JPH05222525A - Production of tungsten target for semiconductor - Google Patents

Abstract

PURPOSE:To provide the process for production of the tungsten target for semiconductors which contains impurities at an extremely low ratio and has high density. CONSTITUTION:This process for production of the tungsten target for semiconductors consists in pressurizing tungsten powder to form a molding having >=60% relative density, then heating the molding to >=1400 deg.C, more preferably >=1600 deg.C in an atmosphere contg. hydrogen to form a sintered body having >=90% relative density and further, hot working the sintered body at >=1400 deg.C, more preferably >=1600 deg.C working initiation temp. to obtain >=99% relative density.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-purity tungsten target used for forming electrodes and wiring of semiconductor devices.

[0002]

2. Description of the Related Art Due to the recent demand for higher integration of VLSI, the number of gate electrodes formed on the LSI is increasing and the wiring is miniaturized. The increase in the gate electrode and the miniaturization of the wiring cause the wiring resistance to increase. Such an increase in wiring resistance causes an electrical signal delay. For this reason, studies have been actively conducted to use a material having a lower resistance value as an electrode material or a wiring material. Among them, pure tungsten is highly promising as a material for electrodes and wirings because it has low resistance and its characteristics do not fluctuate even after various heat treatments are carried out in the manufacture of semiconductor devices.

At present, thin films of tungsten electrodes or wirings for LSI are manufactured by using a sputtering method and a CVD (chemical vapor deposition) method. Among them, the sputtering method is a method in which a tungsten target is used and is sputtered with Ar gas to form a tungsten film. As compared with the CVD method, sputtering can be performed with a simpler device and the film formation rate is faster. Moreover, there is an advantage that it is easy to handle. The size of the tungsten target used for semiconductor applications such as LSI tends to increase in proportion to the increase in the size of the processing substrate (Si wafer). Conventionally, the target size of φ200 to 254 mm is standard. Recently, a diameter of φ300 mm or more is required, and in the future φ350
~ 400 mm is believed to be the standard size for pure tungsten targets.

As described above, the tungsten target for semiconductor must have a relatively large size, a high purity and a high density. If you try to manufacture a target with a large size by powder metallurgy, it is difficult to obtain a high density, and if you process it as a sputtering target in a low density state, it will be sintered during the machining process such as grinding to obtain the target shape and the cleaning process. There is a problem that contaminants such as Na ions penetrate into the body and reduce the purity of the target. Further, when a sintered body having a low density is used as a sputtering target, sputter discharge is very unstable and abnormal discharge is likely to occur.
When the abnormal discharge is generated, the target surface is locally melted at the abnormal discharge generation portion, and a splash is generated from the melted portion. For this reason, on the surface of the pure tungsten film formed by using the low-density target, a large number of granular deposits, which are usually called particles, are generated due to such splash.

Most of the particles have a particle size of about 1 to 10 μm. In particular, in the case of a VLSI, the wiring width is usually as narrow as 1 μm or less (submicron), so if particles adhere, shadowing may cause a portion narrower than a predetermined wiring width. Since the resistance value increases in this portion, it causes malfunction of the semiconductor element. Further, the thermal stress generated during the operation of the semiconductor element concentrates on this portion, so that the wiring breakage easily occurs. Therefore, in the case of targets for semiconductor applications,
It is desirable to use a target with less generation of particles on the sputtered film surface, that is, a target having a high density. As a method of manufacturing such a tungsten target, a method of stacking by a CVD method has been put into practical use, but in addition, an electron beam melting method as described in JP-A-61-107728 is used. Melt by making ingot and rolling this ingot hot
A hot rolling method has been proposed. On the other hand, the inventors of the present invention pressure-sintered by hot pressing or hot isostatic pressing (hereinafter referred to as HIP) and hot rolling as described in JP-A-3-150356. A binding-hot rolling method was developed.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
As described in No. 07728, the crystal grain of the pure tungsten ingot by the electron beam melting is likely to be coarsened, and the coarseness of the crystal grain is particularly remarkable when a large ingot is required as in the case of target application for semiconductors. In the case of pure tungsten, if the crystal grains become coarse, it becomes mechanically brittle, and the hot workability becomes extremely poor. Therefore, the electronic
When a pure tungsten ingot melted in the aluminum is used as a material for a hot-rolled material, it easily cracks during processing. In particular, this tendency becomes remarkable as the diameter of the tungsten ingot increases.

