JP3883790B2 - Scanning atomic force microscope - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a scanning probe microscope such as a scanning atomic force microscope (AFM). In particular, the present invention complements the problems of the conventional contact mode and tapping mode, and can stably observe a sample with a very soft surface. In addition, the present invention relates to a scanning probe microscope that can realize measurement with a good SN ratio.
[0002]
[Prior art]
In order to observe a sample with an extremely soft surface in the contact mode, the probe must be brought into contact with the sample surface with a very weak force, and the atomic force acting on the probe must be made very weak. In order to weaken the interatomic force acting on the probe, conventionally, the spring constant of the cantilever has been made extremely small, or the probe has been installed at a position where the interatomic force is weak on the force curve. Yes.
[0003]
However, in the former case, there is a problem that the thickness of the cantilever has to be extremely thin, making it difficult to manufacture, and a problem of degrading the S / N ratio of the detection signal due to the influence of the frictional force acting from the lateral direction. It was. In the latter case, it is necessary to set the probe near the position where the attractive force on the force curve is the strongest (near the region A in FIG. 4). However, in normal feedback control, when a steeply inclined portion of the sample surface is scanned, the probe moves to the C region beyond the position B in FIG. There was a problem that the tracking became unstable. In FIG. 4, the horizontal axis represents the displacement of the probe in the Z-axis direction, and the vertical axis represents the atomic force.
[0004]
On the other hand, a tapping mode is known as another measurement method suitable for observing a sample having a very soft surface. In this measurement method, the cantilever is self-excited and the sample surface is struck lightly, and the shape data of the sample surface is obtained by feeding back the position of the cantilever in the Z direction with a servo system so that the tapping condition is constant. For example, it is disclosed by Unexamined-Japanese-Patent No. 7-270434.
[0005]
In this tapping mode, when the probe approaches a sample with an extremely soft surface, the self-excited vibration of the cantilever attenuates as the probe approaches the sample. For this reason, when the amplitude of the cantilever is reduced and the probe is very close to the sample, there is a problem that the SN ratio of the detection signal is lowered and the feedback control itself becomes unstable.
[0006]
[Problems to be solved by the invention]
As described above, when an attempt is made to observe a sample with a very soft surface using the conventional measurement method, the SN ratio of the detection signal is not good in both the contact mode and the tapping mode, and the feedback control is stable. There was a problem that could not be done.
[0007]
The object of the present invention is to solve the above-mentioned problems of the prior art, and a scanning type capable of observing a sample or the like having a very soft surface by feedback control with high detection signal accuracy (SN ratio) and high reliability. It is to provide a probe microscope.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a scanning probe microscope in which the probe is finely moved in the Z direction and scanned in the XY direction while being close to the sample surface to observe the surface shape as follows. Characterized by the measures taken.
[0009]
(1) a first actuator that drives the probe in the Z direction, and feedback-controls the first actuator while comparing a detection signal output from the detection body of the probe with a target value A second actuator for controlling the second actuator so that a detection signal output from the detection body falls within a predetermined range; and a second actuator for driving the probe in the Z direction. And a control system.
[0010]
(2) Excitation means for vibrating the probe on the sample surface was provided in the second control system.
[0011]
According to the above feature (1), since the second actuator can be driven so that the detection signal output from the detection body falls within a certain value width, the probe is moved by the atomic force on the force curve. It becomes possible to stably install and maintain a weak position. For this reason, while controlling in the same manner as in the contact mode, a sample with an extremely soft surface, which was difficult to observe in the conventional contact mode, can be observed with high accuracy without being damaged.
[0012]
According to the above feature (2), not only the responsiveness and followability of the probe to the shape change of the sample surface is improved, but also only the low frequency component representing the sample surface shape is selectively extracted from the detection signal. As a result, an observation image with a high S / N ratio can be obtained.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram of a first embodiment of a scanning probe microscope to which the present invention is applied, and includes first and second control systems.
[0014]
The first control system includes a detector 11 that detects the amount of bending of the cantilever 10, a low-pass filter 12 that removes a high-frequency component from the detection signal Vs output from the detector 11, and a probe 9 provided at the tip of the cantilever 10. A target value setting unit 13 for setting a target value (set voltage VR) relating to the position of the sensor, a comparator 14 for comparing the detection signal Vs with the target value VR, a PI (proportional integration) control unit 15, and a Z-axis control unit. It comprises a first actuator 16 and a current source 17 that is a driver circuit thereof.
[0015]
The first control system constitutes a feedback control system in the Z direction, and the current source 17 causes the current corresponding to the control signal from the PI control unit 15 to flow through the voice coil 21, and the probe 9 and the sample 31. The position of the deflection amount detector 11 is controlled via the spindle 8 so that the distance from the surface of the substrate is always constant. The PI control unit 15 outputs a control signal corresponding to the displacement of the probe 9 in the Z direction to the current source 17 and the image signal amplifier 18. The signal amplified by the image signal amplifier 18 is sent to a monitor device (not shown), and an observation image of the sample surface is displayed on the screen of the monitor device.
