Analysis of the thickness uniformity improved by using wire masks for coating optical bandpass filters Jin-Cherng Hsu Department of Physics/Graduate Institute of Applied Science and Engineering, Fu Jen Catholic University, 510 Chung Cheng Rd., Hsinchuang, New Taipei City 24205, Taiwan ([email protected]) Received 12 November 2013; revised 9 January 2014; accepted 3 February 2014; posted 3 February 2014 (Doc. ID 201104); published 28 February 2014
Layer uniformity was improved by using a wire mask in the fabrication of a coarse wavelength division multiplexer (CWDM) filter. Theoretical simulations determined the optimal diameter of the wire to be placed just below the substrate, which was rotated at 800 rpm during E-beam evaporation. This simulation also demonstrated the correction of the thickness distribution by the etching effect of ion-assisted deposition. In the corresponding experiment, a distribution uniformity of 0.083% in the radial range from 35 to 65 mm was achieved by the coating of the CWDM filter. © 2014 Optical Society of America OCIS codes: (310.1620) Interference coatings; (310.4165) Multilayer design; (310.1860) Deposition and fabrication. http://dx.doi.org/10.1364/AO.53.001474
1. Introduction
Coarse wavelength division multiplexing (CWDM) filters are designed to multiplex and demultiplex wavelength signals in metropolitan areas. The typical channel spacing is 20 nm, starting at 1271 nm and going up to 1611 nm in all 18 channels proposed by the Full Spectrum CWDM Alliance (FCA). The pass bandwidth is wider than 14 nm at −0.5 dB, and the stop bandwidth is narrower than 26 nm at −30 dB. A CWDM filter is used to separate a channel’s signal from a group of signals by transmitting it while reflecting the others. Moreover, a channel’s signal may be added to a group of other channels by the same filter. The critical specifications of the CWDM filter are a precise central wavelength, low insertion loss, small ripple, wide pass bandwidth, and high channel isolation. The pass bandwidth and isolation points must be matched to the channel spacing of the CWDM system proposed by the FCA. The rigorous specifications of the bandpass filter (BPF) are so strict that the production yield is poor in mass production although the filter is generally 1559-128X/14/071474-07$15.00/0 © 2014 Optical Society of America 1474
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required to be cut into small chips of size 1.4 mm × 1.4 mm × 1 mm. The variation of the thickness across the substrate usually exceeds the tolerance under poor uniformity control. It is difficult to improve the uniformity and the production yield of thin film filters. There are several coating factors in a chamber that influence the layer uniformity, including substrate holder design [1], evaporation source [2,3], monitoring method [4,5], ion-source operation [6], thin film design [7–11], and mask design [12,13]. The mask design is the most significant among the coating factors. In our previous work [6], we studied the ion-source operation to improve the uniformity of an optical narrow bandpass filter (NBPF) for dense wavelength division multiplexing (DWDM). The optical film was successfully deposited by the electron gun and was simultaneously etched by the ion source during ion-assisted deposition (IAD). Although the ion bombardment of IAD increased the densification and refractive index of the growing film, the thickness of the film also was reduced simultaneously. As a result, the optical thickness of the film was almost unchanged if no etching effect was in IAD at low ion-beam voltage. Therefore, we studied the various ion-beam voltages to etch the deposited film and
controlled the direction of the ion beam to etch the film’s thickness of the central part of the substrate and to modify the film’s distribution. The diameter of the circular area was only 50 mm, although the uniformity was better than 0.006%. This result is insufficient for a large useful coating area for CWDM filters. In this study, we have tried to increase the useful area for the purpose of mass production by using a wire mask. 2. Experimental Processes
The vacuum coating system, which is represented by the schematic drawing in Fig. 1, is a 110 cm box coater (NBPF-2, Optorun Co.) equipped with two 10 kW electron beam guns and a 16 cm 500 W Kaufman-type RF ion source, made by Veeco Ion Tech, Inc. The coating materials, Ta2 O5 and SiO2 , were put on two circular hearths in the two electron guns, respectively. The substrate, WMS-02 glass made by OHARA, 100 mm in diameter and 10 mm in thickness, was loaded on a substrate holder. A metallic wire was placed about 1 cm below the substrate. Spinning the holder at a high rotation rate of 800 rpm ensures a symmetrical film distribution during the deposition. The vertical height from the circular hearth to the substrate is 75 cm. The horizontal distances from the two evaporating sources to the center of the substrate are both 30 cm. The ratio of the horizontal distance to the vertical height is only about 0.4. We can predict that the maximum thickness in this deposition system will be located at the center of the rotating flat plate without IAD [14]. To increase the uniformity of the thickness, the wire mask is added to correct the distribution in this study. We pumped the vacuum chamber down to a base pressure lower than 10−4 Pa and heated the substrate to 200°C within an accuracy of 1% for more than 3 h. Before the coating process, the substrate was cleaned using an ion beam for 3 min with a beam
Fig. 1. Schematic drawing of the CWDM filter coating system [6].
