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Differences and Adaptation Logic of SC-1 and SC-2 Solution Ratios Between 7nm and 28nm Process Nodes
In semiconductor chip manufacturing, wet cleaning is a core process to ensure wafer cleanliness and maintain device yield. As the "gold standard" adopted by the industry for more than half a century, the RCA cleaning method’s core components—SC-1 (Ammonia-Peroxide Mixture, APM) and SC-2 (Hydrochloric Acid-Peroxide Mixture, HPM)—have their ratio designs directly determining cleaning effectiveness and wafer surface integrity. With the advancement of process nodes from 28nm to 7nm, the reduction in wafer feature size, increased complexity of surface structures, and enhanced sensitivity to contaminants necessitate precise iterations of SC-1 and SC-2 ratios to adapt to new process requirements. This article will delve into the ratio differences, causes, and regulation strategies of the two solutions across the two process nodes.

I. Basic Characteristics and Standard Ratios of SC-1 and SC-2 Solutions

As the core reagents of the RCA cleaning method, SC-1 and SC-2 have distinct and complementary roles: SC-1 relies on an alkaline environment to remove particles and organic substances through an oxidation-corrosion balance, while SC-2 uses an acidic system to accurately eliminate metal ions and alkaline residues. Their standard ratios, optimized based on traditional process nodes, provide a benchmark for ratio adjustments in advanced nodes.
SC-1 solution is prepared by mixing ammonia water (NH₄OH, typically 28%-29% concentration), hydrogen peroxide (H₂O₂, 30% concentration), and deionized water (DIW) in a volume ratio, with a standard ratio range of 1:1:5 to 1:2:7 and a typical operating temperature of 60-80℃. Its mechanism involves H₂O₂ oxidizing the wafer surface to form a thin SiO₂ layer, while NH₄OH slightly etches this oxide layer to dislodge particles, which are then repelled by electrostatic forces. The dynamic balance between oxidation and corrosion is key to efficient cleaning.
SC-2 solution is composed of hydrochloric acid (HCl, 37% concentration), hydrogen peroxide (H₂O₂, 30% concentration), and deionized water, with a standard ratio of 1:1:6 to 1:2:8 and an operating temperature usually controlled at 65-85℃. It oxidizes metal impurities via H₂O₂, and HCl forms soluble chlorides with metal ions. Additionally, it removes alkaline metal ions (such as Na⁺ and K⁺) introduced by SC-1 and forms a passivation layer to prevent secondary contamination.

II. Comparison of Ratio Differences Between 7nm and 28nm Process Nodes

As a mature process node, 28nm features relatively simple wafer surface structures and higher tolerance to cleaning damage, with ratios focused on "efficient contamination removal". The 7nm process, entering the FinFET/nanosheet era, has feature sizes approaching physical limits, thinner metal interconnect layers, and stringent requirements for surface roughness and metal corrosion rate control. Thus, its ratios shift to the dual core goal of "precision contamination removal + damage control", resulting in significant differences between the two nodes.

(I) SC-1 Solution Ratio Differences

In the 28nm process, SC-1 ratios mostly adopt medium-to-high concentration combinations within the standard range, with common ratios of NH₄OH:H₂O₂:DIW = 1:1:5 or 1:2:7 and operating temperatures maintained at 70-80℃. This ratio achieves an efficient balance between the corrosive capacity of NH₄OH and the oxidizing capacity of H₂O₂, enabling rapid removal of organic residues and micron-scale particles generated during manufacturing, while keeping damage to the silicon substrate within an acceptable range. For 28nm processes with copper interconnects, some production lines fine-tune the ratio to 1:1:8, moderately increasing the water proportion to reduce copper corrosion rate and ensure metal layer integrity.
In the 7nm process, SC-1 ratios exhibit characteristics of "low concentration, wide water ratio, and temperature-controlled deceleration", with typical ratios of NH₄OH:H₂O₂:DIW = 1:2:10 to 1:3:15 and operating temperatures reduced to 50-65℃. On one hand, low-concentration NH₄OH weakens corrosive effects, avoiding excessive etching that would increase wafer surface roughness (needing to be controlled below 0.1nm) to adapt to the complex morphology of FinFET structures. On the other hand, increasing the H₂O₂ ratio to 1:3 maintains strong oxidizability at low corrosion rates, ensuring efficient removal of submicron-scale particles. Meanwhile, the wide water ratio design reduces solution viscosity, enhances penetration into nanoscale gap structures, and combined with megasonic assisted technology, achieves deep cleaning under low chemical damage conditions.

(II) SC-2 Solution Ratio Differences

SC-2 ratios in the 28nm process are mainly based on standard formulas, with a common ratio of HCl:H₂O₂:DIW = 1:1:6 and operating temperature of 70-80℃. This ratio efficiently removes alkaline metal ions and metal impurities such as Fe and Al残留 from SC-1, with stable passivation effects on the wafer surface, meeting the metal contamination control requirements of mature processes (metal impurity content ≤ 10¹⁰ atoms/cm²). For scenarios with severe metal contamination, the HCl ratio can be fine-tuned to 1:1:5 to improve metal dissolution efficiency.
The 7nm process shows an exponential increase in sensitivity to metal contamination; trace metal residues (such as Cu and Au) can reduce carrier mobility and cause transistor leakage. Therefore, SC-2 ratios must balance "precision metal removal" and "no secondary damage". Typical ratios are HCl:H₂O₂:DIW = 1:0.5:8 to 1:1:10, with operating temperatures controlled at 45-55℃. Reducing HCl concentration avoids etching damage to ultra-thin metal interconnect layers, controlling the copper corrosion rate below 0.3nm/min. Increasing the water ratio extends the retention time of metal ions in the solution, preventing their redeposition on the wafer surface. Additionally, low-temperature operation inhibits HCl volatilization, maintains solution concentration stability, and combined with online spectral monitoring technology, achieves trace removal of metal impurities.

