12-inch-wafer-scale cmos active-pixel sensor for digital...

12-inch-wafer-scale CMOS active-pixel sensor for digital mammography

Sung Kyn Heoa, Jari Kosonena, Sung Ha Hwanga, Tae Woo Kima,

Seungman Yunb, Ho Kyung Kimb*

a Sensor Business Division, Vatech Humanray, Co., Ltd., Bora, Giheung, Yongin 446-904, South Korea

b School of Mechanical Engineering, Pusan National University, Busan 609-735, South Korea

ABSTRACT

This paper describes the development of an active-pixel sensor (APS) panel, which has a field-of-view of 23.1 � 17.1 cm and features 70-�m-sized pixels arranged in a 3300 � 2442 array format, for digital mammographic applications. The APS panel was realized on 12-inch wafers based on the standard complementary metal-oxide-semiconductor (CMOS) technology without physical tiling processes of several small-area sensor arrays. Electrical performance of the developed panel is described in terms of dark current, full-well capacity and leakage current map. For mammographic imaging, the optimized CsI:Tl scintillator is experimentally determined by being combined with the developed panel and analyzing imaging characteristics, such as modulation-transfer function, noise-power spectrum, detective quantum efficiency, image lag, and contrast-detail analysis by using the CDMAM 3.4 phantom. With these results, we suggest that the developedCMOS-based detector can be used for conventional and advanced digital mammographic applications.

Keywords: Active pixel sensor, cesium iodide, CMOS, detective quantum efficiency, digital mammography

1. INTRODUCTIONRecent developments in complementary metal-oxide-semiconductor (CMOS) imaging sensors have widely gained attention and spreading rapidly its application on scientific and medical imaging application because of their high-speed readout, low noise and high-spatial resolution. The recent advances in the performance of CMOS APS led the potential for diagnostic radiography. Especially on digital mammography, its unique and own characteristics can be advantage and directly available with current technology. There have been some try to develop CMOS detector for digital mammography1,2. However, they had some limitation on size. To cover the form factor of small field digital mammography (~ 24 x 17 cm), tiling with several small sub-chips was needed. Due to seaming line and blind zone between sub-chips, it has been hard to exchange directly with current digital mammography sensor. Recently large area sensors, up to wafer scale can be produced by stitching technology and it makes seamless large area sensor up to 12 x 15 cm3.

In this study, we have developed a 12 inch-wafer-scale CMOS APS panel of which each pixel element has an N-diode for photodiode and three transistors as shown in Fig. 1(a). It can cover the form factor of small field digital mammography with one chip and seamless. The pixel-to-pixel pitch is 70 �m and the active pixel array format is 3300 �2442 pixels, which provides a field-of-view of ~23.1 � 17.1 cm. The integration time varies from 0.33 to 3.5 sec. The output bit-depth is 14 bits. The detailed description on the development of CMOS APS is addressed with the measurement results of the electrical performance such as readout noise and full-well capacity in this manuscript.

* [email protected]; phone +82 51 510 3511; fax +82 51 518 4613

Medical Imaging 2011: Physics of Medical Imaging, edited by Norbert J. Pelc, Ehsan Samei, Robert M. Nishikawa,Proc. of SPIE Vol. 7961, 79610O · © 2011 SPIE · CCC code: 1605-7422/11/$18 · doi: 10.1117/12.878053

Proc. of SPIE Vol. 7961 79610O-1

A prototype detector was completed by being coupled with three different thickness of CsI:Tl scintillator. By measuring the mean pixel signal and variance as a function of x-ray air kerma, we evaluated the signal and noise characteristics as well as the quantum-limited operation range. In addition, the imaging characteristics of the prototype detector wereevaluated in terms of characteristic curve, modulation transfer function (MTF), noise-power spectrum (NPS), detective quantum efficiency (DQE), image lag and contrast-detail analysis under the International Electrotechnical Commission(IEC) W/Rh imaging condition.

