1、Designation: F1844 97 (Reapproved 2016)Standard Practice forMeasuring Sheet Resistance of Thin Film Conductors ForFlat Panel Display Manufacturing Using a Noncontact EddyCurrent Gage1This standard is issued under the fixed designation F1844; the number immediately following the designation indicates
2、 the 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 () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes methods for measuring t
3、he sheetelectrical resistance of sputtered thin conductive films depos-ited on large insulating substrates (glass or plastic), used inmaking flat panel information displays.1.2 This practice is intended to be used with Test MethodsF673. This practice pertains to a “manual” measurementprocedure in wh
4、ich an operator positions the measuring headon the test specimen and then personally activates the testapparatus. The resulting test data may be tabulated by theoperator, or, alternatively, sent to a computer-based datalogging system. Both Methods I and II of Test Methods F673(paragraphs 3.1 through
5、 3.3.3 of Test Methods F673) areapplicable to this practice.1.3 Sheet resistivity in the range 0.020 to 3000per square(sheet conductance in the range 3 by 104to 50 mhos persquare) may be measured by this practice. The sheet resistanceis assumed to be uniform in the area being probed.NOTE 1Typical ma
6、nual test units, as described in this practice,measure and report in the units “mhos per square”; this is the inverse of“ohms per square.”1.4 This practice is applicable to flat surfaces only.1.5 This practice is non-destructive. It may be used onproduction panels to help assure production uniformit
7、y.1.6 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.7 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 ap
8、pro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2F673 Test Methods for Measuring Resistivity of Semicon-ductor Slices or Sheet Resistance of Semiconductor Filmswith a Noncontact Eddy-Current Gage
9、 (Withdrawn 2003)33. Summary of Practice3.1 This practice describes the preferred means of applyingTest Methods F673 to measure the electrical sheet resistance ofthin films on very large, flat, nonconducting substrates. Thesubstrate, oriented with the conducting thin film up, is placedbetween the tr
10、ansducers of the eddy current sensor assembly atthe point of interest. The test arrangement is illustrated in Fig.1.3.2 Atypical conductance apparatus is described in detail ina paper by Miller, Robinson, and Wiley.4This paper alsodiscusses skin-depth as a function of thickness and resistivity.3.3 A
11、 typical apparatus operates as follows: when a speci-men is inserted into the fixed gap between the two parallelsensing elements, or transducers, in a special oscillator circuit,eddy currents are induced in the specimen by the alternatingfield between the transducers. The current needed to maintainc
12、onstant voltage in the oscillator is determined internally; thiscurrent is a function of the specimen conductance.3.4 Further details are given in Test Methods F673, para-graphs 3.1 through 3.3.3.3.5 This practice includes calibration procedures for usingNIST Silicon Standard Reference Material5to e
13、nsure properoperation before testing panels.1This practice is under the jurisdiction of ASTM Committee F01 on Electronicsand is the direct responsibility of Subcommittee F01.17 on Sputter Metallization.Current edition approved May 1, 2016. Published May 2016. Originallyapproved in 1997. Last previou
14、s edition approved in 2008 as F1844 97(2008).DOI: 10.1520/F1844-97R16.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM
15、 website.3The last approved version of this historical standard is referenced onwww.astm.org.4Miller, G.L., Robinson, D. A. H., and Wiley, J. D., “Contactless Measurementof Semiconductor Conductivity by Radio Frequency-Free-Carrier PowerAbsorption,” Review of Scientific Instruments , Vol 47, No. 7,
16、July 1976.5Available from NIST, 100 Bureau Dr., Stop 3460, Gaithersburg, MD 20899.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14. Significance and Use4.1 Resistivity is a primary quantity for characterization andspecification of co
17、ated glass plates used for flat panel displays.Sheet resistance is also a primary quantity for characterization,specification, and monitoring of thin film fabrication processes.4.2 This practice requires no specimen preparation.4.3 The eddy current method is non-destructive to the thinfilm being mea
18、sured. Special geometrical correction factors,needed for some four-point probe electrical resistivitymeasurements, are not required to derive the true sheetresistance so long as the transducers have a continuous layer ofconductive thin film between them.4.4 Test Methods F673 refers to a testing arra
19、ngement inwhich the transducers and specimen (a semiconductor gradesilicon wafer) are rigidly positioned. Similar apparatus iscommercially available for testing large glass or plasticsubstrates, not envisioned in the scope of Test Methods F673.Ahand held probe can also be used, depending on throat d
20、epthrequired.4.5 For use as a referee method, the probe and measuringapparatus must first be checked and qualified before use by theprocedures of Test Methods F673 (9.1.1 through 9.1.3 and9.1.4.2 through 9.1.4.5), then this practice is used.4.6 For use as a routine quality assurance method, thisprac
