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Wavelength and Thermal Distribution Selectable Microbolometers Based on Metamaterial Absorbers Volume 7, Number 3, June 2015 Kaikai Du Qiang Li Weichun Zhang Yuanqing Yang Min Qiu

DOI: 10.1109/JPHOT.2015.2406763 1943-0655 Ó 2015 IEEE

IEEE Photonics Journal

Microbolometers Based on Absorbers

Wavelength and Thermal Distribution Selectable Microbolometers Based on Metamaterial Absorbers Kaikai Du,1 Qiang Li,1 Weichun Zhang,1 Yuanqing Yang,1 and Min Qiu1,2 1

State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China School of Information and Communication Technology, KTH Royal Institute of Technology, 16440 Kista, Sweden

2

DOI: 10.1109/JPHOT.2015.2406763 1943-0655 Ó 2015 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received February 1, 2015; revised February 17, 2015; accepted February 19, 2015. Date of publication February 24, 2015; date of current version April 23, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61235007, Grant 61205030, Grant 61275030, and Grant 61425023; by Qianjiang River Fellow Fund of Zhejiang Province; by the Scientific Research Foundation for the Returned Overseas Chinese Scholars from the State Education Ministry; by the Open Fund of the State Key Laboratory of Advanced Optical Communication Systems and Networks; by the Doctoral Fund of the Ministry of Education of China under Grant 20120101120128; by the Fundamental Research Funds for the Central Universities under Grant 2014QNA5020; by the Swedish Research Council (VR); and by VR's Linnaeus Center in Advanced Optics and Photonics (ADOPT). Corresponding authors: Q. Li and M. Qiu (e-mail: [email protected]; [email protected]).

Abstract: An uncooled microbolometer based on metamaterial absorbers is investigated. The absorption peak reaches 90%, and the peak wavelength can be tailored from 2.4 to 10.2 m with corresponding bandwidth varying from 0.5 to 1.5 m by tuning the geometric parameters of the absorbers, covering two atmosphere windows (3–5 m and 8–14 m). The thermal distribution in the microbolometer can be adjusted to realize a strong thermal response. In the given situation with a pixel size of 25.07 m, the temperature response of the detector reaches 1.3 K. The microbolometer can be potentially used in thermal imaging at selected wavelengths in the mid-infrared and far-infrared regimes. Index Terms: Detectors, microbolometers, metamaterial absorbers, absorption, photothermal effect.

1. Introduction Mid-infrared and far-infrared detectors play significant roles in military reconnaissance, environmental monitoring, fire safety control, night vision, medical testing, etc. [1]–[5]. In this context, photon detectors are generally adopted in areas where high sensitivity and quick response are demanded [6]–[9]. However, the applications of infrared photon detectors are hampered by the high cost of their indispensable refrigerator. Therefore, extensive attention has been paid to uncooled infrared detectors, including thermistor-based infrared detectors (or microbolometers) and pyroelectric infrared detectors. Microbolometers require an effective absorption of infrared radiation. Traditional microbolometers are considered to be highly responsive to broadband wavelengths; however, in some applications such as target recognition and chemical detection, a high level of spectral selectivity is required [10]. Conventional solutions to this demand are introducing elaborate structures such as optical filters [11], [12] and =4 resonators [13]–[15] to

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Fig. 1. Schematic diagram of the microbolometer based on metamaterial absorbers. (a) Full view of a pixel of the microbolometer. (b) Unit of the periodic absorber structure, where r ¼ 530 nm, a ¼ 1090 nm, t1 ¼ 80 nm, t2 ¼ 65 nm, t3 ¼ 40 nm, t4 ¼ 20 nm, t5 ¼ 20 nm, and t6 ¼ 250 nm.

tailor the spectral response, which inevitably increases thermal mass and, thereby, hinders integration and miniaturization. One way to solve this dilemma is to introduce plasmonic absorbers, including metal-dielectric-metal metamaterial absorbers [16]–[22] and plasmonic antenna absorbers [23]. The metal-dielectric-metal metamaterial absorber can achieve nearly perfect absorption at specific wavelengths over a broad region by tailoring the geometric parameters [16]–[22]. Maier et al. realized tunable absorption peaks from 2.9 um to 7.7 m by changing period and area coverage of the metamaterial absorbers on the microbolometer membrane [20], [21]. However, the resultant photothermal effect after absorption and temperature change of the thermistor of the microbolometer, which are crucial for the microbolometers, are not investigated. Besides, the adoption of Si3 N4 film as the microbolometer membrane disturbs the wavelength selectivity to some extent due to its inherent vibrational mode absorption at wavelength between 10 m to 14 m [24], [25]. In this paper, the wavelength selectivity and thermal distribution tunability of the microbolometers based on metamaterial absorbers are investigated. First, a schematic diagram of the microbolometer based on metamaterial absorbers is presented. Then numerical calculations are performed to investigate the absorption performance and the thermal distribution of the microbolometer. The obtainable peak absorption wavelength from 2.4 m to 10.2 m can be realized by changing the geometric dimensions of the structure. The thermal distribution could be easily tuned by tuning the metal thicknesses. Finally, the temperature response of the microbolometers for the application of pedestrian detection is analyzed.

