Platinum-based sputtering target and method for producing same

Abstract

To provide a platinum-base sputtering target formed from platinum or a platinum alloy, wherein the platinum-base sputtering target can keep in-plane uniformity over time, and a manufacturing method thereof.SOLUTION: The present invention relates to a platinum-base sputtering target formed from platinum or a platinum alloy. The platinum-base sputtering target has a characteristic material composition in the thickness-direction cross-section. Specifically, dividing the thickness-direction cross-section into n equal parts (n=5-20) in the thickness direction and defining a determination region as the region made up of (n-2) sections with their both ends removed, when the average particle diameter of the entire determination region is measured together with the average particle diameter in each section of the determination region, the average particle diameter of the entire determination region is less than or equal to 150 μm, and the coefficient of variation, calculated from the average particle diameters of each section in the determination region, is less than or equal to 15%.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a platinum-based sputtering target made of platinum or a platinum alloy, and more particularly to a platinum-based sputtering target that suppresses fluctuations in the thickness of a thin film formed during use over time and enables the formation of a thin film with better in-plane uniformity and greater stability over a long period of time than conventional methods. [Background technology]

[0002] Platinum is a conductive material with good chemical stability, and its application as a thin-film electrode for semiconductor elements such as FeRAM and DRAM is being considered. Although platinum is a non-magnetic material, it is known that perpendicular magnetic anisotropy is exhibited by alloying with a ferromagnetic material or by multi-layering at the nanometer level. Taking advantage of this phenomenon, thin films made of platinum or platinum alloys are expected to be used as a constituent material for the magnetic recording surface of magnetic recording media. In addition, a sputtering method using a platinum-based sputtering target (hereinafter sometimes simply referred to as a target) made of platinum or a platinum alloy is used to form thin-film electrodes, magnetic recording surfaces, etc.

[0003] When forming thin-film electrodes, magnetic recording surfaces, etc. by sputtering, uniformity in the in-plane distribution of film thickness is required. Therefore, uniformity in the sputtering rate within the sputtering surface is also required for the sputtering target. As a means of ensuring uniformity in the sputtering rate of the sputtering target, it is considered preferable to refine the crystal grains on the target surface. Since the sputtering rate varies depending on the crystal orientation, it is ideal to use a sputtering target with a uniform crystal orientation, but it is difficult to manufacture such a target practically and industrially. Therefore, by refining the crystal grains, the difference in sputtering rate due to the difference in orientation can be alleviated, and a stable sputtering rate can be obtained overall.

[0004] It is known that in platinum-based sputtering targets, grain refinement is attempted to ensure in-plane uniformity of the film thickness. For example, Patent Document 1 discloses a platinum sputtering target in which the average grain size is 50 μm or less, and the tolerance of the grain size in the in-plane direction of the target surface and in the thickness direction of the target is 20% or less. This platinum sputtering target is manufactured by performing a primary forging process and a secondary forging process in a predetermined temperature range on an ingot after melt casting, followed by a cross rolling process in a predetermined temperature range, and then a heat treatment. In this manufacturing process, strain is introduced by the forging process and the cross rolling, and the grain refinement is attempted by recrystallization in the subsequent heat treatment. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6514646 specification Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, the above-mentioned in-plane uniformity of the film thickness of the sputtering target is required to be continuous. Usually, a sputtering target is used repeatedly to supply a thin film to a large number of substrates. Even if in-plane uniformity can be achieved in the early stages of use, if the in-plane uniformity deteriorates over the accumulated time of use, stable production of products is impossible. The importance of this in-plane uniformity over time has increased in recent years, and the standards for it have become stricter.

[0007] For example, in magnetic random access memory (MRAM), which is being developed as a next-generation magnetic recording medium, the magnetic tunnel junction element (MTJ element), which is the memory element, is composed of many thin films including a platinum thin film. In order for this memory element with a multilayer structure to perform its expected function, it is essential that each thin film has a designed thickness. When manufacturing a memory element, a multilayer thin film is formed on one substrate and then divided into individual elements. If a target that cannot maintain in-plane uniformity over time is used, the memory element to be manufactured will have a variation in film thickness. The variation in film thickness leads to a variation in electrical properties such as sheet resistance, i.e., a variation in the sheet-resistance product of the MTJ element, and therefore an element that does not meet the specifications will be manufactured. Such an element that does not meet the specifications not only leads to a decrease in product yield, but also affects the reliability of the entire product. Therefore, stricter in-plane uniformity than ever before is required.

[0008] In a situation where such strict standards for in-plane uniformity over time are required, it is difficult for the above-mentioned conventional platinum sputtering targets to fully meet the requirements. According to the studies of the present inventors, it has been confirmed that the above-mentioned platinum-based sputtering targets can form thin films with good in-plane uniformity in the early stages of use, but the film thickness varies during use and the required standards are not met.

[0009] The present invention has been made under the above-mentioned background, and has an object to provide a platinum-based sputtering target made of platinum or a platinum alloy that can maintain in-plane uniformity over time and meet the above-mentioned strict standards, and a method for manufacturing the same. [Means for solving the problem]

[0010] In order to solve the above problems, the present inventors conducted intensive research and focused on the consumption mode of the sputtering target. The sputtering method is a thin film formation method in which sputter particles such as ionized argon particles are accelerated and collided with a target, and the constituent atoms of the sputtered target are deposited on a substrate by momentum exchange at that time. There are several sputtering methods depending on the acceleration method of the sputter particles, but the consumption of the target due to the progress of sputtering is not uniform. For example, in magnetron sputtering, which is currently the mainstream sputtering method, electrons tend to concentrate near a certain orbit due to the surface magnetic field applied to the target, and consumption tends to be faster near the center of the target. Due to the progress of such uneven consumption, a target that was flat at the beginning of use will have uneven thickness with bumps when thin films are continuously formed. Then, when the thickness of the target becomes uneven, the constituent elements of the target will be generated by sputtering from positions with different thicknesses (depths from the initial surface).

