Wafer boats for supporting semiconductor wafers inside the furnace

Abstract

To provide a semiconductor wafer boat for supporting semiconductor wafers for use in heat treating the semiconductor wafers in a furnace.SOLUTION: In a wafer boat, each finger in a group 42 of fingers 30 includes an elongated segment 50 extending from a vertical rod and a contact protrusion 52 disposed on an elongated segment 50 toward the distal end 34 of the finger 30 and contacting and supporting a semiconductor wafer. The contact protrusion 52 is used to support any diameter of wafer including 200 mm diameter wafers, 300 mm diameter wafers and / or wafers larger than 300 mm diameter.SELECTED DRAWING: Figure 2

Description

Cross-reference of related applications

[0001] This application claims the interests of U.S. Provisional Patent Application No. 63 / 044,700, filed on 26 June 2020, which is incorporated herein by reference in its entirety. [Technical Field]

[0002] The field of this disclosure relates to semiconductor wafer boats for supporting semiconductor wafers, and more specifically, to semiconductor wafer boats for use in the heat treatment of semiconductor wafers in a furnace. [Background technology]

[0003] Semiconductor wafers are typically heat-treated (i.e., annealed) at high temperatures to achieve specific desired properties. For example, the annealing process can be used to create a defect-free silicon layer on a wafer. High-temperature annealing processes are typically carried out in vertical furnaces that expose wafers to temperatures above 1100°C (e.g., between approximately 1200°C and 1300°C).

[0004] Multiple semiconductor wafers can be supported in a vertical furnace by wafer boats or "racks." A wafer boat contains one or more supports on which semiconductor wafers rest. When exposed to high temperatures, especially above 1100°C, wafers temporarily become more plastic, meaning their yield strength decreases. Contact areas on the wafer where the wafer is supported can slip due to localized gravitational and thermal stresses. Slip can introduce contaminants into the wafer. Excessive slip can also lead to plastic deformation of the wafer, resulting in production problems such as photolithography overlay defects, which cause yield losses in device manufacturing.

[0005] The support is intended to hold the semiconductor wafer, but during the heat treatment of the wafer, local gravitational and thermal stresses are minimized to prevent slip and plastic deformation. Conventionally, a wafer boat used in a vertical furnace has three or more rods. The rods generally have laterally extending fingers in a common horizontal plane. This configuration is conventional and is usually suitable for heating smaller diameter wafers such as those less than 200 mm. Larger diameter wafers (e.g., over 200 mm) are subjected to greater local gravitational and thermal stresses than smaller diameter wafers. Such larger diameter wafers are typically mounted on a support ring that provides a larger surface area for supporting the wafer. The support ring increases the time for loading and unloading the wafer onto the wafer boat of the semiconductor wafer.

[0006] There is a need for a wafer boat that includes a support structure that reduces local gravitational and thermal stresses to limit the occurrence of slip while supporting a semiconductor wafer that is exposed to high temperatures during the process, and a wafer boat that has a relatively high throughput for wafer loading and unloading.

[0007] This section is intended to introduce the reader to various aspects of technologies that may be related to various aspects of the present disclosure, which are described and / or claimed below. This discussion is thought to be helpful in providing background information to the reader to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these descriptions are to be read from this perspective and not as an admission of prior art. SUMMARY OF THE INVENTION

[0008] One aspect of the present disclosure relates to a wafer boat for supporting a plurality of semiconductor wafers in a furnace. The wafer boat includes a vertical rod and a set of fingers extending radially inward from the vertical rod along a finger axis. Each finger of the set includes an elongated segment extending from the vertical rod. Contact protrusions are disposed on the elongated segment toward the distal end of the finger for contacting and supporting a semiconductor wafer. At least a portion of the contact protrusions protrudes from the elongated segment. The contact protrusions have a longitudinal contact protrusion axis. The longitudinal contact protrusion axis and the finger axis are angled with respect to each other.

[0009] There are various improvements to the features mentioned in relation to the above aspect of the present disclosure. Further features can also be incorporated into the above aspect of the present disclosure. These improvements and additional features may exist individually or in any combination. For example, the various features discussed below in relation to any of the illustrated embodiments of the present disclosure can be incorporated into any of the above aspects of the present disclosure, either alone or in any combination.

