Abstract:
Disclosed is a process of treating semiconductor substrates, including the production of pure water, a method of producing the pure water for semiconductor fabrication, and a water-producing apparatus. Ammonia is catalytically oxidized in a catalytic conversion reactor to form pure water. The water is then supplied to a semiconductor fabrication process. The water-producing apparatus comprises a housing surrounding a catalytic material for adsorbing ammonia, an ammonia and oxidant source, each in communication with the housing, and an outlet for reaction products. The outlet is connected to a semiconductor processing apparatus. According to preferred embodiments of the invention, the apparatus can be a catalytic tube reactor, a fixed bed reactor or a fluidized bed reactor. This process and apparatus allows the quantity of unreacted excess oxidant to be limited, preventing undesired oxidation of low oxidation resistant metal gate electrodes during semiconductor fabrication processes, such as during wet oxidation processes like source/drain reoxidation. At the same time, the use of ammonia reactants lessens the risk of dangerous explosions and excessive boron diffusion while fabricating surface p-channel semiconductor devices.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation application of, and claims priority to, U.S. patent application Ser. No. 10/125,762, filed Apr. 18, 2002, which is incorporated in its entirety by reference herein, and which is a continuation application of U.S. patent application Ser. No. 09/387,119, filed Aug. 31, 1999, issued as U.S. Pat. No. 6,440,382, and incorporated in its entirety by reference herein. This application is also related to U.S. patent application Ser. No. 10/123,441, filed Apr. 15, 2002, which is incorporated in its entirety by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to semiconductor fabrication. More specifically, the invention relates to producing water for use in the fabrication of semiconductors.  
           [0004]    2. Description of the Related Art  
           [0005]    Ultra-pure water plays an important role in the fabrication of semiconductors. One such role is the thermal oxidation of silicon, wherein a silicon dioxide (SiO 2 ) film is grown on a silicon substrate by oxidizing the surface of the silicon substrate. Thermal oxidation of silicon proceeds much faster in the presence of water. Thus, the advantages are shorter process time and oxidation at a lower temperature as compared to dry oxidation.  
           [0006]    Other examples of uses of ultra-pure water during semiconductor fabrication include repairing gate oxide material damaged during plasma etching via source/drain reoxidation; forming of “hard” oxides or SiO x N y ; accomplishing cell reoxidation of high dielectric materials; wet cleaning/wet etching; in situ chamber cleaning for furnace; LPCVD; PECVD; HDP processing chambers and etch chambers; plasma etching; removing organic material such as in photoresist “ash” applications; and forming silicon oxides or silicon oxynitrides in steam plasma systems.  
           [0007]    Pure water is required in each of these fabrication processes to avoid contaminating the fine integrated circuit devices and wiring. As devices are continually scaled down, purity requirements become even more stringent and important to the fabrication of operable high-speed circuitry.  
           [0008]    Accordingly, there is a need for an efficient and reliable process for producing water having a high degree of purity for use in semiconductor fabrication processes.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with one aspect of the invention, a process of treating semiconductor substrates, including the production of pure water, is disclosed. The process comprises catalytically oxidizing ammonia to form water. The water is then supplied to a semiconductor fabrication process.  
           [0010]    In accordance with another aspect of the invention, a method of producing substantially pure water for semiconductor fabrication is disclosed. The method comprises introducing ammonia to a catalytic conversion reactor. The ammonia is then oxidized to form water.  
           [0011]    In accordance with another aspect of the invention, an apparatus for producing water is disclosed. The water-producing apparatus comprises a housing surrounding a catalytic material for adsorbing ammonia, a source of ammonia in communication with the housing, a source of oxidant in communication with the housing, and an outlet for reaction products, wherein the outlet is connected to a semiconductor processing apparatus.  
           [0012]    A preferred embodiment of the present invention is achieved by reacting, in a water-producing apparatus, ammonia with oxygen in the presence of catalytic material that adsorbs the ammonia and promotes a reactivity of the ammonia with the oxygen, such as metal oxides, ion-exchanged zeolites, noble metals, titanium dioxide, silicon dioxide, and combinations thereof, to form effectively nitrogen and water. The produced water is conveyed to a semiconductor fabrication process to be used by the same. According to a number of embodiments of the invention, the water-producing apparatus can, for example, be a catalytic tube reactor, a fixed bed reactor or a fluidized bed reactor.  
