JP2000146809A - Method for making image of sample surface in probe microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an image of unevenness, etc., on a sample surface with good reproducibility and with high accuracy by performing control in a non- contact and stable manner. SOLUTION: Vibration of 1 nm or less is applied to a tip 11 of a cantilever 1, and the distance between the tip 11 and a specimen 10 is feedback-controlled so that a force gradient becomes a constant negative value in an attractive force region where the tip 11 and the specimen 10 interact with each other while the tip 11 is caused to scan a surface of the specimen 10, and an image of the surface of the specimen 10 is made by using a signal of the feedback control.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for creating an image of a sample in an atomic force microscope, in particular, of a scanning probe microscope. Under the gravitational region, the vibration of the tip is measured very accurately to create an image of the unevenness and morphology present on the surface of the sample, and the distribution of the chemical species. Magnetism, static electricity, piezoelectricity, etc.
It can be used to drive all kinds of cantilevers.

The most important component of an atomic force microscope is a cantilever having a thin tip attached to the tip, and the force acting between the tip and the sample can be calculated by the movement of the free end of the cantilever. Most atomic force microscopes are only used to detect the above forces, but with the atomic force microscope of the present invention it is also possible to apply forces directly from behind the tip.

One of the features of the present invention is that a force for oscillating the cantilever is applied directly to the tip, and the stiffness (force gradient) of the interaction between the tip and the sample is maintained, not the absolute value of the force. However, the image is created by scanning the surface of the sample with the chip. In the present invention, there is novelty in using this stiffness signal in the range between the negative force gradient and the attractive force region. A method using such a stiffness signal is a conventional method for controlling the absolute value of the force. Could not be realized.

[0004]

2. Description of the Related Art A stiffness measurement caused by an interaction between a chip and a sample has been described in literature for a while. Usually for this purpose, the sample is vibrated with a large amplitude or the tip is vibrated with a large amplitude.
In any case, the region in the AC (alternating current) signal where the force acting on the chip can be controlled is the repulsive region and the positive force region (see FIG. 1). In these examples, as shown in FIG. 1, since the stiffness is always measured in the positive force region, there are two points giving a constant stiffness value. Therefore, in this method, it is difficult to use the stiffness signal as a feedback signal.

In the prior art, the approach to the set point is irreversible and hysteresis appears in the distance curve between the tip and the sample surface of the force or force gradient. This hysteresis is indicative of deformation of the sample surface and / or tip. This deformation is caused by the contact between the chip and the sample in the conventional control.

The stiffness of the cantilever is
It is common to keep it low to increase sensitivity.
Although it is possible to choose a negative force set point, it is not possible to reach this point reversibly in the case of a soft cantilever.

[0007] The tapping mode or the intermittent contact mode in which the chip hits the sample surface is performed in one cycle over the entire interaction area between the chip and the sample.
An AC method at or near the resonance point where the tip vibrates with a large amplitude to search. The vibration amplitude of the tip is used as a feedback parameter, and the tip scans the surface of the sample while making contact with the surface of the sample.

The amplitude of the probe vibration at the set point is 1
Above 0 nm, the energy of the lever arm is set to be much greater than the energy lost to hit the sample surface in each cycle. The main purpose of this is to reduce the influence of the force acting from the side when imaging the sample surface with the tip.

[0009] The effects of stiffness and tapping in the state where the tip and sample are in contact have a double effect on the observed value of the amplitude of the tip vibration, and the cantilever in each cycle changes the surface elasticity. Since most data on interaction potential, including deformation and possibly plastic deformation, is collected, it is difficult to extract accurate quantitative information about chip-sample interaction.

[0010] Two imaging modes for oscillating the cantilever at or near the resonance point in an ultra-high vacuum are used.