2. MATERIALS AND METHODS2.1 CMOS APS

The detector was designed in standard 0.090 µm CMOS technology. On digital mammography low noise and high quantum efficiency is required much more than sensitivity. To increase fill-factor for achieving high quantum efficiency, the pixel structure is designed with three transistors instead of four transistors in Fig. 13. One is used for the pixel reset, where the pixel integration start level is set, the second is source follower input transistor for converting the accumulatedcharge on pixel capacitance to voltage and the last one is row selection (muxing). The detector is a 3300 � 2442 CMOS APS array with 70 µm pixel size. The detector is operated in usual rolling shutter way and its readout speed is up to 3 fps.Integration time is vary from 0.3 to 3.5 sec.

2.2 CMOS APS operation principle

After the pixel integration start level is set by reset transistor control the image is captured. During the image capture the

Fig. 1. (a) Schematic diagram describing pixel architecture and (b) the layout of pixel.

Fig. 2. (a) Readout direction in one channel and (b) the timing for sensor.


pixel voltage level rises because of the light signal affecting pn-junction of pixel photodiode. Acquired charge signal is proportional to the incoming light intensity in the pixel. After the integration is finished the sensor is read pixel by pixel by using the row and column transistors for muxing only one pixel from the array at time. The pixel current is converted to voltage signal and then digital signal by analog-to-digital (AD) conversion. Then several digital processing steps are used to produce the final output image.

The rows go from top to bottom and column from right to left as shown in Fig. 2 (a). Column readout is parallel for each horizontal 330-pixel segment to increase the maximum frame rate. Before actual readout, the pixels are reset with the VRLEV signal and starts charge integration after reaching 3.3V of VRLEV signal as on Fig.2 (b). After the end row clock signal the column readout will be started internally in the ASIC (application specific integrated circuit).

2.3 Electrical performance evaluation

The pixel leakage currents mainly consist of the photodiode and the reset transistor leakages4. Also the source follower input transistor gate leakage could cause problems in submicron designs. And the W/L ratio of transistor affect the amount of leakage current. High L value of dimension on transistor decreases the leakage currents. The leakage currents should be small enough for successful charge integration and readout. Major of the integrated charge should be in the pixel when the pixels are read. Leakage current causes also fixed pattern noise, since due to manufacturing defects, the leakage in the pixels is different. Pixel with high leakage due to photodiode or reset transistor defect can be seen in the raw image at different level from the surrounding pixels. Defected pixels can have high leakage or even short circuit, which prevents successful charge integration and signal readout.

The sensitivity of image sensor is highest when the signal is as close to the overflow as possible in the maximum signal case5. The sensitivity can be adjusted by photodiode selection (Nwell/Psub. N+/Psub, P+/Nwell) or by adjusting the junction depth of the particular diode5. Junction depth adjustment is usually not available in standard CMOS process and thus the standard photodiode sensitivities are usually low for some applications.

The pixel sensitivity can be adjusted by the photodiode capacitance which is typically proportional to pixel size. The sensitivity will be higher at lower pixel capacitance. And the photodiode sensitivity is in terms of the ratio of output voltage to input pixel voltage. The pixel capacitance also can be changed with source follower input transistor dimensions (W/L-ratio). High W/L-ratio increases the pixel capacitance. The QE (quantum efficiency) affect the pixel sensitivity. For the photodiodes QE is highest when capacitance is lowest, since low capacitance photodiodes have large depletion width. Also the surface absorption before the depletion region should be minimized in the photodiode manufacturing by keeping the un-depleted surface layer as thin as possible.

