TW202424481A - Gas concentration sensor and method of using the same - Google Patents

Abstract

An optical gas concentration sensor includes 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, a light detector arranged outside the sample cell, 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 different optical transmission characteristics from the second optical waveguide at wavelengths in a wavelength range from 1.5 μm to 18 μm.

Description

Gas concentration sensor and method of use thereof

Embodiments of the present invention relate generally to gas concentration measurement, and more particularly, to the measurement of the concentration of a precursor in a carrier gas.

In the semiconductor manufacturing process, various liquid or solid materials are vaporized by heating to form material gases, and then these material gases are introduced into a vacuum chamber, such as during chemical vapor deposition (CVD) or atomic layer deposition (ALD) processes. Some of these chemical processes use optical gas concentration measurement sensors, such as nondispersive infrared (NDIR) gas concentration sensors, to monitor gas concentration.

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 light transmitted from the IR light source to the IR detector due to absorption by the sample gas can be used to infer the gas concentration in the gas sample cell.

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

Additionally, conventional NDIR gas concentration sensors (and other conventional 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 the degradation byproducts cannot be distinguished using such conventional NDIR or binary gas sensors.

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

One embodiment of the present invention can be broadly characterized as an optical gas concentration sensor, comprising: a sample cell including 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 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 different light transmission characteristics than the second optical waveguide at a wavelength in a wavelength range of 1.5 μm to 18 μm.

The present invention is described with reference to the accompanying drawings. Unless otherwise expressly stated, the size, position, etc. of components, features, elements, etc. and any distance between components, features, elements, etc. in the drawings are not necessarily to scale, but are exaggerated for clarity.

The terms used herein are for the purpose of describing specific examples only and are not intended to be limiting. As used herein, the singular forms "a/an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should be recognized that the term "comprises and/or comprising" when used in this specification specifies the presence of stated features, integers, steps, operations, elements and/or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. Unless otherwise specified, when describing a range of values, the range of values 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 only used to distinguish one element from another. For example, one node may be referred to as a "first node," and similarly, another node may be referred to as a "second node," or vice versa. The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Unless otherwise indicated, the terms "about," "around," and the like mean that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as necessary, reflecting tolerances, conversion factors, rounding, measurement errors, and the like, as well as other factors known to those of ordinary skill in the art.

Spatially relative terms, such as "below," "under," "lower," "above," and "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 depicted in the figures. It should be recognized that spatially relative terms are intended to encompass different orientations in addition to the orientation depicted in the figures. For example, if the object in the figure is turned over, an element described as being "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term "below" may encompass both orientations of above and below. Objects may be oriented in other ways (rotated 90 degrees or in other orientations) and the spatially relative descriptors used herein may be interpreted accordingly.

Like numbers refer to like elements throughout the text. Therefore, the same or similar numbers may be described when referring to other figures, even if such numbers are not mentioned or described in the corresponding figures. Also, even if the elements are not indicated by reference numbers, they may be described with reference to other figures.

It should be understood that many different forms and embodiments are possible without departing from the spirit and teachings of the present disclosure, and therefore the present disclosure should not be interpreted as limited to the exemplary embodiments described herein. Rather, these examples and embodiments are provided so that the present disclosure will be thorough and complete and will convey the scope of the present disclosure to those of ordinary skill in the art.

FIG. 1 is a schematic diagram of an optical gas concentration sensor according to some specific examples of the present invention.

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

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

The housing 102 itself may be disposed at any suitable or desired location along a line for delivering precursor gas to a chamber in which CVD or ALD is to be performed. For example, FIG. 9 schematically illustrates that the housing 102 may be disposed within a gas cabinet 900 in which a precursor ampoule 902 is located. The precursor ampoule 902 may be provided in any suitable manner known in the art (e.g., the precursor ampoule 902 may be coupled to a carrier gas input line 904 and a precursor gas outlet line 906). In this case, the housing 102 may be configured such that the gas sample cell 104 is in fluid communication with the precursor gas profile line 906 via the aforementioned inlet gas flow port 110 and the outlet gas flow port 112 .

Additionally or alternatively, the housing 102 may be located outside the gas cabinet 900, at a location upstream of the precursor gas supply switch 908 (e.g., in fluid communication with the precursor gas outlet 906). In this case, the housing 102 may be configured so that the gas sample cell 104 is in fluid communication with the precursor gas profile line 906 via the aforementioned inlet gas flow port 110 and outlet gas flow port 112.

