The Circuit Design And Implementation Of Phase-Change RAM

Phase-Change memory is a novel super memory.Because of its excellent performance such as low-voltage,low-power,high speed,high density and its compatibility with CMOS process,SIA(Semiconductor Industry Association)considers that it is a most potential candidate of the next generation non-volatile memory and it is possible to replace SRAM,DRAM and FLASH memory.In this dissertation,a series of research work is carried out through the circuit design and implementation of Phase-Change RAM(PCRAM)chip.The major circuit structure of PCRAM chip mainly includes five parts:1R1T cell array,address decoder,logic control,driving circuit and sense amplifier.The key point of this dissertation is to design the driving circuit.It is comprised of bandgap reference,current source,current mirror and clamp circuit.The bandgap reference with high precision leads to several high precision currents and the currents are then used as bias currents in the current mirror.Besides,in order to read out the status of the phase change cell correctly,a clamp circuit is designed.HSPICE simulation result shows that the voltage reference and the current bias both have a high precision with a low temperature coefficient(less than 8ppm/℃)and a high power supply rejection ratio(higher than 50dB).Driving current mirror meets the request of the phase change cell's operational current.Once read operation is put up,the clamp circuit restricts the voltage on the phase change cell below 0.9V effectively.All the design and simulation of the entire circuit and layout is based on the device model of SMIC's 0.18μm standard CMOS logic process,and finally,the 16Kbit experimental PCRAM chip is implemented using the 0.18μm,single-poly, four-layer-metal,logic CMOS process technology with a special GST thin film process technology.The last section of the dissertation deals with the test work of the PCRAM chip briefly, both wafer level and package level.The probing test of wafer level verifies the character of phase change cell,and the test work on packaging units verifies the performance of the CMOS circuit.