Cell Design Considerations for Ovonic Threshold Switch-Based 3-D Cross-Point Array

Abstract

This study presents cell design parameters to be considered for a two-terminal ovonic threshold switch (OTS) and phase-change memory (PCM)-based array, with an example of 3-D cross-point array (XPA). The 1-Mb 3-D XPA in this study was simulated using MATLAB. The array characteristics were analyzed using the Monte–Carlo simulation for the variability of OTS characteristics. We observed that the OTS threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{th}}$ </tex-math></inline-formula> ) variation further accelerates the IR drop, increasing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{th}}$ </tex-math></inline-formula> of cells in real XPAs. This further reduces the read window margin (RWM) and inhibit-fail margin (IFM), creating constraints on 1-Mb normal operation. The IR drop is more significant at RESET (RST) than the selected cell is at SET. Furthermore, the higher the RST resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{\text{rst}}$ </tex-math></inline-formula> ), the more severe it is, increasing the inhibit bias and worsening inhibit fails. Based on the simulation results, the raw bit error rate (RBER) of the 3-D XPA device can be minimized by balancing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{\text{rst}}$ </tex-math></inline-formula> of the PCM to minimize the error bits between failures from insufficient RWMs and inhibit fails, as the two errors are directly related to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{\text{rst}}$ </tex-math></inline-formula> . Finally, we show that the 3-D XPA device can be fabricated when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{\text{rst}}$ </tex-math></inline-formula> of the memory material is sufficiently large to minimize failures from insufficient RWM, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{th}}$ </tex-math></inline-formula> (OTS <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{th}}$ </tex-math></inline-formula> ) of the selector material is adjustable to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{\text{rst}}$ </tex-math></inline-formula> to minimize inhibit fails.