Compositions for improving interconnect metallization...

Details Compositions for improving interconnect metallization... Compositions for improving interconnect metallization...

1999-05-12

2002-10-01

Loke, Steven (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

C257S762000, C257S765000

Reexamination Certificate

active

06459153

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention relates to techniques for improving interconnect metallization performance in integrated circuits.
2. Description of the Related Art
In the manufacture of semiconductor integrated circuits (“ICs”), well-known metallization techniques are used to interconnect devices on different levels of an IC chip. Generally, the performance of the interconnect metallization, or metallization line, (“metallization performance”) involves providing proper conductivity, ease of etching of the interconnect material, minimizing electromigration, and minimizing capital investment and developmental effort in the metallization process.
Thus, in the design of any IC, strong consideration is generally placed on examining the degree of expected electromigration that may occur in view of a metallization line's current carrying requirements. This is typically required because designers know that if too much electrornigration occurs in a given metallization line having a particular width, serious reliability-impacting voids may form. Accordingly, designers are commonly required to increase a metallization line's width when high levels of current are anticipated, such as, for example, in power and ground bus lines. In certain circumstances, the designer is forced to make particular metallization lines exceedingly wide, just to prevent the possibility of excessive voiding from occurring. Widening metallization lines does, however, impose a cost penalty since this will require semiconductor chips to be larger than may be necessary to carry out the IC's designed function.
Electromigration is commonly understood to be the result of an average current flow through a conductor. The flowing electrons transfer a fraction of their momentum to the metal atoms from a scattering process. This momentum transfer in turn causes a movement of the metal atoms (i.e., mass transfer) in the direction of electron flow. Therefore, the amount of momentum transfer, and resulting metal flow, increases with increasing current density. This flow of material is seldom uniform and regions of tensile stress develop where there is a net loss of material over time and regions of compressive stress develop where there is a net increase of material over time. The development of regions of tensile and compressive stress therefore create stress gradients. These stress gradients also cause a flow of metal since stress drives a flow of atoms from regions of high concentration (i.e., compressive stress) to regions of low concentration (i.e., tensile stress). For more information on electromigration and the degrading effects of electromigration, reference may be made to an article entitled “Effects of W-Plug Via Arrangement on Electromigration Lifetime of Wide Line Interconnects,” by S. Skala and S. Bothra, Proceedings of the International Interconnect Technology Conference, San Francisco, Calif., June (1998). This article is hereby incorporated by reference.
Electromigration voids are most commonly formed at the beginning of an interconnect line. This is believed to occur because electromigration degradation is more likely to stop when the sum of electromigration and stress is zero, which will more likely occur at the end of a line. Early observations of electromigration flow and its tendency to stop when a line is relatively short (e.g., a short distance to its terminating end) and continue when a line is relatively long (e.g., a long distance to its terminating end), was first reported by I. A. Blech. The behavior of electromigration defined in terms of the length of a metallization line has thus become widely referred to as the “Blech effect.” That is, when a metallization line is at least as short as a given Blech length for a particular width, electromigration voids will no longer form. For more information on Blech effect and Blech length, reference may be made to an article entitled “Electromigration and Stress-Induced Voiding in Fine Al and Al-alloy Thin-Film Lines” by C. K. Hu, K. P. Rodbell, T. D. Sullivan, K. Y. Lee and D. P. Bouldin, IBM Journal of Research and Development, Vol. 39, No. 4, July 1995, pp. 465-497. This article is hereby incorporated by reference.
Although the Blech length has been widely known, this concept is generally not applicable for many interconnect metallization lines and power buses because such lines are generally required to be longer than the Blech length in order to meet functional specifications. As a result, designers have continued to design certain metallization lines wider than necessary in order to prevent void formation which may introduce open circuits or complete functional failures.
Al—Cu alloys are one type of composition used for metallization lines in multi-level metallization for IC fabrication. However, if too much Cu is used in the alloy, the conductivity of the alloy decreases and it is more difficult to etch the alloy. At the other extreme, although pure Cu can be deposited with a dual damascene process, significant capital investment and developmental effort are required to provide pure Cu metallization in this manner. Pending reduction of such capital and developmental investment, efforts have been made to obtain improved metallization performance using Al-based alloys.
The Al—Cu alloy follows the classic solvus curve in which the solid solution of Al and Cu occurs at low weight percentages of Cu. Thus, the Cu appears primarily inside the grains of Al at such low weight percents of Cu, such that the grains are predominately composed of Al. At higher weight percents of Cu, the alloy composition shifts to the right of the solvus curve. The excess Cu now segregates to the grain boundaries between grains, in the form of Al—Cu precipitates.
In prior attempts to use Al—Cu alloys for improving interconnect metallization, Cu depletion from grain boundaries is a problem. This problem is schematically shown in
FIG. 1A
, where a magnified portion of an interconnect metallization
20
is shown including grains
21
(primarily of Al) separated by grain boundaries
22
(primarily of the precipitate Al
2
Cu). The grains
21
may be large as shown by grains
21
L or may be small as shown by grains
21
s
. Electrical current is conducted through the interconnect metallization
20
, and electron flow is depicted by a reference arrow e
−
. Arrows E represent the electrons flowing between two adjacent grains
21
along a grain boundary
22
. At the onset of electromigration, the electrons e
−
initially carry atoms Cu, which are shown by dots and tiny circles in the grain boundary
22
. The atoms migrating between exemplary grains
21
-
1
and
21
-
2
are shown diverging, which causes a net loss of material over time as the atoms migrate toward a group of the small grains
21
s
. As the atoms migrate to the right past the group of small grains
21
s
, they converge and accumulate as shown by the reference number
23
, such that there is a net increase of material over time. The large grains
21
L, which may grow depending on thermal history, can block flux along the interconnect metallization, and create flux divergence and voiding at the Cu-rich phase. Although proper heat treatment has been proposed as a solution to the goal of optimization of theta phase morphology, thermal cycles in backend fabrication processing and restrictions on elevated temperature processing preclude use of such proper heat treatment.
In more detail,
FIGS. 1B and 1C
show a magnified portion of
FIG. 1A
illustrating opposed grains
21
(e.g., grains
21
-
1
and
21
-
2
) separated by the grain boundary
22
. Arrows C
U
depict Cu atoms migrating away from the grain boundary
22
under the action of the electron e
−
flow, resulting in Cu depletion from the grain boundary
22
. Because the Cu atoms are heavier than the Al atoms, there is a time period (“incubation time”, or &Dgr;t;
FIG. 5
) during which the Cu atoms resist b

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