(51) Int.Cl.6:

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(19)      Federal Republic of Germany

                        [emblem]

            German Patent Office and Trademark Office

 

(12)                                                         Offenlegungsschrift

                                                      [= published patent application]

 

(10)                                                         DE 198 22 634 A1

 

(21)         Application number:            198 22 634.9

(22)         Filing date:                            May 20, 1998

(43)         Disclosure date:                   November 25, 1999

 

(71)         Applicant: Fuchs, Harald, Prof.Dr., 48301 Nottuln, DE

 

(72)         Inventors: Fuchs, Harald, Prof.Dr., 48301 Nottuln, DE;

                Simon, Ulrich, 45141 Essen, DE

 

(56)         Documents taken into consideration for evaluating the patentability:

                WO 98 05 920 A1

                HONGJIE DAI, et al.: Nanotubes as nanoprobes in scanning probe microscopy. in: Nature, Vol. 384, 14 November 1996, pages 147-150.

                ARIE, Takayuki, et al.: Growth of tungsten carbide nano-needle and its application as a scanning tunnelling microscope tip. in: J. Phys. D: Appl. Phys. 31, 1998, pages L49 - L51;

                YOO, M.J., et al.: Scanning Single Electron Transistor Microscopy: Imaging Individual Charges. in: Science, Vol. 276, 25 April 1997, pp. 579-582.

 

 

 

The following information is derived from the documents submitted by the applicant.

 

(54) Nano-Atomic Force Microscope with a Single Electron Transistor

 

DE 198 22 634 A1

 

                                                                      Description

 

                                                                       Prior Art

 

The practical usability of atomic force microscopes depends on two essential parameters:

 

1. The stiffness (sensitivity) of the measurement probe (compliance) has to be chosen adequately low (high) so that, when the measurement probe (tip) makes contact with the surface to be examined, said surface is not destroyed. In the case of typical bonding energies in solids of a few eV, this means an effective interatomic spring constant ranging from 1 to 10 N/m (based on a simple spring model in two dimensions and surface oscillation frequencies of 1012s-1). Therefore, the stiffness of the measurement probe should be < 1 N/m.

 

2. The natural frequency of the sensor (cantilever) should be chosen high in order to (a) guarantee fast imaging of the surface and (b) achieve a large signal-to-noise ratio for vibrations from the environment.

 

Owing to the non-linearly scaling ratio:

 

            w = √ (E/z) (d/L2)                                (natural frequency)

 

            k = (1/4) E  (d4/L3)                               (spring constant, square cross-section).

 

both conditions can be significantly improved by decreasing the size of the structures (thickness d, length l). Modern sensors for atomic force microscopy are made by means of micro-fabrication processes in order to satisfy the two aforementioned requirements simultaneously. The fabrication materials are usually Si, SiO2 and Si3N4. Typical dimensions range from 100 to 200 micrometers (length), from 10 to 20 micrometers in width and 1 to 3 micrometers in thickness. The mechanical natural frequencies of these micromechanical cantilevers ranges from 1 K Hz to 1 MHz.

 

            A significant increase in the natural frequencies must be expected if the linear dimensions of the cantilevers are further reduced. A simple estimate shows that cantilevers having a linear dimension in the nanometer range should reach natural frequencies in the range of a few 100 HMz. The result was that extremely fast and simultaneously soft cantilevers could be manufactured. Owing to the high natural frequencies such cantilevers make it possible to measure the surface faster and with higher sensitivity than is possible with the currently available micromechanical cantilevers.

 

                                                             Approach to a Solution

 

                                                               1. Nano-Cantilevers

 

            A molecular dynamic simulation, designed by D. H. Robertson, Purdue University, and T. White, NRL, showed that multi-wall carbon nano-tubes having a total length of 300 mm and 12,000 atoms exhibited natural frequencies ranging from 1.1 to 1010 HZ (10 GHz). The outside diameter was 1.03 nm (citation: see, for example, T.W.Ebbesen, 'Carbon Nanotubes', Physics Today, p. 26, June 1996). The use of such systems as AFM sensors (nano-AFM) would make it possible to construct sensitive sensors for interatomic forces that are real nano-sensors. The small tube diameter on the order of nanometers makes it possible to use the tubes directly as the measurement tips. That is, there is no need to modify the nano-cantilever with a measurement tip, as is required in the case of micromechanical cantilevers. A slight tilt of the cantilever in relation to the specimen surface suffices to bring the pointed end of the tip in contact with the surface. The amplitudes of the cantilever are on the order of a few nanometers. One end of the nano-cantilever can be mounted on a solid substrate by spontaneous absorption (self assembly) or by targeted chemical interaction of a suitably functionalized cantilever. This substrate can have macroscopic dimensions (micrometers) and can be manufactured by conventional methods, for example, from Si or similar materials.

 

                                                           2. Nano-Cantilever Drive

 

            Conventional AFMs are either operated in static mode - i.e. in contact with the surface - or in dynamic mode by means of an external drive, e.g. a piezo-actuator and/or are excited in their natural resonance and/or higher harmonics by means of the specimen. As Johansmann (A. Roters et al., J. Phys. Condens Matter, Vol. 8, 7561 - 7577, 1996) showed, instead of the external drive, thermal excitation of soft cantilevers can also be used directly (fluctuations-dissipation-theorem). Microscopy exploits the principle that the resonance conditions of the cantilever change as it approaches the surface. Therefore, variations of the amplitude, the phase and optionally the frequency are used. Whereas in micromechanical cantilevers the thermally excited amplitudes are relatively small (a feature that limits the application of this method), further miniaturization of the sensors does result in relatively large amplitudes, generated directly by thermal excitation. Therefore, if a nano-cantilever, like the above described nano-tubes, is used, there is absolutely no need for an external drive for the dynamic mode. The high mechanical natural frequency (typically from 1 to 10 GHz), which would be generated by external means only with difficulty, is produced directly as the molecular deflection oscillation. It is a function of the size of the nano-tube and can be varied by chemical modification, such as embedding metal or metal oxide nano-particles. The results are totally new considerations for measuring interatomic and/or intermolecular forces.

