Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Solid component
Reexamination Certificate
1999-10-22
2001-09-11
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Solid component
C423SDIG005, C423SDIG006, C588S253000
Reexamination Certificate
active
06287528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for passivating combustible, metallurgical dust which accumulates in the off-gas filter of crucible-drawing installations, in particular of crucible-drawing installations for drawing single silicon crystals. The invention also relates to an oxidation device for carrying out the method.
2. The Prior Art
The Czochralski crucible-drawing method is known for the production of high-purity single crystal rods. During the crucible-drawing of crystal rods, in particular of silicon rods using the Czochralski method, the monocrystalline or polycrystalline silicon fragments which are intended to produce the molten silicon are generally placed in a melting crucible, preferably in a quartz crucible. The silicon fragments are heated by means of a heat source, and then a monocrystalline seed is immersed in the molten liquid and drawn upward while being rotated, with the result that a crystal rod grows at the bottom end of the monocrystalline seed.
In order to establish the type of conductivity and the resistance of the crystal which is grown, dopants, such as boron, phosphorus, arsenic and antimony are added to the silicon fragments or the molten material. In addition to the silicon fragments and the dopants, the molten material also contains oxygen. The oxygen is introduced into the molten material from the surface of the quartz crucible by the contact between them. The oxygen is moved within the crucible by the forced convection of the molten material resulting from the rotation of the crucible and the heat convection of the molten material resulting from temperature differences therein.
During this movement, some of the oxygen is evaporated from the surface of the molten material in the form of the oxides of the constituents of the molten material. Depending on the particular dopant, these oxides, in addition to SiO, also include, owing to their high vapor pressure, the oxygen compounds of arsenic and antimony, in particular arsenic trioxide and antimony trioxide. Since an inert gas, preferably argon, generally flows continuously through the crucible-drawing installation, the oxides of the constituents of the molten material have different oxidation numbers in the gas phase and in the off-gas flow from the installation.
Finely divided metal oxide/semimetal oxide mixtures of medium oxidation numbers are highly reactive industrial dust materials with spontaneous combustion ignition temperatures of below 100° C. These dust materials tend, in particular in the downstream off-gas filters, to burn the filters or to cause dust explosions.
SUMMARY OF THE INVENTION
It is an object of the present invention to increase the spontaneous combustion ignition temperatures of combustible, metallurgical dust which accumulates in the off-gas filter of crucible-drawing installations.
This object is achieved according to the present invention by providing a method for passivating combustible, metallurgical dust which accumulates in the off-gas filter of crucible-drawing installations, wherein the dust in the off-gas flow from the crucible-drawing installation is passivated continuously by means of a reaction gas at temperatures of from 50° C. to 500° C.
In the context of the invention, passivating is understood to mean the controlled oxidation of small amounts of dust in the gaseous phase. Reaction gas is understood to mean gasses or gas mixtures which have an oxidizing action.
The object is also achieved according to the present invention by providing an oxidation device for passivating combustible, metallurgical dust which accumulates in the off-gas filter of crucible-drawing installations, which device has a reaction chamber with a heater device, at least one reaction-gas inlet and a control unit which controls the temperature in the reaction chamber and the supply of reaction gas to the reaction chamber as a function of the drawing conditions.
Surprisingly, it was found that by an accurately metered addition of a reaction gas to the off-gas flow from a crucible-drawing installation at a temperature of from 50° C. to 500° C., preferably from 100° C. to 300° C., the gaseous dust is passivated without interfering with the drawing process.
The reaction gas used is preferably air, oxygen, ozone or mixtures of these gases, with air being a particularly preferred reaction gas.
The delivery of gas within the reaction chamber ensures, for example by means of a suitably long residence time, that the temperature prevailing in the reaction chamber is transferred to the gas mixture, comprising the reaction gas and the off-gas flow. The supply of reaction gas and the temperature in the reaction chamber are controlled by means of a control unit. The off-gas flow which flows into the reaction chamber is mixed with the reaction gas by means of a mixing device.
The supply of the reaction gas, the temperature in the reaction chamber and, if appropriate, the use of a catalyst are dependent on the conditions of the dust materials. The morphology and the composition of the dust materials differ depending on the crucible-drawing process, and on the amount of dust, which increases with increasing initial charge of silicon used, process duration and temperature.
The continuous passivation of only small amounts of gaseous dust in the oxidation device according to the invention during the drawing process using small amounts of reaction gas has various advantages. In particular, it results in homogenous passivation of the dust, i.e. there is no formation of particles or clusters which lead to spontaneously igniting smoldering fires or deflagrations. The passivation of in each case only small amounts of dust has the advantage that the reaction-gas partial pressure in the gas chamber of the crucible-drawing installation increases only slightly. Hence, the vacuum conditions required for the crucible-drawing process are therefore not impaired.
The performance of the method and of the oxidation device according to the invention for carrying out the method is made clear by Experiments 2 and 3:
T[20 C.]
Air [1/h]
T
SZ
[° C.]
T
Z
[° C.]
Experiment 1
—
—
155
260
Experiment 2
230
200
220
290
Experiment 3
230
500
220
330
In the experiments, in each case one crystal rod was drawn in a crucible-drawing installation according to the prior art. Exactly identical drawing conditions prevailed, such as for example the flow of inert gas, the melting temperature, the initial charge of silicon, the type of dopant and the amount of dopant in the crucible-drawing installation. In Experiments 2 and 3, the claimed oxidation device was positioned in the off-gas flow from the crucible-drawing installation, between the crucible-drawing installation and the off-gas filter. The reaction chamber of the oxidation device was heated to a temperature T[° C.] of 230° C. by means of a heater device. The off-gas flow which flows through the reaction chamber was mixed with air, which contains approximately 21% by volume of pure oxygen, by means of a mixing device. In Experiment 2, the off-gas flow was mixed with 200 l of air per hour (air[l/h]), and in Experiment 3 with 500 l of air per hour (air [l/h]).
After drawing of the crystal rods, the dust which had accumulated in the off-gas filter was examined. By means of the oxidation device, it was possible to increase the spontaneous combustion ignition temperature T
SZ
[° C.] of the dust from 155° C. (Experiment 1) to 220° C. (Experiments 2 and 3). At the same time, the ignition temperature T
Z
[° C.] of the dust rose from 260° C. in Experiment 1 to 290° C. in Experiment 2 or to 330° C. in Experiment 3.
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paten
Collard & Roe P.C.
Griffin Steven P.
Vanoy Timothy C
Wacker Siltronic Gesellschaft fur Halbleitermaterialien AG
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