• Acta Photonica Sinica
  • Vol. 51, Issue 6, 0631001 (2022)
Jilong TANG1, Xiongwei XU1, Tianxiang CHEN2、*, Na GAO2, Jiewei CAO2, and Lin LI3
Author Affiliations
  • 1State Key Laboratory of High Power Semiconductor Laser,Changchun University of Science and Technology,Changchun 130022,China
  • 2Particle Astrophysics Division,Institute of High Energy Physics,Chinese Academy of Sciences,Beijing 100049,China
  • 3College of Mathematics and Physics,Beijing University of Chemical Technology,Beijing 100029,China
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    DOI: 10.3788/gzxb20225106.0631001 Cite this Article
    Jilong TANG, Xiongwei XU, Tianxiang CHEN, Na GAO, Jiewei CAO, Lin LI. Preparation of Self-supporting Al Filter[J]. Acta Photonica Sinica, 2022, 51(6): 0631001 Copy Citation Text show less

    Abstract

    The self-supporting metal film is very important in the observation of X-ray and extreme ultraviolet bands. When the X-ray and extreme ultraviolet telescopes are used to observe the corresponding bands, the detector is usually disturbed by visible and infrared light and other stray light, they affect its detection accuracy and performance. Therefore, it is necessary to add filters in front of the detector, the filters are usually hundreds of nanometers of metal film. The preparation of self-supporting thin films is usually deposited on some special substrates, and then the substrate is removed to obtain the required self-supporting thin films. At present, there are two main methods for obtaining self-supporting films, one is substrate etching; the other is the release agent. The substrate etching process is complex, and the method of release agent is simple and easy to implement. Therefore, this study uses the method of release agent to prepare the self-supporting Al filter. The release agent used in the past is very easy to dissolve, such as NaCl, CsI, etc., and they are easy to bring defects to the film deposited on the release agent. In this paper, AZ50XT photoresist and polyvinyl alcohol are used as release agents. They have good film-forming properties, stable performance, and are good release agent materials. To make the self-supporting Al film have a regular shape and easy to test, it is necessary to prepare a nickel supporting frame. We copied the nickel frame pattern on clean Si wafer by two mask lithography, and then prepared the nickel frame by the micro-electroforming process. After preparation, the nickel frame was removed for use. The thickness of the nickel frame is about 60~70 μm, the inner diameter is 16 mm, the outer diameter is 28 mm. The interface between the nickel frame and the sample is smooth and easy to bond. In the experiment, AZ50XT photoresist and polyvinyl alcohol release layers were prepared on clean Si substrate by spin coater, and then dried on the heating table. After the sample was cooled to room temperature, placed it in the magnetron sputtering coating machine with background vacuum of 5.0×10-4 Pa to deposit Al film, the deposition thickness was 80 nm. The thickness of Al film was monitored by crystal oscillation film thickness meter, and the step measuring instrument was used to verify whether the thickness of Al film deposited was 80 nm. The test results show the average thickness of Al film is 80.75 nm, it was within the allowable error range. After the deposition of Al film was completed, the nickel frame has adhered to the surface of Al film by epoxy resin adhesive. After the epoxy resin adhesive was completely cured, the sample was put into acetone or deionized water for release, the self-supporting Al filter was obtained. The defects and pinholes on the surface of the filter were analyzed by scanning electron microscopy and a CMOS camera. The analysis results showed that the prepared Al filter surface was uniform and dense, with only a few pinholes. The optical properties of the prepared filter were characterized by Ultraviolet-visible spectrophotometer, soft X-ray transmittance test system and synchrotron radiation device. The transmittance of Al filters prepared by two kinds of release agents is higher than 0.02% in the ultraviolet band, and lower than 0.02% in the visible and infrared bands, they basically meet the requirements of use. The transmittance of Al filter prepared by AZ50XT photoresist is lower than that prepared by polyvinyl alcohol, because the preparation process of photoresist is advanced and the uniformity is better, the surface of the release layer prepared with it is flat and smooth, and there is almost no pinhole on the surface of Al film. The polyvinyl alcohol solution can produce gel during the preparation process, and there is a little gel residue even after repeated filtration, therefore, the prepared release layers have defects, resulting in a few pinholes on the surface of Al film. Compared with the polyimide-aluminum filters commonly used in space X-ray detection, the self-supporting single-layer Al filter has better ability to suppress visible and infrared light. In the 200~400 nm band, the Al filter without polyimide support has higher transmittance to ultraviolet light, so it is more conducive to extreme ultraviolet detection. The soft X-ray transmittance test system is built by our laboratory, the system consists of an X-ray light tube (target is Ag, window is Be), a vacuum chamber, a translation stage and an SDD detector. Under the given tube voltage and tube current, the transmittance of the film in the soft X-ray band is characterized by measuring the ratio of the transmitted light intensity to the incident light intensity. Due to the existence of Be window in the X-ray tube, low-energy photons are shielded, and no low-energy photons are emitted from the X-ray tube. Therefore, the energy section below 1.6 keV can be ignored. The transmittance of the filter is higher than 90% in the energy range of 1.6 ~10 keV. In order to verify the reliability of the test results, we compare the test results with the theoretical calculation results. The transmittance curve is consistent with the theoretical results, which meets the application requirements. Finally, the transmittance of the filter in the energy range of 50~250 eV was measured by synchrotron radiation device. The test results show the highest transmittance of the prepared two filters in this energy range can reach 53% and 35%, respectively. Due to the oxidation of the Al filter surface and the residue of the release agent, the actually measured transmittance is much lower than the theoretical value.