DE102020107167B3 - Holding device for wafers, PECVD deposition device and use of the holding device - Google Patents

Abstract

The invention relates to a holding device for holding a plurality of substrates during plasma-assisted deposition of a layer from the gas phase on the substrates, which is designed as a boat for a PECVD deposition device and has: - carrier plates arranged parallel to one another for supporting the substrates during the deposition, the Carrier plates isolated from one another are alternately connected to connections of an alternating voltage generator, - electrically insulating spacers (11) which are arranged between the carrier plates and are designed to space the carrier plates apart and to isolate them electrically from one another, - the spacers (11) each through a casing (12) with a casing inner diameter (d1) which is larger than an outer diameter (D3) of the spacer (11) which it surrounds, so that a cavity (4) is formed between the casing (12) and the spacer (11). Furthermore, d The invention of a PECVD separation device which has the holding device. The invention also relates to a use of the holding device in the deposition of an electrically conductive layer on a substrate.

Description

The invention relates to a holding device, a PECVD separating device and a use of the holding device. In particular, the invention relates to a holding device for holding a plurality of substrates during plasma-assisted deposition of an electrically conductive layer, in particular doped amorphous silicon, from the gas phase on the substrates, which is designed as a boat for a gas phase deposition device.

Such a holding device is known from WO 2014/194 892 A1. It has several carrier plates arranged parallel to one another for supporting the substrates during the deposition, the carrier plates being isolated from one another and alternately connected to connections of an alternating voltage generator. The carrier plates have a suitable holder such as substrate pockets, holding pins or the like. To hold the substrates, with individual substrates in the holding device having to be kept at a distance from one another in order to allow gases to flow as evenly as possible through all the gaps and the formation of a Allow plasmas between the substrates to ensure a uniform coating of the substrates. In addition, there must be no conductive connection between adjacent carrier plates so that no power loss occurs and the alternating voltage required to ignite the plasma can be applied. The holding device therefore has electrically insulating spacers which are arranged between the carrier plates and are designed to space the carrier plates from one another and to isolate them electrically from one another.

Further describes DE 10 2011 109 444 A1 a spacer element for a clamping unit for plates a holding device in the form of a wafer boat, which has a base body with at least one passage opening which extends longitudinally between the end faces of the base body through the base body, and with at least one connection opening in the base body which extends transversely to the passage opening extends between the at least one through opening and a circumference of the base body. Furthermore, a wafer boat with a plurality of plates is described, which are arranged with one another via a plurality of clamping units substantially parallel to one another, the clamping units each having a plurality of spacer elements of the above-mentioned type and each at least one alternating through openings in the plates and the Have spacers extending clamping element.

In order to coat the substrates in a chemical vapor deposition or a PECVD process, they are first arranged on the carrier plates of the holding device and then coated in a pipe PECVD system.

The substrates are arranged on the carrier plates in such a way that in each case two substrates are electrically insulated from one another and are electrically conductively connected to connections of an alternating voltage generator.

In a pipe PECVD process, a layer is deposited on the substrates that are located in the holding device. During the PECVD process, the layer is not only applied to the substrate surface, but also to the spacers. This is not a problem if the substrates are coated with a non-conductive layer. However, if the substrates are coated with a conductive layer, such as amorphous silicon doped with phosphorus or boron (a-Si (n) / a-Si (p)), or doped silicon nitride (SiNx (n) / SiNx (p)) or doped aluminum oxide (AlOx (n) / AlOx (n)), the spacers are inevitably provided with a conductive layer. This parasitic, conductive coating of the electrically insulating spacers leads to an increasing conductivity of the spacers as the duration of the coating progresses. As a result, the holding device loses its electrical insulation between the carrier plates. This has the result that a deposition rate drops significantly with a number of depositions or coatings and that the electrical resistance between the carrier plates is significantly reduced. Furthermore, there is plasma instability due to the formation of sparks ("arcing") between the holding plates and the coated spacers. In addition, in the case of a very conductive layer on the spacer, a plasma can no longer be ignited. As a result, a power loss occurs via the spacers and reduces the power effectively available for the coating until it is no longer possible to deposit on the substrates.

It is an object of the present invention to provide a holding device and a PECVD separation device in which a loss of functionality is prevented or reduced and which is inexpensive.

