Sheet Resistance Measurement

Definition Sheet Resistance

Sheet resistance (Rs or R), also known as sheet rho, is a mea­su­re for the electrical resistance of thin layers. It is related to the resistivity of both material and layer thick­ness. The sheet resistance value (typically stated in Ω/sq or Ohm/sq or Ohm per squa­re or OPS) provides a measure for the electrical characteristics of conductive and and semiconducting layers. It is the main physical parameter for describing the electrical performance of electrodes. The sheet resistance Rs correlates with the material thickness if the bulk resistivity can be assumed to be constant. The formula is

Sheet resistance describes the ability of a square layer to conduct a certain current. This characteristic is the most important quality parameter for surface electrodes and is determined during layer deposition process or for quality assurance of conductive thin films.

Sheet Resistance Measurement Unit

Even though the correct physical unit of sheet resistance or sheet resistivity is Ohm, the unit most commonly used is Ohm/ sq.

The sheet resistance is specified in Ohm/sq or OPS, in order to achieve a differentiation for the general resistance, which is indicated in Ohm. Very thick layer and highly conductive layers are often described in mOhm/sq and low conductive material is often described using kOhm/sq or MOhm/sq.

Sheet Resistance Measurement Techniques

There are two different modes to measure the sheet resistance-non-contact and contact. Non-contact sheet resistance measurement is possible with the following techniques:

Methods for Sheet Resistance Measurements

Sheet Resistance Measurement by Eddy Current Testing (EC)

Eddy current sheet resistance testing devices drive an alternating current (AC) through coils to generate a (primary) electromagnetic field that induces so called (eddy) currents in conductive materials. The induced currents in the test object operate with the same AC frequency as applied to the induction coils resulting in a secondary field which is opposed to the primary field. The sum of both fields or the change in fields describes the sheet resistance.

Sheet Resistance Measurement by Four-Point-Probe Testing (2PP/ 4PP)

The four-point-probe method works by contacting four equally-spaced, co-linear probes to the material. This method is known as a four-point probe method. A direct current (DC) is driven between the outer two probes whereas the voltage is measured between the inner two probes. Often a geometric correction factor is required when measuring on small samples or close to edges, where current pathways are affected by the sample geometry. The most accurate values can be obtained in the center of samples.

Comparison of 4PP and EC Sheet Resistance Testing

Eddy Current, 4PP, Hall-Effect and Van-der-Pauw methods are electrical testing methods applicable for testing of the electrical parameter sheet resistance. Hall-Effect and Van-der-Pauw measurements are applied on R&D level since both methods typically require sample preparation. Industry is commonly using contact 4PP and non-contact Eddy Current (EC) measurements which do not require sample preparation. The key differences are summarized in the next image.

Eddy Current Testing al­lows ac­cu­ra­te mea­su­re­ment wi­thout im­pacts due to inhomogeneous con­tact qua­li­ty, wi­thout da­ma­ging any sen­si­ti­ve sur­face or in­du­cing ar­ti­facts due to con­tac­ting. Fur­ther­mo­re, it al­lows the ac­cu­ra­te mea­su­re­ment of in­ac­ces­si­b­ly bu­ried or en­cap­su­la­ted layers. Ap­p­ly­ing non-con­tact tech­no­lo­gy, the­re is no we­ar of need­les or tips, which ty­pi­cal­ly cau­ses high re­pla­ce­ment costs in com­mon 4-point-pro­be map­ping sys­tems. A fur­ther si­gni­fi­cant ad­van­ta­ge is the short mea­su­re­ment ti­me. A measurement ta­kes on­ly a few mil­li­se­conds for each mea­su­re­ment and no ti­me for con­tac­ting the sam­ple is nee­ded. This al­so al­lows to mea­su­re in­li­ne du­ring pro­duc­ti­on or “on the fly” in map­ping sys­tems. In re­sult, the eddy current sheet re­si­stan­ce map­ping sys­tems mea­su­re thou­sands of po­si­ti­ons in a cou­ple of se­conds. No in­ter­po­la­ti­on bet­ween mea­su­re­ments points – as ty­pi­cal in 4-point-pro­be map­ping sys­tems – is re­qui­red. Hence, defects and non-uniform areas can be identified.

Sheet Resistance Measurements by Eddy Current

Eddy current gauges are applied for sheet resistance testing since 30 years. Its accuracy and its ability to measure in contactless mode has a special user value. Key benefits of eddy current resistance testing are:

- Non-contact mode

- Ultra-fast (20 ms / measurement)

- High repeatability and accuracy

- Large distances from sensor to substrate

- Transmission mode and reflective mode

- Measurement through encapsulation

- No wearing

- Large measurement range from 0.1 mOhm/sq to 100 kOhm/sq (9 decades)

Sheet Resistances Applications and Measurement Ranges

Sheet resistance is a key quality parameter in architectural glass, photovoltaics, display, OLED, touch panel sensors, packaging, semiconductor and many more industries. The following table provides an overview of typical sheet resistance values across different applications.

Sheet Resistance Measurement Standards

Several industries apply their own measurements standards for sheet resistance measurement using eddy currents devices. Examples are

- SEMI MF673 — Test Method for Measuring Resistivity of Semiconductor Wafers or Sheet Resistance of Semiconductor Films with a Noncontact Eddy-Current Gauge

- SEMI PV28 — Test Method for Measuring Resistivity or Sheet Resistance with a Single-Sided Noncontact Eddy-Current Gauge

- ASTM F1844 - 97(2016) — Standard Practice for Measuring Sheet Resistance of Thin Film Conductors For Flat Panel Display Manufacturing Using a Noncontact Eddy Current Gage

Sheet Resistance Measurement Devices from SURAGUS