ASTM F1711-96(2016)
(Practice)Standard Practice for Measuring Sheet Resistance of Thin Film Conductors for Flat Panel Display Manufacturing Using a Four-Point Probe Method (Withdrawn 2023)
Standard Practice for Measuring Sheet Resistance of Thin Film Conductors for Flat Panel Display Manufacturing Using a Four-Point Probe Method (Withdrawn 2023)
SIGNIFICANCE AND USE
5.1 Applying Test Method F390 to large flat panel substrates presents a number of serious difficulties not anticipated in the development of that standard. The following problems are encountered.
5.1.1 The four-point probe method may be destructive to the thin film being measured. Sampling should therefore be taken close to an edge or corner of the plate, where the film is expendable. Special geometrical correction factors are then required to derive the true sheet resistance.
5.1.2 Test Method F390 is limited to a conventional collinear probe arrangement, but a staggered collinear and square arrays are useful in particular circumstances. Correction factors are needed to account for nonconventional probe arrangements.
5.1.3 Test Method F390 anticipates a precision testing arrangement in which the probe mount and sample are rigidly positioned. There is no corresponding apparatus available for testing large glass or plastic substrates. Indeed, it is common in flat panel display making that the probe is hand held by the operator.
5.1.4 It is difficult, given the conditions cited in 5.1.3, to ensure that uniform probe spacing is not degraded by rough handling of the equipment. The phased square array, described, averages out probe placement errors.
5.1.5 This practice is estimated to be precise to the following levels. Otherwise acceptable precision may be degraded by probe wobble, however (see 8.6.4).
5.1.5.1 As a referee method, in which the probe and measuring apparatus are checked and qualified before use by the procedures of Test Method F390 paragraph 7 and this practice, paragraph 8: standard deviation, s, from measured sheet resistance, RS, is ≤ 0.01 RS.
5.1.5.2 As a routine method, with periodic qualifications of probe and measuring apparatus by the procedures of Test Method F390 paragraph 7 and this practice, paragraph 8: standard deviation, s, from measured sheet resistance, RS, is ≤ 0.02 RS.
SCOPE
1.1 This practice describes methods for measuring the sheet electrical resistance of sputtered thin conductive films deposited on large insulating substrates, used in making flat panel information displays. It is assumed that the thickness of the conductive thin film is much thinner than the spacing of the contact probes used to measure the sheet resistance.
1.2 This standard is intended to be used with Test Method F390.
1.3 Sheet resistivity in the range 0.5 to 5000 ohms per square may be measured by this practice. The sheet resistance is assumed uniform in the area being probed.
1.4 This practice is applicable to flat surfaces only.
1.5 Probe pin spacings of 1.5 mm to 5.0 mm, inclusive (0.059 to 0.197 in inclusive) are covered by this practice.
1.6 The method in this practice is potentially destructive to the thin film in the immediate area in which the measurement is made. Areas tested should thus be characteristic of the functional part of the substrate, but should be remote from critical active regions. The method is suitable for characterizing dummy test substrates processed at the same time as substrates of interest.
1.7 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.8 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.
WITHDRAWN RATIONALE
This practice describes methods for measuring the sheet electrical resistance of sputtered thin conductive films deposited on large insulating substrates, used in making flat panel information displays. It is assumed that the thickness of the conductive thin film is much thinner than the spacing of the contact probes used to measure the sheet resistance.
Formerly under the jurisdiction of Committe...
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This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation:F1711 −96 (Reapproved 2016)
Standard Practice for
Measuring Sheet Resistance of Thin Film Conductors for
Flat Panel Display Manufacturing Using a Four-Point Probe
Method
This standard is issued under the fixed designation F1711; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This practice describes methods for measuring the sheet
F390Test Method for Sheet Resistance of Thin Metallic
electrical resistance of sputtered thin conductive films depos-
Films With a Collinear Four-Probe Array
ited on large insulating substrates, used in making flat panel
information displays. It is assumed that the thickness of the
3. Terminology
conductive thin film is much thinner than the spacing of the
contact probes used to measure the sheet resistance.
3.1 Definitions:
3.1.1 For definitions of terms used in this practice see Test
1.2 This standard is intended to be used with Test Method
Method F390.
F390.
