EP1331280A1 - Method of manufacturing a silver billet and a tubular sputtering target - Google Patents

Abstract

To produce silver blank, with a silver content of at least 99.99 wt.% and an oxygen content of at most 20 microgram/g, the silver is melted together with 100-500 microgram/g calcium or aluminum under inert or reduction conditions. The calcium or aluminum can be added to the molten silver. The molten silver is poured into a mold and allowed to cool under atmospheric conditions. The system can use centrifugal or stand casting. The inert atmosphere uses carbon dioxide or argon and, during smelting, the silver can be covered by a carbon layer.

Description

The invention relates to a method for producing a silver blank with a silver content of ≥ 99.99% by weight and an oxygen content of <20 µg / g as well as a tube target for cathode sputtering systems.

Stand casting has long been a method for producing such a silver blank known, wherein the silver is melted under an inert or reducing atmosphere to to prevent a high oxygen uptake of the silver during melting. The solubility for oxygen in the liquid state is approximately 40 times higher than for silver in the solid state. If a reducing or inert atmosphere is not provided, then During the melting process, oxygen from the air in high concentrations of 1000 up to 3000 µg / g absorbed extremely quickly by the silver melt. In the subsequent Solidification of the silver melt suddenly causes a large part of the oxygen absorbed released again, which leads to a violent reaction - similar to a boiling of the melt. The casting formed then usually shows a volcanic surface and high porosity. In addition, a high proportion of dissolved oxygen in the solidified silver cracks at the grain boundaries, which leads to fragility of the silver. As high Concentrations of dissolved oxygen are values of greater than 50 µg / g, an oxygen concentration in the range from 50 to 100 μg / g is usually present.

DE 199 53 470 discloses the production of silver blanks for tube targets via the Centrifugal casting. It has been shown that the setting of an inert or reducing Atmosphere over the molten silver during the casting process Centrifugal casting is particularly difficult to achieve. The long distances of the casting material from Crucible to the casting mold, the large surface of the silver in the mold and the turbulence in the atmosphere caused by the high rotational speed of the mold lead to a high oxygen uptake of the silver melt with those described above Disadvantages. The cracks formed at the grain boundaries of the tube target cause considerable Problems during a sputtering process because the tube target on one side (inside) with cooling water and on the other side (outside) with vacuum. least Leaks in the silver lead to an interruption of the sputtering process because Cooling water is drawn to the vacuum side.

The problem therefore arises of providing a method for producing a silver blank, in which the uptake of oxygen from the atmosphere is clearly hindered and thereby suppressing the formation of cracks at the grain boundaries in the silver. It posed The problem continues that the silver blank is for use as an atomizing material in a tube target should have the usual purity in the range ≥ 99.99% by weight.

a) Erschmelzen von Silber mit 100 bis 500 µg/g Kalzium oder Aluminium unter inerten oder reduzierenden Bedingungen oder Erschmelzen von Silber unter inerten oder reduzierenden Bedingungen und zugeben von 100 bis 500 µg/g Kalzium oder Aluminium zur Silberschmelze,

b) Gießen der Silberschmelze in eine Gießkokille unter Luftatmosphäre und

c) Abkühlen der Silberschmelze in der Gießkokille unter Luftatmosphäre.

The problem is solved for the procedure by

a) melting silver with 100 to 500 µg / g calcium or aluminum under inert or reducing conditions or melting silver under inert or reducing conditions and adding 100 to 500 µg / g calcium or aluminum to the silver melt,

b) pouring the silver melt into a casting mold under an air atmosphere and

c) cooling the silver melt in the casting mold under an air atmosphere.

The addition of 100 to 500 µg / g calcium or aluminum to the silver melt before the casting could reduce the oxygen uptake to such an extent that no brittleness and no or only slight porosity could be found in the solidified silver structure. This is all the more surprising since classic deoxidizers such as silicon, magnesium, yttrium or zircon used in steel production did not show any positive effect when added to the silver melt. During the casting process of the silver melt mixed with 100 to 500 µg / g calcium or aluminum, the content of calcium or aluminum surprisingly reduced to a value <100 µg / g. In most cases, values of <50 µg / g were achieved. With such a residual amount of calcium or aluminum, the solidified silver meets the commercial specification for tube targets with a silver content of ≥ 99.99% by weight.
The method according to the invention is particularly advantageous if the casting of the silver melt takes place by centrifugal casting. The addition of 100 to 500 µg / g calcium or aluminum to the silver melt before centrifugal casting makes it possible to carry out the casting process in air. Setting an inert or reducing atmosphere during casting is no longer necessary.

But also for all other casting processes, such as. B. the cast, has the invention Process the advantage that inert or reducing conditions during casting no longer apply required are.

Nevertheless, it is advantageous if the melting of the silver or the silver including the Calcium and aluminum are still made under an inert or reducing atmosphere to have sufficient calcium and aluminum in the casting process in the silver. Suitable here themselves as atmospheres, particularly argon or carbon monoxide. It is also advantageous here melting the silver under a cover with coal.

