JP4461257B2 - Electron emission distribution measuring device - Google Patents

Description

  The present invention is applied to a scanning probe microscope such as a scanning tunneling microscope (“STM”) to obtain a surface shape image of a sample usually obtained by a scanning tunneling microscope (herein, “STM image”). ") As well as a high-resolution electron emission distribution image (" FE image "in this specification) in the sample. ) Can be measured at the same time.

  Conventionally, a scanning probe microscope such as a scanning tunneling microscope is known as a device for measuring the state of a sample surface by detecting the magnitude of a current flowing between a probe and a sample and controlling it to be constant (patent) Reference 1).

  In STM, a constant voltage is applied to the sample and the position in the sample vertical direction is feedback controlled so that the current flowing between the probe and the sample is constant using a control device (constant current tunnel mode). By correlating the trajectory in the vertical direction of the sample when scanned in the horizontal direction with the horizontal position information of the sample (the undulation shape of the sample surface), the surface shape image of the sample is rearranged in a two-dimensional plane in the storage device. To get.

  By the way, conventionally, attempts have been made to measure a field electron emission distribution using a scanning probe microscope (Scanning Tunneling Field Emission Microscope: STFEM method) (see Non-Patent Document 1).

  In addition, although the object is different from the present invention, as a related technique, there is known a CITS method (Current Imaging Tunneling Spectroscopy) that makes it possible to measure an electronic state on a solid surface by energy decomposition while maintaining high spatial resolution. (See Non-Patent Document 2).

JP 5-306926 D Frolov, A V Karabutov, V I Konov, S M Pimenov and A M Prokhorov, “Scanning tunnelling microscopy: application to field electron emission studies” Journal of Physics D: Applied Physics, 32, p. 815-819, 1999 R. J. Hamers, R. M. Tromp, and J. E. Demuth, “Surface Electronic Structure of Si (111)-(7 x 7) Resolved in Real Space” Physical Review Letters, 56, p 1972-1975, 1986

  In the STFEM method described in Non-Patent Document 1, in the normal constant current tunnel mode STM measurement, when the applied voltage between the probe and the sample is temporarily increased until field electron emission occurs, the feedback functions, The probe is moved away from the surface so as to keep a constant current against the increase, and the moving distance of the probe in the sample vertical direction at this time is measured.

  However, since the STFEM method does not directly measure the current, it is not only difficult to quantitatively evaluate, but also because the probe moves far away, the distance between the probe and the sample, which is important in field electron emission, can be kept constant. There was a problem that not only high resolution could not be obtained.

  In the CITS method described in Non-Patent Document 2, the voltage is applied to the sample and the STM measurement is performed in the constant current tunnel mode, and the applied voltage is changed in a time division manner. A spatial distribution of density of states is obtained.

  However, in the CITS method, in principle, the field electron emission distribution can be measured by increasing the applied voltage to the field emission region. However, since the change amount of the measured current is remarkably large, the measurement system cannot follow and the intended measurement cannot be realized.

  The present invention has been made for the purpose of solving the above-mentioned conventional problems, and it is an object to realize an electron emission distribution measuring apparatus capable of measuring a high resolution electron emission distribution in a scanning tunneling microscope or the like. It is said.

  In order to solve the above problems, the present invention provides a probe, a piezo element that moves the probe relative to a sample, a bias voltage application circuit that applies a bias voltage to the sample, the probe, and the sample. In a scanning tunneling microscope comprising a preamplifier that converts a current flowing between the preamplifier and an STM control circuit sequentially provided on the output side of the preamplifier, a sample hold circuit is provided between the preamplifier and the STM control circuit. An electron emission distribution measuring device is provided.

  In order to solve the above problems, the present invention provides a probe, a piezo element that moves the probe relative to a sample, a bias voltage application circuit that applies a bias voltage to the sample, the probe, and the sample. In a scanning tunneling microscope comprising a preamplifier that converts a current flowing between the preamplifier and an STM control circuit sequentially provided on the output side of the preamplifier, a sample hold circuit is provided between the preamplifier and the STM control circuit. The bias voltage application circuit includes an STM bias power source, an FE bias power source having a higher voltage than the STM bias power source, an STM bias power source, and an FE bias power source. And a changeover switch for alternately switching, wherein the sample hold circuit is configured to apply a voltage of an FE bias power source to the sample. During the period, the output from the preamplifier to the STM control circuit is held at the output from the preamplifier immediately before the voltage of the FE bias power supply is applied, and is used for measurement of an electron emission distribution image. An electron emission distribution measuring device is provided.

