Bayraktar, Muharrem

Bijkerk, Frederik

Koster, Gertjan

Lee, Christopher James

Rijnders, Augustinus J.H.M.

Rufus Fraanje

van Goor, F.A.

English

University of Twente Research Information

Reflectance Tuning at Extreme Ultraviolet Wavelengths with Active Multilayer Mirrors

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Reflectance tuning at extreme ultraviolet wavelengths with active multilayer mirrors

Smart Multilayer Interactive optics forLithography at Extreme UV wavelengths(SMILE)Applicants: Prof. Fred Bijkerk (FOM Rijnhuizen / University of Twente)(project leader)Dr. R. Sobierajski (FOM Rijnhuizen)Dr. F. van Goor and Dr. C.J. Lee (University of Twente)Prof. D.H.A. Blank, Prof. A.J.H.M. Rijnders (University ofTwente)Researchers: Muharrem BayraktarBudget: 1069keProject outlineWithout doubt, Extreme UV Lithography represents today’s most advanced optical imaging method,operating at the shortest wavelength ever employed for high-resolution and wide field imaging. Thetechnique, being considered indispensible for the fabrication of the next generation of semiconductorcircuits, represents a major challenge for optics development in general. Typically, sub-tenth nanome-ter precision is required for the optics’ accuracy and positi ning, while, simultaneously, kilowatt-power level EUV light sources cause tremendous thermal loads n the optics, leading to distortionsof the fine imaging process. The obvious, though so far unexplored, solution to this challenge is toadd adaptive functionality where it is most effective, namely in the EUV-reflective multilayer coat-ings. These Bragg-reflecting layers, for which the team holdsa world reflectivity record, have enabledthe success of early EUV wafer scanners. Yet, they must now bemodified to include adaptive fig-ure and spectral functionality to reach the required accuracy and stability. We propose a rigorousnew multilayer composition, including piezo- and pyro-electrical materials so that the periodic Braggstructure can be interactively manipulated. Steering suchcontrol layers by external electrical or ther-mal signals will then allow wavefront corrections and localized reflectivity changes. The aim of thisproject is to achieve an integrated system, where adaptive optics specifically suitable to the EUV areindividually manipulated to obtain optimized EUVL system performance. These ’interactive-EUV’multilayer optics will need to be grown with layer-thickness that have a precision well into thesub-nanometer range, while at the same time having chemicaland thermal stability, and atomicallysharp optical index profiles. To meet these requirements, fundamental challenges in optics, materialsscience, and thin-film physics must be resolved. In the SMILEproject, the essential elements areuniquely combined to achieve these goals:• a unique thin film deposition and analysis instrumentation,as well as a proven deposition tech-nology, ideally suited to produce the ultrathin layers fromthe complex piezo materials while24preserving their adaptive properties,• a positive assessment of critical adaptive and multilayer control concepts, showing more thanadequate dimensional change effects for the proposed adaptive multilayer coatings, including apatent on this adaptive EUV optics,• a new analysis technique in the form of state-of-the-art EUVinterferometry with picometerresolution,• substantial support from our industrial partner on design,analysis, and system engineering,• a proven track record of the team in transfer of know-how to industry.Reflectance Tuning at Extreme Ultraviolet Wavelengths withActive Multilayer MirrorsMuharrem Bayraktar1, Chris J. Lee1, Fred A. van Goor1, Gertjan Koster1, Guus Rijnders1,Fred Bijkerk1,21 Laser Physics and Nonlinear Optics Group, MESA+ Research Institute for Nanotechnology,University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands2 FOM Institute for Plasma Physics Rijnhuizen, P.O. Box 1207, 3430 BE, Nieuwegein,The Netherlandsbayraktar@utwente.nlIntroductionExtreme ultraviolet lithography (EUVL) is the next generation process to satisfy the high demands ofthe semiconductor industry. At EUV wavelength (13.5 nm) multilayer mirrors are commonly used.Multilayer mirrors are composed of many bilayers (high refractive index and low refractive index)which are designed according to the Bragg law [1]. The Bragg formula defines the interferencecondition for waves reflected from different layers and relates the layer thickness to the angle ofincidence and the wavelength. The thicknesses of the layersar critical for the reflectance so thatthey are susceptible to environmental changes such as temperature. Here we propose a new activemultilayer structure which can be adjusted to compensate for reflectance changes. This multilayerstructure makes reflectance tuning with an integrated piezoel ctric layer that can change its dimensionwith external voltage.Active multilayer structureIn the reflectance tuning process, the thickness of the piezoel ctric material is of primary interest forthe phase difference and for the reflectance tuning range. Inorder to see the effect of the piezoelectricmaterial thickness on the reflectance let’s define a specific stru ture as in Fig. 4 a) for 13.5 nm whichis illuminated at normal incidence. The reflectance,R(z), is plotted as a function of the piezoelectriclayer (BaTiO3) thickness,z, in Fig. 4 b). Depending on the thickness of the BaTiO3 layer, the degreeof interference changes, so that the total reflectance has maximum and minimum values. In orderto have the maximum reflectance tuning range, first we select aminimum acceptable reflectancevalue (Rmin) and then we find the number of bilayers for upper and lower MLMstacks (N1 andN2respectively) that gives the maximum slope aboveRmin. For example, if we selectRmin = 60% then25Figure 4: a) Active multilayer structure which can be definedfrom top to bottom as: Mo/Si multilayerstack, top electrode, piezoelectric layer, bottom electrode and bottom Mo/Si multilayer stack. Theperiod of the Mo/Si MLM’s, the thickness of Mo and the thickness of Si are 6.9 nm, 2.76 nm and4.14 nm respectively. b) Reflectance as a function of BaTiO3 layer thickness. By applying voltage,the thickness of the BaTiO3 layer can be changed from 2.1 nm to 2.2 nm resulting in a reflectanceincrease from 60.54% to 63.82%.the optimum solution is satisfied withN1 = 25 andN2 = 48. Reflectance tuning range is limitedby the mechanical properties of the material. Maximum thickness change (maximum strain,ǫmax)for BaTiO3 before any plastic deformation is 4.8% [2]. Therefore, if wedefine the initial thicknessof the BaTiO3 layer and the total reflectance aszmin andRmin respectively, then maximum thicknessand corresponding reflectance can be written aszmax = zmin(1 + ǫmax) andRmax respectively whereRmin = R(zmin) andRmax = R(zmax). Using the configuration in the figure, it is possible to changethe thickness of the BaTiO3 from zmin = 2.1 nm tozmax = 2.2 nm resulting in a reflectance tuningrange betweenRmin = 60.54% andRmax = 63.82% as shown in Fig. 4 b). The reflectance tuningrange is 3.28% for this BaTiO3 layer thickness however it is possible to achieve higher reflectancetuning with thicker piezoelectric layer which will be explained in the presentation.In conclusion an active multilayer mirror structure is described to be used at EUV wavelengths.The structure incorporates a piezoelectric layer into the multilayer mirror that allows reflectance tun-ing. Using this idea, it is also possible to design structures for different wavelengths ranges andmulti-element structures for wavefront correction.Bibliography[1] David Attwood, Soft X-Rays and Extreme Ultraviolet Radiation: Principles and Applications(Cambridge University Press, 2007).[2] R. F. Cook, C. J. Fairbanks, B. R. Lawn, Y-W. Mai, ”Crack resistance by interfacial bridging: Itsrole in determining strength characteristics,” 2, pp. 345-56, 1987.26

Reflectance Tuning at Extreme Ultraviolet Wavelengths with Active Multilayer Mirrors

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