Methods for producing a product ingot having low oxygen content

Abstract

This invention discloses a method for producing a product ingot from molten silicon held in a crucible. The method involves evaluating one or more ingot pulling devices to determine whether the devices are capable of producing silicon product ingots with low oxygen content. A sample rod is pulled from the molten silicon and the oxygen content of the sample rod is measured.

Description

Technical Field

[0001] The field of this invention relates to a method for producing a product ingot from molten silicon held in a crucible, and more specifically, to a method for evaluating a pulling apparatus capable of producing silicon product ingots with low oxygen content. Prior Technology

[0002] Single-crystal silicon (the starting material for most processes used in the manufacture of semiconductor electronic components) is commonly prepared using the so-called Czochralski (CZ) process, in which a single seed crystal is immersed in molten silicon and then grown by slow extraction. The molten silicon is contaminated with various impurities, primarily oxygen, during its placement in a quartz crucible. Some applications (such as SOI RF) have relatively low oxygen concentration specifications, such as 5.0 ppma or less.

[0003] Without being bound by any particular theory, it has been found that drawing equipment with the same design (i.e., hot zone design) will exhibit different oxygen capacities (i.e., the ability to grow low-oxygen ingots, such as those with less than 5.0 ppma). A method is needed to characterize these drawing equipment to determine which equipment is best suited for producing low-oxygen silicon ingots.

[0004] This paragraph is intended to introduce to the reader various aspects of the technology that may be associated with the various aspects of the invention described and / or claimed below. It is believed that this discussion helps to provide background information to the reader to facilitate a better understanding of one of the various aspects of the invention. Therefore, it should be understood that these statements should be read in this sense and not as an endorsement of prior art. Summary of the Invention

[0005] One aspect of this invention relates to a method for producing a product ingot from molten silicon held in a crucible of a casting apparatus. Solid silicon is added to the crucible. The solid silicon is heated to cause molten silicon to form in the crucible. A sample rod is pulled from the molten silicon. The sample rod has a diameter of less than 50 mm. The oxygen content of the sample rod is measured. If the oxygen content of the sample rod is below a critical oxygen content, a product ingot is pulled from the molten silicon. The product ingot has a diameter. The diameter of the sample rod is smaller than the diameter of the product ingot.

[0006] Another aspect of the present invention relates to a method for producing a product ingot having an oxygen concentration of less than 5 ppma. Solid silicon is added to a crucible in a first ingot pulling apparatus. The solid silicon is heated in the crucible of the first ingot pulling apparatus to cause a first silicon melt to form in the crucible. A first sample rod is pulled from the first silicon melt. The first sample rod has a first sample rod diameter. The oxygen content of the first sample rod is measured. Solid silicon is added to a crucible in a second ingot pulling apparatus. The solid silicon in the crucible of the second ingot pulling apparatus is heated to cause a second silicon melt to form in the crucible. A second sample rod is pulled from the second silicon melt. The second sample rod has a second sample rod diameter. The oxygen content of the second sample rod is measured. The oxygen content of the first sample rod is compared with the oxygen content of the second sample rod. The product ingot is grown in the ingot pulling apparatus in which the sample rod having a lower oxygen content is grown. The product ingot has a diameter. The diameters of the first sample bar and the second sample bar are each smaller than the diameter of the product ingot.

[0007] Another aspect of the present invention relates to a method for characterizing the oxygen capacity of a pulling apparatus. The pulling apparatus includes a crystal growth chamber and a crucible disposed within the crystal growth chamber. Solid silicon is added to the crucible. The solid silicon is heated to induce the formation of a silicon melt in the crucible. A sample rod is pulled from the melt. The sample rod has a sample rod diameter of less than 50 mm. The sample rod is processed into a sample rod fragment. The sample rod fragment is analyzed using a Fourier transform infrared spectroscopy (FTIR) spectrometer. The oxygen content of the sample rod fragment is measured.

