sensors Article
Recent Enhancements to Interline and Electron Multiplying CCD Image Sensors † Eric G. Stevens 1, *, Jeffrey A. Clayhold 1 , Hung Doan 1 , Robert P. Fabinski 1 Jaroslav Hynecek 2 , Stephen L. Kosman 1 and Christopher Parks 1 1
2
* †
ID
,
ON Semiconductor, 1964 Lake Avenue, Rochester, NY 14615, USA;
[email protected] (J.A.C.);
[email protected] (H.D.);
[email protected] (R.P.F.);
[email protected] (S.L.K.);
[email protected] (C.P.) ON Semiconductor, 2660 Zanker Road, San Jose, CA 95134, USA;
[email protected] Correspondence:
[email protected] This paper is an expanded version of our published paper Stevens, E.G.; Clayhold, J.; Doan, H.; Fabinski, J. Hynecek, R.; Kosman, S.; Parks, C. Recent Enhancements to Electron Multiplying CCD Image Sensors. In Proceedings of the 2017 International Image Sensor Workshop, Hiroshima, Japan, 30 May–2 June 2017.
Received: 26 September 2017; Accepted: 30 November 2017; Published: 7 December 2017
Abstract: This paper describes recent process modifications made to enhance the performance of interline and electron-multiplying charge-coupled-device (EMCCD) image sensors. By use of MeV ion implantation, quantum efficiency in the NIR region of the spectrum was increased by 2×, and image smear was reduced by 6 dB. By reducing the depth of the shallow photodiode (PD) implants, the photodiode-to-vertical-charge-coupled-device (VCCD) transfer gate voltage required for no-lag operation was reduced by 3 V, and the electronic shutter voltage was reduced by 9 V. The thinner, surface pinning layer also resulted in a reduction of smear by 4 dB in the blue portion of the visible spectrum. For EMCCDs, gain aging was eliminated by providing an oxide-only dielectric under its multiplication phase, while retaining the oxide-nitride-oxide (ONO) gate dielectrics elsewhere in the device. Keywords: interline CCD; EMCCD; quantum efficiency; low noise; gain aging
1. Introduction Although applications of complementary metal oxide semiconductor (CMOS) image sensors have greatly expanded since their introduction, charge-coupled device (CCD) image sensors are still used for their excellent image uniformity (i.e., low fixed pattern noise), and high global-shutter efficiency (GSE). For capturing quality images in low light, high quantum efficiency and low noise are key attributes for both types of sensors. Achieving high quantum efficiency can be broken up into the following parts:
• • • •
Improving micro-lens focusing and fill factor Reducing reflection losses Reducing absorption losses Increasing the collection volume of the photodiodes
To reduce noise, one technique employed on both types of imagers is to improve the charge-to-voltage conversion factor of the output structure. More specific to CCDs, electron multiplication CCD (EMCCD) image sensors were conceived as a way to significantly reduce noise [1]. With an EMCCD, care needed to be taken to ensure that gain was only applied under low-lighting conditions to avoid saturating the device. This put a limit on the intra-scene dynamic range.
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More recently, an EMCCD image sensor with an output structure that includes a novel, nonMore recently, EMCCD image sensor with an output structure that includes a novel, destructive floating an gate amplifier gain switching architecture for enabling dynamic range non-destructive floating gate amplifier gain switching architecture for enabling dynamic range improvement has been reported [2]. A block diagram showing one quadrant of the device is shown improvement has1. been reported [2]. A block diagram showing one quadrant of the device ismade shown below in Figure This paper will describe the details of recent process modifications as below in Figure 1. This paper will describe the details of recent process modifications made as reported reported in Reference [3], that were aimed at improving the sensor’s quantum efficiency and smear in [3],photodiode that were aimed improving volume, the sensor’s quantum efficiency and smearartifact via improved viaReference improved (PD)at collection elimination of the gain-aging [2–5], photodiode (PD) collection volume, of the gain-aging artifact [2–5], reduced PD-to-VCCD reduced PD-to-VCCD transfer gateelimination voltage, reduced electronic shutter voltage, and reduced point transfer defects. gate voltage, reduced electronic shutter voltage, and reduced point defects.
