A High Full Well Capacity CMOS Image Sensor for Space Applications

Article A High Full Well Capacity CMOS Image Sensor for Space Applications

Woo-Tae Kim 1 , Cheonwi Park 1, Hyunkeun Lee 1 , Ilseop Lee 2 and Byung-Geun Lee 1,* 1 School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology, Gwangju 61005, Korea; [email protected] (W.-T.K.); [email protected] (C.P.); [email protected] (H.L.) 2 Korea Aerospace Research Institute, Daejeon 34133, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-62-715-3231

Received: 24 January 2019; Accepted: 26 March 2019; Published: 28 March 2019

Abstract: This paper presents a high full well capacity (FWC) CMOS image sensor (CIS) for space applications. The proposed pixel design effectively increases the FWC without inducing overﬂow of photo-generated charge in a limited pixel area. An MOS capacitor is integrated in a pixel and accumulated charges in a photodiode are transferred to the in-pixel capacitor multiple times depending on the maximum incident light intensity. In addition, the modulation transfer function (MTF) and radiation damage effect on the pixel, which are especially important for space applications, are studied and analyzed through fabrication of the CIS. The CIS was fabricated using a 0.11 µm 1-poly 4-metal CIS process to demonstrate the proposed techniques and pixel design. A measured FWC of 103,448 electrons and MTF improvement of 300% are achieved with 6.5 µm pixel pitch.

Keywords: CMOS image sensors; wide dynamic range; multiple charge transfer; space applications; radiation damage effects

1. Introduction Imaging devices are essential components in the space environment for a range of applications including earth observation, star trackers on satellites, lander and rover cameras [1]. In space applications, charge coupled devices (CCDs) have been used for imaging devices because they can achieve low noise and high image quality. However, there are constraints for CCDs because of radiation tolerance, power, and size. The performance of CCDs is very sensitive to radiation in space. Charge transfer efficiency (CTE) of CCDs is degraded by proton irradiation, which causes position shifts of objects in images and leads star trackers to cause uncorrectable errors [2]. In addition, CCDs have a high power consumption because many external components and high supply voltages are necessary for their operation. These facts make CCDs less effective in the space environment. Therefore, CCDs have gradually been replaced by CMOS image sensors (CISs) due to their benefits, e.g., high level of integration, timing and control in a single chip, low power operation, and better radiation tolerance. CISs can produce high quality images by means of technical development, such as pinned photodiode (PPD), correlated double sampling (CDS), and recently developed circuit topology for low noise readout. Moreover, there is a growing interest in small spacecraft and satellites with lower weights and smaller sizes due to lower development costs and shorter lead times [3]. High integration ability of CISs is appropriate for small satellite missions. The main specifications of commercial CISs include quantum efficiency (QE), dynamic range (DR), saturation level, modulation transfer function (MTF), signal to noise ratio (SNR), dark current, image lag, non-uniformity, and non-linearity of the photon response. Among them, DR and MTF are especially important for space applications and these specifications should be maintained in radiation environments.

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The DR is a quantity that describes the range of light intensity in an image. A high-DR image sensor can produce images with low to high light levels in an exposure time. It is defined as the ratio between the maximum saturated pixel output level and noise level in the dark [4]. The DR can be calculated as N Dynamic range (DR) = 20 log sat [dB], (1) Ndark where Nsat is the number of electrons collected by a pixel at saturation level, which is also referred to as full well capacity (FWC), and Ndark is the number of electrons at noise level without illumination. The improvement of DR is an important issue not only for image quality but also for accuracy of space sensor applications [5,6]. For example, star trackers use known star positions in an image taken with CISs on a spacecraft. An algorithm of the star tracker finds centroids of stars by using blurred spots. Because the wide dynamic range (WDR) of CIS makes the blurred spot more informative, the accuracy of the star tracker is increased with rapid determination of centroids [7]. Methods for achieving WDR can be categorized as a non-linear or linear. Previous reports have described DR extension with non-linear methods by several means. A well capacity adjusting method controlling an overflow gate caused low SNR because of the non-linear response of charge collection [8]. A logarithmic active pixel sensor using logarithmic response of MOS transistor in a weak inversion region caused high fixed pattern noise (FPN), which limits SNR because of large threshold voltage variation [9]. Linear-logarithmic pixel was also introduced to compensate the degradation of SNR, but the pixels were based on the three-transistor (3T) pixel structure, which has poor noise performance compared to a four-transistor (4T) pixel structure [10]. The linear method for the WDR is to increase FWC physically. In previous works [11,12], the FWC is required to be larger than 75,000 e− for space applications. To satisfy this requirement, large pixel pitch was simply adopted. If high resolution is required, the chip size of CISs becomes large, so the manufacturing cost of the image sensor is increased and optical components also become bulky, which is not recommended for small satellite applications. Therefore, a demand for high FWC with small pixel pitch has been raised. Modulation transfer function (MTF) is another important specification for quantifying image quality. It defines the ability of an optical system to resolve a contrast at spatial frequency. Higher MTF makes small objects distinguishable on images. For example, earth observation satellites, which observe geographical features, weather, and surveillance, require high MTF to distinguish over several meters or centimeters. The major cause of MTF degradation is the crosstalk between neighboring pixels, especially for small-pitch pixels [13]. There can be both electrical and optical crosstalk depending on the cause. Electrical crosstalk is the phenomenon that photo-generated electrons move to the neighboring pixel through the substrate. This can be mitigated by shallow trench isolation (STI) and deep trench isolation (DTI) processes. Optical crosstalk is caused by undesirable photo generated electrons as a result of light incident in the lateral direction of the pixel. There are two approaches for degradation of optical crosstalk. The first approach prevents light from entering the PPD of adjacent pixels using a light guide, which refracts undesirable directions of light [14]. This significantly reduces the optical crosstalk but increases cost due to the additional process. Another way, which is adopted in this work, is to physically block the lateral light using a metal layer placed around edges of the PPD [15]. While most electrical parts equipped in a spacecraft have radiation shielding using aluminum or other metallic materials, CISs are directly exposed to outer space for image capturing. This requires CISs to have an extremely high level of radiation tolerance. The effects of radiation on CMOS technology, including MOSFET, capacitors, and various pixel structures, have been individually researched previously [16,17], but the pixel with an in-pixel capacitor has not been studied before. In this paper, a modified 4T pixel with a multiple charge transfer technique is proposed to extend the FWC without a non-linear response or SNR degradation. The measurement results show that the pixel can hold more than 100,000 electrons by means of the technique in a 6.5 µm pixel pitch. This value Sensors 2019, 19, 1505 3 of 16 is approximately two times higher than in previously reported works [18–23]. Even though the MTF of a pixel can be improved either by using a specified fabrication process or design, these approaches couldSensors potentially 2019, 19,increase x the radiation damage effects. Therefore, metal shielding [24] was adopted3 of 16 to maintain the radiation tolerance of pixel devices. Two sensor chips were fabricated, one with metal shieldedapproaches pixels and could the potentially other without increase metal the shielding, radiation damage and the effects. measurement Therefore, results metal show shielding that [24] an MTF was adopted to maintain the radiation tolerance of pixel devices. Two sensor chips were fabricated, improvement of approximately 307% was achieved by using metal shielding. In addition, the radiation one with metal shielded pixels and the other without metal shielding, and the measurement results damage effects of an in-pixel MOS capacitor are also investigated by comparing the performance of show that an MTF improvement of approximately 307% was achieved by using metal shielding. In the sensoraddition, before the and radiation after irradiation.damage effects of an in-pixel MOS capacitor are also investigated by Thecomparing remainder the performance of this paper of the is sensor organized before as and follows. after irradiation. We describe the pixel configuration of the proposedThe CISremainder with anof this in-pixel paper MOS is organized capacitor as follows. and its We operation describe in the Section pixel configuration2. The prototype of the CIS implementationproposed CIS results, with an characteristics, in-pixel MOS capacitor and evaluation and its operation results are in presentedSection 2. The in Sectionprototype3. Finally,CIS conclusionsimplementation are given results, in Section characteristics,4. and evaluation results are presented in Section 3. Finally, conclusions are given in Section 4. 2. Sensor Architecture 2. Sensor Architecture Figure1 shows a block diagram of the CIS and simplified readout circuit. The chip is comprised Figure 1 shows a block diagram of the CIS and simplified readout circuit. The chip is comprised of a 3000 (H) × 3000 (V) pixel array, column readout circuits with buffer amplifiers, row shift registers, of a 3000 (H) × 3000 (V) pixel array, column readout circuits with buffer amplifiers, row shift registers, and Tx controllers. and Tx controllers.

