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2278 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 Analyzing the Radiation Degradation of 4-Transistor Deep Submicron Technology CMOS Image Sensors Jiaming Tan, Bernhard Büttgen, and Albert J. P. Theuwissen, Fellow, IEEE Abstract—This paper presents a radiation degradation study on 4-Transistor (4T) complementary metal-oxide-semiconductor (CMOS) image sensors designed in standard 0.18-μm technol- ogy. The significant contribution of this paper is a systematic evaluation of the X-ray radiation effects on image sensors from the individual device level, to the pixel level and to the level of the entire sensor. The major degradation parameters of the sensor have been analyzed. This paper also includes test structures of varying geometries of in-pixel MOSFETs, pinned photodiodes (PPD), and transfer gates (TG). Characterization was performed during different X-ray doses up to 109 krad. The major degradation-an increase in the dark signal-is analyzed by modifying the TG charge transfer time and integration time. The PPD and the TG are the elements most sensitive to the dark signal of the sensor. The radiation-related dimensional effects on the sensors are also evaluated, which show different results compared to 3T pixels. The transfer-gate length influences the dark signal due to not only the electric field variation in the TG channel but also the local defect generations. In-pixel MOSFETs are used to identify the origin of increases in radiation-induced dark signal. Shallow trench isolation (STI) oxides are responsible for the radiation degradation of the sensor. A slight degradation of the quantum efficiency was observed after radiation in the short-wavelength region. Basic hardening-by-design techniques are also presented. The discussion results of the radiation-related dimensional effects on the sensors together with the STI effect can be used as a guideline for future layout designs of radiation- tolerant sensors. Identifying the pixel dark current origin can help to determine where and how to suppress the pixel dark current generation more effectively. Index Terms— Complementary metal-oxide-semiconductor image sensor, dark signal, dose, hardening-by-design, pinned photodiode, shallow trench isolation, transfer gate, X-ray. I. I NTRODUCTION C MOS Image sensors (CIS) are nowadays becoming more popular for medical/space applications thanks to their Manuscript received November 28, 2011; revised January 10, 2012; accepted January 19, 2012. Date of publication January 31, 2012; date of current version April 27, 2012. This work was supported in part by the Dutch Technology Foundation STW Project. The associate editor coordinating the review of this paper and approving it for publication was Dr. Sandro Carrara. J. Tan is with the Electronic Instrumentation Laboratory, Delft University of Technology, Delft 2628 CD, The Netherlands (e-mail: [email protected]). B. Büttgen was with the Electronic Instrumentation Laboratory, Delft University of Technology, Delft 2628 CD, The Netherlands. He is now with MESA Imaging, Zurich CH-8005, Switzerland (e-mail: [email protected]). A. J. P. Theuwissen is with Harvest Imaging, Bree 3960, Belgium, and also with the Electronic Instrumentation Laboratory, Delft University of Technol- ogy, Delft 2628 CD, The Netherlands (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2012.2186287 low-power, low-cost and high-integration capabilities. With the introduction of a pinned photodiode (PPD) into a 4T pixel, the dark current of the photo-sensitive element is reduced to levels comparable to CCDs [1]–[3]. The dark current of 4T is also much lower compared to 3T pixels. CMOS image sensors are inherently tolerant to ionizing radiation and thus are suitable for applications in the fields of medicine and space. The CMOS tolerance to ionizing radiation is due to the thinner gate oxide used in the CIS technology as compared to CCDs. The effects of ionizing radiations on 3T CMOS image sensors have been widely studied [4], [5]. However, the radiation effects on 4T pixels and corresponding in-pixel elementary devices have not yet been very well studied. The previous knowledge gained from 3T pixel studies cannot be directly applied to 4T pixels because of the additional pinned photodiode and transfer gate in the 4T pixel. These devices make the readout operation more complicated and introduce additional dark current generating sources [2]. In 3T pixels, the dark current is mainly contributed by the surface depletion region of the photodiode edge, which is not the case for 4T pixel radiation issues [6]. The transfer gate acting as an extra transistor in 4T pixels has been reported as an additional dark current source [7]. Presently in the fabrication process of 4T pixels, additional implantations and processing steps are being made to reduce the reset noise and improve the photon-electron collections. These additional processing steps in turn affect the radiation hardness performance of the in-pixel devices. These type of radiation effects which are influenced by the 4T pixel additional processing steps, have not been completely quantified yet based on the results from the former technologies, although radiation-induced interface trap gener- ation and shallow trench isolation (STI) oxide trapped charges are still responsible for the increase in the dark signal of the sensor [8], [9]. In this work, the radiation-induced degradation behavior of a 4T pixel, particularly in the PPD and TG area, is presented. In view of the differences with the 3T pixel, the origin of the dark signal is evaluated both before and after X-Ray radiation, and a radiation-hardness design is proposed. II. EXPERIMENTAL SET-UP A. Test Structures According to previous studies, ionizing radiation is known to generate trapped charges and interface states in MOS oxides. It can induce effects such as voltage shifts, leakage current increases, etc. [10]. Therefore, it is necessary to evalu- 1530–437X/$31.00 © 2012 IEEE

2278 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 …harvestimaging.com/pubdocs/182_IEEE_J_sensors_2012b.pdf · 2278 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 Analyzing the

