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 polycrystalline 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.

<CIT> discloses monocrystalline pulling systems and methods for forming ingots of semiconductor or solar material from a melt, including a crucible and conditioning members disposed within a cavity of the crucible to contact the melt.

A need exists for 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.

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.

One aspect of the present invention is directed to a method for growing a single crystal silicon ingot in a continuous Czochralski process. A melt of silicon is formed in a crucible assembly. A batch of quartz buffer members is added to the melt. The batch has a mass M. A surface of the melt is contacted with a seed crystal. A single crystal silicon ingot is withdrawn from the melt. The single crystal silicon ingot includes a main body. There is a time T between adding the batch of buffer members to the melt and start of growth of the main body. The ratio of M/T is controlled to be greater than a threshold M/T of <NUM> grams per hour to reduce void counts in wafers sliced from the single crystal silicon ingot. Solid polycrystalline silicon feedstock is added to the crucible while withdrawing the single crystal silicon ingot to replenish the melt.

One aspect of the present invention is directed to a method for determining a threshold ratio of M/T for growing a single crystal silicon ingot in a continuous Czochralski process. The continuous Czochralski process includes forming a melt of silicon in a crucible assembly, adding a batch of quartz buffer members to the melt with the batch having a mass M, contacting a surface of the melt with a seed crystal, withdrawing a single crystal silicon ingot from the melt, the single crystal silicon ingot comprising a main body, there being a time T between adding the batch of buffer members to the melt and start of growth of the main body, and adding solid polycrystalline silicon feedstock to the crucible assembly while withdrawing the single crystal silicon ingot to replenish the melt. The method for determining the threshold ratio of M/T includes growing a plurality of single crystal silicon ingots with at least two of the ingots being grown with different ratios of M/T. A defect count in one or more wafers sliced from the plurality of single crystal silicon ingots is measured. The ratio of M/T for single crystal silicon ingots from which wafers were sliced having a defect count below a threshold defect count is determined.

Various refinements exist of the features noted in relation to the above-mentioned aspects of the present invention. Further features may also be incorporated in the above-mentioned aspects of the 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 any of the above-described aspects of the 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. Buffer members (e.g., quartz cullets) are added to the melt of silicon prior to formation of the main body of the ingot. The ratio of the mass M of buffer members that are added to the time T between addition of the buffer members and the start of growth of the main body of the ingot is controlled to be greater than a threshold M/T. By controlling the ratio (M/T) of the mass of buffer members to the time until the ingot main body starts to grow to be greater than the threshold M/T, the amount of defects in the resulting silicon wafers may be reduced.

An example ingot puller apparatus <NUM> for producing an ingot <NUM> by a continuous Czochralski process is 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. 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 polycrystalline silicon which may be, for example, granular, chunk, or a combination of granular and chunk, into the outer melt zone <NUM> 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 granular, chunk, or a combination of granular and chunk. 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. As described below, an amount of buffer members may be added to the initial charge <NUM> of polycrystalline silicon in the outer melt zone <NUM> of the crucible assembly <NUM> prior to or during melting the initial charge <NUM> of polycrystalline silicon.

Once polycrystalline silicon (and optionally buffer members) is added to the crucible assembly <NUM> to form a charge <NUM>, the charge <NUM> is heated to a temperature above about 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>.

The ingot pulling apparatus <NUM> includes a pulling mechanism <NUM> (<FIG>) for growing and pulling the ingot <NUM> from the melt 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, 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 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.

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.

