Basic Knowledge of Wafer Surface Cleaning
Created on 10.23
Before the silicon wafer enters the CMOS manufacturing process, its surface must be cleaned to remove any adhered particles and organic/inorganic impurities, and also to remove the natural oxide of silicon. The continuous miniaturization of chip designs makes cleaning technology increasingly important for achieving acceptable product yield. In modern semiconductor device manufacturing, the wafer cleaning process can account for 30%~40% of the entire manufacturing process. Wafer cleaning has a long history in the semiconductor industry. For a more detailed discussion, refer to the discussions and research by Werner Kern and Tadahiro Ohmi on classic methods.
▲Figure 1. Contaminants that may exist on the surface of a silicon wafer
Contaminants on the wafer surface may exist in the form of adsorbed ions, elements, thin films, discrete particles, particle clusters, and adsorbed gases. Figure 1 shows a schematic diagram of the types of contaminants present on the wafer surface before entering the process flow. Table 1 describes the impact of different types of surface contaminants on device performance, and Table 2 lists the cleaning solutions used to remove different contaminants. Since gas is a poor medium for transporting contaminants, most of the cleaning work is done with liquid.
▼Table 1. Surface pollutants and their impact on device performance
▼Table 2. Examples of cleaning solutions for fabricating wafers by CMOS process.
1. Particulate pollution
Particle contamination may arise from dust in the air, with sources including production equipment, process chemicals, inner surfaces of gas lines, wafer transport, and gas-phase nucleation and operation personnel in thin-film deposition systems.Even low-nanometer-sized particles can potentially cause“lethal” defects, either by physically obscuring the formation of critical features in devices (forming patterns, features, and injecting defects), or by creating local electrical weaknesses in insulating films.
The size of the particles needs to be of the order of 1/5 of the characteristic dimension to cause fatal defects. Cleaning schemes for particle contamination include piranha cleaning for total particle (including organic) contamination and SC-1 cleaning for small and strongly adherent particles.piranha solution is a strong acid that can oxidize many surface contaminants on the wafer, producing soluble species that can be removed from the solution.SC-1 solution is designed to remove insoluble particles by oxidizing a thin layer of silicon on the wafer surface, then dissolving it into the solution and taking away the adsorbed particles.SC-1 solution prevents particle re-adsorption by inducing the same zeta potential (a measure of electrostatic repulsion) on the particles and the wafer surface.
All cleaning solutions containing hydrogen peroxide (SPM,SC-1,SC-2,SPOM) leave a thin layer of oxide on the silicon wafer surface, which can be removed in the final step with HF . As an alternative, DIO₃ can be used in combination with HF .
2. Metal pollution
Semiconductor devices are particularly sensitive to metal contaminants because metals have high mobility in the silicon lattice (especially for metals such as gold), so they can easily migrate from the surface into the bulk silicon. Once in the bulk silicon, moderate process temperatures can cause the metal to diffuse rapidly through the lattice until it is trapped at crystal defects. This "decoration" of crystal defects can reduce device performance, resulting in larger leakage currents and lower breakdown voltages.
Acidic cleaners (such as SC-2, SPM, dilute hydrofluoric acid, dilute hydrochloric acid, and their mixtures) can be used to remove metal contaminants from the wafer surface. These solutions react with metals, producing soluble, ionized metal salts that are washed away. A major source of metal contaminants is the previous SC-1, so SC-2 is a follow-up step to SC-1.
3. Chemical contamination
Chemical contamination on the surface of the wafer can be divided into three types: surface adsorption of organic compounds, surface adsorption of inorganic compounds, and natural oxides formed by covalent binding of silicon oxides or hydroxides. Thick (about 1~2nm) natural oxides.
3.1 Organic compoundsDue to the presence of volatile organic solvents or cleaners and outgassing from polymer-containing construction materials, organic compounds contaminate surfaces in the cleanroom through air pollution or photoresist (PR) residues.