The problem is solved for the tube target by using a silver blank as the atomizing material is used, which was prepared by the inventive method. There a such a silver blank has an almost pore-free and dense grain structure, it is for Excellent use as atomization material in a tube target. It is special advantageous that the content of calcium or aluminum in the atomizing material or silver blank <100 µg / g, in particular <50 µg / g, because the commercial specification requires a silver content of ≥ 99.99% by weight for such targets.

Bild 1

einen Gefügeanschliff ( Vergrößerung 100:1 ) der Biegeprobe 1 aus Versuch 3, bei welchem deutlich Risse R an den Korngrenzen zu erkennen sind

Bild 2

einen Gefügeanschliff ( Vergrößerung 50:1) des Silberrohlings aus Versuch 4, 2. Abguss mit 200 µg/g Ca-Dotierung, bei welchem keine Risse vorhanden sind

The following experiments 1 to 4 and Figures 1 and 2 are intended to explain the invention by way of example. It shows

Image 1

a structure grinding (magnification 100: 1) of the bending sample 1 from experiment 3, in which cracks R can clearly be seen at the grain boundaries

picture 2

a micrograph (magnification 50: 1) of the silver blank from experiment 4, 2. casting with 200 µg / g Ca doping, in which there are no cracks

Trial 1:

200 g of silver were melted under a carbon monoxide atmosphere. After that, the silver melt Air-held for 30 seconds and poured into a graphite mold. At the Casting showed a violent reaction of the silver melt, caused by that already described above Escape of oxygen is caused when the silver solidifies and where a volcanic surface is formed on the side of the solidified silver, which is not limited by an inner wall of the graphite mold. An analysis of the oxygen content in the solidified silver showed a value of 58 µg / g.

Trial 2:

To investigate whether there are doping elements that prevent the oxygen uptake of liquid silver limit, were made of silver and one of the elements zirconium (Zr), yttrium (Y), calcium (Ca), silicon (Si), tin (Sn), aluminum (Al), magnesium (Mg), lead (Pb) and zinc (Zn) in an argon atmosphere at 300 mbar gas pressure melted nine master alloys that each had a content of 0.5% by weight of the doping element.

In each case 200 g of silver were made with the help of one of the nine master alloys with 200 µg / g of the doping elements and doped under a carbon monoxide atmosphere. After that the nine different silver melts were kept liquid in air for 30 seconds and poured into a graphite mold.

During casting, the silver melts with the doping elements Zr, Y, Si, Sn, Mg, Pb showed and Zn the same violent reactions as undoped silver (see experiment 1). The volcanic raised surface was unchanged, so that no influence of these elements on the oxygen absorption behavior of the liquid silver could be determined.

The two samples with the doping elements Ca and Al behaved differently. Here was found a significantly less violent reaction during casting, which leads to the formation of a volcanically raised surface hardly appeared.

In order to determine a possible influence of the amount of doping element added, each 200 g silver with the Ca and Al master alloys each with 400 µg / g and doped melted under a carbon monoxide atmosphere. Then the Ca and Al doped Silver melts were kept liquid in air for 30 seconds and each in a graphite mold decanted.

There was no reaction to the cast and there were no castings volcanic surfaces more present, so that clearly a positive - that is called the oxygen absorption limiting - influence of these elements on the oxygen absorption behavior of the liquid silver could be determined. On the surface of the However, castings could be seen in oxide agglomerates, which predominantly consist of oxidized Ca or Al passed. On the casting with Ca doping, a Ca content of measured only 8µg / g, which indicated that the Ca was in air during the hold period and had been almost completely oxidized during the casting process.

Trial 3:

It is a statement about the ductility of undoped, air-spilled silver and of the cabe relationship Al-doped, air-shed silver was obtained from a larger Quantity of test material bending samples produced.

A bending sample 1 was produced from undoped silver by melting 5 kg of silver in a carbon monoxide atmosphere, keeping the silver melt in air for 30 s and pouring the silver melt in air into a graphite mold.

The production of a bending sample 2 from Ca-doped silver with a Ca addition of 400 µg / g was carried out by melting 5 kg of silver and an Ag-Ca master alloy with a content of 6% by weight Ca under a carbon monoxide atmosphere, keeping the silver melt in air over 30 s and pouring the silver melt in air into a graphite mold.

The production of a bending sample 3 from Al-doped silver with an Al addition of 200 µg / g was carried out by melting 5 kg of silver and an Ag-Al master alloy with a content of 6 wt .-% Al in a carbon monoxide atmosphere, keeping the silver melt in air above 30 s and pouring the silver melt in air into a graphite mold.

The middle of the solidified castings or silver blanks became 5 mm thick Sawed out slices and subjected to a bending test.

