Abstract:
An apparatus and method for cathodic magnetron sputtering of a coating onto a temperature-sensitive substrate is disclosed. The apparatus consists of a vacuum chamber having a work-supporting station and a magnetron sputtering target opposite the work-supporting station. The apparatus produces a magnetic field to contain, in an oval pattern, a gas plasma cloud which ejects target material toward the work-supporting station. The temperature of the substrate being coated is controlled by positioning the cooling anode within the sputtering chamber. The position of the cooling anode is adjusted relative to the cathode target to capture primary electrons that would otherwise impinge the substrate. It is in a position with respect to the cathode that does not interfere with the magnetic field.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 09/626,440, filed Jul. 27, 2000, which is incorporated herein by reference, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for the sputtering of coatings from a cathodic magnetron sputtering device upon temperature-sensitive substrates such as plastic ophthalmic lenses. 
     BACKGROUND OF THE INVENTION 
     Plastic ophthalmic lenses such as eyeglass lenses have become popular in recent years, due particularly to their light weight. Coatings commonly are applied to the lenses for any of a number of reasons. Antireflective coatings, for example, reduce the reflected light from the lens surfaces. Other coatings increase the hardness and scratch resistance of plastic lens surfaces. Yet other coatings are used to provide small amounts of color to the lenses, either for cosmetic purposes or to reduce the incidence of radiation of particular wavelengths, e.g., UV light. 
     Ophthalmic lenses commonly are made from lens blanks, and coatings are applied to the lens blanks by applying a coating liquid to the lens blanks as by spraying, dipping, etc. Depending on the coating desired, the coating process may require a series of sequential steps, followed by drying, irradiating with light to cross-link polymers in the coating, etc. As a result, the coating process is relatively slow, and great care must be taken to preserve the desired characteristics of the coating solutions. Antireflective coatings, consisting typically of a series of metal oxide layers having alternating high and low indices of refraction, commonly are applied to lens blanks by batch-processing vacuum deposition techniques in which the temperature of the lens blanks can be controlled. 
     Magnetron sputtering techniques have been used for applying coatings to temperature-resistant substrates such as glass. In ordinary magnetron sputtering techniques, a temperature-resistant substrate to be coated is exposed to a magnetron sputtering target in a high vacuum environment, and molecules of the target material are ejected from the target to impinge upon and coat the substrate. The loss of electrons to the substrate also causes the temperature of the substrate to rise. When temperature-sensitive materials such as polymers are attempted to be coated in this manner, the impingement of electrons on the polymer rapidly heats the substrate. The heat capacity of polymers commonly is substantially less than that of glass. Moreover, plasma temperatures can rise substantially. As a result, magnetron sputtering of coatings onto the surfaces of temperature-sensitive materials such as polymer lens blanks has not gained favor because of the tendency of such materials to soften and deform at elevated temperatures. 
     It would be desirable to provide a magnetron sputtering apparatus that would enable temperature-sensitive materials such as polymer lens blanks to be successfully coated without being unduly heated. 
     SUMMARY OF THE INVENTION 
     I have found that substrate heating in magnetron sputtering devices occurs primarily through the impingement of primary electrons on the substrate, that is, electrons that generally are not trapped in the magnetic field. Moreover, I have found that electron impingement on the substrate can be largely avoided by placing a cooling anode within the sputtering chamber in a position close enough to the cathode target to capture primary electrons not captured in the magnetic field and that otherwise would impinge upon the substrate, but yet oriented, with respect to the cathode, in a field noninterfering position; that is, in a position that does not cause interference with magnetic flux lines and thus produces no significant effect upon sputtering erosion patterns in the target or sputtering uniformity. 
     Thus, my invention in one embodiment provides an apparatus for the cathodic magnetron sputtering of a temperature-sensitive substrate. The apparatus comprises a vacuum chamber having a work supporting station and a magnetron sputtering target in sputtering opposition to the work supporting station. The apparatus produces a magnetic field serving to contain, in an oval pattern, a gas plasma cloud to eject target material toward the work supporting station. 
     The apparatus includes an anode that is carried between the target and the work supporting station sufficiently out of said magnetic field so as to produce no significant effect upon the magnetic field or sputtering erosion patterns but yet sufficiently near the target as to collect electrons not captured by the magnetic field and that otherwise would impinge upon and heat a substrate supported by the work supporting station. In a preferred embodiment, the anode is externally fluid-cooled preferably by an external source of liquid coolant that is supplied to the anode during the coating operation. 