According to the invention, the object is achieved by a holding device with the features of claim 1 and a PECVD separation device with the features of claim 10. Advantageous further developments and refinements are given in the subclaims.

According to the invention it is provided that the spacers are each surrounded by a casing with a casing inner diameter which is larger than an outer diameter of the spacer that they surround, so that there is a cavity between the shell and the spacer.

By means of the sheathing, the parasitic coating of the spacers no longer leads to an electrically conductive connection between opposing substrates due to the geometry and nature of the surface and / or the formation of the conductive coating of the spacers is slowed down to an acceptable level for mass production. Furthermore, the jacket is an inexpensive solution for mass production.

The cover, as well as the cavity between the cover and the spacer, prevent or reduce coating of the spacer during the PECVD process. This prevents or reduces the formation of a conductive channel between adjacent carrier plates by preventing a continuous conductive firm from being deposited on the spacers, which can bring about potential equalization between the carrier plates. The cavity is not or only partially coated. This means that no continuously conductive layer can form between the carrier plates during the PECVD process.

The substrate is preferably a wafer, for example a silicon wafer. A base material of the carrier plates can have graphite or carbon fiber reinforced plastic (CFRP) or carbon fiber reinforced carbon (CFC). It is preferably made of graphite or CFRP. A base material made of graphite has proven particularly useful in practice, particularly in the coating of substrates for the production of semiconductor components such as solar cells, for example. The carrier plates can have recesses for holding the substrates. The carrier plates preferably have retaining pins and / or retaining lugs in order to fix the substrates.

The spacer is preferably formed from a ceramic, for example from aluminum oxide ceramic. It is preferably designed as a hollow cylinder. The spacer can be circular cylindrical and preferably has a constant inside diameter and / or a constant outside diameter. The spacer can be designed in one or more parts. In a preferred embodiment, the spacer is designed in one or two parts. If the spacer has several parts, preferably in two parts, it is preferably divided perpendicular to its direction of extension, for example in the middle.

The sheathing is also particularly advantageous if the spacer is formed from an aluminum oxide ceramic, which is relatively difficult to machine, so that special turning and milling tools are usually necessary for this. The sheathing protects the spacer from being coated during the chemical vapor deposition without the spacer having to be machined to produce the desired geometry. The spacer and the casing can be produced by means of sintering, and the holding device can be provided by simple assembly such as sliding the spacer and the casing over one another. The sheathing preferably partially but not completely surrounds the spacer.

The casing preferably surrounds the spacer in the manner of a sleeve. The sleeve can have any cross section, but is preferably round. The casing is preferably designed as a hollow cylinder. The casing is preferably formed from a ceramic, preferably from an aluminum oxide ceramic. Both the spacer and the casing are preferably designed as hollow cylinders with different inside and outside diameters, the casing inside diameter being greater than the outside diameter of the spacer. As a result, the holding device can be provided by sliding the casing and the spacer onto and / or over one another. The holding device is therefore easy to manufacture and inexpensive, and the cavity is created in a simple manner. The cavity is preferably symmetrical about a longitudinal axis of the preferably cylindrical spacer.

In a preferred embodiment, the casing is spaced from one or two adjacent carrier plates, while the spacer is in mechanical contact with both carrier plates. As a result, the casing is separated from at least one of the carrier plates by means of a gap. On one or both sides of the casing, one or two gaps between the casing and the carrier plate (s) and one or two cavities between the spacer and the casing are formed in such a way that the spacer is not or only partially coated depending on a penetration depth of the plasma becomes. The casing forms a dark room shield. Charge carriers in a plasma have a defined mean free path and - especially if there is a small gap - cannot penetrate the cavity due to the plasma dark room shielding effect. This means that no continuous conductive layer can form between the carrier plates in a PECVD process.

The spacer and the casing preferably extend along an extension direction, a length of the casing along the extension direction being smaller than a length of the spacer along the extension direction. The direction of extension preferably runs along a longitudinal axis of the preferably cylindrical spacer. The length of the spacer is, for example, in the range from 8 to 12 mm, while the length of the casing is, for example, in the range from 5 to 11 mm, while the cavity has, for example, a width perpendicular to the direction of extension that is in the range from 0.5 to 3.0 mm lies.

The casing preferably also has a connecting section. The expression “connecting section” is to be understood as meaning a section of the casing which further extends perpendicularly or essentially perpendicularly to the direction in which the casing extends.

