Method for evaluating performance of aluminum alloy wiring film

A method for evaluating performance of an aluminum alloy wiring film includes the steps of forming a first aluminum alloy film on a substrate, forming a second aluminum alloy film on another substrate, measuring hardness of the both films, and evaluating the superiority of stressmigration of the aluminum alloy films by comparison between the films in hardness.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for evaluating performance of an 
aluminum alloy wiring film and more particularly, to a method for 
evaluating stressmigration tolerance of a wiring film in an integrated 
circuit element. 
2. Description of the Related Art 
The stressmigration of the metal wiring film in the integrated circuit 
element is determined by stress of a protective film on the metal wiring 
film and critical stress of the wiring film. According to a conventional 
method for evaluating performance of a metal wiring film, a protective 
film is laminated on the metal wiring film to be evaluated and aged for a 
long time at approximately 150.degree. C. and then a void is observed by a 
microscope or an existence of burnout is confirmed by measuring resistance 
of the wiring film. 
However, the conventional evaluating method requires a lot of evaluating 
samples and a long time, which is a problem. 
SUMMARY OF THE INVENTION 
The present invention provides a method for evaluating performance of an 
aluminum alloy wiring film, which method comprises the steps of forming a 
first aluminum alloy film on a substrate, forming a second aluminum alloy 
film on another substrate, measuring surface hardness of both films, and 
evaluating the superiority of stressmigration tolerance of the aluminum 
alloy films by comparison between the films in hardness. 
Further, the present invention provides a method for evaluating performance 
of an aluminum alloy wiring film, which method comprises the steps of 
forming a reference aluminum alloy film having a predetermined thickness 
on a substrate, forming an aluminum alloy film to be evaluated on another 
substrate with the same thickness as that of the reference film, measuring 
surface hardness of the both films, forming a protective film on the 
evaluated film if its hardness is higher than that of the reference film, 
and aging it to evaluate stressmigration tolerance of the aluminum alloy 
film having the protective film. 
In addition, the reference film and the film to be evaluated are preferably 
formed on the substrate by sputtering. 
In addition, the substrates preferably have a BPSG film on its surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It is known that there are two following relations between stressmigration 
tolerance and physical properties of a metal wiring film. That is, 
(1) The smaller the grain size of the metal wiring film is, the higher the 
stressmigration tolerance of the metal wiring film is. 
(2) The stressmigration tolerance of the metal wiring film becomes high by 
adding an impurity. 
Meanwhile, the relation between the grain size and the hardness of the 
metal wiring film has been well known by the Hall-Petch relation 
(referring to E. O. Hall, :Proc. Phys. Soc., B649 (1951), p747 and N. J. 
Petch, :J. Iron Steel Inst., 173 (1953), p25), that is, 
EQU t.sub.ys =t.sub.o +k.sub.y *d.sup.-1/2 
where t.sub.ys is the yield strength and t.sub.o, k.sub.y are constant and 
d is the grain size. The yield strength and the hardness have the same 
dimension as ML.sup.-1. Therefore, it could be said that the Hall-Petch 
relation holds between the grain size and the hardness. 
In other words, the hardness becomes high in proportion to the reciprocal 
of the square root of a grain size d. Therefore, it is found that the 
higher the hardness of the metal film is, the higher the migration 
tolerance of the metal wiring film is, from the Hall-Petch relation and 
the relation described in the above (1). In addition, it is also known 
that the more the impurity is applied to the metal wiring film, the higher 
the hardness thereof is. Therefore, it follows that the higher the 
hardness is, the higher the stressmigration tolerance is from the above 
fact and the relation described in the above (2). More specifically, the 
stressmigration tolerance of the metal wiring film can be evaluated by 
measuring its hardness. Therefore, when the hardness of the metal film to 
be evaluated is lower than that of the reference metal film, the 
inferiority of the stressmigration of the metal film is evaluated without 
evaluation by aging, so that the evaluation can be efficiently performed. 
I Description of the Present Invention by Experiment 
The principle of the present invention will be described in reference to 
the result of an experiment. 
As shown in FIG. 1, a BPSG (Borophosphosilicate glass) film 2 having a 
thickness of 0.9 .mu.m is formed on a silicon substrate (bare Si wafer) 1 
by a CVD method. Then, an Al-Si film having a thickness of 1.1 .mu.m is 
formed thereon as a metal film 3 by sputtering. Then, in order to obtain 
samples of the wiring films having different grain sizes, a temperature 
when the Al-Si film is formed is set at a room temperature, at 150.degree. 