Figure 1. Block diagram of one quadrant of the electron-multiplying charge coupled device (EMCCD) Figure 1. Block diagram of one quadrant of the electron-multiplying charge coupled device imager as previously reported in Reference [2]. Each EM section contains 300 pixels, with two (EMCCD) imager as previously reported in Reference [2]. Each EM section contains 300 pixels, multiplication phases per pixel for a total of 1800 gain stages. with two multiplication phases per pixel for a total of 1800 gain stages.
2. Improved Quantum Efficiency 2. Improved Quantum Efficiency Quantum efficiency (QE) is improved by a factor of over 2× in the NIR region of the spectrum Quantum efficiency (QE) is improved by a factor of over 2× in the NIR region of the spectrum by by increasing the photodiode’s collection volume via the use of high-energy MeV implantation for increasing the photodiode’s collection volume via the use of high-energy MeV implantation for both both the deep p-well (up to 6.5 MeV) and deep n-type photodiode regions (up to 10 MeV). the deep p-well (up to 6.5 MeV) and deep n-type photodiode regions (up to 10 MeV). Comparison of Comparison of the electrostatic potential contours of the old and new processes are shown below in the electrostatic potential contours of the old and new processes are shown below in Figure 2. As can Figure 2. As can be seen, the overflow depth is over 6 µ m with the new MeV implant process vs. only be seen, the overflow depth is over 6 µm with the new MeV implant process vs. only around 2.5 µm around 2.5 µ m deep for the old process. The resulting improvement in absolute quantum efficiency deep for the old process. The resulting improvement in absolute quantum efficiency is shown below in is shown below in Figure 3, which is in good agreement with a simple Beer-Lambert absorption law Figure 3, which is in good agreement with a simple Beer-Lambert absorption law [6]. [6].
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(a) (b) (a) (b) Figure 2. Comparison of potential contours for the old and new processes. Images show twoFigure 2. Comparison of potential contours for the old and new processes. Images show dimensional (2-D) cuts from the three-dimensional (3-D) spaceprocesses. of image area pixelshow shown in Figure 2. Comparison of potential contours for the oldmodel and new Images twotwo-dimensional (2-D) cuts from the three-dimensional (3-D) model space of image area pixel shown Figure 1, which hascuts halffrom a vertical-charge-coupled-device (VCCD) on the left, a area photodiode (PD) in dimensional (2-D) the three-dimensional (3-D) model space of image pixel shown in in Figure 1, which half a avertical-charge-coupled-device (VCCD) on the left, a photodiode (PD) in the middle, andhas another VCCD on the right. (a) Old process. (b)on New Figure 1, which has halfhalf vertical-charge-coupled-device (VCCD) the process. left, a photodiode (PD) in the middle, and and another halfhalf VCCD ononthe process.(b) (b)New New process. the middle, another VCCD theright. right.(a) (a) Old Old process. process.
Figure 3. Measured absolute quantum efficiency for the old and new processes. Figure 3. Measured absolute quantum efficiency for the old and new processes.
Figure 3. Measured absolute quantum efficiency for the old and new processes. The data of Figure 3 show that the quantum efficiency improvement is concentrated at longer The dataasofwould Figurebe3 expected show thatfrom the the quantum efficiency improvement concentrated at longer wavelengths Beer-Lambert law. Because theisabsorption coefficient of The data of Figure 3 show the quantum efficiency is concentrated at longer wavelengths aswavelengths would be expected from the law. Because the coefficient of silicon at short isthat quite high, noBeer-Lambert improvement isimprovement anticipated inabsorption the blue from the deep silicon atas short wavelengths is quite high, no improvement is intothe blue fromvariation. the deep of MeV implants. The small QE differences around 420–480 nmlaw. areanticipated merely duethe part-to-part wavelengths would be expected from the Beer-Lambert Because absorption coefficient MeV implants. The small QE differences 420–480 nm are merely due toinpart-to-part variation. silicon at short wavelengths is quite high,around no improvement is anticipated the blue from the deep 3. Masking of High-Energy Implants MeV implants. The small QE differences around 420–480 nm are merely due to part-to-part variation. 