WO1996017345A1

WO1996017345A1 - Method of recording and reading information and information recording device

Info

Publication number: WO1996017345A1
Application number: PCT/JP1995/002407
Authority: WO
Inventors: Hiroyuki Kado; Takao Thoda
Original assignee: Matsushita Electric Industrial Co., Ltd.
Priority date: 1994-11-29
Filing date: 1995-11-24
Publication date: 1996-06-06

Abstract

A method and an information recording device can record, read, erase and overwrite information with a probe (4) at high density. While negative voltage is applied to a conductive probe (4) at the end of a cantilever (5) from a dc voltage power source (7), the conductive probe (4) which is in contact with a recording medium (3) is shifted. At a recording section, positive voltage is applied to the conductive probe (4) from a pulse voltage power source (8), and overwriting is carried out.

Description

DESCRIPTION

METHOD OF RECORDING AND READING INFORMATION AND INFORMATION RECORDING DEVICE

Technical Field This invention relates to a method of recording and reading information applying an atomic force microscope (described as "AFM" hereafter) and an information recording device.

Background Art Practical information recording methods currently include (1) a magnetic recording method of recording and reading information by changing or detecting magnetizing directions of ferromagnetic fine particles of a magnetic tape, a magnetic disc, etc., (2) a optical recording method of picking up pits formed on a disc such as an optical disc by applying a laser beam, (3) a magnetic optical recording method of recording and reading on a vertical magnetizing film having a magnetic optical effect such as the magnetic Kerr effect and the Faraday effect, by applying a laser beam and an outside magnetic field, and (4) a phase-change recording method, utilizing the change in refractive indices between a crystal phase and an amorphous phase and applying heat energy of a laser beam.

Although the above-mentioned methods were designed to improve recording density, in principle they have limitations on the improvement of recording density. For example, in the magnetic recording method, the recording density is limited due to the correlation between a minimized magnetic domain and weakened detecting signals. The diameter of a pit, which can be recorded and read, depends on the focused spot diameter of a laser beam in the light recording method, but the spot diameter is limited by the wavelength of the laser beam. For the same reasons as those mentioned above, limitations are found in the magnetic optical recording method and the phase-change recording method when a laser beam is applied so as to record information.

In order to solve the above-mentioned problems, the ultra- high density recording method applying a scanning tunnel microscope (mentioned as "STM" hereafter) technique has recently been proposed.

Methods for changing the surface shapes and physical conditions of recording media include (1) a mechanical processing method of pressing a probe directly against a recording medium, (2) an electric field evaporation method of depositing probe materials on the surface of a recording medium or removing the materials from the surface of a recording medium by applying a strong electric field between a probe and a substrate, and (3) an electric method of changing the surface conditions of a recording medium by applying voltage between a probe and the medium and utilizing the heat energy of an electric current. It is particularly desirable to apply the method of utilizing phase changes disclosed in Published Unexamined (Kokai) Japanese Patent Application No. Hei 1- 298551.

In the above-mentioned electric method applying an STM, it

2 is possible to raise the recording density higher than lTb/in .

However, a probe has to be spaced from the surface of a recording medium at a distance on the order of nm in the method applying STM, so that many problems in speed of operation are found when the method is applied for practical use. In addition, since the probe and recording medium are apart, a relatively large voltage has to be applied, thus damaging the probe. Moreover, information on concave and convex surfaces of a recording medium is included in a tunnel current when the surface conditions changed by the electric method are read by detecting the tunnel current, so that a complex operation and device are required so as to separate the information.

Disclosure of Invention

It is an object of this invention to solve the above- mentioned conventional problems by providing an information recording and reading method and an information recording device. The method and device use an AFM technique and can erase and overwrite information at high density.

In order to accomplish these objects, the method of recording information includes the recording operation of applying pulse voltage between a conductive probe and a recording medium so as to record data bits. The conductive probe is carried by a cantilever and is set in contact with the recording medium, which includes an amorphous semiconductor thin film. The erasing operation applies voltage having a polarity opposite to the voltage applied between a conductive probe and a recording medium during the recording operation so as to erase the data bits.

It is preferable that the pulse voltage applied between the conductive probe and the recording medium during the recording process has an absolute value larger than an absolute value of the voltage applied between the conductive probe and the recording medium during the erasing operation.

It is also preferable that the voltage applied between the conductive probe and the recording medium during the recording process and the erasing process is controlled by selecting the thickness of the amorphous semiconductor thin film.

