Method, measurement system and software for performing high-throughput electrical measurements, particularly at cryogenic temperatures

Abstract

The invention relates to a method for performing electrical measurements on a sample at low temperatures wherein a sample holder (2) with a sample and at least one probe needle (3) are moved with respect to each other, for performing an electrical measurement on the sample. The invention further relates to a measurement system (100) for performing electrical measurements on a sample, a computer program and a use of the measurement system (100).

Description

[0001] uba109wo 1

[0002] Method, measurement system and software for performing high-throughput electrical measurements, particularly at cryogenic temperatures

[0003] Description:

[0004] The present invention relates to a method, a measurement system, a computer program for performing electrical measurements on a sample and a use of the measurement system at cryogenic temperatures.

[0005] In the field of sample testing and research, particularly semiconductor sample testing and research, probe stations are widely used for electrically characterizing samples with devices at various stages of fabrication at a high throughput. A critical aspect of operating a probe station is the accurate alignment of the probe needles with respect to the sample. In the prior art, this process often presents significant challenges, especially when dealing with increasingly miniaturized and densely packed structures. Misalignment or improper contact between the probe needles and the sample can result in inaccurate measurements, damage to the sample, or breakage of the probe needles themselves.

[0006] Conventional probe stations typically require significant operator skill and time to manually adjust the probe needles into the correct position. The complexity of this process is further compounded when working with high-density integrated circuits or other samples with intricate structures, where the precise alignment of multiple probe needles becomes even more difficult. Additionally, in certain configurations, visualizing the exact location of the probe needles in relation to the sample can be hindered by limited visibility or obstructions in the setup. This can lead to trial-and-error adjustments, causing inefficiencies and potential damage during the testing process.

[0007] Existing systems may also suffer from limitations in the degree of control or automation they provide for needle positioning, further complicating the localization process. As a result, there is a need for improved solutions that simplify the localization of probe needles with respect to the sample, reducing the time and effort required for accurate probe placement, while minimizing the risk of damage and enhancing the overall precision of the testing process.

[0008] The challenges described above are even more significant in the field of low-temperature device testing. Typically, this process involves electrically contacting the sample by wirebonding at room temperature, followed by cooling the sample to low temperatures for testing, and then warming it up again once the investigation is complete. This approach has clear disadvantages, as the number of electrical contacts available in a cryostat is limited, restricting the number of devices that can be tested during a single cool-down. Consequently, only a relatively small number of devices can be tested before the sample needs to be warmed up uba109wo 2 again to bond additional devices or prepare a new sample for cooling. Since cool-downs are time-consuming and expensive, this process is also economically inefficient, further increasing the overall cost of testing.

[0009] Based on this, it is subject of the present invention to provide a method, a measurement system as well as a computer program for performing electrical measurements on a sample that are improved with respect to the technical problems described above.

[0010] This task is solved by a method for performing electrical measurements on a sample with the features of claim 1 , a measurement system for performing electrical measurements on a sample according to claim 21 , a computer program for performing electrical measurements on a sample according to claim 36 as well as a use of the measurement system according to claim 37.

[0011] Advantageous embodiments of the invention are given in the corresponding dependent claims and described in the following.

[0012] A first aspect of the invention relates to a method for performing electrical measurements on a sample, wherein a sample holder with a sample and at least one probe needle are moved with respect to each other, for performing an electrical measurement on the sample.

[0013] The term “electrical measurements” in the context of the present invention particularly refers to current, voltage and / or capacitance measurements which may be used to investigate sample properties. On the other hand, “electrical measurement” in the context of the present invention may also refer to current, voltage and / or capacitance measurements which may be used to localize one or more probe needles with respect to the sample.

[0014] In particular, the sample is at least partially electrically conductive. For example, the sample is a metal sample, a superconductor sample, or a semiconductor sample. More particularly, the sample can comprise a semiconductor device with or without bond pads such as metallic bond pads for contacting the semiconductor device with the at least one probe needle.

[0015] According to an embodiment of the invention, electrical signals generated by an interaction, particularly an electric interaction between the sample and the probe needle that are indicative of a positional information of the at least one probe needle with respect to the sample are detected.

[0016] In particular, the electrical signals comprise one or more of a list consisting of the following: an electrical capacitance between the at least one probe needle and the sample, a capacitive charging current between the at least one probe needle and the sample, an electrostatic influence current evoked in the at least one probe needle. In particular, the electrical signals uba109wo 3 comprise the electrical capacitance, the capacitive charging current and the electrostatic influence current.

[0017] The term “positional information” may particularly include information regarding a displacement and particularly a change of a displacement in time of at least one probe needle with respect to the sample. The displacement between the sample and the at least one probe needle can be indicated along a displacement direction between the sample and the probe needle which may be oriented vertically, that is perpendicularly on a surface of the sample facing the at least one probe needle. However, the displacement between the sample and the at least one probe needle can also be indicated in a horizontal plane spanned by two orthogonal horizontal directions being oriented orthogonally to the (vertical) displacement direction. The term “positional information” may particularly include information regarding a relative horizontal position between the probe needle and the sample, such that the probe needle may be localized in a horizontal plane with respect to the sample.

[0018] Particularly, for the detection of the electrical capacitance and the electrostatic influence current, the at least one probe needle is not in mechanical contact with the sample but displaced from the sample as these electrical signals are based on distant electrostatic effects. For the detection of the capacitive charging current the at least one probe needle may or may not be in contact with the sample. Being indicative of the position of the at least one probe needle with respect to the sample, the detected electrical signals can advantageously be used to determine a positional information of the at least one probe needle with respect to the sample.

[0019] More particularly, the electrical signals comprise at least one transient of the capacitive charging current between the at least one probe needle and the sample. A transient of the capacitive charging current can be understood as a non-steady capacitive charging current that peaks as a function of time, for example due to charging or discharging events. A transient can be a pulse-like signal. For instance, a mechanical contact between the at least one probe needle and the sample can cause a transient capacitive charging current, wherein an increase followed by a decay of the capacitive charging current corresponds to the charging or discharging of the at least one probe needle or the sample, to equalize finite local electrical potential differences between the two. The end of the transient, i.e. when the capacitive charging current drops to zero or below the accuracy of the measurement, indicates that an equilibrium state has been reached, for example due to charge equalization. However, the transient of the detected capacitive charging current does not need to be detected as time- resolved onset, peak and decay of the detected capacitive charging current. For example, at capacitances on the order of a few pF, the transient of the detected capacitive charging current can be detected for instance from moving averages of the detected electrical current between uba109wo 4 the sample and the at least one probe needle, wherein the transient of the detected capacitive charging current can be identified from an event where the moving average is higher than the current noise of the detected capacitive charging current.

[0020] In an embodiment of the invention, the electrical capacitance is detected from a detected electric oscillation frequency of an LC circuit formed by the sample holder with the sample, the at least one probe needle and at least one inductive element. The LC circuit can be understood as an inductance-capacitance resonant circuit. The electric oscillation frequency can be a frequency, at which an electric current or an electric voltage oscillates within the LC circuit. From the inductance of the at least one inductive element and the detected electric oscillation frequency, the electrical capacitance can be determined from the relation f = 1 / (2TTVIC), wherein f is the electric oscillation frequency, L is the inductance of the at least one inductive element and C is the electrical capacitance between the at least one probe needle and the sample. Preferably, f is a resonance frequency at which, when the LC circuit is driven by an external source with an angular frequency, the inductive and capacitive reactances are equal in magnitude. The detection of the electrical capacitance using the LC circuit enables a more accurate measurement even for very small capacitances, since f depends on 1 / Vc and small changes in C produce noticeable shifts in f. This frequency can be determined at high precision using electronic measurement techniques known from the prior art. The inductive element can be electrically connected to the at least one probe needle and / or the sample holder in parallel or in series.

[0021] In an embodiment of the invention, at least for the detection of the electrical capacitance, the at least one probe needle is kept on a reference ground potential and an electrical potential is applied to the sample holder and thus to the sample. By this measure, parasitic capacitances of for example cables, particularly shielded cables in a cryostat that are electrically connected to the at least one probe needle do not affect the measured electrical capacitance, which results in an improved signal compared to the situation where the electrical potential is applied to the probe needle with the sample holder and the sample being on the reference ground potential.

[0022] According to another embodiment of the invention, the movement between the sample and the at least one probe needle is controlled based on the detected electrical signals. For example, the movement between the sample and the at least one probe needle is controlled based on the detected electrical capacitance in that the movement is directed toward areas with increased electrical capacitance, which typically indicates the proximity of metallic structures such as bond pads of the sample. Similarly, the detected electrostatic influence current and its change in time and / or space increases in proximity to metallic structures and can as such for instance indicate the proximity of bond pads, which information can be used for controlling the uba109wo 5 movement between the sample and the at least one probe. Moreover, the movement between the sample holder and the at least one probe needle can be controlled based on the detected electrical capacitance, so as to prevent the probe needle from contact-damaging the sample and / or the probe needle, as decreasing distances between the at least one probe needle and the sample are indicated by increasing detected electrical capacitance. In particular, the movement between the sample and the at least one probe needle is controlled to establish a mechanical and electrical contact between the sample and the at least one probe needle, more particularly to establish mechanical and electrical contacts between multiple probe needles and the sample at different locations on a surface of the sample, such that electrical measurements can be performed by means of current or voltage measurements between different probe needles contacting the sample. Furthermore, the movement between the sample and the at least one probe needle can be slowed down or stopped based on an onset of a capacitive charging current detected by the at least one probe needle, which indicates close proximity and thus an upcoming mechanical contact event between the probe needle and the sample.

