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Home > Blog > Advanced Cleaning Techniques in Semiconductor Fabrication

Advanced Cleaning Techniques in Semiconductor Fabrication

The manufacturing of integrated circuits (IC) demands precise processes to ensure the integrity of silicon wafers. However, contaminants such as particles, organic residues, metals, and oxides pose significant challenges, affecting wafer quality and functionality. This article explores the types of contaminants impacting IC production and delves into advanced cleaning techniques, including wet and dry methods, that optimize contamination removal while enhancing production efficiency and sustainability in cleanroom environments.

Catalog

1. Types of Contaminants Affecting IC Manufacturing
2. Different Approaches to Cleaning

Types of Contaminants Affecting IC Manufacturing

The production of integrated circuits (IC) involves meticulous processes, yet silicon wafers often face the presence of diverse contaminants within clean room environments. These pollutants, such as particles, organic residues, metals, and oxides, can disrupt the structural quality of wafers, a concern that stands at the intersection of technology and material science.

Particle Contaminants

Originating from materials like polymers, photoresists, and etching residues, particles adhere to wafer surfaces primarily through van der Waals forces, challenging subsequent processing stages. Addressing this issue may involve physical interventions like ultrasonic cleaning or chemical techniques, such as solvent washes, to detach particles while preserving wafer integrity. Effectively reducing this type of contamination requires a nuanced comprehension of material interactions and bespoke solutions that reduce adhesion, smoothing the path for removal. Additionally, incorporating sophisticated filtration systems and airflow strategies in production facilities can substantially lessen particle deposition.

Organic Contaminants

Persistent organic residues from skin oils, ambient air, and machine lubricants form barriers that hinder cleaning agent efficiency. These residues compromise both purity and functionality by interfering with essential processing layers. Initial cleaning steps, therefore, focus on the extraction of these organic layers, preparing the stage for subsequent cleaning phases. Techniques like solvent cleaning and low-pressure UV treatment are pivotal, underscoring the necessity of maintaining rigorously controlled environments to avoid recontamination.

Metal Contaminants

While metal interconnects are embedded within semiconductor processes, they also pose contamination challenges. Metals such as aluminum and copper might originate during photolithography and chemical vapor deposition (CVD), complicating the maintenance of wafer purity. Mitigating these risks involves deploying deposition barriers or advanced etching techniques, emphasizing ongoing oversight to ensure contaminants remain below threshold levels. Implementing dual-mode cleaning processes, which adeptly separate and remove metal impurities without damaging other critical structures, is also fundamental.

Oxide Contaminants

Oxide layers generally develop from the oxidation of silicon atoms in oxygen-rich conditions, resulting in native or chemical oxides. A delicate balance must be struck between removing these oxides and preserving the gate oxide's structural integrity. Selective etching methods and buffered oxide etchants are crucial in managing this balance. Innovations in these techniques continue to evolve, driven by an in-depth understanding of material properties and reaction dynamics. Such a delicate equilibrium facilitates advancements in precision during fabrication, guided by a profound comprehension of the microscopic interactions involved.

Different Approaches to Cleaning

Wet Cleaning Techniques

This cleaning approach employs liquid chemical solvents and deionized (DI) water to perform cleaning tasks such as oxidizing, etching, and dissolving contaminants present on the wafer surface. This includes organic matter and metal ions. Commonly applied techniques include RCA cleaning, dilution chemical cleaning, IMEC cleaning, and single wafer cleaning.

RCA Cleaning Procedure

Initially, the approach to silicon wafer cleaning lacked systematic procedures. Developed by the Radio Corporation of America (RCA) in 1965, the RCA cleaning method established a comprehensive process for silicon wafer cleaning during component manufacturing. This technique continues to be a foundational element for many contemporary cleaning processes.

Utilizing solvents, acids, surfactants, and water, RCA cleaning aims to efficiently remove surface contaminants, while preserving the wafer’s characteristics. Comprehensive rinsing with ultrapure water (UPW) follows each chemical application. Below are several cleaning solutions frequently utilized:

- APM (NH4OH/H2O2/H2O at 65–80°C): This solution consists of ammonium hydroxide, hydrogen peroxide, and DI water, effectively oxidizing and etching surface particles, alongside removing some organic and metal contaminants. While the silicon surface oxidizes and etches, surface roughness increases.

- HPM (HCl/H2O2/H2O at 65–80°C): Known as SC-2, this cleaning solution dissolves alkali metal ions and hydroxides of metals like aluminum and magnesium. Chloride ions in HCl react with residual metal ions, forming water-soluble complexes.

