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SEMI E104-0303 © SEMI 2000, 2003 7 wavelength of the incident li ght beam , diameter, s hape, and refractive index of the particle and the suspension fluid, as well as on the geom etrical layout of th e collection optics…

SEMI E104-0303 © SEMI 2000, 2003 6

8.3 Humidity

8.3.1 The LPPD should work correctly at an ambient

humidity range RH as specified in ISO 14644-5. If the

use of an LPPD at an extended ambient humidity range

is required, the specific LPPD should comply with

these conditions.

8.4 Electromagnetic Compatibility

8.4.1 The equipment should comply with SEMI E33.

8.4.2 Sensors will be incorporated into equipment

either as original equipment or retro-fitted. In either

case, the sensors or the equipment of which they are a

part should comply with the current regulations

covering EMC in the country or region where the

equipment or sensor is used.

8.5 Vibrations

8.5.1 Process equipment designers are advised to

consider the impact of vibration on the performance of

the sensor while it is collecting data. Therefore, they

should minimize vibrations. The designers of the LPPD

sensors are also advised to consider the impact of

vibrations on the equipment at a time the sensor is not

collecting any data. It might be possible that these

vibrations are stronger than those occurring while the

sensor is collecting data.

9 Reference Calibration Procedure

9.1 The response of real contamination particles,

typically with refractive indices and shapes different

from calibration particles, will differ slightly from the

results obtained by the procedures in this document. It

is known that LPPDs with different optical design may

not produce the same data from identical aerosol

samples. This may happen even with similar LPPDs if

calibration differences have occurred. Therefore, before

the first use the sensor should be calibrated by the

manufacturer. This calibration should be compliant

with or should be reviewed with the following reference

calibration equipment and procedure. This reference

calibration allows the characterization of the

performance of the LPPD under test. The LPPD should

be recalibrated at regular intervals and also in case of

unusual measurement readings to ensure correct results.

9.1.1 The parameters calibrated for LPPDs with sizing

capability are:

• Sizing calibration,

• Resolution,

• Zero counting,

• Counting efficiency, and

• Sensitivity.

9.1.2 The parameters calibrated for LPPDs with non-

sizing capability are:

• Zero counting, and

• Counting efficiency.

9.1.3 Field calibration may not necessarily require the

calibration of all parameters performed by calibration at

the sensor manufacturers site.

9.1.4 Due to the fact that different LPPDs might be

working with different detection areas A

, the number

of counts should be printed out in counts per mm

detection area. Therefore, a comparison of the

measurement results of different LPPDs is possible.

The size of the detection area A

shall be reported in

the calibration report form. Any changes of calibration

parameters or of the calibration setup shall be reported

in the calibration report form. A copy of the calibration

report form shall be delivered with the sensor.

9.2 Apparatus

9.2.1 Materials

9.2.1.1 Particles

9.2.1.1.1 Calibration particles are polymer spheres

composed of polystyrene or a similar polymer, having a

refractive index of 1.58-1.61+ 0i (absorption coefficient

α = 0), a sizing accuracy of at least 95%, and a size

distribution in which the coefficient of variation is 5%

or less. They should be traceable to a nationally or

internationally recognized standard (e.g. NIST

9.2.1.1.2 The calibration particles are normally

supplied in concentrations too high to be used directly

in aerosol generators. The particles should be dispersed

and diluted in either deionized, distilled water in

accordance with ASTM D1193, Type 1, or Isopropanol.

The diluent should be cleaned using a filter with a pore

size no more than 10% of the size of the particles being

used. The solution should be stored in a clean container.

For generation and dilution of the suspension see

Appendix 3. After generation the particles should be

neutralized to avoid surface charge. For all tests

described in this document, the concentration should be

no more than 25% of the maximum recommended

concentration limit specified by the manufacturer. For

calibration a suitable set of particle sizes shall be used.

This set should contain at least 5 sizes that cover the

LDL size to at least 80% of the specified maximum size

measurement capability of the LPPD.

