Wet Etching vs. Dry Etching

Key Takeaways

• Wet etching utilizing liquid chemicals is generally isotropic and less precise, and is often used for wafer cleaning and substrate preparation.

• Wet etching uses simpler equipment and a range of acids and bases, while dry etching requires more complex machinery like plasma etch systems and uses gases like sulfur hexafluoride for etching.

• Dry etching provides enhanced precision with higher anisotropy and selectivity but poses a risk of surface damage and environmental concerns due to toxic by-products.

Left: Visual example of isotropic etching. Right: Visual example of completely anisotropic etching

Etching is the process used to remove portions of dielectrics, polymers, and metals from a semiconductor wafer to create the required geometry in semiconductor devices. Etching utilizes methods including chemical corrosion, electrochemical electrolysis, or mechanical polishing. The two primary types of etching are wet etching using a chemical solution and dry etching using ionized gases to remove materials.

Wet Etching vs. Dry Etching 

Etching Goals

Etching treatments can lead to one of three distinct results:

• Isotropic etching uniformly removes material in all directions, resulting in a semi-circular cut in the material. This less precise form of etching is typically employed for cleaning wafers or substrates in chips. 

• Completely anisotropic etching exclusively removes material in a vertical direction, producing a rectangular profile when viewed laterally. 

• Anisotropic etching creates shapes that are intermediate between the first two types and is frequently utilized in the crafting of circuit patterns.

Pros and Cons of Wet Etching vs. Dry Etching

Wet etching uses simpler equipment and rapid etch rates, along with high selectivity. However, it requires using large volumes of chemicals, posing safety challenges and offering less control over the etching process. In contrast, dry etching provides enhanced isotropic control and precision, generally ensuring safer operations. Depending on the specific method, dry etching can achieve high etch rates using fewer chemicals. 

Dry etching is predominantly used for creating semiconductors, whereas wet etching, involving chemical baths, is mainly used for cleaning wafers. During etching, parts of the wafer are often shielded by etch-resistant materials, such as photoresists or hard masks like silicon nitride. Dry etching is predominantly anisotropic, meaning it etches more in a downward direction than laterally, making it essential for patterning-related etching. Wet etching, in most cases, etches uniformly in all directions and is typically used for cleaning, residual film removal, and similar applications.

Similarities

Both wet and dry etching techniques are utilized in various industries and activities, including semiconductor fabrication, PCB etching, metal etching, and transistor gate manufacturing. Silicon processing is one of the most frequent applications of these techniques, and anisotropy is specifically one of the most desirable results. 

The Wet Etching Process

Wet etching uses a liquid-phase solution, or etchant, to selectively remove layers of material segments. In wet etching, the material undergoes treatment with an acidic or basic chemical solution, which chemically reacts with and removes undesired materials from the surface. Depending on the desired material for removal, the chemical solution varies. 

Types of Wet Etching

• The dip method involves immersing the component directly into a tank filled with the aforementioned chemical solutions, creating a chemical bath. 

• In the spin/spray method, the etchant is sprayed onto the material undergoing processing. Concurrently, the part may be rotated, and this is paired with an absorption mechanism. However, the rotation aspect of this method is not mandatory, as the effectiveness of material erosion is largely influenced by the crystal planes of the atoms

Diagram of simple wet etching system.

The Dry Etching Process

Dry etching employs beams of high kinetic energy to remove specific regions from a material or part, typically consisting of ionized gases in a plasma state, which dislodge atoms from the substrate's surface, leading to the evaporation of the targeted area. 

Dry etching is useful, as it allows for minimal undercutting creating anisotropic etching. It involves using neutrally charged, high-energy ions to etch a designated area of a substrate. These ions are produced by converting reactive gases into plasma, which gives rise to the alternate term "plasma etching" for this method.

Types of Dry Etching

• Ion Beam Etching (IBE) uses a directed beam of argon ions at the surface of the material, typically with energies ranging from 1 to 3 kilo electronvolts (keV).
• Reactive Ion Etching (RIE) is another dry etching technique that employs chemically reactive ions. The part being etched is placed on an HF electrode, which influences the ions generated in the chamber, aiding in directional control. This setup forms highly anisotropic channels. By adjusting the pressure inside the etching chamber, the mean free path of particles can be altered, leading to either more directional or less directional etching, resulting in channels with varying degrees of anisotropy.
• ICP-RIE Etching, or simply ICP Etching, involves using an inductively coupled plasma source. This plasma is generated using an RF-powered magnetic field, which is integrated into the existing RIE system. This addition enhances control over dry etching process properties.

Selectivity in dry etching processes, including RIE, depends on the precise control of plasma parameters. By choosing suitable etching gases and process conditions, selectivity for particular material combinations is attainable. For instance, fluorine-based plasmas can selectively etch silicon dioxide over silicon, and chlorine-based plasmas can be used for specific metals like aluminum or copper. However, achieving high selectivity in dry etching can be complex due to the nuanced interplay between plasma, reactive species, and substrate, as well as potential issues with process drift and plasma distribution non-uniformities.

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