This article dives into a new wet-etching strategy that finally tackles a stubborn issue in glass micromachining: controlling sidewall angles. By intentionally tweaking the interface between a metal mask and the glass, researchers have turned undercutting—usually a headache—into a precise, predictable tool for making optical and MEMS parts.
A Persistent Challenge in Glass Micromachining
Glass is everywhere in optics, microfluidics, and MEMS. It’s clear, tough against chemicals, and shrugs off heat. But its amorphous structure makes it pretty tricky to shape with precision, especially if you’re using regular wet etching.
Standard wet etching eats away at glass in all directions, so you get those curved, arc-like sidewalls. That really boxes in your design choices. Sure, you can try femtosecond laser machining or silicon-based hard masks to get better profiles, but those methods are expensive and complicated—hardly ideal for mass production.
A Quasi-Anisotropic Wet-Etching Concept
The team rolled out a quasi-anisotropic wet-etching strategy that creates smooth, sloped sidewalls by managing the chemistry at the interface. The trick is a uniform interfacial layer between a chrome/chromium oxynitride (Cr/CrON) mask and the glass below it.
This method is all about competitive etching between two regions:
From Isotropic to Inclined Profiles
When the interfacial layer etches faster than the glass (v2 > v1), you get sloped sidewalls with a clear oblique angle. If the glass etches as fast or faster (v1 ≥ v2), you’re back to those familiar, rounded profiles.
What’s clever is that the sidewall angle depends on the ratio v2/v1, not on how long you etch or how big the features are. Once you dial in the chemistry, the geometry stays consistent.
Interfacial Layer Engineering and Validation
Using transmission electron microscopy and energy-dispersive spectroscopy, the team spotted a transitional interfacial layer about 10 nm thick. This layer has a mix of chromium, silicon, nitrogen, and a boost of oxygen. That chemical cocktail is what makes the controlled undercutting possible.
Scratch tests showed the metal mask sticks well and evenly to the glass. That’s crucial for keeping the etch profiles steady across big surfaces.
Predictive Modeling with COMSOL
The researchers built a COMSOL level-set simulation that factors in the competing etch rates. By tweaking just the glass etch rate, the model matched up with real cross-sections for both soda-lime and borosilicate glass.
This close match between simulation and experiment points to interfacial chemistry as the real driver behind the quasi-anisotropic effect.
Performance Gains and Practical Advantages
The technique brings some clear improvements over older wet-etching methods:
Changing etch time or mask opening size barely budged the sidewall angle, at least in the tested range. That shows the method holds up well in practice.
Scalable Fabrication and Optical Applications
This approach skips lasers and silicon masks. Instead, it uses simple metal masks and standard wet chemistry, so it’s ready for large-scale manufacturing. The authors managed to crank out microprisms, micro-pyramids, and micro-cones with good consistency.
Enabling Next-Generation Optical Devices
Researchers pushed these structures into real-world optical components. They used them in diffuser plates, light couplers, and even waveguides for VR and AR systems.
Instead of seeing undercutting as a flaw, the team treated it as something they could actually control. That shift made a big difference in how we approach glass microfabrication.
For optics, MEMS, and photonics, being able to precisely shape glass sidewalls with scalable wet processes feels like opening a new chapter in device design. It could change the way manufacturers think about glass altogether.
Here is the source article for this story: Quasi-anisotropic wet etching of glass creates inclined microstructures for advanced optical and MEMS devices