Process for drawing a single crystal from silicon
By periodically varying the drawing speed during the thin neck formation in the Czochralski method, the method addresses dislocation issues in single-crystal silicon growth, achieving defect-free crystals with precise control and reduced material loss.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- SILTRONIC AG
- Filing Date
- 2025-11-12
- Publication Date
- 2026-05-21
AI Technical Summary
Existing methods for growing single-crystal silicon using the Czochralski method struggle to produce dislocation-free crystals with the crystal orientation , as dislocations often form and propagate, particularly in the center of the crystal, leading to defects and material loss.
A method involving the periodic variation of drawing speed during the formation of a thin neck, with specific rotational speed controls and crucible alignment, to minimize dislocation propagation, ensuring a dislocation-free crystal growth.
The method achieves dislocation-free single-crystal silicon with the orientation, reducing defects and material loss, and allows for precise control of crystal diameter and shape.
Smart Images
Figure EP2025082731_21052026_PF_FP_ABST
Abstract
Description
[0001] Method for growing a single crystal from silicon
[0002] Technical field
[0003] The present invention relates to a method for growing a single crystal from silicon according to the Czochralski method and a method for producing wafers from single-crystal silicon.
[0004] State of the art
[0005] Single-crystal disks made of semiconductor material (also called wafers) are the basis of modern electronics. They are manufactured in a variety of process steps, including drawing a single-crystal rod from a melt, grinding and sawing the crystal into disks, and surface finishing of the disks.
[0006] Single-crystal semiconductor wafers, particularly silicon wafers, are typically manufactured by first drawing a monocrystalline rod using the Czochralski (CZ) process. The resulting rods are then cut into crystal fragments using suitable saws such as wire saws, internal hole saws, or band saws. These fragments are then usually processed into semiconductor wafers using a wire saw. After further mechanical, chemo-mechanical, and / or chemical steps, a layer can optionally be deposited onto the wafer using chemical vapor deposition (CVD).
[0007] The CZ process typically involves lowering a seed crystal, usually referred to as a seed crystal, into a silicon melt at a temperature above 1410°C until contact with the melt is established. Crystal growth occurs by slowly pulling the seed crystal back up while rotating it, as the melt solidifies at the forming interface. This process first creates a thin neck, then an initial cone of the single crystal with increasing diameter, followed by a cylindrical section with the desired diameter, and finally a final cone with 2024P00023WG / HA.
[0008] 2
[0009] The crystal is pulled with decreasing diameter before being separated from the melt and removed.
[0010] When the seed crystal is brought into contact with the hot molten silicon, stresses arise within it, leading to the formation of dislocations. These dislocations can propagate along the entire length of the crystal rod, but in most cases, they can be removed by forming a thin neck from the single crystal. To form a thin neck, also known as a dash neck, the rotating seed crystal is pulled upwards to reduce the diameter of the essentially cylindrical thin neck to typically 3 to 8 mm. Starting from this dislocation-free thin neck, the diameter of the single crystal rod is then increased again, and a dislocation-free single crystal with the desired diameter can be grown. This process is also known as the dash method or dash-neck method.
[0011] Edge dislocations can generally be avoided by the dash-neck method. This does not apply to the same extent to screw dislocations. EP 1 498517 A1 describes how, with an inoculation sample having the crystal orientation <110> Dislocations in the center of the vaccine recipient cannot simply be removed using the Dash-Neck method. In a vaccine recipient with a <110> -Crystal orientation can cause sliding dislocations to form in the center when the seed is immersed in the melt. These dislocations can propagate axially, i.e., along the drawing direction of the single-crystal rod, over the entire length of the rod, even when the dash-neck method is used. EP 1 498517 A1 describes a method for drawing a single-crystal rod in which the formation of dislocations in the seed is prevented. Thus, no dislocations need to be removed, and the method therefore does not require the dash-neck technique. (See JP2007070131, EP1897976B1, US7641734B2, and...)
