JP2004139638A - Exposure device for master disk of optical recording medium and exposing method for master disk of optical recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exposure device for a master disk of an optical recording medium, by which fine pits are accurately formed and the productivity is remarkably improved, and to provide an exposing method for the optical recording medium. <P>SOLUTION: This exposure device for the master disk of the optical recording medium is provided with a modulation means 3 for modulating the light intensity from an exposure light source 1 corresponding to the recording information and a light converging optical system 9 for converging the light modulated by this modulation means 3 on a photoresist 12 on the master disk 11 of the optical recording medium, to carry out the pattern exposure to the photoresist 12 in accordance with the recording information. The exposure light source 1 is constituted of a super-short pulse laser having the repetition frequency of ≥1, ≤20 times the clock frequency of the recording information, and a frequency dividing optical system 15 is provided for dividing the frequency of laser beams emitted from this super-short pulse laser. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical recording medium master exposing apparatus and an optical recording medium master exposing method for performing pattern exposure corresponding to recording information by irradiating light from an exposure light source onto a photoresist-coated optical recording medium master. .
[0002]
[Prior art]
An optical recording medium master for producing an optical recording medium such as various optical disks such as a CD (Compact Disc), an MD (Mini Disc), a DVD (Digital Versatile Disc) and a magneto-optical disk is, for example, on a disk-shaped master disk substrate. Is coated with a photoresist, and a concavo-convex pattern corresponding to recorded information is formed on the surface by exposure and development, that is, so-called cutting is performed.
A mastering apparatus that performs this cutting, that is, an optical recording medium master exposure apparatus, emits a laser beam having a wavelength of about 400 nm emitted from an exposure light source, for example, an ultraviolet wavelength such as a Kr laser (λ = 413 nm) or an Ar laser (λ = 351 nm). The exposure process is performed by narrowing down and irradiating a minute spot of a diffraction limit on a master on which a resist is applied through an objective lens using a continuous wave solid-state laser light source in a range.
[0003]
In the various optical recording media as described above, there is a demand for miniaturizing the processing size of pits or grooves in accordance with the increase in density for increasing the recording capacity. However, when a size of 0.25 μm or less is required as a processing dimension of the pit, the numerical aperture NA of the condensing lens is close to 1 at the wavelength of the gas laser used in the above-described optical recording medium master exposure apparatus. However, the laser beam cannot be sufficiently narrowed down. For this reason, at present, it is extremely difficult to accurately process the optical recording medium master to a size of 0.25 μm or less in producing the master.
[0004]
For example, an ultraviolet laser beam with a wavelength of 266 nm is generated using a semiconductor laser-excited high-power green laser with a wavelength of 532 nm as a pumping light source and a second harmonic generator having an external resonator structure. An airy spot of 0.36 μm is obtained using an aberration-free objective lens of 0.9 (see, for example, Patent Document 1).
[0005]
In recent years, the tendency of optical discs to have higher densities has been further accelerated, and the formation of finer pits, for example, 0.2 μm or less is now essential, and a light source with a shorter wavelength is required. As a continuous oscillation laser light source having stability, low noise, and beam quality that can be used for an optical recording medium master exposure apparatus, a BaB laser is used in a resonator of an argon gas laser. ₂ O ₄ A deep UV oscillation water-cooled argon gas laser (with an output of 40 mW at a wavelength of 229 nm and an output of 100 mW at a wavelength of 238 nm) provided with a nonlinear optical crystal such as (BBO) is commercially available. However, the wavelength ratio is 229/266 = 0.86, and in order to obtain a resolution of 0.2 μm or less, it is necessary to use an objective lens having a higher NA in combination. In addition, since a large amount of cooling water is used, there is also an inconvenience that a measure for avoiding vibration in the built-in device is required.
[0006]
Further, as a high NA optical system, a technical study of a means using a near field optical system using a solid immersion lens (SIL) with NA> 1 has been conducted. However, the working distance is very narrow, such as 100 nm or less, for example, several tens of nm. Therefore, it is necessary to pay sufficient attention to dust and dirt, the surface smoothness of the optical recording medium master, and the rotational speed of the optical recording medium master. There is a problem that it cannot be raised too high.
[0007]
As another means for increasing the resolution, a micro pit processing method using an electron beam exposure apparatus has recently been proposed (for example, see Patent Document 2).
[0008]
However, an electron beam exposure apparatus requires a vacuum apparatus because an electron beam is used, and is a large-scale apparatus having a high-precision and high-speed rotation mechanism of a glass master in a vacuum.
[0009]
On the other hand, in recent years, several high-output Ti: Sapphire (titanium / sapphire) ultrashort pulse laser light sources having a repetition frequency of about 1 MHz to about 100 MHz of the Kerr-lens mode-locking method are commercially available. This Ti: Sapphire ultrashort pulse laser light source is excited by a semiconductor laser-excited high-output green laser, oscillates at a center wavelength of 760 nm to 840 nm, for example, 800 nm, average output number W, pulse width (FWHM, full width at half maximum) 100 fs. Is obtained stably. The transverse mode of the beam is TEM00 and has excellent performance with a noise of 0.1% or less.
[0010]
In addition, for example, a Tsunami series manufactured by Spectra Physics, a Mira series manufactured by Coherent, and the like, having a repetition frequency of 80 MHz, a pulse width (FWHM) of 100 fs or less, and an average output of 1 W or more, have been put to practical use.
[0011]
In addition, a two-photon absorption process can be generated using such an ultrashort pulse laser light source, and a pattern finer than the diffraction limit can be formed by super-resolution characteristics using a nonlinear optical effect (for example, see Non-Patent Document 1).
[0012]
For example, there has been reported an example in which a dot pattern having a width of 120 nm is formed using a laser having a wavelength of 780 nm, a repetition frequency of 76 MHz, a pulse width of 150 fs, and an objective lens having a numerical aperture of NA 1.4 (for example, see Non-Patent Document 2). .)
However, at present, a technique of forming a modulation signal of recording information for an optical recording medium on a resist by pattern exposure using a two-photon absorption process and using the same for exposure of a master optical recording medium has not been realized.
[0013]
[Patent Document 1]
JP-A-7-98891
[Patent Document 2]
Japanese Patent No. 3233350
[Non-patent document 1]
Edited by Satoshi Kawada, Spectroscopy Society of Japan, Measurement Method Series 38, “Super-Resolution Optics,” Society Press Center, March 20, 1999, Chapter 5, page 79
[Non-patent document 2]
S. Kawata et al, "Fine features for functional microdevices", Nature, 2001, Vol. 412, p. 697)
[0014]
[Problems to be solved by the invention]
The present invention has been made in view of the above-described circumstances, and has an optical recording medium master exposure apparatus and an optical recording medium exposure apparatus capable of forming fine pits with high precision and greatly improving productivity. The purpose is to provide a method.
[0015]
[Means for Solving the Problems]
The present invention relates to a modulating means for performing light intensity modulation of light from an exposure light source corresponding to recording information, and a condensing optical system for condensing the light modulated by the modulating means on a photoresist on an optical recording medium master. An optical recording medium master exposure apparatus for exposing a photoresist in a pattern according to recording information, wherein the exposure light source is an ultrashort pulse having a repetition frequency of 1 to 20 times the clock frequency of the recording information. It is constituted by a laser and provided with a frequency dividing optical system for dividing the laser light emitted from the ultrashort pulse laser.
[0016]
Further, according to the present invention, in the above-described configuration, the repetition frequency of the ultrashort pulse laser is set to an integer multiple of 1 to 20 times the clock frequency of the recording information, and the resonator length of the ultrashort pulse laser is made variable. An external synchronization mechanism for mode-locking the repetition frequency of the pulse laser in synchronization with the clock frequency of the recording information and oscillating pulses is provided.
[0017]
Further, the present invention provides a high-order harmonic generating means for emitting light whose wavelength has been shortened by wavelength conversion using a non-linear optical element with an ultrashort pulse laser light source as an excitation light source between an exposure light source and a frequency dividing optical system. Is provided.