     In another embodiment, my invention provides a method for cathodic magnetron sputtering of target material on the surface of heat-sensitive substrates in a magnetron sputtering apparatus that includes a vacuum chamber having a work-supporting station and a magnetron sputtering target in sputtering opposition to said work supporting station. The apparatus produces a magnetic field serving to contain, in an oval pattern, a gas plasma cloud to eject target material toward the work-supporting station. The method comprises the steps of: 
     a. providing within the vacuum chamber an anode carried adjacent but out of the oval pattern, 
     b. positioning the anode sufficiently out of the magnetic field so as to have no significant effect upon the magnetic field nor the uniformity of the deposition process, but in position with respect to the target as to collect electrons not captured by the magnetic field and that otherwise would impinge upon and heat a substrate supported by the work supporting station, and 
     c. withdrawing energy from the anode. 
     Preferably, the anode is carried generally centrally (but out of) the oval pattern, although the anode can also be carried to the side of the oval pattern. Energy may be withdrawn from the anode by means of external fluid cooling and/or by conduction of electrons away from the anode, as well as by radiation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a broken-away perspective representation of a magnetron sputtering apparatus. 
     FIG. 2 is a broken-away side view, in partial cross section, of a magnetron target together with an externally cooled anode, as used in the apparatus of FIG.  1 . 
     FIG. 3 is a front view, partially broken away, depicting the target and anode of FIG.  2 . 
     FIG. 4 is a partial cross-sectional view through section  4 — 4  of FIG.  3 . 
     FIG. 5 is a graph comparing the temperature rise at a fixed point in the plasma generated within a magnetron sputtering device of the invention with (lower curve) and without (upper curve) an externally fluid-cooled anode. 
     FIG. 6 is a broken-away schematic view of an embodiment of the invention showing the anode outside the oval groove. 
     FIG. 7 is a broken-away schematic view of an embodiment of the invention showing the anode located within the oval groove. 
     FIG. 8 is a side view of the embodiment of FIG.  7 . 
     FIG. 9 is a schematic representation of a continuous coating line. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a coating apparatus  10  is shown comprising a vacuum chamber  12  of known design, the vacuum chamber  12  including pumping devices for producing a vacuum in the chamber and a gas dispensing system for bleeding gases such as argon or oxygen into the vacuum chamber  12  as needed (not shown). Any of a variety of vacuum chamber configurations for magnetron sputtering may be employed, and the vacuum chamber  12  is typical of such sputtering configurations in which the surface of the object to be sputter coated is supported in a generally vertical position, and although the vertical orientation is preferred, chamber configurations in which the substrate is supported in a horizontal or other configuration could be used as well. 
     The vacuum chamber  12 , as shown, may be divided into three aligned enclosures, enter chamber  14 , central process chamber  16  and exit chamber  18 , which communicate with each other. A vertical motor driven platen  20  capable of supporting the object or objects to be coated is mounted on a rail system within the vacuum chamber  12 , and may move from one of the end enclosures through the central process chamber  16  and into the other end enclosure. Although all three enclosures are evacuated during use, sputter coating occurs only in the central process enclosure  16 . When a surface is to have multiple coatings, the vertical motor driven platen  20  may be caused to move multiple times from one end enclosure to the other, with a single coating being deposited each time the platen moves across the central process chamber  16 . Lens blanks to be coated are shown generally as  22 , and are supported on the vertical motor driven platen  20  as in the manner shown in FIG.  1 . 
     The central process chamber  16  is accessible through a doorway  24 , the doorway  24  providing access to the vertical motor driven platen  20  so that the lens blanks  22  can be loaded and unloaded from the vertically motor driven platen  20 . The doorway  24  is closed by a pivoting door  26 , the doorway  24  having an inner wall surface  28  bearing a resilient bead  30  of rubber or other appropriate material which can seal tightly to the outer wall  32  of the central process chamber  16  when the pivoting door  26  is swung closed. Supported on the inner wall surface  28  are a pair of sputtering targets  34 , which may be different or essentially identical, in which case they could be replaced by a single target, if desired. 
     Referring particularly to FIGS. 2 and 3, the planar sputtering target  36  may be generally parallelepiped in shape, with a front surface target face  38  facing the vertical motor driven platen  20  when the door is closed. Planar sputtering targets  36  are of known design, and due to the positioning of magnets (not shown) behind the planar sputtering targets  36  develop an oval or racetrack-shaped groove designated  40  and spaced inwardly slightly from the edges of the planar sputtering target  36 . 
