ES2396331A1 - Positioning device for microscopes operable in cryogenic environments (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

The present invention provides a positioning device that has a high mechanical stability so it can be incorporated in a local probe microscope, being operable in cryogenic environments, especially at low temperatures, at very low temperatures, and at temperatures less than 100 mk, and additionally in the presence of high magnetic fields. The device comprises a solidary base of the local probe microscope, a sample holder, a guiding means between the base and the sample holder and an actuator located outside the low temperature enclosure. Thermally insulating force transmitting means transmit the impulse from the actuator to the sample holder. The friction between the base and the sample holder is minimized to the maximum. (Machine-translation by Google Translate, not legally binding)

Description

OBJECT OF THE INVENTION

The present invention provides a positioning device that has high mechanical stability and can therefore be incorporated into a local probe microscope, being operable in cryogenic environments, especially at low temperatures, at very low temperatures, and at lower temperatures than 100 mK and in the presence of high magnetic fields.

BACKGROUND OF THE INVENTION

Positioning devices are especially useful in those applications that require spatial manipulation of objects in environments where access to components is not possible or desirable.

When these devices are implemented in environments that are at very low pressure (high vacuum p <10-6 mbar, ultra high vacuum p <10-9 mbar, cryogenic vacuum), with high magnetic fields (H-1-20 T) o In cryogenic environments -especially at low temperatures (T <4.2 K) and very low temperatures (T <1.2 K) - adaptation constraints arise in such extreme environments, and devices must be adapted to overcome the specific problems of the same. Hereinafter, "cryogenic temperatures" are those temperatures below 10 K and that include the ranges of low temperatures, very low temperatures and temperatures below 100 mK.

Furthermore, it is necessary to take into account additional conditioning factors in those devices that are useful for their application in the field of local scanning scanning microscopes (SPM), such as effect microscopy. tunnel (English "Scanning Tunneling Microscopy", STM), the family of atomic force microscopies (AFM) or near optical field microscopy (English "Scanning Near-field Optical Micras-copy These microscopy techniques are used to study and characterize samples using the information provided by the local interaction between a probe and a sample. For example, in a typical implementation of an STM the probe is a metal tip and the sample is a conductive surface. Depending on the requirements of the sample to be studied, it is necessary to adapt these instruments to have a high mechanical stability. Thus, applications such as topog Raphy of surfaces with atomic resolution typically need to achieve measurements with picometer precision.

The size of the region that can be studied with these microscopes is limited by the maximum lateral displacement of the probe with respect to the sample allowed by the scanning system (typically on the order of microns). This small region, hereinafter "observation region", comprises a study area whose surface varies from the atomic scale (-10-20 m2) to the range of the micrometers

(-10-12 2

m) depending on the particular implementation of the technique. To carry out this movement, an actuator is used that allows the probe to be moved with respect to the sample, generally performing a sweep. This actuator is usually referred to by the English term "scanner". Said sweeping actuator or scanner is usually implemented by means of tubes of piezoelectric material adapted to provide movement in the three spatial directions, commonly known as "piezotubes".

There is therefore a need to provide a positioning device that can be adapted to position one or more samples over sufficiently large distances such that these samples can be moved within the observation region of a microscope. For this, the positioning devices must offer us the possibility of making paths on the order of mm with at least sub-micrometer precision, high mechanical stability and the possibility of remote control.

In the state of the art it is possible to find positioning devices based on piezoelectric motors. We can distinguish those technical teachings directed at piezoelectric motors that move a small moving part or object, such as a specimen holder, based on its inertia. In these systems, the small moving object moves with respect to its actuator when it is supplied with a voltage that varies over time in two phases. In a first phase the foot-zoelectric deforms a distance at a slow enough rate dragging the moving object with the actuator thanks to the friction force (static) between the actuator and the moving object. Then, in a second phase, the actuator reverses the direction of movement to roll back quickly enough to

the moving object to slide relative to the actuator. Repeating this sequence is

possible to move small objects. Force adjustment is critical in these systems.

of friction between the actuator and moving object. It is important that the friction force

static is significantly less than the dynamic friction force in the range of

5

frequencies at which the actuator is retracted. The use of lubricants, together the choice

of a mass relationship between the moving object and the actuator are key aspects for

build an operating system. Furthermore, these devices can give rise to

step hand that is not easily reproducible, depending on various parameters

such as mass, surface shape, weight orientation, and friction plane

the

tion.

Piezoelectric stepper motors (known by the English term

"stepper") are capable of moving small objects making a plurality of

Steps. This type of stepper motors are operable in environments like the above-

fifteen

cited, being possible to implement suitable positioning systems

for use in local probe microscopy.

By means of a set of piezoelectric actuators it is possible to move the

small-sized moving part or object. For this, a set of actuators is arranged

twenty

they contact the small moving object. Each actuator is supplied

Mentioned by an electrical signal being possible to deform each piezoelectric actuator

individually. In this way, the deformation of each actuator allows implemen-

tar collective movements and individual movements. During the collecting movement

All actuators move at a speed slow enough to

25

the moving object to move with them because the relationship between friction and

inertia allows no slippage between the moving object and the actuator.