[0008] Various improvements are possible to the features described in relation to the above-described embodiments of the invention. Similarly, further features may be incorporated into the above-described embodiments of the invention. These improvements and additional features may exist individually or in any combination. For example, the various features discussed below with respect to any illustrated embodiment of the invention may be incorporated individually or in any combination into any of the above-described embodiments of the invention. Simple Explanation of the Diagram

[0009] Figure 1 is a schematic side view of one of the pulling devices used to form a single crystal silicon ingot;

[0010] Figure 2 shows one of the sample rods grown from silicon melt;

[0011] Figure 3 is a schematic perspective view of one of the sample bars, showing one of the two trimming planes along which a central plate is formed;

[0012] Figure 4 is a schematic perspective view of one of the trimmed sample bars, which includes one of the central plates;

[0013] Figure 5 is a perspective view of one or more coin-shaped objects cut from a sample bar;

[0014] Figure 6 is a block diagram of one embodiment of a method for producing silicon product ingots;

[0015] Figure 7 is a graph showing the relationship between the oxygen content of the sample bars and the oxygen content of the product ingots.

[0016] Figure 8 is a graph showing the correlation between the oxygen content of sample bars and the oxygen content of finished product spindles of 200 mm and 300 mm grown on various spindle pulling machines; and

[0017] Figure 9 is a graph showing the correlation between the oxygen content of sample bars and the oxygen content of finished 200 mm product ingots grown on various drawing machines.

[0018] The diagram is permeated with the corresponding component symbols indicating the corresponding parts. Implementation

[0019] This application claims the right of U.S. Provisional Patent Application No. 63 / 308,877, filed February 10, 2022, the entire contents of which are incorporated herein by reference.

[0020] This invention relates to a method for characterizing the oxygen capacity of a pulling apparatus and a method for producing silicon product ingots with relatively low oxygen content. According to an embodiment of the invention and referring to FIG1, the product ingot is grown using a so-called Chuklaski process, wherein the ingot is extracted from silicon melt 44 held within a crucible 22 of a pulling apparatus 23. The pulling apparatus 23 includes a housing 26 defining a crystal growth chamber 16 and a lifting chamber 20 having a lateral dimension smaller than that of the growth chamber. The growth chamber 16 has a generally dome-shaped upper wall 45 that transitions from the growth chamber 16 to the narrowing lifting chamber 20. The pulling apparatus 23 includes an inlet 7 and an outlet 12 for introducing a process gas (e.g., argon) into the housing 26 during crystal growth and for removing the process gas from the housing 26.

[0021] The crucible 22 inside the drawing device 23 contains the molten silicon 44 from the silicon ingot it is extracted. The molten silicon 44 is obtained by heating the solid silicon that is filled into the crucible 22 to cause it to melt. The crucible 22 is mounted on a turntable 31 for rotating the crucible 22 about one of the central longitudinal axes X of the drawing device 23.

[0022] A heating system 39 (e.g., a resistance heater) surrounds the crucible 22 for melting the silicon charge to produce a melt 44. The heating system 39 may also extend below the crucible, as shown in U.S. Patent No. 8,317,919. The heating system 39 is controlled by a control system (not shown) to precisely control the temperature of the melt 44 throughout the pulling process. A heat shield (not shown) surrounding the heating system 39 reduces heat loss through the housing 26. The drawing device 23 may also be included above the surface of the melt for shielding the ingot from the heat of the crucible 22, thereby increasing the axial temperature gradient at the solid-melt interface—a heat shield assembly (not shown).

[0023] A lifting mechanism (not shown) is attached to a lifting cable 24 extending downward from the mechanism. The mechanism is capable of raising and lowering the lifting cable 24. Depending on the type of puller, the ingot pulling device 23 may have a lifting shaft instead of a cable. The lifting cable 24 terminates in a lifting assembly 58, which includes a seed chuck 32 for holding a seed crystal 6 for growing a silicon ingot. During ingot growth, the lifting mechanism lowers the seed crystal 6 until it contacts the surface of the molten silicon 44. Once the seed crystal 6 begins to melt, the lifting mechanism slowly raises the seed crystal through the growth chamber 16 and the lifting chamber 20 to grow a single crystal ingot. The speed at which the lifting mechanism rotates the seed crystal 6 and the speed at which the lifting mechanism raises the seed crystal (i.e., the lifting rate v) are controlled by a control system.

[0024] A process gas is introduced into the housing 26 through inlet 7 and extracted from outlet 12. The process gas creates an atmosphere within the housing 26, and the molten material and the atmosphere form a molten-gas interface. Outlet 12 is in fluid communication with one of the discharge systems (not shown) of the puller.

