Such performance represents a significant steptowards addressing the most pressing needs of moderncompact electronic systems. Categories and Subject DescriptorsB. General TermsDesign, Reliability. Keywords 3D Integration ; Memristor s. IntroductionThree-dimensional 3D circuits are a natural way ofincreasing integration density to overcome the inevitablelimitations in the lateral scaling of electron devices .
Incomparison with 2D planar circuits, 3D integrated circuits ICs offer the potential benefits of better performance,higher connectivity, reduced interconnect delays, lowerpower consumption, better space utilization, and moreflexible heterogeneous integration   . ApplicationsPermission to make digital or hard copies of all or part of this work forpersonal or classroom use is granted without fee provided that copies arenot made or distributed for profit or commercial advantage and that copiesbear this notice and the full citation on the first page.
As both military andindustry are moving toward multi-core, multi-threaded, andmulti-media applications, the demand for lower latency andhigher bandwidth between computing elements and memoryis growing fast. Beyond these immediate needs, there is tremendouspotential in the field of neuromorphic circuits. Indeed, inmany cases, conventional signal processing using digitalarchitectures are inadequate for real-time applications such aspattern classification including threat detection and facerecognition.
Neuromorphic architectures may be a viablesolution to address challenges associated with inherentrequirements for massive, parallel information processing,provided that dense integration between memory andprocessing elements can be obtained.
Such verticalintegration solutions suffer from power dissipation problems,low interconnect density because of a poor accuracy ofwafer alignment, as compared with that of photolithographicmasks defining features on a single wafer for multiplemetallization layers , low yields due to the lack of theknown-good-die assembly , and poor cost efficiency . Asa result, such solutions cannot provide adequate density andthroughput that will be required, for example, for real timesignal processing from focal plane arrays .
Suchvertical integration of active devices faces severe thermalmanagement, design complexity, and testability challenges. Moreover, multilayer CMOS circuits with thin-filmtransistors have inadequate characteristics for highperformance memory and logic applications. These challenges can be overcome with 3D hybridcircuits that monolithically integrate conventional CMOS circuits with quasi-passive, two-terminal, nanoscale devicesdistributed in additional layers.
HyNano consists of 9 facultymembers from multiple disciplines, including computerengineering, electrical engineering, materials science, andphysics, from four universities: University of California,Santa Barbara the lead institution , Stony Brook University SUNY , University of Michigan at Ann Arbor, andUniversity of Massachusetts at Amherst.
In this paper wegive a brief overview of the main ideas underpinning thisapproach and discuss preliminary results, including theconceptual design, simulations and key experimentalmilestones. Such circuits are based on the combination of a singleconventional CMOS chip and several layers of quasi-passivenanoscale crosspoint devices.
Although, many material systems exhibit resistive switchingand several are compatible with the CMOS processes, thesedevices face their own challenges. In particular, they sufferfrom low yield and high device-to-device variability. Moreover, the device operation physics, which is likelydifferent for different material systems, is still not wellunderstood.
There are three main classes of the materials systems formemristive devices. The first one is based on chalcogenidematerials, with resistance modulation in this materials isinduced by transition from the crystalline state to adisordered amorphous state . While this concept is themost mature and best understood, rather slow set process i. This group includes the most promising candidates forhybrid circuits, due to CMOS compatibility.
The switchingmechanisms in these devices are, however, the leastunderstood, in part due to very rich experimental phenomenaobserved. In solid state electrolytes and some a-Si and SiO 2devices, the most likely mechanism is metallic filamentformation via redox reaction at the interface. Such reactionresults in the penetration of electrode material into theinsulating film, with subsequent diffusion upon biasreversing . While these devices exhibit some of the bestendurance and low reset currents, the main concern here isvolatility due to high mobility cations which may also createproblems for CMOS integration.
A similar mechanism islikely behind switching in some organic films. Such materialsystems may be very attractive because of potentially lowfabrication costs, but their temperature stability is a concern. While there is an abundance of literature on bistable I-Vcurves, statistics and yield data for memristive devices arerarely reported, most likely due to poor results . Still,there are some very encouraging data, e. Also, excellentreproducibility has been reported in  which demonstratesintegration of such devices in a 30 by 30 crossbar structure.
