Semiconductor cleaning: processes, methods and principles

半导体清洗：工艺、方法和原理

Introduction

After the 1950s, with the invention of four basic processes: ion implantation, diffusion, epitaxial growth, and photolithography, semiconductor technology gradually developed. Chips are susceptible to particle and metal contamination, which can lead to failures such as short circuits or open circuits. Therefore, in addition to avoiding external contamination throughout the entire production process, wet or dry cleaning is required during manufacturing processes such as high-temperature diffusion and ion implantation. These cleaning processes involve the use of chemical solutions or gases to remove particles, metal ions, and organic impurities remaining on the wafer surface while maintaining a clean surface and good electrical performance.

Classification of Contaminants

Various organic and inorganic compounds are used in the manufacturing process of ICs. Although the manufacturing process takes place in clean rooms, human intervention can lead to various environmental contaminations of wafers. Contaminants are classified into four categories based on their forms: particulates, organic compounds, metal contaminants, and oxides.

Particulates

Polymers, photoresists, and etching impurities constitute the majority of particulates. Typically, particles adhere to the silicon surface, affecting the geometric features and electrical performance development of subsequent processes. Although the adhesion between particles and surfaces varies, van der Waals force is predominant. Therefore, the primary method to remove particles is to gradually undercut them using physical or chemical means. As the contact area between particles and the silicon surface decreases, they are eventually removed.

Organic Compounds

Organic compounds such as human skin oils, cleanroom air, mechanical oils, organic silicon vacuum oils, photoresists, cleaning solvents, and other organic contaminants can be found in IC processes. Each contaminant affects the process differently, but mainly by forming organic layers to prevent cleaning solutions from reaching the wafer surface. Therefore, the removal of organic compounds is usually the first step in cleaning.

Metal Contaminants

In IC fabrication processes, metal interconnect materials are used to connect individual devices. Photolithography and etching are used to create contact windows on insulating layers, followed by deposition techniques such as evaporation, sputtering, or chemical vapor deposition (CVD) to build metal interconnects. Various metal contaminants can be produced during the construction of interconnects. Proper cleaning steps are necessary to remove metal contaminants.

Oxides

In environments containing oxygen and water, silicon atoms are easily oxidized to form oxide layers, known as native oxide layers. After cleaning with APM and HPM solutions, chemical oxide layers form on the silicon surface. Once wafers are cleaned, surface oxides must be removed to ensure the quality of gate oxide. Oxides produced by CVD in the process, such as silicon nitride and silicon oxide, should also be selectively removed during cleaning.

Classification of Cleaning Methods

Wet Cleaning

Wet cleaning uses liquid chemicals and deionized water to oxidize, corrode, and dissolve surface contaminants, organic debris, and metal ion pollutants on silicon surfaces. Common methods include RCA cleaning, dilute chemical cleaning, IMEC cleaning, and single wafer cleaning.

1.1 RCA Cleaning

Initially, there were no fixed or systematic cleaning methods. The RCA process used for wafer cleaning was invented by the Radio Corporation of America in 1965 and used in device manufacturing. This cleaning method has since become the basis for many cleaning processes, and most manufacturers' cleaning processes today are derived from RCA cleaning.

RCA cleaning utilizes solvents, acids, surfactants, and water to spray, clean, oxidize, etch, and dissolve surface contaminants, organic materials, and metal ion pollutants without damaging wafer surface properties. Thorough rinsing with deionized water is required after each chemical use. The following are some commonly used cleaning solutions:

APM (NH4OH/H2O2/H2O at 75~80℃) is a mixed solution composed of ammonium hydroxide, hydrogen peroxide, and deionized water. APM formula is NH4OH:H2O2:H2O=1:1:5. It removes surface particles through oxidation and mild etching; it can also remove mild organic contaminants and some metal contaminants. Additionally, surface roughness develops synchronously with silicon oxidation and etching.

HPM (HCl/H2O2/H2O at 75~80℃) is a mixed solution composed of hydrochloric acid, hydrogen peroxide, and deionized water. HPM formula is H2O2:H2O=1:1:6. HCl can dissolve alkali metal hydroxides and hydroxides of aluminum, iron, and magnesium. The chlorine ions in HCl undergo complexation reactions with residual metal ions, removing metal contaminants from silicon.

