RU2227819C1 - Method of control of level of melt in crucible in the course of crystal growth - Google Patents

Abstract

FIELD: production of semiconductor materials. SUBSTANCE: control of melt level in crucible is performed by means of video camera which is placed at angle relative to vertical. Angle between vertical and beam passing through focus of optical system of video camera and center of circumference formed by crystal being grown at boundary with melt is determined by continuous video image. Distance between video camera and plane of melt is calculated according to said angle and crucible with melt is raised to required level. EFFECT: simplification of process; enhanced accuracy of control of melt level. 5 dwg, 1 ex

Description

The invention relates to a technology for producing semiconductor materials and can be used in automatic control systems for the process of growing crystals from a melt by the Czochralski method.

The Czochralski method includes the operations of loading raw materials into a crucible, melting and drawing a crystal to seed from the melt. Stable reproduction of the physicochemical and geometric properties of crystals is achieved with a constant value of the melt level relative to an external heater during the entire growing process. A change in the melt level during crystal growth can be caused by a decrease in the volume of the material due to crystallization to seed, fluctuations in the technological parameters and properties of the melt, the diameter of the crystal, and the configuration of the crucible.

To maintain a constant level of the melt, a crucible lift control system is used.

The most effective are non-contact optical systems for measuring and controlling the parameters of the crystal growing process.

A known method of controlling the level of the melt (German patent 3904858, MKI C 30 V 15/20), in which a laser beam is directed to the surface of the melt and its reflection is obtained on a photodetector. The position in which the laser beam hits the photodetector depends on the level of the melt. The photodetector is connected with a system of continuous supply of raw materials to the crucible, which ensures the constancy of the melt level. The disadvantage of this method is the significant complexity and cost of control, low measurement accuracy, due to the use of a signal from the surface of the melt, making oscillatory movements.

Improving the accuracy of the measurement is achieved using additional equipment, for example a set of linear detectors (US patent 5286461, MKI C 30 V 15/28). However, this significantly complicates the management, increases the cost of the control system.

A known device (US patent 6071340, MKI C 30 V 35/00), in which to measure and control the level of the melt using a rangefinder. The light beam is directed to the surface of the melt, its reflection is captured by a reflector, from the surface of which the beam is reflected and makes a return trip to the sensor.

The sensor detects a change in the duration of the light beam when the melt level deviates from the set value. The disadvantage of the measuring system using the range finder is the need to use an additional reflector apparatus, which complicates the measurement and regulation system.

A number of patents use a video camera to control the level of the melt, which is sent to the surface of the melt and a reference point located outside the melt. Regulation is carried out by measuring the distance between them, which is analyzed using a computer. Closest to the proposed one is a method for controlling the level of the melt, (German patent 4231162 C2, MKI C 30 V 15/26), in which a “well” type heat shield is fixed above the melt surface with a slot for flanging in the lower part, at such a distance from the melt so that the slot (mark) is reflected on it. The camcorder is directed to the surface of the melt and the mark, receive an image on the computer display. Based on the different luminosities of the melt surface and the marks, the distance between them is determined by computer processing the resulting image. The calculated value is compared with the set and based on this comparison produce a control action. The disadvantage of the method for regulating the level of the melt described in the patent is the need to use an additional structural element - a heat shield with a mark and its precise fixation at a predetermined distance from the surface of the melt before each crystal growing operation, which can reduce the accuracy of measurement and regulation.

The boundary between the melt and the crystal, which has the form of a bright, meniscus glowing with reflected light meniscus, which has a circle in horizontal section, is also used as a reference line.

The technical result of the invention is to simplify and improve the accuracy of measurement and regulation of the melt level in the process of growing crystals by the Czochralski method by directly measuring the distance from the camera to the surface of the melt.

The specified technical result is achieved by the fact that in the method of regulating the melt level in the process of growing crystals by the Czochralski method by controlling the position of the melt level using a video camera and raising the crucible to achieve a predetermined position of the melt level, the video camera is installed at an angle to the vertical, and the melt level position is controlled by determining using a video image of the surface of the melt and the grown crystal, the angle y between the vertical and the beam passing through the optical focus video camera systems and the center of the circle formed by the grown crystal at the boundary with the melt.

