Sheet Resistance Testing

Carbon Fiber Testing

Eddy Current

Sheet Resistance Fundamentals

1. What is sheet resistance?

Sheet re­si­stan­ce (Rs or R) is a mea­su­re for the elec­tri­cal re­si­stan­ce of a thin layer. It is re­la­ted to the re­sis­ti­vi­ty of the layer ma­te­ri­al and to the thick­ness of the layer. The sheet re­si­stan­ce va­lue (typically stated in Ω/sq or Ohm/sq or Ohm per squa­re or OPS) pro­vi­des a mea­su­re for the elec­tri­cal cha­rac­te­ri­za­ti­on of con­duc­ti­ve, do­ped and se­mi-con­duc­ting layers. It is the main phy­si­cal pa­ra­me­ter to de­s­cri­be the elec­tri­cal per­for­man­ce of elec­tro­des.

 

2. What is resistivity?

The resistivity ρ (Rho) is a measure for describing the electrical resistivity of bulk materials. The unit is ρmm²/m or ρm. The resistivity can be determined multiplying the sheet resistance with the thickness.

 

3. What is conductance?

The electrical conductance G is the reciprocal value of the sheet resistance. The unit is Siemens [S], formerly also Mho (reversed Ohm) was used. 

 

4. What is conductivity?

The conductivity σ (sigma) defines an ability of a material to conduct electrical current. The unit is Siemens/meter [S/m]. SURAGUS offers systems to measure the conductivity of bulk materials or of thin materials with known or constant thickness.

 

5. How do you test metal thickness using Eddy Current?

The EddyCus systems are able to measure the thickness [nm, µm, mil] of films with constant conductivity such as metals. The conversion from sheet resistance into thickness can be achieved by direct thickness calibration or by calculation using the effective conductivity. Metal layers with thicknesses in the lower nanometer range are precisely determined by direct calibration or using stand correlations given in the literature. These features are implemented in the SURAGUS control software.

 

6. How do you measure emissivity?

Emissivityε of architecture glass is measured by measuring the sheet resistance and then converting it into emissivity. There is a direct correlation between emissivity of e.g. Silver and the sheet resistance. Please select emissivity testing in the control software of your EddyCus tool.

 

7. Is there a difference in sheet resistance measured with 4PP or Eddy Current?

There is no difference in sheet resistance measured with 4PP or Eddy Current. Both testing methods measure the sheet resistance as a physical property. This property is independent from its measurement method.

 

8. Why to measure Graphene with Eddy Current?

Graphene as electrode material is very thin and sensitive. Contact testing with 4PP can induce imprints, defects and contaminations. Therefore non-contact eddy current testing is strongly recommended.

Graphene can come as monolayer, bilayer or multilayer material. If there are more than ten layers involved then it is typically referred to as graphite. Monocrystalline and polycrystalline graphene can have very different mechanical and electrical properties. The electrical properties of graphene can be very different and typically reach from 30 to 3,000 Ohm/sq depending on flake size, doping, number of layers and defect density (line defects, folds, gaps).

Transferred graphene layers on non-conductive substrates such as PET, Quartz wafer or glass can be characterized with high accuracy in a huge measurement range across the samples.

 

9. Does Eddy Current testing also work in metal meshes or nanowire structures?

Yes, meshes and nanowires structures are a typical measurement application for Eddy Current testing. Here the non-contact technology is especially advantageous compared to contact methods as achieving a homogenous contact in such structures is very difficult. Hence the Eddy Current testing methods is the standard method for such materials.   

 

10. What are ty­pi­cal sheet re­si­stan­ce ran­ges?

The sheet re­si­stan­ce va­lues of dif­fe­rent func­tio­nal layers can va­ry lar­ge­ly de­pen­ding on the ap­p­li­ca­ti­on. Whi­le den­se me­tal coa­tings provide sheet resistances in the low mOhm/sq re­gi­on, an­ti­sta­tic layers can re­ach up to se­ver­al MOhm/sq.

