Suspended Gate Silicon Nanodot Memory

Abstract

The non-volatile memory market has been driven by Flash memory since its invention more than three decades ago. Today, this non-volatile memory is used in a wide variety of devices and systems from pen drives, mp3 players to cars, planes and satellites. However,the conventional floating gate memory technology in use for flash memory is facing a serious scalability issue, the tunnel oxide thickness cannot be reduced to less than 7nm as pointed out in the latest international technology roadmap for semiconductors (ITRS2010) [1]. The limit imposed on the tunnel oxide layer reduces the programming and erasing times, the scalability and endurance among other parameters. To overcome those inherent issues, this research is focused on the co-integration of nano-electromechanical systems (NEMS) with metal-oxide-semiconductor (MOS) technology in order to generate a new non-volatile and high speed memory. The memory device that we are proposing is a high-speed non-volatile memory structure called the Suspended Gate Silicon Nanodot Memory (SGSNM) cell. This non-volatile memory device features a MOSFET as a readout element, a silicon nanodot (SiNDs) monolayer as the floating gate and a movable suspended control gate isolated from the floating gate by an oxide layer and by an air-gap. The fundamental component in this novel device is the introduction of a doubly-clamped beam as a movable control gate, in which through this element, the programming and erasing operations take place. To understand the behaviour of the doubly-clamped beam structure, it is analysed by using analytical models such as the doubly-plate capacitor model and also by using two- and three-dimensional (2D and 3D) finite element method (FEM) analysis. The programming and erasing operations within the SGSNM occur when the suspended control gate is in contact with the tunnel oxide layer. This is the point at which the quantum-mechanical tunnelling mechanism (Fowler-Nordheim) takes place. Through this mechanism, the electrons are allowed to tunnel from the suspended control gate into the memory node and vice versa as a function of the applied voltage (bias). The tunnelling process is numerically analysed by implementing the Tsu-Esaki equation and the transfer matrix method within a homemade program which calculates the current density as a function of the tunnel oxide material and thickness. Both the suspended control gate and tunnelling process are implemented as analog behavioural models within the SGSNM cell that is simulated by using a commercial circuit simulator. From a transient analysis of the suspended control gate, it was found that the suspended control gate takes 0.8 nsec in pull-in on the tunnel oxide layer for a 1 ?m-long doubly-clamped structure. In contrast, the time that the memory node takes in charge and discharge is 1.7 nsec. Hence, the programming and erasing times are a combination between the mechanical pull-in and the charging time, which is 2.5 nsec due the fact that to both operations are symmetrical. Moreover, the suspended control gate was successfully fabricated and suspended. This process was performed by depositing a thin layer of aluminium (500 nm) over the sacrificial layer (poly-Si) by using an e-beam evaporator, which was patterned with doubly-clamped beam features through the photolithographic process. By using a combination of wet and dry etching processes, the aluminium and the sacrificial layer were successfully removed without affecting the substrate (Si-based) or the suspended control gate beam. In addition, Capacitance - Voltage measurements were performed on a set of doubly-clamped beams from which the pull-in effect was successfully obtained. Finally, the footprints for the memory device fabrication process were developed and sketched within the document as well as the design of three photomasks

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