Towards integrating chalcogenide based phase change memory with silicon microelectronics

The continued dominance of floating gate technology as the premier non-volatile memory (NVM) technology is expected to hit a roadblock due to issues associated with its inability to catch up with CMOS scaling. The uncertain future of floating gate memory has led to a host of unorthodox NVM technologies to surface as potential heirs. Among the mix is phase change memory (PCM), which is a non-volatile, resistance variable, memory technology wherein the state of the memory bit is defined by the resistance of the memory material. This research study examines novel, bilayer chalcogenide based materials composed of Ge-chalcogenide (GeTe or Ge2Se3) and Sn-chalcogenide (SnTe or SnSe) for phase change memory applications and explores their integration with CMOS technology. By using a layered arrangement, it is possible to induce phase change response in materials, which normally do not exhibit such behavior, and thus form new materials which may have lower threshold voltage and programming current requirements. Also, through the incorporation of a metal containing layer, the phase transition characteristics of the memory layer can be tailored in order to obtain in-situ, a material with optimized phase change properties. Using X-ray diffraction (XRD) and time resolved XRD, it has been demonstrated that stacked phase change memory films exhibit both structural and compositional dependency with annealing temperature. The outcome of the structural transformation of the bottom layer, is an annealing temperature dependent residual stress. By the incorporation of a Sn layer, the phase transition characteristics of Ge-chalcogenide thin films can be tuned. Clear evidence of thermally induced Ge, Sn and chalcogen inter-diffusion, has been discerned via transmission electron microscopy and parallel electron energy loss spectroscopy. The presence of Al2O3 as capping layer has been found to mitigate volatilization and metallic Sn phase separation at high temperatures. Two terminal PCM cells employing these bilayers have been designed, fabricated and tested. All devices exhibit threshold switching and memory switching behavior. By the application of suitable voltage programming pulses, RESET state switching can be accomplished in these devices, thus demonstrating single bit memory functionality. A process for integrating bilayer PCM technology with 2 µm CMOS has been designed and developed. The baseline RIT CMOS process has been modified to incorporate 12 levels of photolithography, 3 levels of metal and the addition of PCM as a BEOL process. On electrical testing, NMOS connected PCM devices exhibit switching behavior. The effect of the state (SET/RESET) of the series connected PCM cell on the drain current of the NMOS has also been investigated. It is determined that threshold switching of the PCM cell is essential in order to observe any change in MOS drain current with variation in drain voltage. Thus, successful integration of bilayer PCM with CMOS has been demonstrated

Magnetic Calorimeter Option for the Lynx X-Ray Microcalorimeter

One option for the detector technology to implement the Lynx x-ray microcalorimeter (LXM) focal plane arrays is the metallic magnetic calorimeter (MMC). Two-dimensional imaging arrays of MMCs measure the energy of x-ray photons by using a paramagnetic sensor to detect the temperature rise in a microfabricated x-ray absorber. While small arrays of MMCs have previously been demonstrated that have energy resolution better than the 3 eV requirement for LXM, we describe LXM prototype MMC arrays that have 55,800 x-ray pixels, thermally linked to 5688 sensors in hydra configurations, and that have sensor inductance increased to avoid signal loss from the stray inductance in the large-scale arrays when the detectors are read out with microwave superconducting quantum interference device multiplexers, and that use multilevel planarized superconducting wiring to provide low-inductance, low-crosstalk connections to each pixel. We describe the features of recently tested MMC prototype devices and simulations of expected performance in designs opti- mized for the three subarray types in LXM

Multimode bolometer development for the PIXIE instrument

The Primordial Inflation Explorer (PIXIE) is an Explorer-class mission concept designed to measure the polarization and absolute intensity of the cosmic microwave background. In the following, we report on the design, fabrication, and performance of the multimode polarization-sensitive bolometers for PIXIE, which are based on silicon thermistors. In particular we focus on several recent advances in the detector design, including the implementation of a scheme to greatly raise the frequencies of the internal vibrational modes of the large-area, low-mass optical absorber structure consisting of a grid of micromachined, ion-implanted silicon wires. With $\sim30$ times the absorbing area of the spider-web bolometers used by Planck, the tensioning scheme enables the PIXIE bolometers to be robust in the vibrational and acoustic environment at launch of the space mission. More generally, it could be used to reduce microphonic sensitivity in other types of low temperature detectors. We also report on the performance of the PIXIE bolometers in a dark cryogenic environment.Comment: 10 pages, 7 figure

Prototype Magnetic Calorimeter Arrays with Buried Wiring for the Lynx X-Ray Microcalorimeter

Metallic magnetic calorimeter (MMC) technology is a leading contender for detectors for the Lynx X-ray Microcalorimeter, which is an imaging spectrometer consisting of an array of greater than 100,000 pixels. The fabrication of such large arrays presents a challenge when attempting to route the superconducting wiring from the pixels to the multiplexed readout. If the wiring is designed to be planar, then an aggressive, submicron scale wiring pitch has to be employed, which is technically challenging to design and fabricate on account of the requirements of low inductance, low cross-talk, high critical currents and high yield. An alternative way to achieve large scale, high density wiring is through the use of multiple buried metal layers, planarized by Chemical Mechanical Planarization. This approach is well-suited for connecting thousands of pixels on a large focal plane to readout chips, and also for fabricating sensor meander coils with narrow line widths, which helps in increasing the sensor inductance and thus alleviates stray inductance issues associated with the wiring in large size arrays. In this work we describe the fabrication of high sensor inductance MMC arrays implementing Lynx concepts and incorporating multiple layers of buried Nb wiring. The detector array is composed of three sub-arrays with pixels optimized to meet the different science driven performance requirements of Lynx. In two of the sub-arrays we adopt a thermal multiplexing scheme to read out pixels by coupling 25 absorbers to a single sensor through thermal links of varied thermal conductance. We demonstrate the successful fabrication of multi-absorber MMCs with fine pitch pixels in very large size arrays

