1

Clean Zone Flow and Particle Measurement

Author: Xiaoliang Wang

Instructor: David Pui

TA: Liming Lo

Lab Members: Xiaoliang Wang,

Meghan Kearney,

Bruce Mehdizadeh,

Bob Chenny

Test Date: March 6, 2002

Report Date: March 24, 2002

Lab Location: Microtechnology Laboratory Area 2

2

Table of Content

Clean Zone Flow and Particle Measurement ...................................................................... 3 Abstract ....................................................................................................................... 3 1. Introduction......................................................................................................... 4 2. Background ......................................................................................................... 6 3. Particle Concentration Measurement ................................................................ 10

3.1 Experiment method.................................................................................. 10 3.2 Particle Concentration Measurement Results........................................ 12

3.2.1 LPC Measurement Result and Air Cleanliness Class ....................... 12 3.2.2 CPC Measurement Result and U Descriptor .................................... 15

4. Air Flow Velocity Profile Measurement and Flow Pattern Study.................... 17 4.1 Flow Velocity Profile Measurement....................................................... 17 4.2 Flow Profile Visualization...................................................................... 19

5. Particle Generation and Dissipation.................................................................. 20 6. Conclusions ....................................................................................................... 21 7. Reference: ......................................................................................................... 22 Appendix A Instruments List ................................................................................... 23 Appendix B Clean Zone Testing Technical Report.................................................. 24

3

Clean Zone Flow and Particle Measurement

Abstract

To test the performance of a 100 ft2 clean zone specified as ISO Class 6, we

classified the exact air cleanliness class in the ‘at-rest’ condition based on ISO 14644-

1:1999(E). We also studied the flow velocity profile and flow pattern inside the

cleanroom. Particle generation by people under different conditions was also measured.

The particle concentrations were measured by a TSI 3753 LPC and a TSI 3760 CPC.

The only size considered in classifying cleanliness class was 0.3µm measured by the LPC.

The CPC concentration was used in the U descriptor. We found the particle concentration

inside the clean zone was not uniform. The cleanliness class determined by the dirtiest

sampling location shows that the clean zone didn’t meet ISO Class 6 at 0.3µm. Instead, it

was ISO Class 6.3. The U descriptor was: U (2,000,000; 0.01µm).

Air flow velocity was measured by an anemometer. The results show that flow

velocities ranged from 13 to 64 ft/min due to the non-uniform layout of filters. The flow

pattern was visualized using non-contaminating fog droplets. We found that the positive

pressure in the clean zone prevented the dirty air coming in and disposed contamination

inside the clean zone rapidly. Recirculation was found in the corner area and near the

clean bench. Since the clean bench was not fully under filters, the flow velocity was low,

contamination was median, and recirculations occurred. These were not good for device

fabrication.

Particle emission from people with different actions and different clothes was studied.

Big action without cleanroom garments produced the most particles, and normal

movement with garments produced the least particles for the four cases we studied.

4

1. Introduction

As feature sizes of today’s semiconductor products becomes smaller and smaller, the

requirement of air borne particle control becomes more and more stringent. In order to

minimize the yield lost due to particle contamination, many of these products are

fabricated in cleanrooms or clean zones. To understand the performance of cleanrooms,

we need answer the following questions:

• What is the particle concentration in certain size range inside the cleanroom?

• How particles are generated and dispersed in the cleanroom?

The objective of this experiment is to obtain answers to the questions above for a

clean zone located in the Microtechnology Laboratory Area 2 in the Department of

Electrical and Computer Engineering at the University of Minnesota. The clean zone was

separated from a normal laboratory by cleanroom curtains and had an area of 100 ft2 (10

ft × 10 ft). It was specified as an ISO Class 6 clean zone. Air enters from ceiling inlets

through high efficiency particular air filters (HEPA), and leaves through the gaps

between curtains and the gap between curtains and the floor. Filters cover 40% of the

ceiling. The locations of filters are shown in Figure 1.

