WO2022055654A1

WO2022055654A1 - Apparatus for and method of fluorine measurement

Info

Publication number: WO2022055654A1
Application number: PCT/US2021/045584
Authority: WO
Inventors: James Michael SIMONELLI; Siyu CHEN
Original assignee: Cymer, Llc
Priority date: 2020-09-10
Filing date: 2021-08-11
Publication date: 2022-03-17
Also published as: JP7511744B2; TWI840693B; TW202212785A; JP2023541105A; CN116057795A; KR20230045093A

Abstract

An apparatus for and method of determining a fluorine concentration in a mixed gas includes a detector that supplies a measure of fluorine concentration on the basis of the amount of oxygen generated in a reaction involving the fluorine and in which the amount of fluorine so measured is adjusted according to other factors. The other factors include an amount of oxygen remaining from an immediately prior measurement and the amount of time that has elapsed since the immediately prior measurement.

Description

APPARATUS FOR AND METHOD OF FLUORINE MEASUREMENT

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63/076,681, filed September 10, 2020, titled APPARATUS FOR AND METHOD OF FLUORINE MEASUREMENT, which is incorporated herein in its entirety by reference.

FIELD

[0002] The disclosed subject matter relates to the indirect measurement of fluorine concentration in a mixture of gases including fluorine.

BACKGROUND

[0003] There are many applications in which it is desired to measure the concentration of fluorine in a mixed gas. As an example, a lithographic apparatus applies a desired pattern onto a substrate such as a wafer of semiconductor material, usually onto a target portion of the substrate. A patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the wafer. Transfer of the pattern is typically accomplished by imaging onto a layer of radiation-sensitive material (photoresist, or simply “resist”) provided on the substrate. In general, a single substrate will contain adjacent target portions that are successively patterned.

[0004] The light source used to illuminate the pattern and project it onto the substrate can be of any one of a number of configurations. One configuration is an excimer laser. The excimer laser derives its name from the fact that under the appropriate conditions of electrical stimulation (energy supplied) and high pressure (of the gas mixture), a pseudo-molecule called an excimer arises, which exists only in an energized state and creates amplified light in the ultraviolet range.

[0005] An excimer light source typically uses a combination of one or more noble gases, such as argon, krypton, or xenon, and a reactive species such as fluorine or chlorine. Deep ultraviolet excimer lasers commonly used in lithography systems include the krypton fluoride (KrF) laser at 248 nm wavelength and the argon fluoride (ArF) laser at 193 nm wavelength. These gases are supplied to the laser discharge chambers. The fluorine is depleted during the laser discharge process and must be replenished. It is necessary to sample the gas in the discharge chambers from time to time and measure its F2 concentration to determine if replenishment is necessary.

[0006] One method of measuring F2 concentration involves reacting the sample gas including F2 with a metal oxide such as activated alumina, in a scrubber, and then using a sensor to measure the concentration of the reaction product, 02, to infer what the concentration of F2 was in original sampled gas. This method, unless countermeasures are taken, can yield an indirect measurement of F2 concentration that is lower than the actual concentration of F2 if the sensor has been idle for too long. One countermeasure to prevent this undermeasurement is increasing the size of the scrubber. This, however, creates a problem of the 02 reading failing to converge or predictably evolve to a final steadystate value in the sampling time allowed by the scrubber volume.

[0007] It is in this context that the apparatus and methods disclose herein arise.

SUMMARY

[0008] The following presents a concise summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0009] According to an aspect of an embodiment, disclosed herein is a method of estimating an amount of F2 by measuring the amount of 02 generated by an F2 reaction in a reaction chamber such as a scrubber that generates 02 and using the amount of 02 generated to infer actual F2 concentration in a sample of gas. According to an aspect of an embodiment, an inert gas such as nitrogen is used to push residual 02 in the reaction chamber to the 02 sensor where a residual 02 reading is taken. The reaction chamber is then charged with the gas to be sampled. In the example of a system used in conjunction with semiconductor photolithography, this may be a sample of gas in one of the laser chambers such as the master oscillator chamber. After a second F2 reaction is sufficiently complete, the inert gas is used to push the 02 in the reaction chamber to the 02 sensor where another, ultimate 02 measurement is taken to obtain a final indirect measurement of F2 concentration. The final measurement is adjusted based in part, for example, on the residual 02 reading to obtain a final and more accurate estimate of F2 concentration.

