Photonic Nanojet Sub-Diffraction Nano-Fabrication With in situ Super-Resolution Imaging

In this paper, we report a system that uses a microsphere to induce a photonic nanojet to directly write on a sample surface with sub-diffraction limit resolution, while simultaneously observing the writing processing in situ. Because of diffraction limit, sub-wavelength laser processing resolution has been difficult to achieve. Recently, we have shown that a microsphere-induced photonics nanojet could be used to create an optical light spot size beyond the diffraction limits, i.e., the diameter of the light spot could be reduced to −200 nm or smaller. This capability could allow a laser (with significant reduction in input power) to pattern nano-scale structures on various substrate materials through the use of microspheres. Many researchers have attempted to use this photonic nanojet (PNJ) based technique to process sub-diffraction limit patterns on all types of materials. However, applying this method to process features at precise locations is very difficult, because the technique requires that a sample being “cut” by the PNJ be observed by a common optical microscope, which cannot resolve features smaller than the optical diffraction limit. This disadvantage limits the applicability of this novel method to nano-scale processing; for example, in trimming resistors in an integrated circuit chip, many circuit components on the chip are much smaller than 200 nm, and not being able to see features beyond the diffraction limit will inadvertently destroy many components while PNJ is used to trim resistive elements. We will show in this paper that processing resolution better than 200 nm is achievable using our laser-based photonic nanojet method, whereas a simultaneous imaging resolution of less than λ/2 could be obtained. Therefore, the method presented in this paper has potential applications in processing sub-diffraction features and bio-specimens in real time with optical information feedback.

I. Introduction

Demand for micro/nano devices and structures has increased significantly because of their unique size-related properties. Among micro- and nano-scale fabrication technologies, laser-based systems are some of the most versatile tools for creating 2D and 3D structures [1]. For instance, lasers can be used to write directly and efficiently on a substrate without contact or requiring special chemicals. In addition, direct laser writing is probably the most flexible “material subtraction” micro/nano fabrication process as it does not require photolithographic steps and has been demonstrated to be effective on many types of substrates, including resin [2], glass [3], [4], metal [5], [6], photoresist [7], [8], and etc. A summary of the state-of-the-art laser-based processing techniques and their fabrication resolution limit is shown in Table I below. TABLE ISize of Features Processed by Lasers on Different Substrates With Respective Parameters

Method	Substrate	Light source	Resolution	Imaging Method	Reference
Two-photon ploymerization	SCR500	Femtosecond laser 780 nm	18 nm	SEM	[2]
Laser direct writitng	Porous glass	Femtosecond laser 800 nm	40 nm	SEM	[3]
Nanojoule-femtosecond laser pulses at a high repetition rate	Silicon	Femtosecond laser 690-1060 nm	120-150 nm	SEM	[4]
Laser direct writing	PEDOT-PSS film	Femtosecond laser 780 nm	1.4 ± 0.9 um	AFM	[5]
Femtosecond Laser Bessel Beams	Gold film/graphene	Femtosecond laser 1030 nm	125 nm/400 nm	SEM	[6]
Microsphere lithography	Photoresist	Femtosecond laser 450 nm	300 nm	SEM	[7]
Interference lithography	Photoresist	HeCd laser 325 nm	200 nm	SEM	[8]
Laser-band Photonic Nanojet	Gold electrode	CW laser 532 nm	190 nm	AFM	Our work

Note: SCR500 is a kind of resin; PEDOT-PSS is poly(3,4-ethylene dioxythiophene)-poly(styrenesulfonate).