JP7739744B2 - Yttrium ingot and sputtering target using same - Google Patents

Description

The present invention relates to an yttrium ingot for film formation and a sputtering target using the same.

In semiconductor device manufacturing, microfabrication using dry etching with highly corrosive halogen-based gases such as fluorine-based and chlorine-based gases, or their plasma, is an important process. These corrosive gases and plasmas corrode and damage the components of semiconductor manufacturing equipment, and the resulting particles are known to cause a decrease in device quality. Many of the components of semiconductor manufacturing equipment are consumables, and are replaced regularly to prevent reduced yield and quality due to the damage described above. Downtime associated with component replacement and equipment maintenance reduces equipment utilization rates and reduces productivity, creating problems. Therefore, the semiconductor manufacturing process requires the development of components with excellent plasma and gas corrosion resistance.

As semiconductor devices become increasingly miniaturized, the plasma used in dry etching processes is becoming increasingly dense, and yttrium oxide is attracting attention as a material that can withstand such high-density plasma. Due to factors such as manufacturing costs and size, the mainstream industrial method for manufacturing yttrium oxide components is to form an yttrium oxide film on a substrate by thermal spraying, as described in Patent Document 1. However, because thermal spraying involves melting ceramic powder and rapidly solidifying it to form a film, surface defects and voids exist on the film surface. The presence of such defects not only reduces plasma resistance but also causes particle generation, so a highly efficient method for forming a dense yttrium oxide film is needed.

Here, sputtering can be mentioned as one of the film formation methods other than thermal spraying. Sputtering involves physically colliding positive ions such as Ar ions with a target placed on a cathode, releasing the materials constituting the target with the collision energy, and depositing a film on a substrate placed opposite the target. Examples of sputtering include direct current sputtering (DC sputtering), radio frequency sputtering (RF sputtering), and alternating current sputtering (AC sputtering). Generally, film formation by sputtering allows for film formation at a lower temperature than film formation by thermal spraying, suppresses the generation of defects such as voids, and is thought to be able to form denser films. Furthermore, sputtering film formation can also be performed by reactive sputtering, in which gases such as oxygen and nitrogen are introduced into the sputtering chamber, thereby forming oxide or nitride films. For example, as in Non-Patent Document 1, it is possible to form an yttrium oxide film on a substrate by reactive DC sputtering, in which an yttrium target is subjected to DC discharge and oxygen is introduced during sputtering; however, the quality of the formed film varies significantly depending on the sputtering conditions. Incidentally, in Non-Patent Document 1, a film is formed using a 99.5% pure yttrium target, but sufficient consideration has not been given to the correlation between the physical properties of the sputtering target, such as density and purity, and the sputtering characteristics, and the relationship with the quality of the film formed by sputtering. Therefore, further consideration was needed regarding the physical properties of the yttrium target, the sputtering characteristics, and the characteristics of the film formed.

Yttrium is typically produced by precipitating chlorides through molten salt electrolysis, as described in Patent Document 2. However, this method focuses only on high purity and does not mention additives or structure. Furthermore, chlorides are typically used to achieve high purity, and using fluorides as a raw material has been considered difficult in achieving high purity. Furthermore, separating rare earth elements is difficult, and it has not been possible to achieve both. Furthermore, sputtering targets containing inherent fluorine and oxygen have never been used due to the need for high purity. Furthermore, yttrium oxyfluoride has high resistance and cannot be subjected to DC discharge, so it can only be produced by thermal spraying, as described in Patent Document 3.

Japanese Patent Application Laid-Open No. 2006-307311 WO2013-005349 publication Japanese Patent Application Laid-Open No. 2018-185657

P. Lei et al. Surface & Coatings Technology 276 (2015) 39-46

The object of the present invention is to provide an yttrium ingot for a low-resistivity yttrium sputtering target that has high plasma resistance and can achieve a high film formation rate, and a sputtering target using the same.

As a result of extensive research into yttrium ingots suitable for yttrium sputtering targets, the inventors discovered a yttrium ingot for yttrium sputtering targets that can produce yttrium oxide-based films with high plasma resistance, leading to the completion of the present invention.

