KR20240169718A - Sputtering target and production method for sputtering target - Google Patents

Abstract

A sputtering target is a sputtering target containing B in an amount of 10 to 20 at%, Fe in the remainder, and having an average area of an Fe-B phase of 20 μm ² or less as observed by SEM image observation.

Description

{SPUTTERING TARGET AND PRODUCTION METHOD FOR SPUTTERING TARGET}

The present specification discloses a technology relating to a sputtering target and a method for manufacturing the sputtering target.

Among nonvolatile memories that retain memory without power supply, there is magnetoresistive memory (MRAM), which stores memory by the magnetization state of a magnetic tunnel junction.

In magnetoresistive memory, a material to which boron is added is sometimes used as a vertical magnetization film of a tunnel magnetoresistive element that is a component thereof. Such a vertical magnetization film can be formed by a sputtering method using a sputtering target containing boron and at least one of iron and cobalt.

Examples of sputtering targets containing boron and used for such magnetoresistive memory or other purposes include those described in Patent Documents 1 to 4.

Patent documents 1 and 2 describe "a magnetic material sputtering target comprising a sintered body of one or more elements selected from Co or Fe with a B content of 26 at% or more and 40 at% or less, the remainder being Co or Fe, wherein the target has a high-concentration B phase and a low-concentration B phase, and wherein the magnetic material sputtering target is characterized in that at most one has a maximum inscribed circle diameter of 15 µm or more that can be drawn in the high-concentration B phase." In addition, patent documents 1 and 2 describe "a method for manufacturing a magnetic material sputtering target, characterized in that "a magnetic material target raw material powder having a B content of 26 at% or more and 40 at% or less and a remainder of at least one element selected from Co or Fe is produced by a gas atomization method, the gas atomized raw material powder is sintered to form a target, and the target has a structure in which a high-concentration B phase and a low-concentration B phase exist, and the maximum inscribed circle diameter that can be drawn on the high-concentration B phase is one or less high-concentration B phases of 15 µm or more."

Patent Documents 3 and 4 describe a "CoFeB alloy sputtering target material containing 10 to 50% of B in at.%, the remainder being at least one of Co and Fe and unavoidable impurities, wherein the intensity ratio [I〔(CoFe) ₃ B〕/I〔(CoFe) ₂ B〕] of the X-ray diffraction intensity of (CoFe) ₂ B (200) [I〔(CoFe) ₂ B〕] and the X-ray diffraction intensity of (CoFe) ₃ B (121) [I〔(CoFe) ₃ B〕] is 1.50 or less."

Japanese Patent No. 6037415 Description of United States Patent Application Publication No. 2016/0237552 Japanese Patent Publication No. 2017-57477 Description of United States Patent Application Publication No. 2018/0245211

There was a problem that a sputtering target containing boron as described above, especially one containing a predetermined amount of boron and further containing iron, generated a large number of particles during sputtering, which resulted in a decrease in product yield.

In this specification, a sputtering target capable of effectively reducing particles and a method for manufacturing the sputtering target are disclosed.

The sputtering target disclosed in this specification contains B in an amount of 10 to 20 at%, the remainder being Fe, and has an average area of an Fe-B phase of 20 μm ² or less as observed by SEM image observation.

The method for manufacturing a sputtering target disclosed in this specification includes a sintering process of maintaining a raw material powder containing Fe and B in an amount of 10 to 20 at% while pressurizing at a temperature of 800°C or higher and less than 900°C for 1 to 3 hours.

According to the above-described sputtering target, particles during sputtering can be effectively reduced.

Figure 1 is an SEM image of the sputtering target of Comparative Example 1.
Figure 2 is an SEM image of the sputtering target of Example 1.
Figure 3 is an image obtained by performing image analysis on the SEM image of Figure 2.

Below, the embodiments disclosed in this specification are described in detail.

A sputtering target of one embodiment contains B in an amount of 10 to 20 at%, and the remainder contains Fe, and has an average area of an Fe-B phase of 20 μm ² or less as observed by SEM image observation. The sputtering target is obtained as a sintered body of a given raw material powder after a sintering process in which a given raw material powder is pressurized and maintained at a temperature of 800° C. or higher and less than 900° C. for 1 to 3 hours, as described below, for example.

