Patent Publication Number: US-2023142858-A1

Title: Image sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0155162, filed on Nov. 11, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference in its entirety herein. 
     1. TECHNICAL FIELD 
     The inventive concept relates to an image sensor. 
     2. DISCUSSION OF RELATED ART 
     An image sensor is a device that detects and conveys information used to generate an image. For example, an image sensor may capture a two-dimensional (2D) or three-dimensional (3D) image of an object. An image sensor includes one or more photoelectric conversion elements that convert light into an amount of electrical current that depends on the intensity of light reflected from the object. Examples of image sensors include charge-coupled device (CCD) and a complementary metal-oxide semiconductor (CMOS)-based image sensor. Cameras integrated into small consumer products are typically used CMOS image sensors since they are cheaper and have power consumption in battery powered devices than CCD image sensors. However, CCD image sensors are typically used to broadcast higher quality image data and are typically incorporated into higher end video cameras. 
     SUMMARY 
     At least one embodiment of the inventive concept provides an image sensor having improved resolution. 
     According to an embodiment of the inventive concept, an image sensor is provided. The image sensor includes a substrate including a plurality of photoelectric conversion elements, a variable filter layer disposed on the substrate, and a plurality of microlenses disposed on the variable filter layer. The variable filter layer includes: a plurality of first electrodes extending in a first direction, and having a first width in a second direction, a first electro-optical material layer disposed on the plurality of first electrodes, a light-transmitting electrode disposed on the first electro-optical material layer, a second electro-optical material layer disposed on the light-transmitting electrode, and a plurality of second electrodes disposed on the second electro-optical material layer, extending in the second direction, and having a second width in the first direction. 
     According to an embodiment of the inventive concept, an image sensor is provided. The image sensor includes a substrate including a plurality of photoelectric conversion elements, a variable filter layer disposed on the substrate, a plurality of conducting patterns providing a path for outputting an electrical signal generated by the plurality of photoelectric conversion elements, and an interlayer insulating layer covering the plurality of conductive patterns. The variable filter layer includes a plurality of first electrodes extending in a first direction, and spaced apart from each other in a second direction, a first electro-optical material layer disposed on the plurality of first electrodes, a light-transmitting electrode disposed on the first electro-optical material layer, a second electro-optical material layer disposed on the light-transmitting electrode, and a plurality of second electrodes disposed on the second electro-optical material layer, extending in the second direction, and spaced apart from each other in the first direction. 
     According to an embodiment of the inventive concept, an image sensor is provided. The image sensor includes a substrate including a plurality of photoelectric conversion elements, a variable filter layer disposed on the substrate, a plurality of conducting patterns providing a path for outputting an electrical signal generated by the plurality of photoelectric conversion elements, an interlayer insulating layer covering the plurality of conductive patterns, and a plurality of microlenses focusing external light to the plurality of photoelectric conversion elements. The variable filter layer includes a plurality of first electrodes extending in a first direction, a first electro-optical material layer disposed on the plurality of first electrodes, a light-transmitting electrode disposed on the first electro-optical material layer and configured to receive a reference potential, a second electro-optical material layer disposed on the light-transmitting electrode, and a plurality of second electrodes disposed on the second electro-optical material layer and extending in a second direction. One of a first voltage higher than the reference potential and a second voltage lower than the first voltage and higher than the reference potential is applied to each of the plurality of first electrodes, and one of the first voltage and the second voltage is applied to each of the plurality of second electrodes. 
     According to embodiment of the inventive concept, an operating method of an image sensor is provided. The operating method includes acquiring a first image that is an image of a red visible light band by adjusting a variable filter layer to operate as a red color filter, acquiring a second image that is an image of a green visible light band by adjusting the variable filter layer to operate as a green color filter, and acquiring a third image that is an image of a blue visible light band by adjusting the variable filter layer to operate as a blue color filter. The variable filter layer includes a plurality of photoelectric conversion elements disposed to configure a matrix. The variable filter layer includes a plurality of first electrodes extending in a first direction, and having a first width in a second direction, a first electro-optical material layer disposed on the plurality of first electrodes, a light-transmitting electrode disposed on the first electro-optical material layer, a second electro-optical material layer disposed on the light-transmitting electrode, a plurality of second electrodes disposed on the second electro-optical material layer, extending in the second direction, and having a second width in the first direction. 
     According to an embodiment of the inventive concept, a method of operating an image sensor is provided. The image sensor includes a plurality of photoelectric conversion elements and a variable filter layer disposed on the plurality of photoelectric conversion elements. The variable filter layer includes a plurality of first electrodes extending in a first direction and spaced apart from each other in a second direction, a first electro-optical material layer disposed on the plurality of first electrodes, a light-transmitting electrode disposed on the first electro-optical material layer, a second electro-optical material layer disposed on the light-transmitting electrode, and a plurality of second electrodes disposed on the second electro-optical material layer, extending in the second direction, and spaced apart from each other in the first direction. The plurality of first electrodes, the first electro-optical material layer, the light-transmitting electrode, the second electro-optical material layer, and the plurality of second electrodes are sequentially stacked in a third direction perpendicular to the first and second directions. 
