Silicon flow sensor

A silicon flow sensor is provided having an uncomplicated design and construction, while also exhibiting a desirable level of sensitivity for use in automotive applications. The primary sensing component is preferably formed by a single silicon chip on which associated signal conditioning and compensating circuitry can be provided. The chip includes a base region from which a vane is cantilevered so as to be adapted to deflect in either of two directions when impinged by fluid flow. A beam region is present intermediate the vane and the base region on which a strain sensing element is present, such that strain occurring in the beam region as a result of vane deflection is sensed to indicate the degree to which the vane is deflected. The construction of the flow sensor is such that its sensitivity can be readily modified during its manufacture in order to optimize the sensor for its intended use.

This invention generally relates to sensors of the flow-sensing type. More 
particularly, this invention relates to a silicon flow sensor that can be 
readily up-integrated with other semiconductor sensors and processes, 
wherein the flow sensor is characterized by a rugged structure that is 
capable of surviving in hostile environments, yet whose sensitivity can be 
readily modified during processing of the flow sensor. 
BACKGROUND OF THE INVENTION 
Sensors are used in automotive and various other applications for a variety 
of purposes, such as sensing fluid or air pressure within an automotive 
fuel system and sensing motion as part of a passenger passive restraint 
system. A third type of sensor finding use in automotive applications is 
flow sensors, which serve such purposes as determining the flow rate of 
intake air to an engine. On the basis of reliability and cost, a trend in 
the automotive industry is to fabricate such sensors in the form of 
monolithic semiconductor sensors that are micromachined from silicon 
wafers. 
The prior art has generally relied on flow sensors in the form of hot wire 
devices and pressure sensing diaphragms modified to detect air flow. Hot 
wire devices are generally polysilicon or metal runners formed on a 
silicon chip and indicate fluid flow by sensing the heat removed from the 
hot wire by the fluid, while pressure sensing diaphragms rely on a venturi 
effect as a fluid passes through an opening in the diaphragm to deflect 
the diaphragm, which can then be sensed by piezoresistive or capacitive 
techniques. While such flow sensor designs have found acceptance in the 
industry, their fabrication, size and complicated construction diminish 
their desirability and potentially their ruggedness for use in automotive 
applications. Such shortcomings can be exasperated by the presence of 
signal conditioning or processing circuitry on the same structure that 
supports the sensing element. Furthermore, these prior art sensor designs 
can be impractical for use in sensing flow in applications where the 
direction of fluid flow is not unidirectional, i.e., the fluid may reverse 
its direction of flow. 
Therefore, it would be desirable to provide a semiconductor flow sensor 
that is relatively uncomplicated in its fabrication and construction, can 
be fabricated to have a monolithic structure on which signal conditioning 
and processing circuitry can be provided, and is characterized by an 
efficient use of material so as to minimize the size of the sensor. It 
would particularly be desirable if the sensitivity of such a flow sensor 
could be readily modified during fabrication in order to optimize the 
sensor for the conditions in which in will be used. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a flow sensor suitable 
for automotive applications and manufacturable by automotive production 
techniques. 
It is another object of this invention that such a flow sensor has a 
relatively uncomplicated and rugged construction in which the primary 
sensing component is formed by a single silicon chip, such that the sensor 
can be mass produced using batch processing techniques. 
It is a further object of the present invention that the sensitivity of 
such a flow sensor can be readily modified during fabrication in order to 
optimize the sensor for the flow conditions in which it will be used. 
It is yet a further object of this invention that such a flow sensor 
enables the presence of signal conditioning circuitry on the same silicon 
chip as the primary sensing component. 
In accordance with a preferred embodiment of this invention, these and 
other objects and advantages are accomplished as follows. 
A silicon flow sensor is provided that has an uncomplicated design and 
construction, and yet is characterized as exhibiting a desirable level of 
sensitivity for use in automotive applications. In particular, the primary 
sensing component is preferably formed by a single silicon chip on which 
associated signal conditioning and compensating circuitry can be provided. 
The construction of the flow sensor is such that its sensitivity can be 
readily modified during its manufacture in order to optimize the sensor 
for its intended use. 
