Page 364 - Sami Franssila Introduction to Microfabrication

35

Cleanrooms

Particle size distributions in cleanroom air, process Table 35.1 Simplified Fed. Std. 209D airborne particle
3
gases, DI-water and wet chemicals all have the same cleanliness classes (particles/ft )
basic characteristics: four to eight times more particles
are detected if the detection threshold is halved. Class 1 10 100 1000 10 000
Therefore, if the minimum linewidth is halved, the No. of particles 0.5 µm 1 10 100 1000 10 000
number of particles that are potential killers increases No. of particles 0.1 µm 35 350 3500 35 000 350 000
by four to eight times.
Cleanrooms were initially a solution to particle con-
Table 35.2 ISO standard airborne particle cleanliness
tamination reduction (cleanrooms were not invented for 3
classes (/m )
microelectronics, but for delicate mechanical assem-
bly). Later on, temperature and humidity control for 0.1 µm 0.2 µm 0.3 µm 0.5 µm 1 µm 5 µm
improved reproducibility in lithography was recognized.
Other features have been added over the years, and a ISO class 1 10 2
modern cleanroom is a system of facilities that ensure ISO class 2 100 24 10 4
contamination-free processing under very stable envi- ISO class 3 1000 237 102 35 8
ronmental conditions (Figure 35.1). ISO class 4 10 000 2370 1020 352 83
The main features of cleanrooms are: ISO class 5 100 000 23 700 10 200 3520 832 29
• overpressure (50 Pa) for keeping particles outside;
• filtered air (99.9995% at 0.15 µm particle size); The proper way to specify cleanroom cleanliness is
• heating/cooling/humidification/drying of incoming air; therefore: Class X (at Y µm particle size).
• laminar (unidirectional) air flow in the working areas; The example in Table 35.3 shows that there are a
• materials compatibility; multitude of cleanroom features in addition to particle
• mechanical and electrical interference minimization; specifications. These are related to air quality plus
• working procedures. mechanical and electrical environment.
Cleanliness is defined for three different stages of
cleanroom construction:
35.1 CLEANROOM STANDARDS
Cleanrooms are classified mainly on the basis of particle 1. as-built: cleanroom construction is finished, but no
counts. Older specifications such as Fed. Std. 209 tools installed;
(Table 35.1) specify particles per cubic foot. Newer ISO 2. static: with process tools installed and running, but
standards (Table 35.2) employ units of particles per no personnel;
3
3
cubic metre (conversion factor: 1 m = 35.3 ft ). ISO 3. operational: with people working in the cleanroom.
standard cleanliness class N with particle concentration
3
C n (particles/m ) is calculated as As-built tests should indicate around one class better
N
C n = 10 × (0.1 µm/D) 2.08 (35.1) cleanliness than the designed operational class. Laser
scattering of sampled air is used to measure particle
where D is particle size in micrometres. counts. There are some methodological problems in the
Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)