To answer the first question, we measured the particle concentration at six different

locations in the clean zone using a laser particle counter (LPC) and a condensation

particle counter (CPC). The air cleanliness was evaluated based on ISO 14644-1:1999(E).

5

Figure 1 Filter and sampling location layout of the clean zone

To obtain the answer to the second question, we first measured the airflow

velocity profile was using an anemometer. Then one of the major particle sources:

personnel emission was studied. We measured the particle concentration when the

operator was wearing different clothes and performing different activities. In the end, we

visualized and observed the airflow pattern inside the clean zone. A non-contaminating

fog generator was set up to produce fog droplets for observing the flow pattern and

contamination dissipation.

6

2. Background

The definition of clean zone is: “ the dedicated space in which the concentration of

airborne particle is controlled, and which is constructed and used in a manner to

minimize the introduction, generation, and retention of particles inside the zone, and in

which other relevant parameters, e.g. temperature, humidity, and pressure, are controlled

as necessary.” [1]

Cleanroom are classified by air cleanliness levels. In ISO 14644-1:1999(E), level of

air cleanliness is expressed in terms of an ISO Class N, which represents maximum

allowable concentration (in particles per cubic meter of air) for considered sizes of

particles. The maximum permitted concentration of particles Cn for each considered

particle size D is determined from Equation 1:

Nn D

C 101.0 08.2

×

= (1)

where

Cn is the maximum permitted concentration (particles/ m3) of airborne

particles that are equal to or larger than the considered particle size.

N is the ISO classification number in the ranges from 1 to 9.

D is the considered particles size (µm).

Table 1 presents the selected airborne particulate cleanliness classes and the

corresponding particle concentrations for particles equal to and larger than the considered

size shown [1]

7

Table 1 Airborne particulate cleanliness classes for cleanrooms and clean zones [1]

CLASS LIMITS (particles/m3) Maximum concentration limits (particles/m3 of air) for particles equal to and larger

than the considered sizes shown below ISO classification

number (N) 0.1 um 0.2 um 0.3 um 0.5 um 1 um 5 um

ISO Class 1 10 2

ISO Class 2 100 24 10 4 ISO Class 3 1000 237 102 35 8 ISO Class 4 10000 2370 1020 352 83

ISO Class 5 100000 23700 10200 3520 832 29 ISO Class 6 1000000 237000 102000 35200 8320 293

ISO Class 7 352000 83200 2930 ISO Class 8 3520000 832000 29300 ISO Class 9 35200000 8320000 293000

Note: Uncertainties related to the measurement process require that concentration data with no more than three significant figures be used in determining the classification level.

Flow pattern is very important to remove contaminates. Generally, there are two

kinds of flow patterns in cleanroom design: multidirectional airflow and unidirectional

airflow. As shown in Figure 2, in a multidirectional airflow design, air is supplied

through large ceiling outlets, flows downward and is removed near the floor level. Air

filters are located in the HVAC units [2].

Figure 2 Multidirectional Flow Cleanroom [2]

8

In a unidirectional flow system, air is introduced evenly from one entire surface of

the room through HEPA filters and flows at constant velocity across the room and is

removed through the entire area of an opposite surface or through low sidewall returns.

An example of vertical unidirectional flow clean room is shown in Figure 3 [2].

Figure 3, Vertical unidirectional flow cleanroom [2]

The particle behaviors in these two kinds of cleanrooms are illustrated in Figure 4.

We can see that in multidirectional design, air currents do not follow a predictable path.

Particles can move in all directions, and they can be re-entrained from the floor. In a

unidirectional flow clean room (vertical or horizontal), the air flows through the room

following a predictable path. The increased flow will dilute and carry away any

generation of particles as soon as it is formed [3].

9

Figure 4, Particle behaviors in multidirectional and unidirectional flow cleanrooms [3]

Personnel emission is one of the major particle contamination sources in the

cleanroom. Figure 5 illustrates the particle emission from a person, indicating that

particle from personnel should be prevented from reaching sensitive items [3]. It also

shows that wearing masks can reduce particle emission. The contamination index for

various personnel activities ranges from 100,000 particles per minute to 30,000,000

particles per minutes of 0.3µm in size and larger according to different levels of actions

[3].