[0010] According to another aspect of an embodiment, a compensation method uses a total 02 measurement from the above sequence to obtain the F2 concentration in the gas sampled in the sensor. In particular a compensated fluorine concent rat ion /jt/;/ is determined according to the relationship room

^L J

where k3,kr, and ks are constant coefficients determined from curve fitting on test data, Oz(n) is the measurement of residual 02,

F2(n) is the F2 concentration inferred from the second 02 measurement,

P(n) is the total pressure of F2 containing gas from the chamber charged to sensor while measuring F2(n),

Atldle is the amount of time since the most recent prior measurement in seconds, and

AtPush is the amount of time N2 is used to push residual 02 out of the reaction chamber in seconds. [0012] According to an aspect of an embodiment there is disclosed an apparatus comprising a vessel defining a reaction cavity adapted to be placed in selectable fluid communication with a source of a mixed gas including fluorine to be sampled, the vessel containing a metal oxide arranged to react with the fluorine in the mixed gas to form a product gas including oxygen, the vessel being further adapted to be placed in selectable fluid communication with a source of an inert gas and an oxygen sensor configured to be placed in selectable fluid communication with the vessel to receive the product gas and to sense an amount of oxygen in the product gas. The vessel may also be adapted to be placed in selectable fluid communication with a vacuum source. The metal oxide may comprise alumina. The source of a mixed gas including fluorine to be sampled may comprise a laser discharge chamber. The source of an inert gas may comprise a source of nitrogen.

[0013] According to another aspect of an embodiment there is disclosed an apparatus comprising a laser discharge chamber, a vessel defining a reaction cavity adapted to be placed in selectable fluid communication with the laser discharge chamber to receive a sample of a mixed gas including fluorine to be sampled, the vessel containing a metal oxide arranged to react with the fluorine in the mixed gas to form a product gas including oxygen, a source of an inert gas in selectable fluid communication with the vessel, and an oxygen sensor configured to be placed in selectable fluid communication with the vessel to receive the product gas and to sense an amount of oxygen within the product gas. The vessel may also be adapted to be placed in selectable fluid communication with a vacuum source. The metal oxide may comprise alumina. The source of a mixed gas including fluorine to be sampled may comprise a laser discharge chamber. The source of an inert gas may comprise a source of nitrogen.

[0014] According to another aspect of an embodiment the apparatus may further comprise a gas maintenance system comprising a gas supply system fluidly connected the laser discharge chamber, a control system connected to the gas maintenance system and the detection apparatus and configured to receive the output of the oxygen sensor and estimate a concentration of fluorine in the mixed gas received from the gas discharge chamber, determine whether a concentration of fluorine in a gas mixture from the gas supply system of the gas maintenance system should be changed based on the estimated concentration of fluorine in the mixed gas, and send a signal to the gas maintenance system to change the relative concentration of fluorine in a gas mixture supplied from the gas supply system of the gas maintenance system to the laser discharge chamber during a gas update to the laser discharge chamber. [0015] According to another aspect of an embodiment there is disclosed a method comprising using a first portion of an inert gas to push residual gas in a reaction cavity to a sensor, sensing a first concentration of oxygen within the residual gas using the sensor, supplying to the reaction cavity at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine, reacting the fluorine in the mixed gas portion with a metal oxide in the reaction chamber to form a product gas including oxygen, using a second portion of the inert gas to push the product gas in the reaction cavity to the sensor, sensing a second concentration of oxygen within the product gas, inferring an estimated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based on the sensed second concentration of oxygen, and determining a compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based at least in part on the first concentration of oxygen and the estimated fluorine concentration. The method may further comprise evacuating the reaction cavity between sensing a first concentration of oxygen within the residual gas using the sensor and supplying to the reaction cavity with at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine. The method may further comprise determining a quantity Atldle indicative of an amount of time elapsed since an immediately prior determination of fluorine concentration to the start of a current measurement and a quantity AtPush indicative of an amount of an amount of time the first portion of the inert gas pushes the produced gas mixture to the sensor and determining the compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber may then also be based at least in part on Atldle and AtPush. The residual gas may be from an immediately prior measurement. The metal oxide may comprise alumina. The inert gas may comprise nitrogen. The method may further comprise determining an amount of additional fluorine-containing gas to supply to the mixed gas in the laser discharge chamber based on the estimated fluorine concentration, and supplying the amount of additional fluorine- containing gas to the mixed gas in the laser discharge chamber.

[0016] According to another aspect of an embodiment there is disclosed a method comprising using a first portion of an inert gas to push residual gas in a reaction cavity to a sensor, sensing a first concentration of oxygen within the residual gas using the sensor, supplying to the reaction cavity at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine, reacting the fluorine in the mixed gas portion with a metal oxide in the reaction chamber to form a product gas including oxygen, using a second portion of the inert gas to push the product gas in the reaction cavity to the sensor, sensing a second concentration of oxygen within the product gas, inferring an estimated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based on the sensed second concentration of oxygen, determining a compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based at least in part on the first concentration of oxygen and the estimated fluorine concentration, determining an amount of additional fluorine-containing gas to supply to the mixed gas in the laser discharge chamber based on the estimated fluorine concentration, and supplying the amount of additional fluorine-containing gas to the mixed gas in the laser discharge chamber.

[0017] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the invention by way of example, and not by way of limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. In the drawings, like reference numbers indicate identical or functionally similar elements.

[0019] FIG. 1 is a not-to-scale, partially schematic block diagram of an overall broad conception of a photolithography system according to an aspect of the disclosed subject matter.

[0020] FIG. 2 is a not-to-scale, partially schematic block diagram an apparatus including a detection apparatus configured to measure a concentration of fluorine in a gas mixture within a chamber according to an aspect of the disclosed subject matter.