21、tice may be employed with periodic qualifications of probeand measuring apparatus by the procedures of Test MethodsF673 (9.1.1 through 9.1.3 and 9.1.4.2 through 9.1.4.5). Theparties to the test must agree upon adequate qualificationintervals for the test apparatus.5. Apparatus5.1 Eddy Current Sensor
22、 Assembly See Fig. 1 and TestMethods F673.5.1.1 Different transducer designs may be required to coverthe full range of sheet resistance values.NOTE 2Three transducers will generally cover the ranges of interest.For convenience these are denoted “High” (15 to 3000 per square),“Low” (0.2 to 15 per squ
23、are) and “Extra Low” (0.035 to 0.2 persquare).NOTE 3The usual “High” range transducer diameter is approximately12.7 to 15.2 mm. The “Low” and “Extra Low” diameter is approximately10.1 to 12.7 mm.Avery large transducer, 63.5-mm diameter, may be usedfor all ranges for thicker than normal substrates (u
24、p to approximately 2.54mm) and for calibration and measurement ease.5.2 Electrical Measuring Apparatus The electrical appa-ratus must meet the requirements of Test Methods F673,paragraphs 6.1 through 6.4.5.3 Specimen Support The flat panel to be tested must besupported firmly to ensure that the thin
25、 film is parallel with thetransducer surfaces.5.4 Reagents and Materials in accordance with Test Meth-ods F673, Section 7.6. Test Specimen6.1 The test article shall be a display substrate that has beensputter coated with the conductive thin film of interest or ionimplanted and annealed, or made cond
26、uctive by anotherprocess.6.2 The conductive film must be thick enough that it iscontinuous. Generally this requires that the film be at least15-nm thick.6.3 The area to be tested shall be free of contamination andmechanical damage, but shall not be cleaned or otherwiseprepared.6.4 Note that a sputte
27、red film may also coat the edge of theglass and can coat the back side of the substrate (“overspray”).All overspray, for example, coating on back of glass, must beremoved before measurement.6.4.1 Any remaining overspray will be included in themeasurement, lowering the measured film resistivity.6.4.2
28、 Scribing the substrate near the edge using a glassscribe is not a reliable remedy.6.4.3 Use a simple 2-point probe ohmmeter to verify that theback side of glass or plastic substrate is insulating.7. Interferences7.1 Caution must be taken that the transducer gap is fixed, inaccordance with the recom
29、mendations of the equipment sup-plier. This may be ensured by firmly tightening the gapadjustment screws after checking the spacing with gages. Usecaution, too, that the electrostatic covers (see Miller, et. al.4)are not damaged by the panel under test.The electrostatic covershould be located approx
30、imately 0.02 mm below the supportsurface.7.2 Radial resistivity variations or other resistivity nonuni-formity under the transducer are averaged by this practice in amanner that may be different from that of other types ofresistivity or sheet resistance techniques, which are responsiveto a finite la
31、teral area. The results may therefore differ fromNOTE 1This figure is partially copied from Fig. 1 of Test MethodsF673.FIG. 1 Schematic Diagram of Eddy-Current Sensor AssemblyF1844 97 (2016)2those of four-probe measurements depending on film proper-ties and the four-probe spacing used (see Test Meth
32、ods F673paragraph 5.1).7.3 Spurious currents can be introduced in the test equip-ment when it is located near high-frequency generators. If theequipment is located near such sources, adequate shieldingmust be provided. Power line filtering may also be required.(Note the precautions in Test Methods F
33、673 paragraph 5.4)7.4 Soda Lime Glass SubstratesSpecial care may berequired in measuring the sheet resistance of sputtered thinfilms on soda lime glass substrates. The surface of this glasscan be somewhat electrically conductive (on the order of 1 106 per square) when the ambient relative humidity i
34、s 90 %or higher.7.4.1 The glass conductivity degradation may interfere withthe sheet resistance measurement when specimen sheet resis-tivity is 1000 /square or higher.7.4.2 Ensure that films 1000 /square sheet resistancedeposited on soda lime glass are conditioned at less than 50 %humidity for at le
35、ast 48 hs prior to measurement, and that themeasurement is performed at an ambient relative humidity ofless than 50 %.NOTE 4At relative humidity 50 % of the surface resistance of sodalime glass is on the order of 1 by 1012/square.8. Procedure for Fabrication and Use of Sheet ResistanceReference Spec
36、imens8.1 It is useful to maintain sheet resistance reference speci-mens for use in verifying the proper performance of themeasuring apparatus (see Fig. 2).8.1.1 Rectangular sheets of etched glass nominally 50 by 75mm are suitable substrates.The roughness of the etched surfacegreatly improves abrasio
37、n resistance.8.1.2 The reference film, applied to the substrate, may be anominally 40-nm thick sputtered tin-oxide coating, doped withnominally 5 wt. % antimony or fluorine. This material dem-onstrates good chemical stability and abrasion resistance, andsheet resistance on the order of 1500 /square.
