Fabrication of Thin-Film Nano-Scale Metal–Insulator–Metal (MIM) Tunnel Diode Using Conventional Photolithography

A metal–insulator–metal (MIM) tunnel diode having a response time of less than a picosecond holds great promise to outperform its semiconductor counterparts (i.e., Schottky diode) in mixing and detection of terahertz and infrared radiation. One of the key objectives of this work is to develop fabrication processes which are well-suited for mass production of thin-film MIM tunnel diodes with junction area on the order of 0.01 μm.² Due to the reduced junction capacitance and elimination of the depletion-region capacitance, detection of signals with frequencies up to 28 THz generated by CO₂ laser is possible. A state-of-the-art electron-beam stepper or nano-imprint lithography system of such resolution costs tens of millions of dollars and is not a viable option for low-cost mass production. Hence, standard photolithography and atomic layer deposition (ALD) methods were employed to fabricate a micrometer-wide finger in the second metal layer that is separated from the electrode defined in the first metal layer by an ALD-deposited sidewall dielectric spacer, thus forming a nm-thick vertical tunnel junction. The junction area is defined by the width of the finger and the thickness of the electrode, while the tunnel layer thickness is precisely controlled by the ALD process. For the purpose of evaluating different ALD tunnel layers, MIM diodes with micron-scale self-aligned cross-fingers have been developed and characterized.