JP2551954B2 - Scanning tunneling microscope - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a scanning tunneling microscope for detecting a tunnel current by scanning a sample surface by bringing a probe close to the sample surface by controlling an applied voltage to a piezoelectric element.

[Conventional technology]

In general, when the probe is brought close to the sample until the atom at the tip of the probe and the electron cloud of the atoms of the sample overlap by about 1 nm, and a voltage is applied between the probe and the sample in this state, a current flows. This current is called a tunnel current, which is about 1 to 10 nA when the voltage is several mV to several V. Since the magnitude of the tunnel current changes depending on the distance between the sample and the probe, the distance between the sample and the probe can be measured ultra-precision by measuring the magnitude of the tunnel current. Therefore, if the probe position is known, the surface shape of the sample can be obtained. Further, if the probe position is controlled so that the tunnel current is constant, the surface shape of the sample can be similarly observed from the probe position locus.

In a scanning tunneling microscope (STM) that observes the surface shape of a sample by such a tunnel current, it is necessary to bring a probe made of W (tungsten) or the like close to the sample to detect the tunnel current in order to obtain an uneven image from the sample. There is.
In this case, first, the probe is brought closer to the sample to about 0.1 μm by the coarse movement, and the probe is brought closer to the sample from 0.1 μm to 1 nm by the fine movement. Therefore, control on the nm order is required.

The scanning tunneling microscope is used by being incorporated in, for example, an electron microscope, and an example thereof is shown in FIG. In FIG. 2, a holder 12 for an electron microscope is one in which the STM unit 1 is incorporated and is arranged between the pole pieces 11, and the probe 3 and the sample 2 are simultaneously attached near the optical axis 14, and its cross section is shown. The figure is shown in FIG. 11B, and the figure in which the holder 12 is viewed from above is shown in FIG. In such a configuration, as shown in FIG. 3B, the electron beam 16 is spotted on the surface of the sample 2 at a certain angle by an electron microscope.
The sample surface can be observed by irradiating the sample and projecting a reflected image on the screen 15, and by scanning the probe 3 with a scanning tunneling microscope, the surface details can be observed with high precision. The advantages of incorporating a scanning tunneling microscope into an electron microscope are:
The above-mentioned probe approach can be performed under observation with an electron microscope, and at the same time, it can be used in combination with the backscattered electron diffraction (REM) method, and the STM image at the same location can be observed under the image observation by the REM method. It will be possible.

[Problems to be Solved by the Invention] By the way, when a scanning tunneling microscope is incorporated in an electron microscope, a voltage is applied to the piezoelectric element to move the probe in XY scanning or move in the Z-axis direction. Therefore, an electric field is generated by the applied voltage, and the electric field constantly changes as the probe moves. Since such a piezoelectric element is irradiated with an electron beam, there is a problem that the electric field from the piezoelectric element deflects the electron beam. In addition, due to structural problems that this system is made of an insulator, charge-up or the like occurs, which causes an error in measurement.

The present invention is to solve the above problems, and a scanning tunnel microscope in which the electric field of the piezoelectric element does not affect the electron beam even when the scanning tunnel microscope is incorporated in the electron microscope. It is intended to provide.

[Means for solving problems]

Therefore, the present invention is a scanning tunneling microscope in which a probe is driven to scan a sample surface by controlling an applied voltage to the piezoelectric element to detect a tunnel current. The scanning unit portion of the probe supported by is shielded by a conductor, and the probe is arranged in the electron beam irradiation region of the electron microscope or in the vicinity of the irradiation region.

[Action]

In the scanning tunneling microscope of the present invention, the scanning unit portion to which the piezoelectric element is attached is covered and shielded by a conductor, so that even if the unit is incorporated in the electron microscope, the influence that the electron beam is deflected by the influence of the electric field due to the piezoelectric element is reduced. Can be eliminated. Further, it is possible to avoid the problem that the scanning tunneling microscope unit is charged up even when the electron beam is applied to the sample by the electron microscope.

〔Example〕

 Hereinafter, embodiments will be described with reference to the drawings.

FIG. 1 is a diagram showing one embodiment of a scanning unit section of a scanning tunnel microscope according to the present invention, FIG. 2 is a diagram showing a schematic configuration of an electron microscope incorporating a scanning tunnel microscope, and FIG.
FIG. 4 is a diagram showing a schematic configuration of a scanning tunneling microscope, and FIG. 4 is a diagram showing examples of various shapes of a three-dimensional actuator. In the figure, 1 is an STM unit, 2 is a sample, 3 is a probe, 4
Is a head, 6, 8, 9 and 31 are piezoelectric elements, 5 and 7 are insulating plates, 11 is a pole piece, 12 is a holder, 13 is a drive mechanism,
14 is an optical axis, 15 is a screen, 16 is an electron beam, 21 and 32 are electrodes, 22 is an XY scanning circuit, 23 is a servo circuit, 24 is a tunnel current amplifier, 25 is a bias power supply, 26 is a memory, 27 is an MP
U and 28 denote display devices.