Ty­pi­cal sheet re­si­stan­ce ran­ges, co­ver­ed by SU­RA­GUS non-con­tact mea­su­re­ment tech­no­lo­gy, are:

 

Application	Main sheet resistance range
Architecture glass (LowE)	1 – 10 Ohm/sq
Transparent electrodes in PV and smart glass	5 – 50 Ohm/sq
Transparent electrodes in OLED	5 – 500 Ohm/sq
Non-transparent metal electrodes	0.1 – 1 Ohm/sq
Display	10 – 1,000 Ohm/sq
Touch panel sensor (TPS)	10 – 1,000 Ohm/sq
Packaging foil	0.001 – 3,000 Ohm/sq
Capacitator foils	0.01 – 100 Ohm/sq
Graphene layer	30 – 3,000 Ohm/sq

 

11. What are the different methods for sheet resistance testing?

Se­ver­al me­thods can be ap­p­lied for mea­su­ring the sheet re­si­stan­ce of a thin layer. The four-point-pro­be and van der Pauw me­thod are of­ten ap­p­lied off­li­ne, if the layer is di­rect­ly ac­ces­si­ble for con­tac­ting and in­sen­si­ti­ve for tou­ching. In so­me ca­ses, test struc­tu­res are even spe­ci­fi­cal­ly de­si­gned in or­der to mea­su­re the sheet re­si­stan­ce.

SU­RA­GUS non-con­tact sheet re­si­stan­ce mea­su­re­ment al­lows to mea­su­re mo­re ea­si­ly the sheet re­si­stan­ce wi­thout any con­tac­ting is­su­es, wi­thout tou­ching the layer or ef­fec­ting any da­ma­ge.

 

12. What is the advantage of non-contact sheet resistance measurement over common four-point-probe devices?

SU­RA­GUS non-con­tact mea­su­re­ment so­lu­ti­ons al­low 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 SU­RA­GUS non-con­tact tech­no­lo­gy, the­re is no we­ar of need­les / tips, which ty­pi­cal­ly cau­ses high re­pla­ce­ment costs in com­mon 4-point-pro­be map­ping sys­tems. A fur­ther si­gni­fi­cant ad­van­ta­ge is the short mea­su­re­ment ti­me: SU­RA­GUS EddyCus® se­ries de­vi­ces ta­ke 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 SU­RA­GUS 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. In result defects and non-uniform areas are not missed.

Non-contact eddy current testing	Four Point Probe Measurement / 4PP
Contact-free / higher repeatability as 4PP	Contact / homogeneous contact quality influences measurement quality
Measurement range from 0.1 mOhm/sq to 100 kOhm/sq	Measurement range from 1 mOhm -  10 kOhm/sq
„real-time“ – up to 1,000 Measurements per second /  „on the fly“	Relative long measurement time mainly due to contact establishment (couple seconds)
Imaging and inline solutions with thousands of measurements e.g. 10,000 Measurement in a pitch of 1 mm on a 4 inch sample within 200 seconds.	Imaging solution and inline solutions with small number of measurement points. Interpolation is required
No costs for wearing	Wearing costs (especially relevant for mapping and inline solutions)
No contamination due to contact	Danger of contamination (especially for semiconductor, OLED industry)
No physical impact to sensitive films	Danger of layer damage through physical impact
Measurement of conductive multilayer systems. Layer separation by parallel resistance formula.	Measurement of accessible top layers only
Characterization of hidden and encapsulated films	No measurement of encapsulated films
Applied since 30 years	Applied since 70 years
Calibration by manufacturer or by user	Calibration by manufacturer or by user

 

13. Which ran­ge can be co­ver­ed by a non-con­tact sheet re­si­stan­ce sys­tem?

SU­RA­GUS non-con­tact sheet re­si­stan­ce mea­su­re­ment co­vers a lar­ge ran­ge from be­low 0.1 mOhm/sq up to 100,000 Ohm/sq. This range can be extended for some applications. Alt­hough the mea­su­re­ment de­vi­ce and sen­sor sys­tem will of­ten be ad­ap­ted to its spe­ci­fic ran­ge of ap­p­li­ca­ti­on, the ran­ge of just one ad­van­ced sen­sor pro­be can cover six decades of sheet resistance (e.g. 1 mOhm/sq – 1 kOhm/sq). Ti­me-con­su­ming sen­sor pro­be change by the ope­ra­tor is not re­qui­red.