[0025] In this regard, the pulling device 23 shown in Figure 1 and described herein is exemplary and can be used with other pulling device configurations and setups to pull a single crystal silicon ingot from a melt, unless otherwise stated.

[0026] According to an embodiment of the present invention, after solid silicon is added to crucible 22 and heating system 39 is operated to melt the solid silicon, a sample ingot or rod is pulled from the melt (step 100 in FIG. 6). An exemplary sample rod 5 is shown in FIG. 2. Rod 5 includes a crown 21 in which the rod tapers outward from the seed crystal to reach a target diameter. Rod 5 includes a constant diameter portion 25 or a cylindrical body of a crystal grown by increasing the pulling rate, or simply "body". The body 25 of sample rod 5 has a relatively constant diameter. Rod 5 includes a tail or end taper 29 in which the rod tapers in diameter after the body 25. When the diameter becomes sufficiently small, rod 5 then separates from the melt. Rod 5 has a central longitudinal axis A extending through the crown 21 of the ingot and a terminal 33.

[0027] The growth conditions for sample rod 5 can generally be selected from any suitable growth conditions available to those skilled in this technique. Sample rod 5 can be a single crystal with a sample rod body having zero differential arrangement. Sample rod 5 can be grown using a locked seed lift (i.e., a fixed pull speed with different diameters (such as + / - about 5 mm)) or an active seed lift (with a pull speed varied to maintain the target diameter).

[0028] The sample rod 5 has a diameter smaller than that of a product ingot grown after the sample rod. For example, the diameter of the sample rod may be less than 0.75 times, less than 0.50 times, less than about 0.25 times, or less than 0.1 times the diameter of the product ingot. In some embodiments, the diameter of the sample rod is less than about 150 mm or less than about 100 mm, less than about 50 mm, less than about 25 mm, or less than about 20 mm (e.g., from about 5 mm to about 150 mm, from about 5 mm to about 100 mm, from about 5 mm to about 50 mm, from about 5 mm to about 25 mm, or from about 10 mm to about 25 mm). Generally, the diameter of rod 5 is measured by measuring the rod at several axial positions (e.g., within a constant diameter portion of the rod if the rod has a crown and / or tapering end) and averaging the measured diameters (e.g., measuring and averaging 2, 4, 6, 10, or more diameters along the length). In some embodiments, the maximum diameter of the sample rod is less than about 150 mm or less than about 100 mm, less than about 50 mm, less than about 25 mm or less than about 20 mm (e.g., from about 5 mm to about 150 mm, from about 5 mm to about 100 mm, from about 5 mm to about 50 mm, from about 5 mm to about 25 mm or from about 10 mm to about 25 mm).

[0029] In some embodiments, the rod 5 has a diameter that substantially corresponds to the diameter of the neck of a product ingot grown in the crystal puller. For example, the rod may have a diameter of less than 50 mm, less than 25 mm, or less than 20 mm.

[0030] The sample rod 5 may have any suitable length. In some embodiments, the rod (e.g., after trimming) has a length of less than about 300 mm, less than about 200 mm, or less than about 100 mm (e.g., from about 25 mm to about 300 mm).

[0031] In a second step 110 (Figure 6), after growing the sample rod 5, the sample rod is processed to form a sample rod segment, such as a central plate 40 (Figure 4) or a coin-shaped object 138 (Figure 5). In embodiments where a plate 40 is formed, the crown and tail of the sample rod 5 may be removed, for example, by using a wire saw. As shown in Figure 4, the sample rod 5 is then trimmed to form the plate 40. The sample rod 5 may be trimmed by a benchtop cutter (e.g., a Minitom available from Struers (Westlake, Ohio)) or by using a diamond wire saw (e.g., a DTW wire saw). The sample rod 5 is trimmed along a first trimming plane 42 (Figure 3) and along a second trimming plane 46 to form first and second trimmed portions 49, 52 (Figure 4) and the plate 40. The first trimming plane 42 and the second trimming plane 46 are parallel to each other and parallel to the central longitudinal axis A of the sample rod 5. Plate 40 may have any suitable thickness for oxygen measurement, such as (for example) between about 5 mm and about 0.1 mm, between about 3 mm and about 0.5 mm, or between about 3 mm and about 1 mm. The cross-section of plate 40 may be generally square or rectangular. The first and second sides 62, 64 of plate 40 may be slightly rounded due to the contour of sample rod 5, or plate 40 may be further trimmed to form flat sides 62, 64. The first and second trimmed flat surfaces 57, 59 of plate 40 may be ground to level the surface.