Moreover, the specified angle γ can be determined by extracting from the common video signal pixels belonging to the crystal-melt boundary, and calculating the angle γ according to the equation:

γ = α + β × Vo / Vy,

where α is the angle between the lower side of the angle of the camera in the plane of the angle γ and the vertical;

β is the angle of the video camera in the plane of the angle γ;

Vy is the number of vertical pixels corresponding to the angle β;

Vo is the number of vertical pixels corresponding to the angle β ’between the beam passing through the center of the meniscus and the focus of the camera and the lower side of angle β, common with angle α.

The essence of the proposed method is illustrated by drawings.

Figure 1 shows a General view of the installation for growing a crystal in front and side; figure 2 - the lower chamber with the crucible, the measurement circuit and the control system for the movement of the crucible; figure 3 - scheme for determining the number of pixels and angles β and β ’; figure 4 is a diagram for determining the coordinates of the center of the meniscus; on figa, In - the results of determining the level of the melt in the process of growing the crystal without regulation (A) and when regulating (B) the level of the melt.

Installation for growing crystals (figure 1) includes an upper chamber 1 with a drive for rotating and displacing the upper rod 2 mounted on it, a lower chamber 4 and a block of drives for moving and rotating the lower rod 5. The chambers 1 and 4 are connected by a cap 3, which is placed under camera angle 6, i.e. the optical axis of the camcorder 6 is located at an angle to the vertical. The lower chamber 4 contains thermal shields 7 (Fig. 2), a heater 8, in which the crucible 9 with the melt 10 is placed. The crucible 9 is lifted by moving the rod 11 connected to the drive 18. The upper rod 2 moving block is connected via a cable 13 to the seed crystal 12. On the seed crystal 12 carry out the growth of the crystal 14.

The video camera 6 is located at a horizontal distance L to the installation axis Z, the viewing angle of the video camera 6, depending on the optical system used, is bounded in the vertical plane through the optical axis of the camera and the crystal axis in beams 15 and 16 and is transverse and horizontal section of a rectangle. In the indicated plane passing through the axis of the crystal and the optical axis of the video camera 6, the sector of its review between the rays 15 and 16 has an angle β.

Crystal growth is as follows. Raw materials are loaded into a quartz crucible 9 having a cylindrical and bottom spherical part and melted using a heater 8. A seed crystal 12 is lowered onto the surface of the melt, melted and a crystal is grown from the melt. Cultivation is carried out with continuous rotation of the seed 12 and the crucible 9. The crucible 9 and the crystal 14 are moved up according to a predetermined program, the level of the melt 10 is kept constant with respect to the upper edge of the heater 8.

Measurement and regulation of the level of the melt are as follows.

Before loading the raw materials into the crucible and its heat treatment carry out the alignment of the camera 6, which is as follows. At a distance H (FIG. 2) from the focus of the optical system of the video camera 6, a template with a scale grid is horizontally installed in the lower camera 4 and the distance A is measured between the vertical axis passing through the focus of the video camera 6 and the line of intersection of the lower side of the angle β with the template. Measure the distance B between the vertical axis of the camcorder 6 and the intersection of the second upper opposite side of the angle β with the template. The values of the angle β and the angles α and γ between the vertical and the corresponding lower and upper sides of the angle β are calculated (rays 15 and 16).

The template is removed from the chamber 4, the crucible 9 is installed, the raw materials are loaded and the crystal 14 is grown. The melt level is controlled by continuously measuring the angle γ (Fig. 2) between the vertical and the beam K passing through the focus of the optical system of the video camera 6 and the center О of the meniscus D of the boundary a melt and a growing crystal, which in a horizontal section is a circle. The angle γ, equal to the sum of the angle α and angle β ', is continuously monitored by calculating on the computer 17 the angle β' between the beam K passing through the center O of the meniscus and the focus of the camera, 6 and the lower side of the angle β, common with angle α, based on the proportion (figure 3):

Vy / Vo = β / β ’,

where Vy is the number of vertical pixels corresponding to the angle β;

Vo is the number of vertical pixels corresponding to the angle β ’.

The video signal from the output of the video camera 6 enters the computer 17. The video image is processed and the pixels Dn belonging to the meniscus D of the growing crystal are extracted from the general signal. Since the video camera 6 is located at an angle to the surface of the melt and the imaginary rays of the video camera 6 are not parallel, the selected pixels Dn belonging to the meniscus in the video image in the XY melt plane cannot be directly described by the circle equation, which would simplify the processing of the video image.

