Arrangement and method for moistening a polishing pad

Abstract

The present invention relates to an arrangement and a method for moistening a polishing pad (125). The arrangement comprises a measuring device (101) for determining an electrical resistance between two points on a surface of the polishing pad (125), and a dispensing device (102) for dispensing a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad (125). The invention also relates to a system for polishing a specimen (126).

Description

[0001] Arrangement and method for moistening a polishing pad

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to an arrangement and a method for moistening a polishing pad according to the preambles of the appended independent claims. The invention also relates to a system for polishing a specimen.

[0004] BACKGROUND OF THE INVENTION

[0005] Various polishing systems are known in the prior art for polishing rock, metallographic and other specimens of solid materials for different applications. The polishing systems enable to polish specimens with minimal manual work.

[0006] An exemplary polishing system comprises a rotatable polishing platen and a polishing pad that is attached to the surface of the polishing platen. The polishing system comprises a specimen holder for holding one or more specimens against the surface of the polishing pad. As the polishing platen is rotated, the polishing pad polishes the specimen(s).

[0007] A polishing suspension is used in the polishing system to enhance the polishing action and to prevent overheating during the polishing process. The polishing suspension contains water and / or other suitable liquids, such as ethanol or glycol for moistening, cooling, lubricating, and cleaning the polishing pad and abrasive particles, such as diamond, aluminium oxide, or colloidal silica for polishing of the surface of the specimen. The polishing system comprises a dispenser for dispensing the polishing suspension, typically at a predefined rate, on the surface of the polishing pad.

[0008] A problem associated with the known polishing systems is that, during the polishing, the moisture in the polishing pad can vary considerably as a function of time and as a function of the position on the polishing pad. This has a negative effect on the polishing result. If the polishing pad is too wet, the friction between the specimen and the polishing pad is small, which makes the polishing ineffective and increases the polishing time. If, on the other hand, the polishing pad is too dry, there is a risk that the polishing of the specimen fails due to the large friction between the specimen and the polishing pad.

[0009] OBJECTIVES OF THE INVENTION It is the main objective of the present invention to reduce or even eliminate the prior art problems presented above.

[0010] It is an objective of the present invention to provide an arrangement and a method for moistening a polishing pad. In more detail, it is an objective of the invention to provide an arrangement and a method enabling to provide and maintain a substantially constant and uniform moisture content in the polishing pad during the polishing process. It is also an objective of the invention to provide an arrangement and a method enabling to minimise the waste of a polishing suspension.

[0011] It is also an objective of the present invention to provide a system for polishing a specimen. In more detail, it is an objective of the invention to provide a system enabling to polish a specimen with a uniform and high-quality polishing result.

[0012] In order to realise the above-mentioned objectives, the arrangement and the method according to the invention are characterised by what is presented in the characterising portions of the appended independent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0013] DESCRIPTION OF THE INVENTION

[0014] An arrangement according to the invention for moistening a polishing pad comprises a measuring device that comprises an electrical circuit comprising a first electrical contact element and a second electrical contact element arrangeable in contact with a surface of the polishing pad, and a capacitor having a first terminal and a second terminal, the first terminal of the capacitor being electrically connected to the first electrical contact element, a DC voltage source electrically connected to the electrical circuit, and means for determining an electrical resistance between the first electrical contact element and the second electrical contact element; and a dispensing device configured to dispense a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad.

[0015] The arrangement according to the invention comprises a measuring device and a dispensing device. The measuring device is used for determining an electrical resistance between two points on a surface of a polishing pad, and the dispensing device is used for dispensing a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad.

[0016] The measuring device comprises an electrical circuit that comprises a first electrical contact element (first electrode) and a second electrical contact element (second electrode). The electrical contact elements are intended to be arranged in contact with the surface of the polishing pad. The measuring device may comprise a contact surface for contacting the surface of the polishing pad, wherein the first electrical contact element and the second electrical contact element are exposed at the contact surface. The electrical contact elements can have different shapes and sizes. The electrical contact elements are made of an electrically conductive material, which is preferably also wear and corrosion resistant. Preferable materials for the electrical contact elements are, for example, steel and gold-platinum alloy. The electrical circuit also comprises a capacitor having a first terminal and a second terminal. The first terminal of the capacitor is electrically connected to the first electrical contact element.

