JP7554687B2 - Cu-Bi2O3 sputtering target and its manufacturing method - Google Patents

Description

The present invention relates to a Cu--Bi ₂ O ₃ sputtering target and a manufacturing method thereof, and more particularly to a Cu--Bi ₂ O ₃ sputtering target that can be used to form a dielectric film and a manufacturing method thereof.

In non-volatile memories such as FeRAM (ferroelectric memory), a dielectric film is used in the _capacitor memory element. It is known to use a bismuth oxide ( _Bi2O3 ) film as the dielectric film, and the _{Bi2O3 film} can be formed by sputtering. Here, the sputtering method is a method in which Ar ions are collided with a sputtering target facing a substrate to release molecules and atoms that constitute the sputtering target, which are then deposited on the substrate to form a film.

The _{Bi2O3 film} can be formed by using a _Bi2O3 sputtering target. Since _{the Bi2O3} sputtering target is insulating, it is not possible to perform direct current ( _DC ) sputtering, which allows high-speed film formation. Therefore, the film is formed by RF (radio frequency) sputtering. In addition, since _{the Bi2O3} sputtering target has a low thermal conductivity, there is a problem that the heat is not sufficiently diffused during sputtering, and the sputtering target cracks.

As sputtering targets _{using Bi2O3 ,} for example, the following prior art documents are known. Patent Document 1 describes a metal Bi-Bi oxide composite sputtering target that can suppress abnormal discharge and particle generation during sputtering. Patent Document 2 discloses a sputtering target made by sintering _{Bi2O3 , B2O3 ,} and oxides such as Ge. Patent Document 3 describes a composite sputtering target containing metal oxide A and metal B, in which metal oxide A is Bi oxide, etc. In addition, Patent Document 4 is known.

JP 2016-222971 A JP 2008-210492 A JP 2011-84804 A International Publication No. 2002/039508

Although _Bi2O3 sputtering _targets are useful for forming dielectric films, they have a problem that they are insulating and therefore cannot be used for DC (direct current) sputtering, which allows high-speed film formation, and therefore have poor productivity. The present invention has been proposed to solve the above problems, and the objective of the present invention is to provide a sputtering target that allows DC sputtering.

The present invention has been proposed to solve the above-mentioned problems, and one aspect of the present invention that can solve the problems is _a Cu- _Bi2O3 sputtering target containing Cu and _Bi2O3 , and having a volume resistivity of 1 _mΩ · _cm or _less .

The present invention provides a sputtering target suitable for forming a dielectric film, and because the volume resistivity is low, it can be used for DC (direct current) sputtering, and the film formation speed of sputtering can be increased. This has the excellent effect of improving productivity.

FIG. 2 is a schematic diagram showing measurement points of volume resistivity. 1 is an EPMA photograph of the structure of the sputtering target of Example 1. 1 is an EPMA photograph of the structure of the sputtering target of Comparative Example 1. 1 is an EPMA photograph of the structure of the sputtering target of Example 2. 1 is an EPMA micrograph of the structure of the sputtering target of Example 4.

Bismuth oxide _{( Bi2O3} ) is insulating and can be used as a dielectric film, but has a problem of being unable to be DC sputtered. If DC sputtering were possible, productivity could be improved significantly, and the present inventors conducted intensive research into this problem, finding that by adding a specified amount of Cu and devising a manufacturing method, it is possible to obtain a sputtering target with low volume resistivity that can be DC sputtered while maintaining the function as a dielectric layer, and thus arriving at the present invention.

The Cu- _Bi2O3 sputtering _target according to the embodiment of the present invention (hereinafter, sometimes referred to as the sputtering target) has a volume resistivity of 1 mΩ·cm or less. If the volume resistivity (average value) of the sputtering target is 1 mΩ·cm or less, stable DC sputtering is possible. Although Cu is generally known as a conductive material, the volume resistivity cannot be sufficiently reduced by simply adding _Cu to _Bi2O3 . In this embodiment, by devising a manufacturing method, a conductive path is formed by agglomerating Cu around _{the Bi2O3 particles} , and the volume resistivity is reduced to 1 mΩ·cm or less.