Brief Description of the Drawings

[0010] [Figure 1] It is a perspective view of a wafer boat. [Figure 2] It is a plan view of a wafer boat showing contact protrusions. [Figure 3] It is a detailed view of the contact protrusions of a wafer boat. [Figure 4] It is a detailed view of another embodiment of the contact protrusions. [Figure 5] It is a plan view of a wafer boat having another embodiment of the contact protrusions. [Figure 6] It is a detailed view of the contact protrusions shown in FIG. 5. [Figure 7] It is a detailed side view of contact protrusions supported by pins and gimbals.

[0011] Corresponding reference numerals indicate the corresponding parts throughout the drawing. [Modes for carrying out the invention]

[0012] An example of a wafer boat supporting multiple semiconductor wafers in a vertical furnace is shown in Figure 1, labeled "10". The wafer boat 10 supports multiple semiconductor wafers during a high-temperature heat treatment process (also referred to herein as "annealing"). The wafer boat 10 includes at least one vertical rod 12 (and typically three or more rods) coupled to the top 14 and bottom 16 of the wafer boat 10. The wafer boat 10 has a longitudinal central axis Y extending from the top 14 to the bottom 16. 10 The vertical rod 12 has a longitudinal central axis Y. 10 It is positioned at a distance D1 from (Figure 1).

[0013] Referring to Figures 1-3, the illustrated wafer boat 10 includes three vertical rods 12, specifically a central rod 18 and two forward rods 20. Each of the vertical rods 12 has a longitudinal central axis Y 10 The two forward rods 20 are positioned on a first circle C1 with radius R1, centered at Y. 10 The central rod 18 is spaced at a first angle α1 around each of the two front rods 20 at a second angle α2, with the central rod 18 aligned with the longitudinal central axis Y. 10 The two front rods 20 are spaced apart by an equal circumferential distance so as to be spaced apart around the hub. The two front rods 20 are spaced apart by a first angle α1 such that the entrance 24 is defined between the two front rods 20 (Figure 1). The first angle α1 can be any suitable angle such that the entrance 24 is sized so that a semiconductor wafer can pass through the entrance 24 and be placed within the internal space 26 of the wafer boat 10. For example, the first angle α1 is 180° or more, and the second angle α2 is 90° or less.

[0014] The wafer boat 10 includes fingers 30 that extend radially inward from the vertical rod 12 to support a semiconductor wafer (i.e., the boat does not include a support ring for supporting the wafer). The two front vertical rods 20 each include first and second sets 36, 38 of fingers 30. The central vertical rod 18 includes a third set 40 of fingers 30. The first set 36 of fingers 30 extends along a first finger axis A 36 ; the second set 38 of fingers 30 extends along a second finger axis A 38 ; and the third set 40 of fingers extends along a third finger axis A 40 . Each finger axis A 36 , A 38 , A 40 extends from the proximal end 32 to the distal end 34 of the finger 30. The proximal end 32 of the finger 30 is close to the vertical rod 12, and the distal end 34 of the finger 30 is disposed in the internal space 26 of the wafer boat 10. The finger 30 may be integrally formed with the vertical rod 12; for example, a cut may be made in an elongated integral structure to form the finger 30. Alternatively, the finger 30 may be formed separately and coupled to the vertical rod 12.

[0015] The finger axes A 36 , A 38 of the first and second sets 36, 38 of fingers 30 extend on respective chords X 36 , X 38 of a first circle C1. The chords X 36 , X 38 do not intersect the longitudinal central axis Y 10 of the wafer boat 10 (i.e., the first and second sets 36, 38 of fingers are not centered such that they point to the longitudinal central axis Y 10 ). The finger axis A 40 of the third set 40 of fingers 30 extends on a chord X 10 of the first circle C1 that intersects the longitudinal central axis Y 40 .

[0016] The groups 42 of fingers 30 extending from each of the vertical rods 12 are in the same, substantially horizontal plane so that the groups 42 of fingers 30 can support a semiconductor wafer. The distal ends 34 of the fingers 30 of group 42 lie on a second circle C2. The second circle C2 has a radius R2 and a longitudinal central axis Y 10 The radius R2 is centered at the longitudinal central axis Y of the wafer boat 10, from the distal end 34 of the finger 30 to the longitudinal central axis Y of the wafer boat 10. 10 It extends to the longitudinal central axis Y. 10 They are separated by an angle β1 defined on the second circle C2 centered on the first circle. In the illustrated embodiment, the angle β1 is 120°.

