Evaluation of high voltage generator MOSFET with green laser annealing for M3D based Peri-on-Cell flash memory

Abstract

Due to the recent development of the 4th Industrial Revolution, the demand for memory semiconductors is rapidly increasing. In the case of flash memory, continuous bit capacity scaling could be achieved with a paradigm shift in the structure from a two-dimensional planar structure to a three-dimensional stack. However, there must be structural innovation for continuous integration, such as the problem of interconnection with peripheral circuits due to continuous stacking and the problem of deterioration of peripheral circuit performance during the process. To resolve this, this thesis newly proposes a peri-on-cell structure in which a peripheral circuit is placed on a memory cell and implements a device used in a charge pumping circuit in the peripheral circuit with a monolithic 3D integrated structure. Peri-on-cell memory with monolithic 3D integrated structure is expected to bring great innovation in terms of input/output speed and bit integration compared to existing memory structure. However, to form a peripheral circuit over the cell array, high temperature heat treatment is crucially required. Therefore, in this thesis, the top tier device was selectively heated using a green laser. MOSFET gate stack was designed to endure laser annealing process using a recrystallized polysilicon gate and an atomic layer deposition silicon oxide film. A device fabricated by green laser has improved electrical characteristics compared to RTA process, and this is because the pn junction characteristics are improved due to the laser nanosecond annealing method. In addition, a high voltage generator circuit which is suitable for the peri-on-cell structure of the monolithic 3D structure was designed. Due to applying the mesa-isolation method and the gate-drain-body junction diode structure, body-effect of the MOSFET and pn junction breakdown were suppressed by applying proper body bias at each stage. Finally, with applying the MOSFET device characteristic to the charge pumping circuit simulation, the voltage required for the currently commercialized flash memory could be achieved.