RU2100868C1 - Tunnel current and clearance control device (variants) - Google Patents

Abstract

FIELD: electrical engineering. SUBSTANCE: device has voltage source, piezoelectric element with probe fixed to it, and terminal for connection of specimen which forms tunnel clearance together with probe. In addition, device has fixed resistor connected in parallel to piezoelectric element inserted between probe and voltage source connected to terminal. Piezoelectric element connection polarity is such that voltage rise across piezoelectric element provides for probe displacement away from specimen. EFFECT: more reliable control. 2 cl, 4 dwg

Description

 The device can be used both in scanning tunneling microscopy and in nanotechnology devices where automatic maintenance of tunneling current and gap is necessary. Of particular importance is the use of this device for multi-probe systems, where independent automatic control of a large number of probes is required simultaneously.

 A known scheme for controlling the tunneling current and the gap in the microscope [1] It consists of an input unit, a current-voltage converter, the input of which is connected to the probe and the sample, and a feedback circuit unit, the input of which is connected to the output of the input unit, and the output is connected to the piezoelectric transducer. The feedback circuit block consists of a reference voltage source, a regulator for manually setting the voltage at the gap, an input voltage inverter, a tunnel current comparison subunit and an error signal amplification, an analog storage device, an integrator, a Butterworth high-pass filter, an inverter, and a high-voltage amplifier.

 As a prototype, a tunnel current and gap control scheme in a tunneling microscope was selected [2]. A control device block diagram consists of three main components: a tunnel gap (probe, sample), an electronic control unit, and a piezoelectric element executing device. Due to the complex amplitude-frequency characteristics of the piezoelectric element (the presence of resonances), rather strict requirements are imposed on the electronic unit to prevent self-excitation along the feedback loop. The electronic unit consists of an active rectifier, a logarithmic amplifier, an integrator, an inverting amplifier, a summing amplifier with the highest possible output voltage. This feedback scheme is quite typical for analog control devices in tunneling microscopes and has several disadvantages: an electronic unit configured to work with a certain type of piezoelectric elements must be rebuilt if another type of piezoelectric elements is used. The design is quite complex, contains a large number of elements, which reduces its reliability due to the higher probability of parameter drift or failure of one of the elements. The use of this design for multi-probe devices, where independent control of a large number of probes is required, will lead to high cost, to a significant decrease in reliability, and an increase in the size of the structure.

 The technical task is to increase the reliability and resistance to self-excitation, increase speed by eliminating delays associated with transients in the external electronic unit, as well as the possibility of miniaturization of the proposed device.

 Two versions of the device for controlling the tunneling current and the tunneling gap are proposed, comprising a piezoelectric element, a tunneling probe, a resistor, a voltage source, and a sample connection terminal.

 In option 1, these elements are interconnected as follows: the tunnel probe is rigidly fixed to the piezoelectric element, the constant resistor is connected in parallel with the piezoelectric element, which is connected in series with the voltage source and the probe, while the piezoelectric element and the voltage source are mutually connected taking into account the polarity of both elements, namely bipolar, and the voltage source is connected to the terminal for the measured sample.

 In option 2, the tunnel probe is also rigidly attached to the piezoelectric element, a constant resistor, voltage source, and piezoelectric element are connected in series, forming a closed loop, and the probe and terminal for the measured sample, where the tunnel gap is formed, are connected in parallel with the piezoelectric element. The mutual inclusion of the piezoelectric element and the voltage source is carried out taking into account the polarity of these elements (unipolar).

 According to option 1, the polarity of switching on the voltage source and the piezoelectric element is bipolar and such that an increase in tension on the piezoelectric element leads to the probe moving in the direction from the sample, and a decrease in voltage leads to the probe moving in the direction toward the sample.

 In option 2, the polarity of the inclusion, namely, unipolar inclusion, should be such that an increase in the voltage on the piezoelectric element causes the probe to move toward the sample, and the decrease in the opposite direction. It must be emphasized that both in option 1 and in option 2, it is precisely the mutual inclusion of the piezoelectric element and the voltage source, i.e. if the inclusion is correct then, when replacing the polarity of the voltage source and the piezoelectric element on the opposite, the circuit remains operational, and the tunneling current will change direction.

 In FIG. 1 shows a circuit diagram of the inclusion, option 1; in FIG. 2 circuit diagram of inclusion, option 2; in FIG. 3 equivalent electrical circuit, option 1; figure 4 equivalent circuit diagram, option 2.

 In FIG. 1 and 2 the following designations are accepted: 1 resistor, 2 - piezoelectric element, 3 probe, 4 sample, 5 voltage source.

In FIG. 3 and 4, the designations are accepted: 6 inductance L, 7 equivalent loss resistance, 8 electric capacitance C _k , 9 parasitic capacitance of the piezoelectric element.

где R_о•exp[D-(U_з -U_о)•R]

есть сопротивление зазора, а D-(U_з U_о)•R d величина зазора между острием зонда и поверхностью образца;
U_з напряжение на зазоре;
U_п напряжение питания;
R_к величина сопротивления;
R чувствительность пьезоэлемента;
U₀ напряжение на зазоре, когда его величина равна нулю;
Ф работа выхода электрона;
R_о сопротивление зазора, когда величина зазора равна нулю;
D величина для отсчета зазора.The equation describing the stationary state of the control system looks like this:

where R _o • exp [D- (U _s -U _o ) • R]

is the resistance of the gap, and D- (U _З U _о ) • R d is the gap between the tip of the probe and the surface of the sample;
U _{s the} voltage at the gap;
U _p supply voltage;
R _{to the} value of resistance;
R sensitivity of the piezoelectric element;
U _{0 the} voltage at the gap, when its value is zero;
F electron work function;
R _{about the} resistance of the gap, when the gap is zero;
D value for counting the gap.

