JPH07326316A - Fine processing method of sample surface and recording / reproducing method of information - Google Patents

Abstract

(57) [Abstract] [Purpose] Perform fine reworking of the sample surface and recording / reproduction of information with a scanning tunneling microscope with good reproducibility. [Structure] After moving the probe to the specified machining position on the sample surface, the feedback circuit sample / hold signal 1 is turned on to interrupt the control of the probe-sample distance, and the probe-sample voltage 2 is momentarily set. The machining success rate is improved by changing the voltage between the probe and the sample a plurality of times during one distance control interruption. When the hole is formed by the first probe / sample voltage change, the position of the probe remains fixed, and the shape of the hole is unlikely to change due to the second probe-sample voltage or later.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for finely processing a sample surface using a scanning probe microscope, and more particularly to a method for recording / reproducing information using the same.

[0002]

2. Description of the Related Art Conventionally, a scanning tunneling microscope (hereinafter referred to as ST
(Referred to as M) is proposed. The STM is used not only for observation but also for microfabrication on the order of nm by utilizing the interaction between the probe and the sample. The microfabrication by STM is expected to develop to high density recording. Since the bit area of an optical disk that has been put into practical use as a high-density, large-capacity recording medium is about 1 μm ² , if the holes and ridges with a diameter of several nm formed by STM are made to correspond to 1 bit, the recording density Will be improved tens of thousands of times. In addition, microfabrication by STM is also expected for the production of a quantum effect element that appears by forming a fine structure having a coherent length of electrons or less, and research is being advanced.

Conventionally, in such a microfabrication method, an STM probe is fixed at a specified place on the sample surface, and a pulse voltage of 1 ms or less is applied between the probe and the sample to form a minute hole. Has been taken. "Applied Physics Letter 61" (13) (1992), pp. 1528-1530 (Appl.Phys.Le
According to tt.61 (13) (1992) 1528-1530), during normal STM observation, feedback control is performed so as to keep the distance between the probe and the sample constant. Move the needle, stop the feedback control, apply a voltage calculated from the sensitivity of the piezoelectric body to the piezoelectric body, fix the probe closer to the sample by 0.5 nm than when observing and fix it, and apply a pulse voltage between the sample probe As a result, holes with a diameter of 1 nm or less are formed.

Although the principle of microfabrication is not clear, it is said that a high electric field is instantaneously generated just below the probe on the sample surface, and the substance on the sample surface is stripped off by this electric field. This phenomenon is called field evaporation. There is also a theory that the time variation of the electric field at the rise of the pulse voltage has an effect ("Applied Physics Letter 60" (19) 2338).
Page-2340 (1992) (Appl.Phys.Lett.60 (19) 2338-2340 (1)
992))).

[0005]

However, such a microfabrication method has a drawback of poor reproducibility.
Therefore, when the information is recorded by this method, it is intended that the hole or the ridge is formed, but the hole or the ridge is not actually formed, and there is a possibility that the information may be erroneously reproduced. Regarding the method of bringing the probe closer to the sample during processing, in the conventional example, after moving the probe to the processing point, the voltage of the piezoelectric body corresponding to the approach distance is calculated and added from the sensitivity. Since the initial value of the distance between the needle samples is unknown, there is a risk that the probe may contact the sample.

In view of the above point, the present invention has an object of improving the reproducibility of fine processing by a scanning tunneling microscope and the reliability of recording / reproducing of information using the same.

[0007]

In order to achieve the above object, the present invention uses a scanning tunneling microscope to interrupt the control of the distance between the probe and the sample, and to temporarily change the voltage between the probe and the sample. A method for microfabrication of a sample surface by a step of changing and returning to a step of microfabrication of a sample surface, in which a step of temporarily changing and returning the voltage between the probe and the sample during one distance control interruption is repeated. And a method of recording / reproducing information using the method.

Further, the first step of moving the probe to the processing point while controlling the distance between the probe and the sample so as to keep the tunnel current at the set value, and temporarily changing the voltage between the probe and the sample. The second step of recording the information by changing the structure or property of the fine region of the sample surface by returning
Information recording comprising a third step of reproducing information by scanning the sample surface with the probe while controlling the distance between the probe and the sample so as to keep the tunnel current at a set value smaller than the set value in the step -It is a reproduction method.