           [0013]    The preferred embodiments provide a method and apparatus for advantageously producing water for use in semiconductor fabrication processes without the risk of hydrogen explosions while permitting the production of low overall electrical resistance materials comprised of low oxidation-resistant metal gate electrodes while the semiconductor device is being fabricated. Moreover, the preferred embodiments advantageously do not subject a surface p-channel semiconductor device to excessive boron diffusion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a side cross-sectional illustration of a combustion-pipe type apparatus for producing water for use in semiconductor fabrication.  
         [0015]    [0015]FIG. 2 is a side cross-sectional illustration of a catalytic apparatus for producing water for use in semiconductor fabrication.  
         [0016]    [0016]FIG. 3 is a side cross-sectional illustration of another catalytic apparatus for producing water for use in semiconductor fabrication.  
         [0017]    [0017]FIG. 4 is a side cross-sectional illustration of still another catalytic apparatus for producing water for use in semiconductor fabrication.  
         [0018]    [0018]FIG. 5 is a system block diagram showing an embodiment of the method of producing water and using the water in a semiconductor fabrication process in accordance with the invention.  
         [0019]    [0019]FIG. 6 is a flow chart showing a method of producing water and supplying the water to a semiconductor fabrication process in accordance with the present invention.  
         [0020]    FIGS.  7 - 8  are partial, cross-sectional representations of a partially fabricated integrated circuit, generally illustrating source/drain reoxidation in accordance with the prior art.  
         [0021]    [0021]FIG. 9 is a partial, cross-sectional view of a catalytic tube reactor, illustrating a first embodiment of a water-producing apparatus in accordance with the invention.  
         [0022]    [0022]FIG. 10 is a partial, cross-sectional view of a packed bed reactor, illustrating a second embodiment of the water-producing apparatus in accordance with the invention.  
         [0023]    [0023]FIG. 11 is a partial, cross-sectional view of a fluidized bed reactor, illustrating a third embodiment of the water-producing apparatus in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Current methods for producing ultra-pure water for use in semiconductor fabrication include distillation and deionization. These methods, while effective, are slow and cumbersome, representing high costs. Moreover, the water produced by these methods tend to contain residual dissolved oxygen. Another manner of providing pure water is through catalytic conversion of hydrogen gas (H 2 ) and oxygen gas (O 2 ) which is traditionally conducted with an excess of H 2  or O 2  to prevent explosion.  
         [0025]    [0025]FIG. 1 shows one combustion-pipe-type apparatus  100  that can be used to produce water for use in semiconductor fabrication. H 2  enters a quartz furnace  102  via a hydrogen gas nozzle  104 . O 2  enters the quartz furnace  102  via an oxygen gas nozzle  106 . The vicinity near a tip end of the hydrogen gas nozzle  104  inside the quartz furnace  102  attains a high temperature (e.g., 1,800° C. to 2,000° C.) due to flames of combustion from a heater (not illustrated) that heats this area. Water vapor produced by the combustion exits the quartz furnace  102  via an outlet  108 .  
         [0026]    [0026]FIGS. 2 through 4 illustrate catalyst-reaction-type apparatuses that can be used to produce water via H 2  and O 2  for use in semiconductor fabrication. In FIG. 2, a water-producing apparatus  200  includes a plurality of pipes  202  made of material that serves as a catalyst to encourage hydrogen or oxygen reactivity. A mixture of H 2 , and O 2  is introduced into the water-producing apparatus  200  via an inlet  204 . Water produced by catalysis exits the water-producing apparatus  200  via an outlet  206 .  
         [0027]    In FIG. 3, a water-producing apparatus  300  includes a plurality of supported plates  302  made of catalytic material. A mixture of H 2  and O 2  is introduced into water-producing apparatus  300  via an inlet  304  and contacts the plates  302 . Water produced by catalysis exits the water-producing apparatus  300  via an outlet  306 .  
         [0028]    In FIG. 4, a water-producing apparatus  400  includes a plurality of particulate elements  402  made of catalytic material. A mixture of H 2  and O 2  is introduced into water-producing apparatus  400  via an inlet  404  and contacts the particulate elements  402 . Water produced by catalysis exits the water-producing apparatus  400  via an outlet  406 . The water-producing apparatus  400  in FIG. 4 is illustrated as a fixed bed reactor, but it may be a fluidized bed reactor.  
