Abstract:
A liquid vaporizing and feeding apparatus, having high heat transfer efficiency and capable of producing a large flow of vaporized gas from a small amount of liquid, utilizes an inclined vaporization plate which is held at a set temperature by a heater. Liquid, discharged by a nozzle in the vicinity of the upper end of the vaporization plate, is vaporized as it flows down an inclined upper face of the vaporization plate. When the volume of the liquid collected on the vaporization plate  20  reaches a designated amount, a level sensor  40  stops the inflow of liquid through the nozzle. Then, as vaporization continues, and the liquid level falls, the sensor causes inflow of liquid to resume. The cycle of stopping and resuming flow of liquid is repeated to maintain a constant liquid level.

Description:
FIELD OF THE INVENTION 
     This invention relates to the vaporization of liquids and to an apparatus for feeding a special gas, produced by the vaporization of a liquid, to a semiconductor manufacturing line or the like. 
     BACKGROUND OF THE INVENTION 
     There are two principal kinds of conventional liquid vaporizing and feeding apparatus. In one such apparatus, a vaporizer, in which a vaporizing portion is built into a control valve, is combined with a flow meter for liquid or a flow meter for gas. In another such apparatus, a vaporization container is combined with a sensor for responding to the amount of liquid, a liquid temperature controller, a gas flow controller, conduits for the inflow and outflow of liquid, and an open-and-shut valve. These components are all located in a temperature-controlled container. 
     A vaporizing and feeding apparatus which vaporizes liquid in a liquid container is not subject to the problems of pyrolysis of the liquid and foaming in the container, since the apparatus is heated from the outside of the container, and the liquid is present in the container in the form of a pool. However, since the container wall, which is typically made of stainless steel or the like, is situated between the heater and the liquid, the apparatus has a low vaporization efficiency. 
     A vaporizer in which a vaporizing portion is built into a control valve is disclosed in Japanese Unexamined Patent Publication No. 156055/2001. This apparatus improves vaporization efficiency by providing a uniquely shaped mixing portion for liquid and carrier gas. 
     Although not used for semiconductor manufacture, an apparatus which vaporizes and feeds liquid raw materials for glass is disclosed in Japanese Unexamined Patent Publication No. 256036/1994. This liquid material vaporizing and feeding apparatus improves vaporization efficiency by providing convexities and concavities inside a vaporization container, thereby increasing the heating area. 
     The vaporizer of Japanese Unexamined Patent Publication No. 156055/2001, in which the control valve has a vaporization function, is small in size. However, particles and contamination are generated in the vaporizer, since liquid materials are readily decomposed by partial heating. This vaporizer has other problems, in that its ability to vaporize liquid deteriorates, and proper control is not achieved because of foaming taking place in the liquid within the container. 
     Although the vaporization apparatus of Japanese Unexamined Patent Publication No. 256036/1994, has high vaporization stability, the apparatus itself is large, and the amount of liquid in its tank is also large. Therefore, since the space required for its installation is large, the apparatus cannot easily be built into semiconductor manufacturing equipment. Moreover, since the amount of liquid utilized in the apparatus is large, and liquid exchange is required for maintenance, the cost of the expensive liquid materials required for maintenance is high, and a large amount of time is required for removal of liquid when the maintenance is carried out. 
     SUMMARY OF THE INVENTION 
     A liquid vaporizing and feeding apparatus in accordance with the invention, which addresses the aforementioned problems, comprises a vaporization container and a vaporization plate within the container, preferably located at the bottom of the container. The plate is inclined relative to the horizontal. Means are provided for heating the vaporization plate, and means are provided for feeding liquid onto the vaporization plate adjacent the upper end thereof. 
     A liquid level detection means is responsive to the level of liquid collected on the vaporization plate, and means responsive to the liquid level detection means controls the feed of liquid onto the vaporization plate by the feeding means. 
     The liquid level detection means includes a capillary tube, a lower end of which is arranged to be contacted by the surface of the liquid collected on the vaporization plate when the volume of liquid reaches a desired volume. The capillary tube is associated with a thermal sensor, and provides liquid level detection with a high degree of sensitivity. 
     The flow of vaporized gas produced by the vaporizing and feeding apparatus is controlled by a mass flow controller, which, in a preferred embodiment, is a pressure type mass flow controller. 
     In the invention, since the vaporization plate is held at a certain temperature which is in accordance with the vaporizing condition of liquid materials, the influence of pyrolysis of liquid materials may be reduced and relatively large amount of vaporization flow may be obtained with a small vaporization area. 
