1.  Field of the Art
This invention relates to a method and apparatus for transferring a pattern of minute structures formed on a transfer surface of a mold, onto a resin layer coated on a substrate plate, and to an annular recording medium produced by that method. 
2.  Prior Art
For example, for the purpose of increasing a recording density, lately a nano-pattern imprinting technology is applied in the manufacture of storage media like annular optical discs which are in use as data recording media.  In nano-pattern imprinting, a pattern of infinitesimally minute structures which are formed on a transfer mold is pressed on a thin curable resin layer coated on a substrate plate or sheet to imprint the minute structures on the resin layer.  A nano-pattern consists of a multitude of minute structures in the form of dents and projections of a linear shape or of a pillar-like shape, ranging from several tens to several hundreds nanometers in size. 
A nano-pattern is imprinted on an energy ray-setting resin, e.g., a thermosetting resin, a photo-setting resin or an electron ray-setting resin.  Typical of energy ray-setting resins in general use are ultraviolet curing resins.  Photo-setting resins including ultraviolet resins can be used at room temperature and have been in wide use in the fields of pattern transfer because of their fast hardening ability. 
In a pattern transfer process of this sort, a resin in a softened or liquid state is coated on a surface of a substrate plate, and a master pattern on a transfer mold is pressed on the coated resin layer under a predetermined pressure to imprint minute structures of the master pattern on the resin layer.  The imprinted resin layer is then hardened by irradiation with ultraviolet rays.  A transfer mold with a high ultraviolet transmittivity is used in case ultraviolet rays are irradiated through the transfer mold.  Normally, minute structures of a master pattern are formed on a transfer surface of the mold by lithography or the like. 
Minute structures of a master pattern are imprinted on a resin layer on a substrate plate which is held in pressed contact with a transfer mold.  Thus, after hardening the imprinted resin layer, the substrate plate needs to be separated from the transfer mold, for example, by mechanically separating the substrate plate from the transfer mold as described in Japanese Laid-Open Patent Application H6-876. 
Namely, in this prior art, for transferring a pattern from a mold onto a substrate plate, as a transfer mold a stamper is set on an attracting table with a magnet adaptor, passing a tubular member through the magnet adaptor and fixing a stamper by means of an internal chuck ring which is formed on the tubular member.  A center pin is passed through the tubular member thereby to hold the stamper fixedly in position.  An upper end portion of the center pin is flanged to provide a guided positioning portion of a cylindrical shape.  Thus, a substrate plate which is fitted on the positioning portion of the center pin is automatically guided into an aligned position relative to the stamper.  In this state, a master pattern on the stamper is imprinted on a resin layer which is coated on the substrate plate.  After hardening the imprinted resin layer, the substrate plate is separated from the stamper.  As a separating mechanism, a flange portion is provided around the outer periphery of the above-mentioned positioning portion at the lower end thereof.  When a substrate plate is set in position in abutting engagement with the stamper, the flange portion is retained in a position where it is slightly protruded into the stamper.  Therefore, as the center pin is pushed up, the separator flange portion comes into engagement with the inner periphery of the substrate plate to separate same from the stamper. 
Above-mentioned Japanese Laid-Open Patent Application H6-876 concerns an apparatus for manufacturing optical discs of the sort which has an allowance for alignment errors between the stamper and a substrate plate at the time of pattern transfer from the stamper to the substrate plate.  That is to say, the positioning portion of the center pin to be brought into fitting engagement with a substrate plate can afford a certain degree of fitting allowance relative to the latter.  However, in this regard, far more strict accuracy is required in nano-pattern imprinting involving transfer of infinitesimally minute structures.  Therefore, it becomes difficult to comply with high positioning accuracy as required in nano-pattern imprinting due to accumulation of errors in a case where a pattern transferring side (e.g., a stamper or a transfer mold) and a pattern receiving side (e.g., a substrate plate) are set in position at and by different portions of a coaxially located positioning member, and in a case where a pattern transferring side and a receiving side are positioned by engagement with separate fitting members.  Thus, the technology of Japanese Laid-Open Patent Application H6-876 cannot be applied to a field involving transfer of infinitesimally minute structures like nano-pattern imprinting.  Besides, in this case, the separating flange portion is arranged to hitch the inner periphery of a substrate plate at the time of separation from a stamper, so that it may cause damages to substrate plates.  In addition, in a pressing stage, the separating flange which is protruded between the stamper and a substrate plate can cause adverse effects on transfer accuracy.