Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1995-04-06
2001-06-19
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192320, C204S298310, C204S298160, C204S298370
Reexamination Certificate
active
06248219
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a process and apparatus for sputtering a surface of a workpiece to be sputter-etched or for sputtering a surface of a target for sputter-coating a workpiece, both referred to as a “sputtering object”.
More specifically, the present invention is directed to such processes and apparatus, whereat RF sputtering is performed in a vacuum recipient which is filled with a working gas at a selected gas pressure.
Even more specifically, the present invention is directed to the application of magnetic fields in such processes and apparatus.
2. Description of Prior Art
In specific art mentioned above, the law of KOENIG as disclosed for instance in H. R. Koenig and L. I. Meissel, IBM Journal Research Development 14, p. 168 (1970), and H. R. Koenig, U.S. Pat. No. 3,661,761 (1969), is well-known. It defines that the ratio of drop of time-averaged electric potential adjacent to electrode surfaces between which an RF plasma discharge is generated, is given by the inverse ratio of respective electrode surface areas raised to the fourth power. This law is only valid under specific conditions:
The discharge space of the RF plasma discharge is confined by only two electrode surfaces between which RF energy is applied. No further electrode surface is exposed to the plasma which is loaded with an RF current. The confinement of the RF plasma discharge space by the two electrode surfaces whereat RF energy is applied may only have gaps or holes which are of such small extent that the plasma discharge may not spread out of the confinement and couple RF currents to other parts of a vacuum chamber. This means e.g. that the minimal diameter of any gaps in such a two electrode confinement must substantially be not larger than the dark space distance at the working gas pressure maintained during RF plasma discharge. Further spacings between the two electrodes may e.g. be bridged by dielectric material which also prevents spreading of the discharge.
If a sputtering object or a workpiece to be sputter-coated is disposed within the confined discharge space, the said condition is further fulfilled only if the sputtering object is either electrically floating or is disposed on the electric potential of one of the two electrode surfaces to which RF energy is applied.
If these conditions are taken in consideration and RF energy is applied at a frequency of above 3 MHz and below about 90 MHz, then the KOENIG law mentioned above will at least approximately be fulfilled.
In a strongly simplified consideration, positive ions out of the RF plasma discharge are accelerated to the respective electrode surfaces at a kinetic energy predominantly given by the drop of time-averaged electric potential adjacent the electrode surface considered. Depending on the material to be sputtered and the kind of ions and thus of the working gas, sputtering starts at a given ion accelerating drop of time-averaged electric potential adjacent an electrode surface considered.
The “sputtering rate” defined as mass of material sputtered off a surface per time unit depends predominantly upon two largely independent entities:
a) the average kinetic energy of the positive ions, given by the said drop of time-averaged electric potential across a dark space region,
b) “plasma density” in such dark space region given by the density of electrically charged particles in said space.
The sputtering rate may be increased by increasing the average kinetic energy of ions and/or by increasing the number of ions impinging on the surface to be sputtered. Thereby increasing of the plasma density will only then increase the sputter rate if the average energy of the ions suffices for sputtering at all.
The law of KOENIG only considers the ratio of averaged kinetic energies in relation to ratio of electrode surfaces at homogenous plasma density.
It thus becomes evident that according to the law of KOENIG, when the two electrode surfaces are equal, both these electrode surfaces will be subjected to sputtering at equal kinetic energy, because in the adjacent dark spaces of both electrodes equal drops of time-averaged electric potential will occur. If one of the two electrodes is made smaller than the other, this will result in an increased ion accelerating drop of time-averaged electric potential adjacent the smaller electrode surface and across its dark space and, accordingly, to diminution of such ion accelerating drop adjacent the larger electrode surface and across its dark space region.
As was mentioned, this phenomenon is known to prevail if the conditions mentioned above are considered.
From a DC plasma sputtering technique, wherein an electric DC field is applied between two electrode surfaces, it is known to provide on one of the two electrode surfaces to be sputtered, here clearly the cathode, a tunnel-shaped magnetic field to improve plasma density adjacent the cathode by the well-known electron trapping effect of magnetic force lines aligned perpendicularly to the electric force lines.
Several successful approaches have become known to apply an e.g. so-called magnetron technique, known from DC sputtering technique, also to RF sputtering techniques, with the object of, as in the DC sputtering case, improving the sputter rate by rising plasma density adjacent the surface to be sputtered. For simultaneously improving sputtering homogenity along a surface to be sputtered, it further became known to provide a relative movement between an applied magnetic field pattern and the surface to be sputtered.
The present invention, as will be described below, is based on a new recognition made by the inventors at systems for which the law of KOENIG is principally valid and which, thus, fulfil the above mentioned conditions by inventively applying specific magnetic fields: It becomes possible to realize average kinetic energy of the ions impinging upon the electrode surfaces which are in opposition to those predicted by the KOENIG law. This inventively recognized deviation of the distribution of the said energy at the two electrode surfaces from that predicted by KOENIG is especially pronounced at electrode surfaces which are of the same order of extent.
From the U.S. Pat. No. 4,278,528 patent (Kuehnle) it is known to provide in an extended vacuum chamber a multitude of targets to be sputtered by RF plasma discharge. Between the multitude of targets and a workpiece band to be continuously sputter-coated a mask in a form of a metallic and grounded plate with respective slits is provided, and provides for sputter-coating a specific line pattern on the moving workpiece band. The RF plasma discharge spaces are formed between respective targets and “anode” electrodes, whereby the RF plasma may spread laterally outwards along the surfaces of the targets. This is because the targets and “anodes” do not confine the respective discharge spaces laterally. Thus, the plasma discharge spaces are primarily confined or bordered by the overall vacuum chamber wall, targets and counter-electrodes named “anodes”. Tunnel-shaped magnetic fields are applied either on the target or, opposite to the target, to the “anode” electrode surfaces, so as to prevent electrons from heating the workpiece band which may consist of paper or plastic material.
If this arrangement is considered under the law of KOENIG, then it must be considered that the discharge space is confined on one hand by the target electrode surfaces and, on the other hand, by the “anode”electrode surfaces plus all metallic surfaces exposed to the inside of the overall vacuum chamber. As was mentioned above, the law of KOENIG is further only valid if the discharge is bordered by surfaces on only the two electrode potentials respectively externally applied and on no third potential externally applied and the discharge is thus generated in a so-called “diode arrangement”.
In the above mentioned U.S. Pat. No. 4,278,528 patent the diode arrangement condition is only fulfilled if the wall of the overall chamber is at the same electric potential a
Hoefler Karl
Wegmann Urs
Wellerdieck Klaus
Cantelmo Gregg
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Nguyen Nam
Unaxis Balzers Aktiengesellschaft
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