Gas concentration sensor and method of use thereof

Abstract

The present invention provides an optical gas concentration sensor comprising: a sample cell comprising an inlet port and an outlet port through which a gas can flow; a light source configured to emit light into an interior of the sample cell; the light detector is arranged outside the sample cell element; and an optical waveguide assembly optically coupling an interior of the sample cell to the detector. The optical waveguide assembly includes a first optical waveguide coupled to the sample cell, a second optical waveguide coupled to the detector, and an optical coupler optically coupling the first optical waveguide to the second optical waveguide. The first optical waveguide has a different light transmission characteristic than the second optical waveguide at a wavelength in a wavelength range of 1.5 [mu] m to 18 [mu] m.

Description

Technical Field

[0001] Specific examples of the present invention generally relate to gas concentration measurement, and more particularly, to the measurement of the concentration of precursors in a carrier gas. Background Art

[0002] In semiconductor manufacturing processes, various liquid or solid materials are vaporized by heating to form material gases, and these material gases are then introduced into a vacuum chamber, for example, during chemical vapor deposition (CVD) or atomic layer deposition (ALD) processes. Some of these chemical processes utilize optical gas concentration measurement sensors, such as non-dispersive infrared (NDIR) gas concentration sensors, to monitor gas concentration.

[0003] A typical NDIR gas concentration sensor includes a gas sample cell, an infrared (IR) light source, an IR detector, and an electronic assembly that receives information from the IR detector. The IR light source emits light through the gas sample cell and reaches the IR detector, and the change in the light transmitted from the IR light source to the IR detector due to the absorption of the sample gas can be used to infer the gas concentration within the gas sample cell.

[0004] However, high-temperature environments, such as those found in many CVD and ALD processes, can significantly limit the lifespan of such NDIR gas concentration sensors as well as many other types of gas sensors, because many electronic components experience significantly shortened lifespans when exposed to high temperatures (such as temperatures above 100 °C (e.g., 200 °C, or thereabouts)). Additionally, the performance stability of well-known IR detectors typically degrades at high temperatures, especially at temperatures above 100 °C. Furthermore, electronic components capable of operating at temperatures above 100 °C may undesirably be expensive.

[0005] In addition, traditional NDIR gas concentration sensors (and other known binary gas sensors, such as ultrasonic sensors) are susceptible to environmental changes and long-term drift factors (such as window contamination over time). Some precursor gases also degrade over time, and it is not possible to distinguish degradation by-products using such well-known NDIR or binary gas sensors.

[0006] Therefore, there is a need in the art for a new optical gas concentration sensor that addresses some of the current drawbacks, particularly those related to measuring the concentration of gases, such as precursor gases, in high-temperature environments. Summary of the Invention

[0007] A specific example of the present invention can be broadly characterized as an optical gas concentration sensor, comprising: a sample cell that includes an inlet port and an outlet port through which a gas can flow; a light source configured to emit light into an interior of the sample cell; a light detector disposed outside the sample cell; and an optical waveguide assembly that optically couples the interior of the sample cell to the detector. The optical waveguide assembly includes a first optical waveguide coupled to the sample cell, a second optical waveguide coupled to the detector, and an optical coupler that optically couples the first optical waveguide to the second optical waveguide. The first optical waveguide has different light transmission characteristics from the second optical waveguide at wavelengths within a wavelength range of 1.5 μm to 18 μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 FIG. shows a schematic diagram of an optical gas concentration sensor according to some specific examples of the present invention.

[0009] Figure 2 FIG. shows according to a specific example of the present invention Figure 1 a schematic diagram of the light source shown in.

[0010] Figure 3 and Figure 10 FIG. shows according to some specific examples of the present invention Figure 1 a schematic diagram of the detector shown in.

[0011] Figure 4 FIG. shows according to a specific example of the present invention Figure 3 a schematic diagram of the filter assembly shown in.

[0012] Figure 5 FIG. shows a graph of the radiant emittance detected by the detector shown in Figure 3 as a function of wavelength.

[0013] Figure 6 FIG. shows a graph of the absorption spectra of different materials at different wavelengths.

[0014] FIG. 7 shows a graph representing the output of a known NDIR gas concentration sensor as a function of time.

[0015] Figure 8 FIG. shows a graph representing Figure 1 the output of the gas concentration sensor shown in

[0016] Figure 9 FIG. shows according to some specific examples of the present invention a system by which the housing shown in Figure 1 can be incorporated. DETAILED DESCRIPTION

[0017] Exemplary embodiments are described herein with reference to the accompanying drawings. Unless otherwise explicitly stated, in the drawings, the sizes, positions, etc. of components, features, elements, etc. and any distances between components, features, elements, etc. are not necessarily to scale, but are exaggerated for clarity.

[0018] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to be limiting. As used herein, unless the context clearly dictates otherwise, the singular forms "a / an" and "the" are also intended to include the plural forms. It should be recognized that the term "comprises and / or comprising", when used in this specification, specifies the presence of the stated features, integers, steps, operations, elements, and / or components, but does not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Unless otherwise specified, when reciting a range of values, the range includes both the upper and lower limits of the range and any sub-ranges therebetween. Unless otherwise indicated, terms such as "first", "second", etc. are used only to distinguish one element from another. For example, one node may be referred to as "the first node", and similarly, another node may be referred to as "the second node", or vice versa. The section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described.

