JP2016222520A - Silicon single crystal ingot for solar cell and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon single crystal ingot having high lifetime due to a small number of oxygen deposits, cut into a wafer having excellent conversion efficiency as solar cells and having excellent processability preventing cracks or chips from occurring when the wafer is cut.SOLUTION: In the straight body part of a silicon single crystal ingot 7 for solar cells, the existing amount of an oxygen deposit having 0.5 nm or more of a diameter or maximum diagonal length measured by a transmission electron microscope is 1×10pieces/cmor less, and the existing amount of the oxygen deposit having less than 0.3-0.5 nm of a diameter or maximum diagonal length is 1×10pieces/cmor less. The concentration of oxygen dissolved in a crystal is preferably 12.0×10-20.0×10atoms/cm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a silicon single crystal ingot for solar cells, that is, a silicon single crystal ingot for cutting out a silicon wafer for solar cells, and a method for manufacturing the same.

  A silicon single crystal is widely used as a solar cell wafer material because high conversion efficiency is easily obtained stably. Here, as a method for producing a silicon single crystal wafer, a silicon melt is accommodated in a crucible, and a silicon seed crystal is brought into contact with the silicon melt to pull up the single crystal (CZ method; Czochralski method). A method of cutting a wafer from the obtained silicon single crystal ingot is general.

One of the performances required for such a silicon single crystal wafer for solar cells is a high minority carrier lifetime (hereinafter referred to as “LT”). That is, LT means the time until the generated holes (minority carriers) are recombined by applying energy such as light and thermoelectricity to the n-type silicon single crystal, for example. More specifically, when an Ar laser is irradiated on a part of the surface of a silicon single crystal, electrons are excited from the valence band of silicon to the conduction band by the energy of the laser beam to generate a hole-electron pair. . The holes which are minority carriers are recombined with electrons and disappear after a time of about 10 ⁻³ to 10 ⁻⁶ seconds. The time until the minority carrier concentration is reduced to 1 / e at the time of generation becomes recombination LT. In this case, the crystal defect forms a level in the band gap, and the minority carriers are easily bound, so that recombination is easy. As a result, when there is a crystal defect, the lifetime of the excited minority carrier is short, and therefore its LT is also short. When the difference in LT between a single crystal body having no crystal defects and a single crystal body having crystal defects is determined, the recombination LT decreases as the number of crystal defects increases and the conversion efficiency decreases. Therefore, by measuring the LT, it can be determined whether the wafer is excellent in conversion efficiency.

  The generation of crystal defects in a silicon single crystal is related to the amount of dopant for adjusting specific resistance, the amount of metal impurities, the number of dislocations, etc., but the influence of the amount of oxygen precipitates is particularly large. Here, when a silicon single crystal wafer is used for a semiconductor, the surface area that is an active region is required to be defect-free, but oxygen precipitates may act as a gettering site for heavy metal impurities inside the wafer. However, it is not always required to prevent the generation. On the other hand, when the wafer is for a solar cell, the entire area inside the wafer contributes to power generation. Therefore, there is a great demand for reducing the oxygen precipitates in order to improve the LT.

  Thus, oxygen precipitates are mainly used in the production of silicon single crystal ingots by the CZ method. Oxygen atoms dissolved from the material of the crucible (quartz) at high temperatures exceed the solid solubility limit during the cooling process. Is generated by solidification, and it is inevitable that a considerable amount is usually contained in the ingot. Moreover, the production | generation state changes with the cooling conditions after a single crystal body pulling in the manufacturing process of an ingot.

  For this reason, various attempts have been made to keep the amount of the oxygen precipitates low by specifically controlling the temperature history of the cooling process during the production of the ingot. That is, in the manufacture of a silicon single crystal ingot for a solar cell, in order to prioritize productivity and accelerate crystal growth, in general, the cooling rate is increased and crystals are grown in the V region of Boronkov theory. There are many. On the other hand, for example, cooling is performed so that the cooling rate in the temperature range of 1200 ° C. or lower and 1000 ° C. or higher is 1.0 ° C./min or lower, and the cooling rate in the temperature range lower than 1000 ° C. and 800 ° C. or higher is 1.0 There has been proposed a method in which the cooling is performed so that the temperature is less than or equal to ° C./min and rapid cooling is started in a temperature range of less than 800 ° C. and 700 ° C. or more (see claim 11 of cited document 1). As another method, as described above, after cooling the cooling rate in the temperature range of 1200 ° C. or lower to 1000 ° C. or higher to be 1.0 ° C./min or lower, the subsequent temperature range up to 500 ° C. is all at once. A method of rapid cooling has been proposed. (See claim 3 of cited document 2).

  Incidentally, in the low temperature range from about 500 ° C. to about 450 ° C. where the temperature further decreases, by passing this for a long time, fine defects in which several oxygen atoms aggregate are generated, and this is an electrically active quasi- It is known to form a position and act as a donor (thermal donor). Such a thermal donor is desirably reduced as much as possible in order to change the resistance value of the crystal. However, in the state of a single crystal ingot, even if the low temperature region is rapidly cooled, the cooling rate is insufficient and the generation cannot be completely suppressed. Therefore, after the wafer is cut out from the ingot, the heat treatment is performed at about 650 ° C. to eliminate the elution. In general, so-called donor killer annealing is performed and used for manufacturing a device for a solar cell (see, for example, Non-Patent Document 1).

