Scalable 3-D vertical chain-cell-type phase-change memory with 4F2 poly-Si diodes

Abstract

A three-dimensional (3-D) vertical chain-cell-type phase-change memory (VCCPCM) for next-generation large-capacity storage was developed. The VCCPCM features formation of memory holes in multi-layered stacked gates by using a single mask and a memory array without a selection transistor. As a result of this configuration, the number of process steps for fabricating the VCCPCM is reduced. The excellent scalability of the VCCPCM's new phase-change material makes it possible to reduce the cell size beyond the scaling limit of flash memory. In addition, a poly-silicon selection diode makes it possible to reduce the cell factor to 4F 2 . Consequently, relative cost of the VCCPCM compared to 3-D flash memory is reduced to 0.2. IntroductionThe most important requirement for the storage-memory market is reduction of bit cost, and that requirement has been met by reducing the cell size of flash memory. However, high-voltage operation of flash memory makes it difficult to further reduce cell size. It has recently been reported that the bit-cost reduction can be continued by utilizing 3-D flash memory [1]. 3-D flash memory needs fewer process steps compared to simple stacking of flash memory, but reducing cell size is difficult for two reasons. Firstly, a 20-nm-thick ONO layer in the memory hole is needed and, secondly, a vertical poly-silicon selection MOS transistor needs a cell factor of 6F 2 [1]. In this work, a vertical chain-cell-type phase-change memory (VCCPCM), which can overcome these problems concerning 3-D flash in view of bit cost, is proposed. The key technologies of this VCCPCM are (1) a vertical chain cell for reducing the number of process steps, (2) a scalable new phase-change material for reducing cell size, and (3) a poly-Si XY-selection diode for reducing cell factor to 4F 2 . A poly-Si diode [2] and a lateral chain-cell-type PCM [3] were previously developed. Relative bit cost of both 3-D flash memory and VCCPCM is shown in Fig. 1. By virtue of technologies (1) to (3), the relative bit cost of the VCCPCM compared to 3-D flash memory is reduced to 0.2. Table 1 compares characteristics of 3-D flash memory and VCCPCM. In the present study, set, reset, and reading operations of the VCCPCM were confirmed. Moreover, off-current variation of the poly-Si diode was suppressed by short-time annealing.2. Device structure and operation method The structure of the VCCPCM is shown in Fig. 2. The poly-Si selection diode and VCCPCM are connected serially and positioned at the cross points between the bit and word lines. The structure and equivalent circuit of a VCCPCM are shown in Fig. 3. The gate oxide, channel poly-silicon, and the phase-change material are formed on the side of the holes in the stacked gates. Each memory cell consists of a poly-silicon transistor and a phase-change layer connected in parallel. The memory cells are connected serially in the vertical direction. In the set/reset operations, an off-voltage is applied to the gate at the selected cell, and a positive on-voltage is applied to the unselect...

A Review of Three‐Dimensional Resistive Switching Cross‐Bar Array Memories from the Integration and Materials Property Points of View

“…Phase changing (sometimes threshold switching or even resistance switching) chalcogenides, such as Ge‐Sb‐Te compounds, and elemental metals used as electrode materials, such Ru, are one of the examples. Although the Ge‐Sb‐Te compounds are the primary candidate for the phase change memory, it has been adopted in vertical RRAM‐type devices recently utilizing a NAND‐like chain architecture (vertical chain‐cell‐type phase change memory (VCCOCM)) . While several previous reports mostly adopted the CVD‐like behaviors of metal‐organic precursors, a genuine ALD of Ge 2 Sb 2 Te 5 was reported only recently by Pore et al, which has been hindered by the lack of an appropriate chemical‐reaction route for the ALD‐type reaction.…”

Phase-Change and Redox-Based Resistive Switching Memories

“…The major problem adapting such process for PCM cells is the difficulty to integrate the required selector element inside this 3-D structure. A combination of a poly-Si-based vertical chain cell with an x-y poly-Si selection diode was proposed to build a true 3-D PCM [38]; see Fig. 9.…”

“…Three-dimensional PCM scheme based on vertical poly-Si transistor chain cell (reproduced with permission from[38]). …”

Memory Select Devices

“…Both single-crystal Si [21] and poly-Si diodes [22][23][24] have been developed as a select device for PCM arrays. To operate PCM with diode selectors, diodes need to provide ON-current density above 8 MA/cm 2 and OFF-current density below 100 A/cm 2 .…”

“…It is believed that Si diode can be scaled beyond 20 or 10 nm. Poly-Si diode select devices have been integrated in PCM crossbar arrays [22], 3D vertical chain-cell type PCM [23], and a 1 Gb PCM test chip [24]. A major challenge of Si diodes is the high processing temperature (above 1000°C) required to crystallize Si to reduce contact resistivity and OFF-current.…”