Gas separation: processes – Deflecting – Tortuous flow path
Reexamination Certificate
1998-10-22
2001-01-23
Smith, Duane S. (Department: 1724)
Gas separation: processes
Deflecting
Tortuous flow path
C055S444000, C055SDIG002, C055S434400, C095S282000
Reexamination Certificate
active
06176901
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention pertains to a device and a method for the precipitation of dust which is created in particular during the manufacture of a crystal grown from a melt and which can be added as a dust mixture to a transport gas for diverting it from the melt chamber.
In the growth of crystals from a melt, such as in the Czochralski or Bridgeman method, or other methods for melting or remelting the starting crystalline material, the melting process takes place within processing chambers separated from the external atmosphere. In this case, the melt is held in a crucible which is located inside an evacuated vessel. Depending on the vapor pressure and the temperature of the molten material, a vapor cloud composed of the molten material forms above the melt crucible. This cloud contains particles which have diameters of less than one micrometer and which tend to agglomerate into larger particles of dust. Sufficiently large dust particles can then drop back into the melt, so that the crystallization process is disrupted, and consequently, the growth of the monocrystal must be interrupted.
In order to prevent the disruption caused by the dust, the dust particles are usually removed from the process chamber by means of a continuous gas stream. The gas used in this case is introduced through a gas inlet into the processing chamber, directed over the opening of the melt crucible, and is then vacuumed off together with the collected dust by a vacuum pump and output through an exhaust opening. Due to the highly abrasive nature of the pumped dust, which may consist of SiO, for example, in the growth of Si-monocrystals, the dust is usually filtered out in dust filters connected in series with the vacuum pumps. In this regard, these filters have porous surfaces to separate and collect the dust.
One problem in the use of these known filters is that their capacity for collecting dust is quickly reached when dust generation is heavy, even when they are generously proportioned with respect to the total generated quantity of dust, and these porous surfaces must then be replaced with new ones. Usually either the entire filter arrangement must be shut down to remove the filter and the gas stream must be shunted through a second, separate filter arrangement, or the filter must be designed for the maximum quantity of dust likely to be encountered. Both alternatives are less than optimal from a design standpoint and are associated with added expense. Moreover, the manual handling of these dust-laden filter elements is associated with health risks, which, in turn, can only be avoided by additional apparatus expense.
Therefore it is an object of the present invention to enable the cleaning and purification of scavenging gas used in the growth of crystals, which is easy to implement and economical to apply.
It is a further object of the present invention to provide a suitable filter apparatus to remove the dust admixed into the scavenging gas, that has a high capacity, that is easy to produce in a simple manner, and that also allows simple handling.
SUMMARY OF THE INVENTION
The above and other objects of the present invention can be achieved by precipitating the dust included in the scavenging gas, by a dust precipitator which has at least one chamber with a cooling wall surface to which the scavenging gas laden with dust and pumped from the process chamber flows past in such a manner that the dust particles remain adhered to the chamber wall upon contact with the wall, and thus are removed from the flow of gas. With the precipitation device according to the invention, even large quantities of dust can be extracted from the gas stream, since any plugging of the filter and exhaustion of the filter effect such as that in a porous screen filter cannot occur. The removed dust is held to the cooled wall surface and grows slowly in layers on this surface. Thus preferably the operating time between two required cleaning steps will be significantly increased with respect to a conventional filter dust apparatus.
The precipitation chamber itself has a chamber wall preferably cooled on all sides and a gas inlet for entry of the dust-laden gas stream and a gas outlet for exit of the purified gas. According to another feature of the invention, the adjoining chamber walls are positioned with respect to each other such that the gas control geometry allows the gas stream to flow past at least one cooled chamber wall surface to which the dust has adhered.
In order to obtain an effective cooling of the wall surface, the extraction chamber is produced with a double-walled cooling jacket whose inner chamber wall is used as a flow surface for the dust-laden scavenging gas. The cooling jacket has one coolant entry, through which the coolant flows into the cooling jacket which cools it upon contact with the wall surface, and one coolant outlet. The coolant is then shunted across a drain channel connected to the coolant outlet. The coolant inlet and the coolant outlet are connected via a locking valve to the coolant inlet line and the coolant drain, respectively.
The precipitation chamber is preferably manufactured as a single part so that it can be removed entirely from the gas exhaust line, for example, for cleaning of the cooling surfaces laden with dust. A replacement of individual filter elements installed in a filter chamber, which usually leads to undesirable swirling of dust, is thus prevented.
In order to allow the replacement of the chamber without implementing additional activities with regard to the coolant inlet itself, a valve is installed, preferably a blocking valve, in both the coolant inlet line and the coolant outlet line which automatically interrupt the flow of coolant upon removal of the dust from the chamber. In this regard, the valves are designed according to the properties of the invention.
Another advantage of the invention is obtained by placement of at least two chambers, one behind the other, such that the gas stream is first directed through the first chamber and is then directed through the second chamber. Due to the doubling of the cooled dust deposition surfaces, the service life of the individual chambers between two required cleanings is doubled. The individual chambers can be connected preferably manually together and can preferably be removed individually and completely from the precipitation device for cleaning without the filtered dust escaping into the environment such that the inlet of coolant to the other chambers is advantageously not interrupted. The subsequent cleaning of the flow surfaces can then occur, according to the invention, outside the vicinity of the crystal growth system and will have no effect on its operation.
For an appropriate use of the precipitation device according to the invention, the dust produced during crystal growth in the melt (e.g., dust produced during growth of Si-monocrystals) is first picked up by a scavenging gas introduced into the processing chamber and together with the scavenging gas according to the gas control geometry in the reaction space, which is defined by the position of the gas inlet and the gas outlet with regard to the melting crucible, is flushed out from the processing space. As scavenging gas, preferably noble gases are used, such as argon, which do not themselves react chemically with the molten material. The dust-gas mixture is pumped through an exhaust line by means of a vacuum pump from the processing space, and the dust precipitation device of the invention, positioned between the intake opening of the vacuum pump and the gas outlet of the melt chamber, is used to filter the gas. According to the invention, the dust-laden gas is directed to the precipitation device against preferably cooled chamber surfaces onto which the dust particles adhere.
In order to avoid turbulence and swirling dust within the deposition device at rough flow surfaces, the cooling surfaces have smooth, preferably polished, surfaces. Due to the herein before mentioned, preferably low, temperature of the surface and its low r
Leybold Systems GmbH
Smith Duane S.
Smith , Gambrell & Russell, LLP
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