voltage of 500 V and a beam current of 400 mA. Oxygen was introduced into the ion source as a working gas by a mass flow controller set at 15 sccm; the working pressure was 1.8 × 10−2 Pa. According to the results of the previous study on coating NBPF, the films were deposited by the electron guns and simultaneously etched by the ion source during IAD. The deposition profiles, i.e., the layer distributions of the films formed by alternately evaporating the two materials, Ta2 O5 and SiO2 , were controlled by the sweeps of the electron beams. An off-axial source coating system is generally used to achieve coating uniformity. The film thickness at the center of the substrate is, however, thicker than that at the other positions on the substrate. Thus, the center wavelength of the deposited NBPF in the area of the small radius on the substrate was longer than that of the large radius. The surplus thickness in the central area of the as-deposited film was reduced to achieve a more uniform layer distribution. In our previous study, the IAD process satisfied this requirement with the etching effect created by a Kaufman-type ion source, which was set at a beam current density of ∼340 μA∕cm2 and a beam voltage of 500 V. In typical CWDM specifications, the pass bandwidth of the filter is wider than 14 nm at −0.5 dB, and the stop bandwidth is narrower than 26 nm at −30 dB in the range of available wavelengths from 1260 to 1610 nm. The top-side multilayer was designed and coated by the following scheme: Air∕HL2 H 6L H LH2 L H LHL2 H 4L H LH3 L HL3 H 6L H LH3 5 L HL3 H 4L H LH3 L HL2 H 4L H LH2 ∕Sub; where H indicates a quarter-wave thickness of Ta2 O5 (refractive index 2.13 at a wavelength of 1550 nm), L represents a quarter-wave thickness of SiO2 (refractive index 1.45 at a wavelength of 1550 nm), and Sub stands for the WMS-02 substrate (refractive index 1.656 at a wavelength of 1550 nm). The back side of the substrate was then coated with antireflective layers to prevent etalon-effect-induced transmission ripples from affecting the optical measurements. When sequentially fabricating the multilayer, a quartz monitor was used to monitor the deposition rate. An optical thickness monitor was operated at the monitoring wavelength of 1390 nm with a bandwidth of 0.12 nm to control the optical thickness with the turning value method [15]. In order to increase the available ring area of the CWDM filter, the monitoring position was set to a position 45 mm away from the center of the rotating substrate. To illustrate the uniformity in this study, we found the maximum and minimum values of the central wavelength in the ring-shaped area from the substrate radius of 35 mm to the radius of 65 mm, based on the spectra that were within the specification requirements, and defined the uniformity as the 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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difference between the maximum and minimum divided by the maximum. An Agilent 8164A 81600B − 140 lightwave measurement system consisting of a tunable laser and a HP 81632A power sensor module was used to measure the filter spectra. This system allows measurement in the wavelength range from 1370 to 1640 nm with an absolute wavelength accuracy of 15 pm and a wavelength resolution of 0.1 pm. The system included a gradient-index fiber to focus the laser beam to a diameter of 0.5 mm on the CWDM filter, which was translated by an X–Y stage over a range of 100 mm with a resolution of 0.02 mm. The system provided stable, accurate, and repeatable spectrum measurements for the CWDM filters.
from the surface element to the source, such that r2 h2 ρ2 R2 − 2ρR cos ψ. If the density of the film is denoted by μ, and the thickness by t, then dM μtdS. In this study, the distribution of the material emitted from the electron beam source is expressed as dM m∕π cosn θ dω [16]. Then Eq. (1) becomes dM
tρ
Thickness Distribution in Vacuum Deposition
The WMS-02 substrate for coating in this study was a flat plate rotating about the central axis, as shown in Fig. 2, where R is the horizontal distance from the axis to the evaporation source. Holland and Steckelmacher have predicted the distributions of layer thickness on flat substrates rotated about the center of the coating chamber using point sources for evaporation, as well as a directed surface source [14]. If the directed surface source has the intensity of a layer distribution falling off as a cos θ function, the distribution of material emitted from the source is expressed as dM m∕π cos θ dω, where m is the total mass of material instantaneously emitted from the source in all directions, dM is the amount passing through the solid angle dω, and θ is the angle from the normal of the source to the direction of the substrate. If the surface element dS of the substrate has its normal at the angle θ to the direction of the source from the element, the amount of the material deposited on the surface is then given by dM
m cos2 θ · 2 dS; π r
(1)
where cos θ h∕r, h is the vertical height from the circular hearth to the substrate, and r is the distance
(2)
The instantaneous deposited thickness at the point P on the circle of radius ρ at the angle ψ, as shown in Fig. 2, is expressed by
3. Theory A.
m cosn1 θ · dS: π r2
m hn1 · 2 : πμ h ρ2 R2 − 2ρR cos ψn3∕2
(3)
Assuming that the flat substrate rotates stably and quickly for a sufficiently long time, the average thickness, tav , at any point around the ring of radius ρ can be expressed as tav
m 1 · πμ 2π
n1
mh π2μ
Z
2π 0
Z
π 0
hn1 dψ h2 ρ2 R2 − 2ρR cos ψn3∕2
dψ : h2 ρ2 R2 − 2ρR cos ψn3∕2 (4)
Then, the thickness at the center of the substrate, t0 , where ρ 0, is given by t0
mhn1 1 · 2 : πμ h R2 n3∕2
(5)
The average thickness is then normalized by dividing by t0, to give the relative thickness tr , i.e., tr
h2 R2 n3∕2 π Zπ dψ × : 2 2 2 n3∕2 0 h ρ R − 2ρR cos ψ
(6)
B. Thickness Distribution in Ion Etching
Fig. 2. Diagram showing the geometry of the evaporation from a symmetrically offset source onto a rotating flat substrate. 1476
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When depositing the layer with IAD, however, the thickness distribution is affected by the etching effect from the ion source. The ion source has three molybdenum grids whose ion beam converges into the convergence point. The divergence angle θ0 is the half-angle from the central axis of the ion source to the direction through the convergence point toward the substrate, as shown in Fig. 1. The distribution of the material etched by the ion source over the solid angle dω0 can be expressed as dM 0 −m0 ∕π 0 cosn θ0 dω0 , which is similar to the distribution of the material emitted from the electron beam source, where m0 is the total reduced mass. The negative sign
means that the film thickness is reduced by the etching effect. If t0av is the average reduced thickness at any point around the ring of radius ρ and t00 is the value at the center of the substrate, the relative etched thickness, t0r , is equal to t0av ∕t00 , i.e., 0
t0av
h02 R02 n 3∕2 π Zπ dψ × : 0 02 2 02 h ρ R − 2ρR0 cos ψn 3∕2 0
(7)
