Post-etch cleaning

As the feature sizes of integrated circuits continue to shrink and three-dimensional structures become increasingly widespread, the importance of post-etch cleaning processes has grown significantly. Residues generated during etching not only affect the electrical performance of devices but can also lead to failures in subsequent process steps. Therefore, gaining a deeper understanding of the composition and behavior of post-etch residues and developing efficient, non-destructive cleaning strategies have become critical challenges in semiconductor manufacturing.

I. Composition and Properties of Residues Remaining After Etching

The residues left after etching are not a single substance but rather a complex mixture whose composition and morphology are closely related to the etching material, gas chemistry, and process conditions. They can be broadly categorized into the following three types:

1. Fluorine-containing/carbon-containing polymers

• Source: Primarily originates from the pyrolysis and recombination of photoresist in a plasma environment, as well as the redeposition and polymerization reactions of fluorine-containing etching gases (such as CHF₃ and C₄F₈) on the sidewalls of the structure. In metal etching, metal-organic complexes are also formed.
• Characteristics: These polymers feature a highly cross-linked chemical structure and exhibit extreme chemical inertness, similar to that of Teflon (PTFE). As a result, they have very poor solubility and reactivity in conventional organic solvents, making them among the most stubborn components to remove during cleaning.

1.2 Lateral Wall Re-deposits

• Source: Under high-energy ion bombardment, the etching products sputtered off (including the material being etched and the hard mask layer material, among others) fail to be promptly carried away by the gas flow and instead redeposit onto the sidewalls of the pattern.
• Main ingredients:

Metal/halide semimetals (such as AlF₃, TiF₄, WF₆, etc.).

Sputtered particles from hard mask materials (such as SiO₂ and SiN debris).

In the etching of multilayer film structures, sputtered particles from different material layers mix with each other, forming complex composite residues with a diverse composition.

• Characteristics: Its composition is directly related to the underlying material and may exhibit either an amorphous or microcrystalline structure. It adheres to the sidewalls, influencing the morphology and dimensions of the structure.

1.3 Plasma Damage Layer

• Source: In the etching process, high-energy plasma causes physical and chemical damage to the exposed surface materials (typically silicon substrates or dielectric layers).
• Features:

Structural damage: Lattice defects, amorphization, and dangling bonds are generated within a surface layer ranging from several nanometers to tens of nanometers.

Elemental contamination: The surface layer is highly enriched with halogen elements (F, Cl), with atomic percentage concentrations reaching 10–20%.

Changes in surface properties: The damaged layer exhibits electrical properties and surface energy that differ from those of the bulk material, resulting in enhanced hydrophobicity and severely compromising the uniformity of subsequent thin-film deposition or wetting processes.

II. Challenges and Mechanisms of Polymer Removal

1. Time dependence of dissolution kinetics

Polymer removal is not a simple dissolution process; its kinetics involve several consecutive steps: penetration of the cleaning solution (1–10 seconds), swelling of the polymer and rupture of molecular chains (10–60 seconds), and diffusion and removal of dissolved products (10–30 seconds). In practice, the effective cleaning time typically ranges from 60 to 180 seconds.

2. Mass-transfer limitations in high aspect-ratio structures

In structures with aspect ratios greater than 20:1, the cleaning fluid is hindered by surface tension, and the reaction products diffuse slowly from the bottom of the pores outward. The characteristic diffusion time can exceed 5 minutes, severely affecting the uniformity and efficiency of the cleaning process.

3. Dynamic Changes in Surface Energy

During the cleaning process, the surface properties undergo significant changes: initially, the surface is hydrophobic (contact angle of 70–90°); in the intermediate stage, it exhibits a coexistence of hydrophilic and hydrophobic characteristics; and finally, it transforms into a hydrophilic surface (contact angle of 10–20°). This change directly affects the wetting and penetration behavior of the cleaning solution.

III. Cleaning Strategies and Procedures in Engineering Practice

In response to the challenges mentioned above, this paper proposes a combined cleaning approach featuring “mild oxidation + complexing and detachment.” The specific process is as follows:

1. Step 1: Mild Oxidation Treatment

• Formula: H₂O₂ 1–3% + NH₄OH 0.1–0.5%
• Process parameters: Temperature 40–60°C, time 30–60 seconds.
• Function: Break C-C cross-link bonds, reduce the molecular weight of the polymer, and enhance its subsequent solubility.

2. Step 2: Complexation Dissolution

• Formula: Contains amine- or alcohol-based solvents
• Process parameters: Temperature 30–50°C, time 60–120 seconds.
• Function: Promote polymer swelling and detachment from the substrate.

3. Step 3: Ultrasonic Enhancement (optional)

• Parameters: Frequency 1 MHz, power 0.5–1 W/cm²
• Effect: Enhances mass transfer efficiency by 3–5 times, particularly suitable for structures with high aspect ratios.

4. Step 4: Rinse thoroughly

• Method: Multi-step rinsing with deionized water (3–5 steps)
• Objective: To prevent chemical residues from crystallizing and ensure surface cleanliness.

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