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

1 , the sensor 100 may also include a light source 114, a light detector 116, an analyzer 118, and a controller 120. The housing 102 is constructed of one or more thermally insulating materials to prevent heat from being transferred from the interior of the housing 102 to areas outside the housing 102 (e.g., areas where the light detector 116, the analyzer 118, and the controller 120 are located).

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

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

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 so) to 18 µm (or so). This wavelength range can be understood as occupying 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 high light flux at the detector, which will result in high concentration measurement sensitivity. For a typical black body emitter (also called a thermal light source), such as a globar (also called a "glowbar" in this art), the emitter temperature needs to be greater than 1500°C.

In one specific example, the light source 114 is provided as a thermal light source (e.g., a black body emitter) operable to emit measurement light at a temperature greater 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 therefore include one or more radiation elements formed from materials such as silicon, silicon carbide, chromium nickel alloys, 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(s) are hermetically sealed (e.g., to prevent or otherwise minimize corrosion or oxidation of the radiation element(s)).

In another specific example, the light source 114 is provided as one or more mid-IR light emitting diodes (LEDs). Typically, when the LEDs are operated in a continuous wave mode, the light emitted by the mid-IR LEDs is not as bright as the light emitted by thermal light sources such as those described above. However, when operated in a pulsed mode, there are some benefits to using an LED-based light source. When operated in a pulsed mode, LEDs can emit light at very high powers, and if placed on a thermoelectric cooler, the output light level can be more stable than a conventional black body emitter. Additionally, LEDs consume less power and are more compact than conventional black body emitters, making them more suitable for more compact designs. In this specific example, the light source 114 is provided as one or more mid-IR LEDs having an emission spectrum covering the 1 to 8 μm wavelength range of the electromagnetic spectrum. Alternatively, the light source 114 may be provided as a plurality of mid-IR LEDs having different overlapping emission spectra in the mid-IR wavelength range of the electromagnetic spectrum. For example, the light source 114 can be provided as the light source 200 shown in Figure 2, which includes a first LED 202 (e.g., emitting light in a first mid-IR wavelength range), a second LED 204 (e.g., emitting light in 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.) that is configured to combine the light emissions output by the first LED 202 and the second LED 204.

In another example, the light source 114 may 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 arranged in close proximity to one another so that they can emit light into a common delivery fiber (e.g., a multi-mode large core, large NA fiber) in such a manner that the light emitted by the LED is injected into the core of the delivery fiber. Optionally, one or more lenses or curved mirrors may be arranged between the LED and the delivery fiber to focus the light emitted by the LED onto the core of the delivery fiber. In either case, depending on the spectral needs of the sensor 100, the precise position of the cores may be aligned to capture equal amounts of power from the LEDs, or the precise position of the cores may be aligned to capture more light from one LED than another. Because 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 desirable to use LEDs that are able to emit light to the edge of their respective emitting surfaces.

1 , the photodetector 116 is optically coupled to the sample cell 104 via the optical waveguide assembly 122 so as to optically communicate with the interior of the sample cell 104 (e.g., behind the first optical coupler 106). Typically, the first optical coupler 106 is configured to guide 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, thereby preventing the gas introduced into the sample cell 104 (e.g., through the inlet gas flow port 110) from reaching the optical waveguide assembly 122. In one specific example, the first optical coupler 106 may include a window (e.g., substantially transparent to at least the measurement light) hermetically 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 specific example, the first optical coupler 106 may include a lens (e.g., substantially transparent to at least the measurement light) hermetically coupled to the sample cell 104 and also configured to focus the measurement light transmitted through the sample cell 104 onto the first end of the optical waveguide assembly 122 (thereby eliminating the need for the aforementioned window).

The optical waveguide assembly 122 may include a first optical waveguide 124 and a second optical waveguide 126, each of which is configured to transmit 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). Therefore, 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 position 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 end of each of the first optical waveguide 124 and the second optical waveguide 126 may be connected to the optical connector 128 in such a manner that the first optical waveguide 124 and the second optical waveguide 126 are in optical communication with each other. Generally, the optical connector 128 can be provided as any known or otherwise suitable connector, which is configured to allow the first optical waveguide 124 and the second optical waveguide 126 to optically communicate with each other when connected thereto. In addition, the optical connector 128 is mounted to the housing 102 at an opening (not shown) in the housing.

