ASTM F1711-96(2002)
(Practice)Standard Practice for Measuring Sheet Resistance of Thin Film Conductors for Flat Panel Display Manufacturing Using a Four-Point Probe
Standard Practice for Measuring Sheet Resistance of Thin Film Conductors for Flat Panel Display Manufacturing Using a Four-Point Probe
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 F 390.
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.
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Designation: F 1711 – 96 (Reapproved 2002)
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 F 1711; 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.
1. Scope Films With a Collinear Four-Probe Array
1.1 This practice describes methods for measuring the sheet
3. Terminology
electrical resistance of sputtered thin conductive films depos-
3.1 Definitions:
ited on large insulating substrates, used in making flat panel
3.1.1 For definitions of terms used in this practice see Test
information displays. It is assumed that the thickness of the
Method F390.
conductive thin film is much thinner than the spacing of the
contact probes used to measure the sheet resistance.
4. Summary of Practice
1.2 This standard is intended to be used with Test Method
4.1 This practice describes the preferred means of applying
F390.
TestMethodF390tomeasuretheelectricalsheetresistanceof
1.3 Sheet resistivity in the range 0.5 to 5000 ohms per
thinfilmsonverylargeflatsubstrates.Anarrayoffourpointed
square may be measured by this practice. The sheet resistance
probesisplacedincontactwiththefilmofinterest.Ameasured
is assumed uniform in the area being probed.
electrical current is passed between two of the probes, and the
1.4 This practice is applicable to flat surfaces only.
electrical potential difference between the remaining two
1.5 Probe pin spacings of 1.5 mm to 5.0 mm, inclusive
probes is determined. The sheet resistance is calculated from
(0.059 to 0.197 in inclusive) are covered by this practice.
the measured current and potential values using correction
1.6 The method in this practice is potentially destructive to
factors associated with the probe geometry and the probe’s
the thin film in the immediate area in which the measurement
distance from the test specimen’s boundaries.
is made. Areas tested should thus be characteristic of the
4.2 The method of F390 is extended to cover staggered
functional part of the substrate, but should be remote from
in-lineandsquareprobearrays.Inallthedesigns,however,the
critical active regions. The method is suitable for characteriz-
probe spacings are nominally equal.
ing dummy test substrates processed at the same time as
4.3 This practice includes a special electrical test for veri-
substrates of interest.
fying the proper functioning of the potential measuring instru-
1.7 The values stated in SI units are to be regarded as the
ment (voltmeter), directions for making and using sheet resis-
standard. The values given in parentheses are for information
tance reference films, an estimation of measurement error
only.
caused by probe wobble in the probe supporting fixture, and a
1.8 This standard does not purport to address all of the
protocol for reporting film uniformity.
safety concerns, if any, associated with its use. It is the
4.4 Two appendices indicate the computation methods em-
responsibility of the user of this standard to establish appro-
ployed in deriving numerical relationships and correction
priate safety and health practices and determine the applica-
factors employed in this practice, and in Test Method F390.
bility of regulatory limitations prior to use.
5. Significance and Use
2. Referenced Documents
5.1 Applying Test Method F390 to large flat panel sub-
2.1 ASTM Standards:
strates presents a number of serious difficulties not anticipated
F390 Test Method for Sheet Resistance of Thin Metallic
in the development of that standard. The following problems
are encountered.
This practice is under the jurisdiction ofASTM Committee F01 on Electronics
and is the direct responsibility of Subcommittee F01.17 on Sputter Metallization.
Current edition approved Dec. 10, 2002. Published May 2003. Originally
approved in 1996. Last previous edition approved in 1996 as F1711–96.
Annual Book of ASTM Standards, Vol 10.04.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F 1711 – 96 (2002)
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, R,is# 0.01 R .
S S
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, R,is#
S
0.02 R .
S
6. Apparatus
6.1 Probe Assembly:
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
FIG. 1 Four-Point Probe Configurations
gravity probe pin loading are acceptable.