 

                                                                      3. Readout

 

            In order to read out the sensor signal, conventional atomic force microscopes with micromechanical force sensors use optical, capacitive or tunnel transitions. The most wide-spread are optical light spots. However, these methods cannot be used owing to the small dimensions of the nano-tubes, since an adequate signal-to-noise ratio cannot be achieved with these methods. One exception is electron tunnelling. This approach shall be pursued in the method proposed here.

 

                                                          a) (Quasi) Static Operation

 

            When semiconducting and/or metallic cathode ray tubes are used, it is possible to implement a quasi-static operation with the use of conductive specimens. When the sensor approaches the conductive surface, the result is a tunnel contact (< 0.4 nm: point contact) at intervals of < 1 nm. A measurable tunnel current can flow over this tunnel contact. The intensity of this current can be regarded (assuming a constant charge density of the surface) as the distance gauge and, thus, as the topography signal.

 

                                                             b) Dynamic Operation

 

            The nano-AFM is especially suitable for a dynamic operating mode, since it does not require any external mechanical source of excitation for the dynamic operating mode (direct excitation by means of thermal fluctuations). However, for the so-called non-contact operating mode, the tunnel contact over a conductive surface can no longer be used. One solution to this problem is to use fast amplifiers, which detect directly the local deflection of the cantilever. The conventional current amplifiers that are used for electron tunnelling microscopy are ruled out, because high gains (1010) with FET / bipolar technology are associated with large time constants (RC constant). Suitable are amplification methods that allow a direct resolution and measurement of single electrons when an oscillating measurement probe approaches the readout sensor.

 

            As a solution to the problem, we propose a readout sensor on the basis of single electron transistors (English: single electron transistor, SET). Owing to the small characteristic intrinsic capacities (typically 1018 F), these quantum components permit extremely fast switching actions and the direct detection of single electrons. According to the invention, the steering element shall be the nano-cantilever, which approaches (thermally excited) in an oscillating manner the control gate of a suitable SET. In a suitable design the upper or lower reversal point of the nano-cantilever, which is brought into proximity with the sensor, shall be switched by one or more electrons in the SET. The corresponding time constants are on an order of the oscillations of the cantilever. Charged (polar) chemical groups that on approaching the gate of the SET trigger the switching action and that are located on the end of the nano-cantilever are suitable for enhancing the sensitivity of the sensor. Suitable chemical groups can be attached on the end of the cantilever in the aforementioned way by means of chemical functionalization of the cathode ray nano-tubes. Another possibility for detection is to use fully conjugated linear organic compounds having charged side groups, whose charge state and, thus, the electric conductivity of the entire chain is affected by the presence of the cantilever.

 

            The proposed method is presented as a schematic drawing in Figure 1. In this drawing the reference numeral 1 denotes the nano-tube structure, one end of which is clamped in a suitable suspension 7 and the left end of which can oscillate freely in the direction of the arrow. Therefore, the bottom edge of the nano-tube reaches the surface structure 4 that is to be measured and that can be moved on a suitably scannable device, which can be scanned in the x and y direction (see crossed arrows). The movement can be moved by means of a nano-tube 8 in the nanometer range and below up to a few 100 micrometers. The adjustment with the nano-tube 8 can be carried out in both the horizontal and vertical direction. The sensor consists of the SET detector 6, which can be built up with suitable semiconductor structures, e.g. nano-clusters. The SET detector is coupled to the macroscopic environment by means of suitable electrode structures 5. When the nano-tube moves oscillatingly, the single electron detector in the vicinity of the upper reversal point of the tube is steered by varying the potential, for example by means of a charged group 2. Thus, an electron and/or a few electrons inside the electron structures 5 is/are moved each time, with the result that the oscillation is detected. The detector (SET sensor) is brought into proximity with the oscillating nano-tube by means of suitable measures, in particular piezo-ceramics.

 

                                                                    Patent Claims

 

1. Atomic force microscope comprising a cantilever and a readout sensor, which can detect the deflections of the cantilever, characterized by a single wall or multi-wall nano-tube as the cantilever.

 

2. Atomic force microscope, as claimed in claim 1, characterized by a single electron transistor (SET) as the readout sensor.

 

3. Atomic force microscope, as claimed in claim 1 or 2, characterized in that the free end area of the nano-tube on the side facing the readout sensor exhibits charged (polar) chemical groups, whose approach is detectable by the readout sensor, and that preferably when a single electron transistor is used as the readout sensor, the approach of the chemical group triggers a detectable switching action.

 

4. Nano-AFM, characterized in that, as claimed in claims 1 and 2, nano-tubes made of transition metal oxides, like VOx or BN, are used.

 

5. SET, as claimed in claim 2, characterized by components, comprising ligand-stabilized metal or semiconductor clusters, as the SET element.

 

6. SET, as claimed in claim 2, characterized by SiO-SET components as the SET element.

 

7. SET, as claimed in claim 2, characterized by claso-carboranes or their derivatives as the SET element components.

 

8. SET, as claimed in claim 2, characterized in that fullerenes are used.

 

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