1、Designation: F 1711 96 (Reapproved 2002)Standard Practice forMeasuring Sheet Resistance of Thin Film Conductors forFlat Panel Display Manufacturing Using a Four-Point ProbeMethod1This standard is issued under the fixed designation F 1711; the number immediately following the designation indicates th
2、e year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes methods for measuring the
3、 sheetelectrical resistance of sputtered thin conductive films depos-ited on large insulating substrates, used in making flat panelinformation displays. It is assumed that the thickness of theconductive thin film is much thinner than the spacing of thecontact probes used to measure the sheet resista
4、nce.1.2 This standard is intended to be used with Test MethodF 390.1.3 Sheet resistivity in the range 0.5 to 5000 ohms persquare may be measured by this practice. The sheet resistanceis assumed uniform in the area being probed.1.4 This practice is applicable to flat surfaces only.1.5 Probe pin spaci
5、ngs 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 tothe thin film in the immediate area in which the measurementis made. Areas tested should thus be characteristic of thefunctional part of the subst
6、rate, but should be remote fromcritical active regions. The method is suitable for characteriz-ing dummy test substrates processed at the same time assubstrates of interest.1.7 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.
7、8 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Doc
8、uments2.1 ASTM Standards:F 390 Test Method for Sheet Resistance of Thin MetallicFilms With a Collinear Four-Probe Array23. Terminology3.1 Definitions:3.1.1 For definitions of terms used in this practice see TestMethod F 390.4. Summary of Practice4.1 This practice describes the preferred means of app
9、lyingTest Method F 390 to measure the electrical sheet resistance ofthin films on very large flat substrates. An array of four pointedprobes is placed in contact with the film of interest. A measuredelectrical current is passed between two of the probes, and theelectrical potential difference betwee
10、n the remaining twoprobes is determined. The sheet resistance is calculated fromthe measured current and potential values using correctionfactors associated with the probe geometry and the probesdistance from the test specimens boundaries.4.2 The method of F390 is extended to cover staggeredin-line
11、and square probe arrays. In all the designs, however, theprobe spacings are nominally equal.4.3 This practice includes a special electrical test for veri-fying the proper functioning of the potential measuring instru-ment (voltmeter), directions for making and using sheet resis-tance reference films
12、, an estimation of measurement errorcaused by probe wobble in the probe supporting fixture, and aprotocol for reporting film uniformity.4.4 Two appendices indicate the computation methods em-ployed in deriving numerical relationships and correctionfactors employed in this practice, and in Test Metho
13、d F 390.5. Significance and Use5.1 Applying Test Method F 390 to large flat panel sub-strates presents a number of serious difficulties not anticipatedin the development of that standard. The following problemsare encountered.1This practice is under the jurisdiction of ASTM Committee F01 on Electron
14、icsand is the direct responsibility of Subcommittee F01.17 on Sputter Metallization.Current edition approved Dec. 10, 2002. Published May 2003. Originallyapproved in 1996. Last previous edition approved in 1996 as F 1711 96.2Annual Book of ASTM Standards, Vol 10.04.1Copyright ASTM International, 100
15、 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5.1.1 The four-point probe method may be destructive to thethin film being measured. Sampling should therefore be takenclose to an edge or corner of the plate, where the film isexpendable. Special geometrical correction
16、 factors are thenrequired to derive the true sheet resistance.5.1.2 Test Method F 390 is limited to a conventional col-linear probe arrangement, but a staggered collinear and squarearrays are useful in particular circumstances. Correction factorsare needed to account for nonconventional probe arrang
17、e-ments.5.1.3 Test Method F 390 anticipates a precision testingarrangement in which the probe mount and sample are rigidlypositioned. There is no corresponding apparatus available fortesting large glass or plastic substrates. Indeed, it is common inflat panel display making that the probe is hand he
18、ld by theoperator.5.1.4 It is difficult, given the conditions cited in 5.1.3, toensure that uniform probe spacing is not degraded by roughhandling 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 follow-ing
