High resolution large formatted CMOS flat panel sensors for X-ray
ABSTRACT This paper describes world largest monolithic CMOS flat panel sensor (FPS) and high speed amplifier with columnar CsI scintillator directly deposited onto the active area. These elements have been employed as a compact digital camera module for dynamic X-ray imaging will be used in digital mammography and scientific image measurement. The FPS has a diagonal size of 11 inches and 4,416 × 3,520 pixel array (15,544,320 pixels), which features a high resolution of 10 lp/mm with single pixel drive. The key material of the FPS is monocrystalline silicon optimized for use in digital radiography. The sensor utilizes a direct deposited scintillator layer made from high resolution, high luminance CsI crystals grown into a needle structure onto a large formatted photodiode array for indirect detection of X-ray images. The CsI scintillation spectrum well matches the spectral response range of the photodiode. The CsI has the advantages of high sensitivity and high resolution compared to GOS phosphors screen. These image sensors are manufactured in a 0.15μ 12 inches CMOS process allowing a high fill factor of 76% for 50μ pixel. Only one chip is taken from one 12 inches wafer in order to realize a seamless active area. The monolithic amplifier blocks have 4,416 channels of charge amplifiers with internal CDS (correlated double sampling) circuit has an optimal design yielding a high gain of 0.26 μV per electron and a data transfer speed of 17.8 Mbytes per second in sufficient low noise. This paper discusses the system design, dynamic range, resolution, detective quantum efficiency (DQE) and imaging performance using ACR mammography phantom.
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ABSTRACT: A 4-sides buttable CdTe-ASIC hybrid module suitable for use in an X-ray flat panel detector (FPD) has been developed by applying through silicon via (TSV) technology to the readout ASIC. The ASIC has 128 times 256 channels of charge integration type readout circuitry and an area of 12.9 mm times 25.7 mm. The CdTe sensor of 1 mm thickness, having the same area and pixel of 100 mum pitch, was fabricated from the Cl-doped CdTe single crystal grown by traveling heater method (THM). Then the CdTe pixel sensor was hybridized with the ASIC using the bump-bonding technology. The basic performance of this 4-sides buttable module was evaluated by taking X-ray images, and it was compared with that of a commercially available indirect type CsI(Tl) FPD. A prototype CdTe FPD was made by assembling 9 pieces of the 4-sides buttable modules into 3 times 3 arrays in which the neighboring modules were mounted on the interface board. The FPD covers an active area of 77 mm times 39 mm. The results showed the great potential of this 4-sides buttable module for the new real time X-ray FPD with high spatial resolution.IEEE Transactions on Nuclear Science 09/2009; · 1.22 Impact Factor
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ABSTRACT: This paper presents a new low-power compact CMOS active pixel circuit specifically optimized for full-field digital mammography. The proposed digital pixel sensor (DPS) architecture includes self-bias capability at ±10% accuracy, up to 15 nA of dark-current autocalibration, built-in test mechanisms, selectable e(-)/h(+) collection, 10-b lossless charge-integration analog-to-digital conversion, more than 1 decade of individual gain tuning for fixed pattern noise cancellation, and a 50-Mb/s digital-only input/output interface. Experimental results for a 70-μm pitch 8-μW DPS cell example are reported in 0.18-μm CMOS 1-polySi 6-metal technology.IEEE Transactions on Biomedical Circuits and Systems 10/2011; 5(5):481-489. · 2.74 Impact Factor
- Journal of the Korean Society for Precision Engineering. 01/2008; 25(1).
The 8th International Conference of the Slovenian Society for Non-Destructive Testing
»Application of Contemporary Non-Destructive Testing in Engineering«
September 1-3, 2005, Portorož, Slovenia, pp. 165-172
LARGE FORMATTED AND HIGH RESOLUTION CMOS FLAT
PANEL SENSORS FOR X-RAY
H. Mori, R. Kyuushima, K. Fujita, M. Honda
HAMAMATSU Photonics, 1126-1 Ichino-cho, Hamamatsu-city, 435-8558 Japan
The CMOS panel sensors developed for detecting high quality X-ray images, feature large
formatted active area and high resolution. The purpose of this study was to assess the
performances of imager have a diagonal size of 6.9 inches (non-tiled monolithic chip) and made
of monocrystalline silicon optimized for use in digital radiography associated with two kind of
thallium doped cesium iodide scintillator. The C7942CA-02 has 50 um pixels, 2400 × 2400 pixel
array (5,760,000 pixels) features high resolution of 8 lp/mm with single pixel drive. The
C7943CA-02 has 100 um pixels, 1248 × 1248 pixel array delivers in a high frame rate of 30
frames/sec achieved by means of 4×4 binning readout. Both sensors utilize an FSP (flipped
scintillator plate) made from high resolution, high luminance CsI crystals grown into a needle
structure and coupled to a photodiode array for indirect detection of X-ray images.
