Successful suppression of dielectric relaxation inherent to high-k NAND from both architecture and material points of view

Abstract

High-k materials, such as HfO 2 and Al 2 O 3 , are known to have dielectric relaxation effect (i.e. slow polarization) [1] [2]. It is reported for the first time in this work that Al 2 O 3 , used as a blocking layer of MANOS NAND flash memory cells, causes modulation of channel current through its dielectric relaxation, resulting in severe transient threshold voltage shift as much as ~ 0.8V. This V th drift cannot be controlled by bit-by-bit verify method, and will severely deteriorate multi-level functionality of NAND flash memory cells. In this work we propose two solutions for this issue. The first one is to give appropriate pre-bias to the word lines of a NAND string before a reading pulse sequence so that the V th drift due to dielectric relaxation can be compensated. The second one is to implement an Al 2 O 3 /SiO 2 /Al 2 O 3 (AOA) stacked blocking layer ( Fig.1(b)) instead of an Al 2 O 3 single layer ( Fig.1(a)). With these solutions, the transient V th drift due to dielectric relaxation can be eliminated entirely. IntroductionMONOS-type NAND flash memory is one of the candidates for future storage systems because of its low cell-to-cell interference [3] and good applicability to 3-dimensional stacked memories [4] [5]. It is common that high-k material such as Al 2 O 3 is used as a blocking layer for suppression of leakage current. However, high-k materials causes dielectric relaxation (slow polarization), and its influence on memory cell characteristics has not been investigated yet. In this work, we evaluate the impact of slow polarization on the V th read-out in a NAND string through measurements of transient drain current in MANOS cells. We also propose solutions to this issue from both architecture and material points of view. ExperimentalIn a NAND string, threshold voltages of each cell are read sequentially by applying a read voltage (V read ) while pass voltages (V pass ) are applied to all the other cells in the same string. Thus, the number of pass-voltage pulses before the read-out event is different from one cell to another. As a good approximation, the state of each cell in a NAND string can be monitored with a single memory cell by applying V pass pulses and measuring drain current as a function of pulse number (Fig.2). Since the reading events repeat in ~10μs periods, high speed I/V converters are connected to the source/drain [6] of a single MONOS transistor, and the source/drain current during gate-pulse application are recorded with an oscilloscope, as shown in Fig.3. Typical samples in the measurement are TaN-gate MANOS transistors with film thicknesses of SiO 2 (4nm)/Si 3 N 4 (5nm)/Al 2 O 3 (15nm), as listed in Table.1. Transient Drain Current in MANOS Transistors due toDielectric Relaxation Transient drain-current characteristics of MANOS memory cells are measured after a step voltage (V meas ) is applied, as shown in Fig.4. This measurement is equivalent to application of 100% duty-ratio pulses, and the maximum impact of dielectric relaxation upon V th drift can be estimated. Fig.5(...

Charge Trapping and Reliability Properties of MONOS Memory with High-k Blocking Layer

“…20(a)) are influenced through the change in EOT. We have observed transient behavior in the channel current of MONOS transistors with a high-k (Al 2 O 3 ) blocking layer [42]. In Fig.…”

Charge-Trapping Non-Volatile Memories

“…Most commonly used high-K dielectric has been the Al 2 O 3 owing to its promise of high band offset (conduction band offset to Si~2.8 eV) and reasonably high dielectric constant (Fujiki et al 2009). In practice, however, the high conduction band offset has been quite elusive.…”

Electron trapping in amorphous Al2O3

“…22 Also, a dielectric relaxation due to a slow polarization of the Al 2 O 3 layer was reported. 23 Additionally, PDA at a high temperature generates a trap-rich transition region between the SiO 2 tunnel layer and the Si 3 N 4 trapping layer. 24 Thus, it is not clear yet whether Al 2 O 3 layer (as-deposited amorphous or crystallized) has better electric properties.…”