Patent Application
20060062686
The invention pertains to physical vapor deposition (PVD) target support members, including backing plates, and methods of making PVD target support members.
Physical Vapor Deposition (PVD) includes a variety of processes with broad applicability. In the context of electronic materials, particularly those used in integrated circuits, device scaling to ever decreasing dimensions prompts continuous review of PVD for improvements in performance. With aggressive competition, cost improvements are continuously sought. Generally, improvement efforts focus on sputtering components, for example, target materials and accessories used directly in the PVD process. However, given the high level of performance already obtained for many sputtering components and their compositions, focus is beginning to include support members of PVD targets.
Support members, which may include backing plates, were previously viewed as less significant in seeking performance enhancements since they typically do not contribute directly to the quality of deposited materials. Even so, PVD target support members may now present opportunities for improvement in PVD performance. As such, a need may exist for identification of support member properties that may produce improvement in deposition performance. Correspondingly, new compositions for and methods of manufacturing support members may result from identification of properties targeted for enhancement.
In one aspect of the invention, a PVD target support member includes an alloy containing at least 90 weight percent (wt %) of a first metal and also containing a second metal and a third metal. The second metal increases electrical resistivity compared to an otherwise identical alloy lacking the second metal. The third metal increases tensile and/or yield strength compared to an otherwise identical alloy lacking the third metal. By way of example, the alloy of the first, second, and third metal may exhibit a thermal stability during diffusion bonding to a target. The thermal stability may meet or exceed thermal stabilities of the otherwise identical alloy lacking the second metal and the otherwise identical alloy lacking the third metal.
In another aspect of the invention, a PVD target support member includes an alloy containing at least 90 wt % copper and also containing titanium and silver. The alloy may consist of copper, titanium, and silver.
In a further aspect of the invention, a PVD target support member includes a target mounting surface configured to receive and to contact a target. The mounting surface includes an alloy containing at least 90 wt % copper and also containing titanium and silver. By way of example, the mounting surface may consist of the alloy. The support member may be a backing plate. The support member may be diffusion bonded at the mounting surface with a target. Exemplary targets include those containing a refractory metal.
Preferred embodiments of the invention are described below with reference to the following accompanying drawings.
Observation has indicated that PVD target assemblies having a backing plate combined with a target, such as a sputtering target, may develop performance problems during some types of PVD processes as a result of eddy currents occurring in the assembly during deposition. The term “eddy current” refers to an induced electric current circulating wholly within a mass of conductive material, as known to those of ordinary skill, and might also be referred to as a Foucault current. Eddy current problems may be particularly evident during self-ionized plasma PVD (SIP PVD), a conventional PVD method. According to aspects of the invention described herein, one way to reduce the occurrence of an eddy current is to increase resistivity of a support member in the PVD target assembly, such as a backing plate.
Conventionally, conductivity and/or resistivity may be gauged by comparison to the International Annealed Copper Standard (IACS) by expressing conductivity as a percentage of the conductivity value for pure copper (i.e., the annealed copper standard, 0.580 microOhm−1-centimeters−1 (μOhm−1-cm−1)). Accordingly, a material with a rating greater than 100% IACS is more conductive than the copper standard. A material with a rating less than 100% IACS is less conductive than the copper standard. As the IACS percentage rating decreases, it follows that the material becomes more resistive. In addressing the problem of eddy currents in PVD target assemblies, a support member may have an IACS rating of 90% IACS or less, or preferably less than about 50% IACS. The optimum IACS rating may vary depending upon magnet design in a PVD apparatus. With the principles described herein, those of ordinary skill may optimize IACS rating to a particular PVD apparatus. Of course, those of ordinary skill can also easily convert the IACS ratings to other resistivity units. Preferably, eddy currents may be inhibited even under PVD apparatus magnet speeds of 60 to 90 RPM.
