Tungsten Wire

The present invention relates to a tungsten wire.

Patent Literature (PTL) 1 discloses a tungsten wire having a tensile strength of at least 4800 MPa.

The workability of the above-described conventional tungsten wire in drawing (wire drawing) using a die deteriorates with an increase in tensile strength and an increase in the amount of additive. For this reason, the die is easily worn, which causes a deterioration in the roundness of the tungsten wire.

In view of the above, an object of the present invention is to provide a tungsten wire having an improved roundness.

In accordance with an aspect of the present disclosure, a tungsten wire includes tungsten as a major component, wherein 4758×D−7258.3×D+5275.5≤T≤4758×D−7258.3×D+6100 is satisfied where T (MPa) is a tensile strength of the tungsten wire, D (mm) is a diameter of the tungsten wire, and a roundness of the tungsten wire is at most 2.0%.

According to the present invention, it is possible to produce a tungsten wire having an improved roundness.

Hereinafter, a tungsten wire and a metal mesh according to embodiments of the present invention will be described in detail with referenced to the drawings. It should be noted that each of the embodiments described below shows a specific example of the present invention. As such, the numerical values, shapes, materials, structural components, the arrangement and connection of the structural components, steps, the processing order of the steps, and so on, shown in the following embodiments are mere examples, and therefore do not limit the present invention. Among the structural components in the embodiments described below, those not recited in the independent claims will be described as optional structural components.

In addition, each diagram is a schematic diagram and not necessarily strictly illustrated. Accordingly, for example, scale sizes, etc. are not necessarily exactly represented. In each of the diagrams, substantially the same structural components are assigned with the same reference signs, and redundant descriptions will be omitted or simplified.

In addition, a term representing a relationship between the components, a term representing a shape of a component, and a numerical range are used in the present description. Such terms and range are each not representing only a strict meaning of the term or range, but implying that a substantially same range, e.g., a range that includes even a difference as small as few percentages, is connoted in the term or range.

First, a tungsten wire according to an embodiment will be described with reference to.is a schematic diagram illustrating an external appearance and a cross section of tungsten wireaccording to the present embodiment.

As illustrated in, tungsten wireis wound around winding frameand stored. Winding framemay be referred to as a bobbin, reel, spool, drum, or the like in some instances. Tungsten wirehas a total length on the order of kilometers, for example, from 50 km or more to 300 km or less.

Tungsten wireillustrated inis utilized in the manufacture of tungsten products. For example, tungsten wireis used as the core of a saw wire. Specifically, the saw wire is a fixed-abrasive-grain wire. The saw wire includes tungsten wireas its core and further includes abrasive grains such as diamond particles or cubic boron nitride (CBN) particles. The abrasive grains are made to adhere to the surface of tungsten wire. The adhesion may be electrodeposition or adhesion with a resin bond. Alternatively, the saw wire may be a loose-abrasive wire. For example, the saw wire may be tungsten wireitself rather than tungsten wirewith abrasive grains.

For example, the saw wire is used to cut a semiconductor ingot such as a silicon (Si) or silicon carbide (SiC) ingot. By slicing the semiconductor ingot with the saw wire, semiconductor wafers can be manufactured. At this time, a smaller diameter of the saw wire allows a smaller cutting margin, reducing losses, and thus it is possible to increase the number of wafers that can be obtained. It should be noted that a cutting object is not limited to a semiconductor ingot and may be glass, concrete, rock crystal, ceramic, or the like.

Tungsten wirecontains tungsten (W) as a major component. The term “major component” means that the content (content percentage) of an element is more than 50 wt %. For example, the content of the tungsten in tungsten wireis at least 97 wt %. The content of the tungsten in tungsten wiremay be at least 99 wt %, at least 99.9 wt %, or at least 99.99 wt %. Tungsten wiremay contain, in addition to additives described later, inevitable impurities which are inevitably mixed therein through the processes of manufacturing.

Tungsten wirecontains, for example, rhenium (Re). The content of the rhenium in tungsten wireis, for example, at least 0.1 wt % and at most 3 wt %. Rhenium and tungsten form their alloy (solid solution).

When the content of the rhenium is high, it is possible to increase the tensile strength of tungsten wire. On the other hand, when the content of the rhenium is excessively high, it is difficult to render tungsten wirethinner while maintaining a high tensile strength thereof. More specifically, wire breakage is likely to occur, which makes it difficult to perform drawing in long lengths. The workability of tungsten wirecan be enhanced by reducing the content of the rhenium and setting the content of the tungsten to at least 97 wt %. In addition, since rhenium is rare and expensive, it is possible to mass-produce in long lengths tungsten wirewhich is inexpensive, by reducing the content of the rhenium.

