Patent Application
20190064061
Not applicable.
Not applicable.
This invention relates to a gas sensing device.
Gas sensors require high sensitivity and high specificity, two factors are often in opposition, since a very sensitive system will likely be sensitive to many gasses. But high sensitivity is extremely important, for example exposure 1 part per million of CO in the atmosphere will cause headaches in 10 minutes and irreversible brain damage 60 minutes.
Chemical receptor systems that provide very high sensitivity to CO, often have a low level sensitivity to CO2, which is far more abundant. Thus distinguishing between harmful and benign gases is a problem. Chemical receptor systems can become contaminated, which causes a loss in sensitivity and risk to personnel requiring protection.
Gas sensors fall into several categories, based on the detection mechanism that they employ. Generally, these include the following:
Chemical receptors are available for specific capture of the target molecule. Here, a chemical compound is covalently bonded to a substrate, which is part of the sensor. Within the molecular structure of this chemical compound is a functional group of atoms that will link with a specific type of target molecule. If a target molecule attaches to the chemical receptor, a change is recorded in the substrate voltage, current, temperature, conductivity, magnetic moment, optical absorbance or reflection. These signals are often very weak and similar levels of a given signal can arise from a variety of molecular species. This leads to inaccuracies, false positives and false negatives.
Resonant beam structures may determine the mass of the molecule. The resonant beam structure is often used with the chemical receptor. These beams are very low in mass so the attachment of a population of target molecules can significantly affect the mass and thus significantly change the frequency of its fundamental mechanical resonance. The shift in resonant frequency is generally small and the quantity of target molecules and their chemical structure are factors that are difficult to separate. This measurement can thus also lead to incorrect identification. Finally, strongly adhered target molecules can be difficult to desorb following the sensing measurement.
Sensors can combust the target species and measure its exothermicity. Most gaseous molecular species can be oxidized, which means they can be burned or combusted in an oxygen environment, such as air. A few notable exceptions (non-combustible compounds) include O2, N2, CO2, Ar, and H2O, which are the constituents of air. Therefore, the heat generated during the chemical reaction of combustion is a means of sensing combustible trace impurities in air. In other words, because the dominant constituents of air do not combust, the trace species can more easily be detected without interference by measuring the heat generated. This measurement can be used to identify the impurity. For example, the heat generated during combustion of CO is far less than the heat generated by burning benzene, heptane, or tri-nitro-toluene (TNT), for example. All of the latter, however, have similar heat generation per unit mass of target species. Also the quantity of heat generated is extremely low, making accurate detection difficult. To ignite the target molecules, the system for combustion must operate at very high temperature, thus leading to very short lifetime and poor reliability.
Chemical Field Effect Transistors (Chem-FET). If the gate electrode of a field effect transistor (FET) is replaced by a population of covalently-bonded chemical receptors, the trans-conductance of the FET will change in the presence of molecules that attach to the receptors. Intrinsically the FET provides gain and thus this method can be very sensitive. However, attached target molecules can be difficult to detach following a sensing episode, since high temperature, which can adversely affect the FET, is required to desorb attached molecules. Strongly bonded contaminants can survive even the highest temperatures that can be practically applied in the field of use.
Combinations of the above. Due to the limitations outlined above, it has been found useful to combine those methods into a system. This can greatly reduce the risk of false positive and false negative responses, although the cost and complexity of the system are increased.
All of these share, to some extent, the following draw backs
1) Contamination
2) Probable false readings (inaccuracy)
3) Low sensitivity
4) Low specificity
Accordingly, a new technology is needed for sensing these dangerous compounds in homes, offices and industrial settings. Ideally this technology is small, inexpensive, robust and highly sensitive.
Gases are composed of low molecular weight molecules, since only small molecules are gaseous at ambient temperatures. In the gas phase, these small molecules are constantly tumbling and vibrating in highly precise quantum states. The energy levels of these quantum states are extensively cataloged and the transition energies between states are known to an astounding precision of nine significant digits (one part per billion or ppb). These precise transition frequencies can be used to sense and identify gases for applications such as air pollution monitoring, automotive engine performance optimization, industrial chemical synthesis control, automotive passenger compartment CO2 sensing, home carbon monoxide sensing, fermentation process control, and indoor agriculture.
The device and method described here uses high resolution infrared spectroscopy to detect and identify small gas molecules. The spectrum of thousands of small molecules is well documented. These spectra provide a finger print of each that can be used to unambiguously identify each, with no chance of falsely assigning the measured spectrum.
