A new process for the fabrication of planar antenna coupled Ni–NiOx–Ni tunnel junction devices Filiz Yesilkoy ⇑ , Sunil Mittal, Neil Goldsman, Mario Dagenais, Martin Peckerar Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742, USA article info Article history: Available online 23 July 2012 Keywords: MIM Tunnelling diodes RF energy scavenging IR energy harvesting EBL abstract Antenna coupled metal–insulator–metal (ACMIM) tunnel junctions are fast electromagnetic wave detec- tors shown to respond to radiation of wavelength as short as 1.6 lm. In the design and fabrication of these devices, it is crucial to keep the RC time constant of the tunnel junction small to achieve the req- uisite cut-off frequency and adequate rectification efficiency. Junction geometry and the properties of the insulation layer between the metal antenna parts play an important role in determining both the time constant and the rectification effectiveness. In this paper we have designed and fabricated Ni–NiOx–Ni devices and performed three oxidation processes to optimize the insulation layer. Ease of manufacture over large areas is also a requirement for successful implementation. We detail two simple and low cost processes for the fabrication of Ni–NiOx–Ni tunnel junctions. These methods allow for the large area array implementations of the antenna coupled MIMs for the application in low-cost energy harvesting. Ó 2012 Published by Elsevier B.V. 1. Introduction Antenna coupled metal–insulator–metal (ACMIM) tunnel junction devices are in active development as infrared detectors and energy harvesters [1–4]. Detection wavelengths as short as 1.6 lm may be possible. In addition to high-frequency sensitivity, these devices offer the possibility of cheap, large area manufacture. Given these benefits, these structures may receive and rectify the infrared portion of the out-of-visible solar spectrum as well as radiation from the cooling earth ( 10 lm in wavelength). In this paper our primary application goal is IR–RF energy scavenging. As shown here, design simplicity, cheap processes that can be real- ized over large areas, low cost material, and efficient zero bias operation move ACMIM junctions closer to reality as energy har- vesting devices. ACMIM tunnel junctions utilize a fast tunneling mechanism that enables the rectification of terahertz signal coupled to the micro-scale antenna. Although the electron transport through the insulator is extremely fast (10–15 fs) [5], the junction time con- stant ðR j C j Þ defines a cut-off frequency. It is crucial to decrease both the resistance and the capacitance for the high frequency opera- tion. However, when the junction is formed by overlapping elec- trodes, junction resistance and capacitance are linked together by geometry: increasing the junction area decreases junction resis- tance, but it also increases capacitance. It is not possible to de- crease the time constant by simple junction area modulation. In our design, we utilize a geometrically asymmetric field enhancing technique in a completely planar device. The planarity significantly reduces the parasitic capacitance effects, thereby reducing the time constant. The asymmetry in the geometry en- hances the junction electric field, reducing the effective junction resistance. This enables zero bias rectification operation. 2. Background In the early development of thin film antenna coupled MIM diodes, the main challenge was to pattern micro-scale antennas that respond to terahertz frequencies [6]. Developments in elec- tron beam lithography (EBL) have made micro-scale patterning straightforward. Currently, the main fabrication challenges are the fabrication of an asymmetric tunnel junction and small area junction formation. Two new methods aimed at overcoming the resolution limits of the EBL have been offered recently. Bean et al. [4] developed a sin- gle step EBL technique where a double layer resist is used in con- junction with a high voltage, high resolution e-beam. The e-beam lithography was followed by precise angled deposition. Hobbs et al. [2] also used single step EBL followed by angled deposition. These methods provide successful results, but they cannot be ap- plied for the large area fabrication. In order to eliminate the parasitic capacitance that forms be- tween the overlapping electrodes we have developed two fabrica- tion methods that create completely planar structures where the tunneling occurs only in the lateral direction. This removes the need to complicate the process for the purpose of minimizing the overlap and opens the way to large area manufacturing. 0167-9317/$ - see front matter Ó 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.mee.2012.07.078 ⇑ Corresponding author. E-mail address: filizy@umd.edu (F. Yesilkoy). Microelectronic Engineering 98 (2012) 329–333 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee