The flow meter (10), particularly useful in monitoring flow in
semiconductor manufacturing operations, measures mass flow rate
of fluids and is fabricated by providing spaced webs (16) deposited
on a silicon substrate (11) and fluid flow grooves (13) etched across
a substrate surface and extending under the spaced webs. The webs
16 include a low thermal conductivity layer deposited in spaced
aligned portions of the substrate and electrically resistive pathways
deposited on the layer portions (61). The webs act as temperature
sensors and/or heaters. Heat is added to the flowing fluid and a
differential temperature is measured on a bridge circuit as is known
in the art to measure flow rate. A cover (12) has etched grooves
(14a) which match the grooves (13) of the substrate to form a fluid
passageway in which the web bisects the passageway forming an "air-foil"
like bridging member extending completely across (or cantilevered
across) in transverse relation to the flat sides of the substrate
on either side of the groove. Normally multiple series of webs and
multiple grooves are employed to provide redundancy of the flow
meter channels in the event contaminants plug a particular channel.
1. A thermal mass flow meter for fluids comprising a first substrate;
a continuous elongated fluid-flow channel having side walls and
a bottom extending on a surface of said first substrate from one
edge of the first substrate to an opposed edge of the first substrate
for conveying a fluid to be measured through said channel, said
first substrate having at least two integral members spaced from
each other and extending transversely across at least a portion
of said channel and spaced from the channel bottom such that said
fluid passes transversely of and under and over said members;
means for forming a temperature sensor on each of said members;
means on at least one of said members for heating said fluid to
provide a temperature difference between said sensors when fluid
is being conveyed.
2. The invention set forth in claim 1 in which said sensor means
are resistive patterns extending on said members.
3. The invention set forth in claim 2 in which said integral members
extend from one channel edge to another spaced channel edge transverse
of said channel.
4. The invention set forth in claim 2 in which said integral members
extends cantilevered from one channel edge transverse of said channel.
5. The invention as set forth in claim 1 including means forming
a reduced cross-sectional area in said members for minimizing thermal
conductivity from the members to said first substrate.
6. The invention as set forth in claim 5 in which said means forming
a reduced cross-sectional area includes aperture means adjacent
a connection between said members and said channel.
7. The invention as set forth in claim 1 where said members are
of low thermal conductivity material.
8. The invention as set forth in claim 1 comprising multiple continuous
elongated channels extending from one edge to an opposite edge of
said first substrate and integral members extending across each
of said multiple elongated channels.
9. The invention as set forth in claim 1 including two integral
members each mounting one of said temperature sensors and a third
integral member between said two members and spaced therefrom for
mounting said means for heating said fluid.
10. The invention as set forth in claim 9 in which said three members
include means for forming resistive patterns thereon forming said
sensors and said heating means.
11. The invention as set forth in claim 1 wherein said first substrate
comprises a first panel including said channel and said integral
members and a second panel having an elongated channel corresponding
to said channel of said first panel, said panels being bonded together
such that said integral members bisect a flow path formed by said
12. A thermal mass flow meter comprising:
a flow conduit;
a flow-splitting substrate member extending transversely across
a portion of said conduit and containing active flow sensors;
means for providing passive by-pass fluid flow paths in said conduit;
said substrate member including:
means for forming an elongated channel extending from one edge
of the substrate member to an opposite edge of the substrate member;
means for forming a first temperature-sensing member integral with
said substrate member extending transversely across said channel;
a second temperature-sensing member on said substrate member; and
means in the channel for differentially heating at least one of
said first and second temperature-sensing members to indicate the
flow rate of said fluid by comparison with output signals from said
first and second temperature sensing members.
13. The invention as set forth in claim 12 in which said substrate
member extends bilaterally across said conduit.
14. The invention as set forth in claim 12 including multiple elongated
active channels in said substrate member, each of such channels
situated in a common plane, multiple ones of said channels including
a first temperature-sensing member and said means to differentially
heat said first temperature-sensing member, such that multiple readings
of fluid flow rate are obtained across said conduit.
15. The invention as set forth in claim 14 further including:
means for monitoring the flow rate in each channel; and
means for disregarding a low flow rate sensing reading in a channel
indicative of an obstructed channel.