3. Masking of High-Energy Implants 3.1. Patterning 3. Masking of High-Energy Implants 3.1. Patterning One of the challenges of using high-energy implantation is masking. A chemically-amplified 3.1. Patterning of the challenges of using high-energy implantation masking. A chemically-amplified thick One photoresist is used for the high aspect ratio masking of bothisMeV implants. The photoresist was thick photoresist is used for low the high masking of both MeV implants. The photoresist was exposed using a wide-field, N.A.,aspect i-line ratio stepper, and developed with a metal-ion-free developer. One of the challenges of using high-energy implantation is masking. A chemically-amplified exposed using a wide-field, low N.A., i-line stepper, the andresist developed with a metal-ion-free developer. Alignment measurement was improved by thinning over the measurement structures with thick aAlignment photoresist is used for the high aspect ratio masking of both MeV implants. The photoresist was measurement wasFigure improved by thinning resistratio overthick the measurement structures with second low-dose exposure. 4a shows a 4.25:1the aspect resist image for the masking exposed using a wide-field, low N.A., i-line stepper, and developed with a metal-ion-free developer. a second low-dose exposure. Figure 4a shows a (PDext) 4.25:1 aspect ratiowithin thick resist image for the masking of the deep MeV n-type photodiode extension implants the pixel array of Figure 1. Alignment was improved bythe thinning the resist over the measurement with a of the measurement deep n-type photodiode (PDext) implants within theofpixel arraystructures of Figure 1. Figure 4b is MeV an SEM of the sidewall extension of thick photoresist at the edge the deep MeV p-well second low-dose exposure. Figure 4a shows athick 4.25:1 aspect ratio thick image for the masking Figure 4b pattern, is an SEM of the sidewall of thelithographic photoresist at the edgeresist of the deepoptimized MeV p-well (PWELL) created using the same processes. Stepper focus was to pattern, created using theextension same lithographic Stepper the focus wasarray optimized to of the(PWELL) deep MeV n-type photodiode (PDext) processes. implants within pixel of Figure 1.
Figure 4b is an SEM of the sidewall of the thick photoresist at the edge of the deep MeV p-well (PWELL) pattern, created using the same lithographic processes. Stepper focus was optimized to minimize the
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reentrance of the top and foot of the photoresist sidewall. The PDext design was optimized by the Sensors 2017, 17, 2841 4 of 10 minimize the reentrance top and foot of the photoresist sidewall. The PDext design was addition of serifs. Figure 5 isofa the comparison of top-down SEM images, showing the impact of serifs optimizedon by the the addition ofthe serifs. 5 is aofcomparison of Dimensional top-down SEM images, showing the minimize the reentrance the topFigure and foot sidewall. The PDextof design was and chamfers shape ofof PDext opening inthe thephotoresist resist. control the opening is impact of serifs and chamfers on theFigure shape 5ofisthe PDext opening in the resist. Dimensional controlthe of optimized by the addition of serifs. a comparison of top-down SEM images, showing important because it affects several key performance attributes of the photodiode. the opening is important because it affects key performance attributes of the photodiode. impact of serifs and chamfers on the shape several of the PDext opening in the resist. Dimensional control of the opening is important because it affects several key performance attributes of the photodiode.
(a) (b) (a) (b) Figure 4. Cross-sections of photoresist patterns. (a) Cross-section of the photodiode extension (PDext) Figure 4. Cross-sections of photoresist patterns. (a) Cross-section of the photodiode extension (PDext) resist pattern within theof pixel array. (b) Cross-section of the deepofp-well (PWELL) extension resist at the array Figure 4. Cross-sections photoresist patterns. (a) Cross-section the photodiode (PDext) resist pattern within the pixel array. (b) Cross-section of the deep p-well (PWELL) resist at the edge. pattern within the pixel array. (b) Cross-section of the deep p-well (PWELL) resist at the array resist array edge. edge.
Figure 5. Effects of chamfers and serifs on the dimensions of the photoresist used to mask the photodiode extension (PDext) implant. Theonopenings are approximately 2.5 μm byused 3.3 μm. Figure 5. Effects chamfers and serifs the dimensions photoresist to mask the Figure 5. Effects of of chamfers and serifs on the dimensionsofofthethe photoresist used to mask the photodiode extension (PDext) implant. The openings are approximately 2.5 μm by 3.3 μm.
