Method for producing a monocrystalline rod piece from silicon

Abstract

A method for producing a single crystal from silicon in a crystal-pulling apparatus comprising the following steps: (a) melting polycrystalline silicon to a melt; (b) contacting a seed crystal with the surface of the melt; (c) pulling the seed crystal to a thin neck of a diameter; (d) expanding the diameter of the thin neck to a target diameter; (e) pulling a substantially cylindrical subsection of the single crystal to said target diameter, wherein geometric parameters are measured as a function of the crystal length and stored as a numerical sequence together with at least one feature from the pulling process from a list of features including pulling speed and crystal rotation; (f) terminating the pulling of the substantially cylindrical subsection; (g) separating the single crystal from the melt and removing the single crystal from the crystal-pulling apparatus;(h) cutting the substantially cylindrical section into substantially cylindrical rod pieces at fixed rod positions, characterized in that a zero position is defined at an axial position on the single crystal, the diameter of the single crystal is measured along its length in relation to the zero position and the values ​​obtained are correlated with the series of geometric parameters, so that features from the drawing process are obtained relative to the zero position which serve to define the rod positions.

Description

[0001] The invention relates to a method for producing a monocrystalline rod piece made of silicon, comprising melting polysilicon, drawing a single crystal, and dividing the cylindrical part into rod pieces.

[0002] Single-crystal silicon, which is used as the starting material for most processes for the production of electronic semiconductor devices, is usually produced using the so-called Czochralski process (“CZ”) or the so-called zone drawing process, also called the float zone process (“FZ”).

[0003] In both processes, polycrystalline silicon (“polysilicon”) is melted, a seed crystal is brought into contact with the molten silicon, and a single crystal is grown by slow extraction or slow pulling.

[0004] In the Czochralski process, the crucible is typically made of a silicon dioxide-containing material such as quartz. It is usually filled with fragments and / or granules of polycrystalline silicon, which is melted using a side heater arranged around the crucible and a bottom heater located underneath. After a period of thermal stabilization of the melt, a single-crystal seed crystal is immersed in the melt and lifted. Silicon then crystallizes at the end of the seed crystal wetted by the melt. The rate of crystallization is primarily influenced by the rate at which the seed crystal is lifted (crystal lift rate) and by the temperature gradient at the interface where molten silicon crystallizes.By appropriately controlling these parameters, a section called the "neck" is first extracted to remove dislocations, followed by a conical section of the single crystal (initial cone), and finally a cylindrical section from which the semiconductor wafers are later separated. Sometimes, a so-called ring heater is used for heating, in which the heater is heated by an electrical current. This ring heater is described, for example, in EP 2 546 392 A1.

[0005] Zone growth of single crystals, on the other hand, is known according to the state of the art (J. Bohm et al.: "Handbook of Crystal Growth", Ed.: DTJ Hurle, Vol. 2, Part A, 213-257, 1994) and is used on an industrial scale for the production of single-crystal materials. In this process, the starting material, in the form of polycrystalline silicon, is melted in the zone using an induction coil carrying a high-frequency current. As the material is pulled vertically, it solidifies as a single crystal, which is usually rotated during this process. Depending on the specific design of the process, the single crystal can be pulled upwards or downwards. The electromagnetic field of the induction coil generates a flow with a double-vortex structure in the molten zone. This flow is always directed inwards in the center of the zone, while near both ends of the molten zone, the flow is always directed radially outwards.The resulting flow in the melting zone is generated not only by electromagnetic forces but also by buoyancy and Marangoni forces, as well as by the rotation of the rod or crystal. The geometry of the solidifying phase boundary adjusts itself according to the temperature distribution prevailing in the rod, which in turn is influenced by the flow conditions.

[0006] When growing single crystals from semiconductor materials, the diameter of the single crystal must generally be monitored, i.e., determined and, if necessary, influenced. The change in the diameter of the single crystal, which depends on the radial growth of the single crystal at the crystallization boundary, can be influenced, for example, by selectively changing the pull-through rate of the single crystal and / or the temperature of the melt in the region of the crystallization boundary.

[0007] The diameter of a single crystal can typically be determined using optical instruments such as a camera. This is done by capturing, for example, three points on a bright ring surrounding the single crystal in the region of the crystallization boundary and calculating the diameter from these points. This bright ring is a reflection from the glowing wall of the crucible containing the melt, which forms a meniscus in the region of the crystallization boundary.

[0008] Towards the end of the drawing process, a so-called end cone is typically drawn on the single crystal; that is, after a long cylindrical section, the single crystal tapers conically to a small diameter. This prevents any glide or dislocations that occur during the separation of the single crystal from the melt from propagating far back into the single crystal, especially into the cylindrical section, which is subsequently used, for example, in the production of wafers.