III. Analysis of Core Causes for Ratio Differences

The differences in SC-1 and SC-2 ratios between 7nm and 28nm process nodes essentially reflect the reconstruction of the "efficacy-damage" balance in cleaning driven by process requirement upgrades, with core driving factors manifested in three dimensions.
Firstly, the reduction in feature size drastically lowers surface damage tolerance. 28nm wafers have larger line widths and gaps, allowing slight etching by SC-1 to be corrected by subsequent processes. In contrast, the feature size of 7nm nodes is only a quarter of that of 28nm; excessive etching leads to line width deviation and excessive surface roughness, directly causing device failure. Thus, it is necessary to reduce the NH₄OH concentration in SC-1 to weaken corrosion, while compensating for the loss of cleaning capacity by optimizing the H₂O₂ ratio and auxiliary physical technologies.
Secondly, there are refined changes in contaminant types and particle sizes. The main contaminants in the 28nm process are micron-scale particles, conventional organic residues, and metal impurities. The 7nm process, with more complex lithography and etching processes, has contaminants dominated by submicron-scale particles, polymer residues, and trace precious metal impurities. SC-1 requires an increased H₂O₂ ratio to enhance the oxidative decomposition capacity of organic substances, and a wide water ratio design to adapt to contaminant removal in nanoscale gaps. SC-2, meanwhile, relies on low-concentration HCl and temperature-controlled operation to precisely target trace metals, avoiding secondary contamination from excessive reactions.
Thirdly, there are process adaptation requirements for metal interconnect structures. The 28nm process mostly adopts copper interconnect + silicide structures, with higher corrosion tolerance. The 7nm process has thinner metal interconnect layers and introduces high-k dielectric/metal gate (HKMG) structures, which are significantly more sensitive to acidic and alkaline solutions. The low-concentration and wide water ratio designs of SC-1 and SC-2 effectively reduce erosion of metal and dielectric layers, ensuring interconnect reliability.

IV. Key Technologies for Ratio Regulation and Quality Assurance Measures

With the advancement of process nodes, a single fixed ratio can no longer meet requirements. It is necessary to combine dynamic regulation technologies and precise monitoring methods to achieve real-time adaptation between ratios and process states, ensuring the stability and consistency of cleaning effects.
In terms of dynamic ratio regulation, a "concentration-temperature-time" linkage strategy is adopted: in the 7nm process, SC-1 uses a stepwise cooling method, first performing rapid cleaning at 65℃ and then naturally cooling to 50℃ to maintain passivation, reducing high-temperature damage. For SC-2, online ORP (Oxidation-Reduction Potential) sensors monitor solution activity, with real-time H₂O₂ supplementation to compensate for decomposition losses. The 28nm process does not require complex stepwise regulation but relies on mass flow meters to precisely control reagent injection volume, ensuring a ratio deviation of ≤ ±2%.
Quality assurance relies on a multi-dimensional monitoring system: laser interferometers monitor wafer surface etching rate, Raman spectroscopy detects residual groups, and TXRF (Total X-Ray Fluorescence) technology quantifies metal impurity content, ensuring wafers meet cleanliness standards of each node after cleaning. Meanwhile, a closed-loop feedback system is established, using machine learning models to analyze historical data and build correlation curves between ratios and surface quality, realizing adaptive optimization adjustments. For volatile reagents (such as NH₄OH and HCl), nitrogen protection and temperature-controlled jacket designs are adopted to maintain solution concentration uniformity and avoid ratio imbalance.

V. Development Trend: Evolution from Traditional Ratios to Precision and Environmental Friendliness

As processes advance to 5nm and below, SC-1 and SC-2 ratios will further develop toward "ultra-low concentration + physical assistance", such as diluted SC-1 with a ratio of 1:10:130 combined with megasonic cavitation for damage-free cleaning. Meanwhile, environmental requirements drive the research and development of low-volatility, low-corrosion reagents; some production lines have begun testing weak alkaline amines as substitutes for NH₄OH to reduce exhaust emissions and equipment corrosion. In addition, single solution ratios will be gradually replaced by "customized formula libraries", which dynamically call optimal ratios based on wafer material, contaminant type, and structural characteristics to achieve full-process precision adaptation of cleaning processes.

VI. Conclusion

The optimization of SC-1 and SC-2 solution ratios is a typical embodiment of "details determining success or failure" in the evolution of semiconductor process nodes. The shift from efficient standard ratios at 28nm to low-damage precision ratios at 7nm essentially represents the upgrade of cleaning processes from "large-scale contamination removal" to "precision cleanliness". In the future, as semiconductor manufacturing imposes increasingly high requirements on cleanliness and reliability, ratio regulation will rely more on intelligent technologies and interdisciplinary collaboration. The traditional RCA cleaning method will continue to play a core role through iterations, providing solid support for breakthroughs in advanced processes.