Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2001-09-14
2003-06-03
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S014000, C117S201000, C117S202000, C117S208000, C117S217000
Reexamination Certificate
active
06572699
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method and apparatus for detecting the surface height (melt level) of a raw-material melt in a Czochralski monocrystal pulling apparatus, and more particularly to a method and apparatus for detecting the melt level of molten silicon.
BACKGROUND ART
Necessity of Detecting Melt Level
The Czochralski method (CZ method) is a method for pulling out monocrystalline ingots from melts of raw materials such as silicon in a crucible. In order to make crystals grow well, it is necessary to accurately detect the surface level (hereinafter “melt level”) of a raw-material melt and to adjust the same.
Detecting and adjusting melt levels properly in a CZ monocrystal pulling apparatus is useful in controlling the relative positions of a heat shield and the melt level, or the relative positions of a heater and the melt level so as to promote stable crystal growth.
In particular, in currently existing CZ silicon monocrystal pulling apparatuses, normally, the thermal radiation from the heater and silicon melt are controlled and also a heat shield (or gas rectifying tube) for rectifying a gas flowing inside the furnace is installed. By appropriately performing the feedback control described above, and controlling the relative positions of the lower surface of the heat shield and the melt level (that is, the distance between them), the thermal hysteresis of the pulled silicon monocrystal and the impurity concentrations (oxygen concentration, etc.) therein can be made constant.
Melt Level Detection Apparatuses
In terms of conventional technology for melt level detection apparatuses, there are apparatuses such as that disclosed in Japanese Patent Publication (Kokoku) No. 3-17084. That conventional apparatus detects melt levels based on the principle of triangulation. More specifically, with that apparatus, as illustrated in
FIG. 19
(
FIG. 1
in this Publication), a laser beam
34
is projected onto the melt surface at an angle &thgr;, the regularly reflected beam
38
is condensed by a lens
44
, and the position of convergence
46
is detected by a photosensor
48
. In this conventional apparatus, the enlarged projection
30
of the laser beam is received for the purpose of averaging the measurement variation caused by very small ripples
22
that develop in the surface of the melt surface
20
.
However, there are other factors in the melt surface
3
that impair the flatness of that melt surface
3
besides the very small ripples noted above. Specifically, as illustrated in FIG.
2
(
b
), in the portion of the melt surface
3
near a crystal
15
, a meniscus
28
develops due to the surface tension near the growth surface of the crystal
15
. Also, due to the rotation of the crucible
14
and the rotation of the crystal
15
being pulled, the surface becomes inclined in a parabolic shape across the entirety of the melt surface
3
. As illustrated in FIG.
2
(
c
), furthermore, when a gas rectifying tube
16
is brought close to the melt level
3
, due to the discharge pressure of the inactive gas, there are cases where a depression is formed in the melt surface
3
near the bottom portion of the gas rectifying tube
16
. Such inclinations as these in the melt surface
3
cause the direction of the regularly reflected beam from the laser beam used for measuring as noted earlier to be shifted (the inclination in the melt surface
3
being indicated by the angle &PSgr; in the figure), making effective reception of that beam difficult.
The above-described conventional discloses that the position where the photodetector unit is disposed is moved so as to catching the shift in the direction of the regularly reflected laser beam, caused by inclinations in the melt surface. The inclination &PSgr; in the melt surface
3
is closely related to the rotational speeds of the crucible
14
and of the crystal
15
and to the height of the gas rectifying tube
16
above the melt surface
3
. Therefore, every time these pulling conditions are changed, the amount to move the position of the photodetector unit must be adjusted. Not only is that a troublesome task, but it is very difficult to precisely reproduce movement settings for the photodetector unit. When the movement adjustment is not done well, furthermore, it is possible that the regular reflected beam from the laser will not be able to be received and that melt surface detection will be disabled.
Furthermore, as illustrated in
FIG. 20
(which corresponds to
FIG. 3
in the Publication mentioned above, being a diagram illustrating changes in the regularly reflected laser beam relative to changes in the melt level), when the melt level changes (&Dgr;L) largely, the positional shift (&Dgr;Y) in the regularly reflected beam becomes large, whereupon it becomes necessary to shift the light detector greatly, by that measure. In conjunction therewith, moreover, installation constraints develop, such as the necessity of having an observation port
40
of a size that corresponds with that large shift, or the need to provide enough space for the light detector to be able to move.
Furthermore, because the melt surface
3
reflects light as does a mirror surface, another problem arises in that, as illustrated in FIG.
15
(
a
), scattered light at the emission port
29
of the laser source
1
is reflected by the melt surface
3
, which is picked up in the photosensor
7
as a ghost
30
(a phenomenon which occurs frequently under conditions where the regularly reflected beam
4
is received through a lens
5
) resulting in a deterioration in position detection precision. In this connection, in the conventional technology noted above, a band-pass filter is employed which passes only laser beam wavelengths, as a measure for distinguishing between the radiated light from the melt surface
20
and the received laser beam with good contrast. However, the wavelength of the scattered light at the emission port
29
of the laser source
1
and the wavelength of the laser beam that is regularly reflected by the melt surface
3
are mutually identical, wherefore the picking up of the scattered light at the emission port
29
by the photosensor
7
cannot be avoided with a band-pass filter.
DISCLOSURE OF THE INVENTION
An object of the present invention, which was devised with the problems noted above in view, is to provide a melt level detection apparatus and melt level detection method wherewith melt levels can be detected simply and precisely.
In order to resolve such problems as those noted above, in the melt level detection apparatus and melt level detection method according to the present invention, use is made of the shape of the surface that regularly develops on the surface of the melt, causing that to function as a kind of reflecting body (that is, a kind of reflecting body for causing the projected laser beam to be accurately guided to a photodetector attached at a prescribed location), and thus performing melt level detection accurately. As means to that end, scanning is performed in the radial directions of the crucible to search out a position at which the projected laser beam will be accurately guided to the photodetector.
The present invention, furthermore, was devised on the basis of certain points of view, namely that, in a CZ furnace, due to the rotation of the crucible itself and/or the rotation of the crystal being pulled out, a swell develops in the shape of a concentric circle centered on the axis of rotation of the crystal being pulled out, which is a regularly occurring phenomenon, and that, because the cross-section of that swell is parabolic, if scanning is done in the radial directions of the crucible, a position will always be found at which the projected laser beam will be guided accurately to the photodetector.
More specifically, the present invention provides a melt level detection apparatus and method such as those described below.
(1) A melt level detection apparatus comprising a laser beam projector and a photodetector at prescribed positions in a CZ furnace, wherein the laser beam projector projects
Hanamoto Tadayuki
Kotooka Toshirou
Mimura Kazuhiro
Moriya Masato
Hiteshew Felisa
Komatsu Denshi Kinzoku Kabushiki Kaisha
Varndell & Varndell PLLC
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