JP-A-11-92274 JP-A-7-223893

Kashima Kaichi, “Heat Treatment and Oxygen Behavior”, Silicon Science, Realize, 1996, p. 540

  According to these cooling methods, it is possible to considerably reduce the amount of oxygen precipitates contained in the obtained ingot. That is, in both methods, the cooling rate in the temperature range of 1200 ° C. or lower and 1000 ° C. or higher is small, and this high temperature range is removed, so that the formed atomic vacancies are removed in the initial ingot. Aggregation can be performed, and void aggregates (voids) involved in oxygen precipitation can be reduced in subsequent cooling. As a result, the oxygen precipitates can be effectively reduced.

  However, in these conventional cooling methods, oxygen precipitation starts and the cooling rate conditions below 1000 ° C. are not sufficient, and the amount of precipitation cannot be reduced to a level that can be satisfied one step further. For example, in the method of the above-described conventional method in which the temperature range of less than 1000 ° C. and 800 ° C. or more is removed in the same manner as the temperature range of 1200 ° C. or less and 1000 ° C. or more, the nucleus of the generated oxygen precipitate is removed. As described later, the amount of oxygen precipitates having a large diameter (diameter or maximum diagonal length of 0.5 nm or more), which is a main factor of LT reduction, as described later, could not be suppressed at a satisfactory level.

  Furthermore, when all the oxygen precipitates generated in the ingot are regarded as the cause of the physical property deterioration and the reduction is uniformly pursued, in the obtained silicon single crystal ingot, the workability in wafer cutting in the future may be reduced. It was found. That is, in the method of the latter method in the prior art described above, in which the temperature range from less than 1000 ° C. to 500 ° C. is rapidly cooled at once, the entire oxygen precipitate including the large-diameter oxygen precipitate can be highly reduced. In this case, however, it was found that the obtained ingot has accumulated thermal stress, and if this is sliced, cracks and chips are likely to occur. This was the same even when the sample was further rapidly cooled to about 450 ° C. where the generation of the thermal donor was activated.

  From the above, silicon with a high LT, high LT, and excellent conversion efficiency for solar cells, as well as good workability that does not easily cause cracking or chipping during wafer cutting. Developing a single crystal ingot was a major challenge.

  The present inventors have continued intensive studies in view of the above problems. As a result, in the production of a silicon single crystal ingot, in the cooling step of the pulled single crystal, a temperature range of 1200 ° C. or lower and 1000 ° C. or higher, a crystal temperature range of less than 1000 ° C. and 800 ° C. or higher, and a temperature of 800 ° C. or lower and 700 ° C. or higher. It has been found that an ingot that satisfies the above requirements can be obtained by setting the temperature range to a specific cooling rate, and the present invention has been completed.

That is, according to the present invention, the abundance of oxygen precipitates having a diameter or maximum diagonal length of 0.5 nm or more, as measured with a transmission electron microscope, is 1 × 10 ¹⁴ pieces / cm ³ or less in the straight body portion of the ingot. And a silicon single crystal ingot for solar cells, wherein the abundance of oxygen precipitates having a diameter or maximum diagonal length of 0.3 nm or more and less than 0.5 nm is 1 × 10 ¹⁴ pieces / cm ³ or more. is there.

Further, the present invention provides a method for producing a silicon single crystal ingot for a solar cell, wherein the molten silicon obtained by heating a silicon raw material is cooled and crystallized to produce a silicon single crystal ingot. In the torso,
A) The cooling rate in the temperature range of 1200 ° C. or less and 1000 ° C. or more is set to 0.1 ° C./min or more and 1 ° C./min or less,
B) The cooling rate in the temperature range of less than 1000 ° C. and 800 ° C. or more is set to 2 ° C./min or more and 20 ° C./min or less,
C) Less than 800 ° C. 700 ° C. or higher A cooling rate in a temperature range of 0.1 ° C./min or more and 1 ° C./min or less is also provided.

  Furthermore, the present invention provides a method for producing a solar cell wafer from the solar cell silicon single crystal ingot, wherein the wafer is cut out from the solar cell silicon single crystal ingot, and the obtained wafer is 600 ° C. or higher and lower than 900 ° C. There is also provided a method of performing a heat treatment at a temperature of 30 minutes or more and 120 minutes or less and then cooling at a cooling rate of 2 ° C./second or more.

In the present invention, the oxygen precipitates present in the silicon single crystal ingot for solar cells have different effects on LT when processed into a wafer for solar cells depending on the size, and all the oxygen precipitates in the ingot There is no need to suppress the generation, and if it can be suppressed for a specific large diameter, it has been made based on the knowledge that there is substantially no problem in use as a solar cell wafer. That is, the silicon single crystal ingot of the present invention has a large diameter (diameter or maximum diagonal length of 0.5 nm or more) oxygen precipitate that is strongly involved in minority carrier trapping and functions as a recombination center in the straight body portion. Is less than 1 × 10 ¹⁴ / cm ³ .

On the other hand, oxygen precipitates having a smaller diameter than this are 1 × 10 ¹⁴ in a slightly larger size than the thermal donor (specifically, the diameter or the maximum diagonal length is 0.3 nm or more and less than 0.5 nm). / Cm ³ or more. And although the oxygen precipitate of this size does not show donor property, it participates in the capture of minority carriers, and if it is present when used as a solar cell wafer, it significantly affects the LT reduction. However, in actuality, when such a small-diameter oxygen precipitate is used for a solar cell from a wafer cut out from an ingot, its abundance is greatly reduced, and this is not a problem in practical use.

  That is, as described above, the wafer from the ingot is inevitably subjected to the heating process in the subsequent device manufacturing process for solar cells. Specifically, in the donor killer annealing step, in order to eliminate the thermal donor, heat treatment is performed at a temperature of about 650 ° C., in some cases at a higher temperature, for 30 minutes to 120 minutes. In the diffusion layer forming step, heat treatment is performed at a temperature range of 800 ° C. to 900 ° C. for about 30 minutes to 120 minutes.