19、 levels. Otherwise acceptable precision may be degraded byprobe wobble, however (see 8.6.4).5.1.5.1 As a referee method, in which the probe andmeasuring apparatus are checked and qualified before use bythe procedures of Test Method F 390 paragraph 7 and thispractice, paragraph 8: standard deviation,
20、 s, from measuredsheet resistance, RS,is# 0.01 RS.5.1.5.2 As a routine method, with periodic qualifications ofprobe and measuring apparatus by the procedures of TestMethod F 390 paragraph 7 and this practice, paragraph 8:standard deviation, s, from measured sheet resistance, RS,is#0.02 RS.6. Apparat
21、us6.1 Probe Assembly:6.1.1 The probe assembly must meet the apparatus require-ments of F 390, 5.1.1-5.1.3.6.1.2 Four arrangements of probe tips are covered in thispractice:6.1.2.1 In-Line, Collinear, Probe Tips, with current flowingbetween the outer two probes (see Fig. 1A). This is theconventional
22、arrangement specified in Test Method F 390.6.1.2.2 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 theresults of a conventional collinear measurement (see 6.1.2.1).6.1.2.3 Square Arra
23、y, with current conducted between twoadjacent 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). Thisarrangement has the advantage of averaging out errors causedby unequal probe spacing.6.1.3 Probe Support The probe s
24、upport shall be designedin such a manner that the operator can accurately lower theprobes perpendicularly onto the surface and provide a repro-ducible probe force for each measurement. Spring loading orgravity probe pin loading are acceptable.6.2 Electrical Measuring Apparatus The electrical appa-ra
25、tus must meet the apparatus requirements of Test MethodF 390, 5.2.1 through 5.2.4.6.3 Specimen Support The substrate to be tested must besupported firmly.6.4 Additional Apparatus:6.4.1 If measurements will be made within a distance of 20times the probe spacing from an insulating or highly conduc-tiv
26、e edge or corner (20 3 Si, where i = 1, 2, 3, or 4, withreference to Fig. 1), an instrument capable of measuring thedistance from the probe array position to the insulating orhighly conductive boundary within 60.25 mm (60.010 in) isrequired. In most instances a vernier depth gage is suitable.6.4.2 T
27、oolmakers Microscope, capable or measuring incre-ments of 2.5 m.7. Test Specimen7.1 The test article shall be either a display substrate that hasbeen sputter coated with the thin film of interest, or, alterna-tively, a dummy plate coated in the same operation as thesubstrate of interest.7.2 The cond
28、uctive film must be thick enough that it iscontinuous. Generally this requires that the film be at least 15nm (150) thick.FIG. 1 Four-Point Probe ConfigurationsF 1711 96 (2002)27.3 The area to be tested shall be free of contamination andmechanical damage, but shall not be cleaned or otherwiseprepare
29、d.7.4 Note that a sputtered film may also coat the edge of theglass and can coat the back side of the substrate (“over spray”).Thus the edge of the glass cannot be automatically assumed tobe insulating. If sheet resistance determinations will be madewithin a distance of 20 times the probe spacing to
30、 an edge ofthe substrate it is necessary to ensure that the film terminates atthe edge.7.4.1 To eliminate over spray error in compensating foredge effects at an insulating boundary (see 10.2.2), either makea fresh cut of the substrate, grind the edge to remove anyresidual film, or etch the film from
31、 the edge.7.4.2 Scribing the substrate near the edge using a glassscribe is not a reliable remedy.7.4.3 Use a simple 2-point probe ohmeter to verify that thesubstrate edge is insulating.7.5 Soda Lime Glass SubstratesSpecial precautions maybe required in measuring the sheet resistance of sputtered th
32、infilms on soda lime glass substrates. The surface of this glasscan be somewhat electrically conductive (on the order of1 3 106V2) when the ambient relative humidity is about 90 %or higher.7.5.1 The glass conductivity degradation may interfere withthe sheet resistance measurement when specimen sheet
33、 resis-tivity is 1000 V/square or higher.7.5.2 Ensure that films 1000 V/square sheet resistancedeposited on soda lime glass are conditioned at less than 50 %humidity for at least 48 h prior to measurement, and that themeasurement is performed at an ambient relative humidity lessthan 50 %.7.5.3 Note
34、that at relative humidity less than 50 % thesurface resistance of soda lime glass in on the order of 1 3 1012V/ square.8. Suitability of Test Equipment8.1 Equipment QualificationThe probe assembly and theelectrical equipment must be qualified for use as specified inTest Method F 390, paragraphs 7.1
