JPH04186825A - Vapor phase growth equipment - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is mainly directed to a metal organic chemical vapor deposition (MOCVD) apparatus for growing a compound semiconductor film such as GaAs on a substrate. Depending on the configuration of the device, the susceptor, which is stacked in stages so that the surface on which the film-forming substrate is attached is horizontal, is placed in a box-shaped or cylindrical reaction vessel, and a reaction gas is introduced and caused to flow horizontally. , relates to a vapor phase growth apparatus for growing a thin film on the film-forming substrate by heating a susceptor.

;Prior Art] FIG. 7 shows an example of a conventional configuration of this type of MOCVD apparatus. A graphite susceptor 2 formed in the shape of a disk or a square plate to which a substrate 1 is attached horizontally is attached inside a cylindrical reaction vessel 3 made of quartz glass. A reaction gas (in the MOCVD device, a mixed gas of an alkylated product of a group I element and a hydride of a group V element, which is gasified by bubbling hydrogen gas, hereinafter referred to as the reaction gas) is introduced into the end of the reaction vessel 3. The reaction gas is introduced from the formed reaction gas inlet 6. Also, high frequency induction heating coil 4
is wound around the outside of the reaction vessel 3, and the graphite susceptor 2 is heated by passing a high frequency current through it.

Reaction gas introduced through the reaction gas inlet 6 at the end of the reaction vessel 3: Gas that is thermally decomposed on the surface of the substrate 1 attached to the susceptor 2, heated to a high Ji, and decomposed on the substrate surface. Molecules are deposited to form a film.

The gas that has passed through the substrate surface is exhausted from the exhaust port 7 of the exhaust flange 5 attached to the end opposite to the inlet.

[Problem that the invention seeks to solve: The above-mentioned device has the following problems.

(1) When forming a film, the reactive gas is consumed to form a film on the upstream side of the substrate surface, and the density decreases on the downstream side, and the thickness of the antinode that grows on the substrate is increased upstream of the reactive gas. Furthermore, due to the cross-sectional shape of the flow path around the susceptor, the thickness of the film differs between the center of the substrate and the left and right sides.

(2) In order to obtain good film quality, it is desirable that the flow of the reaction gas be laminar near the substrate, but since the reaction gas introduced from the reaction gas inlet of the reaction vessel hits the side of the susceptor, The flow of the reactant gas is disturbed in the vicinity, making it impossible to obtain a laminar flow near the substrate.

In order to eliminate the drawbacks of the above-mentioned [1, 1, (2+ terms), in Japanese Patent Application Laid-open No. 132543/1983, a susceptor that is induction heated in a quartz glass reaction vessel around which an induction heating coil is wound is proposed. , two shapes are cut out to an appropriate thickness at an appropriate position in the direction of the rotational axis of the spheroid, and a substrate is attached to the end face with a smaller area, so that the gas that hits the susceptor is caused to flow along the inclined surface of the susceptor. The susceptor is rotated around the axis of rotation of the spheroid, so that it rises while maintaining a laminar flow state and passes over the substrate surface in a laminar state, and by rotating the susceptor around the rotation axis of the spheroid, a good quality and thick film is produced on the substrate surface. A reaction vessel equipped with a one-rotation drive mechanism that forms a uniform thin film is disclosed. However, this reaction vessel is based on the premise that the sassefuri is heated by induction, and there is All of the rotating mechanism members in the publication are made of quartz glass, and in this publication, a quartz glass bevel gear with a vertical axis is attached to the bottom side of the susceptor, and the quartz glass bevel gear that meshes with this bevel gear is placed inside the reaction vessel. A configuration is shown in which the drive shaft is attached to the tip of a horizontally running quartz glass drive shaft, and the drive shaft is driven from outside the reaction vessel to rotate the filter.'/force)'-1 In this way. , the susceptor is anti-2
When it is rotated through gears arranged in the container, quartz powder produced by the abrasion of the tooth surfaces adheres to the substrate surface and becomes Il! This results in the problem of degrading quality. In addition, reaction vessels that are designed for induction heating require a large-volume, high-cost high-frequency power source to supply high-frequency current to the induction heating coil for induction heating, which poses economical problems. .

