ASTM F1438-93(1999)
(Test Method)Standard Test Method for Determination of Surface Roughness by Scanning Tunneling Microscopy for Gas Distribution System Components
Standard Test Method for Determination of Surface Roughness by Scanning Tunneling Microscopy for Gas Distribution System Components
SCOPE
1.1 The purpose of this test method is to define a method for analyzing the surface texture of the above-mentioned components using a scanning tunneling microscope (STM). STM is a noncontact method of surface profiling that can measure three-dimensional surface features in the nanometer size range, which can then be used to represent the surface texture or to provide figures of merit. Application of this test method, where surface texture is used as a selection criterion, is expected to yield comparable data among different components tested.
1.2 Limitations:
1.2.1 This test method is limited to characterization of stainless steel surfaces that are smoother than Ra = 0.25 [mu]m, as determined by a contact-stylus profilometer and defined by ANSI B46.1. The magnifications and height scales used in this test method were chosen with this smoothness in mind.
1.2.2 Intentional etching or conductive coating of the surface are considered modifications of the gas-wetted surface and are not covered by this test method.
1.2.3 This test method does not cover steels that have an oxide layer too thick to permit tunneling under the test conditions outlined in 11.3.
1.3 This technique is written with the assumption that the STM operator understands the use of the instrument, its governing principles, and any artifacts that can arise. Discussion of these points is beyond the scope of this test method.
1.4 The values stated in SI units are to be regarded as the standard.
1.5 This standard does not purport to address all of the safety problems, 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.
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Designation:F 1438–93 (Reapproved 1999)
Standard Test Method for
Determination of Surface Roughness by Scanning
Tunneling Microscopy for Gas Distribution System
Components
This standard is issued under the fixed designation F 1438; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Semiconductor clean rooms are serviced by high-purity gas distribution systems. This test method
presentsaprocedurethatmaybeappliedfortheevaluationofoneormorecomponentsconsideredfor
use in such systems.
1. Scope responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 Thepurposeofthistestmethodistodefineamethodfor
bility of regulatory limitations prior to use.
analyzing the surface texture of the above-mentioned compo-
nents using a scanning tunneling microscope (STM). STM is a
2. Referenced Documents
noncontact method of surface profiling that can measure
2.1 ASTM Standards:
three-dimensionalsurfacefeaturesinthenanometersizerange,
E 691 Practice for Conducting an Interlaboratory Study to
which can then be used to represent the surface texture or to
Determine the Precision of a Test Method
providefiguresofmerit.Applicationofthistestmethod,where
2.2 ANSI Standard:
surface texture is used as a selection criterion, is expected to
ANSI B.46.1-85, “Surface Texture (Surface Roughness,
yield comparable data among different components tested.
Waviness, and Lay),” ANSI/ASME, 1985
1.2 Limitations:
1.2.1 This test method is limited to characterization of
3. Terminology
stainlesssteelsurfacesthataresmootherthan R =0.25µm,as
a
3.1 Definitions of Terms Specific to This Standard:
determined by a contact-stylus profilometer and defined by
3.1.1 artifact—anycontributiontoanimagefromotherthan
ANSI B46.1.The magnifications and height scales used in this
true surface morphology. This could include such examples as
test method were chosen with this smoothness in mind.
vibration, electronic noise, thermal drift, or tip imperfections.
1.2.2 Intentional etching or conductive coating of the sur-
3.1.2 center line (graphical center line)—lineparalleltothe
faceareconsideredmodificationsofthegas-wettedsurfaceand
directionofprofilemeasurement,suchthatthesumoftheareas
are not covered by this test method.
contained between it and the profile contained on either side
1.2.3 This test method does not cover steels that have an
are equal (see Calculation Section).
oxide layer too thick to permit tunneling under the test
3.1.3 cutoff length (l )—for profiles in this context, the
c
conditions outlined in 11.3.
sampling length, that is, the length of a single scan, in
1.3 This technique is written with the assumption that the
nanometers (see Calculation Section).
STM operator understands the use of the instrument, its
3.1.4 current— in this context, the tunneling current (ex-
governing principles, and any artifacts that can arise. Discus-
pressed in nanoamperes) that flows in either direction between
sion of these points is beyond the scope of this test method.
the tip and surface, under the conditions specified.
1.4 The values stated in SI units are to be regarded as the
3.1.5 feature height—datum (height in the z-direction) of
standard.
any point in the scan area, relative to the lowest point in the
1.5 This standard does not purport to address all of the
scan area, as derived from tunneling current during tip raster-
safety concerns, if any, associated with its use. It is the
ing.
