Wet Etching: Techniques and Applications

A close up of a wet etched channel in a glass microchip


What is wet etching?

Wet-etching is a precise technique used in microfabrication to create intricate patterns on glass wafers. It begins with photolithography, a process that uses light to transfer geometric shapes onto a light-sensitive material. This material acts as a protective layer for the glass wafer surface. The wafer is then exposed to hydrofluoric acid (HF), which selectively dissolves the unprotected areas. The depth and width of the etched channels increase until the desired dimensions are achieved, at which point the wafer is removed from the HF bath. To finalize the structure, the etched wafer can be fusion bonded with another layer, creating sealed microfluidic chips. This technique allows for the creation of highly detailed and functional microstructures essential for various applications.

Why wet-etching of glass?

Glass is the preferred material for wet-etching due to its exceptional optical properties and chemical resistance. It offers a smooth surface with roughness in the angstrom range, ensuring high precision in microfabrication. Glass is also fully transparent, which is crucial for applications requiring optical clarity. Wet-etching on glass achieves highly accurate structures, with an etch depth accuracy of around 5%, significantly better than the typical 10%. The initial mask openings, defined by photolithography, ensure extreme precision. Additionally, glass's inherent shape stability and flatness make it ideal for producing ultra-flat products. Furthermore, multiple wafers can be processed simultaneously in a single etching step, making the process efficient and scalable for industrial applications.

Types of wet-etching

Micronit applies wet chemical etching techniques to create structures in a desired material, for this you have two techniques - isotropic and anisotropic.    

Isotropic Etching Methods (Glass) 

Our wet etching of glass is performed using fluorine-based solutions in combination with photolithography. Wet chemical etching results in isotropic characteristics, in effect exposed glass material is removed in all directions. 

The wet isotropic wet etching of glass is a highly controlled process within Micronit’s many established capabilities. Utilizing spray etch processing technologies, highly precise, uniform, and reproducible geometries over 150- and 200-mm glass substrates are obtained. 

 Anisotropic Etching Methods (Silicon) 

For silicon we can offer wet chemical etching based on anisotropy. In this process the etching takes place along the crystal direction of the silicon, and the final etched shape is related to the etching speed of the different crystal planes in the monocrystalline silicon.  

Using a hard mask (from either silicon dioxide or silicon nitride) atop the silicon, patterned with photolithography, complex and precisely defined structures with predictable etch angles are obtained. For anisotropic wet chemical etching, spray etching is applied, using alkali hydroxides, towards highly unform etching of up to 150- and 200-mm substrates. 

 

SEM image of anisotropic etched silicon using a hardmask providing 54° sidewalls according to the silicon crystallinity.  

Things You Should Know About Wet-Etching

Understanding the fundamentals of wet-etching is crucial for achieving the desired microfabrication results. One key aspect is that glass is etched isotropically, meaning it is removed at the same rate in all directions. Consequently, both the depth and width of the etched channels increase simultaneously. This relationship can be described by the formula: channel width = 2 * etch depth + initial mask opening. This formula highlights the importance of carefully planning the initial mask dimensions to achieve the precise channel width and depth needed for your application

table of wet-etching depth

 

 

Single Depth Etching vs. Double Depth Etching

When it comes to wet-etching, understanding the difference between single depth etching and double depth etching is essential for achieving the desired microfluidic structures. Single depth etching involves creating channels of a uniform depth across the entire wafer. This method is simpler and faster, as it only requires one etching step, making it ideal for applications where consistent depth is critical. Single depth etching is commonly used for straightforward designs where uniform channel dimensions are sufficient.

In contrast, double depth etching, or multi-level etching, involves multiple etching steps to create channels of varying depths on the same wafer. This technique allows for more complex and functional designs, enabling the integration of different microfluidic components within a single chip. Double depth etching starts with an initial etching step to achieve the first depth, followed by additional photolithographic masking and etching steps to reach the second, deeper level. This approach is beneficial for applications requiring intricate features, such as valves, pumps, or complex fluid routing systems, providing greater design flexibility and functionality. Understanding when to use single versus double depth etching helps optimize the manufacturing process for specific microfluidic applications.

Getting started 

At Micronit, we specialize in assisting our customers from concept to manufacturing. Do you have a project that would benefit from our etching solutions? Our experts are ready to work with you and turn your ideas into reality! Contact us and find out how Micronit can support your projects with our advanced etching techniques! 

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