[0011] However, even if the sputtering target has an uneven wear pattern as described above, it is considered that there is little effect on the in-plane uniformity if the state of the crystal grains is uniform in the thickness direction. However, according to the consideration and investigation of the present inventors, the uniformity of the state of the crystal grains in the thickness direction is not sufficient in conventional sputtering targets. In particular, it is considered that there is a factor that causes variation in the in-plane uniformity near the center of the sputtering target in the thickness direction.

[0012] From the above-mentioned investigation, the inventors have considered that the cause of the deterioration of the in-plane uniformity over time of conventional platinum-based sputtering targets is the uniformity of the state of the crystal grains in the thickness direction in addition to the consumption mode of the target. In this regard, there have been some studies on the state of the crystal grains in the thickness direction of sputtering targets. For example, the above-mentioned Patent Document 1 also specifies the tolerance in the thickness direction for the crystal grain size of the sputtering target. However, the fact that the in-plane uniformity of such a sputtering target can deteriorate over time shows that the regulations in the conventional technology cannot cope with this.

[0013] Therefore, the present inventors have investigated a manufacturing method different from the conventional one in order to find a sputtering target having stable sputtering characteristics even when used for a long time. As a result, they have found a sputtering target in which the crystal grain size in the thickness direction is in a strict state, and have arrived at the present invention.

[0014] That is, the present invention is a platinum-based sputtering target made of platinum or a platinum alloy, characterized in that a cross section along the thickness direction is divided into n equal parts (n = 5 to 20) along the thickness direction, and a region consisting of (n-2) parts excluding both ends is set as a judgment region, and when the average grain size of each part and the average grain size of the entire judgment region are measured for the judgment region, the average grain size of the entire judgment region is 150 μm or less, and the coefficient of variation calculated from the average grain size of each part of the judgment region is 15% or less.

[0015] As described above, the platinum-based sputtering target according to the present invention specifies the material structure in the thickness direction cross section. Specifically, a predetermined region in the thickness direction cross section is set as a judgment region for judging the in-plane uniformity, and the average grain size of the entire region is specified. In addition, the cross section is divided into a plurality of sections, the average grain size of each section is measured, and the variation coefficient obtained from the measurement is strictly limited. Each configuration of the present invention will be described below.

[0016] The platinum-based sputtering target according to the present invention is composed of platinum (pure platinum) or a platinum alloy. The platinum alloy may be one containing any of Pd (palladium), Rh (rhodium), Ir (iridium), Ru (ruthenium), Co (cobalt), Mn (manganese), Ni (nickel), and W (tungsten) as an additive element. The platinum alloy may be one containing 0.1 atomic % or more and 30 atomic % or less of the additive element.

[0017] In the present invention, the thickness direction is a direction approximately perpendicular to the sputtering surface. The sputtering surface is a surface on which inert gas ions mainly collide to release atoms that constitute the target. The cross section is an arbitrary cut surface in the thickness direction. In a target that has undergone a rolling process, the cross section includes a rolling cross section (RD) that is parallel to the rolling direction and a rolling perpendicular cross section (TD) that is perpendicular to the rolling direction. However, in the present invention, the parallel to the rolling direction and the perpendicular to the rolling direction include a tolerance of ±20°.

[0018] Furthermore, the rolling direction is the rolling direction in the final rolling process. As described later, the platinum target according to the present invention may employ cross rolling in the rolling process performed after forging. Cross rolling is a method of rolling the material in the width direction (vertical direction) in addition to the length direction (longitudinal direction). Therefore, for example, if the rolling direction in the final rolling is the length direction, the cross section parallel to the length direction is the rolling cross section (RD), and the rolling perpendicular cross section perpendicular to the length direction is the rolling cross section (TD). In the present invention, both the rolling cross section and the rolling perpendicular cross section are required to satisfy the above-mentioned criteria for the overall average grain size and the coefficient of variation in each judgment region.

[0019] (A) Structure of the platinum-based sputtering target according to the present invention (i) Judgment area In the present invention, the cross section of the target is divided into n equal parts (n=5-20) along the thickness direction, and the (n-2) parts excluding both ends are set as judgment regions, and the average grain size value and the coefficient of variation in these parts are specified. Then, the in-plane uniformity of the target over time is judged based on these values. The reason for setting the number of divisions to 5 or more and 20 or less is that if the number of divisions is less than 5, each division becomes too wide and statistical reliability is poor. In this case, even if the coefficient of variation of the average grain size of each division of the judgment region meets the conditions of the present invention, it is difficult to say that the grain size variation is suppressed. Therefore, it may not be a target that can maintain in-plane uniformity over time. Also, even if the number of divisions exceeds 20, the area of ​​each division becomes too small, the number of crystal grains contained in each division becomes small, and statistical reliability is low. Therefore, the number of divisions n is set to 5 or more and 20 or less.

[0020] As an index for setting the number of divisions n to ensure statistical reliability, it is preferable that each division contains about 150 to 200 crystal grains. Specifically, since the overall average grain size in the cross section of the target of the present invention is 150 μm or less (preferably 40 μm or less) as described later, it is preferable to set the number of divisions based on the average grain size and the plate thickness of the target.

[0021] The reason why the two end sections are excluded from the judgment region in the present invention is that the end section on the front side (sputtering surface side) of the target is used in the early stage of the sputtering process and is unnecessary when considering the in-plane uniformity over time. On the other hand, the end section on the back side is excluded from the judgment region because this region is not used. Also, since a sputtering target is usually used with a backing plate bonded to the back side, it is not necessary to consider the section near the back side.

[0022] (ii) Average particle size across the entire test area In the platinum-based sputtering target according to the present invention, the average grain size in the judgment region is taken as the average grain size of the entire target, and this value is 150 μm or less. The present inventors also recognize the usefulness of fine grains in order to achieve stable sputtering characteristics in sputtering targets. The sputtering target of the present invention is composed of fine grains with an average grain size of 150 μm or less. This average grain size is preferably 40 μm or less.