     The method includes applying a first voltage to a first portion of the plurality of first electrodes, applying a second voltage lower than the first voltage to a second portion of the plurality of first electrodes, applying the first voltage to a third portion of the plurality of second electrodes, and applying the second voltage to the a fourth portion of the second electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the inventive concept will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG.  1    is a block diagram illustrating an image sensor according to an example embodiment; 
         FIG.  2    is a circuit diagram illustrating a pixel included in an image sensor according to an example embodiment; 
         FIG.  3    illustrates a layout of a pixel array of an image sensor according to an example embodiment; 
         FIG.  4    is a cross-sectional view taken along line I-I′ of  FIG.  3   ; 
         FIG.  5    is a diagram for describing an operation of a pixel array according to an example embodiment; 
         FIG.  6    is a flowchart illustrating an operation of a pixel array according to an example embodiments 
         FIGS.  7 A to  7 C  are views illustrating the operation of the pixel array  10  according to an example embodiment; 
         FIG.  8    is a cross-sectional view illustrating a pixel array of an image sensor according to an example embodiment; and 
         FIG.  9    is a cross-sectional view illustrating a pixel array of an image sensor according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the inventive concept are described in detail with reference to the accompanying drawings. The same reference numerals are used for the same components in the drawings, and redundant descriptions thereof are omitted. In the following drawings, the thickness or size of each layer may be exaggerated for convenience and clarity of description, and thus may be slightly different from the actual shapes and ratios. 
       FIG.  1    is a block diagram illustrating an image sensor  1  according to an example embodiment. 
       FIG.  1    is a block diagram schematically showing the image sensor  1  according to an example embodiment of the present disclosure. 
     The image sensor  1  according to an example embodiment of the present disclosure may be mounted on an electronic device having an image or light sensing function. For example, the image sensor  1  may be applied to electronic devices such as a camera, a smartphone, a wearable device, the Internet of Things (IoT), a tablet personal computer (PC), a personal digital assistant (PDA), a portable multimedia player (PMP), a navigation device. In addition, the image sensor  1  may be employed in vehicles, furniture, manufacturing equipment, door, various measurement devices, and the like. 
     The image sensor  1  may include a pixel array  10 , a row driver  20  (e.g., a driver circuit), an analog-to-digital conversion (ADC) circuit  30 , a timing controller  40  (e.g., a control circuit), and an image signal processor  50 . 
     The pixel array  10  may receive an optical signal reflected from an object incident through a lens LS and convert the optical signal into an electrical signal. The pixel array  10  may be implemented with a complementary metal oxide semiconductor (CMOS), but is not limited thereto. The pixel array  10  may be part of a charge coupled-device (CCD) chip. 
     The pixel array  10  may include a plurality of row lines RL, a plurality of column lines CL (or referred to as output lines), and a plurality of pixels P 11 , P 12 , P 13 , . . . , P 1 N, P 21 , P 22 , . . . P 2 N, P 31 , . . . PM 1 , PM 2  PM 3 , . . . PMN, (hereinafter, P 11  to PMN) connected to the row lines RL and the column lines CL and arranged in M rows and N columns. In the present example, the number of the plurality of pixels P 11  to PMN may be M×N. 
     Each of the plurality of pixels P 11  to PMN may sense a received optical signal using a photoelectric conversion element. For example, each of the pixels P 11  to PMN may include one or more photoelectric conversion elements. The plurality of pixels P 11  to PMN may detect the amount of light of the optical signal and output an electrical signal indicating the detected amount of light. 
     The row driver  20  may generate a plurality of control signals capable of controlling an operation of the pixels P disposed in each row under the control of the timing controller  40 . The row driver  20  may provide a plurality of control signals to each of the pixels P 11  to PMN of the pixel array  10  through the plurality of row lines RL. In response to the control signals provided from the row driver  20 , the pixel array  10  may be driven in units of rows. 
     The pixel array  10  may output a plurality of sensing signals through the column lines CL under the control of the row driver  20 . 
     The ADC circuit  30  may convert each of the sensing signals received through the column lines CL from an analog form into a digital form. The ADC circuit  30  may include an ADC corresponding to each of the column lines CL, and the ADC may convert a sensing signal received through a corresponding column line into a pixel value. According to an operation mode of the image sensor  1 , the pixel value may indicate the amount of light sensed by the plurality of pixels P 11  to PMN. 
     The ADC may include a correlated double sampling (CDS) circuit for sampling and holding the received signal. The CDS circuit may double-sample a noise signal when the plurality of pixels P 11  to PMN are in a reset state and a sensing signal, and output a signal corresponding to a difference between the sensing signal and the noise signal. The ADC may include a counter, and the counter may count a signal received from the CDS circuit to generate a pixel value. For example, the CDS circuit may be implemented as an operational transconductance amplifier (OTA), a differential amplifier, or the like. The counter may be implemented as, for example, an up-counter and arithmetic circuit, an up/down counter, and a bit-wise inversion counter. 
     The timing controller  40  may generate timing control signals for controlling operations of the row driver  20  and the ADC circuit  30 . The row driver  20  and the ADC circuit  30  may drive the pixel array  10  in units of rows as described above based on the timing control signals from the timing controller  40 , and may convert a plurality of sensing signals received through the plurality of column lines CL into a pixel value. 
     The image signal processor  50  may receive first image data IDT 1 , e.g., raw image data, from the ADC circuit  30 , and perform signal processing on the first image data IDT 1  to generate second image data IDT 2 . The image signal processor  50  may perform signal processing such as black level compensation, lens shading compensation, crosstalk compensation, and bad pixel compensation. 
     The second image data IDT 2  output from the image signal processor  50 , e.g., signal-processed image data, may be transmitted to a processor  60 . The processor  60  may be a host processor of an electronic device on which the image sensor  1  is mounted. 
       FIG.  2    is a circuit diagram illustrating pixels included in the image sensor  1  according to an example embodiment. 
