Broadband polarization-selective uncooled infrared sensors using tapered plasmonic micrograting absorbers

Sensors and Actuators A 269 (2018) 563–568

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Broadband polarization-selective uncooled infrared sensors using tapered plasmonic micrograting absorbers Shinpei Ogawa a,∗ , Yousuke Takagawa b , Masafumi Kimata b a b

Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan College of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan

a r t i c l e

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Article history: Received 14 July 2017 Received in revised form 1 December 2017 Accepted 13 December 2017 Available online 14 December 2017 Keywords: Plasmonics Metamaterial Uncooled Infrared sensor Polarimetric

a b s t r a c t Polarization-selective uncooled infrared (IR) sensors are promising for IR polarimetric imaging with respect to enhancement of the object recognition ability. Polarization-selective absorbers that operate in the broadband wavelength region and have simply fabricated small structures are strongly required for high-performance IR polarimetric imaging. In the present study, plasmonic micrograting absorbers (PMGAs) were designed as broadband polarization-selective absorbers. The PMGAs have an Au-based one-dimensional periodic grating structure. Numerical calculations demonstrated that tapered-sidewalls with deep groove depths produce broadband polarization-selective absorption with wavelengths larger than the grating period. Microelectromechanical systems-based uncooled IR sensors with tapered-PMGAs were fabricated using complementary metal oxide semiconductor and micromachining techniques. Tapered-sidewalls were formed by isotropic etching using reactive ion etching equipment. The responsivity could be selectively enhanced according to the polarization angle, and an absorption wavelength longer than the surface period and broadband absorption were realized due to the effect of the tapered-sidewalls. The PMGAs enable detection wavelengths longer than the grating period and broadband polarization-selectivity by control of the taper angle and the grating depth. The results obtained in this study will contribute to the advancement of polarimetric IR imaging. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Recent advances in uncooled infrared (IR) sensors have been remarkable according to the progress of microelectromechanical system (MEMS)-based mass production technology. These sensors are widely used for security, night vision, health care, gas detection, and industrial monitoring [1–4]. Mega-pixel focal plane arrays and small pixels for high-resolution are also being developed [5,6]. At the same time, single pixel or small array formats are being developed for low-cost applications [7]. Advanced functional uncooled IR sensors with wavelength- or polarization-selectivity are drawing significant attention to expand their potential and their field of application. In particular, the function of polarization-selectivity is highly promising for the enhancement of recognition ability such as improved target recognition [8], material discrimination [9], human object recognition in the natural environment [10], contrast enhancement [11], and facial recognition [12,13]. However, other components such as

∗ Corresponding author E-mail address: [email protected] (S. Ogawa). https://doi.org/10.1016/j.sna.2017.12.029 0924-4247/© 2017 Elsevier B.V. All rights reserved.

filters or polarizers are required to realize these functions [14–16]. This not only leads to high cost but also degrades sensor performance because these components themselves emit IR radiation to the sensors and increase noise. To address this challenge, we have developed wavelength or polarization-selective uncooled IR sensors using plasmonic metamaterial absorbers [17]. Plasmonic metamaterial absorbers [18,19] can impart uncooled IR sensors with wavelength- or polarization-selectivity, simply by controlling the surface geometry of the absorbers without the use of filters or polarizers. Absorbers composed of plasmonic metamaterials are roughly classified into three types: plasmonic crystal [20–25], metal-insulator-metal (MIM) [26–29], and mushroom structures [30–32]. MIM absorbers realize broadband polarizationselectivity with multi-size antenna structures [29]. However, MIM absorbers require precise control for antenna size with dimensions on the scale of 100 nm. Plasmonic crystal absorbers (PCAs) have two-dimensional (2D) periodic dimples or one-dimensional (1D) gratings, which are simple structures that are easily fabricated. Moreover, PCAs have the advantage of robustness for structural fluctuation and are thus suitable for commercial use. Polarization selectivity can be realized by introducing asymmetric surface structures such as elliptical dimples [23] and 1D gratings

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Fig. 1. (a) Schematic diagram of the uncooled IR sensor (thermopile) with Au-based PMGA. (b) Definition of the electric field polarization angle with respect to the grating direction and structural dimensions of the PMGA.

