Pressure sensor having multiple pressure cells and sensitivity estimation methodology

A pressure sensor (20) includes a test cell (32) and sense cell (34). The sense cell (34) includes an electrode (42) formed on a substrate (30) and a sense diaphragm (68) spaced apart from the electrode (42) to produce a sense cavity (64). The test cell (32) includes an electrode (40) formed on the substrate (30) and a test diaphragm (70) spaced apart from the electrode (40) to produce a test cavity (66). Both of the cells (32, 34) are sensitive to pressure (36). However, a critical dimension (76) of the sense diaphragm (68) is less than a critical dimension (80) of the test diaphragm (70) so that the test cell (32) has greater sensitivity (142) to pressure (36) than the sense cell (34). Parameters (100) measured at the test cell (32) are utilized to estimate a sensitivity (138) of the sense cell (34).

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to pressure sensors. More specifically, the present invention relates to a pressure sensor having multiple pressure cells of differing sensitivities and methodology for measuring sensitivity of the pressure sensor.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS) devices are semiconductor devices with embedded mechanical components. MEMS devices include, for example, pressure sensors, accelerometers, gyroscopes, microphones, digital mirror displaces, micro fluidic devices, and so forth. MEMS devices are used in a variety of products such as automobile airbag systems, control applications in automobiles, navigation, display systems, inkjet cartridges, and so forth. Capacitive-sensing MEMS devices designs are highly desirable for operation in miniaturized devices due to their low temperature sensitivity, small size, and suitability for low cost mass production.

A microelectromechanical systems (MEMS) pressure sensor typically uses a pressure cavity and a membrane element, referred to as a diaphragm, that deflects under pressure. In some configurations, a change in the distance between two plates, where one of the two plates is the movable diaphragm, creates a variable capacitor to detect strain (or deflection) due to the applied pressure over the area. Process variation on critical design parameters, such as the width of a MEMS pressure sensor diaphragm, can affect the sensitivity of a pressure sensor. For example, a small difference in the width of a MEMS pressure sensor diaphragm can result in a large difference in sensitivity, relative to the predetermined nominal, or design, sensitivity for the pressure sensor. Accordingly, the sensitivity of each MEMS pressure sensor is typically calibrated individually. The equipment used for this calibration can be costly and difficult to maintain. Additionally, calibration can be slow due to the imposition of a physical pressure stimulus on the pressure sensor in order to calibrate the pressure sensor. Individual calibration of MEMS pressure sensors by imposing a physical pressure stimulus undesirably increases costs associated with the pressure sensor and/or can introduce error in pressure measurements.

DETAILED DESCRIPTION

Embodiments of the present invention entail a pressure sensor and methodology for estimating the sensitivity of the MEMS pressure sensor. The pressure sensor includes multiple pressure sensor cells on a single die having different sensitivities. Sense signals from one set of the pressure sensor cells (i.e., test cells) may be utilized to estimate the sensitivity of another set of the pressure sensor cells (i.e., sense cells). These different sensitivities can be achieved by fabricating the test cells with a greater diaphragm width than the sense cells. The widths of the test and sense diaphragms can vary slightly from design specifications due to process variation. Thus, the widths of the test and sense diaphragms are only approximately known for the test and sense cells. However, the difference between the two widths is well known regardless of process variation. Knowledge of the difference in widths of the test and sense diaphragms is utilized herein to provide an estimate of the sensitivity of the sense cells relative to the test cells in order to determine the sensitivity of the pressure sensor. Such a pressure sensor and methodology can reduce test costs, provide improved feedback for process control, and enable sensitivity estimation without imposing a physical stimulus calibration signal.

Referring now toFIGS. 1 and 2,FIG. 1shows a simplified top view of a MEMS pressure sensor20in accordance with an embodiment, andFIG. 2shows a side sectional view of pressure sensor20along section lines2-2ofFIG. 1. Pressure sensor20generally includes a sense structure22and a reference structure24. Sense structure22and reference structure24may be fabricated on an insulating layer26, such as a nitride layer, formed on a surface28of a substrate30. Insulating layer26can comprise any suitable insulative or dielectric material layer selected according to the requirements of a given pressure sensor implementation.

