JP2000039390A - Non-contact atomic force microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To alleviate an operating load of an operator by monitoring a correlation of reciprocating images in the case of scanning a sample, and adding a function of automatically deciding optimum conditions in a non-contact mode measurement, thereby preventing a damage of a probe end. SOLUTION: First, resonance characteristics of a cantilever 1 are measured, and an amplitude value of the cantilever 1 is set as a set point. A sample stage 6 is moved until it arrives at the set point, and a probe 1a is approached to a sample. Thereafter, scanning is started. The set point is reduced while monitoring a correlation of reciprocating AFM signals (AFM images), and the probe 1a is approached to the sample until the correlation is taken between the reciprocating signals by a piezo-scanner 3. Eventually, whether a discontinuous contact state is existed or not is checked. If an RMS value of an error signal is a specified level or less, the set points are increased and the probe 1a is remotely separated from the sample. In the case of the following, an AFM measurement is actually started.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an atomic force microscope (AFM) for measuring a surface shape of a sample in a non-contact mode.

[0002]

2. Description of the Related Art First, the configuration of a scanning probe microscope will be described with reference to FIG. That is, it comprises a cantilever 1 having a probe 1a near the tip, a holder 2 having a piezo for cantilever excitation, a piezo scanner 3, a support 4, an optical system 5, a sample stage 6, a wafer loader 7, a frame 8 and a table 9. The deflection of the cantilever 1 is detected by an optical lever method using a semiconductor laser mounted on the holder 2 and an optical detector. The piezo scanner 3 utilizes a piezo element, and includes x, y, and z.
Each direction can be driven by several tens of microns.

Next, the principle of the non-contact mode of the atomic force microscope (AFM) will be described with reference to FIGS. As the sharp probe near the tip of the cantilever approaches the sample surface, the potential of the probe with respect to the sample changes like a potential curve 21 as shown in FIG. Here, in the hatched area (attraction area 22), an attractive force acts between the probe and the sample. Further, when the probe is brought closer to the sample, a repulsive force starts to act between them (repulsive force region). Consider the case where the cantilever is excited by, for example, a laminated piezo element. When excitation of a constant intensity is applied to the cantilever while changing the frequency, the resonance characteristic shown in FIG. 3 is obtained. When the excitation frequency matches the resonance frequency 26 of the cantilever, the cantilever vibrates most. Here, when the cantilever approaches the sample, when the attractive force acts between the cantilever and the sample, the resonance frequency of the cantilever shifts to a lower side, and the resonance characteristics (attraction force shown in FIG. (Resonance characteristics when received). At this time, the excitation frequency 27 is set higher than the resonance frequency 26. That is, although the vibration is performed at the amplitude represented by the point 28a on the downward-sloping curve, the vibration amplitude of the cantilever is reduced to the amplitude shown at the point 28b due to the shift of the resonance frequency of the cantilever. According to the above principle, the attractive force acting on the cantilever and the distance between the probe and the sample can be detected. When the vibration amplitude of the cantilever is large, it is difficult to perform the non-contact operation. This will be described with reference to the cantilever shakes 23a and 23b in FIG. When the vibration amplitude of the cantilever is large, for example, the cantilever vibrates in a range indicated by reference numeral 23a, and in this case, the cantilever can receive an attractive force only for a part of time. Further, since the region where the attractive force that can be actually detected acts is narrow, when the vibration amplitude is large, the vibration range easily reaches the repulsion region, and the discontinuous contact mode occurs. Therefore, it is desired to reduce the vibration amplitude of the cantilever in order to perform the non-contact operation. Conventionally, when measuring the surface shape of a sample in a non-contact mode, the operator has attempted to optimize parameters such as the excitation intensity of the cantilever and the distance between the probe and the sample while viewing the AFM measurement shape (AFM image). . In addition, since it is relatively difficult to perform AFM measurement in a completely non-contact mode, the imaging is usually performed in a discontinuous contact mode in which a cantilever forcedly vibrated near a resonance frequency contacts a sample surface at regular time intervals. Had gone.

[0004]

In a discontinuous contact state in which the cantilever contacts the sample surface at regular time intervals, the tip of the probe continuously hits the sample surface, and the tip of the probe is damaged. This damage becomes a problem particularly when a probe sharpened by, for example, FIB processing is used, and there is a problem that the life of the probe capable of obtaining a good image is extremely shortened. For this reason, imaging in a non-contact state is required. However, since the non-contact mode requires delicate adjustment of parameters, it has conventionally depended on the know-how of an operator. The present invention has been made in view of the above problems, and adds a function to an atomic force microscope to automatically determine an optimal condition in a non-contact mode measurement to prevent damage to a tip of a probe and to prevent an operator from suffering damage. The purpose is to reduce the burden associated with measurement operations.

