Material delivery system and method

A method includes applying a first amount of heat to a vapor region of a precursor canister, measuring an indication of saturated vapor pressure within the vapor region during the applying the first amount of heat, and applying a second amount of heat to the vapor region of the precursor canister, the second amount of heat being adjusted from the first amount of heat based on the indication of saturated vapor pressure.

BACKGROUND

Often, semiconductor manufacturing processes utilize precursor materials in a gaseous phase as part of the manufacturing process. However, these precursor materials may arrive at the semiconductor manufacturer in various forms, such as liquid raw materials or even solid raw materials. To use these liquid or solid raw materials in the actual manufacturing processes, these raw materials may need to be changed into a gaseous phase in order to be properly controlled and delivered to the various processing chambers where they can react or otherwise be utilized in the semiconductor manufacturing process.

In order to achieve the gaseous forms, the solid or liquid raw materials may be placed into a raw material canister. Once in the raw material canister the raw material may begin to change phase into a gaseous form based in part on the material's equilibrium between itself and an overlying ambient. One such measure of equilibrium is the raw material's saturated vapor pressure, which is dependent at least in part on the temperature of the material within the raw material canister. When in use, the raw material within the raw material canister may be heated until the raw material meets the desired saturated vapor pressure based upon an equation of the raw material's saturated vapor pressure and temperature. The heating may be performed, for example, using heating elements.

DETAILED DESCRIPTION

The making and using of embodiments are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the embodiments, and do not limit the scope of the embodiments.

The embodiments will be described with respect to embodiments in a specific context, namely a precursor canister for a semiconductor manufacturing process. The embodiments may also be applied, however, to other precursor delivery systems.

With reference now toFIG. 1, there is shown a precursor delivery system100that may be used to deliver precursor materials to a semiconductor processing chamber102. The semiconductor processing chamber102may be a chamber utilized to deposit materials onto a semiconductor wafer104. In a particular embodiment, the semiconductor processing chamber102may be utilized in an atomic layer deposition (ALD) process to form a layer of tantalum nitride (TaN—not individually shown separate from the semiconductor wafer104) onto the semiconductor wafer104.

In an ALD process thin films such as the layer of TaN may be formed on the semiconductor wafer104using a self-limiting process, such that atomic layers of material are deposited sequentially using a series of pulses of precursor materials. For example, a first precursor may be introduced into the semiconductor processing chamber102and a layer of this first precursor may be adsorbed and reacted onto the semiconductor wafer104. Excess first precursor may be pumped out and a second precursor may be introduced to react with the first precursor on the semiconductor wafer104to form a monolayer of the desired material (e.g., the layer of TaN) via a self-limiting reaction. This process may be repeated to build up successive monolayers until a desired thickness is achieved.

However, as one of ordinary skill in the art will recognize, the ALD process to form the layer of TaN is merely an illustrative example of a process that may utilize the semiconductor processing chamber102. Other processes that utilize precursor materials may be performed in the semiconductor processing chamber102, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), various types of etching processes, combinations of these, or the like, may alternatively be utilized. These processes and any other process that utilizes gaseous phase precursor materials may alternatively be performed within the semiconductor processing chamber102, and all such processes are fully intended to be included within the scope of the embodiments.

The precursor delivery system100supplies the desired precursor materials to the semiconductor processing chamber102through a final delivery line101. The precursor delivery system100may also help to control the rate of delivery and pressure of the semiconductor processing chamber102by controlling the inflow of gas through the final delivery line101. Furthermore, while only a single precursor delivery system100is illustrated inFIG. 1, this is done for simplicity, as more than one precursor delivery system100may be attached to the semiconductor processing chamber102in order to provide the different number and types of precursor materials desired for the desired process.

In an embodiment the precursor delivery system100may include a carrier gas supply103, a flow controller105, and a precursor canister107. The carrier gas supply103may supply a gas that may be used to help “carry” the precursor gas to the semiconductor processing chamber102. The carrier gas may be an inert gas or other gas that does not react with the precursor material or other materials within the system. For example, the carrier gas may be helium (He), argon (Ar), nitrogen (N2), hydrogen (H2), combinations of these, or the like, although any other suitable carrier gas may alternatively be utilized.

