Method for manufacturing a semiconductor device with a metallic interconnection layer

A method for manufacturing a semiconductor device having a metallic interconnection layer is described. The method includes the steps of providing a substrate having an insulating film, forming at least one contact hole in the insulating film, forming a first metallic interconnection layer on the insulating film so that the contact hole is filled with the interconnection layer, and forming a second metallic interconnection layer on the first layer to provide a builtup structure. The second layer may be formed by a high temperature sputtering method wherein a substrate temperature is 400.degree. C. or over or by a procedure which includes forming a second metallic interconnection layer by an ordinary sputtering method and heating the substrate to a temperature not lower than 450.degree. C. to cause the second layer to be reflown. By this, the second has a smooth surface irrespective of the presence of irregularities on the surface of the first layer.

BACKGROUND OF THE INVENTION 
This invention relates to a method for manufacturing a semiconductor device 
having a metallic interconnection layer. More particularly, the invention 
relates to a method for manufacturing a semiconductor device which has a 
metallic interconnection builtup layer structure including a metallic 
layer, such as a blanket-W filled in a contact hole and a metallic layer, 
such as Al, deposited thereof so that the layer structure has good surface 
smoothness. 
For the interconnection in semiconductor devices, there have been hitherto 
widely used Al materials such as Al, Al--Si and the like which are 
inexpensive. 
On the other hand, semiconductor devices have been recently designed as 
miniaturized, so that the size of contact holes provided through an 
insulating layer, such as SiO.sub.2, on a substrate becomes small. 
However, since the insulating layer has little change in its thickness, 
the aspect ratio of the contact hole inevitably increases. 
When, however, the interconnection layer having a large aspect ratio is 
formed of an Al material alone which has not a good step coverage, the 
contact hole is apt to undergo conduction failure, thus lowering the 
reliability of the resultant semiconductor device. 
On the other hand, there has been proposed a so-called selective W-CVD 
method in Japanese Laid-open Patent Application No. 62-229959 which 
corresponds to U.S. Pat. No. 4,824,802. In the method, WF.sub.6 is 
subjected to reduction reaction to selectively form a W film in the 
contact hole alone. However, it is difficult to attain complete 
selectivity according to the selective W-CVD method. In addition, there is 
the problem that it is not possible principally to simultaneously fill 
contact holes having different depths. 
Moreover, there has also been proposed a so-called blanket W-CVD method 
(Japanese Laid-open Patent Application No. 62-229959) wherein after 
formation of contact holes, a W film is deposited over the entire surface 
of an insulating layer to fill the contact holes therewith, followed by 
etching the W film to leave the W film only in the contact holes. 
According to this method, it is possible to fill the contact holes with 
different depths at the same time. 
According to this blanket W-CVD method, a TiON layer may be formed on the 
SiO.sub.2 insulating film as a bonding layer for improving the bonding 
between the SiO.sub.2 insulating film and the W film, followed by filling 
the W film. Where the W film is formed on the SiO.sub.2 insulating film 
according to the blanket W-CVD method, W which has a high melting point of 
338.degree. C. has to be formed as a film at relatively high temperatures. 
The TiON layer also serves as barrier layer for both SiO.sub.2 insulating 
layer and the W film, thereby inhibiting W from entering into the 
SiO.sub.2 film, ensuring good electric characteristics. 
However, even when the contact hole is filled at a good coverage according 
to the blanket W-CVD method, the specific resistance of the W film is 10 
.mu..OMEGA..cm which is higher than the specific resistance of Al--Si of 
2.9 .mu..OMEGA..cm. Accordingly, where the metallic interconnection layer 
is formed of the W film alone, the sectional area of the layer has to be 
increased undesirably. 
To solve the above problem, it may occur that a W plug is formed only at a 
contact portion and a low resistance interconnection layer such as Al--Si 
is deposited thereon to provide a builtup structure. More particularly, 
there is considered a method wherein a W film is formed as filling the 
contact portion according to blanket W-CVD method, the thus formed W film 
is etched back to form a plug, and a metallic interconnection layer such 
as of Al--Si is formed on the W plug. 