 Consider the principle of the device, option 1.

The resistor 1 and the gap between the tip of the probe and the surface of the sample form a voltage divider. When the gap is large, there is no current, the voltage at the gap is equal to the supply voltage, and the voltage at the piezoelectric element 2 and resistor 1 is 0. When the gap is reduced, the field strength increases, the current and voltage at the piezoelectric element 2 and resistor 1 appear. At the same time, the piezoelectric element 2 is deformed, taking probe at a distance only slightly less than that the sample passes, and because almost entire portion of the dynamic range, the voltage U across the gap _of substantially proportional to the gap d, that follows from the theory is in good agreement with exp ments. After a certain transitional section, probe 3 begins to respond little to the approach of sample 4 and there is a touch.

 To assess the stability and speed of transients, unsteady equations are considered both taking into account resonances (in the approximation of a piezoelectric element - a piezoresonator), and without taking into account resonances, taking into account only the parasitic capacitance of the piezoelectric element (Fig. 3, Fig. 4). An analysis of the equations showed that the resonance properties of the piezoelectric element with this inclusion do not have a big impact on the stability of the device, and this indicates the possibility of using various types of piezoelectric elements in it.

 The proposed device, unlike analogues, does not contain an external electronic unit, so that a system is formed that is in stable equilibrium. Moreover, with external exposure, namely, a random approximation of the sample, a change in the height of the relief during scanning, the system goes into a new stable state, while the gap changes by an amount that is much less than the magnitude of the external impact.

 The device variants differ in that if the power source is switched off accidentally during operation, then in the first version the probe may “stick” into the sample, and in the second it will move away from the sample. In other words, in the first embodiment, in order to exit the gap tracking mode, as in conventional electronic tracking devices, it is necessary to remove the substrate from the probe (or the probe from the substrate). In the second embodiment of the proposed device, it is enough to disconnect the voltage source.

 The authors do not know the use of devices that regulate the maintenance of the tunnel gap and the tunnel current, namely, providing feedback in the control systems of the scanning tunneling microscope, without the use of external electronic units. We can conclude that the proposed device meets the criterion of inventive step.

Execution example. For experimental studies, a prototype was assembled on the mechanical base of a tunneling microscope with a tubular piezoelectric element 2 made of polarized ZTS-19 ceramics with a sensitivity of 5 nm / V. The needle probe 3 was made of tungsten using an electrochemical etching process. For the optimal dependence of the operation parameters of the device from R _to , a constant resistor 1 with a nominal value of 1 MΩ was used. As a voltage source 5, both stabilized power supplies and batteries were used. The gap voltage was measured using both a high voltmeter and an oscilloscope. After mounting the structure, the entrance to the operating mode is as follows. The probe using an optical device is installed at a distance of 10 15 microns from the surface of the sample. After that, the power is turned on both in option 1 and in option 2 and a smooth approximation of the probe and the sample begins using the electromechanical drive of the microscope until the specified current appears, i.e. voltage on piezoelectric element 2 and resistor 1 in option 1, and voltage on resistor 1 in option 2. food immediately.

 The proposed invention has a wide range of applications: a) tunneling microscopy; b) tunnel effect sensors; c) multi-probe systems for nanotechnological purposes; d) multi-probe systems for superdense information recording devices.

 All these possible applications are among the most modern and promising branches of science and technology. At the same time, the proposed device has such an important advantage as practically maximum compactness, economy, high reliability, noise immunity due to the absence of active elements, as well as the absence of delay in transients in the external electronic unit. The device is much cheaper, which again is very important for creating multi-probe devices.

Literature
1. Troyanovsky A. M. Feedback circuit and control of a scanning tunneling microscope. Instruments and experimental technique, 1989, N1, p. 165 170.

 2. DiLella D. Wandess J. Coulton R. An electron microscope control system with a tubular piezoelectric transducer. Instruments for scientific research, 1989, N6, p. 15 21. (Rev. Sci. Instrum. 60, N 6, 997 1002).

Claims (2)

 1. A device for controlling the tunneling current and the gap, containing a voltage source, a piezoelectric element, a probe rigidly fixed on it and a terminal for connecting a sample forming a tunnel gap together with the probe, characterized in that the device further comprises a constant resistor connected in parallel with the piezoelectric element connected between the probe and the voltage source connected to the terminal, while the polarity of the piezoelectric element is such that increasing the voltage on it ensures that the probe moves in the direction from the sample.  2. A device for controlling the tunneling current and the gap, containing a voltage source, a piezoelectric element, a probe and a terminal for connecting a sample forming a tunnel gap together with the probe, characterized in that the device further comprises a constant resistor connected in series with the voltage source and piezoelectric element, forming a closed loop, and the circuit, the elements of which are the probe and terminal, is connected in parallel with the piezoelectric element, while the polarity of the piezoelectric element is such that Vyshen voltage thereon delivers the probe moving toward the sample.

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