The invention relates to a method of copying a relief structure from the external surface of a first cylindrical drum to the external surface of a second cylindrical drum.

An important application of such a method is in the copying of a roller stamper for use in the manufacture of optical registration tape. Such a stamper is described, for example, in International Patent Application WO 96/03743 and International Patent Application WO 9714142.

A method as specified in the opening paragraph is known from European

Patent Application EP 0 481 753. In the procedure therein described, the first cylindrical drum is rolled over a major surface of a (planar) flexible sheet of embossable material so as to endow that surface with a relief pattern (which will be a negative of the relief pattern on the outside surface of the first drum). The sheet can then be cut to size, wrapped around the external surface of the second cylindrical drum in such a manner that the relief pattern on the sheet faces outward, and affixed to the drum using, for example, an adhesive, clips, screws, etc. If the material of the sheet is such that it can be hardened (e.g. thermally or using actinic radiation) after it has been embossed, then the second drum may thus be provided with a durable relief surface. The known method has a number of disadvantages. For example, because the relief pattern is first created in a planar sheet which is then rolled up, a certain amount of distortion of the transferred relief pattern (due to bending and stretching) is inevitable. Furthermore, once the sheet has been wrapped around the second cylindrical drum, the two adjoining ends of the sheet must be fused seamlessly, which is extremely difficult. In addition, the cylindrical surface of the second drum around which the (thin) embossed sheet is wrapped must be exceptionally smooth, since any surfacial roughness will otherwise protrude through to the embossed surface of the sheet. These drawbacks are particularly serious in the case of the relief pattern in an optical recording stamper, in view of the microscopic dimensions and critical specifications of the pattern in such a case.

It is an object of the invention to alleviate these problems. In particular, it is an object of the invention to provide a relief-copying method as stated in the opening paragraph, which method is not subject to as many sources of distortion as the known procedure. In addition, it is an object of the invention that the new method should not incur joining and seaming problems in the transferred relief pattern. Moreover, it is an object of the invention that the said method should be less sensitive to the presence of surfacial roughness on the surface of the drum onto which the relief pattern is copied. These and other objects are achieved according to the invention in a method as specified in the opening paragraph, characterized in that it comprises the following steps:

(a) Placing the first drum within a cylindrical sleeve in such a manner as to leave a first gap between the external surface of the first drum and the internal surface of the sleeve;

(b) Filling the first gap with a first hardenable liquid resin;

(c) Hardening the resin in the first gap so as to form a solid relief layer on the internal surface of the sleeve, whereby this resin contracts slightly and withdraws from the external surface of the first drum; (d) Sliding the first drum out of the sleeve;

(e) Passivating the hardened resin layer on the internal surface of the sleeve;

(f) Inserting the second drum into the sleeve so as to leave a second gap between the external surface of the second drum and the relief layer on the internal surface of the sleeve; (g) Filling the second gap with a second hardenable liquid resin;

(h) Hardening the resin in the second gap so as to form a solid relief layer on the external surface of the second drum, whereby this resin contracts slightly and withdraws from the relief layer on the internal surface of the sleeve; (i) Sliding the second drum out of the sleeve. An immediate advantage of the method according to the invention is that it does not involve a transition between planar and cylindrical geometries (sheet and drum), but is instead restricted to cylindrical geometry only (drum and sleeve); as a result, the intrinsic distortion resulting from rolling a planar sheet into a cylindrical form is absent from the inventive method _r and it becomes unnecessary to form a joint in the copied pattern. In

addition, because contraction of the hardened resin in steps (c) and (h) occurs uniformly in a radial direction, it also occurs seamlessly. Moreover, even if the surface of the second drum is relatively rough at the start of step (0, the liquid resin employed in steps (g) and (h) will accommodate (i.e. "absorb") such roughness before hardening out, so that the roughness will not be pressed through to the patterned side of the resin layer.