10

a b c

Figure 5 a. Droplet dispersion after a violent, unstifled sneeze; b. Enunciating the letter

“F.” Consonants are more difficult than vowels to pronounce without forming droplets.

Note larger droplets; c. A sneeze through a mask of the type worn by surgeons [3]

3. Particle Concentration Measurement

3.1 Experiment method The particle counting instruments used to measure the particle concentrations in

this test were a TSI Model 3753 cleanroom LPC and a TSI Model 3760 CPC. The

experiment setup is shown in Figure 6. These two instruments were calibrated using

monodisperse polystyrene latex (PSL) and sodium chloride (NaCl) particles on February

13, 2002 [4]. Table 2 lists the specifications of these two instruments. The TSI 3760 CPC

has a minimum detection limit of 0.01µm, and the measured result is used in U descriptor.

The TSI 3753 LPC has two size bins: 0.30µm~3.0µm and >3.0µm. The concentration of

particles bigger or equal to 0.3µm measured by the LPC is considered in classifying the

air cleanliness class.

11

Figure 6 Particle concentration measuring setup

Table 2 Specifications of the CPC and LPC used in this test

Instrument Minimum detectable size (µm) Flowrate (cfm) Flowrate (lpm)

TSI 3760 CPC 0.01 0.05 1.4

TSI 3753 LPC 0.30 0.1 2.8

Since only one particle size (0.3µm) is considered, and the specified air

cleanliness classification of this clean zone was ISO Class 6, the maximum permitted

airborne particle concentration for particles =0.3µm can be calculated using Equation (1).

In this case, D = 0.3µm and N = 6, therefore,

608.2

103.01.0

×

=nC

=101763 rounded to 102000 particles/m3

The single sample volume, Vs, is calculated using Equation (2):

196.01000102000

201000

20

,

=×=×=mn

s CV liter. (2)

However, ISO 14644-1:1999 (E) requires that the volume sampled at each location

should be at least 2 liters, with a minimum sampling time at each location of 1minute. To

fulfill this requirement, we set the sampling time to 2 minutes, and the sampling volumes

12

were 2.8 and 5.6 liters for the CPC and LPC, respectively. At each location, three sample

volumes were taken.

The number of sampling locations are derived according to equation (B.1) in ISO

14644-1:1999 (E):

05.3)305.010( 2 =×== AN L (rounded to 4).

However, there were a clean bench and a cabinet inside the clean zoon as shown in

Figure 1, and the cleanliness in the workstation area was of big interest. Hence we added

two more sampling points to obtain more information about the workstation and cabinet

area. The locations of the six sampling points are also shown in Figure 1. Sampling point

6 was on the clean bench. The sampling probe was positioned pointing to the airflow at a

height of about 1 meter above the floor.

3.2 Particle Concentration Measurement Results

3.2.1 LPC Measurement Result and Air Cleanliness Class The counts obtained from LPC 3753 and the sampling time of measurements at each

location are recorded in Table 3.

Table 3 Raw data (particle counts) of LPC measurement

Sample No 1 2 3 Location time (s) counts (>=0.3µm) time (s) counts (>=0.3µm) time (s) counts (>=0.3µm)

1 123 38 122 31 123 45 2 123 389 122 351 122 467 3 122 953 123 1064 123 886 4 123 54 123 43 122 35 5 122 58 123 54 122 33 6 123 382 123 351 122 436

Particle concentration for each measurement is listed in Table 4. The concentration is

obtained by dividing the measured counts by sampling time and LPC flow rate (2.8 lpm).

13

Also listed in Table 4 are the sample average concentrations. They are the average

concentrations of the three samples at each location.