[0019] Unless otherwise indicated, the terms "about", "approximately", etc. mean that the quantity, size, formulation, parameter, and other quantities and characteristics are not and need not be exact, but may be approximate and / or larger or smaller as desired, thereby reflecting tolerances, conversion factors, rounding, measurement errors, and the like, and other factors known to those of ordinary skill in the art.

[0020] Spatial relative terms, such as "below", "beneath", "lower", "above", "upper", and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element or feature, as illustrated in the figures. It should be recognized that the spatial relative terms are intended to cover different orientations in addition to the orientation depicted in the figures. For example, if the object in the figure is flipped, the element described as "below" or "beneath" another element or feature will then be oriented "above" the other element or feature. Thus, the exemplary term "below" can cover both orientations of above and below. The object may be oriented in other ways (rotated 90 degrees or in other orientations) and the spatial relative descriptors used herein may be interpreted accordingly.

[0021] Like reference numerals throughout the specification refer to like elements. Thus, the same or like numbers may be described when referring to other figures, even if such numbers are not mentioned or described in the corresponding figure. Also, elements not indicated by reference numerals may be described with reference to other figures.

[0022] It is to be understood that many different forms and specific examples are possible without departing from the spirit and teachings of the present disclosure, and thus the present disclosure should not be construed as limited to the exemplary specific examples set forth herein. The fact is that providing such examples and specific instances makes the present disclosure will be exhaustive and complete, and will convey the scope of the present disclosure to those of ordinary skill in the art.

[0023] Figure 1 Schematic diagram showing an optical gas concentration sensor according to some specific examples of the present invention.

[0024] Refer to Figure 1 , an optical gas concentration sensor, such as sensor 100, may be disposed within a thermally insulated housing 102. Sensor 100 may include a gas sample cell 104 located within thermally insulated housing 102, and may have one or more optical couplers (e.g., a first optical coupler 106 and a second optical coupler 108) and one or more gas flow ports (e.g., an inlet gas flow port 110 and an outlet gas flow port 112).

[0025] Constructed as described above, gas sample cell 104 may be configured to receive a gas, such as a high-temperature processed gas (e.g., also simply referred to herein as "gas", which may be provided as a gas mixture containing a carrier gas and one or more precursor gases), through inlet gas flow port 110, and discharge the gas through outlet gas flow port 112. When the gas flows through sample cell 104, the temperature within housing 102 is typically higher than 100 °C (e.g., equal to or greater than about 120 °C, 150 °C, 200 °C, 220 °C, etc., or any temperature between such values). Although not shown, a temperature control element (e.g., a heater) may be provided (e.g., within the housing) to heat the interior of housing 102. The operation of the temperature control element may be controlled by a thermostat (not shown in the figure) to maintain the interior of the housing at a constant or substantially constant temperature. Maintaining at least a substantially constant temperature within housing 102 will facilitate consistent operation of sensor 100.

[0026] Housing 102 itself may be disposed at any suitable or desired location along a pipeline for delivering a precursor gas to a chamber in which CVD or ALD will be performed. For example, Figure 9Schematically illustrated is that the housing 102 can be disposed within the gas cabinet 900, and the precursor ampoule 902 is located in the gas cabinet. The precursor ampoule 902 can be provided in any suitable manner known in the art (for example, the precursor ampoule 902 can be coupled to the carrier gas input pipeline 904 and the precursor gas outlet pipeline 906). In this situation, the housing 102 can be configured such that the gas sample cell 104 is in fluid communication with the precursor gas profile pipeline 906 via the aforementioned inlet gas flow port 110 and outlet gas flow port 112.

[0027] Additionally or alternatively, the housing 102 can be located outside the gas cabinet 900, at a position upstream of the precursor gas supply switch 908 (for example, in fluid communication with the precursor gas outlet 906). In this situation, the housing 102 can be configured such that the gas sample cell 104 is in fluid communication with the precursor gas profile pipeline 906 via the aforementioned inlet gas flow port 110 and outlet gas flow port 112.

[0028] Additionally or alternatively, the housing 102 can be located outside the gas cabinet 900, at a position downstream of the precursor gas supply switch 908. For example, the housing 102 can be configured such that the gas sample cell 104 is in fluid communication with the precursor gas supply pipeline 912 via the aforementioned inlet gas flow port 110 and outlet gas flow port 112.

[0029] Returning to Figure 1 , the sensor 100 may also include a light source 114, a photodetector 116, an analyzer 118, and a controller 120. The housing 102 is constructed of one or more thermally insulating materials to prevent heat from transferring from the interior of the housing 102 to the area outside the housing 102 (for example, the area where the photodetector 116, the analyzer 118, and the controller 120 are located).

