Abstract
A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising a first temperature sensing element located on or within a first dielectric membrane and a second temperature sensing element located on or within a second dielectric membrane. An output circuit is configured to measure a differential signal between the first temperature sensing element and the second temperature sensing element. The fluid sensor comprises a first region configured to be exposed to the fluid, and a second region configured to be isolated from the fluid, where the first dielectric membrane is located in the first region, such that in use, the first dielectric membrane is exposed to the fluid, and wherein the second dielectric membrane is located in the second region such that in use, the second dielectric membrane is isolated from the fluid.
Claims
1. A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first substrate portion and a second substrate portion, the first substrate portion comprising a first cavity and the second substrate portion comprising a second cavity; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane provided on the first cavity and a second dielectric membrane provided on the second cavity; a first temperature sensing element located on or within the first dielectric membrane and a second temperature sensing element located on or within the second dielectric membrane; an output circuit configured to measure a differential signal between the first temperature sensing element and the second temperature sensing element, wherein, the fluid sensor comprises a first region configured to be exposed to the fluid, and a second region configured to be isolated from the fluid, wherein the first dielectric membrane is located in the first region, such that in use, the first dielectric membrane is exposed to the fluid, and wherein the second dielectric membrane is located in the second region such that in use, the second dielectric membrane is isolated from the fluid.
2. A fluid sensor according to claim 1, wherein the fluid sensor comprises: a first set of temperature sensing elements each located on the first membrane, wherein the first set of temperature sensing elements comprises the first temperature sensing element and a third temperature sensing element; and a second set of temperature sensing elements each located on the second membrane, wherein the second set of temperature sensing elements comprises the second temperature sensing element and a fourth temperature sensing element, and wherein the output circuit is configured to measure a differential signal between the first set of temperature sensing elements and the second set of temperature sensing elements.
3. A fluid sensor according to claim 2, wherein the output circuit comprises a Wheatstone bridge, and wherein the first set of temperature sensing elements operate as the first and second legs of the Wheatstone bridge and wherein the second set of temperature sensing elements operate as the third and fourth legs of the Wheatstone bridge.
4. A fluid sensor according to claim 3, wherein the Wheatstone bridge is formed on a same chip as the fluid sensor; and optionally further comprising electrical connections between the first set of temperature sensing elements and the second set of temperature sensing elements, and wherein said electrical connections are on or over the dielectric region.
5. A fluid sensor according to claim 1, further comprising: a first fluid sensitive layer located on or over the first dielectric membrane; a second fluid sensitive layer located on or over the second dielectric membrane; a first electrode in contact with the first fluid sensitive layer; and a second electrode in contact with the second fluid sensitive layer.
6. A fluid sensor according to claim 5, further comprising a control unit configured to receive a resistive or capacitive reading from the first electrode and/or the second electrode, and wherein the control unit is configured to calibrate the sensor using the received resistive or capacitive reading.
7. A fluid sensor according to claim 5, wherein the fluid sensor comprises: a first set of electrodes in contact with the first fluid sensitive layer, wherein the first set of electrodes comprises the first electrode and a third electrode separated from each other by at least a portion of the first fluid sensitive layer; and a second set of electrodes in contact with the second fluid sensitive layer, wherein the second set of electrodes comprises the second electrode and a fourth electrode separated from each other by at least a portion of the second fluid sensitive layer; and optionally wherein a resistance or capacitance of the first fluid sensitive layer and the second fluid sensitive layer is dependent on humidity.
8. A fluid sensor according to claim 1, wherein the semiconductor substrate comprises separate first and second semiconductor substrate sections; or wherein the semiconductor substrate comprises integral first and second substrate sections, and wherein the first dielectric membrane and the second dielectric membrane comprise integral sections of a dielectric layer.
9. A fluid sensor according to claim 1, wherein the first temperature sensing element and the second temperature sensing element are configured to operate as heating elements.
10. A fluid sensor according to claim 9, comprising a controller configured to: drive one or more of the heating elements in an AC mode to modulate the temperature of the heating elements to vary the differential signal; monitor the differential signal at the modulation frequency using a lock-in amplifier and/or based on a Fourier transform-based technique; and selectively differentiate between different fluid components and/or determine the concentration of the different fluid components based on the differential signal.
11. A fluid sensor according to claim 9, comprising two DC current sources configured to independently generate DC currents with alternating polarities; and a controller configured to: drive one or more of the heating elements or a sensing element adjacent to the heating element in using DC pulses from the two DC current sources; and monitor the differential signal based on a two point or a three point DC reversal-based technique; and selectively differentiate between different fluid components and/or determine the concentration of the different fluid components based on a differential signal; or comprising a control unit configured to drive one or more of the heating elements in an AC bias or a pulse bias, and further configured to determine the concentration and type of gas or gases present based on the frequency content of a resulting signal.
12. A fluid sensor according to claim 9, comprising one or more current sources with alternating polarities, and a controller configured to: drive one or more of the heating elements using a current from the one or more current sources; and monitor the differential signal based on a two point or a three point DC reversal-based technique; and selectively differentiate between different fluid components and/or determine the concentration of the different fluid components based on a differential signal; and optionally wherein one or more of the heating elements is driven by a single current source, and wherein the fluid sensor comprises switches configured to change the direction of a current in terminals of the heating element or the sensing element.
13. A fluid sensor according to claim 1, further comprising a first ambient temperature sensor close or adjacent to the first dielectric membrane and a second ambient temperature sensor close or adjacent to the second dielectric membrane, and wherein the fluid sensor is configured to determine a measurement of the fluid concentration or composition based on at least one of: (i) a signal from the first ambient temperature sensor; (ii) a signal from the second ambient temperature sensor; and/or (iii) a differential signal between the first ambient temperature sensor and the second ambient temperature sensor.
14. A fluid sensor according to claim 1, further comprising a first ambient temperature sensor and a second ambient temperature sensor, wherein the first ambient temperature sensor is located in the first region, and wherein the second ambient temperature sensor is located within the second region; and wherein the fluid sensor is configured to determine a measurement of the fluid concentration or composition based on at least one of: (i) a signal from the first ambient temperature sensor; (ii) a signal from the second ambient temperature sensor; and/or (iii) a differential signal between the first ambient temperature sensor and the second ambient temperature sensor.
15. A fluid sensor according to claim 1, wherein the fluid sensor comprises a measurement system configured to: store one or more variables; receive a reading from one or more of the temperature sensing elements; determine the fluid composition based on the reading and the one or more variables; and update the one or more variables based on the reading and/or the determined fluid composition.
16. A fluid sensor according to claim 1, wherein the second region comprises a sealed region comprising a fluid of a known composition.
17. A fluid sensor according to claim 1, wherein the semiconductor substrate further comprises a third substrate portion comprising third cavity, and wherein the dielectric region comprises a third dielectric region provided on the third cavity, and wherein the fluid sensor comprises a third temperature sensing element located on or within the third dielectric membrane; and optionally wherein the third dielectric membrane is located in a third region configured to be isolated from the fluid, and wherein the third region comprises a sealed region comprising a vacuum; and optionally comprising an output circuit configured to measure a signal from the third temperature sensing element, and wherein the fluid sensor is configured to calibrate the differential signal using the measured signal from the third temperature sensing element; or wherein a first surface of the third dielectric membrane is exposed to the fluid and wherein a second surface of the dielectric membrane is isolated from the fluid.
