Cleaving of Delicate Nanostructures

Table of Contents

Introduction
The Challenge
Cleaving
LatticeAx® 420 Cleaving System
The Outcome
Conclusion

Introduction

This article shows how LatticeAx was used by researchers devising innovative technologies for quantum-information and photonic applications to deal with one of the most significant challenges faced during sample preparation: precision cleaving of small samples without causing any damage to the delicate fabricated nanostructures.

The Challenge

Researchers of the Niels Bohr Institute of the University of Copenhagen’s Quantum Photonics Group have been performing groundbreaking research to analyze the quantum interactions between light and nanophotonic semiconductor materials. They have been conducting experiments to control the properties of light with the aim of developing innovative devices that will be of major significance for the next-generation computing and other information applications.

In this study, the researchers have devised processes that involve the use of GaAs for fabricating suspended photonic crystals and other nanostructures. The thicknesses of the nanostructures range from 160 to 180 nm. In order to perform experiments on these structures, it was necessary for the researchers to cleave small samples without causing any damage to the delicate and small devices fabricated by them. They were also looking for a technique that reduced sample handling to further minimize the probability of damage to the nanostructures.

Cleaving

Cleaving is performed to dice the large chip with the fabricated nanostructures into individual chips, and even to carry out experiments on the nanostructures. The definitive geometry of specific nanostructures mandates the coupling of light into the sample from the side — a method called “butt-coupling” of optical fiber. Cleaving is thus vital to obtain a device including a coupler at or protruding from the edge (Figure 1).

Cleaving

Figure 1. Optical microscope image from the LatticeAx shows the cleaved sample with the nanophotonic structures protruding from the edge of the chip. Courtesy of the Quantum Photonics Group, Niels Bohr Institute, University of Copenhagen.

Initially, the researchers used a handheld tool and scriber for cleaving and scribing. They made a tiny scratch at the sample’s edge at the location for the cleave. The cleave was carried out with the help of a small, stepped block matched with the scratch, and pressure was applied over the scratch and the block with the fingers. Although exact alignment was achieved with the help of micrometer-scale screws and a small camera, the quality of the cleave was largely dependent on the quality of the scratch, and the scratch was not always straight or clean. Furthermore, it was at times not feasible to regulate the direction of propagation of the cleave with this technique.

LatticeAx® 420 Cleaving System

The LatticeAx was gentle on our very delicate samples containing fragile nanostructures. The microline indent method unique to the LatticeAx is very effective for producing high quality, straight cleaves, without altering material properties or introducing dust and artifacts at the cleaved edge.

Tommaso Pregnolato, PhD Fellow, The University of Copenhagen

Tommaso Pregnolato headed the study of other cleaving solutions that would allow the researchers to more reliably achieve the high quality of cleaves mandated by their research, while simultaneously taking more efforts to prevent the potential damage caused during sample handling. When he was looking online for prospective solutions, he chanced upon LatticeGear and decided that the LatticeAx® 420 cleaving system was the most promising for satisfying their requirements (Figure 2).

Scanning Electron Microscope image shows one of the nanostructures after cleaving with the LatticeAx 420

Figure 2. Scanning Electron Microscope image shows one of the nanostructures after cleaving with the LatticeAx 420. The photonic structure is formed by a suspended nanobeam waveguide (green square) which connects a photonics-crystal waveguide (red square) to a tapered outcoupler (yellow square). The taper is still suspended and protruding from the edge of the sample (black dashed line). Courtesy of the Quantum Photonics Group, Niels Bohr Institute, University of Copenhagen.

In their study, the LatticeAx 420 was used at two stages. First, it is used for downsizing the bare 2” or 3” wafer prior to fabrication of the nanostructures. Cleaving of the wafers is carried out along the crystallographic direction. Then, each wafer is positioned on the LatticeAx with the help of the cleaving bar ruler for precise positioning of the wafer for the microline indent. After the indent, the wafer is cleaved on the LatticeAx. This process was repeated for further downsizing of the wafer until small pieces with the desired final size were created. Then, the small pieces are used as the substrate for fabricating the nanostructures.

