Extreme ultraviolet (EUV) detectors play an irreplaceable role in the fields of electronics manufacturing, space exploration, and basic science research. In electronics manufacturing, EUV lithography offers new possibilities for realizing circuit patterns in smaller sizes. The application of EUV detectors in EUV lithography is crucial because reliable detectors in EUV lithography systems can realize accurate monitoring of light source power and exposure dose to ensure the accuracy and consistency of chip production. In space exploration, the EUV radiation released by solar activity changes the density and ionization of Earth’s thermosphere and ionosphere, which will accordingly affect the performance of ground-based communication systems and spacecraft in low Earth orbit. EUV detectors can be effectively used to study solar activity, which can facilitate investigations on how solar changes affect Earth and technological systems in space missions, thereby improving the prediction capabilities. In basic science, as EUV light has a short wavelength and high energy, relevant studies of its characteristics and interaction mechanisms can deepen the understanding of the photon behavior and electronic structure of atoms and molecules. In this field, EUV detectors are a key component to calibrate the wavelength and intensity of light sources, providing a means to deeply explore the microstructure and basic laws of the material world (Fig.1). Starting with the important application scenarios of EUV detectors in various fields, this review aims to provide a systematic introduction to the advantages and research progress of EUV detectors.
As science and technology progress, various application scenarios have put forward different performance requirements for EUV detectors. This paper reviews the research progress of EUV detectors based on different detection media and working mechanisms, including the gas monitor detector (GMD), scintillator, micro-channel plate, and semiconductor-based photodetector.
GMDs can obtain information by detecting the ionization process after the absorption of EUV radiation. The stable real-time monitoring of the photon fluxes of high-power EUV light sources, such as synchrotron radiation and free-electron lasers (FEL), will affect subsequent tests, making the calibration of these light sources essential. Semiconductor diodes are typically used to measure the absolute photon fluxes; however, they may get damaged under high-power EUV radiation, increasing the uncertainty in EUV detection. In contrast, as GMDs can overcome the performance degradation induced by radiation and realize real-time monitoring of photon fluxes, it has been employed in several free electron laser devices (such as FLASH 2, SwissFEL, European XFEL, and LCLS II). Compared with the calibration of semiconductor diodes, GMDs have the advantages of low deviation, high stability, and long service life. Further, it is more effective in detecting high-power EUV radiation (Fig.2).
Scintillators have been developed based on the down-conversion effect, which converts invisible EUV light into visible light to be collected by a back-end photodiode or photomultiplier tube. The scintillator is generally a fast and efficient photoluminescent material with sufficient size and is an ideal element for high-speed EUV detection and imaging. Scintillators exhibit high-yield luminescence, a fast response to EUV light, and a sufficiently high absorption coefficient. Ce∶YAG, ZnO, and sodium salicylate are scintillators that have attracted significant attention in the field of EUV detection, among which the sodium salicylate scintillators have been commercialized (Fig.3).
The micro-channel plate is a type of large-area electron multiplier detector that converts EUV photons into electrons through the external photoelectric effect that has the advantages of high spatial resolution and low noise. Micro-channel plate EUV detection technology has made great progress in the past few decades, including improvements in detection efficiency, response speed, and image reading technology. It has been widely used in the detection of the EUV band in aerospace missions, thus providing strong support for space science research. Micro-channel plates have been commercialized, mainly by the Hamamatsu Company (Fig.4).
The semiconductor-based photodetector is a type of low-power miniaturized detector that utilizes the internal photoelectric effect. Its types include those based on silicon and wide-bandgap semiconductors, with the advantages of small size, light weight, and easy integration. Silicon-based photodetectors have been applied in a wide spectrum range, from X-rays to visible light, and the test data are considered the absolute calibration standard for EUV detection technology (Fig.5). However, they are prone to face the problems of accelerated aging or even getting damaged under harsh conditions such as high temperatures and radiation. Considering the reliability and operating conditions of EUV detection, wide-bandgap semiconductor materials are preferred in such situations. Owing to its material characteristics, the EUV photodetector based on wide-bandgap semiconductors typically has a higher radiation damage threshold, stronger chemical and physical stability, and a lower intrinsic carrier concentration, which can ensure stable performance under high irradiation intensity. At present, some wide-bandgap semiconductor materials, such as SiC (Fig.6), AlGaN (Fig.7), and diamond (Fig.8), have been used to manufacture EUV photodetectors. Detectors based on those materials have exhibited a longer service life under the same irradiation conditions and greater advantages in relation to EUV lithography light source power and dose monitoring when compared with silicon-based photodetectors. Therefore, wide-bandgap semiconductors have important research significance with the ability to provide new avenues for the development of EUV detection technology.
This paper introduces the development and research status of EUV detection of the GMD, scintillator, micro-channel plate, and semiconductor-based photodetector (with the advantages and disadvantages listed in Fig.9), particularly focusing on the EUV photodetector based on wide-bandgap semiconductors. All these types of detectors have been constantly optimized to meet the needs of different application scenarios. A deeper understanding is expected to be achieved in the future by dealing with the unsolved scientific problems in current EUV detection technology, such as irradiation-resistant EUV power monitoring, high-resolution EUV imaging, and high-rejection-ratio detection for weak EUV light. This in-depth research will provide more advanced technical means or methods for electronics manufacturing, space exploration, and basic science to promote the development of related fields.