The gas sensor described here creates a long absorption path by lithographically forming small gas channels in a substrate. Two substrates can be positioned to form a serpentine long path. The channel sides may be coated with a reflective film, for example gold (Au). An emitter, for example a vertical cavity surface emitting laser (VCSEL) diode, may be coupled into the channel, and the radiation transmitted down the channel by reflection off the film. A detector may be provided at the end of the channel.
Accordingly, the gas sensing device may include at least one substrate with at least one cavity formed lithographically therein, and with a reflective film coating sidewalls of the at least one cavity, and wherein the at least one cavity is configured for multiple passes of a ray of light within the cavity, a sample gas filling the lithographically formed cavities; a radiation source coupled to the at least one substrate that launches radiation into the lithographically formed channels, and a detector coupled to the at least one substrate, that detects radiation transmitting the lithographically formed channels.
Within the gas sensing device, the at least one substrate may comprise of at least two substrates with at least one cavity formed on each substrate, and wherein the substrates are arranged such that the cavities partly overlap and form passages that interconnect, to form a longer optical path through the interconnected cavities.
A method is also disclosed, wherein the method may include filling a lithographically formed gas channel with a sample gas, wherein the channel is formed by bonding at least two substrates, wherein the substrates are arranged in a staggered fashion and bonded together to form a longer lithographically formed channel, launching radiation from a VCSEL down the lithographically formed gas channel, and detecting the radiation after transiting the lithographically formed channel.
These and other features and advantages are described in, or are apparent from, the following detailed description.
Various exemplary details are described with reference to the accompanying drawings, which however, should not be taken to limit the invention to the specific embodiments shown but are for explanation and understanding only.
It should be understood that the drawings are not necessarily to scale, and that like numbers maybe may refer to like features.
Gases are composed of low molecular weight molecules, since only small molecules are gaseous at ambient temperatures. In the gas phase these small molecules are constantly tumbling in highly precise quantum vibrational and rotational states. The energy levels of these quantum states are extensively cataloged and the transition energies between states are known to an astounding precision of nine significant digits (one part per billion or ppb). These precise transition frequencies can be used terrestrially to sense and identify gases for applications such as air pollution monitoring, automotive engine performance optimization, industrial chemical synthesis control, automotive passenger compartment CO2 sensing, home carbon monoxide sensing, fermentation process control, and indoor agriculture.
The device and method described here uses high resolution infrared spectroscopy to detect and identify small gas molecules. The spectrum of thousands of small molecules is well documented. These spectra provide a finger print of each that can be used to unambiguously identify each, with no chance of falsely assigning the measured spectrum.
To provide high sensitivity the spectrometer must possess several aspects:
The device and method described here uses high resolution infrared spectroscopy to detect and identify small gas molecules. The spectrum of thousands of small molecules is well documented. These spectra provide a finger print of each that can be used to unambiguously identify each, with no chance of falsely assigning the measured spectrum.
To provide high sensitivity the spectrometer must possess a long absorption path. The long absorption path may be formed lithographically in one or more substrates. The substrates may be lithographically etched to form at least one cavity and with a reflective film coating sidewalls of the at least one cavity. The at least one cavity may be configured for multiple passes of a ray of light within the cavity by reflection of off the sidewalls of the cavity. A plurality of cavities may be formed in two substrates, and the substrates oriented such that the cavities overlap. This may result in an open passageway interconnecting the cavities, such that a ray of light may, through multiple reflections from the sidewalls, leave the VCSEL and ultimately impinge upon the detector.
The path may be created by bonding a plurality of Si wafers, each with numerous KOH etched trenches. As mentioned, these trenches overlap to form an enclosed channel, or waveguide, which is coated with a high reflectance metal such as Au for high infrared transmittance. As a second embodiment, these channels can be made in glass using an isotropic HF etch, followed by a similar Au coating and bond.
Accordingly, the device described here may include a bright, tunable, narrow band light source at the spectral region of interest. VCSELs throughout the infrared spectrum are available. This light source must have a bandwidth roughly similar or less than the molecular absorption spacing. It may also include a sensitive infrared detector. Numerous semiconductor detector technologies exist today. The device may make use of a digital spectral database that can be quickly compared to the observed spectrum for identification and quantification.
