U.S. patent application number 09/141322 was filed with the patent office on 2001-11-22 for device and method for the determination of diameters of crystals.
Invention is credited to ALTEKRUGER, BURKHARD, AUFREITER, JOACHIM, BRUSS, DIETER, KALKOWSKI, KLAUS.
Application Number | 20010043733 09/141322 |
Document ID | / |
Family ID | 26039641 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010043733 |
Kind Code |
A1 |
ALTEKRUGER, BURKHARD ; et
al. |
November 22, 2001 |
DEVICE AND METHOD FOR THE DETERMINATION OF DIAMETERS OF
CRYSTALS
Abstract
A device and process for the determination of the diameters of a
crystal that is pulled from a liquified material. In this
connection several video cameras are provided, each of which
reproduces its own section along the vertical axis of the crystal
or in a direction vertical to it. The angles of image of the camera
are laid out in such a way that the object to be reproduced
completely fills the entire picture plane - - - at least in one
direction. For objects with a small diameter; e.g., the crystal
neck, a camera with a small angle of image is used, while for
objects with a large diameter; e.g., the crystal body, a camera
with a large angle of image is used.
Inventors: |
ALTEKRUGER, BURKHARD;
(ALZENAU, DE) ; AUFREITER, JOACHIM; (ALZENAU,
DE) ; BRUSS, DIETER; (BRUCHKOBEL, DE) ;
KALKOWSKI, KLAUS; (GRUNDAU, DE) |
Correspondence
Address: |
BEVERIDGE DEGRANDI WEILACHER & YOUNG
1850 M STREET NW
SUITE 800
WASHINGTON
DC
20036
|
Family ID: |
26039641 |
Appl. No.: |
09/141322 |
Filed: |
August 27, 1998 |
Current U.S.
Class: |
382/141 |
Current CPC
Class: |
G01B 11/08 20130101;
C30B 15/26 20130101; Y10T 117/1012 20150115; Y10S 117/90 20130101;
Y10T 117/1008 20150115; Y10T 117/1004 20150115 |
Class at
Publication: |
382/141 |
International
Class: |
G06K 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 1997 |
DE |
197 38 438.2 |
Apr 21, 1998 |
DE |
198 17 709.7 |
Claims
We claim:
1. A device for determining the diameters of a crystal that is
pulled from a liquified material, comprising a plurality, of
cameras for the reproduction of m areas of the crystal, whereby
m.gtoreq.2, wherein said cameras have angles of the image (.gamma.,
.delta.) matched to each of the areas of the crystal reproduced by
them.
2. The device according to claim 1, wherein the area that is
reproduced fills the entire picture plane of a camera in at least
one dimension.
3. The device according to claim 1, wherein a first camera
reproduces a thin neck of the crystal.
4. The device according to claim 1, wherein a second camera
reproduces a cylindrical part of a crystal.
5. The device according to claim 1, wherein a third camera
reproduces areas with different diameters and whose angle of image
matches the larger diameter.
6. The device according to claim 1, wherein a camera reproduces the
meniscus of the crystal.
7. The device according to claim 3, wherein a camera reproduces a
seed crystal.
8. The device according to claim 1, wherein the longitudinal axis
of at least one of the m cameras is aligned vertically with the
longitudinal axis of the crystal.
9. The device according to claim 1, wherein the longitudinal axis
of at least one of the m cameras forms an angle .alpha. with the
longitudinal axis of the crystal, with
0.degree.<.alpha.90.degree..
10. The device according to claim 1, wherein different cameras
reproduce partial areas and the same objects.
11. The device according to claim 1, wherein the object is the body
of the crystal.
12. The device according to claim 1, wherein the longitudinal axis
of a first camera is positioned so it is offset by 180.degree. from
the longitudinal axis of a second camera.
13. The device according to claim 1 wherein m cameras are
positioned next to each other.
14. The device according to claim 1, wherein an automatic
calibration for the cameras is provided for.
15. The device according to claim 1, wherein elements, which are
part of a casing that surrounds a crucible in which the liquified
material is located, are provided for the calibration.
16. Device according to claim 1, wherein a calibration plate is
provided for and a crucible is positioned temporarily.
17. The device according to claim 16, wherein the calibration plate
is positioned on the probable surface of the liquified material in
the crucible.
18. The device according to claim 1 for the determination of
diameters of a crystal by using m cameras wherein the cameras are
used one after another, but during the same process.
19. The device according to claim 1 for the determination of
diameters of a crystal by using m cameras wherein the cameras are
used at the same time during a process.
20. The device according to claim 18, wherein m cameras reproduce
different positions in the vertical longitudinal axis of the
crystal.
21. The device according to claim 19, wherein m cameras reproduce
different positions horizontal to the crystal.
