Reproducing apparatus

Abstract: A reproducing apparatus for reproducing information from recording elements formed with very small pits on a planar surface. The reproducing apparatus includes a probing unit (1) positioned at a very small distance apart from the surface of the recording element (2) and a scanning unit (3, 5) for scanning the probing unit at a very small distance apart from the surface of the recording element (2). The reproducing apparatus also includes a detecting unit (7) connected to the probing unit and the recording element respectively, for detecting interactions generated between the probing unit and the surface of the recording element and a regeneration unit (8) connected to the detecting unit for inputting a signal output from the detecting unit and for regenerating information by detecting a pattern of the pits by the input signal.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to apparatus for reproducing information by reading pits formed on a surface of a recording medium. 2. Description of the Related Art There is a magnetic reproducing apparatus for recording information in magnetization direction with respect to a magnetic recording medium by means of a magnetic head and for reproducing information by detecting the magnetization direction by means of the magnetic head. The Applicants know an optical reproducing apparatus for recording information from pits with minor irregularities formed on the surface of a substrate and for reproducing information by a method in which the pits are radiated with light beams and the presence of pits is detected by variations in the intensity of the reflected light. The Applicants also know a photo-magnetic (magneto-optic) reproducing apparatus for arranging portions having a fixed magnetization direction in a recording medium (recording bits) and for reproducing information by a method in which the recording medium is radiated with light beams and the arrangement of the recording bits is detected by variations in the polarization direction of the reflected light. The above-mentioned reproducing apparatuses have problems in that their ability to read out information using a magnetic head or light beams is limited, and the recording density cannot be increased markedly since pits and recording bits have a length measured in sub-microns at the minimum. For further background art reference is made to GB-A-2 182 480. SUMMARY OF THE INVENTION An object of the invention is to provide a reproducing apparatus capable of reproducing information from a recording medium having a high recording density. In one aspect the invention, as defined by claim 1, provides apparatus for reproducing information by reading pits formed on a surface of a recording medium, said apparatus comprising: probing means for generating a tunnel current; a scanning means having first piezoelectric means for scanning said probing means over said surface of the recording medium, relatively; a tunnel current detecting means for detecting a tunnel current flowing between said probing means and said pits of the recording medium; a servo-circuit controlling second piezoelectric means according to the detected tunnel current, for driving said probing means in a direction towards/away from said surface of the recording medium to maintain a preselected distance between said probing means and said surface so as to keep said tunnel current substantially constant; and a transforming means for transforming voltage supplied to said second piezoelectric means by said servo-circuit into an information signal characterizing said surface of the recording medium; characterized by: a controlling means for controlling said scanning means by measuring lateral displacement of said probing means over said surface of the recording medium and compensating for hysteresis in the voltage drive of said first piezoelectric means for said scanning of the probing means. In another aspect the invention, as defined by claim 5, provides apparatus for reproducing information by reading pits formed on a surface of a recording medium, said apparatus comprising: a probing means; and a scanning means having first piezoelectric means for scanning said probing means over said surface of the recording medium, relatively; characterized by: said probing means being mounted on a cantilever for detecting an atomic interaction force between said probing means and said pits of the recording medium; a laser interferometer for measuring a displacement of said cantilever; a servo-circuit controlling second piezoelectric means according to the measured displacement of said cantilever, for driving said probing means in a direction towards/away from said surface of the recording medium to maintain a preselected distance between said probing means and said surface so as to keep said atomic interaction force substantially constant; and a transforming means for transforming voltage supplied to said second piezoelectric means by said servo-circuit into an information signal characterizing said surface of the recording medium. The combinations of features recited in the preambles of claims 1 and 5 are known from GB-A-2 182 480. According to the invention, the reproduction of pits formed with not only irregularities of the order of micro-metre (µm) but also irregularities of the order of nanometre (nm), and ultimately pits formed of one irregular atom is made possible. Therefore, information can be reproduced by using recording elements having a recording density several digits more than that of the recording elements used in the related art. