Method for manufacturing a semiconductor device

A Ti film is formed on a semiconductor substrate having an element formed on the surface thereof by a sputtering method by using an ordinary DC magnetron sputtering unit under the conditions where Ar gas pressure is 1 mTorr and DC power is 4.4 kW. Under these conditions, the Ti film is formed as a continuous film within one second from the start of discharge so that if the Ti film is charged with secondary electrons generated by plasma used in sputtering, local charge-up does not occur. Thereafter, sputtering is continued and a Ti film of about 300 .ANG. in thickness is formed on the entire surface. Hence, secondary plasma electrons are prevented form causing the breakdown of an insulating film of the element.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for manufacturing a semiconductor 
device capable of preventing the breakdown of an insulating film formed on 
a semiconductor substrate in the manufacturing process of the 
semiconductor device and thereby manufacturing a highly reliable 
semiconductor device with a high yield. 
2. Description of the Related Art 
As semiconductor integrated circuits such as LSI become smaller in size, 
efforts to make elements smaller are attempted. Such efforts involve, for 
example, forming a shallower impurity diffusion layer serving as a 
source-drain region with a smaller area and forming narrower wirings 
connecting elements. Subsequently, electric resistances of the impurity 
diffusion layer and wiring increase, which adversely influences or hampers 
the high-speed operation of elements. To avoid this, in the conventional 
semiconductor device, a high melting point metal silicide layer is formed 
on the surface of the impurity diffusion layer so as to decrease the 
resistance of the impurity diffusion layer and thereby to increase the 
operation speed of the elements. An example of a semiconductor device 
intended to increase operation speed of element is a semiconductor device 
using a Ti silicide layer (U.S. Pat. No. 4,855,798). 
FIGS. 1A through 1D are cross-sectional views in the order of manufacturing 
steps showing a method of manufacturing a conventional semiconductor 
device intended to increase operation speed by using a Ti silicide layer. 
As shown in FIG. 1A, an element separation film 11 consisting of an 
insulating film is selectively formed on the surface of a semiconductor 
substrate 20 thereby to demarcate an element region. Next, an oxide film 
(not shown) and a polysilicon film (not shown) are sequentially formed on 
the surface of the element region. Thereafter, the oxide film and the 
polysilicon film are patterned into a gate shape by lithography and dry 
etching, thereby forming a gate oxide film 14 consisting of the oxide film 
and a gate electrode 13 consisting of the polysilicon film. An oxide film 
(not shown) is then formed on the entire surfaces thereof and etched back, 
thereby forming a sidewall insulating film 12 consisting of the remaining 
oxide film on the sidewall of the gate electrode 13. After that, ions are 
implanted from above and heat treatment is conducted to the substrate 20, 
thereby to selectively form a diffusion layer 15. 
A natural oxide film (not shown) formed on the gate electrode 13 and the 
diffusion layer 15 is then removed by wet etching using dilute 
hydrofluoric acid and the like. As shown in FIG. 1B, a Ti film 19b of 
about 300 .ANG. in thickness is formed thereon. 
As shown in FIG. 1C, heat treatment is conducted to the substrate and a Ti 
silicide layer 17 of C49 layer consisting of high resistance TiSi.sub.2 is 
formed in a region in which the Ti film 19b and the gate electrode 13 
contact with each other and a region in which the Ti film 19b and the 
diffusion layer 15 contact with each other in a self-aligned manner manner 
(see FIG. 1B). Since the heat treatment is conducted under nitrogen 
atmosphere, a TiN layer 18 of about several tens .ANG. in thickness is 
formed on the surface of the unreacted Ti film 19c. 
Next, as shown in FIG. 1D, the unreacted Ti film 19c and TiN film 18 on the 
element separation film 11 and on the sidewall insulating film 12 are 
removed. Heat treatment is then conducted to the substrate under nitrogen 
atmosphere, thereby transferring the high resistance Ti silicide film 17 
to a Ti silicide layer of C54 film consisting of low resistance 
TiSi.sub.2. Thus, in the conventional semiconductor device, the resistance 
of the surface of the diffusion layer 15 is decreased in an effort to 
increase the operation speed of elements. 