[0018]
In the method of exposing an optical recording medium master according to the present invention, exposure is performed using the above-described optical recording medium master exposure apparatus.
That is, the light from the exposure light source is subjected to light intensity modulation corresponding to the recording information, and the light modulated by the modulating means is focused on the photoresist on the optical recording medium master, and the photoresist is converted into the recording information. An exposure method for an optical recording medium master for performing pattern exposure in accordance with the method described above, wherein the exposure light source comprises an ultrashort pulse laser having a repetition frequency of 1 to 20 times the clock frequency of the recording information, A frequency division optical system for dividing the emitted laser light is provided.
[0019]
Further, according to the present invention, in the above-described exposure method, an external synchronization method in which the repetition frequency of the ultrashort pulse laser is set to an integer multiple of 1 to 20 times the clock frequency of the recording information and the resonator length of the ultrashort pulse laser is variable. A mechanism is provided to lock the mode of the repetition frequency of the ultrashort pulse laser in synchronization with the clock frequency, thereby causing pulse oscillation.
[0020]
Further, according to the present invention, in each of the above-described exposure methods, the light emitted from the exposure light source is shortened by high-order harmonic generation means using the exposure light source as an excitation light source by wavelength conversion using a nonlinear optical element. The light is emitted and made incident on the frequency dividing optical system.
[0021]
As described above, the optical recording medium master exposure apparatus and the optical recording medium master exposure method according to the present invention use an ultrashort pulse laser light source as an exposure light source and set the repetition frequency to at least one time the clock frequency of the recording information. The frequency is reduced to 20 times or less, and a frequency dividing optical system for dividing the frequency of the laser light emitted from the ultrashort pulse laser is provided.
[0022]
The clock frequency of the recording information signal of the optical disk is 4.3 MHz for a CD and 26 MHz for a DVD. In recent years, a so-called Blu-ray Disc, which has been attracting attention as a high-density disc and has been developed with a wavelength λ of reproduction light of 405 nm and an NA of an objective lens of 0.85, is 66 MHz. For example, in the case of a Blu-ray Disc, the frequency is 66 MHz, which is almost the same as the repetition frequency of the ultrashort pulse laser. Therefore, it is necessary to match the timing of the information data signal and the pulse oscillation of the laser.
[0023]
However, according to the present invention, as described above, the interval between the pulses is reduced by providing a frequency dividing optical system that makes the pulse repetition frequency of the ultrashort pulse laser twice, four times, eight times. Is superimposed on the recording information, it is possible to reduce the exposure deviation per pulse, to avoid the occurrence of a pattern defect, and to suppress the increase in, for example, the jitter of the reproduction signal due to the pattern defect. Therefore, by using an ultrashort pulse laser, it is possible to form a fine pattern with higher precision than in the past.
By setting the repetition frequency to 20 times or less, it is possible to avoid a decrease in pattern formation accuracy due to a decrease in the peak output of the ultrashort pulse laser. That is, when the laser beam emitted from the ultrashort pulse laser light source is frequency-divided and the wavelength is shortened by the high-order harmonic generation means, the peak output decreases, but the repetition frequency is set to 20 times or less. Can sufficiently secure the output of the ultrashort pulse laser, efficiently generate a two-photon absorption process described later, and perform pattern exposure with a fine spot size.
[0024]
In the present invention, when the repetition frequency of the ultrashort pulse laser is set to an integral multiple of 1 to 20 times the clock signal of the recording information, an external device for adjusting the resonator length of the ultrashort pulse laser light source used as the exposure light source is used. A synchronization mechanism is provided, whereby the resonator length is adjusted, the repetition frequency can be synchronized with the clock signal, and the jitter of the reproduction signal can be suppressed to 10% or less.
By synchronizing in this way, pattern exposure synchronized with information signals recorded on various optical recording media such as CDs, DVDs, and Blu-ray Discs is reliably performed using light from an ultrashort pulse laser light source as an exposure light source. be able to.
[0025]
In another aspect of the present invention, a high-order harmonic generation unit is provided between the exposure light source and the modulation unit, and the wavelength is shortened by wavelength conversion using a non-linear optical element with an ultrashort pulse laser light source as an excitation light source. By emitting the light, an exposure light source having a shorter wavelength can be obtained.
[0026]
Thus, according to the optical recording medium master exposure apparatus and the optical recording medium master exposure method according to the present invention, an exposure light source that emits an ultrashort pulse laser beam having a high repetition frequency that can be regarded as being equivalent to pseudo continuous light, or The ultrashort pulse laser light whose wavelength has been shortened by the high-order harmonic generation means having the excitation means is emitted, and the light modulated by the light intensity modulation means is converted into a diffraction-limited spot size by a predetermined focusing optical system. By condensing and irradiating the photoresist, it is possible to perform exposure of a concave and convex pattern such as a pit having a finer pattern than in the related art.
[0027]
In the above-described present invention, the frequency dividing optical system is constituted by a polarizing beam splitter and an optical delay circuit. When the generating means is provided, by adopting a configuration in which the frequency dividing optical system is provided at the subsequent stage, it is possible to use laser light with high efficiency and little power loss.
[0028]
That is, the average output power of the higher harmonic light is proportional to the square of the fundamental wave power. Therefore, high-order harmonics are generated with high efficiency by injecting ultrashort pulse laser light with a relatively low repetition frequency to obtain high output, and then this pulse train is divided several times to obtain the same input fundamental wave power. As a result, high-output and high-repetition-frequency pulse oscillation can be performed, and it is not necessary to adopt a device configuration in consideration of the clock timing of the recording information.
[0029]
In this case, the high-order harmonic generation means for shortening the wavelength is a non-linear optical process, whereas the process through the frequency dividing optical system is a linear optical process, so that the output loss can be reduced. Has advantages.
[0030]
Further, according to the present invention, in the above-described optical recording medium master exposure apparatus or method, the photoresist is exposed by a two-photon absorption process. By using an ultra-short pulse laser light source with very high peak output (peak output) as an exposure light source and condensing the beam with a condensing optical system, the two-photon absorption process occurs very efficiently in the resist. . For example, when the repetition frequency is 1 GHz and the average power of the laser after emission from the objective lens is 10 mW, the light intensity in the beam spot on the surface of the photoresist is 100 GW / cm as a peak output. ² Range.
[0031]
The two-photon absorption process is one of nonlinear optical phenomena, and the exposure of the resist is given by the square of the intensity distribution of the beam spot. The two-photon absorption cross section of the resist is 10 ⁻⁴⁶ -10 ⁻⁴⁷ cm ⁴ The value is as small as about s / photon, and the sensitivity of the resist is low, but several% absorption occurs.
[0032]
In order to cause two-photon absorption with high efficiency, the peak output of the ultrashort pulse laser beam must be high.
In the present invention, a pulse oscillation having a high repetition frequency is used, and the pulse width (FWHM) is set to at least 1 ps (1 × 10 ^-12 Seconds) or less, but by defining the pulse width in this way, two-photon absorption can be efficiently caused.
[0033]
When the resist is exposed, the light absorption distribution of the light source in the resist surface is proportional to the beam intensity distribution in the case of normal absorption, and proportional to the square of the beam intensity distribution in the case of two-photon absorption. FIG. 6 shows the light absorption distribution. In FIG. 6, I indicates a beam intensity distribution, which corresponds to the case of normal absorption. I2 represents the square of the beam intensity distribution, which corresponds to the case of two-photon absorption. Airy spot diameter d is
d = 1.22λ / NA
It becomes. When the numerical aperture NA of the objective lens is NA = 0.9 and the wavelength λ = 267 nm, the spot size is 0.36 μm. In the case of two-photon absorption, the beam spot is approximately 1 / √2 = 0.7 times. That is, it corresponds to the beam spot size at the time of normal exposure using an exposure light source having a wavelength of 190 nm. As a result, the recording linear density is about 1.4 times that in the case of one-photon (normal) exposure.