     Referring to  FIGS.  1  and  2   , the pixel array  10  may include a plurality of pixels P 11 , P 12 , P 21 , and P 22 . The pixels P 11 , P 12 , P 21 , and P 22  may be arranged in a matrix form. For convenience of illustration, only four pixels P 11 , P 12 , P 21 , and P 22  are illustrated in  FIG.  2   , but descriptions thereof may be similarly applied to each of the plurality of pixels P 11  to PMN included in the pixel array  10 . 
     In an example embodiment, each of the pixels P 11 , P 12 , P 21 , and P 22  includes a transfer transistor TX and logic transistors RX, SX, and DX. Here, the logic transistors RX, SX, and DX may include a reset transistor RX, a selection transistor SX, and a drive transistor DX. 
     A photoelectric conversion element PD may generate and accumulate photo charges in proportion to the amount of externally incident light. The photoelectric conversion element PD may be a photo-sensing element formed of an organic material or an inorganic material, such as an inorganic photodiode, an organic photodiode, a perovskite photodiode, a phototransistor, a photogate or pinned photodiode, and an organic photoconductive film. 
     A transfer transistor TX may transfer charges accumulated in the photoelectric conversion element PD to a floating diffusion region FD based on a transfer signal TG. Photo charges generated by the photoelectric conversion element PD may be stored in the floating diffusion region FD. The drive transistor DX may be controlled by the amount of photo charges accumulated in the floating diffusion region FD. The transfer signal TG may be applied to a gate terminal of the transfer transistor TX. 
     The reset transistor RX may periodically reset charges accumulated in the floating diffusion region FD based on a reset signal RG. The reset signal RG maybe applied to a gate terminal of the reset transistor RX. A drain electrode of the reset transistor RX may be connected to the floating diffusion region FD, and a source electrode thereof may be connected to a node receiving a power voltage VDD. When the reset transistor RX is turned on, the power voltage VDD applied to the source electrode of the reset transistor RX may be transferred to the floating diffusion region FD. Accordingly, when the reset transistor RX is turned on, charges accumulated in the floating diffusion region FD may be discharged and the floating diffusion region FD may be reset. 
     In the drive transistor DX, each of the pixels P 11 , P 12 , P 21 , and P 22 , and a constant current source may configure a source follower buffer which amplify a potential change in the floating diffusion region FD, and output the amplified potential change to the output line Lout. 
     The selection transistor SX may select the pixels P 11 , P 12 , P 21 , and P 22  from which to read a photoelectric signal value sensed in units of rows based on a selection signal SG. The selection signal SG may be applied to a gate terminal of the selection transistor SX. When the selection transistor SX is turned on, the power voltage VDD may be transferred to a source electrode of the drive transistor DX. 
       FIG.  3    shows a layout of the pixel array  10  of the image sensor  1  according to an example embodiment. 
       FIG.  4    is a cross-sectional view taken along line I-I′ of  FIG.  3   . 
     Referring to  FIGS.  3  and  4   , the pixel array  10  of the image sensor  1  (see  FIG.  1   ) may include a substrate  101 , a photoelectric conversion element PD, a gate electrode  115 , an insulating layer  110 , a contact via  116 , conductive patterns  111 , an interlayer insulating layer  120 , first and second device isolation layers  130  and  135 , a passivation layer  140 , a planarization layer  180 , a variable filter layer CFL, and a plurality of micro-lenses  190 . 
     The substrate  101  may include a first surface  101   a  and a second surface  101   b  that face each other. The first surface  101   a  of the substrate  101  may be a front surface of the substrate  101 , and the second surface  101   b  of the substrate  101  may be a rear surface of the substrate  101 . 
     Two directions substantially parallel to the first surface  101   a  and substantially perpendicular to each other are defined as an X direction and a Y direction, and a direction substantially perpendicular to the first surface  101   a  is defined as a Z direction. The X direction, the Y direction, and the Z direction may be perpendicular to each other. 
     A plurality of pixels P 11 , P 12 , P 13 , P 14 , P 21 , P 22 , P 23 , P 24 , P 31 , P 32 , P 33 , P 34 , P 41 , P 42 , P 43 , P 44  (hereinafter, P 11  to P 44 ) are formed in the substrate  101 . The plurality of pixels P 11  to P 44  may be arranged in a matrix form in a plan view. 
     A plurality of dummy pixels may be formed in the substrate. According to an example embodiment, the plurality of pixels P 11  to P 44  may be disposed at the center of the matrix, and the dummy pixels may be disposed at the edges of the matrix. 
     According to an example embodiment, the first device isolation layer  130  may extend in the X and Y directions between the plurality of pixels P 11  to P 44  to horizontally separate the pixels P 11  to P 44  adjacent to each other. In an example embodiment, the second device isolation layer  135  may be disposed between the first device isolation layer  130  and the pixels P 11  to P 44 . 
     The first device isolation layer  130  may include a material having excellent gap fill performance, for example, polysilicon (poly-Si). According to an example embodiment, the first device isolation layer  130  may be doped with a P-type dopant, e.g., boron (B), but is not limited thereto. According to an embodiment, the first device isolation layer  130  has substantially the same length as that of the substrate  101  in the Z direction to separate the plurality of pixels P 11  to P 44  and the dummy pixels from each other. 
     The second device isolation layer  135  may include an insulating material. According to an example embodiment, the second device isolation layer  135  includes a material having a high dielectric constant, but is not limited thereto. 
     Here, the substrate  101  and the first device isolation layer  130  may act as electrodes of a capacitor, and the second device isolation layer  135  may act as a dielectric layer of the capacitor. Accordingly, a voltage difference between the substrate  101  and the first device isolation layer  130  may be maintained substantially constant. 