It is further preferable that the amorphous semiconductor thin film includes at least one element selected from the group consisting of Ge, Si, Sb and Te.

It is preferable that the cantilever has a spring constant less than lN/m.

The method of recording information includes the recording operation of applying pulse voltage between a conductive probe and a recording medium while the conductive probe, which is carried at the end of a cantilever, is set in contact with the recording medium, which includes an amorphous semiconductor thin film, so as to record data bits. The overwrite operation applies dc voltage having a polarity opposite to the voltage applied during the recording operation between a conductive probe and a recording medium while the conductive probe is shifted along and relative to the surface of the recording medium so as to overwrite the data bits.

It is preferable that the pulse voltage applied between the conductive probe and the recording medium during the recording process has an absolute value larger than an absolute value of the dc voltage applied between the conductive probe and the recording medium during the erasing operation. It is also preferable that the voltage applied between the conductive probe and the recording medium during the recording process and the erasing process is controlled by selecting the thickness of the amorphous conductor thin film. It is further preferable that the amorphous semiconductor thin film includes at least one element selected from the group consisting of Ge, Si, Sb and Te.

It is preferable that the cantilever has a spring constant less than lN/m. The method of reading information includes the steps of: shifting a conductive probe along and relative to the surface of a recording medium while dc voltage is applied between the conductive probe and the recording medium; and detecting electric current flowing between the conductive probe and the recording medium, thus reading data bits.

It is preferable that the dc voltage applied between the conductive probe and the recording medium is the same polarity as the voltage applied during a recording process and has an absolute value smaller than an absolute value of the voltage applied during the recording process.

The information recording device includes a recording medium containing an amorphous semiconductor thin film, a conductive probe carried by a cantilever, a means to shift the conductive probe relative to the recording medium while dc voltage is applied between the conductive probe and the recording medium, and a means to superimpose a pulse voltage on the dc voltage when the conductive probe reaches a recording section.

It is preferable that the device further includes a means to detect electric current flowing between the conductive probe and the recording medium.

It is also preferable that the amorphous semiconductor thin film includes at least one element selected from the group consisting of Ge, Si, Sb and Te.

It is further preferable that the cantilever has a spring constant less than lN/m.

The method of recording information includes the recording operation of applying pulse voltage between a conductive probe and a recording medium while the conductive probe, which is carried by a cantilever is set in contact with the recording medium including an amorphous semiconductor thin film, and the erasing operation of applying voltage having a polarity opposite to the voltage applied during the recording operation between a conductive probe and a recording medium. Therefore, unlike the method employing an STM, recording can be carried out with relatively low voltage in this method since the conductive probe fixed to the end of the cantilever is in contact with the surface of the recording medium. When the conductive probe is carried at the end of the cantilever, no large force acts on the probe even if it collides with the convex surfaces of the recording medium. As a result, neither the conductive probe nor the recording medium is damaged. Therefore, since it is unnecessary to control the position of the conductive probe relative to the minute concave and convex surfaces of the recording medium so as to prevent the collision of the probe with the recording medium, information is recorded and read at high speed. Moreover, by applying voltage with opposite polarity to the voltage applied during the recording process, the value of resistance on the surface of the recording medium can be increased, thus erasing recorded data bits.

The method of recording information includes the recording operation of applying pulse voltage between a conductive probe and a recording medium while the conductive probe, which is carried at the end of a cantilever is set in contact with the recording medium including an amorphous semiconductor thin film, and the overwrite operation of applying dc voltage having a polarity opposite to the voltage applied between a conductive probe and a recording medium during the recording operation while the conductive probe is shifted along and relative to the surface of the recording medium. Thus, an overwrite operation can be carried out with only one conductive probe. In the overwrite operation, new data bits are recorded while previously recorded data bits are erased. In the methods, the pulse voltage applied between the conductive probe and the recording medium during the recording process has an absolute value larger than an absolute value of the dc voltage applied between the conductive probe and the recording medium during the erasing operation, so that information can be erased stably.

The voltage applied between the conductive probe and the recording medium during the recording process and the erasing process is controlled by selecting the thickness of the amorphous semiconductor thin film in the methods. Therefore, information can be recorded and erased stably with low voltage.

Electric current flowing between the conductive probe and the recording medium is detected while the conductive probe is shifted relative to and along the surface of the recording medium with the application of dc voltage between the conductive probe and the recording medium. As a result, the recording conditions (difference in conductivity) of the recording medium can be detected by sensing electric current flowing between the conductive probe and the recording medium, thus reading easily recorded data bits.