[0017] The measuring device comprises a DC voltage source that is electrically connected to the electrical circuit. The DC voltage source applies a DC voltage between the second electrical contact element and the second terminal of the capacitor. When the first electrical contact element and the second electrical contact element are in contact with the surface of the polishing pad, electrical current can flow between the electrical contact elements and the capacitor is charged. The charging speed of the capacitor depends on the electrical resistance between the first electrical contact element and the second electrical contact element. The larger the electrical resistance, the lower the charging speed. The electrical resistance depends on the moisture in the polishing pad. The wetter the polishing pad, the smaller the electrical resistance.

[0018] The measuring device comprises means for determining an electrical resistance between the first electrical contact element and the second electrical contact element. The means for determining an electrical resistance is used when the first electrical contact element and the second electrical contact element are in contact with the surface of the polishing pad.

[0019] The dispensing device is configured to dispense a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad. The value of the determined electrical resistance can be compared to a predefined reference value or profile in order to determine the amount of the polishing suspension to be dispensed. The predefined reference value or profile corresponds to the desired moisture content in the polishing pad. The amount of the polishing suspension to be dispensed on the polishing pad can be, for example, less than 5 pl / cm2, less than 2 pl / cm2or 0.5-2 pl / cm2, or 0.1-100 ml / min. The dispensing device may comprise a container for the polishing suspension and a nozzle that is connected by a hose to the container. The dispensing device may comprise a valve or a pump arranged in connection with the container, the hose or the nozzle, and a control unit for controlling the operation of the valve or the pump. The control unit may comprise a processor and a memory including computer program code, the memory and the computer program code being configured to, with the processor, cause the valve or the pump to dispense suitable amounts of the polishing suspension on the surface of the polishing pad.

[0020] The polishing suspension enhances the polishing action and prevents overheating during the polishing process. The polishing suspension contains water and / or other suitable liquids, such as alcohols (e.g. ethanol, glycol or isopropyl alcohol) for moistening, cooling, lubricating and cleaning the polishing pad. The polishing suspension may also contain abrasive particles, such as diamonds, aluminium oxide, colloidal silica or cerium oxide for polishing of the surface of a specimen. Typically, the polishing suspension (i.e. slurry or fluid) is a mixture of liquid(s) and abrasive(s).

[0021] The measuring device can be arranged inside a housing having a contact surface for contacting a surface of a polishing pad, wherein the first electrical contact element and the second electrical contact element are exposed at the contact surface. The housing can be made of a wear and corrosion resistant ceramic, metal, polymer or composite material, or other suitable material. The housing can be a hollow cylinder having the contact surface at one of its ends. The contact surface is preferably planar. The housing can be dimensioned in such a manner that it can be inserted into a specimen cavity of a specimen holder. The dispensing device can be arranged into the same housing with the measuring device.

[0022] The measuring device and the dispensing device can be arranged into different housings. One or both housings can be dimensioned in such a manner that the housing can be inserted into a specimen cavity of a specimen holder.

[0023] The measuring device can be connected to the dispensing device using a wired or wireless connection. This connection enables the determined electrical resistance values to be communicated to the dispensing device.

[0024] The arrangement may comprise more than one dispensing device. The dispensing devices can be configured to dispense different polishing suspensions, based on the determined electrical resistance, on the surface of the polishing pad. An advantage of the arrangement according to the invention is that it can provide and maintain a substantially constant and uniform moisture content in a polishing pad during the polishing process. Another advantage of the arrangement according to the invention is that it enables to achieve a uniform and high-quality polishing result of a specimen. Yet another advantage of the arrangement according to the invention is that it enables to minimise the waste of a polishing suspension.

