Abstract:
The method and device are used to plasma-treat workpieces. The workpiece is inserted into a chamber of a treatment station that can be at least partially evacuated. The plasma chamber is bounded by a chamber bottom, a chamber cover, and a lateral chamber wall. The method process is optically monitored at least at times. In the optical monitoring, spectral lines of the radiation of the plasma above 500 nanometers are evaluated. Preferably, the evaluation is performed for frequencies above 700 nanometers.

Description:
The present application is a 371 of International application PCT/DE2011/000234, filed Mar. 3, 2011, which claims priority of DE 10 2010 012 501.6, filed Mar. 12, 2010, the priority of these applications is hereby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for plasma treatment of workpieces in which the workpiece is placed in a plasma chamber, and in which, subsequently, under the influence of negative pressure after the ignition of plasma, a coating is precipitated on the workpiece, and in which the process sequence is optically monitored at least temporarily. 
     Moreover, the invention also relates to a device for plasma treatment of workpieces which has at least one evacuatable plasma chamber for receiving the workpieces, and in which the plasma chamber is arranged in the area of a treatment station, and in which the plasma chamber is defined by a chamber floor, a chamber cover as well as a lateral chamber wall, and in which the plasma chamber is coupled to a device for the optimum monitoring of a process sequence. 
     Such methods and devices are used, for example, for providing synthetic materials with surface coatings. In particular, also already known are devices of this type for coating inner and outer surfaces of containers which are intended for packaging liquids. Moreover, devices for plasma sterilization are known. 
     PCT/WO 95/22413 describes a plasma chamber for the internal coating of bottles of PET. The bottles to be coated are lifted through a movable bottom into a plasma chamber and are connected to an adapter in the area of a bottle opening. An evacuation of the bottle interior can be effected through the adapter. Moreover, a hollow gas lance is introduced into the interior of the bottles in order to supply process gas. An ignition of the plasma takes place with the use of a microwave. 
     It is also known from this publication to arrange a plurality of plasma chambers on a rotating wheel. This supports a high production rate of bottles per unit of time. 
     In EP-OS 10 10 773 a supply device is explained for evacuating the interior of the bottle and supplying process gas. PCT-WO 01/31680 describes a plasma chamber into which the bottles are inserted by a movable cover which previously had been connected to the mouth portion of the bottles. 
     PCT-WO 00/58631 also already shows the arrangement of plasma stations on a rotating wheel and describes for such an arrangement an assignment of negative pressure pumps and plasma stations in groups, in order to support a favorable evacuation of the chambers as well as the inner spaces of the bottles. Moreover, the coating of several containers in a common plasma station or a common cavity is mentioned. 
     Another arrangement for carrying out an internal coating of bottles is described in PCT-WO 99/17334. In this case, especially an arrangement of a microwave generator above the plasma chamber, as well as a vacuum and operation medium through a bottom of the plasma chamber, is described. 
     In DE 10 2004 020 185 A1 a gas lance is already described which can be moved into the interior of a preform to be coated and for supplying process gas. The gas lance can be positioned in the longitudinal direction of the container. 
     In a predominant number of the known devices, container layers of silicone oxides produced by the plasma having the general chemical formula SiO x  are used for improving the barrier properties of the thermoplastic material. Such barrier layers prevent a penetration of oxygen into the packaged liquids, as well as a discharge of carbon dioxide in the case of CO 2  containing liquids. 
     Because of the chemical elements contained in the plasma, the plasma has characteristic spectral lines. Therefore, an optical monitoring for carrying out a plasma coating is already described in U.S. Pat. No. 6,117,243. In this case, the evaluation of the radiation emission is carried out within the range of a wave length of 425 nanometers. 
     EP 1 948 846 explains another method for monitoring a plasma coating. In that case, an evaluation in a spectral range of 800 nanometers to 950 nanometers takes place. An evaluation of a difference between signals of a first band width as the reference and the signals of a second band width takes place. 
     The previously known methods and devices are not yet sufficiently suitable for making available a process monitoring which is reliable, on the one hand, as well as a process monitoring which is robust with respect to changing border conditions. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the object of the invention to improve a process of the above-mentioned type in such a way that reliable process monitoring is reinforced. 
     In accordance with the invention, this object is met in that, during optical monitoring, wave lengths of the emission radiation of the plasma above 500 nanometers are evaluated. 
