Loïc Henry
email: loic.henry@cea.fr

Nicolas Bernier
email: nicolas.bernier@cea.fr

Martin Jacob
email: martin.jacob@cea.fr

Gabriele Navarro
email: gabriele.navarro@cea.fr

Laurent Clément
email: laurent-renaud.clement@st.com

Jean-Luc Rouvière
email: jean-luc.rouviere@cea.fr

Eric Robin
email: eric.robin@cea.fr

Studying Phase Change Memory devices by coupling Scanning Precession Electron Diffraction and Energy Dispersive X-ray analysis

Keywords: EDXS, Electron diffraction pattern, Phase Change Memories, Spectral decomposition, Transmission Electron Microscopy

A new simultaneous acquisition software for Precession Electron Diffraction (PED) and Energy Dispersive X-ray Spectroscopy (EDX) mapping has been developed. This approach has been used to study the RESET operation in Ge-rich Ge-Sb-Te (Ge-rich GST) non-volatile Phase-Change Memory (PCM). Such an operation consists in the local amorphization of the device's active volume, which defines the memory state. Using PED patterns indexing, we were able to segment the amorphous area, and access its composition through EDX quantification. Moreover, both the spatial distribution and the chemical composition of the two crystalline phases overlapped in the chalcogenide layer were retrieved by using the spectral unmixing approach and PED patterns indexing. It evidences that Ge and Ge2Sb1Te2 (GST-212) crystals are separated in the studied devices. Despite this, we show that Ge-enrichment is preserved in the active volume of a functional device, which is crucial to

Introduction

In the last two decades, Phase-Change Memory (PCM) technology has shown outstanding progress. In a general context marked by the need of a next-generation of Non-Volatile Memory (NVM) [START_REF] Burr | Phase change memory technology[END_REF], Ge-Sb-Te (GST) based PCM has proven its reliability in terms of scalability [START_REF] Bez | Chalcogenide PCM: a memory technology for next decade[END_REF], programming speed [START_REF] Loke | Breaking the Speed Limits of Phase-Change Memory[END_REF] and endurance [START_REF] Wuttig | Phase-change materials for rewriteable data storage[END_REF]. PCM is considered as the most mature memory technology among the emerging ones, and it can be applied in both embedded and stand-alone systems.

Among the stable compositions investigated along the Sb2Te3-GeTe tie-line, Ge2Sb2Te5 has been intensively studied for its advantageous tradeoff between crystallization speed and activation energy [START_REF] Cheng | A high performance phase change memory with fast switching speed and high temperature retention by engineering the GexSbyTezphase change material[END_REF]. However, its low crystallization temperature of about 170 °C is not compatible with embedded automotive applications, requiring stability for several years at 150 °C. With the aim of improving stability at high temperature, Ge-enrichment [START_REF] Cheng | A high performance phase change memory with fast switching speed and high temperature retention by engineering the GexSbyTezphase change material[END_REF], and nitrogen or carbon doping have been proposed [START_REF] Cheng | A thermally robust phase change memory by engineering the Ge/N concentration in (Ge, N)xSbyTezphase change material[END_REF][START_REF] Navarro | N-Doping Impact in Optimized Ge-Rich Materials Based Phase-Change Memory[END_REF][START_REF] Betti Beneventi | On Carbon doping to improve GeTe-based Phase-Change Memory data retention at high temperature[END_REF]. Later on, an optimized Ge-rich Ge-Sb-Te (Ge-rich GST), referred as T-alloy, has been presented [START_REF] Zuliani | Overcoming Temperature Limitations in Phase Change Memories With Optimized GexSbyTez[END_REF] for automotive market applications. This optimized alloy fits the challenging automotive specifications, ensuring both code integrity and data retention in high temperature environments. Integrating this Ge-rich GST alloy into state-of-the-art PCM, an innovative 28 nm FD-SOI e-NVM technology for automotive application has recently been presented [START_REF] Arnaud | Truly Innovative 28nm FDSOI Technology for Automotive Micro-Controller Applications embedding 16MB Phase Change Memory[END_REF]. In such devices, the control of GST stoichiometry is crucial in order to accurately meet the strict target requirements. Ge-rich GST refers to the "as-deposited" composition which is amorphous and homogenous. However, during annealing of full sheet deposited layers, a phase separation between Ge and a GST phase occurs [START_REF] Zuliani | Overcoming Temperature Limitations in Phase Change Memories With Optimized GexSbyTez[END_REF][START_REF] Agati | Chemical phase segregation during the crystallization of Ge-rich GeSbTe alloys[END_REF][START_REF] Privitera | Atomic diffusion in laser irradiated Ge rich GeSbTe thin films for phase change memory applications[END_REF][START_REF] Nam | Phase separation behavior of Ge2Sb2Te5 line structure during electrical stress biasing[END_REF], leading to chemical and crystalline modifications. That could appear as well during the Back-End-Of-Line (BEOL) process fabrication, which consists in a standard CMOS integration process. Moreover, composition variations in the active region of the PCM cell have been reported after activation operation [START_REF] Sousa | Operation fundamentals in 12Mb Phase Change Memory based on innovative Ge-rich GST materials featuring high reliability performance[END_REF][START_REF] Palumbo | Forming operation in Ge-rich GexSbyTez phase change memories[END_REF] and cycling [START_REF] Crespi | Modeling of Atomic Migration Phenomena in Phase Change Memory Devices[END_REF], due to local Ge migration. If such mechanism were to trigger a failure in standard Ge-Sb-Te alloys [START_REF] Padilla | Voltage polarity effects in Ge2Sb2Te5-based phase change memory devices[END_REF][START_REF] Yang | Atomic migration in molten and crystalline Ge2Sb2Te5 under high electric field[END_REF][START_REF] Valery | Advanced TEM-based characterizations for bits fail analysis In 28 nm Phase Change Memory Test Vehicle[END_REF], it is intrinsic and necessary in Ge-rich GST for the functionality of the device.

In this study, we focus on the understanding of the origin of the extrinsic RESET programming failure of a Ge-rich GST based PCM device after BEOL process. To do that, we have characterized both structure and chemistry of GST wall-structure PCM devices [START_REF] Bez | Chalcogenide PCM: a memory technology for next decade[END_REF] used in 28 nm FDSOI e-NMV technology developed by STMicroelectronics [START_REF] Arnaud | Truly Innovative 28nm FDSOI Technology for Automotive Micro-Controller Applications embedding 16MB Phase Change Memory[END_REF]. We applied the Automated Crystal Orientation and phase Mapping technique developed for Transmission Electron Microscopy (ACOM-TEM), also known as ASTAR. This technique was used for the characterisation of phases, while EDX was applied for chemical quantification of both crystalline and amorphous phases. Through the use of a simultaneous PED/EDX acquisition, we report the benefits of an exact correlation between both datasets.

Material and Methods

Material

A scheme of the PCM stacking with a sectional view of the structure is shown in Fig. 1. The active device is built on a stack comprising, from bottom to top, a silicon substrate, a 25 nm-thick buried oxide, a <100> orientation Si thin film and a tungsten bottom electrode. The bottom electrode is connected to the GST layer by a thin TiN element, named "heater". This conductive material is deposited by etching into an insolent silicon nitride layer, and allows to obtain a localized Joule heating of the GST. We remind that PCM relies on the transition between an amorphous resistive state (RESET state) and a crystalline conductive state (SET state) of a small volume of GST above the heater. Both RESET and SET programming are achieved by applying adequate current pulses. On the top of the device, a 20 nm-thick TiN encapsulation layer is deposited onto the chalcogenide layer.

The first RESET programming operation of the PCM includes an initial activation procedure, also named "forming" operation. It allows to switch the cell from a highly resistive virgin state to a lower and suitable resistive state for embedded memory application [START_REF] Sousa | Operation fundamentals in 12Mb Phase Change Memory based on innovative Ge-rich GST materials featuring high reliability performance[END_REF][START_REF] Palumbo | Forming operation in Ge-rich GexSbyTez phase change memories[END_REF]. Such operation leads to the formation of a conductive path between the heater and the top electrode, along which the material gets melted. This melted area then constitutes the programming region of the cell, also called active area. At the end of the BEOL process fabrication, the PCM test vehicle is submitted to a high temperature bake in order to explore the RESET programming reliability after extreme BEOL conditions. We remind that we focus our analyses on two cells, one programmed successfully in the RESET state (RESET-Pass), and the other failing at the RESET programming (RESET-Fail) being its resistance below the target one (i.e. 1MΩ). We analysed a region on interest on each cell (Fig. 1).

