Technical field
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This invention relates to micro relays. The term micro relay is used for microscopic
electric switches in which a movable contact piece and a further contact piece can
be brought into and out of contact with each other. The movable contact piece is
driven by an electrostatic, electromagnetic, piezoelectric or an alternative drive
mechanism. The term micro relay can be explained in that these switches
resemble conventional relays but are much smaller and usually produced by
means of techniques similar to those of semiconductor technology and/or micro
technology. Very often, micro relays are realized in Si-based technologies.
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The present invention relates to a micro relay with a new improved structure. This
micro relay has an electrostatic drive, i.e. a drive by means of an electric capacitor.
Prior art
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Micro relays with electrostatic drives and having a movable first contact piece and
a second contact piece for switching operation are known. The movable contact
piece is supported in an elastic manner and connected to one actuation electrode
of the electrostatic drive. An electrostatic force between this first actuation
electrode and a second actuation electrode is used to deform the elastic support in
order to move the first contact piece to close and open the micro relay.
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The electrostatic type drive has the advantage of a low actuation power, however,
the disadvantage of comparatively small closing forces between the contact
pieces. This applies namely to so-called wedge-type micro relays in which the
movable contact piece is fixed at the end of a bent silicon beam. This silicon beam
constitutes the first actuation electrode whereas the second actuation electrode is
located on the substrate that also carries the second contact piece. Applying a
driving voltage between the actuation electrodes leads to an unrolling of the bent
silicon beam so that an air wedge between this beam and the substrate reduces
and moves towards the beam end until the contact pieces come into contact with
each other. In this micro relay, the silicon beam constitutes the elastic support of
the first contact piece as well as the first actuation electrode.
Summary of the invention
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The object of the present invention is to provide an improved structure of a micro
relay device with electrostatic drive.
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According to the invention, a micro relay device comprises:
- a substrate;
- a membrane mounted on said substrate and being essentially parallel to
said substrate,
said membrane having at least one slit,
said slit starting from a peripheral border of said membrane and defining a
deformable suspension beam being part of said membrane,
said membrane further comprising an actuation portion carrying a first
actuation electrode for electrostatic actuation and being separated from
said suspension beam by said slit and being connected to said suspension
beam at an end of said slit,
said membrane carrying a first contact piece for switching operation;
said micro relay device further comprising:
- a second contact piece for cooperating with said first contact piece; and
- a second actuation electrode for cooperating with said first actuation
electrode;
said actuation electrodes being adapted to deform said suspension beam
as a result of an electrostatic actuation force, so that said first contact piece
can be moved essentially perpendicular to said substrate to come into
contact with and to be separated from said second contact piece.
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Further, the invention relates to embodiments of said micro relay device as defined
in the dependent claims. Finally, the invention also relates to a method for
producing such micro relay devices as defined in claim 10.
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According to the invention, instead of a flexible beam carrying the first movable
contact piece as well as the first actuation electrode, a membrane is used. This
membrane is mounted on the substrate and includes a slit defining at least one
suspension beam for mounting the membrane. The slit has the function to allow a
deformation of the suspension beam that is independent from the membrane. E.g.
the suspension beam could be defined between said slit and a peripheral border of
the membrane or between two such slits.
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Further, the membrane comprises a portion that is used for actuation and thus
carries the first actuation electrode. Also the actuation portion is adjacent to said
slit and separated from said suspension beam by the slit. Thus, the actuation
portion and the suspension beam are connected with each other at an end of the
slit. Applying an actuation voltage creates an electrostatic force between the first
actuation electrode and a second actuation electrode that is also a part of the
micro relay device. This actuation force is applied to the first actuation electrode
and thus to the actuation portion. The slit already described allows the suspension
beam to be flexed independently from the actuation portion so that the suspension
beam serves as the elastic suspension.
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Consequently, the micro relay device comprises a first and a second contact piece
wherein the first contact piece is movable and carried by the membrane. It is clear
that the first contact piece should not be arranged at or near that end of the
suspension beam that is mounted to the substrate. The first contact piece could be
arranged at another part of the suspension beam or, preferably, at a part of the
actuation portion or another part of the membrane moving with the actuation
portion. Thus, by applying an electrostatic force by means of the actuation
electrodes, the movable first contact piece can be moved along with an elastic
deformation of the suspension beam. The second contact piece is arranged in a
manner that allows a closing and opening switching operation by a movement of a
first contact piece into and away from an electric contact to the second contact
piece.