[0032] Generally speaking, the central plate 40 includes at least a portion of the central axis A of the untrimmed sample bar 5. In some embodiments, the trimming method may be variable, taking into account the axial non-uniformity of the sample bar diameter to allow the central plate 40 to capture as many axisymmetric central lines of the bar 5 as possible. For example, the plate 40 may include at least about 10% of the central axis A of the sample bar 5 (i.e., the sample bar just before trimming to form the central plate), or at least about 25%, at least about 50%, at least about 75%, or at least about 90% of the central axis A of the sample bar 5. In some embodiments, after trimming, the central axis A of the trimmed sample bar 5 extends through the entire length of the plate 40 (e.g., from the first end 54 to the second end 56 of the plate 40).

[0033] In some embodiments, the rod segment (e.g., plate 40 or the coin-shaped object 138 described below) is cleaned, etched (e.g., mixed acid etching for at least 3 minutes), and dried before measuring the oxygen content of plate 40. In some embodiments, such as when the resistivity of the rod segment is also measured, the rod segment may be subjected to a rapid thermal annealing at a temperature between 500°C and about 1000°C (e.g., before or after oxygen measurement).

[0034] In some embodiments, one or more coin-shaped objects 138 (FIG. 5) are formed from a sample bar (e.g., trimmed from a cross section of one of the sample bars) instead of forming a central plate 40. Similar to a plate, one or more coin-shaped objects 138 may be ground, cleaned, and / or etched before measuring oxygen content. Multiple coin-shaped objects may be cut from the same bar (e.g., seed end and opposite end) and compared with a threshold value (e.g., averaged and compared with the threshold value). One or more coin-shaped objects may each have a thickness between about 5 mm and about 0.1 mm, between about 3 mm and about 0.5 mm, or between about 3 mm and about 1 mm.

[0035] Once the plate 40 or coin 138 has been processed, the oxygen content of the plate 40 or coin 138 is measured in step 120 (Figure 6). For example, the oxygen content can be measured by Fourier transform infrared spectroscopy (e.g., by analyzing a rod fragment using a Fourier transform infrared spectrometer). FTIR spectrometers are commercially available, such as the QS-300 spectrometer from Bio-Rad (Hercules, California).

[0036] Once the oxygen content of the sample bar is measured, it is compared to a critical oxygen content in step 130 (Figure 6). The critical oxygen content is lower than one of the maximum oxygen contents (e.g., a previously established maximum value) that can produce a product ingot with a relatively low oxygen content. Sample ingots with oxygen contents higher than this maximum value may not be suitable for forming low-oxygen ingots (although growth parameters are controlled to a lower oxygen content) and may be used to form a product ingot other than a low-oxygen ingot. For example, the critical oxygen content of the sample bar (e.g., measured from a plate or coin-shaped object) is one of the maximum oxygen contents (e.g., measured by ASTM 80) of 5.0 ppma, 4.0 ppma, 3.5 ppma, 3 ppma, or even less than 2.5 ppma.

[0037] If the oxygen content is below the critical oxygen content, a product ingot is pulled from the melt (i.e., from the same melt from which the sample rod was grown) in step 140 (Figure 6). The product ingot has a diameter larger than that of the sample rod (i.e., the diameter of the constant diameter portion of the sample rod is smaller than the diameter of the constant diameter portion of the ingot). The product ingot may have a diameter of 150 mm or greater, or, as in other embodiments, 200 mm or greater, or 300 mm or greater (e.g., 450 mm or greater). The product ingot has a relatively low oxygen content, such as less than 5.0 ppma, 4.0 ppma, 3.5 ppma, 3 ppma, or even less than 2.5 ppma. The product ingot may have any suitable resistivity, and in some embodiments, a relatively high resistivity, such as at least 1000 ohm-cm or even at least 7,500 ohm-cm.

[0038] In some embodiments (e.g., in a batch process), polycrystalline silicon is not added during the growth of the ingot. In other embodiments (e.g., in a continuous Chuklaski process), polycrystalline silicon is not added to the melt while growing the product ingot.

