Patent Description:
Continuous Czochralski (CCz) is well suited to form <NUM> or <NUM> diameter single crystal silicon ingots such as ingots that are relative heavily doped with arsenic or phosphorous. Continuous Czochralski methods involve forming a single crystal silicon ingot from a melt of silicon while continuously or intermittently adding solid-state silicon to the melt to replenish the melt while the ingot is grown. The methods may involve forming multiple ingots from the same melt while the hot zone remains at temperature (i.e., with a melt continuously being present in the crucible assembly while the plurality of ingots is grown).

Customers increasingly specify that wafers sliced from ingots grown by continuous Czochralski methods have a low void count (e.g., less than <NUM> defects per wafer) for both <NUM> and <NUM> ingots. Continuous Czochralski methods may involve a crucible assembly that includes at least two and often three melt zones that are separated by physical barriers - an outer melt zone into which solid polycrystalline silicon is fed, a middle melt zone in which the melt stabilizes, and an inner melt zone from which the silicon ingot is pulled. Addition of solid polycrystalline silicon to the melt causes inert gas bubbles (e.g., argon bubbles) to form in the melt which impacts the void count.

In some conventional methods, buffer members such as quartz cullets have been added to the melt to reduce formation of inert gas bubbles. The quartz cullets cushion the polysilicon that falls into the melt. The cullets also promote dissipation of the inert gas bubbles. However, adding quartz cullets adds complexity to the crystal growth process. Cullets also dissolve relatively quickly. Gaps may form between groups of cullets which limits their effectiveness.

A need exists for alternative methods for forming silicon ingots which reduce the defect count in silicon wafers sliced from the ingot and/or in which inert gas bubble formation in the melt is reduced or which promote dissipation of the inert gas bubbles.

<CIT> and <CIT> describe Continuous Czochralski processes of the prior art.

This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention.

The present invention is directed to a method for growing a single crystal silicon ingot in a continuous Czochralski process. A charge of polycrystalline silicon is added to a crucible assembly. The crucible assembly includes a weir and a sidewall that define an outer melt zone between the weir and the sidewall. One or more plates is added to the outer melt zone, the one or more plates each including one or more openings that extend through the plate to allow silicon to enter the melt. A melt of silicon is formed in the crucible assembly. A surface of the melt is contacted with a seed crystal. A single crystal silicon ingot is withdrawn from the melt. Solid polycrystalline silicon feedstock is added to the outer melt zone while withdrawing the single crystal silicon ingot to replenish the melt. The one or more plates at least partially cover the melt in the outer melt zone, wherein the polycrystalline silicon melts and falls through the one or more openings that extend through the one or more plates and enters the melt.

Various refinements exist of the features noted in relation to the above-mentioned present invention. Further features may also be incorporated in the above-mentioned present invention 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 the above-described present invention, alone or in any combination.

The present invention relates to methods for growing a single crystal silicon ingot in a continuous Czochralski (CCz) process. One or more plates (e.g., quartz plates) are added to the outer melt zone of the crucible assembly prior to formation of the ingot. During ingot growth, solid-state silicon (e.g., polycrystalline silicon) is added to the outer melt zone. The polycrystalline silicon falls onto the plates. The solid polycrystalline silicon melts and falls through openings formed in the plates and enters the silicon melt.

An example ingot puller apparatus <NUM> for producing an ingot <NUM> by a continuous Czochralski process is shown in <FIG>. As shown in <FIG>, the ingot puller apparatus <NUM> includes a crucible assembly <NUM> that contains a melt <NUM> of semiconductor or solar grade silicon material. A susceptor <NUM> supports the crucible assembly <NUM>. The crucible assembly <NUM> has a sidewall <NUM> and one or more fluid barriers <NUM>, <NUM> or "weirs" that separate the melt into different melt zones. In the illustrated embodiment, the crucible assembly <NUM> includes a first weir <NUM>. The first weir <NUM> and sidewall <NUM> define an outer melt zone <NUM> of the silicon melt (and of the crucible assembly <NUM>). The crucible assembly <NUM> includes a second weir <NUM> radially inward to the first weir <NUM> which defines an inner melt zone <NUM> of the silicon melt. The inner melt zone <NUM> is the growth region from which the single crystal silicon ingot <NUM> is grown. The first weir <NUM> and a second weir <NUM> define a middle melt zone <NUM> of the silicon melt in which the melt <NUM> may stabilize as it moves toward the inner melt zone <NUM>. The first and second weirs <NUM>, <NUM> each have at least one opening defined therein to permit molten silicon to flow radially inward towards the growth region of the inner melt zone <NUM>.