18. A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first cavity and a second cavity; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first cavity of the semiconductor substrate, and a second dielectric membrane located over the second cavity of the semiconductor substrate; two temperature sensing elements on or within the first dielectric membrane and two temperature sensing elements on or within the second dielectric membrane; an output circuit configured to measure a differential signal between the two temperature sensing elements of the first dielectric membrane and the two temperature sensing elements of the second dielectric membrane; wherein the first dielectric membrane forms a first sealed region that, in use, is exposed to the fluid via one or more holes; and wherein the second dielectric membrane forms a second sealed region, wherein the second sealed region is isolated from the fluid when in use.
19. A method of testing a fluid sensor according to claim 1, wherein the second region comprises a sealed region and wherein the sealed region comprises a first fluid composition, the method comprising: placing the fluid sensor in a known atmosphere of a second fluid composition for a predetermined period of time, wherein the second fluid composition is different to the first fluid composition; obtaining a differential signal between the first temperature sensing element and the second temperature sensing element after the predetermined period of time; comparing the obtained differential signal to a predetermined expected signal, wherein the predetermined expected signal represents a differential signal expected when no fluid is exchanged between the sealed region and the known atmosphere; determining whether a leak is present in the fluid sensor using the comparison between the obtained different signal and the predetermined expected signal.
20. A method of manufacturing a fluid sensor for sensing a concentration or composition of a fluid, the method comprising: forming a semiconductor substrate comprising a first substrate portion and a second substrate portion, the first substrate portion comprising a first etched portion and the second substrate portion comprising a second etched portion; forming a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane provided on the first etched portion and a second dielectric membrane provided on the second etched portion; forming a first temperature sensing element located on or within the first dielectric membrane and a second temperature sensing element located on or within the second dielectric membrane; forming an output circuit configured to measure a differential signal between the first temperature sensing element and the second temperature sensing element, wherein, the fluid sensor comprises a first region configured to be exposed to the fluid, and a second region configured to be isolated from the fluid, wherein the first dielectric membrane is located in the first region, such that in use, the first dielectric membrane is exposed to the fluid, and wherein the second dielectric membrane is located in the second region such that in use, the second dielectric membrane is isolated from the fluid.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0240] Some embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings, in which:
[0241] FIG. 1 shows a top view of a thermal conductivity fluid sensor with two membranes, a heating element on each membrane, two temperature sensing elements on each membrane and two temperature sensing elements outside the membrane alongside each membrane;
[0242] FIG. 2 shows a cross-section of a thermal conductivity fluid sensor with two membranes, where one of the membranes is isolated from the environment;
[0243] FIG. 3 shows the cross-section of a thermal conductivity fluid sensor with two membranes formed by a front side etch, where one of the membranes is isolated from the environment;
[0244] FIG. 4 shows the cross-section of a thermal conductivity fluid sensor with two membranes formed by an isotropic front side etch, where one of the membranes is isolated from the environment;
[0245] FIG. 5 shows the cross-section of a thermal conductivity fluid sensor with two membranes formed by a timed front side etch, where one of the membranes is isolated from the environment;
[0246] FIG. 6 shows the cross-section of a thermal conductivity fluid sensor with two membranes packaged in a flip chip method;
[0247] FIG. 7 shows the cross-section of a thermal conductivity fluid sensor with two membranes packaged in a flip chip method where the membrane exposed to the environment has an encapsulation with holes;
[0248] FIG. 8 shows a top view of a thermal conductivity fluid sensor with two membranes, a heating element on each membrane, two temperature sensing elements on each membrane and an ambient temperature sensor outside the membranes;
[0249] FIG. 9 shows a top view of a thermal conductivity fluid sensor with two membranes, a heating element on each membrane, two temperature sensing elements on each membrane and an ambient temperature sensor outside the membranes with no slots on the membranes;
[0250] FIG. 10 shows a top view of a thermal conductivity fluid sensor with two membranes, a heating element on each membrane, two temperature sensing elements on each membrane and an ambient temperature sensor outside the membranes with an array of holes on the membranes;
[0251] FIG. 11 shows a top view of a thermal conductivity fluid sensor with two membranes, a heating element on each membrane, two temperature sensing elements on each membrane and an ambient temperature sensor outside the membranes, where the temperature sensing elements within each membrane are different;
[0252] FIG. 12 shows a circuit diagram of a thermal conductivity sensor comprising two Wheatstone bridges;
[0253] FIG. 13 shows a circuit diagram of a thermal conductivity sensor comprising a single Wheatstone bridge;
[0254] FIG. 14 shows a circuit diagram of a thermal conductivity sensor where a balance voltage can be used to balance the Wheatstone bridge;
[0255] FIG. 15 shows a circuit diagram of a thermal conductivity sensor with a feedback circuit to control the heater;
[0256] FIG. 16 shows the top view of a thermal conductivity fluid sensor with two membranes and two resistors on each membrane;
[0257] FIG. 17 shows the circuit for a thermal conductivity sensor where there are two resistors on each membrane acting as heating elements and temperature sensing elements;
[0258] FIGS. 18 shows the circuit for a thermal conductivity sensor where there are two temperature sensing elements on each membrane, and one of the temperature sensing elements on each membrane can be switched to also operate as a heating element;
[0259] FIG. 19 shows the circuit for a thermal conductivity sensor where the voltage on one of the branches of the Wheatstone bridge is controlled by a feedback circuit;
[0260] FIGS. 20 shows the circuit for a thermal conductivity sensor where the voltage on one of the branches of the Wheatstone bridge is controlled by a feedback circuit which also takes input from an external sensor;
[0261] FIG. 21 shows the top view of a thermal conductivity sensor where there are two membranes and each membrane has two diodes that are temperature sensing elements;
[0262] FIG. 22 shows the top view of a thermal conductivity sensor where there are two membranes and each membrane has two temperature sensing elements, one of which is a resistor and one is a diode;
[0263] FIG. 23 shows the circuit for a thermal conductivity sensor where the temperature sensing elements are diodes;
[0264] FIG. 24 shows the circuit for a thermal conductivity sensor where the temperature sensing elements are diodes and resistors;
[0265] FIG. 25 shows the top view of a thermal conductivity fluid sensor where there are two pairs of membranes;
[0266] FIG. 26 shows the cross-section of a thermal conductivity fluid sensor package with a sealed region and an exposed region, and two sensor chips and an ASIC chip;
[0267] FIG. 27 shows the cross-section of a thermal conductivity fluid sensor package with a sealed region and an exposed region, and one sensor chip and an ASIC chip;
[0268] FIG. 28 shows the cross-section of a thermal conductivity fluid sensor package with a sealed region and an exposed region, and two sensor chips and an ASIC chip with one sensor chip stacked on the ASIC chip;
[0269] FIG. 29 shows the cross-section of a thermal conductivity fluid sensor package with a sealed region and an exposed region, and two sensor chips and an ASIC chip with one sensor chip stacked on the ASIC chip, and the sensor chips attached in a flip chip method;
[0270] FIG. 30 shows the cross-section of a thermal conductivity fluid sensor package in a chip scale package with the ASIC chip as part of the package;
[0271] FIG. 31 shows the cross-section of a thermal conductivity fluid sensor with two ambient temperature sensors in the sealed region, and two in the exposed region;
[0272] FIG. 32 show an alternate circuit for a thermal conductivity fluid sensor where the ambient temperature sensors are also in the bridge;
[0273] FIG. 33 shows an alternate circuitry for a thermal conductivity fluid sensor where the read out circuit also measures two ambient temperature sensors;
[0274] FIG. 34 shows an alternate circuitry for a thermal conductivity fluid sensor where the heater also acts as a temperature sensor on the membrane, and the ambient temperature sensors are also measured in a bridge;
[0275] FIG. 35 shows an alternate circuitry for a thermal conductivity fluid sensor where the differential signals from the on-membrane and off-membrane temperature sensing elements go through a further differential amplifier;
[0276] FIG. 36 shows a flow diagram of a method to better improve the sensitivity of the measured signal;
[0277] FIG. 37 shows a 3D schematic of a thermal conductivity fluid sensor.
