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WO2016181391A1 - Capteur d'image et son procédé de fabrication - Google Patents

Capteur d'image et son procédé de fabrication Download PDF

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Publication number
WO2016181391A1
WO2016181391A1 PCT/IL2016/050495 IL2016050495W WO2016181391A1 WO 2016181391 A1 WO2016181391 A1 WO 2016181391A1 IL 2016050495 W IL2016050495 W IL 2016050495W WO 2016181391 A1 WO2016181391 A1 WO 2016181391A1
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Prior art keywords
image sensor
sensor according
signal processing
extractor
light detector
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PCT/IL2016/050495
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English (en)
Inventor
Meir Orenstein
Gad Bahir
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Technion Research and Development Foundation Ltd
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Technion Research and Development Foundation Ltd
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Publication of WO2016181391A1 publication Critical patent/WO2016181391A1/fr
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F30/00—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors
    • H10F30/20—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors
    • H10F30/21—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation
    • H10F30/288—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices being sensitive to multiple wavelengths, e.g. multi-spectrum radiation detection devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
    • H10F39/80—Constructional details of image sensors
    • H10F39/803—Pixels having integrated switching, control, storage or amplification elements
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/10—Semiconductor bodies
    • H10F77/14—Shape of semiconductor bodies; Shapes, relative sizes or dispositions of semiconductor regions within semiconductor bodies
    • H10F77/146—Superlattices; Multiple quantum well structures

Definitions

  • the present invention in some embodiments thereof, relates to imaging and, more particularly, but not exclusively, to an imaging sensor having monolithic active pixels made of different material systems.
  • CMOS imagers offer improvements in functionality, power and cost over charge-coupled-device (CCD) based imagers.
  • a typical CMOS type image sensor includes a focal plane array of pixel cells, each including a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate.
  • a readout circuit is connected to each pixel cell and includes an output transistor, formed in the substrate, and a charge transfer section connected to the gate of the output transistor.
  • the CMOS imager typically also includes a transistor for performing reset operation.
  • CMOS imagers utilize less power, have lower fabrications costs and offer high system integration compared to CCD based imagers. Additionally, CMOS imagers have the advantage that they can be manufactured using similar processes employed to those commonly used to manufacture logic transistors, such that the CMOS imager support functions can be fabricated on the same chip.
  • U.S. Patent No. 9,123,605 describes an infrared image sensor that combines an infrared light receiving device (sensor chip) having a two-dimensional array of pixels with a CMOS device forming a read-out circuit.
  • the sensor chip is flip-chip connected via indium bumps to the CMOS device.
  • a photocurrent generated in each pixel of the sensor chip is output as a voltage via an amplifier in the read-out circuit, processed by an external field-programmable gate array (FPGA), and then output as a digital signal.
  • FPGA field-programmable gate array
  • CMOS have also been integrated with GaN high electron mobility transistors (HEMTs) for the purpose of fabricating a current mirror circuit [Hoke et al., Journal of Vacuum Science & Technology B 30, 02B 101 (2012)].
  • HEMTs high electron mobility transistors
  • an image sensor comprises an array of active pixel cells on a substrate.
  • Each active pixel cell of the present embodiments has: a light detector, monolithically integrated with the substrate; and a signal processing circuit, monolithically integrated with the substrate in a region at least partially surrounding the light detector, and being in electronic communication with the light detector.
  • the light detector and the signal processing circuit are formed of different material systems, and in various exemplary embodiments of the invention a lattice mismatch between the light detector and the substrate is at least 10%.
  • the light detector occupies a cavity formed in or on the substrate.
  • the light detector is grown epitaxially on a base of the cavity.
  • the light detector comprises a quantum cascade detector.
  • the quantum cascade detector comprises a plurality of quantum well layers selected to absorb light by intersubband electronic transitions.
  • the quantum cascade detector comprises an active quantum well layer selected to absorb light by intersubband electronic transitions.
  • the quantum cascade detector comprises an extractor adjacent to the active quantum well layer.
  • At least a thickness of the extractor is selected to allow absorption of infrared light. According to some embodiments of the invention at least a thickness of the extractor is selected to allow absorption of near infrared light. According to some embodiments of the invention at least a thickness of the extractor is selected to allow absorption of short-wave infrared light. According to some embodiments of the invention at least a thickness of the extractor is selected to allow absorption of medium-wave infrared light. According to some embodiments of the invention at least a thickness of the extractor is selected to allow absorption of long-wave infrared light. According to some embodiments of the invention at least a thickness of the extractor is selected to allow absorption of any light having wavelength of from about 1 ⁇ to about 100 ⁇ .
  • the image comprises a polarization rotating optical element deposited or formed on the light detector.
  • an imaging system comprising an image sensor as delineated and optionally and preferably as described below.