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Page 1: 2278 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 …harvestimaging.com/pubdocs/182_IEEE_J_sensors_2012b.pdf · 2278 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 Analyzing the

2278 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012

Analyzing the Radiation Degradation of4-Transistor Deep Submicron Technology

CMOS Image SensorsJiaming Tan, Bernhard Büttgen, and Albert J. P. Theuwissen, Fellow, IEEE

Abstract— This paper presents a radiation degradation studyon 4-Transistor (4T) complementary metal-oxide-semiconductor(CMOS) image sensors designed in standard 0.18-µm technol-ogy. The significant contribution of this paper is a systematicevaluation of the X-ray radiation effects on image sensors fromthe individual device level, to the pixel level and to the levelof the entire sensor. The major degradation parameters ofthe sensor have been analyzed. This paper also includes teststructures of varying geometries of in-pixel MOSFETs, pinnedphotodiodes (PPD), and transfer gates (TG). Characterizationwas performed during different X-ray doses up to 109 krad. Themajor degradation-an increase in the dark signal-is analyzed bymodifying the TG charge transfer time and integration time.The PPD and the TG are the elements most sensitive to the darksignal of the sensor. The radiation-related dimensional effectson the sensors are also evaluated, which show different resultscompared to 3T pixels. The transfer-gate length influences thedark signal due to not only the electric field variation in the TGchannel but also the local defect generations. In-pixel MOSFETsare used to identify the origin of increases in radiation-induceddark signal. Shallow trench isolation (STI) oxides are responsiblefor the radiation degradation of the sensor. A slight degradationof the quantum efficiency was observed after radiation in theshort-wavelength region. Basic hardening-by-design techniquesare also presented. The discussion results of the radiation-relateddimensional effects on the sensors together with the STI effectcan be used as a guideline for future layout designs of radiation-tolerant sensors. Identifying the pixel dark current origin canhelp to determine where and how to suppress the pixel darkcurrent generation more effectively.

Index Terms— Complementary metal-oxide-semiconductorimage sensor, dark signal, dose, hardening-by-design, pinnedphotodiode, shallow trench isolation, transfer gate, X-ray.

I. INTRODUCTION

CMOS Image sensors (CIS) are nowadays becoming morepopular for medical/space applications thanks to their

Manuscript received November 28, 2011; revised January 10, 2012;accepted January 19, 2012. Date of publication January 31, 2012; date ofcurrent version April 27, 2012. This work was supported in part by the DutchTechnology Foundation STW Project. The associate editor coordinating thereview of this paper and approving it for publication was Dr. Sandro Carrara.

J. Tan is with the Electronic Instrumentation Laboratory, Delft Universityof Technology, Delft 2628 CD, The Netherlands (e-mail: [email protected]).

B. Büttgen was with the Electronic Instrumentation Laboratory,Delft University of Technology, Delft 2628 CD, The Netherlands. Heis now with MESA Imaging, Zurich CH-8005, Switzerland (e-mail:[email protected]).

A. J. P. Theuwissen is with Harvest Imaging, Bree 3960, Belgium, and alsowith the Electronic Instrumentation Laboratory, Delft University of Technol-ogy, Delft 2628 CD, The Netherlands (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2012.2186287

low-power, low-cost and high-integration capabilities. With theintroduction of a pinned photodiode (PPD) into a 4T pixel,the dark current of the photo-sensitive element is reducedto levels comparable to CCDs [1]–[3]. The dark current of4T is also much lower compared to 3T pixels. CMOS imagesensors are inherently tolerant to ionizing radiation and thusare suitable for applications in the fields of medicine andspace. The CMOS tolerance to ionizing radiation is due tothe thinner gate oxide used in the CIS technology as comparedto CCDs.

The effects of ionizing radiations on 3T CMOS imagesensors have been widely studied [4], [5]. However, theradiation effects on 4T pixels and corresponding in-pixelelementary devices have not yet been very well studied. Theprevious knowledge gained from 3T pixel studies cannot bedirectly applied to 4T pixels because of the additional pinnedphotodiode and transfer gate in the 4T pixel. These devicesmake the readout operation more complicated and introduceadditional dark current generating sources [2]. In 3T pixels,the dark current is mainly contributed by the surface depletionregion of the photodiode edge, which is not the case for 4Tpixel radiation issues [6]. The transfer gate acting as an extratransistor in 4T pixels has been reported as an additionaldark current source [7]. Presently in the fabrication processof 4T pixels, additional implantations and processing stepsare being made to reduce the reset noise and improve thephoton-electron collections. These additional processing stepsin turn affect the radiation hardness performance of the in-pixeldevices. These type of radiation effects which are influencedby the 4T pixel additional processing steps, have not beencompletely quantified yet based on the results from the formertechnologies, although radiation-induced interface trap gener-ation and shallow trench isolation (STI) oxide trapped chargesare still responsible for the increase in the dark signal of thesensor [8], [9]. In this work, the radiation-induced degradationbehavior of a 4T pixel, particularly in the PPD and TG area,is presented. In view of the differences with the 3T pixel, theorigin of the dark signal is evaluated both before and afterX-Ray radiation, and a radiation-hardness design is proposed.