The distance from the convergence point to the substrate is r0 , and r02 h02 ρ02 R02 − 2ρ0 R0 cos ψ, where h0 is the vertical distance from the convergence point of the source to the substrate and R0 is the horizontal distance from the convergence point to the rotating axis of the substrate. Assuming the etching rate is A, the relative thickness distribution at any point around the ring of radius ρ is expressed by t, which is equal to tr − At0r , can be expressed as t
h2 R2 n3∕2 π
Z
π
0
dψ h2 ρ2 R2 − 2ρR cos ψn3∕2
0
h02 R02 n 3∕2 −A π Zπ dψ × : 02 2 02 0 n0 3∕2 0 h ρ R − 2ρR cos ψ
(8)
C. Correction of the Thickness Distribution Using a Wire Mask
If correcting the radial distribution by using the wire mask with a wire diameter of d, the relative thickness distribution then becomes t
h2 R2 n3∕2 π Z π−Δ dψ × h2 ρ2 R2 − 2ρR cos ψn3∕2 Δ
to sweep the surface of the deposited material, which is evaporated by the heating from the electron-beam power. Therefore, the index of the profile, the superscript of cos θ function in Eq. (2), n, depends on the electron-beam power, the electron-beam sweep, and the electron-beam melting of the deposited material in the hearth. In this study, the average profile index n was found to be about 2 by evaluating the two materials [16], Ta2 O5 and SiO2 , deposited on a flat plate without IAD. However, the thickness distribution is affected by the etching effect of IAD from the ion source whose convergence point is a distance below the substrate of about 14 cm, i.e., the value of h0, and R0 , the horizontal distance to the rotating axis of the substrate, is 7.7 cm. The n0 value at the above deposition parameters was estimated to be 2.9, using methods described in our previous research on this characteristics of the ion source [6]. Moreover, the Ta2 O5 and SiO2 films were etched at the rates of 0.008 and 0.02 nm∕s by the ion source, respectively, when the two materials were evaporated at 0.3 and 1.0 nm∕s, as monitored by a quartz monitor. The ratio of the etching rate to the deposition rate for Ta2 O5 is about 2.7%; the ratio for SiO2 is about 2%. The average ratio, A in Eqs. (8) and (9), of the two etching rates is about 2.3%. Then, the results of the relative thickness from a radius of 0 to a radius of 75 mm against various wire diameters d from 0 mm (without the mask) to 1.6 mm are shown in Fig. 3. The value of the relative thickness, deposited without IAD or a mask, at the center of the substrate is unity. Then the value was reduced to 0.977 when it was simulated under the etching effect at the central position of the substrate without a mask during IAD. Moreover, the values of the other curves in the central position of the substrate decrease quickly. That is, the Δ value at the smallest radius appears to be too large to increase the value of the integral. Moreover, the
0
h02 R02 n 3∕2 −A π Z π−Δ dψ × : 0 h02 ρ2 R02 − 2ρR0 cos ψn 3∕2 Δ
(9)
When the angle is less than Δ sin−1 d∕2ρ, which is illustrated in Fig. 2, and larger than π–Δ, no evaporant evaporates toward the substrate. The integral of Eq. (9) is therefore limited to the range of integration between Δ and π–Δ. The integration is carried out on homemade simulation software. 4. Results and Discussion A.
Simulation Results
In order to melt the deposited material uniformly by E-gun, the electron beam is always controlled by the lateral and longitudinal magnetic fields in the E-gun
Fig. 3. Relative thickness distributions are theoretically simulated by using corrective wire masks of various diameters from 0 mm (without the mask) to 1.6 mm. All thicknesses are normalized by the thickness, t0 , deposited at the center of substrate without the mask and IAD. The black bars show the maxima of the curves. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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relative thickness decreases and the maximum of the loci increases with increasing wire diameter. When evaluating the radius region between 35 and 65 mm, the locus of the 1 mm wire diameter has a flatter curve, which has a better uniformity of about 0.087%, as shown in Fig. 4. B.
Experimental Results
An optical CWDM filter was fabricated by using a 1 mm diameter wire and was measured with a spectrometer. The spectra, in the same radius range from 35 to 65 mm, have a passband width of 18.09 0.07 nm at 10.5 dB, a stopband width of 22.92 0.08 nm at −30 dB, and a small ripple in the passband range, as shown in Fig. 5. The spectra become irregular and their central wavelength decrease when the radii are less than 30 mm. The uniformity of the ring area for the radii between 35 and 65 mm is 0.083%, when analyzing their corresponding central wavelength values in Fig. 6. The maximum central wavelength is located at a radius of about 45 mm. The locus approximately agrees with that of the theoretical simulation. We evaluated the useful ring area, where the spectra and the center wavelength of the filter meet the required specification, from the radii between 35 and 65 mm, increased about a factor of 3 with respect to the useful area from the radii between 0 and 30 mm without the mask.
Fig. 5. Transmittance spectra of CWDM filters at 2 mm intervals across the substrate.
Typical masks are stationary and are placed symmetrically just in front of the rotating substrate holders in mass production. Their shapes are designed to modify the radial distribution of thickness according to theoretical simulations or experimental results. Stationary masks are rather sensitive to the characteristics of the evaporating sources and the mask’s positions. Therefore, it is very unstable to allow the attainment of a very high uniformity of vacuum coating (e.g., a uniformity of 0.1% over an area with a 20 cm diameter). It is also difficult to correct the
central position of the chamber where the mask width goes to zero [2]. Figure 3 illustrates this difficulty as the thickness decreases quickly at the central position although the wire diameter is quite small. In addition, the deposition rates and E-gun sweeps of the evaporating sources must be very stable; the masks must be stably arranged at the right positions and their shapes must be very precise. That is why we use the wire masks to improve these situations in this study. It is unnecessary to correct the mask shape in the experimental process because there are many diameters of wire that can be used in the deposition processes. A mask must be cleaned in mass production when a large amount of coating material becomes attached to the mask surface and slightly changes its edge dimension during deposition. However, the flexible metallic wire mask is easily replaced and arranged at the right position. In practice, the wire diameter can be determined from previous experimental data regarding the central wavelength distribution to match the theoretical results of the variation of the relative thickness in Fig. 4. For instance, a larger-diameter wire is chosen
Fig. 4. Uniformity (red dot) and variation (black square) of the relative thickness between that at the radius 35 mm and that at 65 mm [between that at 0 and 30 mm labeled by (0–30 mm)] are simulated by using various diameter of the wire mask, where the 0 mm data are simulated without the mask.
