Wafer support table with ceramic substrate including core and surface layer

A ceramic heater includes a ceramic substrate including, on an upper surface, a wafer mount surface that receives a wafer, and a heater electrode embedded in an inside of the ceramic substrate. The ceramic substrate includes a core portion and a surface layer portion disposed on a surface of the core portion. The surface layer portion has volume resistivity higher than volume resistivity of the core portion. The core portion has thermal conductivity higher than thermal conductivity of the surface layer portion. The surface layer portion is disposed over an area of at least one of a side surface of the core portion and an upper surface of the core portion, the area being not covered with the wafer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer support table.

2. Description of the Related Art

A wafer support table that supports a wafer that is to be subjected to a plasma treatment is known as a component for a semiconductor manufacturing device. Such a known wafer support table includes a ceramic substrate that includes, on an upper surface, a wafer mount portion that receives a wafer, and a heater electrode that is embedded in an inside of the ceramic substrate. A preferable example known as the material of the ceramic substrate is a sintered aluminium nitride having high thermal conductivity and high corrosion resistance. A sintered aluminium nitride, however, significantly degrades its volume resistivity in a high-temperature range. Thus, the current that leaks from the heater electrode increases, and may hinder wafer processing. In view of this point, PTL 1 uses a sintered aluminium nitride with an average grain diameter of smaller than or equal to 4 μm to which 0.3 to 10 percent by mass of an yttrium oxide is added so that the material keeps high volume resistivity also in the high-temperature range.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

However, a ceramic substrate according to PTL 1 has no highly resistive layer on its side surface, so that an electrode is coupled with plasma. Although the material can keep high volume resistivity in the high-temperature range, the material has low thermal conductivity, and may fail to uniformly heat the wafer.

The present invention is made to address the above problem, and mainly aims to prevent a heater electrode and plasma from being coupled together and to improve the uniformity in heating a wafer.

The present invention provides a wafer support table that includes

a ceramic substrate including, on an upper surface, a wafer mount portion that receives a wafer; and

a heater electrode embedded in an inside of the ceramic substrate.

The ceramic substrate includes a core portion and a surface layer portion disposed on a surface of the core portion.

The surface layer portion has volume resistivity higher than volume resistivity of the core portion.

The core portion has thermal conductivity higher than thermal conductivity of the surface layer portion.

The surface layer portion is disposed over an area of at least one of a side surface of the core portion and the upper surface of the core portion, the area being not covered with the wafer.

In this wafer support table, the ceramic substrate includes a core portion and a surface layer portion disposed on the surface of the core portion. The surface layer portion is disposed over an area of at least one of the side surface of the core portion and the upper surface of the core portion, the area being not covered with the wafer. Here, the surface layer portion has volume resistivity higher than volume resistivity of the core portion. This structure can prevent, when a plasma treatment is performed on the wafer, coupling of the heater electrode and plasma, which may hinder the plasma treatment on the wafer. The core portion has thermal conductivity higher than thermal conductivity of the surface layer portion. Thus, the ceramic substrate has relatively high thermal conductivity as a whole, and improves the performance of uniformly heating the wafer.

In the present specification, “upward” and “downward” are used to refer to a relative positional relationship instead of an absolute positional relationship between each other. Thus, depending on the orientation of the ceramic heater, “upward” and “downward” may be changed to “leftward” and “rightward”, “frontward” and “rearward”, or “downward” and “upward”.

In the wafer support table according to the present invention, the surface layer portion may be further disposed on an undersurface of the core portion. This structure can further prevent plasma coupling from occurring through the undersurface of the ceramic substrate.

In the wafer support table according to the present invention, the surface layer portion may be further disposed on an area of the upper surface of the core portion that is covered with the wafer. Specifically, the surface layer portion may be disposed throughout the upper surface of the core portion. This structure can prevent current from leaking from the heater electrode to the wafer.

In the wafer support table according to the present invention, the surface layer portion may be disposed to surround all the surfaces of the core portion. This structure can further prevent the heater electrode and plasma from being coupled together and current from leaking from the heater electrode.

In the wafer support table according to the present invention, at least one of an electrostatic electrode and a RF electrode may be embedded in an inside of the ceramic substrate to serve as an electrode other than the heater electrode. When the electrostatic electrode or the RF electrode is embedded in an inside of the ceramic substrate, current may leak from the electrode. However, in the wafer support table according to the present invention, such leak current can be reduced.

