CN117590080A

CN117590080A - Device and method for determining sheet resistance of a sample

Info

Publication number: CN117590080A
Application number: CN202311018815.8A
Authority: CN
Inventors: F·科尔绍什; A·托特; P·蒂特
Original assignee: Semirab Ltd
Current assignee: Semirab Ltd
Priority date: 2022-08-12
Filing date: 2023-08-14
Publication date: 2024-02-23
Also published as: HUP2200327A1

Abstract

The subject of the invention is a device and a method for determining the sheet resistance of a sample. The device comprises a measuring head comprising at least one light source (2) whose light is applied to the surface of the sample (S) and at least two potential sensors (3). The device further comprises an amplifier (4) connected in signal communication with the potential sensor, and a processing unit (6) connected in data communication with the amplifier, the processing unit being adapted to determine the sheet resistance based on the difference signal of the potential sensor. The device is characterized in that the amplifier is adapted to generate and amplify a difference signal of at least two potential sensors. According to the method of the invention, a sample to be examined is irradiated with at least one point, the potential due to the irradiation in the vicinity of the irradiated point is measured by at least two potential sensors at least two locations, and then the sheet resistance is calculated from the measured potential using a predetermined function. The essence of this method is to determine and amplify the potential difference before determining the sheet resistance.

Description

Device and method for determining sheet resistance of a sample

Technical Field

The present invention relates to an apparatus and a method for determining sheet resistance. More specifically, the subject of the invention is an apparatus and a method for determining the sheet resistance of layers separated by volume charge regions based on photovoltage measurements.

Background

In an industrial environment, direct contact and non-contact techniques are used to determine the sheet resistance of thin conductive or semiconductive layers. So-called four-point sheet resistance measurement techniques requiring direct contact are highly accurate, but relatively slow, and also involve sample degradation. Thus, for on-line integrated sheet resistance measurements where each sample is targeted for scanning, non-contact techniques are more preferred. However, non-contact technology has lower accuracy, which is not sufficient for all applications.

There is an increasing need in the industry for instruments and methods that enable high precision and rapid non-contact measurements, as they can greatly reduce the proportion of waste products by filtering out and removing defective feedstock from the manufacturing process, and by rapidly detecting and eliminating technical problems in the manufacturing process. Some non-contact techniques, i.e. methods based on eddy current generation and measurement and methods based on infrared reflection, are basically capable of measuring the combined resistance of the whole layer structure. On the other hand, based on a method of measuring the propagation of the voltage generated by the light falling onto the p-n junction, the so-called junction photovoltage "JPV", which is parallel to the layer, i.e. lateral, it is possible to measure the resistance of the top conducting layer (called emitter layer) completely independently of the resistance of the bottom conducting layer and the semiconducting layer, since they are separated by an electrical insulation charge region.

Patent US 5,442,297 discloses a method based on JPV measurement for determining sheet resistance. According to the solution described in this patent, the sample is illuminated with a light source in a small spot and the potential, which changes due to the influence of charge carriers (i.e. electron-hole pairs) generated by said illumination around the illuminated site, is measured by means of an annular potential sensor arranged concentrically around the illuminated site, which potential changes with respect to ground potential. From the different potentials measured by the sensors located at different distances from the irradiation site, the sheet resistance of one or more upper layers of the sample, which are electrically isolated from the underlying layer or layers, e.g. by volume charge regions, can be determined in the volume portion between the sensors. The main disadvantage of this solution is the relative lack of positioning, i.e. measuring the potential in the annular region below the sensor, so that the potential difference also exists in the spatial region enclosed by the two sensor rings, so that the position of the difference or material defect detected by the measurement can be determined with relatively low accuracy. Another disadvantage of this solution is that, due to the relatively large size of the potentiometric sensor, in a high-speed, continuous production line, vibrations caused by the high-speed operation of the sample will cause some parts of the sensor to be closer to the sample than others. Therefore, the sensor measurement accuracy is significantly reduced. For high speed transported samples, the electrostatic charge of the sample and the ground potential fluctuations due to the electrostatic charge introduced to ground from the electrostatically charged machine also significantly reduce the measurement accuracy.