The carrier gas supply103may be vessel, such as a gas storage tank, that is located either locally to the semiconductor processing chamber102or remotely from the semiconductor processing chamber102. Alternatively, the carrier gas supply103may be a facility that independently prepares and delivers the carrier gas to the flow controller105of the precursor delivery system100and elsewhere, such as other precursor delivery systems (not separately shown). Any suitable source for the carrier gas may be utilized as the carrier gas supply103, and all such sources are fully intended to be included within the scope of the embodiments.

The carrier gas supply103may supply the desired carrier gas to the flow controller105through a first line113. The flow controller105may be utilized to control the flow of the carrier gas to the precursor canister107and to the semiconductor processing chamber102, thereby helping to control the pressure within the semiconductor processing chamber102. The flow controller105may be, e.g., a proportional valve, a modulating valve, a needle valve, a pressure regulator, a mass flow controller, combinations of these, or the like. However, any suitable method for controlling and regulating the flow of the carrier gas to the semiconductor processing chamber102may be utilized, and all such methods are fully intended to be included within the scope of the embodiments. In an embodiment the carrier gas supply103may control the flow of carrier gas to between about 100 sccm and about 1300 sccm, such as about 800 sccm.

The flow controller105may supply the controlled carrier gas to the precursor canister107through a second line106. The precursor canister107may be utilized to supply a desired precursor to the semiconductor processing chamber102and may be located between a first valve108and a second valve110that may be used to isolate the precursor canister107from inflowing and outflowing streams. By isolating the precursor canister107from the process streams, the precursor canister107may be removed from the process, either physically or functionally, so that maintenance, replacing the precursor material, or other off-line work may be performed on the precursor canister107while it is not actively connected to the rest of the precursor delivery system100. A third valve112may be connected between the second valve110and the semiconductor processing chamber102in order to make sure that atmospheric gases do not enter the semiconductor processing chamber102while the precursor canister107has been removed.

FIG. 2illustrates in greater detail the precursor canister107between the flow controller105and the semiconductor processing chamber102. The precursor canister107may comprise a chamber201with a vapor region203and a raw material region205. In an embodiment a raw material204may be placed into the raw material region205of the chamber201. Once in the raw material region205of the chamber201, thermodynamic equilibrium may be used to drive portions of the raw material204into the gaseous phase and enter the vapor region203of the chamber201, where it may be picked up and carried by the carrier gas from the flow controller105as the carrier gas flows around baffles219(discussed further below with respect toFIG. 3) located within the vapor region203of the chamber201.

The chamber201may be any desired shape that may be suitable for vaporizing (if the raw material204is a liquid) or sublimating (if the raw material204is a solid) the raw material204. In the embodiment illustrated inFIG. 2andFIG. 3(described below), the chamber201has a cylindrical sidewall and a bottom. However, the chamber201is not limited to a cylindrical shape, and any other suitable shape, such as a hollow square tube, an octagonal shape, or the like, may alternatively be utilized. Furthermore, the chamber201may be surrounded by a housing207made of material that is inert to the various process materials. As such, while the housing207may be any suitable material that can withstand the chemistries and pressures involved in the process, in an embodiment the housing207may be steel, stainless steel, nickel, aluminum, alloys of these, combinations of these, and like.

The chamber201may also have a lid209to enclose the chamber201. The lid209may be attached to the housing207utilizing, e.g., a seal such as an o-ring, a gasket, or other sealant in order to prevent leakage from the chamber201while at the same time allowing the lid209to be removed for access to the chamber201within the interior of the housing207. Alternatively, the lid209may be attached by welding, bonding, or adhering the lid209to the housing207in order to form an air-tight seal and prevent any leakage.

An inlet port215and an outlet port217may provide access to the chamber201in order to receive carrier gas from the flow controller105(seeFIG. 1) and output a carrier gas/precursor gas mixture to the semiconductor processing chamber102, respectively. The inlet port215and outlet port217may be formed in the lid209of the chamber201(as illustrated inFIG. 2) or may alternatively be formed through the sidewalls of the chamber201. In an embodiment the inlet port215and outlet port217may also include various valves and fittings (not shown for clarity) to facilitate removal and replacement of the chamber201.

