Apparatus and method for depositing materials onto microelectronic workpieces

Abstract

Reactors for vapor deposition of materials onto a microelectronic workpiece, systems that include such reactors, and methods for depositing materials onto microelectronic workpieces. In one embodiment, a reactor for vapor deposition of a material comprises a reaction chamber and a gas distributor. The reaction chamber can include an inlet and an outlet. The gas distributor is positioned in the reaction chamber. The gas distributor has a compartment coupled to the inlet to receive a gas flow and a distributor plate including a first surface facing the compartment, a second surface facing the reaction chamber, and a plurality of passageways. The passageways extend through the distributor plate from the first surface to the second surface. Additionally, at least one of the passageways has at least a partially occluded flow path through the plate. For example, the occluded passageway can be canted at an oblique angle relative to the first surface of the distributor plate so that gas flowing through the canted passageway changes direction as it passes through the distributor plate.

Description

TECHNICAL FIELD

The present invention is related to the field of thin film deposition in the manufacturing of micro-devices.

BACKGROUND

Thin film deposition techniques are widely used in the manufacturing of microelectronic devices to form a coating on a workpiece that closely conforms to the surface topography. The size of the individual components in the devices is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) is increasing. The size of workpieces is also increasing to provide more real estate for forming more dies (i.e., chips) on a single workpiece. Many fabricators, for example, are transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms and corners in deep depressions that have very small openings.

One widely used thin film deposition technique is Chemical Vapor Deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a thin solid film at the workpiece surface. The most common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction.

Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then a high workpiece temperature is needed to achieve a reasonable deposition rate. Such high temperatures are not typically desirable because heating the workpiece can be detrimental to the structures and other materials that are already formed on the workpiece. Implanted or doped materials, for example, migrate in the silicon substrate when a workpiece is heated. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the substrate. This is not desirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used. Thus, CVD techniques may not be appropriate for many thin film applications.

Atomic Layer Deposition (ALD) is another thin film deposition technique that addresses several of the drawbacks associated with CVD techniques. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules A_x coats the surface of a workpiece W. The layer of A_x molecules is formed by exposing the workpiece W to a precursor gas containing A_x molecules, and then purging the chamber with a purge gas to remove excess A_x molecules. This process can form a monolayer of A_x molecules on the surface of the workpiece W because the A_x molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of A_x molecules is then exposed to another precursor gas containing B_y molecules. The A_x molecules react with the B_y molecules to form an extremely thin solid layer of material on the workpiece W. The chamber is then purged again with a purge gas to remove excess B_y molecules.

FIG. 2 illustrates the stages of one cycle for forming a thin solid layer using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor A_x, (b) purging excess A_x molecules, (c) exposing the workpiece to the second precursor B_y, and then (d) purging excess B_y molecules. In actual processing several cycles are repeated to build a thin film on a workpiece having the desired thickness. For example, each cycle may form a layer having a thickness of approximately 0.5-1.0 Å, and thus it takes approximately 60-120 cycles to form a solid layer having a thickness of approximately 60 Å.

FIG. 3 schematically illustrates an ALD reactor 10 having a chamber 20 coupled to a gas supply 30 and a vacuum 40. The reactor 10 also includes a heater 50 that supports the workpiece W and a gas dispenser 60 in the chamber 20. The gas dispenser 60 includes a plenum 62 operatively coupled to the gas supply 30 and a distributor plate 70 having a plurality of holes 72. In operation, the heater 50 heats the workpiece W to a desired temperature, and the gas supply 30 selectively injects the first precursor A_x, the purge gas, and the second precursor B_y as shown above in FIG. 2. The vacuum 40 maintains a negative pressure in the chamber to draw the gases from the gas dispenser 60 across the workpiece W and then through an outlet of the chamber 20.