  Thus, the wafer cut out from the ingot must be heated at a temperature of 600 ° C. or higher and lower than 900 ° C. for a considerable time. By such heat treatment, not only fine oxygen precipitates hitting the thermal donor but also a diameter slightly larger than this. Most of the oxygen precipitates are eluted. As a result, the abundance of the small-diameter oxygen precipitates is greatly reduced, and the influence on LT reduction is reduced. Therefore, even when a certain amount of the small-diameter oxygen precipitate is present in the production stage of the silicon single crystal ingot, it does not cause a substantial problem when used as a solar cell wafer.

  In addition, the presence of a certain amount of the small-diameter oxygen precipitates at the ingot production stage has an unexpected effect of improving the workability at the wafer cutting stage from the ingot. This is considered to be related to the fact that in the ingot, in the single crystal cooled under conditions where a large number of small-sized oxygen precipitates are generated, thermal stress is likely to occur at the center and outside of the single crystal. Occurrence of cracks and chips during wafer slicing is significantly reduced.

  Here, paying attention to the difference in thickness required according to the use in the silicon wafer, it is usually about 0.50 to 1.00 mm in the semiconductor use, whereas in the solar battery use, it is 0.08. Since the thickness is about 0.25 mm, the wafer mounted thereon is required to be thinner, and a thickness of 0.10 to 0.22 mm is usually preferable. For this reason, cracks and chips when cutting a wafer from the ingot have not been exposed so much in semiconductor applications, but problems become apparent in solar cell applications in which the demand for such thin wafers is high.

  Thus, according to the present invention, since there are few oxygen precipitates, it is possible to cut out a wafer with high LT and excellent conversion efficiency for solar cells, and it is difficult for cracks and chips to occur during wafer cutting. An excellent silicon single crystal ingot is provided.

FIG. 1 is a schematic view showing a typical embodiment of a pulling apparatus for a silicon single crystal ingot equipped with a CZ furnace.

  In the present invention, a silicon single crystal ingot refers to a silicon single crystal produced by cooling molten silicon. A rod-like shape is common, and it is often composed of a straight body portion in which crystals are stably grown, and a top portion (expanded diameter portion) and a tail portion (reduced diameter portion) located at each end thereof. The length of the straight body part of the silicon single crystal ingot produced by the CZ method is usually 900 to 1800 mm, and the diameter is usually 150 to 230 mm.

  In the silicon single crystal, the conductivity type may be n-type or p-type, but the n-type is preferable for the reason that it is easy to obtain a material excellent in LT. Examples of the impurity to be doped include phosphorus (P) as an n-type dopant and boron (B), arsenic (As), and antimony (Sb) as a p-type dopant.

  In the present invention, the amount of oxygen precipitates present in the silicon single crystal ingot is confirmed by the following method. That is, the upper end wafer sample cut out from the upper end of the straight body portion of the ingot with a thickness of 2.5 mm below and the lower end wafer sample cut out from the lower surface of the straight body portion of the ingot with a thickness of 2.5 mm above are prepared. For each of these, an observation flake having a thickness of 100 nm is prepared from a site to be measured, each oxygen precipitate is measured by a method described later, and the amount of oxygen precipitates present in the entire straight body is determined from these results. Is the method. In both of the upper end wafer sample and the lower end wafer sample, if each of the large-diameter and small-diameter oxygen precipitates satisfies the requirements defined by the present invention, the silicon single crystal ingot is the entire straight body part. Therefore, it can be considered that the above-mentioned requirements of the present invention are substantially satisfied.

  More specifically, in the CZ method, the amount of oxygen precipitated on the ingot pulled up from the melt is the highest at the top of the ingot and decreases for a while as the pulling proceeds from there. This is because, as described above, the source of oxygen precipitated in the ingot is mainly from the material of the crucible (quartz), and therefore, the elution of oxygen from the crucible has a large amount of molten silicon filled in the crucible. (In other words, the contact area between the molten silicon and the inner surface of the crucible is increased), and the initial pulling becomes more intense.

  From this, if the upper end wafer sample is cut out and the amount of oxygen precipitates is measured, the maximum abundance of large diameter oxygen precipitates in the entire straight body portion can be confirmed. Furthermore, if the lower end wafer sample is cut out and the amount of oxygen precipitates is measured, the maximum abundance of small diameter oxygen precipitates in the straight body portion can be confirmed. Thus, by combining these two measurement results, it can be determined whether the target ingot substantially satisfies the requirements for the oxygen precipitate in the present invention in the entire straight body portion.

  In addition, in the middle of the straight body part from the upper surface to the lower surface, the requirements for each of the large diameter and small diameter oxygen precipitates deviate from the requirements stipulated in the present invention as an extremely narrow region due to a sudden change in pulling conditions, etc. However, the ingot is allowed to be within the scope of the present invention as long as it is a small region that does not affect the effect of the invention. Preferably, each of the 2.5 mm-thick wafer samples is sequentially cut out from the whole of the straight body portion, and when the oxygen precipitation state of each is measured, each of the large-diameter and small-diameter oxygen precipitates defined by the present invention is measured. It is preferable that the number of wafers that deviate from the requirements of the object is 3% or less with respect to the total number of cut wafer samples.