[0030] The light source 114 can be disposed within the sample cell 104 (for example, behind the second optical coupler 108) and is operable to emit light (in response to one or more power or command signals output by the controller 120 via the link 115) into the gas sample cell 104 via the second optical coupler 108. The link 115 can be fed into the interior of the housing 102 by any known or other suitable means through a thermally insulating access port (not shown in the figure) formed in the wall of the housing 102. In a specific example, the controller 120 is configured to operate the light source 114 in a pulsed mode (for example, where the light source 114 is strobed or otherwise intermittently flashed), in a continuous mode, or the like, or any combination thereof.

[0031] Typically, the second optical coupler 108 is configured to direct the light emitted by the light source 114 into the sample cell 104. The second optical coupler 108 is sealingly coupled within the sample cell 104 to prevent the gas introduced into the sample cell 104 (e.g., via the inlet gas flow port 110) from reaching the light source 114. In one particular example, the second optical coupler 108 may include a window (e.g., substantially transparent at least for the measurement light) sealingly coupled to the sample cell 104 and a concave curved mirror or lens disposed between the window and the light source 114 (e.g., configured to focus or collimate the light emitted by the light source 114 through the window and focus or collimate it into the sample cell 104). In another particular example, the second optical coupler 108 may include a lens (e.g., substantially transparent at least for the light emitted by the light source 114), which is sealingly coupled to the sample cell 104 and is also configured to focus or collimate the light emitted by the light source 114 into the sample cell 104 (thus eliminating the need for the aforementioned window).

[0032] The light emitted by the light source 114 (also referred to herein as "measurement light") has one or more wavelengths in the range of 1.5 µm (or thereabouts) to 18 µm (or thereabouts). This wavelength range can be understood to occupy the mid-infrared (IR) range of the electromagnetic spectrum. The light source 114 is provided as a high-brightness mid-IR source to overcome the light attenuation caused by the mid-IR optical waveguide to achieve a high light flux at the detector, which will result in a high concentration measurement sensitivity. For a typical blackbody emitter (also known as a thermal light source), such as a globar (also known as a "glowbar" in this technology), the emitter temperature needs to be greater than 1500 °C.

[0033] In one particular example, the light source 114 is provided as a thermal light source (e.g., a blackbody emitter) operable to emit measurement light at a temperature higher than 1500 °C (e.g., greater than or equal to 1600 °C, 1700 °C, 1800 °C, etc., or between any of these values). The thermal light source may thus include one or more radiation elements formed of materials such as silicon, silicon carbide, nichrome, tungsten, ceramics, or the like or any combination thereof. However, in order to obtain a meaningful operating life of the thermal light source, the radiation element is hermetically sealed (e.g., to prevent or otherwise minimize the corrosion or oxidation of the radiation element).

[0034] In another specific example, the light source 114 is provided as one or more mid-IR light-emitting diodes (LEDs). Generally, when an LED operates in continuous-wave mode, the light emitted by the mid-IR LED is not as bright as that emitted by a thermal light source such as those thermal light sources described above. However, when operating in pulsed mode, there are some benefits to using an LED-based light source. When operating in pulsed mode, the LED can emit light at extremely high power, and if placed on a thermoelectric cooler, the output light level can be more stable than that of a well-known blackbody emitter. Additionally, compared to well-known blackbody emitters, LEDs consume less power and are more compact, making them more suitable for a more compact design. In this specific example, the light source 114 is provided as one or more mid-IR LEDs having an emission spectrum that covers the 1 to 8 µm wavelength range of the electromagnetic spectrum. Alternatively, the light source 114 can be provided as several mid-IR LEDs having different overlapping emission spectra within the mid-IR wavelength range of the electromagnetic spectrum. For example, the light source 114 can be provided as Figure 2 the light source 200 shown in Figure 2 , which includes a first LED 202 (e.g., emitting light within a first mid-IR wavelength range), a second LED 204 (e.g., emitting light within a second mid-IR wavelength range that overlaps with the first mid-IR wavelength range), and a beam splitter 206 (e.g., a dichroic beam splitter, a 50 / 50 beam splitter, etc.), which is configured to combine the light emissions output by the first LED 202 and the second LED 204.

[0035] In another example, the light source 114 can be provided as one or more LEDs (e.g., the aforementioned first LED 202 and second LED 204) mounted on a common substrate (e.g., a PCB) and configured to be close to each other such that they can emit light into a common delivery fiber (e.g., a multimode large core, a large NA fiber) in such a way that the light emitted by the LEDs is injected into the core of the delivery fiber. Optionally, one or more lenses or curved mirrors can be configured between the LEDs and the delivery fiber to focus the light emitted by the LEDs onto the core of the delivery fiber. In either case, depending on the spectral requirements of the sensor 100, the exact position of the core can be aligned to capture an equal amount of power from the LEDs, or the exact position of the core can be aligned to capture more light from one LED than from another. Since the core size of the delivery fiber limits the amount of light that can be coupled, the LEDs should be as close as possible. Therefore, it may be necessary to use LEDs that can emit light to the edges of their respective emission surfaces.