The raw material region205may be utilized to store and prepare the raw material204from which a desired process gas may be formed. The raw material204may be any suitable raw material that can generate a process precursor in a gaseous form either through vaporization or sublimation. For example, if the raw material204is a solid material, the raw material204may sublimate from the solid phase to a gaseous phase or melt and then vaporize to a gaseous phase. Alternatively, if the raw material204is a liquid, the raw material204may simply vaporize to a gaseous phase.

In the embodiment wherein an ALD process is utilized to form a layer of TaN, the raw material204may be a solid material such as pentakis(dimethylamido) tantalum (PDMAT) and the raw material204may be placed within the raw material region205of the chamber201. While the raw material204rests in the raw material region205, the raw material204may sublimate to a gaseous form and accumulate within the vapor region203located over the raw material region203. As such, the solid PDMAT may provide a process gas for the carrier gas to pick up and utilize in the semiconductor processing chamber102(as described in greater detail below).

However, as one of ordinary skill will recognize, utilization of solid PDMAT is not the only raw material204that is solid and that may be placed within the raw material region205of the chamber201. Any solid precursor that may generate a gaseous process gas that may be used for any suitable semiconductor manufacturing process may also be utilized, and such solid precursors may include, e.g., xenon difluoride, nickel carbonyl, tungsten hexacarbonyl, and the like. These and any other suitable raw material204that is solid and that can generate a gaseous process precursor are fully intended to be included within the scope of the embodiments.

Furthermore, the scope of the embodiments is not intended to be limited to a raw material204that is solid as a raw material204of any suitable phase that may be used as a precursor material within a semiconductor manufacturing process may alternatively be placed within the raw material region205. In other embodiments the raw material204may comprise a liquid raw material such as tetrakis(diethylamido) titanium (TDMAT), tertbutylimino tris(diethylamido) tantalum (TB TDET), pentakis(ethylmethylamido) tantalum (PE-MAT), and the like. These and any other suitable liquid raw materials that can generate a gaseous phase precursor are fully intended to be included within the scope of the embodiments.

A heater213controlled by a controller211(discussed further below with respect toFIG. 4) may be placed around the chamber201in order to adjust the thermodynamic equilibrium of the raw material204and help drive the raw material204into a gaseous phase and into the vapor region203of the chamber201. The amount of the raw material204that may be transferred to the desired gaseous phase and the rate at which it may be transferred to the gaseous phase is related to the thermodynamic equilibrium and may be represented by a saturated vapor pressure of the raw material204itself. In the embodiment in which the raw material204is PDMAT, the initial saturated vapor pressure (P, in mmHg) of PDMAT may have the following relation to the temperature (T, in Kelvin) as expressed in Equation 1:

Log10⁢P(mmHg)=11.30-4125T(K)Eq.⁢1
As illustrated, by increasing the temperature of the raw material204, the saturated vapor pressure of the raw material204may be increased and more of the raw material204may be driven into the gaseous phase and the vapor region203of the chamber201, thereby providing more gaseous process gas.

Additionally, the heater213may be configured to generate a temperature gradient within the chamber201. For example, the heater213may be utilized to generate a temperature gradient with a higher temperature in the vapor region203and a lower temperature in the raw material region205. As solid materials tend to condense back from the gaseous phase in colder regions of the chamber201, this temperature gradient may be utilized to help any raw material204that phase changes back to a solid or liquid phase condense in the raw material region205instead of condensing in the vapor region203. This helps to keep the vapor region203clear of condensing solids and liquids, thereby keeping more gaseous vapors in the vapor region203.

The temperature gradient may be generated by configuring the heater213to generate more heat for the vapor region203and less heat for the raw material region205. For example, in an embodiment in which the heater213is a resistive heater, the heater213may be configured to have a higher resistance adjacent to the vapor region203, thereby leading to a larger generation of heat adjacent to the vapor region203than adjacent to the raw material region205. In an embodiment the temperature gradient between the vapor region203and the raw material region205may be between about 5° C. and about 30° C., such as about 15° C. In a specific embodiment the heater213may be utilized to provide a temperature of about 73° C. to the raw material region205and a temperature of about 88° C. to the vapor region203.

Additionally, the heater213may have temperature sensors221in order to provide heating information to the controller211. The temperature sensors221may be, e.g., a thermocouple installed within the heater213to monitor the temperature of the heater213adjacent to the vapor region203and adjacent to the raw material region205of the chamber201. However, any suitable type of sensor may alternatively be utilized to measure the temperature of the heater213and transmit that measurement to the controller211.

