JP2009109377A - Scanning probe microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning probe microscope capable of automatically correcting the shift of the characteristic frequency of a cantilever due to temperature changes. <P>SOLUTION: The scanning probe microscope, which performs AFM observation based on the cantilever vibration amplitude value of an atomic force microscope, includes a correction means for correcting the shift of the characteristic frequency of the cantilever due to temperature changes when the configuration of a sample surface is observed as temperature is changed. On a forward path, measurements are made as Z-axis feedback control is performed by the correction means; on a return path, the sample 3 is separated from the cantilever 1 and the cantilever 1 is adjusted to correct the shift of the characteristic frequency. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a scanning probe microscope, and more particularly to a scanning probe microscope that can correct a natural frequency of a cantilever due to a temperature change or the like.

  The scanning atomic force microscope detects the atomic force acting between the tip of the cantilever and the sample surface, and measures the sample surface while feedback-controlling the Z axis of the scanner so that this atomic force is constant. There are many measurement methods for this scanning atomic force microscope. For example, in the AC-AFM (dynamic AFM, also called tapping mode AFM) measurement method, the cantilever is vibrated by forcibly exciting it with an external oscillator, and feedback control is performed so that the attenuation due to the atomic force is constant. is doing.

  In this type of scanning atomic force microscope, there are an AM detection method and an FM detection method in order to control the natural frequency of the cantilever. The AM detection method detects a change in force received from the sample as a change in the amplitude of the cantilever (ΔA), and the FM detection method detects a change in force received from the sample as a deviation (ΔF) in the resonance frequency of the cantilever. Is the method.

(AM detection method)
The cantilever driven by the cantilever drive signal interacts with the surface shape of the sample, causing a change in the vibration amplitude of the cantilever. This vibration amplitude is monitored by a light intensity detector and converted into a direct current by an RMS (effective value) circuit. The RMS value of the cantilever amplitude is compared with the cantilever amplitude value set before observation, and an error signal is generated. This error signal is amplified and sent to the Z-axis piezo element driving circuit, and feedback control is performed so that the amplitude value of the cantilever becomes the amplitude value set before observation. This error signal becomes a tapping mode image.

(FM detection method)
In the measurement and observation of a sample using a non-contact atomic force microscope (non-contact AFM), because of the spatial change (force gradient) of the interaction force between the sample and the cantilever probe, the vibration amplitude and vibration frequency of the cantilever Changes. For this reason, a feedback circuit that keeps the drive vibration amplitude and frequency of the cantilever constant and a signal processing circuit that detects a frequency component are provided. In this circuit, an oscillator control amplifier that performs cantilever vibration control is provided. A detection signal of the cantilever vibration is input to the oscita control amplifier and finally fed back to the cantilever vibration drive circuit. In this case, the FM demodulator circuit that receives the input of the oscillator control amplifier detects a frequency change.

  In the AM detection method used in a conventional scanning probe microscope, the cantilever is vibrated with a constant natural frequency. Before the observation, the frequency-amplitude curve of the cantilever is measured to determine the natural frequency, and the excitation voltage for setting the amplitude Vo is set. This value usually cannot be changed during observation of one field of view. If the temperature of the cantilever decreases from room temperature, the natural frequency increases, and if it increases, the natural frequency decreases.

  If there is a change in mass increase due to adhesion of gas or the like, the natural frequency will decrease. When these changes in the natural frequency occur continuously, Vo changes under an excitation voltage having a constant frequency used in the AM detection method. As a result, an image is obtained by force detection during observation of one field of view, and measurement under uniform conditions is not possible. In the worst case, when the set value V is larger than Vo, feedback cannot be performed and image collection cannot be performed. Even in the phase image, a change in the natural frequency changes the phase value, which causes a background change in the information on the viscoelasticity of the substance. A number shift component is detected and observed as an AFM image.

  As a conventional device of this type, a detection signal of a displacement detecting means is input, a frequency is converted into a voltage and output, and at the same time, the voltage is converted into a frequency signal by a voltage control oscillator and fed back to the input side Conversion means, control means for controlling the excitation means by the frequency signal of the voltage controlled oscillator, feedback means for feeding back the output voltage of the voltage controlled oscillator to the Z motion control of the scanning means, and switching for switching the voltage controlled oscillator to a fixed frequency There is known a technique in which the feedback loop of the frequency-voltage conversion means can be switched to an open state, and FM switching is changed to phase feedback before the highest resolution by the switching means (for example, Patent Document 1).