[0005]

Means for Solving the Problems The present invention (claim 1) provides:
"In a non-contact atomic force microscope that scans the surface of a sample and measures its shape by feedback control so as to keep the oscillation amplitude near the resonance frequency of the cantilever constant, the correlation between the reciprocating images in the scanning is monitored. And a search mechanism for automatically searching for the optimum measurement condition. "

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 5A to 5D show AFM images (AF) during reciprocal scanning according to an embodiment of the present invention.
(M signal), (e) and (f) are diagrams showing error signals. FIG. 4 is a schematic diagram of a standard sample according to the embodiment of the present invention. As shown in the figure, the standard sample is provided with, for example, triangular irregularities continuously in the x direction, and has a step of irregularities of 300 nm and a period of 2 μm. FIG. 5 (a) to (d)
FIG. 4 shows an AFM signal and an error signal obtained when FIG. 4 is scanned in the x direction under some conditions. FIG.
(A) is an example of the reciprocating AFM signal when the sample is separated from the cantilever, and the solid line indicates scanning from the left and the dotted line indicates scanning from the right. In this case, no force acts between the probe and the sample, and the AFM signal is completely independent of the shape of the sample. Therefore, no correlation is seen between the reciprocating AFM images. In such a case, the probe is brought closer to the sample by reducing the set point (the amplitude value of the cantilever to be kept constant by the feedback system). FIG. 5B shows an example of an AFM signal when the cantilever and the sample are slightly separated from each other but tracing the shape for the time being. The solid line indicates scanning from the left.
Dotted lines indicate scanning from the right. In this case, since the force acting between the probe and the sample is weak, the feedback system cannot follow the shape of the sample. In such a case, the gain of the feedback system is increased or the scanning speed is reduced. FIG. 5C shows the AFM signal when the shape of the sample is traced, and the reciprocating signals match well. In such a case, it is necessary to determine whether the state is the non-contact state or the discontinuous contact state. This can be monitored by an error signal (a signal indicating a deviation from the set value of the cantilever amplitude). In the case of the non-contact state, FIG.
The error signal is very stable as shown in FIG. On the other hand, in the case of the discontinuous contact state, the error signal is unstable and the noise level is large as shown in (f). This is considered to be attributable to the variation in adsorption force when the cantilever separates from the sample. If the cantilever is in discontinuous contact, increase the set point (increase the cantilever amplitude setting) to move the probe away from the sample, or reduce the excitation intensity to reduce the cantilever amplitude. Let it. Also, whether the cantilever is in a non-contact state or a discontinuous contact state can be determined by monitoring the phase of the vibration of the cantilever,
It is also possible to monitor according to the direction of the shift of the resonance frequency. FIG. 5D shows an AFM signal when the feedback system is in an oscillation state. This state can be monitored, for example, by detecting the amplitude of the signal after passing the AFM signal through a high-pass filter. In such a case, the gain of the feedback system is reduced. Less than,
6 (a), 6 (b) and 6 (c), the process for determining the optimum condition according to the present embodiment will be described. In FIG. 6A, the cantilever has a V-shape and has a resonance frequency of about 40.
0 kHz, an alternating voltage having a constant amplitude with respect to the piezo for excitation is applied while continuously shifting the frequency,
By detecting the cantilever amplitude corresponding to the frequency, the resonance characteristics of the cantilever as shown in FIG. 3 are measured (resonance characteristic measurement 51 of the cantilever). The excitation intensity is set so that the vibration amplitude at the resonance frequency of the cantilever becomes a set value, for example, the amplitude is 20 nm, and the vibration amplitude is 18 nm on the frequency side higher than the resonance frequency of the cantilever.
Set the excitation frequency to Also, the cantilever amplitude is set to 10 nm as a set point (excitation frequency and excitation intensity setting, set point setting 5).
2). The sample stage 6 is moved until the probe 1a and the sample approach each other until the set point set in the previous step is reached (auto approach 53). At this time, since the vibration amplitude of the cantilever decreases due to the influence of air damping,
The auto-approach 53 where the probe is still away from the sample ends. Thereafter, scanning in the x direction is started at a scanning speed of 1 Hz and a scanning range of 8 μm, for example (scan start 5
4). The set point is reduced while monitoring the correlation between the reciprocating AFM signals, and the probe is moved closer to the sample by the piezo scanner 3 until the correlation between the reciprocating AFM signals is obtained (approaching operation 55). Here, the correlation may be obtained not with the reciprocating AFM signal but with a known sample shape. Further, the correlation state is obtained by taking the AFM signal into the control computer by the AD converter and performing software for determining the correlation state. Next, the process of reducing the vibration amplitude of the cantilever while maintaining the correlation between the reciprocating AFM images will be described with reference to FIG. A round trip
The FM signal is constantly monitored to check for a correlation (correlation check 61). If there is no correlation, the set point is decreased and the probe is brought closer to the sample (set point decrease 62). If there is a correlation, the excitation intensity is reduced (the excitation intensity is small 63). Here, the excitation intensity is a set value,
For example, while being compared with the amplitude of 1 nm (comparison operation 64), the amplitude is reduced until the set value is reached. When the set value is reached, the process of decreasing the vibration amplitude ends (end operation 65). In this process, feedback control is always performed so that the gain of the feedback system is set to the maximum value within a range where oscillation does not occur. This feedback system is shown in FIG. AF
By passing the M signal through a high-pass filter and performing amplitude detection, a signal proportional to the oscillation state is obtained. This signal is compared with a set value. If the signal is equal to or more than the set value, the gain is decreased, and if the signal is equal to or less than the set value, the gain is increased. Finally, it is checked whether a discontinuous contact state has occurred and the scanning speed is adjusted. This process will be described with reference to FIG. Whether the discontinuous contact state is established is checked by comparing the variation of the error signal with a specified level (error signal check 71). Specifically, the error signal is taken into the control computer by the AD converter,
Its RMS value is calculated. If the RMS value of the error signal is greater than the specified level, the set point is increased (set point increase 72) to move the probe away from the sample. Thereafter, the correlation between the reciprocating AFM signals is measured (correlation check 73). If the correlation is small, the scanning speed is reduced (measurement speed decrease 74). The reason why the scanning speed is reduced is to correct that the feedback system cannot completely follow. When the RMS value of the error signal is equal to or lower than the specified level, the AFM measurement is actually started (measurement start 75).