However, this method requires an etching-back step of the W film, thus 
complicating the formation step of the interconnection layer. In addition, 
the Ti-based bonding layer formed between the SiO.sub.2 insulating film 
and the W film is also etched back. When a chlorine gas is used for the 
etching back, the Ti-based bonding layer is etched at a rate higher than 
the W film, thereby forming a recess at the upper portion of the side 
walls of the contact hole. Moreover, at the time of the formation of the W 
film, W deposits from the bottom and the side walls in the inside of the 
contact hole. The contact hole after completion of the filling has a seam 
at the central portion thereof established by the contact between the W 
films grown from the bottom and the side walls. The seamed portion becomes 
weak in strength. Accordingly, if the W film after completion of the 
filling is etched back, the seamed portion is more likely to be etched, 
thereby forming a recess. When the etched-back W film has the recess, the 
coverage of the interconnection layer formed thereon degrades. Thus, there 
arises the problem that the reliability of the interconnection layer 
lowers. 
To cope with these problems, we have proposed a method wherein a W film 
(first metallic layer) is filled in a contact hole according to the 
blanket W-CVD method, after which the W film is etched back, and an Al 
interconnection layer (second interconnection layer) is formed. This 
method is set out in Japanese Patent Application No. 04-326128 which 
corresponds to U.S. Ser. No. 08/149,946 filed Nov. 10, 1994. The 
application is assigned to the assignee of the present application and is 
incorporated herein by reference. 
However, in the method wherein the W film is formed according to the 
blanket W-CVD method, after which an Al-based interconnection layer is 
formed on the W film without etching back, if the coverage in the contact 
hole of the W film is improved, irregularities (e.g. approximately 20 nm 
in the height between the peak and the valley of the irregularities) are 
undesirably formed on the surfaces of the W film. Accordingly, when the 
Al-based interconnection layer is formed on the W film, the resultant 
interconnection layer has its surface emphasized with the irregularities 
of the W film, thus presenting the problem that the coverage of the 
Al-based interconnection layer lowers. Moreover, there arises the problem 
that when an interconnection structure is further formed on the Al-based 
interconnection layer, many adverse influences are produced. For instance, 
when the interconnection layer which has not a smooth surface is patterned 
according to a lithographic method, a desired pattern cannot be formed by 
the influence of the surface reflection. Moreover, there is another 
problem that where a contact plug is formed at the interconnection layer, 
the contact resistance between the contact plug and the interconnection 
layer increases. 
On the other hand, if the CVD conditions are so controlled so that the 
surface irregularities of the W film are reduced, there arises the problem 
that the coverage in the contact hole of the W film lowers. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a method for 
manufacturing a semiconductor device with a metallic interconnection layer 
which overcomes the problems involved in the prior art methods. 
It is another object of the invention to provide a method for a 
semiconductor device with a metallic interconnection layer wherein a 
contact hole is filled with a metallic layer such as of blanket-W on which 
a metallic layer such as Al is formed to provide an interconnection layer 
having a builtup structure, the interconnection layer having improved 
surface smoothness. 
We have found that the above objects can be achieved by filling a contact 
hole with a first metallic interconnection layer such as of blanket-W and 
forming a second metallic interconnection layer such as of Al is formed on 
the first layer whereupon the second metallic interconnection layer is 
heated to not lower than 400.degree. C., i.e. the second metallic 
interconnection layer may be formed according to a high temperature 
sputtering method or the substrate is heated after the formation of the 
second metallic layer by ordinary sputtering is heated to cause the second 
metallic interconnection film to be reflown thereby forming a final second 
metallic interconnection layer. 
More particularly, the invention provided a method for manufacturing a 
semiconductor device with a metallic interconnection layer which comprises 
the steps of: 
forming an insulating layer on a substrate; 
forming a contact hole in the insulating layer to expose a selected portion 
of the substrate; 
depositing a first metallic interconnection layer to fill the contact hole 
laterally above the insulating layer; and 
forming a second metallic interconnection layer on the first metallic 
interconnection layer while the substrate is heated at a temperature of 
400.degree. C. or above.

PREFERRED EMBODIMENTS OF THE INVENTION 
Reference is now made to the accompanying drawings wherein like reference 
numerals indicate like parts or members. 
FIG. 1 shows a fundamental embodiment of a method for forming a metallic 
interconnection layer of a semiconductor device. In this embodiment, as 
shown in FIG. 1A, a substrate 1 having an insulating film 2 is provided 
and is formed with a contact hole a in the insulating film 2 so that the 
substrate 1 is exposed. The substrate 1 is not critical and may be a Si 
substrate formed, for example, with a diffusion layer 3. The insulating 
film 2 formed on the substrate 1 may be any layer insulting film formed by 
any ordinary method and is, for example, a SiO.sub.2 film formed according 
to a CVD method. The method for making the contact hole a may be a 
photolithographic method, like prior art methods. 