[0023] There are no particular limitations on the determination of the crystal grains (determination of the grain boundaries) in the target cross section, and the measurement and calculation of the crystal grain size and the average grain size. For example, the target may be cut at an arbitrary cross section, appropriately etched, and the structure observed, and the grain size of all the crystal grains in the observation area may be measured and their average value may be calculated. In addition, a plurality of crystal grains in the observation area may be arbitrarily extracted, and their grain sizes may be measured and their average value may be calculated. In these grain size measurements, the average value of the long diameter and the short diameter may be adopted as a method of calculating the grain size. Furthermore, the line segment method is also known as a method of measuring the average grain size. In the line segment method, a plurality of lines are arbitrarily drawn on the structure observation result (photograph). Then, the average grain size in the line segment is calculated from the number of points (intersections) where the line segment and the grain boundary intersect and the length of the line, and this is performed for each line segment, and the overall average value is taken as the crystal grain size. The line segment method is a method that can relatively easily calculate the average grain size.

[0024] The average grain size can also be measured by cutting the cross section of the target and then using appropriate analytical tools and image processing. A suitable analytical tool is electron backscatter diffraction (EBSD). EBSD is an analytical method that can quickly obtain information on the orientation of crystal grains. Then, by processing with appropriate image analysis software, it is possible to identify the grain boundaries, measure the grain size value, and calculate the average grain size.

[0025] As described above, there are several known methods for measuring the average grain size of a target, and there is no particular limitation. However, it is preferable to use a unified measurement method for measuring the average grain size of the entire cross section and the average grain size of each judgment region.

[0026] In the present invention, the average grain size in the surface direction of the surface (sputtering surface) of the platinum-based sputtering target is not particularly specified. However, as described later, in the manufacturing process of the target in the present invention, a process is performed in which the grains are refined by recrystallization throughout the material. Therefore, the material structure with refined grains is observed on the target surface as well. Therefore, it is preferable that the average grain size on the target surface is also 150 μm or less, preferably 40 μm or less.

[0027] In the present invention, the target is divided into n equal parts in the thickness direction, and the two end sections are excluded from the judgment region, so the average grain size of the crystal grains included in these sections is not taken into consideration. However, it is of course acceptable for the average grain size throughout the entire thickness of the target, including the two end sections, to be 150 μm or less, preferably 40 μm or less.

[0028] (iii) Coefficient of variation of the average particle size in the judgment region In the present invention, the coefficient of variation (CV) of the average particle size of each section included in the judgment region is calculated based on the average particle size in each section within the judgment region. The coefficient of variation is calculated by measuring the average particle size for each section within the judgment region, calculating the standard deviation, and dividing the standard deviation by the average particle size of the entire judgment region.

[0029] The specific procedure for measuring the average particle size and coefficient of variation for each section is to divide the cross section of the platinum-based sputtering target into n equal parts, set judgment regions consisting of (n-2) sections excluding both ends, and observe and measure the average particle size in each section. Then, the standard deviation, which is the square root of the variance (unbiased variance) of the average particle size for each section, is calculated as follows. The coefficient of variation of the average particle size for each section is calculated by dividing this standard deviation by the overall average particle size.

[0030]

number

[0031]

number

[0032] The platinum-based sputtering target according to the present invention is required to have a coefficient of variation calculated from the average grain size of (n-2) sections included in the judgment region of the thickness direction cross section of 15% or less. If the coefficient of variation exceeds 15%, it will contain crystal grains that are not preferable for ensuring in-plane uniformity over time, and the problem of the present invention cannot be solved. The standard for this coefficient of variation is preferably 10% or less, and more preferably 7% or less.

[0033] As described above, the platinum-based sputtering target according to the present invention has a strict regulation for the average grain size of the crystal grains in the thickness direction cross section, thereby ensuring the in-plane uniformity over time during the target use process. Meanwhile, according to the study by the present inventors, in order for the target to exhibit stable sputtering characteristics over time, it is preferable to regulate the shape of the crystal grains as well as regulate the refinement of the crystal grain size and the coefficient of variation of the average grain size.

[0034] That is, in the platinum-based sputtering target according to the present invention, in the above-mentioned judgment region, the proportion of crystal grains having an aspect ratio of 3 or more based on the number of particles is preferably 20% or less, and the proportion of crystal grains having an aspect ratio of 5 or more based on the number of particles is preferably 9% or less. The aspect ratio in the present invention is calculated as the ratio of the maximum diameter to the minimum diameter (maximum diameter / minimum diameter) for each crystal grain. Therefore, according to the criteria of the present invention, the aspect ratio is calculated to be 1 or more, and the larger the value, the more flat the crystal grains are. In the present invention, in order to maintain the in-plane uniformity over time during film formation, it is preferable that the shape of the crystal grains in the cross-sectional structure is also uniform. In particular, it is preferable that the proportion of flat crystal grains having an aspect ratio of 3 or more and 5 or more is low. Therefore, the above conditions are preferable. In addition, the aspect ratio of the crystal grains is more preferably 18% or less, and even more preferably 7% or less, based on the number of particles, for the proportion of crystal grains having an aspect ratio of 3 or more. The proportion of crystal grains having an aspect ratio of 5 or more based on the number of grains is more preferably 3% or less, and even more preferably 1% or less.

[0035] The aspect ratio of crystal grains can be measured by observing the cross-sectional structure and measuring the dimensions of each crystal grain from the observed photographs and images, in the same way as when measuring the average grain size. Image processing and software can also be used. The ratio based on the number of grains is the ratio based on the number of grains within the observation area that are the subject of the aspect ratio measurement. The crystal grains that are the subject of the aspect ratio measurement may be selected from multiple grains within the observation area, or all of the crystal grains within the observation area may be measured.

