Flow Sensors

Abstract

A reliable flow sensor (100) having a housing (110, 410) with predictable thermal variations and reduced mechanical tolerances for more consistent fluid flow and more consistent flow measurements. The thermal variations can be made predictable by using etched structures in the silicon block (110, 410). The mechanical tolerances can be reduced using photolithography and high precision semiconductor manufacturing equipment and techniques.

Description

Technical Field

[0001] The present invention relates to flow sensors having predictable thermal variations and reduced mechanical tolerances to achieve more consistent fluid flow and more consistent flow measurements. Background Art

[0002] There is a great need for small, reliable flow sensors. MEMS devices based on the concept of heat transfer are a commercially successful technology. Despite their success in the market, currently available devices have similar designs and therefore share some of the same performance and manufacturing drawbacks.

[0003] The flow sensor may be referred to as a hot-wire anemometer and may be composed of at least two resistors. One resistor may be made of a refractory material that heats to a controllable temperature when current is passed through it. The second resistor may be a sensing resistor and may be unheated. Current may be driven through the sensing resistor and the resulting voltage measured to calculate its resistance.

[0004] When the flow sensor is operating, the current in the refractory resistor causes the refractory resistor to heat up. When the gas or other fluid flows through the refractory resistor to the sensing resistor, heat can be transferred from the refractory resistor to the sensing resistor. This causes the sensing resistor to heat up, which in turn causes the resistance of the sensing resistor to change. This change in resistance can be measured and the flow rate can be determined based on this change.

[0005] The flow rate of a gas or other fluid in a flow sensor can be determined in part by the shape and contour of the housing that houses the flow sensor. But the size and shape of these housings can change as the housing temperature changes. These temperature-related changes can be problematic, especially when they are unpredictable. In addition, the size and shape of these housings are subject to manufacturing tolerances. These changes can result in changes in the flow being measured and inconsistent measurements.

[0006] A problem to be solved is to provide a reliable flow sensor having a housing with predictable thermal variations and reduced mechanical tolerances to achieve more consistent fluid flow and more consistent flow measurement. Summary of the invention

[0007] This problem is solved by providing a reliable flow sensor having a housing with predictable thermal variations and reduced mechanical tolerances to achieve more consistent fluid flow and more consistent flow measurement.

[0008] Illustrative embodiments may provide flow sensors with improved housings. These housings may include channels for gases or other fluids. The channels may be formed using photolithography and high-precision semiconductor manufacturing equipment and techniques to reduce their mechanical tolerances. This may reduce or reduce variations in the size, shape, and volume of the channels, resulting in improved consistency in flow measurements.

[0009] These and other embodiments of the present invention can provide a channel in a flow sensor housing formed from a monolithic silicon block. Using a silicon block can provide a channel with highly predictable and repeatable thermomechanical properties that can be easily interpreted, thereby providing more consistent measurements.

[0010] In these and other embodiments of the present invention, a first silicon block may be attached to a second silicon block to form a channel. The first silicon block may include one or more refractory resistors and one or more sensing resistors. These resistors may be circular, polygonal, or other shapes. Each of them may be formed as a spiral, such as an Archimedean spiral, an involute, a Fermat spiral, or other types of spirals. Each of them may form a straight line or a straight line array. They may have the same or similar shapes, or may have different shapes. A refractory resistor may be formed around an internal sensing resistor. In these and other embodiments, a sensing resistor may be formed around an internal refractory resistor. An outlet channel for fluid flow may be formed as a hole through the second silicon block, wherein the sensing resistor and the refractory resistor are formed around the outlet channel. The channel in the second silicon block and the outlet channel in the first silicon block may form a flow path for gas or other fluids to pass through the flow sensor, wherein the fluid flows through the channel and out of the outlet channel. In these and other embodiments of the present invention, the outlet channel may be an inlet, and the fluid may flow into the inlet and out of the channel. In these and other embodiments of the present invention, there may be no outlet channel or inlet in the first silicon block. In this configuration, the fluid flow may pass through the channel in the second silicon block.

[0011] These and other embodiments of the present invention may provide a bidirectional flow sensor. In one case, a fluid may flow from a refractory resistor through a sensing resistor. This may cause the temperature of the sensing resistor to increase, thereby causing its resistance to increase. The increase in resistance may be measured. Alternatively, a fluid may flow from the sensing resistor through the refractory resistor. In this case, the temperature of the sensing resistor may decrease as the gas or fluid heated by the refractory resistor is carried from the sensing resistor to the refractory resistor.