  • the system is a mobile device.
  • the mobile device is selected from the group consisting of a cellular phone, a smartphone, a tablet device, a mobile digital camera, a wearable camera, a personal computer, a laptop, a portable media player, a portable gaming device, a portable digital assistant device, and a portable navigation device.
  • a method of imaging comprises capturing an image using an imaging system as delineated and optionally and preferably as described below, and displaying said image on a display and/or transmitting said image over a communication network.
  • a method of fabricating an image sensor comprises: forming on a substrate a plurality of signal processing circuits; for each signal processing circuit, monolithically growing on the substrate a light detector, such that the signal processing circuit is in electronic communication with the light detector and is located in a region at least partially surrounding the light detector; wherein the light detector and the signal processing circuit are formed of different material systems, and wherein a lattice mismatch between the light detector and the substrate is at least 10%.
  • the method comprises forming an array of cavities in or on the substrate, wherein the growing comprises growing a light detector epitaxially on a base of each cavity.
  • the growing comprises growing a stack of quantum well layers forming a quantum cascade detector.
  • the signal processing circuit comprises silicon, and the light detector comprises nitride.
  • the signal processing circuit is devoid of nitride.
  • the signal processing circuit is a MOS circuit.
  • the signal processing circuit is a CMOS circuit.
  • Implementation of the method and/or system of embodiments of the invention can involve performing or completing selected tasks manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of embodiments of the method and/or system of the invention, several selected tasks could be implemented by hardware, by software or by firmware or by a combination thereof using an operating system.
  • a data processor such as a computing platform for executing a plurality of instructions.
  • the data processor includes a volatile memory for storing instructions and/or data and/or a non-volatile storage, for example, a magnetic hard-disk and/or removable media, for storing instructions and/or data.
  • a network connection is provided as well.
  • a display and/or a user input device such as a keyboard or mouse are optionally provided as well.
  • FIGs. 1A and IB are schematic illustrations of a top view (FIG. 1A) and a cross-sectional view (FIG. IB) of an image sensor, according to some embodiments of the present invention
  • FIG. 2 is a schematic illustration of a signal processing circuit, according to some embodiments of the present invention.
  • FIGs. 3A-F are schematic illustrations exemplifying the principles and operations of a light detector which can be used according to some embodiments of the present invention
  • FIG. 4 is a schematic illustration of an image sensor in embodiments of the invention in which the image sensor comprises two or more types of active pixels cells;
  • FIG. 5 is a schematic illustration of an imaging system, according to some embodiments of the present invention.
  • FIG. 6 is a schematic illustration of an image sensor, exemplifying use of silicon on insulator (SOI) technology
  • FIGs. 7A-B are schematic illustration of a light detector device used in experiments and computer simulations performed according to some embodiments of the present invention.
  • FIGs. 8 A and 8B show back and front illumination photoctirrent spectra (FIG. 8A) and perpendicular to the surface electrical field intensity (FIG. 8B), obtained in experiments and computer simulations performed according to some embodiments of the present invention.
  • FIGs. 9A and 9B show photocurrent spectra for different temperatures (FIG. 9A) and Johnson noise limited detectivity, for back and front illumination (FIG. 9B) obtained in experiments and computer simulations performed according to some embodiments of the present invention.
  • the present invention in some embodiments thereof, relates to imaging and, more particularly, but not exclusively, to an imaging sensor having monolithic active pixels made of different material systems.
  • FIGs. 1A and IB are schematic illustrations of a top view (FIG. 1A) and a cross-sectional view (FIG. IB) of an image sensor 10, according to some embodiments of the present invention.
  • Image sensor 10 optionally and preferably comprises an array of active pixel cells 12 arranged on a substrate 14.
  • the array is a two-dimensional array.
  • active pixel cells 12 form a rectangular array, but other geometries are also contemplated.
  • Image sensor 10 can be configured for sensing back illumination 32 and/or front illumination 34, as illustrated in FIG. IB.
  • each active pixel cell 12 comprises a light detector 16 and a signal processing circuit 18 in electronic communication with light detector 16.
  • the light detector 16 of each active pixel cell optionally and preferably has a lateral area of at most 25 ⁇ 2 more preferably at most 20 ⁇ 2 more preferably at most 16 ⁇ 2 . Lateral areas larger than 25 ⁇ 2 are also contemplated.
  • Both light detector 16 and signal processing circuit 18 are optionally and preferably monolithically integrated with substrate 14.
  • signal processing circuit 18 is integrated in a region 20 that laterally surrounds, at least partially, light detector 16.
  • detector 16, circuit 18 and region 20 are only designated for one of cells 12, by in various exemplary embodiments of the invention the active pixel cells are all identical in their structure, except that each cell is at a different lateral location over substrate 14.