     Even if the amount of liquid introduced into the vaporization container is not large, adequate vaporization flow may be obtained by inclining the vaporization plate relative to the horizontal. Therefore, the apparatus can be miniaturized, and the time and cost of maintenance, which requires liquid exchange, can be decreased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view which illustrates the structure of a liquid material vaporizing and feeding apparatus according to the invention; 
     FIG. 2 is an enlarged schematic view illustrating in detail the manner in which a drop of liquid discharged from a nozzle becomes round as a result of surface tension; 
     FIG.  3 ( a ) is a schematic view of the liquid feed portion of a liquid material vaporizing and feeding apparatus according to the invention; 
     FIG.  3 ( b ) is a cross-sectional view of the liquid feed portion of FIG.  3 ( a ), illustrating how the influence of surface tension in liquid discharged from the nozzle is weakened; 
     FIG. 4 is schematic view of the liquid feed portion of a liquid material vaporizing and feeding apparatus according to a further embodiment of the invention; 
     FIG. 5 is a block diagram which illustrates a liquid material vaporizing and feeding apparatus according to an embodiment of the invention in which a thermal type mass flow controller is used as a flow control means for vaporized gas; and 
     FIG. 6 is a block diagram which illustrates a liquid material vaporizing and feeding apparatus according to an embodiment of the invention in which a pressure type mass flow controller is used as a flow control means for vaporized gas. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The liquid material vaporizing and feeding apparatus  10 , as shown in FIG. 1, comprises a vaporization plate  20 , a vaporization container  30 , a liquid level sensor  40 , and one or more nozzles  50 , which serve as liquid feed means. The vaporization plate  20  is located at the bottom of the vaporization container  30 , and its upper surface, which is preferably planar, is inclined relative to the horizontal. The vaporization plate is held at a certain temperature by a heating means (not shown). It is desirable that the angle of inclination be in the range of 2 to 5°, preferably 3°. The temperature of the vaporization plate is set according to the physical and chemical properties, and the vaporizing condition of, the liquid that is fed to the vaporization plate through the nozzles  50 . 
     The nozzles  50  are provided in the vicinity of the upper end of the vaporization plate  20 , and a plurality of nozzles is preferred so that the vaporization area is increased. 
     When operation starts, liquid is discharged from the nozzles  50 , and flows down the inclined surface of the vaporization plate  20 . As the liquid flows down the inclined surface, it is vaporized. The instantaneous rate at which liquid is fed to the plate through the nozzles is set to be greater than the rate at which the liquid vaporizes. Consequently, the liquid level rises gradually after the operation starts. 
     When the volume of liquid in the vaporization container  30  reaches a prescribed amount, a liquid level sensor  40  sends a signal causing the inflow of liquid through the nozzles  50  to stop. 
     As vaporization continues, and the liquid level falls, the liquid level sensor  40  causes inflow of liquid to resume. The liquid level sensor, and the liquid flow controlling apparatus that responds to the sensor signals, cycle repeatedly, and maintain the volume of the liquid collected on the vaporization plate within a prescribed narrow range. 
     The liquid level sensor  40  comprises a tube  42 , and a heat sensor  44  which is attached to the tube  42 . When the liquid rises to a level such that the lower end of the tube  42  touches the liquid surface, liquid rises in the tube as a result of the capillary phenomenon. The liquid level sensor  40  can be made smaller than a conventional float-type liquid level sensor, and therefore allows the liquid material vaporizing and feeding apparatus to be miniaturized. Moreover, the sensor is capable of detecting the liquid level with greater sensitivity than conventional float-type liquid level sensor. 
     FIG. 2 illustrates how, when the surface of the vaporization plate  20  is a mirror finished surface, a drop of liquid  60  being discharged from a nozzle  50  becomes round as a result of surface tension. The effect of surface tension, as illustrated in FIG. 2, impairs the spread of the flow of liquid. Accordingly, measures may be taken to weaken the influence of surface tension on the shape of the liquid drops discharged from the nozzles. 
     One way in which to weaken the effect of surface tension is to make the surface of the vaporization plate  20  a scratch-brush finished surface. 
     FIG.  3 ( a ) illustrates another measure for weakening the influence of surface tension. In liquid feed portion of the apparatus, a round bar  70  is provided on the upper part of the vaporization plate  20 , and the opening of the nozzle  50  is positioned at a very short distance from the round bar. As shown in FIG.  3 ( b ), the liquid  60  moves from the nozzle  50  to the round bar  70 , the influence of the surface tension of the liquid  60  is weakened by the round bar  70 , and the flow and spread of the liquid  60  are improved as a result. 
