Abstract:
The invention concerns a method which consists in a process known per se in producing, on the curved substrate ( 10 ) to be treated, a film of material derived from a specific material source ( 13 ). The invention is characterized in that it consists in inserting, between the curved substrate ( 10 ) and the material source ( 13 ), a mask ( 19 ) relative to the curved substrate ( 10 ), preferably selecting as mask ( 19 ), a mask comprising a ring-shaped part ( 20 ). The invention is particularly useful for providing lenses with antiglare treatment.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates in a general manner to the vacuum treatment of any substrate of the kind in which, using a sputtering technique, the deposition of a layer of material coming from a given source of material is carried out, everywhere, in a chamber, on this substrate by subjecting, for example, this source of material, which then serves as a cathode, to a suitable ion bombardment, the result of which is that particles are torn off it and these are then deposited on the substrate. 
     The invention is aimed more particularly at the case in which the treated substrate is a curved substrate, or even a highly curved substrate, as is the case, at least, in certain spectacle lenses, particularly those of high power. 
     The expression “curved substrate” should therefore be understood to mean here, more generally, a substrate which, in the manner of a spectacle lens, has a curvature accentuated to a greater or lesser extent in at least one transverse plane. 
     Thus, as is known, it is usual to apply a vacuum treatment to certain spectacle lenses in order to give them particular properties, for example antireflection properties. 
     One of the problems to be solved in terms of material is to ensure the complete thickness uniformity desirable for the layer of material thus deposited. 
     The term “thickness” should be understood to mean here, and throughout the following, the optical thickness of the deposited layer, that is to say the physical thickness of this layer multiplied by the refractive index of the material of which it is composed. 
     Should good uniformity of this thickness be lacking, any possible residual reflection runs the risk of resulting in undesirable iridescence. 
     Now, the thickness of the deposited layer of material is inversely proportional to the distance of the spectacle lens, or more generally of the treated substrate, from the corresponding source of material. 
     When, as is the case here, the substrate is curved, this distance is greater at its periphery than its centre and the thickness of the coating obtained is therefore smaller at this periphery than at its centre. 
     This thickness difference may become significant when, for practical reasons, the curved substrate is placed a short distance from the source of material, the difference in distance between its periphery and its centre with respect to this source of material then becoming appreciable because of its very curvature. 
     In practice, this thickness difference may commonly be as much as 20%, depending on the substrates and the chamber employed. 
     SUMMARY OF THE INVENTION 
     The subject of the present invention is, in a general manner, an arrangement allowing this thickness difference to be reduced very simply. 
     More specifically, the subject of the invention is firstly a process for the vacuum treatment of any curved substrate, of the kind in which a layer of material coming from a given source of material is deposited on this curved substrate, this process being generally characterized in that a mask is interposed between the curved substrate and the source of material at a certain distance from the curved substrate, the said mask being stationary with respect to the curved substrate; the subject of the invention is also any mask suitable for implementing such a process. 
     Admittedly, it is already known to employ, for various reasons, a mask during the vacuum treatment of a curved substrate, as is the case, for example, in U.S. Pat. No. 5,225,057. 
     However, in this U.S. Pat. No. 5,225,057, the treated curved substrate is driven in rotation about itself, so that the mask is not stationary with respect to it. 
     Furthermore, this mask is employed only for limiting the beam angle of the beam by means of which the desired deposition is carried out, without it being intended for there to be any effect on the actual thickness of the layer of material obtained in terms of this deposition. 
     Admittedly, it is also already known, especially from U.S. Pat. No. 5,389,397, to use, during the vacuum treatment of a substrate, a mask which is stationary with respect to this substrate. 
     However, in this U.S. Pat. No. 5,389,397, the treated substrates are flat. 
     With regard to the deposition peculiarities involved when the substrate is a curved substrate, it was impossible to imagine a priori that a stationary mask could also be suitable for such a curved substrate. 
     Furthermore, in this U.S. Pat. No. 5,389,397, the mask used is solid. 
     Now, somewhat surprisingly, it turns out that, with the mask according to the invention, significant results are obtained when this mask includes an annular part. 
     Without there being any certainty in this regard, it is conceivable, for example, that, at the working pressures employed, which are generally greater than 0.1 Pa, since the mean free path of the coating particles involved is of the order of a few millimeters, these coating particles are subject to multiple reflections from the atoms of the plasma gas, that, because of these multiple reflections and because of the configuration of the chamber employed, a greater flux of particles is normally created in the direction of the centre of the substrate than in the direction of its periphery and that, with the interposition of a mask according to the invention, and more particularly of a mask including an annular part, between the source of material and the substrate, this flux of particles is probably less directional, this flux then being at least partially limited at the centre of the substrate, which is the point where, in the absence of such a mask, a surplus of deposited material is usually observed. 