The first method is to vibrate the entire tip of the cantilever using a piezo element and measure the amplitude of the vibration of the cantilever excited near the resonance point. The amplitude of the vibration is generally 10 nm or more, but there are reports that images were created with a free lever amplitude of 15 Å (between peaks). However, in addition to the shift of the resonance frequency, the variation of the Q-value also affects the amplitude of the vibration, so that a clear relationship cannot be determined between the amplitude of the vibration and the force gradient of the interaction.

Since this method uses the vicinity of the resonance frequency, a new problem arises because the Q-value of most cantilevers becomes high in an ultra-high vacuum. In this case, the dissipation is small, and as a result, the relaxation time is on the order of seconds, so that the attenuation of the vibration amplitude is extremely slow, so that an appropriate scanning time cannot be obtained.

The second method is a method using a frequency modulation method, in which the cantilever becomes a factor for determining the frequency of the constant amplitude oscillator. The frequency of the oscillator output is immediately modulated by the fluctuation of the force gradient acting on the cantilever. In this method, a vibration amplitude of 20 nm (peak-to-peak) or more is generally used.

[0014]

Although both of the above methods provide images with atomic resolution, the sampling range of each vibration of the chip is large, so that accurate information regarding the interaction between the chip and the sample surface is obtained. It is difficult to obtain quantitative information. For the same reason, it is difficult to definitively elucidate the imaging mechanism or determine the minimum distance between the tip and the sample at the turning point of the vibration. This area is still under discussion.

[0015]

According to the method of the present invention, the stiffness of the cantilever is set larger than the maximum force gradient (MAX point in FIG. 1), and the set point is determined from the amplitude of the vibration of the cantilever. Has been resolved. This is because the stiffness of the cantilever is MA in FIG.
If the value is smaller than the point X, two stable points exist at the same time (for example, a line indicating the non-contact mode sampling region in FIG. 1), so that the point becomes mechanically unstable and becomes uncontrollable. Most importantly, the set point can be reached without entering the positive force region of the interaction between the tip and the sample. The required sensitivity is thus obtained using the direct tip vibration method.

Therefore, the present invention applies a vibration of 1 nm or less to the tip of the cantilever, so that the force gradient between the tip and the sample becomes a negative constant value in the attractive region of the interaction between the tip and the sample. Providing a method of creating an image of a sample surface in a probe microscope, wherein the distance is feedback-controlled, a chip is scanned on the sample surface, and an image of the sample surface is created using the feedback control signal at that time. Things.

Further, the present invention provides a method in which a tip of a cantilever is provided with a vibration of 1 nm or less, and a force gradient between the tip and the sample is set to a negative constant value in an attractive region of interaction between the tip and the sample. Feedback control of the distance of the tip, feedback control of the amplitude or phase of the vibration so that the amplitude or phase of the tip vibration becomes constant, and feedback control of the amplitude or phase at that time by scanning the tip over the sample surface An object of the present invention is to provide a method of creating an image of a sample surface in a probe microscope, wherein an image of the sample surface is created using a signal.

The present invention also provides that when the force gradient is controlled to a negative constant value, the interaction between the tip and the sample is attractive and the distance between the two is controlled to only one point. An object of the present invention is to provide a method of creating an image of a sample surface in a probe microscope, which is a feature of the present invention.

Further, the present invention is characterized in that, in the control for setting the force gradient to a negative constant value, an image of the sample surface is created by using a current that changes depending on the distance between the chip and the sample. An object of the present invention is to provide a method for creating an image of a sample surface in a probe microscope.

Further, the present invention provides a sample for a probe microscope, characterized in that the tip of the tip is pointed to one atom by the vibration of the tip and one atom is fixed to the tip, and measurement is performed in that state. It provides a method for creating a surface image.