An exemplary PVD assembly having a backing plate and target is shown in
One trend in PVD target designs includes manufacturing the target with decreasing thickness in relation to its diameter. One of several motivations for the thin target design includes the increasing cost of high purity target materials. However, PVD target assemblies incorporating thin targets may be more prone to deformation problems during deposition as the target erodes away. One conventional solution to the deformation problems associated with thin targets includes a laminated construction that provides a stiff intermediate support member between a backing plate (the primary support member) and a target. Accordingly, the added intermediate support member provides the strength otherwise lacking from the conventional backing plate.
According to aspects of the invention described herein, a PVD target support member, such as a backing plate, is provided with increased tensile and/or yield strength so as to provide sufficient mechanical strength and stiffness. It will be appreciated that an intermediate support member in a laminated construction may instead or additionally include an alloy providing the improved mechanical strength and stiffness.
A further problem with conventional PVD target assemblies includes deleterious changes in physical and/or mechanical properties of support members during diffusion bonding. Depending upon the particular materials included in the support member and target, diffusion bonding conditions may involve temperatures ranging from about 300° C. for copper targets to about 900° C. for tantalum targets. At lower temperatures in the diffusion bonding range, changes in support member properties typically are a less significant concern. However, in the upper range of bonding temperature, reduction in strength and stiffness of materials is likely to occur. For some materials, loss of strength/stiffness may be relatively minor such that a PVD target assembly may still perform adequately. Losses in other support member materials may be significant enough to preclude their use with targets diffusion bonded at high temperatures. Accordingly, a further aspect of the invention described herein includes support member materials exhibiting sufficient thermal stability to allow diffusion bonding even at high temperatures. Bond strength in excess of 20 ksi may thus be achieved even with tantalum targets.
According to one aspect of the invention, a PVD target support member includes an alloy containing at least 90 weight percent (wt %) of a first metal and also containing a second metal and a third metal. The second metal increases electrical resistivity compared to an otherwise identical alloy lacking the second metal. The third metal increases tensile and/or yield strength compared to an otherwise identical alloy lacking the third metal. By way of example, the alloy of the first, second, and third metal may exhibit a thermal stability during diffusion bonding to a target. The thermal stability may meet or exceed thermal stabilities of the otherwise identical alloy lacking the second metal and the otherwise identical alloy lacking the third metal.
Observation has indicated that the desirable properties described above can be obtained for at least some metals typically used for PVD target support members by adding relatively small amounts of the second and/or third metal. For example, the alloy may contain at least 98.5 wt % of the first metal. Such circumstance is clearly the case where the first metal consists of copper. For copper, and perhaps other metals, the alloy may contain no more than 1.5 wt % of the second metal and no more than 1.0 wt % of the third metal. Preferably, the alloy contains no more than 0.3 wt % of the second metal and no more than 0.2 wt % of the third metal. The alloy may consist of the first metal, the second metal, and the third metal.
Alternatively, it is conceivable that the alloy may contain the three described metals to provide the desired properties while perhaps including other materials, even nonmetals, perhaps to address other properties desired for a PVD target support member. In the context of the present specification, the term “metal” includes all of the elements listed in the Periodic Table known as metallic as well as elements listed as semi-metallic, which includes boron, silicon, arsenic, selenium, tellurium, and astatine. All other elements in the Periodic Table are considered nonmetals.
The second metal may consist of titanium which has been discovered as an element that significantly increases electrical resistivity of copper when added in relatively small amounts. Titanium might exhibit similar properties in other metal alloys. The third metal may consist of silver which has been discovered to significantly increase tensile and/or yield strength of copper when added in relatively small amounts. Silver may exhibit similar properties in other metals. Notably, silver does not significantly affect electrical resistivity when added to copper.
Observation has indicated that addition of titanium to copper contributes to tensile and/or yield strength in the absence or presence of silver. However, titanium is added to regulate electrical resistivity and further improvements in tensile and/or yield strength augment the improvement otherwise obtained by adding silver. This presents a highly advantageous circumstance wherein material properties can be finely tuned to particular applications. Generally speaking, resistivity of the alloy may be tuned from 100 to 10% IACS by varying titanium content respectively from 0 to 1.5 wt %.
