Ceramic susceptor

Abstract

Provided is a ceramic susceptor capable of reducing an influence on adsorption performance or plasma characteristics while incorporating not only a heater circuit and a thermocouple insertion path but also an internal electrode such as an ESC electrode and an RF electrode. This ceramic susceptor includes: a disk-shaped ceramic plate assembly including an upper ceramic plate and a lower ceramic plate bonded to each other at a bonding surface, and having a first surface opposite to the bonding surface of the upper ceramic plate and a second surface opposite to the bonding surface of the lower ceramic plate; at least one internal electrode embedded in the upper ceramic plate in parallel with the first surface and selected from the group consisting of an RF electrode and an ESC electrode; a first heater circuit embedded in the lower ceramic plate in parallel with the first surface; and a thermocouple insertion groove provided on the bonding surface side of the upper ceramic plate or the lower ceramic plate and constituting a thermocouple insertion path together with the bonding surface.

Description

Ceramic Susceptor

[0001] The present disclosure relates to ceramic susceptors.

[0002] In film deposition apparatuses for semiconductor manufacturing processes, ceramic heaters are used as support stages for uniformly controlling the temperature of wafers. A widely used ceramic heater includes a ceramic plate on which the wafer is placed and a cylindrical ceramic shaft attached to the ceramic plate. The ceramic plate typically has a ceramic base made of aluminum nitride (AlN) or other material with excellent heat and corrosion resistance, with internal electrodes embedded therein, such as a heater electrode, an RF electrode, and an electrostatic chuck (ESC) electrode.

[0003] A ceramic heater provided with a thermocouple insertion path for inserting a thermocouple for controlling the peripheral temperature is known.

[0004] Patent Document 1 (Japanese Patent No. 7181314) discloses a ceramic heater including a disk-shaped ceramic plate having a wafer mounting surface, an outer peripheral resistance heating element built into the ceramic plate and arranged in a circular outer peripheral zone with multiple folded portions, and an outer peripheral thermocouple with a temperature measuring portion at its tip for measuring the temperature of the outer peripheral zone. When viewed from the wafer mounting surface, the temperature measuring portion is located in a position in the outer peripheral zone excluding the portion where the folded portions of the outer peripheral resistance heating element face each other. A thermocouple path is provided within the ceramic plate, parallel to the wafer mounting surface. This thermocouple path is configured to extend from an insertion port opening in the center of the ceramic plate on the side opposite the wafer mounting surface to a terminal position just before the outer peripheral surface of the ceramic plate.

[0005] Patent Document 2 (JP 2021-174586 A) discloses a ceramic heater including a disk-shaped ceramic base having a wafer mounting surface, a resistance heating element embedded in the ceramic base, a cylindrical shaft supporting the ceramic base from its underside, a thermocouple passage, and a thermocouple insertion hole communicating with the thermocouple passage. The thermocouple passage is provided between the resistance heating element and the wafer mounting surface, extending from a starting position on the center side of the interior of the ceramic base to a terminal position on the outer periphery. The thermocouple passage opens into an inner shaft region surrounded by the cylindrical shaft on the underside of the ceramic base, and is provided so as to communicate with the thermocouple passage.

[0006] Patent No. 7181314 JP 2021-174586 A

[0007] However, when a ceramic heater having a thermocouple insertion path is further equipped with an internal electrode such as an ESC electrode or an RF electrode to enhance its functionality, the presence of the thermocouple insertion path or the thermocouple inserted therein can have an undesirable effect on the adsorption performance or plasma characteristics brought about by the ESC electrode or the RF electrode, which may result in the ceramic heater being unable to maximize its desired performance.

[0008] The present inventors have now discovered that the influence on adsorption performance or plasma characteristics can be reduced by arranging the thermocouple insertion groove at a depth position between the internal electrode and the first heater circuit in the thickness direction of the ceramic plate.

[0009] Therefore, an object of the present invention is to provide a ceramic susceptor that can incorporate not only a heater circuit and a thermocouple insertion path but also internal electrodes such as ESC electrodes and RF electrodes while minimizing the influence on adsorption performance or plasma characteristics.

