ASTM E2088-00
(Practice)Standard Practice for Selecting, Preparing, Exposing, and Analyzing Witness Surfaces for Measuring Particle Deposition in Cleanrooms and Associated Controlled Environments
Standard Practice for Selecting, Preparing, Exposing, and Analyzing Witness Surfaces for Measuring Particle Deposition in Cleanrooms and Associated Controlled Environments
SCOPE
1.1 This practice is intended to assist in the selection, preparation, exposure, and analysis of witness surfaces for the purpose of characterizing particle deposition rates in cleanrooms and associated controlled environments, particularly for aerospace applications.
1.2 Requirements may be defined in terms of particle size distribution and count, percent area coverage, or product performance criteria such as optical transmission or scatter. Several choices for witness surfaces are provided.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
1.3 The values stated in SI units are to be regarded as the standard.
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Designation:E2088–00
Standard Practice for
Selecting, Preparing, Exposing, and Analyzing Witness
Surfaces for Measuring Particle Deposition in Cleanrooms
and Associated Controlled Environments
This standard is issued under the fixed designation E 2088; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 2.3 Government Standards:
Fed-Std-209 Airborne Particulate Cleanliness Classes in
1.1 This practice is intended to assist in the selection,
Cleanrooms and Clean Zones
preparation, exposure, and analysis of witness surfaces for the
MIL-STD-1246 Product Cleanliness Levels and Contami-
purpose of characterizing particle deposition rates in clean-
nation Control Program
rooms and associated controlled environments, particularly for
aerospace applications.
NOTE 1—The Institute of Environmental Sciences and Technology has
1.2 Requirements may be defined in terms of particle size several Recommended Practices which may also be useful.
distribution and count, percent area coverage, or product
3. Terminology
performance criteria such as optical transmission or scatter.
Several choices for witness surfaces are provided. 3.1 Definitions:
3.1.1 bidirectional reflectance distribution function
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the (BRDF)—the scattering properties of light reflected off sur-
faces, expressed as the ratio of differential outputs of radiance
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- divided by differential inputs of radiance. Surface contami-
nants scatter the incident radiation in all directions and with
bility of regulatory limitations prior to use.
1.4 The values stated in SI units are to be regarded as the variable intensities. The BRDF is a method to quantify the
spatial distribution of the scattered energy.
standard.
3.1.2 cleanliness level—an established maximum allowable
2. Referenced Documents (Note 1)
amount of contamination in a given area or volume, or on a
2.1 ASTM Standards: component.
E 1216 Practice for Sampling for Surface Particulate Con- 3.1.3 cleanroom—an environmentally conditioned area in
tamination by Tape Lift which temperature, humidity, and airborne contaminants are
F 24 Method for Measuring and Counting Particulate Con- controlled by design and operation. High-efficiency particulate
tamination on Surfaces air (HEPA) filters or better are usually required to achieve the
F 50 Practice for Continuous Sizing and Counting of Air- air cleanliness level. Air particulate cleanliness is classified in
borne Particles in Dust-ControlledAreas and Clean Rooms accordance with FED-STD-209 or ISO 14644-1.
Using Instruments Capable of Detecting Single Sub- 3.1.4 contaminant—unwanted molecular and particulate
Micrometer and Larger Particles matter that could affect or degrade the performance of the
F 312 Test Methods for Microscopical Sizing and Counting components upon which they reside.
Particles from Aerospace Fluids on Membrane Filters 3.1.5 contamination—a process of contaminating.
2.2 ISO Standard: 3.1.6 contamination control—organized action to control
ISO 14644-1 Cleanrooms and Associated Controlled the level of contamination.
Environments—Part 1: Classification of Air Cleanliness 3.1.7 controlled area—an environmentally controlled area,
operated as a cleanroom, but without the final stage of HEPA
(or better) filters used in cleanrooms.
This practice is under the jurisdiction of ASTM Committee E-21 on Space
3.1.8 critical surface—any surface of an item or product
Simulation and Applications of Space Technology and is direct responsibility of
which is required to meet established cleanliness level require-
Subcommittee E21.05 on Contamination.
ments.
Current edition approved May 10, 2000. Published July 2000.
Annual Book of ASTM Standards, Vol 15.03.
Discontinued; see 1993 Annual Book of ASTM Standards, Vol 15.03.
Annual Book of ASTM Standards, Vol 14.02.
