oxygen flowing through ~thick black curve! or not
flowing through ~thick gray curve! the ion gun and directed towardhe substrate. The fine solid curve is the spectrum for bare glassubstrate.
data for the neutral oxygen molecules introduced di-rectly into the substrate without ionization are in-cluded for comparison. When the oxygen flow rate is20 sccm, the obtained Ta2O5 film is more transparentfor the oxygen flowing through the ion gun than forthat flowing directly to the substrate as shown in Fig.2. This indicates that the reactivity of the ionizedoxygen is stronger than that of neutral oxygen mol-ecules to get the stoichiometric oxide. However, notmuch difference is observed in the transmission spec-tra for SiO2 films deposited at the oxygen flow rate of8 sccm as shown in Fig. 3. The calculated depositionrates of the films from the measured transmissionspectra shown in Figs. 2 and 3 are 0.195 and 0.056nmys for Ta2O5 and SiO2, respectively, which areunaffected by the oxygen flowing or not flowingthrough the ion gun. This result indicates that theionized oxygen has little influence on the depositionrates of both films.
Figure 4 shows the target voltage versus the oxy-gen flow rate as a function of oxygen flowing or notflowing through the ion gun. The input powers ofthe sputter gun for Ta2O5 and SiO2 are kept at con-stants of 550 and 300 W, respectively. The indepen-dence of the deposition rate on the ionization ofoxygen is further evidenced inasmuch as the curvesfor oxygen flowing or not flowing through the ion gunare nearly the same. However, the oxygen flow hasa great influence on the deposition rates of films.Figure 4 shows that the target voltages for both ma-terials increase with increasing oxygen flow rate, in-dicating that the deposition rates decrease withincreasing oxygen flow rate, which is the result ofoxidation of the target surfaces.15 The refractive in-dices of Ta2O5 and SiO2 films are 2.1 and 1.45, re-spectively, at the wavelength of 1550 nm, andcalculated from Figs. 2 and 3 on the basis of themethod shown in Ref. 14. Little or no absorption for
Fig. 2. Transmission spectra of Ta2O5 films prepared by reactiveion-assisted bipolar dc magnetron sputtering from the tantalumtarget with the oxygen flowing through ~thick black curve! or notflowing through ~thick gray curve! the ion gun and directed towardhe substrate. The fine solid curve is the spectrum for bare glassubstrate.
Fig. 4. Target voltage versus the oxygen flow rate as a function ofoxygen flowing through ~solid curve! or not flowing through~dashed curve! the ion gun and directed to the substrate. Theinput powers of the sputter gun for Ta2O5 and SiO2 are kept atconstants of 550 and 300 W, respectively.
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all films in the 400–2000-nm range is observed.This is consistent with the result of Edlou et al.,10 inwhich the refractive indices of Ta2O5 and SiO2 filmsdeposited by reactive sputtering are 2.11 and 1.44,respectively, at the wavelength of 1200 nm.
The excellent moisture-resistant properties ofTa2O5 and SiO2 films prepared by reactive ion-assisted bipolar dc magnetron sputtering from tan-talum and silicon targets, respectively, are shown inFig. 5. No shift of the transmission spectra for ei-ther film is observed within the resolution limit of thespectrophotometer, indicating that the films pre-pared by this technique posses high packing density.The surface topographies of Ta2O5 and SiO2 filmsnalyzed by atomic force microscopy are shown inig. 6. The root-mean-square surface roughnessesf single-layer Ta2O5 and SiO2 films deposited on
BK7 glass substrates are 0.282 and 0.315 nm, respec-tively, which are very close to that of the bare glasssubstrate of 0.3 nm, indicating that the films pre-pared by ion-assisted sputtering have very smoothsurfaces. The featureless surface morphologies ofthe films shown in Fig. 6 imply that the amorphousstructures of the as-deposited films, which are alsoidentified by the x-ray diffraction. The excellentmoisture-resistant properties and the ultrasmoothsurfaces of the amorphous films prepared by reactiveion-assisted bipolar dc magnetron sputtering indicate
Fig. 5. Transmission spectra of Ta2O5 ~upper! and SiO2 ~lower!films prepared by reactive ion-assisted bipolar dc magnetron sput-tering from tantalum and silicon targets, respectively, before~black curve! and after ~gray curve! being subjected to the water-immersion test at ambient temperature for 2 h.
596 APPLIED OPTICS y Vol. 40, No. 10 y 1 April 2001
that these films are suitable for use as high- andlow-index layers for high-quality NB filters.
B. Narrow-Bandpass Filters
Figure 7 shows the transmission spectra of three-cavity NB filters with high-index half-wave spacerand 78 layers of Ta2O5 and SiO2 prepared by reactiveion-assisted bipolar DC magnetron sputtering. Thedesign of the 78-layer NB filters is glassy@~HL!6 2H~LH!6 L#3yair, where H and L correspond to quarter-wave layers of Ta2O5 and SiO2 at the reference wave-lengths of 1537.40, 1543.73, 1550.12, and 1556.55nm. An antireflection coating with a reflection of,0.5% is coated on the backsurface of the glass sub-strate. The measured transmittances of the 78-layer NB filters are higher than 90% with the centerwavelengths of 1536.53, 1542.88, 1549.31, and1555.77 nm, respectively. Except for a small blueshift of the center wavelength relative to the refer-ence wavelength, the measured data shown in Fig. 7are consistent with the model results. The blue shift
Fig. 6. Surface topographies of Ta2O5 ~upper! and SiO2 ~lower!films analyzed by atomic force microscopy. Layer thicknesses ofTa2O5 and SiO2 are ;600 and 200 nm, respectively.