[0025] According to an embodiment of the invention the means for determining an electrical resistance between the first electrical contact element and the second electrical contact element is configured to determine a time constant of the electrical circuit, and to determine the electrical resistance between the first electrical contact element and the second electrical contact element based on the determined time constant.

[0026] By a time constant of the electrical circuit is meant a time that is required to charge the capacitor from an initial charge voltage of zero to approximately 63.2 % (1 - e-1) of the value of the applied DC voltage. For determining the time constant of the electrical circuit, the means for determining an electrical resistance between the first electrical contact element and the second electrical contact element may comprise means for monitoring the voltage across the capacitor. After the time constant of the electrical circuit has been determined, the electrical resistance R between the first electrical contact element and the second electrical contact element can be calculated by the following equation: where T denotes the time constant, C denotes the capacitance of the capacitor, and R2denotes the electrical resistance of an optional series-connected resistor in the electrical circuit. The means for determining an electrical resistance between the first electrical contact element and the second electrical contact element may comprise a processor and a memory including computer program code, the memory and the computer program code being configured to, with the processor, calculate the electrical resistance by using the above equation.

[0027] According to an embodiment of the invention the measuring device is configured to if the determined time constant is larger than a predetermined first threshold, replace the capacitor with a capacitor having a smaller capacitance, and if the determined time constant is smaller than a predetermined second threshold, replace the capacitor with a capacitor having a larger capacitance. Decreasing the capacitance value will decrease the time constant and increasing the capacitance value will increase the time constant for the next measurement and thus enables to keep the time constant within a desired range. The first threshold can be, for example, less than 3 s or less than 2 s, or in the range of 0.05-2 s or 0.1-1 s, and the second threshold can be, for example, less than 100 ms or less than 50 ms, or in the range of 0.5 ps- 50 ms or 1 ps-10 ms.

[0028] According to an embodiment of the invention the electrical circuit comprises means for controlling the charging and discharging of the capacitor.

[0029] According to an embodiment of the invention the means for controlling the charging and discharging of the capacitor comprises a voltage-controlled switch. The voltage- controlled switch can be configured to change its state based on a voltage at a node between the capacitor and the first electrical contact element. The duration of the conducting state of the voltage-controlled switch can be, for example, 1 ps-10 s or 1 ms-1 s, and the duration of the non-conducting state can be, for example, 1 ps- 10 s or 1 ps-10 ms.

[0030] According to an embodiment of the invention the capacitance of the capacitor is 1 nF-1 pF. The capacitance of the capacitor is preferably 5 nF-500 nF and more preferably 10 nF-100 nF.

[0031] According to an embodiment of the invention the voltage of the DC voltage source is 1 mV-10 V. The voltage of the DC voltage source is preferably 50 mV-8 V and more preferably 100 mV-5 V.

[0032] The present invention also relates to a system for polishing a specimen. The system according to the invention comprises a polishing platen rotatable about an axis perpendicular to its plane, a polishing pad attached to the polishing platen, and a specimen holder for holding the specimen against a surface of the polishing pad, and an arrangement according to the invention for moistening the polishing pad.

[0033] The polishing system according to the invention can be used for polishing one or more specimens (i.e. objects) of a solid material, such as a piece of a natural (e.g. rock, mineral, ore or meteorite) or artificial (e.g. metal, concrete, ceramic, composite, semiconductor or polymer) material. The specimen to be polished may comprise a substrate on which a semiconductor material, such as silicon or germanium, various lll-V compound semiconductor materials, or the like, have been deposited. In this text, the term polishing means polishing, lapping, fine-grinding and grinding processes with a polishing pad and a polishing suspension. The specimen holder may comprise a plurality of specimen cavities for holding one or more specimens against the surface of the polishing pad. The specimen to be polished is inserted into one of the specimen cavities and held against the surface of the polishing pad with the specimen holder. The specimen holder can be configured to hold one or more specimens against the polishing pad. The number of specimen cavities in the specimen holder can be, for example, 2-5, 5-10, 10-50, 50- 200, or more than 200. The specimen can be secured in the specimen cavity by clamping. The specimen holder can be attached to a polishing head that presses the specimen(s) with an adjustable pressing force against the polishing pad.