Figure 1.FigureCMOS 1. CMOS image image sensor sensor (CIS) (CIS) block block diagram. diagram. CSR, CSR, column column shift shift register; register; ADCs, ADCs, analog analog to to digital converters. digital converters.

ThereThere are 10are channel 10 channel readout readout circuits circuits in in the the CIS.CIS. TheThe one channel channel of of column column readout readout circuits circuits consistsconsists of a switchedof a switched capacitor capacitor amplifier amplifier and and unitunit gaingain buffer, which which use use a high a high gain gain operational operational amplifier.amplifier. In one In channel, one channel, 300 sets 300 ofsets two of capacitorstwo capacitors each each were were used used for samplingfor sampling reset reset and and signal signal values out ofvalues the 300-pixel out of the columns. 300-pixel A columns. column A shift column register shift (CSR)register sequentially (CSR) sequentially selects selects one of one 300 of capacitor 300 sets, andcapacitor the sampled sets, and values the sampled are outputted values inare the outputt formed of analogin the form voltage. of analog The output voltage. voltages The output from the voltages from the 10 channels are converted to digital values by ten off-chip analog to digital 10 channels are converted to digital values by ten off-chip analog to digital converters (ADCs). A row converters (ADCs). A row shift register (RSR) approaches each row in sequence starting at the first. shift registerThe multiple (RSR) charge approaches transfer each is controlled row in sequence by the Tx starting controllers, at the which first. is The similar multiple to RSR. charge All pixel transfer is controlledcontrol signals by the are Tx buffer controllers,ed by digital which drivers. is similar to RSR. All pixel control signals are buffered by digital drivers.

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2.1. ProposedSensors Pixel 2019, 19 Design, x with Multiple Charge Transfer 4 of 16 Figure2.1.2 Proposeda,b show Pixel a simpliﬁed Design with schematicMultiple Charge and Transfer layout of the proposed pixel. The pixel is similar to a Sensors 2019, 19, x 4 of 16 conventionalFigure 4T pixel 2a,b except show a thatsimplified a single schematic MOS transistorand layout of is the added proposed to the pixel. ﬂoating The pixel diffusion is similar (FD) to node as a charge2.1. aProposed conventional storage Pixel capacitor Design4T pixel with forexcept Multiple the that multiple aCharge single charge TransferMOS transistor transfer technique.is added to the floating diffusion (FD) node as a charge storage capacitor for the multiple charge transfer technique. Figure 2a,b show a simplified schematic and layout of the proposed pixel. The pixel is similar to a conventional 4T pixel except that a single MOS transistor is added to the floating diffusion (FD) node as a charge storage capacitor for the multiple charge transfer technique.