[0023] Particularly, the movement between the sample and the at least one probe needle is controlled based on the detected electrical signals in a feedback loop. That is, the detected electrical signals are used to move and to adapt the movement between the sample and the at least one probe needle based on the detected electrical signals.

[0024] In particular, an approach between the sample and the at least one probe needle along a displacement direction defined by the displacement between the at least one probe needle and the sample is controlled based on the capacitive charging current and / or the electrostatic influence current detected during the approach.

[0025] In the context of the present invention, the term “approach” can be understood as the process of reducing the displacement between one or more probe needles and the sample, particularly until one or more, particularly all probe needles are in mechanical and electrical contact with the sample.

[0026] In another embodiment of the invention, a mechanical contact event between the sample and the at least one probe needle is detected from the detected electrical capacitance, the detected capacitive charging current and / or the detected electrostatic influence current; particularly from a capacitive charging current that exceeds a predetermined threshold value.

[0027] Particularly, the mechanical contact event between the sample and the at least one probe needle is detected from at least one transient of the detected capacitive charging current. That is, the mechanical contact event between the sample and the at least one probe needle can be detected by a rapid onset of the capacitive charging current, followed by a decay of the uba109wo 6 capacitive charging current, reflecting a charging or discharging event of the sample or the at least one probe needle as a consequence of the mechanical contact between the two. In the time domain, the transient of the capacitive charging current indicating the mechanical contact event can be on the order of the RC time constant, wherein R is the resistance of the at least one probe needle, including cabling and contact resistance and C the capacitance between sample and probe needle. Due to its characteristic transient nature, this type of signal can be detected in a simple and reliable manner. However, particularly at capacitances on the order of a few pF, the transient of the detected capacitive charging current can be detected for example from said moving averages of the detected electrical current between the sample and the at least one probe needle, wherein the transient of the detected capacitive charging current and thus the mechanical contact event can be identified from an event where the moving average is higher than the current noise of the detected capacitive charging current. In this case, the mechanical contact event can be clearly identified as well. The detected capacitive charging current can be amplified by increasing an electrical potential difference between the sample holder and the at least one probe needle, particularly wherein the sample holder is at a finite electrical potential with respect to a ground potential of the at least one probe needle.

[0028] In yet another embodiment of the invention, the sample is a semiconductor sample and the semiconductor sample is exposed to electromagnetic radiation in order to render the semiconductor sample conductive, such that the detected electrical signals are enhanced, wherein the electromagnetic radiation is repeatedly switched on and off during the approach, such that the contact event between the sample and the at least one probe needle can be detected from an alternation of the detected capacitive charging current between two distinct current levels, wherein a first current level is obtained when the electromagnetic radiation is switched on and a second current level is obtained when the electromagnetic radiation is switched off. The alternating electromagnetic radiation repeatedly induces transient capacitive charging current flows for those probe needles that are in electrical and particularly mechanical contact with the sample, while those probe needles that are not in electrical and particularly mechanical contact with the sample will not show repeatedly induced transient capacitive charging current flows. This information can be used to confirm if individual, multiple or all probe needles are in contact with the sample.

[0029] In another embodiment of the invention, the sample is a semiconductor sample and the semiconductor sample is exposed to electromagnetic radiation with associated photon energies below a bandgap of the semiconductor sample. For example, for a silicon-based semiconductor sample, the semiconductor sample is exposed to infrared electromagnetic radiation as disclosed herein. This allows for an optical inspection of the sample and particularly the at least one probe needle without causing a carrier accumulation in the uba109wo 7 semiconductor sample, which typically negatively affects the sample stability or damages the sample.

[0030] In particular, the method according to the first aspect of the invention is a method for performing electrical measurements on a sample at low temperatures, that is, cryogenic temperatures, for instance at temperatures at or below 77K, particularly at or below 4.2K, more particularly at or below 1 K.

[0031] According to an embodiment of the invention, the method according to the first aspect of the invention including the determination of said electrical signals to determine positional information of the at least one probe needle with respect to the sample is used for a sample at low temperatures. This embodiment advantageously allows to contact different areas of the sample, for instance different bond pads of the sample in situ at low temperature, which provides a particularly efficient way to investigate the sample by means of electrical measurements at low temperature in a fast and economic fashion. In contrast, conventional methods and measurement systems typically require an electrical contacting by means of wirebonding at room temperature followed by a cool-down of the sample to low temperatures, which is warmed up once the sample has been investigated. This process consumes a lot of time and resources, as only a few devices can be investigated in a single cooldown.

[0032] Particularly, the sample comprises a plurality of devices, particularly semiconductor devices. Each device can form a separate electric circuit, for instance comprising quantum dots to be characterized. As such, the method according to the first aspect of the invention may be used to investigate multiple devices during one and the same cool-down, which substantially improves the throughput and efficiency of the sample investigation.

[0033] In another embodiment of the invention, spatial information of the sample is determined from the electrical capacitance, the capacitive charging current and / or the electrostatic influence current detected at different relative positions between the at least one needle and the sample.

[0034] In the context of the present invention, the term “spatial information of the sample” may for example include a composition of the sample, i.e. the localization of regions of the sample comprising specific materials such as metallic structures like bond pads, and their geometry.

[0035] In yet another embodiment of the invention, the sample and the at least one probe needle are moved with respect to each other along at least one horizontal direction perpendicular to said displacement direction and wherein said positional information of the at least one probe needle with respect to the sample is determined from the electrical capacitance and / or the electrostatic influence current detected at different relative positions along the at least one horizontal direction. For example, the sample and the at least one probe needle may be moved with uba109wo 8 respect to each other in a two-dimensional plane spanned by the first and the second horizontal direction while detecting said electrical signals in order to scan these areas of the sample.

[0036] According to another embodiment of the invention: a spatial electrical capacitance map of the sample is determined from the detected electrical capacitance and / or a spatial electrostatic influence current map of the sample is determined from the detected electrostatic influence current detected at different relative positions between the sample and the at least one probe needle along the at least one horizontal direction. This map can reflect a two-dimensional representation of the sample and at least parts of its composition. For example, the map can depict the location and spatial extent such as the geometry of one or more metallic structures such as bond pads of the sample. The map can be generated by processing the electric signals on a computer as disclosed herein.

[0037] In particular, positional information of at least a one, for instance a first metallic structure on the sample, particularly of a first bond pad on the sample, is derived based on the electrical capacitance map and / or based on the spatial electrostatic influence current map of the sample.

[0038] Moreover, spatial information of at least a second metallic structure on the sample, particularly of a second bond pad on the sample, can be derived based on the spatial information of the at least one first metallic structure as well as a spatial layout of the metallic structures of the sample, particularly a spatial layout of bond pads of the sample.

[0039] Particularly, the positional information of the first and / or the second metallic structure is further derived based on a pattern recognition technique. For instance, the pattern recognition technique may derive positional information of said second metallic structure and / or further metallic structures based on said first metallic structure. This can particularly be achieved by providing the pattern recognition technique and / or a computer executing the pattern recognition technique with a sample layout that comprises the individual metallic structures and their spatial arrangement on the sample.

[0040] In particular, at least one first probe needle is moved to or onto the first metallic structure and / or wherein at least one second probe needle is moved to or onto the second metallic structure, based on the electrical capacitance map and / or based on the spatial electrostatic influence current map of the sample. In other words, the localization of only a first metallic structure by means of the pattern recognition technique, particularly using the known sample layout, can be sufficient to localize further metallic structures relative to said first metallic structures. This information can be used to drive individual probe needles toward specific metallic structures based on the localization of just a single (first) metallic structure. uba109wo 9

[0041] At least two probe needles can be brought into contact with the sample, particularly wherein the first probe needle is brought into contact with the first metallic structure and the second probe needle is brought into contact with the second metallic structure, and an electrical measurement signal is detected between the at least two probe needles contacting the sample, particularly between the first and the second metallic structure of the sample. For example, the electrical measurement signal can comprise or be an electrical current flowing through the sample between the first probe needle and the second probe needle, when the first and the second probe needles are in contact with the metallic structures of the sample. The electrical measurement signal can be used to derive a sample resistance. The sample resistance may in turn be used to assess sample characteristics.

[0042] According to another embodiment of the invention, the positional information of the at least one probe needle with respect to the sample is determined based on both the electrical signals disclosed herein or corresponding electrical data as well as recorded images or their corresponding image data.

[0043] In another embodiment, positional information of at least one metallic structure of the sample, particularly of a bond pad of the sample, is derived based on an optical map of the sample determined from recorded images of the sample, particularly wherein the images are recorded using said photon energies below the bandgap of the semiconductor sample.

