Laser speckle as a new tool for optical lithography

Laser speckles form due to the interference of randomly scattered waves. Over the past few decades, speckle patterns (made up of speckles of random size and position) have been the subject of active research, with applications in metrology, diagnostic monitoring, and non-destructive testing, among other areas. However, researchers have yet to study in detail the properties of individual speckles and the potential applications they can offer. In most optical systems, speckles are considered to be noise since they reduce the contrast in lithography and imaging techniques. Therefore, previous work in these areas focused on suppressing their presence.¹ On the other hand, speckle patterns are natural random intensity ensembles, where the average size of an individual speckle is tunable. Hence, associating these speckle patterns to photolithography opens up the possibility of writing the random interference patterns on a photosensitive medium, such as a photoresist. This answers the key question of how to fabricate predictable random structures, which has applications in the field of disordered photonics.

Our group recently proposed a new approach to fabricate a predictable random medium, tunable in size and density.² We demonstrated our idea using a simple optical lithography setup where a unit—composed of a diffuser (ground glass), a lens, and a circular aperture—plays the key role. In this setup, a laser beam of specific diameter generates speckles through the diffuser that are collected by the lens, and then projected onto a substrate through the aperture. The key parameters required to tune size and density are the beam diameter and the diffuser-lens distance, respectively. In addition, we can form quasi-random patterns by replacing the circular aperture with a multiple-aperture pupil with a specific geometry. This means we can fabricate a wide variety of quasi-random structures simply by changing the geometry of the aperture. Moreover, we found that by writing the speckle pattern on a silicon substrate coated with a photoresist, we can create, in a controlled fashion, black (low-reflectivity) silicon structures after dry etching. Since previous work showed that black silicon can improve solar harvesting and IR detection,³ this highlights the versatility and wide range of applications of the laser-speckle lithography method.

We have exploited the properties of speckle patterns for different metrological applications, such as the measurement of surface deformation, position, roughness, displacement, and the shape of objects. Further, speckle patterns find applications in the fields of imaging, vibration and flow measurements, medicine, astronomy, and defense. Most of these applications are based on the particulate nature of speckles (size, intensity, and distribution) and the collective wave phenomena of speckle patterns (speckle pattern interference and diffraction). As far as single speckles are concerned, only their size and intensity are relevant parameters from the application point of view. However, the wave properties of individual speckles have not yet been studied in detail, showing that there is unexplored potential in this area.

We recently proposed and illustrated the use of single-speckle diffraction.⁴ Specifically, we demonstrated this phenomenon at the sharp edges of a circular aperture of particular diameter (4mm). In this setup, each speckle forms its respective grating pattern, where grating size depends on the size of individual speckles. The fabricated structure is demonstrated as a random grating that shows the diffraction properties, with each micro-grating giving its respective diffraction patterns. These types of optical elements are excellent candidates for enhancing overall absorption in solar cells. In Figure 1, we show scanning electron microscope images of the fabricated micro- and nanoscale random patterns, as well as microscale random gratings and their diffraction.

Figure 1. Scanning electron microscope (SEM) images of (a) micro and (b) nano random patterns. The inset in panel (b) shows the closer view of nanospots, the largest of which is about 1μm in diameter. (c) SEM image of a quasi-random pattern achieved by elliptical aperture. (d) SEM image of the micro-grating pattern. The inset shows the diffusive diffraction pattern by random grating.

In conclusion, we have demonstrated that speckle patterns can be used as optical tools for photolithography to fabricate tunable and controlled random and quasi-random structures, with a variety of applications. In addition, we fabricated random gratings by demonstrating diffraction of individual speckles. In the future, we will further explore the potential applications of our techniques in fields such as solar harvesting, IR detection, and disordered photonic elements and devices. Furthermore, due to the robustness and ability to generate a variety of surface textures, we will extend the demonstrated speckle lithography for the fabrication of biomimetic-based structures. We anticipate such elements to find application in random surface photonics and related plasmonics studies.

The authors acknowledge the financial support received from the Singapore Ministry of Education through grant RG 98/14.