(542j) Electric Field-Induced Ion Locking in Polymer Electrolytes for Hardware Security

The global integrated circuit supply chain introduces hardware-level vulnerabilities that make electronic devices susceptible to malicious circuitry insertions or modifications, known as hardware trojans. This is due to semiconductor manufacturing practices that set the device polarity during fabrication, which provides opportunity for untrusted third parties to compromise the devices. One way to prevent the implementation of hardware trojans is the use of polymorphic transistors, because their polarity can be set on-demand once the devices are safely in the hands of the end-user. Electric double layer transistors (EDLTs) are potential candidates for polymorphic circuits because their doping can be adjusted or reversed on-demand. They contain an electrically insulating, ionically conductive electrolyte that modulates channel current by applying a program gate bias (V_PG) to electrostatically dope the channel with ions. Depending on the channel material, EDLTs can function as ambipolar transistors to enable reversible doping of the device as either n-type or p-type. However, when the V_PG is removed, the ions relax back into the electrolyte, and the channel returns to its undoped state. The constant application of V_PG is thus required to maintain the channel doping, unless ions are “locked” at the channel interface, for example, by thermal or UV-activated crosslinking reactions. However, such ion-locking methods are impractical for circuit reconfiguration during operation and do not permit individual device programming, which is a necessary requirement for hardware security applications. This has motivated the development of a non-volatile channel doping mechanism in which ions are immobilized at the electrolyte/channel interface by taking advantage of the large electric fields (EFs) that form at the EDL (~V/nm) to induce crosslinking reactions. EFs catalyze reactions by orienting polarizable functional groups within the electrolyte into favorable positions to enhance reactivity. Unlike thermal or UV-activated crosslinking, EF-induced crosslinking could offer advantages in hardware security where devices could be individually programmed on-demand and do not require doping to be set at the factory.

As a first step, we synthesized an EF-sensitive polymer electrolyte to undergo crosslinking only when V_PG is set above the threshold to induce reactivity. It is a methacrylate-based copolymer containing tertiary amine side chains to undergo EF-induced crosslinking and polyethylene glycol (PEG) side chains to impart ion mobility when the polymer is not crosslinked. The copolymer is mixed with a dihalide crosslinker and LiClO₄ to create a polymer electrolyte that is drop-casted onto a graphene field effect transistor (FET) and left to solidify by solvent evaporation. Doping is achieved by applying a V_PG = +5 V to both form the EDL (i.e., dope the device n-type) and perform crosslinking using the Menshutkin reaction, which was chosen due to the polarizability of the amine side chains and dihalide crosslinker. Electrical transport measurements reveal non-volatile, n-type channel doping of ~2.9x10¹² cm^-2 estimated by a permanent Dirac shift even after grounding the gate contact. Non-volatile doping is achieved using electrolytes containing both high and low salt concentrations (20:1 and 350:1 ether oxygen to lithium molar ratio) with evidence of EF crosslinking across the entire concentration range. Crosslinking is characterized using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), where a signature C-N⁺ stretching peak appears when the polymer is crosslinked due to the formation of quaternary ammonium groups. This stretching peak appears in both EF and thermally programmed devices (i.e., thermally crosslinked control sample), but is absent in unprogrammed devices. Further, we hypothesize that ion locking is a result of decreased segmental motion of the polymer chains, which occurs due to the crosslinking reaction and will be directly measured using broadband dielectric spectroscopy (BDS) to quantify the ion conductivity before and after programming. These results demonstrate a new approach to on-demand, non-volatile channel doping - the first step towards reconfigurable programming of individual devices for hardware security applications.