The sensor of this invention is generally composed of a silicon chip having 
a base region from which a vane is cantilevered so as to be adapted to 
deflect in either of two directions when impinged by fluid flow. A beam 
region is present intermediate the vane and the base region on which a 
strain sensing element is present, such that strain occurring in the beam 
region as a result of vane deflection is sensed to indicate the degree to 
which the vane is deflected. The ruggedness of the sensor can be promoted 
by providing stops that limit deflection of the vane relative to the base 
region. 
A particularly advantageous aspect of the sensor is that it is configured 
to enable its sensitivity to be affected by various modifications 
achievable during processing. For example, sensitivity can be affected by 
the presence of one or more through-holes in the vane. The sensitivity and 
linearity of the sensor can be further modified by fabricating the chip to 
include a frame disposed along a peripheral edge of the vane, such that a 
gap having a predeterminable width is present therebetween. The 
sensitivity of the sensor can be also readily effected by modifying the 
length and thickness of the beam region. 
Another advantageous aspect of this invention is that the strain output 
resulting from deflection of the vane is extremely linear, which 
simplifies the need for further signal processing. Yet, the chip can be of 
sufficient size to accommodate signal conditioning or temperature 
compensation circuitry, as well as other sensing elements including 
pressure sensing diaphragms. 
In view of the above, it can be seen that the sensor of this invention is 
relatively uncomplicated and rugged, making the sensor particularly well 
suited for automotive applications. The primary and essential component of 
the sensor is a single silicon chip, which can be readily manufactured 
using batch processes so as to be cost effective, and then packaged within 
a housing or sensor module without significantly complicating the 
manufacture of the sensor. The reliance on a single sensing structure 
makes possible a small sensor, such that the presence of the sensor can 
have a minimal effect on the flow dynamics of the fluid being sensed, 
though it is foreseeable that multiple sensors could be defined in a 
single chip in order to extend the flow sensing range capability. In 
addition, the silicon vane of the sensor is adapted to be bidirectionally 
deflected, which enhances the versatility of the sensor. Furthermore, the 
ability to selectively alter the sensitivity of the sensor during 
processing also enhances the versatility of the sensor, enabling a basic 
sensor design to be adapted to various applications having significantly 
different flow conditions. 
Other objects and advantages of this invention will be better appreciated 
from the detailed description thereof, which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A flow sensor 10 in accordance with a first embodiment of this invention is 
shown in cross-section in FIG. 1. The sensor 10 is generally represented 
as being formed by a single silicon chip 12 whose outer dimensions are 
typical for silicon wafer chips, generally on the order of about 4000 
micrometers by about 6000 micrometers by about 380 micrometers in 
thickness. The chip 12 has a base 14 secured by a housing 16, which can 
generally be of any suitable construction and material capable of 
adequately supporting and anchoring the chip 12 within its intended 
operating environment. With additional reference to FIG. 2, which is a 
plan view the chip 12 as seen when isolated from the housing 16, the chip 
12 can be seen to define a vane 18 cantilevered from the base 14. The vane 
18 is adapted to be deflected in response to fluid flow that impinges 
either of its broader surfaces 18a and 18b, such that the sensor 10 is 
capable of bidirectional operation. As shown, one of the broader surfaces 
18a of the vane 18 is contiguous and coplanar with a surface of the base 
14, though other configurations are foreseeable. 
The vane 18 is preferably micromachined from the chip 12 using any suitably 
micromachining technique so as to be surrounded by a frame 22, such that a 
gap 24 having a predeterminable width is present therebetween. In 
addition, the vane 18 is preferably micromachined such that its thickness 
in the direction of fluid flow is thinner than that of the base 14. A beam 
20 is defined by a transition region of the chip 12 where the thickness of 
the chip 12 changes between the vane 18 and the base 12. This transition 
reduces stress risers in the chip 12, and establishes a region in the beam 
20 in which deflection of the vane 18 will be localized, making the beam 
20 highly suited as a location for sensing deflection of the vane 18 with 
a strain sensing element 26, as shown in FIGS. 1 and 2. The beam 20 is 
preferably protected with a barrier 16a from being directly impinged by 
the fluid being sensed by the vane 18, as shown in FIG. 1, so as to avoid 
erosion of the beam 20 and the strain sensing element 26. While FIG. 2 
illustrates a single-beam configuration for the sensor 10, other beam 
designs could be employed, such as four-beam, two-beam and folded-beam 
designs. 