[0044] In particular, the positional information of the at least one metallic structure of the sample, particularly of the bond pad of the sample, is derived based on the electrical capacitance map, the spatial electrostatic influence current map and / or the optical map of the sample. Based on the positional information, the at least one probe needle can be aligned with the metallic structure of the sample. More particularly, mechanical contact between the at least one probe needle and the at least one metallic structure can be established based on the positional information.

[0045] A second aspect of the invention relates to a measurement system for performing electrical measurements, comprising a probe station, wherein the probe station comprises: a sample holder configured to hold a sample at least one probe needle for performing an electrical measurement on the sample and an actuator unit configured to move the sample holder and the at least one probe needle with respect to each other, for performing an electrical measurement on the sample.

[0046] In particular, the measurement system according to the second aspect of the invention is configured to execute the method according to the first aspect of the invention. To this end, uba109wo 10 the measurement system may comprise a computer configured to send instructions to the actuator unit that cause the actuator unit to execute said relative movements between the at least one probe needle and the sample holder.

[0047] The embodiments of the first aspect of the invention can be applied to the second aspect of the invention, and vice versa.

[0048] In another embodiment of the invention, the measurement system comprises an electrical measurement unit configured to detect electrical signals generated by an interaction between the sample and the at least one probe needle that are indicative of a position of the at least one probe needle with respect to the sample. In particular, the electrical signals comprise one or more of a list consisting of the following: an electrical capacitance between the at least one probe needle and the sample, a capacitive charging current between the at least one probe needle and the sample, an electrostatic influence current evoked in the at least one probe needle, such that the position of the at least one probe needle with respect to the sample can be determined from the detected electrical signals. Said computer can receive data encoding the electrical signals from the electrical measurement unit. Particularly, said computer is configured to determine said spatial electrical capacitance map and / or said spatial electrostatic influence current map of the sample from the detected electrical capacitance and / or the detected electrostatic influence current.

[0049] In another embodiment of the invention, the measurement system comprises an optical microscope arranged and configured to capture an image of a region comprised by the sample holder and, if the sample holder holds a sample in said region, of the sample, such that a position of the at least one probe needle with respect to said region or the sample can be determined from said image. In particular, the microscope can be configured to record said image or multiple images and to generate associated image data that can be sent to the computer.

[0050] If a sample is held by the sample holder, the optical microscope may be arranged and configured to capture an image showing the entire sample or a portion of it, which particularly depends on the size of the sample and the field of view of the microscope.

[0051] According to another embodiment of the invention, the microscope comprises a source of electromagnetic radiation for exposing the sample to electromagnetic radiation.

[0052] In particular, the electromagnetic radiation comprises or consists of white-light electromagnetic radiation. White-light electromagnetic radiation can comprise wavelengths between 400 nm and 800 nm.

[0053] Alternatively or additionally, the electromagnetic radiation can comprise or consist of nearinfrared electromagnetic radiation. Infrared electromagnetic radiation can comprise uba109wo 11 wavelengths between 1100 nm and 1900 nm. Using infrared electromagnetic radiation is particularly advantageous for investigating silicon-based samples, as the associated photon energies are below the bandgap of silicon. This allows for an optical inspection of the sample without causing a carrier accumulation, which typically negatively affects the sample stability or damages the sample.

[0054] According to an embodiment of the invention, the probe station comprises at least two probe needles, wherein the electrical measurement unit is configured to detect an electrical measurement signal between at least two probe needles contacting the sample. The electrical measurement signal can for example be used to derive a sample resistance which comprises valuable information for assessing sample characteristics.

[0055] In an embodiment of the invention, the measurement system is configured to operate at cryogenic temperatures, for instance at temperatures at or below 77K, particularly at or below 4.2K, more particularly at or below 1 K. In particular, the probe station is configured to operate at cryogenic temperatures. More particularly the sample holder, the at least one probe needle and the actuator unit are configured to operate at cryogenic temperatures. To this end, for example, the actuator unit can comprise piezo and / or electromagnetic motors to realize the movement of the sample holder and / or the probe needle. To operate at cryogenic temperatures, for example, the sample holder can comprise or consist of copper or ceramic and / or the at least one probe needle can comprise or consist of tungsten or beryllium copper. Particularly, the actuator unit can comprise ceramic bearings such that the actuator can be operated at cryogenic temperatures.

[0056] In another embodiment of the invention, the measurement system comprises a cryostat for cooling the sample.

[0057] The cryostat can comprise a first temperature stage configured to be cooled to a first temperature and a second temperature stage configured to be cooled to a second temperature, wherein the second temperature is lower than the first temperature. Although the second temperature is lower than the first temperature, the second temperature stage provides less cooling power, as the cooling power decreases significantly with decreasing temperature. The first temperature stage can provide cooling power to cool components of the measurement system, such as the actuator unit, to the low temperature regime. Moreover, in the low temperature regime, the first temperature stage can be used to provide cooling power to the second temperature stage and / or an intermediate temperature stage as disclosed herein In case of a dilution refrigerator as the cryostat, for example, the first temperature stage can be a still stage that operates at between 600 mK and 1.5 K, particularly at between 600 mK and 1 K, and the second temperature stage can be a mixing chamber stage that can operate at temperatures of below 20 mK in the low temperature regime. uba109wo 12

[0058] The sample holder can be connected to the second temperature stage, wherein a thermal insulator is arranged in a heat flow path between the sample holder and the second temperature stage. The heat flow path is formed once the sample holder is cooled to the second temperature of the second temperature stage, which is lower than the first temperature of the first temperature stage, and the insulator advantageously reduces the amount of heat flowing from the first temperature stage to the sample holder due to the temperature gradient.

[0059] In another embodiment, the cryostat comprises an intermediate temperature stage configured to be cooled to an intermediate temperature between the first temperature of the first temperature stage and the second temperature of the second temperature stage. In case of a dilution refrigerator as the cryostat, for example, the intermediate temperature stage can be a cold plate that operates for example at between 50 mK to 200 mK, particularly between 60 mK and 100 mK in the low temperature regime. This temperature is between the typical operation temperature of a still stage and the mixing chamber stage of a dilution refrigerator.

[0060] The sample holder can be connected to the intermediate temperature stage, wherein a thermal insulator is arranged in a heat flow path between the sample holder and the intermediate temperature stage.

[0061] In particular, the first temperature stage is configured to cool the actuator unit to the first temperature.

[0062] More particularly, the cryostat comprises a helium-based cooling system configured to cool the second temperature stage by at least partially and / or temporarily heating the first temperature stage, and wherein a heater unit for heating the first temperature stage comprises the actuator unit. This enables cooling of the second temperature stage and / or the intermediate temperature stage by using the heat generated by the actuator unit to heat the first temperature stage,. That is, the actuator unit can realize a movement of the sample holder or the probe needle as well as a heating of the second temperature stage at the same time. As the cooling power of the second temperature stage, particularly the mixing chamber, increases by heating the second temperature stage, the heat generated under movement of the sample holder and / or the at least one probe needle realized by the actuator unit contributes to the cooling power of the second temperature stage. Besides the actuator unit, the heater unit can comprise further heating elements for heating the first temperature stage, particularly when the actuator unit is not generating heat by moving the sample holder and / or the at least one probe needle.

[0063] The actuator unit can be connected to the first temperature stage. The connection can be a direct mechanical connection between the actuator unit and the first temperature stage. Additionally or alternatively, the actuator unit and the first temperature stage can be connected by thermal straps, for example made of copper. As such, the connection between the actuator uba109wo 13 unit and the first temperature stage enables excellent heat conduction between the actuator unit and the first temperature stage, which allows both an efficient cooling of the actuator unit to low temperatures as well as an efficient contribution of the actuator unit to the heating of the first temperature stage at low temperatures, if the actuator unit is in operation.

[0064] The thermal insulator can be arranged in a heat flow path between the actuator unit and the sample holder, wherein the sample holder is connected to the second temperature stage or the intermediate temperature stage.

[0065] Particularly, the sample holder can be connected to the second temperature stage, and a first thermal insulator and a second thermal insulator separated from each other by a metal element are arranged in a heat flow path between the sample holder and the actuator unit, wherein the metal element is connected to the intermediate temperature stage. By splitting the thermal insulator into the first and second thermal insulator separated by the metal element, the metal element creates a short connection for heat loads conducted from the actuator unit connected to the still stage to the sample holder connected to the mixing chamber. As such, in case more heat than the cooling power of the mixing chamber can handle is conducted toward the sample holder, this heat can advantageously drain into the cold plate.

[0066] For example, the helium-based cooling system is a Helium-3 cryostat, wherein the first temperature stage is a sorption pump stage and / or a 1 K-pot stage for cooling the second temperature stage, particularly a Helium-3 pot. Alternatively, the helium-based cooling system can be a dilution refrigerator, wherein the first temperature stage is a still stage, a 1 K-pot stage or a 4K stage, the second temperature stage is a mixing chamber stage and / or the intermediate temperature stage is a cold plate.