[0009]

In the conventional microfabrication method for such a scanning probe microscope as described above, while the distance control between the probe samples is interrupted once,
There was only one step to change the voltage between the samples. 1
If the probability that the structure of the sample surface is changed by the change of the voltage for the number of times is p, the probability is improved to n × p for the change of the physical quantity for the number of times of n. In the present invention, the reproducibility can be improved because the probe-sample voltage is changed a plurality of times during one distance control interruption.

In the present invention, both the time for which a high electric field is applied to the sample surface and the time for the electric field at the rise of the voltage change are increased, so the probability that the structure of the sample surface changes is improved.

The formation of fine holes by STM will be described in more detail as an example. In STM, the probe is attached to a piezoelectric body that moves in a three-dimensional direction. The feedback circuit applies a voltage to the piezoelectric body so as to keep the tunnel current constant, and controls the distance between the probe and the sample. When the voltage between the probe and the sample is increased, the current is also increased at the same time. Therefore, when the distance is controlled, the probe is separated from the sample surface, and the electric field is not effectively applied to the sample surface. Therefore, the distance control is interrupted, the position of the probe is fixed three-dimensionally, and then the voltage between the probe and the sample is changed.

FIG. 2 is a diagram showing the time change of the sample / hold signal and the voltage between the probe and the sample in the conventional method. The sample and hold signal 21 is a signal for controlling the output of the feedback circuit, and when it is turned on, the previous output is held regardless of the input. After turning on the sample and hold signal 21, turn the probe-sample voltage 22 to 1
The conventional method is to change only once.

FIG. 1 is a diagram showing a time change of a sample / hold signal and a voltage between a probe and a sample in the present invention. It is a feature of the present invention that the probe-sample voltage 2 is changed in a pulsed manner a plurality of times while the sample-hold signal 1 is ON.

In order to improve the reproducibility, it is possible to repeat the conventional method a plurality of times. However, in the repetition of the conventional method, when a hole is formed at the first pulse voltage change, there is a risk that the size of the hole is increased by the second and subsequent pulse voltage changes. Fig. 3 shows (a) the sample-and-hold signal and the time variation of the voltage between the probe and the sample according to the conventional example.
(b) Time change of the positional relationship between the probe and the sample is shown. Time ta
Then, the sample / hold signal 31 is turned on, and then the probe-sample voltage 32 changes in a pulsed manner. At time tb immediately after the pulse voltage change, the distance between the probe 33 and the sample 34 is larger than that at time ta because the sample and hold signal 31 is still ON. However, after this, the sample and hold signal 31 turns off once and the probe 3
Since 1 approaches the sample 34, when the sample / hold signal 31 is turned ON again at the time tc, the distance between the probe 33 and the sample 34 becomes the same as that at the time ta. Since the second pulse-like voltage change is performed at this distance, the size of the hole further increases at time td after the change.

However, in the present invention, since the voltage is changed in a pulsed manner a plurality of times while the position of the probe is fixed, it is unlikely that the size of the hole becomes large. According to the invention in FIG.
(a) Time change of sample-hold signal and voltage between probe and sample, (b) Time change of positional relationship between probe and sample. At time te, the sample / hold signal 41 is turned on, and then the probe-sample voltage 42 changes in a pulsed manner. At time tf immediately after the pulse voltage change, the distance between the probe 43 and the sample 44 is larger than that at time te because the sample and hold signal 41 is still ON. Even at the time tg when the second pulse-like voltage change is performed,
Since the sample / hold signal is still ON, the distance between the probe 43 and the sample 44 is larger than that at the time te, and the electric field generated between the probe 43 and the sample 44 at the time of pulse-like voltage change is smaller than the time te. . Therefore, there is a feature that the size of the hole does not change significantly by the second pulse-like voltage change.

When a hole cannot be formed on the surface of the sample due to the previous pulse-like voltage change, the distance between the probe and the sample does not change, so it is possible to form the hole by applying the pulse voltage again. .

Further, in the present invention, when the hole can be formed as described above, the distance between the probe and the sample becomes large and the tunnel current becomes small. Therefore, it is possible to detect the change in the structure of the sample surface from the change in the tunnel current.