Further, according to the present invention, at the same time as controlling the force gradient to be a negative constant value, a torsional vibration is applied to the tip to generate a lateral motion, and the frictional force gradient at that time is detected. A method of creating an image of a sample surface in a probe microscope, wherein the method creates an image of the sample surface using the method.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below. FIG. 2 shows a force gradient in a negative range in an interaction between a cantilever tip and a sample in a probe microscope. It can be seen that the minimum value of the force curve can be reversibly passed in an attractive region in which the sample and the sample act in the suction direction. This indicates that the negative force gradient and the attractive force region described above can be effectively used because little or no permanent deformation is left on the sample surface.

In this region, since the interaction between the tip and the sample has a steep gradient, it is expected that an image having an atomic level resolution can be obtained in this mode if an appropriately sharp tip is used. .

In particular, FIG. 1 shows a new imaging area in which the imaging method of the present invention can be implemented with a negative range of force gradients in the interaction between the tip and the sample.

In order to utilize this region in a stable manner, the stiffness between the tip and the sample in the experimental setup must be greater than the highest force gradient encountered during approach (points in FIGS. 1 and 2). MAX). This can be achieved by using levers with large mechanical stiffness or levers with enhanced stiffness in the feedback.

To confirm that the tip is actually in the attractive region with a negative force gradient, the free end of the lever is vibrated and the phase or amplitude of the vibration is measured. If this oscillation is lower than the resonance frequency of the lever and the resonance shift due to the interaction potential does not affect the amplitude, the measured amplitude is A / A0 = k / (k-dF / dz)
Is directly related to the interaction force gradient.

Where A is the measured value of the lever amplitude, A0 is the amplitude of the free lever, k is the stiffness of the lever, dF
/ Dz is the force gradient.

The above-described direct relationship greatly simplifies the quantitative analysis of the measured data, and greatly improves the detection imaging method based on the frequency modulation method.

At a distance from the resonance frequency, the vibration can be suppressed even under an ultra-high vacuum where the Q value of the lever becomes extremely large. This method is unique in that it is the only off-resonance imaging technique demonstrated under ultra-high vacuum.

Another advantage of using a cantilever having a large stiffness as a force sensor is that the distance between the chip and the sample is externally increased even when the thermal vibration is extremely small and the amplitude of the driven vibration is less than angstrom. It is determined by the applied vibration. This micro-vibration technique is also useful for measuring second distance-dependent parameters, such as tunneling and electrical conduction.

The amplitude or phase of the vibration in the cantilever can be monitored using a lock-in amplifier shown in FIG. The use of dynamic measurements significantly improves the signal / noise ratio and sensitivity. A signal proportional to the amplitude or phase of the vibration can be compared to a set point, and the position of the cantilever or sample can be adjusted using a feedback signal generated from the difference between the set point and the measured value.

FIG. 4, shown as a photograph, shows an image of a stable sample of the gold surface (deposited gold sputtered / annealed on mica under ultra-high vacuum) created at the set points of FIG. First
The second image, which enlarges a selected portion of the images, shows that the new imaging technique produces a reproducible image.

FIG. 3 is a diagram schematically showing a configuration example for carrying out the method of the present invention. In the figure, cantilever 1
Has a tip 11 and a magnet 12 at the tip of a lever 13. The coil (electromagnet) 9 with an iron core generates a magnetic field by a control current from the coil driver 5, applies a force to the magnet 12 with the magnetic field, and reciprocates the tip 11 with respect to the sample 10 with an amplitude of 1 nm or less. Vibrate. The coil driver 5 outputs a control current based on the vibration signal generated by the function generator 3. The piezo tube 16
The control current from the piezo driver 8 causes a minute fluctuation in the XYZ directions to control the distance between the sample 10 placed at the tip and the chip 11 and to scan the chip 11 on the surface of the sample 10.

