Abstract:
A coating method can use a coating device containing at least one recipient which can be evacuated and which is adapted to accommodate a substrate, at least one gas supply device which is used to introduce at least one gaseous precursor into the recipient and at least one heatable activation element which has a definable longitudinal extension and which is fastened by means of at least one associated mechanical fastening device. Electric current can be supplied to the activation element via at least two contact elements and the contact elements are fastened so as to be virtually immobile relative to the recipient. The activation element is arranged so as to be mobile relative to the recipient.

Description:
BACKGROUND 
       [0001]    The invention relates to a coating device, comprising at least one recipient ( 10 ), which can be evacuated and which is adapted to receive a substrate ( 30 ), at least one gas supply device ( 20 ,  21 ,  22 ), being adapted to introduce at least one gaseous precursor into the recipient ( 10 ), and at least one heatable activation element ( 40 ), which has a predeterminable longitudinal extent and is fastened by means of at least one dedicated mechanical fastening device. The invention also relates to a corresponding coating method. 
         [0002]    Coating devices of the type mentioned are intended according to the prior art for coating a substrate by means of a hot-wire activated chemical vapor deposition. The deposited layers may, for example, comprise carbon, silicon or germanium. Correspondingly, the gaseous precursors may, for example, comprise methane, monosilane, monogermanium, ammonia or trimethylsilane. 
         [0003]    K. Honda, K. Ohdaira and H. Matsumura, Jpn. J. App. Phys., Vol. 47, No. 5, discloses using a coating device of the type mentioned at the beginning for depositing silicon. For this purpose, silane (SiH 4 ) is supplied as a precursor by means of the gas supply device. According to the prior art, the precursor is disassociated and activated at the heated tungsten surface of an activation element, so that a layer of silicon or a layer comprising silicon can be deposited on a substrate. 
         [0004]    However, a disadvantage of the cited prior art is that an undesired reaction of the material of the activation element with the precursor takes place, particularly at the colder clamping points of the activation element. 
         [0005]    For example, the use of a silane compound as a precursor may lead to the formation of silicide phases on the activation element. 
         [0006]    The silicide phases occurring during the reaction generally lead to changes in volume of the activation element, are brittle in comparison with the starting material and cannot withstand such great mechanical forces, and they often exhibit a changed electrical resistance. This has the effect that the activation element is often already destroyed after being in operation for a few hours. For example, the activation element may be used under mechanical prestress in the recipient and rupture under the influence of this mechanical prestress. In order to prevent rupturing of the activation element under mechanical prestress, the prior art proposes flushing the clamping points with an inert gas. Although the prior art does show that the service life is extended to a limited extent, this is still insufficient when performing relatively long coating processes or for carrying out a number of shorter coating processes one directly after the other. Furthermore, the inert gas that is used influences the coating process. 
         [0007]    The invention is consequently based on the object of extending the service life of an activation element in a coating device for hot-wire activated chemical vapor deposition without disadvantageously influencing the coating process. The object of the invention is also to increase the stability of the process and/or to simplify the control of the process. 
       SUMMARY 
       [0008]    According to the invention, it is proposed in a way known per se to introduce a substrate to be coated into a recipient which can be evacuated. The recipient may, for example, consist of aluminum, high-grade steel, ceramic and/or glass. The recipient is closed off in a substantially airtight manner in a way known per se, for example by sealing elements of metal or polymer or by welded and soldered or brazed connections. The recipient may be evacuated by means of a vacuum pump. 
         [0009]    At least one gaseous precursor with a predeterminable partial pressure is introduced into the recipient by way of a gas supply device. For example, the precursor may comprise methane or other hydrocarbons, silanes, germaniums, ammonia, trimethylsilane, oxygen and/or hydrogen. 
