CHEMICAL SENSOR AND METHOD OF FORMING THE SAME

A chemical sensor that includes a first semiconductor substrate. The chemical sensor may also include a second semiconductor substrate. The chemical sensor may further include one or more metal layers between the first semiconductor substrate and the second semiconductor substrate such that the first and second semiconductor substrates and the one or more metal layers form a cell including a cavity, the cavity having a depth of any value equal to or less than 100 μm. The chemical sensor may also include an optical source. The chemical sensor may additionally include an optical detector such that light emitted by the optical source passes through the cell to the optical detector. The first and second semiconductor substrates and the one or more metal layers may also define at least one inlet for fluid to flow into the cavity and at least one outlet for fluid to flow out of the cavity.

TECHNICAL FIELD

Various aspects of this disclosure relate to a chemical sensor. Various aspects of this disclosure relate to a method of forming a chemical sensor.

BACKGROUND

Attenuated total reflection (ATR) is typically used to measure high concentrations for liquids by optical means. The reason is that at high concentrations, most of the ultraviolet (UV)/visible/infrared (IR) light would be absorbed by the analyte and the remaining light would not be able to provide information on the identity or quantity of the analyte. Even with ATR, establishing the quantity of the analyte could be challenging as the path length, lsor lpcould vary depending on the overall refractive index, n2, of the medium. The path length ls, or lpmay be provided by:

where s and p represent perpendicular and parallel polarization respectively, n1represents refractive index of the total internal reflection crystal, and θ represents the incident angle.

Since the refractive index of the medium would change also depending on the different ratio and concentration of chemicals, the effective path length is not easily calculated.

In order to measure the concentrations of gases, a gas cell is normally used. Gas cells available in the market have an optical pathlength of at least 1 cm. Using Beer-Lambert law as seen below, transmittance (t) may be provided by:

where σ is the absorption cross-section of the target gas species, n is the concentration of the attenuating species and l is the path length of the beam of light through the gas cell. At room temperature and pressure, the concentration by volume of x % may be provided by n=x %*2.5*1019cm−3. Then, assuming σ of 2*10−18cm2and path length of 1 cm, the change in transmittance from 60% concentration to 59.5% concentration may be provided by

where Δt is the change in transmittance, t59.5%is the transmittance at 59.5% concentration, and t60%is the transmittance at 60% concentration.

Equation (4) implies strong limitations on the signal-to-noise ratio of the system for a gas cell of 1 cm. This type of gas cell is also described and found in other works

In another type of non-dispersive infrared gas sensor, the source and detector are also in the gas cell. It would be even harder in this gas cell to achieve short pathlengths due to required placement of the source and the detector inside the gas cell.

SUMMARY

Various embodiments may provide a chemical sensor. The chemical sensor may include a first semiconductor substrate. The chemical sensor may also include a second semiconductor substrate. The chemical sensor may further include one or more metal layers between the first semiconductor substrate and the second semiconductor substrate such that the first semiconductor substrate, the second semiconductor substrate and the one or more metal layers form a cell including a cavity, the cavity having a depth of any value equal to or less than 100 The chemical sensor may also include an optical source. The chemical sensor may additionally include an optical detector such that light emitted by the optical source passes through the cell to the optical detector. The first semiconductor substrate, the second semiconductor substrate and the one or more metal layers may also define at least one inlet for fluid to flow into the cavity and at least one outlet for fluid to flow out of the cavity.

Various embodiments may provide a method of forming a chemical sensor. The method may include forming one or more metal layers between a first semiconductor substrate and a second semiconductor substrate such that the first semiconductor substrate, the second semiconductor substrate and the one or more metal layers form a cell including a cavity, the cavity having a depth of any value equal to or less than 100 The method may also include providing an optical source. The method may further include providing an optical detector such that light emitted by the optical source passes through the cell to the optical detector. The first semiconductor substrate, the second semiconductor substrate and the one or more metal layers may also define at least one inlet for fluid to flow into the cavity and at least one outlet for fluid to flow out of the cavity.

DETAILED DESCRIPTION

Embodiments described in the context of one of the methods or chemical sensor are analogously valid for the other methods or chemical sensors. Similarly, embodiments described in the context of a method are analogously valid for a chemical sensor, and vice versa.

The device as described herein may be operable in various orientations, and thus it should be understood that the terms “top”, “bottom”, etc., when used in the following description are used for convenience and to aid understanding of relative positions or directions, and not intended to limit the orientation of the sensor.

