WO2021018502A1

WO2021018502A1 - Method for drawing a silicon monocrystal from a melt using the czochralski method

Info

Publication number: WO2021018502A1
Application number: PCT/EP2020/068489
Authority: WO
Inventors: Alexander Molchanov; Matiss PLATE; Andreas Sattler; Dirk Zemke
Original assignee: Siltronic Ag
Priority date: 2019-08-01
Filing date: 2020-07-01
Publication date: 2021-02-04
Also published as: TW202106935A; DE102019211609A1

Abstract

The invention relates to a method for drawing a silicon monocrystal from a melt using the Czochralski method, said method comprising: pulling at least one end section of a starting taper and of a cylindrical portion of the monocrystal with a pulling speed which follows a predefined course and repeated performance of the following steps a) to c) while regulating the diameter of the end section of the starting taper and of the cylindrical portion of the monocrystal: a) measuring the current diameter of the monocrystal; b) determining the deviation of the current diameter from a target diameter of the monocrystal; and c) using the heat output of an annular resistor heater, which has a diameter that is greater than the current diameter of the monocrystal, and which is arranged concentrically with the starting taper over the melt, as a control variable for regulating the diameter of the monocrystal to the target diameter of the monocrystal.

Description

Method for pulling a single crystal of silicon according to the Czochralski

Method from a melt

The invention relates to a method for pulling a single crystal of silicon according to the Czochralski method from a melt, comprising

pulling an initial cone and a cylindrical portion of the single crystal.

State of the art / problems

A single crystal is pulled from a melt according to the Czochralski method, usually by melting polycrystalline material to a melt in a crucible, dipping a single crystal seed crystal into the melt and pulling it away from the melt. In the course of this process, a single crystal is pulled, which is divided into an initial cone, a cylindrical section and a

End cone can be divided. The cylindrical section of the single crystal is of particular interest because it is further processed into semiconductor wafers, which in turn are the basic material for the production of electronic components.

According to US 2003/0033972 A1, it is desirable to have an initial cone

To pull single crystal of semiconductor material in such a way that the initial cone is given a desired shape and that the single crystal has desired properties with regard to the defect distribution of intrinsic point defects and their agglomerates already at the beginning of the drawing of the cylindrical section.

The method described in WO 00/56956 A1 provides for regulating the

Diameter of a single crystal during the pulling of a first section of the single crystal to vary the pulling speed, and to pull the single crystal at a predetermined pulling speed during the pulling of a second section.

In US 2011/0126757 A1 it is proposed to pull the cylindrical part of a single crystal made of semiconductor material at a predetermined pulling speed and to regulate the diameter of the cylindrical part by means of the heating power of a heating source which is arranged above the melt. The course of the pulling speed is chosen with knowledge of the thermal conditions when pulling the cylindrical part of the single crystal in such a way that the quotient v _p / G of pulling speed v _p and axial temperature gradient G at the phase boundary between the melt and the growing single crystal, which is decisive for the defect distribution maps intrinsic point defects, remains in a specified range of values. Faults that manifest themselves in deviations from a target diameter can be counteracted by regulation using the heating source.

The pulling speed v _p in the quotient mentioned above is only an approximation for the crystallization speed v at which the single crystal grows. And this approximation is especially important in the transition from

The initial cone to the cylindrical section is comparatively imprecise. The

Crystallization rate v is in particular through in this phase

Influenced temperature fluctuations in the melt. This often leads to the dynamic equilibrium being achieved with a desired quotient v / G

The specification of the quotient v _p / G cannot be realized at the beginning of the pulling of the cylindrical section of the single crystal. The consequence of this is a loss of yield because a first part of the cylindrical section of the single crystal does not have the desired material properties with regard to the defect distribution of intrinsic point defects.

The method according to WO 2018/069051 A1 provides, when pulling the initial cone, the pulling speed v _p and the heating power of the heating source, which is above the

Melt is arranged to predetermine iteratively. However, this method is comparatively prone to interference.

The object of the present invention is to effectively limit the mentioned loss of yield by providing a comparatively robust method.

The object of the invention is achieved by a method for drawing a

Single crystal of silicon according to the Czochralski method from a melt, comprising pulling at least one end section of an initial cone and a cylindrical section of the single crystal at a pulling speed that follows a predetermined course and repeatedly performing the following steps a) to c) in the course of regulating the diameter of the end section of the

Initial cone and the cylindrical section of the single crystal:

a) measuring an actual diameter of the single crystal;

b) determining the deviation of the current diameter from one

Target diameter of the single crystal; and

c) using the heating power of a ring-shaped resistance heater, which has a diameter which is larger than the current diameter of the single crystal and which is arranged concentrically to the initial cone above the melt, as a control variable for controlling the diameter of the single crystal

Target diameter of the single crystal.