22. A process for the determination of the diameter of a crystal
that is pulled from a liquified material, using an evaluation
control unit comprising the following steps: (a) correction of the
picture of the crystal for problems that arise from being taken at
a slant so that, once the calibration is taken into consideration,
the elliptical form of a crystal circumference in the reproduction
is converted into a circle, (b) scanning of the circumference that
has been converted into a circle for its diameter, whereby the two
corner points, which mark the diameter, are detected, (c) searching
for a third point on the circle, (d) calculating whether the two
corner points and the third point lie on a common ideal circle.
23. The process according to claim 22 for the determination of the
diameter of a crystal that is pulled from a liquified material,
comprising the following steps: (a) reproduction of the meniscus by
means of a camera on a picture plane; (b) scanning the meniscus for
its diameter, whereby the two outer corner points are detected; (c)
search for the lowest point of the meniscus; (d) calculation of an
ellipse due to the discovered point; (e) conversion of the ellipse
into a circle in accordance with a known algorithm.
24. Process according to claim 22 wherein the objects reproduced by
the two cameras are evaluated one after another.
25. The process according to claim 22 wherein edges of the crystal
or meniscus ring are determined by the formation and evaluation of
gradients.
26. The process according to claim 22, wherein the criteria of
relevance for ruling out irrelevant picture contents and picture
objects are used.
Description
[0001] The present invention relates to a device for determining
the diameters of a crystal that is pulled from a liquified material
and method for accomplishing the same.
[0002] Such a device is used, for example, for measuring the
diameter of crystals when pulling single crystals in accordance
with the Czochralski method.
[0003] In the field of crystal growth, a number of different
methods are known; e.g., crystal growth from a gas phase, from a
solution, or from a liquified material. The various methods for
crystal growth from a liquified material have attained preeminence
among the growth methods due to their highly developed processing
technology and their production quantity.
[0004] The best known methods for crystal growth from a liquified
material are the Kyropoulus method, the Bridgman method, and the
Czochralski method. While by the Kyropoulus method a cooled seed
crystal is dipped into a liquified material and by the Bridgman
method a crucible is lowered vertically into a temperature
gradient, by the Czochralski method a crystal is pulled from a
liquified material.
[0005] With the Czochralski method the original material melts in a
crucible as is also the case with the Kyropoulus method. A seed
crystal is submersed in the liquified material and is wetted by it
and, in so doing, fused. Then the seed crystal is pulled
continuously upwards out of the liquified material while the
temperature is lowered. In so doing, the crystal and crucible
rotate counter-current. The speed of drawing it and the temperature
of the liquified material are controlled in such a way that the
crystal grows with a constant diameter after developing a shoulder.
The orientation of the growing crystal corresponds to the seed
crystal. This procedure is known in the art. See for example,
Bonora, "Czochralski Growth of Single-Crystal Silicon - - - A
State-of-the-Art Overview." Microelectronic Manufacturing and
Testing (September 1980), pp. 44-46 the entire disclosure of which
is relied on and incorporated herein by reference.
[0006] The target diameter of the single crystal pulled in
production today is geared toward the wafer size processed in
semiconductor technology - - - a size that has been taking on
larger and larger values due to reasons of economy in spite of the
advanced miniaturization of the electronic structural components,
and thus today it is predominantly at 150 to 200 mm. There are,
however, plans for wafers with a diameter of 300 to 400 mm. Given
these dimensions the crystal structure and purity and especially
the regularity of the diameter along the cylinder-shaped crystal
play an important role in a flawless single crystal. The smoother
the cylinder wall is, the smaller the expected expenditure on
processing and the loss of materials. For this reason controlling
the diameter during the target method is an important criterion for
economy.
[0007] In practice, one comes up against considerable obstacles
when trying to determine and control exactly the actual diameter of
the crystal in all phases of the growing process.
[0008] To overcome these difficulties mechanical, electrical, and
optical solutions have already been proposed.
[0009] In the case of a mechanical solution the weight of the
crystal is monitored and the diameter inferred from this weight
(GB-PS 1 457 275). In so doing a signal is produced that
corresponds to the effective inert mass of the crystal when pulled
out. In each case this signal is compared to the calculated
expected value. If the two signals deviate from each other, the
pulling speed is changed to match the actual crystal's diameter to
the target diameter via a control system intervention. A
disadvantage of this method is that it is subject to various
uncertain interferences as a result of the slow crystal growth.
[0010] In a refinement of this solution, a method is proposed with
which the effect of the heat delay is compensated for during the
crystal formation (DE-OS 25 13 924).
[0011] Another known solution to the problem of measuring the
diameter of a crystal pulled out of a liquified material based on
mechanical principles makes use of the torsional moment that occurs
because of the relative rotation between the crystal and the
liquified material; see DE-OS 36 40 868.
