WO2007036614A1

WO2007036614A1 - Measuring system

Info

Publication number: WO2007036614A1
Application number: PCT/FI2006/050420
Authority: WO
Inventors: Marko Pudas; Kari Laasonen; Juha Röning
Original assignee: Oulun Yliopisto
Priority date: 2005-09-30
Filing date: 2006-09-29
Publication date: 2007-04-05
Also published as: FI20055526A0

Abstract

The invention relates to a measuring system that comprises at least two probes (102A, 102B) between which there is a first biasing potential difference, the probes (102A, 102B) being configured to provide tunneling of charges between the probes (102A, 102B) in the vicinity of the object to be studied. The measuring system enables study of objects that are electrically non-conductive or weakly conductive, on the basis of tunneling of charges between the probes (102A, 102B) and the effect of the object to be studied on the tunneling.

Description

MEASURING SYSTEM

FIELD

[0001] The invention relates to a measuring system for studying properties of matter.

BACKGROUND

[0002] In scanning tunneling microscopy (STM) a stylus-like measuring head is brought in the vicinity of an object to be studied, such as an electrically conductive surface, and the measuring head enables tunneling of charges between the measuring head and the object to be studied. By transferring the measuring head and performing determination of tunneling at several points in the object to be studied it will be possible to determine a microscopic profile of the object to be studied and possibly atomic properties of the matter constituting the object to be studied.

[0003] The prior art scanning tunneling microscopy has a drawback that it is ill-applicable for analysing objects that are electrically non-conductive or weakly conductive. Hence, it is useful to examine alternative techniques for studying the properties of matter.

BRIEF DESCRIPTION

[0004] The object of the invention is to provide a measuring system that is applicable for studying the properties of electrically non-conductive and/or weakly conductive matter. This is achieved by a measuring system for studying the properties of matter, the measuring system comprising at least two probes, between which there is a first biasing potential difference, the probes being configured to generate tunneling of charges between the probes in the vicinity of the object to be studied.

[0005] Preferred embodiments of the invention will be disclosed in the dependent claims.

[0006] The invention is based on the idea that charges are tunnelled between two probes that are positioned in the vicinity of the matter to be studied. The atomic and molecular structure of the matter to be studied affects the tunneling in a manner that is characteristic to the atomic or molecular structure, and consequently it is possible to determine properties of the matter to be studied by analysing the tunneling measurement. [0007] Several advantages will be achieved with the measuring system of the invention. An advantage of the measuring system is to enable the study of the properties of electrically non-conductive and/or weakly conductive matter utilizing the tunneling effect between the probes and the influence of the matter to be studied on the tunneling effect.

LIST OF DRAWINGS

[0008] In the following, the invention will be described in greater detail in connection with preferred embodiments, with reference to the attached drawings, in which

Figure 1 is a first example of the structure of a measuring system;

Figure 2 is an example of the structure of probes;

Figure 3 is a second example of the structure of the measuring system;

Figure 4 is an example of a measuring unit;

Figure 5 is a first example of the structure of a precision-drive mechanism; and

Figure 6 is a second example of the structure of the precision -drive mechanism.

DESCRIPTION OF THE EMBODIMENTS

[0009] With reference to the example of Figure 1 , the measuring system 100 comprises at least two probes 102A, 102B, a precision-drive mechanism 104A and a control unit (CNTL) 120.

[0010] Typically, the probes 102A, 102B are stylus-shaped structures comprising atomic-sharp tip parts 132A, 132B, between which charge tunneling takes place as the tip parts 132A, 132B are brought at a tunneling distance from one another and as a first biasing potential difference is provided between the tip parts 132A, 132B. A typical tunneling distance is in the order of nanometres, the first biasing potential difference being in the order of 1 mV - 5V and the first tunneling current being within the range of a microampere and a fraction of picoampere.

[0011] The tip part 132A, 132B is made of electrically conductive material that is solid in use conditions. At least one of the tip parts 132A, 132B typically has an atomic-sharp tip that is most susceptible to provide a tunneling channel between the tip parts 132A, 132B of the probes. [0012] The tip part 132A, 132B may be made of tungsten or iridium- platinum alloy without the disclosed solution being limited to the presented metals.

[0013] In an embodiment, at least one probe 102A, 102B is a carbon nano-tube.

[0014] In an embodiment, the precision-drive mechanism 104A adjusts the distance between the probes 102A, 102B for controlling the charge tunneling between the probes 102A, 102B. It is possible to stop the probes 102A, 102B at a desired distance from one another as the charge tunneling satisfies given criteria, such as the value of a first tunneling current. The precision-drive mechanism 104A may comprise, for instance, a piezoelectric component, whose crystal structure and physical dimensions can be controlled by subjecting the piezoelectric component to an electric field. Depending on the geometry of the probes 102A, 102B, the piezoelectric component allows the tip parts 132A, 132B to achieve a travel of 10 μm, the hit precision being in the order of hundredth parts of nanometres. The disclosed solution is not restricted, however, to the presented travel and hit precision, but the travel and the hit precision may vary depending on the employed precision-drive mechanism 104A.