The dominant noise source of CMOS APS is reset noise6. The pixel reset noise Qn,reset, which is caused by the thermal noise when the pixel-reset switch is off, is defined as

pixBn, reset CTkQ �� , (1)

where, kB is the Boltzmann constant, Cpix is the pixel capacitance, and T is absolute temperature of CMOS chip. The reset kTC noise was measured to lower than the expectation given by (1). This is because of the large fall time of reset line. The reset-control-pulse waveform causes damping in the high frequency noise components. Thus kTC noise is usually lower than the given (1). Leakage currents also induce noise in a pixel. This shot noise is defined as

ereadintegleakleakagen, shot qTTIQ �� )(, . (2)

The square root of the number of electrons due to the leakage currents are integrated during the integration and readouttime. The detector readout causes also noise because of the thermal noise, row and column transistors. Usually this noise is much lower than the previous noise mechanisms so that the achieved signal-to-noise ratio will not decrease significantly by the readout. Signal crosstalk between the photodiodes and EMI (electromagnetic interference) can also increase noise in some conditions. Crosstalk appears in the pixels when the signal is integrated or when the pixels are reset in one row at a time. Photodiodes have relatively high capacitve coupling to each other. During the integration the coupling capacitance interferes the normal integration. This could be avoided by covering the photodiodes with transparent conductors, such as ITO (Indium Tin Oxide) or ZnO (Zinc Oxide), and by connecting this layer to power supply (Ground or Vdd).


22.4 X-ray per formance evaluation

The detector is made of CMOS APS which is coupled with scintillator of CsI:Tl. The tube had a tungsten target with a beryllium window and 0.05mm rhodium filter. To perform the physical x-ray characterization, the beam quality W/Rh was chosen from the IEC62220-1-2 which requires a tube voltage of 28kV with 2mmAl additional filtration, giving list to the half-value layer of 0.75 mmAl. The x-ray spectrum from a 0.3-mm focal spot-sized x-ray tube (XM1016T, IAE.CO., LTD.) was used under W/Rh condition. The x-ray dose measurement was performed with a calibrated Victoreen Nero mAx 8000 equipment. The detector was investigated in terms of the characteristic curve, pixel variance, MTF, NPS, DQE, image lag and contrast-detail analysis.

To evaluate the detector response, uniform images were acquired at different x-ray air kerma. The average value of a center area of 11 � 11 cm2 in output uniform images as a function of x-ray air kerma means the detector response and calls the characteristic curve of the detector. In digital x-ray imaging system, the mean pixel value is directlyproportional to input x-ray air kerma on the detector surface for a specified x-ray spectrum7. All image quality metrics of x-ray imaging system including MTF, NPS and DQE, are applicable only to linear range of the detector system.

To analyze noise performance on x-ray images, the signal variance of the images which were used for characteristic curve calculation was evaluated. The pixel variance on image was measured as a function of x-ray air kerma.

The pre-sampling MTF was measured by the slanted-edge method described in IEC 62220-1-2. A slanted 1-mm-thick tungsten edge phantom was placed on the detector surface aligned with x-ray beam center. The slanted degree was approximately 3 ~ 5�. It determines the number of over-sampling. Over-sampled edge-response function was acquired from edge images. The use of fourth-order, Gausian-weighted, moving polynormial fit reduce noise on edge spread function. To obtain line-spread function, noise removed edge-response function was differentiated with a standard central-difference algorithm8. The MTF was then obtained by fast Fourier transform (FFT) of the line-spread function. The MTF was normalized to its DC components. The data for MTF were gain-offset corrected. The pre-sampling MTF should be fully finely sampled to avoid aliasing error due to finite pixel spacing.

The NPS may be thought of as the variance of image intensity divided among the various frequency components of the image. The measurement of NPS was performed according to IEC62220-1-2. In principle, the NPS measurement is very simple. At a particular x-ray air kerma, it was computed by taking the square of the magnitude of a 2D fast Fourier transform of a region-of-interest in number of flat-field images (gain-offset corrected). To avoid variation and erroneous of the result, the NPS was calculated with at least 16 million data. The linearized data in a region of 256 � 256 pixels which cover the center region of x-ray field were evaluated for NPS. One-dimensional (1D) -NPS was extracted byradially averaging the 2D -NPS assuming circular symmetricity9.

The DQE is defined with the calculated MTF and NPS as

qKNPSMTFG

SNRSNRDQE

ain

out��

��

2

2

2 )(, (3)

where, q, Ka and G denote x-ray quantum fluence per unit dose, the measured x-ray air kerma on detector surface and the linearized gain (sensitivity) of the detector, respectively. The value of x-ray quantum fluence was taken from IEC 62220-1-2.

