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Title:
MICROCANTILEVER SENSORS FOR COMBINED MICROSCOPY
Document Type and Number:
WIPO Patent Application WO/2018/015835
Kind Code:
A1
Abstract:
The invention relates to microcantilever sensors for atomic force microscopy (AFM), which are particularly suitable for use in combined systems exploiting simultaneously AFM and other type of microscopy, like: optical or electron, as well as a method of local micro / nano processing. The sensor of the invention comprises a body with a microcantilever extending from it, comprising flexible base portion and probe portion. The sensor contains in the probe portion an opening with symmetrical connecting flexure elements around it. To ensure the visibility to the area of interaction between the probe and the sample and to the probe tip, the base of the probe ends in the opening and the radius r and height h of the base of the probe satisfy the relationship: arctg (r/h)<α, where α is the apparatus dependant tilt angle of the sensor to the plane of the sample. The spring constants of the both portions of the invented sensor can be set separately and the sensor may further comprise: strain sensitive elements for detecting the microcantilever bending and additional openings in the base part; bimorph actuator, galvanic connecting tracks and /or other functional elements.

Inventors:
STAVROV VLADIMIR (BG)
Application Number:
PCT/IB2017/054103
Publication Date:
January 25, 2018
Filing Date:
July 07, 2017
Export Citation:
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Assignee:
AMG TECH LTD (BG)
STAVROV VLADIMIR (BG)
International Classes:
G01Q60/38; G01Q70/10
Foreign References:
US5959200A1999-09-28
JPH10332717A1998-12-18
US5367165A1994-11-22
EP0472342B11997-12-03
US5444244A1995-08-22
US5386720A1995-02-07
Other References:
AKIYAMA T ET AL: "Fast driving technique for integrated thermal bimorph actuator toward high-throughput atomic-force microscopy", REVIEW OF SCIENTIFIC INSTRUMENTS, AIP, MELVILLE, NY, US, vol. 73, no. 7, 1 July 2002 (2002-07-01), pages 2643 - 2646, XP012040103, ISSN: 0034-6748, DOI: 10.1063/1.1488148
A. D. SLATTERY ET AL.: "Atomic force microscope cantilever calibration using a focused ion beam", NANOTECHNOLOGY, vol. 23, no. 28, XP020225639, DOI: doi:10.1088/0957-4484/23/28/285704
M. TORTONESE ET AL.: "Atomic force Microscopy Using A Piezoresistive Cantilever", G. L. REPORT, vol. 91, no. 4821, March 1991 (1991-03-01)
M. TORTONESE ET AL.: "Atomic resolution with an atomic force microscope using piezoresistive detection", APPL. PHYS. LETT., vol. 62, 1993, pages 834, XP000338616, DOI: doi:10.1063/1.108593
Attorney, Agent or Firm:
YORDANOVA, Svetla (44 bl. 76 app, 1220 Sofia, BG)
Download PDF:
Claims:
CLAIMS

1. A microcantilever sensor to be used in scanning probe microscopy at apparatus dependent tilt angle a relative to the surface of the sample, the sensor comprising a body from which a microcantilever of length L and width w extends, said microcantilever has a base portion and a probe portion, first end of the base portion being fixed to the body and second end being connected to the probe portion, and the base and probe portions of the microcantilever being flexible and have different resonant frequencies, and the resonance frequency of the probe portion is larger than the resonance frequency of the base portion, wherein an opening in the probe portion with symmetrical surrounding flexure elements is formed, characterized in that:

to provide direct visibility to the area of interaction between the probe (4) and the sample (5) and to the tip (14) of the probe (4) through the opening (12), the base (4') of the probe (4) being circular with radius r, ends in the opening (12) whereas said radius r and height h of the probe (4) satisfy the relation:

arctg (r/h) < a,

and the spring constants of the base portion (10) and the probe portion (11) are set separately by selecting the parameters length / and thickness d of the base portion (10), heights a and b, widths gi and gi, and thickness t of the flexure elements (13) of the probe portion (11).

2. A microcantilever sensor according to claim 1, characterized in that to provide a high resolution and accuracy of the sensor, the shape of the tip (14) of the probe (4) is symmetrical conical.

3. A microcantilever sensor according to claims 1 and 2, characterized in that the base (4') of the probe (4) has a symmetrical conical shape with a height hb and a flat top section, while the tip (14) has a height ha, and the following relation:

is in effect for the heights ha and hb and the radius r on the base (4') of the probe (4).

4. A microcantilever sensor according to claims 1 to 3, characterized in that the height hb of the probe base (4'), the thickness d of the base part (10) and the thickness t of the probe portion (11) satisfy the relation:

5. A microcantilever sensor according to claims 1 to 4, characterized in that the absolute value of the height ha of the tip (14) is in the range from Ο.ΟΙμιη to 0.5μιη, preferably from 20nm to lOOnm.

6. A microcantilever sensor according to claims 1 to 5, characterized in that the tip (14) is located at or at a controlled distance δ from, the axis of symmetry of the probe base (4') and the material of the tip of the probe (4) is homogeneous or different from the material of the probe base (4').

7. A microcantilever sensor according to claims 1 to 6, characterized in that the probe (4) further contains an element (14') with cylindrical shape and a conical tip (14), which element (14') is disposed in the concave part (4") in the base (4') of the probe (4).

8. A microcantilever sensor according to claims 1 to 7, characterized in that at least one additional opening (17) is formed in the base portion (10) of the microcantilever (3) to increase the sensitivity of the sensor.

9. A microcantilever sensor according to claims 1 to 8, characterized in that the additional openings (17, 17') are two or three.

10. A microcantilever sensor according to claims 1 to 9, characterized in that elements (20) for galvanic connection of the probe (4) with external measuring instruments are additionally formed on the surface of the microcantilever (3).

11. A microcantilever sensor according to claims 1 to 10, characterized in that elements (21) for bimorph actuation of the microcantilever (3) are additionally formed on the surface of the base portion of the microcantilever (3).

12. A microcantilever sensor according to claims 1 to 11, characterized in that for detecting the bending of the microcantilever (3) strain sensing elements (9) are further provided at the fixed end of the base portion (10).

