Growth of multiple sample rods to determine impurity accumulation during the production of single crystal silicon ingots

Abstract

A method for modeling impurity concentrations in a single crystal silicon ingot is provided. A first sample rod is pulled from a silicon melt disposed in a crucible. The first sample rod has a first sample rod diameter of less than 100 mm. A first sample rod parameter related to the impurity content of the first sample rod is measured. A second sample rod is pulled from the silicon melt. The second sample rod has a first sample rod diameter of less than 100 mm. A second sample rod parameter related to the impurity content of the second sample rod is measured. An impurity concentration of the single crystal silicon ingot is determined based at least in part on the measured first sample rod parameter and the measured second sample rod parameter.

Description

[Technical field]

[0001] This application claims priority to U.S. Patent Application No. 16 / 020,701, filed June 27, 2018, the entire disclosure of which is incorporated herein by reference in its entirety.

[0002] The field of the disclosure relates to a method for forming a single crystal silicon ingot in which a plurality of sample rods are grown from a melt, and a parameter related to the impurity concentration of the melt or ingot is measured. In some embodiments, the sample rods each have a diameter smaller than the diameter of the product ingot. [Background technology]

[0003] Single crystal silicon, the starting material for most processes in the manufacture of semiconductor electronic components, is usually prepared by the so-called Czochralski (CZ) process, in which a single seed crystal is immersed in molten silicon and slowly extracted and grown. The molten silicon, while contained in a quartz crucible, is contaminated with various impurities, mainly oxygen. Some applications, such as advanced wireless communication applications, insulated gate bipolar transistors (IGBTs), and low-power, low-leakage devices, require wafers with relatively high resistivity, such as 1500 ohm-cm (Ω-cm) or higher.

[0004] High-purity polysilicon is used for high-resistivity ingot production. High-purity polysilicon may be contaminated with impurities from the crucible, hot zone configuration, and / or process gases circulating throughout the ingot puller. These impurities complicate the production of high-resistivity ingots and may vary over the length of the run, reducing the portion of the ingot with the lowest target resistivity (e.g., reducing the "prime" portion of the ingot).

[0005] A need exists for a method of preparing high resistivity silicon ingots that allows for relatively rapid sampling of the impurity concentration or resistivity of a polysilicon starting material over time and / or allows for relatively rapid measurement of impurities with relatively small amounts of silicon consumed.

[0006] This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, 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 disclosure. As such, it should be understood that these statements are to be read in this light, and not as admissions of prior art. Summary of the Invention

[0007] One aspect of the disclosure relates to a method for producing a single crystal silicon ingot from a silicon melt held in a crucible. Polycrystalline silicon is added to the crucible. The polycrystalline silicon is heated to form a silicon melt in the crucible. A first sample rod is pulled from the melt. The first sample rod has a first sample rod diameter. A first sample rod parameter related to a quality of the first sample rod and / or the silicon melt is measured. A second sample rod is pulled from the melt. The second sample rod has a second sample rod diameter. A second sample rod parameter related to a quality of the second sample rod and / or the silicon melt is measured. A product ingot is pulled from the melt. The product ingot has a diameter. The diameter of the first sample rod and the diameter of the second sample rod are smaller than the diameter of the product ingot.

[0008] Another aspect of the disclosure relates to a method for modeling an impurity concentration of a single crystal silicon ingot. A first sample rod is pulled from a silicon melt disposed in a crucible. The first sample rod has a first sample rod diameter less than 100 mm. A first sample rod parameter related to an impurity content of the first sample rod is measured. A second sample rod is pulled from the silicon melt. The second sample rod has a first sample rod diameter less than 100 mm. A second sample rod parameter related to an impurity content of the second sample rod is measured. An impurity concentration of the single crystal silicon ingot is determined based at least in part on the measured first sample rod parameter and the measured second sample rod parameter.