[0012] Measuring the diameter of crystals with the help of an
electrical method is also already known (DD-PS 145 407). In this
connection the electrical resistance of the growing crystal is
measured while the DC or AC voltage flows through the crystal or
through the system of the heatable crucible, liquified material,
crystal, and pulling objects. To measure the electrical resistance
of the crystal a floating contact area, which does not effect a
reaction with the liquified material or influence the thermal
conditions of the boundary surface between the liquified material
and the crystal due to the specificity of its material and its
structural peculiarities, is located on the surface of the
liquified material.
[0013] In the case of another known method for pulling single
crystal rods with a uniform diameter from a liquified material
contained in a crucible, optical agents are used to measure the
crystal diameter (DE-PS 16 19 969). In so doing, changes in the rod
diameter are constantly balanced out by using a control system that
consists of mechanical servo components and one or several emission
detectors that send emissions onto the liquified material. The
emission detectors are adjusted in such a way that they capture the
emission energy produced by a small surface area of the liquified
material directly near the growing crystal in the near infrared and
the visible spectral region, and its optical path and the crystal
axis form an acute angle.
[0014] Also known is an optical method for measuring the diameter
of a semiconductor rod produced through zone melting; by this
method the rod is filmed by a TV camera in the area of the zone
melting, the camera signal is transformed into a binary video
signal by comparing it with a variable threshold value, and the
diameter of the rod is measured at the site at which a jump in
brightness is determined that characterizes the solid-fluid
boundary that occurs in the axial direction (Journal of Crystal
Growth, 13/14 (1972), pp. 619-23).
[0015] In an improvement of this method the site of the phase
transition between the liquified material and the semiconductor
crystal growing out of it is determined more accurately by taking
more pictures with different threshold values and by examining the
video signals obtained with the various threshold values to see
whether a zone of a prespecified minimum width exists that extends
over the rod cross section and that is darker than a neighboring
take-off area (DE-OS 33 25 003).
[0016] The precise determination of the actual diameter of a
crystal by using the optical method is, however, subject to various
interferences during the growth process that can falsify the
results in such a way that carrying out the method accurately is no
longer possible. As a result, the quality and results of the growth
process can be strongly jeopardized. Included as interferences are,
among other things, strongly variable brightness and contrast
ratios on the objects to be measured; i.e., on the crystal, the
liquified material, or the luminous meniscus ring around the
crystal - - - and interfering reflections on the liquified material
or the unsteadiness of the object to be measured caused by
mechanical interference.
[0017] Moreover, to a certain extent the geometrical form of the
corrected crystal can deviate significantly from the ideal form of
a cylinder with a circular cross section. By varying the crystal
diameter, visibility on the entire diameter of the crystal and the
luminous meniscus ring belonging to it is considerably limited.
Moreover, the components and devices for optimizing the temperature
distribution limit the visibility of the crystal and further
jeopardize it.
[0018] Even with the present normal crystal diameters of about 150
to 200 mm these problems with the detection and control of the
crystal diameter can lead to considerable disadvantages with the
growth process. For the future generation of 300- to 400-mm
crystals the problem of the reliable control of the crystal
diameter will continue to intensify.
[0019] Also known is an optical system or process for controlling
the growth of a silicon crystal in which the aforementioned
problems are solved and in which the diameter of the silicon
crystal is measured with the help of a TV camera, whereby the
surface of the liquified material presents a meniscus, which is
visible as a light area near the silicon crystal (EP 0 745 830 A2).
In this system, first a test pattern of a part of the light area
near the silicon crystal is photographed by the camera. Then the
characteristics of the test pattern are detected. A valid
characteristic of a test pattern is, for example, the intensity
gradient. After this an edge of the light area is defined as a
function of the detected characteristics. Then a contour, which
includes the defined edge of the light area, is defined, and
finally the diameter of the defined contour is determined, whereby
the diameter of the silicon crystal is determined as a function of
the desired diameter of the defined contour.
[0020] A disadvantage with this system is that the accuracy in some
applications is not yet high enough and, more particularly,
external interferences are not sufficiently taken into
consideration.
[0021] Moreover, a device and method for the pulling of single
crystals according to the Czochralski method, in which the diameter
of a crystal is determined by an evaluation control unit, is known
(DE 195 48 845 A1). In this connection two cameras are provided
whose picture axes form an angle of 90 degrees. With these cameras
different points are detected on one and the same object; e.g., a
meniscus ring. The focal distance and the angle of image of these
cameras are identical, as a result of which it is not possible to
take a picture of areas of different sizes of an object on the
picture plane while guaranteeing that at least one coordinate is
completely filled.
[0022] Finally, there is a transition-angle identification system
in which two cameras can be used whose picture axes form an angle
in a vertical plane of a crystal (US-PS 4 943 160, FIG. 4). By the
term transition angle one is to understand an angle that forms a
meniscus surface with the vertical axis of a crystal and with an
axis parallel to this vertical crystal axis. Other areas of the
crystal are not picked up by the camera. The cameras also do not
have a focal distance or angle of image that is adjusted to the
diameter of various crystal parts.
