WO2014055046A1

WO2014055046A1 - Method for performing the local charge transient analysis

Info

Publication number: WO2014055046A1
Application number: PCT/SK2013/000014
Authority: WO
Inventors: Štefan LÁNYI; Vojtech NÁDAŽDY
Original assignee: Fyzikálny Ústav Sav; Centrum Vedecko-Technických Informácii Sr
Priority date: 2012-10-05
Filing date: 2013-10-02
Publication date: 2014-04-10
Also published as: SK782012A3; SK288589B6

Abstract

Method of implementation of local charge transient analysis by the probe of scanning transient microscope is characterized by the fact that the probe is located and moved in short distance from the imaged surface, at the selected point an appropriate distance of the sensor from the surface is set, and the power supply for controlling the distance of the probe from the surface is switched off, a local charge transient spectroscopic analysis is realized, and subsequently, the power supply of the probe for controlling the distance of probe from the surface switches-on. Reliable analysis of transients is possible by separation of transient current from the current powering the sensor for the control of distance of the probe from the surface, namely by separation of the step of setting the position of the probe from the step of quantity measurement.

Description

Method for performing the local charge transient analysis Field of the Invention

The technical solution refers to a specific method of implementation of scanning probe microscopy, namely scanning transient microscopy, using a charge transient spectroscopy for the analysis of materials on the microscopic level.

State of the Art

The DLTS method (Deep Level Transient Spectroscopy) has become probably the most successful method of analysis of electrically active deep defects in semiconductor structures. In conventional DLTS method, the rate of emission of charges trapped in defects is changing by heating. Such an approach is impractical in microscopy, where the data must be recorded from many points, because it would require repeated cycles of heating and cooling, which would pose extreme requirements on the reproducibility of relative position of probe device and analyzed sample at temperature changes, and had large time requirements for the duration of the analysis.

The solution known up to now is described for example in EP 2325657, where a scanning microscope is described that contains an oscillating circuit generating a signal indicating the phase of the excitation signal from which the excitation signal is created. From the signal of deviation, the complex signal is generated. The calculation circuit calculates the argument of the complex signal. The output signal corresponding to the magnitude of interaction between the probe and the sample is obtained by a phase comparator, which detects the phase difference of the argument and excitation signals. Moreover, by supplementing with a loop filter it is possible to create a phase locked loop and create a frequency signal, which reflects the change in resonant frequency of the probe.

The patent US6094971 relates to the scanning probe microscope for the detection of interaction between the sample surface and the probe tip, while the probe is not in a direct contact with the surface of the sample. The circuitry of the microscope is using a phase-sensitive detector for the detection of phase difference between the excitation signal and the output of voltage amplifier, where the output of said phase sensitive detector is the input of the voltage controlled oscillator, by which a phase locked loop connection is formed, where the interaction between the probe tip and the sample manifests itself as a shift in mechanical resonance frequency of the crystal oscillator.

In turn, in the patent EP0551814 a device for the observation of material surfaces and a method of observation is described. The impact of forces on the vibrating probe, contact-less scanning the surface of the material, is sensed by several detectors In alternative solution, for the evaluation is used a phase locked loop circuit with the detection of the phase difference between two signals.

In addition to displaying the surface relief, claims formulated in mentioned examples include the usage of described devices for various analysis methods, for example EFM (electrostatic force microscopy), MFM (magnetic force microscopy), KPM (Kelvin probe microscopy), based on influence of force on the probe. Other claims are based on setting the distance of the probe from the surface for performing the analysis using the method of near-field optical microscopy (SNOM, NSOM) and capacitance microscopy (SCM).

The DLTS method (patent US3859595) is used for the analysis of deep defects in semiconductors. Deep are called defects (traps), separated from the conduction or valence band edge by multiples of the product kT, where k is the Boltzmann constant and T is the absolute temperature, as a result of which the charge carriers after capture persist in such defects for a longer time. It is usually applied to samples (diodes, capacitors) with electrodes area of 0.1 to 1 mm². More difficult situation occurs when DLTS is applied to structures representing a capacitor with very low capacitance. Then the solution requires an increase of sensitivity by more orders of magnitude. On small size transistor structures the problem was solved by applying the excitation pulses at the input and utilising the transistor gain in the measurement of output current or the channel conductance. However, such an option is not applicable to simple thin films, which as measured objects represent a two-terminal network.

