Process for fabricating a semiconductor device

A metal layer exposed by dry etching a phosphosilicate glass (PSG) layer covering the metal layer is insufficiently reliable when an electrical connection is made by bonding wire to the metal layer or depositing another metal layer on the metal layer. This insufficiency is due to the presence of a phosphorous residue on the surface of the exposed metal layer. The phosphorous residue is preferably removed by treating the surface of the metal layer with an alkaline solution so that the PSG layer, the metal layer, and the semiconductor device in which the connection is made are not affected.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for fabricating a semiconductor 
device. In particular, it relates to treatment of the surface of a metal 
layer exposed by dry etching part of a phosphosilicate glass (PSG) layer 
covering the metal layer, the exposed metal layer being used for an 
electrical connection. 
Many semiconductor devices having various types of circuit elements formed 
in a semiconductor substrate, include a metal interconnection layer which 
connects an impurity doped layer, a gate electrode, or another component 
in the substrate, to the exterior of the substrate or to another circuit 
element in the substrate. Such a metal interconnection layer is often 
covered with a PSG layer as a protective layer or an insulating layer 
between metal layers. Such a PSG layer covering a metal layer is partly 
removed by etching so as to expose the underlying metal layer and to form 
a connection between the exposed area of the metal layer and the exterior 
or another circuit element of the substrate. 
Recently, dry etching processes such as parallel plane-type plasma etching, 
ion beam etching, sputter etching, and so on have been used for etching 
PSG layers in order to increase the processing precision, simplify the 
treatment, or accomplish other purposes. However, a metal layer exposed by 
dry etching a PSG layer covering the metal layer may result in an 
insufficient electrical connection or may increase the contact resistance 
when a wire is bonded onto it or a connection is made to it. If such an 
exposed metal surface is used as an electrode pad for wire bonding, the 
metal of the wire does not sufficiently alloy with the metal of the pad, 
sometimes resulting in separation of the wire from the pad. If a second 
metal layer is deposited on an insulating PSG layer covering a first metal 
layer as well as in a through hole formed by dry etching part of the PSG 
layer, i.e., the exposed area of the first metal layer, the contact 
between the two metal layers may not be electrically sufficient and may 
result in a disconnection with the passage of time. 
It has been proposed to treat the surface of a metal layer with a 
hydrofluoric acid solution, etc. after exposing by dry etching a silicon 
dioxide (SiO.sub.2) layer covering the metal layer. However, this method 
is not very effective and, in addition, concerns a SiO.sub.2 layer, not a 
PSG layer. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide excellent 
wire bondings and connections by way of a through hole at the surface of a 
metal layer exposed by dry etching a PSG layer covering the metal layer. 
This and other objects, features, and advantages of the present invention 
are accomplished by a process for fabricating a semiconductor device, 
comprising the steps of: forming a metal layer on a semiconductor 
substrate; forming a PSG layer on the metal layer; dry etching part of the 
PSG layer on the metal layer to expose at least part of the metal layer; 
and treating the exposed surface of the metal layer with an alkaline 
solution. 
The present invention was made on the basis of the discovery by the 
inventor that after a metal layer is exposed by dry etching a PSG layer 
covering the metal layer, a phosphorous residue remains on the surface of 
the metal layer. An excellent electrical connection at the surface of the 
metal layer exposed by dry etching a PSG layer covering the metal layer 
can be made by removing the phosphorous residue from the surface of the 
metal layer. For example, in the case of reactive ion etching using 
fluoroform (CHF.sub.3), a phosphorous residue is formed in the manner 
described below. The etching process is carried out by decomposing a PSG 
layer into silicon tetrafluoride (SiF.sub.4), oxygen (O.sub.2), and a 
phosphorous-containing substance whose chemical formula is not certain, 
all of the substances flying out of a workpiece to be etched. When the 
phosphorous flies out of the workpiece, it flies into a plasma atmosphere 
surrounding the workpiece, the atmosphere ions being directed toward the 
workpiece. As a result, the phosphorous is changed into plasma and is 
forced back toward the workpiece, and, consequently, a phosphorous residue 
is formed on the surface of the workpiece. The phosphorous residue 
includes not only phosphorous but also a small amount of carbon, fluorine, 
etc. The formation of a phosphorous residue on a metal layer by etching a 
PSG layer covering the metal layer occurs in the case of dry etching, for 
example, parallel plane-type plasma etching, ion etching, or sputter 
etching. 