The vibration displacement of the chip 11 is measured by the minute fluctuation measuring device 2 composed of the light emitting diode 21 and the light receiving diode 22, and the measured data is output to the lock-in amplifier 4. The lock-in amplifier 4 calculates a force gradient (differentiation of vibration displacement) based on the measurement data from the minute fluctuation measuring device 2 and the vibration signal from the function generator 3, and outputs the force gradient to the regulator 6.
The set point S of the force gradient is set in the regulator 6 in advance as a negative constant value (point S in FIG. 1 and set point in FIG. 2). It compares the force gradient data from the amplifier 4 and outputs a control signal corresponding to the difference to the piezo driver 8 as a feedback control signal. With this feedback control signal, the force gradient is controlled to a constant negative value (point S), and the tip 11
The distance between the sample and the sample 10 is also kept at the distance D (FIG. 1).

As described above, the force gradient has a negative constant value (point S).
In this case, the interaction between the tip 11 and the sample 10 remains in the attractive region, and the distance between them is controlled to only one point (point C in FIG. 1). The point O in FIG. 1 is a point at which the distance between the chip 11 and the sample 10 becomes “0”.

When the above-described force gradient control is performed at each point on the surface of the sample 10 by scanning, the feedback control signal at that time comes to represent unevenness information on the surface of the sample 10. Therefore, an image of the surface of the sample 10 can be created using the feedback control signal.

In the above description, the feedback control signal is output to the piezo driver 8. However, the feedback control signal may be output to the coil driver 5 at the same time.

That is, the regulator 6 compares the force gradient set point S, which is set in advance as a negative constant value, with the force gradient data from the lock-in amplifier 4, and feedback-controls a control signal corresponding to the difference. The signal is output to the piezo driver 8 as a signal, and the distance between the chip 11 and the sample 10 is controlled so that the force gradient becomes a constant negative value. Further, the regulator 6 outputs to the coil driver 5 a control signal corresponding to a difference between the amplitude data or phase data of the chip 11 from the lock-in amplifier 4 and a predetermined amplitude or phase value. Feedback control is performed so that the amplitude or the phase becomes constant, and the chip 11 is scanned on the surface of the sample 10. Then, an image of the surface of the sample 10 is created using the feedback control signal of the amplitude or phase at that time.
In this case, the distance between the chip 11 and the sample 10 is as shown in FIG.
, And the amplitude or phase of the chip 11 is controlled to a constant value. Therefore, this feedback control signal includes not only the information on the irregularities on the surface of the sample 10 but also the information on the adhesiveness, and it is possible to accurately obtain the information on the non-contact adhesiveness which has been difficult to obtain conventionally. it can.

As described above, since the position of the chip 11 is controlled at the point C in the attractive region just before entering the repulsive force region under the vibration of a small amplitude, highly sensitive control can be performed. In addition, the set point S in this case is set to a negative constant value of the force gradient (point S), and such a set point S is simply set over the entire distance between the tip 11 and the sample 10. Since only one point exists, high-sensitivity control at a small distance D between the chip 11 and the sample 10 is possible from this point as well.

In the above-described feedback control of the vibration of the tip 11, if a tip having a large tip is used, the area of interaction becomes large, and the attractive force becomes extremely large. Contact between the sample and the surface of the sample 10 (FIG. 5)
(A) (b)). In this case, the tip 11 is forcibly pulled back by the feedback control signal (FIG. 5).
(C). At that time, the atom group of the contact portion continues to grow like a so-called gum, the vicinity of the tip becomes gradually sharper, and at the moment when the tip is finally cut off, the chip 11 in which one atom 11a actually stays at the tip is completed (FIG. 5). (D), that is,
In this embodiment, since vibration of 1 nm or less is continuously applied to the tip 11, even if the tip of the tip 11 is round and large as described above, finally, only one atom 11a is added to the tip. Can stay. Thereafter, as the tip of the tip 11 becomes smaller, the attractive force becomes appropriate, and the measurement can be continued without contact. Therefore, the measurement can be performed with higher accuracy at the atomic level.