In the present invention, the amount of oxygen precipitates on the sample wafer may be measured by preparing a thin observation piece having a thickness of 100 nm from each measurement target portion on the wafer and observing the surface with a transmission electron microscope (TEM). The measurement target parts are the upper surface side surface of the upper end wafer sample and the lower surface side surface of the lower end wafer sample at five locations (centers) with a uniform interval at the radius (r) connecting the center and an arbitrary point on the outer periphery. Part, r / 4 place, 2r / 4 place, 3r / 4 place, outer peripheral vicinity place). About the said observation thin piece produced from these each place, the amount of each oxygen precipitate of a large diameter and a small diameter is measured, and the average of these 5 places is calculated | required and it is set as the abundance of each oxygen precipitate amount in the sample wafer. The surface observation by TEM of each observation thin piece was measured for a visual field of 1.4 μm × 2.0 μm to 400 nm × 600 nm at a magnification of 4,000 to 40,000 for a large-diameter oxygen precipitate, and cm ³ Calculated as the number of units. On the other hand, in the case of oxygen precipitates having a small diameter, the field of view of 40 nm × 60 nm is measured at a magnification of 400,000 times or more and obtained as the number in cm ³ units.

  Here, the diameter of the oxygen precipitation part means a value measured as a diameter or a maximum diagonal length. It is known that oxygen precipitates take various shapes such as a spherical shape, an octahedron, and a plate shape depending on the generated temperature range, heat treatment conditions, the presence or absence of carbon, and the like. Therefore, in the case of a spherical precipitate, the diameter may be measured to obtain the precipitate diameter, but in the case of a plate-like precipitate or polyhedral precipitate, the maximum diagonal length is measured and set as the precipitate diameter.

In the silicon single crystal ingot of the present invention, the abundance of oxygen precipitates having a diameter or maximum diagonal length of 0.5 nm or more is 1 × 10 ¹⁴ pieces / cm ³ or less in the straight body portion, which is extremely small. Such large-diameter oxygen precipitates remain substantially as they are even if the wafer is cut out and subjected to the heat treatment in the solar cell device manufacturing process, which causes a decrease in LT, but the amount is greatly reduced in the present invention. A solar cell having high LT and excellent conversion efficiency can be obtained. Considering the good effect, the abundance of large-diameter oxygen precipitates is more preferably 1 × 10 ¹⁰ to 1 × 10 ¹³ pieces / cm ³ , and more preferably 2 × 10 ¹⁰ to 1 × 10 ^11. Most preferably, pieces / cm ³ .

In the silicon single crystal ingot of the present invention, the abundance of oxygen precipitates having a diameter or maximum diagonal length of 0.3 nm or more and less than 0.5 nm in the straight body portion is 1 × 10 ¹⁴ pieces / cm ³ or more. There is a need. As described above, such small-diameter oxygen precipitates also significantly affect LT reduction. However, in the heat treatment inevitably introduced in the device manufacturing process after wafer processing, as in the case of the thermal donor, Many of them disappear. Therefore, even if it is contained in such a considerable amount, deterioration of the conversion efficiency of the solar cell can be suppressed. In addition, the presence of such small-diameter oxygen precipitates provides a secondary effect of improving the processability of cutting out wafers from ingots.

Considering these effects, the amount of the small-diameter oxygen precipitates is more preferably 2 × 10 ^{14 to} 5 × 10 ¹⁵ pieces / cm ³ .

The silicon single crystal ingot of the present invention in the state of the oxygen precipitate has an oxygen concentration dissolved in the crystal of 10.0 × 10 ^{17 to} 20.0 × 10 ¹⁷ atoms / cm ^3. It is preferable from the viewpoint that the above effect is satisfactorily exhibited. Further, the dissolved oxygen concentration is particularly preferably 12.0 × 10 ^{17 to} 18.0 × 10 ¹⁷ atoms / cm ³ . Here, the oxygen concentration dissolved in the crystal is a value measured by a Fourier transform infrared spectrophotometer and derived by ASTM F121-79.

In addition, in a silicon single crystal ingot, generally, the greater the amount of carbon contained, the easier it is for oxygen precipitates to be formed. In a silicon single crystal ingot, the carbon concentration is 1 × 10 ^{13 to} 5 × 10 ¹⁶ atoms / cm ³ , particularly 5 × 10 ¹³ to 1 × 10 ¹⁶ atoms / cm ^3. In such an ingot having a carbon concentration, it is preferable that the requirement for the amount of oxygen precipitates is satisfied. In the present invention, the carbon concentration with respect to the silicon single crystal ingot is measured with a Fourier transform infrared spectrophotometer, and is a value derived by ASTM F123-86.

  Next, the manufacturing method of the silicon single crystal ingot of the present invention will be described. In the present invention, the silicon single crystal ingot is not limited to a specific method as long as the requirements for the oxygen precipitates are satisfied, and the molten silicon obtained by heating the silicon raw material is cooled and crystallized. Although what was manufactured by what kind of method may be used, it is usually preferable that it was manufactured by CZ method. FIG. 1 is a schematic view of a typical embodiment of a pulling apparatus for a silicon single crystal ingot equipped with a CZ furnace.

  The main chamber 1 is provided with a quartz crucible 3 for housing the molten silicon 2 and a graphite crucible 4 for supporting it. Around these, a heater 5 is provided, and the silicon raw material accommodated in the quartz crucible 3 is heated to a melting point of 1414 ° C. or higher and is melted. After immersing the seed crystal 6 in the silicon melt, the seed crystal is slowly pulled up to grow the crystal. The ingot is cooled as it is pulled up above the main chamber 1, and a silicon single crystal ingot 7 having a predetermined size is obtained. Manufacturing.