In the above wafer support table, the electrode other than the heater electrode may be embedded between the upper surface of the ceramic substrate and the heater electrode. This structure has a small gap between the electrode other than the heater electrode and the upper surface of the ceramic substrate, and is more likely to cause leak current. Thus, an application of the present invention is significant.

In the wafer support table according to the present invention, the core portion and the surface layer portion preferably contain an aluminium nitride as a main component. This is because an aluminium nitride has high thermal conductivity and high corrosion resistance.

To manufacture a ceramic substrate constituting a wafer support table according to the present invention, for example, (1) a powder material for the core portion may be shaped and then fired to manufacture the core portion, and then a powder material for the surface layer portion may be fed to a predetermined surface of the core portion, shaped, and fired, or (2) a compact of a powder material for the core portion, and a compact of a powder material for the surface layer portion may be separately formed by mold casting, and they may be integrated together and then fired.

DETAILED DESCRIPTION OF THE INVENTION

Preferable embodiments of the present invention will be described below with reference to the drawings.FIG.1is a perspective view of a ceramic heater10,FIG.2is a cross-sectional view of the ceramic heater10inFIG.1taken along line A-A, andFIGS.3A to3Dare manufacturing processes of a ceramic substrate20.

The ceramic heater10is an example of a wafer mount table of the present invention, and includes a ceramic substrate20and a hollow cylindrical shaft30.

The ceramic substrate20includes, on an upper surface21, a wafer mount surface22that receives a wafer W. The ceramic substrate20has a diameter of, for example, approximately 300 mm, and a thickness of, for example, approximately 20 mm. The wafer mount surface22is a recessed portion disposed at the center of the upper surface21of the ceramic substrate20. The upper surface21has an annular surface23, which surrounds the wafer mount surface22at a position higher than the wafer mount surface22. A bank24, which is a slope, is disposed between the wafer mount surface22and the annular surface23. Specifically, the upper surface21includes the wafer mount surface22, the annular surface23, and the bank24.

As illustrated inFIG.2, the ceramic substrate20includes a core portion20a, which has substantially the same shape as the ceramic substrate20, and a surface layer portion20b, disposed on the surface of the ceramic substrate20. The core portion20aforms a center of the ceramic substrate20. The surface layer portion20bsurrounds all the surfaces of the core portion20a, that is, an upper surface20a1, a side surface20a2, and an undersurface20a3of the core portion20a. The surface layer portion20bhas volume resistivity higher than the volume resistivity of the core portion20a. The core portion20ahas thermal conductivity higher than the thermal conductivity of the surface layer portion20b. The core portion20aand the surface layer portion20bcontain an aluminium nitride as a main component. Examples of the material for the core portion20ahaving low volume resistivity and high thermal conductivity include a sintered aluminium nitride having a large grain diameter. Examples of the material for the surface layer portion20bhaving high volume resistivity and low thermal conductivity include a sintered aluminium nitride having a small grain diameter. Table 1 shows examples of the materials of the core portion20aand the surface layer portion20b. Although not limited to particular ones, the surface layer portion20bmay have a thickness of, for example, 1 to 5 mm.

A heater electrode26and an RF electrode28are embedded in the core portion20aof the ceramic substrate20. The heater electrode26is a coil whose main component is Mo and that is unicursaly wired throughout the entire surface of the ceramic substrate20. Both ends of the heater electrode26are connected to a power feeding member (not illustrated). The power feeding member extends through the hollow space inside the hollow cylindrical shaft30to be connected to an external power source (not illustrated). The RF electrode28is a disc-shaped thin layer electrode having a diameter slightly smaller than the ceramic substrate20, and is a mesh sheet formed by weaving thin metal wires containing Mo as a main component. The RF electrode28is embedded in an inside of the ceramic substrate20between the heater electrode26and the wafer mount surface22. The heater electrode26and the RF electrode28are made of Mo, because Mo has a coefficient of thermal expansion closer to that of an aluminium nitride forming the ceramic substrate20, and is less likely to be cracked during manufacturing of the ceramic substrate20or with repetition of a heat cycle. Power feeding members (not illustrated) are connected to a portion near the center of the RF electrode28. The RF electrode28is used to cause plasma.