photodiode 3.2. Radiationextension (PDext) implant. The openings are approximately 2.5 µm by 3.3 µm. 3.2. Radiation One of the concerns presented by high-energy implants in the MeV range is potential radiation
3.2. Radiation
activity. materials, including photoresist, can become activated. is aradiation primary One Target of the concerns presented bythe high-energy implants in the MeV range Carbon is potential
31P12C constituent of many photoresists. During phosphorus implantation, the coulomb barrier One of the concerns presented by high-energy implants in theactivated. MeV range is potential radiation activity. Target materials, including the photoresist, can become Carbon is afor primary 31 12 nuclear activation is nearly 40 MeV, much higher than the the implanter. So, there isprimary no constituent ofmaterials, many photoresists. During phosphorus implantation, the coulomb barrier for C activity. Target including the photoresist, cancapability become of activated. Carbon is aP11B-12C coulomb 31 radiation safety risk from this interaction. However, for boron implantation, the nuclear activation is nearly 40 MeV, much higher than the capability of the implanter. So, there is no constituent of many photoresists. During phosphorus implantation, the coulomb barrier for P-12 C 11B-12Cconcern. barrier is only about 10 MeV, which is easily attainable and therefore presents a the radiation radiation safety from interaction. However, for boron implantation, coulomb nuclear activation isrisk nearly 40this MeV, much higher than the capability of the implanter. So, there is 11B-12C as characterized by different A iscareful examination of theisradiation activityand fortherefore barrier only about 10 MeV, which easily attainable presents a radiation concern. 11 no radiation safety risk from this interaction. However, for boron implantation, the B-12 C coulomb 11B12Cenergy institutions wasexamination compiled by of Nick in Reference the approaches the coulomb A careful theWhite radiation activity [7]. for As as characterized by different barrier is only about 10 MeV, which is easily attainable and therefore presents a radiation concern. barrier, radiation activity increases by aninorder of magnitude perenergy MeV. approaches The recommended safe institutions was compiled by Nick White Reference [7]. As the the coulomb 11 B-12 C as characterized by different institutions A careful examination the radiation activity exposure rate limitactivity for aof human operator about for 0.6magnitude μS/h. This limits the boron energy to just over barrier, radiation increases by anis order of per MeV. The recommended safe was compiled by aNick White in photoresist Reference AsAccordingly, the0.6energy approaches the coulomb barrier, radiation 7 MeV using carbon mask. we capped our boron implant at exposure rate limit forbased a human operator[7]. is about μS/h. This limits the boron energy to energy just over activity increases an activity order per MeV. The recommended safe rate limit MeV. Radiation ismagnitude not expected toAccordingly, be a problem. to our Table 1. exposure Ifimplant a higher atomic 76.5 MeV using aby carbon basedof photoresist mask. we Refer capped boron energy at for a human operator is about 0.6 µS/h. This limits theaboron energy over using a carbon 6.5 MeV. Radiation activity is not expected to be problem. Refertotojust Table 1. 7If MeV a higher atomic based photoresist mask. Accordingly, we capped our boron implant energy at 6.5 MeV. Radiation
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activity is not expected to be a problem. Refer to Table 1. If a higher atomic number were to be used for the mask material, a higher boron implant energy would be possible. Because the implanter beam stop Sensors 2017, 17, 2841 5 of 10 Sensors 2017, 17, 2841 use graphite, the same 11 B-12 C radiation activity concern applies, independent 5 of 10 and aperture typically of thenumber choice were of photoresist. to be used for the mask material, a higher boron implant energy would be possible. number were to be used for the mask material, a higher boron implant energy would be possible. Because the implanter beam stop and aperture typically use graphite, the same 1111B-1212C radiation Because the implanter beam stop and aperture typically use graphite, the same B- C radiation Table 1.applies, Estimated radiation activity after implanting 6.5 MeV 11 B into photoresist. activity concern independent of the choice of photoresist. activity concern applies, independent of the choice of photoresist. −2 TableBoron 1. Estimated radiation activity after2implanting 6.5for MeV into photoresist. Bq 1501111Bmm Dose/cm Bq/cm Table 1. Estimated radiation activity after implanting 6.5 MeV B intoWafer photoresist.
