Method of forming mother for use in optical disc

A photoresist is applied onto a substrate, the photoresist-coated substrate is exposed to a laser beam of 300 nm or less in wavelength to form a latent image indicative of an information signal, and the photoresist on the substrate is developed to form a pit/groove pattern indicating the information signal. The photoresist has a mean value of 0.1 or less between extinction coefficients before and after the exposure to the laser beam.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method of forming a so-called mother to
 produce a so-called master used to mold an optical disc.
 2. Description of Related Art
 An optical disc comprises a transparent substrate having formed thereon a
 pattern of very fine concavities/convexities including pits and grooves
 indicative of information signals, a reflective layer provided on the
 substrate and formed from a metal film such as aluminum film or the like,
 and a protective layer provided on the reflective layer to protect the
 reflective layer from moisture and oxygen in the atmosphere.
 For producing such an optical disc, a manufacturing process is required in
 which the optical disc can be replicated instantly with a high fidelity
 using a high-precision stamper.
 To meet the above requirement, a photoresist, namely, a photosensitive
 resin, is applied onto a glass substrate, the photoresist is exposed to a
 laser beam to form a latent image corresponding to an information signal,
 and the photoresist on the glass substrate is developed to form a pattern
 of pits and grooves in the photoresist, thereby producing a mother. The
 pit/groove pattern is transferred from the mother to the surface of a
 metal substrate by electroforming or any other suitable method. The metal
 substrate thus processed is used as a stamper.
 More particularly, a glass substrate having a thickness of about a few
 millimeters, for example, is applied uniformly on the surface thereof with
 an ultraviolet ray-sensitive photoresist by a spinner to produce a
 photoresist layer of 0.1-0.2 .mu.m. Next, while the glass disc is being
 spun, the photoresist applied thereon is exposed to a spot of a laser beam
 such as Ar (ion) laser, Kr (ion) laser or the like of 350-460 nm in
 wavelength generated in the blue or near ultraviolet zone and which is
 turned on and off correspondingly to an information signal to be written
 onto a final optical disc, thereby forming a latent image on the glass
 substrate. The photoresist on the glass substrate is developed to complete
 the pit/groove pattern in the photoresist, thereby forming a mother. Then,
 the pits/grooves pattern is transferred from the mother to the surface of
 a metal substrate by nickel plating. The metal substrate thus produced is
 used as a stamper.
 Optical disc substrates can be replicated by the stamper in a large
 quantity by injection molding of a thermoplastic resin such as
 polycarbonate.
 The amount of information recordable in an optical disc depends on the
 density at which pits or grooves can be formed in the disc. That is to
 say, the amount of information recordable in an optical disc depends upon
 how fine the pit/groove pattern can be formed by a so-called cutting
 process in which a photoresist layer is exposed to a laser beam to have a
 latent image formed thereon.
 A stamper used to produce a read-only digital video disc (DVD-ROM), for
 example, has pit rows including shortest pits of 0.4 .mu.m in length
 spirally formed thereon with a track pitch of 0.74 .mu.m. An optical disc
 of 12 cm in diameter produced using this stamper has a storage capacity of
 4.7 GB on one side thereof.
 A Kr ion laser of 413 nm in wavelength is used for cutting the digital
 video disc. The length P of a shortest pit which can be formed in the disc
 can be determined from the relation (1) below:
EQU P=K(NA/.lambda.) (1)
 where .lambda. is the wavelength of laser beam, NA is the numerical
 aperture, and K is the process factor (depends upon the properties of a
 photoresist used, and usually takes a value of 0.8-0.9).
 Therefore, for a digital video disc, putting .lambda.=413 nm, NA=0.9 and
 K=0.9 in the relation (1) will provide a shortest pit length of 0.4 .mu.m.