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

[0010] [Figure 1] FIG. 1 is a schematic side view of a pulling apparatus for forming a single crystal silicon ingot. [Diagram 2] A sample rod grown from a silicon melt. [Diagram 3] A sample rod having a planar segment formed on its surface. [Figure 4] This is a measuring device for measuring the resistivity of a sample rod. [Diagram 5] 1 is an IV curve used to measure the resistivity of a sample rod. [Figure 6] FIG. 1 is a scatter plot of the calculated boron / phosphorus content of silicon melts for several short ingots. [Figure 7] FIG. 13 is a scatter plot of resistivity of sample rods at various positions from the seed end. [Figure 8] FIG. 13 is a scatter plot of resistivity of the phosphorus-doped short ingot, sample rod, and product ingot of Example 4. [Figure 9] FIG. 13 is a scatter plot of resistivity of the boron-doped short ingot, sample rod, and product ingot of Example 4. [Figure 10] FIG. 13 is a scatter plot of resistivity of a short ingot, a sample rod, and a product ingot of Example 4 in which the type was changed from P type to N type. [Figure 11] FIG. 13 is a scatter plot of resistivity of the first sample rod, the short ingot, and the second sample rod of Example 5. [Figure 12] 13 is a trend of phosphorus contamination time calculated based on the resistivity of the first sample rod, the short ingot, the second sample rod, and the product ingot of Example 5.

[0011] Corresponding reference characters indicate corresponding parts throughout the drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Provisions of the present disclosure relate to a method of producing a single crystal silicon ingot by the Czochralski method, in which a plurality of sample rods are grown to determine one or more parameters related to melt impurities over time (e.g., impurities added by melting in a crucible). In some embodiments, the parameters are used to predict impurities in a subsequently grown product ingot. The sample rods have a smaller diameter than the product ingot (e.g., a diameter of less than about 150 mm, less than about 100 mm, less than about 50 mm, or less than about 25 mm).

[0013] 1, in accordance with an embodiment of the present disclosure, the product ingot 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 a smaller lateral dimension 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 narrowed 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 gases into and from the housing 26 during crystal growth.

[0014] A crucible 22 within the ingot puller 23 contains a silicon melt 44 from which silicon ingots are drawn. The silicon melt 44 is obtained by melting a charge of polycrystalline silicon in the crucible 22. The crucible 22 is mounted on a turntable 31 for rotating the crucible 22 about a central longitudinal axis X of the ingot puller 23.

[0015] A heating system 39 (e.g., an electrical resistance heater) surrounds the crucible 22 to melt the silicon charge to produce a melt 44. The heating system 39 can 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 can reduce the amount of heat lost through the housing 26. The ingot puller 23 can also include a heat shield assembly (not shown) above the melt surface to shield the ingot from the heat of the crucible 22 to increase the axial temperature gradient at the solid-melt interface.

[0016] A pulling mechanism (not shown) is attached to a pull wire 24 that extends downward from the mechanism. The mechanism can raise and lower the pull wire 24. The ingot puller 23 can have a pull shaft rather than a wire, depending on the type of puller. 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. In 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 raises the seed crystal through the growth chamber 16 and pulls the 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 pulling mechanism raises the seed crystal (i.e., the pulling speed v) are controlled by a control system.

[0017] 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, and the melt and the atmosphere form a melt-gas interface. The outlet port 12 is in fluid communication with the ingot puller's exhaust system (not shown).

[0018] In this regard, the ingot puller 23 shown in FIG. 1 and described herein is exemplary, and other crystal puller configurations and arrangements can be used to pull single crystal silicon ingots from the melt, unless otherwise specified.

[0019] According to an embodiment of the present disclosure, after polycrystalline silicon is added to the crucible 22 and the heating system 39 is operated to melt the polycrystalline silicon, a number of sample ingots or rods are pulled from the melt. Generally, the number of sample rods have a smaller diameter than the product ingot that will subsequently be grown (e.g., less than about 150 mm, less than about 100 mm, less than about 50 mm, less than about 25 mm). An example of a sample rod 5 is shown in FIG. 2. The rod 5 includes a crown 21 where the rod transitions and tapers outward from the seed to reach a target diameter. The rod 5 includes a constant diameter portion 25 or cylindrical body or simply "body" of the 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 where the rod tapers in diameter after the body 25. Once the diameter is small enough, the rod is separated from the melt. The rod 5 has a central longitudinal axis A that extends through the crown 21 and end 33 of the ingot.