II. EXPERIMENTAL SET-UP

A. Test Structures

According to previous studies, ionizing radiation is knownto generate trapped charges and interface states in MOSoxides. It can induce effects such as voltage shifts, leakagecurrent increases, etc. [10]. Therefore, it is necessary to evalu-

1530–437X/$31.00 © 2012 IEEE

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TAN et al.: ANALYZING THE RADIATION DEGRADATION OF 4-TRANSISTOR DEEP SUBMICRON TECHNOLOGY CMOS IMAGE SENSORS 2279

(a)

(b) (c)

Fig. 1. (a) Cross section of in-pixel elementary devices. (b) Regular layoutof a MOSFET. (c) Enclosed layout of a MOSFET.

ate the post-irradiation performance of the in-pixel MOSFETs,TG and PPD in order to have a better understanding of theirindividual contributions to the total dark current of the pixel.Moreover, for the use of these elementary devices inside thepixel, the substrate doping profiles are modified and someextra implantations are added. Their radiation performancemay vary from previous CMOS device knowledge. As anillustration, Figure 1 shows a cross section and a composi-tion of the in-pixel elementary devices, including a pinnedphotodiode, a transfer gate and a reset transistor, the dopinglayers are also shown. This test structure is used in thefollowing measurements. Figure 1 additionally presents thedifferent layout designs of in-pixel MOSFETs. In order tostudy the behavior of individual elements of the pixel, severaltest structures are designed with varying geometry of pinnedphotodiodes, transfer gate transistors, reset transistors and in-pixel MOSFETs. The transistor geometry (W/L) ratios arevaried to study the post-irradiation channel dimension effects.Enclosed layout transistors (ELT) are designed and tested inorder to be compared with a regular transistor layout.

The sensors used for radiation performance characterizationhave an off-chip ADC (Analog-to-Digital Converter) and anon-chip CDS (Correlated Double Sampling). The CDS is usedfor the offset and kTC noise suppression. Inside the pixelarray, there are several design variations with different transfergate lengths and pinned photodiode lengths. The photodiodelength is defined in the same direction as the length of thetransfer gate.

A sub-pixel-array of 6×4 is used for each variation. Thedifferent PPD lengths used in the sub-pixel-array are 1.2 µm,3.2 µm, 5.2 µm, 7.2 µm and 9.2 µm. The TG length variesfrom 0.7 µm to 1.0 µm, 1.5 µm and 2 µm. However, thereis one array of 137×197 pixels. A 4T pixel consists of areset transistor (RST), a source follower (SF), a row selectortransistor (RS), a TG and a PPD. Figure 2 shows the schematicof a pixel together with a cross section of the PPD and TG.The measurement timing is shown as well.

Fig. 2. Pixel schematic and measurement timings.

B. Measurement Details and Radiation Doses

The radiations were all performed by an X-Ray sourceat Philips Healthcare at room temperature with a dose rateof 0.32 rad/s. The average energy of this X-Ray source is46.2 keV. During the irradiation, none of the devices wereelectrically biased.

The elementary in-pixel devices (MOSFETs, PPD and TG)were irradiated to total ionizing doses (TID) of 31 krad,86 krad, 106 krad, 109 krad and 137 krad after 3-turnradiation for different samples. The measurements on the in-pixel devices were implemented with a four-probe test-benchtogether with a semiconductor parameter analyzer after eachradiation session.

All sensors were irradiated to 60 krad except one, whichreceived a maximum dose of 109 krad. The controlling signalsfor the pixel readout were programmed using an FPGA. Thelow level voltage of the TG signal was used to increase ordecrease the dark signal. Furthermore, the variation in theVRST was used for measuring the PPD pinning voltage.

To reduce the random noise in the dark signal measure-ments, an average of 20 continuous frames was used.

III. RADIATION DEGRADATION ON CMOS IMAGE

SENSORS AND IN-PIXEL ELEMENTARY DEVICES

A. In-Pixel Devices and Radiation Performance

Unlike a 3T pixel, a 4T pixel employs a transfer gate forcharge transfer and a PPD to reduce dark current. Meanwhile,the reset transistor works at a low threshold voltage. It is

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Fig. 3. Transfer characteristic of the in-pixel reset transistor with differentvoltages applied to the transfer gate.

interesting to take a first look at the dark signal contributioncoming from the TG and the PPD in terms of the in-pixeldevice leakage current.

Since the PPD operates in the charge domain and cannotbe fabricated individually, the test structure shown in Fig. 1(a)(a reset transistor with a TG and a PPD) was manufacturedfor leakage current measurement. The transfer characteristicof the reset transistor is measured using the floating diffusionnode (FD) as a drain node. The TG node voltage is sweptfrom −1 V to 3 V for each measurement.

Figure 3 shows the increase in the drain leakage current ofthe in-pixel reset transistor (W/L = 0.5/0.6) with the increasingtransfer gate voltage before radiation. Here the drain node issupplied with 0.05 V and the source node is connected to0 V. The substrate is biased due to the diode on the wholechip, which has been proven to have no influence on theresult. There is a high electric field in the region of PPD-TGduring the charge transfer due to the local doping profile ofthe p+ layer and n-well, which is also shown using the devicesimulation [11][12]. The overlap between the transfer gateand the p+ pinning layer can further strengthen the electricfield.

