Improvement of RRAM Device Performance Through On-Chip Resistors
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Improvement of RRAM Device Performance Through On-Chip Resistors Siddharth Gaba, Shinhyun Choi, Patrick Sheridan, Ting Chang, Yuchao Yang and Wei Lu Electrical Engineering and Computer Science, University of Michigan, Ann Arbor ABSTRACT Research in non-volatile memories (NVM) has intensified in the past few years due to the ever increasing demand for information storage and the near ubiquity of handheld electronics. Resistive memory is a leading contender in this NVM market due to its high endurance, random accessibility, scalability and low programming voltage. The addition of an external series resistor or imposing current compliance is often used to limit the current through RRAM devices and to prevent “over-programming” and stuck-at-one (SA1) errors. Here, we demonstrate that utilizing an external series resistor is not efficient in preventing over-programming and an on-chip resistor is more desirable. Poly-silicon bottom electrode based devices (with the poly-silicon electrode acting like an on-chip resistor) and metal bottom electrode devices were fabricated and tested. The presence of the on-chip resistor is shown to enhance the endurance of the RRAM device. This technique of including an on-chip resistor prevents stored current discharge through the device as the device transitions from a high resistance to a low resistance state. A SPICE simulation is also employed to illustrate the benefit of this approach. INTRODUCTION Resistive random access memory (RRAM) has attracted much attention for use in high density non-volatile memory applications. Many research groups have demonstrated RRAM devices that are competitive to the current mainstay of the non-volatile memory market – FLASH [1] [2]. However, several issues remain (such as a clear understanding of the switching mechanism, the optimization of the resistive switching (RS) materials, improvement in yields, and improvement of device reliability) in order for RRAM to transition to the commercial arena. In this letter, we study the effect of the transient currents on the endurance of the device and why the use of off-chip resistors is inefficient and why on-chip resistors are needed. Devices with different bottom electrodes (BEs) – poly-silicon vs. metal- but with otherwise identical structures are fabricated and electrically tested. Endurance numbers for both types of devices are compared and a possible explanation for the observed differences is proposed. Further, we use a SPICE simulation to corroborate our theory. (SPICE model of the RRAM device is based on custom code as described in an earlier publication [3].) EXPERIMENT Device Fabrication The memory devices studied here are based on a cross-point structure, as illustrated in Fig. 1a, with a top electrode(100nm silver) and a bottom electrode (either 70nm p+ poly-silicon, resulting in an on-chip series-resistance of ~ 50 kΩ, Case A or 70nm tungsten, Case B) sandwiching an insulating layer of amorphous silicon. All devices are fabricated on a silicon substrate with
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100nm thermal oxide using elect
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