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic configurational view of a reproducing apparatus using the measurement principles of STM (Scanning Tunnelling Microscope); Fig. 2 is an explanatory view of one embodiment of the invention in which a recording element is moved two-dimensionally while a probe-head is moved one-dimensionally; Fig. 3 is an explanatory view illustrating another embodiment of the invention in which the recording element is formed in the shape of a disk and is rotatable; Fig. 4 is a graph illustrating the displacement of a piezoelectric element used in the driving parts with respect to an applied voltage; Fig. 5 is a graph illustrating the displacement of a piezoelectric element used in the driving parts with respect to a set voltage by feedback control; Fig. 6 is an explanatory view illustrating one embodiment of the invention in which the position control in a horizontal direction in a probe-head is performed by means of a laser interferometer; Figs. 7A and 7B are graphs of illustrating the change in an output signal (surface shape) with respect to the movement of the probe-head; Fig. 8 is an explanatory view of a recording method in which information is recorded from pits formed with irregular individual atoms; Fig. 9 is a schematic configurational view of one embodiment of the invention using the measurement principles of AFM (Atomic Force Microscope); and Fig. 10 is a schematic configurational view of a laser interferometer used in the invention for detecting the displacement of a cantilever. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the invention will be explained hereinunder with reference to Figs. 1 to 8. The reproducing apparatus of this embodiment utilizes the measurement principles of a scanning tunnelling microscope (STM). As shown in Fig. 1, it uses a metallic material as a probe-head 1, a conductive material as a recording element 2, and utilizes a tunnel-current generated between atoms on the probe-head 1 and the recording element 2 when the probe-head 1 is a very small distance apart from the recording element 2. The surface of the recording element 2 has a planar form and on this planar surface pits with recesses and projections are arrayed in a fixed direction at an atomic level. For the probe-head 1, the tip of a metallic wire made of metallic atoms Pt, W, Ir, or Au 1 to 0.01 mm or thereabouts in diameter electrolytically polished or mechanically polished and machined to a tip of 0.1 µm in radius is used. On the tip of the probe-head 1 are furthermore small irregular atoms, and several of the most protruding of these irregular atoms form the actual probe-head. The probe-head 1 is so disposed that it can move a very small distance by means of a three-dimensional actuator 3 in a Z direction which is axial of the probe-head 1, in a X direction which is perpendicular to this Z direction and in a Y direction which is perpendicular to the Z and X directions respectively. The three-dimensional actuator 3 comprises an X-axis driving part 3a for driving in the X direction, a Y-axis driving part 3b for driving in the Y direction and a Z-axis driving part 3c for driving in the Z direction. The driving parts 3a, 3b and 3c are formed of piezoelectric element which expand and contract in response to an applied voltage. The probe-head 1 is connected with a voltage applying means 6 which is connected to the recording element 2 via a tunnel-current detector 4 as a detecting means. The tunnel-current detector 4 is so arranged that it can detect interactions generated between atoms of the probe-head 1 and the atoms of recording element 2 when the probe-head 1 is a very small distance apart from the recording element 2. The tunnel-current detector 4 is connected with a servo-circuit 5 which is arranged to control the voltage applied to the three-dimensional actuator 3 on the basis of the output from the tunnel-current detector 4. A scanning means comprises the three-dimensional actuator 3 and the servo-circuit 5, and is arranged to scan with the probe-head 1 a very small distance apart from the recording element 2. The servo-circuit 5 is connected with a voltage detecting means 7 which picks up variations in a voltage applied to the three-dimensional actuator 3, and the voltage detecting means 7 is connected to a regeneration means 8. The regeneration means 8 is arranged so as to regenerate information by detecting variations in irregularities formed on the surface of the recording element 2 by the input of a signal from the tunnel-current detector 4, which is a detecting means, via the servo-circuit 5 and the voltage detecting means 7, namely, on the basis of a voltage variation picked up by the voltage detecting means 7. In the above-mentioned arrangement, the distance between the probe-head 1 and the recording element 2 can be adjusted to 1 nm or thereabouts by applying a voltage of 1 V between the probe-head 1 and the recording element 2 by means of the voltage applying means 6 and by driving the probe-head 1 by the Z-axis driving part 3c, with the result that a tunnel-current J T flows by quantum effect. The tunnel-current J T is very sensitive to a variation in the distance between the probe-head 1 and the recording element 2. To keep the tunnel-current J T constant, a voltage applied to the Z-axis driving part 3c is controlled by the servo-circuit 5 to drive the probe-head 1 in the Z direction, and while the distance between the probe-head 1 and the recording element 2 is kept constant, the probe-head 1 is made to scan by the X-axis driving part 3a and the Y-axis driving part 3b in the X and Y directions. At this time, the variations in a voltage applied to drive the Z-axis driving part 3c are picked up by the voltage detecting means 7. The input of the picked-up signal to the regeneration means 8 enables the pattern of arrayed pits formed of irregularities on the surface of the recording element 2 to be detected and that information is reproduced. When irregularities on the recording element 2 are detected by using tunnel effects as described above, resolution of about 10⁻³ nm (10⁻² Å) in the vertical direction and about 10⁻¹ nm in the horizontal direction can be obtained. Ultimately, the reproduction using the recording element 2 having a high density in which individual irregularities of atoms are made pits, can be formed. In the above-mentioned embodiment, reproduction in a constant current mode in which tunnel-current J T is