One drawback of ALD processing is that it is difficult to avoid mixing between the first and second precursors in the chamber apart from the surface of the workpiece. For example, a precursor may remain on surfaces of the gas dispenser or on other surfaces of the chamber even after a purge cycle. This results in the unwanted deposition of the solid material on components of the reaction chamber. The first and second precursors may also mix together in a supply line or other area of a reaction chamber to prematurely form solid particles before reaching the surface of the workpiece. Thus, the components of the ALD reactor and the timing of the A_x/purge/B_y/purge pulses of a cycle should not entrap or otherwise cause mixing of the precursors in a manner that produces unwanted deposits or premature reactions.

Another drawback of ALD processing is that the film thickness may be different at the center of the workpiece than at the periphery. To overcome this problem, the center of some distributor plates do not have any holes 72. In practice, however, this may cause the film at the center of the workpiece to be thinner than the film at the periphery. Moreover, the center portion of such plates may become coated with the solid material because it is difficult to purge all of the precursors from this portion of the gas dispenser 60 during normal purge cycles. Therefore, there is a need to resolve the problem of having a different film thickness at the center of the workpiece than at the periphery.

SUMMARY

The present invention is directed toward reactors for deposition of materials onto a micro-device workpiece, systems that include such reactors, and methods for depositing materials onto micro-device workpieces. In one embodiment, a reactor for depositing a material comprises a reaction chamber and a gas distributor that directs gas flows to a workpiece. The reaction chamber can include an inlet and an outlet, and the gas distributor is positioned in the reaction chamber. The gas distributor has a compartment coupled to the inlet to receive a gas flow and a distributor plate including a first surface facing the compartment, a second surface facing the reaction chamber, and a plurality of passageways. The passageways extend through the distributor plate from the first surface to the second surface. Additionally, at least one of the passageways has at least a partially occluded flow path through the plate. For example, the occluded passageway can be canted at an oblique angle relative to the first surface of the distributor plate so that gas flowing through the canted passageway changes direction as it passes through the distributor plate.

The compartment of the gas distributor can be defined by a sidewall, and the distributor plate can extend transverse relative to the sidewall. In one embodiment, the distributor plate has an inner region, an outer region, and a peripheral edge spaced laterally inward from the sidewall to define a gap between the peripheral edge and the sidewall. In other embodiments, the peripheral edge of the distributor plate can be coupled to the sidewall.

The distributor plate can have several different embodiments. The distributor plate, for example, can have a first plurality of passageways in the inner region that are canted at an oblique angle relative to the first surface of the distributor plate, and a second plurality of passageways in the outer region that are generally normal to the first surface of the distributor plate. In another embodiment, all of the passageways through the distributor plate can be canted at an angle. The size of the passageways can also vary across the distributor plate. In one embodiment, a first plurality of passageways in the inner region have a cross-sectional dimension of approximately 0.01-0.07 inch, and a second plurality of passageways in the outer region have a cross-sectional dimension of approximately 0.08-0.20 inch. In still other embodiments, a first plurality of passageways in the inner region are canted at a first oblique angle relative to the first surface of the distributor plate, and a second plurality of passageways in the outer region are canted at a second oblique angle relative to the first surface of the distributor plate. The canted passageways are generally angled downward and radially outward from the first surface to the second surface to direct the gas flow radially outward across the surface of the workpiece. For example, the canted passageways can extend at an angle of approximately 15 degrees to approximately 85 degrees relative to the first surface of the distributor plate. The passageways, however, can be angled at different angles or canted in different directions in other embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic cross-sectional views of stages in atomic layer deposition processing in accordance with the prior art.

FIG. 2 is a graph illustrating a cycle for forming a layer using atomic layer deposition in accordance with the prior art.

FIG. 3 is a schematic representation of a system including a reactor for vapor deposition of a material onto a microelectronic workpiece in accordance with the prior art.

FIG. 4 is a schematic representation of a system having a reactor for depositing a material onto a micro-device workpiece in accordance with one embodiment of the invention.

FIG. 5 is an isometric, cross-sectional view illustrating a portion of a reactor for depositing a material onto a micro-device workpiece in accordance with an embodiment of the invention.

FIG. 6 is a cross-sectional view of a reactor for depositing a material onto a micro-device workpiece in accordance with another embodiment of the invention.

