IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 56, NO. 8, AUGUST 2008 2649 Absorbing Frequency-Selective-Surface for the mm-Wave Range Fadi Sakran, Yair Neve-Oz, Amichai Ron, Michael Golosovsky, Senior Member, IEEE, Dan Davidov, and Avraham Frenkel, Member, IEEE Abstract—We report on a millimeter-wave (mm-wave) absorber based on the frequency selective surface. It consists of a periodic array of resistive patches on a grounded dielectric layer. By varying the shape of the patches and the distance between them, the device can be tuned to absorb in a given frequency band. We designed and fabricated several devices consisting of square arrays of Nichrome circles or rings. The design was targeted to the mm-wave range, 75–110 GHz. Our measurements of the mm-wave reflection from these devices show good agreement with computer simulations. We discuss the use of our device as a microbolometer array. Index Terms—Absorber, frequency selective surfaces (FSS), mil- limeter-wave (mm-wave). I. INTRODUCTION F REQUENCY selective surfaces (FSS) are considered as powerful and practical tools in the filtering of microwaves and millimeter waves. They have been used in many fields such as radar systems, telecommunication, military and security ap- plications. They also play an important role in electromagnetic shielding [1], [2]. The FSS are usually fabricated from conduc- tive patches which enable electromagnetic wave transmission or reflection at certain bandwidths [3]. Conventional FSS have been used as band-stop or band-pass filters [4]. FSS made of lossy resistive patches have been investigated as selective mi- crowave absorbers [5], [15]. Such absorbers are expected to be useful for indoor and wireless local area network (LAN) [6]. Al- though this concept has been used extensively in the microwave range [7], few studies were carried out at the millimeter-waves (mm-waves) [8]. The most popular FSS is based on a periodic array of resistive elements backed by a grounded thick dielectric spacer [5], [9], [15]. This follows the concept of the Salisbury screen where a uniform resistive sheet above a grounded dielectric operates as Manuscript received April 26, 2007; revised January 29, 2008. Published August 6, 2008 (projected). This work was supported in part by the Israeli Ministry of Science, Culture and Sport and in part by the Israeli Ministry of Industry, Trade and Labor. F. Sakran was with the Racah Institute of Physics, Hebrew University of Jerusalem, 91904 Jerusalem, Israel. He is now with Intel Electronics, Kiryat Gat 82109, Israel. Y. Neve-Oz, A. Ron, M. Golosovsky, and D. Davidov are with the Racah Institute of Physics, Hebrew University of Jerusalem, 91904 Jerusalem, Israel (e-mail: golos@vms. huji.ac.il). A. Frenkel is with Anafa-ElectroMagnetic Solutions Ltd., Kiriat Bialik 27000, Israel (e-mail: avri@anafa-em.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2008.924701 matched absorber [10], [11]. Here, minimal reflection (and, cor- respondingly, maximal absorption) is obtained when the sheet resistance of the resistive layer is equal to , the impedance of free space. However, fabrication of such resistive layers presents se- vere difficulties. Indeed, consider a thin conducting film with impedance . Since the sheet resistance of a thin con- ducting film is , where is the resistivity and d is the film thickness, the thickness of the matched layer is . The resistivity of most metals and alloys lies within narrow limits: from (Cu) to (Ti). The thickness of a metallic film with is therefore exceed- ingly small (assuming that the thin film resistivity is equal to bulk resistivity). In particular, it should be few monolayers for Cu and 1 nm for Ti. The fabrication of continuous films with such thickness is still possible but not practical. However, if we replace continuous metallic film with a FSS consisting of metallic patches, these patches may be considerably thicker and more practical for fabrication. Moreover, the incorporation of the FSS in the Salisbury screen can produce a larger bandwidth as compared to the uniform resistive layer. In this paper we report on mm-wave absorbers based on frequency selective surfaces made of patterned metallic films, in particular, a periodic array of thin Nichrome circles or rings deposited on a grounded dielectric. This design has some similarity to the two-dimensional metamaterials and photonic crystals [12]. Our primary goal is the development of the microbolometer array based on this FSS absorber. The design goals here are somewhat different with respect to absorber used for electromagnetic shielding since perfect absorption is not required. A very important design goal in the context of bolometric applications is the low cross-coupling between the patches. This consideration is not relevant for the electromag- netic shielding applications. Our design was optimized by computer simulations using Ansoft HFSS and ePhysics software. We calculated electromag- netic absorption and corresponding thermal heating. We found that our absorber may effectively absorb the mm-waves in a fre- quency bandwidth of few GHz. Our design can be tuned to be a perfect absorber at a specific resonance frequency. While this is of utmost importance for the mm-wave absorbers used for electromagnetic shielding, this is of minor importance for the bolometric applications. II. DESIGN At the first design step we considered the FSS made of a per- fect conductor. We believe that the interaction of this FSS with the electromagnetic wave is the strongest in the frequency range 0018-926X/$25.00 © 2008 IEEE