(a) (b) Figure 2. FigureProposed 2. Proposed pixel for pixel multiple for multiple charge transfercharge transfer technique: technique: (a) Schematic (a) Schematic of the of pixel; the pixel; (b) Simplified (b) pixel layout.Simplified pixel layout. (a) (b) The pixel maintains beneficial properties of the conventional 4T pixel, such as low noise The pixel maintains beneficial properties of the conventional 4T pixel, such as low noise operation operationFigure 2. andProposed high fill pixel factor, for unlikemultiple other charge specified transfer pixel technique: architectures (a) Schematic for WDR of operation the pixel; [10,25]. (b) and highAlso, fillSimplified factor, the pixel pixel unlike routing layout. other is simpler specified than pixel in other architectures WDR pixels for because WDR no operation additional [10 control,25]. Also,signal the is pixel routing isrequired. simpler In than general, in other the FWC WDR is pixels limited because by PPD nocapacitance, additional which control depends signal on isthe required. PPD size Inand general, the FWCdoping isThe limited pixel profile bymaintains [26,27]. PPD capacitance,The beneficial PPD capacitance properties which can depends ofbe increasedthe conventional on theby expanding PPD 4T size pixel, its and size dopingsuch and adjustingas profilelow noisethe [ 26 ,27]. The PPDoperationshape capacitance ofand the high photodiode can fill be factor, increased itself unlike [4]. byHowever, other expanding specified p-n junc its pixeltion size capacitance andarchitectures adjusting of photodiodefor the WDR shape operation per of unit the area photodiode[10,25]. is itself [Also,4].not However, the sufficient pixel routing p-n to increase junction is simpler the capacitance considerable than in other ofamount photodiode WDR of pixelsFWC within perbecause unit the no arealimited additional is notpixel sufficient size.control Increasing signal to increase is required.the doping In general, profile theof the FWC PPD is increaseslimited by the PPD PPD capacitance, capacitance whichand FWC depends by reducing on the the PPD depletion size and the considerable amount of FWC within the limited pixel size. Increasing the doping profile of the dopingdepth profile between [26,27]. the ThePPD PPD and capacitance the substrate. can However, be increased only by controlling expanding the its sizePPD and doping adjusting cannot the PPD increases the PPD capacitance and FWC by reducing the depletion depth between the PPD and shapesufficiently of the photodiode increase the itself FWC [4]. for However, space applications. p-n junc tion capacitance of photodiode per unit area is the substrate.not sufficientIn However,this to work, increase onlyinstead the controlling considerableof directly theincreasing amount PPDdoping ofthe FWC PPD cannotwithin capacitance, the sufficiently limited the FWC pixel increase is size. enhanced Increasing the FWCby for spacethe applications. transferringdoping profile photo-charges of the PPD to increases the enlarged the PPDFD node capacitance multiple and times. FWC The by use reducing of an in-pixel the depletion MOS Indepth thiscapacitor between work, increases insteadthe PPD the of andcapacitance directly the substrate. increasingof the FDHowever, no thede and PPD only photo-generated capacitance, controlling the thecharge PPD FWC can doping isbe enhancedlinearly cannot by integrated on the FD node without information loss. transferringsufficiently photo-charges increase the FWC to the for enlargedspace applications. FD node multiple times. The use of an in-pixel MOS Figure 3 illustrates a potential diagram of the pixel for the multiple charge transfer. After the In this work, instead of directly increasing the PPD capacitance, the FWC is enhanced by capacitorreset increases of the PPD the and capacitance FD node as shown of the in FD Figure node 3a, the and integration photo-generated of charges generated charge canby photon be linearly transferring photo-charges to the enlarged FD node multiple times. The use of an in-pixel MOS integratedis started. on the Before FD node the integrated without informationcharges of the loss.PPD are overflowed, the charges are transferred to the capacitor increases the capacitance of the FD node and photo-generated charge can be linearly Figurestorage3 illustrates capacitor, aas potential shown in Figure diagram 3b,c. Figure of the 3d pixel illustrates for the the multiple output signal charge level after transfer. the charge After the integrated on the FD node without information loss. reset of thetransfer PPD is and repeated FD node three as times. shown The in multiple Figure 3cha,arge the transfer integration improves of charges the SNR generated because signals by photon Figure 3 illustrates a potential diagram of the pixel for the multiple charge transfer. After the is started.generated Before from the integratedthe photo charges charges tend of to thebe correlated PPD are whereas overflowed, noise is the uncorrelated. charges areFor transferredexample, to resetthe of signal the PPD and and noise FD are node increased as shown by a infactor Figure of N 3a and, the √ integration𝑁, respectively, of charges where thegenerated N is the by number photon the storage capacitor, as shown in Figure3b,c. Figure3d illustrates the output signal level after the is started.of charge Before transfer. the integratedAs a result, charges SNR is increasedof the PPD by are a factor overflowed, of √𝑁. the charges are transferred to the chargestorage transfer capacitor, is repeated as shown three in times. Figure The3b,c. multiple Figure 3d charge illustrates transfer the output improves signal the level SNR after because the charge signals generatedtransfer from is repeated the photo three charges times. tend The tomultiple be correlated charge√ transfer whereas improves noise is the uncorrelated. SNR because For signals example, the signalgenerated and noisefrom the are photo increased charges by atend factor to be of correlated N and Nwhereas,√ respectively, noise is whereuncorrelated. the N For is the example, number of chargethe transfer. signal and As noise a result, are increased SNR is increased by a factor by of a N factor and √ of𝑁, respectively,N. where the N is the number of charge transfer. As a result, SNR is increased by a factor of √𝑁.

(a) (b)

Figure 3. Cont.

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FigureFigureFigure 3. 3. PotentialPotential 3. Potential diagram diagram diagram of of the theof themultiple multiple multiple charge charge charge transfer: transfer:transfer: ( (a aa)) ) InitialInitial Initial potentialpotential potential of of ofpinned pinned pinned photodiode photodiode photodiode (PPD)(PPD) and(PPD) and floating ﬂoating and floating diffusion diffusion diffusion (FD) (FD) (FD) node; node; node; ( (bb) () bIntegration Integration) Integration of ofof photo-gene photo-generatedphoto-generatedrated charges charges charges in in PPD; in PPD; PPD; (c) (Chargec (c) )Charge Charge transfertransfertransfer to to the the toFD FD the node; node; FD node; ( (dd)) Accumulated Accumulated(d) Accumulated charges charges charges in in in FD FD FD node nodenode afterafter multiple multiple charge charge charge transfer. transfer. transfer.

2.2.2.2. Pixel Pixel2.2. Operation Pixel Operation Operation for for Multiple Multiplefor Multiple Charge Charge Charge Transfer Transfer Transfer Figure 4a and b show the timing diagrams of single and triple charge transfer operations for the FigureFigure 4a4a and and b b show show the the timing timing diagrams diagrams of ofsingle single and and triple triple charge charge transfer transfer operations operations for the for pixel control signals, respectively. The pixel readout process during TREAD will be explained in Section the pixel control signals, respectively. The pixel readout process during TREAD will be explained in pixel control2.4. First signals, of all, one respectively. of the 3000 rowsThe pixel in the readout pixel array process is selected during by usingTREAD the will row be selectexplained signal in (SEL), Section Section 2.4. First of all, one of the 3000 rows in the pixel array is selected by using the row select signal 2.4. Firstand of in all, case one of ofsingle the charge3000 rows transfer, in the the pixel photo-ge arraynerated is selected charge by in usingthe PPD the is row transferred select signal to the FD(SEL), and(SEL), incapacitor case and of in casesingle when of thecharge single transfer transfer, charge gate control transfer, the photo-ge signal the (Tx) photo-generatednerated is high. charge The pixel in charge thesignal PPD in value the is transferred PPDis then is sampled transferred to the by FD to capacitorthe FDa sampling capacitor when the capacitor when transfer the in gate transferthe controlreadout gate signalcircuit control (Tx)when signal is thehigh. signal (Tx) The is samplingpixel high. signal The signal pixelvalue (PSIG signalis) thenis high. value sampled After is then by asampled samplingsampling by capacitor a sampling the pixel in signal capacitorthe readoutvalue, in th the ecircuit FD readout node when is circuit reset the to whensignal an initial the sampling signalvoltage sampling whensignal the (P signalRSTSIG) isis (P high.high.SIG )The isAfter high. samplingAfterreset sampling the value pixel theof thesignal pixel FD node signalvalue, is value,finallythe FD sampled the node FD is node using reset is a to resetsampling an initial to an capacitor initialvoltage voltagein when the readout whenthe RST circuit the is RST high. for isan high.The resetThe resetvalueanalog value of delta the of FDreset the node sample FD is node finally (DRS) is ﬁnallyoperation sampled sampled [28]. using Starting a using sampling from a sampling the capacitor first row, capacitor in this the procedure readout in the readout is circuit repeated for circuit an analogfor anuntil delta analog the reset last delta samplerow reset is processed. (DRS) sample operation (DRS) operation [28]. Starting [28]. from Starting the first from row, the ﬁrstthis procedure row, this procedure is repeated is untilrepeated the last until row the is last processed. row is processed.