     In an example embodiment, a predetermined potential may be applied to the substrate  101  through the contact via  116 . According to an embodiment, the potential of the substrate  101  may be a ground potential, but is not limited thereto. According to an example embodiment, a potential different from that applied to the substrate  101  may be applied to the first device isolation layer  130 . According to some embodiments, because the first device isolation layer  130  is doped polysilicon, the first device isolation layer  130  may have substantially the same potential as a whole. 
     According to an example embodiment, by applying a voltage lower than that of the substrate  101  to the first device isolation layer  130 , an energy barrier between the first device isolation layer  130  and the substrate  101  may be increased to reduce dark current. Accordingly, the reliability of the image sensor  1  may be improved. 
     In an example embodiment, the photoelectric conversion element PD, for example, a photodiode, may be formed in the substrate  101 . The gate electrode  115  may be disposed to be spaced apart from each other on the first surface  101   a  of the substrate  101 . The gate electrode  115  may be, for example, any one of a gate electrode of a charge transistor TX of  FIG.  2   , a gate electrode of the reset transistor RX, and a gate electrode of the drive transistor DX of  FIG.  2   . 
     Although the gate electrode  115  is illustrated as being disposed on the first surface  101   a  of the substrate  101  in  FIG.  3   , the inventive concept is not limited thereto. For example, the gate electrode  115  may be buried in the substrate  101 . 
     The interlayer insulating layer  120  and the conductive patterns  111  may be disposed on the first surface  101   a  of the substrate  101 . The conductive patterns  111  may be covered by the interlayer insulating layer  120 . The conductive patterns  111  may be protected and insulated by the interlayer insulating layer  120 . 
     The interlayer insulating layer  120  may include, for example, silicon oxide, silicon nitride, silicon oxynitride, or the like. The conductive patterns  111  may include, for example, aluminum (Al), copper (Cu), tungsten (W), cobalt (Co), ruthenium (Ru), and the like. 
     The conductive patterns  111  may include a plurality of stacked wirings at different levels. In  FIG.  3   , the conductive patterns  111  are illustrated as including three sequentially stacked layers, but are not limited thereto. For example, conductive patterns  111  of two layers or four or more layers may be formed in the interlayer insulating layer  120 . 
     The insulating layer  110  may be disposed between the first surface  101   a  of the substrate  101  and the interlayer insulating layer  120 . The insulating layer  110  may cover the gate electrode  115  disposed on the first surface  101   a  of the substrate  101 . In example embodiments, the insulating layer  110  may include an insulating material such as silicon oxide, silicon nitride, silicon oxynitride, or the like. 
     The passivation layer  140  may be disposed on the second surface  101   b  of the substrate  101 . In an example embodiment, the passivation layer  140  may be in contact with the second surface  101   b  of the substrate  101 . According to an example embodiment, the passivation layer  140  may include, but is not limited to, an amorphous high-k material. 
     A variable filter layer CFL and a planarization layer  180  covering the variable filter layer CFL may be formed on the passivation layer  140 . The planarization layer  180  may include, for example, an oxide layer, a nitride layer, a low-k material, and a resin. According to an example embodiment, the planarization layer  180  may include a multi-layer structure. 
     The variable filter layer CFL may be disposed on the passivation layer  140 . The variable filter layer CFL may be disposed on the second surface  101   b  of the substrate  101 . The variable filter layer CFL may include first electrodes  151 ,  152 ,  153 , and  154 , second electrodes  161 ,  162 ,  163 , and  164 , a first electro-optical material layer  171 , a light-transmitting electrode  173 , and a second electro-optical material layer  175 . 
     The variable filter layer CFL may be configured to transmit light of the same or different wavelength bands to each of the plurality of pixels P 11  to P 44 . In an example embodiment, a thickness of the variable filter layer CFL in the Z direction is in a range of about 100 nm to about 200 nm. 
     According to an example embodiment, the first electrodes  151 ,  152 ,  153 , and  154  may be disposed on the passivation layer  140 . The first electrodes  151 ,  152 ,  153 , and  154  may contact the passivation layer  140 . The first electrodes  151 ,  152 ,  153 , and  154  may extend in the X direction. The first electrodes  151 ,  152 ,  153 , and  154  may be spaced apart from each other in the Y direction. The passivation layer  140  may cover side surfaces of the first electrodes  151 ,  152 ,  153 , and  154 . Accordingly, adjacent ones of the first electrodes  151 ,  152 ,  153 , and  154  may be spaced apart from each other with a portion of the passivation layer  140  therebetween. However, the inventive concept is not limited thereto, and the passivation layer  140  may be disposed entirely below the first electrodes  151 ,  152 ,  153  and  154 , and an additional insulating layer or a portion of the first electro-optical material layer  171  may be between adjacent ones of the first electrodes  151 ,  152 ,  153 , and  154 . In an embodiment, a width W 1  of the first electrodes  151 ,  152 ,  153 , and  154  in the Y direction is substantially the same as a width of the plurality of pixels P 11  to P 44  in the Y direction. 
     In an example embodiment, the first electrodes  151 ,  152 ,  153 , and  154  include a conductive material. According to an example embodiment, the first electrodes  151 ,  152 ,  153 , and  154  include a metal material such as silver (Ag), but is not limited thereto. The first electrodes  151 ,  152 ,  153 , and  154  may include at least one metal selected from the group consisting of copper (Cu), aluminum (Al), nickel (Ni), gold (Au), platinum (Pt), tin (Sn), lead (Pb), titanium. In an embodiment, the first electrodes  151 ,  152 ,  153 , and  154  include at least one metal selected from the group consisting of (Ti), chromium (Cr), palladium (Pd), indium (In) and zinc (Zn), metal alloys thereof, or carbon (C). 
