Abstrict An integrated semiconductor circuit including at least two mutually
separated wafer parts (3 4) of semiconductor material and conductors
(7) for establishing electrical conection between the wafer parts
is disclosed with a thermally insulating jointing substance (9)
which is applied across the gap between the wafer parts (3 4) for
holding these parts together. The jointing substance (9) is mechanically
supporting for holding the wafer parts (3 4) together, and the
conductors (7) are so dimensioned that they have a negligible supporting
function as compared with the jointing substance (9). A method for
producing such a cirucit and the use of the circuit for providing
a flow meter for measuring the flow velocity of a flowing gaseous
or liquid medium are also disclosed.
Claims I claim:
1. A flow meter for measuring the flow velocity of a flowing gaseous
or liquid medium, said flow meter comprising an integrated semiconductor
circuit having a first wafer part (4) with a plurality of thin conductors
thereon and adapted to be heated and disposed in the flowing medium,
a second wafer part (3) with a plurality of thin electrical conductors
thereon and adapted to be disposed in the flowing medium without
being heated, a thermally insulating jointing substance (9) joining
the waver parts (3 4) for holding said first waver part (4) and
said second wafer part (3) together such that the first wafer part
(4) is supported by the second wafer part (3) by way of the jointing
substance, and a plurality of thin electrical conductors (7) connecting
electrical conductors on said first wafer part with electrical conductors
on said second wafer part and having a limited cross-sectional area
in the gap (8) to restrict thermal conduction by the conductors
(7) from the first wafer part (4) to the second wafer part (3).
2. A flow meter as claimed in claim 1 wherein said jointing substance
(9) is mechanically supporting for holding said wafer parts (3
4) together and that said conductors (7) are so dimensioned that
they have a negligible supporting function as compared with said
jointing substance (9).
3. A flow meter as claimed in claim 1 wherein said jointing substance
(9) is so applied in the gap (8) between the narrow sides (10) of
said wafer parts which are facing each other that said wafer parts
(3 4) and said jointing substance (9) form a unit of substantially
uniform thickness.
4. A flow meter as claimed in claim 1 in which said conductors
(7) consist of flat metal conductors extending between said wafer
parts, and in which said conductors directly engage said jointing
substance (9).
5. A flow meter as claimed in claim 3 wherein said jointing substance
(9), apart from being applied over said gap, is applied as a thin
protective layer over said conductors (7) and on part of one flat
side of said wafer parts (3 4).
6. A flow meter as claimed in claim 1 wherein said jointing substance
(9) is an organic material, such as polyimide.
7. A flow meter as claimed in claim 1 wherein at least one of
said wafer parts in the circuit has means (R) for heating said wafer
part (4), said means (R) being driven via said conductors (7) connected
to said wafer part (3).
8. A flow meter as claimed in claim 1 wherein at least one of
said wafer parts in the circuit has means (D1 D2) for measuring
the temperature of said wafer part.
9. A flow meter for measuring the flow velocity of a flowing gaseous
or liquid medium, said flow meter comprising an integrated semiconductor
circuit having a first wafer part (4) having a plurality of thin
conductors thereon and adapted to be heated and disposed in the
flowing medium, a second wafer part (3) having a plurality of thin
electrical conductors thereon and adapted to be disposed in the
flowing medium without being heated, a thermally insulating jointing
substance (9) in the gap (8) between the wafer parts (3 4) for
holding said first wafer part (4) and said second wafer part (3)
together such that the first wafer part (4) is supported by the
second wafer part (3) and the jointing substance, and a plurality
of thin electrical conductors (7) connecting electrical conductors
on said first wafer part with electrical conductors on said second
wafer part and having a total cross-sectional area in the gap (8)
substantially equal to the cross-sectional areas of the connected
electrical conductors to restrict thermal conduction by the conductors
(7) from the first wafer part (4) to the second wafer part (3).