The term "resin" as here employed should be interpreted as encompassing such substances as lacquers, varnishes, pastes, emulsions, putties, etc. , and is intended to refer to any organic liquid material which can be hardened into a room-temperature solid body. Steps (a) and (f) should preferably be performed in such a way that the cylindrical axes of the sleeve and the relevant drum at least substantially coincide, thereby resulting in a gap of substantially uniform radial width. However, this is not essential to the successful operation of the inventive method.

A particular embodiment of the method according to the invention is characterized in that the average depth of the relief structure lies in the range 50-250 nra, and that the average spacing of the relief structure lies in the range 250-5000 nm. As here employed, the term "depth" refers to the height-difference between upper and lower parts of the surfacial relief pattern, whereas the term "spacing" refers to the shortest in-plane separation of neighbouring upper or lower parts of the pattern. An example of such an embodiment is in the copying of stampers for producing optical registration tape, in which digital data are represented by series of elongated surfacial bumps or pits which generally have a depth of about 130 nm, a width of about 500 nm and an average length of the order of about 1500 nm.

In a special case of the method according to the invention, the depth of the relief structure is zero, i.e. the external cylindrical surface of the first drum is (optically) smooth. Such an embodiment of the inventive method is useful for copying blank master drums, i.e. the precision-machined, optically smooth cylinders on which a hardened, cross- linked photoresist relief structure can later be provided. This is an attractive possibility, since it removes the need to individually machine such blank drums at high cost, and instead allows relatively rough drums (second drums) to be provided with a smoothing surface layer of hardened resin, to meet optical smoothness levels.

In a particular embodiment of the method according to the invention, at least one of the first and second hardenable liquid resins is photo-hardenable, and the cylindrical sleeve is transparent to the employed hardening radiation (typically ultra-violet

light). Examples of suitable UV-curable resins in this category include, for example, tripropyleneglycol diacrylate (TPGDA), trimethylolpropane triacrylate (TMPTA), N-vinyl-2- pyrrolidone (NVP), 1,6-hexanediol diacrylate (HDD A) and their mixtures, with the optional presence of α.α-dimethoxy-α-phenylacetophenone (DMPA) as a photoinitiator. A suitable sleeve material in this case is quartz. A particularly neat contraction of the embossed, hardened resin from the embossing surface (steps (c) and (h)) is obtained if the source of curing radiation is approximately circumferential and co-axial with the sleeve, and is scanned (swept) along the sleeve in a direction parallel to its cylindrical axis.

An alternative embodiment of the method according to the invention is characterized in that at least one of the first and second hardenable resins is thermally hardenable. Suitable resins in this category include the epoxide-amines, a particular example being UHU+ (a two-component glue manufactured by UHU GmbH, Buhl, Germany). The sleeve in this case may, for example, be comprised of metal.

An advantageous embodiment of the method according to the invention is characterized in that the width of the first and second gaps (in the radial direction) lies in the range 10-500 μm. The term "width" as here employed is intended to refer to the closest radial separation of features on the internal surface of the sleeve and the external surface of the drum. Substantially smaller gap-sizes result in an extremely thin resin layer, with its attendant susceptibility to breaches and its decreased ability to mask surface roughness. On the other hand, an overly large gap will result in an unnecessarily thick resin layer (at least in the case of an optical stamper), which tends to retard the resin-hardening process. In addition, an overly thick resin layer is more susceptible to internal stress, which may lead to crackling effects.

The inventors have observed that, upon hardening in steps (c) and (h), the resin tends to contract by approximately 10-20% (in volume). In most cases, this is sufficient to allow straightforward removal of the drum from the surrounding sleeve (by coaxial displacement), at least for relatively shallow relief patterns (i.e. patterns having a small depth), such as in the case of relief patterns for optical registration purposes.

