Abstract:
Lock for a vacuum chamber which lock has a passage comprising an outer sealing element and an inner sealing element which outer sealing element can be maintained in the closed position under the influence of the pressure difference caused by the vacuum in the vacuum chamber, and can be opened against the force caused by the pressure difference, which inner sealing element under pretension can be maintained in the opened position and comprises closing element for closing the inner sealing element against the pretension, the closing element comprise at least one electromagnet.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a lock for a vacuum chamber which lock has a passage comprising an outer sealing means and an inner sealing means which outer sealing means can be maintained in the closed position under the influence of the pressure difference caused by the vacuum in the vacuum chamber, and can be opened against the force caused by said pressure difference, which inner sealing means under pretension can be maintained in the opened position and comprises closing means for closing the inner sealing means against said pre-tension. 
     By means of such a lock products can be transferred between the inner space of the vacuum chamber and the surrounding thereof, namely in such a manner that the vacuum is maintained. In the vacuum chamber various operations can be performed onto the product. More specifically one may think of applying a metal layer by means of a sputtering operation onto a substrate such as a CD-disc or DVD-disc. 
     In the known lock the inner sealing means is maintained in the closed position in a mechanical manner. Therefore, a pushing rod has to be applied which has to be operated from the outside through the wall of the vacuum chamber. Such a way of handling will lead to problems, which can influence the operation of the vacuum manner in a disadvantageous way. It appears not to be easy to displace the inner sealing means plan parallel in the correct way such that the required seal is indeed obtained for maintaining the vacuum. A further problem is that the seal of the pushing rod in relation to the wall of the vacuum chamber cannot always be maintained. 
     SUMMARY OF THE INVENTION 
     Object of the invention is now to provide a lock of the above-mentioned type, which does not have said disadvantages. Said object can be reached in case the closing means do comprise an electromagnet. 
     By means of the electromagnet the inner sealing means can be operated without the necessity of a mechanical transfer through the wall of the vacuum chamber. Therefore, sealing problems are eliminated. Furthermore the required parallel movement of the inner sealing means is obtained by the co-operation of the electromagnet with for instance a pole piece. For that purpose the electromagnet may extend around said passage and the pole piece may extend over the inner sealing means in a corresponding way. Especially the passage and the outer and inner sealing means can be cylindrical. 
     To improve the co-operation between the electromagnet and the pole piece, the pole piece has at the surface directed to the electromagnet least one annular ridge or groove, and the area around the passage has at least a correspondingly situated groove respectively ridge, which ridge can be inserted into said groove in case the passage is closed by means of the inner sealing means. The ridge and groove will take care for a correct mutual positioning of the inner sealing means and the passage. 
     Instead of a pole piece also a second electromagnet or permanent magnet can be provided. 
     The outer sealing means comprises an annular O-seal, which under the influence of the vacuum is maintained pressed on, the inner sealing means has an annular O-seal which under the influence of the activated electromagnet can be maintained pressed on. 
     The invention furthermore relates to a vacuum chamber, such as a sputter device, comprising at least one lock, which lock has a passage comprising an outer sealing means and an inner sealing means, which outer sealing means can be maintained in the closed position under the influence of the pressure difference caused by the vacuum in the vacuum chamber and can be maintained opened against the force generated by said pressure difference, which inner sealing means can be maintained in the open position under pretension and comprises locking means for closing the inner sealing means against said pretension, which vacuum chamber comprises a table, which table is displaceable along the passage and carries said inner sealing means. The closing means do comprise an electromagnet and a pole piece, a second electromagnet or a permanent magnet. 
     In a preferred embodiment said vacuum chamber comprises a drum-shaped housing with cylindrical surrounding wall as well as two walls at both ends of said cylindrical surrounding wall in one of which a cylindrical passage is made with an outer vacuum cap, in which housing a table with circular surrounding is mounted in a coaxial rotatable manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Hereinafter, the invention will be explained in more detail with reference to an embodiment, which is illustrated in the figures. 
     FIG. 1 shows an upper view on a sputtering device with a lock according to the invention. 
     FIG. 2 illustrates a transversal cross section according to II—II in FIG. 1 of the lock. 
     FIG. 3 illustrates an electrical circuit to prevent remanent magnetism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 an upper view is provided of the vacuum device which in general is indicated by  23 , such as a sputtering device, as well as a load and emptying device  24 . This last-mentioned device comprises gripping means  25  each able to grip a disc  16  and transport said disc between said sputter device and other, not illustrated stations. In the provided embodiment a pair of gripping devices  25  is positioned above a pair of locks according to the invention, ready to insert a disc into the sputtering device or to remove the disc therefrom. After inserting the discs, the discs are transported over 180° in the sputter device  23  until they are underneath the sputtering apparatuses  29 . After the operation of the discs is performed they are again transported over 180° and removed by means of a pair of gripping devices  25 . 
     Inserting and removing the discs  16  is done through a pair of locks, which are present in the upper wall  13  of the vacuum chamber  6 . In respect to the production rate it is desirable to maintain the vacuum in the vacuum chamber even when the discs are changed. For that purpose the locks are present. 
     The lock illustrated in FIG. 2 forms part of the vacuum chamber  6 , of which only part of the upper wall  13  as well as the turning table  12  are indicated in cross section. The turning table  12  comprises a circular circumferential edge  14  and is at its center  21  (see FIG. 1) rotatably mounted in the vacuum chamber  6 . 
     The vacuum chamber  6  comprises in the upper wall  13  a passage, which is closed by means of the vacuum cap  2 . This vacuum cap  2  operates by means of the O-seal  9  together with the area of the upper wall  13  around the passage  1 . The vacuum cap  2  can be brought in the opened or closed position by (not illustrated) operating means. 
