Redundant circuit of semiconductor device and method of producing same

A redundant circuit of a semiconductor device comprises a fuse (73) for laser trimming to connect between aluminum interconnections (6). The fuse (73) has a two-layer structure comprising a first film (3a) of polysilicon and a second film (7a) formed on the film (3a) of metal silicide, the line width l.sub.0 of the first film (3a) being shorter than the line width l.sub.1 of the second layer (7a). In addition, a PSG film (4) is formed to cover the fuse (73), and the laser beam is irradiated on the PSG film (4) in disconnecting the fuse (73). Accordingly, the first film (3a) having a short line width is uniformly fused and expanded, the fuse (73) is uniformly disconnected, and an opening (10) formed after explosion and splash thereof becomes smaller.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a redundant circuit of a semiconductor 
device and a method of producing the same, and more particularly relates 
to a redundant circuit of a semiconductor device including a fuse for 
laser trimming for replacing a circuit, and a method of producing the 
same. 
2. Description of the Prior Art 
A conventional redundant circuit of a semiconductor device utilizing a 
laser trimming, system is described in, for example, an article by Robert 
T. Smith et al., entitled "Laser Programmable Redundancy and Yield 
Improvement in a 64K DRAM," IEEE Journal of Solid-State Circuits, Vol. 
SC-16, No. 5, Oct. 1981. 
FIG. 1A is a plan view showing an example of a conventional redundant 
circuit of a semiconductor device, FIG. 1B is a cross sectional view taken 
along a line B--B in FIG. 1A, and FIG. 1C is a cross sectional view taken 
along a line C--C in FIG. 1A. 
Referring now to FIGS. 1A to 1C, a method of producing the conventional 
redundant circuit of a semiconductor device is described. First, a field 
oxide film 2 is formed on a silicon substrate 1, and a fuse 3b of 
polysilicon is formed on the field oxide film 2. Then, a PSG (Phospho 
Silicate Glass) film 4 is deposited on the field oxide film 2 and the fuse 
3b by a CVD (Chemical Vapour Deposition) process. Subsequently, the PSG 
film 4 is selectively etched away utilizing a photoresist film (not 
shown), so that a part of the fuse 3b is exposed to form a contact hole 5. 
An aluminum interconnection 6 is then formed on the PSG film 4 and through 
and over the contact hole 5 so as to be connected to the fuse 3b, whereby 
the redundant circuit of a semiconductor device is completed. 
FIG. 2 is a cross sectional view in which the fuse 3b of the redundant 
circuit shown in FIGS. 1A to 1C is disconnected. A circuit in a 
semiconductor device is ordinarily replaced by disconnecting a fuse 
included in a redundant circuit by irradiation of the laser beam. In FIG. 
1C, laser beam energy irradiated on the PSG film 4 is absorbed in the fuse 
3b. Consequently, the fuse 3b is fused and expanded, so that it is 
exploded and splashed with the PSG film 4 and disconnected as shown in 
FIG. 2. As a result of explosion and splash thereof, an opening 9 is 
formed in the PSG film 4 as shown in FIG. 2, the base width of which is 
identical to the line width 1.sub.1 of the fuse 3b. 
When the redundant circuit as shown in FIGS. 1A to 1C is used, the line 
width 1.sub.1 of the fuse 3b must be kept large to some extent to reliably 
irradiate the laser beam. However, when the line width 1.sub.1 of the fuse 
3b is large, the opening 9 formed after explosion thereof also becomes 
large, so that high integration of a semiconductor device becomes 
difficult. In addition, when the line width 1.sub.1 is large, the 
temperature difference under heat becomes large between the central 
portion and the end of the fuse 3bso that it becomes difficult to reliably 
disconnect the fuse 3b. 
SUMMARY OF THE INVENTION 
Briefly stated, the present invention is directed to a redundant circuit of 
a semiconductor device including a first metal interconnection layer, a 
second metal interconnection layer formed apart from the first metal 
interconnection layer, a fuse for electrically connecting the first and 
second metal interconnection layers, and a glass film formed to cover the 
fuse, the fuse having a two-layer structure comprising a first linear film 
of a first material and a second linear film formed on the first film of a 
second material different from the first material, and the line width of 
the first film being narrower than that of the second film. 
In accordance with another aspect of the present invention, a method of 
producing the redundant circuit of a semiconductor device comprises the 
steps of preparing a semiconductor substrate, forming a field oxide film 
on the semiconductor substrate, forming a first layer of a first material 
on the field oxide film, forming a second layer of a second material 
different from the first material on the first layer, etching the first 
and second layers to form a fuse having a two-layer structure comprising a 
first linear film of the first material and a second linear film of the 
second material having a wider line width than that of the first film, 
forming a glass film to cover the fuse, forming.a contact hole in a 
predetermined position of the glass film, and forming first and second 
metal interconnection layers to be connected to the fuse through the 
contact hole. 