1.3 Sheet resistivity in the range 0.5 to 5000 ohms per
4. Summary of Practice
square may be measured by this practice. The sheet resistance
4.1 This practice describes the preferred means of applying
is assumed uniform in the area being probed.
Test Method F390 to measure the electrical sheet resistance of
1.4 This practice is applicable to flat surfaces only. thinfilmsonverylargeflatsubstrates.Anarrayoffourpointed
probesisplacedincontactwiththefilmofinterest.Ameasured
1.5 Probe pin spacings of 1.5 mm to 5.0 mm, inclusive
electrical current is passed between two of the probes, and the
(0.059 to 0.197 in inclusive) are covered by this practice.
electrical potential difference between the remaining two
probes is determined. The sheet resistance is calculated from
1.6 The method in this practice is potentially destructive to
the measured current and potential values using correction
the thin film in the immediate area in which the measurement
factors associated with the probe geometry and the probe’s
is made. Areas tested should thus be characteristic of the
distance from the test specimen’s boundaries.
functional part of the substrate, but should be remote from
critical active regions. The method is suitable for characteriz-
4.2 The method of F390 is extended to cover staggered
ing dummy test substrates processed at the same time as
in-lineandsquareprobearrays.Inallthedesigns,however,the
substrates of interest.
probe spacings are nominally equal.
4.3 This practice includes a special electrical test for veri-
1.7 The values stated in SI units are to be regarded as the
fying the proper functioning of the potential measuring instru-
standard. The values given in parentheses are for information
ment (voltmeter), directions for making and using sheet resis-
only.
tance reference films, an estimation of measurement error
1.8 This standard does not purport to address all of the
caused by probe wobble in the probe supporting fixture, and a
safety concerns, if any, associated with its use. It is the
protocol for reporting film uniformity.
responsibility of the user of this standard to establish appro-
4.4 Two appendices indicate the computation methods em-
priate safety and health practices and determine the applica-
ployed in deriving numerical relationships and correction
bility of regulatory limitations prior to use.
factors employed in this practice, and in Test Method F390.
This practice is under the jurisdiction ofASTM Committee F01 on Electronics
and is the direct responsibility of Subcommittee F01.17 on Sputter Metallization. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2016. Published May 2016. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1996. Last previous edition approved in 2008 as F1711–96(2008). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/F1711-96R16. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1711−96 (2016)
5. Significance and Use 5.1.4 It is difficult, given the conditions cited in 5.1.3,to
ensure that uniform probe spacing is not degraded by rough
5.1 ApplyingTestMethodF390tolargeflatpanelsubstrates
handlingoftheequipment.Thephasedsquarearray,described,
presents a number of serious difficulties not anticipated in the
averages out probe placement errors.
development of that standard. The following problems are
5.1.5 This practice is estimated to be precise to the follow-
encountered.
inglevels.Otherwiseacceptableprecisionmaybedegradedby
5.1.1 Thefour-pointprobemethodmaybedestructivetothe
probe wobble, however (see 8.6.4).
thin film being measured. Sampling should therefore be taken
5.1.5.1 As a referee method, in which the probe and
close to an edge or corner of the plate, where the film is
measuring apparatus are checked and qualified before use by
expendable. Special geometrical correction factors are then
the procedures of Test Method F390 paragraph 7 and this
required to derive the true sheet resistance.
practice, paragraph 8: standard deviation, s, from measured
5.1.2 Test Method F390 is limited to a conventional col-
sheet resistance, R,is ≤ 0.01 R .
S S
linear probe arrangement, but a staggered collinear and square
5.1.5.2 As a routine method, with periodic qualifications of
arraysareusefulinparticularcircumstances.Correctionfactors
probe and measuring apparatus by the procedures of Test
are needed to account for nonconventional probe arrange-
Method F390 paragraph 7 and this practice, paragraph 8:
ments.
standard deviation, s, from measured sheet resistance, R,is ≤
5.1.3 Test Method F390 anticipates a precision testing S
0.02 R .
S
arrangement in which the probe mount and sample are rigidly
positioned. There is no corresponding apparatus available for
6. Apparatus
testinglargeglassorplasticsubstrates.Indeed,itiscommonin
flat panel display making that the probe is hand held by the
6.1 Probe Assembly:
operator.
