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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 appro ved by full letter ballot procedures on July 28…

SEMI E104-0303 © SEMI 2000, 2003 15

A2-4 Counting Efficiency

A2-4.1 The counting efficiency for a specific particle

size is defined as the ratio of the detected concentration

of particles to the concentration actually present. The

determination of the counting efficiency of an LPPD

requires particles of known size and concentration in

the aerosol. The counting efficiency is affected by

several factors.

• First, the counting efficiency is dependent on a

specific particle size.

• The effect of particle concentration is addressed

under zero counting and coincidence.

• Due to inhomogeneous light intensity within the

detection area of the particle counter, not all small

particles near the lower detection limit of the

instrument are detected.

• If the sampling flow is not completely contained

within the defined detection area, then some

portions of the aerosol will not be counted.

A2-4.2 Ideally, the counting efficiency of a particle

counter covering 100% of the line cross-section area as

detection area would consist of a step function at the

point of the LDL. Real particle detectors have a gradual

transition (efficiency curve) instead of a step function.

The point with 50% detection probability of all particles

of a given size moving through the detection area is

often used as a reference point. The corresponding

particle size is called the minimum detectable particle

size (see sensitivity). If the slope of the efficiency curve

for two detectors with the same specified 50%

efficiency point is different, the two detectors may

show different particle counts for the same polydisperse

aerosol. To define an efficiency curve, more than one

particle size, e.g. 5 particle sizes around the LDL

covering 0% to 100% efficiency, should be used.

Acceptable counting efficiency for single particle

counting instruments is 50% ± 20% at the minimum

detection size and 100% ± 10% for all particles larger

than 1.5 times the minimum size.

A2-4.3 The detection area of most LPPDs comprises

only a small portions of the cross-section area of the

line. Therefore, it is not possible to specify a 50%

efficiency point. For this, the commonly used definition

for sensitivity could not be used for such instruments.

A2-4.4 The actual particle concentration is determined

with a reference particle counter with a 100% counting

efficiency for the employed particle size. It is important

that there are no differences between the concentration

within the detection area of the LPPD and the

concentration monitored by the RPC.

A2-5 Sensitivity

A2-5.1 The particle size corresponding to the point

with 50% counting efficiency of the particles of a given

size is defined as the minimum detectable particle size.

This is valid only for particle counters capable to reach

100% counting efficiency for a specific particle size.

This does not apply to most LPPDs.

A2-6 Particle Concentration Effects

A2-6.1 All optical particle detectors are able to operate

accurately only within a limited range of particle

concentration. It is not the scope of this document to

define how this limit is measured or how to verify the

specification.

A2-6.2 High Concentration Effects

A2-6.2.1 The upper limitation of the particle

concentration is chiefly based on coincidence effects.

A2-6.2.2 One effect is optical coincidence. If the

particle concentration is too high, more than one

particle is present within the detection area of the

instrument. The reported particle concentration will be

less than the true value, and the reported particle size

distribution will be shifted towards the indication of

larger particles than in reality.

A2-6.2.3 The other effect is electronic coincidence. It

is defined as the inability of the electronic pulse

processing system to detect and size individual pulses

that are too closely spaced. If there are so many pulses

that they cannot completely return to the baseline, they

become superimposed. Electronic saturation occurs.

Electronic coincidence introduces errors in both particle

size and counts as well. Normally, this problem is not

critical for modern electronic systems. Optical

coincidence can become a problem at particle levels

well below the point where electronic saturation occurs.

A2-6.2.4 Another error can occur when high

concentrations of particles just smaller than the lower

detection limit of the instrument are present. Even

though no single one of these particles will be detected,

scattering-light levels from these particles can increase

the background optical noise level. So, errors might be

produced in particle count data in the lower particle size

ranges reported by the counter.

A2-6.3 Low Concentration Effects

A2-6.3.1 It is obvious that it is necessary to collect

sufficient data to determine number and size of particles

within an acceptable confidence limit.

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.

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.