[0017] Referring to Figures 2 and 3, each of the fingers 30 includes an elongated segment 50 extending from the vertical rod 12 and a contact projection 52 positioned on the elongated segment 50 toward the distal end 34 of the finger 30. The contact projection 52 contacts and supports the semiconductor wafer. The contact projection 52 is raised relative to the elongated segment 50. The contact projection 52 has a first end 54, a second end 56, and a longitudinal contact projection axis A extending through the first and second ends 54, 56 of the contact projection 52. 52 The contact projection 52 can be used to support wafers of any diameter, including wafers with a diameter of 200 mm, wafers with a diameter of 300 mm, and / or wafers with a diameter greater than 300 mm.

[0018] Regarding the first and second sets 36 and 38 of the finger 30, the longitudinal contact projection axis A 52 and each finger axis A 36 , A 38When viewed from above (Figure 2), they form an angle with respect to each other at an angle λ1. Longitudinal contact projection axis A 52 and each finger axis A 36 , A 38 The angle λ1 between is at least 5°, or, as in other embodiments, at least 10°, at least 15°, at least 20°, 5° to about 90°, or 5° to about 75°. For the third set of fingers 30, when viewed from above (Figure 2), the longitudinal contact projection axis A 52 and finger axis A 40 The two points form an angle λ2 with respect to each other. The angle λ2 is the axis A of the longitudinal contact projection. 52 and finger axis A 30 The angles are 90° so that they are perpendicular to each other. Each longitudinal contact projection axis A 52 The contact projection 52 intersects the third circle C3, and the longitudinal central axis Y of the wafer boat 10 is at this position. 10 It is tangent to the third circle C3 centered at [the specified point]. In this configuration, the contact projection 52 is aligned with the longitudinal central axis Y 10 They are arranged in a rotationally symmetric pattern with respect to (i.e., the contact protrusions are on the central axis Y) 10 (They are similarly spaced apart and angled around it.)

[0019] The illustrated contact projections 52 are rounded (for example, their height increases from each side to the midpoint of the side). The rounded contact projections 52 include a surface that is substantially cylindrical in shape. Each contact projection 52 has a longitudinal contact projection axis A 52 The wafer includes a length L1 (Figure 3) extending along the longitudinal axis A of the contact protrusion. 52 It rests on a rounded contact projection 52 that closely follows the curve.

[0020] In some embodiments, the contact projection 52 has a width W1 of 10 mm to 25 mm and a length L1 of 20 mm to 40 mm. Alternatively or additionally, the longitudinal central axis Y of the contact projection 52 is extended from the contact projection 52. 10The distance to the contact projection may be between 75 mm and 125 mm. In some embodiments, the contact projection 52 has a width W1 of 8 mm and a length L1 of 30 mm, and the longitudinal central axis Y from the contact projection 52. 10 The distance to it is 100 mm.

[0021] In some embodiments, such as the embodiment shown in Figure 4, the contact projection 52 is flat. Components shown in Figure 4 that are similar to the components in Figures 1-3 are indicated by adding "100" to the corresponding reference numerals in Figures 1-3 (for example, part 52 becomes part 152). The flat contact projection 152 is located along the longitudinal contact projection axis A 152 Includes a contact surface 153 defined by and longitudinal contact projection axis A 152 The contact surface 153 includes a surface area defined by a length L1 extending along the surface and a width W1 perpendicular to the length L1. The contact surface 153 as a whole is in substantially contact with the semiconductor wafer resting on the flat contact projection 152.

[0022] Alternatively, the projection 52 may be generally dome-shaped, such that the semiconductor wafer is placed on a relatively small, point-like region.

