Semiconductor device equipped with a heat-fusible thin film resistor and production method thereof

A fuse fusible type semiconductor device capable of reducing energy required for fusing and a production method of the semiconductor device. In a semiconductor device equipped with a heat-fusible thin film resistor, the thin film resistor formed on a substrate 1 through an insulating film 2 is made of chromium, silicon and tungsten, and films 7 and 8 of a insulator including silicon laminated on the upper surface of the fusing surface, aluminum films 5 are disposed on both sides of the fusing surface and a barrier film 4. This semiconductor device is produced by a lamination step of sequentially forming a first insulating film 2, a thin film resistor 3, a barrier film 4 and an aluminum film 5 on a substrate 1 for reducing drastically fusing energy, an etching step of removing the barrier film 4 and the aluminum film 5 from the fusing region 31 of the thin film resistor 3, and an oxide film formation step of depositing the insulator including silicon films 7 and 8.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a semiconductor device equipped with a 
heat-fusible thin film resistor, and to a method of producing such a 
semiconductor device. 
2. Description of the Related Art 
A semiconductor device including a thin film resistor consisting of a 
chromium silicon (CrSi) type film and covered with a insulator including 
silicon film is known as a semiconductor device equipped with a 
heat-fusible thin film resistor (refer to Japanese Unexamined Patent 
Publication (Kokai) No. 3-106055). A semiconductor device having a 
structure including thin film resistor consisting of a chromium silicon 
(CrSi) type film and a metal oxide layer for lowering a fusing temperature 
of this thin film resistor, laminated on the thin film resistor, is also 
known (refer to Japanese Unexamined Patent Publication (Kokai) No. 
6-61353). 
The semiconductor device described above, wherein the thin film resistor 
consisting of the chromium silicon (CrSi) type film is covered with the 
insulator including silicon film, has excellent characteristics as a fuse 
fusible type semiconductor device including that it exhibits a small 
volume change at the time of fusing. 
In such a fuse fusible type semiconductor device, however, energy required 
for fusing is great, and thermal losses such as cracks occurring in the 
insulation film including silicon covering the surface of the 
semiconductor device, and deterioration of thermal characteristics, are 
likely to occur. Energy necessary for fusing can be lowered by laminating 
a metal oxide on the thin film resistor, but the resulting fusing 
temperature is not sufficiently lowered and further lowering is necessary. 
In view of the technical background described above, the present invention 
is directed to provide a fuse fusible type semiconductor device which 
requires less energy for fusing than conventional fuse fusible type 
semiconductor devices, but which does not cause thermal losses and 
deterioration of thermal characteristics such as cracks in an insulator 
including silicon covering the surface. 
SUMMARY OF THE INVENTION 
As a result of intensive studies in search for various fuse materials, the 
inventors of the present invention have discovered a thin film resistor 
capable of drastically reducing fusing energy when used as a thin film 
resistor, and have completed the present invention. 
The semiconductor device, according to the present invention, comprises a 
silicon substrate; a first insulator film formed on said silicon 
substrate, being made of a insulator including silicon; a thin film 
resistor formed on said first insulator film, as a fuse, comprising 
chromium, silicon and tungsten; a wiring portion formed on said thin film 
resistor, being made of aluminum or an alloy thereof; and a passivation 
film formed in contact with said wiring portion and said thin film 
resistor, being made of at least one compound selected from a silicon 
nitride and a insulator including silicon. 
A method of producing a semiconductor device according to the present 
invention comprises the steps of: a lamination step of sequentially 
forming, on a semiconductor substrate through a first insulator film, a 
thin film resistor comprising chromium, silicon, and tungsten as a fuse 
and a film for wiring made of aluminum or an alloy thereof; an etching 
step of removing said film for wiring laminated on said thin film resistor 
by etching; and a passivation step of depositing a passivation film on the 
surface of the laminate subjected to said etching treatment, said 
passivation film being made of at least one compound selected from a 
silicon nitride and an insulator including silicon. 
One of the characterizing features of the semiconductor device according to 
the present invention resides in that the heat-fusible thin film resistor 
is made of chromium, silicon and tungsten. When tungsten is added to a 
thin film resistor made of chromium-silicon, an amorphous ternary alloy is 
formed and its melting point lowers. For this reason, the thin film 
resistor made of chromium, silicon and tungsten can drastically reduce the 
thermal energy required to fuse the thin film resistor. As a result, the 
thermal stress applied to the insulation film including silicon covering 
the upper surface of the thin film resistor can be reduced, and the 
occurrence of cracks and the deterioration of thermal characteristics can 
be prevented. 
