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Advanced features of layered-structure organic-photoconductive-film CMOS image sensor: Over120 dB wide dynamic range function andphotoelectric-conversion-controlled global shutterfunctionTo cite this article: Kazuko Nishimura et al 2018 Jpn. J. Appl. Phys. 57 1002B4

 

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Advanced features of layered-structure organic-photoconductive-film

CMOS image sensor: Over 120dB wide dynamic range function

and photoelectric-conversion-controlled global shutter function

Kazuko Nishimura*, Sanshiro Shishido, Yasuo Miyake, Hidenari Kanehara, Yoshiaki Sato, Junji Hirase, Yoshihiro Sato,Yuko Tomekawa, Masayuki Yamasaki, Masashi Murakami, Mitsuru Harada, and Yasunori Inoue

Technology Innovation Division, Panasonic Corporation, Moriguchi, Osaka 570-8501, Japan

*E-mail: nishimura.kazuko@jp.panasonic.com

Received June 1, 2018; revised July 12, 2018; accepted July 15, 2018; published online September 14, 2018

We have developed organic photoconductive film (OPF) CMOS image sensors with pixel structures different from those of a conventional siliconimage sensors, in which, the organic thin film for photoelectric conversion and the charge storage part for signal charge accumulation are completelyindependent. In this paper, we focus on two unique features of the OPF image sensor: (1) technology that realizes over 120 dB simultaneous-capturewide dynamic range, (2) global shutter technology achieving high saturation signals per unit square that is 10 dB higher than that of silicon imagesensors with the global shutter function, without sacrificing pixel area. In addition, we have developed a test chip that realizes a high resolution of8K4K, a high frame rate of 60 fps, a high saturation signals of 450 ke%, and the global shutter function simultaneously. These features of the OPFimage sensor will contribute to significant progress in imaging and sensing fields. © 2018 The Japan Society of Applied Physics

1. Introduction

Recently, image sensors are increasingly becoming keydevices in various applications; in-vehicle, surveillance, robotvision, broadcasting, medical, and many camera systems. Torealize the best possible imaging and sensing performances,there are growing demands for wide dynamic range, highsensitivity, and high frame rate. In particular, the demand forhighly robust sensing technology is increasing rapidly, suchas expanding the dynamic range to reliably sense, even underbacklit condition, and overcoming the LED flicker problem.

In addition, CMOS image sensors have features such ashigh resolution, high frame rate, low power consumption,and high functional processing, and they have become themainstream imaging and sensing devices instead of CCDs.However, CMOS image sensors use a rolling shutter (RS)operation in which exposure and shutter operations aresequentially scanned row by row, not all pixels at the sametime. To perform the global shutter (GS) operation that doesnot cause image distortion as in the case of CCDs, it isnecessary to add in-pixel memories and transfer circuits,which however give rise to the problems of reduction of thesensitivity and the saturation signals, because the lightreceiving and memory areas are sacrificed.

To solve these problems, a CMOS image sensor with anorganic photoconductive film (OPF) laminated on pixelcircuits becomes one optimum candidate. Because the OPFimage sensor has a unique structure, in which the photo-electric conversion part and the charge storage and signalreadout part are completely independent. Therefore, byoptimally designing each part, the sensor performance canbe markedly improved compared with conventional siliconimage sensors. In this paper, we describe the unique structureand two strong features of the OPF sensors: (1) expandeddynamic range technology and (2) global shutter technologybased on this structure.

2. Features of the OPF image sensor

In this section, we describe the unique structure and strongfeatures of the OPF image sensors. In Sect. 2.1, we explainthe superiority of the OPF sensor structure. From Sects. 2.2

to 2.4, we explain about the advantageous techniques onthe basis of the OPF sensor structure, contributing to theimprovement of sensing capability. In previous OPF imagesensors, the full well capacity was 4 times larger than thatof common silicon images sensors, but the reset noise levelwas also higher.1,2) Therefore, we developed technologiesthose expand both sides of the dynamic range; much furtherexpanded dynamic range technique, dual-sensitivity pixel(DS-Pixel) and noise cancellation technique, capacitive-coupled noise canceller (CCNC). In Sect. 2.2, we describethe DS-Pixel technology and in Sect. 2.3, we describe theCCNC technologies. In Sect. 2.4, we explain the unique GStechnologies of the OPF sensor based on its photoelectricconversion characteristics. Finally, in Sect. 2.5, we introducea new 8K4K sensor fabricated using the above technologies.2.1 OPF image sensor pixel structureFigure 1 shows the pixel structures of a common siliconimage sensor and the novel OPF image sensor.1–3) The pixelstructure of the OPF image sensor is completely different fromthat of a common silicon image sensor. In a silicon imagesensor, both photoelectric conversion and charge storage areexecuted by the photodiode. This means that sensitivity andfull well capacity are limited by size and quantum efficiencyof the photodiode. On the other hand, in the OPF imagesensor, incident light enters through the on-chip micro lensesand color filters, and is then absorbed in the OPF layer. Thecharge generated in the OPF is extracted to the pixel elec-trode side according to the electric field intensity applied tothe OPF and accumulated in a floating diffusion (FD) formedon a silicon substrate as a signal charge. The sensitivity of theOPF can be controlled by the voltage applied to the trans-parent electrode (indium tin oxide: ITO) at the top of theOPF, as shown in Fig. 2. The photoelectric conversion part isthe OPF and the charge storage part is the charge storagecapacitance of the FD node. These two parts are completelyindependent. Therefore, if we prepare high-functional circuits,such as, a large charge-storage capacitor at a circuit area,higher saturation can be realized.

