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Abstract
Structured scintillation screen based on oxidized Si micropore array template can effectively improve the spatial resolution of X-ray imaging. The purpose of this study is to investigate the effect of SiO2 layer thickness on the light guide and X-ray imaging performance of CsI scintillation screen when the structural period is as small as microns. Cylindrical micropores with a period of 4.3 µm, an average diameter of 3.3 µm and a depth of about 40 µm were prepared in Si wafers. SiO2 layer was formed on the pore walls after thermal oxidation. Increasing SiO2 layer thickness would be beneficial to the propagation of scintillation light along the cylindrical channels. What was not previously anticipated was that the pore size gradually shrank as the SiO2 layer thickened. The pore shrinkage would reduce the filling rate of CsI in the templates and thus would reduce the production of scintillation light. The structured CsI scintillation screens with different SiO2 layer thicknesses were fabricated by filling CsI scintillator into the oxidized silicon micropore array template. The morphology, crystallinity, X-ray excited optical luminescence, and X-ray imaging performance of the screens were studied. The results show that the spatial resolutions of X-ray images measured using the structured CsI scintillation screens with different SiO2 layer thicknesses are close to each other, and they are all about 110 lp/mm. However, the X-ray excited optical luminescence of the screen and detective quantum efficiency of X-ray imaging vary with the thickness of the SiO2 layer. The optimal thickness is about 350 nm.
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1. Introduction
As the kernel of indirect X-ray imaging detectors, scintillation screens have been widely used in non-destructive testing, astronomy, macromolecular crystallography, and basic scientific research [1–3]. To achieve better imaging quality, it is expected to develop scintillation screens with both higher spatial resolution and detection efficiency [4–7]. Increasing the thickness of scintillation screen can improve the X-ray detection efficiency, but reduce the spatial resolution of X-ray imaging because of lateral spread of scintillation light. To solve this problem, the idea of developing a scintillation screen with columnar structure was proposed. The microcolumns arranged perpendicular to the screen surface facilitate the propagation of scintillation light along the column channels. Compared with the other scintillation screens with columnar structure, the structured scintillation screen based on oxidized Si micropore array template shows its unique advantages. This structured scintillation screen is usually made by melting scintillation material into the micropores of the template [8,9]. The Si walls of micropores can suppress the lateral spread of scintillation light through light absorption, while the SiO2 total reflective layer grown on the Si wall surfaces can guide the scintillation light with an incident angle greater than the critical angle along the microcolumns [10]. Therefore, optical isolation and good light guide performance can be achieved.
CsI is a valuable material for preparing scintillation screen because of its high light yield, fast decay time and suitable density [11–13]. In addition, it has a low melting point and a higher refractive index than SiO2, so it is especially suitable for preparing the structured scintillation screen based on oxidized Si micropore array template. In recent years, the research of the structured scintillation screen is developing towards the direction of small period to achieve higher X-ray imaging resolution. Structured CsI and CsI(Tl) scintillation screens with a 4 µm pore array period were prepared [6,10]. The spatial resolution of X-ray imaging achieved using them exceeds 100 lp/mm. It should be noted that when the array period of the structured scintillation screen is as small as the order of microns, the pore wall should not be too thick, usually no more than 1 µm, otherwise the area ratio of the filled scintillator to the entire screen will be insufficient. Meanwhile, in view of the optical tunneling effect [14] of the reflective layer and the optical absorption coefficient of the absorption layer [15,16], the SiO2 and Si layers in the pore wall need to have a certain thickness to achieve good light guide and optical isolation effects. To accommodate the two requirements, the effect of SiO2 reflective layer thickness on X-ray imaging performance of the structured CsI(Tl) scintillation screen in the case of limited wall thickness has been simulated by us [17]. The results can provide a guidance for balancing the thickness relationship between SiO2 and Si layers to achieve better X-ray imaging performance.
