Sputtering target and alloy thin film

Abstract

Provided is a novel target that makes it possible to suppress the occurrence of cracks during sputtering and suppress the adhesion of particles to a thin film. Also provided is a novel alloy thin film that maintains excellent surface smoothness even after heating.　The target is composed of Ta, Cr, and Fe, has a relative density of 97% or more and an average coercive force of 500 MPa or more, preferably has an average Vickers hardness of 550 HV or less, and more preferably comprises 10-40 atom% of Cr and 15-50 atom% of Fe, with the balance being Ta and inevitable impurities. In addition, the alloy thin film is composed of Ta, Cr, and Fe, has a crystallite diameter of 1.50 nm or less, preferably has a crystallite diameter of 1.60 nm or less after heating at 650°C and 700°C, and comprises 10-40 atom% of Cr and 15-50 atom% of Fe, with the balance being Ta and inevitable impurities.

Description

Sputtering targets and alloy thin films

[0001] The present invention relates to a sputtering target (hereinafter also simply referred to as "target") used for forming an alloy thin film by sputtering, and to an alloy thin film using the same that can be applied to, for example, magnetic recording media.

[0002] Hard disks are used as external storage devices for computers and other devices. Hard disks can be recorded using either longitudinal or perpendicular magnetic recording methods, with perpendicular magnetic recording currently the most common. Furthermore, with the aim of improving recording density, active research is being conducted into microwave-assisted recording and thermally-assisted recording.

[0003] For example, Patent Document 1 discloses a magnetic recording medium comprising a laminate in which an adhesion layer, a soft magnetic underlayer, a seed layer, an intermediate layer, a magnetic recording layer, and a protective layer are sequentially stacked on a substrate. The thin films in each layer of the laminate are formed by, for example, a sputtering method. Patent Document 2 also discloses a target containing Ta and Cr, with the remainder being unavoidable impurities, as a target that is less likely to crack when forming the adhesion layer by a sputtering method.

[0004] On the other hand, Patent Document 3 discloses a thermally assisted magnetic recording medium comprising a substrate, an adhesive layer, a soft magnetic underlayer, an orientation control underlayer, a heat sink layer, a magnetic recording layer, a protective layer, etc. Patent Document 3 also discloses that the substrate needs to be heated before the magnetic recording layer is formed, and that it is desirable to form the magnetic recording layer at as high a substrate temperature as possible. Furthermore, Patent Document 4 discloses a soft magnetic underlayer film that has excellent surface smoothness, in which surface irregularities due to amorphous crystal precipitation are suppressed even after undergoing the heating process required in the manufacture of thermally assisted magnetic recording media.

[0005] Patent No. 4499044 Patent No. 6814758 Patent No. 6009055 Patent No. 7238527

[0006] The TaCr target disclosed in Patent Document 2 is a useful technology in that it is less likely to crack during sputtering to form an adhesion layer. Patent Document 2 states that the formation of an intermetallic compound layer during target fabrication can cause a decrease in strength. The target is fabricated, for example, by pressure sintering metal powder. While lowering the temperature and shortening the pressure sintering time is one way to reduce the intermetallic compound layer, this can result in insufficient sintering and a lower relative density of the target. A low target relative density can lead to cracking due to a decrease in thickness caused by repeated use. Furthermore, a low target relative density can promote the generation of nodules during high-power or long-term sputtering, which can increase the erosion rate and cause particles to adhere to the formed thin film, potentially degrading the properties of the thin film or laminate.

[0007] Furthermore, the thermally assisted magnetic recording medium disclosed in Patent Document 3 is an excellent technology that achieves higher recording density than conventional magnetic recording methods. Patent Document 3 states that substrate heating to 450°C or higher is required before forming a magnetic recording layer. This substrate heating may crystallize the layer having an amorphous structure already formed, thereby impairing surface smoothness, or may cause atomic diffusion between the thin films that make up the laminate, impairing the original thin film properties. Patent Document 4 further discloses the need for a heating process in which the substrate is heated to 600-700°C, a higher temperature than Patent Document 3, during film formation. Therefore, there is a need for materials that can maintain the amorphous structure and thin film properties at higher temperatures.

