WO2001013457A1 - Electrical switches - Google Patents

Abstract

An electrical switch (40) comprises: electrical contacts (18, 20, 24) and an actuator (26) for moving the contacts relative to each other between an open state and a closed state. The actuator (26) includes a deformable member (14), preferably a cantilever, which includes a piezoelectric material (26) which is operable to undergo a dimensional change in response to an electric field applied thereacross for moving the contacts (24) relative to each other. The deformable member (14) is microfabricated from a first substrate (46) which is joined to a second substrate (12) including at least some of the contacts (18, 20) to form an assembly.

Description

ELECTRICAL SWITCHES

This invention relates to electrical switches and is more especially, although not

exclusively, concerned with switches for switching microwave signals in for example

phased array antenna systems as used in communications and radar applications.

Switching of microwave signals is necessary in many applications such as cellular

communication systems, satellite direct broadcast, terrestrial broadcast and systems

which utilise phased array antenna systems. For example, in the amplification stages of

a phased array system for radar or communication, microwave power is switched through

a power amplifier during transmission and through a low noise amplifier during

reception.

Another example of switching of microwave signals is in steering a beam produced by

a phased array antenna using time delay phase shifting in which microwave signals are

switched along transmission lines of different lengths. Each transmission line leading up

to an antenna element of the array is divided into a number of sections, each section

having a relatively short transmission length and a relatively long transmission length.

A switch is provided for each section such that a microwave signal can be switched to

be transmitted through either the short length or the long length. By selectively operating

the switches along the transmission line, its transmission length can be varied thus

varying its transmission time and thereby introducing a phase delay to the microwave

signal. A beam produced by the array can be steered by controlling the phase delays in

a number of such lines. It is known to switch microwave signals using solid state switches based on MESFETs

(Metal Semiconductor Field Effect Transistor) or PIN diodes which can be connected in

series or in shunt with the transmission line. Although solid state switches can provide

rapid switching times of the order of a few nanoseconds problems exist with such

switches. Typically they cause a loss of 0.5 to 2 dB for signals in the X-Band (8-12GHz),

and due to their non-linear characteristics they exhibit power compression characteristics

at relatively low power levels (of the order 27-30 dBm for MESFETs). As a consequence

the use of solid state switches is limited to applications where significant losses can be

tolerated and to low power applications. Furthermore switches based on diodes consume

a relatively high amount of d.c. power, typically of the order of 20mA per diode which

can amount to an appreciable power consumption in applications such as phased array

antennas where several thousand such switches may be required.

It is also known to switch microwave signals using electrostatically actuated mechanical

switches. One such switch comprises a polysilicon cantilever beam which is supported

above a substrate such that a conductor at the free end of the beam overlays a gap in a

transmission line provided on the substrate. The substrate and cantilever are provided

with complementary electrodes which are used to actuate the switch. When an actuating

voltage is applied across the electrodes the electrostatic force of attraction between the

electrodes causes the cantilever to bend such that the conductor bridges the gap in the

transmission line thereby closing the switch. Electrostatic switches have also been proposed which comprise a membrane which is supported by a peripheral frame. The

switch is activated by the electrostatic force of attraction between the membrane, which

may be metallic, and a corresponding electrode on the substrate. Although electrostatic switches, due to the physical separation of the contacts, provide a higher electrical

isolation (up to 120dB) compared with that of solid state switches, they require a high

operating voltage, typically of the order of 20 to 100V, making them TTL (Transistor

Transistor Logic) and CMOS (Complementary Metal Oxide Semiconductor)

incompatible.

The present invention has arisen in an endeavour to provide an electrical switch which

is capable of switching microwave signals and which at least in part overcomes the

limitations of the known switches.

According to the present invention an electrical switch comprises: electrical contacts and

an actuator for moving the contacts relative to each other between an open state and a

closed state, and is characterised in that the actuator includes a deformable member

comprising a piezoelectric material operable to undergo a dimensional change in response

to an electric field applied thereacross for moving the contacts relative to each other and

the deformable member is microfabricated from a first substrate which is joined to a

second substrate including at least some of the contacts to form an assembly. A

particular advantage of a switch in accordance with the invention is that it only requires

low voltages, of the order of a few volts, to actuate the switch making it compatible with

TTL and CMOS logic. Being a simple mechanical device it provides a high isolation

(typically 25dB at 50GHz ) in the open state and a low-loss contact typically in the order

of about O.ldB when switching signal over a frequency range of d.c. to 50 GHZ.