[0015] The precision-drive mechanism 104A may adjust the distance between the probes 102A, 102B in a 3-dimensional, 2-dimensional or 1- dimensional space. In an embodiment, the precision-drive mechanism 104A is configured to adjust the position of the probes 102A, 102B in relation to the object 112 to be studied.

[0016] In an embodiment, one precision-drive mechanism 104B operates in x dimension, while another precision-drive mechanism 104B operates in y and/or z dimension.

[0017] In an embodiment, the distance between the probes 102A, 102B is constant. The distance may be adjusted to correspond to the properties of the object 112 to be studied.

[0018] The measuring system 100 may also comprise a coarse- drive mechanism 114, by means of which the probes 102A, 102B may be positioned in a desired place with respect to the object 112 to be studied and which allows the probes 102A, 102B to perform a scan on the object 112 to be studied. The coarse-drive mechanism 114 may be based on the use of, for instance, piezoelectric components, a micro solenoid and/or a micromechanical screw shifter. The coarse-drive mechanism 114 may position the probes 102A, 102B in 3-dimensional, 2-dimensional or 1 -dimensional space. The coarse- drive mechanism 114 may comprise one or more components, such as an x- shifter of two different accuracies for optimising the travel and the precision.

[0019] The coarse-drive mechanism 114 may be secured to the frame 110A of the measuring system or to a structure 110B that is fixed with respect to the frame 110A. The coarse-drive mechanism 114 moves the probes 102A, 102B. A sample constituting the object 112 to be studied may be connected to the structure 110B that is fixed with respect to the frame 110A of the measuring system. The frame 110A of the measuring system and the fixed structure 110B may be integral parts of the same structure.

[0020] In an embodiment, the frame 110A of the measuring system and the fixed structure 110B may be adjusted such that a working allowance for the precision-drive mechanism 104A will be provided between the object 112 to be studied and the probes 102A, 102B.

[0021] In an embodiment, the fixed part 110B is a structure that is rotatable with respect to the probes 102A, 102B and at the same time it rotates the object 112 to be studied with respect to the probes 102A, 102B. The embodiment set forth enables tunneling measurement of the object 112 to be studied from a variety of directions.

[0022] A control unit 120 may comprise a drive controller (D-CNTL) 122, a first voltage source (VS#1) 126 and a first current measuring unit (CMU#1 ) 124. The control unit 120 may be implemented using analog-to- digital converters, logic control and/or a digital processor with software.

[0023] The control unit 120 may be implemented, at least in part, in the immediate vicinity of the probes 102A, 102B. In that case, the length of galvanic conductors conducting weak tunneling currents are shortened and the quality of the measuring signal is enhanced.

[0024] The drive controller 122 generates a control signal 106A for the precision-drive mechanism 104A. In the given example, the control signal 106A may include various components of the control current for the piezoelectric component, each component being able to control the piezoelectric component in the x, y or z direction. In addition, the drive controller 122 may generate a control signal for the coarse-drive mechanism 114, which signal may include x, y and z components. [0025] The first voltage source 126 produces a first biasing potential difference between the probes 102A, 102B, which difference is applied to the probes 102A, 102B by means of a conductor arrangement 128A, 128B. The first biasing potential difference may be millivolts, volts or a voltage therebetween. In an embodiment, the first biasing potential difference is 5 volts, but the disclosed solution is not restricted to the presented potential differences. The first voltage source 126 may be implemented using a commercial voltage source.

[0026] In an embodiment, the first voltage source 126 comprises a frequency generator which produces a variable biasing potential difference. The biasing potential difference may oscillate within a frequency range of 1 Hz to 1 THz, without the disclosed solution being limited to that frequency range. In an embodiment, the frequency generator is a microwave source. Variable biasing potential difference enables spectroscopic measurements to be directed to the object 112 to be studied. The first current meter may then comprise a frequency separator, which determines an oscillation frequency relating to the first tunneling current.

[0027] The first biasing potential difference causes tunneling of charges between the probes 102A, 102B. The tunneling of charges appears as a first tunneling current, the magnitude of which may be measured in a first current measuring unit (CMU#1 ) 124. The first tunneling current may be, for instance, within the range of 0.1 pA - 1 μA, depending on the distance between the probes 102A, 102B, the sample, the measuring environment and the first biasing potential difference used, for instance. The measuring environment may be a liquid, a gas or a vacuum. The first current measuring unit 124 may be implemented using a commercially available current measuring system.