When the crystal structure of the cross section of the thin film resistor 
fused by the feed of power is analyzed by a transmission electron 
microscope, it is observed that an intermetallic compound having a high 
melting point, i.e. Cr.sub.3 Si, precipitates in the case of the 
conventional chromium-silicon thin film resistor. In the case of the 
chromium-silicon-tungsten thin film resistor according to the present 
invention, on the other hand, an intermetallic compound having a low 
melting point, i.e. CrSi.sub.2, precipitates. It is presumed from this 
fact that when tungsten is added to chromium-silicon to form an amorphous 
ternary alloy, a crystalline intermetallic compound having a low melting 
point is formed by heating, and cutoff of the current occurs at a portion 
of fusion or sublimation of this intermetallic compound. As a result, the 
total energy required for fusing is believed to drastically drop. 
It could be understood from the above description that the thin film 
resistor of the present invention can reduce the thermal energy required 
for fusion and can decrease the thermal stress imparted to the insulator 
including silicon such as the silicon nitride film covering the upper 
surface of the thin film resistor, thereby preventing deterioration, in 
comparison with the conventional chromium-silicon thin film resistor. 
The chromium-silicon-tungsten thin film resistor contains at least 20 to 50 
atm % of chromium, at least 1 to 20 atm %, preferably 2 to 14 atm %, of 
tungsten, and the balance of silicon. 
This chromium-silicon-tungsten film preferably has a composition capable of 
precipitating an intermetallic compound having a low melting point at the 
time of heat-fusion, from the aspect of its object. Small quantities of 
additives such as oxygen, nitrogen, and so forth, may be contained in this 
chromium-silicon-tungsten film. 
Besides the silicon oxide film (SiO.sub.x), PSG (phosphosilicate glass), 
BSG (borosilicate glass), BPSG (borophosphosilicate glass), etc., can be 
used as the insulator including silicon film to be formed on the upper 
surface of the thin film resistor. Further, SiN (silicon nitride) can be 
employed, too. Though this insulator including silicon film is preferably 
disposed on both upper and lower surfaces of the thin film resistor in 
contact therewith, it may be disposed on at least one of the surfaces. 
The semiconductor device according to the present invention can be produced 
by sequentially forming the thin film resistor, the barrier film made of a 
tungsten alloy, and the aluminum film for wiring on the substrate through 
a first insulating film, then removing the barrier film and the aluminum 
film on the fusing region of the thin film resistor by etching, and 
forming the insulator including silicon film on the surface of the 
laminate subjected to the etching treatment. 
The barrier film made of the tungsten alloy and disposed on both sides of 
the fusing opening portion of the upper surface of the thin film resistor 
preferably uses an alloy containing at least 5 to 50 atm % of tungsten and 
the balance of a metal. Small amounts of other additives may be contained 
in the barrier film. 
In the semiconductor device according to the present invention, the 
chromium-silicon-tungsten film constituting the thin film resistor can be 
heat-fused at a lower level of energy. Though this fusing mechanism has 
not yet been clarified, the intermetallic compound having a low melting 
point, i.e. CrSi.sub.2, is found formed from the observation of the 
section of the thin film resistor after heat-fusion. On the other hand, an 
intermetallic compound having a high melting point, i.e. Cr.sub.3 Si, 
precipitates as revealed through the observation of the cross section of 
the chromium-silicon thin film resistor according to the prior art. It is 
presumed that the existence of tungsten promotes the formation of an 
amorphous alloy in the thin film resistor which turns into the 
intermetallic compound of the low melting product by heating, so that the 
fusing can be performed at a lower level of fusing energy. As a result, 
fusing energy required for fusing the fuse can be drastically reduced in 
comparison with the chromium-silicon thin film resistor according to the 
prior art. Since the required fusing energy is small, the semiconductor 
device according to the present invention has small thermal defects such 
as cracks of the protective film, has high reliability and long 
durability, and is easy to handle because the range of the fusing voltage 
is broad.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a sectional view of a semiconductor device equipped with a 
heat-fusible thin film resistor to which the present invention is applied. 
This semiconductor device comprises a silicon substrate 1, a silicon oxide 
film 2 formed on this silicon substrate 1, a fuse 3 of a thin film 
resistor consisting of a chromium-silicon-tungsten film formed on the 
silicon oxide film 2, a barrier metal portion 4 and an aluminum wiring 
portion 5 formed in lamination on both sides of a fusing region 31 of this 
fuse 3, a PSG film 7 formed on these barrier metal portion 4, aluminum 
wiring portion 5 and fusing region 31 of the fuse 3, and a silicon nitride 
film (SiN) 8 for passivation, formed on this PSG film 7. 
The semiconductor device equipped with this thin film resistor was produced 
through the following production steps. 
