Wiring structure for semiconductor device and fabrication method therefor

A metal wiring for semiconductor devices having a double-layer passivation film structure consisting of an intermetallic compound layer formed on a copper thin film and made of a metal reacting with copper to form an intermetallic compound and a metal nitride layer formed over the intermetallic compound. This double-layer passivation film structure is obtained by depositing a metal layer, capable of reacting with copper to form an intermetallic compound, over the copper wiring, and annealing the metal layer in a nitrogen atmosphere, thereby forming an intermetallic compound layer over the copper wiring. By virtue of the double-layer passivation film structure, the copper wiring has a great improvement in the reliability. A metal silicide layer is formed between a diffusion region and a diffusion barrier layer in the contact hole of the semiconductor device. The diffusion barrier layer, which is formed on an insulating layer doped with nitrogen ions, is changed into a metal nitride film. Accordingly, a reduced ohmic contact resistance and an improved passivation reliability are achieved.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to wiring techniques for semiconductor 
devices, and more particularly to a wiring structure for highly integrated 
semiconductor devices having a passivation film disposed on a copper thin 
film for the wiring and made of an intermetallic compound formed by a 
reaction with copper to improve the conductivity and reliability of the 
copper wiring. 
2. Description of the Prior Art 
As semiconductor devices recently developed require a more rapid operation 
speed thereof because their integration degree has been abruptly 
increased. Due to such a requirement, a large amount of current flows 
through wirings of the semiconductor devices. 
The increased integration degree of such semiconductor devices involves 
inevitably a decrease in line width of wirings, thereby increasing the 
density of current flowing through the wirings. 
To this end, inexpensive aluminum exhibiting a good ohmic contact 
characteristic and a high conductivity have been used to make wirings or 
via lines. Where pure aluminum is used to make wirings, subsequent 
processes to be carried out after the formation of pure aluminum wirings 
should be those of low temperature. Moreover, such pure aluminum wirings 
involve a junction spike or an electromigration. 
Due to such disadvantage of pure aluminum, aluminum alloys added with Si, 
Cu, Ni or Cr have been mainly used to make wirings of semiconductor 
devices, in place of pure aluminum. 
Considering that the high integration of semiconductor devices are 
progressing continuously, such aluminum alloy wirings also have many 
disadvantages in terms of resistance and reliability, as in the pure 
aluminum wirings. For solving this problem, there has been proposed use of 
copper exhibiting a better conductivity than that of aluminum alloy by two 
times or greater. 
In case of copper wirings, diffusion of copper to silicon substrates may 
occur. Moreover, the copper may be easily oxidized or etched by air 
existing in the atmosphere or process atmospheres used in process steps. 
Copper also has a poor adhesion to insulating layers containing oxygen. 
Many researches and developments have been made in order to overcome the 
above-mentioned problems. One is disclosed in U.S. Pat. No. 4,742,014 
relating to fabrication of copper wiring structures for semiconductor 
devices. Referring to FIGS. 1A to 1C, this method will be described. 
As shown in FIG. 1A, a substrate 1 made of single crystalline silicon is 
formed with a diffusion region 2 near its surface. An insulating layer 3 
is then coated over the surface of silicon substrate 1. 
In order to expose the diffusion region 2, the insulating layer 3 is then 
partially removed at its portion disposed over the diffusion region 2, 
thereby forming a contact hole 4. 
Over the entire exposed surface of the resulting structure including the 
surface of insulating film 3 and the surface of exposed diffusion region 
2, a molybdenum thin film 5 is deposited using a sputtering method or a 
chemical vapor deposition method, as shown in FIG. 1B. 
Thereafter, a copper thin film 6 is coated over the molybdenum thin film 5. 
The copper thin film 6 and molybdenum thin film 5 are sequentially 
patterned in this order. 
As shown in FIG. 1C, a tungsten thin film 7 is then selectively deposited 
over only the patterned copper thin film 6 using a selective tungsten 
deposition method, thereby passivating the copper thin film 6. 
Accordingly, the copper thin film 6 is used as a conduction layer whereas 
the molybdenum thin film 5 is used as a lower diffusion barrier layer for 
the copper thin film 6. On the other hand, the tungsten thin film 7 
deposited on the upper and side surfaces of copper thin film 6 serves as 
an upper and side diffusion barrier layer for the copper thin film 6. 