[0021] FIG. 3 is a flow chart of a procedure performed by the detection apparatus for measuring a concentration of fluorine in the gas mixture of the chamber according to an aspect of the disclosed subject matter.

[0022] FIG. 4 is a flow chart of a procedure performed by the detection apparatus for measuring a concentration of fluorine in the gas mixture of the chamber according to another aspect of the disclosed subject matter.

[0023] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein.

DETAILED DESCRIPTION

[0024] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the present invention. The scope of the present invention is not limited to the disclosed embodiment! s). The present invention is defined by the claims appended hereto.

[0025] The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” “an exemplary embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0026] Before describing specific embodiments in more detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented. Referring to FIG. 1, a photolithography system 10 includes an illumination system 100. As described more fully below, the illumination system 100 includes a light source that produces a pulsed light beam 20 and directs it to a photolithography exposure apparatus or scanner 30 that patterns microelectronic features on a wafer 40. The wafer 40 is placed on a wafer table 50 constructed to hold wafer 40 and connected to a positioner configured to accurately position the wafer 40 in accordance with certain parameters.

[0027] The photolithography system 10 uses a light beam 20 having a wavelength in the deep ultraviolet (DUV) range, for example, with wavelengths of 248 nanometers (nm) or 193 nm. The minimum size of the microelectronic features that can be patterned on the wafer 40 depends on the wavelength of the light beam 20, with a lower wavelength permitting a smaller minimum feature size. When the wavelength of the light beam 20 is 248 nm or 193 nm, the minimum size of the microelectronic features can be, for example, 50 nm or less. The bandwidth of the light beam 20 can be the actual, instantaneous bandwidth of its optical spectrum (or emission spectrum), which contains information on how the optical energy of the light beam 20 is distributed over different wavelengths. The scanner 30 includes an optical arrangement having, for example, one or more condenser lenses, a mask, and an objective arrangement. The mask is movable along one or more directions, such as along an optical axis of the light beam 20 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables the image transfer to occur from the mask to the photoresist on the wafer 40. The illumination system 100 adjusts the range of angles for the light beam 20 impinging on the mask. The illumination system 100 also homogenizes (makes uniform) the intensity distribution of the light beam 20 across the mask.

[0028] The scanner 30 can include, among other features, a lithography controller 60, air conditioning devices, and power supplies for the various electrical components. The lithography controller 60 controls how layers are printed on the wafer 40. The lithography controller 60 includes a memory that stores information such as process recipes. A process program or recipe determines the length of the exposure on the wafer 40 based on, for example, the mask used as well as other factors that affect the exposure. During lithography, a plurality of pulses of the light beam 20 illuminate the same area of the wafer 40 to constitute an illumination dose.

[0029] The photolithography system 10 also for some embodiments preferably includes a control system 70. In general, the control system 70 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 70 also includes memory which can be read-only memory and/or random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks.

[0030] The control system 70 can also include one or more input devices (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices (such as a speaker or a monitor). The control system 70 may also include one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by one or more programmable processors. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processors receive instructions and data from the memory. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). The control system 70 can be centralized or be partially or wholly distributed throughout the photolithography system 10.

[0031] Referring to FIG. 2, an illumination system 100 includes a detection apparatus 105 that is configured to measure or estimate a concentration of fluorine (F2) in a gas mixture 107 within a laser discharge chamber 110. The concentration of fluorine molecules F2 in the laser discharge chamber 110 may be in a range that is too high to permit a direct detection of the fluorine. For example, the concentration of fluorine in the laser discharge chamber 110 may be greater than about 500 parts per million (ppm) and can be around 1000 ppm or up to about 2000 ppm. However, commercially-available fluorine sensors typically saturate at 10 ppm, thus making it impractical to use such a commercially- available fluorine sensor to directly measure the concentration of fluorine in the laser discharge chamber 110. Instead, the detection apparatus 105 relies on a chemical reaction that generates 02 with the amount of 02 generated being related in a known way to the amount of fluorine that participated in the reaction. The amount of 02 can then be measured with a commercially-available 02 sensor 115. The detection apparatus 105 thus can supply the basis for a calculation of how much fluorine was present before the beginning of the chemical reaction based on the amount of 02 present after the chemical reaction (as supplied from the 02 sensor 115) and based on information about the chemical reaction. Here and elsewhere, the terms “amount” and “concentration” are used interchangeably as the context permits, it being understood that one is directly calculable from the other.

[0032] In order for this estimate to be accurate, operation of the detection apparatus 105 is based on the assumption that the chemical reaction that consumes fluorine and releases the 02 is a linear reaction in which there is a direct known correlation between the concentration of the fluorine before the beginning of the chemical reaction and the concentration of the 02 at the end of the chemical reaction. In some instances, operation is based on the assumption that the consumption of the fluorine is complete and thus, there is no residual molecular fluorine F2 in the gas after the chemical reaction. [0033] The photolithography system 10 (FIG. 1) includes a gas maintenance system 120 that includes at least a gas supply system fluidly connected to the laser discharge chamber 110 via a conduit system 127. The gas maintenance system 120 includes one or more supplies of gases and a control unit (that also includes a valve system) for controlling which of the gases from the supplies are transferred into or out of the laser discharge chamber 110 via the conduit system 127.