When the voltage on the TG node increases, the electric fieldin the PPD-TG overlap region also increases. This increase inthe electric field turns the carriers passing through the overlapregion into hot carriers [12]. These hot carriers will thenbombard the interface beneath the transfer gate. In this course,interface traps are generated due to impact ionization inducedby these hot carriers and a leakage path is formed between theFD node and the PPD [13]. Furthermore, when the voltage onthe TG reaches 3 V, a charge transfer channel beneath theTG is formed which further contributes to the drain leakagecurrent. From the pixel point of view, a lower voltage levelon the TG can help to reduce the pixel dark current, which isdiscussed further below.

Besides the leakage current evaluation of the in-pixel PPDand TG presented above, it is also important to study thepost-irradiation leakage current performance of the in-pixelMOSFET, since it is the origin of the elementary pixel darkcurrent after radiation. Several single in-pixel MOSFETs werefabricated with different layouts. These can be used to study

Fig. 4. Radiation performance of the transfer characteristic of an nMOSFETwith a regular layout, as shown in Fig. 1(b).

Fig. 5. Radiation performance of the transfer characteristic of an nMOSFETwith an enclosed layout.

the basic radiation degradation mechanism of the in-pixeldevices, which will help in the design of a radiation hardnesspixel.

Figure 4 shows the ionizing radiation effects on a transistorwith a regular layout (a stripe-shape gate), as shown inFig. 1(b). A large post-irradiation increase in the drain leakagecurrent is observed. The threshold voltage (Vth) does not shiftdue to the thin gate oxide in the technology used to fabricatethe image sensor. The STI oxide used to isolate the devicesin this technology node can trap some holes generated fromradiation. Due to these trapped charges, a lateral leakage pathis formed between the source and drain node by a parasiticfield oxide transistor [14], [15], which consequently leads toa large increase in drain leakage current in Fig. 4.

Compared to a regular layout, the result from the enclosedlayout transistor in Fig. 5 shows a much slower drain leakagecurrent increase. The Vth does not shift, neither. The ELThas an edgeless drain/source node and thus it has less area incontact with the STI oxide than a regular layout does. Trappedcharges in the STI have less influence on the device charac-teristics as well. As a result, the ELT drain leakage currentincrease is much lower compared to the regular layout afterradiation [14][16]. However, the small post-irradiation drainleakage current increase of the ELT transistor can be attributedto the interface trap generation at the Si-SiO2 interface. These

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TAN et al.: ANALYZING THE RADIATION DEGRADATION OF 4-TRANSISTOR DEEP SUBMICRON TECHNOLOGY CMOS IMAGE SENSORS 2281

Fig. 6. Radiation effects on a pMOSFET with a regular layout.

donor-like interface traps are mostly located in the lower halfof the band gap. They mainly contribute by increasing thedrain leakage current even though they have no effect on thesub-threshold slope and threshold voltage shift [17].

Besides the results from nMOSFETs, a measurement ofa pMOSFET fabricated with this 0.18 µm technology wasperformed as well. There is no noticeable parameter degra-dation, such as a leakage current increase or Vth shift, up to86 krad. Due to the p-doped active region of pMOSFETs,the positive trapped charges in the STI oxide do not help toform a lateral parasitic leakage path through an STI-basedfield oxide transistor as they do in nMOSFETs. The depletionregion expansion beneath the STI in an n-well is inhibitedby the trapped positive charges. Furthermore, these trappedcharges are finally located far away from the Si-SiO2 interfacestates and have less effect on the interface performance.Thus pMOSFETs in this technology are inherently radiation-tolerant [18]. Future pixel designs with pMOSFETs can bepromising for radiation applications.

From the measurements in this section, it can be seen thatthe ELT layout and pMOSFETs are more radiation-tolerantbecause they present a low post-irradiation lateral parasiticleakage current increase. As in-pixel devices, they contributeless to the pixel dark current.

B. Pixel Dark Signal Origin and Radiation Degradation

Now that the radiation performance of the elementaryin-pixel devices has been evaluated, the dark signal originof the pixel and the radiation performance is studied in thissection by means of the transfer gate. The transfer gate of a4T pixel can be used to disconnect the PPD from the otherthree transistors. The measurement with the TG switched offcan be used to study the dark signal behavior of the in-pixelMOSFETs. When the TG is switched on, the dark signalperformance of the entire pixel can be obtained.

Figure 7 shows the dark signal histogram of the137×197 pixel array. Radiation doses of 30 krad and 60 kradare considered. It can be seen in Fig. 7 that when the TGis turned on, the histogram shifts to the right side and itsdistribution becomes wider, which is due to the dark signalincrease introduced by the PPD and the TG. After radiation

Fig. 7. Dark signal histogram with radiation doses and with the TG on andoff.

doses of 30 krad and 60 krad, the histogram shifts furtherto the right and shows a wider distribution, indicating theincrease of radiation induced dark signal. After the radiation,the trapped charges in the STI oxide will help to form a lateralleakage path not only within one MOSFET but also amonginter-devices, which to some extent increases the dark signal[19]. Meanwhile, the post-irradiation trapped charges in theSTI oxide around the PPD cause its nearby depletion region atthe diode boundaries to expand. This increase in the depletionregion will induce some extra generation centers and cause thedark signal to increase [18]. Post-irradiation defect generationunder the TG channel together with the high electric field at thePPD-TG region is another contribution to the large histogramshift when the TG is switched on.

