Jun Hyung Bae

A Pixelated CsI (Tl) Scintillator for CMOS-based X-ray Image Sensor

Direct and indirect imaging detectors for digital radiography have been developed during the past a few years. Among scintillators, columnar CsI(Tl) screens are used for indirect digital X-ray imaging for industrial and medical radiography. By the help of recent developments CMOS (Complementary Metal Oxide Semiconductor) is replacing the CCD (Charge Coupled Devices) in X-ray image sensor because of low operating power, standard CMOS manufacturing process, and low cost. An X-ray imaging detector consisting of pixelated CsI(Tl) scintillator and CMOS imaging sensor for application in dental radiography was developed. The manufactured CMOS imaging sensor has 128 by 128 pixels with the pitch size of 50 mum and gap width of 5 mum. A 0.5 mum CMOS process was used for sensor fabrication. Pixel structured CsI(Tl) scintillator was directly deposited on the patterned CMOS imaging sensor by thermal evaporation and photo-lithography process. The fabricated monolithic X-ray imaging sensor showed dramatic improvement in spatial resolution due to enhancement of the scintillation light guiding effect and reduction of the light cross-talk between scintillator pixels.

Development and characterization of CMOS-based monolithic X-ray imager sensor

We proposed a new design of CMOS-based X-ray image sensor with monolithically grown pixelated CsI(Tl) on photosensor area for securing the maximally achievable spatial resolution for a given sensitivity determined by the CsI(Tl) thickness at a certain X-ray energy. The test version of a CMOS image sensor (CIS) was designed and fabricated using AMIS 0.5 mum standard CMOS process. The chip includes an 128times128 CMOS active pixel sensor(APS) array with 50 mum pitch, a row/column decoder, a sample & hold circuit with buffers, an analog signal processor(ASP) and a 10 bit pipe-lined analog-to-digital converter(ADC). A data acquisition system (DAS) for operating the image sensor and for processing digital signals was also developed. Then the X-ray image sensor was fabricated by depositing thermally evaporated CsI(Tl) blocks of 50 mum thick on each pixel sensor of CIS using a photo-resist pattern as a basis of CsI(Tl) deposition. Linearity of the response, dark current noise, dynamic range and spatial resolution etc. were measured with bare and scintillator-coupled CIS image sensors by green LED light and X-ray beam respectively. The mean charge generation rate per pixel measured at room temperature in the dark was about 45,600 electrons/sec which results in the electronic noise of 210 electrons for 1 sec exposure time. Therefore the dynamic range is ~67 dB if we consider the statistical noise together. The spatial resolution of scintillator-coupled CMOS image sensor was measured to be ~7 lp/mm by a lead line pattern and 80 kVp X-ray. This works was supported by Nuclear R&D Program of MOST, Korea through KOSEF.

Development and evaluation of a high resolution CMOS Image Sensor with 17 μm × 17 μm pixel size for X-ray imaging

In this research, we designed and fabricated a CIS (CMOS Image Sensor) with 17 μm × 17 μm pixel size and 190 × 190 pixels using 0.25 μm standard CMOS process as a testversion sample for developing high resolution X-ray image sensors. Active pixel sensors, area efficient sample and hold circuits, and a switched capacitor amplifier are integrated in a single chip. A unit pixel of the sensor consists of a photodiode and a 3-transitor active pixel structure. A sample and hold circuit is designed to reduce silicon area by including only one capacitor. Finally, a current mirrored operational transconductance amplifier is used to construct a switched capacitor amplifier. Also, in order to analyze the characteristics of the CIS and obtain images, we developed a data acquisition system. The system is responsible for communicating with a personal computer as well as controlling the CIS and an external ADC. The evaluation procedure of the CIS is divided into two categories: one is to investigate the performance of the CIS itself, and the other is to evaluate the quality of the obtained image. We measured not only the linearity, sensitivity and charge-to-voltage conversion gain of the CIS, but also the spatial resolution of the X-ray image acquired by the CIS coupled with a CsI(Tl) scintillator.

Fabrication and comparison Gd 2 O 2 S(Tb) and CsI(Tl) films for X-ray imaging detector application

During the last decade, digital X-ray imaging systems have been replacing analog X-ray imaging systems of conventional X-ray film-screen combination for radiography applications. Indirect detection methods consisted of an X-ray converter (or a scintillator film) and photodiode arrays are more widely used in medical diagnoses and industrial fields. Two major scintillation materials such as terbium doped gadolinium oxysulfide (Gd 2 O 2 S:Tb, GOS) and thallium doped cesium iodide (CsI:Tl) are commonly used. In this work, GOS scintillator films were manufactured by mixing and thermal hardening of Gd 2 O 2 S:Tb powder, dispersion agent, hardening agent, and other organic additives. And CsI:Tl scintillator films with columnar structure were also fabricated by the thermal evaporation method. The scintillation properties, such as emission spectrum and light yield etc., of the GOS and CsI:Tl films were measured by X-ray luminescence and photo-luminescence (PL) methods. The maximum luminescent intensity of both scintillators was observed at 540–560nm wavelength. In order to investigate the imaging performances of both GOS and CsI:Tl films as converters of X-ray imaging detectors, both scintillator films were coupled with an CCD sensor. The light response to X-ray dose, signal-to-noise ratio (SNR), spatial resolution were measured and analyzed under the same X-ray conditions. As X-ray dose increases, the SNR curves showed linear relationship. And the spatial resolution of two scintillator films was resolved at 7∼8lp/mm.