kept constant is described. As shown in Fig. 2, it may be arranged that the probe-head 1 is moved by the Z-axis driving part 3c in the Z direction on one hand and a table on which the recording element 2 is deposited is moved by the X-axis driving part 3a and the Y-axis driving part 3b in the X and Y directions on the other hand. Further, as shown in Fig. 3, it may be arranged that the recording element 2 is formed in a disk form and trains of pits are formed in a voluted or concentric circle form, and while the disk-like recording element 2 is rotated, the position of reproduction is gradually shifted. In the above-mentioned reproducing apparatus, the control of the distance between the probe-head 1 and the recording element 2, i.e., the positional control in the Z direction of the probe-head 1, is made by detecting the tunnel-current J T . However, it has the drawback that reliability of tracking is low, since tracking by the probe-head 1 in the lateral direction (in the X or Y direction) is made only by applying a voltage to the X-axis driving part 3a and the Y-axis driving part 3b, and therefore, an actual movement is not confirmed. In other words, as shown in Fig. 4, the applied voltage - displacement characteristics of a piezoelectric element constituting each of the driving parts 3a, 3b and 3c have hysteresis, thereby causing an error. To solve this error, lateral position control of the probe-head 1 is made by a laser interferometer. That is, as shown in Fig. 5, to measure a relative distance between the probe-head 1 and the recording element 2, laser beams are irradiated toward a table on which a supporting section of the probe-head 1 and the recording element 2 are deposited. Its interference light is obtained and while the position of the probe-head 1 is detected, a voltage applied to a piezoelectric element is set by performing feedback control. As a result, the piezoelectric element can be displaced linearly and the probe-head 1 (or the recording element 2) can be positioned accurately in a lateral direction on the order of nanometer (nm), as shown in Fig. 6. A specific arrangement of the laser interferometer 10 will be described in detail in other embodiments to be mentioned later. In cases where there is no undulation on the surface of the recording element 2, and the recording element 2 is positioned not on an inclination as shown in Figure 7A, a regeneration signal shows the same waveforms as that of the irregularities formed on the recording element 2. When there are undulations on the recording element 2, or the recording element 2 is positioned on an inclination as shown in Figure 7B, components of the undulation and inclination are contained in a regeneration signal. However, the frequency of the above-mentioned components of the undulation or inclination is three digits smaller than that of a regeneration signal of irregularities, on account of which the components of the undulation or inclination can be eliminated by making them pass through a high-pass filter. In the above explanation, the reproduction of pits due to irregularities on the surface of the recording element 2 is explained. Recording of pits using a device shown in Fig. 1 is also possible. To be specific, as shown in Fig. 8, the operation such that one atom 1a, constituting the tip of the probe-head 1, is added to any position on the recording element 2, or any one of atoms 2a on the recording element 2, is removed, is performed by employing an electric-field evaporation process which is considered to be the ultimate in refinement process technology. The tip of the probe-head 1 is brought into an unstable state and atoms are electric-field evaporated as positive ions from the anode side and attached on the cathode side (the reverse is true of the case of negative ions), with the result that pits can be recorded by means of individual irregularities of atoms. Another embodiment of the invention will be explained hereinunder with reference to Figs. 9 and 10. Those parts having the same functions as those of the previous embodiment are given the same reference number, and the explanation is omitted. The reproducing apparatus of the present embodiment uses the measurement principles of an atomic force microscope (AFM). As shown in Fig. 9, to detect the atomic force between the atoms on the tip of the probe-head 1 and the atoms on the surface of the recording element 2, the probe-head 1 is supported by means of the cantilever 9. The cantilever 9 is so arranged that it receives a repulsion or an attraction generated when the probe-head 1 is placed close to the recording element 2 and is elastically deformed easily. The atomic force is detected by the elastic deformation of the cantilever 9. While the position of the probe-head 1 is adjusted so as to keep the atomic force constant, the probe-head 1 is made to scan and the irregularities on the surface of the recording element 2 are detected in the same manner as in the above-mentioned embodiment. According to this embodiment, conductive materials do not need to be used as the probe-head 1 and the recording element 2, and use of insulating materials or the like is made possible. For the measurement of the displacement of the cantilever 9, for example, a laser interference method may be employed. As shown in Figure 10, a laser beam of two-cycle cross linear polarized light is radiated from a Zeeman laser 11 in the laser interferometer 10. Part of this laser beam is reflected by a half-mirror 12 and the other part penetrates it. The resulting beams diverge in two different directions. One of the diverged beams of light enters a photo detector 14 via a polarizing plate 13. The other of the diverged light enters a Wollaston prism 15. The laser beam is divided into two components, different in frequency, intersecting each other at right angles. One of the divided beams of light penetrates a lens 16, after which it is reflected by the cantilever 9. The other of the divided beams of light penetrates the lens 16, after which it