     FIG. 4 illustrates still another measure for weakening the influence of surface tension. In this case, in the liquid feed portion of the apparatus, an elongated recess  80  is provided in the upper part of vaporization plate  20 . As in the case of the round bar of FIG.  3 ( a ), the elongated recess  80  weakens the effect of surface tension and improves the flow and spread of the liquid. 
     Other measures for weakening the influence of surface tension and improving the flow and spread of the liquid, include the attachment of a-mesh to the surface of the plate by adhesive or other suitable means, or etching of the surface of the plate. Experiments have established that vaporization efficiency can be improved by about 20% by providing a mesh on the plate. 
     In the thermal type mass flow controller shown in FIG. 5, the feed of liquid from a source entrance is controlled by opening and shutting an air-operated valve  60 . When the air-operated valve  60  is opened, liquid flows through a feed conduit into a vaporization container  30 . The amount of fed liquid is limited by providing a limiting device  70  either at the end or at an intermediate location along the feed conduit. An orifice, a nozzle, or the like are preferred as limiting devices. Alternatively, a mass flow controller can be used for controlling the flow of liquid through the feed conduit. However, the use of a mass flow controller for this purpose is expensive. 
     Liquid in the vaporization container  30  flows down the inclined surface of a vaporization plate  20 , which is heated by a heater  80 , such as an electrothermal heater, a heat pipe, or the like. The temperature of the vaporization plate  20  is detected by a temperature sensor  90 , and a temperature control circuit  110 , responsive to the temperature sensor  90 , controls the heater  80  to maintain the temperature of the vaporization plate at a constant level. 
     The liquid level in the vaporization container  30  is detected by a liquid level sensor  40 , and a detection signal is transmitted to a comparison circuit  170 , which compares the voltage of the detection signal with an independently set voltage. An air valve make-and-break circuit  180  is driven by the output of the comparison circuit to open and shut the air-operated valve  60 . 
     A sensor  120 , which is preferably a thermal sensor, detects the flow of vaporized gas. The signal delivered by the thermal sensor is amplified by an amplification circuit  130 . The output of the amplification circuit is read as an indication of the flow of vaporized gas. It is also compared with a set value for vaporized gas flow by a comparison circuit  140 . A valve drive circuit  150  is operated by an output signal from the comparison circuit  140 , and the delivery of vaporized gas is controlled by the valve  160  so that the flow of gas corresponds to the set value. 
     The overall temperature of the apparatus is regulated by a temperature sensor  200 , a temperature control circuit  210 , which is responsive to the sensor  200 , and a heater  190 , which is responsive to an output of the control circuit. 
     Space is provided inside the vaporization container so that reliquefaction of the vaporized liquid, and rapid vaporization of liquid which adheres to the sidewalls of the vaporization container, can be prevented. Although it is desirable that the space be defined by a partition wall in the vaporization container  30 , a partition is unnecessary, provided that the capacity of the vaporization container  30  is sufficiently large. 
     In the pressure type mass flow controller of FIG. 6, as in FIG. 5, a liquid level sensor  40  is connected to a comparison circuit  170 , and an air valve  60  is opened and shut. On the other hand, sensor  120 ′ is a pressure sensor, which is arranged to respond to pressure on the upstream side of a nozzle or orifice  72 . The pressure controlled by a control valve  160 ′, which is provided upstream of the pressure sensor  120 ′. Thus the flow of vaporized gas through the nozzle or orifice  72  is controlled by maintaining a constant pressure upstream of the nozzle or orifice  74  Since the pressure sensor output is amplified and transformed and in effect becomes an output corresponding to flow, the operation of the pressure-type mass flow controller of FIG. 6 in response to a flow setting, is similar to that of the thermal type mass flow controller of FIG.  5 . 
     In still another variation (not illustrated) pressure sensors can be provide both on the upstream side and on the downstream side of the nozzle or orifice  72 , in which case the flow of (vaporized gas is controlled in accordance with a flow output derived from the pressure drop across the nozzle or orifice. 
     Other features of the pressure-type mass flow controller of FIG. 6 are the same as those of the thermal mass flow controller of FIG.  5 . 
     The liquid vaporizing and feeding apparatus according to the invention reduces the influence of pyrolysis of the liquid, and achieves improved vaporization efficiency, since the vaporization plate is held at a temperature appropriate for the vaporization of the particular liquid which is used. 
     Moreover, since the inclination of the vaporization plate results in adequate vaporization flow even when relatively small amounts of liquid are flowing into the vaporization container, the apparatus can be miniaturized, and the cost and time required for maintenance can be reduced.