[0067] In another embodiment of the invention:

[0068] - the second temperature stage is arranged below the first temperature stage along a longitudinal axis of the cryostat, or wherein temperature stage is arranged below both the first temperature stage and the intermediate temperature stage along the longitudinal axis of the cryostat, and

[0069] - the probe holder and / or the sample holder is directly or indirectly connected to the second temperature stage or the intermediate temperature stage via pillars extending from second temperature stage or the intermediate temperature stage through openings or recesses of the first temperature stage or through the openings or the recesses of the first temperature stage and openings or the recesses of the intermediate temperature stage to the probe holder and / or the sample holder.

[0070] This arrangement allows for optical inspection of the sample on the sample holder from above, if a sample is arranged on the sample holder. uba109wo 14

[0071] In another embodiment of the invention, the probe station is at least partially enclosed by a radiation shield, and wherein a top face of the radiation shield arranged above the sample holder and the first temperature stage along the longitudinal axis of the cryostat comprises an opening for optical inspection of the sample on the sample holder if a sample is arranged on the sample holder.

[0072] According to another embodiment of the invention, the measurement system comprises: the optical microscope arranged and configured to capture an image of a region comprised by the sample holder and, if the sample holder holds a sample in said region, of the sample, such that a position of the at least one probe needle with respect to said region or the sample can be determined from said image and the cryostat for cooling the sample.

[0073] In particular, the microscope is at least partially comprised by or integrated in the cryostat. For example, at least one optical element, particularly one lens, one mirror and / or one beamsplitter of the microscope is comprised by the cryostat. To this end, the at least one optical element can be attached to the cryostat, for instance to a temperature stage of the cryostat as disclosed herein or to other structures such as rods comprised by the cryostat.

[0074] The embodiments of the first and / or the second aspect of the invention can be applied to the third aspect to the invention, and vice versa.

[0075] According to an embodiment of the invention, the cryostat of the measurement system comprises a first temperature stage for cooling the actuator unit to a first temperature and a second temperature stage for cooling a sample on the sample holder to a second temperature, wherein the second temperature is lower than the first temperature and wherein a thermal insulator is arranged between the sample holder and the actuator unit of the probe station. For example, the thermal insulator comprises a ceramic material, particularly a glass ceramic, more particularly fluorophlogopite glass-ceramic; or a metal with a thermal conductivity below 0.1 or 0.01 W / (mK) at a temperature of 0.1 K, for example titanium or steel, particularly stainless steel.

[0076] In an embodiment of the invention, the measurement system comprises the sample.

[0077] A third aspect of the invention relates to a computer program comprising instructions which, when executed on a computer, cause the computer to control the measurement system according to the second aspect of the invention to execute the method according to the first aspect of the invention. The computer can form part of the measurement system according to the second aspect of the invention. uba109wo 15

[0078] A fourth aspect of the invention relates to a use of the measurement system according to the second aspect of the invention at cryogenic temperatures, for instance at temperatures at or below 77K, particularly at or below 4.2K, more particularly at or below 1 K. To this end, for example, the measurement system according to the second aspect of the invention that does not comprise a cryostat can be attached to a cryostat or components of a cryostat, and a subsequently be used at cryogenic temperatures established by a cooling system of the cryostat.

[0079] Exemplary embodiments are described below in conjunction with the Figures. The Figures are appended to the claims and are accompanied by text explaining individual features of the shown embodiments and aspects of the present invention. Each individual feature shown in the Figures and / or mentioned in the text of the Figures may be incorporated (also in an isolated fashion) into a claim relating to the first aspect, the second aspect and / or the third aspect according to the present invention.

[0080] Fig. 1 shows a measurement system and a method for performing electrical measurements on a sample according to an embodiment of the invention;

[0081] Fig. 2a-c show schematically electrical signals detected by the system sand method shown in Fig. 1 , specifically an electrical capacitance (Fig. 2a), a capacitive charging current (Fig. 2b) and an electrostatic influence current (Fig. 2c);

[0082] Fig. 3 shows an embodiment of a measurement system according to the invention, comprising a microscope;

[0083] Fig. 4 shows another embodiment of a measurement system according to the invention, comprising a microscope and a cryostat;

[0084] Fig. 5 shows a low temperature section of a cryostat of a measurement system according to an embodiment of the invention;

[0085] Fig: 6 shows a low temperature section of a cryostat of a measurement system according to another embodiment of the invention that does not require a frame as the embodiment of Fig. 6; and

[0086] Fig. 7 shows a low temperature section of a cryostat of a measurement system according to another embodiment of the invention, wherein the sample can be cooled to an intermediate temperature between a first uba109wo 16 temperature of a still stage and a second temperature of a mixing chamber of the cryostat;

[0087] Fig. 8 shows a low temperature section of a cryostat of a measurement system according to another embodiment of the invention, wherein the sample can be cooled to the second temperature of a mixing chamber of the cryostat; and

[0088] Fig. 9 shows a low temperature section of a cryostat of a measurement system according to another embodiment of the invention, wherein the insulator between the actuator unit and the sample holder is split into a first and a second insulator with a metal element separating the first and the second insulator.

[0089] Fig. 1 shows a measurement system 100 and a method for performing electrical measurements on a sample 1 according to an embodiment of the invention. The measurement system 100 comprises a probe station 10 with a sample holder 2 configured to hold a sample 1 , at least one probe needle 3 for performing an electrical measurement on the sample 1 and an actuator unit 12 configured to move the sample holder 2 and the at least one probe needle 3 with respect to each other. By means of an example, Fig. 1 shows just a single probe needle 3. However, the probe station 10 may comprise a plurality of probe needles 3. The sample 1 and the probe needle 3 can be displaced with respect to each other by means of the actuator unit 12, which is configured to move the sample holder 2 and thus the sample 1 both vertically along a displacement direction z between the sample 1 and the probe as well as horizontally, i.e. perpendicular to the displacement direction z. The horizontal movement can be realized in two orthogonal horizontal directions, a first horizontal direction designated x and a second horizontal direction designated y. For example, the actuator unit 12 comprises piezo- and / or electromagnetic motors to realize the movement.

[0090] The relative movement between the sample 1 and the at least one probe needle 3 allows to bring the sample 1 and the probe needle into mechanical and electrical contact with each other, such that an electrical measurement can be performed on the sample 1. In the present embodiment, the sample holder 2 is metallic and holds a sample 1 comprising a semiconducting silicon substrate 1a covered by an oxide layer 1 b. The sample 1 can be characterized by electrical measurements performed using the present probe station 10 by establishing mechanical and electrical contact between the probe needles 3 and the sample 1 . To this end, a plurality of metallic bond pads 1c are deposited on the oxide layer 2. uba109wo 17

[0091] The measurement system according to the present embodiment further comprises an electrical measurement unit 11 which serves at least the following purposes: Firstly, the electrical measurement unit 11 is configured to record currents flowing between at least two probe needles 3, particularly once the at least probe needles are in contact with the sample 1. To this end, the measurement unit 11 can be configured to apply a voltage to at least one of the probe needles 3 to trigger a current flow to be recorded. Particularly, the electrical measurement unit 11 can be configured to determine for example a sample resistance of the sample 1 from the current recorded for a predefined voltage applied between two probe needles 3. The sample resistance can in turn be used to assess sample characteristics such as functionalities of a semiconductor device integrated in the sample 1. For example, the probe station 10 comprises at least four probe needles 3 such that the sample characteristics can be assessed from a four terminal measurement performed on the sample 1 , wherein a current is set between two terminals and a voltage drop along the current flow is measured between the other two terminals. However, for performing such an electrical measurement, the sample 1 and the probe needles 3 have to first establish mechanical and electrical contact with each other, which can be challenging due to typically small dimensions of bond pads 1c and fragility of the sample 1. Moreover, the optical access to the sample can be obstructed by cryostat components or limited in case no microscope is present and / or if no sufficient illumination is available, which can particularly be the case if the sample 1 is located in a cryostat at low temperatures. To ensure an accurate and reliable positioning of the probe needles 3 on the sample 1 under such conditions, the electrical measurement unit 11 is further configured to detect electrical signals that are indicative of positional information of the at least one probe needle 3 with respect to the sample 1. These signals comprise at least one of the following: an electrical capacitance between the at least one probe needle 3 and the sample 1 , a capacitive charging current between the at least one probe needle 3 and the sample 1 and / or an electrostatic influence current evoked in the at least one probe needle 3. Fig. 2 schematically explains the positional information comprised by these electrical signals in more detail.

[0092] Optionally, the measurement system 100 according to the present embodiment can comprise a source of electromagnetic radiation 23, particularly for electromagnetic radiation in the whitelight range between 400 nm and 800 nm and / or in the near-infrared range between 1100 nm and 1900 nm, to avoid charge carrier accumulation.

[0093] As further shown in Fig. 1 , the measurement system 100 according to the present embodiment further comprises a computer 13 which is configured to receive signal data indicative of the electrical signals detected by the electrical measurement unit 11 and to send control data for controlling the movement of the sample holder 2 caused by the actuator unit 12 and optionally for controlling the generation and emission of electromagnetic radiation by the source of uba109wo 18 electromagnetic radiation 23. Particularly, the movement between the sample 1 and the at least one probe needle 3 is controlled by the computer 13 based on the detected electrical signals or recorded images of the sample in a feedback loop. That is, the detected electrical signals are used to move and to adapt the movement between the sample 1 and the at least one probe needle 3 based on the detected electrical signals, particularly for determining positional information of the at least one probe needle 3 with respect to the sample 1 and / or to bring the sample 1 and the at least one probe needle 3 into mechanical and electrical contact with each other.