[0012] In these and other embodiments of the present invention, a second silicon block forming a channel can be attached to the first silicon block. The outlet channel may be absent or omitted in the first silicon block. The first silicon block may include one or more refractory resistors and one or more sensing resistors. These resistors may be circular, polygonal or other shapes. Each of them can be formed into a spiral, such as an Archimedean spiral, an involute, a Fermat spiral or other types of spirals. Each of them can form a straight line or a straight line array. They can have the same or similar shapes, or they can have different shapes. The refractory resistor can be formed on the first side of the sensing resistor. In these and other embodiments, the sensing resistor can be formed on the first side of the refractory resistor. The channel can have an inlet at a first end and an outlet or outlet channel at a second end, and the fluid flows from the inlet to the outlet or outlet channel.

[0013] In these and other embodiments of the present invention, a first silicon block forming a channel may be attached to a second silicon block, a plate, a housing, or other substrate. The first silicon block may include one or more refractory resistors and one or more sensing resistors. These resistors may be formed in the channel of the first silicon block. These resistors may be circular, polygonal, or other shapes. Each of them may be formed as a spiral, such as an Archimedean spiral, an involute, a Fermat spiral, or other types of spirals. Each of them may form a straight line or a straight line array. They may have the same or similar shapes, or may have different shapes. A refractory resistor may be formed on a first side of a sensing resistor. In these and other embodiments, a sensing resistor may be formed on a first side of a refractory resistor. The channel may have an inlet at a first end and an outlet or outlet channel at a second end, and a fluid may flow from the inlet to the outlet or outlet channel.

[0014] Several variations of this basic design can be implemented, such as concentric ring resistors or resistors mounted on a film to reduce thermal mass, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0016] Figure 1 A portion of a flow sensor according to an embodiment of the present invention is shown;

[0017] Figure 2 A top view of a portion of a flow sensor according to an embodiment of the present invention is shown;

[0018] Figure 3 shows the gas flow in the flow sensor according to an embodiment of the present invention;

[0019] Figure 4 is an exploded view of a flow sensor according to an embodiment of the present invention;

[0020] Figure 5 is a connection view of a flow sensor according to an embodiment of the present invention;

[0021] Figure 6 Another flow sensor according to an embodiment of the present invention is shown; and

[0022] Figure 7 A portion of another flow sensor according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0023] Conventional flow sensors can provide flow measurements that rely on the movement of gas over a sensing resistor and a reflected resistor. This movement of gas, in turn, depends on the geometry of the housing in which the sensor is located. Within the range of dimensions involved, the thermal expansion coefficient of the housing wall and its dimensional tolerances can significantly affect the flow value being measured, as can any external torque or other mechanical effects. Therefore, embodiments of the present invention can provide a device architecture to reduce measurement uncertainties associated with thermal and mechanical factors affecting the housing.

[0024] Figure 1 FIG. 1 shows a portion of a flow sensor according to an embodiment of the present invention. Figure 1 As such, it is shown for illustrative purposes and does not limit the possible embodiments or claims of the present invention.

[0025] The first silicon block 110 may be a part of the flow sensor 100 (eg, Figure 5 ) The first silicon block 110 may include an outlet channel 120, around which one or more refractory resistors 140 and one or more sensing resistors 130 may be arranged. The pads 150 may be used to electrically connect the refractory resistors 140 and sensing resistors 130 on the first silicon block 110 and other components to other electronic circuits outside the flow sensor 100.

[0026] The outlet channel 120 can be defined and manufactured by photolithography and high-precision semiconductor manufacturing equipment, such as using deep reactive ion etching (DRIE) or other techniques. As a result, the size, shape, and volume of the outlet channel 120 can be tightly controlled. This high degree of precision and control means that flow measurement readings are consistent between different flow sensors because they should have very similar dimensions.

[0027] Furthermore, because the outlet channel 120 can be cut into a monolithic first silicon block 110, the thermomechanical properties of the outlet channel 120 are both simple and well understood, and therefore predictable. This predictability can allow compensation for these effects. For example, because the expansion of silicon with temperature is well known, the effects of this expansion can be easily predicted. This can allow the flow measurement to be corrected as a function of temperature.