That is, the aspects of the present invention are as follows.
(1) A yttrium ingot characterized by having a fluorine atom content of 0.05 wt % or more and 10 wt % or less and a relative density of 96% or more.
(2) The yttrium ingot according to (1), which contains yttrium oxyfluoride.
(3) A yttrium ingot according to (1) or (2), wherein the content of rare earth elements is 98≦100−RE<99.999, where RE is the wt% content of rare earth elements.
(4) A yttrium ingot according to any one of (1) to (3), having an average particle diameter (D50) of 100 μm or less.
(5) A yttrium ingot according to any one of (1) to (4), having a volume resistivity of 1 Ω cm or less.
(6) A yttrium sputtering target comprising the yttrium ingot according to any one of (1) to (5).
(7) The sputtering target according to (6), which comprises a backing plate and an yttrium ingot.
(8) The yttrium sputtering target according to (7), wherein the adhesion rate between the backing plate and the yttrium ingot is 90% or more.
(9) A method for producing an yttrium oxide film, comprising sputtering using the yttrium sputtering target according to any one of (6) to (8).

The present invention is described in detail below.

The yttrium ingot of the present invention is characterized by a fluorine atom content of 0.05 wt% or more and 10 wt% or less. The inclusion of fluorine atoms makes it possible to form a yttrium-based film with high plasma resistance. More preferably, the content is 0.05 wt% or more and 8 wt% or less, 0.05 wt% or more and 5 wt% or less, 0.1 wt% or more and 4 wt% or less, and even more preferably, 0.2 wt% or more and 3 wt% or less. If the content exceeds 10 wt%, the resistivity of the ingot increases, making it impossible to achieve highly productive DC discharge. Furthermore, if the content is less than 0.05 wt%, the plasma resistance will not improve due to the effect of adding fluorine.

Here, the fluorine atom content refers to the weight ratio of fluorine atoms to the entire yttrium ingot, and can be measured using GDMS (glow discharge mass spectrometry) or ICP (inductively coupled plasma (ICP)).

The relative density is 96% or higher, preferably 98% or higher, more preferably 99% or higher, and even more preferably 99.8% or higher. If the density is less than 96%, large ingots in particular are prone to cracking, making it impossible to produce ingots with a high yield. Furthermore, when high power is applied during sputtering using such ingots, cracks are likely to occur during discharge, which is undesirable as it reduces the productivity of the film formation process.

The yttrium ingot of the present invention preferably contains yttrium oxyfluoride. The inclusion of yttrium oxyfluoride can prevent the inability to generate yttrium oxyfluoride due to compositional deviations during sputtering. Many compounds exist as yttrium oxyfluoride (YOF), but it is preferable that it exists primarily as trigonal YOF. Trigonal YOF is highly stable, resulting in excellent final mechanical properties. Furthermore, its abundance ratio, in terms of area ratio, is preferably 0.1% to 35% inclusive, more preferably 0.1% to 20% inclusive, and even more preferably 0.1% to 10% inclusive. Setting the abundance ratio within this range enables stable discharge and makes it possible to produce a film containing yttrium oxyfluoride.

The yttrium ingot of the present invention contains a specific amount of fluorine, and with respect to the content of impurities other than fluorine, when the content of rare earth elements is RE wt%, it is preferable that 98≦100−RE<99.999, more preferably 99≦100−RE<99.999, and even more preferably 99.9≦100−RE<99.999. Reducing the amount of rare earth impurities and increasing the purity of the yttrium target can suppress abnormal discharge and particle generation. Increasing the purity is undesirable because the purification process becomes more complex and production costs increase. The inventors investigated the correlation between impurity amount and discharge characteristics within the above range and determined a purity level suitable for use in sputtering film formation. Here, rare earth elements are Ce, Pr, Nd, Sn, Eu, Gd, Tb, Py, Ho, and Er.

The average particle size (D50) of the yttrium ingot is preferably 100 μm or less, more preferably 0.1 μm to 100 μm, more preferably 0.1 μm to 75 μm, and particularly preferably 0.1 μm to 20 μm. By uniformly dispersing a yttrium fluoride compound in the yttrium ingot, the average particle size is refined, resulting in an yttrium ingot that maintains high strength even when it contains yttrium oxyfluoride. Sputtering targets using such ingots enable high-speed film formation using high power. Furthermore, because the fluorine is uniformly dispersed, it is also possible to reduce compositional variation within the film.

Next, the volume resistivity of the yttrium ingot in the present invention is preferably 1 Ω·cm or less, more preferably 0.00001 Ω·cm or more and 1 Ω·cm or less, and even more preferably 0.00001 Ω·cm or more and 0.001 Ω·cm or less. Yttrium is very susceptible to oxidation, and oxidation progresses naturally in the atmosphere. The yttrium oxide formed by oxidation is an insulator, and can cause abnormal discharge during sputtering discharge, particularly when forming a film using DC discharge. By keeping the volume resistivity within the above range, stable discharge characteristics can be obtained in DC sputtering, RF sputtering, and AC sputtering.

