Patent Description:
Flow sensors, which can be referred to as hot-wire anemometers, can consist of at least two resistors. One resistor can be made of a refractory material that heats to a controllable temperature when current is passed through it. The second resistor can be a sense resistor, and can be unheated. A current can be driven through this sense resistor and the resulting voltage measured in order calculate its resistance. Examples of flow sensors comprising refractory and sense resistors can be found in <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

When the flow sensor is operating, a current in the refractory resistor can cause the refractory resistor to heat up. As a gas or other fluid flows over the refractory resistor to the sense resistor, heat can be transferred from the refectory resistor to the sense resistor. This can cause the sense resistor to heat up, which in turn can cause the resistance of the sense resistor to change. This change in resistance can be measured, and from this change the flow rate can be determined.

The flow of a gas or other fluid in a flow sensor can be partially dependent on the shape and contours of an enclosure housing the flow sensor. But the size and shape of these enclosures can change as the temperature of the enclosure changes. These temperature-dependent variations can be problematic, particularly when they are unpredictable. Also, the size and shape of these enclosures can be subject to manufacturing tolerances. These variations can lead to changes in the flow rate to be measured and to measurement inconsistencies.

The problem to be solved is providing reliable flow sensors with enclosures that have predictable thermal variations and reduced mechanical tolerances for a more consistent fluid flow and more consistent flow measurements.

The problem is solved by providing reliable flow sensors according to claim <NUM> with enclosures that have predictable thermal variations and reduced mechanical tolerances for a more consistent fluid flow and more consistent flow measurements.

An illustrative embodiment can provide flow sensors having improved enclosures. These enclosures can include channels for a gas or other fluid. The channels can be formed using lithography and high-precision semiconductor manufacturing equipment and techniques to reduce their mechanical tolerances. This can tighten or reduce the variations in the size, shape, and volume of the channels, thereby leading to flow rate measurements having an improved consistency.

These and other embodiments of the present invention can provide channels in flow sensor enclosures that are formed of monolithic blocks of silicon. Using a silicon block can provide a channel with highly predictable and repeatable thermomechanical properties that can readily be accounted for, thereby providing more consistent measurements.

In these and other embodiments of the present invention, a first silicon block is attached to a second silicon block, thereby forming a channel. This first silicon block includes one or more refractory resistors and one or more sense resistors. These resistors can have a circular, polygonal, or other shape. They each can be formed as a spiral, such as an Archimedean spiral, involute, Fermat, or other type of spiral. They each can be formed, not according to the claimed invention, as one or an array of lines. The can have the same or similar shapes as each other, or they can have different shapes. A refractory resistor can be formed around an inner sense resistor. In these and other embodiments, a sense resistor can be formed around an inner refractory resistor. An exit passage for fluid flow is formed as a hole through the first silicon block, where the sense and refractory resistors are formed around the exit passage. The channel in the second silicon block and the exit passage in the first silicon block can form a flow path for gas or other fluid through the flow sensor, where fluid flows through the channel and out the exit passage. In these and other embodiments of the present invention, the exit passage can be an inlet and fluid can flow in the inlet and out the channel. In these and other examples not according to the claimed invention, there might not be an exit passage or inlet in the first silicon block. In this configuration, fluid flow can be through the channel in the second silicon block.

These and other embodiments of the present invention can provide a bidirectional flow sensor. In one case, the fluid can flow from the refractory resistor over the sense resistor. This can result in an increase in the temperature of the sense resistor, and therefore in its resistance. This increase in resistance can be measured. Alternatively, fluid can flow from the sense resistor over the refractory resistor. In this case, the temperature of the sense resistor can be reduced as gas or fluid heated by the refractory resistor is being carried away from the sense resistor towards the refractory resistor.

In these and other examples not according to the claimed invention, a second silicon block forming a channel can be attached to a first silicon block. The exit passage can be absent or omitted from the first silicon block. This first silicon block can include one or more refractory resistors and one or more sense resistors. These resistors can have a circular, polygonal, or other shape. They each can be formed as a spiral, such as an Archimedean spiral, involute, Fermat, or other type of spiral. They each can be formed as one or an array of lines. They can have the same or similar shapes as each other, or they can have different shapes. A refractory resistor can be formed on a first side of a sense resistor. In these and other embodiments, a sense resistor can be formed on the first side of a refractory resistor. The channel can have an inlet on a first end and an outlet or exit passage on a second end, where fluid flows from the inlet to the outlet or exit passage.

