Surface Inspection Equipment

Abstract

To provide a surface inspection device capable of detecting defects over a wide range of sizes.SOLUTION: The surface inspection device detects defects on the surface of magnetic disks based on the amplitude of a difference signal of the light intensity in the first interference region and the light intensity in the second interference region detected by a split photodetector.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a surface inspection device that detects defects on the surface of a magnetic disk by using a laser beam. [Background technology]

[0002] Hard disk drives (HDDs) are used as external storage devices for computers, and their storage capacity is on the rise. In HDDs, a magnetic disk, which is an aluminum (or glass) substrate coated with a magnetic material, is rotated at high speed, and magnetic heads are floated and run above the surface to record and reproduce magnetic data.

[0003] As the areal recording density of magnetic disks increases, the flying height of the magnetic head, i.e., the distance between the head and the disk, has been reduced to ensure reliability in recording data to the miniaturized recording magnetic domains and in reproducing weakened magnetic signals, and this distance is currently at a level of less than 10 nm. For this reason, the flatness of the magnetic disk surface is important for stable flying of the magnetic head, and the challenge for further increases in areal recording density is to reduce minute unevenness defects and waviness (wavy defects). Therefore, there is an increasing demand for surface inspection equipment that can measure the height of such defects with nanometer-order accuracy.

[0004] For example, Patent Document 1 discloses a conventional magnetic disk surface inspection device that irradiates a magnetic disk, which is a sample to be inspected, with a laser, receives reflected and scattered light from the magnetic disk surface with multiple detectors, and classifies micro defects according to the light receiving conditions of each detector. It also judges the planar continuity of the detected micro defects, classifies them according to their length, and classifies them into linear defects and clump defects. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2000-180376 A Summary of the Invention [Problem to be solved by the invention]

[0006] One of the problems with surface inspection devices that detect defects using the above-mentioned scattered light intensity is the lack of a measurement dynamic range. As an example, the scattered light intensity from a cylindrical convex defect with a height of 5 nm was calculated assuming a light wavelength of 810 nm and aluminum as the material of the substrate and defect, and in the range of cylinder diameters of 0.1 to 1.0 μm, the scattered light intensity increases in proportion to approximately the 3.2 power of the diameter. That is, the scattered light intensity for a diameter of 1.0 μm is approximately 1500 times greater than that for a diameter of 0.1 μm. This exceeds the dynamic range (8 bits) of the detection system of a general surface inspection device, and it is not possible to detect all defects over a wide range of diameters from 0.1 to 1.0 μm.

[0007] An object of the present invention is to provide a surface inspection apparatus capable of detecting defects over a wide range of sizes. [Means for solving the problem]

[0008] A surface inspection device of one embodiment of the present invention comprises a magnetic disk, a laser light source, an objective lens that focuses laser light emitted from the laser light source and irradiates it onto the surface of the magnetic disk, a split photodetector that detects the light intensity of a first interference region of 0th order light and +1st order diffracted light of reflected light from the magnetic disk on the pupil plane of the objective lens, and the light intensity of a second interference region of 0th order light and -1st order diffracted light of the reflected light, respectively, and a control unit, wherein the control unit detects defects on the surface of the magnetic disk based on the amplitude of a difference signal between the light intensity of the first interference region and the light intensity of the second interference region detected by the split photodetector. Effect of the Invention

[0009] According to one aspect of the present invention, it is possible to provide a surface inspection device capable of detecting defects over a wide range of sizes. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram showing an example of the configuration of a defect detection optical system for implementing a surface inspection method according to the present invention; [Diagram 2] 1 is a schematic diagram showing an example of the configuration of a surface inspection device according to the present invention; [Diagram 3] 1 is a diagram showing the principle of detecting defects present on a magnetic disk surface using a push-pull method; [Figure 4] FIG. 1 is a schematic diagram showing a push-pull signal for a convex defect. [Diagram 5] FIG. 1 is a schematic diagram showing a push-pull signal in a concave defect. [Figure 6] FIG. 1 is a schematic diagram showing a push-pull signal in a wavy defect. [Figure 7] 1A and 1B are diagrams illustrating the principle of detecting a focus error, that is, the amount of deviation of the focal point of an illumination light beam from the disk surface, using an astigmatism method. [Figure 8] 11 is a graph showing a calculation example of the relationship between the swell period and the push-pull signal amplitude. [Figure 9] FIG. 13 is a diagram showing a calculation example of a push-pull signal for a convex defect. [Figure 10] FIG. 13 is a diagram showing a calculation example of a push-pull signal for a concave defect. [Figure 11] FIG. 13 is a diagram illustrating another example of the configuration of the defect detection optical system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. EXAMPLES

[0012] <Configuration of defect detection optical system> FIG. 1 is a schematic diagram showing an example of the configuration of a defect detection optical system 2 for implementing an optical signal detection method according to the present invention. The defect detection optical system of the first embodiment guides light emitted from a semiconductor laser to a magnetic disk, where it is reflected, and detects the reflected light with a detector.