In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

FIG.1is a general illustration of a chemical sensor according to various embodiments. The chemical sensor may include a first semiconductor substrate102. The chemical sensor may also include a second semiconductor substrate104. The chemical sensor may further include one or more metal layers106between the first semiconductor substrate102and the second semiconductor substrate104such that the first semiconductor substrate102, the second semiconductor substrate104and the one or more metal layers106form a cell including a cavity, the cavity having a depth of any value equal to or less than 100 μm. The chemical sensor may also include an optical source108. The chemical sensor may additionally include an optical detector110such that light emitted by the optical source108passes through the cell to the optical detector110. The first semiconductor substrate102, the second semiconductor substrate104and the one or more metal layers106may also define at least one inlet for fluid to flow into the cavity and at least one outlet for fluid to flow out of the cavity.

In other words, the chemical sensor may include a cell which is formed by bonding or adhering the first semiconductor substrate102and the second semiconductor substrate104together via one or more metal layers106. The chemical sensor may also include an optical source and an optical detector such that light from the optical source108passes through the cell or cavity, to the optical detector110.

For avoidance of doubt,FIG.1serves to illustrate some features of a chemical sensor according to various embodiments, and is not intended to limit the shape, size, arrangement, orientation etc. of the various features.

Various embodiments may relate to very short pathlength gas cells which can be precisely fabricated in order to measure accurately the difference in concentration at high concentrations. For example, in order to measure the concentration change of a gas between 60% and 59.5%, a pathlength of 100 um may enable a change in transmittance of 0.002 (Δt=0.002), meaning almost 11 orders of magnitude higher in change compared to a 1 cm pathlength gas cell. Furthermore, with even higher number densities such as at higher pressures or liquids, the pathlength required may be even smaller to enable higher transmittance change to be recorded. Pathlengths on the order of microns cannot be manufactured accurately with current, conventional gas cells.

Various embodiments may relate to or include wafer level gas or chemical cells. Various other embodiments may relate to or include chip level gas or chemical cells.

In various embodiments, the cavity may have a depth of any value less than 100 μm.

In various embodiments, the chemical sensor may have a first non-metal layer at a first inner surface adjoining the cavity. The first non-metal layer may, for instance, include an oxide (e.g. silicon oxide SiO2), a nitride (e.g. silicon nitride Si3N4), or parylene. The chemical sensor may have a second non-metal layer at a second inner surface adjoining the cavity. The second non-metal layer may, for instance, include an oxide (e.g. silicon oxide SiO2), a nitride (e.g. silicon nitride Si3N4), or parylene.

In various embodiments, the first semiconductor substrate102and the second semiconductor substrate104may include a suitable semiconductor material such as silicon or germanium.

In various embodiments, the one or more metal layers106may include a first adhesion layer in contact with at least a portion of the first semiconductor substrate102. The one or more metal layers106may also include a first bonding layer in contact with the first adhesion layer. The one or more metal layers may additionally include a second bonding layer in contact with the first bonding layer. The one or more metal layers may further include a second adhesion layer in contact with at least a portion of the second semiconductor substrate104.

The first adhesion layer and the second adhesion layer may include any suitable metal or metals. In various embodiments, the first adhesion layer and the second adhesion layer may include chromium or titanium. In various other embodiments, the first adhesion layer and the second adhesion layer may be seed layers for electroplating, and may include chromium or titanium together with copper.

The first bonding layer and the second bonding layer may include any suitable metal or metals. In various embodiments, the first bonding layer and the second bonding layer may include gold. In various other embodiments, the first bonding layer and the second bonding layer may include aluminum or copper.

In various embodiments, the first semiconductor substrate102, the second semiconductor substrate104and the one or more metal layers106may form a further cell including a further cavity.

In various embodiments, the further cavity may be sealed. The chemical sensor may not have an inlet and an outlet connected to the further cavity. The further cell may be used as a reference cell, while the cell may be used as a measurement cell.

In various other embodiments, the first semiconductor substrate102, the second semiconductor substrate104and the metal layer106may also define at least one further inlet for fluid to flow into the further cavity and at least one further outlet for fluid to flow out of the further cavity. The further cell may also be used as a measurement cell.

In various embodiments, the chemical sensor may include one or more measurement cells. The chemical sensor may or may not include one or more reference cells.