In the above description, the image is created using the feedback control signal for controlling the vibration of the chip 11, but the surface of the sample 10 is changed using the current that changes depending on the distance between the chip 11 and the sample 10. May be created. The current can be detected by using a highly conductive cantilever and a tip, and measuring the current flowing through the tip and the sample surface in the same manner as in a normal scanning tunneling microscope. At this time, it is important that the amplitude of the chip vibration is small enough that the current continues to flow throughout the entire vibration period. If the amplitude of the chip vibration is small, the current can be reduced using a lock-in amplifier. It is detected as an average over one cycle. As described above, by using the current to create an image, it is possible to obtain information on the distribution state and non-uniformity of substances having different electric conductivity.

In the above description, the tip 11 is vibrated in the direction of reciprocating movement with respect to the sample 10. In addition to the reciprocating movement, the cantilever 1 is subjected to torsional vibration by thermal vibration, electrostatic force, and magnetic force. , The lateral motion is caused in the tip 11, the gradient of the frictional force at that time is detected, and an image of the sample surface may be created using the gradient of the frictional force. In order to simultaneously apply the reciprocating vibration and the torsional vibration, another magnet whose magnetic pole is orthogonal to the magnetic pole of the magnet 12 in FIG. 3 is provided on the lever 13 near the chip 11, and the interaction between the magnet 12 and the electromagnet 9 is performed. , A reciprocating vibration is generated, and torsional vibration is generated by an interaction between another magnet and the electromagnet 9. In this case, as shown in FIG. 3, the photodiode 22 is composed of four combinations of A, B, C, and D, and the detection signal of each photodiode is used. ) − (C + D)}, and the torsional vibration can be detected and calculated by {(A + C) − (B + D)}.

As described above, by using the frictional force gradient for creating an image, it is possible to obtain physical property information on the friction on the surface of the sample 10.

[0044]

As described above, the present invention provides a scanning probe microscope, particularly an atomic force microscope, in which an oscillating force is directly applied to a tip to maintain a constant stiffness of interaction between the tip and a sample. It is possible to scan the surface of the sample with the tip.

Further, a vibration of 1 nm or less is applied to the tip of the cantilever, and the distance between the tip and the sample is set so that the force gradient becomes a constant negative value in the attractive region of the interaction between the tip and the sample. Since the feedback control is performed, the chip and the sample do not come into contact with each other, so that the control can be performed in a stable state. In addition, since the distance between the two is controlled to only one point, control in a stable state is possible from this point as well, and non-contact is maintained, so that images such as unevenness on the sample surface can be created with high reproducibility and high accuracy. It can be carried out.

Further, a vibration of 1 nm or less is applied to the tip of the cantilever, and the distance between the tip and the sample is fed back so that the force gradient becomes a constant negative value in the attractive region of the interaction between the tip and the sample. Control, and furthermore, the feedback control of the vibration amplitude or phase is performed so that the vibration amplitude or phase of the chip becomes constant, similar to the case where the distance between the chip and the sample is feedback-controlled as described above. Therefore, the chip and the sample do not come into contact with each other, so that control can be performed in a stable state. In addition, since the distance between the two is controlled at only one point, control in a stable state is possible, and since non-contact is maintained, images of irregularities on the sample surface can be created with high reproducibility and high accuracy. Further, the feedback control signal includes not only the information on the unevenness of the sample surface but also the information on the adhesiveness, so that it is possible to accurately obtain the information on the adhesiveness in the non-contact area, which was difficult to obtain conventionally. it can.

Further, since an image of the sample surface is created by using an electric current which varies depending on the distance between the chip and the sample, information on the distribution state and non-uniformity of substances having different electric conductivity can be obtained. Can be.

In addition, a chip in which one atom is attached to the tip can be formed, and the measurement is performed using this chip, so that the measurement can be performed with higher accuracy and at the atomic level.

Further, since the torsion vibration is simultaneously applied to the cantilever and an image of the sample surface is created using the frictional force gradient at that time, it is possible to obtain physical property information on the friction of the sample surface.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a typical operating range of a stiffness measurement, a tapping mode, and a non-contact mode in a curve in which a force interaction is plotted with respect to a distance between a tip and a sample.