In the Cu- _{Bi2O3 sputtering} target according to the embodiment of the present invention, it is preferable that the content ratio of Cu and Bi satisfies Cu/(Cu+Bi)≧0.3 in atomic ratio. If Cu/(Cu+Bi) is less than 0.3, it may be difficult to ensure sufficient electrical conductivity in some cases. More preferably, Cu/(Cu+Bi)≧0.4, and even more preferably, Cu/(Cu+Bi)≧0.6. On the other hand, if Cu/(Cu+Bi) exceeds 0.8, it may be difficult to ensure the function as a dielectric layer in some cases. Therefore, it is preferable that Cu/(Cu+Bi)≦0.8, and more preferably, Cu/(Cu+Bi)≦0.7.

The Cu-Bi ₂ O ₃ sputtering target according to this embodiment preferably has a variation coefficient (standard deviation/average value×100) of the volume resistivity within the target surface of 15% or less. If the resistance is low in most of the sputtering target surface, DC sputtering is possible. In other words, if the target surface is measured and the average value is 1 mΩ·cm or less, DC sputtering is possible even if there are areas with high resistance, and the object of the present invention can be achieved. On the other hand, if the variation in the volume resistivity within the target surface becomes large, it may affect the stability of sputtering, so it is preferable to reduce the variation in the volume resistivity within the target surface. More preferably, it is within 10%.

The Cu- _Bi2O3 sputtering _target according to this embodiment preferably has a relative density of 95% or more, more preferably 98% or more. A high-density sputtering target can suppress abnormal discharge and particle generation. The sputtering target according to this embodiment may have a relative density of more than 100%. This is because the phases mixed in the raw material and the phases obtained by sintering are different, and as a result, the theoretical density value used in the true density is different from the original theoretical density value.

(Method of manufacturing sputtering target)
A method for producing a sputtering target according to an embodiment of the present invention will be described below. However, the following production method and its conditions are not limited to the disclosed scope, and some omissions and modifications may be made.
Copper (Cu) powder and bismuth oxide ( _Bi2O3 ) powder are prepared as raw material powders. Next, the raw material powders are dry-pulverized using zirconia beads, and then weighed and mixed to obtain a desired composition ratio (volume ratio ₎ . The particle sizes of the Cu powder and _{Bi2O3 powder} to be mixed are adjusted by their respective volume ratios.
In the present invention, the connection between Cu atoms is important in a sputtering target, and when the volume ratio of Cu is relatively large, the connection between Cu atoms is good, so there is no problem even if the particle size of the Cu powder is larger than the particle size of Bi ₂ O _3. On the other hand, as the volume ratio of Cu becomes smaller, the connection between Cu atoms decreases, so it is important to make the particle size of the Cu powder smaller than the particle size of Bi ₂ O ₃ .
Furthermore, when the volume ratio of Cu becomes smaller, it is desirable that the particle size of _Bi2O3 does not exceed 250 μm. In this way, as the volume ratio of _Cu becomes smaller, it is desirable that the particle size of Cu powder is smaller than that of _{Bi2O3 powder} , and the particle size of _Bi2O3 does not exceed 250 _μm .
As described above, it is important to adjust the particle size ratio of _{Bi2O3 powder} to Cu powder and the particle size of _Bi2O3 powder, so that Cu can be aggregated around _{the Bi2O3 particles} in the sintered body to _form conductive paths, thereby enabling the reduction of resistance.

Next, _{the Bi2O3} powder with the adjusted particle size and Cu powder are mixed, and then a sintered body is obtained by hot pressing (uniaxial pressure sintering) in which pressure and sintering are performed simultaneously. The hot pressing conditions are preferably a pressure of 200 to 300 kgf/ ^cm2 , a sintering temperature of 600°C to 650°C, and sintering in an argon atmosphere. The obtained sintered body is then subjected to lathe processing such as cutting and polishing to produce a sputtering target.

Various physical properties of the sputtering target and the like can be analyzed using the following measurement methods. Note that the following measurement methods are presented to facilitate understanding of the present embodiment. It should be understood that there are other alternative measurement methods, but it is not intended to permit the use of such alternative measurement methods to disguise the present invention in a different form.
(Composition analysis of sputtering targets)
Equipment: SII SPS3500DD
Method: ICP-OES (inductively coupled plasma optical emission spectrometry)
The composition of the sputtering target can be measured by ICP-OES (inductively coupled plasma optical emission spectrometry), but the composition ratio (atomic ratio) of the sputtering target according to this embodiment may be considered to be equivalent to that calculated by using the mixture ratio of the raw materials, because the above-described method for producing a sputtering target does not include any process that affects the composition ratio, and the change in the composition ratio is small.

