Photoelectric conversion element, imaging device, and method for manufacturing photoelectric conversion element

Abstract

This photoelectric conversion element is provided with: a photoelectric conversion layer that converts light into signal charges; a first electrode that contains a metal nitride, that has a first surface and a second surface which is closer to the photoelectric conversion layer than the first surface, and that collects signal charges; and a second electrode that faces the first electrode 2 with the photoelectric conversion layer interposed therebetween. The oxygen element concentration on the second surface of the first electrode is 20 atom% or less.

Description

Photoelectric conversion element, imaging device, and method for manufacturing photoelectric conversion element

[0001] The present disclosure relates to a photoelectric conversion element, an imaging device, and a method for manufacturing a photoelectric conversion element.

[0002] An imaging device such as an image sensor is an element that includes, for example, a photoelectric conversion element that generates a signal charge according to the amount of incident light, a plurality of pixels arranged one-dimensionally or two-dimensionally, a signal detection circuit that detects the signal charge, and various elements for driving them. A stacked image sensor is an imaging device that has pixels including a photoelectric conversion element with a structure in which, from the substrate side, a lower electrode that collects signal charge, a photoelectric conversion layer, and an upper electrode are stacked. An example of a stacked image sensor is disclosed in Patent Document 1.

[0003] JP 2011-71469 A

[0004] S. Logothetidis et al., “Room temperature oxidation behavior of TiN thin films”, Thin Solid Films, 1999, Vol. 338, pages 304-313

[0005] In photoelectric conversion elements and imaging devices, variations in characteristics are desirable because they cause noise and reduced reliability. Furthermore, if variations in characteristics cause defects, the manufacturing yield will also decrease.

[0006] Therefore, the present disclosure provides a photoelectric conversion element and the like that can reduce the variation in characteristics.

[0007] A photoelectric conversion element according to one aspect of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal nitride and having a first surface and a second surface closer to the photoelectric conversion layer than the first surface, the first electrode collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the oxygen element concentration in the second surface of the first electrode is 20 atom % or less.

[0008] An imaging device according to one aspect of the present disclosure includes the photoelectric conversion element described above and a charge accumulation region electrically connected to the first electrode.

[0009] A method for manufacturing a photoelectric conversion element according to one embodiment of the present disclosure is a method for manufacturing a photoelectric conversion element including a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal nitride and collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, the method including forming the first electrode, etching a surface of the first electrode, and after the etching, forming one or more layers including the photoelectric conversion layer on the surface of the first electrode without exposing the surface of the first electrode to the atmosphere, and forming the second electrode on the one or more layers.

[0010] According to the present disclosure, the variation in characteristics can be reduced.

[0011] FIG. 1 is a diagram for explaining concerns arising from the transport of wafers in the atmosphere between an inorganic material process and an organic material process. FIG. 2 is a schematic cross-sectional view showing the configuration of a photoelectric conversion element according to an embodiment. FIG. 3 is an exemplary energy band diagram of a photoelectric conversion element according to an embodiment. FIG. 4 is a diagram showing the relationship between the work function and the element concentration of a contained element with respect to the position in the thickness direction of a titanium nitride film. FIG. 5 is a schematic cross-sectional view showing the configuration of a photoelectric conversion element according to a modified embodiment. FIG. 6 is a diagram showing the circuit configuration of an imaging device according to an embodiment. FIG. 7 is a cross-sectional view schematically showing a cross section of a device structure of a pixel of an imaging device according to an embodiment. FIG. 8 is a flowchart of a method for manufacturing a photodetector according to an embodiment. FIG. 9 is a cross-sectional view for explaining a method for manufacturing a photodetector according to an embodiment. FIG. 10 is a cross-sectional view for explaining a method for manufacturing a photodetector according to an embodiment. FIG. 11 is a cross-sectional view for explaining a method for manufacturing a photodetector according to an embodiment. FIG. 12 is a block diagram showing an example of the configuration of a camera system according to an embodiment.

[0012] (How One Aspect of the Present Disclosure Was Achieved) Before describing the embodiments of the present disclosure in detail, the inventors' viewpoint that led to the achievement of one aspect of the present disclosure will be described.

[0013] Silicon photodiodes are often used for photoelectric conversion elements. However, in photoelectric conversion elements used in stacked image sensors and the like, materials other than silicon may be used as the photoelectric conversion material for forming the photoelectric conversion layer. For example, when an organic material is used as the photoelectric conversion material for the photoelectric conversion layer, the organic material may not be able to withstand the cleaning process and heating temperatures previously used in semiconductor device fabrication. Therefore, factors that make it difficult to control conditions, such as exposure to air during photoelectric conversion element fabrication, associated with the separation of inorganic material processes that handle inorganic materials such as semiconductor substrates and electrodes from organic material processes that handle organic materials, are expected to cause variations in the characteristics of photoelectric conversion elements. Note that the present disclosure is applicable to photoelectric conversion elements in general and is not limited to cases where an organic material is used as the photoelectric conversion material.

[0014] FIG. 1 illustrates concerns about transporting a wafer 200 between an inorganic material process and an organic material process in the atmosphere. For example, if a wafer 200 with electrodes for collecting signal charges formed on its substrate is fabricated using an inorganic material process and then transferred to an organic material process, exposure to the atmosphere during transport can cause a native oxide film to form on the surface of the electrode. For example, it is known that titanium nitride, used as an electrode material, requires approximately 80 hours for the growth of the native oxide film formed on the surface to saturate (see, for example, Non-Patent Document 1). Changes in the formation state of the native oxide film can also fluctuate the work function of the electrode, leading to variations in the characteristics of the photoelectric conversion element. On the other hand, controlling the exposure time of the wafer 200 to the atmosphere to control the composition of the electrode material and then transferring it to the organic material process is not practical. Furthermore, waiting until the growth of the native oxide film saturates before transferring it to the organic material process also poses a risk of atmospheric contaminant adhesion and reduced mass production capacity, so a different solution is needed. Furthermore, introducing an additional layer on the electrode to uniformize characteristics can also complicate the process and reduce the responsiveness of the sensor.

[0015] The present disclosure has been made based on the above-mentioned viewpoint of the inventors of the present application, and provides a photoelectric conversion element and the like that can reduce the variation in characteristics.

[0016] (Summary of the Present Disclosure) As an overview of one embodiment of the present disclosure, examples of a photoelectric conversion element, an imaging device, and a method for manufacturing a photoelectric conversion element according to the present disclosure will be described below.

[0017] (First Aspect) For example, a photoelectric conversion element according to a first aspect of the present disclosure includes: a photoelectric conversion layer that converts light into signal charges; a first electrode that contains a metal nitride and has a first surface and a second surface that is closer to the photoelectric conversion layer than the first surface, and that collects the signal charges; and a second electrode that faces the first electrode with the photoelectric conversion layer interposed therebetween, wherein the oxygen element concentration in the second surface of the first electrode is 20 atom % or less.

[0018] This suppresses variations in the work function of the first electrode, thereby reducing variations in the characteristics of the photoelectric conversion element. Specifically, in a first electrode containing a metal nitride, the work function fluctuates when the oxygen element concentration changes due to the formation of a natural oxide film. The fluctuation in the work function increases as the oxygen element concentration at the surface of the first electrode increases. Therefore, by keeping the oxygen element concentration at the surface of the first electrode at 20 atom % or less, variations in the work function of the first electrode can be suppressed, thereby reducing variations in the characteristics of the photoelectric conversion element.

[0019] Second Aspect Furthermore, for example, in the photoelectric conversion element according to the first aspect, the oxygen element concentration on the second surface of the first electrode may be 15 atom % or less.

[0020] This makes it possible to further suppress variations in the work function of the first electrode.

[0021] (Third Aspect) Furthermore, for example, in the photoelectric conversion element according to the first or second aspect, the nitrogen element concentration on the second surface of the first electrode may be equal to or greater than the oxygen element concentration on the second surface of the first electrode.