     The first electro-optical material layer  171  may be disposed on the first electrodes  151 ,  152 ,  153 , and  154 . The first electro-optical material layer  171  may cover the first electrodes  151 ,  152 ,  153 , and  154 . The first electro-optical material layer  171  may contact each of the first electrodes  151 ,  152 ,  153 , and  154 . 
     The first electro-optical material layer  171  may include a material having a refractive index changing according to an applied voltage. According to an example embodiment, the refractive index of the first electro-optical material layer  171  may be adjusted by a potential difference between the first electrodes  151 ,  152 ,  153 , and  154  and the light-transmitting electrode  173 . 
     According to example embodiments, the first electro-optical material layer  171  may include one of 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate, lithium niobide (LiNbO 3 ), lithium tantalate (LiTaO 3 ), potassium titanyl phosphate (KTP), and beta (β)-barium borate (BBO). 
     The light-transmitting electrode  173  may be disposed on the first electro-optical material layer  171 . The light-transmitting electrode  173  may cover the first electro-optical material layer  171 . The light-transmitting electrode  173  may contact the first electro-optical material layer  171 . 
     The light-transmitting electrode  173  may be transparent to a visible light band. For example, visible light may pass through the light-transmitting electrode  173 . The light-transmitting electrode  173  may include a conductive material. A reference potential GND may be applied to the light-transmitting electrode  173 . For example, the image sensor  1  may include a voltage generator that applies the reference potential GND. The light-transmitting electrode  173  may extend in the X-direction and the Y-direction over the entire pixel array  10 . However, the inventive concept is not limited thereto, and a plurality of light-transmitting electrodes  173  disposed at the same level may divide and cover the pixel array  10 , and the reference potential GND may be applied to each of the light-transmitting electrodes  173 . 
     According to example embodiments, the light-transmitting electrode  173  may include one of a transparent conducting oxide (TCO) such as indium tin oxide (ITO), a silver nanowire, a carbon nanotube (CNT), graphene, and a conducting polymer. 
     The second electro-optical material layer  175  may be disposed on the light-transmitting electrode  173 . The second electro-optical material layer  175  may cover the light-transmitting electrode  173 . The second electro-optical material layer  175  may contact the light-transmitting electrodes  173 . 
     In an embodiment, the second electro-optical material layer  175  includes a material having a refractive index changing according to an applied voltage. According to an example embodiment, the refractive index of the second electro-optical material layer  175  is adjusted by a potential difference between the second electrodes  161 ,  162 ,  163 , and  164  and the light-transmitting electrode  173 . 
     According to an example embodiment, the second electro-optical material layer  175  includes one of the materials included in the first electro-optical material layer  171 . 
     The second electrodes  161 ,  162 ,  163 , and  164  may be disposed on the second electro-optical material layer  175 . The second electrodes  161 ,  162 ,  163 , and  164  may contact the second electro-optical material layer  175 . The second electrodes  161 ,  162 ,  163 , and  164  may extend in the Y direction. The second electrodes  161 ,  162 ,  163 , and  164  may be spaced apart from each other in the X direction. 
     The planarization layer  180  may cover top surfaces and side surfaces of the second electrodes  161 ,  162 ,  163 , and  164 . Accordingly, adjacent ones of the second electrodes  161 ,  162 ,  163 , and  164  may be spaced apart from each other with a portion of the planarization layer  180  therebetween. In an embodiment, a width W 2  of the second electrodes  161 ,  162 ,  163 , and  164  in the X direction is substantially the same as a width of the plurality of pixels P 11  to P 44  in the X direction. 
     In an example embodiment, the second electrodes  161 ,  162 ,  163 , and  164  include a conductive material. According to example embodiments, the second electrodes  161 ,  162 ,  163 , and  164  may include the materials described above in relation to the first electrodes  151 ,  152 ,  153  and  154 . 
     In an example embodiment, a portion of the variable filter layer CFL overlapping the pixels P 11  to P 44  is a color filter of the overlapping the plurality of pixels P 11  to P 44 . 
     For example, the first electrode  151 , the second electrode  161 , and a portion of the first electro-optical material layer  171  between the first electrode  151  and the second electrode  161 , a portion of the light-transmitting electrode  173  between the first electrode  151  and the second electrode  161 , and a portion of the second electro-optical material layer  175  between the first electrode  151  and the second electrode  161  may be a color filter of the pixel P 11 . The first electrode  153 , the second electrode  164 , and a portion of the first electro-optical material layer  171  between the first electrode  153  and the second electrode  164 , a portion of the light-transmitting electrode  173  between the first electrode  153  and the second electrode  164 , and a portion of the second electro-optical material layer  175  between the first electrode  153  and the second electrode  164  may be a color filter of the pixel P 34 . The first electrode  154 , the second electrode  162 , and a portion of the first electro-optical material layer  171  between the first electrode  154  and the second electrode  162 , a portion of the light-transmitting electrode  173  between the first electrode  154  and the second electrode  162 , and a portion of the second electro-optical material layer  175  between the first electrode  154  and the second electrode  162  may be a color filter of the pixel P 42 . 
     In a color filter including an organic material, a wavelength band of light received for each pixel is determined at the time of initial manufacturing. Accordingly, an image including information on red visible light, green visible light, and blue visible light may be stored with a single photographing, but each pixel does not obtain information on red visible light, green visible light, and blue visible light, so resolution of the image is low. In addition, fine patterning is difficult due to the nature of organic material patterning. 