If a semiconductor device is manufactured by the manufacturing method 
illustrated in FIGS. 1A through 1D, the following problems arise. During a 
sputtering process, secondary electrons contained in plasma and the like 
may pass through the Ti film 19b to the gate oxide film 14 and flow into 
the substrate 20. If an electric current flows between the Ti film 19b and 
the substrate 20, the dielectric breakdown of the gate oxide film 14 
occurs thereby to cause withstand voltage failure. Due to this, the 
reliability of the semiconductor device greatly deteriorates and the 
manufacturing yield of the semiconductor device is lowered. 
As a way to prevent the breakdown of the gate oxide film 14 charged with 
secondary electrons, there has been proposed a method of forming a Ti film 
not by normal sputtering but by collimate sputtering. FIG. 2 is a typical 
view showing the normal sputtering method using a DC magnetron. FIG. 3 is 
a typical view showing the collimate sputtering method. As shown in FIGS. 
2 and 3, a target 31 is provided in a sputtering unit (not shown) and a 
cathode magnet 36 is provided above the target 31. A substrate 33 is 
disposed on a stage 34 provided below the target 31. Thereafter, plasma 32 
is generated between the substrate 33 and the target 31 and a Ti film is 
formed on the substrate 33. The Ti film formation method is applied to 
both the normal sputtering and the collimate sputtering. 
However, as shown in FIG. 2, if the normal sputtering method is employed, 
the plasma 32 is generated right above the substrate 33. Due to this, 
secondary electrons tend to fly into the substrate 33. In the collimate 
sputtering method shown in FIG. 3, by contrast, a collimator 35 is 
disposed between a substrate 33 and the plasma 32. The collimator 35 is 
provided with a plurality of holes passing through the thickness direction 
of the collimator 35 in parallel. If the plasma 32 passes through the 
holes of the collimator 35, secondary electrons from the plasma 32 are 
trapped by the collimator 35. It is, thus, possible to prevent the 
dielectric breakdown of the gate oxide film 14 shown in FIGS. 1A through 
1D from occurring. 
If the collimate sputtering method is employed as shown in FIG. 3, a Ti 
film attaches to the collimator 35 and the diameter of the collimator 35 
is decreased. Owing to this, it is necessary to correct film formation 
rate as the target is consumed, which disadvantageously makes maintenance 
difficult. Besides, due to the attachment of the Ti film onto the 
collimator 35, target consumption efficiency deteriorates thereby to push 
up production cost. They are grave disadvantages to the mass production of 
semiconductor devices. The collimate sputtering is originally designed to 
form a film on the bottom surface of a hole in a good coating state even 
if the aspect ratio indicating the depth of a hole formed in the surface 
of a substrate to the diameter of the hole is high. Considering the above, 
it is less advantageous to use the collimate sputtering in the step of 
forming a Ti film which does not require a high coating state. 
In view of the mass production of semiconductor devices, it is desirable to 
form a Ti film for forming a Ti silicide film by using the normal 
sputtering method. If the normal sputtering method is actually employed, 
it is required to provide a method for manufacturing a semiconductor 
device without the dielectric breakdown of the gate insulating film 14. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a method 
for manufacturing a semiconductor device capable of preventing the 
breakdown of an insulating film of an element by secondary electrons of 
plasma in a case where a metal film is formed on elements on the surface 
of a semiconductor substrate. 
A method for manufacturing a semiconductor device according to the present 
invention comprises the steps of forming an element on a silicon 
substrate; and forming a metal film on the element. The step of forming 
the metal film is conducted under a condition to allow a continuous metal 
film to be obtained by one second after the start of film formation. 
The metal film may be formed by sputtering. In this case, the metal film is 
preferably formed under condition that DC power for forming the metal film 
is 4 to 10 kW. The metal film can be also formed by a plasma chemical 
vapor deposition method. 
Further, the metal film may consist of a metal for forming a metal silicide 
by reacting with silicon. The metal film may consist of at least one metal 
selected from the group consisting of, for example, Ti, Co, Ni, Mo, W and 
Ta. 
