Method of locally providing metal on a surface of a substrate

A method of locally providing metal on a surface of a substrate, in which the substrate is provided at the surface with an electrocatalythic image. The surface is then brought into contact with an electroless metal-plating solution. The electrocatalythic image is capable of binding metal to it from the metal-plating solution. Due to the fact that the image is brought into contact with the metal-plating solutin, the image is gradually strengthened with metal from the solution. With images smaller than 10 .mu.m, this strengthening is prevented, however, due to the fact that oxygen is present in the solution. Because oxygen is reduced, a reduction of metal ions in the solution cannot occur, as a result of which no metal deposition takes place on the image. According to the invention, the reduction of oxygen in the solution is counteracted at least relatively so that also in the case of images smaller than 10 .mu.m metal ions are reduced without hindrance to the metal, which is then deposited on the image. This images smaller than 10 .mu.m can be strengthened too.

The invention relates to a method of locally providing metal on a surface 
of a substrate, in which the substrate is provided at the surface with an 
electrocatalytic image, which is brought into contact with an electroless 
metal-plating solution and is strengthened by an eleotroless method. An 
eleotrocatalytic image is to be understood in this connection to mean a 
part of the substrate surface, which is capable of separate metal out of 
the metal-plating solution when the surface is brought into contact with 
said solution. 
The image can be provided in many ways. For example, it is possible to 
locally provide an electrocatalytic compound on the surface, but it is 
also possible to convert the substrate at its surface locally in such a 
manner that it locally obtains an electrocatalytic effect. In the case of 
a substrate of silicon, this can e.g. be achieved by local silicidation 
with a suitable metal. It is further possible to provide the image in that 
a substrate, which is electrocatalytic throughout its whole surface, is 
locally covered with a masking layer. 
During the manufacture of micro-electronic circuits, masks are used which 
generally comprise a transparent substrate, on which an opaque patterned 
metal layer is provided. During the manufacture of such photolithographic 
masks, however, defects may occur in the metal layer. These defeots can be 
distinguished into transparent and non-transparent defects and provided 
for a high rejection percentage of manufactured masks. A transparent 
effeot is meant if at a given area a part of the metal layer fails. 
Non-transparent effects are meant if at a given area metal is present on 
the substrate, while it should not be present there. The method of the 
kind mentioned in the opening paragraph is particularly suitable for 
restoring transparent defects in photolithographic masks. 
The invention is further also suitable, for example, to provide metal 
wiring on a semiconductor device and to fill contact holes with metal for 
forming, for example, interconnections in a multilayer metallization. 
A method of the kind mentioned in the opening paragraph is known U.S. Pat. 
No. 4,426,442. In the method described therein, the surface is provided 
with the image by locally forming on the surface of the substrate an 
electrocatalythic compound. For this purpose, the substrate is provided at 
its surface with a photosensitive compound, for example titanium oxide or 
zinc oxide, which after exposure is capable of separating out of a 
solution of metal ions this metal in the form of nuclei. Subsequently, the 
surface is provided with the electrocatalythic image in that it is 
immersed in a solution of metal ions, such as, for example, copper and 
gold ions, and is exposed according to a given pattern with a focused 
laser beam. As a result, the photosensitive compound is activated 
according to a given pattern and the metal ions are deposited on the 
surface in the form of metal nuclei. The metal nuclei are then intensified 
by an electroless method, in which the surface is brought into contact 
with an electroless metal-plating solution. In a given case, the 
metal-plating solution can also be used to deposit therefrom the metal 
nuclei. For example, for the whole treatment a solution may be used which 
comprises potassium gold (I) cyanide, copper sulphate tetra-Na-salt of 
ethylene diamino-tetra-acetic acid, sodium hydroxide aud formaldehyde. 
lt has been found that the known method is not satisfactory for depositing 
metal images having a size smaller than 10 .mu.m on a substrate. However, 
during the manufacture of semiconductor circuits, nowadays masks are used 
in which transparent defects having a diameter of a few micrometres are 
already inadmissible. In general, in the semiconductor technology there is 
a need to provide submicron metal images. It stands to reason that with 
the gradually advancing miniaturization of semiconductor circuit elements 
this need merely increases. 
