Patent Description:
Silicon crystal silicon ingots may be prepared by the Czochralski method in which a single crystal silicon seed is contacted with a silicon melt held within a crucible. The single crystal silicon seed is withdrawn from the melt to pull a single crystal silicon ingot from the melt. The ingot may be prepared in a batch system in which a charge of polycrystalline silicon is initially melted within the crucible and the silicon ingot is withdrawn from the melt until the melted silicon within the crucible is depleted. Alternatively, the ingot may be withdrawn in a continuous Czochralski method in which polysilicon is intermittently or continuously added to the melt to replenish the silicon melt during ingot growth.

In a continuous Czochralski method, the crucible may be divided into separate melt zones. For example, the crucible assembly may include an outer melt zone in which polycrystalline silicon is added and melted to replenish the silicon melt as the silicon ingot grows. The silicon melt flows from the outer melt zone to an intermediate zone within the outer melt zone in which the melt thermally stabilizes. The silicon melt then flows from the intermediate zone to a growth zone from which the silicon ingot is pulled.

Crystal pulling systems may include a heat shield disposed above the crucible and the silicon melt. The heat shield includes a passage through which the silicon ingot passes as it is drawn vertically from the silicon melt. The heat shield protects and shields the drawn ingot from radiant heat from the melt.

During the melting phase, a temperature gradient may be created within the crystal pulling system. The temperature gradient creates thermal stress in the crucible resulting in damage, and in some cases, destruction of the crucible.

A need exists for crystal pulling systems that maintain a more uniform temperature gradient during meltdown to reduce crucible damage during meltdown.

This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below.

<CIT> describes a method for melting solid silicon in a crucible, comprising impeding the emission of thermal radiation through an upper opening of the crucible during the melting of the solid silicon and making the emission of thermal radiation through the upper opening of the crucible easier after the solid silicon has become a melt. <CIT> describes a method for melting silicon wherein a plate made of reflective material is covering the opening of the crucible in part. <CIT> describes a radiation shielding for a silicon crystal pulling system. <CIT> describes a crystal growth apparatus including a crucible, a heating device, a thermal insulation cover, and a driving device. <CIT> describes a single crystal pulling apparatus comprising a first heat shield disposed so as to be opposite to the surface of the melt in the crucible, and a heating means which is disposed below the first heat shield and heats the polycrystalline raw material. <CIT> describes a single crystal pulling apparatus wherein a radiation heat shielding member is suspended in a radiation screen. <CIT> describes a method for single crystal growth including separating the molten crystalline material, controlling the flow of the molten crystalline material and defining an annular space with respect to sidewalls of a heat shield in the chamber.

The subject-matter of the present invention is set out in the appended set of claims.

Various refinements exist of the features noted in the independent claims. Further features may also be incorporated in the claimed subject-matter as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present invention may be incorporated into any of the appended independent claims, alone or in any combination.

Provisions of the present disclosure relate to a crystal pulling system for producing monocrystalline (i.e., single crystal) silicon ingots (e.g., semiconductor or solar-grade material) from a silicon melt by the continuous Czochralski (CZ) method. The systems and methods disclosed herein may also be used to grow monocrystalline ingots by a batch or recharge CZ method. With reference to <FIG>, the crystal pulling system is shown schematically and is indicated generally at <NUM>. The crystal pulling system <NUM> includes a pull axis Y<NUM> and a housing <NUM> defining a growth chamber <NUM>. A crucible assembly <NUM> is disposed within the growth chamber <NUM>. The crucible assembly <NUM> contains the silicon melt <NUM> (e.g., semiconductor or solar-grade material) from which a monocrystalline ingot <NUM> is pulled by a pulling mechanism <NUM> as discussed further below.

The crystal pulling system <NUM> includes a heat shield <NUM> (sometimes referred to as a "reflector") that defines a central passage <NUM> through which the ingot <NUM> passes during ingot growth. In accordance with the method of the present invention, prior to the ingot <NUM> being drawn from the melt <NUM>, during an initial melting phase, a cover member <NUM> (<FIG>) is lowered to at least partially cover the solid charge of polycrystalline silicon to reduce heat that radiates through the central passage <NUM> during meltdown. The cover member <NUM> is moveable within the heat shield <NUM> along the pull axis Y<NUM>.