[0278] FIG. 38 shows the top view and cross-section of a thermal conductivity fluid sensor comprising two sealed cavities, with one containing holes.
[0279] FIG. 39 shows a graph with thermal conductivities of air and different gases at different temperatures.
[0280] FIG. 40 shows an alternate circuit diagram for a thermal conductivity fluid sensor.
[0281] FIG. 40a shows the current through the heater for the circuit in FIG. 38.
[0282] FIG. 41 shows an alternate circuit diagram for a thermal conductivity fluid sensor.
[0283] FIG. 41a shows the current through the heater and the voltage across it, as well as the reading computation for the circuit in FIG. 43.
[0284] FIGS. 42 and 43 show alternate circuit diagrams for a thermal conductivity fluid sensor based on FIG. 41a.
[0285] FIG. 44 shows a comparison between DC and current-reversal DC voltage measurements.
[0286] FIG. 45 shows a circuit schematic for driving the sensors in who different current directions.
[0287] FIG. 46 shows a table of thermal properties of different gases.
[0288] FIG. 47 shows a top view of a thermal conductivity fluid sensor with two membranes and two temperature sensors on each membrane. The temperature sensors are connected together in a bridge configuration located on the sensor chip.
[0289] FIG. 48 shows a circuit diagram of a Wheatstone bridge driven in constant current.
[0290] FIG. 49 shows a cross-section schematic of a thermal conductivity fluid sensor with an additional layer between the dielectric layer and the substrate, provided with a cavity portion.
[0291] FIG. 50 shows schematically a cross-section of a further thermal conductivity fluid sensor having a fluid sensitive material and electrodes connected to the fluid sensitive material.
[0292] FIG. 51 shows a flow chart of a method for testing a thermal conductivity fluid sensor as described herein.
[0293] FIG. 52 shows schematically a cross-section of a thermal conductivity fluid sensor having three dielectric membranes. In use, a first reference membrane is sealed from the atmosphere, a second membrane has holes and is exposed to the atmosphere, and a third reference membrane is exposed to the atmosphere from a top surface of the membrane, but is sealed on a lower surface of the membrane.
[0294] FIG. 53 shows schematically a cross-section of a thermal conductivity fluid sensor having three dielectric membranes. In use, first and second reference membranes are sealed from the atmosphere, and a third membrane is exposed to the atmosphere.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0295] Some examples of the disclosed device are given in the accompanying figures.
[0296] FIGS. 1 & 2 show the top view and cross-section of a thermal conductivity fluid sensor. It comprises a chip 1 made of a semiconductor substrate 11 and a dielectric layer 10, with a first dielectric membrane 4, and a second dielectric membrane 4A suspended over etched portions of the semiconductor substrate. The first dielectric membrane 4 has a heating element 2 and two resistive temperature sensing elements 8 and 8A. Tracks 7 provide electrical connection from the bond pads to the heating element 2. There are also recessed regions 12 in the shape of slots within the membrane. Similarly the second dielectric membrane 4A has a heating element 2A and two resistive temperature sensing elements 8B and 8C, and recessed regions 12 in the shape of slots. There are a further 4 temperature sensing elements 3, 3A, 3B, 3C on the chip. The chip is attached to a base 101, and a cap 110 is placed over the second dielectric membrane 4A resulting in a sealed region 200, where the fluid (typically but not limited to a gas) is trapped, and the composition of the trapped fluid 200 doesn't change with the change in fluid or environment around the sensor. The first dielectric membrane 4 is exposed to the environment and the thermal behaviour of the membrane changes with change in fluid around the sensorfor example is the fluid composition changes to change the thermal conductivity of the fluid. Hence the power consumption required by the heating element 2 on membrane 4 is affected by change of composition of the surrounding fluid, while the power consumption required by the heating element 2A in membrane 4A does not change.
[0297] The membranes 4, 4A are formed by back side etching using DRIE resulting near vertical sidewalls.
[0298] For example if the encapsulated fluid 200 is air, and the surrounding fluid is also air, then if the concentration of carbon dioxide in the surround fluid/air increases, the overall thermal conductivity of the surrounding fluid will decrease as the thermal conductivity of carbon dioxide is smaller than air. In that case the heating element 2 on membrane 4 will have slightly lower thermal losses as compared to heating element 2A on membrane 4A. So heating element 2 will require slightly less power to maintain membrane 4 at a target temperature as compared to the power required by heating element 2A to maintain membrane 4A at the same temperature. Alternately if the same bias (e.g. current, voltage, power) is applied to both heating elements 2,2A, then membrane 4 will reach a slightly higher temperature than membrane 4A.
[0299] Circuitry is used to drive both the membranes to a high temperature. An output circuit uses at least the temperature sensing elements 8, 8A, 8B, 8C to determine the change in fluid composition or the target gas. The circuitry might drive both the heaters 2,2A in a constant bias current, voltage or power. In this case the temperature of membrane 4A will stay the same if there is a change in fluid composition, but the temperature of membrane 4 will change. Using a differential output circuit such as a Wheatstone bridge will then give an output based on change in the fluid composition. Using two temperature sensors from each membrane rather than a single temperature sensor from each membrane in the Wheatstone bridge means that the sensitivity is doubled as it becomes a half bridge rather than a quarter bridge. Other drive methods can also be used to keep the Wheatstone bridge outputting zero volts while adjusting the bias on one of the heating elements 2,2A, and using the bias required to keep the zero output from the Wheatstone bridge to determine the fluid composition.
[0300] The encapsulated fluid 200 can be air, dry air, synthetic air, an inert gas such as nitrogen or argon. Alternately 200 could be a vacuum or a partial vacuum.
[0301] Circuitry to measure the sensor can also comprise two Wheatstone bridges, the first Wheatstone bridge comprising temperature sensing elements 8, 8A, 3, 3A, and the second Wheatstone bridge comprising temperature sensing elements 8B, 8C, 3B, 3C.