[0012] DE112012006260 describes the use of high boron doping of the melt and seed to reduce dislocation mobility, which, however, limits the electrical resistance in the single crystal to a specific range. In US 7,226,506 B2, EP 1 498516 A1, and US 5,769,941 B2, tilted seed crystals are used to grow single-crystal rods from silicon with the orientation <110> used. The tilting results in the drawing direction, and thus the axis of the growing single-crystal rod, not being perpendicular to the crystal faces with a crystal orientation. <110> is aligned. With increasing tilt, the probability of a dislocation emerging increases. However, to disks with a crystal orientation <110> In order to produce it, the tilting of the single crystal rod must be compensated for by grinding, so that increasing tilting is accompanied by increasing material loss.
[0013] US 4,002,523 describes a method for drawing a single crystal rod with a crystal orientation <110> This method involves drawing a thin-necked die with alternating thin and thick sections, gradually shifting dislocations parallel to the growth direction (drawing direction) of the single crystal away from the crystal's axis and to the periphery. This increases the probability that the dislocations will terminate at a surface and thus disappear from the crystal. However, this elaborate and complex procedure requires a thin-neck diameter of 2.5 mm, reducing the die's stability and making it more difficult to extract large crystal rods. In US 5,911,823 and US 6,652,824, the diameter of the die is even reduced to less than 2 mm, further decreasing its stability.
[0014] In JP 2007-84358 A, JP 2011-57460 A, and JP 2023-65064 A, a thin-necked rod with alternating thin and thick sections is drawn. JP 2007-84358 A describes a thin-necked rod with alternating thin and thick sections, where the diameter at the thickest points is at least three times the diameter at the thinnest points of the rod. JP 2023-65064 A describes that the sections where the diameter increases are shorter than the sections where the diameter decreases. The methods described above are relatively complex and only inadequately suited for drawing a single-crystal rod with the crystal orientation <110> , which is free of dislocations.
[0015] Technical problem of the invention and its solution
[0016] The present invention is based on the objective of providing an efficient method for growing a dislocation-free single crystal from silicon according to the Czochralski method. In particular, the objective of the present invention is to provide an efficient method suitable for growing a dislocation-free single crystal from single-crystal silicon with the crystal orientation <110> to produce.
[0017] This problem is solved by the inventive method for growing a single crystal from silicon according to the Czochralski method. The method according to the first aspect of the present invention comprises the following steps in this order:
[0018] (i) the provision of a crucible containing a silicon melt and a seed crystal of single-crystal silicon above the silicon melt;
[0019] (ii) lowering the seed crystal until contact is established between the seed crystal and the surface of the silicon melt;
[0020] (iii) drawing the seed crystal into a thin neck with a maximum diameter of not more than 20 mm by lifting the seed crystal while rotating it;
[0021] (iv) widening the diameter of the thin neck to a target diameter;
[0022] (v) drawing a cylindrical section of the single crystal with the target diameter;
[0023] (vi) reducing the target diameter to a final cone; and
[0024] (vii) the separation of the single crystal from the melt;
[0025] characterized in that in step (iii) during the drawing of a part of the thin neck the drawing speed is periodically changed for not less than 5 periods and not more than 30 periods such that the diameter of the thin neck in each period initially increases linearly from an initial value, is then held constant, and subsequently decreases linearly again to the initial value;
[0026] in each period, the axial length of the section of the thin neck where the diameter decreases linearly (L2) is at least three times the axial length of the section of the thin neck where the diameter increases linearly (L1); and
[0027] The crucible and the seed crystal rotate in the same direction during the drawing of the part of the thin neck in which the drawing speed is periodically changed, and the rotational speeds of the crucible and the rotational speeds of the seed crystal are independent of each other and are not less than 0.5 and not more than 15 revolutions per minute.
[0028] According to the second aspect, the present invention relates to a method for producing wafers from single-crystal silicon. This method comprises the following steps:
[0029] Pulling a single crystal from silicon according to the method according to the first aspect of the invention;
[0030] Grinding of the single crystal;
[0031] Sawing the polished single crystal into slices;
[0032] Grinding and / or lapping of the discs;
[0033] chemical etching and / or cleaning of the discs; and
[0034] Polishing the discs.
