Change of Damage Mechanism by the Frequency of Applied Pulsed DC in the Ge[sub 2]Sb[sub 2]Te[sub 5] Line

Abstract

We investigated the damage on the Ge 2 Sb 2 Te 5 line structure by pulsed-dc stressing with various frequencies. The line immediately burnt out due to Joule heating under constant dc stress 2.5 MA/cm 2 . However, when pulsed dc 2.5 MA/cm 2 was stressed at the frequency of 5 MHz, failure due to thermal fatigue damage was observed. At higher frequency such as 10 MHz, no noticeable damage was observed, yet the compositional change in constitutive elements by electromigration was detected. The change of damage mechanism by varying of frequency is explained by the difference in thermal cycling extent in response to the pulsedcurrent operation at various frequencies, which is computed using a finite-difference method.Phase-change random access memory ͑PRAM͒ is one of the most promising candidates for the next generation of nonvolatile electrical memory due to its high scalability, high density, high endurance, fast operation speed, and low power consumption. Ge 2 Sb 2 Te 5 ͑GST͒ is most widely employed for phase-change materials in PRAM cells, and the phase of GST is reversibly switched by electrical-resistive Joule heating between amorphous with high resistivity and crystalline with low resistivity. Therefore, a high electrical current density of 10 6 -10 7 A/cm 2 is required to locally heat up GST for phase change. 1 Because of this switching mechanism, PRAM has suffered from severe thermal cycling and significant volume changes in the active region which result in large mechanical stress. This can cause interfacial delamination or compositional alteration in the device cells, which can be the origin of device failures. 2-5 Because these failures occurred in small-scale devices with complicated structure, limitations exist in identifying the physical origin of operational failures.We have studied the damage behaviors in GST line structure under repetitive pulsed-dc stressing with various frequencies. Because the line structure differs from the conventional structure of the PRAM cell, there may be a difference in damage phenomena from a real PRAM cell. However, the line structure can be used as a model system to simplify the physical origin of failures because current density and temperature profile are more uniform in this structure. The relatively wide line structure used in this study makes it easy to observe the change in microstructure and composition by the current stressing. Pulsed-dc stressing is the most appropriate testing method in investigating the reliability of PRAM which employs pulsed currents in actual operation. The frequency of an electrical pulse determines the duration time of electrical pulse and affects the extent of thermal cycling and mechanical stress. To identify the extent of thermal cycling under the testing conditions, we calculated the temperature profile of the line using the finite-difference method ͑FDM͒. 6 ExperimentalGST strip line test structures 50 m long, 2 m wide, and 300 nm thick were made using the lift-off method ͑Fig. 1a͒. The GST film was deposited at room temperature using...

Phase change memory technology

“…The results of these experiments, together with the earlier failure analysis data on mushroom devices [213,214], sketch out a convincingly consistent story for Ge 2 Sb 2 Te 5 devices: Te moves towards the positive electrode (anode), while Sb moves toward the negative electrode (cathode) [54,213,214,[220][221][222][223][224]. This motion is attributed to the higher electronegativity (5.49eV) of Te compared to Ge and Sb (4.6eV and 4.85eV) [224].…”

“…A number of groups have been using techniques such as Energy Dispersion Spectroscopy (EDS), Secondary Ion Mass Spectrometry (SIMS), and Energy Dispersive Xray (EDX) spectrometry to perform elemental analysis on failed cells [54,[213][214][215][216][217][218][219][220][221][222][223][224]. Measurements on mushroom cells built from Ge 2 Sb 2 Te 5 material [213,214] tend to show agglomeration of Antimony (Sb) at the bottom electrode at the expense of Tellurium (Te).…”

“…Early versions of these experiments were affected by lingering uncertainties related to the role of metallic electrodes at the failure point [220], and to the difficulty in understanding material desegregation during long-term cycling by studying the aftermath of a singlepulse "blown-fuse" failure [54,220,221]. However, the most recent experiments have investigated the cycling failure of tapered bridge structures located far from any metal electrodes [222], and the controlled fast melting of large symmetric bridge devices [223,224].…”

Electronic structure and conductivity of off-stoichiometric and Si-doped Ge2Sb2Te5 crystals from multiple-scattering theory

“…36 Finally, even in devices based on a perfectly stoichiometric Ge 2 Sb 2 Te 5 layer, electromigration of the different chemical species, both in the amorphous and the crystalline phases, can be responsible for nonuniform deviations from the ideal composition and for a finite gradient of the Ge, Sb and Te concentrations. 47 Several methods have been considered to dope Ge 2 Sb 2 Te 5 crystals, the most important one consisting in using nitrogen atoms as the dopant chemical species. Such doping enhances the thermal stability of Ge 2 Sb 2 Te 5 , increases its crystallization temperature, crystallization time, electrical resistivity and modifies the optical band gap.…”

High Current Density Effect on In-situ Atomic Migration Characteristics of a BiTe Thin Film System

“…35,36) Electrical failure mechanism of multicomponent chalcogenide materials under high current density seems to be closely related to the atomic migration characteristics during electric current stressing. In our knowledge, there have been only a few ex-situ studies on the atomic migration behavior of under high current stressing condition of multicomponent chalcogenide materials such as GeSbTe (GST) and doped GST [36][37][38] while few reports on BiTe systems. Therefore, in-situ observation of real-time microstructural evolution during current stressing is necessary in order to understand the fundamental atomic migration mechanism of BiTe systems.…”