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BACKGROUND OF THIS INVENTION
The present invention concerns a micropump with at least two valves and at
least one pushing member, while said valves and said pushing members
contain which is electrically controlled. A known micropump of the prior
art is described in the article "An Electronically Controlled
piezoelectric Insulin Pump and Valves." by W. J. Spencer, e.a., IEEE
Transactions on Sonics and Ultrasonics, vol SU-25, No 3 May 1978,
p153-156.
This known pump of which the stated purpose is the administering of insulin
to diabetes patients, has some drawbacks. It is made on the basis of a
well known principle, namely the use of an inlet valve and an outlet
valve, while said inlet valve and said outlet valve can be opened and
closed selectively. The valves and the pushing member are constructed as
socalled bimorphs (two polarized piezoelectric elements joined together
and mounted between electrodes in such a way, that when an electric
voltage is applied, the constellation bends into a concave or convex
shape, depending on the polarity of the electric voltage), or as single
piezoelectric elements attached to e.g. a metal membrane. Said single
piezoelectric elements attached to a metal membrane are called
enakemesomorphs. Said enakemesomorphs also have the property that when an
electric field is applied to them, while they are in a mechanically stable
equilibrium position, e.g. a flat position, they assume a position,
deflected from the original equilibrium position, which is either convex
or concave, albeit that the deflection is less for a given electric
voltage, than that of a bimorph of the same dimensions and containing
identical piezoelectric material.
Said pump, hereafter referred to as the Spencer pump, has the disadvantage
that special precautions have to be taken to ensure that the valves, which
are surrounded by a liquid, are electrically isolated from the liquid to
prevent hydrolysis. Further, this pump has been made using conventional
assembly techniques, resulting in a pump which is rather large and heavy
and unsuitable for implantation in a patient. Moreover, these assembly
techniques are not very economical, leading to pumps which are rather
expensive.
The present invention has the purpose of providing a means in which small
amounts of a liquid or a gas can be pumped from a reservoir into a
recipient medium. The liquid could be a medical drug, a coolant or a
refrigerant, a fuel in a fuel consuming engine, or a liquid used in a
chemical or biological process. Many other applications can be conceived.
The present invention has been stimulated by the need for an implantable
insulin pump for diabetes patients, but it has a wider field of
application. As the interior of the pump consist of silicon and glass,
while the silicon is oxidized, we can state that the parts of the pump
which are exposed to the medium which has to be pumped, are hardly or not
at all susceptible to corrosion when acids, solvents, or aqueous solutions
are present in its interior. Exceptions are some alkaline materials, and
hydrogenfluoride, which will etch the silicondioxide and the glass.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a semiconductor micropump with
microvalves and a process for the fabrication thereof, which overcomes
many of the disadvantages of the previously mentioned Spencer pump.
An objective of the invention is to design a silicon micropump in such a
manner that it can be fabricated by using techniques particularly oriented
towards mass-production
An other objective of the pump with its valves is to design it so that can
be manufactured in a simple fashion, to achieve an economical product.
A third objective of the pump with valves is that it can be used as an
accurate dosage unit.
A fourth objective of the invention is to create a pump which is very
suitable to miniaturization. This is of special importance for medical
purposes, when space and weight requirements are very tight.
A fifth objective of the invention is to provide a silicon pump, which can
be totally integrated: as the mechanical parts of the pump are made of
silicon, the electronic control system of the pump can also be made in the
same silicon, by using the conventional integrated circuit techniques.
DESCRIPTION OF THE INVENTION
This invention provides a micropump, made in silicon, by etching a channel
at the front or backside of a wafer, that terminates at the valves, or at
the pushing member. A distinctive feature of the pump is that the valves
and the pushing member are physically identical, they are arranged in
series, while they are all individually controllable by the electronic
control circuit.
A pump like this can be made in particular as existing mainly of a wafer
with an etched first channel, that reaches the surface of the wafer at a
valve/pushing member and terminates there. A second channel starts at said
valve/pushing member, but is not an extension of said first channel The
connection between said first channel and said second channel is made by
means of the valve/pushing member, which can be open or closed by an
electronic control voltage.
A preferred embodiment of the pump contains a silicon wafer, in which the
channel has been etched at the backside. Said channel consists of an
etched trough, which has been closed at its open side by the attachment to
the silicon of a plate of a suitable type of glass. Said attachment in the
preferred embodiment is obtained in the process which is known as anodic
bonding or Mallory bonding.
