Abstrict A thermal mass flow meter includes a comb-shaped structure having
a plurality closely spaced septa carried from a common spine and
disposed within the flow stream to be measured for partitioning
the flow therein into a plurality of generally parallel elongated
ribbon-shaped flow stream portions. Temperature-dependent fluid
flow sensing resistor means is coupled in heat-exchanging relation
predominantly with the narrow face of at least one of said ribbon-shaped
flow stream portions for measuring the fluid flow. In a preferred
embodiment, the comb-shaped flow partitioning structure is fabricated
by anisotropically etching the parallel flow channels through the
(110) face of diamond cubic material such as silicon. In another
embodiment, a thermal loading member is disposed in heat-exchanging
relation with the temperature dependent fluid flow sensing resistor
means for thermally loading same and pushing the critical flow rate
up to a higher flow to allow operation of the flow sensor in one
mode of operation over a wider range of flow rates.
Claims What is claimed is:
1. In a method for thermal measurement of fluid flow the steps
of:
partitioning the fluid flow to be measured into a plurality of
generally parallel elongated ribbon-shaped partitioned flow stream
portions, each ribbon-shaped stream portion having in cross-section
a broad dimension and a narrow dimension for providing rectified
flow conditions over a relatively wide range of fluid flow rates;
and
disposing a thermal flow sensor across the said narrow dimensions
of a plurality of said partitioned ribbon-shaped flow portions and
in heat-exchanging relation predominantly with the narrow dimensions
of a plurality of said ribbon-shaped flow stream portions for measuring
the fluid flow.
2. The method of claim 1 wherein the step of partitioning the flow
into a plurality of ribbon-shaped flow stream portions includes
the step of:
anisotropically etching a member to define a comb-shaped structure
having a plurality of vane-shaped septa each having major faces
and carried from a common spine member, and disposing the comb-shaped
structure in the flow stream for partitioning the flow stream into
the plurality of ribbon-shaped flow stream portions flowing inbetween
adjacent speta.
3. The method of claim 2 wherien the anisotropically etched member
comprises a wafer of diamond cubic material having (110) major faces,
with the major faces of the vane-shaped septa being defined by (111)
planes of the diamond cubic wafer material.
4. The method of claim 2 wherein the step of disposing the thermal
fluid flow sensor in heat-exchanging relation predominantly with
the narrow face of at least one of the ribbon-shaped flow stream
portions includes the step of supporting the thermal fluid flow
sensor upon a web extending across the fluid flow stream portions
flowing inbetween adjacent septa.
5. The method of claim 1 wherein the step of disposing the thermal
fluid flow sensor in heat-exchanging relation predominantly with
the narrow dimensions of a plurality of said ribbon-shaped flow
stream portions includes the steps of:
disposing first and second sensing resistors serially to the narrow
dimensions of the partitioned flow stream portions with the first
resistor being upstream of the second resistor;
deriving an electrical output signal from said first and second
sensing resistors which is a function of the temperature difference
between said sensing resistors produced by the flow of fluid in
the fluid stream;
said derived output signal being characterized by a critical flow
rate defined as the flow rate at which the amplitude of the temperature
difference between said first and second sensing resistors changes
from a direct function of flow rate to an inverse function of flow
rate with increasing flow rate; and
thermally loading said first and second sensing resistors as arranged
relative to said partitioned flow stream by increasing the loss
of heat from said resistors to their surrounds exclusive of said
flow stream portions so as to substantially increase the fluid flow
rate corresponding to the critical flow rate, whereby the flow regime
wherein the temperature difference between said first and second
resistors is a direct function of flow rate is extended substantially
to higher flow rates.
6. The method of claim 5 wherein the step of thermally loading
said first and second sensing resistors includes the step of coupling
a thermally conductive element in heat-exchanging relation with
said first and second resistors.