A theoretical idea needs to be realized and verified by experiment. Moreover, the factors influencing the experiment are often more complex than the initial theoretical prediction, and it needs to be further optimized according to something new emerged from experiment. In this study, structured CsI scintillation screens with different SiO2 reflective layer thicknesses were prepared. The morphology, elemental composition, crystallinity, X-ray excited optical luminescence (XEOL) and X-ray imaging performances of the structured CsI scintillation screens were investigated. In addition, the light output and modulation transfer function (MTF) that can reflect the light guide and optical isolation effects were simulated after further consideration of something new emerged from experiment. This study can provide more solid support for optimizing the thickness of the SiO2 layer to enhance the X-ray imaging performance of the structured CsI scintillation screen.
2. Experimental and simulation methods
2.1 Sample preparation
The guide pits with hexagonal array were patterned using photolithography on p-type (100) silicon wafers with a resistivity of 30–50 Ω·cm and etched using inductively coupled plasma (ICP) technique. Then arrays of empty pores in the silicon wafer were prepared by electrochemical etching using hydrofluoric acid solution. To study the effect of SiO2 layer thickness on X-ray imaging performance of the structured CsI scintillation screen in the case of limited pore wall thickness, the silicon templates were placed in a muffle furnace to oxidize by thermal oxidation at 1000°C for different periods of time.
High-purity CsI powder (99.999%) was used as filling material. The structured CsI scintillation screens were prepared by filling CsI melt into the oxidized Si micropore array templates by means of vacuum melting and gas pressure injection method using a homemade device. The specific preparation process was the same as that described in our previous study [6] except that the melting temperature and its holding time were 660°C and 20 minutes respectively. The procedure is illustrated in Fig. 1.
2.2 Characterization
The morphologies of the oxidized silicon micropore template and structured CsI scintillation screen were examined using a field emission scanning electron microscope (FE-SEM, ZEISS Gemini 300). The distribution of elements including oxygen and silicon in the template was measured by an energy dispersive X-ray spectrometer (EDS, OXFORD Xplore 30). More accurate elemental composition of the pore wall, including the oxidized and non-oxidized layers, was measured by a focused ion beam-scanning electron microscope (FIB-SEM, FEI Scios2 HiVac) and a transmission electron microscope-EDS elemental analyzer (TEM, FEI Talos F200X G2; EDS, Super-X). The crystalline structure of the sample was analyzed using an X-ray diffractometer (XRD, DX-2700, Haoyuan Instrument Co., Ltd.) with Cu Kα radiation (λ= 0.15405 nm) operated at 40 kV and 30 mA. The XEOL was performed using a homemade X-ray excited spectrometer, where an X-ray tube with a tungsten anode (F30-III, Shanghai Nucl. Med. Instrum. Co., Ltd.) was used as the excitation source operated at 80 kV and 4 mA, a monochromator (SBP300, Zolix Instruments Co., Ltd.) and a photomultiplier (PMTH-S1-CR 131, Hamamatsu Photonics K.K.) were used as the spectral recorder.
The performance of structured CsI scintillation screen in X-ray imaging was characterized by the MTF and detective quantum efficiency (DQE). The spatial resolution of the imaging system was characterized in terms of MTF, which was measured using a standard edge measurement [18,19]. The value of the MTF was normalized to unity at zero frequency. The spatial resolution of the detector configuration was defined as the frequency at which the MTF decreased to 0.1. The DQE is the frequency-domain spectral representation of the signal-to-noise characteristics of a given detector configuration. In this study, it was measured according to the International Standard IEC 62220-1 [20]. The measurements were performed on X-ray Imaging and Biomedical Application Beamline (BL13HB) at Shanghai Synchrotron Radiation Facility by using an X-ray beam with energy of 20 keV. The experimental system and process are the same as that described in our previous study [6], where a visible light lensing system, which consists of a lens and a mirror, was used to couple the image on the scintillation screen to CMOS. The lens is Olympus PlanApo N 2x microscope objective with magnification of 2 × . The working distance from the screen to the lens was set at 6.2 mm. The lens used in X-ray imaging of a resolution plates is Olympus UPlanSApo 20x microscope objective. The magnification and working distance are 20 × and 0.65 mm, respectively. The incident X-ray photon flux density and exposure time were controlled at 1.48 × 106 phs·s-1·mm-2 and 10 s, respectively. The parameters used for the calculation of incident X-ray photon flux are shown in Table S1 of Supplement 1.