[0022] This makes it possible to further suppress variations in the work function of the first electrode.

[0023] (Fourth Aspect) Furthermore, for example, in the photoelectric conversion element according to any one of the first to third aspects, the metal nitride may be titanium nitride.

[0024] This makes it possible to suppress variations in the work function of the first electrode containing titanium nitride.

[0025] (Fifth Aspect) Furthermore, for example, in the photoelectric conversion element relating to the fourth aspect, the nitrogen element concentration on the second surface of the first electrode may be greater than the titanium element concentration on the second surface of the first electrode, and the titanium element concentration on the second surface of the first electrode may be greater than the oxygen element concentration on the second surface of the first electrode.

[0026] This makes it possible to further suppress variations in the work function of the first electrode.

[0027] (Sixth Aspect) Furthermore, for example, in the photoelectric conversion element according to the fourth or fifth aspect, the first electrode may include a first layer containing the titanium nitride and a second layer containing titanium, facing the photoelectric conversion layer with the first layer interposed therebetween.

[0028] This allows the second layer to suppress the diffusion of metal (e.g., copper used for wiring) from the underside of the first electrode into the first layer, and also suppresses the generation of columnar crystals when forming the titanium nitride first layer, allowing the formation of an amorphous titanium nitride film with few gaps.

[0029] (Seventh Aspect) Furthermore, for example, in the photoelectric conversion element according to any one of the first to sixth aspects, the photoelectric conversion layer may contain an organic semiconductor material.

[0030] This makes it possible to suppress variations in the work function of the first electrode even when the photoelectric conversion layer contains an organic semiconductor material.

[0031] (Eighth Aspect) Also, for example, an imaging device according to an eighth aspect of the present disclosure includes a photoelectric conversion element according to any one of the first to seventh aspects and a charge accumulation region electrically connected to the first electrode.

[0032] As a result, the charge accumulation region is connected to the first electrode in which the variation in work function of the photoelectric conversion element is suppressed, and therefore the variation in the characteristics of the imaging device can be reduced.

[0033] (Ninth Aspect) Furthermore, for example, a manufacturing method of a photoelectric conversion element according to a ninth aspect of the present disclosure is a manufacturing method of a photoelectric conversion element including a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal nitride and collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, the method including: forming the first electrode; etching a surface of the first electrode; and, after the etching, forming one or more layers containing the photoelectric conversion layer on the surface of the first electrode without exposing the surface of the first electrode to the atmosphere; and forming the second electrode on the one or more layers.

[0034] As a result, even if a native oxide film is formed on the surface of the first electrode, a photoelectric conversion layer or the like is formed on the first electrode from which the native oxide film has been removed by surface etching, thereby suppressing variations in the work function of the first electrode due to the native oxide film, and making it possible to manufacture photoelectric conversion elements with reduced variations in characteristics.

[0035] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0036] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components not described in the independent claims are described as optional components.

[0037] Furthermore, each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, for example, the scales of the figures do not necessarily match. Furthermore, in each figure, substantially the same components are given the same reference numerals, and redundant explanations are omitted or simplified.

[0038] Furthermore, in this specification, terms indicating the relationship between elements, such as vertical, terms indicating the shape of elements, such as rectangle, and numerical ranges are not expressions that only express a strict meaning, but are expressions that also include a substantially equivalent range, for example, a difference of about a few percent.

[0039] Furthermore, in this specification, the terms "upper" and "lower" do not refer to the upward (vertically upward) and downward (vertically downward) directions in absolute spatial recognition, but are used as terms defined by a relative positional relationship based on the stacking order in the stacked structure. Specifically, the light-receiving side of the photoelectric conversion element and the imaging device is referred to as "upper," and the side opposite the light-receiving side is referred to as "lower." Similarly, the "upper surface" and "lower surface" of each component refer to the surface facing the light-receiving side of the photoelectric conversion element and the imaging device as the "upper surface," and the surface facing the side opposite the light-receiving side as the "lower surface." Note that the terms "upper," "lower," "upper surface," and "lower surface" are used solely to specify the relative arrangement of components and are not intended to limit the orientation of the photoelectric conversion element and the imaging device during use. Furthermore, the terms "upper" and "lower" apply not only to cases where two components are arranged with a gap between them and another component is present between them, but also to cases where two components are arranged closely together and in contact with each other.

[0040] In this specification, electromagnetic waves in general, including visible light, infrared light, and ultraviolet light, are referred to as "light" for convenience.

[0041] (Embodiment) [Photoelectric Conversion Element] A photoelectric conversion element according to this embodiment will be described. The photoelectric conversion element according to this embodiment is a charge readout type photoelectric conversion element. The photoelectric conversion element according to this embodiment is used in, for example, an imaging device such as a stacked image sensor. Fig. 2 is a schematic cross-sectional view showing the configuration of a photoelectric conversion element 10 according to this embodiment.

[0042] 2, photoelectric conversion element 10 is supported by support substrate 1, and includes a pair of electrodes, an upper electrode 4 and a lower electrode 2, a photoelectric conversion layer 3 located between upper electrode 4 and lower electrode 2, and an intermediate layer 5 located between photoelectric conversion layer 3 and lower electrode 2. Photoelectric conversion element 10 also includes a protective layer 6 disposed opposite photoelectric conversion layer 3 with upper electrode 4 interposed therebetween. In this embodiment, lower electrode 2 is an example of a first electrode, and upper electrode 4 is an example of a second electrode.

[0043] The photoelectric conversion element 10 is used in a position where, for example, light that has passed through the upper electrode 4 and the protective layer 6 is incident on the photoelectric conversion layer 3 .

[0044] Hereinafter, each component of the photoelectric conversion element 10 according to this embodiment will be described.

[0045] The support substrate 1 may be any substrate that is commonly used to support a photoelectric conversion element, and may be, for example, a glass substrate, a quartz substrate, a semiconductor substrate, or a plastic substrate.

[0046] The lower electrode 2 includes a metal element. The lower electrode 2 includes, for example, a metal or a metal nitride containing the metal element, and is formed from the metal or the metal nitride. Examples of the metal element include titanium, tungsten, and tantalum. The metal nitride may be titanium nitride, tungsten nitride, or tantalum nitride.

[0047] The lower electrode 2 is, for example, a metal nitride film from which a native oxide film has been removed on a surface 2s of the lower electrode 2 facing the photoelectric conversion layer 3. An intermediate layer 5 is formed on the surface 2s of the lower electrode 2 from which the native oxide film has been removed. In the example shown in Fig. 2, the surface 2s is the upper surface. Note that the native oxide film does not need to be completely removed from the surface 2s of the lower electrode 2, and the surface 2s of the lower electrode 2 may contain oxygen elements as long as the oxygen element concentration is equal to or less than a predetermined value.

[0048] The upper electrode 4 faces the lower electrode 2 with the photoelectric conversion layer 3 therebetween. The upper electrode 4 is, for example, a transparent electrode formed from a transparent conductive material. Examples of the material for the upper electrode 4 include transparent conductive oxide (TCO), indium tin oxide (ITO), indium zinc oxide (IZO), aluminum-doped zinc oxide (AZO), fluorine-doped tin oxide (FTO), and SnO. ２ and TiO ２ The upper electrode 4 may be made of a TCO, and metal materials such as aluminum (Al) and gold (Au) either singly or in combination, depending on the desired transmittance.

[0049] Various methods are used to fabricate the upper electrode 4 depending on the material used. For example, when ITO is used, methods such as an electron beam method, a sputtering method, a resistance heating vapor deposition method, a chemical reaction method such as a sol-gel method, or the application of a dispersion of indium tin oxide may be used. In this case, to fabricate the upper electrode 4, after forming the ITO film, UV-ozone treatment, plasma treatment, or the like may be further performed.