During individual movement an actuator or group of actuators perform a

fast enough speed in which the small moving object is unfolded

Use with respect to each actuator or group of actuators. Each actuator or group of

30

actuators act individually while the rest remain static proportionate

citing an opposition due to static friction between the moving object and the rest of

actuators.

Other engines that use piezoelectrics known as "lnchworms"

35

they use a cylindrical piezoelectric element. These motors are based on a sequence

of steps comprising the contraction of a cylindrical piezoelectric element to

catch a moving object, usually located coaxially. In a next step

an axial deformation is induced in the cylindrical piezoelectric element parallel to its generatrix that translates a distance in the direction of the generatrix to the moving object. Next, an axial deformation occurs that increases the diameter of the cylindrical piezoelectric element that allows the mobile object to be released. By carrying out this sequence of steps, it is possible to move said object reproducibly along a direction parallel to the generatrix of the piezotube.

All these motors are based on the deformation of a piezo-electric element. However, the piezoelectric elements present losses, intrinsic to the piezoelectric materials. These losses are greater in piezoelectric ceramics, which are the materials that have the highest technological value a priori to implement a positioning device since they have a higher value of piezoelectric coefficient, and therefore are capable of providing the largest range of displacements.

The most relevant losses for an actuator are produced by applying an electric field inside the piezoelectric to cause its deformation. These losses translate into energy dissipation in the form of heat. Thus, in the positioning devices present in the state of the art, each step causes heating in an area close to the sample. These losses result in heating when moving the moving part or object, which increases with the number of steps to be performed. In positioning devices present in the state of the art, the power dissipated in the form of heat can usually reach values in the range of tens of milliwatts. These dissipated power values are much higher than the cooling capacity of the refrigeration techniques that allow cryogenic temperatures to be reached. Thus, when it is required to carry out observation at cryogenic temperatures, heating due to this dissipation in the form of heat translates into a significant increase in the temperature of the sample temperature.

The displacement that a piezoelectric actuator is capable of providing depends on the piezoelectric coefficient, and decreases with temperature depending on the material used as the piezoelectric. Thus, as the operating temperature is decreased, the maximum displacement provided by each actuator is decreased to a greater or lesser extent depending on the piezoelectric and the actuator configuration. For example, the ratio of displacement to applied voltage in a shear actuator can typically drop to a quarter of what is observed at room temperature at low temperatures, further decreasing at very low temperatures. This phenomenon makes it necessary to increase the number of steps to be taken to move the object the same distance, which increases the heat dissipated by the actuator. Therefore, the losses are more important when less is the maximum displacement provided by each actuator.

To reach these temperatures, it is necessary to use cryogenic techniques that involve the use of a refrigerator. The cold focus of a refrigerator is kept below a certain temperature provided that the heat generated per unit of time is less than a value, called "cooling power". When a body or instrument is placed in thermal contact with the cold focus, it acquires in equilibrium a temperature that depends on the thermal conductivity of the thermal contact, the heat generated in the instrument and the power of refrigerator cooling. It is necessary to provide a thermal contact that minimizes the temperature difference between the cold focus and the body or instrument in thermal contact. This process is called "thermalization". Typically, proper thermalization implies that the temperature difference between the cold focus and the instrument is less than 1% of the cold focus temperature. The cooling power and thermalization of a body or an instrument decreases as the cold focus temperature decreases, depending on the refrigeration technique used, and therefore the thermalization of the instrument worsens. A typical upper bound would be a cooling power of the order of 1 mW for a cold focus at 100 mK and a power of tens of ~ W for temperatures less than 100 mK.

One type of refrigerator capable of cooling objects at these temperatures is the 3 He to 4 He dilution refrigerator. With a dilution cooler it is possible to reach very low temperatures and temperatures below 100 mK having an operating base temperature typically in the tens of millikelvin range. This base temperature depends both on its design and on the mode of operation, with a typical lower elevation of 5 m K. The refrigeration power of these dilution refrigerators does not decrease as abruptly with temperature compared to other cryogenic refrigeration techniques. in the range of very low temperatures, being able to provide higher cooling or cooling powers than other types of coolers below about 400 m K.

The operation of a dilution refrigerator can be prolonged in principle for long periods of time as it is a closed cycle. However, a very typical design of a dilution refrigerator incorporates a liquid 4He bath. In this case, the operating time at cryogenic temperatures is often limited by the amount of 4He and the consumption of 4He in the refrigerator. This 4He consumption is increased if high magnetic fields (H -1-20 T) are used, provided by a magnet in the superconducting state that is also at low temperatures submerged in the 4He bath. Therefore it is desirable to provide a design that reduces the time required to thermalize those components to be cooled to cryogenic temperatures (cool down time, Te).

These power limitations make it necessary to use designs and materials in the devices that are operable at low temperatures, very low temperatures and temperatures below 100 mK, adapted to allow adequate thermalization of the devices with the cooling power supplied by the device. Cold focus from the refrigerator. The objects to be cooled must have an adequate heat capacity to the refrigerator, and must be in thermal contact with the cold source. In the state of the art it is recommended for these applications the use of good heat conducting materials, such as copper, silver and those alloys that are especially suitable at low temperatures, at very low temperatures, and at temperatures lower than 100 mK . This selection is based on the fact that the thermal conductivity is the highest possible and the specific heat is the lowest possible.

