Abstract
1. Introduction
For many years, low-temperature plasmas emitting extreme ultraviolet (EUV) radiation (photon energy: , wavelength: ) have been the object of strong interest and intense development and study by both industry and academia. In fact, EUV radiation, thanks to its short wavelength, its short penetration length in matter (typically a few tens of nm), and the availability of high-reflectivity ( at ) normal-incidence multilayer mirrors, allows patterning at a high spatial resolution, for example, on photoresists and on photonic materials. This led to the development of high-average-power plasma sources for EUV-radiation-based lithography systems for microelectronics, and also to low- to medium-power sources for metrology and tests on mirrors, innovative materials etc. Basically, two kinds of EUV plasma sources have been developed, laser-produced plasmas (LPPs) and discharge-produced plasmas (DPPs). For an extensive review on plasma sources for EUV lithography, see Ref. [
An EUV DPP source is currently operating at the ENEA Frascati Research Centre. It was originally realized within a collaboration between ENEA and the Physics Department of the University of L’Aquila in the frame of the National FIRB-EUVL Project[
2. EUV plasma source description
A picture of the ENEA DPP source and the related scheme are reported in Figure
Sign up for High Power Laser Science and Engineering TOC. Get the latest issue of High Power Laser Science and Engineering delivered right to you!Sign up now
A low-inductance 50-nF glycol cylindrical capacitor is charged to and then rapidly discharged through a spark-gap switch and a thin-metallic-tube electrode in low-pressure (0.5–1.0 mbar) Xe gas, which fills a short alumina capillary tube, towards the ground annular electrode. This produces the main discharge (11-kA peak current, 240-ns half-period), which follows the low-current (20–30 A, almost constant) long-duration () pre-ionization phase, and leads to high-temperature plasma formation and pinching towards the capillary axis thanks to the resulting magnetic field ( at the capillary surface). Consequently, the plasma resistance rises and the temperature increases up to 30–40 eV, where the hot plasma emits radiation peaked in the EUV region before relaxation and cooling. In typical operating conditions, the DPP source emits EUV pulses having an energy of in the range, with a pulse duration of 100 ns. The temporal behaviour of the high voltage on the glycol capacitor, as detected by using an appropriate calibrated resistive divider, is shown in Figure
The minimum transverse dimension of the EUV source is , measured by a Zr-filtered 100--diameter pinhole camera and a Gafchromic dosimetric film. Ionic debris are considerably deflected by a dipole magnet, allowing the exploitation of clean radiation. An extensive description of the DPP source can be found in Ref. [
3. Absolute calibration of PIN diodes
When using a plasma source to carry out controlled irradiation, an accurate evaluation of the released fluences (i.e., energy per unit area) in a given spectral interval is needed. The IRD AXUV series photodiodes have near-theoretical quantum efficiencies for EUV photons, which can be predicted by the expression , where is the photon energy in eV[
In order to determine the fluence on samples during exposures, it was necessary to take into account the presence of Xe gas in the vacuum chamber during plasma source operation, due to a dynamic equilibrium between the Xe injection flux and the vacuum pumping speed. Accurate measurements of the reference PIN diode signal have been performed changing both the Xe pressure and the PIN diode-to-source distance. It is also crucial to take into account some spurious effects, such as a possible nonuniformity of the residual Xe gas pressure in the vacuum chamber or EUV grazing reflections on the chamber walls, which can easily generate artefacts. The Xe pressure uniformity has been confirmed by measurements taken at different points of the vacuum chamber. Undesired grazing reflections have been detected on dosimetric film and eliminated by proper screening of the PIN diode.