 Along with the great progresses of recent technology of information
 communications and image processing, the aforementioned optical disc has
 been required to have a capacity several times larger than ever. For
 example, a DVD of 12 cm in diameter is required to have a storage capacity
 of 15 GB on one side thereof. To meet these requirements, the shortest pit
 length should be further reduced to 0.22 .mu.m and track pitch be to 0.41
 .mu.m.
 As seen from the above-mentioned relation (1), a decreased shortest
 wavelength of laser beam and an increased numerical aperture (NA) of an
 objective lens used are required for forming pits with such a high
 density. However, the currently available objective lens NA of 0.9 is
 nearly the upper limit because of the lens precision which can be attained
 in the design and manufacture of the objective lens. For example, when an
 ultraviolet laser of 250 nm in wavelength is used, a shortest pit length
 of 0.23 .mu.m is derived by using a process factor K of 0.8 in the
 relation (1).
 Therefore, by cutting a photoresist having similar sensitivity and
 resolution to the conventional ones with respect to a far ultraviolet
 laser, it is possible to produce an optical disc having a storage capacity
 of 15 GB.
 However, the conventional photoresist generally used in manufacture of
 optical discs, for example, novolak type photoresist, is conditioned to
 have a molecular design optimized for use in an exposure equipment using g
 ray of 436 nm in wavelength in the conventional photolithography
 originally used for manufacture of semiconductor devices. The light
 absorption will abruptly increase with a wavelength of less than 300 nm.
 Thus, when the novolak type photoresist is cut using a far ultraviolet
 laser, the contrast value (.gamma. value) upon which the resolution
 depends is deteriorated due to a considerable light absorption in the
 photoresist, so that pits thus formed will have each a poor edge
 definition or a sloped edge profile. Further, since the photoresist is
 less sensitive to the far ultraviolet laser for the same reason, the
 cutting efficiency will considerably decrease. So, it is very difficult to
 cut the conventional photoresist as it is with a far ultraviolet laser.
 SUMMARY OF THE INVENTION
 Accordingly, the present invention has an object to overcome the
 above-mentioned drawbacks of the prior art by providing a method of
 forming a mother for use in manufacture of an optical disc having a high
 density and large storage capacity, wherein a photoresist can be cut to
 have a high-density pit/groove pattern including extremely fine pits and
 grooves.
 The above object can be accomplished by providing a method of forming a
 mother for use in an optical disc manufacturing process, comprising the
 steps of applying a photoresist onto a substrate; exposing the
 photoresist-coated substrate to a laser beam of 300 nm or less in
 wavelength to form a latent image corresponding to an information signal;
 and developing the photoresist on the substrate to form a pattern of pits
 and grooves in the photoresist; the photoresist having a mean value of 0.1
 or less between extinction coefficients before and after exposure to the
 laser beam.
 According to the present invention, the photoresist used has a mean value
 of 0.1 or less between an extinction coefficient before it is exposed to a
 laser beam of 300 nm or less in wavelength and a one after exposure to the
 laser beam, so that the photoresist can be cut with a high precision to
 have formed therein a pattern of pits and grooves each having a
 well-defined or steep edge profile. Thus, very fine pits and grooves can
 be formed, and thus a high-quality mother can be formed with a high
 productivity to permit to produce an optical disc having a storage
 capacity of 15 G for example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 For optically cutting a mother for use in the optical disc manufacturing
 process, a glass substrate of 220 mm in outside diameter and 6 mm in
 thickness is prepared which has a precision-polished surface. Then the
 glass substrate is applied with a photoresist to a layer thickness of 0.1
 .mu.m by a spinner to form a uniform photoresist layer or film.
 As shown in FIG. 1, a light or laser source 1 is used to generate a laser
 beam of 300 nm or less in wavelength, for example, a frequency-quadrupled
 YAG laser of 266 nm wavelength. The laser beam generated from the light
 source 1 is pulse modulated with a digital signal by an acousto-optical
 element 2 or the like and focused through an objective lens 3 having an NA
 of 0.9 to a spot of 0.25 .mu.m on the photoresist layer (not shown) on a
 main side 4a of the glass substrate 4 as shown with an arrow L.sub.1 in
 FIG. 1. Thus a latent image (pit/groove pattern) is formed spirally.