In various embodiments, the chemical sensor may include a further optical detector such that light emitted by the optical source108passes through the further cell to the further optical detector. In various embodiments, the optical source108and the optical detector110may be unpackaged dies bonded to the cell using wafer to wafer bonding methods or chip to wafer bonding methods. In various embodiments, the further optical detector may also be an unpackaged die bonded to the further cell. The optical source108may also be bonded to the further cell.

FIG.2is a general illustration of a method of forming a chemical sensor according to various embodiments. The method may include, in202, forming one or more metal layers between a first semiconductor substrate and a second semiconductor substrate such that the first semiconductor substrate, the second semiconductor substrate and the one or more metal layers form a cell including a cavity. The cavity may have a depth of any value equal to or less than 100 μm. The method may also include, in204, providing an optical source. The method may further include, in206, providing an optical detector such that light emitted by the optical source passes through the cell to the optical detector. The first semiconductor substrate, the second semiconductor substrate and the one or more metal layers may also define at least one inlet for fluid to flow into the cavity and at least one outlet for fluid to flow out of the cavity.

In other words, the method may include forming a cell with a cavity by bonding or adhering the first semiconductor substrate and the second semiconductor substrate together using one or more metal layers. The cavity may have a depth of any value equal to or less than 100 μm. The method may also include providing an optical source and an optical detector such that light from the optical source passes through the cell or cavity, to the optical detector.

For avoidance of doubt, the steps shown inFIG.2may or may not be in sequence. For instance, step206may be after, before or at the same time as step204.

In various embodiments, the cavity may have a depth of any value less than 100 μm.

In various embodiments, the first semiconductor substrate and the second semiconductor substrate may include a suitable semiconductor material such as silicon or germanium.

In various embodiments, the first semiconductor substrate may be patterned before forming a first adhesion layer of the one or more metal layers in contact with the first semiconductor substrate and forming a first bonding layer of the one or more metal layers in contact with the first adhesion layer. The second semiconductor substrate may be patterned before forming a second adhesion layer of the one or more metal layers in contact with the first semiconductor substrate, and forming a second bonding layer of the one or more metal layers in contact with the second adhesion layer. The method may further include bonding the first bonding layer and the second bonding layer to form the cell including the cavity.

The first adhesion layer and the second adhesion layer may include any suitable metal or metals. In various embodiments, the first adhesion layer and the second adhesion layer may include chromium or titanium. In various other embodiments, the first adhesion layer and the second adhesion layer may be seed layers for electroplating may include chromium or titanium together with copper.

The first bonding layer and the second bonding layer may include any suitable metal or metals. In various embodiments, the first bonding layer and the second bonding layer may include gold. In various embodiments, forming the first bonding layer and the second bonding layer may including depositing the suitable metal or metals via sputtering. In various other embodiments, forming the first bonding layer and the second bonding layer may including depositing the suitable metal or metals via electroplating.

In various embodiments, the first semiconductor substrate, the second semiconductor substrate and the one or more metal layers may form a further cell including a further cavity.

In various embodiments, the further cavity may be sealed. The chemical sensor may not have an inlet and an outlet connected to the further cavity. The further cell may be used as a reference cell, while the cell may be used as a measurement cell.

In various other embodiments, the first semiconductor substrate, the second semiconductor substrate and the metal layer may also define at least one further inlet for fluid to flow into the further cavity and at least one further outlet for fluid to flow out of the further cavity. The further cell may also be used as a measurement cell.

In various embodiments, the chemical sensor may include one or more measurement cells. The chemical sensor may or may not include one or more reference cells.

In various embodiments, the method may include providing a further optical detector such that light emitted by the optical source passes through the further cell to the further optical detector.

