Laser Fabrication of Nanobump Arrays on Si Substrate Via Optical Near-Field Enhancement

By: X.C. Wang, H.Y. Zheng, C. W. Tan, F. Wang, H. Y. Yu, K. L. Pey

Singapore Institute of Manufacturing Technology (SIMTech), Singapore
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore

Modern technology requires the ability of creating smaller and smaller devices. Whether it is for information storage, high definition displays or sensor arrays, there are increasing demands for techniques able to define features smaller than micrometers or even just a few nanometers across. Several technologies such as electron beam lithography, X-ray lithography, nanoimprinting, and optical near-field lithography etc, have widely been investigated for nanofabrication. However, they all have some disadvantages especially in terms of cost flexibility, and complexity. Electron beam lithography is characterized by low sample throughput, high sample cost, modest feature shape control and excellent feature size control, whereas x-ray lithography is characterized by initial high capital costs but high sample throughput. Conventional optical lithography is relatively cheap and adaptable, but, its minimum feature size is limited by optical diffraction.

Motivated by the problem of structure damage in dry laser cleaning of submicron particles on surfaces, a new approach involving the illumination of micro/nanometer sized spheres using a laser beam was recently demonstrated to produce nanoscale pattern on a solid surface, where a micro or nano spherical particles act as lenses and intensify the incoming laser beam. With this approach, most of the works have been on fabrication of nanoscale pits or dents.

In this paper we report on the fabrication of silicon nanobumps on an n-doped (100) Si wafer with silica microspheres. With a single shot 248 nm excimer laser irradiation on a monolayer of self-organized hexagonally close-packed silica microspheres formed on the Si surface, a regular array of Si nanobumps surrounded by a ring-shaped trench was fabricated on Si substrate. The morphologies of the features produced were characterized by an atomic force microscope (AFM) and a scanning electron microscope (SEM). The fabricated nanobumps were verified to be Si based with EDX. Nanobumps formation can be explained qualitatively by the optical near field enhancement of the silica microspheres, the anomalous behavior of the Si density in solid and liquid states, and the competition between the thermocapillary force and chemicapillary force. The fabricated Si nanobump arrays could be optimized and have potential application for fabrication of high-efficient Si-based solar cells, sensitive detectors, displays, and field-emitter arrays.

Fig.1 Laser fabricated Si nanobump arrays on a Si (100) surface, (a) SEM image (b) Magnified AFM image