FIG. 7 is a partial cross-sectional view of a distributor plate for use in a reactor for depositing a material onto a micro-device workpiece in accordance with another embodiment of the invention.

FIG. 8 is a schematic representation of a system including a reactor for depositing a material onto a micro-device workpiece in accordance with another embodiment of the invention.

DETAILED DESCRIPTION

The following disclosure is directed toward reactors for depositing a material onto a micro-device workpiece, systems including such reactors, and methods for depositing a material onto a micro-device workpiece. Many specific details of the invention are described below with reference to depositing materials onto micro-device workpieces. The term “micro-device workpiece” is used throughout to include substrates upon which and/or in which microelectronic devices, micromechanical devices, data storage elements, and other features are fabricated. For example, micro-device workpieces can be semiconductor wafers, glass substrates, insulative substrates, and many other types of materials. The term “gas” is used throughout to include any form of matter that has no fixed shape and will conform in volume to the space available, which specifically includes vapors (i.e., a gas having a temperature less than the critical temperature so that it may be liquified or solidified by compression at a constant temperature). Additionally, several aspects of the invention are described with respect to Atomic Layer Deposition (“ALD”), but certain aspects may be applicable to other types of deposition processes. Several embodiments in accordance with the invention are set forth in FIGS. 4-8 and the related text to provide a thorough understanding of particular embodiments of the invention. A person skilled in the art will understand, however, that the invention may have additional embodiments, or that the invention may be practiced without several of the details in the embodiments shown in FIGS. 4-8.

A. Deposition Systems

FIG. 4 is a schematic representation of a system 100 for depositing a material onto a micro-device workpiece W in accordance with an embodiment of the invention. In this embodiment, the system 100 includes a reactor 110 having a reaction chamber 120 coupled to a gas supply 130 and a vacuum 140. For example, the reaction chamber 120 can have an inlet 122 coupled to the gas supply 130 and an outlet 124 coupled to the vacuum 140.

The gas supply 130 includes a plurality of gas sources 132 (identified individually as 132a-c), a valve assembly 133 having a plurality of valves 134 (identified individually as 134a-c), and a plurality of gas lines 136 and 137. The gas sources 132 can include a first gas source 132a for providing a first precursor gas “A,” a second gas source 132b for providing a second precursor gas “B,” and a third gas source 132c for providing a purge gas P. The first and second precursors A and B can be the constituents that react to form the thin, solid layer on the workpiece W. The p-urge gas P can a type of gas that is compatible with the reaction chamber 120 and the workpiece W. The first gas source 132a is coupled to a first valve 134a, the second gas source 132b is coupled to a second valve 134b, and the third gas source 132c is coupled to a third valve 134c. The valves 134a-c are operated by a controller 142 that generates signals for pulsing the individual gases through the reaction chamber 120 in a number of cycles. Each cycle can include a first pulse of the first precursor A, a second pulse of the purge gas, a third pulse of the second precursor B, and a fourth pulse of the purge gas.

The reactor 110 in the embodiment illustrated in FIG. 4 also includes a workpiece support 150 and a gas distributor 160 in the reaction chamber 120. The workpiece support 150 can be a plate having a heating element to heat the workpiece W to a desired temperature for catalyzing the reaction between the first precursor A and the second precursor B at the surface of the workpiece W. The workpiece support 150, however, may not be heated in all applications.

The gas distributor 160 is positioned at the inlet 122 of the reaction chamber 120. The gas distributor 160 has a compartment or plenum 162 that is defined, at least in part, by a sidewall 164. The compartment or plenum 162 can be further defined by a chamber lid 166. The gas distributor 160 further includes a distributor plate 170 having a first surface 171a facing the compartment 162, a second surface 171b facing away from the compartment 162, and a plurality of passageways 172 (identified by reference numbers 172a and 172b). As explained in more detail below, a gas flow F in the compartment 162 flows through the passageways 172a-b and through a gap 180 between the sidewall 164 and the distributor plate 170. As explained in more detail below, this particular embodiment of the distributor plate 170 performs the following functions: (a) directs the gas flow F to provide a more uniform film thickness across the workpiece W; and (b) limits areas in the reaction chamber where the precursors can adduct and mix prematurely before contacting the workpiece.

