US20050061969A1 - Mems-based spectrophotometric system - Google Patents
Mems-based spectrophotometric system Download PDFInfo
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- US20050061969A1 US20050061969A1 US10/667,937 US66793703A US2005061969A1 US 20050061969 A1 US20050061969 A1 US 20050061969A1 US 66793703 A US66793703 A US 66793703A US 2005061969 A1 US2005061969 A1 US 2005061969A1
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Images
Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/02—Details
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/02—Details
- G01J3/0202—Mechanical elements; Supports for optical elements
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/02—Details
- G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
- G01J3/0229—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using masks, aperture plates, spatial light modulators or spatial filters, e.g. reflective filters
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
- G01J3/433—Modulation spectrometry; Derivative spectrometry
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
Definitions
- the present invention relates to sensors and, more specifically, to optical gas detectors.
- Gas analyzers are used in environmental monitoring, industrial process control, geologic exploration, and medical, analytical, and military applications.
- gases such as hydrogen sulfide and sulfur dioxide accompany natural gas and are often present in industrial environments. Since exposure to these gases is a health hazard, low concentrations, typically in the range of parts per million (ppm), need to be detected.
- optical spectrophotometry is based on the absorption of electromagnetic radiation by the sample in a selected spectral range and is generally recognized to have relatively high sensitivity and speed of analysis.
- a spectrophotometric system typically has (i) a light source; (ii) a monochromator, which filters the light from the light source so that only a limited wavelength range is allowed to irradiate the gas sample contained in a sample cell; and (iii) a photo-detector, which measures the amount of light transmitted through the sample.
- a system of the invention has an optical grating, an array of photo-detectors, and a MEMS device having a movable plate positioned between the grating and the array. Light transmitted through a gaseous sample is dispersed by the grating and is imaged onto the movable plate, which has a plurality of openings corresponding to selected absorption lines of the target substance.
- a small-amplitude oscillation is imparted onto the plate such that the openings periodically move in and out of alignment with the corresponding intensity features in the image, which modulates electrical signals generated by the corresponding photo-detectors.
- a lock-in signal processor analyzes the modulation pattern by comparing it to the pattern expected in the presence of the target substance. When a positive correlation between the patterns is established, the system warns the user about the presence of the target substance.
- FIG. 1 shows a schematic diagram of a spectrophotometric system according to one embodiment of the present invention
- FIG. 4 shows a top view of a MEMS device that can be used in the system of FIG. 1 according to one embodiment of the present invention.
- FIG. 1 shows a schematic diagram of a spectrophotometric system 100 according to one embodiment of the present invention.
- a beam of light generated by a broadband light source 102 is directed through a cell 104 containing a sample to be analyzed.
- Light transmitted through cell 104 is dispersed in wavelength by a grating 106 and is imaged by a cylindrical lens 108 onto a movable plate 110 .
- the image produced at the surface of plate 110 is a stripe of light, in which different locations along the length of the stripe correspond to different wavelengths.
- Plate 110 has a plurality of openings 112 and is preferably a part of a MEMS device 120 , which also includes a support structure 122 and one or more springs 124 connected between the support structure and the plate.
- Device 120 further includes an electrostatic actuator (not shown) adapted to controllably move plate 110 with respect to structure 122 as indicated by the double-headed arrow in FIG. 1 .
- Plate 110 allows light to pass through openings 112 and substantially blocks the light impinging onto other portions of the plate.
- An array of photo-detectors 130 located behind plate 110 converts the transmitted light into electrical signals, which are then applied to a signal processor 140 .
- IR absorption of gases is typically characterized by the presence of relatively narrow absorption lines, each of which usually corresponds to a particular rotational and/or vibrational mode of the gas molecule.
- Each of these absorption lines produces a corresponding intensity feature in the image at plate 110 .
- a relatively strong absorption line will produce a corresponding black (i.e., having substantially no light) area at plate 110 while the areas adjacent to the black area will have a relatively high light intensity.
- Position of each opening 112 within plate 110 is chosen such that it can be aligned with the intensity feature corresponding to a selected absorption line of the target gas to be detected. It is also preferred that, for each target gas, plate 110 has two or more openings 112 .