[0028] Figure 2 FIG. 2 shows a top view of a portion of a flow sensor according to an embodiment of the present invention. Figure 3 The first silicon block 110 may be provided with a plurality of silicon electrodes 120 and 130. The plurality of silicon electrodes 120 and 13 ...

[0029] The first silicon block 110 may include one or more refractory resistors 140 and one or more sensing resistors 130. For example, in these and other embodiments of the present invention, the sensing resistor 130 may be located on each side of the refractory resistor 140. In this example, the two resistors 130 and 140 are shown as concentric rings, but in these and other embodiments of the present invention, any one or both of these resistors may have different shapes. Any one or both of these resistors 130 and 140 may have a circular, polygonal or other shape. Any one or both of the resistors 130 and 140 may be formed as a spiral, such as an Archimedean spiral, an involute, a Fermat spiral or other types of spirals. Each of them may form a straight line or an array of straight lines. They may have the same or similar shapes, or may have different shapes. In these and other embodiments of the present invention, one or both of these resistors may be divided into a series of two or more arcs, or two or more straight lines, or two or more arbitrary shapes. In these and other embodiments of the invention, there may be a single refractory resistor 140 and multiple sense resistors 130, there may be multiple refractory resistors 140 and a single sense resistor 130, there may be one refractory resistor 140 and one sense resistor 130, or there may be multiple of each sense resistor. As shown in this example, the refractory resistor 140 may be formed around an internal sense resistor 130, or again, the sense resistor may be formed around an internal refractory resistor.

[0030] Either or both of the resistors 130 and 140 may be made of implanted or diffused silicon, in which case they are flush with the surface of the first silicon block 110, or either or both of the resistors 130 and 140 may be made of a refractory metal such as platinum or tungsten. They may also be made of doped polysilicon.

[0031] Since these devices rely on sensitivity to temperature, temperature sensitivity can be improved through various enhancements. One approach is to mount the sensing resistor 130 on a thin film, reducing the surrounding thermal mass, resulting in more rapid temperature changes. A second approach is to etch trenches around the resistors 130 and 140, thereby reducing the thermal mass of the surrounding area.

[0032] Figure 3 1 shows how gas or fluid flows in a flow sensor according to an embodiment of the present invention. Figure 5 When the gas or other fluid 300 is heated by the heated refractory resistor 140, then by the sensing resistor 130, and through the outlet channel 120, it is forced through the outlet channel 120 at or near the center of the first silicon block 110. This creates a lateral flow 310 from the outer edge to the center of the first silicon block 110. This flow direction can cause the temperature of the internal sensing resistor 130 to increase as the gas or other fluid 300 flows through the heated refractory resistor 140, then flows through the sensing resistor 130, and through the outlet channel 120. In these and other embodiments of the present invention, the flow of the gas or other fluid 300 can flow out of the back side of the first silicon block 110 and out of the top of the first silicon block 110.

[0033] More specifically, a current can be driven through the refractory resistor 140, thereby heating the refractory resistor 140. The refractory resistor 140 can heat the gas or other fluid 300. The heated gas or other fluid 300 can then pass through the sensing resistor 130, thereby heating the sensing resistor 130. A current can be driven through the sensing resistor 130, and the resulting voltage can be measured. The resulting voltage divided by the current is the value of the sensing resistor 130. The current in the sensing resistor 130 can be small to reduce self-heating. The value of the sensing resistor 130 can be used to determine the flow rate of the gas or fluid 300. The gas or other fluid 300 can flow through the outlet channel 120 and out of the bottom of the first silicon block 110.

[0034] Figure 4 4 is an exploded view of a flow sensor according to an embodiment of the present invention. The figure shows a flow sensor 100 in which a second silicon block 410 provides a flatter flow across the surface. In the flow sensor 100, the second silicon block 410 having an etched channel 420 can be attached to the top surface of the first silicon block 110. The resulting configuration can provide a flow sensor 100 in which, during operation, a gas or other fluid 300 (such as Figure 3 ) can be forced to move laterally across the top surface of the first silicon block 110 to reach the low-pressure outlet channel 120.

[0035] The channel 420 can be defined and manufactured by photolithography and high precision semiconductor manufacturing equipment, such as using deep reactive ion etching (DRIE) or other techniques. As a result, the size, shape and volume of the channel 420 can be tightly controlled. This high degree of precision and control means that flow measurement readings are consistent between different flow sensors because they should have very similar dimensions.

