Apparatus for producing water for use in manufacturing semiconductors

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.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor fabrication. More specifically, the invention relates to producing water for use in the fabrication of semiconductors.

2. Description of the Related Art

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 (SiO2) 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.

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 SiOxNy; 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.

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.

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

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.

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.

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.

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.

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.

DETAILED DESCRIPTION OF THE INVENTION

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 (H2) and oxygen gas (O2) which is traditionally conducted with an excess of H2or O2to prevent explosion.

FIG. 1shows one combustion-pipe-type apparatus100that can be used to produce water for use in semiconductor fabrication. H2enters a quartz furnace102via a hydrogen gas nozzle104. O2enters the quartz furnace102via an oxygen gas nozzle106. The vicinity near a tip end of the hydrogen gas nozzle104inside the quartz furnace102attains 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 furnace102via an outlet108.

FIGS. 2 through 4illustrate catalyst-reaction-type apparatuses that can be used to produce water via H2and O2for use in semiconductor fabrication. InFIG. 2, a water-producing apparatus200includes a plurality of pipes202made of material that serves as a catalyst to encourage hydrogen or oxygen reactivity. A mixture of H2, and O2is introduced into the water-producing apparatus200via an inlet204. Water produced by catalysis exits the water-producing apparatus200via an outlet206.

InFIG. 3, a water-producing apparatus300includes a plurality of supported plates302made of catalytic material. A mixture of H2and O2is introduced into water-producing apparatus300via an inlet304and contacts the plates302. Water produced by catalysis exits the water-producing apparatus300via an outlet306.

InFIG. 4, a water-producing apparatus400includes a plurality of particulate elements402made of catalytic material. A mixture of H2and O2is introduced into water-producing apparatus400via an inlet404and contacts the particulate elements402. Water produced by catalysis exits the water-producing apparatus400via an outlet406. The water-producing apparatus400inFIG. 4is illustrated as a fixed bed reactor, but it may be a fluidized bed reactor.

However, one problem with the known methods illustrated inFIGS. 1 through 4is 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.

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.

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.

FIG. 5is a block diagram showing a system for producing water in accordance with a preferred embodiment of the invention. Ammonia gas (NH3) is supplied to a mixing section500via an ammonia gas line502. An oxidant is supplied to the mixing section500via a gas line504. The mixing section500may 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.

The molar ratio of ammonia to oxidant gas can be set and altered using mass flow controllers506aand506band valves508aand508b.The ammonia gas and oxidant gas mixture is preferably supplied via a common inlet line510into a water-producing apparatus512. The water-producing apparatus512of various embodiments is depicted in greater detail inFIGS. 9 through 11.

In the illustrated embodiment, the oxidant gas comprises oxygen (O2) gas. An ammonia and oxygen gas mixture is catalytically converted into reaction products comprising nitrogen gas N2, water vapor H2O, any excess oxygen gas or ammonia gas, and nitrogen-based oxides, according to the following equation:
4NH3+3O2→6H2O+2N2+aNxOy(1)

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 apparatus512, 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.

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.

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 (V2O5), tungsten oxide (WO3), ion-exchanged zeolites (e.g., HZSM-5), titanium dioxide (TiO2), and silicon dioxide (SiO2). 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-5Catalyst, 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.

A heater514placed around the water-producing apparatus512preferably 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 outlet516are undesirably high and can effect consumption of the catalyst.

A preferred gas pressure for the water-producing apparatus512is selected from about 10−8torr 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 inlet510ranges from about 350 Torr to about 1,000 Torr, i.e., roughly in the atmospheric order of magnitude.

In the illustrated embodiment, the mixture of ammonia and oxidant are supplied to the water-producing apparatus512in 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 section500then supplied to the water-producing apparatus512, the skilled artisan will readily appreciate that it is also possible to supply ammonia gas and oxygen gas independently to the water-producing apparatus512, and to mix them in the water-producing apparatus512.

Optionally, a preheating section518can be provided along the inlet line510to the water-producing apparatus512, and the mixture gas can be preheated therein. By providing the mixture gas preheating section518along the inlet line510, 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.

The reaction products exit the water-producing apparatus512via an outlet line516and preferably enter a process equipment520which enables the water vapor to be conveyed to and used in semiconductor fabrication processes. For example, process equipment520can include selective catalytic reduction (SCR) equipment whereby the nitrogen-based oxides are removed from the outline line516or condensing equipment whereby produced water vapor is condensed to liquid water to be used in rinsing of semiconductor devices. In an alternative embodiment, process equipment520can itself be semiconductor fabrication process equipment, the nature of which is described in further detail with respect toFIG. 6.

FIG. 6is 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 block600, 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 block602, water produced from the catalyzed reaction between the ammonia and the oxidant exits the catalytic converter. In block604, 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.