Another method for determining sheet resistance based on JPV measurements is described in patent US 9,921,261. Here, the irradiation site is only semi-surrounded by the semicircular electrode of the potential sensor. This solution allows better resolution than using a full annular ring of the same radius, but due to the asymmetric sensor arrangement the sheet resistance measured on the same surface element of the sample may be different depending on the orientation of the sensor, i.e. the measurement is highly anisotropic. This is disadvantageous for evaluation of the measurement results and detection of potential surface defects. Furthermore, the calibration of the measuring device requires more samples specially prepared for this purpose and more complex functions for evaluating the measurement results, which means that errors are more likely to occur, i.e. eventually the accuracy and reliability of the measurement is reduced.

The JPV type noncontact sheet resistance sensor is particularly useful for integration in solar cell production lines. In recent years, the speed of the production line has increased significantly, which increases the vibration of the samples running on the conveyor and the electrical noise due to friction. Thus, existing measuring heads have become unreliable.

Disclosure of Invention

In view of the above, the present invention aims at alleviating and/or eliminating the drawbacks of the known solutions, in particular by providing an apparatus and a method based on a differential JPV method, which is adapted to determine the sheet resistance of layers separated by volume charge regions, in particular indium tin oxide layers, zinc oxide layers, other transparent conductive layers or polysilicon layers in a tunneling current contact cell (tunnel current contact cell), with high accuracy, speed and resolution in at least one direction.

The object of manufacturing the above-mentioned device has been achieved by a device according to claim 1. Possible preferred exemplary embodiments of the device according to the invention are set forth in claims 2 to 10. The object of developing the method according to the invention is achieved by devising a method according to claim 11, possible preferred exemplary embodiments of which are defined in claims 12 to 24.

The invention is based in part on the finding that the accuracy of sheet resistance measurement can be significantly improved by forming the difference between the signals from the two potential sensors in an analog manner, in particular before amplifying the signals, and determining the sheet resistance from the difference signals, to eliminate measurement noise due to ground potential (potential) fluctuations.

The invention is also based on the following findings: by arranging the potential sensors and the excitation light source in such a manner that two potential sensors of the same shape and size are arranged substantially in line with the irradiation position of the light source, the resolution of sheet resistance measurement can be improved while errors in sheet resistance measurement due to vibration can be reduced.

The invention is also based on the finding that by arranging two light sources in the above-described measuring arrangement on opposite sides of the sensor in correspondence with the potential sensor, the accuracy of the sheet resistance measurement can be further improved.

The invention is also based on the surprising finding that by using two light sources and two in-line potential sensors and switching the light sources alternately at a suitable frequency, the differential signal of the potential sensors, for example in the case of Transparent Conductive Oxides (TCO) and polysilicon layers, shows a perfect proportion to the sheet resistance of the layer to be measured in the range of sheet resistances, so that the measurement can be calibrated with high precision even with samples from the production process itself, contrary to the prior art solutions requiring specially prepared samples for calibration.

The invention is also based on the finding that the accuracy of sheet resistance measurement can be further improved by continuously measuring the distance of the potentiometric sensor relative to the sample surface and using the measured distance to reduce the effect of sample vibration on the measurement result, wherein the measured distance is used in order to:

a) Compensating the measurement result given the dependence of the measured potential on the actual distance between the potential sensor and the sample surface; or alternatively

b) The distance between the potentiometric sensor and the sample is maintained within a prescribed error range by moving the sample and the measuring head containing the potentiometric sensor relative to each other.

Drawings

The solution according to the invention will be described in detail below with reference to the attached drawings, in which:

fig. 1 schematically shows a preferred exemplary embodiment of an apparatus according to the present invention;

fig. 2a to 2c show bottom views of preferred exemplary arrangements of potential sensors and light sources in the device according to the invention; and

fig. 3 is a block diagram of a preferred variant of the method according to the invention.