[0094] Figs. 2a-c show schematically electrical signals detected by the measurement system 100 and method shown in Fig. 1 , specifically an electrical capacitance (Fig. 2a), a capacitive charging current (Fig. 2b) and an electrostatic influence current (Fig. 2c). The electrical signals may be detected and recorded by an electrical measurement unit 11 controlled by and communicating with a computer 13 as described in Fig. 1 and corresponding text.

[0095] Fig. 2a depicts schematically the electrical capacitance detected between the sample 1 and the probe needle 3 during an approach between the sample 1 and the probe needle 3, i.e. during a relative movement along said displacement direction z. As the electrical capacitance scales with 1 / |z|, wherein |z| denotes the absolute value of the distance between the sample 1 and the probe needle 3, the electrical capacitance can be used as an indicator for the distance or the change of the distance between the probe needle 3 and the sample 1. For larger distances, the detected capacitance changes slower with |z| compared to lower distances, such that the capacitance and / or its derivative can be used to predict an upcoming contact event between the sample 1 and the probe needle 3. At the same time, the electrical capacitance is indicative for the sample composition in proximity to the probe: for example, at a given distance |z| between the sample 1 and the probe needle 3, the electrical capacitance is higher if the probe needle 3 is located above a metallic structure such as a bond pad 1c compared to an insulating structure such as a semiconductor at low temperatures. This information can be used to localize bond pads 1c on the sample 1 and particularly to bring the sample 1 and the probe needle 3 into contact with each other at a bond pad 1c localized in this fashion.

[0096] In particular, for the detection of the electrical capacitance, the at least one probe needle 3 is kept on a reference ground potential and an electrical potential controlled by the measurement unit 11 is applied to the sample holder 2 and thus to the sample 1. By this measure, parasitic capacitances of cables, particularly shielded cables in a cryostat that are electrically connected to the at least one probe needle 3 do not affect the measured electrical capacitance, which results in an improved signal compared to the situation with the electrical potential being uba109wo 19 applied to the probe needle 3 with the sample holder 2 and the sample 1 being on the reference ground potential.

[0097] Fig. 2b shows schematically transients of the capacitive charging current detected between the sample 1 and the probe needle 3 during an approach between the sample 1 and the probe needle 3, i.e. during a relative movement along said displacement direction z. The capacitive charging current corresponds to a charging current of a capacitor, which current flows between the sample 1 and the probe needle 3. The capacitor may be realized for example by the probe needle 3 and electrical cables connected to the probe needle 3, wherein a voltage is applied between the sample 1 and the probe needle 3. As can be understood from the plot, no capacitive charging current is flowing as long as the distance between the sample 1 and the probe needle 3 is too large to realize an electrical contact between the two. However, as the electrical and particularly mechanical contact is established, the capacitive charging current features a pronounced, transient peak that thus indicates an onset of and / or a mechanical contact event between the sample 1 and the probe needle 3.

[0098] Fig. 2c illustrates transients of the electrostatic influence current detected between the sample 1 and the probe needle 3 during a horizontal relative movement between the sample 1 and the probe needle 3 along the first horizontal direction x. The movement may be performed at a constant distance |z| between the sample 1 and the probe needle 3, for example at a distance between 20 - 50 pm. For the detection of the electrostatic influence current, a potential difference can be applied between the sample 1 and the probe needle 3 and the electrostatic influence current is detected during a relative movement between the sample 1 and the probe needle 3 along the first horizontal direction x. As can be understood from Fig. 2c, the electrostatic influence current, which scales with the derivative of the electrical capacitance, shows pronounced bipolar current peaks at positions of the probe needle 3 near metallic bond pads 1c that cause a substantially increased electrostatic influence current in the probe needle 3, compared to positions away from conductive areas of the sample 1. Hence, the electrostatic influence current allows to localize metallic structures such as bond pads 1c of the sample, which information may be used to establish contact between the probe needle 3 and the sample 1 , particularly at a bond pad 1c. The potential difference between the sample 1 and the probe needle 3 may correspond to a voltage of between 1 mV and 1 V. A horizontal velocity of the relative movement between the probe needle 3 and the sample 1 in the horizontal directions x,y may be between 100 pm / s and 1000 pm / s. At these relatively high velocities, the transient peaks of the electrostatic influence current are particularly pronounced due to the fast horizontal displacement between the probe needle 3 and the sample 1 .

[0099] Fig. 3 shows an embodiment of a measurement system 100 according to the invention. The present measurement system 100 comprises a probe station 10 with a plurality of probe uba109wo 20 needles 3 for contacting a sample 1 on a sample holder 2, which can be moved with respect to the probe needles 13 in three orthogonal directions x,y,z by means of an actuator unit 12. The probe needles 3 protrude from a probe card 4 toward the sample holder 2 with the sample 1 for contacting the sample 1 , which is enclosed by a housing 10a of the probe station 10. To perform electrical measurements on the sample 1 as well as to localize individual or all probe needles 3 with respect to the sample 1 by means of said electric signals, the probe needles 3 are electrically connected with an electrical measurement unit 11 via the probe card 4 and an electrical connector 5. The sample holder 2 and the sample 1 electrically connected to the sample holder 2 are likewise electrically connected to the measurement unit 11. The computer 13 and the measurement unit 11 may thus allow for the same functionalities as described in Fig. 1 and Fig. 2.

[0100] The measurement system 100 according to the present embodiment of Fig. 3 additionally comprises a microscope 20 for optical inspection of and / or access to the probe needles 3 and the sample 1. The microscope 20 shown here may be part of a measurement system 100 with a probe station 10 for performing electrical measurements on the sample 1 at room temperature or it may be implemented in a low temperature measurement system in combination with a cryostat 30, for example as shown in Fig. 4.

[0101] As best seen in Fig. 3, the microscope 20 comprises a microscope head 20a with a source of electromagnetic radiation 23 facing a first lens 21 collimating the electromagnetic radiation as it propagates toward a beam splitter 26. A fraction of the collimated electromagnetic radiation is reflected downward along the z-direction toward a second lens 22 for focusing the electromagnetic radiation onto the sample 1. The second lens 22 may be mounted to the microscope head 20a, to the probe station 10 or an intermediate structure arranged between the microscope head 20a and the probe station 10. The probe card 4 comprises an opening 4a for access of the electromagnetic radiation onto the sample 1. The source of electromagnetic radiation 23 can be configured to generate and emit near-infrared electromagnetic radiation with wavelengths between 1100 nm and 1900 nm and / or white-light electromagnetic radiation with wavelengths between 400 nm and 800 nm. In particular, nearinfrared electromagnetic radiation may be used for samples 1 comprising silicon such as silicon substrates to prevent carrier accumulation particularly at low temperature while still allowing for optical inspection of the sample 1. Carrier accumulation is known to impose a risk of instability or damage to semiconducting samples 1 at low temperature. The electromagnetic radiation reflected back upward along the z-direction from the sample 1 is collimated by means of said second lens 22 and focused onto a camera 24 by a third lens 25 which focus can be adjusted by moving it along the z-direction. The camera 24 is configured to record corresponding images of the sample 1 and the probe needles 3 that may be forwarded in the uba109wo 21 form of corresponding image data to the computer 13. As such, the microscope 20 permits for example to optically verify whether individual probe needles 3 are in mechanical contact with the sample 1 , particularly with bond pads 1c of the sample, after a relative movement between the sample 1 and the probe needles 3 aiming at a contact event. The recorded images or image data of the sample 1 and the probe needles 3 comprise, just like said electrical signals, positional information of the at least one probe needle 3 with respect to the sample 1 , such that they can likewise be used to localize the relative positions between the probe needle 3 and the sample 1 and / or to compare the localization determined based on the detected electrical signals. The measurement system 100 can comprise additional optical elements, such as lenses and / or mirrors to improve the performance of the microscope 20, particularly for field of view adaption and / or correction of image aberrations.

[0102] As can further be seen in Fig. 3, the microscope head 20a comprises a kinematic stage 27 for changing an orientation of the microscope head 20a with respect to the probe station 10 and particularly the sample 1. To this end, the kinematic stage 27 comprises a plurality, preferably three adjustment screws 27a mounted to a platform 27c of the kinematic stage 27. Each adjustment screw 27a comprises a hemispherical screw tip 27b for contacting an adapter structure 28. By screwing individual adjustment screws 27a clockwise or anticlockwise, particularly a tilt angle of the microscope head 20a with respect to the probe station 10 and the sample 1 can be adjusted in order to control a beam path of the electromagnetic radiation propagating toward the sample 1. The adapter structure 28 can be used to create an interface between the microscope head 20a and the probe station 10 or structures between the microscope head 20a and the probe station 10, such as sections of a cryostat 30 as depicted in Fig. 4.