[0016]
The second control system includes a deflection amount detector 11 that detects the deflection amount of the cantilever 10, a second actuator 41 that drives the cantilever 10 in the Z direction, and an upper limit of the detection signal Vs output from the deflection amount detector 11. In addition to monitoring the value and the lower limit value, the lower limit value monitoring unit 42, the drive signal generating unit 43 for generating a drive signal for changing the detection signal in the reverse direction when the detection signal reaches the upper limit value or the lower limit value, and the switch means 44 It is composed of
[0017]
The second actuator 41 is disposed closer to the detection body 11 than the first actuator 16. The response of the second actuator 41 is faster than the response of the first actuator 16, and the Q value (resonance selectivity) of the second actuator 41 is much higher than the Q value of the first actuator 16.
[0018]
When the switch means 44 is on, the drive signal generated from the drive signal generator 43 is supplied to the second actuator 41 composed of, for example, a piezo element. The second actuator 41 is installed at a position closer to the cantilever 10 than the first actuator 16, and has a higher frequency characteristic than the first actuator 16, that is, a quick response, and an extremely high resonance frequency than the first actuator 16. Q value of The upper and lower limits are set in conjunction with the set voltage VR set by the target value setting unit 13.
[0019]
The upper and lower limit monitoring units 42 and the drive signal generating unit 43 serve to limit the detection signal Vs, and can be realized by a software configuration or a hardware configuration. FIG. 2 is a flowchart in the case of software configuration. In step S1, it is determined whether or not the switch means 44 is turned on, and if it is off, it waits until it is turned on. When the switch means 44 is turned on, it is determined in step S2 whether or not the detection signal VS is less than or equal to a preset upper limit value VH. If this determination is affirmative, it is determined in step S3 whether or not the detection signal VS is greater than or equal to a preset lower limit value VL. If this determination is affirmative, the drive signal generator 43 outputs a 0-voltage drive signal V0 to the second actuator 41 in step S4.
[0020]
On the other hand, if the determination in step S2 is negative, the process proceeds to step S5, where the drive signal generator 43 outputs a drive signal V1 (for example, + 10V). If the determination in step S3 is negative, the process proceeds to step S6, where the drive signal generator 43 outputs a drive signal V2 (for example, -10V).
[0021]
FIG. 3 is a circuit diagram in the case where the upper and lower limit monitoring units 42 and the drive signal generation unit 43 are realized by a hardware configuration. The circuit of FIG. 3 detects whether or not the detection signal Vs has reached the upper limit value, and outputs the drive signal VP0 (= V1) when the upper limit value is reached, and the detection signal Vs has the lower limit value. And a second system circuit that outputs a drive signal VP0 (V2) when the lower limit value is reached.
[0022]
The first system circuit constitutes a clamper of, for example, + 10V, with a first comparator 51a in which the detection signal VS is input to the non-inverting input terminal and the upper threshold value VH (= VR + ΔVH) is input to the inverting input terminal. 1 zener diode 52 a, first buffer amplifier 53 a, and adder circuit 54. The second system circuit constitutes a clamper of, for example, -10V, with a second comparator 51b in which the detection signal VS is input to the inverting input terminal and the lower limit threshold VL (= VR-.DELTA.VL) is input to the non-inverting input terminal. The second Zener diode 52b, the second buffer amplifier 53b, and the adder circuit 54 are included.
[0023]
The output of the adder circuit 54 is amplified by the drive amplifier 55 to become a drive signal VP0 and supplied to the second actuator 41. The ΔVH and ΔVL are determined in conjunction with the set voltage VR.
[0024]
Next, the operation of the present embodiment will be described with reference to FIGS. In the target value setting unit 13 of FIG. 1, a setting voltage VR for setting the target position of the probe 9 to the position S point where the atomic force is weak on the force curve of FIG. 4 is registered. Further, the upper limit threshold value VH and the lower limit threshold value VL corresponding to the positions S−ΔS and S + ΔS before and after the point S are registered as shown in the figure. S + ΔS must not exceed point B.
[0025]
Next, when the first control system is operated with the switch means 44 turned off, the probe 9 is first subjected to rough Z movement, and the gap between the probe 9 and the sample 31 gradually approaches, and the cantilever is moved. 10 detects an atomic force from the sample 31 sensed by the probe 9. When the cantilever 10 detects that the probe 9 has reached the target position S on the force curve, the Z coarse movement stops.