Detailed Description

Fig. 1 schematically shows a preferred exemplary embodiment of the device according to the present invention. The device according to the invention comprises at least one light source 2 for illuminating the sample S. In the embodiment shown in the figures, the light source 2 comprises a secondary light source, for example a mirror, a lens or an optical fiber, preferably a glass rod. In this case, the light source 2 is illuminated by a primary light source 1, preferably a device capable of emitting light in a relatively narrow spectral range and preferably compact. The primary light source 1 is preferably a Light Emitting Diode (LED) or a laser, but the device according to the invention may also be implemented with other primary light sources 1, such as halogen lamps or incandescent lamps, and by applying a suitable spectral filter. Alternatively, the light source 2 is constituted by a primary light source 1 whose light falls directly on the sample S, in which case it is particularly advantageous to use a compact light source, such as an LED, as the primary light source 1. The device preferably further comprises means for continuously measuring and controlling the luminous intensity of the primary light source 1, e.g. an LED. The arrangement of such a device is easier if the primary light source 1 does not directly illuminate the sample S but is positioned at a distance therefrom and the light of the primary light source is transmitted to the sample S by the light source 2.

The intensity of light transmitted to the sample is periodically varied at a modulation frequency. This may be achieved by suitably adjusting the primary light source 1 and/or by suitable optical means, such as opto-electronic means or a light chopper, arranged between the primary light source 1 and the sample S. In a particularly preferred embodiment, the primary light source 1 comprises a light emitting diode, i.e. LED, driven by an electrical signal, preferably a sinusoidal or square wave signal, particularly preferably a square wave, which is modulated at a frequency equal to the modulation frequency. Basically, the intensity of the optical signal emitted by the main light source 1 varies according to the waveform of the driving electrical signal. If the light signal is already a sine wave or square wave signal suitable for use, the additional optical elements present in the optical path (if any) are selected so that the light signal reaching the sample is still a sine wave or square wave signal. The modulation frequency should be selected to fall within the range of 0.1Hz and 500kHz depending on the sheet resistance of the layer of the sample S to be measured and the capacitance of the volume charge region. For most samples, the modulation frequency is preferably 4-32kHz, which allows for rapid measurements. However, we have also found that for some samples with static electricity in some steps of the manufacturing process, measurements in the frequency range of 4 to 32kHz are extremely useful. To overcome this problem, in another preferred embodiment, a modulation frequency below 500Hz is used.

The device according to the invention uses 2 light sources with wavelengths of 400-900nm, preferably 600-900nm, depending on the intended use. The optimum wavelength depends on the material, purpose and manufacturing technique of the sample S to be measured and on the steps in the manufacturing process after which the measurement is performed using the device, i.e. when the thickness of the layer is to be inspected. However, finding the ideal wavelength is not an obvious task for the person skilled in the art, since, contrary to expectations, the complexity of the layer structure and the composition variations within the sample (the material composition of the individual layers, in particular the doping amounts and thicknesses of the individual layers may vary equally within the same sample) often prevent the analytical method from finding a solution describing the behaviour of the sample, and the results obtained from numerical simulations do not exactly match the real behaviour of the sample.

Our measurements made have led to the surprising conclusion that, unlike the results of preliminary calculations, in the sensor for inspection during the manufacture of solar cell structures, the wavelength of the light source 2 or of the primary light source 1 is preferably 470nm, whereas in the generic device it is preferably green light, more preferably 520nm. In order to produce solar cells with newer, more efficient so-called "tunnel oxide passivation contact (TOPCon)" layers and thin amorphous silicon passivation layers obtained by heterojunction technology (HJT), longer wavelengths above 600nm, even in the near infrared range, are preferred. Wavelengths greater than 600nm are also advantageous for testing passivated emitter and back cell (PERC) solar cell structures, which remain dominant in the marketplace, when it is desired to control layer thickness prior to etching a temporarily applied phosphosilicate glass (PSG) or borosilicate glass (BSG) layer. Thus, in a particularly preferred embodiment, the primary light source 1 comprises a light emitting diode that can operate at a wavelength of 470nm, 520nm, 600-700nm or 750-900 nm.

