Abstract
We present the first experimental demonstration of a novel type of narrowband and wavelength-tunable multilayer transmission filter for the extreme ultraviolet (EUV) region. The operating principle of the filter is based on spatially overlapping the nodes of a standing wave field with the absorbing layers within the multilayer structure. For a wavelength with a matching node pattern, this increases the transmission as compared to neighboring wavelengths where anti-nodes overlap with the absorbing layers. Using Ni/Si multilayers where Ni provides strong absorption, we demonstrate the proper working of such anomalous transmission filter. The demonstration is carried out at the example of 13.5 nm wavelength and at normal incidence, providing a 0.27 nm-wide transmission peak. We also demonstrate wavelength tunability by operating the same Ni/Si filter at different wavelengths by varying the angle of incidence. As the multilayer filter is directly deposited on the active area of an EUV-sensitive photodiode, this provides an extremely compact device for easy spectral monitoring in the EUV. The transmission spectrum of the filter is modeled and found to be in good agreement with the experimental data. The agreement proves that such filters and compact monitoring devices can be straightforwardly designed and fabricated, as desired, also for other EUV wavelengths, bandwidths and angles of incidence, thereby showing a high potential for applications.
Highlights
The development of extreme ultraviolet (EUV) lithography tools demands improved and simplified approaches for on-line spectral monitoring of available EUV sources, as well as for source metrology and standardized characterization
For the experimental demonstration of wavelength-tuning, the measurements were performed at three different angles of incidence (0◦, 10◦ and 20◦), the expectation being, as in Fig. 4, that the anomalous transmission (AT) peak will shift toward shorter wavelengths when the angle of incidence is increased. 5.1
We presented the first experimental demonstration of anomalous transmission using a multilayer stack made of alternating thin films of amorphous materials
Summary
The development of extreme ultraviolet (EUV) lithography tools demands improved and simplified approaches for on-line spectral monitoring of available EUV sources, as well as for source metrology and standardized characterization. A related problem of high importance for EUV lithography is that the overall amount of radiation and the shape of the overall spectrum is critically dependent on maintaining specific excitation conditions, such as for the drive laser intensity and pulse duration [6, 7], or the spatial distribution and density of the target material [8]. Thereby, even smaller changes in these conditions can lead to significant variations of the in-band power over time. This cannot be tolerated because maintaining a stable power in the 13.5 nm-band is crucial to avoid over or under-exposure of the resist leading to loss of resolution [9]. In-situ spectrally resolved monitoring of the power emitted across any relevant EUV ranges and, an output monitoring at 13.5 nm wavelength might be used for a tight control of the plasma excitation parameters
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