High performance PRAM cell scalable to sub-20nm technology with below 4F2 cell size, extendable to DRAM applications

Abstract

A PRAM cell with great scalability and high speed operation capability with excellent reliability below 20nm technology was demonstrated. This has the meaning of the potential applicable to the technology area of scaling limitation of DRAM cell.We fabricated a confined PRAM cell with 7.5nmx17nm of below 4F 2 . In particular, Sb-rich Ge-Sb-Te phase change material was employed for high speed operation below 30nsec. The excellent writing endurance performance was predicted to maintain up to 6.5E15cycles by reset program energy acceleration. Its data retention was 4.5 years at 85 o C which is enough for DRAM application.

Compositionally matched nitrogen-doped Ge2Sb2Te5/Ge2Sb2Te5 superlattice-like structures for phase change random access memory

“…The black regions above and below the SLL are the TiW electrodes. Diffusion length calculations based on reported endurance switching of PCRAM devices 5,19,20 indicate that Ge, Sb, and Te elements require $1.26 Â 10 4 cycles before significant intermixing of atoms between the layers and this is consistent with our TEM image. It is interesting to note that diffusion constants of GST derived from molecular simulations 21 range from 3.78 Â 10 À5 to 4.67 Â 10 À5 cm 2 /s and this may result in a diffusion length of $20 nm after a 20 ns pulse.…”

“…It is interesting to note that diffusion constants of GST derived from molecular simulations 21 range from 3.78 Â 10 À5 to 4.67 Â 10 À5 cm 2 /s and this may result in a diffusion length of $20 nm after a 20 ns pulse. The large diffusion lengths might not allow GeSbTe based devices to switch 10 9 to 10 11 times, and so the diffusion constants were estimated from actual endurance data 5,19,20 ($10 À15 cm 2 /s). The reason for the large difference in magnitude may be due to the different conditions considered in the two cases.…”

Exploring latency-power tradeoffs in deep nonvolatile memory hierarchies

“…PCRAM (and other NVM technologies) suffer from write endurance; a memory cell has limited lifetime. Most authors assume endurance of 10 8 write cycles, but recently developed fully-confined PCRAM cells have much longer lifetime (more than 10 11 write cycles) [7]. We can also combine wear-leveling [20], failure tolerance [23,29,24,16] and write buffers (SRAM or DRAM) to cope with write endurance.…”

Phase change memory

“…Compared with flash memory, PCM demonstrates superior endurance (up to 10 8 -10 10 cycles) [45,46]. Moreover, write endurance in PCM is strongly correlated to programming energy: every order-of-magnitude increase in programming energy results in a three orders-of-magnitude reduction in endurance [47]. In a confined a PCM cell, the endurance was predicted to be similar to that of DRAM (∼10 15 cycles by reset program energy acceleration) [47].…”