In accordance with embodiments of the present invention, before the ingot <NUM> is grown, a batch <NUM> (<FIG>) of buffer members <NUM> (e.g., quartz cullets) are added to the silicon melt <NUM> and, in particular, to the outer melt zone <NUM>. The buffer members <NUM> may be less dense than the melt <NUM> of silicon such that the buffer members <NUM> float within the melt <NUM> (i.e., a portion is disposed on the surface of the melt <NUM>). Suitable buffer members <NUM> which may be added to the outer melt zone <NUM> include, for example, solid materials which prevent the polysilicon added through the feeding tube <NUM> from directly entering the melt <NUM> and/or that provide surface area for dissipation of inert gas bubbles. The buffer members <NUM> may form gaps between the buffer members <NUM>. The buffer members <NUM> may be free to move (e.g., when impacted by falling polycrystalline feedstock). The buffer members <NUM> include quartz such as quartz cullets. When quartz cullets are used, the cullets may have any suitable shape (e.g., cylindrical) and any suitable size (e.g., about <NUM> to <NUM> in diameter and/or about <NUM> to about <NUM> in length when cylindrical cullets are used).

After the batch <NUM> of buffer members <NUM> is added to the melt <NUM>, the ingot <NUM> is pulled from the melt <NUM>. In accordance with embodiments of the present invention, the ratio of the mass M of the batch <NUM> of buffer members <NUM> added to the melt <NUM> to the time T between adding the batch <NUM> of buffer members <NUM> to the melt <NUM> and when the ingot main body <NUM> (<FIG>) begins to grow is controlled such that the ratio of M/T is greater than a threshold ratio of M/T to reduce void counts in wafers sliced from the single crystal silicon ingot. Generally, the time T corresponds to the time at which the batch <NUM> of buffer members <NUM> has been fully added and when the ingot main body <NUM> begins to grow.

In some embodiments, the ratio of M/T is controlled to be greater than a threshold M/T such that wafers sliced from the single crystal silicon ingot have a void count of less than <NUM> defects of a size of <NUM> or more or even have a void count of less than <NUM> defects of a size of <NUM> or more. The threshold M/T may vary depending on the hot zone design of the ingot puller apparatus. To determine the threshold M/T, a threshold defect count (e.g., a maximum defect count desired by the manufacturer and/or customer such as less than <NUM> defects, less than <NUM> defect or less than <NUM> defects of a size of <NUM> or more) is established. A plurality of single crystal silicon ingots are grown in which at least two of the ingots (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> ingots) are grown with different ratios of M/T. The defect count in one or more wafers sliced from the plurality of single crystal silicon ingots is measured (e.g., with an SP1 inspection tool). The ratio of M/T for single crystal silicon ingots from which wafers were sliced that have a defect count below the threshold defect count is determined based on the measured defect counts (i.e., a threshold M/T is determined based on M/T values in which the defect count was at or below the defect threshold count).

The threshold M/T at which M/T is controlled to be greater than is <NUM> grams per hour. In other embodiments, the threshold M/T is <NUM> grams per hour or even <NUM> grams per hour. In some embodiments, the threshold M/T at which M/T is controlled to be greater than is <NUM> grams per hour. In yet other embodiments, the threshold M/T at which M/T is controlled to be greater than is <NUM> grams per hour. The threshold M/T (and actual M/T used in the ingot puller apparatus to grow an ingot) may be bound by the practical limits of the ingot growth process (e.g., without inhibiting flow of solid polysilicon into the melt such as when solid polysilicon begins to mound on top of the buffer members). For example, M/T can be controlled to be above a threshold M/T listed above and less than <NUM> grams per hour or even less than <NUM> grams per hour.

As shown in <FIG> and in accordance with some embodiments of the present invention, the batch <NUM> of buffer members <NUM> may be sufficiently large such that the buffer members <NUM> continuously extend from the sidewall <NUM> of the crucible assembly <NUM> to the first weir <NUM>.

In this regard, the mass M of the batch <NUM> of buffer members <NUM> (e.g., quartz cullets) generally excludes any buffer members that were added before the initial charge <NUM> (<FIG>) was melted down (i.e., excludes an initial charge of buffer members added to the solid polycrystalline charge).

To control the ratio of M/T such that the ratio of M/T is greater than the threshold M/T, the mass M of the batch <NUM> of buffer members <NUM> added to the outer melt zone <NUM> may be increased or the time T between addition of buffer members and growth of the main body <NUM> of the ingot <NUM> may be decreased (e.g., by adding buffer members later, i.e., closer to when the ingot main body <NUM> begins to grow). It should be noted that controlling M/T to be "greater than" a threshold M/T generally includes any method in which a minimum M/T is chosen or established for use in an ingot growth process (i.e., includes embodiments in which M/T in the ingot growth process is "equal to" or greater than a minimum or, in other words, the threshold M/T is a unit below the minimum M/T that is chosen such that M/T is greater than the threshold).