- plate 110 may have one or more openings 112 corresponding to selected strong absorption lines of air, which may be used for calibration purposes as further detailed below.
- system 100 operates as follows. First, device 120 is wavelength calibrated. During calibration, the actuator of device 120 is biased to align openings 112 corresponding to absorption lines of air with the appropriate intensity features (e.g., black areas) of the image at plate 110 . Proper alignment of plate 110 is achieved when the intensity profile measured by detectors 130 has the expected intensity pattern corresponding to air. Next, detection of a target gas is performed as follows. When the target gas is present in cell 104 , the intensity profile measured by detectors 130 , in addition to the air pattern, will also include an intensity pattern corresponding to the target gas.
- intensity features e.g., black areas
- openings 112 corresponding to the target gas are aligned with the corresponding intensity features (if present) in the image at plate 110 resulting from light absorption by the target gas.
- these intensity features may be relatively weak.
- the actuator in device 120 is configured to impart onto plate 110 a small-amplitude oscillation, e.g., in the kHz frequency range, about the equilibrium position set during the calibration. Due to the oscillation, each opening 112 corresponding to the target gas periodically moves in and out of alignment with the respective intensity feature, thereby changing in a periodic fashion the amount of light transmitted through the opening and therefore modulating the signal generated by the corresponding detector 130 .
- Processor 140 measures the appropriate ac components having a frequency corresponding to the oscillation frequency of plate 110 in the signals generated by detectors 130 .
- processor 140 is preferably configured to verify correlations expected between the signals generated by different detectors 130 corresponding to the same target gas. Such correlations may include (i) simultaneous presence of the appropriate ac components and/or (ii) proper relative amplitudes of said components in accordance with relative magnitudes of the corresponding absorption lines in the absorption spectrum of the target gas.
- processor 140 may optionally be configured to determine the concentration of the target gas using the measured ac amplitudes.
- HCN hydrogen cyanide
- air has certain spectral “transparency” windows (i.e., spectral regions where absorption is relatively weak), which are located between strong absorption bands of individual air components.
- one such transparency window is between 5700 and 6600 cm ⁇ 1 .
- One or more spectral transparency windows may be used to detect optical absorption of target gases.
- one or more relatively narrow gaps (not discernible in FIG. 2 ) between absorption lines composing a selected strong absorption band may similarly be used to detect optical absorption of target gases.
- FIGS. 3 A-B show optical absorption spectra of air and HCN, respectively, in the spectral range from 6480 to 6500 cm ⁇ 1 , which is within the spectral transparency window indicated above.
- a spectral line 302 corresponding to water vapor is selected as a calibration feature.
- plate 110 ( FIG. 1 ) has opening 112 -cal indicated in FIG. 3A by the two vertical lines near spectral line 302 .
- plate 110 is positioned such that opening 112 -cal aligns with line 302 as shown in FIG. 3A . Additional openings 112 -cal not shown in FIG. 3A may be used to achieve and/or verify the alignment.
- a spectral line 304 corresponding to HCN is selected as a signature feature of that gas.
- plate 110 FIG. 1
- plate 110 has opening 112 -hc indicated in FIG. 3B by the two vertical lines near spectral line 304 .
- opening 112 -cal is aligned with line 302 as shown in FIG. 3A
- opening 112 -hc aligns with line 304 as shown in FIG. 3B .
- the horizontal double-headed arrow in FIG. 3B indicates a representative oscillation amplitude for plate 110 ( FIG. 1 ). Due to this oscillation, the signal generated by detector 130 located behind opening 112 -hc is modulated with the frequency twice the oscillation frequency.
- Processor 140 FIG.
- system 100 processes the modulated signal, measures its amplitude, and preferably verifies that this modulated signal properly correlates with other modulated signals corresponding to additional openings 112 -hc (not shown in FIG. 3B ).
- system 100 warns the user about the presence of HCN and optionally uses the modulation amplitudes to determine and display its concentration.
- cell 104 is a multi-pass cell having an effective optical path length of about 1 to 10 m.
- a representative multi-pass cell that may be used in one embodiment of system 100 is described in U.S. Pat. No. 5,173,749, the teachings of which are incorporated herein by reference.