[0013] Laser light emitted from a laser diode 101, which is a light source, is collimated by a collimator lens 111 to become a parallel beam, and then the beam diameter is restricted to a predetermined size by an aperture 114 to become an illumination light beam 102. The illumination light beam 102 passes through a λ / 2 plate 121 and enters a polarizing beam splitter 131, where its traveling direction is changed by 90 degrees.

[0014] The polarizing beam splitter has a function of transmitting almost 100% of p-polarized light (hereinafter referred to as horizontally polarized light) incident on the separation surface and reflecting almost 100% of s-polarized light (hereinafter referred to as vertically polarized light). By adjusting the rotation angle of the λ / 2 plate 121 around the optical axis with respect to the illumination light beam 102, the polarization direction of the light passing through the λ / 2 plate 121 can be arbitrarily changed, and the intensity ratio of the transmitted light and the reflected light at the polarizing beam splitter 131 can be arbitrarily adjusted. In this embodiment, the rotation angle of the λ / 2 plate 121 is adjusted so that the polarization direction of the light passing through the λ / 2 plate 121 becomes vertically polarized light, and the illumination light beam 102 is reflected almost 100% by the polarizing beam splitter 131.

[0015] The illumination light beam 102 reflected by the polarizing beam splitter 131 is converted into circularly polarized light by passing through the λ / 4 plate 122, and then focused by the objective lens 112 and irradiated onto the magnetic disk 3, which is the object to be measured. The beam reflected by the magnetic disk 3 rotated by the rotary motor 201 (hereinafter, the reflected light beam 103) is converted back into parallel light by the objective lens 112 and converted back into linearly polarized light by the λ / 4 plate 122, but since the direction of rotation of the circularly polarized light is inverted by reflection on the disk surface, the direction of the linear polarization becomes horizontally polarized light perpendicular to the original light. Therefore, the horizontally polarized reflected light beam 103 transmitted through the λ / 4 plate 122 passes through the polarizing beam splitter 131 and heads toward the non-polarizing beam splitter 132.

[0016] A non-polarizing beam splitter has a function of reflecting and transmitting light incident on the splitting surface at a predetermined ratio regardless of the polarization direction. The non-polarizing beam splitter 132 of this embodiment reflects and transmits incident light at a ratio of 50:50.

[0017] The first reflected light beam 103a reflected by the non-polarizing beam splitter 132 is collected by the collecting lens 113 and enters the four-segment photodetector 11. A push-pull signal, which will be described later, is generated from the output signal of the four-segment photodetector 11. The second reflected light beam 103b transmitted through the beam splitter 132 is collected by the collecting lens 116, given astigmatism by the cylindrical lens 117, and enters the four-segment photodetector 12. A focus error signal (FES), which will be described later, is generated from the output signal of the four-segment photodetector 12.

[0018] <Overall configuration of surface inspection device> FIG. 2 shows an example of the overall configuration of a surface inspection device according to the present invention.

[0019] The surface inspection device 1 includes a defect detection optical system 2 and a rotary motor 201 , and the magnetic disk 3 can be rotated by the rotary motor 201 .

[0020] The defect detection optical system 2 serves to detect defects on the surface of the magnetic disk 3 by irradiating the magnetic disk 3 with light and detecting the reflected light. The reflected light detected by the defect detection optical system 2 is converted into a current-voltage (IV) signal and then input to a signal processor 203. The signal processor 203 generates a push-pull signal and a focus error signal, which are sent to a controller 202, which is a control unit. The controller 202 controls a servo controller 204 based on the focus error signal.

[0021] A servo control circuit 204 controls the position of the objective lens of the defect detection optical system 2 described later, an access control circuit (Access controller) 205 controls the position of the rotary motor 201 by a slider 210, and an automatic position controller (Position controller) 206 controls the rotation of the magnetic disk 3 by the rotary motor 201. This allows the light spot 102s to be positioned at an arbitrary position on the magnetic disk 3. The controller 202 also controls a laser driver 207 to cause a laser diode included in the defect detection optical system 2 to emit light with an appropriate power.

