中文责编:方 圆; 英文责编:木 南
Lei Yaohu, Huang Jianheng, Liu Xin, Li Ji, Guo Jinchuan, and Niu HanbenCollege of Optoelectronic Engineering, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen 518060, Guangdong Province, P.R.China
为制作高质量的铋吸收光栅以提高X射线微分相衬成像系统性能,针对表面改性环节对微铸造技术进行改进,以Bi2O3取代SiO2作为改进的浸润层.在周期为3 μm、深度为150 μm的分析光栅结构上制作了具有高填充率的分析光栅,获得了该光栅的扫描电子显微镜照片.为进一步表明改进微铸造技术制作分析光栅的优越性,与改进前的分析光栅进行对比.将两种光栅先后置于X射线微分干涉相衬成像系统中,分别获取了系统莫尔条纹.从最终的条纹对比度上判断,改进后的微铸造技术制作的分析光栅填充率明显提高.
To fabricate high performance Bi absorption gratings for use in X-ray differential phase-contrast imaging(DPCI), the micro-casting method was improved via surface modification. Bi2O3 was used in place of SiO2 as an improved wetting layer. A scanning electron microscopy showed that a high filling-ratio analyzer grating with a period of 3 μm and a depth of 150 μm was obtained. Furthermore, the superiority of the presented analyzer grating to the one fabricated using the traditional micro-casting method was revealed through a comparison of moiré patterns. These tests demonstrate an enhancement to the micro-casting method for the fabrication of absorption gratings.
Since Pfeiffer et al. proposed the use of an X-ray tube in X-ray differential phase-contrast imaging(DPCI), the technology has garnered more attention than other X-ray phase-contrast methods. As a result, DPCI is expected to find applications in product inspection, medical imaging, and other non-destructive observation systems[2-5].
The quality of a phase-contrast image depends strongly on the visibility of the moiré fringe[6-7]. The main factor influencing the fringe visibility is the transmission of X-ray through the absorption grating metal. For the device presented in this work, the transmission is determined primarily by the height of Bi in the absorption grating. This paper reports an improvement to the micro-casting process, which is used to fabricate an analyzer grating with a higher Bi filling-ratio to enhance moiré fringe visibility. Differing from LIGA(lithography, electroplating and molding)and David's process[10-13], the micro-casting method has proven to be a low-cost way to fabricate Bi absorption gratings. The fabrication process as previously described involves the formation of high aspect-ratio(HAR)grating structures, surface modification, and the filling of these structures with molten Bi. However, in previous efforts, the molten Bi did not fully fill these structures(period of 3 μm and depth of 150 μm), especially close to the bottom. It was concluded that insufficient wettability between the layer of the SiO2 and the molten Bi might have been the issue. To address this issue, Bi(NO3)3 in acetone was used to fill the grating structures and to cover the side walls under vacuum pressure and ultrasonic treatment. Bi(NO3)3 then decomposed to Bi2O3 before the molten Bi filling to avoid the agglomerate of unwanted Bi on the upper surface of the analyzer grating. Furthermore, to quantify the improved performance of the presented analyzer grating in comparison to the previously produced one, a comparison between their moiré fringes is given.
For the first step of micro-casting, the analyzer grating structure was fabricated using photo-assisted electrochemical etching(PAEE)[16-17]. A structure was obtained with a period of 3 μm and a depth of 150 μm in a 5-inch n-type <100> silicon wafer. This process has been reported previously[15,18]. Note that the side walls of the analyzer grating were reinforced by the vertical walls, giving the analyzer grating the appearance of a pore array, as shown in Fig.1.
