The cleanroom

It is easy to understand that when the fine structures become smaller and smaller, dust particles and contaminations can increasingly affect the fabrication of small structures. In the air in an urban environment, there are about 35 million particles per cubic meter that are larger than half a micrometer. Not only that, the air also contains a lot of water vapor and sticky and fatty fumes. Particles and sticky gases tend to adhere easily to any surface, creating a dirty layer. Also, people shed a lot of particles from their hair, skin, and clothes. To avoid contamination during the fabrication or investigation of ever-smaller products, special laboratories have been developed clean rooms.

A clean room is a special room in which the incoming air is heavily filtered. Often the airflow is designed such that the clean air comes in from the top and flows down to the floor. The air is not only filtered, but also temperature and humidity controlled to ensure constant conditions for all the chemical processes performed in the clean room.

The conditioning of the air in the clean room requires a lot of infrastructure, which may take up more space than the clean room itself. Most of the air is used again filtered and pumped back into the room. For example, the airflow units of the 1,250 square meter clean room of the MESA+ Institute at the University of Twente can handle up to 120,000 cubic meters per hour. The people working in a clean room create most of the dust particles. Therefore, the clean room staff passes through an airlock into the clean room. This room is often used also as a changing room.

The clean user has to wear a special coverall protective clothing– boots, a hood, a face mask, and often special glasses.

Clean rooms are classified in terms of the number of particles of a certain size per cubic meter. Open air corresponds to ISO class 9, with 35 million particles per cubic meter. Starting from ISO class 6, one also defines the maximum number of particles larger than 100 nanometers to be less than 1 million per cubic meter, while, at the most, 35,000 particles larger than half a micrometer are allowed. Typical clean rooms are of ISO class 4 to 6. The strictest clean room classification, ISO class 1, allows only ten 100- nanometer sized particles per cubic meter and no larger particles. In such rooms, there are hardly any people, and the work is largely robotized.

Inside the clean room, the critical operations are performing in clean benches, with a separate airflow in and out of the clean bench. In these benches, the ISO class can be one to two classes better than in the room.

Working in the nanometer domain also requires a very low vibration level. Vibrations can be caused by passing traffic on a nearby road. Also, the wind or heavy rain can make a building move. Vibrations are also caused by the people passing in the building or nearby corridors opening and closing doors. To avoid these vibrations to reach the vibration-sensitive equipment, one often uses room constructions that are completely separated from the outside construction of the building– a box in a box. Further, the laboratories have a heavy floor with a deep foundation– a floor that has no mechanical connections with the foundation of the outer building.

For example, the MESA+ laboratory has a 90 centimeter thick concrete floor founded on 366 19-meter long pillars, and it is separated by 10 centimeter wide expansion joints. Vibration levels are measured in average velocity of the vibration. In a workshop, it can be as high as 1,000 micrometers per second, while in a residential area, it is about 200 micrometers per second. The lowest vibration levels achieved nowadays– as, for example, in the MESA+ laboratory, are less than 1 micrometer per second. From the above, you will understand that research, development, and production of nano sized objects may require large investments in special buildings.