Further, in the case of the pressure sintering method using hot pressing, it is general to fill a graphite mold with pure tungsten powder and perform pressure sintering in a vacuum or in an inert gas atmosphere. The pressurizing pressure at this time is 2 because of the strength problem of the graphite type.
It is limited to 00 to 300 kgf / cm ² . for that reason,
In order to obtain a high-density tungsten target, it is necessary to raise the temperature of hot pressing, but pure tungsten has a tendency to form carbides when the temperature exceeds 1200 ° C. Therefore, it is necessary to use a graphite mold in hot pressing. The carbon content of the obtained sintered body is on the order of several hundred ppm, which is not preferable for semiconductor applications in which it is necessary to reduce impurities as much as possible. On the other hand, in pressure sintering using HIP, it is general that after filling a metal capsule with pure tungsten powder, the inside of the capsule is degassed in vacuum, placed in a HIP device, and then pressure sintering is performed. .. The pressure sintering condition by HIP is as follows: pressure 2000 kg / cm ² , temperature 2000.
It is possible up to ℃, but when using metal capsules, the material is restricted. That is, at a high temperature, there is a problem that the tungsten and the metal capsule react to contaminate the sintered body. Further, by simply sintering the above powder, alkali metals such as Na and K and alkaline earth metals such as Ca, which are unavoidably contained in the powder in an amount of about 3 to 4 ppm, remain in the target as they are. This is not desirable. An object of the present invention is to provide a method for manufacturing a tungsten target for semiconductors, which has extremely low impurity content and high density.

[0008]

As a result of various studies on the production of a high-purity and high-density tungsten target, the present inventor has carried out compaction molding by pressurizing tungsten powder to obtain a compact body adjusted to a specific density or more. It has been found that by performing hot working after sintering in an atmosphere containing hydrogen, a tungsten target having an extremely high purity and a high density can be obtained. That is, in the manufacturing method of the present invention, after pressing a tungsten powder to form a molded body having a relative density of 60% or more, the molded body is heated to 1400 ° C. or higher, preferably 1600 ° C. or higher in an atmosphere containing hydrogen. It is heated to a sintered body having a relative density of 90% or more, and further the processing start temperature of the sintered body is 1400 ° C. or more, preferably 16
It is a method for manufacturing a tungsten target for a semiconductor, which is characterized by obtaining a relative density of 99% or more by hot working at 00 ° C or more.

In the step of pressurizing the tungsten powder, the temperature and atmosphere are not particularly limited, but in order to reduce contamination and to efficiently perform the next step of heating and sintering in an atmosphere containing hydrogen, it is necessary to cool the step. It is good to do it in warm weather. Further, in the step of pressurizing the tungsten powder to obtain a green body before sintering, it seems desirable to keep the relative density at 60 to 90%. That is, when the density is set to be slightly lower than the true density, hydrogen permeability in the next step is improved, and a more efficient reaction with hydrogen can be expected. As the atmosphere containing hydrogen, various combinations such as a pure hydrogen atmosphere, a mixture of hydrogen and a neutral gas, a mixture of hydrogen and a reducing gas, and the like can be considered. Is.