However, the dark signal variations when the TG is switchedon and off are obviously different. When the TG is off, the darksignal is mainly contributed by the in-pixel MOSFETs, whichdemonstrates a slight post-irradiation dark signal histogramshift. However, the measurement with the TG switched onshows a much larger post-irradiation histogram shift due tothe additional contribution from the PPD and TG. Therefore,the influence of the post-irradiation leakage current increaseof in-pixel MOSFETs on the dark signal degradation of thesensor is relatively small. As a conclusion, the PPD and TGhave a major effect on the dark signal increase after radiation.

From the above result in Fig. 7, it already shows the PPDtogether with the TG are the main contributors to the pixel darksignal compared to the in-pixel MOSFETs, particularly afterradiation. In order to further distinguish whether the majordark signal source is the PPD or the TG, charge transfer timeand integration time are manipulated. Figure 8 shows a largerelative increase in dark signal induced by a TG charge transfertime extension. In order to minimize the effects of the PPD,a very short integration time is used and the relative increasein dark signal is calculated. A constant electrical stress exitswhen the TG gate is biased and the transfer time is long. As aresult, at the PPD-TG overlap area, the impact ionization willoccur, generating more defects. Therefore, as shown in Fig. 8,by extending the TG pulse from 20 µs to 100 ms, a 100 ms TGpulse will induce an approximate 1000% relative increase in

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2282 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012

Fig. 8. Relative dark signal increase induced by TG charge transfer timeincrease with an integration time of 5.05 ms.

Fig. 9. Relative dark signal increase induced by PPD integration time increasewith a TG charge transfer time of 5 ms.

dark signal. By contrast, Fig. 9 shows a lower relative increasein dark signal coming from the PPD integration time increase.Comparing Fig. 8 and Fig. 9 shows that if the charge transfertime and integration time are both extended from 10 ms to100 ms the induced relative increase in dark signal fromthe TG is around 200 times larger. Therefore, the TG is amajor dark signal source in a 4T pixel, while the dark signalcontribution from the PPD is relatively small.

C. Radiation Effects on PPD and TG

According to the previous studies, the 4T pixel dark signalis mainly attributed to the TG and PPD. In order to furtherinvestigate their radiation behavior regarding several aspects, amore detailed radiation study on PPD and TG will be presentedin this section. This includes the measurement of the PPDpinning voltage, the PPD and TG dimensional effect and theinfluence of the TG and RST signal modifications on the darksignal. These measurements are performed over different sizesof TGs and PPDs, wherein each variation is implemented overa small pixel array of 6×4.

The pinning voltage is measured mainly in order to evaluatethe post-irradiation degradation of the PPD bulk depletionregion. The PPD surface depletion region may show a different

Fig. 10. Pinning voltage measurement with radiation doses.

dimensional effect on the pixel dark signal from the 3T pixelbecause of the pinning layer. This surface depletion region isproportional to the PPD length. Therefore, as with the darksignal, the dark electrons are measured with different PPDlengths.

Based on the results above, the TG is a major dark signalsource. The dark signal generation from the TG is sensitiveto the variation in the TG channel electric field and channeldefect generation when the TG length is changing. Therefore,in order to evaluate the dark signal degradation mechanismdue to the TG length, the dark electrons are also measuredwith different TG lengths. The dark electron measurementsare performed at different integration times in order to obtainmultiple evaluation points. As mentioned in section III (A),the voltage on the TG correlates to the pixel dark current.Moreover, the voltage on the TG is set at the low value of theTG clock signal for most of the measurement time. Therefore,the dark electrons are measured as a function of the low valueof the TG clock signal with radiation.

Figure 10 shows the post-irradiation output voltage of onepixel type as a function of the low reset voltage, VRST. Thismeasurement is a tool to extract the PPD pinning voltage. Thelow reset voltage is used to allow the PPD to accept a certainamount of charge within the range of its pinning voltage. WhenVRST is connected to Vdd, the previous amount of charge canbe read out with conventional timing. As soon as the low resetvoltage reaches a value higher than the PPD pinning voltage,the PPD will no longer receive charge from VRST and theoutput voltage of the sensor remains low. The knee voltageshown in Fig. 10 therefore is equal to the pinning voltage.

Figure 10 also shows a slight increase in the pinning voltageafter radiation, with almost no variation. Therefore, neitherthe shallow surface nor bulk depletion regions of the PPDare largely expanded by the increasing trapped charges in thesurrounding STI oxide, which is induced by radiation [6].The PPD depletion region is mainly determined by a lowerdepletion region of n-well/p-epi due to the doping profile,which is deeper than the STI [11]. Thus, the radiation has lesseffect on the pinning voltage. However, when the low resetvoltage exceeds the pinning voltage, the pixel output voltageslightly goes up with the radiation doses. This can be attributed

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TAN et al.: ANALYZING THE RADIATION DEGRADATION OF 4-TRANSISTOR DEEP SUBMICRON TECHNOLOGY CMOS IMAGE SENSORS 2283

Fig. 11. Pinning voltage measurements with the variation in TG length andradiation doses.

to the radiation-induced PPD dark signal increase because theVRST does not introduce any extra charges.