NOTE 4: Most of the ISPM sensors are based on light

scattering. The intensity of the scattered light detected by a

photodetector depends on intensity, polarization state, and

9 National Institute of Standards and Technology, 100 Bureau Drive,

Gaithersburg, MD 20899-0001, USA

SEMI E104-0303 © SEMI 2000, 2003 7

wavelength of the incident light beam, diameter, shape, and

refractive index of the particle and the suspension fluid, as

well as on the geometrical layout of the collection optics and

detector. In the particle size range near the wavelength (0.1λ

< d

< 10λ), large oscillations can be seen in the intensity

curve of the light scattered by spherical particles as a function

of all these parameters. This phenomenon should be taken

into account when selecting a suitable set of particle sizes for

calibration. The used calibration particles should be within a

monotonic response range of the LPPD response curve.

9.2.1.2 Zero Gas

9.2.1.2.1 Clean air or nitrogen filtered with a ULPA

filter.

NOTE 5: The calibration will be executed under atmospheric

pressure and zero gas. In semiconductor manufacturing,

pressure and process gases will differ from the calibration

conditions. This will affect the refractive index ratio of the

particles and the process gas and consequently the scattering

from the particles.

9.2.1.3 Surfaces

9.2.1.3.1 The materials of pump lines and other

components should be conductive to minimize

electrostatic interaction with the particles.

9.2.2 Instrumentation (see Figure 1) — Some LPPDs

may require specialized equipment not generally

available. Please contact the LPPD manufacturer.

9.2.2.1 Fan System

9.2.2.1.1 The fan or pump system should be adjustable

to transport the aerosol and the zero gas within the

stipulated velocity range v

L, min

... v

L,max

, specified by

the manufacturer of the LPPD.

9.2.2.2 Filter System

9.2.2.2.1 The ULPA filter system is used for gener-

ation of zero gas. The filter system should be capable of

removing particles at the minimum size detectable by

the LPPD or the reference particle counter.

9.2.2.3 Aerosol Generator

9.2.2.3.1 An atomizer converts the monodisperse

particle suspension to an aerosol by using compressed

zero gas for generation and transportation of the

particles. The aerosol generator should generate

monodisperse particles as defined in Section 9.2.1.1 in

constant and reproducible concentration C

under

constant and reproducible volume flow rate V´

. The

variation in particle concentration shall be no more than

10% as measured by the reference particle counter over

a time period of 10 times or more of the sample

measurement time. The generation should comply with

the German VDI-Richtlinie 3491, or an equivalent

standard in other countries.

9.2.2.4 Aerosol dryer

9.2.2.4.1 The monodisperse polymer spheres in the test

aerosol shall be thoroughly dry to avoid that the

particles have a water layer which would increase their

size. A diffusion dryer, another appropriate instrument,

or adequately dry dilution air should be used to dry the

particles. The diffusion dryer uses silica gel desiccant to

remove the moisture. The desiccant shall either be new

or freshly regenerated. The design flow rate of the

aerosol dryer shall at least match the output flow rate of

the aerosol generator.

NOTE 6: Some diffusion dryers may precipitate polymer

spheres and add other particles when the aerosol gets in direct

contact with the silica gel.

9.2.2.5 Neutralizer

9.2.2.5.1 An aerosol neutralizer should be connected in

line with the dryer to reduce electrostatic charges on the

dry polymer spheres and to avoid electrostatic

interaction with each other or the line wall. The design

flow rate of the aerosol neutralizer shall at least match

the output flow rate of the aerosol generator.

NOTE 7: Some electrostatic neutralizers may produce a large

number of ultra-fine particles which will combine with the

calibration aerosol.

9.2.2.6 Aerosol Size Separator

9.2.2.6.1 In the case of calibration with ultra-fine

particles, a system should be used to separate single

polymer spheres from the residual particles resulting

from vaporization of solutions and aggregate particles

consisting of several spheres. The size separation could

be achieved with an electrostatic classifier or a

diffusion battery.

9.2.2.7 Aerosol Dilution

9.2.2.7.1 If the particle concentration behind the

aerosol generator is too high, the particle flow shall be

diluted to achieve the required concentration and to

avoid coincidence errors. The dilution should comply

with the German VDI-Richtlinie 3491, or an equivalent

standard in other countries.

9.2.2.8 Aerosol Line System

9.2.2.8.1 The system consisting of

• the aerosol generator,

• the aerosol dryer,

• the neutralizer,

• the particle size separator,

• the dilution stage, and

• tubing connecting the devices with each other and

the filtered, dried and compressed zero gas.