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The micro relay device according to the invention is improved compared to the
above described prior art in that the actuation portion and the suspension beam
can be designed and optimised independently from each other. It is merely
necessary that they are connected to each other at the end of the slit. However,
they are not combined in one and the same flexible beam as in the prior art.
Further, according to the slit membrane design, it is possible to use comparatively
long suspension beams and also activation portions of a comparatively large area
without increasing the overall size and length of the micro relay device too much.
This is due to the fact that both are separated from each other by the slit which
can be relatively narrow. Thus, the complete design can be very compact and
economical as regards the substrate area occupied.
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By using not too short suspension beams, small elasticity constants of the
suspension beam can be realized without using too thin and thus delicate
membrane thicknesses. Further, the suspension beam can be very narrow for
similar reasons. However, in contrast to the prior art, the actuation portion can be
much broader in order to increase the active area of the actuation electrodes. In
the conventional design described above, the width of the beam was constant so
that a broad beam increasing the electrode area simultaneously increased the
elasticity constant of the elastic suspension. Thus, with the conventional design,
the only means to combine relatively large electrostatic forces with not too large
elasticity constants are very thin material and/or a substantial length of the beam
in total. However, beams being too long are not economical with regard to the
substrate area and thus to the costs of the micro relay device.
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Finally, the invention allows to take advantage of elastic properties also of the
actuation portion that can be completely different from those of the suspension
beam, as explained below.
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In summary, the invention allows an individual optimisation of the actuation portion
and the suspension beam independently from each other since they are
"decoupled" and not combined. On the other hand, by using a slit membrane, the
overall technology remains very simple.
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In a preferred version, the first contact piece is arranged neither at the suspension
beam nor at the actuation portion but at that part of the membrane connecting
both. Thus, the contact piece is near the end of the slit. In this way, the complete
actuation portion can be used for the actuation electrode. Further, its elasticity
properties can be taken advantage of as explained below.
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Namely, since the actuation portion is a part of the membrane, it will be elastic in
some sense in most cases. If the elasticity constant with regard to a flexing
movement in the direction perpendicular to the substrate (as for the suspension
beam) is higher than that of the suspension beam but still small enough to be able
to be flexed by the electrostatic forces, it can be used to increase the closing force
of the contact pieces by choosing a design in which during a closing movement
the contact pieces contact first and afterwards the actuation area can be moved
and flexed still further, e.g. until the actuation electrodes contact each other. Thus,
relatively high deformation forces of the actuation portion act as contact forces for
the closed contact of the micro relay.
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A preferred way to achieve adequate elasticity constants is by choosing adequate
widths of the suspension beam and the actuation portion whereas the membrane
thickness is constant. This allows a very simple technology in which the elasticity
design can be done by the two-dimensional design of the membrane, e.g. by the
slit structure and the form of the membrane. It is referred to the embodiments for
illustration.
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In order to achieve the above-described effect of increased closing force by means
of flexing of the actuation portion, it is preferred to mount the second contact piece
on the substrate in an elevated position compared to the second actuation
electrodes. Here, elevation is to be understood in the sense of the movement
perpendicular to the substrate. Compare the embodiments below.
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According to a further preferred version of the invention, the membrane is flat as
long as the actuation electrodes are voltage-free. Thus, the actuation forces are
used to deform the membrane from the flat condition into a flexed condition. Again,
reference is made to the embodiments.
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Preferred designs of the invention comprise at least two suspension beams and at
least two actuation portions. Further, at least four slits are preferred in order to
allow a symmetric structure of the complete membrane. Thus, two respective slits
define one respective suspension beam and separate it from two respective
actuation portions. Two examples are shown as embodiments below.
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A membrane can be made of semiconducting material, e.g. Si. This allows using
doped semiconducting regions as conducting paths and thus for wiring the first
actuation electrode and the first contact piece. The actuation electrode itself can
also be implemented by means of doped semiconducting regions. The first contact
piece itself should preferably be made of metal or at least be metal-coated.
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Of course, also insulating or metal membranes could be used. Insulating
membranes could be manufactured of SiO2 or Si3N4 or other materials. Wiring and
electrodes on the membrane could be implemented by metal conducting paths-even
on semiconducting membranes in order to increase the conductivity. Si can
be a preferred material for merely mechanical reasons and could be covered e.g.
by an insulating surface layer.