  In order to solve the above problems, the present invention provides a probe, a piezo element that moves the probe relative to a sample, a bias voltage application circuit that applies a bias voltage to the sample, the probe, and the sample. In a scanning tunneling microscope comprising a preamplifier that converts a current flowing between the preamplifier and an STM control circuit sequentially provided on the output side of the preamplifier, a sample hold circuit is provided between the preamplifier and the STM control circuit. The bias voltage application circuit includes an STM bias power source, an FE bias power source having a higher voltage than the STM bias power source, an STM bias power source, and an FE bias power source. A selector switch for switching between the preamplifier and the STM bias power source during the period when the voltage of the STM bias power source is applied to the sample. Is input to the STM control circuit, and during the period in which the voltage of the FE bias power supply is applied to the sample, the output from the preamplifier to the STM control circuit is applied to the STM control circuit. There is provided an electron emission distribution measuring apparatus characterized by holding and outputting the input from the preamplifier immediately before the output, and for measuring an electron emission distribution image.

  In order to solve the above problems, the present invention provides a probe, a piezo element that moves the probe relative to a sample, a bias voltage application circuit that applies a bias voltage to the sample, the probe, and the sample. In a scanning tunneling microscope comprising a preamplifier that converts a current flowing between the preamplifier and an STM control circuit sequentially provided on the output side of the preamplifier, a sample hold circuit is provided between the preamplifier and the STM control circuit. The bias voltage application circuit includes an STM bias power source, an FE bias power source having a higher voltage than the STM bias power source, an STM bias power source, and an FE bias power source. And a changeover switch for alternately switching, wherein the sample and hold circuit is connected to the timing signal generator in parallel with each other. A sample hold circuit and a second sample hold circuit, each performing a sampling operation to output an input from the preamplifier by a sample signal sent from the timing signal generator, and from the preamplifier immediately before the hold signal input by a hold signal The power supply changeover switch alternately supplies power to be applied to the sample to the STM bias power supply or FE power supply in response to a switch changeover signal sent from the timing signal generator. An electron emission distribution measuring apparatus characterized by switching is provided.

  During the period in which the voltage of the FE power supply is applied to the sample, the holding operation is performed in the first sample hold circuit, but when the sample signal is applied to the second sample hold circuit during the period, It is preferable that the output from the preamplifier is used for measurement of the electron emission distribution image.

  The preamplifier is preferably composed of two-stage amplifiers having different multiplication factors.

  The electron emission distribution measuring apparatus according to the present invention has the following effects. In other words, high-resolution electron emission distribution and its absolute value can be measured simply by adding a simple circuit to the STM to detect and control the current flowing between the probe and the sample such as a scanning tunneling microscope. Become. In addition, this circuit can realize the target function without adding any special modifications just by adding to the normal STM.

  The best mode for carrying out the electron emission distribution measuring apparatus according to the present invention will be described below with reference to the drawings.

(principle)
A potential diagram when the probe and the sample are brought close to each other is shown in FIGS. The work function is a physical property value specific to a material, and is an energy difference between a Fermi level (described as Ef) of a solid and a vacuum level (described with a broken line) which is a potential of an electron at infinity. That is, it is the minimum energy required for taking out electrons from the solid body into the vacuum. The work function value ΦHfC in FIGS. 1A and 1B is the work function of the HfC (hafnium carbide) thin film (described later) used for the sample, and Φw is the work function of the probe when a tungsten probe is used. is there. The values (3.9 eV, 4.5 eV) shown in FIGS. 1A and 1B are shown in literature values by HBMichaelson (HBMichaelson, J. Appl. Phys. 22, 4729 (1977)). Value).

  In STM measurement, as shown in FIG. 1A, when the potential difference between the probe and the sample is lower than the work function, the following Equation 1 is established, and electrons tunnel through the barrier. When this tunneling current (tunnel current) is J, Formula 1 is as follows.

In Equation 1, e is an electron charge, m is an electron charge, Φ ₀ is a work function, and s is a distance between probe samples.

