Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1999-10-14
2001-05-08
Diamond, Alan (Department: 1753)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C204S192130, C204S192150, C204S192170, C204S298020, C204S298120, C204S298130, C164S418000, C164S419000, C164S271000, C164S047000, C164S459000, C164S076100, C029S527500, C029SDIG005, C029SDIG002, C148S437000, C148S438000, C148S549000, C148S551000
Reexamination Certificate
active
06228186
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates generally to physical vapor deposition (PVD) of metal films. The invention relates more specifically to DC magnetron sputtering of metals such as aluminum (Al) or aluminum alloys onto semiconductor substrates and the like for forming fine pitch metallization such as the electrically-conductive interconnect layers of modern integrated circuits.
2. Cross Reference to Related Patents
The following U.S. patent(s) is/are assigned to the assignee of the present application, and its/their disclosures is/are incorporated herein by reference:
(A) U.S. Pat. No. 5,242,566 issued Sep. 7, 1993 to N. Parker; and
(B) U.S. Pat. No. 5,320,728 issued Jun. 14, 1994 to A. Tepman.
3. Description of the Related Art
The electrically-conductive interconnect layers of modern integrated circuits (IC) are generally of very fine pitch (e.g., 10 microns or less) and high density (e.g., hundreds of lines per square millimeter).
A single, small defect in the precursor metal film that ultimately forms a metallic interconnect layer of an IC can be so positioned as to seriously damage the operational integrity of the IC. As such it is desirable to form metal films with no defects or as few, minimally sized defects as possible.
The metal films of integrated circuits are typically formed by physical vapor deposition (PVD). One low cost approach uses a DC magnetron apparatus such as the Endura™ system available from Applied Materials Inc. of California for sputtering aluminum (Al) or aluminum alloys onto semiconductor wafers.
Although such DC_magnetron PVD systems generally produce high quality metal films with relatively low defect densities, heretofore unexplained ‘blobs’ of extra material are occasionally observed in the deposited metal. These blobs can interfere with device formation and disadvantageously reduce mass production yield of operable devices.
The present inventors have isolated such blobs in DC_magnetron-formed aluminum films, have analyzed the composition and physical structures of such blobs, and have developed methods for minimizing the formation of such undesirable blobs.
SUMMARY OF THE INVENTION
The above-mentioned problems are overcome in accordance with the invention by providing an improved target for use in magnetron sputtering of aluminum, or of aluminum alloys or of like metals where the formed metal films having low defect densities.
It has been determined that the microscopic make up of the target in a DC_magnetron PVD system plays an integral role in the mechanisms that lead to blob formation.
More particularly, nonhomogeneous structures within the target such as dielectric inclusions (e.g., Al
2
O
3
precipitates) and nonconductive voids (e.g., formed by trapped gas bubbles), when exposed as part of the target surface, are believed to create corresponding distortions in the electric fields that surround the target surface during the sputtering process. It is believed that large-enough distortions can evolve into points of field breakdown through which arcs of high current flow between the plasma and the target. Such arcing currents can result in localized melting of the target material and in the production of relatively large blobs of liquid material that splatter onto the wafer surface. The splattered material apparently draws back together on contact with the wafer surface, due to surface tension effects, and solidifies into the undesirable blob.
In accordance with a first aspect of the invention, targets are manufactured so as to minimize the sizes and numbers of dielectric inclusions (e.g., Al
2
O
3
precipitates) and nonconductive voids (e.g., formed by trapped hydrogen bubbles).
Blob formation is additionally believed to be due to stress-induced breakdown of the target material when the sputtering plasma is struck. The electric fields and currents which develop near the surface of the target as the plasma is ignited tend to generate mechanical stresses within the target material. Localized breakdown due to poor mechanical strength of the target local material is believed to be another source of blob generation.
In accordance with a second aspect of the invention, targets are manufactured so as to homogeneously maximize the strength of the target material and thereby inhibit blob generation due to localized mechanical breakdown.
A target in accordance with the invention essentially excludes dielectric inclusions such as metal oxides (Al
2
O
3
), nitride precipitates, carbide precipitates, of sizes larger than about 1 micron in concentrations greater than 5,000 such inclusions per gram of target material. A target in accordance with the invention alternatively or further essentially excludes voids such as those caused by entrapped gas of sizes larger than about 1 micron in concentrations greater than 5,000 such voids per gram of target material. A target in accordance with the invention alternatively or further has an essentially homogeneous distribution of metal grain size in the range of about 75 micron and 90 micron. A target in accordance with the invention alternatively or further has an initial surface roughness of less than about 20 microinches.
A DC_magnetron PVD system in accordance with the invention comprises a target having one or more of the following characteristics: (a) essentially no dielectric inclusions such as metal oxides (Al
2
O
3
), nitride precipitates, carbide precipitates, of sizes larger than about 1 micron in concentrations greater than 5,000 such inclusions per gram of target material; (b) essentially no voids such as those caused by entrapped gas of sizes larger than about 1 micron in concentrations greater than 5,000 such voids per gram of target material; (c) an essentially homogeneous distribution of metal grain size in the range of about 75 micron and 90 micron; and (d) an initial surface roughness of less than about 20 microinches. A DC_magnetron PVD system in accordance with the invention further comprises means for ramping plasma power at a rate of 2 Kw per second or less.
A target manufacturing method in accordance with the invention comprises one or more of the following steps of: (a) obtaining purified aluminum having less than about 1 ppm of hydrogen and less than about 10 ppm oxygen; (b) casting the purified aluminum using a continuous-flow casting method wherein the melt skin is not exposed to an oxidizing atmosphere; (c) working the cast metal so as to produce an essentially homogeneous distribution of metal grains of diameters less than or equal to 100&mgr; and second phase precipitates of diameters in the range of about 1 to 10&mgr; and more than about 50% material having <200> texture; (d) smoothing the initial target surface to an average roughness of no more than about 20 microinches; (e) using ultrasonic cleaning to remove arc-inducing contaminants from the initial target surface; and (f) shipping the cleaned target in an inert gas pack.
A method for operating a DC_magnetron PVD system in accordance with the invention comprises the steps of: (a) installing a new target having one or more of the following characteristics: (a) essentially no dielectric inclusions such as metal oxides (Al
2
O
3
), nitride precipitates, carbide precipitates, of sizes larger than about 1 micron in concentrations greater than 5,000 such inclusions per gram of target material; (b) essentially no voids such as those caused by entrapped gas of sizes larger than about 1 micron in concentrations greater than 5,000 such voids per gram of target material; (c) an essentially homogeneous distribution of metal grain size in the range of about 75 micron and 90 micron; and (d) an initial surface roughness of less than about 20 microinches. A DC_magnetron PVD operating method in accordance with the invention further comprises ramping plasma power at a rate of no more than 2 Kw per second or less.
Other aspects of the invention will become apparent from the below detailed description.
REFERENCES:
patent: 3279039 (1966-10-01), Nippert
patent: 4964962 (1990-10-01), Nobutani et al.
patent: 5196916 (199
Hansen Keith J.
Mori Glen
Narasimhan Murali
Nulman Jaim
Pavate Vikram
Applied Materials Inc.
Diamond Alan
Fliesler Dubb Meyer & Lovejoy LLP
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