[0010] According to the present disclosure, the following aspects are provided: [Aspect 1] A ceramic susceptor comprising: a disc-shaped ceramic plate assembly including an upper ceramic plate and a lower ceramic plate joined together at their joining surfaces, the upper ceramic plate having a first surface opposite to the joining surface of the upper ceramic plate and a second surface opposite to the joining surface of the lower ceramic plate, at least one internal electrode selected from the group consisting of an RF electrode and an ESC electrode embedded in the upper ceramic plate parallel to the first surface, a first heater circuit embedded in the lower ceramic plate parallel to the first surface, and a thermocouple insertion groove provided on the joining surface side of the upper ceramic plate or the lower ceramic plate and constituting a thermocouple insertion path together with the joining surface, whereby the thermocouple insertion groove is located at a depth position between the internal electrode and the first heater circuit in the thickness direction of the ceramic plate assembly. [Aspect 2] The ceramic susceptor according to Aspect 1, further comprising a second heater circuit embedded in the upper ceramic plate parallel to the first surface, whereby the thermocouple insertion groove is located at a depth between the first heater circuit and the second heater circuit in the thickness direction of the ceramic plate assembly. [Aspect 3] The ceramic susceptor according to Aspect 2, wherein the second heater circuit is provided at a depth farther from the first surface than the internal electrode. [Aspect 4] The ceramic susceptor according to Aspect 2 or 3, wherein the ceramic plate assembly, when viewed from above, includes an inner zone defined as a circular region within a predetermined distance from the center of the ceramic plate assembly, and an outer zone defined as an annular region outside the inner zone, and the first heater circuit is located in the outer zone and the second heater circuit is located in the inner zone. [Aspect 5] The ceramic susceptor according to Aspect 4, further comprising: a jumper embedded in the inner zone of the lower ceramic plate and connected to the first heater circuit; and a power supply rod having one end connected to the jumper and the other end extending from the second surface to an outside of the ceramic plate assembly, such that power can be supplied to the first heater circuit via the jumper.[Aspect 6] The ceramic susceptor according to Aspect 4 or Aspect 5, further comprising a thermocouple insertion hole formed as a vertical hole extending from the inner zone of the second surface through the lower ceramic plate to the upper ceramic plate. [Aspect 7] The ceramic susceptor according to Aspect 6, wherein the thermocouple insertion hole reaches a depth position closer to the first surface than the second heater circuit. [Aspect 8] The ceramic susceptor according to any one of Aspects 1 to 7, further comprising a cylindrical ceramic shaft attached to the second surface of the ceramic plate assembly. [Aspect 9] The ceramic susceptor according to any one of Aspects 1 to 8, further comprising a first thermocouple for the outer zone inserted in the thermocouple insertion path. [Aspect 10] The ceramic susceptor according to any one of Aspects 6 to 9, further comprising a second thermocouple for the inner zone inserted in the thermocouple insertion hole. [Aspect 11] The ceramic susceptor according to Aspect 10, wherein the second thermocouple reaches a depth position closer to the first surface than the first thermocouple.

[0011] The ceramic susceptor according to the present invention is a ceramic susceptor having a ceramic shaft, a ceramic shaft portion, a ceramic shaft portion, and a ceramic shaft portion.

[0012] The ceramic susceptor according to the present invention is a ceramic platform for supporting a wafer, and is used in a film formation apparatus or an etching apparatus, particularly a film formation apparatus or an etching apparatus for a semiconductor manufacturing process. For example, the ceramic susceptor according to the present invention may be a ceramic heater for a semiconductor film formation apparatus or an electrostatic chuck for a semiconductor etching apparatus. Alternatively, the ceramic susceptor may be an electrostatic chuck heater that combines heater and electrostatic chuck functions. Typical examples of film formation apparatuses include CVD (chemical vapor deposition) apparatuses (e.g., thermal CVD apparatuses, plasma CVD apparatuses, photo-assisted CVD apparatuses, and MOCVD apparatuses) and PVD (physical vapor deposition) apparatuses.