[24,25,33] to PCAs. However, PCAs require periodicity, which leads to large pixel size and operation only at the wavelength defined by the period [23], which is disadvantageous for polarization selectivity because broadband operation is required for high-performance polarimetric imaging applications. If polarization-selective PCAs with simple 1D periodic structures operated in the broadband IR wavelength region and small pixel size could be realized, it would be a significant benefit for high-performance IR polarimetric imaging. We have reported on polarization-selective uncooled IR sensors that use plasmonic micrograting absorbers (PMGAs) for broadband operation [24]; however, the detailed principle for broadband operation has not yet been investigated. Here, we report on the origin of broadband operation using PMGAs with detailed measurement results. 2. Absorber design Fig. 1(a) and (b) show schematic illustrations of an uncooled IR sensor with a PMGA and a cross section of the PMGA, respectively. Thermopiles [34] were used as an uncooled IR sensor. Thermopiles convert IR radiation to a voltage using thermocouples in thermal isolation legs. Fig. 1(b) shows the structural parameters, where the period, width, depth of the grooves, and the inclination angle of sidewalls are defined as p, w, d, and ϕ, respectively. The polarization angle of the electric field (E) in the incident IR radiation of k-vector direction is defined as  according to the x- and y-axes, which correspond to the perpendicular and parallel directions with respect to the grating direction. p was fixed as 5.0 ␮m, considering the fabrication limit of the photolithography system employed. The incident angle was fixed as normal to the absorber plane to investigate the basic sensor performance.

Fig. 2. Calculated absorbance as a function of the wavelength and w for d = (a) 0.5 and (b) 1.5 ␮m. The absorbance scale is given as a color-map at the right.

The absorption properties were calculated using the rigorouscoupled wave analysis (RCWA) method [35]. RSoft DiffractMOD software [36] was used for the RCWA calculations. The optical constant of Au was taken from reference [37]. The absorbance for  = 90◦ was confirmed to be zero in the following calculations, which indicates that the PMGAs exhibit polarization-selectivity. Hence, we present the calculated results only for  = 0◦ . Fig. 2(a) and (b) show the calculated absorbance as a function of the wavelength and w for d = 0.5 and 1.5 ␮m with fixed ϕ = 0◦ , respectively. Fig. 2(a) indicates that strong wavelength-selective absorption over 80% occurs in PMGAs with shallow d of 0.5 ␮m and the wavelength is almost equal to p, regardless of w, which are similar characteristics to those for 2D PCAs [20–22]. On the other hand, Fig. 2(b) shows that the absorbance of PMGAs with a deep d value of 1.5 ␮m was not obvious. The strong absorption was observed only at w narrower than 400 nm with the wavelength at 8 ␮m and not 5 ␮m, which is demonstrated in 1D plasmonic nanogratings [25]. The effect of ϕ was then investigated. Fig. 3(a) and (b) show the calculated absorbance as a function of the wavelength, and ϕ for d = 0.5 and 1.5 ␮m with w fixed at 3.0 ␮m, respectively. w was defined as 3.0 ␮m, considering fabrication due to the resolution limit of the microfabrication systems employed. Fig. 3(a) shows that the main absorption wavelength of the PMGA with a shallow d of 0.5 ␮m was fixed at 5.0 ␮m. This absorption is due to the period of the PMGA, which is same as that of the vertical sidewalls, as shown in Fig. 2(a). The dispersion also appeared according to the increase in ϕ. The increase in ϕ corresponds to the change in the incidence angle to the sidewalls, which generates this dispersion relation as in the case of flat metals. On the other hand, the absorbance of the PGMA with a deep d of 1.5 ␮m was

S. Ogawa et al. / Sensors and Actuators A 269 (2018) 563–568

Fig. 3. Calculated absorbance as a function of wavelength, and ϕ for d = (a) 0.5 and (b) 1.5 ␮m. The absorbance scale is given as a color-map at the right.

significantly changed from the vertical sidewalls and the broadband absorption was achieved by the tapered-grating structures with ϕ larger than 10◦ , as shown in Fig. 3(b). Although 1D-PMGAs are simple structures, the dominant parameter that defines the absorption wavelength changes according to the relation between d, w, and p. In contrast, p mainly defines the absorption wavelength in 2D-PCAs. The dominant parameter is d with a narrow w dimension of 100 nm [25,38], and is changed to p with a wide w that has dimensions of a few micrometers and a shallow d. This is because the plasmonic resonance in the depth direction is dominant in the former structure and the inplane direction is dominant in the latter structure. Both plasmonic resonances can occur with deep d and wide w. Figs. 2(a) and 3(a) show that the effect of the tapered sidewalls has less impact on the absorbance for shallow d and wide w because the plasmonic resonance in the absorber plane is dominant, and p defines the absorption wavelength. In contrast, as shown in Fig. 3(a) and (b), the effect of the tapered-sidewalls has a significant impact on the absorbance for deep d and wide w because the mixed plasmonic resonances occur in both the depth and in-plane directions, which produces multi-wavelength plasmonic resonance and results in a broadband absorption. 3. Fabrication MEMS-based uncooled IR sensors with tapered-PMGAs were developed. The tapered-sidewalls were obtained by isotropic etching with reactive ion etching (RIE) on a SiO2 mold. RIE generally produces tapered-sidewalls rather than vertical sidewalls. The RIE equipment used in this study can achieve ϕ of approximately 20◦ , which is suitable for broadband absorption according to the