The invention has inter alia for its object to provide a method by which it 
is possible to provide on the substrate surface metal images which are 
smaller than 10 .mu.m. 
In order to achieve the object aimed at, according to the invention the 
method of the kind mentioned in the opening paragraph is characterized in 
that the surface is brought into contact with an electroless metal-plating 
solution in which a reduction of oxygen is counteracted at least 
relatively. 
An electroless metal-plating solution is generally a solution of a metal 
salt and a reduction agent. In order to oxidize the reduction agent, a 
given activation energy must be overcome. Ihe electrocatalythic surface 
makes it possible to achieve this. If this activation energy is very low, 
the term "physical developer" is also used instead of the term 
"electroless metal-plating solution". However, within the scope of the 
invention, this distinction is not made so that in the present Application 
the term "an electroless metal-plating solution" is also to be understood 
to mean a physical developer. 
If the reduction agent is oxidized, electrons are released. These eleotrons 
in turn reduce metal ions from the solution to the metal, which is then 
deposited. The oxidation of the reduction agent does not occur 
spontaneously, however. It only takes place at a surface electrocatalytic 
for the reduction agent. If the substrate is immersed in the metal-plating 
solution, the metal is therefore deposited on the substrate surface only 
at the area at which it is provided with the electrocatalythic image. 
Thus, this image is intensified with the metal from the metal-plating 
solution. 
If, however, oxygen is present in the metal-plating solution, a competitive 
reaction occurs. The oxygen present in the solution is in fact reduced 
much more readily by the electrons than the metal ions so that, if both 
are present, only or substantially only a reduction of oxygen occurs. 
With larger images, this competitive reduction of oxygen does not play such 
an important part because the catalythic surface is sufficiently large and 
thus the number of generated electrons is also sufficiently large to 
deplete the oxygen present in the proximity of this surface. A sufficient 
number of electrons is then still left for the reduction of the metal 
ions. 
However, with smaller images, the situation is different. In the first 
place, a correspondingly smaller number of electrons is generated, but 
moreover the aforementioned competition of oxygen is relatively much 
larger. 
If oxygen is reduced, the oxygen concentration in the proximity of the 
image consequently decreases. As a result, diffusion of oxygen from 
farther remote parts of the solution to the image will occur. The 
diffusion flow density of oxygen to the image is in inverse proportion to 
the size of the surface of the image. This means that with small metal 
images a comparatively large quantity of oxygen is present. Ihe already 
small number of generated electrons is therefore insufficient with small 
metal images to fully reduce the large quantity of oxygen. All the 
generated electrons are consumed by the oxygen present and a reduction of 
metal ions cannot or substantially cannot occur. It is therefore not 
possible to provide metal images by the known method if the size of the 
metal images lies below a given value. Experiments have shown that this 
value amounts to approximately 10 .mu.m. 
If however, in the metal-plating solution according to the invention the 
reduction of oxygen is counteracted, substantially all the electrons 
generated by the reduction agent can be used for the reduction of the 
metal ions. Irrespective of the size of the electrocatalythic image, there 
is a sufficient number of electrons available to reduce metal ions to the 
metal, which is then deposited at the area of the image. In practice, 
electrocatalythic images having a diameter of 1 .mu.m and smaller are thus 
successfully intensified. 
The reduction of oxygen can be counteracted by expelling the oxygen from 
the metal-plating solution. According to an embodiment of the invention, 
this is effected by passing an inert gas is passed through the solution. 
In the solution only a limited quantity of gas can be dissolved. When the 
inert gas is passed through the solution, the solution becomes saturated 
therewith so that no place is left any more for oxygen. 