[0050] The photoelectric conversion layer 3 converts light into signal charges. The photoelectric conversion layer 3 includes, for example, a donor semiconductor material and an acceptor semiconductor material. The donor semiconductor material and the acceptor semiconductor material may be mixed to form a single layer, or a layer of the donor semiconductor material and a layer of the acceptor semiconductor material may be stacked. The photoelectric conversion layer 3 can be fabricated using, for example, a wet method such as a coating method using spin coating, or a dry method such as a vacuum deposition method. The vacuum deposition method is a method in which layer materials are vaporized by heating under vacuum and then deposited on a substrate. The intermediate layer 5 can also be fabricated using a method similar to that used for the photoelectric conversion layer 3.

[0051] The photoelectric conversion layer 3 also contains, for example, an organic semiconductor material. The photoelectric conversion layer 3 is, for example, a mixed film with a bulk heterostructure containing a donor organic semiconductor material and an acceptor organic semiconductor material. Specific examples of donor organic semiconductor materials and acceptor organic semiconductor materials are given below.

[0052] Examples of donor organic semiconductor materials include triarylamine compounds, benzidine compounds, pyrazoline compounds, styrylamine compounds, hydrazone compounds, triphenylmethane compounds, carbazole compounds, polysilane compounds, thiophene compounds, phthalocyanine compounds, naphthalocyanine compounds, subphthalocyanine compounds, cyanine compounds, merocyanine compounds, oxonol compounds, polyamine compounds, indole compounds, pyrrole compounds, pyrazole compounds, polyarylene compounds, fused aromatic carbocyclic compounds, and metal complexes having nitrogen-containing heterocyclic compounds as ligands.

[0053] Examples of the fused aromatic carbocyclic compounds include naphthalene derivatives, anthracene derivatives, phenanthrene derivatives, tetracene derivatives, pyrene derivatives, perylene derivatives, and fluoranthene derivatives.

[0054] Examples of the acceptor organic semiconductor material include fullerene, fullerene derivatives, fused aromatic carbon ring compounds, 5- to 7-membered heterocyclic compounds containing a nitrogen atom, an oxygen atom, or a sulfur atom, polyarylene compounds, fluorene compounds, cyclopentadiene compounds, silyl compounds, and metal complexes having a nitrogen-containing heterocyclic compound as a ligand.

[0055] Examples of fullerenes include C60 fullerene and C70 fullerene.

[0056] The fullerene derivative is, for example, PCBM (phenyl C ６１ butyric acid methyl ester) and ICBA (indene C ６０ Bis-adducts), etc.

[0057] Examples of 5- to 7-membered heterocyclic compounds containing a nitrogen atom, an oxygen atom, or a sulfur atom include pyridine, pyrazine, pyrimidine, pyridazine, triazine, quinoline, quinoxaline, quinazoline, phthalazine, cinnoline, isoquinoline, pteridine, acridine, phenazine, phenanthroline, tetrazole, pyrazole, imidazole, thiazole, oxazole, indazole, benzimidazole, benzotriazole, benzoxazole, benzothiazole, carbazole, purine, triazolopyridazine, triazolopyrimidine, tetrazaindene, oxadiazole, imidazopyridine, pyrrolidine, pyrrolopyridine, thiadiazolopyridine, dibenzazepine, and tribenzazepine.

[0058] The donor organic semiconductor material and the acceptor organic semiconductor material are not limited to the above examples. Low-molecular-weight compounds and high-molecular-weight compounds may be used as the donor organic semiconductor material and the acceptor organic semiconductor material constituting the photoelectric conversion layer 3, as long as they are organic compounds that can be formed into a film as a photoelectric conversion layer by either a dry method or a wet method.

[0059] The photoelectric conversion layer 3 may also contain semiconductor materials other than organic semiconductor materials as donor and acceptor semiconductor materials. The photoelectric conversion layer 3 may also contain, as semiconductor materials, silicon semiconductors, compound semiconductors, quantum dots, perovskite materials, carbon nanotubes, or the like, or a mixture of two or more of these.

[0060] Here, the generation and movement of signal charges in the photoelectric conversion element 10 will be described.

[0061] The photoelectric conversion layer 3 generates excitons therein upon irradiation with light. The generated excitons diffuse within the photoelectric conversion layer 3 and are separated into electrons and holes at the interface between the acceptor semiconductor material and the donor semiconductor material. The separated electrons and holes move toward the lower electrode 2 or the upper electrode 4, respectively, in accordance with the electric field applied to the photoelectric conversion layer 3. When a voltage is applied between the upper electrode 4 and the lower electrode 2 so that the potential of the upper electrode 4 is higher than the potential of the lower electrode 2, the electrons move toward the upper electrode 4 and the holes move toward the lower electrode 2. When the photoelectric conversion element 10 is used in an imaging device, for example, the holes are collected by the lower electrode 2 and stored as signal charges in a charge storage node electrically connected to the lower electrode 2. The charge storage node is at least a part of a charge storage region that stores the signal charges collected by the lower electrode 2. Furthermore, when a voltage is applied between the upper electrode 4 and the lower electrode 2 so that the potential of the upper electrode 4 is lower than the potential of the lower electrode 2, holes move toward the upper electrode 4 and electrons move toward the lower electrode 2. When the photoelectric conversion element 10 is used in an imaging device, for example, electrons are collected by the lower electrode 2 and stored as signal charges in a charge storage node electrically connected to the lower electrode 2. In this way, the photoelectric conversion layer 3 converts light into signal charges, and the lower electrode 2 collects the signal charges generated in the photoelectric conversion layer 3. The signal charges collected by the lower electrode 2 are stored in, for example, the charge storage node. Furthermore, the upper electrode 4 collects charges of the opposite polarity to the signal charges.

[0062] Furthermore, in order to suppress the movement of signal charges to the lower electrode 2, for example, a voltage that reduces the potential difference between the upper electrode 4 and the lower electrode 2 is applied between the upper electrode 4 and the lower electrode 2. This causes the signal charges to recombine with charges of the opposite polarity to the signal charges before they reach the lower electrode 2, and they are substantially not stored in the charge storage node.

[0063] The intermediate layer 5 has a function of transporting signal charges generated in the photoelectric conversion layer 3 to the lower electrode 2. The intermediate layer 5 contains, for example, an organic semiconductor material. As the organic semiconductor material, for example, the donor semiconductor material or acceptor semiconductor material listed above is used. The material contained in the intermediate layer 5 is not limited to an organic semiconductor material, and may be an inorganic semiconductor such as an oxide semiconductor or a nitride semiconductor, or an insulator. Note that the photoelectric conversion element 10 does not necessarily have to include the intermediate layer 5. In this case, for example, the photoelectric conversion layer 3 is formed on the surface 2s of the lower electrode 2.

[0064] Next, the energy level configuration of the photoelectric conversion element 10 will be described.

[0065] 3 is an exemplary energy band diagram of the photoelectric conversion element 10 shown in FIG. 2. In FIG. 3, the energy band of each layer is indicated by a rectangle. Also, in FIG. 3, the Fermi level of each electrode is indicated by a line. Note that the energy level configuration shown in FIG. 3 is an example, and the energy level configuration in the photoelectric conversion element 10 is not limited to the example shown in FIG. 3.

[0066] Here, of the electron-hole pairs generated by the photoelectric conversion layer 3 absorbing light, the material that donates the electrons to the other material is called a donor material, and the material that accepts the electrons is called an acceptor material. In this embodiment, the donor semiconductor material is the donor material, and the acceptor semiconductor material is the acceptor material. When two different types of organic semiconductor materials are used, which one becomes the donor material and which one becomes the acceptor material is generally determined by the relative positions of the HOMO (Highest-Occupied-Molecular-Orbital) and LUMO (Lowest-Unoccupied-Molecular-Orbital) energy levels of the two types of organic semiconductor materials at the contact interface. In FIG. 3 , the upper end of the rectangle showing the energy band is the LUMO energy level, and the lower end is the HOMO energy level. The energy difference between the vacuum level and the LUMO energy level is referred to as the electron affinity. In this specification, the electron affinity is expressed as the absolute value of the energy difference between the vacuum level and the LUMO energy level. The energy difference between the vacuum level and the HOMO energy level is referred to as the ionization potential. In this specification, the ionization potential is expressed as the absolute value of the energy difference between the vacuum level and the HOMO energy level. In Figure 3, the electron affinity and ionization potential are greater the lower you are located.