[0034] The polishing pad can be made of different materials, such as a fibrous cloth (e.g. real silk, polyester or acetate) or synthetic non-fibrous (e.g. neoprene, polyurethane or composite) material. The polishing pad is preferably disc-shaped, i.e. it has a flat circular shape. The polishing pad is preferably porous, enabling it to absorb the polishing suspension. The diameter of the polishing pad can be, for example, less than 500 mm, preferably 200-400 mm. In some applications, the diameter of the polishing pad can be 400-1500 mm. The thickness of the polishing pad can be, for example, less than 10 mm, less than 3 mm, preferably less than 2 mm, and more preferably less than 1 mm.

[0035] The polishing pad is attached, preferably releasably, to the surface of the rotatable polishing platen. As the polishing platen is rotated, the polishing pad polishes the specimen(s). The polishing platen can be made of a heat conductive and corrosion resistant material, such as aluminium, bronze or stainless steel, or of a plastic, polymer or composite material. The polishing platen is preferably disc-shaped and heat conductive. The diameter of the polishing platen can be, for example, less than 500 mm, preferably 200-400 mm. In some applications, the diameter of the polishing platen can be 400-1500 mm. Preferably, the diameters of the polishing platen and the polishing pad are essentially the same. The thickness of the polishing platen can be, for example, 20-50 mm, or 50-100 mm. The optimal thickness of the polishing platen depends on the diameter and the material of the polishing platen.

[0036] The polishing system may comprise an actuator, such as an electric motor, for rotating the polishing platen. The actuator can be configured to rotate the polishing platen, for example, at a rotational speed of 0-1200 rpm, 0-1000 rpm, 0-800 rpm, 0-600 rpm, or 0-300 rpm.

[0037] In addition to the polishing platen, also the specimen holder can be rotatable. The specimen holder can be rotatable about an axis, which is parallel to the rotation axis of the polishing platen. The polishing system may comprise an actuator, such as an electric motor, for rotating the specimen holder. The actuator can be configured to rotate the specimen holder, for example, at a rotational speed of 0-300 rpm, 0-200 rpm, 0-150 rpm, 0-100 rpm, or 0-75 rpm.

[0038] The measuring device and / or the dispensing device of the arrangement according to the invention can be inserted into one or two specimen cavities of the specimen holder. The arrangement may comprise more than one dispensing device, wherein each of the dispensing devices can be inserted into a specimen cavity of the specimen holder.

[0039] An advantage of the polishing system according to the invention is that it can provide and maintain a substantially constant and uniform moisture content in the polishing pad during the polishing process. Another advantage of the polishing system according to the invention is that it enables to achieve a uniform and high-quality polishing result of the specimen. Yet another advantage of the polishing system according to the invention is that it enables to minimise the waste of a polishing suspension.

[0040] The present invention also relates to a method for moistening a polishing pad. The method according to the invention comprises arranging a first electrical contact element and a second electrical contact element in contact with a surface of the polishing pad, the first electrical contact element being electrically connected to a first terminal of a capacitor, applying a DC voltage between the second electrical contact element and a second terminal of the capacitor, determining an electrical resistance between the first electrical contact element and the second electrical contact element, and dispensing a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad.

[0041] An advantage of the method according to the invention is that it can provide and maintain a substantially constant and uniform moisture content in a polishing pad during the polishing process. Another advantage of the method according to the invention is that it enables to achieve a uniform and high-quality polishing result of a specimen. Yet another advantage of the method according to the invention is that it enables to minimise the waste of a polishing suspension.

[0042] According to an embodiment of the invention the step of determining an electrical resistance between the first electrical contact element and the second electrical contact element comprises determining a time constant, and determining the electrical resistance between the first electrical contact element and the second electrical contact element based on the determined time constant.