2.3 Simulation methods
The light output and MTF that can reflect the light guide and optical isolation effects were simulated after further considering some variation in the micropores caused by the growth of SiO2 reflective layer. X-ray imaging using a scintillation screen involves two physical processes. The first one is the generation of scintillation light in the screen under X-ray excitation. In this process, the photoelectric effect, Compton scattering, coherent scattering, ionization, multiple scattering, bremsstrahlung, and de-excitation should be considered. This process was simulated using the Geant4 (version 10.0) Monte Carlo simulation toolkit [21,22]. The second one is the propagation of the scintillation light. The light in the structured screen will undergo a series of reflections, refractions, and absorptions before it reaches the top of the screen. This process was simulated using the Monte Carlo ray-tracing method [23] and finite-difference time-domain (FDTD) method [24]. The FDTD method was mainly used to simulate the process of the scintillation light at the structural interface because the closeness of the Si and SiO2 layer thicknesses to the wavelength of the scintillation light requires the wave nature of the light to be considered.
The scintillation light yield, optical attenuation coefficient, and refractive index of the CsI scintillator were obtained from the Refs. [25,26]. The refractive indices and optical attenuation coefficients of Si and SiO2 were obtained from the Refs. [16,17,27,28]. The energy and number of the incident X-rays are the same as those set experimentally. The area of the scintillation screens was set to 0.5 × 0.5 mm2. To ensure the accuracy of the simulation, the relative ray power threshold was set to 1% in the Monte Carlo ray-tracing method, which indicates that the ray will be terminated if the ray power drops below 1% of the original power. The mesh size in the FDTD method was set to 1/34 of the wavelength. These parameters are sufficiently small such that the simulation results converge to a stable state.
3. Results and discussions
3.1 Morphology, elemental composition and crystallinity
The morphology of a prepared Si micropore array template is shown in Fig. 2, in which Fig. 2(a) and (b) are the top view and side view respectively. It can be seen that the micropores of the template are arranged in a designed hexagonal array with a period of 4.3 µm. The pores are cylindrical with an average diameter of 3.3 µm and a depth of about 40 µm. The morphologies of the micropore array templates and element mappings of Si and O in the templates after thermal oxidation of different time are shown in Fig. 3. The five rows from top to bottom in the figure show the template states after thermal oxidation of 0.0, 0.5, 1.0, 5.0 and 12.0 h, respectively. The pictures in the middle column are the SEM images of different templates. It can be seen that with the increase of oxidation time, the micropore wall thickens and the micropore diameter shrinks. The pictures in the right column are the element mappings of Si (green) and O (pink) in the templates. It can be seen that oxide layers have been formed on the surfaces of the micropore walls after thermal oxidation. The longer the oxidation time, the more serious the oxidation on the surface of the micropore walls. It should be noted that the small amounts of pink color spots emerged in the element mapping of the sample corresponding to the oxidation time of 0 h were not from the oxygen elements but from the X-ray continuum background [29]. The pictures in the left column are the enlarged views of different dotted boxes in the middle column. The oxide layers (marked with double arrow lines) are not hard to see in the pictures. Their thicknesses increase with the increase of oxidation time. The specific data are shown in Table 1. The morphologies of the micropore array templates with a larger scale after thermal oxidation of different time are shown in Fig. 4. By randomly measuring the diameters of a group of 50 micropores in each template, the average diameters obtained are also shown in Table 1. It can be seen that the longer the oxidation time, the more obvious the shrinkage of the micropore diameter. The shrinkage will affect the CsI filling rate in the template, which in turn will affect the light output of the scintillation screen. The extent of the effect is hard to predict without experiment.
