Retaining member

Abstract

This invention provides a technology that can suppress the diffusion of silica into a high-purity alumina layer having a mounting surface. [Solution] A holding member for holding an object, comprising: a low-purity alumina layer containing alumina as the main component and silica; a high-purity alumina layer having a mounting surface on which the object is placed, having a higher purity of alumina than the low-purity alumina layer and a lower mass ratio of silica than the low-purity alumina layer; and a bonding layer disposed between the low-purity alumina layer and the high-purity alumina layer, bonding the low-purity alumina layer and the high-purity alumina layer, with a mass ratio of less than or equal to that of the low-purity alumina layer and a mass ratio of greater than or equal to that of the high-purity alumina layer.

Description

Technical Field

[0001] This disclosure relates to a holding member.

Background Art

[0002] Among holding members that hold objects such as wafers, a ceramic fired body mainly composed of alumina or the like may be used as a member having a mounting surface on which the object is placed. For example, Patent Document 1 discloses an electrostatic chuck in which a first ceramic layer and a second ceramic layer mainly made of alumina are joined via an adhesive layer made of an inorganic adhesive as an example of a holding member provided with a ceramic fired body.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In such a ceramic fired body, glass may be added as a sintering aid. The addition of glass promotes densification at a low temperature of the ceramic fired body. On the other hand, since glass is easily corroded by plasma, in the layer of the ceramic fired body having a mounting surface on which the object is placed, the reduction in plasma resistance is suppressed by increasing the purity of alumina. However, during the heat treatment in the manufacturing process of the holding member, the plasma resistance of the layer having the mounting surface may decrease due to the diffusion of glass from the lower layer side than the layer having the mounting surface of the holding member. For this reason, a technique capable of suppressing the diffusion of glass into the layer having the mounting surface in the holding member has been desired. Note that in Patent Document 1, the diffusion of glass from the lower layer to the layer including the mounting surface due to heat treatment was not considered.

[0005] This disclosure aims to provide a technology that can suppress the diffusion of silica into a high-purity alumina layer having a mounting surface. [Means for solving the problem]

[0006] This disclosure is made to solve at least some of the problems described above and can be implemented in the following forms.

[0007] (1) According to one embodiment of the present disclosure, a holding member is provided. The holding member is a holding member for holding an object, and comprises: a low-purity alumina layer containing alumina as the main component and silica; a high-purity alumina layer having a mounting surface on which the object is placed, having a higher purity of alumina than the low-purity alumina layer and a lower mass ratio of silica than the low-purity alumina layer; and a bonding layer disposed between the low-purity alumina layer and the high-purity alumina layer, bonding the low-purity alumina layer and the high-purity alumina layer, with the mass ratio being less than or equal to that of the low-purity alumina layer and the mass ratio being greater than or equal to that of the high-purity alumina layer.

[0008] With this configuration, the diffusion of silica, which is glass contained in the low-purity alumina layer, into the high-purity alumina layer due to heat treatment during the manufacturing process of the holding member can be suppressed by the bonding layer placed between the low-purity alumina layer and the high-purity alumina layer. Therefore, the reduction in the plasma resistance of the high-purity alumina layer having a mounting surface due to the diffusion of silica from the low-purity alumina layer can be suppressed.

[0009] (2) In the holding member of the above form, the mass ratio of the high-purity alumina layer may be 0.5% by mass or less. With this configuration, the mass ratio of silica in the high-purity alumina layer is kept low, which further suppresses the decrease in plasma resistance of the high-purity alumina layer having a mounting surface.

[0010] (3) In the holding member of the above form, the porosity of the bonding layer may be less than or equal to the porosity of the low-purity alumina layer. This configuration makes it possible to realize a retaining member with a bonding layer that has improved strength, insulation resistance, and airtightness.

[0011] (4) In the holding member of the above form, the hardness of the high-purity alumina layer may be higher than the hardness of the low-purity alumina layer. With this configuration, the hardness of the high-purity alumina layer having a mounting surface is relatively high, which improves the wear resistance of the mounting surface. Therefore, it is possible to realize a holding member in which the generation of particles from the mounting surface is suppressed.