In the illustrated embodiment, the first weir <NUM>, second weir <NUM> and sidewall <NUM> each have a generally annular shape. The first weir <NUM>, second weir <NUM> and sidewall <NUM> may be part of three nested crucibles which are joined at the bottom or floor <NUM> of the crucible assembly <NUM> (i.e., the first and second weirs <NUM>, <NUM> are the sidewalls of two crucibles nested within a larger crucible). The crucible assembly configuration depicted in <FIG> is exemplary. In other embodiments, the crucible assembly <NUM> has a single layer floor (i.e., does not have nested crucibles) with the weirs extending upward from the floor <NUM>. Optionally, the floor <NUM> may be flat rather than curved and/or the weirs <NUM>, <NUM> and/or sidewall <NUM> may be straight-sided. Further, while the illustrated crucible assembly <NUM> is shown with two weirs, in other embodiments the crucible assembly may have a single weir or even no weirs.

A feeding tube <NUM> feeds solid-state silicon which may be, for example, polysilicon chips, granular polysilicon, or chunk polysilicon, or a combination thereof, into the outer melt zone <NUM>. Chuck polysilicon is generally larger in size than chip polysilicon which is larger in size than granular polysilicon. For example, chuck polysilicon may generally have an average nominal size of at least <NUM> (e.g., ranging from <NUM> to <NUM>) while chip polysilicon may have an average nominal size from <NUM> to <NUM>. The solid-state silicon is added at a rate sufficient to maintain a substantially constant melt elevation level and volume during growth of the ingot <NUM>.

Generally, the melt <NUM> from which the ingot <NUM> is drawn is formed by loading polycrystalline silicon into a crucible to form an initial silicon charge <NUM> (<FIG>). In general, an initial charge is between about <NUM> kilograms and about <NUM> kilograms of polycrystalline silicon, which may be chip, chunk, granular, or a combination thereof. The mass of the initial charges depends on the desired crystal diameter and hot zone design. Initial charge does not reflect the length of the ingot crystal, because polycrystalline silicon is continuously fed during crystal growth.

A variety of sources of polycrystalline silicon may be used including, for example, granular polycrystalline silicon produced by thermal decomposition of silane or a halosilane in a fluidized bed reactor or polycrystalline silicon produced in a Siemens reactor. While the solid-state silicon is typically polysilicon, an amount of single crystal silicon (e.g., portions discarded from a cropped ingot) may also be used.

Once polycrystalline silicon is added to the crucible assembly <NUM> to form a charge <NUM>, one or more plates <NUM> (<FIG>) are added to the charge <NUM> in the outer melt zone <NUM>. In the illustrated embodiment, a plurality of plates <NUM> (<FIG>) are added to the outer melt zone <NUM> (e.g., at least two, at least three, at least four, at least five, at least eight, at least ten or at least <NUM> plates or more) are added to the outer melt zone <NUM>. In other embodiments, a single plate <NUM> is added (e.g., a plate that circumscribes the entire circumference of the outer melt zone <NUM>).

In embodiments in which a plurality of plates <NUM> are added to the outer melt zone <NUM>, the plates may be free-floating and not connected to each other. In other embodiments, the plates <NUM> may be connected. The plates <NUM> may be sized to minimize gaps between adjacent plates <NUM>.

The plates <NUM> may be made of quartz or other material that allows the plates <NUM> to operate as described herein. The plates <NUM> are generally less dense than the melt <NUM> of silicon such that the plates float within the melt <NUM> after formation of the melt <NUM>.