[0036] Returning to Figure 1, the photodetector 116 is optically coupled to the sample cell 104 via the optical waveguide assembly 122 for optical communication with the interior of the sample cell 104 (e.g., behind the first optical coupler 106). Generally, the first optical coupler 106 is configured to direct the measurement light emitted by the light source 114 and transmitted through the second optical coupler 108 and the gas within the sample cell 104 into the optical waveguide assembly 122. The first optical coupler 106 is sealingly coupled to the sample cell 104 to prevent the gas introduced into the sample cell 104 (e.g., via the inlet air flow port 110) from reaching the optical waveguide assembly 122. In one particular example, the first optical coupler 106 may include a window (e.g., substantially transparent at least for the measurement light) sealingly coupled to the sample cell 104 and a concave curved mirror or lens disposed between the window and the optical waveguide assembly 122 (e.g., configured to focus the measurement light transmitted through the window onto the first end of the optical waveguide assembly 122). In another particular example, the first optical coupler 106 may include a lens (e.g., substantially transparent at least for the measurement light) that is sealingly coupled to the sample cell 104 and is also configured to focus the measurement light transmitted through the sample cell 104 onto the first end of the optical waveguide assembly 122 (thus eliminating the need for the aforementioned window).

[0037] The optical waveguide assembly 122 may include a first optical waveguide 124 and a second optical waveguide 126, each configured to transmit the measurement light. The first optical waveguide 124 is optically coupled to the second optical waveguide 126 (e.g., by means of an optical connector 128). Thus, the first end of the first optical waveguide 124 (i.e., the aforementioned first end of the optical waveguide assembly 122) may be connected to the sample cell 104 (e.g., at a location behind the first optical coupler 106), and the first end of the second optical waveguide 126 may be connected to the detector 116. The second ends of each of the first optical waveguide 124 and the second optical waveguide 126 may be connected to the optical connector 128 such that the first optical waveguide 124 and the second optical waveguide 126 are in optical communication with each other. Generally, the optical connector 128 may be provided as any known or otherwise suitable connector configured to optically communicate the two waveguides when the first optical waveguide 124 and the second optical waveguide 126 are connected thereto. Additionally, the optical connector 128 is mounted to the housing 102 at an opening (not shown in the figure) in the housing.

[0038] The first optical waveguide 124 is disposed within the housing 102 and is provided to maintain suitable optical characteristics (e.g., maintain acceptably high transmission of the measured light) and mechanical characteristics (e.g., have a suitably high glass transition temperature) at elevated temperatures within the housing 102. Examples of waveguides that can be used as the first optical waveguide 124 include optical waveguides (e.g., optical fibers, fiber bundles, etc.) that include ZrF4 optical fibers (e.g., ZBLAN, etc.), indium fluoride (e.g., InF3) optical fibers, chalcogenide infrared (CIR) optical fibers (e.g., As2S3 core / AsS cladding), polycrystalline infrared (PIR) optical fibers (e.g., silver halide PIR optical fibers), hollow core optical fibers or the like or any combination thereof.

[0039] The second optical waveguide 126 can be provided as any optical waveguide (e.g., optical fiber, fiber bundle, etc.) that can suitably transmit the measured light at low temperatures (e.g., at the temperature in the ambient environment near the exterior of the housing 102). In one particular example, the second optical waveguide 126 has a higher light transmittance of the measured light than the first optical waveguide 124; however, the length of the first optical waveguide 124 is shorter than that of the second optical waveguide 126. Accordingly, the difference in light transmission loss between the first optical waveguide 124 and the second optical waveguide 126 can be minimized or otherwise reduced. Generally, the first optical waveguide 124 will only have a length necessary to connect between the sample cell 104 and the housing 102. Typically, the first optical waveguide 124 will have a length that is less than or equal to 60 cm (e.g., less than or equal to 55 cm, 40 cm, 30 cm, 20 cm, 10 cm, 5 cm, etc., or any value between these values). The second optical waveguide 126 can have a length greater than any of the foregoing lengths of the first optical waveguide 124 (e.g., can have a length up to one meter or longer).

[0040] Although the optical waveguide assembly 122 has been described above as including two optical waveguides, it should be understood that the optical waveguide assembly 122 can include more than two optical waveguides optically connected to each other, or can include a single optical waveguide (e.g., provided as described above with respect to the first optical waveguide 124).

[0041] Typically, detector 116 is operable to generate a detection signal and output the detection signal to analyzer 118 when the metrology light transmitted by optical waveguide assembly 122 impinges thereon (e.g., at its active region), where the detection signal represents the amount of light detected at detector 116. In a specific example where light source 114 operates in a pulsed mode (e.g., as described above), detector 116 may be provided as an InAsSb or MCT type detector. Additionally, in a specific example where light source 114 operates in a pulsed mode (e.g., as described above), detector 116 may operate synchronously with the pulsed mode operation of light source 114 (e.g., in response to one or more control signals output by controller 120) to optimize the SNR of the detection signal.