     According to example embodiments, as described with reference to  FIGS.  5  to  7 C , the variable filter layer CFL may operate as a Bayer pattern color filter that simultaneously receives red visible light, green visible light and blue visible light or may operate as a red filter that receives only red visible light, a green filter that receives only green visible light, and a blue filter that receives only blue visible light by controlling a voltage applied to the first and second electrodes  151 ,  152 ,  153 ,  154 ,  161 ,  162 ,  163 , and  164 . Accordingly, the resolution quality of the image generated by the pixel array  10  may be improved. 
     In addition, because the variable filter layer CFL is provided through patterning of an insulating material and a metallic material, its dimensions may be reduced, as compared to prior organic-type color filters. For example, the width W 1  of each of the first electrodes  151 ,  152 ,  153 , and  154  in the Y direction and the width W 2  of each of the second electrodes  161 ,  162 ,  163  and  164  in the X direction may be reduced, compared with prior organic-type color filters, and accordingly, the plurality of pixels P 11  to P 44  may be reduced in size. 
     Furthermore, a dimension of prior organic-type color filters in the Z direction is about 500 nm, whereas a dimension of the variable filter layer CFL according to an example embodiment in the Z direction may range between about 100 nm and 200 nm. Thus, the variable filter layer CFL according to example embodiments may reduce the dimension of the pixel array  10  in the Z-direction. 
     The plurality of microlenses  190  may be disposed on the planarization layer  180 . The plurality of microlenses  190  may be formed of an organic material such as a photosensitive resin or an inorganic material. The plurality of microlenses  190  may condense incident light to the photoelectric conversion element PD. 
     Each of the plurality of microlenses  190  may vertically overlap a corresponding one of the photoelectric conversion elements PD. Accordingly, one of the plurality of microlenses  190  and one of the photoelectric conversion elements PD may be disposed in each of the pixels P 11  to P 44 . 
       FIG.  5    is a diagram illustrating an operation of the pixel array  10  according to an example embodiment. 
     Referring to  FIGS.  4  and  5   , a first voltage V 1  may be applied to the first electrodes  151  and  153 , a second voltage V 2  may be applied to the first electrodes  152  and  154 , the first voltage V 1  may be applied to the second electrodes  161  and  163 , and the second voltage V 2  may be applied to the second electrodes  162  and  164 . A ground potential GND may be applied to the light-transmitting electrode  173 . In an embodiment, a voltage generator within the image sensor  1  is configured to generate the first voltage V 1  and the second voltage V 2 . 
     Accordingly, the first voltage V 1  may be applied to a portion of the first electro-optical material layer  171  overlapping the pixels P 11 , P 13 , P 31 , and P 33 , and the first voltage V 1  may be applied to a portion of the second electro-optical material layer  175  overlapping the pixels P 11 , P 13 , P 31 , and P 33 . 
     In addition, the first voltage V 1  may be applied to a portion of the first electro-optical material layer  171  overlapping the pixels P 12 , P 14 , P 32 , and P 34 , and the second voltage V 2  may be applied to the second electro-optical material layer  175  overlapping the pixels P 12 , P 14 , P 32 , and P 34 . 
     In addition, the second voltage V 2  may be applied to a portion of the first electro-optical material layer  171  overlapping the pixels P 21 , P 23 , P 41 , and P 43 , and the first voltage V 1  may be applied to a portion of the second electro-optical material layer  175  overlapping the pixels P 21 , P 23 , P 41 , and P 43 . 
     In addition, the second voltage V 2  may be applied to a portion of the first electro-optical material layer  171  overlapping the pixels P 22 , P 24 , P 42 , and P 44 , and the second voltage V 2  may be applied to a portion of the second electro-optical material layer  175  overlapping the pixels P 11 , P 13 , P 31 , and P 33 . 
     Accordingly, a refractive index of the portion of the first electro-optical material layer  171  overlapping the pixels P 11 , P 13 , P 31 , and P 33  may be a first refractive index, and a refractive index of the portion of the second electro-optical material layer  175  overlapping the pixels P 11 , P 13 , P 31 , and P 33  may be the first refractive index. 
     In addition, a refractive index of the portion of the first electro-optical material layer  171  overlapping the pixels P 12 , P 14 , P 32 , and P 34  may be the first refractive index, and a refractive index of the portion of the second electro-optical material layer  175  overlapping the pixels P 12 , P 14 , P 32 , and P 34  may be the second refractive index. 
     In addition, a refractive index of the portion of the first electro-optical material layer  171  overlapping the pixels P 21 , P 23 , P 41 , and P 43  may be the second refractive index, and a refractive index of the portion of the second electro-optical material layer  175  overlapping the pixels P 21 , P 23 , P 41 , and P 43  may be the first refractive index. 
     In addition, a refractive index of the portion of the first electro-optical material layer  171  overlapping the pixels P 22 , P 24 , P 42 , and P 44  may be the second refractive index, and a refractive index of the portion of the second electro-optical material layer  175  overlapping the pixels P 22 , P 24 , P 42 , and P 44  may be the second refractive index. 