(a) (b)

Figure 4. Timing diagram: (a) Single charge transfer; (b) Multiple charge transfer.

The control signals (a) for the multiple charge transfer operation are the same(b) as the ones for the single charge transfer operation except that the Tx becomes high multiple times. As exemplified for triple chargeFigureFigure transfer 4. 4. TimingTiming in Figure diagram: diagram: 4b, (thea (a) )Single photo-char Single charge chargege transfer; is transfer; transferred (b (b) )Multiple Multiple twice by charge charge making transfer. transfer. the Tx high two times before the SEL becomes high, and the last charge transfer is performed when the pixel row is TheTheselected, control control as withsignals signals the forsingle for the the charge multiple multiple transfer charge charge operation. transf transfer er operation operation are are the the same same as as the the ones ones for for the the singlesingle charge charge transfer transfer operation operation except except that that the the Tx Tx becomes becomes high high multiple multiple times. times. As As exemplified exempliﬁed for for tripletriple charge charge transfer transfer in in Figure Figure 4b,4b, the photo-char photo-chargege is transferred twice by making the Tx high two timestimes before before the the SEL SEL becomes becomes high, high, and and the the last last char chargege transfer transfer is is performed performed when when the the pixel pixel row row is is selected,selected, as as with with the the single single charge charge transfer transfer operation. operation.

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Notice that the main difference from the conventional 4T pixel operation is that the operation order Sensors 2019, 19, x 6 of 16 for the pixel reset and the photo-charge transfer is switched. While the photo-charge is transferred to the FD nodeNotice after that the the reset main in difference the conventional from the 4T conventi pixel, theonal charge 4T pixel is transferredoperation is before that the the operation pixel reset, whichorder allows for the for pixel multiple reset charge and the transfer. photo-charge transfer is switched. While the photo-charge is transferred to the FD node after the reset in the conventional 4T pixel, the charge is transferred before 2.3.the Pixel pixel Design reset, for which Modulation allows for Transfer multiple Function charge transfer. The MTF, which can be measured in vertical and horizontal directions, is degraded by crosstalk 2.3. Pixel Design for Modulation Transfer Function between adjacent PPDs. One of effective ways to minimize crosstalk is to maximize the distance betweenThe the MTF, neighboring which can be PPDs. measured However, in vertical this and requires horizontal smaller directions, PPD is area degraded for a by limited crosstalk pixel size,between resulting adjacent in smaller PPDs. ﬁll One factor of effective and quantum ways to efﬁciency minimize (QE).crosstalk Figure is to5a,b maximize show top-down the distance and cross-sectionbetween the views neighboring of the twoPPDs. adjacent However, pixels. this Inrequires Figure smaller5a, the PPD PPD, area active for devices,a limited and pixel in-pixel size, capacitorresulting are in placedsmaller to fill maximize factor and the quantum PPD area. effici Theency control (QE). Figure signals 5a,b are show routed top-down horizontally and cross- not to interferesection with views the of incidentthe two adjacent light for pixels. high QE. In Figure In this 5a, design, the PPD, the active vertical devices, MTF isand much in-pixel better capacitor than the horizontalare placed MTF to maximize because the the distance PPD area. between The control the adjacent signals are PPDs routed in the horizontally vertical direction not to interfere is much with larger thanthe that incident in the light horizontal for high direction QE. In this due design, to the the active vertical devices MTF and is much in-pixel better MOS than capacitor the horizontal placed MTF under because the distance between the adjacent PPDs in the vertical direction is much larger than that in the PPD. the horizontal direction due to the active devices and in-pixel MOS capacitor placed under the PPD.

(a) (b)

FigureFigure 5. Layout5. Layout of of pixels pixels and and cross cross section section view:view: ((aa)) Proposed pixel pixel layout layout including including MOS MOS capacitor capacitor withoutwithout metal metal shielding; shielding; (b) ( Crossb) Cross section section view view without without metal metal shielding; shielding; (c) ( Proposedc) Proposed pixel pixel layout layout with metalwith shield metal for shield enhancing for enhancing MTF; (MTF;d) Cross (d) Cross section section view withview metalwith metal shielding. shielding.

AsAs shown shown in in Figure Figure5 b,5b, the the horizontal horizontal MTF is is det deterioratederiorated by by lateral lateral light, light, and and this thiscannot cannot be beimproved improved by by using using STI. STI. In order In order to improve to improve the horizontal the horizontal MTF without MTF without modifying modifying the pixel thedesign pixel design(especially (especially the PPD the PPDsize sizeand andshape, shape, which which were were already already optimized optimized for for optical optical and and radiation radiation performances),performances), metal metal shielding shielding [24 [24]] has has been been applied applied asas shownshown in Figure 55c,d.c,d. InIn this this work, work, the the layout layout of of PPD, PPD, active active devices,devices, andand MOS capacitor capacitor cannot cannot be be changed changed due due to to considerationconsideration of of other other parameters parameters and and radiation radiation damagedamage effects. Figure Figure 5c5c shows shows a a proposed proposed method method of metalof metal routing routing for for enhancing enhancing MTF MTF performance. performance. The cross-section view view in in Figure Figure 5d5d shows shows the the mechanism for blocking the lateral light by the shielding metals. mechanism for blocking the lateral light by the shielding metals. Since M1 was already used for the signal and power routings, as shown in Figure 5b,d, the space for shielding was insufficient. Even though the use of a stacked metal shielding (M2 and M3) helps

