Page 267 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH18.qxd 04/04/2005 19:58 Page 18.8

WET CLEANING

18.8 WAFER PROCESSING

TABLE 18.2 Design Guidelines for Equipment Using Liquid Chemicals
Hazard Design considerations
Chemical exposure Secondary containment able to hold a minimum of 110 percent of the stored
to operator chemical volume
Exhausting of chemicals
Leak sensors that are part of a hard-wired safety system
Interlocks on access points to ensure depressurization and shutdown
of moving equipment
Barriers or electronic monitoring for point of operation hazards
Chemical exposure to Depressurization of chemical systems upon failure, interlock activation,
maintenance personnel or normal shutdown
Transparent doors and view ports that allow visual inspection of the area
before opening access panels
Systems that allow automated purging and/or flushing of chemicals
Locate system components that are accessible and easy to service
Equipment and Materials of construction that are compatible with chemicals present in system
component failure
Design pressurized systems that are able to withstand 150 percent of the
maximum foreseeable pressure or provide appropriately sized relief valves
Chemical leak Use appropriate chemical storage containers
Incorporate visual pressure indicators
Design and build pressurized vessels and piping to recognized standards
Verification system pressure prior to dispense of chemicals
Incorporate normally closed valves on distribution lines
Overfill sensors on tanks and baths
Monitor for excessive flow on filling systems
Source: “SEMI S2-0703,” Appendix 3 (2003).

fabrication process. The material will be presented as moving from one wet clean to the next disre-
garding the intermediate process steps. In addition, multiple repetitions of the same wet clean will not
be represented although this is often the case with today’s complex wafer fabrication techniques.

18.2.1 Front End-of-Line Processes
Wafer Cleans. The pre-gate wafer clean, used before a wafer is subjected to the furnace, was the
first wafer clean developed. The most common process used for pre-gate is the RCA clean developed
12
by Kern and Puotinen. Since the 1970s when the RCA clean was developed, there have been a num-
ber of variants of the RCA clean as well cleans that completely depart from the RCA chemistries.
The RCA clean process utilizes the chemical mixtures of Standard Clean 1 (SC1) and Standard Clean
2 (SC2). The original RCA process steps are—SC1, ultrapure water, SC2, ultrapure water, and dry. SC1
is a mixture of NH OH/H O /H O in a ratio of 1:1:5 at 70 to 80°C. It primarily removes organic residues
4 2 2 2
and particles by forming and dissolving hydrous oxide films. SC2 is a mixture of HCl/H O /H O in a
2 2 2
ratio of 1:1:6 at 70°C. It removes alkali metal and hydroxides with the exception of Cl residues. 13
Modified RCA processes have been developed to deal with specific problems that have been
encountered as the importance of removing smaller particles increases. These modifications typically
add additional process steps. One additional step that is often added after SC2 is dilute HF (DHF) in
FEOL because the thin oxide left on the wafer surface after the traditional clean cannot be tolerated.
This “HF last” process removes the thin oxide and leaves the wafer with a hydrophobic surface that
requires special attention in the subsequent rinse and dry steps. A DHF step is also utilized before
SC1 when the process is modified to include sulfuric peroxide (SPM) as the initial step. SPM is used
to remove bulk organic material and consists of H SO /H O in a ratio of 4:1. 14
2 4 2 2

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