Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
1999-01-13
2001-11-20
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298090, C204S298170, C204S298040, C204S192110
Reexamination Certificate
active
06319372
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a permanent magnet electron cyclotron resonance plasma source for treating or coating surfaces by sputtering.
BACKGROUND ART
In an electron cyclotron resonance source, ions are obtained by ionizing a gaseous medium composed of one or more gases or metallic vapors in an enclosed chamber such as a hyperfrequency cavity. This ionization is the result of an interaction between the gaseous medium and a number of electrons that have been highly accelerated by electron cyclotron resonance. This resonance is obtained by combining a microwave electromagnetic field injected into the chamber with a magnetic field obtaining in the said chamber.
Electron cyclotron resonance can be used to create dense plasmas at low pressure without using filaments or cathodes. In an embodiment described in reference document No. 1 (see end of description), an electron cyclotron resonance source composed of two rectangular wave guides is used to produce a powerful flow of ions for use in synthesizing materials by sputtering.
Injecting a micro-wave source at a frequency of 2.45 GHz into a plasma chamber comprising a zone of electron cyclotron resonance at 0.0875 Tesla causes ionization of a gas under low pressure at 10
−4
to 10
−3
mbar. The ions and electrons thereby created are diffused along the lines of the magnetic field until they reach a negatively-charged target. Sputtering is carried out at −500 Volts on silicon or quartz substrates. The height of the plasma is 20 cm at a width of 5 cm. Electron densities measured by interferometer at 27 GHz reach 4×10
11
e/cm
3
for argon and krypton. The density of the ion current is 40 mA/cm
2
for oxygen plasma.
In contrast with magnetrons, the independence of the plasma gun from the target makes it possible to give continuous production of a wide range of deposits:
layers of magnetic material (iron) on quartz have been obtained at a speed of 200 nm/min (nanometers/minute) at low pressure;
using reactive sputtering of oxygen and nitrogen plasmas, it has been possible to created oxide layers of transparent Al
2
O
3
and stoichiometric Cr
2
O
3
, and nitride layers of AlN at speeds of 10 to 20 nm/min; using a suitable argon-oxygen mixture to overcome the problem of oxidation of the target with aluminum it is possible to achieve speeds of 100 nm/min. Unlike magnetrons, the procedure requires no RF polarization supply. A simple DC power supply can be used to carry out all deposits;
low pressure deposits (10
−4
mbar) reduce the proportion of gas included in the layers and induce increased density of the material (approximately 7 g/cm
3
for chrome deposits on silicon);
diamond-type carbon layers have been achieved by sputtering a carbon target using a polarized substrate or methane dissociation;
unlike magnetrons, target wear is uniform for the entire sputtered surface.
For several decades, electron cyclotron resonance plasma sources have been widely used for synthesizing materials. Particularly in Japan and the USA, special attention has been devoted to sputtering using electron cyclotron resonance plasmas and several types of equipment have been constructed depending on the type of material and the size of the substrate. A description and brief bibliography are given in reference document No. 1 (see end of description).
The most commonly used electron cyclotron resonance microwave sources for deposition by sputtering consist of a cylindrical plasma chamber, magnetic field coils that create an absorption zone at 875 Gauss and enable the plasma to be diffused onto a negatively-polarized target. The main drawbacks of these sources is that they require solenoids that have a high level of power consumption, they use cylindrical sputtering targets on the periphery of the plasma giving a low level of interaction between the plasma and the target, and the microwave leaktight windows have a tendency to be obliterated by sputtered metal atoms. Sources of this type are described in reference documents 2, 3 and 4.
Electron cyclotron resonance plasma apparatuses have been constructed using permanent magnets fastened to a 90° microwave injection elbow and a wave absorption zone located inside a conical sputter target. The main drawback of these apparatuses is the fact that the microwave window is located near the sputtering zone and very quickly obliterated. Moreover, deposition speeds are limited. This type of apparatus is described in reference document 5.
Permanent magnet electron cyclotron resonance plasma sources have been developed in Japan using slot-guided microwave injection systems (slot antennas). The main drawbacks of these sources are the limited power of the injected microwaves and the ensuing limited ionic density at approximately 10 mA/cm
2
, together with obliteration of the microwave windows. This type of source is described in reference document 6.
Electron cyclotron resonance plasmas have been created using the magnetic structures of magnetrons by alternating the polarity of the permanent magnets behind the sputter target and then injecting a microwave source. The main drawbacks of these structures are the difficulty in sputtering magnetic materials due to the target forming a barrier, the uneven target wear (targets become unusable once 30% worn) and the oxidation of the target to produce oxides (RF polarization). These types of structures are described in reference documents 7 and 8.
Electron cyclotron resonance apparatuses have been developed using a sputter target inside the diffusion plasma (at an angle of 45°) with two solenoid assemblies together with a strongly magnetic microwave window, the creation of an 875 Gauss absorption zone, and diffusion and compression of the plasma on the target; this produces powerful flow of ions at low pressure (25 to 30 mA/cm
2
at 10
−4
mbar on extrapolated surfaces. Tests of this system were carried out using two wave guides for a height of 20 cm. A microwave window is located in a 90° elbow where it is protected from metallic deposition. The drawbacks of these apparatuses are the high power consumption of the solenoids and the angle of the target relative to the substrate. This type of apparatus is described in reference documents 1 and 9.
It is the aim of the present invention to overcome the drawbacks of the apparatuses of the prior art described above.
DISCLOSURE OF THE INVENTION
The present invention relates to a linear microwave source comprising:
a leaktight chamber;
means for creating a magnetic field in the chamber and for generating a plasma stream;
means for coupling the microwave source to the plasma stream inside the chamber;
a sputtering target that is located inside the plasma stream and electrically insulated from the chamber and charged with negative polarity;
pump means for creating negative pressure inside the chamber;
means for injecting gas for controlling the ionic species of the plasma stream such that the ions accelerated towards the target cause ejection of sputtered atoms onto a substrate;
characterized by the fact that the said coupling means comprise a microwave injection guide followed by a 90° elbow opening perpendicularly into the chamber, a leaktight microwave window located between the microwave injection guide and the 90° elbow such that they cause ionization of the gas in a zone of electron cyclotron resonance located a few centimeters inside the elbow and under negative pressure, and by the fact that the means for creating a magnetic field comprise first and second permanent magnets disposed either side of said window, said magnets being installed with alternating polarity.
The target is advantageously cooled by means of a cooling circuit comprising pipes that supply and drain liquid coolant.
In a first embodiment a third magnet is disposed on the opposite side of the chamber from the first magnet and at the same height as the said first magnet, the poles of the first and third magnets being disposed in series.
In a second embodiment the third magnet is disposed behind the target, the p
Burns Doane , Swecker, Mathis LLP
Commissariat A l'Energie Atomique
McDonald Rodney G.
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