(3) When film formation is performed, the inside of the reaction container becomes dirty due to the adhesion of reaction products, so it is necessary to remove the reaction container and clean it, but the reaction container is designed to maintain airtightness. The disadvantage of installation is that removal is a difficult task.

(4) The reaction gas thermally decomposes and becomes a reaction product that adheres to the inside of the container, and the amount of undecomposed reaction gas molecules that contribute to film formation decreases on the downstream side of the substrate surface. There is a drawback that decomposed gas molecules are not effectively utilized for film formation.

(5) High-purity quartz glass is used for the reaction vessel to prevent impurities from entering the film during film formation and to withstand high temperatures. However, quartz glass has low tensile strength and impact strength, so They have the disadvantage that they can be damaged if the pressure difference between the inside and outside of the container becomes large, or if a shock is accidentally applied.

Furthermore, as the size of the substrate increases, the reaction vessel also needs to be made larger, but quartz glass has the disadvantage of having limitations in manufacturing.

An object of the present invention is to provide a vapor phase growth apparatus in which the above-mentioned problems are solved.

[Means to solve the problem]

In order to solve the above problems, the present invention takes the following measures in response to the problems in each of the above sections.

(1) Corresponding to problem No. 1, the vapor phase growth apparatus with the configuration targeted by the present invention, that is, the susceptor installed so that the surface to which the film-forming substrate is attached is horizontal, is shaped like a box. Alternatively, a heating means for heating the susceptor in a vapor phase growth apparatus that grows one film on the film-forming substrate by heating the susceptor by introducing a reaction gas into a cylindrical reaction vessel and flowing it in a horizontal direction. is a resistive heater, and the heater and the susceptor are mounted on the substrate of the susceptor so as to be rotatable around an axis perpendicular to the plane.

(2) Corresponding to problem No. 2 and No. 3 rJ,
In the apparatus of 2. above, the apparatus further includes a desorption device shaped so that the entire amount of the reaction gas introduced into the reaction vessel flows through a desired flow path cross section at least in the vicinity of the susceptor, in an area extending from the upstream side of the susceptor to the downstream side of the susceptor. A possible quartz glass inner vessel is installed within the reaction vessel and H2 gas, N2 gas or inert gas is allowed to flow between the reaction vessel and the inner vessel.

(3) Corresponding to problem No. 4, in the apparatus of No. 2 above, the quartz glass inner container is shaped so that the upper surface is inclined so that the upper surface approaches the upper surface of the susceptor as the downstream side of the reaction gas approaches. do.

(4) Corresponding to problem item 5, in the apparatus of item 1, item 2, or item 3 above, the reaction container is a metal container.

(5) Furthermore, in order to solve the problems associated with the use of a metal container for the reaction container in the apparatus of item 2 or 3 above, the reaction gas inlet of the reaction container and the reaction gas in the internal container are The structure is such that the inlet and the inlet are brought into close contact with each other over the entire circumference of the inlet using a spring.

[Effect]

By applying the above-described measures to the vapor phase growth apparatus having the structure targeted by the present invention, the following actions or effects are produced.

(1) By means 1, the substrate surface rotates in the same plane as the flow direction of the reaction gas, and the film thickness distribution of the film formed on the substrate surface changes between the upstream side and the downstream side of the substrate surface. This is greatly improved without causing any difference, and in particular, the non-uniformity of the film thickness distribution in the circumferential direction of the substrate surface is completely eliminated.

In addition, since a resistance heater is used as the heating means for the susceptor, there are fewer restrictions on the material of the rotating mechanism members, such as when heating the susceptor by induction, and it is easier to form the rotating mechanism inside the reaction vessel in terms of structure and material. becomes. Therefore, it is possible to easily eliminate the fear of dust generation when using gears in the rotation mechanism.

Furthermore, since there is no induction heating coil surrounding the pig, the rotation axis of the heater can be guided out through the peripheral wall of the reaction vessel perpendicularly with a length of 11 L, which simplifies the device configuration including the rotation mechanism. Ru.

(2) According to means 2, for example, the internal container made of quartz glass is formed so that the cross section of the gas flow path is rectangular, and the bottom surface of the rectangle is flush with the surface on which the susceptor is attached to the substrate. The reaction gas flow can pass through the substrate surface in a completely laminar flow state regardless of the cross-sectional shape of the reaction container, and the film quality is greatly improved.

