Finite Element Simulation And Analysis On Read And Write Performance Of Phase Change Memory Cell

As a strong candidate for emerging non-volatile solid-state memory technology,phase-change memory(PCM)has recently been attracting extensive attention by virtue of its characteristics of good read/write performance,good endurance,promising scalability,and capability of performing complicated multifunction.At present,PCM has received great researches,but still remain many problems,such as high wirting ‘0' current and long wirting ‘1' time.PCM cell operations rely on thermally induced phase transitions between conductive crystalline and highly resistive amorphous states of phase-change material,and the temperature profiles are the key of the read/write performance researches.This paper mainly studies the read/write performance of PCM cell based on the temperature profiles and the corresponding phase distributions through a three-dimensional finite element method simulation of PCM cell,and explores the influence of structure and material parameters,thermoelectric effects and pulse parameters in order to decrease the wirting ‘0'current and wirting ‘1' time,and then promotes the read/write performance of PCM cell.First,three-dimensional finite element method simulation incorporating electrical,thermal and phase change models is carried out under the consideration of thermoelectric effects,the read/write operation of PCM cell and the influence of pulse polarity on thermoelectric effects are studied based on the temperature profiles and the corresponding phase distributions.Second,the influence of structure and material parameters on the read/write performance of PCM cell is studied by simulation,which focus on the change of thermoelectric effects during the write operation.Finally,the influence of pulse shape on the writing “1” operation and the feasibility of multilevel storage by the 2-step pulse are studied.In summary,(1)The three-dimensional finite element method simulation incorporating electrical,thermal and phase change models is carried out under the consideration of thermoelectric effects,the electrical and thermal and phase change models use different grids,the coefficient matrix is stored by direct method and the solution of equations is based on the over-relaxed asynchronous iteration,and then the simulation is more close to the actual cell,the simulation precision is higher and the simulation time is shorter.(2)The wirte operation of PCM cell is related closely with the cooling rate of GST close to the heater.Under the positive polarity pulse,thermoelectric effects can improve the heating efficiency and then decrease the programming current and the energy consumption,the Peltier effect is more influential.(3)The decrease of the heater size and isotropic scaling of PCM cell can decrease the programming current and the energy consumption,but also can enhance the thermal diffusion which induces the weaken of thermoelectric effects,the weekend of Peltier effect is more influential,and the Thomson effect remains unchanged during the writing ‘0'operation.The decrease of the PCM cell size can decrease the wirting ‘1' time,and the isotropic scaling is more influential,the shortest pulse width is deceased by58%.(4)For the PCM cell with different size,the electrical resistivity increase and thermal conductivity decrease of phase change material and heater can decrease the wirting ‘0'current.The decrease of thermal conductivity can reduce thermal diffusion and then enhance thermoelectric effects.The electrical resistivity increase of phase change material reduces thermoelectric effects,while the electrical resistivity increase of heater reduces thermoelectric effects only when the heater size decreases.Increasing the difference of Seebeck coefficients and the temperature dependence of GST material Seebeck coefficients can enhance the thermoelectric effects.(5)The 2-step pulse including high-amplitude and low-amplitude pulses can decrease the temperature difference in PCM cell and then reduce the wirting ‘1' time and the energy consumption by 33% and 19%.The decrease of the heater size reduces the influence of the2-step pulse,and the isotropic scaling of PCM cell enhances the influence.The resistance distribution of multilevel operations by the 2-step pulse are more even,which more suitable for multilevel storage.