The hollow cylindrical shaft30is a ceramic cylindrical member containing an aluminium nitride as a main component. The hollow cylindrical shaft30has a first flange31around an upper opening, and a second flange32around a lower opening. The end surface of the first flange31is bonded to an undersurface25of the ceramic substrate20by solid-state welding or diffusion welding.

Subsequently, an application example of the ceramic heater10will be described. The ceramic heater10is disposed in a chamber, not illustrated, and the wafer W is mounted on the wafer mount surface22. Then, an alternating-current high-frequency voltage is applied to the RF electrode28to cause plasma between parallel-plate electrodes, which include a parallel-opposed electrode, which is disposed at an upper portion in the chamber and not illustrated, and a RF electrode28embedded in the inside of the ceramic substrate20. The plasma is used to perform chemical vapor deposition (CVD) or etching on the wafer W. In addition, the temperature of the wafer W is measured based on a detection signal of a thermocouple, not illustrated, and the voltage applied to the heater electrode26is controlled to bring the temperature to a predetermined temperature.

Subsequently, an example for manufacturing a ceramic substrate20constituting the ceramic heater10will be described.FIGS.3A to3Dillustrate manufacturing processes of the ceramic substrate20. Firstly, a first compact50afor manufacturing the core portion20aof the ceramic substrate20is formed by mold casting (refer toFIG.3A). The first compact50ahas the heater electrode26and the RF electrode28incorporated therein. The first compact50ais formed by aluminium nitride powder not containing additives such as titania or magnesia. Mold casting is a known method also referred to as gel casting, and disclosed in detail in, for example, Japanese Patent No. 5458050. Japanese Patent No. 5458050 describes a method for manufacturing a ceramic compact in which two layers of electrodes are embedded. In accordance with this method, the first compact50ais formed and degreased. Subsequently, a lower half50b1and an upper half50b2of a second compact50bfor forming the surface layer portion20bof the ceramic substrate20are formed by mold casting, and degreased (refer toFIG.3B). Aluminium nitride powder to which a small amount (for example, smaller than or equal to 1% by mass) of an additive such as titania or magnesia is added is used to form these halves50b1and50b2. Subsequently, these halves50b1and50b2and the first compact50aare integrated into an integrated compact50(refer toFIG.3C). This integrated compact50is fired by hot pressing, so that the first compact50abecomes a core portion20a, and the second compact50bbecomes a surface layer portion20b, to obtain the ceramic substrate20(refer toFIG.3D). The core portion20ais a sintered compact of aluminium nitride powder not containing an additive, whereas the surface layer portion20bis a sintered compact of aluminium nitride powder containing titania or magnesia, which hinders grain growth. Thus, a sintered aluminium nitride of the surface layer portion20bhas a smaller grain diameter than a sintered aluminium nitride of the core portion20a. A sintered aluminium nitride with a smaller grain diameter has higher volume resistivity and lower thermal conductivity. Thus, the surface layer portion20bhas higher volume resistivity than the core portion20a, and the core portion20ahas higher thermal conductivity than the surface layer portion20b.

In the ceramic heater10described above in detail, the surface layer portion20bhaving higher volume resistivity is disposed over the upper surface20a1, the side surface20a2, and the undersurface20a3of the core portion20ahaving lower volume resistivity. Specifically, the surface layer portion20bis disposed to surround all the surfaces of the core portion20a. This structure can thus prevent, when a wafer W is subjected to a plasma treatment, coupling between the heater electrode26and plasma, which may hinder a plasma treatment on the wafer W. On the other hand, the core portion20aat the center of the ceramic substrate20has higher thermal conductivity than the surface layer portion20bof the ceramic substrate20. Thus, the ceramic substrate20has relatively high thermal conductivity as a whole, and thus improves the performance of uniformly heating the wafer W.

Particularly, the surface layer portion20bis disposed over the entirety of the upper surface21of the ceramic substrate20. Specifically, the surface layer portion20bis disposed on the upper surface21of the ceramic substrate20over an area covered with the wafer W (the wafer mount surface22), besides the area not covered with the wafer W (the annular surface23and the bank24). This structure can thus reduce leak current from the heater electrode26or the RF electrode28to the wafer W. The surface layer portion20bis also disposed over the entirety of the undersurface25of the ceramic substrate20. This structure can thus prevent plasma coupling from occurring through the undersurface of the ceramic substrate20.