First, a 1.2 .mu.m-thick silicon oxide film 2 was formed as a base 
insulating film on the silicon substrate 1 by an oxidization process. The 
silicon dioxide film 2 may be formed by a CVD process in place of the 
oxidization method. Next, a 0.015 .mu.m-thick chromium-silicon-tungsten 
film was formed on this silicon dioxide film 2 by a PVD process and was 
then etched into a predetermined shape so as to obtain the fuse 3 
(heat-fusible thin film resistor). 
A composite insulating film consisting of a silicon nitride layer as a 
lower layer and a silicon oxide film as an upper layer may be used as the 
base insulating film, and boron or phosphorus may be doped into the 
silicon oxide film. The chromium-silicon-tungsten film has a composition 
consisting of 29 atm % of Cr, 65 atm % of Si and 6 atm % of W. 
Next, a 0.15 .mu.m-thick titanium tungsten (TiW) alloy film 40 was formed 
by the PVD process, and a 1.1 .mu.m-thick aluminum film 50 was formed on 
the TiW alloy film 40 by the PVD process. FIG. 2 shows the cross section 
of the resulting laminate. By the way, the titanium tungsten alloy film 40 
has a composition consisting of 90 atm % of Ti and 10 atm % of W. 
A photoresist was disposed on the aluminum film 50, and only the titanium 
tungsten alloy film 40 and the aluminum film 50 were wet etched by using a 
mask obtained by opening the resist by photolithography. 
In this way, the aluminum wiring portions 5 were formed on both end 
portions of the fuse 3 in such a manner as to interpose the barrier metal 
portion 4 made of titanium tungsten between them (see FIG. 3). 
Next, as shown in FIG. 1, a 0.4 .mu.m-thick PSG film 7 was formed by the 
CVD process and a 0.5 .mu.m-thick silicon nitride (SiN) film 8 was formed 
by a plasma CVD process. Pad portions (not shown) were then formed by 
selective opening of these films 7 and 8, and wire bonding was carried out 
to the pad portions. The semiconductor device of this embodiment was 
produced through a series of these process steps. 
COMATIVE EXAMPLE 1 
A semiconductor device of Comparative Example 1 having the same structure 
as that of the semiconductor device of this embodiment was produced in the 
same way as described above with the exception that a chromium silicon 
film was used for the fuse 3 of the thin film resistor in place of the 
chromium-silicon-tungsten film. 
Evaluation 
Energy necessary for fusing was measured for the semiconductor device of 
this embodiment and the semiconductor device of Comparative Example 1, and 
fusing performances of both the fuses 3 were comparatively examined. FIG. 
4 shows the result of this measurement test. The fusing region 31 of each 
of these semiconductor devices had a thickness of 0.015 .mu.m, a length of 
9.6 .mu.m and a width of 6.4 .mu.m. 
The ordinate in FIG. 4 represents input energy per unit area of the fusing 
region expressed by input power (fusing voltage.times.feed 
current.times.pulse feed time.times.number of pulses) which was measured 
by a power meter in the test. The pulse feed time was kept constant (here, 
1 microsecond). The abscissa represents a fusing voltage. 
It can be seen from FIG. 4 that input energy drops when the fusing voltage 
is increased, but at a low fusing voltage, the energy necessary for fusing 
is extremely smaller in the product of this embodiment than in the product 
of Comparative Example 1. Accordingly, the product of this embodiment can 
by far greatly reduce the energy necessary for fusing than the product of 
Comparative Example 1. 
The section of the fusing region 31 of each of these two semiconductor 
devices was observed by a transmission electron microscope so as to 
analyze the crystal structure. As a result, precipitation of an 
intermetallic compound, i.e. CrSi.sub.2, having a low melting point was 
observed in the fuse 3 of the semiconductor device of this embodiment. On 
the other hand, precipitation of an intermetallic compound, i.e. Cr.sub.3 
Si, having a high melting point was observed in the fuse 3 of the 
semiconductor device of Comparative Example 1. 
In consideration of the observation result of the cross section by the 
transmission electron microscope, etc., it is estimated that such lowering 
of the melting point of the fuse 3 results from the following fact. 
Namely, the intermetallic compound precipitated upon heating due to the 
feed of power to the fuse 3 has a lower melting point in the product of 
this embodiment due to the tungsten content than that in the product of 
Comparative Example, and this remarkably reduces energy at the time of 
fusion and evaporation in comparison with the product of Comparative 
Example 1. 
As described above, the product of this embodiment can by far greatly 
reduce the energy necessary for fusion than that required by the product 
of Comparative Example 1. Therefore, the product of this embodiment can 
drastically reduce the thermal stress imparted to various films 
constituting the semiconductor device, particularly, the SiN film 8, and 
it is expected that cracks of the SiN film 8, etc., can be drastically 
reduced. 
