High Precision Capillary Printing of Polymer Patterns for The Area-Selective ALD of Transparent Conducting Oxide

Thin films of transparent conducting oxides (TCO) are used as electrodes for optoelectronic devices and microdisplays. However, these oxides are often difficult to pattern with standard lithography and etching processes especially in the case of ZnO, which is very sensitive to water. This problem becomes very limiting when it is necessary to reduce the characteristic dimensions of the devices (typically for structures below 5 mm). An alternative approach is the area selective deposition (ASD) of these oxides. More precisely, one way consists in using polymeric film that can act as ALD inhibiting layer and prevent deposition in certain areas.<br/><br/>In this work, polymeric thin films were printed by an innovative direct nanoprinting technique [1]. This high precision capillary printing (HPCAP) method is directly derived from AFM technologies, replacing scanning probes by nano capillaries. These “nanopens” are filled up with a solution containing the material to deposit and can draw complexes geometries at the surface of the substrate with resolution between 50 nm and tens of µm and positioning precision as low as 20 nm.<br/><br/>Different patterns with poly(methyl methacrylate), poly(vinyl pyrrolidone) or an epoxy-based resist (SU-8) were printed on a silicon substrate. Then, TCO films were deposited on the patterned polymers by low-temperature ALD. The film growth and material structure were analyzed using several characterization techniques including ellipsometry, atomic force microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy.<br/><br/>Our results show that it is possible to produce regular micrometric polymer patterns using this HPCAP printing technique. Furthermore, this approach combines with ALD can effectively allow to localize the growth of an oxide such as ZnO. An exclusion zone around the polymer in which ZnO is not deposited is also observed depending of the polymer used. Finally, in the favorable configurations, the polymer can be selectively removed without impacting the ZnO layer. Thus, an array of pixels with 10 mm width has been successfully produced.<br/>In conclusion, a solution combining the direct nanoprinting of a polymer pattern followed by the ALD of a transparent conducting oxide appears to be a promising path toward the realization of a matrix of pixels sub-10 mm.<br/><br/>[1] M. Pascual, N. Bigan, A. M'Barki, R. Mental, I. Allegro, U. Lemmer, SPIE Opto 2023, 124330E