     Nevertheless, it is observed, and this is borne out by tests, that, with a mask having, according to the invention, an annular part, the measured thickness difference, for the coating obtained, between the periphery of the treated curved substrate and the centre of the latter, instead of being of the order of 20%, may advantageously be reduced to less than 15%, or even less than 10%, all other conditions being the same. 
     Even more satisfactory results may be observed when, in a development of the invention, the mask employed has, inside its annular part, at least one crosspiece which connects two regions of this annular part together, for example along a diameter of the latter, with, optionally in this case, in a complementary development of the invention, on the outside of the annular part, at each of the ends of such a crosspiece, an arm which extends radially with respect to the annular part in the extension of this crosspiece. 
     In such a case, the thickness difference observed between the periphery of the treated curved substrate and the centre of the latter may, advantageously, be reduced to less than 5%, all other conditions being the same. 
     In all cases, the results obtained with a mask according to the invention are all the more surprising since, in practice, this mask may advantageously have, with respect to the treated curved substrate, a relatively small extension, which, if desired, makes it possible to limit the overall size of the assembly to that of the curved substrate alone, and which thereby makes it possible, as a corollary, to avoid possible shadowing difficulties. 
     In particular, especially favourable results may be obtained with a mask whose projection on a plane in a direction perpendicular to this plane has an area of less than 10%, or even less than 5%, of the area of the projection of the curved substrate on this same plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics and advantages of the invention will, moreover, emerge from the description which follows, given by way of example, with reference to the appended schematic drawings in which: 
         FIG. 1  is a perspective view of a curved substrate to be treated, illustrated in place on a substrate, with the mask with which it is associated, according to the invention; 
         FIG. 2  is an axial sectional view of the assembly, on the line II—II in  FIG. 1 ; 
         FIG. 3  is, taken from  FIG. 1 , but on a different scale, a perspective view of the mask according to the invention, illustrated in isolation; 
         FIG. 4  is, on a larger scale, a partial axial sectional view of this mask, on the line IV—IV in  figure 3 ; 
         FIG. 5  is a partial perspective view, similar to that in  FIG. 3 , for an alternative embodiment; 
         FIG. 6  is also a perspective view, similar to that in  FIG. 3 , for another alternative embodiment; 
         FIG. 7  is, on a larger scale, a partial axial sectional view of this other alternative embodiment, on the line VII—VII in  FIG. 6 ; and 
         FIGS. 8 ,  9  and  10  are perspective views which, also being similar to that in  FIG. 3 , each relate, respectively, to other alternative embodiments of the mask according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     These figures illustrate, by way of example, the application of the invention to the case in which the curved substrate  10  to be treated is a spectacle lens or, more specifically, a disc of circular contour from which such a spectacle lens is subsequently cut out. 
     Let D 1  be the diameter of this curved substrate  10  along its contour. 
     This diameter D 1  is usually between 65 mm and 80 mm. 
     In the embodiment illustrated, the curved substrate  10  is, for example, concavo-convex. 
     When a vacuum treatment has to be applied to it, such a curved substrate  10  is usually supported, around its periphery, by a support  11  suitable for holding it. 
     Since this support  11  is well known per se and does not pertain at all to the present invention, it will not be described here. 
     Furthermore, it is also for the sake of convenience that it is illustrated in  FIGS. 1 and 2  in the form of a flat disc of circular contour. 
     In fact, this support  11  may have very diverse configurations, or indeed may even be perforated. 
     Nevertheless, for the desired vacuum treatment, and in a manner known per se, a layer of material coming from a suitable source of material  13  is deposited on the curved substrate  10  thus supported by a support  11 , in a chamber  12  shown schematically by the broken lines in  FIG. 2 , the source of material also being shown schematically in broken lines in this  FIG. 2 . 
     A machine used, for example, for this purpose is sold by Applied Vision Ltd under the brand name PLASMACOAT AR.10™. 
     This machine is also described in International Patent Application No. WO-A-92/13114. 
     For example, and as shown schematically at  14  in  FIG. 2 , the source of material  13  is placed at a negative potential, in order to form a cathode, and introduced into the chamber  12  are, on the one hand, for example via a nozzle  15 , an inert gas, for example argon, and, on the other hand, for example via a nozzle  16 , an active gas, for example oxygen. 
     In general, a gas pressure of greater than 0.1 Pa is used. 
