WO2020180248A1 - Biodegradable threshold swithcing device and method of fabrication - Google Patents

Biodegradable threshold swithcing device and method of fabrication Download PDF

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Publication number
WO2020180248A1
WO2020180248A1 PCT/SG2020/050101 SG2020050101W WO2020180248A1 WO 2020180248 A1 WO2020180248 A1 WO 2020180248A1 SG 2020050101 W SG2020050101 W SG 2020050101W WO 2020180248 A1 WO2020180248 A1 WO 2020180248A1
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biodegradable
accordance
depositing
oxide
nanometers
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PCT/SG2020/050101
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French (fr)
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XingLong JI
Shuai ZHONG
Zhao RONG
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Singapore University Of Technology And Design
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    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/54—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using elements simulating biological cells, e.g. neuron
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06N—COMPUTING ARRANGEMENTS BASED ON SPECIFIC COMPUTATIONAL MODELS
    • G06N3/00—Computing arrangements based on biological models
    • G06N3/02—Neural networks
    • G06N3/06—Physical realisation, i.e. hardware implementation of neural networks, neurons or parts of neurons
    • G06N3/063—Physical realisation, i.e. hardware implementation of neural networks, neurons or parts of neurons using electronic means
    • G06N3/065—Analogue means
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
    • G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
    • G11C13/0007—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements comprising metal oxide memory material, e.g. perovskites
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
    • G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
    • G11C13/0009—RRAM elements whose operation depends upon chemical change
    • G11C13/0014—RRAM elements whose operation depends upon chemical change comprising cells based on organic memory material
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
    • G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
    • G11C13/0021—Auxiliary circuits
    • G11C13/003—Cell access
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
    • G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
    • G11C13/0021—Auxiliary circuits
    • G11C13/004—Reading or sensing circuits or methods
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N70/00—Solid-state devices having no potential barriers, and specially adapted for rectifying, amplifying, oscillating or switching
    • H10N70/20—Multistable switching devices, e.g. memristors
    • H10N70/24—Multistable switching devices, e.g. memristors based on migration or redistribution of ionic species, e.g. anions, vacancies
    • H10N70/245—Multistable switching devices, e.g. memristors based on migration or redistribution of ionic species, e.g. anions, vacancies the species being metal cations, e.g. programmable metallization cells
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N70/00—Solid-state devices having no potential barriers, and specially adapted for rectifying, amplifying, oscillating or switching
    • H10N70/801—Constructional details of multistable switching devices
    • H10N70/821—Device geometry
    • H10N70/826—Device geometry adapted for essentially vertical current flow, e.g. sandwich or pillar type devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N70/00—Solid-state devices having no potential barriers, and specially adapted for rectifying, amplifying, oscillating or switching
    • H10N70/801—Constructional details of multistable switching devices
    • H10N70/841—Electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N70/00—Solid-state devices having no potential barriers, and specially adapted for rectifying, amplifying, oscillating or switching
    • H10N70/801—Constructional details of multistable switching devices
    • H10N70/841—Electrodes
    • H10N70/8416—Electrodes adapted for supplying ionic species
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N70/00—Solid-state devices having no potential barriers, and specially adapted for rectifying, amplifying, oscillating or switching
    • H10N70/801—Constructional details of multistable switching devices
    • H10N70/881—Switching materials
    • H10N70/883—Oxides or nitrides
    • H10N70/8833—Binary metal oxides, e.g. TaOx
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N1/00—Electrotherapy; Circuits therefor
    • A61N1/02—Details
    • A61N1/04—Electrodes
    • A61N1/05—Electrodes for implantation or insertion into the body, e.g. heart electrode
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N1/00—Electrotherapy; Circuits therefor
    • A61N1/18—Applying electric currents by contact electrodes
    • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
    • A61N1/36—Applying electric currents by contact electrodes alternating or intermittent currents for stimulation
    • A61N1/372—Arrangements in connection with the implantation of stimulators
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06N—COMPUTING ARRANGEMENTS BASED ON SPECIFIC COMPUTATIONAL MODELS
    • G06N3/00—Computing arrangements based on biological models
    • G06N3/02—Neural networks
    • G06N3/04—Architecture, e.g. interconnection topology
    • G06N3/049—Temporal neural networks, e.g. delay elements, oscillating neurons or pulsed inputs
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C2213/00—Indexing scheme relating to G11C13/00 for features not covered by this group
    • G11C2213/10—Resistive cells; Technology aspects
    • G11C2213/15—Current-voltage curve
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C2213/00—Indexing scheme relating to G11C13/00 for features not covered by this group
    • G11C2213/50—Resistive cell structure aspects
    • G11C2213/52—Structure characterized by the electrode material, shape, etc.
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C2213/00—Indexing scheme relating to G11C13/00 for features not covered by this group
    • G11C2213/50—Resistive cell structure aspects
    • G11C2213/55—Structure including two electrodes, a memory active layer and at least two other layers which can be a passive or source or reservoir layer or a less doped memory active layer
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C2213/00—Indexing scheme relating to G11C13/00 for features not covered by this group
    • G11C2213/70—Resistive array aspects
    • G11C2213/76—Array using an access device for each cell which being not a transistor and not a diode

Definitions

  • the present invention generally relates to biodegradable electronic devices, and more particularly relates to biodegradable threshold switching devices and methods for their fabrication.