An advantageous embodiment of the method according to the invention is characterized in that, before filling a gap with resin as in steps (b) and (g), the gap is temporarily caused to narrow by heating at least one of the cylindrical surfaces delimiting it. For example, if the coefficient of thermal expansion of the material of the sleeve (e.g. quartz) is lower than that of a drum (e.g. copper-nickel) inserted within it, then heating the drum and/or the sleeve will cause the drum to expand radially towards the sleeve, thereby

causing the gap between them to narrow (bearing in mind that the thin layer of resin in the gap is thermally conducting). After the resin has been hardened, subsequent cooling of the heated cylindrical surface(s) will cause the gap to widen once again, thus further facilitating separation of drum and sleeve. The passivation operation in step (e) serves to prevent the second liquid resin administered in step (g) from permanently adhering to the hardened first resin layer resulting from step (c). Such passivation can, for example, be carried out using the following procedure: subjecting the hardened first resin layer to an ultra-violet ozone cleaning (— 10 minutes) or a corona discharge (- 30 seconds); introducing the hardened resin layer into a vacuum chamber, together with an open vessel of a liquid passivating agent such as octadecyl triethoxysilane or octadecyl trichlorosilane; evacuating the chamber, so as to cause the passivating agent to evaporate and form a vapour atmosphere around the hardened resin layer; leaving the hardened resin layer for several hours in this atmosphere, before removing it from the vacuum chamber.

Besides its application in the duplication of optical recording stampers, the method according to the invention may also, for example, be used in the duplication (manufacture) of spiral groove bearings. Another possible application is in the copying of optical tracking markings between the axles of precision lathes; such markings can be used, in combination with a laser tracking device connected to an actuator, to finely translate the lathe axis back and forth along its cylindrical axis according to a desired pattern, thereby allowing the lathe to cut complex forms which are not circularly symmetric. The method according to the invention as hereabove described can be performed in two distinct phases, which respectively comprise steps (a) to (d) and steps (g) to (i). These phases are the subjects of Claims 8 and 9, respectively.

The invention and its attendant advantages will be further elucidated using an exemplary embodiment and the accompanying schematic drawings, whereby:

Figure 1 renders a perspective view of a drum-shaped stamper for use in the manufacture (embossing) of optical registration tape;

Figure 2 illustrates step (a) of the method according to the invention,

whereby the subject of Figure 1 is being inserted into a transparent cylindrical sleeve;

Figure 3 shows the subject of Figure 2, after complete coaxial insertion of the stamper into the sleeve;

Figure 4 shows the sleeve after enaction of step (d) of the method according to the invention;

Figure 5 illustrates step (f) of the method according to the invention, whereby a blank drum is being inserted into the subject of Figure 4;

Figure 6 shows the drum after enaction of step (i) of the method according to the invention, the surface of which drum has now been provided with a relief pattern.

Embodiment 1

Figures 1-6 depict various aspects of a particular embodiment of the method according to the invention. Corresponding features in the various Figures are denoted by the same reference numerals.

Figure 1 renders a perspective view of a drum-shaped stamper 1 (first cylindrical drum) for use in optical tape manufacture. The drum 1 has a cylindrical axis 3 and a cylindrical surface 5 (external surface). In this particular example, the surface 5 carries a helical pattern of lanes 7, each lane 7 containing a relief structure 9 running along the length of the lane 7 (only one such relief structure has here been depicted). The relief structure 9 comprises a smooth surface 5 which is provided with series of elongated pits representing binary data. All of these pits have the same depth ( » 130 nm) and width ( » 500 nm), but varying lengths (of the order of 1500 nm).

The bulk of the drum 1 is comprised of brass, with a thin, optically smooth surfacial layer of nickel ( « 0.5 mm thick). This nickel layer is in turn covered by a hardened, cross-linked photoresist layer containing the relief pattern 9. The diameter of the drum 1 is 150 mm, its length is 200 mm, and the (in-plane) width of each lane 7 is 8 mm. If the drum 1 is rolled over a sheet of embossable material (as in

International Patent Application WO 9714142, for example), that sheet will be embossed with bumps corresponding to the pits in the relief structure 9. After metallizing the embossed sheet, it can be spliced into strips of tape, each tape having the same width as a lane 7 (as in International Patent Application WO 96/03743, for example). The bumps in such a tape can

be optically "read" by scanning a focused laser beam along it.