6.1.1 The probe assembly must meet the apparatus require-
ments of F390, 5.1.1 – 5.1.3.
6.1.2 Four arrangements of probe tips are covered in this
practice:
6.1.2.1 In-Line, Collinear, Probe Tips, with current flowing
between the outer two probes (see Fig. 1A). This is the
conventional arrangement specified in Test Method F390.
6.1.2.2 Staggered Collinear Probe Tips, with current flow-
ing between one outer and one interior probe (see Fig. 1B).
This arrangement is sometimes used as a check to verify the
results of a conventional collinear measurement (see 6.1.2.1).
6.1.2.3 Square Array, with current conducted between two
adjacent probe tips (see Fig. 1C).
6.1.2.4 Phased Square Array, with current applied alter-
nately between opposite pairs of tips (see Fig. 1D). This
arrangement has the advantage of averaging out errors caused
by unequal probe spacing.
6.1.3 Probe Support— The probe support shall be designed
in such a manner that the operator can accurately lower the
probes perpendicularly onto the surface and provide a repro-
ducible probe force for each measurement. Spring loading or
gravity probe pin loading are acceptable.
6.2 Electrical Measuring Apparatus— The electrical appa-
ratus must meet the apparatus requirements of Test Method
F390, 5.2.1 through 5.2.4.
6.3 Specimen Support— The substrate to be tested must be
supported firmly.
6.4 Additional Apparatus:
6.4.1 If measurements will be made within a distance of 20
times the probe spacing from an insulating or highly conduc-
tive edge or corner (20× S, where i =1, 2, 3, or 4, with
i
reference to Fig. 1), an instrument capable of measuring the
distance from the probe array position to the insulating or
highly conductive boundary within 60.25 mm (60.010 in) is
required. In most instances a vernier depth gage is suitable.
6.4.2 Toolmaker’s Microscope, capable or measuring incre-
FIG. 1 Four-Point Probe Configurations ments of 2.5 µm.
F1711−96 (2016)
7. Test Specimen films on soda lime glass substrates. The surface of this glass
can be somewhat electrically conductive (on the order of
7.1 Thetestarticleshallbeeitheradisplaysubstratethathas
6 2
1×10 Ω ) when the ambient relative humidity is about 90%
been sputter coated with the thin film of interest, or,
or higher.
alternatively, a dummy plate coated in the same operation as
the substrate of interest. 7.5.1 The glass conductivity degradation may interfere with
the sheet resistance measurement when specimen sheet resis-
7.2 The conductive film must be thick enough that it is
tivity is 1000 Ω/square or higher.
continuous. Generally this requires that the film be at least 15
7.5.2 Ensure that films >1000 Ω/square sheet resistance
nm (150Å) thick.
deposited on soda lime glass are conditioned at less than 50%
7.3 The area to be tested shall be free of contamination and
humidity for at least 48 h prior to measurement, and that the
mechanical damage, but shall not be cleaned or otherwise
measurementisperformedatanambientrelativehumidityless
prepared.
than 50%.
7.4 Note that a sputtered film may also coat the edge of the
7.5.3 Note that at relative humidity less than 50% the
glassandcancoatthebacksideofthesubstrate(“overspray”).
surface resistance of soda lime glass in on the order of
Thus the edge of the glass cannot be automatically assumed to
1×10 Ω/ square.
be insulating. If sheet resistance determinations will be made
within a distance of 20 times the probe spacing to an edge of
8. Suitability of Test Equipment
thesubstrateitisnecessarytoensurethatthefilmterminatesat
the edge.
8.1 Equipment Qualification—The probe assembly and the
7.4.1 To eliminate over spray error in compensating for
electrical equipment must be qualified for use as specified in
edgeeffectsataninsulatingboundary(see10.2.2),eithermake
Test Method F390, paragraphs 7.1 through 7.2.3.3 on suitabil-
a fresh cut of the substrate, grind the edge to remove any
ity.
residual film, or etch the film from the edge.