The light source 2 is used to illuminate the sample S with a single spot of light. At least two potentiometric sensors 3 arranged close to the point measure the photo-induced voltage at least at two different distances from the illuminated point. Preferably, the at least one light source 2 and the at least two potentiometric sensors 3 are arranged such that the orthogonal projections of the illuminated spot and the sensing surface of the potentiometric sensor 3 on the surface of the sample are aligned, i.e. the centers of their plan views are on the same line. The sensing surfaces of the at least two potential sensors 3 shown preferably have the same shape and size. In particular, the number of the potential sensors 3 is most preferably two. The shape of the sensing surface of the three potential sensors is preferably circular, diamond-shaped or rectangular, more preferably rectangular. In another particularly preferred embodiment, the rectangular sensing surfaces together with the area between them cover an approximately square area, for example such that the rectangular sensing surfaces have an aspect ratio of about 1:2, the longer sides of the rectangles facing each other and towards the light source 2, and the distance between the rectangles is smaller than the length of the shorter sides of the rectangles. An advantage of this arrangement is that sheet resistance along the surface of the sample S can be measured in a substantially isotropic manner. The point illuminated on the sample S by the light source 2 is preferably not farther from the edge of the orthogonal projection of the rectangular sensing surface of the potentiometric sensor 3 on the sample surface than the length of the short side of the rectangle. This provides a relatively large difference between the signals measured by the potentiometric sensor 3.

The at least two potential sensors 3 are connected to an amplifier 4, preferably a differential amplifier, which is designed to generate and amplify differential signals of the at least two potential sensors 3.

In a particularly preferred embodiment of the device shown in fig. 1, the chopped light signal is generated by at least one light source 2, preferably by driving the primary light source 1 of the light source 2 with a chopped electrical signal generated by a frequency generator 7. The chopped optical signal may of course also be generated by a continuous light source and an optical chopper or suitable optoelectronic element. When using a chopped optical signal, the amplified difference signal generated by the amplifier 4 is preprocessed by a lock-in analyzer 5 which is connected in signal transmission (signal transmission) with said amplifier 4, wherein the lock-in analyzer 5 is connected in data transmission (data transmission) with the processing unit 6 and optionally with the frequency generator 7. The application of the locking technique results in a better signal-to-noise ratio, i.e. a significantly improved measurement accuracy, than if the locking technique was not applied. Optionally, further electronics for pre-amplification and/or pre-filtering may also be provided between the potentiometric sensor 3 and the amplifier 4, the processing unit 6 determining the sheet resistance from the measurement results and performing further operations based on the measurement results, for example saving it in a database or indicating suitable further devices for marking and/or moving the sample. The processing unit 6 may be, for example, a simple desktop computer, optionally capable of receiving signals, digitizing signals and buffering signals through units specifically tailored to the task. Preferably, the processing unit 6 is operatively connected to the frequency generator 7 and the primary light source 1, that is to say the operation of the latter can be controlled by the processing unit 6.

In a preferred embodiment the lock-in analyzer 5 is connected to a frequency generator 7, whereby the frequency generator 7 provides a reference signal to the lock-in analyzer 5 for checking the signal from the differential amplifier 4. Alternatively, the lock analyzer 5 may be equipped with an internal clock signal generator, the frequency of which may be set to or equal to the modulation frequency of the light.

The processing unit 6 is also connected to an actuator 8 which is able to move at least one of the sample S and/or the measuring head containing the potentiometric sensor 3 closer to or further away from the other. The actuator 8 is preferably provided by a motorized stage holding the sample S and moving and holding the sample S at a distance suitable for performing the measurement.

