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SEMI E104-0303 © SEMI 2000, 2003 17 A3-5 Aerosol Neutralization A3-5.1 Wh en dispers ing th e suspension, the particles are charged. This su rface charge should be rem oved after drying the aerosol fl ow t o avoid electr…

SEMI E104-0303 © SEMI 2000, 2003 16

APPENDIX 3

AEROSOL GENERATION AND AEROSOL DILUTION

NOTICE: The material in this appendix is an official part of SEMI E104 and was approved by full letter ballot

procedures on July 28, 2000 by the European Regional Standards Committee.

A3-1 Monodisperse Particle Suspension

A3-1.1 Monodisperse suspensions of particles (PSL)

are available in sizes from 0.02 µm. These particles are

grown by emulsion polymerization and are stabilized in

aqueous suspensions with an anionic surfactant. They

carry a negative charge which contributes to their

stability. The suspension normally contains 10% solids

and 2% dissolved, highly viscous stabilizer. The solid

contains up to 8% emulsifier and inorganics.

A3-1.2 After evaporation, these solids and the

stabilizer will both increase the diameter of the single

particle and generate a residual nucleus out of an empty

droplet. These residual particles are called secondary

aerosol. The diameter of residual nucleus could be up to

0.25 µm according to the PSL size and dilution of the

suspension. Most of these secondary aerosol particles

might be too small to be detectable with commonly

used particle detectors, but they will have an influence

on the noise level because of there high number.

A3-1.3 Out of a high-concentration suspension it is not

possible to aerosolize only individual particles.

Droplets containing more than one of the suspended

particles will become undesirable agglomerates upon

evaporation.

A3-1.4 For this, an adequate dilution of the suspension

is necessary to avoid the formation of aggregates and

the enlargement of the particles.

A3-2 Generation and Dilution of PSL

Suspension

A3-2.1 A diluted PSL suspension for particle

generation in an aerosol generator can be created by the

following procedure:

• Shaking and/or ultrasonic treatment of the PSL

bottle distributed by the PSL manufacturer.

• Placing one drop of PSL in one liter of deionized

or destilled water or Isopropanol. The water could

be cleaned using a filter with a pore size no more

than 10% of the size of the particles being used.

• Shaking and/or ultrasonic treatment of the

suspension to disperse the particles.

A3-2.2 The suspension will probably need to be diluted

further to provide a required particle concentration.

This is necessary to avoid the formation of

agglomerates and to restrict the size of the secondary

aerosol particles. An equation to calculate the required

particle concentration is given in “The Generation of

Aerosols of Fine Particles” by O. Raabe (see Section

11).

A3-2.3 The particle suspensions distributed by the PSL

manufacturer shows no detectable variation in particle

characteristics when they are stored in a cool place over

the years. Diluted suspensions for atomization feature

an aging process. Diluted suspensions of polymer

spheres smaller than 1 µm should not be stored for

more than one week.

NOTE A3-1: Care should be taken to avoid contamination of

the polymer spheres and the suspension.

A3-3 Aerosol Generation

A3-3.1 The diluted suspension is nebulized in the

aerosol generator. Great account is taken of

concentration and size distribution of the generated

droplets and of the volume flow rate V´

. To achieve a

constant aerosol production, the volume flow rate and

the droplet size distribution should be constant and

independent of the supplies of suspension in the

reservoir. The variation in particle concentration should

be no more than 10% as measured by the reference

particle counter over a time period of 15 minutes or

more.

NOTE A3-2: Care should be taken to avoid contamination of

the aerosol generator.

A3-4 Aerosol Drying

A3-4.1 The water of the generated droplets will

completely evaporate when the relative humidity of the

aerosol flow behind the nebulizer is lower than 70%.

The volume flow of the suspension in the nozzle

increases the humidity of the filtered compressed air

which has a relative humidity of 10%–15% in spite of

predrying. The use of a diffusion dryer or another

appropriate instrument allows a higher relative

humidity of the aerosol flow behind the aerosol

generator. The diffusion dryer uses silica gel desiccant

to remove the moisture. The desiccant shall either be

new or freshly regenerated. It is not wise to get the

aerosol in direct contact with the desiccant. Polymer

spheres will be precipitated and additional particles out

of the desiccant will change the particle size

distribution of the aerosol.