With the desired electrical resistivity established in the copper-titanium alloy, silver may be added to achieve a desired tensile and/or yield strength while maintaining the desired electrical resistivity.
In addition, the CuTiAg alloy has been observed to exhibit a thermal stability during heat treatment that exceeds thermal stability of copper-titanium alloy and of copper-silver alloy. With the improved thermal stability, high temperature diffusion bonding processes may be relied upon to attach PVD targets to support members containing CuTiAg alloys. Although indirectly related to thermal stability, synergistic effects have been observed with regard to grain size properties and hardness of CuTiAg.
Significant knowledge exists in the art regarding improvement of strength and stiffness in PVD target support member alloys, such as copper alloys and aluminum alloys. However, increasing tensile and/or yield strength along with increasing electrical resistivity by the addition of specific elements has not been demonstrated. In addition, the improvement in thermal stability during diffusion bonding that meets or exceeds thermal stabilities of the otherwise identical alloys lacking one or the other of the two added elements has not been demonstrated.
Thus, an alloy according to the present aspect of the invention possesses significant advantage for use generally as a PVD target support member but, specifically, possesses an advantage for use in a backing plate. Copper is commonly used as backing plate material and most or all of the properties motivating use of copper as a backing plate material may be retained while adding small amounts of a second and third metal to increase electrical resistivity and to increase tensile and/or yield strength. For example, properties of the alloy such as density and thermal conductivity appear similar to conventional copper alloy backing plate materials presently in use. In addition, the alloy may exhibit improved thermal stability during diffusion bonding.
Turning to
Turning to
Second, at temperatures below about 350° C., CuTiAg exhibits a lower hardness reading than the CuTi alloys, all of which contain more wt % Ti. However, it appears that CuTiAg will exhibit greater thermal stability in strength/stiffness than any of the CuTi alloys since the higher temperatures do not affect CuTiAg hardness as significantly as CuTi hardness. An important factor in maintaining CuTiAg hardness observed below 350° C. appears to be the significant increase in hardness at about 400° C. None of the CuAg or CuTi alloys display a significant hardness increase. Accordingly, the hardness readings exhibited by CuTiAg cannot be accounted to Ti or Ag alone or be viewed as merely an additive effect. Rather, synergism appears to produce the observed advantages.
Matching coefficient of thermal expansion (CTE) of a support member to a target may also be advantageous. Alloys with high CTE, such as CuZn shown in
According to another aspect of the invention, a PVD target support member includes an alloy containing at least 90 wt % copper and also containing titanium and silver. The alloy may consist of copper, titanium, and silver. In a further aspect of the invention, a PVD target support member includes a target mounting surface configured to receive and to contact a target. The mounting surface includes an alloy containing at least 90 wt % copper and also containing titanium and silver. By way of example, the mounting surface may consist of the alloy. The support member may be a backing plate. The support member may be diffusion bonded at the mounting surface with a target. Exemplary targets include those containing a refractory metal.
It will appreciated by those of ordinary skill that perhaps the simplest manner of incorporating the alloys described herein into a PVD target assembly is to fabricate a backing plate entirely from the alloy. However, it will also be appreciated that a variety of alternative configurations are possible where the alloy may instead be incorporated only into a mounting surface of a backing plate that contacts a target or provided in an intermediate support member positioned between a backing plate and target in a laminated construction. Other configurations are conceivable.
The second and third metals may be incorporated into a first metal by conventional alloy forming methods. Similarly, support members, such as backing plates, may be fabricated using the alloy in accordance with conventional methods. Even so, according to a still further aspect of the invention, a method of making a PVD target support member includes selecting a first metal to form at least 90 wt % of an alloy also containing a second metal and a third metal. The second metal is selected to increase resistivity compared to an otherwise identical alloy lacking the second metal. The third metal is selected to increase tensile and/or yield strength compared to an otherwise identical alloy lacking the third metal. A PVD target support member is formed including the alloy of the first, second, and third metals.
The invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims.
