Page 266 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH18.qxd 04/04/2005 19:58 Page 18.7

WET CLEANING

WET CLEANING 18.7

where ω is the acoustic frequency (2πf ) and ν is the viscosity of the liquid, while the hydrodynamic
boundary layer is given by

 n  17 /
d = 016 x
.
H  Ux 
where x is the distance from the leading edge of the wafer and U is the fluid velocity.
For a typical 8 in wafer in water with a velocity of approximately 4 m/s the hydrodynamic
boundary layer is approximately 1000 µm at the wafer’s center. For the same wafer with a megasonic
frequency of 850 kHz, the acoustic boundary layer is 0.61 µm. Therefore, the acoustic field allows
for much smaller particles to be exposed on the wafer surface and higher cleaning efficiencies. 11

18.1.4 Overview of Wet Cleaning Equipment
Specialized equipment are used for the implementation of the aforementioned technologies. The
three major classes of equipment typically used for wet cleaning are—immersion, placing the wafers
in a tank filled with a chemical; spray, using a nozzle to spray the wafer(s) with a chemical; and dis-
pense, directing a stream of an appropriate chemical to the wafer. These can be further categorized
by the number of wafers treated at a time—batch or single wafer. Figure 18.2 shows a generic dia-
gram that is the basis of a wet cleaning system. All wet cleaning equipment have a means of getting
the chemical to the wafer(s) to be treated in a specialized area (process vessel) and a method of trans-
ferring the wafer(s) to the process vessel.
General Requirements of Wet Cleaning Systems. Since the majority of wet cleans utilize haz-
ardous liquid chemicals, the equipment must incorporate the following general design considerations:
• Ensure separation of chemicals to eliminate cross contamination.
• Rinse chemicals from the wafer surface.
• Dry wafers after chemical treatment.
• Use materials of construction that are compatible with the chemicals used, and do not add contam-
ination to the wafer.
• Provide laminar flow (to minimize particle transport from the equipment to the wafer) in wafer
handling and process areas (if open).
• Control static charge.

In addition, the systems need to ensure that their use does not create a hazardous condition for man-
ufacturing personnel, other equipment, or the general public. Equipment design guidelines to minimize
exposure to manufacturing personnel and to contain chemicals within the system are listed in Table 18.2.
Protection of the general public is accomplished by monitoring and treatment of liquid waste
streams to ensure that effluents are within applicable limits for contaminants. Equipment should be
interlocked to ensure that an appropriate exhaust is present prior to dispense of chemicals, and man-
ufacturing exhaust systems need to incorporate scrubbers to ensure neutralization of airborne conta-
minants prior to release in the atmosphere.

18.2 TYPICAL SEMICONDUCTOR MANUFACTURING
WET CLEANING PROCESSES

Wet cleaning processes can be divided into two major categories—front end-of-line (FEOL) and back
end-of-line (BEOL). The division between these two categories is generally accepted to be at the met-
allization of the wafer. The following sections are organized according to a typical semiconductor

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