Table 4 Particle concentrations at each sampling location

concentrations (partilces/m3) for particles >=0.3µm Location sample 1 sample 2 Sample 3 Sample Average

1 6620 5445 7840 6635 2 67770 61651 82026 70482 3 167389 185366 154355 169037 4 9408 7491 6148 7682 5 10187 9408 5796 8464 6 66551 61150 76581 68094

From the last column of Table 4, we find that the concentration at each location

varies a lot. Furthermore, we notice that concentration at location 3 exceeds the

maximum permitted concentration (102000 particles/m3), suggesting that the clean zone

didn’t meet ISO Class 6. According to the average concentration measured at location 3

(169037 particles/m3), we can calculate the actual cleanliness class by transforming

Equation 1:

−=

DCN n

1.0log08.2log

−=

3.01.0

log08.2)169037log(

= 6.22 rounded to 6.3

The maximum permitted concentration for ISO Class 6.3 clean zone is:

3.608.2

103.01.0

×

=nC

=203042.9 rounded to 203000 particles/m3

14

To confirm that the air cleanliness of this clean zone is ISO Class 6.3 rather than

Class 6, the 95% upper confidence limit is calculated below.

The overall mean of the averages is calculated using Equation 3:

( )miii xxxm

x ,2,1,1

+++= L (3)

where

x is the overall mean of the location averages

1,ix to mix , are individual location averages (listed in the last column of Table 4)

m is the number of individual location averages (m = 6 in this test).

Therefore,

( )680948464768216903770482663561

+++++=x

33039461

×=

=55065.6 rounded to 55066 particles/m3.

The standard deviation of the location averages is calculated with Equation 4:

1

)()()( 2,

22,

21,

−

−++−+−=

m

xxxxxxs miii L

(4)

Plugging in numbers to Equation (4), we get

−+−+−+

−+−+−=

222

2222

)5506668094()550668464()550667682(

)55066169037()5506670482()550666635(51

s

7201592589651

×=

= 4031851793.3 rounded to 4031851793

15

63496.94031851793 ==s rounded to 63497 particles/ m3.

Using Equation 5, we can determine the 95% upper confidence limit for the overall

mean:

+=

ms

txUCL 95.0%95 (5)

where t0.95 represents the 95th percentile of the t distribution, with m-1 degrees of freedom.

For this test, m = 6, and t0.95 = 2.0. So

×+=≥

663497

0.255066)3.0(%95 µmUCL

=109360.5 rounded to 109361 particles/ m3.

We can see that 95% UCL (=0.3µm) exceeds the concentration limit for ISO Class 6

(102000 particles/m3), but it is less than the maximum concentration for ISO Class 6.3

(203000 particles/ m3). Therefore, the air cleanliness class for this clean zone was ISO

Class 6.3 instead of ISO Class 6 at 0.3µm for the data collected on March 6, 2002.

3.2.2 CPC Measurement Result and U Descriptor As mentioned earlier, a TSI model 3760 CNC with a minimum detection limit of

0.01µm was used in this test to measure ultrafine particle concentrations. The measured

particle counts raw data and sampling time at each location are listed in Table 5. The

concentration of each measurement and the sample average concentrations are listed in

Table 6.

16

Table 5 Raw data (particle counts) of CPC measurement

Sample No 1 2 3

Location time (s) counts (>0.01µm) time (s) counts (>0.01µm) time (s) Counts (>0.01µm) 1 122 40 123 30 122 36 2 122 377 123 291 123 300 3 122 714 123 705 123 572 4 123 84 122 40 123 39 5 123 32 123 52 123 26 6 123 253 123 217 122 265

Table 6 CPC Particle concentrations at each sampling location

Location sample 1 sample 2 sample 3 Sample Average 1 14052 10453 12646 12384 2 132436 101394 104530 112786 3 250820 245645 199303 231922 4 29268 14052 13589 18970 5 11150 18118 9059 12776 6 88153 75610 93091 85618

The overall mean of the averages is:

( )8561812776189702319221127861238461

+++++=x

47445661

×=

= 79076 particles/m3.