[0036] Furthermore, because the channel 420 can be cut into a monolithic second silicon block 410, the thermomechanical properties of the channel 420 are both simple and well understood, and therefore predictable. This predictability can compensate for these effects. For example, because the expansion of silicon with temperature is well known, the effects of this expansion can be easily predicted. This can allow the flow measurement to be corrected as a function of temperature.

[0037] Figure 5 4 is an oblique view of a flow sensor according to an embodiment of the present invention. The second silicon block 410 can be connected to the first silicon block 110 at the connection portion 412 to form the flow sensor 100. The etched channel 420 can guide the airflow into the device and through the surface of the first silicon block 110 and into the outlet channel 120, thereby improving the planar or laminar flow of the flow sensor.

[0038] Additional circuitry may be located on either or both of the first silicon block 110 or the second silicon block 410 (both of which may be referred to as dies), or on other structures associated with the first silicon block 110 or the second silicon block 410. The circuitry may include a current generator for providing and controlling a heating current for the refractory resistor 140. The circuitry may also include a current generator for providing a sensing current for the sensing resistor 130. An analog-to-digital converter and other circuitry may be included to convert the voltage across the sensing resistor 130 into a digital value. Signal conditioning or processing circuitry may also be included.

[0039] In these and other embodiments of the invention, the outlet channel 120 may be missing or omitted from the first silicon block. In these configurations, the flow path may be through one or more channels in the second silicon block. An example is shown in the figure below.

[0040] Figure 6Another flow sensor according to an embodiment of the present invention is shown. A first silicon block 610 can be part of a flow sensor 600. The first silicon block 610 can include one or more refractory resistors 614 and one or more sensing resistors 612. For example, in these and other embodiments of the present invention, the sensing resistors 612 can be located on each side of the refractory resistor 614. Pads 618 can be used to electrically connect the refractory resistors 614 and the sensing resistors 612 and other components on the first silicon block 610 to other electronic circuits external to the flow sensor 600.

[0041] The channel 622 may be formed in the second silicon block 620. The gas or other fluid 300 (eg Figure 3 614. As shown in the figure, the gas or other fluid 300 can be driven into the channel 622 and pass through the refractory resistor 614. A current can be driven through the refractory resistor 614, thereby heating the refractory resistor 614. The refractory resistor 614 can heat the gas or other fluid 300. The heated gas or other fluid 300 can then pass through the sensing resistor 612, thereby heating the sensing resistor 612. A current can be driven through the sensing resistor 612 and the resulting voltage can be measured. The current in the sensing resistor 612 can be very small to reduce self-heating. The resulting voltage divided by the current is the value of the sensing resistor 612. The value of the sensing resistor 612 can be used to determine the flow rate of the gas or fluid 300. The gas or other fluid 300 can flow out of the back side of the flow sensor 600 from the channel 622 (as shown, at a point close to the pad 618). Either or both of the sensing resistor 612 and the refractory resistor 614 can be positioned parallel, perpendicular, or at another angle to the direction of fluid flow provided by the channel 622.

[0042] The channel 622 can be defined and manufactured by photolithography and high-precision semiconductor manufacturing equipment, such as using deep reactive ion etching (DRIE) or other techniques. As a result, the size, shape, and volume of the channel 622 can be tightly controlled. This high degree of precision and control means that flow measurement readings are consistent between different flow sensors because they should have very similar dimensions.

[0043] Furthermore, because the channel 622 can be cut into a monolithic second silicon block 620, the thermomechanical properties of the channel 622 are both simple and well understood, and therefore predictable. This predictability can compensate for these effects. For example, because the expansion of silicon with temperature is well known, the effects of this expansion can be easily predicted. This can allow the flow measurement to be corrected as a function of temperature.

[0044] In this example, the refractory resistor 614 and the sensing resistor 612 are shown as parallel resistors formed as straight lines, but in these and other embodiments of the present invention, any one or both of these resistors may have different shapes. Any one or both of these resistors 612 and 614 may have a circular, polygonal or other shape. Any one or both of the resistors 612 and 614 may be formed as a spiral, such as an Archimedean spiral, an involute, a Fermat spiral or other types of spirals. Each of them may form a straight line. They may have the same or similar shapes, or may have different shapes. In these and other embodiments of the present invention, any one or both of these resistors may be divided into a series of two or more arcs, or two or more straight lines, or two or more arbitrary shapes. In these and other embodiments of the present invention, there may be a single refractory resistor 614 and multiple sensing resistors 612, there may be multiple refractory resistors 614 and a single sensing resistor 612, there may be one refractory resistor 614 and one sensing resistor 612, or there may be multiple of each sensing resistor.