16. The invention as set forth in claim 12 including multiple substrate
members extending in parallelism across said conduit, each substrate
member having multiple active channels containing said flow measuring
differential temperature sensors therein.
17. A thermal mass flow meter for fluids comprising:
a continuous elongated flow channel having side walls and a bottom
extending on a surface of said substrate from one edge of the substrate
to an opposed edge of the substrate;
a membrane having a top side and under side and being integral
with said substrate, said membrane extending transversely across
at least a portion of said channel and spaced from said channel
bottom such that fluid passes through said channel transversely
of and parallel to both the top side and under side of said membrane;
a first temperature sensor on said membrane;
a second temperature sensor on said substrate spaced from said
first temperature sensor; and
means for heating at least one of said first and second temperature
sensors to provide a temperature difference between said first temperature
sensor and said second temperature sensor indicative of mass fluid
18. The flow meter as set forth in claim 17 in which said membrane
bisects said channel.
19. The flow meter as set forth in claim 17 in which said substrate
a first substrate including a first flow groove having side walls
extending into said first substrate, said first flow groove extending
from one edge to an opposite edge of the first substrate, said membrane
integrally extending transversely of said first flow groove bridging
said side walls; and
a second substrate coextensive with said first substrate and including
a second flow groove extending from one edge to an opposite edge
of the second substrate, said second substrate being connectedly
sealed to said first substrate such that the said first flow groove
and second flow groove are in face-to-face relation and said membrane
divides an overall channel formed by said facing first and second
20. A thermal mass flow meter for fluids comprising a substrate;
an elongated channel extending on a surface of said substrate for
conveying a fluid, said substrate having at least two integral members
spaced from each other and extending transversely across at least
a portion of said channel;
means for forming a temperature sensor on said of said members;
means in said channel for heating said fluid to provide a temperature
difference between said sensors when fluid is being conveyed; and
wherein said substrate comprises a first element including said
channel and said integral members and a second element having an
elongated channel corresponding to said channel on said first element
said elements being bonded together whereby said integral members
divide a flow path formed by said corresponding channels.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal mass flow meter including sensors
capable of measuring the mass flow rate of liquid or gaseous fluids.
A thermal mass flow sensor operates by adding heat and measuring
a heat transfer function that is dependent upon the mass flow rate
of the fluid through the sensor. Known configurations include 1-,
2- and 3-element types. An example of the 1-element type is the
well known hot wire anemometer. The 2-element configuration has
one element upstream of the other. Both elements are heated by electric
power and cooled by the fluid. The upstream element is cooled by
the flow more than the downstream element and the measured temperature
difference is a function of fluid mass flow rate. The 3-element
configuration has a heated element situated between upstream and
downstream temperature-sensing elements. Again, the temperature
difference is the measure of flow. We are concerned herein only
with 2- and 3- element types.
2. Prior Art
A number of thermal mass flow sensor arrangements have been developed
over the past many years. Basically, these have operated on the
principle of adding heat energy to a flowing fluid and measuring
a heat transfer function and or thermal mass transport function
in two sensors spaced in the flowing fluid or on a passageway wall.
The measured temperature difference between the upstream and downstream
sensors is a function of fluid mass flow. The specific nature of
the relationship between mass flow rate and temperature difference
is complex, and depends upon fluid properties as well as sensor
geometry. The design variables include: sensor style (2- or 3-element);
sizes and shapes of heat transfer surfaces; solid thermal conductivities
such as those from element-to-element, and from element-to-wall;
and flow passage geometry affecting local velocities over the heat
transfer surfaces. Fluid properties affecting the flow signal include
those dependent upon composition (viscosity, conductivity, specific
heat, etc.) but not those dependent upon state (temperature, pressure)
because the device is intended to measure mass flow rate independent
An early version of Thomas for large flow applications employed
a flow pipe of multi-inch diameter in which two spaced sensor loops
were inserted through ports into a circular flow channel with a
separate heating element therebetween forming a three-element device.