[0010]

In the present invention, the molded body having a relative density of 60% or more is heated at a temperature of 14% in an atmosphere containing hydrogen before hot working.
A major feature is that it is heated to 00 ° C. or higher to remove oxygen and other impurities and activate the sintered surface to accelerate sintering. Another major feature of the present invention is to heat the pressure-molded body in an atmosphere containing hydrogen, and to add hot working to a density which was not sufficient only by heating in an atmosphere containing hydrogen. This makes it possible to obtain a relative density of 99% or more. By heating in an atmosphere containing hydrogen, firstly oxygen is mainly removed from the tungsten grains due to the reducing action of hydrogen in the atmosphere, the surface is activated and sintering proceeds, and hot rolling is possible. It is possible to obtain a sintered body having a relative density of 90% or more, and secondly, oxygen which is inevitably contained in the raw material powder unfavorable for semiconductor use, and alkali metals and alkaline earth metals such as Na, K and Ca. Can be reduced. By setting the heating temperature at this time to 1400 ° C. or higher, preferably 1600 ° C. or higher, the density of the obtained sintered body rises rapidly and the relative density at which hot working is possible is 90%.
With reaching the above, the oxygen content is 30 ppm or less,
The total content of Na, K, and Ca can be reduced to 1 ppm or less.

In the present invention, the first step of pressure molding is
This is to create a molded body that can be handled during the subsequent step of sintering in an atmosphere containing hydrogen. As such a method of pressure molding, a mechanical press or a rubber press method using a cold isostatic press can be used. Especially when the rubber pressing method using a cold isostatic press is used for pressure molding, there is little cracking due to isotropic compression, and a uniform structure is obtained, and since it is molded at room temperature, hot pressing or Unlike HIP, preferable molding can be performed without contamination from molds or capsules. First, the relative density of the molded body after pressure molding was defined as 60% or more.
%, Low-density compacts are more likely to crack,
Secondly, it is difficult to handle in the next step. Secondly, if the compact density is too low, it is difficult to obtain a hot workable relative density of 90% or more even if heating is performed in an atmosphere containing hydrogen in the next step. is there. Further, since pure tungsten tends to become brittle rapidly at a temperature of 1200 ° C or lower, the processing start temperature must be 1400 ° C or higher, preferably 1600 ° C or higher in order to maintain good workability during hot working. There is.

[0012]

【Example】

(Example 1) Total content of Na, K, Ca 3.2 pp
m, carbon content 11 ppm, oxygen content 260 ppm,
Tungsten powder with an average particle size of 3.2 μm was
It was filled in 0 mm rubber and pressure-molded by a cold isostatic pressing device. Table 1 shows the relationship between the molding pressure of the cold isostatic press and the density (relative density) of the obtained molded body.

[0013]

[Table 1]

As shown in Table 1, the molding pressure is 1000 k.
Sample No. of gf / cm ² In 5, the compact density is 50%
In some cases, the molded body was cracked, resulting in defective molding.
Molded bodies having different densities that did not crack were heat-treated in a 100% hydrogen reducing atmosphere at 1800 ° C. for 30 hours to obtain sintered bodies. The obtained sintered body density (relative density) is shown in Table 1. Even under heating conditions of 1800 ° C. and 30 hours, sample No. which is a comparative example having a compact density of 60% or less.
6, the obtained sintered body density is as low as 85%,
It was confirmed that the compact density was required to be 60% or more.
Also, the heating atmosphere was changed to 75% hydrogen and 25% nitrogen and the same heat treatment was performed, but no difference from the case of 100% hydrogen atmosphere was observed.

Next, with the molding pressure of 3000 kgf / cm ² , the sample No. Using a compact having a density of 65% produced in the same manner as in No. 3, the heat treatment temperature, the density of the obtained sintered body and N
The relationship between the contents of a, K and Ca was investigated. The heating time was 30 hours and the atmosphere was 100% hydrogen. FIG. 1 shows the density of the obtained sintered body and the total content of Na, K, and Ca. As shown in FIG.
By setting the temperature to 0 ° C. or higher, 90% or more of the sintered body was obtained, and the contents of Na, K and Ca could be remarkably reduced. However, as shown in FIG. 1, even if the heating temperature was raised at 1400 ° C. or higher, the density did not significantly increase. Therefore, hot working was examined in order to further increase the density of the sintered body. The sample No. of Table 1 is shown in FIG. Relative density 95 obtained in 5
The results of investigating the relationship between the heating temperature and the tensile strength of the pure tungsten sintered body by the hot tensile test apparatus using the sintered body of 10% are shown. This is to estimate an appropriate hot working temperature. As shown in FIG. 2, the pure tungsten sintered body has 1
Tensile strength is remarkably reduced in the high temperature range over 200 ° C,
It was speculated that the ductility would increase with this.