Fig. 6. Central-wavelength distributions of the transmittance spectra of the CWDM filter and the distribution curve predicted by the theoretical simulation of the 1 mm wire mask.
C.
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to replace the mask if the central wavelength of the spectrum in the inner useful ring area is more than that in the outer, which corresponds to the variation of the thickness ratio being positive, and a smallerdiameter wire is chosen if the central wavelength in the inner ring is less than that in the outer ring, which corresponds to the variation being negative. This procedure is used for symmetrically correcting the thickness distribution of the ring to increase the amount of useful area that has an acceptable uniformity. If the variation of the relative thickness approaches zero, the film has a better uniformity of 0.087%. However, the uniformity of the experimental result of 0.083% is somewhat better than that of the simulation. For the 100-GHz DWDM filter, the channel spacing is 0.8 nm at wavelengths from 1271 to 1611 nm in the ITU standard. Depending on the particular module technology, the central wavelength tolerance is around 0.15–0.25 nm. The layer uniformity should be better than 0.01% at a wavelength of 1550 nm for such tolerances. Although the wire mask increases usable area in the region with a uniformity of 0.083%, the specification still does not meet the requirement specifications of the DWDM filters. Otherwise, using Eq. (8) to simulate the case of deposition with R0 4.6 cm and without a wire mask, the uniformity of 0.002% in a diameter of 50 mm is achieved in the same tooling factor profile. Actually, the uniformity of 0.006% in the deposition of an optical DWDM filter [6] was somewhat larger than the simulation value, as shown in Fig. 7. The DWDM curve matches the curve with the etching rate of 2.3% very well within a radius of 14 mm and becomes thicker at the larger radius due to the decreased etching. Although the two tooling factor profiles of the coating materials, Ta2 O5 and SiO2 , were able to be
partially controlled by the E-gun deposition parameters and by etching of the ion source, the profiles still varied with respect to the different electronbeam melting of the two materials and affected the uniformity of the multilayer thickness. For the same reason that a mask was used during deposition, it was difficult to completely adapt to the two different tooling factor profiles by using only a wire mask. Besides, this predicament was exacerbated at the larger radius, where we wish to increase the useful area. On the other hand, when simulating the thickness distribution, it is usually assumed that the molecules of the evaporant travel in straight lines until they collide with a surface. This assumption is not always strictly correct because a few of the molecules in the stream of evaporant collide with the edge surface of the wire mask and then are reflected to the substrate. The behavior of these molecules somewhat disturbs the uniformity and the packing density of the layer through the distance between the wire mask and the substrate. The distance is to avoid colliding with the high-speed-rotating substrate holder. The uniformity may be affected by the longer distance due to increase of the disturbance. Fortunately, this disturbance can be neglected at the 1 cm distance according to the above experimental results of the CWDM filter. The uniformity requirement in CWDM filters may be not very severe and it is possible to neglect this unpredictable factor, but the uniformity requirement is still insufficient for coating of NBPFs, such as optical DWDM filters, under strict specification requirements. Based on the above reasons, we suggest (1) not using a mask for fabricating a high-performance NBPF, like an optical DWDM filter and (2) using two different masks alternately for the two different evaporating sources. Because the small variation of the ion etching rate has a strong influence on the uniformity, as shown in Fig. 7, setting different working parameters of the ion source for coating the two materials may become more uniform. 5. Conclusion
Fig. 7. Curve (solid line) of the relative thickness for an optical DWDM filter with a uniformity of 0.006% [6] and the simulated curves (dashed and dotted lines) of the relative thicknesses with respect to the various etching rates (ranging from 2.2% to 2.6%) without a wire mask versus the radius (for radii less than 25 mm). All curves are normalized to the value tc at the center of the substrate.
In this study, the relative thickness of the film deposited by using a directed surface source of E-gun and etched by using an ion source was theoretically simulated. In order to improve the uniformity of coatings, we simulated the correction of the thickness distribution using wire masks of various diameters to find the best uniformity. When we chose the wire mask with the optimal diameter of 1 mm and placed it symmetrically just below a rotating flat substrate, we successfully deposited the CWDM filters in the ring area between the radii of 35 and 65 mm within the specification requirement. The useful ring area had a layer uniformity of 0.083%, which was about three times that deposition without a mask. However, the wire mask cannot be used to correct the thickness distribution at the central position of the substrate or to correct a NBPF under strict specifications. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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The author would like to thank the National Science Council of Taiwan (Grant No. NSC102-2221-E030-011) for financially supporting this study and Apogee Optocom Co. for experimental assistance.
8. 9.
References 1. C. Guo, M. Kong, C. Liu, and B. Li, “Optimization of thickness uniformity of optical coatings on a conical substrate in a planetary rotation system,” Appl. Opt. 52, B26–B32 (2013). 2. H. A. Macleod, Thin Film Optical Filters, 4th ed. (CRC Press, 2010), pp. 598–604. 3. G. I. Abzalova and R. S. Sabirov, “Depositing uniformthickness coatings on large surfaces by means of electronbeam evaporation in vacuum,” J. Opt. Technol. 72, 799–801 (2005). 4. H. A. Macleod and E. Pelletier, “Error compensation mechanisms in some thin-film monitoring systems,” Opt. Acta 24, 907–930 (1977). 5. C. C. Lee, K. Wu, and M. Y. Ho, “Reflection coefficient monitoring for optical interference coating depositions,” Opt. Lett. 38, 1325–1327 (2013). 6. J. C. Hsu, C. C. Lee, C. C. Kuo, S. H. Chen, J. Y. Wu, H. L. Chen, and C. Y. Wei, “Coating uniformity improvement for a densewavelength-division-multiplexing filter by use of the etching effect,” Appl. Opt. 44, 4402–4407 (2005). 7. C. Schmitz, H. Ehlers, and D. Ristau, “Advanced error identification in deposition of complex optical layer systems
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10. 11. 12. 13.
