1、Designation: F 1844 97 (Reapproved 2002)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 F 1844; the number immediately following the designation indicat
2、es 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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes methods for measurin
3、g the 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 MethodsF 673. This practice pertains to a “manual” measurementprocedure i
4、n which 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 F 673(paragraphs 3.1 th
5、rough 3.3.3 of Test Methods F 673) areapplicable to this practice.1.3 Sheet resistivity in the range 0.020 to 3000 V per 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 1T
6、ypical manual 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
7、uniformity.1.6 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. R
8、eferenced Documents2.1 ASTM Standards:2F 673 Test Methods for Measuring Resistivity of Semicon-ductor Slices or Sheet Resistance of Semiconductor Filmswith a Noncontact Eddy-Current Gage3. Summary of Practice3.1 This practice describes the preferred means of applyingTest Methods F 673 to measure the
9、 electrical sheet resistanceof thin films on very large, flat, nonconducting substrates. Thesubstrate, oriented with the conducting thin film up, is placedbetween the transducers of the eddy current sensor assembly atthe point of interest. The test arrangement is illustrated in Fig.1.3.2 A typical c
10、onductance apparatus is described in detail ina paper by Miller, Robinson, and Wiley.3This paper alsodiscusses skin-depth as a function of thickness and resistivity.3.3 A typical apparatus operates as follows: when a speci-men is inserted into the fixed gap between the two parallelsensing elements,
11、or transducers, in a special oscillator circuit,eddy currents are induced in the specimen by the alternatingfield between the transducers. The current needed to maintainconstant voltage in the oscillator is determined internally; thiscurrent is a function of the specimen conductance.3.4 Further deta
12、ils are given in Test Methods F 673, para-graphs 3.1 through 3.3.3.3.5 This practice includes calibration procedures for usingNIST Silicon Standard Reference Material4to ensure properoperation before testing panels.4. Significance and Use4.1 Resistivity is a primary quantity for characterization and
13、specification of coated 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
14、 thinfilm being measured. Special geometrical correction factors,needed for some four-point probe electrical resistivity mea-surements, are not required to derive the true sheet resistance1This practice is under the jurisdiction of ASTM Committee F01 on Electronicsand is the direct responsibility of
15、 Subcommittee F01.17 on Sputter Metallization.Current edition approved Dec. 10, 2002. Published May 2003. Originallyapproved in 1997. Last previous edition approved iin 1997 as F 1844 97.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at servicea
16、stm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Miller, G.L., Robinson, D. A. H., and Wiley, J. D., “Contactless Measurementof Semiconductor Conductivity by Radio Frequency-Free-Carrier Power Absorp-tion,” Review of Scien
17、tific Instruments, Vol 47, No. 7, July 1976.4Available from NIST, 100 Bureau Dr., Stop 3460, Gaithersburg, MD 20899.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.so long as the transducers have a continuous layer of conduc-tive thi
18、n film between them.4.4 Test Methods F 673 refers to a testing arrangement inwhich the transducers and specimen (a semiconductor gradesilicon wafer) are rigidly positioned. Similar apparatus iscommercially available for testing large glass or plastic sub-strates, not envisioned in the scope of Test
19、Methods F 673. Ahand held probe can also be used, depending on throat depthrequired.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 F 673 (9.1.1 through 9.1.3 and9.1.4.2 through 9.1.4.5), then this prac
20、tice is used.4.6 For use as a routine quality assurance method, thispractice may be employed with periodic qualifications of probeand measuring apparatus by the procedures of Test MethodsF 673 (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 qualific
21、ationintervals for the test apparatus.5. Apparatus5.1 Eddy Current Sensor AssemblySee Fig. 1 and TestMethods F 673.5.1.1 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
22、these are denoted “High” (15 to 3000 V per square),“Low” (0.2 to 15 V per square) and “Extra Low” (0.035 to 0.2 V 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. A very large transducer,
23、63.5-mm diameter, may be usedfor all ranges for thicker than normal substrates (up to approximately 2.54mm) and for calibration and measurement ease.5.2 Electrical Measuring ApparatusThe electrical appa-ratus must meet the requirements of Test Methods F 673,paragraphs 6.1 through 6.4.5.3 Specimen Su