FIG.3Ashows a cut-away perspective view of a gas or chemical cell according to various embodiments.FIG.3Bshows an external perspective view of a bonded gas or chemical cell according to various embodiments.FIG.3Cshows a chemical or gas sensor including the chemical or gas cell shown inFIG.3Band including an optical or light source308and an optical detector310according to various embodiments. The wafer-level gas or chemical cell may be made of two wafers patterned302,304with specific depth and geometry to allow for gas to flow into the cell and also for light to be guided in this short region. In this case, as shown inFIG.3A, a circular cavity may be etched in the wafer302in which the etch depth of 50 μm represents half of the desired optical path length of 100 μm. The circle may be 8 mm in diameter and the sides may include 1 mm width rectangular channels to enable gas to flow in from the sides to enter the cell. This four-fold rotational symmetry may enable easy placement of the cell. Also, a metal layer306such as gold (Au) may be coated on the top surface where the metal layer will be in contact with another similar metal layer306(e.g. gold) to bond both wafers302,304together. Gold (Au) may be used to perform thermocompression bonding without requirement to prepare the surface since gold would not be oxidized. Thermocompression bonding may help to seal the wafers302,304together to prevent fluid or gas from leaking out and interacting with the source or detector or other electrical components. This may be crucial since the fluid or gas maybe corrosive. Gold may also be resistant to many chemicals and gases, thus protecting the underlying substrate. At the same time, the gold may be also deposited at the sidewalls to also help guide the light from one end to the other end.

FIG.3Balso includes the path for the optical light to pass through, and path for the fluid or gas to flow and interact with the optical light.FIG.3Cshows the placement of the optical source308and the optical detector310relative to the gas cell. The chemical or gas sensor shown inFIG.3Cmay be a non-dispersive infrared chemical or gas sensor. In various embodiments, both the optical source308and the optical detector310in TO-Cans may be bonded directly to the gas or chemical cell to create a vacuum seal and improve the signal-to-noise ratio (SNR) of the system. The optical source and detector may also be unpackaged dies that can be bonded to the gas or chemical cell using wafer to wafer bonding methods or chip to wafer bonding methods, which would enable low cost, mass production of the sensors.

The substrates302,304of the gas or chemical cell may include materials such as silicon (Si) or germanium (Ge), which may be selected to ensure maximum transparency in the electromagnetic wavelength(s) of interest. The electromagnetic wavelength(s) of interest may correspond to the resonance frequency of the vibrational or rotational bond of the target species. Gold (Au) may be selected to act as a guiding layer for the electromagnetic waves, and also to serve as a bonding layer to enable wafers/chips to be bonded together.

FIGS.4A-Hillustrate the fabrication of a gas or chemical cell according to various embodiments by physical deposition of the metals.FIG.4Ais a cross-sectional schematic illustrating the deposition of silicon oxide (SiO2) on one side of a substrate402according to various embodiments. The deposition of silicon oxide (SiO2) to form a silicon oxide (SiO2) layer412on a backside of the substrate402may be optional. The silicon oxide (SiO2) layer412may serve to act as either an anti-reflection coating or chemical coating. The substrate402may include silicon (Si) or germanium (Ge).

FIG.4Bis a cross-sectional schematic illustrating the patterning and etching of the substrate402according to various embodiments. A hole may be formed on the substrate402.

FIG.4Cis a cross-sectional schematic illustrating the deposition of silicon oxide (SiO2) on an opposing side of a substrate402according to various embodiments. The deposition of silicon oxide (SiO2), followed by subsequent patterning and etching to form a silicon oxide (SiO2) layer414on a front side of the substrate402, i.e. within the hole, may be optional. The silicon oxide (SiO2) layer414may serve to act as either an anti-reflection coating or chemical coating. In various other embodiments, instead of SiO2, another non-metal material such as a nitride (e.g. silicon nitride Si3N4), another oxide, or parylene may be deposited to form the layer414.

FIG.4Dis a cross-sectional schematic illustrating the deposition of chromium/titanium (Cr/Ti) and gold (Au) according to various embodiments. Chromium/titanium (Cr/Ti) may be deposited first, followed by gold (Au). The deposition of chromium/titanium (Cr/Ti) and gold (Au) may form metal layers406aon the front side of the substrate402.

FIG.4Eis a cross-sectional schematic illustrating the deposition of a layer of photoresist (PR)416according to various embodiments. Portions of the layer of photoresist (PR)416may then be patterned to expose portions of the underlying metal layers406a.

FIG.4Fis a cross-sectional schematic illustrating the patterning and etching of gold (Au) according to various embodiments. In addition to the etching of gold, the underlying Cr/Ti may also be etched. The exposed portions of the metal layers406a, i.e. the Cr/Ti and Au layers, not covered by the photoresist416may be etched.

FIG.4Gis a cross-sectional schematic illustrating the removal of the patterned photoresist layer416according to various embodiments.