[0012] (5) In the holding member of the above form, the volume resistivity of the high-purity alumina layer may be higher than the volume resistivity of the low-purity alumina layer. In this configuration, when the holding member holds the object by electrostatic attraction, the charge tends to dissipate quickly when the electrostatic attraction disappears, making it easier to release the object from the mounting surface. As a result, the efficiency of the work required to process the object using the holding member can be improved.

[0013] Furthermore, this disclosure can be implemented in various forms, for example, in the form of a holding member, an electrostatic chuck equipped with an electrostatic electrode that generates an electrostatic attraction force on the holding surface of the holding member, a vacuum chuck, a ceramic heater, a semiconductor manufacturing apparatus, and components equipped therewith, and methods for manufacturing these. [Brief explanation of the drawing]

[0014] [Figure 1] This is an explanatory diagram illustrating the cross-sectional configuration of a holding member according to an embodiment of the present invention. [Figure 2] This is a flowchart of the manufacturing process for ceramic components. [Figure 3] This is an explanatory diagram showing a fired body used in the manufacture of ceramic components. [Figure 4] This is an explanatory diagram showing a fired body with paste printed on it. [Figure 5] This is an explanatory diagram showing a laminate. [Modes for carrying out the invention]

[0015] Figure 1 is a schematic diagram illustrating the cross-sectional configuration of a holding member 1 as an embodiment of the present invention. The holding member 1 is an electrostatic chuck that holds a semiconductor wafer W, which is the object to be held, by electrostatic attraction. The arrows shown in Figure 1 indicate the direction in which the semiconductor wafer W is attracted to the holding member 1. The holding member 1 is used, for example, to fix a semiconductor wafer W in a vacuum chamber of a semiconductor manufacturing apparatus. The holding member 1 comprises a ceramic member 10, a base member 20, an adhesive member 30, an electrostatic electrode 40, and a heater electrode 50.

[0016] The ceramic member 10 is a disc-shaped member and is mainly composed of alumina. The main component refers to the component with the largest mass percentage (for example, the component with a mass percentage of 90 wt% or more). The ceramic member 10 has a mounting surface 10f. The mounting surface 10f is the surface on which the semiconductor wafer W is placed when the semiconductor wafer W is adsorbed and held by the holding member 1.

[0017] The base member 20 is a disc-shaped member with a larger diameter than the ceramic member 10, and is formed from a metal or various composite materials. Preferably, the metal used to form the base member 20 is Al (aluminum), Ti (titanium), or alloys thereof. Preferably, the composite material used to form the base member 20 is a composite material formed by melting and pressurizing an aluminum alloy, primarily composed of aluminum, into a porous ceramic mainly composed of silicon carbide (SiC). The aluminum alloy contained in the composite material may contain Si (silicon) or Mg (magnesium), or other elements to the extent that they do not affect the properties.

[0018] Inside the base member 20, a refrigerant flow path 21 is formed. When refrigerant (for example, fluorine-based inert liquid, water, etc.) flows through the refrigerant flow path 21, the base member 20 is cooled. At this time, the ceramic member 10 is also cooled by heat transfer (heat extraction) between the base member 20 and the ceramic member 10 through the adhesive member 30, and the semiconductor wafer W adsorbed on the mounting surface 10f of the ceramic member 10 is also cooled.

[0019] The adhesive member 30 is disposed between the ceramic member 10 and the base member 20 and adheres the ceramic member 10 and the base member 20. The adhesive member 30 is composed of an adhesive such as a silicone-based resin, an acrylic-based resin, an epoxy-based resin, etc.

[0020] The ceramic member 10 includes a low-purity alumina layer 10L, a bonding layer 10P, and a high-purity alumina layer 10H. The low-purity alumina layer 10L is disposed on the side of the ceramic member 10 that contacts the adhesive member 30. The low-purity alumina layer 10L is a layer containing alumina as a main component and containing silica. The high-purity alumina layer 10H is disposed on the side of the ceramic member 10 that holds the semiconductor wafer W. That is, the high-purity alumina layer 10H is the layer having the above-described mounting surface 10f. The high-purity alumina layer 10H is a layer having a higher purity of alumina and a lower mass ratio of silica than the low-purity alumina layer 10L. The purity of alumina means the mass ratio (wt%) of alumina. In other words, the high-purity alumina layer 10H is a layer having a higher mass ratio of alumina and a lower mass ratio of silica than the low-purity alumina layer 10L. Each of the low-purity alumina layer 10L and the high-purity alumina layer 10H contains silica in an amorphous state. Amorphous silica is a kind of glass.