The sensors were made of a material such as platinum, nickel or
nichrome which had a temperature-dependent electrical resistivity
which varied over a moderate temperature range. When the fluid at
one temperature having passed by the first upstream sensor is then
heated to a higher temperature, the resistivity of the downstream
second sensor is changed, the measured temperature difference between
the sensors being the measure of flow. The temperature rise in the
gas is a function of the amount of heat added, the sensor geometry
and conductivity, the mass flow rate and the properties of the gas.
More recently, a two-element mass flow meter and controller has
been developed by Tylan Corp. of Carson, Calif. in which a pair
of equally heated upstream and downstream resistance sensors made
of Nichrome wire are externally wound around a sensor flow tube.
When fluid (gas) is flowing in the tube, heat is transferred along
the line of flow from the upstream to downstream thermometers/sensors
producing a signal proportional to the gas flow. The higher the
flow, the greater the differential between the sensors. Each sensor
forms part of a bridge and amplifier circuit that produces a zero
to 5 V DC signal proportional to gas flow. This signal is compared
to a command voltage from a potentiometer or the like and an error
signal generated which can adjust a valve to change the gas flow
until a preset set point is reached. The sensor flow tube is in
this device approximately 0.010" ID.times.a length greater
than 1" which will handle flow rates of only a few standard
cc's per minute. Higher flow rates are accommodated by passing additional
gas flow around the sensor through a flow splitter designed to maintain
a constant and known bypass ratio.
A mass flow controller sold by Brooks Instruments, Hatfield, Pa.,
utilizes a horizontal bypass sensor tube with upstream and downstream
sensor coils exterior of the tube and a heater element similarly
wound between the sensors on the tube exterior. A bridge detects
the temperature difference caused by the greater heat input to the
downstream sensor and an amplifier provides an output to control
circuitry. The sensor tubes have an internal diameter of from 0.010"
to 0.060" inches. In each of the last two devices, heat conduction
is through the tube wall resulting in relatively slow long response
times, i.e., several seconds. Such devices also generally require
heating of the fluid to 100.degree.-200.degree. C. greater than
the ambient of the incoming fluid to give satisfactory performance.
In many gaseous applications, this may be above the safe temperature
limit of the gas or cause decomposition of the gas or reaction with
contaminants. Further, for each gas composition and flow range,
the device must be calibrated because of nonlinearities and inconsistent
SUMMARY OF THE INVENTION
The present invention is directed to a thermal mass flow meter
in which the heat sensors and heater of either the two-element or
three-element configuration are formed in-situ on an integral member
bridging across or extending within a channel of a substrate. The
bridging member is formed by a deposition and etching method followed
by etching a series of channel grooves in a substrate to form the
flow passages. The sensors and heating members are deposited on
the bridging members which are in thermal contact with the fluid
stream. In the preferred embodiment, the bridging member bisects
a flow path structure formed by the channel-containing substrate
and a matching channel-containing cover. Redundancy is provided
by employing multiple channels in the substrate/cover or additionally
in providing a series of stacked substrates/covers. The invention
also contemplates having a passive flow path in an overall flow
conduit into which a substrate/cover or substrate/substrate assemblies
are placed so that only a sample portion of the fluid passes through
the active substrate channels.
More particularly, the invention is directed to a flow meter of
fast response time having relatively small flow passageways containing
internal sensors in highly redundant configurations capable of accurate
measurement even in relatively low fluid flow ranges. The flow sensors
and passageways are preferably made by utilization of silicon substrates,
micro-etching and film deposition techniques. They find particular
utility in gas flow systems in semiconductor equipment for chemical
vapor deposition, thermal oxidation and plasma etching of semiconductor
The mass flow meter configurations of this invention summarized
above provide a highly accurate measurement of flow rates with adequate
sensitivity even over very low flow ranges of about 0.2 to 10 standard
cc/minute (SCCM). Larger flow rates may be provided for by paralleling
the necessary number of channels. For example, a 5000 SCCM unit
may be made by paralleling 500 sensors of a 10 SCCM capacity each.
