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Direct X-ray and electron-beam lithography of halogenated zeolitic imidazolate frameworks

Metal-organic frameworks (MOFs) offer disruptive potential in micro- and optoelectronics because of their chemical versatility and high porosity. For instance, the low dielectric constant (low-k) resulting from their porosity makes MOFs competitive candidates for high-performance insulators in future microchips. Both the MOF and microelectronics communities have been striving towards integrating MOFs in microchips, which requires two key engineering steps: thin film deposition and lithographic patterning. However, conventional lithography techniques use a sacrificial layer, so-called photoresist, to transfer a pattern into the desired material. The use of photoresist complicates the process, and might induce contamination of the highly porous MOF films. 
A group of researchers from KU Leuven (Belgium) coordinated by Rob Ameloot has used the deep X-ray lithography (DXRL) beamline at Elettra to demonstrate that MOFs can be patterned by X-ray lithography without the use of resist layer. The method is based on selective X-ray exposure of the MOF film, which induces chemical changes that enable its removal by a common solvent. This process completely avoids the resist layer, thus significantly simplifying patterning while maintaining the physicochemical properties of patterned MOFs intact. The grazing-incidence small-angle X-ray scattering (GISAXS), carried out at the small-angle X-ray scattering (SAXS) beamline of Elettra, demonstrates that the crystal structure of non-irradiated MOF film is maintained throughout the XRL process (Figure 1a). The scanning electron microscopy (SEM) image confirms the features of the mask are replicated with high fidelity in the MOF thin layer (Figure 1b). The researchers extended the same strategy to electron beam lithography (EBL) to further improve the resolution because the electron beam provokes similar effects as the photoelectrons generated by the X-rays. The resulting high-quality patterns have an excellent sub-50-nm resolution, the smallest feature size reported to date for MOFs (Figure 1c).

Figure 1.  a, Synchrotron GISAXS patterns of MOF films before and after XRL and comparison with the calculated MOF powder X-ray diffraction pattern. SEM image of the XRL (b) and EBL (cand d) patterned MOF films. 

To leverage MOFs in solid-state devices, patterning should not alter their physicochemical properties, especially their porosity. The gas sorption study of the MOF-coated quartz crystal microbalance (QCM) substrate illustrates that the MOF pattern fully retains its porosity (Figure 2a). Adsorption in the MOF pores makes it possible to concentrate analyte molecules for chemical sensing. The team demonstrated the potential use of the micropatterned MOF films with periodic structures as a responsive diffraction grating for vapor detection. Vapor uptake into the pores of the MOF causes an increase of refractive index, leading to a modification of the diffraction efficiency that can be monitored with a camera (Figure 2b, c).
The compatibility of XRL and EBL with micro- and nanofabrication provides a new perspective on the potential of MOFs as high-performance dielectrics, coatings for more selective and sensitive sensors, luminescent pixels for display technology, and so on. Looking ahead, such integrations could be accelerated through extreme ultraviolet lithography, a state-of-the-art method in which a solubility switch mechanism similar to the one reported here is expected.

Figure 2.  a, Methanol adsorption isotherms of MOF-coated QCM substrates before and after XRL. The inset shows a photograph of the MOF pattern on a QCM substrate. b, Scheme of the evaluation of MOF patterns as diffraction grating vapor sensors. c, Evolution of the refractive index of a ZIF-71 film measured by ellipsometry (red) and the normalized intensity of the first-order diffraction spot (I1) of the MOF grating (blue) as a function of methanol vapor pressure.


This research was conducted by the following research team:

Min Tu1, Benzheng Xia1, Dmitry E. Kravchenko1, Max Lutz Tietze1, Alexander John Cruz1,2, Ivo Stassen1, Tom Hauffman2, Joan Teyssandier3, Steven De Feyter3, Zheng Wang4, Roland A. Fischer4, Benedetta Marmiroli5, Heinz Amenitsch5, Ana Torvisco5, Miriam de J. Velásquez-Hernández5, Paolo Falcaro6,7 and Rob Ameloot1


Centre for Membrane Separations, Adsorption, Catalysis, and Spectroscopy for Sustainable Solutions (cMACS), KU Leuven, Leuven, Belgium. 
Research Group of Electrochemical and Surface Engineering, Department of Materials and Chemistry, Vrije Universiteit Brussel, Brussels, Belgium. 
Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven, Leuven, Belgium. 
Catalysis Research Centre, Technical University of Munich, Garching, Germany. 
Institute of Inorganic Chemistry, Graz University of Technology, Graz, Austria. 
Institute of Physical and Theoretical Chemistry, Graz University of Technology, Graz, Austria. 
School of Physical Sciences, Faculty of Sciences, University of Adelaide, Adelaide, South Australia, Australia.

Contact persons:

Benedetta Marmiroli, email:  
Heinz Amenitsch, email:


M. Tu, B. Xia, D. E. Kravchenko, M. L. Tietze, A. J. Cruz, I. Stassen, T. Hauffman, J. Teyssandier, S. De Feyter, Z. Wang, R. A. Fischer, B. Marmiroli, H. Amenitsch, A. Torvisco, M. de J. Velásquez-Hernández, P. Falcaro and R. Ameloot, “X-ray and electron-beam lithography of halogenated zeolitic imidazolate frameworks”, Nature Materials (2020), DOI: 10.1038/s41563-020-00827-x.

Last Updated on Monday, 30 November 2020 15:50