[0023] Therefore, an object of the present invention is to
determine the diameter of a crystal with great precision by using
an optical detection system.
SUMMARY OF THE INVENTION
[0024] In achieving the above and other objects, one feature of the
invention resides in a device and process for the determination of
the diameters of a crystal that is pulled from a liquified
material. In this connection a plurality of video cameras are
provided, each of which reproduces its own section along the
vertical axis of the crystal or in a direction vertical to it. The
angles of image of the camera are laid out in such a way that the
object to be reproduced completely fills the entire picture plane,
at least in one direction. For objects with a small diameter, for
example, the crystal neck, a camera with a small angle of image is
used. For objects with a large diameter, as for example, the
crystal body, a camera with a large angle of image is used.
[0025] More particularly, the present invention features a device
for determining the diameters of a crystal that is pulled from a
liquified material, employing a plurality of cameras for the
reproduction of a corresponding number of areas of the crystal, and
where the angles of the image (.gamma., .delta.) of the plurality
of cameras are matched to each of the areas reproduced by them.
[0026] In further detail, it is a feature of the present invention
that the device for the determination of diameters of a crystal by
using a plurality of cameras, the cameras are used one after
another, but during the same process.
[0027] According to a still further feature of the present
invention, the above and other objects are achieved by a process
for the determination of the diameter of a crystal that is pulled
from a liquified material, comprising carrying out the following
steps using the evaluation control unit:
[0028] (a) correction of the picture of the crystal for problems
that arise from being taken at a slant so that, once the
calibration is taken into consideration, the elliptical form of a
crystal circumference in the reproduction is converted into a
circle,
[0029] (b) scanning of the circumference that has been converted
into a circle for its diameter, whereby the two corner points,
which mark the diameter, are detected,
[0030] (c) searching for a third point on the circle,
[0031] (d) calculating whether the two corner points and the third
point lie on a common ideal circle.
[0032] The advantage realized by the invention consists, in
particular, of the fact that the camera resolution can be adjusted
to each measurement problem by using at least two cameras. In the
case of large crystal diameters the resolution can be doubled, for
example, by using two cameras that make a composite picture
possible. In addition, the problems that occur due to varying light
strengths and contrast ratios can be eliminated. By taking into
consideration the different relevance conditions for valid
measurements, a stable signal and consequently an improved method
of carrying out the process are also obtained. In addition to this,
an automatic absolute-value calibration is possible, and the
crystal diameter can be measured in such phases of the process in
which a meniscus does not occur.
BRIEF DESCRIPTION OF DRAWING
[0033] The present invention will be further understood with
reference to the accompanying drawings, wherein:
[0034] FIG. 1 is an elevational cross-section of a crystal-pulling
device with two cameras lying one over the other, according to the
invention;
[0035] FIG. 2 is a partial cross-section view of a crystal-pulling
device of the invention with two cameras positioned next to each
other;
[0036] FIG. 3 is an elevational cross-section view of a
crystal-pulling device according to the invention during
calibration of the camera;
[0037] FIG. 4 is an enlarged representation of a crystal pulled
from the liquified material;
[0038] FIG. 5a-5d are diagrammatic representations of crystal
growth;
[0039] FIG. 6 is a schematic representation of a crystal and a seed
crystal, which are photographed by three cameras,
[0040] FIG. 7 is a perspective view of a crystal to explain the
process for determining the diameter of a crystal according to the
invention;
[0041] FIG. 8 is a flow diagram for the method of determining the
diameter of the crystal,
[0042] FIG. 9 is a diagrammatic view from above onto two cameras
and a crystal;
[0043] FIG. 10 is a diagrammatic view from above onto three cameras
and a crystal; and
[0044] FIG. 11 is a part of a flow diagram when using two cameras
in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0045] In FIG. 1 a device 1 is shown with which it is possible to
determine the diameter of a crystal 2 by using an optical method in
accordance with the invention. The underside 3 of the crystal 2
rests on the surface 4 of a liquified material 5 in this case,
while the crystal is rotated in the direction of arrow 6. The
liquified material 5 is located in a crucible 7, which is driven by
a shaft 8 of an electric motor 9. The shaft 8 and motor 9 are
connected by the flanges 10, 11. The crucible 7 is located in a
casing, which consists of an upper part 12, a middle part 13, and a
lower part 14, and which can be rotated in the direction of arrow
15. An electrical heating device 16, which supplies electrical
energy from a device 17, is located around the crucible 7.
[0046] The rotation of the crystal 2 takes place by using a pole
18; e.g., a threaded pin that is driven by an electric motor 19.
This motor is also supplied with electrical energy from the device
17.
[0047] In the upper part 12 of the casing two cameras 20, 21 are
located whose longitudinal axes form an angle .alpha. or .beta. to
the vertical axis 22 of the pole 18 and the crucible 7.
