TABLE OF CONTENTS Clean wafers are essential at all stages of the fabrication process but are especially necessary before any of the operations are performed at high temperature. Up to 20 percent of all process steps relate to wafer cleaning.28 The cleaning techniques described here are used throughout the wafer fabrication process.
The story of semiconductor process development is in many respects the story of cleaning technology keeping pace with the increasing need for contamination-free wafers. Wafer surfaces have four general types of contamination. Each represents a different problem on the wafer, and each is removed by different processes. The four types are:
Particulates
Organic residues
Inorganic residues
Unwanted oxide layers
In general, a wafer cleaning process, or series of processes, must (a) remove all surface contaminants (listed above), (b) not etch or damage the wafer surface, (c) be safe and economical in a production setting, and (d) be ecologically acceptable. Cleaning processes are generally designed to accommodate two primary wafer conditions. One, called the front end of the line (FEOL), refers to the wafer fabrication steps used to form the active electrical components on the wafer surface. During these steps, the wafer surface, and particularly the gate areas of MOS transistors, are exposed and vulnerable. A critical parameter in these cleans is surface roughness. Excessive surface roughness can alter device performance and compromise the uniformity of layers deposited on top of the devices. Surface roughness is measured in nanometers as the root mean...