As shown in FIG. 1B, a first metallic interconnection layer 5 is formed on 
the insulating film to fill the contact hole a therewith. Preferably, a 
bonding layer 4 is preliminarily formed on the insulating layer 2 so as 
not only to improve the bonding properties of the first metallic 
interconnection layer 5, but also to serve as a barrier between the 
insulating film 2 and the first metallic interconnection layer 5. The 
first metallic interconnection layer 5 is formed on the bonding layer 4. 
The bonding layer 4 should preferably be formed of Ti materials according 
to sputtering or reactive sputtering. The bonding layer 4 may be 
constituted of a single layer or of a builtup structure of a plurality of 
layers. For instance, the bonding layer 4 may have a builtup structure of 
a Ti layer and a TiON layer. 
The first metallic interconnection layer 5 is a metallic layer which is 
formed of W, Mo, Ti, Pt, Cu or silicides thereof, or an Al metal or alloys 
according to a CVD method. Of these, it is preferred to form a W film 
according to a blanket W-CVD method. Where an Al-based metallic layer is 
formed as the first metallic layer 5 according to the CVD method, the 
Al-based material used may be an organoaluminum compound such as 
(CH.sub.3).sub.3 Al. 
The thus formed first metallic interconnection layer 5 is formed as well 
filled in the contact hole a, whereupon irregularities are inevitably 
formed on the surface thereof as shown in FIG. 1B. To avoid this, in the 
practice of the invention, a second metallic interconnection layer 7 is 
formed on the first metallic interconnection layer 5, preferably through 
the bonding layer 6 as will be described hereinafter, by high temperature 
sputtering wherein the substrate temperature is maintained at 400.degree. 
C. or over, thereby forming a metallic interconnection layer having a 
builtup structure of the first and second metallic layers. 
The substrate temperature at the time of the high temperature sputtering 
for the second metallic interconnection layer may, more or less, depend on 
the type of metal for the second metallic interconnection layer 7 and is 
preferably in the range of from 400.degree. to 600.degree. C., more 
preferably from 450.degree. to 500.degree. C. In this manner, when the 
second metallic layer 7 is formed according to the high temperature 
sputtering wherein the substrate temperature is higher than under ordinary 
sputtering conditions, the second metallic layer 7 can be formed as having 
a smooth surface as shown in FIG. 1C. When the substrate temperature at 
the time of sputtering is 300.degree. C. or below, the second metallic 
layer 7 does not become smooth on the surface thereof in a satisfactory 
manner. Over 600.degree. C., surface roughness inherent to the high 
temperature sputtering takes place in the second metallic layer 7. 
Alternatively, the surface smoothness may be attained using, in place of 
the high temperature sputtering, a procedure wherein the second metallic 
interconnection layer is formed at an ordinary substrate temperature and 
then, the substrate temperature is raised to 450.degree. C. or higher 
thereby causing the second metallic layer to be reflown on the substrate. 
According to this procedure, the substrate temperature at which the second 
metallic interconnection layer is formed can be made lower than that of 
the high temperature sputtering, making it possible to form the second 
metallic layer having better surface smoothness. 
Where the second metallic layer is reflown after the formation of the 
layer, the substrate temperature at the time of reflowing should 
preferably be within a range of 450.degree. to 500.degree. C. When the 
substrate temperature is lower than 450.degree. C., reflow does not 
proceed satisfactorily. On the contrary, when the substrate temperature 
exceeds 600.degree. C., the second metallic layer melts with some 
possibility that the smoothness may lower. 
It will be noted that when the contact hole a is filled with an Al-based 
metal or alloy such as Al--Si according to the high temperature sputtering 
without filling with the first metallic interconnection layer 5 such as a 
W film, the substrate temperature at the time of sputtering should be 
heated to approximately 500.degree. C. In this connection, according to 
this embodiment of the invention, since the contact hole a has been 
already filled with the first metallic interconnection layer 5 prior to 
the formation of the second metallic layer 7, the second metallic layer 7 
is expected only to smooth the surface irregularities of the preliminarily 
formed first metallic layer 5 such as a W film. Accordingly, the substrate 
temperature at the time of the high temperature sputtering or reflowing 
may be at 400.degree. C. or over which is lower than a substrate 
temperature required for filling the contact hole a by the high sputtering 
temperature, i.e. approximately 500.degree. C. Thus, since the high 
sputtering can be performed at a substrate temperature which is lower than 
in the case of high temperature sputtering for filling the contact hole a 
directly with an Al-based metal or alloy, the thermal history of the 
substrate can be mitigated, not necessitating any specific barrier metal 
structure. 