[0020] After the first sample rod is pulled from the melt, a first sample rod parameter related to the quality of the first sample rod and / or the silicon melt is measured. The measured parameter may be related to the impurity composition of the rod, such as phosphorus concentration, boron concentration, total impurity concentration, resistivity of the sample rod, or other impurity concentrations (e.g., aluminum, gallium, arsenic, indium, antimony, etc.).

[0021] According to an embodiment of the present disclosure, after the first rod is produced, a second sample rod is pulled from the melt. The second sample rod may have the same diameter and / or length as the first rod, or a rod of a different diameter and / or length may be produced. A second sample rod parameter related to the quality of the second sample rod and / or the silicon melt is measured (e.g., a parameter related to impurities). The first sample rod parameter and the second sample rod parameter may be the same parameter, or, as in other embodiments, may be different parameters (e.g., the impurity content of the first rod is measured and the resistivity of the second rod is measured).

[0022] The growth conditions for the first and second sample rods can generally be selected from any suitable growth conditions available to one of skill in the art. The sample rods can be grown with locked seed lift (i.e., a fixed pull rate for various diameters, such as + / - about 5 mm) or active seed lift (a pull rate that varies to maintain a target diameter).

[0023] In some embodiments, polycrystalline silicon is not added to the crucible after the first sample rod is grown, before the second sample rod is grown, and / or before the product ingot is subsequently grown. Alternatively, or in addition, in some embodiments, a product ingot (e.g., an ingot having a diameter larger than the sample rod, such as a 200 mm, 300 mm, 450 mm diameter ingot) is not grown between the growth of the first and second sample rods.

[0024] In some embodiments, the diameter of both the first and second sample rods 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 tapered end) and averaging the measured diameters (e.g., measuring and averaging 2, 4, 6, 10 or more diameters over the entire length). In some embodiments, the maximum diameter of the rod sample 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).

[0025] The sample rod 5 can 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).

[0026] In some embodiments, the first and / or second sample rod parameters related to the quality of the sample rod and / or silicon melt can be parameters related to impurities of the melt, for example, boron concentration, phosphorus concentration, total impurity concentration, or resistivity of the sample rod can be measured.

[0027] In some embodiments, the impurity concentration is measured by Fourier transform infrared spectroscopy (e.g., low temperature FTIR) or photoluminescence. Such methods can measure impurities typical of high or ultra-high resistivity applications (e.g., 1×10 12 atoms / cm 3Relatively low impurity concentrations can be measured, such as concentrations of impurities less than 1000 nm. The method may involve the determination of the overall net carrier concentration (i.e., resistivity) or the concentration of a specific impurity (e.g., boron or phosphorus).

[0028] In some embodiments, the resistivity of the first and / or second sample rods is measured. The rod 5 is removed from the ingot puller 23 and processed so that the resistivity can be measured. The crown and tail of the ingot can be removed, such as by using a wire saw. In some embodiments, the trimmed ends of the rod 5 are polished to make the ends flat. The ends of the rod can be etched (e.g., mixed acid etch). The rod 5 can be modified to include ohmic contacts, such as ohmic contacts, at its first and second ends 15, 17. For example, the cut ends 15, 17 of the rod 5 can be painted with colloidal silver paint and dried.

[0029] A planar segment 11 (FIG. 3) is formed on a surface of the rod 5. The planar segment 11 can extend axially along the rod 5. In some embodiments, the planar segment 11 extends axially from the first end 15 to the second end 17 of the rod 5. In other embodiments, the planar segment 11 extends only partially along its length.

[0030] The planar segment 11 can be formed by grinding the surface of the sample rod 5, such as by using a grinding pad (e.g., a diamond grit pad). In some embodiments, the planar segment has a width (e.g., about 2-4 mm) sufficient to allow contact with a voltage probe. The planar segment 11 can be cleaned, such as by rinsing with deionized water and drying, prior to resistivity measurements.