[0008] The present invention aims to provide a novel target that can suppress cracking of the target during sputtering and particle adhesion to the thin film. Another object of the present invention is to provide a novel alloy thin film that has an amorphous structure and exhibits excellent surface smoothness even after undergoing the heating process required for the manufacture of thermally assisted magnetic recording media.

[0009] The target of the present invention is composed of Ta, Cr, and Fe, has a relative density of 97% or more, and an average transverse rupture strength of 500 MPa or more.

[0010] The target of the present invention preferably has an average Vickers hardness of 550 HV or less.

[0011] The target of the present invention preferably contains 10 to 40 atomic % of Cr, 15 to 50 atomic % of Fe, and the balance being Ta and unavoidable impurities.

[0012] The alloy thin film of the present invention is composed of Ta, Cr, and Fe, and has a crystallite diameter of 1.50 nm or less.

[0013] The alloy thin film of the present invention preferably has a crystallite diameter of 1.60 nm or less after heating at 650°C.

[0014] The alloy thin film of the present invention preferably has a crystallite diameter of 1.60 nm or less after heating at 700°C.

[0015] The alloy thin film of the present invention preferably contains 10 to 40 atomic % of Cr, 15 to 50 atomic % of Fe, and the balance being Ta and unavoidable impurities.

[0016] The target of the present invention is less likely to crack during sputtering and can suppress particle adhesion to the thin film, making it a useful technology for, for example, the manufacture of external recording devices. Furthermore, the alloy thin film of the present invention does not undergo coarse crystal grain precipitation even after heating and has excellent surface smoothness, making it a useful technology for, for example, a novel alloy thin film having an amorphous structure that constitutes a laminate such as a thermally assisted magnetic recording medium that requires a heating process.

[0017] First, the target of the present invention will be described. The target of the present invention is composed of Ta, Cr, and Fe and has a relative density of 97% or more. As a result, the target of the present invention achieves a constant erosion formation rate, suppresses particle adhesion to the thin film due to the generation of nodules, and maintains the properties of the thin film or laminate. For the same reasons as above, the relative density of the target according to the embodiment of the present invention is preferably 98% or more, more preferably 99% or more, and even more preferably 100% or more. Note that the relative density in the present invention is a value expressed as a percentage, which is the ratio of the measured density to the theoretical density. Here, the theoretical density is the value obtained by dividing the weighted average of the masses of the individual elements constituting the target based on the atomic ratio of the constituent elements by the weighted average of the volumes of the individual elements based on the atomic ratio of the constituent elements.

[0018] The target of the present invention has an average transverse rupture strength of 500 MPa or more at room temperature. This makes the target of the present invention less susceptible to cracking during high-power or long-term sputtering. For the same reasons as above, the transverse rupture strength of the target according to the embodiment of the present invention is preferably 600 MPa or more, more preferably 650 MPa or more, and most preferably 700 MPa or more. On the other hand, the transverse rupture strength of the target according to the embodiment of the present invention is preferably 900 MPa or less, more preferably 800 MPa or less, at room temperature, from the viewpoint of suppressing a decrease in compressive strength, which is a strength that is in a trade-off relationship with transverse rupture strength (i.e., toughness). This prevents damage to the target body during handling when chucking the target into a cutting machine or when installing it into a sputtering device. The transverse rupture strength in this invention refers to the maximum load measured by a three-point bending test, in which a test specimen is placed on two supports spaced 20 mm apart, a pressure plate is placed on the center of the test specimen, and a load is applied at a moving speed of 0.1 mm / min to break the test specimen. The transverse rupture strength is determined by cutting out test pieces for measuring the transverse rupture strength, each measuring 3 mm x 4 mm x 25 mm, from any three positions on the target, and measuring the transverse rupture strength at room temperature (20°C ± 15°C as specified in JIS Z 8703). The average value is used.

[0019] The target according to the embodiment of the present invention preferably has an average Vickers hardness of 550 HV or less. This allows the target according to the present invention to reduce the amount of wear on the tips of, for example, milling machines and lathes. That is, as cutting progresses, the cutting depth of the target according to the present invention gradually decreases as the tips wear, thereby preventing the target from becoming too large in size between the start and end of cutting, and also preventing tip breakage. For the same reasons as above, the target according to the embodiment of the present invention preferably has a Vickers hardness of 500 HV or less, and even more preferably 400 HV or less. Furthermore, the target according to the embodiment of the present invention preferably has a Vickers hardness of 250 HV or more, from the viewpoint of preventing damage to the target body during handling when chucking the target into a cutting machine and installing it in a sputtering device. The average Vickers hardness value referred to in the present invention is the average value calculated from the measured values ​​in a Vickers hardness test at five random positions on the target, with the measurement intervals being such that the indentations are not affected by the measurement, under a load of 9.8 N for a pressure time of 10 seconds.