B. Gas Distributors and Distributor Plates

FIG. 5 illustrates a particular embodiment of the gas distributor 160 and the distributor plate 170 in greater detail. In this embodiment, the distributor plate 170 has an inner region 173a with a first plurality of passageways 172a and an outer region 173b with a second plurality of passageways 172b. The first passageways 172a extend from the first surface 171a to the second surface 171b, and at least a portion of each of the first passageways 172a is at least partially occluded along a flow path to the plate 170. In this particular embodiment, the first passageways 172a are occluded by being canted at an oblique angle relative to the first surface 171a and/or the plane defined by the plate 170. The term “occluded,” as used herein, is not limited to an obstruction that blocks the passageways 172, but rather means that some of the gas molecules flowing through the first passageways 172a cannot flow through the plate 170 along a direct “line-of-sight” between the first surface 171a and the second surface 171b normal to the plane defined by the plate 170. It will be appreciated that canting the first passageways 172a at an oblique angle relative to the plate 170 can either fully or at least partially block the direct line-of-sight to the workpiece while still allowing gas to flow through the first passageways 172a. The first passageways 172a can be canted at an angle of approximately 15° to approximately 85° relative to the plane defined by the plate 170. The second passageways 172b extend through the plate 170 generally normal to the first surface 171a such that they provide a direct line-of-sight to the workpiece throughout the full cross-sectional dimension of the second passageways 172b. The second passageways 172b can also have bevels 176 at the first surface 171a and/or the second surface 171b.

The distributor plate 170 is carried by a number of retainers 177 that are coupled to the lid 166 or another component of the reaction chamber 120. The retainers 177 are brackets, posts, or other suitable devices that can hold the distributor plate 170 relative to the inlet 122 and the sidewall 164. In this embodiment, the distributor plate 170 has a peripheral edge 175 spaced apart from the sidewall 164 by an annular gap 180. In operation, therefore, the gas flow F has a first component F₁ that flows through the first passageways 172a, a second component F₂ that flows through the second passageways 172b, and a third component F₃ that flows through the gap 180. The first passageways 172a direct the first flow component F₁ downward and radially outward to prevent over-saturating the center portion of the workpiece with the precursors. The second passageways 172b direct the second flow component F₂ downward and generally normal to the plate 170 to provide more gas molecules to an outer region of the workpiece. The gap 180 also provides an enhanced flow of gas at the outer and peripheral regions of the workpiece.

Several embodiments of the distributor plate 170 are accordingly expected to provide more uniform saturation of the workpiece W with the first and second precursors A and B to provide a more uniform layer of material on the workpiece. Additionally, because the inner region 173a of the plate 170 includes the first plurality of passageways 172a, the surface areas upon which the first and second precursors A and B can adduct is reduced compared to conventional plates that do not have any openings in the inner region. This is expected to reduce the build up of the deposited material on the first surface 171a of the distributor plate 170. It is also expected that such a reduction in the surface area will enhance the ability to control the uniformity of the deposited layer and the endpoints of the gas pulses for better quality depositions and enhanced throughput.

The first passageways 172a can also have a different cross-sectional dimension than the second passageways 172b as shown in the particular embodiment illustrated in FIG. 5. The first passageways, for example, can have openings of approximately 0.01-0.07 inch, and the second passageways 172b can have openings of approximately 0.08-0.20 inch. In a particular embodiment, the first passageways 172a at the inner region 173a have a circular opening with a diameter of approximately 0.03 inch, and the second passageways 172b in the outer region 173b have a circular opening with a diameter of approximately 0.10 inch. It will be appreciated that the cross-sectional size of the first and second passageways 172a-b can be the same, or that they can have cross-sectional dimensions that are different than the ranges set forth above.