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. 7illustrates a closer view of a gate electrode700, which has been grown out of a single-crystal silicon substrate702, and physical damage (thinning) resulting from gate oxide704exposure to the plasma etch. Generally, the gate electrode700comprises a patterned gate polysilicon layer706, silicide layer708, and metal straps710, as shown. Damage to the gate oxide704caused by plasma etching may induce punchthrough or tunneling current leakage, particularly at or near edges712of the gate electrode700. In turn, junction leakage results in increased threshold voltage and unreliable circuit operation.

It should be understood that damage to the chemical integrity of the gate oxide704also 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 edges712as well as over source/drain regions.

Typically, the source/drain reoxidation step involves wet oxidation at temperatures above 800° C. for a relatively long period (more than 30 minutes).FIG. 8illustrates the gate electrode700after source/drain reoxidation, showing a reoxidized gate oxide800with a slight bird's beak802under the gate corner712. The thickened bird's beak802serves to round the gate corners712and reduce lateral electric field strength in active areas adjacent the gate, thereby reducing hot electron injection to the gate oxide800during transistor operation.

Unfortunately, the presence of an oxidant, such as oxygen, during source/drain reoxidation contributes to oxidation of exposed gate materials, as shown inFIG. 8. Thus, for the illustrated example, a layer of tungsten oxide804forms around the tungsten metal strap710. Similarly, an oxide layer806comprised substantially of tungsten oxide (WO2) and silicon oxide (SiO2) form around the silicide strap708, while a thin layer of silicon oxide808grows out of the gate polysilicon706.

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.

The preferred embodiments of the water-producing apparatus ofFIG. 5are shown inFIGS. 9 through 11.

FIRST EMBODIMENT

FIG. 9is a partial, cross-sectional view of a catalytic tube reactor900, illustrating a first embodiment of the present invention. The illustrated reactor900preferably comprises a heat-resistant, corrosion-resistant alloy housing902, such as Hastelloy™, that houses a plurality of pipes904made of the preferred catalytic material. The pipes904desirably activate the ammonia reactivity. Preferably, both inner and outer surfaces of the catalytic pipes904serve as catalysts. The catalytic pipes904preferably 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.

An inlet stream906, comprising a mixture of ammonia gas and oxygen gas, enters the reactor900and reacts upon the surfaces of the catalytic pipe904to form an effluent stream908. The effluent stream908can include water vapor, nitrogen gas, nitrogen-based oxides, and any unreacted ammonia, all of which exit the reactor900.

A preferred gas pressure for the reactor900is selected from about 10−8torr 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 inlet906ranges from about 350 Torr to about 1,000 Torr.

SECOND EMBODIMENT

FIG. 10is a partial, cross-sectional view of a packed bed reactor1000, illustrating a second embodiment in accordance with the present invention. The packed bed reactor1000preferably comprises a heat-resistant, corrosion-resistant alloy housing1002, such as Hastelloy™, that houses a bed1004of particles1006. The particles1006are preferably made of the preferred catalytic material. The particles1006preferably range in diameter from about 5 mm to about 10 mm. An inlet stream1008, comprising a mixture of ammonia and an oxidant, enters the reactor1000and reacts with the catalytic particles1006to form an effluent stream1010. The effluent stream1010can include water vapor, nitrogen gas, nitrogen-based oxides, and any unreacted ammonia, all of which exit the reactor1000.

A preferred gas pressure for the reactor1000is selected from about 10−8torr 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 inlet708ranges from about 500 Torr to about 1,150 Torr.

The skilled artisan will recognize other methods of increasing catalytic surface area of the particles1006. For example, the particle1006may 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.

Moreover, while in the illustrated embodiment reactor1000is shown as a packed bed, column reactor, the skilled artisan will readily appreciate that reactor1000could also be a continuous, fixed bed tubular reactor (not illustrated).

THIRD EMBODIMENT

FIG. 11is a partial, cross-sectional view of a fluidized bed reactor1100, illustrating a third embodiment in accordance with the present invention. The fluidized bed reactor1100preferably comprises a heat-resistant, corrosion-resistant alloy housing1102, such as Hastelloy™, that houses a bed1104of particles1106. The particles1106are preferably made of the preferred catalytic material. The particles1106preferably have a diameter from about 3 mm to about 5 mm. An inlet stream1108, comprising a mixture of ammonia and an oxidant enters the reactor1100and reacts with the catalytic particles1106to form an effluent stream1110. The effluent stream1110can include water vapor, nitrogen gas, nitrogen-based oxides, and any unreacted ammonia, all of which exit the reactor1100.

A preferred gas pressure for the reactor1100is selected from about 10−8torr 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 inlet1108ranges from about 700 Torr to about 1,350 Torr.

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.

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 inFIG. 5. For example, in the illustrated embodiment the mixing section500precedes the preheating section518. However, a skilled artisan will recognize that the preheating section518may be incorporated into the mixing section500or even precede the mixing section500.

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.