Abstract:
Method and apparatus generate a mixture of the vapor of an organic liquid such as tetraethylorthosilicate (TEOS) and an inert gas such as helium. The ratio of organic vapor to inert gas in the mixture is accurately and continuously controlled as required in semiconductor manufacturing. The apparatus encloses a bubbler chamber which is filled with an organic liquid (e.g., TEOS) to a set level that is automatically maintained. The liquid is also maintained at an exact temperature (e.g., 75° C.). Inert gas (e.g., helium) flows into the bubbler chamber at a controlled rate and continuously evaporates some of the liquid therein. The flow of liquid into the bubbler chamber is monitored by a liquid control circuit, and flow of gas is controlled by a gas control circuit. A feedback signal from the liquid control circuit to the gas control circuit incrementally adjusts gas flow into the bubbler chamber to keep the liquid therein at the set level.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view, with some portions not shown, of apparatus provided according to the invention for generating a precisely controlled mixture of organic vapor and inert gas such as used in semiconductor processing; and 
     FIG. 2 is a block diagram schematically showing the apparatus of FIG. 1 including its control circuitry, and illustrating how it is operated to produce a vapor-gas mixture with the ratio of vapor to gas being automatically held within tight limits. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is shown an apparatus  10  embodying features of the invention for generating an organic vapor-inert gas mixture having a closely controlled ratio of organic vapor to inert gas, such as is particularly useful in semiconductor manufacturing. The apparatus  10 , portions of which are not shown here, comprises a housing  12 , a bubbler chamber  14  (shown in dashed outline), a liquid input pipe  16 , an inert gas input pipe  18 , a vapor and gas output pipe  20 , an input liquid control solenoid valve  22 , an input gas control solenoid valve  24 , a gas bypass solenoid valve  26 , an output vapor-gas mixture control solenoid valve  28 , and heat exchanger fins  30 . Electronic control circuits, liquid level sensor, and gas and liquid flow monitors which are part of the apparatus  10  and important to its operation are not shown in FIG.  1 . They will be described in complete detail shortly in connection with FIG.  2 . 
     Still referring to FIG. 1, the housing  12  is thermally insulated. Its interior and the bubbler chamber  14  are accurately maintained at a desired temperature (e.g., 75° C.) by a solid-state thermal unit (not shown) which operates on the Pellitier principle. The outside of the thermal unit is connected to the heat exchanger fins  30  which exchange thermal energy from the atmosphere and help the thermal unit accurately maintain the interior temperature of the housing at a desired value somewhat above ambient (e.g., 75° C.). 
     The bubbler chamber  14  is filled to a predetermined or set level with an organic liquid such as TEOS. During operation of the apparatus  10 , the quantity of liquid in the chamber  14  is automatically maintained at this set level by an automatic liquid level control mechanism (not shown) associated with the bubbler chamber  14 . This automatic mechanism acts to incrementally increase or decrease the flow of liquid into the chamber  14  as required to maintain the liquid at the set level, and is well known in the art. Liquid from the pipe  16  is fed through the solenoid valve  22  into the chamber  14  via an internal pipe  32  (shown in dashed outline). This automatic operation will be described in greater detail hereinafter. 
     Inert gas, such as helium, is fed into the apparatus  10  via the pipe  18 , the solenoid valve  24  and an internal pipe  34  (shown in dashed outline), an end of which extends down into the bubbler chamber  14 . When gas is bubbling through the end of the pipe  34  into and through the liquid in the bubbler chamber  14 , the flowing gas causes continuous evaporation of some of the liquid. An internal pipe  36  (shown in dashed outline) at the top of the chamber  14  exhausts, via the output solenoid valve  28  to the output pipe  20 , a mixture of vapor and gas being generated within the chamber  14  by the bubbling action of the gas. The flow of gas into the chamber  14  via the pipe  18  is automatically controlled by the solenoid valve  24 , and this in turn controls a mixture of vapor and gas in a predetermined ratio flowing out of the chamber  14  to the output pipe  20 , as will be further explained shortly. The solenoid valve  26  serves to bypass the inert gas from the input pipe  18  directly to the output pipe  20 , without the inert gas flowing through the bubbler chamber  14 , whenever it is desired to purge the output pipe  20  of the apparatus  10  of the vapor and gas mixture. A unit comprising the housing  12 , bubbler chamber  14 , and associated elements such as shown in FIG. 1 is commercially available, for example from the vendor Icon Dynamics, LLC, New York. 
     Referring now to FIG. 2, there is shown a schematic block diagram  40  illustrating portions of the apparatus  10 . The diagram  40  shows the bubbler chamber  14 , the input liquid pipe  16  (represented as a solid line), the input gas pipe  18  (represented as a solid line), a liquid flow control circuit  42  (shown in block form), a gas flow control circuit  44  (shown in block form), the internal liquid pipe  32  (represented as a solid line), the internal gas pipe  34  represented as a solid line), the output pipe ( 36 )  20  (represented as a solid line), a first electrical signal path  46  (shown as a dashed line), a second electrical signal path  48  (shown as a dashed line), and an electrical feedback signal path  50  (shown as a dashed line) between the circuits  42  and  44 . 