The first optical waveguide 124 is disposed in the housing 102 and is provided to maintain suitable optical properties (e.g., maintain an acceptably high transmission of the measurement light) and mechanical properties (e.g., have a suitably high glass transition temperature) at high temperatures in the housing 102. Examples of waveguides that can be used as the first optical waveguide 124 include optical waveguides (e.g., optical fibers, optical fiber bundles, etc.) including ZrF ₄ optical fibers (e.g., ZBLAN, etc.), indium fluoride (e.g., InF ₃ ) optical fibers, chalcogenide infrared (CIR) optical fibers (e.g., As ₂ S ₃ 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.

The second optical waveguide 126 may be provided as any optical waveguide (e.g., an optical fiber, an optical fiber bundle, etc.) that is capable of properly transmitting the measurement light at low temperatures (e.g., at temperatures in the ambient environment near the exterior of the housing 102). In one specific example, the second optical waveguide 126 has a higher light transmittance of the measurement light than the first optical waveguide 124; however, the length of the first optical waveguide 124 is shorter than the second optical waveguide 126. Therefore, the difference in light transmission loss between the first optical waveguide 124 and the second optical waveguide 126 may be minimized or otherwise reduced. Typically, the first optical waveguide 124 will have only the length necessary to connect between the sample cell 104 and the housing 102. Typically, the first optical waveguide 124 will have a length 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 of these values). The second optical waveguide 126 can have a length greater than any of the aforementioned lengths of the first optical waveguide 124 (e.g., can have a length of up to one meter or more).

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 may include more than two optical waveguides optically connected to each other, or may include a single optical waveguide (eg, as provided above with respect to the first optical waveguide 124).

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

As exemplarily shown in FIG. 3 , the detector 116 may include a photodetector 300 and a filter assembly 302. The photodetector 300 is sensitive to mid-IR light and is operable to generate and output a detection signal as discussed above. The 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 the photodetector 300 (i.e., the region of the photodetector 300 that converts photons into current). Therefore, the measurement light transmitted by the optical waveguide assembly 122 will be transmitted by the optical filters of the filter assembly 302 before propagating to the photodetector 300. In one specific example, the filter assembly 302 can be provided as a filter assembly that is movable (e.g., rotatable, as indicated by arrow 304, etc.) relative to the photodetector 300 so as to selectively position a single filter in front of the active region of the photodetector 300. It should be understood that the detector 116 can therefore include a motor and appropriate mechanical linkage to achieve movement of the filter assembly 306, as is known in the art. Although not shown, an optional focusing lens can be disposed between the optical waveguide assembly 122 and the filter assembly 302 and configured to focus the measurement light emitted from the optical waveguide assembly 122 (e.g., to a position at or near the active region of the filter assembly 302 or the photodetector 300).

FIG. 4 illustrates an example embodiment of a filter assembly 302. Referring to FIG. 4, the filter assembly 302 may be provided as a filter wheel 400 having a wheel body 402 (e.g., which may include an axle hole 401, etc.) and a plurality of windows defined therein. An optical filter is affixed to each window, the optical filter being 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 of which is affixed to a respective window formed in the wheel body 402. The first optical filter 404 may be configured to transmit a first wavelength band, the second optical filter 406 may be configured to transmit a second wavelength band, and the third optical filter 408 may be configured to transmit a third wavelength band. The dashed circle 410 represents the measurement 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 exemplarily shown in FIG. 4).

Typically, the center wavelength of the first wavelength band is between the center wavelength of the second wavelength band and the center wavelength of the third wavelength band. In addition, the first wavelength band does not overlap or abut the second wavelength band or the third wavelength band. See, for example, FIG. 5 , a graph 500 of emissivity versus wavelength that can be detected by the detector 300, wherein 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 be adjacent to the second wavelength band 504 and/or the third wavelength band 506.

As exemplarily shown in FIG. 5 , the first wavelength band 502 may cover a wavelength range from 3.42 µm (or above) to 3.65 µm (or above), the second wavelength band 504 may cover a wavelength range from 3.8 µm (or above) to 4.00 µm (or above), and the third wavelength band 506 may cover a wavelength range from 3.2 µm (or above) to 3.3 µm (or above). However, it should be understood that the ranges of the first, second, and third wavelength bands may be selected based on the precursor gas to be monitored, any degradation byproducts of one or more precursor gases that may be present in the sample cell 104, contaminants that may accumulate on the first optical coupler 106 or the second optical coupler 108, expected changes in the emission spectrum of the light source 114, expected changes in the transmission spectrum of the optical waveguide assembly 122, expected changes in the detection spectrum of the detector 116, or the like or any combination thereof.

