Process for forming multilayer wiring

The present invention relates to a method for filling small via holes provided to insulating film on a wafer to expose parts of the underlayer of the wafer by metal by means of CVD, and an apparatus therefor. The gist of the present invention lies in that, before CVD is conducted, a surface cleaning treatment of small via hole bottom underlayer surface and a stabilization treatment of insulating film surface activated thereby are carried out successively or simultaneously and optionally an anti-corrosive treatment is applied to underlayer surface, and then the CVD treatment is conducted without exposing the underlayer metal subjected to above treatments to the air. The present invention provides an effect of enabling via filling by metal which shows good selectivity and gives a low interfacial resistance between underlayer metal and filled metal.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for filling small via holes 
provided to insulating film to expose parts of the underlayer on a wafer 
by metal by means of selective CVD (chemical vapor deposition) and an 
apparatus therefor. In particular, it relates to a method for via filling 
by metal suitable for filling small via holes by metal which is capable of 
both securing a satisfactory selectivity and providing a good low contact 
resistance, and to an apparatus therefor. 
With the recent trend toward more highly integrated LSIs, difficulties in 
wiring design for connection between elements and wiring or between 
respective wirings are becoming more serious, and multilayer wiring, so 
called multilevel metallization, has become an indispensable technique for 
overcoming the difficulties. In order to connect a lower layer wiring and 
an upper layer wiring provided thereon with an insulating film provided 
therebetween, there is used a method which comprises providing minute via 
holes to the insulating film and filling the via holes by a metal. Several 
methods are known for filling via holes. Among them, one of the most 
promising methods for use in practice is selective CVD of a metal 
(particularly tungsten) because it shows a good via filling ability even 
when the diameter of via hole is very small. 
The method of selective CVD of tungsten comprises introducing a mixture of 
tungsten fluoride (WF.sub.6) gas and hydrogen (H.sub.2) gas over a sample 
heated to above 250.degree. C. and making them contact with each other, 
and thereby making a tungsten (W) film grow on the underlayer metal 
(exemplified herein by aluminum) based on either of the following 
reactions. 
EQU WF.sub.6 +2Al.fwdarw.W+2AlF.sub.3 ( 1) 
EQU WF.sub.6 +3H.sub.2 .fwdarw.W+6HF (2) 
On an insulating film such as SiO.sub.2 film, the reaction (1) does not 
take place and also the reaction (2) does not proceed at a temperature not 
higher than 700.degree. C., so that tungsten grows selectively on 
aluminum, whereby via filling can be achieved. 
References which give description of selective CVD of tungsten include J. 
Electrochem. Soc. 131, 1427-1433 (1984) and Proc. of VLSI Multilevel 
Interconnection Conference (Jun. 15-16, 1987) p 132-137. However, even 
when the methods described in above references are used, such insulating 
substances as oxide film present on Al underlayer and AlF.sub.3 formed by 
the reaction (1) remain at the interface between tungsten and aluminum in 
the via hole, making it difficult to obtain a sufficiently low contact 
resistance at the via hole part. Methods which have been proposed to solve 
the above problem include one comprising forming the tungsten film while 
heating the wafer above 380.degree. C. as disclosed in Proc. of VLSI 
Multilevel Interconnection Conference (Jun. 15-16, 1987) p. 208-215, and a 
method comprising attaching a thin (about 500 .ANG. thick) MoSi.sub.2 film 
onto aluminum and thereby inserting MoSi.sub.2 between aluminum and 
tungsten, whereby the tungsten film can be formed without leaving aluminum 
surface oxide film and AlF.sub.3 behind at the interface, as disclosed in 
Toshiba Review, Vol. 41, No. 12, p. 988-991. 
Recently, a method has been reported in which SiH.sub.4 or a similar gas is 
used in place of H.sub.2 as a reducing gas, as described, for example, in 
Technical Digest IEDM (1987), P213-P219. The use of this method enables a 
high rate film growth at a wafer heating temperature as low as 
250.degree.-320.degree. C. In this method, at a temperature not below 
340.degree. C., the selectivity is lost and selective via filling cannot 
be achieved. 
In the prior techniques mentioned above, however, no sufficient 
consideration is given regarding the treatment of underlayer metal surface 
on which tungsten is to be grown by selective CVD, resulting in the 
following problems. That is, the contact resistance at the via hole is 
not-sufficiently low or, even when the contact resistance at the via hole 
is low, the resistance of wiring itself increases; or tungsten grows also 
on the insulating film as the result of surface cleaning treatment of via 
hole underlayers, leading to occurrence of short circuit between adjacent 
through holes. 
The surface of underlayer metal after formation of small via holes is 
contaminated by fouling originated from a photoetching process applied for 
providing the small via holes, or the metal surface is formed of a oxide 
film (Al.sub.2 O.sub.3 etc. when the underlayer metal is aluminum, for 
example) purposely provided as an anti-corrosive treatment against a 
halogen-containing gas used as an etching gas. Therefore, the underlayer 
has no clean metal surface exposed, and impurities which increase contact 
resistance remain at the interface between underlayer metal and tungsten 
even after tungsten film has been grown. The oxides etc. which remain at 
the interface are, when a wafer temperature of 380.degree. C. or more is 
used in growth of tungsten, sometimes decreased in amount through the 
etching reaction of WF.sub.6 and diffusion within film during heating, 
resulting in a sufficiently low contact resistance. However, when the 
underlayer surface conditions of small via holes differ from wafer to 
wafer, a sufficiently low contact resistance cannot always be obtained 
with good reproducibility. To solve the above problems, namely poor 
reproducibility of low contact resistance and increased contact resistance 
of the tungsten/aluminum interface formed by the selective CVD of tungsten 
mentioned above, there has been proposed a method which uses as an 
underlayer an aluminum of a laminate film formed by attaching a MoSi.sub.2 
film (about 500 .ANG. thick) onto aluminum. In this method, since the 
remaining oxygen at the interface is decreased by making the exposed part 
of the small via hole constituted of MoSi.sub.2 more difficultly 
oxidizable than aluminum and further since the formation of insulative 
AlF.sub.3 with a low vapor pressure according to the above-mentioned 
equation (1) does not take place at the interface, the contact resistance 
of the W/MoSi.sub.2 /Al is lower than that of the W/Al interface. However, 
this method is accompanied by a problem of requiring etching for forming a 
wiring of laminated film of MoSi.sub.2 and Al and of increasing wiring 
resistance due to the use of MoSi.sub.2 having a high resistivity. The 
above-mentioned increase in resistance poses no problem in the case of 
MOSLSI such as DRAM and SRAM because the film thickness of MoSi.sub.2 is 
very thin as compared with that of aluminum. In the case of such LSIs as 
bipolar and biCMOS which capitalize on their high speed, however, even a 
slight increase in resistance raises a serious problem. 
On the other hand, several methods have been tried in which aluminum is 
used as the underlayer and the via hole is filled by tungsten after 
cleaning the surface of the underlayer (including removal of Al.sub.2 
O.sub.3 on the aluminum surface). Methods used for cleaning the underlayer 
surface include a wet etching which uses a solution containing hydrogen 
fluoride (HF) or a compound thereof such as ammonium fluoride (NH.sub.4 F) 
and a sputter-etching which uses Ar ions. In the former treatment, 
however, a slight amount of fluorine remains even after washing and drying 
steps and causes the corrosion of aluminum underlayer. The latter 
sputter-etching treatment enables exposure of a clean underlayer surface 
since it physically removes the outer surface of the underlayer, and is 
hence in use as the method of pretreatment of underlayers in multilevel 
interconnection of sputtered aluminum. In this method, however, it has 
been revealed as described below that selectivity is lowered in the 
selective CVD, caused by the fact that the insulating film is 
sputter-etched simultaneously. When an insulating film is sputter-etched, 
the composition of the surface layer of insulator changes owing to the 
difference in sputtering yield between elements. In a SiO.sub.2 film, for 
example, since O atom is more susceptible to sputtering than Si atom, the 
surface layer comes to have a composition rich in Si. In other words, 
active si atoms come to exist at the insulating film surface. This 
phenomenon has been studied by means of X-ray photoelectron spectroscopy 
(XPS or ESCA) and discussed, for example, in J. Vac. Sci. Technol., A 3(5) 
(1985), pp 1921-1928 and J. Phys. D: Applied Phys., 20 (1987), pp 
1091-1094. 
When the selective CVD of tungsten is carried out under such conditions, 
the growth of tungsten proceeds presumably as the result of the following 
reaction: 
EQU WF.sub.6 3/2Si.fwdarw.W+3/2SiF.sub.4 ( 3) 
Accordingly, tungsten grows also on SiO.sub.2, resulting in lowering of 
selectivity. This applies also to the selective CVD of other metals than 
tungsten. Thus, since selective CVD is based on the difference in chemical 
activity between respective surface parts, when a part at which no growth 
is desired, namely the surface of insulating film, is sputtered and 
activated, selectively is lowered resultantly. When a metal grows on 
insulating film, it gives rise to a possibility of short circuit with 
adjacent through holes and, further, since the metal film formed on the 
insulating film is apt to peel off, it remains as dirt on the wafer and 
causes the lowering of yield. 
In the foregoing, description was made with reference to a via hole using 
an aluminum wiring as an underlayer because the surface oxide film on 
aluminum are generally known as the representative of the most difficultly 
removable materials. However, in case of via filling of a via hole with Si 
contact, which is called a contact hole occasionally, where doped Si, 
various barrier metals (e.g., WSi.sub.2, MoSi.sub.2, TiSi.sub.2, PtSi, and 
TiW) are used as underlayer, there arise problems different from those in 
via holes using an aluminum wiring as an underlayer mentioned above. 