[0034] The detection apparatus 105 interfaces with a controller 130 that receives the output from the 02 sensor 115 and calculates how much fluorine was present before the beginning of the chemical reaction to estimate the amount of fluorine in the gas mixture 107. The controller 130 may be part of control system 70 (FIG. 1), a standalone controller, or part of some other control system in photolithography system 10 or distributed among or all some of these. The controller 130 uses this information to determine whether a concentration of fluorine in the gas mixture 107 needs to be adjusted. The controller 130 therefore determines how to adjust the relative amounts of gases in the supplies of the gas maintenance system 120 that are to be transferred into or out of the laser discharge chamber 110 based on the determination. The controller 130 sends a signal to the gas maintenance system 120 to adjust the relative concentration of fluorine in the gas mixture 107 during a gas update to the laser discharge chamber 110.

[0035] The detection apparatus 105 includes a reaction vessel 135 that defines a reaction cavity 140 that houses a metal oxide 145 such as alumina. The reaction cavity 140 may also be referred to as a scrubber. The reaction cavity 140 is fluidly connected to the laser discharge chamber 110 via a conduit 137 to receive a mixed gas 150 including fluorine from the laser discharge chamber 110. A fluid control device 138 (such as a valve) can be placed in the conduit 137 to regulate the timing, under the control of the controller 130, of when the mixed gas 150 is supplied to the reaction cavity 140 as well as to control a rate of flow of the mixed gas 150 into the reaction vessel 135. In this way, the reaction cavity 140 enables the chemical reaction between the fluorine of the received mixed gas 150 and the metal oxide 145 to form a new gas mixture 155. The interior of the reaction vessel 135 that defines the reaction cavity 140 should be made of a non-reactive material so as not to interfere with or alter the chemical reaction between the fluorine of the received mixed gas 150 and the metal oxide 145. For example, the interior of the reaction vessel 135 can be made of a non-reactive metal such as stainless steel or Monel metal.

[0036] The 02 sensor 115 is fluidly connected to receive, i.e., in fluid communication with, the new gas mixture 155 and arranged to sense an amount of 02 within the new gas mixture 155. The 02 sensor 115 can be a commercially-available 02 sensor that is able to detect a concentration of 02 in a range of concentrations that are expected due to the chemical reaction in the reaction cavity 140. For example, the 02 sensor 115 senses 02 within the new gas mixture 155 in a range of 200-1000 ppm, in some embodiments. [0037] One example of an 02 sensor that is suitable for this range of concentrations is an 02 analyzer that utilizes a precision zirconia oxide sensor for the detection of 02. The zirconia oxide sensor includes a cell made of a high purity, high density, stabilized zirconia ceramic. The zirconia oxide sensor produces a voltage signal indicative of the 02 concentration of the new gas mixture 155. Moreover, the output of the zirconia oxide sensor is analyzed (for example, converted and linearized) by a high-speed microprocessor within the 02 sensor 115 to provide a direct digital readout for use by the controller 130. A conventional zirconium oxide cell includes a zirconium oxide ceramic tube plated with porous platinum electrodes on its inner and outer surfaces. As the sensor is heated above a specific temperature (for example, 600 C or 1112°F), it becomes an 02 ion-conducting electrolyte. The electrodes provide a catalytic surface for the change in 02 molecules, 02, to oxygen ions, and oxygen ions to 02 molecules. 02 molecules on the high concentration reference gas side of the cell gain electrons to become ions which enter the electrolyte.

[0038] Simultaneously, at the inner electrode, oxygen ions lose electrons and become released from the surface as 02 molecules. When the 02 concentration differs on each side of the sensor, oxygen ions migrate from the high concentration side to the low concentration side. This ion flow creates an electrical imbalance resulting in a DC voltage across the electrodes. This voltage is a function of the sensor temperature and the ratio of 02 partial pressures (concentrations) on each side of the sensor. This voltage is then analyzed by the high-speed microprocessor within the 02 sensor 115 for direct readout by the controller 130.

[0039] The 02 sensor 115 can be inside a measurement cavity 175 of a measurement vessel 170. The measurement cavity 175 is fluidly connected to the reaction cavity 140 via a conduit 177. According to an embodiment, one or more fluid control devices 178 (such as valves) can be placed in the conduit 177 to regulate the timing, under the control of the control unit 130, of the timing of when the new gas mixture 155 is directed to the measurement cavity 175 as well as to control a rate of flow of the new gas mixture 155 into the measurement vessel 170.