[0013] FIG. 1 shows an example of a ceramic susceptor. The ceramic susceptor 10 shown in FIG. 1 includes a ceramic plate assembly 12, an internal electrode 14, a first heater circuit 16, and a thermocouple insertion groove 18. The ceramic plate assembly 12 is disk-shaped and includes an upper ceramic plate 12a and a lower ceramic plate 12b joined together at their joining surfaces 12c. The ceramic plate assembly 12 has a first surface 12d opposite the joining surface 12c of the upper ceramic plate 12a and a second surface 12e opposite the joining surface 12c of the lower ceramic plate 12b. The internal electrode 14 is at least one type selected from the group consisting of an RF electrode and an ESC electrode, and is embedded in the upper ceramic plate 12a parallel to the first surface 12d. The first heater circuit 16 is embedded in the lower ceramic plate 12b parallel to the first surface 12d. The thermocouple insertion groove 18 is provided on the joining surface 12c side of the upper ceramic plate 12a or the lower ceramic plate 12b, and forms a thermocouple insertion path together with the joining surface 12c. As a result, the thermocouple insertion groove 18 is located at a depth position between the internal electrode 14 and the first heater circuit 16 in the thickness direction of the ceramic plate assembly 12. By arranging the thermocouple insertion groove 18 in this manner, it is possible to provide a ceramic susceptor 10 that can incorporate not only a heater circuit and a thermocouple insertion path, but also an internal electrode 14 such as an ESC electrode or an RF electrode, while minimizing the influence on adsorption performance or plasma characteristics.

[0014] As described above, when a ceramic heater having a thermocouple insertion path is further equipped with an internal electrode such as an ESC electrode or an RF electrode to enhance its functionality, the presence of the thermocouple insertion path or the thermocouple inserted therein can undesirably affect the adsorption performance or plasma characteristics provided by the ESC electrode or the RF electrode. As a result, the ceramic heater may not be able to maximize its desired performance. This problem is successfully solved by the configuration of the present invention. Specifically, by locating the thermocouple insertion groove 18 at a depth between the internal electrode 14 and the first heater circuit 16 in the thickness direction of the ceramic plate assembly 12, no other components (such as the first heater circuit 16, the second heater circuit 26 described below with reference to FIG. 2, the thermocouple insertion groove 18, and the first thermocouple 24) that could become an obstacle exist between the first surface 12d of the ceramic plate assembly 12 and the internal electrode 14. This allows the adsorption performance provided by the ESC electrode and / or the plasma characteristics provided by the RF electrode to be maximized without being affected by such other components. As a secondary effect, the above arrangement can ensure a long separation distance between the internal electrode 14 and the first heater circuit 16, thereby reducing the leakage current that may flow from the internal electrode 14 to the first heater circuit 16 due to the potential difference between the internal electrode 14 and the first heater circuit 16. That is, the longer the separation distance between the internal electrode 14 and the first heater circuit 16, the greater the resistance, thereby further reducing the leakage current that may flow between them. Furthermore, the above arrangement has the advantage that the thermocouple insertion groove 18 is located at a depth closer to the first surface 12d than the first heater circuit 16, thereby reducing the discrepancy between the temperature of the first surface 12d and the temperature read by the first thermocouple 24 in the thermocouple insertion groove 18.

[0015] The ceramic plate assembly 12 includes an upper ceramic plate 12a and a lower ceramic plate 12b bonded together at a bonding surface 12c. The upper ceramic plate 12a and the lower ceramic plate 12b may be made of materials having the same physical properties or different physical properties (e.g., volume resistivity and thermal expansion coefficient). In the latter case, for example, the volume resistivity of the upper ceramic plate 12a may be relatively higher than that of the lower ceramic plate 12b, or the volume resistivity of the lower ceramic plate 12b may be relatively higher than that of the upper ceramic plate 12a. In either case, the upper ceramic plate 12a and the lower ceramic plate 12b are not particularly limited except for the arrangement of the first heater circuit 16, the second heater circuit 26 (described later), the thermocouple insertion groove 18, and the thermocouple insertion hole (described later), and may have a configuration similar to that of ceramic plates used in known ceramic susceptors or ceramic heaters. Therefore, the upper ceramic plate 12a and the lower ceramic plate 12b preferably contain aluminum nitride or aluminum oxide, more preferably aluminum nitride, from the viewpoints of excellent thermal conductivity, high electrical insulation, and thermal expansion characteristics similar to those of silicon.