Tungsten film is formed relatively easily on the surface of the 
above-mentioned materials which may possibly constitute the underlayer of 
contact holes, because they do not form an oxide film so strongly as the 
aluminum does. However, in case that a number of different materials come 
to exist as the underlayer of the bottom part of holes, the growth rate of 
tungsten varies depending on the difference of materials of underlayer, 
leading to a situation wherein while via filling by tungsten has been 
completed in holes of a certain underlayer, the growth of tungsten has 
just begun at other holes. This is attributable to the differences in 
thickness, and/or quality of the oxide film present on the surface of 
underlayers caused by the difference in material of underlayers. It is 
generally considered that in W-CVD (tungsten chemical vapor deposition) 
the growth of tungsten does not begin simultaneously with the introduction 
of raw material gas, but an induction time is present after the 
introduction of gas till the substantial initiation of growth of tungsten, 
because the surface oxide film of the underlayer delays the initiation of 
tungsten film growth. Therefore, the presence of contact holes different 
in thickness and quality of the surface oxide film of underlayers on the 
same wafer leads to difficulty in obtaining uniform film thickness in via 
filling by tungsten. Thus, it is necessary to remove these surface oxide 
films of underlayer in order to obtain a uniform film thickness of via 
filling by tungsten in contact holes. In doped Si and various silicides 
used as the underlayer material of contact holes, wet etching treatment 
with a solution containing hydrogen fluoride or a compound thereof such as 
ammonium fluoride is carried out as a means for cleaning the underlayer 
surface and the surface oxide film is removed thereby because no problem 
of corrosion arises unlike in aluminum. However, even in a wafer subjected 
to such wet etching treatment, there remains a problem unsolved in that a 
surface oxide film is formed during drying wafers or during the time 
preceding the transport to a CVD apparatus, resulting in not uniform film 
thickness in via filling by tungsten. On the other hand, when the 
sputter-etching of a surface oxide film of the underlayers and the growth 
of tungsten film are carried out continuously, no oxide film is formed on 
the underlayer surface of the contact hole bottom before tungsten film 
growth but, as described above with reference to via holes on aluminum 
wiring, there arises the problem of decreased selectivity. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide, in filling a small via 
hole by metal by means of CVD typically represented by tungsten selective 
CVD, a method in which selectivity is not lowered even when a treatment 
for cleaning small via hole underlayer surfaces is carried out, and an 
apparatus therefor. 
The above object can be attained, in via filling of via holes or contact 
holes by CVD of tungsten, by successively conducting the following three 
treatments: 
1-(1) a surface cleaning treatment of small via hole bottom underlayer 
surface (removing treatment of surface oxide layer etc.), 
1-(2) a stabilization treatment of the insulating film surface layer 
activated by said surface cleaning treatment, and 
1-(3) a treatment of filling small via holes by metal by means of CVD of 
tungsten using WF.sub.6 gas and reducing gas (H.sub.2, silane (SiH.sub.4) 
and like compounds used alone or as a gas mixture); or by conducting the 
above treatments (1) and (2) simultaneously and then conducting the third 
treatment, in other words conducting the following two treatments 
successively: 
2-(1) a surface cleaning treatment of small via hole bottom underlayer 
accompanied by no activation of the surface of insulating film on the 
wafer (namely, removing treatment of surface oxide layer etc.), and 
2-(2) a treatment of filling small via holes by metal by means of CVD of 
tungsten using WF.sub.6 gas and reducing gas (H.sub.2, silane (SiH.sub.4) 
and like compounds used alone or as a gas mixture); or, since substances 
corrosive to underlayer produced by surface cleaning treatment sometimes 
remain on wafers depending on the combination of the material of small via 
hole underlayer with the kind of halogen gas used for the cleaning 
treatment, by carrying out the following three treatments successively: 
3-(1) a surface cleaning treatment of small via hole bottom underlayer 
surface accompanied by no activation of the surface of insulating film on 
the wafer (namely, removing treatment of surface oxide layer etc.), 
3-(2) a treatment of removing substances corrosive to small via hole bottom 
underlayer produced by the above surface cleaning treatment (namely, 
anti-corrosive treatment), and 
3-(3) a treatment of filling small via holes by metal by means of CVD of 
tungsten using WF.sub.6 gas and reducing gas (H.sub.2, silane, and like 
compound used alone or as a gas mixture). 
Among the above-mentioned treatments, with respect to the treatment 1-(1), 
sputter-etching treatment using inert gases such as Ar may be used. 
Respecting the stabilization treatment of 1-(2), the insulating film 
surface which has become rich in Si and active can be modified by plasma 
treatment using halogen-containing chemical etching gases such as 
Cl.sub.2, BCl.sub.3, CCl.sub.4, C.sub.2 Cl.sub.4, SiCl.sub.4, NF.sub.3, 
SF.sub.6 and SiF.sub.4 each alone or as a mixture thereof with inert gases 
such as Ar. 
The stabilization treatment of 1-(2) may also be effected by another method 
comprising heating a wafer in N.sub.2 (a pure gas or a mixture containing 
a very small amount of O.sub.2) atmosphere. 
With respect to the treatment 2-(1) wherein said (1) and 1-(2) are carried 
out simultaneously, the surface oxide film of small via hole bottom 
underlayer can be removed, with no accompanying activation of the 
insulating film surface, by plasma treatment using halogen-containing 
chemical etching gases employed in the above stabilization treatment of 
1-(2) such as Cl.sub.2, BCl.sub.3, CCl.sub.4, C.sub.2 Cl.sub.4, 
SiCl.sub.4, NF.sub.3, CF.sub.4, CHF.sub.3, SF.sub.6 and SiF.sub.4 each 
alone or as a mixture thereof with inert gases such as Ar. 
The etch amount necessary in said simultaneous treatments is just to 
correspond to the amount of sputter-etching applied in 1-(1), whereas only 
a slight modification of surface layer is effected in the stabilization 
treatment of 1-(2) described before. When sputter-etching by Ar ions is 
effected by using plasma of Ar gas alone as in the prior art, the surface 
layer of insulating film becomes rich in Si and is activated owing to the 
difference in sputtering yield among elements as described before, whereas 
in the case where halogen gas plasma is used, even when the surface layer 
becomes partly rich in Si, the Si-rich part is immediately removed by 
halogen ions or radicals and the insulating film surface is ultimately not 
activated; rather, selectivity in tungsten film growth is improved as 
compared with a case where no such treatment is applied because active 
parts rich in Si originating from defect etc. developed in forming the 
insulation film are removed. However, depending on the combination of the 
material of small via hole bottom underlayer with the kind of halogen gas 
used for the surface cleaning treatment, there sometimes remain on wafers 
substances corrosive to underlayer produced by the surface cleaning 
treatment. That is, when the underlayer of small via hole bottom part is 
aluminum wiring and a halogen gas containing chlorine is used, there will 
remain such substances as AlCl.sub.3 which will react with moisture 
contained in the air and corrode aluminum wiring when the wafer is taken 
out into the air after tungsten film growth. In such a case, therefore, a 
treatment of removing residual chlorine is necessary as an anti-corrosive 
treatment after the surface cleaning treatment. 
The removal of substances corrosive to small via hole bottom underlayers as 
an anti-corrosive treatment can be effected by oxygen plasma treatment or 
fluorine plasma treatment, to knock out the corrosive substances remaining 
on the wafer with ions or radicals or to form a thin protective film on 
the underlayer surface. A heat treatment is also effective as an 
anti-corrosive treatment which comprises heating the wafer thereby to 
evaporate off remaining corrosive substances. 
The selective tungsten CVD treatment of 1-(2), 2-(2) or 3-(3) can be 
accomplished by using one-stage CVD which comprises passing a gas mixture 
of WF.sub.6 and a reducing gas such as H.sub.2, SiH.sub.4 etc. over a 
heated wafer in a CVD reaction chamber set up for conducting selective 
CVD, by using a two-stage CVD which comprises passing WF.sub.6, alone or 
diluted with inert gases such as Ar, and then passing WF.sub.6 and the 
above-mentioned reducing gas over a heated wafer, or by using a CVD of two 
or more stages which comprises passing WF.sub.6 and H.sub.2 and then 
passing WF.sub.6 and another reducing gas. Reducing gases which can be 
used are, for example, H.sub.2, SiH.sub.4, Si.sub.2 H.sub.6, BH.sub.3, 
PH.sub.3 etc. used alone or in a combination thereof. 
In the plasma treatment apparatus using chemical etching gas used in the 
present invention, plasma is generated by high frequency wave (of 
preferably 10 kHz or more) as in conventional methods. However, if the 
wall etc. of the treatment chamber is sputtered by generated plasma and 
metallic contaminants adhere onto the wafer, tungsten will grow from the 
contaminants serving as nuclei in the subsequent W-CVD, which may result 
in lowering of selectivity. Therefore, it is important for exhibiting the 
effect of the present invention that a cathode coupled type apparatus 
wherein the wafer is negatively charged when high frequency wave is 
applied to the electrode through a blocking capacitance be used as the 
plasma treatment apparatus and the apparatus be constructed such that 
parts which are close to the wafer surface and may possibly become the 
source of metallic contaminants are covered by quartz plates. 