[0026]
When the Z coarse movement is finished, the switch means 44 is turned on, and the second control system is enabled. Then, the voice coil 22 in the X direction and the voice coil (not shown) in the Y direction are operated to scan the probe 9 on the sample 31 in the X and Y directions. Information on the surface shape of the sample 31 is detected as a deflection amount of the cantilever 10 and is output from the deflection amount detector 11 as a detection signal Vs. The detection signal Vs is input to the low-pass filter 12 and the upper / lower limit monitoring unit 42.
[0027]
The upper and lower limit monitoring unit 42 monitors whether the detection signal Vs has reached the upper limit value VH or the lower limit value VL. When the detection signal Vs reaches the upper limit value VH, the cantilever is set so that the detection signal Vs becomes smaller. The drive signal generator 43 is controlled so that the drive signal Vp0 for operating the motor 10 is output to the second actuator 41. As a result, the second actuator 41 operates, and the probe 9 that has reached the position of S−ΔS in FIG. 4 moves in the direction of the target position S. When the probe 9 moves in the direction of the target position S, the condition of the detection signal Vs ≦ VH is satisfied, and the drive signal applied to the second actuator 41 is stopped.
[0028]
On the other hand, when the detection signal Vs reaches the lower limit value VL, the drive signal generator 43 outputs a drive signal Vp0 for operating the cantilever 10 to the second actuator 41 so that the detection signal Vs becomes large. In response to the drive signal Vp0, the second actuator 41 moves the probe 9 that has reached the position of S + ΔS in the direction of the target position S. When the probe 9 moves in the direction of the target position S, the condition of the detection signal Vs ≧ VL is satisfied, and the drive signal applied to the second actuator 41 is stopped.
[0029]
As a result of the above operation, a high frequency signal is superimposed on the detection signal Vs output from the deflection amount detector 11, and the high frequency signal has a cutoff value that sufficiently blocks high frequency components. 12 is input to the non-inverting input terminal of the comparator 14. Therefore, the first control system can perform conventional feedback control.
[0030]
According to the present embodiment, focusing only on the first control system, an operation similar to that in the conventional contact mode is performed, and the control signal output from the PI control unit 15 reflects the unevenness of the sample surface. Signal. Considering the second control system, the probe 9 is placed on the force curve at a position where the interatomic force is weak, that is, within a range of S ± ΔS centering on S in FIG. 9 is in contact with the sample surface stably with a very weak force, that is, without jumping over the point B on the force curve. Therefore, it becomes possible to observe with high accuracy without damaging a sample whose surface is extremely soft.
[0031]
FIG. 5 is a block diagram of a second embodiment of a scanning probe microscope to which the present invention is applied. The same reference numerals as those described above represent the same or equivalent parts.
[0032]
In the present embodiment, a band-pass filter 49 as an excitation unit is provided in the subsequent stage of the drive signal generation unit 43 of the second control system, with the resonance frequency of the second actuator 41 and a frequency in the vicinity thereof as a pass band. ing. According to such a configuration, the probe 9 can be vibrated at an amplitude within the range between the upper limit value VH and the lower limit value VL and at a frequency near the resonance frequency of the second actuator 41.
[0033]
FIGS. 6 and 7 show a case where the stepped portion having a steep slope is scanned without vibrating the probe 9 as in the first embodiment [FIG. 6], and the probe 9 is moved as in the second embodiment. The relative positional relationship between the probe 9 and the sample surface [Fig. 7 (a)] and the waveform diagram of the output signal of the actuator 41 [Fig. 7 (b)] with respect to the case of scanning while vibrating [Fig. 7]. FIG. 3 is a diagram schematically showing the frequency spectrum [FIG. (C)].
[0034]
In the first embodiment, as shown in FIG. 6 (a), the gap between the probe 9 and the sample surface changes irregularly at a relatively low frequency in response to a change in the shape of the sample surface. Therefore, the signal waveform excluding the frequency component corresponding to the stepped portion of the output signal of the actuator 41 is as shown in FIG. For this reason, the frequency spectrum includes a low frequency component below the cut-off frequency fcut of the low-pass filter 12 as shown in FIG.
[0035]
On the other hand, in the second embodiment of the present invention, as shown in FIG. 7A, the probe 9 continues to vibrate at a constant frequency regardless of the shape change of the sample surface. Therefore, the waveform excluding the frequency component corresponding to the step portion in the output signal of the actuator 41 is as shown in FIG. For this reason, the frequency spectrum is based on the center frequency fm of the band-pass filter 49 as shown in FIG. 5C, so that if the cut-off frequency fcut of the low-pass filter 12 is set as shown, noise Can be greatly reduced.
[0036]
In the second embodiment described above, the band pass filter 49 has been described as being incorporated in the second control system as a vibrator that vibrates the probe 9. However, the present invention is not limited to this. Alternatively, a low-pass filter having a large Q value may be used.