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>. In some embodiments, buffer members <NUM> are not added to the melt while the ingot is grown (e.g., neck, crown and/or main body). If buffer members are added during growth of the neck <NUM> and/or crown <NUM> as in other embodiments of the present invention, the mass M of the batch <NUM> of buffer members <NUM> may include any buffer members <NUM> added while the seed crystal <NUM> (<FIG>) is lowered and/or added during growth of the neck <NUM> and crown <NUM> of the ingot <NUM>, as well as any buffer members added prior to lowering of the seed crystal <NUM> (and subsequent to melting the charge of solid polycrystalline silicon and/or subsequent to termination of growth of the previous ingot, if any). In some embodiments of the present invention, buffer members <NUM> are not added while the ingot main body <NUM> is pulled from the melt <NUM>. If buffer members <NUM> are added during growth of the ingot main body <NUM>, such buffer members <NUM> are not considered to be part of the batch <NUM> added prior to growth of the main body <NUM> of the ingot <NUM> (i.e., are not part of the mass M of the batch <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). 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.

After growth of the first ingot <NUM> is terminated and the ingot is removed (e.g., removed from a pull chamber of the ingot puller apparatus <NUM>), a second batch of buffer members may be added to the melt that remains after the first ingot has been removed. A seed crystal <NUM> (i.e., the same seed crystal used to pull the first ingot or a different seed crystal) is lowered to contact the melt. In accordance with embodiments of the present invention, the ratio of the mass M<NUM> of the second batch of buffer members added to the melt to the time T<NUM> between adding the second batch of buffer members to the start of growth of the main body of the ingot is controlled to be greater than the threshold M/T (i.e.. , the threshold M/T referenced above) to reduce void counts in wafers sliced from the second single crystal silicon ingot. In this regard, there may be an amount of the first batch of buffer members that still remain in the melt when the second batch is added. An amount (or the entire amount) of the first batch may be depleted due to dissolution within the silicon melt. The first batch that remains in the melt generally is not part of the mass M<NUM> of the second batch.

The ingot puller apparatus <NUM> may include a buffer member feed system <NUM> (<FIG>) for adding batches of buffer members <NUM> to the outer melt zone <NUM>. The buffer system <NUM> may be configured for autonomous addition of buffer member <NUM> or for manual addition. For example, the buffer member feed system <NUM> may include a storage vessel which contains buffer members (e.g., quartz cullets) and a metering device (e.g., weigh hoppers or metering wheels). The buffer member feed system <NUM> may include a buffer member feeding tube which may be the same tube <NUM> as which polysilicon is added or may be a separate tube. Buffer members <NUM> may be weighed out by an operator or automatically fed to the tube by the buffer member feed system <NUM>.

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. By controlling the ratio of the mass M of the batch of buffer members added to the melt to the time T between adding the batch of buffer members to the melt and when the main body of the single crystal silicon ingot begins to grow to be greater than threshold value of M/T, the void count of wafers sliced from ingots grown in such continuous Czochralski methods may be reduced. For example, such wafers may have less than <NUM> defects per wafer (of a size <NUM> or more and measured by a SP1 inspection tool). 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 buffer members may act to prevent entrapment of the inert gas into the melt by preventing polycrystalline feedstock from dumping directly into the melt. The buffer members 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 buoyant. By increasing the ratio of the mass M of the batch of buffer members added to the melt to the time T between adding the batch of buffer members to the melt and the start of the ingot main body growth to be at least <NUM> grams/hour, the efficiency of the buffer members in reducing inert gas impingement and/or dissipation of inert gas bubbles is increases.