[0034]
Further, according to the present invention, in each of the optical recording medium master exposure apparatus and the optical recording medium master exposure method described above, the spot shape of the laser light emitted from the focusing optical system and focused on the photoresist is changed. It has an elliptical shape extending in the scanning direction.
[0035]
For example, in order to expose a linear pattern such as a groove, the pulse interval (the reciprocal of the repetition frequency) and the scanning speed (the linear speed in the case of a disc-shaped optical recording medium master) must be optimized according to the sensitivity of the resist. No. However, since the pulse interval is uniquely fixed by the clock, it is difficult to expose a linear pattern.
According to the present invention described above, since the beam spot emitted from the focusing optical system and focused on the resist is elongated in the beam scanning direction, the distribution of the irradiated light amount is spread and averaged, and the groove and the like are obtained. Can be easily obtained.
[0036]
As described above, in the present invention, an ultrashort pulse laser is used as an exposure light source, and the beam is narrowed down to the diffraction limit by a focusing optical system, so that two-photon absorption can be performed with high efficiency. Due to the photon absorption process, the photosensitivity of the resist is given by the square of the intensity distribution of the beam spot, which has super-resolution characteristics using the nonlinear effect, and records smaller pits finer than the diffraction limit. Becomes possible.
[0037]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of an optical disk master exposure apparatus according to the present invention will be described with reference to the drawings.
[0038]
FIG. 1 shows a schematic configuration diagram of an example of an exposure apparatus for an optical recording medium master according to the present invention. In this example, a modulating means 3 for performing light intensity modulation of light from an exposure light source 1 corresponding to recording information, and a light modulated by the modulating means 3 is used as a disc-shaped optical recording medium master 11 in the illustrated example. A condensing optical system 9 for condensing light on the upper photoresist 12 is provided, and the photoresist 12 is subjected to pattern exposure according to recording information.
[0039]
In FIG. 1, reference numeral 1 denotes an exposure light source composed of an ultrashort pulse laser, 15 denotes a frequency dividing optical system which will be described in detail later with reference to FIG. 2, 4 denotes a chirp correcting optical system, 5 denotes a beam expander, 6 Denotes an autofocus optical system, an example of which will be described in detail in a later embodiment, 7 denotes a quarter-wave plate, 8a denotes an objective lens, and 8b and 8c denote electromagnetic actuators. 1a, 1b and 1c are mirrors. The optical recording medium master 11 is fixed to the mounting table 10. In this example, the mounting table 10 is rotated by a rotating means 14 as shown by an arrow a, and the condensing optical system 9 is disposed on, for example, a moving optical table (not shown). It is adapted to move in the radial direction so that exposure light can be irradiated over the entire surface of the optical recording medium master.
[0040]
FIG. 2A shows a schematic configuration diagram of an example of the frequency division optical system. In FIG. 2A, reference numerals 60 and 70 denote frequency divider circuits, respectively. Light L11 emitted from an exposure light source composed of an ultrashort pulse laser is split 50:50 by a beam splitter 61. Assuming that this light is s-polarized light, a polarizing beam splitter 67 that transmits light transmitted through the beam splitter 61 is provided. The light reflected by the beam splitter 61 is incident on a half-wave plate 66 via a lens 62, mirrors 63 and 64, and a lens 65, becomes p-polarized light, and is completely reflected by a polarization beam splitter 67.
Similarly, in the frequency dividing optical system 70, the light L12 emitted from the beam splitter 71 and the frequency dividing optical system 60 is split by the beam splitter 71 at 50:50. Assuming that this light is s-polarized light, a polarizing beam splitter 77 that transmits light transmitted through the beam splitter 71 is disposed. The light reflected by the beam splitter 71 enters the half wavelength 76 via the lens 72, the mirrors 73 and 74, and the lens 75, becomes p-polarized light, and is completely reflected by the polarization beam splitter 77.
[0041]
Here, the lenses 62 and 65, 72 and 75 form an afocal (telescope) system, and are arranged so that both beam parameters match. The half-wave plates 66 and 76 are provided with their optical axes rotated by 45 °, and rotate the polarization plane of the incident light by 90 ° so as to be totally reflected by the polarizing beam splitters 67 and 77 as described above. It is.
Further, the mirrors 63 and 64, 73 and 74 can be replaced with a right-angle prism or a corner cube.
[0042]
In the frequency dividing optical system 60, an optical delay circuit is configured by appropriately selecting the difference between the optical path lengths of two lights obtained by splitting the incident light L11 by the beam splitter 61. The difference between the optical path lengths of the beams is precisely adjusted so that the pulse transit time is equal to half the interval between the pulse trains of the incident light L11 (that is, the reciprocal of the repetition frequency of the ultrashort pulse laser). 2B, the incident pulse train p11 can be converted into a pulse train p12 having an interval of 、, that is, a frequency doubled, as schematically shown in FIG. 2B.
[0043]
For example, when the repetition frequency is 66 MHz, the interval between the pulse trains is 15.152 ns (10-9 seconds), and the optical path length is 2.2712 m (light speed = 2.998 × 108 m / s).
Further, instead of the beam splitter 61, a half-wave plate is provided in which the optical axis is arranged to be rotated by 22.5 ° with respect to the polarization plane of the incident pulse laser beam, and the polarization plane is rotated by 45 °, and the polarization beam splitter is rotated. May be combined.
[0044]
Further, in the frequency dividing optical system 70 at the subsequent stage of the frequency dividing optical system 60, the optical path length difference is precisely adjusted to be equal to 1/2 of the interval of the pulse train p12, as shown in FIG. 2B. A pulse train p13 in which the interval between the pulse trains p12 emitted from the frequency dividing circuit 60 is further reduced to １ ／ can be emitted. The optical path difference in this case is 1.1356 m.
[0045]
The pulse train obtained in this way is divided into a pulse train having a peak power of 1/2 when passing through the frequency dividing circuit 60 and a double repetition frequency, and a peak power of 1/4 when passing through the frequency dividing circuit 70. The repetition frequency is divided into quadruple pulse trains. When the aforementioned 66 MHz ultrashort pulse laser is incident, the repetition frequency becomes 264 MHz. However, the polarization planes of the pulses p12 and p13 become horizontal polarization and vertical polarization inclined at 90 ° to each other, but have no direct influence on the exposure apparatus and the exposure method.
[0046]
FIG. 3A shows a schematic waveform of a pulse signal from the ultrashort pulse laser light source and the frequency dividing optical system, and FIG. 3A shows a schematic diagram of a state in which the pulse signal is superimposed on a signal waveform S of recording information by the above-described modulation means 3. The waveform is shown in FIG. 3B. As shown in FIG. 3A, the interval between the pulses P is appropriately selected, and the frequency thereof is set to a frequency of 1 to 20 times the clock signal C of the recording information shown in FIG. As shown in FIG. 3B, it is superimposed on the signal S of the recording information. In FIG. 3B, the pulse waveform is shown as a broken line P ′. As described above, when the repetition frequency of the pulse train is an integer multiple of 1 to 20 times the clock frequency of the recording information, as described below, by providing an external synchronization mechanism in the ultrashort pulse laser, By performing the exposure in synchronization, the pattern exposure of the photoresist can be performed according to the recording information.
[0047]
FIG. 4 is a schematic diagram illustrating an example of an external synchronization mechanism and an exposure light source suitable for use in the embodiment of the present invention. In FIG. 4, reference numeral 30 denotes an ultrashort pulse laser light source using, for example, Ti: Sapphire, and reference numeral 50 denotes an external synchronization mechanism.
First, excitation light Li0 such as a semiconductor laser (not shown) is incident on the ultrashort pulse laser light source 30 via a lens 31 and a spherical mirror 32 to a laser medium 34 such as Ti: Sapphire. The light emitted from the laser medium 34 is reflected by the spherical mirror 33, further reflected by the high reflection mirror 35, and then enters the dispersion compensating prisms 36a and 36b. Then, the light is reflected by the high reflection mirror 38 through the slit 37. After passing through the slit 37 again, the light is returned to the laser medium 34 via the dispersion compensating prisms 36b and 36a, the high reflection mirror 35, and the spherical mirror 33. As the exposure light, the light returned from the laser medium 34 to the spherical mirror 32 is extracted outside as output light Li2 through an output window (output coupler) 39 and a beam splitter 40.