Moreover, the step of forming the element on the silicon substrate may 
include the steps of selectively forming an insulating film on the silicon 
substrate; forming a gate electrode on the insulating film; and forming a 
diffusion layer on a surface of the silicon substrate. 
In the present invention, the continuous film refers to one in a case where 
the measured value of a sheet resistance at sputtering time is 
approximated by a formula: Y=a/X (where a is a constant) in a graph having 
the sheet resistance of the metal film on the Y axis and the sputtering 
time on the X axis. The measured value is not necessarily expressed by the 
above formula in a strict sense and may fall within a range of -40% to 
+40% of the formula. 
The inventors of the present invention conducted various experiments and 
studies in order to prevent the breakdown of an insulating film by 
secondary electrons from plasma. As a result, it was discovered that the 
cause of dielectric breakdown in a case where a metal film is formed on 
elements by a conventional method was the attachment of the metal film 
onto the element surface as a band-shaped discontinuous film in a film 
formation initial period. In other words, as shown in FIG. 4, in a film 
formation initial period from the start of forming a Ti film (or metal 
film) 19a to 2 to 3 seconds after the start, the Ti film 19a attaches to 
the element surface as a band-shaped discontinuous film. After that, a 
continuous Ti film is formed. The discontinuous Ti film 19a in the film 
formation initial period is in a state in which the film floats 
electrically. Due to this, the Ti film 19a is charged with secondary 
electrons contained in, for example, plasma for use in sputtering, and 
charge-up or an phenomenon that high potential is locally occurs. If the 
charge-up potential is increased to exceed a certain threshold value, an 
electric current flows across the substrate 20 after passing through an 
insulating film 14 provided below the Ti film 19a. In case of the 
discontinuous Ti film 19a formed above the gate electrode 13, in 
particular, an electric current passes through the gate oxide film 14 
through the gate electrode 13 and flows across the substrate 20. As a 
result, the dielectric strength of the gate oxide film deteriorates and 
withstand voltage failure occurs in the end. 
Considering the conventional problems, the present invention is designed to 
obtain a continuous film in the film formation initial period. Namely, as 
shown in FIG. 5, according to the present invention, a Ti film is formed 
on elements under conditions for allowing a continuous Ti film (metal 
film) 6a to be formed by one second after the start of forming the Ti 
film. Therefore, even if the Ti film 6a is charged with secondary 
electrons generated from, for example, plasma, local charge-up does not 
occur. Thus, it is possible to prevent the breakdown of the gate oxide 
film 4 and thereby to manufacture highly reliable semiconductor devices 
with a high yield.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A semiconductor device according to the preferred embodiment of the present 
invention will be concretely described with reference to the accompanying 
drawings. FIGS. 6A through 6D are cross-sectional views showing a method 
for manufacturing a semiconductor device in the embodiment of the present 
invention in the order of manufacturing steps. As shown in FIG. 6A, an 
element separation film 1 consisting of an insulating film is selectively 
formed on the surface of a semiconductor substrate 10 thereby to demarcate 
an element region. Next, on the surface of the element region, an oxide 
film (not shown) and a polysilicon film (not shown) are sequentially 
formed and then patterned into a gate shape by lithography and dry 
etching, thereby forming a gate oxide film 4 consisting of the oxide film 
and a gate electrode 3 consisting of the polysilicon film. Thereafter, an 
oxide film (not shown) is formed on the entire surface and then etched 
back, thus forming a sidewall insulating film 2 consisting of the 
remaining oxide film on the sidewall of the gate electrode 3. Ions are 
then implanted from above and heat treatment is conducted to the substrate 
10, thereby selectively forming a diffusion layer 5. 
A natural oxide film (not shown) formed on the gate electrode 3 and the 
diffusion layer 5 is removed by wet etching using dilute hydrofluoric 
acid. Thereafter, as shown in FIGS. 6B and 6C, a Ti film 6a is formed on 
the surfaces thereof by sputtering. In this embodiment, the Ti film 6a is 
formed by using a normal DC magnetron sputtering unit under film formation 
conditions of, for example, Ar gas pressure of 1 mTorr and DC power 
(direct current power) of 4.4 kW. Under these conditions, in a film 
formation initial period from the start of discharge to 1 second after the 
start, the Ti film 6a remains a continuous film as shown in FIG. 6B. Even 
if the Ti film 6a is charged with secondary electrons generated from 
plasma or the like for use in sputtering, no local charge-up occurs. By 
continuing sputtering, the Ti film 6a of about 300 .ANG. in thickness is 
formed on the entire surface. 