In order to evidence the assumption described above, the minimum voltage 
capable of fusing without the occurrence of cracks was examined for the 
product of this embodiment and the product of Comparative Example 1 by 
changing the impressed voltage. The occurrence of cracks was examined by a 
Caros test. By the way, when the impressed voltage to the fuse 3 is 
lowered, the feed current becomes small, and thermal energy occurring at 
the fuse portion per unit time becomes small. Therefore, the temperature 
rise rate of the fuse becomes gentle, and the time necessary for fusing 
becomes elongated. As a result, the quantity of heat of the fuse portion 
transferred to the PSG and the SiN film 8 by heat conduction becomes 
great, and cracks are more likely to occur in the SiN film 8. In other 
words, energy necessary for fusing, which increases due to a low voltage, 
is believed to correspond to energy diffused to the PSG and the SiN film 
8. 
When a high voltage is impressed, on the contrary, the temperature rise 
rate of the fuse 3 is high and fusion occurs within a short time. 
Consequently, the diffusion quantity of the resulting energy to the PSG 
and the SiN film 8 becomes small, and fusion of the fuse 3 can be 
effectively conducted. 
When the application of the present invention to practical devices is taken 
into consideration, it is advantageous that the fusing voltage of the fuse 
is low, because when a large voltage is applied, other devices are likely 
to be destroyed in some cases. 
The results of experiments revealed that the maximum fusible voltage 
without the occurrence of cracks was 30 V in the product of this 
embodiment and was 75 V in the product of Comparative Example 1. 
Accordingly, in the product of this embodiment wherein fusion occurs at a 
low temperature, the occurrence of cracks can be suppressed even when a 
low voltage is impressed for fusing. 
COMATIVE EXAMPLE 2 
In the semiconductor device, the product of Comparative Example 2 is 
produced in the same way as the product of Comparative Example 1 with the 
exception that a tungsten oxide film 6 is interposed between the fuse 3 
and the PSG film 7 instead of directly adding tungsten to the thin film 
resistor. FIG. 5 shows the section of the semiconductor device so 
produced. The fusion characteristics of this Comparative Example 2 is a 
mixture between the characteristics of the product of the embodiment of 
the present invention and those of the product of Comparative Example 1. 
Though energy necessary for fusing is lower than that of the product of 
Comparative Example 1, the product of Comparative Example 2 obviously 
requires greater energy for fusing than the product of the embodiment of 
the present invention. 
The maximum fusible voltage without the occurrence of cracks was 50 V in 
the product of this Comparative Example 2, and was higher than 30 V in the 
product of the embodiment of the present invention. 
In comparison with Comparative Example 2 wherein the tungsten oxide is 
laminated on the chromium-silicon thin film resistor, the ternary alloy 
prepared by adding tungsten to the chromium-silicon thin film resistor of 
this embodiment, that is, the chromium-silicon-tungsten thin film 
resistor, can further reduce energy required for fusing. 
As a result of the studies on the energy reduction effect made by the 
present inventors, it has been found out that in the case of the thin film 
resistor according to this embodiment, the intermetallic compound 
(CrSi.sub.2) having a low melting point starts occuring at arbitrary 
positions of the thin film resistor substantially simultaneously with 
heating. It is therefore presumed that the thin film resistor is heated 
and fused instantaneously at a low level of fusing energy. 
In contrast, the thin film resistor of Comparative Example 2 is not heated 
and fused so instantaneously as in this embodiment. It is presumed that 
the tungsten oxide is molten from the interface of chromium, silicon and 
tungsten oxide and this melting phenomenon greatly affects fusing. Further 
studies on this phenomenon reveal the following fact. In the structure of 
Comparative Example 2, as also disclosed in Japanese Unexamined Patent 
Publication (Kokai) No. 6-61353, the intermetallic compound (CrSi.sub.2) 
having a low melting point starts being formed gradually from near the 
interface to which the tungsten oxide mixes, but a certain period of time 
is necessary before the whole tungsten oxide is molten into the thin film 
resistor. Therefore, lowering of the melting point of the thin film 
resistor is impeded immediately after heating, due to the portions unmixed 
with the tungsten oxide, and this impedes sufficient lowering of the 
melting point of the thin film resistor. 
As described above, the fuse device of this embodiment can be fused at a 
lower level of energy than the prior art devices, and has less occurrence 
of cracks but high reliability. Further, the minimum voltage that can be 
applied is low and the input energy quantity is small. Therefore, the fuse 
device is easier to handle.