Also operates in proximity mode when driving the vibrating probe tip near the surface of the sample to be inspected, and operates in scanning mode when moving the vibrating probe tip along the sample surface to measure the sample properties In the approach mode, the frequency of the tip vibration is preferably changed to maintain a constant phase angle with the movement of the actuator that vibrates the probe tip, and the operation in the scanning mode follows the operation in the approach mode. A technique is known in which the vibration of the probe tip in the scanning mode is the final frequency used in the approach mode (see, for example, Patent Document 2).
JP 2004-226238 A (paragraphs 0012 to 0022, FIGS. 1 and 2) Japanese Patent No. 3484099 (paragraphs 0026 to 0038, FIG. 3)

  In the case of the conventional AC-AFM measurement, if the sample is observed while the temperature is changed, the cantilever near the sample is also warmed. Or it will be cooled, the energy state of the cantilever will change, and the natural frequency will shift. Since the deviation of the natural vibration is detected as a change in amplitude as shown in FIG. 8, accurate measurement is difficult. In that case, the cantilever was adjusted every time the measurement was completed, and the deviation of the natural frequency of the cantilever was corrected.

  FIG. 8 is a diagram showing the amplitude-frequency characteristics of the cantilever. The horizontal axis is frequency and the vertical axis is amplitude. The characteristic indicated by the solid line is the characteristic before the change, and the characteristic indicated by the broken line is the characteristic after the change. In the figure, ΔF is the frequency variation, and ΔA is the amplitude variation.

  Further, in the AM detection method used in the conventional scanning probe microscope, the cantilever is vibrated with a constant natural frequency. Before the observation, the frequency-amplitude curve of the cantilever is measured to determine the natural frequency, and the excitation voltage for setting the amplitude Vo is set. This value usually cannot be changed during observation of one field of view. If the temperature of the cantilever decreases from room temperature, the natural frequency increases, and if it increases, the natural frequency decreases.

  If there is a change in mass increase due to adhesion of gas or the like, the natural frequency will decrease. When these changes in the natural frequency occur continuously, Vo changes under an excitation voltage having a constant frequency used in the AM detection method. As a result, an image is obtained by force detection during observation of one field of view, and measurement under uniform conditions is not possible. In the worst case, when the set value V is larger than Vo, feedback cannot be performed and image collection cannot be performed. Even in the phase image, a change in the natural frequency changes the phase value, which causes a background change in the information on the viscoelasticity of the substance.

  Conventional observation by the AM detection method assumes that the natural frequency of the cantilever is constant. However, if the natural frequency gradually changes due to temperature change or mass change, it could not be handled during image acquisition. As a conventional countermeasure, it was necessary to recalibrate the natural frequency by measuring a frequency-amplitude curve between the image acquisition and the next image acquisition. In the AM detection method, the frequency-amplitude measurement curve of the cantilever is measured before observation, and the natural frequency is measured from the peak position. During image observation, it vibrates with a constant excitation intensity using that frequency, so if there is a change in the natural frequency, it cannot be handled, resulting in an inaccurate shape image or phase image.

  On the other hand, in the FM detection method, since the cantilever self-excited oscillation is caused, the frequency change is made to correspond to even during observation. However, there is a problem that the phase image measured by the AM detection method is not measured and cannot be applied to viscoelastic observation of a polymer sample or the like.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a scanning probe microscope capable of automatically correcting the deviation of the natural frequency accompanying the change in the physical state of the cantilever. Yes.

(1) The invention according to claim 1 is a scanning probe microscope that performs AFM observation based on the cantilever vibration amplitude value of an atomic force microscope. The correction means which corrects the deviation of the natural frequency of the cantilever accompanying the change of
The correction means is characterized in that the forward path is measured while performing Z-axis feedback control, and the return path is to separate the sample and the cantilever and adjust the cantilever to correct the deviation of the natural frequency.

  (2) The invention according to claim 2 is that, in a scanning probe microscope that performs AFM observation based on a cantilever vibration amplitude value of an atomic force microscope, when there is a change in natural frequency caused by a change in physical state, shape image acquisition is performed. Switching to the frequency detection method by the FM detection method in the middle of the process, the natural frequency was automatically detected using the PLL circuit, and the image was collected by the AM detection method using the frequency. Features.