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Fig. 4. Procedure for the fabrication of a MEMS-based thermopile that incorporates a tapered-PMGA.

calculated results shown in Fig. 3(b). Fig. 4 shows the developed fabrication procedure, which is the same as that in our previous study [21–23], except for the formation of tapered-gratings. (i) The devices are fabricated on 6-inch silicon substrates using a standard complementary metal oxide semiconductor (CMOS) process. The thermocouples consist of a series of p- and n-type poly-Si regions, of which the resistivity is controlled by ion implantation. An Al layer is formed under the absorber area to prevent the absorption of incident IR radiation at the backside of the absorber [21]. Etching holes for cavities are formed by RIE. SiO2 layers of two thicknesses (0.5 and 1.5 ␮m) were formed on the absorber area of different wafers. (ii) Tapered-grating structures were then formed only on the SiO2 layer of the IR absorber area by RIE. (iii) 50/250 nm thick Cr/Au layers were sputtered, where the Cr layer acts as an adhesion layer between SiO2 and Au. The 250 nm thick Au layer was significantly thicker than the skin depth in the IR wavelength region but was sufficient to coat all the sidewalls and the bottoms of the grooves, so that the influence of Cr and SiO2 beneath the Au layer was negligible. The Cr/Au layers were selectively etched with a wet etchant to reveal the etching holes covered by the sputtered layers. Scanning electron microscopy (SEM) observations confirmed that the Cr/Au layers were uniformly coated. (iv) The wafers were diced into chips. Si is anisotropically etched through the etching holes using tetramethylammonium hydroxide. The cavity under the IR absorber area is then formed; the thermally isolated freestanding structure is finally completed. Fig. 5(a) and (b) show an SEM image of the developed thermopile with the tapered-PMGA and a magnified SEM image of the taperedPMGA surface, respectively. The thermally isolated floating structures were successfully fabricated, as shown in Fig. 5(a). The detector area (300 × 200 ␮m2 )

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Fig. 5. (a) SEM image of the MEMS-based thermopile with tapered-PMGA developed in this study. (b) Magnified SEM image of the surface of the PMGA. (c) The correspondence of the tapered-sidewalls to the top SEM view. (d) Cross-sectional SEM image of the tapered-sidewalls.

is surrounded by long thermal isolation legs that can reduce the thermal conductance. The tapered-sidewalls are clearly presented on the edge of the grooves in Fig. 5(b). The cross-sectional view is not accurate, because it is extremely difficult to cut the sample exactly vertical to the micrograting direction. Therefore, the ϕ value of approximately 20◦ was estimated, as shown in Fig. 5(c). Fig. 5(d) shows the cross-sectional SEM image for confirming the formation of the tapered-sidewalls. 4. Measurement and results The polarization dependence of the spectral responsivity was measured for the developed sensors. Two types of sensors were prepared: sensor A with d = 0.5 ␮m and sensor B with d = 1.5 ␮m. p, w, and ϕ of 5.0, 3.0 ␮m, and 20◦ were the same for both sensors. Fig. 6 shows the main part of the measurement system used. The sensors were set in a vacuum chamber with a BaF2 window under a pressure of 1 Pa to prevent thermal conduction loss through the atmosphere. IR radiation from a blackbody with a temperature of 1000 K was irradiated to the sample through narrow bandpass filters to select the evaluation wavelengths. The typical full width at half maximum was 80 nm. A polarizer attached onto the angle alignment holder was set in front of the BaF2 window. The control point of the polarizer was adjusted to the sensor position accord-

Fig. 6. Experimental setup used for polarization dependence measurements.

Fig. 7. Polarization dependence of the spectral responsivity for sensors (a) A with d = 0.5 ␮m and (b) B with d = 1.5 ␮m. p, w, and ϕ of 5.0, 3.0 ␮m, and 20◦ were the same for both sensors.

ing to the definition of  shown in Fig. 1(b). A pin-hole was used to restrict the light incident on the tapered-PMGA to exclude the influence of absorption by the sensor except the tapered-PMGA. The spectral responsivity was calculated from the output voltage of the sensor and the input IR radiation power according to the calculation method [20]. Please refer to our previous paper [23] for more details on the experimental setup. The polarization dependence of the spectral responsivity was measured for  = 0, 45, and 90◦ . Fig. 7(a) and (b) show the measurement results for sensors A and B, respectively. The responsivity was normalized according to the peak values of 150 V/W and 120 V/W for each sensor because the wavelength resolution of the measurement system was not sufficient to determine the maximum value. However, the measured peak responsivity for each sensor was at least over 100 V/W, and the difference between the maximum and minimum responsivity was more than 50 V/W, which is sufficient performance for practical use. Both sensors clearly demonstrated polarization-selective detection. The incident electric field perpendicular to the grating direction ( = 0◦ ) produced the maximum responsivity. However, there was a clear contrast in the operation wavelengths of each sensor. Sensor A shows polarization-selective detection only at around 5 ␮m, which corresponds to p, whereas sensor B achieved broadband polarization-selective detection from around 5 to 10 ␮m, which is larger than p. These characteristics coincide well with the calculated absorbance results shown in Fig. 3. Therefore, the broadband operation of sensor B is attributed to the effect of the tapered-sidewall structures.

