Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2000-02-02
2001-12-18
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298120, C420S427000, C148S422000, C428S457000
Reexamination Certificate
active
06331233
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the processing of high-purity tantalum to produce a sputtering target with a microstructure that is desirable for uniform sputtering. In particular, the invention relates to the manufacture of high-purity tantalum with a mean grain size of less than 100 &mgr;m and a uniform, predominately (111)<uvw> crystallographic texture throughout the target thickness.
BACKGROUND OF THE INVENTION
Tantalum is currently used extensively in the electronics industry, which employs tantalum in the manufacture of highly effective electronic capacitors. Its use is mainly attributed to the strong and stable dielectric properties of the oxide film on the anodized metal. Both wrought thin foils and powders are used to manufacture bulk capacitors. In addition, thin film capacitors for microcircuit applications are formed by anodization of tantalum films, which are normally produced by sputtering. Tantalum is also sputtered in an Ar—N
2
ambient to form an ultra thin TaN layer which is used as a diffusion barrier between a Cu layer and a silicon substrate in new generation chips to ensure that the cross section of the interconnects can make use of the high conductivity properties of Cu. It is reported that the microstructure and stoichiometry of the TaN film are, unlike TiN, relatively insensitive to the deposition conditions. Therefore, TaN is considered a much better diffusion barrier than TiN for chip manufacture using copper as metallization material. For these thin film applications in the microelectronics industry, high-purity tantalum sputtering targets are needed.
The typical tantalum target manufacturing process includes electron-beam (EB) melting ingot, forging/rolling ingot into billet, surface machining billet, cutting billet into pieces, forging and rolling the pieces into blanks, annealing blanks, final finishing and bonding to backing plates. The texture in tantalum plate is very dependent on processing mechanisms and temperatures. According to Clark et al. in the publication entitled “Effect of Processing Variables on Texture and Texture Gradients in Tantalum” (
Metallurgical Transactions A,
September 1991), the texture expected to develop in cold-rolled and annealed bcc metals and alloys consists of orientations centered about the ideal orientations, {001}<110>, {112}<110>, {111}<110>, and {111}<112>. Generally, conventionally processed tantalum is forged or rolled from ingot to final thickness, with only one (1) or no intermediate annealing stages. A final anneal is usually applied to the plate simply to recrystallize the material. The direction of the deformation influences the strengths of resulting annealed textures but generally little attention is given to the resulting distribution of textures. In conventionally processed tantalum, significant texture variation exists in the cross-section of the plate, as described by Clark et al. (August 1992), Raabe et al. (1994), Michaluk (1996). Typically the above mentioned textures exist in stratified bands through the thickness of the rolled plate, or form a gradient of one texture on the surface usually {100}<uvw>, with a gradual transition to a different texture at the centerline of the plate, usually {111}<uvw>, Wright et al. (1994). Another cause of texture variation through the target thickness is the non-uniformity of the deformation processes used to form the plate. Texture non-uniformity results in variable sputter deposition rates and sputter surface irregularities, which in turn is believed to be a source of micro-arcing. Micro-arcing is believed to believed to be the principle cause of particle generation and is thus undesirable in the semiconductor industry.
FIG. 1
shows the sputter surface of a mixed-texture tantalum target made by conventional processing methods. The sputter surface reveals regions of two different crystallographic textures; dark areas are {100}<uvw>, lighter areas {111}<uvw>. The type of pattern illustrated in
FIG. 1
is believed to contribute to sputter filn nonuniformities because of the different sputter rates associated with each texture.
FIG. 2
shows severe textural banding in the cross-section of a sputtered tantalum target manufactured according to conventional processes. ‘Textural banding’, refers to a localized concentration of one texture in the cross section strung out over several grains in a matrix of another texture. In tantalum, it is typically {100}<uvw> textures in a matrix of the more prominent {111}<uvw> textures. For example, a series of grains with the same {100}<uvw> texture in a matrix of {111}<uvw> are aligned in an elongated manner over several grains is considered a banded textural feature. Using Electron Backscatter Diffraction, EBSD, imaging the texture in small, localized areas can be determined accurately.
In
FIG. 2
, it can be clearly seen that areas of {100}<uvw> type textures sputter at a greater rate than {111}<uvw> type textures. Thus, any textural non-uniformity at the target surface will produce surface ‘ridges’, which have an increased likelihood of causing micro-arcing.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a processing route for producing high purity tantalum sputtering targets with a mean fine grain size of less than 100 microns and uniform crystallographic texture throughout the target thickness.
The method comprises forging, rolling and annealing high-purity, vacuum-melted tantalum ingots in such a way as to eliminate remnant as-cast grain structure, and produce a homogeneous fine-grain size (mean <100 &mgr;m) microstructure with a uniform, predominately {111}<uvw> texture throughout the thickness of the target. Significant sputtering problems have been reported when the texture of the target is not uniform throughout the target thickness. Sputtering rates and film deposition rates change as a function of target crystallographic texture. This variable sputter rate across a target surface causes film thickness uniformity problems and also produces unwanted surface topography in the form of ‘ridging’, which in turn is believed to cause micro-arcing.
This invention uses a series of deformation techniques, with a minimum of three (3) intermediate, high-temperature inert-atmosphere anneals, preferably vacuum, to produce a combination of fine-grain size (mean <100 &mgr;m) tantalum targets with a uniform, predominately {111}<uvw> texture throughout the target thickness, until now unseen in the industry today. ‘Uniform texture throughout the target thickness’ refers to a homogeneous distribution of textural components with no visible banding at a resolution of 20× from the target surface to at least mid-thickness. ‘Inert’ refers to an atmosphere that is non-reactive with the tantalum-comprising mass.
Experiments associated with this invention also revealed that by controlling the annealing temperature the most desirable texture for collimated sputtering, the (111) texture, can be generated. The (111) texture is the only texture that has one of the close-packed directions aligned normal to the target surface. This direction is a dominant emission direction and is, therefore, the texture required for collimated sputtering.
The high-purity tantalum material of the present invention is preferably 3N5 (99.95%) pure and comprises less than 500 ppm total metallic impurities, excluding gases. The methods of chemical analysis used to derive the chemical descriptions set forth herein are the methods known as glow discharge mass spectroscopy (GDMS) for metallic elements and LECO gas analyzer for non metallic elements.
For the purposes of this invention, the term “sputtering target” covers not only sputtering targets in the traditional sense, but also any other component within the sputtering chamber that is likely
Honeywell International , Inc.
Nguyen Nam
VerSteeg Steven H.
Wells St. John P.S.
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