Further, the gas flow path (flow vA) is not deformed at the susceptor position, the gas flow velocity is equal across the entire vertical cross section of the flow path, and the gas density does not change in the left-right direction of the substrate. Furthermore, since the thickness of the channel on the substrate is the same in the left and right directions of the substrate, there is no difference in the amount of gas applied to film formation in the left and right directions. Therefore, non-uniformity in film thickness does not occur in the left-right direction of the substrate.

Therefore, by rotating the substrate together with the heater for the susceptor 1, it is possible to form a film whose thickness is substantially completely uniform regardless of whether it is on the upstream side, downstream side, or left/right direction of the substrate.

Furthermore, by flowing H2 gas 1N2 gas or an inert gas as a so-called sweep gas between the reaction vessel and the internal vessel, the reaction products will not adhere to the reaction vessel but only to the internal vessel. When the pressure of these sweep gases is made almost the same as that inside the inner container, the airtightness between each part when the inner container is composed of multiple container parts, and the airtightness of the joint between the inner container and the reaction container can be improved. Even if it is not strict, the amount of leakage of the reaction gas to the outside of the internal container or the amount of the sweep gas mixed into the reaction gas will be extremely small, and even if the sweep gas is mixed into the reaction gas, it can be used as a sweep gas. , the same Ht gas or N2 gas as the carrier gas in the reaction gas.

By using an inert gas, I! ! Since the effect of the sieve gas on the quality is negligible, it is possible to form the inner container with emphasis on ease of attachment to and removal from the reaction vessel, which greatly facilitates cleaning operations.

In addition, the reaction gas is introduced into the inner container with a lower ceiling than the reaction container, and the introduced reaction gas is thermally decomposed efficiently, so the amount of reaction gas used for film formation is small, and the raw materials can be used effectively. can be used.

(3) According to means 3, if the quartz glass inner container is tilted in front so that the upper surface approaches the upper surface of the susceptor as the downstream side of the reaction gas flows, it will not disassemble on the upstream side of the substrate surface. , gas molecules moving downstream in an undecomposed state approach the substrate surface as they move, heating the substrate more effectively than in an internal container with a horizontal top surface.

The reactant gas is decomposed and contributes to film formation on the downstream side of the substrate surface, and the reactive gas is used more effectively for film formation.

(4) According to means 1, the heating means for heating the susceptor is a resistance heater, so as shown in means 4, it is possible to use a metal container as the reaction vessel, and the inside of the reaction vessel is There is no risk of the container breaking even if there is a large pressure difference with the outside or if it receives some mechanical or thermal shock. The reaction gas used for -G in film formation is dangerous to the human body, and by using a metal container as the reaction container, the safety of the apparatus is significantly improved.

(5) According to item 5 of the means, the joint on the reaction vessel side that is joined to the inner vessel and allows the reaction gas introduced into the reaction vessel to flow into the inner vessel is simplified, and the metal vessel is not required to maintain heat resistance strength. The container can be uniformly cooled throughout.

〔Example〕

FIG. 1 shows an embodiment of claim 11JI. In the figure, the same reference numerals are attached to the same functional members as in FIG. 7. Inside the cylindrical reaction vessel 3 molded from quartz glass and placed with its axis horizontal, there is a resistor protected and insulated by a sheath at the tip of a hollow vertical shaft 28, here on the lower surface of a molybdenum disk. A resistance heater 8 in which a heating body made of a heating wire formed into a disk shape is fixed, or a resistance heater 8 formed by forming graphite into a disk shape is supported, and the horizontal direction of the susceptor 2 integrated with the resistance heater 8 is supported. A substrate 1 is attached to the top surface.

Inside the reaction vessel 3 is a seal casing 11 that is water-cooled.
The hollow vertical shaft 28 is kept airtight by an internal shaft sealing material 12, and is rotatably supported around its axis by a bearing 10 in a bearing case 9.
It is rotationally driven via the gear 13. The heating current is supplied to the resistance-energizing heater 8 via a conductor running inside the hollow vertical shaft 28, although it is not specifically shown here.