  • Biodegradable threshold switching devices exhibit unique “disappearance” capability at a prescribed time, providing great potential for“green” electronics to address electronic waste as well as providing potential applications in data security and biomedical implantation electronics.
  • Transient electronics is a new type of technology that endows materials, devices, and systems with the capability of degrading into nontoxic products for absorption by its surrounding environment with minimal or nontraceable remains after a period of stable operation.
  • transient electronics with biodegradability are highly desired for biomedical applications, especially in implantable medical diagnostic and therapeutic devices.
  • the transient devices are expected to work for a scheduled time, preforming their predefined function(s), and thereafter get absorbed to the mammalian body to reduce consecutive surgeries.
  • several criteria must be strictly met when developing transient electronics for biomedical applications: (1) the materials should not evoke a sustained inflammatory or toxic response upon implantation in vivo, (2) the degradation products should be nontoxic and easily metabolized and cleared from body; (3) the degradation time should match the healing or regeneration process; and (4) the materials should have appropriate mechanical strength and flexibility. While great efforts have been made in the past few years to develop biocompatible or biodegradable devices using both organic materials and inorganic materials, such stringent requirements place a significant difficulty to material selection.
  • a biodegradable threshold switching device includes a bottom electrode, a top electrode, and a switching layer sandwiched between the bottom electrode and the top electrode.
  • the switching layer includes an oxide biodegradable electrolyte material.
  • a method for fabrication of a biodegradable threshold switching device includes depositing a bottom electrode on a substrate, depositing a biodegradable oxide electrolyte functional switching layer on the bottom electrode, and depositing a top electrode on the biodegradable oxide electrolyte functional switching layer.
  • FIG. 1 is a schematic illustration of a switching mechanism of a threshold switching device in accordance with present embodiments.
  • FIG. 2 depicts microscopy images recording dissolution in phosphate- buffered saline (PBS) of a threshold switching device in accordance with the present embodiments.
  • PBS phosphate- buffered saline
  • FIG. 3 depicts flexibility testing for the biocompatible threshold switching device in accordance with the present embodiments, wherein FIG. 3A is a photograph of flexing of the biocompatible threshold switching device and FIG. 3B is a graph of high resistance states and low resistance states as a function of flexed curvature radius.
  • FIG. 4 depicts the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 4A depicts a metal-insulator-metal structure of the biodegradable threshold switching device, FIG. 4B is an optical image of biodegradable threshold switching arrays, and FIG. 4C is a process flow for fabrication of the threshold switching device.
  • FIG. 5 comprising FIGs. 5A and 5B, depicts graphs evidencing operational parameters of the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 5A depicts a graph depicting an I-V curve of the biodegradable threshold switching device and FIG. 5B depicts a graph of pulse mode testing of the biodegradable threshold switching device.
  • FIG. 6 depicts a selector application scenario for the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 6A is a schematic diagram of a memory crossbar array integrated with the biodegradable threshold switching devices as selectors and FIG. 6B is a graph of normalized read voltage margin versus number of rows (or columns) N for an array integrated with biodegradable selectors.
  • FIG. 7 depicts an artificial neuron application scenario for the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 7A depicts a schematic diagram of an integrate-and-fire neuron and FIG. 7B depicts an artificial neuron simulation with multiple pulse segments demonstrating neuronal tonic spiking.
  • Threshold switching devices also known as volatile resistance switching devices, have application in both memory and neuromorphic devices. Threshold switching is commonly observed in inorganic solid dielectrics with silver (Ag) or copper (Cu) active metal elements. Threshold switching cells could decrease by orders of magnitude when an electric field is applied. However, in addition to the traditional electrochemical metallization mechanism observed in non-volatile cells, its resistance recovers spontaneously upon cessation of the external bias, yielding superior conductance evolution dynamics. Threshold switching devices are not only very important for large-scale memory applications by serving as access elements (i.e., selectors), but also have great potential as computational elements for neuromorphic applications as an artificial neuron.
  • access elements i.e., selectors
  • biodegradable materials such as zinc oxide (ZnO), tungsten oxide (WO 3 ), magnesium oxide (MgO), silicon oxide (S1O 2 ), tungsten (W), and magnesium (Mg), in conjunction with high mobility metal elements (e.g., silver (Ag) and copper (Cu)) are utilized to achieve a biodegradable device with threshold switching behavior which can be used as an access element (selector) for transient electronic systems and a computational element for neuromorphic computing as artificial neuron.
  • the threshold switching behavior can be attributed to the fast diffusion dynamics of metallic ions in oxide solid-state electrolytes.
  • the biodegradable threshold switching devices in accordance with present embodiments demonstrate excellent performance, including ultra-low leakage current ( ⁇ 3 pA), high ON current (> 100 mA), large hysteresis window, large ON/OFF ratio (> 10 9 ), and biocompatibility and biodegradability. Furthermore, the biodegradable threshold switching devices in accordance with present embodiments exhibiting physically transient behavior and good biodegradability by, for example, being soluble in phosphate buffered saline. Good mechanical property and flexibility are also demonstrated through electrical testing under different bending conditions. These results suggest that the biodegradable threshold switching devices in accordance with present embodiments is a promising threshold switching element candidate for constructing transient electronic systems, especially for bio-medical applications.
  • Biocompatible inorganic materials are used to fabricate biodegradable threshold switching devices in accordance with present embodiments.
  • the biodegradable threshold switching devices in accordance with the present embodiments include an inert-metal/active-metal/oxide-solid-state-electrolyte/active- metal/inert-metal multilayer structure to provide stable bipolar threshold switching behavior with good repeatability and device variability, surpassing most known organic threshold switching devices.
