PROCESS FOR OPTICAL MEASURE1"4ENT OF THE THICKNESS OF AN ADHESIVE LAYER, AN OPTICAL DATA-STORING DISC, ITS MANUFACTURING AND ALSO AN ADHESIVE AND ITS USE The present invention relates in general to the manufacture of optical, data-storing discs, in particular to a process for optical control measurement of the thickness of the layer of adhesive between two transparent disc substrates for optical, data- storing discs of so-called DVD type.

When manufacturing DVD discs, two 0.6 mm thick disc substrates of polycarbonate are normally glued together, and at least one of the facing sides of the substrates carries a reflecting information layer containing a large number of short and long depressions with a depth of circa 0.1 pm arranged in a spiral roll around the disc substrate.

In DVD discs of the "SD5" type with a single information layer which is complete- ly reflecting for the intended laser light, the adhesive quality requirements as regards air-bubbles, refractive index and the thickness of the adhesive layer are not critical.

However, very high demands as regards the above-mentioned criteria are placed on DVD discs of"SD9" type, which have two information layers facing each other, where one layer consists of a laser semi-reflecting layer of e.g. Al, Au or SiN, while the other information layer is made semi-reflecting. The adhesive must be entirely free of air-bubbles and have a refractive index which is complementary to the refrac- tive index of the substrates in order not to cause refractive errors when sensing laser light focused on the fully reflecting information layer, i.e. when the laser light passes the adhesive layer.

The third criteria, i.e. the adhesive layer thickness between the information layers, is also essential to assuring correct focusing of the laser light beams on the completely reflecting information layer. According to current specifications, the adhesive thick- ness should be between 40 and 70 ,um, i.e. up to 55 ,um 415 pm, and the adhesive

thickness may vary by i10 pm within the same DVD disc and a maximum ofrt4 llm during one rotation, i.e. at a given radius on the same disc.

One method of control-measuring the gap between the information layers, i.e. the thickness of the adhesive layer, is to allow a laser pick-up to focus on the various information layers and then calculate the distance between them. This method pro- vides accuracy in measuring the layers, but the measurement takes a long time to carry out.

Another method is to simply measure the total thickness of the finished DVD disc and, based on assumptions concerning the substrate thicknesses, calculate the thick- ness of the adhesive layer. This method is much more rapid but the sources of error are, however, significant.

One purpose of the present invention is to suggest a more rapid and reliable method of accurately determining or checking the thickness of the adhesive layer in DVD discs.

According to a first basic aspect of the invention it is suggested that, when gluing together the substrates, an adhesive should be used in which there is admixed an evenly distributed trace amount of a fluorophore, i.e. a fluorescent agent. It is thus possible, with the aid of a beam of light of a predetermined, suitable wave-length, to excite the fluorophore in the beam path through the adhesive layer. By detecting at the same time a fluorescent light emitted by the excitation from this beam path in the adhesive layer and having a longer wave-length, it is possible, on the basis of the strength of the detected emitted fluorescent light, to calculate the thickness of the adhesive layer and to check that this thickness lies within a given tolerance range.

In accordance with the above-mentioned principles, the following categories of claims are encompassed by the present invention:

a) a process for optical control measurement of the thickness of an adhesive layer of an optical, data-storing disc by fluorescent light measurement; b) a process for manufacturing an optical, data-storing disc with an adhesive layer containing a trace amount of a fluorophore; c) use of an adhesive containing a trace amount of a fluorophore when gluing together disc substrates for an optical, data-storing disc; d) an adhesive containing a trace amount of a fluorophore for gluing together disc substrates for an optical, data-storing disc; and e) an optical, data-storing disc comprising an adhesive layer containing a trace amount of a fluorophore.

These categories are based on a common, unified inventive concept.

Further features of the invention will be described in more detail below with reference to the accompanying drawing and are defined in the subsequent patent claims.

In the attached drawing: Fig. 1 shows schematically a cross-section of a so-called DVD disc with two infor- mation layers, a control measurement of the thickness of a fluorophore containing an adhesive layer between two disc substrates; Fig. 2 shows a DVD disc which is illuminated by a linear beam; and Fig. 3 shows a diagram showing the strength of an emitted fluorescent light of a predetermined wave-length as a function of the angular position on the disc.

10 in Fig. 1 designates an optical, data-storing disc of so-called DVD type, and in particular a DVD disc with two information layers of a type which is commonly designated "SD9".

