3D reservoir to improve electromigration resistance of tungsten plug

A new method of metallization using a three-dimensional aluminum reservoir to increase the electromigration lifetime of a tungsten plug in the fabrication of integrated circuits is achieved. An insulating layer is provided covering semiconductor device structures in and on a semiconductor substrate. Aluminum lines are formed over the insulating layer. An intermetal dielectric layer is deposited overlying the aluminum lines. Via openings are made through the intermetal dielectric layer to the aluminum lines. Aluminum is selectively deposited into the via openings to form aluminum reservoirs in the bottom of the via openings wherein the aluminum does not completely fill the via openings. Tungsten plugs are formed within the via openings overlying the aluminum reservoirs wherein the aluminum reservoirs provide a source for electrons to replenish electrons lost through electromigration to complete formation of tungsten plug metallization with increased electromigration lifetime in the fabrication of integrated circuits.

BACKGROUND OF THE INVENTION
 (1) Field of the Invention
 The invention relates to a method of metallization in the fabrication of
 integrated circuits, and more particularly, to a method of increasing the
 electromigration lifetime of a tungsten plug in the manufacture of
 integrated circuits.
 (2) Description of the Prior Art
 Performance and cost constantly drive attempts to design increased
 performance while simultaneously shrinking feature sizes, particularly in
 interconnects. An example of increased design performance is the addition
 of a reservoir to the cathode end of an interconnect to increase
 electromigration lifetime by relying on the well known dependence of
 electromigration average lifetime with reservoir area. U.S. Pat. No.
 5,506,450 to Lee et al, U.S. Pat. No. 5,864,179 to Koyama, and U.S. Pat.
 No. 5,760,477 to Cronin disclose extended end portions of the top Aluminum
 line over the tungsten plug to act as an aluminum reservoir. U.S. Pat. No.
 5,654,216 to Adrian teaches a double aluminum line over a tungsten plug.
 U.S. Pat. No. 5,834,369 to Murakami et al discloses a diffusion preventing
 film over a tungsten plug to allow for an alignment margin.
 SUMMARY OF THE INVENTION
 A principal object of the present invention is to provide an effective and
 very manufacturable method of forming an improved electromigration
 resistant tungsten plug metallization in the fabrication of integrated
 circuits.
 Another object of the invention is to provide a three-dimensional aluminum
 reservoir to increase the electromigration lifetime of a tungsten plug in
 the fabrication of integrated circuits.
 Yet another object is to provide a method of achieving electromigration
 resistance performance without decreasing chip density in the fabrication
 of integrated circuits.
 A still further object of the invention is to provide a method of
 increasing chip density without decreasing electromigration resistance
 performance.
 Yet another object of the invention is to provide a method of improving
 electromigration resistance performance of an unlanded via.
 In accordance with the objects of this invention a new method of
 metallization using a three-dimensional aluminum reservoir to increase the
 electromigration lifetime of a tungsten plug in the fabrication of
 integrated circuits is achieved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Elecromigration failure results when too many electrons are lost from the
 tungsten plug without replenishment. This occurs when the landing area;
 that is the metal area underlying the via, is too small. It is desired to
 form an aluminum reservoir to act as a source of replenishment for
 electrons lost to electromigration.
 FIGS. 1A through 1C illustrate top views of aluminum reservoir test
 structures. The line width is 0.9 .mu.m for all test structures shown.
 FIG. 1A illustrates a borderless via contact. Aluminum line 30 and
 tungsten plug 20 are shown. The reservoir area 40 in this structure is
 0.63 .mu.m.sup.2. FIG. 1B illustrates another test structure having
 aluminum line 32, tungsten plug 20 and a reservoir area 42 of 1.30
 .mu.m.sup.2. Fig 1C illustrates aluminum line 36 and tungsten plug 20
 having a reservoir area 44 of 5.76 .mu.m.sup.2. The larger the reservoir,
 the better the electromigration resistance. However, using structures 1B
 or 1C to improve electromigration requires sacrificing the density of the
 layout.
 The present invention provides a method for forming an aluminum reservoir
 which will increase electromigration lifetime, but will not decrease chip
 density. The aluminum reservoir is formed in a portion of the via
 underlying the tungsten plug. Thus, a structure as small as that shown in
 FIG. 1A can be used in the process of the invention to improve
 electromigration resistance without sacrificing layout density.
 The process for forming the novel reservoir of the present invention will
 be described with reference to FIGS. 2 through 6.
 Referring now more particularly to FIG. 2, there is illustrated a portion
 of a partially completed integrated circuit. There is shown a
 semiconductor substrate 10, preferably composed of monocrystalline
 silicon. Semiconductor device structures, including, for example, gate
 electrodes and associated source and drain regions and subsequent levels
 of metallization with intervening dielectric layers are formed in and on
 the semiconductor substrate. All of these structures, not shown, are
 represented by layer 12.
 Now, metal lines 14 are patterned over the surface of the substrate. The
 metal lines are preferably aluminum or an aluminum alloy. The metal lines
 14 are patterned with a highest density rule.
 An intermetal dielectric layer 16 is deposited over the metal lines, as
 shown in FIG. 3. Via openings 17 are made through the dielectric layer 16
 to contact the underlying aluminum lines 14. A misalignment of the via
 pattern is shown in FIG. 4. This misalignment will not cause a problem.
 The via openings have a depth of between about 7000 and 8000 Angstroms.
 Now, referring to FIG. 5, aluminum is selectively deposited into the via
 openings 17 to form a reservoir 18 having a thickness of between about
 1000 and 2000 Angstroms. The aluminum is deposited by a hot-cold method or
 by a reflow physical vapor deposition (PVD) process. In the hot-cold
 method, a seed layer of aluminum is deposited at a low temperature. The
 bulk of the aluminum then is deposited at a higher temperature, above
 450.degree. C.
 Now, a tungsten plug 20 is formed within the via openings over the aluminum
 reservoirs 18, as shown in FIG. 6. The tungsten plug formation, by
 tungsten deposition and chemical mechanical polishing (CMP), for example,
 is conventional. Another set of patterned aluminum lines 22 may now be
 fabricated overlying the tungsten plugs, as shown. This is followed by
 another dielectric layer deposition 24.
 It will be understood by those skilled in the art that a number of steps
 have been omitted from the description in order to simplify the process.
 For example, aluminum lines 14 and 22 may comprise a multi-layer stack,
 including, for example, barrier layers and antireflective coating layers.
 Likewise, tungsten plug 20 may have an underlying glue and/or barrier
 layer formed within the via opening.
 The process of the present invention provides a method for forming an
 aluminum reservoir for increasing electromigration lifetime. Since the
 reservoir is formed at the bottom of the tungsten plug via opening, no
 additional space is required for the reservoir, thus maintaining high chip
 density. The aluminum reservoir underlying the tungsten plug provides a
 source for electrons to replenish those lost to electromigration thus
 preventing holes within the tungsten plug. Aluminum reservoirs according
 to the process of the invention may be formed at any or all levels of
 tungsten plug metallization, as needed.
 While the invention has been particularly shown and described with
 reference to the preferred embodiments thereof, it will be understood by
 those skilled in the art that various changes in form and details may be
 made without departing from the spirit and scope of the invention.