2. Design, Results, and Discussion Fig. 1 presents the schematic diagram of the microbolometer based on metamaterial absorbers. The absorber is composed of periodic 80-nm-thick gold disks and a 40-nm-thick gold film. A 65-nm-thick silicon layer is spaced between the gold disks and the gold film. The total thickness of the absorber is just 185 nm, which is beneficial to thermal response. The period of the gold disks is 1.09 m. The VOx (20 nm thick) is adopted as thermistor for its excellent compatibility with circuits and high temperature coefficient of resistance, which leads to enhanced sensitivity. A thin film of Si3 N4 (20 nm) is sandwiched between the metamaterial absorber and the VOx thermistor. It is designed to insulate the electric contact between the gold film and the VOx thermistor so as to maximize the electrical response of the circuit. In the meantime, the Si3 N4 film delivers heat from the metamaterial absorber to the VOx thermistor. All these structures are supported by a 250-nm-thick Si3 N4 bridge suspended above the substrate, which not only provides considerable strength, but also reduces the thermal conductivity between the thermistor and the substrate. Since the gold film is thick enough, incident infrared light is prevented from penetrating into the layers beneath the gold film. Compared with the microbolometer based on metamaterial absorbers presented in [20] and [21], a thick-enough gold film here prevented the incident infrared light from penetrating into the Si3 N4 layers; therefore, the wavelength selectivity of the microbolometer is not disturbed by the absorption of the Si3 N4 layers. The photothermal response is numerically analyzed by commercial COMSOL multiphysics software. Since the gold film of the absorber is optically thick enough, only the gold-silicon-gold

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Fig. 2. (a) Absorption as a function of wavelength. a ¼ 1090 nm, r ¼ 530 nm, t1 ¼ 80 nm, t2 ¼ 65 nm, t3 ¼ 40 nm, t4 ¼ 20 nm, t5 ¼ 20 nm, and t6 ¼ 250 nm. (b) Absorption as a function of wavelength and radius of the gold disk. (c) Electric field distribution. (d) Magnetic field distribution.

structure is considered in the absorbing process. The permittivities of gold and silicon are taken from Palik's handbook [26]. The simulation is based on the case that the periodic absorber is illuminated by a beam of plane wave. By changing the radius of the gold disk, the absorption peak could be tuned. Fig. 2(a) shows the absorption at normal incidence when the disk radius is 530 nm. The absorption peak appears at the wavelength of 9.6 m, which is equivalent to the peak wavelength of human body radiation. Fig. 2(b) provides the dependence of absorption at normal incidence on the disk radius. When the radius increases from 100 nm to 540 nm, the absorption peak shifts from 2.4 m to 10.2 m while the peak absorption keeps around 90%. Two absorption peaks appear at the two atmosphere windows (3 5 m and 8 14 m, respectively) for the radius above 500 nm, facilitating simultaneous detection in both mid-infrared and far-infrared regimes. In this letter, the second atmosphere window is mainly considered for the application of night vision. Although the peak absorption wavelength is significantly dependent on the disk radius, the disk period also affect the absorption wavelength to a certain extent. The disk period put an upper limit on the achievable peak absorption wavelength since the maximum disk radius is geometrically limited by the disk period ð2r G aÞ. Besides, the peak absorption decreases slightly with increased disk period, which can be made up for by thickening the dielectric

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Fig. 3. Absorption spectra of the microbolometer as functions of incident angles for TE and TM polarizations.