Similar to the LPC calculation, the standard deviation and 95% upper confidence

limit are 86020 particles/m3 and 149311 particles/m3, respectively.

According to ISO 14644-1:1999 (E), the ultrafine particle concentration listed in the

U descriptor should not be less than the particle concentration limit applicable to the

considered size of 0.1µm for the specified ISO class. The maximum permitted

concentration for particles =0.1µm for ISO Class 6 and Class 6.3 are:

17

608.2

101.01.0

×

=nC

=1,000,000 particles/m3

and

3.608.2

101.01.0

×

=nC

=1995262.3 rounded to 2,000,000 particles/m3,

respectively. We find the CPC concentration at each location is less than these two limits.

By choosing the Class 6.3 limit, the U descriptor can be expressed as follows:

U (2,000,000; 0.01µm)

In summary, the air cleanliness in the clean zone under test does not meet ISO Class

6. Instead it is ISO Class 6.3 at 0.3µm for the data collected on March 6, 2002. The U

descriptor is: U (2,000,000; 0.01µm).

4. Air Flow Velocity Profile Measurement and Flow Pattern Study

As indicated before, 40% of the clean zone ceiling was covered by HEPA filters. Air

entered the clean zone vertically from the ceiling inlets through filters, and left through

the gaps between curtains and the low side curtains. Because filters were not evenly

distributed, and air did not leave through the floor, the flow was not truly uniformly

vertical. This was confirmed by flow velocity profile measurement and flow pattern study.

4.1 Flow Velocity Profile Measurement The flow velocity was measured at each sampling location using a TSI VelociCalc

Anemometer. The result is listed in Table 7.

18

Table 7 Air flow velocity profile of the clean zone

Sampling point 1 2 3 4 5 6

Flow velocity (ft/min) 64 18 13 35 40 14

From Table 7, we find that the flow profile was not uniform and this was consistent

with the filter layout except location 2. Sampling location 1, 4, 5 were under filters, and

air velocities were higher. Surprisingly, the flow velocity at location 2 was less than a

half of the velocity at location 5, although they were under the same filter. This indicates

that the filter above location 2 and 5 was not working properly. While location 3 and 6

were at the corner of the clean zone and there were no filters above it, the flow velocities

were pretty low.

1

10

100

1000

10000

100000

1000000

0 10 20 30 40 50 60 70Flow velocity (ft/min)

con

cen

trat

ion

(#/

cc)

LPC (>=0.3µm)LPC(>3µm) CPC (>0.01µm)

Figure 7 Particle concentrations at various flow velocities

Figure 7 shows the particle concentration measured by the two instruments under

different flow velocities at the 6 sampling locations. The velocity profile also explains the

non-uniform distribution of particle concentrations, (also listed in the right-most column

of Table 6). Location 3 had lowest flow velocity and therefore had highest particle

19

concentration; while Location 1 had highest flow velocity and therefore had lowest

particle concentration. Sampling point 6 was on the clean bench. Because there was no

filter above the sampling point, the flow velocity was low and particle concentration was

high.

4.2 Flow Profile Visualization The objective of this part of experiment is to visually observe the air flow pattern

inside the clean zone. A cleanroom fogger was used to produce fog droplets, which were

release to the clean curtain edges, the entrance area to the clean zone and the workbench,

etc.

As a rule of design, usually there is a positive pressure of 0.1inch water between the

clean zone and the outside environment. The function of increased pressure is to force

clean air out of any cracks or leaks, thus preventing contaminated outside air from

entering the clean zone. It also prevents the entry of outside air when the door is open.

The benefits of positive pressure were observed in this experiment. When fog was leased

near the curtain, it was pushed to outside of the clean zone through the gaps between

curtains. When the door was open, we saw flux of fog left the clean zone through the

door.

However, because the ceiling was not all covered by filters, we saw fog recirculation

in areas that are at the edge of filters and under ceiling panels, such as sampling location

3. We also observed recirculation above and under the workbench. It was because the

workbench was not fully under the filter, and the table itself will block air. This was not

ideal for operation. So I would suggest moving the workstation totally under filters.