     Preferably, this gas pressure is between 0.2 Pa and 2 Pa. 
     As a corollary, the support  11  is usually, and as illustrated, an individual support and, together with other supports  11  of the same type, each supporting a curved substrate  10  to be treated, it is fitted, as shown schematically by broken lines in  FIG. 2 , onto a collective support  18 , for example in the form of a plate, mounted so as to rotate in the chamber  12 . 
     As a variant, the support  11  may just as well move in a linear fashion beneath the source of material  13 . 
     Whatever the case, the inert gas introduced via the nozzle  15  ionizes on entering the chamber  12 , forming as it were a plasma therein, and the positive ions which thus arise therefrom bombard the source of material  13 , which forms a target, and they thus tear off from this source of material  13  particles which, while combining with the active gas introduced via the nozzle  16 , become deposited on the curved substrate  10 , forming on its surface the desired layer of material. 
     The above arrangements are well known per se and will therefore not be described in detail here. 
     According to the invention, a mask  19  is interposed between the curved substrate  10  and the source of material  13 , this mask being stationary with respect to the curved substrate  10 . 
     Preferably, and as illustrated, a mask having an annular part  20  is chosen as the mask  19 . 
     In practice, this annular part  20  has a circular contour. 
     For example, and as illustrated, a mask whose annular part  20  has, on the outside, a diameter D 2  of less than twice the diameter D 1  of the curved substrate  10  is chosen for the mask  19 . 
     More specifically, a mask whose annular part  20  has, on the outside, a diameter D 2  of between one quarter of the diameter D 1  of the curved substrate  10  and twice this diameter D 1  is preferably chosen for the mask  19 . 
     Moreover, it is possible to indicate, in this regard, by way of numerical example, but without this being able to result in any limitation of the invention, that, with a curved substrate  10  whose diameter D 1  is about 65 mm, whose power is six dioptres and whose radius of the convex front face is 62.13 mm, satisfactory results have been obtained with a mask  19  whose annular part  20  has, on the outside, a diameter D 2  of between 20 mm and 130 mm. 
     Whatever the case, since the curved substrate  10  is supported, as indicated above, by a support  11 , the mask  19  is fastened, for example, to this support  11 . 
     In the embodiment illustrated, the mask  19  is, in practice, attached to the support  11  by a bracket  22  and extends in cantilever fashion from the end of the cross-arm  23  of this bracket  22 . 
     Of course, it is preferred to make this bracket  22  as small as possible so as to minimize the shadowing that it may cause during deposition of material on the curved substrate  10 . 
     Nevertheless, and as illustrated, the mask  19  is preferably placed at a distance from the curved substrate  10  and approximately parallel to the latter. 
     Let d be this distance, measured between the mask  19 , at its base, and the highest point of the curved substrate  10 , as indicated in  FIG. 2 . 
     For example, this distance d is less than twice the diameter D 1  of the curved substrate  10 . 
     Preferably, it is between one tenth of the diameter D 1  of the curved substrate  10  and half this diameter D 1 . 
     Moreover, it is possible to indicate in this regard, by way of non-limiting example, and under the same conditions as above, that satisfactory results have been obtained with a distance d between the mask  19  and the curved substrate  10  of less than 130 mm and preferably between 7 mm and 30 mm. 
     In the embodiments illustrated in  FIGS. 1 to 5 , the mask  19  is reduced to its annular part  20 . 
     For example, and as illustrated, the cross section of this annular part  20  is generally rectangular. 
     Let H be its height, measured along the axis of the curved substrate  10 , and therefore perpendicular to the support  11  which supports it, and let E be its radial thickness, measured parallel to this support  11 . 
     Preferably, and this is the case in the embodiments illustrated, the annular part  20  of the mask  19  has, in cross section, a height H greater than its radial thickness E. 
     For example, this height H is less than 15 mm. 
     Preferably, it is between 1 mm and 15 mm. 
     Moreover, it is possible to indicate in this regard, by way of non-limiting example, and under the same conditions as above, that satisfactory results have been obtained with a height H of between 1 mm and 10 mm. 
     Likewise, satisfactory results have been obtained with a radial thickness E of less than 2 mm. 
     For example, this radial thickness E is less than 1 mm. 
     Moreover, it is possible to indicate in this regard, by way of non-limiting example, and under the same conditions as above, that satisfactory results have been obtained with a radial thickness E of about 0.1 mm. 
     Moreover it appears that, in order to obtain satisfactory results, it is desirable to take into account the diameter D 1  of the curved substrate  10  to be treated when choosing the diameter D 2  of the annular part  20  of the mask  19  employed. 
     To do this, according to the invention, care is taken to ensure that at least one of the following formulae, and preferably each of them, is satisfied: 
                     d   +   H     =     A   ·       D   2     2               (   I   )               d   =     B   ·       D   2     2               (   II   )               D 1 =C.D 2   (III) 
in which d, H, D 1  and D 2  are the parameters already mentioned above, in which:
 