 At this time, the glass substrate 4 is spun at a constant linear velocity
 (CLV) and the objective lens 3 is slid radially in such a manner as to
 maintain a predetermined track pitch from the inner to outer perimeter of
 the glass substrate 4. There are provided between the light source 1 and
 objective lens 3 a lens to focus the laser beam generated from the light
 source 1 onto the acousto-optical element 2, a lens 7 to render the
 focused beam to parallel beams, and a beam expander 8 to increase the beam
 diameter nearly to the incident iris diameter or to more than that. As
 also seen from FIG. 1, there are provided between the light source 1 and
 lens 6 mirrors 9a and 9b to deflect the optical path of the laser beam
 from the light source 1, and between the beam expander 8 and objective
 lens 3 a mirror 10 to deflect the optical path of the laser beam from the
 beam expander 8. Thus, the laser beam from the light source 1 is passed
 through the lens 6 and incident upon the acousto-optical element 2 as
 indicated with an arrow L.sub.2 in FIG. 1, and further focused onto the
 photoresist layer through the lens 7, beam expander 8 and then the
 objective lens 3.
 Then, the photoresist layer in which the latent image is formed by the
 above-mentioned cutting, is developed. The latent image will appear as an
 etched pit/groove pattern.
 According to the present invention, the photoresist layer has a mean
 extinction coefficient k[=(k.sub.1 +k.sub.2)/2] of 0.1 or less derived by
 averaging the sum of an extinction coefficient k.sub.1 before exposure to
 the laser beam of 266 nm in wavelength and a one k.sub.2 after the
 exposure to the laser beam.
 In the above relation, the mean extinction coefficient k is indicative of a
 light absorption of the photoresist. As shown in FIG. 2, when a light
 having a wavelength .lambda. and intensity T.sub.0 is incident upon a
 layer 20 of d .mu.m in thickness and made of a photoresist having a light
 absorption coefficient a and a light having an intensity T.sub.1 outgoes
 from the photoresist layer 20, the photoresist layer 20 will have a light
 transmittance T.sub.1 /T.sub.0 =e.sup.-ad and an extinction coefficient k
 l=(.lambda./4.pi.)a.
 By cutting a photoresist having a mean extinction coefficient k of 0.1 or
 less, a latent image of a pit/groove pattern corresponding to a recording
 modulation signal can be formed accurately with definition of a steep pit
 edge profile. Namely, by etching in the process of development the
 pit/groove pattern latent image having been formed in the photoresist
 layer by the cutting, a mother having a pattern of pits and grooves each
 having a well-defined or steep bit edge profile can be prepared for
 producing an optical disc.
 Further, a sputtering or electroless plating is used to form a nickel layer
 of a few ten nm in thickness, for example, on the photoresist layer having
 the pit/groove pattern formed thereon, and then a nickel layer of about
 300 .mu.m in thickness is further formed as a conductive layer on the
 former nickel layer by electroforming using an electroplating apparatus.
 The layers thus formed on the glass substrate are separated from the
 latter to provide a stamper. The stamper will be further adjusted in
 inside and outside diameters thereof, polished at the rear side and worked
 at the ends thereof to have a required shape for installation to an
 optical disc molder.
 The procedure of preparing a mother for use to produce an optical disc has
 briefly been described in the foregoing. Using a far ultraviolet
 laser-sensitive photoresist having a mean extinction coefficient k of 0.1
 or less (k=(k.sub.1 +k.sub.2)/2 where k.sub.1 : extinction coefficient
 thereof before exposure to the laser of 300 nm or less in wavelength and
 k.sub.2 : extinction coefficient after exposure to the laser), it is
 possible to form a convexity/concavity pattern including pits each having
 a size of 0.2 .mu.m and steep edge profile and thus form with a high
 productivity a quality mother permitting to produce a high density, high
 storage capacity optical disc. This was proved through experiments.