[0040] Metal oxide 145 is selected to react with the fluorine in the mixed gas 150 because the chemical reaction between the fluorine and metal oxide is stoichiometrically simple and easy to implement and control. Moreover, the controlled stoichiometric ratio of the chemical reaction is fixed. Additionally, the chemical reaction between the fluorine and the metal oxide is a stable chemical reaction. A chemical reaction can be stable if the chemical reaction is not simultaneously proceeding in the opposite direction and the components of the new gas mixture do not react with anything else in the new gas mixture to form fluorine. One suitable chemical reaction between the fluorine of the mixed gas 150 and the metal oxide 145 that is stable and has a controlled stoichiometric ratio is discussed next.

[0041] In some implementations, the metal oxide 145 is in a powder form. Moreover, the metal oxide 145 in powder form can be closely packed into the reaction vessel 135 (which can be a tube) so that there is no movement of the particles in the powder of the metal oxide 145. The area or volume in the space outside the powder of the metal oxide 145 and within the reaction vessel 135 is considered as pores and by using the metal oxide 145 in a powder form, it is possible to ensure that there is a large surface area to allow a thorough chemical reaction between the metal oxide 145 and the fluorine. In some implementations, and depending on the specific metal oxide, the metal oxide 145 and the reaction vessel 135 are maintained at room temperature and the reaction between the metal oxide 145 and the fluorine proceeds without the need for a catalyst.

[0042] The metal oxide 145 may include a metal such as aluminum. Moreover, the metal oxide 145 preferably lacks an alkali metal, an alkaline earth metal, hydrogen, and carbon. Thus, the metal oxide 145 can be alumina (which is aluminum oxide or A12O3). The alumina is in a powder and solid form with enough pores to provide for enough surface area to facilitate the chemical reaction with the fluorine in a gas. The space between the particles of the powder is large enough to permit the flow of fluorine - containing gas into the alumina to enable the chemical reaction. For example, the alumina can be in the form of a powder or grains that are packed in a column and have a total pore volume of at least 0.35 cubic centimeters per gram. The mixed gas 150 is passed (for example, flowed) through or across the metal oxide 145 to enable the chemical reaction between the fluorine and the alumina.

[0043] In the presence of the fluorine in the mixed gas 150, the following chemical reaction occurs: [0044] 6F2 + 2A12O3 = 4A1F3 + 302.

[0045] Thus, for every six molecules of fluorine that interact with two molecules of the metal oxide (A12O3), four molecules of an inorganic fluoride compound (aluminum fluoride or A1F3) and three molecules of 02 (02) are output. This chemical reaction is a linear and stoichiometrically simple reaction. Thus, to focus just on the fluorine and the 02, for every two molecules of fluorine F2 input into the chemical reaction, one molecule of 02 is output from the chemical reaction. Thus, if the concentration of fluorine F2 that is input into the chemical reaction is 1000 ppm, then a concentration of 500 ppm of 02 is released after the chemical reaction and is detected by the 02 sensor 115. Thus, for example, because interpreting the results of the measurement in the detection apparatus 105 assumes that the ratio of consumed fluorine to produced 02 is 2: 1 in this chemical reaction, if 600 ppm of 02 is detected by the 02 sensor 115, then that means that 1200 ppm of fluorine was present in the gas mixture 107. In other implementations, the detection apparatus 105 can operate based on the assumption that the conversion of the fluorine in the sample is complete (and thus, there is no residual molecular fluorine F2 in the gas after the chemical reaction). For example, this assumption can be a valid assumption if enough time has passed after the beginning of the chemical reaction.

[0046] In some implementations, the reaction between the metal oxide 145 and the fluorine in the mixed gas 150 occurs under one or more specifically designed conditions. For example, the reaction between the metal oxide 145 and the fluorine in the mixed gas 150 can happen under the presence of one or more catalysts, which are substances that change the rate of the chemical reaction, but are chemically unchanged at the end of the chemical reaction. As another example, the reaction between the metal oxide 145 and the fluorine in the mixed gas 150 can happen in a controlled environment such as a temperature-controlled environment or a humidity-controlled environment.

[0047] The gas mixture 107 includes a gain medium that includes a noble gas and a halogen such as fluorine. During operation, the fluorine in the gas mixture 107 (which provides the gain medium for light amplification) is depleted over time. This reduces the amount of light amplification and thus changes characteristics of the light beam 20 (FIG. 1) produced by the illumination system 100. See U.S. Patent No. 5,978,406, titled “Fluorine Control System for Excimer Lasers”, issued November 2, 1999, the specification of which is hereby incorporated by reference in its entirety. The gas maintenance system 120 seeks to maintain a concentration of fluorine within the gas mixture 107 in the laser discharge chamber 110 to within a certain tolerance of a target value compared to a concentration of the fluorine that is set at an initial gas refill procedure. Fluorine will react with the walls and other laser components which results in a relatively regular depletion of the fluorine. The rates of depletion are dependent on many factors, but for a given laser at a particular time in its useful life, the rates of depletion depend primarily on the pulse rate and load factor if the laser is operating. Because of this, additional fluorine is added to the laser discharge chamber 110 on a regular basis and under the control of the gas maintenance system 120. The amount of fluorine consumption varies from gas discharge chamber to gas discharge chamber, so closed loop control is used to determine the amount of fluorine to push or inject into the laser discharge chamber 110 at each opportunity. Thus, this injection of fluorine into the laser chamber can occur when the laser is operational and generating pulses. The detection apparatus 105 is used to determine the concentration of fluorine in the gas mixture in the laser discharge chamber 110, and thus is used in an overall scheme to determine the amount of fluorine to push or inject into the laser discharge chamber 110.