[0302] In this figure the heating elements are shown as wire heaters, but can be any other shape such as meander, ring, multi ring, circular etc. Similarly the temperature sensing elements can be any shape as well. In this figure the temperature sensing elements and the heating elements are all made from the same material layer and laterally spaced from each other. But they can also be made in different layers, and of different materials, and be either laterally or vertically spaced from each other, and can also be vertically stacked, or a combination. The resistors maybe made of a CMOS metal such as aluminium, tungsten, titanium or copper, or a non-CMOS metal such as gold or platinum, or from polysilicon or single crystal silicon. Similarly the membranes are shown as square with rounded corners, but can be square, rectangle or circular.
[0303] FIG. 3 shows the cross-section of an alternate thermal conductivity fluid sensor where the membranes 4,4A are formed by a front side anisotropic etch such as KOH or TMAH. In this case the etched portions of the substrates do not extend all the way to the bottom of the substrate, rather they stop at the crystal planes of the substrate. Such an etching usually results in a suspended membrane or micro-bridge, which not supported by the substrate along its entire perimeter, but rather is suspended by one or more beams.
[0304] FIG. 4 shows the cross-section of an alternate thermal conductivity fluid sensor where the membrane 4,4A are formed by a front side isotropic etch which results in curved surface of the substrate etched portion.
[0305] FIG. 5 shows the cross-section of a thermal conductivity fluid sensor where the membrane is created by a front side anisotropic etch, but the etch process is a timed etch, so as to result in a trapezoid shaped cavity or etched portion within the substrate.
[0306] FIG. 6 shows the cross-section of a thermal conductivity fluid sensor where the chip is packaged in a flip-chip method. In this case the chip is attached to a PCB 35, and electrically connected by the use of solder balls 36. Membrane 4A is sealed by a lid 110 to seal the top, and also a sealant 140 to seal the fluid between the membrane and the PCB. The sealant 140 can be a polymer. It can also be a mixture of components and materials such as a rubber/glass/metal ring coated with a polymer.
[0307] FIG. 7 shows the cross-section of another example of a thermal conductivity fluid sensor in a flip-chip package, where the lid 110 extends to the cavity portion of the exposed membrane as well, but there are holes in that portion of the lid to allow exchange with the surrounding fluid. The holes can help protect the membrane during handling, and also dust or moisture. Additional filters may also be added to protect against particles or moisture.
[0308] FIG. 8 shows the top view of another example of a thermal conductivity fluid sensor there is only one temperature sensing element 130 outside the membrane. The temperature sensing element 130 is used to measure the ambient temperature and helps provide a temperature compensation to the measured value of the fluid composition. The temperature sensing element shown in this example is a resistive temperature sensor, but can also be a diode, transistor a VPTAT or IPTAT circuit.
[0309] FIG. 9 shows the top view of another example of a thermal conductivity fluid sensor where there are no recessed regions within the membrane.
[0310] FIG. 10 shows the top view of another example of a thermal conductivity fluid sensor where the recessed regions 12 are an array of holes rather than slots. It should be noted that while FIGS. 8 and 10 show two examples or recessed regions many other shapes and sizes of recessed regions are possible. The recessed regions can also be in different locations on the membrane.
[0311] FIG. 11 shows the top view of another example of a thermal conductivity fluid sensor where the temperature sensing elements 8 and 8A have a different shape and resistance value. Similarly temperature sensing elements 8B and 8C are also different. But element 8 is identical to element 8C and element 8A is identical to element 8B. In this case although the temperature sensing elements within membrane 4 are different, they are identical to the corresponding temperature sensing elements of membrane 4A. The output circuit can still measure the differential signal across these temperature sensing elements as they are identical in the different membranes.
[0312] FIG. 12 shows circuitry for driving the thermal conductivity fluid sensor and measuring the output from the thermal conductivity fluid sensor shown in FIG. 1. Heating elements 2, 2A are both driven by a constant current source. The output circuit comprises two Wheatstone bridges. One bridge comprises temperature sensing elements 8, 8A from the first membrane, and temperature sensing elements 3,3A. The other bridge comprises temperature sensing elements 8B, 8C from the second membrane. Instrumentation amplifier 210A gets inputs from the left branch of both the Wheatstone bridges, while instrumentation amplifier 210B get inputs from the right branches. These then feed into instrumentation amplifier 111.
[0313] FIG. 13 shows circuitry for driving the thermal conductivity fluid sensor and measuring the output from the thermal conductivity fluid sensor shown in FIG. 8. Heating elements 2, 2A are driven by constant current sources. The output circuit comprises a Wheatstone bridge with temperature sensing elements 8,8A, 8B, 8C, and an instrumentation amplifier 55. If an identical current source is applied to both heating elements 2, 2A, then in normal conditions the temperature of both the membrane 4,4A will be the same, resulting identical resistance of the temperature sensing elements 8,8A, 8B, 8C, giving a zero-volt output on the output circuit. However if the surround fluid changes, for example to have a higher concentration of carbon dioxide, then the thermal conductivity of the surround fluid will decrease and power losses from membrane 4 will decrease, resulting in a slightly higher temperature than membrane 4A. In this case the temperature sensing elements 8,8A will have a slightly higher resistance than temperature sensing element 86,8C, resulting in a misbalance in the Wheatstone bridge and the output circuitry will give a non-zero output that can be used to determine the concentration of carbon dioxide present.
[0314] FIG. 14 shows another example of circuitry to drive and measure the output from the thermal conductivity fluid sensor. Except in this case both arms of the bridge have a different bias voltage. One arm is kept at a constant voltage of Vref, while the voltage to the other arm Vbal can be varied. This can serve many purposes, the main one being in calibration. Due to manufacturing tolerances there may be some mismatch between the resistive temperature sensors 8, 8A, 8B, 8C and when Vref and Vbal are equal the bridge might still give a non-zero output in normal conditions. So during calibration in a known environment, Vbal is adjusted until the output becomes zero. This value of Vbal is stored, and is also applied when making a measurement.
[0315] FIG. 15 show another example of circuitry to drive and measure the output from the thermal conductivity fluid sensor. In this case there is always a constant bias applied to the heating 2A, but the heating element 2 is controlled by a feedback loop from the Wheatstone bridge and amplifier circuit. In this case a heater control circuit varies the bias to heating element 2 until the output from the Wheatstone bridge and amplifier is zero, and the bias needed to drive the heating element is then used to determine the composition of the surrounding fluid.
[0316] FIG. 16 shows the top view of another example of a thermal conductivity fluid sensor comprising two membranes 4, 4A with two temperature sensing elements 8, 8A on membrane 4, and two temperature sensing elements 8B, 8C on membrane 4A. Unlike FIG. 8, there is no additional heating element on either of the membranes, instead one, or both of the temperature sensing elements can be used as the heating element.
[0317] FIGS. 17 to 20 give some examples of circuits to drive and measure the output from the thermal conductivity fluid sensor. In FIG. 17 both the temperature sensing elements on both the membranes, 8, 8A, 8B, 8C all act as heating elements as well. A bridge circuit incorporates all four elements, and the voltage bias on the Wheatstone bridge also provides the bias for heating up the membranes, and also allows a differential measurement between them.