[0048]
In this external synchronization mechanism, a part of the output light is detected by a beam splitter 40 by a photodetector 41 composed of a high-speed photodiode or the like. Then, the phase of the output from the photodetector 41, that is, the electric signal by laser pulse oscillation and the output of the clock signal generator 42 of the information signal output device to be recorded on the optical recording medium are compared by the phase detector 43. Here, when the clock signal is set to an integer multiple of 2 or more, the phase of the clock signal is compared with that of the clock signal generated by the clock signal generator 42. Then, the signal output from the phase detector 43 is input to a control unit 44 including a PLL (Phase Lock Loop) circuit or the like, and the control signal converted into a predetermined control amount is input to the piezo drive unit 45, By slightly moving the piezo element 46 fixed to the high reflection mirror 38 in the optical axis direction, the resonator length of the laser resonator can be finely adjusted. Note that the resonator length in this example is the length of the optical path from the spherical mirror 32 to the high reflection mirror 38.
[0049]
With this configuration, the jitter between the clock signal of the recording information and the oscillation pulse of the laser can be reduced to 1 ps or less.
Since the optical modulator drive signal of the information recording signal is also transmitted in synchronization with the clock signal, the timing is synchronized with the pulse oscillation of the ultrashort pulse laser. When the repetition frequency of the ultrashort pulse laser light source is one time of the clock signal, that is, when the clock signal is synchronized, for example, when recorded using a (1,7) modulation code in the photoresist, 2T is irradiated to the 2T shortest pit. Is done. When externally synchronizing with a frequency twice as high as the clock signal, ie, 132 MHz in the case of the Blu-ray Disc, an ultrashort pulse laser is used so that the resonator length R is R = c / 2L = 1136 mm (c is the speed of light) An optical system in the apparatus may be provided, and the shortest pit of 2T is irradiated with four pulses as shown in FIG. 3B described above.
[0050]
If the repetition frequency of the laser oscillation is increased, the peak output of the pulse is reduced even if the average output is the same, so that the efficiency of generation of higher-order harmonics in the subsequent stage and absorption of two-photon by the resist is reduced. Therefore, the repetition frequency of the ultrashort pulse laser light source is selected to be 20 times or less the frequency of the clock signal.
When the repetition frequency is set to an integral multiple of 10 times or more of the frequency of the clock signal, when the deviation from the clock is set to 1/10 or less of the cycle and the jitter of the reproduction signal can be set to 10% or less, Exposure can be performed without providing an external synchronization mechanism.
[0051]
Further, according to the present invention, in the above-described configuration, as shown in FIG. 1, between the exposure light source 1 and the modulating means 3, the wavelength is shortened by wavelength conversion using a non-linear optical element with an ultrashort pulse laser light source as an excitation light source. And a high-order harmonic generating means 2 for emitting the generated light.
[0052]
FIG. 5 shows a schematic configuration of an example of the high-order harmonic generation means 2.
In FIG. 5, 26 denotes a second harmonic (SHG) generator, 27 denotes a delay line unit, and 28 denotes a third harmonic (THG) generator. The light Li that has entered the second harmonic generation unit 26 enters the nonlinear optical crystal 20 via the condenser lens 19a, is reflected by the harmonic separator 21a via the condenser lens 19b, and is extracted as L2-1. Or when the harmonic separator 21a is not provided, the light enters the delay line unit 27.
[0053]
The light incident on the delay line unit 27 is split into a fundamental wave L1 and a second harmonic L2-2 by the harmonic separator 21b. The fundamental wave is reflected by the mirrors 22a and 22b and is incident on the third harmonic generation unit 28, and the second harmonic L2-2 is reflected by the mirrors 22c, 22d and 21c via the half-wave plate 23. The light is incident on the third harmonic generator 28.
[0054]
For example, F. Roterund, et al: "Generation of the fourth harmonic of a femtosecond Ti: Sapphire laser" Optics Letters, July 1, 1998, Vol. 23, no. 13, p1040, a nonlinear optical crystal LiB using a Ti: Sapphire ultrashort pulse laser having a center wavelength of 800 nm (repetition frequency 82 MHz, pulse width 85 fs, average output 1.9 W). ₃ O ₅ By using a second harmonic generation (SHG) device using (LBO) type I critical phase matching, the center wavelength is 400 nm, and the pulse width is slightly widened by group velocity dispersion, but is, for example, 100 fs and average output 600 mW. Short pulse laser light can be obtained.
[0055]
When the type I phase matching is used in the generation of the second harmonic, the polarization planes of the fundamental wave and the second harmonic are rotated by 90 °, so that, for example, as shown in FIG. The polarization plane of the second harmonic L2-2 can be adjusted to the fundamental wave by providing the half-wave plate 23 for aligning with the fundamental wave L1 before entering the second nonlinear optical crystal 24 using.
[0056]
Further, since the second harmonic L2-2 is emitted with a delay from the fundamental wave L1 due to the wavelength dispersion in the first nonlinear optical crystal 20, the second nonlinear optical crystal 24 is transmitted to the second nonlinear optical crystal 24 by the delay line unit 27 described above. Before the incidence, the fundamental wave L1 is delayed. The delay is achieved by separating the two waves by the harmonic separator 21b, lengthening only the optical path length of the fundamental wave L1 by a length corresponding to the delay time, and combining the waves again.
[0057]
Then, as shown in FIG. 5, in the third harmonic generation unit 28, the combined wave is made incident on the nonlinear optical crystal 24, and the third harmonic L3 is emitted to the outside by sum frequency mixing. 19c and 19d are condenser lenses, 21d is a mirror, 25 is a beam stopper, and Lo is unnecessary light.
The lenses 19a to 19d are arranged to increase the beam intensity in the crystal and improve the conversion efficiency.
[0058]
In the case of ultrashort pulse laser light, the peak output is very high and the conversion efficiency of the second harmonic generation, which is the second-order nonlinear optical phenomenon, increases in proportion to the laser intensity. High efficiency can be obtained even if the optical path is set to pass once through the optical crystal. However, in the case of high-order harmonic generation using an ultrashort pulse laser, the group velocity dispersion of the nonlinear optical crystal causes a group velocity mismatch when the crystal is thick, so that effective wavelength conversion is not performed. For example, the crystal length of LBO needs to be 1.5 mm or less when the pulse width is 100 fs and the center wavelength is 800 nm.
[0059]
Further, in the above-described third harmonic generation unit 28, for example, by sum frequency mixing (SFM) of a fundamental wave having a center wavelength of 800 nm and a second harmonic having a center wavelength of 400 nm emitted from the high-order harmonic generation means, for example. An ultrashort pulse laser beam having a center wavelength of 267 nm, a pulse width of 115 fs, and an average output of about 150 mW can be obtained. This sum frequency mixing is a second-order nonlinear optical phenomenon like the second harmonic generation. For example, the type I critical phase matching of the nonlinear optical crystal BBO can be used. .3 mm or less.
[0060]
Further, by adding a nonlinear optical crystal (BBO) for sum frequency mixing, the fourth harmonic can be generated, and a light source having a wavelength of 204 nm can be obtained. A pulse width of 340 fs and an average output of 15 mW are obtained. Therefore, it can be used as an exposure light source when applied to the optical recording medium master exposure apparatus and the optical recording medium master exposure method of the present invention with sufficient average output power up to the fourth harmonic light.
[0061]
As described above, in the present invention, when pulse oscillation with a high repetition frequency is used, the pulse width (FWHM) is set to at least 1 ps (1 × 10 ^-12 Seconds) or less, but by defining the pulse width in this way, two-photon absorption can be efficiently caused.
[0062]
In the present invention, the following effects can be obtained by setting the absorption peak wavelength of the photoresist to half or less of the wavelength of the exposure light source.