[0020] The target of the present invention is composed of Ta, Cr, and Fe. The Cr and Fe contents can be adjusted as needed to achieve a relative density of 97% or higher and an average transverse rupture strength of 500 MPa or higher without significantly impairing mechanical properties, processability, magnetic properties, etc. Adding 10 atomic % or more of Cr to Ta can lower the melting point of Ta, facilitating sintering and contributing to an improvement in relative density. Furthermore, adding Cr to Ta can sometimes form brittle Ta-Cr compounds during sintering. Therefore, by limiting the amount of Cr to 40 atomic % or less, the volume fraction of intermetallic compounds formed during sintering can be suppressed, resulting in a target with high transverse rupture strength. The preferred lower limit of Cr is 15 atomic %, more preferably 18 atomic %. The preferred upper limit of Cr is 35 atomic %, more preferably 32 atomic %. Adding 15 atomic % or more of Fe to Ta can lower the melting point of Ta, facilitating sintering and contributing to an improvement in relative density. Furthermore, adding Fe to Ta may result in the formation of brittle Ta-Fe compounds during sintering. Therefore, by limiting the amount of Fe added to 50 atomic % or less, the volume fraction of intermetallic compounds formed during sintering can be suppressed, resulting in a target with high transverse rupture strength. Furthermore, Fe contributes to improving the heat resistance and amorphous nature of the adhesion layer described above. The preferred upper limit of Fe is 45 atomic %, and the preferred lower limit is 20 atomic %. Furthermore, by adding Cr and Fe to Ta in combination, the melting point can be significantly lowered, significantly improving sinterability and relative density. However, the combined addition of Cr and Fe may also form Cr-Fe compounds, resulting in a decrease in transverse rupture strength. Therefore, in targets according to embodiments of the present invention, the total content of Cr and Fe is preferably 75 atomic % or less, and more preferably 73 atomic % or less. The target of the present invention is composed of Ta, Cr, and Fe, with the remainder consisting of unavoidable impurities. Examples of the unavoidable impurities include Al, C, Si, Zr, N, and O, and the contents of these impurities are preferably as small as possible, with Al, C, Si, Zr, and N preferably being 100 ppm by mass or less, and O preferably being 1000 ppm by mass or less.

[0021] The target of the present invention can be produced, for example, by pressure sintering a metal powder consisting of a single final composition or a mixed powder obtained by mixing multiple metal powders to achieve the final composition. The raw material powder may be a powder of an intermetallic compound in addition to a powder of a pure metal. Methods for pressure sintering that can be used include hot isostatic pressing, hot pressing, spark plasma sintering, and extrusion press sintering. Among these, hot isostatic pressing is preferred because it can stably achieve the pressure sintering conditions described below.

[0022] The sintering temperature is preferably 1100° C. or higher and 1400° C. or lower. By setting the sintering temperature to 1100° C. or higher, a target with a high relative density can be obtained. Furthermore, by setting the sintering temperature to 1400° C. or lower, the formation of intermetallic compounds can be suppressed and a target with a high transverse rupture strength can be obtained.

[0023] The pressing pressure is preferably 100 MPa or more and 200 MPa or less. By setting the pressing pressure to 100 MPa or more, the progress of sintering is promoted and the generation of voids is suppressed, thereby obtaining a target with a high relative density. Furthermore, by setting the pressing pressure to 200 MPa or less, a general-purpose sintering device can be used.

[0024] The sintering time is preferably 0.5 hours or more and 10 hours or less. By setting the sintering time to 0.5 hours or more, the progress of sintering can be promoted and the generation of voids can be suppressed. Furthermore, by setting the sintering time to 10 hours or less, the formation of intermetallic compounds can be suppressed and a target with high bending strength can be obtained.