In a further preferred embodiment the device according to the invention comprises distance measuring means (not shown in the figures) adapted to measure the distance between the sample S and the potential sensor 3, which distance measuring means are capacitive or optical in nature and may be integrated in the measuring head or they may form a separate external unit. The measured distance value may be used to correct the measured potential value, to correct the sheet resistance value determined by the potential value, and/or to control the actuator 8 so that the distance between the sample S and the potential sensor 3 remains constant within a predetermined error range. Both the correction and the active control of the distance significantly increase the accuracy and reliability of the sheet resistance measurement.

In a further advantageous embodiment, the device according to the invention further comprises means for measuring the surface reflectivity of the sample S, which means are preferably connected in a data transmission with the processing unit 6, so that the result of the reflectivity measurement performed can be used to correct the value of the sheet resistance when calculating the sheet resistance.

Fig. 2a to 2c show some particularly advantageous arrangements of the potential sensor 3 and the light source 2 arranged in the measuring head in a usual arrangement of the measuring head and the sample, wherein the sample is below and the measuring head is above, viewed from the sample direction, i.e. from below. Meanwhile, fig. 2a to 2c essentially correspond to the orthogonal projection of the sensing surface of the potentiometric sensor 3 on the surface of the sample S and the spot illuminated by the light source 2. In each of the embodiments shown in fig. 2a to 2c, i.e. when the light source 2 and the potential sensor 3 are arranged in a line, the sensing surface of each potential sensor 3 preferably forms a plane plate which is symmetrical about an axis connecting the light source 2 and perpendicular to the axis connecting the light source 2, i.e. about an axis indicated by a dashed line parallel to the X and Y directions shown in the figures. The symmetry of the sensing surface about the X-axis ensures that the upper and lower parts of the potential sensor 3 as shown in the figure provide the same resolution and enable measurements to be made with the same signal-to-noise ratio. The symmetry of the sensing surface about the Y-axis ensures that the potential sensor 3 detects the potential due to the illumination of the light source 2 on opposite sides of the potential sensor 3 with similar resolution and signal-to-noise ratio. The device according to the invention may also be implemented symmetrically only along the X-axis or the Y-axis, or not at all, but the calibration of the device and the evaluation of the measurement results will be significantly more complicated. In a particularly preferred embodiment of the device according to the invention, the at least one light source 2 and the potential sensor 3 are arranged relative to each other such that the centers of their orthogonal projections on the surface of the sample S are centrally symmetrical with respect to the center C.

Each of the potential sensors 3 measures an average potential over an area covered by its sensing surface. Thus, smaller sensor areas allow measurements to be made with higher resolution, but smaller sensor areas may cause higher random noise in the measurements. The potential difference measured by the potential sensor 3 is proportional to the sheet resistance along the X direction of the area covered by the potential sensor 3 and between the potential sensors 3. It is therefore particularly advantageous that the dimensions of the selected potential sensors 3 are relatively small but not too small, and that their shape and spacing are selected such that the area covered by said potential sensors 3 is approximately square, or at least that the aspect ratio of said area is not too large. In this way, a high resolution and a relatively isotropic sheet resistance measurement can be achieved.

Fig. 3 is a block diagram of a method of determining sheet resistance in accordance with the present invention. In the method, a sample S to be measured is irradiated 510 with at least one light spot, the potential resulting from the irradiation is measured 520 at least two positions near the irradiated light spot, then the difference of the potentials is generated and amplified 530, and then the sheet resistance is calculated 540 from the measured potential using a predetermined function. Preferably, the predetermined function is a calibration function determined by the measuring device for performing the method of the invention based on potential measurements of samples having different known layer resistivities and the known layer resistivity itself. By forming the differences of the analog signals of the measured potentials, amplifying the analog difference signals, and digitizing the differences only subsequently for processing, a more accurate determination of the sheet resistance can be achieved compared to the prior art.

The illumination step 510 and the potentiometric step 520 are preferably performed in a relatively small neighborhood of three points located on the same line, thereby achieving a higher resolution compared to the prior art.