FIG. 2 is an experimentally measured force gradient curve according to the present invention, in which a set point at which imaging was performed is indicated on the force gradient curve, and a cycle of approach / retreat is plotted; FIG. 2 is a conceptual diagram showing that reversible probing can be performed without hysteresis.

FIG. 3 is a block diagram showing an embodiment of a feedback circuit for creating an image according to the present invention.

FIG. 4 is a photomicrograph showing an image of gold on mica created in a new imaging mode as an example of a feedback method.

FIG. 5 is an explanatory diagram of a situation in which one atom stays at the tip of a chip.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 cantilever 2 minute fluctuation measuring device 3 function generator 4 lock-in amplifier 5 coil driver 6 regulator 8 piezo driver 9 coil with core (electromagnet) 10 sample 11 chip 12 magnet 13 lever 16 piezo tube 21 light emitting diode 22 light receiving diode

Continuation of the front page (71) Applicant 3990009531 International Business Machines Corporation INTERNAL BUSINESS ESS MASCHINES CORPORATION 10504, Armonk, NY 10504 United States of America (No address) (72) Inventor Susan Philippa Jarvis 1-1-4 Higashi, Tsukuba City, Ibaraki Pref. Institute of Industrial Science and Technology, Research Institute of Industrial Technology Integration Technology Research Association, Angstrom Technology Research Institute (72) Inventor Urs Durrick Swiss C 8803 Ruschlicon Zoimersstrasse 4 International Business Machines Corporation Research Division Zurich Institute (72) Inventor Mark Alfred Lands Tsukuba, Ibaraki East 1-1-4 Agency of Industrial Science and Technology Industrial Science and Technology, Interdisciplinary Institute in technology research union Angstrom technology research mechanism within the (72) inventor Tokumoto Hiroshi Higashi, Tsukuba, Ibaraki, 1-1-4 Agency of Industrial Science and Technology fusion region Institute

Claims (6)

[Claims] 1. A vibration of 1 nm or less is applied to a tip of a cantilever, and a distance between the tip and the sample is fed back so that a force gradient of the interaction between the tip and the sample becomes a negative constant value. A method for creating an image of a sample surface in a probe microscope, comprising controlling and scanning a chip on the sample surface, and creating an image of the sample surface using a feedback control signal at that time. 2. A vibration of less than 1 nm is applied to the tip of the cantilever, and the distance between the tip and the sample is fed back so that the force gradient becomes a constant negative value in the attractive region of the interaction between the tip and the sample. Control, and feedback control of the amplitude or phase of the vibration so that the amplitude or phase of the vibration of the chip is constant, and scanning the chip on the sample surface and using the feedback control signal of the amplitude or phase at that time A method for creating an image of a sample surface in a probe microscope, wherein the method creates an image of the sample surface. 3. When the force gradient is controlled to a negative constant value, the interaction between the tip and the sample is attractive, and the distance between the two is controlled to only one point. Item 3. The method for creating an image of a sample surface with a probe microscope according to Item 1 or 2. 4. The method according to claim 1, wherein in the control of setting the force gradient to a negative constant value, an image of a sample surface is created using a current that changes according to a distance between the chip and the sample. 3. The method for creating an image of a sample surface in the probe microscope according to any one of 3. 5. The method according to claim 1, wherein the tip of the tip is pointed at one atom by the vibration of the tip, and one atom is held at the tip, and the measurement is performed in that state. 2. A method for creating an image of a sample surface using a probe microscope according to item 1. 6. At the same time as controlling the force gradient to be a negative constant value, a torsional vibration is applied to the tip to generate a lateral motion, and a frictional force gradient at that time is detected. The method for creating an image of a sample surface in a probe microscope according to any one of claims 1 to 5, wherein an image of the surface is created.