 

14. What is the ma­xi­mum sam­ple thick­ness for SU­RA­GUS non-con­tact de­vi­ces?

All SU­RA­GUS non-con­tact sheet re­si­stan­ce mea­su­re­ment de­vi­ces can be ad­ap­ted to a lar­ge ran­ge of sam­ple thick­nes­ses, de­fi­ning the mea­su­re­ment gap. The stan­dard ran­ge is ty­pi­cal­ly bet­ween 1 and 20 mm. Spe­ci­fic con­fi­gu­ra­ti­on al­lows to in­cre­a­se the mea­su­re­ment gap to 60 mm and mo­re. All substrate thicknesses fitting into the sensor gap can be characterized by the system.

 

15. Can the mea­su­re­ment gap si­ze be chan­ged by the user?

A fi­xed and sta­ble sen­sor pro­be ad­just­ment is cru­ci­al for the ac­cu­ra­cy of non-con­tact sheet re­si­stan­ce mea­su­re­ment. Hence, the mea­su­re­ment gap will be set and fi­xed in the fac­to­ry to the ma­xi­mum ex­pec­ted sam­ple thick­ness. A change of the mea­su­re­ment gap can be pro­vi­ded by SU­RA­GUS ser­vice and ty­pi­cal­ly re­qui­res re-ca­li­bra­ti­on of the mea­su­re­ment de­vi­ce.

 

16. Can the sys­tem mea­su­re en­cap­su­la­ted con­duc­ti­ve films?

Yes, SU­RA­GUS non-con­tact sheet re­si­stan­ce mea­su­re­ment does not need to con­tact the thin con­duc­ti­ve layer. The layer can be mea­su­red ac­cu­ra­te­ly even if bu­ried or en­cap­su­la­ted be­low ano­ther diel­ec­tric layer (e.g. op­ti­cal or pro­tec­ti­ve layer).

 

17. Does rough­ness af­fect the mea­su­re­ment qua­li­ty?

No, other than for con­tac­ting mea­su­re­ment tech­no­lo­gies, rough­ness of the layer does not af­fect the qua­li­ty or ac­cu­ra­cy of the SU­RA­GUS non-con­tact mea­su­re­ment. SU­RA­GUS non-con­tact de­vi­ces are well and com­mon­ly ap­p­lied for mea­su­ring rough or sen­si­ti­ve layers.

 

18. Is the sys­tem ap­p­li­ca­ble to mul­ti-layer sys­tems?

SU­RA­GUS Ed­dy­Cus® se­ries de­vi­ces mea­su­re through the com­ple­te stack of all layers. Mul­ti­ple con­duc­ti­ve films in one stack elec­tri­cal­ly be­ha­ve as par­al­lel re­sis­tors and can be se­pa­ra­ted using ac­cor­ding stan­dard for­mu­las. Hence, mul­ti­ple con­duc­ti­ve layers can be se­pa­ra­ted by mea­su­ring af­ter each coa­ting step.

 

19. What is the measurement pitch / measuring point distance for inline or mapping solutions?

Inline measurements and mapping systems use a lateral measuring point distance of 250 microns to 10 mm (400 mil) depending on the application. The standard distance for property imaging is 1 mm.

 

20. What is the spot size of a system?

The sensitivity of the measurement system is highest in the center of the sensor and decreases towards the outside and then no longer contributes to the characterization. The high sensitive zone (HSZ) of sheet resistance measuring systems ranges from 5 to 25 mm depending on the setup. Some systems with 100 mm HSZ were realized for wide coverage in the past as well. This HSZ diameter is primarily defined by the distance to the sample and some sensor characteristics. Smaller distances and sheet resistances enable smaller measurement spot sizes.

Structure and defect monitoring systems utilize a HSZ from 0.5 to 5 mm. Additionally, differential sensors with very high sensitivities are used for the detection of local defects and variations.

 

21. What is the spatial resolution?

The spatial resolution is determined by the contrast of the measuring effect, the measurement point distance and the spot size. For example, a wafer mapping can be obtained using a distance (gap) of 2 mm. The high sensitive zone (HSZ) then has a diameter of about 5 mm. Sheet resistance fluctuations of 4% can be measured when affecting about 25% of the 5 mm HSZ. Defects that cause a higher contrast can be detected when affecting smaller areas. For instance, cracks with only a few microns width can be easily detected as the contrast and measurement effect is very high.  