Furthermore a switch according to the invention is relatively simple in construction and so can be microfabricated in miniature form having a typical dimension of 2mm x 2mm. It is also robust and reliable.

Preferably the piezoelectric material comprises lead zirconate titanate (PZT). A

particular advantage in using a piezoelectric material is that its dimensional change is

substantially linear with applied electric field. Preferably the actuator comprises a film

of said material and two or more electrodes for applying an electric field thereacross. The

piezoelectric film or similar is preferably deposited using a sol-gel technique. Preferably

the piezoelectric material is in layered form to maximise deflection while minimising the

electric field. The field applied to each layer is preferably applied in opposite directions

to maximise deflection.

In one arrangement the deformable member is planar in form such as a cantilever.

Alternatively the deformable member comprises a strip supported at opposite edges or

a membrane supported around its periphery.

In a preferred embodiment the contacts on the first or second substrate are configured

such as to provide an electrical path having a break therein and the corresponding contact

on the second substrate or first substrate is configured to electrically bridge said break

when the switch is in a closed state. Alternatively the switch further comprises a

dielectric layer on at least one of said electrical contacts such that an electrical signal is

capacitively coupled between said contacts when the switch is in a closed state.

Advantageously the first substrate and its associated deformable member and the second

substrates comprise different materials such as bulk single-crystal silicon and gallium arsenide respectively when it is intended to use the switch to switch microwave frequency

signals.

In a particularly preferred arrangement the second substrate is planer in form and

preferably comprises an integrated circuit to which the first substrate is bonded by, for

example, flip chip solder bonding. Thus according to a further aspect of the invention

an integrated circuit incorporates one or more switches as described above.

A switch in accordance with the invention find particular applications in a phased array

antenna system for transmitting and/or receiving beams of microwave radiation. For

example they can be used to switch different transmission lengths to provide variable

phase delays. Alternatively they can be used to switch an antenna between transmission

and reception modes or to switch between a transmission antenna or a reception antenna.

Phased array antennas are known in which a common antenna is connected to a

circulator which, in turn, is connected to both the power amplifier and the low noise

amplifier. Replacing the circulator with a plurality of switches according to the invention

provides improved isolation of the low noise amplifier and, due to reduced insertion loss,

less waste of transmitted power and a reduced overall noise figure.

According to yet a further aspect of the invention a phased array antenna incorporates one or more electrical switches as described above. In order that the invention may be better understood three electrical switches in

accordance with the invention will now be described by way of example only with

reference to the accompanying drawings in which:

Figure 1 is a schematic isometric representation of a cantilever switch;

Figure 2 is a side view of the cantilever switch of Figure 1;

Figure 3 is an end view of the cantilever switch of Figure 1;

Figure 4 is an isometric partial cut away of second cantilever switch in accordance with

the invention;

Figure 5 is a cross section along the line AA of the switch of Figure 4;

Figure 6 is a cross section along the line BB of the switch of Figure 4;

Figure 7 is a schematic representation of a membrane switch in accordance with the invention in an "open" state;

Figure 8 shows the membrane switch of Figure 7 in a "closed" state; and

Figure 9 is a schematic representation of a switching arrangement for use in a phased

array antenna system. A switch in accordance with a first embodiment of the invention is shown in Figures 1,

2 and 3 which show a single pole single throw (SPST) cantilever switch 10 which

comprises an electrically insulating base or substrate 12 upon which a cantilever 14 is

mounted in fixed relation and is separated from the substrate 12 by a gap 16. The

substrate 12 comprises a material which is insulating at the frequency of the signals the

switch is intended to switch. Thus when the switch is intended for switching microwave

signals, such as for example for frequencies up to 50 GFIZ, the substrate comprises a

material having a low loss at these frequencies such as for example gallium arsenide

(GaAs), gallium nitride (GaN), aluminium nitride (AIN), sapphire, quartz,

silico germanium or a ceramic material such as alumina or a low temperature co-fired

ceramic (LTCC). For operations with lower frequency signals the substrate preferably

comprises silicon (Si). The cantilever 14 can comprise the same material as the substrate

12 though for ease of manufacture it is preferably made of silicon, most preferably bulk

single-crystal silicon.

Upon the surface of the base 12 opposite the cantilever 14 there are provided electrically

conducting tracks 18, 20 which are configured to form an electrically insulating gap 22 which is located under the cantilever 14. A complementary and opposing electrically

conducting contact 24 is provided on the cantilever 14 such as to overlay the gap 22.