[0022] <Illumination light beam and reflected light beam> Laser light emitted from a laser diode 101 with a wavelength of 660 nm mounted in the defect detection optical system 2 is collimated by a collimator lens 111 to become a parallel beam, and then the beam diameter is limited by an aperture 114 to 6.0 mm, which is slightly larger than the effective diameter (5.1 mm) of the objective lens, to become an illumination light beam 102. The illumination light beam 102 passes through a λ / 2 plate 121 to become vertically polarized light, and is reflected by a polarizing beam splitter 131, changing its traveling direction by 90 degrees.

[0023] The illumination light beam 102 reflected by the polarizing beam splitter 131 is converted into circularly polarized light by passing through the λ / 4 plate 122, and then condensed by the objective lens 112 with NA 0.43 and effective diameter 5.1 mm, and is irradiated onto the magnetic disk 3 to form a light spot 102s on the disk surface. The light spot diameter is 0.66 μm / 0.43=approximately 1.5 μm, which is suitable for detecting uneven defects with diameters in the range of 0.1 to 1.0 μm.

[0024] The beam reflected by the magnetic disk 3 (reflected light beam 103) becomes horizontally polarized light after passing through the objective lens 112 and the λ / 4 plate 122. The reflected light beam 103, now horizontally polarized, passes through a polarizing beam splitter 131, and 50% of the light amount is reflected by a non-polarizing beam splitter 132, while the remaining 50% is transmitted.

[0025] The first reflected light beam 103a reflected by the non-polarizing beam splitter 132 is focused by the condenser lens 113 and enters the four-segment photodetector 11. The second reflected light beam 103b transmitted through the non-polarizing beam splitter 132 is focused by the condenser lens 116, given astigmatism by the cylindrical lens 117, and enters the four-segment photodetector 12.

[0026] <Push-pull signal detection> FIG. 3 is a diagram showing the principle of detecting defects present on a magnetic disk surface using the push-pull method.

[0027] The push-pull method is applied to optical disk drives as a means for measuring the amount of deviation of a light spot in order to control the light spot to follow a guide groove (track) on which data is recorded.

[0028] When the illumination light beam 102 is focused and irradiated onto the surface of the magnetic disk 3 by the objective lens 112 to form a light spot 102s, if there is a step 301 due to a defect in the light spot 102s, the reflected light is diffracted. As a result, on the pupil plane 321 of the objective lens 112, a first interference region 314 where the 0th order light 311 and the +1st order diffracted light 312 overlap and interfere, and a second interference region 315 where the 0th order light 311 and the -1st order diffracted light 313 overlap and interfere are formed.

[0029] The light intensities of these interference regions change according to the phase difference between the zeroth-order light and the first-order diffracted light overlapping it, and this phase difference changes depending on the relative positions of the light spot 102s and the step 301. Specifically, when the light spot 102s is located at the center of the step 301, the phase differences in the first interference region 314 and the second interference region 315 are equal, and the light intensities of these two interference regions are equal to each other. When the light spot 102s is located at the left edge of the step 301, the balance of the phase differences between the two interference regions changes, and the light intensity of the first interference region 314 becomes greater than that of the second interference region 315. On the other hand, when the light spot 102s is located at the right edge of the step 301, the relationship between the phase differences between the two interference regions is reversed, and the light intensity of the first interference region 314 becomes smaller than that of the second interference region 315.

[0030] Here, the light intensity of the first interference region 314 and the second interference region 315 are detected by the two-division detector 331, the difference between the two is calculated, and a signal normalized by the sum of the light amounts of the two is called a push-pull signal. The calculation formula for the push-pull signal is shown in Equation 1.

[0031]

number

[0032] The value of the push-pull signal when the light spot 102s crosses the step 301 is zero when the light spot 102s is at the center of the step 301, is a maximum value (positive peak) when the light spot 102s is at the left edge of the step 301, and is a minimum value (negative peak) when the light spot 102s is at the right edge of the step 301. Between both edges of the step 301, the value changes as a sine function with respect to the amount of deviation of the light spot from the center of the step.

[0033] In addition, the amplitude of the push-pull signal when the light spot 102s crosses the step 301, i.e., the difference between the maximum and minimum values, is maximum when the height of the step is 1 / 8 of the wavelength of the illumination light, and changes roughly in proportion to the height of the step in the range where the height is less than that and sufficiently smaller than the wavelength.

[0034] According to the above principle, the height of a defect is detected by scanning a light spot on the magnetic disk surface and measuring the push-pull signal amplitude when the light spot crosses over the defect.

