Significance and Use
5.1 Applying Test Method F390 to large flat panel substrates presents a number of serious difficulties not anticipated in the development of that standard. The following problems are encountered.
5.1.1 The four-point probe method may be destructive to the thin film being measured. Sampling should therefore be taken close to an edge or corner of the plate, where the film is expendable. Special geometrical correction factors are then required to derive the true sheet resistance.
5.1.2 Test Method F390 is limited to a conventional collinear probe arrangement, but a staggered collinear and square arrays are useful in particular circumstances. Correction factors are needed to account for nonconventional probe arrangements.
5.1.3 Test Method F390 anticipates a precision testing arrangement in which the probe mount and sample are rigidly positioned. There is no corresponding apparatus available for testing large glass or plastic substrates. Indeed, it is common in flat panel display making that the probe is hand held by the operator.
5.1.4 It is difficult, given the conditions cited in 5.1.3, to ensure that uniform probe spacing is not degraded by rough handling 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 following levels. Otherwise acceptable precision may be degraded by probe wobble, however (see 8.6.4).
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, RS, is ≤ 0.01 RS.
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, RS, is ≤ 0.02 RS.
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 F390.
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.