[0031] In some embodiments, the sample rod 5 is subjected to a rapid thermal anneal before measuring the resistivity. The rapid thermal anneal may act as a thermal donor kill cycle (i.e., annihilation of the thermal donor) by dissociating the interstitial oxygen clusters. In some embodiments, the anneal is performed at a temperature of about 500° C. or higher, about 650° C. or higher, or about 800° C. or higher (e.g., from about 500° C. to about 1000° C., from about 500° C. to about 900° C., or from about 650° C. to about 1100° C.) for at least about 5 seconds, at least about 30 seconds, at least about 1 minute, or at least about 3 minutes or more (e.g., from about 5 seconds to 15 minutes, from about 5 seconds to about 5 minutes, or from about 5 seconds to about 3 minutes).

[0032] The resistivity of the rod 5 can be measured from the planar segment 11. In some embodiments of the present disclosure, a current is passed through the rod 5 and a resistivity probe is contacted at one or more locations along the length of the rod 5. The current can be applied to the rod 5 through one of the ends 15, 17.

[0033] In some embodiments, the rod 5 is fixed in a measuring device, such as the device 43 shown in FIG. 4. The measuring device 43 includes a clamp 51 that fixes the rod 5. The clamp 51 has a first support 53 that fixes the rod 5 towards the first end 15 and a second support 55 that fixes the rod 5 towards the second end 17. The supports 53, 55 are configured to hold the rod (e.g., threaded for adjustment and clamping). The supports 53, 55 may contact ohmic contacts on the trimmed end of the rod 5. A probe tip 61 is brought into contact with the rod 5 on a flat planar segment of the rod. A current is passed through the supports 53, 55 and a voltage is measured by the probe tip 61. The probe tip 61 is manually moved down the axis of the neck 5 with the current / voltage applied at each point being measured. In the illustrated device 43, the probe tip 61 is moved manually. In other embodiments, the probe tip 61 is moved by an actuator.

[0034] 4 is an exemplary apparatus, and any suitable apparatus for fixing and / or measuring the resistivity of a rod may be used unless otherwise specified. Use of a rod (e.g., a rod of generally small diameter, such as less than 100 mm, 50 mm, or less than 25 mm) and measuring apparatus 43 allows the resistivity to be measured without slicing the rod into wafers or slugs.

[0035] The resistivity probe can be a two-point probe with both probe tips in contact with the planar segment 11. A voltage difference is measured between the two probe tips. For example, resistivity can be measured with a two-point probe in accordance with SEMI Standard MF397-0812, entitled "Test Method for Resistivity of Silicon Bars Using a Two-Point Probe", which is incorporated herein by reference for all relevant consistent purposes. To determine the crystal type (i.e., N-type or P-type), a two-terminal or three-terminal rectification method can be used. Such type determination can be performed in accordance with SEMI Standard MF42-0316, entitled "Test Method for Conductivity Type of Extrinsic Semiconductor Materials", which is incorporated herein by reference for all relevant consistent purposes. Both the two-terminal and three-terminal rectification methods are robust methods for very high resistivity silicon.

[0036] The voltage can be measured at various points along the length, and the measured voltage, along with the length and average diameter of the sample, can be used to calculate the resistivity, such as by determining the slope of the current-voltage curve (e.g., Example 1 below).

[0037] After the first and second sample rods are analyzed (e.g., a parameter such as impurity concentration or resistivity is measured), the measured parameters of the first and second sample rods can be compared. The difference in the parameter provides time-dependent information about the quality of the melt. For example, the rate at which the parameter changes can be measured (e.g., change in impurity concentration or resistivity over time).

[0038] Additional sample ingots can be grown to provide further time-related information related to growth conditions (e.g., impurities in the melt). In some embodiments, a third, fourth, fifth, sixth, seventh, eighth, ninth and / or tenth sample rods are pulled from the melt (e.g., each rod has a diameter of less than about 150 mm, less than about 100, less than about 50 mm, less than about 25 mm, or less than about 20 mm). A sample rod parameter related to the impurity content of each sample rod pulled from the melt can be measured.

