Solid state device with conductors having chain-shaped grain structure

Aluminum electrically conducting patterns for integrated circuits are achieved with narrow lines and unexpectedly-high electromigration characteristics by making the crystal grains of the pattern into a chain-shaped structure with {111} orientation. A process for achieving the structure is also described.

BACKGROUND OF THE INVENTION 
One of the limitations to the realization of very dense semiconductor 
integrated circuit arises from the fact that aluminum and aluminum alloy 
electrical conductors, which are required to carry high currents, develop 
open circuit conditions during use even after testing. These open circuit 
problems have been attributed to electromigration-induced failures as is 
now well understood in the art. Electromigration damage has been found to 
be held to tolerable limits if conductor widths are held to seven microns 
or greater for current density levels required by practical devices. But 
such a limitation constrains the effort to reach higher and higher packing 
densities. 
Conductor width dependence of electromigration life in Al-Cu, Al-Cu-Si, and 
Ag conductors is discussed in an article, so entitled by G. A. Scoggan, B. 
N. Agarwala, P. P. Peressini, and A. Brouillard appearing on page 151 in 
The Proceedings of the 13th Reliability Physics Symposium, IEEE, New York, 
1975, page 151. That reference describes the effect of varying grain size 
and line width on lifetime and illustrates that as the conductor line 
width is decreased the electromigration lifetime approaches a saturation 
level or even exhibits a weak minimum. Thus it concludes that large grain 
size leads to improved lifetimes for short, narrow lines. On the other 
hand, it is clear from the publication that lines even as narrow as one 
micron would contain structural defects or divergent sites causing 
electromigration failures. Consequently, even lines with large grain size 
are limited in their usable lengths. The above-mentioned publication notes 
also the desirability of an ideal bamboo or chain geometry for the grain 
structure in electrical conductors. The Journal of Applied Physics, Vol. 
41, No. 10, Sept. 1970, at page 3954 et seq. documents the decreasing 
electromigration life with increasing length. Therefore, the combination 
of long lines (&gt;1 cm) with narrow widths presents a formidable obstacle to 
the use of fine-line Al. 
The present invention is directed at the problem of achieving fine-line 
width conductor patterns with markedly improved resistance to 
electromigration related open circuits. 
BRIEF DESCRIPTION OF THE INVENTION 
An aluminum or aluminum alloy electrical conductor pattern with one micron 
line width and length of one centimeter or more and characterized by 
commercially attractive electromigration properties is achieved herein by 
a process which produces grains essentially all oriented so that the {111} 
crystal plane is in the plane of the deposition surface. In an 
illustrative embodiment, a SiO.sub.2 layer is formed on a silicon 
substrate by heating the substrate in an oxidizing atmosphere. Next a 
layer of, for example, Al 1/2% Cu is deposited by e-gun evaporation. The 
pattern is formed by etching through a mask with BCl.sub.3 +Cl.sub.2 (10% 
by volume) in a glow discharge plasma. Thereafter, the pattern is annealed 
in hydrogen at 450 degrees C. for one-half hour. The resulting pattern is 
characterized by a chain-like grain structure, having grains most of which 
are oriented with {111} axis perpendicular to the plane of the deposition 
surface.

