HISTORY OF CONTAMINATION CONTROL

At the start of the millennium Hal Smith takes a look back at the history of contamination control.

The field of contamination control is a recent technology coming to age as part of the Atomic Age and greatly enhanced in the Space Age. However, its roots go back hundreds of years to Swiss watchmakers who, to prevent dust from falling on their sensitive timepieces when they were not being worked on, covered them with a small bell jar.

The American Civil War (1860 to 1865) gave impetus to the next phase in cleanliness. Due to the immense loss of life from bullet wounds, surgeons recognized the need for steps to prevent post traumatic infection. Lister and others developed methods to achieve this end and sterility became a watchword in operating suites.

In 1945 the need to test gas mask filters against particulate and biological materials led to the development of the aerosol particle counter.

The final step, the development of the High Efficiency Particulate Air (HEPA) filter was developed at Sandia Labs for the Atomic Energy Commission after World War II. Nuclear particles which would have been deadly to personnel were isolated in process areas with HEPA filters while allowing air to circulate in them. Willis Whitfield pioneered the clean bench, a means of conducting clean work in a small space. White rooms had been used during W.W.II to assemble contamination-sensitive parts such as gyroscopes. These rooms used filtered air but not as highly filtered as HEPA air.

Finally, the Space Age with its requirements for both particulate and film cleanliness promoted contamination control technology. Through the 60's documents such as FED STD 209a, NASA SP-5076, "Contamination Control Handbook", NASA SP-4074, "Clean Room Technology", NASA SP-5045, "Contamination Control Principles", and AF-TO-00-25-203, "Contamination Control of Aerospace Facilities, US Air Force" were written. These documents stated principles which are still relevant today.

CLEANROOMS (Part 1)

A cleanroom is basic to contamination control. Without cleanrooms, contamination sensitive parts could not be manufactured in large numbers. In Federal Standard 209, a cleanroom is defined as a room in which air filtration, air distribution, utilities, materials of construction, and equipment. Operating procedures are specified and regulated to control airborne particle concentrations to meet appropriate particulate cleanliness classifications. This general statement is simplistic compared to the very detailed requirements involved in planning, designing, and construction of a cleanroom. ISO/TC209 14644-1 is the international designation of cleanroom cleanliness and incorporates the metric measurements used in other parts of the world.

Cleanrooms are classified according to the particle concentration of the air required to meet cleanliness criteria for the products made therein. The higher the classification, the lower the particle concentration. Initially, cleanrooms were classified according the number of particles/ft3, N0.5 micrometers (µm). However, as the SI system is dominant and dimensions of integrated circuits were reduced below 0.2 µm, the number of particles/m3 N0.3, N0.2, and N0.1 µm/ft3 are used outside of the US

The determination of the cleanroom class is a process based on both experience and measurement. For instance, if products with smaller geometries are being produced in a new cleanroom and the older cleanroom masking similar products with larger geometries, were ISO class 3, then consideration in the design of the new cleanroom would tend towards classifying it as ISO class 2. Tests in the pilot operation for the new cleanroom would verify whether or not to raise the cleanliness level.

The following table shows the latest cleanroom classifications. Note that ISO Class 2 is equivalent to 209 Class 10

AIRBORNE PARTICULATE CLEANLINESS CLASSES