  From Equation 1, it can be seen that the tunnel current greatly depends on the distance between the probe and the sample. When observing the material surface in the constant current tunnel mode, the distance s between the probe samples is controlled so as to make the tunnel current constant based on the above formula, and the movement distance is measured as an STM image.

  On the other hand, as shown in FIG. 1B, when the potential difference is larger than the work function of the probe, the barrier is thinned by the magnitude of the electric field, and electrons are emitted into the vacuum as real particles. This state is defined as field electron emission, and Equation 2 is established. Note that F is an electric field.

  Further, when the applied voltage is high, the second term of Equation 2 becomes a negligible value and Equation 3 is established. This equation is called the Fowler-Nordheim equation.

  In Equation 3, the electric field F is included in the exponential function argument, and it can be seen that the magnitude of the electric field is large and affects the emission current. The platinum belonging to the highest value of the work function of solid metal is about 5.7 eV. Accordingly, it is possible to measure the electron emission current by applying a voltage of about 6 V or more, measure the electron emission distribution, and obtain an electron emission distribution image.

(Constitution)
The electron emission distribution measuring apparatus according to the present invention basically has an STM that obtains an STM image as a main body, and holds the position of the probe (hold) to measure the electron emission distribution state. It is characterized by a configuration to which means for obtaining an emission image (hereinafter referred to as “electron emission distribution measurement / holding means”) is added, thereby obtaining an electron emission distribution image and an STM image simultaneously. It is possible.

  The STM, which is the main body, applies a voltage A between the probe and the sample, moves the probe so as to keep the flowing tunnel current constant, and maps it on the two-dimensional surface by the moving distance. This is an apparatus for obtaining an STM image. In this specification, a mode in which the tunnel current flowing between the probe and the sample is kept constant and an STM image of the sample surface is obtained is referred to as a “constant current tunnel mode” (also referred to as “STM mode”).

  The outline of the electron emission distribution measurement / hold means is the bias voltage application circuit that switches between the bias voltage for normal constant current tunnel mode and the bias voltage capable of electron emission, and the bias voltage capable of electron emission is applied to the sample. The two types of sample and hold circuits that hold the probe so as not to move in accordance with the current are provided, and the timing generation circuit uses the constant current tunnel mode and the electron emission distribution image acquisition mode. This is a means capable of alternately switching the emission distribution measurement mode (also referred to as “FE mode”).

  When switching to the constant current tunnel mode by means of electron emission distribution measurement / holding means, the voltage A is applied between the probe and the sample, and after a set waiting time, the tunnel current is measured and the feedback function is used. A process for obtaining the height information is performed to obtain an STM image, and a process for holding the position of the probe in the height direction is performed.

  Then, the electron emission distribution measurement / hold means is used to switch to the electron emission distribution measurement mode while maintaining the height position of the probe, and a bias voltage (sample voltage) sufficient to cause electron emission in this electron emission distribution measurement mode. And an electron emission distribution image is obtained by a current emitted after a standby time set under the bias voltage. Then, it returns to the constant current tunnel mode again. This provides a measurement method that enables simultaneous acquisition of an electron emission distribution image and an STM image.

(overall structure)
FIG. 2 is a diagram for explaining an embodiment of the electron emission distribution measuring apparatus according to the present invention, and is a circuit block diagram of the electron emission distribution measuring apparatus 1. As shown in the circuit block diagram of FIG. 2, the electron emission distribution measuring apparatus 1 of this embodiment includes a detection unit 2 and an STM control circuit 3 as main components of a basic STM. Furthermore, as a characteristic configuration of the present invention, an electron emission distribution measurement / hold means 4 for switching between a constant current tunnel mode and an electron emission distribution measurement mode is provided.

  The detection unit 2 includes a probe 6 that can be moved vertically toward and away from a sample by a piezoelectric element 5 that is a piezoelectric body. An STM bias power supply 9 is connected to the sample 7 placed opposite to the probe 6 via a wiring 8, and an STM bias voltage (a normal STM mode bias voltage) is applied. The STM bias power source 9 is configured so that the STM bias voltage can be selectively set to a predetermined value in advance.

A preamplifier 10 is connected to the probe 6. The preamplifier 10 converts the current flowing between the sample 7 and the probe 6 into a voltage (unit: V / A) and amplifies it 10 ⁷ to ⁸ times. The output side of the preamplifier 10 is connected to a first sample hold circuit 11 and a second sample hold circuit 12 which will be described later. The first sample hold circuit 11 is connected to a log amplifier 13 which will be described later.