Severe contamination of organic compounds, such as when photoresist is not completely removed, can form residual carbon at high temperature processes, affecting yield. These carbon-based species can form atomic nuclei, which are manifested as particulate contaminants, and small amounts of residual metal in the photoresist can be captured on the surface of the residues. Therefore PR carbon residues need to be removed using SPM cleaning.
The ubiquitous presence of volatile air pollutants that cause organic contamination also needs to be removed from the wafer surface. The presence of these pollutants hinders DHF solutions from removing the native oxide (Fig.2), resulting in an indefinite interface between the gate oxide and the substrate and gate. Poor interface quality can severely reduce the integrity of the gate oxide. Additionally, the presence of surface organic compounds can affect the initial rate of thermal oxidation and CVD processes, introducing unwanted unknown variations in the film thickness.
▲Figure 2. Removal of natural oxides and hydrogen-terminated surface characteristics
SC-1 Cleaning is performed by oxidation and solvent action using hydrogen peroxide and ammonia. SC-1 Cleaning slowly removes the natural oxide, and the oxidation by hydrogen peroxide replaces this layer with a new one.
In recent years, ozone (DIO₃) dissolved in deionized water has been increasingly used as an alternative to SPM and SC-1 cleaning solutions, removing organic pollutants in a more green and safer way. Depending on the nature of the organic pollution, combinations of SPM with ozone (SPOM) or SOM (sulphur ozone mixture) are also used. The allowed material loss and surface roughness are important parameters in the choice of the appropriate cleaning method.
3.2 Inorganic compounds
Inorganic compounds containing dopant elements such as boron and phosphorus may be present on the wafer surface due to the outgassing of phosphorus flame retardants or dopant residues from production tools. If not removed from the wafer surface before high-temperature processing, these elements may migrate into the substrate, thereby changing the resistivity.
Other types of volatile inorganic compounds, such as alkaline compounds like amines and ammonia and acidic compounds like sulfur oxides (SOx), can also introduce defects in semiconductor devices if present on the wafer surface. Acids and alkalis can cause changes in the pH of photoresists, leading to problems with pattern generation and photoresist removal. These compounds are highly reactive and can easily combine with other volatile environmental chemicals to form particles and mist on the wafer surface, leading to the formation of chemical salts. The combined action of SC-1 and SC-2 clean can remove adsorbed acidic and alkaline substances from the wafer surface.
3.3 Natural oxides
Like many elements of the solid state, silicon naturally forms a thin layer of oxide on its surface by reacting with oxygen and moisture in the ambient air. The chemical composition of this layer is not well defined, and is a more or less random assembly of Si-O-Si, Si-H and Si-OH.
The presence of this natural oxide on the silicon surface can cause problems in the manufacturing of semiconductor devices, as it can increase the difficulty of forming a very thin thermal oxide thickness. Any natural oxide present on the wafer during the formation of the thin gate oxide will weaken the electrical properties of the gate oxide through dopant incorporation of hydroxyl groups.Furthermore, if there is a natural oxide on the silicon surface of the contact pad, it will increase the contact resistance.
In the past 50 years, our understanding of the properties of silicon native oxides and their impact on device performance has greatly increased. These studies have found that the use of deionized water, dilute hydrofluoric acid, or ammonium fluorides, polishing oxide etchants (BOE) can completely remove silicon native oxides, leaving hydrogen-terminated silicon surfaces (Fig. 2).
1970,Werner Kernand his colleagues at RCA developed the first successful wet cleaning process for silicon wafers that was applied to the front end of line (FEOL).RCA cleaning has since been greatly developed and successfully modified, and RCA cleaning is still the major pre-cleaning process in the semiconductor industry.
RCA cleaning process is a combination of the above different steps. It consists of continuous SC-1 and SC-2 solutions, later diluting with HF solution or BOE to remove natural or chemical oxides. Some are RCA cleaning is based on the development of adding SPM or other steps modification.
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