by Laura Peters, contributing editor
November 19, 2010 - Phase-change memory (PCM) devices require high current density (5-20MA/cm2) to melt the phase-change material and change its state. During an investigation of the impact of current density and current polarity on cycling endurance, researchers from the IBM/Macronix PCRAM Joint Project discovered electromigration-induced failures when PCM cells are reverse-stressed. The team will report on this finding at the upcoming International Electron Devices Meeting (IEDM, 12/6-8 in San Francisco, CA).
Most PCM cell designs are asymmetrical, such as the mushroom cell design used in the IBM/Macronix cycling experiments. The structure consists of a TiN, cup-shaped bottom electrode contact (BEC) with interconnect, germanium-antimony-telluride (Ge2Sb2Te5 or GST) phase-change layer, TiN top electrode contact and copper back-end-of-line. Two cell connections (Figure 1), allowed either conventional bit-line-at-TEC (top electrode contact) cell operation or bit-line-at-BEC (bottom electrode contact) cell operation. By controlling the DC bias on the bit line and source line, current travels through the cell in either forward (TEC-to-BEC) or reverse (BEC-to-TEC) direction using positive pulses on the word line.
Cycling in forward-reset/forward-set mode, devices operated reasonably up to 108 cycles, while failures occurred in the reverse-reset/reverse-set testing after only 104 cycles. The impact of stressing at low current (40μA) also led to early failures when reverse-stressed cells became stuck open after 3 × 104 seconds. The high resistance of the stuck cells remained after several hours of baking at 165°C, indicating a failure similar to electromigration. The effect of varying pulse current (Figure 2) indicates endurance is more strongly affected by pulse current in the reverse-stressed cells.
|Figure 1: Two operating modes of the bit-line at top electrode contact (BL at TEC) forward array (top) and bit-line at bottom electrode contact (BL at BEC). With the bit line positively biased at VBL and VSL=0V, current through the phase-change element is controlled by the gate pulse.
TEM and EDX analysis of failed samples showed that for forward-stressed cells, there is significant depletion of Sb, Ge and Te, and Sb especially migrates toward the GST/BEC interface. In the reverse-stressed cells, Sb, Ge and Te depletion occurs, but located in a region on top of the bottom electrode contact. Since GST is a p-type semiconductor, in the reverse-stressed condition, holes flow from the BEC into the GST, causing potential voids.
The IBM/Macronix team suggests that PCM designs follow an asymmetric design and PCM operation should ensure current crowding only occurs in the direction of electron flow.
|Figure 2: In forward-stressing operation, endurance is reduced with higher pulse current, and the effect saturates when current is increased further (left). In reverse-stressed devices, endurance is much more susceptible to pulse current. High hole current may cause voids that cannot be repaired by the next melting (right).