The passageways 172 can accordingly be configured to further enhance or restrict the gas flow to particular areas of the workpiece by canting, or otherwise occluding selected passageways, and/or varying the sizes of the cross-sectional dimensions of the passageways. In the embodiment shown in FIG. 5, for example, the smaller cross-sectional dimension of the first passageways 172a inhibits gas molecules from contacting the central region of the workpiece W, and the larger cross-sectional dimension of the second passageways 172b enhances the number of gas molecules that contact the outer region of the workpiece. Therefore, the cross-sectional dimensions and the angles of inclination of the passageways can be used either separately or together to provide the desired distribution of gas to the surface of the workpiece.

FIG. 6 is a cross-sectional view of a distributor plate 670 in accordance with another embodiment of the invention. Several components of the distributor plate 670 are the same as the distributor plate 170, and thus like reference numbers refer to like components in FIGS. 4-6. The distributor plate 670 can include a plurality of passageways 172 that are canted at an oblique angle relative to the plane defined by the plate 670. In this embodiment, all of the passageways 172 are canted at the same angle. The angle of inclination can be approximately 15 degrees to approximately 85 degrees. In operation, the embodiment of the distributor plate 670 shown in FIG. 6 has a first flow component F₁ that flows radially outwardly and downward from the plate 670, and a second flow component F₂ that flows through the gap 180. The passageways 172 can have the same cross-sectional dimensions, or they can have different cross-sectional dimensions similar to the plate 170 described above.

FIG. 7 is a partial cross-sectional view of a distributor plate 770 in accordance with another embodiment of the invention. The distributor plate 770 is similar to the distributor plate 170, and thus like reference numbers refer to like components in FIGS. 4, 5 and 7. In this embodiment, the first passageways 172a at the inner region 173a are canted at a first angle α, and the second passageways 172b in the second region 173b are canted at a second angle β. The angle α is generally less than the angle β relative to the plane P-P defined by the plate 770. As such, the first passageways 172a have a first occlusion area A₁ in which there is no direct line-of-sight through the plate 770 to the workpiece W along a path normal to the plate 170. The second passageways 172b, however, have a smaller occlusion area A₂ because the higher angle β allows gas to pass completely through a portion of the second passageways 172b along a path normal to the plate 770 or the workpiece W. By increasing the size of the occlusion area A₁ for the first passageways 172a relative to the occlusion area A₂ for the second passageways 172b, fewer gas molecules are likely to be deposited on the central region C of the workpiece W. It will be appreciated that the distributor plate 770 can have variable canting of the passageways 172 from the center to the perimeter of the plate along a continuum or throughout several regions in which the angle of incline increases toward the periphery of the plate 770. Accordingly, in other embodiments, the distributor plate 770 can have more than two regions in which the passageways are canted at different angles.

C. Additional Deposition Systems

FIG. 8 is a schematic illustration of another embodiment of a system 800 for depositing a material onto a microelectronic workpiece. The system 800 is similar to the system 100, and thus like reference numbers refer to like components in FIGS. 4 and 8. The difference between the system 800 and the system 100 is that the system 800 includes a gas distributor 860 with a distributor plate 870 that extends to the sidewall 164 to eliminate the gap 180 shown in FIG. 4. It will be appreciated that the distributor plate 870 can include any of the distributor plates explained above with reference to FIGS. 4-7. Therefore, other aspects of the invention include a completely enclosed compartment or plenum 862.

From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

Claims

1. A reactor for vapor deposition of a material onto a microelectronic workpiece having an inner region and an outer region radially outward from the inner region, comprising: a reaction chamber having an inlet and an outlet; anda gas distributor in the reaction chamber, the gas distributor having a plenum through which a gas can flow along a flow path, the plenum having a distributor plate transverse to the flow path, and the distributor plate having a plurality of passageways extending through the plate, the passageways including first passageways having a first physical parameter and extending through the plate at an oblique angle relative to a plane defined by the plate and second passageways having a second physical parameter different than the first physical parameter and extending through the plate at an angle substantially perpendicular to the plane, and wherein the first passageways are in an inner region of the plate to supply the gas to the inner region of the microelectronic workpiece and the second passageways are in an outer region of the plate to supply the gas to the outer region of the microelectronic workpiece.