The connecting section is preferably formed at one of the two ends of the casing.

Alternatively, the connecting section is preferably designed centrally or centrally if the spacer is designed in two parts. If the two-part spacer is connected by means of the connecting section of the casing, this has the advantage that two cavities are thus created with a firm connection between the spacer and the casing. The connecting section is preferably positioned centrally along the direction of extent. It is preferably clamped between the two parts of the spacer.

In a first variant, the connecting section is preferably connected to or in contact with the spacer which the casing surrounds. The connecting section is preferably arranged in the middle or, more preferably, at one end of the casing. The casing preferably has a hollow cylinder section and the connecting section extending perpendicularly or essentially perpendicularly thereto. As a result, it is designed as a hollow cylinder with a bottom. The connecting section preferably has a connecting section inside diameter which is equal to an outside diameter of the spacer, while the hollow cylinder inside diameter is the casing inside diameter.

Such a holding device can be provided, for example, in that the casing with the hollow cylinder section and the connecting section with their respective inner diameters is pushed over the spacer, so that the cavity is formed due to the fact that the connecting section diameter is essentially the same as the outer diameter of the spacer and the shell inside diameter is larger than the outside diameter of the spacer. Such a holding device can, however, be provided, for example, in that a sheathing precursor, which has the connecting section and the connecting section inner diameter along its direction of extension, is pushed over the spacer and the hollow cylinder section is then milled or chamfered so that the sheathing inner diameter is formed, which is larger than the outer diameter of the spacer. When the holding device is operated in a gas phase separation device or PECVD separation device such as a pipe PECVD system, an electrically conductive layer only forms at the edges of the cavity between the casing and the spacer, since the coating plasma does not completely penetrate the cavity due to the principle the dark room shield.

As an alternative, the connecting section is preferably arranged between the spacer which the casing surrounds and one of the carrier plates, which this spacer separates from a further carrier plate. The second variant corresponds to the first variant with the difference that the connecting section is not arranged on the spacer, but rather adjacent to it. Even when the holding device according to the second variant is operated with a PECVD deposition, an electrically conductive layer only forms at the edges of the cavity, since the coating plasma does not completely penetrate the cavity due to the principle of dark room shielding.

In a preferred embodiment, the casing has a varying casing inside diameter. The casing inner diameter can be designed to be constantly increasing or decreasing, that is to say in a ramp shape, or can vary in an undulating manner. The casing inner diameter preferably tapers in the direction of the connecting section when the casing has the connecting section. More preferably, the shell inner diameter varies between a diameter that is substantially equal to an outer diameter of the spacer that the shell surrounds and at least one diameter that is greater than the outer diameter of the spacer. If the casing has the connecting section and the hollow cylinder section, the hollow cylinder inner diameter preferably varies, while the connecting section diameter is preferably constant.

In a preferred embodiment, the holding device furthermore has a connecting element which connects the casing and the spacer to one another. Such a holding device is realized, for example, in that the connecting element is attached to the spacer and the casing is attached to the connecting element. For example, the connection is realized by sliding the named components one over the other. The connecting element preferably has a connecting element inside diameter and a connecting element outside diameter, wherein the connecting element inside diameter is preferably essentially the same as the outside diameter of the spacer and the connecting element outside diameter is essentially the same as the sheathing inside diameter.

The connecting element is preferably designed as a ring. The ring preferably has a length along the extension direction of the casing and the spacer which is smaller than the length of the casing along the extension direction, which is furthermore preferably smaller than the length of the spacer along the extension direction. The ring is preferably arranged in the middle or centered on the spacer, so that two cavities are formed between the casing and the spacer, which are separated from one another by the ring, and two gaps are formed between the casing and the carrier plates, which are spaced apart by the spacer. Alternatively, the ring is preferably arranged adjacent to one of the two carrier plates spaced apart by the spacer, so that a cavity is formed between the casing and the spacer and a gap is formed between the casing and the other carrier plate that does not adjoin the ring. When the holding device is operated in a PECVD separation device such as a pipe PECVD system, an electrically conductive layer only forms at the edges of the cavity or cavities, since the coating plasma does not completely penetrate into the cavity or cavities due to the principle of dark room shielding.