This preferred embodiment allows the usage of several well established I.C.
processing techniques. These are known techniques, which are generally
applied in semiconductor manufacturing operations, which makes these
techniques attractive, because their possibilities and limitations are
extensively described in the technical literature.
An advantage of the preferred embodiment is that in the silicon of which
the pump has been made, the entire control circuit or a part thereof can
be integrated, because the same techniques are used for the fabrication of
the mechanical parts as for the fabrication of the eletronic circuit.
In the preferred embodiment a valve is made mainly as a surface depression,
which can be circular or rectangular, or have other shapes, the topview of
the depression is not of great importance, while in said depression a
silicon rim in the form of a closed loop has been erected or has not been
depressed. This rim extends substantially to the same height as the
original surface of the wafer.
Mainly concentric with said rim, although not necessarily so, a first hole
has been etched all the way through the wafer, or at least in such a way
that this hole reaches the first connecting channel.
A similar second hole has been etched in the area between the edge of the
depression and the rim, said second hole etched in such a way that it
connects to the second channel.
In the preferred embodiment the top of the pump is closed by a glass plate
by the same process as used for closing the backside, namely anodic
bonding.
Other methods of closing can also be used, e.g. a method in which the
valves are individually treated as opposed to the integrated method of
closing can be envisioned.
In the preferred embodiment in which the top side of the wafer is covered
with glass, it has to be avoided that the glass cover will be bonded to
the rim, because this will prohibit the operation of the pump.
To ascertain that the glass plate would not be bonded to the rim, the
technique of selective bonding has been developed. In this technique, an
area of which bonding to the glass plate is not desired, is left with a
coating of SiO2. As the bonding effect is accomplished by the presence of
a strong electric field in the gap between the glass and the silicon, a
decrease of the electric field due to the presence of the SiO2 coating on
the rim prevents the bonding of the rim to the glass.
When the glass is bonded to the wafer, the adjustment of the electric field
has to be done carefully such that the bare silicon area does bond, while
the coated area does not.
The displacement of the liquid or the gas can be accomplished by means of
piezoelectric bimorphs, as described in the paper by Spencer. In that case
the bimorphs are attached substantially at their edge or rim. Otherwise,
displacement can also be reached by means of a so called enakemesomorph
(from Greek ena ke meso=one a half). An enakemesomorph consist of a
piezoelectric element attached to a non piezoelectric material. If the
piezoelectric material expands due to an electric field, the
non-piezoelectric material resists the expansion, and as a result a moment
builds up, which bends the entire structure. An enakemesomorph can be used
to advantage, in this case because its requires no additional shielding of
the bottom electrode of the bending element, because the bottom element
now can be chosen to be glass, the glass that is used to cover the top of
the wafer.
When an enakemesomorph of which the top element is the piezoelectric
element, must bend upward, it is sufficient to apply a positive voltage to
said piezoelectric element. Under said positive voltage said element will
expand, while its nonpiezoelectric counterpart resists the expansion, and
the resulting moment will cause the bending. Associated with the bending
there is an expansion of both elements, which is beneficial for the
bending process if the element is clamped at its edges, because the bent
shape requires the beam to be longer then the flat shape. In the case of
an enakemesomorph in the form of a substantially circular or rectangular
plate, clamped at its contour, the diameter should be longer in order to
bend.
In the case that an enakemesomorph must bend downward, it is not sufficient
to apply a negative voltage to the electrode of the piezoelectric top
element, because that would cause said element to contract, and with it,
it would require the bottom element to contract too, decreasing the total
length of the enakemesomorph, which will resist the tendency of this
combination to bend downward if the enakemesomorphh is clamped at its
edges.
To avoid this problem we have invented two improvements of the
enakemesamorph.
First the solid enakemesomorph with a separate inner and outer electrodes
configuration. The inner and outer electrode are electrically in parallel
when the enakemesomorph has to bend upward, but are given the opposite
polarity when the enakemesomorph has to bend downward. As a result, the
outer electrodes, in the downward bending mode, cause the enakemesomorph
at its outer rim to expand, resulting in a convex area at the outer edge,
while the inner electrodes cause the enakemesomorph to contract in the
area between said inner electrodes, causing it to bend concave at the
location of the inner electrodes.
The area of the enakemesomorph that is contracted is now reduced from the
entire area to the area between the inner electrodes, which gives the
enakemesomorph a greater length, which facilitates the bending and allows
a greater deflection.