Fig. 4. Normalized thermal distribution (resistive loss) in the absorber at peak absorption wavelength 9.6 m. The geometrical parameters are the same as those shown in Fig. 1.

and metallic layers. By increasing the disk period, a larger disk radius is allowed to tune the peak absorption wavelength beyond 10.2 m. At a ¼ 2000 nm, t2 ¼ 160 nm and t3 ¼ 100 nm, both tunable spectra covering the second atmosphere window (8–14 m) and nearly unity peak absorption can be achieved simultaneously. To reveal the physics behind the absorption, the electromagnetic field distributions at 9.6 m peak wavelength when the gold particle radius is 530 nm are presented in Fig. 2(c) and (d). The electric field mainly focuses in the gap between the disks. The magnetic field is confined in the dielectric layer, demonstrating a strong magnetic resonance responsible for the high absorption. For applications of infrared detection, the detected light usually comes from different directions. Therefore, further simulations are performed to investigate the absorption properties at oblique incidence. The absorption spectra for both TE and TM polarizations as functions of incident angles are presented in Fig. 3. For both polarizations, their absorption wavelengths are the same and angle-insensitive. For TE polarization, the peak absorption drops from 89.1% to 63.2% as the incident angle increases from 0 to 60 ; for TM polarization, the peak absorption rises from 89.2% to 100% as the incident angle increases from 0 to 60 . The absorbed infrared light turns into heat. The thermal distribution (resistive loss) resulting from the infrared absorption is presented in Fig. 4. The photothermal conversion occurs only in the gold layers since the silicon spacer layer is transparent in the mid-infrared and far-infrared regimes. The resistive loss in the gold film, which is attached to the Si3 N4 spacer layer and the VOx thermistor, is of vital importance to the sensitivity of the detector. Fig. 5 shows the total absorption of the absorber and heat efficiency of the gold film (defined as the total absorbed heat power in the gold film layer divided by the incident power) as functions of the three layers of the

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Fig. 5. Absorption of the absorber (a)–(c) and heat efficiency of the gold film (d)–(f). For (a) and (d), the gold disk thickness is changed. For (b) and (e), the silicon film thickness is changed. For (c) and (f), the gold film thickness is changed. The other parameters are a ¼ 1090 nm, r ¼ 530 nm, t4 ¼ 20 nm, t5 ¼ 20 nm, and t6 ¼ 250 nm, if not specified.

absorber at normal incidence. For a 40-nm-thick gold film and 80-nm-thick gold disks, the peak absorption increases and the peak wavelength decreases with the silicon thickness. Heat efficiencies in both gold film and gold disks increase correspondingly. However, the thick silicon film can result into enhanced thermal capacity to some extent, which can compromise the sensitivity of the microbolometer. A silicon thickness of 65 nm is adopted for the performance of the microbolometer. For a 40-nm-thick gold film and a 65-nm-thick silicon layer, both the absorption of the absorber and the heat efficiency of the gold film increase with the gold disk thickness. For 80-nm-thick gold disks and a 65-nm-thick silicon layer, the absorption is less sensitive to the gold film thickness and the heat efficiency of the gold film even decreases with increasing gold film thickness. In this sense, thick gold disks and a thin gold film can enhance the heat efficiency of the gold film. The microbolometers based on the metamaterial absorbers can be potentially applied in infrared human or animal images. According to Planck's law of radiation, the spectral radiant exitance of human body with a surface temperature at 305 K can be calculated. Imaging on the microbolometer and the photothermal response on the detector are investigated further by assuming that a human with radiation surface area of S stands d meters away from the microbolometer. The schematic diagram of this imaging process is simply presented in Fig. 6(a). Normal incidence is investigated in this application due to the angle insensitivity of the absorber. Polarizations have no influence on normal incidence so the incident infrared light is considered to be non-polarized. The focusing system with an aperture diameter of D is placed in front of the microbolometer. Assume the microbolometer has n n pixels and a view field S1 at a distance d , the image area of the human being on the detector S 0 can be calculated by S0 ¼ S

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Sdetector S1

(1)

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Fig. 6. (a) Schematic diagram of the imaging process with labeled parameters (radiation area S, image area S 0 , view field area S1 , detector area Sdetector , focus system aperture diameter D, distance d ). (b) Temperature change as a function of the pixel size. The green line is the temperature change in the metamaterial-based microbolometer, and the red line is the temperature change in the resonant cavity-based microbolometer.

where Sdetector is the area of the detector. The focused thermal power on the image area can be written as W ¼S

S2 X M d2

(2)

where S2 ¼ D 2 =4 is the lens cross-sectional area, and M is the spectral radiant exitance of the human body. Then, the power density of the heat source (w1 and w2 for the gold disks and the gold film, respectively) in the image area can be calculated by X S2 M 1 V1 d 2 X S2 w2 ¼ S M 2 : 2 V2 d w1 ¼ S