In these and other examples not according to the claimed invention, a first silicon block forming a channel can be attached to a second silicon block, a board, housing, or other substrate. The first silicon block can include one or more refractory resistors and one or more sense resistors. These resistors can be formed in the channel in the first silicon block. These resistors can have a circular, polygonal, or other shape. They each can be formed as a spiral, such as an Archimedean spiral, involute, Fermat, or other type of spiral. They each can be formed as one or an array of lines. The can have the same or similar shapes as each other, or they can have different shapes. A refractory resistor can be formed on a first side of a sense resistor. In these and other embodiments, a sense resistor can be formed on the first side of a refractory resistor. The channel can have an inlet on a first end and an outlet or exit passage on a second end, where fluid flows from the inlet to the outlet or exit passage.

Several variations of this basic design can be implemented, such as concentric-ring resistors or resistors mounted on thin membranes to reduce thermal mass, among others.

Conventional flow sensors can provide flow rate measurements that are dependent on the movement of gases over sense and refractory resistors. This movement of gas can in turn depend on the geometry of an enclosure in which the sensor is held. At the dimensional scales involved, thermal coefficients of expansion of the walls of the enclosure, as well as tolerances in their dimensions, can meaningfully affect measured flow rate values, as can any external torqueing or other mechanical effects. Accordingly embodiments of the present invention can provide a device architecture to reduce measurement uncertainty associated with thermal and mechanical factors influencing the enclosure.

<FIG> illustrates a portion of a flow sensor according to an embodiment of the present invention. This figure, as with the other included figures, is shown for illustrative purposes and does not limit either the possible embodiments of the present invention or the claims.

First silicon block <NUM> can be a portion of flow sensor <NUM> (shown in <FIG>. ) First silicon block <NUM> includes exit passage <NUM>, around which can be arrayed one or more refractory resistors <NUM> and one or more sense resistors <NUM>. Bondpads <NUM> can be used to electrically connect refractory resistor <NUM> and sense resistor <NUM> and other components on first silicon block <NUM> to other electronic circuits external to flow sensor <NUM>.

Exit passage <NUM> can be defined by and manufactured using lithography and high-precision semiconductor manufacturing equipment, for example using a deep-reactive-ion etch (DRIE) or other technique. As a result, a size, shape, and volume of exit passage <NUM> can be tightly controlled. This high degree of precision and control can mean that flow measurement readings can be consistent from one flow sensor to another, since they should have very similar dimensions.

Also, since exit passage <NUM> can be cut into monolithic first silicon block <NUM>, the thermomechanical properties of exit passage <NUM> can be both simple and also well understood and therefore predictable. This predictability can allow for compensation of these effects. For example, since the expansion of silicon over temperature is well understood, the effects of this expansion can be readily predicted. This can allow flow rate measurements to be corrected as a function of temperature.

<FIG> illustrates a top view of a portion of a flow sensor according to an embodiment of the present invention. Gas or other fluid <NUM> (shown in <FIG>) can be driven by a pressure gradient from the top surface of the device through the exit passage <NUM> in first silicon block <NUM> and out the bottom (not shown) of first silicon block <NUM>. The outer resistor ring in this case can be the refractory resistor <NUM> and the inner resistor ring can be the sense resistor <NUM>. In these and other embodiments of the present invention, a sense resistor can be positioned as resistor <NUM> and a refractory resistor can be positioned as resistor <NUM>. Exit passage <NUM> can be located at or near a center of first silicon block <NUM>.

First silicon block <NUM> can include one or more refractory resistors <NUM> and one or more sense resistors <NUM>. For example, in these and other embodiments of the present invention, sense resistors <NUM> can be located on each side of refractory resistor <NUM>. In this example, two resistors <NUM> and <NUM> are shown as concentric rings, though in these and other embodiments of the present invention, either or both of these resistors can have different shapes. Either or both of these resistors <NUM> and <NUM> can have a circular, polygonal, or other shape. Either or both resistors <NUM> and <NUM> can be formed as a spiral, such as an Archimedean spiral, involute, Fermat, or other type of spiral. They each can be formed, not according to the claimed invention, as one or an array of lines. The can have the same or similar shapes as each other, or they can have different shapes. In these and other embodiments of the present invention, either or both of these resistors can be split into a series of two or more arcs, or, not according to the claimed invention, two or more lines, or two or more arbitrary shapes. In these and other embodiments of the present invention, there can be a single refractory resistor <NUM> and a multiplicity of sense resistors <NUM>, there can be a multiplicity of refractory resistors <NUM> and a single sense resistor <NUM>, there can be one refractory resistor <NUM> and one sense resistor <NUM>, or there can be a multiplicity of each. Refractory resistor <NUM> can be formed around inner sense resistor <NUM>, as shown in this example, or again, a sense resistor can be formed around an inner refractory resistor.

