JP2014187214A - Sample holder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sample holder having a sample holding surface which can be uniformly heated.SOLUTION: A sample holder 1, made of ceramic, comprises: a sample holding surface 10a at an outer surface thereof; and a fluid passage 11 for a heating medium therein. When viewed in a cross-section orthogonal to a traveling direction of the heating medium, the fluid passage 11 has a projection 30 and a recess 31, facing each other, at inner surfaces thereof. Thus, the flow of the heating medium becomes turbulent, and powder, generated by wear, is prevented from being accumulated inside the fluid passage 11. It is thereby possible to suppress reduction in thermal conduction between the heating medium and the sample holder 1.

Description

  The present invention relates to a sample holder used when holding each sample such as a semiconductor wafer, which is used in a manufacturing process of a semiconductor integrated circuit or a manufacturing process of a liquid crystal display device.

  Semiconductor wafers such as silicon wafers used in the manufacture of semiconductor integrated circuits and plate-like samples such as glass substrates used in the manufacture of liquid crystal display devices are placed on the support bases of the manufacturing equipment or inspection equipment in those manufacturing processes. Is held, and processing or inspection is performed. In a manufacturing process, it is common to use a plurality of manufacturing apparatuses and inspection apparatuses, and means for holding a sample such as a silicon wafer on a support base are the types of manufacturing apparatuses and inspection apparatuses in the manufacturing process and the following: Various types of devices have been proposed according to the type of the conveying device for conveying to the first device.

  Taking a semiconductor integrated circuit as an example, demands for miniaturization and higher density of the semiconductor integrated circuit have been further increased in recent years. Along with this, the sample holder is further required to have thermal uniformity on the surface holding the sample.

  Patent Document 1 discloses an electrostatic chuck composed of a plurality of ceramic layers, and a flow path for allowing a coolant to flow through an intermediate ceramic layer is formed in the electrostatic chuck. Thus, by adjusting the balance between heating and cooling in each part, it is possible to improve the thermal uniformity on the surface of the electrostatic chuck.

Japanese Patent Laid-Open No. 3-108737

  However, as in the electrostatic chuck described in Patent Document 1, when the substrate is composed of a laminate of ceramic layers, contact with the heat medium when a heat medium such as a refrigerant flows through the flow path. As a result, the ceramic layer may deteriorate, and the inner surface of the flow path may be worn by the heat medium. In some cases, the heat generated between the heat medium and the substrate is partially reduced by the powder generated by the wear being deposited inside the flow path. As a result, the thermal uniformity may decrease on the surface of the electrostatic chuck.

  This invention is made | formed in view of such a situation, The objective is to improve the thermal uniformity of the surface of an electrostatic chuck.

  The sample holder according to one aspect of the present invention is a sample holder made of ceramics, having a sample holding surface on the upper surface and having a heat medium flow path therein, and is seen in a cross section orthogonal to the traveling direction of the heat medium. The flow path has a convex portion and a concave portion facing each other on the inner surface.

According to the sample holder of one aspect of the present invention, since the flow path has the convex portion and the concave portion facing each other on the inner surface, the vortex of the heat medium can be generated in the convex portion and the concave portion, respectively. it can. Furthermore, since the convex portion and the concave portion are positioned so as to face each other, the vortices of these heat media can be overlapped, and a large vortex can be generated. Therefore, since the flow of the heat medium can be made turbulent, it is possible to suppress the powder generated by wear from being accumulated inside the flow path as described above. Thereby, it can suppress that the heat conduction between a heat carrier and a sample holder falls. As a result, it is possible to suppress a decrease in heat uniformity on the sample holding surface.

It is a figure which shows the external appearance of the sample holder which is an example of embodiment of this invention. It is a schematic diagram which shows arrangement | positioning of the flow path inside a sample holder planarly. It is sectional drawing of the A-A 'cross section of the sample holder shown in FIG. FIG. 4 is a partially enlarged cross-sectional view in which one channel cross section in the cross-sectional view shown in FIG. 3 is enlarged. It is a partial expanded sectional view of the B-B 'cross section of the sample holder shown in FIG. It is a partial expanded sectional view which shows the modification 1 of the sample holder of this invention. It is a partial expanded sectional view which shows the modification 2 of the sample holder of this invention. It is a partial expanded sectional view which shows the modification 3 of the sample holder of this invention. It is a partial expanded sectional view which shows the modification 4 of the sample holder of this invention.