The disc 10 comprises two transparent disc substrates 12 and 14 of polycarbonate, for example, with a normal thickness of circa 0.6 mm and a diameter of circa 120 mm. The disc substrates 12 and 14 each have an individual information layer 16 and 18, respectively, on their facing sides having very compact spiral grooves, contain- ing relatively short and long depressions with a depth of circa 0.1 pm. The informa- tion layer 16 on one disc substrate 12 is coated with a layer of Al, Au or SiN, for example, which is semi-reflective for laser light, while the information layer 18 on the other disc substrate 14 is made in a similar manner but completely reflective.

The two disc substrates 12,14 are held together by means of a thin layer 20 of adhesive, the index of refraction of which should be complementary to the index of refraction of the substrates (polycarbonate) in order not to give rise to refractive errors when a laser light of a predetermined wave-length is to read the upper infor- mation layer 18. The adhesive can, for example, be of a suitable epoxy type or be acrylate-based.

In addition to the requirement of the adhesive being absolutely free of air-bubbles and having a correct index of refraction, it is of the utmost importance that the thick- ness of the adhesive layer not vary outside a predetermined range of 40 to 70 pm between different DVD discs. The thickness can vary between different discs i15 pm at a mean thickness of 55 pm. Furthermore, the adhesive thickness may vary by zt 10 ,um within a single disc and a maximum of+ 4 llm during a single revolution, i.e. at a specific radius within the same disc. If these tolerances are not maintained, the laser pick-up of the DVD player will have difficulty in correctly focusing on the fully reflecting information layer 18.

In accordance with the present invention there is now suggested a rapid and reliable method of accurately determining and checking that the adhesive thickness lies within a predetermined interval and that the variations in the adhesive layer thick- ness lie within predetermined tolerance values. Thus, the core of the invention can be considered to reside in admixing and evenly distributing a small trace amount of

a fluorophore, e.g. fluorescein, in an adhesive with a suitable index of refraction for laser light and which are suitable for bonding together information-carrying disc substrates, and that this trace amount of a fluorophore is used as an aid in determin- ing and checking the thickness of the adhesive layer or thickness variations. The fluorophore may be present in an amount of some tenth of a microgram per liter adhesive. The adhesive can, for example, be of suitable epoxy type or be acrylate- based.

In an illustrative example, shown schematically in Fig. 1, of the invention, a beam of light L having a wave-length of circa 488 nm is directed through the disc substrates 12 and 14 and through the adhesive layer 20, and the fluorophore molecules struck by the light beam L in the beam path through the adhesive layer 20 will be excited and emit a spectrum of fluorescent light emitted in various directions.

However, the light beam L can in practice with advantage be a linear beam A formed by a large number, e.g. 200-300, of tightly spaced light sources (not shown) arranged in a line as indicated in Fig. 2, the linear beam A extending substantially axially through the disc 10 in a radial formation so that the entire adhesive layer between the information layers 16,18 can be rapidly illuminated during one rotation of the disc 10. In a similar manner, a row of detectors (not shown) can be placed on the same side of the DVD disc as the light sources but arranged to detect fluorescent light E, emitted in the beam path through the adhesive layer 20, in a direction differ- ent from the direction for the incident light (Fig. 1), the detectors sensing the strength ofthe emitted fluorescent light E having a longer wave-length, e.g. circa 530 nm.

A blocking filter 22 can be arranged in the beam path for the selected, detected fluorescent light E in order to filter out interfering light reflections which can arise from the exciting light beam L in the adhesive layer 20, when it strikes possible con- taminating particles in the adhesive.

Since the thickness of the thin information layers 16,18 of Au and Al (circa 180-220 A) can vary somewhat from disc to disc, and this affects the value of the detected reflective light from the light path through the information layers 16,18 and the adhesive layer 20, a calibration of the light reflection and the light transmission through the semi-reflecting information layer 16 and the completely reflecting infor- mation layer 18 can precede the measuring procedure ofthe detected reflective light.

This can be done, for example, by test illumination at the center portion of the disc of exposed, spaced test surfaces of the respective information layers 16,18, the rele- vant basic data (constants) being obtainable for the information layers for direct subsequent calculation of the thickness of the adhesive layer.

The strength of the selected emission light E from the illuminated adhesive gap can, in the case described, be related to the thickness of the adhesive layer in a directly proportional manner. When the cross-section of the exciting light beam L can be kept constant, the signal strength of the detected light E will only depend on the thickness of the adhesive layer.

Fig.3 illustrates a curve showing the variation in intensity of the detected fluorescent light E during one rotation of the DVD disc 10 shown in Fig.2. The interval between the intensity levels I1 and I2 can in this case be calculated to correspond to a varia- tion oft 4 pm in the thickness of the adhesive layer. In the case shown, the varia- tion in thickness of the adhesive layer lies within the specified tolerance range.