Abstract:
An alternating-current source (2) is connected to two cathodes (6,7) which cooperate electrically with targets which are sputtered in a gas discharge while a process gas is introduced in a vacuum chamber (15). A network formed of a transformer (3) and additional coils (5, 12, 13) and condensers (4, 8, 9, 10, 11) acts as a filter to assure a stable coating process.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an apparatus for coating substrates in a vacuum, consisting of an alternating-current source which is connected to two cathodes disposed in an evacuable coating chamber, which cooperate electrically with targets which are to be sputtered, and whose sputtered particles deposit themselves on the substrate, while a process gas can be supplied the coating chamber and between the alternating-current source and the cathode pair a network is disposed which acts as a filter and is formed of a transformer, coils, and condensers. 
     A sputtering apparatus is already known for producing thin coatings (DD 252 205), consisting of a magnet system and at least two electrodes of the material to be sputtered arranged over them, these electrodes being connected electrically so that they are alternately cathode and anode of a gas discharge. The electrodes are for this purpose connected to a sinusoidal alternating voltage of preferably 50 Hz. 
     This known sputtering apparatus is suitable especially for depositing dielectric coatings by reactive sputtering. Operating the apparatus at about 50 Hz is intended to prevent the formation of flakes on the anode and, in the case of metallic coating, resulting in electrical short-circuits (known as arcing). 
     DE 39 12 572 discloses an apparatus for applying thin films by sputtering, wherein the rate at which layers of different materials are deposited is controllable. In order to achieve thin packets of layers, at least thin different kinds of counter-electrodes are provided on the cathode side. U.S. Pat. No. 5,169,509 discloses a method and an apparatus for the reactive coating of a substrate with an electrically insulating material, silicon dioxide (SiO 2 ) for example, consisting of an alternating-current source, which is connected to magnetron cathodes in a coating chamber. Two non-grounded outputs of the alternating-current source are each connected with a cathode bearing a target, each target approximately the same distance away from the opposite substrate. The effective value of the discharge voltage is measured by an effective-voltage detector and fed as direct current through a conductor which by means of a control valve directs the reactive gas flow from the container to the distribution line such that the measured voltage will agree with a set voltage. 
     U.S. Pat. No. 5,240,584 discloses an apparatus for the reactive coating of a substrate in which a magnetron cathode is electrically separated from the vacuum chamber and has two parts electrically separated from one another. The target base body with yoke and magnets as the one part--with the insertion of a capacitor--is connected to the negative pole of a direct-current voltage supply. The target is connected as the other part through a choke and a parallel resistance and through a capacitor to the positive pole of the power supply, and through a resistor to an anode at ground potential. In series with the low-induction capacitor an inductance is inserted in the branch line to the resistor and to the choke, and the value of the resistance is typically between 2,000 ohms and 10,000 ohms. This known apparatus is so constructed that it suppresses a predominant number of the arcs occurring during a coating process, and reduces the energy of the arcs and improves the reignition of the plasma after an arc. 
     The practicality of a sputtering process stands and falls with the stability of the process. That is, the system must be stable over long periods of time (300 h) and assure the electrical requirement (cathode current, power) and the optical properties of the coating (index of refraction, thickness, and thickness distribution). 
     The greater the length of a cathode becomes, the greater becomes the power required and the more difficult it becomes to satisfy the stability requirements. This has determined the physical background, that the voltage is determined by pressure, kind of gas, and the quality of the target surface, while the current increases in proportion to the length. Small changes in the quality of the target surface produce small voltage variations which, however, multiplied by the high current resulting from the great cathode length, lead to high power fluctuations, which can be so great that the target can melt locally. 
     It is especially problematic to maintain the degree of oxidation of the deposited coating at high powers. This problem is the result of the fact that, for the large surface coating of architectural glass, oxides, among other things, are deposited when sputtering of a metal target is performed with an oxygen-argon mixture in order to obtain a particular oxide modification. This oxide modification is needed in order, for example, to set the color of the coating system or the corrosion resistance. 
     If the supply of the oxygen is increased, the cathode tilts to the oxide mode. If the oxygen content is increased only above the tilt point, then the cathode can be brought back into the metal mode by increasing the power. 
     Two basic ways are known for the cathodes to perform: 
     Type 1: The resistance of the cathodes decreases with increasing reactive gas content. 
     Type 2: The resistance of the cathodes increases with increasing reactive gas content. 
     Which type is present depends on the material and on the kind of reactive gas. 
     Most applications use as the &#34;working point&#34; a ratio of oxygen content to power that is close to the &#34;tilt point&#34;. The reason for this is that in this range the highest oxide coating rates are achieved. For the operation of the system this working point is very unfavorable, because slight fluctuations in power, oxygen, or the cathode environment can lead to a change in the mode so that product rejects result. 