Furthermore, needle structure CsI was directly deposited upon the sensor chip to accomplish
optimum resolution. The CsI scintillation spectrum well matches the spectral response range of
the large formatted photodiode array. These image sensors are manufactured in a standard
CMOS process rule allowing a high fill factor of 79 % for the C7942CA-02 and 87 % for the
C7943CA-02. The monolithic amplifier blocks have 2400 channels of charge amplifiers with
internal CDS (correlated double sampling) circuit has an optimal design, yielding a high gain of
1.07 uV per electron and a data transfer speed of 23 M bytes per second in sufficient low noise.
Keywords: CMOS flat panel sensors, X-ray
Over the last 10 years, research and development into various methods for capturing X-ray
image with solid-state image sensors have been carried out, and some remarkable results have
been recently achieved. Solid-state image sensors are expected to replace conventional image
acquisition methods based on ortho film or imaging tubes, also ideal for digital imaging.
Outdated X-ray image intensifiers have a photoelectric conversion and amplification section for
converting X-rays into photoelectrons and then accelerating the electrons to allow high
sensitivity and measurement of moving images. However, since these are vacuum tube devices,
their structure is fragile and bulky, performance variations tend to occur, image distortion
appears, has an effect on magnetic field. Moreover, their dynamic range is narrow and the
detector downsizing (especially for thin unit structures) is not possible. Meanwhile, investment
of large sums of fund has led to development of solid-state image sensors using amorphous
silicon material - and an array device with the large active area required , . The
current process technology used for amorphous silicon unfortunately exhibits an extremely long
signal decay time. This is because a high-concentration metastable level is present between the
conduction band and forbidden band, consequently the decay (after image or image lag) often
lasts from several hundred microseconds to few seconds. This prevents amorphous silicon
devices from delivering images with a high resolution and high frame rate. Another drawback is
that fill factor is poor because of the wide electrode width and switch size, so extremely small
pixels cannot be archived. Amorphous silicon devices have a pixel size of about 150 um and a
fill factor of 50 to 60 %. On the other hand, direct conversion types using materials such as a-Se
,  and CZT  are not subject to light diffusion from the phosphor material so that high
resolution image sensors can be developed. However, these direct conversion types require bump
bonding with TFTs, making it nearly impossible to fabricate megapixel image sensors having a
pixel size of several tens of microns yet small defective pixels.
In recent years, large formatted CMOS solid state image sensors of monocrystalline material
have become available for digital radiography and dynamic imaging. Authors have developed
two types of flat panel sensors that show the tremendous potential offered by large size image
sensor devices implemented in a 0.6 micron standard CMOS process. The C7942CA-02 is a
high-resolution type flat panel sensor with a pixel size of 50 um in a 2400×2400 pixel array. The
C7943CA-02 has a 1248×1248 pixel array with a pixel size of 100 um, realized high speed of 30
frames per second by means of 4x4 binning operation. These devices have an anti-blooming
facility (overflow drain), a correlative double sampling (CDS) circuit, external frame start,
binning functions and 12-bit digital data transfer at high speeds of 23 MHz. The devices are
externally comprised of a thin flat panel of 28 mm.
When the high-sensitivity photodiode matrix receives scintillation light from the newly
developed FSP (flipped scintillator plate), it generates carriers and accumulates them in the
junction capacitance of each pixel. All pixels have an overflow drain function so that blooming
in adjacent pixels is suppressed even when a portion of the pixel saturate. Our standard CMOS
process implemented with a photodiode matrix with 2×2 and 4×4 binning functions achieves a
high ratio of fill factor of 79 % for 50×50 um pixels and 87 % for 100 um pixels (See Fig. 1.)
Supplied shift pulses by the vertical scanning shift resistor for sequential scanning, one line of
photodiodes flows the carrier into each data line. The on-chip 2400 channels and 1248 channels
of extremely low noise charge amplifier array for 50 um and 100um pixel with CDS circuit is
somewhat complicated in terms of structure and operation, however the offset components can
be cancelled out by finding the differential between the accumulated charge and zero level. This
makes a huge improvement in image output uniformity. Generally, some kinds of corrections are
made to an acquired image before it is displayed, but the flat panel sensor was designed to have a
high level of image quality before making those corrections. The output noise is mainly
determined by the charge amplifier itself and the data line capacitance. The ENC (total
equivalent noise charge) is given by the following equations.
where: Cd - junction capacitance of photodiode, Cp - data line capacitance, Cf - feedback
capacitance of the charge amplifier, K - Boltzmann’s constant 1.3806×10-23, T - absolute
temperature, Vout: output voltage and Q - signal charge.