7.4.2 Scribing the substrate near the edge using a glass
8.2 Voltmeter Malfunctions—Modern solid state voltmeters
scribe is not a reliable remedy.
using field effect transistors in the signal input circuitry are
7.4.3 Use a simple 2-point probe ohmeter to verify that the
electrically fragile; failure of a field effect transistor degrades
substrate edge is insulating.
theinputimpedance.Thisfailuremodeisaparticularhazardif
inputprotectionisnotprovidedandiffilmswithstaticcharges
7.5 Soda Lime Glass Substrates —Special precautions may
areprobed.Itisrecommendedthattheerrorfromthevoltmeter
be required in measuring the sheet resistance of sputtered thin
input impedance be checked periodically using the test circuit
illustrated in Fig. 2.
8.2.1 Input Impedance Error—To measure the input imped-
ance error, set the constant current, I, and take the voltage
reading, V. Then, without changing I, make a second reading,
V , with R shorted (close switch IMP, Fig. 2). The impedance
d d
error for R >> R is approximately as follows:
imp v
E 5 @~V 2 V!/V # 3100 (1)
imp d d
where:
E = the percentage voltage error contributed by the finite
imp
voltmeter input impedance.
8.2.2 Common Mode Rejection Error—State of the art
voltmeterstypicallyhavehighcommonmoderejection(onthe
order of 90 dB), but this may be degraded by the failure of a
field effect transistor in the input circuit (8.2). Reduction of
common mode rejection will cause errors in measuring sheet
resistance if unequal probe contact resistances contribute high
NOTE 1—Set R =approximately the resistance measured on the speci-
v
commonmodevoltages.Commonmoderejectionerrormaybe
men film of interest as follows:
measured using the test circuit shown in Fig. 2.
R =R =R
a b v
R =100× R .
d v
8.2.2.1 To measure the common mode rejection error, set
NOTE 2—Set I approximately the same as used for measurement of the
the constant current, I, and take the voltage reading, V. Then,
specimen film of interest, typically 0.05 to 0.50 mA, so that V is
withoutchanging I,makeasecondreading, V ,with R shorted
comparable to that obtained in performing the sheet resistance determi- a a
nation.
(close switch CMR ), and finally complete a third reading, V ,
a b
NOTE 3—If R is set equal to a multiple of In2/2π for the in line probe
v with R shorted(open CMR ,close CMR ).Thecommonmode
b a b
of Fig. 1A, or In2/2π for a square array, then the magnitude of V is the
error is approximately as follows:
sheet resistance value for an equivalent film measurement.
2 2 1/2
FIG. 2Voltmeter Test Circuit E 5 1/2 V 2 V 1 V 2 V /V 3100 (2)
$ @~ ! ~ ! # %
cm a b
F1711−96 (2016)
where:
E = the percentage voltage error contributed by common
cm
mode voltages. The voltmeter must be repaired or
replaced if E exceeds 0.5%.
cm
8.3 Voltage Limited Constant Current Supply—In cases of
high sheet resistance or high contact resistance, the voltage at
theconstantcurrentsourcemaynotbehighenoughtodrivethe
setcurrent.Thisconditioncausesverylargeerrorsincomputed
sheet resistance.
8.3.1 Ensure that the measuring circuit contains a direct
readingammeter(seeTestMethodF390,5.2.4),permittingthe
operator to verify the true current flow.
8.3.2 Alternatively, provide electronic means to divide the
measured voltage by the measured current. This ratio may be
provideddigitallyorbyadual-slopeintegratingvoltmeterwith
reference voltage inputs.
8.4 Avoid Arcing On the Film—Astheprobesaremakingor
breakingcontactwiththefilm,thevoltagedrivingtheconstant
current source can cause arcing damage to the film and the
FIG. 3 Sheet Resistance Reference Specimen
probes. To avoid arcing, keep the constant current supply
voltage low or provide switching preventing application of
current supply voltage until after contact is made with the film
electrode for reference films of 20 Ω per square or greater.
under test.
Referencefilmslessthan20Ωpersquareshouldhaveacopper
wire soldered to the lengths of the bus electrodes, or should
NOTE 1—Ten-volt potential typically does not cause visible arcing
damage, but 100 volt potential often does.
have the thickness of the copper film electrodes increased
proportionately.
8.5 Fabrication and Use of Sheet-Resistance Reference
8.5.4.2 The sheet resistance of the reference film may be
Specimens—It is useful to maintain sheet-resistance reference
cali
...