  In pulling up and cooling the silicon single crystal ingot 7, the atmosphere in the main chamber 1 is preferably an inert gas atmosphere in order to prevent oxygen from reacting with the carbon member.

In the production of a silicon single crystal ingot by such a CZ method, voids are generated in the initial ingot pulled from the melt during the solidification process of silicon. In addition, the solid solubility limit of oxygen in a single crystal at 1300 ° C. is 11.0 × 10 ¹⁷ atoms / cm ³ , and it is considered that oxygen close to this is dissolved. Then, after the solidification process, as the ingot is cooled, excess oxygen is deposited with respect to the solid solubility limit. Thus, in order to manufacture the silicon single crystal ingot of the present invention, the cooling of the straight body part after the silicon solidification process from the silicon melt can be controlled to the following specific cooling rate. is important.

That is,
A) A temperature range of 1200 ° C. or lower and 1000 ° C. or higher (cooling step A) is a cooling rate of 0.1 ° C./min or higher and 1 ° C./min or lower,
B) The temperature range (cooling step B) of less than 1000 ° C. and 800 ° C. or more is set to a cooling rate of 2 ° C./min or more and 20 ° C./min or less,
C) Less than 800 ° C. 700 ° C. or more The temperature range (cooling step C) is required to be controlled to a cooling rate of 0.1 ° C./min or more and 1 ° C./min or less.

  Here, the cooling rate in each of the above temperature ranges is a temperature drop per minute based on the time required for the temperature to drop to the lower limit temperature after reaching the upper limit temperature in the ingot cooling step. Asking. Thus, since it is obtained as an average value from the entire temperature range, the actual temperature drop value per minute may occur if the period deviating from the range defined by the present invention is also short. If the average value obtained from the entire temperature range is within the range of the present invention, the influence on the oxygen precipitation state is small and acceptable in the present invention. For example, in each temperature range, in the vicinity of the upper limit temperature and the lower limit temperature, switching to the cooling rate of the adjacent temperature range is possible, but there is a possibility that the range of the cooling rate in the original temperature range may be out of range. . However, at such actual cooling rate per minute, the total amount of time outside the scope of the present invention is preferably suppressed to 5% or less with respect to the time required for each of the cooling steps A to C. .

  In manufacturing a silicon single crystal ingot by the CZ method, the temperature measurement location in the ingot radial direction for obtaining the cooling rate of the ingot is the central portion of the ingot. However, it is difficult to measure the temperature of the central part of the ingot in the cooling process by actual CZ method manufacturing. In the present invention, the cooling rate of the ingot is the same size as the ingot actually manufactured. Separately, this is obtained by the following method by exposing to the same pulling environment and measuring the temperature change.

  More specifically, in the pulled silicon single crystal ingot, the temperature state is not uniform with respect to the pulling direction of the straight body portion, and is easily cooled down. Therefore, the temperature measuring ingot is prepared for measuring the temperature at the upper end of the straight body part and for measuring the temperature at the lower end of the straight body part, and at the upper end of the straight body part for the former, and directly for the latter. Each of the R thermocouples is embedded in the lower end of the body part at a depth reaching the center of the ingot. In the case of the temperature measuring ingot at the upper end of the straight barrel part, it is immersed in molten silicon contained in a quartz crucible from above, dissolved to a position 30 mm below the upper end of the straight barrel part, inverted in this state, and this is actually The silicon single crystal ingot is pulled up under the same conditions as the pulling conditions. Along with this pulling, the temperature change in the cooling process is measured by a thermocouple embedded in the upper end of the straight body portion, and from this result, the cooling rate in each cooling temperature region at that location is obtained instead. On the other hand, in the case of the temperature measuring ingot at the lower end of the straight body part, the molten silicon in the quartz crucible is melted to a position 30 mm below the lower end of the straight body part in the tail part. The same operation as in the above case is performed, and the cooling rate in each cooling temperature region at the lower end of the straight body portion is obtained instead.

  If the cooling rates in the cooling steps A to C of the straight barrel portion upper end and the straight barrel portion lower end obtained as described above satisfy the cooling rates in the respective cooling steps defined in the present invention, this silicon single crystal ingot In this manufacturing, it can be considered that the cooling requirement is satisfied in the entire region from the upper end to the lower end of the straight body portion to be pulled up.

  In the cooling method, the silicon melt is cooled to 1200 ° C., and the cooling rate at the start of pulling is not particularly limited, but it is preferable to remove the cooling at the same cooling rate as the adjacent cooling step A. In the cooling step A, the cooling rate is set to 0.1 ° C./min or more and 1 ° C./min or less. By removing the high temperature region in this way, atomic vacancies formed in the ingot at the beginning of cooling can be agglomerated, and vacancy point defects related to oxygen precipitation can be reduced in subsequent cooling. it can. As a result, oxygen hardly precipitates in the obtained silicon single crystal ingot. From the viewpoint of better exhibiting the effect for reducing the oxygen precipitates, the cooling rate of the cooling step A is particularly preferably 0.3 ° C./min or more and 0.8 ° C./min or less, 0.4 Most preferably, it is not lower than ℃ / min and not higher than 0.8 ° C / min. Here, when the cooling rate of the cooling step A is less than 0.1 ° C./min, the temperature control becomes difficult, and a special heater or the like for heating to 1200 to 1000 ° C. must be provided inside the apparatus. It will disappear and a large capital investment will be required. On the other hand, when the cooling rate of the cooling step A exceeds 1 ° C./min, the vacancy point defects of the precursor are frozen before the coherent vacancy defects (voids) are formed. The formation of precipitates is greatly promoted.