[0103] Fig. 4 shows another embodiment of a measurement system 100 according to the invention, comprising a probe station 10 as well as a microscope 20 partially integrated into a cryostat 30. The probe station 10 shown in Fig. 4 may be realized for example according to Fig. 1 or Fig. 3. Optionally, the measurement system 100 according to the present embodiment may additionally comprise an electrical measurement unit 11 and a computer 13 as shown in Fig. 1 or 3, such that the measurement system 100 may be likewise configured for the same functionalities regarding the localization of individual probe needles 3 with respect to the sample 1 based on the electrical signals presented in Fig. 2.

[0104] The microscope 20 according to the present embodiment comprises a microscope head 20a that may feature the components shown in detail in Fig. 3. The microscope head 20a is connected via an adapter structure 28 to a room temperature stage 35 of a cryostat 30 such as for example a probe stick, a helium 3 system or a dilution refrigerator. A first lens 21 of the microscope 20 for exposing the sample 1 in the probe station 10 to electromagnetic radiation uba109wo 22 is arranged inside the microscope head 20 (cf. Fig. 3) which can be attached to the room temperature stage 35 of the cryostat 30. However, as can be seen in Fig. 4, a second lens 22 of the optical microscope 20 is mounted to rods 34 and / or a first temperature stage 31 of the cryostat 30, such that the microscope is partially integrated in the cryostat 30. If the cryostat 30 features a single low temperature stage, for example in case of a probe stick that can be cooled to the boiling point of a cryogenic in a dewar filled with said cryogenic liquid such as liquid nitrogen or helium, the second lens 22 may be attached anywhere on rods 34 of the probe stick such that it is configured and arranged to focus the electromagnetic radiation onto the sample 1.

[0105] If the cryostat 30 features multiple temperature stages that can be cooled to different temperatures, such as said first temperature stage 31 at a first temperature and a second temperature stage 32 at a second temperature that is lower than the first temperature, as is the case for example in helium 3 systems or dilution refrigerators, the second lens 22 can be particularly attached to the first temperature stage 31. This temperature stage 31 may be for example at the boiling point of the cryogenic liquid used for cooling, in case of helium-4 at around 4.2K. In contrast, at least the actuator unit 12 of the probe station 11 , particularly the sample 1 , is attached to the second temperature stage 32 at lower temperature.

[0106] The present embodiment according to Fig. 4 advantageously allows to localize probe needles 3 on a sample 1 and particularly to move probe needles 3 to selected areas of the sample 1 , such as its bond pads 1c, while keeping the sample 1 at cryogenic temperatures. This allows to investigate particularly samples 1 with multiple devices including a plurality of different electric circuits such as transistors, quantum dots, or superconducting circuits during one and the same cool-down, which substantially improves the throughput and efficiency of the sample investigation.

[0107] Fig. 5 shows a low temperature section of a cryostat 30 of a measurement system 100 according to an embodiment of the invention. This section of the cryostat 30 comprises a first temperature stage 31 configured to be cooled to a first temperature and a second temperature stage 32 configured to be cooled to a second temperature that is lower than the first temperature. For example, the two temperature stages 31 ,32 are part of a dilution refrigerator, with the first temperature stage 31 being cooled to temperatures of approx. 4.2K, 1 K or 200mK and the second temperature 32 being cooled to the lowest base temperature of the dilution refrigerator, for instance to 20 mK. These types of helium-based cooling systems use a mixture of Helium-3 and Helium-4 to cool samples 1 down to the mK-range. To this end, as is known to the skilled person, an endothermic mixing process is used, which is based on the crossing of Helium-3 atoms from a concentrated phase containing nearly pure Helium-3 into a dilute phase containing mostly Helium-4. The absorption of energy caused by the crossing of Helium- uba109wo 23

[0108] 3 atoms from the concentrated into the diluted phase delivers the cooling power of the dilution refrigerator. Dilution refrigerators comprise a first temperature stage 31 , typically known as still or evaporator still, that is at least partially and / or temporarily heated to increase the vapor pressure of Helium-3 in the first temperature stage 31 , which drives evaporation of Helium-3 and thus circulation of Helium-3 through the system. This circulation allows continuous dilution at a second temperature stage 32, typically known as the mixing chamber, where the cooling power arises from the enthalpy of mixing of Helium-3 and Helium-4. As can be seen in Fig. 5, the first temperature stage 31 is connected to the actuator unit 12 of the probe station 10 by a frame 37 of the cryostat. The frame 37 preferably comprises a thermally conductive material such as copper, such that the actuator unit 12 can be cooled efficiently to the temperature of the first temperature stage 31 , which typically provides relatively high cooling powers at temperatures of 4.2K, 1 K or 200mK. In contrast, as can also be understood from Fig. 5, the housing 10a of the probe station 10 extends through openings 37a of the frame 37 of the first temperature stage 31 and is itself connected to the second temperature stage 32. The sample holder 2 and particularly the sample 1 on the sample holder 2 are thermally anchored to the second temperature stage 32 via a cold finger 36 such as copper braid or a flexible copper thermal strap 36a and the housing 10a. A thermal insulator 33 separates the actuator unit 12 connected to the first temperature stage 31 from the sample holder 2 and particularly the sample 1 connected to the second temperature stage 32. This measure thermally decouples the sample from the actuator unit 12 and advantageously allows to reduce the cooling power necessary to cool the sample 1 to the lowest possible temperature. The actuator unit 12, particularly piezo and / or electromagnetic actuators comprised by the actuator unit 12, generate heat during the actuation. This heat is typically larger than the cooling power of the second temperature stage 32. The thermal decoupling ensures that the heat generated by the actuator unit 12 dissipates towards the first temperature stage 31 , which advantageously decreases the lowest temperature that can be reached by the second temperature stage 32 of the cryostat 30.

[0109] Fig. 6 shows a low temperature section of a cryostat 30 of a measurement system 100 according to another embodiment of the invention. The low temperature section shown in Fig. 6 can be part of a helium-based cooling system of a dilution refrigerator 30 and is shown in a cross-sectional view.

[0110] The cryostat 30 comprises a first temperature stage 31 operating at a first temperature and a second temperature operating at a second temperature 32. For example, the second temperature stage 32 is a mixing chamber 32 of the cryostat, i.e. the coldest structure of the cryostat 30 that can be cooled down to 20 mK. The first temperature stage 31 can be the still stage 31 or an intermediate temperature stage 330 operating at an intermediate temperature uba109wo 24 between the first temperature of the still stage 31 and the second temperature of the second temperature stage 32 of the dilution refrigerator 30.

[0111] As shown in Fig. 6, the first temperature stage 31 and the actuator unit 12 are thermally anchored to each other by means of a direct mechanical connection between the first temperature stage 31 and the actuator 12 and by copper thermal straps 36a. Besides the direct mechanical connection between the actuator unit 12 and the first temperature stage 31 , the thermal straps 36a provide additional heat flow paths for a heat flow from the actuator unit 12 to the first temperature stage 31.

[0112] If the first temperature stage 31 is the still stage 31 , heating of the first temperature stage 31 enables continuous dilution of the Helium-3 and Helium-4 mixture in the second temperature stage 32 of the cryostat 30, which is in the present embodiment the mixing chamber 32 of the dilution refrigerator 30. Since the actuator unit 12 and the still stage 31 are in direct heat exchange, heat generated by the use of the actuator unit 12 during operation, that is, when it is moving the sample holder 2 with respect to the probe card 4 with its plurality of probe needles 3, causes a heating of the still stage 31. In particular, the still stage 31 can be heated to and kept at a first temperature within a temperature window, in which the heated still stage 31 enables continuous dilution of the Helium-3 and Helium-4 mixture into the mixing chamber 32. As such, the actuator unit 12 can form part of a heater unit for heating the still stage 31. If the actuator unit 12 is not in operation, other heating elements of the heater unit can be used to heat the still stage 31. For example, the still stage 31 is heated by means of the heater unit, particularly by the actuator unit 12, to between 0.6 K and 1 K, which typically requires a few mW, This enables cooling of the mixing chamber 32 to a second temperature of for example 20 mK and below. As such, in the present embodiment, the actuator unit 12 contributes to continuous operation of the dilution refrigerator 30, since the actuator unit 12 fulfills a double function of moving the sample holder 2 and heating the still stage 31.

[0113] Compared to the embodiment of Fig. 5, the embodiment of Fig. 6 does not require a frame 37 and the mixing chamber 32 or a plate in thermal contact with the mixing chamber 32 essentially serves as a holder for the probe card 4. The sample 1 on the sample holder 2 is also in thermal contact with the mixing chamber 32 via copper thermal straps 36a, such that the sample 1 can be cooled to said second temperature.

[0114] For example, the cryostat 30 shown in Fig. 6 is an inverted dilution refrigerator, wherein the mixing chamber 21 is located at the top rather than at the bottom of the cryostat 30. The basic thermodynamic principle remains the same as in a conventional design: cooling arises from the endothermic mixing of Helium 3 and Helium 4 at the interface between the concentrated and dilute phases. However, instead of relying on gravity to maintain the phase separation, the uba109wo 25 inverted configuration can use for example capillary confinement to hold the two phases in place. This allows the system to operate independently of gravity and making it ideal for cryogen-free (“dry”) setups such as those used in quantum computing experiments.