When selecting the material for microscopy applications, an additional restriction must be taken into account. Many of the materials, such as Cu, have the best thermal characteristics in the cryogenic temperature range but, on the contrary, have mechanical properties that make them inadvisable from the point of view of mechanical stability. When providing mechanical stability conditions, such as ~ required in local probe microscopies where it may be necessary to accurately resolve distances of at least tens of picometers, it is advisable to use materials that allow parts of the local probe microscope to be manufactured so that they are not affected by the vibrations of the environment. These considerations are linked to the difficulties in implementing passive damping systems in extreme environments, making it necessary to design the parts of the microscopes that provide said mechanical stability.

Finally, the positioning devices present in the state of the art, including those described above, require lubricants to regulate the friction between the moving parts. When these devices operate in cryogenic environments, they make use of solid lubricants such as graphite or molybdenum disulfide that can reduce friction between moving parts even at cryogenic temperatures.

In summary, the positioning devices in the state of the art based on piezoelectric motors dissipate energy in the form of heat as a consequence of the losses that occur in a piezoelectric material. This dissipation increases with the voltage supplied to the piezoelectric actuator. When operating by varying the voltage across the device repeatedly, as in those that require multiple cycles to provide displacement of an object, the dissipation increases. Consequently, piezoelectric motors have obvious disadvantages when control over the temperature of the object is required in environments where the power available to stabilize the temperature is limited.

This dissipation of energy in the piezoelectric-based positioner is especially relevant when it is necessary to operate in the low temperature range, being critical to operate at very low temperatures, and even more critical as the temperature drops to temperatures below 100 m. K. This is because the available cooling power with cooling techniques decreases with temperature. This trend in cooling power is more abrupt below 1.2 K and is further accentuated below 100mK.

It is therefore necessary to provide a positioning device capable of operating in extreme environments, that minimizes heat dissipation, that is capable of providing an object to move at cryogenic temperatures - including the ranges of low temperatures, very low temperatures and temperatures below 100 mK-, and suitable for local probe microscopes.

DESCRIPTION OF THE INVENTION

The present invention provides a positioning device capable of operating in a local probe microscope at cryogenic temperatures. In a first inventive aspect, a positioning device defined by independent claim 1 is provided, which is incorporated by reference to the description.

"Positioning device for local probe microscopes, operable at cryogenic temperatures to position a sample in an observation region"

The positioning device is adapted for use under the conditions that require its use in a local probe microscope, such as an STM or an AFM. Thus, the positioning device is suitable for locating a sample in an "observation region" of a local probe microscope. This region of observation, as defined above, is a region of space where a local probe microscope is operable to carry out measurements of different magnitudes that characterize this instrument. Thus, an observation region may comprise, in an example, the area of a scan of the probe with respect to the surface of the sample that allows its topography to be acquired.

This positioning device has the ability to operate in environments where cryogenic temperatures are reached, being suitable for use at low temperatures, very low temperatures, and temperatures less than 100 mK.

Preferably, this device can reach an operating temperature in the range of very low temperatures by means of the thermal contact between the cold focus of the refrigerator and the positioning device. Even more preferably the positioning device is suitable for use in a cryostat equipped with a 3 He dilution refrigerator in

I have.

For this purpose it is necessary to check that the cooling power of the refrigerator is adequate for the heat capacity of the positioning device to be cooled. It is also necessary to take into account that the thermal conductivity between the cold focus of the refrigerator and the positioning device is adequate for the thermalization of the device to take place. The lower the heat capacity and the higher the thermal conductivity of the materials, the shorter the time required for the positioning device to cool down.

The positioning device comprises:

• "a base, adapted to be attached to the microscope in solidarity

local probe,

•

a sample holder,

•

guiding means between the base and the sample holder. "

The base is adapted so that a joint connection is possible that allows it to be integrated into a local probe microscope, preferably in a removable way.

The sample holder, an object that in an embodiment that will be shown later is small in mass, is adapted to house samples for study with a local probe microscope, preferably in a removable manner.

The guiding means direct or guide the sample holder, determining its path of movement with respect to the base. This path is adapted so that the sample holder, which is an object smaller than the base to which it is attached, reaches a position that allows the sample to be placed within the "observation region".

According to the present invention, the positioning device has:

•

"guiding means between the base and the sample holder comprising a first friction surface arranged on the base and a second friction surface arranged on the sample holder such that said second friction surface is adapted to slide on the first friction surface of the base,

•

an actuator for driving the sample holder from the base. "

The base comprises a first friction surface and the sample holder comprises a second friction surface. These friction surfaces are adapted to be operable at cryogenic temperatures. In these environments, friction is critical because the occurrence of displacement causes heat dissipation of energy, which due to the limitations imposed by the operation at cryogenic temperatures, is counterproductive for the purpose of the invention. . Thus, the first friction surface and the second friction surface are adapted so that the friction between the sample holder and the base is minimized, at cryogenic temperatures.

Furthermore, the guiding means is suitable to allow the use of lubricants capable of reducing friction between the first friction surface and the second friction surface at cryogenic temperatures.

Polished friction surfaces, such as a plane, a sphere or a cylinder, and more preferably flat, edgeless surfaces are preferably provided.

An actuator provides the drive to move the specimen holder relative to the base.