[0016] The internal electrode 14 is an electrode embedded in the upper ceramic plate 12a parallel to the first surface and includes at least one selected from the group consisting of an RF electrode and an ESC electrode. The RF electrode enables film deposition by a plasma CVD process when high-frequency waves are applied to it. The ESC electrode is an abbreviation for electrostatic chuck (ESC) electrode and is also called an electrostatic electrode. The ESC electrode is preferably a circular thin-layer electrode with a diameter slightly smaller than that of the ceramic plate assembly 12. For example, it may be a mesh electrode formed by weaving thin metal wires into a net shape into a sheet. The ESC electrode may also be used as a plasma electrode. That is, by applying high-frequency waves to the ESC electrode, the ESC electrode can also be used as an RF electrode, allowing film deposition by a plasma CVD process. A terminal rod 20 is connected to the internal electrode 14 and is connected to an external power source (not shown). When the internal electrode 14 is an ESC electrode, the ESC electrode chucks a wafer placed on the surface of the ceramic plate assembly 12 by the Johnsen-Rahbek force when a voltage is applied by an external power supply.

[0017] The first heater circuit 16 is embedded in the lower ceramic plate 12b parallel to the first surface 12d. The first heater circuit 16 is not particularly limited, but may be, for example, a conductive coil wired in a single stroke across the entire surface of the lower ceramic plate 12b or across a predetermined region (typically, an outer zone Z2, described later with reference to FIG. 2 ). Power supply rods 22 are connected to both ends of the first heater circuit 16 for power supply, and the power supply rods 22 are connected to a heater power supply (not shown). When power is supplied from the heater power supply, the first heater circuit 16 generates heat to heat a wafer placed on the first surface 12d. The first heater circuit 16 is not limited to a coil, and may be, for example, a ribbon (a thin, elongated plate), a mesh, or a print.

[0018] The thermocouple insertion groove 18, which together with the joining surface 12c constitutes a thermocouple insertion path, is provided on the joining surface 12c side of the upper ceramic plate 12a or the lower ceramic plate 12b. The thermocouple insertion groove 18 or thermocouple insertion path allows a first thermocouple 24 to be inserted or accommodated therein, enabling temperature measurement at a predetermined position (typically, an outer peripheral portion such as the outer zone Z2 described below with reference to FIG. 2) of the ceramic plate assembly 12 or the internal electrode 14. The thermocouple insertion groove 18 is preferably provided in the upper ceramic plate 12a as shown in FIG. 1, but may also be provided in the lower ceramic plate 12b. The thermocouple insertion groove 18 is preferably arranged linearly from a thermocouple insertion opening 18a, which is a vertical hole formed in the second surface 12e, to a thermocouple insertion path end 18b. The thermocouple insertion path end 18b is preferably closed for accurate temperature measurement.

[0019] FIG. 2 shows a ceramic susceptor 10′ according to a preferred embodiment of the present invention. In addition to the above-described configuration, this ceramic susceptor 10′ further includes a second heater circuit 26. The second heater circuit 26 is embedded in the upper ceramic plate 12a parallel to the first surface 12d, thereby positioning the thermocouple insertion groove 18 at a depth between the first heater circuit 16 and the second heater circuit 26 in the thickness direction of the ceramic plate assembly 12. In this case, the second heater circuit 26 is preferably located at a depth farther from the first surface 12d than the internal electrode 14. This ensures that no other components (such as the first heater circuit 16, the second heater circuit 26, or the thermocouple insertion groove 18) that could become obstacles are present between the first surface 12d of the ceramic plate assembly 12 and the internal electrode 14. This allows the adsorption performance provided by the ESC electrode and / or the plasma characteristics provided by the RF electrode to be maximized without being affected by such other components. The second heater circuit 26 is also not particularly limited, and may be, for example, a conductive coil wired in a single stroke across a predetermined region of the upper ceramic plate 12a (preferably the inner zone Z1 described below). Power supply rods 30 are connected to both ends of the second heater circuit 26 for power supply, and the power supply rods 30 are connected to a heater power supply (not shown). When power is supplied from the heater power supply, the second heater circuit 26, together with the first heater circuit 16, generates heat and heats the wafer placed on the first surface 12d. The second heater circuit 26 is not limited to a coil, and may be, for example, a ribbon (a thin, elongated plate), a mesh, or a print.

