Manufacturing method for ingot products with low oxygen content

Abstract

A method for producing an ingot product from a silicon melt held in a crucible is disclosed. The method includes evaluating one or more ingot pulling apparatus to determine whether the apparatus is capable of producing a low oxygen content silicon ingot product. A sample rod is pulled from the silicon melt and the oxygen content of the sample rod is measured.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 308,877, filed February 10, 2022, which is incorporated by reference in its entirety.

[0002] The field of the disclosure relates to methods for producing ingot products from a silicon melt held in a crucible, and methods for evaluating ingot pulling equipment capable of producing ingot products of low oxygen content silicon. [Background technology]

[0003] Single crystal silicon, the starting material for most manufacturing processes of semiconductor electronic components, is commonly prepared by a method called the Czochralski (CZ) process, in which a single seed crystal is immersed in molten silicon and slowly withdrawn to grow. While contained in a quartz crucible, the molten silicon becomes contaminated with various impurities, primarily oxygen. Some applications, such as SOI RF, have relatively low specifications for oxygen concentration, such as 5.0 ppma or less. Summary of the Invention [Problem to be solved by the invention]

[0004] Without being bound to a particular theory, it has been found that ingot pullers having the same design (i.e., hot zone design) are characterized by different oxygen capabilities (i.e., ability to grow low oxygen ingots, such as ingots less than 5.0 ppma). A need exists for a method to characterize such silicon pullers to determine which ingot puller is best suited for producing low oxygen silicon ingots.

[0005] This section is intended to introduce the reader to various aspects of technology that may be related to various aspects of the disclosure 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 current disclosure. As such, it should be understood that these statements are not admissions of prior art, and are to be read in this light. [Means for solving the problem]

[0006] One aspect of the present disclosure is directed to a method of producing an ingot product from a silicon melt held in a crucible of an ingot pulling apparatus. Solid silicon is added to the crucible. A silicon melt is formed in the crucible by heating the solid silicon. A sample rod is pulled from the melt. The sample rod has a sample rod diameter that is less than 50 mm. An oxygen content of the sample rod is measured. When the oxygen content of the sample rod is below an oxygen content threshold, an ingot product is pulled from the melt. The ingot has a diameter. The sample rod diameter is less than the diameter of the ingot product.

[0007] Another aspect of the present disclosure is directed to a method of producing an ingot product having an oxygen concentration of less than 5 ppma. Solid silicon is added to a crucible of a first ingot puller. The solid silicon is heated in the crucible of the first ingot puller to create a first silicon melt in the crucible. A first sample rod is pulled from the first silicon melt. The first sample rod has a first sample rod diameter. An oxygen content of the first sample rod is measured. Solid silicon is added to a crucible of a second ingot puller. The solid silicon in the crucible of the second ingot puller is heated to create a second silicon melt in the crucible. A second sample rod is pulled from the second silicon melt. The second sample rod has a second sample rod diameter. An oxygen content of the second sample rod is measured. The oxygen content of the first sample rod is compared to the oxygen content of the second sample rod. The ingot product is grown in an ingot puller in which the low oxygen content sample rod is grown. The ingot product has a diameter. The first sample rod diameter and the second sample rod diameter are each smaller than the diameter of the ingot product.

[0008] Yet another aspect of the present disclosure is directed to a method of characterizing the oxygen capability of an ingot puller. The ingot puller includes a crystal growth chamber and a crucible disposed within the crystal growth chamber. Solid silicon is added to the crucible. A silicon melt is formed in the crucible by heating the solid silicon. A sample rod is pulled from the melt. The sample rod has a sample rod diameter that is less than 50 mm. The sample rod is processed into sample rod segments. The sample rod segments are analyzed by Fourier transform infrared spectroscopy. The oxygen content of the sample rod segments is measured.

[0009] There are various refinements of features associated with the above-mentioned aspects of the disclosure. Additional features may also be incorporated into the above-mentioned aspects of the disclosure. These refinements and additional features may exist individually or in any combination. For example, various features described below in relation to any of the described embodiments of the disclosure may be incorporated alone or in any combination into any of the above-mentioned aspects of the disclosure. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic side view of an ingot pulling apparatus for forming a single crystal silicon ingot.

[0011] [Diagram 2] FIG. 2 shows a sample rod grown from the silicon melt.

[0012] [Diagram 3] FIG. 3 is a schematic perspective view of a sample rod showing the two cut faces where the rod is cut to form the central slab.