Although the phosphorus residue can be removed, for example, by treating it 
with an acid or hydrogen peroxide, such processes are not suitable for 
practical application since a large part of the metal layer is also 
removed simultaneously with the removal of the phosphorous residue due to 
the corrosive properties of the metal, such as aluminum. Thus, an alkaline 
solution should be used, in accordance with the present invention, to 
remove the phosphorous residue from the metal layer. Alkalis such as 
ammonia; organic amines, for example, diethylamine (Et.sub.2 NH), 
triethylamine (Et.sub.3 N), dimethylamine (Me.sub.2 NH), trimethylamine 
(Me.sub.3 N), buthylamine (H.sub.2 N.C.sub.3 H.sub.7), choline [HOCH.sub.2 
CH.sub.2 N.sup.+ (CH.sub.3).sub.3 ]OH.sup.- ; and salts of organic amines, 
for example, diethylamine hydrochloride (Et.sub.2 N.sup.+ H.sub.2 
Cl.sup.-), triethylamine hydrochloride (Et.sub.3 N.sup.+ HCHl.sup.-), 
trimethylamine hydrochloride (Me.sub.3 N.sup.+ HCl.sup.-), buthylamine 
hydrochloride (H.sub.2 N.sup.+.C.sub.3 H.sub.7 HCl.sup.-), are preferably 
used in the process of the present invention. Hydroxides of an alkali 
metal, such as sodium hydroxide and potassium hydroxide, and hydroxides of 
an alkali earth metal, such as calcium hydroxide, may also be used to 
remove the phosphorous residue from the metal layer. However, these 
hydroxides are disadvantageous since solutions of them may cause 
contamination of a semiconductor device, as is known. 
Metals which can be advantageously used in the process of the present 
invention include aluminum, aluminum-based alloy, molybdenum, 
molybdenum-based alloy, tungsten, tungsten-based alloy, titanium, 
titanium-based alloy, niobium, and niobium-based alloy.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is an example of a semiconductor device including a metal-oxide 
semiconductor field-effect transistor and a wire bonding. In a silicon 
substrate 1, a source or drain 2 was formed by diffusion and the surface 
of the substrate 1 was covered with a silicon dioxide film 3 on which a 
gate 4 was formed. Then an aluminum layer 5 having a thickness of 
approximately 1 .mu.m was deposited on the silicon dioxide film 3 and was 
patterned for use as an interconnection line including a portion of an 
electrode pad 6. PSG layer 7 having a thickness of approximately 1-2 .mu.m 
formed as protective layer to cover the aluminum layer 5 and a 100 
.mu.m.times.100 .mu.m opening 8 was made on the electrode pad 6 by dry 
etching. The dry etching in this example was reactive ion etching in which 
the reactive gas was CHF.sub.3 and the pressure was 0.2 Torr. By carrying 
out dry etching, the PSG layer 7 was precisely etched, resulting in the 
formation of the precise opening 8. 
A plan view of a portion of the electrode pad 6 at this stage, i.e., after 
dry etching, is shown in FIG. 2, and a sectional view of the electrode pad 
6 taken along the line III--III in FIG. 2 is shown in FIG. 3. After the 
reactive ion etching, a phosphorous residue 9 remained in the opening 8 
and on the surface of the electrode pad 6. The presence of the phosphorous 
residue 9 was confirmed by carrying out Auger analysis on a portion of the 
electrode pad 6. The amount of phosphorous residue on the surface of the 
electrode pad 6 was about 10 to 17 times that on the surface of an 
electrode pad fabricated in a manner similar to the above example but in 
which wet etching was carried out instead of dry etching. The surface of 
the electrode pad 6 was then cleaned by dipping the silicon wafer in a 3.5 
wt% aqueous solution of ammonia at a room temperature of about 22.degree. 
C.-23.degree. C. for about 30 seconds. The phosphorous residue was removed 
from the surface of the electrode pad 6, for example, according to the 
following chemical reaction: 
EQU H.sub.3 PO.sub.4 +3NH.sub.4 OH.fwdarw.(NH.sub.4).sub.3 PO.sub.4 +3H.sub.2 O 
The etching rate of aluminum when an ammonia solution is used is slow, 
about 50 .ANG. per minute. Thus, the electrode pad 6 having a thickness of 
1 .mu.m was not affected by the above-mentioned short dipping time. 
After the removal of the phosphorous residue from the surface of the 
electrode pad 6, a gold wire 10 was bonded onto the pad 6. The gold wire 
10 alloyed well with the aluminum electrode pad 6, creating a highly 
reliable electrical connection between the gold wire 10 and the pad 6. 
To ascertain the effect of the present invention, the purple plague test 
was carried out. The purple plague test was conducted by depositing a 1500 
.ANG.-thick gold layer on the aluminum electrode pad 6 and annealing the 
gold layer and the pad 6 at 350.degree. C. for 15 minutes. 