[0048] The upper part 12 of the casing is connected to a pipe 23,
which surrounds the pole 18 and has a gas inlet opening 24. In the
bottom part 14 of the casing, gas outlet openings 25, 26 are
provided. The pole 18 can not only be rotated by using the electric
motor 19, it can also be raised. Control of the electric motors 9,
19 takes place via the device 17, which receives information from
the cameras 20, 21 and evaluates this information.
[0049] A third camera, which is not shown in FIG. 1, can be located
behind the pipe 22 or at another site.
[0050] The cameras 20, 21 have fixed focal distances and angles of
image and reproduce a particular area of the crystal-neck, body,
and the like. In this connection it is established that the
reproduced object completely fills the picture plane at least in
one coordinate so that optimal resolution is attained. Because the
distance of the camera objective from the object to be reproduced
is known, the focal distance is chosen in such a way that optimal
conditions exist. By the term picture plane one is to understand
the plane of the camera in which a picture is reproduced; i.e.,
when using conventional cameras the 24.times.36 film plane or when
using electronic cameras the surface of the light-sensitive
chips.
[0051] If the cameras are integrated into the system so they are
fixed in the casing 12, preferably objectives with fixed focal
distances are used, because the distance between camera and
objective is a constant. It is, however, also possible; e.g., with
a camera that is to observe the neck of the crystal, to design the
camera with the growing, upward-moving crystal so it rotates in the
casing 12 and to provide it with a zoom objective that is
controlled automatically in such as way that the picture plane of
the camera is constantly optimally utilized by the reproduced
object. The constantly changing distance between the objective and
object is thus taken into consideration through the varying focal
distance.
[0052] With the help of the camera each diameter of the crystal
area is determined and compared to the corresponding target values.
The difference between the actual value and target value can then
be made use of to change the crystal-pulling conditions.
[0053] In FIG. 2 the upper part of the device shown in FIG. 1 is
represented again, but now the two cameras 20, 21 are positioned
next to each other. The pole 18 in this case is run underneath - -
- it is the beginning of the pulling process - - - and has a seed
crystal 31. The camera 21, which has a fixed focal distance, is
aligned with the seed crystal 31; i.e., its angle of image is
.gamma.. In so doing the seed crystal can be reproduced in such a
way that it fills the total picture plane. This, in turn, allows
one to examine the contours and other properties of the seed
crystal 31 with precision.
[0054] The second camera 20 has an objective with another fixed
focal distance and picks up the larger crystal 2, which is
represented by dotted lines in FIG. 2, through the angle of image
6. Thus the larger crystal 2 can be reproduced in such a way that
it fills the total picture plane of camera 20.
[0055] Through the use of two cameras 20, 21 the crystal can
consequently be monitored completely in all of its growth phases.
The cameras 20, 21 can be used at the same time, although
preferably they are used one after another, on which occasion
shifting from one camera to the other takes place automatically by
control. It is understood that the successive use of the two
cameras takes place within the same crystal-pulling process.
[0056] The use of only one camera with a zoom objective, which is
already known (Japanese patent application Sho 62-87482 of Oct. 9,
1985), would be a disadvantage when using an evaluation algorithm
and is not comparable to two cameras, each of which has a fixed
focus. With a fixed-focus camera the angle of image can be designed
in such a way that the entire object to be photographed fills the
film plane entirely. With a zoom camera, however, only parts of the
film plane are covered by the object. If, for instance, the zoom
objective is focused on parts and the seed crystal 31 fills the
entire film plane, the seed crystal will cover only a part of the
film plane when there is a wide-angle focus. Of course, the entire
crystal 2 may cover the entire film plane with a wide-angle focus,
but that does not change the fact that the seed crystal 31 takes up
only a small part of the film plane and can, therefore, be examined
with less accuracy. With two fixed-focus cameras of different focal
distances, however, two different areas of a crystal can be
reproduced and evaluated at the same time or one after another. At
any rate, two different areas of a crystal can be reproduced and
evaluated during one uniform process. One can use two zoom cameras
instead of two fixed-focus cameras, but problems with calibration
will result, for it requires focusing with 100% accuracy.
[0057] FIG. 3 shows how such a calibration can take place. For
reasons of simplicity, only one camera 20 is shown; it is to be
understood, however, that the calibration of a second or third
camera can take place in a corresponding way.
[0058] Two variations of the calibration are shown in FIG. 3. In
the case of the first variation, two demarcations 27, 28 are
designated that can also be parts of a circular ring. The edges 29,
36 of these demarcations serve as markers for the target positions.
They are photographed by the camera and then evaluated in a
picture-evaluation unit 37. This picture-evaluation unit 37 is
connected to a mass memory 38 for storing the calibration data. The
information from the picture-evaluation unit 37 is fed to a control
system 38, which can also be loaded with the data of an operating
unit 39.