Experimental techniques

The sample was prepared along the bit-line direction by Ga + Focused Ion Beam (FIB) milling using a FEI Helios 450 machine. A tungsten protection layer was deposited to protect the two regions of interest from the tails of the ion beam and the specimen was deposited onto a molybdenum TEM grid. Then, we used a 30 kV operation voltage for the initial thinning and finished with a low beam energy in the range 5-8 kV to reduce FIB-induced damage. Both cells were observed at 200 kV using a probe corrected Titan Themis FEI microscope equipped with the Super-X detector system for Energy Dispersive X-ray (EDX) spectrometry and a Gatan US1000 CCD camera for diffraction patterns acquisition. The Super-X system comprises four 30mm² windowless silicon drift detectors placed at an elevation angle of 18° from the horizontal with a symmetrical distribution along the beam axis (45°, 135°, 225° and 315° azimuthal angles) and a 0.64 ± 0.06 sr total solid angle [START_REF] Kraxner | Quantitative EDXS: Influence of geometry on a four detector system[END_REF]. EDX was used for chemical characterization whereas ASTAR [START_REF] Rauch | Automated crystal orientation and phase mapping in TEM[END_REF] technique was used to automatically index diffraction patterns for phase identification.

Simultaneous acquisition of EDX spectrum and diffraction patterns was performed in precession mode (PED-EDX) using a homemade Digital Micrograph (DM) script. This script synchronizes (i) the beam shift between the acquisition points and the collection of (ii) the diffraction patterns and (iii) the EDX spectra, while the precession is activated using the precession option available in the user interface of the microscope. It only differs from Scanning Precession Electron Diffraction in that EDX spectra are collected simultaneously with the diffraction patterns. Note that, by performing a simultaneous PED-EDX acquisition, we ensure that the datasets have an exact pixel correlation. Hence, it allows an accurate correlation between the structure and the composition. A schematic of the simultaneous acquisition and datasets is shown in Fig. 2.

The TEM was used in microprobe mode with a gun lens of 1, a spot size of 4, a 30µm C2 aperture, a probe current of 150-200 pA and a precession angle of 0.5°. Considering the Full Width at Half Maximum (FWMH), we measured an electron probe size of 4.0 nm on a monocrystalline [110]-oriented silicon of a 100-nm thick lamella. We also determined a beam convergence semiangle of 1.2 mrad using a diffraction pattern obtained on the same [110]oriented silicon. Fig 1b presents a TEM image showing the two acquisition areas, one on the RESET-fail cell and one on the RESET-Pass cell. Both cells were scanned over a 102 x 92 nm² area using a 2.6 nm pixel size and a 0.9 s dwell time. For each acquisition, both diffraction patterns and EDX spectra were recorded as 40 x 36 pixel images.

Data analysis

Ge K-, Sb L-and Te L-lines were extracted from EDX spectra using the Kramer background subtraction method implemented in Gatan Microscopy Suite 3 (GMS 3). The kfactors were determined for GST quantification using the theoretical expression of kij, the kfactor of element i relative to element j:

k = M M Q ω α Q ω α ε ε
Where , are the atomic masses, , are the ionization cross-sections (cm 2 ), , are the fluorescence yields, , are the relative intensities of the X-ray lines for elements i and j, and

, are the detection efficiencies for these lines. Using the net X-ray intensities of the Ka+Kb and La+Lb+Lg lines for GST quantification, k-factors were determined relative to silicon with , =1. From the NIST database [START_REF] Chantler | X-Ray Form Factor, Attenuation and Scattering Tables (version 2.0)[END_REF], the total emission cross sections (Q.ω) were evaluated at 9.34 × 10 cm², 19.01 × 10 cm², 19.09 × 10 cm² and 8.46 × 10 cm² for the Ge K-, Sb-L, Te-L and Si-K lines, respectively. We assumed an Al contact and a Si dead layer of 30 nm each to evaluate the detector efficiency at 96%, 99%, 99% and 97% for the Ge K-Sb L-, Te L-and Si-K lines, respectively. Therefore, the theoretical k-factors are 2.36, 1.89 and 1.97 for the Ge K-Sb L-and Te L-lines, respectively.

The two EDX maps, obtained on the RESET-Pass and RESET-Fail cells, were combined into one to increase the number of spectra for statistical analysis. The acquisition conditions being the same, we were able to concatenate the two images into a single dataset. Note that this operation is not possible in case EDX acquisition conditions are varying as it would result in non-comparable datasets and then an incorrect statistical analysis. Using the open source Hyperspy toolbox [START_REF] De La Peña | hyperspy/hyperspy[END_REF], scikit-image [START_REF] Van Der Walt | scikit-image: image processing in Python[END_REF] and sci-kit-learn packages [START_REF] Pedregosa | Scikit-learn: Machine learning in Python[END_REF], Principal Component Analysis (PCA) was applied as a denoising method with a weighting pre-treatment for Poisson noise normalization on the merged datasets [START_REF] Potapov | Enhancement of noisy EDX HRSTEM spectrumimages by combination of filtering and PCA[END_REF]. This method consists in identifying the components that yields most of the variance of the data, and hence meaningful information, separating them from noise. The PCA scree plot was used to determine the number of components that explains most of the variability in the dataset. Six components were found, which is in agreement with the five expected chemical phases (titanium nitride, silicon nitride, silicon oxide, GST and Ge), plus the EDX zero peak frequency detected as a component. We then used Spectral Unmixing techniques to express the spectrum in each pixel of our images as a linear combination of a given number of spectral endmembers. We used Vertex Component Analysis (VCA) [START_REF] Nascimento | Vertex component analysis: a fast algorithm to unmix hyperspectral data[END_REF], to extract the spectral signatures of five components present in the data. This approach differs from other multivariate statistical analysis already used for EDX data such as Independent Components Analysis (ICA) [START_REF] Rossouw | Multicomponent Signal Unmixing from Nanoheterostructures: Overcoming the Traditional Challenges of Nanoscale X-ray Analysis via Machine Learning[END_REF], or Non-negative Matrix Factorization (NMF) [START_REF] Shiga | Sparse modeling of EELS and EDX spectral imaging data by nonnegative matrix factorization[END_REF], because VCA have a pure-pixel hypothesis ensuring that the extracted components are present in the original data. In our case, the five components considered as pure pixels were in agreement with the five expected chemical phases mentioned above. We then used the Sparse Unmixing by variable Splitting and Augmented Lagrangian (SUNSAL) algorithm [START_REF] Bioucas-Dias | Alternating direction algorithms for constrained sparse regression: Application to hyperspectral unmixing[END_REF] to estimate their abundance fractions. Both VCA and SUNSAL algorithms were performed using the python implementation proposed here [START_REF] Lagrange | [END_REF].

Finally, the diffraction patterns were acquired and converted to the blockfile format of the ASTAR software without applying any filter. Based on XRD analysis, two crystalline phases were identified to characterize the chalcogenide layer: the Ge cubic phase (space group n°227) and the Ge2Sb2Te5 cubic phase (space group n°225). We generated Ge2Sb2Te5 from XRD observations realized on the bulk sample, also reported in [START_REF] Valery | Advanced TEM-based characterizations for bits fail analysis In 28 nm Phase Change Memory Test Vehicle[END_REF], and in agreement with [START_REF] Zuliani | Overcoming Temperature Limitations in Phase Change Memories With Optimized GexSbyTez[END_REF][START_REF] Kim | Nitrogen-Doping Effect on Ge2Sb2Te5 Chalcogenide Alloy Films during Annealing[END_REF] Information File (CIF) ID 9008567, while for Ge2Te2Te5 the CIF file was generated from XRD observations reported in [START_REF] Valery | Advanced TEM-based characterizations for bits fail analysis In 28 nm Phase Change Memory Test Vehicle[END_REF], and in agreement with [START_REF] Zuliani | Overcoming Temperature Limitations in Phase Change Memories With Optimized GexSbyTez[END_REF][START_REF] Kim | Nitrogen-Doping Effect on Ge2Sb2Te5 Chalcogenide Alloy Films during Annealing[END_REF].