[0035] In single crystals of silicon with a <110> Dislocations frequently occur in crystal orientations determined using the Czochralski method. A typical type of dislocation is a central defect. Central defects are linear combinations of different dislocations, such as multiple screw dislocations and / or edge dislocations, connected by kinks. Central defects arise in the 2024P00023WQ / HA
[0036] 6
[0037] Dislocations occur when the seed crystal comes into contact with the melt and propagates through the thin neck. To propagate through the thin neck, these defects must extend from the center of the seed crystal, across the thin neck, to the center of the growing single-crystal rod. Any deviation of the growth direction from the desired crystal orientation causes the dislocations to move away from the center as the single-crystal rod grows longer. Therefore, with increasing distance from the thin neck, the dislocations propagating through the single-crystal rod can move further and further away from the center of the growing rod.
[0038] Brief description of the characters
[0039] Fig. 1 shows the diameter profile of a portion of the thin neck drawn according to the inventive method in the exemplary embodiment. In each of the eight periods of the thin neck, the axial length L2 of the section of the thin neck where the diameter decreases linearly is more than three times the axial length L1 of the section of the thin neck where the diameter increases linearly. The minimum diameter of a period is designated D1 and the maximum diameter of a period D2. The arrow indicates the drawing direction, i.e., the direction of the crystal growth.
[0040] Fig. 2 shows (i) the rotational speed R1 of the growing single crystal Is / Im seed crystal during the drawing of a thin neck (top), and (ii) the rotational speed R2 of the crucible during the drawing of a thin neck (bottom). The identical signs of the rotational speeds of the seed crystal and the crucible indicate that they rotate in the same direction. The drawing speed was varied periodically for 12 periods over a length of approximately 100 mm to approximately 400 mm.
[0041] Detailed description of the invention
[0042] In the inventive method according to the first aspect, in step (i) a crucible containing a silicon melt and a seed crystal made of 2024P00023WQ / HA is first prepared.
[0043] 7
[0044] Single-crystal silicon is provided above the silicon melt. The terms seed crystal and seedling are used synonymously. The seed crystal is preferably rotationally symmetric and is arranged above the silicon melt such that its axis of rotation is vertical, i.e., aligned along the drawing direction. Particularly preferably, the seed crystal is cylindrical and its cylinder axis is aligned parallel to the drawing direction. The crucible is preferably a quartz crucible.
[0045] The seed crystal preferably has a diameter of not less than 5 mm and not more than 25 mm. The axial length of the seed crystal preferably corresponds to at least five times, and particularly preferably to at least eight times, the diameter of the seed crystal.
[0046] The silicon melt and the seed crystal made of single-crystal silicon consist essentially of silicon. In a preferred embodiment, the silicon content in the melt and in the seed crystal is not less than 99.0 wt.% in each case, more preferably not less than 99.8 wt.% in each case, and most preferably not less than 99.9 wt.% in each case. The silicon concentrations in the melt and in the seed crystal can be varied independently of one another.
[0047] The silicon melt and / or the seed crystal may optionally contain dopants, for example, of the p-type and / or n-type. The dopant is, for example, at least one element selected from boron, aluminum, gallium, indium, carbon, germanium, nitrogen, phosphorus, arsenic, antimony, hydrogen, and oxygen.
[0048] The silicon melt preferably has a temperature of not less than 1410°C, preferably not less than 1420°C. The melting point of a silicon melt is approximately 1410°C. In one embodiment, the melt is superheated. This means that the temperature of the melt is higher than its melting point. The measuring unit for the temperature can be, for example, a pyrometer, a thermal imaging camera, or a thermocouple. The measuring unit for the temperature is preferably a pyrometer. The temperature of the melt at a specific measuring point can be measured with an accuracy of 0.1°C and controlled with an accuracy of 1.0°C. The temperature of the melt is controlled by one or more heating elements, preferably one or more resistance heaters, which are arranged, for example, around the crucible.The heating elements are controlled via a programmable control unit, which is connected to the measuring unit and regulates the temperature of the melt, in particular the temperature at the surface of the melt at the measuring point, depending on the measured temperature with an accuracy of 1.0°C.