"where the base and the sample holder are inside a very low temperature housing and in thermal contact with a thermal element_ characterized in that the actuator is located outside the very low temperature housing, said actuator being linked to the sample holder by means of force transmission means that are included in the positioning device, such that the actuator drives the sample holder through the force transmission means, said force transmission means being of low thermal conductivity. "

This positioning device is adapted to be housed in a cryostat, within a very low temperature enclosure that is thermally isolated from the environment. This very low temperature enclosure is delimited by a very low temperature shield or casing. Although the physical barrier to the passage of heat to the very low temperature enclosure is called a very low temperature casing, this physical barrier can be determined by a set of parts. Thus, the thermalization of the device by means of a refrigerator is made possible with the cooling power limitations inherent in these coolers as the temperature decreases.

The actuator is located outside the very low temperature enclosure and the housing. Preferably, the actuator is located outside the cryostat.

This makes it possible for the cooling power to be supplied to the device and the sample, which reduces the cooling time, improving the operation of the device in cryogenic temperatures.

Force transmission means are used to transmit the drive impulse to the sample holder. In operating mode these force transmission means are characterized in that they are of low thermal conductivity. This means that the thermal conductivity of the transmission medium is low enough so that the heat transmission through said transmission medium allows the cooling of the positioning device, isolating the part of the device to cryogenic temperatures, and allowing an operating temperature to be reached preferably in the low temperature range, more preferably in the very low temperature range, and even more preferably at temperatures less than 100 mK. The selection of these parameters is adjusted to the cooling power that the refrigerator can supply through cold spots in the stages at cryogenic temperatures, typically at 4.2 K and 1.2 K. The maximum height of the thermal conductivity KT, max between a focus at a temperature T1 and a focus at a temperature T2, depends on the maximum power of the focus at T2 (P2) and the temperature difference. For the section that runs at very low temperatures, although it depends on the re-cooling technique used, a maximum height is of the order of KT, max = 10 ~ W K- \ and more preferably a value less than KT, max = 1 ~ WK-1 for better insulation.

Along with the above, it must have some mechanical properties that allow the transmission of this impulse between the actuator and the sample holder, which results in the application of such a sample holder force.

Preferably, in an exemplary embodiment, together with this force transmission means there is a first elastic means for operating in opposition, which cooperates in the positioning of the sample holder with respect to the base. Preferably the first elastic means is attached to the base and to the sample holder, so that it can undergo elastic deformation in the direction of application of the force T by the transmission means when the sample holder moves relative to the base. The elastic deformation suffered by this first elastic means provides a restoring force that has at least one component in the same direction of movement of the sample holder.

DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will become more clearly apparent from the following detailed description of a preferred embodiment, given solely by way of illustrative and non-limiting example, with reference to the accompanying figures.

Figure the

This figure shows a schematic plan representation of

an example of embodiment of a positioner.

Figure lb

This figure shows a schematic representation of a section

tion of the embodiment example of the positioner shown in the figure

the.

Figure 2a

This figure shows a sectional view of the sample holder at scale.

after according to an embodiment of the invention.

Figure 2b

This figure shows the AA section of the anterior view to scale.

sample holder.

Figure 2c

In this figure I know shows a perspective view of the sample holder

cut by the BB section along with the base.

Figure 3

This figure shows a schematic representation of a refrigerator

dilution generator and a local probe microscope comprising a

example of realization of the positioner.

Figure 4

This figure shows a topographic profile of the surface of a

NbSe2 sample at a temperature of 100 mK. Panel (a) corresponds

it gives a zero field surface (H = O T) and panel (b) a surface

cie in the presence of a magnetic field (H = 8T).

Figure 5

This figure shows a spectrum of the mechanical noise recorded in

an exemplary embodiment of the invention.

DETAILED STATEMENT OF THE INVENTION

The present invention comprises a positioning device (1) of which an example of a detailed embodiment is described below with reference to the figures. In figures la and lb a schematic diagram of the device is shown and figures 2a, 2b and 2c correspond to the same embodiment where the structure of the device is more clearly perceived. Figure 3 shows a diagram of the positioning device (1) that allows controlling the relative position between a base

(1.1) and a sample holder (1.2) integrated in a tunnel effect microscope (2) inside a refrigerator (5).

The positioning device (1) according to the embodiment example comprises a base (1.1), a sample holder (1.2), a force transmission means (1.3), an actuator (1.4) and a first elastic means (1.5).

The base (1.1) is a part on which guide means are incorporated that provide a path for the sample holder (1.2), preferably adapting a part in the base (1.1) to form a rail (1.1.2). Preferably this lane

(1.1.2) is a straight rail that provides a substantially straight guide direction, simplifying remote manipulation of the positioning device (1).

Preferably, the sample holder (1.2) is inscribed in a parallelepiped with dimensions 9mmx8mmx2mm. As can be seen in Figures 2a, 2b and 2c, it has a plurality of threaded holes (1.2.2) to attach samples to the same support using screws to fix a sample (3) in a removable way.

The first friction surface (1.1.1) of the base (1.1) and the second friction surface (1.2.1) of the sample holder (1.2) are polished surfaces, without roughness, to provide a displacement minimizing friction.