35、through 7.2.3.3 on suitabil-ity.8.2 Voltmeter MalfunctionsModern solid state voltmetersusing field effect transistors in the signal input circuitry areelectrically fragile; failure of a field effect transistor degradesthe input impedance. This failure mode is a particular hazard ifinput protection i
36、s not provided and if films with static chargesare probed. It is recommended that the error from the voltmeterinput impedance be checked periodically using the test circuitillustrated in Fig. 2.8.2.1 Input Impedance ErrorTo measure the input imped-ance error, set the constant current, I, and take th
37、e voltagereading, V. Then, without changing I, make a second reading,Vd, with Rdshorted (close switch IMP, Fig. 2). The impedanceerror for Rimp Rvis approximately as follows:Eimp5 Vd2 V!/Vd# 3 100 (1)where:Eimp= the percentage voltage error contributed by thefinite voltmeter input impedance.8.2.2 Co
38、mmon Mode Rejection ErrorState of the artvoltmeters typically have high common mode rejection (on theorder of 90 dB), but this may be degraded by the failure of afield effect transistor in the input circuit (8.2). Reduction ofcommon mode rejection will cause errors in measuring sheetresistance if un
39、equal probe contact resistances contribute highcommon mode voltages. Common mode rejection error may bemeasured using the test circuit shown in Fig. 2.8.2.2.1 To measure the common mode rejection error, setthe constant current, I, and take the voltage reading, V. Then,without changing I, make a seco
40、nd reading, Va, with Rashorted(close switch CMRa), and finally complete a third reading, Vb,with Rbshorted (open CMRa, close CMRb). The common modeerror is approximately as follows:Ecm5 $1/2Va2 V!21 Vb2 V!2#1/2%/V 3 100 (2)where:Ecm= the percentage voltage error contributed by com-mon mode voltages.
41、 The voltmeter must be re-paired or replaced if Ecmexceeds 0.5 %.8.3 Voltage Limited Constant Current SupplyIn cases ofhigh sheet resistance or high contact resistance, the voltage atthe constant current source may not be high enough to drive theset current. This condition causes very large errors i
42、n computedsheet resistance.NOTE 1Set Rv= approximately the resistance measured on the speci-men film of interest as follows:Ra=Rb=RvRd= 100 3 Rv.NOTE 2Set I approximately the same as used for measurement of thespecimen film of interest, typically 0.05 to 0.50 mA, so that V iscomparable to that obtai
43、ned in performing the sheet resistance determi-nation.NOTE 3If Rvis set equal to a multiple of In2/2p for the in line probeof Fig. 1A, or In2/2p for a square array, then the magnitude of V is thesheet resistance value for an equivalent film measurement.FIG. 2 Voltmeter Test CircuitF 1711 96 (2002)38
44、.3.1 Ensure that the measuring circuit contains a directreading ammeter (see Test Method F 390, 5.2.4), permitting theoperator to verify the true current flow.8.3.2 Alternatively, provide electronic means to divide themeasured voltage by the measured current. This ratio may beprovided digitally or b
45、y a dual-slope integrating voltmeter withreference voltage inputs.8.4 Avoid Arcing On the FilmAs the probes are making orbreaking contact with the film, the voltage driving the constantcurrent source can cause arcing damage to the film and theprobes. To avoid arcing, keep the constant current supply
46、voltage low or provide switching preventing application ofcurrent supply voltage until after contact is made with the filmunder test.NOTE 1Ten-volt potential typically does not cause visible arcingdamage, but 100 volt potential often does.8.5 Fabrication and Use of Sheet-Resistance ReferenceSpecimen
47、sIt is useful to maintain sheet-resistance referencespecimens for use in verifying the proper performance of themeasuring apparatus.8.5.1 Rectangular sheets of etched glass nominally 50 by 75mm (2.0 by 3.0 in) are suitable substrates. The roughness of theetched surface greatly improves abrasion resi
48、stance.8.5.2 The reference film, applied to the substrate, may be anominally 40 nms (400 ) thick sputtered tin oxide coatingdoped with nominally 5 weight % antimony or fluorine. Thismaterial demonstrates good chemical stability and abrasionresistance, and sheet resistance on the order of 1500 V/squa
49、re.8.5.2.1 Tin oxide is a photo conductor with very long carrierlifetimes. Thus the lighting conditions must be controlled toprevent exposure to direct light, or the film must be recali-brated (see 8.5.4.2) before each use.8.5.3 A double layer of nominally 100-nm (1000-) sput-tered indium-tin oxide at 90/10 composition ratio covered with40 nm (400) doped tin oxide (see 8.5.2) for abrasionresistance forms a satisfactory reference film in the 25V/square sheet resistance range. The photo conductive effect isnegligible, but films may exhibit long term resistiv
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