Thus, the copper thin film 6 is encapsulated by the molybdenum thin film 5 
of high melting point metal and the tungsten thin film 7. 
Where the insulating layer 3 is an oxygen-containing insulating layer, the 
tungsten thin film 7 serves to prevent the copper thin film 6 from being 
oxidized by the oxygen of the insulating layer 3. In this case, the 
tungsten thin film 7 also prevents the copper atoms of the copper thin 
film 6 from being diffused in the insulating film 3. 
Meanwhile, the molybdenum thin film 5 serves to not only reduce the ohmic 
contact resistance between the copper thin film 6 and the diffusion region 
2, but also to prevent the copper atoms of the copper thin film 6 from 
being diffused in the insulating film 3. 
However, this method has a problem that a complicated fabrication is 
required to form the lower diffusion barrier layer, the conduction layer, 
namely, copper layer, and the upper and side diffusion barrier layer. 
Furthermore, this method is difficult to prevent the copper thin film from 
being formed at the surface thereof with a natural oxide film in the 
atmosphere or a tungsten depositing atmosphere because it uses the 
selective tungsten deposition technique that is not a completely developed 
technique. For this reason, the method has a great difficulty to 
selectively form the tungsten thin film on the copper thin film. 
As a means of settling the formation of the natural oxide film on the 
copper thin film, there has been proposed a method for removing the 
natural oxide film using a sputtering etch process in an in-situ fashion, 
prior to the deposition of the tungsten thin film, in the same CVD 
equipment as that used for the deposition of the tungsten thin film. 
However, this method has a difficulty to completely remove the residue of 
natural oxide film left on the side surfaces of copper thin film because 
it uses the sputtering etch technique having the anisotropic etch 
characteristic. As a result, it is difficult to form a satisfactory 
selective tungsten thin film due to the natural oxide film left on the 
side surfaces of copper thin film. 
In this regard, the above-mentioned conventional techniques for passivating 
wirings of copper thin films using the selective tungsten deposition 
method need more researches in order to achieve the application of copper 
thin film wirings to highly integrated semiconductor devices. 
Another conventional method for fabricating copper wirings of semiconductor 
devices is disclosed in U.S. Pat. No. 5,130,274. By referring to FIGS. 2A 
to 2C, this method will be described. 
As shown in FIG. 2A, a copper thin film 11 is coated over the entire 
surface of a single crystalline silicon substrate (not shown) and then 
patterned to form a first wiring. 
An insulating film 13 comprised of, for example, an oxide film is then 
deposited over the entire surface of the resulting structure including the 
surface of the patterned copper thin film 11 and the surface of the 
silicon substrate exposed after the patterning. 
Thereafter, the insulating film 13 is partially removed at its portion 
corresponding to a region where the copper thin film 11 will be in contact 
with a copper thin film (not shown) for a second wiring to be subsequently 
formed, thereby forming a via hole 14. 
A thin film 15 made of copper alloy added with aluminum or chromium is then 
deposited over the entire surface of the resulting structure including the 
surface of insulating film 13 and the surface of copper thin film 11 
exposed through the via hole 14, by using the sputtering method or CVD 
method. 
Subsequently, the copper alloy thin film 15 is etched back so that it is 
left only in the via hole 14, thereby forming a plug, as shown in FIG. 2B. 
The plug comprised of the remaining copper alloy thin film 15 is then 
annealed in an oxygen atmosphere. During the annealing of the plug, the 
aluminum or chromium atoms of the plug migrates to the surface of plug and 
reacts with the oxygen of the insulating film 13 at the interface between 
the plug and the insulating film 13, thereby forming an oxide film 17 of 
Al.sub.2 O.sub.3 or Cr.sub.2 O.sub.3, as shown in FIG. 2C. 
After completing the annealing, the plug buried in the via hole 14 provides 
a pure copper wiring 16 passivated at its upper and side surfaces by the 
oxide film 17. 
However, this method has a problem that the oxide film provides lower 
copper diffusion barrier effect than nitride films. Since the aluminum or 
chromium atoms contained in the copper alloy reacts with oxygen supplied 
at the surface of the insulating film to form the oxide film in this 
method, the volume of the pure copper wiring is reduced as the thickness 
of the oxide film increases. This results in an undesirable increase in 
the resistance of the pure copper wiring. 