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Since M1 was already used for the signal and power routings, as shown in Figure5b,d, the space for shielding was insufficient. Even though the use of a stacked metal shielding (M2 and M3) helps to reduceSensors the 2019 optical, 19, x crosstalk, it also reduces the QE [29]. Therefore, only M2 was used for shielding7 of 16 in order to maintain the QE. The shielding metals can help to prevent generation of photo-charges near to reduce the optical crosstalk, it also reduces the QE [29]. Therefore, only M2 was used for shielding the PPD edges by blocking lateral light, so the horizontal MTF is improved. In addition, photo-charges in order to maintain the QE. The shielding metals can help to prevent generation of photo-charges that are generated in an active area can affect the pixel performance, so the active area is fully covered near the PPD edges by blocking lateral light, so the horizontal MTF is improved. In addition, by metal layers to prevent the incident light. The measurement results show that the horizontal MTF photo-charges that are generated in an active area can affect the pixel performance, so the active area hasis improved fully covered bythree by metal times layers from to 0.131 prevent to 0.402 the in withcident metal light. shielding. The measurement results show that the horizontal MTF has improved by three times from 0.131 to 0.402 with metal shielding. 2.4. Readout Circuit Design and Operation Procedure 2.4.Figure Readout6a Circuit describes Design a readout and Operation circuit Procedure of a single channel and its control signals. One set of capacitorsFigure consisting 6a describes of two a samplingreadout circuit capacitors of a sing (CSIGle andchannel CRST and) is usedits control to sample signals. the One pixel set signal of andcapacitors reset values, consisting respectively, of two sampling from one-pixel capacitors column. (CSIG and Three CRST hundred) is used to sets sample of sampling the pixel capacitors signal and in onereset channel values, process respectively, the pixel from values one-pixel that are column. generated Three from hundred 300 pixel sets columns.of sampling A switched-capacitorcapacitors in one amplifierchannel holds process the the sampled pixel values pixel valuesthat are for genera the outputted from buffer 300 whilepixel columns. also working A switched-capacitor as a variable gain amplifieramplifier (VGA). holds Athe two-phase sampled pixel non-overlapped values for the clockoutput generator buffer while generates also working the clock as a signals variable (P gain1 and P2)amplifier for the switched-capacitor (VGA). A two-phase amplifier. non-overlapped In order cl toock sequentially generator generates transfer the the clock sampled signals charges (P1 and on P2 the) 300for sets the of switched-capacitor sampling capacitors, amplifier. the clock In order signals to se arequentially controlled transfer by thethe sampled column shiftcharges register on the (CSR), 300 whichsets isof alignedsampling with capacitors, the CSR theIN. clock signals are controlled by the column shift register (CSR), which is Thealigned circuit with operation the CSRIN is. as follows. As explained in Section 2.2, the pixel signal and reset values are sampledThe circuit on the operation CSIG and is C asRST follows., respectively, As explained while thein Section PSIG and 2.2, P RSTthe pixelare high. signal This and is reset illustrated values in are sampled on the CSIG and CRST, respectively, while the PSIG and PRST are high. This is illustrated in Figure6b,c. After sampling is completed, the output enable signal (O EN) becomes high and this shorts Figure 6b,c. After sampling is completed, the output enable signal (OEN) becomes high and this shorts the bottom plates of the CSIG and CRST, as shown in Figure6d. Then, the C SEL signal bus consisting the bottom plates of the CSIG and CRST, as shown in Figure 6d. Then, the CSEL signal bus consisting of of 300 control signals sequentially connects the top plates of the CSIG and CRST from the first set of 300 control signals sequentially connects the top plates of the CSIG and CRST from the first set of sampling capacitors up to the 300th set to the input nodes of the amplifier. This makes the sampled sampling capacitors up to the 300th set to the input nodes of the amplifier. This makes the sampled charges on the CSIG and CRST move to the feedback capacitor (CF) and generate differential voltage charges on the CSIG and CRST move to the feedback capacitor (CF) and generate differential voltage outputs as follows. outputs as follows.

(a)

Figure 6. Cont.

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(b) (c)

(d) (e)

FigureFigure 6. 6.Readout Readout circuit circuit and and timing timing diagram: diagram: (a )( Overalla) Overall readout readout circuit circuit and and timing timing diagram; diagram; (b) Signal(b) sampling;Signal sampling; (c) Reset ( valuec) Reset sampling; value sampling; (d) Initializing (d) Initializing the switched the switched capacitor capacitor ampliﬁer; amplifier; (e) Differential (e) outputDifferential generation. output generation.

V V V V V =V − V ,V = V . − V V = RST 2 SIG ,V = −2 RST SIG .(2) (2) OUTP 2 OUTN 2 Finally,Finally, the the differential differential outputsoutputs areare buffered by by the the two two unit-gai unit-gainn amplifiers ampliﬁers and and routed routed to the to the off-chipoff-chip ADC. ADC. NoticeNotice that that the the proposed proposed switched-capacitorswitched-capacitor amplifier ampliﬁer performs performs a asingle-ended single-ended to todifferential differential signalsignal conversion. conversion. The The differential differential signalingsignaling is is extremely extremely important important for for space space application application because because the the off-chipoff-chip ADCs ADCs are are often often placedplaced farfar fromfrom the CIS in in a a shielded shielded location location due due to to the the radiation radiation damage damage effect.effect. Therefore, Therefore, the the output output signal signal traces traces become become very very long long and and are are susceptible susceptible to to common common noises noises and interferences.and interferences. The differential The differential signaling signaling helps tohe removelps to remove common common noises andnoises interferences and interferences effectively effectively and maintain the signal integrity. and maintain the signal integrity.

2.5.2.5. Radiation Radiation Damage Damage Effects Effects onon thethe ProposedProposed Pixel Structure Structure The effects of energetic particle radiation on CMOS devices and circuits in space can be The effects of energetic particle radiation on CMOS devices and circuits in space can be categorized categorized as total ionizing dose (TID) and displacement damage dose (DDD). The TID effect causes as total ionizing dose (TID) and displacement damage dose (DDD). The TID effect causes a build-up a build-up of trapped charge in the gate oxide of a CMOS device. The DDD effect occurs when high of trapped charge in the gate oxide of a CMOS device. The DDD effect occurs when high energy energy particles penetrate a silicon substrate. The energetic particles collide with silicon atoms and particles penetrate a silicon substrate. The energetic particles collide with silicon atoms and generate generate the dislocation of the silicon atoms which causes Frenkel pair defects. These effects cause a thethreshold dislocation voltage of the shift silicon in CMOS atoms devices which and causes result Frenkel in an pairexcessive defects. leakage These current effects or cause a variation a threshold in voltagedevice shiftoperating in CMOS voltage. devices and result in an excessive leakage current or a variation in device operatingIn a voltage.CIS, the radiation damage effects deteriorate the optical performances, such as dark current, temporalIn a CIS, noise, the radiationand random damage telegraph effects noise deteriorate [30] or the result optical in the performances, generation suchof permanent as dark current, hot temporalpixel [31,32]. noise, It and is known random that telegraph the radiation noise [30 damage] or result effect in theon generationCMOS devices of permanent is decreased hot as pixel CMOS [31,32 ]. Ittechnologies is known that shrink the radiationto deep sub-micron damage effect regimes on an CMOSd the oxide devices thickness is decreased becomes as thinner. CMOS technologiesHowever, shrinkpixel todevices deep do sub-micron not benefit regimes from advanced and the technologies oxide thickness because becomes a thick thinner. gate oxide However, is still used pixel in devices the dopixel not beneﬁtdevice. from advanced technologies because a thick gate oxide is still used in the pixel device. In a conventional 4T pixel, the size of the transfer gate and PPD affects the dark current performance under radiation [33–35]. While small PPD size and short gate length help to reduce the radiation damage effect on the dark current, the FWC also decreases as a result of the reduced PPD capacitance. Moreover, the leakage current from the PPD to FD node can increase. In general, the