  FIG. 1 is a view showing an appearance of a sample holder 1 which is an example of an embodiment of the present invention. FIG. 1A is a perspective view of the sample holder 1, and FIG. 1B is a plan view of the sample holder 1.

  The sample holder 1 includes a base body 10 having a sample holding surface 10a on the outer surface (here, upper surface) and having a flow path 11 therein, and an electrode layer 20 provided inside the base body 10. The sample holder 1 is used so as to hold a sample such as a silicon wafer on the sample holding surface 10a of the substrate 10 by electrostatic force by applying a voltage to the electrode layer 20 provided on the substrate 10.

  In the present embodiment, the base body 10 is composed of a laminated body in which a plurality of ceramic layers are laminated. A flow path 11 for flowing a heat medium is provided inside the base body 10. The sample holder 1 can heat, cool, or keep the sample held on the sample holding surface 10 a by flowing a heat medium through the flow path 11. The heat medium flowing in the flow path 11 may be a substance that can exchange heat with the held sample through the ceramic layer and the electrode layer 20 from the flow path 11 of the substrate 10 to the one main surface serving as the sample holding surface 10a. Any heat medium may be used. As such a heat medium, various fluids, for example, an aqueous medium such as hot water, cold water or steam, an organic medium such as ethylene glycol, a gas containing air, or the like can be used.

  As shown in FIG. 1A, the flow path 11 has an opening 11 a that opens to the external space on the end surface of the base 10. Although not shown in FIG. 1A, the end face on the opposite side of the opening 11a also has an opening leading to the external space. The heat medium flowing in the flow path 11 flows into the flow path 11 from, for example, the opening 11a serving as a supply port, and is discharged from the opening on the opposite side of the opening 11a. When the sample holder 1 is used in a semiconductor manufacturing apparatus or inspection apparatus, a supply pipe extending from a supply apparatus for supplying a heat medium is connected to the opening 11a, and a flow path is supplied from the supply apparatus at a predetermined flow rate and flow rate. 11 is supplied with a heat medium. A discharge pipe is connected to the discharge port on the side opposite to the opening 11 a, and the heat medium that flows through the flow channel 11 and exchanges heat with the sample is discharged from the flow channel 11. Alternatively, a return pipe may be connected to the discharge port, and the heat medium that has flowed through the flow path 11 and exchanged heat with the sample may be discharged from the flow path 11 and returned to the supply device to circulate the heat medium. .

  FIG. 2 is a schematic diagram illustrating the arrangement of the flow path 11 inside the substrate 10 in a plan view. In order for the heat medium flowing through the flow channel 11 to efficiently exchange heat with the sample held on the sample holding surface 10a, the flow channel 11 is preferably formed in a wide range corresponding to the sample holding surface 10a. In addition, it is important that the flow path 11 is formed in a wide range from the viewpoint of heat uniformity over the entire sample holding surface 10a. Therefore, in the sample holder 1 of the present embodiment, as shown in FIG. 2, the flow path 11 from the opening 11a to the opening 11b located on the opposite side of the opening 11a extends over the entire sample holding surface 10a. It has a meandering shape. By arranging the flow path 11 in this way, the width of the flow path 11 is increased, or the radius of curvature of the curved portion when the flow path 11 is meandered is reduced, so that Heat exchange can be performed more efficiently. In addition, if the distance of the folded portion between the straight portions when meandering is made too short, the portion that becomes the side wall of the flow path 11 becomes thin and the mechanical strength is lowered. However, it is preferable to form the flow path 11.

  In the example illustrated in FIG. 2, the flow path 11 has a meandering shape, but the arrangement shape of the flow path is not limited thereto. For example, the flow path 11 may have a spiral shape, or may have a shape in which a plurality of concentric circles and a straight line extending in the radial direction connecting the circles are combined.

  The shape of the channel 11 when viewed in a cross section perpendicular to the main surface of the substrate 10 can be a square shape or a circular shape. In particular, a rectangular shape is preferable from the viewpoint of ease of production.

  The base 10 is made of, for example, ceramic (ceramic sintered body) whose main component is silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, or the like. Among these, an aluminum nitride sintered body is particularly preferable.

