Overview

Cleanrooms can be very large. Entire manufacturing facilities can be contained
within a cleanroom with factory floors covering thousands of square meters.
They are used extensively in semiconductor manufacturing, biotechnology,
the life sciences and other fields that are very sensitive to environmental
contamination.

The air entering a cleanroom from outside is filtered to exclude dust, and the air
inside is constantly recirculated through high efficiency particulate air (HEPA)
and ultra low penetration air (ULPA) filters to remove internally generated
contaminants.

Staff enter and leave through airlocks (sometimes including an air shower stage), and wear protective clothing such as hats, face masks, gloves, boots and cover-alls.

Equipment inside the cleanroom is designed to generate minimal air contamination. There are even specialised mops and buckets. Cleanroom furniture is also designed to produce a low amount of particles and to be easy to clean.

Common materials such as paper, pencils, and fabrics made from natural fibers are often excluded; however, alternatives are available. Cleanrooms are not sterile (i.e., free of uncontrolled microbes) and more attention is given to airborne particles. Particle levels are usually tested using a particle counter.

Some cleanrooms are kept at a positive pressure so that if there are any leaks, air leaks out of the chamber instead of unfiltered air coming in.

Cleanroom HVAC systems often control the humidity to low levels, such that extra precautions are necessary to prevent electrostatic discharges.

Low-level cleanrooms may only require special shoes, ones with completely smooth soles that do not track in dust or dirt. Entering a cleanroom usually requires wearing a cleanroom suit.

In cheaper cleanrooms, in which the standards of air contamination are less rigorous, the entrance to the cleanroom may not have an air shower. There is an anteroom, in which the special suits must be put on, but then a person can walk in directly to the room (as seen in the photograph on the right).

Some manufacturing facilities do not use fully classified cleanrooms, but use some cleanroom practices together to maintain their cleanliness requirements.

REASON WHY CLEAN ROOM IS NECESSARY

In the high technological industries such as electronics, optical and biological industries, the newly developed commodities are manufactured with precise fabrication as a basic process. Both production yield and product quality are, therefore, affected by invisible airborne particles or dust. For example in manufacturing of Semiconductor, the design rule of ULSI is now minimized to the line width of submicron. In this case, the size of particle to be eliminated in the clean room is smaller than 0.05 micron meter, namely, 1/5 — 1/10 of the design rule, which means that only a super clean environment can be accepted for its production process.

Also, in food processing, medical or pharmaceuticals, how to control the micro-organism is a key point to maintain the quality, which can be performed by clean room.

 WHAT IS CLEAN ROOM

Clean Room is defined as "a specially constructed room in which the air supply, air distribution, dust and airborne particle. room pressure, temperature and humidity are environmentally regulated to meet appropriate cleanliness level".

Particle referred here means a particle as small as 1/1000 of human hair in diameter, which is equivalent to 1/100 of the smallest visible limit. These small particles affect to production yield and product quality to a great extent.

Air filters being used in common HVAC system can never collect these particles. Also many factors other than air supply are to be carefully managed to make clean environment condition. Clean Room is what solves these subjects.

Clean Room is divided into two kinds by application; one is Industrial Clean Room and the other Biological Clean Room. The latter, together with clean room function, controls contamination density of micro-organism and plays an important role in clinical research and examination, development of medicines, research and improvement of cultivation cycle and plant breeding, etc.

The clean air technology development has taken further steps with the progress of APOLO Program by NASA. During this progress, a high efficient filter has put into practical use, and its development has now advanced from 0.3 micron meter HEPA Filter (High Efficient Particulates Air Filter) to ULPA Filter (Ultra Low Penetration Air Filter) which can eliminate 0.1-0.2 micron meter particles in diameter at so high efficiency as99.999%.

BASIC PRINCIPLE OF CLEAN ROOM 1

Not only visible dust but also invisible airborne particles are kind of enemy for quality control.

In order to maintain cleanliness in Clean Room, "FOUR PRINCIPLES" shown in Fig. 1 must be observed. Namely, "Not to Bring", "Not to Generate", "Not to Accumulate" and "To Remove quickly" any dust. Fig. 2 illustrates Clean Room composition. The part enclosed by thick line shows Clean Room.

Dust and Particles to be controlled in Clean Room

The dust and particles controlled in Clean Room are very small and invisible as shown in Fig. 3. Visible particle is generally larger than 10 micron meter, which is removed out at the stage of Pre-Filter. In case of Clean Room, socalled submicron particle, smaller than 1 micron meter, is the subject to control. According to the United States Federal Standard the cleanliness is defined on the basis of 0.5 micron meter particle indiameter.ln Fig. 3, around 0.5 micron meter particles are smog, oil mist, cigaret smoke, etc. These are in sight only when they are condense, but not discretely. Like this, in Clean Room, the subjects are such small particles that they can not be visible without using microscope.

HEPA Filter used in Clean Room can remove particles larger than 0.3 micron meter in diameter with more than 99.97% collection efficiency. ULPA Filter now being used in Super Clean Room for semiconductor industries has more than 99.999% for 0.1 micron meter particle, and is able to make the supply air free from particle.

BASIC PRINCIPLE OF CLEAN ROOM 2

"Class" of Clean Room is expressed by the value of X of 10x/M3 which shows the concentration of Particle of 0.1 micron meter and larger in diameter contained in one cubic meter of the air"

Clean Room technology was introduced to Japan from the United States, and therefore, American standards have mainly been applied in Japan.

In recent years, however, through a work of international standardization of various standards by ISO, Clean Room is also discussed and JIS Class (JIS B 9920 ‘89) tends to be its standards. Not exception in the American Standards, the recent Federal Standards 209E (‘92/09) applies the value of X of 10x/M3 to expression of Class of Clean Room, same as in JIS, although the size of objective particle still remains unchanged as 0.5 micron meter and larger in diameter.

Table 1 shows the comparison of Class in JIS B 9920 and U.S. Federal Standards 209E, both of which are most applied in Industrial Clean Room.

Fig. 4 shows the distribution of particle size by each Class in JIS B9920.

It must be added, however, that, for expression of Class of cleanliness, the concentration of particle of 0.1 micron meter and larger in diameter in one cubic foot of the air is still applied by usage in Japan. Also main items to control Clean Room function, such as temperature, humidity, room pressure, supply air velocity etc., are freely set up according to the needs.

For Class of Biological Clean Room of pharmaceuticals, hospitals or test animal breeding facilities, NASA Standard, only in which micro organism is regulated, is used. Table 2 shows the outline of U.S. NASA Standard (NHB-5340-2).

COMPARISON OF CONTAMINATION DENSITY

Let us compare the numbers of airborne particle or microorganism in Clean Room with our surrounding environment.

The numbers of particle in general environment vary time to time so that any fixed number can not be determined, but roughly classified as shown in Fig.5. From this Figure, you will see that such a clean condition in the highest class Clean Room can not be found in the natural world, even in the upper area of stratosphere. Also, even in the center of Pacific Ocean, the cleanliness level of the air is lower than that of middle class of Clean Room. In other words, Clean Room is an ultra clean space where airborne particles or microorganism are extremely eliminated so much as we can never experience in our normal life.

DUST GENERATION FROM HUMAN BODY AND ITS QUANTITY

Human body is one of the biggest sources of dust generation, and its quantity are different each other depending upon the person, kind of his movement, kind of his garment and others, and can hardly be determined, but the rough figures are shown in Table 3.