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: F1711 − 96 (Reapproved 2016)
Standard Practice for
Measuring Sheet Resistance of Thin Film Conductors for
Flat Panel Display Manufacturing Using a Four-Point Probe
Method
This standard is issued under the fixed designation F1711; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This practice describes methods for measuring the sheet
F390 Test Method for Sheet Resistance of Thin Metallic
electrical resistance of sputtered thin conductive films depos-
Films With a Collinear Four-Probe Array
ited on large insulating substrates, used in making flat panel
information displays. It is assumed that the thickness of the
3. Terminology
conductive thin film is much thinner than the spacing of the
contact probes used to measure the sheet resistance.
3.1 Definitions:
3.1.1 For definitions of terms used in this practice see Test
1.2 This standard is intended to be used with Test Method
Method F390.
F390.
1.3 Sheet resistivity in the range 0.5 to 5000 ohms per
4. Summary of Practice
square may be measured by this practice. The sheet resistance
4.1 This practice describes the preferred means of applying
is assumed uniform in the area being probed.
Test Method F390 to measure the electrical sheet resistance of
thin films on very large flat substrates. An array of four pointed
1.4 This practice is applicable to flat surfaces only.
probes is placed in contact with the film of interest. A measured
1.5 Probe pin spacings of 1.5 mm to 5.0 mm, inclusive
electrical current is passed between two of the probes, and the
(0.059 to 0.197 in inclusive) are covered by this practice.
electrical potential difference between the remaining two
probes is determined. The sheet resistance is calculated from
1.6 The method in this practice is potentially destructive to
the measured current and potential values using correction
the thin film in the immediate area in which the measurement
factors associated with the probe geometry and the probe’s
is made. Areas tested should thus be characteristic of the
distance from the test specimen’s boundaries.
functional part of the substrate, but should be remote from
critical active regions. The method is suitable for characteriz-
4.2 The method of F390 is extended to cover staggered
ing dummy test substrates processed at the same time as
in-line and square probe arrays. In all the designs, however, the
substrates of interest. probe spacings are nominally equal.
1.7 The values stated in SI units are to be regarded as the 4.3 This practice includes a special electrical test for veri-
fying the proper functioning of the potential measuring instru-
standard. The values given in parentheses are for information
ment (voltmeter), directions for making and using sheet resis-
only.
tance reference films, an estimation of measurement error
1.8 This standard does not purport to address all of the
caused by probe wobble in the probe supporting fixture, and a
safety concerns, if any, associated with its use. It is the
protocol for reporting film uniformity.
responsibility of the user of this standard to establish appro-
4.4 Two appendices indicate the computation methods em-
priate safety and health practices and determine the applica-
ployed in deriving numerical relationships and correction
bility of regulatory limitations prior to use.
factors employed in this practice, and in Test Method F390.
This practice is under the jurisdiction of ASTM Committee F01 on Electronics
and is the direct responsibility of Subcommittee F01.17 on Sputter Metallization. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2016. Published May 2016. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1996. Last previous edition approved in 2008 as F1711 – 96(2008). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/F1711-96R16. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1711 − 96 (2016)
5. Significance and Use 5.1.4 It is difficult, given the conditions cited in 5.1.3, to
ensure that uniform probe spacing is not degraded by rough
5.1 Applying Test Method F390 to large flat panel substrates
handling of the equipment. The phased square array, described,
presents a number of serious difficulties not anticipated in the
averages out probe placement errors.
development of that standard. The following problems are
5.1.5 This practice is estimated to be precise to the follow-
encountered.
ing levels. Otherwise acceptable precision may be degraded by
5.1.1 The four-point probe method may be destructive to the
probe wobble, however (see 8.6.4).
thin film being measured. Sampling should therefore be taken
5.1.5.1 As a referee method, in which the probe and
close to an edge or corner of the plate, where the film is
measuring apparatus are checked and qualified before use by
expendable. Special geometrical correction factors are then
the procedures of Test Method F390 paragraph 7 and this
required to derive the true sheet resistance.
practice, paragraph 8: standard deviation, s, from measured
5.1.2 Test Method F390 is limited to a conventional col-
sheet resistance, R , is ≤ 0.01 R .