C. and 300.degree. C. Then, as the metal film 3, samples are formed of an 
Al-Si-Pd film and an Al-Si-Cu film instead of the Al-Si film in the same 
manner as above. 
Next, the hardness of each of these metal films of samples is measured 
using a microhardness meter (MHPIB made by Carl Zeiss Foundation). 
Then, using a method of boundary-etching (referring to E. G. Solley et al.; 
Solid State Technology, Japanese, March, 1990, p.31), the particles of the 
metal film can be embossed on the surface and then its grain size d is 
found by a scan type electron microscope. 
An example of the result of measurement of the hardness and the grain size 
d is shown in FIG. 2. As can be seen from FIG. 2, the larger the grain 
size is, the lower the hardness is, which is not contradictory to the 
above Hall-Petch relation. 
In addition, it is found that the hardness of the Al-Si-Pd film or the 
Al-Si-Cu film is higher than that of the Al-Si film and the hardness is 
increased by adding an impurity. 
Next, a stressmigration test is preformed. These aluminum films are 
patterned on TEG (Test Element Group), and interconnection lines are 
formed with a width of 0.8 .mu.m. A method of the test is that sample 
wafers are kept for 2.5 hours at high temperature of 420.degree. C. and 
open failures of interconnection lines are checked. The relations between 
the cumulative failure rates and Knoop hardness are shown in FIG. 3. It 
can be seen from FIG. 3 that Knoop hardness corresponds well to the 
cumulative failure rates. We can say that the higher the hardness is, the 
smaller the failure rate is in the same kind of alloy, that is, a harder 
film has better tolerance of stressmigration. 
In addition, it is also found from the results that the stressmigration 
tolerance of the metal film of the Al-Si-Pd film or the Al-Si-Cu film is 
superior to that of the Al-Si film. 
Then, the thus formed metal films 3, that is, the Al-Si film, the Al-Si-Pd 
film and the Al-Si-Cu film are patterned and then an SiO.sub.2 film having 
a thickness of 3000 .ANG. is formed thereon as a protective film by the 
CVD method to form samples. The stressmigration tolerance of each of the 
metal films having the protective films is evaluated by the conventional 
evaluating method by aging the samples. As a result, it is confirmed that 
the stressmigration tolerance of the Al-Si-Pd film or the Al-Si-Cu film is 
superior to that of the Al-Si film. 
II Evaluating Procedure 
Procedure for evaluating performance of the aluminum alloy wiring film will 
be described in detail. 
First, a film to be evaluated such as a BPSG film is formed on an Si 
substrate with a thickness of 1.1 .mu.m. 
At this time, as a reference film, an Al-Si-Cu film or the like which has 
been already used as a metal wiring material is deposited by sputtering on 
the substrate. 
Then, hardness of each of the films is measured. When the hardness of the 
film to be evaluated is higher than that of the reference film (for 
example, it is greater than Knoop hardness of 200 (referring to FIG. 2)), 
a protective film is formed on the film to be evaluated and then evaluated 
by the conventional aging evaluating method. 
When the hardness of the film to be evaluated is smaller than that of the 
reference film, since the stressmigration tolerance is considered to be 
inferior, the aging evaluation is not performed. 
Thus, since the film to be evaluated is selected before the aging 
processing, evaluation can be efficiently performed. 
In addition, when a degree of the influence of the stressmigration 
tolerance upon the metal film of the protective film is known or its 
influence is very small, the stressmigration tolerance of the metal film 
is evaluated by only comparing the hardness. Thus, the step of forming the 
protective film and the aging processing can be omitted, whereby time 
required for the evaluation can be further reduced. 
In the above embodiment, the AlSi film, the Al-Si-Pd film, the Al-Si-Cu 
film are formed by Al-1 wt % Si, Al-1 wt % Si-0.3 wt % Pd, Al-1 wt % 
Si-0.5 wt % Cu, respectively. 
As described above, according to the present invention, it is possible to 
evaluate the performance of the metal wiring film for a short time using 
few samples. 
While only certain presently preferred embodiments have been described in 
detail, as will be apparent with those skilled in the art, certain changes 
and modifications can be made without departing from the scope of the 
invention as defined by the following claims.