(Volume resistivity of sputtering target)
The measurement device and measurement points for the volume resistivity of the sputtering target are as follows:
Equipment: NPS resistivity meter Σ-5+
Method: constant current application method Method: DC four-probe method As shown in Figure 1, the volume resistivity is measured at one location in the center and four locations at 90 degree intervals near the periphery on the front and back surfaces of the sputtering target, for a total of 10 locations, and the average value and standard deviation are calculated.

(Relative Density of Sputtering Target)
The relative density was calculated using the following formula:
Relative density (%) = (Archimedes density) / (true density) x 100
(Archimedes density): A small piece was cut out from the sputtering target, and the Archimedes density of the small piece was measured using an apparatus (AG204 manufactured by METTLER TOLEDO).
(True density): Using a composition analyzer (ICP-OES), the atomic percentages of Cu and Bi are analyzed, and from these atomic percentages the Cu weight is calculated as a (wt%) and the weight of Bi converted _{into Bi2O3} is calculated as b (wt%). Using the theoretical densities of Cu and _Bi2O3 as _{dCu and dBi2O3} respectively, the true density (g/ ^cm3 ) = 100/(a/ _dCu + b/ _dBi2O3 ) can be calculated.
The theoretical density of Cu was d _Cu =8.92 g/cm ³ , and the theoretical density of Bi ₂ O ₃ was d _Bi2O3 =8.90 g/cm ³ .

(Crystalline phase of sputtering target)
The crystal phase of the sputtering target was analyzed using an X-ray diffraction analyzer (Ultima IV, manufactured by Rigaku Corporation).

The following is a description based on examples and comparative examples. Note that these examples are merely examples and are not intended to limit the present invention in any way. In other words, the present invention is limited only by the scope of the claims, and includes various modifications other than the examples included in the present invention. In addition, the composition ratios (atomic ratios) and relative densities of the sputtering targets in the examples and comparative examples were calculated based on the mixture ratios of the raw materials.

Example 1
_{The Bi2O3} raw powder was heat-treated at 750°C in a heating furnace to produce a lump. The lump was crushed and pulverized in a dry ball mill to obtain a powder with a particle diameter _D50 of 86 μm. For the Cu powder, a powder with a particle diameter _D50 of 5 μm was prepared as the raw _powder . These powders were weighed so that _Bi2O3 :Cu=88vol%:12vol% (50mol%:50mol%), and then mixed. Next, the obtained mixed powder was hot-pressed under an argon atmosphere under the conditions of a pressure of 250 kgf/ ^cm2 and a sintering temperature of 600°C to produce a Cu- _{Bi2O3 sintered} body. After that, the sintered body was machined to be finished into a sputtering target shape.

A microstructure photograph of the sputtering target obtained in Example 1 observed with an EPMA (electron probe microanalyzer) is shown in Figure 2. As shown in Figure 2, it can be confirmed that Cu aggregates around the _{Bi2O3 grains} to form a network structure.
In addition, the volume resistivity of the sputtering target of Example 1 was measured, and the average of the volume resistivity at 10 points was 84 μΩ cm, with a standard deviation of 8 μΩ cm. In addition, the relative density of the sputtering target was 98.7%, which was a high density. The results are shown in Table 1.
In addition, the results of analyzing the components of the sputtering target using an inductively coupled plasma (ICP) emission spectrometer confirmed that the ratio was almost the same as when the raw materials were charged. The same was true for Examples 2 to 4 and Comparative Example 1 described below.

Example 2
_{The Bi2O3} raw powder was heat-treated at 750°C in a heating furnace to produce a lump. The lump was crushed and pulverized in a dry ball mill to obtain a powder with a particle diameter _D50 of 64 μm. For the _Cu powder, a powder with a particle diameter _D50 of 5 μm was prepared as the raw powder. These powders were weighed in the ratio of _Bi2O3 :Cu=69 vol%:31 vol% (23 mol%:77 mol%) and then _mixed . Next, the obtained mixed powder was vacuum hot pressed under an argon atmosphere under the conditions of a pressure of 250 kgf/ ^cm2 and a sintering temperature of 600°C to produce a Cu- _Bi2O3 sintered body. After that, the sintered body was machined to be finished into a sputtering target shape.