The following discussion presents a plurality of exemplary embodiments of the novel gas sensor. The following reference numbers are used in the accompanying figures to refer to the following:
10 VCSEL source
20 gas inlet
30 detector
40-48 lithographically defined cavities
80 ray trace
110 lid wafer
115 gold coating
It should be understood that the designation of “first”, “second”, “upper” and “lower” are arbitrary, that is, the cavity may also be formed on an upper substrate and bonded to a lower substrate, or vice-versa. The terms “wafer” and “substrate” are used interchangeably herein, to refer to a supporting member, generally flat and circular, often of a material such as silicon or glass, as is well known in the art. Finally, the terms “cavity,” “channel” and “trench” are used interchangeably to refer to a depression made by removing material in an area of the substrate.
In
As the term is used herein, a VCSEL refers to a vertical-cavity surface-emitting laser, which is a type of semiconductor laser diode with laserbeam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer. VCSEL applications include fiber optic communications, precision sensing, computer mice, laser printers and augmented reality. VCSELS are typically narrow band, and emit into a rather narrow cone compared to diode lasers. VCSELs use epitaxial layers grown on the wafer to create mirrors on the surface with a LED sandwiched in between, perfect for coupling to fibers and for the instant application.
VCSELS can be designed to emit at certain wavelengths, and to be adjustable about that wavelength. In general, the tuning mechanism is heating through current or voltage, which tends to lengthen the wavelength because of thermal expansion of the device. In any case, VCSELS may be designed to emit at about 2300 wavenumbers, which is a spectral range of particular interest in spectroscopy, as discussed below.
As shown in
The detector 30 may be any of a number of photo-sensitive devices, such as a diode, microchannel plate, CCD camera, photomultiplier tube, and the like. The detector need only be sensitive to the appropriate range of frequencies and put out a signal in response to the reception of the radiation.
The substrates 100 and 200 may be silicon, and the cavities 40, 44 and 48 may be formed in the substrate 100 by exposing the silicon material to a potassium hydroxide (KOH) etchant. As is well known in the art, the KOH may perform anisotropic etching on the silicon material, such that the <100> plane is etched preferentially, because the <111> plane etches at a much slower rate than the other planes. Thus, a groove, channel or depression will be formed as a result of wet etching of silicon, making an angle of 54.74 between the <100> and the <111> plane. As a result of this anisotropy, the walls of the trench formed by KOH etching may have an incline of about 54 degrees with respect to horizontal. To perform this etching at between 50° C. and 70° C., a KOH solution may be prepared by adding the required amount of deionized water to a standard 45 wt % solution.
The cavities 44 and 48 may be formed simultaneously with cavity 40 in the first substrate 100. Thus, cavities 44 and 48 may also be formed using a KOH anisotropic etch, such that it may also have 54 degree sidewalls. In fact, three similar cavities 40, 44 and 48 may be formed in the first substrate 100 adjacent to one another and simultaneously, by putting a mask on the surface with slotted apertures, and applying a KOH solution to the exposed surface.
The source cavity 10, gas inlet cavity 20 and detector cavity 30 may be formed by KOH etching in the second substrate 200. A gas inlet may be formed as another cavity 20 in the second substrate 200. Similarly, a photodetector 30 may be disposed at the bottom of the this cavity 48.
Besides the source, inlet and detector cavities 10, 20 and 30, the two additional larger cavities 42 and 46 may be formed in an opposing substrate 200. Note that the first substrate 100 and the second substrate 200 are not identified in
Cavities 10, 20, 30, 42 and 46 may also be formed using a KOH anisotropic etch, such that they may also have 54 degree sidewalls. Accordingly, two similar cavities 42 and 46 may be formed in the second substrate 200 and disposed laterally adjacent to one another. Alternatively, cavities 10, 20, 30, 42, and 46 can be formed in the second substrate, while cavities 4044, and 48 are formed in the first substrate.
Cavities 10, 20, 30, 42, and 46 may all be etched simultaneously with a single masking layer until the etch reaches a depth of approximately 400 microns. At this time, the etch process may be halted and a second mask layer may be applied that masks further etching on cavities 42 and 46. The KOH etch is then resumed until cavities 10, 20 and 30 etch through the entirety of the 500 um substrate. At this time the masking layers may be stripped and the gold is deposited everywhere to enable the Au-Au thermocompression bond.
The starting substrates may be silicon, 500 microns thick for example. The etched cavities 40, 42, 44, 46 and 48 may be about 400 microns deep. The cavities may be about 1 mm to 1 cm in height and about 500 microns in width. It should be understood that these dimensions are exemplary only, and that many other shapes and placements of cavities may be possible, depending on the application.