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In order to optimise the suspension beam and the actuation portion or two or more
suspension beams and two or more actuation portions, it is preferred to use a
much larger area ratio of the membrane for the actuation portion than for the
suspension beam (or a multitude of them) in order to increase the electrostatic
forces and to decrease the electricity constants of the suspension beams. This
goes along with the above mentioned increased width of the actuation portion
compared to the suspension beam in view of a higher elasticity constant of the
former compared to the latter.
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A preferred manufacturing step for producing a flat membrane being fixed to the
substrate at predetermined locations and freely supported over the substrate in its
remaining parts, is to use an Si-substrate and an Si-membrane and removing a
buried oxide under the membrane, e.g. by chemical means (liquid etching or
reactive ion etching or the like).
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However, manufacturing by buried oxide etching could be limited in view of the
device height. Especially for power switching it can be preferred to produce the
membrane separately, e.g. by back-etching of a separate wafer in order to arrive
at a thin Si-membrane or a surface SiO2 or Si3N4 membrane. The membrane could
be structured before or after etching of the wafer. A substrate of the device could
also be insulating (i.e. glass or oxide surface). The second contact piece and
actuation electrodes could be electro-plated by standard processes in order to
arrive at high contact pieces. Such standard processes could work with moulds
made with standard optical lithography in order to arrive at high aspect ratio
electroplated microstructures ("HARM", high aspect ratio microstructures). Another
example are LIGA-processes using X-ray lithography with highly collimated
synchrotron sources. Also connecting leads going through the substrate are
feasible. The membrane could be bonded to mounting posts on the substrate.
Description of preferred embodiments
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In the following, three illustrative embodiments of the invention are shown. They
are not intended to narrow the scope of the claims but rather to support the
understanding of the technical concept by means of concrete examples. Features
disclosed in the embodiments can also be preferred in other combinations.
- Figure 1 shows a first embodiment in top view;
- figure 2 shows a second embodiment in top view;
- figure 3 shows a section through the first embodiment; whereas
- figure 4 shows a section through a similar third embodiment.
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Figure 1 shows a top view onto a first embodiment of the invention. Shown is an
Si-membrane having a square shape and eight slits 2 in total being arranged in
pairs in which slits 2 are spaced from each other. Each pair starts from the
peripheral border of membrane 1 near one corner; one of the slits 2 in each pair
on each side of each corner; and extends essentially in a diagonal direction for
approximately two thirds of the distance to the centre of membrane 1. Thus, each
pair of slits 2 defines one respective suspension beam 3.
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Each suspension beam 3 is fixed to an elevated structure on the substrate and
thus to the substrate itself as will become apparent from the explanation of figure
3.
Between one respective slit 2 defining one suspension beam 3 and a next
neighbour slit 2 defining another suspension beam 3, a respective actuation
portion 4 is defined. Thus, membrane 1 of figure 1 comprises eight slits 2, four
suspension beams 3 and four actuation portions 4. Between opposed suspension
beams 3 and between opposed actuation portions 4 as well as between the ends
of slits 2, and in the centre of the square form of membrane 1, a contact portion 5
is situated. Contact portion 5 carries a first contact piece, which is symbolized by
the sketched line in figure 1. The contact piece is a metallized part of the lower
side of membrane 1. This contact piece is contacted over metal wiring on
suspension beams 3, contact piece and wiring being coated on an SiO2-layer of
membrane 1. Details are not shown. Further, actuation portions 4 comprise similar
metal areas over essentially their complete area in order to serve as first actuation
electrodes.
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An alternative membrane design is shown in figure 2. Identical reference numerals
are used for corresponding parts. The differences to the first embodiment of figure
1 are the circular shape of membrane 1 and the threefold symmetry of its structure
compared to the fourfold symmetry of figure 1. I. e. there are three pairs of two
respective slits 2 (under 120° instead of 90° in figure 1). Thus, there are three
suspension beams 3 and three actuation portions 4. Again, the centre area is used
as contact portion 5.
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It is apparent that in both designs the overall design is highly symmetrical and
comprises actuation portions 4 constituting the main part of the overall area of
membrane 1. Further, actuation portions 4 are much broader than suspension
beams 3. Since membranes 1 have a substantially constant thickness, the elastic
properties of actuation portions and suspension beams 3 are different from each
other.
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Figures 3 and 4 show sections illustrating the function of such membrane designs.