Referring now to <FIG>, each plate <NUM> has one or more openings or slots <NUM> that extend through the thickness of the plate <NUM>. Once solid-state silicon is discharged through the feeding tube <NUM> into the outer melt zone <NUM> of the crucible assembly <NUM> and onto the one or more plates <NUM>, the silicon melts and falls through the openings <NUM> and enters the melt <NUM>. In the illustrated embodiment, the openings <NUM> are slots having a major axis that is generally parallel to the longitudinal axis A of the plate <NUM> (i.e., the openings <NUM> are spaced radially) Generally, the openings <NUM> may have any shape that allows the plates <NUM> to operate as described herein. Generally, the openings <NUM> may be sized to be smaller than the size of the type of polysilicon introduced into the outer melt zone (e.g., chunk, chips or granular).

The plates <NUM> have an inner edge <NUM> and an outer edge <NUM>. The edges <NUM>, <NUM> are rounded to match the contours of the outer melt zone <NUM> (i.e., the area bound by the first weir <NUM> and sidewall <NUM>). The outer edge <NUM> is longer than the inner edge <NUM>. First and second sides <NUM>, <NUM> extend between the inner and outer edges <NUM>, <NUM>.

Each plate <NUM> has a width W<NUM>. The width W<NUM> of each plate <NUM> is less than the width W<NUM> (<FIG>) of the outer melt zone <NUM> to allow the plates <NUM> to be disposed within the outer melt zone <NUM> without contacting the first weir <NUM> or the sidewall <NUM> (e.g., during meltdown and/or during ingot growth).

Once polycrystalline silicon is added to the crucible assembly <NUM> to form a charge <NUM> and the plates <NUM> have been added to the outer melt zone <NUM>, the charge <NUM> is heated to a temperature above the melting temperature of silicon (e.g., about <NUM>) to melt the charge, and thereby form a silicon melt <NUM> (<FIG>) comprising molten silicon. The silicon melt <NUM> has an initial volume of molten silicon and has an initial melt elevation level, and these parameters are determined by the size of the initial charge <NUM>. In some embodiments, the crucible assembly <NUM> comprising the silicon melt <NUM> is heated to a temperature of at least about <NUM>, at least about <NUM> or even at least about <NUM>. One the initial melt <NUM> is formed, the plates <NUM> float on the melt <NUM> in the outer melt zone <NUM>.

The ingot pulling apparatus <NUM> includes a pulling mechanism <NUM> (<FIG>) for growing and pulling the ingot <NUM> from the melt <NUM> within the inner melt zone <NUM>. The pulling mechanism <NUM> includes a pulling cable <NUM>, a seed holder or chuck <NUM> coupled to one end of the pulling cable <NUM>, and a seed crystal <NUM> coupled to the seed holder or chuck <NUM> for initiating crystal growth. One end of the pulling cable <NUM> is connected to a lifting mechanism (e.g., driven pulley or drum, or any other suitable type of lifting mechanism) and the other end is connected to the chuck <NUM> that holds the seed crystal <NUM>. In operation, the seed crystal <NUM> is lowered to contact the melt <NUM> in the inner melt zone <NUM>. The pulling mechanism <NUM> is operated to cause the seed crystal <NUM> to rise along pull axis A. This causes a single crystal ingot <NUM> to be pulled from the melt <NUM>.

Once the charge <NUM> (<FIG>) of polycrystalline silicon is liquefied to form a silicon melt <NUM> (<FIG>) comprising molten silicon with the plates <NUM> floating above the melt <NUM>, the silicon seed crystal <NUM> (<FIG>) is lowered to contact the melt <NUM> within the inner melt zone <NUM>. The silicon seed crystal <NUM> is then withdrawn from the melt <NUM> with silicon being attached thereto to form a neck <NUM> thereby forming a melt-solid interface near or at the surface of the melt <NUM>.