The above-mentioned surface cleaning treatment of small via hole bottom 
underlayers has an effect of removing by a physical treatment of 
sputter-etching oxide films present on the underlayer surface and organic 
residues left behind in preceding processes for small via hole formation 
including etching process and resist-removing process, and thereby 
cleaning the underlayer surface and lowering the contact resistance at the 
small via hole bottom part. On the other hand, the stabilization treatment 
of insulating film layer has an effect of preferentially removing by 
plasma of halogen-containing gas the activated parts including Si radical 
etc. of insulating film surface layer produced by the above treatment and 
thereby preventing the growth of tungsten on insulating film in succeeding 
selective CVD. 
The surface cleaning effect and the effect of stabilizing insulating film 
surfaces mentioned above have been confirmed by the following experimental 
results. Samples shown in Table 1 were prepared and examined for SiO.sub.2 
surface activation by Ar sputter-etching and for surface stabilization 
effect by Cl.sub.2 plasma with an X-ray photoelectron spectroscopy (ESCA 
or XPS) apparatus. First, the content ratio of O to Si (O/Si) at the 
surface was determined. The results are shown in Table 2, which all show a 
value larger than 2 (namely, the surface is richer in O than in 
SiO.sub.2). This is because since the samples were exposed to the air 
before analysis, the surfaces contain adsorbed O.sub.2. As is apparent 
from Table 2, in Ar sputtering, O gives a higher sputtering efficiency 
than Si and resultantly the surface of the sample (#2) treated by Ar 
sputter-etching is richer in Si than that of the untreated sample (#1). In 
Cl.sub.2 plasma treatment, on the other hand, presumably Cl.sup.+ (or 
radicals of Cl atom etc.) attacks Si first and, in consequence, the 
surface of the sample (#3) treated by Cl.sub.2 plasma is richer in O than 
that of the untreated sample. For further verification, spectra of 
Si.sub.2p peak were determined at an X-ray incident angle on the SiO.sub.2 
surface of 30.degree. and 90.degree.. These are shown in FIGS. 13 to 15. 
The smaller the incident angle of X-ray is, information on parts closer to 
surface layer can be obtained. Further, when stable Si-O bonds are severed 
and unbonded Si and Si-Si bonds are formed, the peak undergoes a chemical 
shift toward lower energy side. In the untreated SiO.sub.2 surface the 
peaks at 30.degree., and 90.degree. coincide entirely with each other, 
whereas in Ar-sputtered surface the peak at 30.degree. which represents a 
part closer to surface layer, shifts toward lower energy side and becomes 
broad. On the other hand, in a sample treated additionally by Cl.sub.2 
plasma the peak shows almost no change as in the untreated sample, which 
is attributable to removal of unbonded Si and Si-Si bonds formed by Ar 
sputtering. The peaks at 30.degree. representing a part closer to surface 
layer were compared for above samples and shown in FIG. 16, which 
demonstrates the stabilization effect exhibited by Cl.sub.2 plasma 
treatment. On more detailed inspection, the peak in the Cl.sub.2 
plasma-treated sample somewhat shifts as compared with that in the 
untreated sample. This is attributable, when the data on O/Si content 
ratio is taken into consideration, to chemical shift caused by unbonded O 
or O-O bonds formed. Further, to examine the effect of the pretreatment 
according to the present invention on an aluminum surface, profiles by 
ESCA in the direction of depth at the tungsten/aluminum interface were 
also determined. Depth-direction profiles obtained with samples shown in 
Table 1 are shown in FIGS. 17 and 18. To examine halogens (F and Cl) 
present at the interface with good sensitivity, growth of tungsten was 
conducted at a temperature (370.degree. C.) lower than that (450.degree. 
C.) used in practical processes. The amount of Cl remaining at the 
tungsten/aluminum interface was found to be less than detection 
sensitivity even after Cl.sub.2 plasma treatment. Further, it was 
confirmed that the amount of O remaining at the tungsten/aluminum 
interface had been reduced by Ar sputter-etching. Just like as the effect 
of stabilizing a SiO.sub.2 surface was shown above for the case of using 
Cl.sub. 2 plasma, the effect of stabilizing a SiO.sub.2 surface was 
obtained in exactly the same manner for the case of using NF.sub.3 plasma. 
Also, in heating a wafer in N.sub.2 atmosphere, the effect of stabilizing 
a SiO.sub.2 surface was confirmed by ESCA. However, since it cannot be 
expected that unbonded Si or Si-Si bonds are removed with N.sub.2 and 
since no Si-N bond was detected, it is presumed that either a trace amount 
of O.sub.2 contained in N.sub.2 had oxidized Si or Si-Si bonds or even 
when they had changed into Si-N bonds their amount were below detection 
sensitivity. Though a N.sub.2 gas of a purity of 5N or more was used, 
there is a possibility of leakage of O.sub.2 from piping, so that it 
cannot be judged which of the above two presumptions represents the fact. 
In the foregoing, description was made of action and effect in a method 
wherein surface cleaning treatment and stabilization treatment are 
conducted successively. However, since a halogen-containing gas plasma 
alone exerts itself a physical sputter-etching action, the stabilization 
treatment and the surface cleaning treatment may also be conducted 
simultaneously. In this case, physical etching effect (namely, surface 
cleaning effect) can be enhanced by incorporation of inert gases such as 
Ar. Even when the plasma is formed from a gas mixture consisting of a 
large excess of an inert gas and a slight amount of halogen-containing 
gas, energy transfer occurs through collision with gases and ions in the 
plasma and hence chemical etching effect is not lost, so that physical 
surface cleaning action and stabilizing action on insulating film proceed 
simultaneously. The above-mentioned surface cleaning effect was confirmed 
by the following experimental results. Samples were prepared by forming 
Al.sub.2 O.sub.3 (alumina) film on Si wafers by electron beam vapor 
deposition. The resulting samples were treated by plasma of various gases 
and the film thickness of Al.sub.2 O.sub.3 before and after the plasma 
treatment were determined with an ellipsometer, to examine the etch rate 
in various gas plasma treatments. The results thus obtained are shown in 
FIG. 12. When Ar sputtering, which uses a plasma of Ar gas alone, and a 
plasma treatment using gas mixture comprising Ar gas and a small amount of 
NF.sub.3, Cl.sub.2 or BCl.sub.3 gas incorporated therein are compared, the 
etch rate in the latter plasma of gas mixture is higher than in the plasma 
of Ar gas alone even at the same pressure. This is attributable to the 
combination of physical effect with additional chemical effect. 
Particularly in plasma using Ar/NF.sub.3 gas mixture containing NF.sub.3 
incorporated therein and plasma using Ar/BCl.sub.3 gas mixture containing 
BCl.sub.3 incorporated therein, the etch rate is as high as about 6 times 
that in Ar sputter-etching, revealing that chemical etching effect is 
playing a predominant role. 
Since the underlayer material of small via hole bottom parts is Al wiring 
(sometimes containing a small amount of Si, Cu etc.), the use of 
chlorine-containing gas in surface cleaning treatment will leave such 
substances as AlCl.sub.3 behind which will react with the moisture 
contained in air and corrode the aluminum wiring when a wafer is taken out 
into the air after growth of tungsten film. In such a case, the treatment 
of removing substances corrosive to small via hole bottom underlayers 
(namely, anticorrosive treatment) is desirably introduced to remove the 
above corrosive substances before the wafer is taken out into the air and 
thereby to prevent corrosion from occurring even when the wafer is taken 
out into the air. Since corrosion of small via hole bottom underlayers may 
cause breaking of wire to lower the reliability of products, prevention of 
corrosion is of great importance in some cases. 
Subsequently to the pretreatments described above, selective CVD using 
WF.sub.6 and a reducing gas such as H.sub.2, SiH.sub.4 etc. is carried 
out, whereby tungsten grows only on the small via hole bottom underlayers 
through a reaction with the underlayer or with the reducing gases, while 
no tungsten grows on insulating film. Thus, selective CVD process exerts 
an action of filling only small via holes by tungsten and thereby leveling 
the parts. The "underlayer" of small via holes in the present invention 
refers to all those underlayers which directly react with WF.sub.6 or on 
which reducing gases are adsorbed and dissociated to reduce WF.sub.6, 
including wiring layers comprising aluminum or mainly aluminum, doped or 
not doped Si layers, and barrier layers of MoSi.sub.2, WSi.sub.2, 
TiSi.sub.2, PtSi, TiW, TiN, W, Mo and the like. The "insulating film" 
refers to all the insulating films used in LSI including inorganic 
insulating films such as thermally oxidized film, thermally nitrided film, 
PSG, BPSG, plasma-oxidized film and plasma-nitrided film, and organic 
insulating films such as SOG and PIQ.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some preferred embodiments of the present invention, classified into 5 
cases, will be described below with reference to drawings. First, 
embodiments wherein, among the methods of selective filling of via holes 
by tungsten according to the present invention, Ar sputter-etching is 
conducted as a pretreatment and then a stabilization treatment of 
insulating film surface was carried out by using plasma of a 
halogen-containing gas are shown as Examples 1-1 to 1-6. Nextly, an 
embodiment wherein a stabilization treatment of insulating film surface 
succeeding to sputter-etching is effected by a heat treatment in nitrogen 
(pure nitrogen gas or nitrogen containing a slight amount of oxygen) at 
atmospheric pressure is shown as Example 2. Then, among embodiments 
wherein a plasma etching treatment using a gas mixture of Ar and a 
halogen-containing gas is conducted as a pretreatment, one wherein a 
fluorine-containing gas is used as a halogen gas is shown as Example 3 and 
embodiments wherein a chlorine-containing gas is used as a halogen gas and 
an anticorrosive treatment is conducted in addition are shown as Examples 
4-1 and 4-2. Finally, an embodiment wherein, in selective filling of 
contact holes by tungsten, a plasma etching treatment using a gas mixture 
of Ar and a halogen-containing gas is conducted as a pretreatment is shown 
as Example 5. 