13. A microcantilever sensor according to claims 1 to 12, characterized in that said strain sensing elements (9) are piezoresistors; piezoelectric elements; thin metal film strain-sensors; discontinuous thin metal layers; thin film resistors of polyvinylidene difluoride, graphene or molybdenum disulfide.

14. A microcantilever sensor according to claims 1 to 13, characterized in that the probe portion (11) further contains a solid area reflecting a light beam of a width w equal to the width of the microcantilever (3) and a length a that exceeds 3η times the maximum size λ of the cross-section of the light beam used to detect the bending of the microcantilever (3), where η is a dimensionless apparatus factor ranging from 1.0 to 3.0, preferably from 1.3 to 2.5.

Description:
MICROCANTILEVER SENSORS FOR COMBINED MICROSCOPY

Technical Field

[0001] This invention relates to microcantilever sensors for scanning probe microscopy, especially atomic force microscopy (AFM). In particular, it relates to microcantilever sensors, suitable for application in systems for combined simultaneous use of AFM and other methods, such as optical, electron or other type of microscopy, or simultaneously with a method of local micro/nanoprocessing. In addition, sensors of the present invention may find use in all embodiments of the scanning probe microscopy.

Background Art

[0002] The scanning probe microscopy (SPM), in particular, scanning atomic force microscopy (AFM) is a powerful method to investigate the surface morphology of various samples with very high accuracy and resolution along the Z axis, which is perpendicular to the sample surface. The other known microscopy methods such as optical and electron microscopy, have high accuracy and resolution in the XY plane, but parameters along the Z axis are insufficient. Therefore, combined systems comprising AFM and another microscopy method are especially useful and desired by the researchers.

[0003] Specific consumables are used for AFM - microcantilever sensors 1, which comprise a body 2 and a microcantilever 3 extending from the body, as shown in Fig. la- le. Two preferred shapes of microcantilevers are known from practice - single beam and triangular (also known as "V-shape"). A probe 4 whose tip has a minimal radius of curvature, is disposed at the free end of the flexible microcantilever 3. When the microcantilever 3 is of single beam type, the free end of it around the probe is narrowed to ensure access of the probe 4 to the sample 5. In working position for performing an AFM imaging, the microcantilever is tilted at a specific angle a, which is apparatus dependant, regarding to the surface of the sample 5. Surface of the sample is scanned in a predefined grid of points and the response of the flexible microcantilever 3 (i.e., its bending due to the deflection of the free end) as a result of the local interaction between the atoms of the probe 4 with the atoms of the sample 5 is recorded at each point of the grid. [0004] Upon detection of this interaction, microcantilevers with a smaller value of the spring constant k provide larger deflection amplitude and, accordingly, a higher sensitivity. In practice, the interaction of the microcantilever 3 is detected by two preferred methods - optical (laser) reflection and piezoresistive. In view of the layout of the microcantilever 3, there is an important difference between the above mentioned two detection methods: the laser beam is reflected by the solid area on rear side of the cantilever in the vicinity of probe 4, while the piezoresistive detection is performed in the front side of the region where the microcantilever 3 is connected (attached) to the body 2, which is most distant from the probe. The mode of carrying out the measurement with AFM with laser detection is shown schematically in Fig. la. In this mode, the light beam from a stationary source 6 is reflected by the microcantilever 3 and it is detected by a stationary position-sensitive photo detector 7. In that, the sample surface 5 is scanned along the axes X and Y by moving the sample, while by means of a controlled movement of the entire sensor 1 along the Z axis, a constant signal level of the reflected light from the rear side of the microcantilever 3 is provided, as illustrated in the figure.

[0005] Fig. lb shows a well-known embodiment of the AFM sensor with an overall length L, thickness t and width w of the microcantilever that is constant except for the area of the probe 4, which is narrowed. A longitudinal section of such a sensor along the axis of symmetry is shown in Fig. lc. The thickness of the rigid body 2 of the sensor lis in the range of about 50 μιη to about 500 μιη and the microcantilever 3 has a thickness in the range of about 0.5 μιη to about 10 μιη. The height h of the probe 4 is selected in accordance with the specifics of the sample surface and usually the probes of the sensors are pyramidal shaped and the tip has triangular or quadrangular cross section. A small percentage of the AFM sensors have conical shape of the probe and circular cross section of the apex.

[0006] As the light beam is reflected from the rear solid side of the microcantilever 3 in the region of probe 4, neither the area where the probe interacts with the sample cannot be observed directly, nor the probe tip, can be seen. The lack of direct visibility is shown schematically in Fig. la and this causes inconveniences, for example, when samples with periodic surface patterns are investigated. When there is no direct visibility of the probe tip, the position accuracy of the elements on the sample surface is limited due to the uncertainty of the tip position. Thus, usually AFM measurements provide information about the distance between the elements, but their absolute position cannot be determined. Additional calibration measurement with the same sensor is needed to determine the elements' position, which is burdensome and restrictive.

[0007] Another inherent problem of AFM is related to the large time needed for imaging the analyzed surface. To reduce the time, fast scanning microscopes were developed, which use fast microcantilever sensors with high resonance frequency f c . As the AFM sensors usually have homogeneous thickness t, the fast oscillating sensors possess high value of the spring constant k. This makes these sensors unsuitable for investigation of particularly important group of soft materials such as the biological tissues. Thus, for microcantilevers with homogeneous thickness to achieve both high resonance frequency fc and small value of the spring constant k, it is imperative to use ones with very small dimensions L, w and t. This in turn causes new difficulties and disadvantages known to those skilled in the art.

[0008] Another common embodiment of a sensor 1 for AFM with laser detection and triangular shape of the microcantilever 3 is shown in Fig. Id. Usually, in such microcantilever 3 a trapezoidal opening 8 is formed. However, because of the need for light reflection, the area close to the probe 4 is solid and there is no direct view to the area of its interaction with the sample 5.

[0009] Device with opening is disclosed in European Patent N° EP0472342B1, where a micro-displacement type probe device including: a substrate; a first elastic membe supported by the substrate; a second elastic member supported by the first elastic member, is disclosed. In that, the resonance frequency of the second part is higher than the resonance frequency of the first one, which determines the operating mode of the sensor. Although in an embodiment, shown in Fig. 7a, a sensor for AFM with opening in the second elastic member is shown the function of the opening is not described nor is any purpose disclosed.