  Specifically, the preamplifier 10 has a two-stage amplifier incorporated therein. That is, the first preamplifier 14 connected to the probe 6 and the second preamplifier 15 and the third preamplifier 16 connected in parallel to the output side branch circuit 17 of the first preamplifier 14 respectively. ing.

The first preamplifier 14 performs current-voltage conversion of the tunnel current from the probe 6 by 10 ⁷ (V / A) times. The second preamplifier 15 is connected to the first sample-and-hold circuit 11 and further amplifies the output voltage from the first preamplifier 14 by 10 ¹ times, and amplifies the output voltage 10 ⁸ (V / A) times as a whole. Do. The output voltage from the second preamplifier 15 is input to the STM control circuit 3 via the first sample hold circuit 11 to enable highly accurate control of the distance between the probe 6 and the sample 7. Furthermore, the output of the feedback circuit 20 is output to the circuit system 18 for STM measurement, and an STM image can be obtained.

The third preamplifier 16 amplifies the output voltage from the first preamplifier 14 at the same magnification, amplifies it 10 ⁷ (V / A) as a whole, and measures a maximum emission current of about 1.5 μA. Is possible. The output from the third preamplifier 16 is output to the electron emission distribution measurement circuit system 19 and is measured as an electron emission current to obtain an electron emission distribution image. Note that the output to the circuit system for obtaining the electron emission distribution image can correspond to various measurement conditions by changing the amplification factor, and measurement under appropriate conditions can be realized.

  The STM control circuit 3 is provided with a log amplifier 13 on its input side, and a feedback circuit 20 is connected on its output side. Further, a piezo drive circuit 21 is connected to the output side of the feedback circuit 20, and its output side is connected to the piezo element 5. Further, a circuit system 18 for STM measurement (such as an STM image forming unit that forms an STM image by measuring the undulation on the surface of the sample 7 based on the output of the feedback circuit) is connected to the output side of the feedback circuit 20. Yes.

  The output of the feedback circuit 20 is an output for controlling the probe 6 with the piezo element 5 in order to make the tunnel current flowing between the sample 7 and the probe 6 constant. The output determines the movement of the probe 6. If this output is output to the circuit system 18 for STM measurement and used, an STM image can be measured.

  The circuit system 18 for STM measurement itself has the same configuration as that used in the well-known STM, and the circuit system 19 for electron emission distribution measurement has almost the same configuration as the circuit system 18 for STM measurement. These will be briefly described below.

  FIG. 3 is a diagram for explaining the circuit system 18 for STM measurement and the circuit system 19 for electron emission distribution measurement. In practice, a computer 29 is used. The horizontal coordinate axes of the surface of the sample 7 are defined as the X axis and the Y axis, and the surface vertical direction of the sample 7 is defined as the Z axis. In FIG. 3, an XY position information generating unit 30 that generates position information (position signal) for XY control, an STM image forming unit 31, and an electron emission distribution image forming unit 32 are provided in a computer 29 (actually, The computer CPU functions and is configured).

  The STM image forming unit 31 is connected to the feedback circuit 20 via the AD converter 33, and the electron emission distribution image forming unit 32 is connected to the second sample hold circuit 12 via the AD converter 34. . The STM image forming unit 31 and the electron emission distribution image forming unit 32 are each connected to the XY position information generating unit 30.

  The XY position information generation unit 30 is connected to an XY direction piezo controller 36 via a DA converter 35. The XY direction piezo controller 36 is an XY driving piezo element that drives the probe 6 in the XY direction (a piezo element provided separately from the piezo element 5 for moving the probe 6 vertically toward and away from the sample). ).

  In FIG. 3 showing the above configuration, the circuit system 18 for STM measurement includes an AD converter 33, an STM image forming unit 31 and an XY position information generating unit 30 in the computer 29. The circuit system 19 for electron emission distribution measurement includes an AD converter 34, an electron emission distribution image forming unit 32 and an XY position information generation unit 30 in the computer 29.