The yttrium ingot of the present invention preferably has a number of pores with a diameter of 100 μm or more of 0.1/ ^cm2 or less, more preferably 0.01/cm2 or ^less , and even more preferably 0.005/ ^cm2 or less. If the number of pores with a diameter of 100 μm or more is more than 0.1/ ^cm2 , this may cause abnormal discharge or particles during sputtering.

The surface roughness of the yttrium ingot of the present invention is important in terms of the surface roughness of the sputtering surface when used as a sputtering target. When used as a sputtering target, the surface roughness of the sputtering surface is preferably 10 nm or more and 2 μm or less, more preferably 10 nm or more and 1 μm or less, and even more preferably 10 nm or more and 0.3 μm or less. The sputtering surface refers to the area from which sputter particles are actually emitted (erosion area).

The yttrium ingot of the present invention can be ground into a plate shape using a machining machine such as a surface grinder, cylindrical grinder, lathe, cut-off machine, or machining center.

There are no particular limitations on the method for manufacturing the yttrium ingot of the present invention, but in order to incorporate a certain amount of fluorine, it is preferable to use a manufacturing method that involves reduction treatment using yttrium fluoride, followed by melting and solidifying, such as vacuum melting or EB melting.

Yttrium fluoride is a relatively stable substance, and can be further reduced to produce yttrium oxyfluoride. By controlling the reduction conditions during this process, the necessary amount of fluorine remains, making it possible to synthesize yttrium ingots containing a specific amount of fluorine, with the fluorine uniformly dispersed. Using chloride as the starting material is not preferable, as it is not possible to achieve a uniform fluorine content.

When using melting and solidification methods such as vacuum melting and EB melting, coarse pores of 100 μm or larger tend to form due to vaporization during melting, making it difficult to obtain an ingot with few pores. Therefore, it is preferable to compress the ingot produced by the melting method using hot isostatic pressing (HIP) to eliminate the pores. However, because yttrium is a material that oxidizes easily, it is preferable to seal the outer periphery with metal. The HIP temperature is preferably 1000°C or lower. Furthermore, because yttrium is relatively brittle, it is preferable that the pressure be 100 MPa or lower. This makes it possible to obtain a yttrium ingot with few pores of 100 μm or larger.

The yttrium ingot of the present invention can be used as a sputtering target. The sputtering target can be manufactured by bonding a backing plate or backing tube made of oxygen-free copper, titanium, or the like using indium solder, etc., as needed, to obtain a sputtering target. Among these, a sputtering target made of a backing plate and yttrium ingot is preferred.

The surface roughness of the yttrium ingot side of the bonding surface between the yttrium ingot and the backing plate (bonding surface roughness) is preferably 10 nm or more and 2 μm or less, more preferably 10 nm or more and 1 μm or less, and even more preferably 10 nm or more and 0.3 μm or less. Setting it to 2 μm or less reduces the specific surface area of the surface layer, reducing the surface oxygen of the easily oxidized yttrium and preventing peeling at the oxidized areas during bonding. Furthermore, treating the bonding surface in an oxidized state makes treatment impossible due to peeling of the oxide layer, ultimately resulting in a lower adhesion rate. Setting it to 10 nm or more improves the engagement between the surface and the treated surface, improving adhesion and enabling higher-power discharge. However, since surface oxidation progresses, it is preferable to polish the surface after target formation.

The backing plate is used to efficiently attach the ingot, which is the film material portion of the sputtering target, to the sputtering device, and is cooled with water or other cooling methods to prevent the ingot portion from overheating during sputtering. The adhesive material used is indium or an indium alloy, which has high thermal conductivity and is easy to use as solder.

There are no particular restrictions on the material of the backing plate, and copper, stainless steel, titanium, etc. can be used.

For sputtering targets consisting of a backing plate and an yttrium ingot, the adhesion rate between the yttrium ingot and the backing plate is preferably 90% or higher, more preferably 95% or higher, and even more preferably 98% or higher. By achieving this adhesion rate, it is possible to quickly dissipate the heat generated by the target during sputtering, preventing the sputtering target from overheating and causing the solder material to melt.

The adhesion rate between the yttrium ingot and the backing plate can be determined, for example, by ultrasonic flaw detection measurement. When determining the adhesion rate by ultrasonic flaw detection measurement, it is preferable to adjust the measurement conditions using a pseudo-defect sample with a pseudo-void hole in the center of a plate material of a specified size. The measurement sensitivity is adjusted so that the area of the detected defect matches the area of the specified void hole. The material of the pseudo-defect sample is preferably the same as the material of the sputtering target. It is preferable that the distance from the ultrasonic incident surface of the pseudo-defect sample to the bottom of the void hole be the same as the distance from the ultrasonic incident surface of the sputtering target to the bonding layer.

