Laser Sub-Micron Patterning of Rough Surfaces by Micro-Particle Lens Arrays

Introduction

Within the last two decades research in novel manufacturing techniques on sub-micron, nano and even atomic scales has been accelerated by the increasing demand for miniaturized devices. Ever smaller devices can only be realized with modern precision manufacturing techniques which are also economical. With the reducing size of these devices surface patterning is also gaining more importance. Lasers have been extensively used for surface patterning at micron scale (Bäuerle, 2000; Hon et al., 2008; Pena et al., 2009). Laser is a tool which is widely utilized for manufacturing because of the advantages of being a non contact process, capable of generating complicated structures without the need of photomask and able to work in air, vacuum or water. These advantages have earned lasers a reputable position in the manufacturing industry. Moreover the laser can easily be focused down to a micrometer scale which makes it a tool of choice in the micro device fabrication.

Laser assisted Lithography is one of the commonly used industrial technique for submicron and nano production. However, lithography is also reaching its limit. Although smaller features can be generated by using F₂ 157 nm and Extreme Ultra Violet lithography (EUV) but high resolution comes with the drawback of high cost, low output and unstable light intensity. Also these lasers need to carry the process in vacuum or high purity dry nitrogen because of the high absorption of the laser in air. Moreover there is a need of special reflective mirrors, and need very high power to achieve intensities suitable for lithography (Bjorkholm et al., 1990; Ito et al., 2000; Chong et al., 2009). These limitations have thus restricted the use of EUV for industrial production. Lithography techniques and their limitation have summarized by Ito et al. (Ito et al., 2000). The direct use of laser in sub-micron patterning is limited because of the fact that light cannot be confined to a lateral dimension smaller than half its wavelength called the diffraction limit of light (Abbe, 1873). However, this limitation can be overcome by utilizing near-field enhancement. Laser processing in the near-field has been successfully utilized to generate features with sizes smaller than 100 nm (Chong et al., 2009). Several techniques for utilizing the advantages of near-field have been developed including Near-field Scanning Optical Microscope (NSOM) patterning (Betzig et al., 1992; Chong et al., 2009), Plasmonic Lithography (Srituravanich et al., 2004; Liu et al., 2005; Chong et al., 2009) and Laser in combination with Scanning Probe Microscopy (SPM) for tip patterning (Chimmalgi et al., 2003; Chong et al., 2009; Miyashita et al., 2009), Microlens array (MLA) nanolithography.