Fabrication of clad hollow cathode magnetron sputter targets

A method is provided for forming clad hollow cathode magnetron sputter targets that are lighter in weight and/or less expensive than monolithic targets. A plate of sputter target material is bonded to a sheet of cladding material that is lighter in weight and/or less expensive than the sputter target material. This clad target assembly is then formed into a hollow cathode magnetron sputter target, such as by deep drawing. The clad hollow cathode magnetron further provides greater percent utilization of sputter target material than monolithic targets.

FIELD OF THE INVENTION
 This invention relates to a method of fabricating hollow cathode magnetron
 sputter targets for use in the physical vapor deposition of thin films on
 semiconductor devices.
 BACKGROUND OF THE INVENTION
 In the semiconductor industry, there is a constant need for faster and
 smaller integrated circuit chips to manage an ever-growing assortment of
 sophisticated applications. Thus, the semiconductor industry is edging
 toward mass production of circuit devices with sub-0.25 .mu.m features. To
 produce these devices, significant changes in all aspects of manufacturing
 are needed, including new materials and manufacturing technologies. The
 sub-0.25 .mu.m features, along with their increased aspect ratios,
 compared to current devices, present a significant challenge to current
 physical vapor deposition (PVD) and metallization technologies.
 A new development in physical vapor deposition (PVD) technology that
 addresses the challenges of the sub-0.25 .mu.m devices is the hollow
 cathode magnetron (HCM), which is a high density plasma device for use as
 an ionized PVD sputter target source for semiconductor device fabrication.
 This HCM sputtering source in an ionized PVD application is a low-cost,
 high-performance alternative to traditional PVD technologies.
 HCM technology, when used in an ionized PVD environment, facilitates more
 efficient production of ions of target material that are directed at right
 angles to a substrate being coated for efficient via filling. This
 technology provides a highly directional deposition that is relatively
 unaffected by feature width. It offers excellent bottom coverage of
 high-aspect-ratio features without the use of a collimator. The HCM
 sputter target, as currently designed, has a cup shape. HCMs are an
 attractive alternative to planar cathodes because the cup design enables
 superior filling of high aspect ratio features. A special arrangement of
 oriented permanent magnets is mounted on the exterior wall of the target,
 which creates a high-density plasma inside the target region. This HCM
 design allows deposited neutrals (i.e., neutral polarity atoms) to be
 recycled until they are ionized by the high-density plasma. A longer
 target lifetime results from this recycling effect. Other HCM advantages
 include longer shield life, extended maintenance intervals and
 significantly lower cost of ownership than other sputtering technologies.
 This deposition technology meets and exceeds the demands of the
 semiconductor industry for sub-0.25 .mu.m devices. Specifically, HCM
 ionized source technology enables high quality Ta, TaN, Cu, Ti, TiN, and
 other films to be deposited into sub-0.25 .mu.m dual damascene structures.
 HCM targets have generally been fabricated as monolithic targets by casting
 a billet of target material and then forming the billet into the specially
 designed HCM target by known metal forming techniques, such as forging or
 deep drawing. The formed target is then machined to final dimensions. The
 target materials are generally quite expensive and heavy. As target
 dimensions continue to increase to meet industry demands, the monolithic
 targets are becoming more expensive, heavier and more difficult to handle.
 Furthermore, erosion of particles from the sputter target surface generally
 occurs in a relatively narrow ring-shaped region, called the "racetrack
 region." Only the portion of the total target material in the racetrack
 region is consumed before the target must be replaced. For monolithic HCM
 sputter targets, typically only 25% or less of usable target material
 falls within the racetrack region and is therefore actually sputtered. The
 large amount of remaining expensive, usable target material is either
 wasted or must be recycled.
 There is thus a need to develop a method for fabricating inexpensive,
 lightweight HCM targets to accommodate the continuing need for increased
 target dimensions, and further to develop such a target having a higher
 percentage utilization of sputter target material.
 SUMMARY OF THE INVENTION
 The present invention provides a clad HCM sputter target having a sheet of
 lightweight and/or inexpensive, low purity cladding material bonded to a
 plate of sputter target material, the sputter target material preferably
 having a fine, uniform microstructure. This clad HCM sputter target is
 lighter in weight and/or less expensive than monolithic HCM sputter
 targets and provides a greater percentage utilization of sputter target
 material. To this end, and in accordance with the principles of the
 present invention, a sputter target material is formed into a plate, such
 as by pressing and/or rolling, and preferably heat treated to develop a
 fine, uniform microstructure. This plate of target material is then bonded
 to a sheet of lightweight and/or inexpensive cladding material, such as by
 diffusion bonding, explosion bonding, friction welding, epoxy bonding,
 soldering or brazing, to form a clad target assembly having a lighter
 weight and/or costing less than monolithic sputter targets of equal
 dimensions. Preferably, the bonding method is one in which the
 microstructure of the target material is not substantially altered, such
 as explosion bonding.