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In a conventional 4T pixel, the size of the transfer gate and PPD affects the dark current performance under radiation [33–35]. While small PPD size and short gate length help to reduce the radiation damage effect on the dark current, the FWC also decreases as a result of the reduced PPD Sensors 2019, 19, x 9 of 16 capacitance. Moreover, the leakage current from the PPD to FD node can increase. In general, theradiation radiation damage damage effect effect of a ofpixel a pixel device device is redu isced reduced as the as device the device size shrinks, size shrinks, and hence, and the hence, theincrement increment of dark of dark current current due to due irradiation to irradiation is alleviated is alleviated [36]. Recent [36]. pixel Recent design pixel demands design smaller demands smallerpixel size pixel in order size in to order increase to increasepixel resolution pixel resolution for a fixed for chip a ﬁxedarea. chipTherefore, area. pixel Therefore, design pixel requires design requirescareful consideration careful consideration of the trade-off of the trade-offbetween optical between performance optical performance and radiation and tolerance. radiation tolerance. InIn this this work, work, the the pixel pixel design design followed followedprevious previous researchesresearches forfor aa conventionalconventional 4T4T pixelpixel except for thefor in-pixel the in-pixel MOS MOS capacitor. capacitor. It has It has been been reported reported that that an an amount amount of of charges charges that that cancan bebe trapped in in an interfacean interface of SiO of SiO2/Si2/Si depend depend on on the the MOS MOScapacitor capacitor size and and radiation radiation dose dose [37]. [37 The]. The trapped trapped charges charges causecause a a shift shift in in threshold threshold voltagevoltage and variation of of capacitance capacitance [17,37–42]. [17,37–42 ].However, However, the the radiation radiation damagedamage effect effect onon aa MOSMOS capacitorcapacitor used inside inside a a pixe pixell has has not not been been studied studied yet. yet. In Inorder order to analyze to analyze thethe effect effect of of radiation radiation onon anan in-pixelin-pixel MOS MOS capacito capacitor,r, two two different different pixels pixels having having different different sizes sizes of of capacitorscapacitors has has been been fabricatedfabricated and tested. The The pi pixelxel performance performance under under a radiation a radiation environment environment has has beenbeen analyzed, analyzed, and and thethe analysisanalysis resultsresults are presented presented in in the the next next section. section.

3.3. Experimental Experimental ResultsResults

3.1.3.1. Overall Overall Performance Performance withwith Enhanced Full Full Well Well Capacity Capacity TheThe prototype prototype CISCIS waswas fabricated using using a a 0.11 0.11 μµmm 1P4M 1P4M CIS CIS process. process. The The chip chip microphotograph microphotograph andand the the captured captured image image of of the the ISO ISO 12233 12233 chart chart areare shownshown in Figure 77a,b,a,b, respectively.

(a) (b)

FigureFigure 7. 7.Chip Chip microphotographmicrophotograph and captured captured image: image: (a ()a Chip) Chip microphotograph microphotograph including including a pixel a pixel array,array, pixel pixel control control block,block, andand switched capacitor capacitor amplifiers ampliﬁers of of 10 10 channels; channels; (b) ( bCaptured) Captured image image (ISO (ISO 1223312233 chart). chart).

FigureFigure8a 8a shows shows the the measured measured pixel pixel output output voltages voltages versus versus light intensitylight intensity with differentwith different amounts ofamounts charge transfer.of charge It transfer. is clearly It shownis clearly that shown the saturationthat the saturation voltage voltage of the pixel of the output pixel output linearly linearly increases asincreases the amount as the of amount charge transferof charge increases. transfer increases. The FWC The of 103,448 FWC of e 103,448− with thee− with pixel the readout pixel readout voltage of 1voltage V was achievedof 1 V was by achieved transferring by transferring the charges the three charges times three and times the measured and the measured read noise—which read noise— was mainlywhich generatedwas mainly by generated the off-chip by the ADCs off-chip and ADCs measurement and measurement board—was board—was 24.7 e−. For24.7 the e−. readFor the noise measurement,read noise measurement, the integration the integration time of the time CIS wasof the set CIS to was 52 µ sets and to 52 the μ VGAs and gainthe VGA to 4 forgain minimizing to 4 for theminimizing dark and the shot dark noise. and Then, shot noise. two dark Then, images two dark were images captured, were and captured, the standard and the deviation standard was calculateddeviation fromwas calculated the difference from ofthe the difference two images of the for two removing images for the removing ﬁxed pattern the fixed noise. pattern The read noise. noise The read noise was calculated from the standard deviation considering ADC resolution, VGA gain, and conversion gain. The DR was achieved 72.4 dB resulting in the read noise. The DR could be further improved by using on-chip ADCs, which achieve much lower read noise. For example, a 106.7 dB DR can be achieved with 0.48 e- read noise, which is reported in [43].

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was calculated from the standard deviation considering ADC resolution, VGA gain, and conversion gain. The DR was achieved 72.4 dB resulting in the read noise. The DR could be further improved by using on-chip ADCs, which achieve much lower read noise. For example, a 106.7 dB DR can be Sensorsachieved 2019, with19, x 0.48 e− read noise, which is reported in [43]. 10 of 16

(a) (b)

FigureFigure 8. 8. Measured pixelpixel characteristic:characteristic: (a )(a Readout) Readout voltages voltages with with a different a different amount amount of charge of transfer;charge transfer;(b) Quantum (b) Quantum efﬁciency efficiency chart with chart various with wavelengths.various wavelengths.

2 WithWith a a6.5 6.5 um um pixel pixel pitch, pitch, the the PPD PPD size size is 28.31 is 28.31 um2 umand theand fill the factor ﬁll factoris 67%. is As 67%. shown As in shown Figure in 8b,Figure the measured8b, the measured QE varies QE from varies 13.6% from to 13.6% 68.4% to for 68.4% the spectrum for the spectrum ranging from ranging 375 from nm to 375 875 nm nm, to and875 the nm, peak and QE the is peak 68.4% QE at is 550 68.4% nm. at Performance 550 nm. Performance of the CIS of is thesummarized CIS is summarized in Table 1. in Table1.

TableTable 1. 1. CharacteristicsCharacteristics of of the the CIS. CIS.

