Time for a common language system

Devon Kinkead, President & CEO

Extraction Systems Inc.

Franklin, MA

I read your article on HEPA/ULPA filter advancements and airborne molecular contamination (AMC) (“Particle Trap,” CleanRooms, January 1999, page 14) and thought that I would share some of my experiences over the past 15 years in this business.

First, I think any discussion of AMC should be prefaced with a discussion of SEMI Standard F21-95. This classification standard makes sense out of an otherwise unmanageable body of information; the Merck Index. A discussion of the topic without using the language presented in this standard quickly becomes diffuse and, in many cases, leads the reader to inappropriate conclusions.

For example, the term “organic contamination” is very misleading

because to the “unstandardized eye,” it means all the chemicals that pop up when a TVOC (total volatile organic compounds) analysis is performed on a trap sample of cleanroom air. This is an erroneous conclusion because many of the VOCs present in fab air do not stick to, nor condense upon, the wafer surface. Rather, “condensables,” a subset of VOCs, are the organic contaminants of greatest concern because of their relatively high boiling point and consequently high sticking coefficient when impacting the wafer surface. Thus, your DOP (dioctyl phthalate) discussion should fall within the classification confines of a condensable discussion whereas your boron discussion should fall under the subheading “Dopants.” The A (acids), B (bases), C (condensables), and D (dopants) language spelled out in this standard is not only easy to remember but tends to lead discussions of AMC in the right direction and to appropriate conclusions.

On a final note, significant progress has been made in the field of real-time monitoring for molecular bases and most fabs using deep ultraviolet (DUV) lithography in mass production have either a total molecular base real-time monitor and/or ion mobility spectrometers for ammonia and NMP. These devices are capable of measuring molecular bases with a lower detection limit of 1 part per billion molar. AMC measurement and control is a rapidly evolving field, particularly in the area of molecular base measurement and control but, the industry will never be efficient in its thinking and actions if we do not adopt a common language system; SEMI F21-95, published in 1995, is that common language system and the time has come to use it.