Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 340

7.8.1. Cleaning

A clean surface is essential for device reliability and performance. 220 It becomes
critical as the dimensions of devices become smaller and smaller as a result of ever-
increasing integration and complexity. It has been estimated that over 50% of yield
losses in integrated circuit fabrication are due to microcontamination. 203 Currently,
tremendous efforts are devoted to contamination control and better cleaning procedures,
thus resulting in a very active research field and generating a large volume of techni-
cal literature. This section is not an attempt to summarize the results of these ongoing
research activities but to present cleaning as an application of etching and its impor-
tance to the electrochemical properties discussed in the rest of this topic.
Today, a typical process flow for advanced ICs consists of 300 to 500 steps, 30%
of which are wafer cleaning steps. 407 Many process steps during IC fabrication may
introduce contamination, which must be cleaned before the next process step. For
example, in processes such as steam oxidation, resist etching, and ion implantation,
metallic contamination typically introduces a surface concentration of to
The need for wafer cleaning can be separated into three areas: (1) prepa-
ration of the wafer surfaces for oxidation, diffusion, deposition, and metallization; (2)
preparation for the application of photoresist; and (3) removal of photoresist after the
etching process. 464,669
Cleaning prior to thermal process steps such as gate oxidation, dopant diffusion,
and epilayer deposition is especially critical in ensuring yield and reliability of the
finished devices. 407,466 Trace amounts of impurities such as sodium ions, metals, and
particles are especially detrimental when present on silicon surfaces during high-
temperature processing (thermal oxidation, diffusion, epitaxial growth) because they
spread and diffuse into the semiconductor interior. Also, metallic contaminants on
submicrometer devices are detrimental. When present, heavy metals often form
midband gap states which act as generation-recombination centers, increasing the
leakage current. They also tend to precipitate and decorate extended defects leading to
junction shorting and degradation of gate oxide integrity. 407
Contaminants on silicon surfaces can be classified as molecular, ionic, and atomic
or as hydrocarbons, metals, and particles. 242,472,884 Typical molecular contaminants are
waxes, resins, oils, and organic compounds which are commonly generated in the
processes after sawing or from human skin and plastic containers. They are usually
attached to the surface by weak electrostatic forces. Ionic contaminants such as
are present after etching in HF or alkaline etchants. They may be pre-
cipitated on the surface by physical adsorption or chemical adsorption. The atomic con-
taminants of concern are metals such as gold, iron, copper, and nickel originating from
acid etchants. The metallic impurities can only be effectively removed by wet clean-
ing processes.
large extent from wet processes and cleaning baths. These particles are attracted by the

99,488 The particles such as

and C originate to a

electrical field generated by the wafers, which are negatively charged in water. 242 The
contaminants may be physically located in different phases at the silicon/electrolyte
interface depending on whether the surface is covered with an oxide film and whether
the contaminants are adsorbed onto the surface before, during, or after the formation
of the oxide film as illustrated in Fig. 7.58.