Thus, it is not necessary to separately optimize thermal and mechanical
design for a variety of sensor sizes, thus saving costs. Further,
larger sizes have slower response times and heater power input would
be excessive at high flows. This invention thus provides paralleled
sensors, all with identical flow .DELTA.P characteristics and with
each carrying the same fraction of total flow, thus providing ideal
flow splitting and sampling. With these mass-produced and inexpensive
sensor elements a high-flow unit is provided with multiple sensors
which with suitable checking logic provides failure detection and
The construction of the bridging members is such as to insure substantial
thermal isolation of the fluid temperature sensors and heaters deposited
thereon from other structures within the overall flow conduit. This
is accomplished by reducing the bridging members cross-sectional
area at or near its connection with the substrate and fabrication
of the bridging members from low thermal conductivity materials
such as silicon dioxide or silicon nitride deposited on a silicon
substrate. The sensors and heaters themselves normally fabricated
of nickel or refractory metals are deposited on the bridging members.
Suitable interconnects are added by film deposition techniques,
and a passivating nonreactive film deposited over all surfaces in
contact with the measured fluid. Effects of external natural heat
convection are substantially eliminated by locating the heater internally
in the fluid channel. The resultant device thus has reduced attitude
Multiple sensor outputs are compared with each other over time
to detect changes in their relative output ratios and sense calibration
shifts. Accurate flow measurement can continue by disregarding low
output sensors (caused by plugging), and relying on the high output
sensors which have maintained their original output ratios. Fluid
flow characteristics between sensor flow passages and bypass flow
passages are matched by maintaining a specific value of ##EQU1##
for all flow passages or using a ##EQU2## of 1.0 or above to assure
sampling accuracy over a wide range of temperatures, pressures,
and gas compositions. L/D is the length over diameter ratio. Re
is the flow passage Reynolds number. The ##EQU3## ratio is indictative
of the relative importance of the linear laminar flow pressure drop
compared with the nonlinear entrance and exit losses.
Flow measurement errors due to environmental temperature gradients
are reduced through the use of a substrate with high thermal conductivity,
which maintains a uniform flow passage wall temperature within a
sensor flow passage and between flow passages. Higher heat transfer
efficiency to the measured fluid and a lower sensor element mass,
produces much faster response times than sensors with wire elements
wrapped externally on sensor tubes. Higher heat transfer efficiency
reduces sensor response time and minimizes the influence of environmental
The above attributes of this invention allow its use in batch or
continuous processing operations in the semiconductor industry wherein
deposition of various materials by chemical vapor deposition or
the etching of substrates or wafers necessitates very accurate control
over small liquid or gaseous flow rates. The fluids to be measured
may be highly reactive. Many individually controlled delivery points
may be needed to achieve uniform processing of individual wafers.
Some processing times are very short--measured in seconds--and thus
flow measuring systems must have short stabilization times and fast
response times. The present invention gives fast response times
because of the improved heat transfer and orders of magnitude lower
thermal mass of the web or bridging member than a metal sensing
tube with external coils. Response times of several milliseconds
as compared to several seconds in the prior art are contemplated.
The heat sources and sensors are in intimate contact with the gas
stream resulting in higher efficiency and lower operating temperature.
The device can effectively operate with as small as 25.degree. C.
.DELTA. temperature differential while many prior art devices have
a differential of from 100.degree.-200.degree. C. Due to the small
size of a typical embodiment of the improved flow meter, e.g., 1
in.sup.3 the overall process equipment system size may be reduced.
Prior art devices of which we are aware range from 16-66 in.sup.3.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isomeric cutaway partial view of the mass flow meter
sensor and heating structure.
FIG. 2 is a circuit schematic and block diagram of the flow meter
and a flow controller.
FIG. 3 is a cross-sectional view of a mass flow meter integrated
with a fluid bypass in a fluid conduit.
FIG. 4 is a longitudinal cross-sectional view of the flowmeter
and conduit taken on the line 4--4 in FIG. 3.
FIG. 5 is a blown-up plan view of a typical sensor deposited on
a substrate web.
FIG. 6A is a partial cross-sectional view taken on the line 6--6
of FIG. 5 of a sensor mounted on a substrate prior to channel etching.
FIG. 6B is a partial cross-sectional view taken on the line 6--6
of FIG. 5 of a sensor mounted on a substrate after channel etching.