[0025] Next, the alloy thin film of the present invention will be described. As described above, the alloy thin film of the present invention is composed of Ta, Cr, and Fe, and has a crystallite diameter of 1.50 nm or less. This allows the alloy thin film of the present invention to have excellent surface smoothness and to suppress the impairment of the properties of the thin film formed thereon. For the same reasons as above, the crystallite diameter of the alloy thin film according to the embodiment of the present invention is preferably 1.40 nm or less, more preferably 1.35 nm or less, and even more preferably 1.30 nm or less. The crystallite diameter referred to in the present invention is a value calculated from the Scherrer equation using the peak top angle and half width of the X-ray diffraction profile measured by the in-plane method using CuKα radiation as the X-ray source.

[0026] The alloy thin film of the present invention preferably has a crystallite diameter of 1.60 nm or less after heating at 650° C. Thus, for example, by using the alloy thin film of the present invention in a laminate of a thermally assisted magnetic recording medium, the surface smoothness is excellent even after the substrate is heated to 650° C. before the magnetic recording layer is formed, and the deterioration of the properties of the thin film formed on the upper layer can be suppressed. For the same reasons as above, the crystallite diameter of the alloy thin film according to the embodiment of the present invention after heating at 650° C. is preferably 1.50 nm or less, more preferably 1.40 nm or less, and even more preferably 1.35 nm or less.

[0027] The alloy thin film of the present invention preferably has a crystallite diameter of 1.60 nm or less after heating at 700°C. As a result, for example, by using the alloy thin film of the present invention in a laminate of a thermally assisted magnetic recording medium, the surface smoothness can be excellent even after heating the substrate at 700°C before forming the magnetic recording layer, and the deterioration of the properties of the thin film formed on the upper layer can be suppressed. For the same reasons as above, the crystallite diameter of the alloy thin film according to the embodiment of the present invention after heating at 700°C is preferably 1.50 nm or less, more preferably 1.40 nm or less, and even more preferably 1.35 nm or less.

[0028] The alloy thin film of the present invention is composed of Ta, Cr, and Fe. The Cr and Fe contents can be appropriately adjusted within a range that does not significantly impair atomic diffusion barrier properties, as long as the crystallite diameter is within a range of 1.50 nm or less. Adding 10 atomic % or more of Cr to Ta can lower the liquidus temperature of Ta, contributing to improved amorphous formation. Furthermore, adding Cr to Ta can reduce the amount of Ta, which has a large atomic weight, relatively, which may result in a decrease in diffusion barrier properties. For this reason, the amount of Cr added is preferably 40 atomic % or less. The preferred lower limit of Cr is 15 atomic %, more preferably 18 atomic %. The preferred upper limit of Cr is 35 atomic %, more preferably 32 atomic %. Adding 15 atomic % or more of Fe to Ta can lower the liquidus temperature of Ta, contributing to improved amorphous formation. Furthermore, when Fe is added to Ta, the amount of Ta, which has a large atomic weight, becomes relatively small, which may result in a deterioration in diffusion barrier properties. Therefore, the amount of Fe added is preferably 50 atomic % or less. The preferred upper limit of Fe is 45 atomic %, and the preferred lower limit is 20 atomic %. Furthermore, in the alloy thin film according to the embodiment of the present invention, the liquidus temperature can be significantly lowered by adding Cr and Fe to Ta in combination, and the amorphous forming ability can be significantly improved. However, the combined addition of Cr and Fe may result in a relative decrease in Ta, which has a large atomic weight, which may result in a deterioration in diffusion barrier properties. Therefore, in the alloy thin film according to the embodiment of the present invention, the total content of Cr and Fe is preferably 75 atomic % or less, and preferably 73 atomic % or less.

[0029] The alloy thin film of the present invention can be obtained, for example, by sputtering using the target of the present invention. Alternatively, it can be obtained by simultaneous sputtering using targets of pure Ta, pure Cr, pure Fe, or binary or ternary alloys containing these.

[0030] Examples of the present invention are shown below. However, the raw materials and production methods are merely specific examples and do not limit the scope of the present invention.

[0031] Ta powder, Cr powder, and Fe powder were weighed and thoroughly mixed to obtain a mixed powder containing 20 atomic % Cr, 24 atomic % Fe, and the remainder consisting of Ta and unavoidable impurities. This mixed powder was filled into a mild steel pressure vessel and sealed under vacuum. The vessel was then sintered by hot isostatic pressing under the conditions of a sintering temperature of 1250°C, a pressure of 146 MPa, and a sintering time of 1 hour to produce a sintered body. This sintered body was then machined to obtain a target having a diameter of 180 mm and a thickness of 4 mm, which is Example 1 of the present invention.

