T. Bizjak, T. Mitra, O. Homburg, D. Hauschild, L. Aschke, LIMO GmbH, Dortmund, Germany

Arrays of refractive micro-optics are ideal homogenizing elements in illumination systems for exposure and metrology tools. Such arrays can provide profiles with uniformity significantly better than 1% peak-to-valley at numerical apertures above 0.35 and also produce custom intensity distributions.

Optical lithography in the deep ultraviolet (DUV) region is being pushed to reach the limits of resolution. The effort in achieving the 32nm or even 22nm node with this technology creates very hard requirements for the illumination optics. Extremely high uniformity is needed over large areas of mask illumination in exposure tools and inspection systems. Depending on the demagnification of the projection optics, the mask has to be illuminated with a numerical aperture of 0.2 to 0.4 [1]. In such an NA regime, the aberration effects in the illumination system are usually minimized with a complex system of aspherical macro-optical field lenses. By using asymmetrically shaped micro-optical homogenizers, it is possible to create extremely uniform intensity distributions for high NA with a simple spherical field lens, or without any field lens at all.

Exposure tools use different illumination modes to get better imaging of certain mask structures. The beam shaping used to create these illumination modes is achieved mostly with diffractive elements. Most of the current modes can also be created with arrays of refractive micro-optics. Their advantage lies in efficiency, which comes close to 90%. They can also be manufactured from fused silica and calcium fluoride to withstand higher laser intensities at 193nm.

In the following section, the beam shaping principle of refractive lens arrays used to homogenize illumination is explained. Each cylindrical lens of the array can be designed individually and can also be shaped asymmetrically. These homogenizing elements can provide a custom-designed intensity distribution and offer the
possibility to correct failures of other optical elements in the illumination system. Lens arrays of various asymmetric profiles have also been manufactured. Novel refractive micro-optics based on free-form surfaces, such as asymmetric homogenizers and multipole illumination optics, have also been demonstrated. Another kind of free-form lens array provides a homogeneous field in the plane nonorthogonal to the optical axis or generates off-axis
(i.e., dipole or quad-pole) illumination.

Principle of classic refractive homogenizers

The beam profiles of multimode lasers, e.g., excimer lasers, highpower diode lasers, and various solid state lasers, are usually homogenized and transformed via the principle of beam mixing. Such a setup consists of one or two cylindrical lens arrays and a subsequent field lens [2].

The operating principle is shown in Fig. 1 for the vertical direction (subscript v). The same principle applies for the horizontal direction (employed on the back sides of the plates shown). Due to the incoherent image superposition of all the lenslets, a uniform profile is generated and the intensity fluctuations of the input beam are averaged. A homogeneity better than 1% PV (peak-to-valley) and an illumination numerical aperture >0.3 is regularly achieved. The same principle can also be applied if separate laser sources illuminate different areas of the micro-lens arrays.

Since the cylindrical lenses for the horizontal and vertical directions can be designed independently, homogeneous rectangular illumination fields or lines with different aspect ratios can be produced. Due to the monolithic nature of micro-lens arrays, the whole surface is well-defined and its total area contributes to the illumination of the homogeneous field.

Production of free-form surfaces

To get the best performance from beam shaping optics, the theoretically optimized surfaces have to be transferred onto a real substrate with minimum deviations between the designed and manufactured structures. For over 15 years, LIMO has developed and refined its unique computer-aided design and production technology for cylindrical micro-lenses on the wafer scale. It is possible to manufacture any surface where the profile can be described with odd or even poly-nomial terms. Each cylindrical lens of the array can be designed and produced individually. An example of two crossed micro-lens arrays mounted in a holder is shown in Figure 2.

For larger NAs, spherical lenses show considerable aberration, which can be corrected by aspherical lenses with a conic constant. The higher order polynomial terms can be used to create small adaptations of the flat top as well as completely different light distributions such as hexagons [3] and multipoles.

Since no etching processes are involved at all, a large range of materials, including glasses (fused silica, BK7, S-TIH53), crystals (calcium fluoride), and semiconductors (Si, Ge, ZnSe) can be processed with a sagittal depth from the micrometer to the millimeter range. Wafers exceeding 200mm edge length with surface accuracies on the order of 10?100nm can be structured. The lens apertures cover a range from about 50µm to several millimeters, surface radii from 50µm up to several 100mm with relative focal length variations well below 1% [4].

Free-form cylindrical micro-lens arrays

The free-form surface has been implemented up to now mainly by symmetric lens arrangements with aspherical terms such as conic constant or even polynomial terms. Micro-lens technology by LIMO also enables manufacturing asymmetric surfaces, which can be described with odd polynomial terms and/or an asymmetric cut-off from an even polynomial surface.