[0037]
【The invention's effect】
As described above, according to the present invention, the following effects are achieved.
(1) Since the first actuator is feedback-controlled and the second actuator is driven so that the detection signal output from the detection body is within a certain value range, the probe is placed on the force curve. It will be possible to stably install and maintain at a position where the atomic force is weak. For this reason, it is possible to observe a sample having a very soft surface, which is difficult to observe in the conventional contact mode, with high accuracy without damaging the sample, although the control is similar to the contact mode.
(2) Since the detection body is not self-excited vibration, the vibration amplitude does not change even if it is very close to the detection body sample. For this reason, even when the probe is very close to the sample, the SN ratio of the detection signal does not deteriorate, and the detection characteristics can be stabilized.
(3) Due to the function of the second control system, even if a highly viscous substance adheres to the sample surface, the frequency response of the detection body changes, the first control system becomes unstable, or control It is possible to prevent the loop gain from being increased, so that the sample can be observed stably.
(4) The follow-up error in the first control system that occurs during scanning of the sample surface, that is, during observation is absorbed by the function of the second control system, so that the followability of the entire control system is improved. For this reason, compared with the conventional contact mode or tapping mode, high-speed scanning becomes possible and observation time can be shortened.
(5) Scanning the sample surface while vibrating the probe improves the probe's responsiveness (trackability) to changes in the shape of the sample surface. Furthermore, only low-frequency components that represent the sample surface shape are used. Since it can be selectively extracted, an observation image with a high S / N ratio can be obtained.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a first exemplary embodiment of the present invention.
FIG. 2 is a flowchart showing a specific example of a lower limit monitoring unit and a drive signal generation unit in the upper part of FIG.
3 is a circuit diagram showing another specific example of the lower limit monitoring unit and the drive signal generation unit in the upper part of FIG. 1. FIG.
FIG. 4 is a diagram showing a force curve.
FIG. 5 is a block diagram showing a configuration of a second exemplary embodiment of the present invention.
FIG. 6 is a signal waveform diagram showing the operation of the first embodiment.
FIG. 7 is a signal waveform diagram showing the operation of the second embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 9 ... Probe, 10 ... Cantilever, 11 ... Deflection detector, 12 ... Low pass filter, 13 ... Target value setting part, 14 ... Comparator, 15 ... PI control part, 16 ... 1st actuator, 17 ... Current source , 41... 2 actuator, 42... Upper and lower limit value monitoring unit, 43... Drive signal generating unit, 44.

Claims (10)

In a scanning atomic force microscope that observes the surface shape by finely moving the probe provided on the cantilever in the Z direction and moving it close to the sample surface, the target value for the position of the probe is set. a target value setting unit that, first an actuator, the first actuator while the detection signal output from the deflection amount detector of the cantilever compared to the target value for driving the probe in the Z direction A detection signal output from the deflection amount detection body, and a first control system that feedback-controls, and a second actuator that drives the probe in the Z direction and has a faster response than the first actuator. There scanning atomic force microscope being characterized in that and a second control system for controlling the second actuator to fit within a predetermined range before and after said target value. 2. The scanning type according to claim 1, wherein the target value is in the vicinity of a position where the interatomic force on the force curve works most weakly, and the predetermined range is a range that does not exceed the position where the interatomic force works weakest. Atomic force microscope. 2. The scanning according to claim 1, wherein the first control system includes a low-pass filter that receives the detection signal, and sets the target value for a signal output from the low-pass filter. Type atomic force microscope. The second control system compares the detection signal with a predetermined upper limit value and a lower limit value, and the second actuator causes the detection body to approach the sample so that the detection signal does not fall below the lower limit value. The scanning atomic force microscope according to claim 1, wherein the detection body is moved away from the sample so that a detection signal does not exceed an upper limit value. It said second actuator is a scanning atomic force microscope according to any one of 4 to claims 1, characterized in that it is located closer to the deflection amount detecting body than the first actuator. Said second actuator Q value (resonance selectivity) of the first than the Q value of the actuator, wherein the very high claims 1 to scanning atomic force microscope according to any one of 4. 2. The scanning atomic force microscope according to claim 1, wherein the second control system includes a vibrating unit that vibrates the probe with an amplitude within the predetermined range. 8. The scanning atomic force microscope according to claim 7 , wherein the vibration means vibrates the probe at a frequency near the resonance frequency of the second actuator. The scanning atomic force microscope according to claim 7 or 8 , wherein the excitation means is a low-pass filter having a large Q value (resonance selectivity). Said vibrating means, said second actuator scanning atomic force microscope according to claim 7 or 8, characterized in that the vicinity of the resonance frequency is a band-pass filter having a pass band of.

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