The actuator (1.4) is located outside the very low temperature enclosure (5.12.1), preferably outside the cryostat (5.14), at room temperature, similar to 300 K. This allows using various known actuators, and more preferably a linear actuator , to actuate the force transmission means (1.3). The force transmission means (1.3) allow the actuator (1.4) to move the sample holder

(1.2)

with respect to the base (1.1) when the actuator is located outside the enclosure where low temperatures, very low temperatures and temperatures less than 100 mK are reached, the transmission means (1.3) of force being materials of low thermal conductivity, i.e. thermally insulating. Along with the above, the force transmission means (1.3) must have mechanical properties that allow the transmission of this impulse between the actuator (1.4) and the sample holder.

(1.2)

resulting in the application of a force (T) to the sample holder (1.2). An example of force transmission means (1.3) is a thermally insulating tubular rod.

Preferably, in this embodiment, at least one first cable (1.3.1) is provided as force transmission means (1.3). In this case, first elastic means (1.5) are arranged in opposition to the force T that is transmitted from the actuator (1.4) to the sample holder (1.2), which allows operating with the first cable (1.3.1) that is maintained always in traction. The first elastic means (1.5) are implemented as a first spring whose deformation occurs substantially along the longitudinal direction x of the rail (1.1.2). When a tensile force T is applied to the cable, this first elastic means (1.5) provides a restoring force opposite to the tensile force T. Preferably, the elastic constant of this spring is within the range of 10 N / m to 100 N / m, preferably between 20 N / m and 10 N / m. Thus, it becomes possible for the sample holder (1.2) to recover its position, and therefore allows the positioning operation of the sample holder (1.2) to be carried out by operating remotely, and in particular by ensuring displacement in an x direction.

In this exemplary embodiment, a second elastic means (1.6) is provided. This second elastic means (1.6) is deformed by providing a second restoring force that keeps at least part of the first friction surface (1.1.1) in contact with the second friction surface (1.2.1). Preferably, this second elastic means (1.6) is a spring that is fixed to the base (1.1), passes through a slot (not shown) and is fixed at its other end to the sample holder (1.2) in a hole of fixation (1.2.3). The elastic constant of this spring is within the range of 10 N / m to 100 N / m, preferably between 20 N / m and 10 N / m. Preferably, the restoring force of the second elastic means (1.6) is applied at least in a perpendicular direction to movement through the guiding means. In this example, you have a component along the z direction, perpendicular to the x direction. This feature allows solving the problem of providing a positioning device capable of acting in a variety of directions, regulating the friction between the sample holder (1.2) and the base (1.1), and which is also suitable for use. in local probe microscopes. The use of the second elastic means (1.6) enables the mounting of the positioning device (1) in directions other than vertical, which would be natural by gravity. Likewise, the second elastic means (1.6) regulates friction between the first friction surface (1.1.1) and the second friction surface (1.2.1) and improves mechanical stability since it not only restricts movement in this direction z it makes the set more rigid.

More preferably, both the first friction surface (1.1.1) and the second friction surface (1.2.1) have an alumina coating. In a preferred embodiment, the rail (1.1.2) has flat faces and the coating is carried out using alumina plates, with a maximum thickness of approximately 0.5 mm. This alumina coating is fixed to the base (1.1) using an epoxy resin suitable for use at low temperatures between the rail faces (1.2.1) and said alumina plate. This solution using plates facilitates the support of the sample holder

(1.2) on the basis (1.1). Other embodiments are possible to provide such as those having a plane in front of a bar (or bars) or a sphere (or spheres) of materials exhibiting reduced friction as with sapphire or ruby.

The use of this second elastic medium (1.6) offers a rigidity of the positioning device (1) that has a high mechanical stability performance when used in a local probe microscope (2), where this mechanical condition is critical because the operating distance between probe and surface is on the order of the Angstrom (10-10 m). This mechanical stability is even more necessary in measurements that resolve distance-dependent properties, such as topographic measurements. Especially when it becomes necessary to build a microscope capable of achieving atomic resolution in the properties under study. As an example, the acquisition of the topography of a surface with atomic resolution requires a typical precision of at least tens of picometers (<10-11 m). This resolution must be maintained when the probe is moved through the scanner (2.2) when scanning an area of the sample (3).

Preferably, the first friction surface (1.1.1) and the second friction surface (1.2.1) are in turn coated with a lubricant suitable for use at cryogenic temperatures. Typically, the usual liquid lubricants at room temperature freeze and become useless in this temperature range. However, in these temperature ranges it is crucial to minimize dissipation due to the limited available cooling power. Advantageously, graphite and other solid lubricants such as molybdenum disulfide are preferably used to lubricate the first friction surface (1.1.1) and the second friction surface (1.2.1) at low temperatures, very low temperatures and temperatures less than 100 mK. The use of these lubricants, particularly graphite on alumina plate, together with an adequate selection of the first friction surface (1.1.1) and second friction surface (1.2.1), allows to minimize friction and heating of those parts of the positioning device (1) to be cooled and, more importantly at cryogenic temperatures, the thermalization of the sample is accelerated (3), since the heat generated when moving the sample near the sample holder (1.2 ) is minimized. In addition, it improves performance by using flat support surfaces that, although they increase the area of the first friction surface (1.1.1) and the second friction surface (1.2.1), result in a more stable support. This support is especially improved when used in conjunction with a second elastic medium (1.6).