  • a schematic illustration 100 depicts an exemplary switching mechanism of a threshold switching device 102 in accordance with the present embodiments.
  • the threshold switching device 102 includes an oxide-solid-state-electrolyte functional layer 104 composed of, for example, magnesium oxide (MgO), sandwiched between an inert-metal top electrode (TE) 106 and an inert-metal bottom electrode (BE) 108.
  • the TE 106 and the BE 108 may be composed of tungsten (W).
  • a thin active-metal layer of, for example, silver (AG) may be formed on the BE 108, the TE 106 or both on the TE 106 and the BE 108 in order to provide the active metal (e.g., silver 110) in metallic form and in metallic ion form for creating and rupturing a filament structure 112 to define three states of the switching mechanism of the threshold switching device 102: an initial state 120, a SET process 140, and a RESET process.
  • AG active-metal layer of, for example, silver
  • the initial state 120 no charge is applied to the TE 106 and the BE 108 and ions of the active metal can be found in the functional layer 104.
  • a positive charge is applied to the TE 106 and a negative charge is applied to the BE 108 which causes the filament 112 to form of, for example, metallic silver (Ag) 110.
  • the RESET process 160 the charges on the TE 106 and the BE 108 are switched which ruptures the filament 112.
  • a solubility test in phosphate-buffered saline (PBS) indicates the device exhibits physically transient behavior and good biodegradability.
  • a first microscopy image 200 depicts the threshold switching device 102 in accordance with the present embodiments at an initial state.
  • microscopy images depict the dissolution of the threshold switching device 102 in PBS wherein a microscopy image 200 depicts the threshold switching device 102 after less than a minute in the PBS; a microscopy image 210 depicts the threshold switching device 102 after four hours in the PBS; a microscopy image 220 depicts the threshold switching device 102 after eight hours in the PBS; a microscopy image 230 depicts the threshold switching device 102 after twelve hours in the PBS; a microscopy image 240 depicts the threshold switching device 102 after sixteen hours in the PBS; a microscopy image 250 depicts the threshold switching device 102 after twenty hours in the PBS; and a microscopy image 260 depicts the threshold switching device 102 totally dissolved in the PBS after twenty -four hours.
  • FIG. 3A a photograph 300 depicts flexing of the biocompatible threshold switching device 310 in accordance with the present embodiments.
  • FIG. 3B depicts a graph 350 plotting resistance along the y-axis 352 and curvature radius along the x-axis 354.
  • the graph 350 illustrates high resistance states 360 and low resistance states 365 as a function of flexed curvature radius 354 where good mechanical properties and flexibility were demonstrated through electrical testing under different bending conditions 370, 375, 380, 385.
  • an exploded view 400 depicts a vertical metal- insulator- metal structure of the biodegradable threshold switching device 405.
  • An oxide biodegradable electrolyte material 410 serves as a switching layer and is sandwiched between metal layers 412, 414 and formed on a substrate 416.
  • the metal layers 412, 414 serve as top and bottom electrodes for the biodegradable threshold switching device 405.
  • the metal in the metal layers 412, 414 can be tungsten (W), magnesium (Mg), silver (Ag), combinations thereof (e.g., Mg/Ag or Ag/W), or similar metals.
  • an optical image 430 depicts a first biodegradable threshold switching array 432 formed on a silicon oxide or silicon substrate 433 and a second biodegradable threshold switching array 440 formed on a polyethylene terephthalate (PET) substrate 442.
  • the biodegradable threshold switching arrays 432, 440 include bottom electrodes 434, 444 and top electrodes 436, 446 with biodegradable oxide layers (bio-oxide layers) 438, 448 therebetween.
  • a processing flow 460 for fabrication of the biodegradable threshold switching device 405 is depicted.
  • a bottom electrode is formed having a thickness of between twenty nanometers and one hundred nanometers by depositing 462 an inert metal film and depositing 464 an active metal layer one after the other on a Si/SiCh substrate by, for example, magnetron sputtering.
  • the inert metal film is deposited 462 to a thickness of approximately fifty nanometers and the active metal layer is deposited 464 to a thickness of approximately five nanometers.
  • a biodegradable oxide layer is deposited 466 to a thickness of between ten naometers and eighty nanometers, preferably sixty nanometers, as an oxide solid- state electrolyte functional layer to provide a migration medium for metallic ions.
  • a top electrode is formed having a thickness of between twenty nanometers and one hundred nanometers by depositing 468 an active metal layer and depositing 470 an inert metal film layer through a shadow mask over the oxide functional layer.
  • the inert metal film is deposited 470 to a thickness of approximately fifty nanometers and the active metal layer is deposited 468 to a thickness of approximately five nanometers. All of the processes in the flow 460 can be performed at room temperature and are fully thermal compatible to back-end-of-line (BEOL) processes.
  • BEOL back-end-of-line
  • a graph 500 plots current along the y-axis 502 and voltage along the x-axis 504.
  • the graph 500 depicts dual-sweep current-voltage (I-V) curves 510, 520 of the biodegradable threshold switching device which exhibit bipolar characteristics with good nonlinearity and high ON/OFF ratio.
  • I-V current-voltage
  • V hoid holding voltage
  • the difference between V lh and V hoid forms a large hysteresis window, which allows the biodegradable threshold switching device to avoid a partial RESET and enable a novel operation scheme without stringent matching requirement.