In the cooling method, in the subsequent cooling step B, the cooling rate is set to 2 ° C./min or more and 20 ° C./min or less. In this temperature range, the precipitation of oxygen starts, and by rapidly cooling this region, the effect of suppressing the growth of nuclei of the oxygen precipitates generated here is exhibited. As a result, in the obtained silicon single crystal ingot, the content of large-diameter oxygen precipitates can be effectively suppressed, and a small amount of 1 × 10 ¹⁰ to 1 × 10 ¹⁴ pieces / cm ³ can be realized. . From the viewpoint of better exhibiting the effect for reducing oxygen precipitates, the cooling rate in the cooling step B is particularly preferably 5 ° C./min or more and 18 ° C./min or less. Here, when the cooling rate of the cooling step B is less than 2 ° C./min, the formed oxygen precipitates grow rapidly to generate secondary defects such as dislocations, and the lifetime is greatly reduced. On the other hand, when the cooling rate of the cooling step B exceeds 20 ° C./min, smooth transition to each cooling rate in the adjacent cooling steps A and C becomes difficult.

Furthermore, in the cooling method, in the subsequent cooling step C, the cooling rate is set to 0.1 ° C./min or more and 1 ° C./min or less. This temperature range is a range in which precipitation of oxygen is particularly activated, and the oxygen deposited here remains as a small-diameter oxygen precipitate in the obtained silicon single crystal ingot. Thus, this temperature range is intentionally removed in the present invention, so that such small-diameter oxygen precipitates are present in the ingot in a quantity of 1 × 10 ¹⁴ pieces / cm ³ or more. Thus, even if a small amount of oxygen precipitates are contained in the ingot in a certain abundance, the wafer cut out from the wafer as described above has no substantial effect on the deterioration of the conversion efficiency in the solar cell application. By adding a small-diameter oxygen precipitate to the wafer, the processability of cutting out the wafer is improved, which is convenient.

  In addition, in the case where the oxygen precipitates present in the ingot have a large diameter of 0.5 nm or more, the heat treatment during device fabrication after cutting into such a wafer does not change much in the presence state. As described above, most of the residue remains and LT is lowered.

  From the viewpoint of better exhibiting the effect of causing the small-diameter oxygen precipitates to be present in the ingot, the cooling rate in the cooling step C is particularly preferably 0.2 ° C./min to 0.8 ° C./min. Here, when the cooling rate of the cooling process C is less than 0.1 ° C./min, a part of the oxygen precipitate grows to a size that cannot be eluted in the subsequent process, thereby reducing LT. On the other hand, when the cooling rate in the cooling step C exceeds 1 ° C./min, formation of oxygen precipitates having a size that can be eluted in the subsequent step is insufficient, and cracks and chips are likely to occur during wafer cutting.

  Further, in the cooling method, the cooling to 400 ° C. following the cooling step C is not particularly limited, and the cooling may be performed at the same cooling rate as in the cooling step C. As a result, in this temperature range, thermal donors, which are finer oxygen precipitates, precipitate, but disappear as a result of heat treatment such as donor killer annealing after being cut into the wafer. Moreover, in order not to generate such a thermal donor as much as possible, it may be quenched as in the cooling step B.

  By the above cooling method, the silicon single crystal ingot in the above-described oxygen precipitation state of the present invention can be obtained. In carrying out this cooling method, the respective cooling rates in each temperature region may be adjusted by appropriately applying known cooling means and temperature holding means in the ingot manufacturing apparatus. For example, when the ingot manufacturing apparatus is a CZ furnace as shown in FIG. 1 described above, it may be adjusted by the installation position of the heater 7 or the heating temperature setting. It is also effective means to provide a heat insulating wall 8 or an after heater in the upper space (cooling area of the pulled ingot) in the main chamber 1 or to wind a cooling pipe around the upper outer wall of the main chamber. . What is necessary is just to adjust suitably the pulling-up speed of an ingot, etc. to the cooling means and temperature holding means in these ingot manufacturing apparatuses.

  The cutting of the wafer from the silicon single crystal ingot of the present invention may be appropriately performed according to a conventional method using a peripheral blade, a wire saw, a band saw or the like, using a cutting fluid or the like as necessary. In general, the thickness of the wafer to be cut out can be appropriately selected from the range of 0.5 to 1.0 mm or less. A thin film having a thickness of 0.08 to 0.25 mm is suitable for use in a solar cell. When this thin wafer is cut out, good workability in the ingot of the present invention is remarkably exhibited, which is preferable.

  The silicon single crystal wafer thus cut from the ingot is used for a solar cell. The device fabrication may be performed according to a known method, but as described above, in the process, it is usually performed at a temperature of 600 ° C. or more and less than 900 ° C. for 30 minutes or more and 120 minutes or less in a donor killer annealing or diffusion layer forming step. The heat treatment is performed. As described above, as a result of this heating, many of the small-diameter oxygen precipitates present in the ingot disappear and the LT of the wafer is lowered. From the viewpoint of sufficiently exhibiting the disappearance effect of such small-diameter oxygen precipitates, the heat treatment performed after the wafer cutting is provided with a treatment period at a temperature of 700 ° C. or more and less than 900 ° C. within the above time. Particularly preferred.

  Examples of the present invention will be described below, but it goes without saying that the present invention is not limited by the description of these examples.

  In Examples and Comparative Examples, various physical property values were measured by the following methods.