Though reproducibility of crosspoint memristive devicesis perhaps still the most critical challenge at the moment, insome cases even relative large amount of device-to-devicevariations might be mitigated by using approach proposed in. The most natural way of sustaining the density ofsingle devices is to integrate them into a passive crossbarstructures, which are implemented with mutuallyperpendicular layers of parallel wires electrodes with thinfilm sandwiched between two layers Fig.
This is whycrossbars can be built with advanced patterning techniquessuch as nanoimprint lithography, so that F nano may bepotentially scaled down to just a few nanometers essentiallylimited by quantum mechanical tunneling , which farexceeds the limits of conventional optical lithography.
Finally, fabrication of many thin-film, memristive devicesdoes not require high temperatures, thus enabling back-endmonolithic integration of multiple layers on a CMOS base Fig. Passive crossbar array: a A schematic of the structureand the idea of b writing and c reading a particular bit . The basic operation i. Assuming digital mode operation, in the ONstate representing logic 1 memristive device is essentially adiode, so that the application of a voltage V READ , with V t 1 is adimensionless number that depends on the cell size i.
The decoding scheme in CMOL is based on two separateaddress arrays one for each level of wire in the crossbar sothat there are a total of 4N edge channels to provide access totwo different via controllers one 'blue' and one 'red' in eachof N 2 addressing cells in the CMOS plane.
In turn, each via isconnected to a wire fragment in the crossbar. This implementation features an area-distributedCMOL-like interface with tilted crossbar fabricated withnanoimprint technology Figure 5. To make alignmentbetween the CMOS and crossbar layers feasible,interconnects between the memristor layer and the CMOS layer have been implemented using larger contact padsconnected the nanowires to the tungsten vias in the CMOS substrate. The most natural applications of the 3D hybrid circuits are embedded memories and stand-alonememory chips, with their simple matrix structure.
Suchmemories are an extension of the so-called resistivememories RRAM . Memories based on phase changedevices are already in the development stage . This iswhy implementing strong nonlinearity in the I-V is one of themost important goals for the resistive switching devices inthe context of passive crossbar memories.
Another concern is the effect of the defective crosspointdevices on the memory performance. The defect density formemristive devices is likely to be much higher than that ofconventional CMOS technology so that some novel defectand fault tolerance schemes must be considered. Both analyses rely on the combination of two majortechniques for increasing their defect tolerance: the memorymatrix reconfiguration the replacement of some rows andcolumns with the largest number of bad memory cells forspare lines , and error correction codes.
As a result, CMOL memories may be the firsttechnology to reach the terabit frontier. Reconfigurable Circuits. To implement logic operations,each cell or supercell hosts some CMOS logic gate inaddition to configuration pass transistor circuitry Fig. During the reconfiguration stage, all logic gatesare disabled and any memristive device can be configured tothe high or low resistive state, similar to the operation of theCMOL memory. Once configured, memristive devices donot change state and those set to the low resistive staterepresent electrical connection between logic gates.
The cells thatare not within the connectivity domain of each other can beconnected by dedicating some CMOS cells for routingpurposes, e. Preliminary theoretical results show a very substantialdensity advantage on the average, about two orders ofmagnitude over the purely CMOS circuits, and aconsiderable leading edge over alternative hybrid circuitconcept, so-called nanoPLA .
The situation may be rather different in customlogic circuits, were CMOL may lose a part of its densityadvantage, but become considerably faster than CMOS. Anexample of such a circuit, performing fast, parallel imagingprocessing, was presented in . This time has to be compared with estimated 3, usfor a CMOS circuit based on the same design rules.
As a result, thecommunication delays are cut to the bone. Slightly modified original CMOL FPGA circuits havepotentials to perform massively parallel high throughputpattern matching far exceeding the state-of-the-artconventional implementations  . It is worth mentioningthat this work is perhaps the first ever successfuldemonstration of using this novel technology at such scale. Bio-inspired Information Processing. Perhaps the mostexciting application of CMOL circuits is in bio-inspiredinformation processing — for example, in the field of artificialneuromorphic networks ANN.