EXAMPLE 1-1 
FIG. 1 is a diagram showing, among the methods of filling a via hole by 
tungsten according to the present invention, a process flow wherein Ar 
sputter-etching is carried out as a pretreatment and then a stabilization 
treatment of insulating film surface by Cl.sub.2 plasma is conducted. FIG. 
8 shows a selective CVD apparatus for via filling by metal used in the 
present invention. The present Example will be described by way of FIG. 1 
with concurrent reference to FIG. 8. 
A wafer 9 was loaded in a loadlock chamber 1 shown in FIG. 8 and then the 
loadlock chamber 1 was evacuated to about 10.sup.-5 Torr. Thereafter, the 
wafer 9 was heated to about 200.degree. C. with a heating lamp (not shown 
in the Figure) provided in the chamber, to burn off moisture attaching to 
the wafer 9. After confirming the stop of pressure increase in the 
loadlock chamber 1 due to burning off of moisture from the wafer 9 during 
the heating (the stop occurred after the lapse of about two minutes), the 
heating was stopped, gate valves 4 and 5 were opened, and the wafer 9 was 
transported through a wafer transport mechanism (not shown in the Figure) 
into an etching chamber 3 and placed therein. The etching chamber 3 was 
evacuated beforehand to about 10.sup.-7 Torr by means of a cryopump. In 
order that oxidation in sputter-etching may be prevented from occurring, 
the leak rate of the etching chamber 3 needs to be suppressed to below 
10.sup.-5 Torr.multidot.l/sec. The lower the leak rate value is the more 
desirable. The etching chamber 3 is so constructed that, to prevent 
metallic contaminants, which might come from the inner wall of the etching 
chamber 3 and an electrode 8, from adhering to the wafer 9, the wafer side 
electrode 8 is of a cathode coupled type and a bias of negative potential 
is applied to the wafer side during discharge, whereby sputtering of the 
inner wall of etching chamber 3 by ions during discharge may be kept to 
minimum and only the wafer 9 placed on the cathode electrode 8 side may be 
sputtered by ions. The cathode electrode 8 is provided with a quartz cover 
(not shown in the Figure) to suppress as much as possible metallic 
contamination from the exposed part of electrode 8 at the periphery of the 
wafer 9. 
The pressure in the etching chamber 3 increased somewhat when the gate 
valve 5 was opened in company with the transport of the wafer 9 into the 
etching chamber 3 but recovered in a moment when the gate valve 5 was 
opened after the wafer 9 had been placed. After confirming the pressure 
recovery, Ar gas was introduced into the etching chamber 3. Then, a high 
frequency power was applied by a high frequency power source 13 to the 
cathode electrode 8 in the etching chamber 3 to start discharge and to 
produce Ar plasma. After a predetermined time of discharge, application of 
high frequency power was once stopped, then Cl.sub.2 gas was introduced 
and discharge was started again to generate Cl.sub.2 plasma. After a 
predetermined time of discharge, introduction of Ar gas and Cl.sub.2 gas 
and application of high frequency power are stopped and discharge was 
stopped. After confirming that the pressure in the etching chamber 3 had 
recovered again to about 10.sup.-7 Torr, the gate valve 5 was opened and 
the wafer 9 was transported through a wafer transport mechanism (not shown 
in the Figure) into a CVD reactor 2 evacuated to a pressure below 
10.sup.-5 Torr beforehand. After the wafer 9 was transported into and 
placed in the CVD reactor 2, the gate valve 5 was closed, Ar was 
introduced to the back of the wafer 9, the gate valve 4 was closed 
simultaneously with beginning of gradual increase of the pressure in the 
CVD reactor 2, and H.sub.2 gas was introduced into the CVD reactor 2. In 
the CVD reactor 2, the wafer 9 was irradiated by infrared rays with a 
halogen lamp 6 for heating the wafer through a quartz window 14 and was 
heated to a predetermined temperature. The power of the wafer heating 
halogen lamp 6 is controlled with a thermo-couple 7 provided between the 
quartz window 14 and the back of the wafer 9 and with a pyrometer 10 so 
provided as to be able to measure the temperature of wafer 9 by monitoring 
the infrared rays radiated from the surface of wafer 9 through a calcium 
fluoride (CaF.sub.2) window 15. The inner wall of the CVD reactor 2 is 
cooled with water, whereby the temperature of the inner wall of the CVD 
reactor 2 excepting the surface of the wafer 9 is decreased to a 
sufficiently low temperature (about 120.degree. C. or below) at which 
substantially no growth of tungsten film proceeds even at the time of 
heating the wafer 9. After the wafer 9 was heated to a predetermined 
temperature, WF.sub.6 was introduced in addition to H.sub.2 to effect 
selective growth of tungsten. After the tungsten film was grown to a 
predetermined thickness, introduction of H.sub.2 and WF.sub.6 was stopped, 
and simultaneously therewith the wafer heating lamp 6 was turned off and 
the CVD reactor was evacuated. The gate valve 4 was opened and the wafer 9 
was transported to the loadlock chamber 1. The gate valve 4 was closed, 
N.sub.2 was introduced, the wafer was cooled simultaneously therewith and, 
after the internal pressure of the loadlock chamber 1 reached to 
atmospheric pressure, the wafer 9 was taken out. Thus, via filling by 
tungsten was completed. 
An example of the treatment conditions in the above-described process is 
shown in the column of Example 1-1 of Table 3 together with those for 
other Examples. The wafer used is a test wafer prepared by forming 
SiO.sub.2 film 23 on a Si wafer 21 upon an underlayer aluminum wiring 22 
by means of plasma CVD etc. and then forming a large number of small via 
holes of 1 .mu.m square (1.2 .mu.m in depth) by photoetching. FIG. 9 (a) 
shows an enlarged sectional view of the small via hole of the wafer not 
yet subjected to the treatments shown in FIG. 1. In FIG. 9 (a), 21 
indicates a Si wafer, 22 underlayer aluminum wiring, 23 plasma CVD 
SiO.sub.2 film, and 24 surface oxide film on aluminum wiring at the 
opening part. FIG. 9 (b) shows a sectional view of a via hole after 
subjected to Ar sputter-etching treatment. In FIG. 9 (b), the aluminum 
surface oxide film 24 seen in FIG. 9 (a) has been removed, and a thin 
aluminum film 25 adhered by resputtering from hole bottom by Ar is 
observed on the via hole side wall. As described before, at the SiO.sub.2 
surface there exists a surface layer 26 rich in Si containing free 
radicals of Si activated by Ar sputtering or Si-Si bonds. FIG. 9 (c) shows 
a sectional view of a via hole subjected to Cl.sub.2 plasma treatment. In 
FIG. 9 (c), the activated layer 26 of SiO.sub.2 surface observed in FIG. 9 
(b) has been removed. FIG. 9 (d) shows a sectional view of a via hole 
subjected to via filling treatment by tungsten. In FIG. 9 (d), tungsten 
film 27 has grown directly on aluminum wiring 22 and the via hole has been 
filled by tungsten. During this time, tungsten film grows, with aluminum 
serving as nuclei, simultaneously at the hole bottom part and at the side 
wall part, so that via filling by tungsten is completed at a thickness of 
grown tungsten film of about 0.5 .mu.m for a via hole of 1 .mu.m square 
section. 
Then the wafer subjected to via filling treatment by tungsten film 
according to the present invention was evaluated for selectivity and 
contact resistance at the via hole part. The results of evaluation are 
shown in Table 4 together with those in other Examples and Comparative 
Examples described later. For evaluation of selectivity, a wafer 9 after 
via filling by 1 .mu.m thick tungsten was observed in dark field with a 
metallographic microscope at a magnification of 2000, and the number of 
tungsten particles formed on a certain region of insulating (SiO.sub.2) 
film was counted and expressed in terms of number of particles per unit 
area (herein 1 cm.sup.2). In the test wafer used are formed via holes at a 
density of about two millions/cm.sup.2. Thus, when 100 tungsten particles 
are present per cm.sup.2 on SiO.sub.2, for example, on a region of 
SiO.sub.2 in which 20,000 through holes are present there exists one 
tungsten particle on the average. The criteria for selectivity were 
classified from the above viewpoint into .circleincircle. (50 
particles/cm.sup.2) to xx (growth of tungsten over the whole surface). In 
the present Example, the selectivity was excellent. For evaluation of 
contact resistance, when an upper layer aluminum wiring is formed after 
via filling by tungsten of the above-mentioned test wafer has been 
finished, the series resistance of a continuous chain of 4000-200,000 via 
holes can be measured; aluminum wiring resistance portion was subtracted 
from the measured resistance and the balance was divided by the number of 
through holes, the resulting value being taken as the contact resistance 
at the tungsten/aluminum interface. In the present Example, the contact 
resistance showed a very good value of 0.10-0.15 
.OMEGA./.mu.m.sup..quadrature.. The value showed scarcely any change after 
a treatment at 475.degree. C. for 90 minutes. 