The inventors have found that while the

At the beginning of the cone, the diameter must be regulated which has as little influence as possible on the quotient v / G. So it can be arranged that the

Deviation between the specified pulling speed v _p and the

Crystallization rate v remains sufficiently small that the mentioned

Loss of yield can be avoided. Obviously, it is not sufficient to begin regulating the diameter in the manner described only with the pulling of the cylindrical section of the single crystal. Rather, it must be ensured that this regulation is started before the cylindrical section of the single crystal is pulled. It is particularly preferred to control the

Diameter of the single crystal by means of the ring-shaped resistance heating, as long as at least 15 mm length of the initial cone remains to be drawn. If one proceeds according to the invention, a desired defect distribution of intrinsic point defects in the cylindrical section can be achieved with no or only a slight loss of yield.

The course of the drawing speed is dependent on the desired

Defect distribution of intrinsic point defects given, taking into account the known fact that the ratio of pulling speed and axial Temperature gradients G at the phase boundary between the melt and the single crystal significantly influence this defect distribution. In addition, it is preferred to take into account the influence of stress fields in the single crystal on point defects and their movement, as well as that of temperature fields in the single crystal on the radial diffusion of point defects. How this can be done is described in US 2008/0187736 A1, for example. The radial profile of the axial temperature gradient G at the phase boundary between the melt and the single crystal during the pulling of the cylindrical section of the single crystal is determined in advance, preferably by means of simulation calculations.

To regulate the diameter of the single crystal during the pulling of at least one end section of the initial cone of the single crystal and during the pulling of the cylindrical section of the single crystal, a feedback control is set up, with a specified target diameter as the reference variable, the current diameter as the controlled variable and the heating power of the annular

Resistance heating as a manipulated variable. The heating power of a main heating source for heating the crucible from the side follows a predetermined profile and is preferably readjusted if necessary in order to relieve the annular resistance heating. The diameter of a first part of the initial cone is regulated up to the end section of the initial cone, preferably with the drawing speed v _p as the manipulated variable.

To measure the current diameter (actual diameter) of the single crystal, images from a camera system are preferably evaluated by means of automatic image processing, which depict the bright ring that is on the melt around the

can be seen growing single crystal around. The diameter of the single crystal can be deduced from the diameter of the bright ring. In principle, any known method for measuring the current diameter can be used. The controller of a control loop according to the invention is designed as a PID controller, preferably as a PD controller.

A desired defect distribution of intrinsic point defects preferably comprises a distribution in which such point defects are indeed present in the single crystal, but only in an amount below a concentration at which they are in an agglomerated state. When the preferred distribution is present, agglomerates in particular in the form of COP defects (crystal originated particles), OSF defects (oxidation induced stacking faults), B-band defects and Lpit defects (large pits) cannot be detected. The intrinsic point defects are preferably distributed in such a way that semiconductor wafers made of silicon obtained from the cylindrical section of the single crystal either only have silicon interstitials or only vacancies

(vacancies) as point defects, but no detectable agglomerates thereof with a size greater than 5 nm. Accordingly, the course of the pulling speed v _{p is} preferably specified such that the concentrations of vacancies and silicon interstitials remain below critical concentrations. The critical concentrations can be determined by comparison with the occurrence of larger agglomerates.

A silicon single crystal pulled by the method according to the invention preferably has a diameter of at least 300 mm.

The invention is further described below with reference to drawings.

Brief description of the figures

1 shows an apparatus for pulling a single crystal made of silicon according to the CZ method.

FIG. 2 shows the course of the deviation in the pulling speed as a function of the position in the single crystal as a result of two jumps in the pulling speed.

FIG. 3 shows the change in the concentration of point defects in the single crystal as a result of the cracks shown in FIG. 4 shows the course of the pulling speed as a function of the crystal position when using a method according to the invention and when using a regulation of the diameter of the single crystal that is not according to the invention. 5 shows the course of the deviation of the diameter of a single crystal from a nominal diameter as a function of the position in the single crystal when using the method according to the invention and when using a counter-example not according to the invention. List of reference symbols used

1 drawing chamber

2 crucibles

3 melt

4 camera system

5 single crystal

6 wave

7 resistance heating

8 pulling mechanism

9 heat shield

10 annular resistance heating

12 optical axis

The device according to FIG. 1 comprises a pulling chamber 1, in which a crucible 2 for receiving the melt 3 is accommodated, and a camera system 4 for

Observation of a phase boundary between the melt 3 and a growing single crystal 5 along an optical axis 12. The crucible 2 is carried by a shaft 6 which can be raised, lowered and rotated. A resistance heater 7 is arranged around the crucible 2, with the aid of which solid silicon is melted to form the melt 3. The growing single crystal 5 is pulled from the melt via a pulling mechanism 8. A heat shield 9 and an annular resistance heater 10, which surround the growing single crystal 5, are arranged above the crucible 2. About the resistance heater 10 can a Shielding (not shown) may be provided which restricts the upward transport of heat.