[0043] According to an embodiment of the invention the method comprises if the determined time constant is larger than a predetermined first threshold, replacing the capacitor with a capacitor having a smaller capacitance, and if the determined time constant is smaller than a predetermined second threshold, replacing the capacitor with a capacitor having a larger capacitance.

[0044] According to an embodiment of the invention the method comprises discharging the capacitor, and repeating the steps of applying a DC voltage between the second electrical contact element and a second terminal of the capacitor and determining an electrical resistance between the first electrical contact element and the second electrical contact element.

[0045] According to an embodiment of the invention the method comprises repeating the steps of applying a DC voltage between the second electrical contact element and a second terminal of the capacitor, determining an electrical resistance between the first electrical contact element and the second electrical contact element, and discharging the capacitor at predetermined time intervals.

[0046] The present invention also relates to use of an arrangement according to the invention for moistening a polishing pad and to use of a system according to the invention for polishing a specimen.

[0047] The exemplary embodiments of the invention presented in this text are not interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this text as an open limitation that does not exclude the existence of also unrecited features. The features recited in the dependent claims are mutually freely combinable unless otherwise explicitly stated.

[0048] The exemplary embodiments presented in this text and their advantages relate by applicable parts to the arrangement, the system as well as the method according to the invention, even though this is not always separately mentioned.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Fig. 1 illustrates a schematic cross-sectional view of an arrangement according to an embodiment of the invention, figs. 2A-2B illustrate examples of an electrical circuit that can be used in a measuring device, and fig. 3 illustrates a schematic side view of a system according to an embodiment of the invention.

[0051] DETAILED DESCRIPTION OF THE DRAWINGS

[0052] The same reference signs are used of the same or like components in different embodiments.

[0053] Fig. 1 illustrates a schematic cross-sectional view of an arrangement according to an embodiment of the invention for moistening a polishing pad.

[0054] The arrangement comprises a measuring device 101 and a dispensing device 102. The measuring device 101 is used for determining an electrical resistance between two points on a surface of the polishing pad, and the dispensing device 102 is used for dispensing a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad.

[0055] The measuring device 101 comprises an electrical circuit 103 (surrounded by dashed lines) of which only some electrical components are shown in fig. 1 . Detailed examples of the electrical circuit 103 are shown in figs. 2A and 2B. The electrical circuit 103 comprises electrical contact elements 104 and 105 that can be arranged in contact with the surface of the polishing pad. The measuring device 101 has a contact surface 106 on which the electrical contact elements 104 and 105 are arranged. The electrical circuit 103 comprises a capacitor 107 that is electrically connected to the electrical contact element 104.

[0056] The measuring device 101 comprises a DC voltage source 108 that is electrically connected to the electrical circuit 103. The DC voltage source 108 applies a DC voltage between the electrical contact element 105 and the capacitor 107. When the electrical contact elements 104 and 105 are in contact with the surface of the polishing pad, electrical current can flow between the electrical contact elements 104 and 105, as a result of which the capacitor 107 is charged. The charging speed of the capacitor 107 depends on the electrical resistance between the electrical contact elements 104 and 105. The electrical circuit 103 comprises means (not shown in fig. 1 ) for controlling the charging and discharging of the capacitor 107, said means enabling the capacitor 107 to be charged and discharged repeatedly. The measuring device 101 comprises a measuring unit 109 that is configured to measure the voltage across the capacitor 107 and to determine a time constant of the electrical circuit 103. The time constant is a time that is required to charge the capacitor 107 from an initial charge voltage of zero to approximately 63.2 % (1 - e-1) of the value of the applied DC voltage.

[0057] The measuring device 101 comprises a processing unit 110 that is connected to the measuring unit 109 with a wired connection 111. The processing unit 110 is configured to receive the time constant from the measuring unit 109 and to calculate the electrical resistance between the electrical contact elements 104 and 105 based on the time constant.

