The Unexpected Champion: How One Crystal Orientation Unlocks Flawless Aluminum

This post is about the paper:-
X. Sun, W. D. A. Rickard, O. S. Burrow, E. Riis, and C. N. Ironside, “Investigation of nanofabrication on (111), (100) and (110) surfaces of single-crystal aluminium,” Next Research, p. 101202, 2025/12/06/ 2025,

https://www.sciencedirect.com/science/article/pii/S305047592501067X?via%3Dihub

The Unexpected Champion: How One Crystal Orientation Unlocks Flawless Aluminum
1.0 Introduction: The Hidden Problem with Everyday Aluminum
Aluminum is one of the most common and useful metals in the world. We find it everywhere from kitchen foil to aircraft fuselages. But for the demanding world of high-tech optics, the standard form of this familiar metal has a major flaw. When scientists and engineers try to shape and pattern regular, or “polycrystalline,” aluminum for advanced applications, they run into a persistent problem: surface roughness.
This roughness, a natural result of the metal’s microscopic crystal structure, limits its use in high-precision components like mirrors and gratings for visible, ultraviolet, and extreme ultraviolet (EUV) light. For technologies like quantum computing and advanced sensors, even a few nanometers of roughness can ruin performance. This raises a critical question for material science.
What if we could create perfectly smooth aluminum surfaces? What secrets would we have to unlock at the atomic level to do it? A new study investigates this very problem, exploring how different fabrication techniques interact with aluminum’s fundamental atomic structure to either create a flawlessly smooth surface or a chaotically rough one.
2.0 Takeaway 1: Not All Aluminum is Created Equal
1. Your Kitchen Foil Isn’t Smooth Enough for a Quantum Computer
The aluminum in your kitchen is polycrystalline, meaning it’s composed of countless microscopic crystal grains all oriented in random directions. When you try to sculpt this type of aluminum on a nanoscale, these random grains create a messy, uneven surface. In contrast, single-crystal aluminum is a pristine, perfectly ordered material where all the atoms are aligned in a single, continuous lattice.
This fundamental difference becomes starkly clear when subjected to nanofabrication techniques like Focused Ion Beam (FIB) milling or Reactive Ion Etching (RIE). The research highlights the “well-known rough surface morphology issue with polycrystalline Al alloys.” Visual evidence from the study’s microscope images shows the stark difference: the polycrystalline aluminum surface appears coarse and pebbled after processing, while the single-crystal surfaces are dramatically cleaner. This proves that the key to a perfect surface begins with a perfect crystal.
3.0 Takeaway 2: The Crystal’s Orientation is the Key to Perfection
2. A Crystal’s “Face” Determines Its Fate
Even within a perfect single crystal, not all surfaces are the same. The arrangement of atoms can be exposed in different ways, creating distinct planes or “faces.” The researchers investigated three of the most important crystallographic orientations, designated as (111), (100), and (110). Think of these as looking at the crystal’s atomic structure from three different angles.
The study discovered that the results of the fabrication process were “strongly dependent on the crystallographic orientation of the Al.” The speed of the process and the final smoothness of the surface could be changed dramatically simply by choosing a different crystal face to work on. This finding revealed that controlling the final surface quality isn’t just about using a single crystal, but about choosing the right face of that crystal.
4.0 Takeaway 3: And the Winner Is… The (110) Surface
3. The Unexpected Champion for Smoothness and Speed
After testing all three orientations with different techniques, the research identified one clear winner. When using the chemical Reactive Ion Etching (RIE) method, the single-crystal aluminum with a (110) orientation vastly outperformed the others.
The data points tell a compelling story. The Al (110) surface delivered an unparalleled surface roughness of just 2±0.5 nm. To put that in perspective, this result is 2 to 4 times smoother than the other surfaces tested with the same technique: the (111) face (4±0.5 nm), the (100) face (6±0.5 nm), and standard polycrystalline aluminum (8±0.5 nm). On top of its winning smoothness, it also had the highest etch rate at 15±2 nm/min.
This combination of speed and smoothness makes the (110) orientation a highly promising substrate for creating the next generation of “low-loss diffractive optical elements,” where even the slightest imperfection can scatter light and degrade performance. The study suggests a reason for this success: the (110) surface has the highest surface energy. Think of surface energy as a measure of how “unstable” or reactive the atoms on the surface are. The higher energy of the (110) face makes its atoms more susceptible to the chemical etching process, allowing them to be removed more quickly and cleanly.

4. A Warning: The Precision Tool That “Poisons” the Metal
Focused Ion Beam (FIB) milling is a powerful tool in nanofabrication. It acts like a microscopic sandblaster, using a precise beam of gallium ions (Ga+) to carve out tiny, intricate patterns, making it excellent for “rapid prototyping.” On certain crystal faces, its performance is remarkable; for instance, on the Al (111) surface, FIB milling is incredibly fast (52 nm/min) and produces a very smooth surface (2 nm).
However, this performance comes at a cost: the unavoidable “poisoning” of the surface with gallium. The high-energy gallium ions don’t just carve the aluminum; they get implanted into it. This contamination causes “liquid metal embrittlement,” which fundamentally weakens the material. Specifically, the embedded gallium disrupts the formation of the naturally protective layer of aluminum oxide. This naturally-forming oxide layer is aluminum’s armor, protecting it from corrosion. By disrupting it, the FIB process fundamentally compromises the long-term stability and integrity of the material, a crucial flaw for sensitive optical components. This creates a critical choice for engineers between initial fabrication speed and the final material’s integrity, where RIE emerges as the safer, cleaner option for high-performance optics.
6.0 Conclusion: From a Simple Metal to a High-Tech Future
This research provides a clear and practical roadmap for fabricating ultra-smooth aluminum surfaces for advanced optical devices. By pairing the right material—single-crystal aluminum—with the right atomic orientation—the (110) face—and the right technique—Reactive Ion Etching—scientists can overcome the long-standing challenge of surface roughness.
The impact of this work is best summarised by :
This enhanced understanding of how crystal orientation affects surface quality is expected to contribute significantly to the advancement of low-loss diffractive optical elements across the infrared to extreme ultraviolet spectral ranges.