  • light detector 16 is made of a material system that includes nitride optionally with an active layer in which the dopant is silicon and an extractor which is devoid of silicon
  • substrate 14 is made of a material system that includes silicon and optionally and preferably does not include nitride.
  • light detector 16 can comprise a group Ill-nitride heterostructure
  • substrate 14 can be made of silicon, e.g. , a silicon having a crystal orientation of (111) along the cry stallo graphic axis.
  • Group-Ill nitrides are composed of nitrogen and at least one element from Group III of the Periodic Table of Elements, e.g. , aluminum (Al), gallium (Ga) and indium (In).
  • Representative examples of group-Ill nitrides suitable for the present embodiments include, without limitation, GaN, A1N, InN, GaAIN, and GaAlInN.
  • the advantage of using a group-Ill nitride is that by changing the composition of the group III element within the group-Ill nitride, the group-Ill nitride can be tuned along the electromagnetic spectrum.
  • a group Ill-nitride hetero structure suitable for detector 16 includes, without limitation, a GaN/AIN hetero structure. Also contemplated are other heterostructures such as, but not limited to, GaN/AlGaN, AlGaN/InGaN, AlInN/GalnN, AlGalnN/GalnN.
  • light detector 16 and signal processing circuit 18 are also formed of different material systems.
  • light detector 16 is made of a material system that includes nitride optionally with an active layer in which the dopant is silicon and an extractor which is devoid of silicon
  • signal processing circuit 18 is made of a material system that includes silicon and optionally and preferably does not include nitride.
  • light detector 16 can comprise a group Ill-nitride heterostructure, e.g. , a GaN/AIN heterostructure
  • signal processing circuit 18 can be made of silicon, e.g., a silicon having a crystal orientation of (100) along the crystallographic axis.
  • both signal processing circuit 18 and substrate 14 are made of a material system that includes silicon and optionally and preferably does not include nitride
  • light detector 16 is made of a material system that includes nitride optionally with an active layer in which the dopant is silicon and an extractor which is devoid of silicon.
  • light detector 16 can comprise a group Ill-nitride heterostructure, e.g. , a GaN/AIN heterostructure
  • signal processing circuit 18 can be made of silicon, e.g. , a silicon having a crystal orientation of (100) along the crystallographic axis
  • substrate 14 can be made of silicon, e.g. , a silicon having a crystal orientation of (11 1) along the crystallographic axis.
  • light detector 16 comprise a group Ill-nitride heterostructure, e.g., a GaN/AIN heterostructure, and both signal processing circuit 18 and substrate 14 are made of silicon having the same crystal orientation (e.g. , (I l l)) along the crystallographic axis.
  • group Ill-nitride heterostructure e.g., a GaN/AIN heterostructure
  • both signal processing circuit 18 and substrate 14 are made of silicon having the same crystal orientation (e.g. , (I l l)) along the crystallographic axis.
  • detector 16 In operation, detector 16 generates an electronic signal in response to light interacting with detector 16, and signal processing circuit 18 receives the electronic signal from detector 16 and executes initial processing operations such as, but not limited to, amplification, supply of reset signal and readout, as known in the art.
  • signal processing circuit 18 is a MOS circuit or a CMOS circuit.
  • signal processing circuit 18 comprises a plurality of transistors, optionally and preferably a plurality of field effect transistors (FETs).
  • FETs field effect transistors
  • a reset transistor Ql supplies a reset signal from a voltage source VDD when a reset signal RST is received at the gate of the transistor Ql.
  • the detector 16 is connected to Ql, preferably to the drain terminal, so that the transistor Ql resets the detector 16 to an initial voltage level.
  • a readout transistor Q2 is connected to a common current source Q r0 w, outside the area of pixel cell 12, to form a source-follower type amplifier controllable by a bias voltage Vbias.
  • An additional transistor Q3 serves as a switch to accommodate multiplexing.
  • the pixel is optionally and preferably read in two phases.
  • a first phase is a reset phase, where the detector 16 is connected to a supply voltage VDD through the transistor Ql.
  • a second phase is an integration phase, where the detector 16 accumulates photo-generated charge.
  • the Q3 switch turns on, connecting the transistor Q2 to the external current source Q r0 w.
  • the output voltage V ou t is then sampled in a sampling circuit, which is also outside the area of pixel cell 12.
  • Other types of circuits such as, but not limited to, those described in Brouk et al., IEEE Transactions on Electron Devices, (2007), 54(3), pp. 468-475, are also contemplated,
  • light detector 16 optionally and preferably occupies a cavity 22 formed in or on substrate 14.
  • light detector 16 can be is grown epitaxially on a buffer layer 24 formed on a base of cavity 22, directly on the material system forming substrate 14.
  • Buffer layer 24 is optionally and preferably made of a material system that reduces the lattice mismatch between detector 16 and substrate 14.
