Production Area and Wafer Logistics

Cleanroom cleanliness is classified primarily according to ISO 14644-1. The classification considers the concentration of airborne particles whose diameter is equal to or greater than a specified size. A lower ISO number indicates a cleaner environment.

Maximum permitted concentration of particles ≥ 0.5 µm

The formerly common designations Class 1, Class 10 and Class 100 originate from the withdrawn US standard FED-STD-209E and referred to particle counts per cubic foot. Therefore, Class 1 is not the same as ISO 1: with respect to the concentration of particles 0.5 µm and larger, Class 1 corresponds approximately to ISO 3.

The ISO classification can also be determined for different operating states of a cleanroom: for the completed but empty room, for an operational room without personnel, and for the actual production state. A cleanroom class must therefore always be considered together with the conditions under which it was measured.

It would make little technical or economic sense to maintain every cubic meter of a large semiconductor fab at the highest achievable cleanliness level. Requirements differ depending on the process and on whether the wafer is openly exposed to the surrounding environment.

Particularly sensitive areas are therefore protected more strongly on a local level. Process tools contain enclosed handling zones, and closed wafer carriers together with tool interfaces form so-called mini-environments. In these areas, the immediate environment of the wafer can be controlled much more tightly than the entire production room.

A FOUP should therefore not simply be assigned a specific ISO cleanroom class such as ISO 1 or ISO 2. It is a closed wafer carrier and part of a controlled transport system. What matters is that the wafers have as little contact as possible with the general cleanroom air during transport, storage and transfer to a process tool.