  The aluminum nitride sintered body can have a thermal conductivity at room temperature of 150 W / (m · K) or higher, and has a higher thermal conductivity than other ceramic materials. For this reason, even when heat is locally applied to the held sample, the heat of the sample can be conducted by the base 10 and can be dissipated, so that the sample is hardly distorted due to thermal expansion. This can reduce deterioration of exposure accuracy due to distortion of the sample due to heat generation, for example, in the exposure process in the semiconductor manufacturing process.

  The thermal conductivity at room temperature is a value of thermal conductivity measured at a measurement atmosphere temperature within a room temperature range of 22 ° C. to 24 ° C., and heat measured at any set temperature within this temperature range. The conductivity is 150 W / (m · K) or more. Furthermore, the aluminum nitride sintered body can maintain a high thermal conductivity even in an environment exceeding room temperature. Specifically, for example, the thermal conductivity at an ambient temperature of 600 ° C. or higher can be 60 W / (m · K) or higher.

  The aluminum nitride sintered body preferably has an average crystal grain size in the range of 3 to 10 μm. When the average crystal grain size is 3 μm or more, the crystal grains in the aluminum nitride sintered body are relatively sufficiently filled, and the mechanical properties of the sintered body are made relatively good. In addition, by making crystals having an average crystal grain size of 10 μm or less, it is possible to reduce the residual voids (voids) existing between the crystals. Therefore, the average crystal grain size is preferably in the range of 3 to 10 μm. A more preferable range of the average crystal grain size is 3 to 7 μm.

Returning to FIG. 1, the electrode layer 20 is provided inside the substrate 10 and is composed of one or two separated electrodes 21 and 22. The electrode layer 20 is provided for electrostatic adsorption. One of the electrodes 21 and 22 is connected to the positive electrode of the power supply, and the other is connected to the negative electrode. As an example, the electrode connected to the positive electrode is referred to as electrode 21 (hereinafter referred to as “positive electrode 21”), and the electrode connected to the negative electrode is referred to as electrode 22 (hereinafter referred to as “negative electrode 22”). The electrode layer 20 may have the electrode 21 connected to the negative electrode and the electrode 22 connected to the positive electrode.

  The positive electrode 21 and the negative electrode 22 are each formed in a substantially semicircular shape, and are disposed inside the base 10 so that the semicircular chords face each other. The two electrodes of the positive electrode 21 and the negative electrode 22 are combined, and the outer shape of the entire electrode layer 20 is substantially circular. The substantially circular center that is the outer shape of the entire electrode layer 20 is set to be the same as the circular center that is the outer shape of the substrate 10.

  The positive electrode 21 and the negative electrode 22 are respectively provided with a connection terminal 21a and a connection terminal 22a for electrical connection with an external power source. In the present embodiment, each of the positive electrode 21 and the negative electrode 22 is provided with the connection terminal 21a and the connection terminal 22a so as to extend along the string at a portion where the arc and the string intersect. The connection terminal 21 a provided on the positive electrode 21 and the connection terminal 22 a provided on the negative electrode 22 are spaced from each other by the same distance between the semicircle strings of the positive electrode 21 and the semicircle strings of the negative electrode 22. And extend to the outer peripheral surface of the base body 10 along the extension lines of these semicircular strings. Further, the connection terminal 21 a and the connection terminal 22 a are provided so that a part thereof is exposed on the end surface of the base 10. The positive electrode 21 and the negative electrode 22 are connected to an external power source through a portion where the connection terminal 21a and the connection terminal 22a are exposed.

  The electrode layer 20 is made of, for example, a conductive material such as tungsten or molybdenum, and is formed so as to be positioned between the ceramic layers of the substrate 10 by screen printing of a paste containing the conductive material. The thickness of the electrode layer 20 of this embodiment is about 1-100 micrometers, for example.

  FIG. 3 is a cross-sectional view of the sample holder 1 taken along a cutting plane line A-A ′ shown in FIG. 2. FIG. 4 is a partially enlarged cross-sectional view in which the cross section of one channel 11 in the cross-sectional view shown in FIG. 3 is enlarged. The base 10 is formed of a laminate in which four ceramic layers 12, 13, 14, and 15 are laminated, and an electrode layer 20 is provided therein. The ceramic layers 12, 13, 14, and 15 here do not necessarily need to be made of one green sheet. Specifically, the ceramic layers 12, 13, 14, and 15 may each be a laminate of a plurality of green sheets. In the present embodiment, the electrode layer 20 is provided on the one main surface (sample holding surface 10a) side that holds the sample with respect to the flow path 11.