  (3) In the invention according to claim 3, when the natural frequency is detected, when scanning with the X scanning range wider than the recording range, the natural vibration is generated in a difference area between the X scanning range and the recording range. It is characterized by performing number detection.

  (4) The invention described in claim 4 is characterized in that the natural frequency is detected for each pixel of the image when the natural frequency changes drastically.

  (1) According to the invention described in claim 1, the forward path is measured while performing Z-axis feedback control, and the return path is to separate the sample and the cantilever and adjust the cantilever to correct the deviation of the natural frequency. Therefore, it is possible to automatically correct the deviation of the natural frequency accompanying the temperature change of the cantilever, and to provide a scanning probe microscope capable of stable observation.

  (2) According to the invention described in claim 2, the frequency is switched to the frequency detection method by the FM detection method in the middle of collecting the shape image, the natural frequency is detected using the PLL circuit, and the frequency is used. By performing image collection by the AM detection method, it is possible to automatically correct the deviation of the natural frequency accompanying the temperature change of the cantilever, and to provide a scanning probe microscope capable of stable observation.

  (3) According to the invention described in claim 3, when the natural frequency is detected, the natural frequency is detected in a difference area between the X scanning range and the recording range, thereby obtaining a time-efficient image. Can be performed.

  (4) According to the invention described in claim 4, when the natural frequency changes drastically, a good image can be obtained by detecting the natural frequency for each pixel of the image.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 is a cantilever, 2 is a probe attached to the tip of the cantilever 1, 3 is a sample, 4 is a scanner that scans the sample 3 in a three-dimensional direction, and 5 is a laser beam applied to the back surface of the cantilever 1. A laser 6 is a photodetector that receives the laser beam reflected from the cantilever 1 and converts it into an electrical signal. Although the figure shows an example in which the photodetector 6 is divided into four parts, it is not necessary to be limited to four parts, and it may be divided into two parts.

  7 is a signal input circuit that receives the output of the photodetector 6 and separates signal components, 8 is a processor that receives the output of the signal input circuit 7 and performs predetermined arithmetic control, and 9 is an output of the processor 8 This is a scanner control circuit that drives a scanner 4 that scans a sample 3 in a three-dimensional (X, Y, Z) direction. Reference numeral 11 denotes a heater that heats or cools the sample 3, and 12 denotes a temperature control circuit that controls the temperature of the heater 11. The temperature control circuit 12 receives data such as a set temperature and a control signal from the processor 8.

  Reference numeral 13 denotes a stage control circuit that drives a stage in response to a control signal from the processor 8, and reference numeral 14 denotes a stage. On the stage 14, the scanner 4 and the sample 3 are placed. The operation of the apparatus configured as described above will be described as follows.

  In the scanning forward path, measurement is performed while the processor 8 feeds back the Z axis of the scanner 4 through the scanner control circuit 12 so as to keep the distance between the sample 3 and the probe 2 constant as shown in FIG. To do. As a result, the light from the laser 5 incident on the back surface of the cantilever 1 is reflected, detected by the photodetector 6, and input to the signal input circuit 7. The signal input circuit 7 receives the input electric signal which is the output of the photodetector 6, converts it into digital data, and gives it to the processor 8. The processor 8 receives the output of the input photodetector 6, drives the scanner control circuit 9 so that the distance between the sample 3 and the probe 2 is constant, and controls the Z-axis direction. When the input of the scanner control circuit 9 at this time is output as an image, an uneven image of the sample 3 can be obtained. During this period, the sample 3 is maintained at a predetermined temperature by the heater 11 controlled by the temperature control circuit 12.

  In the scanning return path, a control signal is sent from the processor 8 to the scanner control circuit 9 as shown in FIG. 2B, the sample 3 is lowered in the Z-axis direction, and the sample 3 is pulled away from the probe 2 of the cantilever. Then, the deviation of the natural frequency of the cantilever 1 is corrected until the cantilever 1 is returned to the initial position. According to this embodiment, the deviation of the natural frequency accompanying the temperature change of the cantilever 1 can be adjusted at the scanning stage, and stable observation is possible.