FIG.4His a cross-sectional schematic illustrating the bonding of substrates402,404to form a gas or chemical cell according to various embodiments. Another substrate404may be formed in a similar manner as substrate402shown inFIGS.4A-G. A silicon oxide (SiO2) layer418may be formed on a backside of the substrate404, while another silicon oxide (SiO2) layer420may be formed on a frontside of the substrate404. Metal layers of chromium/titanium (Cr/Ti) and gold (Au) may also be formed on the front side of the substrate404. The metal layers406aformed on the front side of the substate402and the metal layers formed on the front side of the substrate404may be bonded together. The holes on substrates402,404may together form a cavity. As shown inFIG.4F, the metal layers406aformed on the front side of the substate402and the metal layers formed on the front side of the substrate404may collectively be referred to as metal layers406. The metal layers406may extend from between the substrates402,404to the sidewalls of the cavity. In addition, the silicon oxide layer414may be on a first inner surface of the cavity, while the silicon oxide layer420may be on a second inner surface of the cavity opposite the first inner surface. The metal layers406on sidewalls of the cavity may join silicon oxide layer414to silicon oxide layer420.

FIGS.5A-Fillustrate the fabrication of a gas or chemical cell according to various embodiments by electroplating.FIGS.5A-Fmay followFIGS.4A-Cwhich illustrate the initial steps of fabrication of the gas or chemical cell, and which are also applicable to forming the gas or chemical cell by electroplating.

FIG.5Ais a cross-sectional schematic illustrating the deposition of chromium/titanium (Cr/Ti) and copper (Cu) over the front side of the substrate according to various embodiments. The deposition of chromium/titanium (Cr/Ti) and copper (Cu), e.g. via sputtering, may form metal layer506a, which may be a seed layer.FIG.5Amay follow immediately fromFIG.4C. Substrate502inFIG.5Amay correspond to substrate402inFIG.4C, SiO2layer512inFIG.5Amay correspond to SiO2layer412inFIG.4C, and SiO2layer514inFIG.5Amay correspond to SiO2layer414inFIG.4C.

FIG.5Bis a cross-sectional schematic illustrating the deposition of a layer of photoresist (PR)516according to various embodiments. Photoresist may be deposited over the entire front surface, before being patterned to form photoresist layer516within the hole. The photoresist layer516may be away from the sidewalls of the hole to allow for subsequent electroplating on the sidewalls.

FIG.5Cis a cross-sectional schematic illustrating the electroplating of gold (Au) according to various embodiments. A gold layer506bmay be formed on the seed layer506aafter electroplating.

FIG.5Dis a cross-sectional schematic illustrating the removal of the photoresist layer516according to various embodiments. The removal of the photoresist layer516may expose a portion of the seed layer502aunderlying the photoresist layer516.

FIG.5Eis a cross-sectional schematic illustrating the removal of the portion of the seed layer502aunderlying the photoresist layer516according to various embodiments. The exposed portion of the seed layer502a, i.e. Cr/Ti and Cu, may be removed by a wet etch. The portion of the seed layer502aunder the gold layer502bmay be protected by the gold layer502b.

FIG.5Fis a cross-sectional schematic illustrating the bonding of substrates502,504to form a gas or chemical cell according to various embodiments. Another substrate504may be formed in a similar manner as substrate402,502shown inFIGS.4A-C,FIGS.5A-E. A silicon oxide (SiO2) layer518may be formed on a backside of the substrate504, while another silicon oxide (SiO2) layer520may be formed on a frontside of the substrate504. Metal layers of chromium/titanium (Cr/Ti), copper (Cu) and gold (Au) may also be formed on the front side of the substrate504. The metal layers506a-bformed on the front side of the substate502and the metal layers formed on the front side of the substrate504may be bonded together. The holes on substrates502,504may together form a cavity. As shown inFIG.5F, the metal layers506aformed on the front side of the substate502and the metal layers formed on the front side of the substrate504may collectively be referred to as metal layers506. The metal layers506may extend from between the substrates502,504to the sidewalls of the cavity. In addition, the silicon oxide layer514may be on a first inner surface of the cavity, while the silicon oxide layer520may be on a second inner surface of the cavity opposite the first inner surface. The metal layers506on sidewalls of the cavity may join silicon oxide layer514to silicon oxide layer520.