<FIG> shows a portion of the crystal pulling system <NUM> with the cover member <NUM> arranged within the central passage <NUM> during an initial phase in which the charge is melted (i.e., meltdown phase), prior to the ingot <NUM> being drawn. The crucible assembly <NUM> includes a bottom <NUM> and an outer sidewall <NUM> that extends upwards from the bottom <NUM>. The crucible assembly <NUM> includes a central weir <NUM> and an inner weir <NUM> that extends upward from the bottom <NUM>. The central weir <NUM> is disposed between the outer sidewall <NUM> and the inner weir <NUM>. The crucible assembly <NUM> includes a crucible melt zone <NUM> disposed between the outer sidewall <NUM> and the central weir <NUM>. The crucible assembly <NUM> also contains an intermediate zone <NUM> disposed between the central weir <NUM> and the inner weir <NUM>. The crucible assembly <NUM> also contains a growth zone <NUM> disposed within the inner weir <NUM>. The crucible assembly <NUM> may be made of, for example, quartz or any other suitable material that enables the crystal pulling system <NUM> to function as described herein. Further, the crucible assembly <NUM> may have any suitable size that enables the crystal pulling system <NUM> to function as described herein. The crucible assembly <NUM> may also include three "nested" crucibles which have separate bottoms that together make a bottom and in which the sidewalls of the crucibles are the weirs <NUM>, <NUM> described above.

During ingot growth, polycrystalline silicon is added to the crucible melt zone <NUM> where the silicon melts and replenishes the silicon melt. Silicon melt flows through a central weir opening <NUM> and into the intermediate zone <NUM>. The silicon melt then flows through an inner weir opening <NUM> to the growth zone <NUM> disposed within the inner weir <NUM>. The various silicon melt zones (e.g., melt zone <NUM>, intermediate zone <NUM> and growth zone <NUM>) allow the ingot to be grown in accordance with continuous Czochralski methods in which polycrystalline silicon is continuously or semi-continuously added to the melt while an ingot <NUM> is continuously pulled from the growth zone <NUM>. The silicon melt <NUM> within the growth zone <NUM> is contacted with a single seed crystal <NUM> (<FIG>). As the seed crystal <NUM> is slowly raised from the melt <NUM>, atoms from the melt <NUM> align themselves with and attach to the seed to form the ingot <NUM>.

The crucible assembly <NUM> is supported by a susceptor <NUM> (<FIG>). The susceptor <NUM> is supported by a rotatable shaft <NUM>. A side heater <NUM> surrounds the susceptor <NUM> and crucible assembly <NUM> for supplying thermal energy to the system <NUM>. One or more bottom heaters <NUM> are disposed below the crucible assembly <NUM> and susceptor <NUM>. The heaters <NUM>, <NUM> operate to melt an initial charge of solid polycrystalline silicon feedstock, and maintain the melt <NUM> in a liquefied state after the initial charge is melted. The heaters <NUM>, <NUM> also act to melt solid polycrystalline silicon added through feed tube <NUM> (<FIG>) during growth of the ingot. The heaters <NUM>, <NUM> may be any suitable heaters that enable system <NUM> to function as described herein (e.g., resistance heaters).

The crystal pulling system <NUM> includes a gas inlet (not shown) for introducing an inert gas into the growth chamber <NUM>, and one or more exhaust outlets (not shown) for discharging the inert gas and other gaseous and airborne particles from the growth chamber <NUM>. The gas inlet supplies suitable inert gases such as argon.

The system <NUM> includes a cylindrical jacket <NUM> disposed with the heat shield <NUM>. The jacket <NUM> is fluid-cooled and includes a jacket chamber <NUM> that is aligned with the central passage <NUM>. The ingot <NUM> is drawn along the pull axis Y<NUM>, through the central passage <NUM> and into the jacket chamber <NUM>. The jacket <NUM> cools the drawn ingot <NUM>.

The heat shield <NUM> is generally frustoconical in shape. The heat shield <NUM> includes an outer surface <NUM> which faces the crucible assembly <NUM> and the melt <NUM>. The heat shield <NUM> may be coated to prevent contamination of the melt. In some embodiments, the heat shield <NUM> is made of two graphite shells that include molybdenum sheets therein. The surface <NUM> may be coated (e.g., SiC) to reduce contamination of the melt.

