Paper EM-WeM12
Low-k/Cu Resistive 2-Level PROM Memory Collocated with CMOS Back-End Metallization

Wednesday, October 21, 2015, 11:40 am, Room 210E

Building nonvolatile memory directly into a CMOS low-k/Cu interconnects would reduce latency in connectivity constrained computational devices and reduce chip’s footprint by stacking memory on top of logic. NVM memory includes two flavors: i) random-access memory, and ii) programmable read-only memory (PROM). The paper investigates suitable choice of materials for an integration of PROM compatible with manufacturing of CMOS back-end. Three capacitor-like MIM structures Al/Ti/I/Cu, with dielectrics I=SiOC:H, SiC:H, SiCN:H, all 25 nm thick, have been selected among samples manufactured by Intel Inc and investigated for resistive switching properties. The samples have been subjected to set and reset operations applied customarily to resistive switching devices with TiAl electrode being grounded and a positive bias applied to Cu electrode. For SiOC:H devices, a sharp transition from R_OFF=200MΩ to R_ON(1)=120kΩ at threshold V_set(1)= 0.9V - 1.2 V is observed. When the set device is subsequently subjected to a linear voltage ramp, a secondary sharp set transition from R_ON(1)=120kOhm to R_ON(2)=2-10Ω is observed at |V_set(2)|=1.0-1.3V and high compliance currents I_cc≈100mA, independent of the bias polarity. R_ON(2) can be controlled by the level of I_cc (@I_cc≈10mA R_ON(2)=34Ω). Both transitions are irreversible and the low resistance states are stable. The 1^st transition is likely to be caused by formation of a Cu conductive filament (CF). Because of the weak diffusion/migration stopping power of Ti for Cu, the resulting Cu CF is of a cylindrical form instead of a conical with the former being very difficult to rupture. The 2^nd set transition leads to a dramatic decrease of R_ON by a factor 10⁵. To ascertain the nature of the CF, we have measured the temperature coefficient of resistance α of the CF and obtain unusually high values, typically α=0.04K^-1, which is 10X larger than a for bulk Cu, α=0.0039K^-1, or for Cu CF in Cu/TaO_x/Pt, α=0.0033K^-1) and 40X higher than a for oxygen vacancy defects CF, α=0.001K^-1. The secondary set from R_ON(1) to R_ON(2) is attributed to some, presently unknown, dramatic phase transformation. Structures with SiC:H (same metal electrodes) show different behavior, but result in the same low resistivity state R_ON(2). They require high V_set= 3-4V and display volatile behavior at low I_cc values. At higher I_cc they set into a stable and very low R_ON≈5Ω, constituting thus only 1-level PROM. Similarly, devices with SiCN:H are setting permanently only at high I_cc currents (50-100mA) and display also very low final resistance of about 10Ω. The paper discusses the properties of the highly conductive, metallic CFs with the uncharacteristically high α.