- cell 104 is an airtight cell, which has valves connected to a pump (not shown in FIG. 1 ) configured to reduce pressure in the cell prior to or after sample injection.
- cell 104 has heaters to vaporize liquid samples.
- cell 104 has vents enabling unencumbered gas exchange with ambient air.
- FIG. 4 shows a top view of a MEMS device 420 that can be used as MEMS device 120 according to one embodiment of the present invention.
- Device 420 comprises a movable plate 410 supported on a wafer 422 by four serpentine springs 424 a - d . In different embodiments, a different number of springs may be used.
- Wafer 422 preferably comprises at least three layers: a substrate layer (not shown), an overlayer 402 , and a thin insulating layer (not shown) located between the substrate layer and the overlayer. The insulating layer electrically isolates overlayer 402 from the substrate layer. Overlayer 402 and the substrate layer may be silicon and the insulating layer may be silicon oxide.
- Plate 410 is formed using overlayer 402 while the underlying portions of the substrate and insulating layers are removed to enable the plate mobility.
- Plate 410 has six slots 412 a - f , two of which may be calibration slots corresponding to selected absorption lines of air and the remaining four slots may be slots corresponding to absorption lines of one or more target gases.
- Device 420 further comprises a comb-drive actuator 406 including (i) a mobile portion 406 a connected to plate 410 and (ii) an immobile portion 406 b attached to wafer 422 .
- Portion 406 a of actuator 406 is formed using layer 402 and is detached from the underlying substrate and insulating layers to permit in-plane motion of that portion and plate 410 .
- Portion 406 b of actuator 406 is electrically connected to a contact pad 408 using a contact track 404 .
- Portion 406 b of actuator 406 , track 404 , and pad 408 are electrically isolated from the rest of the device structure using the underlying insulation of the insulating layer and the surrounding grooves in overlayer 402 .
- portion 406 a of actuator 406 is in electrical contact with overlayer 402 , e.g., via springs 424 c - d .
- a voltage differential can be applied between portions 406 a - b of actuator 406 .
- layer 402 may be connected to a negative terminal of a voltage source (e.g., ground), whereas pad 408 may be connected to a positive terminal of that voltage source configured to apply voltage between portions 406 a - b of actuator 406 .
- Contact pad 408 may be metal-plated as known in the art for better ohmic contact with a wire lead (not shown).
- device 420 may be operated as follows. First, a suitable dc voltage is applied between portions 406 a - b of actuator 406 , e.g., as explained above, to align calibration slots 412 with the appropriate absorption lines of air. Spring force generated by deformed springs 424 counterbalances the attractive electrostatic force generated between portions 406 a - b of actuator 406 . Then, in addition to the dc voltage, a small ac voltage, for example, having a frequency of about 20 kHz, is applied between portions 406 a - b . The ac voltage causes plate 410 to oscillate at that frequency about the position corresponding to the dc voltage.
- a suitable dc voltage is applied between portions 406 a - b of actuator 406 , e.g., as explained above, to align calibration slots 412 with the appropriate absorption lines of air.
- Spring force generated by deformed springs 424 counterbalances the attractive electrostatic force generated between portions 406 a -
- an ac voltage producing the oscillation amplitude of about 10 ⁇ m will effectively move slots 412 in and out of alignment with the corresponding absorption features of the image at plate 410 .
- Device 420 may be fabricated from a silicon-on-insulator (SOI wafer) using different fabrication techniques. For example, an etch fabrication method may be used. It is known that silicon etches significantly faster than silicon oxide using, e.g., reactive ion etching (RIE). Similarly, silicon oxide etches significantly faster than silicon using, e.g., fluorine-based etchants. Relatively deep cavities in a relatively thick substrate layer may be defined using a standard, anisotropic etching technique, such as deep RIE. Deep RE stops automatically at the oxide layer acting as an etch stop. Various parts of device 420 may be mapped onto the corresponding layer using lithography. Additional description of various etching steps may be found, for example, in U.S. Pat. Nos. 6,201,631, 5,629,790 and 5,501,893, the teachings of all of which are incorporated herein by reference.