The heat shield <NUM> includes a bottom <NUM> (<FIG>). The central passage <NUM> of the heat shield <NUM> has a diameter D<NUM> at the bottom <NUM> of the heat shield <NUM>. The heat shield <NUM> is disposed above the crucible assembly <NUM>, such that the central passage <NUM> is arranged directly above the growth zone <NUM> so that the ingot drawn from the melt <NUM> may be pulled through the central passage <NUM>. The passage diameter D<NUM> is sized to accommodate the diameter of the ingot <NUM> (e.g., <NUM> or <NUM> or other diameter ingots).

The outer surface <NUM> may be coated with a reflective coating which reflects radiant heat back towards the melt <NUM> and the crucible assembly <NUM>. As such, the heat shield <NUM> assists in retaining heat within the crucible assembly <NUM> and the melt <NUM>. In addition, the heat shield <NUM> aids in maintaining a generally uniform temperature gradient along the pull axis Y<NUM>.

During the initial melting phase, an initial amount of solid polycrystalline silicon is loaded to a crucible melt zone <NUM>, intermediate zone <NUM> and growth zone <NUM>. In other embodiments, solid polycrystalline silicon is added to only one or two of the zones selected between the crucible melt zone <NUM>, intermediate zone <NUM> and growth zone <NUM>. During meltdown, the cover member <NUM> is lowered to cover at least a portion of the silicon charge while the initial charge is melted (i.e., by occluding the central passage <NUM> of the heat shield <NUM>). The pulling mechanism <NUM> raises and lowers the cover member <NUM>.

In accordance with embodiments of the present invention, the cover member <NUM> is lowered to within less than <NUM> from the bottom <NUM> of the heat shield <NUM> (i.e., from below or above the bottom <NUM>), or less than <NUM>, less than <NUM>, or less than <NUM> from the bottom <NUM> of the heat shield <NUM>. In some embodiments, the cover member <NUM> is lowered such that it is aligned with the bottom <NUM> of the heat shield <NUM>. In some embodiments, the cover member <NUM> is lowered to within <NUM> to <NUM> of the surface of the charge during melt down.

After the initial amount of silicon charge has been melted, a secondary amount of polycrystalline silicon may be added to the crucible melt zone <NUM> (e.g., continuously added until the entire secondary amount is added). In accordance with some embodiments of the present invention, the cover member <NUM> covers the central passage <NUM> while this secondary amount of polycrystalline silicon is added to the melt zone <NUM> and melted down. After the secondary charge has melted, the cover member <NUM> is raised by the pulling mechanism <NUM>. In other embodiments, the cover member <NUM> is not used while the secondary amount of polycrystalline silicon is added.

An embodiment of the cover member <NUM> is shown in <FIG>. The cover member <NUM> includes a first plate <NUM> and a second plate <NUM> (which may also be referred to herein as "lower plate <NUM>" and "upper plate <NUM>", respectively). Each plate <NUM>, <NUM> has a central axis that is generally parallel to the pull axis Y<NUM> (<FIG>). The first plate <NUM> and the second plate <NUM> are generally parallel. The second plate <NUM> is disposed above the first plate <NUM>.

The first plate <NUM> includes a first annular wall <NUM> (<FIG>) and the second plate <NUM> includes a second annular wall <NUM> (<FIG>). Referring now to <FIG>, the first wall <NUM> includes a first shoulder <NUM> and a first lip <NUM>. The second wall <NUM> includes a second shoulder <NUM> and second lip <NUM>. When assembled, the second shoulder <NUM> rests on the first lip <NUM> and the second lip <NUM> rests on the first shoulder <NUM>. A cover member chamber <NUM> (<FIG> and <FIG>) is disposed between the first and second plates <NUM>, <NUM>.

An insulation layer <NUM> (<FIG>) is disposed within the chamber <NUM> formed between the first and second plates <NUM>, <NUM>. The insulation layer <NUM> has a thickness of T<NUM> (e.g., <NUM> to about <NUM>). The insulation layer <NUM> may be compressed between the first plate <NUM> and the second plate <NUM>. The insulation layer <NUM> may include several stacked layers of insulation or may be a single layer. The insulation layer <NUM> may include an opening <NUM> formed therein.

The insulation layer <NUM> is made of felt. The felt may be composed of natural or synthetic fibers. The felt may be purified felt (e.g., with a max ash of <NUM> ppm). The insulation layer <NUM> may generally be composed of any material that includes suitable insulating properties.