 

22. What is the smal­lest sam­ple that can be mea­su­red?

As a ru­le of thumb, a mi­ni­mum sam­ple si­ze is about 25 x 25 mm² is re­com­men­da­ble for an ex­pec­ted sheet re­si­stan­ce of up to 300 Ohm/sq. For lar­ger sheet re­si­stan­ce than 300 Ohm/sq a mi­ni­mum sam­ple si­ze of 50 x 50 mm is re­com­men­ded.

In some application SURAGUS has realized solutions for much smaller samples (e.g. 10 x 10 mm) or wires or printed stripes. Please contact our team to discuss this point in further detail.

 

23. What is the ma­xi­mum sam­ple si­ze?

SU­RA­GUS of­fers dif­fe­rent ty­pes of the non-con­tact sheet re­si­stan­ce mea­su­re­ment de­vi­ces. A list sharing maximum sample sizes is shown below.

System	Maximum sample sizes
Single point measurement
EddyCus® portable 1010	from 150 x 150 mm (6 inch)
EddyCus® lab 2020SR	up to 200 x 200 mm (8 inch)
EddyCus® lab 4040SR	up to 400 x 400 mm (16 inch)
EddyCus® lab 4040HS	up to 400 x 400 mm (16 inch)
EddyCus® lab 4040A	up to 400 x 400 mm (16 inch)
Imaging devices
EddyCus® map 2525SR	up to 250 x 250 mm (10 inch)
EddyCus® map 5050SR	up to 500 x 500 mm (20 inch)
EddyCus® map 6060	up to 600 x 600 mm (24 inch)
Inline systems
EddyCus® inline	from 1 mm (depending on sheet resistance range)

 

24. Do you offer sensors for measurement in vacuum?

Yes, SURAGUS offers sensors for measurement in vacuum. Please refer to our inline “in-vacuo” sensors. All inline sensors are available for in-vacuo application.  

 

25. Do you offer systems to measure optical characteristics such as optical density?

Yes, SURAGUS offers systems that measure the optical transparency (OT) or optical density (OD) next to the electrical characterization. Both Optical Transparency (OT) and Optical Density (OD) are common parameters for describing the transmission of light through objects. The conversion is done applying the following formula:

 

26. Do you ha­ve a hand­held de­vi­ce for sheet re­si­stan­ce mea­su­re­ment?

Yes, SU­RA­GUS provides a state of the art hand­held-de­vi­ce. The Ed­dy­Cus® por­ta­ble 1010 is dedicated to fast and ac­cu­ra­te con­tact sheet re­si­stan­ce mea­su­re­ments. The hand­held de­vi­ce al­lows to ac­cu­ra­tely mea­su­re the sheet re­si­stan­ce of freely accessible and also en­cap­su­la­ted layers in a stack of diel­ec­tric layers. Setups that measure through the backside of a glass, e.g. 4 mm glass, are also available on request. 

 

27. What is the dif­fe­rence bet­ween sheet re­si­stan­ce sin­gle-point mea­su­re­ment and sheet re­si­stan­ce map­ping?

SU­RA­GUS non-con­tact sin­gle point mea­su­re­ment de­vi­ces pro­vi­de an ac­cu­ra­te sheet re­si­stan­ce mea­su­re­ment on one spot of a sam­ple. The user can manually collect values at certain positions of a sample to get a better understanding of the layer homogeneity.

SU­RA­GUS au­to­ma­ted map­ping de­vi­ces such as EddyCus® map 2530SR al­low to scan a com­ple­te sam­ple in or­der to ob­tain a full scan/“C-scan” or sheet re­si­stan­ce image of the sam­ple. The imaging systems benefits from an automatic edge effect correction, which enables a measurement 2 mm away from the edge.