  In the STM mode, the circuit system 18 for STM measurement converts the output of the feedback circuit 20 into a digital value using the AD converter 33. Then, in synchronization with the XY position information signal for driving the probe 6 output from the XY position information generating unit 30 on the XY coordinates, mapping is performed using the digital value as position information in the Z-axis direction. A STM image is created (measured) by creating a dimensional image.

  In the FE mode, the electron emission distribution measurement circuit system 19 converts the output of the second sample hold circuit 12 into a digital value using the AD converter 34. Then, in synchronization with the XY position information signal for driving the probe 6 output from the XY position information generating unit 30 on the XY coordinates, mapping is performed using the digital value as position information in the Z-axis direction. The electron emission distribution image is created (measured) by dimensional imaging.

(Electron emission distribution measurement / holding means)
The electron emission distribution measurement / hold means 4 includes a sample hold circuit 22 and a timing signal generator 23. Further, as will be described later, the electron emission distribution measuring / holding means 4 includes a changeover switch 24 and an FE bias power supply 25 that constitute a bias voltage application circuit. In the sample and hold circuit 22, the first sample and hold circuit 11 and the second sample and hold circuit 12 are provided in parallel in the branch circuit 17 on the output side of the preamplifier 10.

  The output of the first sample and hold circuit 11 is connected to the log amplifier 13. The output side of the second sample and hold circuit 12 is connected to an electron emission distribution measurement circuit system 19 (means such as an electron emission distribution image forming circuit for measuring an electron emission distribution image). In this electron emission distribution image, the current flowing between the sample 7 and the probe 6 is converted into a voltage by the preamplifier 10, sampled by the second sample hold circuit 12, and an electron emission distribution image is obtained by this sampling output.

  The first sample hold circuit 11 and the second sample hold circuit 12 are connected to the timing signal generator 23 via the first signal line 26 and the second signal line 27. The timing signal generator 23 is connected to an oscillator 28, and an STM mode timing signal A for controlling the first sample hold circuit 11 and the second sample hold circuit 12 using the oscillation signal from the oscillator 28, respectively. An FE mode timing signal B is generated. The STM mode timing signal A and the FE mode timing signal B are composed of a sampling signal and a hold signal, respectively.

  Then, the timing signal generator 23 transmits the STM mode timing signal A and the FE mode timing signal B to the first sample hold circuit 11 and the second sample hold circuit 12, respectively.

  FIG. 4 shows the time series of the bias voltage applied to the sample 7, the STM mode timing signal A input to the first sample hold circuit 11, and the FE mode timing signal B input to the second sample hold circuit 12. It is a timing chart which shows various changes in relation. The STM mode timing signal A and the FE mode timing signal B are alternately changed to a sampling signal (High signal in FIG. 4) and a hold signal (Low signal in FIG. 4) at timings as shown in FIG.

  The first sample hold circuit 11 and the second sample hold circuit 12 each output the same voltage as the input voltage (output voltage of the preamplifier 10) when a sampling signal is applied.

  When the hold signal is applied, the first sample hold circuit 11 and the second sample hold circuit 12 hold (hold) the input voltage at the time of the immediately preceding sampling signal, and the same voltage as this input voltage. Is output. Therefore, when the hold signal is applied, even if the input voltage (the output voltage of the preamplifier 10) changes, the output voltage is not affected.

  The electron emission distribution measuring / holding means 4 is provided with a changeover switch 24 of the bias voltage application circuit and an FE bias power supply 25 (power supply having a higher voltage than the STM bias power supply 9). The FE bias power supply 25 is connected to apply the FE bias voltage to the sample 7 via the changeover switch 24, and is configured so that the FE bias voltage can be selectively set to a predetermined value in advance. ing.

   The changeover switch 24 of the bias voltage application circuit is connected to the timing signal generator 23, and the switching operation is controlled by the switch changeover signal C from the timing signal generator 23. By this switching control, the wiring 8 is selectively switched between the STM bias power supply 9 and the FE bias power supply 25 in the bias voltage application circuit, and the STM bias voltage or the FE bias voltage is selectively applied to the sample 7. Can do.

  As shown in the timing chart of FIG. 4, the timing signal generator 23 switches the switch switching signal C and the STM mode timing signal A so that the bias voltage, the STM mode timing signal A, and the FE mode timing signal B are synchronized with each other. , And FE mode timing signal B are generated and output to each.

