Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
1998-12-29
2001-05-01
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C392S416000, C118S050100, C373S136000
Reexamination Certificate
active
06225602
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a vertical furnace for the treatment of semiconductor substrates under reduced pressure, comprising a treatment chamber delimited by a first liner made of refractory material, around which a second liner made of quartz material is arranged essentially concentrically, and wherein a heating element is arranged outside the second liner, which heating element is enclosed by refractory materials.
A furnace of this type is disclosed in U.S. Pat. No. 4,738,618. Said patent describes a furnace having two spaced liners, the inner liner being closed off at the top and arranged such that it is movable in the vertical direction.
Processing at reduced pressure is deemed to be a treatment at a pressure of less than 2 Torr, more particular 1 Torr, and more specificly in the range between 100 and 150 milli Torr.
As a result it is not possible to feed a process gas in the vertical direction, as is required for certain treatments of semiconductor substrates, through a furnace of this type.
U.S. Pat. No. 5,312,245 describes a furnace with which such a treatment is possible. The furnace has an inner and an outer liner, the liner which delimits the treatment chamber being made of quartz material.
In the prior art two materials are used for furnaces which operate at high temperature and in which semiconductor substrates are treated. In by far the majority of cases, quartz material is used because this provides a good seal of the treatment chamber with respect to the environment and can relatively simply be provided with appendages, such as nozzles and other components. Furthermore, a quartz liner of this type is relatively inexpensive to produce.
The disadvantage of a quartz liner, however, is that, during the process, material which inevitably deposits from the process gas gives rise to appreciable stresses in the quartz liner. Such deposits are particularly difficult to remove and lead, after a limited number of treatments, to the need for replacement of the quartz tube. With average continuous operation of such a furnace it is necessary to clean the quartz tube after approximately two weeks and after cleaning two to three times a quartz tube of this type is no longer usable.
Another material used in the prior art for other applications is silicon carbide. This is not used in vacuum furnaces because it is porous, with the result that it is not possible to provide a seal with respect to the environment, so that it is no longer possible to guarantee that, under the critical conditions of elevated temperature and low pressure, only process gases move through the furnace. Furthermore, it is difficult to make vacuum seals on the basis of O-rings because of the good thermal conductivity of silicon carbide. Silicon carbide is a particularly expensive material when it is shaped into diverse forms, such as fitting nozzles, flanges and the like on liners.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a vertical furnace which can be cleaned more simply and with which the operating period between two maintenance periods can be appreciably lengthened, that is to say with which the liner concerned has a longer life.
This aim is achieved with a vertical furnace as described above having a SiC liner. The SiC liner and the second liner each have a nozzle at the top, wherein the nozzle of the second liner is arranged concentrically around the nozzle of the SiC liner. A free end of at least one nozzle is connected to the remainder of the furnace via a sealing which permits movement, wherein a gap between the liners can be put under reduced pressure and wherein a second liner is sealed relative to the surroundings.
It has been found that silicon carbide forms a surface providing good adhesion for materials which deposit from the process gases. Moreover, it has been found that the coefficient of expansion of silicon carbide and the deposited materials have a relationship relative to one another such that good adhesion is obtained. This is in contrast to quartz, where an appreciable difference in coefficient of expansion exists, which leads in the case of greater layer thicknesses to failure of the tube occurring under stresses generated by temperature. Failure of this type has not been detected in the case of a silicon carbide tube. Problems associated with the porosity of silicon carbide are solved according to the invention in that a quartz tube is positioned around a silicon carbide tube of this type. That is to say, the treatment chamber is sealed with respect to the environment. It has been found that the porosity of the silicon carbide is so low that a negligible quantity of process gas enters the gap between the two tubes. Moreover, this ingress of process gases can be further prevented if the gap between the two liners is flushed with an inert flushing gas which displaces the process gas or leads to a lower partial pressure of the process gas.
With this construction it is no longer necessary regularly to carry out maintenance on the liner, that is to say regularly to remove deposits from the liner and/or to replace the liner. As a result, the somewhat higher costs associated with the production of the boundary of the treatment chamber from a silicon carbide material are fully justified.
From the abstract of the Japanese patent application 59 189 622 a method is known for coating of a silicon carbide tube. This tube is provided with a coating inside a furnace having a furnace liner of quartz material. Around the silicon carbide tube to be processed a graphite liner is provided. The tube to be processed is loosely provided in the furnace and is not part of the furnace as is the liner according to the invention.
From the abstract of the Japanese patent application 03 194933 a furnace operating at atmospheric pressure is known having a double liner between which a gas is passed. A second liner comprises silicon carbide. If such furnace would to be operated under vacuum conditions, it would be possible to seal the second liner relative to the surroundings, which is not possible in view of the good thermal conductivity of silicon carbide with rubber seals or other material in practice.
The first liner according to the invention comprising silicon carbide material has to be provided with a feed for process gas at the top. As indicated, it is not simple to produce a spout with tight concentric tolerance on a silicon carbide tube, and it is therefore proposed, according to one embodiment of the invention, to fit an attachment on the top of the silicon carbide liner, which attachment is tubular and provides a link to the connection for a process gas. Because such an attachment is subjected to very much lower stresses and completely different conditions prevail therein, it is also possible to produce said attachment from relatively inexpensive quartz material. During operation, any small openings between the attachment and the liner made of silicon carbide material will be sealed immediately by deposits from the process gas. Such gaps will be present the first time such a furnace is started up because of the different coefficients of expansion and mechanical tolerances. However, deposits will settle from the process gas flowing through said gaps. A connection of this type will be broken again when a furnace cools. This will take place in the colder section where there is no or hardly any deposit growth. The coupling between the tubular attachment and the silicon carbide liner can be of any shape known from the prior art. However, preference is given to constructing this coupling in a ball-and-dish shape.
Preferably, the spout of the liner made of quartz material is fitted around a coupling tube of this type. This spout can be in one piece with the quartz liner, as is generally known in the prior art. A flushing gas can be introduced into the gaps between the two tubes, which gas is then distributed in the gaps between the two liners.
It is necessary to fit sealing means, in the vicinity of the top of the nozzles, which permit some mo
Buijze Jacobus Pieter
De Ridder Christianus Gerardus Maria
Stohr Hubertus Johannes Julius
Stoutjesdijk Jeroen Jan
Advanced Semiconductor Materials International N.V.
Fuqua Shawntina T.
Knobbe Martens Olson & Bear LLP
Walberg Teresa
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