The second type, the doughnut type enakemesomorph, has an annular
piezoelectric body, and has a hole in the middle. The electrodes are again
separated in an outer set and an inner set.
In this configuration, the glass plate is essentially in a clamped-clamped
position, and an expansion of the piezoelectric body will not
straightforwardly result in bending in either direction, because of the
ambiguity of bending up or down, it may "oilcan", in a direction which can
not be predicted. This ambiguity can be removed, by exciting the inner and
outer electrodes different. To bend it upward, the outer area is first
electrically contracted, without any voltage to the inner set of
electrodes. This results in a concave shape of the outer area, and a small
upward deflection of the enakemesomorph. Next the inner area is expanded,
which will cause a convex form of the inner area, and increase the
deflection of the bender. Following this, the voltage on the outer area is
reversed, which causes this area to expand too, thereby increasing the
deflection.
To bend it downward, the outer area is first expanded, which will cause a
small downward deflection of the bender, after which the inner area is
expanded too, resulting in a larger downward deflection. The hole in the
center of the piezoelectric disk reduces the bending resistance, resulting
in an even larger deflection than without the hole.
It is useful to make the internal surfaces of the pump which are in contact
with the medium which is pumped, as good as possible chemically inert.
This extends the lifetime of the pump, and minimizes leakages due to
reduced wear of critical surfaces, such as sealing rings. In addition, it
reduces the possibility that the medium which is pumped is contaminated by
the material of which the pump has been manufactured.
The invention will now be described in further detail by way of example
only, and with reference to the accompanying drawings. FIG. 1 shows an
embodiment of an early prototype of the pump according to this invention,
not in integrated form, partly in perspective, and partly in exposed view.
FIG. 2 shows a perspective view of a variation of the early prototype of
the pump.
FIG. 3 shows partially a cross section and partially a perspective view of
a part containing a valve of the pump according to a preferred embodiment
of the present invention;
FIG. 4 is a cross section of an enakemesomorph, presented here to
facilitate an explanation of its operation
FIG. 5 is a drawing of a partially exposed perspective view of an other
embodiment;
FIG. 6 shows partially a cross section and partially a perspective view of
a further embodiment;
FIG. 7 is schematic drawing showing a completely integrated pump according
to this invention.
FIG. 1 shows a pump 1, which consists of a body 2, which contains a
plurality of channels 3 which comes to the surface 4 at three locations,
5, 6 and 7. The channel 3 is connected to an inlet tube 8 and an outlet
tube 9.
Hermetically connected to the surface 4, at 5, 6 and 7, piezoelectric
bimorphs are located, which can be moved by electrical means, from the
stable equilibrium position in which they are resting flat against surface
4, into a deflected position, in which said bimorphs have a convex shape.
The embodiment according to FIG. 1 operates in the following manner:
First the piezoelectric bimorph 10 is deflected electrically into a convex
form, which sucks the medium which is to be pumped from the inlet tube 8
into the cavity formed under the convex shape of the bimorph. This cavity
is now in an open connection with the channel 3 and the inlet tube 8.
Next the element 11 is deflected into a convex shape similar to element 10.
This allows said medium to flow into the cavity formed under element 11.
Next element 10 is closed by shorting the voltage of this element to zero
or by giving this element a negative voltage. An amount of said medium is
now stored under element 11.
Then element 12 is deflected, which brings channel 3 in open connection
with the outlet tube 9. The voltage of element 11 is removed, and element
11 will move downward, pushing said amount of said medium into channel 3,
from which it is free to flow past the opened element 12 into the outlet
tube 9. Finally, element 12 is put into its rest position, by removing the
voltage required to maintain it deflected. The three elements are now
closed again, and one complete cycle of the pump has been finished. A new
pumping cycle can start from this point.
This very short explanation may suffice to indicate the principle of
operation of the piezoelectric micropump according to the present
invention.
FIG. 2 shows a pump 13 of the same type as 1 according to FIG. 1. Pump 13
is different from pump 1 in that it is not rectangular in shape, but
substantially triangular, which will make it easier to fit the pump into a
circular package with essentially a low unused volume in the package.