The contact 24 is dimensioned such that when the switch is actuated, as described below,

it bridges the gap 22 between the conducting tracks 18, 20 and electrically connects the tracks 18, 20. An actuator, generally denoted 26, is provided on an upper surface of the cantilever 14.

The actuator 26 comprises a film or thin layer of piezoelectric material 28, most

preferably lead zirconate titanate (PZT), which is sandwiched between metallic electrode

layers 30, 32. The actuator 26, more specifically the piezoelectric material layer 28, is

coupled to the cantilever 14 such that deformation of the layer 28 will cause a

corresponding deformation of the cantilever 14.

In operation of the switch 10 when a potential difference is applied between the

electrodes 30, 32 the resultant electric field across the piezoelectric film 28 causes its

length to increase relative to the length of the cantilever 14 and this causes the cantilever

14 to bend in a direction towards the base 12. The bending of the cantilever 14 brings

the contacts 24 into electrical contact with the conducting tracks 18, 20, electrically

bridging the gap 22 thereby "closing" the switch. Removal of the electric field allows

the cantilever 14 to straighten and thus "opens" the switch. To cause the switch to

"open" more rapidly the direction of the electric field can be reversed rather than being

removed thereby causing the length of the piezoelectric layer to contract. It will be

appreciated that in alternative arrangements the actuator can be located elsewhere on the

switch such as for example on the underside of the cantilever in which case it is operable to "close" the switch through a contraction in the length of piezoelectric layer.

Furthermore it will be appreciated by those skilled in the art that alternative switch

configurations can be readily devised which "open" on application of electric field and "close" on its removal or reversal.

A cantilever switch in accordance with a second embodiment of the invention is shown in Figures 4, 5 and 6 which show a SPDT cantilever switch 40 for switching the signals

from d.c to 50GHz. For consistency the same reference numerals are used to denote like

parts to those of the switch of Figure 1 to 3.

The switch 40 and its method of construction is now described by way of reference to

Figures 4, 5 and 6. The switch 40 is of hybrid construction in which the base 12 is of

suitable low loss microwave substrate, most preferably GaAs, and the cantilever is made

of bulk single-crystal silicon. On the GaAs substrate 12 the conducting tracks 18, 20,

which preferably comprise gold, are deposited using a suitable method such as metal

deposition evaporation, sputtering or plating up. The conducting tracks 18, 20 are

flanked by coplanar parallel conducting tracks 42, 44 deposited in a like manner. The

tracks 42, 44, which are unbroken, provide ground lines whilst the tracks 18, 20 provide

the switchable signal line. Typically each of the tracks 18, 20, 42, 44 is approximately

50μm wide and spaced apart by approximately 60μm. The gap 22 between the tracks 18,

20 is of the order of 200μm which, in conjunction with a 5-1 lμm gap between the

cantilever and the substrate 12, provides an electrical isolation in the "open" state of

approximately 30dB.

The cantilever 14, which is of silicon, is fabricated separately to that of the GaAs

substrate. The cantilever 14 is micromachined into a silicon substrate 46 by firstly

etching downwards from the surface to create a rectangular pit 48 having sloping sides

and a thin bottom 50 of the order of 20μm thickness. A gap 52 is then etched around

three of the edges of the bottom 50 to provide the cantilever 14 having a free end 54. In the context of this patent application micromachining is to be interpreted as meaning those processes which are used to create three dimensional structures at a μm scale such

as for example those used in the fabrication of silicon based integrated circuits and

include for example bulk chemical etching, vapour deposition, reactive ion beam sputter

deposition/etching or milling, laser machining and ion implantation.

The contact 24 and electrode 30 are deposited on the planar surface of the cantilever 14

and a film 28 (typically this is of order lμm thickness) of PZT deposited over the

electrode 30. Most conveniently the PZT film is deposited by sol-gel (solvent-gel)

deposition by spinning a layer of sol-gel over the planar surface of the substrate 46,

sintering the substrate and then etching away unwanted areas of PZT. Although the

actuator 26 can be provided on the upper surface of the cantilever it is preferred to

provide it on the lower planar surface due to its ease of fabrication. Finally the second

electrode 32 of the actuator 26 is deposited.

