Process Control and Safety in SPM

SPM (sulfuric acid–hydrogen peroxide mixture), owing to its strong oxidizing power and high‑efficiency photoresist removal capability, is widely used in semiconductor wafer‑cleaning processes. Its key reaction involves the formation of Caro’s acid from H₂SO₄ and H₂O₂, which subsequently oxidizes and decomposes organic contaminants. However, in practical applications, SPM faces challenges such as a narrow process window, high chemical consumption, and elevated operational risks.

I. Control of Key Process Parameters

1. Temperature Control

Temperature is the most critical parameter affecting SPM cleaning performance. It is typically maintained within the range of 120–150°C.

• Too low a temperature: residual photoresist removal is incomplete, and organic residues can adversely affect subsequent processes.
• Excessive temperature: H₂O₂ decomposition is accelerated, and its oxidative activity is rapidly lost, resulting in ineffective cleaning.

In actual production, a high-precision temperature control system must be installed, and the bath solution temperature fluctuations should be monitored in real time.

2. Proportioning and Oxidizing Strength

The volume ratio of H₂SO₄ to H₂O₂ directly affects both the yield of caroic acid and the oxidizing power of the resulting mixture.

• The higher the H₂SO₄ concentration, the higher the temperature of the mixture and the stronger its oxidizing power.
• However, an excessively low H₂O₂ concentration will reduce the formation of carboxylic acid and decrease cleaning efficiency.

The typical molar ratio of H₂SO₄ to H₂O₂ ranges from 2:1 to 4:1, with the exact value to be optimized based on the type of contaminant and process requirements.

3. Stability of H₂O₂ and Bath Life

H₂O₂ is unstable at elevated temperatures and has a short half-life. The bath solution typically retains effective activity for only a few hours. To ensure consistent cleaning performance, the following measures are required during production:

• Regularly replenish with H₂O₂ or replace the solution entirely.
• Determine the optimal replacement timing through online concentration monitoring or periodic sampling and analysis.

4. Single-Wafer Processing Mode in Advanced Manufacturing Processes

Traditional槽式 SPM cleaning suffers from high chemical consumption and poor uniformity. In advanced manufacturing processes, the Single Wafer processing mode has been introduced:

• Precisely control the SPM dosage and temperature on each wafer.
• Significantly reduce overall chemical consumption and lower wastewater discharge.
• Enhance process repeatability and particle removal efficiency.

II. Safety Operating Procedures

1. The stringent sequence of mixed operations

The SPM mixing process releases substantial heat, which can easily lead to bumping or splashing. The following procedures must be strictly followed:

• Add H₂O₂ slowly to H₂SO₄ while stirring continuously.
• Reverse addition (i.e., adding H₂SO₄ to H₂O₂) is strictly prohibited, as it can result in an instantaneous, violent exothermic reaction and splashing.

2. Personal Protective Equipment and Emergency Facilities

Operators must wear the following when handling SPM:

• Acid-resistant gloves, protective face shield, chemical protective suit.
• Use a positive-pressure respirator when necessary (in high-concentration acid mist environments).
• Equipment and work areas must be equipped with:
• Emergency shower and eyewash station.
• Fume hoods and exhaust systems must be equipped with dedicated acid–alkali scrubbers to prevent the direct discharge of acid mist.

III. Wastewater Treatment and Environmental Protection Technologies

1. Pre-treatment of waste liquid before discharge

SPM waste liquid is highly acidic and strongly oxidizing; direct discharge can severely corrode pipelines and contaminate water bodies. It must:

• First, cool the waste liquid to room temperature.
• Dilute with a large volume of water, then adjust the pH to neutral using a neutralizing agent such as NaOH or lime milk.
• Only after confirming that oxidative residues have been reduced may they be discharged into the wastewater treatment system.

2. Recycling and Regeneration Technology

To reduce chemical consumption and effluent discharge, semiconductor fabs are exploring SPM recycling and regeneration technologies:

• Electrochemical methods are employed to regenerate H₂O₂ from waste solutions.
• H₂SO₄ is recycled through filtration and purification steps.
• This technology can reduce the procurement of fresh chemicals by 30% to 50%, while also alleviating environmental pressures.

Conclusion

SPM cleaning processes are indispensable in semiconductor manufacturing; however, their hallmark—“the stronger the cleaning power, the higher the control requirements”—necessitates integrated management of process performance, safety, and environmental protection. Optimizing temperature settings and chemical ratios, along with adopting a single-wafer processing mode, can enhance both cleaning effectiveness and efficiency. Strict adherence to proper mixing sequences and personal protective guidelines helps prevent accidents, while standardized waste‑liquid treatment and the widespread adoption of recycling and regeneration technologies contribute to sustainable manufacturing. Looking ahead, as online monitoring and automated replenishment systems become more prevalent, SPM processes will evolve toward greater intelligence, enhanced safety, and improved environmental sustainability.

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