[0021] The bonding layer 10P is positioned between the low-purity alumina layer 10L and the high-purity alumina layer 10H in the ceramic member 10, and is a layer that bonds the low-purity alumina layer 10L and the high-purity alumina layer 10H. Furthermore, the bonding layer 10P is a layer in which the mass percentage of silica is less than or equal to that of the low-purity alumina layer 10L, and the mass percentage of silica is greater than or equal to that of the high-purity alumina layer 10H. In this embodiment, the bonding layer 10P contains a trace amount of silica. In addition, in this embodiment, the bonding layer 10P is also a layer in which the purity of alumina is greater than or equal to that of the low-purity alumina layer 10L, and the purity of alumina is less than or equal to that of the high-purity alumina layer 10H.

[0022] The electrostatic electrode 40 is a disc-shaped component located inside the low-purity alumina layer 10L of the ceramic component 10, and is made of a conductive material such as tungsten or molybdenum. The electrostatic electrode 40 generates an electrostatic attraction on the mounting surface 10f when power is supplied from an external power source (not shown). The semiconductor wafer W is held on the mounting surface 10f by being attracted toward the mounting surface 10f by this electrostatic attraction.

[0023] The heater electrode 50 is located inside the low-purity alumina layer 10L of the ceramic member 10 and is made of a conductive material such as tungsten or molybdenum. The heater electrode 50 generates heat when power is supplied from an external power source (not shown). This heat warms the ceramic member 10, and the semiconductor wafer W adsorbed on the mounting surface 10f of the ceramic member 10 is also warmed.

[0024] Figure 2 is a flowchart of the manufacturing process for the ceramic member 10. The manufacturing process for the ceramic member 10 is included in the manufacturing process for the holding member 1. Figure 3 is an explanatory diagram showing the fired body L and fired body H used in the manufacturing of the ceramic member 10. The manufacturer of the ceramic member 10 first prepares the fired body L, which will be the basis for the low-purity alumina layer 10L, and the fired body H, which will be the basis for the high-purity alumina layer 10H (step S1). Electrostatic electrodes 40 and heater electrodes 50 are provided inside the fired body L. The fired body H has a higher alumina purity than the fired body L, and a lower silica mass ratio than the fired body L. Next, the manufacturer polishes the fired body L and the fired body H (step S2). This polishing process creates a flat surface on both the fired body L and the fired body H.

[0025] Next, as shown in Figure 4, the manufacturer screen prints a paste P (not shown), which will form the bonding layer 10P, onto one surface of the fired body L and one surface of the fired body H (step S3). Figure 4 is an explanatory diagram showing the fired body L and the fired body H with the paste P printed on them. In this embodiment, the paste P does not contain silica. After the paste P is printed on the fired body L and the fired body H, degreasing is performed.

[0026] Next, the manufacturer prepares a laminate M in which fired bodies L and H are laminated together via contact between the surface of fired body L printed with paste P and the surface of fired body H printed with paste P, as shown in Figure 5 (Step S4). Figure 5 is an explanatory diagram showing the laminate M. Next, the manufacturer performs hot pressing on the laminate M along the lamination direction (Step S5). The arrows shown in Figure 5 indicate the direction of the pressure applied to the laminate M by hot pressing. At this time, the silica, which is glass contained in fired body L, diffuses into paste P. After hot pressing, the manufacturer processes the laminate M after hot pressing, such as adjusting its shape (Step S6), to produce the ceramic member 10 (see Figure 1). That is, the fired body L, fired body H, and paste P after hot pressing (Step S5) and processing (Step S6) correspond to the low-purity alumina layer 10L, the high-purity alumina layer 10H, and the bonding layer 10P, respectively. The silica contained in the bonding layer 10P originates from the low-purity alumina layer 10L.

[0027] In this embodiment, the mass percentage of silica in the high-purity alumina layer 10H is 0.5% by mass or less. That is, the fired body H prepared in step S1 of the manufacturing process of the ceramic member 10 described above is prepared so that, after going through step S6, it becomes a high-purity alumina layer 10H with a silica mass percentage of 0.5% by mass or less. The silica mass percentage tends to depend on the silica mass percentage contained in the fired body H.