[0013] [Figure 4] FIG. 4 is a schematic perspective view of a cut sample rod including a central slab.

[0014] [Diagram 5] FIG. 5 is a perspective view of one or more coins cut from the sample rod.

[0015] [Figure 6] FIG. 6 is a block diagram of one embodiment of a method for producing a silicon ingot product.

[0016] [Figure 7] FIG. 7 is a graph of the oxygen content of the sample rod versus the oxygen content of the ingot product.

[0017] [Figure 8] FIG. 8 is a graph showing the correlation between the oxygen content of sample rods of ingot product and the oxygen content of the ingot product for 200 mm and 300 mm grown on various ingot pullers.

[0018] [Figure 9] FIG. 9 is a graph showing the correlation between the oxygen content of sample rods of 200 mm ingot product grown on various ingot pullers and the product oxygen content of the ingots.

[0019] Corresponding reference characters indicate corresponding parts throughout the drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present disclosure provides a method for characterizing the oxygen capacity of an ingot puller and for producing a relatively low oxygen silicon ingot product. In accordance with an embodiment of the present disclosure, and with reference to FIG. 1, an ingot product is grown by the so-called Czochralski process, in which an ingot is pulled from a silicon melt 44 held in a crucible 22 of an ingot puller 23. The ingot puller 23 includes a housing 26 that defines a crystal growth chamber 16 and a pull chamber 20 that has smaller lateral dimensions than the growth chamber. The growth chamber 16 has a generally dome-shaped upper wall 45 that transitions from the growth chamber 16 to the narrower pull chamber 20. The ingot puller 23 includes an inlet port 7 and an outlet port 12 that can be used to introduce and remove process gas (e.g., argon) from the housing 26 during crystal growth.

[0021] The crucible 22 in the ingot pulling apparatus 23 contains a silicon melt 44 from which a silicon ingot is pulled. The silicon melt 44 is obtained by heating and melting solid silicon filled in the crucible 22. The crucible 22 is attached to a turntable 31 for rotating the crucible 22 about the central longitudinal axis X of the ingot pulling apparatus 23.

[0022] A heating system 39 (e.g., an electrical 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. Pat. No. 8,317,919. The heating system 39 is controlled by a control system (not shown) so that the temperature of the melt 44 is precisely controlled throughout the pulling process. Insulation (not shown) surrounding the heating system 39 may reduce the amount of heat lost through the housing 26. The ingot puller 23 may also include a heat shield assembly (not shown) above the melt surface to protect the ingot from the heat of the crucible 22 and increase the axial temperature gradient at the solid-melt interface.

[0023] A pulling mechanism (not shown) is attached to a pull wire 24 that extends downwardly from the mechanism. The mechanism can raise and lower the pull wire 24. The ingot puller 23 may have a pull shaft rather than a wire, depending on the type of pull. The pull wire 24 terminates in a pulling assembly 58 that includes a seed chuck 32 that holds a seed crystal 6 used to grow a silicon ingot. When growing an ingot, the pulling mechanism lowers the seed crystal 6 until it contacts the surface of the silicon melt 44. Once the seed crystal 6 begins to melt, the pulling mechanism slowly pulls the seed crystal through the growth chamber 16 and the pulling chamber 20 to grow a single crystal ingot. The speed at which the pulling mechanism rotates the seed crystal 6 and the speed at which the seed crystal is pulled (i.e., the pull speed v) are controlled by a control system.

[0024] Process gas is introduced into the housing 26 through inlet port 7 and withdrawn through outlet port 12. The process gas creates an atmosphere within the housing 26, where the melt and the atmosphere form a melt-gas interface. The outlet port 12 is in fluid communication with the ingot pulling exhaust system (not shown).

[0025] In this regard, the ingot pulling apparatus 23 shown in FIG. 1 and described herein is exemplary and other crystal pulling configurations and arrangements may be used to pull single crystal silicon ingots from the melt, unless expressly indicated.

[0026] According to an embodiment of the present disclosure, after solid silicon is added to the crucible 22 and the heating system 39 is activated to melt the solid silicon, a sample ingot or rod is pulled from the melt (step 100 in FIG. 6). An example of a sample rod 5 is shown in FIG. 2. The rod 5 includes a crown portion 21 that transitions from the seed and tapers outward to reach a target diameter. The rod 5 includes a constant diameter portion 25 or cylindrical body or simply "body" of a crystal that is grown by increasing the pulling rate. The body 25 of the sample rod 5 has a relatively constant diameter. The rod 5 includes a tail or end cone 29 that tapers in diameter after the body 25. Once the diameter is small enough, the rod 5 is separated from the melt. The rod 5 has a central longitudinal axis A that passes through the crown portion 21 and the terminal end 33 of the ingot.

