R. Dasaka

A low power phase change memory using thermally confined TaN/TiN bottom electrode

Application of phase change memory (PCM) has been limited by the high power required to reset the device (changing from crystalline to amorphous state by melting the phase change material). Utilizing the poor thermal and electrical conductivity of TaN we have designed a simple structure that thermally insulates the bottom electrode and thus drastically reduces the heat loss. A 39nm bottom electrode with a TaN thermal barrier and 1.5nm of TiN conductor has demonstrated 30µA reset current, representing a 90% reduction. The benefit of thermal insulation is understood through electrothermal simulation, and the benefit is demonstrated in a 256Mb test chip. The low reset current also improves the reliability and excellent cycling endurance >1E9 is observed. This low power device is promising for expanding the application for PCM.

On the dynamic resistance and reliability of phase change memory

A novel characterization metric for phase change memory based on the measured cell resistance during RESET programming is introduced. We show that this dasiadynamic resistancepsila (Rd) is inversely related to the programming current (I), as Rd = [A/I] + B. While the slope parameter A depends only on the intrinsic properties of the phase change material, the intercept B also depends on the effective physical dimensions of the memory element. We demonstrate that these two parameters provide characterization and insight into the degradation mechanisms of memory cells during operation.

Understanding amorphous states of phase-change memory using Frenkel-Poole model

A method based on Frenkel-Poole emission is proposed to model the amorphous state (high resistance state) in mushroom-type phase-change memory devices. The model provides unique insights to probe the device after amorphizing (RESET) operation. Even when the resistance appears the same under different RESET conditions, our model suggests that both the amorphous region size and the defect states are different. With this powerful new tool, detailed changes inside the amorphous GST for MLC operation and retention tests are revealed.

Mechanisms of retention loss in Ge 2 Sb 2 Te 5 -based Phase-Change Memory

Data retention loss from the amorphous (RESET) state over time in Phase-Change Memory cells is associated with spontaneous crystallization. In this paper, the change in the threshold voltage (VT) of memory cells in the RESET state before and after heating is used as a probe into the nature of the retention loss mechanisms. Two mechanisms for the retention loss behavior are identified, responsible for the main distribution and the tail distribution, respectively. Experimental results suggest that (i) an optimized RESET operation produces a fully amorphized Ge2Sb2Te5 (aGST) active region, with no crystalline domains inside, (ii) cells in the tail distribution fail to retain their RESET state due to spontaneous generation of crystallization nuclei and grain growth, and (iii) cells in the main distribution fail due to grain growth from the amorphous/crystalline GST boundary, instead of nucleation within the active region.

Dynamic Resistance—A Metric for Variability Characterization of Phase-Change Memory

The resistance of phase-change-memory (PCM) cells measured during RESET programming (dynamic resistance, Rd) is found to be inversely proportional to the amplitude of the programming current, as Rd = [A/I] + B. We show that parameters A and B are related to the intrinsic properties of the memory cell, and demonstrate by means of experimental data that they could be used to characterize the cell-to-cell process-induced variability of PCM cells.

Characterizing the effects of etch-induced material modification on the crystallization properties of nitrogen doped Ge2Sb2Te5

The chemical and structural effects of processing on the crystallization of nitrogen doped Ge{sub 2}Sb{sub 2}Te{sub 5} is examined via x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), time resolved laser reflectivity, and time resolved x-ray diffraction (XRD). Time resolved laser reflectivity and XRD show that exposure to various etch and ash chemistries significantly reduces the crystallization speed while the transition temperature from the rocksalt to the hexagonal phase is increased. XPS and XAS attribute this to the selective removal and oxidization of N, Ge, Sb, and Te, thus altering the local bonding environment to the detriment of device performance.

Etch planarization - A new approach to correct non-uniformity post chemical mechanical polishing

The introduction of 3D devices and new materials at sub 28 nm nodes presents challenges for within-wafer and wafer-to-wafer CMP thickness uniformity control that are critical for device yield and performance. Upon CMP the typical thin film uniformity across the whole wafer is unable to meet the target of less than 2 nm 3σ variation. Furthermore, wafer-to-wafer uniformity variation requires a wafer by wafer approach to uniformity correction. In this work, a novel etch planarization approach is presented that combines a conventional production-proven etch process that is temperature sensitive on an inductively coupled plasma reactor with die level thermal controlled electrostatic chuck (ESC). Improved process control enables cost effective uniformity improvements in excess of 85%. In addition, the approach provides wafer-to-wafer tuning capabilities.

Integrated metrology's role in Gas Cluster Ion Beam etch

Shrinking process windows for advanced processing of complex devices, sub-14nm, require advanced topography control. Within-wafer topography variations impact the uniformity of subsequent layers and can affect yield. Process tools can generally control global uniformity across wafer, but are not well equipped to fine-tune the local (reticle-to-reticle) topography. Advanced new process tools such as the Gas Cluster Ion Beam (GCIB) offer a promising path for improving topography within wafer and from wafer-to-wafer [1, 2]. GCIB uses a highly localized focus beam controlled by location specific processing (LSP) algorithms to achieve the needed planarization corrections. In order to be effective, the LSP algorithms require wafer-specific knowledge on the incoming topography distribution. The beam dwells in different locations on the wafer for different amounts of time in order to vary the amount of material removed. Therefore, the performance of local planarization tools depends on the availability and quality of the metrology data. Integrated scatterometry-based metrology (IM) is the workhorse metrology enabler for inline APC (Advanced Process Control) and monitoring solutions for the Chemical Mechanical Planarization (CMP) and Reactive Ion Etching (RIE) processes. While maintaining equivalent performance to their standalone (SA) scatterometry siblings, IMs are typically mounted on the GCIB tool and dedicated to provide pre and post-process measurements for wafers processed on the tool. They enable real-time per-wafer adjustments to within-lot process knobs due to their proximity to the process.

Influence of Bottom Contact Material on the Selective Chemical Vapor Deposition of Crystalline GeSbTe Alloys

Selective Chemical Vapor Deposition of Crystalline Ge-Sb-Te alloys initiating at the bottom metal contact of vias of various sizes has been accomplished. The method is based on selecting Sb and Te precursors which do not decompose on dielectric surfaces in the utilized temperature range.