The first plate <NUM> includes a hub <NUM> (<FIG>) that protrudes upward for connecting a shaft <NUM>. The second plate includes a second plate opening <NUM> (<FIG>). The hub <NUM> extends through the opening <NUM> of the second plate <NUM> and through the insulation opening <NUM> (<FIG>). The hub <NUM> includes a ledge <NUM> (<FIG>) with the second plate <NUM> being seated on the ledge <NUM>. The hub <NUM> includes a hub opening <NUM> (<FIG>) through which the shaft <NUM> extends. The hub opening <NUM> has a profile that matches the profile of the shaft <NUM> (e.g., square or rectangular as in the illustrated embodiment or other shape such as circular). The hub <NUM> includes a hub chamber <NUM> having a top wall <NUM>.

The cover member <NUM> is generally in the shape of a circular segment having a circular portion <NUM> (<FIG>) including a center X and a circumference <NUM> and having a linear edge <NUM>. Specifically, the first and second plates <NUM>, <NUM> have the shape of a circular portion with a segment along a chord that has been removed. The first and second plates <NUM>, <NUM> include a major length L<NUM> and a minor length L<NUM>. The major length L<NUM> is a diameter of the circular portion <NUM> and the minor length L<NUM> extends from the linear edge <NUM>, through the center X to the circumference <NUM> of the circular portion <NUM>. The first and second plates <NUM>, <NUM> are shaped as a circular segment to allow the charge/melt to be viewed. In other embodiments, the cover member <NUM> is fully circular.

In some embodiments, the diameter of the cover member <NUM> is at least <NUM> times the diameter of the central passage <NUM> at the bottom <NUM> of the heat shield <NUM> or, as in other embodiments, at least <NUM> times, at least <NUM> times, at least <NUM> times, or at least <NUM> times the diameter of the central passage <NUM> at the bottom <NUM> of the heat shield <NUM>.

In some embodiments, the first plate <NUM> and second plate <NUM> are made of graphite. The graphite may be coated with silicon carbide (SiC). The first and second plate <NUM>, <NUM> may be composed of other suitable materials. The first and second plates <NUM>, <NUM> have any suitable thickness T<NUM>, T<NUM> (<FIG> and <FIG>) that prevents thermal stresses which result in cracking or damage of the first and second plates <NUM>, <NUM> (e.g., thickness between <NUM> and <NUM>).

With reference to <FIG>, the cover member <NUM> includes a shaft <NUM> that supports the cover member <NUM>. The shaft <NUM> may be connected to the first and/or second plates <NUM>, <NUM> in any suitable coupling arrangement. In the illustrated embodiment, the shaft <NUM> includes an elongated rectangular portion <NUM> and a collar <NUM>. The collar <NUM> has a diameter D<NUM> less than a diameter of the hub chamber <NUM> (<FIG>) and greater than the width of the hub opening <NUM>. The first plate <NUM> rests on the collar <NUM>. The insulation layer <NUM> and second plate <NUM> (<FIG>) are supported by the first plate <NUM>. Alternatively, the shaft <NUM> may be formed integrally with either or both of the first plate <NUM> and/or second plate <NUM>.

With reference to <FIG>, the pulling mechanism <NUM> includes a chuck <NUM> that is raised and lowered along the pull axis Y<NUM>. The chuck <NUM> may be connected to a pull wire or cable <NUM> that is raised and lowered by a drive motor (i.e., the pull wire or cable and motor are part of the pulling mechanism <NUM>). The cover member <NUM> is removably connectable to the chuck <NUM>. For example, the shaft <NUM> and the chuck <NUM> may be connected using a pin lock. The shaft <NUM> includes a recess <NUM> (<FIG>). The shaft <NUM> is inserted into a bore <NUM> within the chuck <NUM>, such that the recess <NUM> is contained within the bore <NUM>. The chuck <NUM> includes an opening <NUM> that extends generally perpendicularly to the bore <NUM>, passing through the chuck <NUM> and opening into the bore <NUM>. A pin <NUM> is inserted through the opening <NUM> and into the bore <NUM> such that the pin <NUM> becomes engaged with the recess <NUM> of the shaft <NUM> that is disposed within the bore <NUM>. In this manner the shaft <NUM> and the chuck <NUM> are coupled. The shaft <NUM> and chuck <NUM> may include any alternative and/or additional features to couple the cover member <NUM> to the chuck <NUM>.