When bonding, it is preferable to polish the bonding surface of the yttrium ingot and quickly perform surface treatment. The surface of the yttrium ingot oxidizes over time, and this oxide film makes it difficult to bond to the solder material. Therefore, the oxide layer on the yttrium surface is removed before bonding, and the surface is quickly treated. There are no particular restrictions on the treatment method, but it is preferable to use methods such as vapor deposition of a metal that adheres well to the solder material, plating, or treatment with an ultrasonic soldering iron. This allows the solder and yttrium to bond without peeling. It is preferable to complete the surface treatment within three hours after oxidation treatment.

Yttrium oxide films can also be produced by sputtering using the resulting yttrium sputtering target.

Yttrium sputtering targets using the yttrium ingot of the present invention can produce highly plasma-resistant yttrium-based films with high productivity.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited thereto. Measurements in the examples were carried out as follows.
(1) Relative Density The relative density of the yttrium ingot portion was determined by measuring the bulk density by the Archimedes method in accordance with JIS R 1634 and dividing it by the true density of metallic yttrium (4.47 g/cm ³ ).
(2) Pore ratio measurement The entire image was measured using an X-ray transmission image, and pores of 100 μm or more were extracted from the image, and the number and size of pores were measured, and converted into pores/cm ² from the measured area.
(3) Volume resistivity: The volume resistivity was measured at three or more points by the four-probe method and the average was obtained.
(4) Average Particle Diameter After mirror polishing, the particles were observed under a scanning electron microscope - electron backscatter diffraction (SEM-EBSD) (SEM: manufactured by JEOL Ltd., EBSD: manufactured by Oxford University Press), and the arithmetic average particle diameter was measured from the obtained image. At least three arbitrary points were observed. Here, particles whose crystal orientation was tilted by 5° or more were counted as one particle, and the diameter was calculated after spherical approximation. The average value here refers to the 50% particle diameter.
(5) Included Crystal Phases and Abundance Ratios Mirror polishing was performed and the specimens were observed with a scanning electron microscope - electron backscatter diffraction (SEM-EBSD). From the identified crystal phases, the included crystal phases were identified, and their abundance ratios were calculated as area ratios.
(6) Method for Measuring Adhesion Rate Measurement was performed using an ultrasonic flaw detector, and the adhesion rate was calculated.
(7) Measurement of Surface Roughness (Ra) The surface roughness Ra was measured using a surface roughness measuring device manufactured by Mitutoyo.
(8) Analysis of Metal Content After sintering, the surface of the yttrium ingot was ground to a depth of 1 mm or more, and the analytical values of a sample cut out from an arbitrary portion were used as measurement data.

Measurement method: Glow discharge mass spectrometry (GDMS)
Example 1
Yttrium fluoride was subjected to a reduction treatment using Li-Mg to obtain an yttrium ingot containing yttrium oxyfluoride. This ingot was then further subjected to a pore reduction treatment to prepare a yttrium ingot, and measurements were carried out, yielding favorable results. The properties of the yttrium ingot are shown in Table 1. Furthermore, the results for the metal impurities contained are shown in Table 2.

Relative density: 100.3%
Fluorine atom content: 0.6 wt%
Before bonding, the bonding surface was polished to the specified roughness, and then indium solder was quickly applied using an ultrasonic soldering iron. After surface treatment, the target was attached to a backing plate using indium solder. The yttrium target characteristics are shown in Table 1.

(Examples 2 and 3)
Yttrium ingots and sputtering targets of Examples 2 and 3 containing yttrium oxyfluoride were prepared in the same manner as in Example 1, except that the amount of yttrium fluoride added was changed. The characteristics of the yttrium ingots and targets are shown in Table 1.

Example 4
Yttrium powder (3N bulk product, manufactured by Nippon Yttrium Co., Ltd.) and yttrium oxyfluoride powder (Grade: 5LW230, manufactured by Nippon Yttrium Co., Ltd.) were charged into a Cu crucible in a weight ratio of 90:10. The mixture was heated and melted in an arc melting furnace, and then cooled to obtain a yttrium ingot containing yttrium oxyfluoride. The obtained yttrium ingot was processed into a predetermined shape and then attached to a backing plate in the same manner as in Example 1 to produce a sputtering target. The characteristics of the yttrium ingot and target are shown in Table 1.