         [0029]    However, one problem with the known methods illustrated in FIGS. 1 through 4 is that the use of hydrogen and hydrogen/oxygen mixture gases presents a high risk of explosion, endangering the safety of technicians, the in-process semiconductor product and the tool itself. Uses of diluting gases, such as argon, to decrease the risk of dangerous hydrogen explosions lowers the overall rate of water producing reaction, extending processing times and increasing the size of the water-producing apparatus, ultimately increasing the cost of producing the pure water. Moreover, even if hydrogen and oxygen are provided in ratios to completely react, the product gas is a mixture gas of water and argon, rather than pure water. Another problem is that any excess hydrogen causes increased boron diffusion during thermal processing phases in excess of 650° C., which are common in semiconductor fabrication.  
         [0030]    Because of the disadvantages posed by the use of hydrogen in the production of water with respect to semiconductor fabrication, the hydrogen reactant is typically arranged to be completely oxidized. As a result, these methods do not allow for precise control of the concentration of excess, unreacted oxygen. Precise control of excess oxygen is important in a number of semiconductor fabrication processes, including selective oxidation of silicon in the presence low oxidation-resistant metal gate electrodes, such as tungsten (W), cobalt (Co), molybdenum (Mo), titanium (Ti) and platinum (Pt). The oxidation of exposed, oxidizable metal gate electrodes form oxides that are insulating and thus detract from electrode conductivity, rendering a slower and less responsive device. Some metals, such as tungsten are so readily oxidized that overall resistance is increased beyond tolerable levels, rendering them impractical for use as gate electrode metals.  
         [0031]    Accordingly, a need exists for processes for producing water for use in semiconductor fabrication, which minimizes the risk of oxidizing metal gates while reducing the risk of hydrogen explosions and excessive boron diffusion.  
         [0032]    [0032]FIG. 5 is a block diagram showing a system for producing water in accordance with a preferred embodiment of the invention. Ammonia gas (NH 3 ) is supplied to a mixing section  500  via an ammonia gas line  502 . An oxidant is supplied to the mixing section  500  via a gas line  504 . The mixing section  500  may simply be a regular in-pipe gas mixing system or a gas mixing mechanism designed to discharge ammonia gas into the oxidant gas in a swirling stream to uniformly mix the reactant gases.  
         [0033]    The molar ratio of ammonia to oxidant gas can be set and altered using mass flow controllers  506   a  and  506   b  and valves  508   a  and  508   b . The ammonia gas and oxidant gas mixture is preferably supplied via a common inlet line  510  into a water-producing apparatus  512 . The water-producing apparatus  512  of various embodiments is depicted in greater detail in FIGS. 9 through 11.  
         [0034]    In the illustrated embodiment, the oxidant gas comprises oxygen (O 2 ) gas. An ammonia and oxygen gas mixture is catalytically converted into reaction products comprising nitrogen gas N 2 , water vapor H 2 O, any excess oxygen gas or ammonia gas, and nitrogen-based oxides, according to the following equation:  
         4NH 3 +3O 2 →6H 2 O +2N 2   +a N x O y   (1)  
         [0035]    A preferred mole ratio of ammonia to oxygen ranges from about 1:9 to about 9:1. The resultant reaction products can include water, nitrogen, any excess unreacted ammonia and oxidant, and nitrogen-based oxides. More preferably, excess ammonia is supplied to the water-producing apparatus  512 , i.e., greater than a 4:3 ammonia to oxygen mole ratio, so as to render the quantity of unreacted excess oxygen in the reaction products negligible. An ammonia to oxygen mole ratio of between about 5:3 and 14:3 is most preferable. Accordingly, excess oxygen is avoided, thereby minimizing the risk of oxidizing metal elements exposed to the process to which the water is fed. In particular, metal layers in transistor gate stacks, particularly tungsten, are not subjected to excess oxygen gas. Desirably, the ratio of water to byproducts such as nitrogen and nitric oxide is also kept low. In particular, the ratio of water to nitrogen is preferably about 6:2 (in accordance with the above Equation (1)), while the ratio of water to nitric oxide is preferably less than about 1:1.  
         [0036]    Performing the reaction shown in Equation (1), rendering the amount of post-reaction, unreacted oxygen species to a negligible quantity, advantageously lowers the risk of undesired oxidation of oxidation-susceptible metal gate electrodes during subsequent wet fabrication processes, such as wet thermal oxidation of silicon substrates. However, excess unreacted ammonia does not pose as high a danger as excess unreacted hydrogen of explosion or excessive boron diffusion during thermal treatments at greater than about 650° C. such as during source/drain anneals.  