Regarding the method of bringing the probe closer to the sample during processing, there is no risk of contact because the probe is brought closer while monitoring the tunnel current. Further, since the probe is moved to the next processing point in the state of being approached in advance, it is not necessary to re-approach it each time it moves to the processing point, so one process can be reduced.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. First, FIG. 5 shows a block diagram of an STM used in implementing the present invention.

As shown in FIG. 5, in the STM, the probe 51 is fixed to the probe fine movement mechanism 52 made of a piezoelectric material, and the probe 51 faces the sample 58. Control computer 5
Reference numeral 6 applies a scanning signal to the probe fine movement mechanism 52 through the piezo amplifier 55 to scan the probe 51 in a direction parallel to the sample surface. The current flowing between the probe 51 and the sample 52 is converted into a voltage by the current-voltage converter 53 and input to the feedback circuit 54. The feedback circuit 54 outputs a voltage corresponding to the difference between the input value and a preset value to the piezo amplifier 55. The output voltage of the piezo amplifier 55 is applied to the probe fine movement mechanism 52 to control the distance between the probe 51 and the sample 58.

The output or input (tunnel current) of the feedback circuit 54 is sent to the control computer 56 to be imaged and also sent to the oscilloscope 57 so that its time change can be observed. The potential of the probe 51 is fixed to the ground potential, and the sample 58 is kept at the offset potential generated by the pulse voltage generator 59. Control computer 5
6 sends a sample hold signal to the feedback circuit 54, and when the sample hold signal is ON, the feedback circuit 54 holds the output regardless of the input. The control computer 56 can send a trigger signal to the pulse voltage generator 59 to instantaneously change the potential of the sample 58.

The sample was cleaved immediately before molybdenum disulfide, and the probe was a fine wire of platinum iridium alloy (Pt80at% Ir20%) sharpened by electrolytic polishing, and the tip radius of curvature was 100n.
The thing below m was used. The sample potential and tunnel current were set to 1.0 V and 0.1 nA during processing and observation, respectively.

Comparative Example Here, for comparison, the above STM is used.
An example of processing by the conventional method using is shown. First, when the surface roughness of a 1 μm × 1 μm region of the sample was measured by STM, it was about 0.5 nm. Next, 500 machining positions were designated to the control computer 56, and machining was performed by applying a pulse voltage.

The control computer 56 moves the probe 51 to the designated position and then outputs the sample hold signal 5
A trigger signal was sent to the pulse voltage generator 59 with the microsecond turned on.

FIG. 6 is a diagram showing the time variation of the sample / hold signal and the voltage between the probe and the sample in the conventional method. As shown in FIG. 6, the pulse voltage generator 59 outputs a single pulse having a height of 5.4 V and a width of 300 nsec with a delay of about 1 μsec from the time when the sample hold signal is turned on. When the STM observation was performed again after the application of the pulse voltage at all the specified points was completed, the diameter at 495 points out of 500 points was measured.
Holes having a depth of 10 nm and a depth of 1 to 2 nm were formed, but no holes were formed at the remaining 5 points. When the above-mentioned processing of 500 points was repeated 20 times, the probability that holes were not formed at the designated points was 1% on average.

Example 1 Next, a processing example according to the present invention will be shown. The STM device, sample, and probe used were the same as those of the comparative example. The sample potential and tunnel current were set to 1.0 V and 0.1 nA during processing and observation, respectively. As in the comparative example, after observing the area of 1 μm × 1 μm and confirming that the surface roughness of the sample is sufficiently small, the control computer 56 specifies the machining positions of 500 points and performs the machining by applying the pulse voltage. I went. The control computer 56 is the probe 5.
After moving 1 to the specified position, the pulse voltage generator 59
Sent a trigger signal to.