In the second step, a wetting layer was established on the surfaces, including the side walls. The improved surface modification can be divided into two stages: filling with low and high concentration solutions; and the transformation from Bi(NO3)3 to Bi2O3. First, Bi(NO3)3 was dissolved in highly volatile acetone to form low(4%)and high(29%)concentration solutions. Experiments revealed that different concentration Bi(NO3)3 solutions showed different physical characteristics on the structure surfaces. A lower concentration solution results in a smaller contact angle, which is beneficial to overcoming surface tension. Therefore, after the structure was filled with the low concentration solution, a thin Bi(NO3)3 layer was deposited on the surface of side walls. A higher concentration solution results in a larger contact angle, which can make it difficult(or impossible)to enable complete filling. However, a small contact angle would appear if the high concentration solution contacts with the thin layer formed by the low concentration solution. As a result, more Bi(NO3)3 was introduced. Therefore, successive application of the low and the high concentration solutions in a vacuum environment ensured full application of the Bi(NO3)3 along all surfaces. It should be noted that the solution flowed freely into the structures if sonicated at a frequency of 80 kHz. The Bi(NO3)3 collects unwanted Bi agglomerates on the upper surface of the analyzer grating after the third step, but the Bi2O3 leads to a clean surface. Therefore, for the second stage, after the solvent was completely dispersed, the structure was heated to 600 ℃ to transform the Bi(NO3)3·5H2O in the structure to a Bi2O3 powder.
For the third step, the analyzer grating structure was filled with molten Bi by use of a high temperature and pressure furnace, which has been described previously. The structure was immersed into the container, which is full of molten Bi. Due to the wetting layer and the applied pressure(0.7 MPa), the molten Bi flowed freely into the HAR structure until it reached the bottom. Then, the structure was removed from the container and allowed to cool to room temperature.
A SEM cross-section back-scattered image of the analyzer grating is shown in Fig.2. The bright lines correspond to the filled Bi. It should be noted that during the preparation of the sample, breakage can occur in a filled Bi line, as shown in the black outlines in Fig.2. The interruptions of some Bi lines result in the observation of some dark regions. The result shows that the molten Bi reached the bottom of the grating structure. Compared to existing results, the advantage of the improved micro-casting method is clear. Additionally, a source grating was also fabricated using the improved micro-casting method, and the SEM cross-section image is shown in Fig.3. The black and the grey regions correspond to the silicon and the filled Bi, respectively.
Fig.3 SEM cross-section image of the source grating used in DPCI, fabricated using the improved micro-casting method
To evaluate performance, an X-ray DPCI system was constructed to observe the improvement of the analyzer gratings from the contrast of moiré fringes. Two absorption gratings(a source grating with period of 42 μm and depth of 150 μm and an analyzer grating)and a phase grating(period of 5.6 μm and depth of 41 μm)were used, as shown in Fig.4. The distance between the source grating and the phase grating was 1 470 cm, and the distance between the phase grating and the analyzer grating was 10.5 cm. The conventional tungsten target X-ray tube was run at 60 kV/ 2 mA. A CsI(Tl)scintillator is coupled to a cooled CCD camera(ANDOR 2 048×2 048, 13.5 μm/pixel)through a fiber tape with a magnification of 0.5 as the image detector. Through adjustment of the angle between the phase grating and the analyzer grating, the moiré fringes were registered by the detector, as shown in Fig.5(a). The moiré fringe pattern shown in Fig.5(b)corresponds to the analyzer grating fabricated using the previous micro-casting method. The normalized intensities along the lines in Fig.5(a)and Fig.5(b)are shown in Fig.5(c)and Fig.5(d). From this measurement, the improvement percentage of the fringe visibility was determined to be 31.6%.
Fig.4 Schematic diagram of the constructed DPCI system and the moiré fringe registered by the X-ray detector
An improved micro-casting method for the fabrication of absorption gratings was developed. The micro-casting process was improved through surface modification. By covering the grating structure with a layer of Bi2O3, the molten Bi could flow freely into the HAR microstructures, resulting in a higher filling ratio. The comparison of the visibility of moiré fringes in DPCI demonstrates that the improved micro-casting method can provide the Bi analyzer gratings with enhanced performances.
Acknowledgments:We thank Mrs. Xu Guiwen in College of Optoelectronic Engineering, Shenzhen University, for her help of the SEM images.