Magnetron for low pressure, full face erosion

Details Magnetron for low pressure, full face erosion Magnetron for low pressure, full face erosion

1999-03-02

2001-05-08

Huff, Mark F. (Department: 1756)

Chemistry: electrical and wave energy

Apparatus

Coating, forming or etching by sputtering

C204S298220, C204S298190, C204S298160, C204S298170, C204S298120

Reexamination Certificate

active

06228235

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to magnetrons (e.g. planar magnetrons) such as are used in plasma sputtering systems.
The term magnetron refers to magnets that are placed behind the cathode (e.g. the target) in the plasma system. The magnets produce B fields in the plasma discharge in front of the target. The term planar means that the magnets are aligned in a plane parallel to the target surface.
The magnetron source acts to increase the electron density in the plasma discharge. The magnets that are placed behind the target generate magnetic field lines, a portion of which are somewhat parallel to the face of the target. These magnetic field lines, along with E fields of the electric circuit create forces acting on the electrons which tend to trap them next to the surface of the target. This increases the probability of collisions with the gas (e.g. argon) atoms thereby producing more gas ions to bombard the target.
If the magnets are placed behind the target, nonuniform erosion of the target is likely to occur. Indeed, the erosion pattern that appears on the face of the target will identify the locations of the magnets behind the target. To prevent such nonuniform erosion patterns from developing, the magnet assembly is typically moved over the backside of the target so as to produce on average a substantially uniform magnetic field over the entire face of the target from its center to its outer edges. Achieving uniform erosion is particularly desirable for a number of reasons including prolonging target life and producing uniform bottom coverage during deposition of metal into high aspect ratio contact holes. Additionally, uniform sputtering ensures that all regions of the target are continuously eroded, thereby preventing material from back sputtering onto non-active (i.e., non-sputtered) areas and then flaking off.
Since the magnetron produces an increased sputtering efficiency, the chamber pressure can be reduced as compared to a system which does not use a magnetron source. The electrons trapped adjacent to the front of the target along magnetic field lines spaced from the target make up for the smaller number of potentially ionized atoms present at low pressure by colliding with, and ionizing numerous of the gas atoms in the low pressure atmosphere. Low pressure operation can be particularly desirable because of its many positive benefits, including better contact hole-filling due to lower gas scattering of the sputtered material and improved film properties, e.g. lower resistivity, higher density, greater stability, less contamination, etc.
Low pressure operation and uniformity in target erosion, particularly at the target's edge are, however, competing objectives. Traditionally, low pressure operation is achieved by accepting less uniform target erosion. Similarly, improved uniformity in target erosion typically results in having to operate at higher chamber pressures.
SUMMARY OF THE INVENTION
In general, in one aspect, the invention is a method for controlling the operation of a magnetron source for sputtering a surface of a target in a vacuum chamber. The method includes the steps of: (1) during a low pressure phase of sputtering in which the chamber is at a first pressure, causing a magnetic field generated by the magnet assembly to be confined primarily to an inner region of the surface of the target so as to reduce leakage of electrons away from the target during sputtering; and (2) during a subsequent high pressure phase of sputtering in which the chamber is at a second pressure, causing the magnetic field generated by the magnet assembly to extend into an outer region surrounding the inner region so as to sputter material from the outer region of the surface of the target.
In preferred embodiments, the low pressure phase uses a chamber pressure of less than 1 mTorr and the high pressure phase uses a chamber pressure of greater than 1 mTorr. The step of causing the magnetic field to be confined primarily within the inner region of the target's surface involves sweeping the magnet assembly within a first area behind the target, wherein the first area is smaller than the area of the target's surface, and the step of causing the magnetic field to extend into the outer region of the target's surface involves sweeping the magnet assembly into a region that extends beyond the first area. The step of causing the magnetic field to extend into the outer region of the target involves causing the magnetic field to spend a larger portion of time in the outer region than it spends in the inner region. The low pressure phase of sputtering and the subsequent high pressure phase of sputtering are performed on the same substrate. Alternatively, the low pressure phase of sputtering is performed on a succession of different substrates before performing the high pressure phase of sputtering.
Also in preferred embodiments, the low pressure sputtering is performed on a substrate and the method further involves after the low pressure sputtering onto the substrate, inserting a shutter between the substrate and the target; and then performing the high pressure sputtering onto the shutter.
In general, in another aspect, the invention is a magnetron source for use in a plasma system for sputtering a surface of a target. The magnetron source includes a first magnet assembly which during use generates a magnetic field which over an area in front of the target is confined primarily to an inner region of that area, and a second magnet assembly which during use generates a magnetic field which over the area in front of the target is confined primarily to the outer region of the surface of the target.
Preferred embodiments have the following features. The second magnet assembly includes a plurality of magnets positioned around a perimeter of the first magnet assembly, and it includes a lift mechanism to which the plurality of magnets are connected. During operation, the lift mechanism raises and lowers the plurality of magnets so as to decrease and increase, respectively, the magnetic field in the outer region. The lift mechanism includes one or more actuators to which the plurality of magnets are connected. The magnetron source also includes a motor which rotates the first magnet assembly during operation. The first magnet assembly includes a shaft by which the first magnet assembly is rotated and output of the motor is coupled to the shaft through a drive belt or direct drive.
In accordance with the invention, the magnetron source which utilizes a magnet assembly that is smaller than the area of the target, has two modes of operation. During an initial deposition mode of operation (e.g. to coat the bottoms of the contact holes), which is performed at low total chamber pressure (e.g. <1 mTorr), the magnet assembly is moved so that the magnetic field sweeps over and is confined within an inner region of the target. Since the magnetic field is thereby kept away from the grounded shield and other hardware near the perimeter of the target, stable low pressure operation is possible. During a subsequent deposition of cleaning mode, which is performed at high total chamber pressure (e.g. >1 mTorr), the magnet assembly is moved so that the magnetic field sweeps out to the edge of the target (i.e., into the outer region surrounding the inner region). Since a higher chamber pressure is used, the magnetic field can be brought closer to the edge of the target without destabilizing the plasma. Thus, in the high pressure mode of operation, it becomes possible to sputter the outer region of the target. The high pressure mode of operation can be used to create a more uniform thickness of the deposited layer on the substrate or, alternatively, it can be performed as a target cleaning step on a shutter after the substrate has been removed from the chamber.
By operating in the low pressure mode followed by the high pressure mode, it becomes possible to achieve a full target erosion while still obtaining the advantages of a low pressure plasma deposition.
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