- RIE reactive ion etching
- device 420 has been described in the context of using silicon/silicon oxide SOI wafers, other suitable materials may similarly be used.
- the materials may be appropriately doped as known in the art.
- Various surfaces may be modified, e.g., by metal deposition and/or by ion implantation.
- differently shaped plates, actuators, and/or support structures may be implemented without departing from the scope and principle of the invention.
- Different embodiments may include differently shaped and/or configured springs, where the term “spring” refers in general to any suitable elastic structure that can recover its original shape after being distorted.
- a spectrophotometric system of the invention employing said device has a size suitable for portable applications. Further miniaturization may be achieved by implementing the signal processor (e.g., processor 140 ) and the array of photo-detectors (e.g., detectors 130 ) in a single integrated circuit.
- processor 140 , detectors 130 , and MEMS device 120 (or device 420 ) may be implemented as a system on a chip (SOC).
- System 100 may be designed to use spectral features other than absorption lines, e.g., an edge of a relatively wide absorption band or an emission line.
- light source 102 is a laser adapted to excite fluorescence in the sample contained in cell 104 and openings 112 correspond to selected fluorescence lines of the sample. Openings 112 may correspond to two or more target substances.
Abstract
Description
- The subject matter of this application is related to that of U.S. patent application Ser. No. 10/153,294 filed on May 22, 2002 and entitled “Monolithic In-Plane Shutter Switch,” the teachings of which are incorporated herein by reference.
- 1. Field of the Invention
- The present invention relates to sensors and, more specifically, to optical gas detectors.
- 2. Description of the Related Art
- Gas analyzers are used in environmental monitoring, industrial process control, geologic exploration, and medical, analytical, and military applications. For example, gases such as hydrogen sulfide and sulfur dioxide accompany natural gas and are often present in industrial environments. Since exposure to these gases is a health hazard, low concentrations, typically in the range of parts per million (ppm), need to be detected.
- Different analytical techniques such as optical spectroscopy, mass spectrometry, gas chromatography, electrochemical analysis, etc., may be used for detection of substances present in relatively low concentrations. One particular technique, optical spectrophotometry, is based on the absorption of electromagnetic radiation by the sample in a selected spectral range and is generally recognized to have relatively high sensitivity and speed of analysis. A spectrophotometric system typically has (i) a light source; (ii) a monochromator, which filters the light from the light source so that only a limited wavelength range is allowed to irradiate the gas sample contained in a sample cell; and (iii) a photo-detector, which measures the amount of light transmitted through the sample. Representative prior-art spectrophotometric systems operating in either ultraviolet (UV) or infrared (IR) parts of the spectrum are described, for example, in U.S. Pat. Nos. 5,936,250 and 6,344,648, the teachings of both of which are incorporated herein by reference. However, one problem with prior-art spectrophotometric systems is that they are relatively difficult to adapt for portable, particularly hand-held, applications.
- Problems in the prior art are addressed, in accordance with the principles of the present invention, by a portable spectrophotometric system for detecting one or more target substances. In a representative embodiment, a system of the invention has an optical grating, an array of photo-detectors, and a MEMS device having a movable plate positioned between the grating and the array. Light transmitted through a gaseous sample is dispersed by the grating and is imaged onto the movable plate, which has a plurality of openings corresponding to selected absorption lines of the target substance. A small-amplitude oscillation is imparted onto the plate such that the openings periodically move in and out of alignment with the corresponding intensity features in the image, which modulates electrical signals generated by the corresponding photo-detectors. A lock-in signal processor analyzes the modulation pattern by comparing it to the pattern expected in the presence of the target substance. When a positive correlation between the patterns is established, the system warns the user about the presence of the target substance.
-
FIG. 1 shows a schematic diagram of a spectrophotometric system according to one embodiment of the present invention; -
FIG. 2 graphically shows optical absorption for mean latitude summer air at pressure p=1 atm. and temperature T=296 K; - FIGS. 3A-B compare absorption spectra of air and hydrogen cyanide at p=0.1 atm. and T=296 K; and
-
FIG. 4 shows a top view of a MEMS device that can be used in the system ofFIG. 1 according to one embodiment of the present invention. - Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.
