This work presents an analysis of Dark Current Random Telegraph Signal (DC-RTS) in CMOS Image Sensors (CIS). The objective is to provide new insight on RTS in modern CIS by determining the localization of DC-RTS centers and the oxide interfaces involved. It is shown that DC-RTS centers are located near the transfer gate. In particular, it is demonstrated that both gate oxide and Shallow Trench Isolation (STI) contribute to this parasitic dark current variation

Durnez, Clémentine

Goiffon, Vincent

Magnan, Pierre

Rizzolo, Serena

Rubaldo, Laurent

Virmontois, Cédric

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(2017) In: 2017 International Conference on Noise and Fluctuations (ICNF), 20 June 2017 - 23 June2017 (Vilnius, Lithuania).Localization of Dark Current Random TelegraphSignal Sources in Pinned Photodiode CMOS ImageSensorsClémentine Durnez, Vincent Goiffon,Serena Rizzolo, and Pierre MagnanISAE-SUPAERO, Toulouse, FranceEmail: clementine.durnez@isae.frCédric VirmontoisCNESToulouse, FranceLaurent RubaldoSOFRADIRVeurey-Voroize, FranceAbstract—This work presents an analysis of Dark CurrentRandom Telegraph Signal (DC-RTS) in CMOS Image Sensors(CIS). The objective is to provide new insight on RTS inmodern CIS by determining the localization of DC-RTS centersand the oxide interfaces involved. It is shown that DC-RTScenters are located near the transfer gate. In particular, it isdemonstrated that both gate oxide and Shallow Trench Isolation(STI) contribute to this parasitic dark current variation.Index Terms—Random Telegraph Signal, CMOS image sen-sors, Pinned Photodiode, Dark current, RTS, RTN, RandomTelegraph Noise, Transfer Gate, STI, Shallow Trench IsolationI. INTRODUCTIONDARK Current Random Telegraph Signal (DC-RTS) is aparasitic random process which limits the performancein many modern solid state photodetector (silicon based andothers) in low light conditions. Indeed, as the noise impact isreduced and CIS are able to detect low flux, RTS phenomenontends to be highlighted. Such signal which trend is given inFig. 1, corresponds to discrete variations of the photodiodeleakage current (see Sec. II for the photodiode cross section),and leads to random blinking pixels. This junction leakagecurrent may be similar to the variable retention time observedin DRAM [1][2].This parasitic fluctuation is different from the well-knownRTS mechanism in MOSFET [3][4], which has alreadybeen widely analyzed and is due to a channel carrier cap-ture/emission by a trap. The RTS signal studied here is differ-ent, because the metastable states can be observed directly atthe output of the CIS (it is not the case of MOSFET RTS dueto the correlated double sampling stage [5]), it is proportionalto the integration time, and time constants between transitionsare far longer (the order of magnitude at room temperatureis about 120 s [6]). It has been demonstrated that in modernCIS, DC-RTS comes from metastable generation centers whichprobably change their geometrical configuration with timeand induce discrete levels in the dark signal [6]. PreviousThe authors would like to thank the ISAE Image Sensor Team for their help,especially Romain Molina for the design of the image sensor, Jean-BaptisteLincelles and Alexandre Le Roch for their help in the data analysis.work have shown that RTS centers are most likely situated atoxide interfaces [7] [8] in unirradiated devices. However, theprecise location of these sources and the influence of oxidecomposition on the phenomenon remain unclear.Thus, two CIS with several pixel designs (change in thetransfer gate length and shape) have been realized to determineRTS centers position and the different oxides contribution.Fig. 1. Typical RTS signals of a same pixel acquired every 0.2 s and 0.5 sduring 4500 s in dark conditions at 60°C.II. EXPERIMENTAL DETAILSThe two studied CIS are constituted of 256x256 4T-PinnedPhotoDiode (PPD)[9] pixels and manufactured using a com-mercially available 0.18 µm process. They contain respectively10 and 24 areas of 2560 to 6528 pixels, but for the sake ofclarity, only 9 designs will be discussed.Fig. 2 represents the architecture of a 4T-PPD pixel. It showsthat the source of the well known SF-RTS (which is not studiedin this work) is located at the Source Follower transistor andmay be modeled by a resistance because of the variation ofthe channel conductance due to the trap capture/emission rate.Fig. 2 also depicts the cross section of the photodiodeconsidered as the reference one. The photosensitive element,which corresponds to the N-doped volume, collects chargesduring the integration time.The applied gate voltage is thusFig. 