ParameterParameter Value Value Technology 0.11 µm CMOS TechnologyPixel Pitch0.11 6.5μmµ mCMOS× 6.5 µm Pixel PitchResolution 6.5 μ 3000m ×× 6.53000 μm pixels Chip size 22 mm × 22 mm ResolutionFill factor 3000 × 3000 67% pixels Output voltage swing V (gain = 1) 1 V Chip size sat 22 mm × 22 mm Modulation transfer function (MTF) at Nyquist 0.4 Fill factorConversion gain 67% 8.55 µV/e− FD capacitance 19.6 fF Output voltage swingFull wellVsat (gain=1) capacity 1 103,448 V e− − Modulation transfer functionCIS read (MTF) noise at 24.7 e Dynamic range0.4 72.4 dB QuantumNyquist efﬁciency @ 550 nm 68.4% Conversion gain 8.55 μV/e− 3.2. MTF Measurement Results FD capacitance 19.6 fF The MTF measurements have been made using the ISO 23333 slanted-edge method. The maximum Full well capacity 103,448 e− output voltage of the bright pixel near the edge was limited to 50% of the saturated pixel output and − the knife-edge was usedCIS as read a target noise with a 5 degree tilting angle [24,44 24.7]. Thee MTF was measured in the vertical and horizontalDynamic directions range and was expressed at Nyquist frequency. 72.4 dB Two different CISs utilizing pixels with and without metal shielding have been fabricated in order Quantum efficiency @ 550 nm 68.4% to compare the MTF performance. Table2 shows the MTF measurement results, and it clearly shows that the horizontal MTF has been increased from 0.131 to 0.402, resulting in a 300% improvement. 3.2. MTF Measurement Results The measured MTF curves with spatial frequency are also shown in Figure9. The MTF measurements have been made using the ISO 23333 slanted-edge method. The maximum output voltage of the bright pixel near the edge was limited to 50% of the saturated pixel output and the knife-edge was used as a target with a 5 degree tilting angle [24,44]. The MTF was measured in the vertical and horizontal directions and was expressed at Nyquist frequency. Two different CISs utilizing pixels with and without metal shielding have been fabricated in order to compare the MTF performance. Table 2 shows the MTF measurement results, and it clearly shows that the horizontal MTF has been increased from 0.131 to 0.402, resulting in a 300% improvement. The measured MTF curves with spatial frequency are also shown in Figure 9.

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SensorsSensors 20192019,, 19,, x 1505 1111 of of 16 16

(a) (b)

Figure 9. Measured(a) MTF at 550nm wavelength: (a) Horizontal MTF; ( b()b Vertical) MTF.

Figure 9. Measured MTF at 550nm wavelength: (a) Horizontal MTF; (b) Vertical MTF. Figure 9. MeasuredTable MTF 2. Measured at 550nm wavelength:MTF results with(a) Horizontal metal covering. MTF; (b) Vertical MTF. Table 2. Measured MTF results with metal covering. Table 2. Measured MTF MTF (Horizontal) results with metal covering. MTF (Vertical) Metal covering MTF (Horizontal)0.402 MTF (Vertical) 0.406 MTF (Horizontal) MTF (Vertical) Without covering 0.131 0.406 Metal coveringMetal covering 0.402 0.406 0.406 ImprovementWithout (%) covering 0.131307 0.406 0 Without coveringImprovement (%) 0.131 307 0 0.406 Improvement (%) 307 0 3.3. Radiation 3.3. Radiation 3.3. RadiationFigure 10 shows a simplified block diagram and actual pictures of the radiation test setup. The TID andFigure DDD 10 testshows setups a simpliﬁed are simila blockr except diagram for radiation and actual source pictures and of exposure the radiation time. test In setup. the radiation The TID Figure 10 shows a simplified block diagram and actual pictures of the radiation test setup. The chamber,and DDD the test CIS setups chip are is similarplaced exceptat a set for distance radiation from source the andradiation exposure source, time. which In the radiationis determined chamber, by TID and DDD test setups are similar except for radiation source and exposure time. In the radiation thethe amount CIS chip of is radiation placed at delivered a set distance to the from chip the per radiation hour and source, electrical which devices is determined except for by the the CIS amount chip chamber, the CIS chip is placed at a set distance from the radiation source, which is determined by areof radiationprotected delivered from radiation to the chipby lead per hourblocks. and For electrical the TID devices test, gamma except forrays the generated CIS chip by are cobalt-60 protected the amount of radiation delivered to the chip per hour and electrical devices except for the CIS chip (60fromCo) radiationwere used by as lead a radiation blocks. Forsource the TIDand test,the amount gamma of rays radiation generated exposed by cobalt-60 to the chip (60Co) was were set usedto 236 as are protected from radiation by lead blocks. For the TID test, gamma rays generated by cobalt-60 a radiation source and the amount of radiation exposed10 to the chip was2 set to 236 rad per hour. Proton rad60 per hour. Proton particles with a fluence of 1.48ⅹ10 particles/cm are accelerated with an energy ( Co) were used as a radiation source10 and the amount2 of radiation exposed to the chip was set to 236 ofparticles 45 MeV withby a aMC-50 ﬂuence cyclotron, of 1.48 × and10 theparticles/cm CIS chip was areexposed accelerated to the accelerated with an energy proton of 45particles MeV byfor a rad per hour. Proton particles with a fluence of 1.48ⅹ1010 particles/cm2 are accelerated with an energy theMC-50 DDD cyclotron, test. and the CIS chip was exposed to the accelerated proton particles for the DDD test. of 45 MeV by a MC-50 cyclotron, and the CIS chip was exposed to the accelerated proton particles for the DDD test.

(a) (b)

(a) Figure 10. Cont. (b)

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(c)

(d) (e)

FigureFigure 10. Radiation 10. Radiation test setup: test setup: (a) Simpliﬁed (a) Simplified block block diagram; diagram; (b) Displacement (b) Displacement damage damage dose (DDD)dose (DDD) test withtest metalwith shieldingmetal shielding to protect to the protect other electricalthe other parts; electrical (c) DDD parts; test ( setupc) DDD without test setup metal shielding;without metal (d) Frontshielding; view of(d) total Front ionizing view of dosetotal (TID)ionizing test dose setup (TID) using test lead setup blocks; using (leade) Overall blocks; TID (e) Overall test setup TID test includingsetup radiation including source. radiation source.