     A further object of the present invention is to make available a device of the above-mentioned type in such a way that a reliable process monitoring is achieved. 
     This object is met in accordance with the invention in that the device for optical monitoring is constructed for evaluating spectral lines emitted by the plasma above 500 nanometers. 
     The method according to the invention and the device according to the invention are particularly suitable for monitoring the sequence of a coating process for bottles of synthetic material. To this end, especially an inner coating of these bottles with a layer of SiO x  takes place, wherein the adherence of the layer of SiO x  on the synthetic material can be improved by an intermediate layer which is constructed as an adhesion promoter. The coating process is preferably carried out as a PICVD plasma process (plasma impulse chemical vapor deposition). In such a method, the plasma is ignited by means of an impulse of a microwave. The impulses can be controlled, with respect to their pulse width, the pulse spacing as well as the pulse height. 
     By using optical monitoring, it is possible to monitor the gas composition of the plasma as well as the microwave power which has been introduced into the plasma. 
     This entry of microwave power is in direct dependence on the adjustment of the microwave system as well as of the energy introduced into the magnetron. Therefore, monitoring of the energy introduced into the magnetron is necessary for being able to evaluate the total system. 
     In accordance with the prior art, optical monitoring of a coating process in the range of visible light has the advantage that the coated workpieces can be distinguished by color and that the transmission properties with respect to visible light cannot be significantly influenced as a result. 
     Therefore, in accordance with the invention, an optimum emission spectroscopy takes place above 500 nanometers. 
     Preferably, it is contemplated to evaluate wave lengths above 600 nanometers. 
     In particular, it has also been found advantageous to evaluate spectral lines above 700 nanometers. 
     A particularly safe evaluation can be obtained by considering two different wave lengths. 
     For achieving a simple construction of the device, at least a portion of the monitored emission radiation of the plasma is transmitted by at least one light wave conductor. 
     Also, it has been found particularly advantageous for process monitoring that the light wave conductor is coupled to at least one photo element whose signal is amplified in a local vicinity to the photo element. 
     A particularly good sensitivity to problems is achieved by integrating at least a portion of the signal pattern, determined by optical monitoring, over at least a predeterminable period of time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the drawings: 
         FIG. 1  shows a principle sketch of a plurality of plasma chambers which are arranged on a rotating plasma wheel and in which the plasma wheel is coupled to input and output wheels, 
         FIG. 2  shows an arrangement, similar to  FIG. 1 , in which the plasma stations are each equipped with two plasma chambers, 
         FIG. 3  is a perspective illustration of a plasma wheel with a plurality of plasma chambers, 
         FIG. 4  is a perspective illustration of a plasma station with one cavity, 
         FIG. 5  is a front view of the device according to  FIG. 4  with closed plasma chamber, 
         FIG. 6  is a cross sectional view along sectional line VI-VI in  FIG. 5 , 
         FIG. 7  shows an optical connection of a quartz glass disc, facing the process to be monitored, to a photo element with the use of a cable for conducting light waves, 
         FIG. 8  is a cross sectional view, on a larger scale, through the cable for conducting light waves, 
         FIG. 9  shows a typical emission spectrum of plasma for SiOx coating of PET bottles with the application of an adhesive layer, 
         FIG. 10  shows an emission spectrum, similar to  FIG. 9 , for applying a barrier layer, and 
         FIG. 11  is an illustration of the transmission characteristics of the light wave conductor used. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The illustration of  FIG. 1  shows a plasma module  1 , which is provided with a rotating plasma wheel  2 . Along a circumference of the plasma wheel  2 , a plurality of plasma stations  3  are arranged. The plasma stations  3  are provided with cavities  4 , or plasma chambers  17 , for receiving the workpieces  5  to be treated. 
     The workpieces  5  to be treated are supplied to the plasma module  1  in the area of an input  6 , and are further conveyed to a transfer wheel  8  through a separating wheel  7 , wherein the transfer wheel  8  is equipped with positionable support arms  9 . The support arms  9  are arranged so as to be pivotable relative to a base  10  of the transfer wheel  8 , so that a change of the distance of the workpieces  5  relative to each other can be carried out. This causes a transfer of the workpieces  5  from the transfer wheel  8  to an input wheel  11  with a spacing of the workpieces  5  which is increased relative to the separating wheel  7 . The input wheel  11  transfers the workpieces  5  to be treated to the plasma wheel  2 . After a treatment has been carried out, all treated workpieces  5  are removed from an output wheel  12  from the area of the plasma wheel  2  and are transferred into the area of an output section  13 . 