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of the center wavelength is probably due to the sub-strate heating effect16 and the monitoring-thicknessrror during the coating process.17 The bandwidths
of the filters at 3 and 20 dB are 3.87 and 7.97 nm,respectively, which are compliant with Bellcore GR-1209-CORE requirements. The radial distributionof the transmission spectra for 78-layer NB filterswith a reference wavelength of 1543.73 nm is shownin Fig. 8. Not much change to transmittance curveshape at various radii is observed. Most of the peaktransmittances are .90%. The center wavelengthof the transmittance curves shifts toward short wave-length with increase of the radius from 1542.88 to1539.32 nm over a sample radius of 8 mm, indicatingthat the thickness maximum is at the center of thesubstrate and decreases with increasing radius.The thermal stability of the filter is evaluated bymeasurement of the center wavelength shift beforeand after heating of the sample from room tempera-ture to 100 °C as shown in Fig. 9. Two stages for thetemperature-dependent wavelength shift are ob-tained. The first one is 2.54 3 1023 nmy°C at tem-
peratures of ,80 °C, and the second is 3.64 3 10nmy°C at temperatures ranging from 80 to 100 °C.The total wavelength shift is ,0.13 nm when thesample is heated from room temperature to 75 °C,indicating that the thermal stability of a NB filterprepared by reactive ion-assisted dc magnetron sput-tering is excellent.
Similar results are also obtained for NB filters withthe narrower bandwidth prepared by reactive ion-assisted bipolar dc magnetron sputtering. Figure 10shows the transmission spectra of a three-cavity NBfilter with high-index half-wave spacer and 102 lay-ers of Ta2O5 and SiO2. The design of the filter isglassy@~HL!8 2H ~LH!8 L#3yair, where H and L corre-spond to quarter-wave layers of Ta2O5 and SiO2 at areference wavelength of 1546.92 nm. The maximumtransmittance of the filter is 82.2% at a wavelength of1546.12 nm. The bandwidths of the filter at 3 and20 dB are 1.06 and 1.93 nm, respectively, which arealso compliant with the Bellcore GR-1209-CORE re-quirement. The surface roughness of the filter ana-lyzed by atomic force microscopy is ;1.32 nm as
Fig. 7. Transmission spectra of three-cavity NB filters with high-index half-wave spacer and 78 layers of Ta2O5 and SiO2 preparedy reactive ion-assisted bipolar dc magnetron sputtering.
Fig. 8. Radial distribution of the transmission spectra for 78-layer NB filters with reference wavelength of 1543.73 nm.
Fig. 9. Temperature-dependent wavelength shift of 78-layer NBfilter with center wavelength at 1536.53 nm.
Fig. 10. Transmission spectra of three-cavity NB filters withhigh-index half-wave spacer and 102 layers of Ta2O5 and SiO2
prepared by reactive ion-assisted bipolar dc magnetron sputtering.
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shown in Fig. 11. The measured temperature-dependent wavelength shifts of the 102-layer NB fil-ter are 2.13 3 1023 nmy°C at temperatures of ,50 °Cand 3.15 3 1023 nmy°C at temperatures rangingrom 50 to 75 °C, which are nearly the same as thosef the 78-layer filter shown in Fig. 9.
4. Conclusion
The characteristics of three-cavity NB filters withhigh-index half-wave spacer and 78–102 layers ofTa2O5 and SiO2 prepared by reactive ion-assisted bi-polar dc magnetron sputtering have been investi-gated. Pure argon and pure oxygen were used as thesputtering gas and the reactant, respectively. Toavoid the oxidation of tantalum and silicon targets,oxygen gas was introduced and ionized by the ion gunand toward the unheated BK7 glass substrate. Theobtained single-layer Ta2O5 and SiO2 films exhibitedsmooth surface and excellent moisture resistance.The optical properties of the films deposited at theunheated glass substrate were quite stable and arecomparable with those of other ion-assisted coatingprocesses. The bandwidths at 3 and 20 dB were 3.87and 7.97 nm for 78-layer NB filters, respectively, and1.06 and 1.93 nm for 102-layer NB filters, which werecompliant with Bellcore GR-1209-CORE require-ments. The center wavelengths of the 78-layer filterchanged from 1542.88 to 1539.32 nm over a samplewidth of 16 mm, and most of the peak transmittanceswere greater than 90%. The temperature-dependent wavelength shifts of the filters were ,3 3023 nmy°C at temperatures of ,75 °C, indicating
that the thermal stability was excellent. The aboveresults indicate that the reactive ion-assisted dc sput-tering is a promising coating technique for the man-
Fig. 11. Surface topography of three-cavity NB filter with 102layers of Ta2O5 and SiO2 analyzed by atomic force microscopy.
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ufacturing of thermally stable optical coatings withlarge uniformity.
We thank our colleagues F. C. Ho, M. S. Wu, C. T.Wei, C.-Y. Huang, and C. C. Lee from National Cen-tral University (NCU) for their assistance in this re-search. This project was supported by the EconomicAffairs of the Republic of China and by Plank Opto-electronics Inc., Hua Eng Wire and Cable Co. Ltd.,and the Walsin Lihwa Co. under contracts 3NC2300,5NC2000, and 5UR1300, respectively.
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