is reflected by a reference mirror 17. After those reflections, the beams penetrate the lens 16 and are overlapped on the Wollaston prism 15. The interference light by this overlap is reflected by a reflection mirror 18, after which it enters a photo detector 20 via a polarizing plate 19 and a beat signal having the frequency of the difference between the two lights overlapped on the interference light is obtained. Since the phase of the beat signal is varied by the deformation of the cantilever 9, the amount of the displacement of the cantilever 9 can be measured by detecting the phase variation. The cantilever 9 and the laser interferometer 10 constitute a detecting means for detecting the interactions generated when the probe-head 1 is a very small distance apart from the surface of the recording element 2. The displacement of the cantilever 9 is the variation in irregularities on the recording element 2. Therefore, by picking up the amount of the displacement of the cantilever 9 as a voltage variation, a pattern of arrayed pits are detected by a regeneration means (not shown) and information is reproduced. According to this reproducing apparatus, piezoelectric elements or the like shown in the above-mentioned embodiment can be used as a scanning means of the probe-head 1 and the recording element 2. The shape of pits are not needed to be measured accurately over the entire surface by making the number of the scanning lines large, as when the shape of the surface of a specimen is measured by STM or AFM, and the presence of pits can be detected by scanning one time per one pit train. Many widely different embodiments of the present invention may be constructed without departing from the scope of the invention. It should be understood that the invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

Claims: Apparatus for reproducing information by reading pits formed on a surface of a recording medium (2), said apparatus comprising: probing means (1) for generating a tunnel current; a scanning means having first piezoelectric means (3a,3b) for scanning said probing means (1) over said surface of the recording medium, relatively; a tunnel current detecting means (4) for detecting a tunnel current flowing between said probing means (1) and said pits of the recording medium; a servo-circuit (5) controlling second piezoelectric means (3c) according to the detected tunnel current, for driving said probing means (1) in a direction towards/away from said surface of the recording medium to maintain a preselected distance between said probing means (1) and said surface so as to keep said tunnel current substantially constant; and a transforming means (7,8) for transforming voltage supplied to said second piezoelectric means (3c) by said servo-circuit (5) into an information signal characterizing said surface of the recording medium (2); characterized by: a controlling means (10,5) for controlling said scanning means (3a,3b) by measuring lateral displacement of said probing means (1) over said surface of the recording medium and compensating for hysteresis in the voltage drive of said first piezoelectric means (3a,3b) for said scanning of the probing means. Apparatus according to claim 1, wherein said transforming means (7,8) is capable of directly transforming high frequency components of said voltage supplied to said second piezoelectric means (3c) into said information signal characterizing said surface of the recording medium. Apparatus according to claim 1, wherein said transforming means (7,8) includes a high-pass filter capable of transforming high frequency components of said voltage supplied to said second piezoelectric means (3c) into said information signal characterizing said surface of the recording medium. Apparatus according to any one of claims 1 to 3, wherein said controlling means (10,5) includes a laser interferometer (10) for measuring said lateral displacement of said probing means (1). Apparatus for reproducing information by reading pits formed on a surface of a recording medium (2), said apparatus comprising: a probing means (1); and a scanning means having first piezoelectric means (3a,3b) for scanning said probing means (1) over said surface of the recording medium, relatively; characterized by: said probing means (1) being mounted on a cantilever (9) for detecting an atomic interaction force between said probing means (1) and said pits of the recording medium; a laser interferometer (10) for measuring a displacement of said cantilever (9); a servo-circuit (5) controlling second piezoelectric means (3c) according to the measured displacement of said cantilever (9), for driving said probing means (1) in a direction towards/away from said surface of the recording medium to maintain a preselected distance between said probing means (1) and said surface so as to keep said atomic interaction force substantially constant; and a transforming means (7,8) for transforming voltage supplied to said second piezoelectric means (3c) by said servo-circuit (5) into an information signal characterizing said surface of the recording medium (2). Apparatus according to claim 4 or claim 5, wherein said laser interferometer (10) includes a Zeeman laser light source (11). Apparatus according to any one of the preceding claims, wherein said probing means is a probe-head (1) and said first piezoelectric means comprises an X-axis driving part (3a) capable of driving said probe-head in the X direction and a Y-axis driving part (3b) capable of driving said probe-head in the Y direction in a rectangular coordinate system. Apparatus according to claim 7, wherein said second piezoelectric means comprises a Z-axis driving part (3c) capable of driving said probe-head in the Z direction in a rectangular coordinate system.

Inventors: KOBAYASHI SHOZOU, OHTA KENJI, SATO HIDEAKI, UEYAMA TETSUO, KOBAYASHI, SHOZOU, OHTA, KENJI, SATO, HIDEAKI, UEYAMA, TETSUO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G11B9
IPC codes: G11B11, B82B3, G01N37, G11B9, G01Q10, G01Q20