[0042] As Figure 3 Illustratively shown in, detector 116 may include a photodetector 300 and a filter assembly 302. Photodetector 300 is sensitive to mid-IR light and is operable to generate and output the detection signal as discussed above. Filter assembly 302 includes a plurality of optical filters, each of which is configured to transmit a different wavelength band and is disposed in front of the active region of photodetector 300 (i.e., the region of photodetector 300 that converts photons into an electric current). Thus, the metrology light transmitted by optical waveguide assembly 122 will be transmitted by the optical filters of filter assembly 302 before propagating to photodetector 300. In one specific example, filter assembly 302 may be provided as a filter assembly that is movable (e.g., rotatable, as indicated by arrow 304, etc.) relative to photodetector 300 to selectively dispose a single filter in front of the active region of photodetector 300. It should be understood that detector 116 may thus include a motor and appropriate mechanical linkages to effect the movement of filter assembly 306, as is known in the art. Although not shown, an optional focusing lens may be disposed between optical waveguide assembly 122 and filter assembly 302 and is configured to focus the metrology light exiting optical waveguide assembly 122 (e.g., to a position at or near the active region of filter assembly 302 or photodetector 300).

[0043] Figure 4 Illustrates an exemplary specific example of filter assembly 302. Refer to Figure 4, the filter assembly 302 can be provided as a filter wheel 400 having a wheel body 402 (e.g., which can include a shaft hole 401, etc.) and a plurality of windows defined therein. Optical filters are fixedly attached to each window, and the optical filters are configured to transmit a specific wavelength band. For example, the wheel body 402 is illustrated as including a first optical filter 404, a second optical filter 406, and a third optical filter 408, each fixedly attached to a respective window formed in the wheel body 402. The first optical filter 404 can be configured to transmit a first wavelength band, the second optical filter 406 can be configured to transmit a second wavelength band, and the third optical filter 408 can be configured to transmit a third wavelength band. The dashed circle 410 represents the amount of light transmitted by the optical waveguide assembly 122 and incident on the filter assembly 302 along an axis perpendicular to the optical filter (e.g., at an optical filter such as the first optical filter 404, as Figure 4 illustrated exemplarily in

[0044] Generally, the central wavelength of the first wavelength band is between the central wavelengths of the second wavelength band and the third wavelength band. Additionally, the first wavelength band does not overlap or abut the second wavelength band or the third wavelength band. See, for example, Figure 5 the graph 500 of radiant power versus wavelength that can be detected by the photodetector 300, where the first wavelength band (also referred to herein as the "signal band") is identified at 502, the second wavelength band (also referred to herein as the "first reference band") is identified at 504, and the third wavelength band (also referred to herein as the "second reference band") is identified at 506. However, in other specific examples, the first wavelength band 502 can abut the second wavelength band 504 and / or the third wavelength band 506.

[0045] As Figure 5 exemplarily illustrated in

[0046] , the first wavelength band 502 can cover a wavelength range from 3.42 µm (or thereabouts) to 3.65 µm (or thereabouts), the second wavelength band 504 can cover a wavelength range from 3.8 µm (or thereabouts) to 4.00 µm (or thereabouts), and the third wavelength band 506 can cover a wavelength range from 3.2 µm (or thereabouts) to 3.3 µm (or thereabouts). However, it should be understood that the ranges of the first, second, and third wavelength bands can be selected based on: the precursor gas to be monitored, any degradation by-products of one or more precursor gases that may be present within the sample cell 104, contaminants that may accumulate on the first optical coupler 106 or the second optical coupler 108, changes in the emission spectrum of the light source 114 that can be expected to occur, changes in the transmission spectrum of the optical waveguide assembly 122 that can be expected to occur, changes in the detection spectrum of the detector 116 that can be expected to occur, or the like or any combination thereof.The emission spectrum of the light source 114 can be altered due to one or more factors such as the temperature of the light source 114 (e.g., if the light source 114 is provided as a thermal light source), the aging of the light source 114 (e.g., if the light source 114 is provided as an LED), or the like or any combination thereof. The transmission spectrum of the optical waveguide assembly 122 can be altered due to one or more factors such as a change in temperature of one or more components of the optical waveguide assembly 122 (e.g., the first optical waveguide 124 and / or the second optical waveguide 126), movement of one or more components of the optical waveguide assembly 122 (e.g., flexure of the first optical waveguide 124 and / or the second optical waveguide 126), the presence of contaminants that can accumulate on the optical surfaces of the optical waveguide assembly 122, or the like or any combination thereof. The detection spectrum of the detector 116 can be altered due to one or more factors such as a change in temperature of the detector 116, the presence of contaminants that can accumulate on the active area of the detector 116, the aging of the detector 116, or the like or any combination thereof.

[0047] However, generally, the ranges of the first, second, and third wavelength bands should fall within the mid-IR range of the electromagnetic spectrum because most of the precursors used in the semiconductor industry have strong absorption in the range of 2 to 7 μm. The changes in the emission spectrum of the light source 114, the changes in the transmission spectrum of the optical waveguide assembly 122, and the changes in the detection spectrum of the detector 116 can generally and / or collectively be referred to as "baseline variations".