Application of capacitance version of DLTS in microscopy has been described in the paper of C. K. Kim, I. T. Yoon, Y. Kuk and H. Lim, "Variable-temperature scanning capacitance microscopy: A way to probe charge traps in oxide or semiconductor," Applied Physics Letters, 78, 613 (2001) and in the work of A.L. Toth, L. Dozsa, J. Gyulai, F. Giannazzo and V. Raineri, "SCTS: scanning capacitance transient spectroscopy", Materials Science in Semiconductor Processing 4, 89 (2001). The disadvantage of capacitance DLTS is that it is applicable only to semiconductor materials with sufficiently high conductivity. It is not suitable for example for the analysis of dielectric films or organic semiconductors. Wider applicability offers the charge version of DLTS, described in the paper of T. J. Mego, "Improved feedback charge method for quasistatic CV measurements in semiconductors", Review of Scientific Instruments, 57, 2798 (1986).

Summary of the Invention

Deficiencies of present devices solves the way of controlling the microscop^es probe, the advantage of which is that it allows a microscopic analysis of defects by transient spectroscopy also in low conductive semiconductor and dielectric films. Another advantage is that the probe is not in contact with the analyzed surface, does not damage it and at the same time does not wear out.

Method of implementation of local charge transient analysis by the probe of the scanning transient microscope is characterized by that, that the probe is placed and moved in short distance from imaged surface, in the selected point the appropriate distance of the sensor from the surface is set, the power supply for controlling the distance of the probe from the surface is switched off, the local charge transient spectroscopic analysis is carried out, and next the power supply of the probe for controlling the distance of the probe from the surface is switched-on. Reliable analysis of transients is made possible by separation of analysed transient current from the current powering the sensor for the control of distance of the probe from the surface, namely by separation of the step of setting the position of the probe from the step of quantity measurement.

The advantage of the solution is that it allows the connection of probe, formed by a miniature resonator with attached tip sensing the analysed variable to the broadband amplifier without a need of an additional lead, which would complicate the realization of the probe and reduce its mechanical quality factor Q, and thus the sensitivity of sensing the interaction with the surface. Local charge transient analysis is carried out after setting the probe tip to selected distance from the surface, sensed by the resonator. The current driving the resonator is amplified by an amplifier, which also serves for amplifying the transient currents (transients). Connecting both signals at the same time to a single amplifier would lead to mutual interaction and difficulties with their reliable separation after the amplification. Therefore the invention also solves the method of separation of both signals.

Figure 1 shows the implementation of scanning charge transient microscope for sensing the force acting between the probe tip 2 and analyzed surface of the sample 12, which uses sensing of phase shift between the supply voltage and the deformation of tuning fork I, while keeping the selected distance constant is realized by stabilization of the oscillation frequency of tuning fork i by a phase locked loop circuit 6, the output of which is connected to the actuator 4 that adjusts the position of the probe in perpendicular direction to the surface of the analyzed sample 12 to ensure constant frequency of oscillation of the tip 2, to which corresponds a constant distance of probe tip 2 from the surface 12.

After setting a selected distance, a dedicated circuit 9 remembers and keeps constant the voltage on actuator 4 and turns off the powering of the tuning fork. After stopping the oscillation, current or light pulses are applied to the analyzed sample 12 and the excited current transients are integrated, averaged if necessary, and analyzed by a suitable method. Subsequently after the finishing of analytical phase the powering of the tuning fork 1 is restored, after stabilization of the amplitude of its oscillation and of the frequency of control voltage of the voltage controlled oscillator .11, the connection of the output with the actuator 4 is restored, by which the correction of the distance of probe from the surface is enabled in case that it changed during the analytical phase.

If necessary, the probe is moved to next point and the process is repeated.

Brief Description of Drawings

Figure No.l shows the block diagram of the probe device.

Figure No.2 shows the block diagram of the probe as an example of particular realization of scanning microscopy.

Figure No.3 shows the usual configuration, in which the angle between the surface of the tuning fork and the analyzed surface is smaller than 15 degrees.