[0049] In step (ii), the seed crystal is lowered until contact is established between the seed crystal and the silicon melt. The contact, as defined in the present invention, can be at least one of the following: contact, thermal contact, or electrical contact between the underside of the seed crystal and the surface of the melt. A portion of the seed crystal can also be immersed in the silicon melt. Preferably, the seed crystal is immersed no more than 5 mm, and particularly preferably no more than 2 mm, in the silicon melt. The seed crystal can be attached to a draw shaft or a cable, by means of which it can be moved downwards in a vertical direction along the draw axis and brought into contact with the melt.
[0050] Step (ii) may additionally include preheating the seed crystal above the melt during lowering and before contact is established between the seed crystal and the silicon melt. The preheating step is preferably carried out by holding the seed crystal at a distance of at least 5 mm and at least 50 mm above the melt for a period of at least 5 minutes. Preheating allows the temperature of the seed crystal to be raised slowly, thus minimizing thermal stresses and consequently dislocations within the seed crystal. Lowering and contacting the seed crystal with the melt in step (ii) is preferably carried out at a rate of at least 30 mm / min. 2024P00023WQ / HA
[0051] 9
[0052] After lowering and contacting the seed crystal with the surface of the silicon melt in step (ii), optionally, a submerged portion of the seed crystal can be remelted. This allows the portion of the seed crystal immersed in the silicon melt to be melted in order to remove the part of the seed crystal exhibiting the most thermal stresses and thus dislocations.
[0053] The seed crystal is aligned along the drawing direction. The axis of rotation of the seed crystal, which is preferably rotationally symmetric, therefore runs in a vertical direction. The seed crystal preferably has a <110> -Crystal orientation. In the case of a <110> - Crystal orientation: the crystal plane runs perpendicular to the drawing direction with a Miller index of (110), and thus, in the case of a cylindrical seed crystal, perpendicular to the cylinder axis of the seed crystal. The desired crystal orientation within the meaning of the present invention is also present if the angle between the plane running perpendicular to the drawing direction and the plane with the desired crystal orientation is no more than 5°. This means that a plane perpendicular to the crystal plane with the desired orientation, for example, a <110> -Crystal orientation, arranged axis may deviate by up to 5° from the orientation of the drawing axis, i.e. the drawing direction.The drawing direction therefore deviates from the desired crystal orientation by no more than 5°. Preferably, the drawing direction deviates from the desired crystal orientation by no more than 4°, particularly preferably by no more than 3°, and most preferably by no more than 2°. In a further particularly preferred embodiment, the drawing direction deviates from the desired crystal orientation by no more than 1.5°. The desired crystal orientation is preferably a <110> -Crystal orientation.
[0054] By varying the drawing speed, also known as crystal stroke, the rotation speed, also known as crystal rotation, and the temperature of the melt, the growing, rotating crystal rod reaches the desired diameter. With suitable control, the diameter of the crystal rod can be very precisely controlled until the end of the drawing process. The exact position and shape of the seed crystal relative to the melt, the phase boundary, and the shape of the growing crystal rod, especially the thin neck, are preferably determined using a camera system. 2024P00023WQ / HA
[0055] 10
[0056] A conventional apparatus for growing a single crystal using the Czochralski method comprises a growing chamber containing a crucible for receiving the melt, a growing shaft or pulley, a measuring unit for measuring the temperature, and a camera system for observing a phase boundary and the shape of the growing single crystal. For example, a crystal growing system of type EKZ 3000 from the manufacturer PVA TePla can be used to carry out the method according to the invention.
[0057] In step (iii) of the method according to the first aspect of the present invention, the seed crystal is pulled into a thin neck with a maximum diameter of no more than 20 mm by lifting the seed crystal while rotating it. The diameter of the thin neck in step (iii) is preferably controlled by the pulling speed at which the seed, or the growing single crystal, is lifted. The thin neck preferably has a length of no less than 100 mm and no more than 600 mm, more preferably a length of no less than 150 mm and no more than 450 mm. The minimum diameter of the thin neck is preferably no less than 3 mm.
[0058] Preferably, the temperature of the silicon melt in step (iii) is reduced compared to the temperature of the silicon melt in steps (i) and (ii), preferably by not less than 2°C and not more than 20°C, before the thin neck is drawn. The temperature of the silicon melt during step (iii) of the process according to the invention is preferably above 1390°C.