Results

Fig. 1b shows a standard High-angle Annular Dark-field (HAADF) image of the RESET-Fail (on the left) and RESET-Pass (on the right) cells. Such images, based on the collection of incoherently scattered electrons at high angles, are very sensitive to mass thickness variations (Z-contrast) [START_REF] Crewe | Visibility of Single Atoms[END_REF]. Within the GST layer, we observe multiple contrast variations which are likely to reveal an inhomogeneous chemical distribution. Such variations are compatible with either the phase separation mechanism described in Ge-rich GST materials [START_REF] Privitera | Atomic diffusion in laser irradiated Ge rich GeSbTe thin films for phase change memory applications[END_REF] or the void formation observed in [START_REF] Navarro | N-Doping Impact in Optimized Ge-Rich Materials Based Phase-Change Memory[END_REF]. However, both chemical and structural information are required to distinguish between these two mechanisms. Fig. 3a-c show an example of the template matching method of the ASTAR indexing, which consists in correlating the recorded PED patterns with simulated templates [START_REF] Rauch | Automated crystal orientation and phase mapping in TEM[END_REF]. The ASTAR correlation index (Q) reflects the degree of matching. We present here an example of PED pattern recorded on the RESET-Fail device and its indexation according to the Ge2Sb2Te5 template (correlation index Q = 833). Note that, on the same diffraction pattern, the Ge template has a much lower correlation index (correlation index Q = 492). Despite the structures similarities between Ge and GST structures, we show that phase identification can be achieved with a comfortable percentage difference of the matching indexes (41%), which is the best parameter to estimate the reliability of the indexing [START_REF] Rauch | Automated crystal orientation and phase mapping in TEM[END_REF]. Usually, values above 15% are sufficient to confirm the validity of the detected phase [START_REF] Rauch | Automated crystal orientation and phase mapping in TEM[END_REF]. Fig. 3a-c also illustrate the importance of using precession to extend the visible reflections on the collected diffraction patterns. Due to Ge and GST structures similarities, the presence of high-order reflections is crucial to enable a reliable pattern indexing. Fig. 4a presents the ASTAR index correlation map obtained on the RESET-Pass cell. Firstly, we observe an amorphous dome located above the TiN heater. Indeed, diffraction patterns extracted from this area (see inset in Fig. 4b) present characteristic halo ring patterns revealing an amorphous material. Thus, Fig. 4a enables to measure a dome diameter of 8 ± 2 nm. Note that crystalline phases are likely to partially cover the amorphous area, leading to an underestimation of the dome size. In order to better highlight the chemical and morphological aspects of the amorphous dome, Fig. 4b shows a view overlapped with the STEM-EDX hyperspectral map. To segment the active dome, ASTAR correlation index was used to distinguish amorphous from crystalline areas. Indeed, as halo ring patterns do not match Ge and GST templates, both correlation indices are low (≤ 50). Then, an X-rays intensities-based mask was applied to extract the amorphous GST dome from the amorphous nitride, as summarised in the schematic flow chart in Fig. 2. Fig. 4c shows the results of the amorphous dome spectra quantification. We observe that the compositions remain close to the Sb2Te3-Ge tie-line which is investigated in the frame of high thermal stability GST development by increasing the Ge content in GST materials [START_REF] Cheng | A high performance phase change memory with fast switching speed and high temperature retention by engineering the GexSbyTezphase change material[END_REF][START_REF] Zuliani | Engineering of chalcogenide materials for embedded applications of Phase Change Memory[END_REF]. Such Ge-rich GST alloys, dedicated to achieve the automotive market requirement [START_REF] Zuliani | Engineering of chalcogenide materials for embedded applications of Phase Change Memory[END_REF], are the ones of interest in this study. Hence, the agreement between the type of Ge-rich alloy studied and the composition displayed in Fig. 4c supports the quantification reliability. Due to grains overlapping along the lamella thickness [START_REF] Meng | Three-dimensional nanostructure determination from a large diffraction data set recorded using scanning electron nanodiffraction[END_REF][START_REF] Bergh | Nanocrystal segmentation in scanning precession electron diffraction data[END_REF], this method could not be applied to determine the GST phase stoichiometry. Indeed, since the EDX signal is averaged by the grain distribution in each pixel, the spectra recorded are combinations of Ge and GST signals which have to be unmixed for proper GST phase stoichiometry determination. This spectral unmixing step, performed using VCA, is described in the following section. Fig. 5 shows the five endmembers signatures extracted by VCA and their abundance maps in insets. As mentioned, the number of components chosen is in agreement with the scree plot and the number of phases expected in the acquisition area (Fig. 1). Then, by comparing spectral signatures, abundance maps and device architecture (Fig. 1), endmembers can be clearly identified. Endmembers 3, 4 and 5 corresponds to titanium nitride, silicon nitride and silicon oxide, respectively. Endmembers 1 and 2, which are distributed in the GST layer, can be identified as GST and Ge pure phases, respectively. No significant Sb L-nor Te L-lines are observed in endmember 2 spectral signature, indicating that pure Ge crystal phases are present, as shown by XRD results. The same GST phase is found in both RESET-Pass and RESET-Fail cells, showing that no difference in the stoichiometry is detected. VCA was also performed on both datasets separately to confirm that the same GST stoichiometry was found.

In each spectral signature Si K-, N K-, Mo L-and Cu K-lines are visible. Firstly, it is known that X-ray emission from the pole piece of the microscope and the specimen holder causes Mo L-and Cu K-lines. Moreover, the silicon nitride which surrounds the cells cannot be completely removed during FIB milling without deteriorating the device (as shown in 3D architecture reported in [START_REF] Arnaud | Truly Innovative 28nm FDSOI Technology for Automotive Micro-Controller Applications embedding 16MB Phase Change Memory[END_REF]). Hence, a thin amorphous silicon nitride layer remains on both sides of the TEM lamella, producing the Si K-and N K-lines in each EDX spectrum. However, as this layer distribution is homogeneous along the studied areas (no more components are detected by PCA), it has no influence on the endmembers extraction. The estimate of the GST phase composition, shown in Fig. 5a, was determined by quantifying the spectra with more than 90% of the highest abundance of GST phase. Taking into account the uncertainties related both to the statistical uncertainty and the use of theoretical k-factors, the composition measured is compatible with Ge2Sb1Te2 (GST-212) [START_REF] Cheng | A thermally robust phase change memory by engineering the Ge/N concentration in (Ge, N)xSbyTezphase change material[END_REF]. Fig. 5e displays, in a ternary plot, both experimental GST and GST-212 phases. It should be noted that a larger statistical sample would be required to definitely confirm the GST stoichiometry found. A larger scanned area would provide such a large statistical sample, but it was not performed in this study.

Fig. 6a-b show the index correlation and ASTAR phase maps obtained on the RESET-Pass cell. We first observe that, despite the Ge and GST structures similarities, both phases are detected. Thereby, this result is in agreement with the XRD analysis and the phase separation described in Ge-rich GST materials [START_REF] Privitera | Atomic diffusion in laser irradiated Ge rich GeSbTe thin films for phase change memory applications[END_REF]. Also, comparing ASTAR phase map to abundance maps of both GST and Ge endmembers presented in Fig. 6c-d, a good agreement can be observed. Fig. 6e-f show the index correlation and ASTAR phase maps obtained on the RESET-Fail cell. As in Fig. 6b, Ge and GST grains are detected within the polycrystalline GST layer. However, we notice that a larger GST grain is located above the titanium nitride heater with respect to previous analysis, and that a lower Ge abundance is measured in this area with respect to the active volume of the RESET-Pass cell. Thus, the active area of the RESET-Fail cell shows a composition closer to GST-212. Also, the GST layer either contains an amorphous dome too small to be detected or it remains fully crystalline. In both cases, this observation corroborates the low resistance value measured for the RESET-Fail cell. Despite disagreements for a few pixels between ASTAR and VCA results, the phase indexed by ASTAR is detected as the most abundant by VCA in most cases. We thus observe an excellent agreement by comparing the ASTAR phase with the VCA abundance maps. It supports that, despite structure similarities and grains overlapping, ASTAR indexation and VCA unmixing provide reliable results. Note that more details can be found in the Supplementary Information accompanying this contribution. All these experimental results are discussed in the following section.

Discussion

Fig. 7 summaries the main findings of our study. It displays (i) the GST crystalline stoichiometry found by VCA for both cells, (ii) the amorphous dome composition of the RESET-Pass cell, calculated by taking the average of compositions shown in Fig. 4c, and (iii) the composition of the RESET-Fail cell active area occupied by the large GST grain. While the core volume of the RESET-Fail cell is occupied by a large GST-212 grain, the GST-212 phase is enriched in Ge-content in the active volume of the RESET-Pass cell.

We remind that, as explained in [START_REF] Sousa | Operation fundamentals in 12Mb Phase Change Memory based on innovative Ge-rich GST materials featuring high reliability performance[END_REF][START_REF] Palumbo | Forming operation in Ge-rich GexSbyTez phase change memories[END_REF], the activation procedure allows to switch the cell from a highly resistive virgin state towards a steady working state. This operation, based on the atomic reorganization that takes place in the active volume of the cell during the activation process, is responsible for the PCM cell reversible and non-volatile resistance state transitions. It leads to the formation of a conductive path between the heater and the top electrode, causing the Ge migration towards the periphery of the melted volume. This halfdome shaped area, visible in Fig. 6d, is the active region of the cell. The higher content in Ge with respect to GST-212 phase found in the amorphous active volume, demonstrates the capability of the cell to feature a higher retention temperature with respect to standard GST-212, required to satisfy automotive specifications. Indeed, an alloy with such Ge-enrichment has a crystallisation temperature higher than 250 °C [START_REF] Palumbo | Forming operation in Ge-rich GexSbyTez phase change memories[END_REF].