4. Improved Reliability
1.0Boron × 1011Dose cm−2 Boron12Dose cm−2 1.0 × 10 1.0 × 101111 1.0 13 × 10 1.0 × 10 1.0 × 101212 1.0 × 10 1.0 × 101313 1.0 × 10
1.0 × 10−227 Bq for 150 mm 1.0 ×Wafer 10−5 Bq/cm mm Wafer Bq/cm −6 Bq for 150 1.0 −4 1.0 × 10 × 10 1.0 × 10−7−7 1.0 × 10−5−5 1.0××10 10−5 1.0 ×1.0 10 × 10−3 1.0 1.0 × 10−6−6 1.0 × 10−4−4 1.0 × 10 1.0 × 10 1.0 × 10−5−5 1.0 × 10−3−3 1.0 × 10 1.0 × 10
4. Improved Reliability
4. Improved Reliability Stability of the multiplication gain in EMCCDs is a known problem, as has been previously Stability of the multiplication gain in EMCCDs is a known problem, as has been previously reported [2–5]. This of gain results from the buildup hot electrons into the gate Stability of loss the multiplication gain in EMCCDs is aof known problem,being as hasinjected been previously reported [2–5]. This loss of gain results from the buildup of hot electrons being injected into the gate reported loss normal of gain results the buildup of hot electrons injectedband into the gate of dielectric over[2–5]. timeThis during devicefrom operation, as illustrated in thebeing conduction diagram dielectric over time during normal device operation, as illustrated in the conduction band diagram dielectric over time during normal device operation, as illustrated in the conduction band diagram Figure 6 below. As an additional complication, the more signal electrons that are present, the more can of Figure 6 below. As an additional complication, the more signal electrons that are present, the more of Figure 6 below. As an additional complication, the signal electrons that are present, the more be injected into theinto dielectric, resulting in this in more gain being a function can be injected the dielectric, resulting in drift this drift in gain being a functionofofsignal signal level. level. can be injected into the dielectric, resulting in this drift in gain being a function of signal level.
Figure 6. Conduction band diagramalong along the EMCCD EMCCD illustrating thethe physical mechanism of gain Figure 6. Conduction band diagram illustrating physical mechanism of gain Figure 6. Conduction band diagram alongthe the EMCCD illustrating the physical mechanism of gain degradation. Note that the conduction band of the buried channel in the silicon is labeled as “BURIED degradation. NoteNote thatthat thethe conduction band channelininthe the silicon is labeled as “BURIED degradation. conduction bandofofthe the buried buried channel silicon is labeled as “BURIED CHANNEL POTENTIAL”. CHANNEL POTENTIAL”. CHANNEL POTENTIAL”.
To get around this problem, we incorporated an oxide-only dielectric structure under the highTo get around this problem, we incorporated an oxide-only dielectric structure under the high-
To getmultiplication around thisgates problem, we incorporated oxide-only structure field, of the EMCCD registers, asan shown above in dielectric Figure 7b. The originalunder gate the field, multiplication gates of the EMCCD registers, as shown above in Figure 7b. The original gate structure is shown for comparison inEMCCD Figure 7a. As can beasseen below in Figure 8, gain aging was high-field, multiplication gates of the registers, shown above in Figure 7b. The original structure is shown for comparison in Figure 7a. As can be seen below in Figure 8, gain aging was eliminated for the new process using an oxide-only dielectric under the high-field, multiplication gate structure shown for comparison Figure 7a. Asdielectric can be seen below in Figure multiplication 8, gain aging was eliminatedisfor the new process usinginan oxide-only under the high-field, gates. for the new process using an oxide-only dielectric under the high-field, multiplication gates. eliminated gates.
(a) (a)
(b) (b)
Figure 7. Comparison of dielectric structures under the multiplication gates: (a) old structure with Figure 7. Comparison of dielectric structures under the multiplication gates: (a) old structure with
Figure 7. Comparison (ONO); of dielectric structures under the multiplication gates: (a) old structure with oxide-nitride-oxide (b) new structure with oxide-only under the high-field gates. oxide-nitride-oxide (ONO); (b) new structure with oxide-only under the high-field gates. oxide-nitride-oxide (ONO); (b) new structure with oxide-only under the high-field gates.
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Figure multiplying gate voltage required for for old and new processes. Figure 8. Shift Shift in multiplyinggate gatevoltage voltagerequired requiredfor for 20× 20× gain versus time for old and new processes. Figure 8. 8. Shift inin multiplying 20 ×gain gainversus versustime time for old and new processes. The slight drift downward in the oxide-only dielectric curve was due to some slight inaccuracy in the The slight drift downward in the oxide-only dielectric curve was due to some slight inaccuracy inin thethe The slight drift downward in the oxide-only dielectric curve was due to some slight inaccuracy temperature controller. temperature controller. temperature controller.