[0067] Of the two types of semiconductor materials contained in the photoelectric conversion layer 3, the one with the smaller electron affinity is the donor material. Furthermore, of the two types of semiconductor materials contained in the photoelectric conversion layer 3, the one with the larger electron affinity is the acceptor material. In this specification, when the photoelectric conversion layer 3 contains a donor material and an acceptor material, such as when the photoelectric conversion layer 3 is a mixed film with a bulk heterostructure, the ionization potential of the photoelectric conversion layer 3 is the ionization potential of the donor material, and the electron affinity of the photoelectric conversion layer 3 is the electron affinity of the acceptor material.

[0068] The energy difference between the Fermi level and the vacuum level in a conductive material is called the work function. In this specification, the work function is expressed as the absolute value of the energy difference between the Fermi level and the vacuum level. In Figure 3, the work function is larger as the material is positioned lower.

[0069] As shown in FIG. 3 , the work function of the lower electrode 2 is, for example, smaller than the work function of the upper electrode 4. As a result, when the signal charges are holes, even if a voltage is applied between the upper electrode 4 and the lower electrode 2 to suppress the movement of the signal charges to the lower electrode 2, the movement of the signal charges to the lower electrode 2 can be suppressed even if the potential of the lower electrode 2 is lower than the potential of the upper electrode 4. The difference between the work functions of the lower electrode 2 and the upper electrode 4 is, for example, larger than 0.0 eV. The difference between the work functions of the lower electrode 2 and the upper electrode 4 may be 0.2 eV or more. Furthermore, the difference between the work functions of the lower electrode 2 and the upper electrode 4 is, for example, 2.0 eV or less. The difference between the work functions of the lower electrode 2 and the upper electrode 4 may be 0.6 eV or less.

[0070] In this specification, the work function of each electrode is a value measured by ultraviolet photoelectron spectroscopy in a vacuum by irradiating the surface of the electrode facing the photoelectric conversion layer 3 with vacuum ultraviolet light having an energy of 21.22 eV as a main component, namely, HeI lines, while maintaining the oxidation state of the electrode surface unchanged. Therefore, for example, the work function of the lower electrode 2 is the work function of the surface 2s of the lower electrode 2 facing the photoelectric conversion layer 3.

[0071] 3 , the work function of the lower electrode 2 is greater than the electron affinity of the photoelectric conversion layer 3 and the intermediate layer 5. The work function of the lower electrode 2 is smaller than the ionization potential of the photoelectric conversion layer 3 and the intermediate layer 5. The electron affinity of the intermediate layer 5 is smaller than the electron affinity of the photoelectric conversion layer 3. The ionization potential of the intermediate layer 5 is greater than the ionization potential of the photoelectric conversion layer 3.

[0072] [Relationship between Composition of Lower Electrode and Work Function] As described above, the lower electrode 2 is, for example, a metal nitride film from which a native oxide film has been removed on the surface 2s of the lower electrode 2 facing the photoelectric conversion layer 3. The present inventors have found that removing the native oxide film on the surface of the metal nitride can reduce the variation in the work function of the lower electrode 2. Below, the results of the inventors' investigation into the reduction in the variation in the work function of the lower electrode 2 by removing the native oxide film will be described.

[0073] Specifically, the inventors used chemical vapor deposition (CVD) to form a titanium nitride film as an electrode on a substrate and confirmed the relationship between the work function and the element concentration of the contained elements and the thickness direction of the titanium nitride film. To measure the work function and element concentration, a multifunctional X-ray photoelectron spectrometer (VersaProbe, manufactured by ULVAC-PHI, Inc.) was used. Work function and element concentration measurements were performed by repeatedly performing argon sputtering, i.e., plasma etching using argon gas, and varying the sputtering depth, which is the depth etched by argon sputtering. The sputtering depth is the thickness direction from the electrode surface. The titanium nitride film used for the measurements was exposed to air for more than 100 hours. In other words, measurements were performed on titanium nitride films whose surface native oxide growth had saturated. The work function was measured using ultraviolet photoelectron spectroscopy, irradiating the electrode surface with vacuum ultraviolet light, mainly composed of HeI rays with an energy of 21.22 eV. The element concentrations were measured by irradiating the electrode surface with X-rays using X-ray photoelectron spectroscopy.

[0074] FIG. 4 shows the relationship between the work function and the element concentration of the contained elements with respect to the position in the thickness direction of the titanium nitride film. As shown in FIG. 4 , at a sputtering depth of 0 nm, the oxygen element concentration is 33.1 atom %, and the work function is 4.3 eV. The work function and element concentration at a sputtering depth of 0 nm are the work function and element concentration of the surface of the titanium nitride film when the native oxide film has not been removed. The oxygen element concentration decreases as the sputtering depth increases, because the native oxide film is less likely to form. At a sputtering depth of 10 nm or more, the oxygen element concentration remains almost constant. At a sputtering depth of 10 nm, the oxygen element concentration is 7.6 atom %, and the work function is 3.8 eV. The work function and element concentration at a sputtering depth of 10 nm correspond to an example of the work function and element concentration of the surface 2 s of the lower electrode 2 from which the native oxide film has been removed.

[0075] 4, the fluctuation range of the work function in the region where the oxygen element concentration exceeds 20 atom % is approximately 0.7 eV, whereas the fluctuation range of the work function in the region where the oxygen element concentration is 20 atom % or less is approximately 0.4 eV. Furthermore, the fluctuation range of the work function in the region where the oxygen element concentration is 15 atom % or less is approximately 0.2 eV. In other words, in the region where the oxygen element concentration is low, the fluctuation of the work function is suppressed.

[0076] Such variations in the work function due to changes in the oxygen element concentration can occur due to differences in the oxygen element concentration at different positions within the surface 2s of the lower electrode 2, or due to differences in the oxygen element concentration at the surface 2s between production lots of the photoelectric conversion element 10. Furthermore, when multiple lower electrodes 2 are formed on a substrate, variations in the work function can also occur between multiple lower electrodes 2. Therefore, by setting the oxygen element concentration at the surface 2s of the lower electrode 2 on the photoelectric conversion layer 3 side at a predetermined level or less, variations in the work function of the lower electrode 2 can be suppressed, thereby reducing variations in the characteristics of the photoelectric conversion element 10. For example, since the work function of the lower electrode 2 affects the sensitivity characteristics of the photoelectric conversion element 10, variations in the sensitivity characteristics of the photoelectric conversion element 10 can be reduced.

[0077] In the photoelectric conversion element 10, from the viewpoint of reducing the variation in characteristics, the oxygen element concentration on the surface 2 s of the lower electrode 2 on the photoelectric conversion layer 3 side is 20 atom % or less. Furthermore, from the viewpoint of further reducing the variation in characteristics, the oxygen element concentration on the surface 2 s may be 15 atom % or less, or may be 10 atom % or less. In this specification, the element concentration on the surface 2 s of the lower electrode 2 is a value measured by irradiating the surface 2 s with X-rays using X-ray photoelectron spectroscopy.

[0078] Furthermore, the nitrogen element concentration at the surface 2s is, for example, equal to or greater than the oxygen element concentration at the surface 2s. Furthermore, when the lower electrode 2 contains titanium nitride, the nitrogen element concentration at the surface 2s is, for example, greater than the titanium element concentration at the surface 2s, and the titanium element concentration at the surface 2s is, for example, greater than the oxygen element concentration at the surface 2s.