At high NA, it is very difficult to project the homogeneous field without aberration effects. The large angles coming from classic homogenizers demand a very sophisticated system of aspherical field lenses. Our new type of asymmetric homogenizer (Fig. 3) enables a reduction in the complexity and costs of such macro-optical systems. Due to their individual profiles, these lenses can correct aberration effects with a simple spherical field lens or without a field lens at all [4].

Example 1: Homogeneous field on a tilted surface. In many metrology applications, a collinear setup between the camera and the illumination source is not possible. Depending on the angle of observation and illumination, this results in a distortion of the illumination profile and/or detected camera image. LIMO offers a solution based on asymmetric micro-lens arrays, where a homogeneous top hat profile can be generated on the tilted target. The scheme of the illumination setup is depicted in Fig. 4a. Using standard symmetrical micro-lens arrays (profile in Fig. 4b) a pronounced slope of the intensity profile along the tilted surface in the simulated light distribution is observed (Fig. 4d). The intensity fall-off along the axis amounts to 25%. But asymmetric micro-lens arrays as in Fig. 4c compensate for this decline and achieve a uniform profile (Fig. 4e).

Such an asymmetric homogenizer was used, for example, for uniform illumination with a 100W high-power diode laser. A field size of 300×340mm₂ was achieved at a working distance of 1.5m. The angle between the optical axis and illuminated surface was 55°. The homogeneity of the top hat profile was measured at ~ ±5% P-V [4].

Example 2: Dipole and quad-pole illumination patterns generated by refractive micro-optics. Dipole illumination achieved with refractive micro-optics follows a similar principle to the homogenizing system outlined previously. Dipole optics has two micro-lens structures that are perpendicular to each other and manufactured monolithically on one substrate. The first array consisting of symmetrical micro-lenses homogenizes the incoming light in one (horizontal, in the example of Fig. 5) direction.Specially designed lenslets at the second surface divide the laser beam into two homogeneous parts in the other (vertical) direction. After passing the array, there is no light refracted at the angles around 0° to the optical axis (Fig. 5a). This results in no illumination intensity (dark field) in the middle of the far field.

A novel kind of micro-lens profile was manufactured according to the special conic lens shape with several higher-order aspheric parameters. Thus, a rectangular dipole intensity distribution was achieved by illuminating our refractive dipole element (Fig. 5b). Two dipole refractive elements perpendicular to each other result in quad-pole illumination. Due to the special design, the intensity ratio between horizontal and vertical dipoles are adjusted by the position of the input beam on the quad-pole element (Fig. 5c). Some unwanted imaging asymmetry can be corrected by inserting such a quad-pole micro-optics with adjustable intensity ratio.

Conclusion

Refractive micro-lens arrays are key elements in the illumination part of high-performance exposure and inspection systems. They enable hyper NA illumination with an excellent homogeneity, high transmission, preserved polarization, and practically lossless beam shaping. Novel free-formed micro-lens profiles enable several applications of the extreme uniform or specially defined light distributions. They provide homogeneous illumination at high NAs with only a simple spherical field lens or even without a field lens. These asymmetric cylindrical lens arrays also create a highly uniform
distribution at tilted surfaces or even multipole illumination such as dipole or quad-pole with variable intensities.

References

1. H.J. Levinson, “Principles of Lithography,” SPIE Press, 133, 2001.
2. D.M. Brown, F.M. Dickey, L.S. Weichmann, “Multi-aperture Beam Integration Systems,” Laser Beam Shaping, Theory and Techniques, Dickey, F.M., Holswade, S.C. (ed), 2000, pp. 273-311.
3. H. Ganser, M. Darscht, Y. Miklyaev, D. Hauschild, L. Aschke, “High-throughput Homogenizers for hyper-NA Illumination Systems,” Proc. SPIE 6154, 61542N1-N11, 2006.
4. T. Bizjak, T. Mitra, L. Aschke, “Novel High-throughput Micro-optical Beam Shapers Reduce the Complexity of Macro-optics in Hyper-NA Illumination Systems,” Proc. SPIE 6520, 65202X1-X11, 2007.

Tanja Bizjak received her degree in physics from the U. of Zagreb in 2000 and her PhD in physics in 2004 from the Ludwig-Maximilians-U. Munich Contact her at LIMO Lissotschenko Mikrooptik GmbH, Bookenburgweg 4-8, 44319 Dortmund, Germany; ph 49/231-22241-311, fax 49/231-22241-140, e-mail t.bizjak@limo.de.

Thomas Mitra received his diploma and PhD in physics at the U. of Düsseldorf in 1996 and 2002, respectively. In 2006, he became director of micro-optics development at LIMO.

Oliver Homburg received his diploma and PhD in physics. He has been chief product manager at LIMO since 2006.

Dirk Hauschild received his Diplom-Ingenieur in electrical engineering in 1994 and is presently director of stategic marketing at LIMO.

Lutz Aschke received his PhD in physics and is CTO and managing director at LIMO.