FIG. 7 shows a sample according to the embodiment of the present invention.
It is a figure which shows the time change of a hold signal and the voltage between a probe and a sample. As shown in FIG. 7, the pulse voltage generator 59 has about 1 time from the time when the sample hold signal is turned on.
With a delay of μsec, a pulse of 5.4V in height and 300nsec in width is 1μsec.
It was output twice at intervals. When the STM observation was performed again after the pulse voltage application at all specified points was completed, it was 500
Holes having a diameter of 10 nm and a depth of 1 to 2 nm were formed at 498 points, but no holes were formed at the remaining 2 points. When the above-mentioned processing of 500 points was repeated 20 times, the probability that holes were not formed at the designated points was 0.4% on average, which was about 0.6% less than that of the comparative example.

In this embodiment, the number of pulse application times is set to 2
Although the number of times is set to two, the number of times is not limited to two and may be three or more. However, if the number of pulse voltage changes is increased,
The processing time per point becomes long and the throughput deteriorates. Therefore, the change in the tunnel current is detected by the control computer 56 every time the pulse voltage is changed, and when the tunnel current decreases, that is, when the hole is formed and the inter-probe sample distance increases, the position at that position is increased. It is desirable to terminate the pulsed voltage change.

When the polarity of the applied pulse voltage is reversed, the direction of the electric field is reversed, and the substance at the tip of the probe breaks off and adheres to the surface of the sample just below the probe to form a ridge. In this case, as shown in FIG. 8, unlike the case where the hole is formed, the distance between the probe and the sample is almost the same before and after the ridge is formed. There is a risk that it will grow larger than that. Therefore, when a ridge is formed, it is desirable that the control computer detect a change in the tunnel current each time the pulsating voltage changes, and terminate the pulsating voltage change when the ridge is formed.

Similar results were obtained when a layered substance of chalcogenide such as tungsten diselenide, highly oriented graphite, or a gold thin film deposited on a mica substrate was used as a sample in addition to molybdenum disulfide. Was given.

(Embodiment 2) In the present embodiment, the setting value of the tunnel current is increased before performing the fine processing, and the fine processing and the movement between a plurality of processing points are performed with the probe close to the sample, and thereafter, This is an example of observing the sample surface in a state where the set value of the tunnel current is reduced and the probe is separated from the sample. The method of Example 1 was used to form the holes, the sample was cleaved immediately before molybdenum disulfide, and the probe was a platinum iridium alloy (Pt80at% Ir20) sharpened by electrolytic polishing.
%) And the tip curvature radius is 100 nm or less.

While the sample potential was fixed at 1.0 V, the set value of the tunnel current was changed in the recording process and the reproducing process. Further, the height of the pulse voltage was also changed, and the hole forming threshold voltage, which is the minimum voltage for forming holes, was determined. Table 1 shows the setting values of the tunnel current in the recording process and the reproducing process, the size of the hole observed during the reproducing process, and the hole forming threshold voltage.

[0033]

[Table 1]

When the set value of the tunnel current during reproduction was set to 10 nA, it was observed that the end of the hole had a long tail, and noise was large as a whole. When the observation was repeated about 10 times, it was observed that the shape of the hole became large. This is considered to be because the conductivity of molybdenum disulfide in the sample is insufficient and the sample is in contact with the sample surface when the probe is scanned. Stable observation was possible when the tunnel current was set to 0.3 nA.

When the setting value of the tunnel current at the time of recording is 10 nA, the hole forming threshold voltage is smaller than that when it is 0.3 nA,
Also, the variation in the diameter of the holes was small. It is generally said that the tunnel current will increase 10 times when the probe approaches 0.1 nm to the sample, and the set value of the tunnel current during recording will be 0.
By changing from 3nA to 10nA, the probe seems to approach the sample by about 0.2nm. Therefore, it is considered that the electric field for causing the electric field evaporation on the sample surface can be generated with a smaller voltage. From the above, it can be said that it is desirable to set the tunnel current at a small value during reproduction and set the tunnel current at a large value during recording.

(Embodiment 3) This embodiment uses STM and
In this embodiment, a character string binary-coded by the ASCII code is recorded / reproduced. The sample was cleaved molybdenum disulfide immediately before, and the probe was a fine wire of platinum iridium alloy (Pt80at% Ir20%) sharpened by electrolytic polishing, and the tip curvature radius was 100 nm or less. STM used
The device is the same as that used in Example 1. The sample potential was 1.0 V for both recording and reproduction.