[0079] [Modification of Photoelectric Conversion Element] Next, a photoelectric conversion element according to a modification of the embodiment will be described. Fig. 5 is a schematic cross-sectional view showing the configuration of a photoelectric conversion element 110 according to a modification of the embodiment. In the following description of photoelectric conversion element 110, differences from photoelectric conversion element 10 will be mainly described, and descriptions of commonalities will be omitted or simplified.

[0080] As shown in Fig. 5, the photoelectric conversion element 110 has a configuration in which the lower electrode 2 of the photoelectric conversion element 10 is replaced with a lower electrode 112. The lower electrode 112 is an example of a first electrode. In the example shown in Fig. 5, the photoelectric conversion element 110 includes an intermediate layer 5, but the intermediate layer 5 may not be included. In this case, for example, the photoelectric conversion layer 3 is formed on a surface 112s of the lower electrode 112.

[0081] The lower electrode 112 has a first layer 2a containing titanium nitride and a second layer 2b containing titanium, which faces the photoelectric conversion layer 3 with the first layer 2a interposed therebetween.

[0082] The first layer 2a is, for example, a titanium nitride film. The surface of the first layer 2a facing the photoelectric conversion layer 3 is the surface 112s of the lower electrode 112 facing the photoelectric conversion layer 3, and similar to the lower electrode 2, the surface 112s facing the photoelectric conversion layer 3 has, for example, a native oxide film removed. Therefore, the surface 112s of the lower electrode 112 has the same work function and element concentration as the surface 2s of the lower electrode 2.

[0083] The second layer 2b is, for example, a metal titanium film. The second layer 2b is disposed on the side of the first layer 2a opposite the photoelectric conversion layer 3 side. The second layer 2b can suppress the diffusion of metal (e.g., copper used for wiring) from the underlying side of the lower electrode 112, such as the support substrate 1, into the first layer 2a. This can suppress variations in the work function of the surface 112s of the lower electrode 112 facing the photoelectric conversion layer 3. Furthermore, the generation of columnar crystals is suppressed when forming the titanium nitride upper first layer 2a, allowing the formation of an amorphous titanium nitride film with few gaps.

[0084] The second layer 2b may be a metal film made of a metal other than titanium, and the first layer 2a may be a metal nitride film made of the metal other than titanium.

[0085] [Imaging Device] Next, an imaging device including a photoelectric conversion element according to this embodiment will be described. FIG. 6 is a diagram showing the circuit configuration of an imaging device 100 according to this embodiment. The imaging device 100 is, for example, a stacked image sensor. As shown in FIG. 6, the imaging device 100 includes a plurality of pixels 14 and peripheral circuits. The imaging device 100 operates, for example, by a global shutter system in which the exposure periods of all of the plurality of pixels 14 are unified. The imaging device 100 may also operate by a rolling shutter system.

[0086] The plurality of pixels 14 are arranged two-dimensionally, i.e., in a row direction and a column direction, on the semiconductor substrate to form a pixel region. In this specification, the row direction and the column direction refer to the directions in which the rows and columns extend, respectively. In other words, the vertical direction is the column direction, and the horizontal direction is the row direction. Note that the plurality of pixels 14 may also be arranged one-dimensionally, i.e., along one direction. In other words, the imaging device 100 may be a line sensor.

[0087] Each pixel 14 includes a photodetector 10A and a charge detection circuit 25. The charge detection circuit 25 includes an amplification transistor 11, a reset transistor 12, and an address transistor 13. The photodetector 10A is configured, for example, with the photoelectric conversion element 10 or 110 described above. The following describes a case where the photodetector 10A has the same configuration as the photoelectric conversion element 10. In FIG. 6 , of the configuration of the photodetector 10A, the lower electrode 2, the photoelectric conversion layer 3, and the upper electrode 4 are representatively shown, and the intermediate layer 5 and the protective layer 6 are omitted. In the imaging device 100, the lower electrode 2 is also referred to as a pixel electrode, and the upper electrode 4 is also referred to as a counter electrode.

[0088] The imaging device 100 includes a voltage control element for applying a predetermined voltage to the upper electrode 4. The voltage control element controls the voltage applied to the upper electrode 4 to be within a specified range so that the electric signal generated in the photodetector 10A can be correctly detected by the charge detection circuit 25. Alternatively, when a current flows from the lower electrode 2, the voltage control element suppresses charging of the photodetector 10A by passing an equal amount of current to the upper electrode 4.

[0089] The voltage control element includes, for example, a voltage control circuit, a voltage generation circuit such as a constant voltage source, and a voltage reference line such as a ground line. The voltage applied by the voltage control element is also referred to as a control voltage. In this embodiment, the imaging device 100 includes a voltage control circuit 30 as a voltage control element. The voltage control circuit 30 is an example of a voltage supply circuit. The voltage control circuit 30 may generate a constant control voltage or multiple control voltages of different values. For example, the voltage control circuit 30 may generate control voltages of two or more different values, or may generate a control voltage that changes continuously within a predetermined range. The voltage control circuit 30 determines the value of the control voltage to be generated based on instructions from the operator operating the imaging device 100 or instructions from another control unit or the like included in the imaging device 100, and generates the control voltage of the determined value. The voltage control circuit 30 is provided outside the photosensitive region, for example, as part of a peripheral circuit. The photosensitive region is substantially the same as the pixel region.

[0090] The voltage control circuit 30 is electrically connected to the upper electrode 4 and applies a potential difference between the lower electrode 2 and the upper electrode 4. For example, the voltage control circuit 30 generates two or more different control voltages and applies the control voltages to the upper electrode 4 to change the spectral sensitivity characteristics of the photoelectric conversion layer 3. This change in spectral sensitivity characteristics also includes a spectral sensitivity characteristic in which the sensitivity of the photoelectric conversion layer 3 becomes zero to light to be detected. As a result, for example, in the imaging device 100, while the pixels 14 read out detection signals row by row, the voltage control circuit 30 applies to the upper electrode 4 a control voltage that makes the sensitivity of the photoelectric conversion layer 3 zero, thereby substantially eliminating the influence of incident light during readout of the detection signals. Therefore, even when the detection signals are read out row by row, a global shutter operation can be achieved.

[0091] In this embodiment, as shown in FIG. 6 , the voltage control circuit 30 applies a control voltage to the upper electrode 4 of each pixel 14 via the counter electrode signal line 16. This changes the voltage between the lower electrode 2 and the upper electrode 4, thereby switching the spectral sensitivity characteristics of the photodetector 10A. Alternatively, the voltage control circuit 30 realizes an electronic shutter operation by applying a control voltage so as to obtain a spectral sensitivity characteristic in which sensitivity to light becomes zero at a predetermined timing during imaging. Note that in FIG. 6 , the upper electrode 4 of each pixel 14 is connected to the voltage control circuit 30 via the counter electrode signal line 16 provided for each row of pixels 14. However, as described below, the upper electrode 4 may be configured as a single upper electrode 4 across all pixels 14. Therefore, for example, the control voltage may be applied from the voltage control circuit 30 to the upper electrodes 4 of all pixels 14 collectively.

[0092] In order to have the lower electrode 2 collect, as signal charges, holes generated when light is irradiated onto the photodetector 10A, the control voltage of the lower electrode 2 is set to a lower potential than that of the upper electrode 4. In addition, in order to have the lower electrode 2 collect, as signal charges, electrons generated when light is irradiated onto the photodetector 10A, the control voltage of the lower electrode 2 is set to a higher potential than that of the upper electrode 4.

[0093] The lower electrode 2 is connected to the gate electrode of the amplifier transistor 11, and signal charges collected by the lower electrode 2 are stored in a charge storage node 24 located between the lower electrode 2 and the gate electrode of the amplifier transistor 11. The charge storage node 24 is an example of a charge storage region.