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FIG. 1 shows a schematic diagram of aspectrophotometric system 100 according to one embodiment of the present invention. A beam of light generated by abroadband light source 102 is directed through acell 104 containing a sample to be analyzed. Light transmitted throughcell 104 is dispersed in wavelength by agrating 106 and is imaged by acylindrical lens 108 onto amovable plate 110. The image produced at the surface ofplate 110 is a stripe of light, in which different locations along the length of the stripe correspond to different wavelengths.Plate 110 has a plurality ofopenings 112 and is preferably a part of aMEMS device 120, which also includes asupport structure 122 and one ormore springs 124 connected between the support structure and the plate.Device 120 further includes an electrostatic actuator (not shown) adapted to controllablymove plate 110 with respect tostructure 122 as indicated by the double-headed arrow inFIG. 1 .Plate 110 allows light to pass throughopenings 112 and substantially blocks the light impinging onto other portions of the plate. An array of photo-detectors 130 located behindplate 110 converts the transmitted light into electrical signals, which are then applied to asignal processor 140. - As will be further illustrated below, IR absorption of gases is typically characterized by the presence of relatively narrow absorption lines, each of which usually corresponds to a particular rotational and/or vibrational mode of the gas molecule. Each of these absorption lines produces a corresponding intensity feature in the image at
plate 110. For example, a relatively strong absorption line will produce a corresponding black (i.e., having substantially no light) area atplate 110 while the areas adjacent to the black area will have a relatively high light intensity. Position of eachopening 112 withinplate 110 is chosen such that it can be aligned with the intensity feature corresponding to a selected absorption line of the target gas to be detected. It is also preferred that, for each target gas,plate 110 has two ormore openings 112. In addition,plate 110 may have one ormore openings 112 corresponding to selected strong absorption lines of air, which may be used for calibration purposes as further detailed below. - In a representative configuration,
system 100 operates as follows. First,device 120 is wavelength calibrated. During calibration, the actuator ofdevice 120 is biased to alignopenings 112 corresponding to absorption lines of air with the appropriate intensity features (e.g., black areas) of the image atplate 110. Proper alignment ofplate 110 is achieved when the intensity profile measured bydetectors 130 has the expected intensity pattern corresponding to air. Next, detection of a target gas is performed as follows. When the target gas is present incell 104, the intensity profile measured bydetectors 130, in addition to the air pattern, will also include an intensity pattern corresponding to the target gas. After calibration,openings 112 corresponding to the target gas are aligned with the corresponding intensity features (if present) in the image atplate 110 resulting from light absorption by the target gas. However, due to the unknown and possibly low concentration of the target gas, these intensity features may be relatively weak. To enable detection of weak absorption, the actuator indevice 120 is configured to impart onto plate 110 a small-amplitude oscillation, e.g., in the kHz frequency range, about the equilibrium position set during the calibration. Due to the oscillation, eachopening 112 corresponding to the target gas periodically moves in and out of alignment with the respective intensity feature, thereby changing in a periodic fashion the amount of light transmitted through the opening and therefore modulating the signal generated by thecorresponding detector 130. -
Processor 140, preferably using lock-in detection techniques, measures the appropriate ac components having a frequency corresponding to the oscillation frequency ofplate 110 in the signals generated bydetectors 130. To discriminate against false positives,processor 140 is preferably configured to verify correlations expected between the signals generated bydifferent detectors 130 corresponding to the same target gas. Such correlations may include (i) simultaneous presence of the appropriate ac components and/or (ii) proper relative amplitudes of said components in accordance with relative magnitudes of the corresponding absorption lines in the absorption spectrum of the target gas. In addition,processor 140 may optionally be configured to determine the concentration of the target gas using the measured ac amplitudes. -