[0036] (iv) Other components (purity, relative density) The platinum-based sputtering target according to the present invention is preferably made of high-purity platinum or platinum alloy in order to ensure the quality of the electrode film or magnetic film. Specifically, the platinum sputtering target made of pure platinum is preferably one having a platinum purity of 99.99% by mass or more. In addition, the platinum alloy sputtering target made of the above-mentioned platinum alloy is preferably one having a total purity of platinum and any one of the additive elements Pd, Rh, Ir, Ru, Co, Mn, Ni, and W of 99.9% by mass or more. The upper limit of the purity of platinum or platinum alloy is preferably 100% by mass, but considering inevitable impurities, it is practical to set it to 99.999% by mass or less.

[0037] Inevitable impurities in a platinum sputtering target made of pure platinum include elements such as Au, Ag, Pd, Rh, Ir, Ru, Os, Al, As, B, Bi, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Sb, Si, Sn, Ti, Zn, and W, as well as gas components such as O (oxygen), N (nitrogen), C (carbon), and S (sulfur). In addition, in a platinum alloy sputtering target made of a platinum alloy, the gas components and elements of the above element group other than the additive elements of the platinum alloy may be unavoidable impurities. The total content of these unavoidable impurities is preferably 100 ppm or less.

[0038] Furthermore, as described below, the platinum-based sputtering target according to the present invention is a material manufactured by a melt casting method. Platinum-based sputtering targets are also known that are manufactured by a so-called powder metallurgy method and are composed of a sintered body of platinum powder or platinum alloy powder, but the present invention is distinguished from these sintered targets. Specifically, the platinum-based sputtering target according to the present invention has a relative density of 99.5% or more when the theoretical density of platinum or platinum alloy of the same composition is used as the standard.

[0039] There is no particular limitation on the shape and dimensions of the platinum-based sputtering target according to the present invention. The shape is generally a circular or rectangular plate shape, but is not particularly limited to these. There is also no particular limitation on the dimensions, such as the planar dimensions (diameter, long side, short side) and thickness.

[0040] (B) Method for producing a platinum-based sputtering target according to the present invention Next, a method for manufacturing a platinum-based sputtering target according to the present invention will be described. The platinum-based sputtering target according to the present invention is basically manufactured by the same manufacturing process as that of a conventional target. As a conventional manufacturing process for a platinum-based sputtering target, it is known that an ingot is manufactured by melt casting, which is then forged to manufacture an ingot, which is then rolled to manufacture a rolled material close to the product dimensions, and the rolled material is then heat-treated. The final heat treatment process is a process for causing recrystallization, and is a process for adjusting the material structure by refining crystal grains using lattice defects such as dislocations introduced by the processing history up to that point as a driving force.

[0041] The manufacturing process of the platinum-based sputtering target according to the present invention also includes the above-mentioned melt casting step, forging step, rolling step, and recrystallization heat treatment step. However, in the present invention, in the material structure of the cross section in the thickness direction, it is necessary to adjust the distribution of the average grain size more strictly than before, and it is also preferable to optimize the shape (aspect ratio) of the crystal grains.

[0042] According to the study by the inventors, it is difficult to form the material structure defined in the present invention by the conventional manufacturing method. In particular, it is difficult to provide a coefficient of variation of the average grain size in the region near the center of the plate thickness and to make the aspect ratio of the crystal grains favorable. The reason for this is considered to be that the cast structure cannot be completely destroyed to the center of the ingot in the forging process after melt casting in the conventional process. In an ingot in which even a small amount of cast structure remains in the center, dislocation introduction by the rolling process is also insufficient. In such a state, it is difficult to obtain a homogeneous material structure in the thickness direction even if a recrystallization heat treatment is performed.

[0043] Therefore, in order to ensure uniform progress of recrystallization during heat treatment, particularly uniformity in the thickness direction, the inventors decided to forge the ingot so that no cast structure would remain in the center of the ingot, and to heat the ingot before rolling to make the material structure of the ingot homogenous overall, before carrying out heat treatment.

[0044] The method according to the present invention, including this homogenization heat treatment, is a method for producing a platinum-based sputtering target, comprising a forging process step of forging an ingot made of platinum or a platinum alloy after melt casting at least once to produce an ingot, a rolling process step of rolling the ingot at least once to produce a rolled material, and a recrystallization heat treatment step of heat treating the rolled material, in which a homogenization heat treatment is performed after the forging process step and before the rolling process step, in which the ingot is heated at a temperature of 850° C. to 950° C., and further, the heating temperature of the rolled material in the recrystallization heat treatment step is 600° C. to 700° C. Each step of this production method is described below.

[0045] (i) Melt casting process The melt casting process is a process in which the raw materials platinum metal and additive element metal are melted, cast into a mold, and cooled to obtain an ingot made of platinum or a platinum alloy. There is no particular difference in this process from the conventional technology. The raw metal is of high purity according to the purity of the product to be manufactured. The raw metal is heated in a high-frequency melting furnace, an electric melting furnace, or a plasma melting furnace, and is preferably melted in an inert gas atmosphere or a vacuum atmosphere. The mold used is a square or round mold in consideration of the product shape. Cooling after casting the molten metal may be performed at a slow cooling rate by furnace cooling or air cooling. The ingot made of platinum or a platinum alloy after melt casting may be cut or machined for the purpose of adjusting the dimensions or removing inhomogeneous parts that may be present at the end. There is no limitation on the shape of the ingot produced here, and it may be any of a rectangular parallelepiped shape, a cube shape, and a cylindrical shape.

