Fabrication and bonding of copper sputter targets

Details Fabrication and bonding of copper sputter targets Fabrication and bonding of copper sputter targets

2000-12-05

2002-11-12

Ip, Sikyin (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Heating or cooling of solid metal

C148S682000

Reexamination Certificate

active

06478902

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of fabricating high purity copper sputter targets to both decrease particle generation in the sputtering process and enhance film uniformity on substrates, such as semiconductor wafers used in the manufacture of semiconductor devices and circuits.
BACKGROUND OF THE INVENTION
Sputtering refers to a process that involves the coating of a semiconductor wafer or other substrate mounted within a processing chamber. This chamber contains an inert gas ionized by an electric field and a sputter target spatially opposed to the wafer. The sputter target contains an electrical bias to the wafer. Ions from the gas bombard the target and dislodge atoms from the target to deposit target material onto the wafer.
In the manufacture of sputter targets used in the semiconductor industry, and more particularly sputter targets used in physical vapor deposition (PVD) of thin films onto complex integrated circuits, it is desirable to produce a sputter target that will provide: 1) film uniformity; 2) high deposition rates; 3) minimal particle generation during sputtering; and 4) good conductivity for connecting transistors. For example, sputtering aluminum and aluminum alloy sputter targets deposits thin electrically conductive films on integrated circuits for interconnect purposes. Copper, however, has the potential to become an alternative to aluminum in interconnect technology. Copper has both a higher electrical conductivity and a higher resistance to electromigration than aluminum. Other potential benefits of using copper interconnect films include the reduction of both power dissipation and interconnect signal delay.
Larger and less uniform grain sizes decrease target performance. Moreover, it is known that crystallographic orientation of the sputter target and the distribution of material ejected from the target affect film uniformity and sputter deposition rate. It is also known that the sputtering of atoms from the target occurs preferentially along the close packed directions of the target material and that a near random grain orientation provides better uniformity of the sputtered films.
Prior processes for producing aluminum or copper targets provide either (200) or (220) oriented crystalline structures. But targets having a strong (200) or (220) crystalline orientation, however, generate films having poor uniformity. Thus, it is desirable to have a target with a random or weak orientation.
To control grain size, copper sputter targets may contain second phase alloy precipitates of up to 10 &mgr;m in size. But poor conductivity of the large second phase precipitates can generate localized arcing during sputtering and deposit disadvantageous high density or large particles. Furthermore, the use of a second phase to control grain size does not provide effective control for copper targets having a purity of 99.99 percent or higher.
In a conventional target cathode assembly, a single bonding surface attaches the target to a nonmagnetic backing plate, typically an aluminum or copper backing plate. This forms a parallel interface between the sputter target and backing plate in the assembly. The backing plate provides a means for holding the target in the sputtering chamber and provides structural stability for the target. Also, water cooling the backing plate removes heat generated by the ion bombardment of the target. Attaching the target and the backing plate by a technique, such as soldering, brazing, diffusion bonding, clamping, screw fastening or epoxy cementing achieves good thermal and electrical contact between the target and the backing plate. Unfortunately, solder bonds are susceptible to debonding during the sputtering operation. Furthermore, the relatively low joining temperatures associated with the “soft” solders reduce the target's temperature range for sputtering. Thus, solder-bonded assemblies are more costly and time-consuming to the consumer because the target has to be used at a lower power level to prevent separation of the target from the backing plate. This results in a decreased sputtering rate.
Diffusion bonding, particularly with a pre-treated, roughened surface, provides a stronger bond. But preparation for diffusion bonding is time-consuming. More importantly, the high temperatures involved in diffusion bonding change the microstructure obtained during pre-bonding processing. Therefore, even if fine grain size and random orientation can be achieved during the target manufacturing stage, they are lost by current diffusion bonding techniques. For pure copper targets, the diffusion bonding has the effect of nearly doubling the grain size. Thus, debonding and alteration of the microstructural and metallurgical characteristics are significant disadvantages of prior diffusion bonding techniques that make them undesirable for copper target assemblies in which small, uniform grains are desirable in the sputter target.
The alternative of using monolithic sputter targets without backing plates also becomes less feasible in view of the continuing increase in target diameters required for sputtering larger size silicon wafers and the increasing purity requirements of target materials, which both result in an increased cost for monolithic targets.
There is thus a need to provide a method of fabricating pure copper target assemblies having a sputter target of fine, equiaxed, uniform grain structure and random crystallographic structure strongly bonded to a non-magnetic backing plate.
SUMMARY OF THE INVENTION
The method is used to fabricate pure copper sputter targets. It includes first heating a copper billet to a temperature of at least about 500° C. The copper billet has a purity of at least 99.99 percent. Then warm working the copper billet applies at least about 40 percent strain. Cold rolling the warm worked copper billet then applies at least about 40 percent strain and forms a copper plate. Finally, annealing the copper plate at a temperature above about 250° C. forms a target blank. The target blank has equiaxed grains having an average grain size of less than about 40 &mgr;m. The target blank's crystallographic structure contains grains of the (111), (200), (220) and (311) orientations with the amount of the target blank's grains having each of the orientations being less than about 50 percent.
DETAILED DESCRIPTION
Fabricating high purity copper by a process including the steps of heating, warm working, cold working and annealing produces a sputter target having fine, uniform grains in random orientation. This process applies these steps to high purity copper ingots, billets, plates or any other form suitable for subsequent working operations. The high purity copper has a purity of at least 99.99 percent. Advantageously, the copper has a purity of at least 99.999 percent. Most advantageously, the copper has a purity of at least 99.9999 percent for limiting the generation of impurity-containing particles. Most advantageously, explosion bonding this sputter target to a backing plate secures the backing plate without altering the size or orientation of the grains.
First, heating high purity copper to a temperature above about 500° C. prepares the copper for warm working. Advantageously, this step preheats the copper for at least one half an hour to ensure uniform heating. Furthermore, high purity copper billets with a thicker cross section may require a longer preheating time. Advantageously, heating the high purity copper to a temperature in the range of about 500 to 750° C. for a period of about 1 to 6 hours prepares the billet for warm working. Heating the high purity copper to a temperature in the range of about 600 to 700° C. for a period of about 1 to 6 hours improves the final grain size after annealing. Heating the copper billets to temperatures between 625 to 675° C. produces the most advantageous results. The atmosphere in which the material is preheated is not critical. The material may be heated under ambient conditions, or may be heated in a protective atmosphere so as to minimize

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