  FIG. 3 is a diagram illustrating the relationship between the cantilever, the sample, the scanner, and the stage. (A) is the same structure as embodiment shown in FIG. That is, the scanner 4 is installed on the stage 14, and the sample 3 is placed on the scanner 4. The cantilever 1 is configured as a separate body. (B) shows a configuration in which the sample 3 is placed on the stage 14 and the scanner 4 and the cantilever 1 are coupled separately. (C) shows a configuration in which the scanner 4 and the sample 3 are coupled, and the stage 14 and the cantilever 1 are coupled separately. (D) shows a configuration in which only the sample 3 is disposed independently, and the cantilever 1, the scanner 4, and the stage 14 are coupled separately. Thus, the relationship between the cantilever, the sample, the scanner, and the stage according to the present invention is not uniquely determined, and various combinations are conceivable, and the present invention can be applied to these combinations.

  Next, another embodiment of the present invention will be described. When observing a high-temperature or low-temperature sample, the temperature of the cantilever changes due to heat conduction from the sample to the cantilever. In high-temperature observation of a polymer sample or the like, vapor of organic molecules generated from the sample is generated, and the vapor condenses on the surface of the cantilever, increasing the mass.

  In such a case, the natural frequency fo of the cantilever changes. As a result of this change, the amplitude value Vo changes when a constant frequency is applied. For this reason, the force intensity (Vo-V) that is a difference from the set value V changes, and a normal image cannot be obtained. Thus, it is conceivable to automatically track the natural frequency of the cantilever that continues to change.

In this embodiment, the AM detection method is basically used at the time of shape and phase observation, but the changed natural frequency is measured using the FM detection method by lifting the probe halfway. FIG. 4 is a diagram showing a configuration example of the circuit (PLL). In the figure, reference numeral 20 denotes a phase comparator (Phase Comparator) which receives a cantilever natural frequency fo _{in one} of them and a feedback frequency fo _out to the other input, and compares the phases of both signals, and 21 denotes the phase comparison. This is a filter that receives the output of the device 20.

Reference numeral 22 denotes a voltage controlled oscillator (VCO: Voltage Controlled Oscillator) that receives the output of the filter 21 and generates a signal having a frequency corresponding to the input voltage. The VCO 22 is supplied with a control signal for switching on (On) / hold (Hold) from the outside. The output of the VCO 22 becomes the feedback frequency fo _out and is input to the phase comparator 20. The filter 21 outputs a voltage Vfo when the VCO is on and a phase when the VCO is on. The natural frequency is extracted from the output fo _{out of} the VCO 22.

  In the case of frequency matching to the natural frequency by self-excited oscillation, the VCO 22 of the PLL circuit of the FM detection method is turned on. If the self-oscillation is performed, the VCO 22 fixes the input voltage by holding the VCO 22 and outputs a frequency corresponding to the input voltage. That is, a constant frequency held from the VCO 22 is output. Thus, the natural frequency can be measured.

  FIG. 5 is a diagram showing a first X-scan operation of the AM detection method using the natural frequency due to self-excited oscillation of VC (cantilever). The X scanning range is wider than the actual image recording range. At the start of scanning, the cantilever is lifted on and is in a free vibration state. In this state, the VCO of the PLL circuit is turned on and a self-excited oscillation state is entered. In this state, the natural frequency is detected as shown in FIG. Once the natural frequency is obtained, the VCO is held and at the same time the lift is released and scanning continues with the normal AM detection method.

  In the recording range as an image, the shape signal and the phase signal can be measured in the same manner as usual. Further, by repeating the same operation in the Y direction (sub-scanning direction), the shape and the phase image can be observed. Of course, the natural frequency can be obtained by turning on the similar lift and VCO in the return X-scan.

  Thus, according to this embodiment, when frequency control by FM detection is performed in the middle of shape image image collection, the natural frequency is detected using a PLL circuit, and the frequency is used to perform AM. By collecting images by the detection method, it is possible to automatically correct the deviation of the natural frequency accompanying the temperature change of the cantilever, and to provide a scanning probe microscope capable of stable observation. According to this embodiment, when detecting the natural frequency, time-efficient image acquisition is performed by detecting the natural frequency in a difference area between the X scanning range and the recording range. Can do.