The RF electrode28is embedded between the heater electrode26and the upper surface21of the ceramic substrate20. Thus, the distance between the RF electrode28and the wafer mount surface22of the ceramic substrate20(that is, the thickness of a dielectric layer) is reduced. A high voltage is applied to the RF electrode28. Thus, leak current is more likely to occur as the dielectric layer has a smaller thickness. Thus, an application of the present invention is significant.

The core portion20aand the surface layer portion20bof the ceramic substrate20contain an aluminium nitride as a main component, and thus have high thermal conductivity and high corrosion resistance.

The present invention is not limited to any of the above embodiments, and may naturally be embodied in various manners within the technical scope of the present invention.

For example, in the above embodiments, the surface layer portion20bhaving high volume resistivity and low thermal conductivity is disposed over the upper surface20a1, the side surface20a2, and the undersurface20a3of the core portion20aof the ceramic substrate20. However, as illustrated inFIG.4, the surface layer portion20bmay not be disposed over the undersurface20a3of the core portion20a. This is because the heater electrode26and plasma are relatively less likely to be coupled together through the undersurface20a3. Alternatively, as illustrated inFIG.5, the surface layer portion20bmay be disposed over the area of the upper surface20a1not covered with the wafer W (the annular surface23and the bank24), but not be disposed over the area of the upper surface20a1covered with the wafer W (the wafer mount surface22). This is because the wafer mount surface22covered with the wafer W is not exposed to plasma while the wafer W is subjected to a plasma treatment. The wafer mount surface22illustrated inFIG.5is located slightly higher than the inner circumferential edge of the bank24. InFIG.4andFIG.5, components the same as those of the embodiment are denoted with the same reference signs. InFIG.5, the surface layer portion20bmay not be disposed over the undersurface20a3of the core portion20a, as inFIG.4.

In the above embodiment, the recessed portion at the center of the upper surface21of the ceramic substrate20is defined as the wafer mount surface22. However, as illustrated inFIG.6, the wafer mount surface22and the annular surface23may be on the same plane without providing a recessed portion on the upper surface21. Alternatively, as illustrated inFIG.7, the wafer mount surface22may be disposed over the entirety of the upper surface21without providing a recessed portion on the upper surface21. InFIG.6andFIG.7, components the same as those of the above embodiment are denoted with the same reference signs.

In the above embodiment, the RF electrode28is embedded in the core portion20aof the ceramic substrate20, but the RF electrode28may be omitted, or an electrostatic electrode may be embedded in the core portion20ainstead of or in addition to the RF electrode28. In a structure in which an electrostatic electrode is embedded, when a voltage is applied to the electrostatic electrode after a wafer W is mounted on the wafer mount surface22, the wafer W can be electrostatically attracted to the wafer mount surface22. The electrostatic electrode may be embedded between the wafer mount surface22and the heater electrode26.

In the above embodiment, the hollow cylindrical shaft30with a straight shape is illustrated by way of example, but the shape of the hollow cylindrical shaft30is not particularly limited to the straight shape. For example, the hollow cylindrical shaft may have a straight portion from the lower end to a predetermined height, and an enlarged tube portion from the predetermined height to the upper end, having a diameter larger than the diameter of the straight portion. The enlarged tube portion may have a diameter that partially or entirely increases toward the upper end.

In the above embodiment, the heater electrode26is unicursaly wired throughout the ceramic substrate20, but may be disposed on each of zones into which the ceramic substrate20is divided.

In the above embodiment, the core portion20aand the surface layer portion20bof the ceramic substrate20are made of ceramic containing an aluminium nitride as a main component. However, the main component may be, for example, alumina, a silicon nitride, a silicon carbide, or cordierite, instead of an aluminium nitride.

In the above embodiment, a coil is used as the heater electrode26, but a ribbon (flat shape) may be used instead of a coil. When a ribbon is used as the heater electrode26, the heater electrode26can be manufactured by performing printing with metal paste (such as Mo paste).

In the above embodiment, the heater electrode26and the RF electrode28are manufactured with a material containing Mo as a main component. However, the material is not particularly limited to this, and may be another material containing another refractory metal (such as W) as a main component.

The present application is a Continuation Application of PCT/JP2019/011170, filed Mar. 18, 2019, and claims the benefit of U.S. Patent Application No. 62/647,970 filed on Mar. 26, 2018.