         [0010]    For the depositing of a layer, at least one activation element arranged in the space inside the recipient is heated. In some embodiments of the invention, the heating of the activation element may be performed by electron-impact heating and/or electrical resistance heating. The activation element substantially comprises a refractory metal. This may be selected from the group comprising molybdenum, niobium, tungsten, tantalum or an alloy of these metals. In addition, the activation element may comprise further chemical elements, which either represent unavoidable impurities or, as an alloying constituent, adapt the properties of the activation element to the desired properties. The activation element may take the form of a wire, a plate, a tube, a cylinder and/or further, more complex geometries. 
         [0011]    At the surface of the activation element, the molecules of the gaseous precursor are at least partially disassociated or excited. The excitation and/or disassociation may be enhanced by catalytic properties of the surface of the activation element. The molecules activated in this way reach the surface of the substrate, where they form the desired coating. In addition, molecules of the gaseous precursor may be at least partially reacted with the material of the activation element. Depending on the temperature of the activation element, the excitation and/or the disassociation and/or the reaction with the material of the activation element may be suppressed or accelerated. 
         [0012]    In order to supply an electrical current to the activation element, at least two contact elements are provided, by means of which the activation element can be connected to an electrical current or voltage source. In an activation element with a homogeneous material composition and constant cross section, the thermal energy deposited when an electrical current flows through is introduced uniformly along the longitudinal extent. 
         [0013]    On account of the increased thermal conductivity and/or the increased heat dissipation of the contact element, the activation element may have a lower temperature in a portion near the contact element, as compared with a portion at a greater distance from the contact element. In this case, the temperature of the activation element may fall so far in a portion near the contact element that the material of the activation element preferentially undergoes a chemical reaction with the precursor. For example, an activation element comprising tungsten may form a tungsten-silicide phase with a precursor comprising silicon. 
         [0014]    To solve this problem, it is proposed according to the invention to move the activation element by means of a movable fastening device over fixed contact elements, so that these contact the activation element at changing contact points. The relative movement has the effect that the shifting of the contact element is accompanied by a shifting of those points at which the activation element has a reduced temperature. As a result, the activation element does not undergo an undesired phase transformation or acceleration of such a phase transformation at the same place during the entire duration of the coating process. 
         [0015]    In some embodiments of the invention, an initially occurring undesired phase transformation, for example the formation of a carbide or silicide, can also be reversed again if the surface area or partial portion of the activation element that has been transformed in an undesired way is subsequently heated up to an elevated temperature. In this way, the lifetime of the activation element can be increased. 
         [0016]    According to the invention, the formation of changing contact points is brought about by movement of the activation element which is imparted by way of a movement of the fastening device, whereas the contact elements remain immovable in relation to the surrounding recipient. In the sense of the present description, the contact element is also immovable whenever it undergoes a small change in length due to thermal expansion or comprises a holding device which can compensate for a thermal expansion. 
         [0017]    In some embodiments of the invention, an elongate activation element may have a greater length than the region used for activating the precursor. However, the activation element is always arranged completely in the recipient and not for instance accommodated in a reservoir. By displacing the activation element, a respectively changing partial portion of the activation element is heated. Since the contact elements are fixedly arranged, the heated partial portion that is arranged between the contact elements remains constant. This simplifies the electrical control of the power consumption and/or the temperature of the activation element. The displacement of the activation element in relation to the contact element can be imparted by a translation of the fastening device and/or a rotation of the fastening device. In the latter case, the activation element may be formed as an endless ring or a loop. 