  In the following, the outermost ceramic layer 15 is the outermost layer 15, the ceramic layer 12 is provided with the electrode layer 20 between the outermost layer 15, and the upper layer 12 is the ceramic provided on the opposite side of the ceramic layer 13. The ceramic layer 13 sandwiched between the lower layer 14 and the upper layer 12 and the lower layer 14 is referred to as an intermediate layer 13. The names of these layers are given for the sake of convenience of explanation, and the upper layer 12 is not necessarily positioned on the upper side in the vertical direction, and the lower layer 14 is not positioned on the lower side in the vertical direction. Further, as shown in FIG. 2, the heat medium flows from the opening 11a serving as the supply port to the opening 11b serving as the discharge port. Therefore, in FIGS. 3 and 4, the flow direction of the heat medium is perpendicular to the paper surface. .

Here, as shown in FIG. 4, the flow path 11 has a convex portion 30 and a concave portion 31 facing each other on the inner surface when viewed in a cross section orthogonal to the traveling direction of the heat medium. Since the flow path 11 has the convex portion 30 and the concave portion 31 facing each other on the inner surface, the vortex of the heat medium can be generated in the convex portion 30 and the concave portion 31, respectively. Furthermore, since the convex portion 30 and the concave portion 31 are positioned so as to face each other, the vortices of these heat media can be overlapped, and a large vortex can be generated. Therefore, since the flow of the heat medium can be made turbulent, it is possible to suppress the powder generated by the abrasion of the substrate 10 from being deposited inside the flow path 11. Thereby, it can suppress that the heat conduction between a heat carrier and the sample holder 1 falls. As a result, it is possible to suppress a decrease in soaking property on the sample holding surface 10a.

  Furthermore, as shown in FIG. 5, it is preferable that the convex part 30 and the recessed part 31 are long along the advancing direction of a heat carrier. Here, “long” means that the length value in the traveling direction of the heat medium is larger than at least the thickness and width of each of the convex portion 30 and the concave portion 31. Thereby, the turbulent flow of the heat medium can be continuously generated between the convex portion 30 and the concave portion 31. Thereby, heat conduction between the heat medium and the sample holder 1 can be favorably performed. As a result, it is possible to suppress a decrease in soaking property on the sample holding surface 10a.

  Furthermore, as shown in FIG. 6, it is preferable that the tip (upstream end) of the convex portion 30 has a curved shape when viewed in a cross section parallel to the traveling direction of the heat medium. Thereby, it is possible to suppress the tip of the convex portion 30 from being scraped off by the heat medium and depositing ceramic powder inside the flow path 11. Therefore, it can suppress that the heat conduction between a heat carrier and the sample holder 1 falls. As a result, it is possible to suppress a decrease in soaking property on the sample holding surface 10a.

  Further, as shown in FIG. 7, when viewed from the upper surface side (sample holding surface 10 a side), the flow path 11 has a portion formed in an arc shape, and in the portion formed in this arc shape, It is preferable to have the convex portion 30 on the outer inner surface and the concave portion 31 on the inner inner surface. Although the flow rate of the heat medium tends to increase outside the portion formed in the arc shape, the flow velocity can be further increased by providing the convex portion 30 at this portion. In addition, the flow rate of the heat medium tends to be slow inside the arc-shaped portion. By providing the concave portion 31 in this portion, the flow velocity can be further slowed down. As a result, a portion having a high flow velocity and a portion having a low flow velocity are generated in the arc-shaped portion, so that a large vortex flow can be generated.

  Further, as shown in FIG. 8, when viewed in a cross section parallel to the traveling direction of the heat medium, the angle of the convex portion 30 with respect to the inner surface of the flow path 11 may be 45 ° or more and less than 90 °. Thereby, since the convex part 30 will inhibit the flow of a heat medium largely, the flow of a heat medium can be made into a turbulent flow. For this reason, it can suppress that the powder which arises by abrasion of the base | substrate 10 accumulates in the inside of the flow path 11. FIG. Thereby, it can suppress that the heat conduction between a heat carrier and the sample holder 1 falls. As a result, it is possible to suppress a decrease in soaking property on the sample holding surface 10a.

