Research Of High Speed Testing Methods For Phase Change Memory Cells

Phase change memory is a novel nonvolatile memory, it has a good scalability, whichbenefits for high capacity memory chips. Besides, its working speed of phase changememory is higher than the FLASH memory more than10times. As we known, the workingspeed is a key parameter of a memory device, the SET/RESET pulse width can be extremelyshort, the duration of the SET/RESET pulse can be even reduced to nanosecond, especiallythe RESET pulse can be shorter than1ns. Therefore, the research on the high speedoperation of phase change memory cells benefits for widening the application of phasechange memory.This paper is focused on the amorphization of phase change memory cell in RESEToperation using picoseconds electric pulse. The bandwidth and signal integrity issues in highfrequency testing were discussed, a high speed program testing circuit was designed. Phasechange memory cell samples with different thickness of phase change layer of50nm,100nm,150nm were characterized in the testing circuit. The shortest RESET pulse which caninduce the repeatable amorphization in a phase change memory cell was only200ps long. Atesting system suitable for the cell arrays and high frequency signal transmission wasdeveloped, the hardware of the testing system consisted of the semiconductorcharacterization instrument, picoseconds pulse generator, digital oscilloscope, encodingmodule, decoding module, drive module, monostabillity trig delay circuit and high speedsignal channel. The testing program code was written in C language, which runs on thebuilt-in computer of the semiconductor characterization equipment. The testing system wasconnected with GPIB cable.This paper also did transient simulation to the high speed testing circuit using SPICEmodel, and the high speed testing schema was verified to have good performance. Theamorphization of phase change memory cell in RESET operation using picoseconds electricpulse was studied, the phenomenon of amorphization is thought to be independent fromjoule heats. A model of the local intensive electric field induces impact ionization within thephase change material is given, which explains the phenomenon of amorphization in phasechange material with good reasonability.