SUMMARY OF THE INVENTION 
Therefore, an object of the invention is to provide a wiring structure for 
semiconductor devices having an intermetallic compound layer interposed 
between a copper wiring and an upper metal layer disposed over the copper 
wiring, the intermetallic compound layer serving to passivate the copper 
wiring, thereby capable of achieving high conductivity and reliability of 
the copper wiring, and a method for fabricating the wiring structure. 
In accordance with one aspect, the present invention provides a wiring 
structure of a semiconductor device, comprising: a substrate; a conduction 
layer made of a first metal and formed on the substrate, the conduction 
layer having a required pattern; and an intermetallic compound layer 
formed over the conduction layer and adapted to passivate the conduction 
layer, the intermetallic compound layer containing a second metal reacting 
with the first metal to form an intermetallic compound. 
In accordance with another aspect, the present invention provides a wiring 
structure of a semiconductor device, comprising: a substrate; a diffusion 
region formed in the substrate; an insulating layer coated over the 
substrate and provided with a contact hole for exposing the diffusion 
region and an ion-doped layer disposed near an upper surface of the 
insulating layer; a diffusion barrier layer disposed on the diffusion 
region and the insulating film, the diffusion barrier layer having a 
required pattern; a conduction layer made of a first metal and formed over 
the diffusion barrier layer; a passivation layer formed over the 
conduction layer; and a metal silicide layer interposed between the 
diffusion region and the diffusion barrier layer. 
In accordance with another aspect, the present invention provides a method 
for fabricating a wiring structure of a semiconductor device, comprising 
the steps of: forming a conduction layer made of a first metal on a 
substrate such that the conduction layer has a required pattern; and 
forming over the conduction layer a first intermetallic compound layer 
containing a second metal reacting with the first metal to form an 
intermetallic compound. 
In accordance with another aspect, the present invention provides a method 
for fabricating a wiring structure of a semiconductor device, comprising 
the steps of: forming diffusion regions for the semiconductor device in a 
substrate; forming an ion-doped layer extending to a required depth in the 
entire upper surface portion of the insulating layer; removing a portion 
of the insulating layer corresponding to a selected one of the diffusion 
regions, thereby forming a contact hole; forming a diffusion barrier layer 
on the insulating layer and the selected diffusion region in the contact 
hole such that the diffusion barrier layer has a required pattern, and 
then forming a conduction layer made of a first metal over the diffusion 
barrier layer; forming over the conduction layer a first intermetallic 
compound layer containing a second metal reacting with the first metal to 
form an intermetallic compound; and subjecting the first intermetallic 
compound layer to an annealing, thereby changing the first intermetallic 
compound layer into a second intermetallic compound layer and a metal 
nitride layer formed over the second intermetallic compound layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, there is illustrated a wiring structure for 
semiconductor devices in accordance with an embodiment of the present 
invention. 
As shown in FIG. 3, the wiring structure of this embodiment includes a 
substrate 21, an insulating layer 23 coated over the substrate 21, a 
diffusion barrier layer 25 formed on a desired portion of the insulating 
layer 23, a copper thin film 26 formed as a conduction, first metal layer 
over the upper surface of the diffusion barrier layer 25, and a 
passivation layer 27 formed over the entire surface of the copper thin 
film 26. 
The passivation layer 27 consists of a TiCu layer 30 containing Ti as a 
second metal forming an intermetallic compound by reacting with the first 
metal, and a layer 31 made of a metal nitride such as TiN and formed over 
the TiCu layer 30. 
FIGS. 4A to 4E are sectional views respectively illustrating a method for 
fabricating the wiring structure of FIG. 3 in accordance with the present 
invention. In FIGS. 4A to 4E, elements respectively corresponding to those 
in FIG. 3 are denoted by the same reference numerals. 
In accordance with this method, a semiconductor substrate 21 of single 
crystalline silicon already subjected to a front end process is prepared 
first. An insulating film 23 is then coated over the substrate 21, as 
shown in FIG. 4A. 
Over the insulating film 23, a diffusion barrier layer 25 is deposited to a 
thickness of about 500 .ANG.. The diffusion barrier layer 25 serves as a 
barrier against diffusion of copper atoms. 