  • the buffer layer can comprise at least one of zinc telluride, titanium dioxide, zinc oxide, boron phosphide, silicon-germanium, hafnium nitride, and boron- aluminum nitride.
  • the buffer layer can be in the form of a sequence of epitaxially grown AIN/AlGaN layers.
  • each cavity 22 can be layered.
  • each cavity can comprise an insulator layer 26 and semiconductor layer 28 in which circuit 18 is formed, wherein the insulator layer 26 is between semiconductor layer 28 and substrate 14.
  • the insulator layer 26 can be a silicon dioxide layer and the semiconductor layer 28 can be a silicon layer.
  • Light detector 16 optionally and preferably comprises a quantum cascade detector (QCD). Also contemplated, are embodiments in which light detector 16 comprises a Quantum Well Infrared Photodetector (QWIP).
  • QCD quantum cascade detector
  • QWIP Quantum Well Infrared Photodetector
  • a QCD has a plurality of periods, where each period has an active layer, typically a quantum well (QW) layer, where electron excitation occurs, optionally and preferably, but not necessarily, upon photon intersubband absorption in the conduction band, and a stack of quantum well layers serving as an extractor that transfers the charge carrier excited at the active layer to a lower energy level (e.g. , the ground level) of the active layer of the following period.
  • QW quantum well
  • FIGs. 3 A and 3B are schematic illustrations exemplifying the principles and operations of a QCD 16, which can be used according to some embodiments of the present invention.
  • the conduction band energy levels of a period of the QCD at zero bias are presented in FIG. 3A, and a schematic illustration of the layers the QCD are presented in FIG. 3B.
  • the dots in FIG. 3B represent optional repetition of periods.
  • the QCD 16 comprises a plurality of periods 102, between a top contact 110 and a bottom contact 112. Arrows represent optical excitation in the active region of the QCD and transport of electrons from one period of the QCD to the next.
  • the top 110 and bottom 112 contacts can be made, for example, of Si doped group Ill-nitride (e.g., AlGaN).
  • Each of periods 102 has an active layer 104, and a stack 106 of layers forming an extractor that transfers the charge carrier from the active layer 102 of one period to the active layer of the following period.
  • the active layer 104 of each period 102 of QCD 16 is typically the widest QW, containing a ground state denoted el .
  • the period of QCD 16 is optionally and preferably selected such that optically excited carriers are transferred from one QW to the other along the cascade and produce a voltage across each period 102.
  • the QCD can include any number of periods (including a single period) and a measurable voltage is produced between the top 110 and bottom 112 contacts.
  • quantum well layers 102 are selected to absorb light by intersubband electronic transitions.
  • the photo-detection mechanism is via absorption between subbands rather than between the valence and conduction bands.
  • the operation of QCD 16 can be is as follows. Upon light absorption, optionally and preferably intersubband absorption, in the active layer, photo-excited charge carriers dominantly tunnel through the extractor. Charge carriers transferred through the extractor stage experience multiple relaxation via LO-phonon emission between the energy staircase in extractor QWs, towards the lower energy state (e.g. , ground state) of adjacent active QW. A cascade of such stages results in a charge separation (or macroscopic photo voltage) over a large distance, which depends on the number of periods.
  • the thickness of contacts 110 and 112 is typically from about 100 nm to about 1 ⁇ .
  • the thickness of the active layers 104 is typically from about 1 nm to about 7 nm, and the thickness of each layer in the extractor 106 is typically from about 1 nm to about 2nm. In various exemplary embodiments of the invention all the layers in the extractor 106 have approximately the same thickness.
  • each of quantum well layers 102 can be selected to allow absorption of light at any wavelength within a predetermined wavelength range.
  • the wavelength range includes the infrared range (e.g., from about 0.7 ⁇ to about 1000 ⁇ , or from about 1 ⁇ to about 1000 ⁇ , or from about 1 ⁇ to about 70 ⁇ , or from about 1 ⁇ to about 50 ⁇ , or from about 1 ⁇ to about 10 ⁇ ).
  • the wavelength range includes the near infrared range (e.g.
  • the wavelength range includes the short-wave infrared range (e.g., from about 0.7 ⁇ to about 2.5 ⁇ , or from about 1 ⁇ to about 2.5 ⁇ ), in some embodiments of the present invention the wavelength range includes the medium- wave infrared range (e.g. , from about 3 ⁇ to about 5 ⁇ ), and in some embodiments of the present invention the wavelength range includes the long-wave infrared range (e.g. , from about 8 ⁇ to about 12 ⁇ ).
  • the short-wave infrared range e.g., from about 0.7 ⁇ to about 2.5 ⁇ , or from about 1 ⁇ to about 2.5 ⁇
  • the wavelength range includes the medium- wave infrared range (e.g. , from about 3 ⁇ to about 5 ⁇ )
  • the wavelength range includes the long-wave infrared range (e.g. , from about 8 ⁇ to about 12 ⁇ ).