2. Pixel architecture and cross section of the pinned photodiode used asa reference. LTG is the transfer gate length and LPPD is the PPD length.PPDSense NodeL1φ PPDDuring integrationVLOTGe- e- e- e-PPDSense NodeL1φ PPDDuring transferVHITGe-e-e-e-Fig. 3. Potential diagrams of the photodiode during integration time (on theleft) and during charge transfer (on the right).very low in order to separate the sense node and the photo-diode (see left side of Fig.3 for the potential diagram of thiscase). After the integration time, a high voltage is applied tothe transfer gate, in order to decrease the potential barrier andtransfer charges to the sense node so that the signal is collected(see right side of Fig.3 for the potential diagram of this case).The different P-doped areas permit to enhance charge transferperformances, and the P+ implant isolates the photosensitiveelement from the oxides. This technique using different dopingis commonly employed in CIS processes.Fig. 4 represents the behavior of the space charge regionextension when the applied transfer gate voltage VLOTG in-creases is shown. Indeed, as RTS centers are more likely tobe near the oxide interface[7] [8], VLOTG variation permits toplace the depleted volume in contact or not with the Si/SiO2interface, to highlight or not RTS centers contribution.This assessment is illustrated Fig.5 where the mean darkcurrent is plotted as a function of VLOTG for pixels exhibitingthe reference design shown in Fig.2. Indeed, when a negativeVLOTG is applied, the transfer gate is accumulating, leading toa low dark current, and when a positive VLOTG is applied, thetransfer gate is depleting, and dark current increases.The RTS pixels are detected thanks to a dedicated tooldescribed in [10].III. EXPERIMENTAL RESULTSA. Influence of VLOTG, LPPD, LTG and L1The CIS used for this section is the image sensor containing24 areas. Fig. 6 and Fig. 7 represent the percentage ofRTS pixels in 6 different areas for several VLOTG. First ofall, it can be seen that whatever the design is, there is noFig. 4. Cross sectional views of the pinned photodiode drawn in Fig. 2. Thefirst two images show the influence of the transfer gate voltage on the spacecharge region, and the last image is another sectional view A-A. L1 is thedistance where the implant PPPD-TG and the transfer gate overlap. PMD meansPre-Metal Dielectric and STI Shallow Trench Isolation.Fig. 5. Mean dark current as a function of VLOTG at room temperature. Thelayout used for this curve is the reference one given in Fig. 2.RTS contribution for negative voltage (0.075% of RTS pixelscorresponds to 2 pixels in the area). This shows that RTScenters are not located in the bulk, because they participate tothe dark current only when the depleted volume is in contactwith Si/SiO2 interfaces.Moreover, Fig. 6 also reveals that the percentage of RTS pix-els at a given VLOTG does not depend on the pinned photodiodelength. This means that centers are not located at the interfacewith the Pre-Metal Dielectric (PMD), which increases whenthe photodiode size increases too. Consequently, RTS centersseem not to be located near the N doped photodiode, but ratherlocated next to the transfer gate side.Fig. 7 represents the percentage of RTS pixels for 4 designs,changing the transfer gate length, or the overlap between thePPPD-TG implant and the transfer gate (L1). It can be seen thatthe transfer gate length influence on the number of RTS pixelsis not significant. However, there is an influence of L1.Indeed, Fig. 8 represents the cross section of Fig.2 in termsof potential wells. As the PTG-SN implant is less doped thanthe PPPD-TG implant, RTS centers contribution go directly tothe sense node instead of being collecting. Hence, they donot participate to measured DC-RTS even if they still exist.That is why the transfer gate length has no incidence on RTSphenomenon, because only the PTG-SN length is enhanced. Thishypothesis is in agreement with the fact that when the PPPD-TGimplant is extended (L1 increases), the ratio of RTS pixelsFig. 