[0010] A microcantilever sensor with opening is described in the paper A. D. Slattery et al., "Atomic force microscope cantilever calibration using a focused ion beam", Nanotechnology, Volume 23, Number 28. In this paper a method is described for calibrating the spring constant of the microcantilever by precisely controlled removal of a part of its mass by using a focused ion beam, wherein an opening is formed in an area near to the probe, but outside the area for laser detection. Other options to exploit the opening, including sample observation through it, are not considered in this paper.

[0011] Microcantilever sensors for AFM with piezoresistive detection were disclosed first in the publications: M. Tortonese et al. "Atomic force Microscopy Using A Piezoresistive Cantilever" by G. L. Report No. 4821; Transducers, 91 (Mar. 1991) and M. Tortonese et al., "Atomic resolution with an atomic force microscope using piezoresistive detection", Appl. Phys. Lett. 62, 834 (1993), as well as in U.S. Patent N° US5444244. Sensors, which include piezoresistors for detection of microcantilever bending and a probe with a small radius of curvature, are disclosed in these works. Although an embodiment with an opening in the microcantilever is disclosed in the above mentioned patent, neither the use of the opening for observation of the sample is disclosed nor a modification of the opening is discussed.

[0012] The U.S. Patent JV° US5386720 discloses a sensor for AFM with piezoresistive detection, consisting of two microcantilevers extending from the sensor body. The ends of the microcantilevers are connected by a common free end with a triangular shape in which a probe with a sharp tip is formed. Although microcantilevers with an opening are disclosed, this patent neither suggests the use of this opening for observation of the area of interaction, nor the shape of the probe and the opening are designed in a way to enable such observation.

[0013] When piezoresistive detection of microcantilever bending is used, no stationary external elements such as a light source 6 and a photo detector 7 are required. Therefore, with such sensors also said "self-sensing", the both: movable and stationary samples can be analyzed. Consequently, piezoresistive microcantilever sensors are preferred in combined systems for simultaneous use of AFM and other methods, such as: scanning electron microscopy (SEM); transmission electron microscopy (TEM); operation in vacuum or in turbid liquids; at low temperatures, etc., known to those skilled in the area.

[0014] Fig. le shows a state of the art sensor 1 for AFM with typical shape and piezoresistive detection of the bending of the microcantilever which has a length L, a width w and a thickness t. Typically, the detecting resistors 9 are two oriented along the axis of microcantilever 3 and are situated in its base, and by means of metal tracks, not shown in the figure, they are connected in a full bridge configuration together with two additional passive resistors 9'.

[0015] A longitudinal section of such a sensor 1 along the line AA' is shown in Fig. If Like the layout of microcantilever sensors with optical detection, parameters of the body and the piezoresistive microcantilever are similar. In addition, to ensure high sensitivity of the sensor, the following relationship between the depth x j of the piezoresistors 9 and t of microcantilever 3, is necessary to be fulfilled:

Xj< 1/3 . t (1)

[0016] Because a violation of the relationship (1) results in reduction of the sensor sensitivity, the reduction of the thickness t of the microcantilever is limited by the depth xj of piezoresistors. This makes it difficult to produce piezoresistive AFM cantilevers having a small spring constant k, necessary for the analysis of soft materials.

Technical Problem

[0017] Consequently, there is a need for AFM microcantilever sensors which allow direct visibility to the area of interaction of the probe with the sample as such visibility will enable both direct determination of the probe tip position, which the state of the art sensors do not allow, and the simultaneous accomplishment of complementary local microscopy analyses or processing. Such complementary methods of analysis are optical, electronic, or con-focal microscopy, local elemental analysis, etc. Processing methods which can be applied simultaneously with the AFM are e. g. : micromachining with focused electron, ion and laser beams, incl. local deposition, growth or phase transition; application of micro-dispensing, micro-printing, and other means of local processing.

[0018] For this purpose, AFM microcantilever sensors have to comprise a suitable opening in the microcantilever providing visibility of the area of probe-sample interaction and of the tip. It is preferable the microcantilevers to have simultaneously conventional size and meet the up-to-date requirements for high sensitivity and resolution; high resonance frequency f c ; and a small spring constant k of the items bearing the probe.

Summary of the Invention

[0019] The invention relates to a microcantilever sensor for scanning probe microscopy which is used at apparatus dependant tilt angle a regarding to the surface of the sample, and which sensor comprising a body from which a microcantilever with a length L and an width w extends and cantilever comprising base portion and probe portion. One end of the base portion is fixed to the body and the other end is connected to the probe portion, the base portion and the probe portion of the microcantilever being flexible with different resonance frequencies, wherein the resonance frequency of the probe portion is larger than the resonance frequency of the base portion. In addition, an opening in the probe portion with symmetric connecting flexible parts around it is formed, and to provide visibility to the area of the interaction between the probe and the sample and to the tip, the circular base of the probe with radius r ends at the opening and the probe height h satisfy the relationship:

arctg(r/h) < a.

[0020] The spring constants of the base and probe portions of the microcantilever can be separately set by selecting their parameters, like: length / and thickness d of the base portion, heights a and b, widths gi and gi, and thickness t of the flexure elements of the probe portion.

In one embodiment of invention, in order to provide higher resolution and accuracy of the sensor, tip of the probe has symmetrical conical shape.

[0021] In another embodiment of the invention, the base of the probe has symmetric conical shape with a height /¾, and a flat upper part, and its tip has a height h a , where the heights h a and /¾ and the radius r of the base of the probe fulfill the following relationship:

[0022] In a further embodiment of the invention, the height /¾ of the base, the thickness d of the base portion and the thickness t of the probe portion satisfy the relationship:

[0023] In yet another embodiment of the invention, the absolute value of the height h a of the tip is in the range from 0.0 lum to 0.5 μιη, preferably from 20nm to lOOnm.

[0024] In a further embodiment, the tip is located on, or at controlled shift δ from, the axis of symmetry of the probe base, and the material of the probe tip is homogenous or different from the material of the base of the probe.

[0025] In another embodiment of the invention, the probe further comprises an element having a cylindrical shape and a conical tip, which element is accommodated in a concave portion at the base of the probe.

[0026] In another embodiment of the invention, in order to increase the sensitivity of the sensor, at least one additional opening is formed in the base portion of the microcantilever.