Optionally, the chamber201may also include other heating and cooling devices (not shown) that may be utilized to help form the temperature gradient. For example, the chamber201may include a cooling plate located at the bottom of the chamber201to lower the temperature of the raw material region205. Additionally, the first valve108, the second valve110, the inlet port215, and the outlet port217may also be heated with, e.g., resistive heating tape or other heating elements. These and any other type of temperature controls are fully intended to be included within the scope of the embodiments.

FIG. 3illustrates a top down view of the precursor canister107along line A-A′ inFIG. 2(with the outlet port217also shown using a dashed circle for clarity) and also illustrates a path that the carrier gas may travel through the vapor region203of the chamber201. As illustrated, the vapor region203may also contain a number of the baffles219that may be designed to form a longer flow path for the carrier gas through the vapor region203of the chamber201than a straight line between the inlet port215and the outlet port217. By forming a longer flow path, the baffles219may also cause the carrier gas to have a longer residence time within the vapor region203of the chamber201, thereby also increasing the amount of gaseous precursor gas that the carrier gas will pick-up and “carry” out the outlet port217and to the semiconductor processing chamber102.

The precise number and shape of the baffles219and the carrier gas flow path through the vapor region203may be selected to control the vaporization/sublimation and the flow of the gaseous precursor material. For example, more baffles219may be installed to form a longer path through the vapor region203, thereby causing the carrier gas to have a faster speed through the vapor region203, or the specific shape of the baffles219may be designed to affect the vaporization/sublimation of the raw material204and help to control the usage of the raw material204, allowing for more control of the usage of the raw material204. As such, while five baffles219are illustrated inFIG. 3as an illustrative embodiment, there could be any number and shape of baffles219while still remaining within the scope of the embodiments.

The baffles219may be attached to the housing207or lid209of the chamber201, or may alternatively be a stand-alone insert that may be separately placed within the chamber201. Additionally, the baffles219may be placed so as to extend into the raw material region205and also to prevent the flow of materials (e.g. the carrier gas and the raw material204in a gaseous phase) between the baffles219and the lid209of the chamber201. Such a placement will cause the carrier gas and raw material204to flow between the baffles219instead of over or around the baffles219.

FIG. 4illustrates in greater detail an embodiment of the controller211that may be utilized to control the heater213and, through the heater213, control the saturated vapor pressure of the raw material204in the vapor region203of the chamber201. The controller211may be any form of computer processor that can be used in an industrial setting for controlling process machines or may alternatively be a general purpose computer platform programmed for such control of industrial machines. In an embodiment the controller211may comprise a processing unit401, such as a desktop computer, a workstation, a laptop computer, or a dedicated unit customized for a particular application. The controller211may be equipped with a display403and one or more input/output components405, such as sensor inputs, a mouse, a keyboard, printer, combinations of these, or the like. The processing unit401may include a central processing unit (CPU)406, memory408, a mass storage device410, a video adapter414, and an I/O interface416connected to a bus412.

The bus412may be one or more of any type of several bus architectures including a memory bus or memory controller, a peripheral bus, or video bus. The CPU406may comprise any type of electronic data processor, and the memory408may comprise any type of system memory, such as static random access memory (SRAM), dynamic random access memory (DRAM), or read-only memory (ROM). The mass storage device410may comprise any type of storage device configured to store data, programs, and other information and to make the data, programs, and other information accessible via the bus412. The mass storage device410may comprise, for example, one or more of a hard disk drive, a magnetic disk drive, or an optical disk drive.

The video adapter414and the I/O interface416provide interfaces to couple external input and output devices to the processing unit401. As illustrated inFIG. 4, examples of input and output devices include the display403coupled to the video adapter414and the I/O component405, such as sensors (e.g., the temperature sensors221, seeFIG. 2), a mouse, keyboard, printer, and the like, coupled to the I/O interface416. Other devices may be coupled to the processing unit401, and additional or fewer interface cards may be utilized. For example, a serial interface card (not shown) may be used to provide a serial interface for a printer. The processing unit401also may include a network interface418that may be a wired link to a local area network (LAN) or a wide area network (WAN)420and/or a wireless link.