[0039] When the sample bar has an oxygen content higher than the threshold, the first product ingot is not grown in the ingot pulling device (i.e., the first ingot pulling device). Instead, a second product ingot with a higher oxygen specification is grown using the first ingot pulling device (i.e., the second product ingot has an oxygen content greater than that of the first product ingot). For example, the first product ingot may have an oxygen content of less than 5 ppma and the second product ingot may have an oxygen content greater than 5 ppma.

[0040] The first product ingot can then be grown from a second drawing device, which is different from the first drawing device. Before growing the first ingot, a second sample bar can be grown in the second drawing device in step 200 (Figure 6) to determine whether the drawing device is suitable for forming a relatively low-oxygen ingot (i.e., by forming a plate or one or more coin-shaped objects from the sample bar in step 210, measuring the oxygen content of the plate or one or more coin-shaped objects in step 220, and determining whether the oxygen content is less than a threshold value in step 230). If the oxygen content of the sample bar grown from the second drawing device is not less than the threshold oxygen content, subsequent drawing devices can be tested until one of the devices is found to have a sample bar with an oxygen content less than the threshold oxygen content (e.g., testing three, four, five, or more devices until a suitable drawing device is found). If the oxygen content of the sample bar grown from the second drawing device is less than the threshold oxygen content, the product ingot is grown in step 240.

[0041] The critical oxygen content can be determined by growing multiple sample bars in different pulling equipment and comparing the oxygen content of each sample bar (e.g., the average value between the seed end and the opposite end) with the oxygen content of the finished ingot (e.g., the ingot block from the midpoint). The comparison allows for the determination of the maximum oxygen content of a finished ingot based on the sample bars having the required oxygen content (e.g., the upper limit of the specification). One correlation between the oxygen content of the sample bars and the oxygen content of the finished ingot can be shown in Figures 8 and 9 and is discussed below.

[0042] In some embodiments where multiple drawing machines can be used to grow relatively low-oxygen product ingots (e.g., less than 5.0 ppma), sample bars can be grown from a melt in each of the drawing machines (e.g., evaluating two, three, four, or more drawing machines). The oxygen content of each sample bar grown from each drawing machine can be compared. The sample bar with the lowest oxygen content can be used to grow a product ingot (with a relatively low oxygen target) in the drawing machine from which it was grown.

[0043] Once a pulling device is identified with an oxygen content less than the critical oxygen content from one of its sample bars, in addition to a first product ingot with a relatively low oxygen content (e.g., less than 5.0 ppma), the pulling device can also be used to grow additional ingots with relatively low oxygen content (e.g., two or more, three or more, five or more, or ten or more ingots with an oxygen content of 5 ppma or less).

[0044] Compared to conventional methods for producing a single-crystal silicon ingot, the method of this invention has several advantages. Without being constrained by any particular theory, it has been found that pulling equipment with the same design (i.e., hot zone design) will exhibit different oxygen capacities (i.e., the ability to grow low-oxygen ingots, such as those with less than 5.0 ppma) and wide variability. By growing a sample ingot from one or more pulling equipment that are candidates for production of relatively low-oxygen ingots, the oxygen content of the sample rod can be compared with a threshold oxygen concentration, below which low-oxygen ingots are believed to be produced. Alternatively or additionally, the oxygen content between sample rods from different pullers can be compared to determine which pulling equipment can produce the lowest oxygen content in silicon product ingots. Even when process conditions are controlled for low-oxygen production, some pulling equipment still cannot achieve low-oxygen ingots. This method reduces the amount of charge wasted due to the pulling equipment and, if the pulling equipment is found to be unable to produce low-oxygen product ingots, allows the melt to be reused for a different customer product (e.g., above 5.0 ppma, in addition to low oxygen). This method is particularly advantageous when it is desirable to increase the output of low-oxygen ingots by bringing additional pulling equipment online for low-oxygen production. The method identifies whether a pulling equipment is suitable for low-oxygen production within less than 24 hours and with a relatively small amount of charge (e.g., less than 0.1 kg) used for characterization (which includes pulling a sample bar, processing the bar into a segment (such as a plate or coin-shaped piece), and measuring oxygen by FTIR). This is compared with conventional methods in which a completely zero-difference arrangement is grown to determine oxygen capacity. Example

[0045] The procedure of the present invention is further illustrated by the following examples. These examples should not be considered as limiting. [Example 1] [Oxygen content of sample rods as one of the predictive indicators of product oxygen content]

[0046] For four different ingot pulling apparatuses, a sample rod with a diameter of less than 50 mm was grown before growing a product ingot. The oxygen content of the sample rod was measured at the seed end and the opposite end, and the average value was calculated. The oxygen content of the product ingot was measured at the midpoint (i.e., the point where oxygen is most stable and flat, rather than a transient region like the seed end or the opposite end). Ingot pulling apparatuses A to C used 200 mm diameter pullers, and ingot pulling apparatus D used a 300 mm puller.