Moreover, it is possible to select the optimum structuresand materials for the OPF that performs photoelectric con-version. For example, as shown in Ref. 4, there is a method

Japanese Journal of Applied Physics 57, 1002B4 (2018)

https://doi.org/10.7567/JJAP.57.1002B4

STAP ARTICLEPhysics-based circuits and systems

1002B4-1 © 2018 The Japan Society of Applied Physics

of stacking red, green, and blue (RGB) OPFs in the verticaldirection and acquiring three colors on the basis of the factthat individual colors have different wavelengths. However,in this method, shields are required between the respectivefilms and a complex manufacturing process is required. Inaddition, since parts of the G or R lights are absorbed by theB light OPF, it is difficult to maintain color reproducibilityand light transmittance. Therefore, we selected a differentstructure with good color reproducibility, in which one OPFis used and RGB colors are separated by the Bayer pattern ofcolor filters, as shown on the right panel of Fig. 1.

Furthermore, they are not fixed materials such as siliconphotodiodes. For example, the light absorption rate of thecurrent OPF is 10 times higher than that of silicon photo-diodes. Therefore, we can use very thin films, such as thoseof 0.5 µm thickness, and low-crosstalk performance and wideincident light range exceeding 60 deg are realized.

Additionally, in the silicon sensor, the sensitivity of variouswavelengths was determined by the physical properties of thesilicon photodiode, but, in the OPF sensor, by developingmaterials with dramatically improved sensitivity at a specificwavelength and adapting it to OPF, it is possible to performhighly accurate sensing using a specific wavelength.5)

2.2 Simultaneous-capture wide dynamic rangetechniqueWe explain about the new expanded dynamic range tech-

niques to enhance the robustness of sensing. Several methodsto expand the dynamic range have been proposed in theprevious works.6–16) A multiple-exposures method is shownin Ref. 6 and lateral overflow integration capacitor methodsare shown in Refs. 7 and 8. These techniques realized 140,104, and 207 dB dynamic ranges, respectively, by synthesiz-ing the images obtained using multiple exposures. Thesemethods have asynchronous problems that comes from multi-ple exposures of different-time images in high-speed imaging,and their simultaneous-capture wide dynamic ranges (SC-WDR) were up to 100 dB. Figure 3 shows the image failures,such as overexposure of a sunset scene, due to an insufficientdynamic range.

Figure 4 shows a summary of the dynamic range of severalmaterials. The dynamic range of a common silicon imagesensor is about 76 dB; however, the dynamic range of thehuman eyes is said to be 110 dB, including calculation in thebrain. This dynamic range is far below the human eyes’capacity. Therefore, we decided to exceed the human eyes’capacity without the above problems by using the newstructure of the OPF image sensor, to realize accurate andtexture-rich imaging and sensing. We set our SC-WDR targetto over 120 dB.

Fig. 1. (Color online) Cross-sectional image of pixel structures.

Fig. 2. (Color online) Photoelectric characteristics of the OPF.

Fig. 3. (Color online) Imaging failures due to insufficient dynamic range.

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Figure 5 shows a cross-sectional image of the proposedtechnique of expanding the dynamic range using theDS-Pixel of the OPF sensor.3) The DS-Pixel is using theOPF image sensor unique structure, in which sensitivity andsaturation can be set independently. Conventionally, even insilicon image sensors, techniques to realize a wide dynamicrange using dual sensitivity photodiodes have been pro-posed.17) In this structure, sensitivity and saturation cannotbe set independently. The storage capacitor of the high-sensitivity photodiode is large and that of the low-sensitivityphotodiode is small. Therefore, it is possible to receive high-brightness light with low sensitivity, but the saturationcharacteristic is limited by the size of the storage capacitor.