In a preferred embodiment, all of the at least two carrier plates are connected to one another via one or more rods and the spacers are each arranged in the form of a sleeve surrounding the rods. The rods connecting all the carrier plates are preferably also formed from a ceramic, preferably aluminum oxide ceramic. The rods are preferably designed as solid cylinders. They are more preferably designed as a threaded rod which can be provided with a counter-element in order to interact with it to form a clamping unit.

The casing and the spacer are preferably designed in one piece. This one-piece part made of casing and spacer is preferably connected via the connecting section, which can be arranged at one end of the extension direction of the spacer and the casing, but does not have to, but can also be arranged, for example, in the middle or centered. Alternatively, the casing and the spacer are preferably two separately formed parts. The two separately formed parts are either pushed over one another in the holding device so that they are connected, or connected by means of the connecting element. The two-piece design has the advantage over the one-piece design that the casing and the spacer can be assembled to establish the connection between them and do not have to be made by turning or sintering together, so that the holding device can be manufactured more easily. In particular, if the casing and the spacer are made of ceramic, they are difficult to machine, so it is advantageous to manufacture the casing and the spacer as two simple parts, preferably as two sleeves, and to mount them in the holding device Has the connecting section in the form of a sleeve base.

The invention further relates to a PECVD separation device which has the holding device according to one or more of the embodiments described above. The PECVD separation device is preferably a pipe PECVD system.

The invention also relates to a use of the holding device in the deposition of one or more electrically conductive layers on a substrate such as a wafer.

The holding device is preferably intended for use in a PECVD deposition device in which one or more electrically conductive layers are deposited. In a preferred embodiment, the electrically conductive layer to be deposited is a doped layer, more preferably a doped amorphous silicon (aSi (n) or aSi (p)), silicon carbide (Sic (n) or SiC (p)), silicon nitride (SiNx) (n) or SiNx (p)), silicon oxynitride (SiN _x O _y (n) or SiN _x O _y (p)) or aluminum oxide (AlOx (n) or AlOx (p)) layer.

The holding device is preferably used to deposit a layer, the dopant concentration in the layer being> 10 ¹⁹ cm ^-3 . The dopant is preferably phosphorus or boron.

• 1 eine Teil-Querschnittsansicht einer Haltevorrichtung gemäß Stand der Technik;
• 2 eine Teil-Querschnittsansicht einer Haltevorrichtung gemäß einer ersten Ausführungsform;
• 3 eine Teil-Querschnittsansicht einer Haltevorrichtung gemäß einer zweiten Ausführungsform;
• 4 eine Teil-Querschnittsansicht einer Haltevorrichtung gemäß einer dritten Ausführungsform;
• 5 eine Teil-Querschnittsansicht einer Haltevorrichtung gemäß einer vierten Ausführungsform; und
• 6 eine Teil-Querschnittsansicht einer Haltevorrichtung gemäß einer fünften Ausführungsform.

Further properties and advantages of the invention are shown in connection with the figures and described below by way of example. They show schematically and not to scale:

• 1 a partial cross-sectional view of a holding device according to the prior art;
• 2 a partial cross-sectional view of a holding device according to a first embodiment;
• 3 a partial cross-sectional view of a holding device according to a second embodiment;
• 4th a partial cross-sectional view of a holding device according to a third embodiment;
• 5 a partial cross-sectional view of a holding device according to a fourth embodiment; and
• 6th a partial cross-sectional view of a holding device according to a fifth embodiment.

1 shows a partial cross-sectional view of a holding device according to the prior art. The holding device is designed to hold a plurality of substrates during plasma-assisted deposition of a layer from the gas phase on the substrates and as a boat for a PECVD deposition device. It has carrier plates (not shown) arranged parallel to one another for carrying the substrates during the deposition, the carrier plates being isolated from one another and alternately connected to connections of an alternating voltage generator. It also has bars 2 one of which is visible and which connect all the carrier plates together. It also has electrically insulating spacers 1 one of which is shown and which are arranged between the carrier plates and are designed to space the carrier plates from one another and to isolate them electrically from one another. The spacers 11 surround the bars 2 . The poles 2 are designed as full cylinders, while the spacers 11 are designed as a hollow cylinder.

In operation, the holding device is arranged in a PECVD deposition device, with a layer during the PECVD deposition 3 on the spacer 11 is trained.