[0045] Either or both of resistors 614 and 612 may be made of implanted or diffused silicon, in which case they are flush with the surface of first silicon block 610, or either or both of resistors 614 and 612 may be made of a refractory metal such as platinum or tungsten. They may also be made of doped polysilicon.

[0046] Since these devices rely on sensitivity to temperature, temperature sensitivity can be improved through various enhancements. One approach is to mount the sense resistor 612 on a thin film, reducing the surrounding thermal mass, resulting in more rapid temperature changes. A second approach is to etch trenches around the resistors 612 and 614, thereby reducing the thermal mass of the surrounding area.

[0047] Additional circuitry may be located on either or both of the second silicon block 620 or the first silicon block 610 (both of which may be referred to as dies), or on other structures associated with the second silicon block 620 or the first silicon block 610. The circuitry may include a current generator for providing and controlling a heating current for the refractory resistor 614. The circuitry may also include a current generator for providing a sensing current for the sensing resistor 612. An analog-to-digital converter and other circuitry may be included to convert the voltage across the sensing resistor 612 into a digital value. Signal conditioning or processing circuitry may also be included.

[0048] Figure 7A portion of another flow sensor according to an embodiment of the present invention is shown. A first silicon block 710 can be a portion of a flow sensor 700. The first silicon block 710 can include one or more refractory resistors 740 and one or more sensing resistors 730. For example, in these and other embodiments of the present invention, the sensing resistors 730 can be located on each side of the refractory resistor 740. Pads (not shown) can be used to electrically connect the refractory resistors 740 and the sensing resistors 730 and other components on the first silicon block 710 to other electronic circuits outside the flow sensor 700.

[0049] A channel 720 may be formed in the first silicon block 710. One or more refractory resistors 740 and one or more sensing resistors 730 may be formed in the channel 720. A cover 750 formed of a second silicon block, plate, or other substrate may be attached to the surface 712 of the first silicon block 710 by bonding, adhesive, or other methods or substances to form a flow path including the channel 720.

[0050] During operation, a gas or other fluid 300 (such as Figure 3 720 ) can enter the channel 720 at the inlet 722, passing through the refractory resistor 740. A current can be driven through the refractory resistor 740, thereby heating the refractory resistor 740. The refractory resistor 740 can heat the gas or other fluid 300. The heated gas or other fluid 300 can then pass through the sensing resistor 730, thereby heating the sensing resistor 730. A current can be driven through the sensing resistor 730, and the resulting voltage can be measured. The current in the sensing resistor 730 can be very small to reduce self-heating. The resulting voltage divided by the current is the value of the sensing resistor 730. The value of the sensing resistor 730 can be used to determine the flow rate of the gas or fluid 300. The gas or other fluid 300 can be discharged from the channel 720 at the outlet 724. Either or both of the sensing resistor 730 and the refractory resistor 740 can be positioned parallel, perpendicular, or at another angle to the direction of fluid flow provided by the channel 720.

[0051] The channel 720 can be defined and manufactured by photolithography and high precision semiconductor manufacturing equipment, such as using deep reactive ion etching (DRIE) or other techniques. As a result, the size, shape and volume of the channel 720 can be tightly controlled. This high degree of precision and control means that flow measurement readings are consistent between flow sensors because they should have very similar dimensions.

[0052] Furthermore, because the channel 720 can be cut into a monolithic first silicon block 710, the thermomechanical properties of the channel 720 are both simple and well understood, and therefore predictable. This predictability can compensate for these effects. For example, because the expansion of silicon with temperature is well known, the effects of this expansion can be easily predicted. This can allow the flow measurement to be corrected as a function of temperature.

