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?
Emissivity ε 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 TF 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 3 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 3,000 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 m Ohm/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 TF 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 TF Portable 1010||from 150 x 150 mm (6 inch)|
|EddyCus TF lab 2020||up to 200 x 200 mm (8 inch)|
|EddyCus TF lab 4040||up to 400 x 400 mm (16 inch)|
|EddyCus TF lab 4040 Hybrid||up to 400 x 400 mm (16 inch)|
|EddyCus TF lab 4040a||up to 400 x 400 mm (16 inch)|
|EddyCus TF map 2525SR||up to 250 x 250 mm (10 inch)|
|EddyCus TF map 5050SR||up to 500 x 500 mm (20 inch)|
|EddyCus TF map 6060||up to 600 x 600 mm (24 inch)|
|EddyCus TF map 2525 Hybrid||up to 200 x 200 mm (8 inch)|
|EddyCus TF 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 TF 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 TF map 2525SR 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 TF 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 email@example.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: firstname.lastname@example.org.
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.
1. Why carbonfibertesting?
The market for carbonfiber (CF) material or carbonfiber reinforced plastic (CFRP) is constantly growing through the last couple of years. Especially areas, as the aerospace and automotive industries emphasize light and strong materials. However, these composites must meet high quality standards. Therefore
- Non-destructive and
- Contact free measurement systems
are indispensable for these materials.
Carbon fiber reinforced plastic material consists of two main components. Firstly, as the name suggests, carbon fiber (CF) which gives the material its stiffness and tensile strength. And secondly, a resin (matrix) which fixes the carbon fiber in the desired position and does not allow displacement under load.
2. What are non-destructive and non-contact measuring systems for carbon fiber materials?
Measurement systems which investigate composite materials and are able to work non-destructive and contact free are highly important for these security-related areas, such as the aviation industry is. This kind of measurement systems which satisfy these requirements are being excellent high performance systems. Since every material has different properties and the chemical composition of all materials is variable, different measuring systems are required for failure analysis.
The Eddy current measurement system - EddyCus is one of the most efficient and technically sophisticated systems worldwide.
3. Does the Eddy current measurement system – EddyCus measure contact free and non-destructive?
Yes, as these characteristics are essential for the assessment of carbon fiber materials, the eddy current measurement provides the perfect solution for this task.
4. How does eddy current testing work for carbon fiber materials?
In an eddy current measurement system, the physical properties of a coil can be used to create an electromagnetic field within an electrically conductive material. For this purpose, an alternating current is applied to a coil. This coil than generates an electromagnetic field which is spreading wavelike. If this field meets an electrically conductive material such as carbon, eddy currents are being generated within the material. These into the material introduced eddy currents in turn generate a magnetic field that counteracts (according to Lenz’s law) the change within the field. This physical effect is used by the EddyCus testing system which thereby is enabled to work contact free and non-destructive.
As carbon fiber material is conductive it has a very good basis to be tested and can be built up to semi-finished products such as:
- Lay up
- Multiaxial lay up
Each of these semi-finished products has different characteristics such as fiber arrangements and fiber orientation. Within the manufacturing and processing steps these semi-finished products can receive
- Fiber uniformity
- Deviation of Fiber orientation
- Play orientation
- Changes of Isotropy
- Fuzzy Balls
And other deviations or defects.
6. What is the penetration depth for the EddyCus inspection system in carbon fiber material?
The depth of penetration depends on several factors and is influenced accordingly. In general however, the penetration depth is between 2 to 3 mm, but may also be a maximum of 5 mm. If the material is measured on both sides, a penetration depth of max. 20 mm can be achieved.
7. Can the EddyCus penetration depth be adjusted?
Yes, the parameters can be adjusted by changing the voltage applied to the coil frequency. Using eddy current frequency change in intensity, the results can be optimized.
8. How high is the EddyCus measurement speed?
The measurement speed is dependent on actuating elements. The internal measuring rate is several 100 measurements per second.
9. What resolution offers the EddyCus testing system?
The resolution of the eddy current testing device depends largely on the measurement task and the associated necessary depth of penetration or the measuring field size. Basically it can be said that the smallest resolution is 1 mm. The spot size can also vary up to max. 100 mm in area if for example the measurement task is to analyse the fiber areal weight (FAW). In principle it is possible to distinguish between high resolution sensors and sensors with high penetration depth.
10. Also complex carbon fiber composite parts can be measured?
Yes, as SURAGUS has designed a robot solution for complex carbon fiber components with a project partner which was produced and brought to market. Now complex parts can be assessed in all three dimensions. A precise containment of the ROI (region of interest) is of course possible. With the developed SURAGUS EddyEVA software, the evaluation of your 3D scan data can be configured individually.
11. Is inline measurement possible with the EddyCus system?
Yes, depending on the measurement task, the EddyCus measuring system can also be integrated within your production line. If you want to characterize, for example, the fiber areal weight as inline process, sensors are installed on both sides of the carbon fiber material. Accordingly the transmitter and receiver sensor are arranged parallel.
12. Are there any other sensor arrangements possible in addition to the transmission arrangement with the EddyCus system?
Yes, also the one-sided sensor arrangement is feasible. The unilateral sensor array with constant lift-off is possible. For this purpose, a tracking is necessary. A bilateral measurement will be more tolerant.
13. Can the distance to the measurement sample fluctuate?
Yes, at the inline transmission method the web flutter tolerance is around 1-2 mm. Thus a constant measuring is ensured even when the distance to the measured object changes. This makes the EddyCus measurement system a reliable partner even at fluctuations of distance.
14. Is there a difference whether the tested carbon fiber material is infiltrated with thermoplastic or resin?
No, as thermoplastic or resin have comparable electrical properties this circumstance doesn’t make any difference.
15. Is there any difference when measuring dry preform or carbon fiber reinforce plastic?
The difference is hardly noticeable and has no influence on the measurement results.
16. What are the advantages of the EddyCus testing device?
The EddyCus measurement system can be used within every production step where carbon fiber is processed. Starting with the carbon fiber yarn processing through carbon fiber/prepreg, carbon fiber preform, the cured carbon fiber reinforced component, but also the recycled carbon fiber material can be monitored and quality assured with the EddyCus measuring system.
17. Allows the EddyCus system carbon fiber recycling or rather the quality assurance of the process?
Yes, using the eddy current measuring system the process of monitoring as well as quality assurance of recycled carbon fiber materials is made possible. Now the isotropy behaviour of the carbon fiber can be monitored or the fiber areal weight can be measured.