 The clad target assembly is then formed into a HCM sputter target, such as
 by deep drawing, forging, hydroforming, explosive forming, punching, roll
 forming, stretch forming or electromagnetic forming. The HCM sputter
 target is preferably formed by deep drawing, whereby the microstructure of
 the target material is not significantly altered. The total amount of
 sputter target material used to form the clad HCM sputter target of the
 present invention is less than with monolithic assemblies, yet the same
 amount of sputter target material is available in the racetrack region for
 sputtering.
 A flange of relatively inexpensive, commercial grade material may then be
 attached to the HCM sputter target, such as by mechanical joints, E-beam
 welding, brazing or epoxy bonding.
 There is thus provided a HCM sputter target assembly capable of meeting
 increased target dimension requirements without becoming prohibitively
 expensive or heavy. There is further provided a HCM sputter target with
 increased percentage utilization of sputter target material.
 These and other objects and advantages of the present invention shall
 become more apparent from the accompanying drawings and description
 thereof.

DETAILED DESCRIPTION
 A clad HCM sputter target assembly, preferably having fine, uniform grains,
 is fabricated by a process including bonding a plate of sputter target
 material to a lightweight and/or inexpensive cladding material and forming
 the clad assembly into a HCM assembly. Where a fine, uniform
 microstructure is obtained in the sputter target material prior to bonding
 to the cladding material, the steps of bonding the sputter target material
 to the cladding material and forming the clad assembly into a HCM sputter
 target assembly preferably are accomplished by methods which do not
 substantially alter the microstructure of the sputter target material.
 Where a fine, uniform microstructure does not yet exist before bonding,
 the bonding process itself may be used to alter the microstructure, or
 alternatively, the formed HCM sputter target assembly may be
 recrystallization annealed to obtain the desired microstructure.
 To this end, and in accordance with the principles of the present
 invention, a sputter target material is first fabricated into a plate,
 such as by pressing and/or rolling or by any other appropriate, well-known
 metalworking operation, with or without intermediate annealing. Depending
 on the particular target material used, pressing and rolling may be
 performed either at room temperature or elevated temperature. Typical
 dimensions for the fabricated plate for 200 mm diameter semiconductor
 wafer applications are 25 inch by 25 inch by 0.3 inch. Larger dimensions
 are expected for 300 mm diameter semiconductor wafer applications.
 The sputter target material is a metal, metal oxide, metal silicide or
 alloy which is to be deposited onto a semiconductor wafer, and is
 advantageously a highly pure material, preferably having a purity of 99%
 to 99.99999%. These materials include, for example, pure metals, alloys,
 suicides and oxides of tantalum, titanium, tungsten, copper, nickel,
 chromium, aluminum, cobalt, molybdenum, silver, gold, platinum, ruthenium,
 rhodium, palladium, iron, bismuth, germanium, niobium and vanadium.
 Preferred suicides include those of tantalum, titanium, tungsten, nickel,
 chromium, cobalt, molybdenum and platinum. For example, a plate may be
 formed out of tantalum, titanium, tungsten, copper or aluminum for
 deposition of thin films of Ta, TaN, Ti, TiN, W, AlCu and Cu. These
 sputter targets may be used for such applications as deposition of Ti/TiN
 liner/barrier layers for tungsten plug and aluminum fill as well as
 advanced TaN/Cu barrier/seed processes for copper interconnects.
 Referring to FIGS. 1 and 2, the plate 10 of sputter target material is
 bonded to a sheet 12 of cladding material to form a clad target assembly
 14. The cladding material is preferably a lightweight material, such as
 copper, aluminum or titanium, and/or is a less expensive material than
 that used for the sputter target material, such as commercial grades,
 which are substantially lower purity materials. The cladding material is
 preferably a good thermal conductor such that heat generated at the target
 sputter surface 16 is quickly dissipated. It is also preferred that the
 cladding material be ductile and malleable to enable the bonded materials
 to be formed into the cup-shaped structure typical of HCM sputter targets.
 For optimum results, the sheet 12 of cladding material should be of
 similar dimension to the plate 10 of sputter target material.
 Any number of known bonding techniques may be used to bond the plate 10 of
 sputter target material to the sheet 12 of cladding material. These
 techniques include diffusion bonding, explosion bonding, friction welding,
 epoxy bonding, soldering and brazing.