[0009] In the production of a crystal using the CZ or FZ process, the axial position of the crystal in the drawing machine, i.e., the crystal length, is defined, for example, by the position of the drawing shaft in the CZ or FZ drawing machine. In CZ machines, this can also be achieved using a rope.

[0010] After the crystal is grown, it is removed from the growing system and must be cut into crystal pieces. The challenge lies in determining the precise cutting points on the crystal. To ensure that the cutting points correspond to features from the growing process, the growing system's position must be transferred to the crystal as accurately as possible.

[0011] According to the invention, a method for producing a monocrystalline silicon rod with the features of the independent claim is proposed. Advantageous embodiments are the subject of the dependent claims and the following description.

[0012] A method according to the invention for producing a single-crystal rod piece made of silicon is carried out using a crystal pulling system, comprising the following steps.

[0013] Melting polycrystalline silicon into a melt is an essential part of the process. This can be done either continuously or batchwise. In a float zone (FZ) process, for example, a small portion of the polycrystalline silicon is continuously melted while the monocrystalline rod grows. In a second example, the Czochralski (CZ) process, a crucible is filled with a quantity of polysilicon that is completely melted before the production of a monocrystalline rod begins.

[0014] In all embodiments of the invention, it is necessary that a seed crystal be brought into contact with the surface of the melt. This seed crystal is drawn into a so-called thin neck (also referred to as a "dash neck") with a certain diameter. This thin neck serves to allow dislocations that arise during the contacting of the seed crystal to flow out of the crystal, so that after drawing the thin neck, a crystal without dislocations can be formed.

[0015] According to the invention, after drawing the thin neck, the diameter of the thin neck is enlarged to the target diameter of the monocrystalline rod to be produced. Preferably, the target diameter is more than 250 mm and less than 450 mm.

[0016] Once the target diameter is reached, the drawing of the essentially cylindrical section of the single crystal begins. The term "essentially cylindrical" takes into account that the surface of this cylindrical section is never perfectly smooth, i.e., without any variation in diameter. While there are legitimate efforts to produce this section of the rod as smoothly as possible, since any unevenness also means economic losses, it is physically impossible to completely avoid these irregularities. The reason for this lies in the inertia of the control system for diameter regulation during crystal drawing, which applies to both of the crystal drawing technologies mentioned above. The observable, mostly irregular, diameter fluctuations range from less than 3 mm to greater than 0.1 mm.

[0017] In the drawing process according to the invention, the substantially cylindrical section of the single crystal with the aforementioned target diameter is drawn, and geometric parameters are measured as a function of the crystal's length. These parameters are then stored as a numerical sequence together with at least one feature from the drawing process, selected from a list of features including drawing speed and crystal rotation. The geometric parameters in this context are understood to be the current diameter of the crystal or the current crystal angle.

[0018] Preferably, the numerical sequence includes an additional feature from the drawing process from a list of features including crucible rotation, side heater power, and bottom heater power. Particularly preferably, the numerical sequence additionally includes the ring heater power.

[0019] The crystal diameter is measured, for example, with a camera system, which can be used to change parameters of the crystal growing system from the measured diameter or its deviation from the target value in such a way that the amount of the deviation of the current diameter from the target diameter is minimal.

[0020] The crystal angle can be determined, for example, by calculating the difference quotient of two adjacent diameter values ​​for the corresponding measured diameter.

[0021] After the essentially cylindrical section of the crystal to be grown is pulled, an end cone or end piece of the crystal is drawn, which prevents dislocations from being generated when the crystal is separated from the remaining melt. The single crystal is separated from the melt and removed from the crystal growing machine.

[0022] For further processing of the monocrystalline material, the drawn rod is cut into pieces, whereby, according to the inventive method, the essentially cylindrical section is cut into essentially cylindrical rod pieces at defined cutting positions (rod positions).

[0023] The inventors have recognized that, for example, setting a cutting position after reaching the target diameter, i.e., at the beginning of the essentially cylindrical section of the rod, is not sufficiently reproducible if only the geometry of the rod is used to measure or determine the cutting position.

[0024] If the cutting position is set too early, i.e., too close to the non-cylindrical section, semiconductor wafers will later be produced from the cut-off rod piece that do not meet semiconductor manufacturing specifications. These wafers must be discarded later, but still contribute to the production costs. If the cutting position is set too late, material suitable for semiconductor manufacturing is wasted. With conventional cutting position settings, this error can be up to 2 cm, which can represent a significant loss in value.