[0094] The voltage control circuit 30 supplies, for example, a first voltage during a first period and a second voltage different from the first voltage to the upper electrode 4 during a second period. When a global shutter operation is performed, the voltage control circuit 30 supplies, for example, a first voltage to the upper electrode 4 during a first period, which is an exposure period during which signal charges are accumulated in the charge accumulation regions of each pixel 14, and supplies a second voltage different from the first voltage during a second period, which is a non-exposure period different from the exposure period. The first voltage is, for example, a voltage of the same polarity as the signal charges. The absolute value of the first voltage is, for example, 5 V or more and 15 V or less. The second voltage is either 0 V or a voltage of the same polarity as the signal charges. The second voltage may be a voltage of the opposite polarity to the signal charges. The absolute value of the second voltage is, for example, 2 V or less. In this specification, the term "exposure period" refers to a period during which either electrons or holes generated by photoelectric conversion are accumulated in the charge accumulation node 24 as signal charges. In other words, the "exposure period" may also be referred to as the "charge accumulation period." In this specification, a period during operation of the imaging device other than the exposure period is referred to as a "non-exposure period." A "non-exposure period" is, for example, a period during which light is irradiated onto the photodetection unit 10A but no charge is substantially accumulated in the charge accumulation node 24. During the non-exposure period, light may be blocked from entering the photodetection unit 10A.

[0095] The charge storage node 24 is connected to the gate electrode of the amplifier transistor 11, and the signal charge stored in the charge storage node 24 is applied to the gate electrode of the amplifier transistor 11 as a voltage corresponding to the amount of signal charge. The amplifier transistor 11 constitutes part of the charge detection circuit 25 and amplifies the voltage applied to the gate electrode. One of the source electrode and drain electrode of the amplifier transistor 11 is connected to one of the source electrode and drain electrode of the address transistor 13. The address transistor 13 selectively reads out the amplified voltage as a signal voltage. The address transistor 13 is also referred to as a row selection transistor. One of the source electrode and drain electrode of the reset transistor 12 is connected to the lower electrode 2 via the charge storage node 24 and initializes (resets) the signal charge stored in the charge storage node 24. In other words, the reset transistor 12 initializes the potentials of the charge storage node 24, the gate electrode of the amplifier transistor 11, and the lower electrode 2 to an initialization voltage.

[0096] In order to selectively perform the above-described operations in the plurality of pixels 14, the imaging device 100 includes a power supply wiring 21, a reset voltage line 23, a vertical signal line 17, an address signal line 26, and a reset signal line 27. These lines are connected to the pixels 14, respectively. Specifically, the power supply wiring 21 is connected to the other of the source electrode and the drain electrode of the amplification transistor 11. The reset voltage line 23 is connected to the other of the drain electrode and the source electrode of the reset transistor 12. The vertical signal line 17 is connected to the other of the source electrode and the drain electrode of the address transistor 13. The address signal line 26 is connected to the gate electrode of the address transistor 13. Furthermore, the reset signal line 27 is connected to the gate electrode of the reset transistor 12.

[0097] The peripheral circuits include a vertical scanning circuit 15, a horizontal signal readout circuit 20, a plurality of column signal processing circuits 19, a plurality of load circuits 18, and a voltage control circuit 30. The vertical scanning circuit 15 is also called a row scanning circuit. The horizontal signal readout circuit 20 is also called a column scanning circuit. The column signal processing circuit 19 is also called a row signal storage circuit.

[0098] The vertical scanning circuit 15 is connected to address signal lines 26 and reset signal lines 27, selects the multiple pixels 14 arranged in each row on a row-by-row basis, reads out the signal voltage, and initializes the potential of the lower electrode 2. The power supply wiring 21 is connected to a voltage supply circuit (not shown) and functions as a source follower power supply, supplying a predetermined power supply voltage to each pixel 14. The horizontal signal readout circuit 20 is electrically connected to multiple column signal processing circuits 19. The column signal processing circuits 19 are electrically connected to the pixels 14 arranged in each column via vertical signal lines 17 corresponding to each column. The load circuits 18 are electrically connected to each vertical signal line 17. The load circuits 18 and the amplification transistors 11 form a source follower circuit.

[0099] An initialization voltage is supplied to the other of the drain electrode and the source electrode of the reset transistor 12 from a voltage supply circuit (not shown), for example, via a reset voltage line 23. The initialization voltage is, for example, 0 or a voltage of the same polarity as the polarity of the signal charge. The absolute value of the initialization voltage is, for example, lower than the absolute value of the first voltage. The absolute value of the initialization voltage may be 1.0 V or less, or 0.3 V or less. The reset voltage line 23 may be part of a feedback circuit in which a feedback path is formed for negatively feeding back the output of the pixel 14. In this case, a feedback voltage for negatively feeding back the output of the pixel 14 is supplied to the reset voltage line 23 as the initialization voltage.

[0100] The vertical scanning circuit 15 applies a row selection signal, which controls the on / off of the address transistor 13, to the gate electrode of the address transistor 13 via the address signal line 26. This scans and selects the row to be read out. A signal voltage is read out from the pixels 14 in the selected row to the vertical signal line 17. The vertical scanning circuit 15 also applies a reset signal, which controls the on / off of the reset transistor 12, to the gate electrode of the reset transistor 12 via the reset signal line 27. This selects the row of pixels 14 to be subjected to the initialization operation. An initialization voltage is supplied to the charge storage node 24, the gate electrode of the amplifier transistor 11, and the lower electrode 2 of the pixels 14 in the row selected for the initialization operation. The vertical signal line 17 transmits the signal voltage read out from the pixels 14 selected by the vertical scanning circuit 15 to the column signal processing circuit 19.

[0101] The column signal processing circuit 19 performs noise suppression signal processing, such as correlated double sampling, and analog-to-digital conversion (AD conversion).

[0102] The horizontal signal readout circuit 20 sequentially reads out signals from the plurality of column signal processing circuits 19 to a horizontal common signal line 28 .

[0103] [Pixel Configuration] Next, a detailed device structure of the pixel 14 of the imaging device 100 will be described with reference to Fig. 7. Fig. 7 is a cross-sectional view that schematically shows a cross section of the device structure of the pixel 14 of the imaging device 100 according to the present embodiment. Note that in the cross-sectional view shown in Fig. 7, the semiconductor substrate 31 and the interlayer insulating layers 43A, 43B, and 43C are not shaded to indicate cross sections in order to avoid complicating the drawing.

[0104] 7, the pixel 14 includes a semiconductor substrate 31, a charge detection circuit 25, and a photodetector 10A. The semiconductor substrate 31 is, for example, a p-type silicon substrate. The charge detection circuit 25 detects the signal charge collected by the lower electrode 2 and outputs a signal voltage. The charge detection circuit 25 includes an amplification transistor 11, a reset transistor 12, and an address transistor 13, and is formed on the semiconductor substrate 31.

[0105] The amplifier transistor 11 is formed on a semiconductor substrate 31. The amplifier transistor 11 includes n-type impurity regions 41C and 41D that function as a drain electrode and a source electrode, respectively, a gate insulating layer 38B located on the semiconductor substrate 31, and a gate electrode 39B located on the gate insulating layer 38B.

[0106] The reset transistor 12 is formed on a semiconductor substrate 31. The reset transistor 12 includes n-type impurity regions 41B and 41A functioning as a drain electrode and a source electrode, respectively, a gate insulating layer 38A located on the semiconductor substrate 31, and a gate electrode 39A located on the gate insulating layer 38A.

[0107] Address transistor 13 is formed on semiconductor substrate 31. Address transistor 13 includes n-type impurity regions 41D and 41E functioning as a drain electrode and a source electrode, respectively, a gate insulating layer 38C located on semiconductor substrate 31, and a gate electrode 39C located on gate insulating layer 38C.

