Mechanism of High-Temperature SPM Cleaning for Removing Contaminants from Nickel Surfaces

I. Introduction

In fields such as semiconductor manufacturing, microelectromechanical systems, and precision electroplating, nickel and its alloys are widely used due to their excellent electrical and mechanical properties. However, during the fabrication process, the nickel surface is highly prone to residual organic photoresist, metallic impurities, and natural oxide layers. These contaminants can severely compromise the yield and device performance of subsequent processes, including thin-film deposition, photolithography, and bonding. Therefore, it is crucial to develop efficient and controllable surface-cleaning technologies.

Wet chemical cleaning is one of the key technologies for removing surface contamination. Among these, the SPM solution—a mixture of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂)—has emerged as an efficient cleaning agent thanks to its powerful oxidizing and dissolving capabilities. Particularly under high-temperature conditions, the reaction kinetics of the SPM solution are significantly enhanced, enabling it to effectively remove a wide variety of contaminants from nickel surfaces.

II. Chemical Properties and Reaction Mechanisms of High-Temperature SPM

The high-temperature SPM cleaning process for nickel is essentially the result of synergistic effects among multiple chemical reactions, with the key lying in the strong acidity of sulfuric acid and the strong oxidizing power of hydrogen peroxide.

2.1 Main functions of sulfuric acid

Sulfuric acid plays a dual role in the system. First, as a strong acid, it can directly react with the oxide layer on the nickel surface (such as NiO), forming soluble nickel sulfate (NiSO₄) and thereby dissolving the oxide layer. Second, its strong dehydrating property can remove water from organic contaminants (such as polymeric photoresists) and cause them to carbonize, thus creating favorable conditions for subsequent oxidative decomposition. The reaction equation can be expressed as:

NiO + H ₂ SO ₄ →NiSO ₄ +H ₂ O

2.2 Oxidation Mechanism of Hydrogen Peroxide

In high-temperature environments, hydrogen peroxide is unstable and decomposes to produce highly reactive hydroxyl radicals (·OH) and nascent oxygen ([O]). These highly oxidizing species can non-selectively attack the molecular chains of organic pollutants, ultimately oxidizing and breaking them down into carbon dioxide and water, thereby achieving thorough removal of organic substances and preventing carbonization residues.

2H ₂ O ₂ →Δ2H ₂ O+O ₂ ↑(and Accompanied by the generation of reactive oxygen species)

2.3 Synergistic Effect

The synergistic effect of sulfuric acid and hydrogen peroxide is key to the high efficiency of SPM cleaning. The acidic environment created by sulfuric acid not only promotes the decomposition of hydrogen peroxide, generating more reactive oxygen species, but also maintains the ionized state of metallic contaminants, making it easier for them to form soluble salts that can be effectively removed. Meanwhile, the oxidizing action of hydrogen peroxide inhibits the excessive corrosion of the nickel substrate caused by sulfuric acid, striking a balance between selective removal of contaminants and protection of the substrate itself.

III. Cleaning Effect of SPM on Nickel Surfaces

3.1 Removal of Organic Pollutants

For organic substances such as oils and photoresist residues, SPM completely mineralizes them through the oxidation mechanisms described above. For example, in phenolic resin-based photoresists, the benzene ring structure is cleaved by reactive oxygen species, ultimately yielding gaseous CO₂ and water, thereby achieving a clean surface.

3.2 Dissolution of Metal Contaminants and Oxide Layers

As for the oxide layer on nickel itself or foreign metallic contaminants, SPM uses the dissolving action of acids to convert them into soluble ions (such as Ni²⁺). It is worth noting that during the cleaning process, an extremely thin passivation oxide film may form on the nickel surface. However, this thin film is typically unstable and easily removed during subsequent rinsing or treatment with dilute acid, without leaving any lasting impact on the surface properties.

IV. Analysis of Key Process Parameters

4.1 Temperature

Temperature is a key parameter that influences reaction rates. The typical operating temperature range is between 120°C and 150°C. Within this range, an increase in temperature significantly accelerates the decomposition of hydrogen peroxide and the rates of various chemical reactions, thereby enhancing cleaning efficiency. However, when the temperature exceeds 200°C, hydrogen peroxide decomposes too rapidly, leading to its complete consumption before it can reach the surface of the workpiece. This, in turn, reduces the actual cleaning effectiveness and may even cause surface roughening due to the sole action of sulfuric acid.

4.2 Solution Ratio

The typical ratio of SPM (volume ratio of H₂SO₄ to H₂O₂) ranges from 2:1 to 8:1. The choice of ratio depends on the predominant type of pollutant: a higher sulfuric acid ratio (e.g., 4:1 to 8:1) emphasizes enhanced dissolution capacity for metal oxides and particulate matter, whereas a higher hydrogen peroxide ratio (e.g., 2:1 to 3:1) boosts the oxidative removal efficiency for organic compounds. The ratio should be optimized based on the actual pollution conditions.

4.3 Processing Time

The cleaning time is typically controlled between 5 and 15 minutes. Insufficient cleaning time may result in incomplete removal of contaminants, while excessively long cleaning times could lead to slight corrosion of the nickel substrate, increase surface roughness, and even alter the surface morphology. The optimal time window must be determined through experimentation.

V. Process Precautions

• Corrosion Control: To prevent irreversible damage to the nickel substrate, it is essential to precisely control the process parameters. Under specific conditions, a small amount of corrosion inhibitor (such as an organic carboxylic acid) may be added to suppress corrosion.
• Post-processing and Rinsing: After SPM cleaning, it is essential to immediately perform thorough rinsing with a large volume of ultrapure water (DI Water) to completely remove any residual sulfate ions (SO₄²⁻) and metal ions, thereby preventing surface passivation or secondary contamination.
• Waste Liquid Treatment: The cleaning waste liquid contains high-concentration sulfuric acid and dissolved nickel ions, classifying it as hazardous waste. It must be treated professionally in compliance with environmental protection regulations, using methods such as neutralization and precipitation to recover nickel resources and ensure that the treated waste liquid meets discharge standards.

VI. Conclusion

The high-temperature SPM cleaning technology leverages the synergistic effects of sulfuric acid’s dissolving and dehydrating actions, combined with hydrogen peroxide’s powerful oxidizing properties, to efficiently remove both organic and inorganic contaminants from nickel surfaces. This process boasts advantages such as rapid reaction rates and broad applicability. However, its cleaning effectiveness and surface quality are highly dependent on the precise control of parameters such as temperature, concentration ratios, and processing time. Future research could further focus on developing more environmentally friendly alternative chemicals and implementing intelligent closed-loop control of process parameters, thereby enhancing the precision and sustainability of the process.

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