Description BACKGROUND OF THE INVENTION
The present invention generally relates to an integrated semiconductor
circuit and a method of producing it, said circuit comprising at
least two mutually separated wafer parts of semiconductor material,
and conductors for establishing electrical connection between the
wafer parts which are substantially thermally insulated from each
other.
The invention further relates to the use of such an integrated
semiconductor circuit for providing a flow meter for measuring the
flow velocity of a flowing gaseous or liquid medium, said flow meter
being particularly characterized in that it comprises two mutually
separated wafer parts one of which is adapted to be heated and disposed
in the flow and the other of which is also adapted to be disposed
in the flow, however without being heated, the flow velocity of
the medium being calculated on the basis of the dissipation by thermal
convection of the first wafer part to the ambient flowing medium.
Since the invention is especially well suited for providing flow
meters or flow sensors of the above-indicated type, the following
description will be directed to the use of the integrated semiconductor
circuit and the method of producing it especially for such sensors,
but it is evident to anyone skilled in the art that this is not
the only field of application of the invention which may also be
used in all integrated circuits where it is desirable to achieve
thermal insulation between different parts of a semiconductor circuit
and electrical connection between the parts.
In semiconductor technology, a large number of integrated transducers
or sensors of semiconductor material have been developed lately,
such as temperature and flow sensors, which are manufactured along
the same principles as conventional integrated circuits, i.e. are
built up on a layer of monocrystalline semiconductor material on
which the electric components and conductors required for the operation
of the sensor have been integrated according to known techniques.
In addition to the small size of the sensor and the reduced costs
of manufacture of the sensor per se, achieved by producing several
identical units at a time (batch process), a further advantage is
gained, namely that the signal processing electronic circuits or
equivalent components associated with the sensor can be directly
integrated on the sensor in connection with the manufacture thereof,
which further reduces the cost of the sensor and enhances its reliability.
The measuring performance of the entire system is also improved
by directly integrating the signal processing electronic circuits
on the sensor. A first signal gain can then be achieved closer to
the measuring unit proper, which prevents weak signals from disadvantageously
being fed over long signal paths.
A known flow velocity sensor of this type comprises a thin, narrow
silicon beam, a base plate fixedly connected to one end of the silicon
beam and carrying bonding pads required for the operation of the
sensor, and a sensor part, also of silicon, fixedly connected to
the other end of the silicon beam. For using the sensor, said other
end of the beam is inserted through a tube wall or the like into
a flow of gas or liquid, the velocity of which should be measured,
so as to place the sensor part in the flow. The mode of operation
of the sensor, which is based on known techniques, is as follows.
The sensor part is electrically heated by means of a resistor integrated
thereon, to an upper temperature, whereupon the sensor part is allowed
to cool to a lower temperature as a result of the dissipation by
thermal convection, it being possible to repeat this process cyclically.
Both the heating time and the cooling time are indicative of the
flow velocity of the medium. A first temperature-sensitive diode
for compensating for temperature variations in the medium is integrated
in the silicon beam, and a second temperature-sensitive diode is
integrated in the sensor part, it being possible by means of this
second diode and the resistor to provide a temperature feedback
control system for controlling the temperature of the sensor part.
In order to achieve accurate flow measurements with the flow sensor
described above, it is obviously desirable that the sensor part
be thermally insulated from the silicon beam, such that the temperature
of the sensor part is substantially affected by the dissipation
by thermal convection because of the flow and not because of thermal
conduction between the sensor part and the beam.
To this end, the circuit in the above-mentioned previously known
flow sensor has been formed into two physically separated units
or wafer parts which are held together only by the conductors extending
between the wafer parts (sensor part and beam) and also providing
an electrical connection between the wafer parts. This solution
however suffers from a serious drawback. So that the conductors
should have a sufficient supporting capacity, i.e. in order that
the beam in the above-described flow sensor should be able to support
the sensor part, they must have a relatively large thickness, which
means a large conductor cross-sectional area in the joint between
the sensor part and the beam and, thus, entails undesired thermal
conduction by the conductors from the sensor part to the beam, which
in turn adversely affects the sensitivity and speed of the sensor
device. If the conductors are made thinner to prevent such undesired
thermal transfer between the sensor part and the beam, the sensor
device will become more vulnerable to impacts and more easily damaged.