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by a multianalyzing approach,” Appl. Opt. 51, 8203–8210 (2012). A. V. Tikhonravov and M. K. Trubetskov, “Automated design and sensitivity analysis of wavelength-division multiplexing filters,” Appl. Opt. 41, 3176–3182 (2002). S. R. Mallinson, “Wavelength-selective filters for single-mode fiber WDM spectrum using Fabry–Perot interferometers,” Appl. Opt. 26, 430–436 (1987). J. Minowa and Y. Fuji, “Subnanometer bandwidth interference filter for optical filter communication system,” Appl. Opt. 27, 1385–1396 (1988). R. R. Willey, “Achieving narrow bandpass filters which meet the requirements for DWDM,” Thin Solid Films 398, 1–9 (2001). F. Villa, A. Martinez, and L. E. Regalado, “Correction masks for thickness uniformity in large-area thin films,” Appl. Opt. 39, 1602–1610 (2000). C. Liu, M. Kong, C. Guo, W. Gao, and B. Li, “Theoretical design of shadowing masks for uniform coatings on spherical substrates in planetary rotation systems,” Opt. Express 20, 23790–23797 (2012). L. Holland and W. Steckelmacher, “The distribution of thin films condensed on surfaces by the vacuum evaporation method,” Vacuum 2, 346–364 (1952). H. A. Macleod, “Turning value monitoring of narrow-band all-dielectric thin-film optical filters,” Opt. Acta 19, 1–28 (1972). E. B. Graper, “Distribution and apparent source geometry of electron-beam-heated evaporation sources,” J. Vac. Sci. Technol. 10, 100–103 (1973).
Layer uniformity was improved by using a wire mask in the fabrication of a coarse wavelength division multiplexer (CWDM) filter. Theoretical simulations determined the optimal diameter of the wire to be placed just below the substrate, which was rotated at 800 rpm during E-beam evaporation. This simulation also demonstrated the correction of the thickness distribution by the etching effect of ion-assisted deposition. In the corresponding experiment, a distribution uniformity of 0.083% in the radial range from 35 to 65 mm was achieved by the coating of the CWDM filter. © 2014 Optical Society of America OCIS codes: (310.1620) Interference coatings; (310.4165) Multilayer design; (310.1860) Deposition and fabrication. http://dx.doi.org/10.1364/AO.53.001474
1. Introduction
Coarse wavelength division multiplexing (CWDM) filters are designed to multiplex and demultiplex wavelength signals in metropolitan areas. The typical channel spacing is 20 nm, starting at 1271 nm and going up to 1611 nm in all 18 channels proposed by the Full Spectrum CWDM Alliance (FCA). The pass bandwidth is wider than 14 nm at −0.5 dB, and the stop bandwidth is narrower than 26 nm at −30 dB. A CWDM filter is used to separate a channel’s signal from a group of signals by transmitting it while reflecting the others. Moreover, a channel’s signal may be added to a group of other channels by the same filter. The critical specifications of the CWDM filter are a precise central wavelength, low insertion loss, small ripple, wide pass bandwidth, and high channel isolation. The pass bandwidth and isolation points must be matched to the channel spacing of the CWDM system proposed by the FCA. The rigorous specifications of the bandpass filter (BPF) are so strict that the production yield is poor in mass production although the filter is generally 1559-128X/14/071474-07$15.00/0 © 2014 Optical Society of America 1474
APPLIED OPTICS / Vol. 53, No. 7 / 1 March 2014
required to be cut into small chips of size 1.4 mm × 1.4 mm × 1 mm. The variation of the thickness across the substrate usually exceeds the tolerance under poor uniformity control. It is difficult to improve the uniformity and the production yield of thin film filters. There are several coating factors in a chamber that influence the layer uniformity, including substrate holder design [1], evaporation source [2,3], monitoring method [4,5], ion-source operation [6], thin film design [7–11], and mask design [12,13]. The mask design is the most significant among the coating factors. In our previous work [6], we studied the ion-source operation to improve the uniformity of an optical narrow bandpass filter (NBPF) for dense wavelength division multiplexing (DWDM). The optical film was successfully deposited by the electron gun and was simultaneously etched by the ion source during ion-assisted deposition (IAD). Although the ion bombardment of IAD increased the densification and refractive index of the growing film, the thickness of the film also was reduced simultaneously. As a result, the optical thickness of the film was almost unchanged if no etching effect was in IAD at low ion-beam voltage. Therefore, we studied the various ion-beam voltages to etch the deposited film and
controlled the direction of the ion beam to etch the film’s thickness of the central part of the substrate and to modify the film’s distribution. The diameter of the circular area was only 50 mm, although the uniformity was better than 0.006%. This result is insufficient for a large useful coating area for CWDM filters. In this study, we have tried to increase the useful area for the purpose of mass production by using a wire mask. 2. Experimental Processes
The vacuum coating system, which is represented by the schematic drawing in Fig. 1, is a 110 cm box coater (NBPF-2, Optorun Co.) equipped with two 10 kW electron beam guns and a 16 cm 500 W Kaufman-type RF ion source, made by Veeco Ion Tech, Inc. The coating materials, Ta2 O5 and SiO2 , were put on two circular hearths in the two electron guns, respectively. The substrate, WMS-02 glass made by OHARA, 100 mm in diameter and 10 mm in thickness, was loaded on a substrate holder. A metallic wire was placed about 1 cm below the substrate. Spinning the holder at a high rotation rate of 800 rpm ensures a symmetrical film distribution during the deposition. The vertical height from the circular hearth to the substrate is 75 cm. The horizontal distances from the two evaporating sources to the center of the substrate are both 30 cm. The ratio of the horizontal distance to the vertical height is only about 0.4. We can predict that the maximum thickness in this deposition system will be located at the center of the rotating flat plate without IAD [14]. To increase the uniformity of the thickness, the wire mask is added to correct the distribution in this study. We pumped the vacuum chamber down to a base pressure lower than 10−4 Pa and heated the substrate to 200°C within an accuracy of 1% for more than 3 h. Before the coating process, the substrate was cleaned using an ion beam for 3 min with a beam
Fig. 1. Schematic drawing of the CWDM filter coating system [6].