FIGS. 2 and 3 further illustrate the principle of operation ofsystem 100 in a representative embodiment adapted for the detection of hydrogen cyanide (HCN), a highly toxic gas. More specifically,FIG. 2 graphically shows optical absorption for mean latitude summer air at pressure p=1 atm. and temperature T=296 K; and FIGS. 3A-B compare absorption spectra of air and HCN at p=0.1 atm. and T=296 K; the abundance of HCN is 1 ppm. Each value of absorption (A) shown inFIGS. 2 and 3 represents a relative amount of electromagnetic radiation that is absorbed in a gas column of length l and is expressed by the following equation:
A=1−exp(−k λ l) (1)
where kλ is the absorption coefficient at wavelength λ. -
FIG. 2 shows optical absorption for a column (l=1 m) of air having the following composition: 77.393230% of nitrogen; 20.710864% of oxygen; 1.862987% of water; 0.032701% of carbon dioxide; 0.000168% of methane; 0.000032% of nitrous oxide; 0.000015% of carbon monoxide; and 0.000003% of ozone. As indicated by the data inFIG. 2 , air has certain spectral “transparency” windows (i.e., spectral regions where absorption is relatively weak), which are located between strong absorption bands of individual air components. For example, one such transparency window is between 5700 and 6600 cm−1. One or more spectral transparency windows may be used to detect optical absorption of target gases. Alternatively, one or more relatively narrow gaps (not discernible inFIG. 2 ) between absorption lines composing a selected strong absorption band may similarly be used to detect optical absorption of target gases. - FIGS. 3A-B show optical absorption spectra of air and HCN, respectively, in the spectral range from 6480 to 6500 cm−1, which is within the spectral transparency window indicated above. Referring now to
FIG. 3A , in one embodiment, aspectral line 302 corresponding to water vapor is selected as a calibration feature. Accordingly, plate 110 (FIG. 1 ) has opening 112-cal indicated inFIG. 3A by the two vertical lines nearspectral line 302. During calibration,plate 110 is positioned such that opening 112-cal aligns withline 302 as shown inFIG. 3A . Additional openings 112-cal not shown inFIG. 3A may be used to achieve and/or verify the alignment. - Referring to
FIG. 3B , aspectral line 304 corresponding to HCN is selected as a signature feature of that gas. Accordingly, plate 110 (FIG. 1 ) has opening 112-hc indicated inFIG. 3B by the two vertical lines nearspectral line 304. When opening 112-cal is aligned withline 302 as shown inFIG. 3A , opening 112-hc aligns withline 304 as shown inFIG. 3B . The horizontal double-headed arrow inFIG. 3B indicates a representative oscillation amplitude for plate 110 (FIG. 1 ). Due to this oscillation, the signal generated bydetector 130 located behind opening 112-hc is modulated with the frequency twice the oscillation frequency. Processor 140 (FIG. 1 ) processes the modulated signal, measures its amplitude, and preferably verifies that this modulated signal properly correlates with other modulated signals corresponding to additional openings 112-hc (not shown inFIG. 3B ). When a positive correlation is established,system 100 warns the user about the presence of HCN and optionally uses the modulation amplitudes to determine and display its concentration. - In a preferred embodiment,
cell 104 is a multi-pass cell having an effective optical path length of about 1 to 10 m. A representative multi-pass cell that may be used in one embodiment ofsystem 100 is described in U.S. Pat. No. 5,173,749, the teachings of which are incorporated herein by reference. In one embodiment,cell 104 is an airtight cell, which has valves connected to a pump (not shown inFIG. 1 ) configured to reduce pressure in the cell prior to or after sample injection. Optionally,cell 104 has heaters to vaporize liquid samples. In another embodiment,cell 104 has vents enabling unencumbered gas exchange with ambient air. -
FIG. 4 shows a top view of aMEMS device 420 that can be used asMEMS device 120 according to one embodiment of the present invention.Device 420 comprises amovable plate 410 supported on awafer 422 by four serpentine springs 424 a-d. In different embodiments, a different number of springs may be used.Wafer 422 preferably comprises at least three layers: a substrate layer (not shown), anoverlayer 402, and a thin insulating layer (not shown) located between the substrate layer and the overlayer. The insulating layer electrically isolatesoverlayer 402 from the substrate layer.Overlayer 402 and the substrate layer may be silicon and the insulating layer may be silicon oxide.Plate 410 is formed usingoverlayer 402 while the underlying portions of the substrate and insulating layers are removed to enable the plate mobility.Plate 410 has six slots 412 a-f, two of which may be calibration slots corresponding to selected absorption lines of air and the remaining four slots may be slots corresponding to absorption lines of one or more target gases. -