[0023] Other embodiments of the contact projection 52 are shown in Figures 5 and 6. Components shown in Figures 5 and 6 that are similar to the components in Figures 1 to 3 are indicated by adding "200" to the corresponding reference numerals in Figures 1 to 3 (for example, part 52 becomes part 252). The finger 230 includes a first contact projection 252a that rises relative to the elongated segment 250. The first contact projection 252a is along the first longitudinal contact projection axis A 252aThe finger 230 also includes a second contact projection 252b positioned toward the distal end 234 of the finger 230. At least a portion of the second contact projection 252b is raised relative to the elongated segment 250. Each of the first and second contact projections 252a, 252b contacts and supports the semiconductor wafer. The second contact projection 252b has a first end 254, a second end 256, and a second longitudinal contact projection axis A extending between them. 252b This includes the first longitudinal contact projection axis A in the illustrated embodiment. 252a and the second longitudinal contact projection axis A 252b They are almost parallel to each other.

[0024] Regarding the first and second sets 236 and 238 of finger 230, the first and second projection axes A 252a , A 252b This is finger axis A 230 Angle λ 201 The angle can be adjusted. In the third set 240 of finger 230, the first and second projection axes A 252a , A 252b is finger axis A 230 It is almost parallel to the first and second contact projections 252a and 252b, and the contact projections 252a and 252b are aligned along the longitudinal central axis Y. 210 The finger axis A is arranged in a rotationally symmetric pattern around it. 230 Any angle λ 201 It can be arranged in this way.

[0025] The first and second contact protrusions 252a and 252b are located on the axis A of the first and second protrusions. 252a , A 252bThey have an elongated shape with a length L2 extending along the line. The first and second contact projections 252a, 252b can be separated by a distance W2. In some embodiments, W2 is 10 mm to 40 mm, 15 mm to 30 mm, or 24 mm as in other embodiments. The first and second contact projections 252a, 252b may have various shapes such as a cylindrical cross section, a rectangular cross section (i.e., flat contact), or a trapezoidal cross section (e.g., flat top).

[0026] Each group 242 of contact protrusions 252a and 252b, which lie on a common horizontal plane, contacts and supports the wafer. The wafer is placed on each of the contact protrusions 252a and 252b such that the weight of the wafer is distributed approximately evenly across each of the contact protrusions 252. In the illustrated embodiment, each of the six contact protrusions 252a and 252b within group 242 supports 1 / 6 of the wafer's weight.

[0027] Other embodiments of the contact projection 352 are shown in Figure 7. Components shown in Figure 7, which are similar to the components in Figures 1-3, are indicated by adding "300" to the corresponding reference numerals in Figures 1-3 (for example, part 10 becomes part 310). Referring to Figure 7, each of the fingers 330 includes an elongated segment 350 extending from a vertical rod 312. The fingers 330 include a platform 360 that supports the first and second contact projections 352a, 352b. The platform 360 is coupled to the elongated segment 350 by a pin 362, which is pivotally connected to the elongated segment 350 by a cylindrical joint or gimbal 364. Thus, the pin 362 and gimbal 364 allow the platform 360 to pivot relative to the elongated segment 350. The platform 360 supports the first and second contact projections 352a, 352b, each of which contacts and supports a semiconductor wafer.

[0028] The pivoting of the platform 360, enabled by the pins 362 and gimbal 364, counteracts uneven load distribution caused by machining inaccuracies and allows the platform 360 to pivot in response to uneven loads on the contact protrusions 352a and 352b. The platform 360 can pivot to more evenly distribute the wafer weight to each of the contact protrusions 352a and 352b. After the wafer boat 310 is placed in the vertical furnace and exposed to oxidation, the pins 362 and gimbal 364 are fixed in place to prevent the platform 360 from pivoting further during the annealing process.

[0029] The wafer boat of this disclosure has several advantages compared to conventional wafer boats. In embodiments in which the wafer boat includes one or more contact protrusions arranged in a rotationally symmetric pattern, local gravitational and thermal stresses on the semiconductor wafer loaded onto the wafer boat are reduced during annealing, and slip is reduced. In embodiments in which the wafer boat includes pins and gimbals supporting a platform having two or more contact protrusions, the platform can pivot to adjust for uneven loading of the contact protrusions caused by machining inaccuracies.

[0030] example The processes described herein are further illustrated by the following embodiments, which should not be construed as limiting.