ThreeThree important important performance performance measures measures (QE, power (QE, consumption, power consumption, and dark current) and dark are evaluated current) are by theevaluated radiation by test. the While radiation radiation test. While did not radiation affect QE did and not power affect consumption QE and power of the consumption CIS, dark current of the CIS, was significantlydark current increased was significantly depending increased on the size depending of in-pixel on capacitor. the size Theof in-pixel radiation capacitor. damage The effect radiation on an in-pixeldamage MOS effect capacitor on an in-pixel on the darkMOS current capacitor performance on the dark can current be explained performanc withe thecan measurementbe explained with resultsthe shown measurement in Figure 11results. The shown CIS with in two Figure different 11. The sizes CIS of in-pixelwith two MOS different capacitors sizes was of fabricatedin-pixel MOS and wascapacitors tested was for the fabricated TID and and DDD. was Figure tested 11fora the shows TID the and test DDD. results Figure for the11a CISshows with the an test in-pixel results for − MOSthe capacitance CIS with an of in-pixel 16.3 fF; MOS the average capacitance dark of current 16.3 fF; of the CISs average was 133.15 dark current e /s before of CISs the was radiation 133.15 e−/sec − test andbefore it increasedthe radiation to 490.5 test andand 622.78it increased e /s afterto 490.5 the and TID 622.78 and DDD e−/sec tests, after respectively. the TID and With DDD an tests, in-pixelrespectively. MOS capacitance With an of in-pixel 19.6 fF, MOS as shown capacitance in Figure of 11 19b,.6 the fF, dark as shown current in significantly Figure 11b, increasedthe dark current to − 931.59significantly and 2585 e increased/s for the to TID 931.59 and and DDD 2585 test, e−/sec respectively. for the TID Only and a DDD 20% increasetest, respectively. in in-pixel Only MOS a 20% capacitanceincrease deteriorates in in-pixel MOS the dark capacitance current performancedeteriorates the by dark about current two times perfor formance the TID by andabout by two four times timesfor for the the TID DDD. and The by possiblefour times reasons for the for DDD. the sharp The possible increase reasons in dark currentfor the sharp are the increase threshold in dark voltage current shift ofare the the MOS threshold capacitor voltage due toshift the of trapped the MOS charges capacitor in the due oxide to the area trapped and Shockley–Read–Hall charges in the oxide (SRH) area and generationShockley–Read–Hall by the radiation (SRH) induced generati defects.on by The the measurementradiation induced results de indicatefects. The that measurement there exists results a criticalindicate trade-off that between there exists the a FWC critical and trade-off the radiation between damage the FWC effect and on the the radiation dark current. damage With effect the on the multipledark charge current. transfer With the technique, multiple the charge FWC transfer can be significantly technique, the enhanced FWC can by be increasing significantly the size enhanced of an by in-pixelincreasing MOS capacitor, the size however, of an in-pixel the dark MOS current capacito performancer, however, after radiation,the dark especiallycurrent performance for the DDD after effect,radiation, can be severely especially worsened. for the Therefore,DDD effect, in-pixel can be capacitance severely worsened. should be Therefore, carefully chosen in-pixel based capacitance on an understandingshould be carefully of the trade-off.chosen based on an understanding of the trade-off.

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(a) (b)

FigureFigure 11. 11.Radiation Radiation damage damage effects effects with with two two different different in-pixel in-pixel MOS MOS capacitors: capacitors: (a )(a 16.3) 16.3fF fF MOS MOS capacitorcapacitor (b ()b 19.6) 19.6fF fF MOS MOS capacitor. capacitor.

TableTable3 compares 3 compares the the radiation radiation performance performance of of this th workis work with with that that of of commercial commercial image image sensors sensors whichwhich were were tested tested with with the the similar similar radiation radiation conditions. conditions. Dark Dark current current increment increment observed observed after after irradiationirradiation of of the the commercial commercial sensors sensors varies varies from aboutfrom about 200% to200% 2700%, to which2700%, is which similar is to similar the ﬁndings to the infindings this work. in this work.

Table 3. Comparison of the dark current with other image sensors (unit: e−/s). Table 3. Comparison of the dark current with other image sensors (unit: e−/s).

ThisThis work [45[45]][ 46][[46] 47][[47] 48] [48] Pre-RadPre-Rad 133 133 66006600 190190 31353135 20 20 TID TID*~~~ * 491~~~491 932~~~932 180,000180,000~~~ 15541554~~~ 61106110~~~ N/A N/A (Percentage(Percentage increase) increase) (368%)(368%) (700%)(700%) (2727%)(2727%) (818%)(818%)(195%) (195%) ProtonProton **~~~ ** 623~~~623 2585~~~2585 500 500~~~ N/AN/A N/AN/A N/A N/A (Percentage(Percentage increase) increase) (468%)(468%) (1941%)(1941%) (2500%)(2500%) * TID: total dose of 7~8 krad of cobalt-60 gamma ray. ** DDD: total proton ﬂuence of 1 × 1010 ~ 1.5 × 1010/cm2 ⅹ 10 ⅹ 10 2 * TID:under total 50 dose MeV. of 7~8krad of cobalt-60 gamma ray. ** DDD: total proton fluence of 1 10 ~ 1.5 10 /cm under 50MeV. 4. Conclusions 4. Conclusions A high FWC CIS for space applications was presented in this paper. The proposed pixel achieves high FWCA high by FWC utilizing CIS for an space in-pixel applications MOS capacitor was presente and multipled in this charge paper. transfer. The proposed In addition pixel achieves to the FWC,high twoFWC important by utilizing performance an in-pixel measures MOS capacitor for space and applications multiple charge (MTF transfer. and radiation In addition tolerance) to the wereFWC, explained two important and a method performance to improve measures MTF for was space suggested. applications The radiation (MTF and damage radiation effect tolerance) on an in-pixelwere explained MOS capacitor, and a whichmethod has to notimprove been reported MTF was in su theggested. other literatures, The radiation was dama studiedge byeffect fabricating on an in- andpixel measuring MOS capacitor, a CIS with which two has different not been size reported of the MOS in the capacitors. other literatures, It was foundwas studied that the by size fabricating of the capacitorand measuring should a be CIS determined with two different with consideration size of the ofMOS the capacitors. radiation damageIt was found effect that because the size a small of the incrementcapacitor in should the MOS be determined capacitance canwith severely consideratio increasen of dark the radiation current under damage a radiation effect because environment. a small Theincrement fabricated in the 3000 MOS× 3000 capacitance CIS achieved can severely an FWC incr ofease 103,448 dark current electrons under and a metal radiation shielding environment. in the horizontalThe fabricated direction 3000 improved × 3000 CIS the MTFachieved by 300%. an FWC of 103,448 electrons and metal shielding in the horizontal direction improved the MTF by 300%. Author Contributions: W.-T.K. and B.-G.L. conceived and designed the CIS; W.-T.K., C.P., and H.L. conducted Author Contributions: W.K. and B.L. conceived and designed the CIS; W.K., C.P., and H.L. conducted measurements of the CIS; I.L. consulted radiation test methodology. measurements of the CIS; I.L. consulted radiation test methodology. Funding: This research was supported by the MSIT (Ministry of Science and ICT), Korea, under the ITRC (InformationFunding: This Technology research Research was supported Center) supportby the MSIT program(IITP-2019-0-01433) (Ministry of Science and supervised ICT), Korea, by the under IITP (Institute the ITRC for(Information Information Technology & communications Research Technology Center) Promotion)support program(IITP-2019-0-01433) and the GIST Research Institute supervised Project by through the IITP a Grant(Institute provided for Information by GIST in 2019.& communications Technology Promotion) and the GIST Research Institute Project Acknowledgments:through a Grant providedThe authors by GIST express in 2019. their sincere thanks to the staff of the MC-50 Cyclotron Laboratory (KIRAMS), for the excellent operation and their support during the experiment and DB HiTek for the CISAcknowledgments: fabrication. The authors express their sincere thanks to the staff of the MC-50 Cyclotron Laboratory (KIRAMS), for the excellent operation and their support during the experiment and DB HiTek for the CIS fabrication.

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Conﬂicts of Interest: The authors declare no conﬂict of interest.

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