Objective Extreme ultraviolet (EUV) radiation plays significant roles in various field applications, such as microscopy imaging, material analysis, and EUV lithography. In particular, EUV lithography is an important technology for manufacturing integrated circuits with a feature size less than 7 nm. Compared with the fuel materials of Li and Xe for EUV radiation, the tin (Sn) plasma EUV radiation has purity, broadband spectra, and high conversion efficiency at 13.5 nm. In addition, the EUV radiation with 2% bandwidth centered at 13.5 nm wavelength can be reflected by Mo and Si multilayer optical devices. The above features make the 13.5 nm EUV radiation of Sn become the source of the EUV lithography system. In practical EUV lithography applications, the Sn droplet target is selected as the source for lithography. However, the difficulties in experiment and complexity are expected when Sn droplet EUV radiation is generated. For simply studying the conversion efficiency of Sn droplet EUV radiation, we can use the metal target to replace the droplet target. In this study, we report the 13.5 nm EUV radiation from laser-produced plasma on the structured target can optimize the conversion efficiency. To the best of our knowledge, no studies have been reported on the effect of spatial constraints on EUV radiation in Sn droplet targets. This work may be helpful for further research on optimizing droplet targets to obtain higher EUV conversion efficiency.

Methods The EUV spectra from plasma are created by an 800 mJ, 10 ns full width at half maximum, and 1064 nm Nd∶YAG laser pulse. The groove structured Sn target is fabricated by laser ablation. The width and depth of various grooves are obtained by adjusting the ablation area and times, respectively. The target is controlled by a translating stage to ensure that each laser pulse can radiate in a fresh position. The laser is focused onto the structured target by a plano-concave lens with a focal length of 400 mm. The laser focal spot diameter is changed by moving the distance between the lens and the target surface. The EUV spectra are measured by a flat-field spectrometer with a charge-coupled device (CCD) camera, which is placed at 45° with respect to the direction of the incident laser beam. Two digital delay generators are employed to control the delay time between the laser pulse and the CCD camera.

Results and Discussions The EUV in-band radiation (2% bandwidth centered at 13.5 nm) from the structured targets is found to be stronger than that from the planar targets. Results show that the enhanced EUV radiation can be obtained due to the plasma expansion restricted by the grooved wall. First, when fixing the groove width, the intensity of in-band radiation at 13.5 nm (2% bandwidth) increases and then drops with increasing groove depth from 50 μm to 200 μm. The optimal groove depth for EUV emission around 13.5 nm is 100 μm. However, when the depth of the groove is larger than 100 μm, part of the EUV radiation is blocked by the wall of the groove. However, when the groove depth is less than 100 μm, the confinement effect of the grooved wall is relatively small (Fig. 2). In addition, the laser energy that corresponds to the highest EUV in-band radiation enhancement is found to be 500 mJ for different groove depths when fixing the groove width at 300 μm. It means that the optimal laser energy may be influenced by the groove width, rather than the groove depth (Fig. 3). Moreover, when fixing the groove depth of 100 μm and the laser energy of 500 mJ, the highest EUV in-band radiation intensity is obtained with the optimal groove width of 300 μm for different groove widths. This attributes that the groove width is larger than 300 μm, and that the confinement effect from the grooved wall is reduced. When the groove width is less than 300 μm, the part of the laser energy cannot be coupled into the groove and interact with the target (Fig. 4). When the groove depth is fixed at 100 μm, the laser energies that correspond to the highest enhancement of the EUV in-band radiation are varied with different groove widths. Meanwhile, the optimal laser energy increases as the groove width increases. It means that the groove width is related to the laser energy (Fig. 5). However, when the focal spot diameter is close to the optimal groove width of 300 μm, the highest in-band radiation enhancement is obtained.

Conclusions In this study, EUV radiation emitted by laser-produced plasma from a structured target is conducted. The results show that the laser energy that corresponds to the optimal in-band intensity of 13.5 nm (2% bandwidth) is 500 mJ, regardless of the groove depth when the groove width is fixed at 300 μm. In addition, the in-band EUV radiation with the groove depth of 100 μm is stronger than that in the other cases. Further, when the groove depth is fixed at 100 μm, for different groove widths, the highest EUV in-band radiation intensity depends on the laser energy. Meanwhile, the optimal laser energy increases as groove width increases. This phenomenon illustrates that the groove wall can effectively restrict plasma expansion. This confinement effect can enhance the EUV radiation. However, the highest in-band EUV radiation is obtained when the focal spot diameter is close to the groove width of 300 μm. In summary, a 1.57-fold enhancement of the in-band EUV emission is obtained using the structured Sn target with 100 μm depth and 300 μm width when the laser energy is 500 mJ. This study is of great significance to improve the EUV radiation intensity and conversion efficiency.