     The oval groove  40  is formed by the loss of target material from the planar sputtering target  36  during sputtering operations in which a corresponding oval-shaped gas plasma cloud supported by the magnetic field serves to eject target material from the planar sputtering target  36  toward the objects to be coated. 
     The sputtering apparatus thus far described in connection with the planar sputtering target  36  is known in the art, and need not be described in greater detail. Moreover, although the magnetron sputtering targets exemplified above are planar targets, it will be evident that cylindrical targets of the type known in the art also can be employed with the invention. 
     Referring now particularly to FIGS.  2 — 4 , an anode is shown generally as  42 . The anode  42  may be made of copper, titanium, tantalum, carbon, or other electrically conductive material, preferably a metal, and, as depicted, is desirably elongated and cylindrical in shape, the length of the anode being such as to enable it to extend through the majority of the length of the target. 
     In this particular embodiment, the anode  42  is positioned generally centrally of the oval-shaped plasma cloud and is spaced from the planar sputtering target  36  in the direction of the substrate. 
     The planar sputtering target  36  itself is supported in an exterior housing  44 . Supporting ground side straps  48  extend forwardly from the top and bottom surfaces of the housing, as shown best in FIG. 2. A pair of similar electrical side straps  50  extend forwardly from the ends of the anode  42 , and an electrical insulating connector  52  joins the forward ends of the ground side strap  48  and the electrical side strap  50  to support the anode  42  in a proper position with respect to the planar sputtering target  36 . The anode  42  is thus electrically insulated from the planar sputtering target  36  or ground, but is connected to the positive side of the power supply. 
     If used without external fluid cooling, the anode preferably is fabricated from a metal that itself has a low sputtering rate. The anode  42  itself may become quite hot, to the point of glowing, and energy is carried away from the anode  42  by the conduction of electrons away from the anode  42 , by radiation, etc., to establish an energy balance over the anode  42  that prevents anode  42  damage. Although a variety of metals may be employed, titanium and tantalum are preferred since they have low sputtering rates and also because they are largely impervious to the erosion effects of the plasma. 
     In a preferred embodiment, the anode  42  is externally cooled by means of a metal water tube that is supplied with cold water or other cooling fluid externally of the coating chamber. The cooling tube, shown as  54 , extends along the length of the anode  42  and may conveniently be formed of copper. Desirably, the surface of the anode  42  is provided with a groove within which the tube may nest, to provide for good heat transfer of heat from the anode body to the cooling tube  54 . As shown in FIGS. 2 and 3, the cooling tube  54  extends from its cooling tube inlet  56  downwardly along the side of the anode  42 , and then doubles back in a U-shaped fashion, as shown in FIG. 3 at upward bend  58  to extend upwardly along the side of the anode  42 , terminating in a cooling tube outlet  60 . It will be understood that a variety of configurations for the cooling apparatus may be employed in addition to those typified above. For example, the anode itself may be provided with one or more interior chambers through which a cooling fluid may flow, or the anode itself may be tubular. 
     The cooling tube inlet  56  and cooling tube outlet  60  of the cooling tube  54  extend outwardly through a sealed, electrically insulated port  62  in the pivoting door  26 . Assuming the cooling tubes  54  are made of metal, electrical contact with the cooling tubes  54  and hence with the anodes  42  can be had by electrically connecting the positive side of an appropriate power supply to the cooling tubes  54  where they exit the pivoting door  26 . The cooling tubes  54  desirably are joined to long, non-conductive plastic tubes  61  of polyethylene or the like, which in turn circulate water or other cooling fluid from a cooling fluid source  63  through the cooling tubes  54  to cool the anodes  42 . If water is used, it is desirably treated with biocidal materials to restrain growth of biological materials. Within a non-conducting plastic tube, water generally loses its ability to conduct electricity when the tube length is on the order of four feet or greater, and hence no special care need be taken to electrically insulate the cold-water source. 
     It will be understood by those skilled in the art that magnetron sputtering devices of the type described are powered by power sources that deliver current to the targets. One may use alternating current power sources, or direct (continuous or pulsed) power sources, or one may use radio frequency (e.g., 13.56 MHz) power sources. The relative potential, and polarity, of the anode and cathode are both controlled by the power source (not shown), such that polarity can be momentarily reversed to clean the anode of sputtered on material by means of bias sputtering. 