The dc voltage applied between the conductive probe and the recording medium is the same polarity as the voltage applied during a recording process and has an absolute value smaller than an absolute value of the voltage applied during the recording process, so that information is read stably.

In the method, the amorphous semiconductor thin film includes at least one element selected from the group consisting of Ge, Si, Sb and Te. Therefore, the change in value of resistance becomes stable and high in speed, and highly reliable recording and reading of information can be carried out at high speed.

Since the cantilever has a spring constant less than lN/m, abrasion generated by scanning the surface of the recording medium with a probe can be reduced. The information recording device includes a recording medium containing an amorphous semiconductor thin film, a conductive probe carried at the end of a cantilever, a means to shift the conductive probe relative to the recording medium while dc voltage is applied between the conductive probe and the recording medium, and a means to superimpose pulse voltage on the dc voltage when the conductive probe reaches a recording section. As a result, the information recording device to use the above- mentioned method can be realized. In other words, the information recording device can overwrite and record at high density.

The information recording device also includes a means to detect electric current flowing between the conductive probe and the recording medium. As a result, the recording conditions (change in conductivity) of the recording medium can be detected by sensing electric current flowing between the conductive probe and the recording medium, thus reading easily recorded data bits.

The amorphous semiconductor thin film of the information recording device includes at least one element selected from the group consisting of Ge, Si, Sb and Te. As a result, the change in value of resistance becomes stable and high in speed, thus highly reliable recording and reading of information can be carried out at high speed.

The cantilever of the information recording device has a spring constant less than IN/m, thus reducing abrasion generated by scanning the surface of the recording medium with a probe.

Brief Description of Drawings

Fig. 1 is a schematic view of an information recording device of one embodiment of the invention.

Fig. 2 A is a diagram, showing an energy band model of an unrecorded region.

Fig. 2 B is a diagram, showing an energy band model of a recorded region.

Best Mode for Carrying Out the Invention

This invention will be described by referring to the following illustrative examples and attached figures. Example 1

Fig. 1 is a schematic view of an information recording device of one embodiment of the invention. As shown in Fig. 1, a 20nm thick GeSb₂Te₄ thin film 2, which is one of the amorphous semiconductor materials in an amorphous condition, is formed on a conductive base material 1, thus preparing a recording medium 3. In other words, recording medium 3 has a large value of resistance in its initial condition. A conductive probe 4, which is placed on recording medium 3, is carried at the end of a cantilever 5. The cantilever 5 is made of a SiN thin film and had O.lN/m spring constant, so as to reduce the damage generated by the collision with the surface of recording medium 3 during the recording or reading processes. In order to add high conductivity, a lOOnm thick gold thin film is coated on the surface of cantilever 5 and conductive probe 4. Cantilever 5 is fixed to an actuator 6 which can drive precisely in X, Y and Z directions, and conductive probe 4 can be shifted precisely over distances less than O.lnm along the surface of recording medium 3. Voltage will be applied between conductive probe 4 and recording medium 3 from a dc voltage power source 7, a pulse voltage power source 8 and a voltage adder 9 for adding dc voltage and pulse voltage. A protective resistor 10 of 1MΩ and a current amplifier 11 for detecting electric current flowing between conductive probe 4 and recording medium 3 are connected to the circuit. The method of recording information is explained by referring to Fig. 1. Conductive probe 4 was shifted in the direction of the arrow at a speed of 2mm/sec by actuator 6 while the conductive probe 4 was in contact with the surface of recording medium 3. As soon as conductive probe 4 reached the position of recording information, the pulse voltage of +3V lasting 10 μ sec was generated by pulse voltage power source 8, and the pulse voltage was applied to conductive probe 4. Resistance of a section 13 of recording medium 3 applied with pulse voltage became lower, thus recording data bits.

The method of reading recorded information is explained below. The value of resistance at a recorded section was different from that at an unrecorded section, so that recorded data bits were detected by measuring the change in resistance. More specifically, conductive probe 4, which was in contact with recording medium 3, was shifted by actuator 6 while +0.5V dc voltage was applied to conductive probe 4 from power source 7, so that electric current was detected through current amplifier 11 and the difference in the value of resistance was found. As a result, recorded data bits were read. Reading voltage during the reading process should be at such a level that the value of resistance of the recording medium does not vary. In this example, The threshold of recorded voltage was 2.7V. Thus, it was necessary to read information by applying a voltage less than 2.7V. Recording section 13 read by this method was a circle with about 20nm diameter. When the size of recording data bits is in such a region, it is ossible to record information

2 with a density of about lTbit/in . The size of recording bits depends on the contacting area between conductive probe 4 and recording medium 3. Thus, by applying a conductive probe having a small radius of edge curvature, density was further improved.