[0318] In FIG. 18 the temperature sensing elements 8 and 8C both act as heating elements as well as temperature sensing elements. To operate as heaters switches 70 and 70A are closed, connecting one of the terminal of elements 8, 8C directly to ground. When a measurement is to be made then switches 70, 70A are opened to connect element 8A and 8B as well, completing the bridge. The voltage bias may be lower during the measurement time. The measurement time should be short compared to the thermal time constant of the membranes so that the measurement does not affect the temperature.
[0319] In FIG. 19 all four elements are used as both heaters and temperature sensing elements, but different voltages can be applied to each branch of the Wheatstone bridge. Voltage V1 is kept constant while a feedback circuit is used to vary V2. Such a circuit can be used in two ways. In one method the feedback circuit is used in calibration to adjust V2 until the amplifier gives a zero-volt output in a known environment, and then the same value of V2 is used always during measurement. In the second method the feedback circuit always varies the value of V2 until the output from the amplifier is zero, and the value of V2 required is used to determine the composition of the fluid.
[0320] FIG. 20 shows another circuit example which is similar to FIG. 19, except the feedback circuit may also use data from one or more external sensors. For example an external measurement of humidity, pressure or temperature can be used to apply an adjustment to the voltage V2 as part of compensating for signal changes due to these factors.
[0321] FIG. 21 shows the top view of a thermal conductivity fluid sensor where the temperature sensing elements 8, 8A, 8B, 8C are diodes instead of resistors. FIG. 22 shows an example where one of the temperature sensing element on each membrane is a resistor and one temperature sensing element is a diode.
[0322] FIG. 23 shows an example circuit for the thermal conductivity sensor shown in FIG. 21. This is similar to the circuit with resistors but the resistors are replaced with diodes.
[0323] FIG. 24 shows an example circuit for the thermal conductivity sensor shown in FIG. 22. Two of the resistors in the Wheatstone bridge are replaced by diodes. However, the connections of the elements are also changed, where elements 8A and 8B are swapped. This is because of the different behaviour of resistors and diodes. During operation if there is more carbon dioxide present, then the temperature of membrane 4 (and hence temperature sensing elements 8, 8A) will decrease. The resistance of temperature sensing element 8A will decrease, while the forward voltage of the diode 8 will increase. In this case, it's advantageous to have them both in the same branch of the Wheatstone bridge as the effect from both will add together.
[0324] FIG. 25 shows an example of a thermal conductivity fluid sensor comprising two pairs of membranes (4, 4A) and (4B, 4C). In this case each pair can be driven at a different temperature and the data can be analysed to improve the selectivity and accuracy of the sensor.
[0325] FIG. 26 shows the schematic cross-section of a thermal conductivity sensor package comprising of two sensor chips 111,112 and an ASIC chip 103. The package comprises a base 101, and a lid 102, where the lid has one or more holes. There is a package wall 110 designed such that the package consists of two regions. One region is either hermetically, or semi-hermetically sealed from the ambient environment. The second region has one or more holes in the lid making it open to the environment. The ASIC 103 and one of the sensor chips 112 are placed in the sealed region, and one of the sensor chips 111 is placed in the region open to the environment. Preferably the sensor chips 111 and 112 are identical. In this way sensor chip 112 is always exposed to a known environment, while sensor chip 111 is exposed to the ambient environment, and a differential signal between the two can be used to determine the concentration of the target gas in the ambient environments. The environment in the sealed region can be 100% target gas, a known quantity of target gas in air, synthetic air, pure nitrogen, an inert gas or any other gas or mixture of gases.
[0326] FIG. 27 shows the schematic cross-section of a thermal conductivity sensor package comprising a sealed region and an exposed region, but only one sensor chip 113. The wall 110 extend from the lid to the top of chip 113 such that one membrane from chip 113 is in the sealed region, and one membrane is in the exposed region. In this way a single sensor chip can be used while having two regions within the package
[0327] FIG. 28 shows the schematic cross-section of a thermal conductivity sensor package comprising a sealed region and an exposed region, an ASIC chip 103 and two sensor chips 111, 112 where one of the sensor chips 112 is assembled on top of the ASIC chip 103. In this case the foot print of the package is smaller, while the height can be higher.
[0328] FIG. 29 show the schematic cross-section of a thermal conductivity sensor package comprising two sensor chips where both the sensor chips 111, 112 are packaged in a flip-chip method. Conductive joints 115 are used to electrically connect the chips to the package or the ASIC. The conductive joints could be solder balls for example. This figure shows the chip 112 above the ASIC, but it could also be side by side with the ASIC and in a flip chip configuration.
[0329] FIG. 30 shows another schematic cross-section of a thermal conductivity package with the ASIC chip as the package base. In this case a lid 117 is formed on top of the ASIC chip, and designed such that it separates the package in to a sealed region and an exposed region. There is one sensor chip 112 attached to the ASIC chip by flip chip in the sealed region, and one sensor chip 111 attached to the ASIC chip in the exposed region. Through Silicon Vias (TSVs) 116 within the ASIC chip 103 allow electrical connection to the base of the package. Although this figure show the sensor chips connected by flip chip, it is also possible that the sensor chips are right side up and electrically connected to the ASIC by wire bonds.
[0330] FIG. 31 shows another schematic cross-section of a thermal conductivity sensor. When compared to FIG. 2, the ambient temperature sensors 3, 3A, 3B, 3C are kept distinctly either within the exposed region or in the sealed region. In this example implementation, the temperature sensors 3 and 3A are in the exposed region of the sensor and the temperature sensors 3B and 3C are in the sealed region of the sensor. During operation, the exposed region and the reference region may have a difference in temperature. Although this difference in temperature is likely to be small, it can still be significant enough to affect the measurement results. If the temperatures of the exposed region and the sealed region are known, then this can be used to account for or cancel out the effects of these temperature changes. There are various circuit topologies that can be used to do this, and FIGS. 32-34 illustrate three such example topologies.
[0331] In FIG. 32, the ambient temperature sensors 3, 3A, 3B, 3C are part of a Wheatstone bridge, the output of which is connected to a differential amplifier 55. The remaining circuit to drive the heater and the on-membrane temperature sensors may be the same as shown in FIG. 13, and comprise a Wheatstone bridge with temperature sensing elements 8,8A, 8B, 8C, and an instrumentation amplifier 55. The outputs from both of the Wheatstone bridges is provided to a readout circuit block 56 that uses both of the inputs to determine the concentration of the target gas. The circuit block can consist of any kind of circuitry including analog or digital circuitry, as well as memory, microcontroller or microprocessor circuits.
[0332] FIG. 33 is an alternate circuit for a device where there is only one temperature sensor 3 in the exposed region, and only one temperature sensor 3B in the sealed region. In this example, the readout circuitry directly reads the temperature from the temperature sensors 3, 3B. The readout circuitry can also include circuitry to measure a differential signal between sensors 3 and 3B.
[0333] FIG. 34 is an alternate circuit to FIG. 32, for a device where the on-membrane temperature sensors 8, 8A, 8B, 8C are configured to also act as heaters. The Wheatstone bridge comprising temperature sensors 8, 8A, 8B, 8C both drives the temperature sensors as a heater, and also allows a differential signal between the exposed and sealed sensors to be measured.