[0031] Example 1: Peak stress of wafers for various boat designs The peak stresses of the wafers were measured using a finite element model for various boat designs with different finger arrangements. One wafer boat included fingers with flat contact protrusions where the contact protrusions of the fingers extending from the forward rods were not angled with respect to the finger axes (i.e., the contact protrusion axes and finger axes were parallel, and the contact protrusions were not rotationally symmetric). A second wafer boat was similar to the first, but the contact protrusions were cylindrical. A third wafer boat included cylindrical contacts arranged as shown in Figure 2 (i.e., fingers extending from two forward rods with contact protrusions angled with respect to the finger axes, and fingers extending from a central rod perpendicular to the central axis so that the contact protrusions were rotationally symmetric). A fourth boat included two contact protrusions on each rotationally symmetric finger, as shown in Figures 5 and 6. The peak stresses for each boat are shown in Table 1 below.

[0032] TIFF0007909581000001.tif36144 Table 1: Peak wafer stress for various boat designs with different contact projection arrangements

[0033] As shown in Table 1, wafer boats with rotationally symmetric contact protrusions (boats 3 and 4) exhibited reduced peak stress, which reduced slippage of the semiconductor wafer annealed on the wafer boat.

[0034] As used herein, the terms “about,” “substantially,” “essentially,” and “nearly,” when used in conjunction with a range of dimensions, concentrations, temperatures, or other physical or chemical properties or characteristics, mean to include variations that may exist at the upper and / or lower limits of the property or characteristic range, including variations resulting from rounding, measurement methods, or other statistical variations.

[0035] When describing elements of this disclosure or its embodiments, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more elements. The terms “comprising,” “including,” “containing,” and “having” are intended to be comprehensive and mean that additional elements other than those listed may exist. The use of terms indicating a particular direction (such as “top,” “bottom,” and “side”) is for explanatory convenience and does not require a specific orientation of the item being described.

[0036] Various modifications can be made to the above-described structures and methods without departing from the scope of this disclosure, so all matters included in the above description and shown in the accompanying drawings are intended to be illustrative and not limiting.

Claims

1. A wafer boat for supporting multiple semiconductor wafers in a furnace, A vertical rod and Includes a set of fingers extending radially inward from a vertical rod along the finger axis, Each finger in the set is, A long, slender segment extending from a vertical rod, A wafer boat comprising: a contact projection positioned on an elongated segment toward the distal end of a finger for contacting and supporting a semiconductor wafer, wherein at least a portion of the contact projection is raised relative to the elongated segment, the contact projection has a longitudinal contact projection axis, the longitudinal contact projection axis and the finger axis are at an angle to each other, and each contact projection is rounded such that its height increases from the side to the apex.

2. The wafer boat according to claim 1, wherein the contact projection has a width of 10 mm to 25 mm and a length of 20 mm to 40 mm, and the distance from the contact projection to the longitudinal central axis of the wafer boat is 75 mm to 125 mm.

3. The wafer boat according to claim 1, wherein the contact projection has a width of 16 mm and a length of 30 mm, and the distance from the contact projection to the longitudinal central axis of the wafer boat is 100 mm.

4. The wafer boat according to claim 1, wherein the angle between the longitudinal contact projection axis and the finger axis is at least 5°.

5. The wafer boat according to claim 1, wherein the angle between the longitudinal contact projection axis and the finger axis is at least 20°.

6. The wafer boat according to claim 1, wherein the angle between the longitudinal contact projection axis and the finger axis is 5° to 75°.

7. The wafer boat according to claim 1, wherein the longitudinal contact projection axis is tangent to a circle centered on the longitudinal central axis of the wafer boat at the position where the contact projection intersects the circle.

8. The wafer boat according to claim 1, wherein the contact projection is a first contact projection, the longitudinal contact projection axis is the longitudinal first contact projection axis, the finger includes a second contact projection toward the distal end of the finger for contacting and supporting a semiconductor wafer, at least a portion of the second contact projection is raised relative to an elongated segment, the second contact projection has a longitudinal second contact projection axis, and the longitudinal second contact projection axis and the finger axis are angled relative to each other.

9. The wafer boat according to claim 8, wherein the longitudinal axis of the second contact projection is parallel to the longitudinal axis of the first contact projection.

10. The wafer boat according to claim 8, wherein the first and second contact projections are supported by a platform, the platform is mounted on a pin pivotable around a gimbal mounted on the distal end of an elongated segment.

11. The wafer boat according to claim 1, wherein the finger axis extends on a chord of a circle centered on the longitudinal central axis of the wafer boat, and the chord does not intersect the longitudinal central axis of the wafer boat.