The method of erasing recorded information is now explained. As in the recording process, conductive probe 4 was shifted to the recorded data bits, and a pulse voltage of -IV lasting 10// sec was applied while conductive probe 4 was set in contact with the recording medium 3. As a result, the value of resistance at a recorded section increases, and the value of resistance returns to the value before the recording process, thus erasing data bits.

The value of resistance of the recording medium increases or decreases by applying voltage from the conductive probe in the above-mentioned method, and this result is due to the change in an energy band structure of GeSbgTe. thin film 2 in an amorphous condition. This mechanism is explained by referring to Figs. 2A and 2B. Positively charged dangling bonds and negatively charged dangling bonds exist in the energy band of an amorphous semiconductor. However, normally (when information is not recorded) the energy band structure is like the one shown in Fig. 2A, so that electrons and electron holes cannot be shifted by the application of reading voltage +V_R between a conductive probe and a conductive base material. However, when a strong electric field is applied (recording process) from conductive probe 4, charged dangling bonds shift toward interfaces. As a result, space charge is formed in the vicinity of the interfaces, and the Schottky- barrier grows. When positive voltage is applied to the conductive probe as recording voltage, an energy band structure becomes as shown in Fig. 2B. With the application of reading voltage +V_R to the structure, electron holes are introduced from the conductive probe due to the tunneling effect, thus lowering the value of resistance. By applying a voltage having a polarity opposite to that of voltage during the recording process, electrons from the conductive probe and electron holes from the conductive base material are injected. The electrons and electron holes recombine to the space charge formed in the vicinity of the interfaces, thus increasing the value of resistance. The threshold of applied voltage for recording depends on the thickness of GeSbgTe. thin film 2 in an amorphous condition. As the film becomes thin, it is possible to record with lower voltage. The recording threshold voltage in this example was 2.7V. However, recording becomes possible with 1.5V when the thickness is lOnm.

The polarities of voltage applied during recording, reading and erasing processes can be totally reversed. In such a case, -3V was applied to the conductive probe during the recording process, - 0.5V was applied during the reading process, and +1V was applied during the erasing process. However, when negative voltage is applied to a conductive probe, gold on the surface of the probe is likely to shift due to an electric field. Thus, it was impossible to increase the voltage during the recording process very much. The composition ratio of Ge-Sb-Te is not limited to GeSb-Te₄.

The amorphous semiconductor thin film is not limited to GeSbTe, and a chalcogenide-based amorphous material can be applied in the invention as long as it contains at least one of Ge, Si, Sb and Te. For example, the material includes GeTe, SiTe, GeTeSn, SbTe, GaSb, SbSe, SnTe, PbTe, SbSe, BiSe, GeSe, Gain, InSbTe, amorphous Ge and amorphous Si . With the application of a cantilever having a spring constant less than IN/m, damage of a probe during scanning is reduced significantly. Example 2

The overwrite operation of information is explained by referring to Fig. 1. From dc voltage power source 7, -IV dc voltage was applied to conductive probe 4. With the application of the voltage, the probe 4 was shifted in the direction of the arrow at a speed of 2mm/sec while the probe 4 was set in contact with the surface of recording medium 3 by actuator 6. As a result, data bits 12 recorded by the application of a pulse voltage of +3V lasting 10 μ sec were erased completely. As soon as conductive probe 4 reached a recording position, a pulse voltage of +3V lasting 10/- sec was applied to the conductive probe 4 by voltage adder 9 and pulse voltage power source 8 which generated the pulse voltage of +4V lasting 10/-; sec. Resistance of section 13 of recording medium 3 applied with pulse voltage became lower, so that data bits were recorded. The overwrite operation was carried out with only one conductive probe.

Even though a positive voltage pulse was applied to conductive probe 4 so as to carry out the overwrite operation in this example, the operation can be carried out with a negative voltage pulse. For instance, with the application of +1V dc voltage from dc voltage power source 7, previously recorded data bits, which was recorded by -3V pulse voltage, are erased while the conductive probe 4 is shifted. As soon as conductive probe 4 reaches a recording position, a pulse voltage of -4V lasting 10 sec is generated by pulse voltage power source 8. By applying pulse voltage of -3V lasting 10/- sec to conductive probe 4 through voltage adder 9, new data bits can be recorded.