[0059] In step (iii), during the drawing of a portion of the thin neck, the drawing speed is periodically varied for no fewer than 5 periods and no more than 30 periods, preferably for no fewer than 8 periods and no more than 20 periods, such that the diameter of the thin neck in each period initially increases linearly from a starting value, is then held constant, and subsequently decreases linearly again to the starting value. In each period, the axial length of the section of the thin neck in which the diameter decreases linearly is at least three times, preferably at least four times, and particularly preferably at least six times, the axial length of the section of the thin neck in which the 2024P00023WG / HA
[0060] 11
[0061] The diameter increases linearly. It has been observed that, with a specific crystal and crucible rotation, the periodic variation of the diameter, with the relatively steep rise and gradual fall of the diameter within a period defined above, results in fewer dislocations remaining in the crystal and able to fully penetrate the thin neck. Thus, a dislocation-free cylindrical section of the single crystal can be obtained. The portion of the thin neck in which the diameter varies periodically, and therefore in which the drawing speed is also varied periodically, preferably has an axial length of not less than 100 mm and not more than 600 mm, more preferably not less than 100 mm and not more than 450 mm, and most preferably not less than 200 mm and not more than 450 mm.
[0062] The relatively steep increase in diameter during the period results in a convex shape at the phase boundary, meaning a downward-curving crystal and a downward-curving silicon melt at the interface. Because the decrease in diameter during the period is relatively gradual, occurring over at least three times the axial length compared to the increase, a concave phase boundary is avoided. In a concave phase boundary, the silicon melt would be curved upwards, and the growing crystal at the interface would also be curved upwards, thus exhibiting a concave shape. At a concave phase boundary, dislocations can migrate to the center of the thin neck, making dislocation removal difficult.The number of dislocations propagating through the thin neck can be further reduced if, in each period, the axial length of the section of the thin neck where the diameter decreases linearly (L2) is at least four times, preferably at least six times, the axial length of the section of the thin neck where the diameter increases linearly (L1).
[0063] The axial length of one period of the thin neck is preferably not less than 10 mm and not more than 50 mm, more preferably not less than 20 mm and not more than 30 mm. The difference between the maximum diameter and the minimum diameter of the thin neck in one period is preferably not less than 0.5 mm and not more than 3.0 mm, more preferably not less than 1.0 mm and not more than 2.0 mm. The minimum diameter of the thin neck in a 2024P00023WQ / HA
[0064] 12
[0065] The period is preferably not less than 3.0 mm, more preferably not less than 3.5 mm and not more than 5.5 mm, and most preferably not less than 4.0 mm and not more than 5.0 mm. The maximum diameter of the thin neck in one period is preferably not more than 15 mm, more preferably not more than 10 mm, particularly preferably not less than 5.5 mm and not more than 10 mm, and most preferably not less than 5.5 mm and not more than 7.0 mm.
[0066] The crucible and the growing single crystal / seed crystal rotate in the same direction during the drawing of the part of the thin neck in which the drawing speed is periodically changed, with the rotational speeds of the crucible and the seed crystal being independent of each other of not less than 0.5 and not more than 15 revolutions per minute.
[0067] Preferably, in step (iii), the rotational speed of the crucible is initially kept constant during the drawing of the portion of the thin neck in which the drawing speed is periodically varied, and then continuously reduced. In a particularly preferred embodiment, in step (iii), during the drawing of the portion of the thin neck in which the drawing speed is periodically varied, the rotational speed of the crucible is initially kept constant at a value of not less than 5 revolutions per minute and not more than 15 revolutions per minute, more preferably not less than 8 revolutions per minute and not more than 12 revolutions per minute, and then continuously reduced.
[0068] Preferably, in step (iii) during the drawing of the part of the thin neck in which the drawing speed is periodically changed, the seed crystal / growing single crystal is first rotated with a constant first rotation speed, then with a reduced constant second rotation speed and finally again with the constant first rotation speed.
[0069] Preferably, the first rotational speed is not less than 8 revolutions per minute and the second rotational speed is not more than 4 revolutions per minute. Particularly preferably, the first rotational speed is not less than 8 and not more than 12 revolutions per minute.