Either or both resistors <NUM> and <NUM> can be made of implanted or diffused silicon, in which case they are flush with the surface of the first silicon block <NUM>, or either or both can be made of refractory metals such as platinum or tungsten. They can also be made of doped polysilicon.

Because the devices rely on sensitivity to temperature, various enhancements are available to improve temperature sensitivity. One method is to mount sense resistors <NUM> on a thin membrane, reducing the surrounding thermal mass and thus giving rise to more rapid temperature changes. A second method is to etch trenches around the resistors <NUM> and <NUM>, thereby, reducing thermal mass in the surrounding area.

<FIG> shows how a gas or fluid can flow in a flow sensor according to an embodiment of the present invention. As gas or other fluid <NUM> flows through flow sensor <NUM> (shown in <FIG>), it can be forced through exit passage <NUM> in or near the center of first silicon block <NUM>. This can create a lateral flow <NUM> from the outer edges towards the center of first silicon block <NUM>. This direction of flow can result in an increase in temperature of the inner sense resistor or resistors <NUM> as gas or other fluid <NUM> flows across heated refractory resistor <NUM>, then over sense resistor <NUM>, and through exit passage <NUM>. In these and other embodiments of the present invention, the flow of gas or other fluid <NUM> can be from the backside of first silicon block <NUM> and out the top of first silicon block <NUM>.

More specifically, a current can be driven through refractory resistor <NUM>, thereby heating refractory resistor <NUM>. Refractory resistor <NUM> can heat the gas or other fluid <NUM>. The heated gas or other fluid <NUM> can then pass over sense resistor <NUM>, thereby heating sense resistor <NUM>. A current can be driven through sense resistor <NUM> and a resulting voltage measured. The resulting voltage divided by the current is the value of the sense resistor <NUM>. The current in sense resistor <NUM> can be small to reduce self-heating. The value of sense resistor <NUM> can be used to determine a flow rate for gas or fluid <NUM>. Gas or other fluid <NUM> can flow through exit passage <NUM> and out a bottom of first silicon block <NUM>.

<FIG> is an exploded view of a flow sensor according to an embodiment of the present invention. This figure shows flow sensor <NUM>, in which a more planar flow across the surface is provided by second silicon block <NUM>. In flow sensor <NUM>, second silicon block <NUM> having etched channels <NUM> can be bonded to a top surface of first silicon block <NUM>. The resulting configuration can provide a flow sensor <NUM>, where during operation gas or other fluid <NUM> (shown in <FIG>) can be forced to move laterally across the top surface of first silicon block <NUM> to reach the lower-pressure exit passage <NUM>.

Channels <NUM> can be defined by and manufactured using lithography and high-precision semiconductor manufacturing equipment, for example using a deep-reactive-ion etch (DRIE) or other technique. As a result, a size, shape, and volume of channels <NUM> can be tightly controlled. This high degree of precision and control can mean that flow measurement readings can be consistent from one flow sensor to another, since they should have very similar dimensions.

Also, since channels <NUM> can be cut into monolithic second silicon block <NUM>, the thermomechanical properties of channels <NUM> can be both simple and also well understood and therefore predictable. This predictability can allow for compensation of these effects. For example, since the expansion of silicon over temperature is well understood, the effects of this expansion can be readily predicted. This can allow flow rate measurements to be corrected as a function of temperature.

<FIG> is an oblique view of a flow sensor according to an embodiment of the present invention. Second silicon block <NUM> can be bonded to first silicon block <NUM> at bond <NUM> to form flow sensor <NUM>. Etched channels <NUM> can guide gas flow into the device and across the surface of first silicon block <NUM> and into exit passage <NUM>, thereby improving planar or laminar flow for the flow sensor.

Additional circuitry can be located on either or both of first silicon block <NUM> or second silicon block <NUM> (both of which can be referred to as die) or other structures associated with first silicon block <NUM> or second silicon block <NUM>. This circuitry can include a current generator for providing and controlling a heating current for refractory resistor <NUM>. This circuitry can also include a current generator for providing a sense current for sense resistor <NUM>. An analog-to-digital converter and other circuits can be included to convert a voltage across sense resistor <NUM> to a digital value. Signal conditioning or processing circuits can also be included.

In these and other examples not according to the claimed invention, exit passage <NUM> can be absent or omitted from a first silicon block. In these structures, the flow path can be through one or more channels in a second silicon block. An example is shown in the following figure.