     For the sake of the following explanation, it is to be assumed that the input liquid solenoid valve  22  (not shown here but see FIG. 1) is part of the liquid flow control circuit  42 , along with an input liquid flow meter (not shown) in the pipe  16 , and associated electronic memory, logic, and control elements (not shown). Similarly, it is to be assumed that the input gas solenoid valve  24  (not shown here but see FIG. 1) is part of the gas flow control circuit  44  along with an input gas flow meter (not shown) in the pipe  18 , and associated electronic memory, logic, and control circuits (not shown). The liquid flow control circuit  42 , including a liquid flow meter (not shown) and the liquid solenoid valve  22 , is commercially available, for example from the vendor Stec, Inc. Similarly, the gas flow control circuit  44 , including a gas flow meter (not shown) and the gas solenoid valve  24 , is commercially available, for example from the vendor Tylan General. 
     To begin gas bubbling operation of the apparatus  10 , electronic signals from a computer (not shown) are applied to the respective signal paths  46  and  48 , and a signal is applied (via a path not shown) to the output solenoid valve  28  to open it. The computer signal applied to the liquid flow control circuit  42  via the signal path  46  opens the input liquid solenoid valve  22 , and provides a bench-mark value for liquid flow via the pipes  16  and  32  into the bubbler chamber  14 . The computer signal applied to the gas flow control circuit  44  via the signal path  48 , together with a feedback signal via the signal path  50  determine the setting of the input gas solenoid valve  24  and hence the flow of inert gas via the pipes  18  and  34  into the bubbler chamber  14 . 
     During gas bubbling operation of the apparatus  10  liquid flow into the chamber  14  is monitored by the liquid control circuit  42  by comparing a signal from the liquid flow meter (not shown) in the pipe  16  with the signal received via the path  46  from the computer. Logic circuitry (not shown) within the liquid control circuit  42  then determines whether liquid flow into the bubbler chamber  14  is above or below a predetermined value desired for it because it is being evaporated at a rate less or greater than that specified by the input computer signal applied to the signal path  46 . In the event that the rate of evaporation differs from that desired, and in order to keep the liquid in the bubbler chamber  14  at precisely the set level for a given liquid flow into the chamber, the liquid control circuit  42  applies a feedback signal via the signal path  50  to the gas control circuit  44 . This feedback signal via the lead  50  and the input computer signal via the signal path  48  together cause logic circuitry (not shown) in the gas control circuit  44  to make fine, incremental adjustments in the setting of the input gas solenoid valve  24 . Thus, slightly less or slightly greater flow of gas into the bubbler chamber  14  will keep the liquid therein at precisely the set level for a desired liquid flow rate. In this way, the ratio of organic vapor to gas in the mixture of vapor and gas flowing out of the bubbler chamber  14  into the pipe ( 36 )  20  is very accurately controlled to the precise value desired. 
     When the apparatus  10  is to be stopped in its gas bubbling operation, signals from the computer are applied via the respective control circuits  42  and  48  to each of the solenoid valves  22 ,  24  and  28  to close them. In the event the output pipe  20  is to be purged of the vapor-gas mixture, the solenoid valve  26  is opened (by a signal from the computer) from its normally closed condition to bypass the solenoid valves  24  and  28 . This permits unmixed gas (e.g., helium) to flow directly from the input gas pipe  18  to the output pipe  20  without flowing through the bubbler chamber  14 . 
     By way of example, in a specific embodiment of the apparatus  10  which has been built and tested, the bubbler chamber  14  had a volume of 150 milliliters, and was filled to a set level with TEOS liquid which was maintained at the set level and at a temperature of exactly 75° C. During operation, helium was flowed into the bubbler chamber  14  at  200  Torr pressure and at a rate of 20 standard liters per minute. Six grams of liquid TEOS per minute were flowed into the bubbler chamber  14  to maintain the liquid therein at the set level. The liquid flow meter and the gas flow meter employed in the apparatus  10  each had very high accuracy, as did the liquid level control mechanism. At the temperature of 75° C. vapor evaporated from the liquid in the chamber  14  remains as vapor in the resulting mixture of vapor and gas. Thus the ratio of vapor to gas in the vapor-gas mixture flowing into the output pipe  20  was automatically and accurately controlled by the control circuits  42  and  44  of the apparatus  10  to the precise ratio desired. 
     Because the apparatus  10  is so compact in size and has such low power consumption, it is easily possible to arrange a plurality of such units as a modular group in order to provide immediately on command a separate, different mixture of organic vapor and inert gas. Thus for example, one apparatus  10  may be supplied with TEOS liquid (as described above), a second apparatus  10  supplied with triethyborate liquid (TEB), and a third apparatus  10  supplied with triethylphosphate liquid (TEPO). The use of such organic materials is well known in the art of semiconductor manufacturing. Inert gasses other than helium may be used where desirable. 
     The above description is intended in illustration and not in limitation of the invention. Various modifications or changes in the embodiment of the invention which is described herein may occur to those skilled the art and can be made without departing from the spirit and scope of the invention as set forth in the accompanying claims. In particular, the apparatus  10  is not limited to use with TEOS liquid and helium, nor to a particular size of bubbler chamber  14 , or to particular flow rates of gas and liquid, or to a given operating temperature. Details of construction and operation of the control circuits  42  and  44  are easily understood by those skilled in the art.