[0048] As mentioned, using the method described above there is a tendency for the resulting indirect measurement of the F2 concentration to be lower than the actual concentration of F2 if the sensor has been idle for too long, i.e., on the order of minutes or hours. Increasing the size of the reaction chamber resists this tendency to some extent but introduces the problem of the 02 reading failing to converge or predictably evolve to a final steady-state value in the sampling time allowed by the reaction chamber volume.

[0049] According to an aspect of an embodiment, in order to avoid these problems, a measurement includes using an inert gas such as nitrogen to push residual 02 in the reaction cavity 140 to the 02 sensor 115 where a residual 02 reading is taken. The reaction cavity 140 is then charged with the gas to be sampled. In the example of a system used in conjunction with semiconductor photolithography, this is a sample of gas in one of the laser discharge chambers such as the master oscillator chamber. After a second F2 reaction is sufficiently close to completion, the inert gas is used to push the resulting 02 in the reaction chamber to the 02 sensor 115 where a second 02 measurement is made to obtain the basis for a final adjusted measurement of F2 concentration. [0050] In order to carry out this procedure, the detection apparatus 105 is selectably connected to an inert gas source 200 arranged to supply an inert gas 220 such as nitrogen to the reaction cavity 140 through a conduit 210. A fluid control device 230 (such as a valve) can be placed in the conduit 210 to regulate the timing, under the control of the control unit 130, of the flow of the inert gas 220 to the reaction cavity 140. Also as shown in FIG. 2, according to some embodiments there is a conduit 410 with a valve 430 selectably placing the reaction cavity 140 in fluid communication with a vacuum source 400 such as a pump or a vessel at near vacuum pressure to evacuate the reaction cavity 140 as part of the process described below in conjunction with FIG. 4.

[0051] FIG. 3 is a flow chart further explaining the process just described. In a step S 10 the inert gas source 200 supplies inert gas to the reaction cavity 140 to push any gas in the reaction cavity 140 to the 02 sensor 115. In a step S20 the 02 sensor 115 measures the 02 concentration in the gas that has been pushed to it. According to an embodiment, these readings are collected and integrated to obtain an 02 measurement. Because this measurement is of a residual amount of 02 that was in the reaction cavity 140 it is referred to herein as a residual 02 measurement.

[0052] In a step S30 the reaction cavity 140 is charged to a fixed pressure with chamber gas which resides there for a period of time known to be sufficient for the reaction in the reaction cavity 140 to proceed substantially to completion.

[0053] In a step S40 the inert gas source 200 supplies a second portion of inert gas to the reaction cavity 140 to push the gas in the reaction cavity 140 to the 02 sensor 115. In a step S50 the 02 sensor 115 measures the 02 concentration in the gas that has been pushed to it. According to an embodiment, readings are collected and integrated to obtain a second 02 measurement. Because this second measurement is of an amount of 02 that was produced in the reaction cavity 140 to measure Fz(n), it is referred to herein as the ultimate 02 measurement. In a step S60, Fj(n) is inferred from the ultimate 02 measurement. In a step S70 the F2 concentration determined, Fj(n), is adjusted based at least in part on the residual 02 measurement. In some embodiments the process continues to a step S80 in which an amount of additional fluorine-containing gas to supply to the mixed gas in the laser discharge chamber is determined based on the estimated fluorine concentration, and then to a step S90 of supplying the amount of additional fluorine-containing gas to the mixed gas in the laser discharge chamber.

[0054] As shown in FIG. 4, according to some embodiments the process can include a step SI 00 in which the reaction cavity 140 is evacuated by placing it in fluid communication with the vacuum source 400 between steps S20 and S30.

[0055] According to another aspect of an embodiment, a compensation method uses a total 02 measurement from the above sequence to obtain the F2 concentration in the gas sampled in the sensor. Here “total 02 measurement” means measurement of 02 not only for uncompensated determination of F2, namely Fj(n), but also a preliminary measurement of residual 02, which is used as the basis for the compensation of the F2 measurement. In particular a corrected or compensated fluorine concentration is determined according to the relationship

where ks, kj, and ks are constant coefficients determined from curve fitting on test data,

Oz(n) is the result of the residual (first) 02 measurement,

F2 1) is the uncompensated F2 concentration inferred from the ultimate (second) 02 measurement, P(n) is the total pressure of chamber gas charged to the sensor during Fj(n) measurement,

Atldle is the amount of time since a most recent, i.e., immediately prior measurement and the start of the current measurement, in seconds, and

AtPush is the amount of time the inert gas, e.g., N2, is used to push residual 02 out of the reaction chamber to obtain the residual 02 measurement at the start of the current measurement, in seconds.