     According to a first example, when the refractive index of the first electro-optical material layer  171  is about 2.7 and the refractive index of the second electro-optical material layer  175  is about 2.7, red visible light passes through the variable color filter CFL. In the first example, green and blue visible light is blocked from passing through the variable color filter CFL. According to a second example, when the refractive index of the first electro-optical material layer  171  is about 1.6 and the refractive index of the second electro-optical material layer  175  is about 2.7, green visible light passes through the variable color filter CFL. In the second example, red and blue visible light is blocked from passing through the variable color filter CFL. According to a third example, when the refractive index of the first optical material layer  171  is about 2.7 and the refractive index of the second electro-optical material layer  175  is about 1.6, green visible light passes through the variable color filter CFL. In the third example, red and blue visible light is blocked from passing through the variable color filter CFL. According to a fourth example, when the refractive index of the first electro-optical material layer  171  is about 1.6 and the refractive index of the second electro-optical material layer  175  is about 1.6, blue visible light passes through the variable color filter CFL. In the fourth example, red and green visible light is blocked from passing through the variable color filter CFL. 
     According to an example embodiment, the Bayer pattern as illustrated may be implemented by adjusting the first and second voltages V 1  and V 2  so that the first refractive index is about 2.7 and the second refractive index is about 1.6. 
     In an embodiment, when the refractive index of the first electro-optical material layer  171  and the refractive index of the second electro-optical material layer  175  are set to a same first value, light of a first visible color passes through the variable color filter CFL and light of second and third visible colors different from the first visible color are prevented from passing through the variable color filter CFL. In an embodiment, when the refractive index of the first electro-optical material layer  171  is a second value different from the first value and the refractive index of the second electro-optical material layer  175  is set to the first value, light of the second visible color passes through the variable color filter CFL and light of the first and third visible colors are prevented from passing through the variable color filter CFL. In an embodiment, when the refractive index of the first electro-optical material layer  171  is set to the first value and the refractive index of the second electro-optical material layer  175  is set to the second, light of the second visible color passes through the variable color filter CFL and light of the first and third visible colors are prevented from passing through the variable color filter CFL. In an embodiment, when the refractive index of the first electro-optical material layer  171  and the refractive index of the second electro-optical material layer  175  are both set to the second value, light of the third visible color passes through the variable color filter CFL and light of the first and second visible colors are prevented from passing through the variable color filter CFL. 
     In the present example, a magnitude of the voltage applied to the first and second electro-optical material layers  171  and  175  is proportional to the refractive index of the first and second electro-optical material layers  171  and  175  and the first voltage V 1  may be greater than the second voltage V 2 , but the inventive concept is not limited thereto. For example, as the voltage applied to the first and second electro-optical material layers  171  and  175  increases, the refractive index of the first and second electro-optical material layers  171  and  175  may decrease, and the first voltage V 1  may be smaller than the second voltage V 2 . 
     In the present embodiment, the first voltage V 1  and the second voltage V 2  may be applied to the first electrodes  151 ,  152 ,  153 , and  154  in a spatially alternative way, and the first voltage V 1  and the second voltage V 2  may be applied to the second electrodes  161 ,  162 ,  163 , and  164  in a spatially alternative way. 
     That is, there may be one first electrode  152  to which the second voltage V 2  is applied between two adjacent first electrodes  151  and  153  to which the first voltage V 1  is applied, and there may be one first electrode  153  to which the first voltage V 1  is applied between two adjacent first electrodes  152  and  154  to which the voltage V 2  is applied. 
     Similarly, there may be one second electrode  162  to which the second voltage V 2  is applied between two adjacent second electrodes  161  and  163  to which the first voltage V 1  is applied, and thereby be one second electrode  162  to which the first voltage V 1  is applied between two adjacent second electrodes  162  and  164  to which the second voltage V 2  is applied. 
       FIG.  6    is a flowchart illustrating an operation of the pixel array  10  according to an example embodiment. 
       FIGS.  7 A to  7 C  are views illustrating an operation of the pixel array  10  according to an example embodiment. 
     Referring to  FIGS.  4 ,  6  and  7 A , a first image of a red visible light band is acquired by applying the first voltage V 1  to the first and second electrodes  151 ,  152 ,  153 ,  154 ,  161 ,  162 ,  163 , and  164  at P 10 . 
     The first voltage V 1  may be applied to each of the first and second electro-optical material layers  171  and  175  in entirety, and accordingly, the refractive index of each of the first and second electro-optical material layers  171  and  175  may be the first refractive index in entirety. 
     According to an example embodiment, when the first voltage V 1  is applied to each of the first and second electrodes  151 ,  152 ,  153 ,  154 ,  161 ,  162 ,  163 , and  164 , the variable filter layer CFL may be a filter that transmits visible red light in entirety. Accordingly, the pixel array  10  may acquire a first image that is an image of a red visible light band. 
     Referring to  FIGS.  4 ,  6  and  7 B , a second image of a green visible light band is acquired by applying the first voltage V 1  to the first electrodes  151 ,  152 ,  153 , and  154  and applying the second voltage V 2  to the second electrodes  161 ,  162 ,  163 , and  164  at P 20 . For example, the first voltage V 1  may differ from the second voltage V 2 . 
     The first voltage V 1  may be applied to each of the first electro-optical material layers  171  in entirety, and accordingly, the refractive index of the first electro-optical material layer  171  may be the first refractive index in entirety. The second voltage V 2  may be applied to each of the second electro-optical material layers  175  in entirety, and accordingly, the refractive index of the second electro-optical material layer  175  may be the second refractive index in entirety. 
     According to an example embodiment, when the first voltage V 1  is applied to each of the first electrodes  151 ,  152 ,  153 , and  154 , and the second voltage V 2  is applied to the second electrodes  161 ,  162 ,  163  and  164 , the variable filter layer CFL may be a filter that transmits green visible light in entirety. Accordingly, the pixel array  10  may acquire the second image that is an image of the green visible light band. For example, the first voltage V 1  may differ from the second voltage V 2 . 