The diffusion barrier layer 25 is made of a material selected from a group 
consisting of a variety of nitrides such as Si.sub.3 N.sub.4, TiN, Ta or 
ZrN, high melting point metals such as Mo, Ti, W, Ta or Zr, conductive 
oxides such as MoO, RuO.sub.2 or Y.sub.2 O.sub.3 and metals, such as La, 
Mg, Pt, Sr or Y, reacting with copper to form an intermetallic compound. 
The diffusion barrier layer 25 is formed by depositing the selected 
material over the insulating film 23 by use of the sputtering method or 
CVD method. 
Subsequently, a thin film 26 made of a first metal such as copper is 
deposited to a thickness of 5,000 .ANG. over the diffusion barrier layer 
25 using the sputtering method or CVD method. 
In place of copper, aluminum may be used as the first metal. 
A photoresist film pattern (not shown) corresponding to a pattern for the 
conduction layer, namely, the copper thin film 26 is then formed on the 
copper thin film 26, as shown in FIG. 4B. Using the photoresist film 
pattern as a mask, the copper thin film 26 is then partially dry etched at 
its portion not masked with the photoresist film pattern in a plasma 
atmosphere containing an SiCl.sub.4 /Cl.sub.2 /N.sub.2 mixture gas. 
Subsequently, the portion of diffusion barrier layer 25 not masked with the 
photoresist film pattern, namely, exposed after the etching of the copper 
thin film 26 is dry etched using a suitable gas selected depending on the 
material of the, diffusion barrier layer 25. After completing the etching, 
the photoresist film pattern is completely removed. 
Over the entire surface of the resulting structure including the surface of 
the remaining copper thin film 26 and the exposed surface of the 
insulating film 23, a layer 28 made of a second metal, such as Ti, 
reacting with copper to form an intermetallic compound is deposited to a 
thickness of 500 .ANG. using the CVD method, as shown in FIG. 4C. 
In place of Ti, the second metal layer 28 may be made of one selected from 
La, Mg, Pt, Sr, Y and Zr. 
Thereafter, the Ti layer 28 is annealed at a temperature of 350.degree. C. 
in an inert gas atmosphere to form a first intermetallic compound layer 29 
of, for example, Ti.sub.2 Cu at its portion being in contact with the 
surface of copper thin film 26, as shown in FIG. 4D. At other portions, 
the Ti layer 28 is still kept as it is, without being changed. 
The Ti layer 28 is then subjected to a wet etch at its portions not changed 
into the first intermetallic compound, as shown in FIG. 4E. This wet etch 
step is carried out using a wet etch solution which is a HCl/HNO.sub.3 
mixture acid solution or a H.sub.2 O-diluted hydrofluoric (HF) acid. 
The Ti.sub.2 Cu layer 29 is then annealed at a temperature of 700.degree. 
C. in a nitrogen atmosphere, thereby forming a passivation film 27 
consisting of the second metallic compound layer, namely, the TiCu layer 
30 and the metal nitride film, namely, the TiN layer 31 formed over the 
TiCu layer 30. 
As the annealing in the nitrogen atmosphere, one is selected from a plasma 
annealing using N.sub.2 or NH.sub.3 gas, a furnace annealing and a rapid 
annealing. 
On the other hand, the TiN layer 31 may be substituted by other metal 
nitride layers. For example, a Ti-Cu-N layer may be used. It is also 
apparent that in case the second metal is Zr, the TiN layer 31 is 
substituted by a Zr layer or a metal nitride layer such as a Zr-Cu-N 
layer. 
Referring to FIG. 5, there is illustrated a wiring structure for 
semiconductor devices in accordance with another embodiment of the present 
invention. 
As shown in FIG. 5, the wiring structure of this embodiment includes a 
substrate 41 provided at its surface portion with a diffusion region 42, 
and an insulating layer 43 coated over the substrate 41. The insulating 
layer 43 has a contact hole 42 for exposing the diffusion region 42 and an 
ion-doped layer disposed near the surface of insulating layer 43. A 
diffusion barrier layer 45 having a desired pattern is disposed on the 
diffusion region 42 and the insulating film 43. The wiring structure 
further includes a copper thin film 46 formed as a conduction, first metal 
layer over the diffusion barrier layer 45, a passivation layer 47 formed 
over the copper thin film 46 and a silicide layer 53 interposed between 
the diffusion region 42 and the diffusion barrier layer 45. 
The passivation layer 47 consists of a TiCu layer 50 containing Ti as a 
second metal forming an intermetallic compound by reacting with the first 
metal, and a layer 51 made of a metal nitride such as TiN and formed over 
the TiCu layer 47. 