[0047] As shown in Figure 7, the oxygen concentration of the product ingot correlates well with the sample bar grown prior to the product ingot. However, with a few exceptions, the oxygen capacity of each puller clusters within a specific window reflecting the capability of the characterization method. Operationally, puller A exhibits the best overall yield and output (i.e., production volume in "high-quality" inches per unit time) relative to the oxygen specifications of RF SOI, while puller D shows the worst yield, thus validating the sample bar response.

[0048] Figure 8 illustrates the relationship between oxygen (sample bar average - ingot) / sample bar average to sample bar average oxygen for an intermediate length product ingot by plotting the ratio of oxygen (sample bar average - ingot) / sample bar average to sample bar average oxygen for all data (200 mm (lifters A to C) and 300 mm (lifter D)). Figure 9 shows a plot for only one of the 200 mm lifters and includes the best-fit trend line.

[0049] The best-fit curve shown in Figure 9 exhibits a fairly high least-squares fit, meaning that the oxygen in the sample bar correlates quite well with the oxygen in the stable oxygen region of the product ingot. Oxygen in this range is primarily governed by the high evaporation of SiO gas and the low solubility of the crucible. Puller A demonstrates the low oxygen levels in both the sample bar and the ingot, meaning that, compared to other pullers, this puller exhibits different hot spots at the crucible wall and a higher SiO gas evaporation rate.

[0050] As shown in Figure 8, the 300 mm crystal data exhibits sample rod oxygen > 7 ppma, and the oxygen difference ratio between the sample rod and the ingot is lower than that of the 200 mm case, while the oxygen levels of both the sample rod and the product ingot are higher than those of the 200 mm case. This means that even if the oxygen in the 300 mm grown crystal is higher than that in the 200 mm crystal, the oxygen in the sample rod is still significantly higher than that in the 200 mm sample rod. This results in a lower oxygen difference ratio in the 300 mm crystal compared to the 200 mm crystal in similar sample rod oxygen conditions. In both ingot diameters, the crucible diameter is the same, meaning the 300 mm ingot grows on a free melt surface less than that of the 200 mm ingot, thus reducing SiO gas evaporation. Consequently, the oxygen in the product ingot increases. Furthermore, the diameter of the reflector above the melt differs between 300 mm and 200 mm, resulting in a slower radial gas velocity above the free melt surface during sample rod growth compared to the 200 mm sample rod, leading to higher oxygen levels in the sample rod.

[0051] As shown in Figure 9, this prediction is valid when the thermal zone configuration and diameter of the product ingot are the same. Furthermore, the relationship between the oxygen content of the sample bar and the oxygen content of the product ingot can be used to confirm and predict the oxygen content under stable conditions in the product ingot. Variations in the data shown in Figures 8 and 9 can be caused by differences in thermal zone degradation (such as heater and insulation). However, maintaining the relationship and high oxygen content in the sample bar indicates high oxygen content in the main body of the product ingot.

[0052] As used herein, when used in conjunction with ranges of size, concentration, temperature or other physical or chemical properties or characteristics, the terms “about,” “substantially,” “basically” and “approximately” mean to cover variations that may exist within the upper and / or lower limits of the range of properties or characteristics, including, for example, variations arising from rounding, measurement methods or other statistical variations.

[0053] When describing elements of the present invention or several embodiments thereof, the articles "a," "an," "the," and "the" are intended to mean that one or more elements are present. The terms "comprising," "including," "containing," and "having" are intended to be inclusive and mean that additional elements may be present in addition to the listed elements. The use of terms indicating a particular orientation (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require any particular orientation of the described items.

[0054] Since various changes can be made to the above construction and methods without departing from the scope of the invention, it is intended that all matters contained in the above description and shown in the accompanying drawings be interpreted in an illustrative and not restrictive sense.