[0028] The mass percentage of silica in the high-purity alumina layer 10H was measured by performing electrical conductivity treatment, such as carbon deposition, on a sample that had undergone CP processing (Cross-Section Polisher processing), and then performing EDS analysis.

[0029] In this embodiment, the hardness (Vickers hardness) of the high-purity alumina layer 10H is higher than the hardness (Vickers hardness) of the low-purity alumina layer 10L. That is, the fired bodies H and L prepared in step S1 of the manufacturing process of the ceramic member 10 described above are prepared so that, after going through step S6, the hardness of the high-purity alumina layer 10H is higher than the hardness of the low-purity alumina layer 10L. The hardness of the alumina layer tends to depend on the purity of the alumina, the grain size of the alumina, and the porosity. In this embodiment, the hardness of the high-purity alumina layer 10H is Hv10 1700 GPa or higher, and the hardness of the low-purity alumina layer 10L is less than Hv10 1700 GPa.

[0030] The hardness of the high-purity alumina layer 10H and the low-purity alumina layer 10L were measured in accordance with JIS R 1610. Specifically, these hardness values ​​were obtained when a sample polished to a mirror finish with diamond abrasive grains was subjected to a load of 10 kgf and a holding time of 15 seconds.

[0031] In this embodiment, the volume resistivity of the high-purity alumina layer 10H is higher than that of the low-purity alumina layer 10L. That is, the fired bodies H and L prepared in step S1 of the manufacturing process of the ceramic member 10 described above are prepared so that, after step S6, the volume resistivity of the high-purity alumina layer 10H is higher than that of the low-purity alumina layer 10L. The volume resistivity of an alumina layer tends to depend on the purity of the alumina. In this embodiment, the volume resistivity of the high-purity alumina layer 10H is 1.0 × 10⁻⁶. 13 The resistivity is greater than Ω·m, and the volume resistivity of 10L of low-purity alumina layer is 1.0 × 10⁻⁶. 13 The resistivity is less than Ω·m. The volume resistivity of the high-purity alumina layer 10H and the low-purity alumina layer 10L are values ​​obtained at a measurement temperature of 400°C.

[0032] The volume resistivity of the high-purity alumina layer 10H and the low-purity alumina layer 10L were measured in accordance with JIS C 2139. Specifically, these volume resistivity values ​​were obtained when a main electrode, guard electrode, and back electrode were formed on the sample, and measurements were taken at an applied voltage of 1000V, a charging time of 60 seconds, and a temperature of 400°C.

[0033] In this embodiment, the porosity of the bonding layer 10P is less than or equal to the porosity of the low-purity alumina layer 10L. That is, the fired body L prepared in step S1 of the manufacturing process of the ceramic member 10 described above and the paste P printed in step S3 are prepared so that, after step S6, the porosity of the bonding layer 10P is less than or equal to the porosity of the low-purity alumina layer 10L. The porosity of each layer tends to decrease as the temperature during the manufacturing process increases.

[0034] The porosity of the bonding layer 10P and the porosity of the low-purity alumina layer 10L were calculated by performing conductivity treatment such as carbon deposition on CP processing (Cross-Section Polisher processing) and then binarizing the images acquired by SEM observation.

[0035] As described above, with respect to the holding member 1 of this embodiment, the diffusion of silica, which is glass contained in the low-purity alumina layer 10L, into the high-purity alumina layer 10H due to heat treatment (e.g., hot pressing) during the manufacturing process of the holding member 1 can be suppressed by the bonding layer 10P placed between the low-purity alumina layer 10L and the high-purity alumina layer 10H. In other words, the bonding layer 10P functions as a glass diffusion prevention layer that prevents the diffusion of glass into the high-purity alumina layer 10H. Therefore, it is possible to suppress the decrease in the plasma resistance of the high-purity alumina layer 10H having a mounting surface 10f due to the diffusion of silica from the low-purity alumina layer 10L.