As described above, normal absorption (one-photon absorption) can be efficiently suppressed by using a material that is transparent in the wavelength region of the exposure light source and has absorption at half the wavelength as the photoresist. .
In two-photon absorption, two photons are simultaneously absorbed, and the electrons of the resist are shifted to a level higher by twice the energy of one photon. In terms of absorption spectrum, this is equivalent to the case of being excited by light (one photon) having a wavelength of half the wavelength of the exposure light source. Therefore, the resist for absorbing two photons should have an absorption peak wavelength not more than half the wavelength of the exposure light source. Accordingly, two-photon absorption can be efficiently generated, and finer pattern exposure can be performed.
[0063]
When exposure is performed over the entire thickness of the photoresist, it is desirable that half of the wavelength of the exposure light source be slightly longer than the absorption peak of the photoresist.
For example, when exposing a master for a CD or the like having a photoresist thickness of about 100 nm, selecting an exposure light source and a photoresist that has an absorption coefficient of about several% with respect to the absorption coefficient at the absorption peak wavelength makes the resist Two-photon absorption is not performed only in the vicinity of the surface, and absorption over the entire thickness can be caused. In the case of a master for a Blu-ray Disc having a photoresist thickness of about 40 nm, by selecting an exposure light source and a photoresist having an absorption coefficient of about 10%, the total thickness of the photoresist is similarly reduced. , And pattern exposure for exposing the surface of the master disk substrate after development can be performed.
[0064]
It should be noted that the bandwidth (FWHM) δλ of the ultrashort pulse laser beam is, for example, 100 fs when the pulse width δt is 100 fs, regardless of which light source is used. ² Δ · δt = 0.315 · λ for a Fourier transform limit pulse of the form ² / C (c: speed of light), and δλ = 6.7 nm. Therefore, when a high NA lens having an NA of 0.5 or more is used, it is necessary to use an achromatic lens such as an apochromatic lens used in a microscope or the like as the objective lens. Further, since chromatic aberration occurs only in a refraction system, the above problem can also be avoided by using a condensing optical system using an aspherical concave mirror.
[0065]
In the present invention, the beam spot emitted from the focusing optical system and focused on the resist is elongated in the beam scanning direction. Thereby, the distribution of the amount of light to be irradiated is expanded and averaged, and a linear pattern such as a groove can be easily obtained.
In order to make the beam spot elliptical, for example, the beam expander 5 described in FIG. 1 is an anamorphic optical system, that is, the beam diameter in the direction perpendicular to the beam scanning direction is further enlarged. Anything can be used.
Specifically, it is desirable to use a cylindrical lens, a cylindrical concave mirror, an anamorphic prism, or the like to make the ratio of the beam expansion ratio about several times.
[0066]
Further, as the modulation means for light intensity modulation described in FIG. 1 described above, an acoustic wave utilizing the Bragg diffraction of light by an ultrasonic wave in an acousto-optic element driven by a piezoelectric element modulated by a recording information signal is used. An electro-optic modulator using an optical effect or a Pockles effect modulated by a recording information signal is suitable.
[0067]
Since all the optical elements such as the lens, the wave plate, and the optical modulator described above have a positive group velocity dispersion, even when the pulse width is adjusted to be minimum at the time of emitting the exposure light source, the light passes through these. When the ultrashort pulse laser beam is applied to the photoresist of the master optical recording medium, it always chirps and the pulse width is widened.
[0068]
Therefore, a chirp correction optical system having a negative group velocity dispersion is used as the chirp correction optical system 4 shown in FIG. 1, and a negative chirp is given to the ultrashort pulse light emitted from the exposure light source in advance, thereby canceling the chirp. Thus, it is necessary to obtain the shortest pulse on the resist. As the chirp correcting optical system 4, a dispersion prism pair, a grating pair, and a chirp mirror can be used.
[0069]
The pulse width required for adjusting the pulse width can be measured by an autocorrelator using a conventional second harmonic generation method.
[0070]
Embodiment 1
Next, an example of the optical recording medium master disc exposure apparatus of the present invention will be described with reference to FIG. In this example, an exposure light source 1 composed of a Ti: Sapphire ultrashort pulse laser light source, a higher harmonic generation means 2 using the ultrashort pulse laser as an excitation light source, and light emitted from the higher harmonic generation means 2 It has a frequency dividing optical system 15 for dividing the light, and a negative group velocity dispersion for correcting in advance a positive group velocity dispersion caused when a pulse output from the frequency dividing optical system 15 passes through various optical components. A chirp correcting optical system 4, a modulating means 3 for modulating light intensity by switching light emitted from the chirp at high speed with an electric pulse signal corresponding to supplied data, and modulating by the modulating means 3. A converging optical system 9 and a beam expander 5 are provided for condensing the obtained light to a diffraction-limited spot size and irradiating the light onto a master optical disc 11 coated with a photoresist 12.
[0071]
The ultrashort pulse laser light source is a Ti: Sapphire laser having a repetition frequency of 128 MHz, which is the same as the above-mentioned Blu-ray Disc clock frequency, a center wavelength of 816 nm, a pulse width of 80 fs, and an average output of 0.8 W, that is, Ti: Sapphire. The ultrashort pulse laser used as the laser medium 34 described in 4 was used.
[0072]
Then, the second harmonic having a wavelength of 408 nm or the third harmonic having a wavelength of 272 m was generated using the high-order harmonic generating means 3 described in FIG. In this example, a type I phase-matched LBO crystal was used as the nonlinear optical crystal 20 of the second harmonic generation unit 26 shown in FIG. A type I BBO was used for the nonlinear optical crystal 24 of the third harmonic generation unit 28. The various lenses 19a to 19d are arranged to increase the beam intensity in the crystal and improve the conversion efficiency. Outgoing light could be extracted with the second harmonic light having an average output of 300 mW and a pulse width (FWHM) of 100 fs, the third harmonic light having an average output of 60 mW and a pulse width of 120 fs or less at 1 ps or less.
[0073]
Then, as the frequency dividing optical system 15, an optical system of frequency dividing by 4 provided with the frequency dividing circuits 60 and 70 described in FIG. By passing the light through the frequency dividing optical system 15, an output having a repetition frequency of 528 MHz and an average power of 15 mW can be obtained. In this embodiment, the pulse laser light is supplied to the clock signal of the recording information without providing an external synchronization mechanism. And pattern exposure could be performed with a good shape.
In this case, the first-stage frequency dividing circuit 60 has an optical path difference of 1.1356 m and the afocal lenses 62 and 65 have a focal length of 300 mm. In the second-stage frequency dividing circuit 70, the optical path difference is 0.5678 m, and the lenses 72 and 75 have a focal length of 160 mm. A half mirror was used to separate the pulse light, and a half-wave plate of the 0th order for the second harmonic of 408 nm and a polarizing beam splitter for multiplexing were used. The adjustment of the optical path length was performed by mounting two reflecting mirrors on a stage with a micrometer. The ultrashort pulse laser was detected by a light-speed photodetector having a band of 1.2 GHz, and while measuring with a spectrum analyzer, the center frequency was adjusted to 528 MHz and the bandwidth became the narrowest.
[0074]
As the chirp correcting optical system 4, a Brewster prism-pair was used.
The light is reflected at 90 ° by the mirror 1 a and sent to the modulating means 3. As the light intensity modulator of the modulating means 3, an electro-optical element EOM having a signal modulation band of 80 MHz was used. The modulation means 3 is supplied with a pit recording signal from a so-called formatter (not shown) which generates electrical pulse signals for data to be recorded on the optical recording medium master. Light is modulated according to this data.
[0075]
This light-modulated light is reflected at 90 ° by the mirror 1b, passes through a quarter-wave plate 7 via a beam expander 5, and a polarizing beam splitter (hereinafter, referred to as PBS) 6a of an autofocus optical system 6, for example. After the light is reflected by 90 ° at 1c, the light is transmitted through an objective lens 8a having a high aperture ratio NA to irradiate the optical recording medium master 11 coated with a photoresist 12 in advance.