     In the embodiment according to  FIG. 2 , the plasma stations  3  are each equipped with two cavities  4  or plasma chambers  17 . Accordingly, always two workpieces  5  can be treated simultaneously. Basically, it is also possible in this connection to construct the cavities  4  so as to be completely separate from each other. However, it is essentially also possible to delimit in a common cavity space only partial areas relative to each other in such a way that an optimum feeding of all workpieces  5  is ensured. In particular, it is intended to delimit the partial cavities relative to each other, at least through separate microwave couplings. 
       FIG. 3  is a perspective illustration of a plasma module  1  with a partially built-up plasma wheel  2 . The plasma stations  3  are arranged on a support ring  14  which is constructed as part of a rotary connection and is supported in the area of a machine base  15 . The plasma stations  3  each have a station frame  16  which support the plasma chambers  17 . The plasma chambers  17  include cylindrical chamber walls  18  as well as microwave generators  19 . 
     In a center of the plasma wheel  2 , a rotary distributor  20  is arranged through which the plasma stations  3  are supplied with drive means as well as energy. For distributing the operating means, particularly ring-shaped lines  21  can be used. 
     The workpieces  5  to be treated are illustrated underneath the cylindrically shaped chamber walls  18 . Bottom parts in each of the plasma chambers  17  are not illustrated for simplicity&#39;s sake. 
       FIG. 4  shows a plasma station  3  in a perspective view. It can be seen that the station frame  16  is provided with guide rods on which is guided a carriage  24  for supporting the cylindrical chamber wall  18 .  FIG. 4  shows the carriage  24  with chamber wall  18  in an elevated state, so that the workpiece  5  is released. 
     The microwave generator  19  is arranged in the upper portion of the plasma station  3 . The microwave generator  19  is connected through a deflection means  25  and an adaptor  26  to a coupling duct  27  which leads into the plasma chamber  17 . The microwave generator  19  can basically be coupled immediately in the area of the chamber cover  31 , as well as through a spacer element at a predeterminable distance from the chamber cover  31  and, thus, in a larger neighboring area of the chamber cover  31 . The adaptor  26  has the function of a transfer element and the coupling duct  27  is constructed as a coaxial conductor. A quartz glass window is arranged in the area of an opening of the coupling duct  27  in the chamber cover  31 . The deflection means  25  is constructed as a hollow conductor. 
     The workpiece  5  is positioned in the area of a duct chamber by a holding element  28  arranged in the area of a chamber bottom  29 . The chamber bottom  29  is constructed as part of a chamber base  30 . For facilitating an adjustment, it is possible to fix the chamber base  30  in the area of the guide rods  23 . Another variation resides in fastening the chamber base  30  directly at the station frame  16 . In such an arrangement, it is also possible to construct the guide rods  23  of two parts in the vertical direction. 
       FIG. 5  shows a front view of the plasma station  3  in accordance with  FIG. 3  in a closed state of the plasma chamber  17 . The carriage  24  with the cylindrically shaped chamber wall  18  is lowered relative to the positioning in  FIG. 4 , so that the chamber wall  18  is placed against the chamber bottom  29 . The plasma coating can be carried out in this state of positioning. 
       FIG. 6  shows, in a vertical sectional view, the arrangement according to  FIG. 5 . It can be especially seen that the coupling duct  27  opens into a chamber cover  31  which includes a laterally projecting flange  32 . Arranged in the area of the flange  32  is a sealing unit  33  which is acted upon by an inner flange  34  of the chamber wall  18 . In a lowered state of the chamber wall  18 , this results in a sealing action of the chamber wall  18  relative to the chamber cover  31 . Another sealing unit  35  is arranged in a lower part of the chamber wall  18  to ensure, also at this location, a sealing action relative to the chamber bottom  29 . 