Figure 11 shows the effect of TG length on the pinningvoltage measurement along with its dark signal before andafter radiation. It proves that the PPD pinning voltage is notcorrelated with the TG length at all since there is no variationin the pinning voltage with the increase in TG length.

Moreover, after 60 krad, the post-irradiation output voltageis not influenced by the TG length, neither, when measuredwith VRST larger than the pinning voltage. Hence, it can befurther confirmed that the tiny post-irradiation output voltageincrease originates mostly from the PPD dark signal. It alsoshows that the effect of PPD on pixel dark signal is small afterradiation.

Besides the above results of the PPD pinning voltageand pixel output voltage, in Fig. 12, the pixel dark signalmeasurement is shown for different sizes of PPDs in orderto study the post-irradiation degradation of the PPD surfacedepletion region. The dark signal is expressed in terms ofelectrons before and after radiation with an integration timeof 4240 ms. In the PPD, the photodiode surface is pinnedby a highly doped p-layer. This eliminates the surface p-njunction depletion region in 4T pixels that usually exists in3T pixels. It is only due to this perimeter-dependent surfacedepletion region that the surface recombination and thermalgeneration in that region contribute so much to the photodiodedark current [6]. Therefore, in 4T pixels this dimensionaleffect is greatly reduced. Furthermore, based on the pinningvoltage measurements, the chance is also small that the 4Tpixel dark signal increase can originate from a defect generatedby the depletion region expansion of the photodiode inducedby the trapped charges [6]. As shown in Fig. 12, after radiationthe pixel dark signal increases with radiation doses, however,the influence of the PPD length effect is negligible.

Besides the above study on the PPD dimensional effect,it is also interesting to evaluate the dimensional effect onthe TG, since it is the major source of pixel dark signal.Figure 13 presents the pixel dark signal increase affected bythe TG length extension before radiation. A high electric fielddistribution exists at the overlap area between TG and

Fig. 12. Pinned photodiode length effect on dark electrons with radiationdoses.

Fig. 13. Dark electrons with integration time for different TG lengths beforeradiation.

PPD [12]. As the TG length increases, the probability alsoincreases of having surface and bulk defects and hot-carriergeneration due to the high electric field induced impactionization at that region. Hence, the dark signal will increasedue to the recombination and thermal generation [7], [12].

However, a longer TG contrarily also poses a lower electricfield distribution under the TG and can raise the potentialbarrier based on the device simulation. Then the dark currentis mitigated with a longer TG. Thus, with the combinationof these two effects–gate length extension-induced defectgeneration and electric field reduction–the dark electron inFig. 13 does not proportionally increase with the TG lengtheven at a different integration time. At the same time, thepre-irradiation increase in dark signal in Fig. 13 is mainlydominated by the TG length extension-induced defect andhot-carrier generation [12], [20].

Contrary to Fig. 13, Fig. 14 shows that the post-irradiationdark signal decreases with the increase in TG length.Nevertheless, the radiation degradation on the PPD and TGstill gives an absolute increase in the dark signal of the sensordue to a large amount of radiation induced defect generationand trapped charges in the STI oxide [19].

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Fig. 14. Transfer gate length effect on dark electrons with radiation doses.

As already mentioned, a shorter TG length induces a higherelectric field under the TG. With a similar amount of post-irradiation defect generation, a higher electric field will makea higher carrier recombination probability if the same numberof defects is generated under the TG [20]. Thus, the relativeincrease in dark signal before and after radiation in the caseof a longer TG is smaller than that of a shorter TG due tothis electric field effect, as shown in Fig. 14. Meanwhile, thepost-irradiation dark signal is declining with the increasing TGlength due to a decrease in the electric field.

Moreover, the pixel dark signal is also related to the voltageon the TG. Figure 15 shows the dark signal variation with thefunction of the low value of the TG clock for two differentTG lengths before and after radiation. With a negative lowvalue of the TG clock, some defects under the TG can befilled by the holes and thus the dark current is reduced [21],[22]. This phenomenon is more obvious with the increasingradiation doses due to more defect fillings.

In this section, the radiation effects on the PPD surface andbulk depletion region are analyzed by measuring the pinningvoltage and the dark electrons vs. the PPD length. The X-Rayradiation has negligible effects on the PPD depletion regiondegradation. A similar pixel size effect study is also conductedon the TG length. However, the TG length effect on the pixeldark signal shows contrary results before and after radiation.Corresponding to section III (A), the negative low value of theTG signal helps to reduce the pixel dark signal by TG-defectfilling, as shown in this section.

D. Dark Random Noise and Quantum Efficiency UnderRadiation

In the previous sections, the radiation effects on the in-pixelindividual devices were evaluated, such as in-pixel MOSFETs,the PPD and the TG. In this section, the radiation studywill be performed through a parameter measurement of theentire sensor output. Two important parameters for the imagesensor–the dark random noise and the quantum efficiency–aremeasured in this section.

The dark random noise of the CIS was measured beforeand after radiation by taking an average of the pixel output

Fig. 15. Dark electrons with a low value for the transfer gate signal voltagebefore and after radiation for TG lengths of 0.7 and 2.0 µm.