According to another embodiment of the method, the reduction of oxygen is 
counteracted at least relatively in that the electroless intensification 
of the nuclei is carried out at elevated temperature. Ihe effect of this 
measure is bilateral: in the first place, with increasing temperature a 
smaller quantity of gas can be dissolved in the solution. By increasing 
the temperature, oxygen is thus expelled from the solution. However, in 
the second place, the oxidation of the reduction agent present in the 
solution is effeoted much more rapidly as the temperature increases. The 
speed of oxidation increases exponentially with an increase of 
temperature. As a result, a considerably larger number of electrons are 
generated at an elevated temperature than at a reduced temperature. lhis 
number is amply sufficient at elevated temperature to deplete the oxygen 
present so that also metal ions can be reduced to the metal, which is then 
deposited on the nuclei. The reduction of oxygen will also be effected 
more rapidly, it is true, but because the speed of reduction depends 
considerably less strongly upon temperature and because, as stated, less 
oxygen is dissolved in the solution at elevated temperature, the reduction 
is nevertheless counteracted relatively by increasing the temperature. 
A particular embodiment of the method is characterized in that the 
reduction of oxygen is counteracted in that the metal-plating solution is 
formulated so that the reduction of oxygen substantially does not occur. 
It has been found, for example, that the reduction of oxygen in a 
nickel-containing metal-plating solution strongly depends upon the degree 
of acidity of the solution. The reduction is found to occur satisfactorily 
only in an alkaline atmosphere. The reduction is therefore strongly 
counteracted by acidifying the metal-plating solution. Ihe oxidation of 
the reduction agent and the reduction of the metal ions are not adversely 
affected by the acid atmosphere, however. 
A further embodiment of the method according to the invention is 
characterized in that the metal-plating solution is provided on the 
surface in the form of a thin film. As stated, the deposition of small 
metal images is rendered difficult especially by the comparatively large 
diffusion current of oxygen to such small metal images. Normally speaking, 
the metal image is surrounded by a spherical diffusion field, within which 
diffusion to the image occurs. With a small metal image, this diffusion 
field is considerably larger than the image itself. When the solution in 
the form of a thin film is brought into contact with the surface, this 
diffusion field is flattened. This may be effected, for example, in that 
the solution is provided on the surface in the form of a drop and is then 
covered by a covering glass, foil or film, as a result of which the drop 
is depressed to a thin film. Consequently, the diffusion field is strongly 
reduced, as a result of which the diffusion of oxygen is strongly reduced. 
The diffusion of the reduction agent and the metal ions is of course also 
reduced by this measure, but because both are present in the solution in a 
considerably concentration, than oxygen: they are not hindered thereby.

The invention will now be described with reference to a few examples. In 
all these examples, the starting material is a quartz substrate covered at 
its surface by a photosensitive titanium oxide layer. 
EXAMPLE I 
The substrate is immersed in a palladium chloride solution of about 
5.6.10.sup.-3 M PdCl.sub.1, 1.2.10.sup.-1 M HCl and 0.4% by weight of 
Tensagex DP-24. The photosensitive titanium oxide layer is then exposed 
with a focused laser beam, as a result of which palladium is deposited in 
the form of nuclei on the surface only at the area of the laser spot. 
These palladium nuclei form the electrocatalythic image, which is 
strengthened in accordance with the invention by an electroless method. 
The size of the electrocatalythic image is thus determined by the diameter 
of the laser spot. For example, an exposure for 4 seconds with a beam 
focused to a spot of 10 .mu.m originating from an A.sup.+ UV laser having 
a power of 5 mW results in an electrocatalythic image of palladium nuclei 
having a diameter of 8 .mu.m. Thus, electrocatalythic images having a 
diameter varying from about 0.5 .mu.m to 40 .mu.m are provided on the 
substrate. 