[0027] The growth conditions of the sample rod 5 may be selected from generally any of the suitable growth conditions available to one of skill in the art. The sample rod 5 may be a single crystal with the body of the sample rod having zero displacement. The sample rod 5 may be grown with a fixed seed lift (i.e., a fixed pull rate where the diameter is varied, such as + / - about 5 mm) or with active seed lift (where the pull rate is varied to maintain a target diameter).

[0028] The sample rod 5 has a smaller diameter than the ingot product that is subsequently 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 ingot product. 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., about 5 mm to about 150 mm, about 5 mm to about 100 mm, about 5 mm to about 50 mm, about 5 mm to about 25 mm, or about 10 mm to about 25 mm). Generally, the diameter of the rod 5 is measured by measuring the rod along several axial positions (e.g., within a constant diameter portion of the rod if the rod has a crown and / or taper) 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 no greater 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 generally corresponds to the diameter of the neck of a crystal puller grown ingot product. For example, the rod may be less than 50 mm, less than 25 mm, or less than 20 mm in diameter.

[0030] The sample rod 5 may be of any suitable length. In some embodiments, the length of the rod (e.g., after trimming) is 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 (FIG. 6), after the sample rod 5 is grown, the sample rod is processed to form a sample rod segment, such as a central slab 40 (FIG. 4) or a coin 138 (FIG. 5). In an embodiment in which a slab 40 is formed, a wire saw may be used to remove the crown and tail of the sample rod 5. As shown in FIG. 4, the sample rod 5 is cut to form the slab 40. The sample rod 5 may be cut by a tabletop cutting machine (e.g., Minitom available from Struers, Westlake, Ohio) or using a diamond wire saw (e.g., DTW wire saw). The sample rod 5 is cut along a first cut surface 42 and cut along a second cut surface 46 to form a first cut portion 49 and a second cut portion 52 (FIG. 4) and a slab 40. The first cut surface 42 and the second cut surface 46 are parallel to each other and to a central longitudinal axis A of the sample rod 5. The slab 40 may have a suitable thickness suitable for oxygen measurements, 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 slab 40 may be generally square or rectangular in cross section. The first and second sides 62, 64 of the slab 40 may be slightly rounded due to the contour of the sample rod 5, or the slab 40 may be further cut away to form flat sides 62, 64. The first and second cut planes 57, 59 of the slab 40 may be polished to provide a flat surface.

[0032] Generally, the central slab 40 includes at least a portion of the central axis A of the untrimmed sample rod 5. In some embodiments, the trimming method may be varied to account for axial non-uniformity in the sample rod diameter so that the central slab 40 captures as much of the axisymmetric centerline of the rod 5 as possible. For example, the slab 40 may include at least about 10% of the central axis A of the sample rod 5 (i.e., the sample rod immediately prior to trimming to form the central slab), 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 rod 5. In some examples, after trimming, the central axis A of the trimmed sample rod 5 extends the entire length of the slab 40 (e.g., from the first end 54 to the second end 56 of the slab 40).

[0033] In some embodiments, the rod segments (e.g., slabs 40 or coins 138 described below) are cleaned, etched (such as with a mixed acid etch for at least 3 minutes), and dried prior to measuring the oxygen content of the slabs 40. In some examples, such as when the resistivity of the rod segments is also measured, the rod segments may be subjected to a rapid thermal anneal at a temperature between 500° C. and about 1000° C. (e.g., before or after the oxygen measurement).

[0034] In some embodiments, rather than forming a central slab 40, one or more coins 138 (FIG. 5) are formed from the sample rod (e.g., from a cross-section cut of the sample rod). As with the slabs, the one or more coins 138 may be polished, cleaned, and / or etched prior to measurement of oxygen content. Multiple coins may be cut from the same rod (e.g., seed end and opposite end) and compared to a threshold (averaged and compared to a threshold). The one or more coins may have a thickness of 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, respectively.

[0035] After the slab 40 or coin 138 has been processed, the oxygen content of the slab 40 or coin is measured in step 120 (FIG. 6). For example, the oxygen content may be measured by Fourier transform infrared spectroscopy (i.e., by analyzing the rod segment with a Fourier transform infrared spectrometer). FTIR spectrometers are commercially available, such as the QS-300 spectrometer from Bio-Rad (Hercules, Calif.).