Results:

Bending test 1: - brittle fracture when the pane is bent by 45 °

• the grain structure showed cracks and widened grain boundaries (see picture 1)

Bending test 2 :: - washer was ductile up to a bend of 90 °

• the grain structure showed a homogeneous structure with finely distributed, small pores
• the Ca content was 98 µg / g

Bending test 3: - Washer was ductile up to a bend of 90 °

• the Al content was 23 µg / g

Experiment 4:

In order to check the influence of the addition of a Ca doping for the centrifugal casting of silver, was pure silver in a first casting and silver with 200 µg / g Ca doping in a second casting in a horizontal centrifugal casting mold with a length of 3000 mm and one Inner diameter of 180 mm (for centrifugal casting see process according to DE 199 53 470) shed.

1. Casting:

Approximately 500 kg of pure silver was melted in an unglazed clay graphite crucible under a cover made of coarse, sulfur-free charcoal in an induction melting plant (750 kW, 300 - 1000 Hz) at 1240 ° C. A reducing atmosphere was also set above the clay crucible using a natural gas flame. The coal cover was skimmed off shortly before casting and the liquid silver was poured into the rotating centrifugal casting mold at a temperature of 1115 ° C.
Analysis of the tubular silver blank produced showed a content of 30-70 µg / g dissolved oxygen in the grain structure.
The porosity in the silver blank rose sharply from the outer tube diameter with values of 1.3 vol.% Porosity in the direction of the inner tube diameter to values of 6.5 vol.% Porosity up to the formation of pronounced cracks in the grain structure.
A leak test with 2 - 3 bar water pressure showed water penetration through the pipe wall, so that the presence of several continuous cracks could be determined.

2. Cast:

Approximately 500 kg of pure silver was melted in an unglazed clay graphite crucible under a cover made of coarse, sulfur-free charcoal in an induction melting plant (750 kW, 300 - 1000 Hz) at 1240 ° C. A reducing atmosphere was also set above the clay crucible using a natural gas flame. The coal cover was skimmed off shortly before casting and the 200 μg / g Ca doping was added in the form of an Ag-Ca master alloy with a content of 6% by weight. The liquid silver, doped with 200 µg / g Ca, was then poured into the rotating centrifugal casting mold at a temperature of 1115 ° C.
The analysis of the tubular silver blank produced showed a content of 5 - 8 µg / g dissolved oxygen and a content of 23 µg / g Ca in the grain structure.
When processing the silver blank, there were no porosity or cracks (see Figure 2). A leak test with 2 - 3 bar water pressure showed no water penetration, the pipe wall was tight.
The silver blank was used as an atomizing material in a tube target for cathode sputtering systems. The behavior of the tube target in the coating of flat glass was flawless.

With regard to the amount of Ca remaining in the silver blank, it is noted that this is strong depends on the amount of molten silver. The smaller the melted batch, the more the ratio of surface area to volume of the melt is less favorable. So with small ones Batches more Ca consumed by oxidation than when melting large batches of Case is. Accordingly, the doping content is in each case dependent on the amount to be melted Adjust silver.

With regard to the oxygen content of the silver, it should be noted that this is not only from the used doping and their addition amounts depends, but that also the rest Experiment parameters have an influence on it. For example, with a fast Cooling of the melt means that less oxygen escapes than with slow cooling. In order to is the oxygen content of a casting with a certain doping and doping amount reproduce only if the other test parameters such as the casting process used, the casting amount, the casting atmosphere, holding times, the casting time or cooling time are largely identical. It has been shown that silver only with an oxygen content ≤ 20 µg / g no longer tends to become brittle and the grain structure is dense and crack-free is.

The general structure of tube targets is known and can, for example, EP 586 809 A1, US 5,354,446, US 5,591,314, FR 2745010, EP 500 774 B1, WO 97/15697 or DE 199 58 666 be removed.

Claims (9)

Process for producing a silver blank with a silver content of ≥ 99.99% by weight and an oxygen content <20 µg / g with the following steps: a) melting silver with 100 to 500 µg / g calcium or aluminum under inert or reducing conditions, or
Melting silver under inert or reducing conditions and adding 100 to 500 µg / g calcium or aluminum to the silver melt, b) pouring the silver melt into a casting mold under an air atmosphere and c) cooling the silver melt in the casting mold under an air atmosphere. A method according to claim 1, characterized in that the casting is carried out by centrifugal casting. A method according to claim 1, characterized in that the casting takes place in the standing casting. Method according to one of claims 1 to 3, characterized in that the melting takes place under an argon atmosphere. Method according to one of claims 1 to 3, characterized in that the melting takes place under a carbon monoxide atmosphere. Method according to one of claims 1 to 3, characterized in that the silver is covered with a layer of carbon during melting. Tube target for cathode sputtering systems with a sputtering material that consists of the silver blank produced according to one of claims 1 to 6 is formed. Tube target according to claim 7, characterized in that a content of calcium or aluminum in the atomizing material is <100 µg / g. Tube target according to claim 8, characterized in that a content of calcium or aluminum in the atomizing material is <50 µg / g.

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