[0046] (ii) Forging process Forging is a process in which an ingot made of platinum or a platinum alloy is compressed and struck to be processed into an ingot of a shape and size that is easy to process in the rolling process described later. The forging process also has an important purpose of destroying the cast structure of the ingot. The forging process basically applies a processing method in the conventional technology. The processing temperature in the forging process may be a temperature that allows deformation for forming and forging the ingot. In the present invention, since there is a homogenization heat treatment process described later, no temperature condition is required to transform the material structure in the forging process. The processing temperature in the forging process can be set to 800°C to 1300°C. In addition, the forging process in this process is performed at least once, and can be performed intermittently multiple times as necessary.

[0047] In the present invention, the destruction of the cast structure of the ingot is emphasized as the purpose of the forging process. In particular, the destruction of the cast structure in the center of the ingot is emphasized. For this purpose, in the forging process of the present invention, it is preferable to form the ingot while forging it sufficiently. As a specific index, it is preferable to forge until the dimension in the direction indicating the maximum dimension of the ingot becomes 50% or less. For example, when forging a rectangular ingot, it is preferable to forge until the maximum side of the ingot becomes 50% or less. This index of processed dimensions is applied at the end stage of the forging process (the stage where the homogenization heat treatment, which is the next process, is performed). When the forging process is completed in one forging process, it is judged by the dimension at the end stage of the process. When multiple forging processes are performed, it is judged by the dimension at the end stage of the final forging process. It is preferable that the lower limit of the maximum side of the ingot by this forging process is 30% or more. In the forging process, the cast structure should be destroyed by processing as much as possible. However, since it is not preferable that the temperature of the ingot becomes too low during the forging process, the lower limit of the maximum side of the ingot may be set to 40% or more. If the ingot is forged until the maximum side of the ingot is 40% or more and 50% or less, the destruction of the cast structure, which is the purpose of the forging process, is possible. In addition, the ingot made of platinum or platinum alloy obtained by the forging process described above may be cut or surface-machined as necessary.

[0048] (iii) Homogenization heat treatment process As described above, the present invention is characterized in that the ingot after the forging process is heat-treated before the subsequent rolling process. In the conventional manufacturing process of platinum-based sputtering targets, the heat treatment at high temperature described below is not performed before rolling. This is because platinum is relatively soft and has good workability compared to other precious metals, and therefore it is not necessary to use a high temperature during rolling. However, in the temperature control in the conventional manufacturing process, the cast structure remains in the ingot, and even if rolling and recrystallization heat treatment are performed in this state, sufficient recrystallization does not occur in the thickness direction, and the distribution of the average crystal grain size cannot be strictly adjusted.

[0049] In the manufacturing method of the present invention, in order to ensure that the effect of the recrystallization heat treatment extends to the entire target, the ingot before the rolling process is heat treated at a high temperature to once become a homogenized material free of cast structure and distortion. This allows homogenous recrystallization to occur based on lattice defects such as dislocations introduced in the subsequent rolling process, making it possible to achieve a favorable distribution of the average grain size in the thickness direction.

[0050] The homogenization heat treatment is performed by heating the ingot at a temperature of 850°C or more and 900°C or less. If the temperature is less than 850°C, it is difficult to obtain the homogenized material described above. If the temperature exceeds 950°C, the distortion in the material is sufficiently released, but the crystal grains become coarse, which is thought to affect the final product characteristics. The heating time of the homogenization heat treatment is preferably 60 minutes or more and 120 minutes or less. The treatment time is controlled by the treatment temperature and the plate thickness of the ingot, but at least 60 minutes or more of heating is required to complete homogenization. On the other hand, even if an excessively long heat treatment is performed, there is no difference in the homogenization effect, so the heating time is set to 120 minutes or less in consideration of manufacturing efficiency.

[0051] (iv) Rolling process The rolling process is a process for processing an ingot made of platinum or platinum alloy after forging into a platinum sheet material of the size and shape required to obtain the final size of the product. In addition, it is a process for introducing lattice defects such as dislocations, which are the driving force of recrystallization for grain refinement, into the homogenized ingot. Therefore, although the rolling process is also an important process, the rolling process itself can be applied under the same conditions as those for processing performed on conventional platinum-based sputtering targets. The rolling process is usually performed by cold rolling, and the temperature of the rolled material is processed at 20°C to 200°C. The rolling process is performed at least once, and can be repeated as necessary. As for the rolling direction, unidirectional rolling may be used, but cross rolling is preferably applied. In the rolling process, various types of rolling are performed according to the respective purposes, such as width rolling, intermediate rolling, finish rolling, and flattening rolling. In these rolling processes, the rolling direction and processing rate appropriate for each rolling process are set. The processing rate of the rolling process for the ingot after the forging process is preferably 90% or more and 95% or less. For example, the plate thickness after the final rolling process is preferably 10% or less and 5% or more of the thickness of the ingot after the forging process. The reason for setting the processing rate of 90% or more in this way is to promote grain refinement by the subsequent recrystallization by introducing a large amount of processing strain.

[0052] (v) Recrystallization heat treatment process By heat treating the rolled material in which lattice defects have been introduced by the rolling process, the grains are refined by recrystallization. In particular, in the present invention, the rolling process is carried out after the homogeneous heat treatment described above, and lattice defects are uniformly introduced throughout the material. Then, the recrystallization heat treatment causes homogeneous grain refinement in the thickness direction, and grains with little variation in average grain size are generated. The aspect ratio of the grains in the thickness direction is also favorable.

[0053] The heat treatment conditions for the recrystallization heat treatment step are a heating temperature of 600°C or more and 700°C or less. If the temperature is less than 600°C, sufficient recrystallization is unlikely to occur. On the other hand, if the heat treatment is performed at a temperature exceeding 700°C, the crystal grains may become coarse, and the overall average crystal grain size may deviate from the range of the present invention. In addition, the coefficient of variation of the average crystal grain size in the judgment region may also increase.