[0036] Once the oxygen content of the sample rod has been measured, in step 130 (FIG. 6), the oxygen content may be compared to an oxygen content threshold. The oxygen content threshold is the maximum oxygen content (e.g., a previously established maximum oxygen content) that can produce a relatively low oxygen content ingot product. A sample ingot having an oxygen content above this maximum may not be suitable for forming a low oxygen ingot (regardless of control of growth parameters to a lower low oxygen content) and may be used to form an ingot other than a low oxygen ingot. For example, the oxygen content threshold for the oxygen content of the sample rod (measured from a slab or coin) may be up to 5.0 ppma, 4.0 ppma, 3.5 ppma, 3 ppma, or even less than 2.5 ppma (measured by ASTM 80).

[0037] If the oxygen content is below the oxygen content threshold, in step 140 (FIG. 6), an ingot product is pulled from the melt (i.e., the same melt from which the sample rod was grown). The diameter of the ingot product is greater than the diameter of the sample rod (i.e., the diameter of the constant diameter portion of the sample rod is less than the diameter of the constant diameter portion of the ingot). The ingot product may be 150 mm or greater in diameter, or in other embodiments, 200 mm or greater, or 300 mm or greater (e.g., 450 mm or greater). The ingot product has a relatively low oxygen content, such as less than 5.0 ppma, less than 4.0 ppma, less than 3.5 ppma, less than 3 ppma, or less than 2.5 ppma. The ingot product may have a suitable resistivity, and in some embodiments may have a relatively high resistivity of at least 1000 ohm-cm, or even at least 7500 ohm-cm.

[0038] In some embodiments, no polycrystalline silicon is added during the growth of the ingot (e.g., in a batch process), while in other embodiments, polycrystalline silicon is added to the melt as the ingot product grows (e.g., in a continuous Czochralski process).

[0039] When the sample rod has an oxygen content above the threshold, a first (initial) ingot product is not grown on the ingot puller (i.e., the first ingot puller). Instead, a second (second) ingot product with a higher oxygen specification is grown on the first ingot puller (i.e., the second ingot product has a higher oxygen content than the first ingot product). For example, the first ingot product may have an oxygen content less than 5 ppma and the second ingot product may have an oxygen content greater than 5 ppma.

[0040] The first ingot product may then be grown from a second ingot puller different from the first ingot puller. A second sample rod may be grown on the second ingot puller in step 200 (FIG. 6) to determine if the ingot puller is suitable for forming a relatively low oxygen ingot prior to growing the first ingot (by forming a slab or one or more coins from the sample rod in step 210 and measuring the oxygen content of the slab or one or more coins in step 220 and determining if the oxygen content is below the oxygen content threshold 230). If the oxygen content of the sample rod grown from the second ingot puller is not below the oxygen content threshold, subsequent ingot pullers may be tested until one is found (testing three, four, five, or more until a suitable ingot puller is found) in which the oxygen content of the sample rod grown from that ingot is below the oxygen content threshold. If the oxygen content of the sample rod grown from the second ingot puller is below the oxygen content threshold, then in step 240 a product ingot is allowed to grow.

[0041] The oxygen content threshold may be determined by growing multiple sample rods on different ingot pullers and comparing the oxygen content of each sample rod (e.g., average between the seed end and the opposite end) with the oxygen content of the ingot product (e.g., slag from the midpoint). The comparison allows for the determination of the maximum oxygen content of the sample rods in the ingot product having the required oxygen content (e.g., upper specification limit). A correlation between the oxygen content of the sample rods and the oxygen content of the ingot product may be formed as shown in Figures 8 and 9 and described below.

[0042] In some embodiments where multiple ingot pullers are available for growing relatively low oxygen ingot products (e.g., less than 5.0 ppma), a sample rod may be grown from the melt of each ingot puller (e.g., evaluating two, three, four or more ingot pullers). The oxygen content of each sample rod grown from each ingot puller may be compared. The ingot product (with the relatively low oxygen concentration target) may be grown in the ingot puller from which the sample rod with the lowest oxygen content was grown.

[0043] Once an ingot puller has been identified in which the sample rod has an oxygen content below the oxygen content threshold in addition to a first ingot product having a relatively low oxygen content (e.g., 5.0 ppma), the ingot puller may be used to grow additional ingots having relatively low oxygen content (2 or more, 3 or more, 5 or more, 10 or more ingots having an oxygen content below 5 ppma).