6.2 Electrical Measuring Apparatus— The electrical appa-
5.1.1 Thefour-pointprobemethodmaybedestructivetothe
ratus must meet the apparatus requirements of Test Method
thin film being measured. Sampling should therefore be taken
F390, 5.2.1 through 5.2.4.
close to an edge or corner of the plate, where the film is
6.3 Specimen Support— The substrate to be tested must be
expendable. Special geometrical correction factors are then
supported firmly.
required to derive the true sheet resistance.
6.4 Additional Apparatus:
5.1.2 Test Method F390 is limited to a conventional col-
6.4.1 If measurements will be made within a distance of 20
linear probe arrangement, but a staggered collinear and square
times the probe spacing from an insulating or highly conduc-
arraysareusefulinparticularcircumstances.Correctionfactors
tive edge or corner (20 3 S, where i =1, 2, 3, or 4, with
i
are needed to account for nonconventional probe arrange-
reference to Fig. 1), an instrument capable of measuring the
ments.
distance from the probe array position to the insulating or
5.1.3 Test Method F390 anticipates a precision testing
highly conductive boundary within 60.25 mm (60.010 in) is
arrangement in which the probe mount and sample are rigidly
required. In most instances a vernier depth gage is suitable.
positioned. There is no corresponding apparatus available for
6.4.2 Toolmaker’s Microscope, capable or measuring incre-
testinglargeglassorplasticsubstrates.Indeed,itiscommonin
ments of 2.5 µm.
flat panel display making that the probe is hand held by the
7. Test Specimen
operator.
5.1.4 It is difficult, given the conditions cited in 5.1.3, to 7.1 Thetestarticleshallbeeitheradisplaysubstratethathas
ensure that uniform probe spacing is not degraded by rough been sputter coated with the thin film of interest, or, alterna-
handlingoftheequipment.Thephasedsquarearray,described, tively, a dummy plate coated in the same operation as the
averages out probe placement errors. substrate of interest.
5.1.5 This practice is estimated to be precise to the follow- 7.2 The conductive film must be thick enough that it is
inglevels.Otherwiseacceptableprecisionmaybedegradedby continuous. Generally this requires that the film be at least 15
probe wobble, however (see 8.6.4). nm (150Å) thick.
F 1711 – 96 (2002)
humidity for at least 48 h prior to measurement, and that the
measurementisperformedatanambientrelativehumidityless
than 50%.
7.5.3 Note that at relative humidity less than 50% the
surface resistance of soda lime glass in on the order of 1 310
12V/ square.
8. Suitability of Test Equipment
8.1 Equipment Qualification—The probe assembly and the
electrical equipment must be qualified for use as specified in
Test Method F390, paragraphs 7.1 through 7.2.3.3 on suitabil-
ity.
8.2 Voltmeter Malfunctions—Modern solid state voltmeters
using field effect transistors in the signal input circuitry are
electrically fragile; failure of a field effect transistor degrades
theinputimpedance.Thisfailuremodeisaparticularhazardif
NOTE 1—Set R =approximately the resistance measured on the speci-
v input protection is not provided and if films with static charges
men film of interest as follows:
areprobed.Itisrecommendedthattheerrorfromthevoltmeter
R =R =R
a b v
input impedance be checked periodically using the test circuit
R =100 3 R .
d v
illustrated in Fig. 2.
NOTE 2—Set I approximately the same as used for measurement of the
specimen film of interest, typically 0.05 to 0.50 mA, so that V is
8.2.1 Input Impedance Error—Tomeasuretheinputimped-
comparable to that obtained in performing the sheet resistance determi-
ance error, set the constant current, I, and take the voltage
nation.
reading, V. Then, without changing I, make a second reading,
OTE 3—If R is set equal to a multiple of In2/2p for the in line probe
N
v
V , with R shorted (close switch IMP, Fig. 2). The impedance
d d
of Fig. 1A, or In2/2p for a square array, then the magnitude of V is the
error for R >> R is approximately as follows:
sheet resistance value for an equivalent film measurement.
imp v
FIG. 2 Voltmeter Test Circuit
E 5[~V 2 V!/V # 3100 (1)
imp d d
where:
7.3 The area to be tested shall be free of contamination and
E = the percentage voltage error contributed by the
imp
mechanical damage, but shall not be cleaned or otherwise
finite voltmeter input impedance.
prepared.