5. 5. Improved Improved Transfer Transfer Efficiency Efficiency
5. Improved Transfer Efficiency
By By reducing reducing the the thickness thickness of of the the photodiode’s photodiode’s p+ p+ pinning pinning layer layer along along with with the the depth depth of of its its
By reducing the thicknesscomponent, of the photodiode’scharge p+ pinning layer along with the depth of its shallow, shallow, shallow, higher higher dose dose n-type n-type component, aa given given charge capacity capacity can can be be achieved achieved with with aa lower lower empty empty higher dose n-type component, a given charge capacity can be achieved with a lower empty photodiode photodiode photodiode potential potential and and nominal nominal substrate substrate bias. bias. The The thin, thin, spike spike pinning pinning layer layer was was formed formed via via lowlowpotential nominal substrate bias.only Thethe thin, spike pinning layer was formed via distribution low-energy is BF2 energy BF whereby steep concentration gradient tail energy and BF22 implantation, implantation, whereby only the steep concentration gradient tail of of the the distribution is implantation, onlyshallow, the steep concentration gradient tail of the distribution is within within silicon. higher dose photodiode implant is via within the the whereby silicon. The The shallow, higher dose n-type n-type photodiode implant is formed formed via As Asthe silicon. The shallow, higher dose n-type photodiode implant is formed via As implantation. A relative implantation. A relative comparison of the doping profiles between the new and old processes are implantation. A relative comparison of the doping profiles between the new and old processes are comparison of the doping9. shown in shown below below in Figure Figure 9.profiles between the new and old processes are shown below in Figure 9.
Figure Figure 9. 9. Comparison Comparison of of doping doping profiles profiles for for old old and and new new shallow shallow photodiode photodiode implants. implants. Figure 9. Comparison of doping profiles for old and new shallow photodiode implants.
The The shallower shallower surface surface layer layer construction construction of of the the photodiode photodiode is is also also found found to to reduce reduce the the parasitic parasitic Theand shallower surface construction the photodiode also found to reduce wells barriers at transfer gate (examples of and can be found wells and barriers at the the layer transfer gate edge edgeof (examples of such suchiswells wells and barriers barriers canthe be parasitic found Reference [8]). By reducing these transfer barriers and the empty diode potential, the transfer-gate wells and barriers at the transfer gate edge (examples of such wells and barriers can be found Reference [8]). By reducing these transfer barriers and the empty diode potential, the transfer-gate voltage charge transfer from PD to the was reduced 33 V, as Reference [8]). By for reducing these transfer barriers and the diode the transfer-gate voltage required required for complete complete charge transfer from the the PD to empty the VCCD VCCD waspotential, reduced by by V, as shown shown below in With combination aa lower empty diode potential and substrate belowrequired in Figure Figure 10. With the thecharge combination offrom lower empty diode potential and nominal nominal substrate voltage for10. complete transferof the PD to the VCCD was reduced by 3 V, as shown bias, the voltage the substrate to the photodiode during shutter bias,in the voltage on the n-type n-type substrate required required to clear clear thediode photodiode during electronic shutter below Figure 10.on With the combination of a lower empty potential andelectronic nominal substrate operation was reduced by 9 V. operation was reduced by 9 V. bias, the voltage on the n-type substrate required to clear the photodiode during electronic shutter
operation was reduced by 9 V.