     As the cathode becomes larger, the power fluctuations greatly increase and the room for stable operation of the cathodes becomes less, so that without additional means, large cathodes for demanding materials such as Si and SiO 2  can no longer be used at all. 
     SUMMARY OF THE INVENTION 
     According to the invention, a network which acts as a filter is provided between the alternating-current source and the cathode pair. The network includes a transformer and additional coils and condensers, and the frequency of the alternating-current source differs from any natural frequency of the filter. The curve of the cathode power as a function of the cathode resistance which the gas discharge offers to the alternating current has a maximum, while the resistance at the maximum is selected such that the cathode resistance at the working point differs from the resistance at the maximum, and the impedance of the network opposes change from one process state to the other. 
     The network stabilizes the system so that reliable operation is assumed even at high powers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the power supply for the cathode pair, and 
     FIG. 2 is a graph of the power over the resistance. 
     The apparatus herein is for sputtering a substrate 14 in an evacuable coating chamber 15. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The energy is obtained from the medium-frequency generator 2 at its resonant circuit. Since due to its construction this circuit is electrically connected to the supply line, an isolating transformer 3 must be included in the network 1. At what point in the network the transformer 3 is inserted is of no importance from here on. 
     Condenser 4 and coil 5 are disposed on the resonant-circuit side of the transformer, and the rest of the components are disposed on the side of the cathodes 6 and 7. Also successfully tried was a circuit in which the transformer 3 was connected directly to the resonant circuit 2 and all condensers and coils were arranged on the cathode side. 
     The size of condenser 8 is selected to be so great that it has little effect on the frequency characteristic of the circuit. It serves to protect the transformer 3 against any direct current which might flow which due to differences in the characteristics of the two cathodes 6 and 7 and thus might impair the operation of the transformer 3. 
     In case the transformer is directly connected to the resonant circuit, the condenser 4 can take over the function of condenser 8. 
     The condensers 9 and 10 serve for arc suppression, by grounding currents of very high frequency. These very high frequency currents can also be produced by switching operations in the plasma. 
     The actual characteristic made possible by the self-stabilization is established by the condensers 4 and 11 as well as the coils 5 and 12 and 13. Formally, coils 12 and 13 can be combined to form one coil. Assembly of coils 12 and 13 of the same size is done in order to obtain good symmetry in the system as a whole. 
     1. The reference value for the calculation is the equivalent resistance of the sputtering section, designated R cat  in the following text. It is formed as the quotient of the effective value V cat  of the voltage between the cathodes 6 and 7 at the instantaneous power and the effective value of the cathode current I cat  at the power in this moment, R cat  =V cat  /I cat . 
     2. The self-stabilization is obtained with cathodes with Type 1 behavior if, in the case of decreasing resistance R cat , the power in cathodes 6 and 7 increases. To provide sufficient leeway for stabilization in the production process the resistance R max  is selected at which the maximum power (M) is reached, and thus the end of the self-stabilization, at R max  &#39;=0.5 . . . 0.75×R cat  at the rated power. See FIG. 2 with the working point (A) at the rated power. 
     3. At the working frequency the input resistance of the network 1 must be inductive, measured on the cathode side. 
     4. The input resistance R in  of the network 1 is great in comparison to R max , and decreases now to a minimum (1st null point) at a frequency below the working frequency. 
     5. At the working frequency the input resistance R in  reaches a value of 1.414×R max , i.e., the imaginary part and the real part of the input resistance R in  are equal to R max . 
     6. As the frequency increases further, the input resistance R in  rises to a maximum and then decreases to a 2nd minimum (2nd null point). The frequency of this 2nd null point is approximately three times the working frequency. 
     Practical values for the components are: 
     Working frequency: 40 kHz 
     C1 (capacitor 4) =0.6 μF, C2 (capacitor 8)=18 μF, C3 (capacitor 11)=108 nF, C4 (capacitor 9)=8 nF, C5 (capacitor 10)=8 nF 
     L1 (coil 5)=16 μH, L2 (coil 12)-L3 (coil 13)=23 μH 
     Transformer ratio: 1:2. 
     The equivalent resistance R cat  of the sputtering section at a rated power of 125 kw amounts to 2 ohms in the mentioned example (FIG. 2); the resistance R max  at maximum power amounts to 1 ohm. 
     The power can be calculated as a function of the equivalent resistance according to Kirchoff&#39;s rules for circuit analysis to yield 
     
         P(R.sub.cat)=R.sub.cat /(b.sub.o +b.sub.2 R.sub.cat.sup.2) 
    
     wherein b o  is the linear component of the filter and b 2  is the component representing all values which are combined with R cat   2 .