Fig. 3. Morphologies of the micropore array templates and element mappings of Si (green) and O (pink) in the templates after thermal oxidation of different time (the rows from top to bottom corresponding to the oxidation time of 0.0, 0.5,1.0, 5.0 and 12.0 h, respectively).
Fig. 4. Morphologies with a larger scale (on the left) and average micropore diameters (on the right) of the templates after thermal oxidation of different time (the rows from top to bottom corresponding to the oxidation time of 0.0, 0.5, 1.0, 5.0 and 12.0 h, respectively).
Table 1. Oxide layer thicknesses and micropore diameters of the templates after thermal oxidation of different time
In order to know exactly whether the oxide layer on the surface of the pore wall is composed of SiO2, a piece of the pore wall after thermal oxidization for 5.0 h was cut off in the direction perpendicular to the pore channel by means of FIB. The SEM and high-resolution TEM images of the sample before and after FIB cutting are shown in Fig. 5(a)-(d). Figure 5(a) shows the cross section of Si micropore array template after thermal oxidization where a single pore wall was deposited with layers of gold and carbon to protect the sample from cracking when a FIB cutting operation was performed. Figure 5(b) shows a piece of sample cut by FIB where the viewing angle has been rotated about 52$^\circ $ compared with Fig. 5(a). The deposited single pore wall in green solid wireframe was separately taken out and thinned. The side and top views of the thinned single pore wall were shown in Fig. 5(c) and (d). It can be seen from Fig. 5(d) that there are obvious boundaries between the surface layers and middle part of the pore wall. The surface layers are considered oxide layers, while the middle part is considered a non-oxide layer. The samples in the red and blue solid wireframes, which represent the oxide layer on the surface and the non-oxide layer inside, respectively, were used for EDS analysis. The EDSs of the surface layer and middle part of the micropore wall are shown in Fig. 5(e) and (f), respectively. The results show that the surface layer of the pore wall is mainly composed of Si and O, with Si accounting for about 32% and O about 68%. This is basically consistent with the stoichiometric ratio of SiO2. This indicates that the oxide layer on the surface of the pore wall can be considered as SiO2, which is indeed the material expected to be used for total reflection. In addition, the middle part of the pore wall is mainly composed of Si, indicating that it can be used for light absorption.
Fig. 5. FIB-SEM, FIB-TEM and FIB-EDS of a piece of the pore wall after thermal oxidization for 5.0 h. (a) SEM of a Si micropore array template after thermal oxidization where a single pore wall was deposited with layers of gold and carbon; (b) FIB-SEM of a piece of the cut sample; (c) side FIB-SEM view of the thinned single pore wall; (d) top FIB-TEM view of the thinned single pore wall; (e) FIB-EDS of the pore wall surface layer; (f) FIB-EDS of the pore wall middle part.
The morphology of a structured CsI scintillation screen is shown in Fig. 6. The left is the top view, and the right is the side view. It can be seen that the template is densely filled with CsI scintillator. The screen surface is smooth and clean, and there are no cracks and voids in the CsI microcolumns.
The X-ray diffraction pattern of a structured CsI scintillation screen is shown in Fig. 7. It can be seen that all diffraction peaks match well with the CsI cubic structure (standard card No. JCPDS 06-0311) except the diffraction peak of the template. Moreover, the ratios of the diffraction peak of (110) crystal plane to other peaks of CsI are higher than those in the XRD pattern of CsI raw powder, which indicates that the scintillator injected into the template is CsI and has a preferred orientation of (110) crystal plane.