  • a graph 550 depicts pulse mode testing to demonstrate the fast switch ON speed of the biodegradable threshold switching device.
  • a waveform 560 that contains three pulses was employed for a single pulse operation. It can be seen that, with a 3.0 V, 500 ps pulse, the threshold switching device can be switched on within ten ps. Two reading pulses with 0.5 V pulse amplitude were also applied before and after the single pulse operation. During the second reading pulse, the near zero current output indicated that the selector spontaneously relaxed to a high resistance state after single pulse operation.
  • the biodegradable threshold switching device 405 in accordance with the present embodiments can be employed as an accessing device in biodegradable memory/neuromorphic crossbar arrays. With ultra-low output current in the OFF state, the threshold switching device 405 can minimize the leakage current in a large scale crossbar array, therefore, enabling larger integration scale.
  • FIG. 6A depicts a schematic diagram 600 of a 1- selector- 1-memristor (1S1M) architecture to enable a memristive array 610 for neuromorphic applications. The large ON/OFF ratio of the biodegradable threshold switching device 405 greatly improves the scale up ability of the memristive array.
  • a read margin criterion with a minimum of ten per cent (10%) is used to determine the maximum array size.
  • the operation scheme in the crossbar array for neuromorphic applications is different from that for memory applications. Either for a training process or an inference process, the cells are updated or read row- by-row. Thus, a training/inference scheme with pulling up all word lines, grounding selected bit line and floating unselected bit lines is employed. This scheme reduces the rigorous variation requirement for selectors.
  • FIG. 6B is a graph 650 of normalized read voltage margin 652 versus number of rows (or columns) N 654 for an array integrated with biodegradable selectors to estimate the maximum array size.
  • the passive array 660 with ON/OFF ratio of ten can only be scaled up to 6 x 6; while the 1S1M array 670 utilizing the biodegradable threshold switching device 405 in accordance with the present embodiments can be scaled up to terabit scale.
  • the 1S 1M structure of the schematic diagram 600 and the read voltage margin 652 indicates that employing the threshold switching device 405 in accordance with the present embodiments as a vacuum gap selector can well support large-scale neuromorphic network architectures.
  • FIG. 7A a schematic diagram 700 of an integrate- and-fire neuron 710 is depicted.
  • the integrate-and-fire neuron 710 integrates the input from a presynaptic neuron and generate neuronal tonic spikes.
  • Neuronal tonic spiking is a basic function of artificial neurons, which requires a device to switch on (fire) after accumulating enough pulses and returning to the initial state after firing.
  • a multi-segment voltage pulse train 712, 714 is employed to trigger the volatile switching of the threshold switching device 405 employed as a biodegradable threshold switching device 716.
  • a graph 750 plots voltage 752 versus time 754 and current 756 versus time 754.
  • the graph 750 depicts a pulse train waveform 760, wherein negative pulses 765 are inserted between each segment to ensure that the atomic switch is returning to the initial state.
  • the successful realization of tonic spiking 770 as shown in the graph 750 indicates that the threshold switching device 405 can be used as the neuron element 716 in the neuromorphic system 710.
  • the present embodiments provides biodegradable threshold switching devices for large scale memory/neuromorphic chip implementation which are also suitable for high density non-volatile memory chips, high density neuromorphic chips, 3D cross point memories, and biocompatible electronic system.
  • a biodegradable device with threshold switching behavior can be used as an access element (selector) for transient electronic systems and a computational element for neuromorphic computing as an artificial neuron.
  • the threshold switching behavior can be attributed to the fast diffusion dynamics of metallic ions in oxide solid-state electrolytes.
  • the biodegradable threshold switching devices in accordance with present embodiments demonstrate excellent performance, including ultra-low leakage current ( ⁇ 3 pA), high ON current (> 100 mA), large hysteresis window, large ON/OFF ratio (> 10 9 ), and biocompatibility and biodegradability.
  • biodegradable threshold switching devices in accordance with present embodiments exhibiting physically transient behavior and good biodegradability by, for example, being soluble in phosphate buffered saline. Good mechanical property and flexibility are also demonstrated through electrical testing under different bending conditions.

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Abstract

Biodegradable threshold switching devices and methods for their fabrication are provided. In accordance with one aspect, a biodegradable threshold switching device includes a bottom electrode, a top electrode, and a switching layer sandwiched between the bottom electrode and the top electrode. The switching layer includes an oxide biodegradable electrolyte material.

Description

BIODEGRADABLE THRESHOLD SWITHCING DEVICE
AND METHOD OF FABRICATION
PRIORITY CLAIM
[0001] This application claims priority from Singapore Patent Application No. 10201901945V filed on 05 March 2019.
TECHNICAL FIELD
[0002] The present invention generally relates to biodegradable electronic devices, and more particularly relates to biodegradable threshold switching devices and methods for their fabrication.
BACKGROUND OF THE DISCLOSURE
[0003] Electronic waste has become an urgent environmental issue emerging from the rapid growth of consumer electronics. Biodegradable threshold switching devices exhibit unique “disappearance” capability at a prescribed time, providing great potential for“green” electronics to address electronic waste as well as providing potential applications in data security and biomedical implantation electronics. Transient electronics is a new type of technology that endows materials, devices, and systems with the capability of degrading into nontoxic products for absorption by its surrounding environment with minimal or nontraceable remains after a period of stable operation.