[0334] FIG. 35 is a further alternate example circuit compared to FIG. 34, where the differential signal from the on-membrane temperature sensor elements 8, 8A, 8B, 8C, 8D, and the differential signal from the off-membrane temperature sensor elements 3, 3A, 3B, 3C are subtracted from each other using a third differential amplifier 55. This allows the effect of temperature differences between the first and second dielectric membranes to be cancelled out in the circuitry without the need for any further circuitry or signal processing.
[0335] FIG. 36 shows an example flow diagram of a method 3600 to improve the accuracy of a sensor. This method may be used in implementations in which the sealed region is not fully sealed (i.e. in which the region only has a partly or semi-hermetic seal), and may account for how much gas may have leaked into or out of the sealed region. In step 3601 a variable X is set to an initial value. The variable X may represent the amount of target gas in the sealed region. In step 3602, a reading is taken from the sensor device, and this reading along with the variable X is then used to determine the concentration of the target gas or chemical in step 3603. Based on the determined concentration, as well as the current value of variable X, the variable X is then updated in step 3604. Steps 3602-3604 may be repeated by getting another reading from the sensor device, but now using the new value of variable X to determine concentration. It will be understood that the name X for the variable is merely provided as an example, and that any variable name is may be used.
[0336] It will be understood that the method may be modified in several ways within the scope of the present disclosure. For example, the method may include more than one variable (e.g. X, Y, Z, . . . ) to be updated and used for determining gas concentration rather than just a single variable (X). Additionally or alternatively, the value stored in the one or more variables may be based just on the determined gas concentration without taking into account the existing values of the variable. In implementations using more than one variable, the updated value may be based on the value of one or more of the other variables (either in addition to or instead of the original value for the variable).
[0337] FIG. 37 shows a 3D schematic of the sensor which is formed by wafer bonding. In this example, there is a sensor chip comprising a semiconductor substrate 11 having two etched portions and covered with a dielectric layer. There are two dielectric membranes within the dielectric layer 4, 4a that are adjacent to the etched portions. The membranes have temperature sensors (not shown in diagram for clarity). There is a further substrate 101 provided below substrate 11 to seal the etched portions of substrate 11. A lid 102 is provided above substrate 11. The lid has open volumes above both membranes 4, 4a. However, in case of membrane 4a the open volume is completely sealed, while for membrane 4 there is a hole 108 in the lid 102 that allows air to diffuse in and out from this volume. Bond pads 6 can be used to make electrical connections to other (e.g. external) circuitry, and these are not covered by the lid 102. Tracks 7 provide electrical connections between the bond pads 6 and the temperature sensors on the membranes 4, 4a.
[0338] FIG. 38 shows another example thermal conductivity fluid sensor. In this design there are two dielectric membranes, a first membrane 4 and a second membrane 4a. The two membranes are identical, apart from the two (or more) holes 5 on membrane 4. Membrane 4a has no holes. Both membranes have identical heaters 2 and 2a, and sensing elements 8 and 8a. The thermal conductivity fluid sensor is located on a base of a package 3, where the cavities 9 and 9a under the membranes 4 and 4a are both sealed. Such a construction can be designed so that the sensing element 8a, which is exposed to the gas or fluid only on one side of the membrane 4a can be used as a reference for the sensing element 8, which is exposed to the gas on both sides of the membrane 4. This design could lead to a faster and more reliable temperature compensation, since both sensing elements are directly exposed to the same environment, however only sensing element 8 is exposed to the higher gas or fluid concentration.
[0339] FIG. 39 shows a graph plotting the thermal conductivity of air and different gases across different temperatures. The graph illustrates that if the heater is driven at around 800K, then air and carbon dioxide have the same thermal conductivities, and any deviation from normal is caused by other effects, such as humidity. The heater can then be run at a lower temperature where in addition to other effects, carbon dioxide also causes a deviation in signal. Using algorithms or a look up table the deviation due to other effects can then be cancelled to determine the deviation solely due to carbon dioxide.
[0340] FIG. 40 shows an example circuit for measuring a thermal conductivity fluid sensor such as that shown in FIG. 38. In this circuit two AC current sources 45 and 45a are used with heaters 2 and 2a respectively. Both current sources can independently generate square wave signals, such as the signal shown in FIG. 40a, with an adjustable intensity and frequency. The differential voltage signal, measured across heaters 2 and 2a, is then processed by a lock-in amplifier 55, or a fast Fourier transform (FFT)-based digital signal processing (DSP) circuit.
[0341] FIG. 41 shows another example circuit for measuring a thermal conductivity fluid sensor such as that shown in FIG. 38. In this circuit two reversible DC current sources 45 and 45a are used with heaters 2 and 2a respectively. Both current sources can independently generate currents with alternating polarities, as shown in FIG. 41a, with adjustable intensity and frequency. In this case the voltage measurements on each heater are performed based on a three-step delta method as detailed in FIG. 41a. This three-step delta method may offer significant advantages over other DC resistance measurement techniques in reducing the impact of or overcoming errors due to changing temperature.
[0342] FIG. 42 shows a variation of the circuit presented in FIG. 41. In this circuit two DC current sources 45 and 45a are used with heaters 2 and 2a respectively, while two reversible DC current sources 46 and 46a, providing a much smaller current, are used with sensing elements 8, 8a and 8b, 8c respectively. The much smaller current level provided by the reversible DC current sources 46 and 46a when compared to e.g. the circuit of FIG. 43, aids in reducing transient effects due to current switching and thus may allow for a faster and more accurate delta reading.
[0343] FIG. 43 shows variation of the circuit presented in FIG. 42. In this circuit the sensing element 8, 8a and 8b, 8c are connected in series and driven by a single reversible DC current source 46. Using a single current source may improve the circuit immunity to common mode noise while simplifying circuit overall.
[0344] FIG. 44 shows a comparison between ?1200 DC voltage measurements of a ?60? heater made with ?8 mA test current taken approximately over 120 seconds. The DC measurements fluctuate with a voltage error of up to 30%, whereas the three-point DC reversal method measurements fluctuate with less than 5% error. These figures can be further significantly improved by using a smaller (e.g. less than a few ?A) test current.
[0345] FIG. 45 shows a circuit schematic for driving the sensors in two different current directions. There is a control 301 that provides the electrical bias to the sensors 302. The sensors are read by a read out circuit 303. The transistors 305, 306, 307 and 308 control the direction of current within the sensors 302. When transistors 305 and 306 are on, and 307 and 308 are off then the current flows in one direction through the sensors. While when the transistors 305 and 306 are off, and transistors 307 and 308 are on, the current flows in the opposite direction. This method can be used to improve the accuracy of measurements using the delta method. The control system 301 can be just a current source, or a voltage source, or a more complex circuit. The sensors 302 may be e.g. a temperature sensing resistor, or may comprise more than one resistor. For example, the more than one resistors may be provided in a bridge configurationwhere either all the branches have active sensors, and/or some branches have fixed resistors. The read out circuit 303 can have a differential amplifier, filter and/or an analog to digital circuit.