[0020] In the ceramic susceptor 10′, the ceramic plate assembly 12 may include an inner zone Z1 and an outer zone Z2. When the ceramic plate assembly 12 is viewed from above, the inner zone Z1 is defined as a circular region within a predetermined distance from the center of the ceramic plate assembly 12, while the outer zone Z2 is defined as an annular region outside the inner zone Z1. In this embodiment, it is preferable that the first heater circuit 16 is disposed in the outer zone Z2, and the second heater circuit 26 is disposed in the inner zone Z1. This allows the temperatures of the inner zone Z1 and the outer zone Z2 to be adjusted separately by the first heater circuit 16 and the second heater circuit 26, respectively, and therefore makes it possible to heat the ceramic plate assembly 12 with a desired temperature distribution profile.

[0021] 2, the ceramic susceptor 10' preferably further includes a jumper 28. The jumper 28 is embedded in the inner zone Z1 of the lower ceramic plate 12b and is connected to the first heater circuit 16. In this case, the power supply rod 22 is provided such that one end thereof is connected to the jumper 28 and the other end thereof extends from the second surface 12e to the outside of the ceramic plate assembly 12. In this way, power can be supplied to the first heater circuit 16 via the power supply rod 22 and the jumper 28.

[0022] As shown in FIG. 2 , the ceramic susceptor 10′ preferably further includes a thermocouple insertion hole 32. The thermocouple insertion hole 32 is a vertical hole that extends from the inner zone Z1 of the second surface 12e through the lower ceramic plate 12b to the upper ceramic plate 12a. By inserting a second thermocouple 34 into the thermocouple insertion hole 32, the temperature of the ceramic plate assembly 12 or the inner zone Z1 of the internal electrode 14 can be measured. In this case, the thermocouple insertion hole 32 preferably extends to a depth position closer to the first surface 12d than the second heater circuit 26. This reduces the discrepancy between the temperature of the first surface 12d and the temperature read by the second thermocouple 34 in the thermocouple insertion hole 32.

[0023] The ceramic susceptor 10' preferably includes a first thermocouple 24 for the outer zone Z2 inserted into the thermocouple insertion path (or thermocouple insertion groove 18). The ceramic susceptor 10' also preferably includes a second thermocouple 34 for the inner zone Z1 inserted into the thermocouple insertion hole 32. A temperature measuring device (not shown) can be connected to the distal end of the first thermocouple 24 or the second thermocouple 34.

[0024] The second thermocouple 34 preferably reaches a depth closer to the first surface 12d than the first thermocouple 24. Specifically, the ratio of the distance B between the first thermocouple 24 and the first surface 12d to the distance A between the proximal end (the tip closer to the first surface 12d) of the second thermocouple 34 and the first surface 12d (i.e., the value of B / A) is preferably 1.4 to 3.0. In this embodiment, the distance A between the proximal end of the second thermocouple 34 and the first surface 12d is preferably 4 to 6 mm. The thickness of the ceramic plate assembly 12 is preferably 20 to 35 mm. According to this embodiment, the temperature of the inner zone Z1 can be accurately measured by the second thermocouple 34, while undesirable effects on the adsorption performance or plasma characteristics (caused by the internal electrode 14, which is an ESC electrode or an RF electrode) of the thermocouple insertion groove 18 for the first thermocouple 24, which has a larger groove area (than the cross-sectional area of ​​the thermocouple insertion hole 32 for the second thermocouple 34), or of the first thermocouple 24 inserted therein, can be more effectively reduced.

[0025] If desired, a cylindrical ceramic shaft 36 may be attached (preferably concentrically) to the second surface 12e of the ceramic plate assembly 12. The ceramic shaft 36 is a cylindrical member having an internal space S and may have a configuration similar to that of ceramic shafts used in known ceramic susceptors or ceramic heaters. The internal space S is configured to accommodate the terminal rod 20, the power feed rod 22, the power feed rod 30 (if present), the first thermocouple 24, and the second thermocouple 34 (if present). The ceramic shaft 36 is preferably made of the same ceramic material as the ceramic plate assembly 12. Therefore, the ceramic shaft 36 preferably contains aluminum nitride or aluminum oxide, more preferably aluminum nitride. The upper end surface of the ceramic shaft 36 is preferably bonded to the second surface 12e of the ceramic plate assembly 12 by solid-state bonding or diffusion bonding. The outer diameter of the ceramic shaft 36 is not particularly limited and may be, for example, approximately 40 mm. The inner diameter of the ceramic shaft 36 (the diameter of the internal space S) is not particularly limited either, and is, for example, about 36 mm.

