Christopher McWilliams

RE-PROGRAMMABLE ANTIFUSE FPGA UTILIZING RESISTIVE CeRAM ELEMENTS

A novel architecture utilizing resistive Correlated Electron Random Access Memory (CeRAM) elements to build re-programmable antifuse Field Programmable Gate Array (FPGA) components is introduced. Unlike traditional antifuse switching elements that can become permanently conducting upon application of the programming voltage, CeRAM antifuse elements can be subsequently switched back to an insulating state upon application of a reset voltage allowing the designer the freedom to re-configure the FPGA. Because the CeRAM element is based on a metal-insulator-metal (MIM) stack structure, it can be easily integrated in the upper metal layers of the fabrication process virtually eliminating the need for routing of channels between logic blocks. By utilizing CeRAM structures in the routing resources and as the block memory, it is possible to enhance the density by as much as 6 times, significantly reducing the size of the array while maintaining the use of existing low power logic styles.

A non-filamentary model for unipolar switching transition metal oxide resistance random access memories

A model for resistance random access memory (RRAM) is proposed. The RRAM under research utilizes certain transition metal oxide (TMO) such as NiO which shows unipolar switching behavior. The existence of metal/insulator states is not explained by filaments but attributed to different Hubbard U values, which stems from an electron correlation effect. Current-voltage formulae are given both on the metal and insulator sides by putting the appropriate solutions of Hubbard model into the mesoscopic Meir-Wingreen transport equation. The RESET phenomenon is explained by a sufficient separation of Fermi levels in the electrodes and hence a Mott transition can be triggered in the anodic region due to a lack of electrons. The SET behavior originates from a tunneling current which removes the insulating region near the anode. Several experimental evidences are also presented to support this model. The model also serves as the theoretical prototype of Correlated Electron Random Access Memory (CeRAM) which is defined to be a TMO RRAM whose working mechanism is based on the strong electron correlation effects.

Material and process optimization of correlated electron random access memories

A method of making transition metal oxide materials that result in resistive switching properties stable over time and temperature is described. We have developed an ultra low temperature (≤450°C) process for carbonyl ligand modified NiO thin films based on the chemical solution deposition (CSD) for correlated electron random access memory (CeRAM) applications. CeRAMs form the general class of devices that use the electron-electron interaction as the primary mode of operation. These devices are fabricated in the conductive state (born-ON), thus, they do not require electroforming to enter the variable resistance state. Several process parameters such as film stoichiometry, thickness, annealing temperature and ambient have been investigated to optimize CeRAMs properties. We present the coordination number ‘fine tuning’ in NiO ultra thin films via carbonyl ligand doping that regulate the number of oxygen vacancies and the surface excess of metal ions. CeRAMs contrary to just standard NiO based resistive mem...

Device characterization of correlated electron random access memories

The switching properties and characterization of correlated electron random Access Memories (CeRAMs) are described herein. High temperature retention, cycle dispersion and optimization, cycle Fatigue, and switching parameter optimization have been investigated. CeRAM’s display initially conductive or “born-ON” behavior without the need for the high electroforming voltages usually required for other transition metal oxide based resistive memories. Nonvolatile data retention at elevated temperatures up to 573 K (300 °C) in addition to a wide operating range from 4 to 423 K for CeRAM has been confirmed. CeRAMs also show exceptional read endurance with no evidence of fatigue out to 1012 cycles. Desirable scaling characteristics for high density memory application have also been shown for CeRAMs due to a widening of the read window and consistent write window as devices are scaled down.

Investigation on Divalent Metal Substituted Bismuth Titanate Ferroelectric Thin Films

Divalent metals Ca, Sr, and Ba were employed to substitute for some of the A-site bismuth in Bi4Ti3O12 (BIT). The corresponding ferroelectric thin films (named CBiT, SBiT and BBiT) were derived by metal organic decomposition. Their remanent polarization (2Pr) values were 17.32 μC/cm2, 8.88 μC/cm2 and 21.34 μC/cm2. The smallest 2Pr value was found in SBiT, where its cation (Sr2+) radius is in the middle of the three. On the other hand, the smallest coercive field was discovered in BBiT. No fatigue improvement was discovered in BBiT compared with BIT, probably due to the lowered charge amount carried by Ba2+.