[0053] In this example, the refractory resistor 740 and the sensing resistor 730 are shown as parallel resistors formed as straight lines, although in these and other embodiments of the present invention, one or both of these resistors may have different shapes. Any one or both of these resistors 730 and 740 may have a circular, polygonal or other shape. Any one or both of the resistors 730 and 740 may be formed as a spiral, such as an Archimedean spiral, an involute, a Fermat spiral or other types of spirals. Each of them may form a straight line. They may have the same or similar shapes, or may have different shapes. In these and other embodiments of the present invention, one or both of these resistors may be divided into a series of two or more arcs, or two or more straight lines, or two or more arbitrary shapes. In these and other embodiments of the present invention, there may be a single refractory resistor 740 and multiple sensing resistors 730, there may be multiple refractory resistors 740 and a single sensing resistor 730, there may be one refractory resistor 740 and one sensing resistor 730, or there may be multiple of each sensing resistor.

[0054] Either or both of resistors 740 and 730 may be made of implanted or diffused silicon, in which case they are flush with the surface of first silicon block 710, or either or both may be made of a refractory metal such as platinum or tungsten. They may also be made of doped polysilicon.

[0055] Since these devices rely on sensitivity to temperature, temperature sensitivity can be improved through various enhancements. One approach is to mount the sense resistor 730 on a thin film, reducing the surrounding thermal mass, resulting in more rapid temperature changes. A second approach is to etch trenches around the resistors 730 and 740, thereby reducing the thermal mass of the surrounding area.

[0056] Additional circuitry may be located on either or both of the first silicon block 710 or the cover 750, or on other structures associated with the first silicon block 710 or the cover 750. The circuitry may include a current generator for providing and controlling a heating current for the refractory resistor 740. The circuitry may also include a current generator for providing a sensing current for the sense resistor 730. An analog-to-digital converter and other circuitry may be included to convert the voltage across the sense resistor 730 into a digital value. Signal conditioning or processing circuitry may also be included.

[0057] The various flow sensors described above and other flow sensors provided by embodiments of the present invention can be driven by analog voltage or current. These flow sensors can provide analog voltage or current output signals. Various signal processing circuits (not shown) that can be specially developed or commercially available can be used to simplify the interpretation of signals from the flow sensors. One embodiment of the flow sensor maintains a constant current flowing through a refractory sensor and reads the change in the resistance of a sensing resistor, which can change with changes in flow. Another method is to force the current in the refractory material to change in a manner that keeps the temperature of the sensing resistor constant. Another method is to provide a current pulse to a refractory resistor to generate a heating pulse, and measure the time when the heating pulse is sensed at the sensing resistor.

[0058] The flow rate can be derived from the resistance of the sensing resistor in a variety of ways. For example, the gas or other fluid flow can be varied in a known manner and the resulting resistance measured. The relationship between the resulting resistance and the gas and fluid flow can be discovered and used during the operation of the flow sensor to determine the gas or fluid flow rate by sensing the resistance. At different temperatures, the relationship between the resulting resistance and the gas and fluid flow can be discovered. During operation, the temperature of the flow sensor can be used to calibrate the measured resistance of the sensing resistor, thereby calibrating the gas or fluid flow rate for temperature.

Claims

1. A flow sensor (100), comprising: The first silicon block (110) comprises: a top surface and a bottom surface, and an outlet passage (120) extending from the top surface to the bottom surface; a sensing resistor (130) circumferentially located around an opening in the top surface formed by the outlet passage (120); and a refractory resistor (140) circumferentially disposed around the opening in the top surface; and The second silicon block (410) comprises: directing a fluid through a channel (420) on a top surface of the first silicon block (110), The outlet channel (120) and the channel (420) form a flow path for fluid to pass through the flow sensor.

2. The flow sensor (100) according to claim 1, wherein: The refractory resistor (140) is one of a plurality of refractory resistors (140) circumferentially positioned around the sensing resistor (130), and the sensing resistor (130) is one of a plurality of sensing resistors (130).

3. The flow sensor (100) according to claim 2, wherein the refractory resistor (140) and the sensing resistor (130) are circular.

4. The flow sensor (100) according to claim 2, wherein the refractory resistor (140) and the sensing resistor (130) have a polygonal shape.

5. The flow sensor (100) according to claim 2, wherein the refractory resistor (140) and the sensing resistor (130) have a spiral shape.

6. The flow sensor (100) according to claim 2, wherein the first silicon block (110) is bonded to the second silicon block (410).

7. The flow sensor (100) of claim 2, wherein the channel (420) is formed using deep reactive ion etching.

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