[0032] Ta powder, Cr powder, and Fe powder were weighed out so that the Cr content was 20 atomic %, the Fe content was 32 atomic %, and the remainder was Ta and unavoidable impurities, and the mixed powder was obtained. Thereafter, a target of Invention Example 2 was obtained in the same manner as Invention Example 1.

[0033] Ta powder, Cr powder, and Fe powder were weighed out so that Cr was 20 atomic %, Fe was 40 atomic %, and the remainder was Ta and unavoidable impurities, and mixed thoroughly to obtain a mixed powder. Thereafter, a target of Invention Example 3 was obtained in the same manner as Invention Example 1.

[0034] Ta powder, Cr powder, and Fe powder were weighed out so that the Cr content was 30 atomic %, the Fe content was 40 atomic %, and the remainder was Ta and unavoidable impurities, and the mixed powder was obtained. Thereafter, a target of Invention Example 4 was obtained in the same manner as Invention Example 1.

[0035] Ta powder, Cr powder, and Fe powder were weighed out so that the Cr content was 30 atomic %, the Fe content was 30 atomic %, and the remainder was Ta and unavoidable impurities, and the mixed powder was obtained. Thereafter, a target of Invention Example 5 was obtained in the same manner as Invention Example 1.

[0036] Ta powder and Cr powder were weighed so that Cr was 20 atomic % and the remainder was Ta and unavoidable impurities, and mixed thoroughly to obtain a mixed powder. Thereafter, a target of Comparative Example 1 was obtained in the same manner as in Example 1 of the present invention.

[0037] The measured density was calculated from the dimensions and mass of each target obtained above, and the relative density was calculated using the method described above. The results are shown in Table 1. It was confirmed that the targets of Invention Examples 1 to 5 all had a relative density of 100% or more. In contrast, the target of Comparative Example 1 had a relative density of less than 97%.

[0038] Three 3 mm x 4 mm x 25 mm test pieces for measuring transverse rupture strength were cut from any position of each target obtained above, and the transverse rupture strengths at room temperature (22°C) were measured and their average values ​​were calculated. The transverse rupture strength was measured by placing the test piece on two supports spaced 20 mm apart, applying a pressure plate to the center, and applying a load at a speed of 0.1 mm / min until fracture occurred. The results are shown in Table 1. It was confirmed that the targets of Invention Examples 1 to 5 all had a transverse rupture strength of 515 MPa or more. In particular, Invention Example 1 achieved a transverse rupture strength of 720 MPa or more.

[0039] For each of the targets obtained above, the Vickers hardness was measured at any five points using an MVK-E manufactured by Akashi Seisakusho Co., Ltd. in accordance with JIS Z 2244, with a distance provided between the indentations so that the measurement would not be affected by the indentations, and the average value of the five points was calculated. The results are shown in Table 1. It was confirmed that the targets of Invention Examples 1 to 5 all had a Vickers hardness of 530 HV or less.

[0040]

[0041] The target according to the present invention has a transverse rupture strength of 500 MPa or more, making it less likely to crack during high-power or long-term sputtering. Furthermore, the target according to the present invention has a relative density of 100% or more, which suppresses cracking during sputtering and particle adhesion to the thin film, thereby enabling the formation of a high-quality thin film. Furthermore, the target according to the present invention has a Vickers hardness of 550 HV or less, which reduces chip wear during cutting processing in target fabrication, suppresses target dimensional variations, and is expected to suppress chip breakage. Furthermore, it is expected that damage to the target body will be suppressed not only during chucking into a cutting machine but also during handling when installing into a sputtering device.

[0042] The targets of Examples 1 to 5 of the present invention prepared in Example 1 were attached to a sputtering device (model number: C3010) manufactured by Canon Anelva Corporation. After that, the chamber was vacuumed to a degree of 5×10 －５ After evacuation to a pressure of 0.6 Pa or less, alloy thin films of 80 nm thickness were obtained as Invention Examples 11 to 15 by DC magnetron sputtering on a 3.5-inch glass substrate under conditions of an Ar gas pressure of 0.6 Pa and an input power of 1000 W. The target of Comparative Example 1 prepared in Example 1 was also attached to a sputtering device (model number: C3010) manufactured by Canon Anelva Corporation. Thereafter, an alloy thin film of Comparative Example 11 was obtained in the same manner as Invention Examples 11 to 15.