In one DS-Pixel of the OPF image sensor, two photoelec-tric-conversion parts, two charge-storage parts, and two typesof noise cancellers are prepared for two cells. The two cellsconsist of Cell 1 (high-sensitivity cell) and Cell 2 (high-saturation cell). In Cell 1, high sensitivity and low noise areimportant for imaging dark objects. Cell 1 does not requirehigh-saturation characteristics because the intensity of light tobe detected is not very high. Therefore, Cell 1 has a largepixel electrode (PE1) for achieving high sensitivity, a smallcharge-storage capacitor for achieving high conversion gainand an additional noise canceller for achieving low noise.Cell 1 consists of four transistors and two capacitors as a newnoise canceller, CCNC. Cell 2 requires high saturation char-acteristics for imaging ultra-bright objects, and no extra noisecancellation because the reset noise is buried in shot noise.On the basis of these requirements, Cell 2 has a small pixel

electrode (PE2) for achieving low sensitivity, a large charge-storage capacitor for achieving high saturation, and a con-ventional noise canceller, because of alleviated noise specs.Cell 2 consists of three transistors as the conventional noisecanceller and a large charge storage capacitor Cs2. In thisstructure, capacitor Cs2 is not used for noise cancellation,only used for charge storage. Therefore, the optimum areaefficiency is realized. And, we designed the charge storagecapacitance ratio between Cell 1 and Cell 2 set to about1 : 10, the sensitivity ratio between PE1 and PE2 to be set toabout 1 : 1=10. The exposure periods are set to be simul-taneous. A SC-WDR that is 100 times wider than those ofcommon silicon sensors is achieved by this DS-Pixel.

Figure 6 shows a cross-sectional image of the DS-Pixelwhen metal–insulator–metal (MIM) is used in pixel. We havedeveloped a high-k MIM capacitor with high capacitance andlow leakage current. We propose in-pixel capacitors in BEOLinstead of DMOS capacitors on a silicon substrate. Theimplementation of capacitors in BEOL enables an easier andmore flexible pixel design. The MIM structure can be changeddepending on the purpose, for example, to realize both highcapacitance and low leakage, the three-dimensional (3D)MIM capacitors in which trenches are located in the BEOLlayer in each pixel, can be prepared. The 3D capacitor consistsof a TiN=HfO2=TiN stack. The 3D structure enables the use ofa thick dielectric film.18) In the OPF image sensor, high-performance devices can be vertically layered between theOPF and the circuit area. This allows the MIM capacitors to belayered as a noise cancellation capacitor (Cs1) and as a chargestorage capacitor (Cs2). Therefore, in the case of using MIMcapacitors instead of DMOS capacitors, 3 µm pixel size,1940H × 1100V resolution can be realized. This means thatthe pixel size can be reduced to a quarter of the original sizeand the resolution can be 4 times higher. If a silicon sensor ofsimilar pixel size could be designed, it would be necessary toreduce the photodiode area and to sacrifice sensitivity andsaturation charge.

Figure 7 shows the performances of the silicon imagesensor and the OPF image sensor. The X-axis is sensitivityand the Y-axis is full well capacity. In silicon image sensors,photodiode characteristics are one point on the gray arrowline, depending on the photodiode size. Therefore, if wewant to set sensitivity to low, full well capacity also becomessmall. On the other hand, in the OPF image sensor, settingboth sensitivity to low and full well capacity to large can berealized. Moreover, the DS-Pixel can combine high perform-ances of the two cells simultaneously. Therefore, the SC-WDR is expanded significantly.2.3 Reset noise cancellation technique2.3.1 Capacitive-coupled noise canceller. We explainabout the new reset noise canceller that enables sharp andtexture-rich imaging when capturing dark objects. First, toexplain about the reset noise, when the reset transistor ofpixels is turned off, reset noise is generated. In the case ofconventional silicon image sensors, fully charge transferringby an embedded photodiode structure enables the use of thecorrelated double sampling (CDS) method. When using theCDS method, reset noise is cancelled and is no longer aproblem. On the other hand, as the OPF image sensor storescharges at the capacitance of the FD, not the photodiode, theCDS method cannot be used and reset noise remains on the

Fig. 4. (Color online) OPF sensor WDR target.

Fig. 5. (Color online) Pixel structure of DS-Pixel.

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FD node. The remaining reset noise becomes a fundamentalproblem, especially to when detecting objects in darksituations. Previously, several techniques were proposed tosuppress reset noise. One example, in Ref. 2, the negativefeedback method with three transistors and a feedbackamplifier was proposed. When the gain of the feedbackamplifier is set to −A, reset noise is suppressed in proportionto the following multiplier.

N / 1=ffiffiffiffiA

pð1Þ

However, A is several tens or several hundreds, so the effectis limited. Therefore, we developed a new noise canceller thatsufficiently suppresses reset noise to improve signal-to-noise(S=N) characteristics of dark regions.