FIG. 7 is a plan view of an alternative embodiment of a cantilevered
bridging member or web.
FIG. 8 is a partial cross-sectional view of a further embodiment
of the invention showing a series of flow meter substrates forming
a redundant array within a fluid conduit.
FIG. 9 is a graph showing the deviation of flow from linear characteristics
as a function of the ratio of the length/diameter of the flow passage(s)
to the flow passage Reynolds number.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the flow meter 10 of this invention which includes
a substrate having a first substrate panel or element 11 containing
a series of grooves 13a-d extending longitudinal thereof and a cover
plate or second element 12 having a corresponding set of grooves
14a etc. which form, upon assembly and frit bonding with substrate
panel 11 a fluid passageway 21. Longitudinal portions of the panel
11 and plate 12 between the grooves abut as at 15 so that adjoining
grooves, e.g., 13a and 13b, are sealed from each other. A pair of
bridging members or webs 16a (upstream) and 16b (downstream) are
formed across the top edge of panel 11 forming a sensor element
extending at least partially across the grooves. As later explained
in detail, the web may be of low heat and electrical conductivity
material such as silicon nitride or silicon oxide with a high thermal
coefficient of resistance material such as nickel deposited on the
web to form a temperature sensing pattern 17 on its surface. "Low
heat conductivity" as used herein means a conductivity lower
than about 0.15 watts/cm/.degree.C. Suitable electrical interconnect
patterns 18a, 18b extend from the sensor pattern to a temperature-sensing
bridge circuit (FIG. 2). Midway of upstream sensor 16a and downstream
sensor 16b is a third web 16c containing a high resistivity pattern
17a functioning as a resistance heater. In operation, heater 17a
differentially heats upstream sensor 16a and downstream sensor 16b
as fluid is conveyed in passage 21 from sensor 16a to sensor 16b.
Each of the webs 16a-c are rectangularly apertured as at 19 20
adjacent their connection with the flat surfaces 15 between the
grooves to reduce the cross-sectional area at the attach point and
limit heat conductivity from the sensor and web to the substrate
panel 11. In the preferred embodiment, each of the webs 16a-c effectively
bisects the passageway 21 formed by the facing grooves 13a-14a,
In a typical embodiment, a 0.3".times.0.3".times.0.020
mil silicon chip is provided with six grooves, each with three webs--two
containing temperature sensors and an intermediate web containing
an electrically powered resistance heater. The channels in panel
11 and cover 12 have a depth of about 2.8 mils so that the height
of the passageway 21 is about 5.6 mils. Width of the channel is
about 23 mils. The webs are about 23 mils in width across the channel,
are about 3 microns in height and extend 7 mils along the channel
length. The webs are spaced apart 35 mils center-to-center. The
channels are spaced apart 43 mils center-to-center.
FIG. 2 illustrates a controller circuit usable with the above-described
sensor. Interconnects carrying signals indicative of temperature
due to the change in sensor resistivity resultant from the differential
temperatures of the sensor patterns 17 imposed by the heater 17a
on web 16c are connected to a conventional bridge circuit 22. Circuit
22 interprets the temperature difference and sends a difference
signal to an amplifier 23 which outputs to control circuitry 26
through an analog-to-digital converter 24. Additional inputs 25a-c
from other groups of sensors in other channels may be connected
to converter 24 to provide a redundancy of flow readings. Controller
26 has a set point input which, when compared to the input from
converter 24 operates a fluid flow control valve 27 to adjust fluid
flow in the fluid conduit and channel(s) to that set point.
Small sensor flow passages are sensitive to particle contamination
which can shift calibration. Fabricating many flow sensors in a
single substrate allows economic sensor redundancy. By independently
mointoring each flow sensor channel in a series of parallel connected
channels, it is possible to compare the relative output of each
sensor combination in a channel. A decrease in output of a specific
sensor combination in a channel relative to the other channels indicates
the low output sensor channel has probably been contaminated or
partially plugged with particulate contamination.