SPM (H2SO4/H2O2 at 100~130℃) is a mixed solution composed of sulfuric acid and hydrogen peroxide. SPM formula is H2SO4:H2O2 =4:1. It is a commonly used cleaning solution for removing organic contaminants. Organic materials can be dehydrated and carbonized by H2SO4, and the carbonized products can be oxidized by H2O2 to produce carbon monoxide or carbon dioxide gas.

DHF (HF/H2O at 20-25℃) is a mixed solution composed of hydrofluoric acid and deionized water. DHF formula is HF:H2O=1:50. It is used to remove oxides and reduce surface metal contamination. After cleaning with APM and HPM solutions, DHF is used to remove native oxide layers on the wafer surface and chemically oxidized layers produced by H2O2. After removing the oxide layer, silicon hydride is produced on the silicon surface, combining to form a hydrophobic surface. RCA cleaning combined with megasonic waves can minimize the consumption of chemicals and deionized water, reduce etching time of wafers in cleaning solutions, reduce the influence of wet cleaning uniformity, and extend the lifespan of cleaning solutions.

1.2 Dilute Chemical Cleaning

When combined with RCA cleaning, diluted chemicals can save chemical and deionized water consumption. Diluted APM (1:1:50) can remove particles and hydrocarbons from the wafer surface. When removing metal contaminants, diluted HPM (1:1:60) and diluted HCl (1:100) are as effective as traditional HPM.

The major advantage of using diluted solutions is that particulates do not settle at low HCl concentrations. When the pH is between 2~2.5, the potentials of silicon and silicon oxide are equal. If the pH is higher than this value, the silicon surface becomes negatively charged; if the pH is lower than this value, the silicon surface becomes positively charged. When the pH of the solution is higher than 2.5, particles have the same negative charge as the silicon surface, resulting in electrostatic shielding between them. This barrier prevents particles from precipitating from the solution and depositing on the silicon surface during etching. However, when the silicon surface becomes positively charged, particles carry a negative charge at a pH below 2, and there is no shielding effect, resulting in particle deposition on the silicon surface during etching. Therefore, controlling the HCl concentration is necessary to prevent particle accumulation on the silicon surface in the solution.

When using the diluted chemical cleaning method, the total chemical consumption is reduced by 14%. Diluting APM, diluting HPM, and DHF with megasonic waves can lower the solution temperature in the tank and optimize the time for various cleaning steps, thereby extending the lifespan of the solution in the tank. Experiments have shown that using hot ultrapure water instead of cold ultrapure water can save 75-80% of ultrapure water usage. Additionally, due to low flow rates and cleaning time requirements, various diluted chemicals can save a significant amount of rinse water.

3.1.3 IMEC Cleaning

Based on the successful experience of using diluted chemicals, IMEC (Interuniversity Microelectronics Centre, Belgium) proposed a simplified ozone oxidation and diluted chemical cleaning method to save chemicals and deionized water usage. The first step eliminates organic pollutants and forms a thin layer of chemical oxide to ensure effective particle removal. Sulfuric acid is usually used, but ozone is used instead due to environmental reasons. This reduces the use of chemicals and deionized water and avoids complex rinsing steps after sulfuric acid baths. It is challenging to completely remove HMDS (hexamethyldisilazane) using ozone-processed deionized water (strictly controlled temperature and concentration parameters) because ozone can dissolve in the solution at high concentrations at ambient temperatures, and slow reaction times result in incomplete removal of HDMS. Reaction rates increase at higher temperatures, but dissolved ozone concentrations decrease, affecting HMDS removal. Therefore, temperature and concentration parameters must be adjusted for more effective organic removal.