Fig. 16. Dark random noise histogram of a 4T CIS before and after radiation.

over 20 continuous frames. The sample was irradiated up to31 krad and 109 krad. Meanwhile, the transfer gate transistorwas turned off in order to exclude the noise influence fromthe PPD and the TG.

As seen in Fig. 16, after the radiation the dark randomnoise histogram shifts right and the width grows. Duringthe radiation, the transistor Si-SiO2 surface is depassivatedand interface traps are generated. The noise histogram after109 krad radiation in Fig.16 exhibits a larger tail comparedto the original one. This can be interpreted as an increasein 1/f noise due to the increase in the number of interfacetraps [7]. Therefore, it can be concluded that post-irradiationnoise performance of the sensor worsens because of theinterface trap generation induced by the X-Ray radiation.Since during the measurements the TG was off and the PPDwas excluded, the post-irradiation noise performance shownin Fig. 16 therefore mostly comes from the 1/f noise of thesource follower. It also results from the reset noise in the 4Tpixel reduced by the on-chip CDS. Nevertheless, the noisecontribution of the reset transistor is small since it remains inthe off-state for most of the measurement time.

The quantum efficiency was measured by a monochromatorwith a 5 nm bandwidth. The number of input photons was

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TAN et al.: ANALYZING THE RADIATION DEGRADATION OF 4-TRANSISTOR DEEP SUBMICRON TECHNOLOGY CMOS IMAGE SENSORS 2285

Fig. 17. Quantum efficiency of a pinned photodiode of 4T CIS.

measured by a calibrated detector. The quantum efficiency isdefined as the pixel output signal (expressed in electrons) overthe number of input photons on a pixel. The measurement wastaken before radiation as well as after the radiation doses of86 krad and 106 krad. As shown in Fig. 17, there is no sig-nificant degradation of the quantum efficiency for most of thewavelengths after the radiation, while there is a small reductionat the short wavelength region between 400 nm and 550 nm.

The pinned-photodiode has a shallow p+ pinning layerwhich passivates the interface in order to reduce the dark cur-rent generation. However, the passivated interface states weredamaged after the radiation. Therefore, some newly generatedinterface traps can attenuate the sensor output by increasing thesurface recombination velocity. Then, the quantum efficiencyis reduced as shown in Fig. 17 [23].

IV. CONCLUSION

An overall analysis of both the ionizing radiation effects onCMOS image sensors and the origin of pixel dark signal waspresented in this work. The measurements were performed onin-pixel elementary test structures and 20 different variationsof pixel arrays.

The dark signal contribution from the PPD and TG isconfirmed by the drain leakage current measurement of anin-pixel reset transistor test structure integrated with a PPDand TG. The measurement shows an increase in the drainleakage current of the in-pixel reset transistor by raisingthe voltage applied to the TG node, because the defectgeneration is enhanced at the overlap region of PPD-TG bythe increasing TG voltage. After exposure to X-Rays, thein-pixel MOSFET gate oxide shows no degradation becausethe transistor threshold voltage does not shift. As for atransistor with a regular layout, a large increase in the leakagecurrent is observed. The trapped charges in the STI helpto form a lateral leakage path and enhance the generationcurrent. From the layout point of view, an ELT is shown tobe more radiation-tolerant. It also shows no radiation-inducedleakage current degradation in pMOSFETs. This is due to thelower possibility of lateral leakage path formation as well asa larger distance between trapped charges and interface statescompared to nMOSFETs.

Compared to the in-pixel MOSFETs, the PPD and the TGare found to be the main dark signal contributors to the pixelbefore and after radiation. The large histogram shift inducedby the TG on/off states confirms that the PPD and TG arethe main contributors. Between the PPD and the TG, the TGcontributes more to the dark signal. This is confirmed byvarying the integration and charge transfer time.

Furthermore, the X-Ray radiation shows no influence onthe PPD pinning voltage because the post-irradiation depletionregion does not expand due to the trapped charges. Theradiation-induced dark signal increase from the PPD is smalland is not proportional to its perimeter in the presence ofthe pinning layer. The effect of TG size on the dark signalshows a different trend as a function of TG length beforeand after radiation. This is because the pre-irradiation defectcreation induced by the TG extension is more dominant thanthe corresponding electric field reduction, while this situationis reversed after radiation. Moreover, with a negative low valueof the TG clock, the holes play an important role in reducingthe dark current by filling in the defects. This function isgetting more effective after radiation.

The post-irradiation interface trap generation of the in-pixeltransistors is indirectly proven by the dark random noisehistogram measurement with the TG off, which shows anobvious shift, a wider distribution, and a larger tail with 1/fnoise after radiation. The increase in surface recombinationvelocity due to the interface trap generation is regarded asthe reason for the quantum efficiency attenuation at a certainwavelength range.

As for future radiation-tolerant CMOS imager design, moreenclosed layouts can be adapted for in-pixel devices. Both thedistance between active region and STI, as well as the area ofthe active region touching the STI should be reduced as muchas possible. A p+ guard ring for the in-pixel device can alsobe helpful.