It should be noted that the controller211may include other components. For example, the controller211may include power supplies, cables, a motherboard, removable storage media, cases, and the like. These other components, although not shown inFIG. 4, are considered part of the controller211.

Operationally, the controller211may be utilized to control the heater213. For example, when the raw material205is initially placed in the raw material region205of the chamber201(and before any degradation or deterioration has occurred), the controller211may control the heater213so as to set the temperature of the heater213(and, through a transfer of heat through the housing207, the temperature of the chamber201) to a desired temperature. In an embodiment which utilizes PDMAT, the desired temperature may be determined in part from the initial saturated vapor pressure equation detailed above with respect to Equation 1. For example, in the embodiment utilizing PDMAT, the controller211may set the heater213to have a temperature gradient, with the temperature adjacent to the vapor region203being between about 85° C. and about 95° C., such as about 88° C., while the temperature adjacent to the raw material region205being between about 68° C. and about 79° C., such as about 73° C. However, the precise temperatures may vary depending upon the material used, the initial vapor pressure, and the desired flow rate of raw material204desired.

Additionally, the controller211may also be configured to automatically tune the temperature (and thereby adjust the saturated vapor pressure of the raw material204) in order to compensate for any degradation and deterioration that may occur over long term, repeated use of the raw material204. In an embodiment, as the saturated vapor pressure of the raw material204decreases at a particular temperature because of deterioration and degradation, the controller211may automatically compensate for the reduced saturated vapor pressure by increasing the process temperature and raising the lowered initial saturated vapor pressure of the raw material204. By raising the saturated vapor pressure through the temperature, the desired concentration of raw material204within the vapor region203and, subsequently, the semiconductor processing chamber102, may be obtained and kept consistent throughout the life span of the raw material204.

The controller211may automatically adjust the temperature of the heater213and adjust the saturated vapor pressure of the raw material204in a number of methods. In a first embodiment a calibration curve may be generated and then implemented within the controller211. In this embodiment an initial sample of raw material204may be placed within the chamber201and used over its life span without compensation from the controller211. By allowing the initial sample of raw material204to degrade and deteriorate, an indication of the saturated vapor pressure of the initial sample of raw material204may be taken each time the initial sample of raw material204is used, and the degradation and deterioration can be charted as the calibration curve.

FIG. 5Aillustrates such a calibration curve in which solid PDMAT is utilized as the initial sample of raw material204. In this type of calibration curve, the indications of the saturated vapor pressure are samples of the vapor pressure or concentration of the raw material204taken from the vapor region203while the initial sample of raw material204was in use. In this calibration curve, a normalized indication of the vapor pressure is illustrated on the y-axis and a normalized number of runs is illustrated on the x-axis.

However, the calibration curve is not limited to be generated by measuring the vapor pressure of the raw material204. Any other suitable indication of saturated vapor pressure may alternatively be utilized.FIG. 5Billustrates another calibration curve which may be generated using another indication of saturated vapor pressure: the variation of the semiconductor wafers104that are formed during the processes. As a reduced saturated vapor pressure leads to a reduced mass flow of precursor material, which in turn can lead to a greater variation in thickness, a larger variation in thickness of a layer may be utilized as an indication that the saturated vapor pressure is being degraded. Given this, the calibration curve may also be generated by utilizing the initial raw sample of raw material204to form layers on the semiconductor wafer104(or series of semiconductor wafers104), and then measuring the variation of the thicknesses of these layers. These data points may then be charted as illustrated inFIG. 5B(using normalized data for the thickness variations and wafers) to generate the calibration curve, with the variability only decreasing after the raw material204has been replaced, such as after 2000 liters.

After the calibration curve has been generated from the actual usage of the initial sample of raw material204, the calibration curve may be then be input and stored into the controller211, which may then use the calibration curve to adjust the heater213for subsequent samples of the raw material204. For example, if similar material and process conditions are utilized for subsequent samples of the raw material204, then the controller may, from the calibration curve, known when and how much the saturated vapor pressure of the raw material204has deteriorated from the initial saturated vapor pressure equation (see, e.g., Equation 1). With the calibration curve, the controller211may automatically adjust the temperature of the heater213in order to compensate for the deterioration and degradation. For example, as the saturated vapor pressure of the raw material204deteriorates from usage, the controller211may, based on the calibration curve, sequentially increase the temperature of the heater213between about 0.5° C. and about 10° C., such as an increase over the life span of the raw material204of about 5° C. In an embodiment in which the temperature gradient for PDMAT is initially set at 88° C.-73° C., the controller211, based on the calibration curve, may sequentially increase the temperature gradient about 5° C., to about 93° C.-78° C., at the end of the life span of the raw material204.