The control circuit 26 determines if the original (uncontaminated)
flow ratios are maintained and if so, averages the readings of all
sensor groups. If the relative flow ratios change, the lower indicating
sensor group readings are disregarded and only the higher readings
from those channels which have maintained the original ratio are
depended upon to indicate flow. An alarm signal indicating contamination
is generated if the original ratio is not maintained.
A proportional/integral/derivative control algorithm is used to
servo the downstream valve and control it to the flow set point.
In prior art devices, individual calibration is needed to adjust
the controller linearity and output specifically for each sensor-bypass
combination and a specific gas composition. In this invention it
has been found that by maintaining closely the fluid flow characteristics
for both the active sensor flow passages and passive bypass passages
(FIG. 3) sampling accuracy is retained over a wide range of temperatures,
pressures and gas composition.
The calibrator 28 includes a microprocessor which plots and stores
calibration table data in a programmable read-only memory (PROM)
and includes a matrix of data points to correct for minor sensor
nonlinearity for the gases being flowed. Conversion constants for
other gases are placed in the Table and the computer will automatically
adjust the amplifier gain when other gases are selected.
Since the sensor sampling accuracy and linearity is maintained,
the output signal level can be scaled from one gas to another or
to mixtures of gases by a sensor amplifier scale factor. The flow,
when scaled, will alter the full scale range and result in changed
values of sensor .DELTA.P and valve .DELTA.P at maximum flow. If
this .DELTA.P is excessive for external reasons, then the full scale
range rating would have to be reduced from the flow rate achievable
by sensor linearity considerations above. For the same linearity
at maximum flow, ##EQU4## Where .rho..sub.s is the density under
standard conditions and Q.sub.s is the flow rate in standard cc
per minute and .mu. is the viscosity. The sensor pressure drop varies
with flow and viscosity, i.e., ##STR1##
The valve wide-open pressure drop is a nonlinear function of flow
because of the density increase at higher .DELTA.P. However, at
low .DELTA.P, the approximate variation is:
so that at the new sensor flow: ##EQU5## The above relationship
is slightly conservative; i.e., at higher valve pressure drop the
actual value would be slightly less than calculated. Since the pressure
drop scaling of the sensor and valve is the same in terms of viscosity
and density, the total pressure drop may be scaled by the same factor.
Valve control in the complete flow control system is independent
of the sensor control described earlier. The sensor and its related
regulator or control is considered to be an independent flow transducer.
The flow control circuit compares the measured flow signal with
an input command and acts to null the difference by manipulating
a valve situated in series with the sensors in the flow stream.
As is known, the dynamic control algorithm will be optimized to
give the best response and steady state accuracy of control, and
will be dependent upon the dynamic characteristics of the sensors
FIG. 3 shows a flow meter 10 as part of a flow tube bypass system.
A flow conduit 40 typically of stainless steel includes passive
bypass passages 41-44 and a flowsplitting flow meter 10 laterally
extending across and centrally mounted in the conduit. The active
sensor passages 21 in two-part substrate 45 sample a portion of
the total flow through all passages inside the wall of conduit 40.
Passages represented by 44-44 are bounded by egg-crate like baffles
or walls extending the length of the sensor 10 and form passive
fluid flow paths. Maintaining a value of ##EQU6## for all active
sensor and passive bypass flow passages assures a constant and linear
L/D is the effective length/diameter ratio of the flow passage
(for either a sensor containing passage or bypass). D is the hydraulic
diameter ##EQU7## (if the passage is not a circular cross-section).
Re is the Reynold's number of flow in the passage. If ##EQU8## becomes
too small (much below 1.0) the flow vs. .DELTA.P curve departs from
linear (laminar) because of entrance and exit loss effects.
If the flow .DELTA.P curve is linear then sensor flow is a constant
fraction of total flow (sensor plus bypass). With different gases
the scaling factor may change, but linearity is not changed as long
as the combination of gas properties and maximum flow rate does
not drive ##EQU9## too low.
If ##EQU10## is too low, the flow is not linear with pressure drop
and nonlinearity will change with pressure, temperature and gas
properties. The sensor and bypass would then be required to be of
identical length and diameter to maintain constant flow split. Otherwise
differences in ##EQU11## would result in different linearity deviations,
such that the calibration would change with gas properties and state.
