Floor space in a Class 1 or better cleanroom costs approximately $2000 to $3000 per square foot to construct. Operating costs for these rooms are equally expensive. It is also true that even the best cleanroom facility may be subverted by sources of contamination within the cleanroom itself.
Due to these factors, many new semiconductor fabs are built with provisions to isolate wafer processing areas from the rest of the factory by means of various enclosures, which may be grouped into the category of wafer isolation technologies. These include cluster tools, clean-tool units mounted directly on equipment (microenvironments), and equipment surrounding clean air enclosures (minienvironments).
For each configuration, critical considerations include wafer transport and loading. Wafer transport mechanisms must keep wafers stringently clean and protect wafers from breakage. Typical methods of wafer cassette transport include standard mechanical interface (SMIF) pods, run boxes, and open cassettes.
This paper examines the relative cleanliness of various modes of accessing microenvironment enclosures at varying cleanliness levels of ambient air surrounding the microenvironment. Ambient air cleanliness tested ranges from Class 10 to Class 10,000.
Wafer cleanliness is measured through the use of a wafer surface scanner to measure particles per wafer pass (PWP). Access methods tested include a swinging door, SMIF robot arm, flexible curtain, and Manual Access Port (MAP). (Manual Access Port and MAP are registered trademarks of Briner/Yeaman Engineering, Inc.)