[0035] <Push-pull signal for uneven defects> Next, measurement of a push-pull signal for a concave-convex defect will be described as a specific defect shape. The concave-convex defect is assumed to have a cylindrical shape and a diameter smaller than the light spot diameter (λ / NA).

[0036] 4 is a diagram showing the light intensity distribution on the four-segment photodetector when the light spot crosses the convex defect in the x direction. Here, the push-pull signal PPx is defined as Equation 2.

[0037]

number

[0038] Push-pull signal PP in the x direction x is calculated as the difference in light intensity between two divided areas corresponding to the x direction of the disk surface in a four-division photodetector, and detects steps in the x direction.

[0039] When the center of the light spot 102s is located at the center of the convex defect 401, the light intensities of the first interference region 411 and the second interference region 412 become equal to each other, and a push-pull signal PP x becomes zero. As the center of the light spot 102s shifts leftward (in the negative direction of the x-axis) from the center of the convex defect 401, the light intensity in the first interference region 411 becomes greater than that in the second interference region 412, and when the shift amount of the spot position is ¼ of the diameter of the light spot 102s, that is, −λ / (4NA) where λ is the wavelength, the difference in light intensity between the two interference regions becomes maximum, and the push-pull signal PP x is the maximum value (positive peak).

[0040] On the other hand, as the center of the light spot 102s shifts to the right (positive direction of the x-axis) from the center of the convex defect 401, the light intensity in the first interference region 411 becomes smaller than that in the second interference region 412. When the shift amount of the spot position is ¼ of the diameter of the light spot 102s, that is, +λ / (4NA) where λ is the wavelength, the difference in light intensity between the two interference regions becomes maximum, and the push-pull signal PP x In this way, for a convex defect with a diameter smaller than the light spot diameter, the interval between the positive and negative peaks of the push-pull signal is approximately equal to half the light spot diameter, regardless of the defect diameter.

[0041] Furthermore, the amplitude of the push-pull signal, i.e., the difference between the maximum and minimum values ​​of the push-pull signal, changes approximately in proportion to the height and area (the square of the diameter) of the defect. Using this, the height of the uneven defect is identified under the condition that the diameter of the uneven defect is approximately constant. Furthermore, a reference size of the defect is determined, and the push-pull signal amplitude for a defect of this reference size is set as a threshold value. When the measured push-pull signal amplitude exceeds the threshold value, it is determined that either the height or the diameter of the measured defect exceeds the reference value.

[0042] The push-pull signal can also be defined in the y direction. y is defined as Equation 3.

[0043]

number

[0044] Push-pull signal PP in the y direction y is calculated as the difference in light intensity between two regions divided in a four-division photodetector in the y direction of the disk surface, and detects steps in the y direction.

[0045] 5 is a diagram showing the light intensity distribution on a four-segment photodetector when the light spot crosses a concave defect in the x direction. As shown in the figure, for a concave defect, the direction of deviation of the light spot from the defect center and the magnitude relationship between the light intensities of the first interference region 411 and the second interference region 412 are reversed from those for a convex defect, and accordingly the polarity of the push-pull signal is also reversed from those for a convex defect.

[0046] <Concave / convex discrimination> As described above, by utilizing the fact that the polarity of the push-pull signal is inverted depending on whether the shape of the uneven defect is convex or concave, it is possible to distinguish whether the defect is concave or concave. In this embodiment, when the light spot crosses the defect in the x direction, if the order of appearance of the peaks of the push-pull signal is positive and negative, the defect is determined to be convex, and if the order of appearance of the peaks is negative and positive, the defect is determined to be concave.

[0047] On the other hand, with conventional methods of detecting defects by measuring the intensity of scattered light, it was difficult to distinguish between concave and convex defects because the scattered light intensity from concave and convex defects of the same size was the same.

[0048] <Dynamic range in the case of push-pull method> The dynamic range when detecting uneven defects using the push-pull method is described below. In the conventional method of detecting defects based on the intensity of scattered light, as shown in the above calculation example, the intensity of scattered light from a cylindrical convex defect with a height of 5 nm increases in proportion to approximately the 3.2nd power of the diameter in the range of the diameter of the cylinder from 0.1 to 1.0 μm. In other words, the intensity of scattered light for a diameter of 1.0 μm is approximately 1500 times greater than that for a diameter of 0.1 μm.