5 6
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E2088–00
3.1.9 demonstrated equivalence—the condition in which a 3.1.21 visibly clean—absence of particulate or molecular
method of measurement has passed a series of tests to show contaminants when viewed from a specified distance with
that it gives equivalent results to those of a standard measure- normal (or corrected to normal) vision with a specified
ment. illumination level.
3.1.10 environmentally controlled area—cleanrooms, con- 3.1.22 witness surface (WS)—a contamination-sensitive
trolled areas, good housekeeping areas, and other enclosures material used instead of direct evaluation of a specific surface
that are designed to protect hardware from contamination. when that surface is either inaccessible or is too sensitive to be
Cleanliness is achieved by controlling air purity, temperature, handled.
humidity, materials, garments, and personnel activities. 3.1.22.1 optical witness surface (OWS)—witness surface
3.1.11 fiber—a particle >100 µm in length with a length to fromwhichcontaminantsmaybeanalyzedbyopticalmethods.
diameter ratio of ten or more. 3.1.22.2 particle witness surface (PWS)—witness surface
3.1.12 image analysis—the measurement of size, shape, from which particulate contaminants may be analyzed by
number, position, orientation, brightness, and other parameters standard optical or electron microscopic methods.
of small objects using the combination of a microscope, an
imaging sensor, and a dedicated computer system. Image 4. Summary of Practice
analysis can be used to perform particle counts or measure
4.1 Particle deposition in controlled environments is deter-
particle dimensions automatically, with far greater accuracy
mined by collecting particles on a clean witness surface for a
than manual techniques.
specified period of time or operational activity, then retrieving
3.1.13 micrometre (µm)—a unit of measurement equal to
the witness surface and quantifying the particle population
one millionth of a metre, or approximately 39 millionths of an
collected.
inch, for example, 25 µm is approximately 0.001 in. The term
4.2 Witness surfaces (WS) are typically surfaces that lend
“micron” has been used but is not a recommended SI unit.
themselves to traditional microscopic or image analysis tech-
3.1.14 nonvolatile residue (NVR)—soluble material re-
niquesforsizingandcountingparticlesonthesurface,butmay
maining after evaporation of a filtered volatile fluid or precipi-
be an optical surface that is evaluated on the basis of the
tate from a gas phase, usually reported in milligrams per unit
changeinitsopticalpropertiesormaybeawitnesssurfacethat
area (or volume).
best represents the surface material of interest which is
3.1.15 particle deposition—the settling of airborne particles
subsequentlyevaluatedbyextractingasamplefromthesurface
onto surfaces resulting from electrostatic or dynamic condi-
and sizing and counting particles removed from the witness
tions, or both, in cleanrooms or other controlled environments.
surface.
3.1.16 particle fallout (PFO)—a standard particle deposi-
4.3 This practice does not address real time particle depo-
tionmethodusedbytheEuropeanaerospacecommunitywhich
sition measurements involving particle counters on site with
uses black glass witness surfaces and measures particle scatter
continuous recording over a specified period of time.
in parts per million.
3.1.17 particle size—(1) the apparent maximum linear di-
5. Significance and Use
mension of a particle in the plane of observation, as observed
5.1 This practice provides a standard approach to mesuring
with an optical microscope; (2) the equivalent diameter of a
particle deposition, or fallout, in cleanrooms and other con-
particle detected by automatic instrumentation. The equivalent
trolled environments. It is based on the use of a witness surface
diameter is the diameter of a reference sphere having known
to collect particles that deposit from the surrounding environ-
properties and producing the same response in the sensing
ment and subsequently sizing and counting the particles by
instrument as the particle being measured; (3) the diameter of
conventional methods. Several options are introduced, with
a circle having the same area as the projected area of a particle,
limitations and guidelines for selecting the best choice for the
intheplaneofobservation,observedbyimageanalysis;(4)the
intended application.
size defined by the measurement technique and calibration
5.2 This practice is applicable across numerous industries
procedure.
including aerospace, microelectronics, and pharmaceuticals.
3.1.18 particulate contamination—discrete mass of solid
matter, size often measured in micrometres (µm), which
6. Selecting Witness Surfaces
adversely affects critical surfaces of component and hence
6.1 ConsiderationsforselectingWSincludeavailablemeth-
system performance.
ods of analysis, precision and accuracy required, size of
3.1.19 percent area coverage (PAC)—fraction of the sur-
particles of concern, actual material of critical surfaces of
face that is covered by particles, reported in percent as total
concern, and cost. Preferably, the WS should be a surface
particle projected area divided by total area of the surface.
material which best represents the actual critical surface and
3.1.20 precision cleaning—cleaning of hardware surfaces
should be analyzed using the method which best represents the
approved by established facility methods or methods specified
actual performance characteristics of interest. Additionally,
or provided by the customer with verification to a specified
certain surfaces may become charged, especially in dry envi-
cleanliness level.
ronments, and this charging can effect the particle deposition.