2 shows a partial cross-sectional view of a holding device according to a first embodiment. In the 2 The first embodiment shown corresponds to that in FIG 1 Embodiment shown according to the prior art with the difference that the spacer 11 of a sheath 12th is surrounded. The sheath 12th has a sheath inner diameter d1 on that is larger than an outside diameter D3 of the spacer 11 is that it surrounds so that there is a cavity 4th between the sheath 12th and the spacer 11 is located. Both the sheath inside diameter d1 as well as the outside diameter D3 of the spacer 11 are constant. Furthermore, the casing has a casing outer diameter d2 that is constant. The spacer 11 has an inside diameter (not shown) that is equal to an outside diameter (not shown) of the rod 2 is. The sheath 12th has a connecting portion 121 and a hollow cylinder portion 122 on. The connecting section 121 is at one end of the hollow cylinder section 122 formed and between the spacer 11 and one of the two carrier plates (not shown), which are the spacers 11 and has a connecting portion inner diameter (not shown) substantially equal to an outer diameter of the rod 2 is. The sheath inside diameter d1 is therefore an inner diameter of the hollow cylinder section 122 . The spacer 11 and the sheath 12th can be designed in one or two pieces. Both the spacer 11 as well as the sheathing 12th extend along an extension direction E. along which the sheath is a length L1 which is smaller than a length L2 of the spacer 11 along the direction of extent E. . This is the sheathing 12th from one of the two through the spacer 11 spaced carrier plates spaced so that there is a gap 5 between the sheath 12th and this carrier plate is located.

During operation, the holding device is arranged in a PECVD separation device, for example a pipe PECVD system, the layer being formed during the PECVD process 3 on the sheath 12th , on one area of the spacer 11 who at the gap 5 adjoins, and at the edges of the cavity 4th forms because the coating plasma does not completely enter the cavity 4th penetrates due to the dark room shielding. This prevents a continuous layer 3 between the carrier plates formed by the spacer 11 are spaced.

3 shows a partial cross-sectional view of a holding device according to a second embodiment. In the 3 The second embodiment shown corresponds to that in FIG 2 shown first embodiment with the difference that the sheathing 12th has a varying sheath inner diameter and that the connecting portion 121 on the spacer 11 - and not between the spacers 11 and one of the carrier plates - is arranged and has a connecting portion inner diameter which is in Substantially equal to the outside diameter (not shown) of the spacer 11 is, the lengths, outer and inner diameters are not shown for the sake of clarity. The sheath inside diameter varies from a diameter substantially equal to the outside diameter of the spacer 11 and a diameter that is larger than the outer diameter of the spacer 11 is, extending in the direction of the connecting portion 121 rejuvenates.

In operation, the holding device is arranged in a PECVD deposition device, the layer being deposited during the PECVD process 3 on the sheath 12th , on one area of the spacer 11 who at the gap 5 adjoins, and at the edges of the cavity 4th forms.

4th shows a partial cross-sectional view of a holding device according to a third embodiment. In the 4th The third embodiment shown corresponds to that in FIG 2 shown first embodiment with the difference that the sheathing 12th with the spacer 11 by means of a connecting element 13th is connected, the lengths, outer and inner diameters not being shown for the sake of clarity. The connecting element 13th is designed as a ring with a connecting element inner diameter D4 and a fastener outer diameter D5 having. The inside diameter of the fastener D4 is substantially equal to the outside diameter (not shown) of the spacer 11 , and the fastener outside diameter D5 is substantially equal to the sheath inner diameter (not shown). The spacer 11 and the sheath 12th are designed in two parts.

In operation, the holding device is arranged in a PECVD deposition device, the layer being deposited during the PECVD deposition 3 on the sheath 12th , on one area of the spacer 11 who at the gap 5 adjoins, and at the edges of the cavity 4th forms.

5 shows a partial cross-sectional view of a holding device according to a fourth embodiment. In the 5 The fourth embodiment shown corresponds to that in FIG 4th shown third embodiment with the difference that the connecting element 13th not at one end of the spacer 11 and the sheath 12th is arranged, but in the middle, the lengths, outer and inner diameters are not shown for the sake of clarity. This creates two cavities 4th by the fastener 13th are separated. The length of the sheath 12th and the length of the spacer 11 are such that the sheathing 12th to those through the spacers 11 spaced support plates (not shown) are spaced so that two columns 5 between the sheath 12th and the two carrier plates are formed.

