The present invention relates generally to a device for vaporizing a liquid at a controlled rate. More specifically, it relates to a device for vaporizing a liquid with a rapid pressure drop and mixing the vaporized liquid with a carrier gas in a manner which allows independent control of the flow rates of the liquid and carrier gas. The invention is particularly suited for supplying vaporized reactants to the reaction chamber of a chemical vapor deposition system.
Chemical vapor deposition (CVD) processes are widely used in the deposition of thin films used in semiconductor devices and integrated circuits. Such processes involve deposition resulting from a reaction of chemical vapors homogeneously or heterogeneously on a substrate. The reaction rate is controlled, e.g., by temperature, pressure and reactant gas flow rates. The use of low vapor pressure liquids as precursors for such processes has several advantages and has become more common.
Prior CVD processes involve transport of low vapor pressure liquid using a bubbler or boiler. In these processes, a carrier gas saturates the liquid and transports the vapor. The amount of vapor transported depends on the downstream pressure, carrier gas flow, vapor pressure in the ampoule holding the source liquid source, and the like. Thus, the amount of vapor transported is not an independent parameter and therefore is difficult to control. As a result, CVD processes using a bubbler or boiler have not demonstrated the ability to consistently control the flow rate of the vaporized reactant, which decreases the quality of films produced by these processes.
An additional shortcoming of CVD processes using bubblers is that these processes have difficulty producing the high reactant flow rate needed to achieve a high film deposition rate. With a bubbler, increasing reactant flow rate requires either increasing the bubbler temperature or the carrier gas flow rate. However, the reliability of downstream hardware limits the use of a bubbler temperature above a certain value, and the adverse effect of excessive carrier gas flow rate on the quality of the deposited film limits the use of high carrier gas flow rates, thus limiting the amount of vapor that can be transported. Thus, the amount of reactant vapor that can be transported is undesirably limited.
In known boilers, the liquid is heated, and the vapor formed is controlled using a high temperature gas flow controller. In this arrangement, the amount of vapor transported depends on the downstream chamber pressure and the boiler temperature. However, the vapor pressure of liquids commonly used in the deposition of semiconductor films (e.g., tetraethylorthosilane TEOS) is very small at normal operating temperatures; as a result, vapor transport limitations occur when a boiler is used in high pressure (e.g., atmospheric pressure) CVD processes. Heating the boiler to the liquid boiling temperature could obviously improve the vapor transport for such processes, but the boiler temperature is limited by the reliability of the downstream hardware.
The above-referenced previously filed U.S. patent application describes a CVD process in which vapor is formed by flowing heated carrier gas over a bead of liquid. The liquid evaporates into the carrier gas, creating reactant vapor for CVD. The evaporation rate is controlled by adjusting the flow rate of liquid into the bead; at high flow rates, the size and surface area of the bead increases until the evaporation rate equals the liquid flow rate. However, above a given limit, increases in liquid flow rate will result in only partial vaporization. An advantage of this process over the bubbler and boiler techniques is that it allows independent control of the liquid flow rate. However, like the bubbler and boiler techniques, this technique relies on heated evaporation to vaporize the liquid, and thus can produce only limited vaporization rates.
A need therefore remains for a reliable and low maintenance liquid vaporizer which can vaporize liquid at high flow rates and additionally allow independent control of liquid and carrier gas flow rates. The present invention addresses that need.
The invention features a vaporizer which accepts a carrier gas and a pressurized liquid. An internal cavity receives the carrier gas through a carrier aperture and combines the carrier gas with vapor formed from liquid received through a liquid aperture. The mixed gas and vapor are exhausted out of the cavity via a third aperture. The liquid is vaporized by the pressure differential between the liquid and vapor: a closure element which is substantially wider than the liquid aperture is disposed adjacent to the liquid aperture so that a pressure gradient forms between the liquid aperture and the remainder of the cavity. The liquid passing through this pressure gradient vaporizes due to expansion.
An advantage of the invention is that the vaporizer forms vapor by expansion in a pressure gradient, rather than evaporation by heating, and therefore can vaporize liquid at high flow rates such as those needed for some semiconductor fabrication processes.
In preferred embodiments, the closure element is a diaphragm movable relative to the liquid aperture to increase or decrease the flow rate of the liquid. The closure element is moved by an electrically controlled actuator such as a piezoelectric element. To control the flow rate of the liquid, a liquid flow meter is connected to measure the flow rate of liquid into the liquid inlet port. A feedback control system compares the measured flow rate to a selected value and controls the piezoelectric actuator so that the flow rate approximates the selected value.
An advantage of this embodiment is that the liquid flow rate is controlled solely by the movement of the diaphragm, so that (unlike the vaporization systems described above) the liquid flow rate is independent of the carrier gas flow rate and therefore can be more accurately controlled.
In further preferred embodiments, a heater heats at least a portion of the value body near to the cavity so as to inhibit the liquid, which has cooled due to expansion, from condensing on the walls of the cavity after it has vaporized.