The measurement results are shown in Figure
4. Visible time-resolved plasma spatial evolution
The plasma spatial evolution, both in the pre-ionization and in the main discharge phases, has been observed in the visible range by a gateable CCD camera (DICAM-2, PCO Computer Optics GmbH) placed at 1 m distance from the source. A schematic of the experimental setup is reported in Figure
The camera, equipped with a telephoto lens, monitored the plasma through a Plexiglass window mounted on the vacuum chamber wall opposite to the source. The acquired plasma images have been elaborated to give them a fixed point of reference: initially, an image of the capillary tube and the HV electrode has been captured under illumination and has been superimposed on all the plasma images, which have been subsequently taken in the dark. In order to better distinguish the two contributions, they have been converted from the original grey levels to false colours – respectively, red for the structure and blue for the plasma column. An example of this image processing is shown in Figure
The plasma formation during the low-current pre-ionization phase is shown in Figure
The evolution of the main discharge was observed using a gate time of 20 ns for each acquisition. Observing the sequence reported in Figure
The sequence of pictures also shows that the plasma collapse and its successive relaxation temporally coincide with the EUV emission rise and fall, respectively, as expected. It is worth noticing that the visible emission power grows both during and after the EUV main pulse, so that the optical density of a neutral filter placed in front of the camera had to be increased for longer delays in order to avoid camera saturation (see Figure
5. Calibrated EUV imaging by dosimetric film
Gafchromic HD-V2 dosimetric film is another powerful tool that can be used to characterize EUV plasma sources. The exposure to ionizing radiation modifies its optical density in a reproducible way depending on the impinging fluence, allowing its use as a quantitative imaging detector. Since the calibration curve provided by the company applies to hard X-rays, the film has been exposed to determined fluences in the EUV region by using the DPP source. It is expected that in this spectral range the film optical density versus fluence follows the photographic thick-film formula, due to the above-mentioned EUV short penetration length:
In Figure
In particular, by placing the film at 12.8 cm from the source (sample exposure position) and at 96 cm from the source (one of the experimental points of Figure
6. EUV plasma source applications
Thanks to the extensive characterization of the EUV radiation produced by the DPP source, several applications have benefited from its controlled operation. In Figure
Also EUV direct exposures of novel photoresists have been carried out in the framework of a project, funded by the CARIPLO Foundation, which involves ENEA and the Universities of Pavia and Padova. An example of EUV exposure of a resist synthesized at Padova University is shown in Figure
Finally, the controlled irradiation of LiF thin films, evaporated at the MNF Laboratory of ENEA, allowed the creation of anticounterfeiting tags by lithographic writing techniques[
7. Diagnostics relevance for LPPs
As mentioned in the Introduction, the development of EUV-emitting plasmas is of paramount interest in the field of next-generation lithography, the choice between LPP and DPP sources still being an open question. As a matter of fact, several critical issues have yet to be overcome and a deeper understanding of the plasma characteristics is needed.
In spite of the evident differences in plasma heating methods between LPP and DPP, many plasma parameters are similar, with regard to temperatures, materials, residual gases in the vacuum chambers, and surrounding environment. The major differences between LPP and DPP are related to the geometry for the collection of the EUV emitted radiation and for the mitigation of the produced debris[
For these reasons, the diagnostics described in this paper are directly applicable to laser-generated plasmas for their temporal and spatial characterization. It is also worth mentioning the particular case of X-ray lasers, both those operating as capillary discharges and those relying on collisional methods, which can particularly benefit, for example, from fast-gated image acquisition, especially for synchronization purposes when used as amplifying media[
8. Concluding remarks
An EUV source, based on a DPP, is operating at the ENEA Frascati Laboratories. Several diagnostics have been exploited to characterize the EUV plasma source: absolute PIN diodes, a gateable CCD camera in the visible range, and a dosimetric film which has been also calibrated in the wavelength interval. The described diagnostics can be applied also to other EUV plasma sources, such as LPPs. Just such an LPP EUV/soft X-ray source is also operating at the ENEA Frascati Laboratories[
References
[1] V. Y. Banine, K. N. Koshelev, G. H. P. M. Swinkels. J. Phys. D Appl. Phys., 44, 253001(2011).
[2] V. M. Borisov. Quantum Electron., 44, 1077(2014).
[3] J. C. Valenzuela. J. Phys. Conf. Ser., 511, 012023(2014).
[4] K. Bergmann, G. Schriever, O. Rosier, M. Muller, W. Neff, R. Lebert. Appl. Opt., 38, 5413(1999).
[5] B. Huang, Y. Takimoto, M. Watanabe, E. Hotta. Japan. J. Appl. Phys., 50, 06GB09(2011).
[8] B. L. Henke, E. M. Gullikson, J. C. Davis. At. Data Nucl. Data Tables, 54, 181(1993).
[10] R. J. Goldston, P. H. Rutherford. Introduction to Plasma Physics(1995).
[11] M. Masnavi, N. Nakajima, K. Horioka. J. Plasma Fusion Res., 79, 1188(2003).
[12] Y. Kobayashi, R. M. Neren. J. Quant. Spectrosc. Radiat. Transfer, 12, 1647(1972).
[13] E. B. Saloman. J. Phys. Chem. Ref. Data, 33, 765(2004).
[16] S. Suckewer, P. Jaeglé. Laser Phys. Lett., 6, 411(2009).
Set citation alerts for the article
Please enter your email address