38、8.1.2.1 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 before each use.8.1.3 A double layer of nominally 100-nm sputteredindium-tin oxide at 90/10 composition ratio
39、covered with40-nm doped tin oxide (paragraph 8.1.2) for abrasion resis-tance forms a satisfactory reference film in the 25-/squaresheet resistance range. The photo conductive effect isnegligible, but films may exhibit long term resistivity drift.Periodic recalibration is required.8.1.4 After applyin
40、g the reference film, highly conductivebus bars, nominally 12.5 mm wide, are deposited over the filmalong two opposite“ short” edges of the substrate, as illustratedin Fig. 2. The free conducting area of film is thus a nominally50 by 50 mm square.8.1.4.1 A sputtered chromium adhesion layer, nominall
41、y100-nm thick, upon which is sputtered a thick copper conduc-tive layer nominally 1000 nm with a sheet resistance of 50milliOhms per square or less is a satisfactory bus electrode forreference films of 20 per square or greater. Reference filmsless than 20 per square should have a copper wire soldere
42、dFIG. 2 Calibration SampleF1844 97 (2016)3to the lengths of the bus electrodes, or should have thethickness of the copper film electrodes increased proportion-ately.8.1.4.2 The sheet resistance of the reference film may becalibrated using a 2-point or 4-point method, using the bus barsas contact lin
43、es. The measured V/I ratio is the sheet resistancefor the square reference sample. No correction factors arerequired.8.1.5 The conditions and precautions prescribed in 7.2 7.4.2 pertain to sheet resistance reference specimens.8.1.6 The probe and associated measuring apparatus arechecked by applying
44、the measuring procedure (Section 10), tothe reference film with the probe near the center of thereference film.9. Calibrations9.1 If using a NIST Standard Reference Material (SRM) (orspecimen as prepared under Section 8), refer to Test MethodsF673 paragraphs 9.1.1 through 9.1.3.9.2 Position the cent
45、er plane of the thickness of the S.R.M.between the eddy current transducers in the same plane as thatoccupied by the conductive film of the flat panel during actualsheet resistance measurements. This is illustrated in Fig. 3.9.3 Continue with Test Methods F673 paragraphs 9.1.4.2through 9.1.4.5.10. M
46、easurement Procedure10.1 Position the flat panel so the surface of the thin film isfacing “up” and is parallel with the flat surfaces of thetransducers, with the thin film plane lined up where thecalibration SRM center was. This is illustrated in Fig. 3.10.2 Measurement Procedure:10.2.1 Because of d
47、ifferences in manufacturers recom-mended procedures for the various flat panel display measuringinstruments, refer to the operators manual for explicit mea-surement instructions for the equipment employed. The stepsfollowing are general statements, intended to outline theprocedure.10.2.2 Manually ze
48、ro the instrument or record theresistivity/conductivity meter reading. If reading the meterwithout zeroing the instrument, this is the “offset.”10.2.3 Place flat panel display on base with the conductivethin film side “up” (in the same position as shown in Fig. 3)sothat the region to be measured is
49、roughly centered in thetransducers “footprint” area.10.3 If previously zeroed, the reading taken after the flatpanel display is placed between the transducer heads is thesheet conductance in mhos per square.10.4 If not previously zeroed, subtract the “offset” from thesecond reading.The result is the sheet conductance in mhos persquare.11. Report11.1 For a referee test the report shall contain the following:11.1.1 Operator name, date, description of test equipment,11.1.2 A description of the specimen, including:11.1.2.1 Type of film,11.1.2.2 Specimen
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