[0054] The heating time of the recrystallization heat treatment is preferably 60 minutes or more and 120 minutes or less. In the present invention, it is necessary to cause sufficient recrystallization in the thickness direction of the target. In particular, sufficient heating is necessary to cause recrystallization up to the center of the plate thickness to optimize the average crystal grain size and the aspect ratio. Therefore, the lower limit of the treatment time is set to 60 minutes. On the other hand, if the heat treatment is performed for more than 120 minutes, the effect is small and there is a risk of partial coarsening.

[0055] (vi) Optional processing steps By the above-mentioned recrystallization heat treatment process, a platinum-based sputtering target having the material structure specified in the present invention can be manufactured. However, as post-processing steps, flattening, facing, cutting, etc. may be performed. Effect of the Invention

[0056] As described above, the platinum-based sputtering target according to the present invention has in-plane uniformity over time due to the stricter regulation of the average grain size in the thickness direction cross section than in the past. According to the present invention, the in-plane uniformity at the beginning of use can be maintained, and a platinum thin film or platinum alloy thin film with a constant thickness can be stably produced over a long period of time. [Brief description of the drawings]

[0057] [Figure 1] Photographs showing the crystal structure of a platinum ingot before homogenization heat treatment (after the forging process) and after the homogenization heat treatment in the manufacturing process of this embodiment. [Diagram 2] FIG. 2 is a diagram for explaining the positions at which samples were taken in each of the Examples and Comparative Examples. [Diagram 3]FIG. 2 is a diagram showing the material structure (EBSD) of the platinum sputtering target of Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] Hereinafter, an embodiment of the present invention will be described. In this embodiment, a platinum sputtering target made of pure platinum was manufactured as a platinum-based sputtering target. Platinum sputtering targets were manufactured under various conditions with reference to the above manufacturing process, and the material structure of the cross section in the thickness direction was observed, and the average grain size and variation coefficient of the crystal grains were measured. Furthermore, a platinum thin film was manufactured using a sputtering device, and the in-plane uniformity of the thin film was evaluated.

[0059] Example 1 [Melt casting process / forging process] Platinum with a purity of 99.99% was melted in a high-frequency plasma melting furnace and cast into a copper mold to produce a platinum ingot (dimensions: 30 mm (thickness) x 75 mm (width) x 205 mm (length)). The ends were cut to obtain a platinum ingot of 30 mm (thickness) x 75 mm (width) x 173 mm (length). This platinum ingot was heated to 1300°C for 30 minutes, and then forged several times in succession to obtain a platinum ingot of 60 mm (thickness) x 78 mm (width) x 82 mm (length). Through the above forging process, the longest side (173 mm) of the platinum ingot was processed to 47% (82 mm). The surface was then machined and molded into a platinum ingot of 55 mm (thickness) x 78 mm (width) x 82 mm (length).

[0060] [Homogenization heat treatment process] Then, the platinum ingot after the forging process was subjected to homogenized heat treatment. In the homogenized heat treatment process, the platinum ingot was heated at 900°C for 60 minutes in an electric furnace in the atmosphere. After heating, the platinum ingot was cooled in the furnace to prepare a platinum ingot for the rolling process. In order to explain the change in the material structure due to the homogenized heat treatment, photographs of the material structure of the platinum ingot before and after the homogenized heat treatment are shown in Figure 1. This material structure observation was obtained by etching the side surface of each target and then observing it with a metal microscope. As can be seen from Figure 1, the material structure of the platinum ingot after the homogenized heat treatment changes significantly from the material structure after the forging process. It can be confirmed that the crystal structure of the platinum ingot is homogenized by the homogenized heat treatment.

[0061] [Rolling process] In the rolling process, the platinum ingot was rolled in the width and length directions until it had a size that could be cut out as a target product. First, width-rolling was performed to make it 16.4 mm (thickness) x 270 mm (width) x 85 mm (length). Then, intermediate rolling was performed to make it 6.77 mm (thickness) x 273 mm (width) x 197 mm (length), and then finish rolling to make it 3.1 mm (thickness) x 273 mm (width) x 427 mm (length). In each rolling process, the workpiece was cooled to 20°C before processing. In addition, a rolled material with a thickness of 3.1 mm was produced from a platinum ingot with a thickness of 55 mm by this rolling process, so the processing rate in the rolling process was about 94%. The platinum plate material after the rolling process was flattened with a roller and then cut to be used as a rolled material for the recrystallization heat treatment process.

[0062] [Recrystallization heat treatment process] In the recrystallization heat treatment process, the platinum rolled material cut after the rolling process was heated at 650°C for 60 minutes. Then, it was flattened again with a roller. Then, a platinum sputtering target was manufactured.

[0063] Example 2 In this example, a platinum ingot was produced by enlarging the mold in the melt casting process to produce a platinum ingot larger than that in Example 1, and forging was performed until the platinum ingot had the same dimensions as Example 1. That is, in Example 2, a platinum sputtering target was produced by performing forging more thoroughly than in Example 1. In the forging process of this example, the platinum ingot was processed until the longest side was 30%. The homogenization heat treatment, rolling process, and recrystallization heat treatment after the forging process were the same as those in Example 1.

[0064] Example 3 In this embodiment, two-stage forging is performed intermittently in the forging process. The same platinum ingot as in Example 1 was manufactured, heated to 1300°C for 30 minutes, and forged to 37 mm (thickness) x 78 mm (width) x 82 mm (length), and then processing was temporarily suspended. The ingot was then heated again to 1300°C for 30 minutes and forged to 60 mm (thickness) x 78 mm (width) x 82 mm (length). The subsequent homogenization heat treatment, rolling processing, and recrystallization heat treatment were the same as in Example 1.

[0065] Comparative Example 1 As a comparative example to the above-mentioned embodiment, a target was manufactured by performing a rolling process and a recrystallization heat treatment without performing a homogenization heat treatment after the forging process. The same as in Example 1 except that the homogenization heat treatment was not performed.

