Fabrication of photonic devices through direct laser writing: how SEM contributes

By Marijke Scotuzzi - Oct 19, 2017

Photonic devices are widely used in the physical sciences for creating, manipulating and detecting light. In the future, the challenge will be to fabricate advanced photonic devices, which will require flexibility and tunability. Fabricating these devices is not easy, as they require an advanced three-dimensional lithographic technique. Direct laser writing (DLW) is an interesting approach that aims to achieve this using a liquid crystalline photoresist as light-sensitive material.

In this blog, we will describe how photoresists are specifically designed and tested for the fabrication of elastomeric light tunable photonic devices — and how imaging with a scanning electron microscope (SEM) helped in the design improvement process.

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Fabrication of photonic devices by direct laser writing

Direct laser writing is a 3D lithography technique that enables the fabrication of structures with a resolution down to, ideally, 100 nm. An ultra-fast laser is focused inside the volume of a transparent material, called the photoresist, that absorbs two or more photons and polymerizes the resist locally. The process flow of DWL is shown in Fig. 1. The laser beam is first focused inside the photoresist material (i) and then scanned according to a pre-defined design (ii). In the development step (iii) the sample is immersed in a proper liquid, revealing the patterned structure (iv). Because it is possible to use a wide variety of materials as photoresist, DLW is increasingly being used in many different laboratories for many different applications. (A. Selimis et al, Microelectronic Engineering, 132 (2015), 83-89).

direct-laser-writing-beam-focusing.png

Fig. 1: The process flow of direct laser writing (DWL): i) beam focusing, ii) laser writing, iii) development and iv) completed structure (A. Selimis et al, Microelectronic Engineering, 132 (2015), 83-89).

Photoresist materials: the design of liquid crystalline photoresist

3D liquid crystalline elastomeric (LCE) structures are photoresist materials that are able to deform under the irradiation of light and combine properties of elastomeric polymers with those of liquid crystals. They are becoming of interest because it is possible to tune their behavior by chemically controlling their molecular structure. S. Nocentini et al. (Materials 2016, 9, 525; doi :10.3390) investigated the fabrication of 3D LCE microstructures using a different liquid crystalline mixture composition. For each photoresist, the writing conditions were studied, and the possibility of fabricating free-standing structures was investigated.

The design of a LCE photoresist requires three components:

  • a mesogen, which is a compound that displays the liquid crystal properties;
  • a cross-linker, that provides a network with an elastic mechanic response;
  • a photoinitiator, that provides polymerization by light absorption.

By changing the ratio between these three components, it is possible to obtain LCE photoresists that have different chemical and physical properties. For instance, the polymerization threshold, which is defined as the lowest power able to create a well-defined polymeric line, can be tuned by varying the writing speed and the laser power.

The structures, patterned by varying these two parameters, are imaged using a scanning electron microscope (SEM), as shown in Fig. 2. While the writing speed has a limited influence on the obtained structures, the laser power has a strong effect on the polymerization threshold.

sem-micrograph-patterned-lines-laser-power-writing-speed.png

Fig. 2: SEM micrograph on the left and zoomed-in insight of the patterned lines, obtained by varying the laser power and the writing speed.

The writing parameters, together with the composition of the photoresist, can also be tuned in order to increase the structure rigidity. This is of utmost importance for the fabrication of suspended lines required for 3D photonic crystals, like woodpiles.

Fig. 3 shows SEM micrographs of 3D structures patterned with photoresists that have a different composition and therefore a different polymerization threshold (PR-20, PR-30, and PR-40). Different light power is also shown. While the structures fabricated using the PR-20 are softer and contain fewer sharp edges, those fabricated with PR-40 are more rigid and straight.

sem-micrograph-structures-patterned-png.pngFig. 3: SEM micrograph of structures patterned using different light power and different photoresist (a-c) and zoomed in images of specific structures (d and e). The scalebar is 10 µm.

The light response of the designed photoresists was also tested using cylindrical structures irradiated with a green-focused laser. The light intensity was then varied using a neutral density wheel filter, while the focal spot covered the whole surface of the cylinder. The deformations, which are visible from the top, are shown in Fig. 4.

sem-image-cylindrical-structures.png
Fig. 4: SEM image of the cylindrical structures, fabricated in order to investigate the light response of the photoresist. The scalebar is 10 µm.


In conclusion, the design of the LCE photoresist is of utmost importance for the quality of the fabricated structures. A fine tuning of the patterning parameters is also necessary in order to obtain rigid and well-defined 3D devices.

As also demonstrated in a previous blog, SEM is a powerful tool that enables the imaging of the patterned structures, giving a valuable insight into the determination of the quality of the photoresist design and the writing process. The high spatial resolution of electron microscopes is essential in imaging and in determining the size of structures in the nanoscale. 

If you are curious about what high-quality images of structures in the nanoscale produced by a SEM look like, you can take our short but fun SEM images quiz. Will you be a SEM Starter, Star or Superstar? Take the quiz now and find out! 

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Topics: Electronics, R&D

About the author

Marijke Scotuzzi is an Application Engineer at Thermo Fisher Scientific, the world leader in serving science. Marijke has a keen interest in microscopy and is driven by the performance and the versatility of the Phenom desktop SEM. She is dedicated to developing new applications and to improving the system capabilities, with the main focus on imaging techniques.

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