Figure 8 shows a schematic of the new noise canceller,capacitive-coupled noise canceller (CCNC), which consistsof four transistors (SF, amplifier transistor; SEL, selecttransistor; RST, reset transistor; FB, feedback transistor),and two capacitors (Cs, stabilized capacitor; Cc, coupledcapacitor) in each pixel and the feedback amplifier (FBAMP)

in each column. The capacitive-coupled structure is veryeffective for suppressing reset noise and robust operation.3)

The scheme of this noise cancellation is as follows. First,the RST and the FB are turned on at the same time and theFD node is set to the reset voltage. Then, the RST and the FBare turned off sequentially. During this time, the reset noisefrom the two transistors are suppressed by using a negativefeedback loop that includes the FB, which is bandwidth-controlled using the gate voltage. When the gain of thenegative feedback loop is set to −A, the reset noise of RSTand FB can be respectively suppressed in proportion to thefollowing multipliers.

NRST / 1=ðA � Cc=CfdÞ ð2ÞNFB / 1=

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA � Cs=Cc

pð3Þ

Total noise is expressed as Eq. (4).

N ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN2

RST þ N2FB

pð4Þ

As a result, by setting A and Cs=Cc to maximum values, totalreset noise can be decreased. Within 5.0 µs, the generatedreset noise 25 e− is suppressed to 1.6 e−, which is 1=15 of thegenerated reset noise. Moreover, the robustness of noisecancellation is improved by controlling the FB, which isseparated from the FD node using the Cc. Consequently, theremaining reset noise becomes small enough for imaging andsensing applications.

Fig. 7. (Color online) Sensitivity and full well capacity characteristics ofDS-Pixel.

Fig. 6. (Color online) Pixel structure of DS-Pixel with MIM capacitors.

Fig. 8. (Color online) Schematic of CCNC.

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1002B4-4 © 2018 The Japan Society of Applied Physics

2.3.2 In-pixel capacitive-coupled noise canceller.Here, we explain different types of noise cancellation circuits.In conventional methods or the method described inSect. 2.3.1, a feedback amplifier is allocated to each columnor each area2,3,19) to suppress the reset noise and thresholdvoltage variation. However, a sensor with a large number ofpixels requires a long suppression time owing to the largetime constant of the vertical signal line. For example, 4K or8K sensor vertical signal lines are two times or four timeslonger than that of the full-high-definition (FHD) sensor,respectively. Thus, we have developed a new in-pixel noisecanceller to shorten the noise suppression time even in high-resolution sensors.

Figure 9 shows a concept and a schematic of the newin-pixel noise canceller, in-pixel capacitive-coupled noisecanceller (IP-CCNC), which consists of four transistors (SF,amplifier transistor; SEL, select transistor; RST, reset tran-sistor; FB, feedback transistor), and two capacitors (Cs,stabilized capacitor; Cc, coupled capacitor) in each pixel andfour switches (S1, S1b, R1, R1b), two bias voltage lines(AVDD, Vbias), and two current sources in each column.20) Toperform high-resolution readout and high-speed noise can-cellation, a FBAMP should be allocated to each pixel, not toeach column, even though the pixel area is limited. Thus, wefurther tried a new ingenuity. To shrink pixel size, a recon-figurable pixel circuit architecture that has two operationmodes has been developed: a FBAMP for noise cancellationand a source follower amplifier for signal readout. During thenoise cancellation period, the in-pixel SF and the columncurrent source configure the in-pixel common-source invert-ing amplifier by setting S1 and R1 to the ON state and S1b andR1b to the OFF state. This allows noise cancelling for eachpixel to be realized. First, the FD node is set to the resetvoltage by simultaneously turning on RST and FB; then RSTand FB are turned off sequentially. During this period, thereset noise caused by RST and FB is suppressed using anegative feedback loop. When the gain of the negative

feedback loop is set to −A, the reset noise of RST and FB canbe suppressed similarly to the CCNC. In this case, the gain ofthe in-pixel FBAMP is smaller than that of the columnFBAMP. To increase this gain, high-capacitance and low-leakage-current MIM capacitors were developed and the Csvalue was set high. This resulted in a reduction in the pixelreset noise from 23 e− to 2.5 e−, within 2.5 µs. During the resetand signal readout period, the in-pixel SF and the columncurrent source configure the source follower amplifier, bysetting S1 and R1 to the OFF state and, S1b and R1b to theON state. The signal readout operation from the FD node isthen realized.

Additionally, this reconfigurable pixel circuit has one moremode. When the amount of incident light is small, this circuitoperates in the noise cancellation mode, but when the amountof incident light is large, this circuit operates in the high-saturation mode by increasing the capacitance of FD toconnect the Cs capacitor to the FD node while setting the gatevoltage of RST Vrst to the ON state. A saturation signals of450 ke− is thus achieved. This saturation signals per unitsquare is 10 dB higher than that obtained when using thesilicon image sensor with the global shutter (GS) functionand high-saturation structure.21) In this architecture, weprepare several current sources, but do not need to prepareseveral column FBAMPs, so there is the significant benefit ofsmaller area and lower power consumption.