[0070] 13
[0071] The first rotational speed is not less than 0.5 and not more than 4 revolutions per minute, and the second rotational speed is not less than 0.5 and not more than 4 revolutions per minute. Particularly preferably, the first rotational speed is not less than 8 and not more than 12 revolutions per minute, and the second rotational speed is not less than 1 revolution per minute and not more than 3 revolutions per minute.
[0072] Particularly preferred in step (iii) during the drawing of the portion of the thin neck in which the drawing speed is periodically varied, both (a) the rotational speed of the crucible is initially kept constant and then continuously reduced, and (b) the seed crystal / growing single crystal is rotated, first at a constant first rotational speed, then at a reduced constant second rotational speed, and finally again at the constant first rotational speed. This preferred embodiment is illustrated in Fig. 2, which shows the rotational speed of the growing single crystal / seed crystal (top) and the crucible (bottom) as a function of the length of the thin neck.
[0073] Following the drawing of the thin neck, step (iv) involves expanding the diameter of the thin neck to a target diameter. The target diameter is preferably not less than 200 mm and not more than 450 mm, particularly preferably not less than 210 mm and not more than 330 mm. Most preferably, the target diameter is not less than 300 mm and not more than 320 mm. In step (iv), the diameter of the growing single-crystal rod is continuously increased until the desired single-crystal diameter is reached. This can be achieved by reducing the drawing speed. The section of the growing single crystal drawn in this step is preferably conical.
[0074] In step (v), a cylindrical section of the single crystal with the target diameter is drawn. This section of the single crystal rod with the target diameter is also referred to as the cylindrical section of the single crystal rod. Within this section, the diameter, in every position along the axial direction (i.e., along the drawing direction), lies within the range of the target diameter. The cylindrical section of the single crystal rod is therefore the section of the single crystal rod suitable for producing wafers with the desired nominal diameter, which is smaller than the target diameter.
[0075] The diameter of the cylindrical section can vary along its axial length, provided that the diameter at any point is at least 1 mm larger than the desired nominal diameter of the semiconductor material wafers to be produced. In one embodiment, the diameter of the cylindrical section varies by less than 20 mm, preferably less than 10 mm, and particularly preferably less than 5 mm, along its axial length, with the variation corresponding to the difference between the largest and smallest diameters along the axial length of the cylindrical section.
[0076] In step (vi) the target diameter is reduced, preferably by increasing the drawing speed, and thus an end cone is formed that tapers downwards.
[0077] In step (vii) the single crystal rod is separated from the melt. The single crystal can then be removed.
[0078] Preferably, in the inventive method for growing a single silicon crystal according to the Czochralski method, a magnetic field is applied, preferably during steps (i) to (vii). The magnetic coils are preferably arranged around the crucible such that a horizontal or a CUSP magnetic field is generated. Preferably, the magnetic field within the crucible has a strength in the range of 500 G to 3000 G.
[0079] The second aspect of the present invention relates to a method for producing wafers from single-crystal silicon, comprising the following steps:
[0080] Pulling a single crystal from silicon according to the method according to the first aspect of the present invention;
[0081] Grinding of the single crystal;
[0082] Sawing the polished single crystal into slices; grinding and / or lapping the slices;
[0083] chemical etching and / or cleaning of the discs; and
[0084] Polishing the discs.
[0085] Preferably, the steps are carried out in the order given above. Additionally, an epitaxial layer of semiconductor material can be deposited on the single-crystal silicon wafer.
[0086] The disks can then be examined for dislocations, for example using SIRD (Scanning Infrared Depolarization). SIRD measurements are performed using a transmission dark-field polariscope, utilizing the effect of stress-induced birefringence. This effect is based on the fact that dislocations generate stress fields. When a material is subjected to shear stress, its refractive index becomes anisotropic and the material birefringent. Incident linearly polarized laser light is thus depolarized by the sample, either through transmission or reflection. Consequently, the resulting depolarization distributions can be interpreted as shear stress distributions.
[0087] Detailed description of an embodiment according to the invention and a comparative example.