FIG. 3 shows a construction of a part of the pump, consisting of a glass
bottom carrier 15, with local elevations 16, which support a silicon wafer
17, which carries a glass membrane 18 on which a piezoelectric disk 19 has
been attached. Said piezoelectric disk 19 has a set of inner top and
bottom electrodes 20 and 21, and a set of outer top and bottom electrodes
22 and 23. The inner electrodes 20 and 21 are connected to the electrical
control circuit by means of two connections, of which only the top one 24
is shown. The outer electrodes are connected to the control circuit by two
connections of which also only the top connection 26 is shown.
The glass membrane 18 has been attached to the top surface 28 of the
silicon wafer 17 by anodic bonding. An inlet channel 29 is located between
the glass carrier 15 and silicon wafer 17. This channel can be made by
etching it either in the wafer 17 or in the glass plate 15 or both. The
inlet channel 29 connects to an orifice 30 which has been etched all the
way through the wafer 17, said orifice 30 is again connected to a
depression 31 in the surface 28 of the wafer. Said depression 31 contains
in its center a ring shaped elevation 32, which separates depression 31
from a substantially concentric depression 33, which is connected with an
orifice 34, which has been etched, like orifice 30, through the entire
thickness of the wafer. Orifice 34 is now connected to an outlet channel
35.
It will be clear, after the explanation that accompanied FIG. 1, that the
inlet channel 29 can be connected to the outlet channel of a previous
pumping element, or to an inlet connection which is connected to a
reservoir which contains the medium which has to be pumped. Similarly, an
outlet channel 35 can be connected to the inlet channel of a following
pumping element, or to an outlet tube, which conducts the medium into the
recipient of the pump.
FIG. 4 shows a cross section of the glass membrane 18, which is anodically
bonded to the surface 28 of the silicon wafer 17, with the piezoelectric
disk 19, substantially concentric with the substantially circular contour,
which defines the non-bonded area of the glass membrane.
As is shown in FIG. 4, we can obtain a convex shape of the combination of
the glass membrane 18 and the piezoelectric disk 19, by applying for
example, given an appropriate polarization direction of the piezoelectric
material, a positive voltage between the electrodes 20 and 21 as well as
between 22 and 23 (electrode 20 and 22 more positive than 21 and 23). If
we neglect the width of the spacing between the inner an outer set, the
result of the application of the voltage will be as if said spacing did
not exist, and electrodes 20 and 22 were connected in parallel with 21 and
23, covering the entire electroded disk.
As a result of said positive voltage on electrodes 20 and 22 with respect
to electrodes 21 and 23, the piezoelectric material between these four
electrodes tends to expand sideways, while at the same time the thickness
of said piezoelectric material will decrease. Said sideways expansion is
hindered by the glass membrane 18, which resists said sideways expansion,
but can only build up sufficient resistance to the stress of the
piezoelectric disk 19 by deforming elastically. If the piezoelectric disk
is not allowed to expand to the contour it would obtain if it were totally
free, it will remain under compressive strain.
In the process of elastic deformation of the glass membrane, a tensile
force builds up in the glass with such a magnitude that an equilibrium is
formed by the compressive forces in the piezoelectric disk and the tensile
forces in the glass membrane. As these forces are not in the same plane,
but at a distance from each other, a moment is created by these forces and
the distance between their points of action.
The resulting moment in the combination of membrane 18 and piezoelectric
material 19 causes that said combination assumes a convex form. The
formation of said convex form is facilitated by the excess length which
the combination of the glass membrane 18 and the piezoelectric disk 19
have obtained, due to the sideways elongation of both of these elements.
On the other hand, if a concave shape of the combination is desired, a
negative voltage to the electrodes may be applied, in an analogous fashion
to when the positive voltage was applied when a convex shape was the goal.
The piezoelectric disk will contract sideways, and increase its thickness,
and again it will force the underlying glass membrane 18 to yield
elastically, and thus build up a moment, which causes it to assume a
concave shape. However, as the sides of the glass membrane 18 are
anodically bonded to the the surface 28 of the silicon wafer 17, a
sideways contraction of the glass will increase the tensile stress in the
glass membrane in the area between the rim of the piezoelectric disk, and
the edge of the area where the glass membrane had been anodically bonded,
and as a result the bending due to the moment will be counteracted by this
tensile stress in forming the concave shape, rather then facilitated as
was the case with the positive voltage when a convex shape was our goal.
To alleviate this counteraction of the tensile stress, the separation of
the inner and outer set of electrodes has been devised. If a concave shape
is required from the combination, a positive voltage is applied to the
outer electrode set 22 and 23, causing the material between these
electrodes to expand, and to bend convex the area of the outer set. In
addition a negative voltage is applied to the set 20 and 21, which causes
the area between the inner set of electrodes to contract, and to bend said
area concave.