(furtherance)

The sputtering target shall contain at least B (boron) and Fe (iron).

The B content is 10 at% to 20 at%, preferably 14 at% to 20 at%, and more preferably 16 at% to 20 at%. If the B content is too low, it is thought that the film formed with the sputtering target cannot exhibit the expected characteristics. On the other hand, if the B content is too high, the sinterability of the powder deteriorates, and particles are generated at a level that cannot be helped by optimizing the sintering conditions alone.

The Fe content is, for example, 5 at% to 80 at%, typically 20 at% to 65 at%.

The sputtering target may contain more Co. In this case, the Co content is preferably 5 at% to 80 at%, and more preferably 20 at% to 65 at%.

The purity, which is the total content of B, Fe, and Co, is suitably 3N (99.9 mass%) or higher. In addition, when Co is not included, the content of Co is 0 (zero) mass%. This is because if the purity is less than 3N, impurities may become the cause of particle generation. It is more preferable that the purity is 3N5 (99.95 mass%) or higher. This purity can be obtained by GDMS analysis.

In addition, the sputtering target may contain impurities such as Si and/or Ni in a total amount of 100 mass ppm or less in addition to the elements described above. Impurities contained in such amounts are acceptable.

(Fe-B phase)

In the sputtering target having the composition described above, an Fe-B phase exists. The Fe-B phase in the sputtering target is observed by a SEM (scanning electron microscope) image of a cross-section perpendicular to the sputtering surface used for sputtering, and its average area is 20 ㎛ ² or less.

Thereby, the surface roughness of the sputtered surface after the sputtering surface has been subjected to a predetermined sputtering is improved, which will be described later. As a result, particle generation during use by sputtering can be effectively reduced.

From this point of view, the average area of the Fe-B phase among the sputtering targets is preferably 15 μm ² or less, and more preferably 10 μm ² or less. In addition, the average area may be, for example, 2 μm ² or more, typically 5 μm ² or more.

The above average area of the Fe-B phase is calculated by analyzing the total area of the Fe-B phase and the number of Fe-B phases in the field of view of the SEM image of a cross-section orthogonal to the sputtering plane, and dividing the total area by the number. In a SEM image, when two or more phases are usually present, a contrast of two or more phases occurs, in which a relatively light element such as B appears black, while a relatively heavy element appears white (see, for example, Figs. 1 and 2). Among the Fe-B phases that appear black in the SEM image, those that are connected to each other in adjacent Fe-B phases are counted as one Fe-B phase as a whole. In addition, when only a part of the Fe-B phase present in the outer portion of the SEM image enters the field of view of the SEM image, only the area of the part entering the field of view is considered, and that part is regarded as one Fe-B phase and counted. Fig. 3 is the result of performing image analysis on the SEM image of Fig. 2 and identifying the Fe-B phase that appears black in the SEM image. By performing image analysis on the SEM image in this way, it is possible to calculate the total area and number of Fe-B phases. In calculating the average area of the Fe-B phase, analysis is performed on one SEM image in the cross-section.

(Surface roughness of sputtering surface after use)

In the above sputtering target, it is preferable that the surface roughness Ra of the sputtering surface after use, which is displayed after performing a predetermined sputtering using it, is small. Specifically, when the sputtering target is set in a sputtering device and used at an output of 600 W and up to 60 kWh, the surface roughness Ra of the sputtering surface after use of the sputtering target is suitably 2.0 µm or less. In addition, as the sputtering device, the model number C-7100GT manufactured by Canon Anerva Co., Ltd. can be used.

Thereby, since the grain boundaries of different Fe-B phases and different phases of the sputtering rate are not prominent, the occurrence of arcing, etc. can be suppressed, and particles can be further reduced. In other words, in a sputtering target having a surface roughness Ra of the sputtering surface after use exceeding 2.0 ㎛, there is a concern about the occurrence of particles due to arcing, etc.

The surface roughness Ra of the sputtered surface after use is more preferably 1.5 ㎛ or less. On the other hand, the surface roughness Ra of the sputtered surface after use may be, for example, 0.4 ㎛ or more.