         [0037]    The catalytic material used to carry out the oxidation of ammonia shown in Equation (1) preferably activates the ammonia by strong adsorption over the catalyst and lowers the activation energy needed to produce water from ammonia and oxygen. More preferably, the catalytic material comprises one or a combination of metallic materials, such as palladium, (Pd), copper (Cu), platinum (Pt), vanadium oxide (V 2 O 5 ), tungsten oxide (WO 3 ), ion-exchanged zeolites (e.g., HZSM-5), titanium dioxide (TiO 2 ), and silicon dioxide (SiO 2 ). Most preferably, the catalytic material comprises platinum or a platinum/palladium alloy. Materials suitable for catalytic oxidation of ammonia are further discussed in A. C. M. van den Broek, J. van Grondelle and R. A. van Santen,  Water - Promoted Ammonia oxidation by a Platinum Amine Complex in Zeolite HZSM -5  Catalyst , Catalysis Letters 55: 79-82 (1998) and M. Ueshima, K. Sano, M. Ikeda, K. Yoshino and J. Okamura,  New Technology for Selective Catalytic Oxidation of Ammonia to Nitrogen , Res. Chem. Intermed. 24: 133-141 (1998), the disclosure of which is incorporated herein by reference.  
         [0038]    A heater  514  placed around the water-producing apparatus  512  preferably maintains a reaction temperature from about 25° C. to about the explosive temperature of the ammonia and oxidant gas mixture, taking into account their respective concentrations and pressure. A more preferred reaction temperature is from about 350° C. to about 410° C. When the reaction temperature is lower than about 180° C., the reaction speed is undesirably low, and as a result, in order to obtain a sufficient decomposition ratio of ammonia gas, a space velocity of the ammonia/oxidant mixture gas in a catalytic reaction zone has to be lowered, making the water production process less economical. When the reaction temperature exceeds about 600° C., nitrogen oxides in the outlet  516  are undesirably high and can effect consumption of the catalyst.  
         [0039]    A preferred gas pressure for the water-producing apparatus  512  is selected from about 10 −8  torr to a pressure not exceeding one that would transition the produced water into the liquid phase for a particular process condition. A more preferred gas pressure at the inlet  510  ranges from about 350 Torr to about 1,000 Torr, i.e., roughly in the atmospheric order of magnitude.  
         [0040]    In the illustrated embodiment, the mixture of ammonia and oxidant are supplied to the water-producing apparatus  512  in a gaseous form, but it will be understood that, in other arrangements, liquified ammonia and liquified oxidant could also be supplied. Furthermore, while in the illustrated embodiment the ammonia gas and oxygen gas are premixed at the mixing section  500  then supplied to the water-producing apparatus  512 , the skilled artisan will readily appreciate that it is also possible to supply ammonia gas and oxygen gas independently to the water-producing apparatus  512 , and to mix them in the water-producing apparatus  512 .  
         [0041]    Optionally, a preheating section  518  can be provided along the inlet line  510  to the water-producing apparatus  512 , and the mixture gas can be preheated therein. By providing the mixture gas preheating section  518  along the inlet line  510 , even under conditions in which there is not a sufficient temperature or flow rate, it is made possible to effectively prevent production of unreacted gas.  
         [0042]    The reaction products exit the water-producing apparatus  512  via an outlet line  516  and preferably enter a process equipment  520  which enables the water vapor to be conveyed to and used in semiconductor fabrication processes. For example, process equipment  520  can include selective catalytic reduction (SCR) equipment whereby the nitrogen-based oxides are removed from the outline line  516  or condensing equipment whereby produced water vapor is condensed to liquid water to be used in rinsing of semiconductor devices. In an alternative embodiment, process equipment  520  can itself be semiconductor fabrication process equipment, the nature of which is described in further detail with respect to FIG. 6.  
         [0043]    [0043]FIG. 6 is a flow chart showing a method of producing water and supplying the water to a semiconductor fabrication process in accordance with the present invention. In block  600 , ammonium and an oxidant is fed into a catalytic converter. Preferred embodiments of the catalytic converter comprise a catalytic tube reactor, a fixed bed reactor, or a fluidized bed reactor. In block  602 , water produced from the catalyzed reaction between the ammonia and the oxidant exits the catalytic converter. In block  604 , the water enters a semiconductor fabrication process. The semiconductor fabrication process is preferably a process of wet oxidation (such as wet thermal oxidation of silicon substrates, source/drain reoxidation and reoxidation of high dielectric materials), wet formation of silicon oxides, nitrides, or oxynitrides, wet bench processing of semiconductor substrates, wet etching, cleaning (such as for furnace, LPCVD, PECVD, and HDP processing chambers and etch chambers), removing organic material, or combinations or these.  