FIG. 2 and FIG. 3 are intended to illustrate how fluctuations in the pulling speed v _{p can change} the concentration C of point defects in the single crystal. In FIG. 2, two jumps in the pulling speed Dn _r as a function of the axial position P in the single crystal are shown with a continuous curve and a dashed curve. The position Po denotes the beginning of the cylindrical section of the single crystal. FIG. 3 shows the direct effect of the jumps as a significant change in the concentration of point defects AC. It can be seen that after the cracks, a significant amount of crystal length must be drawn before

Concentrations of point defects can be achieved that existed before the cracks. Because a regulation of the diameter of the single crystal by means of the

Draw speed as a manipulated variable naturally such fluctuations in the

Containing pulling speed, regardless of their advantages, their use is rather out of the question because of the effect shown, if not only the diameter but also the distribution of point defects in the single crystal are to remain in predetermined areas.

The proposal, which is to be assessed as the state of the art, of regulating the diameter of the single crystal when pulling the cylindrical part of the single crystal

implement the fuselage of an annular, above the melt

arranged resistance heating is used as a manipulated variable, cannot prevent yield losses that occur because of the effect shown in FIG.

According to the invention, it is therefore proposed to begin with such a regulation before the initial cone of the single crystal has been completed. The

The difference is shown in Fig.4. In the example shown according to the invention (solid curve), before the crystal position Po is reached, i.e. during the drawing of the initial cone, there is a change from regulating the diameter by means of the drawing speed v _p as the manipulated variable to regulating by means of the heating power of the annular resistance heater and after the

Change the pulling speed v _p controlled according to a specification which brings the quotient v _p / G into a specified range of values. In contrast to remain according to the opposite example (dashed curve) the fluctuations in the drawing speed due to the control by means of the

Pull speed v _p exist as a manipulated variable up to position Po in the single crystal, with the sequence shown in FIG.

The procedure according to the invention is also advantageous with regard to the deviation of the diameter of the single crystal from the nominal diameter. In Fig.5 are the

Nominal diameter (solid curve) and the diameter according to the example (curve drawn with double dashed lines) and counter-example (curve drawn with single dashed lines) are compared. The diameter d _s designates the nominal diameter in the cylindrical section of the single crystal. When using the method according to the invention, the deviations from

Target diameter already at the beginning of the drawing of the cylindrical part of the

Single crystal comparatively low.

The above description of exemplary embodiments is to be understood as exemplary. The disclosure thus made enables the person skilled in the art, on the one hand, to understand the present invention and the advantages associated therewith, and, on the other hand, also encompasses those which are obvious to the person skilled in the art

Changes and modifications to the structures and procedures described. Therefore, it is intended that all such changes and modifications, and equivalents, be covered by the scope of the claims.

Claims

1. A method for pulling a single crystal of silicon according to the Czochralski method from a melt, comprising

pulling at least one end section of an initial cone and a cylindrical section of the single crystal at a pulling speed that follows a predetermined course and repeatedly performing the following steps a) to c) in the course of regulating the diameter of the end section of the

Initial cone and the cylindrical section of the single crystal:

a) measuring an actual diameter of the single crystal;

b) determining the deviation of the current diameter from one

Target diameter of the single crystal; and

Target diameter of the single crystal.

2. The method of claim 1, comprising

the beginning of the regulation of the diameter of the initial cone of the single crystal with the heating power of the ring-shaped resistance heating as the manipulated variable, as long as at least 15 mm length of the initial cone is to be drawn.

3. The method according to claim 2, comprising the use of a PD controller for controlling the diameter of the single crystal with the heating power of the annular resistance heater as the manipulated variable.

4. The method of claim 1 or claim 2, comprising regulating the

Diameter of a first part of the initial cone to the end section of the initial cone with the pulling speed v _p as the manipulated variable.

5. The method according to claim 1 or claim 2, wherein the annular resistance heater is arranged at a distance of not less than 1 mm and not more than 200 mm above the melt.