8.2.2 Common Mode Rejection Error—State of the art
7.4 Note that a sputtered film may also coat the edge of the
voltmeterstypicallyhavehighcommonmoderejection(onthe
glassandcancoatthebacksideofthesubstrate(“overspray”).
order of 90 dB), but this may be degraded by the failure of a
Thus the edge of the glass cannot be automatically assumed to
field effect transistor in the input circuit (8.2). Reduction of
be insulating. If sheet resistance determinations will be made
common mode rejection will cause errors in measuring sheet
within a distance of 20 times the probe spacing to an edge of
resistance if unequal probe contact resistances contribute high
thesubstrateitisnecessarytoensurethatthefilmterminatesat
commonmodevoltages.Commonmoderejectionerrormaybe
the edge.
measured using the test circuit shown in Fig. 2.
7.4.1 To eliminate over spray error in compensating for
edgeeffectsataninsulatingboundary(see10.2.2),eithermake 8.2.2.1 To measure the common mode rejection error, set
the constant current, I, and take the voltage reading, V. Then,
a fresh cut of the substrate, grind the edge to remove any
residual film, or etch the film from the edge. withoutchanging I,makeasecondreading, V ,with R shorted
a a
7.4.2 Scribing the substrate near the edge using a glass (closeswitch CMR ),andfinallycompleteathirdreading, V ,
a b
scribe is not a reliable remedy. withR shorted(openCMR ,closeCMR ).Thecommonmode
b a b
7.4.3 Use a simple 2-point probe ohmeter to verify that the error is approximately as follows:
substrate edge is insulating. 2 2 1/2
E 5 1/2[~V 2 V! 1 ~V 2 V! # /V 3100 (2)
$ %
cm a b
7.5 Soda Lime Glass Substrates—Special precautions may
be required in measuring the sheet resistance of sputtered thin
where:
films on soda lime glass substrates. The surface of this glass
E = the percentage voltage error contributed by com-
can be somewhat electrically conductive (on the order of cm
mon mode voltages. The voltmeter must be re-
1 310 V )whentheambientrelativehumidityisabout90%
paired or replaced if E exceeds 0.5%.
or higher. cm
7.5.1 The glass conductivity degradation may interfere with 8.3 Voltage Limited Constant Current Supply—In cases of
the sheet resistance measurement when specimen sheet resis- high sheet resistance or high contact resistance, the voltage at
tivity is 1000 V/square or higher. theconstantcurrentsourcemaynotbehighenoughtodrivethe
7.5.2 Ensure that films >1000 V/square sheet resistance setcurrent.Thisconditioncausesverylargeerrorsincomputed
deposited on soda lime glass are conditioned at less than 50% sheet resistance.
F 1711 – 96 (2002)
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
breaking contact with the film, the voltage driving the constant
current source can cause arcing damage to the film and the
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
under test.
NOTE 1—Ten-volt potential typically does not cause visible arcing
damage, but 100 volt potential often does.
8.5 Fabrication and Use of Sheet-Resistance Reference
Specimens—It is useful to maintain sheet-resistance reference
specimens for use in verifying the proper performance of the
FIG. 3 Sheet Resistance Reference Specimen
measuring apparatus.
8.5.1 Rectangularsheetsofetchedglassnominally50by75
mm(2.0by3.0in)aresuitablesubstrates.Theroughnessofthe
8.5.5 Theconditionsandprecautionsprescribedin7.2-7.5.3
etched surface greatly improves abrasion resistance.
pertain to sheet resistance reference specimens.
8.5.2 The reference film, applied to the substrate, may be a
8.5.6 The probe and associated measuring apparatus are
nominally 40 nms (400 Å) thick sputtered tin oxide coating
checked by applying the measuring procedure, Sections 9 and
doped with nominally 5 weight% antimony or fluorine. This
10 to the reference film. Probe near the center o
...
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