The emission spectrum of the light source 114 may change due to one or more of the following factors: the temperature of the light source 114 (e.g., if the light source 114 is provided as a thermal light source), 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 light guide assembly 122 may change due to one or more of the following factors: temperature changes of one or more components of the light guide assembly 122 (e.g., the first light guide 124 and/or the second light guide 126), movement of one or more components of the light guide assembly 122 (e.g., bending of the first light guide 124 and/or the second light guide 126), the presence of contaminants that may accumulate on optical surfaces of the light guide assembly 122, or the like, or any combination thereof. The detected spectrum of the detector 116 may change due to one or more of the following factors: changes in the temperature of the detector 116, the presence of contaminants that may accumulate on the active area of the detector 116, aging of the detector 116, or the like, or any combination thereof.

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

For example, FIG6 shows a graph 600 of 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 byproducts and/or contaminants at 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 degradation byproducts or contaminants are expected to exhibit strong absorption. The first reference band 504 is selected to cover a wavelength band in which baseline variations can be detected (e.g., in a region of the wavelength spectrum in which exemplary precursor gases and expected degradation byproducts and contaminants do not exhibit strong wavelength absorption).

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

As an alternative to the specific example of the detector 116 discussed with respect to FIGS. 3 and 4 , a detector 116 may be provided as exemplarily shown in FIG. 10 , 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 a photodetector 1000) and corresponding filters of a plurality of filters disposed in front of an active region of each photodetector 1000. For example, the first optical filter 404, the second optical filter 406, and the third optical filter 408 may be disposed in front of the active regions of the first photodetector 1000a, the second photodetector 1000b, and the third photodetector 1000c, respectively. Although not shown, a focusing lens may 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 region 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 illuminated by the measurement light simultaneously. Constructed as described above, the detector 116 shown in FIG. 10 is capable of simultaneously generating the aforementioned signal detection signal, the first reference detection signal, and the second reference detection signal, and can output such signals to the analyzer 118.

The analyzer 118 is communicatively coupled to the output of the detector 116 (e.g., receives the detection signal output by the detector 116). Generally, the analyzer 118 is configured to determine or otherwise infer the concentration of the precursor gas received into the gas sample cell 104 based on the detection signal output by the detector 116. In one specific example, the concentration of the precursor gas may 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 the detector 116 can be viewed as a vector, where 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 composition of the precursor gas. In another specific example, the concentration C _P of the precursor gas in the sample cell 104 can be determined or otherwise determined according to the following formula [1]: 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.

The advantages of using three optical filters, each having three different wavelength bands, as compared to the known art of using only two optical filters to provide only two wavelength bands (i.e., a signal band and a single reference band) can be understood by referring to Figure 7 (showing an example of sensor drift that exists when only a signal band and a single reference band are used) and Figure 8 (showing an example of sensor drift that exists when a signal band and two reference bands are used). 7 and 8 show the change in the concentration of the preceding driver gas PDMAT in a gas mixture over time (although the y-axis of these graphs indicates partial pressure, one 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 conventional NDIR gas concentration sensor using only two wavelength bands (i.e., a signal band and a single reference band) changes over time because the use of the signal band and a single reference band cannot accurately detect the aforementioned evidence of baseline change over time. However, by employing a sensor 100 having two reference bands, in accordance with the principles of the present invention, baseline changes can be detected (e.g., at the analyzer 118) and compensated for to eliminate or otherwise reduce drift in the readings of the sensor 100 over time (i.e., attributable to baseline changes), as shown in FIG. 8 .

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

For example, in the specific example discussed above, the sensor 100 is configured to generate reference detection signals for two different reference bands. Therefore, the analyzer 118 can process the first reference detection signal and the second reference detection signal output by the detector 116 (e.g., by performing 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 changes in a manner that minimizes or otherwise reduces the difference between the measured relationship and the calibrated relationship (e.g., by tuning/changing the aforementioned calibration parameters a2 and a3, respectively). The analyzer 118 can then adjust the aforementioned calibration parameter a1 accordingly using any technique suitable or known in the art.

Although the specific example of three optical filters being used to determine the concentration of the pre-driver gas within the sample cell 104 has been discussed above, it should be understood that more than three filters (and therefore, more than three wavelength bands) can be used to produce more measurements, which will improve the stability and accuracy with which the concentration of the pre-driver gas can be determined. For example, using two reference bands as described above can be suitable for removing or otherwise reducing baseline variations that are linear in nature (e.g., baseline variations that occur due to changes in emitter temperature). However, if the baseline variation is not linear (i.e., if higher-order factors are present), three or more reference bands can be used (e.g., so that a high-order polynomial regression can be used to remove/reduce the high-order baseline variations). Additionally, if the gas flow within the sample cell 104 contains multiple chemical species (e.g., precursor gas and degradation byproducts of the precursor gas), and if these chemicals have overlapping absorption characteristics, more signal bands may be provided to separate the readings of each of these chemicals.

