Seal Outgassing in Wafer Processing
Most integrated circuit manufacturing processes operate in high or ultra-high vacuum (UHV) environments. It is important that vacuum levels are maintained within specified limits to insure optimum process efficiency, uniformity and consistency. Specification of an appropriate size vacuum pump for the system is the primary mechanism for maintaining vacuum levels for processing; however, seal outgassing can play a role in the ability to control this processing variable. Even more important is the ability to control the uniformity of the process environment within the chamber. While an adequately sized pump can typically compensate for changes in vacuum level due to seal outgassing, it will not have the capability to control potentially harmful changes to gas uniformity or composition within the chamber. Changes in the partial pressures within the chamber caused by outgassing can cause quality problems in layering processes such as ALD, CVD, oxidation, diffusion, etc.
In many UHV systems, metal seals are used to avoid this issue. However, metal seals are not always effective and/or practical. Some of the difficulties associated with the use of metal seals are:
Therefore, it is important for vacuum system designers and process engineers to understand the outgassing characteristics of elastomeric seals. Currently there is no industry standard for measuring and characterizing elastomer outgassing performance and a limited amount of useful data is available.
This study evaluates and compares outgassing characteristics for three different classes of elastomeric seal materials typically used in semiconductor process environments using a test methodology developed by DuPont.