Thermal Analysis and Structural Design of Phase Change Random Access Memory
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0918-H05-02-G06-02
Thermal Analysis and Structural Design of Phase Change Random Access Memory Rong Zhao1, Ler Ming Lim2, Luping Shi1, Hock Koon Lee1, Hongxin Yang1, and Tow Chong Chong1 1 Optical Materials & Systems Division, Data Storage Institute, DSI Building, 5 Engineering Drive 1 (Off Kent ridge Crescent, NUS), S117608, Singapore 2 Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, S119260, Singapore INTRODUCTION Phase change random access memory (PCRAM) is considered the best candidate as the next generation of non-volatile memory (NVM) to replace current NVM technology. It has almost ideal properties as a NVM: fast accessing time, low power consumption, low cost, high endurance and good data retention [1]. Moreover, PCRAM has a high scalability and more importantly, its performance increases as it is being scaled down. This advantage makes PCRAM unique from other emerging NVM and has the most potential to replace FLASH and all current NVM. PCRAM uses a phase change material which changes from crystalline to amorphous states on the application of a short electrical pulse. This short pulse causes the heating up of the phase change material to its melting temperature (Tm) and followed by a fast quenching. On the other hand, changing from amorphous to crystalline (SET) states requires a smaller but longer electrical pulse to reach its crystallization temperature (Tc). The basis of switching in PCRAM is done through Joule heating and thus understanding of how the temperature distributes within a PCRAM cell is very important. In this paper, thermal modeling based on FEM was used to simulate PCRAM cell structures to understand the temperature distribution within. THERMAL MODELING In this thermal model, the electrical properties of the materials were assumed to be isotropically homogeneous, and both the thermal and electrical properties were assumed to be independent of temperature. Heat source is the phase change layer. The thermal transfer process obeys the standard heat conduction equation: ∇ ⋅ k∇T + Q = ρc
∂T ∂t
(1)
where ∇ is the gradient operator, k, the thermal conductivity, T, the temperature, t, the time, c, the specific heat, ρ, the density, and Q, the heat density. It can be further simplified into a static magnetic analysis since the voltage applied to the electrodes remained unchanged. In the static field analysis, the Joule heating density distribution can be described as the following:
1 n (2) ∑ [σ ]{J i }{J i } n i =1 where n is the number of integration points, [σ] is the resistivity matrix and {Ji} is the total current density in the element at integration point, i. Q=
The local heating power of the structure can be defined by the equation (3) below. P=
V2 R
(3)
where P is the power, V is the voltage across the structure and R is the resistance of the structure. The resistance of the material is defined as: R=
σ .l
(4)
A
where R is the resistance of the material, σ is the resistivity of the material, l is the length of the mate
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