VCA results show that the same GST crystalline phase stoichiometry is found in both RESET-Pass and RESET-Fail cell. However, we can observe that its spatial distribution is different in the two cases. The RESET-Pass cell exhibits multiple fine GST and Ge grains, and an amorphous Ge-rich GST-212 core volume, while the RESET-Fail cell shows a large GST-212 grain above the heater which occupies the majority of the acquisition area. This result confirms that the properties of Ge-rich GST materials do not result from a specific crystallographic phase but from atomic diffusion [START_REF] Agati | Chemical phase segregation during the crystallization of Ge-rich GeSbTe alloys[END_REF], and the spatial distribution of the different phases in the microstructure.

The GST grain formation observed in the RESET-Fail cell, which also corresponds to a larger Ge-depleted volume with respect to RESET-Pass cell, is compatible with an "overshoot" phenomenon. This phenomenon likely happened during the activation operation, when the cells are gradually activated with a staircase up programming voltage [START_REF] Palumbo | Forming operation in Ge-rich GexSbyTez phase change memories[END_REF]. We think that the presence of a region particularly rich in Ge content or containing a pure Ge grain above the heater would require a higher voltage during the activation step with respect to a standard cell. Such a Ge enriched region likely remained after the partial segregation of the Ge-rich GST alloy triggered during the high temperature bake [START_REF] Valery | Advanced TEM-based characterizations for bits fail analysis In 28 nm Phase Change Memory Test Vehicle[END_REF]. It means that the overshoot current generated during activation operation, in such case, leads to a higher temperature in the active volume of the device. Consequently, a higher Ge depletion is created (i.e a lower Ge content in the active volume is achieved). This hypothesis is in agreement with results from [START_REF] Palumbo | Forming operation in Ge-rich GexSbyTez phase change memories[END_REF], where the surface (i.e. volume) of the Ge-depleted region is found to increase linearly with the current applied during the activation procedure. The final composition in the active volume depends on this phenomenon, leaving a material poorer in Ge and richer in Sb content, known to require a higher programming RESET current [START_REF] Navarro | Highly Sb-Rich Ge-Sb-Te Engineering in 4Kb Phase-Change Memory for High Speed and High Material Stability Under Cycling[END_REF]. This could explain the almost absence of amorphous dome in the RESET-Fail cell, that would have required a higher RESET current to be generated than was required for the RESET-Pass device.

As Ge-rich GST materials exhibit both chemical and structure segregation at the nanoscale, an exact correlation between diffraction and EDX signals shows important benefits. Through the application of a simultaneous acquisition system for EDX and PED, coupled with hyperspectral unmixing, we outlined that a better characterisation of PCM cells can be achieved, including amorphous dome characterisation (size and composition) and GST crystalline stoichiometry determination. This new methodology could be applied to study the cycling behaviour of the PCM or to characterize the cells at different steps of the fabrication process.

Conclusion:

In this article, we studied the RESET operation in Ge-rich GST PCM devices by applying a new simultaneous acquisition system for Energy Dispersive X-ray Spectroscopy and Electron diffraction in precession mode. We highlighted the origin of the failure of the RESET operation that could happen after high temperature bake. In particular, we demonstrated the different composition (i.e. lower Ge content) achieved in the active volume of the RESET-Fail cell with respect to a standard RESET-Pass. By using VCA for hyperspectral EDX image unmixing, we detected the same GST-212 crystalline phase in both cells. However, the higher Ge-content with respect to GST-212 phase measured in the amorphous active volume of the RESET-Pass cell demonstrates the capability of the cell to feature a higher retention temperature with respect to standard GST-212, required to satisfy automotive specifications. We also evidenced that a large GST-212 grain occupies the majority of the acquisition area of RESET-Fail cell, suggesting that too much Ge has been expelled during the forming operation. 5. Spectral components extracted by VCA. Endmembers 1 and 2 can be associated to GST and Ge phases, respectively. In order to estimate the GST phase composition displayed in the ternary plot, spectra with more than 90% of the highest abundance of GST phase were quantified. Endmembers 3 to 5 correspond to titanium nitride, silicon nitride and silicon oxide phases, respectively. The ternary plot displays both GST experimental (i.e. endmember 1) and GST-212 phases.

Figures

  Figures

Fig. 1 .

 1 Fig. 1. (a) Schematic drawing of PCM stacking. (b) HAADF image showing the acquisition areas for the RESET-Fail (red) and the RESET-Pass (green) cells.

Fig. 2 .

 2 Fig. 2. Schematic of a simultaneous acquisition of PED and EDX, datasets acquired, and flow chart summarizing the main steps in data processing.

Fig. 3 .

 3 Fig. 3. (a) PED pattern acquired on the GST layer of the RESET-Fail cell and indexed either as (b) Ge (correlation index Q = 492) or (c) as Ge2Sb2Te5 (correlation index Q = 833). In (b) and (c), the experimental PED pattern collected is compared to Ge (green) and Ge2Sb2Te5 (blue) templates generated by ASTAR, respectively. As indicated by the higher correlation index Q and as seen in the superposition, the local crystal has not a crystal Ge structure.

Fig. 4 .

 4 Fig. 4. (a) Index correlation map given by ASTAR software by displaying the correlation index (Q) value of each diffraction pattern acquired, (b) STEM-EDX hyperspectral image. The amorphous dome is highlighted in white by considering pixels with (i) Q <50 to distinguish amorphous from crystalline areas and (ii) a sum of Ge-K, Sb-L and Te-L X-ray net intensities higher than 30 counts to separate the GST from the surrounding layers. A typical diffraction pattern from the dome is displayed as inset. (c) Amorphous dome composition quantification results displayed in a ternary plot.

Fig.

  Fig.5. Spectral components extracted by VCA. Endmembers 1 and 2 can be associated to GST and Ge phases, respectively. In order to estimate the GST phase composition displayed in the ternary plot, spectra with more than 90% of the highest abundance of GST phase were quantified. Endmembers 3 to 5 correspond to titanium nitride, silicon nitride and silicon oxide phases, respectively. The ternary plot displays both GST experimental (i.e. endmember 1) and GST-212 phases.

Fig. 6 .

 6 Fig. 6. (a-d) and (e-h) show results obtained on the RESET-Pass and the RESET-Fail cells, respectively. (a, e) show index correlation maps given by ASTAR software, (b, f) Ge2Sb2Te5, Ge and TiN PED-based phase map,(c, g) and (d, h) abundance maps of the first and second spectral components extracted by VCA, respectively.

Fig. 7 .

 7 Fig. 7. Ternary plot showing the GST compositions of both RESET-Pass and RESET-Fail cells core areas. The composition of the RESET-Pass core area is calculated by taking the average of compositions measured from the amorphous dome (shown in Fig. 3.c). For RESET-Fail cell, we quantified EDX spectra extracted from the GST grain above the heater.

Table 1 :

 1 (Table1). Camera length for ASTAR indexation was calibrated using the silicon substrate. Parameters used for ASTAR theoretical templates creation. For Ge we used Crystallographic

		a, b, c (nm)	Α, β, γ (deg)	Space group	Laue Class
	Ge	5.650	90	f d 3 m	m-3m
	GST	6.027	90	f m 3 m	3-3m

Acknowledgements:

This work, done on the NanoCharacterisation PlatForm (PFNC), was supported by the "Recherches Technologiques de Base" Program of the French Ministry of Research.


Dr Xavier Ayrignac
email: xavier.ayrignac@yahoo.fr

Keywords: Multiple sclerosis, Neuromyelitis optica spectrum disorder, aging, elderly, differential diagnosis Multiple sclerosis, Neuromyelitis optica spectrum disorder, aging, elderly, differential diagnosis

Inflammatory diseases of the central nervous system (CNS) mainly occur during early adulthood and multiple sclerosis (MS) represents the overwhelming majority of these disorders. Nevertheless, MS only rarely begins after 50 years and a diagnosis of lateonset MS should only be done when clinical as well as radiological and biological findings are typical of MS since the probability of misdiagnosis is higher in elderly patients. Indeed, in patients aged over 50 years, along with a relative decrease of MS incidence, other inflammatory diseases of the CNS but also differential diagnoses including neoplastic as well as infectious disorders should be thoroughly searched to avoid diagnostic mistakes and the prescription of inadequate and potentially harmful immunomodulatory/immunosuppressive therapies. Moreover, aging is associated with diverse immune changes also known as immunosenescence resulting in, notably, higher risk of comorbidities (including vascular diseases) and infections which need to be considered when planning medical treatments of elderly patients with inflammatory diseases of the CNS. Herein, therapeutic and diagnostic challenges faced by neurologists are reviewed to ease patient management.

