EUV Optics

After a brief description of the extreme ultraviolet (EUV) radiation peculiarities, the main characteristics of the ENEA Discharge Produced Plasma (DPP) EUV source are described. An efficient debris mitigation system has been developed and tested on the DPP source, to protect delicate objects from plasma emitted debris bombardment. The DPP source has been successfully utilized for pattern generation by EUV irradiation of photonic materials like lithium fluoride crystals and various innovative photoresists. A simple contact EUV lithography technique has been demonstrated to be effective for sub–micrometric resolution patterning on the above mentioned materials. Examples of obtained patterns are presented and discussed.

The design criteria of a Schwarzschild-type optical system are reviewed in relation to its use as an imaging system in an extreme ultraviolet lithography setup. Both the conventional and the modified reductor imaging configurations are considered, and the respective performances, as far as the geometrical resolution in the image plane is concerned, are compared. In this connection, a formal relation defining the modified configuration is elaborated, refining a rather naıve definition presented in an earlier work. The dependence of the geometrical resolution on the image-space numerical aperture for a given magnification is investigated in detail for both configurations.
So, the advantages of the modified configuration with respect to the conventional one are clearly evidenced. The results of a semi-analytical procedure are compared with those obtained from a numerical simulation performed by an optical design program. The Schwarzschild objective based system under implementation at the ENEA Frascati Center within the context of the Italian FIRB project for EUV lithography has been used as a model. Best-fit functions accounting for the behaviour of the system parameters vs. the numerical aperture are reported; they can be a useful guide for the design of Schwarzschild objective type optical systems.

The laboratory-scale Micro-Exposure Tool (MET) for Extreme Ultraviolet projection Lithography (EUVL), realised at the Frascati ENEA Centre within the context of a National Project, was successfully operated in 2008 by achieving a 160-nm resolution imaging of mask patterns onto a polymethylmethacrylate (PMMA) photoresist through 14.4-nm radiation (EPL 84, 58003 (2008)).
The MET uses a laser-produced plasma as EUV radiation source, a couple of twin ellipsoidal mirrors as collecting optics to gather the 14.4-nm radiation from the source to the mask, an efficient combination of ambient gas and mechanical device as Debris Mitigation System and flnally a low-cost Schwarzschild objective as projection optics to image the patterned mask onto the wafer.
The paper gives details of the ENEA MET components and of the aforementioned successful operation along with subsequent related investigations.

Within a National Project on nanotechnologies, a Micro-Exposure Tool (MET) for projection lithography at 14.4 nm, based on a laser-produced plasma source, is being developed at the Frascati ENEA Center. The choice of this “exotic” wavelength is due to the higher efficiency of a Debris Mitigation  System (DMS) working in the interval of
approximately 14 nm < λ < 15 nm. It has to be noted that Mo/Si multilayer mirrors (MLM) can still have a high reflectivity also at these wavelengths.
The solid-tape-target laser-generated plasma is driven by a XeCl  excimer laser, with an optimized intensity of about 3⋅1010 W/cm2, generating an extreme ultraviolet (EUV) source with a diameter of about 0.2 mm. Clearly, this kind of source emits a lot of debris (both atomic and particulate types) and the 7-cm-far collector mirror must be protected against them.
The paper is mostly devoted to the accurate and systematic study of these debris and to their reduction. The results of mitigation efficiency obtained with a DMS prototype are very encouraging and lead to the design and patenting of its improved version.

Schwarzschild objectives are widely used in the extreme ultraviolet (EUV)/soft X-ray spectral region both as reduction and magnification optics, e.g. for small-field projection lithography and microscopy, respectively. When using a Schwarzschild objective as a micro-exposure tool (MET) at high spatial resolution (half-pitch≤0.1 μm), in addition to the tight requirements on the design and surface figure for the single optics, also an accurate alignment between the two mirrors is needed to reach the planned spatial imaging detail. Ideally, at-wavelength alignment should be done in order to overcome limitations due to diffractive effects. While this can be easily performed on synchrotron beam lines, it becomes time expensive (and components consuming) on low-power laboratory plasma sources. In this work we propose and test a new technique to align a EUV Schwarzschild objective by means of ultraviolet light. The aligned objective allowed the attainment of lithographic patterning with edge response of 90 nm, as part of the laboratory-scale MET for EUV projection lithography realized at the ENEA Frascati Research Centre.

While developing a laboratory-scale micro-exposure tool for extreme ultraviolet (EUV) projection lithography which uses a laser-produced plasma emitting EUV pulsed radiation, we faced the problem of suppressing the various debris (ions, neutrals, particulate, clusters, droplets) emitted by the plasma target. The suppression of debris is a crucial task in the frame of EUV projection lithography, mainly because debris seriously limit both lifetime and performance of the expensive optics and filters put close to the plasma source.
In this paper we present the experimental measurements of main debris characteristics (velocity, size, charge, momentum, spectral energy, spatial distribution).  Then, we present the operating results of a patented debris mitigation system (DMS) specifically designed to suppress debris with the measured characteristics. We achieved reduction factors ∼ 800 for atoms and nm-size clusters, and ∼ 1600 for particles larger than 500 nm.  These results are at the forefront in this field.
The excellent performance of our DMS was a breakthrough to achieve a 90-nm patterning on commercial resists by our micro-exposure tool EUV projection lithography.

The performance of the modified Schwarzschild objective configuration in comparison with the ordinary configuration is further investigated, completing the analysis presented
in a previous work, which was concerned with the trend of the image-plane resolution against the numerical aperture.
Here, we compare the third-order aberration coefficients in both configurations, illustrating the dependence of each coefficient on both the on-axis object position and the numerical aperture.
This conveys the tolerances of the two configurations to lateral and mirrors’ axial misalignments. Also, the nonconcentric mirror configuration is considered and the relative performance confronted with that of the modified configuration.

We report the high-resolution patterning achieved by the laboratory-scale micro-exposure tool for extreme ultraviolet projection lithography realized at the ENEA Frascati Research Center in the frame of a National Project. Such a result obtained using a laser-produced-plasma source, a couple of twin ellipsoidal collectors and a low-cost Schwarzschild-type projection optics shows that it is possible to attain a nanometer-scale spatial resolution using relatively inexpensive projection optics