6. Percentage of RTS pixels for several transfer gate voltages and3 designs with different photodiode lengths. Each layout area containsapproximately 2500 pixels.Fig. 7. Percentage of RTS pixels for several transfer gate voltages and 4designs with different transfer gate lengths or changing parameter L1. Eachlayout area contains approximately 2500 pixels.increases too even if the transfer gate length remains the same.Finally, it seems that RTS centers are more likely to belocated under the TG, but the visible contribution to the darksignal comes from the overlap between the transfer gate andthe PPPD-TG implant.B. Analysis of the transfer gate shape at a given VLOTGThe CIS used for this section is the image sensor containing10 areas. In this part, some variations in the transfer gate shapeare analyzed, in order to better understand the precise locationof RTS centers. Fig. 9 represents the three designs that willbe studied. The first is the reference one, the second containsan annular transfer gate (no contact with the Shallow TrenchIsolation (STI)) and the third one has one side of the annulargate which is extended to the STI. Tab. I sums up the designsparameters around the transfer gate. The overlap between thePPPD-TG implant and the transfer gate will be called PTI (PPD-TG Implant). Additionally, ATG-PTI will be the surface sharedby the transfer gate and the PPPD-TG implant, and LSTI-PTI willbe the distance where the STI and the PPPD-TG implant are incontact under the transfer gate. As the STI depth is the samePPDSense NodeL1φ e-e-e-e-PPDFig. 8. Potential diagram of the photodiode when VLOTG is applied to thetransfer gate. Stars corresponds to RTS centers. Since the PTG-SN area is lessdoped than PPPD-TG area, its potential is higher. Thus, electrons generatedby RTS centers located in PPPD-TG (in white) area tend to go to the PPD,and electrons generated by RTS centers located in PTG-SN (in gray) area godirectly to the sense node because they go towards high potentials. Moreover,these electrons encounter a barrier when trying to go towards the PPD.Fig. 9. Three design variations and their sectional views of the photodiodesstudied : the first is the reference design, the second contains an annulartransfer gate (called AnTG), and the third is extended until STI (calledAnTG_STI). The blue lines correspond to the PTI (surface shared by thetransfer gate and the PPPD-TG implant).for each layout, the influence of this parameter (in this case,this would become ASTI-PTI) cannot be analyzed.First of all, there are much more RTS centers in thethird design (AnTG_STI). The main obvious difference is theextension of the transfer gate area and this should play a role inRTS phenomenon as mentioned in Sec.III-A. However, thereare about 2.5 more RTS pixels between AnTG and AnTG_STI,and the ratio of the area parameter (APTI) is about 1.7.Thus,the AnTG_STI contribution seems important but not sufficientto explain the location of RTS centers.Another difference between these two designs (AnTG andAnTG_STI) is the contact of the TG depleted region and theSTI sidewall. Indeed, in the first layout, the transfer gate has nocontact with this oxide. This leads to fewer RTS pixels, butsome remain anyway. Consequently, this interface certainlyplays a role but cannot be the only contribution.Hence, there seems to be a combination of several contribu-tions to RTS phenomenon. In order to estimate the influenceof each of the two interfaces, one can divide the number ofRTS pixels by the total interfaces on the CIS zone :NbRT S = X1×APT Itot +X2×LST I−PT Itot (1)TABLE IPARAMETERS OF THE DIFFERENT DESIGNS USED. ATG-PTI IS THE AREASHARED BY THE TRANSFER GATE AND THE PPPD-TG IMPLANT, LSTI-PTI ISTHE LENGTH WHERE THE STI, THE TRANSFER GATE AND THE PPPD-TGIMPLANT ARE IN CONTACT.THE NUMBER OF RTS PIXELS IS GIVEN ATVLOTG = 0.2 V.APTI (µm2) LSTI-PTI (µm) NbRTSRef 1.1 0.7 40AnTG 2.0 0.0 55AnTG_STI 1.1 0.33 140TABLE IICALCULATED CONTRIBUTION OF APTI AND LSTI-PTI AT VLOTG = 0.2 V.THE SIZE OF EACH DESIGN AREA IS APPROXIMATELY 6500 PIXELS, ANDTOTAL SURFACES ARE GIVEN AS THE SURFACE IN A PIXEL MULTIPLIED BYTHE NUMBER OF PIXELS IN THE AREA.TotalAPTI(µm2)TotalLSTI-PTI(µm)NbRTSpredictedNbRTSmeasuredRef 7000 4600 42 40AnTG 13000 0 59 55AnTG_STI 21700 7200 115 140with X1 and X2 respectively the number of RTS centersper µm2 and per µm for both contributions, and APTItot andLSTI-PTItot respectively the total APTI and LSTI-PTI on all pixelsof the matrix zone (about 6500 pixels).Tab. II represents the results obtained for the different con-tributions. It is found that there are about 0.0045 centers/µm2at the interface between the transfer gate and the PPPD-TGimplant , and 0.0022 centers/µm for the one overlapping thePPPD-TG implant and the STI under the transfer gate (if thedepth of STI is