Page 260 - Semiconductor Manufacturing Handbook

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Source: SEMICONDUCTOR MANUFACTURING HANDBOOK

CHAPTER 18

WET CLEANING

Andrew Machamer
SEZ America, Inc.
Phoenix, Arizona

18.1 OVERVIEW AND BACKGROUND OF WET CLEANING

Wafer cleaning and surface preparation are key areas of technology and development in the semi-
conductor manufacturing industry. Researchers are recognizing that this area of manufacturing, once
thought of as a non-value-added process step, is now an enabling process sequence that is becoming
increasingly more important in wafer processing technology. Some key trends in cleaning that are
critical to future complimentary metal oxide semiconductor (CMOS) technology generations are
toward the use of single-wafer wet cleans, post-damascene copperporous low-k etch clean, post-Cu
CMP cleaning, new material cross-contamination control, and consideration of environmental, safety,
and health factors in development. 1

18.1.1 Contaminants
Wet cleaning and etching are processes that expose a semiconductor wafer to liquid phase chemistries
in order to remove unwanted materials (films and defects) from the surface. Films can be described as
contiguous layers of material across a surface having a thickness of at least one monolayer of atoms.
Defects describe both particle contaminants and physical damage characteristics such as surface scratch-
es and gouges. All defects are generated both inherently as part of the production process and also inad-
vertently due to the manufacturing environment. Particles are bodies with finite mass and structure, but
negligible dimensions. Films may be organic or inorganic in nature. Even with the scrupulously clean
manufacturing environments found in the semiconductor industry, particles are nonetheless generated.
These particle sources include equipment, personnel, and other wafers. Control of wafer contamination
in general requires control of these defect sources. Furthermore, an effective method of defect removal
also needs to exist and wet clean processing provides solutions for these problems.

18.1.2 Theory of Particle Adhesion
There are a number of different interactions that account for the adhesion of a particle to a wafer sur-
face. Understanding these interactions is critical in the development of cleaning processes that ade-
quately remove particles. In this section some of the primary interaction mechanisms are discussed
including molecular and charged particle interactions.
Molecular Interaction. Molecular interaction is attributed to the van der Waals force of adhesion
between the particle and the wafer surface. The strength of adhesion force is a function of (a) the

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