[0028] The first current measuring unit 124 may feed information 130 obtained from the current measuring, such as the value of the first tunneling current, to the drive controller 122. On the basis of the information 130, the drive controller 122 may adjust the precision-drive mechanism 104A, for instance, for optimising the first tunneling current to suit the equipment. The drive controller 122 may record measurement parameters determined during the measuring, such as values of the first tunneling current, location coordinates of the probes 102A, 102B with respect to the object 112 to be studied, values of the first biasing potential difference and/or the distance between the probes 102A, 102B. Said measurement parameters may be used in characterising the atomic properties of the object 112 to be studied.

[0029] The object 112 to be studied may be a surface of solid material, a particle or a DNA-chain spread out to form a surface. For instance, the system makes it possible to study crystal borders, detect charges, detect ions, molecules and/or atoms and to image substances and/or surfaces. However, the disclosed solution is not restricted to the presented measurements.

[0030] In addition, it should be noted that in this connection the object to be studied 112 may refer to the outmost atomic layer of the material, which may be affected by the inner atomic layers.

[0031] With reference to the example of Figure 2, the distance 204 between the probes 102A, 102B may be less than 0.01 nm. The distance 200A, 200B between the probes 132A, 132B and the object 112 to be studied may be less than 0.01 nm.

[0032] The probes 102A, 102B are typically formed such that the tip parts 132A, 132B simultaneously provide a contact between the probes 102A, 102B and a contact between the probes 102A, 102B and the object 112 to be studied. In this connection the contact refers to an effect under which tunneling may take place.

[0033] With reference to the example of Figure 3, at least one probe 102A, 102B is configured to provide charge tunneling between said at least one probe 102A, 102B and the object 112 to be studied. The tip of the probe 102A, 102B may be designed such that the atomic-sharp tip part 132A, 132B is simultaneously directed to the object 112 to be studied and to the tip part 132A, 132B of the second probe. In addition, the probe 102A, 102B is sufficiently tapering so as to restrict the tunneling outside the tip part 132A, 132B and thus to reduce interferences with the measuring. Tunneling outside the tip part 132A, 132B may also be restricted by providing an insulating layer on the surface of the probe 102A, 102B and leaving the tip part 132A, 132B insulation-free.

[0034] In order to provide tunneling between the probe 102A, 102B and the object 112 to be studied it is possible to generate a second biasing potential, for instance, by means of a second voltage source (VS#2) 304, whereby a second tunneling current between the object 112 to be studied and the probe 102A, 102B may be determined in a second current meter 302. Be- tween the second current meter 302 and the object 112 to be studied there is an electrical coupling 310.

[0035] The second current meter 302 may feed measurement parameters 308 relating to the second tunneling current to the drive controller 122, which may adjust the precision-drive mechanism 104A, 104B such that the distance between the probes 102A, 102B and the object 112 to be studied remains desired. The desired distance is, for instance, one that enables tunneling of charges between the object 112 to be studied and the probe 102A, 102B.

[0036] With further reference to the example of Figure 3, the measuring system 300 may comprise probe-specific precision-drive mechanisms 104A, 104B, of which the precision-drive mechanism 104A shifts the probe 102A and the precision-drive mechanism 104B shifts the probe 102B. It is possible to generate a control signal 105A, 106B that is specific to each precision- drive mechanism 104A, 104B and fed to the precision-drive mechanism 104A, 104B.

[0037] In the example of Figure 3, the precision-drive mechanisms 104A, 104B are secured to the frame 110A of the measuring system and the coarse-drive mechanism 114 is secured to a fixed structure 110B. In the given example the object 112 to be studied may be moved by means of the coarse- drive mechanism 114 into the vicinity of the probes 102A, 102B, whereafter the probes 102A, 102B may be moved by means of the precision-drive mechanisms 104A, 104B into the examination zone of the object 112 to be studied.

[0038] In the example of Figure 3, the first voltage source 126, the first current meter 124, the second voltage source 304 and the second current meter 302 may be included in the measuring unit (MU) 306.

[0039] With reference to the example of Figure 4, the measuring unit 400 may be implemented such that the first voltage source 126 and the second voltage source 304 are integrated in the same voltage source (VS) 410.

[0040] The measuring system may comprise a switch system 402, 404 for switching the first biasing potential difference and the second biasing potential difference to the probes 102A, 102B and the object 112 to be studied. The switch system 402, 404 comprises switches 402, 404, wherewith the voltage source 410 may be multiplexed to the probes 102A, 102B and the object 112 to be studied. The switches 402, 404 may be connected to a ground level 412 or another reference potential.

[0041] For instance, as the switch 402 is in a conductive state, the probes 102A, 102B are interconnected. As the switch 404 is in a conductive state, the object 112 to be studied is connected to at least one probe 102A. Figure 4 also represents a first current meter 406 and a second current meter 408.