The second metallic layer formed by the high temperature sputtering or 
reflowing should preferably be made of Al or Al alloys containing at least 
one of Si, Cu and Ti. 
It is preferred that prior to the formation of the second metallic layer 7, 
the bonding layer 6 consisting of Ti, TiON, TiN or TiW is formed on the 
first metallic layer 5. By this, it becomes possible to improve the 
adhesion or bonding between the first and second metallic interconnection 
layers 5 and 7. 
The metallic interconnection layer thus formed can be patterned using 
photolithographic techniques, like known interconnection layers. The 
metallic interconnection layer formed by the method of the invention may 
be built up with other insulating layer or interconnection layer. As a 
matter of course, a contact plug may be formed in association with the 
interconnection layer. 
For instance, as shown in FIG. 2, an antireflection film 8 may be formed on 
the second metallic interconnection layer 7, on which a resist film 9 is 
formed, followed by patterning of the photoresist by photolithography. In 
the case, the since the second metallic layer is smooth on the surface 
thereof, the resist film 9 may be patterned in high accuracy. 
When etched after the patterning of the resist film 9, the metallic 
interconnection layer can be formed in a desired pattern as shown in FIG. 
3. The apparatus used to form the pattern as set out above is not 
critical, any apparatuses which are ordinarily used for fabricating 
metallic interconnection layers may be used in the practice of the 
invention. For instance, there may be used microwave plasma etching 
devices, parallel plane plate etching devices and magnetron RIE devices. 
As will be apparent from the foregoing, according to the invention, a first 
metallic interconnection layer is filled in a contact hole of an 
insulating film formed on a substrate and a second metallic 
interconnection layer formed on the first layer to provide a metallic 
interconnection layer of a builtup structure, whereupon the substrate 
temperature at the time of forming the second metallic layer is maintained 
at 400.degree. C. or over. Alternatively, the second metallic layer may be 
formed by an ordinary sputtering method, after which the substrate is 
heated to cause the second metallic layer to be reflown. By this, even if 
the first metallic film has irregularities on the surface thereof, the 
second metallic layer is formed as having a smooth surface. 
The present invention is more particularly described by way of examples. 
EXAMPLE 1 
(i) Formation of a metallic interconnection layer 
According to the procedure illustrated in FIGS. 1A to 1C, a metallic 
interconnection layer was formed. 
A Si substrate 1 formed with a diffusion layer 3 was formed thereon with an 
insulating layer 2 made of a SiO.sub.2 film according to an ordinary CVD 
method. A resist film was subsequently applied onto the SiO.sub.2 film and 
patterned according to photolithography. The insulating film 2 was 
subjected to a RIE device using the patterned resist film as a mask under 
the following conditions A to make a contact hole a (FIG. 1A). 
Conditions A! 
O.sub.2 /CHF.sub.3 =8/75 sccm 
Pressure=50 mtorr. 
High frequency power=1000 W 
Thereafter, a Ti layer and a TiON layer were successively deposited, as a 
bonding layer 4, on the entire surface of the insulating film 2, followed 
by further deposition of a first metallic interconnection layer 5 on the 
bonding layer 4 using a cold wall-type CVD apparatus under the following 
conditions B. 
Conditions B! 
H.sub.2 /WF.sub.6 (gas ratio)=1/19 
Reaction temperature=400.degree. C. 
Reaction pressure=6.5 torr. 
Thus, a W film 5 was formed in a thickness of not less than 50% of the 
diameter of the contact hole a. The W film 5 at the contact hole had a 
good coverage but was formed on the surface thereof with irregularities 
having a peak-to-valley height of about 200 nm (FIG. 1B). 
The, using a commercially available magnetron sputtering apparatus, a Ti 
layer was formed on the W film as a bonding layer 6 under the following 
conditions C (step 1), followed by raising a substrate temperature to form 
an Al--Si alloy film containing 1% of Si as a second metallic 
interconnection layer 7 (step 2) thereby forming a metallic 
interconnection layer of the invention (FIG. 1C). 
Conditions C! 
First step 
Ar gas pressure=2 mtorr. 
Sputtering power=15 kW 
Substrate temperature=200.degree. C. 