As mentioned, after formation of the cavities 10-48 in the first and second substrates 100 and 200, each of the substrate surfaces, with their cavities, may be coated with a gold reflective layer. Accordingly, the cavities will have highly reflective 54 degree sidewalls which are also coated with a highly reflective material. The thickness of the gold layer may be about 0.1 microns, simply enough to form a continuous, reflective layer.
The second substrate 200 with two cavities 42 and 46 formed thereon, or alternatively with five cavities 42, 46, 10, 20 and 30, may be placed against the first substrate 100 with three cavities 40, 44 and 48 formed thereon. Cavities 42 and 46 may be shifted laterally with respect to cavities 40, 44 and 48, such that the cavities overlap to an extent, forming passageways that interconnect the cavities, and form a longer path length between emitter 10 and detector 30.
The two substrates may be bonded in this position. A gold-gold thermocompression bond may be convenient in this application, as gold may be deposited uniformly over the surfaces as a reflective layer. Thermocompression bonds are well known in the art, and result when two gold surfaces are pressed together and heated.
The gas inlet 20 may be formed in the first substrate 100 or in the second substrate 200 and be a simple aperture or with a with a valve that can introduce a sample gas to the interconnected cavities. The gas then fills the cavities 10, 30 and 40-48.
The radiation emitted by VCSEL 10 may impinge upon the 54 degree walls of the cavity 40, and be reflected at a large percentage. Accordingly, a ray of light may be reflected off the surfaces of the cavities many times before finally reaching the detector 30. By staggering the cavities 42, 44 of the second substrate 200 with respect to those 40, 44 and 48 of the first substrate 100, a passageway is formed between the cavities. Using this passageway, a ray may trace a path from emitter 10 to detector 30 by undergoing a plurality of reflections off of the 54 degree sidewalls. This configuration of cavities may be referred to herein as interconnected cavities, because a passage is created between the cavities that allows a ray of light to traverse the whole sequence of cavities from source 10 to detector 30 by undergoing many reflections and re-directions and meandering through each cavity in succession, until being absorbed by the detector, 30.
Accordingly, upon emission from the VCSEL source, a ray of light may impinge serially on the walls of each cavity 10, 40, 42, 44, 46, 48 and eventually to detector 30. The path of the light from source 10 to detector 30 may as a result, be rather long, and thus the pathlength through the gas sample input at cavity 20 may be quite long. A long path length is advantageous for absorption spectroscopy, because the gas atoms have many opportunities to absorb the radiation. For absorption spectroscopy such as this device, a long pathlength improves the signal to noise of the measurement.
Some energy is removed from the radiation because of absorption by the gas. This occurs at exactly the wavelength of light that matches the energy of the transition. The science of measuring the radiation energy loss as it passes through a sample is known as absorption spectroscopy. The peaks in a spectrum are often referred to as lines due to older spectroscopic methods that used a grating as a monochromator and photographic film as a detector. The light diffracted from the grating would expose a series of lines on the photographic emulsion. Unfortunately, for very high resolution, classical monochromators can be several meters long so that the individual wavelengths can be adequately separated by from each other.
Accordingly, if CO2 is present in the sample, and the wavelength of light is at an absorption peak, the radiation will be strongly attenuated. Thus the presence of CO2 can be determined unambiguously.
As shown in
As before, the light emitted by laser 10 may undergo multiple reflections off of the side wall surfaces of the cavities 40-48. Each of these cavities has 54° side walls as a result of the anisotropic etching procedure. Accordingly, although the route has many turns, because of the high reflectivity of the surfaces and their 54° angle of inclination, a radiation can undergo many, many reflections without losing its amplitude. Accordingly, some fraction of the light emitted by laser 10 will be detected at detector 30, after traversing this long path lights for the serpentine layout.
Again, a substantial reduction in amplitude of transmitted light will be observed when the emission wavelength falls on an absorption feature of a target species. This reduction in amplitude as a function of wavelength will unambiguously identify the target species as being present in the sample.
In addition to the cavities 40, 44, 48, 10 and 30 on the first substrate 100, a second substrate 200 may also have cavities from therein. Substrate 200 may have cavities 42 and 46 form therein, but cavities 42 and 46 are rotated 90 degrees with respect to cavities 40, 44 and 48. As a result of this rotation, the interconnected cavities may transmit the radiation from source 10 to detector 30 with more grazing incidence reflections. Accordingly, these smaller cavities 42 and 46 on second substrate 200 will overlap the larger cavities 40, 44 and 48 on the first substrate 100 to form an interconnected cavity path from source 10 to detector 30. It can be noted that if the cavities 10 and 30 are formed in the second substrate 200, the orientation of these cavities will be rotated in a manner similar to cavities 42 and 46 (as shown in
Therefore,
The cross sectional view of
The distinguishing feature of these embodiments is that the rays of light will sample a large amount of the gas during their transit from source 10 to detector 30. Accordingly the interconnected cavities are an excellent way to have a long path light through the gas and in a still a compact device.