Sense structure22includes sense cells32,34that are configured in an interleaved arrangement, i.e., an alternating arrangement of sense cells32with sense cells34. In general, individual sense cells32,34of sense structure22are sensitive to ambient pressure36, represented by an arrow and labeled P inFIG. 2. Reference structure24includes reference cells38. Unlike sense cells32,34, reference cells38of reference structure24are largely insensitive to ambient pressure36. In alternative embodiments, sense cells32,34need not be interleaved, but may instead be arranged in other structural configurations.

Both sets of sense cells32and sense cells34are sufficiently sensitive to detect ambient pressure36. However, as will be discussed in significantly greater detail in connection withFIGS. 3 and 4, sense cells32are implemented within pressure sensor20to estimate the sensitivity of sense cells34. Once their sensitivity is estimated, sense cells34are used within pressure sensor20to detect and subsequently output a measure indicative of pressure36. Thus, in order to distinguish them, sense cells32will be referred to hereinafter as test cells32and sense cells34will continue being referred to as sense cells34.

Sense structure22includes electrodes40,42,44,46,48, and50formed in or on insulating layer26. Likewise, electrodes52,54, and56of reference structure24may be formed in or on insulating layer26. InFIG. 1, electrodes40,42,44,46,48, and50of sense structure22are illustrated in phantom using dotted lines, due to their location beneath a common electrode58. Likewise, electrodes52,54, and56of reference structure24are also illustrated in phantom using dotted lines, due to their location beneath a common electrode60and a cap layer62.FIGS. 1 and 2are illustrated using various shading and/or hatching to distinguish the different elements produced within the structural layers of the devices, as will be discussed below. These different elements within the structural layers may be produced utilizing current and upcoming surface micromachining techniques of depositing, patterning, etching, and so forth. Accordingly, although different shading and/or hatching may be utilized in the illustrations, the different elements within the structural layers can be formed out of the same material, such as polysilicon, single crystal silicon, and the like.

Electrodes42,46, and50represent a set of sense capacitor bottom plate electrodes for sense cells34, while electrodes40,44, and48represent another set of sense capacitor bottom plate electrodes for test cells32. Since test cells32are interleaved with sense cells34, electrodes40,44, and48are correspondingly configured in an interleaved arrangement with electrodes42,46, and50. In some embodiments, a geometry of individual electrodes of the first set of electrodes42,46, and50may match a geometry (i.e., width, length, and thickness) of individual electrodes of the second set of electrodes40,44, and48, i.e., the geometries are substantially similar. However, matching geometries is not a limitation.

Referring still to sense structure22, common electrode58represents a capacitor top plate electrode for sense cells34and test cells32respectively. Common electrode58is overlying, spaced apart from, and configured in connection with electrodes40,42,44,46,48, and50, to produce sense cavities64for sense cells34and test cavities66for test cells32. Cavities64and66are represented as separate cavities herein. However, in alternative embodiments, cavities64and66may be formed as a common cavity. Cavities64and66may be vacuum chambers or chambers filled with a suitable gas at a given controlled pressure.

Common electrode58anchors to the surface of insulating layer26for establishing portions of common electrode58corresponding to sense diaphragms68for sense cells34and for establishing other portions of common electrode58corresponding to test diaphragms70for test cells32. For example, common electrode58anchors to the insulating layer26about a perimeter72of common electrode58and at desired anchor locations internal to the perimeter, such as indicated by reference numerals74, to establish cavities64and66and to distinguish sense diaphragms68from test diaphragms70.

In general, an area of each of sense diaphragms68is less than an area of each of test diaphragms70. More particularly, each of sense diaphragms68is characterized by a width76and a length78. Likewise, each of test diaphragms70is characterized by a width80and a length82. In an embodiment, length78of each of sense diaphragms68equals length82of each of test diaphragms70. However, width80of each of test diaphragms70is greater than width76of each of sense diaphragms68. Since lengths78and82are equal, and width80of test diaphragms70is greater than width76of sense diaphragms68, it follows that an area of each of test diaphragms70is greater than an area of each of sense diaphragms68.

The greater width80of each of test diaphragms70causes test diaphragms70to deflect more than sense diaphragms68in response to pressure36, thus resulting in a greater sensitivity of test cells32to pressure36than sense cells34. Accordingly, in the illustrated embodiment, widths76and80are critical dimensions that directly affect the sensitivity of sense cells22and test cells24, respectively. In some embodiments, width80may be approximately ten to twenty percent greater than width76so that test cells32are approximately twice as sensitive to pressure36as sense cells34. This greater sensitivity is exploited when estimating the sensitivity of sense cells34, as will be discussed in connection withFIGS. 3 and 4.