Currently, the CCNC and the IP-CCNC can be selectedand used according to the requirements of each application. Ifa more robust operation is required, the CCNC, in which thereference value can be set from outside using FBAMP, can beselected. If we need a high frame rate, a small area, and a lowpower consumption, the IP-CCNC can be selected.2.4 GS technique2.4.1 Photoelectric conversion controlled globalshutter. In this section, we describe the unique GS functionon the basis of the OPF photoelectric conversion character-istics. The GS function is an increasingly powerful technologydriver, not only for solving imaging problems caused byrolling shutter distortion or flash bandings, but also forsensing applications.22–25,27–29) However, conventional siliconimage sensors with the GS function require storage locatednear the photoelectric conversion area,23–25) and the two-stagetransferring pixel structures required to suppress reset noiseneed two storage nodes and extra transistors. In GS pixels, thismakes it difficult to simultaneously decrease the pixel size andenlarge the saturation signals. On the other hand, in the OPFimage sensor, the GS function is realized only by modula-tion of the voltage applied to the OPF.30,31) Therefore, it isunnecessary to add elements, and there is no sensor char-acteristics are sacrificed such as sensitivity, pixel size, andsaturation signals. Moreover, in the OPF image sensor, signalcharges are accumulated in the FD node that can be separatedfrom the OPF.1,2) The FD node and OPF can be verticallylayered, so this structure provides a wide available area ineach pixel. Therefore, the unique features of the OPF imagesensors realize higher saturation compatible with small pixelsize, of approximately 3 µm.

Here, we explain about high-saturation small-size OPFimage sensors with GS function by modulation of a voltageapplied to the OPF. A cross-sectional image of the OPF pixelwith GS function is shown in Fig. 10. The photoelectric

(a)

(b)

Fig. 9. (Color online) Concept and schematic of IP-CCNC. (a) Concept ofIP-CCNC. (b) Schematic of IP-CCNC.

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1002B4-5 © 2018 The Japan Society of Applied Physics

conversion efficiency of the OPF is controlled by modulatinga voltage applied to a transparent electrode (ITO electrode),thus realizing photoelectric-conversion-controlled GS (PCCGS) operation and global sensitivity control. Therefore, whenthe voltage applied to the ITO electrode is high (V1), theshutter is open: optically generated electrons and holes areseparated by an electric field, and holes are collected by apixel electrode. When the voltage applied to the ITO elec-trode is low (V2), the shutter is closed: optically generatedelectrons and holes recombine and are immediately extin-guished. A signal carrier remains stored in FD during PCCGS operation, so there is no need to add storage circuits andcapacitors to the pixel.

Furthermore, the structure of the OPF image sensor hasadvantages in GS operation. The vertical device structure, inwhich the photoelectric conversion area and the storage areaare not placed in the same plane and have a sufficiently highrecombination speed, are advantageous in lowering parasiticlight sensitivity (PLS).

ITO voltage control starts during the vertical blankingperiod. The sensor frame rate is determined by the inverse ofthe sum of exposure time and readout scanning time.2.4.2 Multiple exposure by non-destructive accumu-lation. Figure 11(a) shows a multiple-exposure image ob-tained by global sensitivity control of OPF. PCC GS opera-tion is executed by applying a pulse voltage to ITO, andapplying multiple pulses enables multiple exposures, becausesignal carriers remain stored in the FD node until the FDnode is reset. As shown in Fig. 11(b), changing the pulseduty or pulse voltage in each exposure enables variable-sensitivity multiple exposures.31)

Therefore, in the case of the GS function of the OPF imagesensor, it is possible to capture multiple images for detectingmotion in a single readout image, and it is possible to reducethe amount of data at the processing stage. Moreover, multi-

ple exposures with a fixed sensitivity cannot detect thedirection of motion, but those with variable sensitivity candetect it, on the basis of a change in object brightness.2.5 Development of 8K4K high-resolution high-framerate, high-saturation global shutter image sensorIn this section, we introduce the 8K4K sensor test chip fab-ricated using the OPF technologies described above inresponse to the market demand for high-resolution sensors.Recently, there has been a growing demand for high-resolu-tion and high-reality images for use in broadcasting, surveil-lance, and various camera systems. The conventional papersreported on the promotion of research and development of 8Kultra-high-definition television (UHDTV) systems and thedevelopment of 8K full-resolution cameras26) and 8K 240 fpscameras with layered sensors.32) In these camera systems, aRS method is used for scanning, since a global shutter methodhas an area tradeoff between the photoelectric conversionregion and the charge storage region.21,33,34) However, thisleads to a shutter distortion problem during high-speedimaging and synchronization of multi-viewpoint imaging.