The two cavities 40 and 44 may be formed in the first substrate 100. The second cavity 42 may be formed in the second substrate 200. By overlapping cavity 42 with cavities 40 and 44, a clear path exists for the ray to traverse the cavities 40 to 42 to 44 and to the detector 30.
In the cross-sectional view a
Accordingly, as in the previous embodiments, the cavities in
The etch mask may be made with a slit or a hole formed therein. The etchant is introduced through the slit or hole, and using a timed edge is allowed to remove the portion of the substrate shown in
Instead of the multiple cavity configurations of the previous embodiments, the embodiment illustrated in
Considering first
The walls of the channel in each of these embodiments may be coated with a reflective film, for example gold or silver. The walls of the channel may be smoothly contoured and generally circular with respect to the bottom of the channel, as results from the isotropic etching technique. Alternatively, this embodiment may also be implemented in silicon using anisotropic KOH etching, in which case the sidewalls will have an inclination of 54 degrees. Light directed upward by the 54 degree sidewalls will be reflected by into the cavity 40 by the upper reflective surface 115 on layer 110.
Accordingly, a gas sensing device is described. The gas sensor may include at least one substrate with gas channels formed lithographically therein, and with a reflective film coating the walls, a sample gas filling the lithographically formed channels, a radiation source coupled to the at least one substrate that launches radiation into the lithographically formed channels, and a detector coupled to the at least one substrate, that detects radiation transmitting the lithographically formed channels. The at least one substrate may comprise at least two substrates, wherein the substrates are arranged in a staggered fashion and bonded together to form a longer lithographically formed channel.
The longer lithographically formed channel may be in the shape of a serpentine, with gas filled from a portion of the longer lithographically formed channel in one. The lithographically formed channel may have sidewalls with an incline of between 40 and 60 degrees, and created by the etching of the channel with a liquid anisotropic etchant. The walls of the channel may be coated with a reflective film, which may be gold. The longer lithographically formed channel may be either cylindrical or trapezoidal in cross section, and may be formed by at least one of KOH and HF.
A method of measuring a gas sample is also disclosed. The method may comprise filling a lithographically formed gas channel with a sample gas, wherein the channel is formed by bonding at least two substrates, wherein the substrates are arranged in a staggered fashion and bonded together to form a longer lithographically formed channel, launching radiation from a VCSEL down the lithographically formed gas channel, and detecting the radiation after transmitting the lithographically formed channel.
Within the method, the longer lithographically formed channel may be in the shape of a serpentine, with gas filled from a portion of the longer lithographically formed channel in one. The lithographically formed channel may have sidewalls with an incline of between 40 and 60 degrees, and created by the etching of the channel with a liquid anisotropic etchant. The walls of the channel may be coated with a reflective film, which may be gold.
The longer lithographically formed channel may be either cylindrical or trapezoidal. The lithographically formed channels may be formed by at least one of KOH and HF.
Also disclosed is a method of manufacturing a gas sensor. The method may comprise bonding at least two substrates, wherein the substrates are arranged in a staggered fashion and bonded together to form a longer lithographically formed channel, coating the channels with a reflective film, coupling a VCSEL source to the reflective channel, and coupling a detector to the reflective channel. The longer lithographically formed channel may be in the shape of a serpentine, with gas filled from a portion of the longer lithographically formed channel in one. The lithographically formed channel may have sidewalls with an incline of between 40 and 60 degrees, and created by the etching of the channel with a liquid anisotropic etchant. The walls of the channel may be coated with a reflective film, which may be gold.
The longer lithographically formed channel may be either circular or spherical. The lithographically formed channels may be formed by at least one of KOH and HF.
While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. Furthermore, although the embodiment described herein pertains primarily to an electrical switch, it should be understood that various other devices may be used with the systems and methods described herein. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.
This nonprovisional US Patent Application claims priority to U.S. Provisional Application Ser. No. 62/550570, filed Aug. 25, 2017 and incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62550570 | Aug 2017 | US |