Active mode detection with enhanced pyroelectric sensitivity

A MEMS-less infrared pyroelectric sensor that employs an active detection mechanism based on a strontium bismuth tantalate (SrBi2Ta2O9) ferroelectric sensing material is described and compared to passive modes of operation. A model is based on fundamental performance of ferroelectrics in which the polarization state of the material is actively interrogated enabling improved signal to noise ratio, greater effective pyroelectric coefficient, and chopper-less design. In addition to excellent thermal responsivity in the medium and long wavelength bands and unlimited endurance, the unique design enables selective wavelength tuning of insulating layer and absorber materials to maximize the responsivity at distinct wavelengths.

Operating Current Reduction in Nickel Oxide Correlated Electron Random Access Memories (CeRAMs) Through Controlled Fabrication Processes

Low temperature process for metal organic decomposition (MOD) derived stable and charge compensated thin NiO films is presented. The coordination number ‘fine tuning’ in nickel oxide Correlated Electron Random Access Memories (CeRAMs) is obtained via carbonyl ligand doping. Further regulation of the number of oxygen vacancies and the surface excess of metal ions is realized by selecting the most stable metal-oxide interfaces. Inserting the ultrathin homo- or heterogeneous oxide layers or oxygen diffusion barriers into the top metal-oxide interface is just one example. We have demonstrated the improved performance of the CeRAM cell obtained by a combination of material doping, layer grading, oxygen diffusion barrier and the small cell sizes. It is shown that optimization of various process conditions leads to the endurance parameters improvement and the write current reduction for relatively large device areas with external compliance circuitry. Further current reduction through the cell size scaling and the on-chip compliance circuitry will ensure CeRAMs programming speed sufficient for high-speed performance. These factors combined together unfold a novel opportunity for the fabrication of a memory cell with superior characteristics for universal high-density non-volatile memory.

Advanced Dynamic Pyroelectric Focal Plane Array

The pyroelectric effect has been characterized for single-pixel elements consisting of strontium bismuth tantalate (SBT) ferroelectric material as the sensing elements. These pixels have been integrated into second-generation focal plane arrays. The constituent second-generation pixels include thermal insulating layers and an infrared absorber layer. The MEMS-less arrays are operated in active mode, a technique that eliminates radiation choppers found in other passive pyroelectric IR imagers. This paper addresses the results of precursor 2x2 to 14x14 second-generation arrays of SBT elements, the active detection mechanism, and the unique read-out, interrogation signal, and the synchronization electronics. The second-generation 14x14 pixels array was implemented to demonstrate the performance of an active pyroelectric array as a precursor to larger size arrays using different pixel dimensions. The active mode detection eliminates the use of a chopper, enables the dynamic partition of the array into pixel domains in which pixel sensitivity in the domains can be adjusted independently. This unique feature in IR detection can be applied to the simultaneous tracking of diverse contrast objects. In addition, by controlling the thickness of the absorber material the arrays can be optimized for maximum response at specified wavelengths by means of quarter-wavelength interferometry.

CHARACTERIZATION OF SECOND GENERATION ADVANCED DYNAMIC PYROELECTRIC FOCAL PLANE ARRAY

ABSTRACT The pyroelectric effect has been characterized for single-pixel elements consisting of strontium bismuth tantalate (SBT) ferroelectric material as the sensing element. The pixels include also a thermal insulating layer and an infrared (IR) absorber layer. These MEMS-less devices are operated in active mode, a technique that eliminates the need for a radiation chopper found in passive pyroelectric IR imagers. Test results of the SBT pixels of dimensions 7.5 μm × 7.5 μm to 200 μm × 200 μm have shown high endurance to polar cycling, high responsivity values, and very low noise-equivalent temperature difference for focal plane array applications. This paper describes and analyses the results of precursor 2 × 2 arrays using discrete sensing elements, the active detection mechanism, and its unique read-out electronics. A second-generation 32 × 32 pixels array being implemented to demonstrate the performance of a 1k-pixel array as precursor to larger size arrays is also described. The active mode detection, in addition to eliminating the use of a chopper, enables the dynamic partition of the array into pixel domains in which the pixel sensitivities in each domain can be adjusted independently. This unique feature in IR detection is not readily found in other types of IR imagers and can be applied to the simultaneous tracking of diverse contrast objects. By controlling the absorber material thickness, the arrays can be optimized for maximum response at specified wavelengths by means of quarter-wavelength interferometric technique.