Next, as shown in FIG. 6D, heat treatment is conducted at 700.degree. C. 
for 30 seconds under nitrogen atmosphere by using a lamp annealing 
instrument and a Ti silicide layer 7 of C49 layer consisting of high 
resistance TiSi.sub.2 is formed in a self-aligned manner in a region in 
which the Ti film 6a and the gate electrode 3 contact with each other and 
a region in which the Ti film 6a and the diffusion layer 5 contact with 
each other. Since the heat treatment is being conducted under nitrogen 
atmosphere, a TiN layer 8 of about several tens .ANG. in film thickness is 
formed on the surface of the Ti film 6a. 
Thereafter, the unreacted Ti film 6b and TiN layer 8 on the element 
separation region 1 and the sidewall insulating film 2 shown in FIG. 6D 
are removed by a aqueous solution consisting of ammonia and hydrogen 
peroxide. Heat treatment is then conducted at 850.degree. C. for 10 
seconds under nitrogen atmosphere using the lamp annealing instrument, 
thereby transferring the high resistance Ti silicide layer 7 to a Ti 
silicide layer of C54 layer consisting of low resistance TiSi.sub.2 as 
shown in FIG. 6E. 
In this embodiment, as shown in FIG. 6B, while the Ti film 6a is being 
formed by sputtering, the Ti film 6a is not sporadic in a band-shaped 
state but continuous in the film formation initial period from the start 
of discharge to one second after the start. 
FIG. 7 is a graph showing the relationship between sheet resistance and 
sputtering time for sputtering power of 4.4 kW and 1.1 kW while the 
ordinate axis indicates sheet resistance and the abscissa axis indicates 
sputtering time. In FIG. 7, .largecircle. shows a measured value of a 
sheet resistance at a sputtering time while sputtering power is 4.4 kW and 
.increment. shows a measured value of a sheet resistance at a sputtering 
time while sputtering power is 1.1 kW. Since sputtering time is 
proportional to a film thickness, it is possible to consider that the 
abscissa axis indicates the film thickness in FIG. 7. Normally, the sheet 
resistance of a continuous film and the film thickness satisfies a 
relationship as expressed by the following Formula 1 and sheet resistance 
is inversely proportional to film thickness, where the specific resistance 
is constant. 
(Sheet Resistance)=(Specific Resistance)/(Film Thickness) (1) 
In this embodiment where sputtering power is 4.4 kW, fitting can be 
conducted by the method of least squares using an approximate function of 
Y=a/X corresponding to the Formula 1 above as indicated by solid line 21 
in FIG. 7. Namely, while sputtering power is 4.4 kW, even 0.2 of a second 
after the start of discharge, a continuous Ti film can be formed. If 
fitting is conducted in the same manner by using an approximate function 
Y=a/X while sputtering power is 1.1 kW, measured points are shifted 
greatly from the fitting curve as indicated by solid line 22 in FIG. 7. 
This means that a part of the measured points do not satisfy the above 
Formula 1. 
If fitting is conducted using an approximate function Y=a/(X-b) except for 
the measured point of about 3 seconds after the start of discharge while 
sputtering power is 1.1 kW, the approximate function is consistent with 
the measured points as shown in a broken line 23 as in the case of the 
sputtering power of 4.4 kW. This indicates that the removed measured 
points do not satisfy the above formula. In other words, while sputtering 
power is 1.1 kW, a continuous film is not formed for about three seconds 
after the start of discharge but discontinuous Ti films in band-shaped 
state appear sporadically. Even with the sporadic and discontinuous films, 
a current flows by the tunneling effect with a few volts if the distance 
between adjacent films is several to several tens .ANG., and sheet 
resistance can be measured. In the present invention, therefore, it is 
considered that a continuous film has been formed if a measured value of a 
sheet resistance for sputtering time is approximated by the formula Y=a/X 
(where a is a constant). In this case, however, the measured value is not 
necessarily expressed by the above formula in a strict sense and might 
fall within a range of -40% to +40% of the formula. 