In a particularly preferred embodiment, the illumination step 510 is performed in the neighborhood of two points, i.e. on two spots, and preferably the potential measurement step 520 is performed at a position between two illuminated spots, preferably by alternately illuminating said spots. Performing the measurement in this way allows to eliminate certain types of defects present in the sample remote from the measurement location, as well as artifacts caused by the edges of the sample, for example, such as electrical signals reflected from the edges of the silicon wafer forming the sample, by averaging the signals recorded and analyzed accordingly during irradiation by the two different light sources. In other words, in this way, the measurement accuracy can be further improved. The mode of performing the above-described irradiation step 510, i.e. at the same frequency and opposite phases, provides only a particularly advantageous and convenient variant of the method of compensating for such errors. Alternatively or in combination, different wavelengths, intensities, illumination spot sizes, modulation frequencies or signal shapes may also be used at the two illumination points.

In a further preferred embodiment of the method according to the invention, the distance between the sample S and the potential sensor 3 is measured, and such measured distance is preferably used in determining the sheet resistance, to correct the measured potential value or to correct the sheet resistance value determined by the potential and/or to actively control the distance between the sample S and the potential sensor 3 by moving the sample S and/or the potential sensor 3 based on the measured distance in such a way that the distance remains within a predetermined range. Both the correction and active distance control discussed herein significantly increase the accuracy and reliability of sheet resistance measurements.

In another preferred embodiment of the method according to the invention, the surface reflectivity of the sample S is measured during the method and used to correct the result when the calculation step 540 of sheet resistance is performed.

In the method according to the invention, the irradiation step 510 is preferably performed at a wavelength of 600nm to 900 nm.

In a particularly preferred embodiment of the method according to the invention, the sheet resistance of the transparent conductive layer, in particular of the indium tin oxide layer or the zinc oxide layer, is determined, wherein the irradiation step 510 is preferably performed at a wavelength of 600nm to 700 nm.

In a particularly preferred embodiment of the method according to the invention, the sheet resistance of the polysilicon layer in the tunnel/tunnel current contact cell passivated with an oxide layer is determined, wherein the irradiation step 510 is preferably performed at a wavelength of 750nm to 900 nm.

Accordingly, the above-described invention provides a non-contact apparatus and method for measuring sheet resistance of various samples, particularly sheet resistance of raw materials for manufacturing semiconductor products, and sheet resistance of semi-finished products or finished products made of the raw materials, with high accuracy, high speed and high resolution, and isotropically if necessary. Detecting material defects by the solution according to the invention at an earlier stage of the manufacturing process allows to remove defective raw materials or semi-finished products from the manufacturing process and/or to quickly detect possible anomalies in the manufacturing process. In this way, the proportion of defective finished products produced is reduced, and consequently, ultimately, the cost of producing finished products is reduced, and at the same time, the amount of waste in the manufacturing process is also reduced, which is also environmentally beneficial.

List of reference numerals: s sample

1 main light source

2 light source

3 potential sensor

4 amplifier

5 lock analyzer

6 processing unit

7 frequency generator

8 actuator

510. Irradiation step 520 potentiometric step 530 amplification step 540 calculates the sheet resistance step.

Claims

1. An apparatus for determining sheet resistance of a sample (S), the apparatus comprising:

-a measuring head comprising:

at least one light source (2) configured to transmit light to the surface of the sample (S), an

At least two potential sensors (3),

an amplifier (4) which is arranged in signal transmission connection with the at least two potential sensors (3),

a processing unit (6) arranged in data communication with the amplifier (4) and configured to be able to determine the sheet resistance from a difference signal of the at least two potential sensors (3),

characterized in that the amplifier (4) is configured to generate the difference signal of the at least two potential sensors (3) and to amplify the difference signal.

2. The device according to claim 1, characterized in that the at least two potentiometric sensors (3) and the at least one light source (2) are arranged such that the center of at least one light spot illuminated by the at least one light source (2) on the sample (S) and the center of orthogonal projections of the sensing surfaces of the at least two potentiometric sensors (3) on the surface of the sample (S) are aligned with each other.