After setting the tunnel current to 0.1 nA and observing the 5 μm × 5 μm region of the sample surface with the STM and confirming that the sample surface is sufficiently flat, the tunnel current was set to 10 nA.
Was set, and a ridge having a diameter of 30 nm was formed at one point on the surface of the sample to be the starting point. 50 nm on the right side of the STM scan line including the start point
With the interval as the recording point, the binary code bit sequence is read in sequence and when the bit is 1, a hole is formed and the probe is moved to the next recording point, and when it is 0, the next recording point without fine processing is performed. The probe was moved to. To form the holes, the sample-hold signal and the change in the probe sample voltage as shown in FIG. 7 were applied. After recording in this way, the setting value of the tunnel current was returned to 0.1 nA and STM observation was performed. From the STM image, check the unevenness of the recording place at intervals of 50 nm from the ridge on the scanning line including the ridge, and reproduce the bit string by setting 1 when it is recessed compared to the periphery of the recording place and 0 when it is almost the same height as the surrounding The result of decoding the bit string "123456789"
I was able to get the same string that I recorded. This operation was repeated 100 times, but the probability that a character string was recorded / reproduced by mistake was 0.4%.

[0038]

As described above, according to the present invention, it is possible to improve the reproducibility of fine processing by a scanning tunneling microscope and the reliability of recording / reproduction.

[Brief description of drawings]

FIG. 1 is a diagram showing a time change of a sample hold signal and a voltage between a probe and a sample according to the present invention.

FIG. 2 is a diagram showing a time change of a sample-hold signal and a voltage between a probe and a sample in a conventional example.

FIG. 3A is a diagram showing a time change of a sample-hold signal and a voltage between a probe and a sample of the conventional example, and FIG. 3B is a diagram showing a time change of a positional relationship between the probe and the sample.

FIG. 4A is a diagram showing a time change of a sample-hold signal and a probe-sample voltage of the present invention, and FIG. 4B is a diagram showing a temporal change of a positional relationship between the probe and the sample.

FIG. 5 is a diagram showing a configuration of a scanning tunneling microscope used in the present invention and a conventional example.

FIG. 6 is a diagram showing a time change of a sample-hold signal and a voltage between a probe and a sample in a conventional example.

FIG. 7 is a diagram showing changes over time of a sample / hold signal and a voltage between a probe and a sample in one embodiment of the present invention.

[Explanation of symbols]

 1 Feedback circuit sample-hold signal 2 Probe-sample voltage 41 Sample-hold signal 42 Probe-sample voltage 43 Probe 44 Sample 51 Probe 52 Fine probe mechanism 53 Current-voltage converter 54 Feedback circuit 55 Piezo amplifier 56 control computer 57 oscilloscope 58 sample 59 pulse voltage generator 71 sample and hold signal 72 probe inter-sample voltage

Claims (5)

[Claims] 1. Microfabrication of a sample surface by a step of interrupting control of a distance between a probe and a sample and temporarily changing and returning a voltage between the probe and the sample by using a scanning tunneling microscope. A method for microfabrication of a sample surface, wherein a step of temporarily changing and returning the voltage between the probe and the sample during one distance control interruption is repeated a plurality of times. 2. Discontinuing the process of detecting a change in the distance between the probe and the sample, and temporarily changing the voltage between the probe and the sample and returning the voltage based on the change in the distance. The method for finely processing a sample surface according to claim 1, wherein: 3. Reproducing the recorded information by recording information by the fine processing method of the sample surface according to claim 1 or 2 and observing electronic unevenness on the sample surface using a scanning tunneling microscope. And a method of recording / reproducing information. 4. Recording of information using a scanning tunneling microscope
In the reproducing method, the first step of moving the probe to the processing point while controlling the inter-probe sample distance so as to keep the tunnel current at the set value, and temporarily changing the voltage between the probe and the sample The second step of recording the information by changing the structure or property of the fine region of the sample surface by returning the tunnel current to the set value smaller than the set value in the first step. An information recording / reproducing method comprising a third step of reproducing information by scanning the sample surface with the probe while controlling the distance between the needle and the sample. 5. The method for recording / reproducing information according to claim 4, wherein the second step is the method for finely processing the sample surface according to claim 1 or 2.