EXAMPLE 1-2 
The same apparatus and the same wafer as in Example 1-1 were used and a 
continuous treatment of sputter-etching and Cl.sub.2 plasma treatment was 
carried out. Then the wafer was once taken out into the air, and tungsten 
film growth treatment and subsequent treatments were conducted under the 
same conditions as in Example 1-1 (reference is made to the column of 
Example 1-2 in Table 3). The present Example was conducted with the aim of 
comparing a case wherein continuous treatment was conducted through the 
aid of transport in high vacuum (10.sup.-5 Torr or less) and a case 
wherein such transport was not conducted. As shown in Example 1-2 of Table 
4, the selectivity in the present Example was very good and at exactly the 
same level as in Example 1-1, but, regarding contact resistance 
evaluation, the contact resistance at the tungsten/aluminum interface was 
about 3 times as high as that in Example 1-1. Further, the time required 
for growth of tungsten in filling 1 .mu.m depth was somewhat longer than 
in Example 1-1. This is presumably because when the wafer was once taken 
out into the air, naturally oxidized film was formed on the aluminum 
surface exposed at the via hole bottom. Thus, it is surmised that the 
increase in interfacial resistance is due to increase in oxygen remaining 
at the interface, and the longer time required for tungsten film growth is 
due to a longer nuclei formation time at the initial stage of tungsten 
film growth. The above results show that, from the viewpoint of contact 
resistance characteristic, continuous treatment as in Example 1 is more 
preferable. However, it is needless to say that for LSIs wherein 
specification requirements are not so severe as in bipolar LSIs, e.g. for 
LSIs of MOS type, the pretreatment method according to the present 
invention gives a sufficient effect in improving selectivity even when the 
pretreatment and the selective CVD are conducted discontinuously as in the 
present Example. 
Nextly, three Comparative Examples will be described below to confirm in 
more detail the effects shown in Examples 1-1 and 1-2. 
COMATIVE EXAMPLE 1-1 
The same apparatus and wafer as used in Example 1-1 was employed. The wafer 
was loaded in the loadlock chamber 1, which was then evacuated, then the 
wafer was heated and thereafter transported directly to the CVD reactor 2 
and, without pretreatment, subjected to tungsten film growth treatment and 
subsequent treatments under the same conditions as in Example 1-1 
(reference is made to Table 3). This Example was conducted with the aim of 
confirming the effect of Ar sputtering pretreatment. Resultantly, it was 
found that the selectivity was somewhat lower than in Example 1-1 and the 
contact resistance was about two orders of magnitude or more higher than 
that in Example 1-1. Further, the time required for growth of tungsten in 
filling 1 .mu.m depth was twice or more as long as that in Example 1-1 
(reference is made to Table 4). As the reason for somewhat inferior 
selectivity in the present Comparative Example to that in Example 1, it is 
surmised that metallic contaminants formed on insulating film during via 
hole formation (photoetching, resist removal step, etc.) still remain 
though in a small extent, or during insulating film (plasma CVD SiO.sub.2 
film) formation free radicals of Si or Si-Si bonds have been already 
formed on the SiO.sub.2 surface though in a small amount. In any way, it 
was confirmed that even when such active parts exist from the beginning on 
the surface of testing wafers used, the stabilization treatment by 
Cl.sub.2 plasma mentioned above was effective also for said active parts. 
With respect to contact resistance and film growth time, it is considered 
that surface oxide film present on the aluminum surface is responsible as 
described above. However, whereas a low contact resistance (0.3-0.7 
.OMEGA./.mu.m.sup..quadrature.) was obtained at a tungsten growth 
temperature of 380.degree. C. or more in a publicly known example 
mentioned before, a very high contact resistance was exhibited in the 
present Comparative Example although tungsten film growth was conducted at 
450.degree. C. This is presumably because the preparation process of the 
test wafer used in the present Comparative Example (also in the other 
Examples) involves a step of forming aluminum surface oxide film by 
O.sub.2 plasma at the final stage of etching step of through hole 
formation in order to suppress the corrosion of aluminum wiring. That is, 
it is considered that whereas a fairly low contact resistance can be 
obtained in the case of naturally oxidized film on the aluminum surface 
formed in mere atmospheric air as shown before in Example 1-2, the 
aluminum surface oxide film formed by O.sub.2 plasma is, in respect of 
both film thickness and film quality, stronger and more difficult to 
remove than naturally oxidized film. 
COMATIVE EXAMPLE 1-2 
The same apparatus and the same wafer as in Example 1-1 were used, and the 
wafer was treated under the same conditions as in Example 1-1 except for 
omitting the stabilization treatment by Cl.sub.2 plasma (reference is made 
to Table 3). The process flow diagram of the present Comparative Example 
is shown in FIG. 7. This Comparative Example was conducted with the aim of 
confirming, though already mentioned above in the description of the 
effect of the present invention, what result is obtained when tungsten 
film growth is effected on a SiO.sub.2 surface activated by Ar 
sputter-etching treatment. As expected, the selectivity was very poor, 
tungsten film grew over the whole surface after the lapse of film growth 
time of one minute, and also evaluation of contact resistance was 
impossible (reference is made to Table 4). 
COMATIVE EXAMPLE 1-3 
The same apparatus and the same wafer as in Example 1-1 were used. Unlike 
the above Comparative Example 1-2, the wafer was once taken out into the 
air after sputter-etching treatment, and then subjected to tungsten film 
growth treatment and subsequent treatments under the same conditions as in 
Comparative Example 1-2 (reference is made to Table 3). This is because 
some stabilization effect on the SiO.sub.2 surface was expected even in 
naturally oxidation with O.sub.2 in the air. The results of evaluation of 
selectivity and contact resistance are shown in Table 4. The selectivity 
is considerably better than in Comparative Example 1-2 and tungsten film 
growth over the whole surface does not occur, but a selectivity of a 
practically useful level is not obtained. The contact resistance is about 
the same extent as in Example 1-2 higher than in Example 1-1 corresponding 
to naturally oxidized film formed on the aluminum surface as the result of 
exposure to the air. 
Then, Examples 1-3 to 1-6 wherein the conditions for Cl.sub.2 plasma 
treatment are different will be described below to give further knowledge 
of said conditions. 
EXAMPLE 1-3 AND 1-4 
In Examples 1-3 and 1-4, the base pressure in the CVD reactor 3 was reduced 
one or two orders of magnitude, respectively, as compared with that in 
Example 1-1. This was for the purpose of confirming the effect of 
contamination of Cl.sub.2 plasma by moisture and oxygen in the air, 
because the base pressure was controlled by artificially leaking 
atmospheric air into the reactor. The conditions of Cl.sub.2 plasma 
treatment and the results of evaluation of selectivity, contact resistance 
and appearance by metallographic microscope are shown in Table 6. These 
results reveal that although selectivity is not affected at all by 
reduction of base pressure, discolored parts are developed in some 
portions of wiring and also contact resistance is increased as the base 
pressure is reduced. 
To study the reason for this, an aluminum solid film was subjected to 
Cl.sub.2 plasma treatment under the same conditions as in Example 1-4 and 
then tungsten film growth was conducted thereon in a thickness of 200 
.ANG.. The surface of the sample thus obtained was examined by SEM, and 
the elemental analysis of the tungsten/aluminum interface part was made 
with depth-direction profile of ESCA. 
Resultantly, it was observed that a marked surface roughness developed on 
the aluminum surface and, as shown in FIG. 19, a large amount of F 
remained at the tungsten/aluminum interface though Cl was not detected. 
Thus, it is considered that leakage of H.sub.2 O, O.sub.2 etc. of 
atmospheric air exerted some influence on Cl.sub.2 plasma, developing a 
marked roughness on the aluminum surface and making it difficult for F to 
get out of the aluminum interface during tungsten film growth, whereby 
contact resistance at the tungsten/aluminum interface part was increased. 
The above results show that it is preferable to conduct Cl.sub.2 plasma 
treatment under a sufficient base pressure (10.sup.-6 Torr or less). 
EXAMPLE 1-5 AND 1-6 
In Example 1-5, as shown in Table 5, the conditions of Cl.sub.2 plasma 
treatment used in Example 1 were changed in that the input power of RF was 
increased one order of magnitude and, in company therewith, the etch 
amount was increased 20 times or more. On the contrary, in Example 1-6 the 
input power was reduced to 2/5 and the etch amount was also decreased to 1 
.ANG. or less. The results of evaluation in these Examples are shown in 
Table 6. In both of the above cases, excellent results were obtained with 
respect to selectivity. Surprisingly, a sufficient stabilization effect 
was obtained in stabilization treatment by Cl.sub.2 plasma even when the 
etch amount of SiO.sub.2 surface was 1 .ANG. or less. On the other hand, 
in evaluation of contact resistance and observation of cross-section by 
SEM, in the sample of Example 1-5, loss in film thickness of underlayer 
aluminum wiring was observed and the contact resistance was increased to 
2-20 times that in Example 1-1. This is presumably because though the etch 
amount of SiO.sub.2 by Cl.sub.2 plasma is 70 .ANG., the etch rate is 
substantially higher for aluminum and, although it could not be confirmed 
by SEM, Cl.sub.2 plasma of high RF power gave to the aluminum surface not 
a slight damage (surface roughness, etc.). In contrast, in Example 1-6, 
wherein RF power was reduced below that in Example 1-1, the contact 
resistance was excellent similarly to Example 1-1. 