20

5. Particle Generation and Dissipation

As indicated before, people are big sources of particle contaminations. Various

motions and clothing generate various particles. In this experiment, we studied particle

generation from people for four cases:

• Case 1: A person wearing normal clothes, walking around the sampling location with

no big movement;

• Case 2: A person wearing normal clothes, dancing around the sampling location;

• Case 3: A person wearing cleanroom garments, walking around the sampling location

with no big movement;

• Case 4: A person wearing cleanroom garments, dancing around the sampling location.

Particle counts and concentrations measured by the two instruments are listed in

Table 8 and 9.

Table 8 Particle counts measured for the four cases

Instrument CPC LPC Case time >0.014µm time >=0.3µm >=3µm

1 63 278 63 114 11 2 62 1769 62 338 27 3 63 193 63 82 4 4 63 383 62 220 32

Table 9 Particle concentrations (partilces/m3) measured for the four cases

Instrument CPC LPC Case time >0.014µm time >=0.3µm >=3µm

1 63 189116 63 38776 3741 2 62 1222811 62 116820 9332 3 63 131293 63 27891 1361 4 63 260544 62 76037 11060

21

By comparing the four cases in Table 8 and Table 9, we found that in case 2 when

people wore normal clothes and danced around, the detected concentration was the

highest. One the other hand, in case 3 where people wore cleanroom garments and had no

big movement the detected particle concentration was the lowest. This suggests that

garments do prevent particles from emitting from people. Interestingly, we found that in

case 4, particles bigger than 3.0µm was more than those of case 2. This may be because

the cleanroom garments was not clean or may be because the dancing level was different

since it were performed by different people.

In conclusion, this experiment tells us that big movement should be avoided in the

clean zone, and clean garments are required.

6. Conclusions

In this experiment, we classified the air cleanliness of the clean zone in an at-rest

state. A TSI 3753 LPC with lower detect limit of 0.3µm and a TSI 3760 CPC with lower

detect limit of 0.01µm were used to measure particle concentration. The air cleanliness

classification was based on ISO 14644-1:1999(E). The results show that the clean zone

tested met ISO Class 6.3 at 0.3µm based on the data we collected on March 6, 2002. For

the ultrafine particles, the U descriptor is: U (2,000,000; 0.01µm). A technical report is

attached in Appendix B.

We also measured the air flow velocity profile and visualized the flow pattern inside

the clean zone. We found the flow velocity was not uniform because of the non-uniform

distribution of filters and outlets in the ceiling. This also caused the non-uniform

distribution of particle concentration. We also found velocities at two sampling locations

(2 and 5) under the same filter were quite different, suggesting that the filter was not

22

working fine. The fog visualization showed that the positive pressure inside the clean

zone functioned effectively to prevent outside air entering the clean zone and to expel

contamination out of the clean zone through the gaps between clean curtains and the

lower sidewall. However, we did see re-circulation of air in the clean zone corner and

around the workbench. In order to obtain good cleanliness, we would suggest moving the

workbench fully under filters to make full use of clean air.

Particle generation dissipation for personnel inside the clean zone was also studies.

We measured particle concentrations when a person was walking or dancing around the

sampling probe wearing different clothes. We found that big movement without wearing

cleanroom garments produced most particles, while slight action with wearing garments

produced least particles in the four cases we studies. This suggests that big movement

need to be avoided, and garments are required when working in the cleanroom.

7. Reference:

1. 14644-1:1999(E), I., 1999, Cleanrooms and associated controlled environments---

-Part 1: Classification of air cleanliness

2. Johnson, M., 2002, Cleanroom Design Considerations (ME 5116 class handout)

3. Austin, P.R., Design and Operation of Clean Rooms. 1970, Detroit: Business

News Publishing Company.

4. Wang, X., 2002, Laser Particle Counter and Condensation Particle Counter:

Calibration and Measurement, Lab Report.