It should be noted that the metal used for the tungsten alloy may be osmium (Os), ruthenium (Ru), or iridium (Ir). The content of osmium, ruthenium, or iridium is equivalent to the content of the rhenium, for example. In these cases, the same advantageous effects can be yielded as in the case of a rhenium tungsten alloy. Tungsten wiremay be an alloy wire containing an alloy of tungsten and metal of at least two types other than tungsten.

Tungsten wiremay contain potassium (K). The content of the potassium in tungsten wireis, for example, at least 0.001 wt % and at most 0.01 wt %. Potassium in tungsten wireis present in the grain boundaries of tungsten. Tungsten wirecontaining potassium can also yield a tensile strength higher than a general tensile strength of piano wire.

Tungsten wiremay contain a rare earth element. The content of the rare earth element in tungsten wireis, for example, at least 0.03 wt % and at most 0.3 wt %. The content of the rare earth element in tungsten wiremay be, for example, at least 0.03 wt % and at most 0.09 wt %. The rare earth element is, for example, cerium (Ce), lanthanum (La), yttrium (Y), samarium (Sm), or the like. The rare earth element in tungsten wireis present in the grain boundaries of tungsten. Tungsten wirecontaining the rare earth element can also yield a tensile strength higher than a general tensile strength of piano wire.

As an example, the diameter of tungsten wireis at most 100 μm. Using tungsten wirehaving a smaller diameter as the core of a saw wire enables a reduction in the losses of materials being cut. The diameter of tungsten wiremay be at most 80 μm, at most 70 μm, at most 60 μm, at most 50 μm, at most 40 μm, at most 30 μm, at most 20 μm, or at most 15 μm. In addition, by a manufacturing method described later, tungsten wirehaving an ultrafine diameter of at most 13 μm is also produced. The diameter of tungsten wiremay be at most 10 μm, at most 8 μm, or at most 7 μm. The diameter of tungsten wireis, for example, at least 5 μm, but is not limited to this example.

As an example, the tensile strength of tungsten wireis at least 4800 MPa. The tensile strength may be at least 4900 MPa, at least 5000 MPa, at least 5200 MPa, or at least 5500 MPa. Furthermore, tungsten wirehaving an ultra-high tensile strength of at least 5800 MPa is also produced. The tensile strength of tungsten wiremay be at least 5900 MPa or at least 6000 MPa. The tensile strength can be measured, for example, based on the tensile test of the Japanese Industrial Standards (JIS H 4460 8).

When the tensile strength is denoted as T (unit: MPa), and the diameter is denoted as D (unit: mm), tungsten wireaccording to the present embodiment satisfies a predetermined relationship. Specifically, tungsten wireof which the tensile strength commensurate with its diameter is higher than conventional one is produced. A specific relationship between tensile strength T and diameter D will be described later with a specific working example, with reference to.

In the present embodiment, the roundness of tungsten wireis at most 2.0%. That is, a cross section of tungsten wire(the cross section perpendicular to the axial direction of tungsten wire) has a shape sufficiently close to a perfect circle. It should be noted that the difference between the cross section of tungsten wireand a perfect circle is exaggerated in the illustration of.

Here, the roundness will be described with reference to.is a diagram for describing the roundness. The roundness is an index that indicates the magnitude of the deviation of the cross-sectional shape of tungsten wirefrom a perfect circle. A lower roundness means a smaller deviation, that is, that the cross-sectional shape of tungsten wireis close to the perfect circle. A higher roundness means a larger deviation, that is, that the cross-sectional shape of tungsten wiredeviates from the perfect circle.

Specifically, roundness f (unit: %) is given by Formula (1) shown below.

A and B are as shown in. Specifically, a maximum inscribed circle and a minimum circumscribed circle are first defined for the cross section of tungsten wire. The maximum inscribed circle and the minimum circumscribed circle are defined in such a manner as to make them concentric and minimize a distance between them. That is, when the radius of the minimum circumscribed circle is denoted as r(A), and the radius of the maximum inscribed circle is denoted as r(B), r(A)−r(B) is minimized.