[0059] Instead of the demarcations 27, 28, in a second variation a
plate 41 that is provided with special calibration markings can be
positioned, even at the expected height of the surface of the
liquified material. This plate 41 can be provided with a reference
picture on its surface that contains various "reference diameters"
D.sub.x, D.sub.y. These reference diameters can be determined,
stored in the memory 38, and then later processed. Because the
calibration process generally takes place before the actual
crystal-pulling process, the demarcations 27, 28 and/or the plate
41 can be dismantled after the calibration.
[0060] In FIG. 4 an enlarged representation of the crystal 2 that
is formed from the liquified material 5 in accordance with FIG. 1
is shown. This crystal essentially has a cylindrical form and a
shoulder 30 in the upper area, which passes over into a thin neck
31. This neck 31 corresponds to the seed crystal or seed 31 (FIG.
2) with which the crystal formation process is initiated. The
vertical axis of the crystal is designated as 32, while the
longitudinal axes of the cameras 20, 21 (not shown in FIG. 4) are
designated as 33 and 34 in accordance with FIG. 1. The arrangement
of the cameras and their axes is represented in their general form
in FIGS. 1 and 4. In practice they are aligned with the meniscus so
that the axes 33 and 34 intersect in the point that is also the
intersection point between the surface of the liquified material 5
and the axis 32. Such arrangements are described in detail below.
The diameter D of the crystal 2 can vary independently of the
height, although the crystal essentially has the form of a
cylinder. The lower area of the crystal 2, which lies between the
cylindrical part and the liquified material 5, is designated as 35.
To a certain extent the meniscus 35 is the connecting link between
the crystal 2 and liquified material 5. It represents a narrow
ring-shaped zone between the solidified crystal and the liquid
material, which is clearly brighter than the liquified material
itself. The crystal appears to be surrounded by a light ring on the
boundary surface between crystal and liquified material. The
optical phenomenon, which is not easy to explain, occurs through
the reflection of the radiation emitted chiefly from the crucible
wall onto the concave transition area between the liquified
material and crystal so that an observer looking from above onto
the liquified material and crystal sees a darker crystal, which is
surrounded by a lighter luminous ring at the height of the
liquified material. During the pulling process the ring expands as
the liquified material is lowered into the crucible and as the
existing crystal grows by about threefold. At the end of the
pulling process, this makes it increasingly more difficult to
determine the diameter accurately by reproducing the luminous ring
on a suitable sensor.
[0061] If the cameras 33, 34 are built in so they are stationary,
camera 34 can, e.g., be turned on during one of the first time
segments of the crystal- pulling process. At the beginning of a
second time segment camera 33 is then turned on. The use of such a
method of operation is called, as mentioned above, a successive
method of operation.
[0062] In FIGS. 5a to 5d four views of a growing crystal are
represented from the view of a camera positioned at a slant above
the crystal. One recognizes in this connection only the essential
elements; i.e., liquified material, pole 22, seed crystal 31, the
crystal 2 itself, and the meniscus 35. The crucible 7 is left
out.
[0063] FIG. 5a shows the beginning of the crystal formation when
the liquified material 5, which is connected to the pole 22, comes
into contact with the seed crystal 31. If the pole 22 is then
raised, the situation represented in FIG. 5b results: A slim,
cylindrically formed body 40, which has contact with the liquified
material 5, is then formed. If the pole 22 is, however, raised
further, the formed body 40 is extended and a crystal 2 that has a
circular circumference and that is surrounded by a brightly
radiating meniscus 35 is formed at its end. This situation is
represented in FIG. 5c. When the pole 22 is raised even more, the
cylindrical crystal 2 continues to grow out of the liquified
material 5. With a top view from a camera at a slant, the back part
of the meniscus can no longer be recognized; only the front partial
curve is visible.
[0064] In FIG. 6 the crystal 2 is represented again; it is
nonetheless picked up not by two, but by three cameras (not shown).
The first camera picks up only the left half of the crystal and
reproduces the part framed with a shaded rectangle 42. The second
camera, however, picks up the right half of the crystal 2 and
reproduces the part framed with the shaded rectangle 43.
[0065] The third camera is responsible for reproducing the seed
crystal 31, which is symbolized by the rectangle 44. The
reproductions of the two first cameras can overlap in a boundary
area 45. By using two cameras to reproduce the crystal, the
resolution can be increased, because the entire film plane of a
camera is filled with only one half of the crystal instead of the
entire crystal. The parallel operation of both cameras is therefore
covered in a direction vertical to the longitudinal axis of the
crystal.
[0066] Serial operation is thus possible with the invention; in
this case the cameras reproduce different areas of the crystal
along the vertical axis of the crystal on the reproduction plane in
such a way that the object being reproduced fills the reproduction
plane of the camera in at least one direction - - - the x or y
direction - - - on which occasion the cameras are activated one
after the other. Parallel operation is, however, possible; in this
case the cameras reproduce different areas of the crystal along a
line running perpendicular to the vertical axis of the crystal on
the reproduction plane in such a way that the objects being
reproduced fill the reproduction plane of the camera in at least
one direction, whereby the cameras are activated at the same
time.