23

Appendix A Instruments List • TSI model 3760 cleanroom condensation particle counter

• TSI model 3753 cleanroom laser particle counter

• IBM Personal Computer with appropriate data acquisition boards

• TSI VelociCalc Anemometer

• MSP Model 2000 Cleanroom Fogger

• A Clean Bench

24

Appendix B Clean Zone Testing Technical Report

1

CLEAN ZONE TESTING AND CERTIFICATION*

ME 5116, Department of Mechanical Engineering, University of Minnesota 111 Church St. SE, Minneapolis, MN 55455

March 24, 2002

* The format of this technical report follows Mark Johnson’s class handout

2

Certification of Compliance Microtechnology Laboratory Area 2 Clean Zone,

Department of Electrical and Computer Engineering

University of Minnesota

March 6, 2002

Clean Zone Description Clean Zone Status Clean Zone Class

Clean Zone At-Rest 6.3

Per the results of the particle concentration measurement, the class is considered met if

the measured particle concentration is within the limits specified per ISO 14644-

1:1999(E), page 3, Table 1, and page 7, B6. The considered particle size was 0.3µm. For

ultrafine particles, U descriptor was used.

ME 5116 Group 3 certificates that the above referenced clean zone has been tested and

meets the stated class conditions in accordance with ISO14644-1:1999(E), standard for

cleanrooms and associated controlled environments—Part 1: Classification of air

cleanliness. ME 5116 Group 3 makes no other warranties, stated or implied, concerning

the continued performance, operation or safety in use of the clean zone listed above.

Submitted and Certified by:

ME 5116 Group 3

Xiaoliang Wang

3

Description of the Clean Zone

This clean zone is located in the Microtechnology Laboratory Area 2 at the

Department of Electric Engineering, University of Minnesota. It has an area of 100 ft2 (10

ft × 10 ft) separated from a large normal laboratory by cleanroom curtains. Air enters the

clean zone vertically from the ceiling outlets through high efficiency particular air filters

(HEPA), and leaves through the gaps between curtains and the gap between low side of

curtains and the floor. Filters cover 40% of the clean zone ceiling. The locations of filters

are shown in figure 1.

Figure 1 Filter and sampling location layout of the clean zone

4

Air Cleanliness Class Evaluation

Evaluation Method: The airborne particle concentration was measured at certain size

range to determine the actual particle count level within the facility at the time of the test.

Both cleanliness and U descriptor are used in classification.

This test was performed when the clean zone was in an at-rest condition, where

the installation was complete with all equipment installed and operating in a normal

condition, but with no personnel present. The particle concentration was measured by a

TSI 3760 condensation particle counter (CPC) and a TSI 3753 laser particle counter

(LPC). The concentration of particles bigger or equal to 0.3µm measured by the LPC is

considered in classifying air cleanliness class, while the concentration measured by the

CPC is used in U descriptor. These two instruments were calibrated using monodisperse

polystyrene latex (PSL) and sodium chloride (NaCl) particles on February 13, 2002

[Wang, 2002 #4]. The specifications of these two counters are listed in Table 1.

Table 1 Specifications of the CPC and LPC used in this test

Instrument Minimum detectable size (µm) Flowrate (cfm) Flowrate (lpm)

TSI 3760

CPC 0.01 0.05 1.4

TSI 3753

LPC 0.30 0.1 2.8

In accordance to ISO 14644-1:1999 (E) p6 B.1, the number of sampling point

locations was six, as indicated in Figure 1. At each sampling location, three samples of

two minutes long were taken. The sampling volume of the CPC and LPC were 2.8 liters

and 5.6 liters, respectively. Measurements were taken at 1 meter above the floor.

The 95% upper confidence limit was calculated in accordance with the requirement of

ISO 14644-1:1999 (E).

5

Conclusion: The clean zone didn’t meet the requirements of ISO 14644-1:1999 (E) for

ISO Class 6 at 0.3µm particle size for data taken on March 6, 2002. The exact air

cleanliness class was ISO Class 6.3. For ultrafine particle concentration, the U descriptor

is: U (2,000,000; 0.01µm).