Finally, compared to the 3T pixel, the 4T pixel shows adifferent radiation-induced degradation performance. As anextra transistor in the 4T pixel, the TG becomes the majorsource of dark signal instead of the PPD and the other threein-pixel transistors both before and after radiation. However,most of the dark signal in the 3T pixel is contributed by thephotodiode due to the surface depletion region degradation.The radiation degradation of the PPD in the 4T pixel is smallcompared to the photodiode of the 3T pixel. As a result, thereis less radiation-induced dark current degradation of the entire4T pixel than the 3T pixel, which can be about one magnitudesmaller for a certain radiation dose. As for the dark randomnoise performance with radiation doses, the TG can be themajor noise source in the 4T pixel due to the hot carrier-induced noise increase at the TG-PPD overlap region. Themain noise source in the 3T pixel, on the other hand, can stilloriginate from the photodiode during radiation application.

ACKNOWLEDGMENT

The authors appreciate the involvement of H. Stoutenand T. Poorter from Philips Healthcare, Eindhoven, TheNetherlands, who helped with the irradiation work on the

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samples and MESA Imaging Zurich, Switzerland provided thetest set-ups.

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Jiaming Tan started with the bachelor program in 2002, specializing intechnical physics, at Xidian University. After he received the bachelor’s degreein 2006, he completed the Master degree (Hons.) in microelectronics fromDelft University of Technology (TU Delft), in 2008. In 2008, he graduatedwith honors and decided to pursue the Ph.D. degree.

In 2007, he joined the Thin Film Transistor group at the Department ofElectronic Components, Technology and Materials (ECTM), DIMES, Delft,the Netherlands, where he worked on his master’s thesis project entitled“Reliability Study of Single Grain Silicon Thin Film Transistor With DeviceDegradation Modeling.” Since 2008, he has been with the Image Sensor Groupat the Electronic Instrumentation Laboratory, TU Delft, The Netherlands,supervised by Prof. Albert Theuwissen. His main research interest is radiation-tolerant CMOS image sensor design and radiation effects on Solid-StateCMOS image iensors with pinned-photodiodes.

Bernhard Büttgen received the Diploma degree in electrical engineering fromthe University of Siegen, Siegen, Germany, in 2002, and the Ph.D. degree fromthe University of Neuchatel, Neuchatel, Switzerland, in 2006. He receivedthe Ph.D. degree in 3-D time-of-flight imaging from the Swiss Centre forElectronics and Microtechnology, Zurich, Switzerland.

He has been a Research and Development Engineer with MESA Imaging,Zurich, since 2006, on pixel and system level optimization of 3-D time-of-flight imagers. From 2009 to 2010, he was a Post-Doctoral Researcher with theImage Sensor Group of Prof. A. Theuwissen, Delft University of Technology,Delft, The Netherlands.

Albert J. P. Theuwissen (F’02) was born in Maaseik, Belgium, on December20, 1954. He received the Masters degree in electrical engineering from theCatholic University of Leuven, Leuven, Belgium, in 1977, and the Ph.D.degree in electrical engineering in 1983.

He joined the Micro-Circuits Division, Philips Research Laboratories,Eindhoven, The Netherlands, as a Scientific Staff Member. In 1991, he becamea Department Head of the Division Imaging Devices, including CCD as wellas CMOS imaging activities. In March 2001, he was a part-time Professor withthe Delft University of Technology, Delft, The Netherlands. In April 2002, hejoined DALSA Corp., Eindhoven. After leaving DALSA in September 2007,he started his own company Harvest Imaging, Bree, Belgium, focusing onconsulting, training, teaching, and coaching in the field of solid-state imagingtechnology. In 2006, he co-founded ImageSensors, Inc. (a California non-profitpublic benefit company), to address the needs of the image sensor community,with his peers E. Fossum and N. Teranishi. He is the author or co-author ofover 160 technical papers in the solid-state imaging field and has holds severalpatents. In 1995, he authored a textbook Solid State Imaging with ChargeCoupled Devices and co-edited the book Single Photon Imaging in 2011. In1998 and 2007, he became an IEEE ED and SSCS Distinguished Lecturer.

Dr. Theuwissen was a member of the International Electron DevicesMeeting Paper Selection Committee in 1988, 1989, 1995, and 1996. He isa co-editor of the IEEE TRANSACTIONS ON ELECTRON DEVICES specialissues on Solid State Image Sensors in May 1991, October 1997, January2003, and November 2009, and the IEEE MICRO special issue on DigitalImaging in November and December 1998. He was a General Chair ofthe IEEE International Workshop on Charge-Coupled Devices and AdvancedImage Sensors in 1997, 2003, and 2009. He is a member of the SteeringCommittee of the aforementioned workshop and a founder of the WalterKosonocky Award, which highlights the best paper in the field of solid-state image sensors. He was a Technical Committee Member of the EuropeanSolid-State Device Research Conference and the European Solid-State CircuitsConference for several years. Since 1999, he has been a Technical CommitteeMember of the International Solid-State Circuits Conference (ISSCC), andacted as a Secretary, Vice-Chair, and Chair in the European ISSCC RegionalCommittee, and since 2002, he has been a member of the overall ISSCCExecutive Committee. He was the International Technical Program ChairVice-Chair and Chair for the ISSCC in 2009 and 2010, respectively. In2008, he received the SMPTE’s Fuji Gold Medal for his contributions tothe research, development, and education of others in the field of solid-stateimage capturing. He is an Editorial Board Member of the magazine PhotonicsSpectra and a member of the International Society for Optical Engineering.