A microstructure photograph of the sputtering target obtained in Example 2 observed with an EPMA (electron probe microanalyzer) is shown in Fig. 3. As shown in Fig. 3, it can be confirmed that Cu aggregates around the _{Bi2O3 grains} to form a network structure.
In addition, the volume resistivity of the sputtering target of Example 2 was measured, and the average of the volume resistivity at 10 points was 14 μΩ cm, with a standard deviation of 1 μΩ cm. In addition, the relative density of the sputtering target was 100.5%, which was a high density. The results are shown in Table 1.

Example 3
_{A Bi2O3 powder} having a particle diameter _D50 of 2 μm and a Cu powder having a particle diameter _D50 of ₁₀₇ μm were prepared. These powders were weighed out in a ratio of _Bi2O3 :Cu=59 vol%:41 vol% (16 mol%:84 mol%) and then mixed. The obtained mixed powder was hot pressed under an argon atmosphere, a pressure of 250 kgf/ ^cm2 , and a sintering temperature of 600°C to produce a Cu- _{Bi2O3 sintered} body. The sintered body was then machined to have a sputtering target shape.

The volume resistivity of the sputtering target obtained in Example 3 was measured, and the average volume resistivity at 10 locations was 42 μΩ·cm, with a standard deviation of 6 μΩ·cm. The relative density of the sputtering target was 101.6%. The results are shown in Table 1.

Example 4
_{A Bi2O3 powder} having a particle diameter _D50 of 2 μm and a Cu powder having a particle diameter _D50 of 107 _μm were prepared. These powders were weighed out in a ratio of _Bi2O3 :Cu=49 vol%:51 vol% (12 mol%:88 mol%) and then mixed. The obtained mixed powder was hot pressed under an argon atmosphere, a pressure of 250 kgf/ ^cm2 , and a sintering temperature of 600°C to produce a Cu- _{Bi2O3 sintered} body. The sintered body was then machined to have a sputtering target shape.

The structure of the sputtering target obtained in Example ₄ was observed by EPMA (electron probe microanalyzer) and is shown in FIG. 5. As shown in FIG. 5, it can be confirmed that _Bi2O3 covers Cu and Cu is separated. As a result of measuring the volume resistivity of the sputtering target obtained in Comparative Example 2, the average of the volume resistivity at 10 points was 14 μΩ·cm, and the variation (standard deviation) was 2 μΩ·cm. In addition, the relative density of the sputtering target was 102.7%.

(Comparative Example 1)
_{A Bi2O3 powder} having a particle diameter _D50 of 2 μm and a Cu powder having a particle diameter _D50 of 107 _μm were prepared. These powders were weighed in a ratio of _Bi2O3 :Cu=88 vol%:12 vol% (50 mol%:50 mol%) and then mixed. Next, the obtained mixed powder was hot pressed under an argon atmosphere, a pressure of 250 kgf/ ^cm2 , and a sintering temperature of ₆₀₀ °C to produce a Cu- _Bi2O3 sintered body. After that, the sintered body was machined to be finished into a sputtering target shape.

FIG. 4 shows a structure photograph of the sputtering target obtained in Comparative Example ₁ observed by EPMA (electron probe microanalyzer). As shown in FIG. 4, it can be confirmed that _Bi2O3 covers Cu and Cu is separated. Although an attempt was made to measure the volume resistivity of the sputtering target obtained in Comparative Example 1, the volume resistivity was too high to be measured (over 500 KΩ·cm). The relative density of the sputtering target was 103.6%. The above results are shown in Table 1.

The Cu-Bi ₂ O ₃ sputtering target according to the embodiment of the present invention has a low volume resistivity and is suitable for DC sputtering. Furthermore, it has a high relative density and can suppress the occurrence of breakage or cracks in the target during sputtering. Therefore, it is useful at a practical and commercial level. It is particularly expected to be used for forming a dielectric film of a capacitor memory element in a non-volatile memory such as FeRAM.

Claims (3)

A Cu-Bi ₂ O ₃ sputtering target containing Cu and Bi ₂ O ₃ , characterized in that the volume resistivity is 1 _mΩ ·cm or less , and the content ratio of Cu and Bi satisfies 0.3≦Cu/(Bi+Cu)≦0.8 in atomic ratio _. The Cu-Bi ₂ O ₃ sputtering target according to claim 1 , characterized in that the variation coefficient of volume resistivity within the target surface is within 15%. The Cu-Bi ₂ O ₃ sputtering target according to claim 1 or 2, characterized in that the relative density is 95% or more.