[0004] Besides being environmentally friendly, transient electronics with biodegradability are highly desired for biomedical applications, especially in implantable medical diagnostic and therapeutic devices. For such applications in mammals, the transient devices are expected to work for a scheduled time, preforming their predefined function(s), and thereafter get absorbed to the mammalian body to reduce consecutive surgeries. To prevent potential risks to human health, several criteria must be strictly met when developing transient electronics for biomedical applications: (1) the materials should not evoke a sustained inflammatory or toxic response upon implantation in vivo, (2) the degradation products should be nontoxic and easily metabolized and cleared from body; (3) the degradation time should match the healing or regeneration process; and (4) the materials should have appropriate mechanical strength and flexibility. While great efforts have been made in the past few years to develop biocompatible or biodegradable devices using both organic materials and inorganic materials, such stringent requirements place a significant difficulty to material selection.
[0005] Thus, there is a need for biocompatible, nontoxic, biodegradable materials for transient electronics. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.
SUMMARY
[0006] According to at least one aspect of the present embodiments, a biodegradable threshold switching device is provided. The biodegradable threshold switching device includes a bottom electrode, a top electrode, and a switching layer sandwiched between the bottom electrode and the top electrode. The switching layer includes an oxide biodegradable electrolyte material.
[0007] According to another aspect of the present embodiments, a method for fabrication of a biodegradable threshold switching device is provided. The method includes depositing a bottom electrode on a substrate, depositing a biodegradable oxide electrolyte functional switching layer on the bottom electrode, and depositing a top electrode on the biodegradable oxide electrolyte functional switching layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with present embodiments.
[0009] FIG. 1 is a schematic illustration of a switching mechanism of a threshold switching device in accordance with present embodiments.
[0010] FIG. 2 depicts microscopy images recording dissolution in phosphate- buffered saline (PBS) of a threshold switching device in accordance with the present embodiments.
[0011] FIG. 3, comprising FIGs. 3 A and 3B, depicts flexibility testing for the biocompatible threshold switching device in accordance with the present embodiments, wherein FIG. 3A is a photograph of flexing of the biocompatible threshold switching device and FIG. 3B is a graph of high resistance states and low resistance states as a function of flexed curvature radius.
[0012] FIG. 4, comprising FIGs. 4A, 4B and 4C, depicts the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 4A depicts a metal-insulator-metal structure of the biodegradable threshold switching device, FIG. 4B is an optical image of biodegradable threshold switching arrays, and FIG. 4C is a process flow for fabrication of the threshold switching device. [0013] FIG. 5, comprising FIGs. 5A and 5B, depicts graphs evidencing operational parameters of the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 5A depicts a graph depicting an I-V curve of the biodegradable threshold switching device and FIG. 5B depicts a graph of pulse mode testing of the biodegradable threshold switching device.
[0014] FIG. 6, comprising FIGs. 6A and 6B, depicts a selector application scenario for the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 6A is a schematic diagram of a memory crossbar array integrated with the biodegradable threshold switching devices as selectors and FIG. 6B is a graph of normalized read voltage margin versus number of rows (or columns) N for an array integrated with biodegradable selectors.
[0015] And FIG. 7, comprising FIGs. 7A and 7B, depicts an artificial neuron application scenario for the biodegradable threshold switching device in accordance with the present embodiments, wherein FIG. 7A depicts a schematic diagram of an integrate-and-fire neuron and FIG. 7B depicts an artificial neuron simulation with multiple pulse segments demonstrating neuronal tonic spiking.
[0016] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale.
DETAILED DESCRIPTION
[0017] The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. It is the intent of present embodiments to present fully biodegradable threshold switching devices for memory and neuromorphic devices,
[0018] Threshold switching devices, also known as volatile resistance switching devices, have application in both memory and neuromorphic devices. Threshold switching is commonly observed in inorganic solid dielectrics with silver (Ag) or copper (Cu) active metal elements. Threshold switching cells could decrease by orders of magnitude when an electric field is applied. However, in addition to the traditional electrochemical metallization mechanism observed in non-volatile cells, its resistance recovers spontaneously upon cessation of the external bias, yielding superior conductance evolution dynamics. Threshold switching devices are not only very important for large-scale memory applications by serving as access elements (i.e., selectors), but also have great potential as computational elements for neuromorphic applications as an artificial neuron. Currently, many electrical components have been developed with biocompatible properties for transient electronic system construction, including field-effect transistors, energy harvesters, sensors, radio frequency devices, batteries, and resistive memories. Yet, as a critical component of memory and neuromorphic devices, fully biodegradable threshold switching devices have not received significant attention.
[0019] In accordance with present embodiments, biodegradable materials, such as zinc oxide (ZnO), tungsten oxide (WO3), magnesium oxide (MgO), silicon oxide (S1O2), tungsten (W), and magnesium (Mg), in conjunction with high mobility metal elements (e.g., silver (Ag) and copper (Cu)) are utilized to achieve a biodegradable device with threshold switching behavior which can be used as an access element (selector) for transient electronic systems and a computational element for neuromorphic computing as artificial neuron. The threshold switching behavior can be attributed to the fast diffusion dynamics of metallic ions in oxide solid-state electrolytes. By carefully choosing the materials and optimizing the device structures, the biodegradable threshold switching devices in accordance with present embodiments demonstrate excellent performance, including ultra-low leakage current (< 3 pA), high ON current (> 100 mA), large hysteresis window, large ON/OFF ratio (> 109), and biocompatibility and biodegradability. Furthermore, the biodegradable threshold switching devices in accordance with present embodiments exhibiting physically transient behavior and good biodegradability by, for example, being soluble in phosphate buffered saline. Good mechanical property and flexibility are also demonstrated through electrical testing under different bending conditions. These results suggest that the biodegradable threshold switching devices in accordance with present embodiments is a promising threshold switching element candidate for constructing transient electronic systems, especially for bio-medical applications.