  FIG. 6 is a diagram showing a second operation of the AM detection method using the natural frequency due to the self-excited oscillation of VC. When the change in the natural frequency of the cantilever is significant, the natural frequency is detected for each pixel of the image as shown in FIG. At each image pixel point in the X direction, the cantilever 1 is lifted to turn on the VCO. As a result, the circuit enters the FM detection mode. As a result, self-excited oscillation of the cantilever 1 is performed, and when the natural frequency is obtained, the VCO 22 is held. As a result, the cantilever 1 continues to oscillate with the natural frequency fixed.

  Therefore, the lift is released, and the shape signal and the phase signal are measured by a normal AM detection method. In the recording range to be an image, the shape signal and the phase signal can be measured as usual, and the shape and phase image can be observed by repeating in the Y direction (sub-scanning direction). Of course, the same VCOon for each pixel can be used or not used in the return X-scan. This method can perform AM detection with higher accuracy in order to measure the natural vibration at each pixel position of the image, but requires a waiting time before and after the lift, and thus requires a long time measurement.

According to this embodiment, when the natural frequency changes drastically, a good image can be obtained by detecting the natural frequency for each pixel of the image.
FIG. 7 is a diagram showing a third operation of the AM detection method using the natural frequency due to the self-excited oscillation of VC. The present invention can also be used when the VCO 22 is always on as shown in FIG. With the lift off, the sample 3 and the probe 2 of the cantilever 1 are brought close to each other, and the sample 3 is scanned while the natural frequency of the sample is kept variable. In this case, since the VCO is not held, the output fo _out of the VCO 22 always changes. In this case, the shape signal can be measured, but the phase signal cannot be measured from the output of the VCO 22.

The effects of the present invention described above are listed as follows.
1. In AFM observation by the AM detection method, even when the natural frequency of the cantilever continues to change during observation, it becomes possible to observe the precise shape and phase.
2. Since the cantilever 1 is lifted during image observation and the VCO 22 of the PLL circuit used in the FM detection method is operated, the natural frequency can be automatically obtained, so that image acquisition can be performed automatically as in the conventional case.
3. Since it can cope with the temperature change of the sample, observation at higher and lower temperatures becomes possible.

  As described above, according to the present invention, it is possible to provide a scanning probe microscope capable of automatically correcting the deviation of the natural frequency accompanying the temperature change of the cantilever, and the practical effect is particularly great.

It is a block diagram which shows one embodiment of this invention. It is a figure which shows the state of operation | movement of this invention. It is a figure which shows the relationship between a cantilever, a sample, a scanner, and a stage. It is a figure which shows the structural example of a PLL circuit. It is a figure which shows the 1st operation | movement of the X scan of AM detection method using the natural frequency by self-oscillation of VC. It is a figure which shows the 2nd operation | movement of X scanning of AM detection method using the natural frequency by self-oscillation of VC. It is a figure which shows the 3rd operation | movement of the X scan of AM detection method using the natural frequency by self-oscillation of VC. It is a figure which shows the structural example of a PLL circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cantilever 2 Probe 3 Sample 4 Scanner 5 Laser 6 Photodetector 7 Signal input circuit 8 Processor 9 Scanner control circuit 11 Heater 12 Temperature control circuit 13 Stage control circuit 14 Stage

Claims (4)

In a scanning probe microscope that performs AFM observation using an atomic force microscope cantilever vibration amplitude value,
During the observation of the shape of the sample surface while changing the temperature, a correction means is provided to correct the deviation of the natural frequency of the cantilever accompanying the change in the physical state,
A scanning probe microscope characterized in that the correction means performs measurement while performing Z-axis feedback control on the forward path, and on the return path, the sample and the cantilever are separated, and the cantilever is adjusted to correct the deviation of the natural frequency. In a scanning probe microscope that performs AFM observation using an atomic force microscope cantilever vibration amplitude value,
If there is a change in the natural frequency caused by a change in the physical state, switch to the frequency detection method by the FM detection method in the middle of collecting the shape image, and automatically detect the natural frequency using the PLL circuit, A scanning probe microscope characterized in that images are collected by AM detection using the frequency.   The natural frequency is detected in a difference area between the X scanning range and the recording range when the natural frequency is detected when scanning with an X scanning range wider than the recording range. 2. A scanning probe microscope according to 2.   3. The scanning probe microscope according to claim 2, wherein when the natural frequency changes drastically, the natural frequency is detected for each pixel of the image.

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