As the photoresist 12, for example, an i-line resist (JSR Corporation PFRIX1110G or the like) or a KrF laser mastering resist (Zeon Corporation DVR-100 or the like) can be used.
[0076]
At this time, the objective lens 8a uses an incident light, for example, a lens having a high numerical aperture NA corrected for aberration for a wavelength λ = 267 nm, and irradiates the beam with the beam narrowed down to the diffraction limit. As the objective lens 8a, an achromatizing objective lens made of synthetic quartz, fluorite, or the like, whose material sufficiently transmits light in this wavelength range, was used. The optical recording medium master 11 is fixed on a mounting table 10 which rotates in a direction indicated by an arrow a by rotating means 13 such as a spindle motor.
[0077]
On the other hand, the higher harmonic generation means 2 emits the third harmonic light having the wavelength λ = 272 nm and simultaneously emits the second harmonic light having the wavelength λ = 408 nm. The optical path of this light is also an optical path passing through each of the above-described optical elements, and is applied to the optical recording medium master 11.
The return light reflected from the optical recording medium master 11 is incident on the PBS 6a via the objective lens 8a, the mirror 1c, and the quarter wavelength plate 7. Here, since this return light has passed through the quarter-wave plate 7 twice, it is reflected by the PBS 6a. Thereby, the PBS 6a of the autofocus optical system 6 sends the return light to the focus error amount detection element 6c via the wavelength selection element 6b. The wavelength selection element 6b is used to block the light of the exposure wavelength using a multilayer interference film or the like because a considerable amount of the light of the third harmonic, which is the exposure wavelength, is also reflected by the PBS 6a.
[0078]
The focus error amount detection element 6c optically detects the amount of displacement from the best focus position when the exposure light is focused on the optical recording medium master 11 using, for example, an astigmatism method. The detected amount is converted into an electric signal. The detected electric signal is supplied to a drive control unit 6d which forms a part of the autofocus servo system 6.
Here, the above-mentioned astigmatism method is a method in which a cylindrical lens is arranged behind a detection lens, and a photodetector detects the astigmatism by positively utilizing astigmatism. This cylindrical lens has a lens action only in a single direction, and has only the same action as a parallel plate in a direction orthogonal to this single direction, so it converges at positions other than the focus position of the detection lens and this cylindrical lens. Instead, the focus error signal is detected by forming a thin beam image. By controlling this focus error signal to be zero, the focus of the objective lens is maintained at an optimum position.
[0079]
The drive control unit 11c generates a drive signal for correcting the displacement based on the electric signal, and outputs the drive signal to the electromagnetic actuators 8b and 8c for finely moving the objective lens 8a up and down. The electromagnetic actuators 8b and 8c move the objective lens 8a in the vertical direction indicated by an arrow b by a drive signal, that is, in the direction in which the objective lens 8a approaches or separates from the photoresist, to thereby optimize the focusing position of the optical recording medium master 11. The position can be automatically adjusted to allow exposure with reduced loss.
[0080]
Here, for example, when an aberration-free lens having a numerical aperture NA of 0.9 was used as the objective lens with a laser beam spot size, the airy disc could be reduced to 0.36 μm. Therefore, by generating the two-photon absorption process, exposure corresponding to a spot size of 0.36 × (1√2) ≒ 0.25 (μm) could be performed.
[0081]
At this time, as described above, the beam expander 5 has an anamorphic optical system, and has a spot shape expanded and elongated in the beam scanning direction. As a result, the groove pattern was exposed as a fine pattern.
[0082]
The laser beam formed in this way is rotated and scanned on the optical recording medium master 11 by the rotating means 13, and at the same time, the optical system including the objective lens is moved in the radial direction from the center of the disk (center of the master) to form a spiral beam. Can be scanned on the master to expose the photoresist to form pits with high density.
The photoresist 12 may be a positive resist for g-line or i-line in addition to the above-mentioned i-line. Since the photosensitivity of the resist is photon mode recording, even in the case of ultrashort pulse light having a high repetition frequency, the photosensitivity is determined by the integrated amount of photons per unit area. According to the present invention, unlike the case of continuous light irradiation, there is almost no need to go through the thermal mode. That is, it is possible to suppress an unnecessary expansion and a change in the reaction speed due to an increase in the temperature of the resist, and it is possible to form finer pits.
[0083]
In the first embodiment described above, the case where the center wavelength is 816 nm is described. However, the Ti: Sapphire ultrashort pulse laser can oscillate from 760 nm. In this case, the same means as described above (all the center design wavelengths need to be changed) Thus, the second harmonic light of 380 nm and the third harmonic light of 253 nm can be used. However, since the efficiency is somewhat reduced, it is necessary to increase the output of the excitation green laser for exciting the laser medium of the ultrashort pulse laser light source.
[0084]
Further, by adding a non-linear optical crystal (for example, BBO) for sum frequency mixing, it was possible to generate a fourth harmonic (wavelength around 200 nm). In this case, as the beam spot size, an Airy spot of 0.28 μm was obtained using an aberration-free objective lens having a numerical aperture of NA 0.9. Therefore, exposure corresponding to a spot size of 0.28 × (1 / √2) ≒ 0.2 (μm) can be performed. In this case, a resist for a KrF laser (wavelength 248 nm) or an ArF laser (wavelength 192 nm) can be used as a highly sensitive resist.
[0085]
In the first embodiment, the third harmonic generation means is described as an example of the high harmonic generation means. However, the high harmonic generation means described in FIG. Since the frequency mixing units are independently separated from each other, the second harmonic can be used as an exposure light source. In this case, the conversion efficiency of the second harmonic generation is higher than that of the third harmonic generation, and not only a high exposure power can be obtained with the same excitation laser power, but also the wavelength of the laser light is close to the visible light range, Since various types of glass materials can be used, lens design is easy, and restrictions on optical elements are reduced.
[0086]
Embodiment 2
Next, a second embodiment according to the present invention will be described.
In this example, the configuration of the optical recording medium master exposure apparatus is exactly the same as that of the first embodiment. In this example, the exposure light source has a repetition frequency of 66 MHz, a center wavelength of 816 nm, a pulse width of 80 fs, and an average output of 1.5 W. An ultra-short pulse laser light source using Ti: Sapphire as a laser medium was used. Then, in order to increase the laser power intensity of the higher harmonics, the focal length of the condenser lens in front of each nonlinear optical crystal is made shorter, the beam spot diameter is made smaller, and the wavelength conversion efficiency is increased.
[0087]
In this example, only the frequency-divided optical circuit 5 is used as the frequency-divided optical system 5. That is, only the frequency dividing circuit 60 described in FIG. 2 is provided between the high-order harmonic generating means 2 and the chirp correction optical system 4. The optical path difference of the frequency dividing circuit 60 was set to 2.2712 m, and the focal length of the afocal lenses 62 and 65 was 600 mm. A half mirror was used for separation of the pulse beam, and a half-wave plate of the 0th order for the third harmonic of 267 nm was used, and a polarization beam splitter was used for multiplexing.
[0088]
When an ultrashort pulse light having a wavelength of 272 nm and a pulse width of 1 fps or less and emitted from the third harmonic generation unit 28 and having a pulse width of 1 ps or less is used to irradiate an ArF laser photoresist such as a fluororesin resist as a photoresist, The light intensity in the beam spot on the resist surface is 100 GW / cm as a peak output. ² In addition, two-photon absorption was remarkable, and several percent of absorption, that is, a photoreaction, which is a process of exposing the resist, was able to proceed. In this example, an airy spot of 0.36 μm can be obtained by using an aberration-free objective lens with NA = 0.9 as the objective lens, and a 0.36 × (1 √2) Exposure with an Airy spot size of 0.25 μm could be performed.
[0089]
The photosensitivity of the resist is given by the square of the intensity distribution of the beam spot. In the above-described resist, normal absorption does not occur because it is transparent to light having a wavelength of 272 nm. Only the two-photon absorption process occurs locally only at high intensity distributions. This can be replaced with a resist for ArF laser (193 nm) (for example, ZARF001 of Nippon Zeon Co., Ltd.) or a resist for KrF laser (for example, KRFM89Y of JSR Corporation).