voltage of 500 V and a beam current of 400 mA. Oxygen was introduced into the ion source as a working gas by a mass flow controller set at 15 sccm; the working pressure was 1.8 × 10−2 Pa. According to the results of the previous study on coating NBPF, the films were deposited by the electron guns and simultaneously etched by the ion source during IAD. The deposition profiles, i.e., the layer distributions of the films formed by alternately evaporating the two materials, Ta2 O5 and SiO2 , were controlled by the sweeps of the electron beams. An off-axial source coating system is generally used to achieve coating uniformity. The film thickness at the center of the substrate is, however, thicker than that at the other positions on the substrate. Thus, the center wavelength of the deposited NBPF in the area of the small radius on the substrate was longer than that of the large radius. The surplus thickness in the central area of the as-deposited film was reduced to achieve a more uniform layer distribution. In our previous study, the IAD process satisfied this requirement with the etching effect created by a Kaufman-type ion source, which was set at a beam current density of ∼340 μA∕cm2 and a beam voltage of 500 V. In typical CWDM specifications, the pass bandwidth of the filter is wider than 14 nm at −0.5 dB, and the stop bandwidth is narrower than 26 nm at −30 dB in the range of available wavelengths from 1260 to 1610 nm. The top-side multilayer was designed and coated by the following scheme: Air∕HL2 H 6L H LH2 L H LHL2 H 4L H LH3 L HL3 H 6L H LH3 5 L HL3 H 4L H LH3 L HL2 H 4L H LH2 ∕Sub; where H indicates a quarter-wave thickness of Ta2 O5 (refractive index 2.13 at a wavelength of 1550 nm), L represents a quarter-wave thickness of SiO2 (refractive index 1.45 at a wavelength of 1550 nm), and Sub stands for the WMS-02 substrate (refractive index 1.656 at a wavelength of 1550 nm). The back side of the substrate was then coated with antireflective layers to prevent etalon-effect-induced transmission ripples from affecting the optical measurements. When sequentially fabricating the multilayer, a quartz monitor was used to monitor the deposition rate. An optical thickness monitor was operated at the monitoring wavelength of 1390 nm with a bandwidth of 0.12 nm to control the optical thickness with the turning value method [15]. In order to increase the available ring area of the CWDM filter, the monitoring position was set to a position 45 mm away from the center of the rotating substrate. To illustrate the uniformity in this study, we found the maximum and minimum values of the central wavelength in the ring-shaped area from the substrate radius of 35 mm to the radius of 65 mm, based on the spectra that were within the specification requirements, and defined the uniformity as the 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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difference between the maximum and minimum divided by the maximum. An Agilent 8164A 81600B − 140 lightwave measurement system consisting of a tunable laser and a HP 81632A power sensor module was used to measure the filter spectra. This system allows measurement in the wavelength range from 1370 to 1640 nm with an absolute wavelength accuracy of 15 pm and a wavelength resolution of 0.1 pm. The system included a gradient-index fiber to focus the laser beam to a diameter of 0.5 mm on the CWDM filter, which was translated by an X–Y stage over a range of 100 mm with a resolution of 0.02 mm. The system provided stable, accurate, and repeatable spectrum measurements for the CWDM filters.
from the surface element to the source, such that r2 h2 ρ2 R2 − 2ρR cos ψ. If the density of the film is denoted by μ, and the thickness by t, then dM μtdS. In this study, the distribution of the material emitted from the electron beam source is expressed as dM m∕π cosn θ dω [16]. Then Eq. (1) becomes dM
tρ
Thickness Distribution in Vacuum Deposition
The WMS-02 substrate for coating in this study was a flat plate rotating about the central axis, as shown in Fig. 2, where R is the horizontal distance from the axis to the evaporation source. Holland and Steckelmacher have predicted the distributions of layer thickness on flat substrates rotated about the center of the coating chamber using point sources for evaporation, as well as a directed surface source [14]. If the directed surface source has the intensity of a layer distribution falling off as a cos θ function, the distribution of material emitted from the source is expressed as dM m∕π cos θ dω, where m is the total mass of material instantaneously emitted from the source in all directions, dM is the amount passing through the solid angle dω, and θ is the angle from the normal of the source to the direction of the substrate. If the surface element dS of the substrate has its normal at the angle θ to the direction of the source from the element, the amount of the material deposited on the surface is then given by dM
m cos2 θ · 2 dS; π r
(1)
where cos θ h∕r, h is the vertical height from the circular hearth to the substrate, and r is the distance
(2)
The instantaneous deposited thickness at the point P on the circle of radius ρ at the angle ψ, as shown in Fig. 2, is expressed by