The switch 40 is finally assembled by joining the substrates 12 and 46 using flip chip

solder bonding. This process provides good placement accuracy and height control by

using solder bumps 56 which are located on respective metallic pads which are deposited when the tracks 18, 20, 42, 44 and electrode 32 are deposited.

The operation of the switch is identical to that described above such that electrical

actuation of the PZT film 28 causes the cantilever 14 to bend thus bringing the metal contact 24 into contact with the signal tracks 18, 20 thereby bridging the gap 22 and

closing the switch 40. In the foregoing description the switch 40 is depicted as a discrete

device, it will be appreciated however that a particular advantage of this switch 40 is that it can be readily fabricated as part of an integrated circuit in which the substrate 12 is the integrated circuit. In such an arrangement the conducting tracks 18, 20, 42, 44 and their

interconnection to the circuit are deposited on the surface of the integrated circuit and the

cantilever layer, which is fabricated separately, is then flip chip bonded onto the surface

of the circuit. The electrical connection between the electrode layers 30, 32 of the

piezoelectric actuator 26 and the circuit is conveniently achieved via the solder bumps

56 used in the flip chip bonding.

A particular advantage of a switch having a hybrid construction is that the optimum

materials and fabrication techniques can be utilised for the cantilever and substrate. For

example silicon is an ideal material for the cantilever due to its thermal stability and due

to it being inexpensive and readily machinable using micromachining techniques. Although at microwave frequencies silicon is too lossy its effect is minimised by the

small area of the contact 24 over which such a signal passes. In contrast GaAs provides

an ideal low loss substrate though it tends to be relatively more brittle and difficult to

machine. Furthermore the deposition of temperature of a piezoelectric thin film, such

as PZT, is often greater than the decomposition temperature of GaAs.

A third embodiment of the invention is shown in Figures 7 and 8 which respectively

show a membrane switch 60 in an "open" and "closed" configuration. The switch 60

comprises a base or substrate 62, typically of GaAs for microwave operation, carrying a rectangular frame 64 which circumferentially supports a membrane or diaphragm 66

fabricated of silicon, silicon nitride or silicon oxide. For ease of fabrication the

diaphragm 66 is not supported continuously around its periphery and in one embodiment the membrane is secured to the substrate using flip chip solder bonding such that the

solder bumps comprise the frame. In another embodiment the membrane comprises a

strip supported at opposite ends.

The membrane 66 carries on an upper surface 68 a PZT thin film actuator 70 and on a

lower surface 72 a first metal signal carrying line 74. The lower surface 72 and the signal

carrying line 74 face the base 62. A second metal signal carrying line 76 is provided on

an upper surface 78 of the base. Electrical actuation of the PZT actuator 70 causes the

membrane 66 to deform so that it adopts a bowed configuration (as represented in Figure 8) thereby bringing the signal carrying lines 74 and 76 closer together. In this "closed"

configuration they are electrically coupled together by capacitive coupling through a

dielectric layer 80 present on the second signal carrying line 76. In an alternative

arrangement both the signal carrying lines are provided on the substrate and the gap

between them is bridged by a corresponding contact on the underside of the membrane.

This latter contact arrangement is preferred for switches intended to operate at

microwave frequencies since it reduces the switches loss by minimising the surface area of silicon over which the signal passes.

The switches described in the foregoing can be provided as small discrete packaged

devices (typically a few millimetres square) or mounted onto microwave circuits prior to

packaging. Such micro-switches can be used as broad band switches, for example in the

range d.c to 50GHz. Their simple mechanical action provides low loss (typically less than O.ldB) and high isolation (for example 50dB at 2GHz and 27dB at 50GHz). A

separation between the contacts of 2μm in the "open" state is sufficient to achieve isolation of 25dB. Although switches according to the invention have a relatively low

switching speed (lOμs) compared to conventional PIN diode and MESFET switches

which can operate as quickly as nanoseconds their speeds are still suitable for a wide

range of applications. Piezoelectric actuation requires relatively low drive voltages,

typically in the region of 3 to 5V. Such voltages are much lower than those required for

electrostatic actuation.

The switches are suitable for use in phased array systems. They can be used to replace

a circulator and provide low loss switching of an antenna connection between a power

amplifier in transmission and a low noise amplifier in reception. Such an arrangement

is shown in Figure 9. This shows an antenna arrangement used in a phased array system.

A common transmit/receive line 90 is connected to two SPST switches 92, 94. The

output 96 of the switch 92 is connected to an input of a power amplifier 98 and the output

100 of the second switch 96 is connected to the output of a low noise amplifier 102.

