Editorial Feature

Gas-Assisted Etching in Photonics Fabrication

The accelerated growth of the photonics sector has raised the requirement for efficient and economical fabrication techniques. Etching is a crucial technique used in photonics manufacturing that facilitates the construction of micro- and nanostructures in a variety of materials. In recent years, gas-assisted etching has emerged as a viable replacement for conventional wet etching techniques. 

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Image Credit: l i g h t p o e t/Shutterstock.com

A Brief Overview of Gas-Assisted Etching

Etching is the process of removing material from a surface selectively, leaving behind the required sequence or structure. In the manufacturing of photonics components such as waveguides, attenuators, and optical filters, etching is essential. Various etching techniques exist, including wet etching, dry etching, and gas-assisted etching.

Gas-assisted engraving involves employing a reactive gas to aid in the etching process. The reactive gas is introduced into the etching area, where it reacts with the etched material to produce volatile compounds that are readily removed by ion bombardment. Either a focused ion beam (FIB) or a plasma source can be used to perform gas-assisted etching. The FIB method employs a focused beam of ions to etch the material, whereas the plasma source method generates the reactive gas using plasma.

The Role of Etching in Photonics Manufacturing

Etching is a crucial step in the manufacturing of optoelectronics devices because it enables the development of the intricate micro- and nanostructures required for their operation. Etching is employed to establish the resonant aperture, which is critical for boosting the light signal during resonator production. In the fabrication of optical filters, etching is used to produce the bandpass filter, which is responsible for filtering out specific wavelengths of light.

What are the Different Methods of Gas-Assisted Etching?

There are several methods of gas-assisted etching utilized in photonics fabrication. Three widely utilized methods of gas-assisted ion beam etching include standard focused ion-beam etching (FIBE), reactive-ion beam etching (RIBE), and chemically assisted ion beam etching (CAIBE).

Gas-Assisted Focused Ion Beam Etching Process

The focused ion beam (FIB) is a potent component of technology that has facilitated scientific and technological advancements in the realization and study of micro- and nano-systems in numerous research fields, including nanotech, material science, and the microelectronics industry.

With FIB processing, the production of complex nanostructures can occur via the removal of material initiated by ion milling or via local deposition initiated by the interaction of the ion beam and a gaseous precursor. Remarkably, this technology applies to both material families of interest for nano-photonics, namely conductive metals, as well as the insulating and low-loss dielectrics.

An article published in Micromachines focuses on FIB-induced etching and milling for intrinsic chirality in photonic devices. The research indicates that the focused ion beam (with either heavy ions such as Ga+ or light ions such as He+) may be utilized as a scanning ion sensor for lithography etching in resists, with position and speed regulated by a pattern synthesizer.

During the highly destructive FIB milling procedure, the ion beam examines the material's surface and excavates the desired area. Surface milling can be combined with gas-assisted etching if a gas injection system (GIS) concurrently introduces gaseous natural precursors into the vacuum chamber.

Reactive Ion Beam Etching (RIBE)

The beam in RIBE is made of reactive gases as opposed to inert gases. The process differs from other techniques mainly as the gas interacts with the substrate material to chemically remove it.

Depending on the substrate material, this process can be more accurate and regulated than conventional IBE, which makes it beneficial for etching very thin film coating layers where it is essential to prevent injury to sublayers.

An article published by Neu et al. in Micromachines informs the readers regarding a novelty introduced in the RIBE process for single-crystal diamond devices intended for photonics fabrication.

The innovative method is known as reactive ion beam angled etching (RIBAE) and was created for the homogeneous and adaptable manufacturing of freestanding photonic and electromechanical nanostructures.

To create 3-D nanostructures with this technique, a traditional lithographic pattern is first applied to an etch mask, followed by RIE with the sample affixed orthogonal to the ion beam on a revolving specimen surface. By angling the material with respect to the ion beam, subsequent etching is performed, culminating in consistent etching of the nanostructures beneath the etch mask.

Chemically Assisted Ion Beam Etch (CAIBE) for Photonics Applications

Independent of the ion beam, non-ionized reactive gases are incorporated into the CAIBE process adjacent to the substrate. Ions from inert gases are emitted through the source, similar to IBE. This procedure, like RIBE, generates reacting gases; however, they only occur near the material. When inert species engage with a reactive gas, a chemical reaction takes place, resulting in the removal of material.

The mixture of an inert gas ion beam and the positioning of the reactive gas near the substrate can make the process more efficient than RIBE and more precise than conventional IBE.

In the Journal of Physical Chemistry Letters, researchers utilized a chemically gas-assisted focused-ion beam (GAFIB) for the patterning of perovskite cells. A homogeneous and repetitive submicron perovskite subwavelength grating (SWG) absorber with broadband attenuation and nanoscale accuracy was manufactured using GAFIB etching. The result indicated the use of FIB as a submicron etching method and a surface modification technique (after FIB patterning to mitigate optical loss) for perovskite photonic nanocrystals. Perovskite solar cells can be patterned with SWG absorbers to increase device efficiency by increasing light capture and absorption.

Along with these major techniques, Electron Cyclotron Resonance (ECR) Plasma Etching is also being utilized. Electron cyclotron resonance (ECR) plasma etching is a gas-assisted etching method that generates plasma using a high-frequency magnetic field. The plasma is then used to engrave the manufactured material. ECR plasma etching is an extremely anisotropic etching technique that generates clean sidewalls and well-defined patterns.

In short, gas-assisted etching is a crucial step in the manufacture of photonic devices. It is anticipated that novel gas-assisted etching methods, such as cryogenic etching and atomic layer etching, will play a crucial role in the development of photonic fabrication.

More from AZoOptics: Optical Amplifiers and Modulators: Enabling High-Speed Communication Technologies

References and Further Reading

Denton Vacuum, 2023. 3 Techniques for Ion Beam Etching. [Online]
Available at: https://www.dentonvacuum.com/3-techniques-for-ion-beam-etching/

Manoccio M. et al. (2021) Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures. Micromachines. 12(1). 6. Available at: https://doi.org/10.3390/mi12010006

Rani D, Opaluch OR, Neu E. (2021). Recent Advances in Single Crystal Diamond Device Fabrication for Photonics, Sensing and Nanomechanics. Micromachines. 12(1):36.  Available at: https://doi.org/10.3390/mi12010036

Alias, M. S. et. al. (2016). Enhanced etching, surface damage recovery, and submicron patterning of hybrid perovskites using a chemically gas-assisted focused-ion beam for subwavelength grating photonic applications. The journal of physical chemistry letters7(1), 137-142. Available at: https://doi.org/10.1021/acs.jpclett.5b02558
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Ibtisam Abbasi

Written by

Ibtisam Abbasi

Ibtisam graduated from the Institute of Space Technology, Islamabad with a B.S. in Aerospace Engineering. During his academic career, he has worked on several research projects and has successfully managed several co-curricular events such as the International World Space Week and the International Conference on Aerospace Engineering. Having won an English prose competition during his undergraduate degree, Ibtisam has always been keenly interested in research, writing, and editing. Soon after his graduation, he joined AzoNetwork as a freelancer to sharpen his skills. Ibtisam loves to travel, especially visiting the countryside. He has always been a sports fan and loves to watch tennis, soccer, and cricket. Born in Pakistan, Ibtisam one day hopes to travel all over the world.

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