DETAILED DESCRIPTION 
FIG. 1 shows a typical commercial integrated circuit 10 defined in a chip 
11 of semiconductor material such as silicon. The circuit includes a 
pattern of electrically conducting material extending between active 
semiconductor regions and lands to which external electrical connections 
are made. Illustrative lands are designated 12, 13, 14 and ground (GND) in 
the figure. Attention is directed to illustrative conductor 17 extending 
between land 12 and active region 15. 
FIG. 2 shows a micrograph of the grain structure of a typical conductor 17 
of a standard integrated circuit. Such conductors typically are seven 
microns or greater in commercial circuits. A large number of grains can be 
seen spanning the conductor width. The grain size in this case clearly is 
small compared to the width of the conductor. 
FIG. 3 shows, schematically, a typical grain pattern which corresponds, for 
example, to that of conductor 17. Voids usually form at heterogeneities in 
structure such as, for example, grain boundary triple points, 30 and 31 in 
FIG. 3, by the electromigration of atoms along the direction of current 
flow away from these sites. One such path for electromigration, composed 
of grain boundary segments, may be represented by dotted line 50 in FIG. 
3. Another may be represented by dotted line 51. It is clear that with 
many triple points and many such paths for electromigration, open circuits 
result fairly easily. 
FIG. 4 is a micrograph of the grain structure of one micron wide electrical 
conductors such as 17 of FIGS. 1 and 2 achieved by the process of FIG. 5 
herein. The grain size is generally larger than the width of the 
conductor. Some of these grains are designated 42, 43, 44 and 45 in FIG. 
4. Clearly, no triple points exist. In addition, there are no grain 
boundary heterogeneities which could contribute to void formation and 
failure. A thin film conductor pattern characterized by a chain type grain 
structure would approach an ideal (bamboo) structure as far as 
electromigration characteristics are concerned. Such structures are 
achieved with lengths suitable for commercial use by the processes of FIG. 
5. 
The first step of the process is to form an SiO.sub.2 layer 0.5 microns 
(.mu.m) thick on a processed silicon substrate as indicated by block I of 
FIG. 5. The second step is to deposit Al-0.5% Cu by e-gun evaporation onto 
a hot surface (approximately 300 degrees C.) as indicated by block II. A 
photoresist layer is deposited, exposed, developed and etched to form 
lines of the kind shown in FIG. 2. This step is represented by the block 
of FIG. 3. The etched pattern is annealed in hydrogen at 450 degrees C. 
for about one-half hour to form the bamboo structure of the line shown in 
FIG. 4. 
The grains 42, 43, 44, 45 . . . labelled in FIG. 4 are all oriented with 
the {111} direction perpendicular to the surface of the SiC.sub.2 layer. 
Only with grains in the same direction can lines of the order of a 
centimeter in length have such attractive electromigration 
characteristics. The process of FIG. 5 produces the chain structure, the 
requisite preferred crystal orientation and the resulting attractive 
characteristics. The e-gun evaporation produces a film with atoms 
possessing a high degree of mobility for later recrystallization in a 
preferred direction upon annealing. 
FIG. 6 shows a prior art plot of stripe width in microns versus lifetime 
for two groups of materials A and B taken from the above referenced IEEE 
Proceedings. An increase in the characteristic mean time to failure 
(t.sub.50) below three micron line width is shown for Group B materials. 
The grain size for Group B materials is about one micron as indicated in 
the figure. Improvement seems to be achieved for mean grain sizes of less 
than the line width as well as about equal to the line width. The reason 
for the improvement is offered in connection with FIG. 3. Line widths 
w.sub.1, w.sub.2 and w.sub.3 are indicated by broken lines as seen. A path 
such as 50 or 51 exists in each of the lines w.sub.2 and 2.sub.3 for 
electromigration and resulting void formation. Even in the line of narrow 
width w.sub.1, although single grains may span the entire width of the 
conductor, a number of divergent sites still occur for void formation. 
This is not the case in lines made according to the process of FIG. 5. The 
cleanliness of the SiO.sub.2 surface, the process of metal deposition and 
the final annealing of the pattern cooperate to achieve the {111} 
orientation of the grains and also the requisite chain structure. Others 
who have had a final annealing step in their process for making fine-line 
structures have not realized the chain structure with a like grain 
orientation achieved herein. When grains have a common-poled axis, their 
boundaries are more uniform in properties leading to a reduced number of 
divergent sites for void growth. 
FIG. 7 plots mean time to failure versus line width. The improvement 
achieved herein by lines with bamboo-like structures is represented by 
curve C in FIG. 7. The remaining lines in the figure represent data taken 
for different test materials and deposition techniques according to the 
legend in the figure. Only e-gun evaporated materials showed the 
improvement represented line C. 
FIG. 8 is a plot of the ratio of lifetime at width w to that at 7 microns 
versus line width for prior art lines and for data from experimentation 
with lines made in accordance with the process of FIG. 5. The plot is 
similar to that of FIG. 7, but the data is normalized to the lifeline at 7 
microns so that all curves go to the value 1 (one) as shown. The curve for 
Group B materials in FIG. 6 (from the IEEE Proceedings) is redrawn in FIG. 
8 as indicated. An electromigration lifetime minimum is shown at slightly 
over three microns with a modest increase for narrower lines. Aluminum 
deposition by S-gun evaporation approximately follows the theoretical 
(shown in The IEEE Reliability Physics Symposium 1978, at page 233, by J. 
R. Black) curve so designated in the figure. Lines with like grain 
orientations were characterized by dramatically high lifetimes at narrow 
line widths as shown by a representative curve designated "e-gun Al-0.5% 
Cu". The data for FIG. 8 was determined for nominal device operating 
conditions of 80 degrees C. and a current density of 1.times.10.sup.5 
amperes/cm.sup.2. 
In one specific test arrangement, lines having widths of 1 micron showed an 
unexpected and surprisingly large electromigration lifetime of 
1.times.10.sup.7 h (hours) at 80 degrees C. and a current density of 
1.times.10.sup.5 A/cm.sup.2. These lifetimes were estimated from 
accelerated aging data obtained at 250 degrees C. for a current density of 
2.times.10.sup.6 A/cms.sup.2 and conservatively assuming a thermal 
activation energy of 0.5 ev. It was found generally that films with 
linewidths of 1.5 microns and less had very long lifetimes of 
4.times.10.sup.6 h or greater. 
As is also clear from cure C of FIG. 7 (and the e-gun curve of FIG. 8) much 
higher lifetimes are achieved for narrower lines with chain-shaped grain 
structures herein than for relatively wide lines. Consequently, an 
electrically conducting pattern as shown in FIG. 9 is attractive where 
first and second lands (or one land and an active region) 71 and 72 are 
interconnected by a plurality 73, 74, 75 and 76, of electrically 
conducting lines of say one micron width rather than a single line of say 
seven micron width. 
The embodiments described herein are considered merely illustrative of the 
principles of this invention. Therefore, those skilled in the art can 
devise various modifications thereof within the spirit and scope of the 
invention as encompassed by the following claims. For example, although 
the invention has been described in connection with a {111} grain 
orientation, like grain orientations other than {111} are possible. A 
layer of another metal over the SiO.sub.2 surface is known in this 
connection to permit such other orientation. 
It is to be clear that the invention may be practiced with other solid 
state devices such as magnetic bubble devices.