2. The reactor of claim 1 wherein: the plenum comprises a sidewall; andthe distributor plate has a peripheral edge spaced laterally inward from the sidewall to define a gap between the peripheral edge and the sidewall.

3. The reactor of claim 2 wherein the first passageways are canted at an angle of approximately 15° to approximately 85° relative to the plane defined by the distributor plate.

4. The reactor of claim 2 wherein the first passageways in the inner region have a cross-sectional dimension of approximately 0.01-0.07 inch, and the second passageways in the outer region have a cross-sectional dimension of approximately 0.08-0.20 inch.

5. The reactor of claim 1 wherein: the plenum comprises a sidewall; andthe distributor plate has a peripheral edge coupled to the sidewall.

6. A reactor for vapor deposition of a material onto a microelectronic workpiece having an inner region and an outer region radially outward from the inner region, comprising: a reaction chamber having an inlet and an outlet; anda gas distributor in the reaction chamber that is coupled to the inlet, the gas distributor comprising a plenum having a single compartment through which a gas can flow and a distributor plate, the distributor plate having a first surface facing the compartment, a second surface facing away from the compartment, and a plurality of passageways extending from the first surface to the second surface, wherein the passageways include first passageways located at a central portion of the distributor plate to supply the gas to the inner region of the microelectronic workpiece, the first passageways extending at an oblique angle relative to the first surface of the distributor plate, and wherein the passageways also includes second passageways located in an outer region of the distributor plate to supply the gas to the outer region of the microelectronic workpiece, the second passageways having a physical characteristic different from the first passageways and extending through the plate at an angle substantially perpendicular to the first surface of the distributor plate.

7. The reactor of claim 6 wherein: the compartment comprises a sidewall; andthe distributor plate has a peripheral edge spaced laterally inward from the sidewall to define a gap between the peripheral edge and the sidewall.

8. The reactor of claim 7 wherein the first passageways are canted at an angle of approximately 15° to approximately 85° relative to the first surface of the distributor plate.

9. The reactor of claim 7 wherein the first passageways in the central portion have openings of approximately 0.01-0.07 inch and the second passageways in the outer region have openings of approximately 0.08-0.20 inch.

10. The reactor of claim 6 wherein: the compartment comprises a sidewall; andthe distributor plate has a peripheral edge coupled to the sidewall.

11. A reactor for vapor deposition of a material onto a microelectronic workpiece having an inner region and an outer region radially outward from the inner region, comprising: a reaction chamber having an inlet and an outlet; anda gas distributor in the reaction chamber, the gas distributor having a plenum through which a gas can flow along a flow path, the plenum having a distributor plate transverse to the flow path, and the distributor plate having a plurality of passageways extending through the plate, the passageways including first passageways extending through the plate at an oblique angle relative to a plane defined by the plate to supply the gas to the inner region of the microelectronic workpiece, and second passageways extending through the plate at an angle substantially perpendicular to the plane to supply the gas to the outer region of the microelectronic workpiece, and wherein the first passageways are in an inner region of the plate and the second passageways are in an outer region of the plate.

12. The reactor of claim 11 wherein: the plenum comprises a sidewall; andthe distributor plate has a peripheral edge spaced laterally inward from the sidewall to define a gap between the peripheral edge and the sidewall.

13. The reactor of claim 11 wherein the first passageways have a first cross-sectional dimension and the second passageways have a second cross-sectional dimension different than the first cross-sectional dimension.

14. The reactor of claim 11 wherein the first passageways are canted at an angle of approximately 15° to approximately 85° relative to the plane defined by the distributor plate.

15. The reactor of claim 11 wherein the first passageways in the inner region have a cross-sectional dimension of approximately 0.01-0.07 inch, and the second passageways in the outer region have a cross-sectional dimension of approximately 0.08-0.20 inch.

16. The reactor of claim 11 wherein: the plenum comprises a sidewall; andthe distributor plate has a peripheral edge coupled to the sidewall.