[0057] Thus, using the above apparatus and the above method an improved measurement of F2 concentration can be obtained regardless of the amount of time that elapses between measurements.

[0058] It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.

[0059] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0060] The foregoing description of the specific embodiments will so fully reveal the general nature of the present invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

[0061] Other aspects of the invention are set out in the following numbered clauses:

1. Apparatus comprising : a vessel defining a reaction cavity adapted to be placed in selectable fluid communication with a source of a mixed gas including fluorine to be sampled, the vessel containing a metal oxide arranged to react with the fluorine in the mixed gas to form a product gas including oxygen, the vessel being further adapted to be placed in selectable fluid communication with a source of an inert gas; and an oxygen sensor configured to be placed in selectable fluid communication with the vessel to receive the product gas and to sense an amount of oxygen in the product gas.

2. The apparatus as in clause 1 wherein the vessel is further adapted to be placed in selectable fluid communication with a vacuum source.

3. The apparatus as in clause 1 wherein the metal oxide comprises alumina.

4. The apparatus as in clause 1 wherein the source of a mixed gas including fluorine to be sampled comprises a laser discharge chamber.

5. The apparatus as in clause 1 wherein the source of an inert gas comprises a source of nitrogen.

6. An apparatus comprising: a laser discharge chamber; a vessel defining a reaction cavity adapted to be placed in selectable fluid communication with the laser discharge chamber to receive a sample of a mixed gas including fluorine to be sampled, the vessel containing a metal oxide arranged to react with the fluorine in the mixed gas to form a product gas including oxygen; a source of an inert gas in selectable fluid communication with the vessel; and an oxygen sensor configured to be placed in selectable fluid communication with the vessel to receive the product gas and to sense an amount of oxygen within the product gas.

7. The apparatus as in clause 6 wherein the vessel is also adapted to be placed in selectable fluid communication with a vacuum source.

8. The apparatus as in clause 6 wherein the metal oxide comprises alumina.

9. The apparatus as in clause 6 wherein the source of a mixed gas including fluorine to be sampled comprises a laser discharge chamber.

10. The apparatus as in clause 6 wherein the source of an inert gas comprises a source of nitrogen.

11. The apparatus as in clause 6 further comprising a gas maintenance system comprising a gas supply system fluidly connected the laser discharge chamber, a control system connected to the gas maintenance system and the detection apparatus and configured to receive the output of the oxygen sensor and estimate a concentration of fluorine in the mixed gas received from the gas discharge chamber, determine whether a concentration of fluorine in a gas mixture from the gas supply system of the gas maintenance system should be changed based on the estimated concentration of fluorine in the mixed gas, and send a signal to the gas maintenance system to change the relative concentration of fluorine in a gas mixture supplied from the gas supply system of the gas maintenance system to the laser discharge chamber during a gas update to the laser discharge chamber.

12. A method comprising: using a first portion of an inert gas to push residual gas in a reaction cavity to a sensor; sensing a first concentration of oxygen within the residual gas using the sensor; supplying to the reaction cavity at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine; reacting the fluorine in the mixed gas portion with a metal oxide in the reaction chamber to form a product gas including oxygen; using a second portion of the inert gas to push the product gas in the reaction cavity to the sensor; sensing a second concentration of oxygen within the product gas; inferring an estimated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based on the sensed second concentration of oxygen; and determining a compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based at least in part on the first concentration of oxygen and the estimated fluorine concentration.

13. The method as in clause 12 further comprising evacuating the reaction cavity between sensing a first concentration of oxygen within the residual gas using the sensor and supplying to the reaction cavity with at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine.

14. The method as in clause 12 further comprising determining a quantity Atldle indicative of an amount of time elapsed since an immediately prior determination of fluorine concentration to the start of a current measurement and a quantity AtPush indicative of an amount of an amount of time the first portion of the inert gas pushes the produced gas mixture to the sensor and determining the compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber is also based at least in part on Atldle and AtPush.

15. The method as in clause 12 wherein the compensated fluorine measurement f₂ (n) is determined according to

where k₃, kt, and ks are constant coefficients determined from test data, Oz(n) is the result of sensing a first concentration of 02 within the residual gas using the sensor, F2(n) is an estimated fluorine concentration as measured,

P(n) is a total pressure charged to sensor when obtaining the estimate of F2 in the produced gas mixture,

Atldle is an amount of time since a most recent prior measurement and a start of a current measurement, in seconds; and

AtPush is an amount of time the inert gas is used to push residual 02 out of the reaction chamber to obtain the residual 02 measurement at the start of the current measurement, in seconds.

16. The method as in clause 12 wherein the residual gas is from an immediately prior measurement.

17. The method as in clause 12 wherein the metal oxide comprises alumina. 18. The method as in clause 12 wherein inert gas comprises nitrogen.

19. A method as in clause 12 further comprising determining an amount of additional fluorine- containing gas to supply to the mixed gas in the laser discharge chamber based on the estimated fluorine concentration.