In various embodiments, the chemical sensor may include a measurement gas or chemical cell, and a reference gas or chemical cell. The reference gas or chemical cell may be formed or created at the same time as the measurement gas or chemical cell.FIG.6is schematic showing a perspective view of a chemical sensor including a measurement gas cell, and a reference gas cell according to various embodiments. The wafer-level multi-gas cell may be made of two wafers patterned with specific depth and geometry to allow for gas to flow into the measurement cell and also for light to be guided in this short region. During bonding, the ambient gas may be nitrogen gas (N2). As a result, the reference gas cell may have N2sealed in the reference gas cell. In various other embodiments, any other gases or even vacuum may also be sealed in the reference gas cell. Various embodiments may significantly reduce costs as multiple measurement gas cells along with corresponding reference gas cells may be manufactured at the same time. The sealed ambient gas may be used as reference to reduce or minimize influences and establish baseline. By forming or creating measurement gas cells and reference gas cell at the same time on neighboring locations on the same wafer, the effects for aging and production variation may also be reduced or minimized. Thus, the use of the reference gas cell with the measurement gas cell may be more effective than other known methods.

FIG.7is schematic showing a perspective view of a chemical sensor including a plurality of gas cells according to various embodiments. Various embodiments may enable improved signal noise averaging. Voltage responsivity may improve when the detector active area is decreased although the total optical signal detected is decreased. However, by adding an array of detectors and gas cells, various embodiments may have the same responsivity, while improving the signal-noise ratio (SNR) by applying signal noise averaging. This technique may only be feasible with the design of gas cells and wafer fabrication as described herein, enabling similar gas cells to be manufactured together with minimal variability as seen inFIG.7.

Experimental Results

FIG.8is an image of a wafer including different fabricated and diced gas cell patterns of different designs according to various embodiments. The gas cell patterns may be fabricated based on methods as described herein. The wafer may then be bonded with another corresponding wafer to form different gas cells. The gas cells may be tested with an optical source and an optical detector similar to that shown inFIG.3C. The signal output (peak-to-peak voltage) in volts (V) from the detector was recorded onto the computer.

FIG.9is a plot of voltage (in volts or V) as a function of percentage of Gas Y (in percent or %) showing the gas sensor testing results for Gas Y according to various embodiments. Gas Y is sulfur hexachloride (SF6), and for Gas Y less than 100%, the remaining component (other than SF6) is nitrogen gas (N2). Different concentrations from 0% to 100% of SF6with N2balance were tested. Results show that the chemical sensor according to various embodiments may be able to detect different gas concentration and distinguish them even at higher SF6concentration levels close to 100%.

The chemical sensor may also be tested using liquids. Different concentrations of formic acid from 0% to 7% in methanol solvent were tested. Results show that the chemical sensor may be able to detect different chemical concentrations as seen inFIG.10.FIG.10is a plot of output (in arbitrary units or AU) as a function of time (in seconds or sec) showing the liquid testing results using the chemical sensor according to various embodiments with different concentrations of formic acid in methanol solvent.

Furthermore, due to the small volume of the gas cell at about 5 mm3, a quick response time may be achieved.FIG.11is a plot of signal output (in volts or V) as a function of time (in second or s) illustrating the response time of the chemical sensor when the concentration is changed according to various embodiments. Before t=0, N2was flowing into the sensor, giving an expected output response of 10.26V. At t=0s, the flow of the previous gas was stopped, and a specific concentration of SF6was flowed, which should give an expected signal output of 9.8V.FIG.11indicates a response time of about 11s.

A wafer-level, high concentration gas sensor that enables high gas concentrations to be detected with high resolution and sensitivity has been demonstrated. Various embodiments may overcome the difficulties using traditional machining method that are unable to achieve short pathlengths (10 nm to 1 mm). Furthermore, various embodiments may enable large quantities to be manufactured at the same time with high uniformity. Various embodiments may allow high concentrations to be accurately and reliably measured economically. Various embodiments may also allow the reference gas cells and actual (measurement) gas cells to be made at the same time to reduce variations in production and ensure higher repeatability. Multiple similar gas cells may also be made that are still small in size and with little variations, enabling reduction in detector noise through signal averaging. Various embodiments may have the benefit of withstanding high pressures and reducing the number of joints exposed to high pressures. On the other hand, other methods may require using different materials and multiple joints. Direct growth of filters, anti-reflective coating and chemical coating may be added to enhance the performance of the gas sensor. Various embodiments may be applicable for wafer-scale or chip-scale integrations for optical gas or chemical sensors.