After meltdown, the cover member <NUM> is disconnected from the chuck <NUM> and the seed crystal <NUM> (<FIG>) is connected to the chuck <NUM>. The seed crystal <NUM> may include a similar recess, not shown, such that the seed <NUM> may also be coupled and/or uncoupled to the chuck <NUM>. During the ingot growth process, the seed <NUM> is lowered by the pulling mechanism <NUM> into contact with the melt <NUM> and then slowly raised from the melt <NUM>. The cover member <NUM> and/or the seed crystal are selectively coupled and uncoupled to the chuck <NUM> so that the pull mechanism <NUM> may be used to raise and lower either the cover member <NUM> and/or the seed crystal <NUM>.

Compared to conventional crystal pull systems, the crystal pull systems of embodiments of the present invention have several advantages. Use of a cover member that at least partially covers the charge during meltdown acts to reduce radiant heat loss in the vertical direction which reduces thermal stress in the crucible assembly. In the present invention the cover member includes insulation disposed therein, and thus heat loss through the cover member may be reduced and moreover heater power may be reduced and the lifetime of the crucible can be further increased.

The methods of the present invention are further illustrated by the following Examples. These Examples should not be viewed in a limiting sense.

Internal temperatures of the crucible assembly were modeled during the initial meltdown phase when a cover member similar to that shown in <FIG> was positioned at the bottom of the heat shield. Another cover member similar to that of <FIG> was used but the cover member did not include insulation (i.e., felt). A nested crucible assembly made of three crucibles was used. The cover member with insulation resulted in a lower temperature profile compared to the temperature profile of the crucible assembly when a cover member without insulation was used when the temperature was determined at the outer crucible/sidewall (<FIG>), the central weir/middle crucible (<FIG>), and the inner weir/innermost crucible (<FIG>). The maximum decrease in temperature was <NUM> which occurred in the inner weir (<FIG>). This decrease in temperature reduces damage to the crucible assembly.

The power supplied to the crucible assembly during the initial melt phase (i.e., the power supplied to the heaters of the crystal pulling system) was determined when a cover member similar to that shown in <FIG> was positioned at the bottom of the heat shield and when another cover member similar to that of <FIG> was used but the cover member did not include insulation. The power supplied using the cover member with insulation was less than the power supplied using the cover member without insulation (<FIG>). The maximum power supplied for the cover member without insulation was <NUM> kW greater than the maximum power supplied using the cover member with insulation.

As used herein, the term "about," "substantially," "essentially," and "approximately" when used in conjunction with ranges of dimensions, concentrations, temperatures or other physical or chemical properties or characteristics is meant to cover variations that may exist in the upper and/or lower limits of the ranges of the properties or characteristics, including, for example, variations resulting from rounding, measurement methodology or other statistical variation.

Claim 1:
A crystal pulling system (<NUM>) for growing a monocrystalline ingot (<NUM>) from a silicon melt (<NUM>), the system having a pull axis (Y<NUM>) and comprising:
a housing (<NUM>) defining a growth chamber (<NUM>);
a crucible assembly (<NUM>) disposed within the growth chamber (<NUM>) for containing the silicon melt (<NUM>);
a heat shield (<NUM>) that defines a central passage (<NUM>) through which an ingot (<NUM>) passes during ingot growth; and
a cover member (<NUM>) that is moveable within the heat shield (<NUM>) along the pull axis (Y<NUM>), the cover member comprising:
a first plate (<NUM>) having a first plate axis that is parallel to the pull axis (Y<NUM>), the first plate comprising a hub (<NUM>), the hub having a hub opening (<NUM>);
a second plate (<NUM>) having a second plate axis that is parallel to the pull axis (Y<NUM>), the second plate being disposed above the first plate, the first plate and second plate forming a cover member chamber (<NUM>);
an insulation layer (<NUM>) being disposed between the first plate and the second plate and contacting the first plate and the second plate, the insulation layer being made of felt, the insulation layer being disposed within the cover member chamber; and
a shaft (<NUM>) having an elongated portion and a collar (<NUM>) that extends radially outward from the elongated portion (<NUM>), the shaft extending through the hub opening (<NUM>), the first plate resting on the collar.