Method of locally providing metal on a surface of a substrate

Abstract: 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 solution, the image is gradually strengthened with metal from the solution. With images smaller than 10 µm, 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 taken 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 µm metal ions are reduced without hindrance to the metal, which is then deposited on the image. This images smaller than 10 µm can be strengthened too.

Description: The invention relates to a method of restoring transparent defects in an opaque patterned metal layer provided on a surface of a transparent substrate of a photolithographic mask, in which method the transparent substrate is provided with electrocatalytic images with a dimension less than 10µm, after which said images are brought into contact with an electroless metal plating solution whereby the images are strengthened with metal deposited from said metal plating solution. During the manufacture of photolithographic masks defects may occur in the patterned metal layer provided on the transparent substrate. These defects, which can be distinguished into transparent and non-transparent defects, render the masks unfit for use in the manufacture of micro-electronics circuits. A transparent defect is meant if at a given area a part of the metal layer fails, a non-transparent defect if at a given area metal is present while it should not be present there. The electrocatalytic image can be provided in may 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 a electrocatalytic effect. It is further possible to provide the electrocatalytic image in that a substrate, which is electrocatalytic throughout its whole surface, is locally covered with a masking layer. In US Patent 4 426 442 a method is described, in which the surface is provided with the electrocatalytic image by locally forming on the surface of the substrate a electrocatalytic 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 electrocatalytic 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 strengthened 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 and formaldehyde. It has been found that the known method is not satisfactory for depositing metal images having a size smaller than 10 µm 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. It is the object of the invention to provide a method by which it is possible to provide on the substrate surface metal images which are smaller than 10 µm. Therefore the method according to the invention as claimed in Claim 1 is characterized in that, the electrocatalytic images are brought into contact with the metal plating solution by providing a drop of the metal plating solution on the surface of the substrate which drop subsequently is depressed to a thin film, whereby the diffusion of oxygen present in the metal plating solution is strongly reduced such that all of said images are strengthened with metal. 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 considerable concentration, than oxygen, they are not hindered thereby. It is noted that UP Patent 3 900 599 and JP-A-53-146 933 both disclose a method of electrolessly depositing a metal on a whole surface of a substrate which is catalytic for the deposition by immersing the substrate in a plating bath, in which the bath was stabilized by bubbling an inert gas such as nitrogen through the bath. For similar reasons in the method disclosed in JP-A-61-12 877 the plating bath was degassed. After the whole surface is plated, in the so formed metal layer images in the form of conductors may be formed. 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. The electrocatalytic 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 electrons in turn reduce metal ions from the solution to the metal, which is 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, only metal is therefore deposited on the substrate surface area at which it is provided with the electrocatalytic image. Thus, this image is strengthened 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 catalytic 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 surface of the image. This means that with small metal images a comparatively large quantity of oxygen is present. The 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 µm. 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 metal ions. Irrespective of the size of the electrocatalytic 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, electrocatalytic images having a diameter of 1 µm and smaller are thus successfully strengthened. Said reduction of oxygen is further counteracted in that the electroless strengthening of the nuclei is carried out at elevated temperature. The 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 effected 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. This 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. The reduction of oxygen also is counteracted if the metal plating solution is acidic. 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. The oxidation of the reduction agent and the reduction of the metal ions are not adversely affected by the acid atmosphere, however. 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. 10⁻³ M PdCl₁, 1.2. 10⁻¹ M HCl and 0.4 % by weight of Tensagex DP-24 (registered trademark). 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 electrocatalytic image, which is strengthened in accordance with the invention by an electroless method. The size of the electrocatalytic 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 µm originating from an Ar⁺UV laser having a power of 5 mW results in an electrocatalytic image of palladium nuclei having a diameter of 8 µm. Thus, electrocatalytic images having a diameter varying from about 0.5 µm to 40 µm 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 litre about 30 g of NiCl₂.6H₂O, 10 g of NaH₂PO₂.H₂O and 30 g of glycine. The palladium nuclei are for this solution electrocatalytic. After about 15 minutes, the substrate is taken from the solution, is rinsed with demineralized water and is electrocatalytic images having a diameter of 25 µm were considerably strengthened with nickel, but that the smaller images did not exhibit any nickel deposition at all. Example II . In this example, being not in accordance with the invention, 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 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 deposited not only on electrocatalytic images having a diameter of 25 µm and larger, but also on images having a diameter between 1 µm and 25 µm. Example III Two similar substrates are provided in the same manner as in the preceding examples with electrocatalytic images of palladium nuclei. Subsequently, both substrates are immersed separately in a metal-plating solution commercially available: Niposit 468 (registered trademark), marketed by Shipley. A first substrate is immersed in a solution held by means of a thermostat at the indicated temperature of 65°C, while the solution of the other substrate is held at a temperature of about 85°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 µm and larger, while with the other substrate images having a diameter of 4 µm and larger have been strengthened with nickel. Example IV After a substrate has been provided with electrocatalytic images in the manner described in the first example, it is immersed in a metal-plating solution of 0.025 M N-methyl-p-aminophenol, 0.01 M AgNO₃ and 0.1 M 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 strengthened 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 electrocatalytic image. Moreover, it is possible to provide in a quite different manner an electrocatalytic image on a substrate surface. A few examples thereof have been mentioned hereinbefore. Further, there is a wide choice with respect to the electroless metal-plating solution, Instead of 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 litre 10 g of CuSO₄.5H₂O, 50 g of Rochelle salt, 10 g of NaOH and 25 ml of (37 %) formaldehyde or of a solution containing per litre 10 g off CuSO₄.5H₂O, 32.6 g of Na₄EDTA.4H₂O, 4.8 g of NaOH and 7.5 ml of (37 %) formaldehyde. For the deposition of gold, a solution may be used containing per litre 1.44 g of KAu(CN)₂, 6.5 g of KCN, 8 g of NaOH and 10.8 g of KBH₄.

Claims: A method of restoring transparent defects in an opaque patterned metal layer provided on a surface of a transparent substrate of a photolithographic mask, in which method the transparent substrate is provided with electrocatalytic images with a dimension less than 10µm, after which said images are brought into contact with an electroless metal plating solution whereby the images are strengthened with metal deposited from said metal plating solution, characterized in that the electrocatalytic images are brought into contact with the metal plating solution by providing a drop of the metal plating solution on the surface of the substrate which drop subsequently is depressed to a thin film, whereby the diffusion of oxygen present m the metal plating solution is strongly reduced such that all of said images are strengthened with metal. A method as claimed in Claim 1, characterized in that the electroless strengthening is carried out at an elevated temperature. A method as claimed in Claim 1, characterized in that the metal plating solution is acidic.

Inventors: JACOBS JOHANNES WILHELMUS MARI, RIKKEN JOHANNES MATHIAS GERARD, JACOBS, JOHANNES WILHELMUS MARIA, RIKKEN, JOHANNES MATHIAS GERARDUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1988
Main classification: C23C18
IPC codes: C23C18, H05K3