3. Theory A.
m cosn1 θ · dS: π r2
m hn1 · 2 : πμ h ρ2 R2 − 2ρR cos ψn3∕2
(3)
Assuming that the flat substrate rotates stably and quickly for a sufficiently long time, the average thickness, tav , at any point around the ring of radius ρ can be expressed as tav
m 1 · πμ 2π
n1
mh π2μ
Z
2π 0
Z
π 0
hn1 dψ h2 ρ2 R2 − 2ρR cos ψn3∕2
dψ : h2 ρ2 R2 − 2ρR cos ψn3∕2 (4)
Then, the thickness at the center of the substrate, t0 , where ρ 0, is given by t0
mhn1 1 · 2 : πμ h R2 n3∕2
(5)
The average thickness is then normalized by dividing by t0, to give the relative thickness tr , i.e., tr
h2 R2 n3∕2 π Zπ dψ × : 2 2 2 n3∕2 0 h ρ R − 2ρR cos ψ
(6)
B. Thickness Distribution in Ion Etching
Fig. 2. Diagram showing the geometry of the evaporation from a symmetrically offset source onto a rotating flat substrate. 1476
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When depositing the layer with IAD, however, the thickness distribution is affected by the etching effect from the ion source. The ion source has three molybdenum grids whose ion beam converges into the convergence point. The divergence angle θ0 is the half-angle from the central axis of the ion source to the direction through the convergence point toward the substrate, as shown in Fig. 1. The distribution of the material etched by the ion source over the solid angle dω0 can be expressed as dM 0 −m0 ∕π 0 cosn θ0 dω0 , which is similar to the distribution of the material emitted from the electron beam source, where m0 is the total reduced mass. The negative sign
means that the film thickness is reduced by the etching effect. If t0av is the average reduced thickness at any point around the ring of radius ρ and t00 is the value at the center of the substrate, the relative etched thickness, t0r , is equal to t0av ∕t00 , i.e., 0
t0av
h02 R02 n 3∕2 π Zπ dψ × : 0 02 2 02 h ρ R − 2ρR0 cos ψn 3∕2 0
(7)
The distance from the convergence point to the substrate is r0 , and r02 h02 ρ02 R02 − 2ρ0 R0 cos ψ, where h0 is the vertical distance from the convergence point of the source to the substrate and R0 is the horizontal distance from the convergence point to the rotating axis of the substrate. Assuming the etching rate is A, the relative thickness distribution at any point around the ring of radius ρ is expressed by t, which is equal to tr − At0r , can be expressed as t
h2 R2 n3∕2 π
Z
π
0
dψ h2 ρ2 R2 − 2ρR cos ψn3∕2
0
h02 R02 n 3∕2 −A π Zπ dψ × : 02 2 02 0 n0 3∕2 0 h ρ R − 2ρR cos ψ
(8)
C. Correction of the Thickness Distribution Using a Wire Mask
If correcting the radial distribution by using the wire mask with a wire diameter of d, the relative thickness distribution then becomes t
h2 R2 n3∕2 π Z π−Δ dψ × h2 ρ2 R2 − 2ρR cos ψn3∕2 Δ
to sweep the surface of the deposited material, which is evaporated by the heating from the electron-beam power. Therefore, the index of the profile, the superscript of cos θ function in Eq. (2), n, depends on the electron-beam power, the electron-beam sweep, and the electron-beam melting of the deposited material in the hearth. In this study, the average profile index n was found to be about 2 by evaluating the two materials [16], Ta2 O5 and SiO2 , deposited on a flat plate without IAD. However, the thickness distribution is affected by the etching effect of IAD from the ion source whose convergence point is a distance below the substrate of about 14 cm, i.e., the value of h0, and R0 , the horizontal distance to the rotating axis of the substrate, is 7.7 cm. The n0 value at the above deposition parameters was estimated to be 2.9, using methods described in our previous research on this characteristics of the ion source [6]. Moreover, the Ta2 O5 and SiO2 films were etched at the rates of 0.008 and 0.02 nm∕s by the ion source, respectively, when the two materials were evaporated at 0.3 and 1.0 nm∕s, as monitored by a quartz monitor. The ratio of the etching rate to the deposition rate for Ta2 O5 is about 2.7%; the ratio for SiO2 is about 2%. The average ratio, A in Eqs. (8) and (9), of the two etching rates is about 2.3%. Then, the results of the relative thickness from a radius of 0 to a radius of 75 mm against various wire diameters d from 0 mm (without the mask) to 1.6 mm are shown in Fig. 3. The value of the relative thickness, deposited without IAD or a mask, at the center of the substrate is unity. Then the value was reduced to 0.977 when it was simulated under the etching effect at the central position of the substrate without a mask during IAD. Moreover, the values of the other curves in the central position of the substrate decrease quickly. That is, the Δ value at the smallest radius appears to be too large to increase the value of the integral. Moreover, the
0
h02 R02 n 3∕2 −A π Z π−Δ dψ × : 0 h02 ρ2 R02 − 2ρR0 cos ψn 3∕2 Δ
(9)
When the angle is less than Δ sin−1 d∕2ρ, which is illustrated in Fig. 2, and larger than π–Δ, no evaporant evaporates toward the substrate. The integral of Eq. (9) is therefore limited to the range of integration between Δ and π–Δ. The integration is carried out on homemade simulation software. 4. Results and Discussion A.
Simulation Results
In order to melt the deposited material uniformly by E-gun, the electron beam is always controlled by the lateral and longitudinal magnetic fields in the E-gun
Fig. 3. Relative thickness distributions are theoretically simulated by using corrective wire masks of various diameters from 0 mm (without the mask) to 1.6 mm. All thicknesses are normalized by the thickness, t0 , deposited at the center of substrate without the mask and IAD. The black bars show the maxima of the curves. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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relative thickness decreases and the maximum of the loci increases with increasing wire diameter. When evaluating the radius region between 35 and 65 mm, the locus of the 1 mm wire diameter has a flatter curve, which has a better uniformity of about 0.087%, as shown in Fig. 4. B.
Experimental Results
An optical CWDM filter was fabricated by using a 1 mm diameter wire and was measured with a spectrometer. The spectra, in the same radius range from 35 to 65 mm, have a passband width of 18.09 0.07 nm at 10.5 dB, a stopband width of 22.92 0.08 nm at −30 dB, and a small ripple in the passband range, as shown in Fig. 5. The spectra become irregular and their central wavelength decrease when the radii are less than 30 mm. The uniformity of the ring area for the radii between 35 and 65 mm is 0.083%, when analyzing their corresponding central wavelength values in Fig. 6. The maximum central wavelength is located at a radius of about 45 mm. The locus approximately agrees with that of the theoretical simulation. We evaluated the useful ring area, where the spectra and the center wavelength of the filter meet the required specification, from the radii between 35 and 65 mm, increased about a factor of 3 with respect to the useful area from the radii between 0 and 30 mm without the mask.
Fig. 5. Transmittance spectra of CWDM filters at 2 mm intervals across the substrate.