Figure 2 illustrates the enaction of step (a) of the inventive method. The drum 1 is being slid into a cylindrical sleeve 11 so that the axis 3 coincides with the sleeve's cylindrical axis 13. The sleeve 11 has a smooth internal surface 15, which is separated from the external surface 5 by a gap 20 (first gap). In this case, the width of the gap 20 is

0.2 mm, and the sleeve 11 is made of quartz, so as to be transparent to ultra-violet radiation. Figure 3 shows the subject of Figure 2, after complete coaxial insertion of the drum 1 into the sleeve 11. Once such insertion has been completed, a first hardenable liquid resin (not depicted) is introduced by capillary action into the gap 20 from a butt-end of the sleeve 11 (step (b)). In this particular case, the resin is UV-hardenable. The actual hardening (step (c)) can be performed by scanning a circumferential array of UV lamps 22 along the length of the transparent sleeve 11 (in the direction 24).

As the resin in the gap 20 hardens, it adheres to the internal surface 15 of the sleeve 11 , but contracts radially and withdraws from the relief pattern 9 on the external surface 5 of the drum 1. In this way, the inside surface 15 of the sleeve 11 becomes endowed with a helical pattern of lanes 17 having a relief structure 19 which is the negative of the relief structure 9, as shown in Figure 4. Because the resin contracts upon hardening, the drum 1 can be slid out of the sleeve 11 (step (d)) without damaging either of the surfaces 5,15. If so desired, or if thought to be necessary, the relief structure 9 may be passivated and/or the internal surface 15 may be subjected to an adhesion-promoting treatment prior to enacting step (a), so as to further promote flawless separation of the drum 1 and sleeve 11 in step (d). Such passivation may, for example, be performed using the procedure already set forth hereabove. On the other hand, adhesion can be promoted by subjecting the surface 15 to an ultra-violet ozone cleaning (— 10 minutes) or a corona discharge (~ 30 seconds).

Subsequent to step (d), the hardened resin relief structure 19 on the inside surface 15 of the sleeve 11 is now passivated (step (e)), e.g. using the procedure already described hereabove. Figure 5 depicts the enaction of step (f) of the inventive method, whereby a drum 21 (second cylindrical drum) is slid coaxially into the sleeve 11 depicted in Figure 4. The drum 21 has a cylindrical axis 23 and a smooth cylindrical surface 25 (external surface). The diameter of the drum 21 is chosen so as to leave a gap 30 (second gap) between the surfaces 25 and 15. In this particular case, the gap 30 has a width of approximately

0.25 mm.

Analogous to the scenario depicted in Figure 3, once insertion of the drum 21 into the sleeve 11 has been completed, a second hardenable liquid resin (not depicted) is introduced by capillary action into the gap 30 from a butt-end of the sleeve 11 (step (g)). In this particular case, the second resin is identical to the first resin employed in step (c). Just as for step (c), step (h) can be performed by scanning a circumferential array of UV lamps along the length of the transparent sleeve 11.

As the resin in the gap 30 hardens, it adheres to the external surface 25 of the drum 21 , but contracts radially and withdraws from the relief pattern 19 on the passivated internal surface 15 of the sleeve 11. In this way, as depicted in Figure 6, the external surface 25 of the drum 21 becomes endowed with a helical pattern of lanes 27 having a relief structure 29 which is the negative of the relief structure 19, and thus the positive of the relief structure 9. Because the resin contracts upon hardening, the drum 21 can be slid out of the sleeve 11 (step (i)) without damaging either of the surfaces 15,25. If so desired, or if thought to be necessary, the external surface 25 may be subjected to an adhesion-promoting treatment prior to enacting step (f), so as to further promote flawless separation of the drum 21 and sleeve 11 in step (i). This may, for example, be achieved by subjecting the surface 25 to an ultra-violet ozone cleaning or a corona discharge.