Introduction

Inflammatory diseases of the central nervous system (CNS) represent a large variety of disorders that preferentially occur during early adulthood (i.e. between 20 and 50 years old) [START_REF] Thompson | Multiple sclerosis[END_REF][START_REF] Thompson | Diagnosis of multiple sclerosis: 2017 revisions of the McDonald criteria[END_REF][START_REF] Reich | Multiple Sclerosis[END_REF]. Multiple sclerosis (MS) is by far the most common of these disorders but it should be noted that diagnosis should only be applied in the presence of typical symptoms reminiscent of MS and in the absence of specific red flags (Table 1) suggesting the possibility of an alternative diagnosis [START_REF] Thompson | Diagnosis of multiple sclerosis: 2017 revisions of the McDonald criteria[END_REF][START_REF] Maggi | The "central vein sign" in patients with diagnostic "red flags" for multiple sclerosis: A prospective multicenter 3T study[END_REF][START_REF] Kelly | Using atypical symptoms and red flags to identify non-demyelinating disease[END_REF][START_REF] Street | CNS inflammation other than multiple sclerosis: How likely is diagnosis?[END_REF]. Indeed, in the presence of such red-flags, other non-MS inflammatory diseases must be carefully searched since their prognosis and treatments markedly differ [START_REF] Hardy | Atypical inflammatory demyelinating syndromes of the CNS[END_REF][START_REF] Papadopoulos | Aquaporin 4 and neuromyelitis optica[END_REF][START_REF] Vincent | Autoantibodies associated with diseases of the CNS: new developments and future challenges[END_REF].

Advanced age is one of the most important red flags against MS [START_REF] Thompson | Diagnosis of multiple sclerosis: 2017 revisions of the McDonald criteria[END_REF][START_REF] Maggi | The "central vein sign" in patients with diagnostic "red flags" for multiple sclerosis: A prospective multicenter 3T study[END_REF]. Although MS can occur in elderly patients, this diagnosis should be considered with caution and no patient should be treated for a presumed MS unless clinical, imaging (including spinal cord magnetic resonance imaging (MRI)) and biological features are typical of MS since the probability of misdiagnosis is higher in this population [START_REF] Bar-Or | Central nervous system inflammation across the age span[END_REF][START_REF] Lavandier | First clinical inflammatory demyelinating events of the central nervous system in a population aged over 70 years: A multicentre study[END_REF][START_REF] Mao | Late-onset neuromyelitis optica spectrum disorder in AQP4-seropositive patients in a Chinese population[END_REF].

Moreover, during their aging, people are exposed to multiple immune changes (namely inflammaging) that result in low-grade inflammation leading to defects of adaptive and innate immune systems [START_REF] Sanai | Aging and multiple sclerosis[END_REF][START_REF] Frasca | Inflammaging decreases adaptive and innate immune responses in mice and humans[END_REF][START_REF] Deleidi | Immune aging, dysmetabolism, and inflammation in neurological diseases[END_REF][START_REF] Fulop | Immunosenescence and Inflamm-Aging As Two Sides of the Same Coin: Friends or Foes?[END_REF]. Owing to inflammaging, patients can be exposed to severe complications of highly effective immune therapies including notably infections and cancer.

Herein, we review the diagnostic and therapeutic challenges associated with inflammatory disease of the CNS and provide diagnostic clues that could help to consider differential diagnoses

Main characteristics of inflammatory demyelinating diseases in the elderly

Multiple sclerosis

MS is the most common chronic inflammatory disease of the CNS with an estimated prevalence of 50-300 per 100000 persons and an average sex ratio of approximately 3 females to 1 male [START_REF] Thompson | Multiple sclerosis[END_REF][START_REF] Reich | Multiple Sclerosis[END_REF]. MS onset usually occurs between the third and fourth decades even if paediatric as well as late-onset cases have been described [START_REF] Waldman | Multiple sclerosis in children: an update on clinical diagnosis, therapeutic strategies, and research[END_REF][START_REF] Tremlett | Is late-onset multiple sclerosis associated with a worse outcome?[END_REF][START_REF] Delalande | Late onset multiple sclerosis[END_REF][START_REF] De Seze | Brain MRI in late-onset multiple sclerosis[END_REF].

In young adults, at the beginning of the disease, MS is usually characterised by a relapsing remitting course (i.e. subsequent relapses with complete or almost complete recovery) in the absence of disease progression between relapses [START_REF] Thompson | Multiple sclerosis[END_REF][START_REF] Lublin | Defining the clinical course of multiple sclerosis: the 2013 revisions[END_REF].

MS onset in patients aged over 50 years had been described under the term lateonset MS (LOMS). According to previous series, LOMS roughly represents 3 to 6% of MS cohorts and patients with onset after 60 years old are described in less than 0.5% of cases (some rare patients with onset > 70 years have been described) [START_REF] Roohani | Late onset multiple sclerosis: Is it really late onset?[END_REF][START_REF] Vukusic | Observatoire Français de la Sclérose en Plaques (OFSEP): A unique multimodal nationwide MS registry in France[END_REF]. In these patients, MS more frequently follows a primary progressive course (from 8% to 55%) with or without superimposed relapses and no clear female to male predominance [START_REF] Roohani | Late onset multiple sclerosis: Is it really late onset?[END_REF][START_REF] D'amico | Late-onset and youngonset relapsing-remitting multiple sclerosis: evidence from a retrospective long-term follow-up study[END_REF][START_REF] Kis | Clinical characteristics of patients with late-onset multiple sclerosis[END_REF][START_REF] Lyon-Caen | Late onset multiple sclerosis. A clinical study of 16 pathologically proven cases[END_REF]. Moreover, MRI, as well as biological findings, can be slightly different in LOMS than in younger patients: indeed, it has been rarely described that LOMS can disclose lower percentage of gadolinium enhancing MRI-scan at onset (7% vs 36%) even if CSF results disclose similar proportion of patients with oligoclonal bands (OCBs) (from 50% to 98%) [START_REF] Kis | Clinical characteristics of patients with late-onset multiple sclerosis[END_REF][START_REF] Harding | LATE-ONSET MULTIPLE SCLEROSIS IN SOUTH-EAST WALES[END_REF][START_REF] De | Late onset multiple sclerosis: concerns in aging patients[END_REF].

Additionally, it is important to understand that, even in patients with inflammatory disease, MRI interpretation is complicated by the association of age-related, vascular hyperintensities visible on T2/FLAIR-weighted imaging. Importantly, diagnosis of MS in patients over 50 years has been demonstrated to be delayed suggesting that MS diagnosis in this very specific population is harder than in younger patients.

A single series described the main characteristics associated with very late onset (>70 years-old) inflammatory disease of the CNS [START_REF] Lavandier | First clinical inflammatory demyelinating events of the central nervous system in a population aged over 70 years: A multicentre study[END_REF]. Only 9/25 patients finally had a MS diagnosis including 5/9 PPMS.

Neuromyelitis optica spectrum disorder (NMOSD)

NMOSD is a rare disease that predominantly affects the optic nerve and spinal cord and is characterised by severe relapses with characteristic MRI findings and OCBs in less than 10% of patents [START_REF] Hardy | Atypical inflammatory demyelinating syndromes of the CNS[END_REF][START_REF] Weinshenker | Neuromyelitis Spectrum Disorders[END_REF][START_REF] Krumbholz | B cells and antibodies in multiple sclerosis pathogenesis and therapy[END_REF]. The identification of specific anti-AQP4 and, more recently, anti-MOG autoantibodies has allowed a more precise delineation of these disorders and an expansion of the phenotype from typical neuromyelitis optica to more complex features (Table 2/ Fig. 1) [START_REF] Lennon | A serum autoantibody marker of neuromyelitis optica: distinction from multiple sclerosis[END_REF][START_REF] Kitley | Myelinoligodendrocyte glycoprotein antibodies in adults with a neuromyelitis optica phenotype[END_REF]. Although the age of onset of NMOSD is typically between the third and fourth decades, late-onset NMOSD is relatively more common than late onset MS [START_REF] Zhang | Comparison of deep gray matter lesions on magnetic resonance imaging among adults with acute disseminated encephalomyelitis, multiple sclerosis, and neuromyelitis optica[END_REF][START_REF] Cobo-Calvo | Clinical spectrum and prognostic value of CNS MOG autoimmunity in adults: The MOGADOR study[END_REF][START_REF] Dubey | Clinical, Radiologic, and Prognostic Features of Myelitis Associated With Myelin Oligodendrocyte Glycoprotein Autoantibody[END_REF][START_REF] Collongues | Characterization of neuromyelitis optica and neuromyelitis optica spectrum disorder patients with a late onset[END_REF].