         [0018]    In some embodiments of the invention, the activation element may comprise at least one wire. In the sense of the present invention, a wire may have a round, oval or polygonal cross section. In this way, the ratio of surface to volume of the activation element can be increased. In some embodiments of the invention, such an activation element may be guided or deflected by way of at least one roller. The roller may be part of a contact element and/or a fastening device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention is intended to be explained in more detail below on the basis of exemplary embodiments and figures, without restricting the general concept of the invention. In the figures: 
           [0020]      FIG. 1  shows the basic structure of a coating device according to the invention, 
           [0021]      FIG. 2  illustrates a plan view of activation elements which are led endlessly through the active zone, 
           [0022]      FIG. 3  shows an exemplary embodiment of an activation element proposed according to the invention, wherein the movement is imparted by a translation of the fastening device, 
           [0023]      FIG. 4  shows an embodiment wherein a movable activation element is led over a number of fixed contact elements. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]      FIG. 1  shows a cross section through a coating device  1 . The coating device  1  comprises a recipient  10 , which is, for example, produced from high-grade steel, aluminum, glass or a combination of these materials. The recipient  10  is closed off from the surroundings in a substantially airtight manner. A vacuum pump (not represented) may be connected by way of a pump flange  103 . For example, the recipient  10  may be evacuated to a pressure of less than 10 0  mbar, less than 10 −2  mbar or less than 10 −6  mbar. 
         [0025]    Inside the recipient  10  there is a holding device  104 , wherein a substrate  30  may be mounted. The substrate  30  may, for example, consist of glass, silicon, plastic, ceramic, metal or an alloy. The substrate may be a semiconductor wafer, an architectural glass or a tool. It may have a planar or curved surface. The materials mentioned are only mentioned here by way of example. The invention does not teach the use of a specific substrate as a principle for providing a solution. During the operation of the coating device  1 , a coating  105  is deposited on the substrate  30 . 
         [0026]    The composition of the coating  105  is influenced by the choice of the gaseous precursor. In one embodiment of the invention, the precursor may comprise methane, so that the coating  105  comprises diamond or diamond-like carbon. In another embodiment of the invention, the precursor may comprise monosilane and/or monogermanium, so that the coating comprises crystalline or amorphous silicon and/or germanium. 
         [0027]    The gaseous precursor is introduced into the interior of the recipient  10  by way of at least one gas supply device  20 . The gas supply device  20  obtains the gaseous precursor from a storage vessel  21 . The amount of precursor taken from the storage vessel  21  can be influenced by way of a control valve  22 . If the coating  105  is made up of a number of different precursors, the storage vessel  21  may comprise a prepared gas mixture, or else a number of gas supply devices  20  may be provided, each introducing a component of the made-up precursor into the recipient  10 . 
         [0028]    The amount of precursor supplied to the gas supply device  20  by way of the control valve  22  is monitored by way of a control device  101 . The control device  101  is supplied with an actual value of a partial or absolute pressure by a measuring device  100 . 
         [0029]    For the activation of the gaseous precursor, an activation element  40  is available. The activation element  40  comprises one or more catalytically active surfaces, for example in the form of a metal sheet or a wire. For example, the activation element  40  may comprise tungsten, molybdenum, and niobium and/or tantalum. During the operation of the activation element, there forms inside the recipient  10  an active zone  50 , wherein disassociated and/or excited constituents of the precursor are detectable. 
         [0030]    The activation element  40  is fastened to at least one fastening device  44 . The fastening device  44  guides the activation element  40  to a predeterminable position and/or with a predeterminable mechanical stress. At least one fastening device  44  may be configured in an electrically insulated manner, in order to bring the activation element at least partially to a predeterminable electrical potential. 
         [0031]    The activity of the surface of the activation element  40  is achieved at an elevated temperature in comparison with room temperature. For the heating of the activation element  40 , it is envisaged according to  FIG. 1  to provide at least two electrical contact elements  43 . At least one contact element  43  may be connected to a power source  107  by means of a vacuum-tight leadthrough  108 . In this case, the heating of the activation element  40  is performed by resistance heating. If the activation element consists of a homogeneous material and has a uniform thickness, the heating power E introduced along the longitudinal extent x of the activation element is constant: 
         [0000]    
       
