Thesis advisor :
Dr. Marc Bocquet, MCF HDR
Tel : +33 (0)4 13 55 40 42
Co- advisors :
Dr. Evangéline Bènevent, MCF
Tel : +33 (0)4 91 28 89 75
Dr. Magali Putero, MCF
Tel : +33 (0)4 91 28 85 67
Laboratory : IM2NP-UMR CNRS 7334
Funding : doctoral contract AMU ED353
The goal is to manufacture resistive memories (CBRAM) with the co-integration of nonlinear selection elements printed on flexible substrate for sensor applications. To achieve this goal, a design of experiment will be defined in order to process on one hand a CBRAM device and on the other hand a selector with a diode behavior for example. The final goal is to merge both devices at large scale, to design and manufacture a crossbar memory array functionalized with an already existing pressure sensor technology. This original research work will outweigh previous solution in the field [3-8] because of the structural simplicity of the device combined to the exclusive use of printing process or low temperature manufacturing techniques.
After a bibliography in order to identify the best materials for the electrolyte and the diode, an experience plan will be built for the selection of the best solution in terms of performance and the ease of technological steps. The PhD student will ensure the design, manufacturing and characterization of the memories and nonlinear selection devices. A proof of concept and a demonstrator of a memory array will be realized. The modeling of the electrical/physical behavior of the memory could also be considered.
Microelectronics is undergoing huge changes that are driven by the end of Moore's Law, as well as by several other evolutions: new alternative manufacturing approaches, fast increase of the exchanged/stored data in the world, or the development of new communicating objects. In such a new ecosystem, the development of sensor arrays on flexible media is booming, because they can cover large surfaces, there are easy to produce and have a low environmental impact.
Because of their simple stack (MIM), resistive memories and especially Conductive Bridge RAM (CBRAM) are compatible with these new concepts: based on the creation/dissolution of conductive filaments within a solid electrolyte from an active electrode, they can be fabricated on flexible substrates, uses active/inert electrodes compatible with inkjet printing techniques, as well as various solid electrolyte that can be fabricated using deposition techniques at ambient temperature and atmospheric pressure. The real benefit of this type of device is also the case of a "crossbar" architecture (Fig.1)  allowing a selection without complex electronics. However, such architecture requires the co-integration of non-linear selector with memory points. The role of the non-linear selectors is to block the parasitic paths of the current during the programming and the reading phases (Fig. 2) .
Fig. 1  : (a) Hybrid integration of a CMOS memory crossbar array
(b) Diagram of the reading/programming circuit.
Fig. 2  : Diagram of parasitic paths in a 4-bit memory matrix
(a) Matrix based on resistive switches (b) Matrix based on resistive switches with rectifier diodes
The development of non-volatile memories associated with a non-linear selection element on a flexible support for pressure sensor applications is thus a key issue and a real challenge at IM2NP. This phD will be supported by the expertise of three research teams of the laboratory – RDI, MCI and MEM (www.im2np.fr) to address the four main topics of this subject: materials, printed electronics, memories and sensors.
Objectives of the thesis :
The aim of the thesis is to fabricate resistive memories (CBRAM) with the co-integration of non-linear selection elements printed on a flexible substrate that will be functionalized with pressure sensors. This original research work is in the context of the work proposed by [3-8]. The goal will be to fabricate both a single memory point and a diode-type non-linear selection element, in order to have a crossbar type memory matrix which will be functionalized by pressure sensors.
Resistive memories have a simple MIM stack (Metal-Insulator-Metal). The memory effect is due to the creation/dissolution of one or several conductive path between the electrodes. The presence or lack of this conductive filament makes it possible to code two logic states corresponding to two different resistance values (resistance measured between the two electrodes), called “LRS” for “Low Resistance State” and “HRS” for “High Resistance State”. Several memory technologies use such concept. The technology used in this study is CBRAM type (Conductive Bridge Random Access Memory) and relies on the growth of conductive filaments from an active electrode (e.g. Silver) up to an inert electrode (e.g. Platinum) through a solid electrolyte (Fig.3) . Such memories have been studied at IM2NP, especially within the ANR Reflex project, within the phD thesis of Charles Rebora  and within the HDR of M. Bocquet .