In FIG. 1, in the STM unit 1, a probe 3 is mounted on a head 4, and the head 4 is supported by insulating plates 5, 7 and piezoelectric elements 6, 8, 9. Piezoelectric elements 6, 8, 9
Is a three-dimensional actuator composed of an X axis, a Y axis, and a Z axis. The piezoelectric element 6 drives the Z axis, the piezoelectric element 8 drives the X axis, and the piezoelectric element 9 drives the Y axis. ,
By driving 8 and 9, it is possible to scan the sample 2 with the probe 3 and obtain an uneven image of the surface of the sample 2 based on the tunnel current and the voltage applied to the piezoelectric elements 6, 8 and 9.

The STM unit 1 and the sample are incorporated in the holder 12 shown in FIG. 2, and the scanning tunneling microscope of the present invention is
By covering and shielding the STM unit 1 with the conductor 10, the electric field from each piezoelectric element is prevented from leaking onto the optical axis.

FIG. 3 shows an overall system schematic configuration in which the piezoelectric elements 6, 8 and 9 are driven while detecting the tunnel current to obtain an uneven image of the surface of the sample 2.

In FIG. 3, a driving voltage is applied to the electrodes 21 arranged on both sides of each of the piezoelectric elements 6, 8 and 9 constituting the three-dimensional actuator, and the voltage between the electrodes is MPU27,
Controlled through memory 26. That is, in the control of the three-dimensional actuator, the XY scanning circuit 22 sweeps the applied voltage to the piezoelectric elements 8 and 9 in the X-axis and Y-axis directions to move the probe 3 in the X-axis and Y-axis directions for scanning. While performing this scanning, the voltage to the Z-axis direction piezoelectric element 6 is controlled through the servo circuit 23 so that the tunnel current becomes constant. Therefore, the surface shape of the sample 2 can be observed by reading this control voltage value into the MPU 27 and displaying it on the display device 28.

As a three-dimensional actuator, in addition to the linear type,
As shown in FIG. 4 (a), a cantilever type in which a piezoelectric element 31 is bonded in a tripot type is most often used, and in order to make the rigidity higher than this, shown in FIG. 4 (b). As described above, the piezoelectric element 31 is also used in which the X axis and the Y axis are supported at both ends and the Z axis is supported at one end. As shown in FIG. 6C, the piezoelectric element is assumed to have higher rigidity.
A cylindrical type 31 is used, and four electrodes 32 are attached to the surface of the cylindrical type 31 to apply a voltage between the back side electrode and the cubic type shown in FIG. ing.

In the electron microscope, the effect of the electric field of these piezoelectric elements on the electron beam cannot be ignored, and for example, from the optical axis
When about 100 V is applied to a place 5 mm away, the electron beam on the optical axis shifts about 2 to 5 nm with an applied voltage of 120 kV. If the distance is constant, the Coulomb force acting on the electron beam is proportional to the voltage applied to the piezoelectric element. Therefore, if the applied voltage is increased to increase the driving distance, the deflection amount naturally increases. This also affects the axis of the observed image when the probe is driven by the electron microscope, so that the image becomes distorted when observed at a high magnification. Further, since the unit portion including the piezo element is an insulator, there is a problem that the electron beam is charged. In the present invention, these problems can be prevented by shielding the scanning unit portion of the probe in which the piezoelectric element is arranged as described above with a conductor. Of course, the shield needs to be grounded.

〔The invention's effect〕

As is clear from the above description, according to the present invention, ST
Since the M unit is surrounded by the conductor and shielded, even when the STM unit is incorporated in the electron microscope, it is possible to eliminate the influence of deflection of the electron beam by the electric field of the piezoelectric element. Further, it is possible to avoid charging up the insulating portion of the STM unit during image observation by electron beam irradiation.

[Brief description of drawings]

FIG. 1 is a diagram showing one embodiment of a scanning tunnel microscope according to the present invention, FIG. 2 is a diagram showing a schematic configuration of an electron microscope incorporating a scanning tunnel microscope, and FIG. 3 is a schematic configuration of a scanning tunnel microscope. And FIG. 4 are diagrams showing examples of various shapes of the three-dimensional actuator. 1 ... STM unit, 2 ... sample, 3 ... probe, 4 ...
Head, 6, 8, 9 ... Piezoelectric element, 5 and 7 ... Insulating plate,
11 …… Pole piece, 12 …… Holder, 13 …… Drive mechanism, 14 …… Optical axis, 15 …… Screen, 16 …… Electron beam.

Claims (1)

(57) [Claims] 1. A scanning tunneling microscope in which a probe is driven by controlling an applied voltage to the piezoelectric element to scan a sample surface to detect a tunnel current, and a head equipped with the probe is provided with an insulating plate and a piezoelectric element. A scanning tunneling microscope, wherein the scanning unit portion of the supported probe is shielded by a conductor, and the probe is arranged in the electron beam irradiation region of the electron microscope or in the vicinity of the irradiation region.