Atomic migration under an electric field, electromigration, in molten and crystalline Ge2Sb2Te5 was studied using a pulsed dc stress to an isolated line structure. Under a single pulse (∼10−3 s), Ge2Sb2Te5 was melted by Joule heating, and an electrostatic force-induced drift of Ge and Sb toward the cathode and Te toward the anode was observed. Effective charge numbers were calculated to be 0.28, 0.38, and −0.29 for Ge, Sb, and Te, respectively. Electromigration in the crystalline state was studied by applying a 10 MHz pulsed dc; constituent elements migrated toward the cathode, which suggests a hole wind-force operating in this phase.

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Damascene Cu interconnects show significant differences in both their microstructural and stress behavior as compared to those of Al interconnects patterned using the etching process. Thermal stresses build up during the successive thermal cycles due to the differences in the coefficients of thermal expansion of the component materials. Other than these thermal stresses, growth stresses originating from grain growth develop in damascene Cu interconnects as well. In this study, the linewidth dependence of the stress in damascene Cu was examined experimentally, as well as by numerical simulation. The stresses of damascene Cu with widths ranging from 0.13to2μm were measured using x-ray diffraction, and the measured hydrostatic stress was found to increase with increasing linewidth, in contrast to the typical behavior of Al interconnects. Microstructure analysis using transmission electron microscopy revealed that the grain sizes increased with increasing line dimensions. The increase in stress in the interconnect with increasing dimensions is attributed to the larger grain size, which induces higher growth stress in addition to the thermal stress. The contribution of the growth and thermal stresses of the damascene lines were quantified based on the grain size data utilizing finite element analysis. In this way, the linewidth dependence of the hydrostatic stress of damascene Cu was clearly explained. Finally, the effect of growth stress on the stress-related reliability is discussed.

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...

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