[0043] For each alloy thin film obtained above, the X-ray diffraction profile before heating was measured using the in-plane method with a Rigaku Corporation X-ray diffractometer SmartLab, using CuKα radiation (wavelength: 1.5418 Å) as the X-ray source. The obtained diffraction profile was smoothed using a cubic equation at seven points using the Savitzky-Golay method, and the background was subtracted using a quadratic approximation curve. The profile was then fitted with a segmented pseudo-Voigt function to obtain the peak top angle and half-width. Using these values, the crystallite diameter was calculated using the Scherrer equation. The results are shown in Table 2. All of the alloy thin films of Inventive Examples 11 to 15 had crystallite diameters of 1.33 nm or less before heating, confirming that the amorphous structure was maintained, resulting in excellent surface smoothness. In particular, Inventive Examples 12 to 15 had crystallite diameters of 1.28 nm or less before heating, resulting in excellent surface smoothness. In contrast, the alloy thin film of Comparative Example 11 had a crystallite diameter of more than 1.50 nm before heating.

[0044] Each of the alloy thin films obtained above was transferred to a heating chamber and subjected to lamp heating. －４ The heating was performed in a vacuum of 0.1 Pa or less, with a power input of 2000 W, and at 650°C and 700°C. The crystallite diameter of the alloy thin films after heating was similarly calculated. The results are shown in Table 2. All of the alloy thin films of Inventive Examples 11 to 15 had crystallite diameters of 1.45 nm or less after heating at 650°C, confirming that the amorphous structure was maintained, resulting in excellent surface smoothness. In particular, Inventive Examples 12 to 15 had crystallite diameters of 1.35 nm or less before heating, resulting in excellent surface smoothness. Furthermore, all of the alloy thin films of Inventive Examples 11 to 15 had crystallite diameters of 1.45 nm or less even after heating at 700°C, confirming that the amorphous structure was maintained, resulting in excellent surface smoothness. In particular, Inventive Examples 12 to 15 had crystallite diameters of 1.35 nm or less before heating, resulting in excellent surface smoothness. In particular, Inventive Example 14 had a crystallite diameter of 1.30 nm or less before heating, resulting in excellent surface smoothness. In contrast, the alloy thin film of Comparative Example 11 had a crystallite diameter exceeding 1.60 nm after heating at 650°C and 700°C.

[0045]

[0046] The alloy thin film according to the present invention has a crystallite diameter of 1.50 nm or less, which can be expected to have excellent surface smoothness. Furthermore, the alloy thin film according to the present invention has a crystallite diameter of 1.60 nm or less after heating at 650 ° C, which maintains an amorphous structure even after heating at 650 ° C, thereby maintaining excellent surface smoothness. Furthermore, the alloy thin film according to the present invention has a crystallite diameter of 1.60 nm or less after heating at 700 ° C, which maintains an amorphous structure even after heating at 700 ° C, thereby maintaining excellent surface smoothness. As a result, the alloy thin film according to the present invention can be expected to be used, for example, in a laminate film of a thermally assisted magnetic recording medium that requires a heating process.

Claims

1. A sputtering target composed of Ta, Cr, and Fe, having a relative density of 97% or more and an average value of flexural strength of 500 MPa or more.

2. The sputtering target according to claim 1, having an average value of Vickers hardness of 550 HV or less.

3. The sputtering target according to claim 1 or 2, containing 10 to 40 atomic% of Cr, 15 to 50 atomic% of Fe, and the balance being composed of Ta and unavoidable impurities.

4. An alloy thin film composed of Ta, Cr, and Fe, having a crystallite size of 1.50 nm or less.

5. The alloy thin film according to claim 4, having a crystallite size of 1.60 nm or less after heating at 650 °C.

6. The alloy thin film according to claim 4, having a crystallite size of 1.60 nm or less after heating at 700 °C.

7. The alloy thin film according to any one of claims 4 to 6, containing 10 to 40 atomic% of Cr, 15 to 50 atomic% of Fe, and the balance being composed of Ta and unavoidable impurities.

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