From ernie@physics.carleton.ca Wed Aug 13 12:01:19 2003 Date: Tue, 12 Aug 2003 16:00:06 -0400 (EDT) From: Ernie Neuheimer To: Kirsten Sachs , Madhu@physics.carleton.ca Subject: Resistivity of Carbon-filled Kapton Measured by Four-point Probe and Resistive Strip Methods. X-IMAPbase: 1426689748 1 Status: O X-Status: X-Keywords: X-UID: 1 Carbon-filled Kapton film, ~1mil thick used as resistive foil in GEM detector readout was measured as to surface resistivity. This film is thought to be Dupont Kapton XC, product number 100XC10E7. 1. Resistivity by Method of Rectangular Strips. -------------------------------------------- Thirteen strips of foil, 2cm. wide and >8cm long were prepared on a paper substrate, Contact to the foil ends was made by a 3M type 1181 electrically conductive adhesive copper tape (contact area about 1.2cm x 2cm). The resistance of the 13 strips was measured after the copper contact ends were rubbed with a rounded, smooth metal point to improve electrical contact between copper and resistive foil. Measured resistances were as follows: Resistance Number of Resistivity sigma^2 Meg. Ohm Squares Ohms per Square 4.28 7.79 549K 49 4.34 8.01 542K 196 4.43 8.08 548K 64 4.61 8.04 573K 289 4.60 7.88 584K 784 4.45 7.99 557K 1 4.32 7.95 543K 169 4.41 8.20 538K 324 4.52 8.39 539K 289 4.11 7.65 537K 361 4.55 8.14 559K 9 4.73 8.24 574K 324 4.82 8.32 579K 529 ---- ---- ----- 4.47 (avg.) 8.05 (avg.) Average Resistivity: 556 +/-16 KOhm per square. -------------------------------------------------------------- 2. Resistivity by Method of Four-point Probe. ------------------------------------------ Four Kapton resistive foil sheets, 4" square were prepared on a grid-paper substrate. A central square of 9 measurements (on 1" square intervals) was selected to minimize edge effects. Thus measurements were made at nine grid locations: (1,1), (2,1), (3,1), (1,2), (2,2), (3,2), (1,3), (2,3), (3,3), at each of the four resistive foil squares. Measurements were made with the four-point probe aligned in both the 'x-x' and 'y-y' directions for a total of 18 measurements per square x 4 squares = 72 measurements in total. The resistivity was measured with a four-point probe (probe pins on 4mm pitch) using a battery powered 100nA current source (current was verified by monitoring the voltage across a series 10K Ohm, 1% resistor). Both voltages were measured with Keithley 2000 DVMs. Tabulated resistance measurements are as follows: Sq.#1: Sq.#2: 'x' dir.: 107K 106K 119K 'x' dir.: 109K 111K 113K 103K 98K 124K 111K 112K 109K 108K 101K 104K 100K 104K 112K Avg.: 108K Avg.: 109K Std.Dev: +/-8K Std.Dev: +/-3K 'y' dir.: 110K 110K 119K 117K 105K 112K 102K 105K 114K 103K 107K 117K 111K 108K 114K 106K 106K 110K Avg.: 110K Avg. 109K Std.Dev: +/-5K Std.Dev: +/-5K Sq.#3: Sq. #4: 'x' dir.: 117K 115K 107K 'x'dir.: 103K 102K 107K 116K 114K 99K 113K 113K 120K 120K 115K 100K 113K 126K 118K Avg.: 111K Avg.: 113K Std.Dec: +/-7K Std.Dev: +/-7K 'y' dir.: 126K 105K 103K 'y' dir.: 106K 104K 108K 120K 108K 98K 108K 116K 116K 119K 117K 100K 115K 116K 128K Avg.: 111K Avg.: 113K Std.Dev: +/-9K Std.Dev: +/-7K Average Resistance: 111 +/- 7 KOhms. Average Resistivity: 503 +/-32 KOhms per square. ---------------------------------------------------------- 3. Repeatability Measurements with Four-point Probe. ----------------------------------------------- The four point probe was used for 10 "same place" measurements at the center of each of the four foil squares. Measurements were made by placing the probe in the center of each square, taking a reading, then lifting the probe and replacing it in the same location, as best as is manually possible, and taking the next reading. Results for each square are as follows: Sq.#1: 106K, 116K, 110K, 103K, 111K, 106K, 115K, 115K, 101K, 106K. Avg.: 109K +/- 5 KOhm. Avg. Resistivity: 494 +/-23 KOhms per square. Sq.#2: 112K, 106K, 110K, 110K, 110K, 112K, 107K, 107K, 105K, 117K. Avg.: 110 +/- 3 Kohm. Avg. Resistivity: 498 +/- 15 KOhms per square. Sq.#3: 116K, 112K, 114K, 114K, 110K, 118K, 104K, 109K, 103K, 113K. Avg.: 111 +/- 5 KOhm. Avg. Resistivity: 502 +/- 22 KOhms per square. Sq.#4: 116K, 116K, 118K, 115K, 113K, 112K, 113K, 109K, 113K, 113K. Avg.: 114 +/- 2 KOhm. Avg. Resistivity: 516 +/- 11 KOhm per square. Average resistivity of above repeatability measurements: 503 +/- 18 KOhms per square. ----------------------------------------------------------- 4. Discussion of Results. --------------------- It is evident that the strip measurements yield a value of surface resistivity that is somewhat higher (556 KOhm vs. 503 KOhm) than the four point probe method, the difference being more than three standard deviations larger. The fact that the four-point probe method resistivity is smaller indicates that a correction for finite sample size which would normally indicate a higher measured value is not the explanation here. (Our ~10cm sample size means our size to probe pitch ratio is about 25. A ratio of 40 results in error less than 1%, while a ratio of 10 means a error of less than 10%. See Jandel: http://four-point-probes.com/correc.html) The explanation was thought to be in the contact resistance of the resistive foil to the copper tape terminals. Accordingly a series of measurements were made using copper tape strips of 1.25cm width mounted transversely on 2cm wide resistive foil, the contact geometry of the above strip measurements. The results are attached in Appendix 1 below. The average value of measured strip resistance was corrected accordingly. 5 Conclusion: Resistivity of the carbon filled Kapton film was measured to be the following: 503 +/- 18 KOhms per square, by four-point probe method and 460 +/- 16 Kohms per square, by method of resistive strips (corrected for contact resistance). The repeatability of the four-point probe technique is about 4%. ------------------------------------------------------------- Appendix 1. Copper Strip/ CF Foil Contact Resistance Measurements. a) Paired Strip Contact Resistance: (values in KOhms) 79, 67, 69, 79, 75, 80, 86, 81, 80, 62, 86, 76, 90, 61, 85, Average Resistance: 77 +/- 8 KOhms. When this value is subtracted from the above measured average strip resistance (4.47 - .77) = 3.7 MOhm, the average surface resistance is 460 KOhms per square, a value now somewhat lower than that of the 503 KOhms measured by the four-point probe method. ---------------------------------------------------------------