[0027] In yet another embodiment of the invention, the additional openings are two or three.

[0028] In a further embodiment of the invention, additional elements on the microcantilever surface are formed to provide galvanic connection of the probe to external measuring equipment.

[0029] In a further embodiment of the invention, on the surface of the base portion of the microcantilever additional elements for bimorph actuation of the microcantilever are formed.

[0030] In another embodiment of the invention, the sensor comprises additional sensor elements for detecting the bending of the microcantilever, which are strain sensitive, located in the fixed end of the base portion of the microcantilever. These sensor elements are elements like: piezoresistors; piezoelectric, thin metal strain sensors; thin discontinuous metal layers; thin-film resistors of polyvinylidene difluoride, graphene or molybdenum disulfide structures.

[0031] In yet another embodiment of the invention, the probe portion comprises additional solid area capable of reflecting a light beam, having a width w, equal to the width of the microcantilever, and a length a which is 3η times larger than the maximum crosssection λ of the light beam used for detection of microcantilever deflection. Here, η is a dimensionless apparatus factor ranging from 1.0 to 3.0, preferably from 1.3 to 2.5.

Brief Description of Drawings

[0032] Fig. la shows a side view of microcantilever sensor for AFM with laser detection in a working position known from the state of the art.

Fig. lb is a plan view of a conventional single beam microcantilever sensor for AFM.

In Fig. lc a longitudinal section of a conventional microcantilever sensor for AFM, is displayed.

Fig. Id shows a state of the art microcantilever sensor for AFM with a triangular shape. Fig. le is a top view of the state of the art AFM microcantilever sensor with single beam shape and built-in piezoresistors.

Fig. If shows a longitudinal section of the state of the art microcantilever device with built-in piezoresistors for detection of the bending.

Fig. 2a shows a top view of a horizontally positioned microcantilever sensor for AFM according to the invention with an opening for observation of the area of the probe- sample interaction and with built-in piezoresistors.

Fig. 2b shows a longitudinal section along line AA' of the microcantilever sensor in Fig. 2a, with different thicknesses of the base portion and probe portion.

Fig. 2c shows a longitudinal section along line AA' of microcantilever sensor in Fig. 2a, with the uniform thickness of the base and probe portions.

Fig. 2d shows a side view of the piezore si stive microcantilever sensor for AFM in working position, with an opening for observation of the area of probe-sample interaction.

Fig. 3a shows a top view of a microcantilever sensor for AFM in working position, with a pentagonal opening for observation of the sample and of the probe tip.

Fig. 3b shows a longitudinal section of a microcantilever in working position, with an opening for observation of the sample and a homogeneous symmetrical probe.

Fig. 3c shows a side view of a longitudinal section of a microcantilever with inhomogeneous symmetric probe tip, located on the axis of symmetry of the probe base.

Fig. 3d shows a side view of a longitudinal section of a microcantilever with inhomogeneous probe and a tip, shifted with respect to the axis of symmetry of the probe base.

Fig. 4a is a schematic side view of a probe with symmetrical tip and a sample comprising analyzed and perturbing nodes.

Fig. 4b is a schematic representation of the influence of perturbing interaction between the probe tip with a triangular cross-section and periodically arranged symmetrical perturbing nodes.

Fig. 4c shows schematically the influence of perturbing interaction between the probe tip with a quadrangular cross-section and periodically arranged symmetrical perturbing nodes.

Fig. 4d is a schematic representation of the influence of perturbing interaction between the probe tip with a circular cross section and periodically arranged symmetrical perturbing nodes.

Fig. 4e is a schematic representation of the influence of perturbing interaction between the probe tip with a circular cross section and periodically arranged asymmetric perturbing nodes.

Fig. 5a is a top view of an embodiment of microcantilever sensor for AFM in horizontal position, provided with an observation opening and an additional opening in the base portion.

Fig. 5b shows a top view of an embodiment of microcantilever sensor for AFM in horizontal position, provided with an opening in the probe portion and two additional openings in the base portion.

Fig. 5c is a top view of an embodiment of a horizontally positioned microcantilever sensor for AFM, with an opening in the probe portion for observation of the sample and three additional openings in the base portion.

Fig. 6a is a top view of the microcantilever sensor for AFM with built-in piezoresistors with a triangular opening for sample observation, three openings in the base portion and conductive tracks between the probe and an external measuring instrument.

Fig. 6b shows a plan view of the microcantilever sensor for AFM with built-in detectors of the microcantilever bending, triangular opening for sample observation, three additional openings and bimorph thermo actuator.

Fig. 7a is a top view of a horizontally positioned microcantilever sensor for AFM with laser detection and a triangular opening for sample observation.

Fig. 7b shows a longitudinal section of a horizontally positioned microcantilever sensor for laser detection of deflection and different thicknesses of the base portion and the probe portion.

Fig. 7c shows a side view of microcantilever sensor for AFM for laser detection with an opening for observation of the area of the probe-sample interaction in working position.

Examples of the Invention

Example 1

[0033] One embodiment of the microcantilever sensor according to the present invention is shown in Fig. 2a, and its longitudinal section along the line AA' is displayed in Fig. 2b. Figure 2a is a top view of a horizontally disposed microcantilever sensor 1 comprising a body 2 and micro cantilever 3 which is symmetrically shaped relative to the longitudinal axis, with a length L and a width w. Preferably, the length L is in the range from 20 μιη to about 500 μιη, ideally from 50 μιη to 425 μιη, and the width w is in the range from 5 μιη to about 150 μιη, ideally from 20 μιη to 120 μιη. Typically, the parameters L and w of the microcantilever are selected according to its application, in a manner known for those skilled in the art. In this example, the length L is 100 μιη and the width w is 40 μιη. The microcantilever 3 comprises two flexible portions - the base portion 10 and the probe portion 11. One end of the base portion 10 is firmly attached to the body 2, while other end is connected to the probe portion 11. At the free end of the probe portion 11 of the microcantilever 3 a tip 4 with a conical shape is located, and adjacent to the tip 4 an opening 12 with surrounding flexure elements 13, is formed. The shape of the opening 12 is symmetrical, constituting a logical difference of a symmetric polygon with an odd number of vertices, in this case five, and the shape of the base 4' of the probe 4, wherein the base 4' ends in the opening 12. Thus, the analysed sample 5 is visible through the opening 12 which makes possible the direct observation of the area of interaction between the probe 4 and the sample 5. When the probe 4 has a symmetrical conical shape, respectively the cross-section of the base 4' is a circle, the XY coordinates of the tip 14 of the probe 4 can be calculated as an average value of the coordinates of any pair of diametrically located points of the periphery of the base 4', seen from the rear side of the microcantilever, preferably, the pair of points located on the longitudinal axis of symmetry of the microcantilever. Therefore, the visibility of a part of the periphery of the base 4' is sufficient condition to calculate accurately the position of the tip 14 of the probe 4, and accordingly, the position of the area of interaction of the probe 4 with the sample 5, even when the probe tip 14 is not directly visible from the rear side of the sensor.

