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Particle resuspension by air jets — application to clean rooms
J. Aerosol Sci. Vol. 30, Suppl. 1, pp. $537-$538, 1999 O 1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/99/5 - see front matter
Pergamon
PARTICLE RESUSPENSION BY A I R J E T S APPLICATION TO CLEAN ROOMS C. Gutfinger and G. Ziskind Faculty of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel
KEYWORDS Clean rooms, particle remov',fl, air jets. CLEAN ROOMS Modern manufacture of electronic microchips and production of ultra clean pharmaceuticals takes place in clean rooms. To assure a high degree of cleanliness the air of the clean room is constantly recirculated through a high efficiency filtration system. The presence of people during the manufacturing process inside clean rooms is a major source for microcontamination by particulates. The recirculating air forming turbulent wakes behind obstructions in the room entrains these particulates. Turbulence may also result from a non-uniform air supply, deflection angle of air velocity, heat sources and motion of the personnel. In the semiconductor industry, quality of the air inside clean rooms becomes critical both for the productivity and the quality of the products. There are a number of traditional clean room designs, including the use of air curtains and air streams to achieve required clean zoning. All suffer from problems related to air turbulence in the recirculating clean air, which may cause an increase of up to 50% in the number of relatively large (dp> 0.3 ~na) particles deposited on the surface of a wafer. Calculations of complex flow and temperature fields inside clean rooms, which involve computational fluid dynamics (CFD), should be validated by detailed experimentation in full-scale clean rooms. In general, the results suggest that the-typical air circulation systems do not produce ideal clean room conditions. In particular, the presence of a worktable in a clean room may cause formation of a low-speed recirculation region, which may result in contaminant accumulation. It is obvious that clean room d~signers do their best to create high quality working environments without making unacceptable demands on the air supply systems. Thus, alternative methods for prevention of deposition should be considered, augmented with ways of particle removal from surfaces. EXPERIMENTAL In the present study we investigated the use of focused high-speed air jets to cause removal of particulates from wafer surfaces. An experimental set-up for particle removal from surfaces by continuous and pulsed air jets has been built. The air supply came from a compressed air system. The pulsed jets were obtained by inserting a chopper that interrupted the flow of the air. Silicon wafers or glass slides, on which the particles were placed, were fixed on a solid and perfectly plane surface. In the first series of experiments a continuous air jet was used. The parameters, affecting particle removal were the impinging angle of the jet and its velocity at the nozzle exit. The optimal impinging angle was found to be about 30 degrees, in agreement with the results of other
S537
$538
Abstracts of the 1999 European Aerosol Conference
investigators. In the second series of experiments we used a pulsed jet with three different air velocities, investigating the effect of changing the frequency of the jet on particle removal. The jet velocities were 120, 140, 170 m/s, while the distance between the nozzle and the surface was 30 mm. The results are shown in figure 1 for different pulse frequencies. The data on particle removal by the pulsed jet are presented in terms of a gain factor, which is the ratio of the removal efficiency for a pulsed jet to that for a continuous jet with the same air velocity. Figure 2 shows a similar behavior of the removal efficiency for different surfaces.
2
i
I
1
0.8
1.5
o
class surface | ........ ~']"~" ......... : ............ ,
'~ 0.6
0.4 c o
............. i... l
0.5
ii
o
o
140 m / s | ....... : ...... ~
'
'7°m]sl i
0
20
...........
i
40 60 80 jet frequency, H z
E ,~ 0.2
.-
~~.............. i ~.............. ~ ~................... ~............i
o 100
120
Figure 1. Efficiency of particle removal for different air velocities.
20
40 60 80 jet frequency, ttz
100
120
Figure 2. Efficiency of particle removal for different surfaces.
RESULTS It was found that particle removal efficiency depends on the frequency of the jet. The efficiency increases with the frequency until the optimal frequency is reached, wherefrom the efficiency begins to fall drastically, reflecting the fact that the jet does not have sufficient time to reach its developed state before the chopper cuts it off once more. The frequency of air pulsation in the experiments was in the range 0-120 Hz. The results show high removal efficiencies for jets at frequencies above 80 Hz, see figure 1. The maximum fraction of the removed particles increases as the jet velocity increases; yet for higher jet velocities the maximal removal efficiency is achieved at lower chopper frequencies. The gain factor, i.e., the ratio of the removal efficiency of the pulsed jet to that of a continuous jet at the same air velocity, is smaller for higher velocities. This is because the continuous jet efficiency is larger for higher air velocities. Jet frequency variation has a similar effect on particle removal efficiency for different surfaces, see figure 2. The removal efficiency from glass and silicon surfaces is not the same, because of the different roughness of these two surfaces. ACKNOWLEDGMENT This work was supported by the Fund for the Promotion of Research at Technion.