1) Measurement of cooling speed of ingot in cooling step According to the method of manufacturing in each example, the straight barrel portion has an 8 inch diameter, a length of 50 mm, a top portion length of 100 mm, and a tail portion length of 200 mm. A temperature measuring ingot and a straight barrel lower end temperature measuring ingot each having a diameter of 8 inches in diameter and 1350 mm in length (same size as a silicon single crystal ingot), a top length of 100 mm, and a tail length of 200 mm are prepared. did. Holes were drilled with drills at the upper end of the straight body portion for the former and at the lower end of the straight body portion for the latter, and an R thermocouple was embedded therein so as to reach the center of the ingot.

  Subsequently, in the quartz crucible, when measuring the temperature of the upper end of the straight body part from the weight of the silicon raw material contained in the embodiment, the silicon raw material in an amount obtained by subtracting the weight of the temperature measuring ingot for the upper part of the straight body part is stored. Electric power was supplied to the graphite heater and heated and melted in an Ar atmosphere. The ingot for measuring the temperature at the upper end of the straight barrel part is immersed in the molten silicon contained in the quartz crucible from above, and is melted to a position 30 mm below the upper end of the straight barrel part. The silicon single crystal ingot was pulled up under the same conditions as the pulling conditions. Along with this pulling, the temperature change in the cooling steps A to C was measured with a thermocouple embedded in the upper end of the straight body portion, and from this result, the cooling rate in each cooling temperature region at the location was substituted and determined.

  On the other hand, the ingot for measuring the temperature at the lower end of the straight body part is also melted in the molten silicon in the quartz crucible to a position 30 mm below the lower end of the straight body part in the tail part, and the above-described straight body is started except that the pulling is started from this state The same operation as in the case of the upper end temperature measuring ingot was carried out, and the cooling rate in each cooling temperature region at the lower end of the straight body portion was obtained instead.

2) Measurement of oxygen and carbon concentration dissolved in silicon single crystal ingot For the silicon single crystal ingot produced in each example, an upper end sample was cut out with a thickness of 2.5 mm below the upper end of the ingot straight body. Similarly, a lower end sample was cut out with a thickness of 2.5 mm upward from the lower end of the ingot straight body portion. The upper end sample and the lower end sample thus obtained were etched in a mixed acid (48 wt% hydrofluoric acid: 60 wt% nitric acid: 100 wt% acetic acid = 1: 10: 1) to a sample thickness of 2.0 mm, and pure After washing twice with water, the water adhering to the surface is sufficiently removed, and then, with a Fourier transform infrared spectrophotometer, an equal interval of 5 at a radius (r) connecting an arbitrary point on the outer periphery with the center portion. Oxygen concentration was measured from locations (center, r / 4 locations, 2r / 4 locations, 3r / 4 locations, locations near the outer periphery), derived by ASTM F121-79, and the average value was taken as the oxygen concentration. Similarly, five points (center, r / 4, 2r / 4, 3r / 4, outer circumference) at a radius (r) connecting an arbitrary point on the outer circumference by a Fourier transform infrared spectrophotometer The carbon concentration was measured from the vicinity) and derived by ASTM F123-86, and the average value was defined as the carbon concentration.

3) Measurement of the amount of oxygen precipitates present in the silicon single crystal ingot “2) Measurement of oxygen and carbon concentration dissolved in the silicon single crystal ingot in the straight body portion of the silicon single crystal ingot produced in each example” For the upper end sample and the lower end sample cut out in step 5, the upper surface in the case of the former, and the lower surface in the case of the latter, five locations at equal intervals in the radius (r) connecting the center and an arbitrary point on the outer periphery ( From the center, r / 4 locations, 2r / 4 locations, 3r / 4 locations, and the vicinity of the outer periphery, observation slices having a thickness of 100 nm were prepared. Sample processing for producing the observation slice was performed by an ion milling method.

Surface observation by T EM of each observation flakes were created to such, for the oxygen precipitates having a large diameter of 1.4μm × 2.0μm~400nm × 600nm in magnification 4,000～40,000 times and measured for the field of view, was calculated as the number of in cm ³ units. On the other hand, with respect to small-sized oxygen precipitates, the field of view of 40 nm × 60 nm was measured at a magnification of 400,000 times or more and obtained as the number in cm ³ units.

4) Processability of wafer cutting from silicon single crystal ingot For silicon single crystal ingot manufactured in each example, the top portion and tail portion are cut off, and then part or all of the side surface is cut off to obtain a straight body portion. This was cut into a wafer having a thickness of 0.20 mm ± 0.01 mm using a slicing apparatus equipped with a multi-wire. 250 wafers were processed from the upper surface and the lower surface (total of 500 wafers), the appearance of each wafer was observed, and the results of counting the number of cracks and chips observed were also shown in Table 1.

5) Evaluation of LT of cut-out wafer In the straight body portion of the silicon single crystal ingot manufactured in each example, the influence of the heat treatment cut out in “4) Processability of cutting out wafer from silicon single crystal ingot” The upper end wafer sample for evaluation and the lower end wafer sample for LT evaluation were taken out one by one from the inner part adjacent to the upper end wafer sample for evaluation and the lower end wafer sample. These LT evaluation wafer samples were subjected to a donor killer annealing treatment at 700 ° C. for 30 minutes to remove thermal donors generated during ingot cooling using an RTA (Rapid Thermal Annealing) apparatus, and then less than 600 ° C. It cooled so that the cooling rate of the temperature range of 450 degreeC or more might be 120 degreeC / min or more. Each of the obtained donor killer annealed wafer samples was subjected to donor killer annealing and then cleaned for 10 minutes using cleaning liquid 1 (ammonia water: hydrogen peroxide water: water = 1: 1: 5) heated to 80 ° C. After that, the substrate was washed for 10 minutes in the washing solution 2 (5% by weight dilute hydrofluoric acid). Then, the process (Chemical Passivation process) which apply | coats a 3 weight% iodine / ethanol solution was performed, and LT was measured with the LT measuring apparatus (WT-2000 by SEMILAB). The measurement results were evaluated as “◎” for 1000 μsec or more, “◯” for less than 1000 μsec, 500 μsec or more, and “×” for less than 500 μsec.