  In the timing chart of FIG. 4, as shown in FIG. 4A, the bias voltage is a high FE bias voltage H in the electron emission distribution measurement mode and a low STM bias voltage L in the STM mode at a constant cycle. They are applied alternately.

  The STM mode timing signal A to the first sample hold circuit 11 and the FE mode timing signal B to the second sample hold circuit 12 are periodically constant as shown in (b) and (c), respectively. Sampling signal and hold signal are added. The switching frequency of these bias voltages, STM mode timing signal A and FE mode timing signal B is about 2 KHz. This frequency can be varied depending on the measurement conditions.

  In the present invention, the timing signal generator generates and outputs the switching signal C, the STM mode timing signal A, and the FE mode timing signal B at the following timing. As shown in the timing chart of FIG. 4, in the STM mode timing signal A, the sampling signal (High signal) is added only at the timing of the STM bias voltage L, and only the hold signal (Low signal) is applied at the timing of the FE bias voltage. .

  Conversely, in the FE mode timing signal B, the sampling signal (High signal) is applied only at the timing of the FE bias voltage, and only the hold signal (Low signal) is applied at the timing of the STM bias voltage.

(Function)
The operation (operation) of the electron emission distribution measuring apparatus 1 of the embodiment having the above configuration will be described. The timing signal generator 23 sends a switch switch signal to the switch 24, and switches the switch 24 so that it is alternately connected to the STM bias power supply 9 and the STM bias power supply 9, as shown in FIG. As described above, the STM bias voltage or the FE bias voltage is alternately applied to the sample 7.

  Further, in synchronization with this bias voltage as shown in FIG. 4, the timing signal generator 23 sends the STM mode timing signal A and the FE mode timing signal B to the first sample hold circuit 11 and the second sample hold circuit 12. The sampling signal and the hold signal are input respectively.

STM mode:
First, an outline of the operation in the STM mode will be described. When an STM bias voltage (low bias voltage) is applied to the sample 7, a tunnel current flows between the sample 7 and the probe 6. Since the probe 6 scans the sample 7, if the distance between the probe 6 and the surface of the sample 7 changes due to the undulation of the sample 7, the flowing tunnel current changes.

  The probe 6 is moved using the piezo element 5 through the feedback circuit 20 so as to eliminate the change, and the distance between the probe 6 and the sample 7 is adjusted so that the tunnel current becomes a preset value. An STM image of the surface of the sample 7 can be obtained by mapping the moving distance on the two-dimensional surface.

  This operation will be described in more detail with reference to FIG. 4, the STM bias voltage L is applied to the sample 7, the sampling signal is input to the first sample hold circuit 11 as the STM mode timing signal A, and the FE mode is input to the second sample hold circuit 12. A hold signal is input as the timing signal B.

  Then, the tunnel current flowing between the sample 7 and the probe 6 is converted into a voltage by the preamplifier 10 by the bias voltage for STM applied to the sample 7, and this is input to the first sample hold circuit 11 and the voltage having the same value. Is output as This output voltage is amplified by the log amplifier 13 and input to the feedback circuit 20, where a feedback voltage is generated such that the tunnel current flowing between the sample 7 and the probe 6 is constant.

  This feedback voltage is applied to the piezo element 5 via the piezo drive circuit 21, and the piezo element 5 is moved through the piezo element 5 so that the tunnel current flowing between the sample 7 and the probe 6 is constant. Move with respect to 7. On the other hand, the feedback voltage is output to the circuit system 18 for STM measurement as a voltage equivalent to the movement of the probe 6 and used for generation of an STM image measurement (STM image).

  When the STM mode timing signal A applied to the first sample and hold circuit 11 changes from the sample signal to the hold signal (see period (b) in FIG. 4B), the output from the first sample and hold circuit 11 is The output voltage immediately before the change is held (held) until the next sample signal is applied, and the piezo element 5 is also maintained in a non-moving state.

Electron emission distribution measurement mode:
An outline of the operation in the electron emission distribution measurement mode will be described first. In response to the switch switching signal from the timing signal generator 23, the selector switch 24 is connected to the FE bias power source 25 (high bias power source), and as shown in the period (b) of FIG. The voltage H is applied to the sample 7, the hold signal is input as the STM mode timing signal A to the first sample hold circuit 11, and the sampling signal is input as the FE mode timing signal B to the second sample hold circuit 12.