There is also a risk that a sensor device of the above-defined type
will be damaged because of the pressure from the ambient flowing
medium.
SUMMARY OF THE INVENTION
In order to solve the problem outlined above, the present invention
has provided an integrated semiconductor circuit of the type mentioned
in the introduction to the specification, in which a thermally insulating
jointing substance is applied across the gap between the wafer parts
for holding these together. The jointing substance then preferably
is mechanically supporting for providing a strong bond between the
wafer parts, and the conductors are preferably so dimensioned that
they have a negligible supporting function as compared with the
jointing substance. Thus, this construction will overcome the above-mentioned
problem and achieve the above-mentioned object, i.e. provide thermal
insulation between the wafer parts, electrical connection therebetween
and mechanical interconnection thereof.
In a preferred embodiment of the semiconductor circuit according
to the invention, the jointing substance is so applied in the gap
between the opposing narrow sides of the wafer parts that the wafer
parts and the jointing substance form a unit of substantially
uniform thickness, and the conductors consist of flat metal conductors
extending between the wafer parts and, optionally by an oxide applied
to the conductors, directly engaging the jointing substance.
In addition to the above-mentioned supporting function, the jointing
substance may also have a protective function. To this end, the
jointing substance, apart from being applied in the gap, is also
applied as a thin protective layer over the conductors and, optionally,
on part of one flat side of the wafer parts.
The jointing substance, which may be any thermally insulating and
preferably mechanically supporting material, preferably consists
of an organic material, such as polyimide, which is a most heat
resistant and mechanically strong material.
When using the integrated semiconductor circuit according to the
invention for making a flow meter of the type described above, the
beam and the sensor part are held together by means of said thermally
insulating jointing substance such that the sensor part is supported
by the beam by the intermediary of the jointing substance. The sensor
part is electrically heated through said conductors, the total cross-sectional
area of which in the gap between the sensor part and the beam is
limited for restricting losses by thermal conduction of the sensor
part to the beam, via the conductors.
In order to produce the integrated semiconductor circuit described
above, the present invention provides a method therefor, in which
the circuit is first integrated with optional components and said
conductors applied in a desired pattern on the front side of the
circuit. The method of the invention is characterized by the steps
of providing a layer applied to the backside of the circuit, removing
the semiconductor material in the gap or gaps between the desired
wafer parts, whereby these parts are held together substantially
by means of said backside layer and whereby the conductors form
bridges over the gap or gaps between the wafer parts, and applying
the thermally insulating jointing substance over the gap or gaps.
By using the backside layer, which may consist of a silicon dioxide
or a metal, two advantages are gained. First, the layer serves to
hold the wafer parts together in that step of manufacture where
the semiconductor material in the gap or gaps has been removed and
the jointing substance has not yet been applied in the gap. Secondly,
in the case where the jointing substance is applied to the circuit
from above, the backside layer prevents the jointing substance from
contacting the backside of the circuit. After the application of
the jointing substance, the backside layer can be removed, whereupon
the jointing substance alone will hold the respective wafer parts
together.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail hereinbelow
with reference to a particularly preferred embodiment of an integrated
multi-part flow meter which is based on the integrated semiconductor
circuit according to the invention, and to a preferred method of
producing the semiconductor circuit according to the invention.
In the accompanying drawings, to which reference is now made,
FIG. 1 is a perspective view of a gas flow meter of a known design,
in which the different semiconductor wafer parts are only held together
by means of conductors.