[0066] Comparative Example 2 The heating temperature in the recrystallization heat treatment step was set to a higher temperature than in Example 1. A platinum ingot was produced in the same manner as in Example 1, and subjected to a homogenization heat treatment and rolling processing, and then heated at 900°C for 60 minutes to perform a recrystallization heat treatment, thereby producing a platinum sputtering target.

[0067] Comparative Example 3 In this comparative example, a platinum sputtering target sample was manufactured without carrying out a recrystallization heat treatment process. In the same manner as in Example 1, a platinum sputtering target was manufactured from a platinum plate material that had been subjected to a melt casting process, a forging process, a homogenization heat treatment process, and a rolling process, without carrying out a heat treatment.

[0068] Table 1 shows a summary of the manufacturing conditions for the platinum sputtering targets of Examples 1 to 3 and Comparative Examples 1 to 3.

[0069] [Table 1]

[0070] In this embodiment, first, the average crystal grain size in the thickness direction cross section was measured while observing the material structure in the thickness direction cross section of the platinum sputtering targets of each Example and Comparative Example. In this study, as shown in FIG. 2, when the platinum plate material after cutting was subjected to recrystallization heat treatment to cut out the platinum sputtering target, two samples were cut out from near the target and evaluated. For each sample, two samples (No. 1 and No. 3) were cut out from a portion near the center in the length direction of the target and a portion near the side. For each sample, a rolling cross section (RD) and a rolling perpendicular cross section (TD) were set, and the samples were cut and embedded in resin so that each surface could be measured (sample dimensions: 5 mm x 10 mm). The resin-embedded samples were manually polished and vibration-polished, and then pretreated by ion milling.

[0071] Then, EBSD was used to analyze the rolled cross section (RD) and the rolled perpendicular cross section (TD) of each sample. The grain size and other parameters were measured based on the profile of each surface obtained by EBSD. In this case, when the angle difference between adjacent grains in the EBSD results was 6° or more, it was determined to be a grain boundary, and all grains within the observation area were identified. The identified grains were then fitted with an ellipse to measure the grain size and aspect ratio of each grain within the observation area. Image processing software (HKL CHANNEL5, manufactured by Oxford Instruments) was used for the above analysis.

[0072] The judgment region for each sample's thickness direction cross section (rolling cross section (RD) and rolling perpendicular cross section (TD)) was set by dividing each cross section into 10 equal parts, and the 8th section excluding both ends was set as the judgment region. The average grain size of each section and the average grain size of the entire judgment region were then measured. Furthermore, the standard deviation of the average grain size of each section was calculated to calculate the coefficient of variation in the judgment region.

[0073] For the platinum targets of each of the examples and comparative examples, the crystal grain size on the surface was also measured. Furthermore, the hardness of each platinum target was also measured in the thickness direction cross section. The hardness was measured using a Vickers hardness tester (load: 0.1 kgf), and the average value of multiple measurement points was calculated.

[0074] The measurement results of the overall average grain size, the coefficient of variation, and the proportion of crystal grains with aspect ratios of 3 and 5 or more in the judgment region of the thickness direction cross section of each of the above samples are shown in Table 2. In addition, as an example of the material structure of the thickness direction cross section of the target analyzed by EBSD, the material structure of the rolled cross section of Sample No. 1 in Example 1 is shown in Figure 3.

[0075] [Table 2]

[0076] From Table 2, in the targets of each embodiment, the overall average grain size in the judgment region of the thickness direction cross section is 150 μm or less. The coefficient of variation of the average grain size in the judgment region is 15% or less. This standard is met in both the rolling cross section (RD) and the rolling perpendicular cross section (TD). This is the same for the two samples (No. 1 and No. 3). It is also confirmed that this result is not dependent on the dimensions of the platinum ingot produced by the melt casting process or the number of forging processes in the forging process.

[0077] On the other hand, in the target manufactured without homogenization heat treatment in Comparative Example 1, the variation coefficient of the average grain size in the sample No. 3 exceeded 15%. The difference between the manufacturing process of Comparative Example 1 and Example 1 is the presence or absence of homogenization heat treatment, and it was confirmed that the homogenization heat treatment improves the uniformity of the crystal grains in the thickness direction. In addition, when the temperature of the recrystallization heat treatment in Comparative Example 2 was set to a high temperature, the average grain size was coarsened overall in the thickness direction. And the variation coefficient was also outside the specified range in either the rolled cross section or the rolled perpendicular cross section. In addition, in Comparative Example 3, the grain boundaries could not be recognized during the analysis and analysis by EBSD. It is considered that Comparative Example 3 is composed of a rolled structure because the final recrystallization heat treatment was not performed.

[0078] In addition, the measurement results of the aspect ratio of the crystal grains in the thickness direction cross section of each platinum sputtering target showed good values ​​in Comparative Examples 1 and 2. Rather, it seems that the proportion of crystal grains with an aspect ratio of 3 or more is higher in Examples 1 to 3. From this, it is considered that crystal grains with a high aspect ratio tend to be formed in the platinum sputtering target manufactured by the manufacturing process characterized by the homogenization heat treatment of the present invention. The cause is not clear. Therefore, it is presumed that it may be preferable to take into consideration the aspect ratio when the regulation regarding the average grain size in the thickness direction cross section in the present invention (variation coefficient in the judgment region of 15% or less) is satisfied. However, considering the results of the film formation test described later, it can be said that the in-plane uniformity of the film thickness can be ensured by setting the proportion of crystal grains with an aspect ratio of 3 or more to 20% or less (more preferably 18% or less) and the proportion of crystal grains with an aspect ratio of 5 or more to 3% or less (more preferably 1% or less) as in each example of the present application.

[0079] Next, the film formation characteristics, particularly the in-plane uniformity over time, of the platinum sputtering targets of Examples 1 to 3 and Comparative Examples 2 and 3 were evaluated. In this evaluation test, each target and a substrate (a 12-inch silicon wafer) were set in a magnetron sputtering device, and after evacuating the device, an inert gas was introduced. Sputtering was performed under two conditions (Condition 1 (low sputtering power) and Condition 2 (high sputtering power)) depending on the sputtering rate.