7. In a method for making a fluid flow sensor, the steps of:
forming a layer of electrically and thermally insulative material
on the major face of a substrate wafer;
forming a temperature dependent resistor on said layer of thermally
insulative material;
selectively etching said substrate material from underneath said
insulative layer so as to provide a web of said layer of thermally
insulative material supported from an unetched portion of said substrate
material;
partitioning the fluid flow to be measured into a plurality of
generally parallel, elongated ribbon-shaped flow stream portions,
each stream portion having in cross-section a broad dimension and
a narrow dimension for providing rectified flow conditions over
a relatively wide range of fluid flow rates; and
disposing said temperature dependent resistor as supported upon
said web of thermally insulative material in heat-exchanging relation
predominantly with the narrow dimensions of a plurality of said
ribbon-shaped flow stream portions for measuring the fluid flow.
8. The method of claim 7 wherein said temperature dependent resistor
includes first and second resistor portions serially disposed relative
to the narrow dimensions of the partitioned flow stream portions,
with the first resistor portion being upstream of the second resistor
portion.
9. The method of claim 8 including the steps of:
deriving an electrical output signal from said first and second
resistor portions which is a function of the temperature difference
between said resistor portions produced by the partitioned flow
of fluid;
said derived output signal being characterized by a critical flow
rate at which the amplitude of the temperature difference between
said first and second sensing resistor portions changes from a direct
function of flow rate to an inverse function of flow rate with increasing
flow rate; and
thermally loading said first and second resistor portions as arranged
relative to said partitioned flow stream by increasing the loss
of heat from said resistor portions to their surrounds exclusive
of said flow stream portions so as to substantially increase the
critical flow rate, whereby the flow regime wherein the temperature
difference between said first and second resistor portions is a
direct function of flow rate is extended substantially to a higher
flow rate.
10. The method of claim 9 wherien the step of thermally loading
said first and second resistor portions includes the step of coupling
a thermally conductive element in heat-exchanging relation with
said first and second resistor portions.
11. In a thermal fluid flow apparatus:
partitioning means for partitioning the fluid flow to be measured
into a plurality of generally parallel, elongated narrow ribbon-shaped
partitioned flow stream portions, each stream portion having in
cross-section a broad dimension and a narrow dimension for providing
rectified flow conditions over a relatively wide range of fluid
flow rates; and
thermal fluid flow sensor means disposed in heat-exchanging relation
predominantly with the narrow dimensions of a plurality of said
partitioned ribbon-shaped flow stream portions for measuring the
fluid flow.
12. The apparatus of claim 11 wherein said partitioning means comprises
a comb-shaped structure having a plurality of vane-shaped septa
each having major faces and carried from a common spine member for
partitioning the fluid flow stream into a plurality of ribbon-shaped
flow stream portions flowing inbetween adjacent septa.
13. The apparatus of claim 12 wherein said comb-shaped structure
is made of diamond cubic material the major faces of said septa
being defined by (111) planes of the diamond cubic material.
14. The apparatus of claim 12 wherein said thermal fluid flow sensor
means includes a web extending across the flow stream portions flowing
inbetween adjacent septa; and
temperature dependent resistor means carried from said web in heat
exchanging relation with the flow of fluid between said septa.
15. The apparatus of claim 11 wherein said fluid flow sensor means
includes first and second sensing resistors serially disposed to
the narrow dimensions of the partitioned flow stream portions with
the first resistor being upstream of the second resistor;
electrical circuit means for deriving an electrical output signal
from said first and second sensing resistors which is a function
of the temperature difference between said sensing resistors produced
by the flow of fluid in the fluid stream;
said derived output signal being characterized by a critical flow
rate defined by the flow rate at which the amplitude of the temperature
difference between said first and second sensing resistors changes
from a direct function of flow rate to an inverse function of flow
rate with increasing flow rate; and
thermal loading means for thermally loading said first and second
sensing resistors as arranged relative to said partitioned flow
stream and for increasing the loss of heat from said sensing resistors
to their surrounds exclusive of said flow stream portions so as
to substantially increase the fluid flow rate corresponding to the
critical flow rate, whereby the flow regime wherein the temperature
difference between said first and second resistors is a direct function
of flow rate is extneded substantially to higher flow rates.
16. The apparatus of claim 15 wherein said thermal loading means
comprises a thermally conductive element disposed in heat-exchanging
relation with said first and second resistors.