<FIG> illustrates another flow sensor according to an example not according to the claimed invention. First silicon block <NUM> can be a portion of flow sensor <NUM>. First silicon block <NUM> can include one or more refractory resistors <NUM> and one or more sense resistors <NUM>. For example, in these and other examples not according to the claimed invention, sense resistors <NUM> can be located on each side of refractory resistor <NUM>. Bondpads <NUM> can be used to electrically connect refractory resistor <NUM> and sense resistor <NUM> and other components on first silicon block <NUM> to other electronic circuits external to flow sensor <NUM>.

Channel <NUM> can be formed in second silicon block <NUM>. Gas or other fluid <NUM> (shown in <FIG>) can enter channel <NUM>, passing over refractory resistor <NUM>. A current can be driven through refractory resistor <NUM>, thereby heating refractory resistor <NUM>. Refractory resistor <NUM> can heat the gas or other fluid <NUM>. The heated gas or other fluid <NUM> can then pass over sense resistor <NUM>, thereby heating sense resistor <NUM>. A current can be driven through sense resistor <NUM> and a resulting voltage measured. The current in sense resistor <NUM> can be small to reduce self-heating. The resulting voltage divided by the current is the value of the sense resistor <NUM>. The value of sense resistor <NUM> can be used to determine a flow rate for gas or fluid <NUM>. Gas or other fluid <NUM> can exit from channel <NUM> out a backside of flow sensor <NUM> (at a point near bondpads <NUM> as shown. ) Either or both sense resistor <NUM> and refractory resistor <NUM> can be positioned in parallel, orthogonal to, or at another angle to the direction of fluid flow provided by channel <NUM>.

Channel <NUM> can be defined by and manufactured using lithography and high-precision semiconductor manufacturing equipment, for example using a deep-reactive-ion etch (DRIE) or other technique. As a result, a size, shape, and volume of channel <NUM> can be tightly controlled. This high degree of precision and control can mean that flow measurement readings can be consistent from one flow sensor to another, since they should have very similar dimensions.

Also, since channel <NUM> can be cut into monolithic second silicon block <NUM>, the thermomechanical properties of channel <NUM> can be both simple and also well understood and therefore predictable. This predictability can allow for compensation of these effects. For example, since the expansion of silicon over temperature is well understood, the effects of this expansion can be readily predicted. This can allow flow rate measurements to be corrected as a function of temperature.

In this example, refractory resistor <NUM> and sense resistor <NUM> are shown as parallel resistors formed as lines, though in these and other embodiments of the present invention, either or both of these resistors can have different shapes. Either or both of these resistors <NUM> and <NUM> can have a circular, polygonal, or other shape. Either or both resistors <NUM> and <NUM> can be formed as a spiral, such as an Archimedean spiral, involute, Fermat, or other type of spiral. They each can be formed as a line. The can have the same or similar shapes as each other, or they can have different shapes. In these and other embodiments of the present invention, either or both of these resistors can be split into a series of two or more arcs, or two or more lines, or two or more arbitrary shapes. In these and other embodiments of the present invention, there can be a single refractory resistor <NUM> and a multiplicity of sense resistors <NUM>, there can be a multiplicity of refractory resistors <NUM> and a single sense resistor <NUM>, there can be one refractory resistor <NUM> and one sense resistor <NUM>, or there can be a multiplicity of each.

Additional circuitry can be located on either or both of second silicon block <NUM> or first silicon block <NUM> (both of which can be referred to as die) or other structures associated with second silicon block <NUM> or first silicon block <NUM>. This circuitry can include a current generator for providing and controlling a heating current for refractory resistor <NUM>. This circuitry can also include a current generator for providing a sense current for sense resistor <NUM>. An analog-to-digital converter and other circuitry can be included to convert a voltage across sense resistor <NUM> to a digital value. Signal conditioning or processing circuits can also be included.

<FIG> illustrates a portion of another flow sensor according to an example not according to the claimed invention. First silicon block <NUM> can be a portion of flow sensor <NUM>. First silicon block <NUM> can include one or more refractory resistors <NUM> and one or more sense resistors <NUM>. For example, in these and other examples not according to the claimed invention, sense resistors <NUM> can be located on each side of refractory resistor <NUM>. Bondpads (not shown) can be used to electrically connect refractory resistor <NUM> and sense resistor <NUM> and other components on first silicon block <NUM> to other electronic circuits external to flow sensor <NUM>.