     At P 20 , alternatively, the second image of the green visible light band may be acquired by applying the second voltage V 2  to the first electrodes  151 ,  152 ,  153 , and  154 , and applying the first voltage V 1  to the second electrodes  161 ,  162 ,  163 , and  164 . 
     Referring to  FIGS.  4 ,  6 , and  7 C , a third image of a blue visible light band may be acquired by applying the second voltage V 2  to the first and second electrodes  151 ,  152 ,  153 ,  154 ,  161 ,  162 ,  163 , and  164  at P 30 . 
     The second voltage V 2  may be applied to each of the first and second electro-optical material layers  171  and  175 , and accordingly, the refractive index of each of the first and second electro-optical material layers  171  and  175  may be the second refractive index in entirety. 
     According to an example embodiment, when the second voltage V 2  is applied to each the first and second electrodes  151 ,  152 ,  153 ,  154 ,  161 ,  162 ,  163 , and  164 , the variable filter layer CFL may be a filter that transmits blue visible light in entirety. Accordingly, the pixel array  10  may acquire the third image that is an image of the blue visible light band. 
     Subsequently, referring to  FIGS.  1 ,  3 , and  6   , a high-resolution image may be acquired by combining the first to third images with each other at P 40 . Combining of the first to third images may be performed by the processor  60 . The first to third images may be images of the same object. 
     In the case of an image sensor using a Bayer pattern, about 66% of light information is absorbed by a color filter, so that quality of an image may be deteriorated in a low illuminance condition. In addition, spatial resolution is low because each pixel does not read light amount information for red, green, and blue visible light and predicts a light amount of neighboring pixels based on a representative value. 
     According to an example embodiment, each of the plurality of pixels P 11  to P 44  receives red visible light when acquiring the first image, receives green visible light when acquiring the second image, and receives blue visible light when acquiring the third image, and therefore, all the pixels P 11  to P 44  may obtain information on the light amount of each of the red visible light, green visible light, and blue visible light, thereby obtaining a high-resolution image. 
       FIG.  8    is a cross-sectional view illustrating a pixel array  11  of an image sensor according to an example embodiment, and shows a portion corresponding to  FIG.  4   . 
     Referring to  FIG.  8   , the pixel array  11  includes a substrate  101 , a photoelectric conversion element PD, a gate electrode  115 , an insulating layer  110 , a contact via  116 , conductive patterns  111 , and an interlayer insulating layer  120 , first and second device isolation layers  130  and  135 , a passivation layer  140 , microlenses  190 , a planarization layer  191 , a variable filter layer CFL, and a planarization layer  192 . 
     The substrate  101 , photoelectric conversion element PD, gate electrode  115 , insulating layer  110 , contact via  116 , conductive patterns  111 , interlayer insulating layer  120 , first and second device isolation layers  130  and  135 , and the passivation layer  140  are substantially the same as those described above with reference to  FIGS.  3  and  4   , and thus redundant description thereof is omitted. 
     According to an example embodiment, the microlenses  190  may be disposed on the passivation layer  140 . 
     The planarization layer  191  covering the microlenses  190  may be disposed on the microlenses  190 . According to an example embodiment, the planarization layer  191  may include a light-transmitting material. According to an example embodiment, the planarization layer  191  may include an insulating material. 
     The variable filter layer CFL may be disposed on the planarization layer  191 . The dimensional, compositional, and structural characteristics of the variable filter layer CFL are similar to those described above with reference to  FIGS.  3  and  4   , except that the variable filter layer CFL is disposed on the planarization layer  191 . 
       FIG.  9    is a cross-sectional view illustrating a pixel array  12  of an image sensor according to an example embodiment, and shows a portion corresponding to  FIG.  4   . 
     Referring to  FIG.  9   , the pixel array  12  may include a substrate  101 , a photoelectric conversion element PD, a gate electrode  115 , an insulating layer  110 , a contact via  116 , conductive patterns  111 , an interlayer insulating layer  120 , first and second device isolation layers  130  and  135 , a variable filter layer CFL, a planarization layer  180 , and microlenses  190 . 
     The substrate  101 , photoelectric conversion element PD, gate electrode  115 , insulating layer  110 , contact via  116 , conductive patterns  111 , interlayer insulating layer  120 , and first and second device isolation layers  130  and  135  are substantially the same as those described above with reference to  FIGS.  3  and  4   , and thus a redundant description thereof is omitted. 
     According to an example embodiment, the pixel array  12  may include a front-illuminated image sensor. That is, light collected by the microlenses  190  may be incident on the photoelectric conversion element PD through the first surface  101   a  of the substrate  101 . 
     The interlayer insulating layer  120 , the contact via  116  covered by the interlayer insulating layer  120 , and the conductive patterns  111  may be interposed between the microlenses  190  and the substrate  101 . The variable filter layer CFL may be spaced apart from the substrate  101  with the interlayer insulating layer  120 , the contact via  116  covered by the interlayer insulating layer  120 , and the conductive patterns  111  therebetween. 
     The variable filter layer CFL may be disposed on the interlayer insulating layer  120 , and the planarization layer  180  may be disposed on the variable filter layer CFL. The dimensional, compositional, and structural characteristics of the variable filter layer CFL are similar to those described above with reference to  FIGS.  3  and  4   , except that the variable filter layer CFL is disposed between the planarization layer  180  and the interlayer insulating layer  120 . 
     While the inventive concept has been particularly shown and described with reference to embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.