Integration of resistive switching devices in crossbar structures (2010)

Conventional CMOS-technology defined by optical lithography will reach its physical limits within the next years together with technologies adopted for data storage. This work presents and combines the alternative concepts of resistively switching devices, usable as nonvolatile memory elements or switches, and nano crossbar architecture, which defer these physical limits sustainably. The nano crossbar architecture consists of a functional component that is integrated between two perpendicularly crossing metallization lines. This configuration allows for a high integration density due to a minimal footprint of 4 F² (F = minimum Feature size). The basic elements are straight metallization lines with excellent scaling capability and fabricated by competitive technologies such as nano imprint lithography. The functional component can be composed of reversibly switching TiO2, which is integrated into metal/ insulator/ metal elements (MIM). This can be operated by corresponding set- and reset- voltages between at least two resistance states, which represent a logic "0" or "1". The state is nonvolatile and can be nondestructively determined by voltages below these programming values. The field of application includes memory matrices, which are also named passive ReRAM (Resistive Random Access Memory), elements of the DRL (Diode-Resistor Logic) and RTL (Resistor-Transistor Logic), as well as router and multiplexer. Because of their passive properties, an active control circuitry, which is currently based upon CMOS, is necessary. For this reason, all materials and fabrication technologies are CMOS compatible. The developed and optimized lift-off metallization in combination with electron beam direct writing is a flexible method to fabricate metallization lines with different metals and with a width of 50 nm. The fabricated devices comprise crossbar arrays with a size of 64 × 64 bit and a 30 nm thermally evaporated electrode of a Pt/ Ti double layer. These were examined in terms of ballistic charge transfer mechanisms, since the dimensions of the conductor were in the range of the electron mean free path. The experimental results could be explained by the models of Fuchs-Sondheimer and Mayadas-Shatzkes. Finally, the metal lines offered a high yield and a good scalability with low resistances per unit length. The TiO2 thin film was reactively sputtered or deposited by ALD (Atomic Layer Deposition). Subsequently, the electrical transfer from the insulating to the switching state, also called electroforming, was examined in detail and allowed for a reliable bipolar switching. The required operating voltages and currents of 100 × 100 nm² large cells are 2 V and several 100 µA, respectively, which are appropriate values for a combination with CMOS technology. Additionally, the gained resistance ratio of more than 100 offers a good detection of the stored information. This information is nonvolatile without any degradation for more than 10^5 s, also at elevated temperatures of 85°C. The switching speed was measured by short voltage pulses and is less than 10 ns. At the same time, it is possible to vary the value of the HRS (High Resistive State) depending on the pulse length and amplitude. The multitude of achievable states enables the development of a multi-bit storage element that increases the storage density without an increase of the technological complexity. The interaction of adjacent functional elements was examined by programming of a set of neighboring junctions inside of an array. The subsequent readout of these elements showed no mutual influence for distances above 200 nm and the switching characteristic was consistent with that one of single elements. Each junction that was not electroformed inhibited parasitic currents in the bypasses of an array. The number of programmable bits is thus limited due to the high electroforming voltages of the examined material system. The combination of both concepts shows the high potential of resistively switching elements in nano crossbar architecture. However, the operation of a complete array with the combination of Pt/TiO2/Ti/Pt has to be optimized in respect of the electroforming. Nevertheless, this material system offers a high potential for future ReRAM applications, first of all in combination with a select transistor.