With this background, our OPF image sensor becomes oneoptimum candidate. In the OPF image sensor structure, thephotoelectric conversion part and the charge storage andsignal readout parts are completely independent. Therefore,by using photoelectric-conversion-controlled global shutterfunction and by incorporating high-functional circuits in awide available area of the readout part, high-resolution,global shutter, high-saturation and high-frame rate can berealized simultaneously. Attaining these at the same time isadvantageous.

By using the OPF sensor, we aimed to develop a CMOSimage sensor with 3 µm pixel size, 8K4K resolution, 60 fpsframe rate, and 450 ke− saturation signals. With this structure,

Fig. 10. (Color online) Cross-sectional image of pixel with GS function.

(a)

Sensitivity modulation by pulse duty Sensitivity modulation by pulse voltage(b)

Fig. 11. (Color online) Multiple exposures by photoelectric conversioncontrol. (a) Multiple-exposure image with variable sensitivities. (b) Drivingmethods for variable sensitivities.

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GS function can be realized without deterioration of thesaturation signals even with a small 3 µm pixel.

In this development, to achieve a readout speed of 60 fpswith a resolution of 8K4K, three potential strategies must berealized: 1) high-speed cancellation of reset noise in singlestorage-type global shutter pixels, 2) high-speed readout witha long vertical signal line, and 3) high-saturation in the globalshutter mode. For these three strategies, we have made thefollowing initiatives.20)

For 1), there is a long noise suppression time due to thelarge time constant of the vertical signal line. Therefore, theIP-CCNC, described in Sect. 2.3.2, has been developed toshorten the noise suppression time even when the lengthof the vertical signal line is four times longer than that witha FHD sensor. Moreover, to utilize the advantages of theOPF image sensors layered structure, high-capacitance MIMcapacitors have been allocated in the metal interconnect area.

For 2), since the photoelectric conversion film is present atthe upper layer, the photoelectric conversion characteristicsare not affected, even when the number of vertical signallines is increased. Therefore, we can form multiple high-speed readout circuits (MHRC); two pairs of quadruplevertical signal lines are prepared in each pixel, and 16sample-and-hold (S=H) capacitors are provided in eachcolumn.

For 3), a high saturation mode structure that can be realizedonly by switching the gate voltage of RST as described inSect. 2.3.2 was realized without increasing the pixel size.

3. Evaluation results

In this section, we report the evaluation results of the WDRsensor with the DS-Pixel and 8K4K high-resolution sensorthat can realize the GS function. A chip micrograph of theWDR sensor is shown in Fig. 12. The OPF image sensoris fabricated using 65 nm 1P3Cu1Al CMOS technology.This sensor is designed with a pixel size of 6 µm and an effec-tive pixel number of 970H × 550V. It realizes 60 fps digitalreadout using the 1.404Gbps sub-LVDS interface. Thesupply voltages are 3.3V for analog and 1.2V for digital.

Figure 13 shows the photoelectric conversion character-istics of the DS-Pixel. The full well capacity in the high-saturation mode is 600 ke−, which is 100 times higher thanthat in the high-sensitivity mode. The random noise of thesensor is 5.4 e−. The sensitivity ratio between two cells is14 : 1. Therefore, a SC-WDR of 123.8 dB is achieved.

Figure 14 shows the calculation result of the S=N char-acteristics of the DS-Pixel. The left part shows the high-sensitivity cell characteristics. Esignal is the number of signalcharges. Enoise is the number of noise charges. We considercircuit noise (Ecircuit-noise), shot noise (Eshot-noise), and photoresponse non-uniformity (PRNU) (EPRNU). Total noise is thesquare root of these and expressed by Eq. (5). The signal tonoise ratio is expressed by Eq. (6).

Enoise ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiE2circuit-noise

pþ E2

shot-noise þ E2PRNU ð5Þ

S=Nhigh-sensitivity ¼ 20 � logðEsignal=EnoiseÞ ð6ÞThe right part shows the high saturation cell character-

istics. The sensitivity ratio between two cells is 14 : 1.Therefore, considering this ratio, the high saturation cellcharacteristics are increased 14 times. The signal to noiseratio is expressed by Eq. (7).

S=Nhigh-saturation ¼ 20 � logðEsignal=Enoise � 14Þ ð7ÞThe signal to noise ratio at the switching point is held at aminimum of 31 dB, because the saturation level of the OPFimage sensor is high, and the sensitivity ratio is only 14 : 1.This S=N characteristic leads to high-robust operation forimaging and sensing.

