At the NCCAVS Thin Film Users Group meeting in November, HP was on the program in the person of Joshua Yang (centre above) who gave a materials centric look at the status of the HP ReRAM (Memristor) program. A colleague passed on the informative set of slides presented at the meeting. Being a former process integration team leader, I was immediately struck by a couple of TEMs (Transmission Electron Micrographs) on the Roadmap slide during my first scan of the presentation! Joining up a few dots and tracking down the reference (a joint paper with SK Hynix at the 2012 VLSI Technology Symposium*), the TEMs are probably from the Hynix/HP collaboration and show a 54nm (half pitch?) cross bar array fabricated over larger technology node CMOS in which “parts of decoders to row/column lines” are implemented. I’d further guess that there is a contact via (containing the transition metal oxides) between the two crossbar array electrodes (i.e. the switching film does not look continuous). The VLSI Technology Symposium Abstract further described the crossbar array as selector-less and as a 2Mb ReRAM test chip. The Roadmap slide shows a progression of applications starting with Flash and SSD and progressing through DRAM, Universal Memory to Neural Computing.
The bulk of the presentation describes and contrasts the properties of TiOx and TaOx based devices. The subscript x is such that the TiOx is close to TiO2 and the TaOx is Ta2O5. The oxides appear to be deposited. Non-linearity is greater with TiOx (see above) in both high and low resistance states leading the author to suggest a ‘broken channel’ filamentary state for the TiOx cells. In contrast, the TaOx cells are described as having a continuous channel in both states with relatively linear IV characteristics. Some very nice Proximal Probe/FIB/TEM/Diffraction/EELS/ work is described where the conduction channel is located in the TaOx cell and identified as being oxygen deficient compared to its immediate surroundings. It turns out the TaOx cells are far superior in terms of endurance (#cycles). The author goes on to describe how HP have engineered the TaOx based cells to have a non-linear characteristic to reduce the current through the sneak paths when the cells are implemented in a crossbar bar array. This further means that the complication (i.e. co$t) of the integration of an additional select device is avoided. A non-linearity (Current at voltage V/Current at voltage V/2) of 100 is quoted which shows improvement over the value from the VLSI Symposium Abstract of ~8. The secret appears to be the deposition of a TiOx layer over the TaOx layer although the HP team believes the additional layer is not involved in the resistance switching.
Data retention is not discussed beyond a claim of >10yrs. But all in all, the presentation provides an interesting view of the HP/Hynix collaboration focusing on how material choices can be made to engineer the performance of a ReRAM cell. As with the other presentations from the NCCAVS meeting they will be published on-line and this post will be updated when that occurs.
Christie Marrian, www.ReRAM-Forum Moderator
*Paper 18.1 from the 2012 Symposia on VLSI Technology and Circuits. Integration of 4F2 Selector-less Crossbar Array 2Mb ReRAM Based on Transition Metal Oxides for High Density Memory Applications, H.D. Lee, S.G. Kim, K. Cho, H. Hwang, H. Choi, J. Lee, S.H. Lee, H.J. Lee, J. Suh, S.-O. Chung, Y.S. Kim, K.S. Kim, W.S. Nam, J.T. Cheong, J.T. Kim, S. Chae, E.-R. Hwang, S.N. Park, Y.S. Sohn, C.G. Lee, H.S. Shin, K.J. Lee, K. Hong, H.G. Jeong, K.M. Rho, Y.K. Kim, S. Chung, J. Nickel, J.J. Yang, H.S. Cho, F. Perner, R.S. Williams, J.H. Lee, S.K. Park, S.-J. Hong, Hynix Semiconductor Inc.