[0020] Biocompatible inorganic materials are used to fabricate biodegradable threshold switching devices in accordance with present embodiments. The biodegradable threshold switching devices in accordance with the present embodiments include an inert-metal/active-metal/oxide-solid-state-electrolyte/active- metal/inert-metal multilayer structure to provide stable bipolar threshold switching behavior with good repeatability and device variability, surpassing most known organic threshold switching devices. Referring to FIG. 1, a schematic illustration 100 depicts an exemplary switching mechanism of a threshold switching device 102 in accordance with the present embodiments. The threshold switching device 102 includes an oxide-solid-state-electrolyte functional layer 104 composed of, for example, magnesium oxide (MgO), sandwiched between an inert-metal top electrode (TE) 106 and an inert-metal bottom electrode (BE) 108. The TE 106 and the BE 108 may be composed of tungsten (W). A thin active-metal layer of, for example, silver (AG) may be formed on the BE 108, the TE 106 or both on the TE 106 and the BE 108 in order to provide the active metal (e.g., silver 110) in metallic form and in metallic ion form for creating and rupturing a filament structure 112 to define three states of the switching mechanism of the threshold switching device 102: an initial state 120, a SET process 140, and a RESET process.
[0021] In the initial state 120, no charge is applied to the TE 106 and the BE 108 and ions of the active metal can be found in the functional layer 104. During the SET process 140, a positive charge is applied to the TE 106 and a negative charge is applied to the BE 108 which causes the filament 112 to form of, for example, metallic silver (Ag) 110. During the RESET process 160, the charges on the TE 106 and the BE 108 are switched which ruptures the filament 112.
[0022] Referring to FIG. 2, a solubility test in phosphate-buffered saline (PBS) indicates the device exhibits physically transient behavior and good biodegradability. A first microscopy image 200 depicts the threshold switching device 102 in accordance with the present embodiments at an initial state. Subsequent microscopy images depict the dissolution of the threshold switching device 102 in PBS wherein a microscopy image 200 depicts the threshold switching device 102 after less than a minute in the PBS; a microscopy image 210 depicts the threshold switching device 102 after four hours in the PBS; a microscopy image 220 depicts the threshold switching device 102 after eight hours in the PBS; a microscopy image 230 depicts the threshold switching device 102 after twelve hours in the PBS; a microscopy image 240 depicts the threshold switching device 102 after sixteen hours in the PBS; a microscopy image 250 depicts the threshold switching device 102 after twenty hours in the PBS; and a microscopy image 260 depicts the threshold switching device 102 totally dissolved in the PBS after twenty -four hours.
[0023] Referring to FIG. 3A, a photograph 300 depicts flexing of the biocompatible threshold switching device 310 in accordance with the present embodiments. FIG. 3B depicts a graph 350 plotting resistance along the y-axis 352 and curvature radius along the x-axis 354. The graph 350 illustrates high resistance states 360 and low resistance states 365 as a function of flexed curvature radius 354 where good mechanical properties and flexibility were demonstrated through electrical testing under different bending conditions 370, 375, 380, 385. These properties suggest that the biocompatible threshold switching device 310 in accordance with the present embodiments is a promising memory element candidate for constructing transient electronic systems, especially for biomedical applications.
[0024] Referring to FIG. 4A, an exploded view 400 depicts a vertical metal- insulator- metal structure of the biodegradable threshold switching device 405. An oxide biodegradable electrolyte material 410 serves as a switching layer and is sandwiched between metal layers 412, 414 and formed on a substrate 416. The metal layers 412, 414 serve as top and bottom electrodes for the biodegradable threshold switching device 405. The metal in the metal layers 412, 414 can be tungsten (W), magnesium (Mg), silver (Ag), combinations thereof (e.g., Mg/Ag or Ag/W), or similar metals.
[0025] Referring to FIG. 4B, an optical image 430 depicts a first biodegradable threshold switching array 432 formed on a silicon oxide or silicon substrate 433 and a second biodegradable threshold switching array 440 formed on a polyethylene terephthalate (PET) substrate 442. The biodegradable threshold switching arrays 432, 440 include bottom electrodes 434, 444 and top electrodes 436, 446 with biodegradable oxide layers (bio-oxide layers) 438, 448 therebetween.
[0026] Referring to FIG. 4C, a processing flow 460 for fabrication of the biodegradable threshold switching device 405 is depicted. Firstly, a bottom electrode is formed having a thickness of between twenty nanometers and one hundred nanometers by depositing 462 an inert metal film and depositing 464 an active metal layer one after the other on a Si/SiCh substrate by, for example, magnetron sputtering. Preferably, the inert metal film is deposited 462 to a thickness of approximately fifty nanometers and the active metal layer is deposited 464 to a thickness of approximately five nanometers.
