2.1.1.1 Classification according to detection method

Many analytical tools are available to detect the presence of contamination. Chapters 8 and 10 discuss the methods for detecting particles on the surface of the wafer and in chemicals and H2O. The following types of contaminants are classified in this chapter according to the analytical techniques discussed in these chapters:

Metallic contamination: contamination consisting of (or containing) metallic species. Although the effect on devices may be dramatically different, usually for in-line monitoring no distinction is made between the different forms in which the contamination can be present (i.e. as an oxide, a silicide, pure metal, etc.). Common methods for detecting metallic contamination are total reflection X-ray fluorescence (TXRF), atomic absorption spectrometry (AA), and inductively coupled plasma–mass spectrometry (ICP-MS).

Particle contamination: contamination that is usually observed using light scattering tools. For this reason, a more correct name is “light-point defects“ (LPD). Recently it has become possible to distinguish between substrate defects that scatter light (crystal originated particles or pits, COP) and actual material deposits. In this chapter, the term “particles“ refers to the latter type of contamination.

Organic contamination: contamination containing a large amount of C or slight amount of C on the surface, plus the bonding structure associated with the C. One method for detecting surface contamination is thermal desorption–MS, other methods include X-ray photoelectron spectroscopy and Auger electron spectroscopy (AES).

Surface defectivity: defectivity associated with roughness of the Si and film surface or line edge roughness. Typically, atomic force microscopy (AFM) and associated techniques are used.

Atmospheric molecular contamination (AMC) and moisture: contamination that has molecular dimensions and can therefore not be removed with normal HEPA (high-efficiency particle air) or ULPA (ultra-low penetration air) filters. AMC is usually monitored with ion mass spectrometry and capillary electrophoresis.

Some contaminants may fall in more than one category. For example, particles may consist of either organic or metallic material, or contain both, and thus arbitrarily be classified as particulate, metallic or organic contamination. Figure 2.1-1 shows the effects of metallic compound particles with a density of 5 g/cm3 and a molecular mass of 100 amu (atomic mass unit) on the average metal concentration in atoms/cm2 on a 300-mm wafer. As can be seen, a high concentration of metals on the wafer surface an result either from large numbers of small particles or from small numbers of large particles. As a result, to minimize particle-derived metal contamination on wafers, both the size and number of contaminant particles must be minimized.

2.1.1.2 Classification according to material

Recent years have seen a diversification of materials that are used in the fabrication of IC devices. Initially, the materials were limited to Si and the dopants necessary to control the semiconducting properties, SiO2-based insulation layers, Si3 N4 barriers, and sacrificial layers. For interconnect metallization, Al or Al/Cu alloys were used. Metal silicides were introduced to lower contact resistance. Later, TiN barriers and W plugs were used followed by Cu metallization and low-κ (low dielectric constant) dielectrics. Metallic oxides and metal gate electrodes replaced the memory cell and the transistor. While the use of these materials improves device performance, their presence in sensitive device regions can have a negative impact, as it takes only a very small number of metal atoms to dramatically change the electrical properties of device structures. Many metal atoms diffuse very rapidly through both Si and SiO2 layers, which highlights the importance of controlling metal contamination during device level processing and maintaining barrier integrity during interconnect processing. Therefore, rather than being based on the detection method, a classification system may be based on the contaminant composition. In general, the contamination may be similar or dissimilar to the substrate material. This has important consequences for the cleaning method that is to be used to remove the contamination.

If the contamination consists of a different material than the substrate then it can, in principle, be removed by a material-selective cleaning method. Such methods may involve selective dissolution of the contaminant, selective dissolution of the substrate surrounding the contaminant, chelation of the contaminant, solution modification to create repulsion between the contaminant and substrate, or chemical reaction to convert the contaminant to a soluble or volatile product. This “chemical cleaning“ relies on the chemical properties of the substrate, the contaminant, and the cleaning agent. In practice, the material selectivity is finite, and side effects can occur that may in some instances be harmful. If the side effects become unacceptable, another cleaning agent must be found, or the chemical cleaning process must be enhanced or replaced by a physical cleaning force. As the wafer surface of the device becomes more complex, for example, when the surface is patterned and multiple materials are exposed or when gate-level features are exposed, the range of acceptable chemical cleaning options is limited.

If the contamination consists of the same material as the substrate, the use of chemical cleaning agents is restricted, as it will not be possible to dissolve contaminants without dissolving substrate material. At the same time, depending on the state of the wafer surface and the type of contamination, it is possible that there are no acceptable chemical cleaning methods. Physical cleaning methods, which are cleaning methods that employ physical forces for removal of contamination, will have to be applied. Many current cleaning methods rely on a combination of chemical and physical methods. An example of such a combined method is undercut cleaning, in which dilute HF is used to mildly etch the wafer surface surrounding an adherent contaminant particle, ultimately leading to particle removal; Chapter 4 discusses this method. During such cleaning processes, the isotropic chemical reaction on the wafer changes the particle-to-wafer contact area. As the contact area is reduced, repulsive electrostatic interactions between the particle and wafer become more important, until finally a condition is reached at which the electrostatic repulsion, a physical force, is strong enough to eject the particle from the surface...