Study of Microwave Circuits Based on Metal-Insulator-Metal (MIM) Diodes on Flex Substrates Amanpreet Kaur, Xianbo Yang, and Premjeet Chahal Department of Electrical and Computer Engineering, Michigan State University kaurama1@msu.edu , yangxian@msu.edu , chahal@egr.msu.edu Abstract This paper demonstrates fabrication and characterization of thin-film Ti-TiO 2 -Pd based Metal- Insulator-Metal (MIM) diodes on flexible substrates. MIM diodes with contact areas of 9 μm 2 and 48 μm 2 were fabricated, and a comparison is made for their DC and RF performances. The current-voltage characteristics of the fabricated diodes show strong non-linearity. The diodes are also tested for microwave circuit applications such as detection, frequency multiplication and mixing over a frequency range of 1 – 18 GHz. The devices show strong second harmonic frequency multiplication for fundamental frequencies of 1-10 GHz. Details of DC characteristics, RF rectification, mixing and multiplication using MIM diodes are presented. Introduction The microwave circuits on flex substrates enable various applications in wireless communication, identification systems, portable/wearable communication devices, wireless power transfer, security and bio-sensing. Flexible electronics have potential advantages over conventional system as they are light weight, large area compatible, enables roll to roll fabrication, low temperature processing and thus very important for low cost manufacturing [1-4]. Over the last decade, significant work has been carried out on the design and fabrication of RF passive devices on flexible substrates [5, 6]. However, direct integration of active components such as diodes and transistors on flex substrates is still a challenge. For applications that require low cost but high-performance RF/Microwave circuitry capable of operating up to several gigahertzs, flexible substrate is a good choice. Many RF applications like portable communications systems require microwave wave antenna arrays integrated with active electronic devices. The cost of assembling passive and active device modules is high as the cost of the individual III-V electronic circuitry is very high. This problem can be solved by integration of passive devices like antenna arrays and high- speed electronics onto large-area, flexible substrates for applications like imaging system. The metal-insulator-metal (MIM) diodes are made entirely of thin film materials i.e. metals and insulators and provide non-linear characteristics dominated by tunneling effects. They can be easily fabricated on wide variety of flexible substrate or even on top of existing CMOS circuitry. Thus RF and microwave devices based on MIM diodes is a promising area of research, as they can operated at very high frequencies, they are simple to implement and process, and a host of metal dielectric combinations can be used to achieve desired diode characteristics. Thus, this allows fabrication of RF circuits onto low cost plastic substrates, which in-turn may be attached to objects of interest like automobile, aircraft or on buildings for sensing and trans-receiving applications. The MIM diodes can also be easily coupled with other thin film components like MIM capacitors, metal inductors, and thin film resistors leading to formation of complex circuits. The high speed and frequency response in comparison to III-V Schottky diodes as well as the possibility to choose flexible substrate due to thin film fabrication makes MIM diodes a good choice for RF flex devices. The MIM diodes can operate at high frequencies with high switching speed and faster response time due to tunneling effect. MIM diodes are also preferred due to their temperature insensitive characteristics [7]. High frequency MIM diodes coupled with antennas/waveguides are increasingly explored for application in microwave circuits as they provide good device scalability for Microwave/mm wave detectors [8, 9]. In the past, point-contact MIM diodes also known as whiskers were used for millimeter wavelength detection and mixing, but they lack reproducibility and stability [10].Therefore thin film MIM diodes are utilized more often as they are more reliable and stable. In a MIM diode, the electrons flow between top and bottom metal electrodes through a very thin insulator layer. Also depending on the thickness of insulator and the barrier height, i.e., difference in work function between the two metals, either quantum tunneling or thermionic emission may dominate [11]. Generally MIM diodes with dissimilar metals electrodes show significant non- linearity. The asymmetry is more pronounced when the work function between the two metal electrodes is large. Various combination of dissimilar metals had been investigated in past such as Ni-NiO-Au [8], Ni-NiO-Ni [12], Ti-TiO 2 -Al [13], Ni- Nio-Cr/Au [14], Al-AlO x -Pt [15] Although MIM devices have been studied in great detail for a range of applications, to date their fabrication and implementation in the design of RF circuits on flex substrates has not been investigated. In this paper, we present the design, fabrication and characterization of MIM based RF/Microwave devices on flex substrate, Polyetheretheretherketone (PEEK). PEEK is a low loss substrate which is also compatible with standard micro-fabrication process (temperature and chemicals). For high frequency operation the contact area should be smaller or thickness of the dielectric layer should be high to keep the capacitance low. But increasing the oxide thickness reduces the tunneling current and thus decreasing the contact area is more attractive. In this paper, we report the fabrication of thin film MIM diodes with two different contact areas of 9 μm 2 (Diode A) and 48 μm 2 (Diode B) using Ti- TiO 2 -Pd. Ti has a work function of 4.3 eV and Pd has work function of 5.2 eV, and thus creating a work function difference of 0.9 eV. This difference in work function has exhibited a higher degree of non-linearity and has been verified by studying the Current–Voltage (I–V) characteristics of the diode. This paper also presents microwave circuits