528 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 11, NO. 5, OCTOBER 2002 Uncooled Multimirror Broad-Band Infrared Microbolometers Mahmoud Almasri, Student Member, IEEE, Zeynep Çelik-Butler, Senior Member, IEEE, Donald P. Butler, Senior Member, IEEE, Alparslan Yaradanakul, Student Member, IEEE, and Ali Yildiz Abstract—A new generation of microbolometers were designed, fabricated and tested for the NASA CERES (Clouds and the Earth’s Radiant Energy System) instrument to measure the radiation flux at the Earth’s surface and the radiant energy flow within the atmosphere. These detectors are designed to measure the earth radiances in three spectral channels consisting of a short wave channel of 0.3 to 5 m, a wide-band channel of 0.3 to 100 m and a window channel from 8 to 12 m each housing a 1.5 mm 1.5 mm microbolometers or alternatively 400 m 400 m microbolometers in a 1 4 array of detectors in each of the three wavelength bands, thus yielding a total of 12 channels. The microbolometers were fabricated by radio frequency (RF) magnetron sputtering at ambient temperature, using polyimide sacrificial layers and standard micromachining techniques. A semiconducting YBaCuO thermometer was employed. A double micromirror structure with multiple resonance cavities was designed to achieve a relatively uniform absorption from 0.3 to 100 m wavelength. Surface micromachining techniques in conjunction with a polyimide sacrificial layer were utilized to create a gap underneath the detector and the Si N bridge layer. The temperature coefficient of resistance was measured to be 2.8%/K. The voltage responsivities were over 10 V/W, detectivities above 10 cm Hz /W, noise equivalent power less than 4 10 W/Hz and thermal time constant less than 15 ms. [759] Index Terms—Broadband, detection, far infrared, infrared, mi- crobolometer, uncooled, yttrium barium copper oxide. I. INTRODUCTION I N THE last few years, uncooled infrared (IR) detection sys- tems have shown considerable progress in cost, size, and performance, both for military and commercial applications. In transportation, IR cameras are used to improve the driver’s ability to see in darkness. In rescue operations, they allow fire- fighters to distinguish victims through the smoke. In earth sci- ence research, IR vision systems are used to measure phys- ical properties of cloud radiation [1], [2]. Microbolometer tech- nology is employed in this paper because it can operate at room temperature and it possesses a relatively flat broadband spec- tral response. Microbolometric devices exhibit a change in re- Manuscript received September 19, 2001; revised March 18, 2002. This work was supported in part by NASA (NAS1-99100), the National Science Foun- dation (ECS-9800062), and Army Research Office (38673PH). Subject Editor G. B. Hocker. M. Almasri and A. Yaradanakul are with the Department of Electrical Engi- neering, Southern Methodist University, Dallas, TX 75275-0338 USA. Z. Çelik-Butler and D. P. Butler were with the Department of Electrical Engi- neering, Southern Methodist University, Dallas, TX 75275-0338 USA. They are now with the Department of Electrical Engineering, University of Texas at Ar- linton, Arlington, TX 76109 USA (e-mail: zbutler@uta.edu; dbutler@uta.edu). Digital Object Identifier 10.1109/JMEMS.2002.803413. sistance with respect to a change of temperature of the sensing material accompanying the absorption of infrared radiation [3]. In our previous works, several different designs of semicon- ducting YBaCuO microbolometers were reported, utilizing: SiO bridges bulk-Si-micromachined to hold the detector arrays; Si N membranes surface-micromachined using a MgO sacrificial layer; and surface micromachined YBaCuO detectors, self-supported by thin titanium electrode arms [4]–[10]. The most commonly used infrared detectors are based on the photovoltaic effect such as HgCdTe detectors, which have achieved detectivity of (1 3) 10 cm Hz /W in long- wavelength infrared applications when cooled to 77 K [11], [12]. However these detectors are limited by their narrow spec- tral bandwidth response and fail to reach to far IR region. On the other hand, broadband spectral response can be achieved easily using thermal detectors. Uncooled thermopile detectors are commonly used for radiometric applications. A specifically designed thermopile detector with an area of 200 200 m operating between 2.5–50 m region of the far IR spectrum has been reported to show responsivity around 400 V/W, detectivity of 1.7 10 cm Hz /W with a response time of 37 ms [13]. In addition, square grid metalized silicon nitride bolometers have been used for far IR detection. These suspended micromesh bolometers have achieved noise equivalent power (NEP) as low as 2.7 10 W/Hz at 304 mK with a response time of 24 ms [14]. Other types of far-IR thermal detectors are the con- ventional high- superconducting transition edge bolometers [15]–[18], and antenna-coupled superconducting microbolome- ters [19], [20]. High- superconducting YBa Cu O detec- tors have exhibited responsivity as high as 1.7 10 V/W and NEP as low as 2.1 10 W/Hz [21]. Similarly, supercon- ducting YBa Cu O microbolometers with bowtie antennas on NdGaO have achieved NEP as low as 1.2 10 W/Hz and thermal response time of 20 ns [19]. Antenna-coupled su- perconducting microbolometers have a faster response time and a comparable NEP to that of conventional superconducting mi- crobolometers. However, the high cost of cooling, and the re- quirement for high deposition temperatures limit the application of high- superconductor bolometers. In this paper, novel semiconducting YBaCuO microbolome- ters are developed for far-infrared wavelength detection. The key feature of the structures is that the detector arrays are smaller and faster than those currently used in CERES, while retaining the requirement for spectral flatness of the total detector system. In addition, the sensitivity of the detectors is improved by an order of magnitude compared to the currently used IR detectors. 1057-7157/02$17.00 © 2002 IEEE