[0090]
Also in this case, the groove width is made larger than that of the conventional one by making the beam expander 5 an anamorphic optical system to make the spot shape expanded and elongated in the beam scanning direction. The groove pattern was exposed as a fine pattern.
[0091]
Further, also in this example, the second harmonic (wavelength: 403 nm) is exposed by using the high harmonic generation means having the configuration shown in FIG. 4 in which the second harmonic generation and the sum frequency mixer are independently separated. It can also be used as a light source. In this case, it is desirable to use a photoresist for KrF (wavelength: 248 nm) or i-line (wavelength: 365 nm) as a photoresist.
[0092]
In addition, since the two-photon absorption cross-section is a very small value, an organic dye having a high two-photon absorption cross-section added to the resist as a sensitizer can be used to enhance the sensitivity of the resist. In addition to the same advantages as in the case of the first embodiment, the selection range of applicable photoresists is widened.
[0093]
In the first embodiment described above, the repetition frequency of the ultrashort pulse laser is set to twice the frequency of the clock signal of the recording information, and furthermore, a four-frequency dividing optical system is provided to reduce the repetition frequency of the pulse. As a result, pattern exposure could be performed with a good shape without providing an external synchronization mechanism.
On the other hand, in the above-described second embodiment, the repetition frequency of the ultrashort pulse laser is set to be one time the frequency of the clock signal of the recording information, and the frequency dividing optical system is a 2-frequency dividing optical system. A pulse laser beam having a frequency twice as high as that of the signal was obtained, and in this case, the pattern exposure could be performed with a good shape by synchronizing the exposure pulse with the clock signal by an external synchronization mechanism.
[0094]
On the other hand, if the repetition frequency is set too high, the peak output decreases, and it becomes difficult to generate two-photon absorption, and pattern exposure with high resolution cannot be performed. Therefore, under the present circumstances, a fine pattern can be reliably exposed by setting the upper limit to about 20 times the clock signal of the recording information of the optical recording medium to be exposed.
[0095]
Further, in the above-described embodiment and each example, a Ti: Sapphire ultrashort pulse laser is taken as an example of the light source means, but various other ultrashort pulse laser light sources can be used.
For example, an Nd: Vanadete ultrashort pulse laser that can be excited by a semiconductor laser, has a center wavelength of 1064 nm employing a semiconductor saturable absorption mirror (SESAM), a pulse width of 7 ps, an average output number W, and a repetition frequency of 25 MHz to 1 GHz is commercially available. I have. One with a center wavelength of 917 nm is also available. As a laser medium, Nd: YAG, Nd: YLF, or the like can be used. An ultrashort pulse laser using a solid laser medium such as Cr: LiSAF or Nd: Glass has a pulse width of 100 fs or less and a center wavelength of 850 nm and 1058 nm.
[0096]
The high-order harmonic generation means includes sum frequency mixing, second harmonic generation, fourth harmonic generation, and the like. Non-linear crystal optical elements include, in addition to BBO, KDP, KTP, LN, and periodic polarization inversion thereof. Type KTP (PPKTP), PPLN, LBO, LiIO ₃ , CBO and the like.
[0097]
Furthermore, although the description has been made by taking the exposure of minute pits and grooves as an example, since it can be handled in the same manner as the continuous light source of the conventional method, not only the formation of minute pits and grooves, but also the acousto-optic effect or A wobbling address can be similarly formed using an optical deflector utilizing the effect.
[0098]
Further, the present invention is not limited to an exposure apparatus and an exposure method for a disc-shaped optical recording medium master, but instead of the rotating means 13 shown in FIG. 1, an XY linear scanning system using a high-precision linear actuator. And a three-dimensional micromachining apparatus having a z-direction slide mechanism in addition to the rotary system and the XY linear scanning system.
[0099]
【The invention's effect】
In the optical recording medium master disc exposure apparatus and the optical recording medium exposure method according to the present invention, the ultra-short pulse laser light output from the exposure light source or the short wavelength laser light output from the higher harmonic generation means using the same as an excitation light source is used. By adjusting the pulse repetition frequency of the ultrashort pulse laser beam in accordance with the clock frequency of the information signal to be recorded on the optical recording medium using a frequency dividing optical system or a frequency dividing optical system and an external synchronization mechanism, the clock is controlled. It is possible to perform pattern exposure of a good shape corresponding to a signal, and to obtain an optical recording medium in which the jitter of a reproduced signal is suppressed to about 10% or less. In particular, by using a pulse laser whose wavelength is shortened by the high-order harmonic generation means, a two-photon absorption process is generated, so that pattern exposure of fine pits of about 0.25 μm or less as compared with the conventional method can be performed with high precision. It can be carried out.
[0100]
In the present invention, the frequency dividing optical system is composed of a polarizing beam splitter and an optical delay circuit, and the frequency dividing optical system is provided after the light source made of an ultra-short pulse laser. Thus, the effective use of the laser beam becomes possible.
When the high-order harmonic generation means is provided, the frequency division optical system is provided at the subsequent stage, so that the high-order harmonic generation means uses a laser beam having a relatively high peak output from an ultrashort pulse laser. And the peak output of the pulse laser light finally obtained as the exposure light can be secured.
[0101]
Further, the pulse width of the ultrashort pulse laser is set to 1 × 10 ^-12 By setting the time to less than seconds, the two-photon absorption process can be generated more efficiently, and finer pits smaller than the diffraction limit of the wavelength of the exposure light source can be formed.
[0102]
Further, in the present invention, by making the beam spot of the laser beam irradiating the optical recording medium master into an elliptical shape, the signal of the pattern extending in the scanning direction such as the groove can be subjected to pattern exposure with a good shape. Can be.
[0103]
According to the present invention, since a far-field optical system can be adopted instead of the near-field optical system using the SIL having a high numerical aperture as described above, a sufficiently large working distance can be obtained, and rotation during exposure can be performed. By increasing the number, productivity can be significantly improved.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an example of an optical recording medium master exposure apparatus.
FIG. 2A is a schematic configuration diagram of an example of a frequency dividing optical system. B is an explanatory diagram of a schematic waveform of an example of a pulse train.
FIG. 3A is a schematic explanatory diagram of a pulse waveform of an ultrashort pulse laser beam. B is a schematic explanatory diagram of the waveform of the information signal and the pulse waveform of the ultrashort pulse laser beam. C is a schematic explanatory diagram of the pulse waveform of the clock signal.
FIG. 4 is an explanatory diagram of an external synchronization mechanism.
FIG. 5 is a schematic configuration diagram of an example of a high-order harmonic generation means.