Figure No .4 shows configuration, in which the angle between the surface of the tuning fork and the surface of the analyzed sample is larger than 15 degrees and smaller than 90 degrees and between the tuning fork and analyzed sample is inserted a shield.

Application Examples

In the description of a particular realization, the analog inputs are without marking, marked with (a) or (b) and the control inputs with (k). Outputs are without marking or are marked with (x), (u) or (v). The sensor sensing the position of the probe with respect to the analyzed surface, is formed by a piezoelectric resonator - quartz tuning fork I , one of the contacts of which is connected to AC signal source, represented by the voltage controlled oscillator 22 and the second is connected with the conductive tip 2 and at the same time connected to the input (a) of amplifier 5. The analyzed sample 12 is galvanically connected with the table— the electrode 3, to which a bias voltage and excitation pulses from the source 8 are connected. The current generated in the sample by the bias voltage and excitation pulses is led through the tip 2 to the input (a) of the amplifier 5, in the particular example a switched integrator. The output of the amplifier 5 is connected with the input (a) of the transient processor 13 and simultaneously also with the input (a) of the phase detector 16. The output voltage of oscillator 22 is connected to the input (b) of the phase detector. The output of the phase detector is connected through the input (a) with the input voltage memory of the controller j_8 and simultaneously through its output with the input of the controller 19, and with the control voltage memory of oscillator 21 , and at the same time through its output with the controlled signal source, in this case a voltage controlled oscillator 22. The current from the output of oscillator 22 is led to the quartz tuning fork 1, which is kept in resonance, eventually in other selected point of resonance curve. By the output voltage of controller 19 is controlled the actuator 4, which provides the necessary movement of the probe in the direction perpendicular to the sample surface. A control impulse from the output (x) of data management processor 7 via the input (k) blocks the status of memories of the input voltage of controller 18 and of the control voltage of oscillator 21 , with delay ensured by delay circuit 14 turns off through the input (k) the signal source 8, and with delay ensured by delay circuit 15 blocks through the input (k) the phase detector 16,.

From the output (v) of data management processor 7, the control impulse is led to the input (k) of the source of excitation pulses 8, which are, together with the bias, connected through the electrode 3 to the analyzed sample 12, and at the same time with the input (k) of amplifier 5 which it blocks for the time required for decay of oscillation of the tuning fork and the duration of the excitation pulse. To enable the decay of oscillation of the tuning fork 1 even in case of short excitation pulses, the excitation pulse is generated with delay, ensured by delay circuit 14 connected to the control input (k) of excitation pulse source 8. After the termination of the time interval for recording and processing the transients, the phase detector 16 is immediately unblocked and the controlled signal source 22 is switched on. After the oscillation of the tuning fork 1 and its tuning to working frequency are restored, with delay ensured by delay circuit 17 the memories of the input voltage of the controller 18 and of the control voltage 2J_ are unblocked, and by means of the actuator 4 and the controller 19 the control of distance of the tip 2 from the surface of the sample is restored. To facilitate the onset of oscillation of the quartz tuning fork, the distance of the tip 2 from the sample surface may be slightly increased. The actuator 4 enables the transfer of the tip 2 to other points above the samples surface, where the entire cycle is repeated. The output of the controller 19 is simultaneously connected to the output 23, by which the topography (relief) of the surface of the sample 12 is imaged.

The connection of the conductive tip with a quartz tuning fork in a different arrangement was included in patent US6094971. In currently used scanning probe microscopes a configuration of the probe approximately parallel with the surface of the sample is commonly used (the probe with the samples surface forms an angle to 15 degrees maximum). Such an arrangement is disadvantageous for large parasitic capacitance between the electrodes of the probe and the sample.

The scheme in Fig. 3 shows a conventional configuration, in which the surface of the tuning fork i with the surface of analyzed sample 12 forms an angle smaller than 15 degrees.

The scheme in Fig. 4 shows the new configuration, in which tines of the tuning fork 1_ forms with the surface of the sample 12 an angle larger than 15 degrees and smaller than 90 degrees, and between the tuning fork 1 and the analyzed sample 12 is inserted a shielding 35.