The deviation from linear characteristics is shown in FIG. 9.
As can be seen in FIG. 4 the webs 16a-c act as a vane or air foil
in an airstream. The web has a thickness in one embodiment of 1-7
microns. The aerodynamic force is such that a small deviation from
null angle of attack creates an unbalanced force tending to increase
the angle of attack. If the rate of change of the aerodynamic force
with angle is greater than the rate of change of mechanical resistance
with deflection (i.e.; the spring rate) then the angle increases
until stall occurs. At that point the aerodynamic force is reduced,
the vane returns to lower angle of attack, stall recovery ensues,
and the process repeated. This cycle, which repeats rapidly, is
This potential problem in the webs may be solved by making the
torsional stiffness of the airfoil-like web and its support great
enough to prevent flutter or by positioning the airfoil's flexural
pivot point ahead of the airfoil center of pressure (25% chord point)
so that the response is in the self-correcting rather than self-exciting
direction. Calculations of the present design show that the support
stiffness can be made large enough to prevent flutter, but that
this portion of the design does require a compromise to be made
with thermal isolation from the walls. Based on the worst condition
(maximum inlet pressure and density, no downstream restriction)
the design with adequate stiffness has a thermal conductivity which
is not negligible, but can be tolerated. The compromise is improved
if the flexural pivot is moved toward the vane leading edge by making
the supports unsymmetrical.
FIG. 5 more clearly illustrates the details of a typical sensor
or heater 50. Web 61a bridges across substrate flats 51 and 52 on
either side of longitudinal channel 65. Rectangular apertures 53
54 are cut out by etching or the like at the connection areas of
web 61a to the flats 51 52 leaving connections 55a, 55b and 56a,
56b of low cross-sectional area. This aids in preventing flow of
heat flux from the web to the substrate itself. The surface of web
61a contains a pattern of reverse bends 62a-62d of high temperature
coefficient of resistance material such as nickel or chromium, on
its surface forming the temperature sensors or heater.
FIG. 6A is a cross-sectional view of the web on the line 6--6 showing
a silicon substrate 60 a silicon nitride (or silicon oxide) deposited
layer forming the bridging member or web 61a, the resistive pattern
62a-d, a further overlay of silicon nitride (or silicon oxide) 63
and a resist layer 64 which protects the web from the subsequent
etching step(s). FIG. 6B illustrates the final form of the web 61a
and patterned sensor 62a-d, the passivation-nonreactive film of
silicon nitride coating 63 thereover and an additional passivation
nonreactive film layer 67 of silicon nitride, Paralyne, trifluorochloroethylene
(Teflon), polyimides, silicon dioxide or the like to protect the
web and substrate from reactants being conveyed through the flow
meter. As a result of conventional etching step(s) groove 65 extends
longitudinally of the substrate under the webs. The webs thus bisects
the passageway formed by the matching substrate and cover grooves.
A typical fabrication sequence for a single-etched flow path silicon
substrate with a single groove, dual sensors and frit seal between
the substrate and cover is as follows: A 5000 .ANG. layer of SiO.sub.2
is thermally oxidized on the substrate. Photoresist is applied to
all sides, the resist is exposed and developed utilizing a mask
for recessing the web and interconnect paths*. The oxide mask is
etched, resist is stripped and the silicon is etched 5 microns deep
(isotropic). A 2000 .ANG. layer of Si.sub.3 N.sub.4 is deposited
by chemical vapor deposition (CVD), photoresist applied, exposed
using a top side via mask and developed*, the mask area is etched*,
resist is stripped, a topside contact pit is anisotropically etched
2.5-3 mil deep, Si.sub.3 N.sub.4 is stripped if necessary, CVD Si.sub.3
N.sub.4 apply photoresist to back side and mask front, expose a
backside via trench and develop resist*, etch mask*, strip resist,
etch bottom trenches through to the topside contact pit approximately
18.5 mils deep*, strip Si.sub.3 N.sub.4 oxidize 30000 .ANG. silicon
to silicon oxide, clean, deposit nickel front and back, apply resist
to front and mask back, expose and develop front interconnect and
resistor mask*, etch front nickel, strip resist, apply resist, expose
web and groove mask, develop resist, etch SiO.sub.2 *, etch silicon
2.8 mils deep, apply photoresist to back side, expose rear interconnect
mask, develop resist*, etch nickel*, strip resist, deposit silicon
nitride, apply resist to back side and mask front side, expose open
rear contact mask, develop resist*, etch nitride to open nickel
bonding pads and strip resist*. The notation "*" indicates
an inspection step. Resists, etchants, reactants and detailed fabrication
techniques are those commonly used in the semiconductor manufacturing
art. The above sequence of steps results in a flow meter construction
shown in FIG. 6B with electrical contacts on the rear side (opposite
the cover-facing side) with thin film nickel leads extending thereto
from the sensor patterns. The leads pass through vias selectively
etched through the silicon wafer.
