Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
1995-01-03
2001-09-04
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S762000, C257S763000
Reexamination Certificate
active
06285082
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a soft metal conductor for use in a semiconductor device and a method of making such conductor and more particularly, relates to a soft metal conductor that has improved hardness in its surface layer for use in a semiconductor device wherein the surface layer consists of metal grains having grain sizes sufficiently large so as to provide a substantially scratch-free surface upon polishing in a subsequent chemical mechanical polishing step.
BACKGROUND OF THE INVENTION
Metal films have been utilized in semiconductor manufacturing to electrically connect together various components formed on a semiconductor wafer. For instance, vias, interconnects, trenches are just a few examples of such applications. Elemental aluminum and its alloys such as aluminum-copper have been used traditionally for these applications. The advantages of using aluminum and its alloys include the low resistivity, the superior adhesion to SiO
2
, the ease of patterning, the high purity and low cost of the materials.
Aluminum and aluminum alloys are not without drawbacks when utilized in semiconductor technology. Two of these drawbacks are the softness of the materials which results in difficulty in polishing and the electromigration phenomenon which results in circuit failure. For instance, the polishing problem has been observed in a process where metal films or metal conductive lines are formed in a damascene process by first filling troughs previously etched in an insulator with a metal and then polishing away metal deposited between the troughs. When a soft metal is used, i.e. aluminum, copper or aluminum-copper alloy, the surface of the metal lines may become scratched in a polishing process. The formation of defects during polishing of scratches, pockets, depressions or erosions in the metal surface significantly increases the line resistance and thus reduces the yield of the semiconductor manufacturing process.
In order to avoid these defects produced in the polishing process of soft metals, capping by hard layers has been tried by others to improve the wear resistance of the surface layer of the metal. However, this is achieved at the expense of higher capacitance as the line thickness increases. It is inherently difficult to improve the hardness of soft metals which requires the processing steps of polishing. Poor polishing results in variations in the line or via resistance.
It is therefore an object of the present invention to provide a soft metal conductor that has improved hardness in its upper-most surface and a method of making the same without the shortcomings of the prior art conductors and the prior art methods.
It is another object of the present invention to provide a soft metal conductor that has improved hardness in its upper-most surface such that a substantially scratch-free surface can be obtained after polishing in a chemical mechanical polishing process.
It is a further object of the present invention to provide a soft metal conductor that has improved hardness in its upper-most surface by simply modifying the processing conditions of the deposition process for the soft metal.
It is yet another object of the present invention to provide a soft metal conductor that has a substantially scratch-free surface upon polishing by depositing a soft metal layer consisting of metal grains having large grain sizes in its upper-most layer.
It is another further object of the present invention to provide an electrically conducting soft metal structure that has a substantially scratch-free surface upon polishing by depositing in the upper-most layer of said structure grains of soft metal not smaller than about 200 nm.
It is still another object of the present invention to provide an electrically conducting soft metal structure that has a substantially scratch-free surface upon polishing for use in a semiconductor device by depositing in the upper-most layer of said structure metal grains having grain sizes not smaller than about 20% of the thickness of the soft metal structure.
It is still another further object of the present invention to provide an electrically conducting soft metal structure that has a substantially scratch-free surface upon polishing for use in a semiconductor device wherein the surface has a layer of at least about 100 nm in thickness of large grain size metal grains deposited therein.
It is yet another further object of the present invention to provide a method of making a soft metal conductor that has a substantially scratch-free surface upon polishing for use in a semiconductor device by a physical vapor deposition or a chemical vapor deposition technique.
SUMMARY OF THE INVENTION
In accordance with the present invention, a soft metal conductor that has a substantially scratch-free upper-most surface upon polishing for use in a semiconductor device and a method of making the same is provided.
In the preferred embodiment, the soft metal conductor is provided by depositing an upper-most layer of the conductor consisting of grains having grain sizes not smaller than about 20% of the thickness of the soft metal conductor. This is achieved by, for instance, depositing an upper-most layer of the soft metal material to a thickness of not less than 100 nm with grains of soft metal not less than 200 nm in grain sizes. The large grains provide a significantly improved hardness in the upper-most layer of the soft metal conductor such that a substantially scratch-free surface upon polishing in a subsequent chemical mechanical polishing process is obtained. By substantially scratch-free, it is meant that a surface is obtained after polishing that has less than five scratches per square centimeter area.
In an alternate embodiment, a layer of soft metal having smaller grains, i.e. a grain size of not larger than 50 nm is first deposited in the soft metal conductor to a thickness of not less than 600 nm, an upper-most layer of large grains having grain sizes not smaller than 200 nm is then deposited on top of the layer of small grains. The large grain size in the upper-most layer provides the desirable scratch-free surface for polishing, while the middle layer of soft metal in small grains provides a layer of material without the thermal voiding problem.
In another alternate embodiment, a layer of soft metal having small grains of less than 50 nm in size is sandwiched between a bottom layer and a top layer of metal consisting of grains of larger than 200 nm in size.
In yet another alternate embodiment, after a large grain soft metal M1 is deposited, a layer of Ti is sequentially deposited on top of the soft metal. The Ti layer deposited at the interface between the via and M1, M2 has a thickness of not higher than 30 nm so as to provide improved anti-electromigration property in the soft metal conductor after the Ti layer is converted to a TiAl
3
layer in a subsequently conducted annealing process at 400° C. M1, M2 are metal stacks of Ti/Al-Cu/Ti/TiN.
The present invention is also directed to a method of making a soft metal conductor that has a substantially scratch-free surface upon polishing by a multi-step deposition process, i.e., first sputtering at 450° C. for 10-15 sec, then at 400° C. for 2 min and followed by 450° C. for 15-20 sec. A soft metal conductor that has improved hardness in its upper-most surface can be obtained
The present invention is further directed to a method of polishing soft metal by following a prescribed equation of processing parameters to obtain optimum volume removal without scratches and erosion.
The present invention is still further directed to a method of forming a substantially scratch-free surface on a soft metal conductor by first depositing a soft metal layer at a low deposition temperature and then annealing the soft metal layer at a higher temperature to increase the grain size of the metal.
REFERENCES:
patent: 4990410 (1991-02-01), Saitoh et al.
patent: 5345108 (1994-09-01), Kikkawa
patent: 472733 (1992-06-01), None
patent: 629405 (1994-04-01), None
Journal of Vacuum Science
Joshi Rajiv Vasant
Tejwani Manu Jamnadas
International Business Machines - Corporation
Potter Roy
Trepp Robert M.
Tung Randy W.
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