[0055] 5: Sample sticks 6: Seed Crystal 7: Entrance Port 12: Export Port 16: Crystal growth chamber 20: Lifting Chamber 21: Crown 22: Crucible 23: Spindle pulling equipment 24: Pulling cable 25: Constant diameter section / main body 26: Outer shell 29: Tail / End Cone 31: Turntable 32: Seed chuck 33: Terminal 39: Heating System 40: Center plate 42: First trimming plane 44: Molten Silicon 45: Generally dome-shaped upper wall 46: Second trimming plane 49: First trimming section 52: Second trimming section 54: First end 56: Second end 57: First trimmed flat surface 58: Lifting Assembly 59: Second trimmed flat surface 62: First side 64: Second side 100: Steps 110: Steps 120: Steps 130: Steps 138: Coin-shaped object 140: Steps 200: Steps 210: Steps 220: Steps 230: Steps 240: Steps A: Central longitudinal axis X: Central longitudinal axis

Claims

1. A method for producing a product ingot having an oxygen concentration of less than 5 ppma, the method comprising: Solid silicon is added to a crucible in a first pulling device; the solid silicon in the crucible of the first pulling device is heated to cause a first silicon melt to form in the crucible; a first sample rod having a first sample rod diameter is pulled from the first silicon melt; the oxygen content of the first sample rod is measured; solid silicon is added to a crucible in a second pulling device; the solid silicon in the crucible of the second pulling device is heated to cause a second silicon melt to form in the crucible; a second sample rod having a second sample rod diameter is pulled from the second silicon melt; the oxygen content of the second sample rod is measured; the oxygen content of the first sample rod is compared with the oxygen content of the second sample rod; and the product ingot is grown in the pulling device in which the sample rod having a lower oxygen content is grown, the product ingot having a diameter, the diameter of the first sample rod and the diameter of the second sample rod being smaller than the diameter of the product ingot.

2. The method of request item 1, further comprising: Solid silicon is added to a crucible in one of the third-stage ingot pulling devices; The solid silicon in the crucible of the third drawing device is heated to induce the formation of a third silicon melt in the crucible; a third sample rod having the same sample rod diameter is drawn from the third silicon melt; the oxygen content of the third sample rod is measured; the oxygen content of the first sample rod is compared with the oxygen content of the third sample rod; and the product ingot is grown in the drawing device in which the first, second and third sample rods have the lowest oxygen content.

3. The method of claim 1, wherein the product ingot is a first product ingot, the method comprising growing a second product ingot in the ingot pulling apparatus in which the sample bar having a higher oxygen content is grown.

4. As in request item 1, where: A first plate is formed from the first sample bar; measuring the oxygen content of the first sample bar includes measuring the oxygen content of the first plate; a second plate is formed from the second sample bar; and measuring the oxygen content of the second sample bar includes measuring the oxygen content of the second plate.

5. The method of claim 1, wherein the oxygen content of the first and second sample bars is measured by Fourier transform infrared spectroscopy.

6. The method of claim 3, wherein the second product ingot has an oxygen content greater than 5 ppma.

7. As in request item 4, where: The first plate includes at least a portion of the central axis of one of the first sample bars, and the first plate has a thickness between about 5 mm and about 0.1 mm; and the second plate includes at least a portion of the central axis of one of the second sample bars, and the second plate has a thickness between about 5 mm and about 0.1 mm.

8. The method of claim 4, which includes etching the first plate before measuring the oxygen content of the first sample bar.

9. The method of claim 8, which includes etching the second plate before measuring the oxygen content of the second sample bar.

10. The method of claim 1, comprising forming one or more coin-shaped objects from the first sample bar, wherein measuring the oxygen content of the first sample bar includes measuring the oxygen content of one or more of the coin-shaped objects.

11. The method of claim 10, which includes etching the one or more coin-shaped objects before measuring the oxygen content of the first sample bar.

12. The method of claim 10, comprising forming one or more coin-shaped objects from the second sample bar, wherein measuring the oxygen content of the second sample bar includes measuring the oxygen content of one or more of the coin-shaped objects.

13. The method of claim 1, wherein the diameter of the first sample bar is less than 50 mm and the diameter of the second sample bar is less than 50 mm.

14. The method of claim 1, wherein the diameter of the first sample rod is between about 5 mm and about 25 mm, and the diameter of the second sample rod is between about 5 mm and about 25 mm.

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