The motivation behindANN comes from the fact that the mammalian brain stillremains much more efficient in power and processingspeed for a number of computational tasks, such as patternrecognition and classification, as compared to conventionalcomputers, despite the exponential progress in theperformance of the latter during the past several decades  . The main reason behind poor performance of theconventional CMOS -based circuits is that they cannotprovide high complexity, connectivity and massive parallelinformation processing which is required typical forbiological neural networks.
Voltages V V A The very structure of CMOL circuits makes themuniquely suited for the implementation ofultrafast ANN   and, in general,mixed-signal information processingsystems  . For example, the CMOS subsystem would implement lessdense but also more complex somaticcells. The area-distributed interface anddense fragmented crossbar structure ofCMOL ensure very rich interconnectamong somas cells which are connectedto ech other via nanowire-memristivedevice — nanowire links, i.
Crude estimates have shownthat artificial synapses smaller than 10nanometres in length could be used to. Very recently, a key operation for analog and mixedsignal processing - multiply-and-add i. To realize such circuitry the memristivedevices implement density-critical configurable weights e. As a result, individual voltages applied to memristors can bemultiplied by the unique weight conductance of memristorand summed up by CMOS amplifier - all in analog fashion.
SummaryThe development of 3D CMOL circuits should greatlybenefit digital memory and logic circuits and may, for thefirst time, enable large scale bio-inspired neuromorphiccircuits. Practical introduction of such circuits will enablenew compact, fast electronic systems which would open up anumber of new applications.
With an assumption of moreaggressive, though still realistic, technology parameters, i. The high memorydensity and aggregate throughout of such circuits openunprecedented information processing capabilities, e. The most critical issues for CMOL technologystill remains yield, reproducibility of the memristive devicesand their viable integration with conventionalcomplementary metal oxide semiconductor technology.
AcknowledgmentsThe authors would like to acknowledge usefuldiscussions with K. Likharev as well as other members of theHyNano team. References D. Frank, R. Dennard, E. Nowak, P. Solomon, Y. Appl Phys A, , — Learning abilities achieved by a single solid-state atomic switch.
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Stochastic phase-change neurons. Efficient in-memory computing architecture based on crossbar arrays. Download references. Correspondence to Wei D. Reprints and Permissions. Ma, W. Neuromorphic computing with memristive devices. China Inf. Download citation. Received : 07 December Accepted : 26 February Published : 15 May Search SpringerLink Search.
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The authors wish to thank Dr. Shiping Wen for helpful discussions. Box , Doha, Qatar. Correspondence to Chuandong Li. Reprints and Permissions. Chen, L. Memristor crossbar-based unsupervised image learning. Download citation. Received : 26 December Accepted : 14 October Published : 02 November Issue Date : August Search SpringerLink Search. Abstract This letter presents a new memristor crossbar array system and demonstrates its applications in image learning.
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Delay dynamics of neuromorphic optoelectronic new platform for tuning, probing. Novel circuit designs of memristor synapse and neuron. PARAGRAPHIET image processing, arXiv NanoModelling nanoscale objects in order to conduct an empirical research into their properties as part of an engineering system. Microelectronics Reliability804Formal Design Space. Analog Integrated Circuits and Signal Processing95 3. Barnaby, Shimeng Yu, Michael N. Nanoscale9 278Memory and Information. Journal of Applied PhysicsApplied Physics Letters20. High performance light emitting memories: multifunctional devices for unveiling information neural networks with mixed time-varying. Advanced Materials30 9 4SiGe epitaxial memory by analogue state and nonlinear.May 15, — A functional hybrid memristor crossbar-array/CMOS system for data storage and neuromorphic Integration of nanoscale memristor synapses in neuromorphic computing architectures. Sheridan P M, Cai F X, Du C, et al. Dec 3, — and more bio-realistic neuromorphic computing systems, will each other. (f) Schematic of 3D integration of a 2 stack crossbar array with CMOS circuitry underneath.  Gaba, S.; Cai, F. X.; Zhou, J. T.; Lu, W. D. Ultralow subnA operating Srinivasa, N.; Lu, W. A functional hybrid memristor crossbar-. Nov 4, — Neuromorphic computing systems (NCSs) built using beyond‐CMOS devices and Naturally, the high‐density memristor crossbar arrays (CBAs) can perform the is a function of Δt. However, the STDP in biological systems is simply At the hardware level, the hybrid CNN system based on memristive.