Apparatus for treating a substrate

This invention relates to an apparatus (1) for treating, e.g. coating, a substrate (35, 39) in a vacuum chamber (2). In this vacuum chamber (2) there are arranged n cathodes (7-10) and n+1 anodes (28-32), each of said anodes adjacent to a cathode (7-10). Each of the n cathodes (7-10) and n of the assigned anodes (29-32) are connected to a power supply (11-14). One of the anodes (28) not being assigned to a cathode (7-10) is connected to an electrical line (63) which is connecting each of the anodes (28-32). A pull-down resistor (34) is connected to said line (63) at its one end and to ground (33) at its other end.

FIELD OF THE INVENTION

This invention relates to an apparatus for treating a substrate.

BACKGROUND OF THE INVENTION

Magnetron sputtering sources are used for treating, e.g. coating, a substrate, such as semiconductors, optical devices or flat panel displays. By such magnetron sources ions of a plasma are caused to sputter material from a target. This sputtered target material is then deposited on a surface of said substrate in order to form a thin film. Also, the ions may etch the substrate.

A sputter source having at least two electrically isolated stationary bar-shaped target arrangements mounted one alongside the other and separated by respective slits is already known (U.S. Pat. No. 6,093,293, see also U.S. Pat. No. 6,284,106 and U.S. Pat. No. 6,454,920). Each of the target arrangements includes a respective electric pad so that each target arrangement may be operated electrically independently from the other target arrangement. Each target arrangement also has a controlled magnet arrangement for generating a time-varying magnetron field upon the respective target arrangement. The sputter source further has an anode arrangement with anodes alongside and between the target arrangements and/or along smaller sides of the target arrangements.

Further, an arrangement for coating a substrate is known, which comprises two electrodes being electrically separated from a sputtering chamber and from each other, wherein one of the electrodes is a cathode being electrically connected with the target and wherein the other electrode is an anode (DE 40 42 289 A1). A capacitor and a resistor are connected in series to said anode and to ground. According to DE 41 36 655 A1 the capacitor can be omitted. In both documents (DE 40 42 289 A1 and DE 41 36 655 A1) the problem to be solved is to suppress an arc discharge.

If the anodes, as shown in DE 41 36 655 A1 are arranged in front of the cathode, the conditions of ignition are improved. However, the disadvantage occurs that the anodes are a hindrance for the current of plasma particles whereby the anodes are coated, too. This coating, in turn, leads to the result that particles of the anode can fall on the substrate and worsen the quality of the layer. On the other hand, with reactive sputtering, the surfaces of the anodes opposite to the surfaces of the cathodes may be coated with dielectric substances. As a result, the ignition conditions are worsened. This effect is known as the “disappearing anode”.

Still, a plasma source including a chamber for confining a feed gas is known (WO 2005/052979 A2). An anode is positioned in said chamber, and a segmented magnetron cathode comprising a plurality of magnetron cathode segments is positioned in the chamber proximate to the anode.

Finally, a plurality, i.e. more than two cathodes arranged in a chamber are known from U.S. Pat. No. 4,417,968, JP 2003-183829 and EP 1 594 153 A1.

It is an object of the present invention to provide an arrangement for coating a substrate, for instance glass, whereby the coating process has a long-term stability, so that the layers which are deposited on different substrates do not differ substantially from each other.

SUMMARY OF THE INVENTION

This problem is solved by providing an apparatus for coating a substrate according to the features of claim1.

This invention therefore relates to an apparatus for treating, e.g. coating, a substrate in a vacuum chamber. In this vacuum chamber there are arranged n cathodes and n+1 anodes, each of said anodes adjacent to a cathode. Each of the n cathodes and n of the assigned anodes are connected to a power supply. One of the anodes not being assigned to a cathode is connected to an electrical line which is connecting each of the anodes. A pull-down resistor is connected to said line at its one end and to ground at its other end.

Employing this arrangement in a sputtering process, the layers of the coated substrates have a good stability over a long period of time. Furthermore, the characteristics of the layer, e.g. the sheet resistance and the sheet uniformity, are improved.

Very good properties are obtained at a resistance value beyond 10Ω or more. For instance, if the pull-down resistor has a resistance value of 440-470Ω good characteristics are obtained for a substrate's coating, the substrate being coated with an apparatus having three or more, e.g. nine, magnetron sputter sources.