Preferably, the most massive parts of the positioning device (1), such as the base (1.1) and the sample holder (1.2), are manufactured, adapting their dimensions depending on the density of the material and Young's modulus to have a adequate mechanical stiffness. Mechanical stiffness is typically characterized by the fundamental resonance frequency of the device. Selecting a fundamental resonance frequency greater than the vibration band, and in particular, greater than 10kHz, a device is obtained that is sufficiently rigid to maintain the relative position of the probe with respect to the sample without it varying, and allows obtaining a stability with a resolution of the order of the picometer. This, in conjunction with the elastic constants of the first elastic means (1.5) and the second elastic means (1.6) being in the range of 10 N / m and 100 N / m, provides greater mechanical stability when making the stiffer structure.

Preferably, the body of the sample holder (1.2), the body of the base

(1.1) or both are made of Macar, aluminum or an alloy of aluminum, titanium or a titanium alloy or any combination thereof. In the case of the use of titanium alloys, when the sample holder (1.2.1) is made of grade 5 titanium and its body is preferably inscribed in a parallelepiped with dimensions 9mmx8mmx2mm, it has a high fundamental frequency of resonance much greater than 10kHz.

When the positioning device (1) according to any previous embodiment example is integrated into a local probe microscope (2) comprising a scanning device such as a piezoelectric scanner (2.2), a local probe microscope (2.2) is provided. ) with a positioning system capable of operating at cryogenic temperatures.

Figure 3 shows an example of embodiment in which it is possible to see the positioning device (1) integrated in a local probe microscope (2) which is a tunnel effect microscope or STM. A sample (4) is fixed on the sample holder (1.2) to be studied using this microscopy technique. In this case, the local probe has been represented as a conductive tip that is located in a scanning element or piezoelectric scanner (2.1). In this exemplary embodiment, the piezoelectric scanning element (2.1) is a piezotube. A superconducting magnet (4) provides in the observation region (2.1) a variable magnetic field, typically between 1T and 10T.

In this embodiment, the low temperature refrigerator (5) is a continuous 3He in 4 He dilution refrigerator (5). A similar embodiment of the invention can also be used in other types of refrigerators such as others.

types of 3 He dilution coolers in He, or a 4 He bath or refrigerators based on thermoacoustic refrigeration technologies (without cryogenic liquids) to achieve low temperatures, close to 4.2K, or a 3He refrigerator, an adiabatic demagnetization refrigerator , which can reach temperature ranges below 1 K.

The dilution refrigerator (5) comprises various parts that are at successively lower temperatures from the ambient temperature of the exterior (-300 K) to the base temperature of the cold source of the refrigerator (2). The various parts of the refrigerator (5) can be seen in the diagram of figure 1. An injection line (5.3) introduces a gas from a tank (5.2) at room temperature

comprising a mixture of the 3He and He isotopes in a first enclosure delimited by the cryostat with a casing (5.14) that thermally isolates a bath (5.1) of 4 He liquid at 4.2 K from the environment at 300 K.

This injection line (5.3) reaches a first container (5.13) submerged in the 4 He bath. This first container (5.13), commonly known as a 4.2 K canister, contains the enclosure at low temperatures (5.13.1). In this low temperature enclosure (5.13.1) there is a second container (5.4) in which 4 He is pumped from the liquid 4 He bath (5.1) to the outside by means of a pump (5.11). In operating mode the approximate temperature of this second container (5.4) is 1.2 K, and is commonly known as 1 K canister. In operating mode, the pressure within the low temperature enclosure (5.13.1) is too low for the Cold components are not in thermal contact with hotter components (eg the helium bath) due to the presence of gas.

The injection line (5.3) is thermally anchored to the first container (5.13) and to the second container (5.4), and reaches the interior of a second room with very low temperatures (5.12.1). This second enclosure with very low temperatures (5.12.1) is delimited by a shield or casing (5.12) with very low temperatures that serves as a shield against radiation. The parts located within this very low temperature enclosure (5.12.1) are in operating mode at temperatures less than 1.2 K. To reach these temperatures, the gas pressure in the injection line (5.3) is increased above of the vapor pressure of 3 He through various impedances (5.5) of the injection line (5.3) and in this way the gas flow is controlled and it is guaranteed that it condenses at the temperature of the 1K bottle. Thus, the 3 He passes through a series of heat exchangers (5.6) until it reaches the mixing chamber (5.7). In said mixing chamber (5.7) the 3 He passes from the concentrated to the diluted phase, the dilution process of 3 He taking place in a mixture of 3 He in 4 He that provides the cooling power of the dilution refrigerator. A return flow through a return line (5.9) passes through the heat exchangers (5.6) until it reaches a distiller (5.8) whose approximate temperature is

0.7 K. The return flow is in thermal contact with the injection line (5.3) to cool the inlet flow. On this distiller (5.8) an external pump (5.10) pumps 3 He of the diluted phase towards the outside. Through the dilution of 3 He in 4 He that takes place in this cycle, the dilution refrigerator (5) supplies a continuous re-cooling power in the cold focus of the refrigerator, which is the mixing chamber (5.7).