FIG. 2 is a schematic side view of a joint corresponding to the
joint between the wafer parts in the flow meter of FIG. 1 but where
the wafer parts are instead joined together by using the semiconductor
circuit and the method according to the invention, and
FIGS. 3A-3E schematically illustrate the method of the invention
for producing the flow meter described in connection with FIGS.
1 and 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The known gas flow sensor or meter as shown perspective in FIG.
1 is made up of three main parts, namely a base plate 1 having five
electrical bonding pads 2 which are applied to the front side of
the base plate and by means of which the sensor can be connected
to external circuits and drive means, a silicon beam 3 extending
from the base plate 1 and having a thickness in the order of 30
.mu.m, and a sensor part 4 in the form of a small silicon chip disposed
at the end of the silicon beam 3 facing away from the base plate
1. The beam 3 and the sensor chip 4 are inserted through an opening
5 in a tube wall 6 or the like defining the flow of the gas, the
velocity of which should be measured by the flow sensor. As shown
in the Figure, the sensor chip 4 is arranged with its flat side
parallel to the direction of flow indicated by the arrow A. A resistor
R and a first diode D1 are integrated on the front side of the sensor
chip 4 and a second diode D2 is integrated on the front side of
the silicon beam 3. These three components R, D1 and D2 are electrically
connected to the bonding pads 2 by four flat metal conductors 7.
The mode of operation, which is based on a known technique, of
the illustrated sensor during gas flow measurements will now be
described in greater detail. The sensor chip 4 is heated by means
of the resistor R to an upper temperature T.sub.1 whereupon the
chip as a result of dissipation by thermal convection to the ambient
flowing gas will cool to a lower temperature T.sub.2. This heating
and cooling process can be cyclically repeated. In the measuring
process, use is made of the fact that a p-n junction in silicon,
i.e. diodes D1 and D2 changes its forward voltage drop by about
-2 mV/.degree.C. at constant current in the forward direction of
the diodes. The first diode D1 which is applied to the sensor chip
4 forms together with the resistor R a temperature feedback control
system for controlling the temperature of the sensor chip 4 in said
cycle. The other diode D2 disposed on the silicon beam 3 is used
for compensating for variations in the gas temperature. By measuring
the dissipation by thermal convection from the sensor chip 4 on
the basis of the different temperature and power consumption processes
of the sensor chip 4 during the heating and cooling cycle, it is
thus possible to obtain a measuring value of the flow velocity of
the gas.
In order to achieve a high accuracy and/or sensitivity and speed
of the sensor, it is obviously desirable to provide thermal insulation
between the beam 3 and the sensor chip 4. In the gas flow meter
of known design shown in FIG. 1 attempts have been made to solve
this problem by designing the beam 3 and the sensor chip 4 as two
physically separated units. The above-mentioned metal conductors
7 are then also used for mechanically holding together the two parts
at the gap 8. In order to obtain a sufficient supporting capacity
of the conductors, these have been reinforced by electroplating
to ensure that the final thickness of the conductors has been in
the order of 10 .mu.m or more. Such a considerable thickness of
the conductors has resulted in that the dissipation by thermal convection
from the sensor chip 4 to the beam 3 via the conductors 7 has had
a relatively great influence when measuring and calculating the
gas flow velocity, this reducing the accuracy of the sensor. As
a result of the larger mass of the conductors 7 the speed of the
sensor is also reduced.
In FIG. 2 which is a side view on a larger scale of the sensor
chip 4 and the beam 3 the above-mentioned problem inherent in the
flow sensor described above has been solved by using the integrated
semiconductor circuit and the method according to the present invention.
In the illustrated embodiment of the invention, the beam 3 and the
sensor chip 4 are still designed as two physically separated units,
but the mechanical joining of the two parts now is not accomplished
by the conductors 7 which merely serve to establish electrical
connection between the beam 3 and the chip 4 but instead by means
of a thermally insulating and mechanically supporting jointing substance
9 which is applied in the gap between the opposing narrow sides
10 of the beam 3 and the chip 4 in such a manner that the beam 3
the jointing substance 9 and the chip 4 form a unit of substantially
uniform thickness. The thickness of the conductors 7 which now
need not have any supporting function, has been reduced to about
1 .mu.m or less in the embodiment illustrated in FIG. 2. The use
of the semiconductor circuit according to the invention and the
method for producing it thus provides a strong mechanical supporting
connection as well as an electrical connection, without any undesired
heat transfer over the gap.