6

Statistical Analysis Clean Zone Location:

Microtechnology Laboratory Area 2 Clean Zone,

Department of Electrical and Computer Engineering

University of Minnesota Test Mode : At-Rest

Table 2 General Information Clean Zone Size 100 ft2 Cleanliness Class 6.3

Number of Locations 6 Particle Size 0.3µm

Table 3 Particle Concentration (particles/ m3) Statistical Analysis

Instruments LPC (==0.3µm) CPC (==0.01µm)

Overall Mean of the Averages 55,066 79,076

Standard Deviation 63,497 86,020

95% Upper Confidence Limit 109,361 149,311

Table 4 Acceptance Criteria Tests

Instruments LPC (==0.3µm) CPC (==0.01µm)

1. Highest Average Particle Concentration

(particles/ m3) At a Location

169,037 231,922

Highest Average Allowed 203,000 2,000,000

2. 95% Upper Confidence Limit 109,361 149,311

Allowable Limit 203,000 2,000,000

Clean Zone Tested Meets

Requirements for ISO Class 6.3 at 0.3µm

For Data Corrected On March 6, 2002

7

Particle Counts and Concentrations Raw Data

Table 5 Raw data (particle counts) of LPC measurement

Sample No 1 2 3 Location time (s) counts (>=0.3µm) time (s) counts (>=0.3µm) time (s) counts (>=0.3µm)

1 123 38 122 31 123 45 2 123 389 122 351 122 467 3 122 953 123 1064 123 886 4 123 54 123 43 122 35 5 122 58 123 54 122 33 6 123 382 123 351 122 436

Table 6 Particle concentrations at each sampling location measured by LPC

concentration (partilces/m3) for particles >=0.3µm Location sample 1 sample 2 Sample 3 Sample Average

1 6620 5445 7840 6635 2 67770 61651 82026 70482 3 167389 185366 154355 169037 4 9408 7491 6148 7682 5 10187 9408 5796 8464 6 66551 61150 76581 68094

Table 7 Raw data (particle counts) of CPC measurement

Sample No 1 2 3

Location time (s) counts (>0.01µm) time (s) counts (>0.01µm) time (s) counts (>0.01µm) 1 122 40 123 30 122 36 2 122 377 123 291 123 300 3 122 714 123 705 123 572 4 123 84 122 40 123 39 5 123 32 123 52 123 26 6 123 253 123 217 122 265

Table 8 CPC Particle concentrations at each sampling location (partilces/m3)

Location sample 1 sample 2 sample 3 Sample Average 1 14052 10453 12646 12384 2 132436 101394 104530 112786 3 250820 245645 199303 231922 4 29268 14052 13589 18970 5 11150 18118 9059 12776

8

6 88153 75610 93091 85618

Flow Velocity Profile Measurement

Evaluation Method: This test was performed to determine the air flow velocity at each

sampling location and uniformity of air flow at 1 meter above the floor of the clean zone.

The flow velocity was measured by a TSI VelociCalc Anemometer. The sampling

locations are the same as the particle concentration measurement as shown in Figure 1.

Result: The measured flow velocities are recorded on Table 9.

Table 9 Air flow velocity profile of the clean zone

Sampling point 1 2 3 4 5 6

Flow velocity (ft/min) 64 18 13 35 40 14

The flow velocity inside the clean zone was not uniform according to Table 9.

The main reason is the air filters were not evenly distributed in the ceiling.

Location 6 was on the workbench. Because it was not directly under the filter, as we can

see from Table 6, 8 and 9, the concentration at this location was high, and the flow

velocity was low.

Locations 2 and 5 were under the same filter, but the flow velocity was quite

different. The reason of this may be the pressure drop across the filter was not uniform.

Recommendation:

• Replace the filter above sampling locations 2 and 5.

• Move the workbench fully under filters to achieve best cleanliness.