Thermal Simulation And Key Technologies Of Chemical Mechanical Planarization (CMP) For Phase Change Memory

Phase change memory(PCM)has been considered as one of the best candidates for next-generation nonvolatile memories(NVMs)due to its advantages including low power consumption,fast access time,high endurance,good compatibility with conventional CMOS processes and so on.This thesis focuses on several key problems of phase change memory that need to be solved and obtained following results;1)In order to optimize the device structure,we have simulated reset and set process of phase change process by electric and heat transfer module coupling method.Results show that the size of bottom electrode is the key factor which effects reset process.The smaller the width of bottom electrode,the more easily the reset process happened.The thickness of phase change material is important for set process.Set process happened more easily if the thickness is smaller.According to the simulation result,we offer some new structures to improve the device performance,such as devices with small phase change area,nano electrode dots and hollow bottom electrode and so on.2)Chemical mechanical planarization(CMP)is necessary for the fabrication of PCM device.This work investigates CMP slurry for phase change material using electrochemistry method for the first time.The influences of pH value,contents of oxidant and BTA in the slurry on the CMP process have been studied systematically.Results reveal that slurry with pH value of 10,oxidant content of 3wt%～5wt% and BTA content of 0.1wt% can form valid passive film on the surface of Ge₂Sb₂Te₅ material.This result can help the preparation of CMP slurry for Ge₂Sb₂Te₅.3)Using the optimized slurry,the polish rate selectivity between Ge₂Sb₂Te₅ and SiO₂ and that between SiSb₂Te₃ and SiO₂ is 6;1 and 9;1,respectively.Combined CMP process and other processes such as etching and thin film growth,the cell arrays with damascene structures have been obtained.Results show that the CMP process can remove the remnant phase change materials including Ge₂Sb₂Te₅ and SiSb₂Te₃ on the dielectric material efficiently.Uniform electric properties of array with Ge₂Sb₂Te₅ have been obtained.Small arrays with SiSb₂Te₃ have been fabricated using electron beam lithography and reversible phase change phenomenon has been observed. 4)In order to reduce the cost of PCM,wet-etching process and relative solutions for phase change material have been investigated.We find out hydrochloric acid with oxide is good wet-etching solution for the etching of Ge₂Sb₂Te₅.Using the above etching solution, CRAM cell array has been fabricated and reversible phase change phenomenon has been observed.5)In order to test the electric properties and reliability of phase change memory,we have investigated and built electric testing system.Through the cooperation of modules,we can characterize the electric properties,temperature characteristics and data retention of phase change memory cells.And this system can also be used to characterize other devices such as floating gate memory,MIS structure etc.