After the substrate has been thoroughly rinsed with demineralized water, it 
is immersed in an electroless metal-plating solution, which contains per 
liter about 30 g of NiCl.sub.2.6H.sub.2 0, 10 g of NaH.sub.2 
PO.sub.2.H.sub.2 O and 30 g of glycin. The palladium nuclei are for this 
solution electrocatalythic. After about 15 minutes, the substrate is taken 
from the solution, is rinsed with demineralized water and is analysed 
under a microscope. It has been found that the electrocatalythic images 
having a diameter of 25 .mu.m were considerably intensified with nickel, 
but that the smaller images did not exhibit any nickel deposition at all. 
EXAMPLE II 
In this example, a substrate is treated in the same manner as in the 
preceding embodiment. However nitrogen gas is passed through the solution 
for a few hours in order to expel oxygen therefrom before the substrate is 
immersed in the metal-plating solution. In order to avoid that oxygen from 
the ambient air penetrates again into the solution, the whole treatment is 
carried out in a closed system. After analysis of the substrate, it has 
been found that nickel has been deposited not only on electrocatalythic 
images having a diameter of 25 .mu.m and larger, but also on images having 
a diameter between 1 .mu.m and 25 .mu.m. 
EXAMPLE III 
Two similar substrates are provided in the same manner as in the preceding 
examples with electrocatalythic images of palladium nuclei. Subsequently, 
both substrates are immersed separately in a metal-plating solution 
commercially available: Niposit 468, marketed by Shipley. A first 
substrate is immersed in a solution held by means of a thermostat at the 
indicated temperature of 65.degree. C., while the solution of the other 
substrate is held at a temperature of about 85.degree. C. After about 15 
minutes, the substrates are taken from the solution and are thoroughly 
rinsed with demineralized water. Under the microscope, it has been found 
that with the first substrate nickel has been deposited only on images 
having a diameter of 20 .mu.m and larger, while with the other substrate 
images having a diameter of 4 .mu.m and larger have been intensified with 
nickel. 
EXAMPLE IV 
After a substrate has been provided with electrocatalythic images in the 
manner described in the first example, it is immersed in a metal-plating 
solution of 0.025M N-methyl-p-aminophenol, 0.01M AgNO.sub.3 and 0.1M 
citric acid. This solution has a degree of acidity which is about pH=2.1 
and is composed so that a reduction of oxygen cannot or substantially 
cannot occur therein. After about 15 minutes, the substrate is removed 
from the solution and is thoroughly rinsed with demineralized water. After 
analysis, it has been found that all the images of palladium nuclei have 
been intensified with silver. 
It should be noted here that the invention is not limited to the examples 
described herein, but may be used in numerous other embodiments. For 
example, instead of titanium oxide, zinc oxide may also be used for the 
photosensitive layer. Besides, instead of palladium nuclei, nuclei of most 
of the other metals may also be deposited in an analogous manner on the 
photosensitive layer to form therewith an electrocatalythic image. 
Moreover, it is possible to provide in a quite different manner an 
electrocatalythio image on a substrate surface. A few examples thereof 
have been mentioned herein before. 
Further, there is a wide choice with respect to the electroless 
metal-plating solution. Instead of a metal-plating solution for 
electroless deposition of nickel or silver, for example, use may also be 
made of a solution for the deposition of copper or gold. In the former 
case, use may then be made of a solution containing per liter 10 g of 
CuSO.sub.4.5H.sub.2 O, 50 g of Rochelle salt, 10 g of NaOH and 25 ml of 
(27%) formaldehyde or of a solution containing per liter 10 g of 
CuSO.sub.4.5H.sub.2 O, 32.6 g of Na.sub.4 EDTA.4H.sub.2 O, 4.8 g of NaOH 
and 7.5 ml of (37%) formaldehyde. For the deposition of gold, a solution 
may be used containing per liter 1.44 g of KAu(CN).sub.2, 6.5 g of KCN, 8 
g of NaOH and 10.8 g of KBH.sub.4.