[0108] The amplifier transistor 11, the reset transistor 12, and the address transistor 13 are each, for example, a metal oxide semiconductor field effect transistor (MOSFET). The amplifier transistor 11, the reset transistor 12, and the address transistor 13 are each an n-channel MOSFET, but may be a p-channel MOSFET.

[0109] The gate insulating layers 38A, 38B, and 38C are each formed using an insulating material, and have, for example, a single layer structure of a silicon oxide film or a silicon nitride film, or a laminated structure of these.

[0110] The gate electrodes 39A, 39B, and 39C are each formed using a conductive material. For example, the gate electrodes 39A, 39B, and 39C are formed using polysilicon to which impurities have been added to make it conductive. Alternatively, the gate electrodes 39A, 39B, and 39C may be formed using a metal material such as copper.

[0111] The n-type impurity regions 41A, 41B, 41C, 41D, and 41E are formed by doping the semiconductor substrate 31 with n-type impurities such as phosphorus (P) by ion implantation or the like. In the example shown in FIG. 7 , the n-type impurity region 41D is shared by the amplifier transistor 11 and the address transistor 13. This connects the amplifier transistor 11 and the address transistor 13 in series. Note that the n-type impurity region 41D may be separated into two n-type impurity regions. In this case, the two n-type impurity regions may be electrically connected via a wiring layer.

[0112] In the semiconductor substrate 31, an element isolation region 42 is provided between adjacent pixels 14 and between the amplification transistor 11 and the reset transistor 12. The element isolation region 42 electrically isolates adjacent pixels 14. The element isolation region 42 also suppresses leakage of signal charge accumulated in the charge storage node 24. The element isolation region 42 is formed, for example, by doping the semiconductor substrate 31 with a high concentration of p-type impurities.

[0113] A multilayer wiring structure is provided on the upper surface of the semiconductor substrate 31. The multilayer wiring structure includes a plurality of interlayer insulating layers, one or more wiring layers, one or more plugs, and one or more contact plugs. Specifically, interlayer insulating layers 43A, 43B, and 43C are stacked on the upper surface of the semiconductor substrate 31 in this order from the semiconductor substrate 31 side.

[0114] A contact plug 45A connected to the n-type impurity region 41B of the reset transistor 12, a contact plug 45B connected to the gate electrode 39B of the amplifier transistor 11, a wiring 46A connecting the contact plug 45A and the contact plug 45B, and a plug 47A are buried in the interlayer insulating layer 43A. As a result, the n-type impurity region 41B functioning as the drain electrode of the reset transistor 12 is electrically connected to the gate electrode 39B of the amplifier transistor 11.

[0115] Furthermore, a wiring 46B and a plug 47B connected to the plug 47A via the wiring 46B are embedded in the interlayer insulating layer 43B. A wiring 46C and a plug 47C connected to the plug 47B via the wiring 46C are embedded in the interlayer insulating layer 43C. The plug 47C is connected to the lower electrode 2. As a result, the charges collected by the lower electrode 2 flow in the order of plug 47C, wiring 46C, plug 47B, wiring 46B, plug 47A, wiring 46A, and contact plug 45A, and are accumulated in the n-type impurity region 41B. In addition to the n-type impurity region 41B, the plug 47C, wiring 46C, plug 47B, wiring 46B, plug 47A, wiring 46A, contact plug 45A, contact plug 45B, and gate electrode 39B constitute a charge storage node 24, each of which functions as a charge storage region.

[0116] The photodetector 10A is provided on a substrate 1A that includes a semiconductor substrate 31 and interlayer insulating layers 43A, 43B, and 43C. Specifically, the photodetector 10A is provided on the interlayer insulating layer 43C. The photodetector 10A includes a lower electrode 2, an intermediate layer 5, a photoelectric conversion layer 3, and an upper electrode 4. The photodetector 10A also includes a protective layer 6 formed on at least a portion of the upper surface of the upper electrode 4.

[0117] 7 , the pixel 14 includes a color filter 60 provided above the upper electrode 4 of the photodetector 10A, and a microlens 61 provided on the color filter 60. Note that the pixel 14 does not necessarily have to include at least one of the color filter 60 and the microlens 61.

[0118] In this embodiment, the photoelectric conversion layer 3 and upper electrode 4 of each pixel 14 are connected to the photoelectric conversion layer 3 and upper electrode 4 of the adjacent pixel 14, respectively, to form an integrated photoelectric conversion layer 3 and upper electrode 4 across all pixels 14. However, at least one of the photoelectric conversion layer 3 and upper electrode 4 may be separated for each pixel 14. Furthermore, at least one of the photoelectric conversion layer 3 and upper electrode 4 may be separated for each block of pixels 14, such as for each row or column of pixels 14 arranged two-dimensionally. In contrast, the lower electrode 2 of each pixel 14 is not connected to the lower electrode 2 of the adjacent pixel 14 and is independent.

[0119] [Method for Manufacturing the Photodetector] Next, a method for manufacturing the photodetector 10A according to this embodiment will be described. The method for manufacturing the photodetector 10A can also be considered as a method for manufacturing the photoelectric conversion element 10.

[0120] Fig. 8 is a flowchart of a method for manufacturing the photodetector 10A according to this embodiment. Figs. 9 to 11 are cross-sectional views illustrating the method for manufacturing the photodetector 10A. In Figs. 9 to 11, detailed illustrations of the interior of the substrate 1A and the application of hatching to represent cross sections are omitted to avoid complicating the drawings. In the following, a case where a titanium nitride film is formed as the lower electrode 2 will be described as an example, but a metal nitride film other than a titanium nitride film can also be formed as the lower electrode 2 by a similar method.

[0121] As shown in FIGS. 8 and 9 , first, a lower electrode 2x is formed on the substrate 1A (step S11). Conventionally known methods can be used to form impurity regions in the semiconductor substrate 31 of the substrate 1A, as well as to form an interlayer insulating layer and a wiring layer. In the example shown in FIG. 9 , a plurality of lower electrodes 2x are formed, the number of which corresponds to the number of pixels 14. To form the plurality of lower electrodes 2x, for example, a titanium nitride film is first formed on the upper surface of the substrate 1A using a chemical vapor deposition (CVD) method, thereby forming a titanium nitride film over the entire upper surface of the substrate 1A. Other thin film deposition processes may also be used to form the titanium nitride film. The formed titanium nitride film is then separated into a plurality of lower electrodes 2x by patterning, and an insulating layer is then filled between the lower electrodes 2x, thereby forming the plurality of lower electrodes 2x shown in FIG. 9 .

[0122] The lower electrode 2x is formed using an inorganic material processing apparatus. The substrate 1A on which the lower electrode 2x is formed is transferred in the atmosphere and exposed to the atmosphere for subsequent processes. During this process, a native oxide film is formed on the upper surface 2sx of the lower electrode 2x.

[0123] Next, as shown in FIGS. 8 and 10 , the surface 2sx of the lower electrode 2x is etched to form the lower electrode 2 (step S12). For example, the substrate 1A on which the lower electrode 2x is formed is loaded into a sputtering apparatus. Then, using the lower electrode 2x as a sputtering target, surface treatment is performed by plasma etching using argon gas to remove the surface 2sx of the lower electrode 2x, thereby forming a lower electrode 2 having a surface 2s from which the native oxide film has been removed. Furthermore, even if contaminants adhere to the surface 2sx of the lower electrode 2x, the contaminants can be removed by etching. The etching depth in this case is, for example, 5 nm or more. The etching depth may be 10 nm or more. The etching depth is, for example, 20 nm or less. The gas used for etching is not limited to argon, but may also be nitrogen or hydrogen. The gas used for etching may also be a mixed gas containing two or more of argon, nitrogen, and hydrogen. After etching, the lower electrode 2 is annealed as necessary to stabilize the surface 2s. The surface 2sx of the lower electrode 2x may be removed by other methods such as wet etching.