[0049] On the other hand, in the method using the push-pull method of the present invention, the push-pull signal amplitude increased in proportion to approximately the 1.6th power of the diameter in the range of cylinder diameters from 0.1 to 1.0 μm. In other words, the push-pull signal amplitude in the case of a diameter of 1.0 μm was approximately 40 times that in the case of a diameter of 0.1 μm, and the change in signal magnitude associated with the change in defect diameter was significantly suppressed. Within the dynamic range (8 bits) of the detection system of a general surface inspection device, it is possible to detect all defects in the diameter range of 0.1 to 1.0 μm.

[0050] <Push-pull signal for waviness defect> Next, as another example of a specific defect form, measurement of a push-pull signal for a wavy defect will be described. The wavy defect has a single period, the shape of the undulations is a sine wave, and the period range of the wavy defect to be detected is 20 to 100 μm. The wavelength of the illumination light is 660 nm, the same as in the case of uneven defects, and the NA of the objective lens is 0.033. The light spot diameter (λ / NA) on the disk surface is 0.66 μm / 0.035=20 μm, which is adjusted to the minimum period within the period range of the wavy defect to be detected.

[0051] FIG. 6 is a diagram showing the light intensity distribution on a four-segment photodetector when a light spot crosses a waviness defect in the direction of the waviness (x direction).

[0052] When the center of the light spot 102s is located at an antinode (peak or valley) of the undulation of the undulation defect 601, a push-pull signal PP x becomes zero. When the center of the light spot 102s is located at the node of the undulation of the waviness defect, the push-pull signal PP x is a maximum or minimum value (positive or negative peak).

[0053] The period and amplitude of the wavy defect are detected by the following procedure. The push-pull signal waveform of the wavy defect measured along the circumferential direction of the disk is Fourier transformed to calculate the amplitude spectrum. Fast Fourier transform (FFT) is used as the Fourier transform algorithm. The horizontal axis of the amplitude spectrum is the wavy period, and the vertical axis is the amplitude. A peak is searched for in the amplitude spectrum, and the period corresponding to the position of the peak is obtained as the period of the wavy defect. The push-pull signal amplitude is also obtained as the height of the peak.

[0054] <Detection of focus error signal> FIG. 7 is a diagram showing the principle of detecting a focus error, that is, the amount of deviation of the focal point of an illumination light beam from the disk surface, using the astigmatism method.

[0055] As the second reflected light beam 103b is converged by the condenser lens 116, astigmatism is generated by the cylindrical lens 117. The cylindrical lens 117 acts as a lens only in a single direction, and in the direction perpendicular to that, it is the same as a parallel plate and does not act as a lens. Therefore, astigmatism occurs because the focal length differs in each direction. In the defect detection optical system 2 of this embodiment, the focal length of the beam seen from a cross section of the cylindrical lens 117 that does not have a lens effect is approximately the same as the focal length (fdet) of the condenser lens 116. On the other hand, the beam seen from a cross section that has a lens effect does not focus there, and a long and thin line image (focal line) is obtained.

[0056] In addition, the beam seen from the cross section of the cylindrical lens 117 with the lens effect converges to the focal length of the combined lens of the condenser lens 116 and the cylindrical lens 117, but the beam seen from the cross section without the lens effect does not focus there and becomes a focal line. The focal length of the combined lens fcomb is expressed by the formula 4, where fcyl is the focal length of the cylindrical lens 117 and Dlens is the distance between the condenser lens 116 and the cylindrical lens 117. The beam becomes circular midway between the two focal lines.

[0057]

number

[0058] FIG. 7 is a conceptual diagram for explaining a method for measuring the deviation (defocus) of the focal point of an illumination light beam from the disk surface by utilizing astigmatism.

[0059] In the optical system for detecting astigmatism, the second reflected light beam 103b passes through a condenser lens 116 and a cylindrical lens 117, and astigmatism occurs due to the action of the cylindrical lens 117. The amount of astigmatism changes according to defocus. Defocus can be measured by utilizing the fact that the beam shape on the four-segment photodetector 12 changes according to the amount of deviation.

[0060] When measuring defocus using astigmatism, the center of the four-segment photodetector 12 is aligned with the position where the reflected light beam from the focal plane of the objective lens 112 becomes circular, as shown in (b) of Fig. 7. The four-segment photodetector 12 is composed of four photodiodes, and the optical signal detected by each cell is converted into an electrical signal. The signal processing circuit 203 calculates a focus error signal FES from the four outputs A, B, C, and D of the four-segment photodetector 12 using Equation 5.

[0061]

number

[0062] When the height of the disc surface coincides with the focal plane (focal length) of the objective lens 112, FES=0. When the disc surface is lower than the focal plane, as shown in (a) of Fig. 7, the beam becomes horizontally elongated at an angle to the left, and FES>0. On the other hand, when the disc surface is higher than the focal plane, as shown in (c) of Fig. 7, the beam becomes horizontally elongated at an angle to the right, and FES<0.