The method according to the invention for producing the integrated
semiconductor circuit will now be described in more detail with
reference to FIGS. 3A-3E illustrating different steps in the manufacture
of the flow sensor described in connection with FIGS. 1 and 2.
According to a known technique, the monocrystalline silicon wafer
is processed, for instance by means of a crystal orientation-dependent
silicon etch, into the shape illustrated in FIG. 3A. In this step,
it is assumed that the conductors 7 the resistor R and the diodes
D1 and D2 are integrated on the top face of the silicon wafer.
In a first step (FIG. 3A) according to the invention, there is
provided a backside layer 11 consisting of a silica layer about
1 .mu.m thick on the backside of the circuit.
In a second step (FIG. 3B), the semiconductor material is removed
by etching at the location where the joint 8 between the silicon
beam 3 and the sensor chip 4 should be formed and at the edges around
these two parts, the beam 3 being fixed at its end facing away from
the sensor chip 4 to a supporting or protective frame 14 (see FIG.
3E). By exposing the gap, the beam 3 and the sensor chip 4 will
be held together substantially by means of the backside layer 11
whereby the conductors 7 extending between the beam 3 and the sensor
chip 4 form bridges over the gap. The backside layer 11 is then
"stretched" between the chip 4 the beam 3 and said protective
frame 14.
In a third step (FIG. 3C) according to the invention, the thermally
insulating and mechanically supporting jointing substance 9 is applied
in the gap from above. The jointing substance 9 preferably consists
of an organic material, such as polyimide, which after thermosetting
provides a strong bond between the sensor chip 4 and the beam 3.
The backside layer 11 then prevents the jointing substance 9 from
penetrating down onto the underside of the circuit. As shown in
FIG. 3C, there is also applied in connection with the application
of the polyimide, a thin layer of the same jointing substance over
the sensor chip 4 the conductors 7 and the beam 3 and, in practice,
this thin layer is formed over the entire semiconductor wafer which
should later be broken up into separate sensors. Part of this layer
can be maintained after completed manufacture in order to provide
a protective layer for the flow sensor. At any rate, it is preferable
that the jointing substance, at least at the location of the joint,
projects slightly over the chip 4 and the beam 3 to ensure a strong
interconnection of these parts. (See FIG. 3D).
In a final, fourth step (FIG. 3D) according to the invention, the
backside layer 11 can be removed from the circuit or flow sensor,
whereby the sensor chip 4 is supplied by the silicon beam 3 by means
of the jointing substance 9 alone.
FIG. 3E is a top plan view of the finished flow sensor which, in
addition to the above-mentioned parts, also includes a protective
frame 12 which is fixed along fracture lines 13 to the base plate
1 at a distance from the silicon beam 3. This protective frame is
intended to be broken apart at said fracture lines prior to using
the flow sensor.
The invention must of course not be considered restricted to the
embodiment described above and illustrated in the drawings, but
may be modified in various ways within the spirit and scope of the
patent protection as claimed. For instance, the flow meter may thus
also be of a type in which the temperature of the sensor chip is
maintained constant and the current flow through the heating resistor
R thus varies in dependence upon the prevailing flow velocity. In
this case, the measuring value is calculated on the basis of power
consumption. Further, the semiconductor material may be other than
silicon, e.g. GaAs, or combinations of different semiconductor materials.
As an alternative, the jointing substance may be applied only over
the top face of the wafer parts as a layer for holding the wafer
parts together, such that no jointing substance is applied in the
gap between the narrow sides of the wafer parts facing each other.
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