[0042] From the current or voltage measurements, or the differences thereof, it is possible to produce, by digital processing, an imaging response that is based on a scan by the coarse-drive mechanism 114, for instance, performed in the vicinity of the surface of the object 112 to be studied.

[0043] With reference to the example of Figure 5, the precision- drive mechanism 500 comprises a piezo layer (PL) 502 that is lined with an electric contact layer (ECL#1 , ECL#2) 504A, 504B. In connection with at least one electric contact layer 504B there is an isolating layer (IL) 506, to which the probes 102A, 102B are secured. The function of the isolating layer 506 is to provide electric isolation between the probes 102A, 102B and the electrically conductive layers 504A, 504B.

[0044] In the example of Figure 5, a potential difference 508A, 508B provided between the electrically conductive layers 504A, 504B bends the piezo layer 502 in the longitudinal direction, whereby the bending makes the tips of the probes 102A, 102B move with respect to one another.

[0045] With reference to the example of Figure 6, the precision- drive mechanism 600 comprises a piezo layer 602, on both sides of which there are electric contact layers 604A, 604B, to which the potential difference 608A, 608B is directed and which are enveloped with an isolating layer 606. In this case the probes 102A, 102B are connected on either side of the piezo layer 602 such that the isolating envelope will be between the electrically conductive layers 604A, 604B and the probes 102A, 102B.

[0046] The isolating layer 506, 606 may be of ceramics, polymer or silicone polymer, for instance. The probes 102A, 102B may be secured to the isolating layer 506, 606 by gluing, for instance, or by joining the probes 102A, 102B to the isolating layer 506, 606 in connection with the polymerization step of the isolating layer 506, 606.

[0047] Figures 5 and 6 show examples of an embodiment, in which the measuring system comprises a common piezo actuator 502, 602, to which the probes 102A, 102B are connected. With the common piezo actuator 502, 602 it is possible to achieve an accurate mutual positioning of the probes 102A, 102B and a simple control mechanism for the piezo actuator. The piezo actuators 502, 602 of Figures 5 and 6 may be implemented, for instance, as 1-, 2- or 3-dimensional actuators.

[0048] With further reference to Figures 1 to 6, the measuring system may be implemented by various probe 102A, 102B, precision -drive mechanism 104A, 104B, 500, 500, coarse-drive mechanism 114 and control unit 120 variations.

[0049] Even though the invention is described above with reference to the example in accordance with the attached drawings, it is apparent that the invention is not restricted thereto, but it may be modified in a variety of ways within the scope of the attached claims.

Claims

1. A measuring system for studying properties of matter, characterized by comprising at least two probes (102A, 102B), between which there is a first biasing potential difference, the probes (102A, 102B) being configured to generate tunneling of charges between the probes (102A, 102B) in the vicinity of the object to be studied.

2. The measuring system of claim 1, characterized by further comprising a precision-drive mechanism (104A, 104B, 500, 600) that is configured to adjust the distance between said probes (102A, 102B) so as to adjust the tunneling.

3. The measuring system of claim 2, characterized in that the precision-drive mechanism (104A, 104B, 500, 600) comprises a common piezo actuator (502, 602), to which the probes (102A, 102B) are connected.

4. The measuring system of claim 1, characterized by further comprising a first voltage source (126) for generating a first biasing potential difference, the first voltage source (126) being coupled to said probes (102A, 102B); and a first current meter (124) for determining a first tunneling current between the probes (102A, 102B).

5. The measuring system of claim 4, characterized in that the first voltage source (126) is configured to generate a variable first biasing potential difference.

6. The measuring system of claim 1, characterized in that at least one probe (102A, 102B) is configured to provide charge tunneling between said at least one probe (102A, 102B) and the object to be studied, with a second potential difference prevailing between said at least one probe (102A, 102B) and the object to be studied.

7. The measuring system of claim 2, characterized in that the precision-drive mechanism (104A, 104B, 500, 600) is configured to adjust the distance between said at least one probe (102A, 102B) and the object to be studied.

8. The measuring system of claim 6, characterized by further comprising: a second voltage source (304) for generating a second biasing potential difference between at least one probe (102A, 102B) and the object to be studied, the second voltage source (304) being coupled to said at least one probe (102A, 102B) and the object to be studied; and a second current meter (302) for determining a second tunneling current between said at least one probe (102A, 102B) and the object to be studied.

9. The measuring system of claims 4 and 8, c h a r a c t e r i z e d in that the first voltage source (126) and the second voltage source (304) are the same voltage source (410); and the measuring system comprises a switch system (402, 404) for switching the first biasing potential difference and the second biasing potential difference to the probes (102A, 102B) and the object to be studied, the switch system (402, 404) being coupled to the same voltage source (410).