20. A method comprising: using a first portion of an inert gas to push residual gas in a reaction cavity to a sensor; sensing a first concentration of oxygen within the residual gas using the sensor; supplying to the reaction cavity at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine; reacting the fluorine in the mixed gas portion with a metal oxide in the reaction chamber to form a product gas including oxygen; using a second portion of the inert gas to push the product gas in the reaction cavity to the sensor; sensing a second concentration of oxygen within the product gas; inferring an estimated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based on the sensed second concentration of oxygen; determining a compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based at least in part on the first concentration of oxygen and the estimated fluorine concentration; determining an amount of additional fluorine-containing gas to supply to the mixed gas in the laser discharge chamber based on the estimated fluorine concentration; and supplying the amount of additional fluorine-containing gas to the mixed gas in the laser discharge chamber.

Claims

CLAIMS:

1. An apparatus comprising: a vessel defining a reaction cavity adapted to be placed in selectable fluid communication with a source of a mixed gas including fluorine to be sampled, the vessel containing a metal oxide arranged to react with the fluorine in the mixed gas to form a product gas including oxygen, the vessel being further adapted to be placed in selectable fluid communication with a source of an inert gas; and an oxygen sensor configured to be placed in selectable fluid communication with the vessel to receive the product gas and to sense an amount of oxygen in the product gas.

2. The apparatus as in claim 1 wherein the vessel is further adapted to be placed in selectable fluid communication with a vacuum source.

3. The apparatus as in claim 1 wherein the metal oxide comprises alumina.

4. The apparatus as in claim 1 wherein the source of a mixed gas including fluorine to be sampled comprises a laser discharge chamber.

5. The apparatus as in claim 1 wherein the source of an inert gas comprises a source of nitrogen.

7. The apparatus as in claim 6 wherein the vessel is also adapted to be placed in selectable fluid communication with a vacuum source.

8. The apparatus as in claim 6 wherein the metal oxide comprises alumina.

9. The apparatus as in claim 6 wherein the source of a mixed gas including fluorine to be sampled comprises a laser discharge chamber.

10. The apparatus as in claim 6 wherein the source of an inert gas comprises a source of nitrogen.

11. The apparatus as in claim 6 further comprising a gas maintenance system comprising a gas supply system fluidly connected the laser discharge chamber, a control system connected to the gas maintenance system and the detection apparatus and configured to receive the output of the oxygen sensor and estimate a concentration of fluorine in the mixed gas received from the gas discharge chamber, determine whether a concentration of fluorine in a gas mixture from the gas supply system of the gas maintenance system should be changed based on the estimated concentration of fluorine in the mixed gas, and send a signal to the gas maintenance system to change the relative concentration of fluorine in a gas mixture supplied from the gas supply system of the gas maintenance system to the laser discharge chamber during a gas update to the laser discharge chamber.

13. The method as in claim 12 further comprising evacuating the reaction cavity between sensing a first concentration of oxygen within the residual gas using the sensor and supplying to the 19 reaction cavity with at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine.

14. The method as in claim 12 further comprising determining a quantity Atldle indicative of an amount of time elapsed since an immediately prior determination of fluorine concentration to the start of a current measurement and a quantity AtPush indicative of an amount of an amount of time the first portion of the inert gas pushes the produced gas mixture to the sensor and determining the compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber is also based at least in part on Atldle and AtPush.

15. The method as in claim 12 wherein the compensated fluorine measurement f₂ (n)is determined according to

where k₃, kj, and ks are constant coefficients determined from test data, Oz(n) is the result of sensing a first concentration of 02 within the residual gas using the sensor,

F2(n) is an estimated fluorine concentration as measured,

16. The method as in claim 12 wherein the residual gas is from an immediately prior measurement.

17. The method as in claim 12 wherein the metal oxide comprises alumina.

18. The method as in claim 12 wherein inert gas comprises nitrogen.

19. The method as in claim 12 further comprising determining an amount of additional fluorine-containing gas to supply to the mixed gas in the laser discharge chamber based on the estimated fluorine concentration.

20. A method comprising: using a first portion of an inert gas to push residual gas in a reaction cavity to a sensor; 20 sensing a first concentration of oxygen within the residual gas using the sensor; supplying to the reaction cavity at least a portion of a mixed gas from a laser discharge chamber, wherein the mixed gas includes fluorine; reacting the fluorine in the mixed gas portion with a metal oxide in the reaction chamber to form a product gas including oxygen; using a second portion of the inert gas to push the product gas in the reaction cavity to the sensor; sensing a second concentration of oxygen within the product gas; inferring an estimated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based on the sensed second concentration of oxygen; determining a compensated fluorine concentration in the portion of the mixed gas from the laser discharge chamber based at least in part on the first concentration of oxygen and the estimated fluorine concentration; determining an amount of additional fluorine-containing gas to supply to the mixed gas in the laser discharge chamber based on the estimated fluorine concentration; and supplying the amount of additional fluorine-containing gas to the mixed gas in the laser discharge chamber.