3.2 X-ray excited optical luminescence
The XEOL of the structured CsI scintillation screens with different SiO2 reflective layer thicknesses are shown in Fig. 8(a). It can be seen that there were two main peaks locating at 315 and 515 nm, respectively. The former was thought to result from Vk + e and H + F type self-trapped excitons (STEs) [13,30], and the latter from lattice defects caused by iodine vacancy or substituted oxygen [30,31]. In terms of spectral intensity, as shown in Fig. 8(b), the light output increases with the increase of the SiO2 layer thickness. This is consistent with what is expected when optical tunneling effect is considered. When the SiO2 layer thickness is 351 nm, the light output is much higher than that without the SiO2 layer. This indicates that the SiO2 layer is very important for light output and it needs to have a certain thickness. However, when the SiO2 layer thickness exceeds 351 nm, the light output decreases with the increase of the thickness. This was not anticipated by the initial theoretical simulations [17]. The reason for this phenomenon is the shrinkage of micropores in the template during thermal oxidation. The shrinkage affects the filling rate of the CsI in the template. After considering the shrinkage effect of micropores, the light outputs of the scintillation screens with different SiO2 layer thickness under X-ray excitation were simulated again. The simulation results are in good agreement with the experimental ones. It indicates that the thickness of the SiO2 layer on the micropore wall is not the thicker the better, and the optimal thickness is about 350 nm.
Fig. 8. (a) X-ray excited optical luminescence of the structured CsI scintillation screens with different SiO2 reflective layer thicknesses; (b) Normalized light output of the scintillation screen as a function of SiO2 reflective layer thickness.
3.3 Performance of X-ray imaging
The imaging photographs of the structured CsI scintillation screens with SiO2 reflective layers of different thicknesses under uniform X-ray exposure are shown in Fig. 9. It intuitively reflects the luminescence of each CsI microcolumn and shows the uniformity and quality of the pores filling. Figure 10 shows the histograms of the scintillation intensity of individual CsI microcolumn on each scintillation screen. The results show that the distribution of the scintillation intensity of the CsI microcolumns on each screen is fairly uniform, and the difference is about or less than 6.6%. Assuming the scintillation light intensity of the CsI screen without SiO2 layer thickness is set as 100 units, the light intensities of the CsI screens with SiO2 layer thicknesses of 118 nm, 205 nm, 351 nm and 503 nm are about 153 units, 239 units, 275 units, and 256 units, respectively. The scintillation intensity of the CsI screen with SiO2 layer thickness of 351 nm is strongest among the screens studied.
Fig. 9. Images of the structured CsI scintillation screens with different SiO2 layer thicknesses under uniform X-ray exposure. (a) 0 nm, (b) 118 nm, (c) 205 nm, (d) 351 nm, (e) 503 nm.
Fig. 10. Histograms of the scintillation light intensity of CsI microcolumns on the structured CsI screens with different SiO2 layer thicknesses under uniform X-ray exposure.
The MTFs simulated using the structured CsI scintillation screens with SiO2 reflective layers of different thicknesses are shown in Fig. 11(a) and (b). The pore shrinkage was not considered in Fig. 11(a) and was considered in Fig. 11(b). All of the MTFs are almost the same. This indicates that the optical isolation effect of the micropore walls is not obviously degraded by the thickening of the SiO2 layer and/or pore shrinkage. The reason is attributed to the synergistic action of adjacent pore walls. The MTFs measured using the structured CsI scintillation screens with SiO2 reflective layers of different thicknesses are shown in Fig. 11(c). A detailed description of the experiment and derivation procedure can be found in S1 of Supplement 1. Similar to the simulation results, all MTFs are almost the same. The corresponding spatial resolutions of X-ray images, as shown in Fig. 11(d), are around 110 lp/mm, and only increase imperceptible with the thickness of the SiO2 layer. This imperceptible improvement is due to the enhanced reflection effect of pore walls with the increase of the SiO2 layer thickness.
Fig. 11. MTFs of X-ray imaging system using the structured CsI scintillation screens with different SiO2 reflective layer thicknesses. (a) Simulation without pore shrinkage; (b) Simulation with pore shrinkage; (c) Experimental MTF; (d) Experimental spatial resolutions.