[0115] Fig. 7 shows a low temperature section of a cryostat 30 of a measurement system 100 according to another embodiment of the invention. The probe station 10 is arranged at least partially within a radiation shield 39 in the cryostat 30, which can be a dilution refrigerator 30, as in Fig. 6. The dilution refrigerator 30 comprises a first temperature stage 31 , the still stage 31 , a second temperature stage 32, the mixing chamber 32, and an intermediate temperature stage 330, the cold plate 330. The first temperature stage 31 could also be another temperature stage of the cryostat operating above the temperatures of the mixing chamber 32 and the intermediate temperature stage 330, such as the 4K stage. As such, compared to the embodiment of Fig. 6, the cryostat 30 of the present embodiment comprises at least three temperature stages 31 ,32,330. By heating the still stage 31 , the mixing chamber 32 can be cooled to a second temperature, which is lower than a first temperature of the still stage 31 . In the same fashion, the cold plate 330 can be cooled to an intermediate temperature between the first and the second temperature. The heating power for cooling the mixing chamber 32 and the cold plate 330 can be provided at least partially by the actuator unit 12, i.e. at least when the actuator unit 12 is moving the sample holder 2. Additional heating elements can heat still stage 31 when the actuator unit 12 is not in operation. For first temperatures of between 0.6 K and 1 K and a cooling power of a few mW, the second temperature of the mixing chamber 32 can be below 20 mK, while the intermediate temperature of the cold plate can be in the range from 60 mK to 100 mK.

[0116] The sample 1 on the sample holder 2 is arranged above still stage 31 and the cold plate 32 and the mixing chamber 330 along a longitudinal axis of the cryostat 30, in order to allow for optical inspection of the sample 1 inside the cryostat 30 from above and to enable the mixture of Helium-3 and Helium-4 to accumulate gravity-driven in the mixing chamber 32 below the cold plate 330 and the still stage 31. The radiation shield 39 comprises an opening 39a for optical inspection of the sample 1 arranged on the sample holder 2 via an optical microscope 20, for example as shown in Fig. 4. A transparent window 42 is arranged in or on the opening 39a, such that the sample 1 on the sample holder 2 can be inspected by the optical microscope 20 via the transparent window 42. The transparent window 42 is optically transparent at least for an optical imaging wavelength used by the microscope 20. For example, the transparent window 42 is made of fused silica. This material transmits electromagnetic radiation well at the wavelength used to illuminate the sample 1 but reflects light at longer wavelengths, for example for wavelengths of around 5 pm and above. This means that unwanted thermal radiation cannot reach the sample 1 and heat it up. uba109wo 26

[0117] To cool the sample 1 to the intermediate temperature of the cold plate 330 in this arrangement, a plurality of pillars 38 extend upwardly from the cold plate 330, that is, against the direction of gravity, beyond the still stage 31 via openings 31a or recesses of the still stage 31. Above the still stage 31 , the pillars 38 are connected to the probe card 4, in order to cool the probe card 4 and the probe needles 3 extending from the probe card 4 toward the sample holder 2 to the intermediate temperature. Additionally, the sample holder 2 is thermally anchored to the pillars 38 and thus the cold plate 330, for example via a flexible thermal straps 36a.

[0118] As the sample 1 arranged on the sample holder 2 is thermally anchored to the cold plate 330, the temperature of the sample 1 can essentially correspond to the intermediate temperature of the cold plate 330, i.e. in the range from 60 mK to 100 mK.

[0119] While the sample 1 is in the present embodiment cooled to the intermediate temperature of the cold plate 330, the mixing chamber 32 below the cold plate 330 can be used at the same time for other low temperature measurements. The mixing chamber 32 can be cooled to said second temperature below the first and the intermediate temperature by cooling it using cooling power provided by the actuator unit 12 and / or additional heater elements.

[0120] Fig. 8 shows a low temperature section of a cryostat 30 of a measurement system 100 according to another embodiment of the invention. In the present embodiment, the cryostat 30 is a dilution refrigerator 30 and comprises three temperature stages 31 ,32,330 as the cryostat 30 of Fig. 7.

[0121] The mixing chamber 32 is arranged below the still stage 31 and the cold plate 330 along a longitudinal axis of the cryostat 30, such that the mixture of Helium-3 and Helium-4 can accumulate in the mixing chamber 32 under the influence of gravity. In operation of the dilution refrigerator 30, the mixing chamber 32 is the coldest structure of the cryostat 30, such that it can be desirable to cool the sample 1 to the second temperature of the mixing chamber 32 for low temperature measurements. However, the sample 1 is arranged on the sample holder 2 and the actuator unit 12 above the cold plate 330 and the still stage 31 , in order to allow for optical inspection of the sample 1 inside the cryostat 30 through the opening 39a of the radiation shield 39 from above. To cool the sample 1 to the second temperature of the mixing chamber 32 in this arrangement, a plurality of pillars 38 extend upwardly from the mixing chamber 32, that is, against the direction of gravity, beyond the cold plate 330 and the still stage 31 via respective openings330a,31a or recesses of the cold plate 330 and the still stage 31 . Here, the pillars 38 are connected to the probe card 4, in order to cool the probe card 4 and the probe needles 3 extending from the probe card 4 toward the sample holder 2. Additionally, the sample holder 2 is thermally anchored to the pillars 38 and thus the mixing chamber 32, for example via a flexible thermal straps 36a, similar as in Fig. 6. uba109wo 27

[0122] Fig. 9 shows a low temperature section of a cryostat 30 of a measurement system 100 according to yet another embodiment of the invention. As a further improvement to the embodiment shown in Fig. 8, instead of the thermal insulator 33 for thermally insulating the actuator unit 12 from the sample holder 2, a first thermal insulator 33a and a second thermal insulator 33b can be used, wherein the first thermal insulator 33a and the second thermal insulator 33b are separated from each other by a metal element 40, particularly a highly thermally conductive copper or silver element. The metal element 40 is thermally connected to the cold plate 330 via thermal straps 36 and metallic rods 41. By splitting the thermal insulator 33 in the first and second thermal insulator 33b separated by the metal element 40, the metal element 40 creates a short connection for heat loads conducted from the actuator unit 12 connected to the still stage 31 to the sample holder 2 connected to the mixing chamber 32. Due to the finite heat conduction of the first thermal insulator 33a, there will always be a finite heat load flowing in a heat flow path from the warmer actuator unit 12 to the colder sample stage 2. The cold plate 330 operates at an intermediate temperature between the first temperature of the still stage 31 and the second temperature of the mixing chamber 32 and provides more cooling power than the mixing chamber 32. As such, in case more heat than the cooling power of the mixing chamber 32 can handle is conducted toward the sample holder 2, this heat can advantageously drain into the cold plate 330.

[0123] uba109wo 28

[0124] List of reference signs

[0125] Sample 1

[0126] Silicone substrate 1a

[0127] Oxide layer 1 b

[0128] Bond pad 1c

[0129] Sample holder 2

[0130] Probe needle 3

[0131] Probe card 4

[0132] Opening of the probe card 4a

[0133] Electrical connector 5

[0134] Probe station 10

[0135] Housing 10a

[0136] Electrical measurement unit 11

[0137] Electrical wiring 11a

[0138] Actuator unit 12

[0139] Computer 13

[0140] Microscope 20

[0141] Microscope head 20a

[0142] First lens 21

[0143] Second lens 22

[0144] Source of electromagnetic radiation 23

[0145] Camera 24

[0146] Third lens 25

[0147] Beam splitter 26

[0148] Kinematic stage 27

[0149] Adjustment screw 27a

[0150] Hemispherical screw tip 27b

[0151] Platform 27c uba109wo 29

[0152] Adapter structure 28

[0153] Cryostat 30

[0154] First temperature stage 31

[0155] Opening of the first temperature stage 31 a Second temperature stage 32

[0156] Thermal insulator 33

[0157] First thermal insulator 33a

[0158] Second thermal insulator 33b

[0159] Rods 34 Room temperature stage 35

[0160] Cold finger 36

[0161] Thermal strap 36a

[0162] Frame 37

[0163] Opening of the frame 37a Pillar 38

[0164] Radiation shield 39

[0165] Opening of the radiation shield 39a

[0166] Metal element 40

[0167] Rod 41 Window 42

[0168] Measurement system 100

[0169] Intermediate temperature stage 330

[0170] Opening of the intermediate temperature stage 330a

[0171] First horizontal direction x Second horizontal direction y

[0172] Displacement direction z

Claims

uba109wo 30Patent claims:

1. A method for performing electrical measurements on a sample (1), wherein a sample holder (2) with a sample (1) and at least one probe needle (3) are moved with respect to each other, for performing an electrical measurement on the sample (1).

2. The method according to claim 1 , wherein electrical signals generated by an interaction between the sample (1) and the at least one probe needle (3) that are indicative of a positional information of the at least one probe needle (3) with respect to the sample (1) are detected.

3. The method according to claim 2, wherein the electrical signals comprise one or more of a list consisting of the following: an electrical capacitance between the at least one probe needle (3) and the sample (1), a capacitive charging current between the at least one probe needle (3) and the sample (1), an electrostatic influence current evoked in the at least one probe needle (3).

4. The method according to claim 3, wherein the electrical signals comprise at least one transient of the capacitive charging current between the at least one probe needle (3) and the sample (1).