[0088] In the exemplary embodiment, a cylindrical seed crystal made of single-crystal silicon was used with a <110> -Crystal orientation and a diameter of 18 mm used to create a crystal rod with <110> -Orientation according to the Czochralski method. The seed crystal was attached to the drawing shaft of a PVATePla EKZ 3000 crystal growing system. A quartz crucible filled with solid, high-purity polysilicon from Wacker Chemie was positioned between a resistance heater in the crystal growing system. After closing, rinsing, and evacuating the crystal growing system, the quartz crucible containing the silicon was heated, thus providing a silicon melt. The crystal growing system also included a camera system for observing a phase boundary and a pyrometer for determining the melt temperature. The exact position and shape of the seed crystal relative to the melt, the phase boundary, and the shape of the 2024P00023WG / HA
[0089] 16
[0090] The growing crystal rod, especially the thin neck, could thus be precisely determined using the camera system.
[0091] The rotational speed of the crucible and the single crystal was set to 10 revolutions per minute. The seed crystal was then lowered, held at a distance of at least 20 mm above the melt for a period of at least 5 m, and finally lowered further until part of the seed was immersed in the melt. After the immersed portion melted back into the melt, the seed crystal was slowly moved upwards while rotating, and a thin neck with a lower diameter of 5 mm was drawn by continuously increasing the drawing speed. Subsequently, a thin neck with a length of 450 mm was drawn.During the drawing of a portion of the thin neck, the drawing speed was periodically varied for 12 periods such that the diameter of the thin neck increased from 5 mm to 6 mm over an axial length of 3 mm in one period with an axial length of 25 mm. The diameter then remained constant for an axial length of 3 mm, before decreasing linearly back to the initial value over an axial length of 19 mm. The portion of the thin neck with the periodic diameter change is shown in Fig. 1. After a thin neck with a length of 200 mm had been drawn, the crystal rotation (rotation of the seed crystal / growing single crystal) was reduced to 1 revolution per minute for a length of 100 mm. Subsequently, the crystal rotation was increased again to 10 revolutions per minute for the remaining drawing of the thin neck.
[0092] Simultaneously, during the drawing of the thin neck, the rotational speed of the crucible was kept constant at 10 revolutions per minute up to a thin neck length of approximately 350 mm and then reduced. The rotational speed profile of the seed crystal / growing single crystal on the one hand and the crucible on the other, as seen in the exemplary embodiment, is shown in Fig. 2.
[0093] The thin neck was drawn to a final length of 450 mm. Subsequently, the diameter of the thin neck was enlarged to the target diameter in the range of 305 to 315 mm, and a cylindrical section with this diameter was drawn. Finally, the target diameter was continuously reduced again, thus forming an end cone with a decreasing diameter. The crystal rod was separated from the melt and removed from the crystal drawing machine.
[0094] The finished crystal rod was ground and sawn into slices with a thickness of 900 pm. These were then cut into discs with a diameter of 200 mm, ground to a thickness of 750 pm, and burnished with an etching.
[0095] These disks were then examined for stresses using SIRD measurement. For the stress test, each disk was placed on a rotating stage of a PSI-type measuring device manufactured by Semilab. The measuring device had a fixed, linearly polarized infrared light source and a rotating stage. The disk rotating on the stage was continuously moved radially during the measurement so that the entire surface of the disk was scanned by the light source. It was shown that the disks exhibited no stresses, in particular no stresses within a 50 mm radius of the disk center. Thus, in this exemplary embodiment, defect-free disks made of single-crystal silicon with a <110> -Crystal orientation achieved without defects near the center.
[0096] The comparative example differed from the method of the exemplary embodiments only in that a thin neck was drawn with a constant drawing speed and constant crystal rotation, i.e. without periodic change of the drawing speed and thus without periodic change of the diameter.
[0097] As in the exemplary embodiment, the crystal rod produced in the comparative example was also removed and sawn into slices with a thickness of 900 pm. From these, slices with a diameter of 200 mm were then produced, ground to a thickness of 750 pm, and burnished. These slices were examined for stresses using SIRD measurements, just like the slices produced in the exemplary embodiment. Stresses, and thus defects near the center of the slice, were detected.