The DQEs measured using the structured CsI scintillation screens with SiO2 reflective layers of different thicknesses are shown in Fig. 12. The experiment and derivation process can be seen in S2 of Supplement 1. It can be seen that the DQE initially increases with the increase of the thickness of the SiO2 layer, but when the thickness of the SiO2 layer exceeds
Fig. 12. DQEs of X-ray imaging system using the structured CsI scintillation screens with different SiO2 reflective layer thicknesses.
351 nm, the DQE decreases with the increase of the thickness of the SiO2 layer. This is because the thicker the SiO2 layer, the better the light guide performance of the micropore wall to the scintillation light. However, when the thickness of the SiO2 layer exceeds 351 nm, the pore shrinkage becomes increasingly important, leading to the decrease of the DQE. This is consistent with the result of the scintillation light output.
The X-ray images of the resolution plate (JIMA RT RC-02, Japan Inspection Instruments Manufacturers’ Association) using the structured CsI scintillation screens with SiO2 reflective layers of different thicknesses are shown in Fig. 13. it can be seen that the image obtained by using the screen with SiO2 layer of 351 nm thickness is the clearest.
Fig. 13. X-ray images of a micro resolution plate (JIMA RT RC-02) using the structured CsI scintillation screens with different SiO2 layer thicknesses. (a) 0 nm, (b) 118 nm, (c) 205 nm, (d) 351 nm, (e) 503 nm.
4. Conclusion
The structured CsI scintillation screens with SiO2 reflective layers of different thicknesses were fabricated. The morphology, elemental composition, crystallinity, luminescence, and X-ray imaging performance of the screens were studied. The results shown that the micropores arranged in a designed hexagonal array with a period of 4.3 µm were prepared in Si wafers by photolithography, ICP etching and electrochemical etching. The pores are cylindrical with an average diameter of 3.3 µm and a depth of about 40 µm. After thermal oxidation for 0.5, 1.0, 5.0 and 12.0 h, oxide layers with average thickness of 118, 205, 351 and 503 nm were formed on the surfaces of the pore walls. At the same time, with the thickening of oxide layer, the template pore size decreased gradually. The corresponding average pore diameter shrank to 3.21, 3.08, 2.07 and 2.87 µm, respectively. Based on the elemental analysis of the oxide layer, it can be identified as SiO2. The morphology of a structured CsI scintillation screen shows that the oxidized Si micropore array template is densely filled with CsI scintillator. The screen surface is smooth and clean, and there are no cracks and voids in the CsI microcolumns. The XEOLs of the structured CsI scintillation screens show that the light output initially increases with the increase of the SiO2 layer thickness. However, when the SiO2 layer thickness exceeds 351 nm, the light output decreases with the increase of the thickness. This change is mainly due to the effect of pore shrinkage. The spatial resolution of X-ray imaging measured using the structured CsI scintillation screen with different thickness of the SiO2 layer is about 110 lp/mm, and only increases imperceptible with the thickness of the SiO2 layer. This indicates that the optical isolation effect of the micropore walls is not degraded by the thickening of the SiO2 layer and/or pore shrinkage. The reason is ascribed to the synergistic action of adjacent pore walls. The DQEs measured using the structured CsI scintillation screens show that the DQE initially increases with the increase of the SiO2 layer thickness, but when the thickness exceeds 351 nm, the DQE decreases with the increase of the SiO2 layer thickness. This is because the thicker the SiO2 layer, the better the light guide performance of the micropore wall to the scintillation light. However, when the thickness of the SiO2 layer exceeds 351 nm, the pore shrinkage becomes increasingly important, leading to the decrease of the DQE. The above results indicate that there is an optimal value for the thickness of the SiO2 layer, which is about 350 nm.
Funding
National Natural Science Foundation of China (11475128, 11675121, 11775160); X-ray Imaging and Biomedical Application Beamline (BL13HB) at Shanghai Synchrotron Radiation Facility.
Disclosures
The authors declare no conflict of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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