The composition ratio of Ge-Sb-Te is not limited to GeSb₂Te₄.

Again, the amorphous semiconductor thin film is not limited to GeSbTe, and a chalcogenide-based amorphous material can be applied in the invention as long as it contains at least one of Ge, Si, Sb and Te. For example, the material includes GeTe, SiTe, GeTeSn, SbTe, GaSb, SbSe, SnTe, PbTe, SbSe, BiSe, GeSe, Gain, InSbTe, amorphous Ge and amorphous Si. With the use of a cantilever having a spring constant less than IN/m, abrasion of a probe during scanning is reduced significantly.

Claims

1. A method of recording information comprising: a recording operation of applying pulse voltage between a conductive probe and a recording medium comprising an amorphous semiconductor thin film, said conductive probe being carried by a cantilever and set in contact with said recording medium, thus recording data bits; and an erasing operation of applying voltage to said conductive probe having a polarity opposite to the voltage applied during said recording operation, thus erasing said data bits.

2. The method of claim 1, wherein the pulse voltage applied between the conductive probe and the recording medium during the recording process has an absolute value larger than an absolute value of the voltage applied between said conductive probe and said recording medium during the erasing operation.

3. The method of claim 1, wherein the voltage applied between the conductive probe and the recording medium during the recording process and the erasing process is controlled by selecting a thickness of the amorphous semiconductor thin film.

4. The method of claim 3, wherein a thickness of the amorphous semiconductor thin film is 20 nm or less.

5. The method of claim 1, wherein the amorphous semiconductor thin film comprises at least one element selected from the group consisting of Ge, Si, Sb and Te.

6. The method of claim 1, wherein the cantilever has a spring constant less than IN/m.

7. A method of recording information comprising: a recording operation of applying pulse voltage between a conductive probe and a recording medium comprising an amorphous semiconductor thin film, said conductive probe being carried by a cantilever and set in contact with said recording medium, thus recording data bits; and an overwrite operation of applying dc voltage having a polarity opposite to the voltage applied during said recording operation between said conductive probe and said recording medium while said conductive probe is shifted along and relative to a surface of said recording medium, thus overwriting said data bits.

8. The method of claim 7, wherein the pulse voltage applied between the conductive probe and the recording medium during the recording process has an absolute value larger than an absolute value of the dc voltage applied between said conductive probe and said recording medium during the erasing operation.

9. The method of claim 7, wherein the voltage applied between the conductive probe and the recording medium during the recording process and the erasing process is controlled by selecting a thickness of the amorphous semiconductor thin film. 10. The method of claim 9, wherein a thickness of the amorphous semiconductor thin film is 20 nm or less.

11. The method of claim 7, wherein the amorphous semiconductor thin film comprises at least one element selected from the group consisting of Ge, Si, Sb and Te. 12. The method of claim 7, wherein the cantilever has a spring constant less than IN/m.

13. A method of reading information comprising the steps of: shifting a conductive probe along and relative to a surface of a recording medium while dc voltage is applied between said conductive probe and said recording medium; and detecting variation in electric current flowing between said conductive probe and said recording medium, thus reading data bits.

14. The method of claim 13, wherein the dc voltage applied between the conductive probe and the recording medium is the same polarity as the voltage applied during a recording process and has an absolute value smaller than an absolute value of the voltage applied during the recording process.

15. An information recording device comprising a recording medium comprising an amorphous semiconductor thin film, a conductive probe carried by a cantilever, a means to shift said conductive probe relative to said recording medium while dc voltage is applied between said conductive probe and said recording medium, and a means to superimpose pulse voltage on said dc voltage when said conductive probe reaches a recording section.

16. The information recording device of claim 15, wherein the conductive probe is mounted at the end of the cantilever. 17. The information recording device of claim 15, further comprising a means to detect electric current flowing between the conductive probe and the recording medium.

18. The information recording device of claim 15, wherein the amorphous semiconductor thin film comprises at least one element selected from the group consisting of Ge, Si, Sb and Te.

