The precision of reflectivity measurements of the extreme ultraviolet (EUV) lithography machine must be greater than 0.06%; therefore, the high-precision measurement of the reflectivity of EUV optical components is particularly important. The influence of various factors in the measurement device, such as the energy stability of the light source, performance of the energy detector, and signal-to-noise ratio (SNR) of the data acquisition module must be minimized to achieve high-precision EUV reflectance measurement. Most high-precision measurements of the reflectivity of the EUV-band optical components conducted worldwide are based on synchronous radiation light sources. However, the cost of synchronous radiation light sources is high and the quantity is small with limited machine time, which cannot meet the production measurement and laboratory application requirements for EUV optical components. Therefore, the development of a reflectometer with a compact size and convenient measurements is necessary. Compared with synchronous radiation light sources, small light sources have poor single-pulse energy stability, and significant fluctuations and attenuation of energy during long-term operation greatly affect the repeatability of reflectivity measurements. Therefore, to eliminate the impact of the energy fluctuations inherent in the light source, normalization is essential during high-precision reflectivity measurements. In this study, a reflectivity testing device based on a discharge plasma (DPP) light source is developed. We analyze the effects of the EUV light source parameters, detector types, and other factors on the reflectometer and propose an effective energy-normalization method. The testing of the reflectivity of the multi-layer mirrors indicate that the impact of light source fluctuations reduces significantly, providing a reference for other EUV-related energy tests.

To improve the repeatability of reflectivity testing, the energy of the light source must be normalized. We adopted a simple method of energy normalization, which introduced an identical aperture B beside the aperture A of the test light. A detector was installed behind it to extract the light near the test beam as a reference beam to monitor the energy of the incident beam. The set-up is shown in Fig. 5, where both the apertures possess a diameter of 2 mm and horizontal distance of 7 mm. A beam of light entered the sample through aperture A for reflectance energy testing, whereas the other beam passed through aperture B as the reference light. The reference beam used for monitoring the EUV beams and the experimental beam used for reflectivity testing passed through the same optical path and optical components before splitting, followed by the experimental and reference detectors. During the reflectivity tests, we first used the experimental and reference detectors to detect the initial signal of the incident beam behind the apertures A and B and then moved it into the sample to be tested. The reflected beam signals of the reference and experimental detectors were tested at a certain angle, and the ratio of the front and back signals of the reference detector was used as the normalization factor of the light source energy to correct the actual reflectivity signal detected by the experimental detector.

The SNR of the incident beam energy to the background noise is approximately 43 dB (Table 1). After the normalization design, the energy fluctuations of the incident beams are tested and studied. The energy changes in the reference and experimental detector test beams is shown in Fig. 6. The energy of the incident beam measured by the reference and experimental detectors fluctuates over time. After normalizing the reference beam, the energy remains stable over time, and the ratio of the energy of the experimental beam to that of the reference beam remains at approximately 0.82. Further statistical results are presented in Table 2. The energy of the incident beam generates fluctuation errors of approximately 2%, 4%, and 6% after 5, 10, and 15 min, respectively. Using the reference detector signal to normalize the experimental detector signal, the energy fluctuation of the incident beam is approximately 0.6% after 5, 10, and 15 min, and the energy fluctuation of the incident beam reduces significantly. After normalizing the experimental detector, the relative deviations of the five measurements significantly decrease (Fig. 7). A comparison of the results of the standard deviation of the multi-layered reflector before and after normalization within the range of the incidence angle of 22° shows that the standard deviation of the peak reflectance measurement results of the normalized sample is 0.69%, and the measurement repeatability of the peak reflectance of the sample improves by 84.1% compared to that before normalization. The accuracy of the experimental device is equivalent to that of foreign counterparts (Table 3).

The influences of the DPP source parameters and different types of detectors are analyzed based on a self-developed compact extreme ultraviolet reflectometer established with a gas discharge plasma source. An energy-normalization method is proposed and applied to the reflectivity measurements of a Mo/Si multilayer mirror at a wavelength of 13.5 nm. The results show that the energy normalization design significantly improves the repeatability of reflectance measurements. The peak reflectance measurement repeatability of multi-layer mirrors exceeds 0.69%, reducing the impact of light source energy fluctuations on the sample reflectance measurement. This result is comparable to those of compact EUV reflectometers reported abroad. Owing to the convenient and ultrahigh-precision characteristics of the EUV reflectometer, it can serve as an important measurement tool for the design optimization of EUV multilayer films and the development of EUV optical components.