[0044] The disclosed method has several advantages over conventional methods for producing single crystal silicon ingots. Without being bound by any particular theory, it has been found that ingot pullers with the same design (i.e., hot zone design) are characterized by different oxygen capabilities (i.e., the ability to grow low-oxygen ingots, such as ingots below 5.0 ppma) with a wide range of variability. By growing sample ingots from one or more ingot pullers that are candidates for a relatively low-oxygen ingot production run, the oxygen content of the sample rods can be compared to the oxygen concentration threshold at which low-oxygen ingots can be produced. Alternatively or additionally, the oxygen content between sample rods from different pullers may be compared to determine which ingot puller can produce the lowest oxygen content in silicon ingot products. Some ingot pullers may not be able to achieve low-oxygen ingots even if the process conditions are controlled for low-oxygen production. This method reduces the wasted charge so that if the ingot puller proves unable to produce low oxygen ingot product, the ingot puller and melt may be reused for another customer product (i.e., other than low oxygen, such as >5.0 ppma). This method is particularly advantageous when it is desired to increase the throughput of low oxygen ingot production by installing additional ingot pullers for low oxygen ingot production. This method identifies if an ingot puller is suitable for low oxygen production in less than 24 hours and with a relatively small charge (e.g., less than 0.1 kg) used for characterization (including pulling a sample rod, processing the rod into rod segments such as slabs or coins, and measuring oxygen by FTIR). This is compared to the conventional method of growing a perfect zero dislocation ingot to determine oxygen capability. EXAMPLES

[0045] The processes of the present disclosure are further illustrated by the following examples, which should not be viewed in a limiting sense. Example 1: Sample Rod Oxygen Content as a Predictor of Ingot Product Oxygen

[0046] For four different ingot pullers, sample rods less than 50 mm in diameter were grown before the product ingots were grown. The oxygen content on the seed side and the opposite side of the sample rods was measured and averaged. The oxygen content of the product ingots was measured at the midpoint (i.e., where the oxygen is mostly stable and flat, rather than at a transition section such as the seed end or the opposite end). Ingot puller AC was a 200 mm diameter puller and ingot puller D was a 300 mm diameter puller.

[0047] As shown in Figure 7, the oxygen concentration of the ingot products correlates well with the sample rods grown before the ingot products. Except for a few data points, the oxygen capabilities of each puller cluster within a window that reflects the capabilities of the characterization method. In terms of operation, puller A has the best overall yield and throughput (i.e., the number of "major" inches produced by the ingot puller per unit time) versus the oxygen specification requirements of RF SOI, while puller D has the worst yield, validating the responsiveness of the sample rods.

[0048] Figure 8 shows the relationship between slag and sample rod oxygen for mid-length ingot products by plotting the ratio of oxygen (sample rod average - slag) / sample rod average versus the average oxygen in the sample rod for all data (200 mm (puller AC) and 300 mm (puller D)). Figure 9 shows the plot for the 200 mm puller and includes the best fit trend line.

[0049] The best fit curves shown in Figure 9 have a fairly high least squares fit, which means that the oxygen in the sample rod correlates fairly well with the oxygen in the slag in the stable oxygen region of the ingot product. The oxygen in this range is mainly determined by the high evaporation of SiO gas and the low melting of the crucible. Puller A has low oxygen in both the sample rod and the slag, which means that this puller has different hot spots on the crucible wall and evaporation rate of SiO gas compared to the other pullers.

[0050] As shown in Figure 8, the 300mm crystal data has more than 7ppma of sample rod oxygen, and the ratio of oxygen difference between the sample rod and slug is lower than that of the 200mm case, while the oxygen of the sample rod and the slug of the ingot product is higher than that of the 200mm case. This means that even though the oxygen taken up in the 300mm grown crystal is higher than that of the 200mm crystal, the oxygen of the sample rod is much higher than that of the 200mm sample rod, so the ratio of oxygen difference of the 300mm crystal will be lower than that of the 200mm crystal with similar sample rod oxygen. In both ingot diameters, the crucible diameter is the same, which means that the 300mm ingot has less free melting surface than the 200mm ingot, which reduces the evaporation of SiO gas. As a result, the oxygen of the slug of the ingot product increased. Furthermore, the diameter of the reflector above the melt was different between 300 mm and 200 mm, and the radial gas velocity above the free surface of the melt during sample rod growth was slower than that of the 200 mm sample rod, resulting in a higher oxygen concentration in the sample rod.