 The experiments were conducted as will be described below. A phenol novolak
 type resin having a basic structure represented by a chemical structural
 formula 1 shown below was used as a base resin for a photoresist. A
 diazonaphthoquinone having a basic structure represented by a chemical
 structural formula 2 shown below was used as a sensitizer. The phenol
 novolak type resin and diazonaphthoquinone were mixed together to be a
 novolak type photoresist. The molecular design of the photoresist was
 optimized to provide a photoresist a having a mean value k=0.08 between
 the extinction coefficients before and after exposure to a laser beam. The
 photoresist a has a spectral transmittance as shown in FIG. 3.
 Chemical Formula 1:
 ##STR1##
 Chemical Formula 2:
 ##STR2##
 As shown in FIG. 1, the photoresist layer made of the photoresist a was cut
 by exposing to a far ultraviolet laser of 266 nm in wavelength to form a
 mother. The mother was used to produce a nickel stamper whose surface
 status is shown in FIG. 4.
 As shown in FIG. 4, when the photoresist a having a mean extinction
 coefficient k of 0.08 was used, it was possible to form pits each having a
 steep edge profile with a cutting power density as low as 25 mJ/cm.sup.2
 or so.
 On the other hand, a photoresist b having a mean value k=0.14 between
 extinction coefficients before and after exposure to a laser was prepared
 from a conventional novolak type photoresist. A photoresist layer made of
 the photoresist b was similarly laser cut (with a laser of 266 nm in
 wavelength) to form a mother. The photoresist b had a spectral
 transmittance as shown in FIG. 5. The mother was used to produce a nickel
 stamper whose surface status is shown in FIG. 6.
 As shown in FIG. 6, the pits formed by the stamper produced using the
 mother made from the photoresist b having the mean extinction coefficient
 k of 0.14 had each a sloped edge profile whose sloped portion is larger
 than the steep portion. The relatively long pits were found irregular in
 shape. This is because the photoresist had a large light absorption
 coefficient and thus the photoresist layer could not be uniformly cut or
 exposed to the bottom. Further, when a relatively long pit is formed,
 namely, when the laser beam is turned on for a longer time, so great a
 heat is generated due to a large light absorption in the photoresist that
 the photoresist is instantly heated up to a temperature at which its
 photosensitivity will be deteriorated. By calculating the temperature
 elevation of the photoresist during the laser cutting through a thermal
 analysis simulation, it was proved that the photoresist is heated up to
 about 120.degree. C.
 Since a photoresist having a mean extinction coefficient k as shown in FIG.
 5 has a low sensitivity to laser beam, a cutting power density of 50
 mJ/cm.sup.2 or so is required.
 As evident from the above, the extinction coefficient k representing the
 light absorption of photoresist has a great influence on the pit shape
 after laser cutting, and the mean value k between extinction coefficients
 before and after exposure to a laser beam should be 0.1 or less. Using a
 photoresist having a mean extinction coefficient k of 0.1 or less, it is
 possible to form a high-density convexity/concavity pattern including pits
 and grooves each having a well-defined or steep edge profile, which will
 be suitable for a higher storage capacity of an optical disc and
 contribute to the improvement of optical disc productivity.
 As in the above, a photoresist having a mean extinction coefficient k of
 0.1 or less, prepared by optimizing the molecular design of the
 conventional novolak type photoresist, can be used with the materials such
 as conventional glass substrates and the equipment, which will be
 industrially advantageous.
 As seen from the foregoing, the method according to the present invention
 uses a photoresist having a mean value of 0.1 or less between the
 extinction coefficients before and after exposure to a laser of 300 nm or
 less in wavelength so that the photoresist can be cut accurately with a
 high density to have formed therein a pattern of extremely fine pits and
 grooves corresponding to a recording modulation signal. Therefore, the
 present invention can form with a high productivity a quality mother
 permitting to produce a high density, high storage capacity optical disc.