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Integrated optical filter arrays fabricated by using the combinatorial etching technique Shao-Wei Wang, Xiaoshuang Chen, and Wei Lu National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
Li Wang, Yonggang Wu, and Zhanshan Wang Institute of Precise Optical Engineering and Technology, Department of Physics, Tongji University, Shanghai 200092, China Received August 11, 2005; revised September 19, 2005; accepted October 6, 2005 A combinatorial etching technique is developed to fabricate integrated narrow bandpass filters on a single substrate. It is highly efficient for fabrication of integrated filter arrays in optical regions. A monolithic filter array has been fabricated by using the technique with a two-step deposition process. The filter contains 32 elements in the near-infrared region. The relative full width at half-maximum (FWHM) ␦ / of the filter elements is less than 0.2%. Such a narrow bandpass filter array can be utilized in many optical applications. © 2006 Optical Society of America OCIS codes: 120.2440, 230.3120, 230.4170, 330.6110, 350.2460.
The optical filter array is one of the most important components in wavelength-division multiplexing,1 multispectral devices,2 and parallel array optics,3 which are widely used in communication and electrooptical systems. Traditionally, optical filters are mounted on a rotating wheel frame or employ tunable filters4,5 for wavelength selection, but neither approach can yield information in different bands simultaneously or be integrated with a large number of bands. There are two types of filter array that acquire information simultaneously by controlling the optical thickness 共nd兲 of the filters’ spacer layer. In one the passband is controlled by the refractive index 共n兲 of the spacer layer,6 and in the other by the physical thickness 共d兲 of the spacer layer. The former has only been proposed theoretically and is hard to fabricate with a large integration number. The latter has been realized in the form of wedge filters2,7 and is already commercially available. However, in principle, it is impossible for wedge filters to have the properties of both large dispersion 共Ⰷ10 nm/ mm兲 and a wide useful spectral region, for they are limited by the wedge angle because of the nonparallel interface effect in the resonant cavity layer. A typical linear variable filter in the 300– 700 nm spectral range is 60 mm in length with dispersion of approximately 5 – 10 nm/ mm.6 Therefore, a wedge filter can hardly be integrated with a large number of centimetersized narrow passbands to match a detector array. The combinatorial approach has shown great promise in materials discoveries such as high-Tc superconductor materials, ferroelectric materials, efficient luminescence, and low-loss dielectrics.8,9 It has already been applied to various material fabrication and optimization processes. Here we introduce a combinatorial approach for the fabrication of a filter array with different narrow bandpass filters and demonstrate a 32⫻ 1 filter array in the NIR region. For a dielectric Fabry–Perot-type filter, the peak wavelength of the passband 共兲 will be 0146-9592/06/030332-3/$15.00
=
2nd k + 共1 + 2兲/2
m = k + 共1 + 2兲/2,
2nd =
m
,
k = 0,1,2 . . . .
This equation indicates that the passband of the filter is determined by the optical thickness 共nd兲 of the spacer layer. A series of filters with different passbands can be obtained just by changing the thickness 共d兲 of the spacer layer. Then the filters can be integrated on a single substrate easily if a series of spacer layers with different thicknesses can be fabricated. The main difficulty is to fabricate spacer arrays with different thicknesses. This difficulty can be readily overcome using the combinatorial etching technique presented here, which produces 32 spacer elements of different thicknesses with a five-step etching process. Figure 1 shows a diagram of the procedure for fabrication of a filter array by use of the combinatorial etching technique. A filter array with 2N elements needs the combinatorial etching processes to be run only N times and the deposition processes two times. First, the lower mirror stack and the spacer layer are deposited on the substrate as shown in Fig. 1(a). Then the spacer layer is etched to be a linear array with a different thickness by the combinatorial etching technique [see Fig. 1 (b1)–(b3)]. During combinatorial etching, a series of masks are used to realize selective etching on different areas of the spacer layer. The window’s shape and the size of the masks are determined by the shape and size of the filter array. The mechanical masks used in this work are valid for millimeter-sized or larger filter elements. Photolithography can be used for smaller filter elements. The total etching thickness of each spacer element is different. After the upper mirror stack is deposited on the resultant structure, a filter array with a series of distinct passbands is completed [Fig. 1(c)]. Its 3D schematic is shown in Fig. 1(d). © 2006 Optical Society of America
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The filter properties are influenced by many fabrication parameters, such as the vacuum of the chamber, the temperature of the substrate, and the deposition time. Unlike conventional fabrication of a single filter, our method involves making two deposits and running the etching process N times. These two extra factors influence the filter properties. To evaluate the influence of running the deposition procedure twice on the filter properties, the same filter structure (LH)3 4L (HL)3, where L and H represent low and high refractive index materials with optical
Fig. 1. (a)–(c) Diagram of the procedure for fabricating a filter array by using combinatorial etching, where (b1)–(b3) are the combinatorial etching processes. (d) 3D schematic diagram of the fabricated filter array.
Fig. 2. Transmittance spectra of the same filter structure (LH)3 4L (HL)3 fabricated by running the deposition procedure twice and the conventional procedure once.
