BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to apparatus and methods for magnetron sputtering, and more particularly to magnetron sputtering apparatus that delivers high peak powers to a sputtering magnetron plasma load with arc handling capability.
2. Brief Description of the Prior Art
It is desirable to coat some substrates by generating metal ions and attracting the ions to the work piece by means of an electrical bias. The utility of this approach includes application of coatings to surfaces with irregularities that would prevent uniform deposition by normal sputtering, which essentially requires line of sight from the sputtering source to the workpiece feature. Coating and even filling high aspect ratio trenches in semiconductor devices is possible by biasing the wafer to attract the ions, as reported by Monteiro in JVST B 17(3), 1999 pg. 1094 and Lu and Kushner in JVST A 19(5), 2001 pg. 2652.
Sputtering deposition may be enhanced by making use of plasmas in a highly ionized state. A technique for generating highly ionized, high density metal plasma by driving conventional sputtering magnetrons with electrical pulses having high peak power and low duty factor has been reported by Kouznetsov, et al. in Surface and Coatings Technology 122 (1999) pg. 290. Additional teachings can be found by Macak, et al, JVST A 18(4), 2000 pg. 1533; Gudmundsson, et al., APL, Vol. 78, No. 22, 28 May 2001, pg. 3427; and Ehiasarian, et al., Vacuum 65 (2002) p. 147.
U.S. Pat. No. 6,296,742 B1 describes a method of producing a fully ionized plasma for use in magnetron sputtering applications. A pulse generator delivers pulses of up to 10 MW to a sputtering target, thereby completely ionizing a sputtering gas. In this method, the sputtering gas is described as first adopting a glow discharge state, then continuing to an arc discharge state, and finally adopting a fully ionized state. As shown in FIG. 1 of that patent, the arc discharge state is described as a break-down condition occurring at current densities beyond those of the abnormal glow discharge region, and is characterized by a sudden drop in plasma impedance, as shown by an abrupt drop in plasma voltage as the current density further increases. In practical systems, this is usually represented by a drop in the voltage across the plasma to at most a few tens of volts and may be accompanied by a discharge between some part of the sputtering target and the chamber. The '742 patent indicates that, under that patent's teachings, the plasma, after passing through this arc region, develops into a fully ionized state.
Part of the appeal of these techniques is the ability to generate a large population of ionized species that can in turn be attracted to the work piece by the application of a bias voltage. The above references on the high peak power techniques appear to use a simple capacitor discharge through an inductor. However, the technique taught by these references does not disclose any arc handling capability, and in fact suggests that it is possible, once the fully ionized state is attained, to achieve thereafter arc-free operation. Unavoidable imperfections in hardware, however, make the physical realization of a completely arc free region of operation (after the initial passing through of the arc state) essentially impossible, even if its existence is suggested by theory. Use of the technique, therefore, without arc handling capability, may make commercial utilization impractical. It may also, at the least, make processing time excessive because of the long time which may be needed to condition the target to operate in near arc-free conditions, and may at the very least prevent operation at the highest power levels due to an inability to condition the target adequately. Thus, it would be desirable to provide apparatus that enables commercial processes using high peak power pulses to magnetrons to produce high density, highly ionized plasmas by minimizing arc energy that in turn keeps product and target damage due to arcing within acceptable limits. In view of the possible damage created by passing through the arc state at the outset of the pulsing, it would also be very desirable to prevent the occurrence of the arc state in the initial establishment of the highly ionized condition.
Typically arc control and arc diverting apparatus have been comprised of circuits that either detect the arc and disconnect the power supply from the load or are comprised of a switching circuit that effectively short circuits the power supply to extinguish the arc. These types of arc handling methods are very costly because they may result in a complete shut down of the process, wasting expensive stock material, or require complete dissipation of all of the energy stored in the power supply circuits. In high power applications, short circuits to the power supply may result in extremely high currents—even enough to cause destruction of the power supply itself—and repetitive dissipation of stored energy in any case requires expensive resistive elements capable of high peak power and high average power, as well as the means for cooling them.
It would also be desirable, then, if there were provided a magnetron sputtering apparatus and method that could deliver peak powers of 1 Megawatt or greater, with arc handling capability for high yield commercial applications. It is an object of this invention to provide a magnetron sputtering plasma system that has the capability both to detect arcs and to take action to limit the energy delivered to the arc. It is a further object of this invention to provide a magnetron sputtering system that creates sputtering plasmas in a highly ionized state without first adopting an arc discharge state, which may cause damage to the chamber, substrate, or target, even if only as a transient condition on each pulse.
SUMMARY OF THE INVENTION
There is provided by this invention an apparatus and method for producing high current pulses suited for delivering high peak power to high-density magnetron plasmas with efficient arc handling capability. In one embodiment of the invention, a pulsing circuit comprised of an energy storage capacitor is repetitively charged and then discharged through an inductor in series with the plasma. The combination of the inductor and capacitor serve to shape the pulse, which accomplishes three functions. First, it has been found that it is possible to avoid the initial arc condition by properly shaping the pulse. This is done by controlling the beginning of the voltage pulse. In one embodiment, a network is added for the purpose of controlling the voltage rate of rise, the unclamped peak amplitude of the voltage pulse in the event that the plasma does not ignite, and the frequency at which the voltage waveform rings, particularly in the case that the plasma does not ignite. This circuit in this embodiment amounts to a resistor in series with a capacitor shunt connected at the output of the pulser, or its equivalent implemented as a distributed circuit with a number of discrete capacitors and resistors, possibly also utilizing parasitic capacitors and resistors in devices and circuit conductors. In addition, a circuit is provided to clamp or limit the voltage pulse to a maximum level, implemented with a diode, normally reverse biased in series with a capacitor held at the clamp voltage, connected to the output of the pulser. This circuit is activated when the amplitude of the voltage pulse exceeds a preset adjustable value and acts to prevent the voltage from exceeding a preset level. This has the benefit of preventing undesirable arcs both inside and outside the vacuum chamber. All of this makes it possible to reach a highly ionized plasma state without first passing through the arc state.
Second, the pulsing network, or mesh, serves to provide an impedance match to the plasma. Third, the network serves to limit the current rate of rise and peak magnitude in the event of a later occurrence of an arc. An arc may be detected by either the fall of the discharge voltage below a preset voltage threshold during a pulse, or an increase in discharge current above a preset current threshold. Note that the arc condition represents a lowering of the impedance of the plasma, which is represented by the ratio of the voltage to the current, so either or both detection methods will serve. When an arc is detected, the energy storage capacitor is disconnected from the series inductor to stop the current rise. The pulsing circuit is then disconnected from the plasma load and the inductor energy is recycled to the energy storage capacitor.
For a typical sputtering plasma in a glow or abnormal glow state, the proportion of ionized species is relatively low, on the order of a few percent at most. Using the present invention, sputtering plasmas in a highly ionized state may be achieved, having ionization fractions of ten percent or more. In sputtering systems wherein only very small ionization fractions are normally present, such as systems for sputtering carbon, a highly ionized plasma may be achieved using the present invention by raising the proportion of ionized species in the plasma by a factor of five or more.
Using the apparatus and method of this invention, a sputtering plasma in a highly ionized state may be created without first adopting an arc discharge state. The arc handling features of the invention serve to mitigate and extinguish any arcs that develop while the sputtering plasma is present.