(3) (4)

V1 ¼ r 2 t1 S 0 =a2 and V2 ¼ t3 S 0 are the volume of gold disks and the gold film in the image area, respectively. 1 and 2 are the heat efficiency of gold disks and the gold film, respectively. Under the above conditions, assuming d ¼ 10 m, S ¼ 0:1 m2 , Sdetector ¼ 5 105 m2 , S1 ¼ 20 m2 , S2 ¼ 2:5 103 m2 , 1 and 2 for the case of t1 ¼ 80 nm, t2 ¼ 65 nm and t3 ¼ 40 nm, the calculated power densities (w1 and w2 for the gold disks and the gold film, respectively) are 5:18 1010 W/m3 and 9:86 1010 W/m3 , respectively. In the microbolometer, the heat radiation and the heat convection are ignored in the heat transfer process. The whole microbolometer structure is supposed to be rooted on a cubic silicon substrate with a side length of 100 m and the bottom temperature is fixed at 293.15 K. The two square prism legs are 2-m-long, and

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their side lengths are 250 nm. The temperature change of the thermistor increases as the pixel size enlarges. The relationship between temperature change and pixel size in microns is showed in Fig. 6(b) (the green line), which can be fitted by a quadratic as T ¼ ð2:1x 2 0:92x Þ 103 ðK Þ:

(5)

The coefficients in (5) are related to thermal conduction and thermal capacity of materials for the microbolometer. The temperature change is linearly proportional to the incident heating power when the pixel size is fixed. The temperature change of the thermistor is determined by the incident power, thermal capacity and thermal conductivity. In this letter, the main body of the microbolometer is suspended in a vacuum and supported by a Si3 N4 bridge with two legs. The two legs are the only channels for the thermal conduction between the absorber and the substrate. The sizes of the supporting legs don't change with the pixel size and therefore the thermal conduction keeps constant. The influence by thermal capacity is much smaller compared with the incident power. The heating power linearly increases with the pixel area. Therefore, the temperature change shows a nearly parabolic dependence on the pixel size of the microbolometer. When the pixel size is 25.07 m, the temperature change of the VOx thermistor is 1.3 K. There are two kinds of resonant cavity structure which is used for microbolometers: dielectric cavity and vacuum cavity [13]. To compare, a vacuum resonant cavity structure is investigated under the same radiation situation [15]. The structure is composed of a Si3 N4 =Ti/Si3 N4 =VOx =Si3 N4 (50 nm/10 nm/50 nm/150 nm/400 nm) multi-layer and a 100-nmthick gold reflector, with a 4.63-m-thick air gap in between. The two regular square prism legs are 2-m-long and the side lengths are 250 nm to support the 660-nm-thick absorber. The temperature change as a function of the pixel size is presented in Fig. 6(b) (the red line). The fitted quadratic curve-fitting equation is T ¼ ð2:26x 2 1:69x Þ 103 ðK Þ:

(6)

It is obvious that the temperature changes for both metamaterial absorber based microbolometers and vacuum resonant cavity based microbolometers are close to each other, even though the former have much narrower absorption bandwidth. The metamaterial absorber based microbolometers also show several advantages compared with resonant cavity based microbolometers. Firstly, for resonant cavity based microbolometers, the supporting bridge Si3 N4 and the thermistor (such as VOx ) exhibit inevitable intrinsic absorption [15], [27]; therefore, the metamaterial absorber based microbolometers show much better spectral selectivity. Secondly, the metamaterial absorber based microbolometers are ultra-thin, thus reducing the heat capacity significantly and consequently enhancing the temperature change. Thirdly, in comparison with the vacuum cavity based microbolometers, the legs supporting the metamaterial absorber based microbolometers are allowed to be located right under the main body and the reflector is not necessary, reducing the complexity of the structure with enhanced duty rate.

3. Summary In summary, an uncooled wavelength and thermal distribution selectable microbolometer based on metamaterial absorbers is proposed. The microbolometer is composed of several thin layers and the total thickness of the absorber is only 185 nm. The absorption wavelength can be tuned from 2.4 m to 10.2 m with the peak absorption of around 90% simply by changing the diameters of the gold disks. In the given situation, the temperature change of the thermistor reaches 1.3 K, showing promise in photothermal imaging systems. Therefore, the metamaterial absorber provides a new way for spectral tailoring and thermal distribution adjustment in microbolometers and thereby can be potentially used in thermal imaging at selected wavelengths in the mid-infrared and far-infrared regime.

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