[0048] For example, Figure 6 FIG. 600 is a graph showing the absorption spectra of various materials at different wavelengths. Specifically, line 602 represents the absorption spectrum of an exemplary precursor gas that can be measured by the optical gas concentration sensor 100, and line 604 represents the absorption spectrum of degradation by-products and / or contaminants on the optical coupler of the sample cell 104. In this example, the signal band 502 is selected to cover the wavelength band in which the exemplary precursor gas exhibits strong absorption. The second reference band 506 is selected to cover the wavelength band in which the expected degradation by-products or contaminants exhibit strong absorption. The first reference band 504 is selected to cover the wavelength band in which baseline variations can be detected (e.g., in the region of the wavelength spectrum in which the exemplary precursor gas and the expected degradation by-products and contaminants do not exhibit strong wavelength absorption).

[0049] When the first optical filter 404 (i.e., the optical filter configured to transmit the signal band) is disposed in front of the active area of the photodetector 300, the detection signal generated by the detector 116 can be referred to as the "signal detection signal". Similarly, when the second optical filter 406 (i.e., the optical filter configured to transmit the first reference band) is disposed in front of the active area of the photodetector 300, the detection signal generated by the detector 116 can be referred to as the "first reference detection signal", and when the third optical filter 408 (i.e., the optical filter configured to transmit the second reference band) is disposed in front of the active area of the photodetector 300, the detection signal generated by the detector 116 can be referred to as the "second reference detection signal". Therefore, in order to generate and output the signal detection signal, the first reference detection signal, and the second reference detection signal, the filter wheel 400 of the detector 116 can be rotated (e.g., as discussed above) one revolution. It should be understood that the filter wheel 400 can be rotated multiple revolutions to repeatedly generate and output the aforementioned signals.

[0050] As an alternative to the specific example of the detector 116 discussed with respect to Figure 3 and Figure 4 a detector 116 can be provided as Figure 10 exemplarily shown in, and the detector includes a plurality of photodetectors (e.g., a first photodetector 1000a, a second photodetector 1000b, and a third photodetector 1000c, each generally referred to as the photodetector 1000) and corresponding filters of a plurality of filters disposed in front of the active area of the respective photodetectors 1000. For example, the aforementioned first optical filter 404, second optical filter 406, and third optical filter 408 can be disposed in front of the active areas of the first photodetector 1000a, the second photodetector 1000b, and the third photodetector 1000c, respectively. Although not shown, an optional focusing lens can be disposed between the optical waveguide assembly 122 and the detector 116 and configured to focus the measurement light emitted from the optical waveguide assembly 122 (e.g., to a position at or near the active area of the filter assembly 302 or the photodetector 300) in such a manner that the first optical filter 404, the second optical filter 406, and the third optical filter 408 are simultaneously illuminated by the measurement light. Constructed as described above, Figure 10 the detector 116 shown in can simultaneously generate the aforementioned signal detection signal, first reference detection signal, and second reference detection signal, and can output such signals to the analyzer 118.

[0051] Analyzer 118 is communicatively coupled to the output of detector 116 (e.g., receiving a detection signal output from detector 116). Generally, analyzer 118 is configured to determine or otherwise infer the concentration of the precursor gas received into gas sample cell 104 based on the detection signal output by detector 116. In one specific example, the concentration of the precursor gas can be determined or otherwise inferred by calculating the relative difference between the value encoded by the signal detection signal (i.e., the "signal band value") and the values encoded by the first and second reference detection signals (i.e., the "first reference band value" and the "second reference band value"). For example, the detection signal output from detector 116 can be regarded as a vector, where the different absorption spectra from different molecules in the precursor gas correspond to different absorption vectors; and spectroscopic tools such as classical least square (CLS), partial least square (PLS), deep learning, or the like can be used to distinguish and quantify the components of the precursor gas. In another specific example, the concentration C of the precursor gas in sample cell 104 can be determined or otherwise determined according to Equation [1] P : C P = - a1 * log (signal band value) + (a2 * log (first reference band value) + a3 * log (second reference band value)), where a1, a2, and a3 are calibration parameters.

[0052] Compared to the well-known technique of using only two optical filters to provide only two wavelength bands (i.e., the signal band and a single reference band), the advantage of using three optical filters each having three different wavelength bands can be understood by referring to FIG. 7 (illustrating an example of sensor drift that exists when only the signal band and a single reference band are used) and Figure 8 (illustrating an example of sensor drift that exists when the signal band and two reference bands are used). FIG. 7 and Figure 8Shows the change over time of the concentration of precursor gas PDMAT in a gas mixture over time (although the y-axis of these graphs indicates partial pressure, those of ordinary skill in the art will understand that the partial pressure of a particular gas in a gas mixture is proportional to the concentration of that particular gas in the mixture). In FIG. 7, the output of a well-known NDIR gas concentration sensor using only two wavelength bands (i.e., a signal band and a single reference band) changes over time because the evidence of the aforementioned baseline change cannot be accurately detected over time using the signal band and the single reference band. However, by employing sensor 100 having two reference bands, in accordance with the principles of the present invention, a baseline change can be detected (e.g., at analyzer 118) and the baseline change can be compensated for to eliminate or otherwise reduce the drift of the readings of sensor 100 over time (i.e., attributable to the baseline change), such as Figure 8 as shown in