The x-ray imaging system based on the scintillator and photodiode arrays is known to have temporal artifact which decays over time. These temporal artifacts are attributed mainly to residual signals or un-evacuated charge on photodiode.The residual signals are due to delays in light generation in the scintillator or incomplete reset of signals. The residual signal from one image affects the next subsequent images. Lag effect was calculated by measuring the relative response or contrast to x-ray impulse on the detector. Images were subsequently acquired with 1.5 sec intervals after x-ray impulse. The lag effect is defined as

initialfinal

afterfinal

SSSS

lag�

�� . (4)

where Sinitial, Sfinal and Safter are the signal levels at equilibrium with x-ray impulse, the signal levels at equilibrium without x-ray exposure and the signal in the #th frame after exposure, respectively.


The contrast-detail analysis was achieved by CDMAM 3.4 phantom on Fig. 3. The phantom placed direct on detector surface under same x-ray beam quality as for the described previously on NPS, MTF and DQE10. It was exposed with various values of air kerma, 96, 241 and 606 µGy. The CDMAM 3.4 consists of an aluminum base with gold-disk of various thickness and diameter, which is attached to a Plexiglas cover (PMMA). The aluminum base consists of Al 1050 (99.5 % pure aluminum) and has a thickness of 0.5mm. 205 square cells are arranged in 16 rows and 16 columns. Each cell has two identical gold-disks (99.99 % pure gold) of various thickness and diameter. One is at the center and the other positioned at randomly selected corner. Disk diameter and disk thickness decrease logarithmically, in each column, from 2.00 to 0.06 mm and, in each row, from 2.00 to 0.03 mm, respectively

Analysis by human observers is time consuming task and could cause significant interobserver errors. The CDCOM software which is able to read the test object automatically was used for contrast-detail analysis. Under each x-ray air kerma, six images of the CDMAM phantom were taken and analyzed by CDCOM. The results from CDCOM were used to determine contrast-detail curve relating the minimal recognizing disk diameter and thickness

3. RESULTS33.1 Electr ical per formance evaluation

The electrical performance of the detector was evaluated in terms of full-well capacity, electrical noise, dark current and leakage currents. The electrical characterization parameters are summarized in table I.

Performance parameter Value Units

Quantum efficiency 45 %

Fill factor 75 %

Full well capacitance 17 M electrons

Conversion Gain 9.3 M Electrons / V

Electrical noise 2.4 K Electrons

Dark current 28 K Electrons / s

Pixel leakage current 6.0 fA

Table I. Summary of electrical characterization parameter.

Fig. 3. The CDMAM (Contrast-Detail Mammography) 3.4 phantom


The performance of dark current of the detector was evaluated in terms of the output dark level as a function of integration time of the detector. The output dark signal with different integration time in Fig. 4 have been plotted in the unit of electrons when full-well capacitance was 17 M electrons. The conversion factor of the detector was 1087electrons per ADU (analog-to-digital unit) at 14-bit depth. The output dark level shape almost linear. The simulated pixel noise without kTC component corresponded to 36 �V and kTC noise was 18 �V. Considering the pixel capacitance 1.2pF and the photodiode bias voltage, the simulated total noise is 377 electrons. On the contrary, the experimentally measured electrical noise of detector was 2429 electrons, which implies that there is another source of additive noise from the readout board, such as active chips and power supply. The additive noise from readout board was measured to 1630 electrons. Therefore, the experimental result of CMOS sensor noise is 1800 electrons and this is much less than the simulation result estimated by (1).

The leakage current of the detector was 6.0 fA which is the simulation result under 27 °C condition. The best way to check the fixed-pattern noise and bad pixel due to leakage current is to evaluate leakage-current map. To generate the leakage-current map of the detector, we acquired two sets of dark data with different integration times. The control of integration time was easily performed by varying delay time of the readout pulse signals. We obtained the leakage-current map by subtracting the dark frame with 0.5 sec of integration time from 3.5 sec. As shown in Fig. 5 (a) we could find some non-uniform response area which is relatively lower than surroundings and fixed spike pixel. Contaminated wafer during process could be main cause of non-uniform response area. The fixed spike pixel is more visible on Fig. 5

Fig. 4. The characterization of dark current of detector was presented in number of electrons of dark signal according to different integration time of the detector, from 0.5 to 3.5 sec. The line is linear fit curve describing linearity of data.