[0346] FIG. 46 provides the thermal properties of different example gases of interest as well as those of dry and wet air (at standard temperature and pressure). The values of nitrogen and oxygen are also provided to demonstrate how sensitive the values can be to the oxygen content of air.
[0347] FIG. 46 also shows the thermal response time relative to dry air for each of these gases. For example, it can now be seen that Helium gas is 8.3? faster than dry air and 7.1? faster than wet air. It can be seen that hydrogen gas is 6.9? faster than methane. Wet air is 1.13 or 13% faster than dry air. Finally, it can be seen that CO2 is 2.1? slower than dry air and 2.4? slower than wet air.
[0348] It is now possible to determine the gas type from the thermal response time and hence, knowing the gas, also determine the concentration of the gas in air. It is also possible to determine the different gases in the mixture because there will be two distinct thermal constants in the TC response. For example, one faster one for H2 and one much slower one (?6.9) for CH4.
[0349] Driving the thermal conductivity heater with an AC signal (or using a pulse) will result in different frequency responses according to the gas type and concentration. The frequency content of the signal (e.g. an FFT) will show which gas is present by a characteristic frequency and the height of the FFT peak will give its concentration. In this way we can determine the type of gas when the gas is unknown and also determine the gases present in a gas mixture as well as their concentrations.
[0350] Finally, it should be noted that the thermal time constant of dry and wet air are similar (12% difference) and very different to that of H2 and CO2. In other words, the relative humidity of the air will not significantly affect the signal at the frequency for CO2 or H2.
[0351] This AC method decouples the thermal signal of the target gas (for example CO2) from that of a variable background gas (for example, other components of air such as nitrogen and/or oxygen), and therefore provides a much more accurate way of measuring a gas concentration or composition than DC techniques.
[0352] FIG. 47 shows a top view of a thermal conductivity fluid sensor with two membranes and two temperature sensors on each membrane. The temperature sensors 8, 8A, 8B, 8C are connected together as a bridge (such as a Wheatstone bridge) on the sensor chip. An additional metal layer 401 may be used to help with the routing of the connections. The additional metal layer 401 and any other connections between the temperature sensors may be formed on or within the dielectric layer or semiconductor substrate.
[0353] Such a configuration on the sensor chip can be used in conjunction with a separate ASIC chip. Alternatively, it can be used along with discrete (component based) circuitry. Forming the Wheatstone bridge connections between the temperature sensing elements on the sensor chip can result in better symmetry within the bridge. Additionally, parasitics and noise can be minimised. Furthermore, reliability may be improved and complexity reduced as a lower component count and less external connections are needed. This example shows separate heating elements and temperature sensing elements. However it will be understood that the heating elements may be omitted and the temperature sensing elements may be used as heating elements while they are connected up in a bridge configuration.
[0354] FIG. 48 shows a circuit diagram of a Wheatstone bridge driven in constant current mode. In this circuit, the temperature sensing elements are used as heating elements, and the heating elements are connected in a Wheatstone bridge configuration. The bridge is driven through a constant current supply, and a differential measurement is taken on the Wheatstone bridge.
[0355] FIG. 49 shows a cross-section schematic of a thermal conductivity fluid sensor with an additional layer between the dielectric layer and the substrate, provided with a cavity portion. In this example, there is an additional layer 402 located between the dielectric layer and the substrate. In this case, there is no cavity within the substrate, but there are cavities 403 and 403A within the additional layer 402, and the dielectric membranes are over the cavities 403 and 403A formed within the additional layer. Such a structure can be formed by bulk micromachining and allows very controlled and thin gaps to be provided below the membrane, resulting in improved sensitivity to thermal conductivity of the target gas.
[0356] Layer 402 may comprise more than one layers, and one or more of the layers may have a cavity within them. Furthermore, in addition to the cavities in layer 402, there may be a further cavity in the substrate, e.g. for increased resistance to manufacturing tolerances.
[0357] FIG. 50 shows schematically a cross-section of a further thermal conductivity fluid sensor having a fluid sensitive material and electrodes in contact with the fluid sensitive material. The thermal conductivity sensor includes electrodes 404 and 404A, and fluid sensing layers 405 and 405A comprising a fluid sensitive material. The electrodes 404, 404A may be made of aluminium, platinum, gold, tungsten or any other metal. The electrodes 404, 404A may be used as a humidity sensor, the sensing materials 405 and 405A may comprise alumina or a different humidity sensitive material. The presence of humidity may change the dielectric constant of layers 405/405A, and hence the capacitance on the electrodes 404, 404A which can be measured to determine the humidity.
[0358] Humidity in the sealed region is constant as long as the atmosphere in the sealed region is not altered. The reading of the humidity sensor located in the sealed region may be used as a reference for the humidity sensor located in the exposed region. The advantage of having humidity sensors is that they allow to cancel any effect of humidity on the fluid sensing measurements.
[0359] Alternately, sensing layers 405 and 405A may comprise a material which is sensitive to other gases. In this case, the impedance (resistance and/or capacitance) of the sensing layers 405 and 405A, may change in the presence of a certain gas at a certain temperature, because of a chemical reaction between the gas and the sensing material and/or a diffusion of the gas into the sensing layer.
[0360] Electrodes 405, 405A may be further used to detect a leakage in the sealed reference chamber, during operation and lifetime of the product, caused by a defect in the seal. Any such leak may affect the accuracy of the sensor reading when the sensor reading is a result of the differential measurement between an active sensing element exposed to a fluid and a reference sensor located in the sealed chamber.
[0361] For example, if the reference sensing element is sealed in nitrogen, a leak in the seal may cause oxygen from the external atmosphere to enter the sealed chamber so changing the composition of the atmosphere in the reference chamber and, in turn, a signal produced by the reference sensing element. If the sensing layers 405 and 405A are sensitive to oxygen, then the leak can be detected, and appropriate actions may be taken, for example applying suitable compensation so that an accurate sensor signal may be produced.
[0362] The sensing layers 405 and 405A may comprise a metal oxide, such as tungsten oxide, which is sensitive to oxygen. The sensing layers 405, 405A may be made of any other suitable material known in the art.
[0363] Alternatively, the sensing layers 405 and 405A may comprise a material which is sensitive to a gas which forms the atmosphere of the reference chamber. In this case if, due to a leak, the gas escapes the reference chamber, its concentration within the chamber drops and this change in concentration may be detected by the sensing layers 405 and 405A. Such configuration allows self-calibration of the device by monitoring the changes in the reference atmosphere.
[0364] FIG. 51 shows a flow chart of a method for testing a thermal conductivity fluid sensor as described herein. The method may be used to test a thermal conductivity sensor with a sealed reference chamber comprising a reference atmosphere of a first fluid composition, for leaks in the seal of the reference chamber.