In this example of embodiment, the body of the STM (2) and the positioning device (1) are in thermal contact with the cold focus of the refrigerator (5). For this, various thermal techniques of both the device and the STM can be used

(2) that depend on the materials used since they make use of the thermal conductivity of the STM body material (2) and the positioning device (1). Generally, this thermal contact is made by means of a thermal element. This thermal element can be a structure (not shown) made in good conductors of heat at cryogenic temperatures, and more preferably in non-magnetic materials when it is desired to enable the presence of a field, such as Cu, Ag or Au. Thus, in this embodiment, a copper structure is in thermal contact with the mixing chamber (5.7) and the local probe microscope (2). In turn, the microscope thermalizes the base (1.1) and the base, (1.1) the sample holder (1.2).

Preferably, as shown in this exemplary embodiment,

zan as a force transmission means (1.3) a cable comprising a first

section (1.3.1) of stainless steel cable and a second section (1.3.2) of multifi-

lar non-metallic. The first section (1.3.1) has a diameter between 0.1 mm and 0.2 mm and

presents adequate mechanical properties to act as a transmission medium

5

operating under a pulling force. Additional tension is provided by

an additional spring (1.7) that increases the tension exerted on the first section

(1.3.1). To minimize heat transfer, the first section (1.3.1) is thermally anchored

by means of a plurality of anchors (1.8) to various points of the refrigerator

(5). Figure 3 shows the anchorage at 1.2 K (1.8.2) and the various anchorages (1.8.1) of

the

Within the very low temperature enclosure (5.12.1) with the distiller (5.8), the inter-

heat exchangers (5.6) and the mixing chamber (5.7) at various temperatures.

These anchors are preferably made using a material with a

high thermal conductivity at low temperatures and very low temperatures. materials

fifteen

such as copper, varnish, silver, etc. can be used. Threads are preferably used

coppermade.

Advantageously it is used for the second section (1.3.2).

metallic, with a diameter between O.lmm and 0.2 mm. This multifilament cable, manufactured preferably

twenty

In Kevlar, it is connected to the first section (1.3.1) of steel cable. In this example-

For example, the second section (1.3.2) is inserted through a guide hole (1.1.3) when it reaches

the base (1.1), to move in the longitudinal direction x of the rail (1.1.2). This work

mo of (1.3.2) non-metallic multi-core cable acts as a buffer, in cooperation

with the additional spring (1.7), to provide a resonator that damps vibrations.

25

external actions.

In particular, light and rigid materials, such as titanium and its alloys.

tions, have excellent mechanical characteristics. This improves stability

mechanics of the positioning device (1). In environments at cryogenic temperatures,

30

This stability is critical, but it has important limitations when implementing

Technically, therefore, mechanical damping mechanisms are very

limited, and ultimately disadvantageous by involving heat dissipation that

finally it will produce a heating. Thus it is possible to manufacture parts of a device

positioner (1) in a material and with dimensions suitable for integration

35

in a local probe microscope (2) operable at low temperatures, very low temperatures

ratios and temperatures less than 100 mK with a fundamental frequency of reso-

nance greater than 10 kHz. This provides a microscope with greater stability.

mechanics by making the structure more rigid.

These material properties and the design of the parts of the device

Positioning (1) must be combined with its thermal properties. It is

S

It is possible to carry out these parts in materials such as copper / beryllium, magnesium alloys,

nesium, ceramics such as Macar or alumina that show good results from the

point of view of the thermalization of the positioning device (1).

In an exemplary embodiment, parts are manufactured out of aluminum parts or

10

in aluminum alloys. In another example parts are preferably manufactured parts

in titanium or titanium alloys, and more preferably 5th grade titanium

grade 3 titanium. Titanium alloys, and especially grade titanium alloys

do 5th grade 3 titanium, have mechanical properties at cryogenic temperatures

unique to manufacture parts that have a high resonance frequency,

fifteen

much higher than 10kHz.

However, the thermal properties of titanium and titanium alloys

Child or aluminum and aluminum alloys vary at cryogenic temperatures. Is

well known that when a material is in the superconducting state pre-

twenty

It has a significantly lower thermal conductivity than in the normal state.

This is because in materials in the superconducting state the contribution

Electronic to thermal conductivity is inhibited. In the supercon-

ductor heat conduction occurs mainly by means of phonons and is

less efficient than in the normal state.

25

The critical temperature that determines the transition from the normal state to the state

superconductor of the titanium alloys of interest for use in microscopy of

local probe is above the critical temperature of titanium (0.4 K in the absence

magnetic field). In particular, for grade 5 titanium alloy the tempera-

30

Critical length is 5 K in the absence of a magnetic field. A similar phenomenon is expected

feel when making these parts out of aluminum or the aluminum alloys that

they present a transition to a superconducting state below their corresponding

you critical temperature.

35

This embodiment has been found to present thermalization results

that allow temperatures of up to 10mK to be reached, well below the

critical criticism of titanium or titanium alloys. So preferably both the

The body of the base (1.1) and the body of the sample holder (1.2) are made of grade 5 titanium. More preferably, the body of the local probe microscope (2) is made of a grade 5 titanium alloy.

These results show the possibility of obtaining improved mechanical stability and better thermalization at cryogenic temperatures by means of the instrument, thanks to the combination of the technical teachings contemplated above.