AQP4-positive NMOSD is mainly characterised by longitudinally extensive transverse myelitis (LETM), severe optic neuritis, NMO or characteristic brain (or brainstem) episodes [START_REF] Wingerchuk | International consensus diagnostic criteria for neuromyelitis optica spectrum disorders[END_REF][START_REF] Kitley | Longitudinally Extensive Transverse Myelitis With and Without Aquaporin 4 Antibodies[END_REF][START_REF] Ramanathan | Radiological differentiation of optic neuritis with myelin oligodendrocyte glycoprotein antibodies, aquaporin-4 antibodies, and multiple sclerosis[END_REF]. On MRI, LETM is characterised by an extensive T2hyperintensity spanning ≥ 3 spinal cord segments and some specific features have been suggested [START_REF] Kitley | Longitudinally Extensive Transverse Myelitis With and Without Aquaporin 4 Antibodies[END_REF][START_REF] Zhang | Aquaporin-4-IgG-positive neuromyelitis optica spectrum disorder with recurrent short partial transverse myelitis and favorable prognosis: Two new cases[END_REF][START_REF] Dubey | Association of Extension of Cervical Cord Lesion and Area Postrema Syndrome With Neuromyelitis Optica Spectrum Disorder[END_REF]… Similarly, optic neuritis is different from MS-associated optic neuritis: it is usually severe and/or bilateral, with classical involvement of the intracranial segment (including the chiasma) of the optic nerve and relative extension to the intraorbital segment [START_REF] Ramanathan | Radiological differentiation of optic neuritis with myelin oligodendrocyte glycoprotein antibodies, aquaporin-4 antibodies, and multiple sclerosis[END_REF][START_REF] Petzold | Aquaporin-4 and myelin oligodendrocyte glycoprotein antibodies in immune-mediated optic neuritis at long-term follow-up[END_REF][START_REF] Martinez-Hernandez | Antibodies to Aquaporin 4, Myelin-Oligodendrocyte Glycoprotein, and the Glycine Receptor α1 Subunit in Patients With Isolated Optic Neuritis[END_REF]. Although NMO has been initially described in patients with normal brain MRI, it is noticeable that brain/brainstem syndrome can occur in AQP4-IgG positive NMOSD: even if supratentorial lesions can occur, they are mainly characterised by area postrema syndrome and diencephalic syndrome [START_REF] Camara-Lemarroy | Area postrema syndrome: A short history of a pearl in demyelinating diseases[END_REF][START_REF] Popescu | Neuromyelitis optica unique area postrema lesions: Nausea, vomiting, and pathogenic implications[END_REF].

Importantly, a comparison between anti-AQP4 IgG positive NMOSD patients with age at onset > 50 years and < 49 did not evidence significant differences between young and late NMOSD [START_REF] Mao | Late-onset neuromyelitis optica spectrum disorder in AQP4-seropositive patients in a Chinese population[END_REF].

Following the identification of MOG-IgG as the specific antigen in series of seronegative NMOSD, MOG-IgG-associated disorders (MOGAD) spectrum has been progressively expanded [START_REF] Cobo-Calvo | Clinical spectrum and prognostic value of CNS MOG autoimmunity in adults: The MOGADOR study[END_REF][START_REF] Dubey | Clinical, Radiologic, and Prognostic Features of Myelitis Associated With Myelin Oligodendrocyte Glycoprotein Autoantibody[END_REF]. It currently represents a broad spectrum of (recurrent) optic neuritis, myelitis, NMOSD, acute disseminated encephalomyelitis (ADEM) as well as other encephalitis (Table 3). Apart from ADEM, that seems to be more common in adolescents, all the different clinical presentations of MOGAD seem to occur at various ages including in the elderly [START_REF] Wang | Encephalitis is an important clinical component of[END_REF][START_REF] Zhou | Myelin oligodendrocyte glycoprotein antibody-assoc[END_REF][START_REF] Dubey | [END_REF]. Since the precise extent of the disease is still expanding, it is important to consider this disease in a large variety of situations [START_REF] Cobo-Calvo | Cranial nerve involvement in patients with MOG antibody-associated disease[END_REF].

Other inflammatory diseases of the CNS (figure 2)

Autoimmune encephalitis

Thanks to the recent discovery of several autoantibodies targeting multiple neuronal and glial antigens, autoimmune encephalitis is an increasingly recognised, and potentially treatable condition [START_REF] Behrman | Autoimmune encephalitis in the elderly: who to test and what to test for[END_REF][START_REF] Dubey | Autoimmune encephalitis epidemiology and a comparison to infectious encephalitis: Autoimmune Encephalitis[END_REF]. While classical causes of paraneoplastic encephalitis (mainly related to intracellular antigens: Yo, Hu, Ri, Ma2…) were initially identified in the context of cancer, most of the recently identified autoimmune encephalitis cases (related to synaptic/cell surface antigens: NMDAr, LGI1, CASPR2, GFAP) are more commonly termed "idiopathic". Autoimmune encephalitis should be suspected in the presence of a subacute neurological deficit with predominant short-term memory loss or psychiatric symptoms associated with either evidence of CSF pleocytosis and/or bilateral medial temporal lobe T2-hyperintensities on brain MRI. In the elderly, although NMDAr encephalitis is less common, one should note that almost all of the autoimmune encephalitis have been described. As a consequence, a broad workup looking for all the autoantibodies (synaptic/cell membrane antibodies in the CSF, intracellular antibodies in the blood) and associated cancer is usually needed [START_REF] Graus | A clinical approach to diagnosis of autoimmune encephalitis[END_REF].Acute disseminated encephalomyelitis (ADEM) is an immune-mediated demyelinating disease of the CNS that preferentially occurs during childhood [START_REF] Koelman | Acute disseminated encephalomyelitis in 228 patients: A retrospective, multicenter US study[END_REF][START_REF] Pohl | Acute disseminated encephalomyelitis: Updates on an inflammatory CNS syndrome[END_REF][START_REF] Callen | Role of MRI in the differentiation of ADEM from MS in children[END_REF].

Clinically, it is mainly characterised by multifocal neurologic symptoms and a variable degree of encephalopathy [START_REF] De Seze | Acute fulminant demyelinating disease: a descriptive study of 60 patients[END_REF]. Imaging features commonly include T2-weighted and FLAIR multiple, asymmetric hyperintensities usually involving white matter as well as cortical and deep grey matter [START_REF] Callen | Role of MRI in the differentiation of ADEM from MS in children[END_REF][START_REF] Ketelslegers | A comparison of MRI criteria for diagnosing pediatric ADEM and MS[END_REF][START_REF] Koelman | Acute disseminated encephalomyelitis: prognostic value of early follow-up brain MRI[END_REF]. Although ADEM usually follows a monophasic course, some patients with multiple, recurrent episodes have been described [START_REF] Pohl | Acute disseminated encephalomyelitis: Updates on an inflammatory CNS syndrome[END_REF]. It should also be noted that AQP4 (rarely) and more frequently MOG-IgG have been found in the context of ADEM and should thus systematically screened [START_REF] Hardy | Atypical inflammatory demyelinating syndromes of the CNS[END_REF][START_REF] López-Chiriboga | Association of MOG-IgG Serostatus With Relapse After Acute Disseminated Encephalomyelitis and Proposed Diagnostic Criteria for MOG-IgG-Associated Disorders[END_REF][START_REF] Miyaue | A case of ADEM-like presentation with anti-MOG antibody following tumefactive demyelinating lesion[END_REF].

More recently, autoimmune encephalitis related to GFAP-IgG autoantibodies have been described in patients with acute/subacute meningoencephalomyelitis [START_REF] Flanagan | Glial fibrillary acidic protein immunoglobulin G as biomarker of autoimmune astrocytopathy: Analysis of 102 patients: Autoimmune GFAP Astrocytopathy[END_REF][START_REF] Fang | Autoimmune Glial Fibrillary Acidic Protein Astrocytopathy: A Novel Meningoencephalomyelitis[END_REF].

Although this entity has only recently been described, GFAP autoimmune astrocytopathy can arise in geriatric patients and is mainly characterised by one or more of encephalitis, myelitis or meningitis [START_REF] Dubey | Autoimmune GFAP astrocytopathy: Prospective evaluation of 90 patients in 1 year[END_REF][START_REF] Iorio | Clinical and immunological characteristics of the spectrum of GFAP autoimmunity: a case series of 22 patients[END_REF]. CSF analysis usually discloses pleocytosis and elevated protein and OCBs are found in > 50% of patients [START_REF] Flanagan | Glial fibrillary acidic protein immunoglobulin G as biomarker of autoimmune astrocytopathy: Analysis of 102 patients: Autoimmune GFAP Astrocytopathy[END_REF]. On MRI a striking pattern of radial perivascular gadolinium enhancement is observed although more recent series suggest a broader spectrum of MRI abnormalities.

