Wednesday, April 22, 2015

Iridium Tantalum oxide based Resistive RAM from Panasonic

This is pretty cool for anybody who has been working with high-k for whatever application. More than 10 years ago there was focused research and development by many on high-k materials for logic, memory and capacitor applications and tantalum oxide was one of the contenders. Then two things happened Samsung went with a hafnium oxide based dielectric for their 90 nm DRAM (MIS stack) in 2004/2005 and Intel later introduced a hafnium oxide based HKMG technology for 45 nm logic in 2007. Since then it has been pretty boring looking at reverse engineering reports like the one below - always the same theme hafnium oxide and zirconium oxide, which due to the bloody lanthanide contraction is basically the same thing. That is why I was so happy to find this one a tantalum oxide based stack in a product for an emerging memory technology! Yay - Tantalum is back! I say back since STMicroelectronics had it all figured out already in 2003 in their Ta2O5 based 3D MIM capacitors and one of the earliest(?) patents on using ALD Ta2O5 in a MIM capacitor was granted in 1992(!) for VTT, Finland. Finally, not to forget the important of the metal electrode and metal in general the total awesomeness of using Iridium top electrodes in the case reported below! Just imagine how irritating this must be for Ruthenium.

The first High-k Ta2O5 MIM application patented 1992(!) and presented by VTT, Finland at SEMICON Europa in 1999. Slide as as given in the presentation "High-k fur Alle"

Introduction of High-k as given in the presentation "High-k fur Alle"

The resistive RAM (ReRAM) product of Panasonic has been investigated by TechInsights examines. The microcontroller (MN101LR series) is fabricated at Panasonic’s former Tonami fab using a 180nm CMOS process. As reported in EE Times:

"The Tonami fab is now operated as a joint venture with TowerJazz. Panasonic uses a binary transition metal oxide (tantalum oxide) as a variable resistance layer sandwiched between an upper electrode (iridium) and a lower electrode (tantalum based electrode). Panasonic’s ‘319 patent further describes the tantalum oxide as having two sub-layers[5], where a bottom tantalum oxide layer is formed by the reactive sputtering process of a Ta target to form an oxygen deficient layer (TaO1.43). This deposited tantalum oxide then undergoes an oxidation process to increase the oxygen content of its upper surface to form TaO2.45, which is close to the stoichiometric Ta2O5."
Panasonic ReRAM cell. (EETimes / TECHINSIGHTS)

This article also showcases details on the ADESTO CBRAM technolgy based on Silver and germanium sulfide, which are both unusual materials for a semiconductor fab. Possibly less of a problem for a fabless business model like ADESTO is using.