     A is a coefficient of between 0.8 and 1, being for example about 0.92; 
     B is a coefficient of between 0.7 and 0.9, being for example about 0.77; 
     and C is a coefficient of between 2 and 3, being for example about 2.5. 
     In the embodiment illustrated in  FIG. 5 , the height H has a value twice the value that it has in the embodiment illustrated in  FIG. 3 . 
     In the embodiments illustrated in  FIGS. 6 to 10 , the mask  19  includes, inside its annular part  20 , at least one crosspiece  24  which links together two regions of this annular part  20 . 
     For example, and as illustrated in  FIGS. 6 to 8 , a single crosspiece  24  is provided and this crosspiece  24  extends along a diameter of the annular part  20 . 
     Furthermore, in these embodiments, the cross section of this crosspiece  24  is generally rectangular and it extends approximately parallel to that of the annular part  20 . 
     In the embodiments illustrated in  FIGS. 6 to 8 , the crosspiece  24  has itself, in cross section, a height H′ equal to the height H of the annular part  20  and a radial thickness E′ equal to the radial thickness E of this annular part  20 . 
     It therefore extends, level with the annular part  20 , both on the side of one of the portions of the latter and on the side of the other of these portions. 
     However, of course, the crosspiece  24  may, as a variant, have, in cross section, a height H′ different from the height H of the annular part  20  and/or a radial thickness E′ different from the radial thickness E of this annular part  20 . 
     This is the case, by way of example, at least for the height H′, in the embodiments illustrated in  FIGS. 9 and 10 , in which, moreover, the mask  19  according to the invention includes at least two crosspieces  24 . 
     For example, and as illustrated, only two crosspieces  24  are thus provided and they are perpendicular to each other, each extending in practice along a diameter of the annular part  20 . 
     For example, these two crosspieces  24  each have a construction similar to that of the previous cross section  24 . 
     However, in the embodiments illustrated, their height H′ is equal to half the height H of the annular part  20 . 
     For example, and as illustrated, they extend halfway up this annular part  20 . 
     Finally, in the embodiment illustrated in  FIG. 10 , the mask  19  includes, on the outside of its annular part  20 , at least one arm  25  which extends radially with respect to this annular part  20 , in a cantilever fashion from the latter. 
     In practice, this arm  25  has a structure similar to that of the crosspieces  24  and it extends along the extension of such a crosspiece  24 . 
     Also in practice there is an arm  25  at each of the ends of a crosspiece  24  and, for both of the crosspieces  24 , the various arms  25  thus used have the same length. 
     It is possible to indicate in this regard, by way of non-limiting example, and under the same conditions as above, that satisfactory results have been obtained with arms  25  having a length L of 10 mm. 
     In  FIG. 6 , the height H of the annular part  20  of the mask  19  is equal to that of the embodiment in  FIG. 3 . 
     On the other hand, in  FIGS. 8 to 10 , this height H has a value equal to that of the embodiment illustrated in  FIG. 5 . 
     However, in both cases this height H may be different. 
     Furthermore, according to an alternative embodiment (not illustrated), this height H of the annular part  20  of the mask  19  varies along the perimeter of the latter, extending, for example, between 2 mm and 15 mm, in order to take into account the relative movement between this mask  19  and the source of material  13 . 
     In all cases, simply because of the structure of the mask  19  according to the invention, the shadow cast by the latter on the treated curved substrate  10  is advantageously particularly small. 
     More specifically, a mask whose projection on a plane in a direction perpendicular to this plane has an area of less than 10% of the area of the projection of the curved substrate  10  on this same plane under the same conditions is chosen, in this regard, according to the invention, for the mask  19 , the plane of projection thus taken into account being, for example, that of the support  11  on which the treated curved substrate  10  rests. 
     Again, more specifically, a mask whose projection, under the conditions indicated above, has an area of less than 5% of the area of the projection of the curved substrate  10  is preferably chosen for the mask  19  according to the invention. 
     In all cases too, and to the extent that this can be confirmed, the material of which the mask  19  according to the invention is composed seems to have no effect on the results obtained. 
     This material may therefore be various materials. 
     For example, it may just as well be paper as stainless steel. 
     The table below summarizes, figure by figure, the results obtained with the various embodiments briefly described above. 
     In this table, Δ gives, in percent, the thickness difference observed for the coating obtained during a given treatment between the periphery of the curved substrate  10  and the centre of the latter. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 D 1   
                 D 2   
                 H 
                 E 
                   