Primary angiitis of the central nervous system (PACNS)

PACNS is a rare disease characterised by vessel wall inflammatory infiltrate restricted to the CNS [START_REF] Salvarani | Primary central nervous system vasculitis: analysis of 101 patients[END_REF][START_REF] De Boysson | Primary Angiitis of the Central Nervous System: Description of the First Fifty-Two Adults Enrolled in the French Cohort of Patients With Primary Vasculitis of the Central Nervous System: Findings in a French Cohort of Patients With PACNS[END_REF][START_REF] De Boysson | Primary angiitis of the CNS and reversible cerebral vasoconstriction syndrome: A comparative study[END_REF]. Diagnosis requires the identification of histological evidence of vascular lesions with angiocentric transmural inflammatory infiltrates and vessel wall damage or angiographic evidence of typical segmental stenoses/dilatations [START_REF] Hajj-Ali | Primary angiitis of the CNS[END_REF][START_REF] Singhal | Reversible cerebral vasoconstriction syndromes and primary angiitis of the central nervous system: clinical, imaging, and angiographic comparison: RCVS and PACNS[END_REF]. Most patients present several clinical manifestations including headaches, transient ischaemic attacks, stroke, seizures of altered cognition. CSF analysis is abnormal in more than 50% of patients revealing an elevated leukocyte count and/or raised protein concentration [START_REF] De Boysson | Primary Angiitis of the Central Nervous System: Description of the First Fifty-Two Adults Enrolled in the French Cohort of Patients With Primary Vasculitis of the Central Nervous System: Findings in a French Cohort of Patients With PACNS[END_REF]. MRI usually shows findings suggestive of PACNS typically evidencing disseminated infarcts usually involving subcortical structures associated with gadolinium enhancing FLAIR hyperintensities [START_REF] Singhal | Reversible cerebral vasoconstriction syndromes and primary angiitis of the central nervous system: clinical, imaging, and angiographic comparison: RCVS and PACNS[END_REF][START_REF] Boulouis | Primary Angiitis of the Central Nervous System n[END_REF]. Susceptibility imaging is of importance since hemosiderin deposits are highly suggestive of a vascular phenomenon [START_REF] Huang | Susceptibility-weighted imaging in the differential diagnosis of autoimmune central nervous system vasculitis and multiple sclerosis[END_REF][START_REF] Albayram | Evaluation of parenchymal neuro-behçet disease by using susceptibility-weighted imaging[END_REF]. Rarely, PACNS can present with an isolated cerebral lesion mimicking brain tumour. Computed tomographic (CT) or magnetic resonance (MR) angiography usually disclose alternating stenoses and dilatations highly suggestive of the diagnosis but, in some cases, histopathological analysis is necessary. Although PACNS is a rare disease, it should be considered in the differential diagnosis of all inflammatory diseases of the elderly since nearly one quarter of the patients are older than 50 years [START_REF] De Boysson | Primary Angiitis of the Central Nervous System: Description of the First Fifty-Two Adults Enrolled in the French Cohort of Patients With Primary Vasculitis of the Central Nervous System: Findings in a French Cohort of Patients With PACNS[END_REF].

It is noticeable that other inflammatory diseases with CNS involvement exist, they mainly include neurosarcoidosis, systemic lupus and Sjogren syndrome [START_REF] Brownlee | Diagnosis of multiple sclerosis: progress and challenges[END_REF][START_REF] Shah | Correlation of MR Imaging Findings and Clinical Manifestations in Neurosarcoidosis[END_REF][START_REF] Joubert | Association of Prognostic Factors and Immunosuppressive Treatment With Long-term Outcomes in Neurosarcoidosis[END_REF]. The main causes are disclosed in Table 2.

Differential diagnosis (Fig. 3)

The spectrum of differential diagnoses of inflammatory diseases of the CNS in the elderly is broad [START_REF] Brownlee | Diagnosis of multiple sclerosis: progress and challenges[END_REF]. Notably, in the context of acute/subacute neurological deficits, it includes CNS tumours and, more rarely, infectious diseases. Indeed, it can be complicated to distinguish inflammatory diseases from CNS lymphoma and brain biopsy is sometimes needed [START_REF] Lucchinetti | Clinical and radiographic spectrum of pathologically confirmed tumefactive multiple sclerosis[END_REF][START_REF] Kim | Distinguishing tumefactive demyelinating lesions from glioma or central nervous system lymphoma: added value of unenhanced CT compared with conventional contrast-enhanced MR imaging[END_REF]. The main radiological clues that help to differentiate these two diseases include CT-scan and diffusion-weighted imaging (DWI) hyperintensities associated with punctate and curvilinear or nodular central enhancement that are rarely found in inflammatory demyelinating disorders [START_REF] Kim | Distinguishing tumefactive demyelinating lesions from glioma or central nervous system lymphoma: added value of unenhanced CT compared with conventional contrast-enhanced MR imaging[END_REF][START_REF] Lu | Utility of proton MR spectroscopy for differentiating typical and atypical primary central nervous system lymphomas from tumefactive demyelinating lesions[END_REF].

Moreover, it has been recently shown that CSF IL-10 levels were elevated in CNS lymphoma and could be of additional diagnostic value in this context [START_REF] Ikeguchi | CSF and clinical data are useful in differentiating CNS inflammatory demyelinating disease from CNS lymphoma[END_REF].

In the context of progressive worsening of gait and/or cognitive disturbance, one of the main issues is to differentiate chronic microangiopathy from inflammatory demyelinating disorders (mainly MS). A thorough analysis of MRI features is mandatory since, in patients over 50 years and especially thosewith vascular risk factors, inflammatory and microangiopathy lesions can coexist [START_REF] Weisfeld-Adams | Differential diagnosis of Mendelian and mitochondrial disorders in patients with suspected multiple sclerosis[END_REF]. This analysis can help to distinguish those two conditions (table 3):

-Central pons, temporal pole, deep basal ganglia and external capsule involvement as well as cerebral microbleeds and calcifications are in favour of a vascular process.

-Juxtacortical, cortical and periventricular lesions (perpendicular to lateral ventricles) as well as open ring gadolinium enhancement strongly argues in favour of MS.

Finally, some very rare diagnoses including progressive multifocal leukoencephalopathy (PML), chronic lymphocytic inflammation with pontine perivascular enhancement responsive to steroids (CLIPPERS) and some other rare diseases can be considered in the differential diagnosis of inflammatory diseases in the elderly [START_REF] Van Schependom | Assessing PML risk under immunotherapy: if all you have is a hammer, everything looks like a nail[END_REF][START_REF] Warnke | PML: The Dark Side of Immunotherapy in Multiple Sclerosis[END_REF][START_REF] Taieb | CLIPPERS and its mimics: evaluation of new criteria for the diagnosis of CLIPPERS[END_REF]. They are listed in Table 3.

What are the main diagnostic clues?

Even though the number of published studies focusing on inflammatory diseases of the CNS in elderly patients is very low, it seems to be clear that the overwhelming overrepresentation of MS among all these diseases does not remain after 50 years old and more so over after 60 or 70 years old [START_REF] Lavandier | First clinical inflammatory demyelinating events of the central nervous system in a population aged over 70 years: A multicentre study[END_REF]. This was well described by Lavandier et al., who presented a series of 25 cases of which only nine were finally diagnosed as MS. It is therefore of importance to perform a complete imaging and biological workup searching for evidence of an alternative diagnosis (Table 3).

After a complete workup is done, there are important imaging diagnostic clues that need to be considered:

-Acute partial myelitis notably involving the external part of the spinal cord, evidence of asymptomatic optic nerve involvement (on visual evoked potential and/or optic coherence tomography) as well as presence of ≥ 2 oligoclonal bands are highly in favour of MS.

-Acute transverse myelitis and severe optic neuritis strongly suggest either AQP4-or MOG-IgG associated disease. If antibody screening is negative, it should be renewed (AQP4) and research should be done within the CSF (MOG) [START_REF] Asnafi | The frequency of longitudinally extensive transverse myelitis in MS: A population-based study[END_REF][START_REF] Bourre | Long-term Follow-up of Acute Partial Transverse Myelitis[END_REF][START_REF] Mariotto | Relevance of antibodies to myelin oligodendrocyte glycoprotein in CSF of seronegative cases[END_REF].

-Susceptibility weighted imaging (either SWI or T2-GRE) and CT-scan can identify microhaemorrhages and/or calcifications that suggest a vascular process. In all the cases, when no cause is obvious, a minimal vascular workup is needed to explore extracranial and intracranial vessels [START_REF] Huang | Susceptibility-weighted imaging in the differential diagnosis of autoimmune central nervous system vasculitis and multiple sclerosis[END_REF].

-Punctate and curvilinear gadolinium enhancement are more frequent in lymphoma, CLIPPERS, vasculitis and systemic diseases [START_REF] Taieb | Punctate and curvilinear gadolinium enhancing lesions in the brain: a practical approach[END_REF]. Their identification should lead to appropriate complementary workup including at least CSF IL10/IL6 assessment, whole body PET-FDG and a minimal workup looking for systemic involvement.