The sideways expansion of the area between the outer set of electrodes can
be offset by the sideways contraction of the area between the inner set of
electrodes, leaving the total length substantially unchanged.
A positive voltage on the outer set that is considerably larger in
magnitude than the negative voltage of the inner set may even increase the
total length and thus facilitate the formation of the concave form.
When the piezoelectric disk has been made in the doughnut shape, as
indicated by the contours of the hole in the doughnut 52, the resistance
to bending in the center of the valve has been decreased, which further
facilitates the downward bending of the glass plate.
After this explanation it will be clear that a configuration with at least
three of these elements according to FIG. 3, a pumping action can be
obtained, which is indicated with arrows 36. Application of the required
voltages in the proper order can result in a pumping action as shown with
arrows 36, but it will be clear that the pumping action can be reversed,
by reversing the sequence of the voltages on the pumping elements.
FIG. 5 shows shows a pump 37 which consists of a series connection of
pumping elements according to FIG. 3.
FIG. 6 displays an integrated variation of the pumping element according to
FIG. 3. Corresponding elements of the pumping membrane are therefore
indicated with corresponding numbers.
The silicon wafer 17 carries at its upper surface 28 a wafer 37 of silicon.
This wafer contains depression 38 such that the bottom 39 of this
depression forms a membrane similar to the glass membrane 18 of FIG. 4.
The piezoelectric disk 19 consists of sputtered, or otherwise deposited
material, like zincoxide, and has again electrodes 20, 21, 22 and 23. The
bottom electrodes 21 and 23 and their connections 40 to the control
circuit 43 are deposited to the topsurface of the membrane 39. The control
circuit may consist of an integrated circuit which has been manufactured
in the same wafer 37 as the membrane, or in wafer 17 which contains the
pumpchambers.
FIG. 7 shows a top view of a fully integrated pump 44 in the form of a
layered disk similar to FIG. 6 in which a channel 46 and an inlet
connection 47 have been made as well as an outlet connection 48. Pump 44
has been provided with ten pumping elements of the type shown in FIG. 6.
The integrated circuits 43 are connected by means of conducting leads to a
central control circuit 51.
It should be noted that all material parts in contact with the liquid which
is to be pumped are chemically relatively stable, and electrically
insulated. With reference to FIG. 5, these are particularly the walls of
the inlet channel 29 and outlet channel 35; referring to FIG. 3 said
surfaces are bordering the orifices 30 and 34, the depression in the
surface 31 and 33, the elevation ring 32; further referring to FIG. 6, it
is and the lower surface of diaphragm 39. Insofar as the surfaces are of
glass they are already relatively chemically inert, except for such acids
as hydrofluoride, while the surfaces of the silicon are oxidized to SiO2
which has similar chemical properties as glass.
A special control circuit can operate the pumping elements in such a manner
that a "string of beads" of fluid are pumped, e.g. in a pump with ten
members the first five members may be consecutively opened to suck in the
fluid and store it under the first five open valves. Then the sixth member
could open, while the first member closes, which would push the liquid
under members two through six. This way the string could be passed on
through the pump. In the event that one valve would be slightly leaking,
this would have little effect on the overall performance of the pump,
because there are always be more than one members closed (in this example
five).
A configuration of two or more pumps, connected in parallel, pumping from
different sources, but into one outlet channel, can achieve mixing of the
two or more fluids pumped by said combination of the two or more pumps.
Said combination may reside physically on one and the same wafer. The
mixing ratio can be prescribed by the pumping rates of the individual
pumps of said combination of pumps.
The pumping rate can be chosen by suitable manipulation of the following
parameters:
1. the controlling voltages on the displacing members/valves,
2. the cycle frequency of the pump
3. the phase relation of the voltages controlling the pumpelements
4. the number of elements under which the fluid is stored
Failsafe characteristics of the pump can be obtained by buiding into the
valves/displacing members a static tensile or compressive stress. In the
case of the pump according to FIG. 6 the bottom of membrane 39 can be
oxidized to SiO2. This causes a compressive stress in the SiO2 film which
will cause the membrane to bend down, even without any voltage applied.
Alternatively, the inner sealing ring 32 can be made a little higher to
ensure that the the membrane in its rest position is pressed firmly
against the sealing ring.
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