  • the quantum well layers 102 are selected to absorb light by interband electronic transitions.
  • the wavelength range includes the UV range (e.g. , from about 100 nm to about 400 nm), in some embodiments the wavelength range includes the UVA range (e.g., from about 315 nm to about 400 nm), in some embodiments the wavelength range includes the UVB range (e.g. , from about 280 nm to about 315 nm), in some embodiments the wavelength range includes the UVC range (e.g. , from about 100 nm to about 280 nm), in some embodiments the wavelength range includes the near UV range (e.g.
  • the wavelength range includes the middle UV range (e.g. , from about 200 nm to about 300 nm), and in some embodiments the wavelength range includes the far UV range (e.g. , from about 120 nm to about 200 nm).
  • QCD 16 also comprises a polarization rotating optical element 108 configured for rotating the polarization of the light impinging on the respective pixel cell, e.g., a 90° rotation, so that the polarization is aligned generally vertical with respect to the layers of periods 102.
  • Optical element 108 can be, for example, a grating or a plasmonic structure such as a two-dimensional metallic holes array. Element 108 can be deposited on the top contact 110. The advantage of element 108 is that it provides the proper polarization for exciting the resonances of the periods 102.
  • FIGs. 3C-E are schematic illustration of light detector 16 in embodiments of the invention in which detector 16 comprises quantum dots 312 forming an optically active region 314.
  • a quantum dot is a semiconductor crystalline structure with size dependent optical and electrical properties. Specifically, a quantum dot exhibits quantum confinement effects such that there is a three-dimensional confinement of electron-hole bound pairs or free electrons and holes.
  • the semiconductor structure can have any shape. Preferably, the semiconductor structure the largest cross-sectional dimension of such structure is of less than about 15 nanometers, e.g. , from about 0.2 nanometers to about 10 nanometers.
  • a quantum dot is structurally different from a quantum well. Unlike a quantum dot in which, as stated, there is a three-dimensional confinement, the electron-hole bound pairs or free carriers in a quantum well are confined only one- dimension and are generally free in the other two-dimensions.
  • quantum dots 312 When quantum dots 312 are irradiated by light from an excitation source (not shown) they reach respective energy excited states.
  • detector 16 is preferably designed such that charge carriers that reach excited states are extracted from region 314 thereby converting the optical energy as manifested by the light to electrical energy as manifested by the motion of charge carriers.
  • the quantum dots include electrons in their conductance band. This can be achieved, for example, using self- assembled quantum dots in region 314.
  • Exemplary materials for use as quantum dots 312 according to some embodiments of the present invention include, but are not limited to GaN or InGaN semiconductors.
  • the absorption spectrum of quantum dots 312 is characterized by one or more peaks that correspond to energy levels characterizing active region 314.
  • energy level also encompasses a range of energies, also known in the literature as “energy band.” Such range is typically characterized by an energy value and an energy width. For example, inhomogeneous dimension of the QDs results in broadening of the absorption and emission peaks hence also to wider energy bands.
  • energy level and energy band are used interchangeably throughout this document.
  • FIG. 3D shows a representative example of a set of energy levels characterizing active region 314.
  • FIG. 3D presents schemes of the conduction band part of the quantum dots. Shown in FIG. 3D is a three-level energy system, where each level is shown as a range of energies. The lowest energy level, also referred to as the ground state s, is typically, but not necessarily, the only occupied level. The energy level above the ground state corresponds to in-plane excitation of the quantum dot. In-plane excitation can be in two directions, conveniently denoted the x and y directions. In the present example, the quantum dots in region 314 are arranged such that in-plane excitations in the x and y directions occur at the same energy, denoted in FIG.
  • the p x , p y level is the intermediate energy level in the three-level system of the present example.
  • the highest energy level corresponds to excitation in which the electric field is parallel to the vertical or growth direction.
  • the vertical or growth direction is conveniently denoted the z direction, and the energy level that correspond to excitation along the z direction is denoted P z .
  • the wavelength selection of the quantum dots in active region 314 depends on the type of excitation, the shape and size of the quantum dots, and the sublevel transition in the conduction bands.
  • the excitation of a quantum dot can be via interband transition (transition of charge carriers between a conductance band and a valence band) or via intraband transition (transition of charge carriers between energy levels that belong to the same energy band).
  • intraband transition from the s state to the p z state corresponds to energy of about 0.8 eV (or, equivalently, wavelength of about 1.5 micron)
  • intraband transition from the s state to the p x state corresponds to energy in the range of from about 0.2 eV to about 0.3 eV (wavelength of about 5 microns)
  • interband transition corresponds to energy of about 3.6 eV (wavelength of 0.345 nm).