19. The information recording device of claim 15, wherein the cantilever has a spring constant less than IN/m. AMENDED CLAIMS

[received by the International Bureau on 13 May 1996 (13.05.96); original claims 3 and 14 cancelled; original claims 1 and 13 amended; remaining claims unchanged (4 pages)]

1. (amended) A method of recording information comprising: a recording operation of applying pulse voltage between a conductive probe and a recording medium comprising an amorphous semiconductor thin film, said conductive probe being carried by a cantilever and set in contact with said recording medium, thus recording data bits; and an erasing operation of applying voltage to said conductive probe having a polarity opposite to the voltage applied during said recording operation, thus erasing said data bits; wherein the voltage applied between the conductive probe and the recording medium during the recording process and the erasing process is controlled by selecting a thickness of the amorphous semiconductor thin film.

3. (canceled)

19 AMENDEDSHEET-RTICXEM 7. A method of recording information comprising: a recording operation of applying pulse voltage between a conductive probe and a recording medium comprising an amorphous semiconductor thin film, said conductive probe being carried by a cantilever and set in contact with said recording medium, thus recording data bits; and an overwrite operation of applying dc voltage having a polarity opposite to the voltage applied during said recording operation between said conductive probe and said recording medium while said conductive probe is shifted along and relative to a surface of said recording medium, thus overwriting said data bits.

9. The method of claim 7, wherein the voltage applied between the conductive probe and the recording medium during the recording process and the erasing process is controlled by selecting a thickness of the amorphous semiconductor thin film.

10. The method of claim 9, wherein a thickness of the amorphous semiconductor thin film is 20 nm or less.

11. The method of claim 7, wherein the amorphous semiconductor thin film comprises at least one element selected from the group consisting of Ge, Si, Sb and Te.

12. The method of claim 7, wherein the cantilever has a spring constant less than IN/m.

13. (amended) A method of reading information comprising the steps of : shifting a conductive probe along and relative to a surface of a recording medium while dc voltage is applied between said conductive probe and said recording medium; and detecting variation in electric current flowing between said conductive probe and said recording medium, thus reading data bits; wherein the dc voltage applied between the conductive probe and the recording medium is the same polarity as the voltage applied during a recording process and has an absolute value smaller than an absolute value of the voltage applied during the recording process.

14. (canceled)

16. The information recording device of claim 15, wherein the conductive probe is mounted at the end of the cantilever.

17. The information recording device of claim 15, further comprising a means to detect electric current flowing between the conductive probe and the recording medium.

19. The information recording device of claim 15, wherein the cantilever has a spring constant less than IN/m.

Statement under Article 19(1)

Amended claim 1 clarified that the voltage applied between the conductive probe and the recording medium during the recording process and the erasing process should be controlled by selecting the thickness of the amorphous semiconductor thin film in the method of recording information of this invention. According to the method of recording information recited in claim 1, information can be recorded and erased stably with a low voltage.

EP, A, 0 305 033 discloses a recording and reproducing device capable of recording and erasing by applying a voltage between the recording medium 1 and the probe electrode 102 (refer to Fig. 1). The recording medium 1 comprises an amorphous semiconductor represented by the composition formula Si.gGe-.AS_ςTe-- (film thickness 200 nm) .

PATENT ABSTRACTS OF JAPAN vol.018 No.278 discloses a recording and reproducing device. An electric probe 6 is carried at the tip of the cantilever 3 and is set contact with the recording medium. By applying a pulse voltage on the electric probe 6, electric charge is impressed to the recording medium and information is recorded.

However, neither EP, A, 0 305 033 nor PATENT ABSTRACTS OF JAPAN vol.018 No.278 discloses a means for controlling the voltage as mentioned above. Therefore, even if these references are combined, effects of the present invention can not be achieved.

Amended claim 13 clarified that the dc voltage applied between the conductive probe and the recording medium should be the same polarity as the voltage applied during a recording process and should have an absolute value smaller than an absolute value of the voltage applied during the recording process. According to the method of reading information recited in claim 13, information can be read stably.

JP, A, 06 300514 discloses a multiplex information regenerating device using technique of a scanning probe microscope. A probe conducts scanning along a surface of a sample 5 in a state where the probe 1 is in contact with the surface of the sample 5, a bias voltage is applied between the probe 1 and the sample 5 and a flowing current is detected.

However, JP, A, 06 300514 fails to teach or suggest that "the dc voltage applied between the conductive probe and the recording medium is the same polarity as the voltage applied during a recording process and has an absolute value smaller than an absolute value of the voltage applied during the recording process". Therefore, effects of the present invention can not be achieved by this reference.