[0067] FIG. 7 shows crystal 2 in the camera reproduction. By using
this reproduction, the way in which the diameter of the crystal 2
is determined will be described in detail. The frame 49 can be
equated with a CCD chip, with which the crystal 2 is reproduced.
The chip is then scanned line for line in the direction of the
arrow 56. In so doing, the bright areas and/or gradients in the
light intensity of the individual areas inside the frame 49 are
detected. The goal is, first, to determine the points P.sub.1 and
P.sub.2, which limit the visible part of the meniscus. These points
P.sub.1 and P.sub.2 are found when a jump in brightness occurs on
two sites of a scanned line. By calculating the distance between
the points P.sub.1 and P.sub.2, one obtains the quasi-diameter of
the crystal. This is, however, not the true diameter, because the
reproduction in FIG. 7 is distorted perspectively. It is, however,
possible to determine the true value by making a comparison with a
distance P.sub.1-P.sub.2 from the calibration. The distorted
distance P.sub.1-P.sub.2 can thus be assigned an undistorted
distance P.sub.1'-P.sub.2' so that one knows the true diameter
P.sub.1'-P.sub.2' in this case.
[0068] This diameter can then be compared with a maximum diameter.
Only when the newly determined diameter is larger than the stored
maximum diameter is the new diameter stored.
[0069] In the next step one determines whether the points
P.sub.1-P.sub.2 lie symmetric to the center line 65. If this is the
case, the distance of the connecting line between P.sub.1 and
P.sub.2 to the lower edge becomes smaller than half the distance
between P.sub.1 and P.sub.2. If this is the case, the measured
diameter P.sub.1-P.sub.2 is the maximum diameter.
[0070] Then the point P.sub.3 is determined. In so doing the
scanning beam runs line for line from below to above. If only one
jump in brightness occurs on a line, then point P.sub.3 has been
found. Because all three spheric points are then known, the circle
and its center point P.sub.M can be determined. In the next step
the angle .alpha. is determined, which is yielded when lines are
drawn from the center point P.sub.M through the points P.sub.1 and
P.sub.2.
[0071] For accuracy the angle .alpha. is reduced by an angle
.gamma. so that
.beta.=.alpha.-.gamma.
[0072] Then a prespecified number of measurement points I-VIII are
distributed on the partial circle defined by the new angle .beta.;
and from these measurement points the circle that is the "best fit"
is determined.
[0073] The measurement points I to VIII must be distributed evenly
over the visible arc, not over a calculated arc, for .alpha. is a
visible arc. The reduction of the angle .alpha. by .gamma. occurs
so that one is sure to be able to determine the measurement points
I and VIII on the edges.
[0074] By using the smallest possible number of measurement points
a high accuracy and resolution can be obtained for the diameter
signal. A compensating curve, to a certain extent, is laid by a
large number of measurement points.
[0075] FIG. 8 shows a flow diagram of the method of determining the
diameter of the crystal. As one can see, first the picture to be
taken is read by a camera. This picture approximately corresponds
to the crystal 2 shown in FIG. 7. It is then corrected by a known
algorithm so the ellipse with the points P.sub.1, P.sub.2, P.sub.3
becomes a circle with corresponding points. This corrected
reproduction, which need not correspond to a concrete optical
representation but which is realized by data stored in the memory,
is then subjected to another process.
[0076] The corrected picture that is read is then searched line for
line for the points P.sub.1 and P.sub.2. In this connection one
begins with the lower line - - - the reason why n is set equal to
1, where n is the number of the line. If two points are found in a
line, the further tests represented in the flow diagram take place.
They are of special meaning for the reliability of the
determination of the diameter.
[0077] One must be sure that the points P.sub.1 and P.sub.2 stem
from the crystal, i.e., from the meniscus and not from sources of
interference - - - e.g., reflections on the liquified material.
[0078] If all the tests are passed with positive results, the value
determined is stored in the variable D.sub.max.
[0079] After the site on the flow diagram, "Is the distance to the
lower edge>D/2?", one can thus choose with "yes" that
D.sub.max=D, and the determined points P.sub.1 and P.sub.2 are
stored. After this the next line (n=n+1) is processed. If in this
line a value for D is found that is larger than the value already
determined for D.sub.max, then the new value for D is stored as
D.sub.max. This process is carried out line for line. After the
last line has been processed, P.sub.1 and P.sub.2 are known and the
distance between P.sub.1 and P.sub.2 exists in the variables
D.sub.max. in the middle between P.sub.1 and P.sub.2 one then
searches for P.sub.3.