Description BACKGROUND OF THE INVENTION
The present invention relates in general to thermal mass flow meters
and, more particularly, to such a miniature device readily fabricated
by semiconductor processing technology.
DESCRIPTION OF THE PRIOR ART
Heretofore, a thermal mass flow meter has been proposed wherein
a semiconductor substrate wafer has been etched to provide a plurality
of flow channel portions therein. Webs of electrically insulative
and thermally insulative material, such a silicon nitride and silicon
dioxide, carrying thereon temperature dependent flow sensing elements
bridged across the channels. The webs with the thermal sensing elements
carried thereon are in direct contact, i.e., immersed in the fluid
flow so as to provide a rapid response. The temperature differential
between the sensing resistors, when the device is in operation,
is indicative of the thermal mass flow rate. An example of such
a thermal mass flow meter is disclosed in European patent application
publication No. 137 687 published Apr. 17 1985 based upon U.S.
patent application Ser. No. 526860 filed Aug. 26 1983 now U.S.
Pat. No. 4542650 and U.S. Pat. No. 4472239 issued Sept. 18
1984.
While the aforementioned semiconductive mass flow meters derive
a measure of mass flow with a rapid response on the order of milliseconds,
the sensing elements sense the mass flow only over a relatively
small percentage of the full range of flow rates which it is desired
to measure, i.e., 0-10 liters/min. Accordingly, in this prior art
device, a flow splitter is employed so that a small fraction of
the flow to be measured passes through the thermal flow sensing
element and the remainder of the flow passes through a flow splitter.
The problem with this kind of an arrangement is that the useful
range of the flow measurement is limited because the division of
flow between the active flow sensing element and the remaining portion
of the flow splitter does not provide uniform splitting of the flow,
as a percentage, over a wide range of flow rates. Moreover, clogging
of the flow splitter element or of the flow sensing channel can
change the flow splitting ratio, thereby producing relatively large
erros in the measured flow rate.
Accordingly, it is desirable to provide an improved mass flow meter,
preferably of a semiconductor type and of a size commensurate with
those of the prior art device, but which directly measure a much
greater percentage of the flow to be measured. It is also desirable
to provide such an improved device with the capability of measuring
the mass flow rate essentially over its entire operating range,
in one operating mode, i.e., where the temperature differential
between upstream and downstream sensing resistors remains a direct
function of flow rate.
SUMMARY OF THE PRESENT INVENTION
The principal object of the present invention is the provision
of an improved thermal mass flow meter and method of making same.
In one feature of the present invention, a fluid flow, which is
to be measured, is partitioned into a plurality of generally parallel
elongated ribbon-shaped flow streams with the thermal flow sensor
disposed in heat-exchanging relation predominantly with the narrow
face or faces of the partitioned flow stream, whereby rectified
flow conditions are maintained over a relatively wide range of fluid
flow rates.
In another feature of the present invention, a fluid flow partitioning
means is fabricated by anisotropically etching a member such as
a (110) silicon wafer to define a comb-shaped structure having a
plurality of vane-like teeth or septa, carried from a common spine
member for partitioning the flow stream into the plurality of ribbon-shaped
flow stream portions.
In another feature of the present invention, the thermal fluid
flow sensing elements are supported upon a web extending across
the narrow dimension of the flow stream portions between adjacent
septa of the flow divider.
In another feature of the present invention, a thermal loading
member is disposed in heat-exchanging relation with the thermal
fluid flow sensing elements whereby the flow regime wherein the
temperature difference between the upstream and downstream fluid
flow sensing elements is a direct function of flow rate is extended
to substantially higher flow rates.