 In one embodiment of the present invention, the plate of sputter target
 material is heat treated prior to bonding to develop a fine, uniform
 microstructure. It is well known that fine, uniform grains increase target
 performance and improve thin film uniformity. In this embodiment, it is
 preferable that the bonding step not significantly alter this
 microstructure. In explosion bonding, the heat generated from detonation
 of the explosives is generated for an insufficient time for heat transfer
 throughout the sputter target material, and thus there is no appreciable
 temperature increase in the target material to produce grain growth. Thus,
 the explosion bonding technique is capable of bonding the sputter target
 plate 10 to the cladding sheet 12 without any significant alteration of
 the microstructure of the sputter target material formed by prior heat
 treating. In friction welding, the plate 10 of sputter target material is
 pressed against the cladding sheet 12 and rotated at high pressure to
 create heat and friction at the interface 18 to melt the materials
 together. Although there is some change of the microstructure in the area
 of the interface 18 of the clad target assembly 14, the change in
 microstructure does not generally permeate throughout the thickness of the
 sputter target material. Thus, where a fine, uniform grain structure is
 achieved in the sputter target material prior to bonding, explosion
 bonding and friction welding are preferred methods for bonding the plate
 10 of target material to the cladding sheet 12.
 In another embodiment of the present invention, the plate of sputter target
 material is not heat treated prior to bonding to the cladding material.
 Rather, the fine, uniform microstructure is obtained via the bonding
 process. In diffusion bonding, for example, high temperatures are used,
 which alter the microstructure of the materials being bonded. The
 temperature and duration of the bonding process may thus be designed so as
 to alter the sputter target material to achieve fine, uniform grains while
 simultaneously bonding the target material to the cladding material.
 Referring to FIG. 3, this clad target assembly 14 is then formed into a
 near net-shaped HCM sputter target 20 by a suitable metalworking
 operation. By near net-shaped is meant that the sputter target 20 is close
 to the final shape with only minimal final machining necessary to achieve
 the desired final dimensions. Suitable metalworking operations include
 deep drawing, forging, hydroforming, explosive forming, punching, roll
 forming, stretch forming and electromagnetic forming. Where a fine,
 uniform microstructure has been obtained in the sputter target material
 either by heat treating the plate of sputter target material prior to
 bonding or by the bonding process itself, deep drawing is a preferred
 forming method because no significant alteration of the microstructure of
 the sputter target material occurs by this method. The deep drawing
 operation may be carried out at room temperature to further ensure that
 the microstructure is not altered, or it may be carried out at elevated
 temperatures if the type of material requires it.
 Again referring to FIG. 3, HCM targets 20 are typically provided with a
 flange 22 for attaching the sputter target 20 to sputtering machines (not
 shown). The flange 22 of known or future-developed configuration may be
 attached to the near net-shaped HCM sputter target 20 by any permissible
 means, including E-beam welding, brazing, epoxy bonding or mechanical
 fastening. The flange 22 may be permanently or releasably attached to the
 HCM sputter target 20. The flange 22 is preferably constructed using a
 commercial grade material having sufficient structural integrity for
 fastening into the sputtering machine, such as commercial grades of copper
 and titanium.
 The HCM sputter target 20 of the present invention can then be machined to
 final dimensions for use in physical vapor deposition of thin films onto
 semiconductor wafers.
 In yet another embodiment of the present invention, where a fine, uniform
 microstructure has not been obtained in the sputter target material either
 by heat treating prior to bonding or by the bonding process, the HCM
 sputter target may be subjected to a recrystallization annealing to refine
 the microstructure. This may occur prior to attaching the flange, prior to
 final machining or after final machining. According to the principles of
 any of the embodiments described herein, the clad HCM sputter target of
 the present invention may be fabricated such the microstructure of the
 sputter target material comprises fine, uniform grains.
 The overall dimensions of the clad HCM sputter target of the present
 invention remain the same as prior monolithic assemblies, but the percent
 utilization of sputter target material increases for the present
 invention. By way of example only, and not intended to represent
 commercially accurate values, a monolithic assembly of desired dimensions
 may contain 20 lbs. of sputter target material, with only 5 lbs. of
 sputter target material available in the racetrack region for sputtering.
 Thus, the percent utilization of sputter target material is only 25%. With
 a clad target assembly of the present invention having the same desired
 dimensions, only 10 lbs. of sputter target material is used for the plate,
 which is bonded to a 7-lb. sheet of cladding material, with the same 5
 lbs. of sputter target material available in the racetrack region for
 sputtering. Thus, the percent utilization of sputter target material is
 increased to 50% by the present invention.
 There is thus provided by the principles of the present invention a HCM
 sputter target comprising a material for sputtering bonded to a
 lightweight and/or inexpensive sheet of cladding material, the overall
 assembly being capable of meeting large target dimension requirements at
 less cost and lower weights than monolithic target assemblies, while
 providing increased percent utilization of target material.
 While the present invention has been illustrated by the description of an
 embodiment thereof, and while the embodiment has been described in
 considerable detail, it is not intended to restrict or in any way limit
 the scope of the appended claims to such detail. Additional advantages and
 modifications will readily appear to those skilled in the art. For
 example, the principles of the present invention may be used for sputter
 targets of any desired material, not just those specifically listed
 herein. The invention in its broader aspects is therefore not limited to
 the specific details, representative apparatus and method and illustrative
 examples shown and described. Accordingly, departures may be made from
 such details without departing from the scope or spirit of applicant's
 general inventive concept.