[0080] The distance from the connecting line P.sub.1-P.sub.2 to the
lower edge of the picture is designated as the distance to the
lower edge. The condition that the distance be smaller than D/2 is
fulfilled when the front area of the meniscus circle is visible.
This is a requirement in this case.
[0081] After P.sub.3, P.sub.M, .alpha., and .beta. are determined,
the measurement points I to VIII (see FIG. 7) are distributed on
the arc. From these measurement points a "best fit circle" is then
determined; i.e., a compensating or corrected curve is determined
by these points.
[0082] In FIG. 9 two cameras 20, 21 and a crystal 2 are shown in a
view from above. The one camera 20 has a large focal distance and
reproduces the neck 40 that has arisen on the seed crystal 31. On
the other hand, the camera 21 has a small focal distance and
reproduces the entire crystal 2. The camera 20 in this connection
serves to reproduce the situation with a process step in accordance
with FIG. 5b, while the camera 21 is intended to reproduce the
situation with the process steps in accordance with FIG. 5c,
5d.
[0083] FIG. 10 shows the use of three cameras 21, 20, 70 in a
diagrammatic view from above. Cameras 21, 70 both have a short
focal distance and in each case reproduce half of the crystal 2.
Their use is activated during the process steps in accordance with
5c, 5d. The cameras 21, 70 are evaluated at the same time so that
the advantage of doubled resolution results. Camera 22 has a long
focal distance and serves to reproduce the neck 40 during the
process step in FIG. 5b.
[0084] In FIG. 11 a part of a flow diagram is shown that is
relevant for the camera constellation in accordance with FIG. 9.
First one checks whether the "neck process phase" exists. If this
is not the case, camera 21 is activated. If, however, the "neck
process phase" exists, camera 20 is activated. The pictures
supplied by the cameras 20, 21 are then corrected by using the
reference dates stored during the calibration. The corrected data
need not be reproduced optically; it is sufficient if they are
deposited in the memory.
[0085] The calculation of the diameter of the crystal from the
corrected representation of the crystal 2 or neck 40 then takes
place according to the guidelines of the flow diagram in FIG.
8.
[0086] The resulting value for the diameter is then determined and
fed to a controller.
[0087] With the invention it is thus possible to carry out an
accurate and trouble-free determination of the actual and
up-to-date crystal diameter in all phases of the growth process - -
- something that is an essential condition for optimal process
control and crystal quality.
[0088] The two or more camera pictures are evaluated in a special
way, as a result of which, e.g., problems are eliminated that occur
due to very different light- intensity and light-contrast
conditions. Thus not only the absolute information about the
brightness of the liquified material, meniscus, and crystal is
evaluated, but the intensity gradients .DELTA.J/.DELTA.X and
.DELTA.J/.DELTA.Y are also made use of in the evaluation. In
addition, the geometrical distortions that result due to the
position and the incline of the picture plane and observation angle
of a camera are corrected. Add to this an automatic absolute-value
calibration of the dimensions for a selectable picture plane by
using a standard model. By forming and evaluating gradients, the
edges of the crystal and meniscus ring are recognized and defined
with certainty. An up-to-date crystal diameter is determined by
using a special, multi-stage search and evaluation algorithm; by
determining the edges, maximum diameter, and relevance criteria; by
setting measurement lines; and by determining the diameter by means
of a best fit from a multitude of measurement points, etc.
[0089] To rule out irrelevant picture contents and picture objects,
various relevance criteria are made use of; e.g., a
process-dependent definition and extraction by scanning of relevant
symmetry conditions; the definition and extraction of relevant
intensity, contrast, and gradient conditions; the definition and
extraction of meaningful upper and lower limits for the dimensions
of the measurement objects; and a comparison with stored typical
model pictures.
[0090] The search and measurement algorithm are continuously
checked according to the aforesaid criteria.
[0091] In this way the exact crystal diameters can be determined
even with limited lighting conditions and with a meniscus ring that
is either only partially visible or partially able to be evaluated.
In this connection it does not matter whether covering takes place
due to a large angle of image with a large crystal diameter or due
to partial covering with a variable diameter or due to a partial
covering by building it into the equipment. The "complete circle"
of the crystal is in such cases limited by computation methods.
[0092] If the meniscus ring is only partially covered, an automatic
optimization of the choice of measurement points takes place.
[0093] Moreover, the invention allows for adjustments in the
respective process conditions, among other things by adjusting
filter constants, geometrical conditions such as symmetry and
circular form, intensity and gradient conditions, window sizes,
and/or lower and upper relevance limits for the crystal
diameter.
[0094] In certain phases of the crystal-pulling process in which a
meniscus does not occur, the crystal diameter can, nonetheless, be
measured directly.
[0095] Further variations and modifications of the foregoing will
be apparent to those skilled in the art and are intended to be
encompassed by the claims appended hereto.
[0096] German priority applications 197 38 438.2 and 198 17 709.7
are relied on and incorporated herein by reference.
* * * * *