     The anode  42  is positioned between the planar sputtering target  36  and the object to be sputtered (lens blank  22  in FIG. 1) but must be so oriented and configured as to be sufficiently out of the magnetic field produced during sputtering as to produce no significant effect upon the magnetic field or sputtering erosion patterns that are formed. On the other hand, the anode must be positioned sufficiently near the target as to collect primary electrons that otherwise would impinge upon and heat the substrate. The oval pattern formed in the front face  38  of a planar target provides a visual indication of the position of the oval-shaped plasma cloud that forms during a sputtering operation. A schematic representation of this cloud is presented in FIG. 4, together with one position of an anode. 
     The positioning of the anode with respect to the magnetic field and with respect to the target is key to the successful operation of the anode. As noted above, the anode must be close enough to the target so as to collect electrons that otherwise would impinge upon and heat the substrate to be coated, but at the same time the anode must be spaced from the magnetic field that is generated so as not to produce any significant effect upon the sputtering erosion patterns; that is, so as to produce substantially no effect upon the magnetic field or upon the uniformity of the sputtered coating. 
     One preferred position of an anode is shown in FIGS. 2,  3  and  4 , in which the anode is positioned generally centrally over the racetrack-shaped magnetic field. Note, in FIG. 2, that the ends of the elongated anode  42  (to which are attached the electrical side straps  50 ) do not extend over the oval groove  40  in the target. From FIG. 3, it will be noted that the sides of the anode are spaced inwardly of the oval sections. 
     Other positions of an anode are shown schematically in FIGS. 6,  7  and  8 . The anodes  42  in these figures are typified as lengths of ¼ inch copper tubing or other metal tubing that are supported from the target exterior housing  44  by electrical insulating connectors  52 . The tubing may be supplied with a cooling fluid at one end, with the cooling fluid flowing through the tubing and being withdrawn at the other end. As with the apparatus of FIGS. 1 through 4, the tubes may lead through a wall of the sputtering chamber and there be connected to a source of coolant. In a simple version, water from a tap, such as depicted in FIG. 1 as  63 , is flowed through a length of plastic tubing having a length greater than about four feet, the plastic tubing being connected to one of the anode tubes externally of the chamber. Another plastic tube may be connected exteriorly to the other end of the anode tubing that protrudes externally from the chamber to receive and discharge slightly heated water. 
     In FIG. 6, the anode  42  is spaced slightly to the side of the oval-shaped erosion pattern in the target, again in position to collect primary electrons but to avoid interference with the magnetic field. This position is preferred when the target is silicon, since generally less heat is generated and hence fewer electrons need to be captured by the anode. In FIG. 7, the anode is positioned directly over, that is, centrally of, the oval-shaped erosion pattern but far enough away from the target so as not to interfere with the magnetic field. This position is preferred for targets of such metals as titanium and niobium inasmuch as these targets generate comparatively greater amounts of heat. A side view of FIG. 7 is shown schematically in FIG.  8 . Electrical insulating connectors  052  are employed to insulate the anode from ground and from the target. Although the invention has been described with respect to a single elongated anode per target, it should be understood that more than one anode could be used, if desired. 
     The positions of the anodes may be adjusted with respect to the nature and chemistry of the coating to be produced in order to optimize the coating process and, of course, to enable the anodes to collect primary electrons while having no effect upon the magnetic fields. A variety of adjustment means may be employed. The supports carrying the anodes may be adjusted to move the anode with respect to the target. The supports may be reshaped. For example, in the embodiment of FIGS. 1-4, the straps supporting the anode may be bent, lengthened or shortened, or the straps may themselves be moved, to reposition the anode with respect to the target. In the embodiments of FIGS. 6-8, the tubes forming the anodes may themselves be bent as desired into a desired position with respect to the target and the magnetic field. It is preferred to first adjust the atmosphere in the sputtering chamber (vacuum, ratio of gases) before making adjustments to the position of the anode. Of course, it will be understood that the shapes of the anodes may be varied as desired. 
     In use, a temperature-sensitive substrate such as ophthalmic lens blanks  22  are mounted to the vertical motor driven platen  20 , using any convenient fasteners such as edge-clips (not shown). The vertical motor driven platen  20  is then moved into one of the end chambers, aligned enclosure enter chamber  14  or exit chamber  18 , the pivoting door  26  is closed and sealed, and an appropriate vacuum is drawn within the chamber. Depending upon the nature of the coating to be deposited, the interior of the vacuum chamber may contain a small amount of argon if the atmosphere within the chamber is to be non-reactive, or may carry a small amount of oxygen, nitrogen, etc. if a reactive atmosphere is to be employed to coat oxides or nitrides of the metals of the target. 
