Cleanrooms

 

According to ISO 14644-1, a cleanroom is “a room in which the concentration of airborne particles is controlled, and which is constructed and used in a manner to minimize the introduction, generation, and retention of particles inside the room and in which other relevant parameters, e.g. temperature, humidity, and pressure, are controlled as necessary."

    

Cleanroom environment is achieved through the use of appropriate HVAC (Heating, Ventilation, Air Conditioning) systems and HEPA (high efficiency particulate air) filters that maintain the correct pressurization, temperature, and humidity of the cleanroom, as well as the air flow of the circulating, continuously filtered air.

   

Since cleanrooms are atmospherically isolated from the external environment, they must be provided with special passageways to allow the transport of people and materials to and from them. ‘Air showers’ and ‘pass-through windows’ are the major modes of transfer between cleanrooms and the outside environment.

    

‘Air showers’, which are special chambers located between the cleanroom and the outside environment, provide the passageway through which people enter the cleanroom. Once a person enters the shower room and closes the door, the air shower bombards the person with moving filtered air to dislodge the particles and other attached foreign objects from his hair, skin and clothing. At the same time, the air inside the shower is being continuously filtered. This ‘air shower’ takes a preset amount of time, with the doors remaining to be locked while the process is ongoing. Thus, air showers must be equipped with interlocking doors, air recirculation systems, and HEPA filters.

   

Pass-through windows are used mainly for material transfer to and from the cleanroom. These are cavities on the walls of the cleanrooms with two doors – one that opens to the outside environment and another that opens into the cleanroom. The two doors interlock to ensure  that only one door can be opened at a time. Materials that need to be transferred into the cleanroom are first put into the cavity through the first door, and then retrieved from the cavity through the second door.

    

Cleanrooms are classified in terms of the number and sizes of particles suspended in its atmosphere.  A particle is defined as a solid or liquid object between 0.001 and 1000 microns in size. Table 1 shows the various cleanroom classes and their corresponding statistically allowable number of particles per cubic foot of air, as defined by Federal Standards 209E.  To illustrate, in a Class 100 cleanroom, a cubic foot of air is only allowed to have 100 particles whose size is 0.5 micron.

   

Table 1. Cleanroom Classes

Class Name

0.1 micron

0.2 micron

0.3 micron

0.5 micron

5 micron

1

35

7.5

3

1

N/A

10

350

75

30

10

N/A

100

N/A

750

300

100

N/A

1000

N/A

N/A

N/A

1000

7

10000

N/A

N/A

N/A

10000

70

100000

N/A

N/A

N/A

100000

700

       

In semiconductor manufacturing, wafer fab processes usually require a sub-Class 1 to Class 10 cleanroom, while assembly processes prior to encapsulation of the die require a Class 10K cleanroom.  A class 100K cleanroom is all that post-encapsulation assembly and test processes typically require.

 

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