Other features and advantages of the present invention will become
apparent upon a perusal of the following specification taken in
connection with the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a thermal mass flow sensor incorporating
features of the present invention,
FIG. 2 is a plan view of the thermal mass flow sensing elements
of the structure of FIG. 1
FIG. 3 is an enlarged detail view of a portion of the structure
of FIG. 1 delineated by line 3--3
FIG. 4 is end view of an alternative embodiment of the mass flow
sensor of the present invention,
FIG. 5 is a view similar to that of FIG. 4 depicting an alternative
embodiment of the present invention,
FIG. 6 is an enlarged plan view of one thermal fluid flow sensing
element disposed extending across a flow channel etched into the
substrate member,
FIG. 7 is an enlarged detail view of a portion of the structure
of FIG. 4 delineated by 7--7
FIG. 8 is a view similar to that of FIG. 7 depicting an alternative
embodiment of the present invention, and
FIG. 9 is a plot of temperature differential .DELTA.T between the
upstream and downstream sensing resistive elements as a function
of flow rate F and depicting the characteristics of the prior art
and of the thermally loaded sensor of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIGS. 1 and 2 there is shown a thermal mass flow
sensor 11 incorporating features of the present invention. The mass
flow sensor 11 includes a pair of semiconductors die or chip members
12 and 13 which have been anisotropically etched to provide a plurality
of ribbon shaped flow channels 14 therein extending from the front
side to the backside of each die member 12 and 13 respectively.
The channels 14 are disposed in mutually opposed transverse registration
so that the channel portions in each of the respective die 12 and
13 provides a respective half of the composite channel 14. Lower
die 13 has a thermally insulative web 15 extending across its upper
major face for supporting and carrying thereon an upstream and downstream
temperature dependent resistors 16 and 17 respectively.
The temperature dependent resistors 16 and 17 as of nickel, meander
back and forth in a serpentine pattern so as to each occupy substantially
a one-third percentage of the area of the web 15. Bonding pads 18
are carried upon the upper face of the web 15 at the edge of the
chip 13 for making electrical connections to the opposite ends of
the resistors 16 and 17 and to a center tap node between the two
resistors 16 and 17 via lead 19. The resistors 16 and 17 are typically
connected into two halves of an electrical bridge for producing
an output proportional to the temperature difference between the
upstream and downstream resistors 16 and 17 respectively. In addition,
a heating current is passed serially through the resistors 16 and
17. The upper chip 12 preferably has a notch 21 anisotropically
etched therethrough in registration with the bonding pads 18 to
facilitate making electrical connection to the pads 18.
In a preferred embodiment, the chips 12 and 13 are fabricated in
wafer form. Thereafter the wafers are bonded together in mutually
opposed relation and diced to provide the composite thermal flow
mass sensor 11.
In a preferred embodiment, the channels 14 are anisotropically
etched through a mask of silicon oxynitride on the (110) major face
of the silicon wafer, said wafer having a thickness as of 100 to
200 mils. The longitudinal side edges of the channels 14 are aligned
with the traces of the (111) crystalographic planes with the (110)
plane. The anisotropic etchant, such as KOH or ethylene diamine
pyrocatecol (EDP) has a much faster etch rate for the (110) crystalographic
planes than for the (111) planes. As a result, the channels 14 can
be etched with a much higher aspect ratio, i.e., depth to width
ratio. In a typical example, the channels 14 in each chip 12 or
13 have a width of 0.005" to 0.050" and a depth of from
0.050" to 0.500".
The anisotropically etched, closely spaced channels 14 define flow
partitioning vanes 22 therebetween having a thickness of 0.005"
to 0.0005". As a consequence, with a reasonably sized composite
silicon die, i.e., 0.300" by 0.300" by 0.300"; the
entire flow range from essentially zero flow to 20 liters per minute
can be measured without the provision of a flow divider, i.e., the
entire flow to be measured flows through channels 14 each including
the thermal mass flow sensing elements 16 and 17. As a consequence,
measurement errors due to the provision of a separate flow divider
are eliminated, and furthermore, flow differences in the various
channels are averaged by the provision of the thermal sensing elements
16 and 17 being common to all of the channels 14. The die 12 and
13 are bonded together by means of any suitable adhesive materials
such as glass frit or epoxy resin.