6) Measurement of the amount of oxygen precipitates after the cut wafer is subjected to donor killer annealing (DKA) In the straight body portion of the silicon single crystal ingot manufactured in each example, “5) Evaluation of LT of cut wafer LT” For each wafer sample that was evaluated, each of the large-diameter oxygen precipitates and the small-diameter oxygen precipitates was measured in the same manner as in “3) Measurement of the amount of oxygen precipitates present in the silicon single crystal ingot”. The number in cm ³ units was determined.

Example 1
A silicon single crystal ingot was manufactured using the CZ furnace shown in FIG. The diameter of the quartz crucible is 22 inches (56 cm), and 120 kg of high-purity polycrystalline silicon is added to this, and 74 ppma (about 3.5 Ω · cm at the top) in the melt when these high-purity polycrystalline silicon is melted. The n-type silicon dopant is adjusted and accommodated so as to be in a dissolved state of phosphorus, and the furnace is depressurized to several kPa with a vacuum pump, electric power is supplied to the graphite heater, and the Ar atmosphere is used. Polycrystalline silicon was heated and melted. A silicon seed crystal having a principal plane orientation of Si <100> is brought into contact with the silicon melt to start pulling up the single crystal. The straight body portion has a diameter of 8 inches (203 cm), a length of 1350 mm, and a top portion. A silicon single crystal ingot having a length of 100 mm, a tail portion length of 200 mm, and a crystal growth orientation of <100> was pulled up.

  Prior to the production of this silicon single crystal ingot, a straight body upper end temperature measuring ingot and a straight body lower end temperature measuring ingot are separately prepared, and according to the method described in 1), the cooling steps A to C are performed. When each temperature change of the straight body part upper end and the straight body part lower end was measured and the cooling rate was calculated | required, it was the result shown in Table 1, respectively. In addition, the cooling rate to 400 degreeC following the cooling process C was rapid cooling conditions of 15 degree-C / min.

  With respect to this silicon single crystal ingot, according to the methods 2) and 3), the oxygen concentration, the carbon concentration, and the abundance of oxygen precipitates at the upper end of the straight barrel portion and the lower end of the straight barrel portion were measured. These results are shown in Table 2.

  Next, with respect to the obtained silicon single crystal ingot, the processability of wafer cutting was evaluated according to the method of 4), and the results are also shown in Table 2. In addition, according to the method of 4), LT is evaluated after donor killer annealing (DKA) is performed on the cut wafer. Further, according to the method of 6), the amount of oxygen precipitates after DKA is evaluated. Each result is also shown in Table 2.

Example 2, Comparative Examples 1-3
In the first embodiment, the cooling conditions were changed so that the cooling rate at the upper end of the straight body portion and the lower end of the straight body portion in the cooling steps A to C became the values shown in Table 1 in the manufacture of the silicon single crystal ingot. The same procedure as in Example 1 was performed except that. The results of measuring the physical properties of 2) to 7) for each obtained silicon single crystal ingot are also shown in Table 2.

1; main chamber 2; molten silicon 3; quartz crucible 4; graphite crucible 5; heater 6; seed crystal 7; silicon single crystal ingot 8;

Claims (5)

In the straight body portion of the ingot, the abundance of oxygen precipitates having a diameter or maximum diagonal length of 0.5 nm or more measured by a transmission electron microscope is 1 × 10 ¹⁴ pieces / cm ³ or less, and the diameter or maximum pair A silicon single crystal ingot for solar cells, wherein the abundance of oxygen precipitates having an angular length of 0.3 nm or more and less than 0.5 nm is 1 × 10 ¹⁴ pieces / cm ³ or more. The concentration of oxygen dissolved in the crystal, 12.0 × a ^{10 17 ~20.0 × 10 17 atoms /} cm 3, silicon for solar cell according to claim 1 single crystal ingot. In a method of manufacturing a silicon single crystal ingot by cooling and crystallizing molten silicon obtained by heating a silicon raw material,
A) The cooling rate in the temperature range of 1200 ° C. or less and 1000 ° C. or more is set to 0.1 ° C./min or more and 1 ° C./min or less,
B) The cooling rate in the temperature range of less than 1000 ° C. and 800 ° C. or more is set to 2 ° C./min or more and 20 ° C./min or less,
C) Silicon single crystal for solar cell according to claim 1 or 2, wherein the cooling rate in the temperature range of less than 800 ° C and less than 700 ° C is 0.1 ° C / min or more and 1 ° C / min or less. Ingot manufacturing method.   A method of cooling and crystallizing molten silicon obtained by heating a silicon raw material is a method of containing a silicon melt in a crucible and bringing a silicon seed crystal into contact with the silicon melt to pull up a single crystal. The manufacturing method of the silicon single crystal ingot for solar cells of Claim 3 which is based on the Larsky method.   A wafer is cut out from the solar cell silicon single crystal ingot according to claim 1 or 2, and the wafer is heated at a temperature of 600 ° C or higher and lower than 900 ° C for 30 minutes or longer and 120 minutes or shorter, and then 2 ° C / sec or higher. A method for producing a silicon single crystal wafer for solar cells that is cooled at a cooling rate of

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