Pergamon
PARTICLE RESUSPENSION BY A I R J E T S APPLICATION TO CLEAN ROOMS C. Gutfinger and G. Ziskind Faculty of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel
KEYWORDS Clean rooms, particle remov',fl, air jets. CLEAN ROOMS Modern manufacture of electronic microchips and production of ultra clean pharmaceuticals takes place in clean rooms. To assure a high degree of cleanliness the air of the clean room is constantly recirculated through a high efficiency filtration system. The presence of people during the manufacturing process inside clean rooms is a major source for microcontamination by particulates. The recirculating air forming turbulent wakes behind obstructions in the room entrains these particulates. Turbulence may also result from a non-uniform air supply, deflection angle of air velocity, heat sources and motion of the personnel. In the semiconductor industry, quality of the air inside clean rooms becomes critical both for the productivity and the quality of the products. There are a number of traditional clean room designs, including the use of air curtains and air streams to achieve required clean zoning. All suffer from problems related to air turbulence in the recirculating clean air, which may cause an increase of up to 50% in the number of relatively large (dp> 0.3 ~na) particles deposited on the surface of a wafer. Calculations of complex flow and temperature fields inside clean rooms, which involve computational fluid dynamics (CFD), should be validated by detailed experimentation in full-scale clean rooms. In general, the results suggest that the-typical air circulation systems do not produce ideal clean room conditions. In particular, the presence of a worktable in a clean room may cause formation of a low-speed recirculation region, which may result in contaminant accumulation. It is obvious that clean room d~signers do their best to create high quality working environments without making unacceptable demands on the air supply systems. Thus, alternative methods for prevention of deposition should be considered, augmented with ways of particle removal from surfaces. EXPERIMENTAL In the present study we investigated the use of focused high-speed air jets to cause removal of particulates from wafer surfaces. An experimental set-up for particle removal from surfaces by continuous and pulsed air jets has been built. The air supply came from a compressed air system. The pulsed jets were obtained by inserting a chopper that interrupted the flow of the air. Silicon wafers or glass slides, on which the particles were placed, were fixed on a solid and perfectly plane surface. In the first series of experiments a continuous air jet was used. The parameters, affecting particle removal were the impinging angle of the jet and its velocity at the nozzle exit. The optimal impinging angle was found to be about 30 degrees, in agreement with the results of other
S537
$538
Abstracts of the 1999 European Aerosol Conference
investigators. In the second series of experiments we used a pulsed jet with three different air velocities, investigating the effect of changing the frequency of the jet on particle removal. The jet velocities were 120, 140, 170 m/s, while the distance between the nozzle and the surface was 30 mm. The results are shown in figure 1 for different pulse frequencies. The data on particle removal by the pulsed jet are presented in terms of a gain factor, which is the ratio of the removal efficiency for a pulsed jet to that for a continuous jet with the same air velocity. Figure 2 shows a similar behavior of the removal efficiency for different surfaces.
2
i
I
1
0.8
1.5
o
class surface | ........ ~']"~" ......... : ............ ,
'~ 0.6
0.4 c o
............. i... l
0.5
ii
o
o
140 m / s | ....... : ...... ~
'
'7°m]sl i
0
20
...........
i
40 60 80 jet frequency, H z
E ,~ 0.2
.-
~~.............. i ~.............. ~ ~................... ~............i
o 100
120
Figure 1. Efficiency of particle removal for different air velocities.
20
40 60 80 jet frequency, ttz
100
120
Figure 2. Efficiency of particle removal for different surfaces.
RESULTS It was found that particle removal efficiency depends on the frequency of the jet. The efficiency increases with the frequency until the optimal frequency is reached, wherefrom the efficiency begins to fall drastically, reflecting the fact that the jet does not have sufficient time to reach its developed state before the chopper cuts it off once more. The frequency of air pulsation in the experiments was in the range 0-120 Hz. The results show high removal efficiencies for jets at frequencies above 80 Hz, see figure 1. The maximum fraction of the removed particles increases as the jet velocity increases; yet for higher jet velocities the maximal removal efficiency is achieved at lower chopper frequencies. The gain factor, i.e., the ratio of the removal efficiency of the pulsed jet to that of a continuous jet at the same air velocity, is smaller for higher velocities. This is because the continuous jet efficiency is larger for higher air velocities. Jet frequency variation has a similar effect on particle removal efficiency for different surfaces, see figure 2. The removal efficiency from glass and silicon surfaces is not the same, because of the different roughness of these two surfaces. ACKNOWLEDGMENT This work was supported by the Fund for the Promotion of Research at Technion.