[0080] The wear depth of the target due to the film formation is estimated while monitoring the sputtering rate. Film formation was performed at each stage of the target's initial use (wear depth of about 0.2 mm), middle use (wear depth of about 0.8 mm), and late use (wear depth of about 1.5 mm). Then, for the platinum thin film of the wafer on which the film was formed at each stage, about 30 to 50 points on the wafer were uniformly sampled, and the film thickness and sheet resistance value of each point were measured. The film thickness was measured by fluorescent X-ray analysis. The sheet resistance was measured by a four-terminal measurement method. The average value and standard deviation were calculated from these values, and the value obtained by dividing the standard deviation by the average value was used as an index of variation. The evaluation results are shown in Table 3. In the evaluation of this embodiment, a case in which the variation in both the film thickness and the sheet resistance is 3.0% or less was judged to be acceptable, and if no clear improvement was observed with respect to this standard, the film formation test was discontinued.

[0081] [Table 3]

[0082] It is clear from Table 3 that the platinum sputtering targets of Examples 1 to 3 enable stable deposition of a film with good in-plane uniformity from the early stage of use to the later stage of use.

[0083] In contrast, in each of the comparative examples in which the coefficient of variation of the average grain size in the thickness cross section is outside the standard, the in-plane uniformity of the film thickness was poor from the beginning of use, and did not change even in the middle of use. Specifically, the target of Comparative Example 3, which was manufactured without undergoing a recrystallization heat treatment process after the rolling process, was in a state where the grain boundaries could not be recognized, and had the poorest in-plane uniformity. In addition, the target of Comparative Example 2, in which the heating temperature in the recrystallization heat treatment process was high, had a large average grain size of the crystal grains and could not clear the in-plane uniformity standard. Furthermore, the target of Comparative Example 1, which was manufactured without undergoing a homogenization heat treatment, had better in-plane uniformity than Comparative Examples 2 and 3, but it was significantly reduced in the later stages of use, so it was not possible to suppress the change in in-plane uniformity over time.

[0084] From the above film formation evaluation, it was confirmed that controlling the material structure in the thickness cross section of the target is important to improve the in-plane uniformity in the film formation process. And, to achieve this, it was confirmed that in the manufacturing process of platinum-based sputtering targets, the application of homogenization heat treatment and appropriate temperature setting in the recrystallization heat treatment process are necessary.

[0085] In addition to the above platinum sputtering target, a sputtering target made of a platinum alloy in which any one of palladium, rhodium, iridium, ruthenium, cobalt, manganese, nickel, and tungsten is added to platinum as an additive element in a composition range of 1 atomic % to 30 atomic % is also useful. The additive element concentration of these platinum alloys is in the composition range of the solid solubility limit, so alloying is relatively easy. Even if any one of these additive elements is added in a composition range of 1 atomic % to 30 atomic %, the platinum alloy has similar workability to the platinum target, so the manufacturing method according to the present invention is applicable. As described above, the manufacturing method according to the present invention imparts good in-plane uniformity and effectiveness in suppressing changes in in-plane uniformity over time to the sputtering target. The platinum alloy sputtering target produced by this manufacturing method is also effective in suppressing in-plane uniformity and changes over time. [Industrial Applicability]

[0086] The platinum-based sputtering target according to the present invention can stably produce a platinum thin film or a platinum alloy thin film having good in-plane uniformity in the film formation process. This is because a strict regulation is set for the average grain size of the crystal grains in the thickness direction cross section. The present invention is useful for thin film electrodes of semiconductor devices and recording films of magnetic recording media, which require high-quality platinum thin films or platinum alloy thin films.

Claims

1. In a platinum-based sputtering target made of platinum or a platinum alloy, A cross section along the thickness direction is divided into n equal parts (n=5 to 20) along the thickness direction, and a region consisting of (n-2) parts excluding both ends is set as a judgment region. The average particle size of each part is measured for the judgment region, and the average particle size of the entire judgment region is measured. The average particle size of the entire determination region is 40 μm or less, The coefficient of variation calculated from the average particle size of each section of the judgment region is 15% or less, The platinum-based sputtering target is further characterized in that, in the judgment region, the proportion of crystal grains having an aspect ratio of 3 or more based on the number of particles is 20% or less, and the proportion of crystal grains having an aspect ratio of 5 or more based on the number of particles is 9% or less.

2. 2. The platinum-based sputtering target according to claim 1, which is made of platinum having a purity of 99.99 mass % or more.

3. As an additive element, any one of palladium, rhodium, iridium, ruthenium, cobalt, manganese, nickel, and tungsten is contained in an amount of 1 atomic % or more and 30 atomic % or less; 2. The platinum-based sputtering target according to claim 1, comprising an alloy having a total purity of platinum and said additive element of 99.9 mass % or more.

4. A method for producing a platinum-based sputtering target according to any one of claims 1 to 3, comprising the steps of: The method includes a forging process for producing an ingot by forging an ingot made of platinum or a platinum alloy after melt casting at least once, a rolling process for producing a rolled material by rolling the ingot at least once, and a recrystallization heat treatment process for heat treating the rolled material, After the forging process and before the rolling process, the ingot is subjected to a homogenization heat treatment in which the ingot is heated at a temperature of 850° C. or more and 950° C. or less; In the rolling process, a processing rate of the ingot after the forging process is set to 90% or more; Furthermore, the present invention provides a method for producing a platinum-based sputtering target, wherein the heating temperature of the rolled material in the recrystallization heat treatment step is 600° C. or higher and 700° C. or lower.

5. 5. The method for producing a platinum-based sputtering target according to claim 4, wherein the heating time of the ingot in the homogenization heat treatment step is 60 minutes or more and 120 minutes or less.

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