- SPM (H2SO4/H2O2/H2O at 100°C): Referred to as SC-3, this solution efficiently removes organic contaminants. Sulfuric acid dehydrates and carbonizes organic material, which hydrogen peroxide then oxidizes into gaseous byproducts.

- HF or DHF (HF:H2O=1:2:10 at 20–25°C): Used for oxide removal in hard-to-reach areas, this solution etches silicon oxides while decreasing surface metal. Following SC1 and SC2 cleaning, it eliminates native oxide layers from the silicon wafer, forming a hydrophobic silicon surface.

- Ultrapure Water: Post-cleaning, ozonated water serves to dilute residual chemicals and rinse wafers.

Incorporating megasonic energy into RCA cleaning reduces chemical and DI water usage, shortens wafer etching time, and, consequently, extends the cleaning solution’s life.

Dilution Chemical Method

The dilution approach for SC1 and SC2 mixtures, when combined with RCA cleaning, conserves chemicals and DI water. It's possible to entirely omit H2O2 from the SC2 solution. The APM SC2 mix, diluted at a 1:1:50 ratio, effectively removes wafer surface particulates and hydrocarbons.

For metal removal, heavily diluted mixtures (HPM 1:1:60 and HCl 1:100) are as effective as traditional SC2 fluids. Maintaining low HCl concentrations offers the advantage of preventing particle settling, and the solution pH, which ranges from 2 to 2.5, influences the silicon wafer’s surface charge. Above this pH, the charged surfaces of both silicon and solution particles form an electrostatic barrier, inhibiting particle deposition. Below this pH, particles deposit on the wafer due to a lack of shielding.

Significant reductions, over 86%, in chemical consumption occur with diluted RCA cleaning. Optimized cleaning steps, which include megasonic agitation with diluted SC1, SC2, and HF solutions, enhance solution longevity and cut chemical usage by 80-90%. Experiments suggest hot UPW usage can cut UPW consumption by 75-80%, and various dilution chemistries can conserve large amounts of flushing water due to lower flow rates and time requirements.

IMEC Cleaning Approach

This method focuses on reducing chemical and DI water usage in wet cleaning, aiming to effectively address organic pollutants in its initial phase. Often sulfuric acid combinations are used; however, ozonated DI water is a viable alternative for environmental benefits and reducing difficult cleaning phases. Adjusting temperature and concentration facilitates efficient organic removal.

The second phase targets oxide layers, particles, and metal oxides. Electrochemical deposition processes become a concern with metal ions in HF solutions. HF/HCl solutions usually suppress metal deposition while efficiently removing oxide coatings. Adding chloride ions strategically can prevent metal plating and enhance solution durability.

In the final stage, the aim is to impart hydrophilicity to the silicon surface, minimizing drying spots or watermarks. Dilute HCl/O3 solutions at low pH make the surface hydrophilic without metal recontamination, while using HNO3 during the rinse decreases Ca contamination.

Comparative analysis shows the IMEC method effectively curtails metal contamination while being economically sensible due to reduced chemical usage.

Single Wafer Cleaning

For large-diameter wafers, established procedures often fall short. Single wafer cleaning, utilizing DI-O3/DHF solutions at room temperature, offers a targeted approach. By etching silicon oxide and removing particles and metals with HF, and forming silicon oxide with DI-O3, satisfactory results can be achieved sans cross-contamination. Rinse with DI water or ozonated water, and avoid stains by drying with isopropyl ethanol (IPA) and nitrogen. Enhanced RCA cleaning shows effectiveness over single-wafer techniques, with DI water and HF recycling during the process further optimizing chemical expenditure and wafer cost.

Dry Cleaning Techniques

Dry cleaning, through vapor-phase chemical means, tackles wafer surface impurities. Commonly, thermal oxidation and plasma cleaning are employed. Procedures entail introducing hot or plasma reactive gases into a reaction chamber, leading to the formation of volatile reaction products that are subsequently evacuated. An oxidation furnace enables CI containment annealing, and Ar sputtering is done before deposition. Plasma cleaning involves converting inorganic gas into plasma active particles, which interact with surface molecules to form gas-phase residues.

The benefits of dry cleaning include localized treatment and no leftover waste liquids. Its etching anisotropy aids in producing fine patterns. However, due to non-selective reactions with surface metals and the specific conditions required for complete metal volatilization, dry cleaning alone doesn’t entirely substitute wet cleaning. Studies reveal notable reductions in metallized contaminants using gas-phase techniques, complemented by wet cleaning in practice.

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