In this case, A denotes the diameter of the minimum circumscribed circle and is given by 2×r(A). B denotes the diameter of the maximum inscribed circle and is given by 2×r(B). As shown in Formula (1) above, roundness f represents, in terms of percentage, a proportion of the difference between the diameter of the minimum circumscribed circle and the diameter of the maximum inscribed circle to the average value of the diameter of the minimum circumscribed circle and the diameter of the maximum inscribed circle. When f=0, the minimum circumscribed circle and the maximum inscribed circle coincide with each other, and thus the cross section of tungsten wireis the perfect circle.

According to the present embodiment, it is possible to produce tungsten wireof which the tensile strength commensurate with its diameter is higher than conventional one and that has an improved roundness (i.e., its cross section is close to a perfect circle). For example, when tungsten wireis used as the core of a saw wire, its improved roundness enables a reduction of variations in the thickness of a sliced object (a wafer). In particular, when the roundness exceeds 2% or 3%, the resulting total thickness variation (TTV), which is a variation in the thickness of a wafer, steeply deteriorates.

Next, a manufacturing method of tungsten wireaccording to the present embodiment will be described with reference toand.andare each a flowchart illustrating an example of the manufacturing method of tungsten wireaccording to the present embodiment.

As illustrated in, first, an additive is added to tungsten (S). For example, only the additive (an element to be doped) or a compound containing the additive (e.g., an oxide or aqueous solution of the additive) is added to tungsten powders in a predetermined amount. The element to be doped is rhenium, potassium or a rare earth element such as cerium or lanthanum. Unnecessary components contained in the compound are removed thereafter by sintering or the like. To the obtained doped tungsten powders, tungsten powders are added in a predetermined proportion to adjust the added amount of the element to be doped. That is, the element to be doped may be added in a content larger than an intended content of element to be doped.

In this manner, after the element to be doped is added to tungsten powders in a small amount, tungsten powders are added to thin the element to be doped (reduce the content of the element to be doped). That is, an amount of the element to be doped that is treated in the addition process of the element to be doped is small, which allows small adding equipment to be used. In addition, a large amount of doped tungsten powders can be obtained by performing the addition process, and thus the number of steps in the addition process can be reduced. Accordingly, it is possible to enhance the productivity of tungsten wire.

Next, a tungsten ingot is prepared by pressing and sintering on the aggregate of the obtained doped tungsten powders (S).

Next, swaging processing is performed on the prepared tungsten ingot (S). Specifically, the tungsten ingot is forged and compressed from around to be extended, thus being formed into a wire-shaped tungsten wire. Rolling processing may be performed instead of the swaging processing.

For example, by repeatedly performing the swaging processing, the tungsten ingot having a diameter of at least about 15 mm and at most about 25 mm is formed into a tungsten wire having a diameter of at least about 3 mm and at most about 4 mm. Annealing treatment is performed in a midcourse process of the swaging processing to keep workability in subsequent treatments. For example, the annealing treatment is performed at a temperature of at least 2000 degrees Celsius and at most 2400 degrees Celsius within the range where the diameter of the tungsten ingot is at least 8 mm and at most 10 mm. However, when the diameter is less than 8 mm, the annealing treatment is not performed in the swaging process so as to keep the tensile strength through crystal grain refining.

Next, the tungsten wire is heated at 900 degrees Celsius before being subjected to heat drawing (S). Specifically, the tungsten wire is heated directly with a burner or the like. Heating the tungsten wire forms an oxide layer on the surface of the tungsten wire so that the tungsten wire does not break in the heat drawing that is subsequently performed.

Next, the heat drawing is performed (S). Specifically, drawing of the tungsten wire, that is, wire drawing (thinning) of the tungsten wire is performed using one or more wire drawing dies while the tungsten wire is heated. The heating temperature is, for example, 1000 degrees Celsius. It should be noted that the higher the heating temperature is, the more the workability of the tungsten wire is enhanced, and the drawing can be performed easily. The heat drawing is repeatedly performed while replacing one of the wire drawing dies with another. A reduction rate in a cross-section area of the tungsten wire made by performing the drawing once with one wire drawing die is, for example, at least 10% and at most 40%. In the heat drawing process, a lubricant made of graphite dispersed in water may be used.

The heat drawing (S) is repeated until the tungsten wire having a desired diameter is obtained (No in S). Here, the desired diameter is a diameter when the number of times to perform the drawing is two. For example, the desired diameter is approximately 150 μm.

It should be noted that, in the repetition of the heat drawing, a wire drawing die used in a certain heat drawing has a smaller bore diameter than a wire drawing die used in an immediately previous heat drawing. Furthermore, in the repetition of the heat drawing, the tungsten wire is heated in a certain heat drawing at a heating temperature lower than a heating temperature in an immediately previous heat drawing. That is, the heating temperature is decreased stepwise. The heating temperature in the final heat drawing is, for example, 400 degrees Celsius so as to make the final heat drawing contribute to crystal grain refining.