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Figure 10. Comparison theminimum minimum voltage to transfer charge from from the photodiode to Figure 10. Comparison ofofthe voltagenecessary necessary to transfer charge the photodiode the vertical shift register (VCCD). The new process shows significant improvement, lowering the to theFigure vertical registerof(VCCD). The new process shows significant improvement, lowering 10. shift Comparison the minimum voltage necessary to transfer charge from the photodiode to the voltage by 3 V. voltage 3 V. shift register (VCCD). The new process shows significant improvement, lowering the theby vertical voltage Point by 3 V.Defects 6. Reduced
6. Reduced Point Defects
Quite independent 6. Reduced Point Defectsof the deep MeV photodiode implants, changes were made to the lower energy photodiode (as shown above in Figure 9) thatchanges also improved imageto quality. It was Quite independentimplants of the deep MeV photodiode implants, were made the lower energy Quite independent of the deep MeV photodiode implants, changes were made to the lower found that modifying shallow photodiode implants, BF 2 to formimage the p+ quality. pinning layer and photodiode implants (as the shown above in Figure 9) that using also improved It was found energy photodiode implants (as shown above in Figure 9) that also improved image quality. It was As for the high-dose shallow photodiode implant, reduced point defectsthe by p+ about 5×, as shown below that modifying the shallow photodiode implants, using BF to form pinning layer and As for 2 BF 2 to form the p+ pinning layer and found that modifying the shallow photodiode implants, using in Figure 11. the high-dose shallow photodiode implant, reduced point defects byby about ×, as asshown shownbelow below in As for the high-dose shallow photodiode implant, reduced point defects about55×, Figure in 11. Figure 11.
Figure 11. Point defects >5 e− at 0 °C for 2 Mp imagers with old and new shallow photodiode (PD) implants. Figure 11. Point defects >5 e− at 0 °C for 2 Mp imagers with old and new shallow photodiode (PD) Figure 11. Point defects >5 e− at 0 ◦ C for 2 Mp imagers with old and new shallow photodiode implants. 7. Reduced Image Smear
(PD) implants.
Image quality was also improved by reducing image smear [9], as shown below in Figure 12. 7. Reduced Image Smear This was accomplished 7. Reduced Image Smear in two ways. At longer wavelengths, smear is reduced via the improved
Image quality was also improved by reducing image smear [9], as shown below in Figure 12. collection efficiency of the photodiode. Because smear is essentially given as a ratio of the undesired
This was accomplished inimproved two ways. by At reducing longer wavelengths, smear is as reduced the improved Image quality was also image smear [9], shownviabelow in Figure 12. signal that leaks into the VCCD while it is being read out to that of the photosignal collected in the collection efficiency ofinthe photodiode. Because smear is essentially givenisasreduced a ratio of via the undesired This was accomplished two ways. At longer wavelengths, smear the improved photodiode, a 2× increase in QE will be followed by a commensurate drop of 6 dB in smear, for signal that leaks into the VCCD while it is being read out to that of the photosignal collected in the collection efficiency of thewavelengths, photodiode.reducing Becausethe smear is essentially given as a ratio of the undesired example. At shorter thickness of the photodiode’s p+ pinning layer a 2× increase in QEwhile will be followed by a commensurate of 6 dB in smear, for signalphotodiode, that leaks VCCD it is being region read out topinning that ofdrop the photosignal collected reduces smear.into Thisthe is because as the quasi-neutral in the layer is made thinner, there in example. At shorter wavelengths, reducing the thickness of the photodiode’s p+ pinning layer will be fewer a photoelectrons there can subsequently diffuse into the VCCD the photodiode, 2× increasegenerated in QE will bethat followed by a commensurate drop of 6[10]. dB From in smear, reduces smear. This is because as the quasi-neutral region in the pinning layer is made thinner, there earlier work on other devices without the MeV implants, we anticipated an improvement of bluelayer for example. At shorter wavelengths, reducing the thickness of the photodiode’s p+ pinning will be fewer photoelectrons generated there that can subsequently diffuse into the VCCD [10]. From smear between 2 and 5 dB, without anyquasi-neutral measureable difference in green, red, or NIR smear from the reduces smear. is because as the region the pinning layer is made thinner, earlier work This on other devices without the MeV implants, we in anticipated an improvement of blue theresmear will be fewer 2photoelectrons generated there that can subsequently diffuse thefrom VCCD between and 5 dB, without any measureable difference in green, red, or NIRinto smear the [10]. From earlier work on other devices without the MeV implants, we anticipated an improvement of blue smear between 2 and 5 dB, without any measureable difference in green, red, or NIR smear from the
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thinner pinning andand shallow n-type should noted that, because the smear thinner pinning shallow n-typephotodiode photodiode implants. implants. ItItshould bebe noted that, because the smear signalsignal is extremely small, no measurable difference in blue sensitivity was found, as expected. is extremely small, no measurable difference in blue sensitivity was found, as expected.
