Successful Cleaving of Delicate Nanostructures

Table of Contents

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

Introduction

This article shows how scientists developing new technologies for photonic and quantum-information applications used the LatticeAx to mitigate one of their biggest sample preparation concerns: precise cleaves on small samples without destroying delicate fabricated nanostructures.

The Challenge

In the Niels Bohr Institute of the University of Copenhagen’s Quantum Photonics Group, scientists are engaged in cutting-edge research to study quantum interactions between nanophotonic semiconductor materials and light. The team has been experimenting with controlling the properties of light with the idea of developing new devices that could have major implications for the future of computing and other information applications.

As part of this research, the scientists have developed processes in which suspended photonic crystals and other nanostructures are fabricated using GaAs. The nanostructures themselves have thicknesses between 160 and 180 nm. To carry out experiments on these structures, the team had to cleave small samples without causing damage to the small and delicate devices that they had fabricated. They also required a method that minimized sample handling in order to further reduce the potential for damage to the nanostructures.

Cleaving

Cleaving is carried out to dice the large chip containing the fabricated nanostructures into separate chips, and also to perform experiments on the nanostructures. The specific geometry of certain nanostructures needs the light to be coupled into the sample from the side, a technique known as “butt-coupling” of optical fiber. The cleave is thus essential to acquire a device with a coupler at or protruding from the edge.

At first, the team made use of a scriber and handheld tool for cleaving and scribing. A tiny scratch at the edge of the sample was made at the location for the cleave. The cleave was performed by using a small, stepped block aligned with the scratch, and pressure was applied over the block and the scratch using their fingers. While precise alignment was accomplished by using a small camera and micrometer-scale screws, the quality of the cleave was highly dependent on the quality of the scratch, and the scratch was not always clean or straight. Moreover, it was sometimes impossible to control the direction of propagation of the cleave using this method.

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, Ph.D. Fellow, University of Copenhagen

Ph.D. fellow Tommaso Pregnolato led the research of other cleaving solutions that would enable them to more reliably attain the high quality of cleaves that their research demanded, while at the same time doing more to eliminate the potential damage caused by sample handling. While searching online for potential solutions, he came across LatticeGear and determined that the LatticeAx® 420 cleaving system looked the most promising for meeting their requirements.

Optical microscope image

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.

Scanning Electron Microscope image

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.

Their research makes use of the LatticeAx 420 at two stages. First, the LatticeAx is used for downsizing the bare 2” or 3” wafer before fabricating the nanostructures. The wafers are cleaved with the crystallographic direction. Each wafer is then positioned on the LatticeAx, by using the cleaving bar ruler to accurately position the wafer for the microline indent. Following the indent, the wafer is cleaved on the LatticeAx. This process was repeated to downsize the wafer further until they had created small pieces with the desired final size. These small pieces are then used as the substrate for fabricating the nanostructures.

The Outcome

The cleaving bar was leveraged to its full advantage; as the bar allows the team to execute a three-point cleave directly on the LatticeAx with a more uniformly distributed pressure on the sample than can be attained when pressing with hands alone. Furthermore, the native “indent-to-cleave” process of the LatticeAx enabled the scientists to precisely reproduce very tiny samples (approximate dimensions of 4 mm x 4 mm).

After the nanofabrication process, the LatticeAx is again used to cleave the small samples. This step demands the most care and precision as the delicate, suspended nanostructures are now in place on the sample. Fortunately, the mechanics of the LatticeAx minimize sample handling and cause no damage to the delicate fabricated devices.

The team made use of an objective microscope and a camera to precisely align the delicate sample with the indenter to create the “weak point” for cleaving. Following indent, the team examined whether the size of the sample and the position of the nanostructures would allow for cleaving with the LatticeAx.

If the sample was smaller than 10 mm x 10 mm, they instead used LatticeGear’s Small Sample Cleaver (SSC) accessory. The SSC features a 0.5 mm gauge that makes it possible to retract the sample by 0.5 mm, therefore enabling them to reliably carry out the 0.5 mm indent each time to attain a stronger weak point, resulting in a more reliable, straight, and precise cleave.

LatticeGear’s Small Sample Cleaver (SSC)

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 the team were extremely satisfied with both the quality and accuracy of the cleaves and also from the fact that it was possible to produce the required samples without causing damage to the fragile nanostructures. The LatticeAx allowed them to attain higher yield in the fabrication of functioning devices, without alteration, damage, or contamination to the surface that can be an artifact of other sample preparation methods. This high achievement was a huge relief. The fabrication process of their nanophotonic devices is costly, so to have only one sample successfully produced in six attempts (on average, depending on the specific nanostructures) was quite detrimental 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.

After just two training sessions, all users could consistently produce the required samples using the LatticeAx. Within the academic environment, research teams usually don’t have a dedicated expert for sample preparation from all the different materials that could be subject to examination. Having a tool that can be easily learned and mastered without extensive experience – and which also considerably lowers the chance of human error – is an important consideration when working with small, delicate and unique 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

LatticeGear

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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