Device 420 further comprises a comb-drive actuator 406 including (i) amobile portion 406 a connected to plate 410 and (ii) animmobile portion 406 b attached towafer 422.Portion 406 a ofactuator 406 is formed usinglayer 402 and is detached from the underlying substrate and insulating layers to permit in-plane motion of that portion andplate 410.Portion 406 b ofactuator 406 is electrically connected to acontact pad 408 using acontact track 404.Portion 406 b ofactuator 406,track 404, and pad 408 are electrically isolated from the rest of the device structure using the underlying insulation of the insulating layer and the surrounding grooves inoverlayer 402. In contrast,portion 406 a ofactuator 406 is in electrical contact withoverlayer 402, e.g., viasprings 424 c-d. Thus, a voltage differential can be applied betweenportions 406 a-b ofactuator 406. In one configuration,layer 402 may be connected to a negative terminal of a voltage source (e.g., ground), whereaspad 408 may be connected to a positive terminal of that voltage source configured to apply voltage betweenportions 406 a-b ofactuator 406.Contact pad 408 may be metal-plated as known in the art for better ohmic contact with a wire lead (not shown). - In one configuration,
device 420 may be operated as follows. First, a suitable dc voltage is applied betweenportions 406 a-b ofactuator 406, e.g., as explained above, to align calibration slots 412 with the appropriate absorption lines of air. Spring force generated by deformed springs 424 counterbalances the attractive electrostatic force generated betweenportions 406 a-b ofactuator 406. Then, in addition to the dc voltage, a small ac voltage, for example, having a frequency of about 20 kHz, is applied betweenportions 406 a-b. The ac voltage causesplate 410 to oscillate at that frequency about the position corresponding to the dc voltage. Assuming that grating 106 has 1100 grooves per mm andlens 108 has a focal lens of about 50 mm, an ac voltage producing the oscillation amplitude of about 10 μm will effectively move slots 412 in and out of alignment with the corresponding absorption features of the image atplate 410. -
Device 420 may be fabricated from a silicon-on-insulator (SOI wafer) using different fabrication techniques. For example, an etch fabrication method may be used. It is known that silicon etches significantly faster than silicon oxide using, e.g., reactive ion etching (RIE). Similarly, silicon oxide etches significantly faster than silicon using, e.g., fluorine-based etchants. Relatively deep cavities in a relatively thick substrate layer may be defined using a standard, anisotropic etching technique, such as deep RIE. Deep RE stops automatically at the oxide layer acting as an etch stop. Various parts ofdevice 420 may be mapped onto the corresponding layer using lithography. Additional description of various etching steps may be found, for example, in U.S. Pat. Nos. 6,201,631, 5,629,790 and 5,501,893, the teachings of all of which are incorporated herein by reference. - Although fabrication of
device 420 has been described in the context of using silicon/silicon oxide SOI wafers, other suitable materials may similarly be used. The materials may be appropriately doped as known in the art. Various surfaces may be modified, e.g., by metal deposition and/or by ion implantation. Also, differently shaped plates, actuators, and/or support structures may be implemented without departing from the scope and principle of the invention. Different embodiments may include differently shaped and/or configured springs, where the term “spring” refers in general to any suitable elastic structure that can recover its original shape after being distorted. - Advantageously, due to the relatively small size of
MEMS device 420, a spectrophotometric system of the invention employing said device has a size suitable for portable applications. Further miniaturization may be achieved by implementing the signal processor (e.g., processor 140) and the array of photo-detectors (e.g., detectors 130) in a single integrated circuit. In addition,processor 140,detectors 130, and MEMS device 120 (or device 420) may be implemented as a system on a chip (SOC). - While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although the invention was described in reference to IR light, it may also be practiced using UV light or other parts of the electromagnetic spectrum.
System 100 may be designed to use spectral features other than absorption lines, e.g., an edge of a relatively wide absorption band or an emission line. For example, in one embodiment,light source 102 is a laser adapted to excite fluorescence in the sample contained incell 104 andopenings 112 correspond to selected fluorescence lines of the sample.Openings 112 may correspond to two or more target substances. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims. - Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.
Claims (25)
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