-New MRI techniques, including the identification of central vein on SWI/FLAIR* will probably help to separate MS from other inflammatory diseases of the CNS but their relevance in elderly patients (notably due to the association of age-related vascular changes) is not known [START_REF] Maggi | The "central vein sign" in patients with diagnostic "red flags" for multiple sclerosis: A prospective multicenter 3T study[END_REF][START_REF] Cortese | Value of the central vein sign at 3T to differentiate MS from seropositive NMOSD[END_REF][START_REF] Maggi | Central vein sign differentiates Multiple Sclerosis from central nervous system inflammatory vasculopathies[END_REF] A careful analysis of all the clinical and radiological findings should normally help to identify the exact diagnosis. Nevertheless, in some cases, brain biopsy can be required to identify the exact mechanism.

Treatment specificities in elderly patients

Aging is associated with immune changes including thymic involution, reduced repertoire diversity, loss in naïve T cells as well as epigenetic changes and circulating cytokines increase that results in chronic low-grade inflammation also named inflammaging [START_REF] Bolton | The influence and impact of ageing and immunosenescence (ISC) on adaptive immunity during multiple sclerosis (MS) and the animal counterpart experimental autoimmune encephalomyelitis (EAE)[END_REF][START_REF] Bueno | Ageing and myeloid-derived suppressor cells: possible involvement in immunosenescence and age-related disease[END_REF]. Moreover, elderly patients are exposed to multiple comorbidities including cardiovascular risk factors [START_REF] Zhang | Effects of physical comorbidities on disability progression in multiple sclerosis[END_REF]. These considerations are of importance in patients who may suffer from advanced disability that can also increase the risk of potentially life-threatening medication-related adverse effects including notably infections [START_REF] Grebenciucova | Infections in Patients Receiving Multiple Sclerosis Disease-Modifying Therapies[END_REF].

In the context of MS for example, most infections (including opportunistic infections) in the context of disease modifying therapies occur preferentially in patients over 50 [START_REF] Carpenter | Cutaneous cryptococcosis in a patient taking fingolimod for multiple sclerosis: Here come the opportunistic infections?[END_REF][START_REF] Brownlee | Opportunistic infections after alemtuzumab: New cases of norcardial infection and cytomegalovirus syndrome[END_REF]. Moreover, natalizumab-associated primary multifocal leukoencephalopathy risk has been shown to increase in elderly patients [START_REF] Prosperini | Age as a risk factor for early onset of natalizumab-related progressive multifocal leukoencephalopathy[END_REF]. Recently, leflunomide (the teriflunomide metabolite) has been shown to be associated with pulmonary arterial hypertension in patients > 50 years [START_REF] Coirier | Pulmonary arterial hypertension in four patients treated by leflunomide[END_REF]. Even if teriflunomide has not been associated with a similar adverse effect, ageing patients should be more closely monitored since they are probably more prone to develop potentially severe adverse effects than younger people. Finally, one should note that patients > 50 years are at higher risk of having previous history of stroke or myocardial infarcts, having atrioventricular blocks and being under treatments that either contraindicate or require specific consideration before considering a treatment like fingolimod [START_REF] Vargas | Fingolimod and cardiac risk: latest findings and clinical implications[END_REF].

Taken together, all these changes that occur in ageing patients absolutely need to be considered when planning medical management (including potentially highly effective immunotherapies) of patients with suspected inflammatory disease.

Conclusion

Whereas MS is by far the most common cause of inflammatory diseases of the CNS in young adults, one should note that, in elderly patients, the probability of having MS is markedly reduced and should prompt complete biological and radiological workup to identify specific clues as well as major red flags that can help to recognise alternative diagnoses.

Owing to immune changes that occur along with ageing exposing patients to higher probability of infections along with the presence of multiple comorbidities, it is also noticeable that therapeutic management in this particular population needs to be thoroughly considered in order to maximize the benefit/risk ratio and reduce the number of potentially severe adverse effects.

In this review, we try to focus on the main diagnostic clues that should help clinicians to accurately identify specific red flags for MS and then reconsider the diagnosis. We also give a rapid overview of the main differential diagnoses of inflammatory disease of the CNS and underline the important need for complete biological and radiological workup that is essential to avoid misdiagnosis and the initiation of potentially inadequate treatments.

TABLES :

Figures legend:Figure 1 :

 1 Figures legend:

Figure 2 :

 2 Figure 2 : MRI abnormalities in other inflammatory diseases of the CNS.

Figure 3 :

 3 Figure 3: Disorders that can mimic inflammatory disease of the central nervous

Table 1 :

 1 Atypical features for MS ("red flags")

	Red flags
	Clinical
	Age at onset > 50 years or age at onset < 20 years
	Optic neuritis: severe, bilateral or poor recovery
	Myelitis: bilateral, severe motor weakness
	Abrupt onset, rapid cognitive decline at onset, headaches
	History of systemic disease, aphtosis, retinal vasculitis, hearing loss
	Laboratory (CSF)
	Absence of OCB
	White blood cell count > 50 cells/mm 3
	Proteinorrhachia > 100 mg/dL
	Imaging
	Atypical morphology/distribution of brain white matter lesions
	Longitudinal extensive transverse myelitis
	Extended/posterior and/or bilateral optic neuritis

Table 2 :

 2 Main features differentiating MS from NMOSD in adults

		MS	AQP4-IgG NMOSD	MOGAD
	Clinically	Acute partial myelitis	Longitudinally extensive	Longitudinally extensive
		Unilateral optic neuritis	transverse myelitis	transverse myelitis
		Typical brain/brainstem	Recurrent, bilateral & severe	Recurrent, bilateral & severe
		syndrome	optic neuritis with limited	optic neuritis,
			recovery	Steroid responsiveness
			NMO	ADEM,
			Brainstem syndrome	Steroid responsive epileptic
				encephalopathies
	Biological	OCBs in > 90%	OCBs in 10-20%	OCBs in +/-5%
		Normal WBC	Slightly elevated WBC count	Markedly elevated WBC
				count
	Brain MRI	Periventricular lesion	Brain lesion around	Fluffy lesions
		centered by a small central	subependymal area,	No lesion adjacent to the
		vein (Dawson's finger)	Area postrema, medulla	lateral ventricle,
		Cortical lesions	oblongata involvement	Pons, thalamus involvement
		≥ 9 lesions
	Optic nerve	Canalicular, unilateral,	Intracranial, chiasmal,	Optic nerve head swelling,
	MRI	Short	Bilateral	Bilateral,
				Perineural gadolinium
				enhancement
	Spinal cord	Short partial myelitis	LETM	LETM
	MRI	Multiple lesions	Bright spotty lesion	Multiple lesions
				H sign,
				Linear sagittal hyperintensity
				Conus involvement

Table 3 :

 3 Other inflammatory disease of the CNS and differential diagnosis: main differentiating features

		Clinical findings	Main MRI findings CSF	Diagnostic clues
	CNS vasculitis	Headaches,	Punctate Gd+	WBC count ↗	Angiography,
		transient ischaemic	lesions,		biopsy
		attacks/strokes,	Small ischemic
		encephalopathy	dots
			Hemorrhages
	Acute disseminated	Multifocal CNS	Multiple FLAIR	OCBs usually	Absence of
	encephalomyelitis	involvement,	WM	absent	clinical/radiological
		encephalopathy	hyperintensities,		evolution after 3
			most of the		months, MOG, AQP4
			lesions Gd+		Abs
	Autoimmune	Limbic encephalitis,	Temporal lobe T2-	Variable	Cell-surface/synaptic
	encephalitis	psychiatric	hyperintensities		Abs (CSF), Intracellular
		symptoms, seizures,			Abs (serum)
		FBDS…
	GFAP encephalitis	Encephalomyelitis,	Radial	OCBs in >50%	GFAP-IgG testing
		Meningitis	enhancement	WBC count ↗	(serum/CSF)
			Punctate and
			curvilinear
			enhancement,
	Neurosarcoidosis	Facial palsy	Predominantly	OCBs rare	PET-scan, thoracic
		(bilateral), systemic	meningeal		imaging, biopsy
		involvement	involvement
	Connective tissue	Predominantly	Variable	OCBs usually	Systemic features
	disorder	systemic		absent
		involvement
	CNS lymphoma	Variable	Punctate or	OCBs usually	FDG PET-Scan,
			nodular Gd+	absent,	Raised IL10/IL6
			lesions	WBC count ↗
	CLIPPERS	Brainstem	Punctate	OCBs sometimes	Brain biopsy
		symptoms	brainstem Gd+	present
		Steroid responsive	lesions
		steroid dependent
	Cerebral	Progressive walking	Variable white		Vascular risk factors
	microangiopathy	and cognitive	matter		(age, HBP)
		difficulties	hyperintensities,
			Small ischemic
			dots,
			haemorrhages

Diagnostic and therapeutic issues of inflammatory diseases of the elderly X Ayrignac 1 , C Carra-Dallière 1 , P Labauge 1 Département de Neurologie, CRC sclérose en plaques, CHU Montpellier, Univ Montpellier, Inserm, Montpellier, France