               
               
                   
                 mm 
                 mm 
                 mm 
                 mm 
                 Δ % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 FIGS. 3, 4 
                 65 
                 26 
                 1 
                 0.1 
                 14 
               
               
                   
                 FIG. 5 
                 65 
                 26 
                 2 
                 0.1 
                 12 
               
               
                   
                 FIGS. 6, 7 
                 65 
                 26 
                 1 
                 0.1 
                 13 
               
               
                   
                 FIG. 8 
                 65 
                 26 
                 2 
                 0.1 
                 7 
               
               
                   
                 FIG. 9 
                 65 
                 26 
                 2 
                 0.1 
                 3.7 
               
               
                   
                 FIG. 10 
                 65 
                 26 
                 2 
                 0.1 
                 3.4 
               
               
                   
                   
               
             
          
         
       
     
     Of course, the present invention is not limited to the embodiments described and illustrated, but encompasses any alternative embodiment and/or implementation. 
     In particular, the contour of the annular part of the mask employed is not necessarily circular. 
     For example, this contour could be elliptical. 
     It could even have a more complex shape, being, for example, in the form of a spiral. 
     Furthermore, it is possible to vary the pressure of the gases inside the chamber employed. 
     For example, by increasing this pressure, it is possible to reverse the thickness difference observed, the thickness of the coating obtained then being less at the centre of the treated curved substrate than at the periphery of the latter.