  • quantum dot 312 when quantum dot 312 reaches an excited state, it can experience a relaxation.
  • the excitation when the excitation is via interband transition the energy that is emitted by the quantum dot upon relaxation corresponds to the respective energy band gap.
  • the excited carriers can be relaxed in different ways, e.g., through the emission of longitudinal optical phonon.
  • system 310 is designed such that the excited carriers are extracted from region 314 before the relaxation. This is preferably achieved by means of a charge carrier extractor.
  • detector 16 comprises a channel region 318 and a charge carrier extractor 316 between active region 314 and channel region 318.
  • Channel region 318 is preferably constituted to form a two-dimensional electron gas therein.
  • Extractor 316 serves for extracting excited charge carriers out of active region 314. Specifically, extractor 316 facilitates transport of charge carriers, via quantum tunneling, from active region 314 to channel region 318.
  • Extractor 316 is characterized by a set of gradually decreasing energy levels between a characteristic excited energy level of active region 314 and a characteristic conductance energy level of channel region 318.
  • Detector 16 typically also comprises lateral contacts 320 contacting the channel 318 for collecting the charge carrier from channel 318. Since the charge in the quantum dots 312 is confined, it is isolated from contacts 320.
  • the energy levels of extractor 316 are selected such as to extract charge carriers excited via intraband transition. Nevertheless, while some embodiments are described with a particular emphasis to detection of light that induces intraband transitions, it is to be understood that more detailed reference to intraband transitions is not to be interpreted as limiting the scope of the invention in any way. Thus, in some embodiments of the present invention the energy levels of extractor 316 are selected such as to extract chare carriers excited via interband transition that excited electron-hole pair into the ground state of quantum dots 314. Extractor 316 can be constituted for extracting either electrons or holes.
  • the energy levels of extractor 316 can be between a characteristic excited energy level of electrons in active region 314 and a characteristic conductance energy level of electrons in channel region 318, or between a characteristic excited energy level of holes in active region 314 and a characteristic conductance energy level of holes in channel region 318.
  • Extractor 316 preferably has a layered structure wherein each layer of the structure corresponds to a different energy level of the set characterizing the extractor.
  • a non-limited example of the energy levels of extractor 316 is illustrated schematically in FIG. 3E, which presents conduction band parts.
  • the characteristic excited energy level of active region 314 is at about 0.3 eV, and the characteristic conductance energy level of cannel region 318 is about -0.4 eV.
  • extractor 316 has four energy levels gradually decreasing from about 0.25 eV near active region 314 to about -0.3 eV near channel 318.
  • a charge carrier that is excited at region 314 is transferred along the extractor 316 until it reaches channel 318 where it is allowed flow substantially freely. From channel 318 the charge carrier can be collected via an electrode.
  • the materials from which extractor 316 and channel region 318 are made depend on the selected material for the quantum dots in the active region. Given the list of materials above for the quantum dots, the ordinarily skilled person would know how to selected appropriate material combination for system 310
  • the GaN ⁇ Al x Gai- x N material combination is used.
  • quantum dots 314 can comprise or be made of GaN
  • extractor 316 can comprise or be made of AlGaN/AIN
  • channel region 318 can comprise or be made of GaN.
  • the energy levels of extractor 316 can be selected to allow the extraction of charge carriers at any of the excited levels of active region 314.
  • the highest energy level of extractor 316 is lower than the highest excited level (e.g. , level P z ) but above the intermediate excited level (e.g. , level P x ,P y ) of active region 314.
  • These embodiments are useful for collecting only charge carriers that are excited to a level which is higher than the intermediate level.
  • these embodiments are useful when it is desired to detect light which is polarized in a transverse magnetic (TM) polarization.
  • TM transverse magnetic
  • the highest energy level of extractor 316 is lower than the intermediate level of region 314 (but above the ground state S).
  • These embodiments are useful for collecting charge carriers that are excited to the intermediate level.
  • these embodiments are useful when it is desired to detect light which is polarized in a transverse electric (TE) polarization.
  • TE transverse electric
  • FIG. 3F is a schematic illustration of the conduction band energy levels of a period of light detector 16, in embodiments of the invention in which detector 16 comprises an alloy extractor QCD.
  • detector 16 comprises an alloy extractor QCD.
  • the multiple QW extractor 106 is replaced by a relatively thick alloy extractor layer.
  • the alloy extractor layer is made of Al x Gai- x N, where x is selected to provide a predetermined internal field that effects a V-shaped potential in the extractor.
  • the alloy layer is optionally and preferably selected to support a plurality of bound states (for example, at least 5 or at least 5 or at least 6 or at least 7 or at least 8 or at least 9 or at least 10 bound states). Some of the excited states are preferably in close resonance with the excited state of the active QW.