MEMS pressure sensor20further includes a conductive runner84electrically coupled to electrodes42,46, and50to provide electrical access external to sense cells34of sense structure22. Another conductive runner86is electrically coupled to electrodes40,44, and48to provide electrical access external to test cells32. Additionally a conductive runner88is electrically coupled to common electrode58.

Referring now to reference structure24presented inFIGS. 1 and 2, electrodes52,54, and56represent a set of sense capacitor bottom plate electrodes for reference cells38. Common electrode60is overlying, spaced apart from, and configured in connection with electrodes52,54, and56to produce reference cavities90for reference cells38. Cavities90are represented as separate cavities herein. However, in alternative embodiments, cavities90may be formed as a common cavity. Common electrode60anchors to the surface of insulating layer26for establishing reference diaphragms92for reference cells38. For example, inFIG. 1, common electrode60anchors to insulating layer26about a perimeter of common electrode60and at desired anchor locations internal to the perimeter, to establish cavities90and to distinguish reference diaphragms92from one another.

Cap layer62is formed in contact with diaphragms92. Cap layer62may be a relatively thick layer of, for example, tetraethyl orthosilicate (TEOS), which makes diaphragms92largely insensitive to pressure. As such, diaphragms92may be referred to hereinafter as reference electrodes92. A conductive runner94is electrically coupled to electrodes52,54, and56of reference structure34to provide electrical access external to sense cells34of sense structure22. Another conductive runner96is electrically coupled to common electrode60. It should be observed inFIG. 1that common electrode60is illustrated in phantom using dashed lines, due to its location beneath cap layer62.

In general, sense cells34form a capacitor between diaphragms68and electrodes42,46, and50. That is, a sense signal, referred to herein as a sense capacitance98, labeled CS1, is produced between sense diaphragms68and electrodes42,46, and50(i.e., the difference between CS1+and CS1−) that varies in response to pressure36. Likewise, test cells34form a capacitor between diaphragms70and electrodes40,44, and48. That is, a test signal, referred to herein as a test capacitance100, labeled CS2, is produced between test diaphragms70and electrodes40,44, and48(i.e., the difference between CS2+and CS2−) that also varies in response to pressure36. A distinction of MEMS pressure sensor20is that the sensitivity of test cells32producing capacitance100is different from the sensitivity of sense cells34producing sense capacitance98. As such, test capacitance100may be greater than sense capacitance98in response to pressure36because width80of diaphragm70of each test cell32is greater than width76of diaphragm68of each sense cell34.

Reference cells38also form a capacitor between each of electrodes92and reference electrodes52,54, and56. Thus, a reference capacitance signal102, CR, is formed between electrodes92and reference electrodes52,54, and56(i.e., the difference between CR+and CR−). However, reference capacitance signal102does not vary in response to pressure36due to the presence of cap layer62. In an embodiment, conductive runner88for sense structure22and conductive runner96for reference structure24are interconnected to form a common node104between sense structure22and reference structure44.

A control circuit106is configured to measure the ratio of sense capacitance signal98to reference capacitance signal102(i.e., CS1/CR). Higher pressure36increases sense capacitance98, CS1, but has little effect on reference capacitance102, CR. Therefore the ratio of sense capacitance98to reference capacitance102(i.e., CS1/CR) increases as pressure36increases. This value can be converted into an output signal108, i.e., a measure indicative of pressure36.

In the views of pressure sensor20shown inFIGS. 1 and 2, sense structure22is illustrated as having three test cells32and three sense cells34. Likewise, reference structure24is illustrated as having three reference cells38. However, it should be understood by those skilled in the art that pressure sensor20may have any suitable quantity of test, sense, and reference cells32,34,38, respectively, and their associated diaphragms/electrodes. Additionally, pressure sensor20may include other features on substrate30such as shield lines, a guard ring, and so forth that are not included inFIGS. 1 and 2for simplicity of illustration.