5. The method according to claim 3 or 4, wherein the electrical capacitance is detected from a detected electric oscillation frequency of an LC circuit formed by the sample holder (2) with the sample (1), the at least one probe needle (3) and at least one inductive element.

6. The method according to one of the preceding claims, wherein for the electrical measurement, particularly for the detection of the electrical capacitance, the at least one probe needle is kept on a reference ground potential and an electrical potential is applied to the sample holder and thus to the sample.

7. The method according to one of the claims 2 to 6, wherein the movement between the sample (1) and the at least one probe needle (1) is controlled based on detected electrical signals.

8. The method according to claims 3 to 7, wherein an approach between the sample (1) and the at least one probe needle (3) along a displacement direction (z) defineduba109wo 31 by the displacement between the at least one probe needle (3) and the sample (1) is controlled based on the electrical capacitance and / or the capacitive charging current, particularly the at least one transient of the capacitive charging current detected during the approach.

9. The method according to one of the claims 3 to 8, wherein a mechanical contact event between the sample (1) and the at least one probe needle (3) is detected from the detected capacitive charging current.

10. The method according to claim 9, wherein the mechanical contact event between the sample (1) and the at least one probe needle (3) is detected from at least one transient of the detected capacitive charging current.

11. The method according to claim 9 or 10, wherein the sample (1) is a semiconductor sample (1) and wherein the semiconductor sample (1) is exposed to electromagnetic radiation in order to render the semiconductor sample (1) conductive, such that the detected electrical signals are enhanced and wherein the electromagnetic radiation is repeatedly switched on and off during the approach, such that the contact event between the sample (1) and the at least one probe needle (3) can be detected from an alternation of the detected capacitive charging current between two distinct current levels, wherein a first current level is obtained when the electromagnetic radiation is switched on and a second current level is obtained when the electromagnetic radiation is switched off.

12. The method according to one of the claims 2 to 11 , wherein the sample (1) and the at least one probe needle (3) are moved with respect to each other along at least one horizontal direction (x,y) perpendicular to said displacement direction (z) and wherein said positional information of the at least one probe needle (3) with respect to the sample (1) is determined from the electrical capacitance and / or the electrostatic influence current detected at different relative positions between the sample (1) and the at least one probe needle (3) along the at least one horizontal direction (x,y).

13. The method according to claim 12, wherein: a spatial electrical capacitance map of the sample (1) is determined from the detected electrical capacitance and / or a spatial electrostatic influence current map of the sample (1) is determined from the detected electrostatic influence currentuba109wo 32 detected at different relative positions between the sample (1) and the at least one probe needle (3) along the at least one horizontal direction (x,y).

14. The method according to claim 13, wherein positional information of at least one metallic structure of the sample (1), particularly of a bond pad (1c) of the sample (1), is derived based on the electrical capacitance map and / or based on the spatial electrostatic influence current map of the sample (1).

15. The method according to one of the preceding claims, wherein the sample (1) is a semiconductor sample (1) and the semiconductor sample (1) is exposed to electromagnetic radiation for optical inspection of the sample (1), wherein photon energies associated with the electromagnetic radiation are below a bandgap of the semiconductor sample (1).

16. The method according to one of the preceding claims, wherein positional information of at least one metallic structure of the sample (1), particularly of a bond pad (1c) of the sample (1), is derived based on an optical map of the sample (1) determined from recorded images of the sample (1), particularly wherein the images are recorded using said photon energies below the bandgap of the semiconductor sample (1).

17. The method according to one of the claims 14 to 16, wherein the positional information of the at least one metallic structure of the sample (1), particularly of the bond pad (1c) of the sample (1), is derived based on the electrical capacitance map, the spatial electrostatic influence current map and / or the optical map of the sample (1).

18. The method according to one of the claims 14 to 17, wherein the at least one probe needle (3) and the at least one metallic structure of the sample (1), particularly the bond pad (1c) of the sample (1), are aligned with respect to each other based on the positional information.

19. The method according to claim 18, wherein a mechanical contact event between the at least one probe needle (3) and the at least one metallic structure of the sample (1), particularly the bond pad (1c) of the sample (1) is established based on the positional information.

20. The method according to one of the preceding claims, executed at cryogenic temperatures.21 . A measurement system (100) for performing electrical measurements on a sample, comprising a probe station (10), wherein the probe station (10) comprises: a sample holder (2) configured to hold a sample (1),uba109wo 33 at least one probe needle (3) for performing an electrical measurement on the sample (1) and an actuator unit (12) configured to move the sample holder (2) and the at least one probe needle (3) with respect to each other, for performing an electrical measurement on the sample (1).

22. The measurement system (100) according to claim 21 , comprising an electrical measurement unit (11) configured to detect electrical signals generated by an interaction between the sample (1) and the at least one probe needle (3) that are indicative of a positional information of the at least one probe needle (3) with respect to the sample (1).

23. The measurement system (100) according to claim 21 or 22, wherein the measurement system (100) is configured to operate at cryogenic temperatures.

24. The measurement system (100) according to one of the claims 21 to 23, comprising a cryostat (30) for cooling the sample (1).

25. The measurement system (100) according to claim 24, wherein the cryostat (30) comprises a first temperature stage (31) configured to be cooled to a first temperature and a second temperature stage (32) configured to be cooled to a second temperature, wherein the second temperature is lower than the first temperature.

26. The measurement system (100) according to claim 25, wherein the sample holder (2) is connected to the second temperature stage (32) and wherein a thermal insulator (33) is arranged in a heat flow path between the sample holder (2) and first temperature stage (31).

27. The measurement system (100) according to claim 25, wherein the cryostat (30) comprises an intermediate temperature stage (330) configured to be cooled to a intermediate temperature between the first temperature of the first temperature stage (31) and the second temperature of the second temperature stage (32), wherein the sample holder (2) is connected to the intermediate temperature stage (330) and wherein a thermal insulator (33) is arranged in a heat flow path between the sample holder (2) and intermediate temperature stage (330).

28. The measurement system (100) according to one of the claims 24 to 27, wherein the cryostat (30) comprises a helium-based cooling system configured to cool the second temperature stage (32) and / or the intermediate temperature stage (330) by at least partially and / or temporarily heating the first temperature stage (31), anduba109wo 34 wherein a heater unit for heating the first temperature stage (31) comprises the actuator unit (12).

29. The measurement system (100) according to claim 28, wherein the actuator unit (12) is connected to the first temperature stage (31).

30. The measurement system (110) according to claim 29, wherein the thermal insulator (33) is arranged in a heat flow path between the actuator unit (12) and the sample holder (2), wherein the sample holder (2) is connected to the second temperature stage (32) or the intermediate temperature stage (330).

31. The measurement system (100) according to claim 30, wherein the sample holder (2) is connected to the second temperature stage (31) and wherein a first thermal insulator (33a) and a second thermal insulator (33b) separated from each other by a metal element (40) are arranged in a heat flow path between the sample holder (2) and the actuator unit (12), and wherein the metal element (40) is connected to the intermediate temperature stage (330).

32. The measurement system (100) according to one of the claims 28 to 31 , wherein the helium-based cooling system is a dilution refrigerator, wherein the first temperature stage (31) is a still stage or 1 K-pot stage , the second temperature stage (32) is a mixing chamber stage and / or wherein the intermediate temperature stage (330) is a cold plate.

33. The measurement system (100) according to one of the claims 25 to 32, wherein: the second temperature stage (32) is arranged below the first temperature stage (31) along a longitudinal axis of the cryostat (30), or wherein temperature stage (32) is arranged below both the first temperature stage (31) and the intermediate temperature stage (330) along the longitudinal axis of the cryostat (30), and the probe holder (4) and / or the sample holder (2) is directly or indirectly connected to the second temperature stage (32) or the intermediate temperature stage (330) via pillars (38) extending from second temperature stage (32) or the intermediate temperature stage (330) through openings (31a) or recesses of the first temperature stage (31) or through the openings (31a) or the recesses of the first temperature stage (31) and the openings (330a) or the recesses of the intermediate temperature stage (330) to the probe holder (4) and / or the sample holder (2).

34. The measurement system (100) according to claim 33, wherein the probe station (10) is at least partially enclosed by a radiation shield (39), and wherein a top faceuba109wo 35 of the radiation shield (39) arranged above the sample holder (2) and the first temperature stage (31) along the longitudinal axis of the cryostat (30) comprises an opening (39a) for optical inspection of the sample (1) on the sample holder (2) if a sample (1) is arranged on the sample holder (2).

35. The measurement system (100) according to one of the claims 21 to 34, comprising an optical microscope (20) arranged and configured to capture an image of a region comprised by the sample holder (2) and, if the sample holder (2) holds a sample (1) in said region, of the sample (1), such that a position of the at least one probe needle (3) with respect to said region or the sample (1) can be determined from said image.

36. A computer program comprising instructions which, when executed on a computer (13), cause the computer (13) to control the measurement system (100) according to one of the claims 21 to 34 to execute the method according to one of the claims 1 to 20.

37. Use of the measurement system (100) of one of the claims 21 to 35, particularly of one of the claims 21 to 23, at cryogenic temperatures.

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