Optionally, after the calibration curve has been generated and implemented within the controller211, the calibration curve may be verified during subsequent manufacturing processes by generating a verification curve such as the verification curve illustrated inFIG. 6. For example, the verification curve may be generated by utilizing similar measurements of an indication of saturated vapor pressure as those described above (e.g., measuring the vapor pressure or concentration or measuring a variation of the thickness of a layer formed from the raw material204). These indications of saturated vapor pressure may be charted and, if the indications of saturated vapor pressure remain consistent over the life span of the raw material204, as illustrated inFIG. 6, the calibration curve may be verified. If the indications of saturated vapor pressure do not remain consistent, modifications to the calibration curve may be made.

By having the controller211utilize the calibration curve, the variation in saturated vapor pressure caused by deterioration and degradation of the raw material204may be avoided. As such, a more constant concentration of the raw material204may be generated over the life span of the raw material204in the vapor region203of the chamber201, and a more even flow of the raw material204may be presented to the semiconductor processing chamber102. By having a more consistent concentration variations in the thickness of the layers formed on the semiconductor wafer104may be reduced, leading to more consistent layers and less chances for problems to occur during the manufacturing of the semiconductor wafer104.

FIG. 7illustrates another embodiment in which the controller211may adjust the temperature of the heater213based on a real-time sensor701instead of a calibration curve. In this embodiment the real-time sensor701may be installed such that it samples the vapor pressure or concentration of the raw material204within the vapor region203of the chamber201and immediately relays that information to the controller211. The real-time sensor701may, e.g., be an optical sensor that can measure a concentration by measuring a reduction in optical radiation (e.g., infrared or ultraviolet radiation) due to the absorption of the radiation by the raw material204in the gaseous form. However, any suitable real-time sensor may alternatively be utilized to measure the concentration of the raw material204as a real-time indication of the saturated vapor pressure of the raw material204.

Once a real-time measurement has been taken by the sensor701, the measurement may be relayed to the controller211. The controller211may take this measurement and compare it to a desired value to determine if there is any deterioration or degradation that has occurred. If deterioration or degradation has occurred, the controller211may adjust the temperature of the heater213to compensate for the deterioration or degradation. Subsequent readings from the sensor701may be compared to the desired value in order to see if further adjustments may be desired.

In accordance with an embodiment, a method of manufacturing a semiconductor device includes applying a first amount of heat to a vapor region of a precursor canister, measuring an indication of saturated vapor pressure within the vapor region during the applying the first amount of heat, and applying a second amount of heat to the vapor region of the precursor canister, the second amount of heat being adjusted from the first amount of heat based on the indication of saturated vapor pressure.

In accordance with another embodiment, a method includes placing a first wafer into a semiconductor processing chamber and performing a first process on the first wafer. Performing the first process includes heating a first supply of precursor material to a first temperature in a precursor canister and measuring a first indication of saturated vapor pressure in a vapor region of the precursor canister. The method also includes placing a second wafer into the semiconductor processing chamber and performing a second process on the second wafer. Performing the second process includes heating a second supply of precursor material to a second temperature in the precursor canister, wherein the second temperature is determined from the first indication of saturated vapor pressure.

In accordance with yet another embodiment, a method includes performing a first series of processes on first multiple semiconductor wafers, measuring multiple indications of saturated vapor pressure of a first supply of precursor material during the first series of processes, and performing a second series of processes on second multiple semiconductor wafers. The second series of processes includes heating a second supply of precursor material to multiple temperatures, wherein the multiple temperatures are determined from the multiple indications of saturated vapor pressure of the first supply of precursor material.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. For example, the heater may be a resistive heater or a steam heater. Additionally, any type of indication of that may be utilized to provide a description of the saturated vapor pressure may alternatively be measured.