IfWSaretomonitoravacuumenvironmenttheymustbemade
of low-outgassing, vacuum-compatible materials and held
The Euramark Model 255 PFO photometer has been found to be satisfactory. securely in vacuum-compatible, low-particle shedding holders.
E2088–00
6.2 Microscopic Evaluation—When microscopic sizing and step.Additionally, a quartz crystal microbalance with adhesive
counting of particles is the planned method of analysis, select surfaces can measure accumulated mass in situ.
one of the following PWS, each of which is easily evaluated
7. Preparation of Witness Surfaces
directly after exposure. Microscopic sizing and counting shall
7.1 Witness Surface Holders—Holders should be designed
be performed in accordance with Method F 24 orTest Methods
to retain the witness surface securely and maximize the surface
F 312.
exposure. They should be made from smooth, cleanable
6.2.1 Membrane Filters , should be gridded for ease in
materialssuchasplastic,anodizedaluminum,orstainlesssteel.
microscopic particle counting and precleaned before exposure.
A noncontact, easily removable, protective cover is required
Amembranefiltercanbepreparedaseitheratackyortack-free
which prevents the collection of particulate contamination
surface. The membrane filter is cleaned and then either (1)
during transport of the surfaces between the test laboratory and
immediately placed in a cleaned petri dish, (2) dipped into
the controlled environment being evaluated. Holders should
trichloroethylene or methyl chloroform first so it will fuse to
have captive fasteners and tethers to prevent the holder or
the plastic petri dish, or (3) dipped into a prefiltered tacky
associated hardware from impacting critical surfaces if
adhesiveanddriedinacleanedpetridish.Thepetridishisthen
dropped. Holders should also be designed to be secured in the
covered and transported to the area being tested.
facility being evaluated in either a vertical or horizontal
6.2.2 Gridded Counting Slides, such as those used in Prac-
orientation.
tice E 1216 may be used as WS. After exposure, a pressure-
7.2 Cleaning of Holders—Holders should be precision
sensitive tape is applied to the slide to encapsulate the
cleaned in accordance with MIL-STD-1246 Level 100 or clean
deposited particles before moving them to a microscope for
before installing the witness surface. It is recommended that
analysis.
cleaningandpackagingbeperformedinaFED-STD-209Class
6.2.3 Stainless or Other Surfaces, other materials may be
100 or better clean bench.
selected asWS based on specific needs for durability or to best
7.3 Cleaning of WS—Membrane filters should be blanked
represent the actual surface materials of interest. For these
or recleaned with filtered fluid before exposure. Tapes should
PWS, particles are subsequently extracted from the surface
be inspected before use or a control of the tape must be taken
with a fluid, filtered to collect the particles on a gridded
to compare the actual surfaces. Glass or polyester film-gridded
membrane, and subsequently analyzed microscopically. Note,
slides should be flushed with a filtered solvent. Silicon wafers
the efficiency of the extraction method must be known or
and disks may be new, repolished, or recleaned with solvent
estimated.
and individually baselined. The PFO black glass is wiped with
6.3 Other Particle Sizing and Counting Methods—Particle
methanol-soaked lint-free lens tissue in a unidirectional man-
characterization can also be performed using optical measure-
ner.
ments other than manual microscopic methods. Highly pol-
7.4 Baselining of OWS—The OWS must be baselined by
ished surfaces serve as WS and are selected based on the
the selected reflectance, transmittance, or scatter measurement
analysis method chosen.
before exposure. With this type of analysis, the baseline value
6.3.1 The PFO instrument uses a smooth black glass plate
is subtracted from the postexposure measurement to determine
40 by 45 mm protected from unintentional sedimentation by a
the net optical degradation as a result of particle deposition on
plate holder. The effective sampling surface is circular with a
the WS.
diameter of 25 mm.
7.5 Protective Packaging—All precision cleaned holders
6.3.2 Silicon wafers or disks shall be selected for image
containing witness surfaces shall be provided with cleanliness
analysis or other surface scanning methods.
protection before leaving the c
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