[0063] The sign of the FES depends on the installation angle with the optical axis direction of the cylindrical lens 117 as the center of rotation. In this embodiment, the lens is installed at an angle of 45 degrees from the vertical direction with the optical axis direction as the center of rotation. The optical axis direction here is defined as the direction in which light propagates.

[0064] In this way, the amount of deviation from the surface of the magnetic disk 3 having irregularities as defects is detected based on the focus error signal. EXAMPLES

[0065] <Method of correcting push-pull signal amplitude for waviness defects> The second embodiment is a configuration example for correcting the measurement value of a push-pull signal for a waviness defect.

[0066] In the measurement results of the waviness period dependency, the push-pull signal amplitude reaches a maximum at a waviness period of 20 μm as expected, and decreases monotonically as the waviness period increases. However, this indicates that the detection sensitivity of the push-pull signal amplitude is not constant in the waviness period range of 20 to 100 μm that is the subject of inspection, and therefore conversion from the push-pull signal amplitude to the waviness amplitude cannot be performed using a uniform relational formula. The compensation method is shown below.

[0067] Figure 8 is a graph in which the relationship between the undulation period and the push-pull signal amplitude is calculated by optical simulation under the condition of a undulation amplitude of 1 nm, and the value normalized by the push-pull signal amplitude when the undulation period is 20 μm (push-pull signal amplitude ratio) is plotted. This push-pull signal amplitude ratio r is stored as a function of the undulation period, and the push-pull signal amplitude PP meas By dividing by the amplitude ratio r corresponding to the detected undulation period, the push-pull signal amplitude PP is obtained, where the difference in detection sensitivity due to the undulation period is corrected. comp This makes it possible to convert the push-pull signal amplitude into the waviness amplitude using a uniform relational expression calibrated with a waviness period of 20 μm.

[0068]

number

[0069] <How to display push-pull signal measurements for uneven defects> The third embodiment is a configuration example in which the measured value of a push-pull signal for a concave-convex defect is visualized and displayed to the user.

[0070] 9 is a diagram showing an example of calculation of a push-pull signal for a convex defect. The shape of the convex defect is a cylinder, with a diameter of 0.2 μm and a height of 2 nm.

[0071] FIG. 9(a) is a diagram showing the two-dimensional distribution of the push-pull signal PPx in a 3×3 μm region centered on a convex defect using contour lines. Here, the solid contour lines indicate positive values, and the dashed contour lines indicate negative values. FIG. 9(b) is a two-dimensional distribution contour line of the push-pull signal PPy in the same region. FIG. 9(d) is a waveform of the push-pull signal PPx along the dashed arrow in FIG. 9(a), and FIG. 9(e) is a waveform of the push-pull signal PPy along the dashed arrow in FIG. 9(b).

[0072] Here, an index S calculated using the push-pull signals PPx and PPy is defined as shown in Equation 7.

[0073]

number

[0074] Here, x and y are the x and y coordinates on the disk surface, respectively, and Δ is 1 / 2 the light spot diameter (λ / NA).

[0075] The first term of the formula 7 is the push-pull signal PP x The two-dimensional data of is shifted by Δ / 2 in the positive direction of the x-axis, and the second term is the same as PP x is obtained by shifting the two-dimensional data of in the negative direction of the x-axis. As mentioned above, the interval between the positive and negative peaks of the push-pull signal for an uneven defect with a diameter smaller than the light spot diameter is approximately equal to half the light spot diameter, so when the data of the first and second terms are overlapped, the positive and negative peaks of the push-pull signal overlap at the same position on the x-axis.

[0076] Here, the difference between the first and second terms results in a distribution with one positive peak, and the height of the peak is approximately equal to the amplitude of the push-pull signal. y The third and fourth terms are obtained by the same operation for , and the difference between the third and fourth terms is PP x The distribution has a single positive peak, which is almost the same as in the case of . Finally, the index S is obtained by taking the sum of these two distributions.

[0077] FIG. 9(c) shows the two-dimensional distribution of the index S using contour lines, and FIG. 9(f) shows the waveform of the index S along the dashed arrow in FIG. 9(c). Thus, the index S has a distribution with a single positive peak, and the height of the peak is PP x Amplitude and PP y The amplitude of the signal is approximately equal to the sum of the amplitudes of the signals.

[0078] Figure 10 shows the result of performing the same calculations as for the convex defect in Figure 9 for a concave defect.