FIG. 7 shows an alternative embodiment of the sensor used. Web
70 is cantilevered from one flat 76 extending on the side of etched
groove 79 out over the groove to a position where its free end is
spaced about 3 mils from the parallel flat 78. In this embodiment
the sensor resistors are shown in parallel having opposed spaced
resistive bus bars 71 73 resistive elements 72a, b, c therebetween
and leads 74 75 extend from opposite ends of the bus bars which
are connected by the vias (contact windows) and interconnect leads
to contact pads connected to the bridge circuit (FIG. 2). The resistive
path shown in FIGS. 5 and 7 may serve also as an integral heating
element while also additionally functioning as a temperature sensor
in a two-element flowmeter design.
FIG. 8 shows a stacked version of the flow meter suitable for handling
larger flow rates. In this mode, inner substrates 82 and 83 may
contain webs and grooves on one side and web-less grooves on the
other which co-act as a cover with its abutting substrate. A FIG.
1-type substrate bottom grooved cover 81 with flat top 86 is used
abutting the webbed-grooved top of substrate 82 and a substrate
84 having a webbed-groove top surface used to abut the groove-only
bottom surface of substrate 83. Substrate 84 has a flat grooveless
bottom surface 87. Any number of grooves across the substrate may
be provided dependent on the desired flow range, redundancy desired
and the conduit 85 diameter. Likewise, additional etched substrates
forming a layer substrate "sandwich" may be stacked one
on another with suitable matching channels.
While the invention has been described in terms of a silicon substrate,
other crystalline or non-crystalline materials such as silicon dioxide,
sapphire or metals such as nickel, monel or stainless steel may
be employed as the substrate or cover. Also, while the web has been
shown at the top level of the substrate, it may be formed at an
intermediate level in the substrate and an ungrooved cover plate
utilized to close the overall fluid passageway. A "web"
as used herein means a deposited support layer and resistive pathway
with or without its associate protective layer. The web may be the
channel bridging member as shown, or may extend upwardly pedestal-like
from a wall surface such as the channel bottom or may be formed
on a channel wall inner surface as a laid down layer thermally isolated
from the substrate such as by undercutting the web area.
In the preferred embodiment when the web bisects the active flow
channel, the preferred thickness or weight of the web (perpendicular
to the fluid flow path) is in the range of about 1 to about 7 microns.
In this embodiment the web thickness preferably represents from
about 1% to about 5% of the total height of the active flow channel.
Further, while the sensor control has been explained in terms of
the temperature difference resultant from the flow of fluid from
equally heated (in non-flow condition) sensors, control may be exercised
in other ways. For example, heater power may be regulated independent
of output temperature difference signal. Variations on this theme
include automatic temperature compensation achievable by the proper
selection of heater resistance temperature coefficient along with
the corresponding appropriate regulated variable (voltage, current,
power, or combination of these). Also, the temperature difference
signal may be regulated by feedback control of the heater. The heater
excitation (voltage or current) them becomes the output signal.
Further, a variable schedule of the regulated parameter in either
of the above schemes may be used as a function of other parameters
(e.g. temperature or output signal level) to achieve compensation
The above description of the advantages and the embodiments of
this invention is intended to be illustrative only, and not limiting.
Other embodiments of this invention will be apparent to those skilled
in the art in view of the above disclosure.