S S
linear probe arrangement, but a staggered collinear and square
5.1.5.2 As a routine method, with periodic qualifications of
arrays are useful in particular circumstances. Correction factors
probe and measuring apparatus by the procedures of Test
are needed to account for nonconventional probe arrange-
Method F390 paragraph 7 and this practice, paragraph 8:
ments.
standard deviation, s, from measured sheet resistance, R , is ≤
S
5.1.3 Test Method F390 anticipates a precision testing
0.02 R .
arrangement in which the probe mount and sample are rigidly S
positioned. There is no corresponding apparatus available for
6. Apparatus
testing large glass or plastic substrates. Indeed, it is common in
flat panel display making that the probe is hand held by the
6.1 Probe Assembly:
operator.
6.1.1 The probe assembly must meet the apparatus require-
ments of F390, 5.1.1 – 5.1.3.
6.1.2 Four arrangements of probe tips are covered in this
practice:
6.1.2.1 In-Line, Collinear, Probe Tips, with current flowing
between the outer two probes (see Fig. 1A). This is the
conventional arrangement specified in Test Method F390.
6.1.2.2 Staggered Collinear Probe Tips, with current flow-
ing between one outer and one interior probe (see Fig. 1B).
This arrangement is sometimes used as a check to verify the
results of a conventional collinear measurement (see 6.1.2.1).
6.1.2.3 Square Array, with current conducted between two
adjacent probe tips (see Fig. 1C).
6.1.2.4 Phased Square Array, with current applied alter-
nately between opposite pairs of tips (see Fig. 1D). This
arrangement has the advantage of averaging out errors caused
by unequal probe spacing.
6.1.3 Probe Support— The probe support shall be designed
in such a manner that the operator can accurately lower the
probes perpendicularly onto the surface and provide a repro-
ducible probe force for each measurement. Spring loading or
gravity probe pin loading are acceptable.
6.2 Electrical Measuring Apparatus— The electrical appa-
ratus must meet the apparatus requirements of Test Method
F390, 5.2.1 through 5.2.4.
6.3 Specimen Support— The substrate to be tested must be
supported firmly.
6.4 Additional Apparatus:
6.4.1 If measurements will be made within a distance of 20
times the probe spacing from an insulating or highly conduc-
tive edge or corner (20 × S , where i = 1, 2, 3, or 4, with
i
reference to Fig. 1), an instrument capable of measuring the
distance from the probe array position to the insulating or
highly conductive boundary within 60.25 mm (60.010 in) is
required. In most instances a vernier depth gage is suitable.
6.4.2 Toolmaker’s Microscope, capable or measuring incre-
FIG. 1 Four-Point Probe Configurations ments of 2.5 µm.
F1711 − 96 (2016)
7. Test Specimen films on soda lime glass substrates. The surface of this glass
can be somewhat electrically conductive (on the order of
7.1 The test article shall be either a display substrate that has
6 2
1 × 10 Ω ) when the ambient relative humidity is about 90 %
been sputter coated with the thin film of interest, or,
or higher.
alternatively, a dummy plate coated in the same operation as
the substrate of interest. 7.5.1 The glass conductivity degradation may interfere with
the sheet resistance measurement when specimen sheet resis-
7.2 The conductive film must be thick enough that it is
tivity is 1000 Ω/square or higher.
continuous. Generally this requires that the film be at least 15
nm (150Å) thick. 7.5.2 Ensure that films >1000 Ω/square sheet resistance
deposited on soda lime glass are conditioned at less than 50 %
7.3 The area to be tested shall be free of contamination and
humidity for at least 48 h prior to measurement, and that the
mechanical damage, but shall not be cleaned or otherwise
measurement is performed at an ambient relative humidity less
prepared.
than 50 %.
7.4 Note that a sputtered film may also coat the edge of the
7.5.3 Note that at relative humidity less than 50 % the
glass and can coat the back side of the substrate (“over spray”).
surface resistance of soda lime glass in on the order of
Thus the edge of the glass cannot be automatically assumed to
1 × 10 Ω/ square.
be insulating. If sheet resistance determinations will be made
within a distance of 20 times the probe spacing to an edge of
8. Suitability of Test Equipment
the substrate it is necessary to ensure that the film terminates at
the edge.
