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
1998-07-09
2002-01-01
Whitehead, Jr., Carl (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S752000, C257S758000, C257S767000
Reexamination Certificate
active
06335569
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 wear resistance 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 and a dual-step deposition method for making such conductor.
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 wear resistance 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 wear resistance in its uppermost 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 wear resistance in its uppermost 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 wear resistance in its uppermost 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 uppermost 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 uppermost 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 uppermost 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 meal conductor that has a substantially scratch-free surface upon polishing for use in a semiconductor device by a physical vapor deposition technique, a chemical vapor deposition technique or a dual-step deposition technique.
SUMMARY OF THE INVENTION
During chem-mech polishing (CMP), overall wear resistance is important. Wear could be due to the combination of chemical and mechanical action and the contribution of each is difficult to separate. It has been experimentally observed in by the inventors that the large grains during the high temperature deposition process or thermal annealing or combination of low temperature deposition followed by high temperature annealing or deposition improves wear resistance. These facts can be technically explained as follows: A. During CMP,“wear” mechanisms can be attributed to chemical wear in combination with adhesion, abrasion and delamination wear. It appears that these components attack the large grains differently.
For mechanical component strength, adhesion (if loose debris are formed they can scratch the metallic layers), hardness etc. are important. For chemical component etchability in the chemical solution, slurry composition (pH), microstructure, etc., play a role in metal removal. Other parameters such as polishing pressure, speed, and pad structure play a significant role. Keeping other parameters constant, large grains with minimal defects may resist overall wear better than the small grains. It can be deduced that the ratio of atoms on the grain boundary and atoms in the grain itself =2×10
−3
/(xd), where x is lattice spacing and d is grain size in microns, assuming a square grain). Thus the larger the grain size smaller is the fraction of atoms on the grain boundary for constant x. Also, smaller grains (having larger surface of grain boundaries) are prone to chemical attack during CMP. Once the grain are loose they can be easily knocked down due to mechanical action during CMP. These debris can scratch the metal. Thus it is possible that small grains can wear faster than large grains. Experimental results support this statement.
In addition, small grains when annealed to form large grains or large grains formed at high temperature deposition process reduces the free volume or defect structure changing the grain boundary (free volume or defects are susceptible to chemical attack). The reduction of these defects during grain growth may improve resistance to chemical attack at the grain boundaries and also improve wear related to adhesion.
B. If impurities are added in the film they would segregate at grain boundaries during grain growth. In the case of alloys (e.g Al—Cu), intermetallic would form at the grain boundaries. These impurities or intermetallic can prevent attack at the grain boundaries or improve wear resistance during CMP. Impurities which impart sufficient wear resistance would be beneficial from the point of view of CMP. In short large grains formed by thermal annealing or high temperature deposition or combination, etc., would improve wear resistance (also improvement of adhesion, less debris during polishing and scratching) based on the reasons given above.
In accordance with the present invention, a soft metal conductor that has a substantially scratch-free uppermost 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 uppermost
Brophy Jamie L.
Jr. Carl Whitehead
Trepp Robert M.
Tung Randy W.
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