     In the positioning illustrated in  FIG. 6 , the chamber wall  18  surrounds the cavity  4 , so that an interior space of the cavity  4 , as well as an interior space of the workpiece  5 , can be evacuated. For supporting a supply of process gas, a hollow gas lance  36  is arranged in the area of the chamber bottom  30 , wherein the gas lance can be moved into the interior space of the workpiece  5 . For carrying out positioning of the gas lance  36 , the gas lance is held by a lance carriage  37  which is positionable along the guide rods  23 . A process gas channel  38  extends within the lance carriage  37 , wherein, in the raised position illustrated in  FIG. 6 , the process gas channel  38  is coupled to a gas connection  39  of the chamber base  30 . As a result of this configuration, hose-like connecting elements at the lance carriage  37  are avoided. 
     As an alternative to the above explained construction, it is also possible in accordance with the invention, to introduce the workpiece  5  into a plasma chamber  17  which is arranged so as to be immovable relative to the assigned support structure. As another alternative to the illustrated coating of the workpieces  5  with their openings directed downwardly in the vertical direction, a coating of the workpieces with their openings directed upwardly in the vertical direction is possible. In particular, it is contemplated to carry out coating of bottle shaped workpieces  5 . Such bottles are also preferably constructed of a thermoplastic material. Preferably, the use of PET or PP is contemplated. In accordance with another preferred embodiment, the coated bottles are serving to receive beverages. 
     In the following, a typical treatment procedure will be explained in connection with an example of a coating procedure and is carried out in such a way that, initially, the workpiece  5  is transported to the plasma wheel  2  with the use of an input wheel  11  and the insertion of the workpiece  5  into the plasma station  3  in an upwardly pushed state of the sleeve-like chamber wall  18  takes place. 
     After the conclusion of the insertion procedure, the chamber wall  18  is lowered into its sealed position and, offset with respect to time, a displacement of the holding element  28  takes place, so that a separation of the inner space of the workpiece  5  relative to the interior space of the cavity  4  is created. Subsequently, the gas lance  36  is moved into the interior space of the workpiece  5 . It is also possible to move the gas lance  36  into the interior of the workpiece  5  already synchronously with the beginning of the lowering of the cavity  4  into the interior space of the workpiece  5 . This is followed by an evacuation of the cavity  4  and of the interior space of the workpiece  5 , either simultaneously or offset with respect to time. After the interior space of the cavity  4  has been sufficiently evacuated, the pressure in the interior space of the workpiece  5  is lowered further. Moreover, it is also contemplated to carry out the positioning movement of the gas lance  36  at least partially already parallel with the positioning of the chamber wall  18 . 
     After reaching a sufficiently low negative pressure, the process gas is conducted into the interior of the workpiece  5  and the plasma is ignited by means of the microwave generator  19 . In particular, it is contemplated to separate out, by means of the plasma, an adhesion promoter to an inner surface of the workpiece  5  as well as the actual barrier layer of silicon oxides. 
     After the conclusion of the coating procedure, the plasma chamber  17 , as well as the interior space of the workpiece  5 , are ventilated. After the ambient pressure within the cavity  4  and the interior space of the workpiece  5  has been reached, the chamber wall  18  is once again raised and the gas lance  36  is once again removed from the interior space of the workpiece  5 . A removal of the coated workpiece  5 , as well as the introduction of a new workpiece  5  to be coated, can now be carried out. 
     Positioning of the chamber wall  18 , of the sealing element  28  and/or of the gas lance  36  can be carried out with the use of different drive units. Basically, the use of pneumatic drives and/or electric drives, in particular in the form of an embodiment as a linear motor, is conceivable. In particular, however, it is contemplated to realize, for supporting an exact coordination of movements, a cam control with a rotation of the plasma wheel  2 . The cam control can be carried out, for example, in such a way that control curves are arranged along a circumference of the plasma wheel  2 , wherein cam rollers are guided along the control curves. The cam rollers are coupled to the structural elements to be positioned respectively. 
       FIG. 7  shows a schematic illustration of a device  40  for optically monitoring a coating procedure. The plasma chamber  17  provided with a window  41  is illustrated schematically. A quartz glass disc  42  is placed in the window  41 . Plasma  43  is schematically illustrated within the plasma chamber  17 . 
     Starting from the quartz glass disc  42 , a light wave conductor extends to a photo element  45 . The photo element  45  is directly connected to an amplifying step  52  which is typically coupled directly to an analog/digital converter  46 . The analog/digital converter  46  is connected to an evaluating unit  47 . 