[0026] 3 is a schematic plan view of the ceramic susceptor 10′ as viewed from the ceramic shaft 36 side. As shown in FIG. 3, the terminal rods 20, the power feed rods 22, the power feed rods 30, the thermocouple insertion holes 32, and the second thermocouples 34 are arranged in a region corresponding to the internal space S surrounded by the sidewall of the ceramic shaft 36 when viewed from the ceramic shaft 36 side. The thermocouple insertion grooves 18 and the first thermocouples 24 housed therein preferably extend linearly from the thermocouple insertion openings 18a in the region corresponding to the internal space S to the thermocouple insertion path ends 18b at predetermined positions corresponding to the outer zone Z2 (preferably positions close to the outer periphery of the ceramic plate assembly 12).

[0027] The ceramic plate assembly 12 or the ceramic susceptor 10, 10′ can be manufactured using a known method. For example, the ceramic plate assembly 12 can be manufactured by bonding together a disc-shaped upper ceramic plate 12a, in which the internal electrode 14 and the second heater circuit 26 are embedded and the thermocouple insertion groove 18 is formed, and a disc-shaped lower ceramic plate 12b, in which the first heater circuit 16 and the jumper 28 are embedded, by applying a known ceramic bonding agent to the surfaces to be joined, and then appropriately firing the plates. Next, the resulting ceramic plate assembly 12 is processed to form the thermocouple insertion opening 18a, the thermocouple insertion hole 32, and various other rod insertion holes, and the first thermocouple 24, the second thermocouple 34, and various other rod insertion holes can be inserted or connected as needed.

Claims

1. A disc-shaped ceramic plate assembly including an upper ceramic plate and a lower ceramic plate joined together at a joint surface, having a first surface on the side opposite to the joint surface of the upper ceramic plate and a second surface on the side opposite to the joint surface of the lower ceramic plate, at least one internal electrode selected from the group consisting of an RF electrode and an ESC electrode embedded in the upper ceramic plate parallel to the first surface, a first heater circuit embedded in the lower ceramic plate parallel to the first surface, and a thermocouple insertion groove provided on the joint surface side of the upper ceramic plate or the lower ceramic plate and constituting a thermocouple insertion path together with the joint surface, whereby the thermocouple insertion groove is arranged at a depth position between the internal electrode and the first heater circuit in the thickness direction of the ceramic plate assembly, a ceramic susceptor.

2. The ceramic susceptor according to claim 1, further comprising a second heater circuit embedded in the upper ceramic plate parallel to the first surface, whereby the thermocouple insertion groove is arranged at a depth position between the first heater circuit and the second heater circuit in the thickness direction of the ceramic plate assembly.

3. The ceramic susceptor according to claim 2, wherein the second heater circuit is provided at a depth position farther from the first surface than the internal electrode.

4. The ceramic plate assembly includes an inner zone defined as a circular region within a predetermined distance from the center of the ceramic plate assembly in a plan view, and an outer zone defined as an annular region outside the inner zone. The first heater circuit is arranged in the outer zone, and the second heater circuit is arranged in the inner zone. The ceramic susceptor according to claim 2 or 3.

5. The ceramic susceptor according to claim 4, further comprising a jumper embedded in the inner zone of the lower ceramic plate and connected to the first heater circuit, and a power supply rod having one end connected to the jumper and the other end extending outside the ceramic plate assembly from the second surface, whereby power can be supplied to the first heater circuit through the jumper via the power supply rod.

6. The ceramic susceptor according to claim 4, further comprising a thermocouple insertion hole formed by a vertical hole that penetrates the lower ceramic plate from the inner zone of the second surface and reaches the upper ceramic plate.

7. The ceramic susceptor according to claim 6, wherein the thermocouple insertion hole reaches a depth position closer to the first surface than the second heater circuit.

8. The ceramic susceptor according to claim 1 or 2, further comprising a cylindrical ceramic shaft attached to the second surface of the ceramic plate assembly.

9. The ceramic susceptor according to claim 1 or 2, further comprising a first thermocouple for the outer zone inserted into the thermocouple insertion path.

10. The ceramic susceptor according to claim 6, further comprising a second thermocouple for the inner zone inserted into the thermocouple insertion hole.

11. The ceramic susceptor according to claim 10, wherein the second thermocouple reaches a depth position closer to the first surface than the first thermocouple.

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