[0027] Next, a biodegradable oxide layer is deposited 466 to a thickness of between ten naometers and eighty nanometers, preferably sixty nanometers, as an oxide solid- state electrolyte functional layer to provide a migration medium for metallic ions. Then, a top electrode is formed having a thickness of between twenty nanometers and one hundred nanometers by depositing 468 an active metal layer and depositing 470 an inert metal film layer through a shadow mask over the oxide functional layer. Preferably, the inert metal film is deposited 470 to a thickness of approximately fifty nanometers and the active metal layer is deposited 468 to a thickness of approximately five nanometers. All of the processes in the flow 460 can be performed at room temperature and are fully thermal compatible to back-end-of-line (BEOL) processes.
[0028] Referring to FIG. 5A, a graph 500 plots current along the y-axis 502 and voltage along the x-axis 504. The graph 500 depicts dual-sweep current-voltage (I-V) curves 510, 520 of the biodegradable threshold switching device which exhibit bipolar characteristics with good nonlinearity and high ON/OFF ratio. During each sweeping 510, 520, the current maintains constantly low at the low voltage regions. When the voltage exceeds the threshold voltage (Vth), the current abruptly jumps to a high value, which is clamped by a compliance current (Icc). Gradually reducing the applied voltage, the current drops sharply at a very low holding voltage (Vhoid) and the device returns to an OFF-state with a high resistance. The difference between Vlh and Vhoid forms a large hysteresis window, which allows the biodegradable threshold switching device to avoid a partial RESET and enable a novel operation scheme without stringent matching requirement.
[0029] Referring to FIG. 5B, a graph 550 depicts pulse mode testing to demonstrate the fast switch ON speed of the biodegradable threshold switching device. A waveform 560 that contains three pulses was employed for a single pulse operation. It can be seen that, with a 3.0 V, 500 ps pulse, the threshold switching device can be switched on within ten ps. Two reading pulses with 0.5 V pulse amplitude were also applied before and after the single pulse operation. During the second reading pulse, the near zero current output indicated that the selector spontaneously relaxed to a high resistance state after single pulse operation.
[0030] The biodegradable threshold switching device 405 in accordance with the present embodiments can be employed as an accessing device in biodegradable memory/neuromorphic crossbar arrays. With ultra-low output current in the OFF state, the threshold switching device 405 can minimize the leakage current in a large scale crossbar array, therefore, enabling larger integration scale. FIG. 6A depicts a schematic diagram 600 of a 1- selector- 1-memristor (1S1M) architecture to enable a memristive array 610 for neuromorphic applications. The large ON/OFF ratio of the biodegradable threshold switching device 405 greatly improves the scale up ability of the memristive array. A read margin criterion with a minimum of ten per cent (10%) is used to determine the maximum array size. The operation scheme in the crossbar array for neuromorphic applications is different from that for memory applications. Either for a training process or an inference process, the cells are updated or read row- by-row. Thus, a training/inference scheme with pulling up all word lines, grounding selected bit line and floating unselected bit lines is employed. This scheme reduces the rigorous variation requirement for selectors.
[0031] Considering worst conditions, the read (inference) voltage margin for a passive memristor array and the memristor array integrated with biodegradable threshold switching device 405 in accordance with the present embodiments is simulated. FIG. 6B is a graph 650 of normalized read voltage margin 652 versus number of rows (or columns) N 654 for an array integrated with biodegradable selectors to estimate the maximum array size. As shown in the graph 650, the passive array 660 with ON/OFF ratio of ten can only be scaled up to 6 x 6; while the 1S1M array 670 utilizing the biodegradable threshold switching device 405 in accordance with the present embodiments can be scaled up to terabit scale. Accordingly, the 1S 1M structure of the schematic diagram 600 and the read voltage margin 652 indicates that employing the threshold switching device 405 in accordance with the present embodiments as a vacuum gap selector can well support large-scale neuromorphic network architectures.
[0032] The unique switching dynamics of the threshold switching device 405 in accordance with the present embodiments opens the possibility of achieving different functions for neuromorphic applications when using the biodegradable threshold switching device 405. Referring to FIG. 7A, a schematic diagram 700 of an integrate- and-fire neuron 710 is depicted. The integrate-and-fire neuron 710 integrates the input from a presynaptic neuron and generate neuronal tonic spikes. Neuronal tonic spiking is a basic function of artificial neurons, which requires a device to switch on (fire) after accumulating enough pulses and returning to the initial state after firing. In order to realize the tonic spiking, a multi-segment voltage pulse train 712, 714 is employed to trigger the volatile switching of the threshold switching device 405 employed as a biodegradable threshold switching device 716.
[0033] Referring to FIG. 7B, a graph 750 plots voltage 752 versus time 754 and current 756 versus time 754. The graph 750 depicts a pulse train waveform 760, wherein negative pulses 765 are inserted between each segment to ensure that the atomic switch is returning to the initial state. The successful realization of tonic spiking 770 as shown in the graph 750 indicates that the threshold switching device 405 can be used as the neuron element 716 in the neuromorphic system 710.
[0034] Thus, it can be seen that the present embodiments provides biodegradable threshold switching devices for large scale memory/neuromorphic chip implementation which are also suitable for high density non-volatile memory chips, high density neuromorphic chips, 3D cross point memories, and biocompatible electronic system.