Figure 3 is a section through the design of figure 1 along a diagonal section line,
i.e. through opposed suspension beams 3. However, figure 3 also shows the
sectional form of actuation portions 4. Figure 4 shows an alternative structure as a
third embodiment.
Suspension beams 3 are fixed to elevated parts of the substrate (with reference
numeral 6 in figure 3 and 4). These elevated parts are not shown in detail but can
be parts of substrate 6 that are used to mount a free membrane 1 over substrate
6.
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Substrate 6 carries a second contact piece 7 which is pressed against the first
contact piece on the lower side of contact portion 5 of membrane 1 in figure 3 and
figure 4. Figures 3 and 4 show two different versions of second contact pieces 7
which can be unitary closed structures as in figure 4 or structures with a centre
opening as in figure 3. Figure 3 also suggests that the micro relays shown in the
embodiments could further be used for multiple switches by using multiple first and
multiple second contact pieces and adequate wiring structures.
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Substrate 6 further comprises second actuation electrodes 8 being electroplated
regions at those locations on which actuation portions 4 are pressed down in
figures 3 and 4.
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Bearing in mind that membrane 1 is flat as long as the actuation electrodes are
voltage-free, the function is as follows: Applying a voltage between the first
actuation electrodes within actuation portions 4 and second actuation electrodes 8
produces an attractive electrostatic force between these parts. Since actuation
portions 4 are much stiffer than suspension beams 3, membrane 1 lowers mainly
by means of a flexing deformation of suspension beams 3 until contact portion 5
touches second contact piece 7. Since second contact piece 7 is somewhat
elevated compared to second actuation electrodes 8 on substrate 6, the above-mentioned
attractive force now tends to deform actuation portions 4 until they
touch substrate 6. A short circuit between the actuation electrodes can be
prevented by oxide layers or the like.
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The relatively high elastic constants of actuation portions 4 provide for a
comparatively large closing force of the micro relay. On the other hand, the
beginning of the closing movement is determined by the relatively small elastic
constants of suspension beams 3, which is imported because of the small
electrostatic forces in case of larger distances between the actuation electrodes.
However, when the deformation of actuation portion 4 takes place, the distance
between the actuation electrodes is already smaller and thus the attractive forces
are much higher. (In a parallel plate capacitor, the attractive force is inversely
proportional to the square of the plate distance.) The initial distance (with flat
membrane 1) between the contact pieces is important for the voltage withstand
capacity of the micro relay. (Thus, the elevation of second contact pieces 7 will not
be very high, usually.) Please note that figures 3 and 4 are not dimensional,
neither in the relations between the different heights nor in the relations between
the heights and the widths of the structures shown. Realistic micro relays will be
much flatter and broader.
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It is clear that a section through the structure of figure 2 would look very similar to
figures 3 and 4 and thus is obsolete here.
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Further, it can be seen that the invention provides very large areas of the actuation
electrodes in compact designs allowing small pull-in voltages, and even
comparatively long suspension beams 3. The actuation electrode areas and the
design of suspension beams 3 can be optimised independently from each other
since they are decoupled parts in the membrane design.
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Finally, since even central contact part 5 of membrane 1 is somewhat flexible, it is
obvious that during the flexing movement of actuation portions 4, there will also be
some deformation of contact portion 5. This results in small lateral movements
between the first and second contact pieces which can be of high importance for
the improvement of the contact resistances since oxides and other surface layers
can be removed and the actual contact area can be increased. It is to be noted
that the real electric contact between the contact pieces only takes place at few
more or less microscopic points. These microscopic contacts can be enlarged and
improved by lateral movements on the contact force because of some plastic
deformation of the contact pieces.
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It appears also from figures 1 and 2 that for embodiments in which the elastic
constants of the actuation portions are not much higher than those of the
suspension beams, it is effectively the sum of the lengths of the suspension
beams and the actuation areas (in radial direction) that appears as effective beam
length compared to conventional designs. Thus, if the above-explained contact
force improvement is not important for a special application, the invention allows
compact designs with long effective beam lengths. In those cases, the first contact
piece could also be included in the areas of the actuation portions.
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A further important advantage of these embodiments is that an initial curvature of
membrane 1 is not required. This simplifies manufacturing substantially.
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Disadvantages can be low natural frequencies because of somewhat larger moved
masses and low elastic constants. Thus, preferred applications are in power
switching more than in signal switching. However, if the design according to the
invention is made small enough, also fast micro relays can be obtained.