Such a solution suppresses the parasitic capacitance between electrodes of the tuning fork and the sample. Although shielding is a common way of mutual capacitance reduction, but in combination with probes of scanning probe microscopes such an arrangement has not been used yet.

Industrial Applicability

Scanning probe microscopy allows to image the relief or other characteristic of the surface with high spatial resolution by using the probe placed and moved in short distance from the displayed surface.

The invention enables a reliable analysis of transients by separation of analysed transient current from the current driving the sensor controlling the distance of the probe to the surface. The method is appropriate for the analysis of materials on microscopic level, and also on nanometer level. The subject of the technical solution can be also used in connection with vibrating probes, which use for driving a different type of actuator, for example a separate piezoelectric element driving the vibrating cantilever of force microscope or a cantilever from ferromagnetic material driven by a variable magnetic field.

Claims

P A T E N T C L A I M S

1. Method of implementation of local charge transient analysis by the probe of scanning transient microscope characterized by the fact that the conductive probe tip (2) is located and moved in short distance from imaged surface of the analyzed sample (12), at the selected point an appropriate distance of the sensor from the surface is set, subsequently the power supply for controlling the distance of the probe from the surface (10) is switched off, then a local charge transient spectroscopic analysis is carried out, and subsequently, the powering of the probe for controlling the distance of the probe from the surface (10) is switched on, whereby a reliable analysis of the transient is ensured by separation of the step of setting the position of the probe before the measurement from the step of the quantity measurement, namely by separation of analyzed transient current from the current supplying the sensor for controlling the distance of probe from the surface.

2. Method of implementation of local charge transient analysis by the probe of scanning transient microscope according to claim 1, characterized by the fact that the realization of scanning charge transient microscope for sensing the force acting between the probe tip (2) and the analyzed surface of the sample (12) uses sensing of the phase shift between the supply voltage and the deformation of tuning fork (1), while maintaining a selected distance is realized by stabilization of frequency of tuning fork oscillation, where the constant distance ensures a constant frequency of oscillation of the tip, whereby after setting a chosen distance, the specified circuit (9) remembers the parameters and keeps a voltage on the actuator (4), the power supply of the tuning fork turns off, subsequently after stopping its oscillation current or light pulses are applied to the analyzed sample (12) and created current transients are integrated, where appropriate averaged, and analyzed by a suitable method, whereby after finishing of the analytical phase the power supply of the tuning fork (1) is restored, and after the stabilization of the amplitude and of the frequency of its oscillation and of control voltage of voltage controlled oscillator, the connection of the output with the actuator (4) is restores, by means of which the correction of the distance of the probe from the surface is allowed in case that it has changed during the analytical phase.

3. Method of implementation of local charge transient analysis by the probe of scanning transient microscope according to claims 1 and 2, characterized by the fact that specific realization of the scanning charge transient microscope for sensing of force between the probe tip (2) and analyzed surface of the sample (12) uses sensing of phase shift between the powering voltage and the deformation of the tuning fork (1), whereby maintaining the selected distance is realized by stabilization of frequency of tuning fork oscillation (1) by means of the circuit of phase locked loop (6), the output of which is connected to the actuator (4), adjusting the position of the probe in a direction perpendicular to the analyzed surface (12) so that a constant frequency of the tip oscillation (2) is ensured, to which a constant distance of the probe tip (2) from the surface (12) is corresponding.

4. Method of implementation of local charge transient analysis by the probe of scanning transient microscope according to claims 1, 2 and 3, characterized by the fact that after the measurement the probe is moved into the next point and the process is repeated.

5. Method of implementation of local charge transient analysis by the probe of scanning transient microscope according to claims 1, 2, 3 and 4, characterized by the fact that the technical solution is used in conjunction with vibrating probes, which for actuation are using a different type of actuator, for example a separate piezoelectric element actuating the oscillating cantilever of the force microscope, or the cantilever from ferromagnetic material vibrated by variable magnetic field.

6. Method of implementation of local charge transient analysis by the probe of scanning transient microscope according to claims 1, 2, 3, 4 and 5, characterized by the fact that the surface of the tuning fork (1) is not parallel to the surface of the analyzed sample (12), but forms with it an angle greater than 15 degrees, less than 90 degrees, and between the tuning fork (1) and analyzed sample (12) a shielding (35) is inserted.