[0036] Furthermore, in the holding member 1 of this embodiment, the mass ratio of silica contained in the high-purity alumina layer 10H is lower than both the mass ratio of silica contained in the low-purity alumina layer 10L and the mass ratio of silica contained in the bonding layer 10P. In this case, if the high-purity alumina layer 10H does not contain silica near the mounting surface 10f, and a small amount of silica is contained in the portion closer to the bonding layer 10P, then the entire portion of the ceramic member 10 excluding the area near the mounting surface 10f contains silica that functions as a sintering aid. In such a case, during the manufacturing of the ceramic member 10, it is possible to promote densification at low temperatures throughout the entire portion while suppressing a decrease in plasma resistance near the mounting surface 10f.

[0037] Furthermore, in the holding member 1 of this embodiment, the mass ratio of silica in the high-purity alumina layer 10H is 0.5% by mass or less. Therefore, because the mass ratio of silica in the high-purity alumina layer 10H is kept low, the decrease in plasma resistance of the high-purity alumina layer 10H having the mounting surface 10f can be further suppressed.

[0038] Furthermore, in the holding member 1 of this embodiment, the hardness of the high-purity alumina layer 10H is higher than that of the low-purity alumina layer 10L. Therefore, since the hardness of the high-purity alumina layer having a mounting surface is relatively high, the wear resistance of the mounting surface 10f can be improved. Consequently, a holding member 1 can be realized in which the generation of particles from the mounting surface 10f is suppressed.

[0039] Furthermore, in the holding member 1 of this embodiment, the volume resistivity of the high-purity alumina layer 10H is higher than that of the low-purity alumina layer 10L. Therefore, in the holding member 1 that holds an object (semiconductor wafer W) by electrostatic attraction, the volume resistivity of the high-purity alumina layer 10H having the mounting surface 10f is relatively high. As a result, when the electrostatic attraction disappears due to the cessation of power supply to the electrostatic electrode 40, the charge is easily dissipated quickly, making it easier to release the object from the mounting surface 10f. Consequently, the efficiency of the work required to process the object using the holding member 1 can be improved.

[0040] Furthermore, in the holding member 1 of this embodiment, the porosity of the bonding layer 10P is less than or equal to that of the low-purity alumina layer 10L. Therefore, it is possible to realize a holding member 1 equipped with a bonding layer 10P that has improved strength, insulation resistance, and airtightness.

[0041] <Modified form of this embodiment> The present invention is not limited to the embodiments described above, and can be implemented in various forms without departing from its spirit, for example, the following modifications are also possible.

[0042] In the above embodiment, the bonding layer 10P contained a small amount of silica, but is not limited to this. The bonding layer 10P only needs to have a silica mass ratio of 10L or less in the low-purity alumina layer and a silica mass ratio of 10H or more in the high-purity alumina layer; therefore, the bonding layer 10P does not need to contain silica.

[0043] In the above embodiment, the paste P did not contain silica in the manufacturing process of the ceramic member 10, but this is not limited to this. Silica may be included in the paste P. The mass ratio of silica contained in the paste P should be set so that the mass ratio of silica contained in the bonding layer 10P is less than or equal to the low-purity alumina layer 10L and greater than or equal to the high-purity alumina layer 10H.

[0044] In the above embodiment, the mass percentage of silica in the high-purity alumina layer 10H was 0.5% by mass or less, but is not limited to this. The mass percentage of silica in the high-purity alumina layer 10H may be higher than 0.5% by mass. Of course, from the viewpoint of suppressing a decrease in the plasma resistance of the high-purity alumina layer 10H, it is preferable that the mass percentage of silica in the high-purity alumina layer 10H be 0.5% by mass or less.

[0045] In the above embodiment, the hardness of the high-purity alumina layer 10H was higher than that of the low-purity alumina layer 10L, but this is not limited to this. The hardness of the high-purity alumina layer 10H may be less than or equal to that of the low-purity alumina layer 10L. Of course, from the viewpoint of improving the wear resistance of the mounting surface 10f, it is preferable that the hardness of the high-purity alumina layer 10H is higher than that of the low-purity alumina layer 10L.

[0046] In the above embodiment, the volume resistivity of the high-purity alumina layer 10H was higher than that of the low-purity alumina layer 10L, but this is not limited to this. The volume resistivity of the high-purity alumina layer 10H may be less than or equal to that of the low-purity alumina layer 10L. Of course, from the viewpoint of making it easier to release the object (semiconductor wafer W) from the mounting surface 10f, it is preferable that the volume resistivity of the high-purity alumina layer 10H is higher than that of the low-purity alumina layer 10L.