[0025] The inventors recognized that it is essential for the invention to first define a zero position at an axial location on the single crystal. The precise location of this zero position on the crystal is irrelevant. Preferably, however, it is located on the substantially cylindrical section of the crystal.

[0026] The inventors also recognized that it is essential for the invention that geometric parameters of the physical single crystal are measured along its length in relation to the zero position. Geometric parameters here are understood to be the diameter of the crystal as a function of the distance to the set zero position, or the crystal angle as a function of the distance to the set zero position.

[0027] The measurement can be easily carried out using a camera with image processing. Alternatively, tactile measurement methods can also be used to determine the aforementioned geometric parameters.

[0028] Ideal cutting positions are determined according to the invention depending on features of the drawing process. For example, a change in the convection of the melt may occur during the drawing process, causing a change in the temperature at the phase boundary of the crystal, which in turn would result in a change in the crystal diameter. However, the change in diameter is compensated for by the diameter-controlling element, the power of a ring heater in the case of a CZ process, or the power of the high-frequency coil in the case of an FZ process.

[0029] Even if no significant diameter change is visible in the finished crystal (apart from minor diameter fluctuations), the corresponding performance deviation is still present. Performance deviations (and the associated temperature properties of the crystal) influence, for example, defects in the material and are therefore quality-limiting. The inventors have therefore recognized that the affected area on the finished crystal must be precisely determined and taken into account separately when cutting the crystal.

[0030] Another ideal cutting position arises when, for example, the drawing speed of the single crystal is changed during the drawing process to deliberately induce different properties of the crystal. In this case, it is important to transfer the precise position at which the change occurred onto the crystal.

[0031] In reality, it has been shown that a discrepancy of more than 1 cm can exist between the ideal cutting position and the actual cutting position. However, in order to find the theoretically ideal cutting position as accurately as possible on the actual rod, the determined physical geometry parameters measured after crystal drawing and the geometry parameters measured during crystal drawing are correlated according to the invention, so that a distance (offset) ΔI between the spatial coordinates of the determined physical geometry parameters and those of the crystal drawing is calculated.

[0032] Mathematically, a correlation function between the two geometry parameters could look like this: K(Δl)=1lmax∫0lmax(g1(l)⋅g2(l−Δl))dl

[0033] The function K(ΔI) gives the correlation function of the geometry parameter g1 with the geometry parameter g2 as a function of the axial position on the crystal, where the function K is maximized and the associated parameter ΔI represents the distance (offset).

[0034] This distance (offset) allows the ideal cutting position on the rod to be determined with significantly higher accuracy (< 2mm). QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 2 546 392 A1

[0004] Cited non-patent literature

[0000] J. Bohm, u.a.:„Handbook of Crystal Growth", Ed.: D.T.J. Hurle, Vol. 2, Part A, 213-257, 1994

[0005]

Claims

[1] Method for producing a single-crystal rod of silicon using a crystal growing system comprising the following steps: (a) the melting of polycrystalline silicon to a melt; (b) bringing a seed crystal into contact with the surface of the melt; (c) drawing the seed crystal into a thin neck with a diameter; (d) widening the diameter of the thin neck to a target diameter; (e) the drawing of a substantially cylindrical section of the single crystal with said target diameter, wherein geometric parameters are measured as a function of the length of the crystal during the crystal drawing process and stored as a series of numbers together with at least one feature from the drawing process from a list of features including drawing speed and crystal rotation; (f) the cessation of the drawing of the substantially cylindrical section; (g) the separation of the single crystal from the melt and removal of the single crystal from the crystal growing apparatus; (h) cutting the substantially cylindrical section into substantially cylindrical bar pieces at bar positions, characterized by , that a zero position is defined at an axial position on the single crystal and geometric parameters of the single crystal are measured along its length in relation to the zero position and the obtained values ​​are correlated with the numerical series of geometric parameters, so that features from the drawing process relative to the zero position are obtained, which serve to define the rod positions. [2] Method according to claim 1, wherein the geometry parameters include at least one parameter from the group consisting of the parameters diameter and crystal angle and parameters derived therefrom. [3] Method according to claim 1, wherein the method for growing a single crystal is carried out using the Czochralski method. [4] Method according to claim 3, wherein the number sequence is stored together with at least one additional feature from the drawing process from a list of features including crucible rotation, crystal lift, side heater power and bottom heater power. [5] Method according to claim 1 or 3, wherein the number sequence is stored together with at least one additional feature from the drawing process from a list of features containing ring heater power. [6] Method according to claim 1, wherein the method for pulling a single crystal is carried out using the zone pulling method.