Example 5
To the yttrium ingot containing yttrium oxyfluoride prepared in Example 4, yttrium oxyfluoride was added so that the final yttrium:yttrium oxyfluoride ratio was 50:50, and the mixture was placed in a Cu crucible. The mixture was heated and melted in an arc melting furnace, and then cooled to obtain a yttrium ingot containing yttrium oxyfluoride. A sputtering target was prepared by mounting the ingot on a backing plate in the same manner as in Example 1. The characteristics of the yttrium ingot and the target are shown in Table 1.

Example 6
To the yttrium ingot containing yttrium oxyfluoride prepared in Example 4, yttrium oxyfluoride was added so that the final yttrium:yttrium oxyfluoride ratio was 40:60, and the mixture was placed in a Cu crucible. The mixture was heated and melted in an arc melting furnace, and then cooled to obtain a yttrium ingot containing yttrium oxyfluoride. A sputtering target was prepared by mounting the ingot on a backing plate in the same manner as in Example 1. The characteristics of the yttrium ingot and the target are shown in Table 1.

(Comparative Example 1)
A fluorine-free yttrium ingot synthesized using a chloride molten salt electrolysis method was prepared and attached to a backing plate to prepare a sputtering target in the same manner as in Example 1. The characteristics of the yttrium ingot and the target are shown in Table 1.

(Comparative Example 2)
An yttrium ingot and a sputtering target of Comparative Example 2 containing yttrium oxyfluoride were obtained in the same manner as in Example 4, except that the mixing ratio of yttrium powder and yttrium oxyfluoride powder in Example 4 was 30:70. The characteristics of the yttrium ingot and the target are shown in Table 1.

The adhesion rates of the sputtering targets of Examples 1 to 6 and Comparative Examples 1 and 2 were measured. The adhesion rates were measured using an ultrasonic imaging inspection device (model: AT LINE, manufactured by Hitachi Construction Machinery Finetech Co., Ltd.) equipped with an ultrasonic flaw detector (model: I3-0508-T). Prior to the measurement, a pseudo sample made of the same material as the sputtering target was used, and the sensitivity was adjusted so that the area of the detected defects matched the area of the pseudo holes in the pseudo sample. The measurement conditions were as follows:

Gain (sound wave strength): 15dB
Measurement pitch: 0.61 mm
Echo level: ≧3.1V
Ultrasonic wave incidence: target side The adhesion rate of the sputtering target was measured using the analysis program attached to the device. The measurement results are shown in Table 1.

The sputtering targets of Examples 1 to 4 were mounted in a DC sputtering apparatus and films were formed on quartz substrates. Annealing was then performed in an oxygen atmosphere to obtain yttrium oxide films. The sputtering conditions were as follows:

Target size: Φ101.6 x 6mmt
Power: 200W
Sputtering gas: Ar
Gas pressure: 0.5 Pa
Film thickness: 5 μm
When the sputtering target of Comparative Example 1 was sputtered, oxidation of the sputtering surface of the yttrium target progressed, and the adhesion rate was low, so that DC discharge was not possible.

When sputtering the sputtering target of Comparative Example 2, DC discharge was not possible due to the high bulk resistivity of the target.

The samples obtained in Examples 1 to 4 were placed in an etching chamber capable of evaluating plasma resistance, and the etching rate was calculated from the change in film thickness before and after plasma exposure. The plasma resistance evaluation conditions were as follows:

Sample size: 20mm x 20mm
Power: 300W
Etching gas: Ar + CF ₄ + O ₂
Treatment time: 4 hours. The results of the plasma resistance evaluation are shown in Table 3.

The yttrium films in Examples 1 to 4 had an extremely low etching rate compared to the quartz substrate, and exhibited excellent plasma resistance.

Claims (9)

A yttrium ingot characterized by a fluorine atom content of 0.05 wt% or more and 10 wt% or less and a relative density of 96% or more. The yttrium ingot according to claim 1, containing yttrium oxyfluoride. The yttrium ingot according to claim 1 or 2, wherein the rare earth element content, RE wt%, satisfies 98≦100−RE<99.999. A yttrium ingot according to any one of claims 1 to 3, having an average particle size (D50) of 100 μm or less. The yttrium ingot according to any one of claims 1 to 4, having a volume resistivity of 1 Ω cm or less . A yttrium sputtering target comprising the yttrium ingot described in any one of claims 1 to 5. The sputtering target described in claim 6, comprising a backing plate and an yttrium ingot. The yttrium sputtering target according to claim 7, wherein the adhesion rate between the backing plate and the yttrium ingot is 90% or more. A method for producing an yttrium oxide film, characterized by sputtering using the sputtering target described in any one of claims 6 to 8.