[0034] The sensor 1 described above can be used in AFM with laser detection of the microcantilever deflection. Optionally, it may also be provided with additional strain sensing elements 9 arranged on the upper surface the base portion 10 of the microcantilever 3 adjacent to the area where it is fixed to the body 2. The said sensing elements 9 are selected from certain known types, like: piezoresistive, thin discontinuous metal film elements, nano-granular tunnelling resistors, piezoelectric, thin metal film strain sensors, thin polyvinylidene difluoride (PVDF) film resistors, patterns made of two-dimensional materials such as graphene, molybdenum disulfide, etc.; in this example, the sensing elements are piezoresistors. They are connected in a full bridge configuration with passive elements 9' by means of conductive metal tracks, not shown in the figure, and elements 9' being identical in layout and parameters to the sensor elements 9, but located on the surface of the rigid body 2. The sensor elements 9 connected in such a configuration generate a voltage signal corresponding to the bending of the microcantilever. Most preferred sensing elements are the piezoresistors that, as shown in Figure 2a, have a length l r , which is smaller than the length / (/?-</) of the base portion 10.

[0035] In the embodiment of the invention shown in Fig. 2a, the pentagonal shape of the symmetrical opening 12 where the base 4' ends, can be represented by a sum of two overlapping members: a rectangular member with a height a and an isosceles triangle shaped member with a height b while the widths of the surrounding flexure elements 13 are gi and g 2 , respectively.

[0036] The thickness of the body 2 shown in Fig. 2b is typically in the range from 50 μιη to about 500 μιη, in this example, the thickness is 270 μιη. The thickness d of the base portion 10 of the microcantilever 3 is in the range from 0.5 μιη to 10 μιη, in this example it is 5 μιη, and is different from the thickness t of the probe portion, which in this example is 1 μιη. In this embodiment of the invention the following relationship is valid: d > t (2)

[0037] At given values of parameters L and w, by selecting the parameters /, d, t, a, b, gi and g2 of the respective portions, a variety of sensors having desired different resonance frequencies and spring constants of the both portions - the base 10 and the probe 11 ones, can be obtained, because all said parameters can be set separately and selectively. In all embodiments of this invention, the resonance frequency of the probe portion 11 is larger than the resonance frequency of the base portion 10; the working frequency of the microcantilever sensor is determined by the lower frequency. Thus, a reduced value of the spring constant of the probe portion 11 can be achieved, without a decrease of the sensitivity of the piezoresistive sensors 9 and of the resonance frequency f c , which are determined by the thickness d of the base portion 10. [0038] Fig. 2c shows a longitudinal section along the line AA' of another embodiment of the microcantilever sensor 1 of the invention. In this embodiment, the thicknesses d of the base portion 10 and t of the probe portion 11 of the microcantilever 3, are equal, i.e.: d = t (3)

[0039] Thereby the spring constant of the base portion 10 can be set by the above mentioned thickness d, length / and width w. While the spring constant of the probe portion 11 is determined by the heights a and b of the flexure elements 13, as well as their widths gi and g2. Thus, even with equal thicknesses d and t of the both portions and given values of the parameters L and w, desired values of the resonance frequencies and the spring constants of the both portions of the microcantilever 3 - the base portion 10 and the probe portion 11, can be set separately by selecting the parameters /, a, b, gi and g2. Thereby parameters, like: high resonance frequency f c , high sensitivity and a "soft" probe portion 11, which does not damage soft samples, can be provided simultaneously with conventional geometrical dimensions L, w and t of the microcantilever.

[0040] The sensor according to the embodiment of the present invention can be used in a manner shown in Fig. 2d. The microcantilever sensor 1 in a working position is tilted at apparatus dependant angle a regarding to the surface of the sample 5, which may be scanned, optionally, by movement of the sample 5 along the axes X and Y, provisionally denoted in the figure by symbols X(o) and Y(o). Said angle a is specific for every scanning microscopy apparatus and is known from the specifications.

[0041] Further, by a controlled movement of the sensor 1 along the axis Z, a constant signal from the resistors 9 and 9' connected in a bridge configuration can be ensured. It is also possible, the sample to be stationary, and its surface to be scanned by moving the microcantilever sensor 1 with the embedded piezoresistors 9 along the axes X and Y. Similarly, by a controlled motion of the sensor 1 along the axis Z, constant signal from the bridge of sensing piezoresistors 9 and passive resistors 9' can be ensured, when scanning the surface of the sample 5. It is known that when the sample 5 is stationary, the positions of its elements is constant and they may be registered in advance, e.g. by a supplementary instrument.

[0042] Opening 12 provides visibility to a part of the base 4' of the probe 4 and if it has a symmetrical shape, the position of the apex of the tip 14 can be calculated accurately as already mentioned. Therefore, besides that the area of interaction of the probe 4 with the sample 5 can be observed, the coordinates (position) of each feature on the AFM imaged surface can be calculated.

[0043] Fig. 3a shows a top view of microcantilever sensor 1 with an opening 12 in working position, according to the examples of the present invention. In particular, an embodiment where the width g2 of the flexure element 13 is smaller than the diameter (g2<2r) of the circular base 4', which centre is located at the intersection of the central lines of the elements 13, is shown in the figure. This ensures that the points of the base 4', which are located on the axis of symmetry of the microcantilever, can be observed from the rear side.

[0044] Moreover, the microcantilever in working position is tilted at an apparatus dependent angle a regarding to the surface of the sample 5 and the tip 14 of the probe 4 can be directly seen through the opening 12 as shown in Fig. 3a. Thus, microscopy analyses in the area of interaction of the AFM probe with the sample and another complementary microscopy, such as: optical, scanning electron, etc., can be performed simultaneously.