(density ratio)

The density ratio of the sputtering target is preferably greater than 99%. This is because, when the density ratio is greater than 99%, internal defects in the target that cause particle generation can be reduced. In this respect, the density ratio is more preferably 99.9% or more. The density ratio of the sputtering target is measured by the Archimedes method.

The density ratio of the sputtering target is calculated from the theoretical density calculated by calculation and the actual density measured by the Archimedes method by the following formula: Density ratio = (actual density measured by the Archimedes method) ÷ (theoretical density) × 100(%). Here, the theoretical density is the density when it is assumed that the constituent components of the sputtering target are mixed without diffusing or reacting with each other, and is calculated by the following formula: Theoretical density = Σ (molecular weight of constituent × molar ratio of constituent) / Σ (molecular weight of constituent × molar ratio of constituent / literature value density of constituent). However, in reality, since each element of FeCoB reacts and exists, the true value of the target sputtering target may be higher than the theoretical density. Therefore, the density ratio calculated using the theoretical density may exceed 100%.

(Manufacturing method)

The sputtering target described above can be manufactured, for example, as follows.

First, a raw material preparation process is performed to prepare raw material powder containing Fe together with B at 10 at% to 30 at%. The raw material powder is prepared by adjusting the content of each element so that the composition of the predetermined sputtering target as described above is obtained. Therefore, the raw material powder further contains Co as necessary. When the raw material powder contains Co, the content of Co can be 5 at% to 80 at%.

When producing raw material powder, it is preferable to use the gas atomization method from the viewpoint of reducing the oxygen content of the raw material powder. In the gas atomization method, for example, high-pressure gas is sprayed onto a molten metal containing B, Fe, Co, etc., in an inert gas atmosphere, and the molten metal is turned into powder.

In addition, it is suitable to set the average particle diameter D50 of the raw material powder to 50 ㎛ to 300 ㎛, for example, by sieving the gas atomized powder. This facilitates sintering of the raw material powder in the subsequent sintering process, and makes it possible to obtain a high-density sputtering target.

Next, a sintering process can be performed in which the above raw material powder is pressurized and maintained at a predetermined temperature for a predetermined period of time. As a result, a predetermined sintered body is obtained.

In addition, this sintering can be performed using a vacuum hot press method, another hot press method, a plasma discharge sintering method, or a hot isostatic pressing method.

Here, it is important to make the temperature during pressurization 800℃ or higher and less than 900℃, and to maintain the temperature for 1 to 3 hours. By making the temperature during pressurization relatively low and shortening the maintenance time to a certain extent, grain growth is suppressed, and the average area of the sputtering target described above can be reduced. In addition, since the structure becomes fine, the surface roughness Ra of the sputtering surface after use described above also becomes smaller.

More specifically, if the temperature is lower than 800°C, the density does not increase sufficiently. On the other hand, if the temperature is higher than 900°C, the average area and surface roughness Ra increase, and as a result, particles increase during sputtering.

If the holding time is less than 1 hour, the density does not increase sufficiently. In addition, if the holding time is longer than 3 hours, the average area and surface roughness Ra increase due to particle growth over time.

From this point of view, the temperature during pressurization is preferably 800°C or higher and less than 900°C, more preferably 850°C or higher and less than 900°C. In addition, the holding time is preferably 1 hour to 3 hours, more preferably 1 hour to 2 hours.

In addition, the heating rate until the above temperature is reached is preferably 5°C/min or more, and more preferably 5°C/min to 10°C/min. In addition, in the cooling after the above temperature is reached, the cooling rate is preferably 1°C/min or more. This is because by increasing the heating rate and cooling rate quickly, the heating time is shortened, and further suppression of grain growth and a significant reduction in the average area and surface roughness Ra can be achieved. In order to achieve a predetermined cooling rate, forced cooling such as air cooling may be employed.

In the sintering process, the pressure and atmosphere can be appropriately determined according to various conditions, but for example, the pressure can be 15 MPa to 30 MPa, and the atmosphere can be a vacuum atmosphere.

