Towards Single Photon Lithography

We propose a completely new concept [1,2] for a future lithography (see Fig. 1a) based on an array of single photon sources which has the potential to simultaneously decrease structure size and to greatly simplify nanostructure realization. Single photon sources can controllably produce a train of single photons with the appropriate energy needed for photochemical reactions in polymer systems conventionally used as photo resists (see for example Fig, 1b). The single photon sources are arranged in arrays of singularly addressable single photon emission (SPE) elements. This concept allows for a very flexible process and renders mask patterning superfluous for structure definition. Additionally it is possible to initialize the photochemical reaction in single molecules with the help of single photons and, in this way, to produce smallest possible structures down to molecular dimensions. The concept, however, is not limited to subnanometer systems. It could also be used also for the production of comparatively large, nanometer and micrometer sized structures. The feature of the technique is made possible by precise control of SPE emission intensity. If desired, a higher intensity would allow an exposure of a large number of molecules, thus influencing the photoresist volume and thus enabling the realization of structures ranging from subnanometers up to several micrometers using a single SPE element. Tunable arrays of singularly addressable SPE elements have the potential to substantially increase the nano device production efficiency and thus induce further progress in semiconductor industry. We will demonstrate how this concept can be realized by presenting the development of a technology for the single photon lithography (SPL) technique. We used an array of nano-LED structures based on a IIInitride layer system (p-GaN/MQW/n-GaN/sapphire) grown by MOVPE as a source of photons. Nano-LED structures (see Fig. 2a) were defined using electronbeam lithography and Ni as the mask to the subsequent Ar-ion beam etching process. All nanostructures were isolated from neighbors using a SiO2 layer. The large bottom (Ti/Al/Ni/Au) and top (Ni/Au) contacts were defined with the help of optical lithography. After device processing the array of nano-LED structures, which serve as sources for photon emission, were optically tested by micro-electroluminescence-mapping at 5 V bias voltage and spectrally characterized. The maximum wavelength emitted from our single nano-LED structures is about 400 nm, which corresponds to a photon energy of ~ 3.1 eV and is sufficient for the initialization of the photochemical reaction. The electroluminescence intensity from all devices is in the same range, and is homogeneously distributed across the entire array (Fig. 2b), proving that we developed a highly reliable and reproducible technology for photon source production. In the next step, the assembly of nano-LEDs was used as the source and the patterning mask simultaneously. Fig. 2c presents the result after lithography and subsequent developing and proves the principal of the SPL. With this alternative concept we fabricated nanostructures with different dimensions by controlling the exposure time. For our approach, SPEs will replace our nano-LEDs in future. Since the SPE arrays will consist of singularly addressable/driven elements, structures and patterns can be designed with the desired size and geometry in a next step. The pattern can be modified instantaneously by driving the elements accordingly. This SPL technique will significantly improve the functionality and flexibility of lithographical processes and has the potential to replace lithographic techniques dependant on classical masks. The technique will help accelerate the development of novel device concepts since the SPE can be driven flexibly by computer controlled systems in a time-saving way. [1] M. Mikulics, H. Hardtdegen, Method for optical transfer of a pattern into a recording medium. Patent DE20121016178 20120816A. [2] M. Mikulics, H. Hardtdegen, Nano-LED array fabrication suitable for future single photon lithography, Nanotechnology 2015, 26, 185302.