Fig. 5. (a) Leakage current map from subtracted dark frames which were acquired at different integration time showed non-uniform response region due to wafer contamination (b) 3D surface plotted of leakage current map gave easy

to recognize fixed spike pixel due to pixel leakage from defects.


(b). The root cause of fixed spike pixel could be high pixel leakage due to defects on photodiode contact region.

33.2 X-ray per formance evaluation

The response performance of the detector is described in Fig. 6 (a) in terms of characteristic curve with different scintillator thickness. The data were analyzed by first-order regression method. The response of detector is proportional to the thickness of CsI:Tl. The curve shows that the pixel response with respect to x-ray air kerma is almost linear. The linearity characteristic of this detector is different with four transistor common source pixel design which was slightly drifts in the range of very low signal3. Constant current source via optimized gate drain capacitance of source follower transistor make the response linear. The pixel variance with different x-ray air kerma is plotted on Fig. 6 (b). The variance gets higher with increasing air kerma, but its amount get lower, especially after 100 µGy.

Measured pre-sampling MTF of detector is displayed in Fig. 7 and is about 15% at the frequency of 7 lp/mm. The data were fitted with Lorenzian function. The MTF of 100, 150 µm thick CsI:Tl at high frequency are slightly higher than one of 200 µm. The difference of MTF performance between various CsI:Tl thickness is less than expectation.

Fig.8 shows the measured 1D NNPS of detector for different x-ray air kerma on 200 �m thick CsI:Tl. The 1D NNPS was extracted by radially averaging of 2D NNPS assuming circular symmetricity. All the curves of NNPS data have the same shape, but differ by a scaling factor that is proportional to x-ray dose. Due to the increased quantum noise, the noise spectral density increases as dose increases. There is a notable x-ray exposure dependence and the NNPS curve can be divided into two main group above and below at 100 µGy. These characteristic follows previous pixel variance performance.

DQE of the detector is plotted in Fig. 9 (a), (b), (c) with various x-ray air kerma (23, 96, 241 µGy) and thickness of

Fig. 6. (a) Characteristic curve of detector as a function of x-ray air kerma with different CsI:Tl thickness. (b) Pixel Variance accordance with x-ray air kerma.

Fig. 7. Pre-sampling MTF curve of detector with different thickness of CsI:Tl. The line is Lorenzian fit curve of MTF data.


CsI:Tl. The data are dependent on x-ray air kerma and CsI:Tl thickness. Over the x-ray air kerma value from 23 to 241 �Gy, DQE data on 200 µm thick CsI:Tl are higher than others at whole frequency. As on Fig. 9 (d), above air kerma value of 100 µGy DQE data at each frequency are almost same.

Lag effects were performed using the method as described by (4). The data for lag-effect test were acquired with every 1.5 sec readout time, which is the typical speed of the detector, under x-ray input dose of 241 �Gy and thus the time interval between each sequential frame is 1.5 sec. The offset data were subtracted before lag-effect test. Fig. 10 shows

Fig. 8. Measured 1D NNPS at different incident x-ray doses on 200 µm thick CsI:Tl.

Fig. 9. Measured DQE with various x-ray air kerma and thickness of CsI:Tl (a) under air kerma value of 23 �Gy (b) under air kerma value of 96 �Gy (c) under air kerma value of 241 �Gy (d) DQE as a function of air kerma on 200

µm thick CsI:Tl.


measured lag effects of the detector as a function of time after the end of x-ray exposure. Lag effect of first frame is0.04% and it is measured on 1.5 sec after the end of x-ray pulse. After 9 sec from the end of x-ray pulse, lag effect isalmost zero.