Sputtering time=4 seconds 
Second step 
Ar gas pressure=2 mtorr. 
Sputtering power=7.5 kW 
Substrate temperature=400.degree. C. 
Sputtering time=40 seconds 
The thus formed Al--Si film 7 had a smooth surface in spite of the 
irregular surface of the W film 5. 
(ii) Patterning of the metallic interconnection layer 
A TiON film was formed on the Al--Si interconnection layer 7 formed in (i) 
as an antireflection film 8 according to a reactive sputtering method 
using an O.sub.2 /N.sub.2 gas, followed by formation of a resist film 9 
and patterning of the resist film 9 according to photolithography. As a 
result, the resist film could be patterned in high accuracy. 
Subsequently, using the pattern of the resist film 9 as a mask, the 
interconnection layer was etched by use of a microwave plasma etching 
apparatus under the following conditions D. 
Conditions D! 
BCl.sub.3 /Cl.sub.2 =60/90 sccm 
Pressure=1.1 Pa (8 mtorr.) 
Microwave current=300 mA 
Substrate bias=40 W 
Under these etching conditions, the TiON film 8, Al--Si film 7 and Ti layer 
6 are etched by means of the plasma of the chlorine gas. However, when the 
etching process to such an extent the W film is exposed, tungsten chloride 
is formed. This tungsten chloride is so low in vapor pressure that etching 
scarcely proceeds. Therefore, when the W film was exposed, the etching 
conditions were changed to the following conditions E wherein discharge 
was effected in a mixed gas of fluorine and chlorine gases. 
Conditions E! 
SF.sub.6 /Cl.sub.2 =80/40 sccm 
Pressure=1.1 Pa (8 mtorr.) 
Microwave current=300 mA 
Substrate bias=30 W 
Under these conditions using the mixed gas, tungsten fluoride was formed 
during the course of etching. The tungsten fluoride was so high in vapor 
pressure that the etching of the W film 5 proceeded readily. Moreover, 
during the etching, there were also formed tungsten chlorides, such as 
WCl.sub.5, WCl.sub.6 and the like, by reaction between chlorine radicals 
and the W film. These tungsten chlorides are removed by the sputtering 
reaction based on the incident ions at the bottom plane for etching. On 
the other hand, at the side walls for the etching plane, the tungsten 
chlorides serve as a kind of protective film for the side walls and 
prevent the side attack reaction of the fluorine radicals. Accordingly, 
the W film could be anisotropically processed under these etching 
conditions. By the etching, the bonding layer 4 serving as an underlying 
layer for the W film 5 could also be etched out. After exposure of the 
SiO.sub.2 insulating film 2 by removal of the W film 5 and the bonding 
layer 4, the etching conditions may be changed to the conditions D using 
no fluorine gas in order to improve the selectivity to the SiO.sub.2 
insulating film 2. 
As a results, the metallic interconnection layer could be patterned as 
shown in FIG. 3. 
EXAMPLE 2 
In the same manner as in Example 1, a SiO.sub.2 insulting film 2 was formed 
in a Si substrate 1 having a diffusion layer 3, followed by making a 
contact hole a, forming a bonding layer 4 and forming a W film 5. 
Thereafter, a Ti layer and an Al--Si film 7a were formed according to an 
ordinary magnetron sputtering at a substrate temperature of 200.degree. C. 
As a result, the surface of the Al--Si film became irregular in a more 
pronounced form of the irregularities of the W film as shown in FIG. 4A. 
Next, while keeping the sputtering apparatus under a reduced pressure, the 
substrate temperature was raised to 500.degree. C. over 90 seconds. By 
this, the Al--Si film 7a was reflown to obtain an Al--Si film whose 
surface was smoothed irrespective of the surface irregularities of the W 
film (FIG.4B). 
Thereafter, the procedure of Example 1 (ii) was repeated to pattern the 
resultant metallic interconnection layer. 
Comparative Example 1 
The general procedure of Example 1 was repeated except that the Al--Si film 
7 was formed according to a conventional sputtering method without use of 
the high temperature sputtering, thereby forming a metallic 
interconnection layer and that a resist film 9 was patterned on the 
interconnection layer. 
As a result, as shown in FIG. 5, the Al--Si film 7a had irregularities in a 
more pronounced form of the surface irregularities of the W film. The side 
walls of the patterned resist film 9 were toughened by the influence of 
the irregular reflection at the time of exposure to light.