[0039] In some embodiments, the first and second sample rods (and optionally each subsequent rod) have a relatively low oxygen content, such as an oxygen content of less than about 5.5 ppma. In other embodiments, the oxygen content of each sample rod is less than 5.2 ppma, less than 5.0 ppma, less than 3.5 ppma, less than about 3 ppma, or even less than about 2.5 ppma. In some embodiments, the sample rod 5 is free of dislocations.

[0040] In some embodiments, the product ingot is grown from the melt after the sample rod is grown. The diameter of the first sample rod and the diameter of the second sample rod are smaller than the diameter of the product ingot. For example, the diameter of the sample rod can 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.

[0041] In some embodiments, the rod 5 has a diameter that corresponds approximately to the diameter of the neck portion of the 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.

[0042] In some embodiments, polycrystalline silicon is not added during the growth of the product ingot (e.g., as in a batch process). Additional sample rods can be grown after the product ingot is grown and the crucible is refilled with polycrystalline silicon. In other embodiments, polycrystalline silicon is added to the melt as the product ingot is grown (e.g., as in a continuous Czochralski process).

[0043] The measured first sample rod parameters and the measured second sample rod parameters can be used to adjust the growth conditions of one or more subsequently grown product ingots (e.g., a second melt formed in a crucible after the first product ingot is grown) pulled from the product ingot and / or other melts. For example, the growth conditions of the product ingots can be altered based at least in part on the measured first sample rod parameters and / or second sample rod parameters. Examples of growth conditions include hot zone configuration, side heater power, heater geometry, crucible liner thickness, dopant concentration, impurity and dopant evaporation, gas flow rate, and / or chamber pressure. In some embodiments, the first sample rod parameters and / or second sample rod parameters are correlated to crucible melting (e.g., impurity input during crucible melting), hot zone contamination, or gas flow impurities.

[0044] In some embodiments, a parameter related to the impurity content of a single crystal silicon ingot (e.g., a product ingot pulled from the same melt as the sample rod or a product ingot pulled from a second melt) is determined (e.g., modeled) at least in part from the first and second sample rod parameters. For example, the impurity concentration or resistivity of the product ingot is determined at least in part from the rate of change of the difference between the first sample rod parameters and the second sample rod parameters. Alternatively, or in addition, a parameter related to the impurity of the product ingot may be determined from a model in which the first sample rod parameters and the second sample rod parameters are input into the model. In general, any model available to one skilled in the art for predicting the impurity of a product ingot may be used, such as a model using the time-dependent change in the impurity of the melt.

[0045] In some embodiments, the measured first sample rod parameters and the measured second sample rod parameters are used to adjust the amount of dopant added to the polycrystalline silicon melt. An amount of dopant can be added to the melt before growing the first sample rod, and an amount of dopant (e.g., the same dopant or a different dopant) can be added after growing the first sample rod and before growing the second sample rod. In some embodiments, an amount of dopant is added after growing the second sample rod and before growing the product ingot. In other embodiments, no dopant is added before growing either the first or second sample rod.

[0046] The polysilicon from which the first and second sample rods and the product ingot are pulled may be semiconductor grade polysilicon. When using semiconductor grade polysilicon, in some embodiments, the polysilicon has a resistivity greater than 4,000 Ω-cm and contains 0.02 ppba or less of boron or phosphorous.

[0047] After the sample rod is pulled and optionally dopants are added to the melt, a product ingot is pulled from the melt. The product ingot has a diameter greater than the diameters of the first and second sample rods (i.e., the diameters of the constant diameter portions of the first and second sample rods are less than the diameter of the constant diameter portion of the ingot). The product ingot may have a diameter of about 150 mm, or, in other embodiments, a diameter of about 200 mm, about 300 mm or more (e.g., 450 mm or more).