12. The vertical rod is the first rod, the set of fingers is the first set of fingers, the string is the first string, and the wafer boat is The second vertical rod, A second set of fingers extending radially inward from a second vertical rod along a second finger axis, wherein the second finger axis extends on a second chord of a circle, and the second chord does not intersect the longitudinal central axis of the wafer boat, and the second set of fingers includes: Each finger in the second set is, A slender segment extending from the second vertical rod, A wafer boat according to claim 11, comprising a contact projection positioned on an elongated segment toward the distal end of a finger for contacting and supporting a semiconductor wafer, wherein at least a portion of the contact projection is raised relative to the elongated segment, the contact projection has a longitudinal contact projection axis, and the longitudinal contact projection axis and the second finger axis are angled relative to each other.

13. A third vertical rod, A third set of fingers extending radially inward from a third vertical rod along a third finger axis, wherein the third finger axis extends along a third chord of a circle, and the third chord intersects the longitudinal central axis of the wafer boat, further comprising: Each finger in the third set is, A slender segment extending from the third vertical rod, A wafer boat according to claim 12, comprising a contact projection positioned on an elongated segment toward the distal end of a finger for contacting and supporting a semiconductor wafer, wherein at least a portion of the contact projection is raised relative to the elongated segment, the contact projection has a longitudinal contact projection axis, and the longitudinal contact projection axis and the third finger axis are orthogonal to the contact projection.

14. The wafer boat according to claim 13, wherein the distal ends of each finger are spaced 120° apart around the wafer boat.

15. The wafer boat according to claim 13, wherein the contact protrusions of the first set, the second set, and the third set are spaced 120° apart around the wafer boat.

16. The wafer boat according to claim 13, wherein the contact protrusions of the first set, the second set, and the third set are arranged such that the contact protrusions of the first set, the second set, and the third set form a rotationally symmetrical pattern.

17. A wafer boat for supporting multiple semiconductor wafers in a furnace, A vertical rod and Includes a set of fingers extending radially inward from a vertical rod along the finger axis, Each finger in the set is, A long, slender segment extending from a vertical rod, A wafer boat comprising: a contact projection positioned on an elongated segment toward the distal end of a finger for contacting and supporting a semiconductor wafer, wherein at least a portion of the contact projection is raised relative to the elongated segment, the contact projection has a longitudinal contact projection axis, the longitudinal contact projection axis and the finger axis are at an angle of 5° to 75° with respect to each other, and the longitudinal contact projection axis includes a contact projection that, at the position where the contact projection intersects the circle, is tangent to a circle centered on the longitudinal central axis of the wafer boat.

18. The wafer boat according to claim 17, wherein the contact projection includes a rounded surface for supporting and contacting a semiconductor wafer.

19. The wafer boat according to claim 17, wherein the contact projection includes a plane for supporting and contacting a semiconductor wafer.

20. The vertical rod is the first vertical rod, the set of fingers is the first set of fingers, the finger axis is the first finger axis, the first finger axis extends on a chord of a circle centered on the longitudinal central axis of the wafer boat, and the chord does not intersect the longitudinal central axis of the wafer boat. The wafer boat further, The second vertical rod, A second set of fingers extending radially inward from a second vertical rod along a second finger axis, wherein the second finger axis extends along a chord of a circle, and the chord does not intersect the longitudinal central axis of the wafer boat, and the second set of fingers includes: Each finger in the second set is, A slender segment extending from the second vertical rod, A contact projection is positioned on an elongated segment toward the distal end of a finger to contact and support a semiconductor wafer, and at least a portion of the contact projection is a contact projection that is raised relative to the elongated segment, The wafer boat further, A third vertical rod, A third set of fingers extending radially inward from a third vertical rod along a third finger axis, wherein the third finger axis extends along a chord of a circle, and the chord intersects the longitudinal central axis of the wafer boat, and the third set of fingers includes: Each finger in the third set is, A slender segment extending from the second vertical rod, A wafer boat according to claim 17, comprising contact projections positioned on elongated segments toward the distal ends of fingers for contacting and supporting a semiconductor wafer, wherein at least a portion of the contact projections is raised relative to the elongated segments, and the first, second, and third sets of contact projections, or fingers, are spaced 120° apart around the wafer boat.

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