Pressure sensor20is illustrated with generally rectangular diaphragms having a width that is less than a length of the rectangular diaphragms. However, the diaphragms need not be rectangular, but may instead be other shapes (e.g., squares, circles, multi-sided elements, and so forth) with test cells32having greater sensitivity than sense cells34in order to provide sensitivity estimation.

FIG. 3shows a shows a simplified block diagram of a device109. Device109includes pressure sensor20, control circuit106, and any other application specific integrated circuit (ASIC)111or ASICs111appropriate for the operation of device109. Device109may be a pressure sensing system for an automotive application such as for airbag pressure sensing, oil pressure sensing, HVAC pressure sensing, and other various automotive pressure sensing applications. Alternatively, device109may be a global positioning system (GPS) unit, smartphone, tablet, sports watch, weather station, or any other industrial application in which pressure sensing may be utilized. Regardless of the particular device109, the sensitivity of pressure sensor20included in device109can be estimated prior to or following its installation within device109without imposition of a physical stimulus calibration signal, and device109can be calibrated as needed.

FIG. 4shows a flowchart of a pressure sensor sensitivity estimation process110in accordance with another embodiment. Pressure sensor sensitivity estimation process110is performed to estimate the sensitivity of sense cells34(FIG. 1) of pressure sensor20(FIG. 1) using the higher sensitivity test cells32(FIG. 1). Estimation process110can be performed under ambient pressure conditions, e.g., standard atmospheric pressure, without imposing a physical pressure calibration stimulus in excess of atmospheric pressure.

Estimation process110begins with a task112. At task112, ambient pressure36(FIG. 1) is measured in the location at which pressure sensor20is being tested. Pressure36may be measured using any suitable and highly accurate pressure measurement device.

Process110continues with a task114. At task114, sense capacitance98(FIG. 1) is determined for sense cells34(FIG. 1).

A task116is performed in conjunction with task114. At task116, test capacitance100is determined for test cells32(FIG. 1)

Sensitivity estimation process110continues with a task118. At task118, the sensitivity of pressure sensor20(FIG. 1), and in particular, sense cells34(FIG. 1) is estimated using sense capacitance98and test capacitance100. The details of estimation task118are discussed in connection withFIG. 5.

Following task118, a task119may be performed. At task119, the results obtained from estimation task118may be utilized to calibrate or otherwise trim pressure sensor20in accordance with known methodologies. Accordingly, following task119, pressure sensor sensitivity estimation process110ends.

FIG. 5shows a set120of equations for deriving the sensitivity of pressure sensor20, and particularly, for estimating the sensitivity of sense cells34(FIG. 1) of sense structure22(FIG. 1) utilizing the higher sensitivity test cells32. Set120reveals that a relationship can be established between sensitivity and the geometry of a pressure sensing cell, e.g., sense cell34and test cell32. Three parameters have a strong effect on the sensitivity of a pressure sensing cell. As shown in the provided figure of an exemplary pressure sensing cell122, these parameters include a width124, W, of a diaphragm126of pressure cell122, a depth128, D, of a cavity130underlying diaphragm126, and a thickness132, T, of diaphragm126. Accordingly, a diaphragm sensitivity133, SENS, can be expressed as a function of width124, depth128, and thickness132as represented by a generalized functional equation134.

Given the relationship between sensitivity and the geometry of a pressure sensing cell, set120further reveals that the value of sensitivity138of sense cells34(FIG. 1) can be derived relative to the value of sensitivity142of test cells32(FIG. 1). Rearranging the terms of equation140yields an equation144, and substituting the appropriate terms of equation144into equation138yields an equation146in which sensitivity138of sense cells34is a function of sensitivity142of test cells32, as well as, diaphragm widths76and80. Rearranging the terms of equation146yields a sensitivity equation148for sense cells34in which sensitivity138is a function of sensitivity142of test cells32and a ratio of diaphragm width76to diaphragm width80. Thus, sensitivity138is related to sensitivity142by two parameters. These parameters include width76of sense diaphragms68(FIG. 2) of sense cells34(FIG. 2) and the difference, ω, between diaphragm width80and diaphragm width76. Width76is known approximately, but not exactly since width76may vary from its design width due to some process variations, e.g., over or under etch. This difference, ω, is well known since this is the difference between diaphragm widths80and76, regardless of process variation resulting in some over or under etch.