According to the purple plague test, if the gold alloys with the aluminum, 
the gold will change to a purple color, indicating the formation of a 
gold-aluminum alloy. In the present test, the entire aluminum electrode 
pad 6, which was cleaned with an ammonia solution according to the present 
invention, turned purple in all the samples while only a small portion of 
an aluminum electrode pad similar to the pad 6 but not having been cleaned 
with an ammonia solution turned purple, i.e., about one tenth of the pad 
turned purple. Further, an aluminum electrode pad similar to the pad 6 but 
which was treated with a hydrofluoric acid solution instead of an ammonia 
solution did not change color at all. These results indicate that an 
excellent and reliable connection can be made between an electrode pad, 
comprising aluminun or another metal and being cleaned with an ammonia 
solution, and a wire comprising gold or another metal. 
In addition to the purple plague test, the phosphorous content of various 
electrode pads was determined by means of Auger analysis. Auger analysis, 
however, reveals the relative amount of phosphorous, not the absolute 
amount. In the following analysis, the resultant amounts of phosphorous 
are expressed as a relative value on the basis of an assumed reference 
amount. 
In the Auger analysis, first, a PSG layer covering the surface of an 
aluminum layer was wet etched with a aqueous hydrofluoric acid solution. 
Phosphorous was present on the surface of the aluminum layer in a relative 
value of 0.5 to 0.7. Next, a PSG layer covering the surface of the 
aluminum layer was reactive ion etched with a CHF.sub.3, argon, or 
nitrogen reaction gas, the etching conditions being maintained for some 
minutes after the completion of the etching of the PSG layer. Phosphorous 
was present on the surface of the aluminum layer in a relative value of 7 
to 8. (It should be noted that most practical semiconductor devices are 
subjected to over etching in order to eliminate, on a metal layer the 
residue of the PSG layer to be etched.) Then the surfaces of the aluminum 
layers which were subjected to the above reactive ion etching process were 
subjected to three types of cleaning treatments. FIrst, the aluminum layer 
was treated with an aqueous hydrofluoric acid solution. The relative value 
of phosphorous present on the surface of the aluminum layer was 1 to 1.5. 
Second, the aluminum layer was subjected to ion etching with an argon gas 
so as to etch the suface of the aluminum layer. The relative value of 
phosphorous present on the surface of the aluminum layer was 0.7 to 1.4. 
Third, the aluminum layer was treated with an aqueous ammonia solution. 
The relative value of phosphorous present on the surface of the aluminum 
layer was 0 to 0.2. 
FIG. 4 shows an example of a semiconductor device wherein a bipolar 
transistor was formed and a connection between two interconnection layers 
was made by way of a through hole. The bipolar transistor was formed by 
growing an N-type epitaxial silicon layer 40 on a silicon substrate (not 
shown in FIG. 4) having a diffused N.sup.+ -type buried layer (not shown 
in FIG. 4) and by carrying out P.sup.+ -type, P-type, and N-type diffusion 
to form an isolation region 41, a base 42, and an emitter 43, 
respectively. The surface of the epitaxial layer 40 was covered with a 
silicon dioxide layer 44. After opening windows in the silicon dioxide 
layer 44 to form electrodes, first aluminum layer 45, approximately 1 
.mu.m thick, was deposited on the silicon dioxide layer 44 and in the 
windows and was patterned so as to make a first interconnection line. A 
PSG layer 46, having a thickness of 1 to 2 .mu.m, was formed on the 
aluminum layer 45 and the silicon dioxide layer 44. A through hole 47 was 
then opened in the PSG layer 46 by parallel plane-type plasma etching, 
using carbon tetrafluoride and oxygen as reaction gases, under a pressure 
of 0.8 Torr. By dry etching, the through hole 47, having a diameter of 3 
.mu.m in the PSG layer 46, could be precisely formed, but a phosphorous 
residue was present on the surface of the aluminum layer 45 and at the 
bottom of the through hole 47. 
Then the surface of the aluminum layer 45 at the bottom of the through hole 
47 was cleaned with an aqueous ammonia solution to remove the phosphorous 
residue from the through hole 47. The chemical reaction which occurred was 
similar to that described previously. After removal of the phosphorous 
residue, a second aluminum layer 48 was deposited on the silicon dioxide 
layer 44 and in the through hole 47. The electrical connection between the 
first and second aluminum layers was excellent and reliable and there was 
no contact failure. In comparison, connections similarly made but in which 
the surface of the first aluminum layer 45 at the bottom of the through 
hole 47 was not cleaned with an aqueous ammonia solution sometimes 
resulted in contact failure with the passage of time. 
It will be understood by those skilled in the art that the advantages of 
the present invention also exist when the type of dry etching is a 
reactive sputter etching as well as and the metal to be cleaned is an 
aluminum-based alloy, molybdenum, a molybdenum-based alloy, tungsten, a 
tungsten-based alloy, titanium, a titanium-based alloy, niobium, or a 
niobium-based alloy and that the foregoing and other changes in form and 
in detail may be made therein without departing from the spirit and scope 
of the present invention.