The second step involves using a mixed dilution solution of HF and HCl. While removing oxide layers and particles, it can suppress the deposition of metal ions such as Cu and Ag. Since metal ions such as Cu and Ag deposit on the silicon surface in HF, this deposition process is an electrochemical process. When removing oxide coatings and particles with HF/HCl solution, metal ions are usually suppressed. Due to the catalytic effect in the Cu2+/Cu+ process, a small amount of chloride ions increases Cu deposition, but a large amount of chloride ions generate soluble high sub-copper chlorides, preventing Cu deposition. The optimized HF/HCl dilution solution successfully prevents the formation of metal coatings and prolongs the solution's lifespan.

To avoid dry spots or watermarks, the third step is to make the silicon surface hydrophilic. To make the silicon surface hydrophilic at low pH without recurring metal contamination, a dilute HCl/O3 solution is typically used, with an increase in HNO3 concentration during the final rinse to reduce Ca surface contamination.

3.1.4 Wafer Cleaning

As critical dimensions in equipment process technologies continue to shrink and new materials are introduced, surface treatment in the front-end-of-line (FEOL) processes becomes increasingly important. The reduction in critical dimensions narrows the cleaning process window, making it challenging to minimize surface and structural damage while meeting cleaning efficiency requirements. Traditional batch cleaning methods are increasingly inadequate for practical applications in wet cleaning. Manufacturing processes require other new cleaning methods to ensure that important equipment specifications, performance, and reliability are not significantly affected by contamination. Therefore, the industry is gradually moving towards single wafer cleaning to reduce the risk of cross-contamination during critical cleaning processes, thereby improving product yield and reducing costs.

Reusing DI-O3/DHF cleaning solution at room temperature is the main step in cleaning. DHF etches silicon oxide while removing particles and metal contaminants, and deionized water (DI-O3) produces silicon oxide. Depending on the etching and oxidation requirements, a brief spraying cycle can achieve satisfactory cleaning results without cross-contamination. The final rinse uses deionized water or ozone-treated deionized water. Isopropanol is mixed with a large amount of nitrogen for drying to avoid water spots.

3.2 Dry Cleaning

Dry cleaning removes impurities from wafer surfaces using chemical vapor techniques. Thermal oxidation and plasma cleaning are the two most common chemical vapor techniques. The cleaning process involves introducing hot chemical gases or plasma reaction gases into the reaction chamber, where the reaction gases chemically combine with the wafer surface, producing volatile reaction products that are pumped away by vacuum pumps. In the oxidation furnace, annealing in a closed environment is a common thermal oxidation process, usually preceded by in-situ argon sputtering before sputter deposition.

Plasma cleaning involves converting inorganic gases into plasma-active particles using lasers, microwaves, thermal ionization, and other methods, which then combine with surface molecules to produce product molecules that are inspected for gas phase residues separated from the surface.

The advantages of dry cleaning are that no waste liquid is left after cleaning and selective local treatment is allowed. Anisotropic dry etching also makes it easier to create delicate lines and geometric shapes. On the other hand, chemical vapor techniques cannot selectively react with surface metal impurities alone, so they can only react with the silicon surface. Different volatile metal components have different vapor pressures, and different metals have different low-temperature volatilities. Therefore, under specific temperature and time conditions, not all metal contaminants can be completely eliminated, so dry cleaning cannot completely replace wet cleaning. Experiments have shown that metal contaminants such as Fe, Cu, Al, Zn, and Ni can be removed using vapor-phase chemical techniques to meet necessary standards. Using chlorine ion-based chemical techniques, Ca can also be successfully vaporized at low temperatures. Typically, a combination of dry and wet cleaning is used in this process.

4. Conclusion

The most common process in chip manufacturing is semiconductor cleaning. The effectiveness of cleaning has a significant impact on IC processes and performance. Improper handling of cleaning solutions can severely pollute the environment, and extensive cleaning cycles will consume large amounts of chemicals and deionized water. Diluted chemical methods, IMEC methods, dry cleaning, as well as combinations of dry and wet cleaning, all contribute to reducing the use of various chemicals and deionized water. Research into more effective cleaning technologies is ongoing. For example, when facing processes with smaller dimensions and higher integration, the effective combination of cleaning with megasonic waves can remove finer particles. In higher-precision manufacturing processes, semiconductor cleaning will face increasingly more challenges.