The contrast-detail analysis evaluated using CDCOM software, determined optimal CsI:Tl thickness on developed detector. The contrast-detail curves are plotted for three different thickness of CsI:Tl at the x-ray air kerma value of 96,241 and 606 �Gy on the log-log graph (Fig. 11). The correct reading minimal disk thickness and diameter on 200 �mthick CsI:Tl is smaller than others at the all kinds of x-ray air kerma value. This trend is exactly same with the results of DQE performance. The results of CDCOM should not be confused to threshold diameter or thickness value that human can observe. Many authors report that all the results from automatic methods are lower than those recognize by human eyes11.

4. DISCUSSION AND CONCLUSIONSThe electrical and x-ray performances of the prototype CMOS APS panel for digital mammography have been presented. The electrical characterization performed in terms of full-well capacitance, dark current analysis and leakage-current map. The dark current was quite small and dark signal of the detector was linearly responded to the integration time. Due to very low-level of pixel leakage current, successful charge integration and readout were achieved. However some response non-uniform region from wafer contamination and spike pixels due to pixel leakage from defects on photodiode contact region or source follower transistor were appeared in the leakage-current map. Low electrical noise (2429electrons), high full well capacitance (17 M electrons) and high quantum efficiency of the photodiode (45% at 540~560 nm) were key benefits of the prototype CMOS detector. The selected buried N-diode as the photodiode and the depth of pn-junction made the optimal pixel response and wide dynamic range. The x-ray performance was evaluated in terms of characteristic curve, pixel variance, MTF, NPS, DQE and contrast-detail analysis with different thickness of CsI:Tl. The detector was linearly responded well and its sensitivity with respect to x-ray dose in linear range was 10316electrons/�Gy on 200 �m thick CsI:Tl. Over 15 %-level of MTF result at high frequencies (7 lp/mm) would be suitable for digital mammography. DQE(0) of the detector was varying from 35 to 80 % proportional to x-ray dose and scored highest value on 200 �m thick CsI:Tl. The image lag of the first frame, which was acquired at 1.5 sec after the end of x-ray exposure, is 0.0427%. The contrast-detail analysis was achieved by CDMAM 3.4 phantom. The contrast-detail curve from CDCOM software revealed that 200 �m thick CsI:Tl performed better than 100 and 150 �m thick CsI:Tl.

Fig. 10. The data for lag test were acquired 1.5 sec integration time. Measured lag effect under x-ray dose level of 241 �Gy with respect to time after end of x-ray exposure.


In this study we have investigated the design, physical performance and feasibility of the prototype CMOS APS panel for digital mammography application. Optimization of its performance has been achieved through experimental results with different thickness of CsI:Tl (100, 150, 200 �m). The 200 �m thick CsI:Tl scored highest value of DQE and lowest disk thickness on contrast-detail curve. These results pointed out the developed CMOS APS panel with 200 �m thick CsI:Tl makes optimal image quality for digital mammography. The performance of high resolution up to 7lp/mm, low noise, wide dynamic range, no lag effect and high DQE was fully adaptable for digital mammography applications. Withthese results, we suggest that the detector has been verified in its applications. However, some performance verificationstill remained about noise source which is difference between simulation and empirical results. And also performance evaluation should be done on fast readout mode. This prototype detector can operate 3 fps of readout speed without

Fig. 11. Contrast-detail curve from CDCOM software under various x-ray air kerma with different thickness of CsI:Tl. Disk thickness is plotted against disk diameter on log-log graph (a) at the air kerma value of 96 �Gy, (b) at the air

kerma value of 241 �Gy and (c) at the air kerma value of 606 �Gy on 200 µm thick CsI:Tl.


binning. The dedicated performance for breast tomosynsthesis also should be approved. Future work includes the performance evaluation and optimization for breast tomosynthesis.

ACKNOWLEDGEMENTS

This work was supported by a Grant-in-Aid for Strategy Technology Development Programs from the Korea Ministry of Knowledge Economy (No. 10032060).

REFERENCES

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12-inch-wafer-scale cmos active-pixel sensor for digital...

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