A Mixed-Signal Interface Circuit for Integration of Embedded 1T1R RRAM Arrays

The system integration of Resistive Random Access Memory (RRAM) as embedded memory is a key issue to enable the usage of this emerging technology. The integration of the individual memory cells in CMOS circuits demands analog operations to realize forming, programming and reading of the individual RRAM cells. The presented circuit bridges this gap between technological research and system design with a circuit capable of these operations while offering a fully-digital interface to allow easy system integration. This enables the usage of RRAM technology as embedded memory without extensive analog knowledge on transistor level.The presented circuit was designed, simulated and layouted using IHP’s MEMRES Module, but can be adapted to other RRAM technologies by changing just a few circuit parameters. Due to its flexibility, it can mitigate the technologies’ intrinsic variability and offers a wide degree of adaptability for different application scenarios. The presented read concept is able to resolve four states with an overall resistance ratio of 3.7. The programming mode enables multi-level set and reset operations with fully-digital control and complete CMOS integration.

I. Introduction

Due to its scalability, non-volatility and multi-level capability, Resistive Random Access Memory (RRAM) is a promising emerging technology for embedded memory applications. It’s scalability allows the integration into different technology nodes, the non-volatility can be taken advantage of in low-power applications and the multi-level capability enhances memory density. Further research is done to improve these properties to further develop RRAM technology [1]–[4]. But besides the technological integration, concepts and circuits for integration on system level are necessary to enable the utilization of these well suited properties. This includes handling of analog forming, read and programming operations as well as a complete digitization and possibility for automation, to reliably embed RRAM in sophisticated systems. There exists a wide variety of different RRAM technologies with diverse structure and electrical behavior [5] [6]. Therefore, a general concept for RRAM integration must be suitable for different technologies, which have different circuit demands due to a variety of resistance ratios, high resistance state (HRS) and low resistance state (LRS) resistances, read and programming voltages or forming methods. Nevertheless, the one-transistor one-resistor configuration (1T1R) is widely common for embedded RRAM memory designs, due to the flexibility, the selection transistor provides. The presented mixed-signal circuit offers a fully digital interface, that enables system designers to utilize the key advantages of 1T1R RRAM technology, while performing the analog forming, programming and read operations internally. By tuning a small amount of components, the circuit can be adapted to other technologies with different electrical properties.