[0051] As shown in Figure 9, this prediction is valid when the hot zone configuration and diameter of the ingot product are the same. Furthermore, the relationship between the oxygen of the sample rod and the oxygen of the ingot product slag may be utilized to confirm and predict the oxygen at steady state of the ingot product. The variability in the data shown in Figures 8 and 9 may be caused by differences in the degradation of the hot zone such as heaters and insulation. However, the relationship holds, with higher oxygen in the sample rod indicating higher oxygen in the main body section of the ingot product.

[0052] As used herein, the terms "about," "substantially," "essentially," and "approximately" when used in connection with a range of dimensions, concentrations, temperatures, or other physical or chemical properties or characterizations are meant to cover variations that may exist at the upper and / or lower limits of the range of the property or characterization, including, for example, variations due to rounding, measurement methods, or other statistical variations.

[0053] When introducing elements of the disclosure or embodiments thereof, the terms "a," "an," "the," and "said" are intended to mean that there are one or more elements. The terms "comprising," "including," "containing," and "having" are inclusive and mean that there may be additional elements other than the listed elements. The use of specific directional terms (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require a particular orientation of the articles being described.

[0054] Because various changes may be made in the above structures and methods without departing from the scope of the present disclosure, all matter contained in the above description and shown in the accompanying drawings is intended to be interpreted in an illustrative sense and not in a limiting sense.

Claims

1. 1. A method for producing an ingot product having an oxygen concentration of less than 5 ppma, comprising: adding solid silicon to the crucible of the first ingot puller; heating the solid silicon in the crucible of the first ingot puller to form a first silicon melt in the crucible; Pulling a first sample rod from the first silicon melt, wherein the first sample rod has a first sample rod diameter; measuring the oxygen content of the first sample rod; adding solid silicon to the crucible of the second ingot puller; heating the solid silicon in the crucible of the second ingot puller to form a second silicon melt in the crucible; pulling a second sample rod from the second silicon melt, wherein the second sample rod has a second sample rod diameter; measuring the oxygen content of the second sample rod; comparing the oxygen content of the first sample rod with the oxygen content of the second sample rod; growing an ingot product in an ingot puller that has grown a sample rod having a low oxygen content, the ingot product having a diameter, and each of the first sample rod diameter and the second sample rod diameter being smaller than the diameter of the ingot product; A method comprising:

2. adding solid silicon to the crucible of the third ingot puller; heating the solid silicon in the crucible of the third ingot puller to form a third silicon melt in the crucible; pulling a third sample rod from the third silicon melt, wherein the third sample rod has a sample rod diameter; measuring the oxygen content of the third sample rod; comparing the oxygen content of the first sample rod with the oxygen content of the third sample rod; growing the ingot product in the ingot pulling apparatus in which the sample rod having the lowest oxygen content among the first, second, and third sample rods was grown; The method of claim 1 further comprising:

3. the ingot product is a first ingot product, and the method includes growing a second ingot product on the ingot puller from which the sample rod having a higher oxygen content was grown. The method of claim 1.

4. a first slab is formed from the first sample rod; measuring the oxygen content of the first sample rod includes measuring the oxygen content of the first slab; a second slab is formed from the second sample rod; Measuring the oxygen content of the second sample rod includes measuring the oxygen content of the second slab. The method of claim 1.

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

6. The second ingot product has an oxygen content of more than 5 ppma. The method of claim 3.

7. The first slab includes at least a portion of the central axis of the first sample rod and has a thickness between about 5 mm and about 0.1 mm; the second slab includes at least a portion of the central axis of the second sample rod and has a thickness between about 5 mm and about 0.1 mm; The method of claim 4.

8. The method of claim 7, further comprising: etching the first slab before measuring the oxygen content of the first sample rod. The method of claim 4.

9. The method of claim 8, further comprising: etching the second slab prior to measuring the oxygen content of the second sample rod. The method of claim 8. forming one or more coins from the first sample rod; measuring the oxygen content of the first sample rod includes measuring the oxygen content of one or more of the coins; The method of claim 1.

11. The method of claim 10, further comprising etching the one or more coins prior to measuring the oxygen content of the sample rod. The method of claim 10. forming one or more coins from the second sample rod; measuring the oxygen content of the second sample rod includes measuring the oxygen content of one or more of the coins; The method of claim 10.

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

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

Images