Figure 12. Improved smear performance with photodiode extension implants and shallower BF 2
Figure 12. Improved smear performance with photodiode extension implants and shallower BF2 pinning layer. pinning layer. 8. Conclusions
8. Conclusions
Recent process changes to improve the performance of our EMCCD devices have been
Recent process changes to improve the performance of our EMCCD devices have been successfully successfully demonstrated. These improvements resulted in increased quantum efficiency, elimination of gain aging, and reduced PD-to-CCD transfer and electronic-shutter voltages. A gain demonstrated. These improvements resulted in increased quantum efficiency, elimination of performance summary is given below in Table 2, and some sample images are shown in Figures A1 aging, and reduced PD-to-CCD transfer and electronic-shutter voltages. A performance summary is A2 in A. some sample images are shown in Figures A1 and A2 in Appendix A. givenand below inAppendix Table 2, and Table 2. Device performance summary table.
Table 2. Device performance summary table. Parameter Old Process pixel size (µ m) 5.5 Parameter Old Process −) charge-to-voltage (µ V/e 44 pixel size (µm) 5.5 −) − charge capacity, nominal (ke 20 charge-to-voltage (µV/e ) 44 − ) 900 nm (%) quantum efficiency 450, 550, 650, 49, 45, charge capacity,atnominal (ke 20 31, 5 electronic shutter quantumabsolute efficiency at 450, 550, 650,voltage 900 nm (V) (%) 49, 45,2731, 5 absolute electronic shutter voltage (V) 27
New Process 5.5 Process New 44 5.5 20 44 48, 52, 43, 2010 48,1852, 43, 10 18
Acknowledgments: The authors would like to thank Imperx for supplying some of the sample images presented, and for useful discussions with Nicholas White regarding high-energy implantation. Author Contributions: For thewould MeV deep implants, E.G.S. developed and simulated thesample process images design, presented, H.D. Acknowledgments: The authors like to thank Imperx for supplying some of the diduseful integration and implant R.P.F.regarding developedhigh-energy photolithography methods, and J.A.C. characterized and for discussions withengineering, Nicholas White implantation. the resulting sensors. E.G.S. conceived of the shallow pin process and developed and simulated the process
Author Contributions: For the MeV deep implants, E.G.S. developed and simulated the process design, H.D. design, S.L.K. did process integration, and J.A.C. characterized the devices. The EMCCD devices were conceived, did integration and implant engineering, R.P.F. developed photolithography methods, and J.A.C. characterized designed,sensors. and characterized by C.P. The mitigation process wasdeveloped conceived by J.H., E.G.S., C.P., the resulting E.G.S. conceived ofgain-aging the shallow pin process and and simulated theand process S.L.K., E.G.S., and C.P. contributed simulation, and S.L.K. performed integration. All authors contributed the design, S.L.K. did process integration, and J.A.C. characterized the devices. The EMCCD devices wereto conceived, manuscript. designed, and characterized by C.P. The gain-aging mitigation process was conceived by J.H., E.G.S., C.P., and S.L.K., E.G.S., and C.P. contributed simulation, and performed integration. All authors contributed to Conflicts of Interest: The authors declare no conflict of S.L.K. interest. the manuscript. Conflicts of Interest: Appendix A The authors declare no conflict of interest. Additional images demonstrating improved sensor performance are shown in this appendix.
Appendix A
Additional images demonstrating improved sensor performance are shown in this appendix.
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(a) (b) (a) (b) Figure A1. Sample images with NIR illuminator and no EMCCD gain. (a) Old process. (b) New process. Figure A1. Sample images with NIR illuminator and no EMCCD gain. (a) Old process. (b) New process. Figure A1. Sample images with NIR illuminator and no EMCCD gain. (a) Old process. (b) New process.
Figure A2. Sample images taken just after dusk on beach in Delray, FL, USA. Captured with Imperx Puma camera. Images fromjust a device withon the old process is shown on Captured the left, the enhanced FigureP1940 A2. Sample images taken after dusk beach in Delray, FL, USA. with Imperx Figure A2. Sampleon images taken just after dusk on beach in Delray, FL, USA. Captured with Imperx NIR process the right. Puma P1940 iscamera. Images from a device with the old process is shown on the left, the enhanced
PumaNIR P1940 camera. from a device with the old process is shown on the left, the enhanced NIR process is on Images the right. process is on the right. References References 1.
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