SEMI E104-0303 © SEMI 2000, 2003 8

9.2.2.8.2 The line system should be smooth, conduc-

tive, and electrically grounded to minimize electrostatic

interaction of the particles with line walls and the parti-

cles themselves. The line should be as short and straight

as possible with no bends with a radius of curvature less

than 100 mm. Leak-free connections of the line and all

devices should be ensured using appropriate fittings.

9.2.2.8.3 Figure 1 illustrates a recommended

calibration aerosol generation system.

9.2.2.9 Flow Control

9.2.2.9.1 The velocity of aerosol and the zero gas

should be within the stipulated range v

L, min

... v

L,max

specified by the manufacturer of the LPPD. Dependent

on the line diameter D

, a stipulated aerosol flow is

necessary. The velocity meter (e.g. thermoanemometer)

or flow meter should be mounted in a line with known

diameter D

NOTE 8: The aerosol drawn by the reference particle counter

influences the velocity or flow measured by the velocity

meter or flow meter if it is mounted behind the probe inlet.

This is taken into account when calculating and adjusting the

flow and velocity v

at the LPPD. If the velocity meter or

flow meter is mounted in front of the probe inlet of the

reference particle counter, the device should have no

influence on the particle size distribution measured by the

reference particle counter.

9.2.2.10 Calibration Line System

9.2.2.10.1 The system consisting of

• a device to inject the particles into the zero gas,

• a device to mix the aerosol sample with the zero

gas to obtain uniform particle concentration,

• a device to adapt the LPPD into the line (diameter

• a device to adapt the velocity meter or the flow

meter,

• a device (diameter D

= D

, same particle

concentration as at the LPPD) to draw a defined,

isokinetic sample from the line for the reference

particle counter (the opening area of the probe inlet

should be reported in the calibration report form),

and

• tubing connecting the devices with each other and

with the fan and filter system.

9.2.2.10.2 The line system should be smooth, conduc-

tive, and electrically grounded to minimize electrostatic

interaction of the particles with line walls and the parti-

cles themselves. The line should be as short and straight

as possible with no bends with a radius of curvature less

than 100 mm. Leak-free connections of the line and all

devices should be ensured using appropriate fittings.

9.2.2.10.3 Figure 1 illustrates a recommended LPPD

calibration system.

NOTE 9: The distance between the device to adapt the LPPD

under test and the device for sample acquisition for the

reference particle counter should be as short as possible to

minimize particle loss and to ensure comparable particle

concentrations.

9.2.2.11 Reference Particle Counter (RPC)

9.2.2.11.1 The reference particle counter is required to

measure the actual concentration of the monodisperse

aerosol and the quality of the zero gas inside the line.

Therefore, the counting efficiency of the reference

particle counter is defined as 100% over the range of

particle sizes used in the test. The resolution should be

better than 10% at the lower detection limit of the

LPPD under test. The values of the measurement

should be printed out in counts per mm

opening area

of the probe inlet. The sample transit line from the

probe inlet and the reference particle counter should be

as short and as straight as possible. Smooth, conductive,

and electrically grounded materials should be used.

9.2.2.12 Sensor Window Temperature

9.2.2.12.1 Some LPPDs have the capability to heat

their sensor windows to avoid coating of the window. A

device to measure and to adjust the sensor window

temperature within the operating range (measurable to

5%) should be installed in the calibration setup. The

thermometer should be calibrated with an accuracy of

0.2° C.

9.2.2.13 Pulse Height Analyzer (PHA)

9.2.2.13.1 The external analyzer is connected with the

analog sensor calibration output. The PHA should have

at least 64 channels and a resolution of at least 1% of

the average voltage that will be measured. The use of a

PHA, which is built into the sensor electronics, is

allowed if this PHA meets the criteria mentioned above.

The required range and speed will depend on the

performance of the LPPD under test. These parameters

should be obtained by the LPPD manufacturer.

9.2.2.14 Environmental Monitoring

9.2.2.14.1 The environmental temperature T

measured by a thermometer calibrated with an accuracy

of 0.2° C.

9.2.2.14.2 The environmental relative humidity RH is

measured by a hygrometer.

9.2.2.14.3 The atmospheric pressure p is measured by a

barometer calibrated with an accuracy of 133 Pa.