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5. Discussion The bandwidth for polarization-selective absorption can be controlled according to the structural parameters of the PMGAs, such as d and ϕ. When d is shallow with the dimensions of w on the scale of a few micrometers (structure A), polarization-selective absorption occurs with the wavelength almost equal to p, and the same characteristics are almost maintained for tapered-sidewalls. However, the weak dispersion appears according to the increase in ϕ, which can be considered to be the same as the change in the incidence angle to a flat metal surface. When d is deeper with the dimensions of w on the scale of a few micrometers (structure B), almost no absorption occurs for the vertical side wall (ϕ = 0◦ ), although the broadband absorption occurs for the tapered-sidewall with a wavelength region that is larger than p. These properties suggested that the plasmonic resonances of 1D-PMGAs are produced according to the relations of the structural parameters because the dominant structural parameter of 1D-PMGAs was not fixed, in contrast to the period of 2D-PCAs [39]. The tapered-sidewalls produce multi-mode plasmonic resonance because the deep d for the tapered-sidewalls produces multi-resonance of the dispersion mode observed with the shallow d in addition to the resonance in the depth direction. In structure A, the period is dominant and plasmonic resonance is produced in the in-plane direction. Therefore, the impact of the plasmonic resonance is less in the depth direction and polarization-selective absorption occurs only at the wavelength defined by p. In contrast, for structure B, there is no dominant plasmonic resonance for the vertical side wall (ϕ = 0◦ ). Therefore, the effect of the tapered-sidewall structures becomes more obvious and enhances the multi-mode plasmonic resonances in the depth direction mixed with the in-plane direction, which results in the broadband polarization-selective absorption. The mixed plasmonic resonances in the in-plane and depth directions produce longer plasmonic resonance wavelengths. Therefore, the absorption wavelength region of structure B is larger than p. The simple controllability of polarization-selective detection is a significant advantage for practical uncooled IR sensors to achieve high responsivity, small pixel size, and wide operation wavelength region. 6. Conclusion Broadband polarization-selective uncooled IR sensors were designed and developed using simple 1D-PMGAs. RCWA calculations demonstrated that PMGAs with tapered-sidewalls provide broadband absorption for larger depths than the period. Thermopiles with tapered-sidewall PMGAs were fabricated using conventional CMOS and MEMS techniques. The tapered-sidewalls were formed by isotropic etching with RIE. The measured polarization dependence of the spectral responsivity confirmed that broadband polarization-selective detection was achieved with a wavelength larger than the period. The sensor properties obtained were well consistent with the calculated results. The proposed tapered-sidewall structures can be simply fabricated by RIE and are compatible to CMOS processes. The principles demonstrated in this paper can be applied to other types of thermal IR sensors, such as bolometers [40] and silicon-on-insulator (SOI) diodes [41]. The results obtained are expected to contribute to the development of high-performance polarimetric imaging with IR sensors. References [1] M. Kimata, IR imaging, in: Y. Gianchandani, O. Tabata, H. Zappe (Eds.), Comprehensive Microsystems, Elsevier, Spain, 2008, pp. 113–163.

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Biographies Shinpei Ogawa received his B.E., M.E., and Ph.D. degrees from the Department of Electronic Science and Engineering, Kyoto University, Japan, in 2000, 2002, and 2005, respectively. He joined Mitsubishi Electric Corporation in 2005. He works on various devices including RF-MEMS switches, through-silicon vias, and advance functional infrared sensors using plasmonics and metamaterials. He is currently a unit leader of plasmonics, metamaterials, and graphene research for novel optoelectronic devices. Yousuke Takagawa received his BE and ME degrees from the College of Science and Engineering, Ritsumeikan University, Japan, in 2013 and 2015, respectively. He has been with Mitsubishi Electric Corporation, Japan, since 2015. Masafumi Kimata received his MS degree from Nagoya University in 1976, and received his PhD degree from Osaka University in 1992. He joined Mitsubishi Electric Corporation in 1976, and retired from Mitsubishi Electric in 2004. Currently, he is a professor of Ritsumeikan University, where he continues his research on MEMSbased uncooled infrared focal plane arrays and type-II superlattice infrared focal plane arrays. He is a fellow of SPIE.