As a raw material gas for the organometallic compound film, a mixed gas of an alkylated product of a group (III) element and a hydride of a group (■) gasified by bubbling of a hydrogen carrier gas is introduced into the reaction vessel 3 through the reaction gas inlet 6. Then, the mixed gas flows horizontally in the reaction vessel 3, is heated by the susceptor 2 and the substrate 1, and is decomposed, and the decomposed gases react with each other and deposit on the substrate, or react on the substrate and form a compound. form. By performing this film formation while rotating the hollow vertical shaft 28, the thickness of the pus is averaged on the upstream side and the downstream side on the substrate surface, and the thickness of the pus is differentiated between the upstream side and the downstream side on the substrate surface. Therefore, a film with good film thickness uniformity is formed. The reaction gas 1 after participating in film formation is discharged to the outside from the discharge lower.

FIG. 2 shows an embodiment of claim 2. The quartz glass reaction vessel 3 is formed into a semicylindrical shape, with a flat bottom and a semicircular cross section at the top. A fan-shaped gas introduction section 3a is formed in which the cross section of the flow path gradually widens in the lateral direction from the reaction gas introduction port 6 toward the inside.
It forms the joint part of the gas flow path with.

The inner container 100 includes an inner container part 15 made of nine quartz glass plates placed on the bottom surface of the reaction container 3, and an inner container made of one quartz glass plate having a thickness such that the top surface is at the same height as the top surface of the susceptor 2. part 16 and inner container part 1
9 and a cover-shaped inner container 17 formed of quartz glass, which is placed on top of a gas flow path having a rectangular cross section. In the space between the inner container 100 and the reaction container 3,
From the gas inlet 3b formed on the bottom of the reaction vessel 3, H
2 gas, Nz gas, or an inert gas is introduced as a sweep gas and is discharged to the outside together with the reaction gas from the discharge lower. Since the reaction gas pressure in the inner container 100 and the sweep gas pressure between the inner container 100 and the reaction container 3 are both Torrf-der vacuum pressure, the inner container portion is placed on the bottom surface of the reaction container 3. It maintains the position at the time of mounting just by placing it there, and also, as mentioned in the section [Function], the inner container parts are strictly connected to each other and the joint between the gas introduction part 3a of the reaction container and the inner container 100. does not require airtightness. Therefore, attachment and detachment of the inner container into the reaction container 3 can be performed extremely easily.

When the apparatus is configured in this way and the reaction gas is introduced into the inner container 100 through the gas introduction part 3a, the upper surface of the susceptor 2 and the upper surface of the inner container portion 16 are in the same plane, so that the reaction gas flows in a laminar flow state. It passes through the substrate surface while maintaining the same, and forms a film of good quality on the substrate surface. In addition, the vertical cross section of the gas flow path is the same one-sided cross section in the range from the upstream side to the downstream side of the susceptor near the susceptor, so the gas μ degree of any part of the flow path cross section within this range is equal. ,
Furthermore, since there is no difference in the thickness of the flow path in the left-right direction of the substrate, there is no difference in the thickness of the film formed on the substrate surface in the left-right direction of the substrate. Therefore, by forming a film while rotating the hollow vertical shaft 28, it is possible to form a film with substantially completely uniform film thickness distribution on the substrate.

In addition, with this equipment configuration, in order to prevent reaction products from adhering to the reaction container during film formation, only the internal container needs to be cleaned.
In addition, this internal container has its structure and installation method.
It can be taken out of the reaction container very easily. It is also extremely easy to reinstall after cleaning.

Furthermore, as seen in the figure, the inner container has a low ceiling;
Since the amount of reaction gas discharged to the outside in an undecomposed state is reduced, the introduced reaction gas can be efficiently used for film formation, and raw materials can be used effectively.

FIG. 3 shows an embodiment of claim 3. 6 The inner container has an upper surface with an angle of 2 to 5°Il so that the ceiling is two degrees lower in the direction of flow.
The gas that is formed on the upper surface of the susceptor or on the upstream side of the substrate surface, resulting in less undecomposed gas, also approaches the substrate surface as it moves downstream, and becomes more concentrated than in an inner container with a horizontal top surface. Undecomposed gas is decomposed and contributes to film formation on the downstream side of the substrate surface.

FIG. 4 shows an embodiment of claim 5. Inner container 10
1 is formed by incorporating the gas introduction part 3a of the reaction vessel 3 in FIGS. 2 and 3 as its component.