[0045] In the detailed side view of the longitudinal section of the probe 4 in working position shown in Fig. 3b, the probe has a cylindrical symmetrical shape, a height h, and the radius of the probe base is r. It was found that at an apparatus dependant tilt angle a, in this example, a = 13°, the tip 14 of the probe 4 can be seen directly through the opening 12 if the following condition is fulfilled:

arctg (r/h) < a (4)

[0046] It will be apparent to those skilled in the art that for a given tilt angle a and a preselected size of the radius r of the base of the probe 4, the relationship (4) may be satisfied when the probe height h is greater than the threshold height ho, defined by the relation:

ho= r/tg a (5)

[0047] In this example, the condition (5) gives ho ~ 4Vs.r. The value of the parameter h can be achieved by selecting the parameters of the plasma etching process, as it is customary for the area.

[0048] In one embodiment of the invention, the probe 4 is homogeneous, the both elements - the base 4' and the tip 14 are made from a homogeneous material, as example silicon, and their cross sections are circles. For this purpose, the said probe elements usually are shaped simultaneously.

[0049] Another embodiment of this invention in which the elements of the probe 4 are formed successively is illustrated in Fig. 3c. In this embodiment, the probe 4 has a cylindrical symmetry and comprises a base 4' with a height /¾, which base has a flat upper surface and a tip 14 with a height h a and the smallest achievable radius of curvature, located in the centre of the flat area. Besides, the height /¾ of the base 4' is in the range from 1 μιη to about 15 μιη, preferably from 2 to 5 μιη, in this example the height is 4 μιη. The size of the flat area is from lOnm to lOOnm, preferably from 15nm to 50nm, in this example is a circle with a diameter of 25nm. Moreover, the height h a is in the range from lOnm to 500nm, preferably from 20nm to 100 nm, in this example it is 60nm, and the height h of the probe is equal to the sum of the two mentioned heights /¾ and h a . In order to have direct visibility to the tip 14 in working position through the opening 12, the sum of the said heights should be greater than or equal to the threshold height ho, according to the relationship:

[0050] In the present invention, the absolute value of the height h a is also in the range of 10 nm to 500 nm, preferably from 20 nm to 100 nm.

[0051] In a preferred embodiment of this invention, when the material of the tip 14 is homogeneous with the material of the base 4' of the probe, and the tip is formed from the base 4' of the said probe with a height /¾ by locally removing a part of the material of the flat surface in a controlled manner, which can be done preferably by an anisotropic process, until obtaining the desired radius of curvature, in this example, the radius is 3nm. Thus, for this embodiment, relative to the initial height /¾, the sign of the height h a is negative (h a <0), in this example h a = -30nm. It is apparent for those skilled in the art that in this embodiment, the probe tip 14 can be formed, e.g. by locally removing material with micro / nano-processing, such as local anisotropic dry etching, ion sputtering, high-temperature oxidation followed by selective etching of silicon dioxide or another conventional process.

[0052] In another preferred embodiment of this invention, on the flat area a tip 14 with the desired height h a , shape and radius of curvature can be formed by material additive method; the added material is usually different from the material of the base 4', deposited along the axis of symmetry of the base 4'. In this example, the height h a is 60nm, the shape of the tip is a straight cone and the radius of curvature is 5nm. This can be done, e.g. by local deposition of material from a gas phase by a decomposition: thermal, stimulated by electron, ion or laser beam; local growth of a eutectic alloy or by phase transition; micro-printing, optionally with subsequent local removal by e.g. ion etching and other local micro / nanoprocessing methods known to those skilled in the art. When the most protruding part of the probe, in this case the base with radius r, can be seen from the apex of the tip 14 in the above described embodiment of invention, at an angle β, satisfying the condition:

β < 2a (7), the tip 14 can be seen in working position through the opening 12 in the microcantilever 3.

[0053] In another preferred embodiment, shown in Fig. 3d, also by adding material, the tip 14 with the desired shape and height h a can be shifted at a controllable distance δ from the axis of symmetry of the base 4'. In this example, the shift δ is 5nm along the axis of symmetry of the micro cantilever, away from the body 2. In such case, the tip 14 is visible through the above mentioned opening 12 in the microcantilever 3 when the angle β/2, at which the effective radius r* = r - δ is seen from the tip 14, satisfies the condition (7). Furthermore, the known distance δ and shift direction of the tip 14 from the axis of symmetry of the base 4', are sufficient conditions to calculate accurately position in XY of the area of interaction of the probe 4 with the sample 5.

[0054] A further embodiment of this invention for an appropriate shaping of the base 4' of the probe 4 is shown in Fig. 3d. The figure is an axonometric view of a probe 4 with a base 4' which has a height /¾ and a concave part 4" is formed, in this case a trench. A pre- prepared element 14', consisting of a portion with a cylindrical shape and a conical tip, which serves as a tip 14 of the probe, is micro positioned and fixed in the trench 4", so that the apex of the tip 14 is located at a height h a above the base 4'. When the angle β, at which the most protruding part of the base 4' is seen from the apex of the tip 14, satisfies the condition (7), there is a direct visibility of the tip through the opening 12. In the most preferred implementation of the embodiments of invention, such as the examples shown in Fig. 3c - 3e, for the height /¾ of the base 4' and the thicknesses d and t, the following relationship is fulfilled:

[0055] In these embodiments of the invention, the base 4' of the probe 4 can be formed, e.g. by selectively removal of the material from the top surface of the microcantilever 3 with the thickness d, until reaching the desired thickness t of the probe portion 11.

[0056] These embodiments are particularly relevant when it is required to perform a correlative in-situ analysis with more than one microscopy methods of a stationary sample. They are also particularly suitable in the case when it is required to perform a high spatial resolution analysis of delicate samples that change their properties in the course of the analyse, or when analysing in-situ synthesized structures.

Example 2

[0057] It was found by the inventor of the present application that the shape of the tip 14 of the probe 4 has an impact on resolution and accuracy of the AFM analysis. As shown in Fig. 4a, the interaction between the tip 14 of the probe 4 with the node 15 of the sample 5 is perturbed by the other nodes 15' located closely to the probed node 15. Such perturbations deteriorate the resolution and accuracy of the AFM analysis, but they can be reduced by the proper selection of the shape of the tip 14 of the probe 4.