8.1 Equipment Qualification—The probe assembly and the
7.4.1 To eliminate over spray error in compensating for
electrical equipment must be qualified for use as specified in
edge effects at an insulating boundary (see 10.2.2), either make
Test Method F390, paragraphs 7.1 through 7.2.3.3 on suitabil-
a fresh cut of the substrate, grind the edge to remove any
ity.
residual film, or etch the film from the edge.
7.4.2 Scribing the substrate near the edge using a glass
8.2 Voltmeter Malfunctions—Modern solid state voltmeters
scribe is not a reliable remedy.
using field effect transistors in the signal input circuitry are
7.4.3 Use a simple 2-point probe ohmeter to verify that the
electrically fragile; failure of a field effect transistor degrades
substrate edge is insulating.
the input impedance. This failure mode is a particular hazard if
input protection is not provided and if films with static charges
7.5 Soda Lime Glass Substrates —Special precautions may
are probed. It is recommended that the error from the voltmeter
be required in measuring the sheet resistance of sputtered thin
input impedance be checked periodically using the test circuit
illustrated in Fig. 2.
8.2.1 Input Impedance Error—To measure the input imped-
ance error, set the constant current, I, and take the voltage
reading, V. Then, without changing I, make a second reading,
V , with R shorted (close switch IMP, Fig. 2). The impedance
d d
error for R >> R is approximately as follows:
imp v
E 5 V 2 V /V 3 100 (1)
@~ ! #
imp d d
where:
E = the percentage voltage error contributed by the finite
imp
voltmeter input impedance.
8.2.2 Common Mode Rejection Error—State of the art
voltmeters typically have high common mode rejection (on the
order of 90 dB), but this may be degraded by the failure of a
field effect transistor in the input circuit (8.2). Reduction of
common mode rejection will cause errors in measuring sheet
resistance if unequal probe contact resistances contribute high
NOTE 1—Set R = approximately the resistance measured on the speci-
v
common mode voltages. Common mode rejection error may be
men film of interest as follows:
R = R = R measured using the test circuit shown in Fig. 2.
a b v
R = 100 × R .
d v
8.2.2.1 To measure the common mode rejection error, set
NOTE 2—Set I approximately the same as used for measurement of the
the constant current, I, and take the voltage reading, V. Then,
specimen film of interest, typically 0.05 to 0.50 mA, so that V is
without changing I, make a second reading, V , with R shorted
comparable to that obtained in performing the sheet resistance determi-
a a
nation. (close switch CMR ), and finally complete a third reading, V ,
a b
NOTE 3—If R is set equal to a multiple of In2/2π for the in line probe
v with R shorted (open CMR , close CMR ). The common mode
b a b
of Fig. 1A, or In2/2π for a square array, then the magnitude of V is the
error is approximately as follows:
sheet resistance value for an equivalent film measurement.
2 2 1/2
FIG. 2 Voltmeter Test Circuit E 5 $1/2@~V 2 V! 1~V 2 V! # %/V 3 100 (2)
cm a b
F1711 − 96 (2016)
where:
E = the percentage voltage error contributed by common
cm
mode voltages. The voltmeter must be repaired or
replaced if E exceeds 0.5 %.
cm
8.3 Voltage Limited Constant Current Supply—In cases of
high sheet resistance or high contact resistance, the voltage at
the constant current source may not be high enough to drive the
set current. This condition causes very large errors in computed
sheet resistance.
8.3.1 Ensure that the measuring circuit contains a direct
reading ammeter (see Test Method F390, 5.2.4), permitting the
operator to verify the true current flow.
8.3.2 Alternatively, provide electronic means to divide the
measured voltage by the measured current. This ratio may be
provided digitally or by a dual-slope integrating voltmeter with
reference voltage inputs.
8.4 Avoid Arcing On the Film—As the probes are making or
breaking contact with the film, the voltage driving the constant
current source can cause arcing damage to the film and the
FIG. 3 Sheet Resistance Reference Specimen
probes. To avoid arcing, keep the constant current supply
voltage low or provide switching preventing application of
current supply voltage until after contact is made with the film
electrode for reference films of 20 Ω per square or greater.
under test.
Reference films less than 20 Ω per square should have a copper
wire soldered to the lengths of the bus electrodes, or should
NOTE 1—Ten-volt potential typically does not cause visible arcing
damage, but 100 volt potential often does.
have the thickness of the copper film electrodes increased
proportionatel
...
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