Design Of A High Precision Current Source For Phase Change Memory Measurement

Phase change memory(PCM), which is based on chalcogenide alloy,is one of emerging nonvolatile memories.It makes use of the Joule’s heat induced by electrical pulses to achieve information storage in phase change material which can reversibly change between the polycrystalline state and amorphous state.Therefore,the pulse current source is a key component which is used to achieve information storage in phase change memory.In this thesis,we use two methods to design precision current sources for phase change memory measurement.According to the principle of the phase change memory and it’s requirement of testing current source,by using modular design method and integrated chips,we design a pulse current source with the core chips of CPLD and the precision programming delay chip, the width of the current source is nanosecond adjustable,and the CPLD is used as a logic control unit.It’s amplitude is controlled by a D/A converter, The current is driven by the laser diode driver chip EL6257which generates current source used for PCM write/erase and read operation.The amplitude and width of pulse current source are controlled by the host computer.According to the measurement requirement of PCM with high speed and low power consumption,a precision, low power current mirror with MOS transistors is analyzed and its outstanding performance has been verified by Pspice simulation in TSMC0.18μm CMOS, BSIM3, Leve149technology.The simulation results show that the circuit can generate highly precise stable current with large broadband,which is suitable for phase change memory measurement.Based on the current source’s specific application, this thesis presents a measurement method for transient programming current of PCM-cell.By utilizing the equivalent circuit model of the measurement circuit,we analyze the factors that affect the measurement.In the end,the transient programming current and R-I property of PCM-cell have been measured. The results show that the width and amplitude of the precision can be precisely adjusted,which lays foundation for phase change memory measurement.