EXAMPLE 2 
FIG. 2 is a diagram showing, among the methods of filling via holes by 
tungsten according to the present invention, a process flow wherein the 
stabilization treatment of insulating film surface is effected by heat 
treatment of a wafer in N.sub.2 atmosphere of atmospheric pressure. The 
same apparatus, the same wafer and the same procedures as in Example 1-1 
were used except that after Ar sputter-etching treatment was conducted in 
the etching chamber 3, the wafer was transported to the CVD reactor 2, in 
which a stabilization treatment of insulating film surface was conducted 
by subjecting the wafer to heat treatment in N.sub.2 atmosphere of 
atmospheric pressure. The treatment conditions and the results of 
evaluation in Example 2 are shown in Tables 3 and 4. An enlarged view 
showing the process of via filling by metal at the cross-section of a via 
hole in Example 2 is exactly the same as FIG. 9 in Example 1-1. 
Table 4 reveals that in the sample of the present Example, a selectivity of 
the same level as in untreated sample can be secured and yet a good 
contact resistance characteristic is exhibited. Thus, it is apparent that, 
also in the present Example, the surface cleaning effect on the inside 
surface of via holes and the stabilization effect on insulating film 
surface are exhibited. Though an N.sub.2 treatment at 300.degree. C. for 2 
minutes was conducted in the present Example, selectivity improving effect 
could be observed at temperatures of 200.degree. C. or more. Further, when 
the extent of deterioration of insulating film surface by Ar 
sputter-etching treatment is large, satisfactory effect can be obtained by 
increasing the treating temperature or increasing the treating time 
according to necessity. 
EXAMPLE 3 
FIG. 3 is a diagram showing, among the methods for filling a via hole by 
tungsten, a process flow wherein a plasma etching using Ar incorporated 
with NF.sub.3 is conducted as a pretreatment, whereby a surface cleaning 
treatment for removing oxide film and fouling present on aluminum wiring 
at the via hole bottom is effected without being accompanied by activation 
of insulating film surface. The same apparatus, the same wafer, and the 
same procedures as in Example 1-1 were used except that pretreatment was 
conducted by one time plasma treatment in the etching chamber 3 by 
introducing Ar and BF.sub.3 simultaneously thereinto (reference is made to 
Table 3). 
The conditions of treatments and the results of evaluation conducted in 
Example 3 are shown in Table 4. The process of via filling by metal at the 
cross-section of via hole in Example 3 was approximately the same as in 
Example 1-1 shown in FIG. 9. Difference from Example 1-1 lies in that, as 
described before, the etch rate of aluminum oxide (Al.sub.2 O.sub.3) film 
shown in FIG. 12 differs greatly when NF.sub.3 is incorporated into Ar 
from that observed when Ar gas is used alone, and not a mere physical 
sputtering but a chemical etching action operates in the present Example. 
Regarding the mechanism for this difference, although it is possible that 
radicals produced in NF.sub.3 plasma directly etch Al.sub.2 O.sub.3, it is 
rather natural to consider that Al.sub.2 O.sub.3 is once converted to 
AlF.sub.3 (fluorine substitution reaction) and, since AlF.sub.3 is more 
susceptible to sputtering than Al.sub.2 O.sub.3, the etch rate is 
increased by addition of NF.sub.3 in consequence. Plasma has almost no 
etching ability against alumina not like as Cl.sub.2 plasma. In both 
selectivity and contact resistance, the results in the present Example are 
excellent, like as those in Example 1-1 (reference is made to Example 3 of 
Table 4). When a fluorine-containing gas is used as a halogen gas added to 
Ar as shown in the present Example, corrosion of aluminum caused by the 
use of chlorine-containing gas shown in the following Example 4 does not 
take place at all. 
EXAMPLE 4-1 
The same apparatus and the same wafer as in Example 1-1 were used. After a 
plasma treatment using a gas mixture of Ar/Cl.sub.2 and plasma treatment 
using O.sub.2 were continuously conducted, tungsten film growth treatment 
and subsequent treatments were conducted under the same conditions as in 
Example 1 (reference is made to Example 4-1 of Table 1). The present 
Example represents an example of a case wherein chlorine-containing gas 
(herein, Cl.sub.2) is used in place of a fluorine-containing gas (e.g., 
NF.sub.3) as plasma etching gas. FIG. 4 is a process flow diagram showing 
the procedures followed in Example 4-1. FIG. 10 shows an enlarged view 
representing the process of via filling by metal at the cross-section of a 
via hole in Example 4-1. Herein, difference from Example 1-1 is as 
follows: since a chemical etching action, not a mere physical sputtering, 
is used in surface cleaning treatment of aluminum surface, aluminum film 
resputtered onto the side wall seen in FIG. 9 (b) is not observed in FIG. 
10 (b) and, in consequence, the form of via filling by tungsten assumes a 
form wherein tungsten grows only from the bottom of the via hole as shown 
in FIG. 10 (c). When this method is used, the flatness after via filling 
by tungsten is improved, but since a tungsten growth thickness just to 
meet the depth of the via hole to be filled is required, the time 
necessary for via filling is longer than in Example 1-1. Further, when a 
chlorine-containing gas (Cl.sub.2) is used in the present Example, such 
substances as AlCl.sub.3 remain on the wafer after surface cleaning 
treatment which will corrode aluminum when the wafer is subjected to 
selective CVD of tungsten and taken out into atmospheric air, unlike in 
using a fluorine-containing gas (NF.sub.3) as in Example 3. Accordingly, 
the wafer should be subjected to an anticorrosive treatment of removing 
corrosive substances such as AlCl.sub.3 therefrom before subjected to 
selective CVD of tungsten. In Example 4-1, an anticorrosive treatment was 
conducted, after plasma treatment using a gas mixture of Ar/Cl.sub.2, by 
once terminating discharge, turning off Ar and Cl.sub.2, evacuating the 
chamber, introducing O.sub.2 and causing O.sub.2 plasma discharge, and 
exposing the wafer to O.sub.2 plasma. As shown in Example 2 of Table 2, 
the selectivity was excellent, being at exactly the same level as in 
Example 1, but, regarding contact resistance evaluation, the contact 
resistance at the tungsten/aluminum interface part was about twice as high 
as that in Example 1. Further, the film growth time required for filling 1 
.mu.m depth was somewhat longer than in Example 1. This is attributable to 
formation of oxide film during O.sub.2 plasma treatment on the aluminum 
surface exposed at the via hole bottom part. That is, it is considered 
that the increase in interfacial resistance is due to increase in residual 
O at the interface and the increase in film growth time required is due to 
longer time required for nuclei formation at the initial stage of tungsten 
film growth. However, it is considered that even when a thin oxide film is 
once formed on the aluminum surface, the oxide decomposes at the initial 
stage of tungsten film growth according to the following equation and 
leaves substantially no O at the tungsten/aluminum interface. 
EQU Al.sub.2 O.sub.3 +3WF.sub.6 .fwdarw.3WOF.sub.4 .uparw.+2AlF.sub.3 (4) 
AlF.sub.3 shown in the above equation is a substance which is formed at the 
aluminum surface also when plasma of fluorine-containing gas is used and 
is considered not to be the cause of any particular problem also form the 
result of contact resistance evaluation. It is also possible that F is 
abstracted by reducing gas at the initial stage of tungsten film growth 
and residual F at the aluminum/tungsten interface is reduced thereby. 
EXAMPLE 4-2 
The process in the present Example was approximately the same as in Example 
1, but the wafer was subjected to heat treatment, after transported to the 
CVD reactor, as an anticorrosive treatment in place of O.sub.2 plasma 
treatment. FIG. 5 is a process flow diagram showing the procedures 
followed in Example 4-2. In Example 4-2, the wafer was subjected to a 
plasma treatment using a gas mixture of Ar/Cl.sub.2, then transported to 
the CVD reactor 2 in the same manner as in Example 3, and subjected to 
heat treatment in H.sub.2 gas at 450.degree. C. for 3 minutes. The heat 
treatment causes corrosive substances (AlCl.sub.3) to evaporate and 
prevents corrosion from occurring when the substrate is taken out into the 
air after completion of the growth of tungsten film. As shown in Example 
4-2 of Table 2, the selectivity was excellent, being at exactly the same 
level as in Example 1-1. As to contact resistance evaluation, an 
approximately the same result as in Example 4-1 was obtained. This is 
presumably because the aluminum surface was oxidized by leaked gas from 
piping or residual oxygen in the CVD reactor. 
As to which of the two forms of via filling by tungsten, namely, (1) via 
filling wherein tungsten growth from the side wall of via hole also occurs 
as in Examples 1-1 to Example 3 shown in FIG. 9 and (2) via filling 
wherein tungsten grows solely from the bottom of via hole as in the 
present Examples 4-1 and 4-2 shown in FIG. 10, is to be selected, it 
depends on the aspect ratio (hole depth/hole diameter) of via holes and to 
what extent does the depth vary among the holes present on the same wafer. 
That is, when a via hole of a small aspect ratio is filled by metal by the 
method of Example 1-1, there arises a situation wherein the via hole is 
not yet fully filled even after another hole with the same depth and a 
larger aspect ratio has already been filled by metal completely. On the 
other hand, when via holes having greatly different depths are present on 
the same wafer and the holes are filled by the method of Examples 4-1 and 
4-2, there comes a situation wherein a deep hole is not yet fully filled 
even after a shallow hole has already been filled. Therefore, it is 
preferable to use these methods properly according to the kind of via 
holes. 