Figure 4 shows a topographic profile taken with a tunnel effect microscope equipped with the positioning device (1) according to the previous embodiment example. The measurement was carried out with an Au tip on a sample of NbSe2 at a temperature of 100 mK. The polarization voltage between tip and sample is 30 mV and the tunnel current is 10 nA. Panel (a) shows the profile of an area in the absence of a magnetic field (H = O T) and panel (b) shows a profile of an area in the presence of a magnetic field (H = 8 T). It is possible to observe in this figure 4 the co-corrugation associated with the atoms of the surface of NbSe2 with a resolution level that allows obtaining an image of the surface (not shown) with atomic resolution in the presence of a magnetic field. The curves in panels (a) and (b) reflect a different value of the apparent height in the atomic corrugation as they correspond to a different region of the surface of NbSe2 •

The resolution not only allows an atomic corrugation to be distinguished but is sufficient to appreciate the charge density waveform as a modulation superimposed on the crystal structure of the material surface (not shown). This is a phenomenon that manifests itself at very low temperatures in this material in the form of a modulation of the charge with an Acow spatial periodicity that is superimposed on atomic corrugation, with a spatial periodicity. They agree well with the values that they are found in the literature (A.c0 w: = :::: 3A.corr). Stability is maintained at these temperatures, even in the presence of an 8T magnetic field, without degrading the performance and resolution of the tunnel effect microscope.

Figure 5 shows the discrete Fourier transform of the tunnel current (FFT) in arbitrary units on a sample of 1024 points for 1 s, performed on a sample of NbSe2 at 100 mK in the presence of a magnetic field H = 5 T The tunnel current presents an exponential decay with distance with a decay length of the order of 0.5 nm in metals. This signal is used

to feed back a control circuit of the distance between tip and sample, which is disconnected when making this measurement. In this case, the time-averaged value of the tunnel current 1.2 nA and the bias voltage between tip and sample 50 mV. Panel (a) corresponds to a microscope with a positioning device according to the state of the art and panel (b) corresponds to the same microscope with the spectrum taken with the positioning device (1). It is possible to appreciate that at low frequency it is highly stable and is not affected by the positioning device (1). In other words, the positioning device (1) is at least as stable as the microscope because it does not degrade low-frequency stability appreciably. The operating temperature of the microscope in this experiment is 50 mK. The microscope with a positioner device in the state of the art is typically heated from the operating temperature to temperatures above 4K when the positioner in the state of the art is operated. However, no changes in the microscope temperature greater than 1% are observed over the operating temperature in the microscope when operated.

15 the positioning device (1) according to the previous embodiment example.

Claims (11)

1.-Positioning device (1) for local probe microscopes (2), operable at cryogenic temperatures, to position a sample (3) in an observation region (2.2) comprising:

•

a base (1.1) adapted to be integrally attached to the local probe microscope (2);

•

a sample holder (1.2);

•

guiding means between the base (1.1) and the sample holder (1.2) comprising a first friction surface (1.1.1) arranged on the base (1.1) and a second friction surface (1.2.1) arranged on the sample holder (1.2), such that said second friction surface (1.2.1) is adapted to slide by sliding on the first friction surface (1.1.1) of the base (1.1); and

•

an actuator (1.4) for driving the sample holder (1.2) with respect to the base (1.1);

where the base (1.1) and the sample holder (1.2) are inside a casing (5.12) with very low temperatures and in thermal contact with a thermal element, characterized in that the actuator (1.4) is located outside the very low temperature casing (5.12), said actuator (1.4) being linked to the sample holder (1.2) by means of force transmission (1.3) that are included in the positioning device (1), such that the actuator (1.4 ) drives the sample holder (1.2) through the force transmission means (1.3), said force transmission means (1.3) being of low thermal conductivity. 2.-Device according to claim 1 characterized in that the force transmission means (1.3) are traction, and by additionally having first elastic means (1.5) such that the transmission means (1.3) of force and said first elastic means (1.5) are adapted to operate in opposition. 3. Device according to claims 1 or 2, characterized in that it comprises second elastic means (1.6) adapted to maintain contact under pressure between the first friction surface (1.1.1) and the second surface (1.2.1). ) friction. 4. Device according to claims 1 to 3, characterized in that the first friction surface (1.1.1), the second friction surface (1.2.1) or a combination thereof are covered by a solid lubricant. 5. Device according to any of claims 1 to 4, characterized in that the fundamental resonance frequency of the positioning device (1) is greater than 10kHz to improve mechanical stability and increase precision in the position of the sample (3). 6. Device according to any of claims 1 to 5, characterized in that the body of the base (1.1) or the body of the sample holder (1.2) is made of titanium or aluminum. 7. Device according to any of claims 1 to 6, characterized in that the actuator (1.4) is a linear actuator. 8. Device according to any of claims 1 to 7, characterized in that the first friction surface (1.1.1) or the second friction surface (1.2.1) are covered with alumina. 9. Device according to any of claims 1 to 8, characterized in that the guiding means are configured to form a rail (1.1.2) that allows linear displacement. 10. Device according to claim 9, characterized in that the first friction surface (1.1.1) and the second friction surface (1.1.2) are flat. 11.- Local probe microscope (2) with the capacity to operate at cryogenic temperatures, comprising a positioning device (1) according to any of the previous claims 1 to 10.