Claims
Claims 1. Method for growing a single crystal from silicon according to the Czochralski method, comprising the following steps in this order: (i) the provision of a crucible containing a silicon melt and a seed crystal of single-crystal silicon above the silicon melt; (ii) lowering the seed crystal until contact is established between the seed crystal and the surface of the silicon melt; (iii) drawing the seed crystal into a thin neck with a maximum diameter of not more than 20 mm by lifting the seed crystal while rotating it; (iv) widening the diameter of the thin neck to a target diameter; (v) drawing a cylindrical section of the single crystal with the target diameter; (vi) reducing the target diameter to a final cone; and (vii) the separation of the single crystal from the melt; characterized by the fact that In step (iii) during the drawing of a part of the thin neck, the drawing speed is periodically changed for not fewer than 5 periods and not more than 30 periods such that the diameter of the thin neck in each period initially increases linearly from an initial value, is then held constant, and subsequently decreases linearly again to the initial value; in each period, the axial length of the section of the thin neck where the diameter decreases linearly (L2) is at least three times the axial length of the section of the thin neck where the diameter increases linearly (L1); and The crucible and the seed crystal rotate in the same direction during the drawing of the part of the thin neck in which the drawing speed is periodically changed, and the rotational speeds of the crucible and the rotational speeds of the seed crystal are independent of each other and are not less than 0.5 and not more than 15 revolutions per minute.
2. A method for pulling a single crystal from silicon according to claim 1, wherein the axial length of one period of the thin neck is not less than 10 mm and not more than 50 mm.
3. Method for drawing a single crystal from silicon according to claim 1 or 2, wherein the length of the part of the thin neck in which the drawing speed is periodically varied is not less than 100 mm and not more than 450 mm.
4. Method for pulling a single crystal from silicon according to any one of claims 1 to 3, wherein the difference between the maximum diameter (D2) and the minimum diameter of the thin neck (D1) in one period is not less than 0.5 mm and not more than 3.0 mm.
5. Method for pulling a single crystal from silicon according to any one of claims 1 to 4, wherein the minimum diameter of the thin neck (D1) in one period is not less than 3.0 mm.
6. Method for pulling a single crystal from silicon according to any one of claims 1 to 5, wherein the maximum diameter of the thin neck (D2) in one period is not more than 15 mm.
7. Method for pulling a single crystal from silicon according to any one of claims 1 to 6, wherein the minimum diameter of the thin neck (D1 ) in one period is not less than 3.5 mm and not more than 5.5 mm.
8. Method for pulling a single crystal from silicon according to any one of claims 1 to 7, wherein the maximum diameter of the thin neck (D2) in one period is not less than 5.5 mm and not more than 10 mm.
9. Method for pulling a single crystal from silicon according to any one of claims 1 to 8, wherein the pulling direction is not more than 5° from the orientation <110> differs.
10. Method for pulling a single crystal from silicon according to any one of claims 1 to 9, wherein in step (iii) during the pulling of the part of the thin neck in which the pulling speed is periodically changed, the rotational speed of the crucible is first kept constant and then continuously reduced.
11. Method for pulling a single crystal from silicon according to any one of claims 1 to 10, wherein in step (iii) during the pulling of the part of the thin neck in which the pulling speed is periodically changed, the rotational speed of the crucible is initially kept constant at a value of not less than 5 and not more than 15 revolutions per minute and is subsequently continuously reduced.
12. Method for pulling a single crystal from silicon according to any one of claims 1 to 11, wherein in step (iii) during the pulling of the part of the thin neck in which the pulling speed is periodically changed, the seed crystal is first rotated at a constant first rotation speed, then at a reduced constant second rotation speed and finally again at the constant first rotation speed.
13. Method for pulling a single crystal from silicon according to claim 12, wherein the first rotational speed is not less than 8 revolutions per minute and the second rotational speed is not more than 4 revolutions per minute.
14. Method for producing wafers from single-crystal silicon, comprising the following steps: Pulling a single crystal from silicon according to the method according to any one of claims 1 to 13; Grinding of the single crystal; Sawing the polished single crystal into slices; Grinding and / or lapping of the discs; chemical etching and / or cleaning of the discs; and Polishing the discs.
15. A method for producing wafers of single-crystal silicon according to claim 14, wherein the method further comprises the following step: Deposition of an epitaxial layer of semiconductor material onto the wafers of single-crystal silicon.