|Low-temperature Preparation of Metal-oxide Nanostructures by DUV Lithography from Metal-oxo Clusters
Fabrice Stehlin, Chun-Cheng Yeh, Dominique Berling, Fernand Wieder, Arnaud Ponche, Arnaud Spangenberg, Olivier Soppera
Institut de Science des Matériaux de Mulhouse (IS2M), CNRS - UMR 7361
Université de Haute Alsace, 15 rue Jean Starcky, Mulhouse, France
Contact author: firstname.lastname@example.org
Topic : Synthesis, integration and assembly of new materials
Abstract—Metal oxo-cluster (MOC) prepared from Zr, Ti or Hf precursors were synthetized and used as photocrosslinkable hybrid building blocks for direct write lithography. Deep-UV irradiation (λ<250 nm) was used to generate micro-nanopatterns by DUV-induced condensation of the MOC for applications in optics, photonics, sensors or nanoelectronics.
Among other functional materials, metal oxides are of highest interest for a wide range of applications like optics, photonics, microelectronics, magnetism, photocatalysis, and photovoltaic,... However, their integration into complex systems or devices usually requires multistep processes, involving vacuum deposition or high temperature processes. A major challenge is to enable nanopatterning of the metal oxide material with a low temperature process, even, if possible room-temperature process. Such characteristic is a key-point for implementing functional inorganic nanostructures in devices comprising organic materials or on plastic substrates.
A versatile, fast and easy route towards metal-oxide nanostructures prepared by full-optical method, at room temperature is presented here. The concept relies on the preparation of photosensitive metal-oxo clusters (MOC) that can be crosslinked and mineralized in a single step process, by Deep-UV (DUV) irradiation (ArF laser emission band at 193 nm). The oxo-clusters are prepared by complexation between metal alkoxides and methacrylic acids followed by a partial hydrolysis. These molecular building blocks are designed to absorb DUV light and they can react from excited stated to give rise to crosslinking reactions.
Preparation of the starting building blocks (MOC precursors) is first described. Photocrosslinking of Ti, Zr and Hf oxo-clusters was investigated by means of in situ FTIR and spectroscopic ellipsometry. Finally, micro and nanostructures obtained by laser direct write are presented.
II.Preparation of MOC
The first step consists in mixing the metal-alcoxides precursors with the MAA in the following ratio M:MAA = 1:8. After 5 minutes of stirring, a volume of 2ml of 1-propanol was added as a solvant. Then, after 10 minutes of stirring, de-ionized water was added with a molar ratio M:H2O = 1:20. An additional volume between 2 to 8ml of 1-propanol was added to tune the viscosity of the formulation for adjusting the thin film thickness by spin-coating. Using such preparation conditions, the formulation could be kept during 2 months without any noticeable modification of its properties (including the photosensitivity). 
Metal oxo-clusters preparation scheme.
Thin films for lithography were deposit by spin-coating. The typical thickness was 100 nm (3000 rpm, 60 sec.) and the films were irradiated without any pre-treatment. Home-made DUV interferometric setup was used for patterning. An ArF laser (Braggstar from Coherent), was used as irradiation source at 193 nm. Several phase masks were used to generate regular patterns with periods ranging from 100 nm to 600 nm (line width resp. from 50 nm to 300 nm). The interferometric head is depicted in Figure 2. After irradiation, the thin films were developed 30s in cyclohexanone to remove the non-irradiated part. A thermal or DUV annealing could be applied after development to mineralize the sample.
Scheme of the interferometer used to generate periodical light patterns in the MOC thin film and application to nanopatterning.
IV.Typical examples of microstructructures
Using such lithographic tool and MOC precursors thin films, it is possible to generate easily micro and nanopatterns over relatively wide areas (cm2) in limited times (few sec). 
Figure 3 presents typical examples accessible with TiOC precursors resins. The case of TiOC was studied with much care because in this case, we could demonstrate that a room-temperature mineralization of the material was possible, leading to amorphous TiO2 structures. Low temperature curing of sol-gel materials is getting increasing importance due to the challenge to develop optical or electronic devices on plastic substrates.
High resolution patterning can also be achieved with structures widths as low as 50 nm  and also, multiple irradiation strategies allows preparing more complex architectures .
These inorganic photoresists open new doors towards room temperature preparation of high-resolution inorganic nanostructures with strong interest for practical applications in electronics, optics, photonics or biology since they can be used in other lithographic setup and they can be easily doped to obtain optical, magnetic or electrical properties.
This concept was extended recently to materials with electronic properties. In particular, we demonstrated that InGaZnO sructures can be achieved by laser lithography. The final material has semi-conductor properties after post-annealing at moderate temperature .
AFM images of 300nm wide structures prepared by DUV interferometric lithography starting from TiOC thin film. Left image corresponds to structure after lithography and development and right images are the same structure after DUV or thermal annealing post-treatment. 
This work was supported by ANR program PHOTOMOC (N° ANR-14-CE26-0039-01). Authors gratefully acknowledge the Regional Council of Alsace for fundings.
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