[0035] In accordance with present embodiments, a biodegradable device with threshold switching behavior can be used as an access element (selector) for transient electronic systems and a computational element for neuromorphic computing as an artificial neuron. The threshold switching behavior can be attributed to the fast diffusion dynamics of metallic ions in oxide solid-state electrolytes. By carefully choosing the materials and optimizing the device structures, the biodegradable threshold switching devices in accordance with present embodiments demonstrate excellent performance, including ultra-low leakage current (< 3 pA), high ON current (> 100 mA), large hysteresis window, large ON/OFF ratio (> 109), and biocompatibility and biodegradability. Furthermore, the biodegradable threshold switching devices in accordance with present embodiments exhibiting physically transient behavior and good biodegradability by, for example, being soluble in phosphate buffered saline. Good mechanical property and flexibility are also demonstrated through electrical testing under different bending conditions.
[0036] While exemplary embodiments have been presented in the foregoing detailed description of the present embodiments, it should be appreciated that a vast number of variations exist. It should further be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, operation, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing exemplary embodiments of the invention, it being understood that various changes may be made in the function and arrangement of steps and method of operation described in the exemplary embodiments without departing from the scope of the invention as set forth in the appended claims.

Claims

CLAIMS What is claimed is:
1. A biodegradable threshold switching device comprising:
a bottom electrode;
a top electrode; and
a switching layer sandwiched between the bottom electrode and the top electrode, wherein the switching layer comprises an oxide biodegradable electrolyte material.
2. The biodegradable threshold switching device in accordance with Claim 1 wherein the oxide biodegradable electrolyte material comprises a material selected from the group comprising zinc oxide (ZnO), tungsten oxide (WO3), magnesium oxide (MgO), and silicon oxide (S1O2).
3. The biodegradable threshold switching device in accordance with either Claim 1 or Claim 2 wherein the top electrode and/or the bottom electrode comprise a high mobility metal element.
4. The biodegradable threshold switching device in accordance with Claim 3 wherein the high mobility metal element comprises a metal selected from the group comprising silver (Ag), copper (Cu), tungsten (W), magnesium (Mg) or any combinations thereof.
5. The biodegradable threshold switching device in accordance with any of the preceding claims further comprising a substrate on which the bottom electrode is formed.
6. The biodegradable threshold switching device in accordance with any of the preceding claims wherein the top electrode is patterned.
7. The biodegradable threshold switching device in accordance with any of the preceding claims wherein the top electrode and/or the bottom electrode is approximately twenty nanometers (20 nm) to one hundred nanometers (lOOnm) thick.
8. The biodegradable threshold switching device in accordance with any of the preceding claims wherein the switching layer is approximately ten nanometers (lOnm) thick to eighty nanometers (80nm).
9. A method for fabrication of a biodegradable threshold switching device, the method comprising:
depositing a bottom electrode to a thickness of between twenty nanometers and one hundred nanometers on a substrate;
depositing a biodegradable oxide electrolyte functional switching layer to a thickness of between ten nanometers and eighty nanometers on the bottom electrode; depositing a top electrode on the biodegradable oxide electrolyte functional switching layer to a thickness of between twenty nanometers and one hundred nanometers.
10. The method in accordance with Claim 9 wherein depositing the bottom electrode on the substrate comprises:
depositing approximately fifty nanometers of an inert metal film on the substrate; and
depositing approximately five nanometers of an active metal layer on the inert metal film.
11. The method in accordance with Claim 10 wherein depositing the inert metal film on the substrate comprises magnetron sputtering the inert metal film on the substrate.
12. The method in accordance with either Claim 10 or Claim 11 wherein depositing the active metal layer on the inert metal film comprises magnetron sputtering the active metal layer on the inert metal film.
13. The method in accordance with any of Claims 9 to 12 wherein depositing the biodegradable oxide electrolyte functional switching layer on the bottom electrode comprises depositing approximately sixty nanometers of the biodegradable oxide electrolyte functional switching layer on the bottom electrode.
14. The method in accordance with any of Claims 9 to 13 wherein depositing the top electrode on the biodegradable oxide electrolyte functional switching layer comprises:
depositing approximately five nanometers of an active metal layer on the biodegradable oxide electrolyte functional switching layer; and depo siting approximately fifty nanometers of an inert metal film on the active metal layer.
15. The method in accordance with Claim 14 wherein depositing the active metal layer on the biodegradable oxide electrolyte functional switching layer comprises patterning the active metal layer.
16. The method in accordance with either Claim 14 or Claim 15 wherein depositing the inert metal film on the active metal layer comprises patterning the inert metal layer.
17. The method in accordance with either Claim 15 or Claim 16 wherein patterning the inert metal layer and/or patterning the active metal layer comprises patterning through a shadow mask.
18. The method in accordance with any of Claims 9 to 17 wherein the biodegradable oxide electrolyte functional switching layer on the bottom electrode comprises depositing a material selected from the group comprising zinc oxide (ZnO), tungsten oxide (WO3), magnesium oxide (MgO), and silicon oxide (S1O2) to form the biodegradable oxide electrolyte functional switching layer.
15. The method in accordance with any of Claims 8 to 14 wherein depositing tungsten on the magnesium oxide comprises depositing approximately seventy nanometers of tungsten on the magnesium to form the top electrode.
16. The method in accordance with any of Claims 8 to 15 wherein depositing tungsten on the magnesium oxide comprises patterning the deposited tungsten.
17. The method in accordance with Claim 16 wherein patterning the deposited tungsten comprises patterning the deposited tungsten using a shadow mask.
19. An integrate-and-fire neuron device comprising the biodegradable threshold switching device of any of Claims 1 to 8.
20. A memristive array for neromophic applications comprising the biodegradable threshold switching device of any of Claims 1 to 8.
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