When the tungsten wire having the desired diameter is obtained, and the number of times to perform the drawing is two (Yes in S), a drawing at room temperature is performed (S). It should be noted that, as illustrated in, electrolytic polishing (S) may be performed before the drawing at room temperature (S). In the drawing at room temperature, the tungsten wire is drawn without being heated, so that further crystal grain refining is implemented. In addition, the drawing at room temperature yields the advantageous effects of aligning crystal orientations in a processing axis direction (specifically, a direction parallel to the axis of tungsten wire.

The room temperature is a temperature within the range of, for example, at least 0 degrees Celsius and at most 50 degrees Celsius. As an example, the room temperature is 30 degrees Celsius. Specifically, the drawing of tungsten wire is performed using a plurality of wire drawing dies having different bore diameters. In the drawing at room temperature, a liquid lubricant such as a water-soluble liquid lubricant. Since the drawing at room temperature does not involve heating, evaporation of the liquid is restrained. Therefore, it is possible to make the liquid lubricant to exert its function sufficiently. In contrast to heat drawing performed at 600 degrees Celsius or higher, which is a conventional, traditional method of processing a tungsten wire, processing a tungsten wire while the tungsten wire is not heated but cooled with the liquid lubricant restrains dynamic recovery and dynamic recrystallization, contributes to crystal grain refining, and can yield a high tensile strength without breakage.

The processing proportion of the drawing at room temperature is, for example, at least 70%. The processing proportion is given by Formula (2) shown below, using Db denoting the diameter immediately before the drawing at room temperature and Da denoting the diameter immediately after the drawing at room temperature.

Formula (2) shows that the more the diameter is reduced by the drawing at room temperature, the higher a value of the processing proportion becomes. For example, when diameter Db immediately before the drawing at room temperature is assumed to be constant, diameter Da immediately after the drawing at room temperature decreases with an increase in the processing proportion. As the processing proportion is increased, the degree of thinning of the tungsten wire by the drawing at room temperature increases. That is, a thinner tungsten wire is obtained. The processing proportion of the drawing at room temperature is at least 70% but may be at least 80%, at least 90%, or at least 95%. The diameter immediately after the drawing at room temperature is roughly within the range of, for example, at least 50 μm and at most 120 μm.

Next, low-temperature hot drawing is performed (S) after the drawing at room temperature. That is, the tungsten wire is subjected to the final drawing while being heated at a low temperature. The temperature at this time is higher than the temperature (the room temperature) of the drawing at room temperature (S) and lower than the temperature of the heat drawing (S). Specifically, the temperature of the low-temperature hot drawing is within the range of at least 100 degrees Celsius and at most 300 degrees Celsius. As an example, the temperature is 200 degrees Celsius or 300 degrees Celsius. The diameter after the low-temperature hot drawing is roughly within the range of, for example, at least 20 μm and at most 100 μm.

Finally, the tungsten wire formed by performing the low-temperature hot drawing is subjected to electrolytic polishing (S) to adjust the diameter. The electrolytic polishing is carried out, for example, as a result of the generation of a potential difference between the tungsten wire and a counter electrode in a state in which the tungsten wire and the counter electrode are bathed into electrolyte such as aqueous sodium hydroxide.

Through the above-described processes, tungsten wireaccording to the present embodiment is manufactured. Immediately after being manufactured through the above-described processes, tungsten wirehas a length of, for example, at least 50 km, which enables industrial use of tungsten wire. Tungsten wireis cut to an appropriate length in accordance with its usage mode and can be used in a needle shape or a rod shape.

It should be noted that the processes shown in the manufacturing method of tungsten wireare performed in-line, for example. Specifically, the plurality of wire drawing dies used in step Sare located in a production line in descending order of bore diameter. In addition, heating equipment such as a burner is located between every adjacent wire drawing dies. In addition, electrolytic polishing equipment may be located between every adjacent wire drawing dies. On a downstream side (post-processing side) of the wire drawing dies used in step S, one or more wire drawing dies used in step Sand one or more wire drawing dies used in step Sare located in descending order of bore diameter, and on a downstream side of a wire drawing die having a smallest bore diameter, electrolytic polishing equipment is located. It should be noted that the processes may be performed individually.

It should be noted that the above-described manufacturing method of tungsten wireis a mere example. The temperature, the diameter, and the like in each process can be adjusted as appropriate.