Typical masks are stationary and are placed symmetrically just in front of the rotating substrate holders in mass production. Their shapes are designed to modify the radial distribution of thickness according to theoretical simulations or experimental results. Stationary masks are rather sensitive to the characteristics of the evaporating sources and the mask’s positions. Therefore, it is very unstable to allow the attainment of a very high uniformity of vacuum coating (e.g., a uniformity of 0.1% over an area with a 20 cm diameter). It is also difficult to correct the
central position of the chamber where the mask width goes to zero [2]. Figure 3 illustrates this difficulty as the thickness decreases quickly at the central position although the wire diameter is quite small. In addition, the deposition rates and E-gun sweeps of the evaporating sources must be very stable; the masks must be stably arranged at the right positions and their shapes must be very precise. That is why we use the wire masks to improve these situations in this study. It is unnecessary to correct the mask shape in the experimental process because there are many diameters of wire that can be used in the deposition processes. A mask must be cleaned in mass production when a large amount of coating material becomes attached to the mask surface and slightly changes its edge dimension during deposition. However, the flexible metallic wire mask is easily replaced and arranged at the right position. In practice, the wire diameter can be determined from previous experimental data regarding the central wavelength distribution to match the theoretical results of the variation of the relative thickness in Fig. 4. For instance, a larger-diameter wire is chosen
Fig. 4. Uniformity (red dot) and variation (black square) of the relative thickness between that at the radius 35 mm and that at 65 mm [between that at 0 and 30 mm labeled by (0–30 mm)] are simulated by using various diameter of the wire mask, where the 0 mm data are simulated without the mask.
Fig. 6. Central-wavelength distributions of the transmittance spectra of the CWDM filter and the distribution curve predicted by the theoretical simulation of the 1 mm wire mask.
C.
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Discussion
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to replace the mask if the central wavelength of the spectrum in the inner useful ring area is more than that in the outer, which corresponds to the variation of the thickness ratio being positive, and a smallerdiameter wire is chosen if the central wavelength in the inner ring is less than that in the outer ring, which corresponds to the variation being negative. This procedure is used for symmetrically correcting the thickness distribution of the ring to increase the amount of useful area that has an acceptable uniformity. If the variation of the relative thickness approaches zero, the film has a better uniformity of 0.087%. However, the uniformity of the experimental result of 0.083% is somewhat better than that of the simulation. For the 100-GHz DWDM filter, the channel spacing is 0.8 nm at wavelengths from 1271 to 1611 nm in the ITU standard. Depending on the particular module technology, the central wavelength tolerance is around 0.15–0.25 nm. The layer uniformity should be better than 0.01% at a wavelength of 1550 nm for such tolerances. Although the wire mask increases usable area in the region with a uniformity of 0.083%, the specification still does not meet the requirement specifications of the DWDM filters. Otherwise, using Eq. (8) to simulate the case of deposition with R0 4.6 cm and without a wire mask, the uniformity of 0.002% in a diameter of 50 mm is achieved in the same tooling factor profile. Actually, the uniformity of 0.006% in the deposition of an optical DWDM filter [6] was somewhat larger than the simulation value, as shown in Fig. 7. The DWDM curve matches the curve with the etching rate of 2.3% very well within a radius of 14 mm and becomes thicker at the larger radius due to the decreased etching. Although the two tooling factor profiles of the coating materials, Ta2 O5 and SiO2 , were able to be
partially controlled by the E-gun deposition parameters and by etching of the ion source, the profiles still varied with respect to the different electronbeam melting of the two materials and affected the uniformity of the multilayer thickness. For the same reason that a mask was used during deposition, it was difficult to completely adapt to the two different tooling factor profiles by using only a wire mask. Besides, this predicament was exacerbated at the larger radius, where we wish to increase the useful area. On the other hand, when simulating the thickness distribution, it is usually assumed that the molecules of the evaporant travel in straight lines until they collide with a surface. This assumption is not always strictly correct because a few of the molecules in the stream of evaporant collide with the edge surface of the wire mask and then are reflected to the substrate. The behavior of these molecules somewhat disturbs the uniformity and the packing density of the layer through the distance between the wire mask and the substrate. The distance is to avoid colliding with the high-speed-rotating substrate holder. The uniformity may be affected by the longer distance due to increase of the disturbance. Fortunately, this disturbance can be neglected at the 1 cm distance according to the above experimental results of the CWDM filter. The uniformity requirement in CWDM filters may be not very severe and it is possible to neglect this unpredictable factor, but the uniformity requirement is still insufficient for coating of NBPFs, such as optical DWDM filters, under strict specification requirements. Based on the above reasons, we suggest (1) not using a mask for fabricating a high-performance NBPF, like an optical DWDM filter and (2) using two different masks alternately for the two different evaporating sources. Because the small variation of the ion etching rate has a strong influence on the uniformity, as shown in Fig. 7, setting different working parameters of the ion source for coating the two materials may become more uniform. 5. Conclusion
Fig. 7. Curve (solid line) of the relative thickness for an optical DWDM filter with a uniformity of 0.006% [6] and the simulated curves (dashed and dotted lines) of the relative thicknesses with respect to the various etching rates (ranging from 2.2% to 2.6%) without a wire mask versus the radius (for radii less than 25 mm). All curves are normalized to the value tc at the center of the substrate.
In this study, the relative thickness of the film deposited by using a directed surface source of E-gun and etched by using an ion source was theoretically simulated. In order to improve the uniformity of coatings, we simulated the correction of the thickness distribution using wire masks of various diameters to find the best uniformity. When we chose the wire mask with the optimal diameter of 1 mm and placed it symmetrically just below a rotating flat substrate, we successfully deposited the CWDM filters in the ring area between the radii of 35 and 65 mm within the specification requirement. The useful ring area had a layer uniformity of 0.083%, which was about three times that deposition without a mask. However, the wire mask cannot be used to correct the thickness distribution at the central position of the substrate or to correct a NBPF under strict specifications. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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The author would like to thank the National Science Council of Taiwan (Grant No. NSC102-2221-E030-011) for financially supporting this study and Apogee Optocom Co. for experimental assistance.
8. 9.
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