Planar magnetron sputtering apparatus

Details Planar magnetron sputtering apparatus Planar magnetron sputtering apparatus

2000-10-13

2002-08-13

Ver Steeg, Steven H. (Department: 1753)

Chemistry: electrical and wave energy

Apparatus

Coating, forming or etching by sputtering

C204S298170, C204S298190

Reexamination Certificate

active

06432285

ABSTRACT:

The present invention relates to a magnetron sputtering apparatus, and, more particularly, to an improved planar magnetron sputtering cathode apparatus.
BACKGROUND
Magnetron sputtering is well known and widely used in production and research applications for the deposition of thin films of various metallic, semiconductor, and ceramic materials on a substrate. A planar target cathode is mounted in a vacuum chamber, and is eroded on one of its surfaces by a DC or RF/AC plasma discharge confined in close proximity to its surface by a closed loop magnetic tunnel. The target, generally a circular or rectangular plate fabricated of the material to be deposited, is electrically connected to the negative side of a DC or biased RF power supply. The positive side of the power supply is connected to a separate anode structure, or may be connected to the vacuum chamber itself if the chamber is electrically conductive. A substrate work piece, the object on which a thin film coating will be deposited, is placed in close proximity to the target cathode. The vacuum chamber is evacuated and a sputtering gas is introduced at low pressure, generally in the range of 10
−2
to 10
−4
Torr, to provide a medium in which a glow discharge plasma can be initiated and maintained. The most common sputtering gas is argon (Ar). In some cases, gases or gas mixtures other than Ar are introduced to the vacuum chamber. For example, in a reactive sputtering process, a deposition compound is synthesized by sputtering a selected target material in the presence of a reactive gas. Deposition of a thin film of TIO
2
on a substrate work piece by sputtering metallic Titanium in the presence of an Ar/O
2
plasma is one common reactive gas sputtering process. When an electric field of direct voltage is produced by the power supply, the electrons generated move under the influence of the electric field, ionizing the introduced gas molecules and thereby producing within the chamber a plasma of positive gas ions, secondary electrons and ions, desorbed gases and photons. The positive gas ions within the plasma are attracted to and impact the target cathode, causing mainly neutral target atoms and secondary electrons to be ejected from the target surface by kinetic energy transfer. The dislodged neutral target atoms impact and condense on the substrate, forming a thin film of the target material or its reactants.
It was recognized early in the development of thin film sputtering processes that utilizing only an electric field (diode sputtering), while producing uniform thin films, resulted in a low rate of deposition. The total number of ions bombarding the target surface during a given time period was relatively low, yielding a low sputtering rate and, consequently, a low rate of deposition. Therefore, this method was not suitable for rapid thin film deposition or to form relatively thick deposition layers.
To improve the deposition rate, the so called magnetron sputtering method has been introduced, wherein a magnetic field is superimposed on the electric field within the sputtering chamber. The magnetic field is created by a magnet assembly comprising permanent and/or electromagnets, usually placed behind and in close proximity to the back surface of the target cathode. The magnets are generally oriented such that their magnetic axes are parallel or perpendicular to the plane of the target. The conventional magnet assembly of the prior art comprises a central core and outer magnets, in opposite magnetic orientation perpendicular to a magnetically permeable, planar base pole piece supporting and physically connecting both magnets. The magnetic flux, exiting from one pole and returning to the opposite pole, crosses the target twice, forming the convexly arched magnetic field. When properly placed and oriented, the magnets produce a closed loop tunnel within and immediately above the sputtering surface of the target. Secondary electrons, ejected from the negatively charged target surface by impacting positive gas ions, are trapped in the closed loop magnetic field tunnel of the prior art. Primarily through collisions between these trapped secondary electrons and chamber sputtering gas molecules, positive ionization is increased. The plasma discharge density and the ionization efficiency of the discharge current produced by the electric field are thereby enhanced. The enhanced plasma density increases the sputtering rate of the target material, since the total number of ions available near the target for impact with its sputtering surface has increased. A correspondingly high rate of deposition is achieved.
As previously described above, in order to enhance sputtering efficiency, it is desirable to produce and confine the ions and electrons in the glow discharge plasma as close as possible to the surface of the target material. It is also, however, desirable that the plasma density in the discharge be uniform over as much of the target surface as possible in order to erode the largest possible fraction of the target volume. Sputtering targets are generally expensive to produce. Although spent targets may be reworked into new targets, any increase in target utilization results in direct savings of target cost, and indirect savings of reduced chamber downtime for target replacement. A significant limitation of the above described magnetron prior art is that erosion of the planar target takes place in a relatively narrow band within the tunnel width and along the closed loop shape of the magnetic field.
Secondary electrons, ejected from the target under the influence of the electric field normal to the target surface plane, are ejected nearly perpendicularly. As is well known by those of ordinary skill in the art, the component of the convexly arched magnetic field extending parallel to the target surface deflects the electron movement along the path of the magnetic tunnel, causing electrons within the glow discharge to gain a net velocity, with the magnitude and direction of the electron velocity vector being given by the vector cross product of the electric field vector E and the magnetic field vector B (known as the E×B drift velocity). In the region just above both poles of the magnet assembly, the arched magnetic field is almost perpendicular to the target surface, resulting in a very small parallel component. Therefore the electrons can easily escape from the magnetic tunnel. As a result, the ionization region is limited to a narrow band across the arched magnetic tunnel width and along its closed-loop path.
Within the tunnel, the interaction of the drift velocity with the component of the magnetic field perpendicular to the target surface causes another force on the electrons, (V×B), in the direction perpendicular to both the magnetic flux lines and the velocity. These lateral forces “pinch” the electrons in the glow discharge toward the center of the arched tunnel from both sides. This pinching causes the plasma density and, therefore, the sputtering erosion of the target to be highest along the center of the closed-loop path of the magnetic tunnel. As the sputtering erosion proceeds into the target volume, the convexly arched magnetic field, and in particular its perpendicular component, becomes increasingly stronger, causing stronger pinching and typically producing an acute V-shaped erosion groove in the target, centered along the width of the closed-loop path. The fraction of the target volume which has been sputtered by the time the bottom of the erosion groove reaches the back of the target, called the target utilization, is rather low (typically around 10 to 20%) for a conventional magnetron sputtering cathode apparatus.
Various devices in the prior art have been introduced to increase target utilization in magnetron sputtering. For example, electromagnetic sources have been employed to reshape the curvature of the plasma confining magnetic tunnel into a less convex shape, thereby reducing the field component perpendicular to the target surface and increasing the parallel field component

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