FIGS. 6A to 6C are sectional views respectively illustrating a method for 
fabricating the wiring structure of FIG. 5 in accordance with the present 
invention. In FIGS. 6A to 6C, elements respectively corresponding to those 
in FIG. 5 are denoted by the same reference numerals. 
In accordance with this method, a diffusion region 42 is formed first in a 
semiconductor substrate 41 of single crystalline silicon, as shown in FIG. 
6A. An insulating film 43 is then coated over the substrate 41. 
Thereafter, nitrogen ions are implanted to a small depth in the insulating 
layer 43 so that they are filed up in a portion of the insulating layer 43 
near the insulating layer surface. 
Thereafter, a contact hole is formed in the insulating layer 43 in order to 
expose a desired portion of the diffusion region 42, as shown in FIG. 6B. 
Over the entire surface of the resulting structure including the exposed 
surface of the diffusion region 42 and the surface of the remaining 
insulating layer 43, a Ti layer is deposited to a thickness of about 500 
.ANG. as a diffusion barrier layer 45. Over the Ti layer, a thin film 46 
made of a first metal such as copper is deposited to a thickness of 1,000 
.ANG.. 
In place of the Ti layer, the diffusion barrier layer 45 may be comprised 
of other layer made of a material, such as Zr, Ta or Co, reacting with 
single crystalline silicon to form a metal silicide and reacting with 
nitride to form a metal nitride. 
In place of copper, aluminum may be used as the first metal. 
A photoresist film pattern (not shown) corresponding to a pattern for the 
conduction layer, namely, the copper thin film 46 is then formed on the 
copper thin film 46. Using the photoresist film pattern as a mask, the 
copper thin film 46 and Ti layer 45 are then sequentially etched at their 
portions not masked with the photoresist film pattern, as shown in FIG. 
6B. After completing the etching, the photoresist film pattern is 
completely removed. 
Thereafter, process steps similar to those shown in FIGS. 4C to 4E are 
carried out to form a first intermetallic compound layer 50 comprised of a 
Ti.sub.2 Cu layer over the remaining copper thin film 46. The Ti.sub.2 Cu 
layer is then changed into a passivation film 47 consisting of a second 
metallic compound layer comprised of the TiCu layer 50 and a metal nitride 
film comprised of a TiN layer 51 formed over the TiCu layer 50, as shown 
in FIG. 6C. 
During the formation of the passivation film 47, a silicide layer 53 made 
of, for example, TiSi.sub.2 is formed between the diffusion region 42 and 
the diffusion barrier layer 45. This silicide layer 53 serves to reduce 
the ohmic contact resistance of the wiring comprised of the copper thin 
film 46. During the formation of the passivation film 47, Ti of the 
diffusion barrier layer 45 also reacts with the nitrogen implanted in the 
insulating layer 43, so that the Ti layer 45 is changed into a TiN layer. 
The TiN layer 51 may be substituted by other metal nitride layers. For 
example, a Ti-Cu-N layer may be used. It is also apparent that in case the 
second metal is Zr, the TiN layer 51 is substituted by a Zr layer or a 
metal nitride layer such as a Zr-Cu-N layer. 
As apparent from the above description, the present invention achieves an 
easy formation of the passivation film on the copper wiring by depositing 
a metal layer, capable of reacting with copper to form an intermetallic 
compound, over the copper wiring, and annealing the metal layer in an 
inert gas atmosphere, thereby forming an intermetallic compound layer over 
the copper wiring. In accordance with the present invention, the 
passivation film has a double-layer structure because a metal nitride is 
formed over the intermetallic compound layer by annealing the 
intermetallic compound in a nitrogen atmosphere. By virtue of the 
double-layer passivation film structure, the copper wiring has a great 
improvement in the reliability. 
In accordance with the present invention, a reduced ohmic contact 
resistance and an improved passivation reliability are achieved by virtue 
of the formation of a metal silicide layer in the contact hole and the 
change of the diffusion barrier layer on the insulating film into a metal 
nitride film. 
Although the preferred embodiments of the invention have been disclosed for 
illustrative purposes, those skilled in the art will appreciate that 
various modifications, additions and substitutions are possible, without 
departing from the scope and spirit of the invention as disclosed in the 
accompanying claims.