This test method is under the jurisdiction of ASTM Committee F-1 on
Electronics and is the direct responsibility of Subcommittee F01.10 on Contamina- Annual Book of ASTM Standards, Vol 14.02.
tion Control. Available fromAmerican National Standards Institute, 13th Floor, 11 W. 42nd
Current edition approved Feb. 15, 1993. Published April 1993. St., New York, NY 10036.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F 1438–93 (1999)
3.1.6 filter—process of modification of surface data for 3.2 Abbreviations:Abbreviations and Symbols:
purposes of numerical analysis or data presentation. Examples
3.2.1 HOPG—highly ordered pyrolytic graphite; used for
include high or low pass filters and plane-fitting.
atomic scale x-y calibration of the scanning tunneling micros-
3.1.7 gold ruled grating—gold surface having uniformly
copy.
spaced grooves of known depth and separation; used for
3.2.2 STM—scanning tunneling microscopy (or micro-
micrometer scale x-y calibration.
scope).
3.1.8 illuminated surface—three-dimensional image repre-
−9
3.2.3 nA—nanoamperes (1 310 amperes).
sentation that simulates a reflective surface illuminated ob-
3.2.4 Pt/Ir—platinum and iridium alloy wire used to make
liquely or from overhead.
tunneling tips.
3.1.9 image—surface topography represented by plotting
feature height as a function of tip position. The feature height 3.2.5 R —see mean roughness.
a
data is derived from the amount of tunneling current flowing
3.2.6 R —maximum height difference between the high-
max
between the tip and surface during rastering.
est and the lowest points on the profile over the length of the
3.1.10 line plot—three-dimensionalimagegivenasside-by-
profile (see Calculation Section).
side surface profiles.
3.2.7 root mean square (RMS)—see algebraic definition in
3.1.11 mean roughness (R )—average deviation from the
a
Calculation Section.
mean of all profile heights (see algebraic definition in the
3.2.8 R —the 10-point mean roughness; that is, the average
z
Calculation Section).
differenceinheightbetweenthefivehighestpeaksandthefive
3.1.12 peak—highest point between two crossing points of
lowest valleys over the length of the profile (see Calculation
a profile and its center line.
Section).
3.1.13 profile—the cross-sectional data that has been high
3.2.9 x-direction—see scan.
pass filtered with a two-pole filter having a gain of 75% at the
cutoff length l (in nanometers).
3.2.10 y-direction—the direction, in the sample plane, over
c
3.1.14 raster—repetitive scanning in the x-direction while
which successive scans are taken, orthogonal to the scan
moving stepwise in the y-direction; also the area defined by
direction.
such action.
3.2.11 z-direction—the direction perpendicular to the
3.1.15 scan—a single, continuous movement in one direc-
sample plane. Also referred to as the feature height direction.
tion (defined as the x-direction) of the tip relative to sample
3.2.12 Z—sameasfeatureheight(seeCalculationSection).
i
surface.
3.2.13 Z —maximum height difference over entire sur-
max
3.1.16 scan area—area covered by successive, side by side
face (see Calculation Section).
scans.
3.2.14 Z —root-mean-square of all surface heights (see
3.1.17 scan length—distance from start to end of a single
rms
Calculation Section).
scan, without moving in the y-direction (see cutoff length).
3.1.18 scan rate—the speed at which the tip moves relative
to the surface. 4. Summary of Test Method
3.1.19 shaded height plot—image representing feature
4.1 In this test method a sharp, conductive tip is scanned
height as dark or light shades (any color) over a two-
over very closely but not in contact with a conductive surface;
dimensional area. Higher features are shaded lighter and lower
thatis,theyareseparatedbyagapofseveralangstroms.Abias
features are shaded darker.
voltage present between them causes a flow of electrons
3.1.20 thermal drift—movement of the surface with respect
through,ratherthanover,theenergybarrierrepresentedbythis
to the tip due to a lack of thermal equilibrium.
tip-surface gap. This flow is referred to as the tunneling
3.1.21 tilted surface—three-dimensional image showing
current. The manner in which the current fluctuates during the
surface tilted away from viewer, as opposed to a topview.
scanning process is used to indicate the surface’s topography.
3.1.22 tip crash—touchingoftiptosurface,duringrastering
Thoughthetiporsamplecanbescanned,themethoddescribed
or attempts to initiate tunneling, usually resulting in damage to
here considers only the tip to be in motion. A more extensive
one or both.
discussion of the operating principles can be found in other
3.1.23 top view—image represented as a surface viewed
literature.
from overhead.
4.2 In this test method, stainless steel tubing is used as an
3.1.24 tunneling—in this context, the flow of current
example of a component surface.An area of the surface is first
betweenthetipandsurface(see current);morediscussioncan
scanned at a width of 500 nA, then 2000 nA. Even though
be found in additional references.
larger areas can be scanned by most instruments, these mag-
3.1.25 valley—lowestpointbetweentwocrossingpointsof
nifications are chosen to show surface texture in a size range
a profile and its center line.
beyond that measured by contact stylus type surface profiling
3.1.26 voltage—bias voltage, expressed in volts (V) or
instruments, but not at an atomic scale. The surface scans are
millivolts (mV), applied between the tip and the surface.
then compared for damage, artifacts, etc. Numerical analysis
canthenproceedusingthesedataforroughnessorsurfacearea
or both, following the model of other standards such asANSI
Binning, G., et al., “Surface Studies by Scanning Tunneling Microscopy,”
Physical Review Letters, Vol 49, No. 1, July 1982, pp. 57–61. B46.1.