Typically, the controller 120 includes one or more processors operable to generate control signals (e.g., after executing instructions or otherwise). The processor may be provided as one or more general purpose computer processors, microprocessors, digital signal processors, or any other suitable form of circuit system, including a programmable logic device (PLD), a field-programmable gate array (FPGA), a field-programmable object array (FPOA), an application-specific integrated circuit (ASIC) (including digital, analog, and hybrid analog/digital circuit systems), or the like or any combination thereof, each of which is operable to execute instructions or otherwise generate control signals. The execution of instructions may be performed on one processor, distributed among multiple processors, performed in 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 tangible media such as computer memory that 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., tapes, hard drives, etc.), optical disks, volatile or non-volatile semiconductor memories (e.g., RAM, ROM, NAND flash memory, NOR flash memory, SONOS memory, etc.), etc., and can be accessed locally, remotely (e.g., across a network), or a combination thereof. Typically, the aforementioned instructions can be stored as computer software (e.g., executable code, files, instructions, etc., library files, etc.) that can be easily authorized by a technician according to the description provided herein, which is written in, for example, C, C++, Visual Basic, Java, Python, Tel, Perl, Scheme, Ruby, assembly language, hardware description language (e.g., VHDL, VERILOG, etc.), etc. Computer software is typically stored in one or more data structures that are transferred from the computer's memory.

As described above, embodiments of the present invention provide numerous advantages over conventional gas concentration sensors. For example, the optical waveguide assembly 122 allows the sample cell 104 to be mechanically decoupled from the rest 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 black body 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 spectroscopy techniques, variations in the spectrum of light transmitted through the optical fiber would significantly degrade the accuracy and stability of the gas concentration measurement.

The foregoing describes specific embodiments and examples of the present invention and should not be interpreted as limiting thereof. Although several specific embodiments and examples have been described with reference to the drawings, it should be readily apparent to those skilled in the art that many modifications to the disclosed embodiments and examples and other embodiments 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 a single time), it should be understood that the sensor 100 may 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 may be provided as a MEMS-based Fabre-Perot filter. In yet another example, the detector 116 may include an optical detector configured to output the optical waveguide assembly 122 (e.g., as described above), but an angle-tunable optical filter may be configured 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 as being optically coupled to the interior of the sample cell 104 via the optical waveguide assembly 122, the optical waveguide assembly 122 may be omitted and the detector 116 may be mounted directly to the sample cell 104 so as to be in optical communication with the interior thereof. Therefore, all such modifications are intended to be included within the scope of the present invention as defined in the scope of the claims. For example, a person with ordinary knowledge in the art should understand that the subject matter of any sentence, paragraph, example or specific example can be combined with some or all of the subject matter in other sentences, paragraphs, examples or specific examples, unless such combinations are mutually exclusive. The scope of the present invention should therefore be determined by the scope of the following patent application, wherein the equivalents of such technical solutions are included in the scope of the present invention.

100: sensor 102: thermally insulating housing 104: gas sample cell 106: first optical coupler 108: second optical coupler 110: inlet airflow port 112: outlet airflow port 114: light source 115: link 116: optical detector 118: analyzer 120: controller 122: optical waveguide assembly 124: first optical waveguide 126: second optical waveguide 128: optical connector 200: light source 202: first LED 204: second LED 206: beam splitter 300: optical detector 302: filter assembly 304: arrow 400: filter wheel 401: Axis hole 402: Wheel body 404: First optical filter 406: Second optical filter 408: Third optical filter 410: Dashed circle 502: First wavelength band/signal band 504: Second wavelength band/First reference band 506: Third wavelength band/Second reference band 600: Curve graph 602: Line 604: Line 900: Gas cabinet 902: Propeller ampoule 904: Carrier gas inlet pipeline 906: Propeller gas outlet pipeline/Propeller gas profile pipeline/Propeller gas outlet 908: Propeller gas supply switch 912: Front drive gas supply pipeline 1000a: First photodetector 1000b: Second photodetector 1000c: Third photodetector

