Tutor: GERACI ANGELO
Advisor: IELMINI DANIELE Major Research topic
:Fabrication of resistive switching memories and their implementation in brain-like architectures and in-memory computing.Abstract:
The research activities focus on the development of memristor devices, in particular on resistive switching memories. The structure is simple, made with an oxide layer sandwiched between two metals. These devices are two-terminal components which behave as resistors, but with a variable value of the resistance: the working window allows to go from a high resistive state (HRS), usually in the teraohm range , to a low resistive state (LRS) in the range of few kiloohms. This characteristic can be exploited in vector-matrix multiplication as accelerators of neural network purposes or can be used in brain-like architectures to store the weight of the neurons.
The fabrication is entirley performed in the Polifab facility with standard CMOS processes, in particolar with 395nm UV light optical lithography and e-beam evaporator. There are three main moments:
- the bottom electrode layer, made with 5nm of titanum as a primer and 20nm of platinum as conductor, both evaporated through e-beam evaporator. Platinum is both a common choice in litterature as a reference material and a stable and inhert metal. The lithography has a minimum detail size of 3um (the width of the lines). ;
- oxide layer and channels; a layer of 70nm of SiO2 separates the bottom layer from the top layer. A mask with 600nm hole-diameter is used as a guide for the plasma etching process, which creates the channels which will host the devices. For smaller diameters the electron-beam lithography is used, reaching 10nm diameters size. ;
- switching layer and top electrode; the last lithographic step creates the top electrode lines and then a suitable stack of oxide and metals is evaporated to fabricate the devices. The last metal is usually gold, as stable material for the bonding and common choice in litterature. ;
Depending on the oxide-metal couple used, it is possible to have volatile memories, where the device spontaneously comes back in his HRS (in case of high atomic mobility metals, Ag Cu Sn) or non-volatile memories, able to maintain the resistance value (Ti, Cr, Ni, Ta, Nb). One of the main problem of this device is the natural passivation of metals, that take place between the desired oxide and the switching metal: this problem brings the increase of the oxide thickness, which is connected to the forming voltage, that increases too.
Thanks to the different electrical behaviours of the devices it is possible to use the volatile memories as synapses, because they spontaneously reset themselves after the signal propagation, and the non-volatile memories as neurons, to store specific weights.