         [0044]    In the most preferred embodiment, the reaction products are fed directly to an oxidation furnace or RTP tool, in which source/drain reoxidation of semiconductor layers is performed. Source/drain reoxidation serves to repair plasma etch damage to a gate dielectric at the corners of the gate dielectric in order to reduce hot carrier injection into the gate dielectric. FIG. 7 illustrates a closer view of a gate electrode  700 , which has been grown out of a single-crystal silicon substrate  702 , and physical damage (thinning) resulting from gate oxide  704  exposure to the plasma etch. Generally, the gate electrode  700  comprises a patterned gate polysilicon layer  706 , silicide layer  708 , and metal straps  710 , as shown. Damage to the gate oxide  704  caused by plasma etching may induce punchthrough or tunneling current leakage, particularly at or near edges  712  of the gate electrode  700 . In turn, junction leakage results in increased threshold voltage and unreliable circuit operation.  
         [0045]    It should be understood that damage to the chemical integrity of the gate oxide  704  also takes place as a result of photon-assisted and other damage during the ion bombardment generally required for anisotropic etching. Etch damage may also extend to the underlying silicon substrate. Aside from the illustrated physical thinning, plasma etching tends to damage oxide bonds, creating charge trap sites. Such structural damage extends laterally under the gate edges  712  as well as over source/drain regions.  
         [0046]    Typically, the source/drain reoxidation step involves wet oxidation at temperatures above 800° C. for a relatively long period (more than 30 minutes). FIG. 8 illustrates the gate electrode  700  after source/drain reoxidation, showing a reoxidized gate oxide  800  with a slight bird&#39;s beak  802  under the gate corner  712 . The thickened bird&#39;s beak  802  serves to round the gate corners  712  and reduce lateral electric field strength in active areas adjacent the gate, thereby reducing hot electron injection to the gate oxide  800  during transistor operation.  
         [0047]    Unfortunately, the presence of an oxidant, such as oxygen, during source/drain reoxidation contributes to oxidation of exposed gate materials, as shown in FIG. 8. Thus, for the illustrated example, a layer of tungsten oxide  804  forms around the tungsten metal strap  710 . Similarly, an oxide layer  806  comprised substantially of tungsten oxide (WO 2 ) and silicon oxide (SiO 2 ) form around the silicide strap  708 , while a thin layer of silicon oxide  808  grows out of the gate polysilicon  706 .  
         [0048]    The oxides formed in consumption of the metal are insulating and so unable to contribute to word line conductivity. Thus, overall resistance may be radically increased by excess oxygen during source/drain reoxidation. Some metals, such as titanium or titanium nitride are so readily oxidized that overall resistance is increased beyond tolerable levels, rendering such metals for use in gate materials. By using ammonia, the present invention can produce water for use in source/drain reoxidation under lean oxidant conditions, most preferably such that the oxidant is completely reacted. As a result, the risk of undesired oxidation of low oxidation-resistant metal gate electrodes is advantageously lowered.  
         [0049]    The preferred embodiments of the water-producing apparatus of FIG. 5 are shown in FIGS. 9 through 11.  
       First Embodiment  
       [0050]    [0050]FIG. 9 is a partial, cross-sectional view of a catalytic tube reactor  900 , illustrating a first embodiment of the present invention. The illustrated reactor  900  preferably comprises a heat-resistant, corrosion-resistant alloy housing  902 , such as Hastelloy™, that houses a plurality of pipes  904  made of the preferred catalytic material. The pipes  904  desirably activate the ammonia reactivity. Preferably, both inner and outer surfaces of the catalytic pipes  904  serve as catalysts. The catalytic pipes  904  preferably have an end diameter between about 1 cm and about 2 cm and a length between about 20 cm and about 30 cm. The skilled artisan will appreciate, however, that these surfaces can be optimized depending upon the desired production rates.  