[0058] The dispensing device 102 comprises a container 112 for the polishing suspension, a nozzle 113 that is connected by a hose 114 to the container 112, and a valve 115 that is arranged in connection with the hose 114. The dispensing device 102 comprises a control unit 116 that is connected to the processing unit 110 with a wired connection 117. The control unit 116 is configured to receive the electrical resistance from the processing unit 110 and to control, based on the electrical resistance, the operation of the valve 115 so that a suitable amount of the polishing suspension is dispensed on the surface of the polishing pad.

[0059] Figs. 2A-2B illustrate examples of an electrical circuit that can be used in a measuring device.

[0060] The electrical circuit of fig. 2A comprises electrical contact elements 104 and 105 for contacting a surface of a polishing pad, a capacitor 107 that is electrically connected to the electrical contact element 104, and a resistor 118 that is electrically connected to the electrical contact element 105. The electrical circuit comprises a voltage-controlled switch 119 that is used for controlling the charging and discharging of the capacitor 107. A DC voltage source 108 is used to supply a DC voltage to the electrical circuit.

[0061] When the electrical contact elements 104 and 105 are arranged in contact with the surface of the polishing pad, and the voltage-controlled switch 119 is open (i.e. in a non-conducting state), the capacitor 107 charges and the voltage at a node 120 between the electrical contact element 104 and the capacitor 107 increases. When the voltage at the node 120 exceeds a first threshold value, the voltage-controlled switch 119 closes (i.e. it goes into a conducting state). As a result of this, the capacitor 107 begins to discharge and the voltage at the node 120 decreases. The voltage-controlled switch 119 stays closed until the voltage at the node 120 falls below a second threshold value. When the voltage at the node 120 falls below the second threshold value, the voltage-controlled switch 119 opens, and the capacitor 107 begins to charge again.

[0062] The electrical circuit of fig. 2B comprises electrical contact elements 104 and 105 for contacting a surface of a polishing pad, a capacitor 107 that is electrically connected to the electrical contact element 104, and a resistor 121 that is electrically connected to the electrical contact element 105. The electrical circuit comprises a voltage-controlled switch 122 that is used for controlling the charging and discharging of the capacitor 107. A DC voltage source 108 is used to supply a DC voltage to the electrical circuit.

[0063] When the electrical contact elements 104 and 105 are arranged in contact with the surface of the polishing pad, and the voltage-controlled switch 122 is closed (i.e. in a conducting state), the capacitor 107 charges and the voltage at a node 120 between the electrical contact element 104 and the capacitor 107 increases. When the voltage at the node 120 exceeds a first threshold value, the voltage-controlled switch 122 opens (i.e. it goes into a non-conducting state). As a result of this, the capacitor 107 begins to discharge and the voltage at the node 120 decreases. The voltage- controlled switch 122 stays open until the voltage at the node 120 falls below a second threshold value. When the voltage at the node 120 falls below the second threshold value, the voltage-controlled switch 122 closes, and the capacitor 107 begins to charge again.

[0064] Fig. 3 illustrates a schematic side view of a system according to an embodiment of the invention for polishing a specimen.

[0065] The system comprises a polishing platen 123 that can be rotated about an axis, which is perpendicular to its plane, with an actuator 124. A polishing pad 125 is attached to the polishing platen 123. The polishing pad 125 is used for polishing a specimen 126 that is held against a surface of the polishing pad 125 with a specimen holder 127. The specimen 126 is arranged in a specimen cavity 128 of the specimen holder 127. The specimen holder 127 can be rotated about an axis, which is parallel to the rotation axis of the polishing platen 123, with an actuator 129. As the polishing platen 123 and the specimen holder 127 are rotated, the polishing pad 125 polishes the specimen 126. The system comprises a measuring device 101 for determining an electrical resistance between two points on a surface of the polishing pad 125, and a dispensing device 102 for dispensing a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad 125. The measuring device 101 and the dispensing device 102 are arranged inside a housing 130 that is arranged in a specimen cavity 131 of the specimen holder 127 in such a manner that the housing 130 is in contact with the surface of the polishing pad 125.