Fig. 1: Photodiode matrix.
Reducing the noise here requires increasing the FET conductance (gm) at the input stage of the
charge amplifier and lowering the Ct. The noise at a pixel size of 100×100 um is approximately
One major application of the large formatted image sensor is in the X-ray imaging field. There
are two methods for to design X-ray devices from monocrystalline silicon. In the direct detection
method, an electron-hole pair is obtained upon absorbing an X-ray energy of approximately 3.6
eV. Silicon with a thickness of 6.5 mm would be required to absorb for example 50 % of an X-
ray energy of 50 keV. However, not only a thickness of 6.5 mm is impossible to obtain using
standard CMOS processes and materials but such a device would tend to have a high cost and
poor performance characteristics.
Fig. 2: CsI emission spectrum and photodiode spectral response.
The other method is the indirect method utilizing a scintillator. A spectral response characteristic
that matches the peak wavelength and spectral range of the scintillator emission is a critical
factor in the photodiode matrix design. (See Fig. 2.) By controlling the impurity profile and anti-
reflection layer, we have successively developed an optimal, high-sensitivity X-ray device that
is ideal for indirect X-ray detection. The needle-like crystal morphology of CsI:Tl (See Fig. 3.)
mounted on the large formatted photodiode array allows to channel the scintillation light
through the fiber-like crystals. This structure offers advantages in light propagation over other
scintillators having grainy crystalline structure. The FSP (flipped scintillator plate) formed on
the photodiode active area also provides higher intensity and better resolution compared to
Gd2OsS:Tb used for medical diagnosis screens.
Fig. 3: Newly developed CsI FSP with needle structure.
3.1 Dynamic Range
A design achieves low noise amplifier and in particular lowers input capacitance as much as
possible in the initial stage of the charge amplifier is essential for attaining a wide dynamic range
at the 12 bit, 4096 gray level. A wide dynamic range is obtained by keeping the output swing
width of the amplifier within the range of the 5 V supply used in the standard CMOS rule and by
optimizing the saturation voltage for the photodiode array. However, improving the gain in the
initial stage of amplifier matched to the fine pixels means reducing the charge amplifier feedback
capacitance Cf. A one line portion of gate switch drain capacitance is also added on the data line
capacitance, along a large active area making up large input capacitance Ct. X-ray image sensors
usually require a large active area, however the value for Ct in formula (2) then becomes too
large so that a format with a large active area and extraordinary small pixels has drawbacks in
terms of noise. However by optimizing the gm of the initial amplifier stage, and using patterning
that miniaturizes the data lines and reduces stray capacitance, the C7942CA-02 and the
C7943CA-02 achieved respective wide dynamic ranges of 2100 and 4300. Since a highly stable
X-ray source is not obtained for measurements, a simulated light system emits a light spectrum
nearly close to the CsI emission spectrum was used. The scintillator was then removed from the
flat panel sensor and placed in a dark box facing the light source. After setting the light source
on a fixed light intensity with a neutral density filter, the distance between the light source and
flat panel were adjusted and the linearity was measured. (See Fig. 4)
Video Output Linearity for C7943CA-02
Incident Light Normalized
Video Output Voltage (V) .
Limit = 2.65V
Dynamic Range =4260
Noise = 0.62mVrms
Fig. 4: Linearity data of C7943CA-02.
An analog video signal before being inputed to the AD converter was measured on a Hewlett
Packard 54845A oscilloscope. The 100 micron C7943CA-02 has over 4200 dynamic range is
good agreement with calculated noise level and simulated saturation voltages of the charge
The CTF characteristics of the C7942CA-02 was measured using a resolution target (Nuclear
Associates #07-553 lead-thickness 0.05 mm, 07-525 lead thickness 0.03 mm), a micro focus X-
ray source (Hamamatsu Photonics L6731-02) and an image grabber card (National Instruments
PCI-1424) at x-ray tube voltages of 40 kVp. After correction between the resolution target image
data and the dark current image data, the shading was normalized under light field image data.
One line of data was extracted from the final image and CTF (contrast transfer function) was
calculated from the contrast ratio.
CTF of C7942 with FSP and Direct Deposition
C7942 with directly
Fig. 5: Resolution curve of FSP and Direct deposition.
The FSP (flipped scintillator plate) used for the panel sensors is manufactured by growing
needle-shaped CsI crystals on a glass substrate with an aluminum reflective film in a resistor
heated vacuum furnace. The furnace well suited to the compound deposition, provide accurate
temperature control to achieve high polycrystalline structure. Thanks to the appropriate rotation
of the substrate and revolution of the fixtures provided uniform and fine morphology of 5-micron