JPH10293135A - Detection circuit for atomic force microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an AFM image in a short time by constituting a detection circuit of a rectifier circuit and an LPF circuit thereby increasing the scanning frequency of an atomic force microscope in noncontact mode or periodic contact mode. SOLUTION: At the time of observation in periodic contact mode, a cantilever is vibrated at the primary resonance frequency. A signal reflecting the dynamic interaction between a probe and a sample, i.e., an AC signal based on the amplitude of the cantilever, is inputted to a full-wave rectifier circuit from the probe. The AC signal is rectified and inputted to an LPF circuit and then converted into a DC signal proportional to the amplitude of the cantilever. A feedback circuit is operated to sustain the DC signal at a constant level and to sustain a constant vibratory state of the cantilever. More specifically, the distance between the probe and the sample is sustained constant. An AFM image is obtained by scanning the sample relatively in the X-Y direction while performing feedback control in Z direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric circuit used in an atomic force microscope.

[0002]

2. Description of the Related Art An atomic force microscope (AFM), which is one of scanning microscopes, forms a secondary observation image of a sample surface by a force acting between materials. AFM is expected to have a wide range of applications because it can observe material surfaces and organic molecules having no electrical conductivity on a nanometer scale (Yamada, Applied Physics, Vol. 59, No. 2, pp. 191-192).

FIG. 3 is a conceptual diagram of a conventionally used AFM periodic contact mode. The AFM comprises a cantilever 2 having a needle-like probe 1 having a small radius of curvature at the tip.
And a displacement detection system (not shown) for measuring the bending of the cantilever, an electric circuit for processing the detected signal, and the like. The sensor unit that detects this displacement is called a probe 3.

An AFM observation method (or measurement method) using the periodic contact mode will be described below. When the cantilever 2 is vibrated near the resonance frequency and the needle-like probe 1 at the tip of the cantilever is brought close to the observation sample by about 10 nm, static electricity, magnetism, van der Waals force, etc. act between the needle-like probe 1 and the observation sample. As a result, the resonance frequency of the cantilever changes, and the amplitude changes accordingly. The change in the amplitude is detected by a displacement detection system to measure the interatomic force. By scanning the sample surface in the XY directions, the force of the sample surface
Dimensional information is obtained. Further, by scanning the sample while performing feedback control on the position of the sample in the height direction (Z direction) so as to keep the amplitude of the cantilever constant,
The microscopic shape of the surface can be known.

As a method for detecting an amplitude change due to an atomic force, an optical lever system, an optical interference system, and a piezoelectric response system are used. In either method, the output signal is an AC signal, so it needs to be converted to a DC signal when inputting to a control system, and a lock-in amplifier is usually used as a means for this.

[0006]

However, in the lock-in amplifier, the conversion time from the AC signal to the DC signal is long, and the integration time is at least 1 msec. This is the biggest obstacle to speeding up the non-contact mode or the periodic contact mode. In the contact mode where there is no need for AC / DC conversion, observation can be performed at a scanning frequency of 5 Hz, whereas when using the non-contact mode or the periodic contact mode, observation can be performed only at a scanning frequency of about 2 Hz, which is twice as long. It was hanging.

An object of the present invention is to provide a detection circuit capable of obtaining an AFM image in a short time by increasing a scanning frequency in a non-contact mode or a periodic contact mode in an atomic force microscope.

[0008]

According to a first aspect of the present invention, there is provided a detection circuit used in an atomic force microscope for observing a sample in a non-contact mode, a periodic contact mode, or a periodic contact mode. The detection circuit has a rectifier circuit and a low-pass filter as means for converting an AC signal detected from a probe into a DC signal. "

According to a second aspect of the present invention, there is provided a detection circuit used in an atomic force microscope for observing a sample in a non-contact mode, a periodic contact mode, or a periodic contact mode, wherein the detection circuit comprises: A synchronous detection circuit and a low-pass filter are provided as means for converting an AC signal detected from the probe into a DC signal. "

[0010]

FIG. 1 is a diagram showing a detection circuit having a full-wave rectifier circuit and a low-pass filter circuit as an AFM detection circuit according to an embodiment of the present invention.
In the observation in the periodic contact mode, the cantilever is vibrated at its primary resonance frequency. A signal reflecting a mechanical interaction between the probe and the sample, that is, an AC signal based on the amplitude of the cantilever is input from the probe to the full-wave rectifier circuit. This AC signal is rectified and then input to a low-pass filter circuit, where it is converted into a DC signal proportional to the amplitude of the cantilever.

In the low-pass filter circuit, the vibration at the resonance frequency of the cantilever is removed. The resonance frequency of the cantilever is about 35 kHz in the present embodiment, and can be sufficiently eliminated if the cutoff frequency of the low-pass filter is 20 kHz or more. In order for this DC signal to be constant,
That is, so that the vibration state of the cantilever is constant,
Activate the feedback circuit. This is to keep the distance between the needle probe and the observation sample constant. As described above, when the needle probe is caused to scan in the XY direction relative to the sample while performing feedback control in the Z direction,
The AFM image shown in FIG. 4 was obtained.

FIG. 4 is an AFM image when a gold deposition film is observed in the periodic contact mode. Scan frequency is 13
Hz, and an AFM image of 256 × 256 pixels could be obtained in 20 seconds. This means that the observation time is reduced by a factor of 5 or more compared to a conventional detection circuit using a lock-in amplifier for AC / DC conversion. FIG. 2 is a detection circuit diagram for an atomic force microscope having a synchronous detection circuit and a low-pass filter as an AFM detection circuit according to the embodiment of the present invention.

Using this circuit, an AFM image of the deposited gold film could be obtained in 20 seconds in the same manner as described above.

[0014]

As described above, according to the AFM detection circuit of the present invention, since the scanning frequency is increased to 13 Hz, 256 ×
An AFM image of 256 pixels can be obtained in a short time of 20 seconds. This means that the observation time has been reduced by a factor of 5 or more compared to a conventional detection circuit using a lock-in amplifier.

[Brief description of the drawings]

FIG. 1 is a diagram showing a detection circuit having a full-wave rectifier circuit and a low-pass filter circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a detection circuit having a synchronous detection circuit and a low-pass filter circuit according to the embodiment of the present invention.

FIG. 3 is a conceptual diagram for explaining a conventional periodic contact mode of an AFM.

FIG. 4 is an AFM image of a deposited gold film obtained in a periodic contact mode according to an embodiment of the present invention.

[Explanation of symbols]

 1 ··· Needle probe 2 · · · cantilever 3 · · · probe

Claims (3)

[Claims] 1. A detection circuit used in an atomic force microscope for observing a sample in a non-contact mode or a periodic contact mode, wherein the detection circuit is configured to convert an AC signal detected from a probe into a DC signal. A detection circuit for an atomic force microscope, comprising a rectifier circuit and a low-pass filter. 2. A detection circuit used in an atomic force microscope for observing a sample in a non-contact mode or a periodic contact mode, wherein the detection circuit is configured to convert an AC signal detected from a probe into a DC signal. A detection circuit for an atomic force microscope, comprising a synchronous detection circuit and a low-pass filter. 3. The detection circuit according to claim 1, wherein the low-pass filter has a cutoff frequency of 20 kHz or more.