FIG. 6 is a diagram showing an absorption amount of an airy spot in a photoresist.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Exposure light source, 1a ... Mirror, 1b ... Mirror, 1c ... Mirror, 2 ... Higher harmonic generation means, 3 ... Modulation means, 4 ... Chirp correction optical system, 5 ... Beam expander, 6 ... Autofocus optical system , 6a: polarization beam splitter, 6b: wavelength selection element, 6c: focus error amount detection element, 6d: drive control unit, 7: quarter wave plate, 8a: objective lens, 8b ..., electromagnetic actuator, 8c: electromagnetic actuator , 9: Condensing optical system, 10: Mounting table, 11: Optical recording medium master, 12: Photoresist, 13: Rotating means, 15: Frequency dividing optical system, 19a: Lens, 19b: Lens, 19c: Lens, 19d ... Lens, 20 ... Nonlinear optical crystal, 21a ... Harmonic separator, 21b ... Harmonic separator, 21c ... Harmonic separator, 21d ... Harmonic Nick separator, 22a high-reflection mirror, 22b high-reflection mirror, 22c high-reflection mirror, 22d high-reflection mirror, 23 half-wave plate (HWP), 24 nonlinear optical crystal, 25 beam stopper, 26 .. Second harmonic (SHG) generator, 27 delay line unit, 28 third harmonic (THG) generator, 31 lens, 32 spherical mirror, 33 spherical mirror, 34 laser medium, 35 laser High reflection mirror, 36a: dispersion compensation prism, 36b: dispersion compensation prism, 37: slit, 38: high reflection mirror, 39: output window, 40: beam splitter, 41: photodetector, 46: piezo element, 60: minute Circuit, 61: Beam splitter, 62: Lens, 63: Mirror, 64: Mirror, 65: Lens, 66: 1/2 wavelength plate, 67: Polarized beam splitter , 70 ... dividing circuit, 71 ... beam splitter 72 ... lens, 73 ... mirror, 74 ... mirror, 75 ... lens, 76 ... 1/2-wave plate, 77 ... polarizing beam splitter

Claims (40)

A modulating means for performing light intensity modulation of light from the exposure light source in accordance with recording information, and a condensing optical system for condensing light modulated by the modulating means on a photoresist on an optical recording medium master are provided. An optical recording medium master exposure apparatus that performs pattern exposure on the photoresist in accordance with the recording information,
The exposure light source comprises an ultrashort pulse laser having a repetition frequency of 1 to 20 times the clock frequency of the recording information,
An optical recording medium master exposure apparatus, comprising: a frequency dividing optical system for dividing a laser beam emitted from the ultrashort pulse laser. The repetition frequency of the ultrashort pulse laser is an integer multiple of 1 to 20 times the clock frequency of the recording information,
An external synchronization mechanism is provided, wherein the resonator length of the ultrashort pulse laser is variable, the repetition frequency of the ultrashort pulse laser is mode-locked in synchronization with the clock frequency, and pulse oscillation is performed. Item 2. An optical recording medium master exposure apparatus according to Item 1. Between the exposure light source and the frequency-dividing optical system, high-order harmonic generation means for emitting light whose wavelength has been shortened by wavelength conversion using a non-linear optical element with the ultrashort pulse laser light source as an excitation light source is provided. 2. The optical recording medium master disc exposure apparatus according to claim 1, wherein: Between the exposure light source and the frequency-dividing optical system, high-order harmonic generation means for emitting light whose wavelength has been shortened by wavelength conversion using a non-linear optical element with the ultrashort pulse laser light source as an excitation light source is provided. 3. The optical recording medium master exposure apparatus according to claim 2, wherein: 2. The apparatus according to claim 1, wherein the frequency dividing optical system includes a polarization beam splitter and an optical delay circuit. 3. The apparatus according to claim 2, wherein the frequency dividing optical system includes a polarization beam splitter and an optical delay circuit. 4. The apparatus according to claim 3, wherein the frequency dividing optical system includes a polarization beam splitter and an optical delay circuit. 5. The apparatus according to claim 4, wherein the frequency division optical system includes a polarization beam splitter and an optical delay circuit. 2. The apparatus according to claim 1, wherein the exposure of the photoresist is performed by a two-photon absorption process. 3. The apparatus according to claim 2, wherein the photoresist is exposed by a two-photon absorption process. 4. The apparatus according to claim 3, wherein the exposure of the photoresist is performed by a two-photon absorption process. 5. The apparatus according to claim 4, wherein the exposure of the photoresist is performed by a two-photon absorption process. 10. The apparatus according to claim 9, wherein a pulse width of the exposure light source is 1 * 10 <^-12> seconds or less. 11. The apparatus according to claim 10, wherein a pulse width of the exposure light source is 1 * 10 <^-12> seconds or less. 12. The apparatus according to claim 11, wherein a pulse width of the exposure light source is 1 * 10 <^-12> seconds or less. 13. The apparatus according to claim 12, wherein a pulse width of the exposure light source is 1 * 10 <^-12> seconds or less. 10. The spot shape of the laser beam emitted from the focusing optical system and focused on the photoresist is an ellipse extending in the scanning direction of the laser beam. Optical recording medium master exposure equipment. The spot shape of the laser beam emitted from the focusing optical system and focused on the photoresist is an ellipse extending in the scanning direction of the laser beam. Optical recording medium master exposure equipment. 12. The spot shape of the laser beam emitted from the focusing optical system and focused on the photoresist is an ellipse extending in the scanning direction of the laser beam. Optical recording medium master exposure equipment. 13. The spot shape of a laser beam emitted from the focusing optical system and focused on the photoresist is an oval extending in a scanning direction of the laser beam. Optical recording medium master exposure equipment. The light from the exposure light source is subjected to light intensity modulation corresponding to the recording information, and the light modulated by the modulating means is focused on a photoresist on an optical recording medium master, and the photoresist is converted to the recording information. Exposure method of the optical recording medium master for pattern exposure according to,
The exposure light source is an ultrashort pulse laser having a repetition frequency of 1 to 20 times the clock frequency of the recording information,
A method for exposing a master optical recording medium, comprising providing a frequency dividing optical system for dividing a laser beam emitted from the ultrashort pulse laser. The repetition frequency of the ultrashort pulse laser is an integer multiple of 1 to 20 times the clock frequency of the recording information,
22. An external synchronization mechanism for changing a resonator length of the ultrashort pulse laser, wherein a mode lock is performed in synchronization with a repetition frequency of the ultrashort pulse laser in synchronization with the clock frequency, and pulse oscillation is performed. 3. The method for exposing an optical recording medium master according to item 1. The light emitted from the exposure light source is made shorter by a wavelength conversion using a non-linear optical element to be emitted by high-order harmonic generation means using the exposure light source as an excitation light source, and is incident on the frequency dividing optical system. 22. The method for exposing a master optical recording medium according to claim 21, wherein: The light emitted from the exposure light source is made shorter by a wavelength conversion using a non-linear optical element to be emitted by high-order harmonic generation means using the exposure light source as an excitation light source, and is incident on the frequency dividing optical system. The method for exposing a master optical recording medium according to claim 22, wherein: 22. The method according to claim 21, wherein the frequency dividing optical system comprises a polarization beam splitter and an optical delay circuit. 23. The exposure method for an optical recording medium master according to claim 22, wherein the frequency dividing optical system comprises a polarization beam splitter and an optical delay circuit. 24. The method according to claim 23, wherein the frequency dividing optical system comprises a polarization beam splitter and an optical delay circuit. 25. The method according to claim 24, wherein the frequency dividing optical system comprises a polarization beam splitter and an optical delay circuit. 22. The method according to claim 21, wherein the exposure of the photoresist is performed by a two-photon absorption process. 23. The method according to claim 22, wherein the exposure of the photoresist is performed by a two-photon absorption process. 24. The method according to claim 23, wherein the exposure of the photoresist is performed by a two-photon absorption process. 25. The method according to claim 24, wherein the exposure of the photoresist is performed by a two-photon absorption process. 30. The method of exposing an optical recording medium master according to claim 29, wherein a pulse width of the exposure light source is set to 1 × 10 ⁻¹² seconds or less. 31. The method of exposing a master optical recording medium according to claim 30, wherein a pulse width of the exposure light source is set to 1 × 10 ⁻¹² seconds or less. 32. The method according to claim 31, wherein a pulse width of the exposure light source is set to 1 * 10 <^-12> seconds or less. 33. The method of exposing a master optical recording medium according to claim 32, wherein a pulse width of the exposure light source is set to 1 × 10 ⁻¹² seconds or less. 30. The spot according to claim 29, wherein the spot shape of the laser beam emitted from the focusing optical system and focused on the photoresist is an ellipse extending in the scanning direction of the laser beam. An exposure method for an optical recording medium master. 31. The laser beam according to claim 30, wherein the spot shape of the laser beam emitted from the focusing optical system and focused on the photoresist is an ellipse extending in the scanning direction of the laser beam. An exposure method for an optical recording medium master. 32. The spot according to claim 31, wherein the spot shape of the laser beam emitted from the focusing optical system and focused on the photoresist is an ellipse extending in the scanning direction of the laser beam. An exposure method for an optical recording medium master. 33. The spot according to claim 32, wherein the spot shape of the laser beam emitted from the focusing optical system and focused on the photoresist is an oval extending in the scanning direction of the laser beam. An exposure method for an optical recording medium master.

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