The pulling mechanism <NUM> may rotate the seed crystal <NUM> and the ingot <NUM> connected thereto. A crucible drive unit <NUM> may rotate the susceptor <NUM> and crucible assembly <NUM>. In some embodiments, the silicon seed crystal <NUM> and the crucible assembly <NUM> are rotated in opposite directions, i.e., counter-rotation. Counter-rotation achieves convection in the silicon melt <NUM>. Rotation of the seed crystal <NUM> is mainly used to provide a symmetric temperature profile, suppress angular variation of impurities and also to control crystal melt interface shape.

After formation of the neck <NUM>, an outwardly flaring seed-cone portion <NUM> (or "crown") adjacent the neck <NUM> is grown. In general, the pull rate is decreased from the neck portion pull rate to a rate suitable for growing the outwardly flaring seed-cone portion <NUM>. Once the seed-cone portion reaches the target diameter, the many body <NUM> or "constant-diameter portion" of the ingot <NUM> is grown. In some embodiments, the main body <NUM> of the ingot <NUM> has a diameter of about <NUM>, at least about <NUM>, about <NUM>, at least about <NUM>, about <NUM>, at least about <NUM>, about <NUM>, or even at least about <NUM>.

While the ingot <NUM> is pulled from the melt <NUM>, solid polysilicon feedstock is added to the outer melt zone <NUM> through the tube <NUM> or other channel to replenish the melt <NUM> in the ingot growth apparatus <NUM>. Solid polycrystalline silicon may be added from a polycrystalline silicon feed system <NUM> and may be continuously or intermittently added to the ingot puller apparatus <NUM> to maintain the melt level. Generally, polycrystalline silicon may be metered into the ingot puller apparatus <NUM> by any method available to those of skill in the art. The solid polysilicon added to the outer melt zone <NUM> may be silicon chips, chunk or granular.

In some embodiments, dopant is also added to the melt <NUM> during ingot growth. Dopant may be introduced from a dopant feed system <NUM>. Dopant may be added as a gas or solid and may be added to the outer melt zone <NUM>.

The apparatus <NUM> may include a heat shield <NUM> disposed about the ingot <NUM> to permit the growing ingot <NUM> to radiate its latent heat of solidification and thermal flux from the melt <NUM>. The heat shield <NUM> may be at least partially conical in shape and angles downwardly at an angle to create an annular opening in which the ingot <NUM> is disposed. A flow of an inert gas, such as argon, is typically provided along the length of the growing crystal. The ingot <NUM> is pulled through a growth chamber <NUM> that is sealed from the surrounding atmosphere.

A plurality of independently controlled annular bottom heaters <NUM> may be disposed in a radial pattern beneath the crucible assembly <NUM>. Annular bottom heaters <NUM> apply heat in a relatively controlled distribution across the entire base surface area of the crucible assembly <NUM>. The annular bottom heaters <NUM> may be planar resistive heating elements that are individually controlled as described in <CIT>. The apparatus <NUM> may include one or more side heaters <NUM> disposed radially outward to the crucible assembly <NUM> to control the temperature distribution through melt <NUM>.

The ingot growth apparatus <NUM> shown in <FIG> and described herein is exemplary and generally any system in which a crystal ingot is prepared by a continuous Czochralski method may be used unless stated otherwise.

As the ingot <NUM> is withdrawn from the melt <NUM>, solid polycrystalline silicon feedstock is added to the crucible assembly <NUM> while withdrawing the single crystal silicon ingot <NUM> to replenish the melt <NUM>. Solid-state silicon falls onto the plates <NUM> which at least partially cover the melt <NUM> in the outer melt zone <NUM>. The heat of the melt <NUM> heats the solid polycrystalline silicon disposed on the plates <NUM> causing the silicon to melt and pass through openings <NUM> that extend through the plate <NUM> or fall over the edges <NUM>, <NUM> and/or sides <NUM>, <NUM> of the plates <NUM>.

