Semi F21-95 is the Standard for the classification of microelectronic clean environments according to their airborne molecular contaminant levels. The Standard is applicable in the specification of semiconductor clean environments which includes process tool environments as well as contamination control and measurement equipment. The Standard specifies four categories of material contaminants, being Acids, Bases, Condensables and Dopants. A description of the four main material contaminant categories is provided in the table below.
 

Classification is according to the maximum allowable total gas phase concentration of each category of material contaminant. The combination of a quantitative class for each of the four categories of contaminant results in a classification that describes the AMC level of that environment. The maximum cumulative gas phase concentration of the four categories may be different.

The classification system can be best explained as follows: Each contaminant category is designated by the letter ‘M’ followed by the letter A, B, C, or D which signifies the material as follows: A = Acids, B = Bases, C= Condensables, D= Dopants. The integer following the material category classification indicates the maximum total gas phase concentration in parts per trillion (ppt). For example, a category MB-10 has a maximum allowable total concentration of 10 parts per trillion for the category Bases.