     In the inner space  15  of the vacuum chamber  6  in which a vacuum can be maintained by means of vacuum means  22  (see FIG.  1 ), the table  12  is rotatably installed. This table  12  carries an inner sealing disc  3  onto which a substrate, such as a plastic blank for a CD or DVD is located. 
     The sealing disc  3  comprises a core  18  around which the annular pole piece  5  is installed. As indicated said pole piece  5  comprises an outstanding section  17  which reaches under the core  18 . The core  18  furthermore carries an O-seal  10 , which in a manner to be described hereinafter is able to co-operate with the wall  13  of the vacuum chamber  6 . 
     In the situation, illustrated in FIG. 2, the vacuum cap  2  is closed and a substrate to be operated is present on the inner sealing disc  3 , especially on the core  18  thereof. During inserting or removing of the disc  16  in or out the chamber  6  the sealing cap  2  has to be removed to make the passage  1  free. To be able to maintain the vacuum in the vacuum chamber  6  the inner sealing disc  3  has to be moved upwards such that the O-seal  10  will sealingly be pressed against the upper wall  13 . For that purpose the upper wall  13  carries an annular electromagnet  4  in a groove  19  in the upper wall  13 . Said electromagnet  4  can be supplied with electric current by means of cables  20 . 
     In the activated position the electromagnet  4  acts in a pulling manner on the pole piece  5  which therefore moves upwards and takes with it the core  18  such that the O-seal  10  is maintained pressed against the upper wall  13 . 
     Thereafter the substrate  16  can be removed or a new substrate can be placed onto the core  18 . 
     After these operations are performed the sealing cap  2  is replaced whereafter the activation of the electromagnet  4  can be stopped. Thereafter the inner sealing disc  3  moves back to its opened position under the influence of the diaphragm  11 , which cooperates with the core  18  and the table  12 . Because the electromagnet, which is necessary for moving the lock, co-operates with a core which is made of a magnetisable material the eventual remanent magnetism may cause problems. This remanent magnetism may be so strong that the normal operation of the lock can be hampered. For this problem two solutions are proposed. 
     According to the first solution the rubber O-seal  10  is selected so thick that in the closed position wherein the O-seal is pressed against the opposite metal parts, a certain air gap is maintained between the upper and lower wall. That implies that the magnetic circuit also comprises, apart from the electromagnet itself and the components which are forming the core, an air gap. The influence of said air gap is that the magnetic flux through the core is restricted so that the maximum attracting force is less than the “spring” force of the O-ring. Because of this restriction the forming of remanent magnetism is also strongly restricted and the eventual small amount will not cause problem. 
     According to a second solution a capacitor is placed parallel to the coil of the electromagnet. Reference is made to FIG. 3 in which the coil of the electromagnet  4  is indicated by  44 . The circuit for activating this coil  44  comprises the DC voltage source  40 , a current restricting resistor  42  and a switch  46 . According to the second solution to avoid remanent magnetism a capacitor  48  is placed parallel to the coil  44 . The influence of this capacitor  48  in the situation wherein the switch  46  is closed is negligible. If it is assumed that the real impedance of the coil  44  is rather small then the charge, which will accumulate on the capacitor  48 , is also rather small and negligible. If the switch  46  is opened then in the first instance the coil  44  will try to maintain its current and will start charging the capacitor  48 . The capacitor  48  is charged in a direction opposite to the polarity of the DC source  40 . As soon as the charging process is stopped the capacitor  48  starts uncharging across the coil  44  with the result that a current in the other direction will run through the coil  44  which therefore will generate an electric field which counteracts the eventual remanent magnetism. In fact the coil  44  and the capacitor  48  are forming a parallel LC-circuit with a certain own frequency of which the value is determined by the impedance values of the coil  44  and of the capacitor  48 . In case the power supply to such a circuit is disrupted, then the circuit starts generating a damped AC oscillation, which after a number of waves will be reduced to zero. Because of this phenomenon no remanent magnetism will develop in the core or other magnetisable parts of the magnetic circuit. 
     A small extension of the electric circuit illustrated in FIG. 3 provides the possibility to detect the position of the movable lock section. This circuit extension comprises the capacitor  50 , the AC voltage source  52  and the AC voltage measuring instrument  54 . The capacitor  50  functions on the one hand as DC block, so that the functioning of the DC circuit  40 ,  42 ,  44 ,  46  is in fact not influenced. Together with the capacitor  48  the capacitor  50  forms an AC voltage divider so that a part of the AC voltage supplied by the source  52  is imprinted on the LC circuit  44 / 48 . 
     As long as the switch  46  is closed a DC current will run through the coil  44 , so that the electromagnet is activated and the door is maintained in its closed position. In that situation the magnetic flux of the electromagnet runs (with the exception of an eventual small air gap) only through magnetisable material so that the self-induction of the coil  44  is rather high. The AC voltage meter  54  indicates a correspondingly high value. 
     If the switch  46  is opened no current will run anymore through coil  44  and the electromagnet will fall off bringing the lock in its opened position. In this position there is a rather large air gap within the magnetic flux circuit so that the impedance of the coil  44  will be rather low. The AC voltage meter  54  will indicate a correspondingly rather low voltage value. 
     If for whatever reason (for instance a remanent magnetism) the lock will be in the closed position although the switch  46  is opened the meter  54  will indicate a rather low value which for the operating personnel of this installation forms a sign that there is something wrong in the installation because the lock is still closed where it has to be opened. The inverse situation will arise in case the lock is open although it should be closed. In fact, the lock can be guarded in this manner by the operating personnel. 
     To avoid energy loss the resistor  42  can be replaced by a coil. In that case a damping element (a diode, a varistor or resistor) should be placed parallel to that coil to dampen the voltage peak which will be generated as soon as the switch  46  will be opened.