Figure 15 shows the characteristics of noise suppressionusing two types noise canceller, the CCNC and the IP-CCNC. The WDR sensor (2M resolution) pixel reset noise issuppressed to 1.6 e− by 5.0 µs when using the CCNC tech-nology. The 8K4K sensor (32M resolution) pixel reset noiseis suppressed to 2.5 e− by 2.5 µs when using the IP-CCNCtechnology. In particular, for the 8K4K sensor, noise sup-pression by up to 2.5 e− has been realized in half the time bythe CCNC, even though it has 4 times the number of rows.

Figure 16 shows the SC-WDR images captured by theOPF image sensor. The upper-left panel in Fig. 16 shows an

Fig. 12. (Color online) Chip micrograph of WDR OPF sensor.

Fig. 13. (Color online) Photoelectric conversion characteristics of WDROPF sensor.

Fig. 14. (Color online) S=N characteristics of WDR OPF sensor.

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image taken with the high-saturation cell. A car and a trafficsignal are imaged clearly. The lower-left panel shows animage taken with the high-sensitivity cell. The colorfulsurrounding blocks can be seen clearly, and the right panelshows a total image from our OPF image sensor. The precisetone in the whole range is reproduced by weighting, usingdata from both cells.

The high-speed images captured by the two types of imagesensors are shown in Fig. 17. The left image is taken with asilicon image sensor and the right image with the OPF imagesensor. In the case of using the conventional silicon imagesensor, a wide dynamic range is realized by synthesizingmultiple exposures. The deviation of these exposure timingscauses motion blur at high-speed imaging, especially, of high-speed moving tires or rear bumpers. On the other hand,motion blur is completely prevented when using the OPFimage sensor, because the exposure times are the samebetween two cells. Additionally, in this structure, no framememory to store the data of multiple exposures is necessary,which enables to shrink the logic circuits area.

Moreover, the OPF image sensor structure is also aneffective solution to resolve the recent LED flicker problemfor in-vehicle, surveillance, and broadcasting cameras.Figure 18 shows images of an LED traffic sign. In the caseof common silicon image sensors, although some lines ofpixels are captured; the LED lights on-time and the imagesensor exposure time did not overlap. Therefore, parts ofsignals cannot be imaged, and LED flicker happens. On theother hand, in the case of the OPF image sensor, the high-saturation cell always captures with low sensitivity, exceptduring the readout period, when capturing pauses. Therefore,

all signals can be imaged, and the LED flicker problem iscompletely prevented.

Figure 19 shows the images captured in the rolling shutterand global shutter modes. Both images are captured by thesame OPF sensor, changing only the driving method. Thehigh-speed moving object in the upper images in Fig. 19, isa fan that rotates at 1000 rpm. In the rolling shutter mode,shutter distortion occurs with the scanning from the top tothe bottom. However, no distortion occurs in the globalshutter mode. As shown in the lower images in Fig. 19, inthe rolling shutter mode, part of the image becomes brightonly when a flash of light is irradiated, and flash bandingoccurs. However, in the global shutter mode, since all thepixels are imaged simultaneously, such a problem does notoccur.

Fig. 15. (Color online) Noise suppression characteristics of CCNC andIP-CCNC.

Fig. 16. (Color online) Image of 123.8 dB SC-WDR.

Fig. 17. (Color online) Images of high-speed object.

Fig. 18. (Color online) Images of LED traffic signal.

Fig. 19. (Color online) Images captured in RS and GS modes.

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1002B4-8 © 2018 The Japan Society of Applied Physics

A chip micrograph of the 8K4K sensor is shown inFig. 20. The 8K4K image sensor is fabricated using 65 nm1P4Cu1Al CMOS technology. This sensor is designed with apixel size of 3 µm, a total pixel number of 8,800H × 4,548V,and an effective pixel number of 8,192H × 4,320V. Itrealizes 60 fps digital readout using the 1.404Gbps sub-LVDS interface. The supply voltages are 3.3V for analogand 1.2V for digital.

Figure 21 shows the photoelectric conversion character-istics. The full well capacity of 45 ke− in the high-sensitivitymode or 450 ke− in the high-saturation mode is achieved bysimply changing RST gate voltage. This full well capacitycan be realized in both the rolling shutter mode and globalshutter mode. The saturation signals per unit squared in thehigh-saturation mode is 20 dB higher than that in the high-sensitivity mode, and 10 dB higher than that when usingthe global shutter type silicon sensor with a special high-saturation structure.

Figure 22 shows images captured by the fabricated OPFimage sensor. High-definition 8K4K resolution images wareobtained. It also shows high-saturation mode ON-state andOFF-state images, rolling shutter mode and global shuttermode images taken using the 8K4K OPF sensor. In thehigh-saturation mode, the fine winding structure of the lampfilaments can be captured clearly. In the global shutter mode,letters on the rotating fan can be read sharply withoutdistortion. This means that, since all the functions can berealized simultaneously imaging and sensing with no con-straints in various environments will be possible.