     The power supply is switched on, causing formation of the appropriate racetrack-shaped plasma cloud, and cooling fluid is flowed through the cooling tubes  54  to cool the anode  42 , which has been carefully positioned in the chamber as described above. The vertical motor driven platen  20  then is passed back and forth from one end enclosure to the other as many times as is necessary to build up the desired thickness of coating on the substrate. If more than one type of coating is involved, the sputtering targets  34  and sputtering targets  36  may be replaced with other targets of the desired material. 
     As an example, eyeglass lens blanks made of, e.g., polycarbonate or high index of refraction plastics, may be mounted to the platen to receive an anti-reflective coating, which is provided by applying alternate layers of materials having substantially differing indices of refraction. For example, utilizing a substrate having an index of refraction of about 1.50, one may apply first a coating of SiO 2 , having an index of refraction of about 1.47, and alternating this layer with layers of TiO 2  which have indices of refraction of about 2.30. The SiO 2  coating may be formed using a silicon target in an atmosphere containing a small amount of oxygen, and the TiO 2  oxide film can be formed utilizing a titanium target in an oxygen-containing atmosphere. In this manner, a typical antireflective coating consisting of (from the lens surface outwardly) SiO 2  (900 Å), TiO 2  (70 Å), SiO 2  (350 Å), TiO 2  (900 Å), and SiO 2  (750 Å) can be formed. As is known in the art, a variety of other sputtering materials may be employed as well, including oxides of niobium, zinc, aluminum, indium and tin, and nitrides of silicon, titanium, tantalum, yttrium, zirconium and vanadium. 
     Although the invention has been described above primarily in connection with planar magnetron targets, cylindrical magnetron sputtering targets also can be employed. These targets carry within them stationary magnets producing a stationary, generally horseshoe-shaped plasma cloud on the outer surface of the cylindrical target. The externally cooled anode of the invention is then mounted adjacent the exterior of the cylindrical target in the manner described above, that is, such that it does not have any significant effect upon the magnetic field, but yet is close enough to the target to collect electrons that otherwise would impinge upon and heat a substrate to be coated. 
     FIG. 9 is a schematic representation of a continuous coating line or inline process in which the substrate to be coated moves from left to right through a plurality of chambers  70  generally, comprising enter lock  72 , and chambers designated as  74 ,  76 ,  78 ,  80 ,  82 ,  84 ,  86 ,  88 ,  90 , buffer  92 , sputtering coating side  2  cambers  94 , buffer zone  96 , and exit lock  98 . The substrate may be a temperature-sensitive material such as plastic ophthalmic lenses mounted on a platen that is movable through the chambers  70 . For example, the platen could be suspended on an overhead rail and movable by means of a motor-driven chain or the like. Preferably, the platen moves continuously, at constant velocity, through the coater. In the embodiment of FIG. 9, the lenses or other substrates are mounted so that each of their opposing surfaces to be coated is exposed to enable that surface to be coated. For example, if the targets are mounted vertically along the walls of the chambers, the substrates may be supported vertically in openings formed in the vertical platen so that each side of the substrates can be coated, as described below. 
     A vacuum is drawn within the chambers of the coating line, except the initial enter lock  72 . Upon entry of the platen into the initial enter lock  72 , a vacuum is drawn in that lock and a gas-tight entryway is opened to permit the platen to move sequentially into chambers  74  and  76  to enable the substrates to be heated and cleaned by plasma etching. As the platen moves sequentially through chambers  70 , one surface of the substrates is sequentially coated with SiO 2 , TiO 2 , SiO 2 , TiO 2  and SiO 2  films in sputter coating side  1  chambers  68 . To increase the thickness of a film, the same film may be applied in two or more successive chambers. For example, chambers  84  and  86  both apply TiO 2  films, and chambers  88  and  90  both apply SiO 2  films. The platen then passes through a buffer zone  92  and into another series of chambers, designated for simplicity as  94 , wherein the same or another coating is applied to the other surface of the substrates in sputter coating side  2  chambers  94 . The series of chambers  94  may, if desired, be the same as the chambers  68 . The platen then moves through buffer zone  96  and exit lock  98 , to emerge from the coating line. In practice, of course, a series of platens would move in succession through the coating line. 
     While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptation and modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.