[0007]

According to the present invention, a means for automatically determining parameters in the non-contact mode AFM measurement is provided, and ultimately the burden of the measurement operation of the operator can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of an atomic force microscope.

FIG. 2 is a schematic diagram illustrating the principle of interaction between a probe and a sample.

FIG. 3 is a schematic diagram illustrating the principle of a non-contact mode AFM.

FIG. 4 is a schematic diagram showing a sample shape according to the embodiment of the present invention.

FIG. 5 is an AFM signal according to the embodiment of the present invention.

FIG. 6 is a flowchart showing a process of determining an optimum condition according to the embodiment of the present invention.

FIG. 7 is a block diagram showing a gain feedback system according to the embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 cantilever 1a probe 2 holder 3 piezo scanner 4 support 5 optical system 6 sample stage 7 wafer loader 8 frame 9

Claims (8)

[Claims] 1. A non-contact atomic force microscope which performs a feedback control so as to keep a vibration amplitude near a resonance frequency of a cantilever constant and scans a surface of a sample to measure a shape thereof. A non-contact atomic force microscope comprising a search mechanism for automatically searching for optimum measurement conditions by monitoring correlations. 2. The non-contact atomic force microscope according to claim 1, wherein the search mechanism searches using a standard sample having a known shape. 3. The method according to claim 2, wherein the standard sample has a one-dimensional repetitive structure, and the same structure is always measured on the sample by scanning in a fixed direction. Characteristic non-contact atomic force microscope. 4. The method according to claim 1, wherein the search mechanism is an AF.
A non-contact atomic force microscope having a feedback system oscillation monitoring function by passing an M signal through a high-pass filter and then performing amplitude detection. 5. The method according to claim 1, wherein the search mechanism has a function of monitoring a transition from a non-contact state to a discontinuous contact state due to an increase in a change amount of a vibration amplitude signal of the cantilever. Contact type atomic force microscope. 6. The method according to claim 1, wherein the search mechanism moves the cantilever closer to the sample by reducing a set value of a cantilever amplitude to be kept constant when there is no reciprocal correlation. Contactless atomic force microscope. 7. The cantilever according to claim 1, wherein the search mechanism reduces the amplitude of the forced vibration of the cantilever when the reciprocal correlation is obtained, and keeps the amplitude constant when the correlation cannot be obtained. A non-contact atomic force microscope, wherein the cantilever is brought closer to the sample by reducing the set value of the amplitude. 8. An AFM image and a known sample shape in a non-contact atomic force microscope that scans the surface of a sample and measures the shape by feedback control so as to keep the oscillation amplitude near the resonance frequency of the cantilever constant. A non-contact atomic force microscope comprising a search mechanism for automatically searching for an optimal measurement condition by monitoring a correlation between the non-contact atomic force microscope.