FIG. 2 illustrates the use of a pair or a set of four piezoresistors formed 
on the beam 20 as the strain sensing elements 26 for sensing deflection of 
the vane 18. As is known in the art, piezoresistive sensing elements are 
well suited for use as the strain sensing element for a silicon pressure 
sensor or motion-sensor. In addition, piezoresistors are highly suitable 
as the strain sensing elements 26 of this invention, in that they can be 
readily integrated with appropriate adjusting circuitry in a monolithic 
silicon integrated circuit. The piezoresistors are preferably formed in a 
doped epitaxial layer formed on the surface of the chip 12, though for 
high temperature applications the piezoresistors can be formed within 
deposited films such as polysilicon or a metal. While piezoresistive 
sensing elements are preferred, other strain sensing elements, such as 
strain gauges, could be used. Furthermore, it is foreseeable that the 
deflection of the vane 18 could be sensed using other techniques, such as 
capacitive sensing. 
Metal bond pads 28 are shown as being provided on the base 14 of the chip 
12 through which the input power and output signals of the strain sensing 
elements 26 are transmitted to and from the sensor 10 for signal 
conditioning and processing. For enhanced reliability and corrosion 
resistance, the bond pads 28 and runners (not shown) interconnecting the 
piezoresistors to the bond pads 28 should be formed from polysilicon or a 
noble metal such as gold, platinum or palladium, or should be shielded 
from the gas or liquid being sensed. To achieve the latter, the housing 16 
can be designed to protect the bond pads 28 and runners, or the sensor 10 
can include a plastic or wax layer that covers the bond pads 28 and 
runners, as is done with piezoresistive pressure sensors known in the art. 
As would be expected, the degree that the vane 18 will deflect in response 
to a given rate of fluid flow is influenced by the size of the vane 18. 
According to this invention, the size of the vane 18 can be readily 
tailored during micromachining of the chip 12 to achieve a size suitably 
adapted for the operating environment of the sensor 10. Additionally, the 
width of the gap 24 surrounding the vane 18 also influences deflection of 
the vane 18. Greater widths will render the vane 18 less sensitive to 
flow, while narrower widths will increase sensitivity. In practice a vane 
18 having planar dimensions of about 1500 by 1500 micrometers has been 
found practicable when used with a gap 24 of about 25 micrometers. 
However, it is foreseeable that the size of the vane 18 and gap 24 could 
vary greatly from these dimensions and still perform well. 
Advantageously, the vane configuration shown in FIG. 2 permits various 
other techniques for modifying its sensitivity. For example, the width and 
thickness of the beam 20 and the number of beams 20 can each be readily 
tailored during processing of the chip 12 to alter the sensitivity of the 
sensor 10. The width of the beam 20 must generally be optimized in 
relation to the size of the vane 18 in order to avoid the vane 18 from 
becoming unstable within the range of flow rates for the fluid to which it 
will be exposed. Furthermore, the thickness of the epitaxial layer in 
which the strain sensing elements 26 are formed can also be tailored to 
have an intentional affect on sensitivity, with thicker epitaxial layers 
enabling the use of the sensor 10 in higher flow applications and with 
more viscous fluids. Finally, as shown in FIG. 2, holes 30 can be 
micromachined directly through the vane 18 in order to further alter 
sensitivity, with fewer and/or smaller holes generally increasing the 
sensitivity of the sensor 10. 
For illustrative purposes, FIG. 3 shows the sensor 10 of this invention 
disposed within an automobile's air intake duct 32. The sensor 10 is shown 
housed within a module 34 that includes a passage into which the vane 18 
projects. The module 34 can be mounted directly to the wall of the duct 
32, and can be readily sized to achieve suitably air flow through the 
passage without unnecessarily restricting air flow through the duct 32. 