[0047] In the above embodiment, the porosity of the bonding layer 10P was less than or equal to the porosity of the low-purity alumina layer 10L, but is not limited to this. The porosity of the bonding layer 10P may be higher than the porosity of the low-purity alumina layer 10L. Of course, from the viewpoint of improving the strength, insulation resistance, and airtightness of the bonding layer 10P, it is preferable that the porosity of the bonding layer 10P is less than or equal to the porosity of the low-purity alumina layer 10L.

[0048] In the above embodiment, the adhesive member 30 bonded the ceramic member 10 and the base member 20, but it is not limited to this. For example, a resin layer made of a highly insulating resin (e.g., polyimide) may be provided between the ceramic member 10 and the adhesive member 30, and the adhesive member 30 may bond the resin layer to the base member 20. Also, the heater electrode 50, which was provided inside the low-purity alumina layer 10L in the above embodiment, may be provided inside the resin layer instead of being provided inside the low-purity alumina layer 10L.

[0049] The embodiments of this specification have been described above based on the embodiments and modifications described above. The embodiments described above are for the purpose of facilitating understanding of this specification and do not limit it. This specification may be modified and improved without departing from its spirit and the scope of the claims, and equivalents thereof are included in this specification. Furthermore, any technical features that are not described as essential in this specification may be deleted as appropriate.

[0050] The present invention can also be realized in the following forms. [Application Example 1] A holding member for holding an object, A low-purity alumina layer containing alumina as the main component and silica, A high-purity alumina layer having a mounting surface on which the object is placed, having a higher purity of alumina than the low-purity alumina layer, and a lower mass ratio of silica than the low-purity alumina layer, A holding member characterized by comprising a bonding layer disposed between the low-purity alumina layer and the high-purity alumina layer, bonding the low-purity alumina layer and the high-purity alumina layer, wherein the mass ratio is less than or equal to that of the low-purity alumina layer and the mass ratio is greater than or equal to that of the high-purity alumina layer. [Application Example 2] The retaining member described in Application Example 1, A retaining member characterized in that the mass ratio in the high-purity alumina layer is 0.5% by mass or less. [Application Example 3] A retaining member as described in Application Example 1 or Application Example 2, A retaining member characterized in that the porosity of the bonding layer is less than or equal to the porosity of the low-purity alumina layer. [Application Example 4] A retaining member as described in any of Application Examples 1 to 3, A retaining member characterized in that the hardness of the high-purity alumina layer is higher than the hardness of the low-purity alumina layer. [Application Example 5] A retaining member as described in any of Application Examples 1 to 4, A retaining member characterized in that the volume resistivity of the high-purity alumina layer is higher than that of the low-purity alumina layer. [Explanation of Symbols]

[0051] 1…Retaining member 10…Ceramic components 10H... High-purity alumina layer 10L... Low-purity alumina layer 10P...Joining layer 10f... Mounting surface 20…Base component 21… Refrigerant flow path 30…Adhesive material 40…Electrostatic electrodes 50... Heater electrode H... Firing body L…Fired body M...Laminated body P... Paste

Claims

1. A holding member for holding an object, A low-purity alumina layer containing alumina as the main component and silica, A high-purity alumina layer having a mounting surface on which the object is placed, having a higher purity of alumina than the low-purity alumina layer, and a lower mass ratio of silica than the low-purity alumina layer, A holding member characterized by comprising a bonding layer disposed between the low-purity alumina layer and the high-purity alumina layer, bonding the low-purity alumina layer and the high-purity alumina layer, wherein the mass ratio is less than or equal to that of the low-purity alumina layer and the mass ratio is greater than or equal to that of the high-purity alumina layer.

2. A retaining member according to claim 1, A retaining member characterized in that the mass ratio in the high-purity alumina layer is 0.5% by mass or less.

3. A retaining member according to claim 1 or claim 2, A retaining member characterized in that the porosity of the bonding layer is less than or equal to the porosity of the low-purity alumina layer.

4. A retaining member according to claim 1 or claim 2, A retaining member characterized in that the hardness of the high-purity alumina layer is higher than the hardness of the low-purity alumina layer.

5. A retaining member according to claim 1 or claim 2, A retaining member characterized in that the volume resistivity of the high-purity alumina layer is higher than that of the low-purity alumina layer.

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