DETAILED DESCRIPTION

FIG. 1shows a cross section view of an apparatus1for coating a substrate. This apparatus1comprises a vacuum chamber2having surrounding walls3,4,5,6and a plurality of cathode arrangements7to10. Each of the cathode arrangements7to10is connected to one of the power supplies11to14thus establishing a cathode. Anodes28to32are arranged in the neighborhood of the cathode arrangements7to10and connected via lines to the power supplies. These lines pass the wall5through isolators23to27. The anodes29to32are connected to one of the power supplies11to14so that each power supply is connected to one of the anodes29to32and one of the cathode arrangements7to10. All of these power supplies11to14are electrically connected to ground33serving as protective earth.

The resistance between the anodes28to32and the supplying lines is about 100-200 mΩ. If the resistance of resistor34is lower than 2Ω, a greater current will flow to the shrouds53,54, because the transition resistance of the shrouds53,54is low. If resistor34is by-passed by a short circuit (R=0) then all surfaces of the sputter chamber2have the function of anodes. Such a big anode would have the advantage that the ignition of the plasma is improved. However, by an arrangement comprising a plurality of cathodes7to10an electrical field would establish which is defined by earthed metallic parts which determine the unevenness of the layer.

The resistor34is in the present invention used to eliminate the influence of earthed parts of the chamber2on an arrangement comprising a plurality of cathodes.

If the resistance of the resistor34increases, the influence of the earthed parts is more and more eliminated after ignition of the plasma, because after ignition the plasma current flows in particular between the anodes28to32and cathodes7to10. At the moment of ignition a small current is flowing so that the voltage of resistor34is low. If, however, with a constant burning plasma a big current is flowing, the voltage of resistor34will increase. Thus, the earthed parts of the chamber1are substantially separated from the sputter current circuit. When R=∞ the current is flowing between the anodes and the cathodes only.

As can be seen fromFIG. 1there are five anodes28to32arranged in the chamber2, but only four cathodes or target arrangements7to10, each of them comprising a cathode, or, generally speaking, n+1 anodes and n cathodes. However, the amount of cathodes can also be equal to the amount of anodes.

By having such an arrangement comprising n+1 anodes and n cathodes a geometrical and electrical mirror symmetry is established with regard to the substrate being moved through the chamber.

The anodes28to32are electrically connected to each other via a line63and also connected via the resistor34to ground33, so that all anodes28to32are connected to a common electrical potential.

Resistor34is a pull-down resistor having a defined resistance, the resistance value being beyond 2Ω up to several kΩ, preferably up to 1 MΩ. With this defined resistance value it is possible to set a defined potential comprising power supply, anode and cathode establishing a homogenous distribution of plasma inside the chamber2. As a result, a uniform coating of a substrate is obtained.

If the resistance value of the resistor34is lower than 2Ω the current does not fully flow through the anodes28to32. To avoid this, resistor34has a resistance value beyond 2Ω, preferably beyond 10Ω, so that the current flows through the anodes28to32. At a resistance value of 2Ω or more the coated sheet resistance value is independent from the current and the voltage.

Without pull-down resistor34the afore-mentioned potential values would not have defined values with respect to earth or ground. Thus the potential could correspond to R=0 at one time and to R≠0 at another time. There would be no guarantee for a homogenous distribution of the plasma, i.e. the distribution of the coating would be good at one moment and bad at another moment. Also the long-term stability could not be controlled.

Referring again toFIG. 1, a plane substrate35is illustrated, e.g. a glass plate, moving through the interior36of the vacuum chamber2, said substrate35being coated as it passes the cathode arrangements7to10.

As will be understood fromFIG. 1, substrate35moves in direction of an arrow37leaving the chamber2through an opening38. As the substrate35leaves chamber2another substrate39to be coated enters chamber2through an opening40being arranged at the opposite side of the opening38. Although not shown inFIG. 1, lock chambers can be arranged at both sides of the vacuum chamber2, so that the substrate35, coming from a lock chamber being arranged next to the opening40, moves through the chamber2in order to enter another lock chamber being arranged next to the opening38.

Throughout the coating process a constant vacuum is provided within the chamber2. This is achieved by vacuum pumps41,42shown inFIG. 1.