Accordingly, a principal object of the present invention is to provide the 
redundant circuit of a semiconductor device capable of high integration 
and a method of producing the same. 
Another object of the present invention is to provide a redundant circuit 
of a semiconductor device having an improved rate of success in replacing 
a circuit by the redundant circuit and a method of producing the same. 
A principal advantage is that an opening formed after exploding and 
splashing the fuse can be decreased, since the first film of the melted 
and expanded fuse has a narrow line width. 
Another advantage of the present invention is that the fuse can be 
uniformly and reliably disconnected by the laser beam, since the 
temperature distribution under heat becomes uniform in the first film of 
the fuse. 
Still another advantage is that the line width of the fuse itself is not 
required to be reduced. 
These objects and other objects, features, aspects and advantage of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 3A to 3D are cross sectional views showing major processes producing 
the redundant circuit of a semiconductor device of an embodiment of the 
present invention. 
Referring now to FIGS. 3A to 3D description is made on a method of 
producing redundant circuit of a semiconductor device of an embodiment of 
the present invention. 
Referring to FIG. 3A, a field oxide film 2 is formed on a silicon substrate 
1, and subsequently, a polysilicon film 3 is formed on the field oxide 
film 2. The polysilicon film 3 is used for forming a first layer of a fuse 
for laser trimming. Then, a film 7 of metal silicide such as molybdenum 
silicide, tungstem silicide, titanium silicide, or tantalum silicide is 
formed on the polysilicon film 3. The metal silicide film 7 is used for 
forming a second layer of the fuse for laser trimming. 
Referring now to FIG. 3B, photoresist (not shown) is formed on the metal 
silicide film 7, and is patterned into a desired shape to form a 
photoresist mask 8. Subsequently, only the metal silicide film 7 is 
selectively etched away utilizing the photoresist mask 8 as a mask so as 
to form a metal silicide film 7a having a width 1.sub.1 of 1 to 2 .mu.m 
and a thickness of 1000 to 3000 .ANG.. 
Referring now to FIG. 3C, the polysilicon film 3 is selectively etched 
using an etching gas having faster etching speed for the polysilicon film 
3 than for the metal silicide film 7a, and utilizing the photoresist mask 
8 as a mask so as to form a polysilicon film 3a. The polysilicon film 3a 
has a narrower line width than that of the metal silicide film 7a by 
.DELTA.1.sub.0 on one side, e.g. a width 1.sub.0 of 0.1 to 0.3 .mu.m and a 
thickness of 500 to 3000 .ANG.. That is, a fuse 73 for laser trimming in T 
shape in cross section is formed of the polysilicon film 3a and the metal 
silicide film 7a. Then, the photoresist mask 8 is removed. 
Referring now to FIG. 3D, a PSG film 4 is deposited to cover the fuse 73 by 
the CVD method. A contact hole 5 is then formed in a predetermined 
position of the PSG film 4, and an aluminum interconnection 6 is formed on 
the PSG film 4 and in the contact hole 5, so that the redundant circuit of 
a semiconductor device including the fuse 73 for laser trimming is 
completed. FIG. 4 is a plan view of the redundant circuit of a 
semiconductor device completed in accordance with the above described 
processes. 
FIG. 5 is a cross sectional view in which the fuse 73 of the redundant 
circuit shown in FIGS. 3D and 4 is disconnected by the laser beam. 
Referring now to FIG. 5, operation of disconnecting the fuse 73 is 
described. 
When a circuit of a semiconductor device is replaced, the laser beam is 
irradiated on the PSG film 4 in FIG. 3D, and laser beam energy is absorbed 
in the fuse 73, in which case the energy distribution is generally the 
Gaussian distribution, so that the temperature difference under heat 
occurs between the central portion and the end of the fuse 73. However, 
since the line width 1.sub.0 of the polysilicon film 3a constituting the 
first layer of the fuse 73 is narrower, by 2 .DELTA.1.sub.0, than the line 
width 1.sub.1 of the conventional fuse 3b shown in FIG. 1A, the 
temperature difference under heat between the central portion and the end 
of the fuse 73 is smaller as compared with the conventional fuse 3b. 
Accordingly, the polysilicon film 3a constituting the first layer of the 
fuse 73 is uniformly and reliably fused and expanded, and the fuse 73 is 
uniformly and reliably disconnected. 
In addition, as shown in FIG. 5, an opening 10 is formed in the PSG film 4 
after explosion and splash thereof. The width 10 of the bottom of the 
opening 10 is narrower by 2.DELTA.1.sub.0 than the width 1.sub.1 thereof 
in the conventional redundant circuit, so that the opening 10 is smaller 
than the conventional opening 9 in FIG. 2. Therefore, the effect on an 
adjacent circuit in disconnecting the fuse by the laser beam is reduced, 
thereby allowing high integration of a semiconductor device. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.