         
           
             
               
                 ∂ 
                 E 
               
               
                 ∂ 
                 x 
               
             
             = 
             
               const 
               . 
             
           
         
       
     
         [0032]    On account of the heat conduction and/or heat radiation of the fastening devices  44  and/or the contact elements  43 , the temperature of the activation element  40  decreases from the geometrical center to the periphery if the heating power is substantially constant over the length of the wire. In this case, a temperature at which the material of the activation element  40  is reacted at an accelerated rate with the gaseous precursor to form undesired phases, for example carbides and/or silicides and/or germanides, may be established near the contact element  43 . 
         [0033]    In order to minimize the harmful influence of the precursor on the activation element, it is proposed according to  FIG. 1  to use an activation element  40  of which the geometrical dimensions are greater than the dimensions of the active zone  50 . The activation element  40  is mounted inside the recipient  10  in an electrically insulated manner by means of the fastening devices  44 . The two contact elements  43  lie in contact with the activation element  40 , for example, by way of rollers, rolls, sliding contacts or similar elements. 
         [0034]    The contact elements  43  are installed substantially fixedly in the recipient. The spacing of the contact elements  43  determines the length of the partial portion of the activation element  40  that is heated by current flow. 
         [0035]    The fastening devices  44  are movable along a transporting direction  49 , which may in some embodiments run along the longitudinal extent of the activation element  40 . The movement of the fastening devices  44  along the transporting direction  49  may be harmonic or anharmonic, and take place continuously or with intermittent breaks. 
         [0036]    The movement of the fastening devices  44  has the effect that the location of lower temperature that forms near the contact point of the contact element  43  on the activation element  40  is locally variable. In this way, the harmful influence of the precursor on the activation element  40  is distributed over a greater surface area or longitudinal portion of the activation element, so that the overall lifetime of the activation element is increased. In some embodiments of the invention, it may additionally be provided that a region that is damaged in the presence of the precursor at low temperatures is regenerated again by increasing the temperature when moving the contact element  43  away, in that the undesired phases of the activation element  40  undergo a renewed reaction. 
         [0037]      FIG. 2  shows a further embodiment of an activation element  40 . In this embodiment of the invention, the active surface of the activation element  40  is formed by a wire  41 , which, for example, comprises tungsten, niobium, molybdenum or tantalum. The cross section may be polygonal or round, so that the visual impression of a strip can be obtained. 
         [0038]    The activation element  40  according to  FIG. 6  comprises three wires  41   a ,  41   b  and  41   c . The ends of each individual wire are connected to one another, for example by spot welding, so that an endless loop is obtained. Each of these endless loops is led over two rollers  46 , which form both the contact element  43  and the holding element  44 . The two rollers  46  dedicated to a wire  41   a ,  41   b  or  41   c  lie at different electrical potentials, so that through the respective wire  41  there flows an electrical current, which heats the wire  41 . In this way there forms between the wires  41   a ,  41   b  and  41   c  and the surface of the substrate  30  once again an active zone  50 , wherein the disassociated and/or excited molecules for the depositing of a layer are detectable. 
         [0039]    On account of the heat removal by way of the rollers  46 , the temperature of the wire  41  decreases from the center of the active zone to the peripheral region with the rollers  46 . In order to slow down or prevent the embrittlement or rupture of the wire  41 , it is proposed to set at least one roller  46  in rotation, for example by means of an electric motor or a spring tensioning means. In this way, the wire  41  cyclically passes through the region with the least heat removal and the highest temperature at the center of the active zone  50  and the region with the greatest heat removal and the lowest temperature at the rollers  46 . In this way, undesired carbide, germanide or silicide phases, which form in the colder region near the rollers  46 , can be reduced again in the center of the active zone  50 . At least, however, the wire  41  undergoes a uniform reaction over its entire length, so that the lifetime thereof to mechanical failure is increased. The uniform aging of the wire has the effect that its electrical properties change only slowly, so that the monitoring of the coating process can be simplified. 
         [0040]    The embodiment according to  FIG. 3  shows an activation element  40 , which comprises a wire  41 , in three different phases of movement. In a first position, which is represented at the top in  FIG. 3 , the fastening devices  44  are in a neutral position, which is illustrated by the lines  200 . Stretched between the fastening devices  44  is the wire  41 , which provides the active surface of the activation element  40 . The spacing of the two fastening devices  44  is substantially constant, but in some embodiments it may be provided that the spacing is variable to a slight extent to compensate for thermal expansion. In this case, a device which ensures a constant mechanical stress of the wire  41 , for example by means of at least one spring, may be integrated. 
         [0041]    As already explained in conjunction with  FIG. 1 , two contact elements  43  are provided, arranged substantially fixedly inside the recipient. In the case represented, one contact element  43  comprises a roller  46 , which is rotatably fastened on a roller mount  47 . 