The structure of this internal container 101 is shown in an exploded perspective view in FIG. A protrusion 16b is formed on the lower surface side of the inner container portion 16, and by inserting this protrusion 16b into the hole 15a of the inner container portion 15, the centers of the holes 16a and 15a are aligned.

Further, a ring-shaped projection 15b is formed on the lower surface side of the inner container portion 15 concentrically with the hole 15a, and by inserting this projection 15b into the opening 31 (FIG. 4) of the reaction container 3, the hole 15a and the opening 31 The centers of the two coincide with each other. As a result, the resistance heater 8. The susceptor 2 and the substrate l can rotate without contacting the inner container 101.

By the way, in FIGS. 2 and 3, the inner container portion 18 formed of 1 quartz glass serves as the gas introduction part 3a.
On the side of the reaction gas introduction port 6 of the reaction vessel 3, a reaction gas introduction port 18a having a structure as shown in FIG. The tapered surface of the gas introduction flange 20 forming the spring 2
In addition to simplifying the structure of the reaction vessel side, the reaction vessel 3 is made of M material and has a rectangular cross section, and cooling plates 19 are provided with cooling pipes 19a along each wall surface in a zigzag pattern. Attach the reaction vessel 3 in close contact with the reaction vessel 3.
It has a structure that cools it uniformly without any parts being undercooled.

The reason why the reaction vessel is cooled is that the softening temperature of M is lower than that of quartz glass (to prevent a decrease in strength when heating the sassefuri).

〔Effect of the invention〕

In the present invention, since the vapor phase growth apparatus to which the present invention is directed is configured as described above, the following effects can be achieved.

(1) In the apparatus of claim 1, the substrate surface rotates within a plane in the same direction as the flow direction of the reaction gas, and the film thickness distribution of the film formed on the substrate surface is different between the upstream side and the downstream side of the substrate surface. In particular, the non-uniformity of the film thickness distribution in the circumferential direction of the substrate surface is substantially completely eliminated.

In addition, since a resistance heater is used as the heating means for the susceptor, there are fewer restrictions on the material of the rotating mechanism members, such as when heating the susceptor by induction, and it is easier to form the rotating mechanism inside the reaction vessel in terms of structure and material. becomes. Therefore, it is possible to easily eliminate the fear of dust generation when using gears in the rotation mechanism.

Furthermore, since there is no induction heating coil surrounding the susceptor, the rotating shaft of the heater can be guided out by vertically penetrating the peripheral wall of the reaction vessel, and the device configuration including the rotating mechanism is simplified.

(2) In the device according to claim 2, for example, the internal container made of quartz glass is formed so that the gas flow path has a rectangular cross section,
The rectangular bottom surface can be installed in the reaction vessel so that it is flush with the surface of the susceptor substrate, and the flow of reaction gas can completely cover the substrate surface, regardless of the cross-sectional shape of the reaction vessel. It can pass through in a flowing state, and the film quality is greatly improved.

In addition, since there is no difference in gas density per gas flow rate in the left and right directions of the substrate, by rotating the substrate together with the susceptor and heater, the film thickness can be virtually completely adjusted regardless of whether it is on the upstream side, downstream side, or left and right direction of the substrate. The film quality and thickness can be easily controlled, such as being able to form a uniform film.

Furthermore, by flowing H2 gas, NZ gas, or an inert gas as a so-called sweep gas between the reaction container and the internal container, the reaction products do not adhere to the reaction container but only to the internal container, and the sweep gas By making the pressure approximately equal to the reaction gas pressure in the inner vessel, it is possible to improve the airtightness between each part when the inner vessel is composed of multiple vessel parts, and the airtightness of the joint between the inner vessel and the reaction vessel. Even if the properties are not strict, the amount of reaction gas leaking out of the inner container or the amount of sweep gas mixed into the reaction gas will be significantly reduced, and even if the sweep gas is mixed into the reaction gas, it will be treated as a sweep gas. , the same uHz gas or ~2 gases as the carrier gas in the reaction gas.

By using an inert gas, the influence of the sweep gas on # quality can be ignored, but the inner container can be formed with emphasis on ease of installation and removal from the reaction container, making cleaning work easier. becomes significantly easier. This also improves the operating efficiency of the device.