3. The device according to claim 1, characterized in that the at least two potentiometric sensors (3) and the at least one light source (2) are arranged such that at least one light spot illuminated by the at least one light source (2) on the sample (S) and the center of orthogonal projection of the sensing surfaces of the at least two potentiometric sensors (3) on the surface of the sample (S) are aligned with each other and are center-symmetrical with respect to a center (C).

4. A device according to any one of claims 1 to 3, characterized in that the device comprises two light sources (2).

5. The device according to claim 4, characterized in that the at least two potential sensors (3) are arranged between the two light sources (2).

6. The device according to any one of claims 4 to 5, characterized in that the two light sources (2) are configured to be operable at different wavelengths, intensities, irradiation spot sizes, drive frequencies, signal waveforms or phases.

7. The apparatus according to any one of claims 1 to 6, further comprising a distance measuring device arranged to be connected in data communication with the processing unit (6).

8. The apparatus according to claim 7, further comprising an actuator (8) connected in data communication with the processing unit (6) and/or the distance measuring device, the actuator (8) being configured to enable at least one of the at least two potential sensors (3) and the sample (S) to move in a direction towards the other.

9. The apparatus according to any one of the preceding claims, further comprising a reflectivity measuring means for measuring the reflectivity of the surface, the reflectivity measuring means being arranged in data communication connection with the processing unit (6).

10. The device according to any of the preceding claims, characterized in that the at least one light source (2) is configured to emit light having a wavelength between 600nm and 900 nm.

11. A method for determining the sheet resistance of a sample (S), wherein

Illuminating (510) a sample (S) to be inspected with at least one light spot,

the potential due to the illumination in the vicinity of the illuminated spot is measured (520) by at least two potential sensors (3) at least two positions,

calculating (540) the sheet resistance from the measured potential using a predetermined function,

it is characterized in that the method comprises the steps of,

the difference in the potential is determined and amplified (530) prior to determining the sheet resistance.

12. The method according to claim 11, characterized in that the irradiation step (510) and the potential measurement step (520) are performed in the vicinity of three points located on the same line.

13. Method according to claim 11 or 12, characterized in that the sample (S) is illuminated with two spots.

14. The method according to claim 13, characterized in that the irradiation step (510) and the potential measurement step (520) are performed in the vicinity of four points located on the same line.

15. The method according to claim 14, characterized in that the irradiating step (510) is performed with two spots, which are located outside a zone defined by two points defining the position of the potentiometric measurement.

16. The method according to claim 14 or 15, characterized in that the irradiating step (510) is performed with two spots at different wavelengths, intensities, irradiation spot sizes, driving frequencies, signal shapes or phases.

17. Method according to claim 15 or 16, characterized in that the method further comprises the step of measuring the distance between the sample (S) and the potential sensor (3) and moving the sample (S) and/or the potential sensor (3) by means of an actuator (8) based on the measured distance such that the distance is within a predetermined range and/or the distance is used to correct the result obtained in the calculation step (540) of sheet resistance.

18. The method according to any one of claims 11 to 17, further comprising a step of measuring the surface reflectivity of the sample (S), the obtained surface reflectivity being used to correct the result of the calculation step (540) of sheet resistance.

19. The method according to any one of claims 11 to 18, characterized in that in the irradiation step (510) light having a wavelength of 600nm to 900nm is used.

20. The method of any one of claims 11 to 19, further comprising determining the sheet resistance of the transparent conductive layer.

21. The method of claim 20, further comprising determining the sheet resistance of the indium tin oxide layer or the zinc oxide layer.

22. The method of claim 21, wherein the irradiating step (510) is performed at a wavelength in the range of 600nm to 700 nm.

23. The method of any one of claims 11 to 19, further comprising determining a sheet resistance of a passivated tunneling current contact polysilicon layer in the cell.

24. The method of claim 23, wherein the irradiating step (510) is performed at a wavelength in the range of 750nm to 900 nm.