  Moreover, as shown in FIG. 9, when viewed in a cross section parallel to the traveling direction of the heat medium, the angle of the convex portion 30 with respect to the inner surface of the flow path 11 may be 90 ° or more and less than 135 ° C. Thus, by making the convex portion 30 have a gentle inclination, it is possible to suppress the convex portion 30 from being scraped off by the heat medium and depositing ceramic powder inside the flow path 11. Therefore, it can suppress that the heat conduction between a heat carrier and the sample holder 1 falls. As a result, it is possible to suppress a decrease in soaking property on the sample holding surface 10a.

  The sample holder 1 may further have a heating resistor (not shown). The heating resistor is preferably provided in the vicinity of the convex portion 30 and the concave portion 31. Thereby, the heat conduction between the heating resistor and the heat medium can be favorably performed. As a result, the temperature of the heating resistor can be easily adjusted. As a result, the sample holding surface 10a can be easily adjusted to a desired temperature. At this time, it is preferable that the heating resistor is located between the sample holding surface 10 a and the flow path 11. As a result, heat can be transferred from the heating resistor to the sample holding surface 10a satisfactorily, and the temperature of the heating resistor can be easily adjusted by the flow path 11.

The ceramic constituting the substrate 10 is preferably a ceramic that is stable against the heat medium flowing in the flow path 11. Thereby, the corrosion by the chemical reaction with a heat medium can be suppressed. As a result, it is possible to reduce deterioration of the thermal uniformity of the sample holding surface 10a due to the accumulation of corrosive powder.

  Here, in order to explain the convex portion 30 and the concave portion 31 on the inner surface of the flow path 11, a method for manufacturing the base body 10 will be briefly described.

  The base 10 of the sample holder 1 of the present embodiment is a laminated body in which a plurality of ceramic layers are laminated as described above, and is obtained by laminating and firing a plurality of green sheets that are pre-formed into a predetermined shape. Is. The green sheet serving as the intermediate layer 13 is formed in a shape corresponding to the flow path 11. Specifically, a through hole penetrating in the vertical direction is provided. Of the surface of the green sheet serving as the upper layer 12 on the side in contact with the intermediate layer 13, the surface 121 facing the through hole provided in the green sheet serving as the intermediate layer 13, and the side contacting the intermediate layer 13 in the green sheet serving as the lower layer 14 The surface 141 facing the through hole provided in the green sheet to be the intermediate layer 13 and the inner surface 131 of the through hole in the green sheet to be the intermediate layer 13 are the inner surface of the flow path 11 after firing. Become. Here, the size and shape of the inner surface are not particularly limited, and are determined by the amount of the heat medium flowing through the flow path 11 or the required cooling rate.

  Here, in order to form the convex part 30 and the concave part 31, the following method can be used. Specifically, the intermediate layer 13 is formed of a plurality of ceramic layers, and the position or shape of the through hole provided in at least one ceramic layer among them is the position or shape of the through hole provided in another ceramic layer serving as the intermediate layer 13. Change it. Thereby, the convex part 30 and the recessed part 31 can be formed in the inner surface of the flow path 11.

  In the above description, the sample holder 1 having the electrode layer 20, that is, the so-called electrostatic chuck has been described as an embodiment of the present invention, but is not limited thereto. Specifically, the present invention can be applied to a vacuum chuck by vacuum suction without providing the electrode layer 20. In the embodiment applied to the vacuum chuck, in addition to the flow path 11, other flow paths are provided in the base 10, and a plurality of suction holes that open to face one main surface of the base 10 and are connected to the other flow paths are provided. The other flow path may be connected to a vacuum pump to place the other flow path in a vacuum state. The channel 11 is configured to flow a heat medium that exchanges heat with the sample in order to cool or heat the sample that is vacuum-adsorbed and held on the sample holding surface 10a, as in the case of the sample holders 1 and 1A. .