The EddyCus® map control soft­wa­re al­lows ana­ly­zing in­ho­mo­ge­nei­ties, de­fects or gra­di­ents in the sheet re­si­stan­ce. Spe­ci­fic are­as of in­te­rest can be se­lec­ted by the ope­ra­tor and cho­sen li­ne scans pro­vi­de in­sight in­to the sheet re­si­stan­ce dis­tri­bu­ti­on along the cho­sen li­ne on the sam­ple. In addition, the high number of measurements on a sample (e.g. 10k or 10M) is used to provide profound statistical parameters and histograms.

 

28. Do I need to change se­ver­al sen­sors to co­ver a lar­ge mea­su­re­ment ran­ge?

The mea­su­re­ment ran­ge of one sensor covers 6 decades of sheet resistance. The required measurement range requested by the application is set during manufacturing of the device. There is no need for annoying sensor changes or system adjustments.

 

29. How can I check if the sys­tem mea­su­res cor­rect­ly?

SU­RA­GUS non-con­tact sheet re­si­stan­ce mea­su­re­ment de­vi­ces are usual­ly de­li­ver­ed with one re­fe­rence sam­ple. The re­fe­rence sam­ple is pro­vi­ded for pe­ri­odic check-up of the sys­tem ac­cu­ra­cy, for ex­am­ple eve­ry three to six months. In ca­se that the mea­su­red va­lue of the re­fe­rence sam­ple de­via­tes from the spe­ci­fied va­lue, the SU­RA­GUS ser­vice ta­kes care of it. In that ca­se, plea­se call us on +49 (0) 351 32 111 555 or send us an email to support@suragus.com.

 

30. In what ca­ses do I need to ca­li­bra­te the sys­tem?

All SU­RA­GUS non-con­tact sheet re­si­stan­ce mea­su­re­ment de­vi­ces are de­li­ver­ed with fac­to­ry ca­li­bra­ti­on and ty­pi­cal­ly do not re­qui­re any fur­ther ca­li­bra­ti­on. If by any ca­se the sys­tem shows mea­su­re­ment va­lues de­via­ting from the de­li­ver­ed re­fe­rence stan­dards, plea­se con­tact the SU­RA­GUS ser­vice. Pho­ne: +49 (0) 351 32 111 555 or E-Mail: support@suragus.com.

Re-ca­li­bra­ti­on of the sys­tem is ve­ry ea­sy and can be per­for­med by any user, for ex­am­ple in gui­dance by the SU­RA­GUS pho­ne sup­port. The gui­ded ca­li­bra­ti­on pro­ce­du­re does not re­qui­re spe­ci­fic pre-know­led­ge and will not ta­ke mo­re than a few mi­nu­tes.

 

31. Do I ha­ve to con­si­der an "ed­ge ef­fect"?

The ed­dy cur­rent-ba­sed mea­su­re­ment re­li­es on an induced elec­tric cur­rent in conductive layers. The currents have a high current density below the sensor that diminishes with increasing distance to the sensor. This effect is also known from 4PP systems, which use the similar physics and cope with the same effect. Typically, there is a correction formula deepening on the distance to the edge in the user manual of 4PP and Eddy Current systems so the user can correct those measurements by multiplying a factor.

SU­RA­GUS map­ping de­vi­ces pro­vi­de an in­te­gra­ted and au­to­ma­ted cor­rec­ti­on of this “ed­ge ef­fect” for ma­ny pre-con­fi­gu­red sam­ple si­zes and al­low an ac­cu­ra­te mea­su­re­ment at any po­si­ti­on of the sam­ple

 

32. Can you mea­su­re struc­tu­red thin films?

Yes, the SU­RA­GUS non-con­tact mea­su­re­ment al­lows ac­cu­ra­te uni­for­mi­ty mea­su­re­ment of both struc­tu­red and un­struc­tu­red coa­tings. For some applications such as characterizing structured electrodes or capacitor metallization structural factors are applied. Please feel free to discuss your application with SURAGUS.    

33. Do vi­bra­ti­ons of the sam­ple ha­ve an in­flu­ence on the mea­su­re­ment?

Depending on the measuring range and the measuring gap different vibrations can be tolerated. The standard tolerance is ± 1 mm. A low sheet resistance and large measuring gap allow tolerating large variances of up to ± 5 mm. For the measurement of curved substrates sensors are used with integrated ultrasonic sensors, the measured values are compensated according to the position in the measuring gap and position-independent accurate measurement values can be determined.