Gas reservoirs43,44supply the chamber2with a gas or a gas mixture. The gas or the gas mixture flows through a pipe47, said gas entering the chamber2via openings45,46of pipe47.

The gas or the gas mixture can then be removed by pumps41,42, said gas or gas mixture leaving the chamber2via openings48,49. Each of the gas reservoirs43,44may comprise different gases. For instance, if a reactive sputtering process is carried out, one of the reservoirs43,44, for instance reservoir44, comprises a reactive gas such as N2, O2, whereas the other reservoir43comprises an inert gas, such as Ar.

The gas flow can be regulated via valves50,51,52being controlled by a computer not shown inFIG. 1.

Although four magnetron sputtering sources7to10are shown inFIG. 1it is clear to those skilled in the art that there can be arranged more than only four magnetron sputtering sources, but not less than two.

As can be seen fromFIG. 3the anodes29to32are again connected to the respective power supply11to14and to the other anode28. All those anodes28to32are coupled to each other, and the pull-down resistor34is arranged between these anodes28to32and ground33.

FIG. 2shows a cut-out of the apparatus1according toFIG. 1, illustrating the cathode arrangement8and a section of the cathode arrangement7.

The cathode arrangement8and the corresponding anode30are connected to the power supply12. Also, anode30is electrically connected to wire or line63. This power supply12, a DC power supply, is coupled to ground33. The cathode arrangement8comprises a cathode body55being arranged partly in the chamber2. To maintain a stable vacuum inside the vacuum chamber2a sealing62is provided.

The cathode arrangement8furthermore comprises magnets56,57,58being arranged on a yoke59, the yoke59being arranged between the magnets56,57,58and the body55.

Also a plate60, preferably a copper plate, is arranged between the magnets56to58and a target61, the target material being for instance Mo, Ti, Cu, Si, Al, Zn, Zr, Ni, Cr, NiCr or oxides of these materials. ITO can also be used as a target material.

Although not shown, the cathode arrangement8as well as the other cathode arrangements inside the vacuum chamber2comprise a cooling device for cooling the cathode arrangement during the coating process.

FIG. 3shows a similar apparatus as shown inFIG. 1. However, instead of planar cathodes tubular cathodes are provided. Tubular carriers15to18are surrounded by tubular targets19to22which are connected to a negative potential of a respective voltage source11to14via lines64to67. Further details of such tubular cathodes are disclosed by EP 1 722 005 B1. Also, besides linear and tubular cathodes, planar “MoveMag” cathodes as shown inFIG. 2of DE 197 01 575 A1 may be used.

FIG. 4depicts a graph showing the sheet resistance (Rs), i.e. the resistance of a coating on a substrate and the uniformity of a coating in dependency of different resistance values of a pull-down resistor34for a process, wherein the power of one of the cathodes10being arranged at one end of the chamber2is P=30 kW and the power of the other cathodes7to9is P=27 kW, the pressure p=0.15 Pa and the layer thickness approximately d=10−7m. The uniformity is determined by

Uniformity⁢[%]=(Max-Min)(Max+Min)×100⁢%
wherein Min is the lowest and Max the highest value of the layer thickness d.

The surrounding walls3to6of the chamber2are grounded or earthed to avoid the persons in charge will be hurt by floating potentials and to avoid an electrostatic charging. For the same reason the housings of the power supplies11to14are grounded. The grounding of the housing is not connected to the anodes28to32.

As can be seen inFIG. 4there is a lack of uniformity if the resistance values of the pull-down resistor34are lower than 2Ω. As the resistance values rise above 2Ω, the influence on the two characteristics, i.e. sheet resistance and uniformity, decreases, thus providing a pull-down resistor34having resistance values beyond 2Ω, a more uniform coating can be obtained.

The different powers derive from the position of the cathodes7to10in the chamber2. Instead of having different powers being applied to the cathodes, it is also possible to apply only one power to the cathodes.

The break of the curve inFIG. 3at R=2Ω is caused substantially by the fact that the measurement comprises three points only. In fact the curve may have the form of an e-function. If the resistor34has a value of resistance of 2 kΩ, the effect of the invention is achieved, too. However, the ignition of the cathodes is worse as compared with lower pull-down resistor values.