In some continuous Czochralski processes, more than one ingot is grown while the hot zone (i.e., lower portion of the apparatus <NUM> such as the crucible assembly <NUM> and the susceptor <NUM>) remains heated with silicon melt <NUM> being continuously within the crucible assembly <NUM>. In such methods, a first ingot is grown to a target length and growth is terminated, the ingot is removed from the ingot puller, and a seed crystal is then lowered into the melt to initiate growth of a second single crystal silicon ingot (i.e., using the same melt from which the first ingot was withdrawn). The plates <NUM> remain in the melt <NUM> while the second and subsequent ingots are grown and while polycrystalline silicon is added to the outer melt zone <NUM> to replenish the melt. The thickness of the plates <NUM> may be selected such that the plates <NUM> do not fully dissolve and remain in the melt <NUM> after the first ingot is formed. In other embodiments, a new set of plates <NUM> is added before growth of each subsequent ingot.

Subsequent ingots may be grown with the hot zone intact and at temperature with a continuous melt of silicon being within the crucible assembly <NUM> (e.g., until one or more components of the hot zone have degraded such as the crucible assembly requiring cool-down and replacement of the degraded component). For example, at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> or more ingots may be grown.

Compared to conventional methods for growing single crystal silicon ingots in a continuous Czochralski (CCz) process, the methods of the present invention have several advantages. Without being bound by any particular theory, it is believed that addition of polycrystalline silicon into the outer melt zone of the crucible assembly creates relatively small bubbles (e.g., less than <NUM>) of the inert gas (e.g., argon) that can be carried by the melt through the openings within each weir which allows bubbles to reach the solid-melt interface. The plates may act to prevent entrapment of the inert gas into the melt by preventing polycrystalline feedstock from being discharged directly into the melt. The plates may also provide surface area and nucleation points for inert gas bubbles to aggregate, thereby increasing the size of the bubbles to allow them to become more buoyant. The plates provide a monolithic layer of quartz on the surface of the melt (e.g. with less gaps relative to quartz cullets). The plates dissolve an amount after melt formation and the dissolved quartz also helps remove inert gas from the melt. The rate of dissolution of the plates is less relative to quartz cullets which increases the durability of the plates relative to cullets. The plates may be placed in the crucible assembly relatively easily before the hot zone is up to temperature (e.g., placed on the initial charge of polycrystalline silicon). In embodiments in which a plurality of plates are used, the plates are less rigid and are permitted to move with the silicon as it moves relative to the crucible which helps ensure the plates do not become submerged in the melt. In embodiments in which the plates have a width less than the width of the outer melt zone, the plates are less likely to sinter to the sides/weirs of the crucible assembly during meltdown.

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

As shown in <FIG>, the first ingot (Batch A) grown during a continuous Czochralski process (<NUM>) typically includes more microvoids (detected by laser light scattering with a size of at least than <NUM>) relative to subsequently grown ingots (Ingots B-G).

<FIG> shows the void count for the first grown ingots (Batch A) during a number of continuous Czochralski runs. The "Test" run (far right box plot) included plates on top of the initial charge of silicon (<FIG>) and on the subsequent melt (<FIG>) as polycrystalline silicon was added to the outer melt zone. The other runs included quartz cullets instead of quartz plates. As shown from <FIG>, the run with plates did not increase void counts to unacceptable levels.

Claim 1:
A method for growing a single crystal silicon ingot in a continuous Czochralski process, the method comprising:
adding a charge of polycrystalline silicon to a crucible assembly, the crucible assembly comprising a weir and a sidewall that define an outer melt zone between the weir and the sidewall;
adding one or more plates to the outer melt zone, the one or more plates each including one or more openings that extend through the plate to allow silicon to enter the melt;
forming a melt of silicon in the crucible assembly;
contacting a surface of the melt with a seed crystal;
withdrawing a single crystal silicon ingot from the melt; and
adding solid polycrystalline silicon feedstock to the outer melt zone while withdrawing the single crystal silicon ingot to replenish the melt, the one or more plates at least partially covering the melt in the outer melt zone while adding solid polycrystalline silicon feedstock to the outer melt zone, wherein the polycrystalline silicon melts and falls through the one or more openings that extend through the one or more plates and enters the melt.