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thickness of a quarter of the designed wavelength 共0 = 2.8 m兲, has been fabricated by running the deposition procedure twice and the conventional procedure once. During the two-time deposition procedure, the sample has been loaded into the vacuum chamber twice to grow the (LH)3, 4L, and (HL)3 separately by using electron-beam evaporation with thickness controlled by an optical monitor. The transmittance spectra of the filters fabricated by these two different procedures, shown in Fig. 2, are measured by a Fourier transform infrared spectrometer (Perkin-Elmer). The passband of the filters fabricated by running the deposition procedure once is located at 2.794 m with a relative FWHM ␦ / of 0.8% and transmittance of 66.0%. For the sample used in the two-time deposition procedure, the center wavelength is at 2.805 m with ␦ / of 1.1% and transmittance of 59.7%. The results show that the filters fabricated by running the deposition procedures once and twice are similar. As for the etching processes, the etching-induced surface roughness will influence the properties of the filters. To check the effect of surface roughness, we investigated a series of surfaces that resulted from different filter array fabrication procedures by use of atomic force microscopy (AFM). Figure 3 presents three AFM pictures of sample surfaces in different processes and their rms values of surface roughness. The pictures show the surfaces [Fig. 3(a)] after deposition of the lower mirror stack and the spacer layer, [Fig. 3(b)] after etching the surface of spacer layer, and [Fig. 3(c)] after deposition of the upper mirror stack on the etched surface. All etching is performed on a home-made Ar-ion beam etching machine. The rate of etching SiO2 is 0.25 nm/ s. The etching time is controlled by hand with a stopwatch. This method is precise enough for such an etching rate. The surface roughness becomes 1.34 nm [Fig. 3(a)] after deposition of the lower mirror stack and the spacer layer on a substrate with a rms roughness of 0.30 nm. The surface roughness decreases to 0.67 nm [Fig. 3(b)] after etching. The spacer layer’s surface has been modified by the etching process. The surface roughness increases to 1.38 nm [Fig. 3(c)] after the upper mirror stack is deposited on the etched spacer layer. The results indicate that the etching process will not induce
Fig. 3. (Color online) AFM pictures of sample surfaces in different combinatorial etching processes: (a) after deposition of the lower mirror stack and the spacer layer, (b) after etching of the surface of the spacer layer, (c) after deposition of the upper mirror stack on the etched surface.
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Fig. 4. (a) Transmittance spectra of each element on the 32⫻ 1 filter array, with element size of 0.37 mm⫻ 12 mm. (b) Peak wavelength of the filter passbands, where the line and points are theoretical and experimental results, respectively.
more roughness than the deposition process. Therefore, it is a good way to combine deposition and etching for fabrication of a filter array. Based on the analyses of the above two extra processes involved in the combinatorial etching technique, a 32⫻ 1 filter array in the NIR region has been fabricated with five etching steps and two deposition processes. Transparent and low absorption materials of Ta2O5 and SiO2 are chosen as high and low refractive index materials with nH and nL of 2.11 and 1.48, respectively. The original filter is designed to have a (LH)10 4.4 L (HL)10 structre with a designed wavelength 0 of 777.4 nm, where H and L are 0 / 共4nH兲 and 0 / 共4nL兲, respectively. The center wavelength of the passband is determined mainly by the optical thickness of the spacer layer and can be adjusted by varying the layer thickness for a constant refractive index. The thickness of the spacer layer can be controlled using the etching process. During the first deposition, the lower (LH)10 mirror stack of and the 4.4 L spacer layer are deposited on the glass substrate. Then the spacer layer is transferred to a 32 ⫻ 1 spacer array with thickness discretely distributed in the range 4.4 L – 3.95 L by running the etching process five times. There is no obvious uniformity problem, as the whole etched area is only 12 mm ⫻ 12 mm. The size of a single element is 0.37 mm ⫻ 12 mm. A 32⫻ 1 filter array has been completed after the upper (HL)10 mirror stack is deposited on the spacer array. The corresponding spectra of each filter element [Fig. 4(a)] were measured by a modified micro-Raman spectroscope (Dilor LabRam-Infinity). The passbands of the 32 filter elements are distributed from 774.7 to 814.2 nm. They are distributed almost linearly in good agreement with the design, except for filter elements 17 and 18 [Fig. 4(b)]. The
deviation of filters 17 and 18 may be a result of the etching process. All the filters’ FWHM are very narrow, less than 1.5 nm, corresponding to ␦ / of each filter of ⬍0.2%. The narrowest FWHM of the integrated filters is 0.8 nm with ␦ / of 0.1% at a wavelength of 794.3 nm. Since the passbands of the fabricated filter array are ultranarrow, they are very sensitive to the fabrication conditions and processes. The transmittance of the passbands is 21.2–32.4%. Most of the passbands’ transmittance is near 30%. The relatively low transmittance is due mainly to the introduction of etching and the two deposition processes. Two-dimensional (2D) filter arrays can also be fabricated simply by repeating the etching processes on the other dimension. A filter array fabricated by use of our technique can be easily designed to match with a 1D or a 2D photodetector array (e.g., a silicon CCD array) and form a compact spectrophotometer. This technique can be extended to other important optical regions. However, the above results are still far from optimum. One can improve the properties of a filter array further by optimizing the filter design and improving the deposition and etching processes. Filter arrays realized by use of such a simple technique will match the demands of multispectral acquisition systems, parallel arrayed optics, flat panel color displays, and so on. We have introduced the fabrication procedure of combinatorial etching for integration of narrow bandpass filters. It is highly efficient and can be applied to most important optical ranges. The larger the integration number, the greater the advantage of this technique over traditional ones. A filter array with 2N elements can be fabricated by running the etching process only N times. As a simple sample, a 32⫻ 1 filter array was demonstrated in the NIR region. This work is sponsored by the Shanghai RisingStar Program (05QMX1459) and the National Natural Science Foundation of China (60508018). Corresponding authors: W. Lu (
[email protected]), S. W. Wang (
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