[0048] In some embodiments, the melt is relatively pure and / or compensated to achieve a relatively high resistivity product ingot, in some embodiments, the product ingot has a resistivity of at least about 1,500 Ω-cm, or, in other embodiments, at least about 2,000 Ω-cm, at least about 4,000 Ω-cm, at least about 6,000 Ω-cm, at least about 8,000 Ω-cm, at least about 10,000 Ω-cm, or from about 1,500 Ω-cm to about 50,000 Ω-cm, or from about 8,000 Ω-cm to about 50,000 Ω-cm. Alternatively, or in addition, the first and second sample rods may have a resistivity of at least about 1,500 Ω-cm, or at least about 2,000 Ω-cm, at least about 4,000 Ω-cm, at least about 6,000 Ω-cm, at least about 8,000 Ω-cm, at least about 10,000 Ω-cm, from about 1,500 Ω-cm to about 50,000 Ω-cm, or from about 8,000 Ω-cm to about 50,000 Ω-cm.

[0049] Compared with conventional methods for producing monocrystalline silicon ingots, the disclosed method has several advantages. By growing first and second sample rods and measuring parameters related to changes in impurities of the melt (e.g., impurities added by crucible melting), time-dependent information (e.g., impurity changes) can be determined over time. By growing a sample rod with a relatively small diameter (e.g., less than about 100 mm, less than about 50 mm, or less than about 25 mm compared to a product ingot with a diameter of 200 mm or more), the addition of polycrystalline silicon during the growth of the rod can be eliminated, and changes to impurities due to the addition of polycrystalline silicon can be eliminated. This allows the impurity contribution effects of crucible melting and / or hot zone contamination to be isolated.

[0050] The relatively small diameter sample rods consume a relatively small amount of melt (e.g., less than 1 kg, less than 0.5 kg, or about 0.25 kg or less, compared to a full diameter short ingot that may consume 15 kg, 20 kg, or 50 kg or more of melt), reducing the accumulation of impurities due to the sampling process (e.g., separation). Multiple sample rods can be produced with negligible variation in melt depth within the crucible. Sample rods can be grown relatively quickly (e.g., about 12, 10, or 5 hours or less, compared to a full size short ingot that may involve growth times of 20, 30, 40, or 50 hours).

[0051] In embodiments where relatively pure polysilicon is used to produce relatively high resistivity product ingots, the impurity content (e.g., boron and / or phosphorus) can be measured using transform infrared spectroscopy or photoluminescence.

[0052] In embodiments where the resistivity of a sample rod is measured to determine the impurity concentration and / or resistivity of the melt and / or product ingot, the resistivity can be measured relatively quickly (e.g., 8 hours or less or 6 hours or less) directly from the rod by a four-point or two-point probe, providing a near real-time indirect resistivity measurement that may change due to impurity accumulation over time. example

[0053] The processes of the present disclosure are further illustrated by the following examples, which should not be viewed in a limiting sense. Example 1: Determining resistivity from an IV curve

[0054] The voltage on the sample rod was measured axially (e.g., using the apparatus of Figure 4) and the applied current and measured voltage were recorded. Figure 5 shows the I-V curve that was generated. Using the sample geometry and the slope of the I-V curve, the resistivity of the sample was determined to be 6139 ohm-cm. Example 2: Estimated boron and phosphorus content of melted and molten crucible walls

[0055] Figure 6 shows the calculated phosphorus / boron content of silicon melts for a number of short ingots produced prior to the production of product ingots. The P / B was calculated based on a model of resistivity behavior by measuring the resistivity of the short ingots, adding known amounts of P / B dopant to the melt, and measuring the resistivity of the product ingots. Using these parameters, the starting P / B concentration from the starting polysilicon charge to the crucible melt can be calculated.

[0056] Of the data points shown in Figure 6, 10-20% of the data points showed additional P / B content to the P / B content of the initial polysilicon charge. This indicates that the boron and phosphorus concentrations are generally affected by the melting of the crucible. The unclear time dependence in Figure 6 indicates that the crucible contribution may vary by lot and process time. Example 3: Comparison of short ingot and sample rod

[0057] A short sample ingot of single crystal ("short ingot") with a diameter approximately the same size as the product rod (e.g., about 200 mm for a 200 mm puller) was grown in a puller similar to Figure 1. The crystal was trimmed and subjected to mixed acid etching (MAE). The crystal slug was rapid thermal annealed at 800 °C for 3 min and lapped. The slug was contacted with a four-point probe to measure resistivity, which was averaged over three measurements.