  • the thickness of the alloy extractor is optionally and preferably selected to provide an energy separation between the ground state of the extractor and that of the next period active QW above the LO-phonon energy. Representative thickness values suitable for the present embodiments include, without limitation, from about 10 nm to 20 nm, e.g. , about 15 nm.
  • FIG. 4 is a schematic illustration of image sensor 10 in embodiments of the invention in which image sensor 10 comprises two or more types of active pixels cells.
  • sensor 10 comprises two types of active pixel cells, shown as cells 12 and cells 40. Each type is constructed for absorption of light within a different wavelength range, so that the dynamic range of sensor 10 is significantly improved.
  • both types of cells are made of the same material systems except that they are constructed to absorb light at different wavelengths.
  • both types of cells can comprise QCDs as further detailed hereinabove wherein the layer thicknesses of the QCDs of cells 12 are selected for absorption of light within a first wavelength range, and the layer thicknesses of the QCDs of cells 40 are selected for absorption of light within a second wavelength range.
  • each type of cells is made of a different material system.
  • cells 12 can comprise QCDs as further detailed hereinabove
  • cells 40 can comprise a photodiode, such as, but not limited to, a silicon photodiode, and a processing circuit made of a material system that is compatible or the same as the material system from which the photodiode is made.
  • the substrate can also be made of a material system that is compatible or the same as the material system from which the photodiode is made, so that it is not necessary for cells 40 to occupy a cavity since there is no lattice mismatch.
  • FIG. 5 is a schematic illustration of an imaging system 500, according to some embodiments of the present invention.
  • System 500 comprises an image sensor, such as, but not limited to, image sensor 10 which generates electrical current in response to light and a processing circuit 504 which generates an image based on the generated current.
  • system 500 operates in the infrared domain so as to allow, e.g. , thermal imaging.
  • System 500 can be mounted on a mobile device, in which case processing circuit 504 can be the part of the circuit of the mobile device.
  • mobile devices suitable for the present embodiments including, without limitation, a cellular phone, a smartphone, a tablet device, a mobile digital camera, a wearable camera, a personal computer, a laptop, a portable media player, a portable gaming device, a portable digital assistant device, and a portable navigation device.
  • a method of fabricating an image sensor comprises forming on a substrate a plurality of signal processing circuits, wherein each signal processing circuit is formed in a predefined area of a pixel cell.
  • the method continues by monolithically growing on the substrate a light detector for each signal processing circuit.
  • the light detectors are formed such that the signal processing circuit is in electronic communication with the light detector and is located in a region at least partially surrounding light detector.
  • the light detector and the signal processing circuit are optionally and preferably formed of different material systems, and the lattice mismatch between the light detector and the substrate is at least 10%, as further detailed hereinabove.
  • the growth of light detectors is preceded by formation of photodiodes in at least some of the predefined pixel cell areas, wherein the photodiodes are made of the same or similar material systems as the signal processing circuit as further detailed hereinabove.
  • compositions, methods or structure may include additional ingredients, steps and/or parts, but only if the additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure.
  • a compound or “at least one compound” may include a plurality of compounds, including mixtures thereof.
  • range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.
  • CMOS imagers are ultra-cheap devices, with very small pixels, and large array formats (tens of megapixels). CMOS imagers are operating at the visible light spectrum and a little beyond (0.8-0.9 micrometer the maximum is the Si bandgap which is 1.1 microns).
  • the present example contemplates growth of GaN based layers on silicon wafers, with CMOS compatibility.
  • the present example also contemplates use of QCD based on the GaN/Al(Ga)N material system.
  • QCD based on the GaN/Al(Ga)N material system.
  • the advantage of using QCD is that, unlike other detectors that detect light at a wavelength governed by the separation of their conduction and valance bands, the wavelength detected by the QCD can be controlled by an appropriate selection of the quantum layer thickness of the QCD.
  • the group Ill-nitride heterostructures optionally and preferably have electronic and optical properties that are suitable for extending the functionality of semiconductor optoelectronics into spectral ranges currently inaccessible with other material systems.
  • the conduction band offset provided by Ill-nitride heterostructures ⁇ e.g., about 1.75 eV for GaN/AIN) allows operating at relatively short wavelengths.
  • group Ill-nitrides exhibit relatively short intersubband absorption recovery times (from about 150 fs to about 400 fs), due to the relatively strong electron-phonon interaction in these materials. This allows the image sensor of the present example to operate in the 0.1-1 Tbit/s bit-rate regime.
  • the GaN/Al(Ga)N material system is also advantageous from the standpoint of wavelength selectability, speed, high power handling capabilities, temperature insensitivity and material hardness. Another advantage is that the thermal load of the detector of the present embodiments is relatively low which is particularly advantageous when the available cooling is limited.
  • the ladder of extractor states of the nitride based QCD of the present example can be achieved by engineering the internal field generated by a polar wurtzite III nitride heterostructures grown along the [0001] crystallographic axis.