[0079] Push-pull signal PP x or PP y The polarity of the peak of or PP is reversed compared to the case of the convex defect, and the calculated index S has a distribution with a single negative peak.

[0080] The index S calculated as described above is displayed to the user as an index indicating the magnitude of the push-pull signal amplitude. Push-pull signal PP x or PP y The advantage when compared to the method of directly displaying PP or PP to the user is that in the distribution data of push-pull signal PP x or PP y it is difficult to identify the center position of the defect because two positive and negative peaks appear around the defect, whereas in the case of index S, only a single peak appears, so the peak appearance position can be identified as the center position of the defect.

[0081] In addition, to discriminate between the concavity / convexity of the defect from the distribution data of push-pull signal PP x or PP y it is necessary to know the rule of the appearance order of the positive and negative peaks of the push-pull signal, whereas in the case of index S, since the peak polarity is positive for convex defects and negative for concave defects, visual discrimination between concavity / convexity becomes easy.

[0082] Thus, in Example 3, a display unit (not shown) is further arranged in the surface inspection apparatus 1 of Figure 2. The control unit (Controller) 2 calculates the two-dimensional distribution data of index S. Then, the display unit visualizes and displays the center position of the defect as the position of the peak in the two-dimensional distribution data of index S, the height of the defect as the height of the peak, and the type of concavity / convexity of the defect as the polarity of the peak.

Example

[0083] <Switching of NA> The fourth embodiment is an example of the configuration of a defect detection optical system that can handle a wide range of diameters of uneven defects.

[0084] 11 is a diagram showing another example of the configuration of the defect detection optical system. The defect detection optical system 21 is configured by adding a pinhole 115 between the λ / 4 plate 122 and the objective lens 112 in the configuration of the defect detection optical system 2. The opening diameter of the pinhole 115 is 1.0 mm. The pinhole 115 is configured to be insertable and removable from the optical path.

[0085] The wavelength of the laser light is 660 nm, the effective diameter and NA (numerical aperture) of the objective lens are 5.1 mm and 0.43, respectively, so that the light spot diameter (λ / NA) when the pinhole 115 is removed from the optical path is 0.66 μm / 0.43=1.5 μm, which is an ideal condition for detecting uneven defects with diameters of 0.1 to 1.0 μm. However, if the diameter of the uneven defect to be detected is expanded to 1.0 μm or more, or even to a range larger than the spot diameter of 1.5 μm, the detected push-pull signal amplitude will decrease significantly, affecting the defect detection accuracy.

[0086] Therefore, when the pinhole 115 is inserted into the optical path, the effective NA of the optical system is reduced to 1.0 mm / 5.1 mm×0.43=0.084 according to the ratio of the aperture diameter of the pinhole 115 to the effective diameter of the objective lens, and the light spot diameter is expanded to 0.66 μm / 0.084=7.8 μm. This state is an ideal condition for detecting irregular defects with diameters ranging from 0.5 to 5.0 μm. Therefore, by switching the NA by inserting or removing the pinhole 115 according to the diameter range of the irregular defects to be detected, it becomes possible to detect irregular defects in a wide range of diameters ranging from 0.1 to 5.0 μm with a single optical system configuration.

[0087] According to the above embodiment, the change in signal magnitude associated with a change in defect diameter is significantly suppressed, making it possible to detect defects over a wide range of diameters even within the dynamic range of the detection system of a typical surface inspection device.

[0088] In addition, it is possible to convert the amplitude of the detected signal into the waviness amplitude using a uniform relational expression, and it is also easy to visually identify the center position of a defect and distinguish between concave and convex.

[0089] Furthermore, by switching the NA depending on the diameter range of the defects to be detected, it becomes possible to detect defects with a wide range of diameters with a single optical system configuration. [Explanation of symbols]

[0090] 1 Surface inspection equipment 2. Defect detection optical system 3. Magnetic Disk 11, 12 Photodetector 101 Laser Diode 102 Illumination light beam 102s Light Spot 103 Reflected Light Beam 103a First reflected light beam 103b Second reflected light beam 111 Collimator lens 112 Objective Lens 113, 116 Condenser lens 114 Aperture 115 Pinhole 117 Cylindrical Lens 121 λ / 2 plate 122 λ / 4 plate 131 Polarizing Beam Splitter 132 Non-polarizing beam splitter 201 Rotary Motor 202 Controller 203 Signal Processing Circuit 204 Servo control circuit 205 Access control circuit 206 Automatic Position Control Unit 207 Laser Driver 208 Non-volatile memory 210 Slider