[0058] Sample rods ("sample rods") were grown in rock seed lift mode on the same pulling apparatus after the short ingots were grown. The diameter of the rods varied throughout their length, ranging from 17 to 23 mm, with an average of 20 mm. The sample rods were trimmed and polished to form flat segments extending from one end of the rod to the other. The rods were rapid thermal annealed at 800 °C for 3 minutes. The resistivity of the ingots was measured using a measuring apparatus similar to that shown in Figure 4 and a two-point probe. The differences in growth conditions are listed in Table 1 below.

[0059] TIFF0007678833000001.tif55135

[0060] Table 1: Growth conditions for sample ingots with a diameter of 200 mm and sample rods with a diameter of approximately 17–23 mm.

[0061] The resistivity measured over the length of the sample rod and the resistivity of the slag from the sample ingot are shown in Figure 7.

[0062] The sample preparation time for the short ingots was 26 h and included trimming, mixed acid etching, rapid thermal annealing, slab cutting, grinding (e.g., with diamond pad), lapping, and four-point probe measurements, whereas the sample preparation time for the sample rods was 6 h and included trimming, mixed acid etching, rapid thermal annealing, flat grinding (e.g., with diamond pad), lapping, and two-point probe measurements. Example 4: Silicon melt resistivity trend

[0063] Short ingots with the same diameter (200 mm) as the product ingots were grown from the silicon melt (doped with phosphorus for N-type). Sample rods with a diameter of about 17 mm were then grown, followed by the growth of the product ingots. The resistivities of the slags of the short ingots and the product ingots were measured with a four-point probe.

[0064] The end of the sample rod was cut to form a planar segment on the rod. The resistivity along the planar segment of the sample rod was measured by a two-point probe. The resistivity of the slug and the change in resistivity along the sample rod are shown in Figure 8.

[0065] Short ingots, sample rods, and product ingots were grown for P-doped melts (boron doped) for which the same resistivity measurements were performed. The resistivity of the short ingot, the change in resistivity along the sample rod, and the resistivity of the product ingot are shown in Figure 9.

[0066] A short ingot, a sample rod, and a product ingot were grown for the second P-doped melt (boron doped) and the same resistivity measurements were performed. The resistivity of the short ingot, the change in resistivity along the sample rod, and the resistivity of the product ingot are shown in Figure 10. As shown in Figure 10, the crystal type was inverted to N-type between the sample rod and the slug of the product ingot.

[0067] As shown in Figures 8-10, the resistivity of the melt is not at a steady state over time, and the resistivity of the product ingot is affected by changes in impurity content (e.g., due to melting of the crucible). The resistivity trends of the sample rods are consistent with the resistivity of the product ingots. The change in resistivity of the sample rods changed over time rather than due to changes in g-value because the sample rods were grown with negligible differences in g-value between both ends of the sample rod. Growing multiple sample rods at different time intervals makes the seed end resistivity target of the product ingot more valid and improves the axial resistivity of the product ingot. Example 5: Phosphorus accumulation shown in multiple sample rods

[0068] A boron doped silicon melt was prepared and multiple sample rods and short ingots were pulled from the melt. A first sample rod (approximately 17 mm) was grown, followed by a short ingot. A second sample rod (approximately 17 mm) was grown after the short ingot. Figure 11 shows the resistivity of the sample rod and the short ingot. A type change occurred between the first sample rod and the short ingot. Figure 12 shows the phosphorus accumulation calculated from each of the first sample rod, the short ingot, the second sample rod, and the product ingot. As shown in Figure 12, the temporal change from the first sample rod to the second sample rod provides a more accurate estimate of the phosphorus accumulation in the melt over time.