The Outcome

The cleaving bar was utilized to its full ability since the bar enabled the researchers to directly perform a three-point cleave on the LatticeAx with a more uniformly distributed pressure on the sample than can be achieved by pressing with only the hands. Moreover, the innate “indent-to-cleave” process of the LatticeAx allowed the researchers to accurately reproduce very small samples (with dimensions of roughly 4 mm x 4 mm).

Following the nanofabrication process, the LatticeAx is once more used to cleave the small samples. This step mandates utmost precision and care since the suspended, delicate nanostructures are now positioned on the sample. Luckily, the LatticeAx mechanics reduce sample handling and prevent damage to the delicate fabricated devices.

The researchers used a camera and an objective microscope for precise alignment of the delicate sample with the indenter to produce the “weak point” for cleaving. After the indent, the researchers probed whether the position of the nanostructures and the size of the sample would enable cleaving with the LatticeAx.

In case the size of the sample was lesser than 10 mm x 10 mm, they rather used LatticeGear’s Small Sample Cleaver (SSC) accessory (Figure 3). The SSC includes a 0.5 mm gauge that enables the sample to be retracted by 0.5 mm, thus allowing the researchers to reliably perform the 0.5 mm indent every time to achieve a stronger weak point, leading to a precise, straight, and more reliable cleave.

LatticeGear’s Small Sample Cleaver

Figure 3. LatticeGear’s Small Sample Cleaver (SSC) was used for samples smaller than 10 mm x 10 mm. The SSC allowed the team to cleave these samples without having to handle them and risk damage to the fragile nanostructures.

Pregnolato and his colleagues were highly satisfied with the accuracy as well as the quality of the cleaves and also due to the fact that the required samples could be produced without causing any damage to the fragile nanostructures (Figure 4). The LatticeAx enabled them to achieve higher yield in the fabrication of functioning devices, eliminating contamination, damage, or alteration of the surface that can be an artifact of other sample preparation techniques. Such a high accomplishment was a huge sigh of relief. Since the fabrication process of their nanophotonic devices is expensive, successful production of only one sample in six attempts (on average, based on the specific nanostructures) was quite disadvantageous to their research.

Figure 4. The Quantum Photonics Group selected the LatticeAx 420 cleaving system to support their research. The LatticeAx 420 delivers cleaving accuracy of 10 μm in <5 minutes, so it is ideal for the lab that values speed and high accuracy while at the same time needing to accommodate a variety of sample sizes, thicknesses, and materials. The compact footprint allows it to be located wherever it is needed in the lab. Courtesy of the Quantum Photonics Group, Niels Bohr Institute, University of Copenhagen.

Following merely two training sessions, the required samples could be consistently produced by all users with the help of the LatticeAx. Inside the academic setting, research groups, in general, do not have a dedicated expert for preparing the samples from all the different materials that could be subject to examination. Access to a tool that can be easily studied and mastered without vast experience — and which also significantly reduces the chance of human error — is a crucial consideration while working with unique, delicate, and small materials samples.

Conclusion

The ability to use the indenter for the ‘indent-to-cleave’ process for cleaving samples makes handling of the samples much simpler. Applying evenly distributed pressure with our fingers on such small samples is almost impossible and much care must be taken to not crush the fabricated structures. Using the LatticeAx and its ability to not only mark the position of the cleave and initiate with the indent, but also perform the cleave, enables us to have a more reliable and reproducible fabrication process. This increased the final yield of functioning samples so we were able to spend less time preparing samples, with less wasted material in each run.

Tommaso Pregnolato

This information has been sourced, reviewed and adapted from materials provided by LatticeGear.

For more information on this source, please visit LatticeGear.

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