  Then, an extremely large field emission current flows between the sample 7 and the probe 6 due to the FE bias voltage applied to the sample 7 as compared with the tunnel current in the STM mode. This field emission current is converted as a larger voltage in the preamplifier 10 than in the STM mode. The output voltage from the preamplifier 10 is input to the first sample hold circuit 11 and the second sample hold circuit 12.

  By the way, when a large field emission current flows between the sample 7 and the probe 6 in this way, if in the STM mode, the output voltage is large so that the probe 6 is separated from the sample 7 by the STM control circuit 3. Moving.

  However, in the electron emission distribution measurement mode, even if the output voltage from the preamplifier 10 is input to the first sample hold circuit 11, the hold signal is applied to the first sample hold circuit 11 and the hold state is maintained. Therefore, the output of the first sample hold circuit 11 does not change, and a large voltage is not output from the preamplifier 10.

  On the other hand, a sampling signal is added to the second sample and hold circuit 12 as the FE mode timing signal B. When a large voltage from the preamplifier 10 is input, the same large voltage as this input is output, and based on this output value The electron emission current is measured by the circuit system 19 for measuring the electron emission distribution (for example, by an electron emission distribution image forming apparatus), and an electron emission distribution image is obtained.

  When the STM mode timing signal A applied to the second sample and hold circuit 12 changes from the sample signal to the hold signal in the period (b) of FIG. 4, the output from the second sample and hold circuit 12 is immediately before the change. The large output voltage is held until the next sample signal is applied.

  Therefore, since the first sample hold circuit 11 holds the output voltage, the piezo element 5 is maintained so as not to move because the first sample hold circuit 11 holds the output voltage. For this reason, in the electron emission distribution measurement mode, an STM image is measured (generated) together with the electron emission distribution image.

  In the period of FIG. 4C, the same operation as in the period of FIG. 4A is performed, the STM bias voltage is applied to the sample 7, and the second sample hold circuit 12 maintains the hold operation. The one sample hold circuit 11 performs a sample operation, and the STM control circuit 3 operates and the STM image so that the tunnel current flowing between the sample 7 and the probe 6 becomes constant according to the undulations on the surface of the sample 7. Is measured (generated).

  In the period shown in FIG. 4 (d), the same operation as in FIG. 4 (b) is performed, the STM bias voltage is applied to the sample 7, and the first sample hold circuit 11 and the probe 6 maintain the hold state. However, the second sample hold circuit 12 performs the sample operation, and a field emission current flows between the sample 7 and the probe 6 in accordance with the undulation of the surface of the sample 7, and this current is based on a large voltage converted by the preamplifier 10. Thus, an electron emission distribution image is measured (generated).

  As described above, according to the electron emission distribution measuring apparatus 1, the period of the STM mode and the period of the FE mode are alternately switched at a frequency of approximately 2 KHz, and as a result, the STM image and the electron emission distribution image are observed simultaneously. it can.

  In addition, even if an FE bias voltage is applied to the sample 7 and a very large current flows between the sample 7 and the probe 6 as compared with the STM mode, the first sample hold circuit 11 holds the signal at this time. In this state, the STM control circuit can measure (generate) the STM image and the electron emission distribution image with extremely high accuracy without causing the probe 6 to move greatly.

(Experimental example)
As an experimental example of the embodiment of the present invention, a measurement experiment was performed using the electron emission distribution measuring apparatus 1 and will be described below. In this experimental example, an HfC polycrystalline thin film deposited on a Si substrate using a sputtering apparatus was used as a sample, and the electron emission distribution image was measured. Since the surface of the HfC polycrystalline thin film, which is a sample, is contaminated by the air, the outermost surface of the sample was first cleaned using an argon ion sputtering apparatus prior to measurement.

  As the probe 6, tungsten produced by electropolishing was used. The set values of the STM bias voltage and the FE bias voltage, and the set tunnel current are arbitrary, and are appropriately set depending on the sample 7 to be measured. In this experimental example, the STM bias voltage was set to -1.0 V, and the tunnel current was set to 5.0 nA. And it measured by setting the bias voltage for FE to -7.0V.