Referring now to FIG. 3 the web 15 and septa 22 are shown in enlarged
scale. More particularly, the web 15 includes a support layer 23
of thermally insulative stress-free material such as a silicon oxynitride
film having a thickness as of 5000 .ANG. to two microns. The temperature-dependent
resistor material 16 or 17 as of nickel, permaloy or the like,
is deposited to a suitable thickness as of 800 .ANG. as by sputtering
upon the layer 23 and then patterned to define the resistive elements
16 and 17 respectively, by conventional photolithography and etching
techniques. Then, the patterned resistors are passivated with a
third layer 24 of stress-free silicon oxynitride material, to a
thickness substantially equal to the thickness of layer 23. These
oxynitride layers are preferably deposited by plasma enhanced chemical
vapor deposition at relatively low temperatures as of 380.degree.
C. utilizing a chemical vapor processing machine of the type disclosed
in U.S. Pat. No. 4223048 issued Sept. 16 1980. As an alternative
to depositing a stress-frame silicon oxynitride layer, the layers
23 and 24 may be made up of alternate layers of silicon dioxide
and silicon nitride proportioned such that the composite layered
structure is stress-free. Silicon nitride layers, are particularly
useful as an etch-stop layer when utilizing the anisotropic etchants
of KOH or EDP.
Referring now to FIG. 6 there is shown one specific embodiment
of a web structure 15 which is thermally decoupled from the supporting
septa 22. In this embodiment, the side edges of the web are defined
by lines paralleling the diagonals of parallelograms defined by
the traces produced by the intersections of the (111) crystalographic
planes with the (110) face of the silicon wafer or die 12 or 13.
In addition, the web 15 is slotted with arrays of slots 26 and 27
also paralleling the diagonals of such parallelograms. The slots
26 and 27 are interrupted and arranged in sets of parallel lines
with the interruption in one line of parallel slots being disposed
in transverse registration with the central regions of slots in
an adjacent parallel line of interrupted slots. In a typical example,
the slots 26 and 27 have a width of 0.005" and a length of
0.002" to 0.010". The resistor 16 or 17 is meandered through
the array of slots. One pattern for such meandering is shown in
FIG. 6.
The side edgtes of the web 15 which are disposed adjacent the
septa 22 need not be serrated, as shown in FIG. 6 but may extend
over and be contiguous with the septs 22. In this case, the slot
array 26 and 27 is merely continued over to the septs 22 such that
the respective slots 26 and 27 terminate along a common (111) trace
forming the sidewall of the septa 22. This embodiment, not shown,
provides increased strength but has increased thermal loss to the
septa 22.
The web 15 may comprise one integral member extending over the
open ends of all of the flow channels 14 in the manner as shown
in FIG. 2. As an alternative, the web 15 may be provided only under
the respective resistors 16 17 and lead 19 with openings or gaps
provided in the web spaces between resistor 16 and lead 19 and lead
19 and resistor 17.
Referring now to FIGS. 4 and 7 there is shown an alternative embodiment
to that of FIG. 1 with the exception that the lower chip member
13 is much thinner, i.e. 0.007" to 0.030". The major face
of the chip 13 is recessed by anisotropically etching through the
slots 26 and 27 as previously described with regard to FIG. 6. However,
the depth of the recess 31 is much shallower than that of the channels
14. In addition, the recesses 31 need not be open on their ends
to the flow of fluid through the channels 14 i.e., merely recessed
wells or tubs. The depth of the recess 31 determines the amount
of thermal loading presented to the sensing resistors 16 and 17.
By decreasing the depth of the recess 31 the amount of thermal
loading can be correspondingly increased.
The flow response characteristic for a thermal mass flow sensor
incorporating an upstream and downstream sensing resistor heated
by a heating current flowing therethrough and subjected to flow
is as shown in FIG. 9 at curve 32. This response of temperature
differential .DELTA.T as a function of flow F is characterized by
a low-flow regime wherein .DELTA.T is a direct function of flow
rate. The expression in the low-flow regime is as follows: ##EQU1##
where P is the injected heating power, F the mass flow, C.sub.p
the specific heat capacity of the gas, K is a system related constant
and N is a correction factor which depends on the molecular structure
of the gas. .DELTA.T increases directly with flow until a critical
point is reached indicated at 33 of FIG. 9 and thereafter the response
becomes an exponential inverse function of flow rate in accordance
with the following relation: ##EQU2## The critical point 33 corresponds
to a condition where the loss of heat to the surrounds is approximately
equal to the loss of heat to the flow. Accordingly, in the embodiment
of FIG. 4 by making the spacing between the resistor 16 and 17
and the bottom of the recess 31 small, the thermal loading is increased
thereby moving the critical point out to higher flow rates as indicated
at 34 of FIG. 9. This will allow one electronic circuit to be utilized
for measuring the flow in the low-flow regime over the entire flow
range of interest.