Therefore, based on this result, the sample No. Relative density of 1, 2, 3, 4, 6 from 85% to 93%
The hot working start temperature is 1200 ° C for the sintered products up to
~ 1800 ℃, the final rolling reduction is 60
The hot rolling was repeated 8 times so that it would be%. After hot rolling, a target of φ300 mm × 6 mm was manufactured by machining. Relative density of the obtained target, N
Table 2 shows the total content of a, K, and Ca, the oxygen content, the carbon content, and the presence or absence of cracking during hot working.

[0017]

[Table 2]

From Table 2, the density of the sintered body is 90% or more, and 1
Sample N manufactured as a target by the manufacturing method of the present invention in which hot working is performed at a hot rolling start temperature of 400 ° C. or higher.
o. 1b, 1c, 2b, 2c, 3b, 3c, 4b, 4c
Does not increase the carbon content in the process of manufacturing the target, the oxygen content, the content of Na, K, and Ca can be significantly reduced compared to the raw material powder, and the target has a theoretical density of 99.5% or more. Became. On the other hand, sample No. whose hot rolling start temperature is lower than 1400 ° C. 1a, 2a, 3a, 4
a and the sample No. having a sintered body density of less than 90%. 6
Nos. a, 6b, and 6c could be used as targets because cracking occurred during hot rolling. Further, a 6-inch wafer was sputtered using the target manufactured by the manufacturing method of the present invention to form a 100 nm thin film. Table 2 also shows the number of particles of 0.5 μm or more generated in the 6-inch wafer. From the number of particles generated in Table 2, it can be seen that the target obtained by the method of the present invention has a small particle generation of 25 particles or less.

[0019]

Industrial Applicability The tungsten target for semiconductors manufactured by the method of the present invention has a low oxygen content, a small amount of alkali metals and alkaline earth metals, and a high density.
Further, since there is no accident of cracking the target during the manufacturing process, it is possible to stably produce the tungsten target. When a thin film is formed by sputtering using the target produced by the manufacturing method of the present invention, the generation of particles is reduced. Therefore, it is possible to reduce the occurrence of defects such as delay of electrical signals due to particles and disconnection of wiring, which are a concern when semiconductor electrodes, wiring, etc. are formed.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a sintered body density and a heating temperature of Na, K, and Ca contents when a pressure-molded body is heated in an atmosphere containing hydrogen.

FIG. 2 is a diagram showing a relationship between a heating temperature and a tensile strength of a sintered body.

Claims (4)

[Claims] 1. A tungsten powder is pressed to form a compact having a relative density of 60% or more, and the compact is heated to a temperature of 1400 ° C. or higher in an atmosphere containing hydrogen to obtain a relative density of 90% or higher. A method of manufacturing a tungsten target for a semiconductor, comprising a sintered body, and further hot working the sintered body at a processing start temperature of 1400 ° C. or higher to obtain a relative density of 99% or higher. 2. The heating in an atmosphere containing hydrogen has a temperature of 16
The method for manufacturing a tungsten target for semiconductor according to claim 1, wherein the temperature is set to 00 ° C. or higher. 3. The method for manufacturing a tungsten target for semiconductor according to claim 1, wherein the hot working start temperature is 1600 ° C. or higher. 4. The method for manufacturing a tungsten target for semiconductor according to claim 1, wherein the pressing of the tungsten powder is performed by a cold isostatic press.