Further, to improve throughput, it is also possible to use, at the initial 
stage of tungsten film growth, a H.sub.2 reduction at 380.degree. C. or 
higher, which can give a low contact resistance at the tungsten/aluminum 
interface, and after thin tungsten film has been formed over the whole 
surface of via hole bottom part, to change the gas to be introduced and 
decrease the wafer temperature to 250.degree.-320.degree. C. thereby to 
conduct a tungsten-film growth treatment by SiH.sub.4 or like reducing 
gases capable of giving a high film growth rate. It is also possible, when 
required specification for contact resistance is not so severe, as in LSIs 
of MOS type, to conduct a tungsten film growth treatment by SiH.sub.4 or a 
like reducing gas, capable of working at low film growth temperature, from 
the initial stage of tungsten film growth treatment. 
Though an etch amount of 250 .ANG. was adopted in the present Example to 
effect surface cleaning of via hole underlayer, selectivity is improved as 
compared with untreated case even at an etch amount of 10 .ANG. or less. 
Thus, it is apparent from the result described already that selectivity 
improving effect can be obtained by using the stabilization treatment of 
insulating film surface by means of Cl.sub.2 plasma treatment as a 
pretreatment for selective CVD, separately from its use as surface 
cleaning treatment of underlayer. 
EXAMPLE 5 
FIG. 6 is a process flow diagram of a method of filling a contact hole, 
i.e. a via hole with Si contact, by tungsten according to the present 
invention in which a surface cleaning treatment of hole bottom part where 
in different kinds of underlayers including doped Si, poly Si and various 
silicides are present mingled with one another and a stabilization 
treatment of insulating film surface are simultaneously conducted by a 
plasma treatment using a gas mixture of Ar and NF.sub.3. The same 
apparatus and the same procedures as in Example 1 were used except that 
the underlayer of test wafer was different, the gas used for plasma 
treatment was changed from Cl.sub.2 to NF.sub.3, and SiH.sub.4 was used as 
reducing gas to avoid the problems of encroachment and wormhole 
characteristic of Si contact in H.sub.2 reduction W-CVD. The phenomena of 
encroachment and wormhole are described in Proc. of ECS Japan Branch First 
Symposium on CVD technology for VLSI (1988), p.48-65, referred to before. 
The conditions of treatments and the results of evaluation conducted in 
the present Example are shown in Tables 3 and 4. An enlarged view of a 
process of via filling at the cross-section contact hole in Example 5 are 
shown in FIG. 11 with a doped Si underlayer as an example. In FIG. 11(a), 
28 is underlayer Si(n.sup.+ -Si and P.sup.+ -Si), 29 is PBSG and 30 is 
naturally oxidized surface film of opening part Si. FIG. 11 (a) shows a 
state of things before application of the treatments of FIG. 6, but the 
surface oxide film of contact hole bottom has been once removed beforehand 
by wet etching using a dilute hydrofluoric acid solution (HF: H.sub.2 
O=1:99). Accordingly, the oxide film present at the contact hole bottom is 
solely a naturally oxidized film formed during the course of from drying 
succeeding to wet etching and water washing to loading the wafer in the 
loadlock chamber. FIG. 11(b) shows a state after application of plasma 
treatment using gas mixture of Ar and NF.sub.3. Similarly to Example 3, 
the insulating film surface keeps a stable condition, adhered film 
(herein, Si film) of underlayer material resputtered onto the side wall 
seen in FIG. 9 (b) (Example 1-1) is not observed here, and resultantly the 
form of via filling by tungsten assumes a form where in tungsten grows 
solely from the contact hole bottom as shown in FIG. 11(c). Though 
SiH.sub.4 was used as reducing gas in W-CVD in the present Example it is 
also possible, to solve the problem of poor adhesion at the 
tungsten/aluminum interface, to use H.sub.2 reduction, which gives a good 
adhesion, at the initial stage of tungsten film growth and, after thin 
tungsten film has been formed over the whole surface of contact hole 
bottom part, to change the gas to be introduced and conduct tungsten film 
growth treatment by SiH.sub.4 or a like reducing gas which gives a high 
film growth rate. In said H.sub.2 reduction in the above method, when the 
film growth temperature is selected at the same temperature of about 
250.degree.-270.degree. C. as in SiH.sub.4 reduction, the problems of 
encroachment and tunneling do not occur. Selectivity after via filling by 
tungsten in 1 .mu.m thickness is excellent as in Example 1-1. This 
signifies that the stabilization effect of SiO.sub.2 surface does not 
particularly depend on the kind of halogen-containing gas, but is 
satisfactorily exhibited so long as the gas exerts an chemical etching 
action. When reactivity with underlayer is taken into consideration, 
however, NF.sub.3 is considered particularly preferable in filling contact 
hole by tungsten. Also, the contact resistances of W/Si(n.sup.+), 
W/Si(p.sup.+), W/WSi.sub. 2, and W/Poly-Si interface formed by said via 
filling by tungsten show excellent values (n.sup.+ indicates doping of 
5.times.10 .sup.5 /cm.sup.2 of As+ by ion implantation at 70 KeV and 
p.sup.+ indicates doping of 1.5.times.10.sup.6 /cm.sup.2 of 
BF.sub.2.sup.+ by ion implantation at 60 KeV). Further, with regard to 
the problem described above that uniform film thickness in via filling by 
tungsten cannot be obtained when contact holes with different underlayers 
are present on the same wafer, since tungsten film growth in the present 
Example was conducted always under conditions where no underlayer surface 
oxide film is present, uniform film thickness in via filling by tungsten 
was obtained, and the effectiveness of the present invention was 
confirmed. 
Although some embodiments of the present invention were described above, 
the present invention is not restricted to the apparatus and conditions 
shown the above Examples, and all of the tungsten selective CVD 
apparatuses having a cold-wall type CVD reactor capable of effecting a 
selective film growth of tungsten, an etching chamber wherein Ar 
sputter-etching treatment free from metallic contamination and 
introduction of halogen-containing gases such as NF.sub.3, Cl.sub.2 and 
BCl.sub.3 are possible, and a transport mechanism capable of 
vacuum-transporting a wafer between the reactor and the chamber may be 
used by properly selecting treatment conditions. Although plasma treatment 
using halogen-containing gas was carried out using high frequency grow 
discharge plasma in the present Example, there may be used other methods 
which are free from metallic contamination, for example those which are 
used, or being studied for use, in semiconductor processes, e.g. microwave 
plasma method and ECR microwave plasma method. Further, though O.sub.2 
plasma treatment or heat treatment was shown as anticorrosive treatment in 
the Examples of the present invention, it is only necessary to remove 
corrosive substances, and other methods which are used, or being studied 
for use, in semiconductor processes, for example allowing a wafer to stand 
in N.sub.2 at atmospheric pressure at high temperature by using a quartz 
tube furnace, may be used. Further, it is needless to say that selective 
CVD systems and metals intended by the present invention are not limited 
to selective CVD of tungsten using WF.sub.6 -H.sub.2 system or WF.sub.6 
-SiH.sub.4 system described in above Examples, and the present invention 
can also be applied to other systems capable of effecting selective CVD, 
for example selective CVD of molybdenum using M.sub.o F.sub.6 -H.sub.2 
system or M.sub.o F.sub.6 -SiH.sub.4 system, and selective CVD of aluminum 
using alkylaluminum as a raw material. 
TABLE 1 
______________________________________ 
Samples for various evaluations 
(for confirmation of the effect of the invention) 
Sample 
No. Material Preparation process*) 
______________________________________ 
#1 SiO.sub.2 SiO.sub.2 solid film, no treatment 
#2 SiO.sub.2 SiO.sub.2 solid film + Ar sputter-etching 
treatment 
#3 SiO.sub.2 SiO.sub.2 solid film + Ar sputter-etching 
treatment + Cl.sub.2 plasma treatment 
#4 W-Al Untreated Al solid film + W 
deposition 
#5 W-Al Al solid film + Ar sputter-etching 
treatment + W deposition 
#6 W-Al Al solid film + Ar sputter etching 
treatment + Cl.sub.2 plasma treatment + W 
deposition 
______________________________________ 
Note: *)Detail treatment conditions 
Ar sputteretching treatment: base pressure = 3 .times. 10.sup.-7 Torr, Ar 
flow rate = 100 sccm, P = 10 m Torr, RF power = 480 W (13.56 MHz), wafer 
bias = -550 V, etch amount = 250 .ANG. (as SiO.sub.2 film) 
Cl.sub.2 plasma treatment: base pressure = 3 .times. 10.sup.-7 Torr, 
Cl.sub.2 flow rate = 3 sccm, Ar flow rate = 100 sccm, P = 10 m Torr, RF 
power = 50 W (13.56 MHz), wafer bias = -70 V, etch amount = about 3 .ANG. 
(as SiO.sub.2 film) 
W deposition: WF.sub.6 flow rate = 3 sccm, H.sub.2 flow rate = 500 sccm, 
Ar flow rate = 4 sccm, Pressure P = 0.66 Torr, wafer temp. T = 370.degree 
C. 
TABLE 2 
______________________________________ 
Results of elements analysis of SiO.sub.2 
surface by ESCA 
Sample No. O/Si content ratio*) 
______________________________________ 
#1 2.181 
#2 2.087 
#3 2.383 
______________________________________ 
Note: *)Including O adsorbed onto surface 
TABLE 3 
__________________________________________________________________________ 
Samples preparation conditions (1) 
Under- Connection betweene 
Example 
layer of pretreatment and W 
No. via hole 
Pretreatment*) deposition W deposition Remarks 
__________________________________________________________________________ 
Example 
Al Ar sputter-etching tratment + Cl.sub.2 
Continuous WF.sub.6 = 3 sccm, H.sub.2 = 
500 Process 
1-1 plasma treatment sccm, Ar = 4 sccm 
flow FIG. 1 
P = 0.66 Torr, 
T = 450.degree. C. 
