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Integrated Crossbar of Microelectromechanical Selectors and Non-Volatile Memory Devices for Neuromorphic Computing

Periodic Reporting for period 1 - SelectX (Integrated Crossbar of Microelectromechanical Selectors and Non-Volatile Memory Devices for Neuromorphic Computing)

Reporting period: 2016-04-01 to 2018-03-31

Manufacturing of ANNs is currently limited, since traditional CMOS hardware cannot provide the high synaptic density of biological neural circuits with reasonable energy consumption. A promising solution is to combine transistor hardware with an alternative technology, like the emerging memristors (or ReRAM), that can deliver the artificial synaptic functionality in an energy efficient way. A memristor has an analog tunable resistance that mimics the basic synaptic functionality, which would otherwise take dozen transistors to reproduce. A memristor stores data in a non-volatile fashion without drawing power and has only two terminals, making it energy efficient, scalable and stackable.

Crossbars of memristors stacked on transistor chips (Fig 1a) could deliver the high density and connectivity required for hardware ANNs. However, the crossbar architecture has the sneak path problem: neighbouring devices create an electrical short around the selected device (Fig 1b). A solution is to use a non-linear selector for each of the memory devices, the most common selector being a transistor. The transistor is a three-terminal device fabricated in the Silicon bulk so it has limited scalability and stackability. Therefore its use as a selector negates the advantages offered by the two-terminal memristor.

This project proposed an alternative selector based on a two-terminal microelectromechanical switch (MEMS) (Fig. 2), thanks to its high nonlinearity.The goal was to develop a crossbar of memristors integrated with MEMS selectors. The success of this project is immediately relevant to hardware ANNs and non-volatile memories and to the industry players in the field (HP, IBM, etc.).
The SelectX project was structured in five work packages (WPs). The WP1 developed the active material used for memristor fabrication in other WPs. The WP2 referred to the fabrication of individual memristors and was pursued in parallel with WP3, the simulation and fabrication of two-terminal MEMS selectors. The results from WPs 2 and 3 were combined in WP4 - fabrication of an integrated memristor-MEMS device. The final WP5 was the fabrication of a crossbar of integrated memristor-MEMS devices.

WP1.Material development
Substoichiometric titanium dioxide (TiO2-x) was the material of choice due to its good memristive properties. The initial plan was to deposit substoichiometric TiO2-x using reactive sputtering, but it was not successful due to experimental difficulties with the sputtering equipment owned by IMT Bucharest. An alternative method - evaporation from TiO2 pellets in vacuum - was pursued with success, using an e-beam deposition tool at IMT. The deposited material was sub-stoichiometric with high initial conductivity.

WP2.Single memristor
Based on the substoichiometric TiO2-x from WP1, memristors as small as 1.5 x 1.5µm^2 (Fig 3a) were fabricated in three photolithography steps using the cleanroom facilities at IMT Bucharest. The device programming was done using a Keithley parameter analyzer, by applying voltage to top electrode and grounding the bottom one. The memristor has low forming (<3V) and switching voltages (~-1.5V for reset and 1.2 - 2V for set) (Fig 3b) and behaves in a reliable digital fashion with an ON/OFF ratio ~ 1000.

WP3.Single MEMS selector
The MEMS selector is the novel concept proposed, so it was extensively investigated in COMSOL simulations and experimental fabrication. MEMS are typically large (~100µm) and have high actuation voltages (>50V), not desirable for a selector. Lee et al. (2013) showed a nanoelectromechanical switch with low actuation voltage (Vth<1V), but its pipe clip structure is prone to stiction. Our design has a metallic beam supported by pillars separated from a fixed bottom electrode by a small air gap. The switch is electrostatically actuated by applying a voltage between the beam line and the bottom electrode. Two beam geometries– solid vs. H-shaped- were simulated in COMSOL (Fig 4). The H-shaped one showed reduced actuation voltage, but slightly higher stress at the corners. For a memristor-compatible fabrication, metal pillars were first patterned to support the beam line. A bottom electrode was patterned in the gaps between the pillars, then the entire structure was covered with a sacrificial SiO2 layer for temporary support of the beam. To ensure that the beam is fabricated on a flat surface, chemical mechanical polishing was done at EPFL during three secondment trips (September 2016, March and September 2017). Devices with protrusions were fabricated to reduce the contact area. Lastly, the SiO2 was removed. The device was tested in series with an external 90MOhms resistor. The current – voltage characteristics (Fig 5) has a sharp transition at ~3V when the switch turns ON, then the series resistor limits the current in the 100nA range. When the voltage drops <1.5V the beam disconnects and the switch returns to OFF.

WP4.Integrated memristor/MEMS selector
The selector design from WP3 was modified to accommodate the memristor from WP2. After the bottom electrode fabrication, another additional e-beam lithography step is performed to deposit the active layer and top electrode of the memristor device. The integrated device then follows the same steps as the single NEMS selector: SiO2 deposition, planarization, beam patterning and release. The integrated memristor (Fig 6) showed improved non-linearity, but future work will focus on hermetic device encapsulation since the devices were affected by humidity and stiction.

WP5.Crossbar of integrated devices
The results from WP4 were expanded in WP5. With the help of two summer interns, a matrix of such integrated devic
The novelty resides in the integration of memristors with MEMS selectors to prevent the sneak path problem. This work is the first memristor/MEMS integrated structure, combining the benefits of both technologies: non-volatile behavior of TiO2 memristors and high nonlinearity of MEMS switchs. The SelectX goal was achieved by developing a 5 x 5 matrix of integrated devices, thanks to extensive development of a two-terminal low voltage MEMS switch in WP3 and its integration with a memristor in WP4. These results will be expanded to other non-volatile memory technologies, like phase change devices, in a continuing collaboration with EPFL Nanolab.

Given the SelectX results, the Researcher has identified two future projects related to radio frequency non-volatile switches and to artificial neuron implementations and is currently writing proposals based on these ideas.