Claims

1. A magnetic disk; A laser light source; The laser light emitted from the laser light source is focused and irradiated onto the surface of the magnetic disk. An objective lens; a split photodetector for detecting the light intensity of a first interference region of the 0th order light and +1st order diffracted light of the reflected light from the magnetic disk on a pupil plane of the objective lens, and the light intensity of a second interference region of the 0th order light and -1st order diffracted light of the reflected light, A control unit, The control unit is detecting defects on the surface of the magnetic disk based on an amplitude of a difference signal between the light intensity of the first interference region and the light intensity of the second interference region detected by the split photodetector; 2. A surface inspection apparatus according to claim 1, wherein said difference signal is normalized by the sum of the light intensity of said first interference region and the light intensity of said second interference region.

2. 2. The surface inspection apparatus according to claim 1, The split photodetector has two split regions, The control unit is A surface inspection apparatus comprising: a first optical detector that detects an intensity of light emitted from the first interference region and a second optical detector that detects an intensity of light emitted from the second interference region;

3. 2. The surface inspection apparatus according to claim 1, The control unit is A surface inspection device for detecting an in-plane size or height of the defect on the surface of the magnetic disk.

4. 2. The surface inspection apparatus according to claim 1, The control unit is a correction coefficient calculated based on a relationship between the period of the defect and the detection sensitivity of the difference signal is stored in advance as a function of the period of the defect; Calculating an amplitude spectrum of the difference signal by frequency analyzing the measured difference signal; A surface inspection apparatus comprising: a correction coefficient for correcting a difference in detection sensitivity of the difference signal due to a period of the defect, the correction coefficient being determined by dividing the amplitude spectrum by the correction coefficient.

5. 2. The surface inspection apparatus according to claim 1, Further comprising a display unit, The control unit is calculating two-dimensional distribution data of index values ​​calculated using the difference signal normalized by the sum of the light intensity of the first interference region and the light intensity of the second interference region; The display unit is The center position of the defect is determined as a peak position in the two-dimensional distribution data of the index value, The height of the defect as the height of the peak, A surface inspection device characterized in that the type of unevenness of the defect is visualized and displayed as the polarity of the peak.

6. The surface inspection apparatus according to claim 5, The control unit is measuring two-dimensional distribution data of a first difference signal corresponding to a direction of a first coordinate axis in a plane of the magnetic disk and a second difference signal corresponding to a direction of a second coordinate axis perpendicular to the direction of the first coordinate axis; calculating first positive shift data obtained by shifting the two-dimensional distribution data of the first difference signal in a positive direction of the first coordinate axis by approximately ½ of the diameter of the light spot, first negative shift data obtained by shifting the two-dimensional distribution data of the first difference signal in a negative direction of the first coordinate axis by approximately ½ of the diameter of the light spot, second positive shift data obtained by shifting the two-dimensional distribution data of the second difference signal in a positive direction of the second coordinate axis by approximately ½ of the diameter of the light spot, and second negative shift data obtained by shifting the two-dimensional distribution data of the second difference signal in a negative direction of the second coordinate axis by approximately ½ of the diameter of the light spot, a surface inspection apparatus for calculating the two-dimensional distribution data of the index value as a sum of a difference between the first positive shift data and the first negative shift data and a difference between the second positive shift data and the second negative shift data.

7. 2. The surface inspection apparatus according to claim 1, a focus error signal detector for generating a focus error signal from the reflected light; The control unit is a focus error signal that indicates a focus error caused by a shift of a focus point of the laser beam from the surface of the magnetic disk, the focus error being detected by an astigmatism method;

8. The surface inspection apparatus according to claim 7, The control unit is a surface inspection apparatus for detecting the amount of deviation from a surface of the magnetic disk having irregularities as the defect based on the focus error signal;

9. 2. The surface inspection apparatus according to claim 1, The control unit is A surface inspection device that detects defects having a diameter within a range of 0.1 to 1.0 μm based on the amplitude of the difference signal.

10. The surface inspection apparatus according to claim 3, a pinhole configured to limit a beam diameter of the laser beam incident on the objective lens and to be insertable into and removable from an optical path of the laser beam; A surface inspection device characterized in that the effective numerical aperture of the objective lens is switched by inserting and removing the pinhole into and from the optical path of the laser light depending on the in-plane size range of the defect.

11. The surface inspection apparatus according to claim 10, The control unit is A surface inspection device that detects defects having a diameter within a range of 0.1 to 5.0 μm based on the amplitude of the difference signal.

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