Alternatively, the sensor 10 could be mounted directly within the duct 32. 
Variations of the sensor 10 shown in FIGS. 1 and 2 are represented in FIGS. 
4 through 6. FIG. 4 illustrates a sensor 110 having the basic vane 
configuration shown in FIG. 2 in combination with a pressure sensor 
diaphragm 36, all of which is formed on the same chip 12. The diaphragm 36 
can be micromachined in the chip 12 in a conventional manner as an 
absolute or differential sensing element, and with or without a capping 
chip (not shown) for enclosing a cavity (not shown) that results from the 
presence of the diaphragm 36 in the chip 12, all of which is conventional 
in the art of silicon pressure sensors. In this embodiment, additional 
metal bond pads 28 are required to serve both the strain sensing elements 
26 of the flow sensor portion of the sensor 110 and strain or 
capacitive-sensing elements required for the pressure sensing portion of 
the sensor 110. 
FIG. 5 represents yet another embodiment of this invention, in which 
wafer-to-wafer bonding is employed to improve the reliability and 
manufacturability of a silicon flow sensor 210 that incorporates the 
sensor components shown in FIGS. 1 and 2. The sensor 210 is shown as 
including a top capping chip 38 and a bottom capping chip 40 that can be 
bonded to the chip 12 in any suitable manner. The top and bottom capping 
chips 38 and 40 define a passage 42 through which fluid flows to and from 
the vane 18 of the sensor 210. The top and bottom capping chips 38 and 40 
also provide motion stops 38a and 40a, respectively, which prevent high 
flow bursts through the passage 42 from damaging the vane 18. 
Advantageously, the passage 42 formed by the capping chips 38 and 40 can 
be tailored to influence the flow to and around the vane 18, and even 
optimize the flow pattern over the vane 18. For example, entrance to the 
passage 42 can be restricted by grooves, slits and/or holes that allow 
fluid to enter the passage 42, and potentially impinge only a limited 
portion of the vane 18, while excluding large fluid-borne particles and 
other undesirable debris. 
Finally, FIG. 6 illustrates a flow sensor 310 having the basic sensor 
configuration shown in FIG. 2 in combination with a thick film integrated 
circuit (IC) 44. The sensor 310 and IC 44 may be formed on the same chip 
12 or, as shown, formed on separate chips housed within a single package 
54. The IC 44 can be a signal processor, temperature compensator, or other 
control chip such as a microprocessor. If piezoresistors are used as the 
strain sensing elements 26, they will typically be connected to a suitable 
signal processing circuitry for measuring the deflection of the vane 18, 
Furthermore, piezoresistors often require calibration to compensate for 
the variations that tend to occur during manufacturing between individual 
strain sensing elements and signal conditioning circuitry during 
manufacturing. To provide for this compensation, it is advantageous to 
include either separately or as part of the integrated circuit, a 
conditioning network that permits customized adjustment of the output 
parameters for the individual flow sensor. Conductors 46 are employed to 
electrically interconnect the sensor 310 with the IC 44. In addition, the 
package 54 is preferably equipped with electrical programming pads 48 by 
which the IC can be appropriately programmed, and plug connectors 50 with 
which the input power can be transmitted to the sensor 310 and the 
conditioned and processed output signal of the sensor 310 can be 
transmitted to the appropriate associated control system. An opening 52 
formed in the package 54 allows fluid to flow through the package and over 
the vane 18. 
Notably, forming the sensor 310 to include the frame 22 makes the sensor 
module assembly process much easier, in that the vane structure is not 
directly handled during the die pick-and-place operation. The frame 22 is 
also used to attach the sensor 310 over the opening 52 in the package 54. 
An adhesive (not shown) is applied round the opening 52 such that the 
frame 22 is directly contacted by the adhesive. As such, the frame 22 
serves to mount the sensor chip in the package 54, thereby preventing the 
adhesive from contacting the vane 18 or beam 20 in order to ensure proper 
operation of the sensor 310. The adhesive should be relatively soft (i.e., 
a low Young's Modulus) and be compatible with the fluid being sensed. A 
low Young's Modulus is required to prevent packaging stresses from 
interfering with the piezoresistors on the beam 20. 