24、pportThe flat panel to be tested must besupported firmly to ensure that the thin film is parallel with thetransducer surfaces.5.4 Reagents and Materials in accordance with Test Meth-ods F 673, Section 7.6. Test Specimen6.1 The test article shall be a display substrate that has beensputter coated wit
25、h the conductive thin film of interest or ionimplanted and annealed, or made conductive 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 andmechan
26、ical damage, but shall not be cleaned or otherwiseprepared.6.4 Note that a sputtered 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
27、 will be included in themeasurement, lowering the measured film resistivity.6.4.2 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 Ca
28、ution must be taken that the transducer gap is fixed, inaccordance with the recommendations 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.3)are no
29、t damaged by the panel under test. The electrostatic covershould be located approximately 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 type
30、s ofresistivity or sheet resistance techniques, which are responsiveto a finite lateral area. The results may therefore differ fromthose of four-probe measurements depending on film proper-ties and the four-probe spacing used (see Test Methods F 673paragraph 5.1).7.3 Spurious currents can be introdu
31、ced 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 673 paragraph 5.4)7.4 Soda Lime Glass SubstratesSpecial ca
32、re 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 3106V per square) when the ambient relative humidity is 90 %or higher.7.4.1 The glass conductivity degradation
33、 may interfere withthe sheet resistance measurement when specimen sheet resis-tivity is 1000 V/square or higher.NOTEThis figure is partially copied from Fig. 1 of Test MethodsF 673.FIG. 1 Schematic Diagram of Eddy-Current Sensor AssemblyF 1844 97 (2002)27.4.2 Ensure that films 1000 V/square sheet re
34、sistancedeposited on soda lime glass are conditioned at less than 50 %humidity for at least 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
35、 1 by 1012V/square.8. Procedure for Fabrication and Use of Sheet ResistanceReference Specimens8.1 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
36、 75mm are suitable substrates. The roughness of the etched surfacegreatly improves abrasion 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 che
37、mical stability and abrasion resistance, andsheet resistance on the order of 1500 V/square.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
38、.3 A double layer of nominally 100-nm sputteredindium-tin oxide at 90/10 composition ratio covered with40-nm doped tin oxide (paragraph 8.1.2) for abrasion resis-tance forms a satisfactory reference film in the 25-V/squaresheet resistance range. The photo conductive effect is negli-gible, but films
39、may exhibit long term resistivity drift. Periodicrecalibration is required.8.1.4 After applying 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 fi
40、lm is thus a nominally50 by 50 mm square.8.1.4.1 A sputtered chromium adhesion layer, nominally100-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 V
41、 per square or greater. Reference filmsless than 20 V per square should have a copper wire solderedto 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-
42、point or 4-point method, using the bus barsas contact lines. 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 7.2-7.4.2pertain to sheet resistance reference specimens.8.1.6 The probe an
43、d associated measuring apparatus arechecked by applying 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 MethodsF
44、 673 paragraphs 9.1.1 through 9.1.3.9.2 Position the center 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
45、Test Methods F 673 paragraphs 9.1.4.2through 9.1.4.5.10. Measurement 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 illustrate
46、d in Fig. 3.10.2 Measurement Procedure:10.2.1 Because of differences in manufacturers recom-mended procedures for the various flat panel display measuringFIG. 2 Calibration SampleF 1844 97 (2002)3instruments, refer to the operators manual for explicit mea-surement instructions for the equipment empl
47、oyed. The stepsfollowing are general statements, intended to outline theprocedure.10.2.2 Manually zero 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 conduct
48、ivethin film side “up” (in the same position as shown in Fig. 3) sothat the region to be measured is 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
49、 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 identification, and11.1.2.3 Brief description of visual appearance and physicalcondition,11.1.3 Dimensions and data, including:11.1.3.1 Length and width of specimen,11.1.3.2 Description of eddy current instrument incl