In a typical example, the flow range of interest is from zero to
10 liters per minute. Thus, the embodiment of FIG. 4 allows measurement
of the entire flow, i.e., there is no bypass, and also permits one
kind of electronics to be utilized for measuring flow over the entire
range of flow of interest. Furthermore, by blocking off the ends
of the recesses 31 etching tubs, the spacing between the web 15
and the bottom of the recess 31 can be very small, minimizing the
possibility of collecting debris in the space between the web 15
and the bottom of the recesses 31.
Referring now to FIG. 5 there is shown an alternative embodiment
of the present invention. The embodiment of FIG. 5 is substantially
the same as that of FIG. 4 with the exception that the lower chip
13 is from a (100) wafer, i.e., the major face of the wafer is the
(100) plane. The web 15 is carried upon the major face of chip 13
over a recess 35 provided under the web 15 preferably by undercut
anisotropic etching through slots in the web 15 similar to the array
of slots 26 and 27 shown in FIG. 6 except that the slots 26 and
27 correspond to the diagonals of parallelograms defined by the
intersection of the (111) planes with the (100) plane at the major
face of the wafer. In the embodiment of FIG. 5 the recess 35 may
be open on the ends to provide a flow channel or may be closed on
the ends to provide tubs underlying the web 15.
In both of the emboidments of FIGS. 4 and 5 the lower chip 13
may also be incorporated into a sandwich structure of the type shown
in FIG. 1 where chip 13 serves as the dividing web 15 between parallel
flow channels 14. In this embodiment, the flow channels 14 above
and below the dividing chip 13 preferably have the same or nearly
the same dimensions. In this latter embodiment, the chip 13 may
be thinned after etching of the recesses 31 so as to reduce perturbation
of the flow caused by the flow dividing chip 13. Also, chip 13 can
be etched from both major faces to provide webs 15 facing into flow
channels 14 in chip 12 on both sides.
Referring now to FIG. 8 there is shown an alternative embodiment
of the present invention wherein the web 15 including the temperature
dependent resistor 16 or 17 is carried from the ends of the septa
22 in one chip 13 or 12 and a thermal-loading member 37 such as
an undercut silicon web 37 is carried as a web over the ends of
the septa 22 in a separate wafer chip 12 or 13. In this embodiment,
the spacing between the thermal loading member 37 and the web 15
can be determined by the thickness of a spacing element 38 or the
like which can be made to have any desired thickness from a few
microns to a mil or more.
The comb-shaped fluid flow partitioning structure 12 or 13 and
22 need not be fabricated by anisotropic etching of silicon or other
diamond cubic material. As an alternative, it may be made by laminating
together alternate sheets of dissimilar materials such as plastic
and metal (acrylic and stainless steel) or aluminum and stainless
steel and dissolving (anisotropically etching) one of the materials
leaving the comb-shaped structure, such as by dissolving out the
plastic or aluminum to leave the stainless steel comb-shaped structure.
The spine of the comb would be made of an etch resistant material
such as epoxy.
The advantages of the flow sensor 11 of the present invention include
the ability to sense a substantial proportion of the flow directly
eliminating, in some cases, the need for a bypass or if the bypass
is utilized, the flow division ratio (total flow divided by flow
through the directly measured channels) is relatively small on the
order of two or three to one. Thus, the errors attributable to changes
in the flow division ratio are not multiplied to large percentage
error signals. Furthermore, by employing the thermal loading structure,
of the present invention, the low-flow regime is pushed out to much
higher flow rates, thus, permitting one kind of electronics to be
utilized for sensing the flow to much higher flow rates. |