         [0042]    The circumferential surface of the roller may incorporate a groove, in order to prevent the wire  41  from running off the roller  46 . A potential difference is applied to two contact elements  46  by means of a power source  107 . The circuit closes by way of the wire  41 , so that the heated partial portion of the activation element  40  comes to lie between the contact elements. 
         [0043]    As can be seen in the middle representation of  FIG. 3 , the fastening devices  44  can be moved to the left from the neutral position along the direction of movement  49 . This has the effect of displacing the heated, and consequently active, partial portion of the activation element. However, the overall length of the partial portion remains constant, so that simple control of the temperature or the power requirement is made possible. In this way, the harmful influence of the precursor on the activation element  40  is distributed over a larger surface area or longitudinal portion of the activation element, so that the overall lifetime of the activation element is increased. In some embodiments of the invention, it may additionally be provided that a region that is damaged in the presence of the precursor at low temperatures is regenerated again by increasing the temperature when moving the contact element  43  away, in that the undesired phases of the activation element  40  undergo a renewed reaction. 
         [0044]    As can be seen in the lower representation of  FIG. 3 , when an end position is reached, the direction of movement can be reversed, so that the fastening devices are moved to the right. This has the effect of displacing the heated, and consequently active, partial portion of the activation element. However, the overall length of the partial portion and the relative position thereof with respect to the recipient remain constant. After reaching the end position represented in the figure, the direction of movement can change once again, so that the positions represented in  FIG. 3  are passed through cyclically. The movement may in this case take place continuously or discontinuously. 
         [0045]    The embodiment according to  FIG. 4  shows an activation element  40 , which comprises a wire  41 , in three different phases of movement. In a first position, which is represented in  FIG. 4   a , the fastening devices  44  are in a neutral position. Stretched between the fastening devices  44  is the wire  41 , which provides the active surface of the activation element  40 . The spacing of the two fastening devices  44  is substantially constant, as explained in conjunction with  FIG. 3 . 
         [0046]    The activation element  40  is led over four contact elements  43  or two pairs of contact elements, which are arranged substantially fixedly within the recipient. In the case represented, one contact element  43  comprises a roller  46 , which is rotatably mounted. The circumferential surface of the roller may incorporate a groove, in order to prevent the wire  41  from running off the roller  46 . A potential difference is respectively applied to two contact elements  43  by means of a power source (not represented), so that a predeterminable electrical power is deposited in each partial portion between two contact elements  43 . The circuit closes by way of the wire  41 , so that the heated partial portion of the activation element  40  comes to lie between the contact elements. Between the respectively outermost contact element  43  and the fastening device  44  there is an unheated partial portion. 
         [0047]    As can be seen in  FIG. 4   b , the fastening devices  44  can be moved to the left from the neutral position along the direction of movement  49 . This has the effect of displacing the heated, and consequently active, partial portion of the activation element. However, the overall length of the partial portion remains constant, so that simple control of the temperature or the power requirement is made possible. Similarly, the position of the partial portion and consequently the position of the active zone  50  within the recipient remain constant. Nevertheless, the harmful influence of the precursor on the activation element  40  is distributed over a larger surface area or longitudinal portion of the activation element, so that the overall lifetime of the activation element is increased. In some embodiments of the invention, it may additionally be provided that a region that is damaged in the presence of the precursor at low temperatures is regenerated again by increasing the temperature when moving the contact element  43  away, in that the undesired phases of the activation element  40  undergo a renewed reaction. 
         [0048]    As can be seen in  FIG. 4   c , when an end position is reached, the direction of movement can be reversed, so that the fastening devices  44  are moved to the right. This has the effect of displacing the heated, and consequently active, partial portion of the activation element. However, the overall length of the partial portion and the relative position thereof with respect to the recipient remain constant. After reaching the end position represented in the figure, the direction of movement can change once again, so that the positions represented in  FIGS. 4   a  to  4   c  are passed through cyclically. The movement may in this case take place continuously or discontinuously. 
         [0049]    In some embodiments, the overall length of the activation element is intended to be arranged within the recipient and to run in a substantially stretched-out manner. This allows the overall available length to be used cyclically for the activation of the vapor phase, without the wire  41  having to be wound up in a reservoir or unwound from a reservoir. The reliability of the device can be increased as a result. 
         [0050]    It goes without saying that the features represented in  FIGS. 1 to 4  may also be combined in order in this way to obtain further embodiments of the activation element according to the invention or of the proposed coating device. Therefore, the above description should not be regarded as restrictive, but as explanatory. The claims which follow should be understood as meaning that a feature which is mentioned is present in at least one embodiment of the invention. This does not exclude the presence of further features. Wherever the claims define “first” and “second” features, this designation serves for distinguishing between two identical features, without giving them any priority.