[0053] Generally, analyzer 118 can detect a baseline change by: 1) processing the reference detection signals output by detector 116 to determine the measured relationship between the reference band values encoded by the output reference detection signals; and 2) comparing the measured relationship with a predetermined calibration relationship to determine the difference. If a difference exists (or if the difference is greater than a certain predetermined threshold value), then the baseline change can be considered to have been detected and analyzer 118 can compensate for (e.g., eliminate or otherwise reduce) the baseline change. In one specific example, analyzer 118 compensates for the baseline change by proportionally adjusting the first reference band value and the second reference band value (e.g., by tuning / varying the aforementioned calibration parameters a2 and a3) in a manner that minimizes or otherwise reduces the difference between the measured relationship and the calibration relationship, and then adjusts the aforementioned calibration parameter a1 accordingly using any suitable or known technique in the art.

[0054] For example, in the specific example discussed above, sensor 100 is configured to generate reference detection signals for two different reference bands. Thus, analyzer 118 can process the first reference detection signal and the second reference detection signal output by detector 116 (e.g., by performing a linear regression on the first reference band value and the second reference band value using any suitable technique known in the art) and compensate for any baseline change (e.g., by tuning / varying the aforementioned calibration parameters a2 and a3) in a manner that minimizes or otherwise reduces the difference between the measured relationship and the calibration relationship. Analyzer 118 can then adjust the aforementioned calibration parameter a1 accordingly using any suitable or known technique in the art.

[0055] Although the foregoing has described specific examples of three optical filters for determining the concentration of precursor gas within the sample cell 104, it should be understood that more than three filters (and thus, more than three wavelength bands) can be used to generate additional measurements, which will improve the robustness and accuracy available for determining the concentration of precursor gas. For example, using two reference bands as described above can be applied to remove or otherwise reduce baseline variations that are essentially linear (e.g., baseline variations that occur due to changes in emitter temperature). However, if the baseline variation is not linear (i.e., if there are higher order factors), then three or more reference bands can be used (e.g., such that higher order polynomial regression can be used to remove / reduce higher order baseline variations). Additionally, if the gas flow within the sample cell 104 contains multiple chemicals (e.g., precursor gas and degradation by-products of the precursor gas), and if these chemicals have overlapping absorption characteristics, then additional signal bands can be provided to separate the readings of each of these chemicals.

[0056] Typically, controller 120 includes one or more processors operable to generate control signals (e.g., after executing instructions or otherwise). The processors may be provided as one or more general-purpose computer processors, microprocessors, digital signal processors, or any other suitable form of circuitry, including programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), field-programmable object arrays (FPOAs), application-specific integrated circuits (ASICs) (including digital, analog, and mixed analog / digital circuitry), or the like or any combination thereof, each of which is operable to execute instructions or otherwise generate control signals. Execution of the instructions may be performed on one processor, distributed among multiple processors, parallel across processors within a device or across a network of devices, or the like or any combination thereof. The controllers described herein may include a tangible medium such as computer memory, which may be accessed by the processor (e.g., via one or more wired or wireless communication links). As used herein, computer memory (or more simply, "memory") includes magnetic media (e.g., magnetic tape, hard disk drive, etc.), optical disks, volatile or non-volatile semiconductor memory (e.g., RAM, ROM, NAND-type flash memory, NOR-type flash memory, SONOS memory, etc.), etc., and may be accessed locally, remotely (e.g., across a network), or in combination. Typically, the aforementioned instructions may be stored as computer software (e.g., executable code, files, instructions, etc., library files, etc.) that can be readily authorized by a person skilled in the art based on the description provided herein, which is written, for example, in C, C++, Visual Basic, Java, Python, Tel, Perl, Scheme, Ruby, assembly language, hardware description language (e.g., VHDL, VERILOG, etc.), etc. The computer software is typically stored in one or more data structures transmitted by the computer memory.

[0057] As described above, specific examples of the present invention offer many advantages over well-known gas concentration sensors. For example, the optical waveguide assembly 122 allows the sample cell 104 to be mechanically decoupled from the remainder of the sensor 100 (e.g., the detector 116, the analyzer 118, and the controller 120). As a result, the sample cell 104 can be compact and easily assembled within the housing 102 while the detector 116, the analyzer 118, and the controller 120 can be safely located outside the housing 102 (e.g., at room temperature). The use of a hermetically sealed high-temperature blackbody emitter or other high-brightness light source 114 and a high-speed detector 116 can optimize the SNR performance of the sensor 100. Spectroscopic techniques are used to improve the stability of the sensor 100; without such spectroscopic techniques, variations in the transmission spectrum through the optical fiber would significantly impair the accuracy and stability of the gas concentration measurement.