To describe the family of points Dn by the equation of the circle, the melt plane is perpendicular to the central axis P of the video camera 6 (Fig. 4). Get a family of points Dn ’in the new coordinate system X’Y’ belonging to the meniscus and described by the equation of a circle centered at point O ’. By reverse transforming the coordinates of the video image from the X’Y ’plane to the XY melt plane, the coordinates of the point O are found - the center of the circle formed by the crystal at the boundary with the melt, i.e. of the meniscus center D. Based on the obtained coordinates of the center, the required angle γ is calculated. The estimated position of the melt level H relative to the horizontal level at which the focus of the camera 6 is located, is calculated by the formula:

H = L / tgγ,

where L is the distance from the Central axis of the crystal to the focus of the camera 6.

As a result, according to the coordinates of the center O of the meniscus D and the angle γ between the vertical and the beam K passing through the focus of the optical system of the video camera 6 and the center of the circumference of the meniscus, the melt level is controlled.

During crystal growth, the position of the melt level is compared with a predetermined level. When changing the position of the melt level, the signal from the computer 17 enters the control system 18, which provides a control signal to the channel for controlling the movement of the crucible 9 (Fig.2), to return the melt level to a predetermined position by changing the speed of movement of the crucible 9.

Experimental data on the regulation of the melt level.

Figure 5 presents the results of measuring the vertical distance from the focus of the optical system of the video camera 6 to the plane of the melt surface of two processes for growing silicon single crystals. Process “A” (FIG. 5A) was carried out without controlling the melt level, process “B” (FIG. 5B) was carried out with the automatic maintenance of the melt level.

Cultivation was carried out from a crucible with a diameter of 406 mm, a loading mass of 43 kg, and a diameter of the grown crystals of 158 mm. The calculated vertical distance from the camera to the surface of the melt at the beginning of the cultivation of the cylindrical part was 617.5 mm In the process “A”, at a crystal length of 750 mm, the average deviation from the initial level of the melt was 4 mm; in the case of process “B”, the average deviation at the same length was 1 mm.

Example. We used a video camera 6 with a 16 mm lens and a video capture card with a resolution of 640 × 480 pixels. The video camera 6 was fixed on the cap 3 of the cultivation unit at a distance of L = 310 mm from the geometric axis Z. The vertical angle of the camera β was 12.4 °, and its angle of inclination α was 16.1 °. In the process of growing a silicon single crystal with a diameter of 158 mm, a video image of the meniscus of the cylindrical part of the growing crystal was obtained. The coordinates of the center of the circle O in the melt plane were (318, 409). The angle γ between the vertical and the beam passing through the focus of the optical system of the video camera 6 and the center of the circle O of the grown crystal at the boundary with the melt were calculated:

γ = 16.1 + 12.4 × 409/480 = 26.66 °.

Determined the distance from the camcorder to the melt plane:

H = 310 / tg26.66 ° = 617.4 mm.

The set distance H was 616.5 mm over the entire growing period.

Crucible 6 was raised to a distance of 102 mm over the same period.

At the final stage, the crystal was separated from the melt according to a predetermined program and the ingot was cooled.

Claims (1)

The method of regulating the melt level in the process of growing crystals by the Czochralski method by controlling the position of the melt level using a video camera and raising the crucible to achieve a predetermined position of the melt level, characterized in that the video camera is installed with an inclination to the vertical, and the control of the position of the melt level is carried out by determining using the video the surface of the melt and the grown crystal, the angle between the vertical and the beam passing through the focus of the optical system of the video camera and the center spine formed by the crystal grown at the interface with the melt, wherein said angle γ is determined by isolation of total video pixels belonging to the boundary of crystal - melt, and the angle γ is calculated according to the equation γ = α + β × Vo / Vy, where α is the angle between the lower side of the angle of the camera in the plane of the angle γ and the vertical; β is the angle of the video camera in the plane of the angle γ; Vу - the number of vertical pixels corresponding to the angle β; Vо is the number of vertical pixels corresponding to the angle β ’between the beam passing through the center of the meniscus and the focus of the camera and the lower side of the angle β.

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