considered to be about 0.3 µm and that half ofthis depth is depleted, this would become 0.014 centers/µm2and in this case, this contribution would be more importantthan APTI). These results are given at VLOTG = 0.2 V. Forhigher transfer gate voltages, both contributions are highersince the depleted volume increases too.It can be seen that this model seems relevant, even ifthere are some differences between the values predicted andmeasured. Indeed, there are not enough statistics to be moreaccurate, because the overall ratio of RTS pixels is low.Finally, Fig.10 gives the summary of this work. RTS centersappear to be located at the Si/SiO2 interfaces, more preciselyunder the transfer gate because the photodiode size has noinfluence. Moreover, as the doping PTG-SN is lower than thePPPD-TG one, the electrons generated there go directly tothe sense node (and do not participate to the dark current).Consequently, only RTS centers located under the transfergate, AND in the PPPD-TG implant contribute to the signalcollected. Eventually, two main edges seem to be at the originof RTS phenomenon : the first is the area APTI and the secondis the distance in contact with the STI LSTI-PTI (it may alsobe an area if the depth is considered, but this parameter isdifficult to estimate correctly in this work).Fig. 10. Cross sections of the photodiode and localization of the main sourcesof RTS centers. The two contributions are shown in purple and blue.IV. CONCLUSIONThe results of this work provide new insight on RTS inunirradiated modern CIS, and allow a better understanding ofnoise and electrical temporal fluctuations in photonic devices.It has been demonstrated that there are two major contributionsto Dark Current RTS : the interface between the STI and thePPPD-TG implant under the transfer gate, and the area of thetransfer gate overlapping the PPPD-TG implant. The first onecan be removed with an annular transfer gate which has nocontact with the STI. The density has been calculated at agiven VLOTG for both contributions and this reveals that thesecond contribution is responsible for twice more RTS pixelsthan the first source (in a standard pixel design for a 7 µmpixel pitch in the studied technology).REFERENCES[1] D.S. Yaney, C.Y. Lu, R.A. Kohler, M.J. Kelly, and J.T. Nelson, “A meta-stable leakage phenomenon in DRAM charge storage-Variable hold time,”Electron Devices Meeting (IEDM), 1987 IEEE International, pp. 336-339,1987.[2] P.J. Restle, J.W. Park, and B.F. Lloyd, “DRAM variable retentiontime,”IEDM Tech. Dig, pp. 807–810,1992.[3] M. J. Kirton et. al, “Noise in solid-state microstructures: A new per-spective on individual defects, interface states and low-frequency (1/ƒ)noise,”Advances in Physics, vol. 38, no. 4, pp. 367–468, 1989.[4] E. Simoen, and C. Claeys, “Random Telegraph Signals in SemiconductorDevices,”IOP Publishing, Online ISBN: 978-0-7503-1272-1, 2016.[5] X. Wang, P.R Rao, A. Mierop, and A.JP. Theuwissen, “Random telegraphsignal in CMOS image sensor pixels,”Electron Devices Meeting (IEDM),2006 IEEE International, pp. 1–4, 2006.[6] C. Durnez, V. Goiffon, C. Virmontois, J. M. Belloir, P. Magnan, andL. Rubaldo, (2016). “In-depth Analysis on Radiation Induced Multi-levelDark Current Random Telegraph Signal in Silicon Solid State ImageSensors,” IEEE Trans. Nucl. Sci., Dec. 2016.[7] V. Goiffon, C. Virmontois, and P. Magnan, “Investigation of dark currentrandom telegraph signal in pinned photodiode CMOS image sensors,”Electron Devices Meeting (IEDM), 2011 IEEE International, pp. 8–4,2011.[8] K. Ackerson, C. Musante, J. Gambino, J. Ellis-Monaghan,D. May-nard,R.J. Rassel, and K. Ogg, M. Jaffe, “Characterization of" blinkingpixels" in CMOS Image Sensors,” Advanced Semiconductor Manufactur-ing Conference., pp. 255-258, May. 2008[9] E.R. Fossum, and D.B. Hondongwa, “A review of the pinned photodiodefor CCD and CMOS image sensors,” IEEE J. Electron Devices Soc.,vol. 2, no. 3, pp. 33-43, May. 2014.[10] V. Goiffon, G. R. Hopkinson, P. Magnan, F. Bernard, G. Rolland, andO. Saint-Pé, “Multilevel RTS in proton irradiated CMOS image sensorsmanufactured in a deep submicron technology,” IEEE Trans. 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Localization of Dark Current Random Telegraph Signal sources in pinned photodiode CMOS Image Sensors

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Localization of Dark Current Random Telegraph Signal sources in pinned photodiode CMOS Image Sensors

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