Rev 37 Operator’s Manual OM-610-1002-1
page 16 SECTION 5 OPERATION PDS-2010
be the case. Outgassing, the slow release of trapped gases and vapors from the interior surfaces and materials
of the vacuum system and the substrate, will act to lengthen the actual pump down time.
Many gas molecules are attached to surfaces and are gradually released as the system pressure lowers. Other
molecules may be imbedded in certain substrate materials (such as rubber, plastics, and fibrous materials)
which, under vacuum, diffuse through the material toward its surface and are eventually released to the
vacuum chamber. Minute cracks and spaces can trap molecules which will not release until lower pressures
are achieved within the chamber. Extremely small leaks will also provide a steady stream of molecules into
the vacuum vessel, which can be difficult to discern from true outgassing. Regardless of the cause, severe
outgassing will result in longer pump down times, and can have detrimental effects on coating quality.
5.2.7 Base Pressure
Base pressure is defined as the lowest attainable steady state pressure of a given vacuum system. There is a
practical limit to its value. Due to variations in vacuum components, monitoring equipment, and substrate
composition, the base pressure of a particular vacuum system can vary over time and from machine to
machine. Refer to Section 5.10 of this manual for the procedures to determine the base pressure.
Pressure Location: The one location in the system where pressure is monitored is the chamber. The
chamber pressure is the pressure that is most critical to the process description. Unless otherwise
specified, any references to pressure by SCS will mean the chamber pressure.
Hot or Cold Base Pressure: The chamber pressure sensor (simply called “gauge tube” here) is heated
during processing so that Parylene does not deposit on it. A thermocouple in it senses the temperature.
This heating does change the tube’s response, but in a small, repeatable, and predictable way. Hot base
pressure of a system is the base pressure read when the gauge tube is heated to its normal processing
temperature. Cold base pressure means that NO component in the system is heated during the
measurement; this does not normally occur except when special testing is being done.
The SCS convention is that whenever base pressure is used, it is understood to be hot base pressure. If
other components in the system are heated, these conditions must be clearly explained and understood.
5.2.8 Coating Pressure
Coating pressure is generally defined as the desired chamber pressure to be maintained during a vacuum
process. With Parylene deposition systems, coating pressure is the most important factor affecting substrate
coating quality. In general, a higher coating pressure will result in a shorter coating cycle, but excessive
coating pressures will result in unacceptable coating quality- such as cloudiness, frosting, or formation of
coating nodules. The process goal is to identify the highest coating pressure that results in an acceptable
coating quality for each application.
5.3 PARYLENE DEPOSITION TECHNOLOGY
5.3.1 Parylene Types
Parylene is the generic name of members of a unique polymer series developed by Union Carbide
Corporation. As the Parylene deposition process matured, a group eventually separated from the parent
company, and SCS was formed.
Parylene is applied in the vacuum chamber at room temperature. Deposition takes place at the molecular
level as the chemical is converted under vacuum and heat from the powdered dimer form to a clear
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