In operation, the holding device is arranged in a PECVD deposition device, the layer being deposited during the PECVD deposition 3 on the sheath 12th , on areas of the spacer 11 attached to the columns 5 adjoins, and at the edges of the cavities 4th forms.

6th Fig. 13 is a partial cross-sectional view of a holding device according to a fifth embodiment. In the 6th The fifth embodiment shown corresponds to that in FIG 2 shown first embodiment with the difference that the connecting portion 121 not at one end of the hollow cylinder section 122 is formed and that the spacer 11 is designed in two parts. The connecting section 121 is between the parts of the two-piece spacer 11 clamped, the lengths, outer and inner diameters not being shown for the sake of clarity. The length of the sheath 12th or the hollow cylinder section 122 and the length of the two-piece spacer 11 are such that the sheathing 12th to those through the spacers 11 spaced support plates (not shown) are spaced so that two columns 5 between the sheath 12th and the two carrier plates are formed.

In operation, the holding device is arranged in a PECVD deposition device, the layer being deposited during the PECVD deposition 3 on the sheath 12th , on areas of the spacer 11 attached to the columns 5 adjoins, and at the edges of the cavities 4th forms.

List of reference symbols

d1

Sheath inner diameter

d2

Sheath outer diameter

D3

Outside diameter of the spacer

D4

Fastener inside diameter

D5

Fastener outer diameter

E.

Direction of extension

L1

Length of the sheath

L2

Length of the spacer

1

Spacers

2

pole

3

electrically conductive layer

4th

cavity

5

gap

11

Spacers

12th

Sheathing

13th

Connecting element

121

Connection section

122

Hollow cylinder section

Claims (10)

Holding device for holding several substrates during plasma-assisted deposition of a layer from the gas phase on the substrates, which is designed as a boat for a PECVD deposition device and has: - Carrier plates arranged parallel to one another for carrying the substrates during the deposition, the carrier plates being insulated from one another and alternately connected to connections of an alternating voltage generator, and - electrically insulating spacers (11) which are arranged between the carrier plates and are designed to space the carrier plates from one another and to isolate them electrically from one another, - wherein the spacers (11) are each surrounded by a casing (12) with a casing inside diameter (d1) which is greater than an outside diameter (D3) of the spacer (11) which it surrounds, so that a cavity ( 4) is located between the casing (12) and the spacer (11). Holding device after Claim 1 , characterized in that the casing (12) is spaced from one or both of the adjacent carrier plates, which the spacer (11) is spaced apart. Holding device according to one of the preceding claims, characterized in that the casing (12) furthermore has a connecting section which is connected to the spacer (11) which the casing (12) surrounds, or which is between the spacer (11) which the Sheath (12) surrounds, and one of the carrier plates is arranged, which this spacer (11) is spaced from the further carrier plate. Holding device according to one of the preceding claims, characterized in that the casing (12) has a varying casing inside diameter (d1), the casing inside diameter (d1) preferably between a diameter which is equal to an outside diameter (D3) of the spacer ( 11), which it surrounds, and at least one diameter which is greater than the outer diameter (D3) of the spacer (11) varies. Holding device according to one of the preceding claims, characterized by a connecting element (13) that connects the casing (12) and the spacer (11) to one another. Holding device after Claim 5 , characterized in that the connecting element (13) is designed as a ring with a connecting element inside diameter (D4) and a connecting element outside diameter (D5), the ring being surrounded by the casing (12) which has a casing inside diameter ( d1), which is essentially equal to the connecting element outer diameter (D5), and wherein the ring on the spacer (11) is arranged at least partially surrounding it and the connecting element inner diameter (D4) is essentially equal to an outer diameter (D3) of the spacer (11). Holding device according to one of the preceding claims, characterized in that all of the at least two carrier plates are connected to one another via one or more rods (2) and the spacers (11) are each arranged in the form of a sleeve surrounding the rods (2). PECVD separation device, comprising a holding device according to one of the preceding claims. Use of a holding device according to one of the Claims 1 to 7th in the deposition of an electrically conductive layer on a substrate. Use after Claim 9 , characterized in that the electrically conductive layer is a doped amorphous silicon, silicon nitride, silicon oxynitride or aluminum oxide layer.

Images