After that, the sintered body is generally machined into a predetermined shape such as a disk using a lathe or flat grinder, and its surface is polished. Thereby, a sputtering target can be manufactured.

Example

Next, the sputtering target as described above was tested and its performance was confirmed, which is described below. However, the description herein is intended as a simple example and is not intended to be limited thereto.

A raw material powder manufactured by the gas atomization method and having the composition shown in Table 1 and adjusted to include B, Co and Fe was prepared. This raw material powder was pressurized in a vacuum atmosphere under the conditions of the temperature, holding temperature and heating rate shown in Table 1, to obtain a sintered body. The pressurization force was 29.42 MPa. The sintered body thus obtained was subjected to predetermined mechanical processing and polishing, and a sputtering target was manufactured.

For each sputtering target manufactured under different sintering conditions as shown in Table 1, the average area of the Fe-B phase, the surface roughness of the sputtering surface after use, and the density ratio were measured by the method described above. The results are also shown in Table 1. In addition, SEM images of the cross-sections orthogonal to the sputtering surface of each sputtering target of Comparative Example 1 and Example 1 are shown in Figs. 1 and 2, respectively.

In addition, using each sputtering target, sputtering was performed under the conditions of power 600 W, Ar flow rate 30 sccm, and target film thickness 20 nm by C-7100GT manufactured by Canon Anerva. As a result, the particle counts shown in Table 1 were obtained.

As can be seen from Table 1, in Examples 1 to 8, which were maintained at a temperature of 800°C or higher and less than 900°C for 1 to 3 hours during sintering, the average area became smaller. As a result, it was obtained that Examples 1 to 8 had a small number of particles during sputtering.

In Comparative Example 1, because the temperature during sintering was high, the average area increased and the number of particles increased. In contrast, in Comparative Example 2, because the temperature during sintering was low, the pores increased and the average area could not be measured. This Comparative Example 2 had a large number of particles.

In Comparative Example 3, the average area was large due to the long temperature maintenance time during sintering, and thus the number of particles was large. In addition, in Comparative Examples 4 and 5, the B content was high, so the average area became unmeasurable due to pores, and the number of particles increased.

Therefore, it can be said that in Examples 1 to 8, particles during sputtering could be effectively reduced.

Claims (10)

A sputtering target, which contains B in an amount of 10 at% to 20 at%, the remainder being composed of Fe and inevitable impurities, or Fe, Co and inevitable impurities, and having a density ratio of 99.9% or more, and an average area of an Fe-B phase as observed by an SEM image of 20 ㎛ ² or less, wherein the average area of the Fe-B phase is calculated by dividing the total area of the Fe-B phase in a field of view of an SEM image of a cross-section orthogonal to a sputtering surface by the number of Fe-B phases. In the first paragraph,
A sputtering target having an average area of Fe-B of 15 ㎛ ² or less. In the second paragraph,
A sputtering target having an average area of Fe-B of 10 ㎛ ² or less. In any one of claims 1 to 3,
A sputtering target having a surface roughness Ra of 2.0 ㎛ or less of the sputtering surface after use when used at up to 60 kWh with an output of 600 W using a sputtering device. In any one of claims 1 to 3,
Sputtering target with purity of 3N or higher. In any one of claims 1 to 3,
A sputtering target containing Co and having a Co content of 5 at% to 80 at%. A method for manufacturing a sputtering target,
A method for manufacturing a sputtering target, comprising a sintering process in which a raw material powder containing Fe and containing B in an amount of 10 to 20 at% is pressurized and maintained at a temperature of 800°C or higher and less than 900°C for 1 to 3 hours to obtain a density ratio of 99.9% or higher. In Article 7,
A method for manufacturing a sputtering target, wherein the temperature in the sintering process is 850°C or higher and less than 900°C. In clause 7 or 8,
A method for manufacturing a sputtering target, wherein, in a sintering process, the heating rate until the temperature is reached is set to 5°C/minute or more, and the cooling rate after the temperature is reached is set to 1°C/minute or more. In clause 7 or 8,
A method for manufacturing a sputtering target, wherein the above raw material powder also contains Co in an amount of 5 at% to 80 at%.