[0124] 8 and 11 , one or more layers including a photoelectric conversion layer 3 are formed on the surface 2s of the etched lower electrode 2 without exposing the surface 2s to the atmosphere (step S13). For example, the substrate 1A on which the lower electrode 2 is formed is transferred from a sputtering apparatus to an organic material processing apparatus without being exposed to the atmosphere. For example, the substrate 1A on which the lower electrode 2 is formed is transferred from the sputtering apparatus to the organic material processing apparatus in a vacuum or a non-oxidizing atmosphere such as a nitrogen or argon atmosphere. Surface treatment by sputtering and an organic material process for forming a thin film of an organic material have a high affinity, making it easy to maintain uniformity in the composition of the surface 2s of the lower electrode 2 during the process transition.

[0125] Then, the intermediate layer 5 and the photoelectric conversion layer 3 are formed in this order on the surface 2s of the lower electrode 2 by vacuum deposition. By forming one or more layers including the photoelectric conversion layer 3 on the surface 2s of the lower electrode 2 in this manner without exposing the surface 2s of the lower electrode 2 to the atmosphere, a native oxide film is not formed on the surface 2s of the lower electrode 2, and a lower electrode 2 with reduced work function variation can be formed. When forming the photoelectric conversion layer 3, for example, the photoelectric conversion layer 3 is formed by co-depositing a donor organic semiconductor material and an acceptor organic semiconductor material at a predetermined volume ratio by vacuum deposition. The method for forming one or more layers including the photoelectric conversion layer 3 is not particularly limited, and for example, at least one of the intermediate layer 5 and the photoelectric conversion layer 3 may be formed by spin coating, inkjet printing, die coating, spray coating, screen printing, or the like.

[0126] 8 and 11, the upper electrode 4 is formed on one or more layers including the photoelectric conversion layer 3 (step S14). For example, the upper electrode 4 is formed by depositing a material for the upper electrode 4, such as ITO, on the photoelectric conversion layer 3 using a sputtering method or the like. Then, for example, Al is deposited on the upper electrode 4 using an atomic layer deposition (ALD) method or the like. ２ O ３ The protective layer 6 is formed by depositing the material of the protective layer 6, such as the above, on the upper electrode 4. In this way, the photodetector 10A is formed on the substrate 1A.

[0127] [Camera System] Next, a camera system including the imaging device according to this embodiment will be described.

[0128] FIG. 12 is a block diagram showing an example of the configuration of a camera system 400 according to this embodiment.

[0129] 12, camera system 400 according to this embodiment includes lens optical system 601, imaging device 602, system controller 603, and camera signal processing circuit 604. Camera system 400 may be, for example, a smartphone, a digital camera, a video camera, or an in-vehicle camera.

[0130] The lens optical system 601 focuses light onto an imaging surface of the imaging device 602. The lens optical system 601 may include, for example, a lens group including an autofocus lens and a zoom lens, and an aperture. As the imaging device 602, for example, the imaging device 100 described above is used.

[0131] The system controller 603 controls the entire camera system 400. The system controller 603 is, for example, a semiconductor integrated circuit, and a specific example is a CPU (Central Processing Unit).

[0132] The camera signal processing circuit 604 has a function of processing an output signal from the image capture device 602. The camera signal processing circuit 604 receives output data from the image capture device 602 and performs processes such as gamma correction, color interpolation, spatial interpolation, and auto white balance. The camera signal processing circuit 604 is, for example, a DSP (Digital Signal Processor). The image capture device 602 and the camera signal processing circuit 604 may be implemented as a single semiconductor device. The semiconductor device may be, for example, a so-called SoC (System on a Chip). This configuration allows for further miniaturization of electronic devices that include the image capture device 602 as a part thereof.

[0133] (Other Embodiments) The photoelectric conversion element, imaging device, and camera system according to the present disclosure have been described above based on embodiments and examples, but the present disclosure is not limited to these embodiments and examples. As long as they do not deviate from the gist of the present disclosure, various modifications that a person skilled in the art could conceive of to the embodiments and examples, as well as other forms constructed by combining some of the components of the embodiments and examples, are also included in the scope of the present disclosure.

[0134] For example, in the above embodiment, the photoelectric conversion element is used in an imaging device, but is not limited thereto. For example, the photoelectric conversion element according to the present disclosure may be used in an optical sensor by extracting charges generated by light as a signal.

[0135] The photoelectric conversion element according to the present disclosure can be used for various applications requiring sensitivity adjustment, such as imaging devices for various applications. The imaging device according to the present disclosure can also be applied to various camera systems and sensor systems, such as medical cameras, surveillance cameras, vehicle-mounted cameras, distance measuring cameras, microscope cameras, drone cameras, and robot cameras.

[0136] 1 Support substrate 1A Substrate 2, 2x, 112 Lower electrode 2a First layer 2b Second layer 2s, 2sx, 112s Surface 3 Photoelectric conversion layer 4 Upper electrode 5 Intermediate layer 6 Protective layer 10, 110 Photoelectric conversion element 10A Photodetector 11 Amplifying transistor 12 Reset transistor 13 Address transistor 14 Pixel 15 Vertical scanning circuit 16 Counter electrode signal line 17 Vertical signal line 18 Load circuit 19 Column signal processing circuit 20 Horizontal signal readout circuit 21 Power supply wiring 23 Reset voltage line 24 Charge storage node 25 Charge detection circuit 26 Address signal line 27 Reset signal line 28 Horizontal common signal line 30 Voltage control circuit 31 Semiconductor substrate 38A, 38B, 38C Gate insulating layer 39A, 39B, 39C Gate electrode 41A, 41B, 41C, 41D, 41E n-type impurity region 42 Element isolation region 43A, 43B, 43C Interlayer insulating layer 45A, 45B Contact plug 46A, 46B, 46C Wiring 47A, 47B, 47C Plug 60 Color filter 61 Microlens 100, 602 Imaging device 200 Wafer 400 Camera system 601 Lens optical system 603 System controller 604 Camera signal processing circuit

Claims

1. A photoelectric conversion element comprising: a photoelectric conversion layer that converts light into signal charges; a first electrode containing a metal nitride, having a first surface and a second surface closer to the photoelectric conversion layer than the first surface, and collecting the signal charges; and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the oxygen element concentration on the second surface of the first electrode is 20 atom % or less.

2. The photoelectric conversion element according to claim 1, wherein the oxygen element concentration on the second surface of the first electrode is 15 atom % or less.

3. The photoelectric conversion element according to claim 1, wherein the nitrogen element concentration on the second surface of the first electrode is equal to or greater than the oxygen element concentration on the second surface of the first electrode.

4. The photoelectric conversion element according to claim 1, wherein the metal nitride is titanium nitride.

5. A photoelectric conversion element as described in claim 4, wherein the nitrogen element concentration on the second surface of the first electrode is greater than the titanium element concentration on the second surface of the first electrode, and the titanium element concentration on the second surface of the first electrode is greater than the oxygen element concentration on the second surface of the first electrode.

6. The photoelectric conversion element according to claim 4, wherein the first electrode includes a first layer containing the titanium nitride and a second layer containing titanium, facing the photoelectric conversion layer with the first layer interposed therebetween.

7. The photoelectric conversion element according to claim 1, wherein the photoelectric conversion layer contains an organic semiconductor material.

8. An imaging device comprising: a photoelectric conversion element according to any one of claims 1 to 7; and a charge accumulation region electrically connected to the first electrode.

9. A method for manufacturing a photoelectric conversion element comprising a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal nitride and collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, the method comprising: forming the first electrode; etching a surface of the first electrode; after the etching, forming one or more layers containing the photoelectric conversion layer on the surface of the first electrode without exposing the surface of the first electrode to the atmosphere; and forming the second electrode on the one or more layers.

Images