[0058] For example, Fig. 4b shows the scan grid of a stationary sample 5 by the sensor probe 4 with a tip having a cross section of triangular shape as depicted in the figure, which is typical for a large part of the conventional sensors. Probe with such a tip shape can be fabricated, e.g. by using anisotropic wet etching of single crystal silicon through a mask with patterns oriented along certain crystallographic directions. When the atoms of the tip are in close proximity to, and interact with the atoms of the probed node 15 of the sample 5, the perturbations from each of the nodes 15' (/ = 1, k, in this example k = 4) which in this case have a symmetrical shape, is also detected. Even with periodic patterns, as are known e.g. in present day microelectronic integrated circuits or samples with periodic (crystal) structure, incl. two-dimensional crystals, etc., in which the nodes 15' are symmetrically situated with respect to the node 15, the position of the centre 16 of the overall perturbing interaction indicated on the figure with a cross, is shifted with respect to the position of the probed node 15 due to the specific shape of the tip. Thus, when using a probe 4 with a tip which has not a cylindrical symmetry, the amplitude and the position of the centre 16 of the overall perturbing interaction is shifted, which leads to inherent measurement inaccuracies of the probed node 15 along each of the axes X, Y and Z.

[0059] Similarly, in the example shown in Fig. 4c, the tip of the probe 4 has a quadrangular shape typical for another large part of conventional sensors. As in Fig. 4b, in this case the above-mentioned inaccuracies in the measurement due to the shape of the tip are also present.

[0060] A further example is shown in Fig. 4d, where the sample 5 is scanned by the probe 4 having a tip with a circular cross-section, schematically indicated by a bright circle. Probe with such a shape of the tip can be done, e.g. by using a plasma etching through a mask with a round shape and size, determining the radius r of the base of the probe 4. In this example, the tip of the probe 4 is positioned adjacent to the node 15 of the sample 5.

[0061] When periodically and symmetrically perturbing features 15' ( = 1, k, in this example k = 4) are arranged around the probed node 15, each of them with a symmetric shape, the position of the centre 16 of the net perturbation coincides with the position of the probed node 15. Therefore, although each of the nodes 15' perturbs and changes the amplitude of interaction, which is registered by the tip of microcantilever 4, the position of the resulting interaction does not change. Thus, the use of a probe 4 with a tip having a circular cross-section ensures keeping of the precision and resolution along the axes X and Y, but does not provide the accuracy along the axis Z.

[0062] Similarly, Fig. 4e shows the scan grid of the sample 5 in which nodes the interaction of the probe 4 with a tip having a circular cross-section, schematically indicated by a bright circle, is recorded. In this embodiment, the individual perturbing nodes 15' (/ = 1, k, k = 4) of the sample 5 have no cylindrical symmetry. In case of regular and symmetrical arrangement of the nodes 15' relative to the probed node 15, the position of the centre 16 of the net perturbation is slightly shifted and is indicated with a cross in the figure. Thus, in case of presence of adjacent non-cylindrically symmetric perturbing features 15', the use of the probe 4 with a tip having a circular cross-section provides a small loss of accuracy and resolution along X and Y axes.

[0063] When the centre 16 of the resulting interaction of the nodes 15', is shifted at a distance from the probed node 15 of less than the resolution of the XY scanning system, the shift cannot be detected. When the effective impact of the perturbing nodes 15' on the magnitude of the microcantilever 3 deflection along the axis Z, is less than the bending resolution of the system, said perturbation cannot be detected.

[0064] Therefore, in case of AFM analysis of a sample 5 with the probed node 15 and in the presence of perturbing nodes 15', the circular cross-section of the tip of the probe 4 is a necessary condition for achieving higher resolution and accuracy. Therefore, this embodiment is preferred, in comparison with the embodiments which use a tip without cylindrical symmetry. This requirement is particularly important when the resolution of AFM system using these sensors is not known in advance. In particular, this concerns sensors intended to be used as a consumable for non-specific AFM apparatus (for general use).

Example 3

[0065] A preferred embodiment of the present invention of a horizontally disposed microcantilever sensor with piezoresistive detection is shown in Fig. 5a. The figure shows a top view of microcantilever sensor 1, similar to the sensor of Fig. 2a.

[0066] In this example, the base portion 10 has a length / and a width w, and is provided with piezoresistive sensor elements 9 with a length l r , located on the upper surface of the base portion 10 adjacent to the area where the microcantilever 3 is connected to the body 2. Furthermore, in the base portion 10 an additional rectangular opening 17 with respective connecting flexible elements 18 is formed and the length h of the opening 17 is chosen to be equal to the length l r of the piezoresistors 9. Moreover, the piezoresistors 9 are arranged along the periphery on the outer sides of the opening 17 in the area of the connecting elements 18. In this embodiment of the invention, a concentration of mechanical stress in the areas of location of piezoresistors 9 is obtained, which results in a sensor with a higher sensitivity. In this embodiment, the position of the connecting flexure elements 18 determines the width w of microcantilever 3 and they retain stability of the microcantilever 3 against in-plane bending.

[0067] In this case, the thickness d of the base portion 10 of the microcantilever 3 and the thickness t of the probe portion 11 are different and satisfy the condition (2). Thus, it becomes possible to obtain optimal values of the parameters of the microcantilever - spring constant k and resonance frequency f c , without a reduction of the thickness d of the base portion 10 and the correlated sensitivity of piezoresistors 9.

[0068] Another preferred embodiment of microcantilever sensor with piezoresistive detection according to the present invention is shown in Fig. 5b. In this case, in the base portion 10 two additional rectangular openings 17 are formed, whose length h is selected to be equal to the length l r of the piezoresistors 9, which are located in the area 19 between the said openings 17. In this embodiment of the invention a particularly very high mechanical stress concentration in the areas of location of piezoresistors 9 is obtained, resulting in a particularly very high sensitivity of the microcantilever sensor 1.