Furthermore, the reaction gas is introduced into the inner container with a lower ceiling than the reaction container, and the introduced reaction gas is efficiently thermally decomposed, so the amount of reaction gas used for film formation can be reduced.
Raw materials can be used effectively.

(3) In the apparatus of claim 3, the gas molecules that do not decompose on the upstream side of the substrate surface and move downstream in an undecomposed state approach the substrate surface as they move, making it more effective than an internal container with a horizontal top surface. The reaction gas is decomposed by heating and contributes to film formation on the downstream side of the substrate surface, and the reaction gas is used more effectively for film formation.

(4) In the apparatus of claim 4, it is possible to use a metal container as the reaction container, and the pressure difference between the inside and outside of the reaction container becomes large or the reaction container is subjected to some mechanical or thermal shock. There is also no risk of the container being destroyed. Since the reaction gas commonly used in film formation is dangerous to humans, the safety of the apparatus is significantly improved by using a metal container as the reaction container. In addition, it becomes easy to manufacture a large-sized reaction vessel corresponding to an increase in the size of the substrate.

(5) In the apparatus of claim 5, the reaction vessel side joint part that is joined to the inner vessel and allows the reaction gas introduced into the reaction vessel to flow into the inner vessel is simplified, and the metal vessel is connected to the metal vessel, which is necessary for maintaining heat-resistant strength. The container can be uniformly cooled throughout.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of an apparatus showing an embodiment of claim 1, and FIG. 2 is a cross-sectional view of the apparatus showing an embodiment of claim 2. FIG. 3 is a longitudinal sectional view of the main part of the device showing an embodiment of claim 3, and FIG. The figure is a cross-sectional view of a device showing an embodiment of claim 5, and is shown in the figure).
Figure 5 is a vertical sectional view, +a+ is a sectional view taken along the line B-B in the same figure, and Figure 5 is an exploded perspective view showing the structure of the internal container used in the device shown in Figure 4. The figure is a vertical sectional view of the main part of the apparatus showing the coupling structure of the reaction gas inlets of the reaction vessel and inner vessel in the apparatus shown in Fig. 4, and Fig. 7 is a conventional vapor phase growth apparatus to which the present invention is directed FIG. 2 is a vertical cross-sectional view showing a configuration example. Substrate (substrate for film formation), 2; Susceptor, 3: Reaction vessel, 6.18a: Reaction gas inlet, 8. resistance heating heater,
20: Gas introduction 7-77, LOo, 101: Internal container. Acting accountant Yamaguchi Gen 1st figure 21st sentence review! ] Dragon Travel Power Figure 5

Claims (1)

[Claims] 1) A susceptor installed so that the surface on which the film-forming substrate is attached is horizontal is placed in a box-shaped or cylindrical reaction container, and a reaction gas is introduced and caused to flow horizontally; In a vapor phase growth apparatus that grows a thin film on the film-forming substrate by heating a susceptor, the heating means for heating the susceptor is a resistance heater, and the heater is connected along with the susceptor to an axis perpendicular to the substrate mounting surface of the susceptor. A vapor phase growth apparatus characterized by having a structure that can rotate around the circumference. 2) In the vapor phase growth apparatus according to claim 1, the entire amount of the reaction gas introduced into the reaction vessel is transferred to a desired flow path, at least in the vicinity of the susceptor, in an area extending from the upstream side of the susceptor to the downstream side of the susceptor. A removable quartz glass inner container shaped to flow in cross section is attached to the reaction container, and H_2 gas, N_2 gas, or inert gas is allowed to flow between the reaction container and the inner container. Vapor phase growth equipment. 3) The vapor phase growth apparatus according to claim 2, characterized in that the quartz glass inner container has an upper surface inclined such that the upper surface approaches the upper surface of the susceptor as the downstream side of the reaction gas approaches. Vapor phase growth equipment. 4) A vapor phase growth apparatus according to claim 1, 2 or 3, characterized in that the reaction vessel is a metal vessel. 5) In the vapor phase growth apparatus according to claim 2 or 3, the reaction container is a metal container, and the reaction gas inlet of the reaction container and the reaction gas inlet of the inner container are connected using springs. A vapor phase growth apparatus characterized in that the two ports are brought into close contact with each other over the entire circumference of the inlet.