         [0051]    An inlet stream  906 , comprising a mixture of ammonia gas and oxygen gas, enters the reactor  900  and reacts upon the surfaces of the catalytic pipe  904  to form an effluent stream  908 . The effluent stream  908  can include water vapor, nitrogen gas, nitrogen-based oxides, and any unreacted ammonia, all of which exit the reactor  900 .  
         [0052]    A preferred gas pressure for the reactor  900  is selected from about 10 −8  torr to a pressure not exceeding one that would transition the produced water into the liquid phase for a particular process condition. A more preferred gas pressure at the inlet  906  ranges from about 350 Torr to about 1,000 Torr.  
       Second Embodiment  
       [0053]    [0053]FIG. 10 is a partial, cross-sectional view of a packed bed reactor  1000 , illustrating a second embodiment in accordance with the present invention. The packed bed reactor  1000  preferably comprises a heat-resistant, corrosion-resistant alloy housing  1002 , such as Hastelloy™, that houses a bed  1004  of particles  1006 . The particles  1006  are preferably made of the preferred catalytic material. The particles  1006  preferably range in diameter from about 5 mm to about 10 mm. An inlet stream  1008 , comprising a mixture of ammonia and an oxidant, enters the reactor  1000  and reacts with the catalytic particles  1006  to form an effluent stream  1010 . The effluent stream  1010  can include water vapor, nitrogen gas, nitrogen-based oxides, and any unreacted ammonia, all of which exit the reactor  1000 .  
         [0054]    A preferred gas pressure for the reactor  1000  is selected from about 10 − 8   torr to a pressure not exceeding one that would transition the produced water into the liquid phase for a particular process condition. A more preferred gas pressure at the inlet  708  ranges from about 500 Torr to about 1,150 Torr.  
         [0055]    The skilled artisan will recognize other methods of increasing catalytic surface area of the particles  1006 . For example, the particle  1006  may comprise a granule, sintered material, thin sheet laminate, honeycomb body, mesh body, sponge body, or fin-shape body whose surfaces are covered with the preferred catalytic material.  
         [0056]    Moreover, while in the illustrated embodiment reactor  1000  is shown as a packed bed, column reactor, the skilled artisan will readily appreciate that reactor  1000  could also be a continuous, fixed bed tubular reactor (not illustrated).  
       Third Embodiment  
       [0057]    [0057]FIG. 11 is a partial, cross-sectional view of a fluidized bed reactor  1100 , illustrating a third embodiment in accordance with the present invention. The fluidized bed reactor  1100  preferably comprises a heat-resistant, corrosion-resistant alloy housing  1102 , such as Hastelloy™, that houses a bed  1104  of particles  1106 . The particles  1106  are preferably made of the preferred catalytic material. The particles  1106  preferably have a diameter from about 3 mm to about 5 mm. An inlet stream  1108 , comprising a mixture of ammonia and an oxidant enters the reactor  1100  and reacts with the catalytic particles  1106  to form an effluent stream  1110 . The effluent stream  1110  can include water vapor, nitrogen gas, nitrogen-based oxides, and any unreacted ammonia, all of which exit the reactor  1100 .  
         [0058]    A preferred gas pressure for the reactor  1100  is selected from about 10 −8  torr to a pressure not exceeding one that would transition the produced water into the liquid phase for a particular process condition. A more preferred gas pressure at the inlet  1108  ranges from about 700 Torr to about 1,350 Torr.  
         [0059]    Using ammonia, rather than hydrogen, as a reactant species to produce water for use in semiconductor fabrication processes advantageously allows the water-production process to be performed under lean oxidant conditions, most preferably such that the oxidant is completely reacted. The risk of undesired oxidation of low oxidation-resistant metal gate electrodes during subsequent wet fabrication processes, such as source/drain reoxidation, is advantageously lowered. However, excess unreacted ammonia, which is a product of limiting the quantity of excess unreacted oxidant, does not pose as high a danger of explosion or boron diffusion, as compared to excess unreacted hydrogen.  
         [0060]    The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. With reference to the information above, a myriad of alternative embodiments which are within the scope of the invention will be readily apparent to one skilled in the art, such as the simple re-arrangement of the blocks shown in FIG. 5. For example, in the illustrated embodiment the mixing section  500  precedes the preheating section  518 . However, a skilled artisan will recognize that the preheating section  518  may be incorporated into the mixing section  500  or even precede the mixing section  500 .  
         [0061]    The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come with the meaning and range of equivalency of the claims are to be embraced within their scope.