Connected to the output of the power amplifier 98 and to the input of the low noise

amplifier 102 are respective SPST switches 104, 106 . The second connection of each

switch 104, 106 is connected to an antenna 108.

Each SPST switch comprises a switch according to the invention as described above. To

transmit a microwave signal from the line 92 to the antenna 108, switches 92 and 104 are

closed and switches 92, 94 are opened (as shown in Figure 9) thus connecting the common line 92 to the antenna 106 via the power amplifier 98 whilst isolating the low

noise amplifier 102. The states of the switches are reversed to receive a microwave

signal from the antenna 108 through the low noise amplifier 102. Alternatively switches according to the invention can be used to switch between short

and long transmission lengths to provide a transmission line having a variable length and

thus phase delay. An arrangement of about 1000 antenna elements each fed by a track

having about five or six sections having different transmission lengths can be used in a phased array system for satellite tracking, for example tracking satellites in a low earth

orbit (LEO). Satellite communications typically occur in the region of 18 GHZ to

40GHz.

It will be appreciated by those skilled in the art that modifications can be made to the

switches described which are within the scope of the invention. For example whilst in

the embodiments described the switch is actuated using a piezoelectric material other

electro-strictive materials can be used, such as for example PMN:PT (Lead Magnesium

Niobate: Lead Titonate). In the context of this patent application an electro-strictive

material is a material which undergoes a dimensional change in response to an applied

electric field. An advantage of a piezoelectric material is that its change in dimension

is substantially linear with applied electric field. It will be further appreciated that

multiple layers of electro-strictive material to which opposing polarity electric fields are

applied can be used to provide a larger deformation for a given actuating voltage.

Furthermore whilst SPDT switches have been described other forms of switches are

envisaged such as single or multiple double throw switches.

Whilst the switch of the present invention is primarily intended for switching electrical signals it is also envisaged that the switch can be used to switch other media such as for

example optical signals. For example in one embodiment the electrically conducting

tracks 18, 20 comprise optical waveguides which are fabricated within the substrate 62

and the contact 24 on the cantilever 14 is operable to act as a shutter which is movable

into and out of the gap 22, thereby blocking or allowing the passage of light between the waveguides.

Claims

1. An electrical switch (10; 40; 60) comprising: electrical contacts (18, 20, 24; 74, 76) and an actuator (26, 70) for moving the contacts relative to each other

between an open state and a closed state, characterised in that the actuator (26,

70) includes a deformable member (14; 66) comprising a piezoelectric material

(28, 70) operable to undergo a dimensional change in response to an electric field applied thereacross for moving the contacts relative to each other and the

deformable member (14; 66) is microfabricated from a first substrate (46) which

is joined to a second substrate (12; 62) including at least some of the contacts (18,

20; 76) to form an assembly.

2. A switch according to Claim 1 in which the piezoelectric material (28, 70)

comprises lead zirconate titanate.

3. A switch according to Claim 1 or Claim 2 in which the piezoelectric material is a sol-gel deposited material.

4. A switch according to any preceding claim in which the deformable member is

a membrane (66) or a cantilever (14).

5. A switch according to any one of claims 1 to 3 in which the deformable member comprises a strip supported at opposite edges.

6. A switch according to any preceding claim in which the first substrate (46) and its associated deformable member (14) comprise bulk single-crystal silicon.

7. A switch according to any preceding claim in which the second substrate (12, 62)

comprises gallium arsenide, gallium nitride, aluminium nitride, sapphire, quartz, silicon: germanium, aluminium or a low temperature co-fired ceramic material.

8. A switch according to any preceding claim in which the contacts (18, 20) on the

first (46) or second substrate (12) are configured such as to provide an electrical path having a break therein and wherein the corresponding contact (24) on the

second substrate (12) or first substrate (46) is configured to electrically bridge

said break when the switch is in a closed state.

9. A switch according to any preceding claim and further comprising a dielectric

layer (80) on at least one of said electrical contacts (76) such that an electrical

signal is capacitively coupled between said contacts (74, 76) when the switch is

in a closed state.

10. An electrical switch according to any preceding claim in which the second substate (12, 62) is planar in form.

11. A switch according to any preceding claim in which the second substrate (12, 62) comprises an integrated circuit.

12. An integrated circuit incorporating one or more switches according to any

preceding claim.

13. A phased array antenna incorporating one or more switches according to any preceding claim.