Figure 3  : SET (A)-(D) and RESET (E) for a CBRAM memory
Starting from the pristine reset state (HRS), the set process occurs if a sufficient positive bias voltage (SET voltage) is applied to the active electrode. The overall set process involves (i) the anodic dissolution of Ag creating Ag+ cations in the electrolyte layer, (ii) the electromigration of the Ag+ cations within the electrolyte layer, (iii) the reduction of Ag+ to Ag on the vicinity of the inert electrode. This process leads to the formation of metallic filaments that bridge both electrodes. At this step, the memory element is in a low resistance state (LRS, SET). By decreasing the applied voltage, the memory element remains in low resistance state unless a sufficient voltage of opposite polarity (RESET voltage) is applied with a subsequent electrodissolution of the metallic filaments that enable switching back the memory element in a HRS (RESET).
Various materials can be used as solid electrolyte: inorganic materials (SiO2, GeS, GeSe, Ta2O5, GeSbTe..)[12-13], polymers (PEO, PVK…) or biodegradable materials . In this thesis, a nanoporous inorganic material will be chosen. Indeed, several studies have shown the interest of nano-pores which serve as diffusion path for the cations migration  : for example, the use of electrodeposited GeSx  allows an accurate control of the Ag doping; fabrication of a CBRAM Ag/Al2O3/CdS/Pt device with good endurance and retention can be obtained using CdS nanoporous thin films (obtained from CdS nanocrystals colloidally synthesized, and a simple drop-drying) . TiOxNy was also successfully tested as porous electrolyte . At IM2NP, the GeSbTe alloy with a 7%vol. porosity was also previously used to fabricated CBRAM cells .
Methodology/ Key issues / IM2NP support
The realization of CBRAM memories and diodes by printing techniques on flexible substrates will be possible by combining several ambient temperature deposition techniques without any masks or vacuum: inkjet printing, sol-gel, electrochemical deposition thin films, spin-coating. The PhD student will rely on the resources and more specific skills of two teams in the laboratory, the MCI (Micro-sensors and Instrumentation) and RDI (Reactivity and Diffusion at Interfaces) teams, and will benefit from the know-how [21-23] acquired thanks to the technological SPRINT platform (flexible printed electronics platform). The three main technological issues to be addressed during the thesis will be:
The design and characterization of the memory cells and selectors will be carried out more specifically with the support of the MEM team and the platform Intelligent Object Lab (IO-LAB).
The thesis work will start with a bibliography review in order to identify potential candidates for the realization of the solid electrolyte layer and diodes. From the few selected materials and associated deposition technologies, a work program will be built in order to select only the best solutions in terms of device performance (memory and non-linear element) and the difficulty of implementing the technological steps. The phD student will then be responsible for the design, production and characterization of the memory cells and the associated selectors. The final objective is the realization of a prototype/demonstrator of a crossbar matrix associating a few memory points and selectors coupled with pressure sensors. The physical modeling of the memories could also be envisaged.
Teams, departments, platforms involved: DETECT, ACSE, MATER, MCI, MEM, RDI, SPRINT, IO-LAB.
Staff involved: Marc Bocquet, Evangéline Bènevent, Magali Putero.
Student in engineering school or Master 2 in microelectronics.
Skills in materials and printed electronics will be appreciated.
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|Title||PhD – Realization of resistive memories (CBRAM) and co-integration of nonlinear selection elements printed on flexible substrate functionalized by pressure sensor|
|Employer||Institut Matériaux Microélectronique Nanosciences de Provence (IM2NP)|
|Job location||Faculté des Sciences de Saint Jérôme - Case 142, Avenue Escadrille Normandie Niemen, F-13397 Marseille Cedex 20, 13397 Marseille|
|Published||February 17, 2020|
|Job types||PhD  |
|Fields||Materials Chemistry,   Electrochemistry,   Inorganic Chemistry,   Polymer Chemistry,   Electronics  |