<Example>
As an example of the present invention, a sample holder 1 in which the base 10 was made of aluminum nitride was produced. The shape of the main surface was set to a circular shape with a diameter of 300 mm, and the thickness was set to 12 mm. A flow path 11 for flowing a heat medium is formed inside the sample holder 1. The dimensions of the flow path 11 were 4 mm in width and 3.5 mm in height when cut in a cross section perpendicular to the traveling direction of the heat medium. Of the ceramic layers constituting the intermediate layer 13, one ceramic layer is provided with a portion protruding about 0.5 mm toward the flow channel 11 as compared with the other ceramic layers, thereby projecting to the inner surface of the flow channel 11. Part 30 was formed. In addition, by providing a portion of the ceramic layer constituting the intermediate layer 13 that is recessed by about 0.5 mm from the flow channel 11 side as compared with the other ceramic layers, one inner surface of the flow channel 11 is provided. A recess 31 was formed on the surface. The length of the convex portion 30 and the concave portion 31 in the traveling direction of the heat medium was about 50 mm.

  In addition, as a comparative example of the present invention, a sample holder was prepared in which the convex portion 30 and the concave portion 31 were not provided and other configurations were the same as those of the above-described embodiment of the present invention.

In the Example, it has confirmed that the flow velocity of the heat medium became quick in the vicinity of the convex part 30 and the recessed part 31. FIG. The following method was used for confirmation of the flow rate. Specifically, a flow rate test was performed using an ultrasonic flow meter US300PM manufactured by Yokogawa Electric Corporation with a constant flow rate. As a result, it was confirmed that in the vicinity of the convex portion 30 and the concave portion 31, the flow velocity was about 1 to 5% faster than other regions.

  Furthermore, in order to evaluate the thermal uniformity of the sample holding surface, the following evaluation was performed. Specifically, using a thermography TH3100MR (device name) manufactured by NEC Corporation, an endurance test for heat distribution measurement was performed in an environment that was not affected by the convection of the surrounding air and heat radiation or radiant heat in the device. I did it. As a result, the temperature difference between the maximum temperature and the minimum temperature in the temperature distribution after the intermittent operation of the durability test for 500 hours on the sample holding surface 10a is 0 as compared with the temperature difference between the maximum temperature and the minimum temperature before the test. .Changed less than 1 degree.

  On the other hand, in the comparative example, when the same test was performed, the temperature difference between the maximum temperature and the minimum temperature in the temperature distribution after the endurance test was compared with the temperature difference between the maximum temperature and the minimum temperature before the test. In the range of 0.1 to 0.2 degrees.

  From these results, it was confirmed that the thermal uniformity of the sample holding surface can be improved by providing the convex portions 30 and the concave portions 31.

DESCRIPTION OF SYMBOLS 1,1A Sample holder 10 Base | substrate 10a Sample holding surface 11 Flow path 11a, 11b Opening part 12 Upper layer 13 Intermediate layer 14 Lower layer 15 Top layer 20 Electrode layer 21 Positive electrode 22 Negative electrode 30 Convex part 31 Concave part

Claims (8)

  A sample holder made of ceramics and having a sample holding surface on the outer surface and a heat medium channel inside, when viewed in a cross section perpendicular to the traveling direction of the heat medium, A sample holder having a convex portion and a concave portion facing each other on a surface.   The sample holder according to claim 1, wherein the convex part and the concave part are long along the traveling direction of the heat medium.   3. The sample holder according to claim 1, wherein when viewed in a cross section parallel to the traveling direction of the heat medium, the tip of the convex portion has a curved shape.   When viewed from the sample holding surface side, the flow path has a portion formed in an arc shape, and in the portion formed in the arc shape, the outer surface has the convex portion. The sample holder according to any one of claims 1 to 3, wherein the concave portion is provided on the inner inner surface.   The angle of the convex portion with respect to the inner surface of the flow path is 45 ° or more and less than 90 ° when viewed in a cross section parallel to the traveling direction of the heat medium. The sample holder according to any one of the above.   The angle of the convex portion with respect to the inner surface of the flow path is 90 ° or more and less than 135 ° when viewed in a cross section parallel to the traveling direction of the heat medium. 5. The sample holder according to any one of 4 above.   The sample holder according to any one of claims 1 to 6, further comprising a heating resistor therein, wherein the flow path includes the convex portion and the concave portion in the vicinity of the heating resistor. Ingredients.   The sample holder according to claim 7, wherein the heating resistor is located between the sample holding surface and the flow path.

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