Page 365 - Sami Franssila Introduction to Microfabrication

344 Introduction to Microfabrication

Supply plenum
Silencer
Hepa ceiling
Flex
Fan + Fan +
system system
Optical floor Vibration
isolator
R.A. R.A
plenum space Silencer

R.A. = Return air
Figure 35.1 Cleanroom: fans generate unilateral airflow from HEPA (high efficiency particle) filter ceiling. Air is highly
purified and temperature- and humidity-controlled. Optical floor, isolated from the rest of the building, prevents vibrations
that would destabilize microlithography and microscopy operations. Source: Cleanroom Design, W. Whyte, 1999,  John
Wiley & Sons, Ltd

Table 35.3 Fed. Std. class 1 cleanroom
Feature Values
3
Cleanliness, process area <35 particles/m ,
>0.10 µm
◦
Temperature, lithography 22 C ± 0.5
Temperature, other areas 22 C ± 1.0
◦
Humidity, lithography 43 ± 2%
Humidity, other 45 ± 5%
Air quality
Total hydrocarbons <100 ppb
NO x <0.5 ppb
SO 2 <0.5 ppb
8
2
Envelope outgassing 6.3 × 10 Torr L/cm /s
Pressure typical 30 Pa relative to
outside Figure 35.2 Fed. std. Class 100 cleanroom with wet
Acoustic noise <60 dB benches. Photo courtesy Ulrika Gyllenberg, VTT Micro-
Vibration <3 µm/s (8–100 Hz) electronics Centre
Grounding resistance 1 Mohm
Magnetic field variation < ±1 mG the outside world but which does not have unilateral air
Charging voltage < ±50 V flow. People enter cleanrooms in stages of increasing
cleanliness: at the entrance, footwear is changed into
Source: Cheng, H.P. & R. Jansen (1996)
cleanroom shoes and hair is covered. In the next stage,
an overall is put on. Depending on the cleanliness class,
best cleanrooms: there are simply too few particles to further protective garments are added: a mouthpiece, a
get good statistics. second layer of headgear and cleanroom boots to cover
The cleanroom must include not only the structure the shoes. Finally, gloves are put on (Figure 35.2). A
itself and airflows, but also procedures for transfer of similar, but somewhat reverse, procedure of increasing
people and materials. Cleanrooms are built with stages cleanliness is applied when new tools, wafer boxes,
of increasing cleanliness: at the heart of the cleanroom sputtering targets or any other material is transported
is the process area, which is surrounded by the service into the cleanroom: in the anteroom, the outermost
area (known as gray area), which is clean compared to layer of packaging is removed and the gadgets are