Example Ar sputter-etching tratment + Cl.sub.2 
Discontinuous 
.uparw. 
1-2 plasma treatment (Wafer in the air) 
Comp. 
Al None -- .uparw. 
Example 
1-1 
Comp. 
Al Ar sputter-etching treatment 
Continuous .uparw. Process 
Example flow FIG. 7 
1-2 
Comp. 
Al Ar sputter-etching treatment 
Discontinuous 
.uparw. 
Example (wafer in the air) 
1-3 
Example 
Al Ar sputter-etching tratment + N.sub.2 
Continuous .uparw. Process 
2 treatment flow FIG. 2 
Example 
Al Plasma treatment using Ar/NF.sub.3 
Continuous .uparw. Process 
3 mixture *.sup. 1) flow FIG. 3 
Example 
Al Plasma treatment using Ar/Cl.sub.2 
Continuous .uparw. Process 
4-1 mixture + O.sub.2 plasma treatment flow FIG. 4 
Example 
Al Plasma treatment using Ar/Cl.sub.2 
Continuous .uparw. Process 
4-2 mixture + heat treatment flow FIG. 5 
Example 
Si(p.sup.+), 
Plasma treatment using Ar/NF.sub.3 
Continuous SiH.sub.4 = 4 sccm, WF.sub.6 = 
Process 
5 Si(n.sup.+) 
mixture *.sup.2) sccm, Ar = 100 
flow FIG. 6 
WSi.sub.2, P = 50 mTorr, T = 270.degree. C. 
poly-Si 
__________________________________________________________________________ 
Note *)Detail pretreatment conditions 
Ar sputteretching treatment: base pressure = 3 .times. 10.sup.-7 Torr, Ar 
flow rate = 100 sccm, P = 10 m Torr, RF power = 480 W (13.56 MHz), wafer 
bias = -550 V, etch amount = 250 .ANG. (as SiO.sub.2 film) 
Cl.sub.2 plasma treatment: base pressure = 3 .times. 10.sup.-7 Torr, 
Cl.sub. 2 flow rate = 3 sccm, Ar flow rate = 100 sccm, P = 10 m Torr, RF 
power = 50 W (13.56 MHz), wafer bias = -70 V, etch amount = about 3 .ANG. 
(as SiO.sub.2 film) 
N.sub.2 treatment: N.sub.2 flow rate = 10 l/min (atmospheric pressure), T 
= 300.degree. C., Treating time = 2 min 
Plasma treatiment using Ar/NF.sub.3 mixture *.sup.1): base pressure = 3 
.times. 10.sup.-7 Torr, NF.sub.3 flow rate = 5 sccm, Ar flow rate = 100 
sccm, P = 10 mTorr, RF power = 50 W (13.56 MHz), wafer bias = -70 V 
Plasma treatment using Ar/Cl.sub.2 mixture: base pressure = 3 .times. 
10.sup.-7 Torr, Cl.sub.2 flow rate = 9 sccm, Ar flow rate = 100 sccm, P = 
10 mTorr, RF power = 150 W (13.56 MHz), wafer bias = -200 V 
O.sub.2 plasma treatment: O.sub.2 flow rate = 100 sccm, P = 10 mTorr, RF 
power = 150 W (13.56 MHz) 
Heat treatment: 450.degree. C., H.sub.2 = 100 sccm, P = 0.66 Torr, 3 min 
Plasma treatment using Ar/NF.sub.3 mixture*.sup.2): base pressure 3 
.times. 10.sup.-7 Torr, NF.sub.3 flow rate = 3 sccm, Ar flow rate = 100 
sccm, P = 10 mTorr, RF power = 150 W (13.56 MHz), wafer bias = -260 V, 
etch amount = about 30 .ANG. (as SiO.sub.2 film) 
TABLE 4 
__________________________________________________________________________ 
Properties of samples of Example (1) 
Selectivity*) Time 
Under- 
[Number of tungsten 
Contact resistance 
required for 
Example 
layer of 
nuclei on insulating 
(W/Underlayer) 
via filling 
No. via hole 
film (number/cm.sup.2)] 
(.OMEGA./.mu.m.sup..quadrature.) 
(min) Remarks 
__________________________________________________________________________ 
Example 
Al .circleincircle. 
0.10-0.15 
3.0 
1-1 [5 .times. 10-5 .times. 10.sup.2 ] 
Example 
Al .circleincircle. 
0.30-0.52 
3.2 
1-2 [5 .times. 10-5 .times. 10.sup.2 ] 
Comp. 
Al .largecircle. 
&gt;20 8.0 
Example [5 .times. 10.sup.2 -5 .times. 10.sup.3 ] 
1-1 
Comp. 
Al XX not determined 
-- Deposition on 
Example [Deposition on whole whole surface 
1-2 surface] of wafer about 
1 min after 
initiation of W 
deposition 
Comp. 
Al X 0.40-0.62 
3.2 
Example [5 .times. 10.sup.3 -5 .times. 10.sup.5 ] 
1-3 
Example 
Al .largecircle. 
0.14-0.18 
3.0 
2 [5 .times. 10.sup.2 -5 .times. 10.sup.3 ] 
Example 
Al .circleincircle. 
0.10-0.15 
3.0 
3 [5 .times. 10-5 .times. 10.sup.2 ] 
Example 
Al .circleincircle. 
0.20-0.40 
5.5 
4-1 [5 .times. 10-5 .times. 10.sup.2 ] 
Example 
Al .circleincircle. 
0.16-0.40 
5.0 
4-2 [5 .times. 10-5 .times. 10.sup.2 ] 
Example 
Si(p.sup.+), 
.circleincircle. 
10-50 (Si(p.sup.+)), 
1.5 
5 Si(n.sup.+) 
[5 .times. 10-5 .times. 10.sup.2 ] 
50-10 (Si(n.sup.+)) 
WSi.sub.2, 1-2 (WSi.sub.2), 
poly-Si 1-2 (poly-Si) 
__________________________________________________________________________ 
Note: *)Selectivity: Excellent .rarw. .circleincircle. .largecircle. X XX 
.fwdarw. Poor 
TABLE 5 
__________________________________________________________________________ 
Conditions of Cl.sub.2 plasma treatment 
Base Wafer 
Etch 
pressure 
RF power 
bias 
amount 
Other 
Example No. 
(Torr) 
(W) (V) (.ANG.) 
conditions 
Remarks 
__________________________________________________________________________ 
Example 1-3 
3 .times. 10.sup.-6 
50 -70 .about.3 
Same as in 
Process flow 
Example 1-1 
FIG. 1 
Example 1-4 
3 .times. 10.sup.-5 
50 -70 .about.3 
.uparw. 
.uparw. 
Example 1-5 
3 .times. 10.sup.-7 
500 -480 
70 .uparw. 
.uparw. 
Example 1-6 
3 .times. 10.sup.-7 
20 -30 .about.1 
.uparw. 
.uparw. 
__________________________________________________________________________ 
Note: *)Calculated in terms of SiO.sub.2 film 
TABLE 6 
__________________________________________________________________________ 
Properties of samples of Examples (2) 
Selectivity*) Time 
Under- 
[Number of tungsten 
Contact resistance 
required for 
Example 
layer of 
nuclei on insulating 
(W/Underlayer) 
via filling 
No. via hole 
film (number/cm.sup.2)] 
(.OMEGA./.mu.m.sup..quadrature.) 
(min) Remarks 
__________________________________________________________________________ 
Example 
Al .circleincircle. 
0.10-0.75 
3.0 Disclosed part 
1-3 [5 .times. 10-5 .times. 10.sup.2 ] 
developed in 
parts of under- 
layer Al wiring 
Example 
Al .circleincircle. 
5.5-8.5 3.0 Disclosed part 
1-4 [5 .times. 10-5 .times. 10.sup.2 ] 
developed in 
parts of under- 
layer Al wiring 
Example 
Al .circleincircle. 
0.20-3.0 3.0 Film thickness 
1-5 [5 .times. 10-5 .times. 10.sup.2 ] 
decrease 
observed in 
underlayer Al 
wiring 
Example 
Al .circleincircle. 
0.10-0.14 
3.0 
1-6 [5 .times. 10-5 .times. 10.sup.2 ] 
__________________________________________________________________________ 
Note: *)Selectivity: Excellent .rarw. .circleincircle. .largecircle. X XX 
.fwdarw. Poor 
As described in the foregoing, in filling small via holes provided to 
insulating film on a wafer to expose parts of the underlayer of the wafer 
by means of selective CVD of metal, according to the method of the present 
invention which essentially comprises applying, before application of CVD, 
a surface cleaning treatment of said underlayer by Ar sputter-etching and 
a stabilization treatment of insulated film surface activated in said 
cleaning treatment successively or simultaneously, and optionally applying 
an anti-corrosive treatment to the underlayer, and then applying a 
selective CVD treatment without exposing the underlayer subjected to above 
treatments to the air, via filling which gives good selectivity and low 
interfacial resistance between underlayer metal and via-filling metal can 
be accomplished. Accordingly, the present invention can contribute to 
improving the reliability of multilayer wiring of LSIs and multilayer 
printed boards of computors where filling of connecting via holes by metal 
is necessary.