What Is Wet Etching?

Wet etching is a microfabrication process in which a substrate material is selectively removed through immersion in, or exposure to, a liquid chemical solution. The etchant dissolves exposed regions of the substrate while a patterned mask, typically photoresist or a deposited hard mask, protects the areas that must remain. The process draws from chemistry, materials science, and process engineering, and it was the dominant patterning technique used in early integrated circuit manufacturing before the semiconductor industry transitioned to plasma-based dry etching for fine feature sizes. Wet etching remains widely used today for applications where high selectivity, large-batch throughput, and surface smoothness take priority over geometric precision at submicron scales.

The mechanism of wet etching involves chemical reactions at a solid-liquid interface: the etchant species diffuses to the surface, reacts with the material to form soluble products, and those products diffuse away into the bulk solution. The three-step sequence of diffusion, reaction, and removal controls the etch rate and uniformity. Temperature, agitation, etchant concentration, and the crystallographic orientation of the substrate all influence the outcome. In semiconductor cleanrooms, wet etch baths are maintained at tightly controlled temperatures and replenished on regular schedules to keep etchant composition within specification.

Isotropic and Anisotropic Etching

Wet etchants are classified by their directionality. Isotropic etchants remove material at the same rate in all directions, producing a rounded undercut profile beneath the mask edges. This undercutting limits the minimum feature sizes achievable with isotropic wet etching, making it unsuitable for deep submicron patterning. Anisotropic wet etchants, by contrast, etch different crystallographic planes at very different rates. Potassium hydroxide (KOH) dissolved in water etches silicon along the (100) crystallographic planes much faster than along the (111) planes, producing V-shaped grooves and pyramidal pits with well-defined angles determined by the crystal geometry rather than diffusion. This crystallographic selectivity is exploited in microelectromechanical systems (MEMS) fabrication to define precise three-dimensional structures such as membranes, cantilevers, and accelerometer proof masses.

Common Etchants and Selectivity

Selectivity, defined as the ratio of the etch rate of the target material to the etch rate of an adjacent material or the mask, is the central metric for choosing a wet etchant. Hydrofluoric acid (HF) etches silicon dioxide at high rates while attacking silicon very slowly, making it the standard etchant for oxide removal and surface cleaning in silicon processing. Buffered oxide etch (BOE), a mixture of HF and ammonium fluoride, provides a more stable etch rate by moderating pH changes as the etchant is consumed. Phosphoric acid at elevated temperatures selectively removes silicon nitride over oxide with a selectivity of approximately 40:1, enabling nitride stripping without significantly attacking underlying gate oxide. Wet-chemical etching of silicon and silicon dioxide provides detailed etch rate data and process guidance for common silicon-family etchants.

Wet Etching in Modern Semiconductor Processing

Although plasma dry etching dominates patterning steps for features below one micrometer in high-volume logic and memory manufacturing, wet etching remains indispensable for cleaning, surface preparation, and selective layer removal. Post-etch cleaning sequences use dilute hydrofluoric acid to remove native oxide before thermal processes, and piranha solution (sulfuric acid with hydrogen peroxide) removes organic residues. In advanced three-dimensional NAND flash fabrication, selective wet etches remove sacrificial silicon nitride layers from high-aspect-ratio stacks to form the word-line gaps in a replacement-gate process. Deep etching of silicon with metal-assisted chemical etching demonstrates that wet chemistry can produce high-aspect-ratio nanostructures in silicon that complement dry etch capabilities for photovoltaic and nanophotonic applications.

Applications

Wet etching has applications in a wide range of industries and device types, including:

• Silicon MEMS sensor and actuator fabrication using crystallographic KOH etching
• Compound semiconductor device manufacturing for photonic and power electronics
• Solar cell texturing to reduce surface reflectance and improve light absorption
• Printed circuit board copper patterning using ferric chloride or persulfate solutions
• Microfluidic channel definition in glass and silicon substrates