[0066] Only advantageous exemplary embodiments of the invention are described in the figures. It is clear to a person skilled in the art that the invention is not restricted only to the examples presented above, but the invention may vary within the limits of the claims presented hereafter. Some possible embodiments of the invention are described in the dependent claims, and they are not to be considered to restrict the scope of protection of the invention as such.

Claims

Claims1. An arrangement for moistening a polishing pad, characterised in that the arrangement comprises:- a measuring device that comprises:- an electrical circuit comprising a first electrical contact element and a second electrical contact element arrangeable in contact with a surface of the polishing pad, and a capacitor having a first terminal and a second terminal, the first terminal of the capacitor being electrically connected to the first electrical contact element,- a DC voltage source electrically connected to the electrical circuit, and- means for determining an electrical resistance between the first electrical contact element and the second electrical contact element, and- a dispensing device configured to dispense a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad.

2. The arrangement according to claim 1 , characterised in that the means for determining an electrical resistance between the first electrical contact element and the second electrical contact element is configured to:- determine a time constant of the electrical circuit, and- determine the electrical resistance between the first electrical contact element and the second electrical contact element based on the determined time constant.

3. The arrangement according to claim 2, characterised in that the measuring device is configured to:- if the determined time constant is larger than a predetermined first threshold, replace the capacitor with a capacitor having a smaller capacitance, and if the determined time constant is smaller than a predetermined second threshold, replace the capacitor with a capacitor having a larger capacitance.

4. The arrangement according to any of the preceding claims, characterised in that the electrical circuit comprises means for controlling the charging and discharging of the capacitor.

5. The arrangement according to claim 4, characterised in that the means for controlling the charging and discharging of the capacitor comprises a voltage-controlled switch.

6. The arrangement according to any of the preceding claims, characterised in that the capacitance of the capacitor is 1 nF-1 pF.

7. The arrangement according to any of the preceding claims, characterised in that the voltage of the DC voltage source is 1 mV-10 V.

8. A system for polishing a specimen, comprising:- a polishing platen rotatable about an axis perpendicular to its plane,- a polishing pad attached to the polishing platen, and- a specimen holder for holding the specimen against a surface of the polishing pad, characterised in that the system comprises an arrangement according to any of the preceding claims for moistening the polishing pad.

9. A method for moistening a polishing pad, characterised in that the method comprises:- arranging a first electrical contact element and a second electrical contact element in contact with a surface of the polishing pad, the first electrical contact element being electrically connected to a first terminal of a capacitor,- applying a DC voltage between the second electrical contact element and a second terminal of the capacitor,- determining an electrical resistance between the first electrical contact element and the second electrical contact element, and- dispensing a polishing suspension, based on the determined electrical resistance, on the surface of the polishing pad.

10. The method according to claim 9, characterised in that the step of determining an electrical resistance between the first electrical contact element and the second electrical contact element comprises:- determining a time constant, and- determining the electrical resistance between the first electrical contact element and the second electrical contact element based on the determined time constant.11 . The method according to claim 10, characterised in that the method comprises:- if the determined time constant is larger than a predetermined first threshold, replacing the capacitor with a capacitor having a smaller capacitance, and- if the determined time constant is smaller than a predetermined second threshold, replacing the capacitor with a capacitor having a larger capacitance.

12. The method according to any of claims 9 to 11 , characterised in that the method comprises:- discharging the capacitor, and- repeating the steps of applying a DC voltage between the second electrical contact element and a second terminal of the capacitor and determining an electrical resistance between the first electrical contact element and the second electrical contact element.

13. The method according to claim 12, characterised in that the method comprises repeating the steps of claim 12 at predetermined time intervals.

14. Use of an arrangement according to any of claims 1 to 7 for moistening a polishing pad.

15. Use of a system according to claim 8 for polishing a specimen.

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