FIG. 8 is a graph showing the rate of failure occurrence in a case where a 
Ti film is formed with different sputtering power while the ordinate axis 
indicates the rate of failure occurrence of gate withstand voltage. It is 
noted that the rate of failure occurrence was compared between a case 
where a Ti film is formed for forming a Ti silicide layer under the 
conditions according to this embodiment (i.e., sputtering power of 4.4 kW) 
under which a continuous film is formed by one second after the start of 
discharge and a case where a Ti film is formed under the conditions of the 
comparison example(i.e., sputtering power of 1.1 kW) under which 
discontinuous, band-shaped Ti films are formed for about 3 seconds after 
the start of discharge. 
A method for measuring the rate of failure occurrence will be described. 
First, after a specimen having the constitution shown in FIG. 6A is formed 
as a test pattern, a Ti film is formed on the surface of the specimen and 
then removed therefrom by sputtering and a voltage of 0 to 12 V is 
variably applied between the gate electrode and the substrate. Voltage at 
which the dielectric breakdown of the gate oxide film occurs and high 
current thereby flow, is measured, and it is determined as a failure if 
the measured voltage is 3 V or less. The ratio of the number of defects to 
the number of measured points is calculated as a rate of failure 
occurrence. The normal gate oxide film has dielectric strength which does 
not deteriorate, depends on its film thickness and area. If, for example, 
the film thickness is about 100 .ANG. and the area is about 32 mm.sup.2, 
the film can withstand voltage of about 10 V. 
As shown in FIG. 8, if a Ti film is formed with sputtering power of 4.4 kW, 
the rate of withstand voltage failure of the gate oxide film is quite low, 
and becomes 0.58%. This is because the Ti film is formed under the 
conditions where a continuous Ti film is formed from the start of forming 
the Ti film to one second after the start. On the other hand, if a Ti film 
is formed with sputtering power of 1.1 kW, band-shaped discontinuous films 
are formed from the start of discharge to about 3 seconds after the start 
and the rate of failure occurrence is as high as 14.53%. The rate 
increases by about 25 times of the rate in the embodiment. If a Ti film is 
formed by sputtering under conditions where a continuous film is formed 
from the start of film formation to one second after the start, then it is 
possible to obtain an advantage of preventing the occurrence of withstand 
voltage failure to the gate oxide film. 
In the embodiment shown in FIGS. 6A to 6E, conditions under which a 
continuous film is formed from the start of film formation to one second 
after the start are set such that the DC power is increased from the 
conventional range to thereby increase the formation rate of a Ti film. In 
the present invention, conditions other than the DC power may be changed. 
It is possible to obtain the same advantage as in the case where the 
sputtering power is 4.4 kW if the method in which another film formation 
parameter such as gas pressure during sputtering is optimized or a cathode 
magnet for use in discharge is optimized is employed. Such a method has to 
satisfy the conditions under which a continuous metal film can be formed 
from the start of film formation to one second after the start. 
In the above embodiment, description has been given to a case where 
sputtering is conducted for forming a Ti film. According to the present 
invention, it is possible to obtain the same advantage even if a Ti film 
is formed by using, for example, the plasma CVD method and the like under 
the conditions specified by the present invention. Further, in the above 
embodiment, description has been given to conditions for forming a Ti film 
to form a silicide layer. According to the present invention, the same 
advantage can be obtained by using, for example, one of Co, Ni, Mo, W and 
Ta as a metal film to form a silicide layer by reacting with silicon. 
As described in detail so far, according to the present invention, 
conditions for forming a metal film on elements is appropriately specified 
to thereby form a continuous film from the start of the formation of the 
metal film to one second after the start. Due to this, it is possible to 
prevent the breakdown of an insulating film caused by the metal film being 
charged, whereby a semiconductor device of higher reliability can be 
manufactured with a high yield.