     The structural elements  45 ,  52   46 ,  47  are thus arranged at a distance from the plasma chamber  17  and are located outside of an immediate influence of the microwave system. The photo element  45  is coupled directly to an amplifying step in order to achieve a low ratio of signal components to noise components. Typical reinforcement factors are in the range of 10 5  to 10 9 . Preferably, a dark current balance is realized in the area of the amplifier for facilitating a compensation of the thermal behavior of the photo diode and of the amplifying stage. Moreover, an amplifying circuit is implemented. This makes it possible to make available a respectively optimum amplification for different light intensities. The output signal of the amplifier is then supplied to the analog/digital converter  46 . The direct coupling of the photo element  45  and the amplifier can be realized, for example, by an arrangement in a common electronic structural group, on a common plate, or on a common semi-conductor chip. 
     In the area of the evaluation unit  47 , it is preferably provided to integrate the signal of the analog/digital converter  46  over a predetermined period of time. The respectively scanned values can be supplied to intermediate storage in order to support a representation of the signal patterns over time. 
       FIG. 8  shows an enlarged cross sectional view of the light wave conductor  44 . The light wave conductor has an optical core  48 , an optical casing  49  as well as an outer sleeve  50 . The core  48  typically has a diameter of about 400 to 600 micrometers. As material for the optical casing  4 , PMMA can be used. The use of ETFE for the casing  50  has been found useful. 
     A typical damping of the light wave conductor  44  in a wave length to be transmitted is about 850 nanometers, at most 8 db/km. The factor NA (Numerical Aperture) is about 0.37. Quartz glass is used as the material for the optical core  48 . 
       FIG. 9  shows a typical transmission pattern for carrying out an inner coating of PET bottles. In this connection, the process step of applying an adhesive layer between the PET material and the layer of SiOx is carried out. In a range above 700 nanometers, three characteristic spectral lines are obtained. These spectral lines are at 777 nanometers, 845 nanometers and 927 nanometers. Monitoring at least two of these spectral lines, preferably all three spectral lines, has been found particularly advantageous for process monitoring and possibly process control. 
       FIG. 10  shows a comparable pattern of spectral lines when the SiOx layer is applied. Also in this case, the three characteristic spectral lines mentioned above are developed to a significant extent. 
       FIG. 11  shows the typical spectral range for the preferred light wave conductor. The light wave conductor explained above in more detail corresponds to the pattern  51 . 
     In  FIGS. 9 to 11 , the respective intensities are shown in counts over the wave length in nanometers. 
     The light wave conductor according to  FIG. 7  has two essential functions. In accordance with a first function, the light wave conductor  44  picks up, through its numerical aperture, the scattered light generated by the plasma  43 . As a result, a special collecting lens is not required. The second function of the light wave conductor resides in its property as spectral filter. Because of the spectral behavior illustrated in  FIG. 11 , different wave lengths (spectral lines) are transmitted with different damping by the light wave conductor  44 . The light wave conductor with the pattern  51  in  FIG. 11  has an optimized property for the transmission of the spectral lines according to  FIG. 9  and  FIG. 10 . 
     An inexpensive photo diode having a spectral sensitivity in a range of 700 nanometers to 1,000 nanometers can be used as the photo element  45 . Basically, it would also be possible to use photo resistors or photo transistors as photo elements  45 . However, photo diodes have a very low inertia. 
     As already mentioned, an integration of the measured signal patterns is carried out in the area of the evaluation unit  47 . The resulting area contents describe the energy contained in the signal. The integration combines a high sensitivity with a high accuracy. 
     Therefore, the signal transmission through the light wave conductor  44 , the selective transmission of wave lengths above 700 nanometers by the optical properties of the light wave conductor  44 , the signal amplification carried out in close local vicinity to the photo element  45 , the switchable signal amplification, and the integration of signal patterns in the area of the evaluation unit  47 , are to be considered the essential components for process monitoring according to the invention. Particularly advantageous is the realization of all five of the above mentioned properties in combination with each other; however, each individual property can also be realized by itself or in combination with only one or with two or three of the additional properties. 
     With respect to the integration, it is particularly contemplated, not only to integrate the signal of an individual spectral line, but also to integrate all intensities in a predeterminable spectral range, for example, from 700 nanometers to 1,000 nanometers. The resulting area is compared to a reference area and upper and lower limits are to be observed in the case of deviations. 
     The oxygen line in the spectral range is especially significant for process monitoring.