  • the presence of the internal field offers an additional degree of freedom, which can be exploited, for example, for fabricating multi-color QCDs.
  • an alloy extractor QCD in which the multiple QW extractor region is replaced by an AlGaN thick layer, whose composition is chosen to engineer the internal field and achieve a graded potential.
  • the QCD comprises a Ti/Au metallic nano-hole array or any other metamaterial structure integrated on top surface of the active region of the QCD.
  • the metallic nano-hole array rotates the polarization of the incoming radiation in the near field and makes it compatible with the dipole selection rules of III- nitrides quantum- wells.
  • GaN Due to its hexagonal crystal structure, GaN exhibits better material properties when grown on Si (111) with its hexagonal array of atoms rather than on other orientations of silicon such as the (100) orientation. For silicon CMOS, however, due to higher trap densities on the Si (111) orientation, the (100) orientation is preferred.
  • modified silicon on insulator (SOI) wafers were used as illustrated in FIG. 6.
  • the (100) orientation is chosen for the surface silicon layer for ease of CMOS fabrication.
  • the (111) orientation for GaN growth is chosen for the handle substrate to reduce thermal path for the GaN transistor. Consequently, GaN is grown in islands on the surface of the handle substrate. This arrangement also has the benefit of resulting in a near planar arrangement of the detector's surface with the CMOS surface, which aids interconnect formation.
  • the processing circuit is a CMOS circuit formed in a (100) Si layer having resistivity of about lQcm.
  • the substrate is a (111) Si substrate having resistivity of more than 1000 Qcm.
  • the CMOS electronic is implemented on the (100) Si layer, and the QCD is grown in a cavity etched in the SOI structure down to the (111) Si substrate.
  • the CMOS circuit is fabricated before the growth of GaN based QCD.
  • the CMOS is fabricated on a flat, virgin wafer with no group III-V materials present. This is advantageous since group III-V may cause wafer bow and pose a contamination risk to a silicon fabrication line.
  • the growth of GaN based QCD can be, for example, by plasma molecular beam epitaxy.
  • both the QCD e.g., GaN QCD
  • the CMOS can be grown on a (111) Si substrate following modification of the CMOS process.
  • MHAs multi-dimensional metallic holes arrays
  • SP surface plasmons
  • the generated SP is a TM mode thus exhibits a dominant electric field component normal to the surface that is the proper polarization for exciting the ISB resonance.
  • FI Front Illumination
  • BI Backside Illumination
  • the QCD structure consisted of 40 active periods sandwiched between Si-doped AlGaN contact layers.
  • the QCD structure was processed in the form of 700x700 ⁇ 2 mesas, with top and bottom Ti/Al/Ti/Au metallic contact layers.
  • the MHA on the mesa top surface was implemented and patterned with periodic holes using Electron-beam lithography.
  • the diameter and periods of holes array were designed using finite difference time domain software (FDTD) to achieve a plasmon peak resonance that overlaps the ISB peak resonance.
  • FDTD finite difference time domain software
  • FIG. 7B The simulated structure unit cell is illustrated shown in FIG. 7B.
  • FIG. 8 A shows the photoresponse spectra at RT (zero bias) of the QCD under normal incidence FI and BI.
  • the peak responsivity, at 1.82 ⁇ at room temperature, increases from 1.77 to 2.72 mA/W by changing the light direction.
  • Figure 8B shows the FDTD simulated Ez-intensity enhancement spectrum. Taking into account that the electric field Ez can be coupled to QCD intersubband resonance, it is averaged over the detecting volume and divided by the averaged electric field intensity of the QCD without MHA.
  • the photoresponse of the QCD at normal incidence, FI as a function of temperature is shown in FIG. 9A.
  • the photoresponse signal intensity at RT is 60% compared to 40 K and the peak energy position remains almost without shift.
  • FIG. 9B shows DJ* (Johnson noise limited detectivity) as a function of temperature for the two different illumination

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Abstract

La présente invention concerne un capteur d'image. Le capteur d'image comprend un réseau de cellules de pixel actif sur un substrat. Chaque cellule de pixel actif selon les présents modes de réalisation comporte : un détecteur de lumière, intégré de façon monolithique avec le substrat ; et un circuit de traitement de signal, intégré de manière monolithique avec le substrat dans une région entourant au moins partiellement le détecteur de lumière, et en communication électronique avec le détecteur de lumière. Le détecteur de lumière et le circuit de traitement de signal sont éventuellement constitués de systèmes matériels différents. Une inégalité des paramètres de maille entre le détecteur de lumière et le substrat est éventuellement d'au moins 10 %.
PCT/IL2016/050495 2015-05-11 2016-05-10 Capteur d'image et son procédé de fabrication Ceased WO2016181391A1 (fr)

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