Gas and liquid contact apparatus – Contact devices – Liquid tank
Reexamination Certificate
1999-04-22
2001-11-06
Barry, Chester T. (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Liquid tank
C261S121100
Reexamination Certificate
active
06311959
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improved method and apparatus for generating a mixture of an organic vapor, such as the vapor of tetraethylorthosilicate (TEOS), and an inert gas such as helium, the mixture having a precisely controlled ratio of vapor-to-gas such as required during semiconductor manufacturing for the deposition of highly uniform layers of insulation (e.g., SiO
2
).
BACKGROUND OF THE INVENTION
In the manufacturing of semiconductor devices, such as integrated circuits, memories, etc., a semiconductive wafer (e.g., a thin disc of single-crystal silicon) is sequentially put through a number of processing steps (well known in the art). One or more of these steps involve exposing the wafer to a mixture of reactive gasses to deposit thin layers of insulation, such as silicon dioxide (SiO
2
), on exposed surfaces of the wafer. The reactive gasses comprise, for example, ozone on the one hand and an organic vapor of a liquid, such as tetraethylorthosilicate (TEOS), in an inert gas such as helium on the other hand. Because ozone and organic vapor such as TEOS immediately begin reacting when mixed together, they are brought separately into close proximity of a wafer on which insulation is to be deposited, and then mixed together. The mixed gases are then flowed immediately as a uniform cloud or dispersion of reactive gas upon and over the wafer thereby depositing a layer of insulation thereon.
It is desirable, for uniform and rapid deposition of an insulating layer on the wafer during a given time interval, that a mixture of TEOS vapor and helium, for example, have a sufficient, and precisely controlled amount of TEOS vapor per standard units of measurement. This in turn requires that the TEOS vapor-helium mixture be generated at a slightly elevated temperature (e.g., somewhat above about 65° C.) so that none of the TEOS vapor in the mixture condenses as liquid and thereby decreases the desired ratio of vapor to helium.
A typical prior art module which generates an organic vapor-inert gas mixture (e.g., TEOS vapor and helium) is somewhat bulky and operates at an elevated temperature. Because of limited space and to minimize thermal buildup it is customary to place such a module a convenient distance (e.g., a few feet) away from where a wafer-processing chamber is located. The gas mixture from the module is then piped to the wafer chamber via a suitable means.
In the past, different ways of generating organic vapor-inert gas mixtures for semiconductor manufacturing have been employed. A first way was to bubble the inert gas through a container of organic liquid (e.g., TEOS). The ratio of vapor to gas in the resulting output mixture was controlled by measuring the amount of organic vapor in the output mixture of vapor and gas. But such vapor measurements are not as accurate as is desirable and the equipment used to generate the vapor-gas mixture was relatively bulky, in part because the quantity of liquid in the container was not controlled to a set amount. Therefore an extra amount of liquid, requiring a larger container, was used to provide for variations in the liquid level during operation.
A second widely used way of generating an organic vapor-inert gas mixture is to inject, by means of an injector head, a fine mist of organic liquid into a stream of inert gas at an elevated temperature (e.g., 120° ). This system works reasonably well but has several disadvantages. The injector head is prone to clogging and this sometimes requires temporary shutdown and servicing of the equipment. Moreover, the organic vapor causes wear within the injector head with the result that it must be frequently replaced (e.g., every six months or so). It is desirable therefore to have a way of generating a mixture of organic vapor and inert gas which provides highly precise and continuous control of the amount of vapor in the mixture and which avoids the problems and costs of previous equipment.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention there is provided a highly effective method of generating an organic vapor-inert gas mixture (e.g., TEOS vapor-helium) with precise and continuous control of the ratio of vapor to gas in the mixture. This in turn, during a given time interval in a semiconductor processing step, insures the deposition of a uniform layer of insulation having a predetermined thickness. Variation in the ratio of vapor to gas in the mixture would otherwise result in different thickness of insulation being deposited during the given time interval, which of course is undesirable.
The method provided by the invention includes filling a relatively small chamber (colloquially termed a “bubbler”) with an organic liquid, such as TEOS, the chamber being filled to a predetermined or set level and thereafter kept at that level. The liquid is maintained at a desired temperature (e.g., 75° C.)somewhat above ambient. An inert gas such as helium, is bubbled through the liquid in the chamber at a controlled rate, and a resulting vapor-gas mixture is continuously exhausted from the chamber. The level or quantity of liquid in the chamber is automatically maintained at the set value by controlled flow of gas into the chamber and by continuous monitoring of the liquid flow rate.
The respective flows of inert gas and of organic liquid into the bubbler chamber are controlled by electronic circuits utilizing digital signals from computer inputs and signals from respective gas flow and liquid flow, all of which are highly accurate. These control circuits and respective signals are connected in a feedback arrangement which insures that the ratio of organic vapor to inert gas in the mixture flowing out of the bubbler chamber is constantly held within very tight limits. This in turn means that the amount of organic vapor delivered by the mixture of vapor and gas per unit time to a semiconductor manufacturing station is controlled with great accuracy.
In accordance with another aspect of the invention there is provided a method of accurately controlling the ratio of liquid vapor to gas in a mixture of the two. The method includes the steps of: flowing liquid into a chamber; flowing gas into the chamber and bubbling it through the liquid; determining whether liquid in the chamber is being evaporated at a rate at variance with that desired; generating a feedback signal in accordance with the variance in the rate of evaporation of the liquid; modifying the flow rate of gas into the chamber in accordance with the feedback signal to maintain the evaporation of the liquid at the desired rate; and evacuating from the chamber a mixture of vapor and gas such that the amount of vapor in the mixture is controlled.
In accordance with still another aspect of the invention there is provided an apparatus for generating a liquid vapor and gas mixture with accurate control of the vapor to gas ratio. The apparatus comprises a housing defining a chamber for containing a liquid to be vaporized, a liquid supply for flowing liquid into the chamber at a predetermined rate, a gas supply for flowing gas into the chamber to evaporate liquid therein, an exhaust for exhausting a mixture of vapor and gas from the chamber, and a control circuit. The control circuit controls the gas supply and generates a feedback signal from the flow of liquid into the chamber in accordance with whether the flow of liquid therein is increasing or decreasing, relative to the predetermined rate, the feedback signal incrementally adjusts the flow of gas into the chamber such that the mixture of vapor and gas exhausted from the chamber has a predetermined ratio of vapor to gas. This in turn means that the amount of organic vapor delivered by the mixture of vapor and gas per unit time to a semiconductor manufacturing station is controlled with great accuracy.
A better understanding of the invention will best be gained from the following detailed description and claims taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 3855974 (1974-12-01), Mayer
patent: 4235829 (1980-11-01), Partus
patent:
Hendrickson Scott
Nguyen Son T.
Applied Materials Inc.
Barry Chester T.
Ostroff & Associates
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