[0058] The foregoing illustrates specific examples and instances of the present invention and should not be construed as limiting thereof. Although several specific examples and instances have been described with reference to the drawings, those of ordinary skill in the art will readily appreciate that many modifications to the disclosed specific examples and instances, as well as other specific examples, are possible without significantly departing from the novel teachings and advantages of the present invention. For example, although the sensor 100 has been described above as having a single-pass sample cell configuration (i.e., where the measurement light traverses the length of the sample cell once), it should be understood that the sensor 100 can have a multi-pass sample cell configuration (i.e., where one or more mirrors are disposed within the sample cell 104 to increase the optical path length of the measurement light within the sample cell, as is known in the art). In another example, the optical filter of the detector 116 can be provided as a MEMS-based Fabre-Perot filter. In yet another example, the detector 116 can include a photodetector that configures the optical output of the optical waveguide assembly 122 (e.g., as described above), but an angle-tunable optical filter can be disposed within the sample cell 104 to filter the light emitted by the light source 114 (e.g., in the manner described in U.S. Patent No. 9,651,422, which is incorporated herein by reference in its entirety). In another example, although the detector 116 has been shown to be optically coupled to the interior of the sample cell 104 via the optical waveguide assembly 122, the optical waveguide assembly 122 can be omitted and the detector 116 can be directly mounted to the sample cell 104 for optical communication with its interior. Accordingly, all such modifications are intended to be included within the scope of the present invention as defined in the claims. For example, those of ordinary skill in the art will understand that the subject matter of any sentence, paragraph, example, or specific instance can be combined with some or all of the subject matter of other sentences, paragraphs, examples, or specific instances, unless such combinations are mutually exclusive. The scope of the present invention is thus to be determined by the following claims, with equivalents of such technical solutions being included within the scope of the present invention.

Claims

1. An optical gas concentration sensor, comprising: A sample cell, which includes an inlet port and an outlet port, wherein gas flows through the inlet port and the outlet port; A light source, which is configured to emit light into the interior of the sample cell; A photodetector, which is disposed outside the sample cell; And An optical waveguide assembly, which optically couples the interior of the sample cell to the photodetector, wherein the optical waveguide assembly includes: A first optical waveguide, which is coupled to the sample cell; A second optical waveguide, which is coupled to the photodetector; and An optical coupler, which optically couples the first optical waveguide to the second optical waveguide, Wherein the first optical waveguide has different light transmission characteristics from the second optical waveguide at wavelengths within the wavelength range of 1.5 μm to 18 μm.

2. The optical gas concentration sensor according to claim 1, further comprising a thermal insulation housing, wherein the sample cell is disposed within the thermal insulation housing.

3. The optical gas concentration sensor according to claim 2, wherein the optical coupler is connected to the thermal insulation housing.

4. The optical gas concentration sensor according to claim 1, wherein the length of the first optical waveguide is less than the length of the second optical waveguide.

5. The optical gas concentration sensor according to claim 1, wherein the first optical waveguide includes at least one optical fiber.

6. The optical gas concentration sensor according to claim 1, wherein the first optical waveguide includes a material selected from the group consisting of ZrF4, InF3, chalcogenide materials, and silver halide materials.

7. The optical gas concentration sensor according to claim 1, wherein the photodetector includes: At least one photodetector; And A plurality of optical filters, which are configured and arranged to transmit different wavelength bands within the IR range in the electromagnetic spectrum to the at least one photodetector.

8. The optical gas concentration sensor according to claim 7, wherein the at least one photodetector includes a single photodetector.

9. The optical gas concentration sensor according to claim 7, wherein the at least one photodetector includes a plurality of photodetectors.

10. The optical gas concentration sensor according to claim 9, wherein different optical filters among the plurality of optical filters are disposed at different photodetectors among the plurality of photodetectors.

11. The optical gas concentration sensor according to claim 7, wherein the plurality of optical filters includes: A first optical filter, which is configured to transmit a first wavelength band within the wavelength range of the electromagnetic spectrum; A second optical filter, which is configured to transmit a second wavelength band within the wavelength range of the electromagnetic spectrum; And A third optical filter, which is configured to transmit a third wavelength band within the wavelength range of the electromagnetic spectrum, Wherein the first wavelength band does not overlap with the third wavelength band.

12. The optical gas concentration sensor according to claim 11, wherein the second wavelength band does not overlap with the first wavelength band or the third wavelength band.

13. The optical gas concentration sensor according to claim 11, wherein the second wavelength band is between the first wavelength band and the third wavelength band.

14. The optical gas concentration sensor according to claim 11, wherein wavelengths in the second wavelength band are more easily absorbed by the gas than wavelengths in the first wavelength band or the third wavelength band.

15. The optical gas concentration sensor according to claim 14, wherein wavelengths in the first wavelength band are more easily absorbed by by-products of the gas or contaminants within the sample cell than wavelengths in the second wavelength band or the third wavelength band.

16. The optical gas concentration sensor according to claim 15, wherein wavelengths in the third wavelength band are less easily absorbed by the gas, by-products of the gas, or contaminants within the sample cell than wavelengths in the first wavelength band or the second wavelength band.

17. The optical gas concentration sensor according to claim 11, further comprising an analyzer coupled to the output of the photodetector and configured to determine the concentration of the gas in the sample cell based on a detection signal output by the at least one photodetector in response to light transmitted by the first optical filter, the second optical filter, and the third optical filter.

18. The optical gas concentration sensor according to claim 1, further comprising an analyzer coupled to the output of the photodetector and configured to determine the concentration of the gas in the sample cell.

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