SEMI E104-0303 © SEMI 2000, 2003 17

A3-5 Aerosol Neutralization

A3-5.1 When dispersing the suspension, the particles

are charged. This surface charge should be removed

after drying the aerosol flow to avoid electrostatic

interactions with each other or the line walls. With the

help of the discharging distance of an electrostatic

neutralizer, the aerosol is exposed to a bipolar ion

source. Some electrostatic neutralizers may produce a

large number of ultra-fine particles which will combine

with the calibration aerosol and affects the signal-to-

noise-ratio of the LPPD and the reference particle

counter.

A3-6 Aerosol Size Separation

A3-6.1 When calibrating with ultra-fine particles, a

particle size separation might be necessary to remove

the residue particles and agglomerates. An electrostatic

classifier is most effective for particle sizes less than 1

µm. This instrument electrically charges the incoming

aerosol. With electrostatic deflection, it separates

selected particles of one mobility. The residue particles

and the larger aggregate particles would be stripped out

of the particle stream consisting of the desired polymer

particles. To charge the aerosol, the electrostatic

classifier uses a radioactive neutralizer. The aerosol

exiting the instrument will contain singly charged

particles. This low charge level makes the use of an

additional aerosol neutralizer unnecessary.

Nevertheless, conductive tubing should be used

between the aerosol generator and the LPPD and

reference particle counter to minimize electrostatic

particle loss.

A3-7 Aerosol Dilution

A3-7.1 The particle concentration of the aerosol

generated by the nebulizer might be too high. The

aerosol should be further diluted to achieve the required

concentration and to avoid coincidence errors. The

dilution could be executed with a mixing chamber in

which the aerosol is mixed with zero gas. The internal

chamber pressure should be stable and very close to

ambient atmospheric pressure at operational flow rates.

The spatial particle distribution in the exiting aerosol

should be as homogenous as possible.

SEMI E104-0303 © SEMI 2000, 2003 18

APPENDIX 4

AEROSOL TRANSPORT AND AEROSOL SAMPLING

NOTICE: The material in this appendix is an official part of SEMI E104 and was approved by full letter ballot

procedures on July 28, 2000 by the European Regional Standards Committee.

A4-1 Aerosol Transport

A4-1.1 Aerosol loss in lines occurs both for small and

larger particles. Larger particles (approximately 5 µm

and larger) are lost as a result of gravitational settling in

horizontal lines and inertial effects in all lines. Smaller

particles are lost to the line walls by diffusion and by

electrostatic charge effects.

A4-1.2 A limitation of particle loss could be achieved

with tubing as short and as straight as possible with no

bends with a radius of curvature less than 100 mm. The

tubing of the calibration setup should be smooth,

conductive, and electrically grounded. Stainless,

polished steel for rigid lines and Polyurethane or

polyvinyl chloride for flexible lines is found acceptable

for handling most aerosols with low electrostatic

particle loss. If long transit lines are required, they

should be sized to permit a Reynolds number in the

range of 5,000 to 25,000 at the sample flow rate to

minimize particle residence time in the tubing without

causing excessive turbulence at high flow rates.

A4-2 Aerosol Sample Acquisition

A4-2.1 The term isokinetic sampling often is used in

aerosol measurement and characterization. Isokinetic

sampling of particles in a moving aerosol is performed

by matching the sample probe inlet velocity (flow speed

and

direction) to the velocity of the moving aerosol. At

velocities less than 15 m/s, anisokinetic sampling errors

are negligible for most particles smaller than

approximately 5 µm. Losses of larger particles might be

significant. The larger the particle, the larger the loss

due to inertial effects. Isokinetic sampling is not

possible in environmental conditions with varying

velocity, motionless air, and turbulent, random, or non-

unidirectional flow. For better sampling inlet efficiency,

isokinetic or at least isoaxial sampling is recommended

at calibration.

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