As further shown in set120, sense capacitance98can be defined as a function of sensitivity138, ambient pressure36, and zero pressure offset, ZPO1, represented by a capacitance equation150. Likewise, test capacitance100can be defined as a function of sensitivity142, ambient pressure36, and zero pressure offset, ZPO2, as represented by a capacitance equation152. Zero pressure offset is the theoretical output of pressure sensor20at zero pressure. Due to their structural configuration, it can be assumed that the zero pressure offset, ZPO2, for test cells32is equal to the zero pressure offset, ZPO1, for sense cells34.

Accordingly, with ZPO2=ZPO1, capacitance equations150and152can be combined and rearranged to derive a sensitivity equation154for test cells34, where sensitivity142is shown to be a function of sense capacitance98, test capacitance100, sensitivity138of sense cells34, and pressure36.

Sensitivity equation154for test cells32can be combined with sensitivity equation148for sense cells34to yield another equation156. Equation156can be mathematically rearranged as represented by a sensitivity equation158in order to derive sensitivity138of sense cells34. Accordingly, sensitivity138can be shown to be a function of width76(approximately known), the difference, ω, between width80and width76(exactly known), sense capacitance98at pressure36(measured), test capacitance100at pressure36(measured), and pressure36(measured). Thus, through the execution of pressure sensor sensitivity estimation process110, the estimated sensitivity138of sense cells34of pressure sensor20can be determined utilizing parameters derived from the higher sensitivity test cells32.

Exemplary equation158is provided herein for illustrative purposes. In practice, however, there may be deviations from the ideal that may call for the inclusion of scaling constants and/or other terms, not shown for simplicity of illustration. Some additional terms may be added to compensate for higher order effects that are not in the theoretical models.

It is to be understood that certain ones of the process blocks depicted inFIG. 4may be performed in parallel with each other or with performing other processes. In addition, it is to be understood that the particular ordering of the process blocks depicted inFIG. 4may be modified, while achieving substantially the same result. Accordingly, such modifications are intended to be included within the scope of the inventive subject matter. In addition, although particular system configurations are described in conjunction withFIGS. 1-2, above, embodiments may be implemented in systems having other architectures, as well. These and other variations are intended to be included within the scope of the inventive subject matter.

An embodiment of a pressure sensor comprises a sense cell having a first electrode formed on a substrate and a sense diaphragm overlying and spaced apart from the first electrode to produce a sense cavity. The pressure sensor further comprises a test cell having a second electrode formed on the substrate and a test diaphragm overlying and spaced apart from the second electrode to produce a test cavity. Each of the sense cell and the test cell are sensitive to pressure, and a first area of the sense diaphragm is less than a second area of the test diaphragm.

An embodiment of a method of determining a sensitivity of a pressure sensor comprises measuring an ambient pressure, determining a first sense signal between a first electrode and a sense diaphragm of a sense cell of the pressure sensor at the ambient pressure, and determining a second sense signal between a second electrode and a test diaphragm of a test cell at the ambient pressure. The sensitivity of the sense cell is estimated using the measured ambient pressure, and the first and second sense signals.

The embodiments of a MEMS pressure sensor and a method of estimating the sensitivity of the MEMS pressure sensor. The pressure sensor includes multiple pressure sensor structures having different sensitivities formed on a single die. Atmospheric pressure (approximately 100 kPa) is sufficient to deflect each diaphragm differently. Each pressure sensor can thus have a different sense signal at atmospheric pressure. The sense signals from a higher sensitivity set of the pressure sensor structures may be utilized to estimate the sensitivity of another set of the pressure sensor structures. Such a pressure sensor and methodology can reduce test costs, provide improved feedback for process control, and enable sensitivity estimation without imposing a physical stimulus calibration signal.

While the principles of the inventive subject matter have been described above in connection with specific apparatus and methods, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the inventive subject matter. The various functions or processing blocks discussed herein and illustrated in the Figures may be implemented in hardware, firmware, software or any combination thereof. Further, the phraseology or terminology employed herein is for the purpose of description and not of limitation.

The foregoing description of specific embodiments reveals the general nature of the inventive subject matter sufficiently so that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the general concept. Therefore, such adaptations and modifications are within the meaning and range of equivalents of the disclosed embodiments. The inventive subject matter embraces all such alternatives, modifications, equivalents, and variations as fall within the spirit and broad scope of the appended claims.