Multimode Bolometer Development for the PIXIE Instrument

The Primordial Inflation Explorer (PIXIE) is an Explorer-class mission concept designed to measure the polarization and absolute intensity of the cosmic microwave background. In the following, we report on the design, fabrication, and performance of the multimode polarization-sensitive bolometers for PIXIE, which are based on silicon thermistors. In particular we focus on several recent advances in the detector design, including the implementation of a scheme to greatly raise the frequencies of the internal vibrational modes of the large-area, low-mass optical absorber structure consisting of a grid of micromachined, ion-implanted silicon wires. With approximately 30 times the absorbing area of the spider-web bolometers used by Planck, the tensioning scheme enables the PIXIE bolometers to be robust in the vibrational and acoustic environment at launch of the space mission. More generally, it could be used to reduce microphonic sensitivity in other types of low temperature detectors. We also report on the performance of the PIXIE bolometers in a dark cryogenic environment

Multimode Bolometer Development for the Primordial Inflation Explorer (PIXIE) Instrument

The Primordial Inflation Explorer (PIXIE) is an Explorer-class mission concept designed to measure the polarization and absolute intensity of the cosmic microwave background [1]. In this work, we report on the design, fabrication, and performance of the multimode polarization-sensitive bolometers for PIXIE, which are based on silicon thermistors. In particular we focus on several recent advances in the detector design, including the implementation of a tensioning scheme to greatly raise the frequencies of the internal vibrational modes of the large-area, low-mass optical absorber structure consisting of a grid of micromachined, ion-implanted silicon wires. With 30 times the absorbing area of the spider-web bolometers used by Planck, the tensioning scheme enables the PIXIE bolometers to be robust in the vibrational and acoustic environment at launch of the space mission. More generally, it could be used to reduce microphonic sensitivity in other types of low temperature detectors. We also report on the performance of the PIXIE bolometers in a dark cryogenic environment

Residual Stress Analysis of Stacked SnTe/Ge2Se3 Phase Change Memory Films using Vantec 2000 Area Detector

Residual stress in novel chalcogenide thin film stacks of polycrystalline SnTe and amorphous Ge2Se3 is measured using X-ray Diffraction (XRD). The as-deposited film stacks are annealed at different temperatures and the thermal dependence of stress is investigated. Stress evaluations are performed by 2 employing the sin ψ technique using a 2D area detector system. As-deposited samples are found to be compressively stressed and exhibit increasingly tensile thermal stress with increasing annealing temperature. Onset of crystallization of the bottom Ge2Se3 layer is indicated by a dip in the stress level in the 270 o C – 360 o C temperature range, due to volume shrinkage associated with the crystallization. Diffraction patterns of samples annealed at different temperatures indicate compositional changes that are attributed to inter-diffusion of ions between the two layers. The XRD profiles of samples annealed at 360 o C and 450 o C indicate the formation of a SnTe-GeSe solid solution. It is suggested that both, residual stress and temperature dependent compositional changes affect the measured d spacings. 1

Influence of Sn Migration on Phase Transition in GeTe and Ge\u3csub\u3e2\u3c/sub\u3eSe\u3csub\u3e3\u3c/sub\u3e Thin Films

Phase transitions in GeTe/SnSe and Ge2Se3/SnTe are investigated using time resolved x-ray diffraction. GeTe exhibits a structural transition from rhombohedral to the cubic phase at 300 °C, which is ∼ 100 °C lower than that of pure GeTe. This is facilitated by incorporation of Sn from SnSe. Sn migration is observed explicitly in Ge2Se3/SnTe by separation of SnSe phase. Amorphous Ge2Se3 is also found to crystallize at a lower temperature of 300 °C resulting in orthorhombic GeSe and monoclinic GeSe2. Thus, inclusion of a Sn containing layer may offer a means to tailor phase transition in Ge-chalcogenide thin films for phase change memory applications

Investigation of Inter-Diffusion in Bilayer GeTe/SnSe Phase Change Memory Films

A metal-chalcogenide layer, SnSe, is inserted between the memory layer GeTe and the top electrode to form a phase change memory cell. The GeTe layer exhibits ovonic threshold switching at a threshold field of ~ 110 V/μm. For subsequent implementation into applications and reliability, material inter-diffusion and sublimation are examined in bilayer phase change films of GeTe/SnSe. Transmission electron microscopy and parallel electron energy loss spectroscopy analyses reveal Sn migration to the GeTe layer, which is responsible for lowering the rhombohedral to cubic structural transformation temperature in GeTe. Incongruent sublimation of SnSe and GeTe is observed at temperatures higher than 500 °C. Severe volatilization of Se results in the separation of a metallic Sn phase. The use of Al2O3 as a capping layer has been found to mitigate these effects