4. Conclusions

Table I shows the performance summary of the WDR OPFimage sensor and 8K4K OPF image sensor we developed.The image sensor for WDR was fabricated using a 65 nmCMOS technology. The pixel size is 6 µm. The number ofpixels is 970H × 550V. 600 ke− high-saturation and 123.8dB SC-WDR were achieved. Pixel reset noise can be reducedto 1.6 e− by 5.0 µs. Both the rolling shutter and global shuttermodes can be operated by simply changing the drivingmethods. In the global shutter mode, the global shutterspeed of 1=400000 s and the PLS of −100 dB are achieved.Furthermore, in the case of using the high-capacitance andlow-leakage-current MIM, the pixel size is 3 µm. The numberof pixels is 1.940H × 1,100V. 489 ke− high-saturation and121 dB SC-WDR were achieved. Other sensor characteristicsare almost similar to those of the 6 µm sensor. The imagesensor for 8K4K was fabricated using a 65 nm CMOS tech-nology. The pixel size is 3 µm. The number of pixels is8800H × 4548V. A high resolution of 8K4K, a high framerate of 60 fps, and a high saturation of 450 ke− are realizedsimultaneously. Pixel reset noise can be reduced to 2.5 e− by2.5 µs. Both the rolling shutter and global shutter modescan be operated by simply changing the driving methods, aswell. In the global shutter mode, the global shutter speed of1=65000 s, and a PLS of −110 dB are achieved. These are thehighest performances ever reported.

In this paper, we introduced two technologies and evalua-tion results, which show the most advantageous features ofthe OPF sensor we have developed. In the future, demand forimage sensors will shift from capturing precise images torecognizing capturing data, predicting the next action, andseeing something invisible. It is predicted that the readoutdata of the image sensor will not be seen by the human eyes,but will be read by machines. Therefore, it is required notonly to increase specs but also to add functions to raiserecognition rate and analytical ability. Therefore, we areaiming to meet such market needs and even to create newmarkets that contribute to industry and human life, throughour approach to design independently the photoelectric con-version part and the charge storage and signal processingparts.

Fig. 20. (Color online) Chip micrograph of 8K4K OPF sensor.

Fig. 21. (Color online) Photoelectric conversion characteristics of 8K4KOPF sensor.

Fig. 22. (Color online) Images captured by fabricated 8K4K OPF sensor.

Jpn. J. Appl. Phys. 57, 1002B4 (2018) STAP ARTICLE

1002B4-9 © 2018 The Japan Society of Applied Physics

Acknowledgments

We would like to thank the engineers of Panasonic Semi-conductor Solutions Co., Ltd., Semiconductor Business Unitand Panasonic Corporation Automotive & Industrial SystemsCompany Engineering Division for support in chip design.

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Table I. Performance summary. NC: noise cancellation mode. HS: high saturation mode.

OPF sensorfor WDR

OPF sensorfor 8K4K

Ref. 26 Ref. 32 Ref. 33 Ref. 34 Ref. 21

Process technology65 nm

1P3Cu1AlCMOS

65nm1P4Cu1AlCMOS

180 nm 1P4MCMOS

45nm 1P4MstackedCMOS

180nmCMOS

130nm 1P4MCMOS

180 nm=90 nmCMOS

Number of pixels total — 8,800 × 4,548 15,488 × 8,776 9,600 × 5,396 — 2,676 × 2,200 —

Number of pixels effective970 × 550

1,940 × 1,100a)8,192 × 4,320 15,360 × 8,640 7,728 × 4,368 7,920 × 6,004 2,592 × 2,054 (1.3Mpixel)

Pixel size (µm × µm)6.0 × 6.03.0 × 3.0a)

3.0 × 3.0 2.45 × 2.45 1.1 × 1.1 4.6 × 4.6 3.4 × 3.4 3.875 × 3.875

Frame rate (fps) 60 60 60 240 30 120, 60 —

ADC resolution (bit) 12 12 14 12 12 12 —

Full well capacity (e−)600k489ka)

45k at NC450k at HS

10k 5.7k 14.1k8k at 120 fps16k at 60 fps

224k

Full well capacity perunit square (e−=µm2)

16.7k54.3ka)

5k at NC50k at HS

1.7k 4.7k 0.7k0.7k at 120 fps1.4k at 60 fps

14.9k

Random noise (e−)5.46.2a)

8.6 7.684.5 at gain13.6 at gain4

8.8 1.8 —

Global shutter function Available Available Unavailable Unavailable Available Available Available

Global shutter speed (s) 1=400000 1=65000 — — — — —

PLS (dB) −100 −110 — — −86 −89 −83

Power (W) — 8.4 11.0 3.0 — 0.45 —

a) In the case using MIM capacity

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