FIGS. 7 through 9 graphically represent the signal output response of flow 
sensors configured in accordance with the embodiment of FIG. 6. FIG. 7 is 
indicative of the extreme linearity of the sensor for air flow rates above 
about 150 cubic centimeters per minute (cc.sup.3 /min). The particular 
sensor tested to produce the results shown in FIG. 7 had a beam thickness 
of about twelve micrometers. FIG. 8 represents similar linearity for a 
sensor configured in accordance with the embodiment of FIG. 6, but adapted 
for higher flow conditions. Excellent linearity is apparent from the 
results represented in FIG. 8 for flow rates of between about 500 and 
about 1500 cc.sup.3 /min. Notably, the results depicted in FIGS. 7 and 8 
also illustrate the desirable bidirectional capability of the sensor 
design of this invention, in which flow impinging either broad surface 18a 
or 18b produces a linear output. 
Finally, FIG. 9 illustrates the extreme linearity of the output signal of a 
flow sensor configured in accordance with FIG. 6. The output signal is 
shown as being highly linear from a flow rate of about 150 cc.sup.3 /min 
to about 2500 cc.sup.3 /min. The maximum flow rate tested was limited only 
be the structural capability of the sensor tested, and not the stability 
of the vane 18 or the linearity of the sensor output. 
From the above, it can be seen that a particularly advantageous aspect of a 
flow sensor configured in accordance with this invention, is that its 
output signal resulting from deflection of the vane 18 is extremely 
linear, which simplifies the need for further signal processing. By 
comparing the results represented in FIGS. 7 and 8, it is also apparent 
that linearity is unaffected by modifications to the sensor in order to 
alter the sensitivity. Advantageously, a sensor configured in accordance 
with this invention can be readily configured to enable its sensitivity to 
be modified during processing of the chip in which the sensor is formed. 
For example, sensitivity can be affected by varying the size of the vane 
18, the width and thickness of the beam 20, the presence of one or more 
through-holes in the vane 18, and by forming the sensor to include a frame 
22 that surrounds the vane 18 such that a gap having a predeterminable 
width is present therebetween. 
Another advantageous aspect of this invention is that a sensor configured 
in accordance with this invention is very small (the size of a typical IC 
silicon chip), relatively uncomplicated and rugged, making such sensors 
particularly well suited for automotive applications. The primary and 
essential component of the sensor is a single silicon chip, which can be 
readily manufactured using batch processes so as to be cost effective, and 
then packaged within a housing or sensor module without significantly 
complicating the manufacture of the sensor. The reliance on the vane 18 as 
the single sensing structure of the sensor makes possible a small sensor, 
such that the presence of the sensor will have a minimal effect on the 
flow dynamics of the fluid being sensed. In addition, the silicon vane of 
the sensor is adapted to be bidirectionally deflected, which enhances the 
versatility of the sensor. Furthermore, the ability to selectively alter 
the sensitivity of the sensor during processing also enhances the 
versatility of the sensor, enabling a basic sensor design to be adapted to 
various applications having significantly different flow conditions. 
Finally, the sensor design of this invention provides a small compact 
assembly which is amenable to automotive production techniques, yet 
enables other sensing structures and signal conditioning or temperature 
compensation circuitry to be incorporated on the same chip that forms the 
flow sensor. As such, sensors configured in accordance with this invention 
are characterized by an efficient use of material and packaging. Because 
of its all-silicon design, sensors of this invention are also useful in 
applications where the use of prior art flow sensors would be 
inappropriate. For example, the silicon flow sensor of this invention can 
be used to sense the flow of combustible fluids, an application 
incompatible with prior art hot-wire flow sensors. 
While our invention has been described in terms of a preferred embodiment, 
it is apparent that other forms of the device could be adopted by one 
skilled in the art. By example, it is apparent that these teachings could 
be used with alternative materials and processing techniques. It is also 
apparent that the specific size, shape and overall appearance of a sensor 
configured within the scope of this invention could vary considerably from 
that shown in the Figures. Accordingly, the scope of our invention is to 
be limited only by the following claims.