[0069] Yet another preferred embodiment of microcantilever sensor with piezoresistive detection according to the present invention is shown in Fig. 5c. In this case, three additional rectangular openings are formed in the base portion 10: a centrally located one 17' and two symmetrically arranged openings 17 whereas the piezoresistors 9 are located between the openings 17 and 17'. [0068] In this embodiment of the invention a maximum concentration of the mechanical stress is obtained in the areas of location of piezoresistors 9, resulting in a maximal sensitivity of the microcantilever sensor 1, while stability of microcantilever against in-plane bending is assured. [0069] In this configuration of the piezoresistors it is possible to achieve the maximum degree of uniformity and reproducibility of their parameters. Furthermore, in the design of microcantilever sensors 1, the widths of the three openings 17 and 17' can be varied, which allows the implementation of various embodiments with optimization of the specific characteristics of the sensors.

[0070] Those skilled in the art will appreciate that the described openings 17 and 17' in the base portion 10 are also applicable to achieve the desired values of the resonance frequencies and spring constants of the microcantilever sensors 1 with laser detection, as described below, and it will become apparent from the following examples.

Example 4

[0071] Fig. 6a shows a further preferred embodiment of microcantilever sensor 1 with piezoresistive detection, and with an opening 12 through the microcantilever 3 for observation of the sample 5, in which opening, the base 4' of the probe 4 ends, similarly to Example 1. In this case, the opening with pentagonal shape is replaced with a triangular one. In addition, the sensor of Fig. 6a is equipped with electrically conducting tracks 20 providing galvanic contact of the probe 4 with external measuring instruments. In this embodiment, the openings 17 secure a reduced capacitive coupling between the conducting tracks 20 and piezoresistors 9.

[0072] Similarly, Fig. 6b shows another embodiment of microcantilever sensor 1 with piezoresistive detection, and a triangular opening 12 for observation of the sample 5 through the microcantilever 3, in which opening the base 4' of the probe 4 ends. The base portion 10 of the microcantilever comprises also conductive tracks 21, which in this case are of material with such a coefficient of temperature expansion that provide bimorph thermo actuation of the micro cantilever 3, known to those skilled in the art. In this case, the openings 17 secure thermal insulation between the thermo actuator with an elevated temperature and the areas of piezoresistors 9 and allow additional convective cooling of the entire device.

[0073] These embodiments are particularly suitable when it is required to perform correlative analysis using signals complementary to the ones resulting from atomic forces interaction between the probe and the sample, such as: electrical conductivity, electrostatic forces, etc., known to those skilled in the art or, at local heating, as well as when integration of an actuator element on the microcantilever in extremely small volume is required.

Example 5

[0074] Another embodiment of microcantilever sensor with laser detection according to the present invention is shown in Fig. 7a. The figure is a top view of a horizontally disposed microcantilever sensor 1, similar to that described in Example 1, with the difference that in the opening 12 in which the base 4' of the probe ends, the pentagon is replaced with a triangle. The opening 12 is with a height b, while the width of the surrounding flexure elements 13 is g, preferably, this width is smaller than the diameter 2r of the base 4' of the probe 4.

[0075] To achieve high sensitivity to deflection of the microcantilever 3 in this embodiment, the laser beam is reflected from the solid area of the probe portion 11 with a length a and width w. It is located adjacent to the opening 12 from the side of the body 2 and is marked in the figure by crosshatching. Preferably, the reflection takes place from the closest to the opening area of a length ½a, which boarder is marked in the figure by a dashed line. Said length a is selected so as to satisfy the condition:

α = 3η. λ (9)

where λ is the longitudinal dimension of the intersection of the laser beam, and η is a dimensionless apparatus factor in a range from 1.0 to 3.0, preferably from 1.3 to 2.5. The area of a rectangular shape with length ½a and width w, is farthest from the body 2 and deflects with the largest magnitude from the neutral position at deflection of the microcantilever 3. This layout ensures high sensitivity of the sensor.

[0076] Fig. 7b shows a longitudinal section of microcantilever sensor 1 along the line AA' of Fig. 7a. Similarly to Example 1, the thickness of the body 2 is within the range between 50 μιη and 500 μιη as it is conventional in the field, in this example the thickness is 270μιη. The thickness of the base portion d of the microcantilever 3 is in the range from 0.5μιηΐο ΙΟμιη, in this example it is 4 μιη, and is different from the thickness of the probe portion t, which in the example is 1 μιη. In this embodiment of the invention the ratio (2) is valid. The spring constant of the base portion 10 is determined by the mentioned thickness d, as well as by its length / and width w. While the spring constant of the probe portion 11 is determined by the thickness t, the width g and the height b of the flexure elements 13. Thus, at fixed values of the parameters L and w, which in the example are respectively 100 μιη and 40μιη, and various thicknesses d and t of the two portions, by means of selection of the parameters b and g, the chosen ratio of the spring constants and the resonance frequencies of the both portions of the microcantilever 3 : base portion 10 and probe portion 11, can be achieved.

[0077] Thus, unintentionally and unexpectedly, besides visibility of the area of interaction of the probe 4 with the sample 5, by selecting the dimensions b and g, the desired low value of the spring constant k of the probe portion 11 at a high value of the resonance frequency f c for the base portion 10 can be achieved. At suitable choice of the parameters d, t, b, and g in a manner known to those skilled in the art, microcantilever sensor 1 with probe 4 according to the present invention can be built, which demonstrates simultaneously: high resonance frequency f c , high sensitivity and resolution, as well as a "soft" probe portion 11, which does not damage the sample during the measurement, even if microcantilevers are with conventional geometric dimensions L, w and d.

[0078] The sensor according to the example of the present invention can be used in the manner shown in Figure 7c. The microcantilever sensor 1 is tilted at an apparatus dependant angle a relative to the surface of the sample 5 and the scanning is done by moving the sample 5 along the axes X and Y. By controlled movement of the sensor 1 along the axis Z, a constant signal from the laser beam reflected by the rectangular area with dimensions a x w, in particular from the rectangular area with dimensions ½a x w, can be obtained.

[0079] Due to the presence of the opening 12 adjacent to the probe 4, part of the periphery of probe base 4' is seen. Therefore, when using a probe 4 having a symmetrical shape in addition to the visibility of the area of interaction of the probe 4 with the sample 5, through the calculation described in Example 1, exact position of the tip 14 can be determined. Additionally, if condition (4) is fulfilled during operation, the tip 14 is visible through the opening 12.




 
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