  In this experimental example, an STM image and an electron emission distribution image were obtained simultaneously with the electron emission distribution measuring apparatus 1, and the STM image is shown in FIG. 5, and the electron emission distribution image is shown in FIG. The bright part in the STM image shown in FIG. 5 is a part having a high shape height. And the bright part in the electron emission distribution image shown in FIG. 6 is a part with a large amount of electron emission. From FIG. 6, it was confirmed for the first time that in this sample 7, many electrons were emitted from the boundaries of the agglomerates.

  The best mode for carrying out the electron emission distribution measuring apparatus according to the present invention has been described based on the embodiments. However, the present invention is not limited to such embodiments, and the scope of the claims is as follows. It goes without saying that there are various embodiments within the technical scope described.

  According to the electron emission distribution measuring apparatus according to the present invention as described above, it is suitable for measuring the electron emission distribution by a device that detects or controls the tunnel current flowing between the probe and the sample such as a scanning tunneling microscope. It is.

It is a figure explaining the principle of this invention, (a) is a conceptual diagram of a tunnel current, (b) is a conceptual diagram of an electron emission current. (A) is a figure explaining the Example of the electron emission distribution measuring apparatus based on this invention, is a circuit block diagram of an electron emission distribution measuring apparatus, (b) is the detail drawing of the one part. It is a figure explaining the circuit system for the Example STM measurement of this invention, and the circuit system for electron emission distribution measurement. FIG. 6 is a timing chart showing the change with time of a bias voltage applied to a sample and an STM mode timing signal and an FE mode timing signal input to a first sample hold circuit and a second sample hold circuit. It is an STM image obtained in an experimental example. It is an electron emission distribution image obtained in the experimental example.

Explanation of symbols

1 Electron emission distribution measuring device
2 detector
3 STM control circuit
4 Electron emission distribution measurement / holding means
5 Piezo elements
6 Probe
7 samples
8 Wiring
9 STM bias power supply
10 Preamplifier
11 First sample and hold circuit
12 Second sample and hold circuit
13 Log amplifier
14 First preamplifier
15 Second preamplifier
16 Third preamplifier
17 Branch circuit
18 Circuit system for STM measurement
19 Circuit system for electron emission distribution measurement
20 Feedback circuit
21 Piezo drive circuit
22 Sample hold circuit
23 Timing signal generator
24 selector switch
25 Bias power supply for FE
26 First signal line
27 Second signal line
28 Oscillator 29 Computer
30 XY position information generator
31 STM image forming section
32 Electron emission distribution image forming section
33 AD converter
34 AD converter
35 DA converter
36 XY direction piezo controller

Claims (2)

A probe, a piezo element that moves the probe with respect to the sample, a preamplifier that converts a tunnel current flowing between the probe and the sample when a bias voltage is applied to the sample, and the preamplifier An electron emission distribution measuring apparatus in a scanning tunneling microscope, comprising: an STM control circuit provided with an STM bias power source for applying a bias voltage to the sample;
A FE bias power source that is higher in voltage than the STM bias power source and applies a bias voltage to the sample; a changeover switch that alternately switches between the STM bias power source and the FE bias power source; and a sample hold circuit. ,
The sample and hold circuit includes a first sample and hold circuit and a second sample and hold circuit connected in parallel to each other to the timing signal generator, and each of the sample and hold circuits receives an input from the preamplifier by a sample signal sent from the timing signal generator. Perform sampling operation to output, hold operation to hold and output the input from the preamplifier immediately before the hold signal input by the hold signal,
The first sample and hold circuit is provided between the preamplifier and the STM control circuit, and the second sample and hold circuit is provided between the preamplifier and a circuit system for electron emission distribution measurement. ,
The selector switch alternately switches the power source to be applied to the sample to the STM bias power source or the FE bias power source by a switch switching signal sent from the timing signal generator,
During the period when the voltage of the FE bias power source is applied to the sample, the first sample and hold circuit performs a holding operation, but the second sample and hold circuit applies a sample signal during the period. An electron emission distribution measuring apparatus for measuring the electron emission distribution image by using an output from the preamplifier. The preamplifier includes a first preamplifier connected to the probe, and a second preamplifier and a third preamplifier connected in parallel with each other to a branch circuit on the output side of the first preamplifier,
The output side of the second preamplifier is connected to a first sample and hold circuit, the output side of the third preamplifier is connected to a second sample and hold circuit,
The electron emission distribution measuring apparatus according to claim 1, wherein the third preamplifier has a multiplication factor different from that of the second preamplifier .

Images