Channel <NUM> can be formed in first silicon block <NUM>. The one or more refractory resistors <NUM> and one or more sense resistors <NUM> can be formed in channel <NUM>. A cover <NUM> formed of a second silicon block, a board, or other substrate can be attached by bonding, adhesive, or other method or substance, to surfaces <NUM> of first silicon block <NUM> to form a flow path including channel <NUM>.

During operation, gas or other fluid <NUM> (shown in <FIG>) can enter channel <NUM> at inlet <NUM>, passing over refractory resistor <NUM>. A current can be driven through refractory resistor <NUM>, thereby heating refractory resistor <NUM>. Refractory resistor <NUM> can heat the gas or other fluid <NUM>. The heated gas or other fluid <NUM> can then pass over sense resistor <NUM>, thereby heating sense resistor <NUM>. A current can be driven through sense resistor <NUM> and a resulting voltage measured. The current in sense resistor <NUM> can be small to reduce self-heating. The resulting voltage divided by the current is the value of the sense resistor <NUM>. The value of sense resistor <NUM> can be used to determine a flow rate for gas or fluid <NUM>. Gas or other fluid <NUM> can exit from channel <NUM> at outlet <NUM>. Either or both sense resistor <NUM> and refractory resistor <NUM> can be positioned in parallel, orthogonal to, or at another angle to the direction of fluid flow provided by channel <NUM>.

Also, since channel <NUM> can be cut into monolithic first silicon block <NUM>, the thermomechanical properties of channel <NUM> can be both simple and also well understood and therefore predictable. This predictability can allow for compensation of these effects. For example, since the expansion of silicon over temperature is well understood, the effects of this expansion can be readily predicted. This can allow flow rate measurements to be corrected as a function of temperature.

In this example, refractory resistor <NUM> and sense resistor <NUM> are shown as parallel resistors formed as lines, though in these and other embodiments of the present invention, either or both of these resistors can have different shapes. Either or both of these resistors <NUM> and <NUM> can have a circular, polygonal, or other shape. Either or both resistors <NUM> and <NUM> can be formed as a spiral, such as an Archimedean spiral, involute, Fermat, or other type of spiral. They each can be formed as a line. The can have the same or similar shapes as each other, or they can have different shapes. In these and other examples not according to the claimed invention, either or both of these resistors can be split into a series of two or more arcs, or two or more lines, or two or more arbitrary shapes. In these and other embodiments of the present invention, there can be a single refractory resistor <NUM> and a multiplicity of sense resistors <NUM>, there can be a multiplicity of refractory resistors <NUM> and a single sense resistor <NUM>, there can be one refractory resistor <NUM> and one sense resistor <NUM>, or there can be a multiplicity of each.

Additional circuitry can be located on either or both of first silicon block <NUM> or cover <NUM>, or other structures associated with first silicon block <NUM> or cover <NUM>. This circuitry can include a current generator for providing and controlling a heating current for refractory resistor <NUM>. This circuitry can also include a current generator for providing a sense current for sense resistor <NUM>. An analog-to-digital converter and other circuitry can be included to convert a voltage across sense resistor <NUM> to a digital value. Signal conditioning or processing circuits can also be included.

The various flow sensors described above, as well as other flow sensors provided by embodiments of the present invention, can be driven by an analog voltage or current. These flow sensors can provide an analog voltage or current output signal. Various signal processing circuits (not shown) that can be specifically developed or commercially available can be used to simplify the interpretation of the signal coming from the flow sensors. One implementation of the flow sensors maintains a constant current flow through the refractory sensor and read out changes in the sense resistor's resistance, which can vary as the flow rate changes. Another method is to force the current in the refractory material to vary in such a manner that the temperature of the sense resistor remains constant. Another method is to provide a current pulse to a refractory resistor to generate a heating pulse and measure the time for the heating pulse to be sensed at the sense resistor.

Claim 1:
A flow sensor (<NUM>) comprising:
a first silicon block (<NUM>) comprising:
a top surface, a bottom surface, a passage (<NUM>) extending from the top surface to the bottom surface and an opening in the top surface formed by the passage (<NUM>);
a sense resistor (<NUM>) located on or in the top surface; and
a refractory resistor (<NUM>) located on or in the top surface and apart from the sense resistor (<NUM>); and
a second silicon block (<NUM>) attached to the first silicon block (<NUM>) to provide a channel (<NUM>) to direct a gas flow across the top surface of the first silicon block (<NUM>);
characterized in that the sense resistor (<NUM>) and the refractory resistor (<NUM>) are circumferentially located around the opening in the top surface, wherein the channel (<NUM>) and the passage (<NUM>) define a flow path for gas or other fluid through the sensor between the channel and the passage.