[0365] The method includes steps of: [0366] i. placing the fluid sensor in a known atmosphere of a second fluid composition for a predetermined time period, wherein the second fluid composition is different to the first fluid composition. This exposes the sensor to the known atmosphere that is different, in composition and/or pressure from the atmosphere of the reference chamber. The first fluid composition and/or the second fluid composition may each include a vacuum, a gas, or a liquid. For example, the reference chamber may be under a vacuum, or may be at any suitable pressure and may comprise dry air, nitrogen, hydrogen, carbon dioxide, argon and/or any other suitable gas. Similarly, the known atmosphere of the second fluid composition may be a vacuum, or it may comprise dry air, nitrogen, hydrogen, carbon dioxide, argon and/or any other gas, at any suitable pressure;
in this way, if a leak is present in the seal of the reference chamber, at least some fluid may be exchanged between the reference chamber and known atmosphere, causing a change in the reference atmosphere; [0367] ii. obtaining a differential signal between the first temperature sensing element and the second temperature sensing element after the predetermined period of time; [0368] iii. comparing the obtained differential signal to a predetermined expected signal, wherein the predetermined expected signal represents a differential signal expected when no fluid is exchanged between the sealed region and the known atmosphere. The differential signal may be a reference signal; [0369] iv. determining whether a leak is present in the fluid sensor using the comparison between the obtained different signal and the predetermined expected signal.
[0370] By taking a differential sensor measurement and comparing it to the value expected in the presence of the known atmosphere if no leaks were present, it is possible to determine whether the reference chamber and, in turn, the device is leaking. If the expected signal and the obtained signal are the same, no leak are present but if they are different, a leak is present.
[0371] The method may include taking one or more differential readings after a predetermined time and comparing them to an expected value. Alternatively, the method may include taking a series of readings and comparing them to previous readings, to detect if there is a progressive change of signal due to a progressive change in the reference atmosphere with time. Taking a series of readings can be used to detect smaller leaks.
[0372] FIG. 52 shows schematically a cross-section of a thermal conductivity fluid sensor having three dielectric membranes. In use, a first reference membrane is sealed from the atmosphere, a second membrane has holes and is exposed to the atmosphere, and a third reference membrane is exposed to the atmosphere from a top surface of the membrane, but is sealed on a lower surface of the membrane.
[0373] A first membrane 4A has holes and is sealed from the atmosphere and acts as a reference membrane. A second membrane 4 has holes and is exposed to the atmosphere from above and below the membrane. A third membrane 4B is exposed to the atmosphere only on the upper side or surface of the dielectric membrane, as it has no holes. This means that the third membrane 4B is sealed from the atmosphere on a lower surface of the membrane. A differential measurement between the temperature sensing elements of membranes 4 and 4A may respond differently to the atmosphere compared to a differential signal between the temperature sensing elements of membranes 4 and 4B, for example there may be a stronger humidity coupling between the elements of membrane 4B and 4 compared to 4A and 4. This means that different information may be obtained when membrane 4 has as reference the membrane 4A compared to when the reference membrane is 4B.
[0374] FIG. 53 shows schematically a cross-section of a thermal conductivity fluid sensor having three dielectric membranes. In use, first and second reference membranes 4A and 4B are sealed from the atmosphere, and a sensing membrane 4 is exposed to the atmosphere. The further sealed reference membrane 4B, which can be sealed with a vacuum atmosphere may be provided to be used as a lifetime monitoring structure during the operation of the sensor.
[0375] The three dielectric membranes are enclosed in three corresponding chambers. Membrane 4 has holes and is exposed to the atmosphere from above and below. Membrane 4A has holes and is sealed from the atmosphere and acts as a reference membrane, e.g. for differential measurements. Membrane 4A is enclosed in a sealed reference chamber. Membrane 4B is similarly sealed from the atmosphere and acts as an additional reference membrane. Membrane 4B is enclosed in an additional reference chamber. If membrane 4A is sealed in a known atmosphere, such as dry air, and membrane 4B is sealed in vacuum (or in a different known atmosphere to the one of membrane 4A), then this configuration may be used to (indirectly) check for leaks in the seal of the reference chamber 4A during lifetime operation.
[0376] During the sensor operation, leaks in the seal of the reference chamber 4B may be detected by monitoring (e.g. periodically) the vacuum level within the vacuum sealed further reference chamber enclosing membrane 4B. This could be done by measuring the temperature of the heating element 2B provided to membrane 4B (or the temperature of a temperature sensing element placed in the vicinity of such heating element 2B) for a given power level applied to the heating element. In the presence of a leak, the temperature of the heating element would gradually decrease (compared to the value expected when no leak is present). This may provide an indication that the seal of the reference chamber, enclosing membrane 4A (sealed in dry air) may have failed too, since they have both been used for the same time period in similar conditions, and, therefore, the heating element provided to membrane 4A may no longer provide a solid reference to the active heating element provided to membrane 4, for a differential or bridge measurement.
[0377] It will be understood that, while the further reference membrane 4B may preferably be sealed in a vacuum, in other embodiments it may be sealed in any other suitable atmosphere that is different from the application atmosphere, to which membrane 4 is exposed, or air. For example, it may be sealed in low pressure air, high pressure air, hydrogen, helium, argon and/or carbon dioxide or any other suitable gas or combination of gases at any suitable pressure. Different pressures or gases may be used as long as they behave in a significantly different way from air, or the application atmosphere, with respect of their interaction with the heater.
[0378] The skilled person will understand that in the preceding description and appended claims, positional terms such as above, overlap, under, lateral, etc. are made with reference to conceptual illustrations of a device, such as those showing standard cross-sectional perspectives and those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to a device when in an orientation as shown in the accompanying drawings.
[0379] Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.
TABLE-US-00001 Reference Numerals 1 Semiconductor chip 7 Tracks 2 Heating element 8, 8A, 8B, 8C, 8D, 8E temperature 2A Heating element sensing element 3, 3A, 3B, 3C temperature sensing 9 Etched portion under membrane element outside the membranes 9a Sealed etched portion under 4 Dielectric membrane membrane 4A Additional dielectric membrane 10 Dielectric layer 4A Further additional dielectric 11 Semiconductor substrate membrane 12 Membrane Recess 6 Bond pads 14 Second dielectric membrane 20 Connecting element 106 Inlet 25 Covering layer 107 Outlet 26 Fluid channel above membrane 108 Hole through package lid 30 Hole through covering layer 110 Lid 35 Printed Circuit Board 111 Instrumentation amplifier 36 Solder balls 115 Solder Balls 40, 41 Additional resistor 116 Through Silicon Vias 42, 43 Additional resistor 117 Lid for a chip scale package 44 Variable resistor 130 Ambient Temperature sensor 45, 45a, 46, 46a, 47 Current source 140, Sealant for flip chip package 50 Reference voltage 200 Sealed cavity 55 Differential amplifier 210A Differential Amplifier 56 Read out Circuitry 210B Differential Amplifier 60 Ground 211 Sensor die in Exposed Region 65 Field Effect Transistor 212 Sensor die in sealed region 70 Switch 213 Combined sensor chip in package 75 Ambient temperature sensing 301 Control Circuitry element 302 Sensor(s) 80 Heater control 303 Read out Circuitry 100 Pair of temperature sensing 305, 306, 307, 308 switching transistors elements 401 Additional metal layer 101 Package base 402 Additional layer 102 Package lid 403 Cavities 103 ASIC 404 Electrodes 104, 105 Wire bonds 405 Fluid sensitive layer