[FIG. 1] shows a schematic diagram of an optical gas concentration sensor according to some specific examples of the present invention. [FIG. 2] shows a schematic diagram of the light source shown in FIG. 1 according to one specific example of the present invention. [FIG. 3] and [FIG. 10] show schematic diagrams of the detector shown in FIG. 1 according to some specific examples of the present invention. [FIG. 4] shows a schematic diagram of the filter assembly shown in FIG. 3 according to one specific example of the present invention. [FIG. 5] shows a graph of the emissivity detectable by the detector shown in FIG. 3 versus wavelength. [FIG. 6] shows a graph of the absorption spectra of different materials at different wavelengths. [FIG. 7] shows a graph showing the output of a known NDIR gas concentration sensor as a function of time. [FIG. 8] shows a graph representing the output of the gas concentration sensor shown in FIG. 1 as a function of time. [FIG. 9] shows a schematic diagram of a system that can be incorporated into the housing shown in FIG. 1 according to some specific examples of the present invention.

100:Sensor

102: Thermal insulation housing

104: Gas sample cell

106: First optical coupler

108: Second optical coupler

110: Inlet air flow opening

112: Exit air flow opening

114: Light source

115: Link

116: Photodetector

118:Analyzer

120: Controller

122: Optical waveguide assembly

124: First optical waveguide

126: Second optical waveguide

128: Optical connector

Claims (18)

An optical gas concentration sensor includes: a sample cell including 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 optically coupling an interior of the sample cell to the detector, wherein 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, wherein the first optical waveguide has different light transmission characteristics from the second optical waveguide at a wavelength in a wavelength range of 1.5 μm to 18 μm. The optical gas concentration sensor of claim 1 further comprises a thermally insulating housing, wherein the sample cell is configured within the thermally insulating housing. An optical gas concentration sensor as claimed in claim 2, wherein the optical coupler is connected to the thermally insulating housing. An optical gas concentration sensor as claimed in claim 1, wherein a length of the first optical waveguide is less than a length of the second optical waveguide. An optical gas concentration sensor as claimed in claim 1, wherein the first optical waveguide comprises at least one optical fiber. The optical gas concentration sensor of claim 1, wherein the first optical waveguide comprises a material selected from the group consisting of ZrF ₄ , InF ₃ , a chalcogenide material, and a silver halide material. An optical gas concentration sensor as claimed in claim 1, wherein the photodetector comprises: At least one photodetector; and A plurality of optical filters configured and arranged to transmit different wavelength bands within the IR range of the electromagnetic spectrum to the at least one photodetector. An optical gas concentration sensor as claimed in claim 7, wherein the at least one photodetector comprises a single photodetector. An optical gas concentration sensor as claimed in claim 7, wherein the at least one photodetector comprises a plurality of photodetectors. An optical gas concentration sensor as claimed in claim 9, wherein different optical filters of the plurality of optical filters are arranged at different photodetectors of the plurality of photodetectors. An optical gas concentration sensor as claimed in claim 7, wherein the plurality of optical filters include: a first optical filter configured to transmit a first wavelength band within the wavelength range of the electromagnetic spectrum; a second optical filter configured to transmit a second wavelength band within the wavelength range of the electromagnetic spectrum; and a third optical filter 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. An optical gas concentration sensor as claimed in claim 11, wherein the second wavelength band does not overlap with the first wavelength band or the third wavelength band. An optical gas concentration sensor as claimed in claim 11, wherein the second wavelength band is between the first wavelength band and the third wavelength band. An optical gas concentration sensor as claimed in claim 11, wherein the wavelengths in the second wavelength band are more easily absorbed by the gas than the wavelengths in the first wavelength band or the third wavelength band. An optical gas concentration sensor as claimed in claim 14, wherein wavelengths in the first wavelength band are more easily absorbed by byproducts of the gas or contaminants within the sample cell than wavelengths in the second wavelength band or the third wavelength band. An optical gas concentration sensor as claimed in claim 15, wherein the wavelengths in the third wavelength band are less easily absorbed by the gas, byproducts of the gas, or contaminants within the sample cell than the wavelengths in the first wavelength band or the second wavelength band. An optical gas concentration sensor as claimed in claim 11, further comprising an analyzer coupled to an output of the detector and configured to determine a concentration of the gas in the sample cell based on a detection signal output by the at least one optical detector in response to receiving light transmitted by the first optical filter, the second optical filter and the third optical filter. The optical gas concentration sensor of claim 1, further comprising an analyzer coupled to an output of the detector and configured to determine a concentration of the gas in the sample cell.

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