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

Single crystal silicon ingots were grown in a continuous Czochralski method in an ingot puller apparatus similar to the apparatus shown in <FIG>. The silicon ingots were grown with a <NUM> main body portion and were doped with red phosphorous. An initial charge of polycrystalline silicon was added to the outer melt zone, middle melt zone and inner melt zone. Quartz cullets (<NUM>) were added to the top of the polycrystalline feedstock in the outer melt zone. After the charge was melted, additional polycrystalline silicon was added through the polycrystalline silicon feed system until the initial charge was fully formed. A batch of quartz cullets (<NUM>) was added to the melt. A seed crystal was lowered and a single crystal silicon ingot was grown from the melt. Subsequent ingots were grown while maintaining the hot zone at temperature (i.e., from the same melt without cooling the hot zone down). A batch (<NUM>) of buffer members (quartz cullets) was added to the outer melt zone before growth of each subsequent ingot. The first run of ingots was growth with the ratio of the mass M of the batch of buffer members added to the melt to the time T between adding the batch of buffer members to the melt and the start of growth of the ingot main body being less than a threshold M/T (in this case less than <NUM> grams/hour). A second run of ingots was grown after the first run with the ratio of M/T being greater than the threshold M/T (i.e., <NUM> grams/hour or more). As indicated, one ingot in the second run was grown with M/T less than the threshold M/T for confirmation of the effect.

The defect counts in wafers sliced from ingots of the first run (M/T less than the threshold M/T) and ingots of the second run (M/T greater than the threshold M/T) are shown in <FIG> and <FIG>, respectively. As may be seen from comparing the figures, increasing M/T to the threshold M/T reduced the defect growth of the wafers to less than <NUM> defects/wafer, thereby increasing the amount of wafers that were within customer specification. <FIG> is a scatter plot showing the defect counts as a function of the M/T ratio (both for the red phosphorous ingots and for other ingot runs which were arsenic doped). As shown in <FIG>, the defect counts were below <NUM> defects/wafer for all runs in which M/T was greater than a threshold M/T.

<FIG> shows the defect counts of wafers sliced along the axis of an ingot grown by the process of Example <NUM> in which M/T was about <NUM> grams/hour. As shown in <FIG>, the defect counts across the entire axis of the ingot were greater than <NUM> defects/wafer. <FIG> shows the defect counts of wafers sliced along the axis of an ingot grown by the process of Example <NUM> in which M/T was about <NUM> grams/hour. As shown in <FIG>, the defect counts across the entire axis of the ingot were less than <NUM>. Ingots grown under both conditions exhibited axial uniformity in defects. This demonstrates that buffer members do not need to be added during growth of the ingot main body.

<FIG> is a scatter plot showing the defect counts as a function of the M/T ratio for wafers sliced from single crystal silicon ingots similar to the apparatus shown in <FIG>. The ingot puller apparatus was a different apparatus than the ones used in Examples <NUM>-<NUM>. As shown in <FIG>, a minimum threshold value of M/T of <NUM> grams/hour resulted in defect counts below <NUM> defects/wafer for all runs in which M/T was greater than the threshold M/T. The threshold M/T (i.e., minimum) for the ingot puller apparatus was determined to be about <NUM> grams/hour.

As used herein, the terms "about" and "substantially" when used in conjunction with ranges of dimensions, concentrations, temperatures or other physical or chemical properties or characteristics are 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 method for growing a single crystal silicon ingot in a continuous Czochralski process, the method comprising:
forming a melt of silicon in a crucible assembly;
adding a batch of buffer members to the melt, the buffer members being made of quartz, the batch having a mass M;
contacting a surface of the melt with a seed crystal;
withdrawing a single crystal silicon ingot from the melt, the single crystal silicon ingot comprising a main body, there being a time T between adding the batch of buffer members to the melt and start of growth of the main body;
controlling the ratio of M/T to be greater than <NUM> grams per hour to reduce void counts in wafers sliced from the single crystal silicon ingot; and
adding solid polycrystalline silicon feedstock to the crucible assembly while withdrawing the single crystal silicon ingot to replenish the melt.