[0069] As used herein, the terms "about," "substantially," "essentially," and "approximately," when used in conjunction with a range of a dimension, concentration, temperature, or other physical or chemical property or characteristic, are meant to cover the variation that may exist at the upper and / or lower limits of the range of the property or property, including, for example, variation due to rounding, measurement method, or other statistical variation.

[0070] When introducing elements of the disclosure or embodiments thereof, the articles "a," "an," "the," and "said" are intended to mean that there are one or more elements. The terms "comprise," "include," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular direction (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require a particular orientation of the items being described.

[0071] Since various changes may be made in the above structures and methods without departing from the scope of the present disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings be interpreted in an illustrative and not a limiting sense.

Claims

1. 1. A method for modeling an impurity concentration in a single crystal silicon ingot, the method comprising: pulling a first sample rod from a silicon melt disposed in a crucible, the first sample rod having a first sample rod diameter of less than 100 mm; measuring a first sample rod parameter related to an impurity content of the first sample rod, the first sample rod parameter being selected from the group consisting of phosphorus concentration, boron concentration, aluminum concentration, gallium concentration, arsenic concentration, indium concentration, antimony concentration, total impurity concentration, dopant concentration, and resistivity of the first sample rod; pulling a second sample rod from the silicon melt, the second sample rod having a second sample rod diameter of less than 100 mm; measuring a second sample rod parameter related to an impurity content of the second sample rod, the second sample rod parameter being selected from the group consisting of phosphorus concentration, boron concentration, aluminum concentration, gallium concentration, arsenic concentration, indium concentration, antimony concentration, total impurity concentration, dopant concentration, and resistivity of the second sample rod; determining an impurity concentration of the single crystal silicon ingot based at least in part on a rate of change of a difference between the measured first sample rod parameter and the measured second sample rod parameter; Including, A method wherein each sample rod is annealed in a thermal donor kill cycle prior to measuring said first and second sample rod parameters.

2. The method of claim 1 , wherein the first sample rod parameter and the second sample rod parameter are the same parameter.

3. 2. The method of claim 1, wherein the first sample rod diameter and the second sample rod diameter are each less than 50 mm.

4. 10. The method of claim 1, wherein the first sample rod diameter and the second sample rod diameter are each less than 25 mm.

5. The method of claim 1 , wherein the first sample rod diameter and the second sample rod diameter are between 5 mm and 100 mm.

6. The method comprises: pulling a third, fourth, fifth, sixth, seventh, eighth, ninth and / or tenth sample rods from the melt, each sample rod having a diameter of less than 100 mm; measuring a sample rod parameter related to the impurity content of each sample rod pulled from the melt; The method of claim 1 further comprising:

7. 2. The method of claim 1, wherein an impurity concentration of a single crystal silicon ingot is determined from a model, and the first sample rod parameters and the second sample rod parameters are input into the model.

8. 10. The method of claim 1 , wherein the method further comprises pulling a product ingot from the melt, and the single crystal silicon ingot for which the impurity concentration is determined is the product ingot pulled from the melt.

9. The melt is a first melt, and the method comprises: pulling a first product ingot from the first melt; pulling a second product ingot from the second melt, wherein the single crystal silicon ingot for which the impurity concentration is determined is the second product ingot pulled from the second melt; The method of claim 1 further comprising:

10. 10. The method of claim 9, wherein the diameter of the first and second sample rods is each less than 0.75 times the diameter of the product ingot.

11. The method of claim 1 , wherein the impurity concentration is an impurity concentration of a portion of a single crystal silicon ingot.

12. 2. The method of claim 1, wherein the impurity concentration is a concentration of one or more impurities over a length of a single crystal silicon ingot.

13. The method of claim 1 , wherein the sample rod is annealed at a temperature of at least 500° C.

14. The method of claim 1 , wherein the annealing is at least 5 seconds in length.

15. The method of claim 1 , wherein the length of the anneal is between 5 seconds and 5 minutes.

16. The method of claim 1 , wherein the sample rod is annealed at a temperature of at least 650° C.

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