F 1438–93 (1999)
5. Significance and Use some other suitable dimensional standard, depending upon the
size range to be used and the accuracy of the standard.
5.1 The use of STM images and data is for purposes of
textural quality assessment and calculation of figures of merit,
10. Conditioning
and for high purity gas system clean room components.
5.2 This test method defines a standard data presentation
10.1 Aconductive path for the sample shall be provided for
format and suggests figures of merit that utilize STM’s ability voltage biasing of the sample with respect to the tip.
to analyze three-dimensional surface features.
10.2 Mount the sample so the tip will scan an arbitrarily
chosen representative area.
6. Interferences
10.3 Bring the sample and microscope to thermal equilib-
6.1 Some(stainlesssteel)componentsurfaceshaveanoxide
rium.
layer that prevents tunneling from occurring under any condi-
tions without affecting tip or surface morphology. This results
11. Procedure
in ambiguous surface data. Such surfaces require the use of
11.1 As stated in 1.2.3, this test method does not cover
other techniques for topographic measurement.
steels that have an oxide layer too thick to permit tunneling
6.2 This test method assumes that the images obtained are
under the test conditions outlined in this test method.
unperturbed by very thin, non-solid layers (for example,
11.2 Make sure that a minimum of 200 data points is
hydrocarbons, moisture) on the surface.
collected in the x-direction, and at least 200 scans per raster is
6.3 Operationwiththesurfaceinair,vacuum,orunderinert
in the y-direction (200 3200=40000 data points).
liquids is permissible. (The liquids must be suitably inert and
11.3 Initiate tunneling between a Pt/Ir tip and the sample in
fluid, so as to not modify the apparent surface topography or
accordance with the manufacturer’s instructions under condi-
introduce artifacts into the image.)Water is not recommended.
tionstoprovideartifactfreeimages.Suggestedstartingvalues:
6.4 The tip shall be made from platinum/iridium or tung-
voltage bias of 1800 mV, and current levels monitored at 0.1
sten.
nA (Standard Test Conditions—Room temperature and ambi-
NOTE 1—Caution:The tip must not have previously touched any
ent pressure (101.3 kPa, 25° 6 2°C)).
surface (see 11.7).
11.4 Scan an area 500-nm across at a rate of approximately
2 s/µm (or slow enough to prevent the tip from touching the
7. Apparatus
surface during rastering), collecting at least 200 data points
7.1 Scanning Tunneling Microscope, capable of the follow-
with each x-direction scan.
ing may be used:
11.5 Collect area scans so that comparisons can be made
7.1.1 Scanning lengths up to at least 50 µm,
between the first two successive rasters. Ensure that the area
7.1.2 Substaining 6 3 V between the tip and sample,
has not drifted more than 10% of the scan width in any
7.1.3 Monitoring tunneling current as low as 0.05 nA,
direction. If it has, discontinue scanning for sufficient time to
7.1.4 Traversing feature height variations as great as 2 µm
allow thermal equilibrium to be obtained. Determine this by
without touching the tip to the surface, and
repeating 11.3 through 11.5.
7.1.5 Providing the surface topography as a shaded height
11.6 Scan the selected 500-nm area for at least five full,
plot or line plot.
successive rasters and then store at least the last (fifth) image
7.2 Inert atmospheres, temperature controls, acoustic isola-
of that scan area. Fig. 1, Fig. 2, and Fig. 3 show an example of
tion, and vibration isolation are to be provided as necessary to
such an area after the fifth successive raster (see data presen-
obtain artifact-free images.
tation section for explanation of format shown).
8. Sampling 11.7 Inconsistent topography from one scan to the next is a
sign of tip crashing. Fig. 4 shows an example of where the tip
8.1 Many components are too large or irregularly shaped to
touched the surface during rastering.
permit STM analysis without cutting a sample from the
11.8 Change the scanned width to 2000 nm across, centered
component. Low speed cutting, preferably without lubricants,
around the same region, and immediately obtain an image of
using a diamond blade saw is recommended over high speed
that area. Fig. 5, Fig. 6, and Fig. 7 show an example of a
abrasive cutting or hacksaws.
2000-nm wide area.
8.2 This sampling must not modify the surface topography,
11.9 Observe whether the 2000-nm image shows evidence
such as effects due to stress, heat, corrosion, or combination
of damage from scanning the previous 500-nm (smaller) area.
thereof, from its condition as found in the component.
This generally appears as a 500-nm square of very different
8.3 Cleaning the surface using an inert fluid to remove
topography or height near the center of the 2000-nm image. If
cutting contamination is permitted.
there is such evidence, the surface may be too oxidized to
9. Calibration
analyze(see6.1).Fig.8showsanexampleofdamageapparent
at 2000 nm, which occurred during the 500 nm-wide rasters.
9.1 Calibration frequency may vary with different instru-
ment manufacturers. It should be performed, at least initial
...
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